A transmission line coupled to an externally driven superconducting quantum interference device (SQUID) can exhibit the dynamical Casimir effect. Employing this setup, we quantize the SQUID degrees of freedom and show that it gives rise to a three-body interaction Hamiltonian with the cavity modes. By considering only two interacting modes from the cavities we show that the device can function as an autonomous cooler where the SQUID can be used as a work source to cool down the cavity modes. Moreover, this setup allows for coupling to all modes existing inside the cavities, and we show that by adding two other extra modes to the interaction with the SQUID the cooling effect can be enhanced.

We theoretically study the influence of quenched outside disturbances in an intermediately long-time limit. We consider localized imperfections, uniform fields, noise, and couplings to an environment within a unified framework using a prototypical but idealized interacting quantum device—the Kitaev honeycomb model. As a measure of stability we study the Uhlmann fidelity of quantum states after a quench. To treat the unperturbed dynamics as a free-fermion model without neglecting evolution of states between flux sectors, we push the flux degree of freedom into the perturbation. For noisy quenches, both gapped and gapless systems exhibit a universal form for the long-time fidelity, $C{e}^{-\alpha t}{t}^{-\beta }$ where the values of $C, \alpha$, and $\beta$ depend on physical parameters such as system size and disturbance strength. Finally, we show that selective filling of the spinon Brillouin zone can be used to greatly increase the fidelity over the ground-state value. Our work provides estimates for the intermediate long-time stability of a quantum device, offering engineering guidelines for quantum devices in quench design and system size.

Magnetic hopfions are three-dimensional localized magnetic topological solitons which can exist in the bulk of magnetic materials. Based on a Ritz model for magnetic hopfions in a chiral magnet, the unpolarized magnetic small-angle neutron scattering (SANS) cross section, the spin-flip scattering cross section, and the chiral function (characterizing the imbalance between the two spin-flip scattering amplitudes) are computed here analytically, while the real-space correlation function is obtained numerically. Features of these functions, specific to magnetic hopfions, are discussed. Our results enable the SANS method to be used for the detection of magnetic hopfions.

Optimal choice of spin polarization enables electron injection into the helical edge state at a precise position, despite the uncertainty principle, permitting access to specific nonlocal Green's functions. We show, within 1D effective description, that this fact facilitates a direct identification of odd-frequency pairing through parity measurement (under frequency reversal) of the nonlocal differential conductance in a setup comprising the Josephson junction on the helical edge state of a 2D topological insulator with two spin-polarized probes tunnel-coupled to the junction region. A 2D numerical simulation has also been conducted to confirm theoretical predictions as well as to demonstrate the experimental feasibility of the proposal.

Recently, two monolayer magnetic materials, i.e., ${\mathrm{FePS}}_3$ and ${\mathrm{NiPS}}_3$, have been successfully fabricated. Despite them having the same atomic structure, the two monolayers exhibit distinct magnetic properties. ${\mathrm{FePS}}_3$ holds an out-of-plane zigzag antiferromagnetic (AFM-ZZ) structure, while ${\mathrm{NiPS}}_3$ exhibits an in-plane AFM-ZZ structure. However, there is no theoretical model that can properly describe its magnetic ground state due to the lack of a full understanding of its magnetic interactions. Here, by combining the first-principles calculations and the newly developed machine learning method, we construct an exact spin Hamiltonian of the two magnetic materials. Different from the previous studies that failed to fully consider the spin-orbit-coupling effect, we find that the AFM-ZZ ground state in ${\mathrm{FePS}}_3$ is stabilized by competing ferromagnetic nearest-neighbor and antiferromagnetic third-nearest-neighbor exchange interactions and combining single-ion anisotropy. In contrast, the often ignored nearest-neighbor biquadratic exchange is responsible for the in-plane AFM-ZZ ground state in ${\mathrm{NiPS}}_3$. We additionally calculate the spin-wave spectrum of the AFM-ZZ structure in the two monolayers based on the exact spin Hamiltonian, which can be directly verified by the experimental investigation. Our work provides a theoretical framework for the origin of the AFM-ZZ ground state in two-dimensional materials.

We experimentally observe nonlinear spin torque in metallic bilayers of platinum and permalloy by means of spin-torque ferromagnetic resonance (ST-FMR) under massive dc current injection. The observed nonlinear spin torque exerted on permalloy magnetization is attributed primarily to nonlinear spin polarization. An additional origin of the nonlinear spin torque is magnon generation (annihilation) followed by shrinkage (expansion) of effective magnetization, which is revealed by ST-FMR and unidirectional spin Hall magnetoresistance measurements. The present paper paves a way to spin Hall effect based nonlinear spintronic devices as well as time-varying nonlinear magnetic metamaterials with tailormade permeability.

A structural orthorhombic-to-tetragonal phase transition was recently discovered in the heavy-fermion compound ${\mathrm{UTe}}_2$ at a pressure $p^{*}\simeq 3-8\mathrm{GPa}$ [Honda et al., J. Phys. Soc. Jpn. 92, 044702 (2023); Huston et al., Phys. Rev. Mater. 6, 114801 (2022)]. In the high-pressure tetragonal phase, a phase transition at $T_x=235\mathrm{K}$ and a superconducting transition at $T_{\text{sc}}=2\mathrm{K}$ have been revealed. In this work we present an electrical-resistivity study of ${\mathrm{UTe}}_2$ in pulsed magnetic fields up to ${\mu }_{0}H=58\mathrm{T}$ combined with pressures up to $p=6\mathrm{GPa}$. The field was applied in a direction tilted by ${30}^{\circ }$ from b to c in the orthogonal structure, which is identified as the direction c' of the tetragonal structure. In the tetragonal phase, the presence of superconductivity is confirmed and signatures of metamagnetic transitions are observed at the fields ${\mu }_0{H}_{x1}=24\mathrm{T}$ and ${\mu }_0{H}_{x2}=34\mathrm{T}$ and temperatures smaller than $T_x$. We discuss the effects of uniaxial pressure, and we propose that a magnetic ordering drives the transition at $T_x$.

We extensively study long-time dynamics and the fate of a topologically ordered state in the toric code model evolving through a projective measurement-only circuit. The circuit is composed of several measurement operators corresponding to each term of the toric code Hamiltonian with magnetic-field perturbations, which is a gauge-fixed version of a ($2+1$)-dimensional gauge Higgs model. We employ a cylinder geometry with distinct upper and lower boundaries to classify stationary states after long-time measurement dynamics. The appearing stationary states depend on measurement probabilities for each measurement operator. The Higgs, confined, and deconfined phases emerge in the time evolution by the circuit. We find that both Higgs and confined phases are separated from the deconfined phase by topological entanglement entropy. We numerically clarify that both Higgs and confined phases are characterized by a long-range order on the boundaries accompanying edge modes, and they shift with each other by a crossover reflecting properties in the bulk phase diagram.

The ferroelectric polarization in van der Waals multiferroic ${\mathrm{NiI}}_2$ is believed to be induced by the helimagnetic spin order. The formation of the helimagnetic ground spin configuration and the properties of its spin excitation are yet to be clarified. In the present work, we explore the proper magnetic ground states with well-defined magnon spectra in a single-layer ${\mathrm{NiI}}_2$, by analyzing the role of different interactions. While the spin frustration due to the ferromagnetic and antiferromagnetic exchange terms stabilizes a helimagnetic phase, the anisotropic Kitaev interaction introduces a canting of the spin rotation plane. We find the modulation vector of the helimagnetic structure can be continuously oriented within the atomic plane by the competition between the Kitaev interaction and the third-nearest-neighbor exchange. The calculation of magnon spectrum reveals anomalous features with soft magnons at finite wave vectors, which is found to be related with the vibration of the canting plane. From the finite-temperature calculation of the magnon spectra, we predict a magnetic phase transition driven by soft magnons, which cause a spatial modulation of the canting plane. Furthermore, a sign change is predicted in the temperature dependence of the transverse magnon thermal conductivity. Our results provide a comprehensive understanding of the spin ground state and the soft magnons in the van der Waals ${\mathrm{NiI}}_2$.

We analyze the recently proposed dipolar background field (BF) theory with couplings to charge and dipole currents. The quasiparticles of the theory are either chargelike or dipolelike, and the mutual braiding statistics between chargelike and dipolelike quasiparticles are dipolar, meaning that it depends on the position of the quasiparticle being encircled. The braiding statistics between two dipolelike quasiparticles is that of ordinary anyons. We further prove that the dipolar BF theory is equivalent to the rank-2 tensor BF theory developed earlier as an effective theory for the rank-2 toric code. Although the two theories are equivalent, the dipolar BF formulation embodies the dipole symmetry explicitly and gives a clean insight into the way the dipole symmetry manifests itself in various conservation laws and the dipolar braiding statistics.

We study the combined effect of the passivating atoms, edge electric fields, and exchange fields on the edge states, conductance, and spin polarization in zigzag stanene nanoribbons (ZSNRs). The distortion direction and amplitude of edge bands significantly depend on the electronegativity difference between the passivating atoms and tin atoms. The conductance dips can reflect the curved characteristic of the edge bands induced by passivating atoms, which can be used to detect the quantum spin-Hall edge states or passivating atoms. Interestingly, the edge antisymmetric electric fields can recover the passivation-induced band structures and transport properties, and an effective spin polarization can be realized by applying an exchange field on the edges of ZSNRs. The results provide a comprehensive understanding of the modulating method by utilizing the passivating atoms and the external fields, which will be of great help for the application of stanene devices.

Topological phonons have attracted tremendous attention in recent years, particularly when topological phonons have nonzero topological numbers and exhibit many peculiar physical phenomena. However, studies on the coexistence of different phonons with nonzero topological charge are quite limited. Here, we reveal a realist material ${\mathrm{K}}_2{\mathrm{Pb}}_2{\mathrm{O}}_3$, which can realize the coexistence of three types of phonons, namely charge-1 Weyl point (WP), charge-2 triple point (TP), and charge-4 WP. We perform symmetry analysis for these points, presenting symmetry conditions to realize these topological phonons, and constructing an effective model to describe them. Meanwhile, we calculate the phonon spectrum, topological surface states, and the constant frequency slice for ${\mathrm{K}}_2{\mathrm{Pb}}_2{\mathrm{O}}_3$, and we perform the Wilson loop method to calculate the Chern number for WPs and TPs in the material. The results indicate that the material ${\mathrm{K}}_2{\mathrm{Pb}}_2{\mathrm{O}}_3$ has charge-1 WPs, charge-2 TPs, and charge-4 WPs at high-symmetry points in the range of 13–19 THz. In particular, the phonon spectrum within this range solely contains the bands responsible for forming these topological band-crossing points. The Fermi arcs emanating from the projections of a charge-1 WP, a charge-2 TP, and a charge-4 WP are connected with each other, forming a long Fermi arc extended in the whole Brillouin zone, which greatly facilitates observation in experiment by neutron scatterings or high-resolution electron energy loss spectroscopy. Our work provides a platform for studying charge-1 WP, charge-2 TP, and charge-4 WP in a phononic system.

We study the interfacial modes across the interface between the two regions of a semi-Dirac type material, which are illuminated by the left and right circularly polarized light, respectively. We show that a smooth boundary between the two regions give rise to a momentum dependent Pööschl-Teller potential well which hosts the metallic and gapless Volkov-Pankratov states. We also note that incident electron can fully pass over the interfacial well without any reflection only at certain light parameters—known as the Ramsauer-Townsend effect. Moreover, we also observe that such modes can even exist across the interface between the irradiated and nonirradiated regions under certain condition.

At the nanoscale, thermal boundary conductance (TBC) and thermal conductivity are not intrinsic properties of interfaces or materials but depend on the nearby environment. However, most studies focus on single interfaces or superlattices, and the thermal transport across heterostructures formed by multiple different materials is still mysterious. In this study, we demonstrate how much the TBC of an interface is affected by the existence of a second interface, as well as how much the thermal conductivity of a material is affected by the nearby materials. Using Si and Ge modeled by classical molecular dynamics simulations, the following phenomena are demonstrated. (1) The existence of a nearby interface can significantly change the TBC of the original interface. For example, by adding an interface after Si/Ge, the TBC can be increased from 400 to $700\mathrm{MW}{\mathrm{m}}^{-2}{\mathrm{K}}^{-1}$. This is because the nearby interface serves as a filter of phonon modes, which selectively allows particular modes to pass through and affect the TBC of the original interfaces. This impact will disappear at the diffusive limit when the distance between interfaces is much longer than the phonon mean free path so that phonon modes recover equilibrium statistics before arriving at the second interface. (2) The thermal conductivity of a material can be significantly changed by the existence of neighboring materials. For example, the thermal conductivity of standalone 30 nm thick Si can be increased from 50 to $280\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$, a more than fourfold increase, beating the bulk thermal conductivity of Si, after being sandwiched between two Ge slabs. This is because the Ge slabs on the two sides serve as filters that only allow low-frequency phonons to transport heat in Si; these phonons carry more heat than optical phonons. This work opens up an area of successive interface thermal transport and is expected to be important for nanoscale thermal characterization and thermal management of semiconductor devices.

In this Letter we suggest a phenomenological theory of photogalvanic phenomena in superconducting materials and structures revealing the diode effect. Starting from a generalized London model including the quadratic nonlinearity in the relation between the supercurrent and superfluid velocity, we show that the electromagnetic wave incident on the superconductor can generate a nontrivial superconducting phase difference between the ends of the sample. Being enclosed in a superconducting loop, such a phase battery should generate a dc supercurrent circulating in the loop. By increasing the electromagnetic wave intensity one can provoke the switching between the loop states with different vorticities.