554 research outputs found
Oxygen vacancies in strained SrTiO thin films: formation enthalpy and manipulation
We report the enthalpy of oxygen vacancy formation in thin films of
electron-doped SrTiO, under different degrees of epitaxial stress. We
demonstrate that both compressive and tensile strain decrease this energy at a
very similar rate, and promote the formation of stable doubly ionized oxygen
vacancies. Moreover, we also show that unintentional cationic vacancies
introduced under typical growth conditions, produce a characteristic rotation
pattern of TiO octahedra. The local concentration of oxygen vacancies can
be modulated by an electric field with an AFM tip, changing not only the local
electrical potential, but also producing a non-volatile mechanical response
whose sign (up/down) can be reversed by the electric field.Comment: Physical Review B (accepted for publication
Electron degeneracy and intrinsic magnetic properties of epitaxial Nb:SrTiO thin-films controlled by defects
We report thermoelectric power experiments in e-doped thin films of SrTiO
(STO) which demonstrate that the electronic band degeneracy can be lifted
through defect management during growth. We show that even small amounts of
cationic vacancies, combined with epitaxial stress, produce a homogeneous
tetragonal distortion of the films, resulting in a Kondo-like resistance upturn
at low temperature, large anisotropic magnetoresistance, and non-linear Hall
effect. Ab-initio calculations confirm a different occupation of each band
depending on the degree of tetragonal distortion. The phenomenology reported in
this paper for tetragonally distorted e-doped STO thin films, is similarto that
observed in LaAlO/STO interfaces and magnetic STO quantum wells.Comment: 5 pages, 5 figure
M-SrFe12O19 and ferrihydrite-like ultrathin nanoplatelets as building blocks for permanent magnets: HAADF-STEM study and magnetic properties
Mixtures of M-type strontium hexaferrite (M-SrFe12O19) and ferrihydrite-like particles were prepared by a microwave-assisted hydrothermal process at 200 °C with heating rates in the range 40–50 °C min-1. The particles exhibited a platelet shape with a diameter comprised between 20 and 200 nm and a thickness between 2 and 5 nm. HAADF-STEM observations and EDS analysis were carried out for a better understanding of nucleation and growth process. EDS showed that most of the particles contained Sr and HAADF-STEM revealed that very thin particles with a hexaferrite core extending over less than a unit cell and with surface disorder crystallized along with well crystallized hexaferrite and defect free ferrihydrite particles. The symmetric multilayer structures (SRS) of the ultrathin particles suggested that the nucleation step of the hexaferrite particles involved clusters containing Sr atoms. In comparison with the M-SrFe12O19 micrometer sized platelets prepared with heating rate of 25 °C min-1, the mixtures of ultrathin hexaferrite- and ferrihydrite-like particles combined after annealing a higher coercivity reaching 465 kA m-1 thanks to the smaller initial particle size and a high magnetization reaching 65 A m2 kg-1 thanks to a limited amount of hematite
Randomized Extended Kaczmarz for Solving Least-Squares
We present a randomized iterative algorithm that exponentially converges in
expectation to the minimum Euclidean norm least squares solution of a given
linear system of equations. The expected number of arithmetic operations
required to obtain an estimate of given accuracy is proportional to the square
condition number of the system multiplied by the number of non-zeros entries of
the input matrix. The proposed algorithm is an extension of the randomized
Kaczmarz method that was analyzed by Strohmer and Vershynin.Comment: 19 Pages, 5 figures; code is available at
https://github.com/zouzias/RE
Reduction of thermal conductivity in ferroelectric SrTiO3 thin films
Bulk SrTiO3 is a quantum paraelectric in which an antiferrodistortive distortion below approximate to 105 K and quantum fluctuations at low temperature preclude the stabilization of a long-range ferroelectric state. However, biaxial mechanical stress, impurity doping, and Sr nonstoichiometry, among other mechanisms, are able to stabilize a ferroelectric or relaxor ferroelectric state at room temperature, which develops into a longer-range ferroelectric state below 250 K. In this paper, we show that epitaxial SrTiO3 thin films grown under tensile strain on DyScO3 exhibit a large reduction of thermal conductivity, approximate to 60% of at room temperature, with respect to identical strain-free or compressed films. The thermal conductivity shows a further reduction below 250 K, a temperature concurrent with the peak in the dielectric constant [J. H. Haeni et al., Nature (London) 430, 758 (2004)]. These results suggest that strain gradients in the relaxor and ferroelectric phase of SrTiO3 are very effective phonon scatterers, limiting the thermal transport in this material
Efficient Triangle Counting in Large Graphs via Degree-based Vertex Partitioning
The number of triangles is a computationally expensive graph statistic which
is frequently used in complex network analysis (e.g., transitivity ratio), in
various random graph models (e.g., exponential random graph model) and in
important real world applications such as spam detection, uncovering of the
hidden thematic structure of the Web and link recommendation. Counting
triangles in graphs with millions and billions of edges requires algorithms
which run fast, use small amount of space, provide accurate estimates of the
number of triangles and preferably are parallelizable.
In this paper we present an efficient triangle counting algorithm which can
be adapted to the semistreaming model. The key idea of our algorithm is to
combine the sampling algorithm of Tsourakakis et al. and the partitioning of
the set of vertices into a high degree and a low degree subset respectively as
in the Alon, Yuster and Zwick work treating each set appropriately. We obtain a
running time
and an approximation (multiplicative error), where is the number
of vertices, the number of edges and the maximum number of
triangles an edge is contained.
Furthermore, we show how this algorithm can be adapted to the semistreaming
model with space usage and a constant number of passes (three) over the graph
stream. We apply our methods in various networks with several millions of edges
and we obtain excellent results. Finally, we propose a random projection based
method for triangle counting and provide a sufficient condition to obtain an
estimate with low variance.Comment: 1) 12 pages 2) To appear in the 7th Workshop on Algorithms and Models
for the Web Graph (WAW 2010
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