Influence of charge and spin degree of freedom on superconductivity in λ-type salts

Master's thesis

The interplay between magnetic order or charge order and superconductivity

The interplay between magnetic order or charge order, on the one hand, and superconductivity, on the other hand, attracts great attention; of particular interest are organic charge-transfer salts, for which the superconducting state is found on the border between metal and insulator. In κ-(BEDT-TTF)2X organic conductors, strong dimerization of BEDT-TTF donor molecules leads to effectively half-filled bands with the main physics governed by the on-site Coulomb repulsion U. The superconducting state for such systems is placed next to the antiferromagnetically-ordered insulating state, and spin-fluctuation driven mechanism of superconductivity was proposed here. In contrast to κ-type, in β"-(BEDT-TTF)2X salts the absence of dimerization leads to three-quarter-filled conduction bands; hence the inter-site Coulomb repulsion V becomes more important and gives rise to a charge-ordered insulating ground state, which is neighboring with the superconducting one. For these systems, superconductivity is mediated by the charge fluctuations and the charge degree of freedom is more important.

λ-(BETS)2GaCl4 organic superconductor takes a special place compared to κ- and β"- type salts. In λ-(BETS)2GaCl4 spin-fluctuations are found that originate from the neighboring spin-density-wave state. Here the study of the λ-(BETS)2GaBrxCl4-x series was crucial because bromine substitution is equivalent to negative chemical pressure [1,2]. Recently we could show that charge fluctuations are also present in λ-(BETS)2GaCl4. This implies that a theoretical description has to consider both on-site and inter-site Coulomb repulsion, U and V. It also means that in λ-type organic conductors both spin- and charge- mediated mechanisms of superconductivity are important. According to theoretical predictions for the certain ratio of U and V on a square lattice at half-filling, spin- and charge-density-wave states can coexist near the superconducting state. The aim is to explain superconductivity in λ-(BETS)2GaCl4 salts by taking into account the presence of both spin and charge fluctuations.

Phase diagrams from 13C NMR measurements [2] and as theoretically proposed [3].

In this project λ-(BETS)2GaBrxCl4x  (x=0,75; 1) salts, which are placed between superconducting and non-magnetic insulating states, will be investigated by means of infrared spectroscopy and X-band ESR technique for understanding the mechanism of superconductivity in λ-type organic conductors.

[1]   H. Tanaka, A. Kobayashi, A. Sato, H. Akutsu, and H. Kobayashi, Chemical control of electrical properties and phase diagram of a series of λ-type BETS superconductors, λ-(BETS)2GaBrxCl4x, J. Am. Chem. Soc. 121, 760 (1999).
[2]   T. Kobayashi, T. Ishikawa, A. Ohnuma, M. Sawada, N. Matsunaga, H. Uehara, and A. Kawamoto, Spindensity wave in the vicinity of superconducting state in λ-(BETS)2GaBrxCl4x probed by 13C NMR spectroscopy, Phys. Rev. Research 2, 023075 (2020).
[3]   S. Onari, R. Arita, K. Kuroki, and H. Aoki, Phase diagram of the two-dimensional extended Hubbard model: Phase transitions between different pairing symmetries when charge and spin fluctuations coexist, Phys. Rev. B 70, 094523 (2004).

This image shows Olga Iakutkina

Olga Iakutkina


Mott insulators

This image shows Martin Dressel

Martin Dressel

Prof. Dr. rer. nat.

Head of Institute

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