12 research outputs found
Ultrafast photocurrents at the surface of the three-dimensional topological insulator
Topological insulators constitute a new and fascinating class of matter with
insulating bulk yet metallic surfaces that host highly mobile charge carriers
with spin-momentum locking. Remarkably, the direction and magnitude of surface
currents can be controlled with tailored light beams, but the underlying
mechanisms are not yet well understood. To directly resolve the "birth" of such
photocurrents we need to boost the time resolution to the scale of elementary
scattering events ( 10 fs). Here, we excite and measure photocurrents in
the three-dimensional model topological insulator
with a time resolution as short as 20 fs by sampling the concomitantly emitted
broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast
surface current response is dominated by a charge transfer along the Se-Bi
bonds. In contrast, photon-helicity-dependent photocurrents are found to have
orders of magnitude smaller magnitude than expected from generation scenarios
based on asymmetric depopulation of the Dirac cone. Our findings are also of
direct relevance for optoelectronic devices based on topological-insulator
surface currents
Singular robust room-temperature spin response from topological Dirac fermions
Topological insulators are a class of solids in which the nontrivial inverted
bulk band structure gives rise to metallic surface states that are robust
against impurity scattering. In three-dimensional (3D) topological insulators,
however, the surface Dirac fermions intermix with the conducting bulk, thereby
complicating access to the low energy (Dirac point) charge transport or
magnetic response. Here we use differential magnetometry to probe spin rotation
in the 3D topological material family (BiSe, BiTe, and
SbTe). We report a paramagnetic singularity in the magnetic
susceptibility at low magnetic fields which persists up to room temperature,
and which we demonstrate to arise from the surfaces of the samples. The
singularity is universal to the entire family, largely independent of the bulk
carrier density, and consistent with the existence of electronic states near
the spin-degenerate Dirac point of the 2D helical metal. The exceptional
thermal stability of the signal points to an intrinsic surface cooling process,
likely of thermoelectric origin, and establishes a sustainable platform for the
singular field-tunable Dirac spin response.Comment: 20 pages, 14 figure
Stable topological insulators achieved using high energy electron beams
Topological insulators are transformative quantum solids with
immune-to-disorder metallic surface states having Dirac band structure.
Ubiquitous charged bulk defects, however, pull the Fermi energy into the bulk
bands, denying access to surface charge transport. Here we demonstrate that
irradiation with swift ( MeV energy) electron beams allows to
compensate these defects, bring the Fermi level back into the bulk gap, and
reach the charge neutrality point (CNP). Controlling the beam fluence we tune
bulk conductivity from \textit{p}- (hole-like) to \textit{n}-type
(electron-like), crossing the Dirac point and back, while preserving the Dirac
energy dispersion. The CNP conductance has a two-dimensional (2D) character on
the order of ten conductance quanta , and reveals, both in
BiTe and BiSe, the presence of only two quantum channels
corresponding to two topological surfaces. The intrinsic quantum transport of
the topological states is accessible disregarding the bulk size.Comment: Main manuscript - 12 pages, 4 figures; Supplementary file - 15 pages,
11 figures, 1 Table, 4 Note
Stable topological insulators achieved using high energy electron beams
Topological insulators are potentially transformative quantum solids with metallic surface states which have Dirac band structure and are immune to disorder. Ubiquitous charged bulk defects, however, pull the Fermi energy into the bulk bands, denying access to surface charge transport. Here we demonstrate that irradiation with swift (B2.5MeV energy) electron beams allows to compensate these defects, bring the Fermi level back into the bulk gap and reach the charge neutrality point (CNP). Controlling the beam fluence, we tune bulk conductivity from p- (hole-like) to n-type (electron-like), crossing the Dirac point and back, while preserving the Dirac energy dispersion. The CNP conductance has a two-dimensional character on the order of ten conductance quanta and reveals, both in Bi2Te3 and Bi2Se3, the presence of only two quantum channels corresponding to two topological surfaces. The intrinsic quantum transport of the topological states is accessible disregarding the bulk size
Topological Lifshitz transition in Weyl semimetal NbP decorated with heavy elements
Studies of the Fermi surface modification after in-situ covering NbP
semimetal with heavy elements Pb and Nb ultrathin layers were performed by
means of angle-resolved photoemission spectroscopy (ARPES). First, the
electronic structure was investigated for pristine single crystals with two
possible terminations (P and Nb) of the (0 0 1) surface. The nature of the
electronic states of these two cleaving planes is different: P-terminated
surface shows spoon and bow tie shaped fingerprints, whereas these shapes are
not present in Nb-terminated surfaces. ARPES studies show that even 1 monolayer
(ML) of Pb causes topological quantum Lifshitz transition (TQLT) in P- and
Nb-terminated surfaces. Deposited Pb 5d electrons have wide extended atomic
orbitals which leads to strong hybridization with Pb-terminated surface and a
corresponding shift in the Fermi energy. Nb has less capability to perturb the
system than Pb because Nb has weaker spin-orbit coupling than Pb. Nb-terminated
surface subjected to surface decoration with approximately 1.3 ML of Nb shows
no dramatic modification in the Fermi surface. In the case of Nb decorated
P-terminated surface, deposition of approximately 1 ML modifies the electronic
structure of NbP and it is on the verge of TQLT. Despite the strong spin-orbit
and strong hybridization of the heavy elements on the surface, it is possible
to observe the TQLT of the surface states thanks to the robustness of the bulk
topology.Comment: 9 Pages, 8 Figure
Erratum: Corrigendum: Singular robust room-temperature spin response from topological Dirac fermions
Electronic properties of topological semimetal investigated by transport and ARPES
We have performed electron transport and ARPES measurements on single
crystals of transition metal dipnictide TaAs2 cleaved along the (
0 1) surface which has the lowest cleavage energy. A Fourier transform of the
Shubnikov-de Haas oscillations shows four different peaks whose angular
dependence was studied with respect to the angle between the magnetic field and
the [ 0 1] direction. The results indicate the elliptical shape
of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis
was carried out, which also reveals at least four types of carriers
contributing to the conductance (two kinds of electrons and two kinds of
holes). ARPES spectra were taken on freshly cleaved ( 0 1)
surface and it was found that bulk states pockets at the constant energy
surface are elliptical, which confirms the magnetotransport angle dependent
studies. First-principles calculations support the interpretation of the
experimental results. The theoretical calculations better reproduce the ARPES
data if the theoretical Fermi level is increased, which is due to a small
n-doping of the samples. This shifts the Fermi level closer to the Dirac point,
allowing to investigate the physics of the Dirac and Weyl points, making this
compound a platform for the investigation of the Dirac and Weyl points in
three-dimensional materials.Comment: 12 pages, 13 figure
Manipulating the Topological Interface by Molecular Adsorbates: Adsorption of Co-Phthalocyanine on Bi 2 Se 3
International audienceTopological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal–organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/Bi2Se3 interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer
Manipulating the topological interface by molecular adsorbates: adsorption of co-phthalocyanine on Bi2Se3
Topological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal–organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/Bi2Se3 interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer