The Ising antiferromagnets on the triangular and on the pyrochlore lattices are two of the most iconic examples of magnetic frustration, paradigmatically illustrating many exotic properties such as emergent gauge fields, fractionalization, and topological order. In this paper, we show that the two instances of frustration can, remarkably, be combined in a single system, where they coexist without inducing conventional long-range ordering. Our results indicate that the system undergoes a first-order phase transition upon lowering the temperature, into a yet different frustrated phase that we characterize to exhibit nematic order. We argue that an extensive degeneracy survives down to zero temperature, at odds with a customary Pauling estimate. Dynamically, we find evidence of anomalous noise in the power spectral density, arising from an effectively fractal anisotropic motion of monopoles at low temperature.

Topological insulator (TI)/ferromagnet heterostructures hold immense application potential for spin-orbitronic memory technologies owing to strong spin-orbit coupling of TIs combined with ultrahigh spin-charge interconversion efficiency. Here, we use all-optical time-resolved magneto-optical Kerr effect magnetometry to demonstrate fluence-modulated spin pumping and ultrafast demagnetization in $\mathrm{Sub}/\mathrm{BiSbT}{\mathrm{e}}_{1.5}{\mathrm{Se}}_{1.5}\left(\mathrm{BSTS}\right)/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}\left(\mathrm{CoFeB}\right)/\mathrm{Si}{\mathrm{O}}_2$ thin films. The effective spin-mixing conductance of the BSTS/CoFeB interface is extracted, and the two-magnon scattering (TMS) effect is isolated to reveal the true spin-pumping contribution to the damping enhancement at various laser pump fluences. The demagnetization time and Gilbert damping are found to be inversely correlated in BSTS/CoFeB due to spin-pumping-driven pure spin-current transport and interfacial spin accumulations. The fluence-dependent enhancement of spin pumping at the BSTS/CoFeB interface, surpassing TMS losses, reveals enhanced spin absorption at higher pump fluences, resulting in a factor-of-two enhancement of the spin-mixing conductance. This systematic study of femtosecond to nanosecond magnetization dynamics and pump-fluence-tunable spin pumping in a BSTS/CoFeB heterostructure can aid in the functionalization of TI-based spin-orbitronic devices and the integration of topological spintronics with femtomagnetism.

We report a magnetization study of the rare-earth-based paramagnet $\mathrm{KEr}{\left({\mathrm{MoO}}_4\right)}_2$ in a magnetic field up to 50 T. A recent observation of massive magnetostriction and rotational magnetocaloric effects in this compound triggered interest in the microscopic mechanism behind these phenomena. We combine several experimental techniques to investigate the magnetization behavior up to its saturation along three main crystallographic directions. The synergy of magnetic torque measurements and vibrating sample magnetometry allowed us to reconstruct parallel and perpendicular components of the magnetization vector, enabling us to trace its evolution up to 30 T. Our experiments reveal the magnetization saturation along all principle axes well below the value, expected from crystal electric field calculations. We argue that an externally applied magnetic field induces a distortion of the local environment of ${\mathrm{Er}}^{3+}$ ions and affects its crystal electric field splitting.

We investigate the phase diagram of a bilayer Kitaev honeycomb model with Ising interlayer interactions, deriving effective models via perturbation theory and performing Majorana mean-field theory calculations. We show that a diverse array of magnetic and topological phase transitions occur, depending on the direction of the interlayer Ising interaction and the relative sign of Kitaev interactions. When two layers have the same sign of the Kitaev interaction, a first-order transition from a Kitaev spin liquid to a magnetically ordered state takes place. The magnetic order points along the Ising axis and it is (anti)ferromagnetic for (anti)ferromagnetic Kitaev interactions. However, when two layers have opposite signs of the Kitaev interaction, we observe a notable weakening of magnetic ordering tendencies and the Kitaev spin liquid survives up to a remarkably larger interlayer exchange. Our mean-field analysis suggests the emergence of an intermediate gapped ${\mathbb{Z}}_2$ spin-liquid state, which eventually becomes unstable upon vison condensation. The confined phase is described by a highly frustrated ${120}^{\circ }$ compass model. We furthermore use perturbation theory to study the model with the Ising axis pointing along the $\widehat{z}$ axis or lying in the $xy$ plane. In both cases, our analysis reveals the formation of one-dimensional Ising chains, which remain decoupled in perturbation theory, resulting in a subextensive ground-state degeneracy. Our results highlight the interplay between topological order and magnetic ordering tendencies in bilayer quantum spin liquids.

Magnons, the quanta of the oscillations of localized electron spins, are a powerful tool for information transport and processing of microwave signals. Owing to the challenge of energy efficient spin-wave control on small time- and space scales, dynamic magnonic crystals have been proposed. Their distinct feature is the possibility to toggle on and off the spatial periodicity of the magnetic waveguide that allows one to realize the unusual signal processing functions. The miniaturization of magnonic circuits, reduction in energy consumption, and fast operation are important possibilities of these artificial crystals. These can be achieved in ferrite-ferroelectric (multiferroic) heterostructures, where strong coupling of magnons and microwave photons constitutes quasiparticles called electromagnons. Using both a theoretical approach and microwave measurements, we report on successful dynamic control of electromagnonic band structures in artificial thin film crystals via application of a voltage to the grid electrode located on a ferroelectric film. A promising functionality of the proposed waveguiding structures arises from two major factors: (i) low energy consumptions due to the thin ferroelectric layer, and (ii) pronounced rejection bands caused by a gradual change of the dielectric permittivity.

In two-dimensional superconductors with a Rashba-type spin-orbit coupling, it is known that an in-plane magnetic field can induce a helical superconducting (SC) state with a phase modulation $e^{i\mathbf{q}·\mathbf{r}}$. Here, we theoretically investigate the stability of a stripe order, a weight-biased superposition state composed of $+\mathbf{q}$ and $-\mathbf{q}$ modes taking the form of ${\mathrm{\Delta }}_{+}{e}^{i\mathbf{q}·\mathbf{r}}+{\mathrm{\Delta }}_{-}{e}^{-i\mathbf{q}·\mathbf{r}}$ with $|{\mathrm{\Delta }}_{+}|\ne |{\mathrm{\Delta }}_{-}|\ne 0$, assuming that the spin-singlet pairing channel is dominant. Based on the Ginzburg-Landau theory, we show that for both $s$- and $d$-wave pairing symmetries, the stripe order can appear in the high-field and low-temperature region inside the helical phase and that the transition between the helical and stripe phases is of second order. It is noteworthy that for the $d$-wave pairing, the stability region of the stripe phase shrinks when the in-plane field is rotated from the nodal direction to the antinodal direction. It is also found that the nonreciprocity of the critical current, the so-called SC diode effect, emerges not only in the helical phase but also in the stripe phase, with no clear nonreciprocity anomaly at the helical-stripe transition due to its second-order nature.

The investigation of the dynamical universality classes of quantum systems is an important, and rather less explored aspect of nonequilibrium physics. In this paper, considering the out-of-equilibrium dynamics of spinless fermions in a one-dimensional quasiperiodic model with and without periodic driving, we report the existence of the dynamical one-parameter Family-Vicsek (FV) scaling of the “quantum surface roughness” associated with the particle-number fluctuations. In the absence of periodic driving, the model is interestingly shown to host a subdiffusive critical phase with anomalous FV scaling exponents, separated by two subdiffusive critical lines and a triple point from other phases. Our analysis on the fate of the critical phase in the presence of (interphase) driving indicates that the critical phase is quite fragile, and has a tendency to get absorbed into the delocalized or localized regimes depending on the driving parameters, especially in the slow driving limit. Interestingly, periodic driving can also conspire to show quantum Kardar-Parisi-Zhang (KPZ)-like superdiffusive dynamical behavior, which seems to have no classical counterpart. We further construct an effective Floquet Hamiltonian, which qualitatively captures this feature occurring in the driven model.

The electronic structure and topological state of an ${\mathrm{MoS}}_2$ monolayer decorated by iron or vanadium adatoms is investigated by first-principles calculations. It is shown that several Chern insulator phases occur in these samples in the presence of external strain or electric field. In particular, Chern number $\mathcal{C}=2$ is achieved by applying 6% biaxial tensile strain on the V-decorated ${\mathrm{MoS}}_2$ monolayer or 10% on the Fe-decorated one. On the other hand, the intrinsic $\mathcal{C}=1$ phase of the iron system is switched to the normal insulator phase under an electric field of 0.8 V/Å, while the vanadium system with intrinsic $\mathcal{C}=-1$ state adopts a $\mathcal{C}=1$ state if the electric field reaches 0.7 V/Å. The induced band inversion may coincide with an abrupt increase of magnetization and a simultaneous change in the adatom adsorption height, or a magnetization decrement without displacement of the adatom. Our findings suggest that applying an in-plane biaxial tensile strain is a promising direction to search high Chern number states.

Recent advances in thermoelectric research have shed light on the promising properties of topological semimetals, which exhibit superior carrier mobility and electrical conductivity compared to traditional thermoelectric materials. Herein, we employ a first-principles method and semiclassical Boltzmann transport theory to investigate the electronic structures, lattice dynamics, and thermoelectric performance of CaCdGe, a representative nodal-line semimetal, along with their response to pressure. Our findings reveal that CaCdGe possesses an ultralow lattice thermal conductivity at room temperature $(0.765 {\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1})$ and exhibits excellent $n$-type thermoelectric behavior in a wide temperature range. Specifically, we identify in-plane (out-of-plane) maximum thermoelectric figures of merit, $Z{T}_{\text{max}}$, reaching 1.02 (0.65) at 700 $\mathrm{K}(1100 \mathrm{K})$. Remarkably, even under extreme conditions of high pressure (up to 50 GPa), CaCdGe maintains its structural stability without undergoing any structural phase transition. Moreover, we explore the impact of spin-orbit coupling and observe a pressure-driven Lifshitz transition of the Fermi surface around 40 GPa. This transition results in CaCdGe undergoing an electronic phase transition from a charge-compensated semimetal to a topological insulator. Intriguingly, the thermoelectric parameters exhibit unconventional pressure response, embodied by the insensitivity to high pressure after electronic phase transition. Notably, even at 50 GPa, the in-plane (out-of-plane) $Z{T}_{\text{max}}$ reaches 1.2 (0.7) at 1500 $\mathrm{K}(500 \mathrm{K})$. Our comprehensive theoretical study demonstrates the immense potential of CaCdGe and related topological semimetals for thermoelectric applications, even in extreme conditions of high pressure and high temperature.

We present a self-consistent Maxwell-Bloch theory to analytically study the interaction between a nanostructure consisting of a metal nanoparticle and a monolayer of transition-metal dichalcogenide. For the combined system, we identify an effective eigenvalue equation that governs the center-of-mass motion of the dressed excitons in a plasmon-induced potential. Examination of the dynamical equation of the exciton-plasmon hybrid reveals the existence of bound states with negative eigenenergies, which we interpret as excitons localized in the plasmon-induced potential. The appearance of these bound states in the potential indicates strong coupling between excitons and plasmons. We quantify this coupling regime by computing the scattered light in the near-field explicitly and identify signatures of strong exciton-plasmon coupling with an avoided crossing behavior and an effective Rabi splitting of tens of meV.

We consider a two-dimensional electron system in the Laughlin sequence of the fractional quantum Hall regime to investigate the effect of strong correlations on the mutual interaction between two levitons, single-electron excitations generated by trains of quantized Lorentzian pulses. We focus on two-leviton states injected in a single period with a time separation $\mathrm{\Delta }t$. In the presence of a quantum point contact operating in the weak-backscattering regime, we compute the backscattered charge by means of the Keldysh technique. In the limit of an infinite period and zero temperature, we show that the backscattered charge for a two-leviton state is not equal to twice the backscattered charge for a single leviton. We present an interpretation for this result in terms of the wave-packet formalism for levitons, thus proposing that an effective interaction between the two levitons is induced by the strongly correlated background. Finally, we perform numerical calculations in the periodic case by using the Floquet formalism for photoassisted transport. By varying the system parameters such as pulse width, filling factor, and temperature we show that the value of the backscattered charge for two-leviton states is strongly dependent on the pulse separation, thus opening scenarios where the effective interaction between levitons can be controllably tuned.

Motivated by recent experiments that observed low-frequency second-order optical responses in doped striped superconductors, here we investigate the nonlinear electrodynamics of systems exhibiting a charge density wave (CDW) order parameter. Due to the Bragg scattering off the CDW order, an incoming spatially homogeneous electric field in addition to zero momentum current generates umklapp currents that are modulated in space at momenta of the reciprocal CDW lattice. In particular, here we predict and microscopically evaluate the umklapp shift current, a finite momentum analog of the regular shift current which represents the second-order optical process that downconverts homogeneous AC electric field into low-frequency, zero momentum current. Specifically, we evaluate real-time response functions within mean-field theory via the Keldysh technique and use the Peierls substitution to compute observables at finite momenta in lattice models. We find that systems with certain lattice symmetries (such as inversion symmetry), where the regular shift current is disallowed, may give rise to the umklapp one. We apply our framework to investigate lattice symmetries in layered materials with helical-like stripes and show that both types of shift currents provide insight into the nature of intertwined phases of matter. Finally, we discuss the relation of our findings to recent experiments in striped superconductors.

The electronic bands formed in moiré systems with twisted bilayer graphene (tBLG) have emerged as a tunable platform for studying many novel concepts of condensed matter physics due to new interaction and topological effects. In particular, the multitude of closely packed flat bands and a sequence of van Hove singularities (vHSs) in minimally tBLG can not only lead to nontrivial topological transport but also the breakdown of conventional Boltzmann transport formalism due to the competition between the scales of energy variation within the system and that of the external parameters such as temperature or electric field. Here, we demonstrate the violation of the semiclassical Mott relation in small-angle tBLG $(\theta \sim 0.{45}^{\circ })$ even at room temperature, which we associate to a narrow diverging density of states. We also show the emergence of nonlinear effects in thermovoltage by exploiting vertical thermoelectric transport in an atomically thin tBLG device. Our results not only point towards the fundamental limitations of the applicability of the semiclassical Boltzmann approach in small-angle tBLG but also outline an experimental approach that can lead to the discovery of different broken-symmetry states.

We investigate quantum transport through a rectangular potential barrier in Weyl semimetals (WSMs) and multi-Weyl semimetals (MSMs), within the framework of Landauer-Büttiker formalism. Our study uncovers the role of nodal topology imprinted in the electric current and the shot noise. We find that, in contrast to the finite odd-order conductance and noise power, the even-order contributions vanish at the nodes. Additionally, depending on the topological charge $\left(J\right)$, the linear conductance $\left(G_1\right)$ scales with the Fermi energy $\left(E_F\right)$ as $G_1^{E_F>U}\propto E_F^{2/J}$, $U$ being the barrier height. We demonstrate that the $E_F$ dependence of the second-order conductance and shot noise power could quite remarkably distinguish an MSM from a WSM depending on the band topology, and may induce several smoking gun experiments in nanostructures made out of WSMs and MSMs. Analyzing shot noise and Fano factor, we show that the transport across the rectangular barrier follows the sub-Poissonian statistics. Interestingly, we obtain universal values of Fano factor at the nodes unique to their topological charges. The universality for a fixed $J$, however, indicates that only a fixed number of open channels participate in the transport through evanescent waves at the nodes. The proposed results can serve as a potential diagnostic tool to identify different topological systems in experiments.

A quantum system constrained to a degenerate energy eigenspace can undergo a nontrival time evolution on adiabatic driving, described by a non-Abelian Berry phase. This type of dynamics may provide logical gates in quantum computing that are robust against timing errors. A strong candidate to realize such holonomic quantum gates is an electron or hole spin qubit trapped in a spin-orbit-coupled semiconductor, whose twofold Kramers degeneracy is protected by time-reversal symmetry. Here we propose and quantitatively analyze protocols to measure the non-Abelian Berry phase by pumping a spin qubit through a loop of quantum dots. One of these protocols allows to characterize the local internal Zeeman field directions in the dots of the loop. We expect a near-term realization of these protocols, as all key elements have been already demonstrated in spin-qubit experiments. These experiments would be important to assess the potential of holonomic quantum gates for spin-based quantum information processing.

In rare-earth intermetallic topological materials, carriers from topological bands mediate the magnetic interactions between local moments, giving rise to a plethora of exotic quantum phenomena. Recently, anomalous magnetic instability, helical spin orders, and skyrmions were found in topological semimetal ${\mathrm{EuAl}}_4$ with tetragonal lattice. Comparing with its counterpart ${\mathrm{EuGa}}_4$, which does not show intricate magnetism, the difference lies in the presence of charge-density wave (CDW) order in ${\mathrm{EuAl}}_4$. Thus, studying the effect of CDW transition on electronic structure is decisive for the final understanding of the intricate magnetism in topological materials. Here, we studied the charge excitations in ${\mathrm{EuAl}}_4$ across the CDW transition through optical spectroscopy and the first-principles calculations. After the CDW transition, a partial gap (60 meV) on the Fermi surface and an enhanced mid-infrared absorption at around 0.4 eV were observed in the optical conductivity. With the magneto-optical spectroscopy, we further observed the evolution of charge excitations alongside the magnetization. Through the first-principles calculations, we have identified that the CDW transition not only partially erodes the Fermi surface contributed by the topological bands but also modulates the high-energy excitations between the bands dominated by Eu $5d$ and Al $3p$ orbitals. In the counterpart ${\mathrm{EuGa}}_4$, the band reconstruction is absent. Since the itinerant carriers and $pd$ hybridizations are usually assigned to mediate the magnetic interactions, our findings offer unprecedented insights to understanding the complex magnetism observed in highly symmetric topological semimetals.

We report the observation of two-threshold voltage-current characteristics accompanied by a peak of broadband current noise between the two threshold voltages in the insulating state at low densities in the 2D electron system in ultrahigh mobility SiGe/Si/SiGe heterostructures. The observed results can be described by a phenomenological theory of the collective depinning of elastic structures, which naturally generates a peak of a broadband current noise between the dynamic and static thresholds and changes to sliding of the solid over a pinning barrier above the static threshold. This gives compelling evidence for the electron solid formation in this electron system and shows the generality of the effect for different classes of electron systems.