Resonant tunneling transport in polar heterostructures is intimately connected to the polarization fields emerging from the geometric Berry-phase. In these structures, quantum confinement results not only in a discrete electronic spectrum, but also in built-in polarization charges exhibiting a broken inversion symmetry along the transport direction. Thus, electrons undergo highly asymmetric quantum interference effects with respect to the direction of current flow. By employing doping to counter the broken symmetry, we deterministically control the resonant transmission through GaN/AlN resonant tunneling diodes and experimentally demonstrate the recovery of symmetric resonant tunneling injection across the noncentrosymmetric double-barrier potential.

Titanium alloys are used in a large array of applications. In this work we focus our attention on the most used alloy, Ti-6Al-4V (Ti64), which has excellent mechanical and biocompatibility properties with applications in aerospace, defense, biomedical, and other fields. Here we present high-fidelity experimental shock compression data measured on Sandia's Z machine. We extend the principal shock Hugoniot for Ti64 to more than threefold compression, up to over 1.2 TPa. We use the data to validate our ab initio molecular dynamics simulations and to develop a highly reliable, multiphase equation of state (EOS) for Ti64, spanning a broad range of temperature and pressures. The first-principles simulations show very good agreement with Z data and with previous three-stage gas gun data from Sandia's STAR facility. The resulting principal Hugoniot and the broad-range EOS and phase diagram up to 10 TPa and ${10}^5$ K are suitable for use in shock experiments and in hydrodynamic simulations. The high-precision experimental results and high-fidelity simulations demonstrate that the Hugoniot of the Ti64 alloy is stiffer than that of pure Ti and reveal that Ti64 melts on the Hugoniot at a significantly lower pressure and temperature than previously modeled.

We propose random tight-binding models that host macroscopically degenerate zero-energy modes and belong to the unitary class. Specifically, we employ the molecular-orbital representation, where a Hamiltonian is constructed by a set of nonorthogonal orbitals composed of linear combinations of atomic orbitals. By setting the coefficients appearing in molecular orbitals to be random U(1) variables, we can make the models belong to the unitary class. We find two characteristic behaviors that are distinct from the random real-valued molecular-orbital model. Firstly, a finite-energy gap opens on top of the degenerate zero-energy modes. Secondly, besides the zero-energy modes, we also argue that the band center of the finite-energy modes is critical, which is inherited from the dual counterpart, namely, the random-phase model on a bipartite lattice. Furthermore, as a by-product of this model-construction scheme, we also construct the random tight-binding model on a composite lattice, where we also find a realization of critical states.

We present a simple but attractive tool to describe and model the spin states of single molecule magnets. It is presented through a generalized Landau-Lifshitz-Gilbert equation with bias-dependent couplings that can be externally controlled. We provide a complete account of the various magnetic couplings [magnetic anisotropy, exchange, Dzyaloshinskii-Moriya interactions (DMI), damping, and magnetic noise's correlations] within a dimer of localized moments. The inversion symmetry breaking, ensued by the bias potential, induces a DMI between the magnetic elements that can be tuned accordingly. Through a calculation of the evolution of the spin, we conclude that such DMI is the dominant interaction during the reversal process. Along with the prescription to describe the time evolution of the spin moments, our results provide a qualitatively complete and integrated picture of various systems of interest in nanomagnetism.

The Floquet time crystal, which breaks discrete time-translation symmetry, is an intriguing phenomenon in nonequilibrium systems. It is crucial to understand the rigidity and robustness of discrete time crystal (DTC) phases in a many-body system, and finding a precisely solvable model can pave a way for understanding of the DTC phase. Here, we propose and study a solvable spin chain model by mapping it to a Floquet superconductor through the Jordan-Wigner transformation. The phase diagrams of Floquet topological systems are characterized by topological invariants and tell the existence of anomalous edge states. The subharmonic oscillation, which is the typical signal of the DTC, can be generated from such edge states and protected by topology and solvability. We also examine the robustness of the DTC by adding symmetry-preserving and symmetry-breaking perturbations. Our results on the topologically protected DTC can provide a deep understanding of the DTC when generalized to other interacting or dissipative systems.

We investigate the time traces of currents induced in five segmented electrodes by the motion of electrons on the surface of liquid ${}^4\mathrm{He}$ at $\sim 0.3$ K, that are placed in a perpendicular magnetic field and exposed to microwave radiation. Nonlinear dynamics methods are utilized to explore the characteristic features of the current oscillations in the electrodes for different electron densities and pressing voltages. The wavelet phase coherence and phase shift are calculated to obtain the coherence relationships between the signals in all five electrodes as functions of the pressing voltage. Coupling analysis of the ridge-extracted instantaneous frequencies revealed the directions of motion of electrons inside the cell and provided evidence of strong phase coupling at a pressing voltage of 4.20 V. These classical methods reveal that the motion is oscillatory, with a varying frequency subject to a constant frequency modulation. High harmonics due to nonlinearity arise at higher frequencies where the resonance condition is satisfied at a pressing voltage of 4.20 V for low-electron density. Our approach provides a platform for investigating these phenomena analytically. We show that slow gravity waves on the helium surface modulate the electronic oscillatory behavior and illustrate that the model in fact produces three-dimensional dynamics. Motion of electrons on the surface of liquid helium is shown to be a paradigmatic example of a chronotaxic system, i.e., a system that undergoes continuous perturbation but is nonetheless capable of maintaining its stability.

As a metallic material of a heavy atom, elemental gadolinium (Gd) has many extraordinary electronic properties associated with the strong correlation and spin-orbital coupling (SOC). Previous studies have shown that the resistivity of ferromagnetic (FM) Gd due to the spin fluctuation scattering is comparable to the intrinsic resistivity limited by electron-phonon (e-ph) scattering even near the room temperature. However, such theoretical explanations cannot account for the experimental observation quantitatively. In the present work, by performing the first-principles calculations, we find that the intrinsic resistivity is insensitive to Hubbard $U$, i.e., the electronic correlation. On the contrary, when the effect of SOC is taken into account, the band structure near the Fermi level is modified greatly. As a result, the calculated intrinsic resistivity increases by nearly 3 times over that without SOC. By further counting in the additional resistivity contributed from spin fluctuation scattering, our theoretical result of resistivity agrees very well with the experimental observation of resistivity of FM Gd as a function of temperature.

Two-dimensional, high-quality perylene single crystals were grown with a space-confined strategy method. The grown films crystallize in the $\alpha$ form, as is confirmed by a combination of techniques. Polarization-dependent optical absorption measurements show a strong anisotropy in very good agreement with the literature data, and the anisotropic mobility data in field-effect transistors document the very high crystalline order. Momentum-dependent studies using electron energy-loss spectroscopy reveal a negative dispersion of the first exciton along the crystal $b$ direction with an exciton bandwidth of 72 meV. We argue that this behavior is a result of charge-transfer exciton coupling between the perylene dimers in the unit cell.

We theoretically study the in-plane acoustic phonons of graphene/hexagonal boron nitride moiré superlattice using a continuum model. We demonstrate that the original phonon bands of individual layers are strongly hybridized and reconstructed into moiré phonon bands consisting of dispersive bands and flat bands. The phonon band structure can be effectively described by a spring-mass network model to simulate the motion of moiré domain walls where the flat-band modes are interpreted as vibrations of independent, decoupled strings. We also show that the moiré phonon has angular momentum due to the inversion symmetry breaking by hBN, with high amplitudes concentrated near the narrow gap region. Finally, we apply the same approach to twisted bilayer graphene and we find a notable difference between the origins of the flat-band modes in G/hBN and TBG, reflecting distinct geometric structures of the domain pattern.

We propose a mechanism to generate dissipationless current in time reversal symmetry breaking Weyl semimetals through the anomalous chiral magnetic effect (ACME). The ACME current is induced by chiral imbalance and flows along the direction of the Weyl nodes separation in momentum space. In contrast to the chiral magnetic effect, the ACME is not related to the chiral anomaly and does not require external magnetic field. In the presence of parallel electric and magnetic fields, the ACME gives rise to an antisymmetric linear magnetoresistance and a Hall as well as planar Hall conductivity, which we estimate to be observable.

We study theoretically the interplay between magnetism and topology in three-dimensional HgTe/MnTe superlattices stacked along the (001) axis. Our results show the evolution of the magnetic topological phases with respect to the magnetic configurations. An axion insulator phase is observed for the antiferromagnetic order with the out-of-plane Néel vector direction below a critical thickness of MnTe, which is the ground state among all magnetic configurations. Defining $T$ as the time-reversal symmetry, this axion insulator phase is protected by a magnetic twofold rotational symmetry $C_2·T$. We find that the axion insulator phase evolves into a trivial insulator as we increase the number of the magnetic MnTe layers, and we present an estimate of the critical thickness of the MnTe film above which the axion insulator phase is absent. By switching the Néel vector direction into the $ab$ plane, the system realizes different antiferromagnetic topological insulators depending on the thickness of MnTe. These phases feature gapless surface Dirac cones shifted away from high-symmetry points on surfaces perpendicular to the Néel vector direction of the magnetic layers. In the presence of ferromagnetism, the system realizes a magnetic Weyl semimetal and a ferromagnetic semimetal for out-of-plane and in-plane magnetization directions, respectively. We observe large anomalous Hall conductivity in the presence of ferromagnetism in the three-dimensional superlattice.

In this work, we present and analyze two tensor network-based influence functional approaches for simulating the real-time dynamics of quantum impurity models such as the Anderson model. Via comparison with recent numerically exact simulations, we show that such methods accurately capture the long-time nonequilibrium quench dynamics. The two parameters that must be controlled in these tensor network influence functional approaches are a time discretization (Trotter) error and a bond dimension (tensor network truncation) error. We show that the actual numerical uncertainties are controlled by an intricate interplay of these two approximations, which we demonstrate in different regimes. Our work opens the door to using these tensor network influence functional methods as general impurity solvers.

The restricted multicanonical (rMUCA) ensemble method is developed and combined with the on-the-fly machine learning potential (MLP) generation scheme. The rMUCA simulation performs a random walk in the potential-energy subspace restricted by the selected collective variables and allows us to sample physically relevant configurations without being trapped in local energy minima. No preliminary simulation runs are required to construct a bias potential. The sample structures for training are collected dynamically from the simulations using the MLP itself where the simultaneous error estimation is utilized to judge whether an updated structure should be added to the sample data set or not. The rMUCA formula can be also used for the saddle-point search with minor modification. The method is applied to the oxidation of carbon monoxide on platinum surfaces. The results show that the rMUCA simulation provides an efficient and accurate way to sample rare events for the MLP construction.

The nontrivial spin texture on the (001) surface of topological crystalline insulator SnTe hosts exotic scientific importance and spintronic applications. Here, we study the effects of weak Floquet optical driving on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities on a doped SnTe(001) surface. Due to peculiar spin-orbit hybridization, we find a noncollinear twisted RKKY interaction with XYZ-Heisenberg, symmetric in-plane, and asymmetric Dzyaloshinskii-Moriya (DM) terms. We see that contributions from the $z$ $\left(x\right)$ component of the XYZ-Heisenberg (DM) interaction are dominant for most parameters. The interactions, including DM terms that are responsible for interesting spin textures, require doping in most cases. We propose to modify the interactions in situ via optical control of band structure, and thereby doping. A notable aspect of this control protocol is breaking of electron-hole symmetry, which stems from the DM interaction.

The magnetic properties of the axion insulator candidate ${\mathrm{EuSn}}_2{\mathrm{P}}_2$ are comprehensively investigated. Under an applied external field, the antiferromagnetic phase below 30 K is driven to a forced ferromagnetic state. The yielded Rhodes-Wolfarth ratio (RWR) is equal to 1.13 (1.12) with $H//ab$ $(H//c)$, indicating the forced ferromagnetism in ${\mathrm{EuSn}}_2{\mathrm{P}}_2$ is weak itinerant. A large magnetocrystalline anisotropy with $K_u=521$ $\mathrm{kJ}/{\mathrm{m}}^3$ at 2 K is evaluated. Analysis on the critical behavior generates a set of critical exponents $\beta =0.281\left(2\right)$ with $T_C=30.2\left(1\right)$ K, $\gamma =0.77\left(3\right)$ with $T_C=30.1\left(1\right)$ K, and $\delta =3.76\left(4\right)$ at $T_C=30$ K, exhibiting a three-dimensional critical behavior. A phase diagram of $H$ versus $T$ is established, where a tricritical point at temperature and field of (30 K, 16.7 kOe) is determined on the boundaries of the paramagnetic, antiferromagnetic, and forced ferromagnetic states. In addition, the magnetic entropy change exhibits anisotropic behavior. The maximal value of the magnetic entropy $-\mathrm{\Delta }{S}_M^{\text{max}}$ reaches 7.36 (6.41) J ${\mathrm{kg}}^{-1}{\mathrm{K}}^{-1}$ with a magnetic field change of 50 kOe in $H//ab$ $(H//c)$, which is comparable with that of well-known materials suitable for magnetic refrigeration with second-order ferromagnetic transitions. The plots of $-\mathrm{\Delta }{S}_M(T,H)$ of high field can be rescaled into a universal curve, verifying the high-field paramagnetic to forced ferromagnetic transition is of a second order. The well-established scaling behavior of the magnetization and the magnetic entropy change further supports ${\mathrm{EuSn}}_2{\mathrm{P}}_2$ as an axion insulator.

The interplay between spin waves and magnetic texture represents the information exchange between the fast and slow dynamical parts of magnetic systems. Here we formulate a set of magnonic torques acting on background magnetic texture, by extracting time-invariant information from the fast precessing spin waves. Under the frame of magnonic torques, we use theoretical formulations and micromagnetic simulations to investigate the spin-wave-driven domain wall motion in two typical symmetry-breaking situations: the rotational symmetry broken by the Dzyaloshinskii-Moriya interaction and the translational symmetry broken by magnetic damping. The torque-based microscopic analyses provide compact yet quantitative tools to reinterpret the magnetic texture dynamics induced by spin waves, beyond the conventional framework of global momentum conservation.

We investigate abilities of various linear response based techniques for extracting parameters of antisymmetric Dzyaloshinskii-Moriya (DM) interactions from first-principles electronic structure calculations. For these purposes, we further elaborate the idea of Sandratskii [Phys. Rev. B 96, 024450 (2017)], which states that the $z$ component of the DM vector can be computed by retaining only the spin-diagonal part of the spin-orbit (SO) interaction. This approximation, which becomes exact to the first order in the SO coupling, greatly simplifies the calculations as it requires only minor extensions in comparison to isotropic parameters in the nonrelativistic case. We start our analysis with the magnetic force theorem (MFT), which relies on additional approximations resulting in the linear dependence of the exchange interactions on the response tensor, and compare it with the exact approach formulated in terms of the inverse response. For the ligand states, which are not primarily responsible for the magnetism but magnetized from the localized states, we propose the downfolding procedure transferring the effect of these ligand spins into parameters of effective interactions between the localized spins. These techniques are applied for the series of ${\mathrm{CrCl}}_3$- and ${\mathrm{CrI}}_3$-based materials, including bulk, monolayer, bilayer, and three-layer systems. Particularly, we discuss how the DM interactions are induced by the inversion symmetry breaking at the surface or by the electric field. As long as the response tensor between the Cr $3d$ states is calculated by taking into account the SO interaction on the heavy ligand sites, the MFT appears to be a good approximation for the DM interactions, being in contrast with the isotropic exchange, for which MFT and the exact method provide quite a different description. Finally, we discuss the relevance of our approach to other techniques proposed for calculations of the DM interactions. Particularly, we argue that the spin-current model for the DM interactions can be derived from the MFT-based expression and is the relativistic counterpart of the double exchange, occurring in metallic systems in the limit of infinite exchange splitting.

Spin transition and its effect on the physical properties of iron-bearing minerals at high pressure-temperature $\left(P\text{−}T\right)$ are of great importance for understanding the structural heterogeneity of Earth's mantle. Here, we investigate the phase diagram and thermoelastic properties of iron oxyhydroxide (FeOOH) across the spin crossover using dynamic high $P\text{−}T$ experiments and theoretical simulations. The Hugoniot equation of state in FeOOH has been measured up to $\sim 90$ GPa and $\sim 2100$ K in a two-stage light-gas gun and exhibits a density discontinuity between 47 GPa (∼950 K) and 61 GPa (∼1150 K) due to the high-low spin transition of ${\mathrm{Fe}}^{3+}$, which is consistent with our first-principles calculations. The $P\text{−}T$ phase diagram indicates that the shock-elevated temperature shifts the spin transition to a higher pressure and broadens the pressure range of mixed spins. The large volume collapse of FeOOH during its spin crossover leads to remarkable elastic anomalies, with ∼60% softening of adiabatic bulk modulus and a negative Poisson's ratio (−0.1) of abnormal auxeticity in the mixed-spin phase. Our results suggest that FeOOH undergoes an unselective spin transition of ferric iron at the corresponding $P\text{−}T$ conditions of the Earth's 1400–1800 km depth and exhibits drastic softening in sound velocities and elastic modulus which may be detected as seismic heterogeneities in subducting slabs of the lower mantle.

Spin noise spectroscopy, as a sort of magnetic resonance technique, uses, for detection of spin precession, spontaneous fluctuations of magnetization revealed as a peak in the Faraday-rotation (FR) noise spectrum at Larmor frequency. In the model of precessing magnetization, the FR noise signal should be the greatest in the Voigt geometry (with magnetic field aligned across the light propagation), and should vanish in the Faraday geometry (with the field along the probe beam). This reasoning employs, implicitly or explicitly, the so-called Van Vleck theorem that establishes, within the limits of certain assumptions, a direct relation between the FR and magnetization of the spin system. We show that violation of these assumptions in crystals with anisotropic paramagnetic centers may qualitatively change the conventional laws of spin noise detection, making, in particular, the FR noise detectable in the Faraday geometry. These conclusions are confirmed by experimental studies of spin-noise spectra of ${\mathrm{CaF}}_2$ crystals with tetragonal centers of ${\mathrm{Nd}}^{3+}$ ions.