Atomically Smooth Ultrathin Films of Topological Insulator Sb_(2)Te_(3)

The growth and characterization of single-crystalline thin films of topological insulators(TIs)is an important step towards their possible applications.Using in situ scanning tunneling microscopy(STM)and angle-resolved photoemission spectroscopy(ARPES),we show that moderately thick Sb_(2)Te_(3)films grown layer-by-layer by molecular beam epitaxy(MBE)on Si(111)are atomically smooth,single-crystalline,and intrinsically insulating.Furthermore,these films were found to exhibit a robust TI electronic structure with their Fermi energy lying within the energy gap of the bulk that intersects only the Dirac cone of the surface states.Depositing Cs in situ moves the Fermi energy of the Sb_(2)Te_(3)films without changing the electronic band structure,as predicted by theory.We found that the TI behavior is preserved in Sb_(2)Te_(3)films down to five quintuple layers(QLs).

Observation of unconventional anomalous Hall effect in epitaxial CrTe thin films

We have studied the magnetic and electrical transport properties of epitaxial NiAs-type CrTe thin films grown on SrTiO3（111） substrates. Unlike rectangle hysteresis loops obtained from magnetic measurements, we have identified intriguing extra bump/dip features from anomalous Hall experiments on the films with thicknesses less than 12 nm. This observed Hall anomaly is phenomenologically consistent with the occurrence of a topological Hall effect （THE） in chiral magnets with a skyrmion phase. Furthermore, the THE contribution can be tuned by the film thickness, showing the key contribution of asymmetric interfaces in stabilizing N6el-type skyrmions. Our work demonstrates that a CrTe thin film on SrTiO3（111） substrates is a good material candidate for studying real-space topological transport.

In situ Raman spectroscopy of topological insulator Bi2Te3 films with varying thickness

Topological insulators （TIs） are a new state of quantum matter with a band gap in bulk and conducting surface states. In this work, the Raman spectra of topological insulator Bi2Te3 films prepared by molecular beam epitaxy （MBE） have been measured by an in situ ultrahigh vacuum （UHV）-MBE-Raman spectroscopy system. When the thickness of Bi2Te3 films decreases from 40 quintuple-layers （QL） to 1 QL, the spectral characteristics of some Raman modes appearing in bulk Bi2Te3 vary and a new vibrational mode appears, which has not been reported in previous studies and might be related to quantum size effects and symmetry breaking. In addition, an obvious change was observed at 3 QL when a Dirac cone formed. These results offer some new information about the novel quantum states of TIs.

Magnetoresistance Oscillations of Ultrathin Pb Bridges

Pb nanobridges with a thickness of less than 10 nm and a width of several hundred nm have been fabricated from single-crystalline Pb fi lms using low-temperature molecular beam epitaxy and focus ion beam microfabrication techniques.We observed novel magnetoresistance oscillations below the superconducting transition temperature(TC)of the bridges.The oscillations which were not seen in the crystalline Pb fi lmsmay originate from the inhomogeneity of superconductivity induced by the applied magnetic fi elds on approaching the normal state,or the degradation of fi lm quality by thermal evolution.

Ambi-chiral anomalous Hall effect in magnetically doped topological insulators

The chirality associated with broken time-reversal symmetry in magnetically doped topological insulators has important implications for the quantum transport phenomena.Here we report anomalous Hall effect studies in Mn-and Cr-doped Bi_(2)Te_(3) topological insulators with varied thicknesses and doping contents.By tracing the magnitude of the anomalous Hall resistivity,we find that the Mn-type anomalous Hall effect characterized with clockwise chirality is strengthened by the reduction of film thickness,which is opposite to that of the Cr-type anomalous Hall effect with counterclockwise chirality.We provide a phenomenological physical picture to explain the evolution of the magnetic order and the anomalous Hall chirality in magnetically doped topological insulators.

Gate voltage control of helicity-dependent photocurrent and polarization detection in(Bi_(1-x)Sb_(x))_(2)Te_(3)topological insulator thin films

Optical helicity provides us with an effective means to control the helicity-dependent photocurrent in the spinmomentum-locked surface states of topological insulators(TIs).Also,the TIs show potential in polarization detection as an intrinsic solid-state optical chirality detector for easier integration and fabrication.However,the complex photoresponses with the circular photogalvanic effect,the linear photogalvanic effect,and the photon drag effect in the TIs prevent them from direct chirality detection of the elliptically polarized light.Here,by fitting with the theoretical models to the measured photocurrents,the microscopic origin of different components of the helicity-dependent photocurrent has been demonstrated.We show a comprehensive study of the helicitydependent photocurrent in(Bi_(1-x)Sb_(x))_(2)Te_(3)thin films of different thicknesses as a function of the light incident angle and the gate-tuned chemical potential.The observation of the light incident angle dependence of the helicitydependent photocurrent provides us with a polarization detection strategy using a TI thin film without the use of any additional optical elements,and the detection accuracy can be enhanced by gate tuning.Additionally,the Stokes parameters can be extracted by arithmetic operation of photocurrents measured with different incident angles and gating voltages for complete characterization of the polarization states of a light beam.Using this means,we realize the polarization detection and the Stokes parameters analysis with a single device.Our work provides an alternative solution to develop miniaturized intrinsic polarization-sensitive photodetectors.