Within an effective Dirac-Weyl theory we solve the scattering problem for massless chiral fermions impinging on a cylindrical time-dependent potential barrier. The setup we consider can be used to model the electron propagation in a monolayer of graphene with harmonically driven quantum dots. For static small-sized quantum dots scattering resonances enable particle confinement and interference effects may switch forward scattering on and off. An oscillating dot may cause inelastic scattering by excitation of states with energies shifted by integer multiples of the oscillation frequency, which significantly modifies the scattering characteristics of static dots. Exemplarily the scattering efficiency of a potential barrier with zero bias remains finite in the limit of low particle energies and small potential amplitudes. For an oscillating quantum dot with finite bias, the partial wave resonances at higher energies are smeared out for small frequencies or large oscillation amplitudes, thereby dissolving the quasibound states at the quantum dot.

We study the equilibration properties of observables in long-range field theories after the mass quench in $d=1$, 2, and 3 dimensions. We classify the regimes where we expect equilibration or its absence in different dimensions. In addition, we study the effect of the initial state in the equilibration properties of our system. In the case of free field theories, we show that whenever we have equilibration, the long-time limit of correlations can be described by the generalized Gibbs ensemble. We prove that all integrals of motion in our system are nonlocal.

Based on the first-principles density functional theory calculations for the electronic band structure and lattice dynamics of ${\mathrm{Li}}_3{\mathrm{B}}_4{\mathrm{C}}_2$, we predict that this material is a strong electron-phonon coupled superconductor with a superconducting transition temperature higher than that for ${\mathrm{MgB}}_2$. ${\mathrm{Li}}_3{\mathrm{B}}_4{\mathrm{C}}_2$ is a layered material which is formed by substituting one-third carbon atoms in the semiconducting compound LiBC with boron atoms, with the remaining carbon atoms forming a regular hexagonal lattice in each boron-carbon layer. Similar to ${\mathrm{MgB}}_2$, ${\mathrm{Li}}_3{\mathrm{B}}_4{\mathrm{C}}_2$ is inherently metallic and possesses two $\sigma$-bonding bands around the Fermi energy. The superconductivity in this material arises from the coupling of these two $\sigma$-bonding bands with the intralayer bond-stretching $E^{\prime }$ modes. From the phonon spectrum and the formation energy, we find that ${\mathrm{Li}}_3{\mathrm{B}}_4{\mathrm{C}}_2$ is dynamically stable and has a high probability to be synthesized in laboratory.

We study nonlinear properties of multilayer metamaterials created by graphene sheets separated by dielectric layers. We demonstrate that such structures can support localized nonlinear modes described by the discrete nonlinear Schrödinger equation and that its solutions are associated with stable discrete plasmon solitons. We also analyze the nonlinear surface modes in truncated graphene metamaterials being a nonlinear analog of surface Tamm states.

We extend the Ginzburg-Landau (GL) theory of atomically rough bcc-liquid interfaces [Wu et al., Phys. Rev. B 73, 094101 (2006)] outside of equilibrium. We use this extension to derive an analytical expression for the kinetic coefficient, which is the proportionality constant $\mu \left(\widehat{n}\right)$ between the interface velocity along a direction $\widehat{n}$ normal to the interface and the interface undercooling. The kinetic coefficient is expressed as a spatial integral along the normal direction of a sum of gradient square terms corresponding to different nonlinear density wave profiles. Anisotropy arises naturally from the dependence of those profiles on the angles between the principal reciprocal lattice vectors ${\vec{K}}_i$ and $\widehat{n}$. Values of the kinetic coefficient for the $\left(100\right),\left(110\right)$, and $\left(111\right)$ interfaces are compared quantitatively to the prediction of linear Mikheev-Chernov (MC) theory [J. Cryst. Growth 112, 591 (1991)] and previous molecular dynamics (MD) simulation studies of crystallization kinetics for a classical model of Fe. Additional MD simulations are carried out here to compute the relaxation time of density waves in the liquid in order to make this comparison free of fit parameters. The GL theory predicts an expression for $\mu$ similar to the MC theory but yields a better agreement with MD simulations for both its magnitude and anisotropy due to a fully nonlinear description of density wave profiles across the solid-liquid interface. In particular, the overall magnitude of $\mu$ predicted by GL theory is an order of magnitude larger than predicted by the MC theory. GL theory is also used to derive an inverse relation between $\mu$ and the solid-liquid interfacial free energy. The general methodology used here to derive an expression for $\mu \left(\widehat{n}\right)$ also applies to amplitude equations derived from the phase-field-crystal model, which only differ from GL theory by the choice of cubic and higher order nonlinearities in the free-energy density.

We show that atomistic first-principles calculations based on real-time propagation within time-dependent density functional theory are capable of accurately describing electronic stopping of light projectile atoms in metal hosts over a wide range of projectile velocities. In particular, we employ a plane-wave pseudopotential scheme to solve time-dependent Kohn-Sham equations for representative systems of H and He projectiles in crystalline aluminum. This approach to simulate nonadiabatic electron-ion interaction provides an accurate framework that allows for quantitative comparison with experiment without introducing ad hoc parameters such as effective charges, or assumptions about the dielectric function. Our work clearly shows that this atomistic first-principles description of electronic stopping is able to disentangle contributions due to tightly bound semicore electrons and geometric aspects of the stopping geometry (channeling versus off-channeling) in a wide range of projectile velocities.

Long range antiferromagnetic (AFM) ordering of Ni spins in ${\text{Ni}}_2{\text{NbBO}}_6$ has been studied with single crystal from spin susceptibility measurement and compared with the ab initio calculation results consistently. Below $T_{\mathrm{N}}\sim 23.5$ K, the $S=1$ spins align along the $a$ direction for edge-shared ${\text{NiO}}_6$ octahedra which form crystallographic armchair chains along the $b$ direction. The isothermal magnetization $M\left(H\right)$ below $T_{\mathrm{N}}$ shows spin-flop transition for magnetic field above $\sim 36$ kOe along the $a$ axis, which indicates the spin anisotropy is along the $a$ direction. The electronic and magnetic structures of ${\text{Ni}}_2{\text{NbBO}}_6$ have also been explored theoretically using density functional theory with generalized gradient approximation plus on-site Coulomb interaction $\left(U\right)$. These calculations support the experimentally observed antiferromagnetism of ${\text{Ni}}_2{\text{NbBO}}_6$. In particular, the long range AFM ordering below ${\mathit{T}}_{\mathrm{N}}$ can be dissected into armchair chains which consists of $S=1$ dimers of $J_2\sim 2.43$ meV with ferromagnetic (FM) intrachain and interchain couplings of size $\lesssim \frac{1}{2}\left|J_2\right|$.

Through the combination of x-ray diffraction, electrical transport, magnetic susceptibility, and heat capacity measurements, we studied the effect of Ce doping in the newly discovered ${\mathrm{SrFBiS}}_2$ system. It is found that ${\mathrm{Sr}}_{0.5}{\mathrm{Ce}}_{0.5}{\mathrm{FBiS}}_2$ undergoes a second-order transition below $\sim 7.5$ K, followed by a superconducting transition with the critical temperature $T_c\sim 2.8$ K. Our transport, specific heat, and dc-magnetization results suggest the presence of bulk ferromagnetic correlation of Ce ions below 7.5 K that coexist with superconductivity when the temperature is further lowered below 2.8 K.

On the basis of the Eilenberger theory, spatial variation of the local NMR relaxation rate $T_1^{-1}$ is quantitatively estimated in the vortex lattice state, to clarify the differences between the $s$-wave and the $d$-wave superconductors. We study the temperature and the magnetic field dependencies of $T_1^{-1}$ inside and outside of the vortex core, including influences of nonmagnetic impurity scatterings in the Born limit and in the unitary limit. These results are helpful to detect detailed characters of local electronic structures in the vortex lattice states via site-selective NMR experiments.

We study superconductors with two order components and phase separation driven by intercomponent density-density interaction, focusing on the phase where only one condensate has nonzero ground-state density and a competing order parameter exists only in vortex cores. We demonstrate there that multibody intervortex interactions can be strongly nonpairwise, leading to some unusual vortex patterns in an external field, such as vortex pairs and vortex chains. We demonstrate that in an external magnetic field such a system undergoes a field-driven phase transition from (broken) $U\left(1\right)$ to (broken) $U\left(1\right)\times U\left(1\right)$ symmetries when a subdominant order parameter in the vortex cores acquires global coherence. Observation of these characteristic ordering patterns in surface probes may signal the presence of a subdominant condensate in the vortex core.

By comparing the photoinduced magnetization dynamics in simple layered systems we show how light-induced modifications of the magnetic anisotropy directly enhance the magnetization. It is observed that the spin precession in ${\left({\text{CH}}_3{\text{NH}}_3\right)}_2{\text{CuCl}}_4$, initiated by a light pulse, increases in amplitude at the critical temperature $T_C$. The phenomenon is related to the dependence of the critical temperature on the axial magnetic anisotropy. The present results underline the possibility and the importance of the optical modifications of the anisotropy, opening different paths toward the control of the magnetization state for ultrafast memories.

A quantitative investigation of spin-pumping-induced spin transport in $n\text{-GaAs}$ was conducted at room temperature (RT). GaAs has a non-negligible spin-orbit interaction, so that electromotive force due to the inverse spin Hall effect (ISHE) of GaAs contributed to the electromotive force detected with a platinum (Pt) spin detector. The electromotive force detected by the Pt spin detector had opposite polarity to that measured with a $\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}/\mathrm{GaAs}$ bilayer due to the opposite direction of spin current flow, which demonstrates successful spin transport in the $n\text{-GaAs}$ channel. A two-dimensional spin-diffusion model that considers the ISHE in the $n\text{-GaAs}$ channel reveals an accurate spin diffusion length of ${\lambda }_{\mathrm{s}}=1.09\mu \mathrm{m}$ in $n\text{-GaAs}$ $(N_{\mathrm{Si}}=4\times {10}^{16}\mathrm{c}{\mathrm{m}}^{-3})$ at RT, which is approximately half that estimated by the conventional model.

We study the possibility of a two-dimensional (2D) skyrmion crystal stabilization in a magnetic film with perpendicular anisotropy in the absence of Dzyaloshinskii-Moriya interaction by creating the regular array of blind holes or stubs. By micromagnetic simulation we demonstrate that skyrmions can be stable in the patterned films with the parameters of ordinary materials such as CoPt, FePt, or FePd. The skyrmion lattices can be initialized in the system by simple magnetization in the uniform external magnetic field. At the zero external field the skyrmion helicity depends on the geometry of the blind hole or stub but also can be tuned by applying the field. The suggested method makes it possible to create dense enough (with the period less than 100 nm) skyrmion lattices which are important to carry out transport measurements.

We have investigated the structural and magnetic transitions in ${\mathrm{CsCoO}}_2$ using calorimetric measurements, neutron powder diffraction, density functional theory calculations, and muon-spin relaxation measurements. ${\mathrm{CsCoO}}_2$ exhibits three-dimensional long-range antiferromagnetic (AFM) order at 424 K, resulting in antiferromagnetic alignment of chains of ferromagnetically ordered Co-Co spin dimers. Although there is no change in magnetic structure around a structural transition at $T^{*}=100$ K, the resulting bifurcation of corner-shared Co-O-Co bond angles causes a weakening of the AFM interaction for one set of bonds along the chains. Consequently, the system undergoes a complex freezing out of relaxation processes on cooling.

The characteristics of Fe-based superconductors are manifested in their electronic, magnetic properties, and pairing symmetry of the Cooper pair, but the latter remain to be explored. Usually in these materials, superconductivity coexists and competes with magnetic order, giving unconventional pairing mechanisms. We report on the results of the bulk magnetization measurements in the superconducting state and the low-temperature specific heat down to 0.4 K for ${\text{BaFe}}_{2-x}{\text{Ni}}_x{\text{As}}_2$ single crystals. The electronic specific heat displays a pronounced anomaly at the superconducting transition temperature and a small residual part at low temperatures in the superconducting state. The normal-state Sommerfeld coefficient increases with Ni doping for $x=0.092$, 0.096, and 0.10, which illustrates the competition between magnetism and superconductivity. Our analysis of the temperature dependence of the superconducting-state specific heat and the London penetration depth provides strong evidence for a two-band $s$-wave order parameter. Further, the data of the London penetration depth calculated from the lower critical field follow an exponential temperature dependence, characteristic of a fully gapped superconductor. These observations clearly show that the superconducting gap in the nearly optimally doped compounds is nodeless.

Exciton-polaritons can condense to a macroscopic quantum state through a nonequilibrium process of pumping and decay. In recent experiments, polariton condensates are used to observe, for a short time, nonlinear Josephson phenomena by coupling two condensates. However, it is still not clear how these phenomena are affected by the pumping and decay at long times and how the coupling alters the polariton condensation. Here, we consider a polariton Josephson junction pumped on one side and study its dynamics within the mean-field theory. The Josephson current is found to give rise to multistability of the stationary states, which are sensitive to the initial conditions and incoherent noises. These states can be attributed to either the self-trapping effect or the parity-time (PT) symmetry of the system. These results can be used to explain the emission spectra and the $\pi$-phase locking observed in recent experiments. We further predict that the multistability can reduce to the self-trapped state if the PT symmetry is broken. Moreover, the polaritons can condense even below the threshold, exhibiting hysteresis.

A theoretical framework for pump-probe photoemission is presented. The approach is based on a general formulation using the Keldysh formalism for the lesser Green's function to describe the real-time evolution of the electronic degrees of freedom in the initial state after a strong pump pulse that drives the system out of equilibrium. The final state is represented by a time-reversed low-energy electron-diffraction state. Our one-step description is related as close as possible to Pendry's original formulation of the photoemission process. The formalism allows for a quantitative calculation of time-dependent photocurrent for simple metals where a picture of effectively independent electrons is assumed to be reliable. The theory is worked out for valence- and core-electron excitations. It comprises the study of different relativistic effects as a function of the pump-probe delay.

In this paper, we apply the formalism of translation invariant (continuous) matrix product states in the thermodynamic limit to $(1+1)$-dimensional critical models. Finite bond dimension bounds the entanglement entropy and introduces an effective finite correlation length, so that the state is perturbed away from criticality. The assumption that the scaling hypothesis holds for this kind of perturbation is known in the literature as finite entanglement scaling. We provide further evidence for the validity of finite entanglement scaling and based on this formulate a scaling algorithm to estimate the central charge and critical exponents of the conformally invariant field theories describing the critical models under investigation. The algorithm is applied to three exemplary models; the cMPS version to the nonrelativistic Lieb-Liniger model and the relativistic massless boson, and MPS version to the one-dimensional quantum Ising model at the critical point. Another new aspect to our approach is that we directly use the (c)MPS induced correlation length rather than the bond dimension as scaling parameter. This choice is motivated by several theoretical arguments as well as by the remarkable accuracy of our results.

The doubly ionized $d^9$ copper ion provides, originally in ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and later in many more compounds, the platform for high temperature superconductivity when it is forced toward higher levels of oxidation. The nearest chemical equivalent is ${\mathrm{Ag}}^{2+}$, which is almost entirely avoided in nature. AgO is an illustrative example, being an unusual nonmagnetic insulating compound with an open $4d$ shell on one site. This compound has been interpreted in terms of one ${\mathrm{Ag}}^{3+}$ ion at the fourfold site and one ${\mathrm{Ag}}^{+}$ ion that is twofold coordinated. We analyze more aspects of this compound, finding that indeed the ${\mathrm{Ag}}^{3+}$ ion supports only four occupied $4d-\mathrm{based}$ Wannier functions per spin, while ${\mathrm{Ag}}^{+}$ supports five, yet their physical charges (as quantified by their spherical radial charge densities) are nearly equal. The oxygen $2p$ Wannier functions display two distinct types of behavior, one type of which includes conspicuous Ag $4d$ tails. Calculation of the Born effective charge tensor shows that the mean effective charges of the Ag ions differ by about a factor of 2, roughly consistent with the assigned formal charges. We analyze the $4d$ charge density and discuss it in terms of recent insights into charge states of insulating (and usually magnetic) transition-metal oxides. What might be expected in electron- and hole-doped AgO is discussed briefly.