Superconductivity In Organic Compounds Research Articles

Spectroscopic evidence of low-energy gaps persisting up to 120 K in surface-dopedp-terphenyl crystals

The possibility of high temperature superconductivity in organic compounds has been discussed since the pioneering work of Little in 1964, with unsatisfactory progress until the recent report of a weak Meissner shielding effect at 120 Kelvin in potassium-doped para-terphenyl samples. To date however, no other signals of the superconductivity have been shown, including the zero-resistance state or evidence for the formation of the Cooper pairs that are inherent to the superconducting state. Here, using high-resolution photoemission spectroscopy on potassium surface-doped para-terphenyl crystals, we uncover low energy gaps that persist to approximately 120 K. Among a few potential origins for these gaps, we argue that the onset of electron pairing within molecules is most likely. And while pairing gaps are a prerequisite for high temperature superconductivity they do not guarantee it. Rather, the development of long-range phase coherence between the paired states on the molecules is necessary, requiring good wavefunction overlap between molecular states--something that is in general difficult for such weakly overlapping molecules.

Unconventional superconductivity pairing induced by antiferromagnetic spin fluctuations in layered organic superconductors

Abstract The unconventional character of the superconductivity in organic compounds κ - ( ET ) 2 X is ascribed to an antiferromagnetic spin fluctuation induced pairing. Since the band structure involves two bands ( ± ), we assume that the large amplitude spin fluctuations arise from the band with the best nesting properties (band + ), while superconductivity pairing occurs in the other band ( - ) . We show that the nesting properties may mimic either a chemical pressure (deuterization) or a hydrostatic one. Indeed, a change of the nesting ratio t 1 / t 2 , according to our model, induces a modification of the Fermi surface topology. The spin fluctuations strength in the system is affected and consequently the calculated effective coupling constant of superconductivity for the even-parity singlet pairing channel. Our theory appears to be qualitatively consistent with major experimental reports.

Superconductivity in Organic Compounds with Pseudo-Triangular Lattice

We study spin fluctuation (SF) mediated superconductivity (SC) in a half-filled square lattice Hubbard model with the transfer matrices -t between nearest neighbor sites and -t' between a half of next nearest neighbor sites neighboring along only one of the <1,1> directions, considering application of this model to organic kappa-(BEDT-TTF)2X compounds. Varying the t'/t value from 0 to 1, one can interpolate between a square and an equilateral triangular lattice, the latter giving frustration to antiferromagnetically (AF) coupled spin systems. Within the fluctuation exchange (FLEX) approximation, we calculate chi(q,omega), Tc and the SC order parameter for various model parameter values and find that both AF and SC are suppressed as one approaches the frustration geometry or |(t'/t)-1| \to 0. The SC phase, however, extends beyond the AF phase boundary fairly close to t'/t=1 for realistic U/t values. The order parameter is of x2-y2-type for t'/t<1 and of xy-type for t'/t>1.

Essential Importance of Dimerization for the Superconductivity in Organic Compounds (ET)2X

Essential Importance of Dimerization for the Superconductivity in Organic Compounds (ET)₂<i>X</i>

Spin fluctuation-induced superconductivity in organic compounds

Spin fluctuation-induced superconductivity in quasi-two-dimensional organic compounds such as κ-(BEDT-TTF) 2X is investigated within a fluctuation exchange approximation using a half-filled dimer Hubbard model with right-angled isosceles triangular lattice. The calculated value for T c is in good agreement with experiment and the ( U/ τ)-dependence of T c compares qualitatively well with the observed pressure dependence of T c. The pairing symmetry is of ( x 2− y 2)-type and the energy gap develops with decreasing temperature below T c more rapidly than in the BCS model.

Spin Fluctuation-Induced Superconductivity in Organic Compounds

Spin fluctuation-induced superconductivity in two-dimensional organic compounds such as \kappa-(ET)_2-X is investigated by using a simplified dimer Hubbard model with right-angled isosceles triangular lattice (transfer matrices -\tau, -\tau^\prime). The dynamical susceptiblity and the self-energy are calculated self-consistently within the fluctuation exchange approximation and the value for T_c as obtained by solving the linearized Eliashberg-type equations is in good agreement with experiment. The pairing symmetry is of d_{x^2-y^2} type. The calculated (U/\tau)-dependence of T_c compares qualitatively well with the observed pressure dependence of T_c. Varying the value for \tau^\prime/\tau from 0 to 1 we interpolate between the square lattice and the regular triangular lattice and find firstly that values of T_c for \kappa-(ET)_2-X and cuprates scale well and secondly that T_c tends to decrease with increasing \tau^\prime/\tau and no superconductivity is found for \tau^\prime/\tau=1, the regular triangular lattice.