Resistance measurements on single crystals of $\mathrm{CaK}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\right)}_4{\mathrm{As}}_4$ $(x=0.024)$ were performed under hydrostatic pressure up to 5.15 GPa. The pressure dependence of the magnetic and superconducting transition temperatures and that of the superconducting upper critical field are reported. Our results show that upon increasing pressure, the magnetic transition temperature $T_N$ is suppressed, whereas the superconducting transition temperature $T_c$ first increases and then decreases, exhibiting a maximum at a pressure $p_c$ corresponding to the intersection of the $T_N\left(p\right)$ and $T_c\left(p\right)$ lines. In addition, a minimum in the normalized slope of the superconducting upper critical field as well as a change in the pressure dependence of the inferred superconducting coherence length are observed at $p_c$, suggesting a difference in the Fermi surface of the paramagnetic and antiferromagnetic states. Finally, $\mathrm{CaK}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\right)}_4{\mathrm{As}}_4$ $(x=0.024)$ likely goes through a half-collapsed tetragonal phase transition at $\sim 4.3$ GPa, further demonstrating that the half-collapsed tetragonal transition pressure in the ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ system is relatively insensitive to transition metal substitution.

We develop a low-energy continuum model for phonons in twisted moiré bilayers, based on a configuration space approach. In this approach, interatomic force constants are obtained from density functional theory (DFT) calculations of untwisted bilayers with various in-plane shifts. This allows for efficient computation of phonon properties for any small twist angle, while maintaining DFT-level accuracy. Based on this framework, we show how the low-energy phonon modes, including interlayer shearing and layer breathing modes, vary with the twist angle. As the twist angle decreases, the frequencies of the low-energy modes are reordered and the atomic displacement fields corresponding to phonon eigenmodes break translational symmetry, developing periodicity on the moiré length scale. We demonstrate the capabilities of our model by calculating the phonon properties of three specific structures: Bilayer graphene, bilayer molybdenum disulfide $\left({\mathrm{MoS}}_2\right)$, and molybdenum diselenide-tungsten diselenide $\left({\mathrm{MoSe}}_2\text{−}{\mathrm{WSe}}_2\right)$.

Magnetic skyrmions are swirling magnetic textures that can be efficiently driven with spin-orbit torques with a deflected trajectory. However, pinning slows skyrmions down and alters their trajectory, which prevents a quantitative comparison to analytical models. Here, we study skyrmions driven by spin-orbit torques at room temperature in ferrimagnetic GdCo thin films, an amorphous material with low pinning. Above a sharp current depinning threshold, we observe a clearly linear velocity increase with current that extrapolates to zero and a constant deflection angle, reaching high velocities up to 200 m/s. The mobility increases and the depinning threshold current decreases with the skyrmion diameter, which we vary using an external magnetic field. An analytical model based on the Thiele equation quantitatively reproduces these findings with a single fitting parameter. This validates the linear flow regime description and shows, in particular, the important role of skyrmion size in its dynamics.

We report a study of the spin dynamics of the frustrated ferromagnetic $J_1\text{−}{J}_2$ chain compound linarite, ${\mathrm{PbCuSO}}_4{\left(\mathrm{OH}\right)}_2$, in applied magnetic fields up to field polarization. By means of an extreme-environment inelastic neutron scattering experiment, we have measured the low-energy spin excitations of linarite in fields up to 8.8 T for $\mathbf{H}\parallel b$ axis. We have recorded the spin excitation spectra along $h$, $k$, and $l$ for the field-induced magnetic phases IV, V, and the field-polarized state close to saturation. By employing first-principles calculations, we estimate the leading magnetic exchanges out of the $bc$ plane, and we model the dispersion relations using linear spin-wave theory. In this way, we find evidence for a (very weak) residual magnetic exchange coupling out of the $bc$ plane. Together with the previously established dominant intrachain couplings $J_1$ and $J_2$ and the interchain coupling $J_3$, we derive an effective set of exchange couplings for a microscopic description of linarite. Further, we find that the peculiar character of phase V manifests itself in the measured spin dynamics.

Nonlinear devices, such as transistors, enable contemporary computing technologies. We theoretically investigate nonlinear effects, bearing a high fundamental scientific and technical relevance, in magnonics with emphasis on superconductor-ferromagnet hybrids. Accounting for a finite magnon chemical potential, we theoretically demonstrate magnonic spin Joule heating, the spin analog of conventional electronic Joule heating. Besides suggesting a key contribution to magnonic heat transport in a broad range of devices, it provides insights into the thermal physics of nonconserved bosonic excitations. Considering a spin-split superconductor self-consistently, we demonstrate its interface with a ferromagnetic insulator to harbor large tunability of spin and thermal conductances. We further demonstrate hysteretic rectification I-V characteristics in this hybrid, where the hysteresis results from the superconducting state bistability.

In europium-based iron pnictides, superconducting Fe planes can be influenced by a Zeeman field originated from the neighboring Eu planes. The field tends to induce spin-density waves with a ferromagnetic average which coexists with the superconducting order by forming complementary patterns of the superconducting and magnetic order parameters in a Fulde-Ferrell-Larkin-Ovchinnikov phase and a two-dimensional textured-superconducting phase. The hard gap around the Fermi energy disappears in these fragile inhomogeneous superconducting states, which features, instead, a V-shaped spin-resolved local density of states. The inhomogeneous states are also competing with either a homogeneous superconducting or a homogeneous ferromagnetic state, manifesting the intertwining influences of the magnetic orders in Fe and Eu planes, the spin-density wave band structure, and the superconducting pairing order.

We report on temperature-dependent size and anisotropy of the Fermi pockets in graphite revealed by magnetotransport measurements. The magnetoresistances (MRs) obtained in fields along the $c$ axis obey an extended Kohler's rule, with the carrier density following the prediction of a temperature-dependent Fermi energy, indicating a change in the Fermi pocket size with temperature. The angle-dependent magnetoresistivities at a given temperature exhibit a scaling behavior. The scaling factor that reflects the anisotropy of the Fermi surface is also found to vary with temperature. Our results demonstrate that temperature-driven changes in Fermi surface can be ubiquitous and need to be considered in understanding the temperature-dependent carrier density and MR anisotropy in semimetals.

Transition metal (TM) trichlorides, with $4d$ or $5d$ electrons, are materials at the forefront of recent studies about the interplay of spin-orbit coupling (SOC) and strong Coulomb interactions. Within our first-principles calculations (DFT $+$ $U$ $+$ SOC) we study the effects of graphene on the electronic and magnetic properties of the monolayers (MLs) of $\alpha \text{−}{\mathrm{RuCl}}_3$ and ${\mathrm{OsCl}}_3$. Despite the spatially inhomogeneous $n$-type doping induced by graphene, we show that the occupancy of Ru-$4d$ $\left(\mathrm{Os}\text{−}5d\right)$ bands of MLs of $\alpha \text{−}{\mathrm{RuCl}}_3\left({\mathrm{OsCl}}_3\right)$ can be tuned through external electric fields and allows the control of (i) metal-insulator transitions and (ii) the magnetic easy-axis and anisotropy energies. Our findings point toward the tuning of electronic and magnetic properties of TM trichloride MLs by using graphene and external electronic fields.

In this paper we introduce a solvable two-orbital (two-band) model with an infinite-range Hatsugai-Kohmoto interaction, which serves as a modified periodic Anderson model. Its solvability results from strict locality in momentum space and is valid for arbitrary lattice geometry and electron filling. A case study of a one-dimensional chain shows that the ground-states have a Luttinger-theorem-violating non-Fermi-liquid-like metallic state, a hybridization-driven insulator, and an interaction-driven featureless Mott insulator. The involved quantum phase transition between metallic and insulating states belongs to the universality of the Lifshitz transition, i.e., a change of topology of the Fermi surface or band structure. Further investigation on a two-dimensional square lattice indicates its similarity with the one-dimensional case, thus the findings in the latter may be generic for all spatial dimensions. We hope the present model or its modification may be useful for understanding novel quantum states in $f$-electron compounds, particularly the topological Kondo insulator candidates ${\mathrm{SmB}}_6$ and ${\mathrm{YbB}}_{12}$.

The latest generations of transistors are nanoscale devices whose performance and reliability are limited by thermal noise in low-power applications. Therefore, developing efficient methods to compute the voltage and current fluctuations in such nonlinear electronic circuits is essential. Traditional approaches commonly rely on adding Gaussian white noise to the macroscopic dynamical circuit laws, but do not capture rare fluctuations and lead to thermodynamic inconsistencies. A correct and thermodynamically consistent approach can be achieved by describing single-electron transfers as Poisson jump processes accounting for charging effects. But such descriptions can be computationally demanding. To address this issue, we consider the macroscopic limit, which corresponds to scaling up the physical dimensions of the transistor and resulting in an increase of the number of electrons on the conductors. In this limit, the thermal fluctuations satisfy a “large deviations principle”, which we show is also remarkably precise in settings involving only a few tens of electrons, by comparing our results with Gillespie simulations and spectral methods. Traditional approaches are recovered by resorting to an ad hoc diffusive approximation introducing inconsistencies. To illustrate these findings, we consider a low-power CMOS inverter, or NOT gate, which is a basic primitive in electronic design. Voltage (resp. current) fluctuations are obtained analytically (semi-analytically) and reveal interesting features.

Coherent excitation of shear phonons in van der Waals layered materials is a nondestructive mechanism to fine-tune the lattice structure and the electronic state of the system. We develop a diagrammatic theory for the displacive Raman force and apply it to the shear phonon's dynamics. We obtain a noticeable light-induced Raman force of the order of $\mathcal{F}\sim 10–100\mathrm{nN}/{\mathrm{nm}}^2$ leading to a large rectified shear displacement $Q_0$ in bilayer graphene. In analogy to the photogalvanic effect, we decompose the Raman force to circular and linear components where the former vanishes due to the lattice symmetry in bilayer graphene. We show that the laser frequency and polarization can effectively tune $Q_0$ in different electronic doping, temperature, and scattering rates. The finite rectified shear displacement induces a Dirac crossing pair in the low-energy dispersion that photoemission spectroscopy can probe. Our systematic formalism of Raman force can simulate the coherent manipulation of stacking order in the heterostructures of layered materials by laser irradiation.

While the prototypical quasi-two-dimensional charge density wave system of $1T\text{−}{\mathrm{TaS}}_2$ has been known as a Mott insulator with a possibility of quantum spin liquid, recent band-structure calculations and spectroscopic works in parallel suggested a metallic system or a spin-singlet insulator due to the interlayer coupling. Here, we carefully reinvestigate the out-of-plane electron dispersion, which reflects the interlayer electronic coupling, with angle-resolved photoelectron spectroscopy. We identify two distinct branches for the topmost valence band, which can be unambiguously related to the surface and the bulk layers with different band gaps. Density functional theory calculations clearly indicate a trivial band insulator due to the interlayer coupling for the bulk but the surface band gap affected substantially by the electron correlation. The surface-bulk electronic dichotomy consistently incorporates most of the theoretical and spectroscopic results reported so far and has wide implications for van der Waals materials with nontrivial interlayer interactions.

We study the properties of one-dimensional topological superconductors under the influence of generic boundary conditions mimicking the coupling with external environments. We identify a general four-parameter classification of the boundary effects and show that particle-hole and reflection symmetries can be broken or preserved by appropriately fixing the boundary parameters. When the particle-hole symmetry is broken, the topological protection of the edge modes is lost due to the hybridization with the external degrees of freedom. We assess the robustness of the edge modes in the various regimes by considering different quantifiers of topological properties. In particular, we investigate the resilience of the long-distance, edge-to-edge quantum mutual information and squashed entanglement, measuring the nonlocal correlations of the Majorana excitations. Besides their relevance for the open dynamics of topological systems, these results may provide a useful guide to the appropriate embedding of low-dimensional topological systems on nanodevices in realistic conditions.

Interfacing topological insulators (TIs) with magnetic insulators (MIs) have been widely used to study the interaction between topological surface states and magnetism. Previous transport studies typically interpret the suppression of weak antilocalization or appearance of the anomalous Hall effect as signatures of the magnetic proximity effect (MPE) imposed to TIs. Here, we report the observation of the nonlinear planar Hall effect (NPHE) in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films grown on MI thulium and yttrium-iron-garnet (TmIG and YIG) substrates, which is an order of magnitude larger than that in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ grown on nonmagnetic gadolinium-gallium-garnet (GGG) substrate. The nonlinear Hall resistance in $\mathrm{TmIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ depends linearly on the external magnetic field, while that in $\mathrm{YIG}/{\mathrm{Bi}}_2{\mathrm{Se}}_3$ exhibits an extra hysteresis loop around zero field. The magnitude of the NPHE is found to scale inversely with carrier density. We speculate that the observed NPHE is related to the MPE-induced exchange gap opening and out-of-plane spin textures in the TI surface states, which may be used as an alternative transport signature of the MPE in MI/TI heterostructures.

We investigate spontaneous entanglement of two qubits mediated by nonreciprocal surface plasmon polaritons (SPPs) in a Weyl semimetal. In the absence of external magnetic fields, the topology of the Weyl semimetal even gives rise to nonreciprocal SPPs that are topologically protected and reside inside the photonic gap. We utilize this nonreciprocal SPP as a mediator of entanglement of two spatially separated qubits. Our two main findings are (1) the nonreciprocal SPP gives better quantum entanglement than the reciprocal one, and (2) the entanglement achieved in the Weyl semimetal is sufficiently long lived compared to the entanglement using SPPs in conventional metals.

Moiré excitons feature tunable Dirac dispersion and spatially dependent optical selection rules. With its long lifetime due to its interlayer nature, it is promising to realize a Bose-Einstein condensation of moiré excitons. Here we study the properties of moiré exciton condensate within the mean-field theory, with special focus on the exciton-exciton interaction effect on the nonlinear Bloch band and Bloch waves of exciton condensate in moiré potential. We find the nonlinear dispersion of the moiré exciton condensate exhibits a complex loop structure induced by exciton-exciton interaction. A nonlinear Dirac cone emerges at the $\mathit{\Gamma }$ point of the Bloch band, with a threefold degenerate nonlinear Dirac point. Each degenerate Bloch wave at the nonlinear Dirac point spontaneously breaks $C3$ rotational symmetry of the moiré potential and itself differs by $C3$ rotations. Since they reside in the light cone, these nontrivial Bloch band structures and broken-symmetry Bloch waves can be experimentally detected by examining light emission from those states. Symmetry breaking of Bloch states implies unusual optical selection rules compared with the single-exciton case: moiré exciton condensate at the $\mathit{\Gamma }$ point emits light with all three components of polarization instead of only left or right circular polarization. We further propose that with a force applied on the exciton center of mass by engineering the potential energy of excitons to drive an initially optically dark exciton condensate towards the light cone, the light polarization of final states as well as their dependence on force direction serves as the smoking gun for experimental observation.

Despite extensive research on the heavy fermion superconductor $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$ in the past three decades, the nature of the hidden order (HO) phase transition occurring at 17.5 K remains ambiguous. Here we report a comparative scanning tunneling microscopy/spectroscopy (STM/STS) study on different terminations of the parent $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$ and Fe-doped samples. A small gap, which was ascribed to the HO parameter by previous STM/STS studies, emerges in both the HO and large moment antiferromagnetic phases on the U terminations, indicating it is not the unique hallmark of the HO parameter. Moreover, a peak-gap-peak structure is observed on the Si terminations. Variations of the two spectral features with Fe concentration and temperature show that they stem from the alteration of $f\text{−}c$ hybridization. The higher vanishing temperatures and larger sizes of the gap in the Fe-substituted samples indicate stronger $f\text{−}c$ hybridization strength compared to $\mathrm{U}{\mathrm{Ru}}_2{\mathrm{Si}}_2$. Our studies demonstrate hybridization is not the driving force for the HO phase transition.

Type-II quantum well heterostructures are considered high-potential candidates for next-generation active semiconductor devices. They promise low fundamental transition energies and suppressed Auger scattering as well as temperature stability in device performance. Understanding their fundamental properties, such as scattering and diffusion, and revealing intricate differences from type-I quantum structures are important steps towards optimized structure design. Using degenerate four-wave mixing spectroscopy, we investigate the phase coherence of excitonic polarizations in a (Ga,In)As/GaAs/Ga(As,Sb) type-II double-quantum-well structure. It is designed to exhibit spectrally isolated resonances in its linear absorption spectrum including a well-resolved charge-transfer exciton resonance. This allows us to study the coherent dynamics of the unperturbed charge-transfer exciton polarization. In addition, the effects of many-body interactions with free charge carriers as well as excitons that are injected by an optical prepulse are revealed. Scattering of charge-transfer excitons with free charge carriers is three times more efficient than scattering of charge-transfer excitons with each other. The comparison with Wannier excitons in a type-I quantum well structure shows that the interaction strength between charge-transfer excitons is about twice as large as for excitons in a type-I quantum well structure.