The rearrangement of a material's electron density during laser irradiation leads to modified nonthermal forces on the atoms that may lead to coherent atomic motions and structural phase transformation on very short time scales. We present ab initio molecular dynamics simulations of a martensitic solid-solid phase transformation in tungsten under conditions of strong electronic excitation. The transformation is ultrafast, taking just over a picosecond, and follows the tetragonal Bain path. To examine whether a solid-solid bcc-fcc phase transformation could occur during laser irradiation, we use two-temperature molecular dynamics (2T-MD) simulations with a specially developed potential dependent on the electronic temperature. Our simulations show that the occurrence of the solid-solid phase transformation is in competition with ultrafast nonthermally assisted melting with the strength of the electron-phonon coupling determining the lifetime of the new solid phase. In tungsten the melting transition is predicted to occur too rapidly for the fcc phase to be detectable during laser irradiation.

Understanding the metal-insulator transition in disordered many-fermion systems, both with and without interactions, is one of the most challenging and consequential problems in condensed matter physics. In this paper, we address this issue from the perspective of the modern theory of the insulating state (MTIS), which has already proven to be effective for band and Mott insulators in clean systems. First, we consider noninteracting systems with different types of aperiodic external potentials: uncorrelated disorder (one-dimensional Anderson model), deterministic disorder (Aubry-André Hamiltonian and its modification including next-nearest-neighbor hopping), and disorder with long-range correlations (self-affine potential). We show how the many-body localization tensor defined within the MTIS may be used as a powerful probe to discriminate the insulating and the metallic phases, and to locate the transition point. Then, we investigate the effect of weak repulsive interactions in the Aubry-André Hamiltonian, a model which describes a recent cold-atoms experiment. By treating the weak interactions within a mean-field approximation we obtain a linear shift of the transition point towards stronger disorder, providing evidence for delocalization induced by interactions.

We theoretically study equilibrium and dynamic properties of nanosized magnetic skyrmions in thin magnetic films with broken inversion symmetry, where an electric field couples to magnetization via spin-orbit coupling. Based on a symmetry-based phenomenology and micromagnetic simulations we show that this electric-field coupling, via renormalizing the micromagnetic energy, modifies the equilibrium properties of the skyrmion. This change, in turn, results in a significant alteration of the current-induced skyrmion motion. Particularly, the speed and direction of the skyrmion can be manipulated by designing a desired energy landscape electrically, which we describe within Thiele's analytical model and demonstrate in micromagnetic simulations including electric-field-controlled magnetic anisotropy. We additionally use this electric-field control to construct gates for controlling skyrmion motion exhibiting a transistorlike and multiplexerlike function. The proposed electric-field effect can thus provide a low-energy electrical knob to extend the reach of information processing with skyrmions.

We present a detailed investigation of the magnetic and superconducting properties of ${\mathrm{Ca}}_{1-x}{\mathrm{Na}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals with $x=0.00$, 0.35, 0.50, and 0.67 by means of the local probe techniques Mössbauer spectroscopy and muon spin relaxation experiments. With increasing Na-substitution level, the magnetic order parameter is suppressed. For $x=0.50$ we find a microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature $T_c$. A systematic comparison with other 122 pnictides reveals a square-root correlation between the reduction of the magnetic order parameter and the ratio of the transition temperatures $T_c/T_N$, which can be understood in the framework of a Landau theory. In the optimally doped sample with $T_c\approx 34$ K, diluted magnetism is found and the temperature dependence of the penetration depth and superfluid density are obtained, proving the presence of two superconducting $s$-wave gaps.

We investigate electron-phonon coupling in the molecular crystals ${\mathrm{CH}}_4, {\mathrm{NH}}_3, {\mathrm{H}}_2\mathrm{O}$, and HF, using first-principles quantum mechanical calculations. We find vibrational corrections to the electronic band gaps at zero temperature of $-1.97$ eV, $-1.01$ eV, $-1.52$ eV, and $-1.62$ eV, respectively, which are comparable in magnitude to those from electron-electron correlation effects. Microscopically, the strong electron-phonon coupling arises in roughly equal measure from the almost dispersionless high-frequency molecular modes and from the lower-frequency lattice modes. We also highlight the limitations of the widely used Allen-Heine-Cardona theory, which gives significant discrepancies compared to our more accurate treatment.

A Josephson junction may be driven through a transition where the superconducting condensate favors an odd over an even number of electrons. At this switch in the ground-state fermion parity, an Andreev bound state crosses through the Fermi level, producing a zero mode that can be probed by a point contact to a grounded metal. We calculate the time-dependent charge transfer between superconductor and metal for a linear sweep through the transition. One single quasiparticle is exchanged with charge $Q$ depending on the coupling energies ${\gamma }_1,{\gamma }_2$ of the metal to the Majorana operators of the zero mode. For a single-channel point contact, $Q$ equals the electron charge $e$ in the adiabatic limit of slow driving, while in the opposite quenched limit $Q=2e\sqrt{{\gamma }_1{\gamma }_2}/({\gamma }_1+{\gamma }_2)$ varies between 0 and $e$. This provides a method to produce single charge-neutral quasiparticles on demand.

Near-field thermal transfer between material bodies is generally obtained by numerical calculation methods. We present simple close-form analytic results for heat transfer between planar structures, derived by explicitly relating the heat transfer to the dispersion relations of the coupled systems. In the conventional case of heat transfer between SiC slabs, the closed-form analytic results agree excellently with numerical simulations. In the case where two graphene sheets are separated by a distance $d$, our theory shows that the heat transfer scales as $1/d$, rather than $1/d^2$ as in the conventional cases, due to the unique dispersion relation of two-dimensional plasmons.

We propose a mechanism for the protection against spin gapped states in doped antiferromagnets. It requires the presence of a Chern-Simons term that can be generated by a coupling between spin and an insulator. We first demonstrate that in the presence of this term the vortex loop excitations of the spin sector behave as anyons with fractional statistics. To generate such a term, the fermions should have a massive Dirac spectrum coupled to the emergent spin field of the spin sector. The Dirac spectrum can be realized by a planar spin configuration arising as the lowest-energy configuration of a square lattice antiferromagnet Hamiltonian involving a Dzyaloshinskii-Moriya interaction. The mass is provided by a combination of dimerization and staggered chemical potential. We finally show that for realistic parameters, anyonic vortex loop condensation will likely never occur and thus the spin gapped state is prevented. We also propose real magnetic materials for an experimental verification of our theory.

Discrete energy levels of ultrasmall metallic grains are extracted in single-electron tunneling spectroscopy experiments. We study the response of these energy levels to an external magnetic field in the presence of both spin-orbit scattering and pairing correlations. In particular, we investigate $g$ factors and level curvatures that parametrize, respectively, the linear and quadratic terms in the magnetic-field dependence of the many-particle energy levels of the grain. Both of these quantities exhibit level-to-level fluctuations in the presence of spin-orbit scattering. We show that the distribution of $g$ factors is not affected by the pairing interaction and that the distribution of level curvatures is sensitive to pairing correlations even in the smallest grains in which the pairing gap is smaller than the mean single-particle level spacing. We propose the level curvature in a magnetic field as a tool to probe pairing correlations in tunneling spectroscopy experiments.

Many numerical techniques for the description of quantum systems that are coupled to a continuous bath require the discretization of the latter. To this end, a wealth of methods has been developed in the literature, which we classify as (i) direct discretization, (ii) orthogonal polynomial, and (iii) numerical optimization strategies. We recapitulate strategies (i) and (ii) to clarify their relation. For quadratic Hamiltonians, we show that (ii) is the best strategy in the sense that it gives the numerically exact time evolution up to a maximum time $t_{\text{max}}$, for which we give a simple expression. For nonquadratic Hamiltonians, we show that no such best strategy exists. We present numerical examples relevant to open quantum systems and strongly correlated systems, as treated by dynamical mean-field theory (DMFT).

The periodized cluster dynamical mean-field theory (PCDMFT) combined with exact diagonalization as the impurity solver has been applied to the half-filled standard Hubbard model on the honeycomb lattice. A correlated Dirac semimetal is found for weak interactions and it transforms into an antiferromagnetic insulating phase for strong interactions via a first-order quantum phase transition, not intervened by a spin liquid phase in between. In this application, the PCDMFT introduces the partial translation symmetry, but cures well the problem caused by the translation symmetry breaking in the cluster dynamical mean-field theory studies for the same model, which gives rise to a spurious insulating phase in the weakly interacting region.

We study theoretically the effects of short-range electron-electron interactions on the electronic structure of graphene, in the presence of substitutional impurities. Our computational approach is based on the $\pi$ orbital tight-binding model for graphene, with the electron-electron interactions treated self-consistently at the level of the mean-field Hubbard model. The finite impurity concentration is modeled using the supercell approach. We compare explicitly noninteracting and interacting cases with varying interaction strength and impurity potential strength. We focus in particular on the interaction-induced modifications in the local density of states around the impurity, which is a quantity that can be directly probed by scanning tunneling spectroscopy of doped graphene. We find that the resonant character of the impurity states near the Fermi level is enhanced by the interactions. Furthermore, the size of the energy gap, which opens up at high-symmetry points of the Brillouin zone of the supercell upon doping, is significantly affected by the interactions. The details of this effect depend subtly on the supercell geometry. We use a perturbative model to explain these features and find quantitative agreement with numerical results.

We describe a method for simulating the real time evolution of extended quantum systems in two dimensions (2D). The method combines the benefits of integrability and matrix product states in one dimension to avoid several issues that hinder other applications of tensor based methods in 2D. In particular, it can be extended to infinitely long cylinders. As an example application we present results for quantum quenches in the 2D quantum [$\left(2+1\right)$-dimensional] Ising model. In quenches that cross a phase boundary we find that the return probability shows nonanalyticities in time.

Using Raman scattering, the quasi-two-dimensional organic superconductor $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2\mathrm{Cu}\left[{\mathrm{N}\left(\mathrm{CN}\right)}_2\right]\mathrm{Br}$ ($T_c=11.8$ K) and the related antiferromagnet $\kappa -{(\mathrm{BEDT}-\mathrm{TTF})}_2\mathrm{Cu}\left[{\mathrm{N}\left(\mathrm{CN}\right)}_2\right]\mathrm{Cl}$ are studied. Raman scattering provides unique spectroscopic information about magnetic degrees of freedom that has been otherwise unavailable on such organic conductors. Below $T=200$ K a broad band at about 500 ${\mathrm{cm}}^{-1}$ develops in both compounds. We identify this band with two-magnon excitation. The position and the temperature dependence of the spectral weight are similar in the antiferromagnet and in the metallic Fermi liquid. We conclude that antiferromagnetic correlations are similarly present in the magnetic insulator and the Fermi-liquid state of the superconductor.

We investigated the pressure effect of ${\mathrm{Cr}}^{4+}/{\mathrm{Cr}}^{3+}$ mixed-valence chromium oxide ${\mathrm{K}}_2{\mathrm{Cr}}_8{\mathrm{O}}_{16}$ using several kinds of high-pressure cells on the ferromagnetic Peierls transition. The resistivity measurements under pressure revealed that the Peierls phase ($T_{\mathrm{MI}}=$ 95 K at 0 GPa) is suppressed but survives even at 13 GPa. Meanwhile, the ferromagnetic phase ($T_{\mathrm{C}}=$ 190 K at 0 GPa) abruptly disappears at a relatively low pressure of 4 GPa. The two phase boundaries between metal/insulator (non-Peierls/Peierls) phases and between ferromagnetic/nonferromagnetic phases seem to cross each other, namely, the four phases seem to meet at a point in the pressure-temperature ($P-T$) phase diagram. At low temperature (Peierls insulating state), we also found an additional phase near the crossing point. These observations might realize the ground states theoretically predicted in the one-dimensional double-exchange system.

Nanomechanical and other mesoscopic vibrational systems typically have several nonlinearly coupled modes with different frequencies and with long lifetime. We consider the power spectrum of one of these modes. Thermal fluctuations of the modes nonlinearly coupled to it lead to fluctuations of the mode frequency and thus to the broadening of its spectrum. However, the coupling-induced broadening is partly masked by the spectral broadening due to the mode decay. We show that the mode coupling can be identified and characterized using the change of the spectrum by weak resonant driving. We develop a path-integral method of averaging over the non-Gaussian frequency fluctuations from nonresonant (dispersive) mode coupling. The shape of the driving-induced power spectrum depends on the interrelation between the coupling strength and the decay rates of the modes involved, providing a means of characterizing these modes. The analysis is extended to the case of coupling to many modes, which because of the cumulative effect can become effectively strong. We also find the power spectrum of a driven mode where the mode has internal nonlinearity. Unexpectedly, the power spectra induced by the intra- and intermode nonlinearities are qualitatively different. The analytical results are in excellent agreement with the numerical simulations.

Phase transitions in nickel-titanium shape-memory alloys are investigated by means of atomistic simulations. A second nearest-neighbor modified embedded-atom method interatomic potential for the binary nickel-titanium system is determined by improving the unary descriptions of pure nickel and pure titanium, especially regarding the physical properties at finite temperatures. The resulting potential reproduces accurately the hexagonal-close-packed to body-centered-cubic phase transition in Ti and the martensitic $\mathrm{B}2\text{−}\mathrm{B}{19}^{\prime }$ transformation in equiatomic NiTi. Subsequent large-scale molecular-dynamics simulations validate that the developed potential can be successfully applied for studies on temperature- and stress-induced martensitic phase transitions related to core applications of shape-memory alloys. A simulation of the temperature-induced phase transition provides insights into the effect of sizes and constraints on the formation of nanotwinned martensite structures with multiple domains. A simulation of the stress-induced phase transition of a nanosized pillar indicates a full recovery of the initial structure after the loading and unloading processes, illustrating a superelastic behavior of the target system.

In this work we present an alternative way to look at electron diffraction in a transmission electron microscope. Instead of writing the scattering amplitude in Fourier space as a set of plane waves, we use the cylindrical Fourier transform to describe the scattering amplitude in a basis of orbital angular momentum (OAM) eigenstates. We show how working in this framework can be very convenient when investigating, e.g., rotation and screw-axis symmetries. For the latter we find selection rules on the OAM coefficients that unambiguously reveal the handedness of the screw axis. Detecting the OAM coefficients of the scattering amplitude thus offers the possibility to detect the handedness of crystals without the need for dynamical simulations, the thickness of the sample, nor the exact crystal structure. We propose an experimental setup to measure the OAM components where an image of the crystal is taken after inserting a spiral phase plate in the diffraction plane and perform multislice simulations on $\alpha$ quartz to demonstrate how the method indeed reveals the chirality. The experimental feasibility of the technique is discussed together with its main advantages with respect to chirality determination of screw axes. The method shows how the use of a spiral phase plate can be extended from a simple phase imaging technique to a tool to measure the local OAM decomposition of an electron wave, widening the field of interest well beyond chiral space group determination.

Ferromagnetic shape memory alloys (FSMAs) have shown great potential as active components in next generation smart devices due to their exceptionally large magnetic-field-induced strains and fast response times. During application of magnetic fields in FSMAs, as is common in several magnetoelastic smart materials, there occurs simultaneous rotation of magnetic moments and reorientation of twin variants, resolving which, although critical for design of new materials and devices, has been difficult to achieve quantitatively with current characterization methods. At the same time, theoretical modeling of these phenomena also faced limitations due to uncertainties in values of physical properties such as magnetocrystalline anisotropy energy (MCA), especially for off-stoichiometric FSMA compositions. Here, in situ polarized neutron diffraction is used to measure directly the extents of both magnetic moments rotation and crystallographic twin-reorientation in an FSMA single crystal during the application of magnetic fields. Additionally, high-resolution neutron scattering measurements and first-principles calculations based on fully relativistic density functional theory are used to determine accurately the MCA for the compositionally disordered alloy of $\mathrm{N}{\mathrm{i}}_2\mathrm{M}{\mathrm{n}}_{1.14}\mathrm{G}{\mathrm{a}}_{0.86}$. The results from these state-of-the-art experiments and calculations are self-consistently described within a phenomenological framework, which provides quantitative insights into the energetics of magnetostructural coupling in FSMAs. Based on the current model, the energy for magnetoelastic twin boundaries propagation for the studied alloy is estimated to be $\sim 150\mathrm{kJ}/{\mathrm{m}}^3$.

We study the interplay effect of spin-orbit coupling (SOC) and an optical lattice on the single-particle physics and superfluid-insulator transition in ultracold Fermi gases. We consider the type of SOC that has been realized in cold atom experiments via two-photon Raman processes. Our analyses are based on the knowledge of a full single-particle spectrum in lattices without relying on any tight-binding approximation. We evaluate existing tight-binding models and point out their limitations in predicting the correct single-particle physics due to the missed high-band contributions. Moreover, we show that the Raman field (creating SOC) can induce band-gap closing in a two-dimensional optical lattice, leading to the intriguing phenomenon of superfluidity reentrance for interacting fermions at integer filling. We present the superfluid-insulator phase diagram in a wide parameter regime of chemical potentials and Raman fields. All these results are far beyond any that a tight-binding model can predict and can be directly probed in current cold atoms experiments.