Vincent Sacksteder

Topological effects on the magnetoconductivity in topological insulators

Three-dimensional strong topological insulators (TIs) guarantee the existence of a two-dimensional (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 (2-DEGs) which are planar. This has several consequences for the magnetoconductivity �σ , a frequently studied measure of weak antilocalization which is sensitive to the quantum coherence time τφ and to temperature. We show that conduction on the TI sidewalls systematically reduces �σ , 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 �σ 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 1/T 2 in 2-DEGs, 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. DOI: 10.1103/PhysRevB.90.235148

Phase structure of the topological Anderson insulator

We study the disordered topological Anderson insulator in a two-dimensional (square not strip) geometry. We first report the phase diagram of finite systems and then study the evolution of phase boundaries when the system size is increased to a very large 1120 x 1120 area. We establish that conductance quantization can occur without a bulk band gap, and that there are two distinct scaling regions with quantized conductance: TAI-I with a bulk band gap, and TAI-II with localized bulk states. We show that there is no intervening insulating phase between the bulk conduction phase and the TAI-I and TAI-II scaling regions, and that there is no metallic phase at the transition between the quantized and insulating phases. Centered near the quantized-insulating transition there are very broad peaks in the eigenstate size and fractal dimension d(2); in a large portion of the conductance plateau eigenstates grow when the disorder strength is increased. The fractal dimension at the peak maximum is d(2) approximate to 1.5. Effective-medium theory (Coherent Potential Approximation, self-consistent Born approximation) predicts well the boundaries and interior of the gapped TAI-I scaling region, but fails to predict all boundaries save one of the ungapped TAI-II scaling region. We report conductance distributions near several phase transitions and compare them with critical conductance distributions for well-known models.

Spin response to localized pumps: Exciton polaritons versus electrons and holes

Polariton polarization can be described in terms of a pseudospin which can be oriented along the

Bulk effects on topological conduction in three-dimensional topological insulators

x,\,y,

Hole spin helix: Anomalous spin diffusion in anisotropic strained hole quantum wells

or

Spin conduction in anisotropic three-dimensional topological insulators

z

Robust topological insulator conduction under strong boundary disorder

axis, similarly to electron and hole spin. Unlike electrons and holes where time-reversal symmetry requires that the spin-orbit interaction be odd in the momentum, the analogue of the spin-orbit interaction for polaritons, the so-called TE-TM splitting, is even in the momentum. We calculate and compare spin transport of polariton, electron, and hole systems, in the diffusive regime of many scatterings. After dimensional rescaling diffusive systems with spatially uniform particle densities have identical dynamics, regardless of the particle type. Differences between the three particles appear in spatially non-uniform systems, with pumps at a specific localized point. We consider both oscillating pumps and transient (delta-function) pumps. In such systems each particle type produces distinctive spin patterns. The particles can be distinguished by their differing spatial multipole character, their response and resonances in a perpendicular magnetic field, and their relative magnitude which is largest for electrons and weakest for holes. These patterns are manifested both in response to unpolarized pumps which produce in-plane and perpendicular spin signals, and to polarized pumps where the spin precesses from in-plane to out-of-plane and vice versa. These results will be useful for designing systems with large spin polarization signals, for identifying the dominant spin-orbit interaction and measuring subdominant terms in experimental devices, and for measuring the scattering time and the spin-orbit couplings magnitude.

Many-body renormalisation of forces in f-materials

affected by bulk physics in the interior of the topological insulator sample. We show that both signatures of the topological metal are robustagainst bulk effects. However the bulk does substantially accelerate the metal’s decay in a magneticfield and alter its response to surface disorder. When the disorder strength is tuned to resonance with the bulk band the conductivity follows the predictions of scaling theory, indicating that conduction is diffusive. At other disorder strengths the bulk reduces the effects of surface disorder and scaling theory is systematically violated, signaling that conduction is not fully diffusive. These effects will change the magnitude of the surface conductivity and the magnetoconductivity.

Many-body renormalization of forces in f-electron materials.

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.

New DMFT capabilities in CASTEP

We present the in-plane optical reflectance measurement on single crystals of URu2Si2. The study revealed a strong temperature-dependent spectral evolution. Above 50 K, the low frequency optical conductivity is rather flat without a clear Drude-like response, indicating a very short transport lifetime of the free carriers. Well below thecoherence temperature, there appears an abrupt spectral weight suppression below 400 cm(-1), yielding evidence for the formation of a hybridization energy gap arising from the mixing of the conduction electron and narrow f-electron bands. A small part of the suppressed spectral weight was transferred to the low frequency side, leading to a narrow Drude component, while the majority of the suppressed spectral weight was transferred to the high frequency side centered near 3500 cm(-1). Below the hidden order temperature, another very prominent energy gap structure was observed, which leads to the removal of a large part of the Drude component and a sharp reduction of the carrier scattering rate. The study revealed that the hybridization gap and the hidden order gap are distinctly different: they occur at different energy scales and exhibit completely different spectral characteristics.