Exotic superconductivity in strongly anisotropic correlatedorganic metals in the vicinity of insulating phases
Final Report Abstract
Thanks to high crystal quality, relatively simple conduction band structures, and very good tunability between various electronic states, organic charge transfer salts can often serve as model systems for studying correlated electron physics. In this project three families of layered organic conductors have been chosen for studying the impact of competing ordering phenomena on the conducting and superconducting properties and searching for possible manifestations of exotic superconductivity. In the salts α-(ET)2 MHg(SCN)4 (M = K, Tl) superconductivity exists in close proximity to a chargedensity-wave (CDW) state. At sufficiently high pressures the CDW ordering is suppressed, giving way to a low-temperature, very anisotropic superconductivity. The ratio between the critical magnetic fields aligned parallel and perpendicular to molecular layers was found to be higher than ever reported for a layered superconductor. As a consequence, the Pauli paramagnetism of charge carriers turns out to be the main limiting mechanism for superconductivity in fields parallel to the layers, by contrast to the orbital mechanism dominating in usual superconductors. One might expect an exotic inhomogeneous superconductivity with a modulated order parameter (Fulde-Ferrel-Larkin-Ovchinnikov state) to emerge at high enough fields. However, no evidence for that has been found so far, probably due to the very large coherence length, ∼ 500 nm, comparable with the mean free path even in very clean crystals. At pressures below critical, a strong evidence in favor of the proposed earlier Fermi surface reconstruction has been obtained. Furthermore, the behavior of the oscillations suggests the existence of narrow domain walls with a suppressed CDW order parameter. Superconductivity coexists with the CDW phase, surviving mainly in the CDW domain walls. Additionally, strong pairing fluctuations seem to emerge directly inside the CDW phase. The fact that they arise already at much higher, than the main transition, temperatures suggests a significant role of the competing CDW ordering in formation of these fluctuations. The κ-(BETS)2 FeY4 salts with Y = Cl, Br can be viewed as perfect natural nanostructures with alternating single-molecular conducting and magnetic layers. At low temperatures the magnetic sublattice of Fe3+ spins in the insulating layers undergoes antiferromagnetic (AFM) ordering, whereas the π-electrons of the organic BETS layers remain metallic and even superconducting. The impact of the AFM ordering on the conducting system is clearly seen in the magnetic quantum oscillations, which reveal a Fermi surface reconstruction and suppression of the Zeeman splitting of conduction electrons in magnetic field. Moreover, in the Br-salt the π-d exchange interaction leads, besides the observed earlier high-field re-entrant superconductivity, to an unusual protection of the low-field superconducting state by the AFM ordering in the fields parallel to the easy axis. The “back action” of π electrons on the magnetic d-electron subsystem can be traced in the shape of the magnetic phase diagram. The stronger role of the π-d exchange in the AFM ordering in the Cl-salt leads, through reducing the inplane anisotropy, to a spin-flop transition absent in the Br-salt. Another natural hybrid multilayered structure, κ-(BETS)2 Mn[N(CN)2 ]3 , turns out to be an example of virtually non-interacting magnetic and conducting subsystems. Here the insulating electronic ground state is fully determined by the Mott instability of the π-electron system. The resulting AFM ordering of π electrons has only a marginal effect on the frustrated paramagnetic d-electron spins in the polymeric anion layers. In the pressurized, metallic state the presence of the localized d-electron spins could only be traced via weak features in the angle-dependent semiclassical and quantum magnetoresistance oscillations. In all the present salts superconductivity is found to coexist with the charge- or spin-ordered states. However, except the case of κ-(BETS)2 FeBr4 , where the conducting and magnetically ordered subsystems are clearly separated in space, superconductivity occupies only a minor volume fraction in the coexistence region. No definite evidence of a non-s-wave superconducting pairing has been found so far, although some indirect hints, like a high sensitivity to the crystal quality and the presence of a 4-fold component in the angular dependence of the critical field, call for further studies in this direction.
Publications
- “Magnetic quantum oscillations in the charge-density-wave state of the organic metals α-(BEDT-TTF)2 MHg(SCN)4 with M = K and Tl.” Low Temp. Phys. 40, 377 (2014)
M. V. Kartsovnik, V. N. Zverev, D. Andres, W. Biberacher, T. Helm, P. D. Grigoriev, R. Ramazashvili, N. D. Kushch and H. Mü ller
(See online at https://doi.org/10.1063/1.4869592) - “Interplay Between Conducting and Magnetic Systems in the Antiferromagnetic Organic Superconductor κ-(BETS)2 FeBr4.” J. Supercond. Nov. Magn. 29, 3075 (2016)
M. V. Kartsovnik, M. Kunz, L. Schaidhammer, F. Kollmannsberger, W. Biberacher, N. D. Kushch, A. Miyazaki, and H. Fujiwara
(See online at https://doi.org/10.1007/s10948-016-3829-z) - “Resistive properties and phase diagram of the organic antiferromagnetic metal κ-(BETS)2 FeCl4.” Phys. Rev. B 94, 205104 (2016)
M. Kunz, W. Biberacher, N.D. Kushch, A. Miyazaki, and M. V. Kartsovnik
(See online at https://doi.org/10.1103/PhysRevB.94.205104) - “Interplay between the d- and π-electron systems in magnetic torque of the layered organic conductor κ-(BETS)2 Mn[N(CN)2]3.” 2017
O. M. Vyaselev, W. Biberacher, N. D. Kushch, and M. V. Kartsovnik
(See online at https://doi.org/10.1103/PhysRevB.96.205154) - “New radical cation salt κ-(BETS)2 Co0.13 Mn0.87 [N(CN)2]3 with two magnetic metals: Synthesis, structure, conductivity and magnetic peculiarities.” Synth. Metals 227, 52 (2017)
N. D. Kushch, O. M. Vyaselev, V. N. Zverev, W. Biberacher, L. I. Buravov, E. B. Yagubskii, E. Herdtweck, E. Canadell, and M. V. Kartsovnik
(See online at https://doi.org/10.1016/j.synthmet.2017.03.008) - “Shubnikov-de Haas oscillations and electronic correlations in the layered organic metal κ-(BETS)2 Mn[N(CN)2]3.” Low Temp. Phys. 43, 239 (2017)
M. V. Kartsovnik, V. N. Zverev, W. Biberacher, S. V. Simonov, I. Sheikin, N. D. Kushch, and E. B. Yagubskii
(See online at https://doi.org/10.1063/1.4976634)
