29 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
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
Inverse melting of the vortex lattice
Inverse melting, in which a crystal reversibly transforms into a liquid or
amorphous phase upon decreasing the temperature, is considered to be very rare
in nature. The search for such an unusual equilibrium phenomenon is often
hampered by the formation of nonequilibrium states which conceal the
thermodynamic phase transition, or by intermediate phases, as was recently
shown in a polymeric system. Here we report a first-order inverse melting of
the magnetic flux line lattice in Bi2Sr2CaCu2O8 superconductor. At low
temperatures, the material disorder causes significant pinning of the vortices,
which prevents observation of their equilibrium properties. Using a newly
introduced 'vortex dithering' technique we were able to equilibrate the vortex
lattice. As a result, direct thermodynamic evidence of inverse melting
transition is found, at which a disordered vortex phase transforms into an
ordered lattice with increasing temperature. Paradoxically, the structurally
ordered lattice has larger entropy than the disordered phase. This finding
shows that the destruction of the ordered vortex lattice occurs along a unified
first-order transition line that gradually changes its character from
thermally-induced melting at high temperatures to a disorder-induced transition
at low temperatures.Comment: 13 pages, 4 figures, Nature, In pres
Fermi-arc diversity on surface terminations of the magnetic Weyl semimetal Co3Sn2S2
Bulk-surface correspondence in Weyl semimetals assures the formation of
topological "Fermi-arc" surface bands whose existence is guaranteed by bulk
Weyl nodes. By investigating three distinct surface terminations of the
ferromagnetic semimetal Co3Sn2S2 we verify spectroscopically its classification
as a time reversal symmetry broken Weyl semimetal. We show that the distinct
surface potentials imposed by three different terminations modify the Fermi-arc
contour and Weyl node connectivity. On the Sn surface we identify
intra-Brillouin zone Weyl node connectivity of Fermi-arcs, while on Co
termination the connectivity is across adjacent Brillouin zones. On the S
surface Fermi-arcs overlap with non-topological bulk and surface states that
ambiguate their connectivity and obscure their exact identification. By these
we resolve the topologically protected electronic properties of a Weyl
semimetal and its unprotected ones that can be manipulated and engineered
Propagation of Avalanches in Mn-acetate: Magnetic Deflagration
Local time-resolved measurements of fast reversal of the magnetization of
single crystals of Mn12-acetate indicate that the magnetization avalanche
spreads as a narrow interface that propagates through the crystal at a constant
velocity that is roughly two orders of magnitude smaller than the speed of
sound. We argue that this phenomenon is closely analogous to the propagation of
a flame front (deflagration) through a flammable chemical substance.Comment: 5 pages, 5 figure
Visualizing near-coexistence of massless Dirac electrons and ultra-massive saddle point electrons
Strong singularities in the electronic density of states amplify correlation
effects and play a key role in determining the ordering instabilities in
various materials. Recently high order van Hove singularities (VHSs) with
diverging power-law scaling have been classified in single-band electron
models. We show that the 110 surface of Bismuth exhibits high order VHS with an
usually high density of states divergence . Detailed mapping
of the surface band structure using scanning tunneling microscopy and
spectroscopy combined with first-principles calculations show that this
singularity occurs in close proximity to Dirac bands located at the center of
the surface Brillouin zone. The enhanced power-law divergence is shown to
originate from the anisotropic flattening of the Dirac band just above the
Dirac node. Such near-coexistence of massless Dirac electrons and ultra-massive
saddle points enables to study the interplay of high order VHS and Dirac
fermions