82 research outputs found
Resolving the Topological Classification of Bismuth with Topological Defects
Bulk boundary correspondence in topological materials allows to study their
bulk topology through the investigation of their topological boundary modes.
However, for classes that share similar boundary phenomenology, the growing
diversity of topological phases may lead to ambiguity in the topological
classification of materials. Such is the current status of bulk bismuth. While
some theoretical models indicate that bismuth possesses a trivial topological
nature, other theoretical and experimental studies suggest non-trivial
topological classifications such as a strong or a higher order topological
insulator, both of which hosts helical modes on their boundaries. Here we use a
novel approach to resolve the topological classification of bismuth by
spectroscopically mapping the response of its boundary modes to a topological
defect in the form of a screw dislocation (SD). We find that the edge mode
extends over a wide energy range, and withstands crystallographic
irregularities, without showing any signs of backscattering. It seems to bind
to the bulk SD, as expected for a topological insulator (TI) with non-vanishing
weak indices. We argue that the small scale of the bulk energy gap, at the time
reversal symmetric momentum , positions bismuth within the critical region
of a topological phase transition to a strong TI with non-vanishing weak
indices. We show that the observed boundary modes are approximately helical
already on the trivial side of the topological phase transition.
This work opens the door for further possibilities to examine the response of
topological phases to crystallographic topological defects, and to uniquely
explore their associated bulk boundary phenomena
Interplay of Anisotropy and Disorder in the Doping-Dependent Melting and Glass Transitions of Vortices in BiSrCaCuO
We study the oxygen doping dependence of the equilibrium first-order melting
and second-order glass transitions of vortices in
BiSrCaCuO. Doping affects both anisotropy and
disorder. Anisotropy scaling is shown to collapse the melting lines only where
thermal fluctuations are dominant. Yet, in the region where disorder breaks
that scaling, the glass lines are still collapsed. A quantitative fit to
melting and replica symmetry breaking lines of a 2D Ginzburg-Landau model
further reveals that disorder amplitude weakens with doping, but to a lesser
degree than thermal fluctuations, enhancing the relative role of disorder.Comment: 4 pages, 4 figure
Equilibrium First-Order Melting and Second-Order Glass Transitions of the Vortex Matter in BiSrCaCuO
The thermodynamic phase diagram of BiSrCaCuO was mapped
by measuring local \emph{equilibrium} magnetization in presence of
vortex `shaking'. Two equally sharp first-order magnetization steps are
revealed in a single temperature sweep, manifesting a liquid-solid-liquid
sequence. In addition, a second-order glass transition line is revealed by a
sharp break in the equilibrium slope. The first- and second-order lines
intersect at intermediate temperatures, suggesting the existence of four
phases: Bragg glass and vortex crystal at low fields, glass and liquid at
higher fields.Comment: 5 pages, 4 figures. To be published in Phys. Rev. Let
Hot Electrons Regain Coherence in Semiconducting Nanowires
The higher the energy of a particle is above equilibrium the faster it
relaxes due to the growing phase-space of available electronic states it can
interact with. In the relaxation process phase coherence is lost, thus limiting
high energy quantum control and manipulation. In one-dimensional systems high
relaxation rates are expected to destabilize electronic quasiparticles. We show
here that the decoherence induced by relaxation of hot electrons in
one-dimensional semiconducting nanowires evolves non-monotonically with energy
such that above a certain threshold hot-electrons regain stability with
increasing energy. We directly observe this phenomenon by visualizing for the
first time the interference patterns of the quasi-one-dimensional electrons
using scanning tunneling microscopy. We visualize both the phase coherence
length of the one-dimensional electrons, as well as their phase coherence time,
captured by crystallographic Fabry-Perot resonators. A remarkable agreement
with a theoretical model reveals that the non-monotonic behavior is driven by
the unique manner in which one dimensional hot-electrons interact with the cold
electrons occupying the Fermi-sea. This newly discovered relaxation profile
suggests a high-energy regime for operating quantum applications that
necessitate extended coherence or long thermalization times, and may stabilize
electronic quasiparticles in one dimension
Multiple Changes of Order of the Vortex Melting Transition in BSCCO with Dilute Columnar Defects
A low concentration of columnar defects is reported to transform a
first-order vortex lattice melting line in BSCCO crystals into alternating
segments of first-order and second-order transitions separated by two critical
points. As the density of CDs is increased, the critical points shift apart and
the range of the intermediate second-order transition expands. A third, low
temperature critical point was also observed in one sample. The measurement of
equilibrium magnetization and the mapping of the melting line down to 27K was
made possible by employment of the shaking technique.Comment: 5 pages, 3 figure
Termination dependent topological surface states of the natural superlattice phase BiSe
We describe the topological surface states of BiSe, a compound in the
infinitely adaptive Bi-BiSe natural superlattice phase series,
determined by a combination of experimental and theoretical methods. Two
observable cleavage surfaces, terminating at Bi or Se, are characterized by
angle resolved photoelectron spectroscopy and scanning tunneling microscopy,
and modeled by ab-initio density functional theory calculations. Topological
surface states are observed on both surfaces, but with markedly different
dispersions and Kramers point energies. BiSe therefore represents the
only known compound with different topological states on differently terminated
surfaces.Comment: 5 figures references added Published in PRB:
http://link.aps.org/doi/10.1103/PhysRevB.88.08110
Interplay between ferromagnetism, surface states, and quantum corrections in a magnetically doped topological insulator
The breaking of time-reversal symmetry by ferromagnetism is predicted to
yield profound changes to the electronic surface states of a topological
insulator. Here, we report on a concerted set of structural, magnetic,
electrical and spectroscopic measurements of \MBS thin films wherein
photoemission and x-ray magnetic circular dichroism studies have recently shown
surface ferromagnetism in the temperature range 15 K K,
accompanied by a suppressed density of surface states at the Dirac point.
Secondary ion mass spectroscopy and scanning tunneling microscopy reveal an
inhomogeneous distribution of Mn atoms, with a tendency to segregate towards
the sample surface. Magnetometry and anisotropic magnetoresistance measurements
are insensitive to the high temperature ferromagnetism seen in surface studies,
revealing instead a low temperature ferromagnetic phase at K.
The absence of both a magneto-optical Kerr effect and anomalous Hall effect
suggests that this low temperature ferromagnetism is unlikely to be a
homogeneous bulk phase but likely originates in nanoscale near-surface regions
of the bulk where magnetic atoms segregate during sample growth. Although the
samples are not ideal, with both bulk and surface contributions to electron
transport, we measure a magnetoconductance whose behavior is qualitatively
consistent with predictions that the opening of a gap in the Dirac spectrum
drives quantum corrections to the conductance in topological insulators from
the symplectic to the orthogonal class.Comment: To appear in Phys. Rev.
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