The Aharonov-Bohm (AB) effect, which predicts that a magnetic field strongly influences the wave function of an electrically charged particle, is investigated in a three-site system in terms of the quantum control by an additional dephasing source. The AB effect leads to a nonmonotonic dependence of the steady-state current on the gauge phase associated with the molecular ring. This dependence is sensitive to site energy, temperature, and dephasing, and can be explained using the concept of the dark state. Although the phase effect vanishes in the steady-state current for strong dephasing, the phase dependence remains visible in an associated waiting-time distribution, especially at short times. Interestingly, the phase rigidity (i.e., the symmetry of the AB phase) observed in the steady-state current is now broken in the waiting-time statistics, which can be explained by the interference between transfer pathways.

We consider a planar two-dimensional system between two media with different dielectric constants and in the presence of a third dielectric medium separated by a nonplanar interface. Extending a perturbative method for solving Poisson's equation, developed by Clinton, Esrick, and Sacks [Phys. Rev. B 31, 7540 (1985)], in the presence of nonplanar conducting boundaries to the situation proposed here, we obtain, up to the first order in terms of the function which defines the nonplanar interface, the effective potential, the effective electrostatic field, and the effective dielectric constant for the planar 2D system. We also point to the existence of an effective external field acting in-plane in the 2D system. Implications of the results to properties of 2D systems are discussed. In the limit of planar surfaces, vacuum-dielectric or vacuum-conducting media, our results are in agreement with those found in the literature.

We theoretically study the proximity spin-orbit coupling (SOC) in graphene on a transition-metal dichalcogenides (TMDC) monolayer stacked with arbitrary twist angles. We find that the relative rotation greatly enhances the spin splitting of graphene, typically by a few to ten times compared to the nonrotated geometry, and the maximum splitting is achieved around ${20}^{\circ }$. The induced SOC can be changed from the Zeeman type to the Rashba type by rotation. The spin splitting is also quite sensitive to the gate-induced potential, and it sharply rises when the graphene's Dirac point is shifted toward the TMDC band. The theoretical method does not need the exact lattice matching and it is applicable to any incommensurate bilayer systems. It is useful for the twist-angle engineering of a variety of van der Waals proximity effects.

The spinel ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ undergoes a transition into a dimerized state at low temperatures that is expected to be a spin singlet. However, no signature of a singlet-triplet transition has been discovered, in part due to the difficulty of predicting the energy of the transition from theory. We find that the dimers of ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ can be described by a Heisenberg model with very small interactions between different dimers. Using high-accuracy first-principles quantum Monte Carlo combined with a model-fitting approach, we predict the singlet-triplet gap of these dimers to be 350(50) meV, a higher energy than previous experimental observations have considered. The prediction is published in advance of experimental confirmation.

We study the effects of bond randomness in the spin-$1/2$ $J_1-J_2$ triangular Heisenberg model using exact diagonalization and density-matrix renormalization group. With increasing bond randomness, we identify a randomness-induced spin-liquid-like phase without any magnetic order, dimer order, spin-glass order, or valence-bond-glass order. The finite-size scaling of gaps suggests the gapless nature of both spin triplet and singlet excitations, which is further supported by the broad continuum of the dynamical spin structure factor. By studying the bipartite entanglement spectrum of the system on cylinder geometry, we identify the features of the low-lying entanglement spectrum in the spin-liquid-like phase, which may distinguish this randomness-induced spin-liquid-like phase and the intrinsic spin-liquid phase in the clean $J_1-J_2$ triangular Heisenberg model. We further discuss the nature of this spin-liquid-like phase and the indication of our results for understanding spin-liquid-like materials with triangular-lattice structure.

The heavy-fermion antiferromagnet ${\mathrm{CeIn}}_3$ exhibits a field-induced anomaly associated with Fermi surface reconstruction at $H^{*}\sim 45$ T, well below the antiferromagnetic (AFM) critical field that exceeds 60 T. In order to explore the origin of this anomaly, we have measured ${}^{115}\mathrm{In}$ NMR spectra in pulsed magnetic fields up to 56 T using high-quality single crystals. To within experimental resolution, we do not detect any change in the NMR spectra that could suggest a modification in either the character of the hyperfine field or the electric field gradient at In sites through $H^{*}$. This strongly suggests that the 45 T anomaly cannot be simply ascribed to a field-induced change in the magnetic structure of the AFM state. Neither do we find any evidence of changes in the crystallographic structure or in the distribution of charge density at $H^{*}$. The field dependence of the NMR shift implies the existence of a field-induced anomaly in the hyperfine coupling above $H^{*}$.

We report on a strong nonlinear behavior of the photogalvanics and photoconductivity under excitation of HgTe quantum wells (QWs) by intense terahertz (THz) radiation. The increasing radiation intensity causes an inversion of the sign of the photocurrent and transition to its superlinear dependence on the intensity. The photoconductivity also shows a superlinear raise with the intensity. We show that the observed photoresponse nonlinearities are caused by the band-to-band light impact ionization under conditions of a photon energy less than the forbidden gap. The signature of this kind of impact ionization is that the angular radiation frequency $\omega =2\pi f$ is much higher than the reciprocal momentum relaxation time. Thus the impact ionization takes place solely because of collisions in the presence of a high-frequency electric field. The effect has been measured on narrow HgTe/CdTe QWs of 5.7 nm width; the nonlinearity is detected for linearly and circularly polarized THz radiation with different frequencies ranging from $f=0.6$ to 1.07 THz and intensities up to hundreds of kW/${\mathrm{cm}}^2$. We demonstrate that the probability of the impact ionization is proportional to the exponential function, $exp(-E_0^2/E^2)$, of the radiation electric field amplitude $E$ and the characteristic field parameter $E_0$. The effect is observable in a wide temperature range from 4.2 to 90 K, with the characteristic field increasing with rising temperature.

Weyl points, i.e., peculiar nodal degenerate points in three-dimensional reciprocal space, carry topological charges, therefore giving rise to many intriguing phenomena including topologically protected surface states and chiral anomaly. More importantly, the concept of the Weyl point can be applied in systems with reduced geometrical dimensionality by means of synthetic dimension. Such generalized synthetic Weyl points therefore provide a versatile platform to explore the Weyl-point physics in one or two-dimensional structures. In this work, we study the synthetic Weyl points in one-dimensional plasmonic-dielectric crystals with broken inversion symmetry and further demonstrate that the synthetic Weyl points can ensure the emergence of topological interface states. Instead of using the Zak phase, the emergence of a Weyl point in synthetic space is helpful to understand the underlying physics and the topological properties of the interface states in one-dimensional crystals with broken inversion symmetry. Due to the opposite sign of the surface impedance for plasmonic and dielectric cover medium, the surface states demonstrate distinct properties and locate in separated parameter space regions. Our results show that the synthetic Weyl point provides a potential approach for studying novel electromagnetic modes and facilitates the formation of topologically protected interface states in systems with reduced geometrical dimensions.

The adsorption of O and H atoms on the ${\mathrm{Fe}}_3{\mathrm{O}}_4$(111) surface was investigated using ultraviolet photoemission spectroscopy (UPS) and first-principles calculations. On the FeA1-terminated surface, H adsorption decreases the work function without changing the density of states near the Fermi level. The density of states near the Fermi level decreases by O adsorption. On the O/FeA1-terminated surface, H adsorption dramatically increases the density of states near the Fermi level, indicating that the Fe $3d$ state is modified. The first-principles calculations showed that the electronic structure of the surface FeA on the FeA1-terminated surface is half metallic with its spin up forming an isolated spin-polarized conductive layer. By O adsorption, the up-spin band at the Fermi level of the surface FeA site is removed, and the Fermi level of the surface FeA moves to the down-spin $t_{2g}$ band. By subsequent H adsorption, electrons are doped to the FeA1 site, and the surface FeA1 layer becomes semiconducting. These results indicate that the charge and spin structure is modulated by O and H adsorption.

We theoretically study the electronic band structure and the Hall effect in the negatively-curved three-dimensional (3D) graphene network in magnetic fields. We found that special energy regions appear above and below the zero-energy Landau level, where the 3D Hall conductivity is quantized even though the spectrum is not fully gapped. These regions are dominated by the chiral snake states traveling along the zero magnetic field contour on the 3D surface, and the quantization of the Hall conductivity crucially depends on the topology of the contour. The exclusive energy region for the chiral states is found to be a universal feature of 3D graphene systems.

We study the localization transitions for coupled one-dimensional lattices with quasiperiodic potential. In addition to the localized and extended phases, there is an intermediate mixed phase that can be easily explained decoupling the system so as to deal with effective uncoupled Aubry-André chains with different transition points. We clarify, therefore, the origin of such an intermediate phase, finding the conditions for getting a uniquely defined mobility edge for such coupled systems. Finally, we consider many coupled chains with an energy shift that compose an extension of the Aubry-André model in two dimensions. We study the localization behavior in this case comparing the results with those obtained for a truly aperiodic two-dimensional (2D) Aubry-André model, with quasiperiodic potentials in any directions, and the 2D Anderson model.

We develop a theoretical framework based on the electron-phonon coupled Boltzmann transport equations (BTEs) for the interpretation of nondiffusive thermal conductivity measurements in metals made via frequency domain thermoreflectance (FDTR). The thermal conductivity of a bulk gold crystal was measured over a temperature range of 23–304 K as a function of FDTR's laser spot size. Our interpretation of these measurements by a two-temperature heat diffusion model finds that the thermal conductivity is suppressed when the laser spot size is comparable to electron mean free paths. Using a simplified spherical geometry that enables analytical solutions, we compare the two-temperature diffusion model with the coupled BTEs to identify a thermal conductivity suppression function. We also conclude that over the timescales of our experiment electron-phonon nonequilibrium is negligible, but that the length scale of heat deposition by hot electrons critically influences our interpretation.

The interfacial Dzyaloshinskii-Moriya interaction (DMI) plays a crucial role in chiral domain wall (DW) motion, favoring fast DW velocities. We explore the effect of interface disorder on DMI and DW dynamics in perpendicular magnetized Ta/CoFeB/MgO thin films. Light ${\mathrm{He}}^{+}$ irradiation has been used to gently engineer interface intermixing on a scale of 0.1 nm. We demonstrate that a slight modification of the Ta/CoFeB interface leads to an increase of the DMI value accompanied by an enhancement of DW velocity in the flow regime. Using micromagnetic simulations based on granular structures, we show that the enhancement of DW velocity is mainly related to an increase in the distribution of magnetic parameters related to the interface. We further infer that the DMI modulation is related to the asymmetric disorder induced by irradiation leading to alloying with the Ta buffer layer. Understanding the role of disorder is therefore crucial for the design of future devices where post-growth interface alloying can be used to finely tune the DMI.

We have used exact numerical diagonalization to study the excitation spectrum and the dynamic spin correlations in the $s=1/2$ next-next-nearest-neighbor Heisenberg antiferromagnet on the square lattice, with additional four-spin ring exchange from higher-order terms in the Hubbard expansion. We have varied the ratio between Hubbard model parameters $t/U$ to obtain different relative strengths of the exchange parameters, while keeping electrons localized. The Hubbard model parameters have been parametrized via an effective ring exchange coupling $J_r$, which have been varied between $0J$ and 1.$5J$. We find that ring exchange induces a quantum phase transition from a $(\pi ,\pi )$ ordered state to a $(\pi /2,\pi /2)$ ordered state. This quantum critical point is reduced by quantum fluctuations from its mean-field value of $J_r/J=2$ to a value of $\sim 1.1$. At the quantum critical point, the dynamical correlation function shows a pseudocontinuum at $q$ values between the two competing ordering vectors.

We show how a charge current through a single antiferromagnetic layer can excite and control self-oscillations. Sustained oscillations with tunable amplitudes and frequencies are possible in a variety of geometries using certain classes of noncentrosymmetric materials that exhibit finite dissipative spin-orbit torque. We compute the steady-state phase diagram as a function of the current and spin-orbit torque magnitude. The anisotropic magnetoresistance causes the conversion of the resulting AF oscillations to a terahertz AC output voltage. These findings provide an attractive and novel route to design terahertz antiferromagnetic spin-orbit torque oscillators in simple single-layer structures.

We propose a methodology for crystal structure prediction that is based on the evolutionary algorithm USPEX and the machine-learning interatomic potentials actively learning on-the-fly. Our methodology allows for an automated construction of an interatomic interaction model from scratch, replacing the expensive density functional theory (DFT) and giving a speedup of several orders of magnitude. Predicted low-energy structures are then tested on DFT, ensuring that our machine-learning model does not introduce any prediction error. We tested our methodology on prediction of crystal structures of carbon, high-pressure phases of sodium, and boron allotropes, including those that have more than 100 atoms in the primitive cell. All the the main allotropes have been reproduced, and a hitherto unknown 54-atom structure of boron has been predicted with very modest computational effort.

Electron backscatter diffraction (EBSD) is a technique to obtain microcrystallographic information from materials by collecting large-angle Kikuchi patterns in the scanning electron microscope (SEM). An important fundamental question concerns the scattering-angle dependent electron energy distribution, which is relevant for the formation of the Kikuchi diffraction patterns. Here we review the existing experimental data and explore the effective energy spectrum that is operative in the generation of backscatter Kikuchi patterns from silicon. We use a full pattern comparison of experimental data with dynamical electron diffraction simulations. Our energy-dependent cross-correlation based pattern matching approach establishes improved constraints on the effective Kikuchi pattern energy spectrum, which is relevant for high-resolution EBSD pattern simulations and their applications.

Topological metamaterials offer new routes for control of waves, which are widely realized by Bragg scattering and/or local resonance. Understanding of topological transition by the interaction between these mechanisms is strongly desired to extend the design degrees of freedom for intriguing wave phenomena. Here, we demonstrate a phononic metamaterial consisting of C-shaped elements that enables us to investigate interaction between Bragg scattering and local resonance. We show that by adding resonance scattering a topological band gap is opened from a Bragg scattering based Dirac cone, and its band gap is controlled by the resonance frequency of the cavities relative to the Dirac cone frequency. In addition, we show that topological band-gap opening induced by the Bragg scattering can be reversed into an ordinary state or vice versa by judicious inclusion of the local resonance. Scattering cross-section analysis elaborates the combined effect of the two mechanisms on topological states. By employing lossy resonant elements, we further demonstrate a lossy topological insulator capable of one-way sound propagation immune to sharp corners, potentially leading to an energy-harvesting topological insulator. Our results provide a critical understanding of topological phenomena involving coupled scattering mechanisms.

Antimonene is a recently discovered two-dimensional semiconductor with exceptional environmental stability, high carrier mobility, and strong spin-orbit interactions. In combination with an electric field, the latter provides an additional degree of control over the material's properties because of induced spin splitting. Here, we report on a computational study of electron-phonon coupling and superconductivity in $n$- and $p$-doped antimonene, where we pay special attention on the effect of the perpendicular electric field. The range of accessible hole concentrations is significantly limited by the dynamical instability, associated with strong Fermi-surface nesting. At the same time, we find that in the case of electron-doping antimonene remains stable and can be turned into a state with strong electron-phonon coupling, with the mass enhancement factor $\lambda$ of up to 2.3 at realistic charge carrier concentrations. In this regime, antimonene is expected to be a superconductor with the critical temperature of $\approx 16$ K. Application of bias voltage leads to a considerable modification of the electronic structure, affecting the electron-phonon coupling in antimonene. While these effects are less obvious in the case of electron-doping, the field effect in hole-doped antimonene results in a considerable variation of the critical temperature, depending on bias voltage.