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Register a new userGate-controlled Spin Extraction from Topological Insulator Surfaces
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Spin-momentum locking, a key property of the surface states of three-dimensional topological insulators (3DTIs), provides a new avenue for spintronics applications. One consequence of spin-momentum locking is the induction of surface spin accumulations due to applied electric fields. In this work, we investigate the extraction of such electrically-induced spins from their host TI material into adjoining conventional, hence topologically trivial, materials that are commonly used in electronics devices. We focus on effective Hamiltonians for bismuth-based 3DTI materials in the ${rm Bi}_2{rm Se}_3$ family, and numerically explore the geometries for extracting current-induced spins from a TI surface. In particular, we consider a device geometry in which a side pocket is attached to various faces of a 3DTI quantum wire and show that it is possible to create current-induced spin accumulations in these topologically trivial side pockets. We further study how such spin extraction depends on geometry and material parameters, and find that electron-hole degrees of freedom can be utilized to control the polarization of the extracted spins by an applied gate voltage.
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The thermoelectric properties of the surface states in three-dimensional topological insulator nanowires are studied. The Seebeck coefficients $S_c$ and the dimensionless thermoelectrical figure of merit $ZT$ are obtained by using the tight-binding Hamiltonian combining with the nonequilibrium Greens function method. They are strongly dependent on the gate voltage and the longitudinal and perpendicular magnetic fields. By changing the gate voltage or magnetic fields, the values of $S_c$ and $ZT$ can be easily controlled. At the zero magnetic fields and zero gate voltage, or at the large perpendicular magnetic field and nonzero gate voltage, $ZT$ has the large value. Owing to the electron-hole symmetry, $S_c$ is an odd function of the Fermi energy while $ZT$ is an even function regardless of the magnetic fields. $S_c$ and $ZT$ show peaks when the quantized transmission coefficient jumps from one plateau to another. The highest peak appears while the Fermi energy is near the Dirac point. At the zero perpendicular magnetic field and zero gate voltage, the height of $n$th peak of $S_C$ is $frac{k_B}{e}texttt{ln}2/(|n|+1/2)$ and $frac{k_B}{e}texttt{ln}2/|n|$ for the longitudinal magnetic flux $phi_{parallel} = 0 $ and $pi$, respectively. Finally, we also study the effect of disorder and find that $S_c$ and $ZT$ are robust against disorder. In particular, the large value of $ZT$ can survive even if at the strong disorder. These characteristics (that $ZT$ has the large value, is easily regulated, and is robust against the disorder) are very beneficial for the application of the thermoelectricity.
We report an electric field-induced in-plane magnetoresistance of an atomically flat paramagnetic insulator|platinum (Pt) interface at low temperatures with an ionic liquid gate. Transport experiments as a function of applied magnetic field strength and direction obey the spin Hall magnetoresistance phenomenology with perpendicular magnetic anisotropy. Our results establish the utility of ionic gating as an alternative method to control spintronic devices without using ferromagnets.
The layered semimetal WTe_2 has recently been found to be a two-dimensional topological insulator (2D TI) when thinned down to a single monolayer, with conducting helical edge channels. We report here that intrinsic superconductivity can be induced in this monolayer 2D TI by mild electrostatic doping, at temperatures below 1 K. The 2D TI-superconductor transition can be easily driven by applying a just a small gate voltage. This discovery offers new possibilities for gate-controlled devices combining superconductivity and topology, and could provide a basis for quantum information schemes based on topological protection.
The discovery of photonic topological insulators (PTIs) has opened the door to fundamentally new topological states of light.Current time-reversal-invariant PTIs emulate either the quantum spin Hall (QSH) effect or the quantum valley Hall (QVH) effect in condensed-matter systems, in order to achieve topological transport of photons whose propagation is predetermined by either photonic pseudospin (abbreviated as spin) or valley. Here we demonstrate a new class of PTIs, whose topological phase is not determined solely by spin or valley, but is controlled by the competition between their induced gauge fields. Such a competition is enabled by tuning the strengths of spin-orbit coupling (SOC) and inversion-symmetry breaking in a single PTI. An unprecedented topological transition between QSH and QVH phases that is hard to achieve in condensed-matter systems is demonstrated. Our study merges the emerging fields of spintronics and valleytronics in the same photonic platform, and offers novel PTIs with reconfigurable topological phases.
Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature, and exciton condensates can be realized at much higher temperature than condensates of atoms because of strong Coulomb binding and small mass. Signatures of exciton condensation have been reported in double quantum wells1-4, microcavities5, graphene6, and transition metal dichalcogenides7. Nonetheless, transport of exciton condensates is not yet understood and it is unclear whether an exciton condensate is a superfluid8,9 or an insulating electronic crystal10,11. Topological insulators (TIs) with massless particles and unique spin textures12 have been theoretically predicted13 as a promising platform for achieving exciton condensation. Here we report experimental evidence of excitonic superfluid phase on the surface of three-dimensional (3D) TIs. We unambiguously confirmed that electrons and holes are paired into charge neutral bound states by the electric field independent photocurrent distributions. And we observed a millimetre-long transport distance of these excitons up to 40 K, which strongly suggests dissipationless propagation. The robust macroscopic quantum states achieved with simple device architecture and broadband photoexcitation at relatively high temperature are expected to find novel applications in quantum computations and spintronics.
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