Qinjun Peng

Distinct Fermi Surface Topology and Nodeless Superconducting Gap in a (Tl0.58Rb0.42)Fe1.72Se2 Superconductor

High resolution angle-resolved photoemission measurements have been carried out to study the electronic structure and superconducting gap of the (Tl0.58Rb0.42)Fe1.72Se2 superconductor with a T(c) = 32  K. The Fermi surface topology consists of two electronlike Fermi surface sheets around the Γ point which is distinct from that in all other iron-based superconductors reported so far. The Fermi surface around the M point shows a nearly isotropic superconducting gap of ∼12  meV. The large Fermi surface near the Γ point also shows a nearly isotropic superconducting gap of ∼15  meV, while no superconducting gap opening is clearly observed for the inner tiny Fermi surface. Our observed new Fermi surface topology and its associated superconducting gap will provide key insights and constraints into the understanding of the superconductivity mechanism in iron-based superconductors.

Robustness of topological order and formation of quantum well states in topological insulators exposed to ambient environment

The physical property investigation (like transport measurements) and ultimate application of the topological insulators usually involve surfaces that are exposed to ambient environment (1 atm and room temperature). One critical issue is how the topological surface state will behave under such ambient conditions. We report high resolution angle-resolved photoemission measurements to directly probe the surface state of the prototypical topological insulators, Bi2Se3 and Bi2Te3, upon exposing to various environments. We find that the topological order is robust even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the exposed surface of the topological insulators. These findings provide key information in understanding the surface properties of the topological insulators under ambient environment and in engineering the topological surface state for applications.

Observation of Fermi arc and its connection with bulk states in the candidate type-II Weyl semimetal WTe2

Chenlu Wang, Yan Zhang, Jianwei Huang, Simin Nie, Guodong Liu1,∗, Aiji Liang, Yuxiao Zhang, Bing Shen, Jing Liu, Cheng Hu, Ying Ding, Defa Liu, Yong Hu, Shaolong He, Lin Zhao, Li Yu, Jin Hu, Jiang Wei, Zhiqiang Mao, Youguo Shi, Xiaowen Jia, Fengfeng Zhang, Shenjin Zhang, Feng Yang, Zhimin Wang, Qinjun Peng, Hongming Weng, Xi Dai, Zhong Fang, Zuyan Xu, Chuangtian Chen and X. J. Zhou1,5,∗ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China. Department of Physics and Engineering Physics, Tulane University, New Orleans, Louisiana 70118, USA Military Transportation University, Tianjin 300161, China. Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. Collaborative Innovation Center of Quantum Matter, Beijing 100871, China. These people contributed equally to the present work. ∗Corresponding author: gdliu arpes@iphy.ac.cn, XJZhou@iphy.ac.cn.

Tunable Dirac Fermion Dynamics in Topological Insulators

Three-dimensional topological insulators are characterized by insulating bulk state and metallic surface state involving relativistic Dirac fermions which are responsible for exotic quantum phenomena and potential applications in spintronics and quantum computations. It is essential to understand how the Dirac fermions interact with other electrons, phonons and disorders. Here we report super-high resolution angle-resolved photoemission studies on the Dirac fermion dynamics in the prototypical Bi2(Te,Se)3 topological insulators. We have directly revealed signatures of the electron-phonon coupling and found that the electron-disorder interaction dominates the scattering process. The Dirac fermion dynamics in Bi2(Te3−xSex) topological insulators can be tuned by varying the composition, x, or by controlling the charge carriers. Our findings provide crucial information in understanding and engineering the electron dynamics of the Dirac fermions for fundamental studies and potential applications.

Nonlinear optical properties of BaAlBO 3 F 2 crystal

We investigated the nonlinear optical properties of new BaAlBO(3)F(2)(BABF) crystal. The high quality BABF is nonhygroscopic and possesses a moderate birefringence suitable for UV light generation. On the basis of its refractive index dispersion curves, it is inferred that BABF has great potential applications nonlinear optical material, notably for UV light generation at 355 nm. In order to characterize its nonlinear optical properties, BABF samples were cut an oriented in phase matching conditions The optical conversion efficiency from 1064 nm to 532 nm was investigated for the first time: up to 49.0% were achieved. The external angular acceptance bandwidth of SHG and THG for 1064 nm pump light was measured.

Real-time gray-scale photolithography for fabrication of continuous microstructure

A novel real-time gray-scale photolithography technique for the fabrication of continuous microstructures that uses a LCD panel as a real-time gray-scale mask is presented. The principle of design of the technique is explained, and computer simulation results based on partially coherent imaging theory are given for the patterning of a microlens array and a zigzag grating. An experiment is set up, and a microlens array and a zigzag grating on panchromatic silver halide sensitized gelatin with trypsinase etching are obtained.

Orbital-selective spin texture and its manipulation in a topological insulator

Topological insulators represent a new quantum state of matter that are insulating in the bulk but metallic on the edge or surface. In the Dirac surface state, it is well-established that the electron spin is locked with the crystal momentum. Here we report a new phenomenon of the spin texture locking with the orbital texture in a topological insulator Bi₂Se₃. We observe light-polarization-dependent spin texture of both the upper and lower Dirac cones that constitutes strong evidence of the orbital-dependent spin texture in Bi₂Se₃. The different spin texture detected in variable polarization geometry is the manifestation of the spin-orbital texture in the initial state combined with the photoemission matrix element effects. Our observations provide a new orbital degree of freedom and a new way of light manipulation in controlling the spin structure of the topological insulators that are important for their future applications in spin-related technologies.

Electronic evidence of temperature-induced Lifshitz transition and topological nature in ZrTe 5

The topological materials have attracted much attention for their unique electronic structure and peculiar physical properties. ZrTe5 has host a long-standing puzzle on its anomalous transport properties manifested by its unusual resistivity peak and the reversal of the charge carrier type. It is also predicted that single-layer ZrTe5 is a two-dimensional topological insulator and there is possibly a topological phase transition in bulk ZrTe5. Here we report high-resolution laser-based angle-resolved photoemission measurements on the electronic structure and its detailed temperature evolution of ZrTe5. Our results provide direct electronic evidence on the temperature-induced Lifshitz transition, which gives a natural understanding on underlying origin of the resistivity anomaly in ZrTe5. In addition, we observe one-dimensional-like electronic features from the edges of the cracked ZrTe5 samples. Our observations indicate that ZrTe5 is a weak topological insulator and it exhibits a tendency to become a strong topological insulator when the layer distance is reduced.

Efficient improvement of laser beam quality by coherent combining in an improved Michelson cavity

The efficient improvement of laser beam quality is demonstrated by coherent combining of two Nd:YVO4 lasers in an improved Michelson cavity. When the two lasers are separately operated with beam qualities of M(x)2 = 1.74 and M(y)2 = 1.34 for laser 1 and M(x)2 = 1.53 and M(y)2 = 1.39 for laser 2, by coherent combining of the two lasers, a single beam output exceeding 2 W with a nearly perfect TEM00 mode (M(x)2 = 1.08, M(y)2 = 1.04) is obtained. Moreover, it can withstand environmental perturbations with no change in beam quality or output power and has the potential for scaling to much higher output power with high beam quality by coherent addition of multiple lasers in this configuration.

Disappearance of nodal gap across the insulator-superconductor transition in a copper-oxide superconductor

The parent compound of the copper-oxide high-temperature superconductors is a Mott insulator. Superconductivity is realized by doping an appropriate amount of charge carriers. How a Mott insulator transforms into a superconductor is crucial in understanding the unusual physical properties of high-temperature superconductors and the superconductivity mechanism. Here we report high-resolution angle-resolved photoemission measurement on heavily underdoped Bi₂Sr₂-xLaxCuO(₆+δ) system. The electronic structure of the lightly doped samples exhibit a number of characteristics: existence of an energy gap along the nodal direction, d-wave-like anisotropic energy gap along the underlying Fermi surface, and coexistence of a coherence peak and a broad hump in the photoemission spectra. Our results reveal a clear insulator-superconductor transition at a critical doping level of ~0.10 where the nodal energy gap approaches zero, the three-dimensional antiferromagnetic order disappears, and superconductivity starts to emerge. These observations clearly signal a close connection between the nodal gap, antiferromagnetism and superconductivity.