856 research outputs found
3D-xy critical properties of YBa2Cu4O8 and magnetic field induced 3D to 1D crossover
We present reversible magnetization data of a YBa2Cu4O8 single crystal and
analyze the evidence for 3D-xy critical behavior and a magnetic field induced
3D to 1D crossover. Remarkable consistency with these phenomena is observed in
agreement with a magnetic field induced finite size effect, whereupon the
correlation length transverse to the applied magnetic field cannot grow beyond
the limiting magnetic length scale L_H. By applying the appropriate scaling
form we obtain the zero-field critical temperature, the 3D to 1D crossover, the
vortex melting line and the universal ratios of the related scaling variables.
Accordingly there is no continuous phase transition in the (H,T)-plane along
the H_c2-lines as predicted by the mean-field treatment.Comment: 8 pages, 4 figure
Normal state bottleneck and nematic fluctuations from femtosecond quasi-particle relaxation dynamics in Sm(Fe,Co)AsO
We investigate temperature and fluence dependent dynamics of the photoexcited
quasi-particle relaxation and low-energy electronic structure in electron-doped
1111-structure Sm(Fe_{0.93}Co_{0.07})AsO single crystal. We find that the
behavior is qualitatively identical to the 122-structure Ba(Fe,Co)_{2}As_{2}
including the presence of a normal state pseudogap and a marked 2-fold symmetry
breaking in the tetragonal phase that we relate to the electronic nematicity.
The 2-fold symmetry breaking appears to be a general feature of the electron
doped iron pnictides
Magnetic field induced 3D to 1D crossover in Sr0:9La0:1CuO2
The effect of the magnetic field on the critical behavior of Sr0:9La0:1CuO2
is explored in terms of reversible magnetization data. As the correlation
length transverse to the magnetic field Hi,applied along the i-axis, cannot
grow beyond the limiting magnetic length LHi, related to the average distance
between vortex lines, one expects a magnetic field induced finite size effect.
Invoking the scaling theory of critical phenomena we provide clear evidence for
this effect. It implies that in type II superconductors there is a 3D to 1D
crossover line Hpi(T). Consequently, below Tc and above Hpi(T) uperconductivity
is confined to cylinders with diameter LHi(1D). Accordingly, there is no
continuous phase transition in the (H,T)-plane along the Hc2-lines as predicted
by the mean-field treatment.Comment: 4 pages, 5 figure
Online Self-Indexed Grammar Compression
Although several grammar-based self-indexes have been proposed thus far,
their applicability is limited to offline settings where whole input texts are
prepared, thus requiring to rebuild index structures for given additional
inputs, which is often the case in the big data era. In this paper, we present
the first online self-indexed grammar compression named OESP-index that can
gradually build the index structure by reading input characters one-by-one.
Such a property is another advantage which enables saving a working space for
construction, because we do not need to store input texts in memory. We
experimentally test OESP-index on the ability to build index structures and
search query texts, and we show OESP-index's efficiency, especially
space-efficiency for building index structures.Comment: To appear in the Proceedings of the 22nd edition of the International
Symposium on String Processing and Information Retrieval (SPIRE2015
Finite-size and pressure effects in YBa_2Cu_4O_8 probed by magnetic field penetration depth measurements
We explore the combined pressure and finite-size effects on the in-plane
penetration depth \lambda_{ab} in YBa_2Cu_4O_8. Even though this cuprate is
stoichiometric the finite-size scaling analysis of \lambda_{ab}^{-2}(T)
uncovers the granular nature and reveals domains with nanoscale size L_{c}
along the c-axis. L_{c} ranges from 33.2 Angstrom to 28.9 Angstrom at pressures
from 0.5 to 11.5 kbar. These observations raise serious doubts on the existence
of a phase coherent macroscopic superconducting state in cuprate
superconductors.Comment: 7 pages, 6 figure
Probing superconductivity in MgB2 confined to magnetic field tuned cylinders by means of critical fluctuations
We report and analyze reversible magnetization measurements on a high quality
MgB2 single crystal in the vicinity of the zero field transition temperature,
T_c=38.83 K, at several magnetic fields up to 300 Oe, applied along the c-axis.
Though MgB2 is a two gap superconductor our scaling analysis uncovers
remarkable consistency with 3D-xy critical behavior, revealing that close to
criticality the order parameter is a single complex scalar as in 4He. This
opens up the window onto the exploration of the magnetic field induced finite
size effect, whereupon the correlation length transverse to the applied
magnetic field H_i applied along the i-axis cannot grow beyond the limiting
magnetic length L_Hi, related to the average distance between vortex lines. We
find unambiguous evidence for this finite size effect. It implies that in type
II superconductors, such as MgB2, there is the 3D to 1D crossover line H_pi and
xi denotes the critical amplitudes of the correlation lengths above and below
T_c along the respective axis. Consequently, above H_pi(T) and T<T_c
superconductivity is confined to cylinders with diameter L_Hi (1D). In
contrast, above T_c the uncondensed pairs are confined to cylinders.
Accordingly, there is no continuous phase transition in the (H,T)-plane along
the H_c2-lines as predicted by the mean-field treatment
Measurement of the penetration depth and coherence length of MgB2 in all directions using transmission electron microscopy
We demonstrate that images of flux vortices in a superconductor taken with a
transmission electron microscope can be used to measure the penetration depth
and coherence length in all directions at the same temperature and magnetic
field. This is particularly useful for MgB, where these quantities vary
with the applied magnetic field and values are difficult to obtain at low field
or in the -direction. We obtained images of flux vortices from an MgB
single crystal cut in the plane by focussed ion beam milling and tilted to
with respect to the electron beam about its axis. A new method
was developed to simulate these images which accounted for vortices with a
non-zero core in a thin, anisotropic superconductor and a simplex algorithm was
used to make a quantitative comparison between the images and simulations to
measure the penetration depths and coherence lengths. This gave penetration
depths nm and nm at 10.8 K in a
field of 4.8 mT. The large error in is a consequence of tilting
the sample about and had it been tilted about , the errors would be
reversed. Thus, obtaining the most precise values requires taking images of the
flux lattice with the sample tilted in more than one direction. In a previous
paper, we obtained a more precise value using a sample cut in the plane.
Using this value gives nm, nm,
nm and nm which agree well with
measurements made using other techniques. The experiment required two days to
conduct and does not require large-scale facilities. It was performed on a very
small sample: microns and 200 nm thick so this method could prove
useful for characterising new superconductors where only small single crystals
are available.This work was funded by the Royal Society. Work at
Eidgenossische Technische Hochschule, ZĂŒrich was supported
by the Swiss National Science Foundation and the National
Center of Competence in Research programme âMaterials
with Novel Electronic Properties.âThis is the accepted manuscript for a paper published in Physical Review B, 91, 054505, 5 February 2015, DOI: 10.1103/PhysRevB.91.05450
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