Anomalous diffusion in some bcc metals is the long-standing topic in material science. In this work, I obtain the temperature dependence of the self-diffusion coefficient in bcc titanium directly from molecular dynamics (MD) calculation. MD simulations indicate that both vacancies and self-interstitials contribute to diffusivity in bcc Ti. The resultant self-diffusion coefficient is non-Arrhenius, but shows less curvature than observed in most experiments.

Spin-torque nano-oscillators (STNO) are microwave auto-oscillators based on magnetic resonances having a nonlinear response with the oscillating amplitude, which provides them with a large frequency tunability including the possibility of mutual synchronization. The magnetization dynamics in STNO are induced by spin-transfer torque (STT) from spin currents and can be detected by changes in electrical resistance due to giant magnetoresistance or tunneling magnetoresistance. The STT effect is usually treated as a dampinglike term that reduces magnetic dissipation and promotes excitation of magnetic modes. However, an additional term, known as Zhang-Li term, has an effect on magnetization gradients such as domain walls, and could have an effect on localized magnetic modes in STNO. Here we study the effect of Zhang-Li torques in magnetic excitations produced in STNO with a nanocontact geometry. Using micromagnetic simulations we find that Zhang-Li torques modify threshold currents of magnetic modes and their effective sizes. Additionally, we show that effects can be controlled by changing the ratio between nanocontact size and layer thickness.

We report the hydrostatic pressure effect on the charge density wave (CDW) and superconductivity (SC) of single-crystal $2H\text{−}\mathrm{Ta}{\mathrm{Se}}_{2\text{−}x}{\mathrm{S}}_x$ by measuring the electrical resistivity and ac magnetic susceptibility under various pressures up to 12.5 GPa. Different from the previously reported phase diagrams of CDW and SC as a function of the Se/S substitution and/or the uniaxial pressure, the superconducting transition temperature ($T_{\mathrm{c}}$) was found to increase monotonously with increasing the pressure at first and then reaches a maximum constant above a critical pressure $P_{\mathrm{c}}$. The maximal ${T_{\mathrm{c}}}^{\max }$ is nearly two times higher than the optimal ones via the S doping and decreases with further increasing the S-doping level. High-pressure ac magnetic susceptibility demonstrates that the superconducting volume quickly increases along with the gradual suppression of CDW and reaches nearly 1 above $P_{\mathrm{c}}$, which provides clear evidences for pressure-induced crossover from CDW to bulk SC. High-pressure x-ray diffraction proves that there is no structural phase transition in $2H\text{−}\mathrm{Ta}{\mathrm{Se}}_2$ up to 20 GPa; although the dependence of $T_{\mathrm{c}}$ on the volume shows distinct behaviors at the initial S doping and the applied physical pressure, they both collapse into a universal curve upon the further volume shrinkage. This indicates that both physical pressure and the S-doping-induced chemical pressure can melt CDW and enhance SC through lattice contraction consistently. In comparison with the effects of hydrostatic pressure, the S doping can destruct the CDW ground state faster due to the presence of chemical disorders, which also restrict the further enhancement of $T_{\mathrm{c}}$ after the collapse of CDW.

We present and implement a parquet approximation within the dual-fermion formalism based on a partial bosonization of the dual vertex function which substantially reduces the computational cost of the calculation. The method relies on splitting the vertex exactly into single-boson exchange contributions and a residual four-fermion vertex, which physically embody, respectively, long- and short-range spatial correlations. After recasting the parquet equations in terms of the residual vertex, these are solved using the truncated-unity method of Eckhardt et al. [Phys. Rev. B 101, 155104 (2020)], which allows for a rapid convergence with the number of form factors in different regimes. While our numerical treatment of the parquet equations can be restricted to only a few Matsubara frequencies, reminiscent of Astretsov et al. [Phys. Rev. B 101, 075109 (2020)], the one- and two-particle spectral information is fully retained. In applications to the two-dimensional Hubbard model the method agrees quantitatively with a stochastic summation of diagrams over a wide range of parameters.

The characterization of helical states can be performed by checking the existence of the reentrant behavior, which appears as a dip in the conductance probed in nanowires (NWs) with strong spin-orbit coupling (SOC) and under a perpendicular magnetic field. Yet puzzling experimental results report the observation of the re-entrant behavior also in the absence of magnetic fields, ascribed to unconventional spin-flipping two-particle backscattering. We theoretically demonstrate that the observation of the conductance dip can be explained through a multichannel scattering mechanism, which causes a reduction of the transmission when an effective attractive potential and coupling between different channels are present. Both ingredients are provided by the SOC in the transport properties of NWs. The relative effect of the sharpness of interfaces and external fields has also been assessed. The reduction of symmetry constraints of the NW is analyzed and proved to be important in the tuning of the reentrant characteristic.

We propose and show that application of light leads to an intriguing platform for controlling exceptional points in non-Hermitian topological systems. We demonstrate our proposal using three different non-Hermitian systems—nodal line semimetals, semi-Dirac semimetals, and Dirac semimetals—and show that using illumination with light one can engineer the positions and stability of exceptional points. We illustrate the topological properties of these models and map out their light-driven topological phase transitions.

Recently, a layered ferroelectric $\mathrm{CuIn}{\mathrm{P}}_2{\mathrm{Se}}_6$ was shown to exhibit domain walls with locally enhanced piezoresponse—a striking departure from the observations of nominally zero piezoresponse in most ferroelectrics. Although it was proposed that such “bright” domain walls are phase boundaries between ferri- and antiferroelectrically ordered regions of the materials, the physical mechanisms behind the existence and response of these boundaries remain to be understood. Here, using Landau-Ginzburg-Devonshire phenomenology combined with a four sublattices model, we describe quantitatively the bright-contrast and dark-contrast domain boundaries between the antiferroelectric, ferroelectric, or ferrielectric long-range-ordered phases in a layered ferroelectric-antiferroelectric ferroics, such as $\mathrm{CuIn}{\mathrm{P}}_2{\left({\mathrm{S}}_{1-y}{\mathrm{Se}}_y\right)}_6$.

We present an investigation of the effect of randomizing exchange coupling strengths in the $S=1/2$ square lattice quasi-two-dimensional quantum Heisenberg antiferromagnet (QHAF) ${\left(\mathrm{QuinH}\right)}_2\mathrm{Cu}{\left({\mathrm{Cl}}_x{\mathrm{Br}}_{1-x}\right)}_4·2{\mathrm{H}}_2\mathrm{O}$ (QuinH $=$ Quinolinium, ${\mathrm{C}}_9{\mathrm{H}}_8{\mathrm{N}}^{+}$), with $0\le x\le 1$. Pulsed-field magnetization measurements allow us to estimate an effective in-plane exchange strength $J$ in a regime where exchange fosters short-range order, while the temperature $T_{\mathrm{N}}$ at which long-range order (LRO) occurs is found using muon-spin relaxation, allowing us to construct a phase diagram for the series. We evaluate the effectiveness of disorder in suppressing $T_{\mathrm{N}}$ and the ordered moment size, and we find an extended disordered phase in the region $0.4\lesssim x\lesssim 0.8$ where no magnetic order occurs. The observed critical substitution levels are accounted for by an energetics-based competition between different local magnetic orders. Furthermore, we demonstrate experimentally that the ground-state disorder is driven by quantum effects of the exchange randomness, which is a feature that has been predicted theoretically and has implications for other disordered quasi-two-dimensional QHAFs.

Atomic orbitals play fundamental roles in the modern theory of magnetism, not only providing local moments via their partial occupation, but also offering exchange interactions through their direct or indirect hybridization. Here, we report atomic-orbital-free intrinsic ferromagnetism in monolayer electrides or electrenes, in which excess electrons act as anions. Taking the honeycomb ${\mathrm{LaBr}}_2$ (${\mathrm{La}}^{3+}{\mathrm{Br}}_2{}^{-}·e^{-}$) as an example, our first-principles calculations, in combination with a low-energy effective model in the basis of the Wannier function and Anderson's superexchange theory, reveal that the excess electron is localized at the center of the hexagon, which leads to the spontaneous formation of a local moment due to the strong Stoner instability of the associated state near the Fermi level ($E_f$). The large off-site Coulomb interaction and extended tails of both wave and Wannier functions indicate a significant spatial extension of the anionic electron state in ${\mathrm{LaBr}}_2$. The overlap of the long tails mediates an extended ferromagnetic direct exchange to second-nearest neighbors (up to 7–8 Å), in sharp contrast to the conventional direct exchange which is short ranged due to the overlap of atomic orbitals with limited spatial extension. The dual nature, localization and extension, of the anionic electron state results in a unique magnetic mechanism in such atomic-orbital-free intrinsic two-dimensional ferromagnets.

In this paper, we explore the properties of dynamical diffraction coefficients in orbital angular momentum resolved electron energy-loss magnetic chiral dichroism spectra, in a scanning transmission electron microscopy setup. We demonstrate that for basic zone axis geometries with fourfold or threefold symmetry the coefficients are constrained to have simplified forms. By exploiting these properties, we show how a dichroism spectrum accessible using this technique is only weakly dependent on sample thickness and, more generally, on dynamical diffraction effects. Our results indicate that in such cases it is possible to determine the orbital and spin components of atomic magnetic moments approximately from experimental spectra without the need for additional dynamical diffraction calculations.

Characterizing the scaling with the total particle number ($N$) of the largest eigenvalue of the one-body density matrix (${\lambda }_0$) provides information on the occurrence of the off-diagonal long-range order (ODLRO) according to the Penrose-Onsager criterion. Setting ${\lambda }_0\sim N^{{\mathcal{C}}_0}$, then ${\mathcal{C}}_0=1$ corresponds in ODLRO. The intermediate case, $0<{\mathcal{C}}_0<1$, corresponds in translational invariant systems to the power-law decaying of (nonconnected) correlation functions and it can be seen as identifying quasi-long-range order. The goal of the present paper is to characterize the ODLRO properties encoded in ${\mathcal{C}}_0$ (and in the corresponding quantities ${\mathcal{C}}_{k\ne 0}$ for excited natural orbitals) exhibited by homogeneous interacting bosonic systems at finite temperature for different dimensions in presence of short-range repulsive potentials. We show that ${\mathcal{C}}_{k\ne 0}=0$ in the thermodynamic limit. In one dimension it is ${\mathcal{C}}_0=0$ for nonvanishing temperature, while in three dimensions it is ${\mathcal{C}}_0=1$ (${\mathcal{C}}_0=0$) for temperatures smaller (larger) than the Bose-Einstein critical temperature. We then focus our attention to $D=2$, studying the $XY$ and the Villain models, and the weakly interacting Bose gas. The universal value of ${\mathcal{C}}_0$ near the Berezinskii-Kosterlitz-Thouless temperature $T_{\text{BKT}}$ is $7/8$. The dependence of ${\mathcal{C}}_0$ on temperatures between $T=0$ (at which ${\mathcal{C}}_0=1$) and $T_{\text{BKT}}$ is studied in the different models. An estimate for the (nonperturbative) parameter $\xi$ entering the equation of state of the two-dimensional Bose gases is obtained using low-temperature expansions and compared with the Monte Carlo result. We finally discuss a “double jump” behavior for ${\mathcal{C}}_0$, and correspondingly of the anomalous dimension $\eta$, right below $T_{\text{BKT}}$ in the limit of vanishing interactions.

Type-II Weyl semimetals are dictated by bulk excitations with tilted light cones, resembling the inside of black holes. We obtain generic boundary conditions for surface boundaries of the type-II Weyl semimetals near Weyl nodes, and show that for a certain boundary condition edge states can escape out of the “black hole” event horizon. This means that for realization of the material “black hole” by the type-II Weyl semimetals, a careful choice of the boundary condition is necessary.

An efficient modulation of backscattered energy is one of the key requirements for enabling efficient wireless communication channels. Typical architectures, based on either electronically or mechanically modulated reflectors, cannot be downscaled to subwavelength dimensions by design. Here we show that integrating high-index dielectric materials with tunable subwavelength resonators allows one to achieve an efficient backscattering modulation, keeping a footprint of an entire structure small. An interference between high-order Mie resonances leads to either enhancement or suppression of the backscattering, depending on a control parameter. In particular, a ceramic core shell, driven by an electronically tunable split-ring resonator, was shown to provide a backscattering modulation depth as high as tens of the geometrical cross section of the structure. The design was optimized toward maximizing the reading range of radio-frequency identification tags and shown to outperform existing commercial solutions by orders of magnitude in terms of the modulation efficiency. The proposed concept of multipole engineering allows one to design miniature beacons and modulators for wireless communication needs and other relevant applications.

Using the generalized gradient approximation plus dynamical mean-field theory ($\mathrm{GGA}+\mathrm{DMFT}$) we confirm the importance of multi-orbital dynamical correlations in determining the paramagnetic insulating state of $\mathrm{Cr}{\mathrm{I}}_3$. While the ferromagnetic phase reveals weak electronic correlation effects due to strong spin-orbital polarization, the Mott insulating state of paramagnetic $\mathrm{Cr}{\mathrm{I}}_3$ crystal is shown to be driven by the interplay between orbital-dependent one-electron lineshape and multi-orbital electronic interactions. To probe the paramagnetic Mott insulating state we performed x-ray absorption spectroscopy (XAS) measurements for the two structural phases of $\mathrm{Cr}{\mathrm{I}}_3$. Our study is relevant to understanding the orbital-selective electronic structure reconstruction of Mott insulators and should be applicable to other van der Waals bonded materials from bulk to the ultrathin limit.

We investigate the lifetime and luminescence quantum efficiency of excitons in extremely high-purity diamond grown by the chemical vapor deposition method. A clear correlation is found between the diffusion coefficient, lifetime, and quantum efficiency. The effect of small strain (magnitude $\approx {10}^{-5}$) assessed using birefringence is also clarified thanks to the effective absence of impurity traps. A surface recombination model, extended for nonradiative recombination at dislocations due to diffusion, is proposed to explain the temperature dependence from 4 to 300 K, which facilitates the derivation of the radiative lifetime. The proposed model is generally applicable to a mixture of excitons and carriers in a wide range of materials with high exciton binding energies.

We derive the Casimir-Polder force and Casimir torque expressions for an anisotropic molecule close to a conducting plane with a tensorial conductivity. We apply our general expressions to the case of a carbon dioxide ${\mathrm{CO}}_2$ molecule close to a plane with pure Hall conductivity and to graphene. We show that the equilibrium position of this linear molecule is with its main axis perpendicular to the surface. We hence conjecture a possible way to exploit the Casimir torque to mechanically improve the performance of ${\mathrm{CO}}_2$ separation membranes useful for an efficient atmospheric ${\mathrm{CO}}_2$ reduction.

We study disorder induced topological phase transitions in magnetically doped ${(\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ thin films by using large scale transport simulations of the conductance through a disordered region coupled to reservoirs in the quantum spin Hall regime. Besides the disorder strength, the rich phase diagram also strongly depends on the magnetic exchange field, the Fermi level, and the initial topological state in the undoped and clean limit of the films. In an initially trivial system at nonzero exchange field, varying the disorder strength can induce a sequence of transitions from a normal insulating to a quantum anomalous Hall, then a spin-Chern insulating, and finally an Anderson insulating state. In contrast, for a system which is initially in the topological phase, a similar sequence can be induced by the disorder, but only starting from the quantum anomalous Hall phase that is also stabilized by the weak disorder. Varying the Fermi level we find a similarly rich phase diagram, including transitions from the quantum anomalous Hall to the spin-Chern insulating state via a state that behaves as a mixture of a quantum anomalous Hall and a metallic state, akin to recent experimental reports.