Certain periodically driven quantum many-particle systems in one dimension are known to exhibit edge modes that are related to topological properties and lead to approximate degeneracies of the Floquet spectrum. A similar situation occurs in spin chains, where stable edge modes were shown to exist at all energies in certain integrable spin chains. Moreover, these edge modes were found to be remarkably stable to perturbations. Here we investigate the stability of edge modes in interacting, periodically driven, clean systems. We introduce a model that features edge modes that persist over timesscales well in excess of the time needed for the bulk of the system to heat to infinite temperatures.

Polar surfaces are known to be unstable due to the divergence of the surface electrostatic energy. Here we report on the experimental determination, by grazing incidence x-ray diffraction, of the surface structure of polar Ti-terminated (111) ${\mathrm{SrTiO}}_3$ single crystals. We find that the polar instability of the $1\times 1$ surface is solved by a pure electronic reconstruction mechanism, which induces out-of-plane ionic displacements typical of the polar response of ${\mathrm{SrTiO}}_3$ layers to an electron confining potential. On the other hand, the surface instability can be also eliminated by a structural reconstruction driven by a change in the surface stoichiometry, which induces a variety of $3\times 3$ (111) ${\mathrm{SrTiO}}_3$ surfaces consisting in an incomplete Ti (surface)–${\mathrm{O}}_2$ (subsurface) layer covering the $1\times 1$ Ti-terminated (111) ${\mathrm{SrTiO}}_3$ truncated crystal. In both cases, the ${\mathrm{TiO}}_6$ octahedra are characterized by trigonal distortions affecting the structural and the electronic symmetry of several unit cells from the surface. These findings show that the stabilization of the polar (111) ${\mathrm{SrTiO}}_3$ surface can lead to the formation of quasi-two-dimensional electron systems characterized by radically different ground states which depend on the surface reconstructions.

Hybrid functional calculations reveal the Zn-O divacancy in ZnO, consisting of adjacent Zn and O vacancies, as an electrically active defect exhibiting charge states ranging from $2+$ to $2-$ within the band gap. Notably, the divacancy retains key features of the monovacancies, namely, the negative-$U$ behavior of the O vacancy and the polaronic nature of the Zn vacancy. The thermodynamic charge-state transition levels associated with the negative-$U$ behavior $\varepsilon (0/2-), \varepsilon (-/2-)$, and $\varepsilon (0/-)$ are predicted to occur at 0.22, 0.42, and 0.02 eV below the conduction band minimum, respectively, resulting in $\mathit{U}=-0.40\mathrm{eV}$. These transition levels are moved closer to the conduction band, and the magnitude of $U$ is lowered compared to the values for the O vacancy. Further, the interaction with hydrogen is explored, and it is shown that the divacancy can accommodate up to three H atoms. The first two H atoms prefer to terminate O dangling bonds at the Zn vacancy, while the geometrical location of the third depends on the Fermi level position. The calculated electrical properties of the divacancy are in excellent agreement with those reported for the $\mathrm{E}4$ center observed by deep-level transient spectroscopy, challenging the O vacancy as a candidate for this level.

The behavior of oxygen in Fe-Cr binary alloys is important to understand for a variety of applications including fuel cell interconnects and nuclear energy systems. In this work, we performed an ab initio investigation of the binding of oxygen in the entire compositional range of Fe-Cr at minima and saddles. This database was subsequently used to parametrize concentration-dependent local cluster expansions which were utilized to examine the kinetics of oxygen transport in the alloy and the thermodynamics of the system. The behavior of oxygen in the alloy system was investigated using our cluster expansion model and a convex hull diagram was recorded. The most favorable composition for the incorporation of oxygen was found to occur at 70% Cr. The dependence of the oxygen solution energy on the Cr occupation of the nearest neighbor shells was carefully characterized. It was determined that the third nearest neighbor Cr occupation was unfavorable and that, generally speaking, the first and second nearest neighbor shell Cr occupation was favored with a few exceptions. Kinetic Monte Carlo models at dilute oxygen levels were performed using two models [a kinetically resolved activation (kRA) model and a cluster expansion model for the saddle points]. The models qualitatively agreed with the observed trends in the literature which reported a decrease in diffusivity at dilute Cr levels, but the models predicted different behaviors beyond the dilute Cr limit. Finally, using grand canonical Monte Carlo, we further examined the thermodynamics of oxygen incorporation into the alloy. Together, our results offer new insight into the behavior of oxygen in Fe-Cr alloys that have ramifications on the early stages of the corrosive behavior of the material.

We study the nonequilibrium dynamics of random spin chains that remain integrable (i.e., solvable via Bethe ansatz): because of correlations in the disorder, these systems escape localization and feature ballistically spreading quasiparticles. We derive a generalized hydrodynamic theory for dynamics in such random integrable systems, including diffusive corrections due to disorder, and use it to study nonequilibrium energy and spin transport. We show that diffusive corrections to the ballistic propagation of quasiparticles can arise even in noninteracting settings, in sharp contrast to clean integrable systems. This implies that operator fronts broaden diffusively in random integrable systems. By tuning parameters in the disorder distribution, one can drive this model through an unusual phase transition, between a phase where all wave functions are delocalized and a phase in which low-energy wave functions are quasilocalized (in a sense we specify). Both phases have ballistic transport; however, in the quasilocalized phase, local autocorrelation functions decay with an anomalous power law, and the density of states diverges at low energy.

We study the resistance states and spin dynamics in a Ag/Fe(8)/MgO(3)/Fe(8)/MgO(5)/Fe/Ag junction based on a first-principles scattering theory. Spin dynamics simulations show that up to four resistance states with noticeable difference can be achieved via manipulating the voltage bias scheme and the position of the reference magnetization. The study indicates possible multiple-valued logic applications in an asymmetric MgO-based double-barrier magnetic tunnel junction.

The BCS model of an isolated superconductor initially prepared in a nonequilibrium state predicts the existence of interesting dynamical phenomena in the time-dependent order parameter as decaying oscillations, persistent oscillations, and overdamped dynamics. To make contact with real systems remains an open challenge as one needs to introduce dissipation due to the environment in a self-consistent computation. Here, we reach this goal with the use of the Keldysh formalism to treat the effect of a thermal bath. We show that, contrary to the dissipationless case, all dynamical phases reach the equilibrium order parameter in a characteristic time that depends on the coupling with the bath. Remarkably, as time evolves, the overdamped phase shows a fast crossover where the superconducting order parameter recovers to reach a state with a well-developed long range order that tends towards equilibrium with the damped Higgs mode oscillations. Our results provide a benchmark for the description of the dynamics of real out-of-equilibrium superconductors relevant for quantum technological applications.

Tin-based vacancy-ordered double perovskite ${\mathrm{Cs}}_2{\mathrm{SnI}}_6$ has emerged as a potential candidate for nontoxic, nonhygroscopic, and high efficient light absorber in the field of photovoltaics. By using lattice-dynamics calculations, we demonstrate that a series of phase transitions in ${\mathrm{Cs}}_2{\mathrm{SnI}}_6$ from cubic ($Fm\overline{3}m$) to tetragonal ($P4/mnc$) and to monoclinic ($P{2}_1/n$) is driven by anharmonic phonon modes, associated with octahedral tilting. For all the anharmonic phonon soft modes, double-well potentials are found with different well depths, which are used to estimate the hopping rate of a structural transition between two symmetry-breaking local minima. Phonon calculations of the lower-symmetry structures reveal that the anharmonic phonon modes are partly or fully eliminated. Furthermore, we calculate Helmholtz free energy difference to predict the phase transition temperatures, which are 44 K for monoclinic-tetragonal and 137 K for tetragonal-cubic transitions.

Silicon disulfide, ${\mathrm{SiS}}_2$, is thought to occur in interstellar dust and is of fundamental interest more generally among the silicon chalcogenides as a comparator to ${\mathrm{SiO}}_2$, an important component of terrestrial planets. However, the high-pressure behaviors of silicon sulfides are unclear. Here, using an efficient structure search method, we systematically explore the structural evolution of different Si-S stoichiometries up to 250 GPa. ${\mathrm{SiS}}_2$ is found to be stable below 155 GPa, above which it decomposes into two compounds, SiS and ${\mathrm{SiS}}_3$. SiS adopts a high-symmetry cubic structure consisting of eightfold-coordinated silicon in face-sharing ${\mathrm{SiS}}_8$ polyhedra, while ${\mathrm{SiS}}_3$ crystallizes in a rhombohedral structure containing ninefold-coordinated ${\mathrm{SiS}}_9$ polyhedra. Analyses suggest that the Si eightfold-coordination environment could be a common feature for group IV–VI compounds under high pressure. Our findings provide insights on the nature of Si-S compounds under ultrahigh pressure.

Motivated by the ongoing effort to search for high-resolution signatures of quantum spin liquids, we investigate the temperature dependence of the indirect resonant inelastic x-ray scattering (RIXS) response for the Kitaev honeycomb model. We find that, as a result of spin fractionalization, the RIXS response changes qualitatively at two well-separated temperature scales, $T_L$ and $T_H$, which correspond to the characteristic energies of the two kinds of fractionalized excitations, ${\mathbb{Z}}_2$ gauge fluxes and Majorana fermions, respectively. While thermally excited ${\mathbb{Z}}_2$ gauge fluxes at temperature $T_L$ lead to a general broadening and softening of the response, the thermal proliferation of Majorana fermions at temperature $T_H\sim 10T_L$ results in a significant shift of the spectral weight, both in terms of energy and momentum. Due to its exclusively indirect nature, the RIXS process we consider gives rise to a universal magnetic response and, from an experimental perspective, it directly corresponds to the $K$-edge of ${\mathrm{Ru}}^{3+}$ in the Kitaev candidate material $\alpha -{\mathrm{RuCl}}_3$.

We propose an approach to detect the peculiarity of Majorana fermions at the edges of Kitaev magnets. As is well known, a pair of Majorana edge modes is realized when a single complex fermion splits into real and imaginary parts which are, respectively, localized at the left and right edges of a sample magnet. Reflecting both of this peculiarity of the Majorana fermions and the ground-state degeneracy caused by the existence of the Majorana edge zero modes, the spins at the edges of the sample magnet are expected to behave as a peculiar “free” spin which exhibits a unidirectional magnetization without any transverse magnetization when applied a sufficiently weak external magnetic field. For the Kitaev honeycomb model, we obtain the expression of the Majorana edge magnetization by relying on standard techniques to diagonalize a free fermion Hamiltonian. The magnetization profile thus obtained indeed shows the expected behavior. We also elucidate the relation between the Majorana edge flat band and the bulk winding number from a weak topological point of view.

Recent experiments showed magnetic moment reversal and pronounced exchange-bias (EB) effect in the vicinity of the compensation temperature $T_{\mathrm{comp}}$ of orthoferrites $R\mathrm{Fe}{\mathrm{O}}_3$ ($R=\mathrm{Nd}$, Sm, Er). Although different orthoferrites exhibit diverse $R$-Fe interactions, $T_{\mathrm{comp}}$ values, and spin-reorientation temperatures, the EB field in the like manner emerges and diverges upon approaching $T_{\mathrm{comp}}$ and changes its sign with crossing $T_{\mathrm{comp}}$. In order to explain these observations, a mean-field theory approach for the representative $\mathrm{ErFe}{\mathrm{O}}_3$ orthoferrite is proposed. The general case of two sublattices, antiferromagnet with exchange anisotropy and rare-earth–iron interactions, is considered. A small applied magnetic field appears to be a source of additional anisotropy, resulting from canting of sublattice moments. This anisotropy leads to an imbalance of free energy for two different types of magnetic domains, causing a spin jump near the $T_{\mathrm{comp}}$. The suggested approach allows reproduction of magnetization reversal and main features of the coercive and exchange-bias fields observed in $\mathrm{ErFe}{\mathrm{O}}_3$.

The temperature dependence of the optical properties of the delafossite ${\mathrm{PdCoO}}_2$ has been measured in the a-b planes over a wide frequency range. The optical conductivity due to the free-carrier (intraband) response falls well below the interband transitions, allowing the plasma frequency to be determined from the f-sum rule. Drude-Lorentz fits to the complex optical conductivity yield estimates for the free-carrier plasma frequency and scattering rate. The in-plane plasma frequency has also been calculated using density functional theory. The experimentally determined and calculated values for the plasma frequencies are all in good agreement; however, at low temperature the optically determined scattering rate is much larger than the estimate for the transport scattering rate, indicating a strong frequency-dependent renormalization of the optical scattering rate. In addition to the expected in-plane infrared-active modes, two very strong features are observed that are attributed to the coupling of the in-plane carriers to the out-of-plane longitudinal optic modes.

Recently, a new group of layered transition-metal tetra-chalcogenides was proposed via first-principles calculations to correspond to a new family of Weyl type-II semimetals with promising topological properties in the bulk as well as in the monolayer limit. In this paper, we present measurements of the Shubnikov-de Haas (SdH) and de Haas-van Alphen effects under high magnetic fields for the type-II Weyl semimetallic candidate ${\mathrm{NbIrTe}}_4$. We find that the angular dependence of the observed Fermi surface extremal cross-sectional areas agree well with our density functional theory (DFT) calculations supporting the existence of Weyl type-II points in this material. Although we observe a large and nonsaturating magnetoresistivity in ${\mathrm{NbIrTe}}_4$ under fields all the way up to $35\mathrm{T}$, Hall-effect measurements indicate that ${\mathrm{NbIrTe}}_4$ is not a compensated semimetal. The transverse magnetoresistivity displays a fourfold angular dependence akin to the so-called butterfly magnetoresistivity observed in nodal line semimetals. We conclude that the magnetoresistivity and its unconventional angular dependence are governed by the topography of the Fermi surface and the resulting anisotropy in effective masses and in carrier mobilities.

In conventional solid-state electron systems with localized states the ac absorption is linear since the inelastic widths of the energy levels exceed the drive amplitude. The situation is different in the systems of cold atoms in which phonons are absent. Then even a weak drive leads to saturation of the ac absorption within resonant pairs, so that the population of levels oscillates with the Rabi frequency. We demonstrate that, in the presence of weak dipole-dipole interactions, the response of the system acquires a long-time component which oscillates with frequency much smaller than the Rabi frequency. The underlying mechanism of this long-time behavior is that the fields created in the course of the Rabi oscillations serve as resonant drive for the second-generation Rabi oscillations in pairs with level spacings close to the Rabi frequency. The frequency of the second-generation oscillations is of the order of interaction strength. As these oscillations develop, they can initiate the next-generation Rabi oscillations, and so on. Formation of the second-generation oscillations is facilitated by the nondiagonal component of the dipole-dipole interaction tensor.

Su-Schrieffer-Heeger (SSH) chains are the simplest model systems that display topological edge states. We calculate high-harmonic spectra of SSH chains that are coupled to an external laser field of a frequency much smaller than the band gap. We find huge differences between the harmonic yield for the two topological phases, similar to recent results obtained with more demanding time-dependent density functional calculations [D. Bauer and K. K. Hansen, Phys. Rev. Lett. 120, 177401 (2018)]. This shows that the tight-binding SSH model captures the essential topological aspects of the laser-chain interaction (while higher harmonics involving higher bands or screening in the metal phase are absent). We study the robustness of the topological difference with respect to disorder, a continuous phase transition in position space, and on-site potentials. Further, we address the question of whether the edges need to be illuminated by the laser for the huge difference in the harmonic spectra to be present.

Light with a chemical potential and no mass is shown to possess a general phase-transition curve to Bose-Einstein condensation. This limiting density and temperature range is found by the diverging in-medium potential range of effective interaction. The inverse expansion series of the effective interaction from the Bethe-Salpeter equation is employed, exceeding the ladder approximation. While usually the absorption and emission with dye molecules is considered, here it is proposed that squeezing can also create such a mean interaction leading to a chemical potential. The equivalence of squeezed light with a complex Bogoliubov transformation of an interacting Bose system with finite lifetime is established, with the help of which an effective gap is deduced where the squeezing parameter is related to an equivalent gap by $\left|\mathrm{\Delta }\right(\omega \left)\right|=\hslash \omega /(coth2\left|z\right(\omega \left)\right|-1)$. This gap phase creates a finite condensate in agreement with the general limiting density and temperature range. In this sense, it is shown that squeezing induces the same effect on light as an interaction, leading to possible condensation. The phase diagram for condensation is presented due to squeezing and the appearance of two gaps is discussed.

Materials with large magnetostriction are widely used in sensors, actuators, microelectromechanical systems, and energy harvesters. Binary Fe-Ga alloys (Galfenol) are the most promising rare-earth-free candidates combining numerous advantages such as low saturation magnetic field $(\sim 200\mathrm{Oe})$, excellent ductility, and low cost, while further improving their performance is imperative for practical applications. Using density functional theory calculation, we report the results of the effect of substituting small amounts of additional elements $X$ (e.g., $X=\mathrm{Ag}$ or Cu) on magnetostriction of Fe-Ga alloys, and we find that it may double the magnetostriction with a substitutional percentage of only $1.6\%$. Moreover, adsorbents with high chemical activity (e.g., O or Os atoms) may affect the surface energy of different face orientations of Fe-Ga alloys, indicating that proper surface treatments are necessary to tune the alignment of Fe-Ga grains to achieve better performance. These results may be helpful to further optimize the magnetostrictive properties of Fe-Ga alloys for device applications.