Search results for "topological insulator"

A topological current divider

2020

We study the transport properties of a hybrid junction made of a ferromagnetic lead in electrical connection with the helical edge modes of a two-dimensional topological insulator. In this system, the time reversal symmetry, which characterizes the ballistic edge modes of the topological insulator, is explicitly broken inside the ferromagnetic region. This conflict situation generates unusual transport phenomena at the interface which are the manifestation of the interplay between the spin polarization of the injected current and the spin-momentum locking mechanism operating inside the topological insulator. We show that the spin polarized current originated in the ferromagnetic region is a…

Observation of a charge delocalization from Se vacancies inBi2Se3: A positron annihilation study of native defects

2016

By means of positron annihilation lifetime spectroscopy, we have investigated the native defects present in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, which belongs to the family of topological insulators. We experimentally demonstrate that selenium vacancy defects $({\text{V}}_{\text{Se1}})$ are present in ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ as-grown samples, and that their charge is delocalized as temperature increases. At least from 100 K up to room temperature both ${\text{V}}_{\text{Se1}}^{0}$ and ${\text{V}}_{\text{Se1}}^{+}$ charge states coexist. The observed charge delocalization determines the contribution of ${\text{V}}_{\text{Se1}}$ defects to the $n$-type conductivity of ${\mathrm{…

Transport in topological insulators with bulk-surface coupling: Interference corrections and conductance fluctuations

2018

Motivated by the experimental difficulty to produce topological insulators (TIs) of the ${\text{Bi}}_{2}{\text{Se}}_{3}$ family with pure surface-state conduction, we study the effect that the bulk can have on the low-temperature transport properties of gated thin TI films. In particular, we focus on interference corrections, namely weak localization (WL) or weak antilocalization (WAL), and conductance fluctuations (CFs) based on an effective low-energy Hamiltonian. Utilizing diagrammatic perturbation theory, we first analyze the bulk and the surface separately and subsequently discuss WL/WAL and CFs when a tunneling-coupling is introduced. We identify the relevant soft diffusion modes of t…

Understanding the Giant Enhancement of Exchange Interaction in Bi2Se3−EuS Heterostructures

2017

A recent experiment indicated that a ferromagnetic EuS film in contact with a topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ might show a largely enhanced Curie temperature and perpendicular magnetic anisotropy [F. Katmis et al., Nature (London) 533, 513 (2016).]. Through systematic density functional calculations, we demonstrate that in addition to the factor that ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ has a strong spin orbit coupling, the topological surface states are crucial to make these unusual behaviors robust as they hybridize with EuS states and extend rather far into the magnetic layers. The magnetic moments of Eu atoms are nevertheless not much enhanced, unlike what was…

2013

The spin texture of a Dirac-type surface state in W(110) lends itself to study spin-dependent effects in electron spectroscopies that show up very clearly. Firstly, we report on spin-resolved photoemission calculations and separate the spin polarization that is attributed to the initial state from that induced by the photoemission process itself. This disentanglement allows one to map spin textures of spin-polarized initial states using circular dichroism, for example from Dirac surface states in topological insulators. Secondly, we demonstrate the mapping of spin-polarized states by spin-dependent two-electron emission. Selecting highly polarized initial states, this spectroscopy can furth…

Electrically Controlled Pumping of Spin Currents in Topological Insulators

2011

Pure spin currents are shown to be generated by an electrically controlled quantum pump applied at the edges of a topological insulator. The electric rather than the more conventional magnetic control offers several advantages and avoids, in particular, the necessity of delicate control of magnetization dynamics over tiny regions. The pump is implemented by pinching the sample at two quantum point contacts and phase modulating two external gate voltages between them. The spin current is generated for the full range of parameters. On the other hand, pumping via amplitude modulation of the inter-boundary couplings generates both charge and spin currents, with a pure charge current appearing o…

Spin texture of time-reversal symmetry invariant surface states on W(110)

2016

AbstractWe find in the case of W(110) previously overlooked anomalous surface states having their spin locked at right angle to their momentum using spin-resolved momentum microscopy. In addition to the well known Dirac-like surface state with Rashba spin texture near the "Equation missing"-point, we observe a tilted Dirac cone with circularly shaped cross section and a Dirac crossing at 0.28 × "Equation missing" "Equation missing" within the projected bulk band gap of tungsten. This state has eye-catching similarities to the spin-locked surface state of a topological insulator. The experiments are fortified by a one-step photoemission calculation in its density-matrix formulation.

Bulk-free topological insulator Bi 2 Se 3 nanoribbons with magnetotransport signatures of Dirac surface states

2018

Many applications for topological insulators (TIs) as well as new phenomena require devices with reduced dimensions. While much progress has been made to realize thin films of TIs with low bulk carrier density, nanostructures have not yet been reported with similar properties, despite the fact that size confinement should help reduce contributions from bulk carriers. Here we demonstrate that Bi2Se3 nanoribbons, grown by a simple catalyst-free physical-vapour deposition, have inherently low bulk carrier densities, and can be further made bulk-free by size confinement, thus revealing the high mobility topological surface states. Magneto transport and Hall conductance measurements, in single n…

Renormalization group flows for Wilson-Hubbard matter and the topological Hamiltonian

2019

Understanding the robustness of topological phases of matter in the presence of interactions poses a difficult challenge in modern condensed matter, showing interesting connections to high energy physics. In this work, we leverage these connections to present a complete analysis of the continuum long-wavelength description of a generic class of correlated topological insulators: Wilson-Hubbard topological matter. We show that a Wilsonian renormalization group (RG) approach, combined with the so-called topological Hamiltonian, provide a quantitative route to understand interaction-induced topological phase transitions that occur in Wilson-Hubbard matter. We benchmark two-loop RG predictions …

Topological Signatures in the Electronic Structure of Graphene Spirals

2013

Topology is familiar mostly from mathematics, but also natural sciences have found its concepts useful. Those concepts have been used to explain several natural phenomena in biology and physics, and they are particularly relevant for the electronic structure description of topological insulators and graphene systems. Here, we introduce topologically distinct graphene forms - graphene spirals - and employ density-functional theory to investigate their geometric and electronic properties. We found that the spiral topology gives rise to an intrinsic Rashba spin-orbit splitting. Through a Hamiltonian constrained by space curvature, graphene spirals have topologically protected states due to tim…