We study the fate of helical edge states in a quantum spin Hall insulators when the whole system is exposed to strong Coulomb interactions. Using dynamical mean-field theory, we show that the dispersion relation of the edge states is strongly affected by Coulomb interactions. In fact, the formerly gapless edge modes become gapped at a critical interaction strength. Interestingly, this critical interaction strength is significantly smaller at the edge than its counterpart in the bulk. Thus, the bulk remains in a topologically nontrivial state at intermediate interaction strengths where the edge states are already gapped out. This peculiar scenario leads to the reconstruction of gapless helical states at the new boundary between the topological bulk and the trivial (Mott insulating) edge. Further increasing the interaction strength triggers the progressive localization on the new boundary, the shrinking of the quantum spin Hall region, and the migration of the helical edge states towards the center of the system. The edge state reconstruction process is eventually interrupted by the Mott localization of the whole sample. Finally, we characterize the topological properties of the system by means of a local Chern marker.

The coupling between the optical mode of an LC cavity and a magnetoplasmon is studied by terahertz transmission spectroscopy. The magnetoplasmons are created by etching a high-mobility two-dimensional electron gas into stripes. As a result, we identify three different regimes, depending on the plasmon frequency relative to the cavity frequency. We find a significant coupling to the cyclotron dispersion even in presence of screening of the electric field by the plasmon.

We consider photon-assisted tunneling (PAT) and the Landau–Zener–Stueckelberg (LZS) interference for double quantum dots induced electrostatically along a semiconducting carbon nanotube. An atomistic tight-binding approach and the time-dependent configuration-interaction method are employed to describe systems of a few confined electrons and holes. We reproduce the patterns of the LZS interference recently observed for quantum double dots describing transport across hole-localized states. Moreover, we indicate that for charge configurations for which the ground state is Pauli-blocked, PAT can be used for resolution of the transitions that involve spin-flip or intervalley transitions without the spin-valley conserving background signal.

We describe a hybrid quantum system composed of a micrometer-sized carbon nanotube (CNT) longitudinally coupled to a flux qubit. We demonstrate the usefulness of this device for generating high-fidelity nonclassical states of the CNT via dissipative quantum engineering. Sideband cooling of the CNT to its ground state and generating a squeezed ground state, as a mechanical analog of the optical squeezed vacuum, are two additional examples of the dissipative quantum engineering studied here. Moreover, we show how to generate a long-lived macroscopically distinct superposition (i.e., a Schrödinger-cat-like) state. This cat state can be trapped, under some conditions, in a dark state, as can be verified by detecting the optical response of control fields.

We consider tunneling of spinless electrons from a single-channel emitter into an empty collector through an interacting resonant level of the quantum dot. When all Coulomb screening of sudden charge variations of the dot during the tunneling is realized by the emitter channel, the system is described with an exactly solvable model of a dissipative qubit. To study manifestations of the coherent qubit dynamics in the collector ac response we derive a solution to the corresponding Bloch equation for the model quantum evolution in the presence of the oscillating voltage of frequency $\omega$ and calculate perturbatively the ac response in the voltage amplitude. We have shown that in a wide range of the model parameters the coherent qubit dynamics results in the nonzero frequencies resonances in the amplitudes dependence of the ac harmonics and in the jumps of the harmonics phase shifts across the resonances. In the first order the ac response is directly related to the spectral decomposition of the corresponding transient current and contains only the first $\omega$ harmonic, whose amplitude exhibits resonance at $\omega ={\omega }_I$, where ${\omega }_I$ is the qubit oscillation frequency. In the second order we have obtained the $2\omega$ harmonic of the ac response with resonances in the frequency dependence of its amplitude at ${\omega }_I,{\omega }_I/2$ and zero frequency and also have found the frequency dependent shift of the average steady current.

We introduce the concept of the hybrid magnetic skyrmion (HMS) by patterning arrays of magnetic nanodisks on top of thin films that possess the Dzyaloshinskii-Moriya interaction (DMI). The practical feasibility of the method is validated by micromagnetic simulations and computed Skyrmion number per unit cell. The HMS has enhanced stability and mobility with respect to existing skyrmion configurations. The skyrmion Hall effect is substantially reduced, in comparison with skyrmion stabilized by the DMI only. We created isolated skyrmions via current pulses and drove them by a continuous current at enhanced speed of 750 m/s.

$\mathrm{S}{\mathrm{r}}_{4-x}\mathrm{C}{\mathrm{a}}_x\mathrm{M}{\mathrm{n}}_2\mathrm{Co}{\mathrm{O}}_9$ oxides ($x=0$ and $x=2$) are found to exhibit magnetic responses typical of single-chain magnets (SCMs) and single-ion magnets (SIMs), two features generally investigated in coordination polymers or complexes. The compound $x=0$ appears to be a genuine SCM, in that blocking effects associated with slow spin dynamics yield remanence and coercivity in the absence of long-range ordering (LRO). In addition, SIM signatures of nearly identical nature are detected in both compounds, coexisting with SCM in $x=0$ and with LRO in $x=2$. It is also observed that a SCM response can be recovered in $x=2$ after application of magnetic field. These results suggest that purely inorganic systems could play a valuable role in the topical issue of the interplay among SIM, SCM, and LRO phenomena in low-dimensional magnetism.

Topological phases of matter are one of the hallmarks of quantum condensed matter physics. One of their striking features is a bulk-boundary correspondence wherein the topological nature of the bulk manifests itself on boundaries via exotic massless phases. In classical wave phenomena, analogous effects may arise; however, these cannot be viewed as equilibrium phases of matter. Here, we identify a set of rules under which robust equilibrium classical topological phenomena exist. We write simple and analytically tractable classical lattice models of spins and rotors in two and three dimensions which, at suitable parameter ranges, are paramagnetic in the bulk but nonetheless exhibit some unusual long-range or critical order on their boundaries. We point out the role of simplicial cohomology as a means of classifying, writing, and analyzing such models. This opens an experimental route for studying strongly interacting topological phases of spins.

We measure and model the homogeneous excitation linewidth of regioregular poly(3-hexylthiophene), a model semicrystalline polymeric semiconductor, by means of two-dimensional coherent photoluminescence excitation spectroscopy. At a temperature of 8 K, we extract a linewidth of $\sim 90$ meV full width at half maximum, which is a significant fraction of the total linewidth. We interpret this homogeneous broadening as a consequence of interchain exciton coupling and discuss it within the context of a weakly coupled aggregate model.

Spin-$\frac{1}{2}$ compounds $A_3{\mathrm{Cu}}_3{\mathrm{AlO}}_2{\left({\mathrm{SO}}_4\right)}_4(A=\mathrm{K},\mathrm{Rb},\mathrm{and}\mathrm{Cs})$ have one-dimensional (1D) inequilateral diamond chains. We analyze the temperature dependence of the magnetic susceptibility and determine the magnetic exchange interactions. In contrast to azurite, a dimer is formed on one of the sides of the diamond. From numerical analyses of the proposed model, we find that the dimer together with a nearly isolated 1D Heisenberg chain characterize magnetic properties including magnetization curve and magnetic excitations. This implies that a dimer-monomer composite chain without frustration is a good starting point for describing these compounds.

Here, we present a study on $\mathrm{Si}\left(111\right)/\mathrm{Ta}\left(150\AA \right)/\mathrm{IrMn}\left(150\AA \right)/\mathrm{NiFe}\left(50\AA \right)/\mathrm{Nb}\left(x\right)/\mathrm{NiFe}\left(50\AA \right)/\mathrm{Ta}\left(50\AA \right)$ spin valves with $x=100-500\AA$. Samples with a Nb layer thickness of $300\AA$ or higher exhibit an unusual magnetization behavior below $T_c$. The superconducting Nb layer contributes strongly to the magnetization, and manifests itself in an anomalous hysteresis loop. While the hysteresis loop is seemingly similar to what is generally expected from hard superconductors and many superconductor/ferromagnet hybrid systems, its direction is inverted when compared to what is generally observed. Thus, the samples show paramagnetic behavior for upsweeping fields and diamagnetic behavior for downsweeping fields. This means that the respective samples exhibit a magnetization, which is contrary to what Lenz's rule dictates.

The compound ${\mathrm{SmB}}_6$ is the best established realization of a topological Kondo insulator, in which a topological insulator state is obtained through Kondo coherence. Recent studies have found evidence that the surface of ${\mathrm{SmB}}_6$ hosts ferromagnetic domains, creating an intrinsic platform for unidirectional ballistic transport at the domain boundaries. Here, surface-sensitive x-ray absorption (XAS) and bulk-sensitive resonant inelastic x-ray scattering spectra are measured at the Sm ${\mathrm{N}}_{4,5}$ edge, and used to evaluate electronic symmetries, excitations, and temperature dependence near the surface of cleaved samples. The XAS data show that the density of large-moment atomic multiplet states on a cleaved surface grows irreversibly over time, to a degree that likely exceeds a related change that has recently been observed in the surface $4f$ orbital occupation.

Density matrix embedding theory (DMET) is a relatively new technique for the calculation of strongly correlated systems. Recently, block product DMET (BPDMET) was introduced for the study of spin systems such as the antiferromagnetic $J_1\text{−}{J}_2$ model on the square lattice. In this paper, we extend the variational Ansatz of BPDMET using spin-state optimization, yielding improved results. We apply the same techniques to the Kitaev-Heisenberg model on the honeycomb lattice, comparing the results when using several types of clusters. Energy profiles and correlation functions are investigated. A diagonalization in the tangent space of the variational approach yields information on the excited states and the corresponding spectral functions.

Clathrates can form a type of guest-host solid structures that, unlike most crystalline solids, have very low thermal conductivity. It is generally thought that the guest atoms caged inside the host framework act as “rattlers” and induce lattice dynamics disorders responsible for the small thermal conductivity. We performed a systematic study of the lattice dynamical properties of type-VIII clathrates with alkali and alkaline-earth guests, i.e., $X_8\mathrm{S}{\mathrm{i}}_{46}$ ($X=\mathrm{Na}$, K, Rb, Cs, Ca, Sr, and Ba). The energy dependent participation ratio (PR) and the atomic participation ratio of phonon modes extracted from density functional theory calculations revealed that the rattler concept is not adequate to describe the effect of fillers as they manifest strong hybridization with the framework. For the case of heavy fillers, such as Rb, Sr, Cs, and Ba, a phonon band gap was formed between the acoustic and optical branches. The calculated PR indicated that the fillers suppress the acoustic phonon modes and change the energy transport mechanism from propagative to diffusive or localized resulting in “phonon-glass” characteristics. This effect is stronger for the heavy fillers. Furthermore, in all cases, the guest insertion depressed the phonon bandwidth, reduced the Debye temperature, and reduced the phonon group velocity, all of which should lead to reduction of the thermal conductivity.

The electronic structure and transport properties of the half-Heusler compound CoVSn are studied systematically by combining first-principles electronic structure calculations and Boltzmann transport theory. The band structure at the valence-band edge is complex with multiple maxima derived from hybridized transition element $d$ states. The result is a calculated thermopower larger than $200\mu \mathrm{V}/\mathrm{K}$ within a wide range of doping concentrations and temperatures for heavily doped $p$-type CoVSn. The thermoelectric properties additionally benefit from the corrugated shapes of the hole pockets in our calculated isoenergy surfaces. Our calculated power factor $S^2\sigma /\tau$ (with respect to an average unknown scattering time) of CoVSn is comparable to that of FeNbSb. A smaller lattice thermal conductivity can be expected from the smaller group velocities of acoustical modes compared to FeNbSb. Overall, good thermoelectric performance for CoVSn can be expected by considering the electronic transport and lattice thermal conductivity.

We study the optical response of a four-level double-V-type quantum system which interacts simultaneously with probe and pump laser fields and is located near a two-dimensional array of metal-coated dielectric nanospheres. By considering different coupling configurations for the pump/probe laser fields and analyzing the resulting probe absorption spectrum we reveal a variety of phenomena, such as huge enhancement of the absorption at the central line, gain without inversion, and a phase-dependent absorption spectrum. We also show that the enhancement of probe absorption or the gain can be controlled by varying the distance of the quantum system from the plasmonic nanostructure, the intensity of the pump field(s), and, when applicable, their relative phase. Our results can find applications in on-chip nanoscale photonic devices.

The ability to probe the electronic structure of individual nano-objects at high energy resolution using momentum- and space-resolved electron energy loss spectroscopy in the scanning transmission electron microscope is demonstrated through the observation of confinement of the $\pi$ plasmon in individual single-wall carbon nanotubes. While confinement perpendicular to the tube axis was identified for all investigated tubes, a variable degree of confinement parallel to the tube axis was attributed to the concentration of topological defects. Spatially resolved valence loss spectra allowed for the identification of a loss peak attributed to a chirality-dependent radial interband transition. Furthermore, the importance of a careful consideration of loss peak momentum dispersions for the interpretation of spatially resolved valence loss spectra is discussed.

The type-II Weyl/Dirac fermions are a generalization of conventional or type-I Weyl/Dirac fermions, whose conic spectrum is tilted such that the Fermi surface becomes lines in two dimensions, and surface in three dimensions rather than discrete points of the conventional Weyl/Dirac fermions. The mass-independent renormalization group calculations show that the tilting parameter decreases monotonically with respect to the length scale, which leads to a transition from two-dimensional type-II Weyl/Dirac fermions to the type-I ones. Because of the nontrivial Fermi surface, a photon gains a finite mass partially via the chiral anomaly, leading to the strong screening effect of the Weyl/Dirac fermions. Consequently, anisotropic type-II Dirac semimetals become stable against the Coulomb interaction. This work provides deep insight into the interplay between the geometry of Fermi surface and the Coulomb interaction.

The electric coupling between surface ions and bulk ferroelectricity gives rise to a continuum of mixed states in ferroelectric thin films, exquisitely sensitive to temperature and external factors, such as applied voltage and oxygen pressure. Here we develop the comprehensive analytical description of these coupled ferroelectric and ionic (ferroionic) states by combining the Ginzburg-Landau-Devonshire description of the ferroelectric properties of the film with Langmuir adsorption model for the electrochemical reaction at the film surface. We explore the thermodynamic and kinetic characteristics of the ferroionic states as a function of temperature, film thickness, and external electric potential. These studies provide a new insight into mesoscopic properties of ferroelectric thin films, whose surface is exposed to chemical environment as screening charges supplier.