In [Phys. Rev. B 92, 115416 (2015)], Velzen and Webb proposed that by structuring a metal film suspended in free space with a periodic array of nanoscale slots, the steady-state optical pressure on the film resulting from an incident plane wave can be substantially enhanced compared to the standard radiation-pressure limit. This result appears to be in violation of the law of momentum conservation, given that the momentum of free-space electromagnetic plane waves is well established. We recalculated the optical fields and forces for the same configuration as Velzen and Webb and found that, although the calculated optical fields and force densities agree with the published result, the resulting total optical forces were not enhanced and, instead, matched the standard radiation-pressure relation. Our calculations imply that the diverging results arise due to different choices of the region used for integration of the force density.

The recently discovered coexistence of superconductivity and charge-density wave order in the kagome systems $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) has stimulated enormous interest. It is predicted that a vanadium-based kagome system may host a flat band, nontrivial linear dispersive Dirac surface states, and electronic correlation. Although much research has been carried out, the origin of the charge-density wave (CDW) order, the relationship between the superconductivity and the CDW, and whether the anomalous Hall effect (AHE) arises primarily from the kagome lattice or the CDW order, remain controversial. Here, we report an extensive investigation of $\mathrm{Cs}{\left({\mathrm{V}}_{1\text{−}x}{\mathrm{Nb}}_x\right)}_3{\mathrm{Sb}}_5$ samples with systematic Nb doping. Our results show that the Nb doping induces apparent suppression of CDW order and promotes superconductivity; meanwhile, the AHE and magnetoresistance (MR) are weakened significantly together with the CDW order. Combining with density-functional calculations, we interpret these effects by an antiphase shift of the Fermi energy with respect to the saddle points near $M$ and the Fermi surface centered around Γ. It is found that the former depletes the filled states for the CDW instability and worsens the nesting condition for CDW order, while the latter lifts the Fermi level upward and enlarges the Fermi surface surrounding the Γ point, and thus promotes superconductivity. Our results uncover a delicate but unusual competition between the CDW order and superconductivity.

We report a time-resolved ultrafast quasiparticle dynamics investigation of cubic boron arsenide ($c$-BAs), which is a recently discovered highly thermally conducting material. The excited-state ultrafast relaxation channels dictated by the electron-phonon coupling (EPC), phonon-phonon scattering, and radiative electron-hole recombination have been unambiguously identified, along with their typical interaction times. Significantly, the EPC strength is obtained from the dynamics, with a value of ${\lambda }_{T_2}=0.008$ (corresponding to $\lambda \langle {\mathrm{\Omega }}^2\rangle =1.18\pm 0.08\mathrm{p}{\mathrm{s}}^{–2}$), demonstrating an unusually weak coupling between the electrons and phonons. As a comparison, an ultraweak EPC strength for graphene is also expected. We propose that preserving an ultrasmall EPC strength may be a prerequisite for exhibiting an ultrahigh thermal conductivity. Our investigation provides insight for searching and designing ultrahigh thermal conductivity materials. Notably, during our analysis we have generalized the fluence-dependence method for obtaining the EPC strength to room temperature, which can be applied to many other types of quantum materials in the future.

Dynamical quantum phase transitions (DQPTs) are criticalities in the time evolution of quantum systems and their existence has been theoretically predicted and experimentally observed. However, how the system behaves in the vicinity of DQPT and its connection to physical observables remains an open question. In this work, we introduce the concept of the Loschmidt amplitude spectrum (LAS), which extends the Loscmidt amplitude - the detector of the transition - by considering the overlap of the initial state to all the eigenstates of the prequench Hamiltonian. By analyzing the LAS in the integrable transverse-field Ising model, we find that the system undergoes a population redistribution in the momentum space across DQPT. In the quasiparticle picture, collective excitations around lower-half $k$ modes become dominant when the system approaches DQPT. The LAS is also applicable to study the dynamics of nonintegrable models where we have investigated the Ising model with next-nearest-neighbor interactions as an example. The time evolution of the system's magnetization is found to be connected to the products of the LAS and there exists a simultaneous overlap of the time-evolved state to pairs of eigenstates of the prequnech Hamiltonian that possess spin configurations of negative magnetization. Our findings provide a better understanding of the characteristics of the out-of-equilibrium system around DQPT.

We present a detailed real-space spin-polarized scanning tunneling microscopy (SP-STM) study of the magnetic domain structure of Gd(0001) films epitaxially grown on W(110). To find optimal preparation conditions, the influence of the substrate temperature during deposition and of the postgrowth annealing temperature was investigated. Our results show that the lowest density of surface defects, such as step edges as well as screw and edge dislocations, is obtained for room-temperature deposition and subsequent annealing at 900 K. SP-STM data reveal small-size magnetic domains at lower annealing temperatures, evidently caused by pinning at grain boundaries and other crystalline defects. The coverage-dependent magnetic domain structure of optimally prepared Gd films was systematically investigated. For low coverage up to about 80 atomic layers (AL), we observe $\mu \mathrm{m}$-size domains separated by domain walls which are oriented approximately along the $\left[1\overline{1}0\right]$ direction of the underlying W substrate. Above a critical film thickness ${\mathrm{\Theta }}_{\text{crit}}\approx (100\pm 20)$ AL, we identify stripe domains, indicative of a spin reorientation transition from in plane to out of plane. In agreement with existing models, the periodicity of the stripe domains increases the further the coverage exceeds ${\mathrm{\Theta }}_{\text{crit}}$. While the orientation of the stripe domains is homogeneous over large distances just above ${\mathrm{\Theta }}_{\text{crit}}$, we find a characteristic zigzag pattern at $\mathrm{\Theta }\gtrsim 200$ AL and irregular stripe domains beyond 500 AL. Intermediate minima and maxima of the magnetic signal indicate the nucleation of branching domains. The results are discussed in terms of various contributions to the total magnetic energy, such as the magnetocrystalline, magnetostatic, and magnetoelastic energy density.

We completed the analytical framework of determining the angular dependence of coercivity for exchange-coupled phases connected with sharp interfaces in one-dimensional micromagnetics. For various thicknesses of soft phase in a hard matrix, the two types of angular dependence of coercivity, with or without a local minimum, were demarcated and illustrated as a phase diagram. The appearance or absence of a local minimum was confirmed as a sign of whether the switching was dominated by the spin rotation or by the depinning of the domain wall. The mechanism could transform from the former to the latter if the coupling ratio between soft and hard phases, ${\varepsilon }_{AM}=A_s{M}_s/A_h{M}_h$, was increased. The results explained the transformation of angular dependence curves reported in Nd-Fe-B-based permanent magnets.

Twisted magnons (TMs) carrying orbital angular momentum (OAM) have attracted growing interest from the magnonic community. The fabrication of such a novel magnon state, however, is still challenging. Here we present a simple method to generate TMs with arbitrary radial and azimuthal quantum numbers through the spin-to-orbital angular momentum conversion. The conversion rate from plane-wave magnons to twisted ones is shown to be insensitive to the quantum index. The spectrum of TMs in thin nanodisks is solved analytically, showing good agreement with micromagnetic simulations. Moreover, we numerically study the propagation of TMs in magnetic nanodisk arrays and obtain the quantitative dependence of the decay length on quantum indices. Our results are helpful for realizing TMs with large OAMs that are indispensable for future high-capacity magnonic communication and computing.

Magnetic frustration in ferromagnetic metallic systems is unusual due to the long-range and symmetric nature of the exchange interactions. In this work we prove that it is possible to obtain a highly frustrated ferromagnetic phase in a multisublattices cubic structure through a fine-tuning of the magnetic interactions. This peculiar state is achieved in Ni-Mn-(In, Sn) Heusler alloys and results in the effective decoupling of their two intertwined ferromagnetic sublattices. One sublattice is ferromagnetic long-range ordered below the macroscopic Curie temperature ($T_C$) whereas the second one remains disordered until a crossover to a polarized state occurs at $T\ll T_C$. This result points out that a fine engineering of the magnetic interactions in metallic systems can lead to interesting novel and emergent phenomena.

Layered kagome metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$ exhibit diverse correlated electron phenomena. They include charge density wave formation and superconductivity, the pairing symmetry of which, however, is controversial due to contradictory experimental evidence. Through calculations based on real-space lattice models at the mean-field level, we investigate the vortex and surface spectra of all competitive pairing propensities suggested for $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ from a weak-coupling analysis of unconventional superconductivity. Chiral $p$-wave pairing emerges as the only option to host Majorana bound states in the vortex core. We find chiral edge states for both $p$-wave and $d$-wave pairing, along with flat Andreev surface bound states for $f$-wave pairing. Our results expand the fingerprint of superconducting pairing and thus will contribute to resolving the nature of superconductivity in $A{\mathrm{V}}_3{\mathrm{Sb}}_5$.

When computing finite-temperature properties of materials with atomistic simulations, nuclear quantum effects are often neglected or approximated at the quasiharmonic level. The inclusion of these effects beyond this level using approaches like the path integral method is often not feasible due to their large computational effort. We discuss and evaluate the performance of a temperature-remapping approach that links the finite-temperature quantum system to its best classical surrogate via a temperature map. This map, which is constructed using the internal energies of classical and quantum harmonic oscillators, is shown to accurately capture the impact of quantum effects on thermodynamic properties at an additional cost that is negligible compared to classical molecular dynamics simulations. Results from this approach show excellent agreement with previously reported path integral Monte Carlo simulation results for diamond cubic carbon and silicon. The approach is also shown to work well for obtaining thermodynamic properties of light metals and for the prediction of the fcc to bcc phase transition in calcium.

We investigate the quantum phase diagram of the $K$-layer Ising toric code corresponding to $K$ layers of two-dimensional toric codes coupled by Ising interactions. While for small Ising interactions the system displays ${\mathbb{Z}}_2^K$ topological order originating from the toric codes in each layer, the system shows ${\mathbb{Z}}_2$ topological order in the high-Ising limit. The latter is demonstrated for general $K$ by deriving an effective low-energy model in $K\mathrm{th}$-order degenerate perturbation theory, which is given as an effective anisotropic single-layer toric code in terms of collective pseudospins 1/2 referring to the two ground states of isolated Ising chain segments. For the specific cases $K=3$ and $K=4$ we apply high-order series expansions to determine the gap series in the low- and high-Ising limit. Extrapolation of the elementary energy gaps gives convincing evidence that the ground-state phase diagram consists of a single quantum critical point and our findings suggest a quantum phase transition in the 3D Ising* universality class for both $K$ separating both types of topological order, which is consistent with former findings for the bilayer Ising toric code.

The instability toward a magnetic skyrmion crystal in centrosymmetric trilayer magnets is investigated based on a spin model with layer-dependent Dzayloshinskii-Moriya interaction. We find various types of skyrmion crystal phases with different skyrmion numbers in a low-temperature phase diagram by performing the simulated annealing. In addition to the Néel skyrmion crystal phase that is expected to emerge in the presence of the polar-type Dzayloshinskii-Moriya interaction, we obtain the skyrmion crystal phases characteristics of the layered system: the twisted surface skyrmion crystal, antiskyrmion crystal, and high-topological-number skyrmion crystal phases. The rich magnetic phases are brought about by the synergy among the layer-dependent Dzayloshinskii-Moriya interaction, interlayer exchange interaction, and an external magnetic field. Our results indicate that the layer degree of freedom at the surface and heterostructures is a good platform to engineer and design the topological spin textures.

AlN and ZnO, two wide band-gap semiconductors extensively used in the display industry, crystallize in the wurtzite structure, which can favor the formation of epitaxial interfaces to close-packed common ferromagnets. Here we explore these semiconductors as material for insulating barriers in magnetic tunnel junctions. In particular, the ab initio quantum transport code smeagol is used to model the $X$[111]/$Y$[0001]/$X$[111] ($X=$ Co and Fe, $Y=$ AlN and ZnO) family of junctions. Both semiconductors display a valance-band top with $p$-orbital character, while the conduction-band bottom exhibits $s$-type symmetry. The smallest complex-band decay coefficient in the forbidden energy-gap along the [0001] direction is associated with the ${\mathrm{\Delta }}_1$ symmetry, and connects across the band gap at the $\mathrm{\Gamma }$ point in 2D Brillouin zones. This feature enables spin filtering and may result in a large tunneling magnetoresistance. In general, we find that Co-based junctions present limited spin filtering and little magnetoresistance at low bias, since both spin subbands cross the Fermi level with ${\mathrm{\Delta }}_1$ symmetry. This contrasts the situation of Fe, where only the minority ${\mathrm{\Delta }}_1$ band is available. However, even in the case of Fe the magnitude of the magnetoresistance at low bias remains relatively small, mostly due to conduction away from the $\mathrm{\Gamma }$ point and through complex bands with symmetry different than ${\mathrm{\Delta }}_1$. The only exception is for the Fe/AlN/Fe junction, where we predict a magnetoresistance of around 1000% at low bias.

The discovery of the quantum spin Hall effect (QSHE) has ignited the field of topological physics with a vast variety of exotic properties. Here we present the emergence of doubled QSHE in two dimensions characterized with a high-spin Chern number of ${\mathcal{C}}_S=2$ and two pairs of helical edge states. Although overlooked and invisible in topological quantum chemistry and symmetry indicator theory, the already experimentally synthesized $\alpha$-antimonene and $\alpha$-bismuthene ($\alpha$-Bi) are revealed as realistic material candidates of predicted topological states with band inversions emerging at generic $k$ points, rather than the high-symmetry momenta. Remarkably, the nontrivial energy gap can be as large as 342 meV for $\alpha$-Bi, indicating the high possibility of room-temperature observation of the doubled QSHE. Moreover, a four-band effective model is constructed to demonstrate further the feasibility of attaining this type of nontrivial topology. Our results not only uncover a novel topological character of antimony and bismuth but also facilitate the experimental characterization of the previously overlooked hidden topology.

We present a systematic investigation of intersubband transitions in nonintentionally doped $m\text{−}\mathrm{plane}$ ZnO/ZnMgO quantum wells (QWs). The investigation is performed using photoinduced absorption spectroscopy at room temperature under optical pumping by a UV laser to generate electron-hole pairs. All samples exhibit TM-polarized intersubbandlike absorption resonances. However, the peak transition energy is largely blueshifted (>100 meV) with expectations from electronic quantum confinement simulations. Based on calculations of the exciton binding energies, we attribute the photoinduced absorption at room temperature to the dissociation of $h{h}_1\text{−}{e}_1$ excitons towards free carriers in the $e_2$ state and not to $h{h}_1\text{−}{e}_1$ to $h{h}_1\text{−}{e}_2$ excitonic transitions induced by the intersubband absorption as previously stated by Olszakier et al. [Phys. Rev. Lett. 62, 2997 (1989)]. This effect is a consequence of the huge binding energy of excitons in the ZnO material system, which is further enhanced in QWs due to the quantum confinement. This may pave the way for a better understanding of semiconductors’ excitonic processes as well as for developing intersubband devices with a blueshifted operating range.

We develop a non-Abelian time evolving block decimation (NA-TEBD) approach to study open systems governed by Lindbladian time evolution, while exploiting an arbitrary number of Abelian or non-Abelian symmetries. We illustrate this method in a one-dimensional fermionic SU(2) Hubbard model on a semi-infinite lattice with localized particle loss at one end. We observe a ballistic front propagation with strongly renormalized front velocity, and a hydrodynamic current density profile. For large loss rates, a suppression of the particle current is observed, as a result of the quantum Zeno effect. Operator entanglement is found to propagate faster than the depletion profile, preceding the latter.

Hexagonal two-dimensional materials with broken inversion symmetry (as BN or transition metal dichalcogenides) are known to sustain chiral phonons with finite angular momentum, adding a further useful degree of freedom to the extraordinary entangled (electrical, optical, magnetic, and mechanical) properties of these compounds. However, because of lattice symmetry constraints, such chiral modes are constrained to the corners of the Brillouin zone, allowing little freedom for manipulating the chiral features. In this paper, we show how the application of uniaxial strain leads to the existence of unique chiral modes in the vicinity of the zone center. We also show that such strain-induced chiral modes, unlike the ones pinned at the K points, can be efficiently manipulated by modifying the strain itself, which determines the position of these modes in the Brillouin zone. The present paper results add a technique for the engineering of the quantum properties of two-dimensional lattices.