Study on the competition between density waves, singlet, and triplet pairing superconductivity in organic conductors (TMTSF)2X

We study the effect of dimerization of TMTSF molecules and the effect of magnetic field (Zeeman splitting) on the phase competition in quasi one-dimensional organic superconductors (TMTSF)₂X by applying the random phase approximation method. As for the dimerization effect, we conclude that due to the decrease of the dimerization, which corresponds to applying the pressure and cooling, spin and charge density wave states are suppressed and give way to a superconducting state. As for the magnetic field effect, we find generally that spin-triplet pairing mediated by a coexistence of 2k_F spin and 2k_F charge fluctuations can be strongly enhanced by applying magnetic field rather than triplet pairing due to a ferromagnetic spin fluctuations. Applying the above idea to (TMTSF)₂X compounds, a magnetic field induced singlet-triplet transition is consistent with above mechanism in (TMTSF)₂ClO₄.     

Cavity size versus anion size in (TMTSF)2X salts: Possible implications for the uniqueness of (TMTSF)2C104

A simple, effective method for deducing the fit of the complex anion, X, within its structural cavity for (TMTSF)₂X salts with tetrahedral anions is presented. Subsequent analysis suggests that an electronic instability is operative over the full range of salts, becoming coincident with or leading to anion ordering and an insulating ground state above some critical fit of the anion within its structural cavity. Moreover, (TMTSF)₂C10₄ can be considered unique in that the perchlorate anion is too small for anion ordering to be the critical instability, yet large enough for anion ordering to occur at a finite temperature.     

Electronic structure and structural phase stability of CuAlX2 (X=S, Se, Te) under pressure

The electronic and structural properties of chalcopyrite compounds CuAlX₂ (X=S, Se, Te) have been studied using the first principle self-consistent Tight Binding Linear Muffin-Tin Orbital (TBLMTO) method within the local density approximation. The present study deals with the ground state properties, structural phase transition, equations of state and pressure dependence of band gap of CuAlX₂ (S, Se, Te) compounds.Electronic structure and hence total energies of these compounds have been computed as a function of reduced volume. The calculated lattice parameters are in good agreement with the available experimental results. At high pressures, structural phase transition from bct structure (chalcopyrite) to cubic structure (rock salt) is observed. The pressure induced structural phase transitions for CuAlS₂, CuAlSe₂, and CuAlTe₂ are observed at 18.01, 14.4 and 8.29 GPa, respectively. Band structures at normal as well as for high-pressure phases have been calculated. The energy band gaps for the above compounds have been calculated as a function of pressure, which indicates the metallic character of these compounds at high-pressure fcc phase. There is a large downshift in band gaps due to hybridatization of the noble-metal d levels with p levels of the other atoms.     

Role of anion ordering in the coexistence of spin-density-wave and superconductivity in (TMTSF)2ClO4

Using various transport and magnetotransport probes, we study the coexistence of spin-density wave and superconductor states in (TMTSF)₂ClO₄ at various degrees of ClO₄ anions ordering. In the two-phase complex state when both superconductivity and spin-density wave are observed in transport, we find prehistory effects, enhancement of the superconducting critical field, and strong spatial anisotropy of the superconducting state. These features are inconsistent with the conventional model of structural inhomogeneities produced by anion ordering transition. We reveal instead that superconductor and spin-density wave regions overlap on the temperature—dimerization gap V phase diagram, where V is varied by anion ordering. The effect of anion ordering on (TMTSF)₂ClO₄ properties is thus analogous to that of pressure on (TMTSF)₂X (X = PF₆ or AsF₆), thereby unifying general picture of the coexistence of superconductivity and spin-density wave in these compounds.     

Metal-insulator transition in the low-dimensional organic conductor (TMTSF)<Subscript>2</Subscript>FSO<Subscript>3</Subscript> probed by infrared microspectroscopy

We present measurements of the infrared response of the quasi-one-dimensional organic conductor (TMTSF)₂FSO₃ along (E ) and perpendicular (E ) to the stacking axis as a function of temperature. Above the metal-insulator transition related to the anion ordering the optical conductivity spectra show a Drude-like response. Below the transition an energy gap of about 1500 cm^-1 (185 meV) opens, leading to the corresponding charge transfer band in the optical conductivity spectra. The analysis of the infrared-active vibrations gives evidence for the long-range crystal structure modulation below the transition temperature and for the short-range order fluctuations of the lattice modulation above the transition temperature. We also report about a new infrared mode at around 710 cm-1 with a peculiar temperature behavior, which has so far not been observed in any other (TMTSF)₂X salt showing a metal-insulator transition. A qualitative model based on the coupling between the TMTSF molecule vibration and the reorientation of electrical dipole moment of the FSO₃ anion is proposed, in order to explain the anomalous behavior of this new mode.     

Electronic properties of chalcopyrite CuAlX2(X=S,Se,Te) compounds

We present results of the band structure and density of states for the chalcopyrite compounds CuAlX₂ (X=S,Se,Te) using the state-of-the-art full potential linear augmented plane wave (FP-LAPW) method. Our calculations show that these compounds are direct band gap semiconductors. The energy gap decreases when S is replaced by Se and Se replaced by Te in agreement with the experimental data. The values of our calculated energy gaps are closer to the experimental data than the previous calculations. The electronic structure of the upper valence band is dominated by the Cu-d and X-p interactions. The existence of Cu-d states in the upper valence band has significant effect on the optical band gap.     

Temperature dependence of the reflectance spectrum of (TMTSF)2ClO4

The near-infrared and visible reflectance spectrum was measured on (TMTSF)₂ClO₄ single crystal at various temperatures in the range of 30K–290K. The reflectance spectrum polarized parallel to the stacking axis was analyzed by Drude model, giving the results that the band width 4t is about 1 eV and m¹_p/m is about 1.3. The band parameters obtained here were almost the same as those of (TMTSF)₂PF₆ reported by Jacobsen et al.     

Anisotropic electrodynamics of low dimensional metals: Optical studies of (TMTSF)2ClO4

Optical experiments are reported for the metallic state of the linear chain compound (TMTSF)₂ClO₄. For the electric field polarized both along the highly (a) and intermediately (b ) conducting directions, a zero energy (ZE) mode and a finite energy mode (FE) are observed. The large anisotropy in the spectral weight of the FE mode is consistent with the band structure, however the spectral weight of the ZE mode is surprisingly isotropic. In the least conducting (c ^*) direction, the low frequency optical conductivity along with the dc conductivity indicate the presence of a (small) Drude component only at temperatures below 10 K. These observations provide evidence for a correlation induced semimetallic state, with a 3D to 2D crossover with increasing temperature. Received 20 May 1999     

Dipole-dipole spin anisotropy energy for spin density waves in highly anisotropic crystals: (TMTSF) 2X

The dipole-dipole spin anisotropy energy for a spin density wave (SDW) is compatible with experimental easy, intermediate, and hard spin polarization axes in the (TMTSF) ₂X class of organic conductors only when electron-hole correlations of the SDW are atomic in size. Magnitudes of the spin anisotropy energy and the spin-flip magnetic field in this case agree with experiment for a SDW amplitude comparable to 10^-2 Bohr magnetons/TMTSF molecule.     

Synthetic metals based on tetramethyltetraselenafulvalene(TMTSF): Varied anisotropy in the low temperature (125K) structures of (TMTSF)2X,X=C104-, PF6-, and AsF6-.

The low temperature (125K) X-ray crystal structures of (TMTSF)₂X, X=C10₄^-, PF₆^-, and AsF₆^- reveal decreases in the intermolecular interand intrastack Se-Se contact distances upon cooling (298K to 125K) which are highly anisotropic and different from one salt to another. The changes in the interstack distances, which are normal to the stacking direction, are approximately twice those involving intrastack Se-Se interactions. These observations establish that the anisotropic structural changes which accompany decreased temperature are common to numerous (TMTSF)₂X radical cation conducting salts.     

Anion-donor coupling in (TMTSF)2X salts: symmetry considerations

A study of the geometry of the structural cavity in which the anion resides in the novel (TMTSF)₂X salts has revealed a stronger than previously realized symmetry basis for the rationalization of the contrasting crystal physics displayed by salts with anions of grossly different molecular symmetries. Moreover, the asymmetry of the anion-cavity interactions in salts with tetrahedral anions leads to the identification of a principal structural driving force for anion ordering. Finally, additional symmetry arguments suggest that salts with dipolar anions with C_2v molecular symmetry are candidates for a low-temperature ferroelectric ground state.     

Paramagnetic contribution to the magnetic susceptibility of Bechgaard salts

Summary The crystal of Bechgaard salt (TMTSF)₂X) is considered as a system of defect-bounded finite-length fragments of the TMTSF stacks. The paramagnetic contribution χ_spin to the susceptibility of the system arises due to the thermal population of the triplet excited states of the fragments and considerably increases with temperature in accordance with experiment. The unusual dependence of the pressure fractional derivative of χ_spin on temperature is explained as well. For the average fragment length flowing to infinity our expression for χ_spin transforms into the known Pauli formula and becomes temperature independent.     

Many-body effects on the width of the band gap in Bi2Te2X (X = Te, Se, S) topological insulators

The spectrum of quasiparticles of Bi₂Te₂X (X = Te, Se, S) three-dimensional topological insulators has been theoretically studied in the GW approximation with the inclusion of the spin-orbit interaction in the construction of the Green’s function and self-energy. It has been shown that many-body corrections to the Kohn-Sham states in Bi₂Te₂X increase the fundamental band gap similar to conventional semiconductors. However, the band gap at the Γ point decreases in this case. Gaps in the quasiparticle spectrum obtained in agreement with the experimental data correspond to the difference between the minimum of the conduction band, which is located on the Γ-Z line, and the maximum of the valence band, which lies beyond the symmetric directions in the mirror plane.     

Molecular Structures of H2X and D2X(X=0, S,Se, Te)

There are two possible configurations for H₂O, linear(D_∞h) or bent(C_2v). For a C_2v′, the three bands ν_1′ ν₂ and ν₃ should appear in both Raman and infrared. For a D_∞h. however, the ν₁, band should appear in only Raman and the ν₂ and ν₃ bands, in only infrared, that is, a principle of mutual exclusion of Raman and infrared should hold. The present author concludes that H₂X and D₂X(X=O, S, Se, Te) have a linear D_∞h. structure, since the obtained spectra show mutual exclusion of Raman and infrared.     

Large triplet pairing mixing in the FFLO state of spin fluctuation mediated superconductivity in Q1D systems

We study the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) state of spin fluctuation mediated superconductivity and focus on the effect of coexisting charge fluctuations. We find that (i) consecutive transitions from singlet pairing to FFLO and further to S_z=1 triplet pairing can generally take place upon increasing the magnetic field when strong charge fluctuations coexist with spin fluctuations and (ii) the enhancement of the charge fluctuations lead to a significant increase of the parity mixing in the FFLO state, where the triplet/singlet component ratio in the gap function can be close to unity. We propose that such consecutive pairing state transition and strong parity mixing in the FFLO state may take place in a quasi-one-dimensional organic superconductor (TMTSF)₂X.     

Ab-initio study of Li based chalcopyrite compounds LiGaX2 (X= S,Se, Te) in tetragonal symmetry: A class of future materials for optoelectronic applications

Structural, electronic, optical, and thermoelectric aspects of chalcopyrite LiGaX₂ (X = S, Se and Te) compounds have been investigated by density functional theory (DFT) based Wien2k simulator. The optimized ground state parameters are calculated by Wu-Cohen generalized gradient approximation (WC-GGA) and electronic structures, which have been further improved by modified Becke-Johnson (mBJ) potential. Moreover, a comparative study is given among the contribution of three anions (S, Se and Te) in the same symmetry of tetragonal phase. The calculated band gaps of the studied compounds are 3.39, 2.83, and 1.96 eV for LiGaS_2, LiGaSe₂ and LiGaTe₂, respectively. The observed band gaps consider the studied compounds are potential materials for optoelectronic devices. In addition, the optical response of the studied materials has been analyzed in terms of dielectric constants, refraction, absorption, reflectivity and energy loss function. We have also reported the thermoelectric properties like Seebeck coefficient, thermal and electrical conductivities, and figure of merit as function of temperatures by using BoltzTrap code. The high thermal efficiency and absorption spectra in the visible region make the studied materials multifunctional for energy applications.     

Physical properties of (TMTSF)2TaF6: Influence of the anion size

The physical properties of (TMTSF)₂TaF₆ are discussed through transport and magnetic measurements. A crossover between two different regimes is observed around 100K and a spin density wave ground state appears below 11 K. The phase diagram under magnetic field is determined. Finally, the properties of this compound are compared with that of the other members of the TMTSF series and the influence of the anion size is discussed.     

Ginzburg-landau slopes of the anisotropic upper critical magnetic field and band parameters in the superconductor (TMTSF)<Subscript>2</Subscript>ClO<Subscript>4</Subscript>

We theoretically determine the Ginzburg-Landau slopes of the anisotropic upper critical magnetic field in a quasi-one-dimensional superconductor and correct the previous works on this issue. By using the experimentally measured values of the Ginzburg-Landau slopes in the superconductor (TMTSF)ClO₄, we determine band parameters of its electron spectrum. Our main result is that the so-called quantum dimensional crossover has to happen in this material in magnetic fields, H = 3–8 T, which are much lower than the previously assumed. We discuss how this fact influences metallic and superconducting properties of the (TMTSF)₂ClO₄.     

Low temperature thermodynamical properties of the organic chain conductor (TMTSF)AsF

We report on specific heat measurements of the quasi-one-dimensional organic salt (TMTSF)₂AsF₆ in its spin density wave state between 75 mK and 7 K. Similarly to (TMTSF)₂PF₆, we find discontinuities in the lattice contribution at 1.9 K an d 3.5 K ascribed to sub-spin density wave phases. Time-dependent effects due to dynamics of low-energy excitations in metastable states occur only below 0.2 K which yields an activation energy for the equilibrium energy relaxation process of 0.34 K, 4-5 times smaller than found for (TMTSF)₂PF₆. Finally the reduction of the low-energy excitations contribution to the specific heat in comparison to PF₆ reveals an intermediate cubic-like regime between 0.25 and 0.5 K that we tentatively describe as the phason contribution of the incommensurate spin density wave modulation. Received: 17 March 1998 / Revised: 27 July 1998 / Accepted: 22 September 1998