I am interested in the fundamental physics of quantum condensed-matter systems. Scattering based spectroscopic methods are used in the investigations of novel quantum phases realized via topological ordering, strong-interaction or geometrical frustration and their combinations. Quantum Hall phases, correlated superconductors and frustrated magnets have profoundly changed our microscopic understanding of interacting quantum matter. My research is focused on the frontier aspects of these areas of fundamental physics such as the quantum Hall-like effect without external magnetic field, non-BCS superconductivity in correlated materials and fractionalized phases in higher dimensions as well as the development of innovative X-ray-based instrumentation capability necessary to address these issues.

I am also interested in the state-of-the-art Synchrotron and next-generation linear-Accelerator based X-ray FELs for their potential use in fundamental condensed-matter physics and biology (see sample publications below). At Princeton, I am also affiliated with the institute for the technology of advanced materials PRISM and the engineering physics program at the school of engineering and quantitative biophysics program at the genomics institute.

Traditionally spectroscopic methods have been used to characterize electronic or spin behavior in matter whereas initial discoveries originated from non-spectroscopic methods. My work focuses on the theme which I often like to call "Spectroscopy for discovering new states of matter". I am currently interested in Quantum spin-textures & axion Berry phases of matter, Topological Insulator, Topological Theta-Vacuum, topological Hall phases and Berry's phases due to Dirac excitations  in strong spin-orbit coupled materials and quantum phase transitions leading to the topological insulators; Emergence of superconductivity in strongly correlated triangular lattice materials (Correlated Nodal Superconductors) and spin-liquid-like behavior in frustrated magnet materials (RVB-type fractionalized phases) in search of direct/unambiguous signatures of electron fractionalization in higher dimensions.

Following our experimental observation of first strong topological insulators in 2007(actual observation dates a few years back in connection to our work in spin-thermoelectrics), we have theoretically predicted (using first-principle calculations)  as well as experimentally discovered/demonstrated by directly measuring the topological character of the edge-spectrum of single-Dirac-cone topological insulators in 2008. This discovery opens up possiblities for a wide range of novel/exotic experiments with topological quantum phenomena. Recently, we have demonstrated that the topological-order can be maintained at room-temperature without any magnetic field, the absence of backscattering or the protected nature of the states and that the materials can be driven to the topological transport regime paving the way for realizing some high-energy-physics-like experiments at table-top settings with wide tunability and systematic control.

Examples of topological phases of quantum condensed-matter include charge quantum Hall effect (Nobel Prizes '85, '98), novel topological insulators, anti-localization effect due to unpaired Dirac fermions, quantum spin Hall effect, helical Dirac modes and topological quantum phase transitions. Examples of strong-interaction physics include Mott phenomena such as electron fractionalization or competing orders such as SDW/CDW vs. superconductivity and examples of geometrical frustration physics include spin-liquids and its competing orders such as quantum dimer phases or bond-frustration.

Topological insulators have recently been proposed as a possible route to fault-tolerant quantum computing and related spin-orbit Dirac materials can potentially provide low-power (energy-saving) spin currents for electronic applications. On the other hand, doped Mott insulators on triangular lattices not only exhibit exotic superconductivity, spin-liquid behavior but also feature high thermopower figure of merit for applications. Currently, there is no obvious application of spin-liquids but some spin-liquids may exhibit “exotic topological order (related to ground-state degeneracy)” and fractionalization which is of much interest to me.

My recent research work has focused in the following areas:

Quantum Spin-Textures, Topological Insulators, Topological Theta-vacuum and Quantum Spin Hall effect: Experimental methods and direct imaging/determination of topological order character of the topological Theta-vacuum, axion-fields and spin Hall phases. Experimental realizations of Quantum Hall effect without Landau levels. Ternary and binary alloys of bismuth and related compounds. Doping of a topological Hall state. Quantum Hall-like effect without external magnetic fields. (Nature 2008, Nature Phys N&V 2008, Nature 2009a, Nature 2009b, Nature N&V 2009, Science09, Science N&V 09), Nature08, Nature09a, Nature09b, PRL09, Science09, PhysicsToday09

Non-cosmological (laboratory-based) realization of Hawking Radiation and related physics: Synchrotron and advanced electronic system integration.

Berry's Phases and spin-helical-Dirac Fermions for Quantum Information: Dirac physics in non-Graphene systems (graphene has a vanishingly small spin-orbit coupling). Domain wall Fermions, Chiral fermions, Parity anomaly without Fermion doubling. Spontaneous Rashba effects etc. Doping of a Dirac spectrum. Material-Physics matrix for topological (fault-tolerant) Quantum Computing. Topological Spin-textures (Science 2009, Nature Physics 2009, Preprints 2009). Science09, Nature-Physics09, SSRL-Science, BerkeleyLabScience, Spin-resolved ARPES/Topo Insulator Demonstration

Electrons on Frustrated Lattices and Novel Superconductors with Exotic Order-Paramerers: Fermiology and quasiparticle dynamics, Collective charge and spin excitations in strongly interacting quantum electron systems. Mott phenomena, metal-insulator transition, charge-order, superconductivity, high thermopower, spin-dependent thermoelectricity, Order-by-Frustration, quantum zero modes. Doped cobaltates, chromates and related compounds. (Phys.Rev.Lett.s 2006a,b,c, preprints 2009). BerkeleyLabScience

Novel routes/mechanism to High T_c Superconductivity: Momentum-dependence of superconducting gap, order-parameter, Fermiology and quasiparticle dynamics, Collective charge and spin excitations in FeAs pnictides. SDW- Superconductor competition, co-existence, Pairing mechanism and quantum magnetism. Quasiparticle scattering with collective modes. Order-by-Frustration, quantum zero modes. Iron pnictides, iron-chalcogenides and related high Tc superconductors. (Phys.Rev.Lett./Bs 2008, 2009). Momentum-dependence of Superconducting Gap in Pnictides 2008, PRL-Perspective: Physics 2009

Competition/Co-existence of Superconductivity and CDW/SDW : Non-nested CDWs, Commensurate CDWs in two dimensions, Excitonic CDW as a competing order to superconductivity, spin-dependent thermoelectricity, Kohn-Overhauser phases, Charge-order and superconductivity: Doped cobaltates, Titanate TMDs and related compounds. (Phys.Rev.Lett.s 2007a,b,c, preprints 2009). Charge-Density-Wave Melts to a Superconductor

Electron Fractionalization and Collective charge excitations coupled to X-rays: Direct detection of spinless collective charge modes in 1D spin-1/2 Mott insulator via full Brillouin zone imaging in inelastic resonant x-ray scattering demonstrated (Phys.Rev.Lett. 2002, IJMPB 2003, preprints 2009). Direct signature of Holons (Electron fractionalization/collective mode) 

Collective Charge Modes in doped Mott insulators via high-resolution X-ray spectroscopy: X-ray Imaging techniques. Development of high resolution bulk-sensitive momentum-resolved X-ray techniques to probe collective charge excitation modes in doped Mott insulators, Cuprates near AFM/SC transition. Full Brillouin zone imaging in inelastic resonant x-ray scattering demonstrated. Electron Fractionalization and Collective excitations: Direct detection of spinless collective charge modes in 1D spin-1/2 Mott insulator via full Brillouin zone imaging in inelastic resonant x-ray scattering demonstrated (Science 2000, Phys.Rev.Lett./Bs 2002-2008, preprints 2009). Mott physics via momentum-resolved Inelastic resonant X-ray scattering demonstrated

Novel X-ray spectroscopic method development: Designs of coherent light based spectroscopic techniques to probe fundamental issues in condensed-matter physics. Measurement of electron-orbit quantization in magnetic fields. (2007-).

Advanced scattering probes (Synchrotron X-ray photons, electrons, neutrons) are used to study order and excitations of correlated electrons in various condensed matter systems. Scattering probes allow one to measure various orders of correlation functions and order parameters and reveal the quantum numbers (energy, momentum or spin) of electrons in crystals which describes the phase (Fermi surface topology, quasiparticle self-energy etc.) or some collective excitations such as magnons, phonons, plasmons or holons/solitons over the entire Brillouin zones (allowing to classify the broken-symmetry phases). Precise experimental measurements of dispersion relations (E vs. k or Q) of these elementary quantum and collective excitation modes provide fundamental insights about the microscopic physics of the complex systems. We use three principal classes of techniques:

• Inelastic, Elastic, Resonant X-ray Scattering (at ALS-LBNL, APS, SSRL, LCLS(future))
• Angle-and Spin-Resolved, UV and X-ray Photoemission (at ALS, SSRL-SLAC, SLS and SRC)
• Neutron Scattering with strong magnetic fields (at NIST, ISIS-Oxford)

Experiments are performed at national and international laboratories (ALS-LBNL, SSRL/SLAC, Argonne, Brookhaven, ESRF/France, NIST, Spring8/Japan, ISIS/Oxford), as well as at the Joseph Henry Labs at Princeton. We are currently developing two novel high resolution (~10-100 meV) state-of-the-art synchrotron X-ray scattering spectrometers - one to work around 1 KeV and another around 100 eV at the Advanced Light Source of LBL (Co-leading the development of MERLIN soft X-ray synchrotron beamline (at Adv. Light Source). We are also scientific members of scattering consortia at APS/ANL, ALS-LBNL, SSRL-SLAC and LCLS.

Research Opportunities in the Hasan Lab: Research opportunities exist for highly motivated graduate and undergraduate students. Interested students are encouraged to contact me (mzhasan@princeton.edu)

Topological Insulators, Quantum Spin-textures, Topological Berry's phases, Quantum Spin Hall effect and Novel routes to Quantum Computing:

• First observation of spin-helical Dirac fermions and topological insulator phases in doped and undoped Bi₂Te₃ demonstrated by spin-ARPES spectroscopy;  D. Hsieh, Y. Xia, D. Qian, L. Wray, J. H. Dil, F. Meier, L. Patthey, J. Osterwalder, A.V. Fedorov, H. Lin, A. Bansil, D. Grauer, Y.S. Hor, R.J. Cava, M.Z. Hasan; http://aps.arxiv.org/abs/0904.1260 
• Topological Insulators: The Next Generation Nature Physics (2009) 
• Helical Dirac Fermions: A tunable topological insulator in the helical Dirac fermion topological transport regime (spin-ARPES evidence of direct detection of  topological order) D. Hsieh, Y. Xia, D. Qian, L. Wray, J. H. Dil, F. Meier, L. Patthey, J. Osterwalder, A.V. Fedorov, H. Lin, A. Bansil, D. Grauer, Y.S. Hor, R.J. Cava, M.Z. Hasan
  Nature 460, 1101 (2009).  
  An Insulator's Metallic side (Topological Insulator):
  Nature 460, 1090 (2009).  
  Light-like Spin-Textured Fermions and Quantum Hall-like Effect (2009) 
• Axions, Spin-Textures & Topo Insulators Physics Today (2009) 
• Absence of Backscattering and Topological Protection: Topological surface states protected from backscattering by Chiral Spin-Textures  (STM+spin-ARPES); P. Roushan, J. Seo, C.V. Parker, Y.S.Hor, D. Hsieh, A. Richardella, D. Qian, M.Z. Hasan, R.J. Cava, A. Yazdani (STM+spin-ARPES)
  Nature 460, 1106 (2009).  
  Nature (Perspective) 2009
• Observation of Unconventional Quantum Spin Textures in a Topological Insulator : Probing the "spin" degrees of freedom in a quantum spin Hall system, D. Hsieh, Y. Xia, L. Wray, D. Qian, A. Pal, J. H. Dil, F. Meier, J. Osterwalder, G. Bihlmayer, C. L. Kane, Y. S. Hor, R. J. Cava and M. Z. Hasan
  Science 323, 919 (2009).
  Fast Electrons Tie Quantum Knots
  Science (Perspectives), J. Zaanen
  Quantum Twist, P.W. Anderson  
  Topological Spin-textures in Momentum-space : Spin-resolved-ARPES
• Observation of time-reversal-protected single-Dirac-cone topological-insulator states in Bi₂Te₃ and Sb₂Te₃; Y. Xia, L. Wray, D. Qian, D. Hsieh, H. Lin, A. Bansil, D. Grauer, Y. Hor, R. J. Cava, M. Z. Hasan
  Physical Review Letters 103, 146401 (2009).   
• Observation of a large-gap topological-insulator class with a single surface Dirac cone; Y. Xia, L. Wray, D. Qian, D. Hsieh, H. Lin, A. Bansil, D. Grauer, Y. Hor, R. J. Cava, M. Z. Hasan
  Nature Physics  5, 398-402 (2009). 
  Topological Insulators: The Next Generation, J.E. Moore 
  Nature Physics 5, 378-380 (2009). 
• A Topological Dirac insulator in a Quantum Spin Hall Phase,  D. Hsieh, D. Qian, L. Wray, Y. Xia, Y. Hor, R.J. Cava and M.Z. Hasan
  Nature 452, 970 (2008).
  An insulator with a twist : Topological Insulator, C.L. Kane
  Nature Physics 4, 348 (2008)
  Topological States of Quantum Matter, S.C. Zhang
  Physics 1, 6 (2008)
  High-energy physics in a new guise (Axion Electrodynamics), M. Franz
  Physics 1, 36 (2008) 
  Observation of a New Phase of Matter : Quantum Hall-like effects w/o Magnetic Field

Novel High T_c Pnictide superconductors : Pairing Mechanism and Quantum Magnetism: 

• Momentum dependence of Superconducting Gap, strong-coupling dispersion Kink, and tightly bound Cooper pairs in the high-T_c (Sr,Ba){1-x}(K,Na)xFe₂As₂ superconductors, L. Wray, D. Qian, D. Hsieh, Y. Xia et.al.,
  Physical Review B 78, 184508 (2008) [Editor's Highlight]
• Determination of electronic groundstate of magnetically ordered parent iron pnictides, D. Hsieh, Y. Xia, L. Wray, D. Qian, G. F. Chen, J. L. Luo, N. L. Wang, M. Z. Hasan 
  Nature, (in review) http://aps.arxiv.org/abs/0812.2289 (2008).
• Fermi Surface Topology and Low-Lying Quasiparticle Dynamics of Parent Fe _{1+ x} Te / Se Superconductor, Y. Xia, D. Qian, L. Wray, D. Hsieh, G. F. Chen, J. L. Luo, N. L. Wang, M. Z. Hasan 
  Physical Review Letters, 103, 037002 (2009).
  Not all iron superconductors are the same: A.V. Balatsky and D. Parker 
  Physics 2, 59 (2009)

Physics of Competing Order : Charge-order and Superconductivity:

• Emergence of Fermi Pockets in a New Excitonic CDW Melted Superconductor Cu_xTiSe_{2 ,} D. Qian, D. Hsieh, L. Wray, Y. Xia, N.L. Wang, E. Morosan, R.J. Cava and M.Z. Hasan
  Physical Review Letters 98, 117007 (2007).
• Evidence for an Overhauser phase –a semimetal-to-semimetal CDW transition in the parent compound of Cu_xTiSe_2, G. Li, W. Hu, D. Qian, D. Hsieh, M.Z. Hasan, E. Morosan, R.J. Cava, N.L. Wang
  Physical Review Letters 99, 027404 (2007).
• Quasiparticle’s quantum coherence and dynamics in the vicinity of metal-insulator phase transition in Na_xCoO₂ D. Qian, L. Wray, D. Hsieh, A. Kuprin, A. Fedorov, D. Wu, J. L. Luo, N.L. Wang, L. Viciu, R.J. Cava and M.Z. Hasan
  Physical Review Letters 96, 046407 (2006).

Correlated electrons on triangular lattices: Quantum Charge Frustration

• Complete d-Band dispersion relation and small Fermion scale in Na_xCoO_2, D. Qian, L. Wray, D. Hsieh, L. Viciu, R.J. Cava, J.L. Luo, D. Wu, N.L. Wang, and M.Z. Hasan
  Physical Review Letters 97, 186405 (2006).
• Low-lying quasiparticle modes and hidden collective charge instabilities in parent cobaltates superconductors Na_xCoO_2, D. Qian, D. Hsieh, L. Wray, Y.-D. Chuang, A. Fedorov, D. Wu, J.L. Luo, N.L. Wang, L. Viciu, R.J. Cava and M.Z. Hasan
  Physical Review Letters 96, 216405 (2006).
• Fermi surface topology and quasiparticle dynamics of host Na_xCoO₂ investigated by ARPES, M. Z. Hasan, Y.-D. Chuang, D. Qian, Y.W. Li, Y. Kong, A. Kuprin, A.V. Fedorov, R. Kimmerling, E. Rotenberg, K. Rossnagel, Z. Hussain, H. Koh, N.S. Rogado, M.L. Foo, and R. J. Cava
  Physical Review Letters 92, 246402 (2004).

Charge Collective Modes and Momentum-resolved Resonant X-ray Scattering:

• Momentum-resolved Charge Modes (Holons) in a Prototype 1-D Mott Insulator Studied by Inelastic Resonant X-ray Scattering, M.Z. Hasan, P.A. Montano, E.D. Isaacs, Z.X. Shen, S. Sinha, Z. Islam, H. Eisaki, N. Motoyama and S. Uchida Physical Review Letters 88, 177403 (2002).
• Electronic Structure of Mott Insulators Studied by Inelastic X-ray Scattering,M.Z. Hasan, E.D. Isaacs, Z.X. Shen, L.L. Miller, K. Tsutsui, T. Tohyama and S. Maekawa Science 288, 1811 (2000).
• X-ray imaging of dispersive charge modes in a doped Mott insulator near the antiferromagnet/superconductor transition, Y.W. Li, L. Wray, D. Qian, D. Hsieh, Y. Xia, H. Eisaki, et.al., Physical Review B 78, 073104 (2008). 

• Instrumentation: Beamline & Spectrometer Development, R. Reininger, Y,-D. Chuang, Z. Hussain et al., MERLIN soft X-ray beamline : A meV Resolution Beamline at the ALS (2008).

• Random-coil behavior and the dimensions of chemically unfolded proteins: A view with X-ray forward scattering , J.E. Kohn, I.S. Millett, J. Jacob, et al.
  Proc. of the Nat. Acad. of Sci, 101, 12491 (2004).
• Images of biological soft tissue using synchrotron X-ray and laser CT systems
  Radiation Measurements 41, 177 (2006);
  Nuc. Instrum. and Methods : Beam Interact. with Mat. 239, 209 (2005). 

David Hsieh (PhD 2009, Pappalardo Fellowship Award, MIT)

L. Andrew Wray

Yu-Qi Xia

Suyang Xu

Jun Xiong

Ram Shanker

Dong Qian (Postdoc)

Yinwan Li (Postdoc)

Liang Fu  (Visitor, Harvard University)

U.S. DOE Physicists at Work