Session J7: Focus Session: Spin Transport & Optical Phenomena in Topological Insulators

Spin-Polarized Tunneling Study on Spin-Momentum Locking in the Topological Insulator Bismuth Selenide

In this talk, we will demonstrate that the helical spin texture on topological insulator (TI) surfaces can be electrically detected using four-terminal tunnel junction devices with ferromagnetic top electrodes. Consistent results are obtained in both the Edelstein and spin-galvanic effect configurations, allowing a quantitative determination of the charge-spin conversion efficiency in bismuth selenide. By applying finite DC biases at the junction, we further extract the energy dependence of the effective spin polarization in bismuth selenide. The observed temperature stability up to 200K suggests that TIs can be highly promising for room-temperature spintronics applications   

Tunable Intrinsic Spin Hall Conductivities in Bi$_2$(Se,Te)$_3$ Topological Insulators

It has been recently shown by spin-transfer torque measurements that Bi$_2$Se$_3$ exhibits a very large spin Hall conductivity (SHC)[1]. It is expected that Bi$_2$Te$_3$, a topological insulator with similar crystal and band structures as well as large spin-orbit coupling, would also exhibit a giant SHC. In this study we have calculated intrinsic spin Hall conductivities of Bi$_2$Se$_3$ and Bi$_2$Te$_3$ topological insulators from a tight-binding Hamiltonian including two nearest-neighbor interactions. We have calculated the Berry curvature, used the Kubo formula in the static, clean limit and shown that both materials exhibit giant spin Hall conductivities, consistent with the results of Ref. 1 and larger than previously reported Bi$_{1-x}$Sb$_x$ alloys[2]. The density of Berry curvature has also been computed from the full Brillouin zone in order to compute the dependence of the SHC in these materials on the Fermi energy. Finally we report the intrinsic SHC for Bi$_2$(Se,Te)$_3$ topological insulators, which changes dramatically with doping or gate voltage. [1] A.R. Mellnik et al., Nature 511, 449 - 451 (2014) [2] C.\c{S}ahin and M.E.Flatt\'e, arXiv:1410.7319   

Inverse spin galvanic effect in topological-insulator based heterostructures

We study the inverse spin galvanic effect in heterostructures formed by a layer of a three dimensional strong topological insulator (TI) and a magnetic material. We consider different configurations for the heterostructure and for the contacts. We carefully treat the effect on the TI bands of the proximity of a magnetic material and take into account both intra-band and inter-band contributions to the current-induced spin polarization of the TI surface states. Finally, we discuss the relevance of our results for recent experiments.   

Low energy electrodynamics of topological insulator thin films

Topological insulators are recently discovered states of matter with robust metallic surface states protected by the topological properties of their bulk wavefunctions. A quantum phase transition from a topological insulator to a conventional insulator and a change in topological class can occur only when the bulk bandgap closes. In this work, we have used time-domain terahertz spectroscopy to investigate the low-frequency conductance in (Bi$_{1-x}$ In$_x$)$_2$Se$_3$ as we tune through this transition by In substitution. Above certain substitution levels we observe a collapse in the transport lifetime that indicates the destruction of the topological phase. We associate this effect with the threshold where states from opposite surfaces hybridize. The substitution level of the threshold is thickness dependent and only asymptotically approaches the bulk limit $x \sim 0.06$ where a maximum in the mid-infrared absorption is exhibited. This absorption can be identified with the bulk bandgap closing and change in topological class. The correlation ength associated with the quantum phase transition appears as the evanescent length of the surface states. The observation of the thickness- dependent collapse of the transport lifetime shows the unusual role that finite-size effects play in this topological quantum phase transition. In even more recent work on bulk insulating films, we have identified a cyclotron resonance feature. At high magnetic fields we identify a anomalous increase of the scattering rate. I will discuss the reasons for this increase and put it in the context of existing theories for charge transport in topological insulators.   

Evidence for surface-generated photocurrent in (Bi,Sb)$_{2}$Se$_{3}$ and (Bi,Sb)$_{2}$Te$_{3}$ thin films

Illumination with circularly polarized light is known produce a helicity-dependent photocurrent in topological insulators such as Bi$_{2}$Se$_{3}$ [e.g. Nature Nanotech. \textbf{7}, 96 (2012)]. However, the exact origin of this effect is still unclear since it is observed with photons well above the bulk band gap. We report measurements of the polarization-dependent photocurrent in a series of (Bi,Sb)$_{2}$Se$_{3}$ thin films with different carrier concentrations and find that the photocurrent is enhanced as we increase the population of the surface states. This finding is supported by a study of helicity-dependent photocurrents in back-gated (Bi,Sb)$_{2}$Te$_{3}$ thin films, where the chemical potential is varied electrostatically. By illuminating our samples at different wavelengths, we show that the helicity-dependent photocurrent is enhanced when the photon energy approaches the energy difference between the lowest and first excited (unoccupied) topological surface states. This leads us to attribute the helicity-dependent photocurrent in topological insulators to optical excitations between these two spin-textured surface states. We will also discuss experiments imaging the spatial variation of these helicity-dependent photocurrents.   

Quantum Spin Hall Effect in Two-Dimensional Transition Metal Dichalcogenides

Quantum spin Hall (QSH) effect materials have an insulating bulk but conducting edge states that are topologically protected from backscattering by time reversal symmetry. However, the small band gap and the lack of efficient on/off switching in materials that have been identified as QSH insulators limit their applications. Here using first-principles calculations we predict a class of large-gap QSH insulators in two-dimensional transition metal dichalcogenides with 1T' structure, namely, 1T'-MX$_2$ with M=(W, Mo) and X=(Te, Se, S)\footnote{Xiaofeng Qian, Junwei Liu, Liang Fu, and Ju Li, arXiv:1406.2749 (2014)}. A structural distortion causes an intrinsic band inversion between chalcogenide-$p$ and metal-$d$ bands. Additionally, spin-orbit coupling opens a gap that is tunable by vertical electric field and strain. We propose a topological field effect transistor made of van der Waals heterostructures of 1T'-MX$_2$ and 2D dielectric layers that can be rapidly switched off by electric field through topological phase transition instead of carrier depletion.   

Observing the Zeeman effect of topological surface state

Dirac fermions in the topological surface state (TSS) have helical spin textures. This is different from those in graphene, which are both valley and spin degenerated. The spin degeneracy can be lifted by Zeeman effect, which is manifested as a spin-splitting of Landau levels (LLs). In the case of TSS, LLs instead should exhibit monotonic shift with magnetic field since the spin degeneracy is lacking. While the Zeeman splitting of LLs in graphene has been successfully observed, the expected features in TSS still lack experimental proof. With scanning tunneling microscopy and spectroscopy, we observed the Zeeman shifting of zeroth LL in the TSS of Bi2Se3 and Sb2Te2Se unambiguously. Moreover, we exclude the extrinsic influence on LL shifting from potential variations and the nonideal band dispersions of TSS in actual materials by modeling. The surface g factor in Sb2Te2Se and Bi2Se3 is estimated as -10 and 16, respectively. This observation indicates that the g factor of TSS is significantly material dependent, which may be related to the atomic orbital character of the compound.   

Optical transitions to second Dirac point in a ferromagnetic topological insulator

It is well known that the bismuth telluride family of 3D topological insulators has a surface state in the bulk gap and that doping with magnetic impurities opens a gap in its Dirac-like dispersion. Recent two-photon ARPES measurements [Sobota et al., PRL, 2013] have revealed the existence of a second Dirac band about 1.8eV above the energy of the surface state node. To probe transitions from the first to second Dirac bands we studied the spectral response of thin films of the magnetic topological insulator Cr:(Bi,Sb)$_2$Te$_3$ using magneto-optic Kerr (MOKE) spectroscopy at visible and near-infrared frequencies. We observe a strong resonant enhancement of the Kerr response near 1.8eV. The enhancement enables measurement of Kerr rotation in ultrathin films, allowing us to explore the predicted [Lu et al., PRB, 2010] transitions between topologically trivial and non-trivial states as a function film thickness.   

Observation of Current induced Optical Kerr Rotation in Topological Insulators

Topological insulators (TIs) are an unusual phase of quantum matter with an insulating bulk gap and gapless spin-momentum locked Dirac surface states (SS), showing exotic topological quantum properties. However, optical identification of the spin-momentum locked SS is still challenging. Here, we report room-temperature, current induced magneto-optical Kerr rotation effect observed from various TI bulk crystals grown by the Bridgman method. The Kerr rotation was measured while a square wave bias current was applied across the samples. We find that the Kerr angle increases linearly with the applied bias current and reverses its sign as the polarity of the current is reversed. Such an observation is consistent with the spin-momentum locking of the surface states. The largest Kerr rotation we measured is 4x10-6 radians/(A/cm2)) from Bi2Te2Se1.   

Electrical Probing of Inherent Spin Polarization in a Topological Insulator with Electrical Gating

The hallmark of a time-reversal symmetry protected three-dimensional topological insulator is the helically spin-textured surface state. Although electrical detection of spin polarization in topological insulators has been demonstrated very recently, there have not been any electrical measurements to demonstrate the entire mapping of the spin polarization throughout the surface state. We report the electrical probing of the spin-polarized surface state using a magnetic tunnel junction as a spin detector while the chemical potential of a topological insulator (Bi,Sb)$_{2}$Te$_{3}$ is tuned by back gating. Hysteretic spin signals were observed as the magnetization of the detector ferromagnet (permalloy) switches with in-plane magnetic field. Changing the direction of bias current through the topological insulator channel flips the direction of the spin polarization, resulting in the reverse of sign of the detected spin signals. We demonstrate the control of the Fermi energy, which has importance not only in further understanding of the spin-momentum locking in the surface state but also in possible electrical tuning of the spin polarization for potential spin-based devices.   

Micro-metric electronic patterning of a topological band structure using a photon beam

The only states crossing E$_F$ in ideal, 3D TIs are topological surface states. Single crystals of Bi$_2$Se$_3$ and Bi$_2$Te$_3$ are too defective to exhibit bulk-insulating behaviour, and ARPES shows topologically trivial 2DEGs at E$_F$ in the surface region due to downward band bending. Ternary \& quaternary alloys of Bi$\big /$Te$\big /$Se$\big /$Sb hold promise for obtaining bulk-insulating crystals. Here we report ARPES data from quaternary, bulk-insulating, Bi-based TIs. Shortly after cleavage in UHV, downward band bending pulls the bulk conduction band below E$_F$, once again frustrating the ``topological only'' ambition for the Fermi surface. However, there is light at the end of the tunnel: we show that a super-band-gap photon beam generates a surface photovoltage sufficient to flatten the bands, thereby recovering the ideal, ``topological only'' situation. In our bulk-insulating quaternary TIs, this effect is local in nature, and permits the writing of arbitrary, micron-sized patterns in the topological energy landscape at the surface.   

Quantum Spin Hall Effect in Ultrasonic Metamaterials

The discovery of topological order without breaking time-reversal symmetry, such as that in Quantum Spin Hall (QSH) effect and Topological Insulators, is one of the most groundbreaking advancements of recent years in condensed matters physics. The approach to topological order without breaking time-reversal symmetry is particularly important in elastics because no natural elastic materials show linear nonreciprocal response. Here we illustrate the first elastic-wave system emulating QSH effect and demonstrate existence of topologically protected elastic edge states. The system represents an elastic metamaterial-based phononic crystal. In this crystal, we achieved degenerate linear dispersion for two sets of modes, classified by one of the system's symmetries. Then, by relaxing and removing that symmetry by deliberately engineering a gauge field emulating a strong spin-orbit coupling of QSH, we observe opening a complete topological bandgap. Finally, the hallmark of the topological order, namely the presence of one-way chiral edge waves insensitive to nonmagnetic defects and disorders, is demonstrated in such elastic metacrystals. We illustrate the unique property of these elastic edge waves to flow around sharp corners without back-reflection or localization.   

Thickness tunable quantum interference between surface phonon and Dirac plasmon states in thin-films of the topological insulator Bi$_{2}$Se$_{3}$

Raman scattering has been applied to study thin films of the topological insulator Bi$_{2}$Se$_{3}$. We observed a more than 100-fold enhancement of Raman responses~if laser photon energy switches from 2.33 eV (532 nm) to 1.58 eV (785 nm), which is due to direct optical coupling to Dirac surface states (SS) at the resonance energy of about 1.5 eV (a thickness-independent enhancement) and due to nonlinearly excited Dirac plasmon (a thickness-dependent enhancement). Owing to the direct optical coupling, we were able to monitor an in-plane phonon mode of hexagonally arranged Se-atoms associated with a continuous network of Dirac SS. This mode revealed a Fano lineshape for films below 15 nm thick, resulting from quantum interference between surface phonon and Dirac plasmon states.