Force-based algorithms for ab initio atomic structure relaxation, such as conjugate gradient methods, usually get stuck in the line minimization processes along search directions, where expensive ab initio calculations are triggered frequently to test trial positions before locating the next iterate. We present a force-based gradient descent method, WANBB, that circumvents the deficiency. At each iteration, WANBB enters the line minimization process with a trial step size capturing the local curvature of the energy surface. The exit is controlled by a nonrestrictive criterion that tends to accept early trials. These two ingredients streamline the line minimization process in WANBB. The numerical simulations on nearly 80 systems with good universality demonstrate the considerable compression of WANBB on the cost for the unaccepted trials compared with conjugate gradient methods. We also observe across the board significant and universal speedups as well as the superior robustness of WANBB over several widely used methods. The latter point is theoretically established. The implementation of WANBB is pretty simple, in that no a priori physical knowledge is required and only three parameters are present without tuning.

There is still a lack of fundamental understanding of the structural properties and thermal conductivity of few-layer SnSe, although it is the current recording material for thermoelectric conversion efficiency. From first-principles molecular dynamics and lattice dynamics, the electronic structure, structural properties, and lattice vibrational properties of few-layer SnSe are thus investigated in this study. The results show that SnSe exhibits semiconducting characteristics with the optical absorption mainly concentrated in the visible and near-ultraviolet regions. The band gap varies from 0.90 to 1.35 eV as the number of atomic layers decreases from bulk to single layer (1L). Four-layer (4L) SnSe shows typical bulk characteristics in terms of phase stability and electronic and vibrational properties. On further reducing the number of atomic layers, the quantum confinement effect becomes significant. At room temperature, the Pnma structural symmetry of 4L SnSe is dominated by the Jahn-Teller effect, consistent with the bulk phase. The second-order phase transition from Pnma to Cmcm driven by the phonon anharmonic effect occurs in 1L SnSe. A competitive picture of these two effects can be observed in 3L and 2L SnSe. At the enhancement temperature, the distribution peak of the phonon density of states between 3.5 and 4.5 THz shows a significant redshift relative to the harmonic case, exhibiting strong phonon anharmonicity. Correspondingly, the Raman-active normal modes $B_{3g}^1$ and $A_g^2$, which contribute to the peak, present significant frequency softening, consistent with the experimental observations. The reduction in the number of atomic layers will aggravate the frequency softening of these modes. Due to the strong phonon anharmonicity and dynamic instability of the Pnma phase, the lattice thermal conductivity of few-layer SnSe at room temperature is significantly lower than that of the bulk phase.

Multiferroic CuO showing two distinct antiferromagnetic (AFM) phases: incommensurate (ICM) spiral and commensurate (CM) collinear AFM phases, is investigated by resonant x-ray diffraction (RXD) at the Cu $L_3$ edge. In the ICM phase where spin-spiral-induced ferroelectricity develops, circular dichroism ascribed to the handedness of the spiral magnetic order is observed at the magnetic Bragg (0.506, 0, −0.483) reflection. By measuring two-dimensional maps of the reflection intensity, the spatial distribution of the ferroelectric domains is clarified and is found to be strongly influenced by crystallographic twin structures. In the CM phase, weaker but substantial circular dichroic signals are observed at the magnetic Bragg (0.5, 0, −0.5) reflection. It is found that the spatial distributions of the circular dichroic signals resemble those in the ICM phase, suggesting some relation between the ICM and CM phases. The origin of the circular dichroic RXD observed in CuO is discussed based on the experimental observations.

We study the phase diagram of the ${\nu }_e=7/3$ state in the $N=1$ Landau level in the presence of band mass anisotropy. Using the density matrix renormalization group on an infinite cylinder geometry, we find a continuous transition from the topologically ordered Laughlin fractional quantum Hall state to a stripe phase with a period of approximately five and a half magnetic lengths. The transition is driven by the condensation of the magnetoroton mode which becomes gapless at the critical point. We interpret the transition within the composite-boson theory as the onset of stripe order in a superfluid background, resulting from the roton mode going soft.

Nieh-Yan anomaly describes the nonconservation of chiral charges induced by the coupling between Dirac fermions and torsion fields. Since the torsion field is beyond general relativity, this effect remains hypothetical and its relevance to our universe is unclear in the context of high-energy physics. In this work, we propose that the phonons can induce a torsion field for the Kramers-Weyl fermions through electron-phonon interaction in a nonmagnetic chiral crystal, thus leading to the occurrence of the Nieh-Yan anomaly. As a consequence, the Nieh-Yan term can strongly influence the phonon dynamics and lead to the helicity of acoustic phonons, namely, two transverse phonon modes mix with each other to form a circular polarization with a nonzero angular momentum and the phonon angular momentum forms a hedgehog texture in the momentum space. The phonon helicity can be probed through measuring the total phonon angular momentum driven by a temperature gradient.

While the spatially nonhomogeneous light emission from synthetic ${\mathrm{WS}}_2$ monolayers is frequently reported in the literature, the nature of this phenomenon still requires thoughtful investigation. Here, we combine several characterization techniques (optical imaging, scanning probe and electron microscopy) along with density functional theory to investigate the presence of substitutional doping localized at narrow regions along the S zigzag edge of ${\mathrm{WS}}_2$ monolayers. We verified that photoluminescence quenching along narrow regions is not related to grain boundaries but to substitutional impurities of lighter metals at the W sites, which modify the radiative and nonradiative decay channels. We also found potential candidates for occupying the W site through ADF-STEM analysis and discussed their impact on photoluminescence quenching by performing density functional theory calculations. Our findings shed light on how atomic defects introduced during ${\mathrm{WS}}_2$ monolayer's synthesis impact the crystalline quality and, therefore, the development of high-performance optoelectronic devices based on semiconducting 2D materials.

For modulation in free space using the linear electro-optic effect, a narrow-linewidth resonance is favorable when a minimal voltage is preferred to fully shift the resonance. Here a method is proposed to engineer the quasibound states in a slab waveguide consisting of electro-optic polymer and metal gratings, in order to achieve narrow-linewidth resonances for electro-optic modulation. This quasibound state features a strong out-of-plane electric field component located at the position of the metal strips constituting the grating, which also work as the interdigitated electrodes, and thus, the optical mode can interact with the electrostatic field efficiently. The radiation damping of the quasibound state can be controlled by changing the relative position of the neighboring metal strips, while the optical mode can maintain the same periodicity as the electrostatic field. This method also offers prospects for engineering narrow-linewidth resonances for other applications.

A quantum dot (QD) system provides various quantum physics of nanostructures. So far, many types of semiconductor QD structures have been fabricated and investigated experimentally and analyzed theoretically. Presently, QD systems have attracted considerable attention as units for the qubit system of quantum computers. Therefore, it is vital to integrate QD systems as measurement devices in addition to qubits. Here, we theoretically investigate the side-QD system as a measurement apparatus for energy levels of the target QDs. We formulate the transport properties of both three and five QDs based on the Green's function method. The effects of the energy difference of two side-QDs on the measurement current are calculated. The tradeoff between the strength of the measurement and the back-action induced by the measurement is discussed. It is found that the medium coupling strength of the three QDs is appropriate for reading out the difference of the two energy levels.

Despite intense experimental efforts, the nature of the unconventional superconducting order parameter of ${\mathrm{UTe}}_2$ remains elusive. This puzzle stems from reports of either a single or a double superconducting transition at ambient pressure as well as a complex pressure-temperature phase diagram. To address this issue, we measured the heat capacity and electrical resistivity of ${\mathrm{UTe}}_2$ under compressive uniaxial stress $\sigma$ applied along different crystallographic directions. We find that the critical temperature $T_{\mathrm{c}}$ of the single observed bulk superconducting transition decreases with $\sigma$ along [100] and [110] but increases with $\sigma$ along [001]. Aside from its effect on $T_{\mathrm{c}}$, $c$-axis stress leads to a significant piezoresistivity. Importantly, an in-plane shear stress ${\sigma }_{xy}$ does not induce any observable splitting of the superconducting transition over a stress range of ${\sigma }_{xy}\approx 0.17\mathrm{GPa}$. This result suggests that the superconducting order parameter of ${\mathrm{UTe}}_2$ may be single component at ambient pressure.

Room-temperature phonon dynamics in crystals of the high-$T_c$ superconductor ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ were probed using Brillouin light scattering spectroscopy. Six distinct bulk acoustic modes were observed and identified: two quasilongitudinal bulk modes and four quasitransverse bulk modes. A peak at a frequency shift of $\sim$95 GHz with behavior reminiscent of an optic phonon was also observed in the spectra. The existence and nature of these modes is a manifestation of the incommensurate structure of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ and suggests that it may be categorized as a so-called composite incommensurate crystal comprised of two weakly interacting sublattices. A mass ratio of $m_1/m_2=2.8$ obtained from the two measured quasilongitudinal acoustic velocities led to sublattice assignments of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{O}}_4$ and ${\mathrm{CaCu}}_2{\mathrm{O}}_4$. Two surface acoustic modes were also observed in the spectra, the number and character of which suggest that the incommensurate structure in the near-surface region is disrupted due to microscopic-scale damage and/or the presence of defects. The Brillouin data also place an upper limit of $\sim$10 GHz on the crossover frequency between commensurate and incommensurate phonon dynamics.

We report a {001} planar defect found in SiGe nanopillars. The defect structure, determined by atomic-resolution electron microscopy, matches the Humble defect model proposed for diamond. We also investigate several possible variants of the Humble structure using first-principles calculations and find that the one lowest in energy is in agreement with the scanning transmission electron microscope images. The pillar composition has been analyzed with electron energy loss spectroscopy, which hints at how the defect is formed. Our results show that the structure and formation process of defects in nanostructured group IV semiconductors can be different from their bulk counterparts.

Observing critical phases in lattice models is challenging due to the need to analyze the finite time or size scaling of observables. We study how the computational topology technique of persistent homology can be used to characterize phases of a generalized Aubry-André-Harper model. The persistent entropy and mean squared lifetime of features obtained using persistent homology behave similarly to conventional measures (Shannon entropy and inverse participation ratio) and can distinguish localized, extended, and critical phases. However, we find that the persistent entropy also clearly distinguishes ordered from disordered regimes of the model. The persistent homology approach can be applied to both the energy eigenstates and the wave packet propagation dynamics.

We study the thermalization of a small $XX$ chain coupled to long, gapped $XXZ$ leads at either side by observing the relaxation dynamics of the whole system. Using extensive tensor network simulations, we show that such systems, although not integrable, appear to show either extremely slow thermalization or even lack thereof since the two cannot be distinguished within the accuracy of our numerics. We show that the persistent oscillations observed in the spin current in the middle of the $XX$ chain are related to eigenstates of the entire system located within the gap of the boundary chains. We find from exact diagonalization that some of these states remain strictly localized within the $XX$ chain and do not hybridize with the rest of the system. The frequencies of the persistent oscillations determined by numerical simulations of dynamics match the energy differences between these states exactly. This has important implications for open systems, where the strongly interacting leads are often assumed to thermalize the central system. Our results suggest that, if we employ gapped systems for the leads, this assumption does not hold.

The explorations of non-Hermiticity have been devoted to investigate the disorder-induced many-body localization (MBL). However, the sensitivity of the spatial boundary conditions and the interplay of the non-Hermitian skin effect with many-body phenomena are not yet clear. For a MBL system in the presence of nonreciprocal tunnelings and random disorder potential, we identify two different complex-real spectral transitions, one is present for both open and periodic boundaries while the other is present only for open boundaries of coupled non-Hermitian chains. The later is driven due to the interchain coupling at weak disorder where the level statistics of the real eigenenergy phase follows a Gaussian orthogonal ensemble. We further characterize wave functions through the (biorthogonal) inverse participation ratio and fractal dimension, which reveal the suppression of skin effect in the non-Hermitian MBL phase. Finally, we demonstrate that the quench dynamics of the local particle density, spin imbalance, and entanglement entropy also signify the hallmark of the boundary effects and nonergodic character of many-body localization.

The impact of ordered vacancies on the dynamic stability of perovskites is investigated under the ab initio framework with a focus on cubic ${\mathrm{BaFeO}}_3$ ($Pm\overline{3}m$) and vacancy-ordered monoclinic ${\mathrm{BaFeO}}_{2.67}$ ($P{2}_1/m$). The harmonic approximation shows that both structures are dynamically unstable at 0 K. For the monoclinic structure, the instability is related to rotational distortions of the Fe coordination tetrahedra near the ordered vacancies. Ab initio molecular dynamics simulations in combination with the introduced structural descriptor demonstrate that both structures are stabilized above 130 K. Our results suggest that the ordered vacancies do not significantly alter the critical temperature at which Ba-Fe-O perovskites are dynamically stabilized. Furthermore, strong anharmonicity for the vacancy-ordered structure above its critical temperature is revealed by a significant asymmetry of the trajectories of O anions near the ordered vacancies.

We present a combination of density functional theory and dynamical mean-field theory (DMFT) for computing the electron transmission through two-terminal nanoscale devices. The method is then applied to metallic junctions presenting alternating Cu and Co layers, which exhibit spin-dependent charge transport and the giant magnetoresistance (GMR) effect. The calculations show that the coherent transmission through the $3d$ states is greatly suppressed by electron correlations. This is mainly due to the finite lifetime induced by the electron-electron interaction and is directly related to the imaginary part of the computed many-body DMFT self-energy. At the Fermi energy, where in accordance with the Fermi-liquid behavior the imaginary part of the self-energy vanishes, the suppression of the transmission is entirely due to the shifts of the energy spectrum induced by electron correlations. Based on our results, we finally suggest that the GMR measured in Cu/Co heterostructures for electrons with energies about 1 eV above the Fermi energy is a manifestation of dynamical correlation effects.

The materials search for Kitaev quantum spin liquids led to the discovery of many honeycomb lattice materials. Much attention has been paid to materials without magnetic order down to the lowest temperatures. The newly synthesized Na-deficient ${\mathrm{Na}}_x{\mathrm{IrO}}_3$ has been found to bear no sign of long-range magnetic order above 1 K from physical property measurements. In this paper, we report momentum- and energy-resolved excitation spectra in Na-deficient ${\mathrm{Na}}_x{\mathrm{IrO}}_3$ measured using a resonant inelastic x-ray scattering spectrometer. Orbital excitation spectra show that the octahedral and trigonal crystal field splittings are larger in ${\mathrm{Na}}_x{\mathrm{IrO}}_3$ than in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. On the other hand, the low-energy spectrum at low temperature shows a wave-vector dispersion and a spectral weight distribution that are similar to those of ${\mathrm{Na}}_2{\mathrm{IrO}}_3$, revealing that the two-dimensional zigzag magnetic correlations in ${\mathrm{Na}}_x{\mathrm{IrO}}_3$ are similar to those in ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ in terms of the ordered magnetic moment direction and three dynamically fluctuating zigzag orders. The azimuth angle dependence of the low-energy spectrum corroborates these results. The two-dimensional zigzag magnetic correlations rapidly weaken until 50 K. At high temperatures, the spectral weight distribution of the low-energy excitation resembles that of the pure Kitaev model, indicating that the Kitaev interaction dominates the dynamic magnetic response at high temperature. We suggest that the larger crystal field and distortion and the weakened longer-range Heisenberg exchange interactions due to the Na deficiency in ${\mathrm{Na}}_x{\mathrm{IrO}}_3$ contribute to bring ${\mathrm{Na}}_x{\mathrm{IrO}}_3$ away from the zigzag long-range magnetic order phase.

In this paper, we demonstrate the emergence and disappearance of topological zero modes (TZMs) in a coupled topolectrical (TE) circuit lattice. Specifically, we consider non-Hermitian TE chains in which TZMs do not occur in the individual uncoupled chains, but emerge when these chains are coupled by interchain capacitors. The coupled system hosts TZMs which show size-dependent behaviors and vanish beyond a certain critical size. In addition, the emergence or disappearance of the TZMs in the open boundary condition spectra for a given size of the coupled system can be controlled by modulating the signs of its inverse decay length. Analytically, trivial and nontrivial phases of the coupled system can be distinguished by the differing ranks of their corresponding Laplacian matrix. The TE circuit framework enables the physical detection of the TZMs via electrical impedance measurements. Our work establishes the conditions for inducing TZMs and modulating their behavior in coupled TE chains.

A systematic analysis of phonon-plasmon coupled excitations in three-dimensional (3D) polar systems is provided through the prism of both raw and integrated electron energy loss spectroscopy (EELS) and phonon spectra in the whole relevant parametric space, spanned by the adiabaticity parameter and the electron-phonon interaction (EPI) strength. We show that the EPI strength plays a prominent role in distributing spectral weights among excitations, providing an experimentally convenient way to estimate it from integrated spectra. By projecting the excitations onto the phonon degree of freedom, we also report for strong couplings large phonon production contributions, which are of very different origins depending on the adiabaticity parameter. In parallel to this thorough spectral weights analysis, excitations' dispersion evolutions, dampings, and various limiting behaviors are qualitatively and quantitatively correctly accounted for in the whole parametric space.