Mark H. Fischer

Spin-transfer torque generated by a topological insulator

Magnetic devices are a leading contender for the implementation of memory and logic technologies that are non-volatile, that can scale to high density and high speed, and that do not wear out. However, widespread application of magnetic memory and logic devices will require the development of efficient mechanisms for reorienting their magnetization using the least possible current and power. There has been considerable recent progress in this effort; in particular, it has been discovered that spin–orbit interactions in heavy-metal/ferromagnet bilayers can produce strong current-driven torques on the magnetic layer, via the spin Hall effect in the heavy metal or the Rashba–Edelstein effect in the ferromagnet. In the search for materials to provide even more efficient spin–orbit-induced torques, some proposals have suggested topological insulators, which possess a surface state in which the effects of spin–orbit coupling are maximal in the sense that an electron’s spin orientation is fixed relative to its propagation direction. Here we report experiments showing that charge current flowing in-plane in a thin film of the topological insulator bismuth selenide (Bi2Se3) at room temperature can indeed exert a strong spin-transfer torque on an adjacent ferromagnetic permalloy (Ni81Fe19) thin film, with a direction consistent with that expected from the topological surface state. We find that the strength of the torque per unit charge current density in Bi2Se3 is greater than for any source of spin-transfer torque measured so far, even for non-ideal topological insulator films in which the surface states coexist with bulk conduction. Our data suggest that topological insulators could enable very efficient electrical manipulation of magnetic materials at room temperature, for memory and logic applications.

Observation of many-body localization of interacting fermions in a quasirandom optical lattice

Making interacting atoms localize Disorder can stop the transport of noninteracting particles in its tracks. This phenomenon, known as Anderson localization, occurs in disordered solids, as well as photonic and cold atom settings. Interactions tend to make localization less likely, but disorder, interactions, and localization may coexist in the so-called many-body localized state. Schreiber et al. detect many-body localization in a one-dimensional optical lattice initially filled with atoms occupying alternating sites. Externally induced disorder and interactions prevented the system from evolving quickly to a state with a single atom on each site. Science, this issue p. 842 Disorder and interactions are tuned to induce nonergodic behavior in an atomic system in a one-dimensional optical lattice. Many-body localization (MBL), the disorder-induced localization of interacting particles, signals a breakdown of conventional thermodynamics because MBL systems do not thermalize and show nonergodic time evolution. We experimentally observed this nonergodic evolution for interacting fermions in a one-dimensional quasirandom optical lattice and identified the MBL transition through the relaxation dynamics of an initially prepared charge density wave. For sufficiently weak disorder, the time evolution appears ergodic and thermalizing, erasing all initial ordering, whereas above a critical disorder strength, a substantial portion of the initial ordering persists. The critical disorder value shows a distinctive dependence on the interaction strength, which is in agreement with numerical simulations. Our experiment paves the way to further detailed studies of MBL, such as in noncorrelated disorder or higher dimensions.

Signatures of Many-Body Localization in a Controlled Open Quantum System

In the presence of disorder, an interacting closed quantum system can undergo many-body localization (MBL) and fail to thermalize. However, over long times, even weak couplings to any thermal environment will necessarily thermalize the system and erase all signatures of MBL. This presents a challenge for experimental investigations of MBL since no realistic system can ever be fully closed. In this work, we experimentally explore the thermalization dynamics of a localized system in the presence of controlled dissipation. Specifically, we find that photon scattering results in a stretched exponential decay of an initial density pattern with a rate that depends linearly on the scattering rate. We find that the resulting susceptibility increases significantly close to the phase transition point. In this regime, which is inaccessible to current numerical studies, we also find a strong dependence on interactions. Our work provides a basis for systematic studies of MBL in open systems and opens a route towards extrapolation of closed-system properties from experiments.

Evidence for superconductivity with broken time-reversal symmetry in locally noncentrosymmetric SrPtAs

P. K. Biswas, H. Luetkens, ∗ T. Neupert, 3 T. Stürzer, C. Baines, G. Pascua, A. P. Schnyder, M. H. Fischer, J. Goryo, 3 M. R. Lees, H. Maeter, F. Brückner, H.-H. Klauss, M. Nicklas, P. J. Baker, A. D. Hillier, M. Sigrist, A. Amato, and D. Johrendt Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Condensed Matter Theory Group, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Institute for Theoretical Physics, ETH Zurich, 8093 Zurich, Switzerland Department Chemie, Ludwig-Maximilians-Universität München, D-81377 München, Germany Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany Department of Physics, Cornell University, Ithaca, New York 14853, USA Institute of Industrial Science, The University of Tokyo, Meguro, Tokyo 153-0041, Japan Physics Department, University of Warwick, Coventry, CV4 7AL, United Kingdom Institute for Solid State Physics, TU Dresden, D-01069 Dresden, Germany Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dresden, Germany ISIS Facility, STFC Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom (Dated: May 5, 2014)

Chiral d-wave superconductivity in SrPtAs

Recent muon spin-rotation (

Dynamics of a Many-Body-Localized System Coupled to a Bath

\ensuremath{\mu}

Topological superconductivity in monolayer transition metal dichalcogenides

SR) measurements on SrPtAs revealed time-reversal-symmetry breaking with the onset of superconductivity [Biswas et al., Phys. Rev. B 87, 180503(R) (2013)], suggesting an unconventional superconducting state. We investigate this possibility via the functional renormalization group and find a chiral

Possible pairing symmetries in SrPtAs with a local lack of inversion center

(d+id)

Spin-torque generation in topological insulator based heterostructures

-wave order parameter favored by the multiband fermiology and hexagonal symmetry of SrPtAs. This

Superconductivity and local noncentrosymmetricity in crystal lattices

(d+id)