The strongly anisotropic adsorbate system Br/Pt(110) exhibits an anomalous order-order phase transition due to competition between interadsorbate repulsion and charge density wave (CDW) correlations. The phase transition is preceded by fluctuations between a normal $c\left(2\times 2\right)$ adsorbate phase on a flat surface and a $\left(2\times 1\right)$ phase involving a periodic lattice distortion of the substrate. Here we study the fluctuations, determine the correlation length in real and reciprocal space, and report its evolution with temperature. This allows us to identify a critical temperature. However, since around the critical temperature the adsorbate mobility freezes out, the transition remains incomplete, resulting in a 2D nematic glass at low temperature. In the temperature range, where the fluctuations occur, the system is demonstrated to be close to a $1\mathrm{D}\to 2\mathrm{D}$ crossover. This unusual phase transition driven by competing interactions exhibits several parallels to phenomena observed in unconventional superconductors.

The neutralization of low energy $\mathrm{N}{\mathrm{a}}^{+}$ and $\mathrm{L}{\mathrm{i}}^{+}$ ions scattered from Au nanoclusters formed by deposition onto oxide surfaces decreases as the cluster size increases. An explanation for this behavior is provided here, which is based on the notion that the atoms in the clusters are not uniformly charged, but that the edge atoms are positively charged while the center atoms are nearly neutral. This leads to upward-pointing dipoles at the edge atoms that increase the neutralization probability of alkali ions scattered from those atoms. As the clusters increase in size, the number of edge atoms relative to the number of center atoms decreases, so that that the average neutralization also decreases. Calculations employing this model are compared to experimental data and indicate good agreement if the strengths of the dipoles at the edge atoms are assumed to decrease with cluster size. This model also explains differences in the neutralization probabilities of scattered $\mathrm{N}{\mathrm{a}}^{+}$ and $\mathrm{L}{\mathrm{i}}^{+}$.

With the help of the numerical renormalization group method, we theoretically investigate the Josephson phase transition in a parallel junction with one quantum dot embedded in each arm. It is found that in the cases of appropriate dot levels and symmetrical dot-superconductor couplings, the Josephson phase transition will be suppressed. This is manifested as the fact that with the enhancement of the electron correlation, the supercurrent only arrives at its ${\pi }^{\prime }$ phase but cannot enter its $\pi$ phase. Moreover, when the dot levels are detuned, one ${\pi }^{\prime }$-phase island appears in the phase diagram. Such a result is attributed to the interplay among the Kondo effect, RKKY effect, and the Cooper pairing correlation. We believe that this work can be helpful in understanding the special roles of the correlation mechanisms of the parallel junction in driving the Josephson phase transition.

All-optical control of surface plasmon polaritons (SPPs) is theoretically demonstrated in arrays of metallic slits. It is shown how the mixing of electromagnetic fields scattered by the slits from a weak beam at $\lambda$ wavelength, with second-harmonic fields generated by a high-intensity $2\lambda$ beam, creates a destructive interference of surface plasmons in one of the two possible directions of emission from the slits, while these are enhanced along the opposite direction. Our method enables spectrally broadband excitation and thus unidirectional launching of SPP pulses as short as $\sim \mathrm{20}$ fs, all-optically controlled. Based on these findings, an ultranarrow bandwidth surface plasmon frequency comb is designed.

The conductance measurement of a half-quantized plateau in a quantum anomalous Hall insulator-superconductor structure has been reported by a recent experiment [Q. L. He et al., Science 357, 294 (2017)], which suggests the existence of the chiral Majorana fermion modes. However, such a half-quantized conductance plateau may also originate from a disorder-induced metallic phase. To identify the exact mechanism, we study the transport properties of such a system in the presence of strong disorders. Our results show that the local current density distributions of these two mechanisms are different. In particular, the current noise measurement can be used to distinguish them with the existing experimental setup.

Spinless fermions on the honeycomb lattice with repulsive nearest-neighbor interactions are known to harbour a quantum critical point at half-filling, with critical behavior in the Gross-Neveu (chiral Ising) universality class. The critical interaction strength separates a weak-coupling semimetallic regime from a commensurate charge-density-wave phase. The phase diagram of this basic model of correlated fermions on the honeycomb lattice beyond half-filling is, however, less well established. Here, we perform an analysis of its many-body instabilities using the functional renormalization group method with a basic Fermi surface patching scheme, which allows us to treat instabilities in competing channels on equal footing also away from half-filling. Between half-filling and the Van Hove filling, the free Fermi surface is holelike and we again find a charge-density wave instability to be dominant at large interactions. Moreover, its characteristics are those of the half-filled case. Directly at the Van Hove filling, the nesting property of the free Fermi surface stabilizes a dimerized bond-order phase. At lower filling, the free Fermi surface becomes electronlike and a superconducting instability with $f$-wave symmetry is found to emerge from the interplay of intra-unit-cell repulsion and collective fluctuations in the proximity to the charge-density wave instability. We estimate the extent of the various phases and extract the corresponding order parameters from the effective low-energy Hamiltonians.

Using a fully ab initio methodology, we demonstrate how the lattice vibrations couple with neutral excitons in monolayer ${\mathrm{WSe}}_2$ and contribute to the nonradiative excitonic lifetime. We show that only by treating the electron-electron and electron-phonon interactions at the same time is it possible to obtain an unprecedented agreement of the zero- and finite-temperature optical gaps and absorption spectra with the experimental results. The bare energies were calculated by solving the Kohn-Sham equations, whereas $G_0{W}_0$ many-body perturbation theory was used to extract the excited-state energies. A coupled electron-hole Bethe-Salpeter equation was solved incorporating the polaronic energies to show that it is the in-plane torsional acoustic phonon branch that contributes mostly to the $A$ and $B$ exciton buildup. We find that the three $A$, $B$, and $C$ excitonic peaks exhibit different behavior with temperature, displaying different nonradiative linewidths. There is no considerable transition in the strength of the excitons with temperature, but the $A$ exciton remains dark in comparison with the $C$ exciton. Further, all the excitonic peaks redshift as temperature rises. Renormalization of the bare electronic energies by phonon interactions and anharmonic lattice thermal expansion causes a decreasing band gap with increasing temperature. The zero-point energy renormalization (31 meV) is found to be entirely due to the polaronic interaction with a negligible contribution from lattice anharmonicities. These results may have a profound impact on electronic and optoelectronic device technologies based on these monolayers.

A quantum dot coupled to an optical cavity has recently proven to be an excellent source of on-demand single photons. Typically, applications require simultaneous high efficiency of the source and quantum indistinguishability of the extracted photons. While much progress has been made in both suppressing background sources of decoherence and utilizing cavity quantum electrodynamics to overcome fundamental limitations set by the intrinsic exciton-phonon scattering inherent in the solid-state platform, the role of the excitation pulse has been often neglected. We investigate quantitatively the factors associated with pulsed excitation that can limit simultaneous efficiency and indistinguishability, including excitation of multiple excitons, multiphotons, and pump-induced dephasing, and find for realistic single-photon sources that these effects cause degradation of the source figures of merit comparable to that of phonon scattering. We also develop rigorous open quantum system polaron master equation models of quantum dot dynamics under a time-dependent drive which incorporate non-Markovian effects of both photon and phonon reservoirs, and explicitly show how coupling to a high-Q-factor cavity suppresses multiphoton emission in a way not predicted by commonly employed models. We then use our findings to summarize the criteria that can be used for single-photon-source optimization.

We analyze a simple implementation of an absorption refrigerator, a system that requires heat and not work to achieve refrigeration, based on two Coulomb-coupled single-electron systems. We analytically determine the general condition to achieve cooling-by-heating, and we determine the system parameters that simultaneously maximize the cooling power and cooling coefficient of performance (COP) finding that the system displays a particularly simple COP that can reach Carnot's upper limit. We also find that the cooling power can be indirectly determined by measuring a charge current. Analyzing the system as an autonomous Maxwell demon, we find that the highest efficiencies for information creation and consumption can be achieved, and we relate the COP to these efficiencies. Finally, we propose two possible experimental setups based on quantum dots or metallic islands that implement the nontrivial cooling condition. Using realistic parameters, we show that these systems, which resemble existing experimental setups, can develop an observable cooling power.

The electronic structure of Si nanocrystals (NCs) doped with B (acceptor) and P (donor) impurities is calculated. A tight-binding approach is employed, allowing us to investigate NCs of up to 10 nm diameter and to compare directly with recent experiments on colloidal Si NCs. The calculations show that the experimental data of optical and electrical spectroscopy are consistent with configurations of full compensation between donors and acceptors. The NC energy gap is narrowed in the presence of codopants and can be even smaller than in bulk Si, but at the same time it varies with NC size under the effect of quantum confinement. Measured energy gaps are compatible with NCs containing a small number $(\sim 2\text{–}5$) of B-P pairs if we assume a random placement of the dopants in the NC core. B-P clusters in which the B and P atoms are nearest neighbors have a marginal influence on the NC energy gap. In the case of Si NCs doped with a single P impurity, the theory predicts the hyperfine splitting of electron spin resonance as a function of the NC size, in excellent agreement with experiments if the dopant is placed at random positions.

Realizing strong coupling between a single quantum emitter (QE) and an optical cavity is of crucial importance in the context of various quantum optical applications. Although Rabi splitting of single quantum emitters coupled to high-$Q$ classical cavities has been reported in numerous configurations, attaining single emitter Rabi splitting with a plasmonic nanostructure remains a challenge. In particular, strong coupling at the single QE regime would open the path for the realization of single-photon nonlinearities. In this paper, we derive a plasmon quantization procedure for systems consisting of a single QE located in the gap of a nanoantenna. This procedure leads to the description of the quantum dynamics by a master equation for the state of the QE and the quantized plasmonic modes, which is crucial to demonstrate the emergence of single-photon nonlinearities. We investigate numerically the optical response and the resulting Rabi splitting in metallic nanoantennas and find the optimal geometries for the emergence of the strong-coupling regime with single QEs. Finally, we demonstrate the saturation of hybridized modes for a chosen configuration. Our results will be useful for implementation of realistic quantum plasmonic nanosystems involving single QEs at room temperature.

By modeling a Rashba nanowire contacted to leads via an inhomogeneous spin-orbit coupling profile, we investigate the equilibrium properties of the spin sector when a uniform magnetic field is applied along the nanowire axis. We find that the interplay between magnetic field and Rashba coupling generates a spin current, polarized perpendicularly to the applied field and flowing through the nanowire even at equilibrium. In the nanowire bulk, such effect persists far beyond the regime where the nanowire mimics the helical states of a quantum spin Hall system, while in the leads, the spin current is suppressed. Furthermore, despite the nanowire not being proximized by superconductors, at the interfaces with the leads we predict the appearance of localized spin torques and spin polarizations, orthogonal to the magnetic field and partially penetrating into the leads. This feature, due to the inhomogeneity of the Rashba coupling, suggests to use caution in interpreting spin polarization as signatures of Majorana fermions. When the magnetic field has a component also along the Rashba field, its collinearity with the spin polarization and orthogonality to the spin current are violated in the nanowire bulk too. We analyze these quantities in terms of the magnetic field and chemical potential for both long and short nanowires in experimentally realistic regimes.

We introduce the concept of entanglement features of unitary gates, as a collection of exponentiated entanglement entropies over all bipartitions of input and output channels. We obtained the general formula for time-dependent $n\mathrm{th}$-Rényi entanglement features for unitary gates generated by random Hamiltonian. In particular, we propose an Ising formulation for the second Rényi entanglement features of random Hamiltonian dynamics, which admits a holographic tensor network interpretation. As a general description of entanglement properties, we show that the entanglement features can be applied to several dynamical measures of thermalization, including the out-of-time-order correlation and the entanglement growth after a quantum quench. We also analyze the Yoshida-Kitaev probabilistic protocol for random Hamiltonian dynamics.

A phenomenological model for magnetodynamics of core-shell nanoparticles is presented. The term core-shell implies, prima facie, that the spin structures (phases) of the particle components are different, no matter whether or not they are chemically identical. The model takes in two main assumptions. First, the interphase boundary is a thin (in comparison with the core and shell scales) transition layer with strongly inhomogeneous spin arrangement. Second, the shell is a granular structure consisting of spin-ordered crystallites with a wide spread of internal anisotropy, including a fraction of ones with extremely high coercivity. The framework developed on that basis, despite its simplicity, provides a unified explanation for the major specific properties of magnetic core-shell particles. Namely, (i) the growth of observed anisotropy field with reduction of the particle size, (ii) the presence of fixed unidirectional anisotropy, and (iii) the existence of rotatable anisotropy that emerges in a threshold manner under an external field. For a nanoparticle with the above-described core-shell architecture, a theory of ferromagnetic resonance with allowance for the superparamagnetic effect is worked out, and the ferromagnetic resonance absorption lines of noninteracting ensembles of such particles are presented. It is demonstrated that all the qualitatively different contributions to the core-shell-induced anisotropy could be evaluated from the data on a temperature sweep of the resonance field. Moreover, to do that, it suffices to use just the samples with random orientation of the particle axes. Preliminary assessment shows that a set of measurements for complete evaluation of the exchange-mediated anisotropies is feasible with the aid of a standard $X$-band spectrometer.

In order to explain and predict the properties of many physical systems, it is essential to understand the interplay of different energy scales. Here we present investigations of the magnetic order in thermalized artificial spin-ice structures, with different activation energies of the interacting Ising-like elements. We image the thermally equilibrated magnetic states of the nanostructures using synchrotron-based magnetic microscopy. By comparing results obtained from structures with one or two different activation energies, we demonstrate a clear impact on the resulting magnetic order. The differences are obtained by the analysis of the magnetic spin structure factors, in which the role of the activation energies is manifested by distinct short-range order. These results highlight the potential of artificial spin-ice structures to serve as model systems for designing various energy-scale hierarchies and investigating their impact on the collective dynamics and magnetic order.

A new version of asymptotically exact theory of spectrum and transverse distribution of magnetization in long-wave magnons is presented. It is based on exact analytical solution of the linearized Landau-Lifshitz equation in a film. We also studied and used symmetry of the Hamiltonian. Our method simplifies all calculations and allows us to obtain analytical results for the range of parameters most important for experiment. The quantization of the transverse wave vector and role of evanescent waves at different values of parameters are studied.

We investigate the impact of spin anisotropic interactions, promoted by spin-orbit coupling, on the magnetic phase diagram of the iron-based superconductors. Three distinct magnetic phases with Bragg peaks at $(\pi ,0)$ and $(0,\pi )$ are possible in these systems: one $C_2$ (i.e., orthorhombic) symmetric stripe magnetic phase and two $C_4$ (i.e., tetragonal) symmetric magnetic phases. While the spin anisotropic interactions allow the magnetic moments to point in any direction in the $C_2$ phase, they restrict the possible moment orientations in the $C_4$ phases. As a result, an interesting scenario arises in which the spin anisotropic interactions favor a $C_2$ phase, but the other spin isotropic interactions favor a $C_4$ phase. We study this frustration via both mean-field and renormalization-group approaches. We find that, to lift this frustration, a rich magnetic landscape emerges well below the magnetic transition temperature, with interesting $C_2,C_4$, and mixed $C_2$–$C_4$ phases. Near the putative magnetic quantum critical point, spin anisotropies promote a stable Gaussian fixed point in the renormalization-group flow, which is absent in the spin isotropic case, and is associated with a near-degeneracy between $C_2$ and $C_4$ phases. We argue that this frustration is the reason why most $C_4$ phases in the iron pnictides only appear inside the $C_2$ phase, and discuss additional manifestations of this frustration in the phase diagrams of these materials.

Recently the influence of antisymmetric spin-orbit coupling has been studied in novel topological superconductors such as half-Heusler compounds and artificial heterostructures. We investigate the effect of Rashba and/or Dresselhaus spin-orbit couplings on the band structure and topological properties of a two-dimensional noncentrosymmetric superconductor. For this goal, the topological helical edge modes are analyzed for different spin-orbit couplings as well as for several superconducting pairing symmetries. To explore the transport properties, we examine the response of the spin-polarized edge states to an exchange field in a superconductor-ferromagnet heterostructure. The broken chiral symmetry causes the unidirectional currents at opposite edges.

In recent years, the fundamental understanding of thermopower in strongly correlated systems as it relates to spin and orbital degeneracy has advanced, but the consequences for determining cation distribution have not been considered. In this work we compare measurements of the electrical conductivity and thermopower in ${\mathrm{Co}}_3{\mathrm{O}}_4$ spinel with different models for the thermopower based on different assumptions of the cation distributions. Using thermopower measurements we calculate the in situ cation distribution according to three different models: case (i) the original Heikes equation, case (ii) Koshibae and co-workers' modified Heikes equation, and case (iii) a new model, using the modified Heikes equation but with contributions from both octahedral and tetrahedral sites. We find that only the modified Heikes equation that includes contributions from both octahedral and tetrahedral sites satisfies the constraints of stoichiometry in the spinel structure. The findings suggest either complete $\left(\sim 100\%\right)$ inversion of the spinel structure if no change in spin state, or a combination of a minimum 40% inversion with a change in spin state of the octahedral ${\mathrm{Co}}^{3+}$ cation.

Eliashberg theory provides a theoretical framework for understanding the phenomenon of superconductivity when pairing between two electrons is mediated by phonons and retardation effects are fully accounted for. BCS theory is often viewed as the weak-coupling limit of Eliashberg theory, in spite of a handful of papers that have pointed out that this is not so. Here, we present very accurate numerical solutions in the weak-coupling limit to complement the existing analytical results and demonstrate more convincingly the validity of this limit by extending the analytical results to first order in the coupling constant.