We present time-resolved synchrotron x-ray diffraction measurements to study kinetics associated with the liquid-solid and solid-solid high-pressure phase transitions in Kr under dynamic loading in a dynamic-diamond anvil cell. The results show a strong compression-rate dependence of the solidification/melting process in liquid Kr. The analysis of the compression-rate dependent melting/solidification, using an Avrami equation with the parameter $n=1$, indicates a spontaneous nucleation and one-dimensional growth mechanism. In contrast, the face-centered-cubic to metastable hexagonal close-packed transition in solid Kr occurs rapidly at ∼0.8 GPa near the melting line, which has negligible compression-rate dependence within the range of compression rates studied $\left(0.004–13 \mathrm{GPa}/\mathrm{s}\right)$.

Using the Aharonov-Casher (A-C) phase, we present a microscopic theory of the Josephson and persistent spin currents in quasiequilibrium Bose-Einstein condensates (BECs) of magnons in ferromagnetic insulators. Starting from a microscopic spin model that we map onto a Gross-Pitaevskii Hamiltonian, we derive a two-state model for the Josephson junction between the weakly coupled magnon-BECs. We then show how to obtain the alternating-current (ac) Josephson effect with magnons as well as macroscopic quantum self-trapping in a magnon-BEC. We next propose how to control the direct-current (dc) Josephson effect electrically using the A-C phase, which is the geometric phase acquired by magnons moving in an electric field. Finally, we introduce a magnon-BEC ring and show that persistent magnon-BEC currents flow due to the A-C phase. Focusing on the feature that the persistent magnon-BEC current is a steady flow of magnetic dipoles that produces an electric field, we propose a method to directly measure it experimentally.

Metallic interfaces between insulating perovskites are often observed in heterostructures combining polar and nonpolar materials. In these systems, the polar discontinuity across the interface may drive an electronic reconstruction inducing free carriers at the interface. Here, we theoretically show that a metallic interface between a Mott and a band insulator can also form in the absence of a polar discontinuity. The condition for the appearance of such a metallic state is consistent with the classical Mott criterion: the metallic state is stable if the screening length falls below the effective Bohr radius of a particle-hole pair. In this case, the metallic state bears a remarkable similarity to the one found in polar/nonpolar heterostructures. On the other hand, if the screening length approaches the size of the effective Bohr radius, particles and holes are bound to each other resulting in an overall insulating phase. We analyze this metal-insulator transition, which is tunable by the dielectric constant, in the framework of the slave-boson mean-field theory for a lattice model with both on-site and long-range Coulomb interactions. We discuss ground-state properties and transport coefficients, which we derive in the relaxation-time approximation. Interestingly, we find that the metal-insulator transition is accompanied by a strong enhancement of the Seebeck coefficient in the band-insulator region in the vicinity of the interface. The implications of our theoretical findings for various experimental systems such as nonpolar (110) interfaces are also discussed.

A comprehensive model of ultrafast laser-induced plasma generation intended for coupling with pulse propagation simulations in transparent solids is introduced. It simultaneously accounts for the changing spectrum of a propagating ultrashort laser pulse while coupling to the evolution of the energy-resolved nonequilibrium free-carrier distribution. The presented results indicate that strong pulse chirps lead to ionization dynamics that are not captured by the standard monochromatic treatment of laser-induced plasma formation. These results have strong implications for ultrafast laser-solid applications that depend on ionization in a strong nonlinear focus.

A grazing incidence fast atom diffraction (GIFAD or FAD) setup, installed on a molecular beam epitaxy chamber, has been used to characterize the ${\beta }_2(2\times 4)$ reconstruction of a GaAs(001) surface at $530{}^{\circ }\mathrm{C}$ under an ${\mathrm{As}}_4$ overpressure. Using a 400-eV ${}^4\mathrm{He}$ beam, high-resolution diffraction patterns with up to eighty well-resolved diffraction orders are observed simultaneously, providing a detailed fingerprint of the surface structure. Experimental diffraction data are in good agreement with results from quantum scattering calculations based on an ab initio projectile-surface interaction potential. Along with exact calculations, we show that a straightforward semiclassical analysis allows the features of the diffraction chart to be linked to the main characteristics of the surface reconstruction topography. Our results demonstrate that GIFAD is a technique suitable for measuring in situ the subtle details of complex surface reconstructions. We have performed measurements at very small incidence angles, where the kinetic energy of the projectile motion perpendicular to the surface can be reduced to less than 1 meV. This allowed the depth of the attractive van der Waals potential well to be estimated as $-8.7$ meV in very good agreement with results reported in literature.

We study the properties of a quantum particle interacting with a one-dimensional structure of equidistant scattering centers. We derive an analytical expression for the dispersion relation and for the Bloch functions in the presence of both even and odd scattering waves within the pseudopotential approximation. This generalizes the well-known solid-state physics textbook result known as the Kronig-Penney model. Our generalized model can be used to describe systems such as degenerate Fermi gases interacting with ions or with another neutral atomic species confined in an optical lattice, thus enabling the investigation of polaron or Kondo physics within a simple formalism. We focus our attention on the specific atom-ion system and compare our findings with quantum defect theory. Excellent agreement is obtained within the regime of validity of the pseudopotential approximation. This enables us to derive a Bose-Hubbard Hamiltonian for a degenerate quantum Bose gas in a linear chain of ions.

We investigate the band structure and the optical absorption spectrum of twisted bilayer graphenes with changing interlayer bias and Fermi energy simultaneously. We show that the interlayer bias lifts the degeneracy of the superlattice Dirac point, while the amount of the Dirac point shift is significantly suppressed in small rotation angles, and even becomes opposite to the applied bias. We calculate the optical absorption spectrum in various asymmetric potentials and Fermi energies, and associate the characteristic spectral features with the band structure. The spectroscopic features are highly sensitive to the interlayer bias and the Fermi energy, and widely tunable by the external field effect.

Motivated by puzzling aspects of spin-glass behavior reported in frustrated magnetic materials, we theoretically investigate effects of magnetoelastic coupling in geometrically frustrated classical spin models. In particular, we consider bond-disordered Heisenberg antiferromagnets on a pyrochlore lattice coupled to local lattice distortions. By integrating out the lattice degree of freedom, we derive an effective spin-only model, the bilinear-biquadratic model with bond disorder. The effective model is analyzed by classical Monte Carlo simulations using an extended loop algorithm. First, we discuss the phase diagrams in detail by showing the comprehensive Monte Carlo data for thermodynamic and magnetic properties. We show that the spin-glass transition temperature $T_{\mathrm{f}}$ is largely enhanced by the spin-lattice coupling $b$ in the weakly disordered regime. By considering the limit of strong spin-lattice coupling, this enhancement is ascribed to the suppression of thermal fluctuations in semidiscrete degenerate manifold formed in the presence of the spin-lattice coupling. We also find that, by increasing the strength of disorder $\Delta$, the system shows a concomitant transition of the nematic order and spin glass at a temperature determined by $b$, being almost independent of $\Delta$. This is due to the fact that the spin-glass transition is triggered by the spin collinearity developed by the nematic order. Although further-neighbor exchange interactions originating in the cooperative lattice distortions result in spin-lattice order in the weakly disordered regime, the concomitant transition remains robust with $T_{\mathrm{f}}$ almost independent of $\Delta$. We find that the magnetic susceptibility shows hysteresis between the field-cooled and zero-field-cooled data below $T_{\mathrm{f}}$, and that the nonlinear susceptibility shows a negative divergence at the transition. These features are common to conventional spin-glass systems. Meanwhile, we find that the specific heat exhibits a broad peak at $T_{\mathrm{f}}$, and that the Curie-Weiss temperature varies with $\Delta$, even in the region where $T_{\mathrm{f}}$ is insensitive to $\Delta$. In addition, we clarified that the concomitant transition remains robust against a substantial external magnetic field. These features are in clear contrast to the conventional spin-glass behavior. Furthermore, we show that the cubic susceptibility obeys a Curie-Weiss–type law and the estimated “Curie-Weiss” temperature gives a good measure of the spin-lattice coupling even in the presence of bond randomness. We also show, by studying single-spin-flip dynamics in the nematic phase, that the glassy spin dynamics may be observed at a rather high temperature in a realistic situation for weak disorder. All these results are discussed in comparison with experiments for typical pyrochlore magnets, such as ${\mathrm{Y}}_2{\mathrm{Mo}}_2{\mathrm{O}}_7$ and ${\mathrm{ZnCr}}_2{\mathrm{O}}_4$.

The Fermi sign structure plays a crucial role for a Landau's Fermi liquid. In this work, we identify the exact sign structure for the bipartite Hubbard model at an arbitrary strength of the on-site Coulomb repulsion $U$. This general sign structure naturally reproduces the conventional fermion signs in the small $U$ limit, and the phase string signs of the $t-J$ model in the large $U$ limit. We focus on the half-filling case as an example to illustrate why such a generic sign structure is important to understand the transition from the weakly correlated Fermi-liquid regime to the strongly correlated Mott regime. In particular, we show that an electron fractionalization scheme with emergent partons and mutual Chern-Simons gauge fields provides a suitable framework to accurately handle the singular sign structure in two dimensions. A ground-state ansatz at half filling with incorporating the sign structure and the specific electron fractionalization is also proposed.

An ab initio study of the dynamical dielectric response of bulk lead is presented. The influence of the $5d$ semicore states on the characteristics of the bulk plasmon is analyzed by means of first-principles pseudopotential calculations. The effects of spin-orbit interactions and local-fields are also studied in detail. The inclusion of the $5d$ semicore states in the valence configuration completely changes the high-energy-transfer dielectric properties of bulk Pb. In particular, it lowers the computed bulk plasmon energy by about 3.5 eV, bringing its frequency to good agreement with experimental data. In general, the high-energy-transfer dielectric response of bulk Pb is found to be shaped mostly by the interplay between the interband transitions involving the semicore $5d$ states and the spin-orbit coupling interaction. Local-field effects are found to affect the relative spectral weight of the high-energy excitations, while leaving their dispersion mostly unaffected.

We perform electronic structure calculations for the recently synthesized iron-based superconductor ${\mathrm{LiFeO}}_2{\mathrm{Fe}}_2{\mathrm{Se}}_2$. In contrast to other iron-based superconductors, this material comprises two different iron atoms in $3d^5$ and $3d^6$ configurations. In band theory, both contribute to the low-energy electronic structure. Spin-polarized density functional theory calculations predict an antiferromagnetic metallic ground state with different moments on the two Fe sites. However, several other almost degenerate magnetic configurations exist. Due to their different valences, the two iron atoms behave very differently when local quantum correlations are included through the dynamical mean-field theory. The contributions from the half-filled $3d^5$ atoms in the ${\mathrm{LiFeO}}_2$ layer are suppressed and the $3d^6$ states from the FeSe layer restore the standard iron-based superconductor fermiology.

We outline a possibility to determine the adatom density of individual nano-objects via measurement of their electronic structure. For this aim, the nonlinear shift of image potential states measured on individual Cu nanoclusters on Ag(100) by low-temperature scanning tunneling spectroscopy is carefully analyzed. The quantitative analysis is confirmed by density-functional theory calculations. A peak width analysis furthermore reveals whether the clusters are purely metallic or alloyed.

A new subsurface growth mode in the Co-Cu system is reported. This mode provides a direct subsurface growth of Co nanoclusters by depositing Co atoms on the Cu(001) surface in a single stage. The resulting subsurface Co nanoclusters are located 2 monolayers (ML) deep below the atomically flat surface of Cu(001). Although these hidden nanoclusters cannot be directly accessed by a scanning tunneling microscopy/spectroscopy (STM/STS) probe, their shape could be deduced using STM/STS via a careful analysis of the local deformation of the Cu(001) surface as well as local variations of surface electron density induced by the subsurface clusters. A strongly asymmetric shape of the nanoclusters is deduced: they are typically 5-10 nm in lateral size but only 2 to 3 ML in thickness. The thickness of the nanoclusters does not evolve significantly under a heat treatment. A simple model is implemented to describe the growth kinetics. The results in this study reveal that intense processes of diffusion, nucleation, and growth take place in a region 1 nm deep, thus defining the near-surface region.

We explore the dynamics of hard-core lattice bosons in the presence of strong nonlocal particle loss. The evolution occurs on two distinct time scales, first a rapid strongly correlated decay into a highly degenerate Zeno state subspace, followed by a slow almost coherent evolution. We analytically solve the fast initial dynamics of the system, where we specifically focus on an initial Mott insulator state, and perform an analysis of the particle arrangements in the Zeno subspace. We investigate the secondary slow relaxation process that follows and find an intricate regime where the competition between dissipation and coherence results in various types of interacting particle complexes. We classify them and analyze their spectral properties in the presence and absence of nearest-neighbor interactions. Under certain circumstances the dispersion relations of the complexes feature flat bands, which are a result of an effective spin-orbit coupling.

In a relativistic description of a real solid the crystal wave functions are never pure but spin-mixed functions. As a direct result of the spin-mixing, scattering mechanisms, which are in principal diagonal in spin-space, as for example the deformation potential part of the electron-phonon scattering potential, may lead to spin flips. Spin-mixing thereby affects damping and lifetimes in the fast and ultrafast regime of magnetization dynamics. The average spin-mixing appears as an important parameter in many models, the most prominent being the Elliott-Yafet model of spin relaxation. The application of an external magnetic field, a laser-field, or a Faraday-field induced by a laser may be considered as a source of magnetic field in a sample. Therefore we will discuss the influence of a homogeneous applied magnetic field on the average spin-mixing calculated in the framework of spin-density-functional theory. It is shown that even high fields do not significantly change the spin-mixing in $3d$ transition metals. Therefore the related dynamics are unaffected, which is in accordance with the experimental data on ultrafast demagnetization's dependence on light polarization.

We present a comprehensive transport investigation of the itinerant antiferromagnet ${\mathrm{Mn}}_3\mathrm{Si}$ which undergoes a spin density wave (SDW) order below $T_N\sim 21.3\mathrm{K}$. The electrical resistivity, the thermal conductivity, and the Hall, Seebeck, and Nernst effects exhibit pronounced anomalies at the SDW transition. At temperatures higher than $T_N$ our data provide strong evidence for a large fluctuation regime which extends up to $\sim 200\mathrm{K}$ in the resistivity, the Seebeck effect, and the Nernst effect. From the comparison of our results with other prototype SDW materials, viz., LaFeAsO and chromium, we conclude that many of the observed features are of generic character.

We have investigated the dc bias current dependence of the nonlocal spin valve signal in CoFeAl/Cu lateral spin valves. The spin signal is found to increase monotonically with the bias current. Surprisingly, the modulation amplitude from $-1$ mA to 1 mA exceeds 30 percent of the spin signal at low bias current. From the analysis based on the one-dimensional spin diffusion model and considering the bias-current heating effect, we find that the contribution of the thermal spin injection is much larger than the influence of the reduction of the spin diffusion length due to the Joule heating. We also show that the second harmonic lock-in signal precisely extracts the contribution of the thermal spin injection from the mixed spin signal.

We establish the Berry-phase formulas for the angular momentum (AM) and the Hall viscosity (HV) to investigate chiral superconductors (SCs) in two and three dimensions. The AM is defined by the temporal integral of the antisymmetric momentum current induced by an adiabatic deformation, while the HV is defined by the symmetric momentum current induced by the symmetric torsional electric field. Without suffering from the system size or geometry, we obtain the macroscopic AM $L_z=\hslash m{N}_0/2$ at zero temperature in full-gap chiral SCs, where $m$ is the magnetic quantum number and $N_0$ is the total number of electrons. We also find that the HV is equal to half the AM at zero temperature not only in full-gap chiral SCs as is well known but also in nodal ones, but its behavior at finite temperature is different in the two cases.

Recently, magnetic phase-change materials have been synthesized experimentally by doping with $3d$ transition metal impurities. Here, we investigate the electronic structure and the magnetic properties of the prototypical phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$ (GST) doped with V, Cr, Mn, and Fe by density functional calculations. Both the supercell method and the coherent potential approximation (CPA) are employed to describe this complex substitutionally disordered system. As regards the first approach, we consider a large unit cell containing 1000 sites to model the random distribution of the cations and of the impurities in doped cubic GST. Such a large-scale electronic structure calculation is performed using the program kkrnano, where the full potential screened Korringa-Kohn-Rostoker Green's function method is optimized by a massively parallel linear scaling (order-$N$) all-electron algorithm. Overall, the electronic structures and magnetic exchange coupling constants calculated by kkrnano agree quite well with the CPA results. We find that ferromagnetic states are favorable in the cases of V and Cr doping, due to the double exchange mechanism, whereas antiferromagnetic superexchange interactions appear to be dominant for Fe- and Mn-doped GST. The ferromagnetic interaction is particularly strong in the case of Cr. As a result, high Curie temperatures close to room temperatures are obtained for large Cr concentrations of 15%.