25 research outputs found
Quantum and Classical Magnetoresistance in Ambipolar Topological Insulator Transistors with Gate-tunable Bulk and Surface Conduction
Weak antilocalization (WAL) and linear magnetoresistance (LMR) are two most commonly observed magnetoresistance (MR) phenomena in topological insulators (TIs) and often attributed to the Dirac topological surface states (TSS). However, ambiguities exist because these phenomena could also come from bulk states (often carrying significant conduction in many TIs) and are observable even in non-TI materials. Here, we demonstrate back-gated ambipolar TI field-effect transistors in (Bi0.04Sb0.96)(2)Te-3 thin films grown by molecular beam epitaxy on SrTiO3(111), exhibiting a large carrier density tunability (by nearly 2 orders of magnitude) and a metal-insulator transition in the bulk (allowing switching off the bulk conduction). Tuning the Fermi level from bulk band to TSS strongly enhances both the WAL (increasing the number of quantum coherent channels from one to peak around two) and LMR (increasing its slope by up to 10 times). The SS-enhanced LMR is accompanied by a strongly nonlinear Hall effect, suggesting important roles of charge inhomogeneity (and a related classical LMR), although existing models of LMR cannot capture all aspects of our data. Our systematic gate and temperature dependent magnetotransport studies provide deeper insights into the nature of both MR phenomena and reveal differences between bulk and TSS transport in TI related materials
Chiral charge density wave and backscattering-immune orbital texture in monolayer 1T-TiTe2
Non-trivial electronic states are attracting intense attention in
low-dimensional physics. Though chirality has been identified in charge states
with a scalar order parameter, its intertwining with charge density waves
(CDW), film thickness and the impact on the electronic behaviors remain less
well understood. Here, using scanning tunneling microscopy, we report a 2 x 2
chiral CDW as well as a strong suppression of the Te-5p hole-band
backscattering in monolayer 1T-TiTe2. These exotic characters vanish in bilayer
TiTe2 with a non-CDW state. Theoretical calculations approve that chirality
comes from a helical stacking of the triple-q CDW components and therefore can
persist at the two-dimensional limit. Furthermore, the chirality renders the
Te-5p bands an unconventional orbital texture that prohibits electron
backscattering. Our study establishes TiTe2 as a promising playground for
manipulating the chiral ground states at the monolayer limit and provides a
novel path to engineer electronic properties from an orbital degree.Comment: 21 pages, 5 figure
Dichotomy of Electronic Structure and Superconductivity between Single-Layer and Double-Layer FeSe/SrTiO3 Films
The latest discovery of possible high temperature superconductivity in the
single-layer FeSe film grown on a SrTiO3 substrate, together with the
observation of its unique electronic structure and nodeless superconducting
gap, has generated much attention. Initial work also found that, while the
single-layer FeSe/SrTiO3 film exhibits a clear signature of superconductivity,
the double-layer FeSe/SrTiO3 film shows an insulating behavior. Such a dramatic
difference between the single-layer and double-layer FeSe/SrTiO3 films is
surprising and the underlying origin remains unclear. Here we report our
comparative study between the single-layer and double-layer FeSe/SrTiO3 films
by performing a systematic angle-resolved photoemission study on the samples
annealed in vacuum. We find that, like the single-layer FeSe/SrTiO3 film, the
as-prepared double-layer FeSe/SrTiO3 film is insulating and possibly magnetic,
thus establishing a universal existence of the magnetic phase in the
FeSe/SrTiO3 films. In particular, the double-layer FeSe/SrTiO3 film shows a
quite different doping behavior from the single-layer film in that it is hard
to get doped and remains in the insulating state under an extensive annealing
condition. The difference originates from the much reduced doping efficiency in
the bottom FeSe layer of the double-layer FeSe/SrTiO3 film from the FeSe-SrTiO3
interface. These observations provide key insights in understanding the origin
of superconductivity and the doping mechanism in the FeSe/SrTiO3 films. The
property disparity between the single-layer and double-layer FeSe/SrTiO3 films
may facilitate to fabricate electronic devices by making superconducting and
insulating components on the same substrate under the same condition.Comment: 19 pages, 4 figure
Phase Diagram and High Temperature Superconductivity at 65 K in Tuning Carrier Concentration of Single-Layer FeSe Films
Superconductivity in the cuprate superconductors and the Fe-based
superconductors is realized by doping the parent compound with charge carriers,
or by application of high pressure, to suppress the antiferromagnetic state.
Such a rich phase diagram is important in understanding superconductivity
mechanism and other physics in the Cu- and Fe-based high temperature
superconductors. In this paper, we report a phase diagram in the single-layer
FeSe films grown on SrTiO3 substrate by an annealing procedure to tune the
charge carrier concentration over a wide range. A dramatic change of the band
structure and Fermi surface is observed, with two distinct phases identified
that are competing during the annealing process. Superconductivity with a
record high transition temperature (Tc) at ~65 K is realized by optimizing the
annealing process. The wide tunability of the system across different phases,
and its high-Tc, make the single-layer FeSe film ideal not only to investigate
the superconductivity physics and mechanism, but also to study novel quantum
phenomena and for potential applications.Comment: 15 pages, 4 figure
Current induced anisotropic magnetoresistance in topological insulator films
Topological insulators are insulating in the bulk but possess spin-momentum
locked metallic surface states protected by time-reversal symmetry. The
existence of these surface states has been confirmed by angle-resolved
photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM).
Detecting these surface states by transport measurement, which might at first
appear to be the most direct avenue, was shown to be much more challenging than
expected. Here, we report a detailed electronic transport study in high quality
Bi2Se3 topological insulator thin films. Measurements under in-plane magnetic
field, along and perpendicular to the bias current show opposite
magnetoresistance. We argue that this contrasting behavior is related to the
locking of the spin and current direction providing evidence for helical spin
structure of the topological surface states
Scanning Tunneling Microscopy Studies of Topological Insulators Grown by Molecular Beam Epitaxy
We summarize our recent scanning tunneling microscopy (STM) study of topological insulator thin films grown by molecular beam epitaxy (MBE), which includes the observation of electron standing waves on topological insulator surface and the Landau quantization of topological surface states. The work has provided valuable information to the understanding of intriguing properties of topological insulators, as predicted by theory