20,259 research outputs found
Decoupling of the superconducting and magnetic (structural) phase transitions in electron-doped BaFe2As2
Study and comparison of over 30 examples of electron doped BaFe2As2 for
transition metal (TM) = Co, Ni, Cu, and (Co/Cu mixtures) have lead to an
understanding that the suppression of the structural/antiferromagnetic phase
transition to low enough temperature in these compounds is a necessary
condition for superconductivity, but not a sufficient one. Whereas the
structural/antiferromagnetic transitions are suppressed by the number of TM
dopant ions (or changes in the c-axis) the superconducting dome exists over a
limited range of values of the number of electrons added by doping (or values
of the {a/c} ratio). By choosing which combination of dopants are used we can
change the relative positions of the upper phase lines and the superconducting
dome, even to the extreme limit of suppressing the upper structural and
magnetic phase transitions without the stabilization of low temperature
superconducting dome
Physical and magnetic properties of Ba(FeRu)As single crystals
Single crystals of Ba(FeRu)As, , have been grown
and characterized by structural, magnetic and transport measurements. These
measurements show that the structural/magnetic phase transition found in pure
BaFeAs at 134 K is suppressed monotonically by Ru doping, but, unlike
doping with TM=Co, Ni, Cu, Rh or Pd, the coupled transition seen in the parent
compound does not detectably split into two separate ones. Superconductivity is
stabilized at low temperatures for and continues through the highest
doping levels we report. The superconducting region is dome like, with maximum
T ( K) found around . A phase diagram of temperature
versus doping, based on electrical transport and magnetization measurements,
has been constructed and compared to those of the
Ba(FeTM)As (TM=Co, Ni, Rh, Pd) series as well as to the
temperature-pressure phase diagram for pure BaFeAs. Suppression of the
structural/magnetic phase transition as well as the appearance of
superconductivity is much more gradual in Ru doping, as compared to Co, Ni, Rh
and Pd doping, and appears to have more in common with BaFeAs tuned
with pressure; by plotting and as a function of changes in unit
cell dimensions, we find that changed in the ratio, rather than changes
in , or V, unify the and phase diagrams for BaFeAs
and Ba(FeRu)As respectively.Comment: 16 pages, 10 figure
Effects of Co substitution on thermodynamic and transport properties and anisotropic in Ba(FeCo)As single crystals
Single crystalline samples of Ba(FeCo)As with
have been grown and characterized via microscopic, thermodynamic and transport
measurements. With increasing Co substitution, the thermodynamic and transport
signatures of the structural (high temperature tetragonal to low temperature
orthorhombic) and magnetic (high temperature non magnetic to low temperature
antiferromagnetic) transitions are suppressed at a rate of roughly 15 K per
percent Co. In addition, for superconductivity is stabilized,
rising to a maximum of approximately 23 K for and
decreasing for higher values. The phase diagram for
Ba(FeCo)As indicates that either superconductivity can
exist in both low temperature crystallographic phases or that there is a
structural phase separation. Anisotropic, superconducting, upper critical field
data () show a significant and clear change in anisotropy between
samples that have higher temperature structural phase transitions and those
that do not. These data show that the superconductivity is sensitive to the
suppression of the higher temperature phase transition
Determination of Intrinsic Ferroelectric Polarization in Orthorhombic Manganites with E-type Spin Order
By directly measuring electrical hysteresis loops using the Positive-Up
Negative-Down (PUND) method, we accurately determined the remanent
ferroelectric polarization Pr of orthorhombic RMnO3 (R = Ho, Tm, Yb, and Lu)
compounds below their E-type spin ordering temperatures. We found that LuMnO3
has the largest Pr of 0.17 uC/cm^2 at 6 K in the series, indicating that its
single-crystal form can produce a Pr of at least 0.6 \muuC/cm^2 at 0 K.
Furthermore, at a fixed temperature, Pr decreases systematically with
increasing rare earth ion radius from R = Lu to Ho, exhibiting a strong
correlation with the variations in the in-plane Mn-O-Mn bond angle and Mn-O
distances. Our experimental results suggest that the contribution of the Mn t2g
orbitals dominates the ferroelectric polarization.Comment: 16 pages, 4 figure
Whole-Page Optimization and Submodular Welfare Maximization with Online Bidders
In the context of online ad serving, display ads may appear on different types of webpages, where each page includes several ad slots and therefore multiple ads can be shown on each page. The set of ads that can be assigned to ad slots of the same page needs to satisfy various prespecified constraints including exclusion constraints, diversity constraints, and the like. Upon arrival of a user, the ad serving system needs to allocate a set of ads to the current webpage respecting these per-page allocation constraints. Previous slot-based settings ignore the important concept of a page and may lead to highly suboptimal results in general. In this article, motivated by these applications in display advertising and inspired by the submodular welfare maximization problem with online bidders, we study a general class of page-based ad allocation problems, present the first (tight) constant-factor approximation algorithms for these problems, and confirm the performance of our algorithms experimentally on real-world datasets.
A key technical ingredient of our results is a novel primal-dual analysis for handling free disposal, which updates dual variables using a “level function” instead of a single level and unifies with previous analyses of related problems. This new analysis method allows us to handle arbitrarily complicated allocation constraints for each page. Our main result is an algorithm that achieves a 1 &minus frac 1 e &minus o(1)-competitive ratio. Moreover, our experiments on real-world datasets show significant improvements of our page-based algorithms compared to the slot-based algorithms.
Finally, we observe that our problem is closely related to the submodular welfare maximization (SWM) problem. In particular, we introduce a variant of the SWM problem with online bidders and show how to solve this problem using our algorithm for whole-page optimization.postprin
Demonstrating Additional Law of Relativistic Velocities based on Squeezed Light
Special relativity is foundation of many branches of modern physics, of which
theoretical results are far beyond our daily experience and hard to realized in
kinematic experiments. However, its outcomes could be demonstrated by making
use of convenient substitute, i.e. squeezed light in present paper. Squeezed
light is very important in the field of quantum optics and the corresponding
transformation can be regarded as the coherent state of SU(1; 1). In this
paper, the connection between the squeezed operator and Lorentz boost is built
under certain conditions. Furthermore, the additional law of relativistic
velocities and the angle of Wigner rotation are deduced as well
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