22 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
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
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.
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
Recommended from our members
Visualization of Topological Boundary Modes Manifesting Topological Nodal-Point Superconductivity
The extension of the topological classification of band insulators to topological semimetals gave way to the topology classes of Dirac, Weyl, and nodal line semimetals with their unique Fermi arc and drum head boundary modes. Similarly, there are several suggestions to employ the classification of topological superconductors for topological nodal superconductors with Majorana boundary modes. Here, we show that the surface 1H termination of the transition metal dichalcogenide compound 4Hb-TaS2, in which 1T-TaS2 and 1H-TaS2 layers are interleaved, has the phenomenology of a topological nodal point superconductor. We find in scanning tunneling spectroscopy a residual density of states within the superconducting gap. An exponentially decaying bound mode is imaged within the superconducting gap along the boundaries of the exposed 1H layer characteristic of a gapless Majorana edge mode. The anisotropic nature of the localization length of the edge mode aims towards topological nodal superconductivity. A zero-bias conductance peak is further imaged within fairly isotropic vortex cores. All our observations are accommodated by a theoretical model of a two-dimensional nodal Weyl-like superconducting state, which ensues from inter-orbital Cooper pairing. The observation of an intrinsic topological nodal superconductivity in a layered material will pave the way for further studies of Majorana edge modes and its applications in quantum information processing.N.A., H.B., and B.Y acknowledge the German–Israeli Foundation for Scientific Research
and Development (GIF grant no. I-1364-303.7/2016). H.B. and N.A. acknowledge the
European Research Council (ERC, project no. TOPO NW), B.Y. acknowledges financial
support by the Willner Family Leadership Institute for the Weizmann Institute of Sci-
ence, the Benoziyo Endowment Fund for the Advancement of Science, the Ruth and Her-
man Albert Scholars Program for New Scientists, and the Israel Science Foundation (ISF
1251/19). G.A.F. gratefully acknowledges partial support from the National Science Foun-
dation through NSF Grant no. DMR-1720595, and DMR-1949701. Y.O. acknowledges
partial support through the ERC under the European Union’s Horizon 2020 research and
innovation programme (grant agreement LEGOTOP No 788715), the ISF Quantum Science
and Technology (2074/19), the BSF and NSF (2018643), and the CRC/Transregio 183. A.K.
acknowledges the Israel Science Foundation (ISF 320/17).Center for Dynamics and Control of Material
Coexisting Boundary States in the Dual Weak and Crystalline Topological Insulator Bi2TeI
Non UBCUnreviewedAuthor affiliation: Weizmann Institute of ScienceResearche