We performed a comprehensive study on structural, electrical, magnetic, and optical properties for iron-based ladder materials ${\mathrm{BaFe}}_2{\left({\mathrm{S}}_{1-x}{\mathrm{Se}}_x\right)}_3(0\le x\le 1)$, which shows pressure-induced superconductivity in the vicinity of the Mott transition at $x=0$ and 1. We obtain a complete electronic phase diagram in a temperature-composition plane, which reveals that the magnetic ground state switches from the stripe-type to the block-type phase without any intermediate phase at $x=0.23$ with increasing $x$. This behavior is in sharp contrast to the filling controlled system ${\mathrm{Ba}}_{1-x}{\mathrm{Cs}}_x{\mathrm{Fe}}_2{\mathrm{Se}}_3$, in which a paramagnetic state down to the lowest temperature is realized between two magnetic ordered states. The structural transition, which is considered to be relevant to the orbital order, occurs far above the magnetic transition temperature. The magnetic and structural transition temperatures exhibit a similar composition dependence, indicating a close relationship between magnetic and orbital degrees of freedom. In addition, we found that charge dynamics are considerably influenced not only by the magnetic order but also by the structural change (orbital order) from the detailed measurements of electrical resistivity and optical conductivity spectra. We discuss the magnetism and orbital order by comparing the experimental results with the proposed theory based on the multiorbital Hubbard model. The relationship between the charge dynamics and the magnetic/orbital order is also discussed.

Time-resolved photoluminescence and light-induced transient grating techniques were applied for the comparative investigation of the evolution of the internal quantum efficiency and of the carrier diffusion in an $\mathrm{A}{\mathrm{l}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$ silicon-doped epilayer and $\mathrm{A}{\mathrm{l}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$ MQWs ($x>0.6$). The experiments were performed between 80 and 550 K under various densities of excitation from $1\mu \mathrm{J}\mathrm{c}{\mathrm{m}}^{-2}$ to $1\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$. The decrease of the photoluminescence efficiency measured at high excitations is quantitatively correlated to the increase of the diffusion coefficients of carriers and to the increase of their nonradiative recombination rate. We evidence the reduction of the density of localized excitons with increasing both excitation density and temperature. The decrease of the excitation-dependent lifetime is less pronounced than the corresponding drop of the time-integrated photoluminescence efficiency. This is dominated by the thermal dissociation of excitons. At the lowest excitation densities, the excitons are captured to vacancy complexes. When increasing the excitation density, the ionization of excitons is produced, which leads to enhancing the nonradiative recombination of holes to aluminum vacancies and this simultaneously quenches the PL efficiency, due to the saturation of the bimolecular free-carrier–plasma recombination. At the initial recombination stages and under high excitation conditions we reveal diffusive recombination on dislocations. After a careful and sophisticated modeling, we establish relevant numbers for the following: (i) the free exciton binding energies, with values of 104 and 140 meV in the MQWs and in the layer respectively; (ii) the exciton localization energies, which are framed in the 12–35-meV range; (iii) the lifetimes of the localized excitons, which sit in the 2–4-ns range; (iv) the free exciton and carrier radiative recombination rate coefficients that are $r_{\mathrm{ex}}=\left(0.6\pm 0.2\right)\times {\left(T/300\right)}^{-1}\times {10}^9{\mathrm{s}}^{-1}$ and $B_{\mathrm{rad}}=\left(7\pm 1\right)\times {\left(T/300\right)}^{-3/2}\times {10}^{-10}\mathrm{c}{\mathrm{m}}^3{\mathrm{s}}^{-1}$, respectively; (v) the capture cross section for excitons to the vacancy complex $\sigma =\left(2\pm 1\right)\times {\left(300/T\right)}^2\times {10}^{-16}\mathrm{c}{\mathrm{m}}^2$. Regarding the electron and hole capture cross sections to aluminum vacancy we found values of $\left(1.5\pm 1\right)\times {10}^{-13}\mathrm{c}{\mathrm{m}}^2$ and $\left(7\pm 1\right)\times {10}^{-13}\mathrm{c}{\mathrm{m}}^2$, respectively. The value of the Coulomb dislocation radius for the free-carrier recombination is established to be 12–15 nm.

Within the Floquet theory of periodically driven quantum systems, we developed the theory of light-induced modification of electronic states in semiconductor materials described by the Luttinger Hamiltonian (the electronic term ${\mathrm{\Gamma }}_8$). Particularly, exact solutions of the Floquet problem are found for the band edge in the cases of linearly and circularly polarized irradiation. It is shown that the irradiation changes electron effective masses near the band edge, induces anisotropy of the electron dispersion, and splits the bands. It is demonstrated that the light-induced band splitting strongly depends on the light polarization. Namely, the circularly polarized light acts similarly to a stationary magnetic field and lifts the spin degeneracy of electron branches, whereas a linearly polarized light does not affect the spin degeneracy and only splits the bands in the center of the Brillouin zone. The present theory can be applied to describe electronic properties of various semiconductor structures irradiated by an electromagnetic field in the broad frequency range.

Spin-momentum locking, a key property of the surface states of three-dimensional topological insulators (3DTIs), provides a new avenue for spintronics applications. One consequence of spin-momentum locking is the induction of surface spin accumulations due to applied electric fields. In this paper, we investigate the extraction of such electrically induced spins from their host TI material into adjoining conventional, hence topologically trivial, materials that are commonly used in electronics devices. We focus on effective Hamiltonians for bismuth-based 3DTI materials in the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ family, and numerically explore the geometries for extracting current-induced spins from a TI surface. In particular, we consider a device geometry in which a side pocket is attached to various faces of a 3DTI quantum wire and show that it is possible to create current-induced spin accumulations in these topologically trivial side pockets. We further study how such spin extraction depends on geometry and material parameters, and find that electron-hole degrees of freedom can be utilized to control the polarization of the extracted spins by an applied gate voltage.

Using a semiclassical approach, we derive a fully analytical expression for the ionization rate of excitons in two-dimensional materials due to an external static electric field, which eliminates the need for complicated numerical calculations. Our formula shows quantitative agreement with more sophisticated numerical methods based on the exterior complex scaling approach, which solves a non-Hermitian eigenvalue problem yielding complex energy eigenvalues, where the imaginary part describes the ionization rate. Results for excitons in hexagonal boron nitride and the $A$ exciton in transition metal dichalcogenides are given as simple examples. The extension of the theory to include spin-orbit-split excitons in transition metal dichalcogenides is trivial.

The study of an external force triggering the many-electron redistribution in nanostructures is of importance for both theoretical and practical interest in condensed matter physics. Hence, the goal of this contribution is to theoretically investigate the influence of an external electric field (EEF) on the ground and excited states of a system of many-electrons confined in GaAs/AlGaAs nanostructures. To deal with the many-body issue, we employ a Yukawa-like potential in a two-electron framework, while it is proposed to explore the challenging EEF contribution by analyzing its effect on the electronic center-of-mass framework. Thus, while the Yukawa-like potential allows us to model the many-electron interaction in the range from ${10}^{17}$ to ${10}^{23}$ electrons/${\mathrm{cm}}^3$, via the adjustment of a simple screening parameter; the center-of-mass approximation also allows us to describe easily the effect of the EEF separately. Internal electronic distributions in nanostructures of various sizes and electronic concentrations are obtained and discussed for EEF of variable magnitude. This alternative approach enables us to decouple the electron-electron and the EEF interactions, which is useful in determining the origin of the observed features. We show that this theoretical framework is well suited to get relevant information in semiconductor nanostructures of practical sizes and realistic doped levels.

Compared to inversion-symmetric systems, the emerging bulk Dirac point (DP) in noncentrosymmetric systems is much harder and its symmetry-protection mechanism is poorly understood. In this Rapid Communication, we propose that orthogonal glide symmetries can protect two distinct types of bulk anisotropic DPs. One is the ordinary anisotropic DP that is doubly degenerate only along one invariant axis, which results from symmetry-protected accidental band crossing. The other one is a symmetry-enforced DP fixed at a non-time-reversal-invariant point, which is doubly degenerate at three orthogonal directions. This unique topological phase is exemplified by ${\mathrm{KSnSe}}_2$ in space group 108. Our work not only unveils a unique symmetry-protection mechanism but also provides a material candidate for exploring multitype Dirac fermions in noncentrosymmetric systems.

Topologically ordered phases of matter can be characterized by the presence of a universal, constant contribution to the entanglement entropy known as the topological entanglement entropy (TEE). The TEE can be calculated for Abelian phases via a “cut-and-glue” approach by treating the entanglement cut as a physical cut, coupling the resulting gapless edges with explicit tunneling terms, and computing the entanglement between the two edges. We provide a first step towards extending this methodology to non-Abelian topological phases, focusing on the generalized Moore-Read (MR) fractional quantum Hall states at filling fractions $\nu =1/n$. We consider interfaces between different MR states, write down explicit gapping interactions, which we motivate using an anyon condensation picture, and compute the entanglement entropy for an entanglement cut lying along the interface. Our work provides new insight towards understanding the connections between anyon condensation, gapped interfaces of non-Abelian phases, and TEE.

We present real-space quantum Monte Carlo (QMC) calculations of the scandate ${\mathrm{LaScO}}_3$ that proved to be challenging for traditional electronic structure approaches due to strong correlation effects resulting in inaccurate band gaps from DFT and $GW$ methods when compared with existing experimental data. Besides calculating an accurate QMC band gap corrected for supercell size biases and in agreement with numerous experiments, we also predict the cohesive energy of the crystal using the standard fixed-node QMC without any empirical or nonvariational parameters. We show that promotion (optical) gap and fundamental gap agree with each other illustrating a clear absence of significant excitonic effects in the ideal crystal. We obtained these results in perfect consistency in two independent tracks that employ different basis sets (plane wave versus localized Gaussians), different codes for generating orbitals (quantum espresso versus crystal), different QMC codes (qmcpack versus qwalk) and different high-accuracy pseudopotentials (ccECPs versus Troullier-Martins) presenting the maturity and consistency of QMC methodology and tools for studies of strongly correlated problems.

We study how a system of one-dimensional spin-$\frac{1}{2}$ fermions at temperatures well below the Fermi energy approaches thermal equilibrium. The interactions between fermions are assumed to be weak and are accounted for within the perturbation theory. In the absence of an external magnetic field, spin degeneracy strongly affects relaxation of the Fermi gas. For sufficiently short-range interactions, the rate of relaxation scales linearly with temperature. Focusing on the case of the system near equilibrium, we linearize the collision integral and find the exact solution of the resulting relaxation problem. We discuss the application of our results to the evaluation of the transport coefficients of the one-dimensional Fermi gas.

Many efforts have been devoted to understanding the origin and effects of magnetic moments induced in graphene with carbon atom vacancy, and light adatoms like hydrogen or fluorine. In the meantime, the large negative magnetoresistance (MR) widely observed in these systems is not well understood, nor has it been associated with the presence of magnetic moments. In this paper, we study the systematic evolution of the large negative MR of in-situ hydrogenated graphene in ultrahigh-vacuum (UHV) environment. We find for most combinations of electron density ($n_e$) and hydrogen density ($n_{\mathrm{H}}$), MR at different temperature can be scaled to $\alpha ={\mu }_{B}B/k_B\left(T-T^{*}\right)$, where $T^{*}$ is the Curie-Weiss temperature. The sign of $T^{*}$ indicates the existence of tunable ferromagneticlike ($T^{*}>0$) and antiferromagneticlike ($T^{*}<0$) coupling in hydrogenated graphene. However, the lack of hysteresis of MR or anomalous Hall effect below $|T^{*}|$ points to the fact that long-range magnetic order did not emerge, which we attribute to the competition of different magnetic orders and disordered arrangement of magnetic moments on graphene. We also find that localized impurity states introduced by H adatoms could modify the capacitance of hydrogenated graphene. This work provides a way to extract information from large negative MR behavior and can be a key to understanding interactions of magnetic moments in graphene.

First-principles calculations of surfaces or two-dimensional materials with a finite surface charge invariably include an implicit or explicit compensating countercharge. We show that an ideal constant-charge counterelectrode in the vacuum region can be introduced by means of a simple correction to the electrostatic potential in close analogy to the well-known dipole correction for charge-neutral asymmetric slabs. Our generalized dipole correction accounts simultaneously for the sheet-charge electrode and the huge voltage built up between the system of interest and the counterelectrode. We demonstrate its usefulness for two prototypical cases, namely, field evaporation in the presence of huge electric fields (20 V/nm) and the modeling of charged defects at an insulator surface. We also introduce algorithmic improvements to charge initialization and preconditioning in the density functional theory algorithm that proved crucial for ensuring rapid convergence in slab systems with high electric fields.

Scattering of waves is present in many areas of physics. Within all these areas, in a great number of systems, the scattering can be separated in an averaged response that crosses rapidly the scattering region and a fluctuating delayed response. This fact is the basis of the optical model; the averaged response, represented by the optical matrix $\langle S\rangle$, is combined with the fluctuating part that can be taken as a random matrix. Although the optical model was developed more than 60 years ago, a theoretical prediction for the optical matrix was obtained only very recently. The validity of such a prediction is experimentally demonstrated here. This is done studying the scattering of torsional waves in a quasi-1D elastic system in which a locally periodic system is built; the full distribution of the scattering matrix is then calculated completely free of parameters. In contradistinction to all previous works, in microwaves and in elasticity, in which the value of $\langle S\rangle$ is obtained from the experiment, here the theoretical prediction is used to compare with the experiment. Numerical simulations show that the theoretical value is still valid when strong disorder is present. Several applications of the theoretical expression for the optical matrix in other areas of physics are proposed. Possible extensions of this work are also discussed.

The thermal stability of atomic-scale skyrmions is of high relevance for potential spintronics applications and validation of theoretical models. We investigated Pd/Fe nanoislands on an Ir(111) substrate as a function of temperature and magnetic field. Utilizing noncollinear magnetoresistance contrast in scanning tunneling microscopy, the thermomagnetic phase space is explored up to 3 T within a temperature range between 1 K to 100 K. Evidence is found for the spin spiral, field-polarized, and fluctuating disordered magnetic phases. Evidence for the presence of atomic-scale skyrmions at up to approximately 80 K is found, irrespective of considerable magnetization dynamics arising from thermal agitation.

Here we report the results of the high-pressure Hall coefficient ($R_H$) measurements, combined with the high-pressure resistance measurements, at different temperatures on the putative topological superconductor ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$. We find the intimate correlation of sign change of $R_H$, a fingerprint to manifest the reconstruction of Fermi surface, with structural phase transition and superconductivity. Below the critical pressure ($P_{\mathrm{c}}$) of 2.7 GPa, our data reveal that the hole-electron carriers are thermally balanced ($R_H=0$) at a critical temperature ($T^{*}$), where $R_H$ changes its sign from positive to negative, and concurrently a tetragonal-orthorhombic phase transition takes place. Within the pressure range from ambient pressure to $P_{\mathrm{c}}$, $T^{*}$ is continuously suppressed by pressure, while $T_c$ increases monotonically. At about $P_{\mathrm{c}}$, $T^{*}$ is undetectable and $T_c$ reaches a maximum value. Moreover, a pressure-induced sign change of $R_H$ is found at $\sim P_{\mathrm{c}}$ where the orthorhombic-monoclinic phase transition occurs. With further compression, $T_c$ decreases and disappears at ∼ 12 GPa. The correlation among the electron-hole balance, crystal structure, and superconductivity found in the pressurized ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ implies that its nontrivial superconductivity is closely associated with its exotic normal state resulting from the interplay between the reconstruction of the Fermi surface and the change of the structural lattice.

The ${\mathrm{BiS}}_2$-based layered materials are characterized by a highly susceptible physical state, revealing a large response to external conditions. A particular case is the ${\mathrm{EuFBiS}}_2$ compound, showing a superconducting transition temperature $T_c\sim 0.3$ K at ambient pressure. Upon increasing external pressure, $T_c$ goes through a large amplification, accompanied by a structural phase transition (SPT) from tetragonal to monoclinic symmetry. Here, we use a combination of Eu $L_3$ - edge x-ray absorption spectroscopy and synchrotron x-ray diffraction to unveil the evolution of the Eu valence and lattice symmetry under high pressure. We find that the average Eu valence increases gradually with pressure, exhibiting a pressure plateau near the SPT, at which the $T_c$ increases sharply. Since in ${\mathrm{EuFBiS}}_2$ the charge carriers are introduced via self-doping induced by the mixed valence of the Eu ions, our findings clearly indicate that the role of the charge doping is marginal in the $T_c$ enhancement. On the other hand, the structural distortions, taking place at the SPT, play a central role in enhancing the superconducting properties of the ${\mathrm{EuFBiS}}_2$ system.

Spinel compounds have exhibited rich functionalities but have rarely shown superconductivity. Here, we report the emergence of superconductivity in the spinel ${\mathrm{MgTi}}_2{\mathrm{O}}_4$, known to be an insulator with a complicated order. The superconductivity is achieved by engineering a superlattice of ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ and ${\mathrm{SrTiO}}_3$. The onset transition temperature in the ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ layer can be tuned from 0 to 5 K in such a geometry, concurrently with a stretched out-of-plane lattice (from 8.51 to 8.53 Å) compared to the bulk material. Such a positive correlation suggests ample room for further enhancement. Intriguingly, the superlattice exhibits an isotropic upper critical field $B_{\mathrm{c}2}$ that breaks the Pauli limit, distinct from the highly anisotropic feature of interface superconductivity. The origin of superconductivity in the ${\mathrm{MgTi}}_2{\mathrm{O}}_4$ layer is understood in combination with the electron energy loss spectra and first-principles electronic structure calculations, which point to the birth of superconductivity by suppressing orbital ordering. Our discovery not only provides a platform to explore the interplay between superconductivity and other exotic states, but also opens another window to realize superconductivity in spinel compounds as well as other titanium oxides.

The spin-$1/2$ Heisenberg chain with a ferromagnetic first-neighbor exchange coupling $J_1$ and an antiferromagnetic second-neighbor $J_2$ has a Haldane dimer ground state with an extremely small spin gap. Thus the ground state is readily altered by perturbations. Here, we investigate the effects of XXZ exchange magnetic anisotropy of both the easy-axis and easy-plane types and an alternation in $J_1$ on the ground state, the spin gap, and magnetic properties of the frustrated ferromagnetic spin-$1/2$ chain. It is found that there are two distinct dimerized spin-gap phases, in one of which the spin gap and the magnetic susceptibility are extremely small around the SU(2) symmetric case and in the other they are moderately large far away from the SU(2) symmetric case. A small alternation in the amplitude of $J_1$ rapidly shortens the pitch of spin correlations towards the four-spin periodicity, as in the limit of $J_1/J_2\to 0$. These effects are not sufficient to quantitatively explain overall experimentally observed magnetic properties in the quasi-one-dimensional spin-gapped magnetoelectric cuprate ${\mathrm{Rb}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$ that exhibits ferroelectricity stabilized by a magnetic field. Our results are also relevant to ${\mathrm{Cs}}_2{\mathrm{Cu}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{12}$, where the ferromagnetic intrachain and antiferromagnetic interchain order has recently been found, in a single chain level. We also reveal the nature of symmetry-protected topological phase transitions in the model by mapping onto effective spin-1 chain models.

The magnetic and superconducting properties of a series of underdoped ${\mathrm{Ba}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ (BNFA) single crystals with $0.19\le x\le 0.34$ have been investigated with the complementary muon-spin-rotation ($\mu \mathrm{SR}$) and infrared spectroscopy techniques. The focus has been on the different antiferromagnetic states in the underdoped regime and their competition with superconductivity, especially for the ones with a tetragonal crystal structure and a so-called double-Q magnetic order. Besides the collinear state with a spatially inhomogeneous spin-charge-density wave (i-SCDW) order at $x=0.24$ and 0.26, that was previously identified in BNFA, we obtained evidence for an orthomagnetic state with a “hedgehog”-type spin vortex crystal (SVC) structure at $x=0.32$ and 0.34. Whereas in the former i-SCDW state the infrared spectra show no sign of a superconducting response down to the lowest measured temperature of about 10 K, in the SVC state there is a strong superconducting response similar to the one at optimum doping. The magnetic order is strongly suppressed here in the superconducting state and at $x=0.34$ there is even a partial reentrance into a paramagnetic state at $T\ll T_c$.

Within the Landauer transport formalism we demonstrate that conductance zeros are possible in bipartite systems at half-filling when leads are contacted to different sublattice sites. In particular, we investigate the application of this theory to graphene quantum dots with leads in the armchair configuration. The obtained conductance cancellation is robust in the presence of any single-site impurity.