Open Fermi Surface Research Articles

The metallic state of doped polyacetylene and polypyrrole

A low frequency Drude model is used to analyze the huge negative dielectric constant at microwave frequency for heavily iodine doped stretched and unstretched Tsukamoto polyacetylene and unoriented polypyrrole containing PF 6. The relatively small plasma frequencies and anomalously long scattering times of these materials indicate the importance of phonon back scattering of the most delocalized conduction electrons, as expected for the open Fermi surface of a highly anisotropic metal. These results are contrasted with the more localized behaviour of the neck and end portions of stretched polyacetylene and of polypyrrole doped with TsO.

Correlations, dimensionality and instabilities in organic superconductors

We discuss the role of Coulombic repulsion in organic superconductors exhibiting quasi-one-dimensional transport properties. A recent investigation in high magnetic fields shows that the charge localization occurring at low temperature in a 1-D half-filled band is suppressed by the 1-D to 2-D (3-D) cross-over in selenium compounds. However, a localization around 30 K in the non-ordered phase can be reactivated by the application of a high transverse magnetic field. This phenomenon provides an interpretation for the large transverse magnetoresistance observed in quasi-1-D conductors with open Fermi surfaces. The intermediate (strong) coupling limit is valid for the spin degree of freedom whereas the charge is governed by the strength of the Umklapp scattering varying by a large factor from sulfur to selenium compounds and under pressure.

Mean-Field Theory of a Quasi-One-Dimensional Superconductor in a High Magnetic Field

At high magnetic field, the semiclassical approximation which underlies the Ginzburg-Landau (GL) theory of the mixed state of type II superconductors breaks down. In a quasi-1D superconductor (weakly coupled chains system) with an {\it open Fermi surface}, a high magnetic field stabilizes a cascade of superconducting phases which ends in a strong reentrance of the superconducting phase. The superconducting state evolves from a triangular Abrikosov vortex lattice in the semiclassical regime towards a Josephson vortex lattice in the reentrant phase. We study the properties of these superconducting phases from a microscopic model in the mean-field approximation. The critical temperature is calculated in the quantum limit approximation (QLA) where only Cooper logarithmic singularities are retained while less divergent terms are ignored. The effects of Pauli pair breaking (PPB) and impurity scattering are taken into account. The Gor'kov equations are solved in the same approximation but ignoring the PPB effect. We derive the GL expansion of the free energy. We obtain the specific heat jump at the transition, the sign of the magnetization and the quasi-particle excitation spectrum. The calculation is extended beyond the QLA taking into account all the pairing channels and the validity of the QLA is discussed in detail.

Thermodynamics and excitation spectrum of a quasi-one-dimensional superconductor in a high magnetic field.

At high magnetic field, the semiclassical approximation which underlies the Ginzburg-Landau theory of the mixed state of type II superconductors breaks down. In a quasi-1D superconductor with an {\it open Fermi surface}, a high magnetic field stabilizes a cascade of superconducting phases which ends in a strong reentrance of the superconducting phase. From a microscopic mean-field model, we determine the thermodynamics and the excitation spectrum of these quantum superconducting phases.

Two-dimensional angular-correlation-of-annihilation-radiation study of kappa -(BEDT-TTF)2Cu(NCS)2.

We report the two-dimensional angular-correlation-of-annihilation-radiation measurement of the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ (pair) momentum distribution in the organic superconductor \ensuremath{\kappa}-(BEDT-TTF${)}_{2}$Cu(NCS${)}_{2}$. Our data, when compared with tight-binding (H\uckel) electronic-band calculations, show features consistent with predicted Fermi surfaces. The structure around Z in our spectrum supports the results from earlier de Haas--van Alphen and Shubnikov--de Haas measurements. In addition, our data show features consistent with the predicted open Fermi surface that runs nearly parallel to YM.

Anisotropic galvanomagnetic effect in the quasi-two-dimensional organic conductor α-(BEDT-TTF)2KHg(SCN)4, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene

The anisotropy of the magnetoresistance has been measured systematically by changing the current and field direction with respect to the crystallographic axes in the quasi-two-dimensional organic conductor \ensuremath{\alpha}-(BEDT-TTF${)}_{2}$KHg(SCN${)}_{4}$, where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene, which shows an antiferromagneticlike ordering as a spin-density wave below ${\mathit{T}}_{\mathit{A}}$=10 K. The open direction of the Fermi surface changes from \ensuremath{\theta}=0\ifmmode^\circ\else\textdegree\fi{} in the normal-metal phase to 30\ifmmode^\circ\else\textdegree\fi{} in the antiferromagneticlike phase (\ensuremath{\theta} is the tilt angle from the c axis in the conductive a-c plane). The open direction in the normal-metal phase is in agreement with a tight-binding band-structure calculation based on the crystal structure at room temperature. Angle-dependent magnetoresistance oscillations (ADMRO's) have been measured in both phases using a double-axis sample rotation system in fields up to 26 T. In the normal-metal phase, the angle-dependent magnetoresistance oscillations show two-dimensional behavior resulting from the closed Fermi surface with cross-sectional area 14% of the Brillouin zone. On the other hand, in the antiferromagneticlike phase, the angle-dependent magnetoresistance oscillations show one-dimensional behavior, the open direction of which is along \ensuremath{\theta}=30\ifmmode^\circ\else\textdegree\fi{} as observed in the current-direction dependence. At 26 T higher than ${\mathit{H}}_{\mathit{A}}$ (T=1.5 K)=23 T, the phase boundary between the antiferromagneticlike phase and the normal-metal phase, we have observed exactly the same two-dimensional ADMRO effect as in the normal-metal phase at higher temperatures. In order to explain these anomalous temperature and field dependences of the Fermi surface, we propose a superlattice model which reconstructs the Fermi surface at the phase transition. This model shows that a pair of open Fermi surfaces in the normal-metal phase is nested to each other by the vector Q(=1/6${\mathbf{k}}_{\mathit{a}}$+1/2${\mathbf{k}}_{\mathit{b}}$+ 1) / 3 ${\mathbf{k}}_{\mathit{c}}$) and the new open Fermi surface is reconstructed in antiferromagneticlike phase by the multiconnected closed Fermi surface.

Superconductivity of quasi-one-dimensional conductors in a high magnetic field.

We determine the phase diagram of a quasi-one-dimensional superconductor (weakly coupled chains system with an open Fermi surface) in a magentic field. A field H(0,H,0) along the y direction (perpendicular the direction x of highest conductivity) tends to confine the electronic motion in the z direction. At low temperature, this effect cannot be neglected and the Ginzburg-Landau theory breaks down. We find that the usual Ginzburg-Landau regime is followed, when the field is increased, by a cascade of superconducting phases separated by first-order transitions, which ends with a strong reentrance of the superconducting phase where the chains interact by Josephson coupling. This high-field superconductivity can survive even in the presence of Pauli pair breaking because the quasi-one-dimensional Fermi surface allows one to construct a Larkin-Ovchinnikov-Fulde-Ferrell state that can exist far above the Pauli-limited field. Moreover, elastic scattering does not destroy the superconducting phases in clean materials with sufficiently large anistropy. We show that the superconducting state evolves from an Abrikosov vortex lattice in weak field towards a Josephson vortex lattice in the reentrant phase. Between these two limits, the order parameter and the current distribution show a laminar-type symmetry. The relevance of our results is discussed for quasi-one-dimensional organic superconductors and quasi-two-dimensional superconductors.

Quasi-one-dimensional superconductors in strong magnetic field

We determine the phase diagram of a quasi-one-dimensional conductor (weakly coupled chains system with an open Fermi surface) in a magnetic field. The superconducting state evolves from an Abrikosov vortex lattice in weak field to a Josephson vortex lattice in very strong field, with a reentrance of the superconducting phase. Between these two limits, there is a cascade of first order transitions between different superconducting phases due to commensurability effects between the crystalline lattice and different superconducting phases due to commensurability effects between the crystalline lattice and the order parameter. In these phases, the order parameter has a laminar structure.

Cyclotron resonance on open orbits in organic materials.

The cyclotron resonance on open orbits is suggested to demonstrate the one dimensional character of electron motion along the main conducting axis in the presence of a perpendicular magnetic field for organic materials with warped open Fermi surfaces. Singular behavior at resonant frequencies in the surface impedance and the helicon waves spectrum are obtained

COLLECTIVE FIELD THEORY TO MAGNETIC-FIELD-INDUCED SPIN-DENSITY-WAVE-STATE IN BECHGAARD SALTS, (TMTSF)2X

A field description of spin-density-wave (SDW) in a quasi-two-dimensional metal with open Fermi surface in magnetic field, is proposed. The SDW transition temperature, T c (H), and the Hall conductivity σxy, are calculated. The dependence T c (H) is found to be different from that of the Bardeen-Cooper-Schrieffer model, in particular, a threshold field, H c , found its natural explanation. It is proved that the quantized Hall conductivity arises from the chiral anomaly terms in the effective action provided there is pinning of chemical potential in the gap of extended states.

Localization, superconductivity and magnetic field in a quasi-1D conductor

We show that the quasi-1D aspect of the electronic motion leads to new developments in the physics of localization. In a conductor with an open Fermi surface, the magnetic field makes the electronic motion more and more one-dimensional, in the sense that it confines the wave functions on the chains. Therefore, the effect of disorder is stronger when the field is increased. As a consequence, a strong magnetic field induces a crossover to a 1D localized state. We also extend the original work of A.G. Lebed' on the phase diagram of BCS quasi-1D superconductors in a magnetic field. The usual Ginzburg-Landau regime is followed, when increasing the field, by a series of superconducting phases. The first order transitions between these phases are related to commensurability effects between the periodicity of the order parameter and the distance between chains. The order parameter and the current distribution of these new phases show a new symmetry (of laminar type) which has never been observed or predicted in other systems.

Dispersion of transverse sound in the model of layered high-T c superconductors

The dynamical interaction of charged carriers and ions is considered for a transverse sound propagation in layered superconductors with open Fermi surfaces. The difference from conventional superconductors follows due to the new Pippard factors for quasi-two-dimensional materials. The transverse electric field could be neglected in high- T c superconductors because of small values of both the carrier plasma frequency and the carrier scattering lifetime. The sound attenuation depends strongly on an angle of a wave propagation with respect to layers. The dynamical change of a sound velocity is always positive for layered superconductors at low temperatures and could be negative only near the transition temperature. The used model for nonequilibrium carrier states could be important for the ultrasonic diagnostic technique.

Quasi-one-dimensional superconductors in strong magnetic field

We determine the phase diagram of a quasi-one-dimensional conductor (weakly coupled chains system with an open Fermi surface) in a magnetic field. The usual Ginzburg-Landau regime is followed, when the field is increased, by a cascade of superconducting phases separated by first order transitions, which ends with a strong reentrance of the superconducting phase. These new phases show a novel kind of symmetry of laminar type. The Zeeman splitting does not completely suppress the reentrance in very strong field, the ground state being in this case a Larkin-Ovchinnikov-Fulde-Ferrell state

Magnetic field dependence ofT c and temperature dependence ofH c2 in layered superconductors with open normal state Fermi surface

A theoretical framework for treating the effects of magnetic fieldH on the pairing theory of superconductivity is considered, where the field is taken in an arbitrary direction with respect to crystal axes. This is applicable to closed, as well as open normal state Fermi surface (FS), including simple layered metals. The orbital effects of the magnetic field are treated semiclassically while retaining the full anisotropic paramagnetic contribution. Explicit calculations are presented in the limits |H| → |Hc2(T)|,T ∼ 0 andT →Tc(|H|), |H| ∼ 0. Effects of weak nonmagnetic impurity scattering, without vertex corrections, have also been taken into account in a phenomenological way. The final results for the case of open FS and layered materials are found to differ considerably from those of the closed FS. For example, an important parameter,h(T=0)=|Hc2(0)|/[-Tδ|Hc2T|δT]T{s0} for the case of a FS open inkz-direction with thekz-bandwidth, 4t3, very small compared to the Fermi energy,EF, is close to 0.5906, compared to 0.7273 for the closed FS, in the clean limit. Analytical results are given for the magnetic field dependence ofTc and the temperature dependence of Hc2 for a model of layered superconductors with widely open FS. For a set of band structure parameters for YBa2Cu3O7 used elsewhere, we find reasonable values for the upper critical fieldHc2(0), the slope (dHc2/dT)Tc0, anisotropic coherence lengths ζi(T=0),i=x, y, z, and (dTc/d|H|)|H| → 0.

On the magnetic breakdown oscillations in organic superconductor k-(BEDT-TTF)2Cu(NCS)2

Abstract From the observed magnetic breakdown oscillations in a title organic superconductor, the energy gap E g which separates the closed and open Fermi surfaces at the zone boundary is evaluated to be about 54±10K on the basis of the Lifshitz-Kosevich theory and the coupled network model for the magnetic breakdown probability. Anomalous enhancement of the Hall resistance below 60–70K is explained from the viewpoint that the effective carrier number might change around T⋍E g .

Field induced spin density waves in quasi-1D conductors

We present a review of the unusual physics of the quasi-1D conductors in a magnetic field. They have an open Fermi surface and dramatic magnetic field effects were a priori not expected in these compounds. But they present a very rich phase diagram in a field. The reason is that, although the field does not quantize the electronic orbits in the metallic phase, it has a spectacular effect on the instability of this metallic phase. As a result, the CDW or SDW ground state has a complex structure made of a cascade of successive subphases with quantization of Hall effect and very unusual thermodynamics. We review the successes and failures of the Quantized Nesting model used to describe these phases.

Crystal and Electronic Structures of (BEDT–TTF)2[MHg(SCN)4](M=K and NH4)

Abstract The crystal structures of (BEDT–TTF)2[MHg(SCN)4] (M=K at 298 and 104 K and M=NH4 at 298 K) were determined by X-ray analyses. A donor layer and an anion sheet stack alternately along the b-axis. The packing pattern of the donor is close to α-phase, while an anion sheet constructs a thick two-dimensional polymer plane parallel to (010). The linkage of (Remark: Graphics omitted.) (or NH4) (Remark: Graphics omitted.) in an anion sheet is very unique. The band structure calculation indicates both a closed and an open Fermi surfaces, resembling that of κ-(BEDT–TTF)2[Cu(NCS)2].

BEDT-TTF Salts with Square-Planar Gold(III) Complex Anions: β-(ET)2AuCl4and (ET)2Au(CN)2Cl2

Abstract We report the synthesis, crystal structures, and band electronic structures of the first 2:1 salts of the organic donor radical cation bis(ethylene-dithio) tetrathiafulvalene (BEDT-TTF or ET) with square-planar anions, i. e., β-(ET)2AuCl4(1) and (ET)2Au(CN)2Cl2 (2). The crystal and the band electronic structures of 1 are related to the β-phase superconductors, β-(ET)2 X (X ˭ I3, IBr2, AuI2). However, β-(ET)2AuCl4 undergoes a destructive phase transition after prolonged cooling to 120 K and is non-superconducting. Temperature-dependent ESR measurements of 2 indicate a very broad metal- to-insulator phase transition between 250 and 300 K. The band electronic structure of 2 indicates 1-d metallic behavior at room temperature with a well nested open Fermi surface. Crystal parameters: β-(ET)2Au(CN)2Cl2: triclinic, P1, a = 6.590(2) A, b = 8.561(2) A. c = 18.319(5)°, α = 93.19(2)°, β = 105.52(2)°, γ = 119.47(2)°, Vc = 845.1(3) A3, Z = 1. (ET)2Au(CN)2Cl2: monoclinic, P2/c, a = 17.726(8) A, b = 6.690(4) A...

Magnetization and high field magneto-resistance in SmCu 2

The magnetization curve of antiferromagnetic SmCu 2 showed a spin flop along the easy axis at 225 kOe. The magnetic moment after the transition is below half of the gJ value. By means of magneto-resistance measurements, it was found that SmCu 2 has an open Fermi surface along the b-axis.

Fermi-surface geometry and pressure effects on the spin-fluctuation contributions to the specific heat: Anisotropic spin-fluctuation model for heavy-fermion UPt3.

On the basis of an uniaxially anisotropic and nonparabolic band model, the effects of Fermi-surface (FS) anisotropy and topology and of hydrostatic pressure on the spin-fluctuation (SF) contributions to the low-temperature specific heat of exchange-enhanced paramagnetic metals are investigated in the framework of paramagnon theory. The model describes different types of SF; for the closed FS a parameter region is found in which there is a coexistence of anisotropic ferromagnetic and antiferrromagnetic SF. In this region and for the open FS a linear relation between the SF-mediated mass enhancement and the Stoner enhancement factor can be obtained which results in the pressure-dependent scaling of the SF parameters and the magnetic susceptibility in terms of only one parameter. For either FS topology the ${T}^{3}$ lnT law holds, where the amplitude is enlarged compared with the parabolic band model and the range of validity increases with FS anisotropy. Comparing theory with experiments, an anisotropic SF model for ${\mathrm{UPt}}_{3}$ is proposed which describes the coexistence of antiferromagnetic SF along the hexagonal axis and ferromagnetic SF in the basal planes and may explain the specific heat, magnetic susceptibility, and neutron scattering experiments.