Non-fermi liquid behavior in nearly charge ordered layered metals

Superconductivity and Charge Ordering in BEDT-TTF Based Organic Conductors with β″-Type Molecular Arrangement

Exotic superconductivity that appears near the charge ordering instability has attracted significant interest since the beginning of superconducting study. The discovery of possible coexistence of charge ordering and superconductivity in cuprates and kagome metals has further fascinated researchers in recent years. In this review, we focus on the BEDT-TTF-based organic superconductor with β″-type molecular packing sequence, which shows the charge ordering transition in the very vicinity of superconducting transition, and summarize the experimental results reported up to the present. At the charge ordering temperature, ultrasonic measurement detects the softening of the crystal lattice, and 13C-NMR measurement shows an increase in nuclear spin-lattice relaxation rate divided by temperature 1/T1T. These results suggest that low-energy dynamics are activated near the charge ordering transition, leading us to invoke the charge-fluctuation mediated superconducting pairing mechanism.

Modern History of Organic Conductors: An Overview

This short review article provides the reader with a summary of the history of organic conductors. To retain a neutral and objective point of view regarding the history, background, novelty, and details of each research subject within this field, a thousand references have been cited with full titles and arranged in chronological order. Among the research conducted over ~70 years, topics from the last two decades are discussed in more detail than the rest. Unlike other papers in this issue, this review will help readers to understand the origin of each topic within the field of organic conductors and how they have evolved. Due to the advancements achieved over these 70 years, the field is nearing new horizons. As history is often a reflection of the future, this review is expected to show the future directions of this research field.

Charge fluctuations and ethylene-group-ordering transition in β''-(BEDT-TTF)4[(H3O)Fe(C2O4)3]⋅Y molecular charge-transfer salts

The polarized infrared reflectance and Raman spectra of the three quasi-two-dimensional β''-(BEDT-TTF)4[(H3O)Fe(C2O4)3]⋅Y bifunctional charge-transfer salts, where BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene and Y = C6H5Br, (C6H5CN)0.17(C6H5Br)0.83, (C6H5CN)0.4(C6H5F)0.6, have been measured as a function of the temperature. Signatures of charge inhomogenity have been found in both Raman and infrared spectra of the β''-(BEDT-TTF)4[(H3O)Fe(C2O4)3]⋅Y superconductors. A 100 K transition to a mixed insulating/metallic state is clearly seen for the first time in the temperature dependence of the electronic spectra of superconducting β''-(BEDT-TTF)4[(H3O)Fe(C2O4)3]⋅C6H5Br. We suggest that this phase transition is due to subtle changes in the ethylene groups ordering, which are related to a structural phase transition in the anionic layer. The infrared and Raman spectra of quasi-two-dimensional metal α-'pseudo-κ'-(BEDT-TTF)4[(H3O)Fe(C2O4)3]C6H4Br2 are also investigated.

Quantum criticality in organic conductors? Fermi liquid versus non-Fermi-liquid behaviour

Organic metals exhibit unusual electronic properties in their charge and spin degrees of freedom that have puzzled physicists for decades. By now this behaviour is established as intrinsic and related to electronic interactions. Like other correlated electron systems, such as heavy fermions or transition-metal oxides, organic conductors are located next to some ordered phase in the spin or charge sectors. Theory predicts quantum fluctuations to become important at low temperatures and quantum critical behaviour present in most physical properties. Here we survey the experimental evidence of quantum criticality in well-established organic model compounds and look for indications of non-Fermi-liquid behaviour.

Bandwidth tuning triggers interplay of charge order and superconductivity in two-dimensional organic materials

We observe charge-order fluctuations in the quasi-two-dimensional organic superconductor β''-(BEDT-TTF)2SF5CH2CF2SO3, both by means of vibrational spectroscopy, locally probing the fluctuating charge order, and by investigating the in-plane dynamical response by infrared reflectance spectroscopy. The decrease of the effective electronic interaction in an isostructural metal suppresses both charge-order fluctuations and superconductivity, pointing to their interplay. We compare the results of our experiments with calculations on the extended Hubbard model.

Quantum critical behavior of electrons at the edge of charge order

We consider quantum critical points in which quantum fluctuations associated with charge rather than magnetic order induce unconventional metallic properties. Based on finite-T calculations on a two-dimensional extended Hubbard model, we show how the coherence scale T(*) characteristic of Fermi liquid behavior of the homogeneous metal vanishes at the onset of charge order. A strong effective mass enhancement reminiscent of heavy fermion behavior indicates the possible destruction of quasiparticles at the quantum critical points. Experimental probes on quarter-filled layered organic materials are proposed for unveiling the behavior of electrons across the quantum critical region.

Antiferromagnetic d-electron exchange via a spin-singlet pi-electron ground state in an organic conductor

Electron spin resonance reveals the spin behavior of conduction (pi) and localized (d) electrons in beta-(BDA-TTP)2MCl4 (M=Fe, Ga). Both the Ga3+(S=0) and Fe3+(S=5/2) compounds exhibit a metal-insulator transition at 113 K with the simultaneous formation of a spin-singlet ground state in the pi electron system of the donor molecules. The behavior is consistent with charge ordering in beta-(BDA-TTP)2MCl4 at the metal-insulator transition. At 5 K, the Fe3+ compound orders antiferromagnetically, even though the pi electrons, which normally would facilitate magnetic exchange, are localized nonmagnetic singlets.

Nonlocal Coulomb correlations in metals close to a charge order insulator transition

The charge ordering transition induced by the nearest-neighbor Coulomb repulsion V in the 1/4-filled extended Hubbard model is investigated using cellular dynamical mean-field theory. We find a transition to a strongly renormalized charge ordered Fermi liquid at V(CO) and a metal-to-insulator transition at V(MI)>V(CO). Short range antiferromagnetism occurs concomitantly with the CO transition. Approaching the charge ordered insulator, V approximately <V(MI), the Fermi surface deforms and the scattering rate of electrons develops momentum dependence on the Fermi surface.