471 research outputs found
Additional Evidence for the Surface Origin of the Peculiar Angular-Dependent Magnetoresistance Oscillations Discovered in a Topological Insulator Bi_{1-x}Sb_{x}
We present detailed data on the unusual angular-dependent magnetoresistance
oscillation phenomenon recently discovered in a topological insulator
Bi_{0.91}Sb_{0.09}. Direct comparison of the data taken before and after
etching the sample surface gives compelling evidence that this phenomenon is
essentially originating from a surface state. The symmetry of the oscillations
suggests that it probably comes from the (111) plane, and obviously a new
mechanism, such as a coupling between the surface and the bulk states, is
responsible for this intriguing phenomenon in topological insulators.Comment: 5 pages, 4 figures, Proceedings manuscript for the 19th International
Conference on the Application of High Magnetic Fields in Semiconductor
Physics and Nanotechnology (HMF-19
Origin of the large thermoelectric power in oxygen-variable RBaCo_{2}O_{5+x} (R=Gd, Nd)
Thermoelectric properties of GdBaCo_{2}O_{5+x} and NdBaCo_{2}O_{5+x} single
crystals have been studied upon continuous doping of CoO_2 planes with either
electrons or holes. The thermoelectric response and the resistivity behavior
reveal a hopping character of the transport in both compounds, providing the
basis for understanding the recently found remarkable divergence of the Seebeck
coefficient at x=0.5. The doping dependence of the thermoelectric power evinces
that the configurational entropy of charge carriers, enhanced by their spin and
orbital degeneracy, plays a key role in the origin of the large thermoelectric
response in these correlated oxides.Comment: 5 pages, 4 figures, accepted for publication in PR
Mobile Broadband Possibilities considering the Arrival of IEEE 802.16m & LTE with an Emphasis on South Asia
This paper intends to look deeper into finding an ideal mobile broadband
solution. Special stress has been put in the South Asian region through some
comparative analysis. Proving their competency in numerous aspects, WiMAX and
LTE already have already made a strong position in telecommunication industry.
Both WiMAX and LTE are 4G technologies designed to move data rather than voice
having IP networks based on OFDM technology. So, they aren't like typical
technological rivals as of GSM and CDMA. But still a gesture of hostility seems
to outburst long before the stable commercial launch of LTE. In this paper
various aspects of WiMAX and LTE for deployment have been analyzed. Again, we
tried to make every possible consideration with respect to south Asia i.e. how
mass people of this region may be benefited. As a result, it might be regarded
as a good source in case of making major BWA deployment decisions in this
region. Besides these, it also opens the path for further research and in depth
thinking in this issue.Comment: IEEE Publication format, ISSN 1947 5500,
http://sites.google.com/site/ijcsis
Room-temperature ferromagnetism in Sr_(1-x)Y_xCoO_(3-delta) (0.2 < x < 0.25)
We have measured magnetic susceptibility and resistivity of
SrYCoO ( 0.1, 0.15, 0.2, 0.215, 0.225, 0.25, 0.3,
and 0.4), and have found that SrYCoO is a room
temperature ferromagnet with a Curie temperature of 335 K in a narrow
compositional range of 0.2 0.25. This is the highest transition
temperature among perovskite Co oxides. The saturation magnetization for
0.225 is 0.25 /Co at 10 K, which implies that the observed
ferromagnetism is a bulk effect. We attribute this ferromagnetism to a peculiar
Sr/Y ordering.Comment: 5 pages, 4 figure
Improved Approximation Algorithms for Segment Minimization in Intensity Modulated Radiation Therapy
he segment minimization problem consists of finding the smallest set of
integer matrices that sum to a given intensity matrix, such that each summand
has only one non-zero value, and the non-zeroes in each row are consecutive.
This has direct applications in intensity-modulated radiation therapy, an
effective form of cancer treatment. We develop three approximation algorithms
for matrices with arbitrarily many rows. Our first two algorithms improve the
approximation factor from the previous best of to (roughly) and , respectively, where is
the largest entry in the intensity matrix. We illustrate the limitations of the
specific approach used to obtain these two algorithms by proving a lower bound
of on the approximation
guarantee. Our third algorithm improves the approximation factor from to , where is (roughly) the largest
difference between consecutive elements of a row of the intensity matrix.
Finally, experimentation with these algorithms shows that they perform well
with respect to the optimum and outperform other approximation algorithms on
77% of the 122 test cases we consider, which include both real world and
synthetic data.Comment: 18 page
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
adde
Ultra-low carrier concentration and surface dominant transport in Sb-doped Bi2Se3 topological insulator nanoribbons
A topological insulator is a new state of matter, possessing gapless
spin-locking surface states across the bulk band gap which has created new
opportunities from novel electronics to energy conversion. However, the large
concentration of bulk residual carriers has been a major challenge for
revealing the property of the topological surface state via electron transport
measurement. Here we report surface state dominated transport in Sb-doped
Bi2Se3 nanoribbons with very low bulk electron concentrations. In the
nanoribbons with sub-10nm thickness protected by a ZnO layer, we demonstrate
complete control of their top and bottom surfaces near the Dirac point,
achieving the lowest carrier concentration of 2x10^11/cm2 reported in
three-dimensional (3D) topological insulators. The Sb-doped Bi2Se3
nanostructures provide an attractive materials platform to study fundamental
physics in topological insulators, as well as future applications.Comment: 5 pages, 4 figures, 1 tabl
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