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  • 1. Abou-Hamad, E.
    et al.
    Babaa, M. -R
    Bouhrara, M.
    Kim, Y.
    Saih, Y.
    Dennler, S.
    Mauri, F.
    Basset, J. -M
    Goze-Bac, C.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Structural properties of carbon nanotubes derived from (13)C NMR2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 16, p. 165417-Article in journal (Refereed)
    Abstract [en]

    We present a detailed experimental and theoretical study on how structural properties of carbon nanotubes can be derived from 13C NMR investigations. Magic angle spinning solid state NMR experiments have been performed on single-and multiwalled carbon nanotubes with diameters in the range from 0.7 to 100 nm and with number of walls from 1 to 90. We provide models on how diameter and the number of nanotube walls influence NMR linewidth and line position. Both models are supported by theoretical calculations. Increasing the diameter D, from the smallest investigated nanotube, which in our study corresponds to the inner nanotube of a double-walled tube to the largest studied diameter, corresponding to large multiwalled nanotubes, leads to a 23.5 ppm diamagnetic shift of the isotropic NMR line position d. We show that the isotropic line follows the relation d = 18.3/D + 102.5 ppm, where D is the diameter of the tube and NMR line position d is relative to tetramethylsilane. The relation asymptotically tends to approach the line position expected in graphene. A characteristic broadening of the line shape is observed with the increasing number of walls. This feature can be rationalized by an isotropic shift distribution originating from different diamagnetic shielding of the encapsulated nanotubes together with a heterogeneity of the samples. Based on our results, NMR is shown to be a nondestructive spectroscopic method that can be used as a complementary method to, for example, transmission electron microscopy to obtain structural information for carbon nanotubes, especially bulk samples.

  • 2.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of YBa2Cu4O8 dominated by phonon-phonon interactions1993In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 48, no 5, p. 3575-3578Article in journal (Refereed)
    Abstract [en]

    The thermal conductivity κ of dense sintered ceramic YBa2Cu4O8 in the range 30–310 K has been measured. At 100 K, κ is 10 W m-1 K-1, approaching the in-plane κ of single crystals of other high-Tc materials. κ decreases rapidly with increasing T to 7.4 W m-1 K-1 at 300 K. Fitting standard models for κ(T) to the data we find that κ is limited mainly by phonon-phonon interactions. Depending on the model used, the best fit is found for effective values of FTHETADebye between 155 and 185 K, characteristic for the acoustic phonon branches, indicating that such phonons carry most of the heat. Finally, we suggest a possible way to test the electron-phonon model for the electrical and thermal conductivities in high-Tc materials.

  • 3.
    Andersson, Britt M.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Niska, John
    Loberg, Bengt
    Thermal conductivity of polycrystalline YBa2Cu4O81994In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 49, no 6, p. 4189-4198Article in journal (Refereed)
    Abstract [en]

    We have measured the thermal conductivity κ and the thermal diffusivity a of a dense bulk ceramic polycrystalline sample of YBa2Cu4O8 (1:2:4) in the temperature range 30–300 K. We find κ≊10 W m-1 K-1 at 100 K, significantly higher than in ceramic YBa2Cu3O7-δ (1:2:3) and approaching the in-plane value for single-crystal 1:2:3, and decreasing to 7.6 W m-1 K-1 at 300 K. The data for this sample can be described by standard theories for phonon thermal conductivity of crystalline materials with boundary, phonon, and electron scattering. The higher κ in 1:2:4 as compared to 1:2:3 is, in this model, due to the smaller point defect scattering in the former. The fitted parameters for the three scattering mechanisms all agree with independent estimates based on simple models; inserting data for electric resistivity, grain size, carrier density, and lattice properties we can predict κ and its T dependence to within about 20%. We also discuss models for the phonon and electron thermal conductivities in some detail, including some second-order effects such as inelastic electron scattering and a T-dependent carrier density.

  • 4.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Chobal, O.
    Rizak, I.
    Rizak, V.
    Sabadosh, V.
    Effects of pressure and temperature on the thermal conductivity of Sn2P2S62011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 13, article id 134121Article in journal (Refereed)
    Abstract [en]

    The thermal conductivity kappa of the ferroelectric, paraelectric, and incommensurate phases of polycrystalline Sn2P2S6 has been measured in the 0.1-0.7 GPa range. The thermal conductivity kappa of the ferroelectric phase decreases with increasing pressure p. This unusual behavior, which is found in only a few other phases, is attributed to a negative Gruneisen parameter. The temperature T dependence of kappa for the ferroelectric phase (kappa similar to T-1) is well described by the Debye model for kappa, with three-phonon Umklapp scattering serving as the dominant scattering mechanism near and above the Debye temperature (similar to 100 K) up to a few tenths of degrees below the ferro- to paraelectric phase transition, where kappa(T) gradually changes and becomes temperature independent upon further heating. The thermal conductivity of the paraelectric and incommensurate phases was temperature independent and indistinguishable. Possible causes for the unusually weak T dependence at high temperatures and implications of the p dependence of kappa are discussed.

  • 5.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Filinchuk, Yaroslav
    Dmitriev, Vladimir
    Quwar, Issam
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Phase coexistence and hysteresis effects in the pressure-temperature phase diagram of NH3BH32011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 2, p. 024115-Article in journal (Refereed)
    Abstract [en]

    The potential hydrogen storage compound NH3BH3 has three known structural phases in the temperature and pressure ranges 110–300 K and 0–1.5 GPa, respectively. We report here the boundaries between, and the ranges of stability of, these phases. The phase boundaries were located by in situ measurements of the thermal conductivity, while the actual structures in selected areas were identified by in situ Raman spectroscopy and x-ray diffraction. Below 0.6 GPa, reversible transitions involving only small hysteresis effects occur between the room-temperature tetragonal plastic crystal I4mm phase and the low-temperature orthorhombic Pmn21 phase. Transformations of the I4mm phase into the high-pressure orthorhombic Cmc21 phase, occurring above 0.8 GPa, are associated with very large hysteresis effects, such that the reverse transition may occur at up to 0.5 GPa lower pressures. Below 230 K, a fraction of the Cmc21 phase is metastable to atmospheric pressure, suggesting the possibility that dense structural phases of NH3BH3, stable at room temperature, could possibly be created and stabilized by alloying or by other methods. Mixed orthorhombic Pmn21/Cmc21 phases were observed in an intermediate pressure-temperature range, but a fourth structural phase predicted by Filinchuk et al. [ Phys. Rev. B 79 214111 (2009)] was not observed in the pressure-temperature ranges of this experiment. The thermal conductivity of the plastic crystal I4mm phase is about 0.6 W m−1 K−1 and only weakly dependent on temperature, while the ordered orthorhombic phases have higher thermal conductivities limited by phonon-phonon scattering.

  • 6.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Johari, Gyan P.
    Collapse of an ice clathrate under pressure observed via thermal conductivity measurements2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, p. 174201-Article in journal (Refereed)
    Abstract [en]

    Irreversible transformation of the tetrahydrofuran ice clathrate at 130 K was studied by measuring thermal conductivity k with increase in pressure p. Initially, k increases slowly with p up to 0.75 GPa where it levels off, is roughly constant up to 0.95 GPa, then decreases up to 1.05 GPa. Pressure collapses the clathrate structure, plausibly beginning with lattice distortion, and k increases at 1.05 GPa in a sharp sigmoid-shape manner due to large densification until the transformation is complete at 1.25 GPa. This is the opposite of that found for ice whose k decreases first slowly with increase in p and then rapidly in an inverted sigmoidshape manner [O. Andersson and H. Suga, Phys. Rev. B 65, 140201 (2002)]. At 1.08 GPa and 131 K, k increases with time t (s) according to exp(t /2945), which is also the opposite of the collapse of ice [G. P. Johari and O. Andersson, Phys. Rev. B 70, 184108 (2004)]. The difference in its behavior is attributed to strong phonon scattering from the tetrahydrofuran guest molecules. k of the collapsed clathrate is 30% less than that for the collapsed ice, which is comparable with the 25% lesser k of the tetrahydrofuran-water solution from k of water at ambient pressure. On depressurizing at 130 K, k decreases progressively more rapidly and k of the collapsed state at 0.3 GPa is slightly lower than that of the as-made clathrate, showing that its original structure is not recovered.

  • 7.
    Andersson, Ove
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity of C60 at pressures up to 1 GPa and temperatures in the range 50-300 K1996In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 54, no 5, p. 3093-3100Article in journal (Refereed)
    Abstract [en]

    The thermal conductivity λ of C60 shows anomalies near 260 K and 90 K which are associated with the well-established phase transition and glass transition, respectively. Both transition temperatures increase with pressure, at the rates 120 K GPa-1 and 62 K GPa-1, respectively. With increasing temperature, λ of the simple cubic (sc) phase increased below 170 K (glasslike behavior) but decreased above. The glasslike behavior of λ is probably due to a substantial amount of lattice defects. Possible reasons for the change of sign of dλ/dT near 170 K are discussed. In the face centered cubic (fcc) phase (T≳260 K at atmospheric pressure) λ was almost independent of temperature, a behavior which is far from that of an ordered crystal (λ∝T-1 for T≳Debye temperature). This result can be attributed to the molecular orientational disorder of the fcc phase. The relaxation behavior associated with the glassy state and its unusually strong dependence on thermal history are discussed briefly, and data which support a previously reported relaxation model are presented. At room temperature, the density dependencies of λ, (∂ lnλ/∂ lnρ)T, were 5.5 and 9.5 for the fcc and sc phases, which are values typical for an orientationally disordered phase and a normal crystal phase, respectively.

  • 8. Araújo, C. Moysés
    et al.
    Ahuja, Rajeev
    Talyzin, Alexandr V
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Pressure-induced structural phase transition in NaBH42005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 72, no 5, p. 054125-Article in journal (Refereed)
    Abstract [en]

    We present a combined experimental and theoretical study of the technologically important NaBH4 compound under high pressure. Using Raman spectroscopy at room temperature, we have found that NaBH4 undergoes a structural phase transformation starting at 10.0 GPa with the pure high-pressure phase being established above 15.0 GPa. In order to compare the Raman data recorded under high pressure with the low-temperature tetragonal phase of NaBH4, we have also performed a cooling experiment. The known order-disorder transition from the fcc to the tetragonal structure was then observed. However, the new high pressure phase does not correspond to this low-temperature structure. Using first-principle calculations based on the density functional theory, we show that the high-pressure phase corresponds to the alpha-LiAlH4–type structure. We have found a good agreement between the measured and calculated transition pressures. Additionally, we present the electronic structure of both the fcc and the high-pressure phases.

  • 9.
    Baek, Seung Ki
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Minnhagen, Petter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kim, Beom Jun
    Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea.
    Kosterlitz-Thouless transition of magnetic dipoles on thetwo-dimensional plane2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 18, p. 184409-5 pArticle in journal (Refereed)
    Abstract [en]

    The universality class of a phase transition is often determined by factorslike dimensionality and inherent symmetry. We study the magneticdipole system in which the ground-state symmetry and the underlying latticestructure are coupled to each other in an intricate way. A two-dimensional(2D)square-lattice system of magnetic dipoles undergoes an order-disorder phasetransition belonging to the 2D Ising universality class.According to Prakash and Henley [Phys. Rev. B {\bf 42}, 6572 (1990)], this can berelated to the fourfold-symmetric ground states which suggests a similarityto the four-state clock model. Provided that this type ofsymmetry connection holds true, the magnetic dipoles on a honeycomb lattice,which possess sixfold-symmetric ground states, should exhibit aKosterlitz-Thouless transition in accordance with the six-state clock model.This is verified through numerical simulations in the present investigation.However, it is pointed out that this symmetry argument does not alwaysapply, which suggests that factors other than symmetry can be decisive forthe universality class of the magnetic dipole system.

  • 10. Basylko, S. A.
    et al.
    Lundow, Per-Håkan
    Condensed Matter Theory, Department of Theoretical Physics, AlbaNova University Center, KTH.
    Rosengren, A.
    One-dimensional Kondo lattice model studied through numerical diagonalization2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 77, no 7, article id 073103Article in journal (Refereed)
    Abstract [en]

    The one-dimensional Kondo lattice model is studied by means of the numerical diagonalization method. By using massively parallel computations, we were able to study lattices large enough to obtain convergent results for electron densities n <= 2/3. For such densities, an additional ferromagnetic region is found inside the paramagnetic phase. Also, a region is found where the localized spins participate in the low-energy dynamics together with the conduction electrons, thus resulting in a large Fermi surface. These results are an independent confirmation of previous density matrix renormalization group results.

  • 11.
    Bychkov, Vitaly
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Jukimenko, Olexy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Modestov, Mikhail
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nonliner dynamics of corrugated doping fronts in organic optoelectronic devices2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 85, no 24, p. 245212-Article in journal (Refereed)
    Abstract [en]

    Recently, it was demonstrated that electrochemical doping fronts in organic semiconductors exhibit a new fundamental instability growing from multidimensional perturbations [ Bychkov et al.  Phys. Rev. Lett. 107 016103 (2011)]. In the instability development, linear growth of tiny perturbations goes over into a nonlinear stage of strongly distorted doping fronts. Here we develop the nonlinear theory of the doping front instability and predict the key parameters of a corrugated doping front, such as its velocity, in close agreement with the experimental data. We show that the instability makes the electrochemical doping process considerably faster. We obtain the self-similar properties of the front shape corresponding to the maximal propagation velocity, which allows for a wide range of controlling the doping process in the experiments. The developed theory provides the guide for optimizing the performance of organic optoelectronic devices such as light-emitting electrochemical cells.

  • 12. Cheng, Benyuan
    et al.
    Li, Quanjun
    Zhang, Huafang
    Liu, Ran
    Liu, Bo
    Yao, Zhen
    Cui, Tian
    Liu, Jing
    Liu, Zhenxian
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics. State Key Laboratory of Superhard Materials, Jilin University, 130012 Changchun, China.
    Liu, Bingbing
    Pressure-induced metallization and amorphization in VO2(A) nanorods2016In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 18, article id 184109Article in journal (Refereed)
    Abstract [en]

    A metallic state enabled by the metal-insulator transition (MIT) in single crystal VO2(A) nanorods is demonstrated, which provides important physical foundation in experimental understanding of MIT in VO2. The observed tetragonal metallic state at ∼28 GPa should be interpreted as a distinct metastable state, while increasing pressure to ∼32 GPa, it transforms into a metallic amorphous state completely. The metallization is due to V 3d orbital electrons delocalization, and the amorphization is attributed to the unique variation of V-O-V bond angle. A metallic amorphous VO2 state is found under pressure, which is beneficial to explore the phase diagram of VO2. Furthermore, this work proves the occurrence of both the metallization and amorphization in octahedrally coordinated materials.

  • 13.
    Edman, Ludvig
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    McRae, Edward
    Litvin-Staszewska, E.
    Electrical resistivity of single crystal graphite under pressure: an anisotropic 3-D semimetal1998In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 57, no 11, p. 6227-6230Article in journal (Refereed)
    Abstract [en]

    The in-plane (ρa) and c-axis (ρc) resistivities of natural single-crystal graphite have been measured from 4 to 450 K at zero pressure and from 77 to 450 K under pressures up to 500 MPa. Data for ρc differ strongly from earlier results obtained on synthetic graphite in showing a strong T dependence of the pressure coefficient, while data for ρa agree well with those from previous studies. Our results can be analyzed quantitatively in a simple free-electron model, and we conclude that in contrast to quasi-two-dimensional synthetic graphite, natural single-crystal graphite is an anisotropic three-dimensional semimetal.

  • 14.
    Iwasiewicz-Wabnig, Agnieszka
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kováts, Éva
    Jalsovszky, Istvan
    Pekker, Sándor
    Polymerization of the rotor-stator compound C60-cubane under pressure.2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 2, p. 024114-Article in journal (Refereed)
    Abstract [en]

    Cubane, C8H8, can be inserted into the octahedral voids of fullerene lattices to create a family of rotor-stator compounds. We have investigated the structural phase behavior of C60 center dot C8H8 by annealing a number of samples for up to 3 h at selected temperatures in the range 380-870 K under pressures up to 2 GPa. The high-pressure treated materials were then investigated under ambient conditions using Raman spectroscopy and x-ray diffraction. C60 center dot C8H8 is found to have at least five different structural phases depending on treatment conditions. In addition to the known cubic and orthorhombic structures observed at atmospheric pressure, we find two polymeric states with pseudocubic and pseudoorthorhombic structures, respectively, based on the two original lattices and created by heating in different pressure ranges. These materials are believed to be copolymers of C60 and decomposition products of cubane. In contrast to the polymeric states of C60 the present polymer structures are determined by the topology of the original lattices rather than by the molecular structure. Above 700 K we find a carbon-rich amorphous state created when the cubane finally decomposes, releasing its hydrogen content in the form of hydrocarbons.

  • 15.
    Jacobsson, Per
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Thermal conductivity and electrical resistivity of gadolinium as functions of pressure and temperatur1989In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 40, no 14, p. 9541-9551Article in journal (Refereed)
    Abstract [en]

    The electrical resistivity ρ and the thermal diffusivity a of gadolinium have been measured as functions of T in the range 45–400 K. The thermal conductivity λ has been calculated from a and experimental data for the specific-heat capacity, cp. λ can be analyzed in terms of simple models for the lattice and electronic components above the Curie temperature TC≃291.4 K. Below TC an additional term, identified as a magnon (spin-wave) thermal conductivity λm, is found. ρ and λ have also been studied as functions of T and P in the range 150–400 K and 0–2.5 GPa. The Lorenz function L=ρλ/T increases by about 20%/GPa under pressure due to a very strong pressure dependence of the lattice thermal conductivity. The pressure coefficients of ρ and λ are -5.1×10-2 and 0.22 GPa-1, respectively, at 300 K (above TC), and 0 and 0.16 GPa-1 at 200 K (below TC). TC and the spin-reorganization temperature Tr≃219 K both decrease under pressure, at the rates -14.0 and -22.0 K/GPa, respectively. Although the magnitude of λm cannot be accurately calculated from the zero-pressure data for λ, the temperature dependence of dλ/dP allows us to distinguish between several models and assign a value of λm≃1.5 W m-1 K-1, or 16.0% of λ, at 200 K.

  • 16. Johari, G.P.
    et al.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Physics.
    Vibrational features of water’s amorph at high pressures2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 73, no 9, p. 094202-Article in journal (Refereed)
  • 17.
    Jukimenko, Olexy
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Modestov, M.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics. Chalmers, Dept Appl Phys, SE-41296 Gothenburg, Sweden.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Magnetic detonation structure in crystals of nanomagnets controlled by thermal conduction and volume viscosity2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 9, article id 094428Article in journal (Refereed)
    Abstract [en]

    Experimentally detected ultrafast spin avalanches spreading in crystals of molecular (nano) magnets [Decelle et al., Phys. Rev. Lett. 102, 027203 (2009)] have recently been explained in terms of magnetic detonation [Modestov et al., Phys. Rev. Lett. 107, 207208 (2011)]. Here magnetic detonation structure is investigated by taking into account transport processes of the crystals such as thermal conduction and volume viscosity. The transport processes result in smooth profiles of the most important thermodynamical crystal parameters, temperature, density, and pressure, all over the magnetic detonation front, including the leading shock, which is one of the key regions of magnetic detonation. In the case of zero volume viscosity, thermal conduction leads to an isothermal discontinuity instead of the shock, for which temperature is continuous while density and pressure experience jump. It is also demonstrated that the thickness of the magnetic detonation front may be controlled by applying the transverse-magnetic field, which is important for possible experimental observations of magnetic detonation.

  • 18.
    Jukimenko, Olexy
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Modestov, Mikhail
    Nordita.
    Dion, Claude M.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Chalmers University of Technology.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Counterpart of the Darrieus-Landau instability at a magnetic deflagration front2016In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 93, no 13, article id 134418Article in journal (Refereed)
    Abstract [en]

    The magnetic instability at the front of the spin avalanche in a crystal of molecular magnets is considered. This phenomenon reveals similar features with the Darrieus-Landau instability, inherent to classical combustion flame fronts. The instability growth rate and the cutoff wavelength are investigated with respect to the strength of the external magnetic field, both analytically in the limit of an infinitely thin front and numerically for finite-width fronts. The presence of quantum tunneling resonances is shown to increase the growth rate significantly, which may lead to a possible transition from deflagration to detonation regimes. Different orientations of the crystal easy axis are shown to exhibit opposite stability properties. In addition, we suggest experimental conditions that could evidence the instability and its influence on the magnetic deflagration velocity.

  • 19.
    Lundin, Anders
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Compressibility of C60 in the temperature range 150 - 335 K up to a pressure of 1 GPa1996In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 53, no 13, p. 8329-8336Article in journal (Refereed)
    Abstract [en]

    We have measured the volume of nominally 99.9% pure C60 as a function of pressure (p) in both the fcc and sc phases up to 1 GPa over the temperature range 150 to 335 K using a highly accurate piston-and-cylinder technique. At zero pressure, the room-temperature bulk modulus B(0) is 6.8 GPa for the fcc phase and about 8.7–9.5 GPa for the sc phase. For the latter phase, B(0) increases strongly with decreasing temperature to 10.4 GPa at 152 K and for both phases B increases rapidly with increasing pressure. The fcc→sc transformation is always smeared over large ranges in p. Surprisingly, above 200 MPa B is lower in the sc phase than in the fcc phase, which we explain as arising from molecular reorientation with increasing pressure. The formation of a mainly ‘‘hexagon’’-oriented, ordered phase above 0.6 GPa at low T gives rise to observable anomalies in B(p).

  • 20. McRae, Edward
    et al.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Jacobsson, Per
    Umeå University, Faculty of Science and Technology, Physics.
    Vangelisti, René
    Lelaurain, Michelle
    Raman light scattering and c-axis resistivity evidence for a pressure-induced stage transformation in PdAl2Cl8 intercalated graphite.2000In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 62, no 20, p. 13757-13766Article in journal (Refereed)
    Abstract [en]

    We have examined several samples of first- to third-stage PdAl2Cl8-intercalated graphite under hydrostatic pressures up to 1 GPa. In stage-1 highly oriented pyrolytic graphite–(HOPG) and single-crystal-graphite-based materials, the c-axis resistivity decreases sharply above a few kilobars; pressure release induces a reversible return to the initial value only in the case of the latter sample. Raman spectra taken in situ under pressure on a HOPG-based material show similarly irreversible effects. Analysis of the spectra taken on higher-stage samples leads to the conclusion that hydrostatic pressure beyond a few kilobars increases the density of the intercalate within the graphitic galleries, transforming the initial sample to a higher-stage material. Since there is no loss of intercalate, the overall intercalate-to-host charge transfer remains constant so that the Raman frequency is approximately the same for both first- and second-stage products. This is an unusual situation in which there is thus an apparent lack of Raman signature in spite of the stage change.

  • 21.
    Minnhagen, Petter
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Kim, Beom Jun
    Bernhardsson, Sebastian
    Umeå University, Faculty of Science and Technology, Physics.
    Cristofano, Gerardo
    Symmetry-allowed phase transitions realized by the two-dimensional fully frustrated XY class2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 18, p. 184432-Article in journal (Refereed)
  • 22.
    Modestov, Mikhail
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Brodin, Gert
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Matyba, Piotr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Model of the electrochemical conversion of an undoped organic semiconductor film to a doped conductor film.2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 8, p. 081203(R)-Article in journal (Refereed)
    Abstract [en]

    We develop a model describing the electrochemical conversion of an organic semiconductor (specifically, the active material in a light-emitting electrochemical cell) from the undoped nonconducting state to the doped conducting state. The model, an extended Nernst-Planck-Poisson model, takes into account both strongly concentration-dependent mobility and diffusion for the electronic charge carriers and the Nernst equation in the doped conducting regions. The standard Nernst-Planck-Poisson model is shown to fail in its description of the properties of the doping front. Solving our extended model numerically, we demonstrate that doping front progression in light-emitting electrochemical cells can be accurately described.

  • 23.
    Modestov, Mikhail
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pulsating regime of magnetic deflagration in crystals of molecular magnets2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 21, p. 214417-214416Article in journal (Refereed)
    Abstract [en]

    The stability of a magnetic deflagration front in a media of molecular magnets, such as Mn12 acetate, is considered. It is demonstrated that stationary deflagration is unstable with respect to one-dimensional perturbations if the energy barrier of the magnets is sufficiently high in comparison with the release of Zeeman energy at the front; their ratio may be interpreted as an analog to the Zeldovich number, as found in problems of combustion. When the Zeldovich number exceeds a certain critical value, a stationary deflagration front becomes unstable and propagates in a pulsating regime. Analytical estimates for the critical Zeldovich number are obtained. The linear stage of the instability is investigated numerically by solving the eigenvalue problem. The nonlinear stage is studied using direct numerical simulations. The parameter domain required for experimental observations of the pulsating regime is discussed.

  • 24. Nagel, Peter
    et al.
    Pasler, Volker
    Lebedkin, Sergei
    Soldatov, Alexander
    Meingast, Christoph
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Persson, Per-Axel
    Umeå University, Faculty of Science and Technology, Physics.
    Tanaka, T.
    Komatsu, K.
    Buga, Sergei
    Inaba, Akira
    C60 one- and two-dimensional polymers, dimers, and hard fullerite: Thermal expansion, anharmonicity, and kinetics of depolymerization1999In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 60, no 24, p. 16920-16927Article in journal (Refereed)
    Abstract [en]

    We report on high-resolution thermal expansion measurements of high-temperature-pressure treated C60 [one-dimensional (1D) and (2D) polymers and “hard fullerite”], as well as of C60 dimers and single crystal monomer C60 between 10 and 500 K. Polymerization drastically reduces the thermal expansivity from the values of monomeric C60 due to the stronger and less anharmonic covalent bonds between molecules. The expansivity of the “hard” material approaches that of diamond. The large and irreversible volume change upon depolymerization between 400 and 500 K makes it possible to study the kinetics of depolymerization, which is described excellently by a simple activated process, with activation energies of 1.9±0.1 eV (1D and 2D polymers) and 1.75±0.05 eV (dimer). Although the activation energies are very similar for the different polymers, the depolymerization rates differ by up to four orders of magnitude at a given temperature, being fastest for the dimers. Preliminary kinetic data of C70 polymers are presented as well.

  • 25.
    Olsson, Peter
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Teitel, Stephen
    University of Rochester, Rochester, NY.
    Unified Phase Diagram for the Three-Dimensional XY Model of a Point    Disordered Type-II Superconductor2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 21, p. 214503-Article in journal (Refereed)
    Abstract [en]

    We carry out extensive Monte Carlo simulations of the three-dimensional uniformly frustrated XY model with uncorrelated randomly perturbed couplings, as a model for the equilibrium behavior of an extreme type-II superconductor with quenched uncorrelated random point vortex pinning, in the presence of a uniform applied magnetic field. We map out the resulting phase diagram as a function of temperature T and pinning strength p for a fixed value of the vortex line density. At low p we find a sharp first-order vortex lattice melting phase boundary separating a vortex lattice from a vortex liquid. As p increases, it appears that this first-order transition smears out over a finite temperature interval due to the effects of the random pinning, in agreement with several recent experiments. At large p we find a second-order transition from vortex liquid to vortex glass.

  • 26.
    Ozana, Marek
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shelankov, Andrei
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Quasiclassical theory of superconductivity: Interfering paths2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, no 1, p. 014510-Article in journal (Refereed)
    Abstract [en]

    We apply the method of a two-point quasiclassical Green's function to geometries where the trajectories include intefering paths and loops. For a system of two superconducting layers separated by a partially transparent interface, corrections to the quasiclassical solutions for the Green's function are explicitly found as well as the deviation from the normalization condition.

  • 27.
    Ozana, Marek
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shelankov, Andrei
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tobiska, J
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bogoliubov-de Gennes versus quasiclassical description of Josephson layered structures2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 5, p. 054508-Article in journal (Refereed)
    Abstract [en]

    The applicability of the quasiclassical theory of superconductivity in Josephson multilayer structures is analyzed. The quasiclassical approach is compared with the exact theory based on the Bogoliubov-de Gennes equation. The angle- and energy-resolved (coarse-grain) currents are calculated using both techniques. It is shown that the two approaches agree in SIS'IS" geometries after the coarse-grain averaging. A quantitative discrepancy, which exceeds the quasiclassical accuracy, is observed when three or more interfaces are present. The invalidity of the quasiclassical theory is attributed to the presence of closed trajectories formed by sequential reflections from the interfaces.

  • 28.
    Rammer, Jörgen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shelankov, Andrei L.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wabnig, Joachim
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Quantum measurement in the charge representation2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, p. 115327-Article in journal (Refereed)
    Abstract [en]

    Counting statistics of charge transfers in a point contact interacting with an arbitrary quantum system is studied. The theory for the charge specific density matrix is developed, allowing the evaluation of the probability of the outcome of any joint measurement of the state of the quantum system and the transferred charge. Applying the method of charge projectors, the master equation for the charge specific density matrix is derived in the tunneling Hamiltonian model of the point contact. As an example, the theory is applied to a quantum measurement of a two-state system: The evolution of the charge specific density matrix in the presence of Nyquist or Schottky noise is studied and the conditions for the realization of a projective measurement are established.

  • 29. Rapp, Östen
    et al.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    The pressure dependence of of the electron-phonon interaction and the normal state resistivity1981In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 24, no 1, p. 144-154Article in journal (Refereed)
    Abstract [en]

    Accurate measurements of the electrical resistance as a function of temperature and pressure are reported for Sn, Zr, dhcp La, and V. These measurements cover a temperature region around room temperature and pressures up to 1.3 GPa. From these data, including also our previous measurements for Al and published results for Pb, the pressure dependence of d ρ / dT (the resistivity-temperature derivative) is obtained. This quantity is found to be a significant factor in the pressure dependence of the electron-phonon interaction parameter λ. For the nontransition metals the relative pressure dependence of d ρ / dT is much larger than the compressibility. Therefore the pressure dependence of the superconducting Tc is quantitatively well accounted for by the resistance data for these metals. For the transition metals the pressure dependence of d ρ / dT is relatively smaller and Tc(p) calculated from the resistance data is, at the best, only qualitatively correct. These differences are discussed. Estimates for the pressure dependence of the plasma frequency are obtained.

  • 30. Robinson, Nate D.
    et al.
    Fang, Junfeng
    Umeå University, Faculty of Science and Technology, Physics.
    Matyba, Piotr
    Umeå University, Faculty of Science and Technology, Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Physics.
    Electrochemical doping during light emission in polymer light-emitting electrochemical cells2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 24, p. 245202-Article in journal (Refereed)
  • 31. Robinson, N.D.
    et al.
    Shin, Joon-Ho
    Umeå University, Faculty of Science and Technology, Physics.
    Berggren, Magnus
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Physics.
    Doping Front Propagation in Light-Emitting Electrochemical Cells2006In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 74, no 15, p. 155210-Article in journal (Refereed)
  • 32.
    Shelankov, Andrei
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ozana, Marek
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Quasiclassical theory of superconductivity: A multiple-interface geometry2000In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 61, no 10, p. 7077-7100Article in journal (Refereed)
    Abstract [en]

    A method is suggested that allows one to study multiple coherent reflection/transmissions by partially transparent interfaces (e.g., in multilayer mesoscopic structures or grain boundaries in high T-c's), in the framework of the quasiclassical theory of superconductivity. It is argued that in the presence of interfaces, a straight-line trajectory transforms to a simple connected one-dimensional tree (graph) with knots, i.e., the points where the interface scattering events occur and pieces of the trajectories are coupled. For the two-component trajectory "wave function" which factorizes the Gor'kov matrix Green's function, a linear boundary condition on the knot is formulated for an arbitrary interface, specular or diffusive (in the many channel model). From the new boundary condition, we derive (i) the excitation scattering amplitude for the multichannel Andreev/ordinary reflection/transmission processes; (ii) the boundary conditions for the Riccati equation; (iii) the transfer matrix which couples the trajectory Green's function before and after the interface scattering. To show the usage of the method, the cases of a him separated from a hulk superconductor by a partially transparent interface, and a SIS' sandwich with finite thickness layers, are considered. The electric current response to the vector potential (the superfluid density rho(s)) with the pi phase difference in S and S' is calculated for the sandwich. It is shown that the model is very sensitive to imperfection of the SS' interface: the low temperature response being paramagnetic (rho(s) < 0) in the ideal system case, changes its sign and becomes diamagnetic (rho(s) > 0) when the probability of reflection is as low as a few percent.

  • 33.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Comment on "Pressure and temperature diagram of polymerized fullerite"1998In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 57, no 5, p. 3164-3165Article in journal (Refereed)
    Abstract [en]

    Fullerenes such as C60 can be polymerized by treatment under high-pressure, high-temperature conditions. I point out that there is a strong correlation between the rotational state of the C60 molecules in the initial (pristine) material and the structure of the resulting polymer.

  • 34.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Electrical resistance of nickel in the range 300-725 K and 0-2 GPa1988In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 38, no 17, p. 12283-12289Article in journal (Refereed)
    Abstract [en]

    The electrical resistance R of Ni has been measured as a function of temperature T and pressure p over the range 300–725 K and 0–2 GPa (0–20 kbar). In contrast to recent reports, we see no anomalies in R(p) at 300 K, and we see no change in sign in dR/dp at the Curie temperature TC. The pressure coefficient of R was -1.82×10-2 GPa-1 at 300 K, increasing with T to a peak value of -2.5×10-2 GPa-1 at TC, above which it rapidly decreased to a value 20% below the room-temperature value. The data are analyzed using simple models; the resistivity at constant volume is found to deviate significantly from the constant-p data above TC and a small anomaly is detected in d(lnR)/dp that could possibly be an effect of band-structure changes at TC.

  • 35.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Point defects and thermal conductivity of C601993In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 48, no 19, p. 14712-14713Article in journal (Refereed)
    Abstract [en]

    Recent literature data for the thermal conductivity κ of single-crystal C60 are analyzed using a standard model. It is shown that in the intermediate range 100–250 K, below the rotational transition, κ can be described extremely well by a model taking into account phonon and point defect scattering only. The magnitude derived for the phonon-phonon scattering term agrees very well with an estimate from a simple model.

  • 36.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Fischer, John E.
    Pressure-temperature phase boundaries in KC24: Evidence for a kinetically-hindered low-temperature staging transition1986In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 34, no 5, p. 3532-3535Article in journal (Refereed)
    Abstract [en]

    The pressure-temperature phase diagram of KC24 was mapped out using in-plane resistivity anomalies to identify phase boundaries. Surprisingly, the 300-K staging transitions at 3.4 and 6.4 kbar are found to connect to the 1-bar ordering-stacking transitions at Tu=123 K and Tl=96 K, respectively, whereas the loci of P, T anomalies cannot be equilibrium phase boundaries since the low-P, low-T transitions do not involve stage transformations. We suggest that this unusual behavior is due to kinetic hindering of the low-T staging transitions, predicted by simple equilibrium models and observed in the analogous Li compound. A possible origin of the slow staging kinetics at low T in KC24 is the pinning of discommensurations at Daumas-Herold boundaries.

  • 37.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Fischer, John E.
    Pressure-temperature phase diagrams of stage-2-4 potassium-graphite intercalation compounds deduced from anomalies in the basal plane resistivity1987In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 35, no 15, p. 8231-8242Article in journal (Refereed)
    Abstract [en]

    The basal-plane resistivities ρ of stage-2–4 potassium-graphite intercalation compounds (K-GIC’s) have been measured as functions of temperature T and pressure p using a contactless method. The pressure range investigated was 0–1.4 GPa. For stage-2 K-GIC (KC24) measurements were made in the temperature range 90–300 K, while for the higher-stage materials the range was mainly limited to 245–300 K. In all cases ρ increases with p through the anomalous regime near the phase transitions, whereas dρ/dp is always negative in the stable high-p (high-stage) phases. Approximate p-T phase diagrams are deduced from anomalies in ρ corresponding to the staging and in-plane order-disorder transitions, respectively. In all cases the slopes dp/dT of the phase boundaries for the high-pressure staging transitions are such that the transitions extrapolate to positive T at atmospheric p, although no such transitions have been reported. The low-p staging-ordering phase boundary of KC24 is particularly complex, involving segments with both dp/dT→0 (125<200 K) and dp/dT→∞ (T≊125 K). The p-T diagrams are compared with recent structural experiments and models. We suggest that the complexity of the KC24 phase diagram is attributable to the weakly incommensurate in-plane structure at low p and T.

  • 38.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Neve, Jesper
    Rapp, Östen
    Pressure dependence of the electron-phonon interaction and Fermi-surface properties of Al, Au, bcc Li, Pb, and Pd1985In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 32, no 4, p. 2200-2212Article in journal (Refereed)
    Abstract [en]

    The electrical resistance of Li, Pd, Au, and Pb is measured as a function of temperature and pressure in the region -20 °C to +30 °C and 0–1.3 GPa. Self-consistent linear muffin-tin orbital band-structure calculations of these elements and of Al are performed at normal and reduced volumes. Results are obtained for the density of states N(EF), the average Fermi ve- locity, the optical mass, the plasma frequency ω, and the volume dependence of these parameters. The pressure dependence of the electron-phonon interaction λ(p) is obtained from these measurements and the calculated ω(p). For the superconducting elements there is good agreement with the measured superconducting Tc(p). Results from our previous measurements and calculations on Al, V, Nb, and La and published results for ω(p) are included in this comparison. λ(p) increases with pressure for bcc Li and decreases for Pd and Au. Pressure is expected to suppress spin fluctuations much faster than λ(p) in Pd. The possibility of inducing superconductivity by pressure in Pd and bcc Li is discussed. The electronic Grüneisen parameter γe is obtained from λ(p) and the volume dependence of N(EF). Comparison with other results for γe generally shows good agreement.

  • 39.
    Sundqvist, Bertil
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Nilsson, E.M. Charlotta
    Umeå University, Faculty of Science and Technology, Physics.
    In-plane electrical resistivity of La2-xSrxCuO4 at constant volume1995In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 51, no 9, p. 6111-6114Article in journal (Refereed)
    Abstract [en]

    Literature data for the in-plane electrical resistivity ρ of single-crystal La2-xSrxCuO4 with 0.07 ≤x≤0.34 have been corrected to constant crystal volume V using literature data for the pVT properties and for the resistivity vs pressure p. The original data at constant p show the usual anomalies observed for ceramic high-transition-temperature superconductors, such as linearity of ρ to extremely high T at optimum doping and an upwards curvature (ρ∝T1.5) for overdoped material. After correction, the data for ρ at constant volume are linear in T for overdoped material only. We show that phonons cannot be ruled out as the major carrier scattering agent.

  • 40.
    Talyzin, Alexandr
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Reversible phase transition in LiAlH4 under high pressure conditions.2004In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 70, no 18, p. 180101(R)-Article in journal (Refereed)
    Abstract [en]

    The structural stability of the hydrogen storage compound LiAlH4 was studied by Raman spectroscopy in the pressure interval 0–6 GPa. A slow phase transition from a-LiAlH4 to a b phase was observed to occur between 2.2 and 3.5 GPa. The phase transition is reversible and a reverse transformation to a-LiAlH4 was observed below 1.4 GPa. Analysis of Raman spectra from b-LiAlH4 shows that they cannot be explained by the theoretically predicted a-NaAlH4-type structure with I41 /a symmetry.

  • 41.
    Wabnig, Joachim
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rammer, Jörgen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shelankov, Andrei L.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Noise spectrum of a tunnel junction coupled to a nanomechanical oscillator2007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Physical Review B, Vol. 75, no 20, p. 205319-13 pArticle in journal (Refereed)
    Abstract [en]

    A nanomechanical resonator coupled to a tunnel junction is studied. The oscillator modulates the transmission of the junction, changing the current and the noise spectrum. The influence of the oscillator on the noise spectrum of the junction is investigated, and the noise spectrum is obtained for arbitrary frequencies, temperatures, and bias voltages. We find that the noise spectrum consists of a noise floor and a peaked structure with peaks at zero frequency, the oscillator frequency, and twice the oscillator frequency. The influence of the oscillator vanishes if the bias voltage of the junction is lower than the oscillator frequency. We demonstrate that the peak at the oscillator frequency can be used to determine the oscillator occupation number, showing that the current noise in the junction functions as a thermometer for the oscillator.

  • 42. Winter, Johannes
    et al.
    Kuzmany, Hans
    Soldatov, Alexander
    Umeå University, Faculty of Science and Technology, Physics.
    Persson, Per-Axel
    Umeå University, Faculty of Science and Technology, Physics.
    Jacobsson, Per
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Charge transfer in alkali-metal-doped polymeric fullerenes1996In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 54, no 24, p. 17486-17492Article in journal (Refereed)
    Abstract [en]

    We present room-temperature Raman measurements of pressure-polymerized C60 and compare them with the spectra of RbC60 in the orthorhombic phase. Although both materials were prepared according to two completely different routes the spectra show a surprising similarity with respect to mode positions and line splitting. We concluded from this that both materials, the uncharged pressure-polymerized C60 and the rubidium-doped orthorhombic compound, have the same overall structure and the AC60 compounds can be considered as the doped species of the C60, polymerized using moderate low pressure and high temperatures. From a detailed comparison between both spectra mode shifting and line broadening as a consequence of the charge transfer was determined and electron-phonon coupling constants were estimated for the high-frequency Hg(7) and Hg(8) modes. The low values for the coupling constants compared to the ones in the K3C60 can explain the lack of superconductivity in the AC60 compounds.

  • 43.
    Wågberg, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Jacobsson, Per
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Comparative Raman study of photopolymerised and pressure-polymerised C60 films1999In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 60, no 7, p. 4535-4538Article in journal (Refereed)
    Abstract [en]

    We have made a Raman study of low-pressure polymerized and photopolymerized C60 films, polymerized in different ways, but otherwise initially identical. Although the structures developing in the films during polymerization are initially very similar, an analysis of characteristic features in the Raman spectra shows that photopolymerized material forms a more complex polymeric structure when the polymerization approaches saturation. We suggest that this complex structure contains branched polymeric chains and in particular that a mode near 1454 cm-1 in photopolymerized C60 is characteristic for this type of structure.

  • 44.
    Wågberg, Thomas
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Raman study of the two-dimensional polymers Na4C60 and tetragonal C60.2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 65, no 15, p. 155421-Article in journal (Refereed)
    Abstract [en]

    We present a Raman investigation of the monoclinic two-dimensionally polymerized phase Na4C60 and a comparison with the pure tetragonal C60 polymer and the doped K3C60 structure. The Raman spectrum of Na4C60 depends on the probing laser wavelength, and high wave-number modes are hard to observe with a low-energy laser excitation. The spectrum for Na4C60 is very rich with a pronounced splitting of the original Hg modes and a large number of new modes. Several modes at low wave numbers support the presence of intermolecular bonds and the line broadening observed for low-frequency Hg modes is typical for electron-phonon coupling in metallic fullerene systems. From the shift of the pentagonal pinch mode we deduce an unexpectedly low charge transfer of approximately three electrons per C60 molecule. The presence of a distinct mode around 980 cm-1 throws doubts on earlier assumptions that modes in this range are connected to vibrations in the intermolecular cyclobutane rings found in C60 polymers. No superconductivity is observed in Na4C60, although the electron-phonon interaction derived from the Raman spectrum is similar to that in K3C60.

  • 45. Yao, Mingguang
    et al.
    Pischedda, Vittoria
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mezouar, Mohamed
    Debord, Régis
    San Miguel, Alfonso
    Pressure-induced transformation in Na4C60 polymer: X-ray diffraction and Raman scattering experiments2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 14, p. 144106-Article in journal (Refereed)
    Abstract [en]

    In this article the alkali metal-intercalated two-dimensional (2D) polymer Na4C60 is studied under pressure up to 41 GPa at room temperature by Raman spectroscopy and x-ray diffraction (XRD) measurements. Two transitions are identified in the studied pressure range. The first one is observed at ∼3 GPa by both diffraction and Raman scattering. A kink in the pressure slope of the cell parameters (especially along the c axis) shows the appearance of a less compressible phase. The decrease in the C60-C60 distance and the Na-C distance, combined with the frequency softening of the Raman modes, leads to a picture of higher electron hopping. The second transition occurs at around 15 GPa, where the distinct Raman peaks of 2D-Na4C60 disappear and become very broad and diffuse. New bands at 200–800 cm−1, 1590 cm−1, and ∼1800 cm−1, exhibit similar features to those of a reported 3D-C60 polymeric structure. The XRD data show that the cell parameters a, b, and c deviate from their early pressure evolution and become almost pressure independent, accompanied by the formation of amorphous material. Both the evolution of the Raman features of Na4C60 at pressures above 15 GPa and the Raman measurements of the samples on decompression indicate that most C60 molecules in the material are preserved after such a high pressure cycle. Our findings are discussed in terms of the formation of the first high-pressure intercalated C60 3D-polymer structure through the random creation of new polymeric bonds between fullerene molecules.

  • 46.
    Yao, Mingguang
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Reversible pressure-driven nanoscale phase separation in Rb4C602009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 8, p. 081403-Article in journal (Refereed)
    Abstract [en]

    Wepresent Raman spectra for Rb4C60 at pressures up to 13GPa. Above 1 GPa we observe a phase separation ofRb4C60 into Rb6C60 and Rb3C60, identified by a splitting ofthe Ag(2) mode and the appearance of characteristic Raman modesof the daughter phases. Remarkably, the phase separation is completelyreversible and the sample returns to a homogeneous Rb4C60 phaseafter pressure release. The reversibility of the phase separation suggeststhat the structural change is associated with small displacements ofRb ions in the unit cell, giving local differences inthe charge state of the C60 molecules. The pressure dependenceof the Raman shifts agrees well with earlier measurements. Ourobservation of a reversible phase separation explains several peculiar resultsreported earlier for Rb4C60.

  • 47.
    Yao, Mingguang
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Wang, Zhigang
    Liu, Bingbing
    Zou, Yonggang
    Yu, Shidan
    Lin, Wang
    Hou, Yuanyuan
    Pan, Shoufu
    Jin, Mingxing
    Zou, Bo
    Cui, Tian
    Zou, Guangtian
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Physics.
    Raman signature to identify the structural transition of single-wall carbon nanotubes under high pressure2008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 20, p. 205411-Article in journal (Refereed)
    Abstract [en]

    Raman spectra of single-walled carbon nanotubes (SWNTs) with diameters of 0.6–1.3 nm have been studied under high pressure. A “plateau” in the pressure dependence of the G-band frequencies was observed in all experiments, both with and without pressure transmission medium. Near the onset of the G-band plateau, the corresponding radial breathing mode (RBM) lines become very weak. A strong broadening of the full width at half maximum of the RBMs just before the onset of the G-band plateau suggests that a structural transition starts in the SWNTs. Raman spectra from SWNTs released from different pressures also indicate that a significant structural transition occurs during the G-band plateau process. Simulations of the structural changes and the corresponding Raman modes of a nanotube under compression show a behavior similar to the experimental observations. Based on the experimental results and the theoretical simulation, a detailed model is suggested for the structural transition of SWNTs, corresponding to the experimentally obtained Raman results in the high-pressure domain.

  • 48.
    Yao, Mingguang
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Effect of high pressure on electrical transport in the Li4C60 fulleride polymer from 100 to 400 K2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 15, p. 155441-155448Article in journal (Refereed)
    Abstract [en]

    In situ resistance measurements have been carried out on Li4C60 under pressures up to 2 GPa at temperatures from below 100 to 400 K. In agreement with recent reports we find an Arrhenius law behavior for the conductivity, which can be interpreted in terms of Li+ ionic conduction with an activation energy near 225 meV. The activation energy decreases with increasing pressure at an initial rate of about −11%/GPa and the room-temperature conductivity increases by a factor of about 6 from 0.1 to 2 GPa. We also observe conductivity terms with a lower excitation energy, most probably associated with conduction by electrons excited from defect-induced states in the main band gap. We discuss this conduction behavior in the context of recent measurements on both Li4C60 and other alkali-metal intercalated phases such as Rb4C60, Na2C60, and Na4C60. After heating to 400 K at 2 GPa the conduction behavior changes drastically, manifested by a change in the slopes of R versus T curves signifying newly created gap states. Postexperimental characterization by Raman spectroscopy and x-ray diffraction indicate the loss of Li especially from the grain surfaces. Finally, high-pressure Raman studies suggest a possible metallization transition above 9 GPa.

  • 49.
    Yao, Mingguang
    et al.
    Université de Lyon, Lab. de Phys. Matière Condenseé et Nanostructures, 69622 Villeurbanne, Lyon, France.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Electrical transport properties of Na2C60 under high pressure2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, no 11, p. 115405-Article in journal (Refereed)
    Abstract [en]

    We present direct resistance measurements on Na2C60 under high pressure up to 2 GPa in the temperature range ~90–500 K. We show that fresh, nominally pure Na2C60 exhibits a semiconductor-type conduction behavior at all experimental conditions, with a 1.2 eV band gap and a conduction strongly influenced by defect gap states. Contrary to Rb4C60 and Na4C60, both band gap and gap states have large pressure dependences. In contrast, a Na2C60 sample preserved in an oxygen-free environment for several months shows conduction by another mechanism, variable range 3D hopping. This is consistent with recent findings of nanoscale phase segregation in Na2C60 and can be understood in terms of conduction by intergrain hopping. Heating the sample at 2 GPa leads to a significant change in the resistivity, suggesting a structural transition. By characterizing the sample after the phase transformation with Raman spectroscopy and x-ray diffraction we identify a phase separation of the material into two main structures, linearly polymerized orthorhombic C60 and two-dimensionally polymerized Na4C60.

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