arXiv subjects

Naoto Nagaosa

Publications and source records attributed to Naoto Nagaosa.

At least 37 records · Page 2Linked to original sources

arXiv2025

Quantum geometric bounds for observables: Linear responses, Drude weight, and orbital magnetization

The quantum geometric tensor (QGT) provides nontrivial bounds among physical quantities, as exemplified by the metric-curvature inequality. In this paper, we investigate various bounds for different observables through certain generalizations of the QGT. First, we demonstrate that bounds hold for all linear responses, which are produced by a QGT extended to many-body states, finite temperature, and general parameter space. As an application, we show the thermodynamic inequality originating from the convexity of free energy can be further tightened. Second, we establish a bound between the Drude weight and the orbital magnetization. The equality is exactly satisfied in the Landau level system, and systems with nearly flat bands tend to approach equality as well. We apply the resulting inequality to two orbital ferromagnets and support that the twisted bilayer graphene system is close to the Landau level system. Moreover, we show that an analogous inequality also holds for a higher-order multipole, magnetic quadrupole. Finally, we discuss the analogy between the QGT and the uncertainty principle, emphasizing that the existence of nontrivial bounds necessarily reflects quantum effects.

cond-mat.mtrl-sci↗

arXiv2025

Quantum Geometry Phenomena in Condensed Matter Systems

Quantum geometry, which describes the geometry of Bloch wavefunctions in solids, has become a cornerstone of modern quantum condensed matter physics. The quantum geometrical tensor encodes this geometry through two fundamental components: the quantum metric (real part) and the Berry curvature (imaginary part). While the Berry curvature gained prominence through its manifestation in the intrinsic anomalous Hall effect, recent advances have revealed equally significant effects arising from the quantum metric. This includes its signatures in nonlinear transport, superfluid density of flat-band superconductors, and nonlinear optical responses. These advances underscore how quantum geometry is reshaping our understanding of condensed matter systems, with far-reaching implications for future technologies. In this review, we survey recent progress in the field, focusing on both foundational concepts and emergent phenomena in transport and optics-with particular emphasis on the pivotal role of the quantum metric.

cond-mat.str-el↗

arXiv2025

Spin-phonon coupling and thermal Hall effect in the Kitaev model

The Kitaev model, which involves bond-direction-dependent spin interactions on a honeycomb lattice, has attracted significant interest due to its exact solvability and potential uses in quantum computing. A key feature of this model is the half-quantized thermal Hall conductivity (HQTHC) under a magnetic field perpendicular to the lattice; however, HQTHC only appears at low temperatures. Here, in the higher temperature range beyond the HQTHC regime, we theoretically suggest an extrinsic phonon contribution to the thermal Hall effect in the Kitaev model through skew-scattering of chiral phonons by scalar spin chirality, previously examined in Mott insulators. We demonstrate the emergence of scalar spin chirality from fluctuating spins, estimate the resulting field strength and its symmetric form applied to chiral phonons, and obtain the associated thermal Hall conductivity in semi-quantitative agreement with existing experiments. This work offers a fundamental understanding of how spin-phonon interactions influence strongly correlated systems.

cond-mat.str-el↗

arXiv2025

Floquet spin textures in optically pumped non-Hermitian surface states

Optical effects of quantum matter with interaction are key to physics and technology. The class of non-Hermitian (NH) phenomena is mostly explored in cold atoms, photonics, and metamaterials out of equilibrium. Effective NH systems due to interaction in equilibrium solids, however, provide a unique opportunity for realizing light-NH matter hybrids via periodic irradiation. Given NH topological surface states with magnetic disorder, here we reveal spectroscopically observable Floquet spin textures for this hybrid quantum matter: merged meron strings, dichroic skyrmions, and domain structure accompanying vortices in energy planes; Bloch lines and topologically twisted vortex rings distinct from Hopfions in energy-momentum space; topological selectiveness of linear polarization occurs besides circular dichroism. Spectroscopic energy evolution and chemical potential dependence are key ingredients of this open hybrid system. With spin-resolved photoemission spectroscopy, this scenario bears prime physical interest by reaching the crossroad between solid-state interaction effects, non-Hermiticity, and light-matter coupling.

cond-mat.mes-hall↗

arXiv2025

Polarons with arbitrary nonlinear electron-phonon interaction

We develop an exact computational method based on numerical X-propagators for solving polaron models with arbitrary nonlinear couplings of local vibration modes to the electron density and magnitude of the hopping amplitude. Our approach covers various polaron models, some of which were impossible to treat by any existing approximation-free techniques. Moreover, it remains efficient in the most relevant but computationally challenging regime of phonon frequencies much smaller than the electron bandwidth. As a case study, we consider the double-well type nonlinear model with quadratic ($g_2<0$) and quartic ($g_4>0$) interactions describing a broad class of technologically important materials, such as quantum paraelectric compounds and halide perovskites. We observe, depending on the model parameters, three qualitatively different regimes: (i) quantum interplay of quartic and quadratic interactions which suppresses effects of the quadratic coupling, (ii) intermediate-coupling regime with exponential $\propto \exp(αg_2 Ω^{-1/4})$ scaling of the quasiparticle weight and mass renormalization, and (iii) strong-coupling asymptotic behavior.

cond-mat.str-el↗

arXiv2025

Nanosculpted 3D helices of a magnetic Weyl semimetal with switchable nonreciprocity

The emergent properties of materials are defined by the symmetries of their underlying atomic, spin and charge order. The explorations of symmetry breaking effects are therefore usually limited by the intrinsic properties of known, stable materials. In recent years, advances in focused ion beam (FIB) fabrication have enabled the nanostructuring of bulk crystals into ultraprecise transport devices [1-4], facilitating the investigation of geometrical effects on mesoscopic length scales. In this work, we expand such explorations into three-dimensional (3D), curvilinear shapes, by sculpting helical nanostructure devices from single crystals of the high-mobility, centrosymmetric magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ [5,6]. The combination of the imposed chiral geometry and intrinsic ferromagnetism yields nonreciprocal electron transport [7-9]. The high coercivity results in an anomalous, reversable diode effect remnant under zero applied magnetic field, which is orders of magnitude larger than can be explained by a classical self-field mechanism. We argue the enhancement originates from the high carrier mobility and the resulting quasi-ballistic transport: the conduction electron mean free path approaches the length scale of the curvature, resulting in increased asymmetrical scattering at the boundaries. We further demonstrate the inverse effect of the nonreciprocal transport: the field-free, current-induced switching of the magnetisation. The results establish the vast potential of 3D nanosculpting to explore and enrich the functionality of quantum materials.

cond-mat.mes-hall↗

arXiv2025

Merons, hedgehogs and magnetoelectric switching in spiral multiferroics

In spiral multiferroics, magnetism induces ferroelectricity, thus holding a promise for novel memory devices where an electric field switches magnetic bits. However, such a switching process, in which magnetic domain walls are moved electrically, is still poorly understood. We find multiferroic domain walls containing arrays of meron (half-skyrmion) strings with a plethora of topological defects, which profoundly affect wall dynamics. Minimum energy walls have alternating meron topological charges and move as relativistic massive particles, with velocity limited by the magnon speed. During domain nucleation, walls with non-alternating meron charges appear. Such defects result in a peculiar non-local dynamics where all the spins in the system rotate, and the wall mobility is suppressed. Meron strings possess 0D hedgehog defects, analogous to Bloch points, that pin the wall to the lattice. This fascinating interplay of magnetoelectric switching with a variety of topological defects and non-local spin dynamics opens a new playground for the electric manipulation of spins.

cond-mat.str-el↗

arXiv2025

Emergent reactance induced by the deformation of a current-driven skyrmion lattice

The interaction between conduction electrons and spin textures gives rise to remarkable phenomena associated with the Berry phase. The Berry phase acquired by conduction electrons acts as an emergent electromagnetic field, facilitating phenomena analogous to classical electromagnetism, such as the Lorentz force and electromagnetic induction. Magnetic skyrmions, spin vortices with non-trivial topology, serve as a key platform for such studies. For example, non-trivial transport responses are recognized as being induced by the emergent Lorentz force and the emergent electromagnetic induction. Despite remarkable progress in skyrmion physics, emergent reactance, in which the phase of an applied AC current is modified by emergent electromagnetism, has not been thoroughly investigated. Here, we report emergent reactance in the prototypical skyrmion-hosting material, MnSi. We observe longitudinal and Hall reactance signals as the skyrmion lattice undergoes creep motion, in which the skyrmions deform while moving. The Hall reactance is attributed to the emergent electric field associated with the inertial translational motion arising from the skyrmion effective mass. In contrast, the longitudinal reactance results from the emergent electric fields generated by the phason and spin-tilting modes excited by their deformation. Our findings shed light on the internal deformation degrees of freedom in skyrmions as a important factor for efficient generation of the emergent electric field.

cond-mat.mtrl-sci↗

arXiv2025

Probing Loop Currents and Collective Modes of Charge Density Waves in Kagome Materials with NV Centers

Recently, the unconventional charge density wave (CDW) order with loop currents has attracted considerable attention in the Kagome material family AV$_3$Sb$_5$ (A = K, Rb, Cs). However, experimental signatures of loop current order remain elusive. In this work, based on the mean-field free energy, we analyze the collective modes of unconventional CDW order in a Kagome lattice model. Furthermore, we point out that phase modes in the imaginary CDW (iCDW) order with loop current orders result in time-dependent stray fields. We thus propose using nitrogen-vacancy (NV) centers to detect these time-dependent stray fields, providing a potential experimental approach to identifying loop current order.

cond-mat.str-el↗

arXiv2025

A Clue on Small-Capacitance Josephson Junction: What to Expect from Cooper Pair Ideal Conductor and Ohmic Resistor in Parallel?

By using analytical and Worldline Monte Carlo approaches, we investigate the effects induced by quantum phase fluctuations combined with quasiparticle subgap and shunt resistances on a small-capacitance Josephson junction. By using the linear response theory in the presence of two biasing schemes, we prove that the ideal conduction, foreseen in the pioneering papers on this topic, is not robust against either quantum phase fluctuations or dissipative effects. By including both of them in the Hamiltonian, we prove that an increase of the Ohmic dissipation strength induces a Berezinskii-Kosterlitz-Thouless quantum phase transition in thermodynamic equilibrium. Then we study charge and phase fluctuations at the thermodynamic equilibrium within the linear response theory. We find that the phase particle motion, in a quantum Josephson junction, does not change from diffusive to localized, resulting in an insulator-superconductor transition, as is commonly believed. At the transition, we prove that: i) the motion of the phase particle changes from ballistic to localized; ii) by turning on the coupling with the environment, a long-lived excitation at finite frequency emerges in the charge response function: it evolves first into a resonance and then disappears at the transition. Consequences beyond the linear response regime are investigated, leading to an alternative comprehensive physical picture for this system: we predict a transition from a dissipative quasiparticle current to a polaronic Cooper pair current.

cond-mat.supr-con↗

arXiv2025

Magnetoelectric control of spin helicity and nonreciprocal charge transport in a multiferroic metal

A multiferroic state with both electric polarization ($P$) and magnetization ($M$) shows the inherently strong $P$-$M$ coupling, when $P$ is induced by cycloidal (Néel-wall like) spin modulation. The sign of $P$ is determined by clockwise or counterclockwise rotation of spin, termed the spin helicity. Such a multiferroic state is not limited to magnetic insulators but can be broadly observed in conductors. Here, we report the current-induced magnetoelectric control of the multiferroics in a helimagnetic metal YMn$_6$Sn$_6$ and its detection through nonreciprocal resistivity (NRR). The underlying concept is the coupling of the current with the toroidal moment $T \sim P \times M \sim (\hat{q} \times χ_v) \times M$ as well as with the magneto-chirality $χ_v \times M$, where $\hat{q}$ and $χ_v$ being the unit modulation wave vector and the vector spin chirality, respectively. We furthermore observe an enhancement of NRR by the spin-cluster scattering via $χ_v$ and its fluctuation. These findings may pave a way to exploration of multiferroic conductors and application of the spin-helicity degree of freedom as a state-variable.

cond-mat.mtrl-sci↗

arXiv2025

Photon-drag photovoltaic effects and quantum geometric nature

The bulk photovoltaic effect (BPVE) generates a direct current photocurrent under uniform irradiation and is a nonlinear optical effect traditionally studied in non-centrosymmetric materials. The two main origins of BPVE are the shift and injection currents, arising from transitions in electron position and electron velocity during optical excitation, respectively. Recently, it was proposed that photon-drag effects could unlock BPVE in centrosymmetric materials. However, experimental progress remains limited. In this work, we provide a comprehensive theoretical analysis of photon-drag effects inducing BPVE (photon-drag BPVE). Notably, we find that photon-drag BPVE can be directly linked to quantum geometric tensors. Additionally, we propose that photon-drag shift currents can be fully isolated from other current contributions in non-magnetic centrosymmetric materials. We apply our theory explicitly to the 2D topological insulator $1T'$-WTe$_2$. Furthermore, we investigate photon-drag BPVE in a centrosymmetric magnetic Weyl semimetal, where we demonstrate that linearly polarized light generates photon-drag shift currents.

cond-mat.mes-hall↗

arXiv2025

Theory of thermopolarization effect

We study the polarization response to the temperature gradient in insulators, known as the thermopolarization effect. We show that this response can be understood through the free energy response function to an electric field gradient, which we call Q-tensor. By using the Q-tensor, we present a unified description of the polarization responses to both electric fields and temperature gradients and derive the generalized Mott relation. Additionally, we draw an analogy with the anomalous Hall and Nernst effects. These effects are observable as the Seebeck effect where the linear size of the system is shorter than the screening length.

cond-mat.mes-hall↗

arXiv2025

Quasi-one-dimensional Supersolids in Luther-Emery Liquids

The supersolid is a long-sought phase in condensed matter physics, characterized by the coexistence of density wave and superfluid orders. This phase is counterintuitive, as different symmetry-breaking orders typically compete with one another. A deeper understanding of how such a state forms in condensed matter systems remains an open question, especially in quasi-one-dimensional correlated systems. In this work, we investigate the emergence of supersolids in Luttinger-Emery liquids using a variational method. As the system consists of coupled Luttinger-Emery liquid chains, we refer to this phase as a quasi-one-dimensional supersolid. Notably, we demonstrate that the quasi-one-dimensional supersolid phase is energetically favorable in chains with finite size or short-range order. Furthermore, we investigate the collective dynamics of these coexisting charge density waves and superconducting states, identifying a quasi-Goldstone mode. Our theory provides valuable insights into both the ground state and the dynamic properties of supersolids in strongly correlated systems.

cond-mat.str-el↗

arXiv2025

Nonreciprocal Nonlinear Responses in Moving Charge Density Waves

The incommensurate charge density wave states (CDWs) can exhibit steady motion in the flow limit after depinning, behaving as a nonequilibrium system with time-dependent states. Since the moving CDW, like an electric current, breaks both time-reversal and inversion symmetries, one may speculate the emergence of nonreciprocal nonlinear responses from such motion. However, the moving CDW order parameter is intrinsically time-dependent in the lab frame, and it is known to be challenging to evaluate the responses of such a time-varying system. In this work, following the principle of Galilean relativity, we resolve this time-dependent hard problem in the lab frame by mapping the system to the comoving frame with static CDW states through the Galilean transformation. We explicitly show that the nonreciprocal nonlinear responses would be generated by the movement of CDW states through violating Galilean relativity.

cond-mat.mes-hall↗

arXiv2024

Nonreciprocal transport in a room-temperature chiral magnet

Chiral magnets under broken time-reversal symmetry can give rise to rectification of moving electrons, called nonreciprocal transport. Several mechanisms, such as the spin-fluctuation-induced chiral scattering and asymmetry in the electronic band dispersion with and without the relativistic spin-orbit interaction, have been proposed, but clear identification as well as theoretical description of these different contributions are desired for full understanding of nonreciprocal transport phenomena. Here, we investigate a chiral magnet Co8Zn9Mn3 and find the nonreciprocal transport phenomena consisting of different contributions with distinct field- and temperature-dependence across the magnetic phase diagram over a wide temperature range including above room-temperature. We successfully separate the nonreciprocal resistivity into different components and identify their mechanisms as spin-fluctuation-induced chiral scattering and band asymmetry in a single material with the help of theoretical calculations.

cond-mat.mtrl-sci↗

arXiv2024

This took us a Weyl: synthesis of a semimetallic Weyl ferromagnet with point Fermi surface

Quantum materials governed by emergent topological fermions have become a cornerstone of physics. Dirac fermions in graphene form the basis for moiré quantum matter, and Dirac fermions in magnetic topological insulators enabled the discovery of the quantum anomalous Hall effect. In contrast, there are few materials whose electromagnetic response is dominated by emergent Weyl fermions. Nearly all known Weyl materials are overwhelmingly metallic, and are largely governed by irrelevant, conventional electrons. Here we theoretically predict and experimentally observe a semimetallic Weyl ferromagnet in van der Waals (Cr,Bi)$_2$Te$_3$. In transport, we find a record bulk anomalous Hall angle $> 0.5$ along with non-metallic conductivity, a regime sharply distinct from conventional ferromagnets. Together with symmetry analysis, our data suggest a semimetallic Fermi surface composed of two Weyl points, with a giant separation $> 75\%$ of the linear dimension of the bulk Brillouin zone, and no other electronic states. Using state-of-the-art crystal synthesis techniques, we widely tune the electronic structure, allowing us to annihilate the Weyl state and visualize a unique topological phase diagram exhibiting broad Chern insulating, Weyl semimetallic and magnetic semiconducting regions. Our observation of a semimetallic Weyl ferromagnet offers an avenue toward novel correlated states and non-linear phenomena, as well as zero-magnetic-field Weyl spintronic and optical devices.

cond-mat.mes-hall↗

arXiv2024

Nonlinear edge transport in a quantum Hall system

Nonlinear transport phenomena in condensed matter reflect the geometric nature, quantum coherence, and many-body correlation of electronic states. Electric currents in solids are classified into (i) Ohmic current, (ii) supercurrent, and (iii) geometric or topological current. While the nonlinear current-voltage $(I$-$V)$ characteristics of the former two categories have been extensive research topics recently, those of the last category remains unexplored. Among them, the quantum Hall current is a representative example. Realized in two-dimensional electronic systems under a strong magnetic field, the topological protection quantizes the Hall conductance in the unit of $e^2/h$ ($e$: elementary charge, $h$: Planck constant), of which the edge transport picture gives a good account. Here, we theoretically study the nonlinear $I$-$V_\text{H}$ characteristic of the edge transport up to third order in $V_\text{H}$. We find that nonlinearity arises in the Hall response from electron-electron interaction between the counterpropagating edge channels with the nonlinear energy dispersions. We also discuss possible experimental observations.

cond-mat.mes-hall↗