Ab initio pseudopotential calculations have been performed to investigate the structural, electronic, and vibrational properties of ${\mathrm{SrPd}}_2{\mathrm{Ge}}_2, {\mathrm{SrPd}}_2{\mathrm{As}}_2$, and ${\mathrm{CaPd}}_2{\mathrm{As}}_2$ crystallizing in the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type body-centered tetragonal structure. Our electronic results show that the density of states at the Fermi level is mainly dominated by the strong hybridization of Pd $d$ states and Ge (or As) $p$ states. The linear response method and the Migdal-Eliashberg approach have been used to calculate the Eliashberg spectral function for all these compounds. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter $\left(\lambda \right)$ is found to be 0.74 for ${\mathrm{SrPd}}_2{\mathrm{Ge}}_2$, 0.66 for ${\mathrm{SrPd}}_2{\mathrm{As}}_2$, and 0.72 for ${\mathrm{CaPd}}_2{\mathrm{As}}_2$. Using the calculated values of $\lambda$ and the logarithmically averaged phonon frequency ${\omega }_{ln}$ the superconducting critical temperature $\left(T_c\right)$ values for ${\mathrm{SrPd}}_2{\mathrm{Ge}}_2, {\mathrm{SrPd}}_2{\mathrm{As}}_2$, and ${\mathrm{CaPd}}_2{\mathrm{As}}_2$ are found to be 3.20, 2.05, and 2.48 K, respectively, which are in acceptable agreement with the corresponding experimental values. The relative differences in the $T_c$ values between the Ge and As compounds have been explained in terms of some key physical parameters.

Fine structures in the tunneling spectra of superconductors have been widely used to identify fingerprints of the interaction responsible for Cooper pairing. Here, we show that for scanning tunneling microscopy (STM) of Pb, the inclusion of inelastic tunneling processes is essential for the proper interpretation of these fine structures. For STM the usual McMillan inversion algorithm of tunneling spectra must therefore be modified to include inelastic tunneling events, an insight that is crucial for the identification of the pairing glue in conventional and unconventional superconductors alike.

Ruthenium hydride was synthesized at a hydrogen pressure of about 14 GPa in a diamond-anvil cell. Energy-dispersive x-ray diffraction was used to monitor the ruthenium crystal structure as a function of hydrogen pressure up to 30 GPa. The hydride formation was accompanied by phase transition from the original hcp structure of the pristine metal to the fcc structure. Our results confirmed the theoretical prediction of ruthenium hydride formation under hydrogen pressure. The standard Gibbs free energy of the ruthenium hydride formation reaction was calculated assuming the pressure of decomposition as the equilibrium pressure.

The magnetization dynamics of individual nanomagnets is studied by spin-polarized scanning tunneling microscopy, combining real-time telegraphic noise analysis with pump-probe schemes. A transition between two Arrhenius regimes is observed as a function of temperature. The switching rates at high temperature are found to be orders of magnitude lower than expected from the extrapolation from the low-temperature regime. A four-state hopping model of magnetization reversal is developed to interpret the experimental results in terms of nucleation, annihilation, and propagation.

We investigate spin currents in microstructured permalloy/platinum bilayers that are excited via magnetic high-frequency fields. Due to this excitation spin pumping occurs at the permalloy/platinum interface and a spin current is injected into the platinum layer. The spin current is detected as a voltage via the inverse spin-Hall effect. We find two regimes reflected by a nonlinear, abrupt voltage surge, which is reproducibly observed at distinct excitation field strengths. Micromagnetic simulations suggest that the surge is caused by excitation of a spin-wave-like mode. The comparatively large voltages reveal a highly efficient spin-current generation method in a mesoscopic spintronic device.

We perform theoretical analysis of polarization-sensitive Raman spectroscopy on ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}, \mathrm{EuFe}2{\mathrm{As}}_2, {\mathrm{SrFe}}_2{\mathrm{As}}_2$, and $\mathrm{Ba}\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right){}_2{\mathrm{As}}_2$, focusing on two features seen in the $B_{1g}$ symmetry channel (in one Fe unit cell notation): the strong temperature dependence of the static, uniform Raman response in the normal state and the existence of a collective mode in the superconducting state. We show that both features can be explained by the coupling of fermions to pairs of magnetic fluctuations via the Aslamazov-Larkin process. We first analyze magnetically mediated Raman intensity at the leading two-loop order and then include interactions between pairs of magnetic fluctuations. We show that the full Raman intensity in the $B_{1g}$ channel can be viewed as the result of the coupling of light to Ising-nematic susceptibility via Aslamazov-Larkin process. We argue that the singular temperature dependence in the normal state is the combination of the temperature dependencies of the Aslamazov-Larkin vertex and of Ising-nematic susceptibility. We discuss two scenario for the resonance below $T_c$. One is the resonance due to development of a pole in the fully renormalized Ising-nematic susceptibility. Another is orbital excitonic scenario, in which spin fluctuations generate attractive interaction between low-energy fermions.

Weyl semimetals are gapless quasitopological materials with a set of isolated nodal points forming their Fermi surface. They manifest their quasitopological character in a series of topological electromagnetic responses including the anomalous Hall effect. Here, we study the effect of disorder on Weyl semimetals while monitoring both their nodal/semimetallic and topological properties through computations of the localization length and the Hall conductivity. We examine three different lattice tight-binding models which realize the Weyl semimetal in part of their phase diagram and look for universal features that are common to all of the models, and interesting distinguishing features of each model. We present detailed phase diagrams of these models for large system sizes and we find that weak disorder preserves the nodal points up to the diffusive limit, but does affect the Hall conductivity. We show that the trend of the Hall conductivity is consistent with an effective picture in which disorder causes the Weyl nodes move within the Brillouin zone along a specific direction that depends deterministically on the properties of the model and the neighboring phases to the Weyl semimetal phase. We also uncover an unusual (nonquantized) anomalous Hall insulator phase which can only exist in the presence of disorder.

Many metals display resistivity saturation—a substantial decrease in the slope of the resistivity as a function of temperature that occurs when the electron scattering rate ${\tau }^{-1}$ becomes comparable to the Fermi energy $E_F/\hslash$ (the Mott-Ioffe-Regel limit). At such temperatures, the usual description of a metal in terms of ballistically propagating quasiparticles is no longer valid. We present a tractable model of a large number $N$ of electronic bands coupled to $N^2$ optical phonon modes, which displays a crossover behavior in the resistivity at temperatures where ${\tau }^{-1}\sim E_F/\hslash$. At low temperatures, the resistivity obeys the familiar linear form, while at high temperatures, the resistivity still increases linearly, but with a modified slope (that can be either lower or higher than the low-temperature slope, depending on the band structure). The high-temperature non-Boltzmann regime is interpreted by considering the diffusion constant and the compressibility, both of which scale as the inverse square root of the temperature.

Electronic topological insulators are one of the breakthroughs of 21st century condensed matter physics. So far, the search for a light counterpart of an electronic time-reversal invariant topological insulator has remained elusive. This is due to the fundamentally different natures of light and matter and the different spins of photons and electrons. Here, it is shown that the theory of electronic topological insulators has a genuine analog in the context of light wave propagation in time-reversal invariant continuous materials. We introduce a gauge invariant ${\mathbb{Z}}_2$ index that depends on the global properties of the photonic band structure and is robust to any continuous weak variation of the material parameters that preserves the time-reversal invariance. A nontrivial ${\mathbb{Z}}_2$ index has two possible causes: (i) the lack of smoothness of the pseudo-Hamiltonian in the $\mathbf{k}\to \infty$ limit and (ii) the entanglement between positive and negative frequency eigenmode branches. In particular, it is proven that electric-type plasmas and magnetic-type plasmas are topologically inequivalent for a fixed wave polarization. We propose a bulk-edge correspondence that links the number of edge modes with the topological invariants of two continuous bulk materials and present detailed numerical examples that illustrate the application of the theory.

We present thermal measurements of large-area (over $1000\mu {\mathrm{m}}^2$) monolayer graphene samples at cryogenic temperatures to study the electron-phonon thermal conductivity of graphene. By using two large samples with areas which differ by a factor of 10, we are able to clearly show the area dependence of the electron-phonon cooling. We find that, at temperatures far below the Bloch-Grüneisen temperature $T_{\mathrm{BG}}$, the electron-phonon cooling power is accurately described by the $T^4$ temperature dependence predicted for clean samples. Using this model, we are able to extract a value for the electron-phonon coupling constant as a function of gate voltage and the graphene electron-lattice deformation potential.

Spectroscopic studies of mechanically exfoliated monolayer $\mathrm{MoT}{\mathrm{e}}_2$ were performed over a wide temperature range from 4.2 to 300 K. At a low temperature, the photoluminescence spectra for monolayer $\mathrm{MoT}{\mathrm{e}}_2$ showed two sharp peaks for excitons and charged excitons (trions). The homogeneous linewidth of the exciton peak broadened linearly as the temperature increased. This linear linewidth broadening was caused by acoustic-phonon scattering of the exciton, i.e., shortening of exciton dephasing. The broadening factor due to exciton–acoustic–phonon interactions was found to be $0.11\mathrm{meV}{\mathrm{K}}^{-1}$. This small value for the exciton–phonon coupling coefficient and the lack of a Stokes shift suggest that exciton–phonon interactions in monolayer $\mathrm{MoT}{\mathrm{e}}_2$ are in the weak coupling regime.

Structural investigations of the ($2\overline{1}0$) and (100) surfaces of ${\mathrm{Al}}_5{\mathrm{Co}}_2$ using experimental ultrahigh vacuum techniques and ab initio computational methods are presented in this work. Both surfaces are identified and show bulk terminations where only specific atoms remain. These atoms can be seen as truncated parts of chemically bonded atomic motifs which have been identified in the bulk in a previous work [M. Meier et al., Phys. Rev. B 91, 085414 (2015)]. Whereas the ($2\overline{1}0$) surface presents a single termination, the structure of the (100) topmost layers is found to be highly dependent on the preparation conditions, especially on the annealing temperature. This behavior, also observed for the (001) surface studied previously, can be partly explained when considering the bonding strength of the truncated motif parts with the subsurface.

We investigate the scattering properties of two-dimensional high-aspect-ratio metal trenches acting as resonators for gap-surface plasmons and show that these resonators are highly efficient scatterers of free waves, reaching at resonance in the perfect-conductor limit the unitary dipolar limit for a two-dimensional scatterer. We construct a simple resonator model which predicts the wavelength-dependent extinction, scattering, and absorption cross section of the trench and compare the model findings with full numerical simulations. Both extinction and scattering cross sections are mainly determined by the wavelength and can reach highly supergeometric values. At wavelengths where the metal exhibits near perfect electrical conductor behavior, such trenches lend themselves to be used as self-normalizing scatterers, as their scattering cross section is independent of their geometry and depend only on the resonance wavelength. For real metals with nonzero absorption, efficient monomaterial absorbers and emitters can be fabricated. We extend the analysis to tapering trenches that can be readily fabricated employing common milling or etching techniques and verify by reflection spectroscopy and two-photon luminescence that the resonant behavior of the vertical trenches is preserved.

We reinvestigate the momentum-resolved single-particle spectral function of the Tomonaga-Luttinger model. In particular, we focus on the role of the momentum dependence of the two-particle interaction $V\left(q\right)$. Usually, $V\left(q\right)$ is assumed to be a constant and integrals are regularized in the ultraviolet “by hand” employing an ad hoc procedure. As the momentum dependence of the interaction is irrelevant in the renormalization group sense, this does not affect the universal low-energy properties of the model, e.g., exponents of power laws, if all energy scales are sent to zero. If, however, the momentum $k$ is fixed away from the Fermi momentum $k_{\mathrm{F}}$, with $|k-k_{\mathrm{F}}|$ setting a nonvanishing energy scale, the details of $V\left(q\right)$ start to matter. We provide strong evidence that any curvature of the two-particle interaction at small transferred momentum $q$ destroys power-law scaling of the momentum-resolved spectral function as a function of energy. Even for $|k-k_{\mathrm{F}}|$ much smaller than the momentum-space range of the interaction the spectral line shape depends on the details of $V\left(q\right)$. The significance of our results for universality in the Luttinger liquid sense, for experiments on quasi-one-dimensional metals, and for recent results on the spectral function of one-dimensional correlated systems taking effects of the curvature of the single-particle dispersion into account (“nonlinear LL phenomenology”) is discussed.

We observed all-in type Nb tetrahedral displacement in nonmagnetic pyrochlore niobates $A_2{\mathrm{Nb}}_2{\mathrm{O}}_7(A={\mathrm{Nd}}_{0.5}{\mathrm{Ca}}_{0.5}$ and ${\mathrm{Y}}_{0.5}{\mathrm{Ca}}_{0.5})$ through the analysis of the neutron pair distribution function and the extended x-ray absorption function spectroscopy. The all-in type Nb tetrahedral displacement, which has the character of a charge singlet state, is driven by the formation of the bonding orbital. The diffuse scattering in the x-ray diffraction, which has the resonant component in the Nb $L_3$ edge, indicates that the all-in type Nb tetrahedral displacement has the periodicity with its short-range correlation.

We report pure carbon-based superlattices that exhibit direct band gaps and excellent optical absorption and emission properties at the threshold energy. The structures are nearly identical to that of cubic diamond except that defective layers characterized by five- and seven-membered rings are intercalated in the diamond lattice. The direct band gaps lie in the range of 5.6–5.9 eV, corresponding to wavelengths of 210–221 nm. The dipole matrix elements of direct optical transition are comparable to that of GaN, suggesting that the superlattices are promising materials as an efficient deep ultraviolet light emitter. Molecular dynamics simulations show that the superlattices are thermally stable even at a high temperature of 2000 K. We provide a possible route to the synthesis of superlattices through wafer bonding of diamond (100) surfaces.

The ground-state electronic configuration of three coupled bidimensional electron gases has been determined using a variational Hartree-Fock approach, at zero magnetic field. The layers are Coulomb coupled, and tunneling is present between neighboring layers. In the limit of small separation between layers, the tunneling becomes the dominant energy contribution, while for large distance between layers the physics is driven by the Hartree electrostatic energy. Transition from tunneling to Hartree dominated physics is shifted towards larger layer separation values as the total bidimensional density of the trilayers decreases. The interlayer exchange stabilizes a “balanced” configuration, where the three layers are approximately equally occupied; most of the experiments are performed in the vicinity of this balanced configuration. Several ground-state configurations are a consequence of a delicate interplay between tunneling and intersubband exchange.

We study the zero-frequency current noise of the interacting resonant level model for arbitrary bias voltages using a functional renormalization group approach. For this we extend the existing nonequilibrium scheme by deriving and solving flow equations for the current-vertex functions. On-resonance artificial divergences of the latter found in lowest-order perturbation theory in the two-particle interaction are consistently removed. Away from resonance they are shifted to higher orders. This allows us to gain a comprehensive picture of the current noise in the scaling limit. At high bias voltages, the current noise exhibits a universal power-law decay, whose exponent is, to leading order in the interaction, identical to that of the current. The effective charge on resonance is analyzed in detail, employing properties of the vertex correction. We find that it is only modified to second or higher order in the two-particle interaction.

We report Ramsey interference in the excitonic population of a negatively charged quantum dot measured in resonant fluorescence. Our experiments show that the decay time of the Ramsey interference is limited by the spectral width of the transition. Applying a vertical magnetic field induces Zeeman split transitions that can be addressed by changing the laser detuning to reveal two-, three-, and four-level system behavior. We show that under finite field the phase-sensitive control of two optical pulses from a single laser can be used to prepare both population and spin states simultaneously. We also demonstrate the coherent optical manipulation of a trapped spin in a quantum dot in a Faraday geometry magnetic field.