Dr. Du’s research area and scholarly interests have been on the physics of low dimensional electron systems, especially on the fractional quantum Hall effect. The research work focuses on fundamental understanding of quantum mechanical properties of electrons in low dimensional and nanoscale semiconductor structures and devices. Here is an outline of current research topics. 2D and 1D electron systems provide both a clean laboratory for many-particle physics and a unique platform addressing emerging issues in spintronics and solid-state quantum information devices. Our recent research has been centered on these two themes and has been supported by DOE, NSF, and DARPA. The materials used are mainly GaAs/AlGaAs heterostructures and quantum wells, but also include Si and InGaAs depending on the physics involved.

1. Many-Particle Physics in Ultracold 2D Electron Systems Experiments in 2D electron system (2DES) can be turned by many “knobs”, and multitude many-particle electronic phases and phase transitions can be studied: fractional charged quantum fluid, Wigner lattices, Luttinger liquid, Stripes and liquid crystal-like phases, and Skyrmions, to name a few. Not unlike in AMO where Bose-Einstein condensation was discovered in ultracold atoms, the frontier of 2DES physics is on ultracold electrons. In collaboration with Columbia (Prof. Horst Stormer), Princeton (Prof. Dan Tsui), Stanford (Prof. Doug Osheroff), and funded by DOE, we have been pursuing a major research project (submillikelvin demagnetization refrigerator for solid state research) for experiments on ultracold electrons in low-D semiconductors and nanostructures. One of the objectives is to searching for exotic many-particle electronic states that obey non-Abelien statistics, but there are much more: cooling electrons to lower than few millikelvin becomes increasingly relevant to general issues of quantum decoherence and phase transitions in electronic systems including mesoscopic metals, quantum dots, and quantum magnets. With the capability to cool 2D electrons to lower temperature, we will develop two important tools for studying many-particle states: tunneling into edge of the quantum fluid and NMR for nuclear spins in the host materials (which in turn reflect the spin states in the 2DES).

2. Novel DC Transport Phenomena and Effect in a AC Driven Quantum Well Electrical transport of a 2DES exposed to a high frequency (from GHz to THz) driving electromagnetic wave is directly relevant to emerging issues concerning spintronics and quantum information processing. Fundamentally new phenomena of magnetoresistance oscillation and zero-resistance/zero-conductance have been discovered in our lab during the recent years (Phys. Today, Search & Discovery April 2003). We will continue to explore this new effect and its physics. One ongoing project is to develop a scanning Hall probe microscopy for simultaneously imaging the electrical current pattern formation and measuring dc transport. If this is successful we will be able to study the domain states proposed by theories in this novel effect.

3. Spin and Magnetic Phenomena Spin and spin-interaction effects are the fundamental properties in 2D electron and 2D hole systems in semiconductors. Controlling spin polarization via electrical gates or microwave excitations is part of the research effort in my group. We have studied some interesting phenomena in magnetic alloys, such as DyAs, and more recently InGaAs.

4. Si-Based Nanoelectronics Supported by a DARPA grant (University of Illinois) “Wavefunction engineering of individual donors for Si-based quantum computers” we have been participating in a quantum information project. The goal of this project is to place individual phosphorous (P) donors into silicon with atomic precision, and to demonstrate electrical control over the coupling between them as a function of their spin states. Our group is responsible for quantum transport measurement of the Si-nanostructures produced by this atom-scale STM patterning technique. We have shown that an ultradense (about 3 orders of magnitude higher than usual 2DES in GaAs) P donor sheets can be formed using this technique, and transport shows a metallic conduction at low temperatures. Recently a nano-size Aharanov-Bohm ring has been fabricated and verified by our magnetotransport. In summary, our research will continue to focus on the many-particle physics in lower dimensional electron systems, one of the most important and challenging areas in condensed matter sciences.

Lingjie Du, Ivan Knez, Gerard Sullivan, and Rui-Rui Du, "Robust Helical Edge Transport in Gated InAs/GaSb Bilayers", Phys. Rev. Lett. 114, 096802 (2015)

Tingxin Li, Pengjie Wang, Hailong Fu, Lingjie Du, Kate A. Schreiber, Xiaoyang Mu, Xiaoxue Liu, Gerard Sullivan, Gábor A. Csáthy, Xi Lin, Rui-Rui Du, Observation of a Helical Luttinger Liquid in InAs/GaSb Quantum Spin Hall Edges, Phys. Rev. Lett. 115, 136804 (2015)

Xing-Jun Wu, Ting-Xin Li, Chi Zhang, and Rui-Rui Du, Landau Level Crossing in Spin-Orbit Coupled Two-Dimensional Electron Gas, Appl. Phys. Lett. 106, 012106 (2015)

Xuebin Wang, Hailong Fu, Lingjie Du, Xiaoxue Liu, Pengjie Wang, L. N. Pfeiffer, K. W. West, Rui-Rui Du, and Xi Lin, Depinning transition of bubble phases in a high Landau level, Phys. Rev. B 91, 115301 (2015)

 I. Knez, C. T. Rettner, S. S. P. Parkin, L.-J. Du, R.-R.Du, and G. Sullivan,  "Observation of Edge Transport in the Disordered Regime of Topologically Insulating InAs/ GaSb Quantum Wells", Phys. Rev. Lett. 112, 026602 (2014)

Xi Lin, Rui-Rui Du, Xincheng Xie, invited review, "Recent Experimental Progress of Fractional Quantum Hall Effect: 5/2 Filling State and Graphene", National Science Review 1, 564 (2014)

Xiaoxue Liu, Yuying Zhu, Lingjie Du, Changli Yang, Li Lu, Loren Pfeiffer, Ken West, and Rui-Rui Du, "2kF -Selected Conductance Oscillations of High-Mobility Two-Dimensional Electron Gas in Corbino Devices", Appl. Phys. Lett. 105, 182110 (2014)

K. Stone, R. R. Du, M. J. Manfra, L. N. Pfeiffer, and K. W. West, Millimeter wave transmission spectroscopy of gated two-dimensional hole systems, Appl. Phys. Lett. 100, 192104 (2012)

Fanming Qu, Chi Zhang, Rui-Rui Du, Li Lu, Coexistence of Bulk and Surface Shubnikov-de Hass Oscillations in Bi2Se3, Journal of Low Temperature Physics 170, 397 (2013)

Ivan Knez, Rui-Rui Du, and Gerard Sullivan, Andreev Reflection of Helical Edge Modes in InAs/GaSb Quantum Spin Hall Insulator, Phys. Rev. Lett. 109, 186603 (2012)

Guangtong Liu, Chi Zhang, D. C. Tsui, Ivan Knez, Aaron Levine, R. R. Du, L. N. Pfeiffer, and K. W. West, Enhancement of the ν=5/2 Fractional Quantum Hall State in a Small In-Plane Magnetic Field, Phys. Rev. Lett. 108, 196805 (2012)

Ivan Knez and Rui-Rui Du, invited review article, Frontiers of Physics, 2011 Springer-Verlag Berlin, "Quantum spin Hall effect in inverted InAs/GaSb quantum wells ." 

Ivan Knez, Rui-Rui Du, and Gerard Sullivan, "Evidence for Helical Edge Modes in Inverted InAs/GaSb Quantum Wells." Phys. Rev. Lett. 107, 136603 (2011)

Yanhua Dai, Kristjan Stone, Ivan Knez, Chi Zhang, R. R. Du, Changli Yang, L. N. Pfeiffer, and K. W. West, "Response of the microwave-induced cyclotron harmonic resistance spike to an in-plane magnetic field ." Physical Review B 84, 241303 (2011)

Chi Zhang, T. Knuuttila, Yanhua Dai, R. R. Du, L. N. Pfeiffer, and K. W. West, ν = 5/2 Fractional Quantum Hall Effect at 10 T: Implications for the Pfaffian State, Phys.  Rev.  Lett. 104, 166801 (2010)

Ivan Knez, R. R. Du, and Gerard Sullivan, "Finite Conductivity in Mesoscopic Hall Bars of Inverted InAs/GaSb Quantum Wells", Physics Review B 81, 201301R (2010).

Jian Zhang, R. R. Du, J. A. Simmons, and J. L. Reno, Resistance Minimum Observed at Landau Level Filling ν = 1/2 in Ultra-High Magnetic Field,  Physics Review B 81, 041308R (2010).

Yanhua Dai, R. R. Du, L. N. Pfeiffer, and K. W. West, "Observation of a Cyclotron Harmonic Spike in Microwave-Induced Resistances in Ultraclean GaAs/AlGaAs Quantum Wells" , Physics  Review Letters 105, 246802 (2010).

C. L. Yang, R. R. Du, L. N. Pfeiffer, and K. W. West, Phys. Rev. B 74, 045315 (2006), "Influence of a Parallel Magnetic Fields on the Microwave-Photoconductivity in a High-Mobility 2D Electron System." 

Jian Zhang, R. R. Du, J. A. Simmons, and J. L. Reno, Phys. Rev. B 81, 041308 (2010), "Resistance Minimum Observed at Landau Level Filling Factor 1/2 in Ultra High Magnetic Fields." 

K. Stone, C. L. Yang, Z. Q. Yuan, R. R. Du, L. N. Pfeiffer and K. W. West, Phys. Rev. B 76 153306 (2007), "Photovoltaic oscillations due to edge-magnetoplasmon modes in a very high-mobility two-dimensional electron gas." 

M. A. Zudov, R. R. Du, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 96, 236804 (2006), "Bichromatic Microwave Photoresistance of a Two-Dimensional Electron System." 

S. J. Robinson, J. S. Kline, H. J. Wheelwright, J. R. Tucker, C. L. Yang, R. R. Du, B. E. Volland, I. W. Rangelow, and T.-C. Shen, Phys. Rev. B 74, 153311 (2006), "Electron transport in laterally confined phosphorus δ layers in silicon." 

Yanhua Dai, Z. Q. Yuan, C. L. Yang, R. R. Du, M. J. Manfra, L. N. Pfeiffer, and K. W. West, Phys. Rev. B 80, 041310 (2009), "Magnetotransport in Zener Tunneling Regime in a High-Mobility Two-Dimensional Hole Gas." 

Z. Q. Yuan, C. L. Yang, R. R. Du, L. N. Pfeiffer, and K. W. West, Phys. Rev. B 74, 075313 (2006), "Microwave Photoresistance of a High-Mobility Electron Gas in a Triangular Antidot Lattice." 

Z. Q. Yuan, R. R. Du, M. J. Manfra, L. N. Pfeiffer, and K. W. West , Appl. Phys. Lett. 94, 052103 (2009), "Landau Level Spectrum in a Two-Dimensional Hole Gas in C-Doped (100) GaAs/Al0.4Ga0.6As Square Quantum Well." 

M. A. Zudov, R. R. Du, L. N. Pfeiffer, and K. W. West, "Bichromatic Photoresistance of a Two-Dimensional Electron System,." Phys. Rev. Lett. 96, 236804 (2006)

"Proceedings of the 16th International Conference on Electronic Properties of Two-Dimensional Systems (EP2DS-16)." Physica E 34, 1-716 92006)

R. R. Du, M. A. Zudov, C. L. Yang, Z. Q. Yuan, L. N. Pfeiffer, and K. W. West, "Oscillatory and Vanishing Resistance States in Microwave Irradiated 2D Electron Systems." Invited, Proceedings of the 16th International Conference on High Magnetic Fields in Semiconductor Physics”, eds. Y.-J. Wang, L. Engel, N. Bonesteel, World Scientific, Singapore, 2005

"Publication Committee, SCES'07 Houston." SCES' 07 Houston, Houston. (July 2007)

"Chair, Int. Program Committee, 16th Int. Conf. on Electronic Properties of Two-Dimensional Systems, Albuquerque, NM, USA, July 10-15, 2005."

"Chair, Int. Workshop on Macroscopic Quantum Phenomena, Chinese Academy of Sciences, Beijing, 7-11 June 2005."

2010 International Workshop on Quantum Matter, May 2010

Nonlinear Transported in Modulated Two-Dimensional Electron Systems, invited Symposium, "Non-Equilibrium Transport in Very High Landau Levels", 2010 APS March Meeting (Portland, Oregon).

"5/2 States in High Electron-Density GaAs/AlGaAs Quantum Wells." 18th International Conference on High Magnetic Field in Semiconductors and Nanotechnolwgies, Sao Pedro, Brazil. (08/03/08-08/08/08) With Chi Zhang, T. Knuuttila, Yuanhua Dai, L. N. Pfeiffer, and K. W. West

Mott Transition in InAs/GaSb, invited talk, Workshop on Fifteen Years of ICQS, Chinese University of Science and Technology, August 2015, Hefei, China  

Helical Luttinger Liquid, invited talk, Int'l Workshop on Interaction and Topology in 1D, Mark Planck Institute, Dresden Germany, Sept. 2015

Helical Luttinger Liquid, invited talk,  IAS Workshop on Topological Matter, HKUST, Hong Kong, July 2015

InAs/GaSb Quantum Spin Hall Insulator and Beyond, invited talk, Max Plank Society Summer School, Stuttgart, July 2015

Robust Helical Transport in InAs/GaSb, Invited SymposiumTalk, APS March Meeting, March 2015, San Antonio, USA

InAs/GaSb Quantum Spin Hall Insulators, invited Talk, American Vacuum Society, Jan. 2015, Salt Lake City, USA

"Helical Edge States and Beyond in InAs/GaSb Bilayers", Workshop on Majorana Zero - Modes and Beyond, Princeton Center for Theoretical Science, Oct. 29-31, 2014

"Superconductor Proximity-Coupled Helical Edges in InAs/GaSb Bilayers", Int'l Conf. on Emerging Phenomena in Quantum Hall Systems – EPQHS-5, July 7-9, 2014, Israel

"Proximity-Coupled InAs/GaSb Quantum Wells as a Platform of Majorana Zero-Modes", Inte'l Workshop, Institute of Advanced Studies, Hong Kong University of Science and Technology, Jan. 9, 2014

"Robust Helical Edge States in InAs/GaSb Bilayers", 21th Int'l Conf. on the Physics of High Magnetic Fields, Aug. 3-8, 2014, Panama City, FL, USA 

"Robust Quantum Spin Hall Effect in Gated InAs/GaSb Bilayers", European Materials Research Society Fall 2014 Conf. Sept. 17, 2014, Warsaw, Poland

"Creating Majorana Fermions on the Edge of InAs/GaSb Quantum Wells", Max-Planck-Institute for Solid-State Research in Stuttgart, Germany,  June 26-28, 2013.

"Creating Majorana Fermions on the Edge of InAs/GaSb", MIT Chez Pierre CM seminar April 22, 2013.

"Quantized Edge States in InAs/GaSb Bilayers", Microsoft Station-Q Meeting, Santa Barbara, December 6, 2013

Symposium on Majorana Physics in Condensed Matter, Ettore Majorana Center, Erice, Italy, July 12 -18, 2013.

Quantum Spin Hall effect in InAs/GaSb Quantum Wells, Rui-Rui Du, Invited talk, International Workshop on Majorana Fermions in Condensed Matter, Lorentz Center, Leiden University, July 2 to July 6, 2012

Quantum Spin Hall effect in InAs/GaSb Quantum Wells, Department of Physics, Stanford University, February 23, 2012 

Helical Edge Modes in Inverted Electron-Hole Bilayers,  Rui-Rui Du, Ivan Knez, and Gerard Sullivan, Invited talk, 31th International Conference on the Physics of Semiconductors, Zurich, Switzerland, July 29 to August 3, 2012

Quantum Hall Physics: From High Magnetic Field to Zero Magnetic Field, Invited talk, Beijing University, June 1, 2012

"A New 2D Topological Insulator from Semiconductors,." Tsinghua University, Beijing. (August 29, 2011)

"Edge Modes and Topological Superconducting States in Inverted InAs/GaSb Quantum Wells,." THE HEBREW UNIVERSITY OF JERUSALEM, THE INSTITUTE FOR ADVANCED STUDIES, The First China-Israel Meeting on STRONGLY CORRELATED ELECTRON MATTER, (November 19, 2011)

"Perfect Andreev Refection of Helical Edge Modes in Nb‐Contacted InAs/GaSb Quantum Wells,." International Workshop “Superconducting hybrids: from conventional to exotic”, Villard de Lans, France. (Sepember 10, 2011)

"Quantum Spi Hall Effect in InAs/GaSb Quantum Wells,." International Workshop "World of Hall", Peking University, Beijing, (June 22, 2011)

"Quantum Spin Hall Effect in InAs/GaSb Quantum Wells,." Institute of Technicial Physics, Chinese Academy of Sciences, Shanghai , (June 27, 2011)

"Quantum Spin Hall Effect in Inverted Electron-Hole Bilayers,." Kavli Institute for Theoretical Physics, University of California, Santa Barbara, (December 12, 2011)

"Topological Insulators Made of Inverted Electron-Hole Bilayers,  ." Institute of Physics, chinese Academy of Sciences, Beijing. (October 17, 2011)

Institute of Physics, Chinese Academy of Sciences, Beijing, June 2010

Institute of Semiconductors, Chinese Academy of Sciences, Beijing 2010

School of Physics, Beijing University, June2010

University of Chicago Beijing Center Opening Workshop, September 2010

"Quantum Hall Topological Phase at 5/2 Landau Level Filling Factor." Internationa Workshop on Quantum Information, Shanghai , July 2009, (7/10/09)

"Quantum Spin Hall Effect in InAs/GaSb Quantum Wells ." International Workshop on Quantum Matter, Hangzhou, 10/12-10/15, 2009, (10/12/09)

"Searching For Quantum Spin Hall Effect in InAs/GaSb Semiconductors." International Workshop on Topological Quantum Phases, (6/23/2009)

"Transport in Inverted InAs/GaSb Quantum Wells." Institute of Physics, Chinese Academy of Sciences, (10/15/09)

"Experimental Aspects of the Quantum Hall Topological Phases." Kavli Institute for Theoretical Physics-Beijing, Beijing, China. (June 2008)

"Experiments on Quantum Hall Topological Phases, Kavli Institute for Theoretical Physics." Kavli Institute of Theoretical Physics, Chinese Academy of Sciences, Beijing. (June 2007)

"Experiments on Quantum Hall Topological Phases." University of Science and Technology, Fudan University, Jiao-Tong University, (2008)

"International Workshop on Quantum Matter." Hong Zhou, October 2009,

"International Workshop on Topological Quantum Phases." Hong Kong, June 2009,

"International workshop on Quantum Information ." Shanghai, July 2009,

"Fractional Quantum Hall Effect." 18th International Conference on High Magnetic Field in Semiconductors and Nanotechnology, Sao Pedro, Brazil. (08/03/08-08/08/08)

"Session Chair, APS March Meeting 2007." Denver. (March 2007)

"International Workshop on Quantum Structures." Institute of Physics, Chinese Academy of Sciences, Beijing, China. (June 2008)

"Workshop on Quantum Spin Hall Effect (QSPIN)." Kavli Institute for Theoretical Physics, University of California at Santa Barbara. (12/08/08-12/19/08)

"Organizer and Chair, Beijing Workshop on Quantum Transport, Chineses Academy of Sciences." Beijing. (December 2006)