In a conventional semiconductor, electrons $\left(e\right)$ scatter efficiently with longitudinal-optical phonons $\left(\mathrm{p}{\mathrm{h}}_{\mathrm{LO}}\right)$ via the Fröhlich interaction, e.g., in polaron formation or hot-electron cooling. Recent studies on lead halide perovskites have suggested that the Fröhlich interaction may be sufficiently screened by the polar lattice. Here we show that hot-electron cooling in a layered ferroelectric semiconductor $\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ slows down by 50% as temperature is increased from 115 to 387 K; this decrease is about four times more than that predicted by the Fröhlich model. We suggest that screening of Fröhlich interaction by polar domains or fluctuations in the ferroelectric semiconductor may be responsible for the reduced $\mathrm{e}\text{−}\mathrm{p}{\mathrm{h}}_{\mathrm{LO}}$ scattering. Such enhanced screening in polar materials featuring anharmonic lattice instability may also reduce carrier scattering with other charge carriers or with charged defects, leading to improved optoelectronic properties.

We show that the finite lateral sizes of ultrathin CdSe nanoplatelets strongly affect both their photoluminescence and optical absorption spectra. This is in contrast to the situation in quantum wells, in which the large lateral sizes may be assumed to be infinite. The lateral sizes of the nanoplatelets are varied over a range of a few to tens of nanometers. For these sizes excitons experience in-plane quantum confinement, and their center-of-mass motion becomes quantized. Our direct experimental observation of the discretization of the exciton center-of-mass states can be well understood on the basis of the simple particle-in-a-box model.

Ge atoms segregating on zirconium diboride thin films grown on Ge(111) were found to crystallize into a two-dimensional bitriangular structure, which was recently predicted to be a flat band material through an embedded kagome lattice. Angle-resolved photoelectron emission experiments together with theoretical calculations pointed out the existence of a nearly flat band in spite of non-negligible in-plane long-range hopping and interactions with the substrate. This provides an experimental verification for the fact that a flat band can emerge from the electronic coupling between atoms and not from the geometry of the atomic structure.

$\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$ is a rare material that offers the opportunity to investigate the complex interplay between charge density wave (CDW) ordering and topology-protected electronic band structure. As both of these phenomena are governed by the structural symmetries, a comprehensive study of the lattice dynamics is highly desirable. In this paper, we present the results of temperature- and pressure-dependent Raman spectroscopy and x-ray diffraction in single crystalline $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$. Our results confirm that Raman spectroscopy is a highly sensitive tool to probe the CDW-ordering phenomenon, particularly the low-temperature second CDW transition in $\mathrm{La}\mathrm{Ag}{\mathrm{Sb}}_2$, which appears as a very weak anomaly in most experiments. The crystal-orientation-dependent measurements provide the evolution of Raman modes with crystallographic symmetries and can be further studied through group symmetry analysis. The low-temperature x-ray diffraction data show the emergence of structural modulations corresponding to the CDW instability. The combined high-pressure Raman spectroscopy and synchrotron x-ray diffraction reveal multiple structural phase transitions through lowering of crystalline symmetries, which are also expected to lead to electronic topological transitions.

Scanning tunneling microscopy and spectroscopy were performed on the (001) cleavage plane of praseodymium hexaboride ($\mathrm{Pr}{\mathrm{B}}_6$). We found three different ordered morphologies, namely, a chainlike ($2\times 1$) reconstruction and two uniform terminations. The chainlike ($2\times 1$) reconstruction is rationalized as parallel Pr rows on top of a complete ${\mathrm{B}}_6$ network. The two uniform terminations are identified as complete Pr or ${\mathrm{B}}_6$ layers. Although the uniform terminations could be expected to be simply ($1\times 1$) reconstructed, one of them shows a rather stripelike atomic corrugation for close tip-sample distances. All morphologies share two spectral features at $-0.2$ and $+0.2$ eV around $E_{\text{F}}$. In addition, one uniform termination shows an additional peak in the differential conductance at $-0.7\mathrm{eV}$. Similarly, the chainlike ($2\times 1$) reconstruction reveals a feature in the differential conductance at $-1.1\mathrm{eV}$ when moving the tip closer to the surface. The distance dependency points towards rather localized electronic states, which we tentatively attribute to a $4f$-related feature.

We show here that the application of potential barriers that induce contributions of hyperboloid subbands (pseudospin filters) in graphene nanoribbons locally breaks the chiral symmetry of the Dirac state, in the sides of the barrier, with the consequent transition from Klein to anti-Klein behavior. With the increased filter potential applied, resonances of Fabry-Pérot type with line widths that are decreasing are generated in the conductance, which is associated with a pseudospin precession located on the sides of the barrier. Interestingly, throughout this process the chiral symmetry of the state is conserved, and pseudospin oscillations (opposite) that increase in intensity are observed on the sides of the filter. This is associated with a gradual loss of the electron-hole correlation, with the increased filter potential, which ends with the formation of a transport gap when the electron-hole symmetry is completely broken. This is an example of how the chiral symmetry can evolve and still be conserved, during a tunneling process. All this leads to the generation of energy gaps, associated with anti-Klein tunneling, which can be controlled using certain spatial configurations of the applied filters. The inclusion of a new type of asymmetry (roughness in the filters) makes it possible to recover Klein's tunneling in the region of induced gaps. The interaction of the hyperboloid subbands with the Dirac band can be observed in density-of-states maps using the pseudospin filters, following the behavior of the van Hove singularities as a function of the filter potential.

Prevailing nanostructuring strategies focus on increasing phonon scattering and reducing the mean-free-path of phonons across the spectrum. In nanoporous Si materials, for example, boundary scattering reduces thermal conductivity drastically. In this work, we identify an unusual anticorrelated specular phonon scattering effect which can result in additional reductions in thermal conductivity of up to ∼80% for specific nanoporous geometries. We further find evidence that this effect has its origin in heat trapping between large pores with narrow necks. As the heat becomes trapped between the pores, phonons undergo multiple specular reflections such that their contribution to the thermal conductivity is partly undone. We find this effect to be wave-vector dependent at low temperatures. We use large-scale molecular-dynamics simulations, wave-packet analysis, as well as an analytical model to illustrate the anticorrelation effect, evaluate its impact on thermal conductivity, and detail how it can be controlled to manipulate phonon transport in nanoporous materials.

The $3d\text{−}5d$ based double perovskites offer an ideal playground to study the interplay between electron correlation ($U$) and spin-orbit coupling (SOC) effect, showing exotic physics. The ${\mathrm{Sr}}_2{\mathrm{FeIrO}}_6$ is an interesting member in this family with ionic distribution of ${\mathrm{Fe}}^{3+}$ ($3d^5$) and ${\mathrm{Ir}}^{5+}$ ($5d^4$) where the later is believed to be nonmagnetic under the picture of strong SOC. Here we report a detailed investigation of structural, magnetic, and electronic transport properties along with electronic structure calculations in ${\left({\mathrm{Sr}}_{1-x}{\mathrm{Ca}}_x\right)}_2{\mathrm{FeIrO}}_6$ series with $x$ from 0 to 1. While the basic interactions such as $U$ and SOC are unlikely to be modified, a structural modification is expected due to ionic size difference between ${\mathrm{Sr}}^{2+}$ and ${\mathrm{Ca}}^{2+}$ which would influence other properties such as crystal field effect and bandwidths. While nonmonotonic changes in lattice parameters are observed across the series, the spectroscopic investigations reveal that $3+/5+$ charge state of Fe/Ir continue till the end of the series. An analysis of magnetic data suggests similar nonmonotonic evolution of magnetic parameters with doping. Temperature dependent crystal structure as well as low temperature (5 K) magnetic structure have been determined from neutron powder diffraction measurements which further indicate site ordered moments for both Fe and Ir. The whole series shows insulating behavior with a nonmonotonic variation in resistivity where the charge transport follows the three-dimensional variable range hopping model. The electronic structure calculations show, SOC enhanced, a noncollinear antiferromagnetic and a Mott-type insulating state is the stable ground state for the present series with a substantial amount of orbital moment, but less than the spin magnetic moment, at the Ir site and the magnetocrystalline anisotropy. The calculations further show the evolution of the spin and orbital magnetic moment components across the series along with the magnetization density. The obtained results imply that the local structural modification with introduction of lower size ${\mathrm{Ca}}^{2+}$ has a large influence on the magnetic and transport properties, further showing a large agreement between experimental results as well as theoretical calculations.

Here, we report an exhaustive study of the frequency-dependent ac magnetic susceptibility of the $D_{2d}$-symmetric Heusler system Mn-Pt(Pd)-Sn that hosts antiskyrmions over a wide temperature range. Magnetic relaxation studies using Cole-Cole formalism reveal a Debye-type relaxation with a nearly negligible distribution in relaxation times. In contrast to the archetypical skyrmion hosts, the high Curie temperature $T_C$ of the present system ensures shorter switching times, and correspondingly, higher frequencies are required to probe the relaxation dynamics. We find a nonmonotonic variation in the characteristic relaxation time with distinct maxima at the phase boundaries via helical $⟶$ antiskyrmion $⟶$ field-polarized states, indicating slower magnetization dynamics over the region of phase coexistence. The temperature-dependent relaxation time across different phases is of the order of ${10}^{-5}–{10}^{-4}$ s and follows the well-known Arrhenius law with reasonable values of the energy barriers. The present study concerning the magnetization dynamics in the antiskyrmion host tetragonal Heusler system is an important contribution towards the basic understanding of the dynamical aspects of antiskyrmions for their potential applications.

Based on a recently developed combination of layer group analysis with order-parameter symmetry, we study the polarity of antiphase domain boundaries (APBs) and ferroelastic twin boundaries (TBs) in $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$. In addition to the celebrated layer group analysis of domain twins, the present method allows us to investigate tensor properties of domain walls also for the case where order-parameter variables other than the spontaneous ones are active. We find that antiphase boundaries in $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ can carry a polarization if in addition to the spontaneous order parameter $(0,0,{\varphi }_s)$ a second component, i.e., $({\varphi }_1,0,{\varphi }_s)$, develops within the domain wall. This result, which is solely based on symmetry arguments, strongly suggests that polarization in APBs is possible if a phase transition from an Ising-type wall to a Néel- or Bloch-like wall occurs. This is in very good agreement with previous calculations based on Landau-Ginzburg free energy expansions including biquadratic ($\propto P_i{P}_j{\varphi }_k{\varphi }_l$) [A. K. Tagantsev et al. Phys. Rev. B 64, 224107 (2001)] and flexoelectric ($\propto P_k\frac{\partial u_{ij}}{\partial \xi }$) coupling terms [A. N. Morozovska et al. Phys. Rev. B. 85, 094107 (2012)]. The present results also unveil a close connection between the recently discovered macroscopic polarization in antiferrodistortive cycloids of ferroelastic domain walls of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ and a mechanism for explaining polarization of APBs.

We utilize simulations of spin-polarized electron scattering by a chain of localized quantum spins to show that energy and linear momentum conservation laws impose strong constraints on the properties of magnetic excitations induced by spin transfer. In turn, orbital and spin dynamics of conduction electrons depends on the dynamical characteristics of the local spins. Our results suggest the possibility to achieve precise control of spin-transfer-driven magnetization dynamics by tailoring the spectral characteristics of the magnetic systems and of the driving electrons.

We investigated the transport characteristics of superconducting Ta thin films with three configurations of radio-frequency radiation filters: no filter, only room-temperature filter, and low- and room-temperature filters. We find that when the room-temperature filter is installed, the entire transition is shifted to higher temperatures, and once the zero-resistance state is achieved at zero magnetic field, no strong radiation effect is observed. When the magnetic field is turned on, the non-zero-resistance saturation at low temperatures is revealed without low-temperature filters, which has previously been considered to be a magnetic-field-induced quantum metallic phase. However, the insertion of the additional low-temperature filters weakens the saturation of the resistance, i.e., the signature of the metallic behavior. This observation suggests that the previously reported anomalous metallic state in Ta films is mainly induced by unfiltered radiation, and, thus, the intrinsic metallic ground state, if it exists, should be limited to a narrow range of magnetic fields near the critical point.

The role of nematic order for the mechanism of high-temperature superconductivity is highly debated. In most iron-based superconductors (IBSs) the tetragonal symmetry is broken already in the normal state, resulting in orthorhombic lattice distortions, static stripe magnetic order, or both. Superconductivity then emerges, at least at weak doping, already from the state with broken $C_4$ rotational symmetry. One of the few stoichiometric IBSs, lithium iron arsenide superconducts below 18 K and does not display either structural or magnetic transition in the normal state. Here we demonstrate, using angle-resolved photoemission spectroscopy, that even the superconducting state in LiFeAs is also a nematic one. We observe spontaneous breaking of the rotational symmetry in the gap amplitude on all Fermi surfaces, as well as unidirectional distortion of the Fermi pockets. Remarkably, these deformations are hardly visible above superconducting $T_c$. Our results demonstrate the realization of the phenomenon of superconductivity-induced nematicity in IBSs, emphasizing the intimate relation between them. We suggest a theoretical explanation based on the emergence of a secondary instability inside the superconducting state, which leads to the nematic order and $s-d$ mixing in the gap function.

We study numerically the superconductor to insulator transition in two-dimensional phase-glass (or chiral-glass) models with varying degree of disorder. These models describe the effects of gauge disorder in superconductors due to random negative Josephson-junction couplings, or $\pi$ junctions. Two different models are considered, with binary and Gaussian distribution of quenched disorder, having nonzero mean. Monte Carlo simulations in the path-integral representation are used to determine the phase diagram and critical exponents. In addition to the usual superconducting and insulating phases, a chiral-glass phase occurs for sufficiently large disorder, with random local circulating currents of different chiralities. A transition from superconductor to insulator can take place via the intermediate chiral-glass phase. We find, however, that the chiral-glass state has a finite phase stiffness, being still a superconductor, instead of the Bose metal, which has been suggested by mean-field theory.

Double-Weyl fermions, as novel topological states of matter, have been mostly discussed in nonmagnetic materials. Here, based on density-functional theory and symmetry analysis, we propose the realization of fully spin-polarized double-Weyl fermions in a family ferromagnetic materials ${\mathrm{X}}_2{\mathrm{RhF}}_6$ ($X=\text{K}$, Rb, Cs). These materials have the half-metal ground states, where only the bands from the spin-down channel present near the Fermi energy. The spin-down bands form a pair of triply degenerate nodal points (TDNPs) if spin-orbit coupling (SOC) is not included. Under SOC, one TDNP splits into two double-Weyl points featuring quadratic dispersion along two momentum directions with the chiral charge of $\pm 2$, and they are protected by the three-fold rotation ($C_3$) symmetry. Unlike most double-Weyl semimetals, the Weyl points proposed here have the type-III dispersion with one of the crossing bands being saddle-shaped. An effective model is constructed, which describes well the nature of the Weyl points. These Weyl points are fully spin-polarized, and are characterized with double Fermi arcs on the surface spectrum. Breaking $C_3$ symmetry by lattice strain could shift one double-Weyl point into a pair of type-II single-Weyl points. The ${\mathrm{X}}_2{\mathrm{RhF}}_6$ materials proposed here are excellent candidates to investigate the novel properties of type-III double-Weyl fermions in ferromagnetic system, as well as generate potential applications in spintronics.

Bulk ceramics and thick nanogranular films of mixed-valence cobaltite ${\left({\mathrm{Pr}}_{1-y}{\mathrm{Y}}_y\right)}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Co}{\mathrm{O}}_3$ were studied using time-domain terahertz transmission spectroscopy and infrared reflection spectroscopy. Thanks to the high probing frequency, the overall increase in the conductivity during the insulator to metal transition was observed both in the ceramics and in the nanogranular films. The terahertz conductivity spectrum shows that the transition is not a simple cross-over between two states. As the charges become delocalized during the insulator to metal transition, a broad resonant mode corresponding to the eigenfrequency of the binding potential appears in the terahertz conductivity spectra in the ceramics, and it further softens as the metallic state prevails. This is interpreted as a consequence of the complexity of charge motion in a temperature-dependent energy landscape, changing dramatically from the tightly bound charges on atomic distance in the insulating phase to almost free charges in the metallic phase. This behavior is not directly observed in the nanogranular films as their nature introduces spectral signatures of an additional localization due to grain boundaries which obscure the pronounced effects observed in the bulk. Nevertheless fine spectral signatures accompanying the phase transition are still resolved. The insulator to metal transition is also accompanied by a weak softening of infrared-active phonon modes, which is related to the expansion of lattice parameters upon phase transition.

Spin-orbit coupling (SOC) is essential in understanding the properties of $5d$ transition metal compounds, whose SOC value is large and almost comparable to other key parameters. Over the past few years, there have been numerous studies on the SOC-driven effects of the electronic bands, magnetism, and spin-orbit entanglement for those materials with a large SOC. However, how the SOC affects the lattice dynamics is less studied and remains an unsolved problem. We therefore measured the phonon spectra of $5d$ pyrochlore ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ over the full Brillouin zone to address the question by using inelastic x-ray scattering. Our main finding is a visible mode dependence in the phonon spectra, measured across the metal-insulator transition at 227 K. We examined the SOC strength dependence of the lattice dynamics and its spin-phonon (SP) coupling, with first-principles calculations. Our experimental data taken at 100 K are in good agreement with the theoretical results obtained with the optimized $U=2.0\mathrm{eV}$ with SOC. By scaling the SOC strength and the $U$ value in the density-functional theory calculations, we demonstrate that SOC is more relevant than $U$ in explaining the observed mode-dependent phonon energy shifts with temperature. Furthermore, the temperature dependence of the phonon energy can be effectively described by scaling SOC. Our work provides clear evidence of SOC producing a non-negligible and essential effect on the lattice dynamics of ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ and its SP coupling.