Condensed Matter

We report on a process for the fiber-coupling of electrically driven cavity-enhanced quantum dot light emitting devices. The developed technique allows for the direct and permanent coupling of p-i-n-doped quantum dot micropillar cavities to single-mode optical fibers. The coupling process, fully carried out at room temperature, involves a spatial scanning technique, where the fiber facet is positioned relative to a device with a diameter of 2 μ m using the fiber-coupled electroluminescence of the cavity emission as feedback parameter. Subsequent gluing and UV curing enables a rigid and permanent coupling between micropillar and fiber core. Comparing our experimental results with finite element method simulations indicate fiber coupling efficiencies of ~46%. The technique presented in this work is an important step in the quest for efficient and practical quantum light sources for applications in quantum information.

Current-induced spin-orbit torques (SOTs) are of interest for fast and energy-efficient manipulation of magnetic order in spintronic devices. To be deterministic, however, switching of perpendicularly magnetized materials by SOT requires a mechanism for in-plane symmetry breaking. Existing methods to do so involve the application of an in-plane bias magnetic field, or incorporation of in-plane structural asymmetry in the device, both of which can be difficult to implement in practical applications. Here, we reported bias-field-free SOT switching in a single perpendicular CoTb layer with an engineered vertical composition gradient. The vertical structural inversion asymmetry induces strong intrinsic SOTs and a gradient-driven Dzyaloshinskii-Moriya interaction (g-DMI), which breaks the in-plane symmetry during the switching process. Micromagnetic simulations are in agreement with experimental results, and elucidate the role of g-DMI in the deterministic switching. This bias-field-free switching scheme for perpendicular ferrimagnets with g-DMI provides a strategy for efficient and compact SOT device design.

Recently, in topological insulators (TIs) the phenomenon of planar Hall effect (PHE) wherein a current driven in presence an in-plane magnetic field generates a transverse voltage has been experimentally witnessed. There have been a couple of theoretical explanations of this phenomenon. We investigate this phenomenon based on scattering theory on a normal metal-TI-normal metal hybrid structure and calculate the conductances in longitudinal and transverse directions to the applied bias. The transverse conductance depends on the spatial location between the two NM-TI junctions where it is calculated. It is zero in the drain electrode when the chemical potentials of the top and the bottom TI surfaces ( μ t and μ b respectively) are equal. The longitudinal conductance is ? -periodic in ? -the angle between the bias direction and the direction of the in-plane magnetic field. The transverse conductance is ? -periodic in ? when μ t = μ b whereas it is 2? -periodic in ? when μ t ??μ b . As a function of the magnetic field, the magnitude of transverse conductance increases initially and peaks. At higher magnetic fields, it decays for angles ? closer to 0,? whereas oscillates for angles ? close to ?/2 . The conductances oscillate with the length of the TI region. A finite width of the system makes the transport separate into finitely many channels. The features of the conductances are similar to those in the limit of infinitely wide system except when the width is so small that only one channel participates in the transport. When only one channel participates in transport, the transverse conductance in the region 0<x<L is zero for μ t = μ b and the transverse conductance in the region x>L is zero even for the case μ t ??μ b . We understand the features in the obtained results.

We investigate the transport properties of magnetic Josephson junctions. In order to capture realistic material band structure effects, we develop a numerical method combining density functional theory and Bogoliubov-de Gennes model. We demonstrate the capabilities of this method by studying Nb/Ni/Nb junctions in the clean limit. The supercurrent through the junctions is calculated as a function of the ferromagnetic Ni thickness, magnetization, and crystal orientation. We identify two generic mechanisms for the supercurrent decay with ferromagnet thickness: (i) large exchange splitting may gap out minority or majority carriers leading to the suppression of Andreev reflection in the junction, (ii) loss of synchronization between different modes due to the significant dispersion of the quasiparticle velocity with the transverse momentum. Our results are in good agreement with recent experimental studies of Nb/Ni/Nb junctions. The present approach opens a path for material composition optimization in magnetic Josephson junctions and superconducting magnetic spin valves.

Confined nanoscale spaces, electric fields and tunneling currents make the molecular electronic junction an experimental device for the discovery of new, out-of-equilibrium chemical reactions. Reaction-rate theory for current-activated chemical reactions is developed by combining a Keldysh nonequilibrium Green's functions treatment of electrons, Fokker-Planck description of the reaction coordinate, and Kramers' first-passage time calculations. The NEGF provide an adiabatic potential as well as a diffusion coefficient and temperature with local dependence on the reaction coordinate. Van Kampen's Fokker-Planck equation, which describes a Brownian particle moving in an external potential in an inhomogeneous medium with a position-dependent friction and diffusion coefficient, is used to obtain an analytic expression for the first-passage time. The theory is applied to several transport scenarios: a molecular junction with a single, reaction coordinate dependent molecular orbital, and a model diatomic molecular junction. We demonstrate the natural emergence of Landauer's blowtorch effect as a result of the interplay between the configuration dependent viscosity and diffusion coefficients. The resultant localized heating in conjunction with the bond-deformation due to current-induced forces are shown to be the determining factors when considering chemical reaction rates; each of which result from highly tunable parameters within the system.

Flat energy bands of model lattice Hamiltonians provide a key ingredient in designing dispersionless wave excitations and have become a versatile platform to study various aspects of interacting many-body systems. Their essential merit lies in hosting compactly localized eigenstates which originate from destructive interference induced by the lattice geometry, in turn often based on symmetry principles. We here show that flat bands can be generated from a hidden symmetry of the lattice unit cell, revealed as a a permutation symmetry upon reduction of the cell over two sites governed by an effective dimer Hamiltonian. This so-called latent symmetry is intimately connected to a symmetry between possible walks of a particle along the cell sites, starting and ending on each of the effective dimer sites. The summed amplitudes of any eigenstate with odd parity on the effective dimer sites vanish on special site subsets called walk multiplets. We exploit this to construct flat bands by using a latently symmetric unit cell coupled into a lattice via walk multiplet interconnections. We demonstrate that the resulting flat bands are tunable by different parametrizations of the lattice Hamiltonian matrix elements which preserve the latent symmetry. The developed framework may offer fruitful perspectives to analyze and design flat band structures.

'Magic'-angle twisted bilayer graphene has received a lot of interest due to its flat bands with potentially non-trivial topology that lead to intricate correlated phases. A spectrum with flat bands, however, does not require a twist between multiple sheets of van der Waals materials, but rather can be realized with the application of an appropriate periodic potential. Here, we propose the imposition of a tailored periodic potential onto a single graphene layer through local perturbations that could be created via lithography or adatom manipulation, which also results in an energy spectrum featuring flat bands. Our first-principle calculations for an appropriate decoration of graphene with adatoms indeed show the presence of flat bands in the spectrum. Furthermore, we reveal the topological nature of the flat bands through a symmetry-indicator analysis. This non-trivial topology manifests itself in corner-localized states with a filling anomaly as we show using a tight-binding model. Our proposal of a single decorated graphene sheet provides a new versatile route to study correlated phases in topologically non-trivial, flat band structures.

Dispersionless bands -- \emph{flatbands} -- provide an excellent testbed for novel physical phases due to the fine-tuned character of flatband tight-binding Hamiltonians. The accompanying macroscopic degeneracy makes any perturbation relevant, no matter how small. For short-range hoppings flatbands support compact localized states, which allowed to develop systematic flatband generators in d=1 dimension in Phys. Rev. B {\bf 95} 115135 (2017) and Phys. Rev. B {\bf 99} 125129 (2019). Here we extend this generator approach to d=2 dimensions. The \emph{shape} of a compact localized state turns into an important additional flatband classifier. This allows us to obtain analytical solutions for classes of d=2 flatband networks and to re-classify and re-obtain known ones, such as the checkerboard, kagome, Lieb and Tasaki lattices. Our generator can be straightforwardly generalized to three lattice dimensions as well.

We investigate the electronic properties in the Bloch electron on a square lattice with vacancies in the uniform magnetic field. We show that a single vacancy site introduced to the system creates a defect energy level in every single innumerable fractal energy gap in the Hofstadter butterfly. The wavefunctions of different defect levels have all different localization lengths depending on their fractal generations, and they can be described by a single universal function after an appropriate fractal scaling. We also show that each defect state has its own characteristic orbital magnetic moment, which is exactly correlated to the gradient of the energy level in the Hofstadter diagram. Probing the spatial nature of the defect-localized states provides a powerful way to elucidate the fractal nature of the Hofstadter butterfly.

We study fractional boundary charges (FBCs) for two classes of strongly interacting systems. First, we study strongly interacting nanowires subjected to a periodic potential with a period that is a rational fraction of the Fermi wavelength. For sufficiently strong interactions, the periodic potential leads to the opening of a charge density wave gap at the Fermi level. The FBC then depends linearly on the phase offset of the potential with a quantized slope determined by the period. Furthermore, different possible values for the FBC at a fixed phase offset label different degenerate ground states of the system that cannot be connected adiabatically. Next, we turn to the fractional quantum Hall effect (FQHE) at odd filling factors ν=1/(2l+1) , where l is an integer. For a Corbino disk threaded by an external flux, we find that the FBC depends linearly on the flux with a quantized slope that is determined by the filling factor. Again, the FBC has 2l+1 different branches that cannot be connected adiabatically, reflecting the (2l+1) -fold degeneracy of the ground state. These results allow for several promising and strikingly simple ways to probe strongly interacting phases via boundary charge measurements.