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

Abstract: Valleytronics, which makes use of the two valleys in graphenes, attracts much attention and the valley filter is expected to be central component in valleytronics. We investigate valley-dependent transport properties of the Stone-Wales (SW) and blister defects of graphenes by density functional theory calculations. It is found that the intervalley transition is perfectly suppressed in some structures although the intravalley scattering occurs by the defect states of the SW or blister defects. Using the tight-binding model, the perfect suppression of the intervalley transition in the SW and blister defects is explained by the sublattice symmetry between the A and B sites of the bipartite honeycomb lattice. In addition, introducing the additional carbon atoms to graphenes to form blister defects, the defect states appear near the Fermi level and the energies where the resonant scattering occurs on the $\mathrm{K}$ and $\mathrm{K}^\prime$ channel electrons split. Making use of this splits, the valley-dependent transport property will be achieved by local application of a gate voltage.

Abstract: Lasers with injected spin-polarized carriers show an outstanding performance in both static and dynamic operation. In addition to the intensity response of conventional lasers, without spin-polarized carriers, both intensity and polarization of light can be exploited for optical communication in spin-lasers. However, the polarization dynamics of spin-lasers under amplitude modulation has been largely overlooked. Here we reveal, analytically and numerically, a nontrivial polarization response that accompanies the well-known intensity dynamics of a spin-laser under amplitude modulation. We evaluate the polarization and intensity response under the same amplitude modulation, and further assess the capability of such a polarization response in digital data transfer with eye diagram simulations. Our results provide a more complete understanding of the modulation response in spin-lasers and open up unexplored opportunities in optical communication and spintronics.

Abstract: Applying a unified approach, we study the integer quantum Hall effect (IQHE) and fractional quantum Hall effect (FQHE) in the Hofstadter model with short range interactions between fermions. An effective field, that takes into account the interaction between fermions, is determined by both amplitude and phase. Its amplitude is proportional to the interaction strength, the phase corresponds to the minimum energy. In fact, the problem is reduced to the Harper equation with two different scales: the first is a magnetic scale with the cell size corresponding to a unit quantum magnetic flux, the second scale determines the inhomogeneity of the effective field, forms the steady fine structure of the Hofstadter spectrum and leads to the realization of fractional quantum Hall states. In a sample of finite size with open boundary conditions, the fine structure of the Hofstadter spectrum consists of the Dirac branches of the fermion excitations and includes the fine structure of the edge chiral modes. The Chern numbers of the topological Hofstadter bands are conserved during the formation of their fine structure. The edge modes are formed into the Hofstadter bands. They connect the nearest-neighbor subbands and determine the conductance for the fractional filling.

Abstract: The model of a spherical quantum dot with several donor impurities on its surface is suggested. The electron energy spectra are studied as a function of the quantum dot radius and the number of impurities. Several cases of the location of impurities on the quantum dot surface are considered. The plane wave functions method has been applied to calculate the electron energy spectrum. The splitting of electron energy levels is analyzed in the cases of different number of impurities. It is shown that the electron energy splitting depends on both the number of impurities on the surface and on their location. The electron binding energy is defined too.

Abstract: A quantum theory of spectral parameters and oscillator strengths of quantum transitions in an active region, which contains cascades of wide quantum wells with a complicated potential profile is developed. A new spatial design of the cascade is calculated and proposed with such an asymmetric arrangement of the wells and barriers, in which, without an applied electric bias, the magnitudes of oscillator strengths are considerable and one-way resonant-tunneling transport of electrons is observed. As a result, it becomes possible to ensure a successful functioning of the broadband photodetector in the far IR range.

Abstract: Magnetic skyrmions in thin films with perpendicular magnetic anisotropy are promising candidates for magnetic memory and logic devices, making the development of ways to transport skyrmions efficiently and precisely of significant interest. Here, we investigate the transport of skyrmions by surface acoustic waves (SAWs) via several modalities using micromagnetic simulations. We show skyrmion pinning sites created by standing SAWs at anti-nodes and skyrmion Hall-like motion without pinning driven by travelling SAWs. We also show how orthogonal SAWs formed by combining a longitudinal travelling SAW and a transverse standing SAW can be used for the precise 2D positioning of skyrmions. Our results also suggest SAWs offer a viable approach to the precise transport of multiple skyrmions along multichannel racetrack.

Abstract: We study a nonlinear spin dynamics of a ferromagnetic ring in a vortex state induced by the spin-polarized current. We also suggest to use the ferromagnetic ring as a free layer of a coreless vortex spin-transfer nano-oscillator. The calculated working frequency is about several GHz, that is much higher than the gyromode frequency of the disk-based vortex oscillator. The response of the vortex-state ring to the spin-polarized current has hysteretic behavior with the reasonable values of the thresholds current densities: ignition threshold is about $10^{8} \text{A}\text{cm}^{-2}$, and elimination current to maintain the oscillations has much lower values about $10^{6} \text{A} \text{cm}^{-2}$. The output signal can be extracted by the help of the inverse spin Hall effect or by the giant magnetoresistance. The output electromotive force averaged over all sample vanishes, and we suggest to use a ferromagnetic ring or disk in a vortex state as a GMR analyzer. For an inverse spin Hall analyser we advise to use two heavy metals with different signs of Spin-Hall angle. The ring-based STNO is supposed to increase the areas of practical application of the STNOs.

Abstract: We theoretically study the Dzyaloshinskii-Moriya interaction (DMI) mediated by band electrons with strong spin-orbit coupling (SOC). We first derive a general formula for the coefficient ${\bm D}_i$ of the DMI in free energy in terms of Green's functions, and examine its variations in relation to physical quantities. In general, the DMI coefficient can vary depending on physical quantities, i.e., whether one is looking at equilibrium spin structure (${\bm D}_i$) or spin-wave dispersion (${\bm D}_i^{(2)}$), and the obtained formula helps to elucidate their relations. By explicit evaluations for a magnetic topological insulator and a Rashba ferromagnet with perpendicular magnetization, we observe ${\bm D}_i^{(2)} \ne {\bm D}_i$ in general. In the latter model, or more generally, when the magnetization and the spin-orbit field are mutually orthogonal, ${\bm D}_i$ is exactly related to the equilibrium spin current for arbitrary strength of SOC, generalizing the similar relation for systems with weak SOC. Among various systems with strong SOC, magnetic Weyl semimetals are special in that ${\bm D}_i^{(2)} = {\bm D}_i$, and in fact, the DMI in this system arises as the chiral anomaly.

Abstract: The Fermi sea topology is characterized by the Euler characteristics $\chi_F$. In this Letter, we examine how $\chi_F$ of the metallic state is inhereted by the topological invariant of the superconducting state. We establish a correspondence between the Euler characteristic and the Chern number $C$ of $p$-wave topological superconductors without time-reversal symmetry in two dimensions. By rewriting the pairing potential $\Delta_{\bf k}=\Delta_1-i\Delta_2$ as a vector field ${\bf u}=(\Delta_1,\Delta_2)$, we found that $\chi_F=C$ when ${\bf u}$ and fermion velocity ${\bf v}$ can be smoothly deformed to be parallel or antiparallel on each Fermi surface. We also discuss a similar correspondence between Euler characteristic and 3D winding number of time-reversal-invariant $p$-wave topological superconductors in three dimensions.

Abstract: Majorana zero modes in superconductor-nanowire hybrid structures are a promising candidate for topologically protected qubits with the potential to be used in scalable structures. Currently, disorder in such Majorana wires is a major challenge, as it can destroy the topological phase and thus reduce the yield in the fabrication of Majorana devices. We study machine learning optimization of a gate array in proximity to a grounded Majorana wire, which allows us to reliably compensate even strong disorder. We propose a metric for optimization that is inspired by the topological gap protocol, and which can be implemented based on measurements of the non-local conductance through the wire.

Abstract: We obtain the numerical ground state of a one-dimensional ladder model with the upper and lower chains occupied by spatially-separated electrons and holes, respectively. Under charge neutrality, we find that the excitonic bound states are always formed, i.e., no finite regime of decoupled electron and hole plasma exists at zero temperature. The system either behaves like a bosonic liquid or a bosonic crystal depending on the inter-chain attractive and intra-chain repulsive interaction strengths. We also provide the detailed excitonic phase diagrams in the intra- and inter-chain interaction parameters, with and without disorder.

Abstract: Traditional topological materials belong to different Altland-Zirnbauer symmetry classes (AZSCs) depending on their non-spatial symmetries. Here we introduce the notion of hybrid symmetry class topological insulators (HSCTIs): A fusion of two different AZSC topological insulators (TIs) such that they occupy orthogonal Cartesian hyperplanes and their universal massive Dirac Hamiltonian mutually anticommute. The boundaries of HSCTIs can also harbor TIs, typically affiliated with an AZSC different from the parent ones. As such, a fusion between planar quantum spin Hall and vertical Su-Schrieffer-Heeger insulators gives birth to a three-dimensional HSCTI, accommodating quantum anomalous Hall insulators and quantized Hall conductivity on the top and bottom surfaces. We extend this construction to encompass crystalline HSCTI and topological superconductors, and beyond three dimensions. Possible (meta)material platforms to harness HSCTIs are discussed.

Abstract: We study topological magnons on an anisotropic square-hexagon-octagon (SHO) lattice which has been found by a two-dimensional Biphenylene network (BPN). We propose the concepts of type-II Dirac magnonic states where new schemes to achieve topological magnons are unfolded without requiring the Dzyaloshinsky-Moriya interactions (DMIs). In the ferromagnetic states, the topological distinctions at the type-II Dirac points along with one-dimensional (1D) closed lines of Dirac magnon nodes are characterized by the $\mathbb{Z}_2$ invariant. We find pair annihilation of the Dirac magnons and use the Wilson loop method to depict the topological protection of the band-degeneracy. The Green's function approach is used to calculte chiral edge modes and magnon density of states (DOS). We introduce the DMIs to gap the type-II Dirac magnon points and demonstrate the Dirac nodal loops (DNLs) are robust against the DMIs within a certain parameter range. The topological phase diagram of magnon bands is given via calculating the Berry curvature and Chern number. We find that the anomalous thermal Hall conductivity gives connection to the magnon edge current. Furthermore, we derive the differential gyromagnetic ratio to exhibit the Einstein-de Haas effect (EdH) of magnons with topological features.

Abstract: This work presents a study on the nonrelativistic quantum motion of a charged particle in a rotating frame, considering the Aharonov-Bohm effect and a uniform magnetic field. We derive the equation of motion and the corresponding radial equation to describe the system. The Schr\"odinger equation with minimal coupling incorporates rotation effects by substituting the momentum operator with an effective four-potential. Additionally, a radial potential term, dependent on the average radius of the ring, is introduced. The analysis is restricted to motion in a two-dimensional plane, neglecting the degree of freedom in the $z$-direction. By solving the radial equation, we determine the eigenvalues and eigenfunctions, allowing for an explicit expression of the energy. The probability distribution is analyzed for varying rotating parameter values, revealing a shift of the distribution as the rotation changes, resulting in a centrifugal effect and occupation of the ring's edges. Furthermore, numerical analysis demonstrates the significant rotational effects on energy levels and optical properties, including optical absorption and refractive coefficients.

Abstract: In this letter, we investigate the dissipative dynamics at the edge of Laughlin fractional quantum Hall (FQH) states starting from the hydrodynamic framework of the composite Boson theory recently developed in arXiv:2203.06516. Critical to this description is the choice of boundary conditions, which ultimately stems from the choice of hydrodynamic variables in terms of condensate degrees of freedom. Given the gapped nature of bulk, one would expect dissipation effects to play an important role only near the FQH edge. Thus, one envisions a scenario where the bulk hydro equations remain unmodified, while the dissipation effects are introduced at the edge via boundary conditions. We have recently shown that the anomaly requirements fix the boundary conditions of the FQH fluid to be no-penetration and no-stress boundary conditions. In this work, we introduce energy dissipation in the no-stress boundary condition leading to charge diffusion at the boundary. The resulting dissipative edge dynamics is quite rigid from a hydro perspective, as it has to preserve the edge charge continuity and the anomaly structure. We show that the diffusive edge dynamics with fluctuation-dissipation relations within a power counting scheme belong to the Kardar-Parisi-Zhang universality class.

Abstract: {\bf Hypothesis:} Introducing charged terminal groups to polymers that graft nanoparticles enables Coulombic control over their assembly by tuning pH and salinity of aqueous suspensions. {\bf Experiments:} Gold nanoparticles (AuNPs) are grafted with poly(ethylene glycol) (PEG) terminated with CH3 (charge neutral), COOH (negatively charged), or NH2 (positively charged) groups. The nanoparticles are characterized using dynamic light scattering, {\zeta}-potential, and thermal gravimetric analysis. Liquid surface X-ray reflectivity (XR) and grazing incidence small-angle X-ray scattering (GISAXS) techniques are employed to determine the density profile and in-plane structure of the AuNP assembly across and on the aqueous surface. {\bf Findings:} The assembly of PEG-AuNPs at the liquid/vapor interface can be tuned by adjusting pH or salinity, particularly for COOH terminals. However, the effect is less pronounced for NH2 terminals. These distinct assembly behaviors are attributed to the overall charge of PEG-AuNPs and the conformation of PEG. The COOH-PEG corona is the most compact, resulting in smaller superlattice constants. The net charge per particle depends not only on the PEG terminal groups but also on the cation sequestration of PEG and the intrinsic negative charge of the AuNP surface. NH2-PEG, due to its closeness to overall charge neutrality and the presence of hydrogen bonding, enables the assembly of NH2-PEG-AuNPs more readily.