1
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Chen L, Zhao W, Xing K, You M, Wang X, Zheng RK. Anomalous Hall effect in Nd-doped Bi 1.1Sb 0.9STe 2 topological insulator single crystals. Phys Chem Chem Phys 2024; 26:2638-2645. [PMID: 38174415 DOI: 10.1039/d3cp05850f] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/05/2024]
Abstract
Topological insulators are emerging materials with insulating bulk and symmetry protected nontrivial surface states. One of the most fascinating transport behaviors in a topological insulator is the quantum anomalous Hall effect, which has been observed in magnetic-topological-insulator-based devices. In this work, we report successful doping of rare-earth element Nd into Bi1.1Sb0.9STe2 bulk-insulating topological insulator single crystals, in which the Nd moments are ferromagnetically ordered at ∼100 K. Benefiting from the in-bulk-gap Fermi level, electronic transport behaviors dominated by the topological surface states are observed in the ferromagnetic region. At low temperatures, strong Shubnikov-de Haas oscillations with a nontrivial Berry phase are observed. The topological insulator with long range magnetic ordering in Nd-doped Bi1.1Sb0.9STe2 single crystals provides a good platform for quantum transport studies and spintronic applications.
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Affiliation(s)
- Lei Chen
- School of Physics and Materials Science, Guangzhou University, Guangzhou 510006, China.
| | - Weiyao Zhao
- Department of Materials Science & Engineering, and ARC Centre of Excellence in Future Low-Energy Electronics Technologies, Monash University, Clayton, VIC 3800, Australia
| | - Kaijian Xing
- School of Physics & Astronomy, Monash University, Clayton, VIC 3800, Australia
| | - Mengyun You
- Institute for Superconducting and Electronic Materials, and ARC Centre of Excellence in Future Low-Energy Electronics Technologies, Innovation Campus, University of Wollongong, NSW 2500, Australia
| | - Xiaolin Wang
- Institute for Superconducting and Electronic Materials, and ARC Centre of Excellence in Future Low-Energy Electronics Technologies, Innovation Campus, University of Wollongong, NSW 2500, Australia
| | - Ren-Kui Zheng
- School of Physics and Materials Science, Guangzhou University, Guangzhou 510006, China.
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2
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Zhu S, Su LL, Ren J. Tunable couplings between location-insensitive emitters mediated by an epsilon-near-zero plasmonic waveguide. OPTICS EXPRESS 2023; 31:28575-28585. [PMID: 37710908 DOI: 10.1364/oe.498569] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/21/2023] [Accepted: 08/05/2023] [Indexed: 09/16/2023]
Abstract
This work demonstrates the efficient tuning of incoherent and coherent coupling between emitters embedded in an epsilon-near-zero (ENZ) waveguide coated with a multilayer graphene. As a result, a tunable two-qubit quantum phase gate based on the ENZ waveguide is realized at the cutoff frequency. Furthermore, due to the vanishingly small permittivity of the ENZ waveguide, all incoherent coupling between any two identical emitters located in the central area of the slit approaches a maximum, enabling near-ideal bipartite and multipartite entanglement. The coherent coupling between emitters is much larger at an operating frequency far from the ENZ resonance frequency than at the cutoff frequency, and the coherent coupling and resulting energy transfer efficiency can also be effectively tuned by the Fermi level of graphene. These results demonstrate an efficiently tunable electro-optical platform for quantum devices.
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3
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Guo B, Miao W, Huang V, Lygo AC, Dai X, Stemmer S. Zeeman Field-Induced Two-Dimensional Weyl Semimetal Phase in Cadmium Arsenide. PHYSICAL REVIEW LETTERS 2023; 131:046601. [PMID: 37566870 DOI: 10.1103/physrevlett.131.046601] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/06/2023] [Revised: 06/18/2023] [Accepted: 06/28/2023] [Indexed: 08/13/2023]
Abstract
We report a topological phase transition in quantum-confined cadmium arsenide (Cd_{3}As_{2}) thin films under an in-plane Zeeman field when the Fermi level is tuned into the topological gap via an electric field. Symmetry considerations in this case predict the appearance of a two-dimensional Weyl semimetal (2D WSM), with a pair of Weyl nodes of opposite chirality at charge neutrality that are protected by space-time inversion (C_{2}T) symmetry. We show that the 2D WSM phase displays unique transport signatures, including saturated resistivities on the order of h/e^{2} that persist over a range of in-plane magnetic fields. Moreover, applying a small out-of-plane magnetic field, while keeping the in-plane field within the stability range of the 2D WSM phase, gives rise to a well-developed odd integer quantum Hall effect, characteristic of degenerate, massive Weyl fermions. A minimal four-band k·p model of Cd_{3}As_{2}, which incorporates first-principles effective g factors, qualitatively explains our findings.
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Affiliation(s)
- Binghao Guo
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
| | - Wangqian Miao
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
| | - Victor Huang
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
| | - Alexander C Lygo
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
| | - Xi Dai
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
- Department of Physics, Hong Kong University of Science and Technology, Hong Kong, China
| | - Susanne Stemmer
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
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4
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Alnaser HF, Sparks TD. BSTS synthesis guided by CALPHAD approach for phase equilibria and process optimization. Sci Rep 2023; 13:3944. [PMID: 36894685 PMCID: PMC9998604 DOI: 10.1038/s41598-023-30976-3] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2022] [Accepted: 03/03/2023] [Indexed: 03/11/2023] Open
Abstract
This work presents a new method for processing single-crystal semiconductors designed by a computational method to lower the process temperature. This research study is based on a CALPHAD approach (ThermoCalc) to theoretically design processing parameters by utilizing theoretical phase diagrams. The targeted material composition consists of Bi-Se2-Te-Sb (BSTS). The semiconductor alloy contains three phases, hexagonal, rhombohedral-1, and rhombohedral-2 crystal structures, that are presented in the phase field of the theoretical pseudo-binary phase diagram. The semiconductor is also evaluated by applying Hume-Rothery rules along with the CALPHAD approach. Thermodynamic modelling suggests that single-crystals of BSTS can be grown at significantly lower temperatures and this is experimentally validated by low-temperature growth of single crystalline samples followed by exfoliation, compositional analysis, and diffraction.
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Affiliation(s)
- Husain F Alnaser
- Department of Material Science and Engineering, University of Utah, Salt Lake City, UT, 84112, USA
| | - Taylor D Sparks
- Department of Material Science and Engineering, University of Utah, Salt Lake City, UT, 84112, USA. .,Chemistry Department, University of Liverpool, Liverpool, United Kingdom.
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5
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Chong SK, Zhang P, Li J, Zhou Y, Wang J, Zhang H, Davydov AV, Eckberg C, Deng P, Tai L, Xia J, Wu R, Wang KL. Electrical Manipulation of Topological Phases in a Quantum Anomalous Hall Insulator. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2023; 35:e2207622. [PMID: 36538624 DOI: 10.1002/adma.202207622] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/20/2022] [Revised: 11/22/2022] [Indexed: 06/17/2023]
Abstract
Quantum anomalous Hall phases arising from the inverted band topology in magnetically doped topological insulators have emerged as an important subject of research for quantization at zero magnetic fields. Though necessary for practical implementation, sophisticated electrical control of molecular beam epitaxy (MBE)-grown quantum anomalous Hall matter have been stymied by growth and fabrication challenges. Here, a novel procedure is demonstrated, employing a combination of thin-film deposition and 2D material stacking techniques, to create dual-gated devices of the MBE-grown quantum anomalous Hall insulator, Cr-doped (Bi,Sb)2 Te3 . In these devices, orthogonal control over the field-induced charge density and the electric displacement field is demonstrated. A thorough examination of material responses to tuning along each control axis is presented, realizing magnetic property control along the former and a novel capability to manipulate the surface exchange gap along the latter. Through electrically addressing the exchange gap, the capabilities to either strengthen the quantum anomalous Hall state or suppress it entirely and drive a topological phase transition to a trivial state are demonstrated. The experimental result is explained using first principle theoretical calculations, and establishes a practical route for in situ control of quantum anomalous Hall states and topology.
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Affiliation(s)
- Su Kong Chong
- Department of Electrical and Computer Engineering, University of California, Los Angeles, Los Angeles, CA, 90095, USA
| | - Peng Zhang
- Department of Electrical and Computer Engineering, University of California, Los Angeles, Los Angeles, CA, 90095, USA
| | - Jie Li
- Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92697, USA
| | - Yinong Zhou
- Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92697, USA
| | - Jingyuan Wang
- Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92697, USA
| | - Huairuo Zhang
- Theiss Research, Inc., La Jolla, CA, 92037, USA
- Materials Science and Engineering Division, National Institute of Standards and Technology (NIST), Gaithersburg, MD, 20899, USA
| | | | - Christopher Eckberg
- Department of Electrical and Computer Engineering, University of California, Los Angeles, Los Angeles, CA, 90095, USA
- Fibertek Inc., Herndon, VA, 20171, USA
- US Army Research Laboratory, Adelphi, MD, 20783, USA
- US Army Research Laboratory, Playa Vista, CA, 90094, USA
| | - Peng Deng
- Department of Electrical and Computer Engineering, University of California, Los Angeles, Los Angeles, CA, 90095, USA
| | - Lixuan Tai
- Department of Electrical and Computer Engineering, University of California, Los Angeles, Los Angeles, CA, 90095, USA
| | - Jing Xia
- Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92697, USA
| | - Ruqian Wu
- Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92697, USA
| | - Kang L Wang
- Department of Electrical and Computer Engineering, University of California, Los Angeles, Los Angeles, CA, 90095, USA
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6
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Lygo AC, Guo B, Rashidi A, Huang V, Cuadros-Romero P, Stemmer S. Two-Dimensional Topological Insulator State in Cadmium Arsenide Thin Films. PHYSICAL REVIEW LETTERS 2023; 130:046201. [PMID: 36763420 DOI: 10.1103/physrevlett.130.046201] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/04/2022] [Accepted: 12/23/2022] [Indexed: 06/18/2023]
Abstract
Two-dimensional topological insulators (2D TIs) are a highly desired quantum phase but few materials have demonstrated clear signatures of a 2D TI state. It has been predicted that 2D TIs can be created from thin films of three-dimensional TIs by reducing the film thickness until the surface states hybridize. Here, we employ this technique to report the first observation of a 2D TI state in epitaxial thin films of cadmium arsenide, a prototype Dirac semimetal in bulk form. Using magnetotransport measurements with electrostatic gating, we observe a Landau level spectrum and quantum Hall effect that are in excellent agreement with those of an ideal 2D TI. Specifically, we observe a crossing of the zeroth Landau levels at a critical magnetic field. We show that the film thickness can be used to tune the critical magnetic field. Moreover, a larger change in film thickness causes a transition from a 2D TI to a 2D trivial insulator, just as predicted by theory. The high degree of tunability available in epitaxial cadmium arsenide heterostructures can thus be used to fine-tune the 2D TI, which is essential for future topological devices.
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Affiliation(s)
- Alexander C Lygo
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
| | - Binghao Guo
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
| | - Arman Rashidi
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
| | - Victor Huang
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
| | - Pablo Cuadros-Romero
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
| | - Susanne Stemmer
- Materials Department, University of California, Santa Barbara, California 93106-5050, USA
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7
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Structural investigations of the Bi2–xSbxTe3–ySey topological insulator. J SOLID STATE CHEM 2023. [DOI: 10.1016/j.jssc.2023.123868] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
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8
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Emergent helical edge states in a hybridized three-dimensional topological insulator. Nat Commun 2022; 13:6386. [PMID: 36302907 DOI: 10.1038/s41467-022-33643-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2021] [Accepted: 09/22/2022] [Indexed: 11/08/2022] Open
Abstract
As the thickness of a three-dimensional (3D) topological insulator (TI) becomes comparable to the penetration depth of surface states, quantum tunneling between surfaces turns their gapless Dirac electronic structure into a gapped spectrum. Whether the surface hybridization gap can host topological edge states is still an open question. Herein, we provide transport evidence of 2D topological states in the quantum tunneling regime of a bulk insulating 3D TI BiSbTeSe2. Different from its trivial insulating phase, this 2D topological state exhibits a finite longitudinal conductance at ~2e2/h when the Fermi level is aligned within the surface gap, indicating an emergent quantum spin Hall (QSH) state. The transition from the QSH to quantum Hall (QH) state in a transverse magnetic field further supports the existence of this distinguished 2D topological phase. In addition, we demonstrate a second route to realize the 2D topological state via surface gap-closing and topological phase transition mechanism mediated by a transverse electric field. The experimental realization of the 2D topological phase in a 3D TI enriches its phase diagram and marks an important step toward functionalized topological quantum devices.
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9
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Llacsahuanga Allcca AE, Pan XC, Miotkowski I, Tanigaki K, Chen YP. Gate-Tunable Anomalous Hall Effect in Stacked van der Waals Ferromagnetic Insulator-Topological Insulator Heterostructures. NANO LETTERS 2022; 22:8130-8136. [PMID: 36215229 DOI: 10.1021/acs.nanolett.2c02571] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/16/2023]
Abstract
The search of novel topological states, such as the quantum anomalous Hall insulator and chiral Majorana fermions, has motivated different schemes to introduce magnetism into topological insulators. A promising scheme is using the magnetic proximity effect (MPE), where a ferromagnetic insulator magnetizes the topological insulator. Most of these heterostructures are synthesized by growth techniques which prevent mixing many of the available ferromagnetic and topological insulators due to difference in growth conditions. Here, we demonstrate that MPE can be obtained in heterostructures stacked via the dry transfer of flakes of van der Waals ferromagnetic and topological insulators (Cr2Ge2Te6/BiSbTeSe2), as evidenced in the observation of an anomalous Hall effect (AHE). Furthermore, devices made from these heterostructures allow modulation of the AHE when controlling the carrier density via electrostatic gating. These results show that simple mechanical transfer of magnetic van der Waals materials provides another possible avenue to magnetize topological insulators by MPE.
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Affiliation(s)
- Andres E Llacsahuanga Allcca
- Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, United States
- Purdue Quantum Science and Engineering Institute and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States
| | - Xing-Chen Pan
- WPI Advanced Institute for Materials Research (AIMR), Tohoku University, Sendai 980-8577, Japan
| | - Ireneusz Miotkowski
- Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, United States
| | - Katsumi Tanigaki
- WPI Advanced Institute for Materials Research (AIMR), Tohoku University, Sendai 980-8577, Japan
- Beijing Academy of Quantum Information Sciences, Beijing 100193, China
| | - Yong P Chen
- Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, United States
- Purdue Quantum Science and Engineering Institute and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States
- WPI Advanced Institute for Materials Research (AIMR), Tohoku University, Sendai 980-8577, Japan
- School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, United States
- Institute of Physics and Astronomy and Villum Center for Hybrid Quantum Materials and Devices, Aarhus University, 8000 Aarhus-C, Denmark
- Center for Science and Innovation in Spintronics, Tohoku University, Sendai 980-8577, Japan
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10
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Intrinsic coupling between spatially-separated surface Fermi-arcs in Weyl orbit quantum Hall states. Nat Commun 2021; 12:2572. [PMID: 33958588 PMCID: PMC8102497 DOI: 10.1038/s41467-021-22904-8] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/06/2020] [Accepted: 04/06/2021] [Indexed: 11/10/2022] Open
Abstract
Topological semimetals hosting bulk Weyl points and surface Fermi-arc states are expected to realize unconventional Weyl orbits, which interconnect two surface Fermi-arc states on opposite sample surfaces under magnetic fields. While the presence of Weyl orbits has been proposed to play a vital role in recent observations of the quantum Hall effect even in three-dimensional topological semimetals, actual spatial distribution of the quantized surface transport has been experimentally elusive. Here, we demonstrate intrinsic coupling between two spatially-separated surface states in the Weyl orbits by measuring a dual-gate device of a Dirac semimetal film. Independent scans of top- and back-gate voltages reveal concomitant modulation of doubly-degenerate quantum Hall states, which is not possible in conventional surface orbits as in topological insulators. Our results evidencing the unique spatial distribution of Weyl orbits provide new opportunities for controlling the novel quantized transport by various means such as external fields and interface engineering. The spatial distribution of the quantized transport due to the presence of Weyl orbits in topological semimetals remains elusive. Here, the authors report concomitant modulation of doubly-degenerate quantum Hall states, evidencing intrinsic coupling between two spatially separated surface states in the Weyl orbits of a Dirac semimetal film.
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11
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Wang J, Gorini C, Richter K, Wang Z, Ando Y, Weiss D. Two-Dimensional-Dirac Surface States and Bulk Gap Probed via Quantum Capacitance in a Three-Dimensional Topological Insulator. NANO LETTERS 2020; 20:8493-8499. [PMID: 33174423 DOI: 10.1021/acs.nanolett.0c02733] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
BiSbTeSe2 is a 3D topological insulator (3D-TI) with Dirac type surface states and low bulk carrier density, as donors and acceptors compensate each other. Dominating low-temperature surface transport in this material is heralded by Shubnikov-de Haas oscillations and the quantum Hall effect. Here, we experimentally probe and model the electronic density of states (DOS) in thin layers of BiSbTeSe2 by capacitance experiments both without and in quantizing magnetic fields. By probing the lowest Landau levels, we show that a large fraction of the electrons filled via field effect into the system ends up in (localized) bulk states and appears as a background DOS. The surprisingly strong temperature dependence of such background DOS can be traced back to Coulomb interactions. Our results point at the coexistence and intimate coupling of Dirac surface states with a bulk many-body phase (a Coulomb glass) in 3D-TIs.
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Affiliation(s)
- Jimin Wang
- Institute of Experimental and Applied Physics, University of Regensburg, 93040 Regensburg, Germany
| | - Cosimo Gorini
- Institute of Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany
| | - Klaus Richter
- Institute of Theoretical Physics, University of Regensburg, 93040 Regensburg, Germany
| | - Zhiwei Wang
- Physics Institute II, University of Cologne, Zülpicher Strasse 77, 50937 Köln, Germany
- Key Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement, Ministry of Education, School of Physics, Beijing Institute of Technology, Beijing 100081, China
| | - Yoichi Ando
- Physics Institute II, University of Cologne, Zülpicher Strasse 77, 50937 Köln, Germany
| | - Dieter Weiss
- Institute of Experimental and Applied Physics, University of Regensburg, 93040 Regensburg, Germany
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12
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Fei F, Zhang S, Zhang M, Shah SA, Song F, Wang X, Wang B. The Material Efforts for Quantized Hall Devices Based on Topological Insulators. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2020; 32:e1904593. [PMID: 31840308 DOI: 10.1002/adma.201904593] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/17/2019] [Revised: 09/09/2019] [Indexed: 06/10/2023]
Abstract
A topological insulator (TI) is a kind of novel material hosting a topological band structure and plenty of exotic topological quantum effects. Achieving quantized electrical transport, including the quantum Hall effect (QHE) and the quantum anomalous Hall effect (QAHE), is an important aspect of realizing quantum devices based on TI materials. Intense efforts are made in this field, in which the most essential research is based on the optimization of realistic TI materials. Herein, the TI material development process is reviewed, focusing on the realization of quantized transport. Especially, for QHE, the strategies to increase the surface transport ratio and decrease the threshold magnetic field of QHE are examined. For QAHE, the evolution history of magnetic TIs is introduced, and the recently discovered magnetic TI candidates with intrinsic magnetizations are discussed in detail. Moreover, future research perspectives on these novel topological quantum effects are also evaluated.
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Affiliation(s)
- Fucong Fei
- National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Physics, Nanjing University, Nanjing, 210093, China
| | - Shuai Zhang
- National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Physics, Nanjing University, Nanjing, 210093, China
| | - Minhao Zhang
- National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Physics, Nanjing University, Nanjing, 210093, China
| | - Syed Adil Shah
- National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Physics, Nanjing University, Nanjing, 210093, China
| | - Fengqi Song
- National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Physics, Nanjing University, Nanjing, 210093, China
| | - Xuefeng Wang
- National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing, 210093, China
| | - Baigeng Wang
- National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, School of Physics, Nanjing University, Nanjing, 210093, China
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