We study the melting of a domain wall, prepared as a certain low-energy excitation above the ferromagnetic ground state of the XY chain. In a well-defined parameter regime the time-evolved magnetization profile develops sharp kinklike structures in the bulk, showing features of a phase transition in the hydrodynamic scaling limit. The transition is of a purely dynamical nature and can be attributed to the appearance of a negative effective mass term in the dispersion. The signatures are also clearly visible in the entanglement profile measured along the front region, which can be obtained by covariance-matrix methods despite the state being non-Gaussian.

In this study, two ideal triple degenerate point (TDP) semimetals were predicted, namely, HfIrAs and HfIrBi, which exhibit clean TDPs around the Fermi level. At the atomic level, the origin of the TDP was analyzed, and when combined with the low-energy effective four-band model, the bulk inversion asymmetry was found to directly determine the formation of the TDP for cubic crystals. The projected surface states and corresponding patterns of the Fermi arcs with and without tensile strain were investigated for two different crystal face orientations [(111) and (110)], both of which could be detected using photoemission spectroscopy. In addition, the disappearance of the spin texture, which was studied in strained HgTe bulk states, was also observed in the surface states. The results of this study indicate that the two materials are unique platforms on which emergent phenomena, such as the interplay between the TDP of semimetals and superconductivity, can be explored.

We investigate the ultrafast dynamic behavior of photoinduced insulator-metal phase transition in an epitaxially grown $\mathrm{SmNi}{\mathrm{O}}_3$ thin film by using time-resolved terahertz (THz) spectroscopy at different excitation fluences. It is found that the relaxation of the photoexcited carriers is characterized by two components with different time scales: (i) an ultrafast one with time constant varying between 0.3 ps (at 4.4 $\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$) and 0.35 ps (at 0.88 $\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$) and (ii) a slow one with a time constant varying between 1.7 ps (at 2 $\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$) and 4.7 ps (at 0.88 $\mathrm{mJ}\mathrm{c}{\mathrm{m}}^{-2}$). These relaxation times could be primarily attributed to the coexistence of conducting and insulating charge-ordering domains. The observed behavior of the transient THz conductivity of $\mathrm{SmNi}{\mathrm{O}}_3$ thin film is well fitted by the Drude-Smith model, revealing that insulating domains emerge between the metallic domains, which in turn increase the carrier confinement during the relaxation process. Furthermore, the ultrafast dynamic behavior at different temperatures suggests that the insulating gap shrinks gradually between the Ni $3d$ and O $2p$ states as the temperature applied to the film increases. Our experimental findings pave the way to scientific opportunities and to technological applications of epitaxially grown $\mathrm{SmNi}{\mathrm{O}}_3$ thin film such as ultrafast optical switching and modulation devices.

Extremely intense few-cycle terahertz pulses exhibit complex nonlinear behavior under interaction with $n$-doped opaque Si having low field transmission of $\sim 0.02\%$. Transmission of a 700-fs pulse with central frequency $\sim 1.5\mathrm{THz}$ through the Si sample increases up to $\sim 8\%$ for the external field strength of 5 MV/cm and then drops twofold at $\sim 20\mathrm{MV}/\mathrm{cm}$. Electro-optical sampling measurements revealed formation of a single-cycle terahertz pulse at this field due to generation of a thin ionized layer by the first intense oscillation of the field. We explained the data obtained from the competition of processes of collision electron-phonon rate fast increase and saturation, as well as ionization under action of an intense THz field.

The onset of distortion in one-dimensional monatomic chains with partially filled valence bands is considered to be well established by the Peierls theorem, which associates the distortion with the formation of a band gap and a subsequent gain in energy. Employing modern total energy methods on the test cases of lithium, sodium, and carbon chains, we reveal that the distortion is not universal but conditional upon the balance between distorting and stabilizing forces. Furthermore, in all systems studied, the electrostatic interactions between the electrons and ions act as the main driving force for distortion, rather than the electron band lowering at the Fermi level as is commonly believed. The main stabilizing force which drives the chains toward their symmetric arrangement is derived from the electronic kinetic energy. Both forces are affected by the external conditions, e.g., stress, and consequently the instability of one-dimensional nanowires is conditional upon them. This brings a perspective to the field of one-dimensional metals and may shed light on the distortion of more complex structures.

A minimal tight-binding model of the two-dimensional topological-insulator bismuthene on SiC that accurately describes its experimentally determined low-energy electronic band structure is presented. Two-terminal electron transport through quantum dots described by this model with Ohmic normal conductor contacts and leads is studied within Landauer theory. Depending on the configuration of the leads and contacts, quantized conductances exactly or approximately equal to the conductance quantum $2e^2/h$, or conductance resonances are found at zero temperature if the Fermi level is within the topological-insulator bulk band gap. Interface states formed at the normal contact–topological-insulator boundary and their role in electron transport are discussed. Disorder along the normal contact–topological-insulator interfaces is found to improve the accuracy of the conductance quantization due to Anderson localization of the interface states.

Tunnel-injection lasers promise various advantages in comparison to conventional laser designs. In this paper, the physics of the tunnel-injection process is studied within a microscopic theory in order to clarify design requirements for laser structures based on quantum dots as active material and an injector quantum well providing excited charge carriers. We analyze how the electronic states of the injector quantum-well and quantum-dot levels should be aligned and in which way their coupling through the tunnel-injection barrier should be adjusted for optimal carrier injection rates into the quantum-dot ground state used for the laser transition. Our description of the tunnel-injection process combines two main ingredients: the tunnel coupling of the wave functions as well as the phonon- and Coulomb-assisted transition rates. For this purpose, material-realistic electronic state calculations for the coupled system of injector quantum well, tunnel barrier, and quantum dots are combined with a many-body theory for the carrier scattering processes. We find that the often assumed longitudinal-optical-phonon resonance condition for the level alignment has practically no influence on the injection rate of carriers into the quantum-dot states. The structural design should provide optimal hybridization of the injector quantum-well states with excited quantum-dot states.

Using the strain-dependent effective Hamiltonian and the geometric phase, Droth et al. [Phys. Rev. B 94, 075404 (2016)] obtain an analytical expression for the electronic contribution to the piezoelectricity of planar hexagonal boron nitride ($h$-BN). Their analytical results of piezoelectric constants for $h$-BN are invalid because of the mistakes in constructing the adiabatic process of the piezoelectricity. In this Comment, we reconstruct a proper adiabatic process for piezoelectricity and formulate a general Berry phase expression for the piezoelectric coefficients of two-dimensional piezoelectric crystals by means of the modern theory of polarization. The corrected analytical results of the piezoelectric constants are in complete consistency with the first-principles calculations and hence manifest the validity and generality of the Berry phase expression of piezoelectric coefficients.

In relation to our original paper [M. Droth et al., Phys. Rev. B 94, 075404 (2016)], the Comment by Li et al. [Phys. Rev. B 98, 167403 (2018)] claims to have identified a “mistake in constructing the adiabatic process of the piezoelectricity.” More specifically, they write that in our original work, “the erroneous usage of the polarization difference formula in Eq. (3) leads to an invalid analytical expression of piezoelectric constant in Eq. (12) of Ref. [1].” We explain below why these and other minor claims in the Comment are unwarranted, and why we maintain that our result is correct and physically sound.

The optical spectra of hydrogen at $\sim 500$ GPa were studied theoretically using a combination of ab initio methods. Among the four most competitive structures, i.e., C2/c-24, Cmca-12, Cmca-4, and I41/amd, only the atomic phase I41/amd can provide satisfactory interpretations of the recent experimental observation, and the electron-phonon interactions (EPIs) play a crucial role. Anharmonic effects (AHEs) due to lattice vibration are nonnegligible but not sufficient to account for the experimentally observed temperature dependence of the reflectance. The drop of the reflectance at 2 eV is not caused by diamond's band gap reducing or interband plasmon, but very likely by defect absorptions in diamond. These results provide theoretical support for the recent experimental realization of metallic hydrogen. The strong EPIs and the nonnegligible AHEs also emphasize the necessity for quantum treatment of both the electrons and the nuclei in future studies.

The iron-containing crystals of the langasite family $A_{3}M\mathrm{F}{\mathrm{e}}_3{X}_2{\mathrm{O}}_{14}(A=\mathrm{Ba},\text{Sr},M=\mathrm{Sb},\text{Nb},\text{Ta},X=\mathrm{Si},\text{Ge})$ are a new class of multiferroics in which ferroelectricity can be induced by magnetic ordering. We have measured Mössbauer spectra of a powder sample $\mathrm{B}{\mathrm{a}}_3\mathrm{NbF}{\mathrm{e}}_3\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_{14}$ of this family in a wide temperature range and developed a physico-mathematical model for analyzing the complex Mössbauer spectra shape. This model allows establishing a helical magnetic structure at all temperatures in the magnetic ordering region $\left(T<T_{\mathrm{N}}\right)$. It is shown that the experimental Mössbauer spectra are well described by the model of a helical magnetic structure with a constant angle of rotation of the moment (rotation step) during its transfer between adjacent planes. The angle value is about 51°, which agrees with the previous results of neutron experiments. The excited states of the magnetic moments that appear at finite (nonzero) temperatures are also taken into account, which makes it possible to study the magnetic dynamics at different temperatures. The value of the angle between the main axis of the electric field gradient (EFG) at the iron nuclei and the crystal axis $c\left(\theta ={36}^{\circ }\right)$, which does not change with the temperature in the region $T<T_{\mathrm{N}}$, is established. This indicates that in the region of magnetic ordering of this langasite no structural transitions occur. The developed model promotes a deeper understanding of the magnetic properties of the multiferroic $\mathrm{B}{\mathrm{a}}_3\mathrm{NbF}{\mathrm{e}}_3\mathrm{S}{\mathrm{i}}_2{\mathrm{O}}_{14}$ and can be used to study the magnetic structure of other compounds where iron ions form a similar helical structure.

The structural and magnetic properties of the $\mathrm{C}{\mathrm{a}}_3\mathrm{C}{\mathrm{o}}_2{\mathrm{O}}_6$ spin-chain compound have been studied by means of neutron and x-ray powder diffraction at pressures up to 6.8 and 32 GPa, respectively. A suppression of the initial spin-density wave state $\left(T_{\mathrm{N}}=25\mathrm{K}\right)$ and stabilization of the collinear commensurate antiferromagnetic (AFM) state at high pressures $(T_{\mathrm{NC}}=26\mathrm{K}$ at $P=2.1$ GPa) were observed. The pressure behavior of the competing intra- and interchain magnetic interactions was analyzed on the basis of obtained structural data and their role in the formation of the magnetic phase diagram is discussed. The pressure behavior of the Néel temperature of the commensurate AFM phase was evaluated within the mean field theory approach and a good agreement with the experimental value $d{T}_{\mathrm{NC}}/dP=0.65\mathrm{K}/\mathrm{GPa}$ was obtained.

Based on large-scale quantum Monte Carlo simulations, we examine the correlations along the edges of two-dimensional semi-infinite quantum critical Heisenberg spin-$1/2$ systems. In particular, we consider coupled quantum spin-dimer systems at their bulk quantum critical points, including the columnar-dimer model and the plaquette-square lattice. The alignment of the edge spins strongly affects these correlations and the corresponding scaling exponents, with remarkably similar values obtained for various quantum spin-dimer systems. We furthermore observe subtle effects on the scaling behavior from perturbing the edge spins that exhibit the genuine quantum nature of these edge states. Our observations furthermore challenge recent attempts that relate the edge spin criticality to the presence of symmetry-protected topological phases in such quantum spin systems.

We report a density-matrix renormalization group study of the lightly doped $t\text{−}J$ model on a four-leg cylinder with doped hole concentrations per site $\delta =5\%\text{–}12.5\%$. By keeping an unusually large number of states and long system sizes, we are able to accurately document the interplay between superconductivity, spin, and charge-density-wave orders. The long-distance behavior is consistent with that of a Luther-Emery liquid with a spin gap and power-law charge-density-wave and superconducting correlations. This is the widest $t\text{−}J$ or Hubbard system in which power-law superconducting correlations have been established.

The compounds ${\mathrm{BaDy}}_2{\mathrm{O}}_4$ and ${\mathrm{BaHo}}_2{\mathrm{O}}_4$ are part of a family of frustrated systems exhibiting interesting properties, including spin-liquid-type ground states, magnetic-field-induced phases, and the coexistence of short- and long-range magnetic order, with dominant one-dimensional correlations, which can be described as Ising $J_1\text{−}{J}_2$ zigzag chains along the $c$ axis. We have investigated polycrystalline samples of ${\mathrm{BaDy}}_2{\mathrm{O}}_4$ and ${\mathrm{BaHo}}_2{\mathrm{O}}_4$ with both neutron diffraction and neutron spectroscopy, coupled to detailed crystalline electric field calculations. The latter points to site-dependent anisotropic magnetism in both materials, which is corroborated by the magnetic structures we determined. The two systems show the coexistence of two different long-range orders: two double Néel $\uparrow \uparrow \downarrow \downarrow$ orders in the $ab$ plane with propagation vectors ${\mathbf{k}}_1=(\frac{1}{2},0,\frac{1}{2})$ and ${\mathbf{k}}_2=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ for ${\mathrm{BaDy}}_2{\mathrm{O}}_4$, and two distinct arrangements of simple Néel $\uparrow \downarrow \uparrow \downarrow$ orders along the $c$ axis, both with the propagation vector ${\mathbf{k}}_0=(0,0,0)$ for ${\mathrm{BaHo}}_2{\mathrm{O}}_4$. The order for both wave vectors in ${\mathrm{BaDy}}_2{\mathrm{O}}_4$ occurs at $T_{\mathrm{N}}=0.48\mathrm{K}$, while in ${\mathrm{BaHo}}_2{\mathrm{O}}_4$ the first order sets in at $T_{\mathrm{N}}\sim 1.3\mathrm{K}$ and the second one has a lower ordering temperature of 0.84 K. Both compounds show extensive diffuse scattering which we successfully modeled with a one-dimensional axial next-nearest neighbor Ising (ANNNI) model. In both materials, strong diffusive scattering persists to temperatures well below where the magnetic order is fully saturated.

A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the $\mathrm{Cu}{\mathrm{O}}_2$ layers and those of the intermediate layers. The large $c$ axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-layer depth resolution of these properties. Applying SW photoemission to $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_{8+\delta }$ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the atomic supermodulation structure, as determined by comparison to density functional theory calculations, by Ca-Sr intermixing, and by correlation effects associated with the Cu $3d\text{−}3d$ Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Measurements of this type for other quasi-two-dimensional materials with large $c$ represent a promising future direction.

Recent observations of robust zero-bias quantized conductance plateaus of height $2e^2/h$ in quantum dot-semiconductor-superconductor nanowire heterostructures have been interpreted as clear evidence for the presence of non-Abelian topologically protected Majorana zero modes (MZMs), since other sources of low-energy conductance are believed to be unable to produce such quantized plateaus. Based on extensive numerical calculations, we show that, in fact, quantized conductance plateaus of height $2e^2/h$ can also arise as a result of partially separated Andreev bound states (ps-ABSs), in which the component Majorana bound states are somewhat shifted in space without being topological MZMs. As ps-ABSs can form rather generically in the topologically trivial phase, even in the absence of disorder, our results conclusively establish that the observation of quantized conductance plateaus of height $2e^2/h$ does not represent sufficient evidence for the existence of topologically protected MZMs localized at the opposite ends of a wire.