The anomalous Floquet Anderson insulator (AFAI) has been theoretically predicted in stepwise periodically driven models, but its stability under more general driving protocols has not been determined. We show that adding disorder to the anomalous Floquet topological insulator realized with a continuous driving protocol in the experiment by Wintersperger et al. [Nat. Phys. 16, 1058 (2020)], supports an AFAI phase, where, for a range of disorder strengths, all the time averaged bulk states become localized, while the pumped charge in a Laughlin pump setup remains quantized.

Spin-state ordering—a periodic pattern of ions with different spin-state configurations along a crystal lattice—is a rare phenomenon, and its possible interrelation with other electronic degrees of freedom remains little explored. Here we perform a structural investigation of the mixed-valence Co homometallic ludwigite ${\mathrm{Co}}_2^{2+}{\mathrm{Co}}^{3+}{\mathrm{O}}_2{\mathrm{BO}}_3$. A superstructure consistent with a long-range ${\mathrm{Co}}^{3+}$ spin-state ordering is observed between $T_4=580$ K and $T_3=510$ K. Intermediate states with mesoscopic correlations are detected below $T_3$ down to $T_1=480$ K with a change of dimensionality at $T_2=495$ K. The spin-state correlations are connected to the charge sector as revealed by the abrupt changes in the electrical resistance at $T_1$ and $T_2$. The evolution of the structural parameters below $T_1$ indicate that the spin crossover is ignited by a moderate degree of thermally induced ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ charge disorder. Charge and spin-state degrees of freedom can be interrelated in mixed-valence spin-crossover materials, leading to sharp transitions involving intermediate spin-state and charge-correlated states at the mesoscale.

Materials exhibiting nontrivial topological spin textures known as merons attract considerable interest for their fascinating underlying physics and promising device applications. The ${\mathrm{PbTiO}}_3$ family of compounds is known to host ferroelectric, ferroelastic, and piezoelectric polar orders and possess topological domain structures that are sensitive to external conditions. Here, we examine the polar states in ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ (PZT) thin films and explore the influence of film thickness and epitaxial strain via phase-field modeling under short-circuit (sc) and open-circuit (oc) boundary conditions. In this paper, we uncovered four distinct meron states in ultrathin films with tensile strains. Under the sc boundary condition, there is an intermediate meron bubble texture comprising a twisted Néel-like and Bloch-like meron texture with a distinct helicity and a meron bubble pair texture combining two intermediate merons along the $z$ axis. Under the oc boundary condition, there is an antimeron bubble texture with negative winding numbers and an achiral topology, and a trimeron bubble texture with two antimeron bubbles and one meron bubble. These simulation results show that strain and film thickness have a major impact on the polarization strength and topology to dictate the formation and transition of meron phases in PZT thin films. The insights obtained in this paper may help elucidate and design materials with tailored and tunable microscopic and mesoscopic polar topological structures.

Crystal structure prediction (CSP) and machine learning potential (MLP) are two fundamental methods for modern computational material discovery. While the former aims at efficient sampling of the potential energy surface (PES) for discovering new materials, the latter focuses on reproducing the PES to accelerate various atomic simulation tasks. In this work, we combine the two methods within a concurrent learning framework in an effort to generate efficient MLP models for accelerating CSP. The proposed scheme explores the PES through the swarm-intelligence calypso method, labels the most representative structures with quantum mechanical calculations, and learns the PES through a deep potential (DP) model. The process proceeds in an iterative, computationally efficient, and automated manner, leading to the collection of a most compact reference training set from which the resulting DP model is proven particularly suitable for accelerating calypso structure prediction. The scheme has been systematically benchmarked on binary magnesium-aluminium (Mg-Al) alloys and ternary lithium-lanthanum-hydrogen (Li-La-H) superhydrides, demonstrating its efficiency and reliability in DP model construction and calypso structure prediction. The proposed scheme represents a promising routine to perform the structure prediction of large or multicomponent systems.

Helicity, one of the degrees of freedom for skyrmions, describing the direction of spin rotation in skyrmions, is of fundamental importance for skyrmion-based potential applications. However, continuously modulating the helicity is challenging. Here, we propose an effective method to control the skyrmion helicity by means of the competition between Dzyaloshinskii-Moriya interaction and exchange frustration in inversion-symmetry-broken frustrated magnets. In the prototypical case of Janus NiClBr, we found a nanometer-sized skyrmion with uniform unconventional helicity of ${139}^{\circ }$. Moreover, a spin texture phase diagram as a function of strain and out-of-plane magnetic field was illustrated and, most importantly, an unexpected linear change in the helicity from $\sim 0^{\circ }$ to $\sim {50}^{\circ }$ and from $\sim {120}^{\circ }$ to $\sim {180}^{\circ }$ was found in the skyrmion lattice region. As such, the skyrmion Hall effect of Janus NiClBr can be reduced under external field for a critical value of the helicity. These results not only give a feasible method to effectively modulating the skyrmion helicity, but also provide an ideal platform for further investigations on skyrmion helicity-related phenomena.

The low-dimensional frustrated antiferromagnet system has been an appealing playground for investigating the exotic magnetic ground states. The $S=1/2$ skew chain ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ attracted considerable interest for its magnetization plateau and the emergent ferroelectricity under high magnetic fields. In this work, we report the comprehensive angular studies of the magnetization plateau and the ferroelectric polarization in ${\mathrm{Co}}_2{\mathrm{V}}_2{\mathrm{O}}_7$ under high fields up to 30 T. The magnetization was studied in detail with the applied field rotated within the $bc$ and $ab$ planes. We find that the 1/2 magnetization plateau is enhanced with the field rotating within the $bc$ plane and becomes most pronounced when the field is titled ${30}^{\circ }$ away from the $b$ axis (denoted as the $b^{*}$ axis). The temperature dependence of ferroelectric polarization was also studied along the $a, c$, and $b^{*}$ axes, respectively. Based on these polarization measurements, the magnetic field temperature phase diagrams have been constructed for the three axes. We propose a model which considers the decoupling of the two inequivalent Co spins to explain the exotic polarization flop and the phase diagrams. This decoupling is considered to be enhanced by the dimerization effect, and that the non-negligible interchain interaction $J_3$ plays a crucial role.

Spin-lattice coupling is one of the microdynamic sources of magnetic-mechanical effect, which can significantly affect the physical properties of materials hosting the skyrmion microstructure. Recent studies found a local lattice distortion is a natural result induced by the spin-lattice coupling and the nonalignment of spin texture, and it can lead to a non-Arrhenius transport behavior of skyrmion spin texture. In this paper, we study the deep insight of the microdynamic nature of this local lattice distortion field (LLDF). Starting with a general atomistic model for the chiral magnetic system containing the intrinsic spin-lattice coupling, we propose an analytical expression of the LLDF morphology. It suggests that the LLDF is an irrational strain-field with source, and is independent of the type of skyrmion spin texture, i.e., either Bloch- or Néel-type. The numerical simulations results based on spin-lattice dynamics support this conjecture. Subsequently, the effects of LLDF are discussed on the transport and stability of the skyrmion spin texture, which are two key issues for the skyrmion engineering and application. From the simulation results, the LLDF is found to bring the negative effects. The presence of LLDF, (1) brings a dragging force so to reduce the moving speed of spin texture for skyrmion transport and (2) enhances the thermal fluctuation of the spin-lattice-coupled system so to reduce the skyrmion lifetime. Our study provides an underlying mechanistic understanding on the control and manipulation of the magnetic skyrmion.

Magnetic skyrmions are topologically protected spin textures with potential applications in memory and logic devices. Skyrmions have been commonly observed in systems with Dzyaloshinskii-Moriya interaction due to broken inversion symmetry. Yet, recent studies suggest that skyrmions can also be stabilized in systems with inversion symmetry, such as Ni-based dihalides due to magnetic frustration. In this article, we employ atomistic simulations to investigate chiral magnetic phases in bilayers of ${\mathrm{NiI}}_2$ and ${\mathrm{NiBr}}_2$. We show that the antiferromagnetic interlayer coupling introduces an additional magnetic frustration and gives rise to a variety of novel spin textures with different topological charges. Specifically, for ${\mathrm{NiI}}_2$, we observe that the skyrmions with the in-plane component of spins wrapping around twice (biskyrmions) have an enhanced stability compared to the monolayer case. We also study the polarization induced by the noncolinear magnetic order in ${\mathrm{NiI}}_2$ bilayers and show that the polarization of the topologically nontrivial phases is negligible compared to the spiral phases. Thus, we conclude that polarization measurements can be an indirect route for detecting skyrmions in upcoming experiments.

The most common allotrope of tungsten, $\alpha \text{−}\mathrm{W}$, has a superconducting transition at a temperature of $\sim 11\mathrm{mK}$. However, two other forms of tungsten have been reported to have superconducting transitions in the temperature range $T_c\sim 2-5\mathrm{K}$ when synthesized as thin films: crystalline $\beta \text{−}\mathrm{W}$ and amorphous W ($a\text{−}\mathrm{W}$). In this work we carry out a systematic study of W films synthesized using dc magnetron sputtering, using transport, low frequency magnetic shielding response, and transmission electron microscopy. Our results show that while $a\text{−}\mathrm{W}$ is indeed a conventional superconductor, $\beta \text{−}\mathrm{W}$ is not a superconductor down to 2.3 K. Superconductivity with $T_c>3\mathrm{K}$ in the putative $\beta \text{−}\mathrm{W}$ films probably originates from an amorphous phase that forms underneath the $\beta \text{−}\mathrm{W}$ phase. Our findings reconcile some of the anomalies earlier reported in $\beta \text{−}\mathrm{W}$, such as the very small superconducting gap and the decrease of $T_c$ with increase in film thickness.

Recent investigations into ${\mathrm{MoB}}_2$ have unveiled a direct connection between a pressure-induced structural transition to a $P6/mmm$ space group structure and the emergence of superconductivity, producing critical temperatures up to 32 K at 100 GPa. This pressure-induced superconducting state underscores the potential of doped ${\mathrm{MoB}}_2$ as a possible candidate for metastable superconductivity at ambient pressure. In this work, we demonstrate that doping by Zr, Hf, or Ta stabilizes the $P6/mmm$ structure at ambient pressure and results in the realization of a superconducting state with critical temperatures ranging from 2.4 up to 8.5 K depending on the specific doping. We estimate the electron-phonon coupling $\lambda$ and the density of states based on resistivity and specific heat data, finding that $\lambda$ ranges from 0.4 to 0.6 for these compounds. Finally, to investigate the role of possible metastable defect structures on the critical temperature, we analyze ${\mathrm{MoB}}_2, {\mathrm{MoB}}_{2.5}$, and Nb/Zr-doped ${\mathrm{MoB}}_2$ using rapid cooling techniques. Notably, splat quenching produces samples with higher critical temperatures and even retains superconductivity in ${\mathrm{MoB}}_2$ at ambient pressure, achieving a critical temperature of 4.5 K.

We investigate the optical properties of laser-excited gold within the Drude-Lorentz framework. Our approach extends the Drude-Lorentz model, initially fitted to experimental data for ambient conditions, to accommodate higher temperatures. It covers a wide range of excitation strengths and frequencies, Fermi-distributed electrons, as well as nonequilibrium scenarios. It is in good qualitative agreement with density functional theory–based calculations capturing key features of the reflectivity and, thus, provides a very efficient alternative to these numerically expensive calculations. When applied to thin films, we find a nonlinear dependence of reflection and absorption on electron temperature, accentuated at lower photon energies and under polarized light. Our highly efficient model can be particularly valuable when analyzing experiments investigating ultrafast processes as the pathways of energy relaxation in laser-excited samples.

The quantum tunneling effect in a two-dimensional (2D) checkerboard lattice is investigated. By analyzing the pseudospin texture of the states in a 2D checkerboard lattice based on the low-energy effective Hamiltonian, we find that this system has a chiral symmetry with chirality equal to 2, although compared with regular chiral fermions, its pseudospin orientation does not vary uniformly. This suggests that the perfect reflection chiral tunneling, also known as the anti-Klein tunneling, is expected to appear in a checkerboard lattice as well. To verify the conjecture, we calculate the transmission probability and find that normally incident electron states can be perfectly reflected by the barrier with hole states inside and vice versa. Furthermore, we numerically calculate the tunneling conductance of the checkerboard nanotube using the recursive Green's function method. The results show that a perfect on-off ratio can be achieved by confining the energy of the incident states within a certain range. It also suggests that, by tuning the barrier, the checkerboard nanotube can work as a perfect band filter or tunneling field effect transistor, which transmits electrons selectively with respect to the pseudospin of the incident electrons.

Motivated by the recent progress in realizing and controlling extended Bose-Hubbard systems using excitonic or atomic devices, the present Letter theoretically investigates the case of a two-band Bose-Hubbard chain with nearest-neighbor interactions. Specifically, this study concentrates on the scenario where, due to the interactions, one band supports a density-wave phase, i.e., a correlated insulating phase with spontaneous breaking of translational symmetry in the lattice, while the other band supports superfluid behavior. Using the density matrix renormalization group method, we show that supersolid order can emerge from such a combination, that is, an elusive quantum state that combines crystalline order with long-range phase coherence. Depending on the filling of the bands and the interband interaction strength, the supersolid phase competes with phase-separation, superfluid order, or Mott insulating density-wave order. As a possible setup to observe supersolidity, we propose the combination of a lower band supporting density-wave order and a thermally excited band that supports superfluidity due to weaker lattice confinement.

We analyze a model of qubits that we argue has an emergent quantum gravitational description similar to the fermionic Sachdev-Ye-Kitaev (SYK) model. The model we consider is known as the quantum $q$-spin model because it features $q$-local interactions between qubits. It was previously studied as a model of a quantum spin glass, and while we find that the model is glassy for $q=2, q=3$, and likely $q=4$, we also find evidence for previously unexpected SYK-like behavior for the quenched free energy down to the lowest temperatures for $q\ge 5$. This SYK-like physics includes power-law correlation functions and an extensive low-temperature entropy, so we refer to the model as Spin SYK. The model is generic in that it includes all possible $q$-body couplings, lacks most symmetries, and has no spatial structure, so our results can be construed as establishing a certain ubiquity of quantum holography in systems dominated by many-body interactions. Furthermore, we discuss a generalized family of models that includes Spin SYK and which provably exhibit SYK-like physics in the solvable limit of large local Hilbert space dimension. We also comment on the implications of a bosonic system with SYK-like properties for the study of holography, Hamiltonian complexity, and related topics.

We present a family of Floquet circuits that can interpolate between noninteracting qubits, free propagation, generic interacting, and dual-unitary dynamics. We identify the operator entanglement entropy of the two-qubit gate as a good quantitative measure of the interaction strength. We test the persistence of localization in the vicinity of the noninteracting point by probing spectral statistics, decay of autocorrelators, and measuring entanglement growth. The finite-size analysis suggests that the many-body localized regime does not persist in the thermodynamic limit. Instead, our results are compatible with an integrability-breaking phenomenon.

Artificial periodic structures including photonic crystals and metamaterials have been widely adopted to manipulate the propagation of light, whereas their effective relative impedance and refractive index are usually coupled together and difficult to design separately. Here, we demonstrate that using the rotational deformations inside the unit cells, the relative impedance $Z_r$ of a two-dimensional photonic crystal can be adjusted flexibly while the refractive index basically is not altered. This corresponds to a transition of effective relative permeability related to the topological property of the lowest band. Hence, zero reflection with $Z_r=1$ is achieved in a wide frequency range near the band edges, which is important for designing high-performance photonic devices.

Spin-split superconductors offer new functionality compared to conventional superconductors such as diode effects and efficient thermoelectricity. The superconducting state can nevertheless only withstand a small amount of spin splitting. Here, we self-consistently determine the spin transport properties and the phase diagram of a spin-split superconductor in the presence of an injected spin accumulation. Energy and spin relaxation are accounted for in the relaxation time approximation via a single effective inelastic scattering parameter. We find that the spin-splitting field in the superconductor enables a spin diode effect. Moreover, we consider the superconducting phase diagram of a system in contact with a spin accumulation and in the presence of spin relaxation, and find that the inclusion of energy and spin relaxation alters the phase diagram qualitatively. In particular, these mechanisms turn out to induce a superconducting state in large parts of the phase diagram where a normal state would otherwise be the ground state. We identify an Fulde–Ferrel–Larkin–Ovchinnikkov-like state even in the presence of impurity scattering, which can be controllably turned on and off via the electrically induced spin accumulation. We explain the underlying physics from how the superconducting order parameter depends on the nonequilibrium modes in the system as well as the behavior of these modes in the presence of energy and spin relaxation when a spin-splitting field is present.