Of late there has been a surge of interest in localized phonon polariton resonators which allow for the subdiffraction confinement of light in the midinfrared spectral region by coupling to optical phonons at the surface of polar dielectrics. Resonators are generally etched on deep substrates which support propagative surface phonon polariton resonances. Recent experimental work has shown that understanding the coupling between localized and propagative surface phonon polaritons in these systems is vital to correctly describing the system resonances. In this paper we comprehensively investigate resonators composed of arrays of cylindrical SiC resonators on SiC substrates. Our bottom-up approach, starting from the resonances of single, free-standing cylinders and isolated substrates, and exploiting both numerical and analytical techniques, allows us to develop a consistent understanding of the parameter space of those resonators, putting on firmer ground this blossoming technology.

The size-dependent submersion of sodium clusters into helium and para-hydrogen droplets has been computationally investigated using continuum models and path-integral molecular dynamics (PIMD) simulations. All-atom explicit potential energy surfaces combining a semiempirical many-body model for the alkali-metal subpart and a pairwise additive repulsion-dispersion contribution for the solvent-alkali-metal interactions parametrized on quantum chemical calculations were employed for the simulations. Direct evidence for the submersion process was found by placing a sufficiently large sodium cluster, $\mathrm{Na}_{55}$, initially at the surface of a ${}^4\mathrm{He}_{300}$ droplet, whereas $\mathrm{Na}_{13}$ spontaneously migrates to the surface when initially placed at the center of this droplet. Under the normal fluid conditions probed by our approach, submersion in larger helium droplets appears thermally activated but the potential of mean force harvested from out-of-equilibrium PIMD trajectories confirms that the submersion transition occurs near the size of 20 atoms, in agreement with earlier investigations. In the case of para-hydrogen media, temperature and the crystalline nature of the cryogenic host were both found to play significant roles: while a single sodium atom migrates to the surface of liquid $p-{\mathrm{H}}_2$ clusters, it remains stuck inside at 2 K. Similarly, a $\mathrm{Na}_{13}$ cluster remains at the surface in a cold $p-{\mathrm{H}}_2$ cluster but becomes readily submerged at 16 K. Our results also indicate that submersion is disfavored in smaller droplets of the cryogenic medium.

We study the electronic structure and transport properties of zigzag and armchair monolayer molybdenum disulfide nanoribbons using an 11-band tight-binding model that accurately reproduces the material's bulk band structure near the band gap. We study the electronic properties of pristine zigzag and armchair nanoribbons, paying particular attention to the edges states that appear within the ${\mathrm{MoS}}_2$ bulk gap. By analyzing both their orbital composition and their local density of states, we find that in zigzag-terminated nanoribbons these states can be localized at a single edge for certain energies independent of the nanoribbon width. We also study the effects of disorder in these systems using the recursive Green's function technique. We show that for the zigzag nanoribbons, the conductance due to the edge states is strongly suppressed by short-range disorder such as vacancies. In contrast, the local density of states still shows edge localization. We also show that long-range disorder has a small effect on the transport properties of nanoribbons within the bulk gap energy window.

Scanning-probe magnetometry is a valuable experimental tool to investigate magnetic phenomena at the micro- and nanoscale. We theoretically analyze the possibility of measuring magnetic fields via the electrical current flowing through quantum dots. We characterize the shot-noise-limited magnetic-field sensitivity of two devices: a single dot in the Coulomb blockade regime, and a double dot in the Pauli blockade regime. Constructing such magnetometers using carbon nanotube quantum dots would benefit from the large, strongly anisotropic and controllable $g$ tensors, the low abundance of nuclear spins, and the small detection volume allowing for nanoscale spatial resolution; we estimate that a sensitivity below $1\mu \text{T}/\sqrt{\text{Hz}}$ can be achieved with this material. As quantum dots have already proven to be useful as scanning-probe electrometers, our proposal highlights their potential as hybrid sensors having in situ switching capability between electrical and magnetic sensing.

We developed a method to predict the height corrugations of moiré structures resulting from varying azimuthal orientations of graphene on hexagonal boron nitride substrates. Our model accounts for the flexural rigidity of graphene and assumes a sinusoidal corrugation of the preferred height of C atoms above the substrate. Using four parameters derived from density functional theory (DFT), the model computes corrugations of incommensurate moiré structures currently inaccessible to DFT with an estimated accuracy on the order of 0.01 Å. We predict that azimuthally aligned graphene domains are energetically favored over rotated ones.

We present a wave-packet dynamical approach to charge transport using maximally localized Wannier functions based on density functional theory including van der Waals interactions. We apply it to the transport properties of pentacene and rubrene single crystals and show the temperature-dependent natures from bandlike to thermally activated behaviors as a function of the magnitude of external static disorder. We compare the results with those obtained by the conventional band and hopping models and experiments.

The prediction of Weyl fermions in the low-temperature noncentrosymmetric $1T^{\prime }$ phase of ${\mathrm{MoTe}}_2$ still awaits clear experimental confirmation. Here, we report angle-resolved photoemission (ARPES) data and ab initio calculations that reveal a surface state arc dispersing between the valence and the conduction band, as expected for a Weyl semimetal. However, we find that the arc survives in the high-temperature centrosymmetric $1T^{\prime \prime }$ phase. Therefore, a surface Fermi arc is not an unambiguous fingerprint of a topologically nontrivial phase. We have also investigated the surface state spin texture of the $1T^{\prime }$ phase by spin-resolved ARPES, and identified additional topologically trivial spin-split states within the projected band gap at higher binding energies.

Surface polaritons, e.g., surface plasmon polaritons, are invaluable tools for deep subwavelength nanophotonics, but have inevitable substantial energy dissipation and decoherence. Here, we search for stable lossless surface polaritons at the interfaces between a loss and a gain medium beyond the compensated surface plasmon polariton picture and the parity-time symmetry limit. We unveil a general picture of “relationship circles” as the conditions for lossless surface polaritons. Tunable light trapping, spin/orbital-angular-momentum winding, energy/momentum fluxes, and intrinsic spin/orbit Hall effects are revealed for the lossless surface polaritons.

We report on nonlinear magnetotransport in a two-dimensional electron gas hosted in a MgZnO/ZnO heterostructure. Upon application of a direct current, we observe pronounced Hall field–induced resistance oscillations (HIRO), which are well known from experiments on high-mobility GaAs/AlGaAs quantum wells. The unique sensitivity of HIRO to the short-range component of the disorder potential allows us to unambiguously establish that the mobility of our MgZnO/ZnO heterostructure is limited by impurities residing within or near the two-dimensional (2D) channel. This demonstration that HIRO can be realized in a system with a much lower mobility, much higher density, and much larger effective mass than in previously studied systems highlights the remarkable universality of the phenomenon and its great promise for use in studies of a wide variety of emerging 2D materials.

We introduce a time evolution algorithm for one-dimensional quantum field theories with periodic boundary conditions. This is done by applying the Dirac-Frenkel time-dependent variational principle to the set of translational invariant continuous matrix product states with periodic boundary conditions. Moreover, the ansatz is accompanied with additional boundary degrees of freedom to study quantum impurity problems. The algorithm allows for a cutoff in the spectrum of the transfer matrix and thus has an efficient computational scaling. In particular we study the prototypical example of an atomtronic system—an interacting Bose gas rotating in a ring shaped trap in the presence of a localized barrier potential.

A system composed of a conducting planar strip with Rashba spin-orbit coupling (RSOC), magnetically coupled to a layer of localized magnetic moments, at equilibrium, is studied within a microscopic Hamiltonian with numerical techniques at zero temperature in the clean limit. In particular, transport properties for the cases of ferromagnetic (FM) and antiferromagnetic (AFM) coupled layers are computed in linear response on strips of varying width. Some behaviors observed for these properties are consistent with the ones observed for the corresponding Rashba helical currents. The case of uncoupled Rashba strips is also studied for comparison. In the case of Rashba strips coupled to an AFM localized order, results for the longitudinal dc conductivity, for small strip widths, suggest the proximity to a metal-insulator transition. More interesting, in the proximity of this transition, and in general at intermediate values of the RSOC, a large spin Hall conductivity is observed that is two orders of magnitude larger than the one for the FM order for the same values of the RSOC and strip widths. There are clearly two different regimes for small and for large RSOC, which is also present in the behavior of Rashba helical currents. Different contributions to the optical and the spin Hall conductivities, according to a new classification of inter- or intraband origin proposed for planar strips in the clean limit, or coming from the hopping or spin-orbit terms of the Hamiltonian, are examined. Finally, the effects of different orientation of the coupled magnetic moments will be also studied.

The low-frequency dynamical response of an Anderson insulator is dominated by so-called Mott resonances: hybridization of pairs of states close in energy but separated spatially. We study the effect of interaction on Mott resonances in the model of spinful fermions (electrons) with local attraction. This model is known to exhibit a so-called pseudogap: a suppression of the low-energy, single-particle excitations. Correspondingly, the low-energy dynamical response is also reduced. However, this reduction has mostly quantitative character. In particular, the Mott formula for frequency-dependent conductivity preserves its functional asymptotic behavior at low frequencies, but with a small numerical prefactor. This result can be explained in terms of Mott resonances for electron pairs instead of single electrons.

We compute the entanglement entropy of the Wilson-Fisher conformal field theory (CFT) in 2+1 dimensions with $\mathrm{O}(N)$ symmetry in the limit of large $N$ for general entanglement geometries. We show that the leading large-$N$ result can be obtained from the entanglement entropy of $N$ Gaussian scalar fields with their mass determined by the geometry. For a few geometries, the universal part of the entanglement entropy of the Wilson-Fisher CFT equals that of a CFT of $N$ massless scalar fields. However, in most cases, these CFTs have a distinct universal entanglement entropy even at $N=\infty$. Notably, for a semi-infinite cylindrical region it scales as $N^0$ in the Wilson-Fisher theory, in stark contrast to the $N$-linear result of the Gaussian fixed point.

Nonlocal effects, requiring wave-vector- $(q$-) dependent dielectric response functions, are becoming increasingly important in studies of plasmonic and metamaterial structures. The phenomenological hydrodynamic approximation is the simplest and most often used model but with limited applicability to problems involving surface plasmons. We show here that the $d$-function formalism, exact to first order in $q$, is a powerful and simple-to-use alternative, which allows for exact nonlocal extensions of local calculation schemes, e.g., finite-difference time-domain methods, without code changes. It is also extendable to order $q^2$, and we demonstrate this by comparing with various earlier ab initio calculations and experiments as well as by performing our own random-phase-approximation calculations (valid for all $q)$ of the surface-plasmon dispersions for simple metals with various electron-gas densities. Finally we show that this hydrodynamic-extended $d$-function formalism can also be applied to arbitrary plasmonic/metamaterial structures as long as the nonflat interfaces can be modeled as effective media films.

We theoretically study Coulomb drag between two helical edges with broken spin-rotational symmetry, such as would occur in two capacitively coupled quantum spin Hall insulators. For the helical edges, Coulomb drag is particularly interesting because it specifically probes the inelastic interactions that break the conductance quantization for a single edge. Using the kinetic equation formalism, supplemented by bosonization, we find that the drag resistivity ${\rho }_D$ exhibits a nonmonotonic dependence on the temperature $T$. In the limit of low $T,{\rho }_D$ vanishes with decreasing $T$ as a power law if intraedge interactions are not too strong. This is in stark contrast to Coulomb drag in conventional quantum wires, where ${\rho }_D$ diverges at $T\to 0$ irrespective of the strength of repulsive interactions. Another unusual property of Coulomb drag between the helical edges concerns higher $T$ for which, unlike in the Luttinger liquid model, drag is mediated by plasmons. The special type of plasmon-mediated drag can be viewed as a distinguishing feature of the helical liquid—because it requires peculiar umklapp scattering only available in the presence of a Dirac point in the electron spectrum.

Using the nonequilibrium Keldysh Green's function formalism, we investigate the effect of defects on the electronic structure and transport properties of two-dimensional topological insulators (TI). We demonstrate how the spatial flow of charge changes between the topologically protected edge and bulk states and show that elastically and inelastically scattering defects that preserve the time-reversal symmetry of the TI lead to qualitatively different effects on the TI's local electronic structure and its transport properties. Moreover, we show that the recently predicted ability to create highly spin-polarized currents by breaking the time-reversal symmetry of the TI via magnetic defects [J. S. Van Dyke and D. K. Morr, Phys. Rev. B 93, 081401 (2016)] is robust against the inclusion of a Rashba spin-orbit interaction and the effects of dephasing, and remains unaffected by changes over a wide range of the TI's parameters. We discuss how the sign of the induced spin currents changes under symmetry operations, such as reversal of bias and gate voltages, or spatial reflections. Finally, we show that the insight into the interplay between topology and symmetry of the magnetic defects can be employed for the creation of intriguing quantum phenomena, such as highly localized magnetic fields inside the TI.

We present a theory of surface plasma waves (SPWs) in metals with arbitrary electronic collision rate ${\tau }^{-1}$. We show that there exists a universal intrinsic amplification channel for these waves, subsequent to the interplay between ballistic motions and the metal surface. We evaluate the corresponding amplification rate ${\gamma }_0$, which turns out to be a sizable fraction of the SPW frequency ${\omega }_s$. We also find that the value of ${\omega }_s$ depends on surface scattering properties, in contrast with the conventional theory.