Renzhong Du

Anisotropic magneto-transport properties of electron gases at SrTiO3 (111) and (110) surfaces

Electron gases at the surfaces of insulating (111)- and (110)-oriented SrTiO3 (STO) single crystals have been created using Ar+-irradiation and their magneto-transport properties are characterized. Fully metallic behaviors with sheet carrier density of ∼1014 cm−2 and low-temperature-mobilities as large as 8600 cm2 V−1 s−1 are obtained. Intrinsic in-plane anisotropic magnetoresistance (AMR) has been obtained by applying current along different crystal axes to subtract the Lorentz force effect. The results yield nearly 6-fold and 2-fold components for the (111)-and (110)-surfaces. A symmetry breaking in AMR for the (111)-surfaces with ordering temperature TO ∼ 30 K is also observed. In contrast, the out-of-plane AMR does not show anisotropy associated with crystal axes, suggesting a two-dimensional nature of the effect. Our results demonstrate that electron gases at (111)- and (110)-oriented STO surfaces are a promising playground for both fundamental research and all-oxide device applications.

Proximity-effect-induced Superconducting Gap in Topological Surface States – A Point Contact Spectroscopy Study of NbSe2/Bi2Se3 Superconductor-Topological Insulator Heterostructures

Proximity-effect-induced superconductivity was studied in epitaxial topological insulator Bi2Se3 thin films grown on superconducting NbSe2 single crystals. A point contact spectroscopy (PCS) method was used at low temperatures down to 40 mK. An induced superconducting gap in Bi2Se3 was observed in the spectra, which decreased with increasing Bi2Se3 layer thickness, consistent with the proximity effect in the bulk states of Bi2Se3 induced by NbSe2. At very low temperatures, an extra point contact feature which may correspond to a second energy gap appeared in the spectrum. For a 16 quintuple layer Bi2Se3 on NbSe2 sample, the bulk state gap value near the top surface is ~159 μeV, while the second gap value is ~120 μeV at 40 mK. The second gap value decreased with increasing Bi2Se3 layer thickness, but the ratio between the second gap and the bulk state gap remained about the same for different Bi2Se3 thicknesses. It is plausible that this is due to superconductivity in Bi2Se3 topological surface states induced through the bulk states. The two induced gaps in the PCS measurement are consistent with the three-dimensional bulk state and the two-dimensional surface state superconducting gaps observed in the angle-resolved photoemission spectroscopy (ARPES) measurement.