Yongqing Li

Tunable Surface Conductivity in Bi2Se3 Revealed in Diffusive Electron Transport

We demonstrate that the weak antilocalization effect can serve as a convenient method for detecting decoupled surface transport in topological insulator thin films. In the regime where a bulk Fermi surface coexists with the surface states, the low-field magnetoconductivity is well described by the Hikami-Larkin-Nagaoka equation for single-component transport of noninteracting electrons. When the electron density is lowered, the magnetotransport behavior deviates from the single-component description and strong evidence is found for independent conducting channels at or near the bottom and top surfaces. The magnetic-field-dependent part of corrections to conductivity due to Zeeman energy is shown to be negligible for the fields relevant to the weak antilocalization despite considerable electron-electron interaction effects on the temperature dependence of the conductivity.

Highly tunable electron transport in epitaxial topological insulator (Bi1-xSbx)2Te3 thin films

Atomically smooth, single crystalline (Bi1−xSbx)2Te3 films have been grown on SrTiO3(111) substrates by molecular beam epitaxy. A full range of Sb-Bi compositions have been studied in order to obtain the lowest possible bulk conductivity. For the samples with optimized Sb compositions (x=0.5±0.1), the carrier type can be tuned from n-type to p-type across the whole thickness with the help of a back-gate. Linear magnetoresistance has been observed at gate voltages close to the maximum in the longitudinal resistance of a (Bi0.5Sb0.5)2Te3 sample. These highly tunable (Bi1−xSbx)2Te3 thin films provide an excellent platform to explore the intrinsic transport properties of the three-dimensional topological insulators.

Observation of Anderson localization in ultrathin films of three-dimensional topological insulators.

Anderson localization, the absence of diffusive transport in disordered systems, has been manifested as hopping transport in numerous electronic systems, whereas in recently discovered topological insulators it has not been directly observed. Here, we report experimental demonstration of a crossover from diffusive transport in the weak antilocalization regime to variable range hopping transport in the Anderson localization regime with ultrathin (Bi_{1-x}Sb_{x})_{2}Te_{3} films. As disorder becomes stronger, negative magnetoconductivity due to the weak antilocalization is gradually suppressed, and eventually, positive magnetoconductivity emerges when the electron system becomes strongly localized. This work reveals the critical role of disorder in the quantum transport properties of ultrathin topological insulator films, in which theories have predicted rich physics related to topological phase transitions.

A Synaptic Transistor based on Quasi‐2D Molybdenum Oxide

Biological synapses store and process information simultaneously by tuning the connection between two neighboring neurons. Such functionality inspires the task of hardware implementation of neuromorphic computing systems. Ionic/electronic hybrid three-terminal memristive devices, in which the channel conductance can be modulated according to the history of applied voltage and current, provide a more promising way of emulating synapses by a substantial reduction in complexity and energy consumption. 2D van der Waals materials with single or few layers of crystal unit cells have been a widespread innovation in three-terminal electronic devices. However, less attention has been paid to 2D transition-metal oxides, which have good stability and technique compatibility. Here, nanoscale three-terminal memristive transistors based on quasi-2D α-phase molybdenum oxide (α-MoO3 ) to emulate biological synapses are presented. The essential synaptic behaviors, such as excitatory postsynaptic current, depression and potentiation of synaptic weight, and paired-pulse facilitation, as well as the transition of short-term plasticity to long-term potentiation, are demonstrated in the three-terminal devices. These results provide an insight into the potential application of 2D transition-metal oxides for synaptic devices with high scaling ability, low energy consumption, and high processing efficiency.

Local photocurrent generation in thin films of the topological insulator Bi2Se3

We report on the optoelectronic properties of thin films of Bi2Se3 grown by molecular beam epitaxy. The films are patterned into circuits with typical extensions of tens of microns. In spatially resolved experiments, we observe submicron photocurrent patterns with positive and negative amplitudes. The patterns are independent of the applied bias voltage, but they depend on the width of the circuits. We interpret the patterns to originate from a local photocurrent generation due to potential fluctuations.

Thickness Dependence of the Quantum Anomalous Hall Effect in Magnetic Topological Insulator Films

The evolution of the quantum anomalous Hall effect with the thickness of Cr-doped (Bi,Sb)2 Te3 magnetic topological insulator films is studied, revealing how the effect is caused by the interplay of the surface states, band-bending, and ferromagnetic exchange energy. Homogeneity in ferromagnetism is found to be the key to high-temperature quantum anomalous Hall material.

Proximity effect between a topological insulator and a magnetic insulator with large perpendicular anisotropy

We report that thin films of a prototype topological insulator, Bi

Aharonov-casher effect in Bi2Se3 square-ring interferometers.

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Chemical potential fluctuations in topological insulator (Bi0.5Sb0.5)2Te3-films visualized by photocurrent spectroscopy

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Current-induced nuclear spin depolarization at Landau level filling factor nu=1/2

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