arXiv subjects

Youichi Yamakawa

Publications and source records attributed to Youichi Yamakawa.

At least 19 recordsLinked to original sources

arXiv2026

Prominent Intrinsic Orbital Hall Effect in the 135 Kagome Metal Family and Orbital Responses in the Loop-Current Phase

The study of electronic orbital degrees of freedom, including the generation and control of orbital currents and orbital angular momentum, has emerged as a vibrant research field. Here, we study the orbital Hall effect (OHE), one of the key mechanisms for orbital current generation, in transition-metal kagome-lattice metals. We propose a large positive OHE in CsTi$_3$Bi$_5$ and negative OHEs in CsV$_3$Sb$_5$ and CsCr$_3$Sb$_5$ models. Orbital-sector decomposition shows that the $\vert l^z_d \vert=2$ $d$-orbital channel gives a positive contribution, whereas the $\vert l^z \vert=1$ $d$- and $p$-orbital channels can give negative contributions. Control calculations suggest a persistent orbital-sector sign tendency near the actual filling, while strong $p$-$d$ hybridization modulates the quantitative balance and total OHE sign. Thus, the compound and filling dependence of the OHE reflects both the sign tendency of each orbital sector and the hybridization-controlled balance among them. Furthermore, we investigate the loop-current phase of CsV$_3$Sb$_5$ and show that it induces finite local atomic orbital angular momentum. We also show that loop-current-related symmetry lowering allows finite symmetric components of the orbital conductivity tensor. This study provides a basis for exploring orbital currents and local orbital-angular-momentum responses in strongly correlated kagome metals.

cond-mat.str-el↗

arXiv2026

Quasi-One-Dimensional Electronic Nature of Ta4SiTe4 Underlying the Giant Thermoelectric Performance

Ta4SiTe4 is a one-dimensional van der Waals material that exhibits an exceptionally large thermoelectric power factor below room temperature. However, since this material has been available only in the form of acicular microcrystals, experimental exploration of the electronic properties responsible for its giant thermoelectric performance has long been challenging. In this study, we quantitatively evaluated the one-dimensional electronic nature of Ta4SiTe4 by combining micro-spot angle-resolved photoemission spectroscopy and transport measurements on focused-ion-beam-processed samples. The angle-resolved photoemission spectroscopy measurements reveal anisotropic band dispersions along and perpendicular to the crystallographic c axis. Consistently, transport measurements demonstrate that the resistivity perpendicular to the c axis is approximately five times larger than that along the c axis at 200 K. These results provide direct experimental evidence for the quasi-one-dimensional electronic character of Ta4SiTe4, which underlies its giant thermoelectric response reported previously, and offer fundamental insights into the role of electronic dimensionality in enhancing thermoelectric performance.

cond-mat.mtrl-sci↗

arXiv2026

Molecular Orbital Degeneracy Lifting in a Tetrahedral Cluster System NbSeI

The lifting of degenerate electronic states, in which multiple electronic states share the same energy, is a fundamental issue in the physics of crystalline solids. In real materials, this problem has been extensively studied in transition metal compounds, where various quantum phenomena arise from the spin and orbital degeneracy of the d electrons on individual transition-metal atoms. In contrast, materials containing high-symmetry clusters composed of multiple transition-metal atoms are expected to exhibit more emergent phenomena due to the entanglement of the electronic degrees of freedom across multiple atoms. Here, we report the discovery of two distinct mechanisms of orbital-degeneracy lifting in NbSeI, which comprises Nb4 tetrahedral clusters with molecular orbital degrees of freedom and whose average crystal structure is predicted to host a flat-band metal. Below 106 K, NbSeI is found to be a nonmagnetic molecular orbital-ordered insulator. Above this temperature, the average structure becomes face-centered cubic without any superlattice, while the orbital degeneracy remains lifted by significant local distortions of Nb4 tetrahedra, which may be associated with a molecular orbital-liquid or orbital-frozen state. This noncooperative Jahn-Teller distortion stabilizes a nonmagnetic insulating state above 106 K, in stark contrast to the flat-band metal predicted from the average structure.

cond-mat.str-el↗

arXiv2025

Giant critical current peak induced by pressure in kagome superconductor RbV$_{3}$Sb$_{5}$

Superconductivity can coexist or compete with other orders such as magnetism or density waves. Optimizing superconductivity requires identifying competing orders that may disrupt Cooper pair coherence. Here, we use the self-field critical current ($I_{\rm c,sf}$) to probe pressure-tuned superconductivity in the kagome superconductor RbV$_3$Sb$_5$. As pressure destabilizes the charge-density wave (CDW) state, $I_{\rm c,sf}$ drastically enhances, peaking near the critical pressure where the CDW state is completely suppressed at zero temperature. Surprisingly, a weaker $I_{\rm c,sf}$ peak emerges within the CDW phase. Near the pressure of the weaker peak, the superconducting phase transition temperature shifts from an increasing trend with pressure to a near plateau. Our analysis suggests the possibility of a sudden change in the CDW pattern or a Lifshitz transition, highlighting the need for microscopic examinations of the CDW state for understanding the pressure evolution of superconductivity in RbV$_3$Sb$_5$.

cond-mat.supr-con↗

arXiv2025

Microscopic origin of period-four stripe charge-density-wave in kagome metal CsV$_3$Sb$_5$

The interplay between unconventional density waves and exotic superconductivity has attracted growing interest. Kagome superconductors $A\rm{V}_3\rm{Sb}_5$ ($A = \rm{K}, \rm{Rb}, \rm{Cs}$) offer a platform for studying quantum phase transitions and the resulting symmetry breaking. Among these quantum phases, the $4a_0$ stripe charge-density-wave (CDW) has been widely observed for $A=\rm{Rb}$ and $\rm{Cs}$ by scanning tunneling microscopy (STM) and nuclear magnetic resonance (NMR) measurements. However, the microscopic origin of the $4a_0$ stripe CDW remains elusive, and no theoretical studies addressing this phenomenon have been reported so far. In this paper, we propose a microscopic mechanism for the emergence of the $4a_0$ stripe CDW. We analyze the CDW instability in the 12-site kagome lattice Hubbard model with the $2\times2$ bond order driven by the paramagnon-interference mechanism by focusing on the short-range magnetic fluctuations due to the geometrical frustration of kagome lattice. We reveal that the nesting vector of the reconstructed Fermi surface, formed by the $2\times 2$ bond order, gives rise to a $4a_0$-period CDW. Remarkably, the obtained stripe CDW is composed of both the off-site hopping integral modulations and on-site potentials. The real-space structure of the stripe CDW obtained here is in good qualitative agreement with the experimentally observed stripe pattern.

cond-mat.str-el↗

arXiv2025

Superconductivity and the quasiparticle mass enhancement near the CDW critical point using Bethe-Salpeter method: Application to cuprates

In recent years, charge-channel order in strongly correlated metals has attracted much attention. Representative examples include electronic nematic order in cuprates and iron-based superconductors, and Star-of-David order in kagome metals. Critical phenomena and unconventional superconductivity arising from fluctuations of such charge-channel orders are central issues today; however, the essential role is played by many-body effects (vertex corrections) beyond the mean-field approximation, and their origin and computational methods have not been established. In this study, we propose the Bethe-Salpeter equation method to evaluate electron-electron interactions in two-dimensional Hubbard models beyond the mean-field approximation. Based on the Baym-Kadanoff conserving approximation, we find that an attractive interaction in the charge channel emerges from the Aslamazov-Larkin vertex corrections that describe the interference processes among spin fluctuations. Applying this method to the square-lattice Hubbard model shows that the cooperation of attractive charge fluctuations and repulsive spin fluctuations yields high-$T_c$ $d$-wave superconductivity together with enhanced effective mass. These results provide a natural explanation for the phase diagram of cuprate superconductors, in which $d$-wave superconductivity is strongly enhanced near the charge-order critical point.The theory can also be applied to iron-based and nickelate superconductors, suggesting broad potential for future applications.

cond-mat.supr-con↗

arXiv2025

Impact of multiband effects on non-Fermi-liquid transport phenomena in bilayer nickelates

Recently discovered high-$T_c$ superconductivity in thin-film bilayer nickelates La$_3$Ni$_2$O$_7$ under ambient pressure has attracted great interest. Non-Fermi-liquid transport behaviors, such as $T$-linear resistivity and a positive Hall coefficient that increases at low temperatures, have been reported in this system. In this study, we analyze the non-Fermi-liquid transport phenomena in the thin-film bilayer nickelate La$_3$Ni$_2$O$_7$ using a multiorbital tight-binding model. In La$_3$Ni$_2$O$_7$, the cold spots composed of Ni $d_{x^2-y^2}$ orbital emerge, since the spin fluctuations cause stronger quasiparticle damping $\gamma$ in the Ni $d_{z^2}$ orbital. Notably, in the present study, we derive a rigorous formula for the Hall coefficient $R_H$ incorporating the $\gamma$ in the quasi-quantum metric (qQM) term. We find that the $T$ dependence of $\gamma$ in the qQM term is important in determining $R_H$. In La$_3$Ni$_2$O$_7$, the $T$ dependence of $R_H$ becomes pronounced due to the competition between the positive contribution from the hole band and the negative contribution from the electron band. Moreover, the qQM term plays an important role in describing the Nernst coefficient and other transport phenomena involving the second derivative velocity $v^{\mu\nu}$.

cond-mat.str-el↗

arXiv2025

Nematic and chiral superconductivity emerging within the loop-current phase in kagome metals

The kagome metals $A$V$_3$Sb$_5$ ($A =$ Cs, Rb, K) host multiple symmetry-breaking phases, including charge-density-wave and loop-current orders, and exhibit highly exotic superconductivity with pronounced nematicity and chirality. Remarkably, even dilute impurities transform this exotic superconducting state into an isotropic $s$-wave state. These observations pose a challenge to existing theoretical scenarios. We show that loop-current order induces nematic chiral $d$-wave superconductivity in kagome metals. The loop-current-induced orbital magnetization (OM) stabilizes one chiral superconducting channels. This OM-chirality coupling mechanism is generic and applies to pairing driven by either attractive or repulsive interactions. Furthermore, coexisting loop-current and bond orders give rise to pronounced nematic chiral superconductivity even for an almost $C_6$-symmetric Fermi surface. For attractive pairing, dilute impurities suppress the chiral state and restore a conventional $s$-wave phase, as observed experimentally. The theory further predicts a robust $2\times2$ pair-density modulation. This study provides key insights into time-reversal-symmetry-breaking exotic superconductivity in kagome metals.

cond-mat.supr-con↗

arXiv2025

Cubic ReSTe as a High-Performance Thermoelectric Material

We report thermoelectric properties of sintered samples of undoped, W-doped, and Sb-doped ReSTe crystallized in a cubic MoSBr-type structure. All samples exhibited p-type thermoelectric properties. ReSTe and Re0.993W0.007STe exhibited the largest dimensionless figure of merit ZT, reaching 0.4 at 660 K. This high performance is attributed to large power factor owing to the degenerate semiconducting state realized by the strong spin-orbit coupling and low lattice thermal conductivity of the sintered samples. Furthermore, electronic band dispersion of ReSTe is almost flat at the bottom of the conduction band, suggesting that n-type ReSTe is expected to exhibit much higher performance than p-type ReSTe.

cond-mat.mtrl-sci↗

arXiv2025

Unified mechanism of charge-density-wave and high-$T_c$ superconductivity protected from oxygen vacancies in bilayer nickelates

Unconventional charge and spin density-wave states are commonly observed in bilayer nickelates, drawing considerable attention due to their proximity to high-$T_c$ superconductivity in various phase diagrams. However, the nature and mechanisms of charge and spin density-waves (DWs) in nickelates remain poorly understood. Numerous experiments have reported that the charge-density-wave (CDW) transition temperature $T_{ cdw}$ and the spin-density-wave (SDW) transition temperature $T_{sdw}$ are closely related but distinct. However, in contrast to these experiments, previous mean-field-type analyses have yielded only a simple SDW phase. To resolve this key problem, this paper demonstrates that sizable CDW instabilities emerge in proportion to the SDW instability in La$3$Ni$2$O$7$.This behavior is driven by the paramagnon-interference (PMI) mechanism, which captures important electron correlations beyond mean-field theory. Therefore, (i) experimental CDW + SDW coexisting state is naturally explained. In addition, (ii) the CDW + SDW fluctuations cooperatively drive high-$T_c$ superconductivity. Notably, the predicted $s$-wave SC state is robust against the inner apical O vacancies. Furthermore, (iii) the CDW instability is highly sensitive to the size of the $d_{z^2}$-orbital hole pocket, allowing for the realization of CDW quantum criticality through carrier-doping and pressure application. We find that the coexistence of charge and spin fluctuations is essential in bilayer nickelates, with both playing a cooperative role in mediating high-$T_c$ superconductivity.

cond-mat.supr-con↗

arXiv2025

Origin of switchable quasiparticle-interference chirality in loop-current phase of kagome metals measured by scanning-tunneling-microscopy

In the kagome superconductors AV3Sb5 (A=Cs,Rb,K), a cascade of correlated electron phases cause exotic symmetry-breaking quantum states. In particular, the dissipationless chiral loop-current phase has been attracting increasing attention. A crucial clue is offered by the chirality of the quasiparticle interference signal observed in scanning tunneling microscopy. However, the connection between loop-current chirality and quasiparticle interference chirality remains poorly understood. Here, we reveal theoretically that a pronounced chiral quasiparticle interference signal emerges in the extremely dilute impurity regime ($lesssim$ 0.1 %). A single impurity at site Z induces a quasiparticle interference chirality $\chi_Z=\pm1$, determined by the direction of the Z3 nematicity, itself set by the relative position of the loop-current order in the star-of-David charge-density-wave phase. Notably, even a small magnetic field can smoothly switch the chirality, leading to field-induced shear lattice strain consistent with recent experiments. Our theoretical study provide key insights into the nature of the loop-current-induced symmetry-breaking states in kagome metals.

cond-mat.str-el↗

arXiv2024

Paramagnon-Interference Mechanism for Three-Dimensional Bond Order in Kagome Metals AV$_3$Sb$_5$ (A=Cs, Rb, K): Analysis by the Density-Wave Equation

The mechanism of CDW and its 3D structure are important fundamental issues in kagome metals. We have previously shown that, based on a 2D model, $2\times 2$ bond order (BO) emerges due to the paramagnon-interference (PMI) mechanism and that its fluctuations lead to $s$-wave superconductivity. This paper studies these issues based on realistic 3D models of kagome metals AV$_3$Sb$_5$ (A=Cs, Rb, K). We reveal that a commensurate 3D $2\times 2\times 2$ BO is caused by the PMI mechanism, by performing the 3D density-wave (DW) equation analysis for all A=Cs, Rb, K models in detail. Our results indicate a BO transition temperature $T_{\rm BO}\sim 100$K within the regime of moderate electron correlation. The 3D structure of BO is attributed to the three-dimensionality of the Fermi surface, while the 3D structure of BO is sensitively changed, since the Fermi surface is quasi-2D. Based on the analysis of the DW equation, by taking into account a finite third-order Ginzburg-Landau (GL) term, (i) shift stacking $2\times 2\times 2$ BO can be realized via a first-order transition below $T_{\rm BO}$. Here, the in-plane BO pattern (tri-hexagonal or star-of-David) is determined by the sign of the third-order GL term, with hole doping tending to favor the tri-hexagonal state. On the other hand, if the third-order GL term is very small, (ii) alternating vertical stacking BO may instead be realized via a second-order transition. The present study enhances our understanding of the rich variety of BOs observed experimentally. It is confirmed that the PMI mechanism is the essential origin of the 3D CDW of kagome metals.

cond-mat.str-el↗

arXiv2024

Quantum-metric-induced giant and reversible nonreciprocal transport phenomena in chiral loop-current phases of kagome metals

Rich spontaneous symmetry-breaking phenomena with nontrivial quantum geometric properties in metals represent central issues in condensed matter physics. In this context, the emergence of chiral loop-current order, accompanied by time-reversal symmetry-symmetry breaking in various kagome metals, has garnered significant attention. Particularly noteworthy is the giant electrical magnetochiral anisotropy (eMChA) in CsV3Sb5, which provides compelling evidence of time-reversal-symmetry and inversion-symmetry breakings. However, the underlying essence of this observation has remained obscured due to the lack of theoretical understanding. Here, we reveal that the loop-current order causes giant and reversible eMChA coefficient, $\gamma_{eM}$, is proportional to the loop-current-induced orbital magnetization $M_{orb}$ times the lifetime of conduction electrons $\tau$. In kagome metals, the derived $\gamma_{eM}$ is substantial and reversible by minute magnetic fields, due to the large $\tau (\gg a_0/v_{Fermi})$ and the field-induced reversal of $M_{orb}$. By considering the experimentally observed stripe charge-density wave, the loop-current state becomes non-centrosymmetric, thereby giving rise to the eMChA. Surprisingly, the quantum-metric, which defines a fundamental geometric aspect of Bloch wavefunctions, acquires significant momentum dependence in the loop-current phase, resulting in a dramatic enhancement of eMChA by $\sim100$ times. This research not only clarifies the fundamental symmetry-breaking states in kagome metals, but also opens a new path for exploring quantum-metric-induced phenomena arising from exotic quantum phase transitions in strongly correlated metals

cond-mat.str-el↗

arXiv2024

Giant Impurity Effects on Charge Loop Current Order States in Kagome Metals

The exotic electronic states in the charge loop current (cLC) phase, in which the permanent charge current breaks the time-reversal symmetry, have been attracting increasing attention in recently discovered kagome metals AV3Sb5 (A = Cs, Rb, K). Interestingly, the cLC state is sensitively controlled by applying a small magnetic field as well as a tiny uniaxial strain. In addition, many experiments indicate that the cLC state is sensitive to the small number of impurities. To understand the impurity effects on the cLC electronic states accurately, we analyze the giant unit-cell (up to 1200 sites) kagome lattice model with single impurity potential. The loop current is found to be strongly suppressed within the current correlation length $\xi_J$ centered on the impurity site, where $\xi_J$ increases as the cLC order parameter $\eta$ decreases. (The cLC order is the pure imaginary hopping integral modulation $\delta t_{i,j}=\pm i\eta$.) In addition, both the uniform orbital magnetization $M_{orb}$ and the anomalous Hall conductivity $\sigma_{xy}$ are drastically suppressed by dilute impurities. Especially, the suppression ratio $R=-\Delta M_{orb}/M_{orb}^0$ can exceed 50% with the introduction of 1% impurities. Unexpectedly, the ratio $R$ is qualitatively insensitive to $\eta$, in highly contrast to a naive expectation that $R$ is proportional to the current suppression area $\pi \xi_J^2$. The resulting giant impurity effect of $M_{orb}$ would originates from the nonlocal contribution of the itinerant circulation of electrons. The present study gives a natural explanation of why the cLC electronic states in kagome metals are sensitive to dilute impurities.

cond-mat.str-el↗

arXiv2024

Superconductivity in Ternary Zirconium Telluride Zr6MTe2 with 3d Transition Metals

We report the synthesis, electronic properties, and electronic states of Zr6MTe2 (M = Cr, Mn, Fe, and Co), which is isostructural to a recently discovered superconductor family Sc6MTe2. Based on the electrical resistivity and heat capacity data measured at low temperatures, Zr6FeTe2 is found to show bulk superconductivity below Tc = 0.76 K. Zr6CoTe2 also exhibited zero resistivity due to superconductivity below 0.13 K. In contrast, Zr6+dMn1-dTe2 does not show superconductivity but instead exhibits strong magnetism, which most likely prevents the formation of superconductivity in this material. The electronic properties and electronic states of Zr6MTe2 are discussed in comparison with those of Sc6MTe2.

cond-mat.supr-con↗

arXiv2023

Real-space Loop Current Pattern in Time-reversal-symmetry Breaking Phase in Kagome Metals

The charge loop current (cLC) state has attracted increasing attention in kagome metals. Here, we calculate the spontaneous currents along the nearest sites $i$ and $j$, $J_{i,j}$, induced by the cLC order that is the imaginary and odd-parity hopping integral modulation $\delta t_{i,j}$. We reveal that the magnitude of $J_{i,j}$ strongly depends on the nearest sites $i$ and $j$ in the $2\times2$ cLC state, where $\eta\equiv |\delta t_{i,j}|$ is equivalent for all nearest sites. The obtained $J_{i,j}$ becomes large near the van-Hove singularity (vHS) filling ($n\sim n_{vHS}$) even when $\eta$ is fixed. Interestingly, the obtained $J_{i,j}$ exhibits the logarithmic divergence behavior at low temperatures for $n\sim n_{vHS}$ with a fixed $\eta$, by reflecting the vHS points that are the characteristic of kagome metals. The present study provides useful information for local electronic state measurements, such as the site-selective NMR and STM experiments.

cond-mat.str-el↗

arXiv2023

Superconductivity in Ternary Germanide ScPdGe and Silicide ScPdSi

The electronic properties of ScPdGe and ScPdSi, crystallizing in the hexagonal ZrNiAl and orthorhombic TiNiSi structures, respectively, are investigated. ScPdGe and ScPdSi are found to show bulk superconductivity below 0.9 and 1.7 K, respectively, based on electrical resistivity and heat capacity data measured using synthesized polycrystalline samples. First principles calculations indicate the presence of large contributions of Sc 3d and Pd 4d electrons at the Fermi energy in both materials. The electronic properties and electronic states of these materials are discussed in comparison with those of several superconductors containing scandium and a 4d transition metal element.

cond-mat.supr-con↗

arXiv2023

Low temperature phase transitions under CDW state in kagome metals AV3Sb5 (A=Cs,Rb,K): Significance of mix-type Fermi surface electron correlations

To understand the multistage phase transitions in V-based kagome metals inside the charge-density-wave (CDW) phase, we focus on the impact of the "mix-type" Fermi surface because it is intact in the CDW state on the "pure-type" Fermi surface. On the mixed-type Fermi surface, moderate spin correlations develop, and we reveal that uniform (q=0) bond order is caused by the paramagnon interference mechanism, which is described by the Aslamazov-Larkin vertex correction. A dominant solution is the E2g-symmetry nematic order, in which the director can be rotated arbitrarily. In addition, we obtain the A1g-symmetry non-nematic order, which leads to the change in the lattice constants without symmetry breaking. The predicted E2g and A1g channel fluctuations at q=0 can be observed by the elastoresistance measurements. These results are useful to understand the multistage phase transitions inside the 2 $\times$ 2 CDW phase. The present theory has a general significance because mix-type band structure universally exists in various kagome lattice models.

cond-mat.str-el↗