arXiv subjects

Yohei Saito

Publications and source records attributed to Yohei Saito.

At least 19 recordsLinked to original sources

arXiv2026

Mott Intermittency at the Metal-Insulator Boundary

The resistivity maximum at a temperature $T=T_{\mathrm{max}}$ is a recurring feature of bandwidth-tuned Mott systems, yet its meaning remains controversial: is it a coherence-incoherence crossover of an electronically homogeneous metal, or does it mark the onset of transport through a mixed landscape of metallic and insulating regions? Even more debated is whether a true phase-coexistence regime survives in the relevant parameter range, or whether apparent inhomogeneity is merely extrinsic. Here we address these questions by moving beyond temperature sweeps and probe charge transport in the time domain. Near $T=T_{\mathrm{max}}$, we find that the resistance of a model system, a quasi-two-dimensional Mott spin liquid material, exhibits clear random-telegraph switching between discrete levels over long timescales. The statistics of the switching - sharp two-level behavior with thermally activated dwell times - point to a mesoscopic "current-controlling" region that dynamically toggles between metallic and insulating states, intermittently opening and closing the dominant conduction channel. This characteristic fluctuating dynamics provides direct evidence for intrinsic metal-insulator coexistence and establishes $T\sim T_{\mathrm{max}}$ as the regime of Mott intermittency, where transport is governed by stochastic domain switching rather than quasiparticle decoherence.

cond-mat.str-el↗

arXiv2025

Charge-localization-driven metal-insulator phase transition in layered molecular conductors

The organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ provides the prime example of a charge-order-driven metal-insulator transition. Restricted chemical substitution of S atoms by Se in the constituent molecules allows us to modify the electronic properties. This not only decreases the transition temperature but, in addition, alters the phase transition mechanism, resulting in the ground state deviating from the charge-ordered insulator state of the parent compound. Employing infrared optical spectroscopy, we investigate changes in the charge dynamics. Furthermore, we demonstrate the absence of charge ordering in the Se-substituted materials and suggest that the phase transition is instead driven by the localization of the itinerant charge carriers due to strong electron-phonon interactions.

cond-mat.str-el↗

arXiv2025

Bouncy particle sampler with infinite exchanging parallel tempering

Bayesian inference is useful to obtain a predictive distribution with a small generalization error. However, since posterior distributions are rarely evaluated analytically, we employ the variational Bayesian inference or sampling method to approximate posterior distributions. When we obtain samples from a posterior distribution, Hamiltonian Monte Carlo (HMC) has been widely used for the continuous variable part and Markov chain Monte Carlo (MCMC) for the discrete variable part. Another sampling method, the bouncy particle sampler (BPS), has been proposed, which combines uniform linear motion and stochastic reflection to perform sampling. BPS was reported to have the advantage of being easier to set simulation parameters than HMC. To accelerate the convergence to a posterior distribution, we introduced parallel tempering (PT) to BPS, and then proposed an algorithm when the inverse temperature exchange rate is set to infinity. We performed numerical simulations and demonstrated its effectiveness for multimodal distribution.

cs.LG↗

arXiv2025

Universal relation between residual resistivity and A coefficient in correlated metals

The effects of strong electronic correlations and disorder are crucial for emergent phenomena such as unconventional superconductivity, metal-insulator transitions, and quantum criticality. While both are omnipresent in real materials, their individual impacts on charge transport remain elusive. To disentangle their respective roles, we have independently varied the degree of randomness and the strength of electronic correlations -- by chemical substitution and physical pressure, respectively -- within the metallic phase nearby a Mott-insulating state. We find a distinct correlation dependence of the disorder-dependent residual resistivity $\rho_0$ in the Fermi-liquid regime $\rho(T)=\rho_0 + A T^2$, where $A\propto (m^{\star}/m)^2$ quantifies the electronic mass enhancement. Contrary to conventional expectations, we observe that at fixed disorder level $\rho_0$ grows linearly with $A$. This scaling can be understood in terms of chemical-potential fluctuations with variance $\sigma_\mu^2$, yielding $\rho_0 \propto A\,\sigma_\mu^2$. By comparing our findings to transport data on other organic Mott systems, oxides, heavy-fermion compounds, and moir\'e materials, we demonstrate that this new relation between residual resistivity and mass enhancement is a universal feature of correlated metals.

cond-mat.str-el↗

arXiv2025

Failed superconductivity in a Mott spin liquid material

A central challenge for understanding unconventional superconductivity in most strongly correlated electronic materials is their complicated band structure and presence of competing orders. In contrast, quasi-two-dimensional organic spin liquids are single-band systems with superconductivity arising near the bandwidth-tuned Mott metal-insulator transition in the absence of other orders. Here, we study chemically substituted $\kappa$-organics in which superconducting fluctuations emerge in the phase coexistence region between the Mott insulator and the Fermi liquid. Using magnetotransport and ac susceptibility measurements, we find that global superconductivity fails to set in as temperature $T\rightarrow 0$. Our results indicate instead the presence of superconducting domains embedded in the metallic percolating cluster that undergo a magnetic field-tuned quantum superconductor-to-metal phase transition. Surprisingly, albeit consistent with the percolation picture, universal conductance fluctuations are seen at high fields in macroscopic samples. The observed interplay of the intrinsic inhomogeneity and quantum phase fluctuations provides a new insight into failed superconductivity, a phenomenon seen in various conventional and unconventional superconductors, including cuprates.

cond-mat.supr-con↗

arXiv2025

Strain as a tool to stabilize the isotropic triangular lattice in a geometrically frustrated organic quantum magnet

Geometric frustration is a key ingredient in the emergence of exotic states of matter, such as the quantum spin liquid in Mott insulators. While there has been intense interest in experimentally tuning frustration in candidate materials, achieving precise and continuous control has remained a major hurdle -- particularly in accessing the properties of the ideally frustrated lattice. Here, we show that large, finely controlled anisotropic strains can effectively tune the degree of geometric frustration in the Mott insulating $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ -- a slightly anisotropic triangular-lattice quantum magnet. Using thermodynamic measurements of the elastocaloric effect, we experimentally map out a temperature-strain phase diagram that captures both the ground state of the isotropic lattice and the less frustrated parent state. Our results provide a new benchmark for calculations of the triangular-lattice Hubbard model as a function of frustration and highlight the power of lattice engineering as a route to realizing perfectly frustrated quantum materials.

cond-mat.str-el↗

arXiv2025

Techniques for Enhancing Memory Capacity of Reservoir Computing

Reservoir Computing (RC) is a bio-inspired machine learning framework, and various models have been proposed. RC is a well-suited model for time series data processing, but there is a trade-off between memory capacity and nonlinearity. In this study, we propose methods to improve the memory capacity of reservoir models by modifying their network configuration except for the inside of reservoirs. The Delay method retains past inputs by adding delay node chains to the input layer with the specified number of delay steps. To suppress the effect of input value increase due to the Delay method, we divide the input weights by the number of added delay steps. The Pass through method feeds input values directly to the output layer. The Clustering method divides the input and reservoir nodes into multiple parts and integrates them at the output layer. We applied these methods to an echo state network (ESN), a typical RC model, and the chaotic Boltzmann machine (CBM)-RC, which can be efficiently implemented in integrated circuits. We evaluated their performance on the NARMA task, and measured information processing capacity (IPC) to evaluate the trade-off between memory capacity and nonlinearity.

cs.LG↗

arXiv2025

Asymptotic evaluation of the information processing capacity in reservoir computing

Reservoir computing (RC) is becoming increasingly important because of its short training time. The squared error normalized by the target output is called the information processing capacity (IPC) and is used to evaluate the performance of an RC system. Since RC aims to learn the relationship between input and output time series, we should evaluate the IPC for infinitely long data rather than the IPC for finite-length data. However, a method for estimating it has not been established. We evaluated the IPC for infinitely long data using the asymptotic expansion of the IPC and weighted least-squares fitting. Then, we showed the validity of our method by numerical simulations. This work makes the performance evaluation of RC more evident.

cs.IT↗

arXiv2023

Combined experimental and theoretical studies on glasslike transitions in the frustrated molecular conductors $θ$-(BEDT-TTF)$_2MM'$(SCN)$_4$

We present results of the coefficient of thermal expansion for the frustrated quasi-two-dimensional molecular conductor $θ$-(BEDT-TTF)$_2$RbZn(SCN)$_4$ for temperatures 1.5 K $\leq T \leq$ 290 K. A pronounced first-order phase transition anomaly is observed at the combined charge-order/structural transition at 215 K. Furthermore, clear evidence is found for two separate glasslike transitions at $T_{\mathrm{g}}$ = 90-100 K and $T_{\mathrm{g}}^\dagger$ = 120-130 K, similar to previous findings for $θ$-(BEDT-TTF)$_2$CsZn(SCN)$_4$ and $θ$-(BEDT-TTF)$_2$CsCo(SCN)$_4$, reported in T. Thomas et al., Phys. Rev. B 105, L041114 (2022), both of which lack the charge-order/structural transition. Our findings indicate that these glasslike transitions are common features for the $θ$-(BEDT-TTF)$_2MM^\prime$(SCN)$_4$ family with $M$ = (Rb, Cs) and $M^\prime$ = (Co, Zn), irrespective of the presence or absence of charge order. These results are consistent with our model calculations on the glasslike dynamics associated with the flexible ethylene endgroups of the BEDT-TTF molecules for various $θ$-(BEDT-TTF)$_2MM^\prime$(SCN)$_4$ salts, predicting two different conformational glass transitions. Moreover, calculations of the hopping integrals show a substantial degree of dependence on the endgroups' conformation, suggesting a significant coupling to the electronic degrees of freedom. Our findings support the possibility that the glassy freezing of the ethylene endgroups could drive or enhance glassy charge dynamics.

cond-mat.str-el↗

arXiv2023

Field-induced effects in the spin liquid candidate PbCuTe$_{2}$O$_{6}$

PbCuTe$_2$O$_6$ is considered as one of the rare candidate materials for a three-dimensional quantum spin liquid (QSL). This assessment was based on the results of various magnetic experiments, performed mainly on polycrystalline material. More recent measurements on single crystals revealed an even more exotic behavior, yielding ferroelectric order below $T_{\text{FE}}\approx 1\,\text{K}$, accompanied by distinct lattice distortions, and a somewhat modified magnetic response which is still consistent with a QSL. Here we report on low-temperature measurements of various thermodynamic, magnetic and dielectric properties of single crystalline PbCuTe$_2$O$_6$ in magnetic fields $B\leq 14.5\,\text{T}$. The combination of these various probes allows us to construct a detailed $B$-$T$ phase diagram including a ferroelectric phase for $B \leq$ $8\,\text{T}$ and a $B$-induced magnetic phase at $B \geq$ $11\,\text{T}$. These phases are preceded by or coincide with a structural transition from a cubic high-temperature phase into a distorted non-cubic low-temperature state. The phase diagram discloses two quantum critical points (QCPs) in the accessible field range, a ferroelectric QCP at $B_{c1}$ = $7.9\,\text{T}$ and a magnetic QCP at $B_{c2}$ = $11\,\text{T}$. Field-induced lattice distortions, observed in the state at $T>$ $1\,\text{K}$ and which are assigned to the effect of spin-orbit interaction of the Cu$^{2+}$-ions, are considered as the key mechanism by which the magnetic field couples to the dielectric degrees of freedom in this material.

cond-mat.str-el↗

arXiv2023

Thermodynamic Properties of the Mott Insulator-Metal Transition in a Triangular Lattice System Without Magnetic Order

The organic system, $κ$-[(BEDT-TTF)$_{1-x}$(BEDT-STF)$_x$]$_2$Cu$_2$(CN)$_3$, showing the Mott transition between a nonmagnetic Mott insulating (NMI) state and a Fermi liquid (FL), is systematically studied by calorimetric measurements. An increase of the electronic heat capacity at the transition from the NMI state to the FL state which keeps the triangular dimer lattice demonstrates that the charge sector lost in the Mott insulating state is recovered in the FL state. We observed that the remaining low-energy spin excitations in the Mott insulating state show unique temperature dependence, and that the NMI state has a larger lattice entropy originating from the frustrated lattice, which leads to the Pomeranchuk-like effect on the electron localization. Near the Mott boundary, an unexpected enhancement and magnetic-field dependence of heat capacity are observed. This anomalous heat capacity is different from the behavior in the typical first-order Mott transition and shows similarities with quantum critical behavior. To reconcile our results with previously reported scenarios about a spin gap and the first-order Mott transition, further studies are desired.

cond-mat.str-el↗

arXiv2022

Charge imbalance in $λ$-(BETS)$_2$GaCl$_4$ and their interplay with superconductivity

The two-dimensional organic superconductor $λ$-(BETS)$_2$GaCl$_4$ exhibits pronounced charge fluctuations below $T \approx 150$~K, in contrast to the sibling compound $κ$-(BETS)$_2$GaCl$_4$ that remains metallic down to milli-Kelvin. Infrared spectroscopy reveals only minor splitting in the vibrational features of the latter compound, common to other strongly dimerized $κ$-salts. When the organic molecules are arranged in the $λ$-pattern, however, a strong vibrational $ν_{27}(b_{1u})$ mode is present, that forms a narrow doublet. %indicating static charge imbalance of about 2\%. Most important, when cooling $λ$-(BETS)$_2$GaCl$_4$ below 150~K, two weak side modes appear due to charge disproportionation that amounts to $2δ=0.14e$. In analogy to the $β^{\prime\prime}$-type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in ł at $T_c=4.7$~K. We discuss the possibility of a charge-density-wave that coexists with the proposed spin-density-wave state.

cond-mat.str-el↗

arXiv2021

Spin liquid and ferroelectricity close to a quantum critical point in PbCuTe$_2$O$_6$

Geometrical frustration among interacting spins combined with strong quantum fluctuations destabilize long-range magnetic order in favour of more exotic states such as spin liquids. By following this guiding principle, a number of spin liquid candidate systems were identified in quasi-two-dimensional (quasi-2D) systems. For 3D, however, the situation is less favourable as quantum fluctuations are reduced and competing states become more relevant. Here we report a comprehensive study of thermodynamic, magnetic and dielectric properties on single crystalline and pressed-powder samples of PbCuTe$_2$O$_6$, a candidate material for a 3D frustrated quantum spin liquid featuring a hyperkagome lattice. Whereas the low-temperature properties of the powder samples are consistent with the recently proposed quantum spin liquid state, an even more exotic behaviour is revealed for the single crystals. These crystals show ferroelectric order at $T_{\text{FE}} \approx 1\,\text{K}$, accompanied by strong lattice distortions, and a modified magnetic response -- still consistent with a quantum spin liquid -- but with clear indications for quantum critical behaviour.

cond-mat.str-el↗

arXiv2021

Involvement of structural dynamics in the charge-glass formation in molecular metals

We present a combined study of thermal expansion and resistance fluctuation spectroscopy measurements exploring the static and dynamic aspects of the charge-glass formation in the quasi-two-dimensional organic conductors $θ$-(BEDT-TTF)$_2$$MM^\prime$(SCN)$_4$ with $M$ = Cs and $M^\prime$ = Co,Zn. In these materials, the emergence of a novel charge-glass state so far has been interpreted in purely electronic terms by considering the strong frustration of the Coulomb interactions on a triangular lattice. Contrary to this view, we provide comprehensive evidence for the involvement of a \textit{structural} glass-like transition at $T_{\text{g}} \sim 90-100\,$K. This glassy transition can be assigned to the freezing of structural conformations of the ethylene endgroups in the donor molecule with an activation energy of $E_{\rm{a}}\approx 0.32\,$eV, and the concomitant slowing down of the charge carrier dynamics is well described by a model of non-exponential kinetics. These findings discolse an important aspect of the phase diagram and renders the current understanding of the charge-glass state in the whole family of $θ$-(BEDT-TTF)$_2MM^\prime$(SCN)$_4$ incomplete. Our results suggest that the entanglement of slow structural and charge-cluster dynamics due to the intimate coupling of lattice and electronic degrees of freedom determine the charge-glass formation under geometric frustration.

cond-mat.str-el↗

arXiv2021

Rise and Fall of Landau's Quasiparticles While Approaching the Mott Transition

Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass $m^{\star}$. Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator. We reveal persistent Fermi-liquid behavior with pronounced quadratic dependences of the optical scattering rate on temperature and frequency, along with a puzzling elastic contribution to relaxation. The strong increase of the resistivity beyond the Ioffe-Regel-Mott limit is accompanied by a `displaced Drude peak' in the optical conductivity. Our results, supported by a theoretical model for the optical response, demonstrate the emergence of a bad metal from resilient quasiparticles that are subject to dynamical localization and dissolve near the Mott transition.

cond-mat.str-el↗

arXiv2018

Decreasing the size of the Restricted Boltzmann machine

We propose a method to decrease the number of hidden units of the restricted Boltzmann machine while avoiding decrease of the performance measured by the Kullback-Leibler divergence. Then, we demonstrate our algorithm by using numerical simulations.

cs.LG↗

arXiv2018

Detuning the Honeycomb of $α$-RuCl$_{3}$: Pressure-Dependent Optical Studies Reveal Broken Symmetry

The honeycomb Mott insulator $α$-RuCl$_3$ loses its low-temperature magnetic order by pressure. We report clear evidence for a dimerized structure at $P>1$ GPa and observe the breakdown of the relativistic $j_{\rm eff}$ picture in this regime strongly affecting the electronic properties. A pressure-induced Kitaev quantum spin liquid cannot occur in this broken symmetry state. We shed light on the new phase by broad-band infrared spectroscopy of the low-temperature properties of $α$-RuCl$_{3}$ and ab initio density functional theory calculations, both under hydrostatic pressure.

cond-mat.str-el↗

arXiv2018

Antiferromagnetic ordering in organic conductor $λ$-(BEDT-TTF)$_2$GaCl$_4$ probed by $^{13}$C NMR

The ground state of $λ$-(BEDT-TTF)$_2$GaCl$_4$, which has the same structure as the organic superconductor $λ$-(BETS)$_2$GaCl$_4$, was investigated by magnetic susceptibility and $^{13}$C NMR measurements. The temperature dependence of the magnetic susceptibility revealed an antiferromagnetic (AF) correlation with $J/k_{\rm B} \simeq$ 98 K. NMR spectrum splitting and the divergence of $1/T_1$ were observed at approximately 13 K, which is associated with the AF transition. We found that the AF structure is commensurate according to discrete NMR peak splitting, suggesting that the ground state of $λ$-(BEDT-TTF)$_2$GaCl$_4$ is an AF dimer-Mott insulating state. Our results suggest that the superconducting phase of $λ$-type salts would be located near the AF insulating phase.

cond-mat.str-el↗