Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session R29: Disordered and Floquet Topological Phases 
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Sponsoring Units: DCMP Chair: Andrew Potter, University of California, Berkeley Room: 328 
Thursday, March 17, 2016 8:00AM  8:12AM 
R29.00001: Spin susceptibility function of helical metal and RKKY interacton Chunxiao Liu, Bitan Roy, Jay Sau Topological insulator is a peculiar material, which is insulating in the bulk, while conducting on the surface due to the topology in momentum space . Here, we study the surface of 3D topological insulator, which is also called helical metal. The Hamiltonian of the surface electrons is $H = \bold{k} \times \sigma$, which has spinmomentum locking and Dirac dispersion. The property that we are interested in is the spin susceptibility function of the helical metal. It describes how the system respond to the external magnetic field in the linear response regime. However, because the dispersion of helical metal is linear in momentum $k$, the spin susceptibility is easily divergent in terms of UV momentum cutoff. The talk we will present is focusing on how we renormalize the spin susceptibility function in the particular renormalization scheme we choose. In the second part of the talk, we will make use of the renormalized spin susceptibility to show how external magnetic impurities on the surface would interact with each other, with the interaction mediated by electrons of helical metal. This interaction is also called RKKY interaction. Treating impurities as classical spins, we show the pattern of ground state with numerical simulations. [Preview Abstract] 
Thursday, March 17, 2016 8:12AM  8:24AM 
R29.00002: NonSymmorphic Symmetry Protected Topological Order in Manybody Localized Systems Khalid Ashraf Manybody localized systems have many interesting physical properties such as localization protected quantum order, symmetry protected topological order, area law in entanglement spectrum etc. [1]. Specifically, it has been shown that closed quantum system in 1D i.e. pwave superconducting wires host localization protected topological order [2]. In this work, we explore the interplay between nonsymmorphic symmetry which protects topological order and localization due to disorder. Using a Bogoliubovde Gennes (BdG) description of pwave superconductors, we study the topological edge states on a 2D nonsymmorphic crystal. We show that a localization protected topological order can exist at high energy in a 2D nonsymmorphic crystal. The system goes between topologically trivial and nontrivial phases based on the degree of disorder and shift between the adjacent atoms in the bipartite lattice. We further explore the nature of this phase transition by calculating the entanglement spectrum of the two phases. Finally, the effect of dimensionality on the realization of these phases are discussed.$\backslash $pard. Nandkishore et al. Annual Rev. Cond. Matt. Phys., vol 6 (2015). 2. Huse et al., Phys. Rev. B 88, 014206 (2013). [Preview Abstract] 
Thursday, March 17, 2016 8:24AM  8:36AM 
R29.00003: Effect of disorder on the decreasing the critical value of magnetic field in proximity induced topological superconductors Yahya Alavirad, ChingKai Chiu, Jay Sau Here we investigate how adding disorder changes the critical value of magnetic field $B_c$ required to observe the topological phase transition in proximity induced topological superconductors .We consider disordered topological superconductors in $D=1,2$ spatial dimensions and use numerical analysis to directly calculate the topological invariant.Qualitatively different behavior are found depending on the dimensionality of the system. In contrast to $D=2$ for $D=1$ we show that adding disorder gradually decreases $B_c$ from the clean the case limit of $B_c=\sqrt{\mu^2+\Delta^2}$ to $B_c=\Delta$. A discussion of why these results are expected is provided. These findings, specially in $D=1$ dimension are of experimental interest since they show that the topological phase transition might be observable at values of magnetic field $B_c$ much smaller than previously expected. [Preview Abstract] 
Thursday, March 17, 2016 8:36AM  8:48AM 
R29.00004: Anderson Chern Insulators James Jun He, Tong Zhou, Z. C. Gu, K. T. Law When a magnetic field is applied to a quantum spin Hall insulator (QSHI) without inversion symmetry, the edge states become gapful due to the breaking of time reversal symmetry (TRS) and the QSHI becomes a trivial spin Hall insulator (SHI) whose Chern number is $N=0$. In this work we show that disorder can drive such a SHI to a Chern insulator (CI) with $N=1$ which supports a gapless chiral edge state. This CI exists in a finite range of disorder strength. Interestingly, the edge state is protected by the bulk mobility gap instead of an energy gap. For this reason, the new phase is called an Anderson Chern insulator (ACI). [Preview Abstract] 
Thursday, March 17, 2016 8:48AM  9:00AM 
R29.00005: Disorderdriven breakdown of topological protection in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ films. Jisoo Moon, Matthew Brahlek, Nikesh Koirala, Maryam Salehi, Liang Wu, N. Peter Armitage, Seongshik Oh In topological insulators (TI), there are unusual metallic states on their surfaces, socalled topological surface states (TSS). They are protected by time reversal symmetry through strong spinorbit coupling, making them immune to disorderrelated localization effects. However, in highlydisordered TIs the crystal momentum becomes illdefined because translational invariance is broken. In those materials, it is doubtful that topological protection is still valid. In this presentation, we show that high level of disorder drives breakdown of topological protection in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ films. TSS in the films are not protected above a critical level of disorder, and the films become trivial insulators. The films are grown by Molecular Beam Epitaxy (MBE), and the level of disorder in the films is controlled by annealing after the films are grown at room temperature. [Preview Abstract] 

R29.00006: ABSTRACT WITHDRAWN 
Thursday, March 17, 2016 9:12AM  9:24AM 
R29.00007: Disorderinduced WALWL transition and anomalous Hall effect in undoped thin Sb$_2$Te$_3$ films Inna Korzhovska, Yury Deshko, Lukas Zhao, Zhiyi Chen, Lia KrusinEbaum, Simone Raoux We examine the effects of disorder on charge transport in thin (2050 nm) films of topological insulator (TI) Sb$_2$Te$_3$, where, uniquely, structural disorder can be controllably tuned over a huge range  from amorphous to crystalline  by a suitable annealing schedule. We report on the observation of disorderinduced transition from weak localizationlike state (WLlike) to weak antilocalization (WAL), at which conductance changes its character from 3D in the WLlike state to 2D in the WAL (crystalline) state. Near the transition, the conductance is $G\approx e^2/h$, suggestive of the transport through a surface channel that is decoupled from the bulk by disorder. Quite remarkably, the onset of the WL state (where bulk transport is of variable range hopping type) is found to be concurrent with the appearance of anomalous Hall signal (AHE) which grows with increased disorder, with Hall resistivity $\rho_{xy}$ scaling as the longitudinal resistivity squared, $\rho_{xy} \propto \rho_{xx}^2$. The nature of spin correlations (probed directly by the arrays of micro Hall sensors) responsible for AHE in disordered TI films in the absence of magnetic dopants will be discussed. [Preview Abstract] 

R29.00008: ABSTRACT WITHDRAWN 
Thursday, March 17, 2016 9:36AM  9:48AM 
R29.00009: Superconductivity and disorder in the potential topological superconductor (Sn,In)Te Matthew Smylie, Bing Shen, Helmut Claus, Alexey Snezhko, Ulrich Welp, WaiKwong Kwok, Morten Eskildsen, Elizabeth De Waard, Michael Susner, Athena Sefat Indoped SnTe has been proposed as a candidate topological superconductor. It has been suggested that the superconducting critical temperature (Tc) is strongly enhanced with impurity scattering in this material, with the pairing mechanism perhaps changing with doping. To access information on the paring symmetry of the superconducting order parameter, ultrasensitive magnetic field penetration measurements have been conducted by means of a TunnelDiodeOscilator (TDO) technique. Particle irradiation with low MeV protons was used as a controllable source of disorder, but no enhancement of Tc was observed in cubicphase material as scattering increased. Detailed characterization measurements and analysis were performed before and after irradiation of the samples. [Preview Abstract] 
Thursday, March 17, 2016 9:48AM  10:00AM 
R29.00010: Characterization {\&} Transport Signatures of Periodically Driven Topological Phases Michel Fruchart, Pierre Delplace, Krzysztof Gawedzki, David Carpentier The discovery of the Quantum Hall Effect in the 80's opened the field of topological phases of matter, which has been renewed by the discovery of a new kind of topological insulator in 2005, this time in a timereversal invariant system. In order to obtain a material with tunable topological properties, research were carried out on outofequilibrium systems subject to a periodic drive. Such periodically driven topological phases turn out to be richer that their equilibrium counterparts. We consider a 2D crystal subject to a drive periodic in time, constrained so that is is timereversal invariant and show that such a system is characterized by Z2 indices attached to a gap (and not to a band), which we explicitly construct. To probe these outofequilibrium phases in a phase coherent regime, we use standard transport measurements. With the help of numerical simulations, we show that the running timeaveraged differential conductances are quantized in a topological gap, and that multiterminal setups enable to probe the chirality of the outofequilibrium topological states. [Preview Abstract] 
Thursday, March 17, 2016 10:00AM  10:12AM 
R29.00011: Floquet topological phases coupled to environments Szabolcs Vajna, Baruch Horovitz We consider the fate of a helical edge state of a spin Hall insulator and its topological transition in presence of a circularly polarized light when coupled to various forms of environments. A Lindblad type equation is developped to determine the fermion occupation of the Floquet bands. We find that nonsecular terms, corresponding to 2photon transitions lead to a mixing of the band occupations, hence the light induced photocurrent is in general not perfectly quantized, although deviations are small in the adiabatic limit. Sharp crossovers are identified at frequencies $\Omega$ and 1/2 $\Omega$ ($\Omega$ is the strength of lightmatter coupling) with the former corresponding to a phase transition on the level of the 2nd order theory. [Preview Abstract] 

R29.00012: ABSTRACT WITHDRAWN 
Thursday, March 17, 2016 10:24AM  10:36AM 
R29.00013: Selective scattering between FloquetBloch and Volkov states in a topological insulator Fahad Mahmood, ChingKit Chan, Zhanybek Alpichshev, Dillon Gardner, Young Lee, Patrick Lee, Nuh Gedik The coherent optical manipulation of solids is emerging as a promising way to engineer novel quantum states of matter. The strong time periodic potential of intense laser light can be used to generate hybrid photonelectron states. Interaction of light with Bloch states leads to FloquetBloch states which are essential in realizing new photoinduced quantum phases. Similarly, dressing of free electron states near the surface of a solid generates Volkov states which are used to study nonlinear optics in atoms and semiconductors. The interaction of these two dynamic states with each other remains an open experimental problem. Here we use Time and Angle Resolved Photoemission Spectroscopy (TrARPES) to selectively study the transition between these two states on the surface of the topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. We find that the coupling between the two strongly depends on the electron momentum, providing a route to enhance or inhibit it. Moreover, by controlling the light polarization we can negate Volkov states in order to generate pure FloquetBloch states. This work establishes a systematic path for the coherent manipulation of solids via lightmatter interaction. [Preview Abstract] 
Thursday, March 17, 2016 10:36AM  10:48AM 
R29.00014: Anisotropic ultrafast dynamics in BiSbTe$_{\mathrm{2}}$S topological insulator investigated by timeresolved photoemission spectroscopy F. Boschini, M. Zonno, E. da Silva Neto, S. Zhdanovich, M. Schneider, B. Zwartsenberg, G. Levy, A. Mills, D. Jones, A. Damascelli, S. Kushwaha, R. J. Cava Topological insulating phases in 3dimensional bulk materials are characterized by the presence of a Diraclike dispersive surface state  with a specific momentumlocked spin structure  localized within the bulk insulating band gap [1,2] Here we will present timeresolved photoemission (TRARPES) experimental results from a new topological insulator, BiSbTe$_{\mathrm{2}}$S. BiSbTe$_{\mathrm{2}}$S exhibits superior chemical stability, as evidenced by the lack of any measurable energy shift of the Dirac point over time. The TRARPES signal (1.55eV pump and 6.2eV probe) reveals a direct optical population/depopulation of the Dirac states followed by slow recombination processes on a pstimescale with a marked dependence of the relaxation time on crystallographic orientation. In addition, we also observe an ultrafast pumpinduced modification of the equilibrium Dirac state energy dispersion. These effects can be ascribed to an anisotropic pumpinduced modification of the phonon population, which in turn leads to an anisotropic electronphonon assisted scattering of the hot electrons populating the unoccupied Dirac states. [1] Y. Xia et al. Nat. Phys. \textbf{5}, 398 (2009) [2] Z.H. Zhu et al. Phys. Rev. Lett. \textbf{112}, 076802 (2014) [Preview Abstract] 
Thursday, March 17, 2016 10:48AM  11:00AM 
R29.00015: Using NonEquilibrium Dynamics to Probe Competing Orders in a MottPeierls System Yao Wang, Brian Moritz, ChengChien Chen, Chunjing Jia, Michel van Veenendaal, Thomas Devereaux The competition between ordered phases and the associated quantum criticality are significant in the study of strongly correlated systems. Here we examine one aspect, the nonequilibrium dynamics of a photoexcited MottPeierls system, using an effective Hubbardbondphonon model and exact diagonalization. Near the quantum phase transition where spin and charge become intertwined, we observe antiphase dynamics and couplingstrengthdependent suppression or enhancement in the static structure factors. The renormalized bosonic excitations coupled to a photoemitted electron can be extracted from the spin and charge dynamics, providing an approach for characterizing the underlying bosonic modes. The results from this analysis for different electron momenta show uneven softening of bosonic modes due to a stronger coupling near $k_F$. This behavior reflects the strong link between the fermionic momenta, the coupling vertices, and ultimately the bosonic susceptibilities near a quantum phase transition. [Preview Abstract] 
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