Zhenqi Hao

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.

Mottness Collapse in 1T−TaS2−xSex Transition-Metal Dichalcogenide: An Interplay between Localized and Itinerant Orbitals

Shuang Qiao, 2 Xintong Li, Naizhou Wang, Wei Ruan, Cun Ye, Peng Cai, Zhenqi Hao, Hong Yao, 4 Xianhui Chen, 5 Jian Wu, 4, ∗ Yayu Wang, 4, † and Zheng Liu 4, ‡ State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, P.R. China Institute for Advanced Studies, Tsinghua University, Beijing 100084, P.R. China Hefei National Laboratory for Physical Science at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Innovation Center of Quantum Matter, Beijing 100084, P.R. China Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China (Dated: April 11, 2017)

Quasiparticle interference and charge order in a heavily overdoped non-superconducting cuprate

One of the key issues in unraveling the mystery of high Tc superconductivity in the cuprates is to understand the normal state outside the superconducting dome. Here we perform scanning tunneling microscopy and spectroscopy measurements on a heavily overdoped, non-superconducting (Bi,Pb)2Sr2CuO6+x cuprate. Spectroscopic imaging reveals dispersive quasiparticle interferences and the Fourier transforms uncover the evolution of momentum space topology. More interestingly, we observe nanoscale patches of static charge order with sqrt(2)*sqrt(2) periodicity. Both the dispersive quasiparticle interference and static charge order can be qualitatively explained by theoretical calculations, which reveal the unique electronic structure of strongly overdoped cuprate.

Visualization of the periodic modulation of Cooper pairing in a cuprate superconductor

In cuprate superconductors, the existence of various intertwined orders associated with spin, charge and Cooper pairs1,2 is an obstacle in understanding the mechanism of Cooper pairing. The ubiquitous charge order is particularly important2–7. Various theories have been proposed to explain the origin of the charge order and its implications for the superconducting phase, including charge stripes8, electronic nematicity8,9 and Fermi surface instability5,10. A highly appealing physical picture is the ‘pair density wave’, a spatially periodic modulation of Cooper pairing, which may also induce a charge order2,11–21. To elucidate the existence and nature of the pair density wave, we use scanning tunnelling microscopy to investigate a severely underdoped cuprate, in which superconductivity just emerges on top of a pronounced chequerboard charge order. We observe a periodic modulation of both the superconducting coherence peak and gap depth, indicating the existence of a density wave order of Cooper pairing based on the empirical relationship between superconducting coherence and superfluid density22–27. The pair density wave has the same spatial periodicity as the charge order, and the amplitudes of the two orders exhibit clear positive correlation. These results shed new light on the origin of and interplay between the charge order and Cooper pairing modulation in the cuprates.A periodic pattern of Cooper pairs is observed at the atomic scale and is shown to be correlated with the local strength of the superconductivity. This reveals a new interplay between different ordered states in the cuprates.

Atomic scale electronic structure of the ferromagnetic semiconductor Cr 2 Ge 2 Te 6

Cr2Ge2Te6 is an intrinsic ferromagnetic semiconductor with van der Waals type layered structure, thus represents a promising material for novel electronic and spintronic devices. Here we combine scanning tunneling microscopy and first-principles calculations to investigate the electronic structure of Cr2Ge2Te6. Tunneling spectroscopy reveals a surprising large energy level shift and change of energy gap size across the ferromagnetic to paramagnetic phase transition, as well as a peculiar double-peak electronic state on the Cr-site defect. These features can be quantitatively explained by density functional theory calculations, which uncover a close relationship between the electronic structure and magnetic order. These findings shed important new lights on the microscopic electronic structure and origin of magnetic order in Cr2Ge2Te6.

AlO x /LiF composite protection layer for Cr-doped (Bi,Sb)2Te3 quantum anomalous Hall films*

We have realized robust quantum anomalous Hall samples by protecting Cr-doped (Bi,Sb)2Te3 topological insulator films with a combination of LiF and AlO x capping layers. The AlO x /LiF composite capping layer well keeps the quantum anomalous Hall states of Cr-doped (Bi,Sb)2Te3 films and effectively prevent them from degradation induced by ambient conditions. The progress is a key step towards the realization of the quantum phenomena in heterostructures and devices based on quantum anomalous Hall system.