Probing the Topological Surface State in Bi₂Se₃Thin Films Using Temperature-Dependent Terahertz Spectroscopy

Abstract

Strong spin-momentum coupling in topological insulators give rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics such as the spin hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23quintuple-thick film of bismuth selenide (Bi2Se3) allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (< 30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics, obtain an optical mobility, exceeding 2000 cm 2 /V•s at 4 K, indicative of a surfacedominated response. A scattering lifetime of ~0.18 ps and a carrier density of 6×10 12 cm -2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ~1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state. Device fabrication and electrical transport:Optical-lithography was used to define a microscale Hall bar of dimensions 1400 µm × 80 µm. This was followed by the deposition of 15 nm Ti and 90 nm Au and a standard lift-off process to obtain ohmic contacts to the mesa. The device was then packaged and measured in a He-3 cryostat with a base temperature of 300 mK. Hall measurements were obtained using an a.c lock-in four-terminal setup with an input current of 1 µA at a frequency of 133 Hz. ASSOCIATED CONTENTSupporting information: XRD and XRR measurements to characterize the quality, phase and thickness of the Bi2Se3 film; field dependence of the longitudinal and Hall resistance measurement at corresponding temperatures; THz time domain response of the (0001)-oriented sapphire substrate at different temperatures.

Systematic Study of Ferromagnetism in CrxSb2−xTe3 Topological Insulator Thin Films using Electrical and Optical Techniques

“…An undoped 20-nm-thick Sb 2 Te 3 sample was also measured which showed WAL [see Figure S3.2(a) of the Supplementary Information]. The WAL phenomenon demonstrates both the Dirac nature of the surface state carriers as well as the strong spin−orbit interaction in pristine TI materials 27 . On doping the Sb 2 Te 3 film with Cr, the WAL response disappears and instead, an increase in the resistance at low fields corresponding to the MR effect is observed [Figure S3.2(b) of the Supplementary Information].…”

“…The carrier damping rate in TIs (typically~10 −13 s −1 ) lies in the THz frequency range (0.1–2.2 THz), making broadband THz spectroscopy a particularly sensitive probe of TI carrier dynamics. Moreover, the energies typical of collective quasiparticles such as optical phonons, have a fingerprint across the energetic range of the THz radiation (0.4–4 meV) making it a powerful technique to study TIs 27 . The time-resolved THz transmission through all Cr x Sb 2−x Te 3 thin films was measured at 4 K as shown in Fig.…”

“…5(e) ], also see Supplementary Figure S5.2 for the time domain measurement. A detailed description of the method used to obtain the THz conductance is discussed elsewhere 27 . The THz conductance spectra in Fig.…”

“…The temperature dependent measurements were carried out using a Janis continuous flow cryostat with optical access capable of reaching a base temperature of 4 K. In the ultrathin-film limit, THz conductance spectra were obtained from the normalized transmission using Tinkham’s theory 43 . A detailed account of the optical setup and measurement is described elsewhere 27 .…”

Electron–phonon coupling in topological insulator Bi2Se3 thin films with different substrates

“…Thus, it is essential to explore the nonequilibrium properties of TIs under light excitation, such as phonon, carrier, as well as spin dynamics of both the SS and the bulk. The relaxation mechanism of photoexcited hot carriers has been intensively studied by various methods, including the optical pump optical probe (OPOP) [12][13][14][15][16][17][18][19] , optical pump terahertz probe (OPTP) [20][21][22][23] , time-resolved angle-resolved photoemission spectroscopy (Tr-ARPES) [24][25][26][27][28] , Raman spectroscopy [29,30] , second harmonic generation (SHG) [31][32][33] , as well as the time-resolved Kerr rotation (TRKR) [31,34] .…”