We report electronic properties of superconductivity in single crystals of topological nodal-line semimetal ${\mathrm{PbTaSe}}_2$. Resistivity, magnetic susceptibility, and specific heat measurements were performed on high-quality single crystals. We observed large, temperature-dependent anisotropy in its upper critical field ($H_{c2}$). The upper critical field measured for $H\parallel \mathbf{ab}$ plane shows a sudden upward feature instead of saturation at low temperatures. The specific heat measurements in magnetic fields reveal a full superconducting gap with no gapless nodes.

By combining DFT-based computational analysis and symmetry constraints in terms of group-subgroup relations, we analyzed the formation of the native crystalline structure of loellingite ${\mathrm{FeAs}}_2$. We showed that the ground state of the material exhibits the ordered patterns of the electronic localization which are mainly associated with iron $3d_{x^2-y^2}$ orbitals and can be characterized in terms of nematiclike ordering. The ordering is the result of the close interplay of the lattice and the electron degrees of freedom. In a structural aspect, the interplay pursues an energy quest to select the orthorhombic crystal lattice attributed to the $Pnnm$ space group. In a charge aspect, the ordering is connected with the valence charge density redistribution that not only provides a high electronic polarizability but also gives rise to an extra-large magnitude of the negative component of the dynamical $p\text{−}d$ charge transfer.

The novel ferroelectric-like structural transition observed in metallic ${\text{LiOsO}}_{\text{3}}$ [Y. Shi et al., Nat. Mater. 12, 1024 (2013)], has invoked many theoretical and experimental interests. In this work, we have performed polarized and temperature-dependent Raman scattering measurements on high-quality single crystal ${\text{LiOsO}}_{\text{3}}$ and identified Raman-active modes in both centrosymmetric phase (300 K, $R\overline{3}c)$ and noncentrosymmetric phase (10 K, $R3c)$. Only four phonon peaks are observed in the former phase, while there are 12 peaks in the latter phase because of the reduction of crystal symmetry. With the help of careful symmetry analysis and first-principles calculations, we can make a systematic assignment for the observed Raman modes in both phases. The significant changes in linewidth and the continuous evolution of Raman frequencies with temperatures were observed for the $E_g$ modes around the transition temperature, which suggests that the ferroelectric-like structural transition is a continuous order-disorder transition. The result sheds light on the coexistence of ferroelectricity and metallicity in the compound.

We have carried out micromagnetic simulations of the gyrotropic resonance mode of a magnetic vortex in the presence of spatially localized and spatially uniform out-of-plane magnetic fields. We show that the field-induced change in the gyrotropic mode frequency is significantly larger when the field is centrally localized over lengths which are comparable to or a few times larger than the vortex core radius. When aligned with the core magnetization, such fields generate an additional confinement of the core. This confinement increases the vortex stiffness in the small-displacement limit, leading to a resonance shift which is greater than that expected for a uniform out-of-plane field of the same amplitude. Fields generated by uniformly magnetized spherical particles having a fixed separation from the disk are found to generate analogous effects except that there is a maximum in the shift at intermediate particle sizes where field localization and stray field magnitude combine optimally to generate a maximum confinement.

We report on microwave measurements of the spectrum and the spatial intensity distribution of the eigenstates for an artificial Penrose tiling. The energy-scaling behavior of the Hamiltonian parameters in this particular system allows us to identify the main patterns that determine the first hierarchical structure of the spectrum and to provide a physical interpretation of the gap labeling. Our results apply to a general class of systems.

Quantum Monte Carlo (QMC) calculations of transition metal oxides are partially limited by the availability of high-quality pseudopotentials that are both accurate in QMC and compatible with major plane-wave electronic structure codes. We have generated a set of neon-core pseudopotentials with small cutoff radii for the early transition metal elements Sc to Zn within the local density approximation of density functional theory. The pseudopotentials have been directly tested for accuracy within QMC by calculating the first through fourth ionization potentials of the isolated transition metal (M) atoms and the binding curve of each M-O dimer. We find the ionization potentials to be accurate to 0.16(1) eV, on average, relative to experiment. The equilibrium bond lengths of the dimers are within 0.5(1)% of experimental values, on average, and the binding energies are also typically accurate to 0.18(3) eV. The level of accuracy we find for atoms and dimers is comparable to what has recently been observed for bulk metals and oxides using the same pseudopotentials. Our QMC pseudopotential results also compare well with the findings of previous QMC studies and benchmark quantum chemical calculations.

Mott insulators with strong spin-orbit coupling have been proposed to host unconventional magnetic states, including the Kitaev quantum spin liquid. The $4d$ system $\alpha -{\mathrm{RuCl}}_3$ has recently come into view as a candidate Kitaev system, with evidence for unusual spin excitations in magnetic scattering experiments. We apply a combination of optical spectroscopy and Raman scattering to study the electronic structure of this material. Our measurements reveal a series of orbital excitations involving localized total angular momentum states of the Ru ion, implying that strong spin-orbit coupling and electron-electron interactions coexist in this material. Analysis of these features allows us to estimate the spin-orbit coupling strength, as well as other parameters describing the local electronic structure, revealing a well-defined hierarchy of energy scales within the Ru $d$ states. By comparing our experimental results with density functional theory calculations, we also clarify the overall features of the optical response. Our results demonstrate that $\alpha -{\mathrm{RuCl}}_3$ is an ideal material system to study spin-orbit coupled magnetism on the honeycomb lattice.

The influence of electronic many-body interactions, spin-orbit coupling, and thermal lattice vibrations on the electronic structure of lithium niobate is calculated from first principles. Self-energy calculations in the $GW$ approximation show that the inclusion of self-consistency in the Green function $G$ and the screened Coulomb potential $W$ opens the band gap far stronger than found in previous $G_0{W}_0$ calculations but slightly overestimates its actual value due to the neglect of excitonic effects in $W$. A realistic frozen-lattice band gap of about 5.9 eV is obtained by combining hybrid density functional theory with the $\mathrm{QS}G{W}_0$ scheme. The renormalization of the band gap due to electron-phonon coupling, derived here using molecular dynamics as well as density functional perturbation theory, reduces this value by about 0.5 eV at room temperature. Spin-orbit coupling does not noticeably modify the fundamental gap but gives rise to a Rashba-like spin texture in the conduction band.

We predict quantized Imbert-Fedorov, Goos-Hänchen, and photonic spin Hall shifts for light beams impinging on a graphene-on-substrate system in an external magnetic field. In the quantum Hall regime, the Imbert-Fedorov and photonic spin Hall shifts are quantized in integer multiples of the fine structure constant $\alpha$, while the Goos-Hänchen ones in multiples of ${\alpha }^2$. We investigate the influence on these shifts of magnetic field, temperature, and material dispersion and dissipation. An experimental demonstration of quantized beam shifts could be achieved at terahertz frequencies for moderate values of the magnetic field.

We study the transient heat current out of a confined electron system into a weakly coupled electrode in response to a voltage switch. We show that the decay of the Coulomb interaction energy for this repulsive system exhibits signatures of electron-electron attraction, and is governed by an interaction-independent rate. This can only be understood from a general duality that relates the nonunitary evolution of a quantum system to that of a dual model with inverted energies. Deriving from the fermion-parity superselection postulate, this duality applies to a large class of open systems.

Direct laser writing of conductive paths in synthetic diamond is of interest for implementation in radiation detection and clinical dosimetry. Unraveling the microscopic processes involved in laser irradiation of diamond below and close to the graphitization threshold under the same conditions as the experimental procedure used to produce three-dimensional devices is necessary to tune the laser parameters to optimal results. To this purpose a transient currents technique has been used to measure laser-induced current signals in monocrystalline diamond detectors in a wide range of laser intensities and at different bias voltages. The current transients vs time and the overall charge collected have been compared with theoretical simulations of the carrier dynamics along the duration and after the conclusion of the 30 fs laser pulse. The generated charge has been derived from the collected charge by evaluation of the lifetime of the carriers. The plasma volume has also been evaluated by measuring the modified region. The theoretical simulation has been implemented in the framework of the empirical pseudopotential method extended to include time-dependent couplings of valence electrons to the radiation field. The simulation, in the low-intensity regime, $I\sim 1\mathrm{TW}/{\mathrm{cm}}^2$, predicts substantial deviation from the traditional multiphoton ionization, due to nonperturbative effects involving electrons from degenerate valence bands. For strong field with intensity of about $50\mathrm{TW}/{\mathrm{cm}}^2$, nonadiabatic effects of electron-hole pair excitation become prominent with high carrier densities eventually causing the optical breakdown of diamond. The comparison of theoretical prediction with experimental data of laser-generated charge vs laser energy density yields a good quantitative agreement over six orders of magnitude. At the highest intensities the change of slope in the trend is explained taking into account the dependence of the optical parameters and the carrier mobility on plasma density.

We report a high-pressure ${}^{75}\mathrm{As}$ NMR study on the heavily hole-doped iron pnictide superconductor ${\mathrm{KFe}}_2{\mathrm{As}}_2$ ($T_c\approx 3.8$ K). The low-energy spin fluctuations are found to decrease with applied pressure up to 2 GPa, but then to increase again, changing in lockstep with the pressure-induced evolution of $T_c$. Their diverging nature suggests close proximity to a magnetic quantum critical point at a negative pressure of $P\simeq -0.6$ GPa. Above 2.2 GPa, the ${}^{75}\mathrm{As}$ satellite spectra split below 40 K, indicating a breaking of As site symmetry and an incipient charge order. These pressure-controlled phenomena demonstrate the presence of nearly critical fluctuations in both spin and charge, providing essential input for the origin of superconductivity.

We formulate a spin-polarized van Leeuwen and Baerends (vLB) correction to the local density approximation (LDA) exchange potential [R. van Leeuwen and E. J. Baerends, Phys. Rev. A 49, 2421 (1994)] that enforces the ionization potential (IP) theorem following T. Stein et al. [Phys. Rev. Lett. 105, 266802 (2010)]. For electronic-structure problems, the vLB correction replicates the behavior of exact-exchange potentials, with improved scaling and well-behaved asymptotics, but with the computational cost of semilocal functionals. The vLB + IP correction produces a large improvement in the eigenvalues over those from the LDA due to correct asymptotic behavior and atomic shell structures, as shown in rare-gas, alkaline-earth, zinc-based oxides, alkali halides, sulfides, and nitrides. In half-Heusler alloys, this asymptotically corrected LDA reproduces the spin-polarized properties correctly, including magnetism and half-metallicity. We also consider finite-sized systems [e.g., ringed boron nitride (${\mathrm{B}}_{12}{\mathrm{N}}_{12}$) and graphene (${\mathrm{C}}_{24}$)] to emphasize the wide applicability of the method.

We point out that the effective channel for the interfacial thermal conductance, the inverse of Kapitza resistance, of metal-insulator/semiconductor interfaces is governed by the electron-phonon interaction mediated by the surface states allowed in a thin region near the interface. Our detailed calculations demonstrate that the interfacial thermal conductance across Pb/Pt/Al/Au-diamond interfaces are only slightly different among these metals, and reproduce well the experimental results of the interfacial thermal conductance across metal-diamond interfaces observed by Stoner et al. [Phys. Rev. Lett. 68, 1563 (1992)] and most recently by Hohensee et al. [Nat. Commun. 6, 6578 (2015)].

Using an atomic force microscope we performed measurements of the Casimir force between a gold- coated (Au) microsphere and doped silicon carbide (SiC) samples. The last of these is a promising material for devices operating under severe environments. The roughness of the interacting surfaces was measured to obtain information for the minimum separation distance upon contact. Ellipsometry data for both systems were used to extract optical properties needed for the calculation of the Casimir force via the Lifshitz theory and for comparison to the experiment. Special attention is devoted to the separation of the electrostatic contribution to the measured total force. Our measurements demonstrate large contact potential ${\mathrm{V}}_0(\approx 0.67\mathrm{V})$, and a relatively small density of charges trapped in SiC. Knowledge of both Casimir and electrostatic forces between interacting materials is not only important from the fundamental point of view, but also for device applications involving actuating components at separations of less than 200 nm where surface forces play dominant role.

$\mathrm{MnTi}{\mathrm{O}}_3$ has been studied for many decades, but it was only in the last few years that its magnetoelectric behavior had been observed. Here, we use neutron scattering on two separately grown single crystals and two powder samples to show the presence of a supercell that breaks $R\overline{3}$ symmetry. We also present the temperature and field dependence of the dielectric constant and pyroelectric current and show evidence of nonzero off-diagonal magnetoelectric tensor elements (forbidden by $R\overline{3}$ symmetry) followed by a polarization flop accompanying the spin flop transition at ${\mu }_0{H}_{\mathrm{SF}}=6.5\mathrm{T}$. Mössbauer spectroscopy on $\mathrm{MnTi}{\mathrm{O}}_3$ gently doped with ${}^{57}\text{Fe}$ was used to help shed light on the impact of the supercell on the observed behavior. Although the full supercell structure could not be solved at this time due to a lack of visible reflections, the full scope of the results presented here suggest that the role of local spin-lattice coupling in the magnetoelectric properties of $\mathrm{MnTi}{\mathrm{O}}_3$ is likely more important than previously thought.

The ground state of the large Hubbard $U$ limit of a honeycomb lattice near half filling is known to be a singlet $d+id$-wave superconductor. It is also known that this $d+id$ superconductor exhibits a chiral $p+ip$ pairing locally at the Dirac cone, characterized by a $2\mathbb{Z}$ topological invariant. By constructing a dual transformation, we demonstrate that this $2\mathbb{Z}$ topological superconductor is equivalent to a collection of two topological ferromagnetic insulators. As a result of the duality, the topology of the electronic structures for a $d+id$ superconductor is controllable via the change of the chemical potential by tuning the gate voltage. In particular, instead of always being a chiral superconductor, we find that the $d+id$ superconductor undergoes a topological phase transition from a chiral superconductor to a quasihelical superconductor as the gap amplitude or the chemical potential decreases. The quasihelical superconducting phase is found to be characterized by a topological invariant in the pseudospin charge sector with vanishing both the Chern number and the spin Chern number. We further elucidate the topological phase transition by analyzing the relationship between the topological invariant and the rotation symmetry. Due to the angular momentum carried by the gap function and spin-orbit interactions, we show that by placing $d+id$ superconductors in proximity to ferromagnets, varieties of chiral superconducting phases characterized by higher Chern numbers can be accessed, providing a platform for hosting large numbers of Majorana modes at edges.

Spin-$S$ Heisenberg quantum antiferromagnets on the kagome lattice offer, when placed in a magnetic field, a fantastic playground to observe exotic phases of matter with (magnetic analogs of) superfluid, charge, bond, or nematic orders, or a coexistence of several of the latter. In this context, we have obtained the (zero-temperature) phase diagrams up to $S=2$ directly in the thermodynamic limit owing to infinite projected entangled pair states, a tensor network numerical tool. We find incompressible phases characterized by a magnetization plateau versus field and stabilized by spontaneous breaking of point group or lattice translation symmetry(ies). The nature of such phases may be semiclassical, as the plateaus at the $\frac{1}{3}\mathrm{th},\left(1-\frac{2}{9S}\right)\mathrm{th}$, and $\left(1-\frac{1}{9S}\right)\mathrm{th}$ of the saturated magnetization (the latter followed by a macroscopic magnetization jump), or fully quantum as the spin-$\frac{1}{2}$ $\frac{1}{9}$ plateau exhibiting a coexistence of charge and bond orders. Upon restoration of the spin rotation $U\left(1\right)$ symmetry, a finite compressibility appears, although lattice symmetry breaking persists. For integer spin values we also identify spin gapped phases at low enough fields, such as the $S=2$ (topologically trivial) spin liquid with no symmetry breaking, neither spin nor lattice.