10 research outputs found
Hole Spin Helix: Anomalous Spin Diffusion in Anisotropic Strained Hole Quantum Wells
We obtain the spin-orbit interaction and spin-charge coupled transport
equations of a two-dimensional heavy hole gas under the influence of strain and
anisotropy. We show that a simple two-band Hamiltonian can be used to describe
the holes. In addition to the well-known cubic hole spin-orbit interaction,
anisotropy causes a Dresselhaus-like term, and strain causes a Rashba term. We
discover that strain can cause a shifting symmetry of the Fermi surfaces for
spin up and down holes. We predict an enhanced spin lifetime associated with a
spin helix standing wave similar to the Persistent Spin Helix which exists in
the two-dimensional electron gas with equal Rashba and Dresselhaus spin-orbit
interactions. These results may be useful both for spin-based experimental
determination of the Luttinger parameters of the valence band Hamiltonian and
for creating long-lived spin excitations
Topological Effects on the Magnetoconductivity in Topological Insulators
Three-dimensional strong topological insulators (TIs) guarantee the existence
of a 2-D conducting surface state which completely covers the surface of the
TI. The TI surface state necessarily wraps around the TI's top, bottom, and two
sidewalls, and is therefore topologically distinct from ordinary 2-D electron
gases (2DEGs) which are planar. This has several consequences for the
magnetoconductivity , a frequently studied measure of weak
antilocalization which is sensitive to the quantum coherence time
and to temperature. We show that conduction on the TI sidewalls systematically
reduces , multiplying it by a factor which is always less than
one and decreases in thicker samples. In addition, we present both an
analytical formula and numerical results for the tilted-field
magnetoconductivity which has been measured in several experiments. Lastly, we
predict that as the temperature is reduced will enter a wrapped
regime where it is sensitive to diffusion processes which make one or more
circuits around the TI. In this wrapped regime the magnetoconductivity's
dependence on temperature, typically in 2DEGs, disappears. We present
numerical and analytical predictions for the wrapped regime at both small and
large field strengths. The wrapped regime and topological signatures discussed
here should be visible in the same samples and at the same temperatures where
the Altshuler-Aronov-Spivak (AAS) effect has already been observed, when the
measurements are repeated with the magnetic field pointed perpendicularly to
the TI's top face
Phase-coherent loops in selectively-grown topological insulator nanoribbons
We succeeded in the fabrication of topological insulator (Bi0.57Sb0.43)2Te3 Hall bars as well as nanoribbons by means of selective-area growth using molecular beam epitaxy. By performing magnetotransport measurements at low temperatures information on the phase-coherence of the electrons is gained by analyzing the weak-antilocalization effect. Furthermore, from measurements on nanoribbons at different magnetic field tilt angles an angular dependence of the phase-coherence length is extracted, which is attributed to transport anisotropy and geometrical factors. For the nanoribbon structures universal conductance fluctuations were observed. By performing a Fourier transform of the fluctuation pattern a series of distinct phase-coherent closed-loop trajectories are identified. The corresponding enclosed areas can be explained in terms of nanoribbon dimensions and phase-coherence length. In addition, from measurements at different magnetic field tilt angles we can deduce that the area enclosed by the loops are predominately oriented parallel to the quintuple layers