10,335 research outputs found
The upper critical field and its anisotropy in LiFeAs
The upper critical field of LiFeAs single crystals has
been determined by measuring the electrical resistivity using the facilities of
pulsed magnetic field at Los Alamos. We found that of LiFeAs
shows a moderate anisotropy among the layered iron-based superconductors; its
anisotropic parameter monotonically decreases with decreasing
temperature and approaches as . The upper
critical field reaches 15T () and 24.2T () at
1.4K, which value is much smaller than other iron-based high
superconductors. The temperature dependence of can be
described by the Werthamer-Helfand-Hohenberg (WHH) method, showing orbitally
and (likely) spin-paramagnetically limited upper critical field for and , respectively.Comment: 5 pages,5 figure
Universality in quantum chaos and the one parameter scaling theory
We adapt the one parameter scaling theory (OPT) to the context of quantum
chaos. As a result we propose a more precise characterization of the
universality classes associated to Wigner-Dyson and Poisson statistics which
takes into account Anderson localization effects. Based also on the OPT we
predict a new universality class in quantum chaos related to the
metal-insulator transition and provide several examples. In low dimensions it
is characterized by classical superdiffusion or a fractal spectrum, in higher
dimensions it can also have a purely quantum origin as in the case of
disordered systems. Our findings open the possibility of studying the metal
insulator transition experimentally in a much broader type of systems.Comment: 4 pages, 2 figures, acknowledgment added, typos correcte
Anderson transition in a three dimensional kicked rotor
We investigate Anderson localization in a three dimensional (3d) kicked
rotor. By a finite size scaling analysis we have identified a mobility edge for
a certain value of the kicking strength . For dynamical
localization does not occur, all eigenstates are delocalized and the spectral
correlations are well described by Wigner-Dyson statistics. This can be
understood by mapping the kicked rotor problem onto a 3d Anderson model (AM)
where a band of metallic states exists for sufficiently weak disorder. Around
the critical region we have carried out a detailed study of the
level statistics and quantum diffusion. In agreement with the predictions of
the one parameter scaling theory (OPT) and with previous numerical simulations
of a 3d AM at the transition, the number variance is linear, level repulsion is
still observed and quantum diffusion is anomalous with . We note that in the 3d kicked rotor the dynamics is not random but
deterministic. In order to estimate the differences between these two
situations we have studied a 3d kicked rotor in which the kinetic term of the
associated evolution matrix is random. A detailed numerical comparison shows
that the differences between the two cases are relatively small. However in the
deterministic case only a small set of irrational periods was used. A
qualitative analysis of a much larger set suggests that the deviations between
the random and the deterministic kicked rotor can be important for certain
choices of periods. Contrary to intuition correlations in the deterministic
case can either suppress or enhance Anderson localization effects.Comment: 10 pages, 5 figure
The magnetoresistance and Hall effect in CeFeAsO: a high magnetic field study
The longitudinal electrical resistivity and the transverse Hall resistivity
of CeFeAsO are simultaneously measured up to a magnetic field of 45T using the
facilities of pulsed magnetic field at Los Alamos. Distinct behaviour is
observed in both the magnetoresistance Rxx({\mu}0H) and the Hall resistance
Rxy({\mu}0H) while crossing the structural phase transition at Ts \approx 150K.
At temperatures above Ts, little magnetoresistance is observed and the Hall
resistivity follows linear field dependence. Upon cooling down the system below
Ts, large magnetoresistance develops and the Hall resistivity deviates from the
linear field dependence. Furthermore, we found that the transition at Ts is
extremely robust against the external magnetic field. We argue that the
magnetic state in CeFeAsO is unlikely a conventional type of spin-density-wave
(SDW).Comment: 4 pages, 3 figures SCES2010, To appear in J. Phys.: Conf. Ser. for
SCES201
Modeling Heterogeneous Materials via Two-Point Correlation Functions: II. Algorithmic Details and Applications
In the first part of this series of two papers, we proposed a theoretical
formalism that enables one to model and categorize heterogeneous materials
(media) via two-point correlation functions S2 and introduced an efficient
heterogeneous-medium (re)construction algorithm called the "lattice-point"
algorithm. Here we discuss the algorithmic details of the lattice-point
procedure and an algorithm modification using surface optimization to further
speed up the (re)construction process. The importance of the error tolerance,
which indicates to what accuracy the media are (re)constructed, is also
emphasized and discussed. We apply the algorithm to generate three-dimensional
digitized realizations of a Fontainebleau sandstone and a boron
carbide/aluminum composite from the two- dimensional tomographic images of
their slices through the materials. To ascertain whether the information
contained in S2 is sufficient to capture the salient structural features, we
compute the two-point cluster functions of the media, which are superior
signatures of the micro-structure because they incorporate the connectedness
information. We also study the reconstruction of a binary laser-speckle pattern
in two dimensions, in which the algorithm fails to reproduce the pattern
accurately. We conclude that in general reconstructions using S2 only work well
for heterogeneous materials with single-scale structures. However, two-point
information via S2 is not sufficient to accurately model multi-scale media.
Moreover, we construct realizations of hypothetical materials with desired
structural characteristics obtained by manipulating their two-point correlation
functions.Comment: 35 pages, 19 figure
Progress and challenges in photocatalytic ammonia synthesis
Photocatalytic ammonia (NH_{3}) synthesis from N_{2} and water driven by solar energy is a sustainable and environmentally friendly technology, which has gained considerable attention in recent years. In this review, the recent development in the fundamental understanding of photocatalytic NH_{3} synthesis and the methods of precise NH_{3} detection are summarized. More importantly the strategy for surface engineering and interface engineering of photocatalysts toward photocatalytic NH_{3} production has been thoroughly analyzed with the aim to stimulate critical thinking about the effective methodology for catalyst modification instead of exploring new materials. At the end the challenges and a few concerns are raised from the current reports and future perspectives in this research field are discussed targeting to clarify the reliability and reproducibility of the photochemical process and to direct the future research direction, such as flow reactor design and in-depth understanding of the underlying reaction pathway
- …