501 research outputs found
Quantum oscillations in a topological insulator Bi_{1-x}Sb_{x}
We have studied transport and magnetic properties of Bi_{1-x}Sb_x, which is
believed to be a topological insulator - a new state of matter where an
insulating bulk supports an intrinsically metallic surface. In nominally
insulating Bi_{0.91}Sb_{0.09} crystals, we observed strong quantum oscillations
of the magnetization and the resistivity originating from a Fermi surface which
has a clear two-dimensional character. In addition, a three-dimensional Fermi
surface is found to coexist, which is possibly due to an unusual coupling of
the bulk to the surface. This finding demonstrates that quantum oscillations
can be a powerful tool to directly probe the novel electronic states in
topological insulators.Comment: 4 pages, 4 figure
FMR Study of Co/Ti Bilayer Thin Films
. We focused on the interaction between two ferromagnetic cobalt layers
through a non-magnetic titanium layer. The magnetic properties of the structure
were characterized by ferromagnetic resonance technique (FMR). The data were
collected as a function of non-magnetic titanium layer thickness. Co/Ti
multilayer (Ti (50 {\AA})/Co(45 {\AA})/Ti(2-40 {\AA})/Co(40 {\AA})/Ti(100
{\AA}))films were grown onto naturally oxidized p-type single crystal Si (100)
substrate at UHV condition with magnetron sputtering system at room
temperature. The thickness of Ti spacer layer ranges from 2 to 40 {\AA} with 2
{\AA} steps. We did not observe usual optic and acoustic modes; instead we had
two broad overlapped peaks for the films ranged from 6 {\AA} to 40 {\AA}. One
interesting result was the high anisotropic resonance field values for these
films. Exchange coupling between ferromagnetic layers causes shift on resonance
field values but these shifts in our samples were much larger than expected.
This large anisotropic behavior is not clear at the moment. Our theoretical
model was not able to determine a value for the exchange coupling parameter.
One reason can be the close thickness values for Co sublayers. The other reason
can be the Ti non-magnetic layer. If titanium did not grow layer by layer on
cobalt, the cobalt ferromagnetic layers may behave as a single layer. As a
result one cannot observe exchange interaction between ferromagnetic layers
through non-magnetic spacer.Comment: 6 pages, 5 figure
Oscillatory angular dependence of the magnetoresistance in a topological insulator Bi_{1-x}Sb_{x}
The angular-dependent magnetoresistance and the Shubnikov-de Haas
oscillations are studied in a topological insulator Bi_{0.91}Sb_{0.09}, where
the two-dimensional (2D) surface states coexist with a three-dimensional (3D)
bulk Fermi surface (FS). Two distinct types of oscillatory phenomena are
discovered in the angular-dependence: The one observed at lower fields is shown
to originate from the surface state, which resides on the (2\bar{1}\bar{1})
plane, giving a new way to distinguish the 2D surface state from the 3D FS. The
other one, which becomes prominent at higher fields, probably comes from the
(111) plane and is obviously of unknown origin, pointing to new physics in
transport properties of topological insulators.Comment: 4 pages, 5 figures, revised version with improved data and analysi
Angular-dependent oscillations of the magnetoresistance in Bi_2Se_3 due to the three-dimensional bulk Fermi surface
We observed pronounced angular-dependent magnetoresistance (MR) oscillations
in a high-quality Bi2Se3 single crystal with the carrier density of 5x10^18
cm^-3, which is a topological insulator with residual bulk carriers. We show
that the observed angular-dependent oscillations can be well simulated by using
the parameters obtained from the Shubnikov-de Haas oscillations, which
clarifies that the oscillations are solely due to the bulk Fermi surface. By
completely elucidating the bulk oscillations, this result paves the way for
distinguishing the two-dimensional surface state in angular-dependent MR
studies in Bi2Se3 with much lower carrier density. Besides, the present result
provides a compelling demonstration of how the Landau quantization of an
anisotropic three-dimensional Fermi surface can give rise to pronounced
angular-dependent MR oscillations.Comment: 5 pages, 5 figure
Large bulk resistivity and surface quantum oscillations in the topological insulator Bi2Te2Se
Topological insulators are predicted to present novel surface transport
phenomena, but their experimental studies have been hindered by a metallic bulk
conduction that overwhelms the surface transport. We show that a new
topological insulator, Bi2Te2Se, presents a high resistivity exceeding 1 Ohm-cm
and a variable-range hopping behavior, and yet presents Shubnikov-de Haas
oscillations coming from the surface Dirac fermions. Furthermore, we have been
able to clarify both the bulk and surface transport channels, establishing a
comprehensive understanding of the transport in this material. Our results
demonstrate that Bi2Te2Se is the best material to date for studying the surface
quantum transport in a topological insulator.Comment: 4 pages, 3 figure
Spin-Orbit Coupling and Anomalous Angular-Dependent Magnetoresistance in the Quantum Transport Regime of PbS
We measured magnetotransport properties of PbS single crystals which exhibit
the quantum linear magnetoresistance (MR) as well as the static skin effect
that creates a surface layer of additional conductivity. The Shubnikov-de Haas
oscillations in the longitudinal MR signify the peculiar role of spin-orbit
coupling. In the angular-dependent MR, sharp peaks are observed when the
magnetic field is slightly inclined from the longitudinal configuration, which
is totally unexpected for a system with nearly spherical Fermi surface and
points to an intricate interplay between the spin-orbit coupling and the
conducting surface layer in the quantum transport regime.Comment: 5 pages, 5 figure
Landau level spectroscopy of surface states in the topological insulator BiSb via magneto-optics
We have performed broad-band zero-field and magneto-infrared spectroscopy of
the three dimensional topological insulator BiSb. The
zero-field results allow us to measure the value of the direct band gap between
the conducting and valence bands. Under applied field in the
Faraday geometry (\emph{k} \emph{H} C1), we measured the presence of
a multitude of Landau level (LL) transitions, all with frequency dependence
. We discuss the ramification of this observation for
the surface and bulk properties of topological insulators.Comment: 7 pages, 8 figures, March Meeting 2011 Abstract: J35.0000
Observations of two-dimensional quantum oscillations and ambipolar transport in the topological insulator Bi2Se3 achieved by Cd doping
We present a defect-engineering strategy to optimize the transport properties
of the topological insulator Bi2Se3 to show a high bulk resistivity and clear
quantum oscillations. Starting with a p-type Bi2Se3 obtained by combining Cd
doping and a Se-rich crystal-growth condition, we were able to observe a
p-to-n-type conversion upon gradually increasing the Se vacancies by post
annealing. With the optimal annealing condition where a high level of
compensation is achieved, the resistivity exceeds 0.5 Ohmcm at 1.8 K and we
observed two-dimensional Shubnikov-de Haas oscillations composed of multiple
frequencies in magnetic fields below 14 T.Comment: 7 pages, 6 figure
Band structure engineering in (Bi1-xSbx)2Te3 ternary topological insulators
Three-dimensional (3D) topological insulators (TI) are novel quantum
materials with insulating bulk and topologically protected metallic surfaces
with Dirac-like band structure. The spin-helical Dirac surface states are
expected to host exotic topological quantum effects and find applications in
spintronics and quantum computation. The experimental realization of these
ideas requires fabrication of versatile devices based on bulk-insulating TIs
with tunable surface states. The main challenge facing the current TI materials
exemplified by Bi2Se3 and Bi2Te3 is the significant bulk conduction, which
remains unsolved despite extensive efforts involving nanostructuring, chemical
doping and electrical gating. Here we report a novel approach for engineering
the band structure of TIs by molecular beam epitaxy (MBE) growth of
(Bi1-xSbx)2Te3 ternary compounds. Angle-resolved photoemission spectroscopy
(ARPES) and transport measurements show that the topological surface states
exist over the entire composition range of (Bi1-xSbx)2Te3 (x = 0 to 1),
indicating the robustness of bulk Z2 topology. Most remarkably, the systematic
band engineering leads to ideal TIs with truly insulating bulk and tunable
surface state across the Dirac point that behave like one quarter of graphene.
This work demonstrates a new route to achieving intrinsic quantum transport of
the topological surface states and designing conceptually new TI devices with
well-established semiconductor technology.Comment: Minor changes in title, text and figures. Supplementary information
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