51 research outputs found
I-V curves and intergranular flux creep activation energy in the magnetic superconductor RuSr2GdCu2O8
A systematic study of I-V characteristic curves for RuSr2GdCu2O8 [Ru-(1212)]
is presented, with magnetic fields up to 3 T and 5 K<T<30 K, in the region of
the superconducting transition. The activation energy E_{a}(H,T) for flux line
depinning was determined by fitting the nonlinear region of the curves using
the flux creep model. E_{a}(H,T) was found to vary linearly with temperature,
while a power-law dependence on the magnetic field was observed up to H=0.1 T,
where an abrupt reduction in its decreasing rate occurs. The extrapolated
value, E_{a}(0,0)=50 meV, is twice the reported value for YBa2Cu3O7, but the
critical current density J_{C}(0,0)=70 A/cm2 is about one order of magnitude
lower. These results are explained as a consequence of the contribution of the
magnetization in the grains to the effective field at the intergranular links
and to a spin-flop transition of the Ru-sub-lattice.Comment: 4 pages, 3 figure
Inter- and Intra-granular flux Pinning in Ba(Fe0.91Co0.09)2As2 Superconductors
AbstractThermally assisted flux flow (TAFF) and flux pinning energiesare studied in a Ba(Fe0.91Co0.09)2As2 (Tc = 25.3K) sample via resistivity and AC susceptibility measurements in magnetic fields up to 18T. The flux pinning energy U(T,H) is determined from the Arrhenius law. The pinning maxima well determined by resistivity measurements ranged from 1724K at 0 T to 585K at 18 T with a sharp drop off so that U(T=Tc) varied with the applied field H as . The pinning activation energies determined from the AC susceptibility data but were by a factor of three higher, which is explained here. Both inter- and intra-granular pinning energies are determined in low fields. The onset of TAFF temperature and the crossover temperature Tx from TAFF to flux flow are determined, showing the limitations of the Anderson-Kim model
Frequency Dependent Flux Dynamics and Activation Energies in Pnictide Bulk (Ba0.56K0.44)Fe2As2 Superconductor
AbstractThermally activated flux de-pinning and flux activation de-pinning energies are studied in a (Ba0.56K0.44)Fe2As2 (Tc=38.5K) bulk superconductor in DC magnetic fields up to 18 T. Ac susceptibility was measured as a function of temperature, DC and AC magnetic fields, and frequency. Ac susceptibility curves shift to higher temperatures as the frequency is increased from 75 to 1997Hz in all fields. We model this data by Arrhenius law to determine flux activation energies as a function of AC and DC magnetic fields. The activation energy ranges from 8822K at μ0 Hdc = 0 T to 1100K at 18 T for Hac =80 A/m. The energies drop quickly in a non-linear manner as DC field rises above 0 T and around 1 T, which we describe as pinning transition field, the drop levels and continues more slowly in a linear like manner as DC field approaches to 18 T. Furthermore, the activation energy drops quickly as AC field increases from 80 A/m to 800 A/m at 0 DC field. As the DC field rises above 0, the activation energy has significantly weaker dependence on the AC field amplitude. Extensive map of the de-pinning, or irreversibility, lines shows broad dependence on the magnitude of the small AC field, frequency, in addition to the DC field
Study of higher-order harmonics of complex ac susceptibility in thin films by the mutual inductive method
We have applied the mutual inductive method to study higher-order harmonics
of complex ac susceptibility for
YB_2C_3O_{7-\delta}nU_cJ_c$ for two samples
both at T=0.Comment: 8 pages, 3 figure
Local origin of the strong field-space anisotropy in the magnetic phase diagrams of CeLaB measured in a rotating magnetic field
Cubic f-electron compounds commonly exhibit highly anisotropic magnetic phase
diagrams consisting of multiple long-range ordered phases. Field-driven
metamagnetic transitions between them may depend not only on the magnitude, but
also on the direction of the applied magnetic field. Examples of such behavior
are plentiful among rare-earth borides, such as RB or RB ( = rare
earth). In this work, for example, we use torque magnetometry to measure
anisotropic field-angular phase diagrams of La-doped cerium hexaborides,
CeLaB ( = 0, 0.18, 0.28, 0.5). One expects that
field-directional anisotropy of phase transitions must be impossible to
understand without knowing the magnetic structures of the corresponding
competing phases and being able to evaluate their precise thermodynamic energy
balance. However, this task is usually beyond the reach of available
theoretical approaches, because the ordered phases can be noncollinear, possess
large magnetic unit cells, involve higher-order multipoles of 4f ions rather
than simple dipoles, or just lack sufficient microscopic characterization. Here
we demonstrate that the anisotropy under field rotation can be qualitatively
understood on a much more basic level of theory, just by considering the
crystal-electric-field scheme of a pair of rare-earth ions in the lattice,
coupled by a single nearest-neighbor exchange interaction. Transitions between
different crystal-field ground states, calculated using this minimal model for
the parent compound CeB6, possess field-directional anisotropy that strikingly
resembles the experimental phase diagrams. This implies that the anisotropy of
phase transitions is of local origin and is easier to describe than the ordered
phases themselves.Comment: To be published in Phys. Rev. B; v2: minor typographic correction
Revealing a 3D Fermi Surface Pocket and Electron-Hole Tunneling in UTe with Quantum Oscillations
Spin triplet superconductor UTe is widely believed to host a
quasi-two-dimensional Fermi surface, revealed by first principal calculations,
photoemission and quantum oscillation measurements. An outstanding question
still remains as to the existence of a three-dimensional Fermi surface pocket,
which is crucial for our understanding of the exotic superconducting and
topological properties of UTe. This 3D Fermi surface pocket appears in
various theoretical models with different physics origins but has not been
detected experimentally. Here for the first time, we provide concrete evidence
for a relatively isotropic, small Fermi surface pocket of UTe via quantum
oscillation measurements. In addition, we observed high frequency quantum
oscillations corresponding to electron-hole tunneling between adjacent electron
and hole pockets. The coexistence of 2D and 3D Fermi surface pockets, as well
as the breakdown orbits, provides a test bed for theoretical models and aid the
realization of a unified understanding of superconducting state of UTe
from the first-principles approach
Magneto-transport and magnetic susceptibility of SmFeAsO1-xFx (x = 0.0 and 0.20)
Bulk polycrystalline samples, SmFeAsO and the iso-structural superconducting
SmFeAsO0.80F0.20 are explored through resistivity with temperature under
magnetic field {\rho}(T, H), AC and DC magnetization (M-T), and Specific heat
(Cp) measurements. The Resistivity measurement shows superconductivity for x =
0.20 sample with Tc(onset) ~ 51.7K. The upper critical field, [Hc2(0)] is
estimated ~3770kOe by Ginzburg-Landau (GL) theory. Broadening of
superconducting transition in magneto transport is studied through thermally
activated flux flow in applied field up to 130 kOe. The flux flow activation
energy (U/kB) is estimated ~1215K for 1kOe field. Magnetic measurements
exhibited bulk superconductivity with lower critical field (Hc1) of ~1.2kOe at
2K. In normal state, the paramagnetic nature of compound confirms no trace of
magnetic impurity which orders ferromagnetically. AC susceptibility
measurements have been carried out for SmFeAsO0.80F0.20 sample at various
amplitude and frequencies of applied AC drive field. The inter-granular
critical current density (Jc) is estimated. Specific heat [Cp(T)] measurement
showed an anomaly at around 140K due to the SDW ordering of Fe, followed by
another peak at 5K corresponding to the antiferromagnetic (AFM) ordering of
Sm+3 ions in SmFeAsO compound. Interestingly the change in entropy (marked by
the Cp transition height) at 5K for Sm+3 AFM ordering is heavily reduced in
case of superconducting SmFeAsO0.80F0.20 sample.Comment: 18 pages text + Figs: comments/suggestions welcome
([email protected]
Intergranular flux pinning in underdoped and overdoped R(1-x)CaxBa2Cu3Oz (R=Y,Gd; x=0,0.2) samples
The temperature dependences of AC magnetic susceptibility at different
magnetic field amplitudes and frequencies are investigated for underdoped and
overdoped R(1-x)Ca(x)Ba(2)Cu(3)O(7-delta)(R=Y; Gd and x=0; 0.2) polycrystalline
samples. The activation energy, Ea, for thermally assisted flux flow (TAFF) in
intergranular region is determined. It was established that a correlation
exists between the intergranular critical current and flux pinning activation
energy. In underdoped samples the intergranular current shows S-I-S behaviour
and the activation energy is small, while in overdoped samples the
intergranular current is changed to S-N-S type and the activation energy
increases. 2D pancake vortices are characteristic of underdoped samples, while
3D vortex system exists in overdoped samples. In fact we demonstrate that Ca
substitution not only increases carrier concentration, but improves
intergranular activation energy for TAFF.Comment: 20 pages including 7 figures and one Table. accepted in Physica
Physical property characterization of single step synthesized NdFeAsO0.80F0.20 bulk 50K superconductor
We report an easy single step synthesis route of title compound
NdFeAsO0.80F0.20 superconductor having bulk superconductivity below 50 K. The
title compound is synthesized via solid-state reaction route by encapsulation
in an evacuated (10-3 Torr) quartz tube. Rietveld analysis of powder X-ray
diffraction data shows that compound crystallized in tetragonal structure with
space group P4/nmm. R(T)H measurements showed superconductivity with Tc (R=0)
at 48 K and a very high upper critical field (Hc2) of up to 345 Tesla. Magnetic
measurements exhibited bulk superconductivity in terms of diamagnetic onset
below 50 K. The lower critical field (Hc1) is around 1000 Oe at 5 K. In normal
state i.e., above 60 K, the compound exhibited purely paramagnetic behavior and
thus ruling out the presence of any ordered FeOx impurity in the matrix. In
specific heat measurements a jump is observed in the vicinity of
superconducting transition (Tc) along with an upturn at below T=4 K due to the
AFM ordering of Nd+3 ions in the system. The Thermo-electric power (TEP) is
negative down to Tc, thus indicating dominant carriers to be of n-type in
NdFeAsO0.80F0.20 superconductor. The granularity of the bulk superconducting
NdFeAsO0.8F0.2 sample is investigated and the intra and inter grain
contributions have been individuated by looking at various amplitude and
frequencies of the applied AC drive magnetic field.Comment: 26pages text + Figures: comments/suggestions welcome
([email protected] & http://www.freewebs.com/vpsawana
Peaceful Returns: Reversing Ethnic Cleansing after the Bosnian War
This article questions the conventional wisdom which claims forced migration is irreversible following massive ethnic cleansing campaigns, by investigating durable returns to pre-conflict home communities in Bosnia-Herzegovina. We formulate a set of novel hypotheses on the demographic determinants of return as well as on the role of social capital, nationalist ideology, integration, and war victimization. We use a 2013 Bosnian representative sample with 1,007 respondents to test our hypotheses. The findings support the expectation that gender and age have a major impact on return. Net of other factors, women and those experiencing wartime victimization are less likely to return. Older Bosnians with positive memories of pre-conflict interethnic relations are more likely to return than younger persons or those with negative memories. Finally, ethnic Bosniacs are more likely to return than ethnic Croats or Serbs. More nationalistic internally displaced persons (IDPs) are less likely to return
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