Hydrodynamic phonon transport was recently predicted as an important regime for phonon transport in graphitic materials. Many of the past studies on hydrodynamic phonon transport have focused on the cases where the hydrodynamic regime is significantly stronger than other regimes such that hydrodynamic features can be clearly observed. However, this often requires stringent conditions of temperature and sample size. In many cases, the transport cannot be characterized by a single regime, but the features of all three regimes—ballistic, hydrodynamic, and diffusive regimes—exist to some extent. Here we assess the extent of three regimes by comparing momentum destruction rates by three different mechanisms, each of which represents a different regime: diffuse boundary scattering without internal phonon scattering (ballistic regime), diffuse boundary scattering combined with normal scattering (hydrodynamic regime), and umklapp scattering (diffusive regime). We solve the Peierls-Boltzmann equation with an ab initio full scattering matrix using a deviational Monte Carlo method. We sample distribution functions of ballistic and scattered particles separately, and thereby compare the momentum destruction rates by the three different mechanisms. Using this framework, we discuss a well-known phenomenon of ballistic-to-hydrodynamic crossover, called the phonon Knudsen minimum.

We analyze time evolution of charge and spin states in a quantum dot coupled to an electric reservoir. Utilizing high-speed single-electron detection, we focus on dynamics induced by the first-order tunneling. We find that there is a difference between the spin and the charge relaxation: The former appears slower than the latter. The difference depends on the Fermi occupation factor and the spin relaxation becomes slower when the energy level of the quantum dot is lowered. We explain this behavior by a theory including the first-order tunneling processes and find a good agreement between the experiment and the theory.

We experimentally find that the substitution of Si for Al in Heusler-type ${\mathrm{Fe}}_2\mathrm{VAl}$ epilayers contributes to the significant reduction in the thermal conductivity, in which ${\mathrm{Fe}}_2\mathrm{VAl}$ is one of the next-generation thermoelectric materials without using toxic elements. Because of the low-temperature growth of the epilayers, the Si substitution induces the decrease in the degree of $L{2}_1$ ordering, giving rise to the formation of V-Si antisite defects in the epilayers. For ${\mathrm{Fe}}_2{\mathrm{VAl}}_{0.57}{\mathrm{Si}}_{0.43}$ epilayers, we can obtain a low thermal conductivity of $\sim 3.9\mathrm{W}/\left(\mathrm{m}\mathrm{K}\right)$, one-third less than Si-substituted ${\mathrm{Fe}}_2\mathrm{VAl}$ bulk samples [Lue et al., Phys. Rev. B 75, 064204 (2007)]. We discuss that a possible origin of the low thermal conductivity is related to the low lattice thermal conductivity due to the presence of the V-Si antisite defects.

We study level statistics of a generalized Sachdev-Ye-Kitaev (SYK) model with two-body and one-body random interactions of finite range by exact diagonalization. Tuning the range of the one-body term, while keeping the two-body interaction sufficiently long ranged, does not alter substantially the spectral correlations, which are still given by the random matrix prediction typical of a quantum chaotic system. However, a transition to an insulating state, characterized by Poisson statistics, is observed by reducing the range of the two-body interaction. Close to the many-body metal-insulator transition, we show that spectral correlations share all features previously found in systems at the Anderson transition and in the proximity of the many-body localization transition. Our results suggest the potential relevance of SYK models in the context of many-body localization. It also offers a starting point for the exploration of a gravity dual of this phenomenon which we speculate to be related to the Hawking-Page transition.

We predict a magnonic resonant tunneling phenomenon in the double-barrier insulating magnon junctions of ${\text{FI}}_1/{\text{AFI}}_1/{\text{FI}}_2/{\text{AFI}}_2/{\text{FI}}_3$, where ${\text{FI}}_{(1,2,3)}$ and ${\text{AFI}}_{(1,2)}$ denote the ferromagnetic and antiferromagnetic insulating layers, respectively. Similar to electron tunneling in well-known double-barrier magnetic tunnel junctions, each antiferromagnetic insulator layer acts as an effective potential barrier for magnons due to its intrinsically large magnon-spectrum gap of the order of THz. Based on the magnon tunneling effect, we further propose a magnon field effect transistor that is capable of realizing a gate-tunable transmitted magnon flow by tuning the resonant tunneling via a gate electric field induced Dzyaloshinskii-Moriya interaction in the middle FI layer. The advantages of such transistors include their broadband frequency width ranging from GHz to THz at room temperature, high scalability, and intrinsic low dissipation without Joule heating loss.

We report on spin-vortex pair dynamics measured at temperatures low enough to suppress stochastic core motion, thereby uncovering the highly nonlinear intrinsic dynamics of the system. Our analysis shows that the decoupling of the two vortex cores is resonant and can be enhanced by dynamic chaos. We detail the regions of the relevant parameter space, in which the various mechanisms of the resonant core-core dynamics are activated. We show that the presence of chaos can reduce the thermally induced spread in the decoupling time by up to two orders of magnitude.

Simulated quantum annealing (SQA) is a classical computational strategy that emulates a quantum annealing (QA) dynamics through a path-integral Monte Carlo whose parameters are changed during the simulation. Here we apply SQA to the one-dimensional transverse field Ising chain, where previous works have shown that, in the presence of disorder, a coherent QA provides a quadratic speedup with respect to classical simulated annealing, with a density of Kibble-Zurek defects decaying as ${\rho }_{\mathrm{KZ}}^{\mathrm{QA}}\sim {\left({log}_{10}\tau \right)}^{-2}$ as opposed to ${\rho }_{\mathrm{KZ}}^{\mathrm{SA}}\sim {\left({log}_{10}\tau \right)}^{-1}, \tau$ being the total annealing time, while for the ordered case both give the same power law ${\rho }_{\mathrm{KZ}}^{\mathrm{QA}}\approx {\rho }_{\mathrm{KZ}}^{\mathrm{SA}}\sim {\tau }^{-1/2}$. We show that the dynamics of SQA, while correctly capturing the Kibble-Zurek scaling ${\tau }^{-1/2}$ for the ordered case, is unable to reproduce the QA dynamics in the disordered case at intermediate $\tau$. We analyze and discuss several issues related to the choice of the Monte Carlo moves (local or global in space), the time-continuum limit needed to eliminate the Trotter-discretization error, and the long autocorrelation times shown by a local-in-space Monte Carlo dynamics for large disordered samples.

First-order reversal curves (FORCs) are a powerful tool to separate microscopic coercivities and interactions in a system without the need for lateral resolution. However, measured FORC densities are not always straightforward to interpret, especially if the system is interaction dominated and the Preisach-like interpretation of the FORC density breaks down. This is why FORC is often seen as a magnetic fingerprint instead of a measurement method yielding quantitative information. To understand additional features arising from the interactions in the system, we purposely designed permalloy microstructures which violate the Mayergoyz criteria. These artificial systems allow us to isolate the origin of an additional interaction peak in the FORC density. Modeling the system as a superposition of dipoles allows us to extract interaction strength parameters from this static simulation. Additionally, we suggest a linear relation between integrated interaction peak volume and interaction strength within the system. The presented correlation could be used to investigate the interaction behavior of samples as a function of structural parameters within a series of FORC measurements. This is an important step towards a more quantitative understanding of FORCs which violate the Mayergoyz criteria and away from a fingerprint interpretation.

We have investigated magnetic and dielectric properties of rhombohedral $3R-{\mathrm{AgFeO}}_2$ and hexagonal $2\mathrm{H}-{\mathrm{AgFeO}}_2$ by using magnetic and dielectric bulk measurements and a neutron diffraction experiment with single crystals grown by hydrothermal synthesis. Magnetic phase transitions occur at $T=14.0$ K and $T=6.0$ K in $3R-{\mathrm{AgFeO}}_2$ and $T=17.0$ K and $T=9.5$ K in $2H-{\mathrm{AgFeO}}_2$ under zero magnetic field. Multistep metamagnetic phase transitions were observed in $3R-{\mathrm{AgFeO}}_2$ in magnetization measurements up to 60 T, while a single phase transition occurs in $2H-{\mathrm{AgFeO}}_2$. The ferroelectric polarization parallel and perpendicular to the triangular lattice plane appears below $T=6.0\mathrm{K}$ in $3R-{\mathrm{AgFeO}}_2$, which is concomitant with the onset of the cycloid magnetic ordering with the propagation vector $\mathit{k}=(-\frac{1}{2},q,\frac{1}{2};q\simeq 0.2)$ and the magnetic point group polar $m{1}^{\prime }$. On the other hand, the ferroelectric polarization is absent even below the lower phase transition temperature in $2H-{\mathrm{AgFeO}}_2$, which can be explained by the proper screw magnetic structure with $\mathit{k}=(0,q,0;q\simeq 0.4)$ and the nonpolar ${2221}^{\prime }$ point group. Although the two-dimensional triangular lattice layers of ${\mathrm{Fe}}^{3+}$ are common in the two polytypes, the magnetic and ferroelectric properties are significantly different. The emergence of ferroelectric polarization which is not confined to be within the plane of the cycloid for $3R-{\mathrm{AgFeO}}_2$ can be explained by the extended inverse Dzyaloshinskii-Moriya effect with two orthogonal components, ${\mathit{p}}_1\propto {\mathit{r}}_{ij}\times [{\mathit{S}}_i\times {\mathit{S}}_j]$ and ${\mathit{p}}_2\propto {\mathit{S}}_i\times {\mathit{S}}_j$. Unlike other delafossite compounds, the ${\mathit{p}}_2$ component is not allowed in the proper screw phase of $2H-{\mathrm{AgFeO}}_2$ due to the symmetry restriction of the parent space group.

This paper demonstrates the realization of an extraordinary beam splitter, exhibiting one-way beam splitting amplification. Such a dynamic beam splitter operates based on nonreciprocal and synchronized photonic transitions in obliquely illuminated space-time-modulated (STM) slabs which impart the coherent temporal frequency and spatial frequency shifts. As a consequence of such unusual photonic transitions, a one-way beam splitting is exhibited by the STM slab. Beam splitting is a vital operation for various communication systems, including circuit quantum electrodynamics, and signal-multiplexing and demultiplexing. Despite that the beam splitting is conceptually a simple operation, the performance characteristics of beam splitters significantly influence the repeatability and accuracy of the entire system. As of today, there has been no approach exhibiting a nonreciprocal beam splitting accompanied with transmission gain and an arbitrary splitting angle. Here, we show that oblique illumination of a periodic and semicoherent dynamically-modulated slab results in coherent photonic transitions between the incident light beam and its counterpart space-time harmonic (STH). Such transitions introduce a unidirectional synchronization and momentum exchange between two STHs with same temporal frequencies but opposite spatial frequencies. Such a beam splitting technique offers high isolation, transmission gain, and zero beam tilting, and is expected to drastically decrease the resource and isolation requirements in communication systems. In addition to the analytical solution, we provide a closed-form solution for the electromagnetic fields in STM structures, and accordingly, investigate the properties of the wave isolation and amplification in subluminal, superluminal, and luminal ST modulations.

Recent scanning tunneling microscopy experiments have revealed the itinerant character of $\pi$ magnetism in defected graphene. Employing the constrained random-phase approximation, I calculate the strength of the effective screened Coulomb interaction (Hubbard $U$) for bare and hydrogen-passivated vacancies in graphene. I find that vacancy defects lead to a reduction in screened Coulomb interactions compared to graphene. Screening strongly depends on the concentration of vacancies, passivation, and the position of vacancies in the graphene lattice. For hydrogen-passivated vacancies $p_z$ orbitals are very well screened, suggesting a strong itinerant character of $\pi$ magnetism. On the basis of local $U$ parameters, I discuss the instability of the paramagnetic state of the defected graphene towards the ferromagnetic within the Stoner model. $\pi$ magnetism survives at high concentrations of vacancies when the ordered vacancies belong to the same sublattices. For completeness, the long-range Coulomb interactions for both passivated and unpassivated vacancies are also reported. For vacancies in the same sublattices, the interactions turn out to be local, and the nonlocal part is strongly screened due to $\pi$ states, making defected graphene a correlated system.

We study a one-dimensional (1D) system that shows many analogies to proposed two-dimensional (2D) deconfined quantum critical points (DQCP). Our system is a translationally invariant spin-$\frac{1}{2}$ chain with onsite $Z_2\times Z_2$ symmetry and time-reversal symmetry. It undergoes a direct continuous transition from a ferromagnet (FM), where one of the $Z_2$ symmetries and the time reversal are broken, to a valence bond solid (VBS), where all onsite symmetries are restored while the translation symmetry is broken. The other $Z_2$ symmetry remains unbroken throughout, but its presence is crucial for both the direct transition (via specific Berry phase effect on topological defects, also related to a Lieb-Schultz-Mattis–type theorem) and the precise characterization of the VBS phase (which has crystalline-symmetry-protected-topological–like property). The transition has a description in terms of either two domain-wall species that “fractionalize” the VBS order parameter or in terms of two partons that “fractionalize” the FM order parameter, with each picture having its own $Z_2$ gauge theory structure. The two descriptions are dual to each other and, at long wavelengths, take the form of a self-dual gauged Ashkin-Teller model, reminiscent of the self-dual easy-plane noncompact ${\mathrm{CP}}^1$ model that arises in the description of the 2D easy-plane DQCP. We also find an exact reformulation of the transition that leads to a simple field-theory description that explicitly unifies the FM and VBS order parameters; this reformulation can be interpreted as a new parton approach that does not attempt to fractionalize either of the FM and VBS order parameters but instead encodes them in instanton operators. Aside from providing explicit realizations of many ideas proposed in the context of the 2D DQCP, here in the simpler and fully tractable 1D setting with continuous transition, our study also suggests a possible line of approach to the 2D DQCP.

We derive a model to describe the interaction of an rf-SQUID (radio frequency superconducting quantum interference device)-based metasurface with free space electromagnetic waves. The electromagnetic fields are described on the base of Maxwell's equations. For the rf-SQUID metasurface, we rely on an equivalent circuit model. After a detailed derivation, we show that the problem that is described by a system of coupled differential equations is well posed and, therefore, has a unique solution. In the small amplitude limit, we provide analytical expressions for reflection, transmission, and absorption, depending on the frequency. To investigate the nonlinear regime, we numerically solve the system of coupled differential equations using a finite element scheme with transparent boundary conditions and the Crank-Nicolson method. We also provide a rigorous error analysis that shows convergence of the scheme at the expected rates. The simulation results for the adiabatic increase of either the field's amplitude or its frequency show that the metasurface's response in the nonlinear interaction regime exhibits bistable behavior both in transmission and reflection.

In this paper we investigate the energy spectrum statistics of fractals at the quantum level. We show that the energy level distribution of a fractal follows a power-law behavior, if its energy spectrum is a limit set of piecewise linear functions. We propose that such a behavior is a general feature of fractals, which can not be described properly by random matrix theory. Several other arguments for the power-law behavior of the energy level spacing distributions are proposed.

The magnetic-field response of the Mott-insulating honeycomb iridate ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ is investigated using torque magnetometry measurements in magnetic fields up to 60 T. A peak-dip structure is observed in the torque response at magnetic fields corresponding to an energy scale close to the zigzag ordering ($\approx 15\mathrm{K}$) temperature. Using exact diagonalization calculations, we show that such a distinctive signature in the torque response constrains the effective spin models for these classes of Kitaev materials to ones with dominant ferromagnetic Kitaev interactions, while alternative models with dominant antiferromagnetic Kitaev interactions are excluded. We further show that, at high magnetic fields, long range spin correlation functions decay rapidly, pointing to a transition to a long-sought-after field-induced quantum spin liquid beyond the peak-dip structure, suggesting this to be a common feature of the family of Kitaev systems. Kitaev systems are thus revealed to be excellent candidates for field-induced quantum spin liquids, similar physics having been suggested in another Kitaev system $\alpha \text{−}{\mathrm{RuCl}}_3$.

We study a new class of non-Hermitian topological phases in three dimensions in the absence of any symmetry, where the topological robust band degeneracies are Hopf-link exceptional lines. As a concrete example, we investigate the non-Hermitian band structures of nodal line semimetals under non-Hermitian perturbations, where the Fermi surfaces can transit from one-dimensional nodal lines to two-dimensional twisting surfaces with Hopf-link boundaries when the winding number defined along the nodal line is $\pm 1$. The linking numbers of these linked exceptional line phases are also proposed, based on the integral of Chern-Simons form over the Brillouin zone.

Asymmetric coupling amplitudes effectively create an imaginary gauge field, which induces a non-Hermitian Aharonov-Bohm (AB) effect. Nonzero imaginary magnetic flux invalidates the bulk-boundary correspondence and leads to a topological phase transition. However, the way of non-Hermiticity appearance may alter the system topology. By alternatively introducing the non-Hermiticity under symmetry to prevent nonzero imaginary magnetic flux, the bulk-boundary correspondence recovers and every bulk state becomes extended; the bulk topology of the Bloch Hamiltonian predicts the (non)existence of edge states and topological phase transition. These are elucidated in a non-Hermitian Su-Schrieffer-Heeger model, where chiral inversion symmetry ensures the vanishing of imaginary magnetic flux. The average value of Pauli matrices under the eigenstate of chiral-inversion-symmetric Bloch Hamiltonian defines a vector field; the vorticity of topological defects in the vector field is a topological invariant. Our findings are applicable in other non-Hermitian systems. We first uncover the roles played by the non-Hermitian AB effect and chiral inversion symmetry for the breakdown and recovery of bulk-boundary correspondence, and develop new insights for understanding the non-Hermitian topological phases of matter.

To gain insight into the peculiar temperature dependence of the thermoelectric material SnSe, we employ many-body perturbation theory and explore the influence of the electron-phonon interaction on its electronic and transport properties. We show that a lattice dynamics characterized by soft highly polar phonons induces a large thermal enhancement of the Fröhlich interaction. We account for these phenomena in ab initio calculations of the photoemission spectrum and electrical conductivity at finite temperature, unraveling the mechanisms behind recent experimental data. Our results reveal a complex interplay between lattice thermal expansion and Fröhlich coupling, providing a rationale for the in silico prediction of transport coefficients of high-performance thermoelectrics.

Monoclinic semimetallic ${\mathrm{Mo}}_8{\mathrm{O}}_{23}$ belongs to a multifunctional series of compounds showing multiple ordering phenomena that have not achieved much attention until now. Previous x-ray studies of this compound have revealed an incommensurate ordering transition at $T_{\mathrm{IC}}\sim 350$ K, followed by a structural transition to commensurate order at $T_{\mathrm{IC}-\mathrm{C}}=285$ K. In addition, an enigmatic resistance maximum is observed at $T_{\mathrm{el}}\sim 150$ K, whose origin has so far proved elusive. Aiming to disentangle these multiple orders we use the polarized transient optical spectroscopy supplemented by Raman spectroscopy to study the electronic relaxation dynamics and lattice vibrational modes in ${\mathrm{Mo}}_8{\mathrm{O}}_{23}$ single crystals. Remarkably, both the coherent vibrational mode response and single-particle response display extrema of damping/relaxation times close to $T_{\mathrm{el}}$ with the concurrent appearance of new coherent vibrational modes and a characteristic polarization asymmetry which saturates below $T_{\mathrm{el}}$. The single-particle relaxation data analysis shows the appearance of a temperature-independent gap in the electronic excitation spectrum below $T_{\mathrm{IC}}$ and additional temperature-dependent gap opening near $T_{\mathrm{el}}$. Concurrently, a low-frequency vibrational mode shows anomalous softening around $T_{\mathrm{m}}\sim 200$ K, far below $T_{\mathrm{IC}-\mathrm{C}}$ and $T_{\mathrm{IC}}$. The observations are interpreted in terms of the appearance of a hidden gapped state below $T_{\mathrm{el}}$ that has so far eluded detection by structural analyses.

Many-body interactions in monolayer transition-metal dichalcogenides are strongly affected by their unique band structure. We study these interactions by measuring the energy shift of neutral excitons (bound electron-hole pairs) in gated ${\mathrm{WSe}}_2$ and ${\mathrm{MoSe}}_2$. Surprisingly, while the blueshift of the neutral exciton $X^0$ in electron-doped samples can be more than 10 meV, the blueshift in hole-doped samples is nearly absent. Taking into account dynamical screening and local-field effects, we present a transparent and analytical model that elucidates the crucial role played by intervalley plasmons in electron-doped conditions. The energy shift of $X^0$ as a function of charge density is computed showing agreement with experiment, where the renormalization of $X^0$ by intervalley plasmons yields a stronger blueshift in ${\mathrm{MoSe}}_2$ than in ${\mathrm{WSe}}_2$ due to differences in their band ordering.