Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: The Witten effect predicts that a magnetic monopole gains a fractional electric charge inside topological insulators. In this work, we give a microscopic description for this phenomenon, as well as an analogous two-dimensional system with a vortex. %solving a ``negatively" massive Dirac equation. We solve a regularized Dirac equation both analytically in continuum and numerically on a lattice, adding the Wilson term to make the sign of the fermion mass well-defined and smearing the singular gauge field in a finite range of radius $r_1$ to make the analysis UV finite. We find that the Wilson term locally gives a positive mass shift with a size of $1/r_1$, which dynamically creates a finite-sized domain-wall around the monopole/vortex. We can identify the chiral edge-localized zero modes sitting on the created domain-wall as the origin of the electric charge. The fact that the charge origin is not a point-like singularity but a small codimension-one domain-wall makes the topological meaning of the zero modes clearer: they are protected by the Atiyah-Singer index theorem on the wall, which is essential to show that only a half of the wave function is captured by the monopole/vortex.

Abstract: Parity-time ($\mathcal{PT}$) symmetry is a cornerstone of non-Hermitian physics as it ensures real energies for stable experimental realization of non-Hermitian phenomena. In this work, we propose $\mathcal{PT}$ symmetry as a paradigm for designing new families of higher-dimensional non-Hermitian states with unique bulk, surface, hinge or corner dynamics. Through systematically breaking or restoring $\mathcal{PT}$ symmetry in different sectors of a system, we can selectively ``activate'' or manipulate the non-Hermitian skin effect (NHSE) in both the bulk and topological boundary states. Some fascinating new phenomena include the directional toggling of the NHSE, an intrinsic hybrid skin-topological effect and the flow of boundary states without chiral or dynamical pumping. Our results extend richly into 3D or higher, with more sophisticated interplay with hybrid skin-topological localizations and $\mathcal{CP}$ symmetry. Based on non-interacting lattices, $\mathcal{PT}$-activated NHSE phenomena can be observed in various optical, photonic, electric and quantum platforms that admit gain/loss and non-reciprocity.

Abstract: Chiral phonons have an angular momentum which represents the microscopic local rotation of atoms in crystals. In this theoretical investigation, we establish a spin-wave model in a ferromagnet with exchange and Dzyaloshinskii-Moriya interactions on a two-dimensional kagome lattice. We then introduce chiral phonons, which modulate spin-spin interactions. In the valley-phonon modes with angular momenta, the microscopic rotational motion of atoms around their equilibrium positions are treated as an adiabatic process and it can dynamically affect the spin configuration of electrons. By means of the adiabatic modulation for the spin-wave model, the number of the magnon excitations due to chiral phonons can be calculated. As a result, the change of the number of magnons induced by chiral phonons is caused by geometrical effects. The chiral phonons with clockwise and counterclockwise modes induce a change of the number of magnons with opposite signs.

Abstract: The effective tight-binding model with compensated ferrimagnetic inverse-Heusler lattice Ti$_{2}$MnAl, candidate material of magnetic Weyl semimetal, is proposed. The energy spectrum near the Fermi level, the configurations of the Weyl points, and the anomalous Hall conductivity are calculated. We found that the orbital magnetization is finite, while the total spin magnetization vanishes, at the energy of the Weyl points. The magnetic moments at each site are correlated with the orbital magnetization, and can be controlled by the external magnetic field.

Abstract: We study experimentally and theoretically the hybridization among intralayer and interlayer moir\'e excitons in a MoSe$_2$/WS$_2$ heterostructure with antiparallel alignment. Using a dual-gate device and cryogenic white light reflectance and narrow-band laser modulation spectroscopy, we subject the moir\'e excitons in the MoSe$_2$/WS$_2$ heterostack to a perpendicular electric field, monitor the field-induced dispersion and hybridization of intralayer and interlayer moir\'e exciton states, and induce a cross-over from type I to type II band alignment. Moreover, we employ perpendicular magnetic fields to map out the dependence of the corresponding exciton Land\'e $g$-factors on the electric field. Finally, we develop an effective theoretical model combining resonant and non-resonant contributions to moir\'e potentials to explain the observed phenomenology, and highlight the relevance of interlayer coupling for structures with close energetic band alignment as in MoSe$_2$/WS$_2$.

Abstract: The electronic properties of graphene can be modified by the local interaction with a selected metal substrate. To probe this effect, Scanning Tunneling Microscopy is widely employed, particularly by means of local measurement via lock-in amplifier of the differential conductance and of the field emission resonance. In this article we propose an alternative, reliable method of probing the graphene/substrate interaction that is readily available to any STM apparatus. By testing the tunneling current as function of the tip/sample distance on nanostructured graphene on Ni(100), we demonstrate that I(z) spectroscopy can be quantitatively compared with Density Functional Theory calculations and can be used to assess the nature of the interaction between graphene and substrate. This method can expand the capabilities of standard STM systems to study graphene/substrate complexes, complementing standard topographic probing with spectroscopic information.

Abstract: A rotating organic cation and a dynamically disordered soft inorganic cage are the hallmark features of hybrid organic-inorganic lead-halide perovskites. Understanding the interplay between these two subsystems is a challenging problem but it is this coupling that is widely conjectured to be responsible for the unique behaviour of photo-carriers in these materials. In this work, we use the fact that the polarizability of the organic cation strongly depends on the ambient electrostatic environment to put the molecule forward as a sensitive probe of local crystal fields inside the lattice cell. We measure the average polarizability of the C/N--H bond stretching mode by means of infrared spectroscopy, which allows us to deduce the character of the motion of the cation molecule, find the magnitude of the local crystal field and place an estimate on the strength of the hydrogen bond between the hydrogen and halide atoms. Our results pave the way for understanding electric fields in lead-halide perovskites using infrared bond spectroscopy.

Abstract: We demonstrate the existence and study in detail the features of chiral bimerons which are static solutions in an easy-plane magnet with the Dzyaloshinskii-Moriya (DM) interaction. These are skyrmionic textures with an integer topological charge and they present essential analogies to the meron configurations introduced in the context of quark confinement in the O(3) nonlinear sigma-model. We employ a Moebius transformation to show that, for weak chirality, bimeron configurations approach Belavin-Polyakov (BP) solutions characterized by tightly bound vortex and antivortex parts of the same size. Stronger chirality induces different vortex and antivortex sizes and also a detachment of merons, suggesting the possibility for a topological phase transition. Exploiting the fact that bimerons of opposite topological charges may exist in the same material, we demonstrate numerically a mechanism to generate meron pairs.

Abstract: Shubnikov-de Haas (SdH) oscillations have served as a paradigmatic experimental probe and tool for extracting key semiconductor parameters such as carrier density, effective mass, Zeeman splitting with g-factor $g^*$, quantum scattering times and spin-orbit (SO) coupling parameters. Here, we derive for the first time an analytical formulation for the SdH oscillations in 2D electron gases (2DEGs) with simultaneous Rashba, Dresselhaus, and Zeeman interactions. Our analytical and numerical calculations allow us to extract both Rashba and Dresselhaus SO coupling parameters, carrier density, quantum lifetimes, and also to understand the role of higher harmonics in the SdH oscillations. More importantly, we derive a simple condition for the vanishing of SO induced SdH beatings for all harmonics in 2DEGs: $\alpha/\beta= [(1-\tilde \Delta)/(1+\tilde \Delta)]^{1/2}$, where $\tilde \Delta$ is a material parameter given by the ratio of the Zeeman and Landau level splitting. We also predict beatings in the higher harmonics of the SdH oscillations and elucidate the inequivalence of the SdH response of Rashba-dominated ($\alpha>\beta$) vs Dresselhaus-dominated ($\alpha<\beta$) 2DEGs in semiconductors with substantial $g^*$. We find excellent agreement with recent available experimental data of Dettwiler ${\it et\thinspace al.}$ Phys. Rev. X $\textbf{7}$, 031010 (2017), and Beukman ${\it et\thinspace al.}$, Phys. Rev. B $\textbf{96}$, 241401 (2017).