Publications (82)428.94 Total impact
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ABSTRACT: We develop a unified framework to classify topological defects in insulators and superconductors described by spatially modulated Bloch and Bogoliubov de Gennes Hamiltonians. We consider a Hamiltonian H(k,r) that varies slowly with adiabatic parameters r away from the defect. Band theories are grouped into ten classes according to the presence or absence of antiunitary symmetries, time reversal 2ˆ=±1 and/or particlehole 2ˆ=±1. Both send kk and rr. Stable classification of topological band theories are characterized by a unified set of integral formulae for all the symmetry classes in any dimensions. Examples that fall into this framework include edge and surface states along an interface, 1D chiral, helical and Majorana modes along a line defect, bound charge and Majorana zero mode at a point defect. This approach also applies to time dependent phenomena, such as the Thouless charge pumb, the Z2 spin pumb and the exchange statistics of Majorana bound states in three dimensions.03/2010; 
Article: Colloquium : Topological Insulators
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ABSTRACT: Topological insulators are electronic materials that have a bulk band gap like an ordinary insulator but have protected conducted states on their edge or surface. These states are possible due to the combination of spinorbit interactions and timereversal symmetry. The twodimensional (2D) topological insulator is a quantum spin Hall insulator, which is a close cousin of the integer quantum Hall state. A threedimensional (3D) topological insulator supports novel spinpolarized 2D Dirac fermions on its surface. In this Colloquium the theoretical foundation for topological insulators and superconductors is reviewed and recent experiments are described in which the signatures of topological insulators have been observed. Transport experiments on HgTe/CdTe quantum wells are described that demonstrate the existence of the edge states predicted for teh quantum spin hall insulator. Experiments on Bi1xSbx, Bi<2Se3, Bi2Te3 and Sb2Te3 are then discussed that establish these materials as 3D topological insulators and directly probe the topology of their surface states. Exotic states are described that can occur at the surface of a 3D topological insulator due to an induced energy gap. A magnetic gap leads to a novel quantum Hall state that gives rise to a topological magnetoelectric effect. A superconducting energy gap leads to a state that supports Majorana fermions and may provide a new venue for realizing proposals for topological quantum computation. Prospects for observing these exotic states are also discussed, as well as other potential device applications of topological insulators.Physics World 01/2010; · 0.45 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We show that three dimensional superconductors, described within a Bogoliubovde Gennes framework, can have zero energy bound states associated with pointlike topological defects. The Majorana fermions associated with these modes have nonAbelian exchange statistics, despite the fact that the braid group is trivial in three dimensions. This can occur because the defects are associated with an orientation that can undergo topologically nontrivial rotations. A feature of three dimensional systems is that there are "braidless" operations in which it is possible to manipulate the ground state associated with a set of defects without moving or measuring them. To illustrate these effects, we analyze specific architectures involving topological insulators and superconductors.Physical Review Letters 01/2010; 104(4):046401. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: 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 novel topological order whose identification requires a spin sensitive measurement. Using SpinresolvedARPES, we probe the spin degrees of freedom and demonstrate that topological quantum numbers are uniquely determined from spintexture Berry Phase imaging measurements. Applying this method to pure antimony (Sb) and BiSb, we identify the origin of its novel Topological Order and the negative value of the mirror Chern number. These results taken together constitute the first observation of surface electrons collectively carrying a topological Berry's phase and definite mirror Chern chirality in pure Antimony (Sb) which are the key electronic properties for realizing topological quantum computing via the interface Majorana fermion framework. This paper contains the details of the above mentioned previously reported (Science \textbf{323}, 919 (2009)) results. Comment: 6 Figures, 10 Pages, RevTex Format, Detailed version of Hsieh et.al., SCIENCE 323, 919 (2009)09/2009;  [Show abstract] [Hide abstract]
ABSTRACT: We propose two experiments to probe the Majorana fermion edge states that occur at a junction between a superconductor and a magnet deposited on the surface of a topological insulator. Combining two Majorana fermions into a single Dirac fermion on a magnetic domain wall allows the neutral Majorana fermions to be probed with charge transport. We will discuss a novel interferometer for Majorana fermions, which probes their Z2 phase. This setup also allows the transmission of neutral Majorana fermions through a point contact to be measured. We introduce a point contact formed by a superconducting junction and show that its transmission can be controlled by the phase difference across the junction. We discuss the feasibility of these experiments using the recently discovered topological insulator Bi2Se3.Physical Review Letters 06/2009; 102(21):216403. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study a quantum point contact in a quantum spin Hall insulator. It has recently been shown that the Luttinger liquid theory of such a structure maps to the theory of a weak link in a Luttinger liquid with spin with Luttinger liquid parameters g_\rho = 1/g_\sigma = g < 1. We show that for 1/2<g<1, the pinchoff of the point contact as a function of gate voltage is controlled by a novel quantum critical point, related to a nontrivial intermediate fixed point found previously in the Luttinger liquid model. We predict that the dependence of the conductance on gate voltage and temperature near the pinchoff transition collapses onto a universal curve described by a crossover scaling function. We compute the conductance, the critical exponents and the scaling function in solvable limits, which include g=1\epsilon, g=1/2+\epsilon and g=1/\sqrt{3}. These results, along with a general scaling analysis provide an overall picture of the critical behavior as a function of g. In addition, we analyze the structure of the four terminal conductance of the point contact in the weak tunneling and weak backscattering limits. We find that different components of the conductance can have different temperature dependence. We identify a skew conductance G_{XY}, which we predict vanishes as T^\gamma with \gamma\ge 2. This behavior is a direct consequence of the unique edge state structure of the quantum spin Hall insulator. Finally, we show that for g<1/2 the presence of spin non conserving spin orbit interactions leads to a novel time reversal symmetry breaking insulating phase. In this phase, the transport is carried by spinless chargons and chargeless spinons. These lead to nontrivial correlations in the low frequency shot noise. Implications for experiments on HgCdTe quantum well structures will be discussed. Comment: 23 pages, 11 figuresPhysical review. B, Condensed matter 04/2009; · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetrybased classification of condensed matter. Exotic spintransport phenomena, such as the dissipationless quantum spin Hall effect, have been speculated to originate from a topological order whose identification requires a spinsensitive 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 textureimaging measurements. Applying this method to Sb and Bi(1x)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 topological quantum computing bits with intrinsic spin Halllike topological phenomena.Science 03/2009; 323(5916):91922. · 31.20 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: 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 novel topological order whose identification requires a spin sensitive measurement, which does not exist to this date in any system (neither in Hg(Cd)Te quantum wells nor in the topological insulator BiSb). Using Mott polarimetry, we probe the spin degrees of freedom of these quantum spin Hall states and demonstrate that topological quantum numbers are uniquely determined from spin texture imaging measurements. Applying this method to the Bi{1x}Sb{x} series, we identify the origin of its novel order and unusual chiral properties. These results taken together constitute the first observation of surface electrons collectively carrying a geometrical quantum (Berry's) phase and definite chirality (mirror Chern number, n_M =1), which are the key electronic properties for realizing topological computing bits with intrinsic spin Halllike topological phenomena. Our spinresolved results not only provides the first clear proof of a topological insulating state in nature but also demonstrate the utility of spinresolved ARPES technique in measuring the quantum spin Hall phases of matter.02/2009; 
Article: Josephson current and noise at a superconductor/quantumspinHallinsulator/superconductor junction
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ABSTRACT: We study junctions between superconductors mediated by the edge states of a quantumspinHall insulator. We show that such junctions exhibit a fractional Josephson effect, in which the current phase relation has a 4π rather than a 2π periodicity. This effect is a consequence of the conservation of fermion parity—the number of electron mod 2—in a superconducting junction and is closely related to the Z2 topological structure of the quantumspinHall insulator. Inelastic processes, which violate the conservation of fermion parity, lead to telegraph noise in the equilibrium supercurrent. We predict that the lowfrequency noise due these processes diverges exponentially with temperature T as T→0. Possible experiments on HgCdTe quantum wells will be discussed.Physical review. B, Condensed matter 01/2009; 79(16). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Experiment has now proved the existence of the predicted threedimensional 'topological insulator' in the semiconducting alloy Bi1xSbx.Nature Physics 05/2008; 4(5):348349. · 19.35 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the electronic surface states of the semiconducting alloy bismuth antimony (Bi1−xSbx). Using a phenomenological tightbinding model, we show that the Fermi surface for the 111 surface states encloses an odd number of timereversalinvariant momenta (TRIM) in the surface Brillouin zone. This confirms that the alloy is a strong topological insulator in the (1;111) Z2 topological class. We go on to develop general arguments which show that spatial symmetries lead to additional topological structure of the bulk energy bands, and impose further constraints on the surface band structure. Inversionsymmetric band structures are characterized by eight Z2 “parity invariants,” which include the four Z2 invariants defined by timereversal symmetry. The extra invariants determine the “surface fermion parity,” which specifies which surface TRIM are enclosed by an odd number of electron or hole pockets. We provide a simple proof of this result, which provides a direct link between the surfacestate structure and the parity eigenvalues characterizing the bulk. Using this result, we make specific predictions for the surfacestate structure for several faces of Bi1−xSbx. We next show that mirrorinvariant band structures are characterized by an integer “mirror Chern number” nM, which further constrains the surface states. We show that the sign of nM in the topological insulator phase of Bi1−xSbx is related to a previously unexplored Z2 parameter in the L point k⋅p theory of pure bismuth, which we refer to as the “mirror chirality” η. The value of η predicted by the tightbinding model for bismuth disagrees with the value predicted by a more fundamental pseudopotential calculation. This explains a subtle disagreement between our tightbinding surfacestate calculation and previous firstprinciples calculations of the surface states of bismuth. This suggests that the tightbinding parameters in the LiuAllen model of bismuth need to be reconsidered. Implications for existing and future angleresolved photoemission spectroscopy (ARPES) experiments and spinpolarized ARPES experiments will be discussed.Physical review. B, Condensed matter 04/2008; 78(4). · 3.77 Impact Factor 
Article: Superconducting proximity effect and majorana fermions at the surface of a topological insulator.
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ABSTRACT: We study the proximity effect between an swave superconductor and the surface states of a strong topological insulator. The resulting twodimensional state resembles a spinless px+ipy superconductor, but does not break time reversal symmetry. This state supports Majorana bound states at vortices. We show that linear junctions between superconductors mediated by the topological insulator form a nonchiral onedimensional wire for Majorana fermions, and that circuits formed from these junctions provide a method for creating, manipulating, and fusing Majorana bound states.Physical Review Letters 04/2008; 100(9):096407. · 7.73 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The alloy Bi1xSbx is a narrow gap semiconductor for .0703/2008;  [Show abstract] [Hide abstract]
ABSTRACT: We study threedimensional generalizations of the quantum spin Hall (QSH) effect. Unlike two dimensions, where a single Z2 topological invariant governs the effect, in three dimensions there are 4 invariants distinguishing 16 phases with two general classes: weak (WTI) and strong (STI) topological insulators. The WTI are like layered 2D QSH states, but are destroyed by disorder. The STI are robust and lead to novel "topological metal" surface states. We introduce a tight binding model which realizes the WTI and STI phases, and we discuss its relevance to real materials, including bismuth.Physical Review Letters 04/2007; 98(10):106803. · 7.73 Impact Factor 
Article: Electron backscattering on singlewall carbon nanotubes observed by scanning tunneling microscopy
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ABSTRACT: Singlewall carbon nanotubes, seamless cylindrical molecules formed from a graphene sheet, are either conducting or semiconducting, depending on the particular "wrapping vector" that defines the waist of the tube. Scanning tunneling microscopy experiments have tested this idea by simultaneously measuring a tube's lattice structure and electronic properties. Here we present a series of STM images of singlewall carbon nanotubes with a strikingly rich set of superstructures. The observed patterns can be understood as due to interference between propagating electron waves that are reflected from defects on the tube walls and ends, or as intrinsic to states propagating on semiconducting tubes. The measured broken symmetries can be used to directly probe electronic backscattering on the tube and provide a key element in the understanding of lowenergy electron transport on these structures.EPL (Europhysics Letters) 01/2007; 47(5):601. · 2.26 Impact Factor  Science 01/2007; 314(5806):16923. · 31.20 Impact Factor
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ABSTRACT: Topological insulators are materials with a bulk excitation gap generated by the spin orbit interaction, and which are different from conventional insulators. This distinction is characterized by Z_2 topological invariants, which characterize the groundstate. In two dimensions there is a single Z_2 invariant which distinguishes the ordinary insulator from the quantum spin Hall phase. In three dimensions there are four Z_2 invariants, which distinguish the ordinary insulator from "weak" and "strong" topological insulators. These phases are characterized by the presence of gapless surface (or edge) states. In the 2D quantum spin Hall phase and the 3D strong topological insulator these states are robust and are insensitive to weak disorder and interactions. In this paper we show that the presence of inversion symmetry greatly simplifies the problem of evaluating the Z_2 invariants. We show that the invariants can be determined from the knowledge of the parity of the occupied Bloch wavefunctions at the time reversal invariant points in the Brillouin zone. Using this approach, we predict a number of specific materials are strong topological insulators, including the semiconducting alloy Bi_{1x} Sb_x as well as \alphaSn and HgTe under uniaxial strain. This paper also includes an expanded discussion of our formulation of the topological insulators in both two and three dimensions, as well as implications for experiments. Comment: 16 pages, 7 figures; published versionPhysical review. B, Condensed matter 11/2006; · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We introduce and analyze a class of one dimensional insulating Hamiltonians which, when adiabatically varied in an appropriate closed cycle, define a "$Z_2$ pump". For an isolated system a single closed cycle of the pump changes the expectation value of the spin at each end even when spin orbit interactions violate the conservation of spin. A second cycle, however returns the system to its original state. When coupled to leads, we show that the $Z_2$ pump functions as a spin pump in a sense which we define, and transmits a finite, though non quantized spin in each cycle. We show that the $Z_2$ pump is characterized by a $Z_2$ topological invariant that is analogous to the Chern invariant that characterizes a topological charge pump. The $Z_2$ pump is closely related to the quantum spin Hall effect, which is characterized by a related $Z_2$ invariant. This work presents an alternative formulation which clarifies both the physical and mathematical meaning of that invariant. A crucial role is played by time reversal symmetry, and we introduce the concept of the time reversal polarization, which characterizes time reversal invariant Hamiltonians and signals the presence or absence of Kramers degenerate end states. For non interacting electrons we derive a formula for the time reversal polarization which is analogous to the Berry's phase formulation of the charge polarization. For interacting electrons, we show that abelian bosonization provides a simple formulation of the time reversal polarization. We discuss implications for the quantum spin Hall effect, and argue in particular that the $Z_2$ classification of the quantum spin Hall effect is valid in the presence of electron electron interactions. Comment: 14 pages, 6 figuresPhysical Review B 06/2006; · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report pumpprobe transient absorption spectroscopy on carbon nanotubes with a high initial excitation density. We find that the recovery of the ground state optical absorption is well described by a 1/t relaxation, indicating that the long time population relaxation is controlled by onedimensional diffusion limited two body recombination.Physical review. B, Condensed matter 01/2006; 74. · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the effects of spin orbit interactions on the low energy electronic structure of a single plane of graphene. We find that in an experimentally accessible low temperature regime the symmetry allowed spin orbit potential converts graphene from an ideal twodimensional semimetallic state to a quantum spin Hall insulator. This novel electronic state of matter is gapped in the bulk and supports the transport of spin and charge in gapless edge states that propagate at the sample boundaries. The edge states are nonchiral, but they are insensitive to disorder because their directionality is correlated with spin. The spin and charge conductances in these edge states are calculated and the effects of temperature, chemical potential, Rashba coupling, disorder, and symmetry breaking fields are discussed.Physical Review Letters 12/2005; 95(22):226801. · 7.73 Impact Factor
Publication Stats
10k  Citations  
428.94  Total Impact Points  
Top Journals
Institutions

1994–2014

University of Pennsylvania
 • Department of Physics and Astronomy
 • Laboratory for Research on the Structure of Matter
Philadelphia, Pennsylvania, United States


2013

University of Illinois, UrbanaChampaign
 Department of Physics
Urbana, IL, United States


2012

Massachusetts Institute of Technology
 Department of Physics
Cambridge, MA, United States


2009–2010

Princeton University
 Department of Physics
Princeton, New Jersey, United States
