SnTe belongs to the recently discovered class of topological crystalline insulators. Here, we study the formation of line defects which break crystalline symmetry by strain in thin SnTe films. Strained SnTe(111) films are grown by molecular beam epitaxy on lattice and thermal expansion coefficient-mismatched CdTe. To analyze the structural properties of the SnTe films, we applied in situ reflection high-energy electron diffraction, x-ray reflectometry, high-resolution x-ray diffraction, reciprocal space mapping, and scanning tunneling microscopy. This comprehensive analytical approach reveals a twinned structure, tensile strain, bilayer surface steps, and dislocation line defects forming a highly ordered dislocation network for thick films with local strains up to 31% breaking the translational crystal symmetry.

We report a type-II intermetallic superconductor ${\mathrm{Y}}_7\mathrm{R}{\mathrm{u}}_4\mathrm{InG}{\mathrm{e}}_{12}$ with a transition temperature $\left(T_{\mathrm{c}}\right)$ of ∼5.8 K, which is confirmed by zero resistivity, diamagnetic magnetic susceptibility, and specific-heat jump. Single crystals of ${\mathrm{Y}}_7\mathrm{R}{\mathrm{u}}_4\mathrm{InG}{\mathrm{e}}_{12}$ were grown from a reactive indium flux. ${\mathrm{Y}}_7\mathrm{R}{\mathrm{u}}_4\mathrm{InG}{\mathrm{e}}_{12}$ crystallizes in the tetragonal space group $P4/m$ and features a $\left[\mathrm{R}{\mathrm{u}}_4\mathrm{InG}{\mathrm{e}}_{12}\right]$ polyanionic network with Y atoms located in three different channels. The upper critical fields of ${\mathrm{Y}}_7\mathrm{R}{\mathrm{u}}_4\mathrm{InG}{\mathrm{e}}_{12}$ at 0 K are determined to be ∼5.3 and 2.4 T along the $c$ axis and the $ab$ plane, respectively. The estimated coherence length along the $c$ axis (∼174 Å) is much larger than the estimated mean free path along the $c$ axis (∼29 Å) in ${\mathrm{Y}}_7\mathrm{R}{\mathrm{u}}_4\mathrm{InG}{\mathrm{e}}_{12}$, placing its superconductivity in the so-called dirty regime. The compound exhibits a large superconducting specific-heat jump $\frac{\mathrm{\Delta }C}{{\gamma }_e{T}_{\mathrm{c}}}\approx 2.4$, significantly well above the weak-coupling Barden-Cooper-Schrieffer theoretical value of 1.43 and pointing to a strong-coupling scenario in ${\mathrm{Y}}_7\mathrm{R}{\mathrm{u}}_4\mathrm{InG}{\mathrm{e}}_{12}$. Density-functional-theory calculations show that the density of states in ${\mathrm{Y}}_7\mathrm{R}{\mathrm{u}}_4\mathrm{InG}{\mathrm{e}}_{12}$ exhibits a broad peak near the Fermi level which mainly derives from $\mathrm{Y}\text{−}4d,\mathrm{Ru}\text{−}4d$, and $\mathrm{Ge}\text{−}4p$ states.

We report a novel quasi-two-dimensional compound of ${\mathrm{EuTe}}_4$ hosting charge density waves (CDW) instability. The compound has a crystallographic structure in a orthorhombic space group Pmmn (No. 59) with cell parameters $a=4.6347\left(2\right)\AA$, $b=4.5119\left(2\right)\AA$, $c=15.6747\left(10\right)\AA$ at room temperature. The pristine structure contains consecutive near-square Te sheets separated by corrugated Eu-Te slabs. Upon cooling, the compound experiences a phase transition near 255 K. X-ray crystallographic analysis and transmission electron microscopy measurements reveal strong structural distortions in the low-temperature phase, showing a $1a\times 3b\times 2c$ superstructure with a periodic formation of Te trimers in the monolayer Te sheets, yielding evidence for the formation of CDW order. The charge transport properties show a semiconducting behavior in the CDW state. Density functional theory calculations reveal a Fermi surface nesting-driven instability with a nesting vector in good agreement with the one observed experimentally. Our finding provides a promising system for the study of CDW driven two-dimensional (2D) semiconducting mechanisms, which would shed a light on exploring 2D semiconductors with collective electronic states.

The antiferromagnetic semiconductor MnTe has recently attracted attention for spintronics and high-performance thermoelectric applications. However, little is known about its vibrational and thermal transport properties and how these might relate to the electronic and magnetic structure, particularly as related to $3d$ Mn orbital correlations. Here, we calculate a physically justified Coulomb correlation parameter within the $\mathrm{DFT}+U$ framework. We couple this framework with the Heisenberg Hamiltonian and first-principles Boltzmann transport to understand the magnetic, vibrational, and phonon thermal transport properties of MnTe. We also perform inelastic neutron and nuclear inelastic x-ray scattering measurements of the total and partial phonon density of states, respectively. Very good agreement is obtained with the measured and calculated phonon density of states, and with available measurements for the band gap, local magnetic moments, Néel temperature, magnon dispersion, thermal conductivity, and phonon dispersion. This study demonstrates that the vibrational and magnetic degrees of freedom are not strongly coupled in MnTe, and provides a more comprehensive picture of this technologically promising material.

Far-from-equilibrium evolution of metallic nanocluster shapes is highly sensitive to the atomistic-level details of surface diffusion for diverse local surface configurations. A stochastic model was developed incorporating realistic values for the multiple diffusion barriers (contrasting previous unrealistic generic prescriptions) based upon insights from homoepitaxial film growth. Kinetic Monte Carlo simulation then elucidates the conversion of Ag nanocubes to Wulff polyhedra mediated by nucleation of new $\left\{100\right\}$ facets, the pinch-off of sufficiently elongated Ag nanorods, and key aspects of sintering for orientationally aligned Ag and Au nanoclusters. The time scale for sintering of Au nanoclusters observed in high-resolution transmission electron microscopy studies was also recovered.

The interfacial coupling between the switchable polarization and neighboring magnetic order makes ferroelectric/ferromagnetic composite structures a versatile platform to realize voltage control of magnetic anisotropy. We report the nonvolatile ferroelectric field effect modulation of the magnetocrystalline anisotropy (MCA) in epitaxial $\mathrm{PbZ}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}{\mathrm{O}}_3$ (PZT)/$\mathrm{L}{\mathrm{a}}_{0.8}\mathrm{S}{\mathrm{r}}_{0.2}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) heterostructures grown on (001) $\mathrm{SrTi}{\mathrm{O}}_3$ substrates. Planar Hall effect measurements show that the in-plane magnetic anisotropy energy in LSMO is enhanced by about 22% in the hole accumulation state compared to the depletion state, in quantitative agreement with our first-principles density functional theory calculations. Modeling the spin-orbit coupling effect with second-order perturbation theory points to the critical role of the $d$-orbital occupancy in controlling MCA. Our work provides insights into the effect of ferroelectric polarization on the magnetic anisotropy at the composite multiferroic interfaces, paving the path for their implementation into high-performance, low-power spintronic applications.

We perform Raman spectroscopy studies on $\alpha \text{−}{\mathrm{RuCl}}_3$ at room temperature to investigate its phase transitions of magnetism and structure under pressure. The Raman measurements resolve two critical pressures at $p_1=1.1$ GPa and $p_2=1.7$ GPa with very different structural and magnetic behaviors. With increasing pressure, a stacking order phase transition of $\alpha \text{−}{\mathrm{RuCl}}_3$ occurs at $p_1$, indicated by the new Raman modes. The appearance of infrared active modes in Raman spectrum usually results from the inversion symmetry breaking, which is confirmed by the second harmonic generation measurement. The second transition at $p_2$ is signaled by the in-plane Ru-Ru bond dimerization accompanied by the Mott collapse and the system becomes a correlated band insulator. Our studies demonstrate the competition among spin-orbit coupling, magnetism, and chemical bondings in Kitaev compounds.

We have experimentally investigated the magnetic properties of ${\mathrm{CoMnO}}_3$(0001) epitaxial films known to exhibit orbital ferrimagnetism. The films were grown by the reactive rf magnetron sputtering method. X-ray magnetic circular dichroism revealed that the spin momenta of Mn and Co ions are oriented opposite to each other and a large orbital momentum comparable to the spin angular momentum emerges only on Co, indicating the orbital ferrimagnetic nature of ${\mathrm{CoMnO}}_3$. The magnetic anisotropy of ${\mathrm{CoMnO}}_3$(0001) thin films was found to have a large negative value of $K_u=-15.6\pm 0.8\mathrm{Merg}/{\mathrm{cm}}^3$ at 300 K. The saturation magnetization was slightly smaller than the reported values of tiny single crystals, due to the presence of a sizable dead layer.

The binary ${\mathrm{Re}}_{1-x}{\mathrm{Mo}}_x$ alloys, known to cover the full range of solid solutions, were successfully synthesized and their crystal structures and physical properties investigated via powder x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. By varying the Re/Mo ratio, we explore the full ${\mathrm{Re}}_{1-x}{\mathrm{Mo}}_x$ binary phase diagram, in all its four different solid phases: hcp-Mg $\left(P{6}_3/mmc\right)$, $\alpha$-Mn $\left(I\overline{4}3m\right)$, $\beta$-CrFe $\left(P{4}_2/mnm\right)$, and bcc-W $\left(Im\overline{3}m\right)$, of which the second is noncentrosymmetric with the rest being centrosymmetric. All ${\mathrm{Re}}_{1-x}{\mathrm{Mo}}_x$ alloys are superconductors, whose critical temperatures exhibit a peculiar phase diagram, characterized by three different superconducting regions. In most alloys, the $T_c$ is almost an order of magnitude higher than in pure Re and Mo. Low-temperature electronic specific-heat data evidence a fully gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling across the series. Considering that several $\alpha$-Mn-type $\mathrm{Re}T$ alloys $(T$ = transition metal) show time-reversal symmetry breaking (TRSB) in the superconducting state, while TRS is preserved in the isostructural ${\mathrm{Mg}}_{10}{\mathrm{Ir}}_{19}{\mathrm{B}}_{16}$ or ${\mathrm{Nb}}_{0.5}{\mathrm{Os}}_{0.5}$, the ${\mathrm{Re}}_{1-x}{\mathrm{Mo}}_x$ alloys represent another suitable system for studying the interplay of space-inversion, gauge, and time-reversal symmetries in future experiments expected to probe TRSB in the $\mathrm{Re}T$ family.

We investigate the diffusive electron-transport properties of charge-doped graphene ribbons and nanoribbons with imperfect edges. We consider different regimes of edge scattering, ranging from wide graphene ribbons with (partially) diffusive edge scattering to ribbons with large width variations and nanoribbons with atomistic edge roughness. For the latter, we introduce an approach based on pseudopotentials, allowing for an atomistic treatment of the band structure and the scattering potential, on the self-consistent solution of the Boltzmann transport equation within the relaxation-time approximation and taking into account the edge-roughness properties and statistics. The resulting resistivity depends strongly on the ribbon orientation, with zigzag (armchair) ribbons showing the smallest (largest) resistivity and intermediate ribbon orientations exhibiting intermediate resistivity values. The results also show clear resistivity peaks, corresponding to peaks in the density of states due to the confinement-induced subband quantization, except for armchair-edge ribbons that show a very strong width dependence because of their claromatic behavior. Furthermore, we identify a strong interplay between the relative position of the two valleys of graphene along the transport direction, the correlation profile of the atomistic edge roughness, and the chiral valley modes, leading to a peculiar strongly suppressed resistivity regime, most pronounced for the zigzag orientation.

We report the molecular beam epitaxial growth, structure, and electronic measurements of single-crystalline LaAuSb films on ${\mathrm{Al}}_2{\mathrm{O}}_3$ (0001) substrates. LaAuSb belongs to a broad family of hexagonal $ABC$ intermetallics in which the magnitude and sign of layer buckling have strong effects on properties, e.g., predicted hyperferroelecticity, polar metallicity, and Weyl and Dirac states. Scanning transmission electron microscopy reveals highly buckled planes of Au-Sb atoms, with strong interlayer Au-Au interactions and a doubling of the unit cell. This buckling is four times larger than the buckling observed in other $ABC\mathrm{s}$ with similar composition, e.g., LaAuGe and LaPtSb. Photoemission spectroscopy measurements and comparison with theory suggest an electronic driving force for the Au-Au dimerization, since LaAuSb, with a 19-electron count, has one more valence electron per formula unit than most stable $ABC\mathrm{s}$. Our results suggest that the electron count, in addition to conventional parameters such as epitaxial strain and chemical pressure, provides a powerful means for tuning the layer buckling in ferroic $ABC\mathrm{s}$.

Using angle-resolved photoemission spectroscopy and density functional theory (DFT) we study the electronic structure of layered ${\mathrm{BaZnBi}}_2$. Our experimental results show no evidence of Dirac states in ${\mathrm{BaZnBi}}_2$ originated either from the bulk or the surface. The calculated band structure without spin-orbit interaction shows linear band dispersions at $X$ along the $X\text{−}M$ high-symmetry line. In addition, the calculations suggest a gapless band crossing point along the $\mathrm{\Gamma }\text{−}M$ high-symmetry line. However, as soon as the spin-orbit interaction is turned on, the band crossing point is significantly gapped out. These observations suggest that the Dirac fermions in ${\mathrm{BaZnBi}}_2$ are trivial similar to the Dirac states observed in grapheme. The experimental observations are in good agreement with the DFT calculations.

$\left(\mathrm{B}{\mathrm{i}}_{1/2}\mathrm{N}{\mathrm{a}}_{1/2}\right)\mathrm{Ti}{\mathrm{O}}_3$-based solid solutions, one of the major systems of lead-free piezoelectric ceramics, exhibit a low thermal depolarization temperature ($T_{\mathrm{d}}\sim 100{}^{\circ }\mathrm{C}$). It was reported that by incorporating 30 mol% ZnO particles to form a ceramic composite of ${\left(\mathrm{B}{\mathrm{i}}_{1/2}\mathrm{N}{\mathrm{a}}_{1/2}\right)}_{0.94}\mathrm{B}{\mathrm{a}}_{0.06}\mathrm{Ti}{\mathrm{O}}_3$/ZnO, the depolarization process can be shifted up to ∼250 °C. In the present work, a variety of advanced transmission electron microscopy techniques, including in situ heating, annular bright-field, high-angle annular dark-field, geometric phase analysis, energy-dispersive spectrum and electron energy-loss spectroscopy, are employed to investigate the underlying mechanisms for the enhanced thermal stability of polarization in the composite. It is found that the abrupt depolarization in ${\left(\mathrm{B}{\mathrm{i}}_{1/2}\mathrm{N}{\mathrm{a}}_{1/2}\right)}_{0.94}\mathrm{B}{\mathrm{a}}_{0.06}\mathrm{Ti}{\mathrm{O}}_3$ at $T_{\mathrm{d}}$ becomes diffused over a wide temperature window up to the temperature at maximum dielectric constant ($T_{\mathrm{m}}$) under the combined actions of the incorporation of Zn into the perovskite lattice, the presence of residual stresses, and the pinning effect on micron-sized domains provided by ZnO particles.

We propose an approach for simultaneously calculating absolute surface energies of polar and semipolar planes without invoking the similarity between hexagonal and cubic structures. Our approach is based on the calculations of total energy differences between two identical wedge-shaped geometries with ideal surfaces using density functional calculations in conjunction with the evaluations of surface dangling bonds for various orientations. The calculations demonstrate characteristic features of the surface stability depending on the crystal orientation: For both GaN and AlN, the surface energy of semipolar $\left(11\overline{2}2\right)$ plane under H-rich condition is lower than that of nonpolar $\left(11\overline{2}0\right)$ plane. The lower surface energy on the $\left(11\overline{2}2\right)$ plane compared with the $\left(11\overline{2}0\right)$ plane implies that the semipolar $\left(11\overline{2}2\right)$ surface is preferentially formed for high ${\mathrm{H}}_2$ pressures during the metal-organic vapor phase epitaxy, consistent with the experimental results of selective-area growth of GaN. Similar trends can also be found in the surface energies of semipolar $\left(1\overline{1}01\right)$ plane in both GaN and AlN, and the absolute surface energy on the $\left(1\overline{1}01\right)$ plane is found to be lower than that on the $\left(11\overline{2}2\right)$ plane. These calculated results suggest that all of the phenomena related to the stability of polar and semipolar planes of group-III nitrides would benefit from these accurate values of surface energies.

Theory and experiment are combined to gain an understanding of the electronic properties of ${\mathrm{OsO}}_{\text{2}}$, a poorly studied metallic oxide that crystallizes in the rutile structure. Hard and soft valence-band x-ray photoemission spectra of ${\mathrm{OsO}}_{\text{2}}$ single crystals are in broad agreement with the results of density-functional-theory calculations, aside from a feature shifted to high binding energy of the conduction band. The energy shift corresponds to the conduction electron plasmon energy measured by reflection electron energy loss spectroscopy. The plasmon satellite is reproduced by many-body perturbation theory.

We have investigated the electronic and optical properties of epitaxial $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Fe}{\mathrm{O}}_3$ for $0\le x\le 1$ prepared by molecular-beam epitaxy. Core-level and valence-band x-ray photoemission features monotonically shift to lower binding energy with increasing $x$, indicating downward movement of the Fermi level toward the valence-band maximum. Combining valence-band photoemission and O $K$-edge x-ray absorption data, we map the evolution of the occupied and unoccupied bands and observe a narrowing of the gap, along with a transfer of state density from just below to just above the Fermi level as a result of hole doping. In-plane transport measurements confirm that the material becomes a $p$-type semiconductor at lower doping levels and exhibits a conversion from semiconducting to metallic behavior at $x=1$. Low-energy optical transitions revealed by spectroscopic ellipsometry are explained based on insight from theoretical densities of states and first-principles calculations of optical absorption spectra.