Electrical transport and optical studies of spin-orbit coupled topological phases in different correlation regimes

Abstract

The interplay of spin-orbit coupling (SOC) and electron-electron correlation in different regimes gave birth to many novel topological phases with exotic properties ranging from quantum transport to superconductivity. One of those phases, topological insulators(TI), attract attention widely due to its promising potential in building next-generation quantum computers. Strong TI (for example, Bi2Se3, BiSbTeSe, etc.), the most popular subclass of TI, has been investigated extensively. Recently, another subclass of TI known as a dual topological insulator(DTI) is being realized as new material. One of the examples of DTI is Bi1Te1, recently discovered as a topological system to host a weak TI state on the sides and topological crystalline insulating (TCI) state on the remaining surfaces. On the contrary, quantum spin liquid (QSL), another new topological phase, emerges due to strong electron-electron correlation. Honeycomb material RuCl3 has been studied as a quantum spin liquid candidate combining the Kitaev model and Jackeli-Khaliullin theory. In the 1st work, we synthesized the single crystals of pure Bi1Te1 and Sb-doped Bi1Te1 (Bi0.88Sb0.05Te1) via the modified Bridgmann method. After characterizing the single crystal by XRD, Raman spectroscopy, XPS, and EPMA, we investigated the electrical transport properties of the devices fabricated out of nanoflakes exfoliated from the as-grown crystals in the presence of out-of-plane and in-plane magnetic fields. We observed weak anti-localization (WAL), an important feature in low-field magneto-conductance to quantify the coherently conducting surface states. We analyzed WAL using Hikami-Larkin-Nagaoka equation. The phase coherence length (Lϕ) vs. T indicates the dephasing mechanism via 2D electron-phonon interaction in both cases: pure Bi1Te1 and Sb-doped Bi1Te1. With increasing thickness of the nanodevice, the dominance of the electron-phonon interaction increases. Hall effect further confirms the negative sign of the majority charge carriers and yields carrier density and mobility for both the crystals. The change in the parameters from the Hall effect is not noticeable much. In-plane field transport gives more information about the intermixing of the surface states with the bulk states owing to defects in Sb-doped system. In the second work, we mainly investigated the optical properties of Kitaev spin liquid candidate RuCl3 via steady-state and time-resolved photoluminescence (PL) and Raman spectroscopy. We have grown RuCl3 via the physical vapor transport technique. We characterized as-grown crystals via XRD, EPMA, and magnetic measurement, confirming the antiferromagnetic transitions at 8 K and 15 K. We evaluated the bandgap to be 1.9 eV, indicating the Mott insulating properties of RuCl3. We explained the PL spectra obtained from the bulk RuCl3 and nanoflake in terms of the Tanabe-Suagno diagram, useful to describe the optical transition in a strongly correlated system with an octahedral crystal structure. Electrical transport on RuCl3 was also carried out in bulk and the thin-film limit. R ~ T curve indicates 2D variable range hopping transport and has a feature at 170 K owing to structural transition. In the third work, we investigated the electrical transport properties of strong 3D TI Bi1Sb1Te1.5Se1.5 coupled to RuCl3. Since RuCl3 is a Mott insulator, we carried out the electrical measurement on the BSTS flake only. We fabricated the BSTS/RuCl3 heterostructure via the hot, dry transfer method in Argon-filled Glove Box followed by e-beam lithography. We extracted information from the perpendicular and in-plane field transport measurement indicating the presence of topological surface states (TSS) and Rashba surface states (RSS). We observed for thicker RuCl3 ( ≥ 100 nm), RSS is well separated from TSS underneath the RuCl3 layer. This effect is clearly visible in the values of the change in the slope in logarithmic temperature-dependence of the sheet conductance plot. This phenomenon is explained in terms of charge transfer from RuCl3 to the BSTS surface layer, causing stronger band bending.