Mott Transition, Antiferromagnetism, and Unconventional Superconductivity in Layered Organic Superconductors

Abstract

The phase diagram of the layered organic superconductor κ-(ET)2Cu[N(CN)2]Cl has been accurately measured from a combination of 1 H NMR and AC susceptibility techniques under helium gas pressure. The domains of stability of antiferromagnetic and superconducting long-range orders in the pressure vs temperature plane have been determined. Both phases overlap through a firstorder boundary that separates two regions of inhomogeneous phase coexistence. The boundary curve is found to merge with another first order line related to the metal-insulator transition in the paramagnetic region. This transition is found to evolve into a crossover regime above a critical point at higher temperature. The whole phase diagram features a point-like region where metallic, insulating, antiferromagnetic and non s-wave superconducting phases all meet.The determination of the conditions giving rise to superconductivity (SC) in layered organic conductors constitute one of the chief objectives in understanding the physics of these strongly correlated electronic materials [1]. Closely bound to the now classical issue of proximity of antiferromagnetism (AF) in the emergence of superconductivity stands the problem of the 'normal' phase which, depending on pressure conditions in these systems, is either a Mott insulator (MI) or an unconventional metal [2]. A pressure-driven metal-insulator transition can be thus revealing of the strong coupling conditions for electrons that are responsible for broken symmetry states [3].In this matter, the phase diagram of the series of layered organic superconductors κ-(BEDT-TTF) 2 X as a function of both hydrostatic and chemical (or anion X substitution) pressures is set to stand out of the debate. By chemical means, the study of anion substituted compounds has allowed few discrete shifts of the pressure scale. Thus for X= Cu[N(CN) 2 ]Br and X= Cu(NCS) 2 , experiments adduce growing evidence for an unconventional metal and a non s-wave SC state [3,4], whereas AF order is shown to become in turn stable on the deuterated X= d n -Cu[N(CN) 2 ]Br compound [3,5]. Among all members of the series κ−(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl, denoted as κ− Cl [6], is the prototype compound of the series showing the complete sequence of states namely, the Mott-insulating, antiferromagnetic, metallic and superconducting states, within a pressure interval of few hundred bars [2,5]. Despite the numerous experimental efforts recently expended on the properties of this salt, the information collected from experiments done under pressure remained until now scattered and limited by the selectivity of the experimental probe used. Regions of stability of the metallic and superconducting phases have been investigated whereas the information about the pressure profile of AF critical point is missing so far [2,3]. Our knowledge on the multicritical structure of opposing phases and the nature of the MI transition under pressure is also partial so that a major part of the phase diagram remained until now grounded on a conjectural rather than an ...

Robust superconducting state in the low-quasiparticle-density organic metalsβ″−(BEDT−TTF)4[(Bangura1, Coldea2, Singleton3 et al. 2005Phys. Rev. B

“…The Fermi surfaces of the salts in this study therefore only comprise one or two pockets, with the larger corresponding to a cross-sectional area ϳ5% of the Brillouinzone cross-sectional area. By contrast, the isostructural ␤Љ-͑BEDT-TTF͒ 4 Band-structure calculations on this family of salts predict two Fermi-surface pockets, of equal area ͑roughly 8.5% of the Brillouin zone͒; 14,37 by contrast, the experimental data shown in Figs. 4͑a͒-4͑c͒ suggest one or two rather smaller pockets, with unequal area.…”

“…The -͑BEDT-TTF͒ 2 X family of salts has been most heavily studied; their low-temperature superconducting phase emerges with increasing pressure from an antiferromagnetic state 4 that exhibits behavior 5 characteristic of a Mott insulator. 6 Subsequent increases of pressure gradually suppress the superconductivity, the suppression being linked to a fall in the quasiparticle density-of-states close to the Fermi energy 7,8 and an increase in the effective dimensionality of the Fermi surface.…”

Chirald-wave superconductivity in doped graphene

“…Many of these organic salts are either antiferromagnets, Mott insulators, or superconductors [196], with the phase determined by pressure [197] and frustration. They thus have strong similarities with the high-temperature cuprate superconductors, but with the distinction that they are situated on an anisotropic triangular lattice [198].…”

Infrared and Terahertz Spectroscopy of Strongly Correlated Electron Systems under Extreme Conditions

“…This behavior is the same as that in -Cl at around 30 MPa. 177,178,183) Additionally, the peak shifts to the higher-wavenumber side with phase changes to the metallic one. The peak shape and wavenumber indicate the pressure-dependent insulator-to-metal transition.…”