Surface optical and bulk acoustic phonons in the topological insulator, Bi2Se2Te

Abstract

We explore the phonon dynamics in thin films of the topological insulator material Bi 2 Se 2 Te using ultrafast pump-probe spectroscopy. The time resolved differential reflectivity in these films exhibit fast and slow oscillations. We have given a careful analysis of variation of phonon frequency as a function of film thickness, which we attribute to the existence of standing acoustic modes. However, no variation in the frequency of the optical phonon modes was found with film thickness. This indicates that the optical phonons intrinsically belong to the surface of topological insulators. The controllability of acoustic phonons by way of varying the film thickness will have tremendous implications in the application of these materials in low power spintronic device operating at room temperature. Keywords: topological insulators, optical phonons, acoustic phononsThe newly discovered 3D topological insulators (TIs) represent a class of exotic quantum state of matter with an insulating bulk and conducting surface states 1,2 . These conducting surface states exhibit a relativistic energy-momentum dispersion owing to the nontrivial Z 2 topology of the bulk electronic wave-function 3 . As a consequence, these surface states form a Dirac cone between bulk valence band and bulk conduction band which has been confirmed by angle resolved photo-emission spectroscopy (ARPES) studies 4,5 . Additionally, the surface states are "topologically" protected from back-scattering. Helical spin texture due to high spin-orbit interaction, a π Berry phase and the presence of odd number of Dirac cones are responsibe for this topological protection. Furthermore, the constraint of time reversal symmetry ensures that the surface electrons are not scattered from non-magnetic impurities and defect states. Apart from these, TI materials also exhibit interesting magneto-optical properties which arise due to the presence of axionic term 6 . These exciting properties have made TIs a subject of intense research both theoretically as well as experimentally over the past few years. TIs also promise important practical application in quantum computation and low power spintronic device applications 7 .

Nanoscale momentum-resolved vibrational spectroscopy

“…The ability to trade off spatial and momentum resolution within the same instrument is a unique feature of this experimental approach. Furthermore, combining the present approach with other more established STEM-EELS techniques ( 23 ) provides for a highly comprehensive atomic-scale structural and chemical characterization of material systems such as topological insulators ( 4 , 5 ), thermoelectric materials ( 6 – 8 ), 2D materials ( 1 , 2 ), and van der Waals heterostructure devices ( 3 ), where the vibrational response depends on nano- to atomic-scale structure and chemistry. As a particularly relevant example, in thermoelectric materials, the interplay between acoustic and anharmonic, so called rattler, vibrational modes is thought to play an essential role in controlling thermal conductivity ( 6 ).…”

“…Acoustic and optical phonons affect fundamental physical properties such as the conduction of sound and heat. For two-dimensional (2D) materials (1,2), van der Waals heterostructure devices (3), topological insulators (4,5), and thermoelectric materials (6)(7)(8), as well as many other materials systems relevant to fields spanning catalysis, biomedical, condensed matter physics, and chemistry, vibrational properties can depend on nano-and atomic-scale structure. However, until now, techniques available to probe the momentum transfer (ℏq) dependence or dispersion of these vibrational modes have been limited to essentially bulk approaches, such as inelastic neutron and x-ray scattering spectroscopies (9,10), reflection electron energy loss spectroscopy (REELS) measuring the average surface response (11,12), or optical techniques limited to q ~0 (where q is the wave vector) (13)(14)(15)(16)(17).…”

Linear magnetoresistance and surface to bulk coupling in topological insulator thin films

“…Unintentional defect formation during sample growth results in undesirable bulk conduction which in turn masks the non-trivial contribution of the surface states. It is essential for a TI device to have a minimal bulk contrib ution to the conduction and in this regard Bi 2 Se 2 Te (BST) is a worthy candidate [5][6][7]. The resistance versus temperature (R-T) profile of these samples yields a bulk insulating response (see figure 1(a)) thus enunciating the fact that the chemical potential lies in the bulk band gap.…”

All-optical observation of giant spin transparency at the topological insulator BiSbTe1.5Se1.5/Co20Fe60B20 interface

“…1 , E g 2 and A 1g 2 modes 48 . These results effectively agreed with previous reports of Raman measurements in PLD-grown BSTS having the same stoichiometry as our films 34 .…”