40 research outputs found
Flux jumps, Second Magnetization Peak anomaly and the Peak Effect phenomenon in single crystals of and
We present magnetization measurements in single crystals of the tetragonal
compound, which exhibit the phenomenon of peak effect as well as
the second magnetization peak anomaly for H 0.5T (H c). At the lower
field (50mT H 200mT), we have observed the presence of flux jumps,
which seem to relate to a structural change in the local symmetry of the flux
line lattice (a first order re-orientation transition across a local field in
some parts of the sample, in the range of 100mT to 150mT). These flux jumps are
also observed in a single crystal of for H c in the field
region from 2 mT to 25 mT, which are compatible with the occurrence of a
re-orientation transition at a lower field in a cleaner crystal of this
compound, as compared to those of . Vortex phase diagrams drawn for
H c in and show that the ordered elastic glass
phase spans a larger part of (H, T) space in the former as compared to latter,
thereby, reaffirming the difference in the relative purity of the two samples.Comment: 11 pages, 14 figure
Evidence of gap anisotropy in superconducting YNi2B2C using directional point contact spectroscopy
We present a study of the anisotropy in the superconducting energy gap in a
single crystal of YNi2B2C (Tc~14.6K) using directional point contact
spectroscopy. The superconducting energy gap at 2.7K, when measured for I||c,
is 4.5 times larger than that for I||a. The energy gaps in the two directions
also have different temperature dependences. Our results support a scenario
with s+g like symmetry.Comment: Ps file with figure
Realization of giant magnetoelectricity in helimagnets
We show that low field magnetoelectric (ME) properties of helimagnets
Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22 can be efficiently tailored by Al-substitution
level. As x increases, the critical magnetic field for switching electric
polarization is systematically reduced from ~1 T down to ~1 mT, and the ME
susceptibility is greatly enhanced to reach a giant value of 2.0 x 10^4 ps/m at
an optimum x = 0.08. We find that control of nontrivial orbital moment in the
octahedral Fe sites through the Al-substitution is crucial for fine tuning of
magnetic anisotropy and obtaining the conspicuously improved ME
characteristics
Anisotropy in spatial order-disorder transformations and the vortex lattice symmetry transition in and
Explorations of the order-disorder transformation in vortex matter in single
crystals of tetragonal structured (c/a 3) borocarbide superconductors,
and , reveal that vortex arrays experience different
effective pinning in different crystallographic directions. We surmise that
correlation exists between the large anisotropy in effective pinning/disorder
and the differences in the (local) symmetry transition from rhombohedral to
(quasi) square vortex lattice(VL). For field along high symmetry directions,
like, c-axis and ab-basal plane, the VL symmetry is close to square and the
ordered state spans a large field interval. When the field is turned away from
the c-axis towards ab-plane, at intermediate angles, the region of ordered
state shrinks, in response to enhancement in effective pinning. At such
intermediate angles the symmetry of the VL would be far from ideal triangular
or square.Comment: 6 pages, 5 figures (Accepted in Euro Phys. Letts.
Magnetocaloric effect and magnetic cooling near a field-induced quantum-critical point
The presence of a quantum critical point (QCP) can significantly affect the
thermodynamic properties of a material at finite temperatures T. This is
reflected, e.g., in the entropy landscape S(T, r) in the vicinity of a QCP,
yielding particularly strong variations for varying the tuning parameter r such
as pressure or magnetic field B. Here we report on the determination of the
critical enhancement of near a B-induced QCP via
absolute measurements of the magnetocaloric effect (MCE), , and demonstrate that the accumulation of entropy around the QCP can be
used for efficient low-temperature magnetic cooling. Our proof of principle is
based on measurements and theoretical calculations of the MCE and the cooling
performance for a Cu-containing coordination polymer, which is a very
good realization of a spin-1/2 antiferromagnetic Heisenberg chain - one of the
simplest quantum-critical systems.Comment: 21 pages, 4 figure
Superconductivity in single crystals of a quasi-one dimensional infinite chain cuprate SrCaCuO at 90 K
Although there is no complete theory of high temperature superconductivity,
the importance of CuO planes in cuprate superconductors is confirmed from
both theory and experiments. Strong Coulomb repulsion between electrons on the
CuO plane makes the resultant electron system highly correlated and a
difficult problem to solve since exact solutions of many-body Hamiltonian in
two dimensions do not exist. If however, superconductivity can arise in
structures having chains rather than planes and having a high critical
temperature, then the high temperature superconductivity problem could become
more tractable since exact solutions in one dimension do exist. In this paper,
we report the observation of bulk superconductivity in single crystals of a
cuprate SrCaCuO at very high critical temperature, T, of
90 K whose structure reveals the presence of infinite double chains of
Cu-O-Cu-O instead of CuO planes, thus, ensuring quasi-one dimensional
superconductivity. Bulk superconducting behaviour was observed in \textit{dc}
magnetisation, \textit{ac} susceptibility as well as resistance measurements.
The observation of bulk superconductivity in SrCaCuO having
chains of Cu-O-Cu-O rather than planes of CuO at a high T of 90 K is
expected to profoundly impact our understanding of high temperature
superconductivity.Comment: 15 pages, 4 figure
Critical points in the Bragg glass phase of a weakly pinned crystal of CaRhSn
New experimental data are presented on the scan rate dependence of the
magnetization hysteresis width ( critical current
density ) in isothermal scans in a weakly pinned single crystal
of CaRhSn, which displays second magnetization peak (SMP)
anomaly as distinct from the peak effect (PE). We observe an interesting
modulation in the field dependence of a parameter which purports to measure the
dynamical annealing of the disordered bundles of vortices injected through the
sample edges towards the destined equilibrium vortex state at a given .
These data, in conjunction with the earlier observations made while studying
the thermomagnetic history dependence in in the tracing of the minor
hysteresis loops, imply that the partially disordered state heals towards the
more ordered state between the peak field of the SMP anomaly and the onset
field of the PE. The vortex phase diagram in the given crystal of
CaRhSn has been updated in the context of the notion of the
phase coexistence of the ordered and disordered regions between the onset field
of the SMP anomaly and the spinodal line located just prior to the
irreversibility line. A multi-critical point and a critical point in the
() region of the Bragg glass phase have been marked in this phase diagram
and the observed behaviour is discussed in the light of recent data on
multi-critical point in the vortex phase diagram in a single crystal of Nb.Comment: To appear in Current trends in Vortex State Studies - Pramana J.
Physic