We compute the electronic band structure and optical properties of boron arsenide using the relativistic quasiparticle self-consistent $GW$ approach, including electron-hole interactions through solution of the Bethe-Salpeter equation. We also calculate its electronic and optical properties using standard and hybrid density functional theory. We demonstrate that the inclusion of self-consistency and vertex corrections provides substantial improvement in the calculated band features, in particular, when comparing our results to previous calculations using the single-shot $GW$ approach and various density functional theory (DFT) methods, from which a considerable scatter in the calculated indirect and direct band gaps has been observed. We find that BAs has an indirect gap of 1.674 eV and a direct gap of 3.990 eV, consistent with experiment and other comparable computational studies. Hybrid DFT reproduces the indirect gap well, but provides less accurate values for other band features, including spin-orbit splittings. Our computed Born effective charges and dielectric constants confirm the unusually covalent bonding characteristics of this III-V system.

The narrow band-gap semiconductor PbTe exhibits a number of striking properties upon thallium (Tl)-doping, including the onset of superconductivity at temperatures $T_c$ which are substantially higher than for materials with equivalent charge carrier concentration. Here we provide a thorough optical investigation of ${\mathrm{Pb}}_{1-x}{\mathrm{Tl}}_x\mathrm{Te}$ over a very broad spectral range and contrast its normal-state, complete excitation spectrum with the optical response of the nonsuperconducting analog Na-doped PbTe. We capture the relevant energy scales shaping their electronic structure and uncover the formation of an impurity band upon doping with Tl, which evolves into a resonant state for large doping. This implies a large density of states and an enhancement of the optical effective mass $m^{*}/m_e$ of the itinerant charge carriers, which is stronger for Tl- than for Na-doping. Since the enhancement of $m^{*}/m_e$ particularly occurs upon crossing a critical concentration $x_c$ in Tl-doped PbTe for which $T_c\ne$ 0, we advance its relevance for the onset of superconductivity.

$\mathrm{Cu}{\left(\mathrm{pz}\right)}_2{\left({\mathrm{ClO}}_4\right)}_2$ (with pz denoting pyrazine, ${\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2)$ is a two-dimensional spin-1/2 square-lattice antiferromagnet with $T_{\mathrm{N}}=4.24$ K. Due to a persisting focus on the low-temperature magnetic properties, its room-temperature structural and physical properties caught no attention up to now. Here we report a study of the structural features of $\mathrm{Cu}{\left(\mathrm{pz}\right)}_2{\left({\mathrm{ClO}}_4\right)}_2$ in the paramagnetic phase, up to 330 K. By employing magnetization, specific heat, ${}^{35}\mathrm{Cl}$ nuclear magnetic resonance, and neutron diffraction measurements, we provide evidence of a second-order phase transition at $T^{★}=294$ K, not reported before. The absence of a magnetic ordering across $T^{★}$ in the magnetization data, yet the presence of a sizable anomaly in the specific heat, suggests a structural order-to-disorder-type transition. NMR and neutron-diffraction data corroborate our conjecture by revealing subtle angular distortions of the pyrazine rings and of ${\mathrm{ClO}}_4^{-}$ counteranion tetrahedra, shown to adopt a configuration of higher symmetry above the transition temperature.

Topological insulators are insulating bulk materials hosting conducting surface states. Their magnetic doping breaks time-reversal symmetry and generates numerous interesting effects such as dissipationless transport. Nonetheless, their dynamical properties are still poorly understood. Here, we perform a systematic investigation of transverse spin excitations of $3d$ and $4d$ single impurities embedded in two prototypical topological insulators (${\mathrm{Bi}}_2{\mathrm{Te}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$). The impurity-induced states within the bulk gap of the topological insulators are found to have a drastic impact on the spin excitation spectra, resulting in very high lifetimes reaching up to microseconds. An intuitive picture of the spin dynamics is obtained by mapping onto a generalized Landau-Lifshitz-Gilbert phenomenological model. The first quantity extracted from this mapping procedure is the magnetic anisotropy energy, which is then compared to the one provided by the magnetic force theorem. This uncovers some difficulties encountered with the latter, which can provide erroneous results for impurities with a high density of states at the Fermi energy. Moreover, the Gilbert damping and nutation tensors are obtained. The nutation effects can lead to a non-negligible shift in the spin excitation resonance in the high-frequency regime. Finally, we study the impact of the surface state on the spin dynamics, which may be severely altered due to the repositioning of the impurity-induced state in comparison to the bulk case. Our systematic investigation of this series of magnetic impurities sheds light on their spin dynamics within topological insulators, with implications for available and future experimental studies.

The cubic, tetragonal, and orthorhombic phase of potassium niobate (${\mathrm{KNbO}}_3$) are studied based on density-functional theory. Starting from the relaxed atomic geometries, we analyze the influence of self-energy corrections on the electronic band structure within the $GW$ approximation. We find that quasiparticle shifts widen the direct (indirect) band gap by 1.21 (1.44), 1.58 (1.55), and 1.67 (1.64) eV for the cubic, tetragonal, and orthorhombic phase, respectively. By solving the Bethe-Salpeter equation, we obtain the linear dielectric function with excitonic and local-field effects, which turn out to be essential for good agreement with experimental data. From our results, we extract an exciton binding energy of 0.6, 0.5, and 0.5 eV for the cubic, tetragonal, and orthorhombic phase, respectively. Furthermore, we investigate the nonlinear second-harmonic generation (SHG) both theoretically and experimentally. The frequency-dependent second-order polarization tensor of orthorhombic ${\mathrm{KNbO}}_3$ is measured for incoming photon energies between 1.2 and 1.6 eV. In addition, calculations within the independent-(quasi)particle approximation are performed for the tetragonal and orthorhombic phase. The novel experimental data are in excellent agreement with the quasiparticle calculations and resolve persistent discrepancies between earlier experimental measurements and ab initio results reported in the literature.

Efficient charge to spin conversion is important for low-power spin logic devices. Spin and charge interconversion is commonly performed using heavy metals and topological insulators, while the field of oxides is not yet fully explored. Strontium iridate thin films were grown, where the different crystal structures form a perfect playground to understand the key factors in obtaining high charge to spin conversion efficiency (i.e., large spin Hall angle). It was found that the semiconducting ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ has a spin Hall angle of $\sim 0.1$ (depending on measurement technique), which is promising for a spin-orbit coupled electronic system and comparable to Pt. In contrast, the perovskite ${\mathrm{SrIrO}}_3$, reported to have a Dirac cone near the Fermi level, has a larger spin Hall angle of 0.3–0.4 degrees. The largest difference between the two materials is a large degree of spin-momentum locking in ${\mathrm{SrIrO}}_3$, comparable to known topological insulators. A simple semiclassical relationship is found where the spin Hall angle increases for higher degrees of spin-momentum locking and it also increases for lower Fermi wave vectors. This relationship is then able to explain the decreased spin Hall angle below 10 nm film thickness in ${\mathrm{SrIrO}}_3$, by relating it to the correspondingly higher carrier concentration (related to the higher Fermi wave vector). Breaking the commonly believed anticorrelation between resistivity and carrier concentration paves a pathway to lower power losses due to resistance while keeping large spin Hall angles.

The MAX phase boride ${\mathrm{Nb}}_2\mathrm{SB}$ and the solid solutions $\mathrm{N}{\mathrm{b}}_2\mathrm{S}{\mathrm{B}}_x{\mathrm{C}}_{1-x}\left(x=0–1\right)$ were synthesized via solid-state methods and characterized by x-ray powder diffraction. All phases crystallize in the hexagonal ${\mathrm{Cr}}_2\mathrm{AlC}$ type with the space group $P{6}_3/mmc$. The lattice parameters increase with the boron content $[a=3.278(1)-3.334(1\left)\AA \right(+1.7\%),c=11.49(1)-11.54(1\left)\AA \right(+0.5\%\left)\right]$ and the distortions of the $\mathrm{N}{\mathrm{b}}_6\left(\mathrm{B},\mathrm{C}\right)$ octahedra slightly decrease. Magnetic susceptibility and dc resistivity measurements confirm that ${\mathrm{Nb}}_2\mathrm{SC}$ is a superconductor while ${\mathrm{Nb}}_2\mathrm{SB}$ shows no superconducting transition above 1.9 K. The solid solutions $\mathrm{N}{\mathrm{b}}_2\mathrm{S}{\mathrm{B}}_x{\mathrm{C}}_{1-x}$ are metals and superconductors for $x=0–0.6$ with critical temperatures of $T_{\mathrm{c}}=4.8–2.6\mathrm{K}$, which decrease with increasing boron content. First-principles density-functional theory calculations confirm the metallic state and a lower electronic density of states at the Fermi energy in the boride. The calculated elastic constants, phonon density of states, and Debye temperatures of ${\mathrm{Nb}}_2\mathrm{SB}$ are similar to ${\mathrm{Nb}}_2\mathrm{SC}$ and are probably not the reason for the absence of superconductivity in the boride. We therefore suggest that the lower $N\left({\varepsilon }_{\mathrm{F}}\right)$ of the boride reduces the interaction strength and thus the superconducting critical temperature.

The complex dielectric constants of $\mathrm{GaA}{\mathrm{s}}_{1-x}\mathrm{B}{\mathrm{i}}_x$ alloys grown by molecular beam epitaxy with $x=0\%$ to 17% have been measured over the spectral range from 0.37 to 9.1 eV using spectroscopic ellipsometry. Critical points in the joint density of states have been analyzed by fitting the line shape of the Van Hove singularities in the dielectric function derived from the ellipsometry data. The critical points generally match similar critical points in the dielectric function of GaAs with at least one alternative critical point. The energy of the critical points involving transitions from the top of the valence band show a strong dependence on Bi concentration similar to the composition dependence of the band gap, while the other critical points have a weaker dependence on Bi concentration. The measured composition dependence of the band gap and the energy of the split-off hole band are in good agreement with density functional calculations. The composition dependence of the index of refraction in the vicinity of the band gap for $\mathrm{GaA}{\mathrm{s}}_{1-x}\mathrm{B}{\mathrm{i}}_x$ alloys is an order of magnitude larger than the composition dependence of the index of refraction in $\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{I}{\mathrm{n}}_x\mathrm{As}$ alloys.

We demonstrate that transport in metallic rare-earth nickelates can be engineered by directly tuning the electronic mean free path. Using irradiation as a tool to induce disorder, we drive this system from a metallic phase into an Anderson insulator. This proceeds via an intermediate regime which shows a thermal crossover from insulating to metallic behavior. We argue that this phase falls within the paradigm of weak localization in three dimensions. We develop a theoretical model for the temperature dependence of resistivity which shows good agreement with our data. The three-dimensional weak localization picture is supported by magnetoconductivity, which scales as $\sim B^2$ up to several tesla. Interestingly, our data indicate that this phase lies in the Mott-Ioffe-Regel regime with the mean free path approaching the lattice constant. Upon further increasing disorder, the charge carriers are localized, leading to insulating behavior at all temperatures. Our results show that irradiation provides a “clean” tuning knob for the mean free path, without altering other system parameters. This suggests promising directions for studies of Anderson localization.

A hexagonal chromium pernitride, ${\mathrm{CrN}}_2$, has been successfully synthesized $via$ a direct chemical reaction between chromium and molecular nitrogen at a pressure of approximately 70 GPa. The advanced synchrotron x-ray diffraction (XRD) measurements after recovery and structural refinement at 0.1 MPa demonstrated that ${\mathrm{CrN}}_2$ crystallizes in the anti-NiAs-type structure with lattice parameters of $a=2.7465\left(3\right)\AA$ and $c=7.3670\left(4\right)\AA$. Micron-sized hexagonal habit compounds were found in the synthesized products, which indicated that the single crystal of hexagonal ${\mathrm{CrN}}_2$ was grown in the supercritical nitrogen at high pressure. High-pressure in situ XRD measurements were performed to determine the zero-pressure bulk modulus, $K_0$, value of 286(6) GPa $\left[{K^{\prime }}_0=2.5\left(7\right)\right]$ and remarkable incompressibility of the $c$ axis. ${\mathrm{CrN}}_2$ is a unique example of the recoverable transition-metal pernitride which presents one-dimensionally aligned N-N dimers in the structure. The one-dimensionally aligned N-N dimers play an important role in controlling the mechanical and electronic properties of ${\mathrm{CrN}}_2$ as predicted by theoretical calculations.

We use coarse-grained molecular dynamics simulations to study the precursor steps for fibril formation in methylcellulose solutions. Simulations of ring stacking between two collapsed methylcellulose chains demonstrate the existence of a capture radius that is much larger than that predicted by polymer diffusion alone. When two rings are in very close proximity, they stack together to form a fibril precursor. Simulations of stacks of such rings suggest that this structure is metastable. In contrast, chains that are within the capture radius but not in close proximity, as well as for systems containing both ringlike and relaxed chains, fibril-like structures form via a distinctly different mechanism. Irrespective of their initial arrangement, the chains undergo two specific conformational changes: (i) a part of either a ring or a randomly coiled chain splays out and (ii) the splayed chain subsequently engulfs a nearby chain if it is within a certain capture distance. The latter results are consistent with recent experimental measurements of fibril formation by short methylcellulose chains, which suggests the formation of a twisted bundle.

Imaging ordered materials with coherent x rays holds great potential to improve our understanding of phenomena in complex materials systems where emergent behavior can arise due to coupling of spin, lattice, and orbital degrees of freedom. Coherent diffractive imaging (CDI) is a lensless imaging technique for probing the structure of materials in three dimensions. Central to the success of the CDI method is the inversion of propagated wave field information to recover a quantitative image of the illuminated crystalline structure. Present challenges faced with existing approaches to image recovery are often due to nonuniqueness of wave propagated forms of the electron density information that can cause prohibitive stagnation of the reconstruction algorithm. Here we report on a major advancement in image recovery that is able to recover the three-dimensional image of a 492 nm gold single crystal undergoing progressive deformation to a highly strained condition without the use of a priori information. Our findings also demonstrate the significance of robust image recovery techniques for revealing high resolution topological structure.

Time-dependent Hartree-Fock (TDHF) and Bethe-Salpeter equation (BSE) methods are benchmarked against configuration interaction (CI) calculations of excitation energies and photoabsorption cross-sections for small closed shell sodium clusters with up to six atoms in several low-energy configurations. The mean absolute deviation of lowest excitation energies of each symmetry in these clusters for TDHF or BSE calculations from CI results is about 0.1 eV. The Thomas-Reiche-Kuhn (TRK) sum rule is satisfied to within numerical accuracy for both TDHF and BSE calculations, however, the Tamm-Dancoff approximations applied to either method for these systems yield optical cross-sections which grossly violate the TRK sum rule. TDHF and BSE calculations for photoabsorption cross-sections of ${\mathrm{Na}}_8$ and ${\mathrm{Na}}_{20}$ clusters are compared to experiment and found to be in good agreement. A feature in the cross-section of ${\mathrm{Na}}_{20}$ above 4 eV is found to be caused by a large density of optical transitions rather than an incipient volume plasmon with a short lifetime.

We study the relationship between antiferromagnetic order and the electronic properties of ${\mathrm{KCuMnS}}_2$ with the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure. We propose two magnetic structures for ${\mathrm{KCuMnS}}_2$ with the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure. Powder samples of ${\mathrm{KCuMnS}}_2$ and ${\mathrm{KLiMnS}}_2$ were prepared for structural studies and magnetization measurements. In both compounds, the ${\mathrm{Mn}}^{2+}$ site is alloyed by either ${\mathrm{Cu}}^{+}$ or ${\mathrm{Li}}^{+}$. We also prepared single crystals of ${\mathrm{KCuMnS}}_2$ for x-ray and neutron-diffraction studies and resistivity measurements. We relate these properties to the electronic structure calculated with density functional theory. Neutron-diffraction studies reveal that ${\mathrm{KCuMnS}}_2$ exhibits long-range magnetic ordering with a Néel temperature near 160 K and a moment of 0.92(2) ${\mu }_B/{\mathrm{Mn}}^{2+}$ at 6 K. In contrast, ${\mathrm{KLiMnS}}_2$ never exhibits long-range magnetic ordering down to 3.5 K. Both sulfides never display a crystallographic phase transition from our temperature-dependent x-ray and neutron-diffraction studies. We discuss the magnetic phases in detail and how they relate to isostructural phases such as iron-based superconductors and related chalcogenides. Electrical resistance measurements indicate that while ${\mathrm{KCuMnS}}_2$ is semiconducting, there is an anomaly around the Néel temperature, which indicates that long-range magnetism influences its electronic structure.

Several oxides, including those of rare-earth metals such as $\mathrm{G}{\mathrm{d}}_2{\mathrm{O}}_3$, have been used to passivate the GaN (0001) surface and are being considered as possible gate oxides in metal-oxide-semiconductor high electron mobility transistor applications. One of the problems in heteroepitaxy of rare-earth oxides (REOs) on GaN is poor wetting related to the unfavorable interface and film surface energy balance. The use of Zintl and Zintl-like intermetallic compounds can offer a solution to overcome this problem. Using density-functional theory, we investigate the bare (0001)-oriented surface of wurtzite GaN and rare-earth Eu, Gd, and Ce adatoms on it including surface mobility, wetting, and electronic structure. We also explore the possibility of forming a Zintl-like transition layer, $\mathrm{EuG}{\mathrm{a}}_2$. Our results provide a microscopic understanding of the intermetallic layer formation and its potential role in the heteroepitaxy of REOs on GaN.

Two 4-cyanoresorcinol-derived achiral bent-core liquid crystalline materials with terephthalate wings terminated by long alkyl chains ${\mathrm{C}}_n{\mathrm{H}}_{2n+1}$, $1/n=\mathbf{1}/\mathbf{16}$ and 1/18, are investigated. Both compounds form a paraelectric smectic phase with uniform (synclinic) molecular tilt in layers of the SmC phase composed of fluctuating polar clusters (polarization randomized ${\mathrm{SmC}}_S{\mathrm{P}}_R$ phase). On cooling, the polar coherence length continuously grows and at the distinct phase transition from ${\mathrm{SmC}}_S{\mathrm{P}}_R$, a synclinic ${\mathrm{SmC}}_S{\mathrm{P}}_F$ phase is formed. The latter turns into a heliconical phase, formed either spontaneously for 1/16 or after application of an ac field for 1/18. The helical structure has a helical axis perpendicular to the layer planes and is stable over a relatively broad range of temperatures. The spontaneously helical phase, formed by achiral molecules, is described as ${\mathrm{SmC}}_S{\mathrm{P}}_F^{hel}$ where $S$, P, $F$, and “$hel$” stand for the synclinic, polar, ferroelectric, and helical, respectively. The helical structure is confirmed by measurements of the birefringence in planar-aligned cells as well as by conoscopy in homeotropic-aligned cells with in-plane electric field. The field applied in a plane at right angles to the helix axis leads to the deformed helical ferroelectric mode, and the field dependency of the apparent tilt angle unequivocally confirms that the phase under study is helical. It is concluded that the structure of this helical phase is similar to that of the ${\mathrm{SmC}}_{\alpha }^{*}$ phase formed by permanently chiral rodlike mesogens. Due to the achiral nature of the molecules, ${\mathrm{SmC}}_S{\mathrm{P}}_F^{hel}$ can alternatively be considered as ${\mathrm{SmCP}}_{\alpha }$ phase. Since the helical pitch is approximately the same for both 1/16 and 1/18, in spite of different molecular lengths and its correspondence of the structure with ${\mathrm{SmCP}}_{\alpha }$ (like ${\mathrm{SmC}}_{\alpha }^{*}$ for chiral molecules) the helical pitch is suggested to be incommensurate with the integral number of the layers. The relationship of this mirror-symmetry-broken polar helical phase with an apolar twist bend smectic phase of mesogenic dimers is also discussed.

The growth mode of strained epitaxial films relies on the interaction strength, the lattice matching, and the mechanical response of the system. The present work focuses on the basic physics of supported nano-islands by examining the characteristics of MgO/Ag(100) taken as a case study. The combination of experiments and simulations highlights the existence of a small size regime in which, despite the largest adhesion and the smallest mismatch, the islands are the least distorted by the substrate. We assign this unexpected behavior to the enhanced island stiffness which makes the cost of elastic distortion prohibitive compared to the associated gain of MgO-Ag interaction energy. The analysis provides a general framework to predict and/or understand nanoscale effects on interfacial pseudomorphism. These are likely to hold whatever the nature of the deposit and substrate under consideration. It may have far reaching consequences on many properties of supported nano-objects.

We demonstrate that the shear elastic constant of misfit-layered dichalcogenide films ${\left[\mathrm{SnSe}\right]}_n{\left[{\mathrm{MoSe}}_2\right]}_n$ with $n=1,2,3$, synthesized by the modulated elemental reactants method, is $c_{44}\approx 1$ GPa, an order of magnitude lower than $c_{44}$ of typical layered crystals that have weak interlayer van der Waals bonding. The films are synthesized by alternating deposition of the elements to a total thickness of $\approx 60$ nm followed by thermal annealing. We determine $c_{44}$ through measurements of the velocities of 700 nm wavelength surface acoustic waves propagating along the surface of Al/${\left[\mathrm{SnSe}\right]}_n{\left[{\mathrm{MoSe}}_2\right]}_n$/Si structures in combination with picosecond acoustics measurements of $c_{33}$ and calculations of the $c_{11}$, $c_{12}$, and $c_{13}$ elastic constants by density functional theory. We attribute the low value of $c_{44}$ to incommensurate interfaces between SnSe and ${\mathrm{MoSe}}_2$ layers and turbostratic disorder within the ${\mathrm{MoSe}}_2$ layers. We conclude that the ultralow shear modulus of disordered layered materials contributes significantly to their exceptionally low thermal conductivity.

The orthovanadates are useful as host matrices for immobilization of radioactive wastes. The thermodynamic stability of these materials is crucial for their applications in high-pressure and high-temperature environment. It is necessary to investigate the phonons in the entire Brillouin zone, beyond the zone-center phonons accessible in previous Raman and infrared experiments. We have carried out extensive neutron inelastic scattering experiments to derive the phonon dispersion relation of ${\mathrm{YVO}}_4$ up to high-energy transfer of 65 meV using a single crystal, which are perhaps reported for the first time in any orthovanadate compound. The measured phonon dispersion relation is in good agreement with our first-principles density functional theory as well as shell-model calculations. The calculated pressure dependence of phonon modes in the zircon and scheelite phases shows unstable modes and violation of the Born stability criteria at high pressure, which may be lead to instability in ${\mathrm{YVO}}_4$ at high pressures. We also calculate large anisotropy in the thermal expansion behavior, which arises from difference in anisotropic elasticity and mode Grüneisen parameters.