Spin Transport in a TwoDimensional Tilted Dirac Electron System
© The Physical Society of Japan
This article is on
Anomalous Spin Transport Properties of Gapped Dirac Electrons with Tilting
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 91, 023708 (2022).
We propose that spin current in an organic conductor, α(BETS)_{2}I_{3}, is an appropriate physical quantity to detect the topological nature of the material whose electrons obey a quasitwodimensional Dirac equation.
In quantum mechanics, electrons are described by wave forms, and thus, their wave functions are characterized by wave numbers in the Fourier space. Recently, the topological nature of the wave functions in the Fourier space (that is, momentum space) has been confirmed to provide various peculiar properties to materials, which are called “topological materials.” However, extracting the topological nature as an observable quantity in a bulk material is difficult. In this paper, we propose that spin current in an organic conductor is an appropriate physical quantity to detect the topological property of the material.
The organic conductors considered in this study are layered materials composed of relatively large molecules, and the electrons can move around hopping on various molecules forming a quasitwodimensional electron system. Material parameters such as the lattice constant, kinetic energy of electrons, and interaction between particles can be rather easily controlled by pressure or by substituting molecules with slightly different molecules. Consequently, organic conductors demonstrate various interesting properties as shown in a wide variety of phase diagrams of parameters. Recently, related to the topological nature, a certain class of organic conductors whose electrons obey the quasitwodimensional Dirac equation has been focused on. The Dirac equation was proposed by Dirac in 1928, which combined the newly born field of quantum mechanics and relativity. It predicts positrons occupying the vacuum. However, an electron–positron pair creation requires very high energy. In an organic conductor, α(BETS)_{2}I_{3}, the firstprinciples band calculation shows that electrons inside obey an equation similar to the Dirac equation, but with a very small energy gap. Therefore, in this material, the corresponding electron–positron (called hole in solids) pair creation requires considerably less energy than that for the original electron–positron pair creation. It can be created using thermal energy even at room temperatures.
Theoretically, the twodimensional Dirac electrons have been shown to have very peculiar topological properties. However, the conventional physical quantities are often written as an integral over the momentum space. Consequently, the topological nature of the Dirac electrons represented by Berry curvature vanishes after integration because the Berry curvature has two oppositesign contributions at the two distinct Dirac points in the momentum space. In this study, we noticed that this cancellation can be avoided by considering spin current, and the topological nature of the Dirac electrons in α(BETS)_{2}I_{3} can be detected in the spin current that can be measured using recent experimental techniques. More explicitly, we calculated the spin Hall conductivity and spin conductivity in a magnetic field based on a linear response theory.
(written by Masao Ogata on behalf of all authors)
Anomalous Spin Transport Properties of Gapped Dirac Electrons with Tilting
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 91, 023708 (2022).
Share this topic
Fields
Related Articles

Violation of FluctuationDissipation Theorem Results in Robustness of Fluctuation Against Localization
Statistical physics and thermodynamics
Electronic transport in condensed matter
Electronic structure and electrical properties of surfaces and nanostructures
2022323
We study equilibrium current fluctuations in systems without timereversal symmetry, violating the fluctuationdissipation theorem. Notably, the offdiagonal fluctuation is insensitive to system imperfections in contrast to other fluctuations and conductivity.

Topological Aspects of a Nonlinear System: The Dimerized Toda Lattice
Electronic structure and electrical properties of surfaces and nanostructures
Electronic transport in condensed matter
2022316
A nonlinear topological phase is shown to emerge in the dimerized Toda lattice. It is experimentally detectable by measuring the voltage propagation in electric circuits.

Unusual Nonlinear Conductivity in ChargeOrdered α(BEDTTTF)_{2}I_{3}
Electronic transport in condensed matter
202239
Unusual nonlinear conduction is detected in organic molecular salt α(BEDTTTF)_{2}I_{3}. The observed thirdorder nonlinear conductivity implies a quadrupole instability hidden in this chargeordered organic salt.

PhosphorousBased Zintl Compounds as Viable Semimetals
Electronic structure and electrical properties of surfaces and nanostructures
Structure and mechanical and thermal properties in condensed matter
Electron states in condensed matter
202224
A blackphosphorusderived Zintl compound, MoP_{4}, is obtained by highpressure synthesis. The material exhibits a nonquadratic large magnetoresistance and semimetallic Seebeck behavior, as predicted by the first principles calculations yielding massive and nonsymmorphic Dirac semimetal states.

New Systematic Analysis Method of Identifying Microscopic Essential Couplings
Electronic transport in condensed matter
Dielectric, optical, and other properties in condensed matter
Crossdisciplinary physics and related areas of science and technology
2022131
We propose a systematic method of identifying essential model parameters for nonlinear response tensors. The analysis results reveal fundamental couplings in nontrivial responses and promote further development of multiferroic responses.