Dimensional Organic Superconductor Research Articles

Seebeck coefficient in correlated low-dimensional organic metals

We study the influence of inelastic electron-electron scattering on the temperature variation of the Seebeck coefficient in the normal phase of quasi-one-dimensional organic superconductors. The theory is based on the numerical solution of the semi-classical Boltzmann equation for which the collision integral equation is solved with the aid of the electronic umklapp scattering vertex calculated by the renormalization group method. We show that the one-loop renormalization group flow of momentum and temperature dependent umklapp scattering, in the presence of nesting alterations of the Fermi surface, introduce electron-hole asymmetry in the energy dependence of the anisotropic scattering time. This is responsible for the enhancement of the Seebeck coefficient with respect to the band $T$-linear prediction and even its sign reversal around the quantum critical point of the phase diagram, namely where the interplay between antiferromagnetism and superconductivity along with the strength of spin fluctuations are the strongest. Comparison of the results with available data on low dimensional organic superconductors is presented and critically discussed.

Vortex Dynamics and Diamagnetic Torque Signals in Two Dimensional Organic Superconductor λ-(BETS)2GaCl4

Resistance and magnetic torque measurements have been performed for a two dimensional organic superconductor λ-(BETS)2GaCl4, where BETS stands for bis(ethylenedithio)tetraselenafulvalene. In magnetic fields along the c-axis (parallel to the conducting layers), the interlayer resistance shows a peak at ∼7 T for large currents in the superconducting state. The peak disappears when the magnetic field is tilted from the layers only by 0.2°, which shows that the peak is caused by the dynamics of the Josephson vortices penetrating in the insulating layers. The current–voltage characteristics show that the depinning current of the Josephson vortices is consistently suppressed at around 7 T. The diamagnetic signal, irreversibility field, and depinning current rapidly decrease with increasing field above ∼10 T at low temperatures. The results suggest that there exists a phase boundary in the superconducting state, likely a transition to a Fulde–Ferrell–Larkin–Ovchinnikov phase.

Antiferromagnetic fluctuations in a quasi-two-dimensional organic superconductor detected by Raman spectroscopy

Using Raman scattering, the quasi-two dimensional organic superconductor k-(BEDT-TTF)2Cu[N(CN)2]Br (T_c=11.8 K) and the related antiferromagnet k-(BEDT-TTF)2Cu[N(CN)2]Cl are studied. Raman scattering provides unique spectroscopic information about magnetic degrees of freedom that has been otherwise unavailable on such organic conductors. Below T=200 K a broad band at about 500 cm-1 develops in both compounds. We identify this band with two-magnon excitation. The position and the temperature dependence of the spectral weight are similar in the antiferromagnet and in the metallic Fermi-liquid. We conclude that antiferromagnetic correlations are similarly present in the magnetic insulator and the Fermi-liquid state of the superconductor.

Investigation of fermionic pairing on tight binding lattice for low dimensional systems – Fermi liquid vs. Luttinger–Tomonaga liquid

Cooper's original one pair problem in continuum is revisited here corresponding to a lattice of tight binding nature, with an aim to investigate superconductivity in low dimensional systems. An electronic type of boson mediated attraction in a passive Fermi sea-like background is considered for the pairing mechanism with the non-trivial energy dependence of the electronic density of states taken into account in the calculation in a rigorous way. Some of the very important electronic and optical properties in the normal phase of quasi one dimensional organic superconductors are used for the development of the formalism and calculations. The results of our calculations show that a realistic fermionic pair formation is indeed possible with some constraints, without any necessity at all of invoking Luttinger–Tomonaga liquid (LTL) theory. Similarities emerge in the physical properties of the electron pair formed from Cooper's treatment corresponding to continuum and ours, excepting the striking difference appearing in the form of occurrences of a maximum allowed band filling for pairing and of an upper bound of the pairing energy found in our approach.

Theory of 'hidden' quasi-1D superconductivity in Sr2RuO4

Is the mechanism of unconventional superconductivity in Sr2RuO4 closer in spirit to superfluid 3He, or to the cuprates, pnictides, and organic superconductors? We challenge prevailing assumptions in this field and using well-controlled perturbative renormalization group calculations, we suggest that superconductivity in Sr2RuO4 resembles more closely the quasi-one dimensional organic superconductors. Our theory has certain phenomenological consequences that are consistent with the experimentally observed phenomena.

Beyond the quantum spin liquid concept in frustrated two dimensional organic superconductors

AbstractThe occurrence of antiferromagnetism (AFM) in κ‐(ET)2X can be understood within an effective 1/2‐filled band with dimers of ET molecules containing one hole each. We argue that while this effective model can describe the presence of AFM, a complete description for these materials requires the correct carrier density of one‐half per molecule. For dimerized and strongly frustrated 1/4‐filled lattices we show that a singlet‐paired statecoexisting with charge ordering occurs that we have termed the paired electron crystal (PEC). Here we investigate the 1/4‐filled model on a dimerized lattice, showing regions where AFM, PEC, and the Wigner‐crystal occur. We point out the need to go beyond quantum spin liquid concepts for highly frustrated materials such as κ‐(ET)2Cu2(CN)3 and β′‐EtMe3Sb[Pd(dmit)2]2 which we believe are PECs at low temperatures.

Organic Superconductors: When Correlations and Magnetism Walk in

This survey provides a brief account for the start of organic superconductivity motivated by the quest for high Tc superconductors and its development since the eighties'. Besides superconductivity found in 1D organics in 1980, progresses in this field of research have contributed to better understand the physics of low dimensional conductors highlighted by the wealth of new remarkable properties. Correlations conspire to govern the low temperature properties of the metallic phase. The contribution of antiferromagnetic fluctuations to the interchain Cooper pairing proposed by the theory is borne out by experimental investigations and supports supercondutivity emerging from a non Fermi liquid background. Quasi one dimensional organic superconductors can therefore be considered as simple prototype systems for the more complex high Tc materials.

Inhomogeneous superconductivity in organic conductors: the role of disorder and magnetic field

Several experimental studies have shown the presence of spatially inhomogeneous phase coexistence of superconducting and non-superconducting domains in low dimensional organic superconductors. The superconducting properties of these systems are found to be strongly dependent on the amount of disorder introduced in the sample regardless of its origin. The suppression of the superconducting transition temperature Tc shows a clear discrepancy with the result expected from the Abrikosov–Gor’kov law giving the behavior of Tc with impurities. On the basis of the time dependent Ginzburg–Landau theory, we derive a model to account for this striking feature of Tc in organic superconductors for different types of disorder by considering the segregated texture of the system. We show that the calculated Tc quantitatively agrees with experiments. We also focus on the effect of superconducting fluctuations on the upper critical fields Hc2 of layered superconductors showing slab structure where superconducting domains are sandwiched by non-superconducting regions. We found that Hc2 may be strongly enhanced by such fluctuations.

Role of charge fluctuation in Q1D organic superconductor (TMTSF) 2ClO 4

We have theoretically investigated the spin and charge fluctuations in the quasi-one dimensional organic superconductor (TMTSF) 2ClO 4. Using the extended multi-site Hubbard model, which contains four sites in a unit cell and the transfer energies obtained by the extended Hückel method, we calculate the linearized gap equation with the random phase approximation, to find novel order parameters of superconductivity due to several kinds of charge fluctuations induced by the anisotropic intersite repulsive interactions. For the singlet state, the order parameter with line nodes appears in the case of the strong charge fluctuation, while the order parameter with anisotropic gap suggested by Shimahara is reproduced in the spin fluctuation. The triplet state is also obtained for the wide parameter range of repulsive interactions due to a cooperation between charge and spin fluctuations.

Susceptibility of the quasi-one-dimensional electrons in a periodic potential

The effects of the strength ( V) of the potential caused by the anion ordering on the field-induced spin density wave (FISDW) in the quasi-one dimensional organic superconductor (TMTSF) 2ClO 4 is studied theoretically. From the result of the susceptibility ( χ 0( Q)) in the absence of the magnetic field, we show that the position of the largest peak of χ 0( Q) at B = 0 continuously changes from Q ≃ Q 0 + q 4 to Q ≃ Q 0 + q 3 upon increasing V, where q 4 = ( 1 ℏ ν F 24 t b ′ 1 + 128 ( t b ′ t b ) 2 + 1 , ± 2 b sin − 1 [ 8 ( t b ′ t b ) 1 + 128 ( t b ′ t b ) 2 + 1 ] ) and q 3 = ( 1 ℏ ν F ( 4 t b ′ + 4 t 4 ) , 0 ) . This change of the maximum peak at B = 0 is related to the stability of FISDW.

On the possible role of antiferro-magnetic short range order in quasi one dimensional organic superconductors

The possible influence on superconductivity of short range antiferromagnetic order observed in some organic conductors is compared with the case of cuprates.

Disorder effect on the superconductivity of the organic superconductor κ-(BEDT-TTF)2Cu(NCS)2partly substituted by the deuterated molecules

The effect of the disorder induced by the deuterated BEDT-TTF (d-BEDTTTF) molecule substitution on the superconductivity has been investigated in the quasi two dimensional organic superconductor κ-[(h-BEDT-TTF)1−x(d-BEDT-TTF)x]2Cu(NCS)2. The scattering time τ obtained by the de Haas-van Alphen effect changes in the form of 1/τ ∝ x(1 − x), which is known as Nordheim relation. Although the scattering time takes minimum around x = 0.5, the superconducting transition temperature Tc increases linearly with x. The relation between the scattering time and Tcis not simply explained by the nonmagnetic impurity effect in the unconventional superconductivity expected in this organic superconductor.

Unconventional Superconductivity in a Quasi-One-Dimensional System (TMTSF)2X

The nature of superconductivity in a quasi-one-dimensional organic system has been controversial since its discovery in the early 80's. Here, we discuss recent developments regarding the issue based mainly on studies of the upper critical field H c2 (T) and the nuclear magnetic resonance (NMR) Knight shift. A strong upward curvature found in the temperature dependent upper critical field and an absence of the shift in the resonant adsorption frequency upon cooling through the superconducting transition strongly support the notion that the nature of the low dimensional organic superconductor is spin triplet.

Suppression of superconductivity by non-magnetic disorder in the organic superconductor $\mathsf{(TMTSF)_{2}(ClO_{4})_{(1-x)}(ReO_{4})_{x}}$

We present a study of the superconducting properties (Tc and Hc2) in the solid solution (TMTSF)2(ClO4)(1-x)(ReO4)x with a ReO-4 nominal concentration up to x = 6%. The dramatic suppression of Tc when the residual resistivity is increased upon alloying with no modification of the Fermi surface is the signature of non-conventional superconductivity . This behaviour strongly supports p or d wave pairing in quasi one dimensional organic superconductors. The determination of the electron lifetime in the normal state at low temperature confirms that a single particle Drude model is unable to explain the temperature dependence of the conductivity and that a very narrow zero frequency mode must be taken into account for the interpretation of the transport properties.

From nodal liquid to nodal charge gap in a frustrated Hubbard model

We investigate the physics of frustrated 3-leg Hubbard ladders in the band limit, when hopping across the ladder's rungs (t$_{\perp}$) is of the same order as hopping along them (t) much greater than the onsite Coulomb repulsion (U). We show that this model exhibits a striking electron-hole asymmetry close to half-filling: the hole-doped system at low temperatures develops a Resonating Valence Bond (RVB)-like d-wave gap (pseudogap close to ($\pi$,0)) coinciding with gapless nodal excitations (nodal liquid); in contrast, the electron-doped system is seen to develop a Mott gap at the nodes, whilst retaining a metallic character of its majority Fermi surface. At lower temperatures in the electron-doped case, d-wave superconducting correlations -- here, coexisting with gapped nodal excitations -- are already seen to arise. Upon further doping the hole-doped case, the RVB-like state yields to d-wave superconductivity. Such physics is reminiscent of that exhibited by the high temperature cuprate superconductors--notably electron-hole asymmetry as noted by Angle Resolved PhotoEmission Spectroscopy (ARPES) and the resistivity exponents observed. This toy model also reinforces the importance of a more thorough experimental investigation of the known 3-leg ladder cuprate systems, and may have some bearing on low dimensional organic superconductors.

Fermi surface in magnetic-field-induced superconductor κ-(BETS) 2FeBr 4

In order to investigate the Fermi surface of the quasi-two dimensional (Q2D) organic superconductor κ-(BETS) 2FeBr 4, the Shubnikov-de Haas (SdH) oscillation and the angular-dependent magnetoresistance oscillation (AMRO) have been measured. In the antiferromagnetic (AF) state, we observed the SdH oscillation with the low frequency of about 62 T that has not been ever reported. This new frequency can be ascribed to a small pocket formed by the reconstruction of the Fermi surface (FS) due to the long range AF order. In the AMRO, clear periodic oscillation as a function of tan θ was found. The oscillation shows the presence of a 2D FS as predicted by band calculations. The analyses of the periodicity indicate that the cross-section of the FS has an elliptic shape. In the magnetic field almost parallel to the layers, a peak in the resistance is observed, suggesting the coherent transport in the interlayer direction.

Strong evidence of field-induced superconductivity and Shubnikov-de Haas oscillation in κ -(BETS)2FeBr4

High field electronic state of the quasi-two dimensional organic superconductor, κ-(BETS) 2 FeBr 4 was investigated. Under the magnetic field exactly parallel to the conducting layers, zero resistance was observed in the region of about 11 ∼ 14 T at 27 mK. This is the strong evidence of the field induced superconductivity in this salt. In the antiferromagnetic state below 5 T, Shubnikov-de Haas oscillation with a low frequency of 62 T was observed. This new frequency can be ascribed to a small pocket formed by the reconstruction of the Fermi surface due to the antiferromagnetic order.

Shubnikov–de Haas effect in the quantum vortex liquid state of the organic superconductorκ−(BEDT−TTF)2Cu(NCS)2

We report the Shubnikov-de Haas (SdH) oscillations observed in the vortex liquid state of the quasi two dimensional organic superconductor $\kappa$-(BEDT-TTF)$_{2}$Cu(NCS)$_{2}$. The SdH oscillations can be observed down to about 5 T at 0.5 K, where the flux flow resistivity becomes as small as about 30 % of the normal state value. Below the upper critical field $H_{\rm c2}$ of about 7 T, the additional damping of the SdH oscillation amplitude appears, as well as that of the de Haas-van Alphen (dHvA) oscillations, with respect to the normal state one which is described with the standard Lifshitz-Kosevich formula. The magnitude of the additional damping near $H_{\rm c2}$ is the same with that observed in the dHvA oscillations and well explained by the theoretical predictions in consideration of fluctuations in the thermal vortex liquid state. In the quantum fluctuation region at lower temperature, however, only SdH effect shows the stronger damping than that of the dHvA oscillations. The different magnetic field dependence of the additional damping of the oscillation amplitude between the SdH and dHvA effects is discussed in connection with the effect of the transport current on the short-range order of vortices in the quantum vortex slush state reported at the same temperature and magnetic field region.

Theoretical study on the tunneling spectrum of quasi-one dimensional organic superconductors (TMTSF) 2X

Pairing symmetries ( p, ‘ d’ and ‘ f’ waves) are analyzed for the organic superconductor (TMTSF) 2 X in terms of the “generic phase diagram” against the spin/charge fluctuations and the magnetic anisotropy. We then propose that the symmetry can be identified from the tunneling spectroscopy (presence/absence of zero bias peaks along with the shape of the gap) by calculating the surface density of states using an extended Hubbard model.

Transverse and in-plane modification of superconductivity and electronic structure in the quasi-two-dimensional organie conductor K-(BEDT-TTF)2CU(SCN)2 by uniaxial stress

We have employed uniaxial stress along the principal axes of the quasi-two dimensional organic superconductor $\kappa $--(BEDT-TTF)$_{2}$Cu(SCN)$_{2}$. The lattice anisotropy is thereby altered, with corresponding changes in the intermolecular transfer energies. The effect of uniaxial stress on the superconducting transition temperature $T_{c}$ and critical field $B_{c2}$ is found to be anisotropic.There is an indication of an increase in $T_{c}$ and $B_{c2}$ for in-plane stress, but both parameters decrease rapidly for transverse (inter-plane) stress. Magnetotransport studies reveal stress-induced changes in the Fermi surface through the observation of the Shubnikov de Haas oscillations. The stress dependence of a resistive anomaly in the magnetoresistance, which is associated with the critical field $B_{c2}$, is also investigated. We discuss the experimental findings in the context of recent phenomenological and theoretical treatments of quasi-two dimensional systems where the anisotropic triangular lattice Hubbard model has been used to treat two-dimensional superconductors.