Floquet engineering is a powerful technique using periodic potentials, typically laser light, to drive materials into regimes inaccessible in equilibrium. Here, we show that Kondo models can be driven to multichannel degenerate points, even when the starting model is single channel. These emergent channels are differentiated by symmetry, and their strength and number can be controlled by changing the light polarization, frequency, and amplitude. Unpolarized light, constructed by polarization averaging, is particularly useful to induce three- and four-channel degeneracies. Multichannel Kondo models host a wide variety of exotic phenomena, including non-Abelian anyons in impurity models and composite pair superconductivity in lattice models. We demonstrate our findings on both a simple square-lattice toy model and a more realistic spin-orbit-coupled model for $J=\frac{5}{2}$ Ce ions in a tetragonal environment, as relevant for the Ce 115 materials, and show that the transition temperature for composite pair superconductivity can be dynamically enhanced.

We report the ground-state symmetry of Ce $4f$ states in the heavy-fermion superconductor ${\mathrm{CeNi}}_2{\mathrm{Ge}}_2$, yielding anisotropic $cf$ hybridization between Ce $4f$ states and conducting electrons. By analyzing linear dichroism in soft x-ray absorption and core-level hard x-ray photoemission spectra, the $4f$ symmetry is determined as $\mathrm{\Sigma }$-type ${\mathrm{\Gamma }}_7$, promoting predominant hybridization with the conducting electrons originating from the Ge site. The band structures probed by soft x-ray angle-resolved photoemission indicate that Ge $4p$ components contribute to band renormalization through anisotropic hybridization effects, suggesting that control of the electronic structures of a Ge orbital helps to achieve the exotic phenomena in ${\mathrm{CeNi}}_2{\mathrm{Ge}}_2$.

The breakdown of Lieb-Robinson bounds in local, non-Hermitian quantum systems opens up the possibility for a rich landscape of quantum many-body phenomenology. We elucidate this by studying information scrambling and quantum chaos in non-Hermitian variants of paradigmatic local quantum spin-chain models. We utilize a mixture of exact diagonalization and tensor network techniques for our numerical results and focus on three dynamical quantities: (i) out-of-time-ordered correlators (OTOCs), (ii) operator entanglement of the dynamics, and (iii) entanglement growth following a quench from product initial states. We show that while OTOCs fail to capture information scrambling in a simple, local, non-Hermitian transverse-field Ising model, the closely related operator entanglement is a robust measure of dynamical properties of interest. Moreover, we show that the short-time growth of operator entanglement can generically detect entanglement phase transitions in these systems while its long-time average is shown to be a reliable indicator of quantum chaos and entanglement phases. This allows us to extend operator entanglement-based diagnostics from previous works on closed and open quantum systems, to the new arena of monitored quantum dynamics. Finally, we remark on the efficacy of these dynamical quantities in detecting integrability/chaos in the presence of continuous monitoring.

This paper presents a comprehensive study of the nonlinear transport properties of magnons electrically emitted or absorbed in extended yttrium-iron garnet (YIG) films by the spin transfer effect across a $\mathrm{YIG}|\mathrm{Pt}$ interface. Our goal is to experimentally elucidate the pertinent picture behind the asymmetric electrical variation of the magnon transconductance, analogous to an electric diode. The feature is rooted in the variation of the density of low-lying spin-wave modes (so-called low-energy magnons) via an electrical shift of the magnon chemical potential. As the intensity of the spin transfer increases in the forward direction (magnon emission regime), the transport properties of low-energy magnons pass through three distinct regimes: (i) at low currents, where the spin current is a linear function of the electric current, the spin transport is ballistic and determined by the film thickness; (ii) for amplitudes of the order of the damping compensation threshold, it switches to a highly correlated regime, limited by the magnon-magnon scattering process and characterized by a saturation of the magnon transconductance. In other words, the reported signal suggests coupled dynamics between numerous modes rather than Bose-Einstein condensation around a single dominant mode. (iii) As the temperature under the emitter approaches the Curie temperature, scattering with high-energy magnons starts to dominate, leading to diffusive transport. We find that such a sequence of transport regimes is analogous to the electron hydrodynamic transport in ultrapure media predicted by Radii Gurzhi, where condensation of magnons. This study, restricted to the low-energy part of the magnon manifold, complements Paper II [Kohno et al., Phys. Rev. B 108, 144411 (2023)] of this series, which focuses on the full spectrum of propagating magnons.

This paper presents a comprehensive study of the spatial dispersion of propagating magnons electrically emitted in extended yttrium-iron garnet (YIG) films by the spin transfer effects across a $\mathrm{YIG}|\mathrm{Pt}$ interface. Our goal is to provide a generic framework to describe the magnon transconductance inside magnetic films. We experimentally elucidate the relevant spectral contributions by studying the lateral decay of the magnon signal. While most of the injected magnons do not reach the collector, the propagating magnons can be split into two fluids: (i) a large fraction of high-energy magnons carrying energy of about $k_B{T}_0$, where $T_0$ is the lattice temperature, with a characteristic decay length in the submicrometer range, and (ii) a small fraction of low-energy magnons, which are particles carrying energy of about $\hslash {\omega }_K$, where ${\omega }_K/\left(2\pi \right)$ is the Kittel frequency, with a characteristic decay length in the micrometer range. Taking advantage of their different physical properties, the low-energy magnons can become the dominant fluid (i) at large spin transfer rates for the bias causing the emission of magnons, (ii) at large distance from the emitter, (iii) at small film thickness, or (iv) for reduced band mismatch between the YIG below the emitter and the bulk due to variation of the magnon concentration. This broader picture complements a companion paper [R. Kohno et al., Phys. Rev. B 108, 144410 (2023)], which focuses solely on the nonlinear transport properties of low-energy magnons.

Quantum transport and localization are fundamental concepts in condensed matter physics. It is commonly believed that in one-dimensional systems, the existence of mobility edges is highly dependent on disorder. Recently, there has been a debate over the existence of an exact mobility edge in a modulated mosaic model without quenched disorder, the so-called mosaic Wannier-Stark lattice. Here, we experimentally implement such disorder-free mosaic photonic lattices using a silicon photonics platform. By creating a synthetic electric field, we could observe energy-dependent coexistence of both extended and localized states in a finite number of waveguides. The Wannier-Stark ladder emerges when the resulting potential is strong enough, and can be directly probed by exciting different spatial modes of the lattice. Our studies provide the experimental proof of coexisting sets of strongly localized and conducting (though weakly localized) states in finite-sized mosaic Wannier-Stark lattices, which hold the potential to encode high-dimensional quantum resources with compact and robust structures.

Discrete Floquet time crystals (DFTCs) are characterized by the spontaneous breaking of the discrete time-translational invariance characteristic of Floquet-driven systems. In analogy with equilibrium critical points, also time-crystalline phases display critical behavior of different order, i.e., oscillations whose period is a multiple $p>2$ of the Floquet driving period. Here, we introduce an experimentally accessible order parameter which is able to unambiguously detect crystalline phases regardless of the value of $p$ and, at the same time, is a useful tool for chaos diagnostic. This p aradigm allows us to investigate the phase diagram of the long-range (LR) kicked Ising model to an unprecedented depth, unveiling a rich landscape characterized by self-similar fractal boundaries. Our theoretical picture describes the emergence of DFTCs phase both as a function of the strength and period of the Floquet drive, capturing the emergent ${\mathbb{Z}}_p$ symmetry in the Floquet-Bloch waves.

Topological insulators and Weyl semimetals have demonstrated a helicity-dependent photocurrent (HDPC). In contrast, Dirac semimetals (DSMs) possess chirality-degenerate Weyl nodes and were thought to forbid HDPC, unless the symmetry is broken by an external stimulus such as strain. Here, we demonstrate HDPC in strain-free DSM ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanobelt field-effect transistors at room temperature. Freestanding single-crystalline ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanobelts grown by chemical vapor deposition allow fabrication of devices without the complication of external strain. HDPC mapping with micrometer resolution reveals HDPC in the vicinity of the metal contact. We attribute the HDPC to the spin-polarized Fermi arcs at the surface, supported by theoretical calculation, as well as photon energy and angle-resolved helical photocurrent mapping. This work provides key insights into light polarization controlled spin manipulation in Dirac materials.

The phenomenon of negative-$U$ behavior, where a defect traps a second charge carrier more strongly than the first, has been established in many host crystals. Here, we report the case of four-carrier transitions for both vacancy defects in ${\mathrm{Sb}}_2{\mathrm{Se}}_3$. A global structure searching strategy is employed to explore the defect energy landscape from first principles, revealing large atomic reconfigurations which facilitate a major charge redistribution. A thermodynamic analysis of the accessible charge states reveals a four-electron negative-$U$ transition ($\mathrm{\Delta }q=4$) for both $V_{\text{Se}}$ and $V_{\text{Sb}}$ which, combined with previous calculations for antisites and interstitials, now demonstrates amphoteric behavior for all intrinsic point defects in ${\mathrm{Sb}}_2{\mathrm{Se}}_3$, with an impact on its usage in solar cells. The unusual behavior is facilitated by valence alternation, a reconfiguration of the local bonding environments, characteristic of both Se and Sb.

More than two decades after the discovery of superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, it is still unclear whether the order parameter has a single component or two degenerate components. For any two-component scenario, application of uniaxial strain is expected to lift the degeneracy, generating two distinct phase transitions. The presence of a second (lower-temperature) transition may be observable by probes that are sensitive to changes in the London penetration depth, $\lambda$, as a function of temperature, $T$. Here, we use scanning SQUID microscopy combined with a uniaxial strain device to test for a second transition under strain. We only observe a single transition. Within the temperature range where a second transition has been suggested by $\mu \mathrm{SR}$ measurements [Grinenko et al., Nat. Phys. 17, 748 (2021)], we further place a tight upper bound of less than 1% on the change in the zero temperature superfluid density $n_s\propto {\lambda }^{-2}\left(0\right)$ due to a second transition, suggesting that such a transition does not occur. These results constrain theories of the order parameter in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

We investigate the thermal Hall effect in the Heisenberg-Kitaev antiferromagnet ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$, where we observe negative thermal Hall conductivity (THC) with thermal Hall angles up to 2% at low magnetic fields, which changes the sign to positive THC at higher fields. Our theoretical calculations, incorporating spin-lattice coupling, reveal that the quantum-geometric Berry curvature of magnon polarons counteracts the purely magnonic contribution, resulting in a reversed sign and an increased magnitude in THC. This finding emphasizes the significance of spin-lattice coupling in understanding the thermal Hall effect.

We present an extension of the quasiparticle self-consistent $GW$ approximation ($\mathrm{QS}\mathit{GW}$) [T. Kotani et al., Phys. Rev. B 76, 165106 (2007)] to include vertex corrections in the screened Coulomb interaction $W$. This is achieved by solving the Bethe-Salpeter equation for the polarization matrix at all $k$ points in the Brillouin zone. We refer to this method as $\mathrm{QS}G\widehat{W}$. $\mathrm{QS}\mathit{GW}$ yields a reasonable and consistent description of the electronic structure and optical response, but systematic errors in several properties appear, notably a tendency to overestimate insulating band gaps, blueshift plasmon peaks in the imaginary part of the dielectric function, and underestimate the dielectric constant ${\epsilon }_{\infty }$. A primary objective of this paper is to assess to what extent including ladder diagrams in $W$ ameliorates systematic errors for insulators in the $\mathrm{QS}GW$ approximation. For benchmarking we consider about 40 well-understood semiconductors, and also examine a variety of less well-characterized nonmagnetic systems, six antiferromagnetic oxides, and the ferrimagnet ${\mathrm{Fe}}_3{\mathrm{O}}_4$. We find ladders ameliorate shortcomings in $\mathrm{QS}GW$ to a remarkable degree in both the one-body Green's function and the dielectric function for a wide range of insulators. New discrepancies with experiment appear, and a key aim of this paper is to establish to what extent the errors are systematic and can be traced to diagrams missing from the theory. One key finding of this work is to establish a relation between the band gap and the dielectric constant ${\epsilon }_{\infty }$. Good description of both properties together provides a much more robust benchmark than either alone. We show how this information can be used to improve our understanding of the one-particle spectral properties in materials systems such as ${\mathrm{SrTiO}}_3$ and FeO.

Friction is a familiar phenomenon to humankind and has long been studied; however, it is fundamentally difficult to understand because of the complex processes that contribute to it. For elucidating friction, it is helpful to simplify the system. In this respect, molecular manipulation, in which a single molecule or atom on a surface is moved by the tip of a scanning probe microscope, is an ideal research target. In this paper, we combine noncontact atomic force microscopy, inelastic electron tunneling spectroscopy, and density functional theory calculations to investigate the molecular manipulation process of a single CO molecule on Cu(110) and Cu(111) surfaces at low temperature. We discovered the presence of a metastable adsorption site that is not occupied when the tip is far from the surface but is engaged for close tip positions. This adsorption site plays the role of an intermediate state in the reaction path of manipulation, and this intermediate is important for understanding the dynamics of manipulation and dynamic friction. We elaborate the process leading to the above conclusions in detail and discuss future perspectives.

Thermoelectrical properties are frequently used to characterize materials and endow the free energy from wasted heat with useful purposes. Here, we show that linear thermoelectric effects in tunnel junctions with Fe-based superconductors not only address the dominance between particle and hole states but also even provide information about the superconducting order-parameter symmetry. In particular, we observe that nodal order parameters present a maximal thermoelectric effect at lower temperatures than for nodeless cases. Furthermore, we show also that superconducting tunnel junctions between Fe-based and BCS superconductors could provide a thermoelectric efficiency ZT exceeding 6 with a linear Seebeck coefficient around $S\approx 800\text{µ}\text{V/K}$ at a few kelvins. These results pave the way to developing novel thermoelectric machines based on multiband superconductors.

Strontium ruthenate ${\text{Sr}}_2{\text{RuO}}_4$ is an unconventional superconductor whose pairing symmetry has not been fully clarified, despite more than two decades of intensive research. Recent NMR Knight shift experiments have rekindled the ${\text{Sr}}_2{\text{RuO}}_4$ pairing debate by giving strong evidence against all odd-parity pairing states, including chiral $p$-wave pairing that was for a long time the leading pairing candidate. Here, we exclude additional pairing states by analyzing recent elastocaloric measurements [Y.S. Li et al., Nature 607, 276 (2022)]. To be able to explain the elastocaloric experiment, we find that unconventional even-parity pairings must include either large $d_{x^2-y^2}$-wave or large $\{d_{xz}\mid d_{yz}\}$-wave admixtures, where the latter possibility arises because of the body-centered point group symmetry. These $\{d_{xz}\mid d_{yz}\}$-wave admixtures take the form of distinctively body-centered-periodic harmonics that have horizontal line nodes. Hence $g_{xy(x^2-y^2)}$-wave and $d_{xy}$-wave pairings are excluded as possible dominant even pairing states.

The concept of “deep thermalization” has recently been introduced to characterize moments of an ensemble of pure states, resulting from projective measurements on a subsystem, which lie beyond the purview of conventional eigenstate thermalization hypothesis (ETH). In this paper, we study deep thermalization in systems with kinetic constraints, such as the quantum East and the PXP models, which have been known to weakly break ETH by slow dynamics and high sensitivity to the initial conditions. We demonstrate a sharp contrast in deep thermalization between the first and higher moments in these models by studying quench dynamics from initial product states in the computational basis: While the first moment shows good agreement with ETH, higher moments deviate from the uniform Haar ensemble at infinite temperature. We show that such behavior is caused by an interplay of time-reversal symmetry and an operator that anticommutes with the Hamiltonian. We formulate sufficient conditions for violating deep thermalization, even for systems that are otherwise “thermal” in the ETH sense. By appropriately breaking these properties, we illustrate how the PXP model fully deep thermalizes for all initial product states in the thermodynamic limit. Our results highlight the sensitivity of deep thermalization as a probe of physics beyond ETH in kinetically-constrained systems.

Enhancement of the Curie temperature $\left(T_C\right)$ has always been a vital problem since the discovery of two-dimensional (2D) van der Waals (vdW) magnets. The interfacial effect from layers with strong spin-orbit coupling (SOC) has been widely investigated to be an effective method. Here, we experimentally show that $T_C$ of vdW $\mathrm{CrP}{\mathrm{S}}_4$ is enhanced from 40 K up to room temperature by the interfacial effect from graphene with weak SOC. The compatible Fermi level between $\mathrm{CrP}{\mathrm{S}}_4$ and graphene enables sufficient interfacial charge transfer across their atomically fine interface, leading to largely elevated $T_C$ of $\mathrm{CrP}{\mathrm{S}}_4$ at the interface without the participation of strong SOC. The enhanced magnetism of $\mathrm{CrP}{\mathrm{S}}_4$ further induces room-temperature exchange splitting in graphene by the magnetic proximity effect, which is shown in the quantum oscillation controlled by field cooling and temperature-dependent resistance. Our work reveals a mechanism for the enhancement of $T_C$ in vdW magnets and the achieved room-temperature all-2D magnetic proximity effect also advances the practical applications of miniaturized low-dissipative 2D spintronics.

The few-body problem (with $N\le 6$ fermionic charge carriers) in isolated moiré quantum dots (MQDs) in transition metal dichalcogenide (TMD) bilayer materials with integer fillings, $\nu \ge 2$, is investigated by employing large-scale full configuration interaction (FCI; also termed exact diagonalization) computations, and by performing a comparative analysis of the ensuing first-order (charge densities) and second-order (conditional probability distributions, CPDs) correlation functions. With parameters representative of bilayer experimental TMD setups, our investigations reveal the determining role of the strong interparticle Coulombic repulsion in bringing about Wigner molecularization, which is associated with many-body physics beyond both that described by the Aufbau principle of natural atoms, as well as by the widely used Hubbard model for strongly interacting condensed-matter systems. In particular, for weak and moderate trilobal crystal-field deformations of the MQDs, the imperative employment of the CPDs brings to light the geometrical polygonal-ring configurations underlying the Wigner molecules (WMs) that remain hidden at the level of a charge-density analysis, apart from the case of $N=3$ when a pinned WM emerges in the charge density due to the coincidence of the $C_3$ symmetries associated with both the intrinsic geometry of the $N=3$ WM and the TMD trilobal crystal field of the confining pocket potential. The FCI numerically exact diagonalization results provide critical benchmarks for assessing and guiding the development of future computational methodologies of interacting strongly correlated fermions in isolated MQDs and their superlattices in TMD materials.

We investigate the ground-state phase diagram of an $\text{SU}\left(N\right)$-symmetric antiferromagnetic spin model on a square lattice where each site hosts an irreducible representation of $\text{SU}\left(N\right)$ described by a square Young tableau of $N/2$ rows and $2S$ columns. We show that negative sign free fermion Monte Carlo simulations can be carried out for this class of quantum magnets at any $S$ and even values of $N$. In the large-$N$ limit, the saddle point approximation favors a fourfold degenerate valence bond solid phase. In the large $S$ limit, the semiclassical approximation points to the Néel state. On a line set by $N=8S+2$ in the $S$ versus $N$ phase diagram, we observe a variety of phases proximate to the Néel state. At $S=1/2$ and $3/2$, we observe the aforementioned fourfold degenerate valence bond solid state. At $S=1$, a twofold degenerate spin nematic state in which the $C_4$ lattice symmetry is broken down to ${\mathrm{C}}_2$ emerges. Finally, at $S=2$ we observe a unique ground state that pertains to a two-dimensional version of the Affleck-Kennedy-Lieb-Tasaki state. For our specific realization, this symmetry-protected topological state is characterized by an SU(18), $S=1/2$ boundary state that has a dimerized ground state. These phases that are proximate to the Néel state are consistent with the notion of monopole condensation of the antiferromagnetic order parameter. In particular, one expects spin-disordered states with degeneracy set by $\text{mod}(4,2S)$.

We studied magnetic states and phase transitions in the van der Waals antiferromagnet $\mathrm{V}{\mathrm{Br}}_3$ experimentally by specific heat and magnetization measurements of single crystals in high magnetic fields and theoretically by the density functional theory calculations focused on exchange interactions. The magnetization behavior mimics Ising antiferromagnets with magnetic moments pointing out-of-plane due to strong uniaxial magnetocrystalline anisotropy. The out-of-plane magnetic field induces a spin-flip metamagnetic transition of first-order type at low temperatures, while at higher temperatures, the transition becomes continuous. The first-order and continuous transition segments in the field-temperature phase diagram meet at a tricritical point. The magnetization response to the in-plane field manifests a continuous spin canting which is completed at the anisotropy field ${\mu }_0{H}_{\mathrm{MA}}\approx 27\mathrm{T}$. At higher fields, the two magnetization curves above saturate at the same value of magnetic moment ${\mu }_{\mathrm{sat}}\approx 1.2{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$, which is much smaller than the spin-only $\left(S=1\right)$ moment of the ${\mathrm{V}}^{3+}$ ion. The reduced moment can be explained by the existence of an unquenched orbital magnetic moment antiparallel to the spin. The orbital moment is a key ingredient of a mechanism responsible for the observed large anisotropy. The exact energy evaluation of possible magnetic structures shows that the intralayer zigzag antiferromagnetic (AFM) order is preferred, which renders the AFM ground state significantly more stable against the spin-flip transition than the other options. The calculations also predict that a minimal distortion of the Br ion sublattice causes a radical change of the orbital occupation in the ground state, connected with the formation of the orbital moment and the stability of magnetic order.