Atomic-scale models of amorphous structures are typically generated using a simulated annealing (SA) quench from a melt simulation protocol. This approach resembles the preparation of bulk glasses, but it may not be suitable for modeling amorphous materials produced using low-energy and low-temperature physical vapor deposition, where a deposited atom induces only local relaxations and no equilibrated melt is formed. To account for such growth conditions, we developed the kinetically limited minimization (KLM) technique, in which an amorphous structure is constructed from a randomly initialized structure in a number of local perturbation-relaxation steps. We compare formation energies as well as short- and medium-range order of KLM- and SA-generated structures of a-${\mathrm{In}}_2{\mathrm{O}}_3$, a-ZnO, and a-Si.

We perform an extensive study of the spin-configurations in a PdFe bilayer on Ir(111) in terms of ab initio and spin-model calculations. We use the spin-cluster expansion technique to obtain spin model parameters, and solve the Landau-Lifshitz-Gilbert equations at zero temperature. In particular, we focus on effects of layer relaxations and the evolution of the magnetic ground state in an external magnetic field. In the absence of a magnetic field, we find a spin-spiral ground state; while applying an external magnetic field, skyrmions are generated in the system. Based on energy calculations of frozen spin configurations with varying magnetic field we obtain excellent agreement for the phase boundaries with available experiments. We find that the wavelength of spin-spirals and the diameter of skyrmions decrease with increasing inward Fe layer relaxation, which is correlated with the increasing ratio of the nearest-neighbor Dzyaloshinskii-Moriya interaction and the isotropic exchange coupling, $D/J$. Our results also indicate that the applied field needed to stabilize the skyrmion lattice increases when the diameter of individual skyrmions decreases. Based on our observations, we suggest that the formation of the skyrmion lattice can be tuned by small structural modification of the thin film.

We use nuclear resonance inelastic x-ray scattering (NRIXS), a relatively new, synchrotron-based, isotope-specific technique in combination with a more traditional one, Raman spectroscopy, to probe the vibrational dynamics of the host frameworks in two Zintl clathrates: ${\mathrm{K}}_8{\mathrm{Zn}}_4{\mathrm{Sn}}_{42}$ (KZS) and ${\mathrm{Ba}}_8{\mathrm{Ga}}_{16}{\mathrm{Sn}}_{30}$ (BGS). From the normalized Sn vibrational density of states obtained from NRIXS, we calculate the stiffness, a mean force constant of the Sn environment, the resilience, a compact way of expressing the temperature dependence of the Sn mean square displacement, and several thermodynamic properties. The stiffness and the resilience are approximately $7\%$ lower in KZS, reflecting its larger unit cell compared to BGS. We emphasize the complementariness between NRIXS and Raman spectroscopy and establish a series of benchmarks for a more quantitative evaluation of the Raman spectra for the numerous clathrates that are still not suitable for NRIXS studies.

The spin resonance observed in the inelastic neutron scattering data on Fe-based superconductors has played a prominent role in the quest for determining the symmetry of the order parameter in these compounds. Most theoretical studies of the resonance employ an RPA-type approach in the particle-hole channel and associate the resonance in the spin susceptibility ${\chi }_S(\mathbf{q},\omega )$ at momentum $\mathbf{Q}=(\pi ,\pi )$ with the spin-exciton of an $s^{+-}$ superconductor, pulled below $2\Delta$ by residual attraction associated with the sign change of the gap between Fermi points connected by $\mathbf{Q}$. Here we explore the effect of fluctuations in the particle-particle channel on the spin resonance. Particle-particle and particle-hole channels are coupled in a superconductor and to what extent the spin resonance can be viewed as a particle-hole exciton needs to be addressed. In the case of purely repulsive interactions, we find that the particle-particle channel at total momentum $\mathbf{Q}$ (the $\pi$ channel) contributes little to the resonance. However, if the interband density-density interaction is attractive and the $\pi$ resonance is possible on its own, along with spin-exciton, we find a much stronger shift of the resonance frequency from the position of the would-be spin-exciton resonance. We also show that the expected double-peak structure in this situation does not appear because of the strong coupling between particle-hole and particle-particle channels, and $\mathrm{Im}{\chi }_S(\mathbf{Q},\omega )$ displays only a single peak.

We apply a diagrammatic Monte Carlo method to the problem of an impurity interacting resonantly with a homogeneous Fermi bath for a quasi-two-dimensional setup. Notwithstanding the series divergence, we can show numerically that the three particle-hole diagrammatic contributions are not contributing significantly to the final answer, thus demonstrating a nearly perfect destructive interference of contributions in subspaces with higher-order particle-hole lines. Consequently, for strong enough confinement in the third direction, the transition between the polaron and the molecule ground state is found to be in good agreement with the pure two-dimensional case and agrees very well with the one found by the wave-function approach in the two-particle-hole subspace.

Electronic interactions in multiorbital systems lead to nontrivial features in the optical spectrum. In iron superconductors, the Drude weight is strongly suppressed with hole-doping. We discuss why the common association of the renormalization of the Drude weight with that of the kinetic energy, used in single-band systems, does not hold in multiorbital systems. This applies even in a Fermi liquid description when each orbital is renormalized differently, as happens in iron superconductors. We estimate the contribution of interband transitions at low energies. We show that this contribution is strongly enhanced by interactions and dominates the coherent part of the spectral weight in hole-doped samples at frequencies currently used to determine the Drude weight.

We study the spin-resolved transport properties of T-shaped double quantum dots coupled to ferromagnetic leads. Using the numerical renormalization group method, we calculate the linear conductance and the spin polarization of the current for various model parameters and at different temperatures. We show that an effective exchange field due to the presence of ferromagnets results in different conditions for Fano destructive interference in each spin channel. This spin dependence of the Fano effect leads to perfect spin polarization, the sign of which can be changed by tuning the dots' levels. Large spin polarization occurs due to Coulomb correlations in the dot, which is not directly coupled to the leads, while finite correlations in the directly coupled dot can further enhance this effect. Moreover, we complement accurate numerical results with a simple qualitative explanation based on analytical expressions for the zero-temperature conductance. The proposed device provides a prospective example of an electrically controlled, fully spin-polarized current source, which operates without an external magnetic field.

We study the nonequilibrium spectral function of the single-impurity Anderson model connecting with multiterminal leads. The full dependence on frequency and bias voltage of the nonequilibrium self-energy and spectral function is obtained analytically up to the second-order perturbation regarding the interaction strength $U$. High- and low-bias voltage properties are analyzed for a generic multiterminal dot, showing a crossover from the Kondo resonance to the Coulomb peaks with increasing bias voltage. For a dot where the particle-hole symmetry is not present, we construct a current-preserving evaluation of the nonequilibrium spectral function for arbitrary bias voltage. It is shown that finite-bias voltage does not split the Kondo resonance in this order, and no specific structure due to multiple leads emerges. Overall bias dependence is quite similar to finite-temperature effect for a dot with or without the particle-hole symmetry.

We measured the spectroscopic response of stripe- and vortex-containing ${\mathrm{ErMnO}}_3$ in order to uncover the electronic properties of the domain walls. We quantify Born effective charge and polarization differences using the lattice behavior, analyze the local rare earth environment from the $f$-manifold excitations, and reveal how shifts in the charge transfer excitations impact the band gap. The increased Born charge, polarization, and band gap in the vortex-containing material are brought together with a discussion of hybridization and wall density effects. The domain wall optical constants are strongly frequency dependent.

We characterize experimentally and theoretically the collective electronic excitations in two prototypical layered transition-metal dichalcogenides, ${\mathrm{NbSe}}_2$ and ${\mathrm{Cu}}_{0.2}{\mathrm{NbS}}_2$. The energy- and momentum-dependent dynamical structure factor was measured by inelastic x-ray scattering (IXS) spectroscopy and simulated by time-dependent density-functional theory. We find good agreement between theory and experiment, provided that Nb semicore states are taken into account together with crystal local-field effects. Both materials have very similar spectra, characterized by two main plasmons at 9 and 23 eV, which we show to both have $\pi +\sigma$ character on the basis of a detailed analysis of the band structure. Finally, we discuss the role of the layer anisotropy in the dispersion of these plasmons.

We report the resistivity and ac susceptibility measurements of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ single crystals at pressures up to 11 GPa. Under highly hydrostatic pressure conditions generated in a multi-anvil high-pressure apparatus, a pressure-induced superconductivity only appears above a critical pressure $P_{\mathrm{C}}\sim 7 \mathrm{GPa}$, coinciding with a structural transition from a rhombohedral to a monoclinic structure. The absence of superconductivity in the rhombohedral phase at pressures below $P_{\mathrm{C}}$, which is contrary to previous studies using the high-pressure apparatus with a solid pressure medium, suggests that the occurrence of superconductivity in the rhombohedral phase is sensitive to pressure conditions such as pressure inhomogeneity or uniaxial stress.

We develop an electrodynamic model based on dyadic Green's functions for analyzing the near-field interactions between a dipolar scatterer (DS) and a plasmonic nanosphere (PN) under external excitation, accounting for multipolar contributions in the evaluation of the scattered fields. In particular, we include all the radiative and nonradiative field interactions between the DS and the PN, particularly the physical mechanism of DS's self-coupling through the PN, which is either neglected or approximated in previous work. Our objective is to show under which conditions self-coupling is important for strong excitation-rate enhancement of the DS and provide a description of the system's properties. We analytically investigate the conditions under which the excitation rate of a DS, such as an organic dye or a quantum dot, is enhanced when located in close proximity to a PN. We show the existence of critical conditions in terms of polarizabilities and distances that lead to large enhancement based on self-coupling and how to predict it.

We use an exact solution of the relaxation-time Boltzmann equation in a uniform ac electric field to describe the nonlinear optical response of graphene in the terahertz (THz) range. The cases of monolayer, bilayer, and $ABA$-stacked trilayer graphene are considered, and the monolayer species is shown to be the most appropriate one to exploit the nonlinear free electron response. We find that a single layer of graphene shows optical bistability in the THz range, within the electromagnetic power range attainable in practice. The current associated with the third harmonic generation is also computed.

While the tunneling conductance between two spherical-like conducting particles depends on the relative interparticle distance, the wave function overlap between states of two rodlike particles, and so the tunneling conductance, depends also on the relative orientation of the rod axes. Modeling slender rodlike particles as cylindrical quantum wells of diameter $D$ and length $L\gg D$, we calculate the matrix element of the tunneling between two rods for arbitrary relative orientations of the rod axes. We show that tunneling between two parallel rods is about $L/\sqrt{D\xi }$ times larger than the tunneling matrix element for perpendicular rods, where $\xi$ is the tunneling decay length. By considering the full dependence of the tunneling conductance on the angle between rod axes, we calculate within an effective medium theory the conductivity of dispersions of rods with different degrees of alignment. We find that for isotropically oriented rods, the effect of orientation in the tunneling processes is marginal for all rod concentrations. On the contrary, for systems of strongly aligned rods, the enhanced tunneling between nearly parallel rods increases significantly the system conductivity in a relatively large concentration range. Next, we consider systems in which short-range attraction between rods is added, as in dispersions of rods with depletion interaction. We find that the strongly anisotropic attraction promotes enhanced tunneling between neighboring parallel rods, increasing the effective medium conductivity by several orders of magnitude compared to the case in which the angular dependence of tunneling is ignored, even for relatively weak attractions.

We present a theoretical study of restoration of superconductivity in the form of the triplet reentrant superconducting phase in a quasi-one-dimensional (Q1D) conductor. Substitution of known band and superconducting parameters of the presumably triplet Q1D superconductor ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ into our theoretical equations shows that such restoration can happen in non-destructive pulsed high magnetic field of the order of $H\simeq 100\text{T}$. We investigate in detail how small inclinations of a direction of a magnetic field from its best experimental geometry decrease the superconducting transition temperature of the reentrant phase, which is important for its possible experimental discovery.

We study the quantum phase transition in a spin chain with variable Ising interactions and position-dependent coupling to a resonator field. Such a complicated model, usually not present in natural physical systems, can be simulated by an array of qubits based on man-made devices and exhibits interesting behavior. We show that, when the coupling between the qubit and field is strong enough, a superradiant phase transition occurs, and it is possible to pick a particular field mode to undergo this phase transition by properly modulating the strength of the Ising interaction. We also study the impact of the resonator field on the magnetic properties of the spin chain and find a rich set of phases characterized by distinctive qubit correlation functions.

We revisit the description of the low-energy singlet sector of the spin-$\frac{1}{2}$ Heisenberg antiferromagnet on kagome in terms of an effective quantum dimer model. With the help of exact diagonalizations of appropriate finite-size clusters, we show that the embedding of a given process in its kagome environment leads to dramatic modifications of the amplitudes of the elementary loop processes, an effect not accessible to the standard approach based on the truncation of the Hamiltonian to the nearest-neighbor valence-bond basis. The resulting parameters are consistent with a $Z_2$ spin liquid rather than with a valence-bond crystal, in agreement with the last density matrix renormalization group results.

Ultrashort optical pulses can be used both to create fundamental quasiparticles in crystals and to change their properties. In noble metals, femtosecond lasers induce bond hardening, but little is known about its origin and consequences. Here we simulate ultrafast laser excitation of silver at high fluences. We compute laser-excited potential-energy surfaces by all-electron ab initio theory and analyze the resulting quantum lattice dynamics. We also consider incoherent lattice heating due to electron-phonon interactions using the generalized two-temperature model. We find phonon hardening, which we attribute to the excitation of $s$ electrons. We demonstrate that this may result in phonon vacuum squeezed states with an optimal squeezing factor of $\sim 0.001$ at the L-point longitudinal mode. This finding implies that ultrafast laser-induced bond hardening may be used as a tool to manipulate the quantum state of opaque materials, where, so far, the squeezing of phonons below the zero-point motion has only been realized in transparent crystals by a different mechanism. On the basis of our finding, we further propose a method for directly measuring bond hardening.

We investigate the relation between thermal spin-transfer torque (TSTT) and the spin-dependent Seebeck effect (SDSE), which produces a spin current when a temperature gradient is applied across a metallic ferromagnet, in nanopillar metallic spin valves. Comparing its angular dependence (aSDSE) with the angle-dependent magnetoresistance measurements on the same device, we are able to verify that a small spin heat accumulation builds up in our devices. From the SDSE measurement and the observed current driven STT switching current of 0.8 mA in our spin valve devices, it was estimated that a temperature difference of 230 K is needed to produce an equal amount of TSTT. Experiments specifically focused on investigating TSTT show a response that is dominated by overall heating of the magnetic layer. Comparing it to the current driven STT experiments we estimate that only $\sim 10\%$ of the response is due to TSTT. This leads us to conclude that switching dominated by TSTT requires a direct coupling to a perfect heat sink to minimize the effect of overall heating. Nevertheless the combined effect of heating, STT, and TSTT could prove useful for inducing magnetization switching when further investigated and optimized.

We have carried out high magnetic field studies of single-crystalline ${\mathrm{Li}}_2{\mathrm{MnO}}_3$, a honeycomb lattice antiferromagnet. Its magnetic phase diagram was mapped out using magnetization measurements at applied fields up to 35 T. Our results show that it undergoes two successive meta-magnetic transitions around 9 T fields applied perpendicular to the ab plane (along the $c^{*}$ axis). These phase transitions are completely absent in the magnetization measured with the field applied along the ab plane. In order to understand this magnetic phase diagram, we developed a mean-field model starting from the correct Néel-type magnetic structure, consistent with our single crystal neutron diffraction data at zero field. Our model calculations succeeded in explaining the two meta-magnetic transitions that arise when ${\mathrm{Li}}_2{\mathrm{MnO}}_3$ enters two different spin-flop phases from the zero field Néel phase.