Papers by Md. Zahid Hasan 182-33-722
Physical Review Letters, 2006
Layered cobaltates embody novel realizations of correlated matter on a spin-1=2 triangular lattic... more Layered cobaltates embody novel realizations of correlated matter on a spin-1=2 triangular lattice. We report a high-resolution systematic photoemission study of the insulating cobaltates. The observation of a single-particle gap opening and band folding provides direct evidence of anisotropic particle-hole instability on the Fermi surface due to its unique topology. Overlap of the measured Fermi surface is observed with the 3 p 3 p charge-order Brillouin zone near x 1=3 but not at x 1=2 where the insulating transition is actually observed. Unlike conventional density waves, charge stripes, or band insulators, the onset of the gap depends on the quasiparticle's quantum coherence which is found to occur well below the disorder-order symmetry breaking temperature of the crystal (the first known example of its kind).
Proceedings of the National Academy of Sciences of the United States of America, Jan 19, 2016
Weyl semimetals have attracted worldwide attention due to their wide range of exotic properties p... more Weyl semimetals have attracted worldwide attention due to their wide range of exotic properties predicted in theories. The experimental realization had remained elusive for a long time despite much effort. Very recently, the first Weyl semimetal has been discovered in an inversion-breaking, stoichiometric solid TaAs. So far, the TaAs class remains the only Weyl semimetal available in real materials. To facilitate the transition of Weyl semimetals from the realm of purely theoretical interest to the realm of experimental studies and device applications, it is of crucial importance to identify other robust candidates that are experimentally feasible to be realized. In this paper, we propose such a Weyl semimetal candidate in an inversion-breaking, stoichiometric compound strontium silicide, SrSi2, with many new and novel properties that are distinct from TaAs. We show that SrSi2 is a Weyl semimetal even without spin-orbit coupling and that, after the inclusion of spin-orbit coupling, ...
Physical Review B, 2015
We discuss the evolution of topological states and their orbital textures in the mixed valence co... more We discuss the evolution of topological states and their orbital textures in the mixed valence compounds SmB6 and YbB6 within the framework of the generalized gradient approximation plus onsite Coulomb interaction (GGA+U) scheme for a wide range of values of U. In SmB6, the topological Kondo insulator (TKI) gap is found to be insensitive to the value of U , but in sharp contrast, Kondo physics in isostructural YbB6 displays a surprising sensitivity to U. In particular, as U is increased in YbB6, the correlated TKI state in the weak-coupling regime transforms into a d − p-type topological insulator phase with a band inversion between Yb-5d and B-2p orbitals in the intermediate coupling range, without closing the insulating energy gap throughout this process. Our theoretical predictions related to the TKI and non-TKI phases in SmB6 and YbB6 are in substantial accord with recent angle-resolved photoemission spectroscopy (ARPES) experiments.
Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due... more Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due to the possibility of being a topological superconductor. Here, we report a systematic high-resolution angle resolved photoemission spectroscopy (ARPES) study of the normal state electronic properties of BiPd. Our experimental results show the presence of a surface state at higher binding energy with the location of Dirac point at around 700 meV below the Fermi level. The detailed photon energy and temperature dependent measurements complemented by our first-principles calculations provide further evidence for the presence of the topological surface state at high binding energy. The absence of topological surface states near the Fermi level negates the possibility of the topological superconducting behavior in the surface of this material. Our first direct experimental discovery of a topological surface state in BiPd provides novel information that will guide the future search for topol...
Physical Review B, 2014
We probe the near Fermi level electronic structure of tunable topological insulator (Bi2Se3)cupra... more We probe the near Fermi level electronic structure of tunable topological insulator (Bi2Se3)cuprate superconductor Bi2Sr2CaCu2O 8+δ (Tc ≃ 91 K) heterostructures in their proximity induced superconductivity regime. In contrast to previous studies, our careful momentum space imaging provides clear evidence for a two-phase coexistence and a lack of d-like proximity effect. Our Fermi surface imaging data identifies major contributors in reducing the proximity-induced gap below the 5 meV range. These results correlate with our observation of momentum space separation between the Bi2Se3 and Bi2Sr2CaCu2O 8+δ Fermi surfaces and mismatch of crystalline symmetries in the presence of a small superconducting coherence length. These studies not only provide critical momentum space insights into the Bi2Se3/Bi2Sr2CaCu2O 8+δ heterostructures, but also set an upper bound on the proximity induced gap for realizing much sought out Majorana fermion condition in this system.
Compared with the semiconductors such as silicon and gallium arsenide which have been used widely... more Compared with the semiconductors such as silicon and gallium arsenide which have been used widely for decades, semimetals have not received much attention in the field of condensed matter physics until very recently. The realization of electronic topological properties has motivated interest of investigations on Dirac semimetals and Weyl semimetals, which are predicted to show unprecedented features beyond the classical electronic theories of metals [1-8]. In this letter for the first time we report the electric transport properties of a robust Weyl semimetal candidate proposed by recent theoretical calculations [9, 10], TaAs. Our study shows that this bulk material manifests ultrahigh carrier mobility (5 × 10 5 cm 2 /V • s) accompanied by an extremely large, unsaturated linear magnetoresistance (MR), which reaches 5400 at 10 Kelvins in a magnetic field of 9 Teslas and 2.47×10 4 at 1.5 Kelvins in a magnetic field of 56 Teslas. We also observed strong Shubnikov-de Haas (SdH) oscillations associated with an extremely low quantum limit (∼8 Teslas). Further studies on TaAs, especially in the ultraquantum limit regime, will help to extend the realization of the topological properties of these exotic electrons.
In this Book Chapter we briefly review the basic concepts defining topological insulators and ela... more In this Book Chapter we briefly review the basic concepts defining topological insulators and elaborate on the key experimental results that revealed and established their symmetry protected (SPT) topological nature. We then present key experimental results that demonstrate magnetism, Kondo insulation, mirror chirality or topological crystalline order and superconductivity in spin-orbit topological insulator settings and how these new phases of matter arise through topological quantum phase transitions from Bloch band insulators via Dirac semimetals at the critical point.
Physics, 2009
The energy-momentum relationship of electrons on the surface of an ideal topological insulator fo... more The energy-momentum relationship of electrons on the surface of an ideal topological insulator forms a cone-a Dirac cone, which, when warped (no longer described by the Dirac equation), can lead to unusual phenomena such as enhanced electronic interference around defects and a magnetically ordered broken symmetry surface. A detailed spin-texture and hexagonal warping maps on Bi2Te3 are presented here.
Science, 2009
A topologically ordered material is characterized by a rare quantum organization of electrons tha... more A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetry–based classification of condensed matter. Exotic spin-transport phenomena, such as the dissipationless quantum spin Hall effect, have been speculated to originate from a topological order whose identification requires a spin-sensitive measurement, which does not exist to this date in any system. Using Mott polarimetry, we probed the spin degrees of freedom and demonstrated that topological quantum numbers are completely determined from spin texture–imaging measurements. Applying this method to Sb and Bi 1–x Sb x , we identified the origin of its topological order and unusual chiral properties. These results taken together constitute the first observation of surface electrons collectively carrying a topological quantum Berry's phase and definite spin chirality, which are the key electronic properties component for realizing topol...
Science, 2011
Two types of bulk insulator are realized in the same family of compounds through chemical doping.
Physical Review Letters, 2012
By combining transport and photo emission measurements on (Bi 1-x In x) 2 Se 3 thin films, we rep... more By combining transport and photo emission measurements on (Bi 1-x In x) 2 Se 3 thin films, we report that this system transforms from a topologically non-trivial metal into a topologically trivial band insulator through three quantum phase transitions. At x ≈ 3-7%, there is a transition from a topologically non-trivial metal to a trivial metal. At x ≈ 15%, the metal becomes a variable-range-hopping insulator. Finally, above x ≈ 25%, the system becomes a true band insulator with its resistance immeasurably large even at room temperature. This material provides a new venue to investigate topologically tunable physics and devices with seamless gating/tunneling insulators.
Physical Review Letters, 2006
We report a state-of-the-art photoemission (ARPES) study of high quality single crystals of the N... more We report a state-of-the-art photoemission (ARPES) study of high quality single crystals of the NaxCoO2 series focusing on the fine details of the low-energy states. The Fermi velocity is found to be small (< 0.5 eV.Å) and only weakly anisotropic over the Fermi surface at all dopings setting the size of the pair wavefunction to be on the order of 10-20 nanometers. In the low doping regime the exchange inter-layer splitting vanishes and two dimensional collective instabilities such as 120 •-type fluctuations become kinematically allowed. Our results suggest that the unusually small Fermi velocity and the unique symmetry of kinematic instabilities distinguish cobaltates from other unconventional oxide superconductors such as the cuprates or the ruthenates.
Physical Review Letters, 2002
We report momentum resolved charge excitations in a one dimensional (1-D) Mott insulator studied ... more We report momentum resolved charge excitations in a one dimensional (1-D) Mott insulator studied using high resolution (∼ 325 meV) inelastic x-ray scattering over the entire Brillouin zone for the first time. Excitations at the insulating gap edge are found to be highly dispersive (momentum dependent)compared to excitations observed in two dimensional Mott insulators. The observed dispersion in 1-D is consistent with charge excitations involving holons which is unique to spin-1/2 quantum chain systems. These results point to the potential utility of inelastic x-ray scattering in providing valuable information about electronic structure of strongly correlated insulators.
Physical Review Letters, 2009
We show that the strongly spin-orbit coupled materials Bi 2 Te 3 and Sb 2 Te 3 and their derivati... more We show that the strongly spin-orbit coupled materials Bi 2 Te 3 and Sb 2 Te 3 and their derivatives belong to the Z 2 topological-insulator class. Using a combination of first-principles theoretical calculations and photoemission spectroscopy, we directly show that Bi 2 Te 3 is a large spin-orbit-induced indirect bulk band gap ($ 150 meV) semiconductor whose surface is characterized by a single topological spin-Dirac cone. The electronic structure of self-doped Sb 2 Te 3 exhibits similar Z 2 topological properties. We demonstrate that the dynamics of spin-Dirac fermions can be controlled through systematic Mn doping, making these materials classes potentially suitable for topological device applications.
Physical Review Letters, 2010
We have investigated several strong spin-orbit coupling ternary chalcogenides related to the (Pb,... more We have investigated several strong spin-orbit coupling ternary chalcogenides related to the (Pb,Sn)Te series of compounds. Our first-principles calculations predict the low temperature rhombohedral ordered phase in TlBiTe2, TlBiSe2, and TlSbX2 (X=Te, Se, S) to be topologically Z2 =-1 nontrivial. We identify the specific surface termination that realizes the single Dirac cone through first-principles surface state computations. This termination minimizes effects of dangling bonds making it favorable for photoemission (ARPES) experiments. Our analysis predicts that thin films of these materials would harbor novel 2D quantum spin Hall states, and support odd-parity topological superconductivity. For a related work also see arXiv:1003.2615v1. Experimental ARPES results will be published elsewhere.
Physical Review B, 2008
By incorporating a long-range Coulomb interaction into the framework of the one-band Hubbard mode... more By incorporating a long-range Coulomb interaction into the framework of the one-band Hubbard model, we delineate how the low-energy plasmon around 1 eV, which is a universal feature of the charge dynamics of the cuprates, manifests itself in the resonant inelastic x-ray scattering ͑RIXS͒ spectra. The long-range Coulomb interaction in the doped system controls the form of the intraband RIXS dispersion near the Brillouin zone center around the ⌫ point. The out-of-plane momentum transfer component q z is found to play a key role in determining whether or not the RIXS spectrum shows a plasmon-related gap at ⌫.
Physical Review B, 2011
We have investigated the nature of surface states in the Bi2Te3 family of three-dimensional topol... more We have investigated the nature of surface states in the Bi2Te3 family of three-dimensional topological insulators using first-principles calculations as well as model Hamiltonians. When the surface Dirac cone is warped due to Dresselhaus spin-orbit coupling in rhombohedral structures, the spin acquires a finite out-of-plane component. We predict a novel in-plane spin-texture of the warped surface Dirac cone with spins not perpendicular to the electron momentum. Our k • p model calculation reveals that this novel in-plane spin-texture requires high order Dresselhaus spin-orbit coupling terms.
New Journal of Physics, 2011
We present first-principles calculations to predict several three dimensional (3D) topological in... more We present first-principles calculations to predict several three dimensional (3D) topological insulators in quaternary chalcogenide compounds which are made of I2-II-IV-VI4 compositions and in ternary compositions of I3-V-VI4 famatinite compounds. Among the large members of these two families, we give examples of naturally occurring compounds which are mainly Cu-based chalcogenides. We show that these materials are candidates of 3D topological insulators or can be tuned to obtain topological phase transition by manipulating the atomic number of the other cation and anion elements. A band inversion can occur at a single point Γ with considerably large inversion strength, in addition to the opening of a bulk band gap throughout the Brillouin zone. We also demonstrate that both of these families are related to each other by cross-substitutions of cations in the underlying tetragonal structure and that one can suitably tune their topological properties in a desired manner.
Nature Physics, 2009
Recent experiments and theories have suggested that strong spin-orbit coupling effects in certain... more Recent experiments and theories have suggested that strong spin-orbit coupling effects in certain band insulators can give rise to a new phase of quantum matter, the socalled topological insulator, which can show macroscopic quantum-entanglement effects 1-7 . Such systems feature twodimensional surface states whose electrodynamic properties are described not by the conventional Maxwell equations but rather by an attached axion field, originally proposed to describe interacting quarks 8-15 . It has been proposed that a topological insulator 2 with a single Dirac cone interfaced with a superconductor can form the most elementary unit for performing fault-tolerant quantum computation 14 . Here we present an angle-resolved photoemission spectroscopy study that reveals the first observation of such a topological state of matter featuring a single surface Dirac cone realized in the naturally occurring Bi 2 Se 3 class of materials. Our results, supported by our theoretical calculations, demonstrate that undoped Bi 2 Se 3 can serve as the parent matrix compound for the long-sought topological device where in-plane carrier transport would have a purely quantum topological origin. Our study further suggests that the undoped compound reached via n-to-p doping should show topological transport phenomena even at room temperature. It has been experimentally shown that spin-orbit coupling can lead to new phases of quantum matter with highly nontrivial collective quantum effects . Two such phases are the quantum spin Hall insulator 4 and the strong topological insulator , both realized in the vicinity of a Dirac point but yet quite distinct from graphene . The strong-topological-insulator phase contains surface states (SSs) with novel electromagnetic properties . It is currently believed that the Bi 1-x Sb x insulating alloys realize the only known topological-insulator phase in the vicinity of a three-dimensional Dirac point 5 , which can in principle be used to study topological electromagnetic and interface superconducting properties . However, a particular challenge for the topological-insulator Bi 1-x Sb x system is that the bulk gap is small and the material contains alloying disorder, which makes it difficult to gate for the manipulation and control of charge carriers to realize a device. The topological insulator Bi 1-x Sb x features five surface bands, of which only one carries the topological quantum number 6 . Therefore, there is an extensive world-wide search for topological phases in stoichiometric materials with no alloying disorder, with a larger gap and with fewer yet still odd-numbered SSs that may
Nature Materials, 2010
Topological insulators realize a novel state of quantum matter that are distinguished by topologi... more Topological insulators realize a novel state of quantum matter that are distinguished by topological invariants of bulk band structure rather than spontaneously broken symmetries. A number of exotic quantum phenomena have been predicted to exist in multiply-connected geometries which require an enormous amount of materials flexibility. We have extended our previous search for TI materials from binary (Bi2X3 series, Xia et.al., Nature Phys. 5, 398 (2009)) to the thermoelectric ternary compounds. We discover that the distorted LuPtSb is the first ternary compound of the "M M ′ X" series harboring a 3D topological insulator state with Z2=-1 whereas TiNiSn is trivial. We also show that the half-Heusler LuPtSb-type series is a natural platform that hosts a range of candidate compounds, alloys and artificial heterostructures (quantum-wells). We also discovered several different paradigms of trivial and non-trivial topological ordering in this class, including an intrinsically metallic nontrivial topological state in YAuPb. Some of these materials are grown (results will be reported separately).
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Papers by Md. Zahid Hasan 182-33-722