日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

This article discusses fluctuating order in a quantum disordered phase proximate to a quantum critical point, with particular emphasis on fluctuating stripe order. Optimal strategies are derived for extracting information concerning such local order from experiments, with emphasis on neutron scattering and scanning tunneling microscopy. These ideas are tested by application to two model systems---an exactly solvable one-dimensional (1D) electron gas with an impurity, and a weakly interacting 2D electron gas. Experiments on the cuprate high-temperature superconductors which can be analyzed using these strategies are extensively reviewed. The authors adduce evidence that stripe correlations are widespread in the cuprates. They compare and contrast the advantages of two limiting perspectives on the high-temperature superconductor: weak coupling, in which correlation effects are treated as a perturbation on an underlying metallic (although renormalized) Fermi-liquid state, and strong coupling, in which the magnetism is associated with well-defined localized spins, and stripes are viewed as a form of micro phase separation. The authors present quantitative indicators that the latter view better accounts for the observed stripe phenomena in the cuprates.

How to detect fluctuating stripes in the high-temperature superconductors

The basic aspects of electrons in graphene (two-dimensional graphite) exposed to a strong perpendicular magnetic field are reviewed. One of its most salient features is the relativistic quantum Hall effect, the observation of which has been the experimental breakthrough in identifying pseudorelativistic massless charge carriers as the low-energy excitations in graphene. The effect may be understood in terms of Landau quantization for massless Dirac fermions, which is also the theoretical basis for the understanding of more involved phenomena due to electronic interactions. The role of electron-electron interactions both in the weak-coupling limit, where the electron-hole excitations are determined by collective modes, and in the strong-coupling regime of partially filled relativistic Landau levels are presented. In the latter limit, exotic ferromagnetic phases and incompressible quantum liquids are expected to be at the origin of recently observed (fractional) quantum Hall states. Furthermore, the electron-phonon coupling in a strong magnetic field is discussed. Although the present review has a dominant theoretical character, a close connection with available experimental observation is intended.

Electronic properties of graphene in a strong magnetic field

A wide range of materials, like d-wave superconductors, graphene, and topological insulators, share a fundamental similarity: their low-energy fermionic excitations behave as massless Dirac particles rather than fermions obeying the usual Schrodinger Hamiltonian This emergent behavior of Dirac fermions in condensed matter systems defines the unifying framework for a class of materials we call "Dirac materials'' In order to establish this class of materials, we illustrate how Dirac fermions emerge in multiple entirely different condensed matter systems and we discuss how Dirac fermions have been identified experimentally using electron spectroscopy techniques (angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy) As a consequence of their common low-energy excitations, this diverse set of materials shares a significant number of universal properties in the low-energy (infrared) limit We review these common properties including nodal points in the excitation spectrum, density of states, specific heat, transport, thermodynamic properties, impurity resonances, and magnetic field responses, as well as discuss many-body interaction effects We further review how the emergence of Dirac excitations is controlled by specific symmetries of the material, such as time-reversal, gauge, and spin-orbit symmetries, and how by breaking these symmetries a finite Dirac mass is generated We give examples of how the interaction of Dirac fermions with their distinct real material background leads to rich novel physics with common fingerprints such as the suppression of back scattering and impurity-induced resonant states

Dirac materials

Intermetallic compounds containing $f$-electron elements display a wealth of superconducting phases, which are prime candidates for unconventional pairing with complex order parameter symmetries. For instance, superconductivity has been found at the border of magnetic order as well as deep within ferromagnetically and antiferromagnetically ordered states, suggesting that magnetism may promote rather than destroy superconductivity. Superconducting phases near valence transitions or in the vicinity of magnetopolar order are candidates for new superconductive pairing interactions such as fluctuations of the conduction electron density or the crystal electric field, respectively. The experimental status of the study of the superconducting phases of $f$-electron compounds is reviewed.

Superconducting phases of f -electron compounds

We show a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 salt, which emerges under uniaxial pressure along the a -axis (the stacking axis of t...

Pressure-Induced Zero-Gap Semiconducting State in Organic Conductor α-(BEDT-TTF)2I3 Salt

We show a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ salt, which emerges under uniaxial pressure along the a-axis (the stacking axis of the BEDT-TTF molecule). The ZGS state is the state in which a Dirac cone with the band spectrum of a linear dispersion exists around the Fermi point connecting an unoccupied (electron) band with an occupied (hole) band. The spectrum exhibits a large anisotropy in velocity, which depends on the direction from the Fermi point. By varying the magnitude of several transfer energies of a tight-binding model with four sites per unit cell, it is shown that the ZGS state exists in a wide pressure range, and is attributable to the large anisotropy of the transfer energies along the stacking axis.

https://arxiv.org/pdf/cond-mat/0601068.pdf

Pressure-Induced Zero-Gap Semiconducting State in Organic Conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ Salt

The electronic states in two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 have been investigated to show the noticeable property of the massless fermions, i.e., the linear dispersion which exists on the contact point between the conduction band and the valence band. These fermions are well known in bismuth and graphite, where the former are described by the Dirac equation and the latter obeys the Weyl equation corresponding to the massless fermion. In the present study, we show that the effective Hamiltonian describing the massless fermions in α-(BEDT-TTF) 2 I 3 contains intrinsically new terms of Pauli matrices σ z and σ 0 in addition to the Weyl equation which consists of σ x and σ y . The new massless fermions are robust against the charge disproportionation, and induce the anomalous momentum-dependence in the charge density.

/pdf/massless-fermions-in-organic-conductor-5ugw7pjtka.pdf

Massless Fermions in Organic Conductor

The transport phenomena of zero-gap conductors are one of the central subjects in condensed matter physics. α-(BEDT-TTF)2I3 [BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalene] is a typical zero-gap conductor consisting of two-dimensional multiple two-dimensional layers. Under high pressures of above 1.5 GPa, it undergoes a phase transition to a zero-gap state, in which it exhibits unusual transport phenomena. This paper reviews the recent progress in clarifying the physics of α-(BEDT-TTF)2I3. In particular, the effect of a magnetic field on the behavior of electrons is investigated. On the basis of the interlayer magnetoresistance, evidence of the Landau level with the index N = 0 was obtained. The collaboration of experiments and theory has opened a new field of exploring the interlayer electron transport in two-dimensional layered zero-gap conductors. Furthermore, these phenomena have been observed by NMR and specific heat measurements in contrast to the case of graphene. The paper also reviews some rec...

Molecular Dirac Fermion Systems — Theoretical and Experimental Approaches —

We theoretically examine superconductivity in quasi-two-dimensional organic conductor, α-(ET) 2 I 3 salt, which has been found under uniaxial pressure and at temperature below the charge ordering. The interplay of many bands and several repulsive interactions gives rise to novel role of the charge ordering (CO), which leads to the coexistence with the superconducting (SC) state. The hole and electron pockets at Fermi surface, which exist in normal state and at ambient pressure, vanish in the charge ordered insulating state due to the formation of the charge gap. Under pressures, the charge gap disappears and the narrow gap semiconductor (NGS) with CO exhibits the density of state with a sharp peak just below the Fermi energy. Based on the mean field of CO state, charge and spin susceptibilities, and the paring interaction are evaluated by using the random phase approximation. The onset for the SC state is calculated in terms of a linearized Eliashberg equation, where the pairing interaction consists of bo...

Akito Kobayashi

Papers

Pressure-Induced Zero-Gap Semiconducting State in Organic Conductor α-(BEDT-TTF)2I3 Salt

Pressure-Induced Zero-Gap Semiconducting State in Organic Conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ Salt

Massless Fermions in Organic Conductor

Molecular Dirac Fermion Systems — Theoretical and Experimental Approaches —

Superconductivity in Charge Ordered Organic Conductor –α-(ET)2I3 Salt–