arXiv subjects

Masafumi Tamura

Publications and source records attributed to Masafumi Tamura.

11 recordsLinked to original sources

arXiv2026

Anomalous Phase-Coherence Scaling in a Quantum-Critical Dirac Semimetal

We have investigated the weak antilocalization (WAL) in the pressurized Dirac semimetal $α$-(BEDT-TTF)$_2$I$_3$ across a correlation-driven quantum phase transition to a charge-ordered insulating state and evaluated the phase coherence length $L_ϕ$ and its temperature scaling under various pressures from the low-temperature magnetoconductivity. In the high-pressure regime, the system exhibits the conventional two-dimensional dephasing behavior ($L_ϕ \propto T^{-p}$ with $p \approx 1/2$), characteristic of electron-electron scattering in diffusive conductors. As the pressure approaches the critical pressure ($P_c \sim 1.2$ GPa), the temperature exponent is suppressed to $p \sim 0.3$, while $L_ϕ$ remains large ($700\text{-}800$ nm at 0.5 K). This anomalous scaling suggests nontrivial inelastic scattering associated with Dirac electrons near the quantum critical point. The persistence of WAL across the transition supports a gapless or nearly gapless quantum phase transition.

cond-mat.str-el↗

arXiv2022

Anomalous scaling law for thermoelectric transport of 2D-confined electrons in an organic molecular system

Confined electrons in low dimensions host desirable material functions for downscaled electronics as well as advanced energy technologies. Thermoelectricity is a most fascinating example, since the dimensionality modifies the electron density of states dramatically, leading to enhanced thermopower as experimentally examined in artificial two-dimensional (2D) structures. However, it is still an open question whether such an enhanced thermopower in low dimensions is realized in layered materials with strong 2D characters such as cuprates. Here, we report unusual enhancement of the thermopower in the layered organic compound $α$-(BEDT-TTF)$_2$I$_3$, where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene. We find that the slope in the Jonker plot (thermopower $S$ vs. logarithm of electrical conductivity $\logσ$) for $α$-(BEDT-TTF)$_2$I$_3$ is significantly larger than that of conventional semiconductors. Moreover, the large slope is also seen in the related layered salt, demonstrating the impact of the 2D-confined carriers in the layered organics on thermoelectricity.

cond-mat.mtrl-sci↗

arXiv2022

Third-order Electrical Conductivity of the Charge-ordered Organic Salt $α$-(BEDT-TTF)$_2$I$_3$

We performed third-order electrical conductivity measurements on the organic conductor $α$-(BEDT-TTF)$_2$I$_3$ using an ac bridge technique sensitive to nonlinear signals. Third-order conductance $G_3$ is clearly observed even at low electric fields, and interestingly, $G_3$ is critically enhanced above the charge-order transition temperature $T_{\rm CO}=136$~K. The observed frequency dependence of $G_3$ is incompatible with a percolation model, in which a Joule heating in a random resistor network is relevant to the nonlinear conduction. We instead argue the nonlinearity of the relaxation time according to a phenomenological model on the mobility in materials with large dielectric constants, and find that the third-order conductance $G_3$ corresponds to the third-order electric susceptibility $χ_3$. Since the nonlinear susceptibility is known as a probe for higher-order multipole ordering, the present observation of the divergent behavior of $G_3$ above $T_{\rm CO}$ reveals an underlying quadrupole instability at the charge-order transition of the organic system.

cond-mat.str-el↗

arXiv2020

Universal Behavior of Magnetoresistance in Organic Dirac Electron Systems

In-plane magnetoresistance for organic massless Dirac electron system (OMDES) $α$-(BEDT-TTF)$_2$I$_3$ and $θ$-(BEDT-TTF)$_2$I$_3$ in addition to possible candidates of the OMDES $α$-(BETS)$_2$I$_3$ and $α$-(BEDT-STF)$_2$I$_3$, was investigated under hydrostatic pressure. We have found the universal behavior of the in-plane magnetoresistance under a low magnetic field perpendicular to two-dimensional plane. As for $α$-(BEDT-TTF)$_2$I$_3$, the universality was examined with the parameters of temperature, magnetic field and its direction. We suggest that the universal magnetoresistance behavior is found even for the gapped state of $α$-(BEDT-TTF)$_2$I$_3$ under intermediate pressure, when the thermal energy exceeds the gap.

cond-mat.str-el↗

arXiv2020

Anomalous 2D-confined electronic transport in layered organic charge-glass systems

To get insight into the nature of the electronic fluid in the frustration-driven charge glasses, we investigate in-plane and out-of-plane charge transport for several quasi-triangular-lattice organic systems, $θ$-(BEDT-TTF)$_2$X [X=RbZn(SCN)$_4$, CsZn(SCN)$_4$ and I$_3$]. These compounds host a charge order, charge glass and Fermi liquid, depending on the strength of charge frustration. We find that the resistivity exhibits extremely two-dimensional (2D) anisotropy and contrasting temperature dependence between in the in-plane and out-of-plane directions in the charge glass phase, qualitatively distinguished from the charge order and metallic states. The experimental features indicate that the frustration-induced charge glass carries an anomalous 2D-confined electronic fluid with possible charge excitations other than conventional quasiparticles.

cond-mat.str-el↗

arXiv2018

Novel constructive method for the quantum dimer model in spin-1/2 Heisenberg antiferromagnets with frustration on a diamond-like-decorated square lattice

We study spin-1/2 Heisenberg antiferromagnets on a diamond-like-decorated square lattice. The diamond-like-decorated square lattice is a lattice in which the bonds in a square lattice are replaced with diamond units. The diamond unit has two types of antiferromagnetic exchange interactions, and the ratio $λ$ of the diagonal bond strength to that of the other four edges controls the frustration strength. For $0.974<λ<2$, the present system has a nontrivial macroscopic degeneracy, which is called the macroscopically degenerated tetramer-dimer (MDTD) states. The MDTD states are identical to the Hilbert space of the Rokhsar-Kivelson (RK) quantum dimer model (QDM). By introducing further neighbor couplings in the MDTD states, we calculate the second-order effective Hamiltonian, which is exactly the same as the square-lattice QDM with a finite hopping amplitude $t$ and dimer-dimer interaction $v$. Furthermore, we calculate $v/|t|$ as a function of the ratio $λ$ in the Heisenberg model and examine which phases of the square-lattice QDM appear in our obtained states. Our obtained QDM has a region where $λ$ exhibits a finite hopping amplitude ($|t|>0$) and repulsive interaction between dimers ($v>0$). This suggests the possibility of realizing the resonating valence bond (RVB) state because the RVB state is obtained at $v=|t|$, which is known as the RK point.

cond-mat.stat-mech↗

arXiv2017

Anomalous Spin Correlations and Mass-Generating Excitonic Instability of Interacting Weyl Fermions

Recent advances in the study of nodal Weyl fermions (WFs), quasi-relativistic massless particles, constitute a novel realm of quantum many-body phenomena. The Coulomb interaction in such systems, having a zero density of states at the Fermi level, is of particular interest, since in contrast to conventional correlated metals, its long-ranged component is unscreened. Here, through nuclear-magnetic-resonance (NMR) measurements, we unveil the exotic spin correlations of two-dimensional WFs in an organic material, causing a divergent increase of the Korringa ratio by a factor of 1000 upon cooling, in striking contrast with conventional metallic behaviors. Combined with model calculations, we show that this divergence stems from the interaction-driven velocity renormalization that almost exclusively suppresses the zero-momentum spin fluctuations. At low temperatures, the NMR rate shows a remarkable increase, which is shown by numerical analyses to correspond to inter-node excitonic fluctuations, precursor of a transition from massless to massive quasiparticles.

cond-mat.str-el↗

arXiv2016

Observation of an anisotropic Dirac cone reshaping and ferrimagnetic spin polarization in an organic conductor

The Coulomb interaction among massless Dirac fermions in graphene is unscreened around the isotropic Dirac points, causing a logarithmic velocity renormalization and a cone reshaping. In less symmetric Dirac materials possessing anisotropic cones with tilted axes, the Coulomb interaction can provide still more exotic phenomena which have not been experimentally unveiled yet. Here, using site-selective nuclear magnetic resonance, we find a non-uniform cone reshaping accompanied by a bandwidth reduction and an emergent ferrimagnetism in tilted Dirac cones that appear on the verge of charge ordering in an organic compound. Our theoretical analyses based on the renormalization-group approach and the Hubbard model show that these observations are the direct consequences of the long-range and short-range parts of the Coulomb interaction, respectively. The cone reshaping and the bandwidth renormalization, as well as the novel magnetism revealed here, can be ubiquitous and vital for many Dirac materials.

cond-mat.str-el↗

arXiv2016

Spin excitations in the quasi-two-dimensional charge-ordered insulator $α$-(BEDT-TTF)$_2$I$_3$ probed via $^{13}$C NMR

The spin excitations from the nonmagnetic charge-ordered insulating state of $α$-(BEDT-TTF)$_2$I$_3$ at ambient pressure have been investigated by probing the static and low-frequency dynamic spin susceptibilities via site-selective nuclear magnetic resonance at $^{13}$C sites. The site-dependent values of the shift and the spin-lattice relaxation rate $1/T_1$ below the charge-ordering transition temperature ($T_{CO} \approx$ 135 K) demonstrate a spin density imbalance in the unit cell, in accord with the charge-density ratio reported earlier. The shift and $1/T_1$ show activated temperature dependence with a static (shift) gap $Δ_S \approx$ 47-52 meV and a dynamic ($1/T_1$) gap $Δ_R \approx$ 40 meV. The sizes of the gaps are well described in terms of a localized spin model, where spin one-half antiferromagnetic dimer chains are weakly coupled with each other.

cond-mat.str-el↗

arXiv2016

Insulating nature of strongly correlated massless Dirac fermions in an organic crystal

Through resistivity measurements of an organic crystal hosting massless Dirac fermions with a charge-ordering instability, we reveal the effect of interactions among Dirac fermions on the charge transport. A low-temperature resistivity upturn appears robustly irrespectively of pressure and is enhanced while approaching the critical pressure of charge ordering, indicating that the insulating behavior originates from short-range Coulomb interactions. Observation of apparently vanishing gap in the charge-ordered phase accords with the theoretical prediction of the non-topological edge states.

cond-mat.str-el↗

arXiv2012

Electron Correlations in the Quasi-Two-Dimensional Organic Conductor $θ$-(BEDT-TTF)$_{2}$I$_{3}$ investigated by $^{13}$C NMR

We report a $^{13}$C-NMR study on the ambient-pressure metallic phase of the layered organic conductor $θ$-(BEDT-TTF)$_{2}$I$_{3}$ [BEDT-TTF: bisethylenedithio-tetrathiafulvalene], which is expected to connect the physics of correlated electrons and Dirac electrons under pressure. The orientation dependence of the NMR spectra shows that all BEDT-TTF molecules in the unit cell are to be seen equivalent from a microscopic point of view. This feature is consistent with the orthorhombic symmetry of the BEDT-TTF sublattice and also indicates that the monoclinic $I_{3}$ sublattice, which should make three molecules in the unit cell nonequivalent, is not practically influential on the electronic state in the conducting BEDT-TTF layers at ambient pressure. There is no signature of charge disproportionation in opposition to most of the $θ$-type BEDT-TTF salts. The analyses of NMR Knight shift, $K$, and the nuclear spin-lattice relaxation rate, $1/T_{1}$, revealed that the degree of electron correlation, evaluated by the Korringa ratio [$\varpropto 1/(T_{1}TK^{2}$)], is in an intermediate regime. However, NMR relaxation rate $1/T_{1}$ is enhanced above $\sim$ 200K, which possibly indicates that the system enters into a quantum critical regime of charge-order fluctuations as suggested theoretically.

cond-mat.mtrl-sci↗