A series of high-pressure measurements including resistivity, ac and dc magnetic susceptibility, neutron powder diffraction, and synchrotron x-ray diffraction were performed to investigate the crystal structure and electronic states of the itinerant ferromagnet $\mathrm{SrCo}{\mathrm{O}}_3$. Two pressure-induced phase transitions were observed at about 1.1 and 45 GPa, corresponding to a spin reorientation and a spin-state transition, respectively, while the cubic crystal structure was stable with pressure up to 60.0 GPa. The origins of these electronic state changes are discussed and rationalized in the light of first-principles calculations.

The photon energy dependence of angle-resolved photoemission spectroscopy (ARPES) in graphene is investigated experimentally and theoretically. By applying light with energy of around $46\mathrm{eV}$, we found an unexpected increase in the ARPES relative intensity of graphene for the $p$ branch (ARPES spectra brightened by the $p$-polarized light) with respect to the $s$ branch (those brightened by the $s$-polarized light). The origin of the enhanced $p$-branch intensity is explained by first-principles calculations, in which we show (1) the optical dipole vector as a function of final-state energies of the excited electron, (2) the absorption intensity as a function of the incident light angle, and (3) the symmetry of the initial and the final states. The calculated results imply that the dipole vector of the excited electron near $46\mathrm{eV}$ has an exceptionally large component in the normal direction of the graphene surface compared to that within the graphene plane, which could be the main reason for the enhancement of the $p$-branch intensity.

Dislocations play an important role in semiconductor devices made of crystalline silicon (Si). They are known to be strongly performance-limiting defects in solar cell applications, since they act as preferred segregation sites for metallic impurities. In this work we investigate the segregation of iron (Fe) to the cores of the ${30}^{\circ }$ and ${90}^{\circ }$ partial dislocations in Si using atomistic calculations based on first-principles density functional theory. Our simulations show that interstitial Fe impurities segregate readily to all investigated cores and the driving force for the segregation increases with impurity concentration. Moreover, our analysis of the electronic structure reveals the existence of deep defect levels within the band gap that can be related to experimental observations by deep-level transient spectroscopy.

We present a theoretical study of the quantum depletion of microcavity polaritons that are excited with a resonant laser pulse. The dynamics of the quantum fluctuations are interpreted in the context of quantum quenches in general and in terms of the dynamical Casimir effect in particular. We compute the time evolution of the first- and second-order correlation functions of the polariton condensate. Our theoretical modeling is based on the truncated Wigner approximation for interacting Bose gases. For homogeneous systems, analytical results are obtained in the linearized Bogoliubov approximation. Inhomogeneous systems are studied numerically by Monte Carlo simulations.

In many electron spin qubit systems coherent control is impaired by the fluctuating nuclear spin bath of the host material. Previous experiments have shown dynamic nuclear polarization with feedback to significantly prolong the inhomogeneous dephasing time $T_2^{*}$ by narrowing the distribution of nuclear Overhauser field fluctuations. We present a model relating the achievable narrowing of the Overhauser field to both the pump rate and the noise magnitude and find reasonable agreement with experimental data. It shows that former experiments on gated GaAs quantum dots were limited by the pump rate of the pumping mechanism used. Here we propose an alternative feedback scheme using electron dipole spin resonance. Sequentially applying two ac electric fields with frequencies slightly detuned from the desired Larmor frequency results in a pump curve with a stable fixed point. Our model predicts that $T_2^{*}$ values on the order of microseconds can be achieved.

${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$ exhibits Kondo semiconducting properties, yet orders antiferromagnetically at a rather high temperature, $T_{\mathrm{N}}=28.5$ K. We have performed break-junction tunneling measurements on single crystals of ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$ and the nonmagnetic Kondo semiconductor ${\mathrm{CeFe}}_2{\mathrm{Al}}_{10}$. Upon cooling ${\mathrm{CeFe}}_2{\mathrm{Al}}_{10}$, two hybridization gaps successively appear in the tunneling spectra, while another gap opens in ${\mathrm{CeOs}}_2{\mathrm{Al}}_{10}$ below $T_{\mathrm{N}}$. For both compounds, the ratio of the two hybridization gap widths is approximately 4, in agreement with the hybridization gap model for the crystal-field ground state of ${\mathrm{Ce}}^{3+}$ with $|J_z\rangle =|\pm 3/2\rangle$. Furthermore, we found that the gap widths are well scaled by the Kondo temperature among Ce-based Kondo semimetals/semiconductors with orthorhombic structures.

The relaxation dynamics of hot H, N, and ${\mathrm{N}}_2$ on Pd(100), Ag(111), and Fe(110), respectively, is studied by means of ab initio molecular dynamics with electronic friction. This method is adapted here to account for the electron density changes caused by lattice vibrations, thus treating on an equal footing both electron-hole ($e-h$) pair and phonon excitations. We find that even if the latter increasingly dominate the heavier is the hot species, the contribution of $e-h$ pairs is by no means negligible in these cases because it gains relevance at the last stage of the relaxation process. The quantitative details of energy dissipation depend on the interplay of the potential energy surface, electronic structure, and kinetic factors.

We report on transport and capacitance measurements of graphene devices in magnetic fields up to 30 T. In both techniques, we observe the full splitting of Landau levels and we employ tilted field experiments to address the origin of the observed broken symmetry states. In the lowest energy level, the spin degeneracy is removed at filling factors $\nu =\pm 1$ and we observe an enhanced energy gap. In the higher levels, the valley degeneracy is removed at odd filling factors while spin polarized states are formed at even $\nu$. Although the observation of odd filling factors in the higher levels points towards the spontaneous origin of the splitting, we find that the main contribution to the gap at $\nu =-4,-8$, and $-12$ is due to the Zeeman energy.

We investigate the ground-state properties of a recently proposed model for a topological Kondo insulator in one dimension (i.e., the $p$-wave Kondo-Heisenberg lattice model) by means of the density-matrix renormalization-group method. The nonstandard Kondo interaction in this model is different from the usual (i.e., local) Kondo interaction in that the localized spins couple to the “$p$-wave” spin density of conduction electrons, inducing a topologically nontrivial insulating ground state. Based on the analysis of the charge- and spin-excitation gaps, the string order parameter, and the spin profile in the ground state, we show that, at half filling and low energies, the system is in the Haldane phase and hosts topologically protected spin-1/2 end states. Beyond its intrinsic interest as a useful “toy model” to understand the effects of strong correlations on topological insulators, we show that the $p$-wave Kondo-Heisenberg model could be experimentally implemented in $p\text{−}\mathrm{band}$ optical lattices loaded with ultracold Fermi gases.

We use the Vienna ab initio simulation package (vasp) for evaluation of the quasiparticle spectra and their spectral weights within Hedin's $\text{GW}$ approximation (GWA) for ${\text{Fe}}_3\text{Si}$ and $\alpha {\text{-FeSi}}_2$ within the non-self-consistent one-shot approximation ${\text{G}}_0{\text{W}}_0$ and self-consistent $\text{scGWA}$ with the vertex corrections in the particle-hole channel, taken in the form of two-point kernel. As input for ${\text{G}}_0{\text{W}}_0$, the band structure and wave functions evaluated within the generalized gradient corrected local-density approximation to density functional theory (GGA) have been used. The spectral weights of quasiparticles in these compounds deviate from unity everywhere and show nonmonotonic behavior in those parts of bands where the delocalized states contribute to their formation. The ${\text{G}}_0{\text{W}}_0$ and scGWA spectral weights are the same within 2%–5%. The scGWA shows a general tendency to return ${\text{G}}_0{\text{W}}_0$ bands to their GGA positions for the delocalized states, while in the flat bands it flattens even more. Variable angle spectroscopic ellipsometry measurements at $T=296$ K on grown single-crystalline $\sim 50$-nm-thick films of ${\text{Fe}}_3\text{Si}$ on $n$-Si(111) wafer have been performed in the interval of energies $\omega \sim (1.3–5)$ eV. The comparison of ${\text{G}}_0{\text{W}}_0$ and $\text{scGW}$ theory with experimental real and imaginary parts of permittivity, refractive index, extinction and absorption coefficients, reflectivity, and electron energy loss function shows that both ${\text{G}}_0{\text{W}}_0$ and $\text{scGW}$ qualitatively describe experiment correctly, the position of the low-energy peaks is described better by the $\text{scGW}$ theory, however, its detailed structure is not observed in the experimental curves. We suggest that the angle-resolved photoemission spectroscopy experiments, which can reveal the fine details of the quasiparticle band structure and spectral weights, could help to understand (i) if the scGWA with this type of vertex correction is sufficiently good for description of these iron silicides and, possibly, (ii) why some features of calculated permittivity are not seen in optical experiments.

By using first-principles calculations, we propose that ZrSiO can be looked at as a three-dimensional (3D) oxide weak topological insulator (TI) and its single layer is a long-sought-after 2D oxide TI with a band gap up to 30 meV. Calculated phonon spectrum of the single layer ZrSiO indicates it is dynamically stable and the experimental achievements in growing oxides with atomic precision ensure that it can be readily synthesized. This will lead to novel devices based on TIs, the so-called “topotronic” devices, operating under room temperature and stable when exposed in the air. Thus a new field of “topotronics” will arise. Another intriguing thing is this oxide 2D TI has the similar crystal structure as the well-known iron-pnictide superconductor LiFeAs. This brings great promise in realizing the combination of superconductor and TI, paving the way to various extraordinary quantum phenomena, such as topological superconductor and Majorana modes. We further find that there are many other isostructural compounds hosting the similar electronic structure and forming a $WHM$ family with $W$ being Zr, Hf, or La, $H$ being group IV or group V element, and $M$ being group VI one.

Placing a two-dimensional sheet of graphene in an external magnetic field the continuous electronic band structure is discretized due to Landau quantization. The resulting optical transitions are subject to a broadening, which can lead to a significant overlap of Landau levels. We investigate the possible microscopic processes that could cause a broadening of the corresponding peaks in the absorption spectrum of Landau-quantized graphene: (i) radiative decay, (ii) Coulomb interaction, (iii) optical phonons, (iv) acoustic phonons, and (v) impurities. Since recent experiments have shown that independent of the magnetic field the resolvable number of Landau levels is constant, we put a special focus on the dependence of the broadening on the external magnetic field $B$ and the Landau level index $n$. Our calculations reveal the impurities to be the crucial broadening mechanism, where different regimes of well separated and densely spaced Landau levels need to be taken into account. Furthermore, carrier-carrier and carrier-phonon scattering give rise to a very specific dependence on the Landau level index $n$ that has not been observed yet.

Density functional theory (DFT) calculations of 58 liquid elements at their triple point show that most metals exhibit near proportionality between the thermal fluctuations of the virial and the potential energy in the isochoric ensemble. This demonstrates a general “hidden” scale invariance of metals making the condensed part of the thermodynamic phase diagram effectively one dimensional with respect to structure and dynamics. DFT computed density scaling exponents, related to the Grüneisen parameter, are in good agreement with experimental values for the 16 elements where reliable data were available. Hidden scale invariance is demonstrated in detail for magnesium by showing invariance of structure and dynamics. Computed melting curves of period three metals follow curves with invariance (isomorphs). The experimental structure factor of magnesium is predicted by assuming scale invariant inverse power-law (IPL) pair interactions. However, crystal packings of several transition metals (V, Cr, Mn, Fe, Nb, Mo, Ta, W, and Hg), most post-transition metals (Ga, In, Sn, and Tl), and the metalloids Si and Ge cannot be explained by the IPL assumption. The virial-energy correlation coefficients of iron and phosphorous are shown to increase at elevated pressures. Finally, we discuss how scale invariance explains the Grüneisen equation of state and a number of well-known empirical melting and freezing rules.

A first-principles study of the $L{1}_2{\text{Co}}_3(\text{Al},\text{W})$ phase was performed to assess phase stability at elevated temperature. Configurational degrees of freedom were treated with a cluster expansion and Monte Carlo simulations, while contributions to the free energy from vibrational excitations were determined within the quasiharmonic approximation accounting for effects of thermal expansion. It was found that while contributions from configurational degrees of freedom are important, vibrational entropy is crucial in making $L{1}_2{\text{Co}}_3(\text{Al},\text{W})$ stable at high temperature in the Co-rich portion of the Co-Al-W ternary. The emergence of $L{1}_2$ at high temperature can be attributed to a large difference in vibrational entropy between the close packed fcc-based $L{1}_2$ phases and the energetically stable, yet vibrationally stiff $B2$ CoAl compound.

The thermodynamic compensation law describing an empirical linear relationship between activation enthalpy and activation entropy has seldom been validated for amorphous solids. Here molecular dynamics simulations reveal a well-defined enthalpy-entropy compensation rule in a metallic glass (MG) over a wide temperature and stress range, spanning the glass transition induced by temperature and/or stress. Experiments on other MGs reproduce this law, suggesting that it applies universally to amorphous solids, so we extend it from crystals to amorphous solids. In the glassy state, the compensation temperature is found to agree with the thermal glass transition temperature $T_{\mathrm{g}}$; whereas in the supercooled liquid region, the compensation temperature matches $\sim 1.4T_{\mathrm{g}}$, at which the diffusion kinetics start to feel the roughness of the free-energy surface.

We study the density of states and magnetotransport properties of disordered Weyl semimetals, focusing on the case of a strong long-range disorder. To calculate the disorder-averaged density of states close to nodal points, we treat exactly the long-range random potential fluctuations produced by charged impurities, while the short-range component of disorder potential is included systematically and controllably with the help of a diagram technique. We find that, for energies close to the degeneracy point, long-range potential fluctuations lead to a finite density of states. In the context of transport, we discuss that a self-consistent theory of screening in magnetic field may conceivably lead to nonmonotonic low-field magnetoresistance.

We study the fate of the Ising model and its universal properties when driven by a rapid periodic drive and weakly coupled to a bath at equilibrium. The far-from-equilibrium steady-state regime is accessed by means of a Floquet mean-field approach. We show that, depending on the details of the bath, the drive can strongly renormalize the critical temperature to higher temperatures, modify the critical exponents, or even change the nature of the phase transition from second to first order after the emergence of a tricritical point. Moreover, by judiciously selecting the frequency of the field and by engineering the spectrum of the bath, one can drive a ferromagnetic Hamiltonian to an antiferromagnetically ordered phase and vice versa.

Quantum tricriticality of a $J_1-J_2$ antiferromagnetic Ising model on a square lattice is studied using the mean-field (MF) theory, scaling theory, and the unbiased worldline quantum Monte-Carlo (QMC) method based on the Feynman path integral formula. The critical exponents of the quantum tricritical point (QTCP) and the qualitative phase diagram are obtained from the MF analysis. By performing the unbiased QMC calculations, we provide the numerical evidence for the existence of the QTCP and numerically determine the location of the QTCP in the case of $J_1=J_2$. From the systematic finite-size scaling analysis, we conclude that the QTCP is located at $H_{\mathrm{QTCP}}/J_1=3.260\left(2\right)$ and ${\mathrm{\Gamma }}_{\mathrm{QTCP}}/J_1=4.10\left(5\right)$. We also show that the critical exponents of the QTCP are identical to those of the MF theory, because the QTCP in this model is in the upper critical dimension. The QMC simulations reveal that unconventional proximity effects of the ferromagnetic susceptibility appear close to the antiferromagnetic QTCP, and the proximity effects survive for the conventional quantum critical point. We suggest that the momentum dependence of the dynamical and static spin structure factors is useful for identifying the QTCP in experiments.

There is growing evidence that a key feature of sufficiently disordered superconductors is the spatial inhomogeneity of the order parameter. However, not much is known analytically about the impact of the inhomogeneity on the global critical temperature that signals the onset of resistance in the superconductor. Here we address this problem in the experimentally relevant case of disordered conventional superconductors characterized by weak multifractality such as quasi-two-dimensional thin films. We compute analytically the superconducting energy gap, the temperature at which it vanishes, and the energy dependence and spatial distribution of the order parameter. The latter is found to be log normal. The global critical temperature, computed by percolation techniques, is much smaller than the temperature at which the energy gap vanishes. We show that disorder might enhance superconductivity but only for very weakly coupled superconductors, such as Al, and for relatively weak phase fluctuations. These results are consistent with experiments where enhancement of the critical temperature is observed in Al thin films but not in more strongly coupled materials.

We study the surface state of a doped topological crystalline insulator in the superconducting state. Motivated by ${\mathrm{Sn}}_{1-x}{\mathrm{In}}_x\mathrm{Te}$, we consider fully gapped pair potentials and calculate the surface spectral function. It is found that mirror-protected zero-energy surface Andreev bound states (SABSs) appear at the (001) surface. The gapless points of these SABSs appear on the mirror-symmetric line on the surface Brillouin zone while the positions of the gapless points depend on the chemical potential. In addition, due to the presence of the Dirac surface states in the normal state, the dispersion of the SABSs drastically changes with the chemical potential.