We calculate the conductances of a three-way junction of spinless Luttinger-liquid wires as functions of bias voltages applied to three independent Fermi-liquid reservoirs. In particular, we consider the setup that is characteristic of a tunneling experiment, in which the strength of electron-electron interactions in one of the arms of the junction (“tunneling tip”) is different from that in the other two arms (which together form a “main wire”). The scaling dependence of the two independent conductances on bias voltages is determined within a fermionic renormalization-group approach in the limit of weak interactions. The solution shows that, in general, the conductances scale with the bias voltages in an essentially different way compared to their scaling with the temperature $T$. Specifically, unlike in the two-terminal setup, the nonlinear conductances cannot be generically obtained from the linear ones by simply replacing $T$ with the “corresponding” bias voltage or the largest one. Remarkably, a finite tunneling bias voltage prevents the interaction-induced breakup of the main wire into two disconnected pieces in the limit of zero $T$ and a zero source-drain voltage.

We investigate the topological properties of Fibonacci quasicrystals using cavity polaritons. Composite structures made of the concatenation of two Fibonacci sequences allow one to investigate generalized edge states forming in the gaps of the fractal energy spectrum. We employ these generalized edge states to determine the topological invariants of the quasicrystal. When varying a structural degree of freedom (phason) of the Fibonacci sequence, the edge states spectrally traverse the gaps, while their spatial symmetry switches: The periodicity of this spectral and spatial evolution yields direct measurements of the gap topological numbers. The topological invariants that we determine coincide with those assigned by the gap-labeling theorem, illustrating the direct connection between the fractal and topological properties of Fibonacci quasicrystals.

Backscattering-immune chiral modes arise along certain line defects in three-dimensional materials. Here, we study the Floquet chiral modes along Floquet topological defects, namely, the defects come entirely from spatial modulations of periodic driving. We define a precise topological invariant that counts the number of Floquet chiral modes, which is expressed as an integral on a five-dimensional torus parametrized by $(k_x,k_y,k_z,\theta ,t)$. This work demonstrates the possibility of creating chiral modes in three-dimensional bulk materials by modulated driving.

Enabling the confinement of light to a scale far below the one of conventional optics, surface plasmon polaritons (SPPs) induced by an electromagnetic field in a metal-dielectric interface supply an ideal system to explore strong quantized light-matter coupling. The fast matter-SPP population exchange reported in previous works makes it a candidate for spin manipulation, but such reversible dynamics asymptotically vanishes accompanying the quantum matter relaxing completely to its ground state. Here, we study the exact dissipative dynamics of a quantum emitter (QE) coupled to SPPs. It is interesting to find that, qualitatively different from conventional findings, the QE can be partially stabilized in its excited state even in the presence of the lossy metal. Our analysis reveals that it is the formation of a QE-SPP bound state which results in such suppressed dissipation. Enriching the decoherence dynamics of the QE in the lossy medium, our result is helpful to understand QE-SPP interactions and apply plasmonic nanostructures in quantum devices.

The effect of the electron-electron coupling of the Hubbard type on the one-dimensional electron system with the spin-orbit interaction (SOI), caused by the deformation of the elastic subsystem, is studied. As a rule, the electron-electron repulsion suppresses the deformation-induced SOI. In particular, at half-filling there is no lattice-induced SOI for the repulsive Hubbard chain. The attraction between electrons, instead, mostly enhances the deformation-induced SOI. The effect of the external magnetic field on such a SOI, caused by the deformation of the lattice, is considered.

We examine the temperature dependence of the visibility of the excitonic Aharonov-Bohm peak in type-II quantum dots. We obtain a functional temperature dependence that is similar to that determined by transport experiments, namely, with the $T^{-1}$ term due to electron-electron collisions and the $T^{-3}$ term due to electron-phonon interactions. However, the magnitude of the latter term is much smaller than that for the transport electrons and similar to the interaction strength of the exciton-phonon coupling. Such suppressed electron-phonon interaction ushers a way for all-optical studies of decoherence processes in semiconductor nanostructures as other dephasing mechanisms become more pronounced.

I show that a nonequilibrium paraelectric to ferroelectric transition can be transiently induced using midinfrared pulses. This relies on a quartic $l{Q}_{{\mathrm{l}}_z}^2{Q}_{{\mathrm{h}}_x}^2$ coupling between the lowest ($Q_{{\mathrm{l}}_z}$) and highest ($Q_{{\mathrm{h}}_x}$) frequency infrared-active phonon modes of a paraelectric material. Density functional calculations show that the coupling constant $l$ is negative, which causes a softening of the $Q_{{\mathrm{l}}_z}$ mode when the $Q_{{\mathrm{h}}_x}$ mode is externally pumped. A rectification along the $Q_{{\mathrm{l}}_z}$ coordinate that stabilizes the nonequilibrium ferroelectric state occurs only above a critical threshold for the electric field of the pump pulse, demonstrating that this is a nonperturbative phenomenon. A first-principles calculation of the coupling between light and the $Q_{{\mathrm{h}}_x}$ mode shows that ferroelectricity can be induced in the representative case of strained ${\mathrm{KTaO}}_3$ by a midinfrared pulse with a peak electric field of $17\mathrm{MV}{\mathrm{cm}}^{-1}$ and duration of 2 ps. Furthermore, other odd-order nonlinear couplings make it possible to arbitrarily switch off the light-induced ferroelectric state, making this technique feasible for all-optic devices.

We present quantum mechanical calculations of the Gilbert damping constant $\alpha$ in ultrathin ${\mathrm{L}1}_0$ ${[\mathrm{Co}/\mathrm{NM}]}_N$ superlattices and (001) fcc ${[\mathrm{Co}/\mathrm{NM}]}_N$ magnetic multilayers built of cobalt and nonmagnetic metals NM = Cu, Ag, Pd, Pt, and Au. The calculations are performed within a realistic nine-orbital tight-binding model of the band structure including spin-orbit interaction. The dependence of $\alpha$ on the stacking number $N$, ferromagnetic and nonmagnetic layer thicknesses as well as the electron scattering rate is investigated. The damping constant is shown to be the sum of a constant term (bulklike) and a $1/N$ term (due to external surfaces) which arise from inter- and intraband electronic transitions, respectively. The calculated $\alpha$ is found to be enhanced in the considered multilayers in comparison with its values for bulk Co and their bilayer counterparts with the same total Co thickness. The origin of this enhancement and the variation of $\alpha$ with the geometric structure of the multilayers are further investigated by analyzing the damping contributions from individual atomic layers. The obtained theoretical results for the damping constant are shown to be in good agreement with previous experimental observations in magnetic multilayers. In particular, the experimentally observed linear dependence on the ratio of NM (Pd or Pt) and Co layer thicknesses is reproduced in the present calculations.

We numerically demonstrate the possibility to manipulate domain walls in magnetoelastic nanostripes by means of uniform mechanical stresses. The symmetry breaking of the magnetic states in unidimensional ferromagnets allows the control of the domain-wall position or velocity in geometrically tailored nanostripes coupled to piezoelectric substrates. We further predict that this approach yields unusual domain-wall configurations with velocities of the same order of magnitude as that induced by magnetic fields or spin-polarized currents, while the energy consumption is considerably smaller.

We investigate the effect of periodic and disordered distributions of pairing centers in a one-dimensional itinerant system to obtain the microscopic conditions required to achieve an end Majorana mode and the topological phase diagram. Remarkably, the topological invariant can be generally expressed in terms of the physical parameters for any pairing center configuration. Such a fundamental relation allows us to unveil hidden local symmetries and to identify trajectories in the parameter space that preserve the nontrivial topological character of the ground state. We identify the phase diagram with topologically nontrivial domains where Majorana modes are completely unaffected by the spatial distribution of the pairing centers. These results are general and apply to several systems where inhomogeneous perturbations generate stable Majorana modes.

We investigate the maximum speed at which a driven superfluid can flow through a narrow constriction with a size on the order of the healing length. Considering dissipation via the thermal nucleation of quantized vortices, we calculate the critical velocity for superfluid ${}^4\mathrm{He}$ and ultracold atomtronic circuits, identify fundamental length, and velocity scales, and are thus able to present results obtained in widely different temperature and density ranges in a universal framework. For ultranarrow channels, we predict a drastic reduction in the critical velocity as the energy barrier for flow reducing thermally activated phase slip fluctuations is suppressed.

The task of finding the smallest energy needed to bring a solid to its onset of mechanical instability arises in many problems in materials science, from the determination of the elasticity limit to the consistent assignment of free energies to mechanically unstable phases. However, unless the space of possible deformations is low-dimensional and a priori known, this problem is numerically difficult, as it involves minimizing a function under a constraint on its Hessian, which is computationally prohibitive to obtain in low-symmetry systems, especially if electronic structure calculations are used. We propose a method that is inspired by the well-known dimer method for saddle point searches but that adds the necessary ingredients to solve for the lowest onset of mechanical instability. The method consists of two nested optimization problems. The inner one involves a dimer-like construction to find the direction of smallest curvature as well as the gradient of this curvature function. The outer optimization then minimizes energy using the result of the inner optimization problem to constrain the search to the hypersurface enclosing all points of zero minimum curvature. We use the method to investigate potential failure mechanisms of graphene and to determine the free energy of mechanically unstable Pt-W and Cu-W alloys.

Angle-resolved photoemission spectroscopy is used to study the scattering rates of charge carriers from the hole pockets near $\mathrm{\Gamma }$ in the iron-based high-$T_c$ hole-doped superconductors ${\mathrm{K}}_x{\mathrm{Ba}}_{1-x}{\mathrm{Fe}}_2{\mathrm{As}}_2$, $x=0.4$, and ${\mathrm{K}}_x{\mathrm{Eu}}_{1-x}{\mathrm{Fe}}_2{\mathrm{As}}_2$, $x=0.55$, and the electron-doped compound $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$, $x=0.075$. The scattering rate for any given band is found to depend linearly on the energy, indicating a non-Fermi-liquid regime. The scattering rates in the hole-doped compound are considerably higher than those in the electron-doped compounds. In the hole-doped systems the scattering rate of the charge carriers of the inner hole pocket is about three times higher than the binding energy, indicating that the spectral weight is heavily incoherent. The strength of the scattering rates and the difference between electron- and hole-doped compounds signals the importance of Hund's exchange coupling for correlation effects in these iron-based high-$T_c$ superconductors. The experimental results are in qualitative agreement with theoretical calculations in the framework of combined density functional dynamical mean-field theory.

The finite-temperature transport properties of FeRh compounds are investigated by first-principles density-functional-theory-based calculations. The focus is on the behavior of the longitudinal resistivity with rising temperature, which exhibits an abrupt decrease at the metamagnetic transition point, $T=T_m$, between ferro- and antiferromagnetic phases. A detailed electronic structure investigation for $T\ge 0$ K explains this feature and demonstrates the important role of (i) the difference of the electronic structure at the Fermi level between the two magnetically ordered states and (ii) the different degree of thermally induced magnetic disorder in the vicinity of $T_m$, giving different contributions to the resistivity. To support these conclusions, we also describe the temperature dependence of the spin-orbit-induced anomalous Hall resistivity and Gilbert damping parameter. For the various response quantities considered, the impact of thermal lattice vibrations and spin fluctuations on their temperature dependence is investigated in detail. Comparison with corresponding experimental data shows, in general, very good agreement.

Superradiance is a signature effect in quantum photonics that explains the collective enhancement of emission power by a factor of $N^2$ when $N$ emitters are placed in subwavelength proximity. Although the effect is inherently transient, successful attempts have been made to sustain it in the steady-state regime. Until recently, the effects of superradiance were not considered to be applicable to thermal emitters due to their intrinsic incoherent nature. Novel nanophotonic thermal emitters display favorable coherent characteristics that enable them to obey principles of superradiance. However, published analytical work on conventional superradiant thermal emitter assemblies shows an anomalous power scaling of $1/N$, and therefore increasing the number of thermal emitters leads to a degeneration of power at resonance. This phenomenon immediately renders the effect of thermal superradiance futile since it cannot outperform noncoupled emitters in the steady-state regime. We propose an alternative assembly of thermal emitters with specific features that improves the power scaling while maintaining the effects of superradiance. In essence, we show that our emitter assembly achieves superior power delivery over conventional noncoupled emitter systems at resonance. Additionally, this assembly has the ability to be tuned to operate at specific resonant frequencies, which is a vital requirement for applications such as photothermal cancer therapy.

A theory for the thermodynamic properties of steps on faceted crystalline surfaces is presented. The formalism leads to the definition of step excess quantities, including an excess step stress that is the step analogy of surface stress. The approach is used to develop a relationship between the temperature dependence of the step free energy $\left({\gamma }^{\mathrm{st}}\right)$ and step excess quantities for energy and stress that can be readily calculated by atomistic simulations. We demonstrate the application of this formalism in thermodynamic-integration (TI) calculations of the step free energy, based on molecular-dynamics simulations, considering $\langle 110\rangle$ steps on the $\left\{111\right\}$ surface of a classical potential model for elemental Cu. In this application we employ the Frenkel-Ladd approach to compute the reference value of ${\gamma }^{\mathrm{st}}$ for the TI calculations. Calculated results for excess energy and stress show relatively weak temperature dependencies up to a homologous temperature of approximately 0.6, above which these quantities increase strongly and the step stress becomes more isotropic. From the calculated excess quantities we compute ${\gamma }^{\mathrm{st}}$ over the temperature range from zero up to the melting point $\left(T_{\mathrm{m}}\right)$. We find that ${\gamma }^{\mathrm{st}}$ remains finite up to $T_{\mathrm{m}}$, indicating the absence of a roughening temperature for this $\left\{111\right\}$ surface facet, but decreases by roughly fifty percent from the zero-temperature value. The strongest temperature dependence occurs above homologous temperatures of approximately 0.6, where the step becomes configurationally disordered due to the formation of point defects and appreciable capillary fluctuations.

We report the observation of two distinct types of magnetoconductance oscillations in six coupled quantum dots (QDs) in the integer quantum Hall regime. By tuning the magnetic field and gate voltage, we find robust conductance peaks and dips on the plateau of one conductance quantum $2e^2/h$. These features fall into two types associated with two different collective quantum states: for the first type, only dips and the crossing behaviors are found, and their traces show an anomalous temperature dependence, named “reversed Coulomb blockade oscillation”, whereas for the second type, the peak traces show both crossing and anticrossing behaviors with resonance-type temperature dependence. Notably, all peaks show clear Coulomb diamonds in their differential conductance. We argue that the observed features reveal the electron addition spectra in an edge-state-mediated QD network, manifesting intricate interdot and dot-edge Coulomb interactions. In the first-type regime, the dots are more isolated from each other and the electron transport is governed by the dot-edge interaction. Conversely, in the second-type regime, the QDs behave as a coupled dot array due to the presence of strong interdot interactions. Our results open a route by using the edge-state-mediated multi-QD system as a laboratory for exploring coherent many-body interactions.

The finite-frequency transport properties of a large-spin molecule attached to ferromagnetic contacts are studied theoretically in the Kondo regime. The focus is on the behavior of the dynamical conductance in the linear response regime, which is determined by using the numerical renormalization group method. It is shown that the dynamical conductance depends greatly on the magnetic configuration of the device and intrinsic parameters of the molecule. In particular, conductance exhibits characteristic features for frequencies corresponding to the dipolar and quadrupolar exchange fields resulting from the presence of spin-dependent tunneling. Moreover, a dynamical spin accumulation in the molecule, associated with the off-diagonal-in-spin component of the conductance, is predicted. This spin accumulation becomes enhanced with increasing the spin polarization of the leads, and it results in a nonmonotonic dependence of the conductance on frequency, with local maxima occurring for characteristic energy scales.

We report the enthalpy of oxygen vacancy formation in thin films of electron-doped ${\mathrm{SrTiO}}_3$, under different degrees of epitaxial stress. We demonstrate that both compressive and tensile strain decrease this energy at a very similar rate and promote the formation of stable doubly ionized oxygen vacancies. Moreover, we also show that unintentional cationic vacancies introduced under typical growth conditions, produce a characteristic rotation pattern of ${\mathrm{TiO}}_6$ octahedra. The local concentration of oxygen vacancies can be modulated by an electric field with an AFM tip, changing not only the local electrical potential but also producing a nonvolatile mechanical response whose sign (up/down) can be reversed by the electric field.