Physics Department, Princeton University

I am interested in the fundamental physics of exotic quantum matter, interacting quantum electron systems (correlated matter) and quantum nature of emergent phenomena in these 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 frontiers and new discoveries of fundamental physics such as the quantum Hall-like effect without external magnetic field, understanding non-BCS superconductivity in correlated materials and fractionalized phases in higher dimensions as well as the development of innovative/advanced instrumentation capability necessary to address these issues. I am interested in the state-of-the-art storage-ring Synchrotrons and next-generation linear-Accelerator based X-ray FELs (ultra-fast, time-resolved phenomena) for their potential use in fundamental physics and biophysics (see sample publications below). At Princeton, I am also affiliated with the institute of technology PRISM and the engineering physics program.

Traditionally spectroscopic methods have been used to characterize electronic or spin behavior in quantum 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 quantum matter. I am currently interested in Quantum spin-textures & Berry phases of matter, Topological Insulator, topological Hall phases, Topological-Superconductors, 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 Superconductors) and spin-liquid-like behavior in frustrated magnets (RVB-type fractionalized phases) in search of direct/unambiguous signatures of electron fractionalization or new fractional phases in higher dimensions.

Topological Surface States : Topological Insulators and Superconductors 
 

Topological Matter  
 

Topological Surface States : Non-quantum-Hall-like topological matter (new type of 2DEG)
(Physics World 2011).

A new experimental approach for studying topological quantum phenomena

Three dimensional topological insulators (3D-TI) are the first example of topological order in the bulk solids (there is no genuine quantum Hall effect in three dimensions). They feature a "protected" metallic Dirac-like surface state (2DEG or planar "topological metal") where electron’s spin and momentum are locked to each other and possess half the degrees of freedom present in an ordinary electron Fermi gas. Strong spin-orbit coupling leads to an insulating bulk and the surface states are protected by time reversal symmetry and belong to the Z2 class (see theory by Fu-Kane'07, Moore-Balents'07; Kane-Mele'05; precursor theory of TR-breaking 2D topological insulator by Haldane in 1988). In experiments, 3D Topological Insulators are an example of non-quantum-Hall-like topological matter experimentally discovered and reported (2007) around the same time, in parallel, as the spin Hall edge-states in Hg(Cd)Te, in 2007. Both the spin quantum Hall effect (Wurzburg team, Science 2007) and 3D Topological Insulator Surface States (my team, Nature 452, 970 (2008), submitted in 2007,  "Search&Discovery" Physics Today, KITP 2007) were reported the same year 2007 (a few months apart) using two independent and unrelated experimental methods. Experimentally, these two are unrelated. The spin quantum Hall effect (QSHE) can be thought of as two copies of well-known IQH (integer quantum Hall) states put together in two dimensions. Since IQH state is a 2D topological insulator, spin quantum Hall effect is also a 2D topological insulator but time-reversal invariant (protected by Z2 invariant). On the other hand, the 3D Topological Insulators are a new and distinct state of matter which cannot be reduced to multiple copies of IQH and there is no spin Hall effect in 3D (the term "Topological Insulators" was originally used exclusively for the novel and unprecendented 3D state since there is no spin Hall like effect there). The 3D state is thus an example of  non-quantum-Hall-like topological matter and the first realization of topologically ordered bulk solid in nature (Physics World 2011). Experimentally, 3D TI did not arise from quantum spin Hall effect. Additionally, transport measurements can not provide a proof of Z2 topology either.

All of the 2D topological insulator examples (IQH, FQH, QSH) including the fractional one (FQH) involving Coulomb interaction are understood in the standard picture of quantized electron orbits in a spin-independent or spin-dependent magnetic field, the 3D topological insulator defies such description and is a novel type of topological order which cannot be reduced to multiple copies of quantum-Hall-like states. In fact, the 3D topological insulator exists not only in zero magnetic field, they also differ from the 2D variety in three very important aspects:

1) they possess topologically protected 2D metallic surfaces (a new type of 2DEG) rather than the 1D edges, 2) they can work at room temperature (300K and beyond, largegap topological insulators) rather than cryogenic (mK) temperatures required for the QSH effects and,3) they occur in standard bulk semiconductors rather than at buried interfaces of ultraclean semiconductor heterostructures thus tolerate stronger disorder than the IQH-like states.

The non-quantum-Hall-like (novel) character and the extremely rich physics (surface 2DEG, helical fermion gas) of the 3D state has led to a world-wide research interest in this topic in general. Also the novel experimental approach in studying topological quantum phenomena demonstrated by us are now being used by many other groups world-wide studying many other topological materials and phenomena.

Why Topological Surface States (TIs) are so exciting?

3DTI is new and unprecendented and cannot be reduced to multiple copies of quantum Hall or spin Hall like states. Most topological states of matter are realized in two or lower dimensions (quantum Hall states, quantum spin Hall effect, non-Fermi liquid chains and wires, quantum spin-liquids etc.). Unlike all others, neither strong electron-electron interactions (necessary for quantum spin liquids), high magnetic fields and low temperatures (necessary for quantum Hall states), nor low dimensionality (needed for quantum Hall states, spin quantum Hall states (QSHE), and non-Fermi liquid spin chains) are needed for the 3D topological insulator. The theoretical and experimental discovery of the 3D TIs – the first example of topological order in bulk solids - has generated much experimental and theoretical efforts to understand and utilize all aspects of these quantum phenomena and the materials that exhibit them.

One of the major challenges in going from quantum Hall-like 2D states to 3D topological insulators is to develop new experimental approaches/methods to precisely probe this novel form of topological-order since the standard tools and settings that work for IQH-state also work for QSH states. The method to probe 2D topological-order is exclusively with charge transport (pioneered by Von Klitzing in the 1980s), which either measures quantized transverse conductance plateaus in IQH systems or longitudinal conductance in quantum spin Hall (QSH) systems. In a 3D topological insulator, the boundary itself supports a two dimensional electron gas (2DEG) and transport is not (Z2) topologically quantized hence cannot directly probe the topological invariants νo or the topological quantum numbers analogous to the Chern numbers of the IQH systems. This is unrelated to the fact that the present materials have some extrinsic or residual/impurity conductivity in their naturally grown bulk. In this paper, we review the birth of momentum- and spin-resolved spectroscopy as a new experimental approach and as a directly boundary sensitive method to study and prove topological-order in three-dimensions via the direct measurements of the topological invariants νo that are associated with the Z2 topology of the spin-orbit band structure and opposite parity band inversions, which led to the experimental discovery of the first 3D topological insulator in Bi-based semiconductors.

Topological Surface States: Experimentally demonstrated, a three dimensional topological insulator (3D-TI) features a protected two-dimensional electron gas on its surface. The high magnetic fields, low temperatures or low dimensionality are not necessary for retaining the topological protection or topological order of a macroscopic 2DEG on the surface of a topological insulator.

Nature 452, 970 (2008) , submitted in 2007, KITP Proc'07, "Search&Discovery" Physics Today (2009) 

Experimental measurements of Z2 topological invariants {vo}: Transport measurements that have been the key to probe topological order in conventional quantum Hall like systems cannot (even in theory!) be used to measure the topological quantum numbers of Z2 TIs (Fu-Kane’s {vo}). We have shown a technique/method for the direct measurement of Z2 topological quantum numbers {vo} for the first time:

Science 323, 919 (2009), later further expanded: Science 332, 560 (2011) see below for details.

Topological Order is more directly manifested in the spin and momentum correlated motion of electrons on the surface. This leads to a new type of 2DEG where the electron’s spin and linear momentum are one-to-one locked. Such a 2DEG only carries half of the total degrees of freedom of a conventional 2DEG and in the vicinity of the Kramers’ point takes the form of a half Dirac gas:

Nature 460, 1101 (2009) and further details in related materials Phys. Rev. Lett. (2009)

Consequence of Z2 topological order: Spin-Momentum locking and pi-Berry's phase lead to the absence of elastic backscattering on the surfaces. By combining spin-ARPES and tunneling (collaboration), we have demonstrated such absence of elastic backscattering:

Nature 460, 1106 (2009) (STM+Spin-ARPES), Berry's phase was shown at Science 323, 919 (2009) 

Discovery of the next generation and “room temperature topological insulators”: the Bi2Se3 class: The advantages of large band gap and simple spin-polarized Dirac cone topology of these spin-orbit insulators led to our observation of topological quantum phenomena at room temperatures without magnetic fields and without high purity semiconductors

Nature Physics 5, 398 (2009) submitted in 2008 and (see above Nature 460, 1101 (2009)); N&V NatPhys (2009) 

Given that the topological insulators are standard bulk semiconductors and their topological characteristics can survive to high temperatures, their novel properties could lead to many exciting applications. An exciting progress along this line is that the Bi2Se3 class of 3D TIs can be turned in to superconductors to form the host material for Majorana Fermions (Nature Physics 6, 855 (2010)). Also both integer and fractional quantum Hall effects have been reported in this Bi2Se3 class of materials by other groups indicating the high mobility of the topological surface state. These developments have unleashed a world-wide experimental effort to understand all aspects of electrical and spin properties leading to a nearly graphene-like revolution in physics (Physics World 2011).

Magnetic Symmetry Breaking: Topological protection and degree of robustness: The Dirac cone materials are probed via the modification of surface potential: How robust the topological properties of a Topological Insulator surface are: (Nature Physics 7, 32 (2011)). This paper reported preliminary results regarding magntism, for a full detail see, Magnetic Topological Insulators : The effect of time-reversal symmetry leads to unconventional spin textures on the surface of a topological insulator. Hedgehog spin texture and Berry’s phase tuning in a magnetic topological insulator was observed in our experiments;

Nature Physics 8, 616 (2012) and additional details in Phys. Rev. B (2012) 

Also see works by other groups Phys. Rev. B (2010) [ our eariler work on Bi2Te2Se at arXiv (2010) ]

Topological Phase transition and Texture inversion: It is believed that a trivial insulator can be twisted into a topological state by modulating the spin-orbit interaction driving the system through a topological quantum phase transition. We reported the observation of such a phase transition in a tunable spin-orbit system where the topological state formation is visualized. In the topological state, vortex-like polarization states are observed to exhibit 3D vectorial textures, which collectively feature a chirality transition as the spin-momentum locked electrons on the surface go through the zero carrier density point. Such phase transition and texture inversion can be the physical basis for observing fractional charge (±e/2) and other fractional topological phenomena

Science 332, 560 (2011) and in a related system Phys. Rev. Lett. (2012)

Topological Crystalline Insulator  (TCI) Phase: Z2 topological insulator protected by time-reversal symmetry is realized via spin–orbit interaction-driven band inversion. The topological phase in the Bi_1−xSb_x system is due to an odd number of band inversions. We experimentally investigated the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb_1−xSn_xTe class of materials that has been theoretically predicted (by Fu et.al.,) to exist in its end compound SnTe. Our observation of the spin-polarized Dirac surface states in the inverted Pb_1−xSn_xTe and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.

Nature Communications 3, 1192 (2012) and Quasiparticle Interference Pattern at arXiv. (2012)  Mirror Chern number measurement method critical for the TCI-phase was shown in  Science 323, 919 (2009). 

Quantum Spin-Textures, Topological matter and Quantum spin Hall phase, Superconductivity in Topological Matter: 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, half-Heuslers and related compounds. Doping of a topological Hall state. Quantum Hall-like effect without external magnetic fields. Superconductivity in doped topological insulators.

Topological 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.

Electrons on Frustrated spin-1/2 Lattices and Novel Superconductors: 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.

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.

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 2010).

Electron Fractionalization (holons) 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 2010). 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 2010). Mott physics via momentum-resolved Inelastic resonant X-ray scattering demonstrated,Science 2000.

Novel routes to topological matter and quantum phenomena:

• Colloquium: Topological Insulators; M.Z. Hasan, C.L. Kane; Rev. Mod. Phys 82, 3045 (2010)  
• Topological Quantization in Topological Insulators; M.Z. Hasan Physics 3, 62 (2010). 
• Review: Three-Dimensional Topological Insulators; M.Z. Hasan, J.E. Moore Ann. Review. Cond. Mat. Physics 2, 55-78 (2010). 

• Topological Crystalline Insulators protected by mirror symmetry:   Observation of Topological Crystalline Insulator phase in Pb1-xSnxTe : Su-Yang Xu, C. Liu, N. Alidoust, D. Qian, M. Neupane, J. D. Denlinger, Y. J. Wang, L. Wray, R. J. Cava, H. Lin, A. Marcinkova, E. Morosan, A. Bansil, M. Z. Hasan Nature Communications  3, 1192 (2012), Preprint (2012)

• Magnetic Topological Insulators: Hedgehog spin texture and Berry’s phase tuning in a magnetic topological insulator; Su-Yang Xu, M. Neupane, C. Liu, D. Zhang, A. Richardella, L. A. Wray, N. Alidoust, M. Leandersson, T. Balasubramanian, J. Sánchez-Barriga, O. Rader, G. Landolt, B. Slomski, J.H. Dil, J. Osterwalder, T.-R. Chang, H.-T. Jeng, H. Lin, A. Bansil, N. Samarth, M. Z. Hasan  Nature Physics 8, 616 (2012).  
• Topological Phase Transition & Texture-Inversion; Topological phase transition and texture inversion in a tunable topological insulator. Xu SY, Xia Y, Wray LA, Jia S, Meier F, Dil JH, Osterwalder J, Slomski B, Bansil A, Lin H, Cava RJ, Hasan MZ.; Science 332, 560 (2011). 
• Superconducting doped Topological Insulator; Observation of unconventional band topology in a superconducting doped topological insulator, Cux-Bi2Se3: Topological Superconductor or non-Abelian superconductor? L.A. Wray, S. Xu, Y. Xia, D. Qian, H. Lin, A. Bansil, Y. Hor, R.J. Cava, M.Z. Hasan Nature Physics 6, 855 (2010); Science Editor's Choice (2010). 
• A new platform for topological quantum phenomena : Topological Insulator states in thermoelectric Heusler-related ternary compounds; H. Lin, L.A. Wray, Y. Xia, S. Jia, R.J. Cava, A. Bansil, M.Z. Hasan; Nature Materials 9, 546 (2010), NaturePhys News (2010).
• A topological insulator surface under strong Coulomb, magnetic and disorder perturbations L.A. Wray, S. Xu, Y. Xia, D. Qian, H. Lin, A. Bansil, Y. Hor, R.J. Cava, M.Z. Hasan Nature Physics (2010). NaturePhys News (2011).  
• Single-Dirac-Cone topological surface states on TlBiSe₂-class Thallium-based III-V-VI2 Ternary Chalcogenides H. Lin, R.S. Markiewicz, L.A. Wray, L. Fu, M.Z. Hasan, A. Bansil; Phys. Rev. Lett. 105, 03640 (2010).  
• A new class of Topological insulators: Single-Dirac-cone Z₂ topological insulator phases in distorted Li₂AgSb-class and related quantum critical Li-based spin-orbit compounds; H. Lin, L.A. Wray, Y. Xia, S.-Y. Xu, S. Jia, R.J. Cava, A. Bansil, M.Z. Hasan; arXiv:1004.0999v1 (2010). 
• Warping the cone on a Topological Insulator 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) 
• Spin-Textures, Topological Order & Topo Insulators Physics Today (2009) 
• Topological Insulators: The Next Generation Nature Physics (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
• Topological Spin-Textures & pi Berry's phase observation: 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 (news), Topological Spin-textures in Momentum-space : Spin-resolved-ARPES
• Spin-polarized Single-Dirac-Cone: 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).   
• Discovery of Bi₂Se₃ class possessing Topological-Order: 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) [Submitted in 2007]
  High-energy physics in a new guise, M. Franz, Physics 1, 36 (2008) 
  Observation of a New Phase of Matter : Quantum Hall-like effects w/o Magnetic Field 

Strongly Correlated (Many-Body) Electrons and Novel High T_c superconductors :  

• Fermi-surface topology and low-lying electronic structure of the iron-based superconductor Ca10(Pt3As8)(Fe2As2)5; M. Neupane, C. Liu, S.-Y. Xu, Y.-J. Wang, Ni Ni, J. M. Allred, L. A. Wray, N. Alidoust, H. Lin, R. S. Markiewicz, A. Bansil, R. J. Cava, and M. Z. Hasan  Physical Review B 85, 094510 (2012).

  Dirac cone in iron-based (pnictide) superconductors, M. Z. Hasan and B. A. Bernevig  
  Physics 3, 27 (2010). (PRL Viewpoint)

  Orbital-Textures in Pnictides: Observation of intertwined Fermi surface topology, orbital parity symmetries and electronic interactions in iron arsenide superconductors; L.A. Wray, D. Hsieh, Y. Xia, S.-Y. Xu, D. Qian, G. F. Chen, J. L. Luo, N. L. Wang, M.Z. Hasan; arXiv:0912.5089v1 (2009).  

• Superconducting Gap in Pnictides: 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]
• Spin-density-Wave (nodal state) in Pnictides: 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
• Fermi Surface Topology and Low-Lying Quasiparticle Dynamics of Parent Fe(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: 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).

Resonant X-ray Scattering and Charge Collective Modes :  

• R. Reininger Co-existence of pseudogap, charge-transfer gap, and Mott-gap energy scales in the resonant inelastic X-ray profile of electron doped cuprate superconductors  S. Basak, T. Das, H. Lin, M.Z. Hasan, R.S. Markiewicz, and A. Bansil
  Physical Review B 85, 075104 (2012). 
• 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 (Resonant 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). 
   
• R. Reininger, Y,-D. Chuang, Z. Hussain et al., MERLIN soft X-ray beamline : A meV Resolution Beamline for X-ray scattering at the ALS-Berkeley (2012)
   

Su-Yang Xu

Nasser Alidoust

Ilya Belopolski

Chang Liu