arXiv subjects

Katsumi Tanigaki

Publications and source records attributed to Katsumi Tanigaki.

At least 19 recordsLinked to original sources

arXiv2026

$\mathcal{PT}$-symmetry as Effective Time Reversal Symmetry for Anderson Localization in a Collinear Antiferromagnet

We show that $\mathcal{PT}$-symmetry acts as the effective time-reversal symmetry governing Anderson localization in the antiferromagnet BaMn$_2$Bi$_2$, whereas the general time-reversal symmetry is broken by its long-range magnetic order. The magnetoconductance follows the laws established from breaking time-reversal symmetry throuhg the orbital coupling of charge carriers to the magnet vector potential, however, the degrees of symmetry breaking are qualitatively modulated by the magnetocrystalline anisotropy stabilizing $\mathcal{PT}$-related sublattices. The quantum interferences and hoping amplitudes governing the phenomena are drastically impaired by transverse magnetic fields, which readily cant $\mathcal{PT}$-related magnetic sublattices. Accordingly, when the $\mathcal{PT}$-related texture is weakly perturbed by longitudinal fields orienting along the sublattices, the localization is minutely affected. The robustness of the $\mathcal{PT}$-enforced degeneracies is thus governed by exchange and magnetocrystalline interactions, in contrast to the exact $\mathcal{PT}$-symmetry of the full Hamiltonian, which is broken by any finite field.

cond-mat.other↗

arXiv2026

Supercritical fluid of quantum electrons in three-dimensional superconducting fullerides

The supercritical fluid (SCF) of quantum electrons at the Mott metal-insulator transition without symmetry breaking is one of the most elusive phenomena in strongly correlated electron physics. Prior studies of Cr-doped V2O3 and organic Mott systems reported discrepant critical exponents. A key limitation is that the scaling analysis relies on a single experimental observable, leaving the roles of phase coexistence, inhomogeneity, and percolation unaddressed. Here we report the first experimental identification of a thermodynamically equilibrated SCF phase and its associated Mott endpoint in the three-dimensional superconducting fullerides CsxRb3-xC60, using two independent probes of electrical conductivity and magnetic susceptibility, which reveal two distinct metal-insulator transition lines converging at a single Mott endpoint. A hypothesis-free two-particle analysis of magnetic susceptibilities yields a metal-insulator coexisting SCF by exhibiting the maximum two-phase mixing entropy, in agreement with a picture of a thermodynamically equilibrated Widom line. Simultaneously, conductivity scaling yields a critical exponent in the regime of quantum critical predictions. Our new dual-probe approach provides a unified microscopic picture of the Mott SCF with a characteristic length scale below current diffraction resolution, in addition to a new interpretation on the origin of superconducting Tc-dome.

cond-mat.str-el↗

arXiv2026

Time-resolved measurement of Seebeck effect for superionic metals during structural phase transition

We propose a new time (t)-resolved method of both vertical- and horizontal-temperature gradients in an orthogonal configuration (t-resolved T(t)-HVOT) to have real interpretations of the enhancement in thermoelectric Seebeck effect (SE) observed during the structural phase transition. We apply our new method to superionic-state semiconductors of p-type Cu2Se and n-type Ag2S. The experimental data differentiate the two types of enhancements during the phase transition: a colossal SE (Scolossal), exhibiting an enormous value of up to 5 mV/K, and a slight enhancement in SE (Sstructure), approximately 1.5-2.0 times larger than those in the absence of the phase transition. We provide critical insights that both enhancements in SE arising during the structural phase transition are not intrinsic phenomena.

cond-mat.mtrl-sci↗

arXiv2025

Longitudinal magneto-thermal conductivity and magneto-Seebeck of itinerant antiferromagnetic BaMn$_2$Bi$_2$

Thermal transport, generally mediated by the direct microscopic exchange of kinetic energy via lattice phonons, can also be modified by contributions from additional quasiparticles, such as electrons and magnons. However, a comprehensive understanding of the magnon influence has yet to be realized and remains an active research area. The most significant roadblock has been a lack of available materials in which these three quasiparticles can be clearly identified and quantitatively examined in order to provide an intrinsic understanding, not only of their independent contributions to thermal conductivity but also of the cross-correlated interactions among them. Itinerant antiferromagnetic (AFM) BaMn$_{2}$Bi$_{2}$ with PT symmetry exhibits Anderson metal-insulator localization, which can be tuned into the metallic regime via an applied magnetic field due to its unique electron-magnon interactions. We identify itinerant AFM BaMn$_{2}$Bi$_{2}$ as an ideal material for scientific investigations into how these quasiparticles participate in thermal conductivity. Here, we present the direct contribution of electrons, phonons, and magnons to thermal conductivity, as well as their interspecies interactions, supported by detailed analyses conducted in the framework of the Boltzmann transport formalism. The comparison of the magneto-thermal conductivity and magneto-electrical conductivity, as well as the magneto-Seebeck effect of itinerant antiferromagnetic BaMn$_{2}$Bi$_{2}$, gives unique insight into how magnons participate in longitudinal thermal-associated phenomena.

cond-mat.str-el↗

arXiv2024

Spontaneously formed phonon frequency combs in van der Waals solid CrXTe$_3$ (X=Ge,Si)

Optical phonon engineering through nonlinear effects has been utilized in ultrafast control of material properties. However, nonlinear optical phonons typically exhibit rapid decay due to strong mode-mode couplings, limiting their effectiveness in temperature or frequency sensitive applications. In this study, we report the observation of long-lived nonlinear optical phonons through the spontaneous formation of phonon frequency combs in the van der Waals material CrXTe$_3$ (X=Ge, Si) using high-resolution Raman scattering. Unlike conventional optical phonons, the highest $A_g$ mode in CrGeTe$_3$ splits into equidistant, sharp peaks forming a frequency comb that persists for hundreds of oscillations and survives up to 100K before decaying. These modes correspond to localized oscillations of Ge$_2$Te$_6$ clusters, isolated from Cr hexagons, behaving as independent quantum oscillators. Introducing a cubic nonlinear term to the harmonic oscillator model, we simulate the phonon time evolution and successfully replicate the observed comb structure. Similar frequency comb behavior is observed in CrSiTe$_3$, demonstrating the generalizability of this phenomenon. Our findings reveal that Raman scattering effectively probes high-frequency nonlinear phonon modes, providing new insight into generating long-lived, tunable phonon frequency combs with applications in ultrafast material control and phonon-based technologies.

cond-mat.mes-hall↗

arXiv2024

Incommensurate Charge Super-modulation and Hidden Dipole Order in Layered Kitaev Material $α$-RuCl$_3$

The magnetism of Kitaev materials has been widely studied, but their charge properties and the coupling to other degrees of freedom are less known. Here we investigate the charge states of $α$-RuCl$_3$, a promising Kitaev quantum spin liquid candidate, in proximity to graphite. We discover that few-layered $α$-RuCl$_3$ experiences a clear modulation of charge states, where a Mott-insulator to weak charge-transfer-insulator transition in the 2D limit occurs by means of heterointerfacial polarization. More notably, distinct signals of incommensurate charge and lattice super-modulations, regarded as an unconventional charge order, accompanied in the insulator. Our theoretical calculations have reproduced the incommensurate charge order by taking into account the antiferroelectricity of $α$-RuCl$_3$ that is driven by dipole order in the internal electric fields. The findings imply that there is strong coupling between the charge, spin, and lattice degrees of freedom in layered $α$-RuCl$_3$ in the heterostructure, which offers an opportunity to electrically access and tune its magnetic interactions inside the Kitaev compounds.

cond-mat.str-el↗

arXiv2024

Molecular Beam Epitaxy Growth and Doping Modulation of Topological Semimetal NiTe$_2$

In this study, high-quality thin films of the topological semimetal phase NiTe$_2$ were prepared using molecular beam epitaxy (MBE) technique, confirmed through X-ray diffraction with pronounced Laue oscillations. Electrical transport experiments reveal that thick films have properties similar to bulk materials. By employing co-deposition, we introduced either magnetic or non-magnetic elements during the growth of thinner films, significantly altering their electrical properties. Notably, magnetic element Cr induces long-range ferromagnetic ordering, leading to the observation of significant anomalous Hall effect in NiTe2 thin films. The Hall conductivity remains nearly constant well below the Curie temperature, indicating the correlation with the intrinsic topological nature of the band structure. Theoretical first principles band calculations support the generation of the Weyl semimetal state in the material through magnetic doping. These findings pave the way for exploring more magnetic Weyl semimetal materials and related low-dimensional quantum devices based on topological semimetal materials.

cond-mat.mtrl-sci↗

arXiv2023

Insulator-to-metal Mott transition facilitated by lattice deformation in monolayer $α$-RuCl$_3$ on graphite

Creating heterostructures with graphene/graphite is a practical method for charge-doping $α$-RuCl$_3$, but not sufficient to cause the insulator-to-metal transition. In this study, detailed scanning tunneling microscopy/spectroscopy measurements on $α$-RuCl$_3$ with various lattice deformations reveal that both in-plane and out-of-plane lattice distortions may collapse the Mott-gap in the case of monolayer $α$-RuCl$_3$ in proximity to graphite, but have little impact on its bulk form alone. In the Mott-Hubbard framework, the transition is attributed to the lattice distortion-facilitated substantial modulation of the electron correlation parameter. Observation of the orbital textures on a highly compressed monolayer $α$-RuCl$_3$ flake on graphite provides valuable evidence that electrons are efficiently transferred from the heterointerface into Cl3$p$ orbitals under the lattice distortion. It is believed that the splitting of Ru $t_{2g}$ bands within the trigonal distortion of Ru-Cl-Ru octahedra bonds generated the electrons transfer pathways. The increase of the Cl3$p$ states enhance the hopping integral in the Mott-Hubbard bands, resulting in the Mott-transition. These findings suggest a new route for implementing the insulator-to-metal transition upon doping in $α$-RuCl$_3$ by deforming the lattice in addition to the formation of heterostructure.

cond-mat.str-el↗

arXiv2023

Superior surface modification layer of poly(styrene) on SiO$_2$ gate insulator in rubrene single crystal field-effect transistor

We conduct comparative research on the density of states of electron- and hole- carrier trap levels (DTR(E)), dispersing inside the energy gap of a rubrene single crystal in a field effect transistor (FET) struction with Ca and Au hetero-electrodes for an ambipolar carrier injection mode, by using polymeric protection-layer materials on a Si substrate. Three different types of polymeric materials, poly(methyl-methacrylate) (PMMA), poly(styrene) (PS) and poly(chloro-styrene) (PCS) are employed. From the temperature (T)-dependent source-drain current and gate voltage (ISD-VG) transfer characteristics, the values of DTR(E) are evaluated. PS exhibits the most efficiently-balanced ambipolar carrier transport, which is superior to PMMA that is most typically used as the standard protection layer on a SiO$_2$/doped-Si substrate. Discussions are made in the framework of a carrier multiple trap and release (CMTR) model.

cond-mat.mtrl-sci↗

arXiv2023

Enhanced ferromagnetism in artificially stretched lattice in quasi two-dimensional Cr2Ge2Te6

In the fundamental understanding of magnetic interactions between atoms in solids, the crystal lattice is one of the key parameters. As the effective tool for controlling the lattice using tensile stress is limited, there are only few demonstrations of the control in magnetic properties with expanding the lattice structure. Here, we observe that the Curie temperature (Tc) of quasi two-dimensional Cr2Ge2Te6 with NiO overlayer doubles from ~60 K to ~120 K, describe a clear correlation of magnetic properties with lattice expansion, which is characterized by several probes and computational approaches, and address on the mechanisms leading to the increase in Tc via the change in exchange interactions.

cond-mat.mtrl-sci↗

arXiv2023

Tunneling spectroscopic signatures of charge doping and associated Mott transition in $α$-RuCl${_3}$ in proximity to graphite

The layered Mott insulator $α$-RuCl${_3}$ has been extensively studied as a potential Kitaev quantum spin liquid candidate. Here, by constructing heterostructures with graphite, we employed electron tunneling measurements on few-layer $α$-RuCl${_3}$ using a scanning tunneling microscopy/spectroscopy. Characteristic tunneling spectra were detected on $α$-RuCl${_3}$ layers in proximity to graphite. In the single-layer $α$-RuCl${_3}$ in direct contact with graphite, distinct states in the Mott-gap regime were observed. The in-gap states are demonstrated to be closely related to the electron orbitals in $α$-RuCl${_3}$ and graphite, and to be sensitive to interfacial coupling, where a hybridization at the heterointerface is hypothesized. The in-gap states are also thought of as a charge reservoir for weakly doping the $α$-RuCl${_3}$ upper-layers. It demonstrated that the weak doping effect causes a considerable decrease in the Mott-gap within the upper-layers, suggesting that an unconventional Mott-transition is occurring in these layers. The results show that the heterostructure comprised of $α$-RuCl${_3}$ and graphite is a good platform for investigating the doping physics in $α$-RuCl${_3}$. Therefore, tunneling into such a doped system is a useful probe for studying otherwise insulating spin-liquid candidates.

cond-mat.str-el↗

arXiv2022

Gate-tunable anomalous Hall effect in stacked van der Waals ferromagnetic insulator - topological insulator heterostructures

The search of novel topological phases, such as the quantum anomalous Hall insulator (QAHI) or the axion insulator, has motivated different schemes to introduce magnetism into topological insulators. One scheme is to introduce ferromagnetic dopants in topological insulators. However, it is generally challenging and requires carefully engineered growth/heterostructures or relatively low temperatures to observe the QAHI due to issues such as the added disorder with ferromagnetic dopants. Another promising scheme is using the magnetic proximity effect with a magnetic insulator to magnetize the topological insulator. Most of these heterostructures are synthesized so far by growth techniques such as molecular beam epitaxy and metallic organic chemical vapor deposition. These are not readily applicable to allow mixing and matching many of the available ferromagnetic and topological insulators due to difference in growth conditions and lattice mismatch. Here, we demonstrate that the magnetic proximity effect can still be obtained in stacked heterostructures assembled via the dry transfer of exfoliated micrometer-sized thin flakes of van der Waals topological insulator and magnetic insulator materials (BiSbTeSe2/Cr2Ge2Te6), as evidenced in the observation of an anomalous Hall effect (AHE). Furthermore, devices made from these heterostructures can allow modulation of the AHE when controlling the carrier density via electrostatic gating. These results show that simple mechanical transfer of magnetic van der Waals materials provides another possible avenue to magnetize topological insulators by magnetic proximity effect, a key step towards further realization of novel topological phases such as QAHI and axion insulators.

cond-mat.mes-hall↗

arXiv2022

Magnetic-field-induced Anderson localization in orbital selective antiferromagnet BaMn$_2$Bi$_2$

We report a metal-insulator transition (MIT) in the half-filled multiorbital antiferromagnet (AF) BaMn$_2$Bi$_2$ that is tunable by a magnetic field perpendicular to the AF sublattices. Instead of an Anderson-Mott mechanism usually expected in strongly correlated systems, we find by scaling analyses that the MIT is driven by an Anderson localization. Electrical and thermoelectrical transport measurements in combination with electronic band calculations reveal a strong orbital-dependent correlation effect, where both weakly and strongly correlated $3d$-derived bands coexist with decoupled charge excitations. Weakly correlated holelike carriers in the $d_{xy}$-derived band dominate the transport properties and exhibit the Anderson localization, whereas other $3d$ bands show clear Mott-like behaviors with their spins ordered into AF sublattices. The tuning role played by the perpendicular magnetic field supports a strong spin-spin coupling between itinerant holelike carriers and the AF fluctuations, which is in sharp contrast to their weak charge coupling.

cond-mat.str-el↗

arXiv2021

Electronic States and Energy Dissipations of Vortex Core in Pure FeSe Single Crystals Investigated by Microwave Surface Impedance Measurements

In order to clarify electronic states and energy dissipations due to a motion of a vortex core in pure FeSe, which is a candidate superconductor possessing a super-clean core, we measured the microwave surface impedance of pure FeSe single crystals under finite magnetic fields. From the magnetic-field dependence of the flux-flow resistivity, we found that a barometer of electronic states inside the vortex core $ω_{0}τ_{\rm core}$ is $1\pm0.5$, suggesting that the vortex core of pure FeSe is in the moderately clean regime contrary to the expectation of the super-clean core. We also found that the mean-free path inside the vortex core is suppressed at the distance of the order of the core radius. Based on observed results and previous reports, we discussed possible origins of rather small $ω_{0}τ_{\rm core}$ value in terms of the multiple-bands nature of FeSe and additional mechanisms producing extra energy dissipations specific to the vortex core in motion.

cond-mat.supr-con↗

arXiv2021

Large negative magnetoresistance in BaMn$_2$Bi$_2$ antiferromagnet

A very large negative magnetoresistance (LNMR) is observed in the insulating regime of the antiferromagnet BaMn$_2$Bi$_2$ when a magnetic field is applied perpendicular to the direction of the sublattice magnetization. High perpendicular magnetic field eventually suppresses the insulating behavior and allows BaMn$_2$Bi$_2$ to re-enter a metallic state. This effect is seemingly unrelated to any field induced magnetic phase transition, as measurements of magnetic susceptibility and specific heat did not find any anomaly as a function of magnetic fields at temperatures above $2\,\mathrm{K}$. The LNMR appears in both current-in-plane and current-out-of-plane settings, and Hall effects suggest that its origin lies in an extreme sensitivity of conduction processes of holelike carriers to the infinitesimal field-induced canting of the sublattice magnetization. The LNMR-induced metallic state may thus be associated with the breaking of the antiferromagnetic parity-time symmetry by perpendicular magnetic fields and/or the intricate multi-orbital electronic structure of BaMn$_2$Bi$_2$.

cond-mat.str-el↗

arXiv2020

Separate tuning of nematicity and spin fluctuations to unravel the origin of superconductivity in FeSe

The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc.

cond-mat.str-el↗

arXiv2019

Towards electrically driven organic semiconductor laser with field-effective transistor structure

Laser is one of the most important discoveries in the 20th century, and inorganic semiconductor lasers (ISCL) are most frequently used in many applications nowadays. Organic semiconductor lasers (OSCL) have many attractive features when they compared to ISCLs, such as flexibility, human friendliness, feasible and inexpensive production process, light weight, and multicolor emission. However, electrically driven OSCLs (el-OSCL) have not yet been realized, although they are possible in an optically driven mode. Here, we report that an el-OSCL can be realized in field-effect transistor (FET) structure. The FET el-OSCL with distributed feedback (DFB) construction is made using a BP3T single crystal as a lasing medium electrostatically laminated on a silicon substrate modified with periodically patterned polystyrene. An emergent sharp-linewidth emission spectrum to the resolution limit of a detector and a non-linear increase in intensity above the threshold current density of ca. 1 kA cm-2 was observed, being indicative of lasing. Discussions on the possible realization of lasing in el-OSCLs can be made from the comparison between optical and electrical-driven mode.

physics.optics↗

arXiv2018

Ultrathin film of 3D topological insulators by vapor-phase epitaxy: Surface dominant transport in wide temperature revealed by Seebeck measurement

Realization of intrinsic surface dominant transport in a wide temperature region for a topological insulators (TIs) is an important frontier research in order to promote the progresses of TIs towards the future electronics. We report here systematic measurements of longitudinal electrical transport, Shubnikov-de-Haas (SdH) quantum oscillations, Hall coefficient (R_H^2D), and Seebeck coefficient as a function of film thickness (d) and temperature using high quality Bi2-xSbxTe3-ySey (BSTS) single crystal thin films grown by physical vapor-phase deposition. The thickness dependence of sheet conductance and Seebeck coefficient clearly show the suppression of semiconducting hole carriers of bulk states by reducing film thickness, reaching to the surface dominant transport at below dc=14 nm. Quantitative arguments are made as to how the contribution of itinerant carrier number (n) can be suppressed, using both R_H^2D (n_Hall^2D) and SdH (n_SdH). Intriguingly, the value of n_Hall^2D approaches to be twice of n_SdH below dc. While R_H^2D shows a negative sign in whole temperature region, a change from negative to positive polarity is clearly observed for S at high temperatures when d is thick. We point out that this inconsistency observed between R_H^2D and S is intrinsic in 3D-TIs and its origin is the large difference in carrier mobility between the bulk and the topological surface. We propose that Seebeck coefficient can become a convenient and powerful tool to evaluate the intrinsic carrier concentration for the topological surface in 3D-TIs even in the absence of magnetic field.

cond-mat.mtrl-sci↗