The change of shape under illumination by visible light, called photostriction, is investigated in the classical ferroelectrics barium titanate and lead titanate. By means of the $\mathrm{\Delta }\mathrm{SCF}$ method, the use of first-principle calculations confirms that the converse piezoelectric effect is the main driving force of the photostriction of the polar axis in those materials. As a result, when compared to barium titanate and bismuth ferrite, lead titanate is a better photostrictive material in the direction of the polar axis, due to its larger longitudinal piezoelectric constant. On the other hand, in directions transverse to the polar axis, photoinduced electronic pressure can also become a sizable contribution that can either compete or cooperate with the piezoelectric effect, depending on the transitions involved. A simple Landau model is further developed and shows reasonable qualitative agreement with results from $\mathrm{\Delta }\mathrm{SCF}$ calculations, which is promising for a fast screening of materials with high photostrictive effects.

This manuscript describes harmonic generation in semiconductor superlattices, starting from a nonequilibrium Green's functions input to relaxation rate–type analytical approximations for the Boltzmann equation in which imperfections in the structure lead to asymmetric current flow and scattering processes under forward and reverse bias. The resulting current-voltage curves and the predicted consequences on harmonic generation, notably the development of even harmonics, are in good agreement with experiments. Significant output for frequencies close to 1 THz (7th harmonic) at room temperature, after excitation by a 141-GHz input signal, demonstrate the potential of superlattice devices for gigahertz to terahertz applications.

We investigate how external screening shapes excitons in two-dimensional semiconductors embedded in laterally structured dielectric environments. An atomic scale view of these elementary excitations is developed using models which apply to a variety of materials including transition metal dichalcogenides. We find that structured dielectrics imprint a peculiar potential energy landscape on excitons in these systems: While the ground-state exciton is least influenced, higher excitations are attracted towards regions with high dielectric constant of the environment. This landscape is “inverted” in the sense that low energy excitons are less strongly affected than their higher energy counterparts. Corresponding energy variations emerge on length scales of the order of a few unit cells. This opens the prospect of trapping and guiding of higher excitons by means of tailor-made dielectric substrates on ultimately small spatial scales.

In their work, Yan et al. [Phys. Rev. B 88, 121403 (2013)] employing density functional perturbation theory (DFPT) calculations, demonstrate that silicene and germanene show weaker Kohn anomalies in the $\mathrm{\Gamma }\text{−}{E}_g$ and $K\text{−}{A}_1$ phonon modes, compared to graphene. Furthermore, the electron-phonon (e-ph) coupling matrix elements were computed using the frozen-phonon approach. They found that in silicene the average e-ph coupling matrix element square over the Fermi-surface ${\langle g_{\mathbf{q}\nu }^2\rangle }_F$ is about 50% of those in graphene, but in germanene is weaker and nearly negligible. However, Yan et al. [Phys. Rev. B 88, 121403 (2013)] argue that the smaller Fermi velocity in silicene compensates the reduced ${\langle g_{\mathbf{q}\nu }^2\rangle }_F$, leading to phonon linewidths (${\gamma }_{\mathbf{q}\nu }$) slightly larger than those in graphene. In this Comment, we show that the DFPT and the frozen-phonon results of Yan et al. [Phys. Rev. B 88, 121403 (2013)] for silicene are inconsistent. Additionally, we have evaluated the e-ph coupling using direct DFPT calculations, analytical relations, and frozen-phonon calculations, and we found systematically that ${\langle g_{\mathbf{q}\nu }^2\rangle }_F$ and ${\gamma }_{\mathbf{q}\nu }$ in silicene are one order of magnitude smaller than in graphene, in contrast to the conclusions of Yan et al. [Phys. Rev. B 88, 121403 (2013)].

Atomistic effective Hamiltonian simulations are used to investigate electrocaloric (EC) effects in the lead-free $\mathrm{Ba}\left({\mathrm{Zr}}_{0.5}{\mathrm{Ti}}_{0.5}\right){\mathrm{O}}_3$ (BZT) relaxor ferroelectric. We find that the EC coefficient varies nonmonotonically with the field at any temperature, presenting a maximum that can be traced back to the behavior of BZT's polar nanoregions. We also introduce a simple Landau-based model that reproduces the EC behavior of BZT as a function of field and temperature, and which is directly applicable to other compounds. Finally, we confirm that, for low temperatures (i.e., in nonergodic conditions), the usual indirect approach to measure the EC response provides an estimate that differs quantitatively from a direct evaluation of the field-induced temperature change.

In current fracture theory, the fracture stress is related to the surface energy on the basis of linear elastic theory. However, the fracture stress does not necessarily exceed the stress required to break atomic bonds. Here, we show that a jump in the inelastic separation energy is generated by fracture, where the inelastic separation energy is the energy between the separation planes measured by excluding the contribution of elastic relaxation, and the stress at the onset of the energy jump is the fracture stress. Analysis of the electronic states of $\beta -\mathrm{SiC}$ (cubic SiC), Ge, and Cu by first-principles tensile tests shows that the electrons redistribute during surface formation in the transition from the onset to the end of the energy jump. Therefore, it is suggested that the inelastic separation energy at the end of the energy jump can be identified with the fracture energy. Also, first-principles shear tests show that an energy jump occurs during shearing for $\beta -\mathrm{SiC}$, but not for Ge and Cu. Thus, an energy jump is a sign of fracture (bond breaking), and an energy jump during shearing is a good indicator estimating the ductile and brittle character. These principles can hold for any solid and will therefore be beneficial for the fundamental understanding of the mechanical properties of solids and for their industrial applications.

We construct a distribution function of the strain-tensor components induced by point defects in an elastically anisotropic continuum, which can be used to account quantitatively for many effects observed in different branches of condensed matter physics. Parameters of the derived six-dimensional generalized Lorentz distribution are expressed through the integrals computed over the array of strains. The distribution functions for the cubic diamond and elpasolite crystals and tetragonal crystals with the zircon and scheelite structures are presented. Our theoretical approach is supported by a successful modeling of specific line shapes of singlet-doublet transitions of the $\mathrm{T}{\mathrm{m}}^{3+}$ ions doped into $\mathrm{AB}{\mathrm{O}}_4$ ($\mathrm{A}=\mathrm{Y}$, Lu; $\mathrm{B}=\mathrm{P}$, V) crystals with zircon structure, observed in high-resolution optical spectra. The values of the defect strengths of impurity $\mathrm{T}{\mathrm{m}}^{3+}$ ions in the oxygen surroundings, obtained as a result of this modeling, can be used in future studies of random strains in different rare-earth oxides.

The density of states of a three-dimensional Dirac equation with a random potential as well as in other problems of quantum motion in a random potential placed in sufficiently high spatial dimensionality appears to be singular at a certain critical disorder strength. This was seen numerically in a variety of studies as well as supported by detailed renormalization group calculations. At the same time it was suggested by a number of arguments accompanied by detailed numerical simulations that this singularity is rounded off by the rare region fluctuations of random potential, and that tuning the disorder past its critical value is not a genuine phase transition but rather a crossover. Here we develop an analytic theory which explains how rare region effects indeed lead to rounding off of the singularity and to the crossover replacing the transition.

The magnetization process of single crystals of the metallic kagome ferromagnet $\mathrm{C}{\mathrm{o}}_3\mathrm{S}{\mathrm{n}}_2{\mathrm{S}}_2$ was carefully measured via magnetization and ac susceptibility. Field-dependent anomalous transitions observed in low fields indicate the presence of an unconventional magnetic phase just below the Curie temperature, $T_{\mathrm{C}}$. The magnetic phase diagrams in low magnetic fields along different crystallographic directions were determined for the first time. The magnetic relaxation measurements at various frequencies covering five orders of magnitude from 0.01 to 1000 Hz indicate markedly slow spin dynamics only in the anomalous phase with characteristic relaxation times longer than 10 s.

The close interplay between superconductivity and antiferromagnetism in several quantum materials can lead to the appearance of an unusual thermodynamic state in which both orders coexist microscopically, despite their competing nature. A hallmark of this coexistence state is the emergence of a spin-triplet superconducting gap component, called a $\pi$ triplet, which is spatially modulated by the antiferromagnetic wave vector, reminiscent of a pair density wave. In this paper, we investigate the impact of these $\pi$-triplet degrees of freedom on the phase diagram of a system with competing antiferromagnetic and superconducting orders. Although we focus on a microscopic two-band model that has been widely employed in studies of iron pnictides, most of our results follow from a Ginzburg-Landau analysis, and as such should be applicable to other systems of interest, such as cuprates and heavy fermion materials. The Ginzburg-Landau functional reveals not only that the $\pi$-triplet gap amplitude couples trilinearly with the singlet gap amplitude and the staggered magnetization magnitude but also that the $\pi$-triplet $d$-vector couples linearly with the magnetization direction. While in the mean-field level this coupling forces the $d$-vector to align parallel or antiparallel to the magnetization, in the fluctuation regime it promotes two additional collective modes—a Goldstone mode related to the precession of the $d$-vector around the magnetization and a massive mode, related to the relative angle between the two vectors, which is nearly degenerate with a Leggett-like mode associated with the phase difference between the singlet and triplet gaps. We also investigate the impact of magnetic fluctuations on the superconducting-antiferromagnetic phase diagram, showing that due to their coupling with the $\pi$-triplet order parameter the coexistence region is enhanced. This effect stems from the fact that the $\pi$-triplet degrees of freedom promote an effective attraction between the antiferromagnetic and singlet superconducting degrees of freedom, highlighting the complex interplay between these two orders, which goes beyond mere competition for the same electronic states.

Based on recent high-resolution angle-resolved photoemission spectroscopy measurement in monolayer FeSe grown on ${\mathrm{SrTiO}}_3$, we constructed a tight-binding model and proposed a superconducting (SC) pairing function which can well fit the observed band structure and SC gap anisotropy. Then we investigated the spin excitation spectra in order to determine the possible sign structure of the SC order parameter. We found that a resonancelike spin excitation may occur if the SC order parameter changes sign along the Fermi surfaces. However, this resonance is located at different locations in momentum space compared to other FeSe-based superconductors, suggesting that the Fermi surface shape and pairing symmetry in monolayer FeSe grown on ${\mathrm{SrTiO}}_3$ may be different from other FeSe-based superconductors.

Fluctuation magnetoconductivity and magnetization above the superconducting transition temperature ($T_c$) are measured on the recently discovered 112 family of iron-based superconductors (IBS), ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{Fe}}_{1-y}{\mathrm{Ni}}_y{\mathrm{As}}_2$, which presents an extra As-As chain spacer-layer. The analysis in terms of a generalization of the Lawrence-Doniach approach to finite applied magnetic fields indicates that these compounds are among the most anisotropic IBS ($\gamma$ up to $\sim 30$) and provides compelling evidence of a quasi-two-dimensional behavior for doping levels near the optimal one.

Motivated by the recent observations of small Fermi energies and comparatively large superconducting gaps, present also on bands not crossing the Fermi energy (incipient bands) in iron-based superconductors, we analyze the doping evolution of superconductivity in a four-band model across the Lifshitz transition including BCS-BEC crossover effects on the shallow bands. Similar to the BCS case, we find that with hole doping the phase difference between superconducting order parameters of the hole bands change from 0 to $\pi$ through an intermediate $s+is$ state, breaking time-reversal symmetry (TRS). The transition, however, occurs in the region where electron bands are incipient and chemical potential renormalization in the superconducting state leads to a significant broadening of the $s+is$ region. We further present the qualitative features of the $s+is$ state that can be observed in scanning tunneling microscopy (STM) experiments, also taking incipient bands into account.

In a superconducting nanostructure, phase fluctuations are prominent and give rise to finite resistance below superconducting transition temperature $T_c$. By using a monolayer polymer/nanosphere hybrid we developed previously, we fabricated a large array of interconnected niobium (Nb) honeycomb lattices with the thinnest interconnected linewidth $d$ ranging from 36 nm to 89 nm. The honeycomb cells form a highly ordered triangular lattice with more than ${10}^8$ unit cells extending over few ${\mathrm{mm}}^2$ area, which enables the detailed transport study at nanometer scales. We found $T_c$ gradually drops with decreasing $d$ due to the phase-slip effect, while the critical field at lower temperature tends to follow that of a continuous Nb thin film. One likely scenario is to consider a model system of numerous superconducting islands interconnected by short phase-slip junctions, where the phase coherence is dictated by the phase slippage in the nanoconstriction. This was strongly supported by the excellent fitting to the thermally activated phase-slip model and also the unusual phenomena of transition width narrowing in high fields.

We study the dynamics of an ordered hexagonal monolayer of polystyrene microspheres adhered to a glass substrate coated with a thin aluminum layer. A laser-induced transient grating technique is employed to generate and detect three types of acoustic modes across the entire Brillouin zone in the $\mathrm{\Gamma }-K$ direction: low-frequency contact-based modes of the granular monolayer, high-frequency modes originating from spheroidal vibrations of the microspheres, and surface Rayleigh waves. The dispersion relation of contact-based and spheroidal modes indicates that they are collective modes of the microgranular crystal controlled by particle-particle contacts. We observe a spheroidal resonance splitting caused by the symmetry breaking due to the substrate, as well as an avoided crossing between the Rayleigh and spheroidal modes. The measurements are found to be in agreement with our analytical model.

The magnetic and thermal properties of single-crystalline ${\mathrm{NiSb}}_2{\mathrm{O}}_6$ are reported. The ${\mathrm{Ni}}^{2+}$ ions exhibit local magnetic order below $\sim 50$ K followed by long-range antiferromagnetic order below $T_N=6.7$ K. Analysis of the magnetic susceptibility data using the one-dimensional Heisenberg model with spin $\mathbb{S}=1$ results in a magnetic exchange coupling $J_{\left|\right|}/k_B\sim 26.0\left(1\right)$ K. $T_N$ is observed to either increase or decrease depending on whether field is applied perpendicular or parallel to the magnetic moment axis. A two-sublattice magnetic structure, with the axis of alignment alternating by $90{}^{\circ }$ between neighboring layers along the crystallographic $c$ axis, is argued to result in two magnetic transition temperatures for certain magnetic field directions, which agrees well with theory. This leads to a highly anisotropic magnetocaloric effect.

In transition metal oxides, quantum confinement arising from a large surface to volume ratio often gives rise to novel physicochemical properties at nanoscale. Their size-dependent properties have potential applications in diverse areas, including therapeutics, imaging, electronic devices, communication systems, sensors, and catalysis. We have analyzed the structural, magnetic, dielectric, and thermal properties of weakly ferromagnetic $\mathrm{SmFe}{\mathrm{O}}_3$ nanoparticles of sizes of about 55 and 500 nm. The nanometer-size particles exhibit several distinct features that are neither observed in their larger-size variants nor reported previously for the single crystals. In particular, for the 55-nm particle, we observe a sixfold enhancement of compensation temperature, an unusual rise in susceptibility in the temperature range 550 to 630 K due to spin pinning, and a coupled antiferromagnetic-ferroelectric transition, directly observed in the dielectric constant.

We employ ab initio fully relativistic electronic structure calculations to study the stability of the Weyl points in the momentum space within the class of the half-metallic ferromagnetic full Heusler materials, by focusing on ${\mathrm{Co}}_2\mathrm{TiAl}$ as a well-established prototype compound. Here we show that both the number of the Weyl points together with their $k$-space coordinates can be controlled by the orientation of the magnetization. This alternative degree of freedom, which is absent in other topological materials (e.g., in Weyl semimetals), introduces functionalities that are specific for the class of half-metallic ferromagnets. Of special interest are crossing points which are preserved irrespective of any arbitrary rotation of the magnetization axis.

We have investigated the response of brickwork artificial spin ice to an applied in-plane magnetic field through magnetic force microscopy, magnetotransport measurements, and micromagnetic simulations. We find that, by sweeping an in-plane applied field from saturation to zero in a narrow range of angles near one of the principal axes of the lattice, the moments of the system fall into an antiferromagnetic ground state in both connected and disconnected structures. Magnetotransport measurements of the connected lattice exhibit unique signatures of this ground state. Also, modeling of the magnetotransport demonstrates that the signal arises at vertex regions in the structure, confirming behavior that was previously seen in transport studies of kagome artificial spin ice.

We quantify and compare the relative weight of inverse spin-Hall and spin-rectification effects occurring in metallic polycrystalline (Py/Pt), epitaxial (Fe/Pt), and insulating (YIG/Pt) bilayers. To distinguish the spin rectification signal from the inverse spin-Hall voltage the external magnetic field is rotated in plane to take advantage of the different angular dependencies of the prevailing effects. We prove that in permalloy anisotropic magnetoresistance is the dominant source for spin rectification while in epitaxial iron the anomalous Hall effect has a comparable strength. The rectification in yttrium-iron-garnet/platinum bilayers reveals an angular dependence imitating the one seen for anisotropic magnetoresistance and anomalous Hall effect caused by spin-Hall magnetoresistance.