Raza et al. recently observed the extraordinarily large energy blueshifts of localized surface plasmon resonances and additional surface plasmon resonances in silver nanoparticles encapsulated in silicon nitride [S. Raza, S. Kadkhodazadeh, T. Christensen, M. D. Vece, M. Wubs, N. A. Mortensen, and N. Stenger, Nat. Commun. 6, 8788 (2015)], which are not fully understood yet. By using the quantum model consisting of two subsystems, respectively, for describing the center of mass and intrinsic motions of conduction electrons of a metallic nanosphere and a coupling occurring between the center of mass and conduction electrons outside the metallic nanosphere, we first deduced the general energy and line broadening size dependence of localized surface plasmon resonances, which removes the divergent defect of usual $1/R$ size dependence. Second, we proposed that the additional surface plasmon resonance in a metallic nanosphere originates from the transition of the first excited state to the ground state of the center-of-mass subsystem with energy levels corrected by degenerate-state pairs of the system composed of the center of mass and intrinsic motions of conduction electrons. Then, we implemented this generation mechanism of additional surface plasmon resonances for silver nanoparticles encapsulated in silicon nitride, and the calculated results are consistent with experimental results. Furthermore, we obtained an energy expression of localized surface plasmon resonances, with which we successfully explained the extraordinarily large energy blueshifts of localized surface plasmon resonances in few-nanometer silver nanoparticles encapsulated in silicon nitride. Finally, we calculated the localized and additional surface plasmon resonance energies of silver nanoparticles resting on carbon films, and the calculated results perfectly explain the experimental measurements of Scholl et al. [J. A. Scholl, A. L. Koh, and J. A. Dionne, Nature (London) 483, 421 (2012)]. Within this quantum model, the optical properties of metallic nanoparticles are completely determined by degenerate-state or nearly degenerate-state pairs of the system composed of the center-of-mass and intrinsic motions of conduction electrons. Our calculations also show that additional surface plasmon resonances play almost an equal role as localized surface plasmon resonances for metallic nanoparticles excited by fast-moving electrons.

Understanding microscopic heat conduction in thin films is important for nano/micro heat transfer and thermal management for advanced electronics. As the thickness of thin films is comparable to or shorter than a phonon wavelength, phonon dispersion relations and transport properties are significantly modulated, which should be taken into account for heat conduction in thin films. Although phonon confinement and depletion effects have been considered, it should be emphasized that surface-localized phonons (surface phonons) arise whose influence on heat conduction may not be negligible due to the high surface-to-volume ratio. However, the role of surface phonons in heat conduction has received little attention thus far. In the present work, we performed anharmonic lattice dynamics calculations to investigate the thickness and temperature dependence of in-plane thermal conductivity of silicon thin films with sub-10-nm thickness in terms of surface phonons. Through systematic analysis of the influences of surface phonons, we found that anharmonic coupling between surface and internal phonons localized in thin films significantly suppresses overall in-plane heat conduction in thin films. We also discovered that specific low-frequency surface phonons significantly contribute to surface-internal phonon scattering and heat conduction suppression. Our findings are beneficial for the thermal management of electronics and phononic devices and may lead to surface phonon engineering for thermal conductivity control.

The edge states of two-dimensional time-reversal topological insulators support a perfect helical conductance on wide ribbons due to the absence of backscattering. Here, we study the changes in the transport properties of topological insulator nanoribbons by introducing a constriction along the ribbon. This setup allows the edge states to hybridize, leading to reflections at the ends of the constriction. We find that the electronic states running along one edge can be reflected back along the opposite edge multiple times, giving rise to Fabry-Pérot resonant vortices within the constriction with well-defined conductance peaks. We show that magnetic barriers allow one to manipulate these peaks and obtain significant changes in the system spin-resolved magnetoconductance.

We measured the second-order correlation function of the cathodoluminescence intensity and investigated the Purcell effect by comparing the lifetimes of quantum emitters with and without metal structure. The increase in the electromagnetic local density of state due to the coupling of a quantum emitter with a plasmonic structure causes a shortening of the emitter lifetime. Since the plasmon-enhanced electric field is confined well below the wavelength of light, the quantum emitter lifetime is changed in the nanoscale range. In this study, we combined cathodoluminescence in scanning (transmission) electron microscopy with Hanbury Brown–Twiss interferometry to measure the Purcell effect with nanometer and nanosecond resolutions. We used nitrogen-vacancy centers contained in nanodiamonds as the emitters and compared their lifetime in different environments: on a thin $\mathrm{Si}{\mathrm{O}}_2$ membrane, on a thick flat silver film, and embedded in a silver film. The lifetime reductions of nitrogen-vacancy centers were observed in the samples with silver. We evaluated the lifetime by analytical calculation and numerical simulations and revealed the Purcell effects of emitters coupled to the propagating and localized surface plasmons.

Stationary waves in the condensate of electron-hole pairs in the $n-p$ bilayer system are studied. The system demonstrates the transition from a uniform (superfluid) to a nonuniform (supersolid) state. The precursor of this transition is the appearance of the roton-type minimum in the collective mode spectrum. Stationary waves occur in the flow of the condensate past an obstacle. It is shown that the roton-type minimum manifests itself in a rather complicated stationary wave pattern, with several families of crests which cross one another. It is found that the stationary wave pattern is essentially modified under variation in the density of the condensate and under variation in the flow velocity. It is shown that the pattern is formed in the main part by short-wave modes in the case of a point obstacle. The contribution of long-wave modes is clearly visible in the case of a weak extended obstacle, where the stationary wave pattern resembles the ship wave pattern.

Research on the anomalous Hall effect (AHE) has been lasting for a century to make clear the underlying physical mechanism. Generally, the AHE appears in magnetic materials, in which the extrinsic process related to scattering effects and intrinsic contribution connected with Berry curvature are crucial. Recently, AHE has been counterintuitively observed in nonmagnetic topological materials and attributed to the existence of Weyl points. However, the Weyl point scenario would lead to unsaturated AHE even in large magnetic fields and contradicts the saturation of AHE in several tesla (T) in experiments. In this work, we investigate the Hall effect of $\mathrm{Zr}{\mathrm{Te}}_5$ and $\mathrm{Hf}{\mathrm{Te}}_5$ thin flakes in static ultrahigh magnetic fields up to 33 T. We find the AHE saturates to $55\left(70\right){\mathrm{\Omega }}^1\mathrm{c}{\mathrm{m}}^1$ for $\mathrm{Zr}{\mathrm{Te}}_5$ ($\mathrm{Hf}{\mathrm{Te}}_5$) thin flakes above $\sim 10\mathrm{T}$. Combining detailed magnetotransport experiments and Berry curvature calculations, we clarify that the splitting of massive Dirac bands without Weyl points can be responsible for AHE in nonmagnetic topological materials $\mathrm{Zr}{\mathrm{Te}}_5$ and $\mathrm{Hf}{\mathrm{Te}}_5$ thin flakes. This model can identify our thin flake samples to be weak topological insulators and serve as a tool to probe the band structure topology in topological materials.

We investigate quantum transport through the Kondo impurity assuming both a large number of orbital channels $\mathcal{K}\gg 1$ for the itinerant electrons and a semiclassical spin $\mathcal{S}\gg 1$ for the impurity. The non-Fermi-liquid regime of the Kondo problem is achieved in the overscreened sector $\mathcal{K}>2\mathcal{S}$. We show that there exist two distinct semiclassical regimes for the quantum transport through impurity: (i) $\mathcal{K}\gg \mathcal{S}\gg 1$, differential conductance vanishes, and (ii) $\mathcal{S}/\mathcal{K}=\mathcal{C}$ with $0<\mathcal{C}<1/2$, differential conductance reaches some nonvanishing fraction of its unitary value. Using the conformal field theory approach we analyze the behavior of the quantum transport observables and residual entropy in both semiclassical regimes. We show that the semiclassical limit (ii) preserves the key features of resonance scattering and the most essential fingerprints of the non-Fermi-liquid behavior. We discuss possible realization of two semiclassical regimes in semiconductor quantum transport experiments.

We report on investigations of phonons and lithium diffusion in $\mathrm{LiAl}{\mathrm{O}}_2$ based on inelastic neutron scattering measurements of the phonon density of states (DOS) in γ-$\mathrm{LiAl}{\mathrm{O}}_2$ from 473 to 1073 K, complemented with ab initio molecular dynamics (AIMD) simulations. We find that phonon modes related to Li vibrations broaden on warming as reflected in the measured phonon DOS and reproduced in simulations. Further, the AIMD simulations probe the nature of lithium diffusion in the perfect crystalline phase (γ-$\mathrm{LiAl}{\mathrm{O}}_2$), as well as in a structure with lithium vacancies and a related amorphous phase. In all three structures, almost liquidlike superionic diffusion is observed in AIMD simulations at high temperatures, with predicted onset temperatures of 1800, 1200, and 600 K in the perfect structure, vacancy structure, and the amorphous phase, respectively. In the ideal structure, the Li atoms show correlated jumps, while simple and correlated jumps are both seen in the vacancy structure, and a mix of jumps and continuous diffusion occurs in the amorphous structure. Further, we find that Li diffusion is facilitated in all cases by a large librational amplitude of the neighboring $\mathrm{Al}{\mathrm{O}}_4$ tetrahedra and that the amorphous structure opens additional diffusion pathways due to a broad distribution of $\mathrm{Al}{\mathrm{O}}_4$ tetrahedra orientations.

We study the transitions between ergodic and many-body localized phases in spin systems, subject to quenched disorder, including the Heisenberg chain and the central spin model. In both cases systems with common spin lengths $1/2$ and 1 are investigated via exact numerical diagonalization and random matrix techniques. Particular attention is paid to the sample-to-sample variance ${\left({\mathrm{\Delta }}_{s}r\right)}^2$ of the averaged consecutive-gap ratio $\langle r\rangle$ for different disorder realizations. For both types of systems and spin lengths we find a maximum in ${\mathrm{\Delta }}_{s}r$ as a function of disorder strength, accompanied by an inflection point of $\langle r\rangle$, signaling the transition from ergodicity to many-body localization. The critical disorder strength is found to be somewhat smaller than the values reported in the recent literature. Further information about the transitions can be gained from the probability distribution of expectation values within a given disorder realization.

In this paper, we include the elastic scattering of longitudinal electromagnetic waves in transport theory using a medium filled with pointlike, electric dipoles. The interference between longitudinal and transverse waves creates two new channels among which one allows energy transport. This picture is worked out by extending the independent scattering framework of radiative transfer to include binary dipole-dipole interactions. We calculate the diffusion constant of light in the new transport channel and investigate the role of longitudinal waves in other aspects of light diffusion by considering the density of states, equipartition, and Lorentz local field. In the strongly scattering regime, the different transport mechanisms couple and impose a minimum conductivity of electromagnetic waves, thereby preventing Anderson localization of light in the medium. We extend the self-consistent theory of localization and compare the predictions to extensive numerical simulations.

While classical spin systems in random networks have been intensively studied, much less is known about quantum magnets in random graphs. Here, we investigate interacting quantum spins on small-world networks, building on mean-field theory and extensive quantum Monte Carlo simulations. Starting from one-dimensional (1D) rings, we consider two situations: all-to-all interacting and long-range interactions randomly added. The effective infinite dimension of the lattice leads to a magnetic ordering at finite temperature $T_{\mathrm{c}}$ with mean-field criticality. Nevertheless, in contrast to the classical case, we find two distinct power-law behaviors for $T_{\mathrm{c}}$ versus the average strength of the extra couplings. This is controlled by a competition between a characteristic length scale of the random graph and the thermal correlation length of the underlying 1D system, thus challenging mean-field theories. We also investigate the fate of a gapped 1D spin chain against the small-world effect.

We consider localized topologically nontrivial magnetic textures (skyrmions, antiskyrmions, and bimerons) in a thin magnetic film with anisotropic interfacial Dzyaloshinskii-Moriya interaction (iDMI). We use micromagnetic simulations and analytical consideration for studying the internal magnetic structure and stability of these textures. Skyrmion and antiskyrmion become elliptic and orient along the main axes of the iDMI tensor even for small anisotropy. In contrast, bimeron (antibimeron) orientation changes fluently with varying the iDMI anisotropy. Depending on the iDMI anisotropy the bimeron may consist of a vortex and antivortex pair or of “hedgehog” state and antivortex. In experiment the considered iDMI anisotropy can be induced by a strain applied to a magnetic film. We develop a phenomenological approach to establish the strain-iDMI relation.

We present a comprehensive first-principles study of the thermophysical properties of liquid molybdenum in the near-critical region. The ab initio estimate of the molybdenum critical-point parameters is also provided. The robustness of our calculations is confirmed by the excellent agreement with available experiments on shock compression and the successive isentropic expansion of porous samples of molybdenum and several isobaric expansion experiments. We also analyze important thermodynamic properties such as the isochoric and isobaric heat capacity, the Grüneisen parameter, and the speed of sound of liquid molybdenum along the critical isobar up to the critical point.

Recent studies on ${\mathrm{Ba}}_2{\mathrm{CoO}}_4$ (BCO) and ${\mathrm{SrRuO}}_3$ (SRO) have unveiled a variety of intriguing phenomena, such as magnetic polarization on oxygens, unexpected nodes in the spin density profile along bonds, and unusual zigzag spin patterns in triangular lattices. Here, using simple model calculations supplemented by density functional theory we explain the presence of nodes based on the antibonding character of the dominant singly occupied molecular orbitals along the transition metal (TM) to oxygen bonds. Our simple model also allows us to explain the net polarization on oxygen as originated from the hybridization between atoms and mobility of the electrons with spins opposite to those of the closest TM atoms. Our results are not limited to BCO and SRO, but they are generic and qualitatively predict the net polarization expected on any ligands, according to the spin order of the closest TM atoms and the number of intermediate ligand atoms. Finally, we propose that a robust easy-axis anisotropy would suppress the competing ${120}^{\circ }$ antiferromagnetic order to stabilize the zigzag pattern order as ground state in a triangular lattice. Our generic predictions should be applicable to any other compound with characteristics similar to those of BCO and SRO.

We have investigated the superconducting properties in a single crystal of the superconductor ${\mathrm{Pb}}_2\mathrm{Pd}$ via various techniques including, magnetization, AC transport, transverse field muon spin rotation and relaxation, and heat capacity. All measurements confirm the superconducting transition temperature $T_C=3.0\pm 0.1\mathrm{K}$. Electronic specific-heat data are well described by the BCS expression, suggesting that ${\mathrm{Pb}}_2\mathrm{Pd}$ opens an isotropic gap on entering the superconducting state. Specific-heat jump and the ${\lambda }_{e\text{−}ph}$ value categorize ${\mathrm{Pb}}_2\mathrm{Pd}$ as a moderately coupled superconductor. Magnetization and transverse field muon spin rotation measurements along with the Ginzburg-Landau parameter, $\kappa$ < $1/\sqrt{2}$ demonstrates that ${\mathrm{Pb}}_2\mathrm{Pd}$ is a type-I superconductor.

The Kitaev model on the honeycomb lattice is a paradigmatic system known to host a wealth of nontrivial topological phases and Majorana edge modes. In the static case, the Majorana edge modes are nondispersive. When the system is periodically driven in time, such edge modes can disperse and become chiral. We obtain the full phase diagram of the driven model as a function of the coupling and the driving period. We characterize the quantum criticality of the different topological phase transitions in both the static and driven model via the notions of Majorana-Wannier state correlation functions and momentum-dependent fidelity susceptibilities. We show that the system hosts crossdimensional universality classes: although the static Kitaev model is defined on a two-dimensional (2D) honeycomb lattice, its criticality falls into the universality class of one-dimensional (1D) linear Dirac models. For the periodically driven Kitaev model, in addition to the universality class of prototype 2D linear Dirac models, an additional 1D nodal loop type of criticality exists owing to emergent time-reversal and mirror symmetries, indicating the possibility of engineering multiple universality classes by periodic driving. The manipulation of time-reversal symmetry allows the periodic driving to control the chirality of the Majorana edge states.

This paper is a continuation and a partial summary of our analysis of the pairing at a quantum-critical point (QCP) in a metal for a set of quantum-critical systems, whose low-energy physics is described by an effective model with dynamical electron-electron interaction $V\left({\mathrm{\Omega }}_m\right)=(\overline{g}/|{\mathrm{\Omega }}_m{\left|\right)}^{\gamma }$ (the $\gamma$ model). Examples include pairing at the onset of various spin and charge-density-wave and nematic orders and pairing in SYK-type models. In previous papers, we analyzed the physics for $\gamma <2$. We have shown that the onset temperature for the pairing $T_p$ is finite, of order $\overline{g}$, yet the gap equation at $T=0$ has an infinite set of solutions within the same spatial symmetry. As the consequence, the condensation energy $E_c$ has an infinite number of minima. The spectrum of $E_c$ is discrete, but becomes more dense as $\gamma$ increases. Here we consider the case $\gamma =2$. The $\gamma =2$ model attracted special interest in the past as it describes the pairing by an Einstein phonon in the limit when the dressed phonon mass ${\omega }_D$ vanishes. We show that for $\gamma =2$, the spectrum of $E_c$ becomes continuous. We argue that the associated gapless “longitudinal” fluctuations destroy superconducting phase coherence at a finite $T$, such that at $0<T<T_p$, the system displays pseudogap behavior of preformed pairs. We show that for each gap function from the continuum spectrum, there is an infinite array of dynamical vortices in the upper half-plane of frequency. For the electron-phonon case, our results show that $T_p=0.1827\overline{g}$, obtained in earlier studies, marks the onset of the pseudogap behavior, while the actual superconducting $T_c$ vanishes at ${\omega }_D\to 0$.