270 research outputs found
Free energy and torque for superconductors with different anisotropies of H_{c2} and lambda
The free energy is evaluated for a uniaxial superconductor with the
anisotropy of the upper critical field, gamma_H = H_{c2,ab}/H_{c2,c}, different
from the anisotropy of the penetration depth gamma_{lambda} =
lambda_c/lambda_{ab}. With increasing difference between gamma_H and
gamma_{lambda}, the equilibrium orientation of the crystal relative to the
applied field may shift from theta = pi/2 (theta is the angle between the field
and the c axis) to lower angles and reach theta = 0 for large enough gamma_H.
These effects are expected to take place in MgB_2.Comment: 4 pages, 3 fig
Temperature-dependent anisotropy in MgB as inferred from measurements on polycrystals
We present data on temperature-dependent anisotropy of the upper critical
field of MgB obtained from the analysis of measurements on high purity, low
resistivity polycrystals. The anisotropy decreases in a monotonic fashion with
increase of temperature
Pressure induced superconductivity in CaFeAs
CaFeAs has been found to be exceptionally sensitive to the
application of hydrostatic pressure and superconductivity has been found to
exist in a narrow pressure region that appears to be at the interface between
two different phase transitions. The pressure - temperature () phase
diagram of CaFeAs reveals that this stoichiometric, highly ordered,
compound can be easily tuned to reveal all the salient features associated with
FeAs-based superconductivity without introducing any disorder. Whereas at
ambient pressure CaFeAs does not superconduct for K and
manifests a first order structural phase transition near K, the
application of kbar hydrostatic pressure fully suppresses the
resistive signature of the structural phase transition and instead
superconductivity is detected for K. For kbar a different
transition is detected, one associated with a clear reduction in resistivity
and for kbar superconductivity is no longer detected. This higher
pressure transition temperature increases rapidly with increasing pressure,
exceeding 300 K by kbar. The low temperature, superconducting dome
is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This
superconducting phase appears to exist when the low pressure transition is
suppressed sufficiently, but before the high pressure transition has reduced
the resistivity, and possibly the associated fluctuations, too dramatically
Signatures of quantum criticality in the thermopower of Ba(Fe(1-x)Co(x))2As2
We demonstrate that the thermopower (S) can be used to probe the spin
fluctuations (SFs) in proximity to the quantum critical point (QCP) in Fe-based
superconductors. The sensitivity of S to the entropy of charge carriers allows
us to observe an increase of S/T in Ba(Fe(1-x)Co(x))2As2 close to the
spin-density-wave (SDW) QCP. This behavior is due to the coupling of low-energy
conduction electrons to two-dimensional SFs, similar to heavy-fermion systems.
The low-temperature enhancement of S/T in the Co substitution range 0.02 < x <
0.1 is bordered by two Lifshitz transitions, and it corresponds to the
superconducting region, where a similarity between the electron and
non-reconstructed hole pockets exists. The maximal S/T is observed in proximity
to the commensurate-to-incommensurate SDW transition, for critical x_c ~ 0.05,
close to the highest superconducting T_c. This analysis indicates that low-T
thermopower is influenced by critical spin fluctuations which are important for
the superconducting mechanism
Field-Dependent Hall Effect in Single Crystal Heavy Fermion YbAgGe below 1K
We report the results of a low temperature (T >= 50 mK) and high field (H <=
180 kOe) study of the Hall resistivity in single crystals of YbAgGe, a heavy
fermion compound that demonstrates field-induced non-Fermi-liquid behavior near
its field-induced quantum critical point. Distinct features in the anisotropic,
field-dependent Hall resistivity sharpen on cooling down and at the base
temperature are close to the respective critical fields for the field-induced
quantum critical point. The field range of the non-Fermi-liquid region
decreases on cooling but remains finite at the base temperature with no
indication of its conversion to a point for T -> 0. At the base temperature,
the functional form of the field-dependent Hall coefficient is field direction
dependent and complex beyond existing simple models thus reflecting the
multi-component Fermi surface of the material and its non-trivial modification
at the quantum critical point
Magnetoresistivity and Complete in
Detailed magneto-transport data on dense wires of are reported for
applied magnetic fields up to 18 T. The temperature and field dependencies of
the electrical resistivity are consistent with behaving like a simple
metal and following a generalized form of Kohler's rule. In addition, given the
generally high values and narrow resistive transition widths associated
with synthesized in this manner, combined with applied magnetic fields
of up to 18 T, an accurate and complete curve could be determined.
This curve agrees well with curves determined from lower field measurements on
sintered pellets and wires of . is linear in over a wide
range of temperature (7 K 32 K) and has an upward curvature for
close to . These features are similar to other high , clean limit,
boron-bearing intermetallics: and .Comment: minor changes in styl
Magnetic field-induced quantum critical point in YbPtIn and YbPtIn single crystals
Detailed anisotropic (Hab and Hc) resistivity and
specific heat measurements were performed on online-grown YbPtIn and
solution-grown YbPtIn single crystals for temperatures down to 0.4 K,
and fields up to 140 kG; Hab Hall resistivity was also measured on
the YbPtIn system for the same temperature and field ranges. All these
measurements indicate that the small change in stoichiometry between the two
compounds drastically affects their ordering temperatures (T
K in YbPtIn, and K in YbPtIn). Furthermore, a field-induced
quantum critical point is apparent in each of these heavy fermion systems, with
the corresponding critical field values of YbPtIn (H around
35-45 kG and H kG) also reduced compared to the analogous
values for YbPtIn (H kG and H kG
Effects of Neutron Irradiation on Carbon Doped MgB2 Wire Segments
We have studied the evolution of superconducting and normal state properties
of neutron irradiated Mg(BC) wire segments as a function
of post exposure annealing time and temperature. The initial fluence fully
suppressed superconductivity and resulted in an anisotropic expansion of the
unit cell. Superconductivity was restored by post-exposure annealing. The upper
critical field, H(T=0), approximately scales with T starting with an
undamaged T near 37 K and H(T=0) near 32 T. Up to an annealing
temperature of 400 C the recovery of T tends to coincide with a
decrease in the normal state resistivity and a systematic recovery of the
lattice parameters. Above 400 C a decrease in order along the c- direction
coincides with an increase in resistivity, but no apparent change in the
evolution of T and H. To first order, it appears that carbon doping
and neutron damaging effect the superconducting properties of MgB
independently
Superconducting MgB2 thin films by pulsed laser deposition
Growth of MgB2 thin films by pulsed laser deposition is examined under ex
situ and in situ processing conditions. For the ex situ process, Boron films
grown by PLD were annealed at 900 C with excess Mg. For the in situ process,
different approaches involving ablation from a stoichiometric target under
different growth conditions, as well as multilayer deposition involving
interposed Mg layers were examined and analyzed. Magnetic measurements on ex
situ processed films show TC of ~39 K, while the current best in situ films
show a susceptibility transition at ~ 22 K.Comment: 3 pages, PD
Direct observation of Fe spin reorientation in single crystalline YbFe6Ge6
We have grown single crystals of YbFe6Ge6 and LuFe6Ge6 and characterized
their anisotropic behaviour through low field magnetic susceptibility,
field-dependent magnetization, resistivity and heat capacity measurements. The
Yb+3 valency is confirmed by LIII XANES measurements. YbFe6Ge6 crystals exhibit
a field-dependent, sudden reorientation of the Fe spins at about 63 K, a unique
effect in the RFe6Ge6 family (R = rare earths) where the Fe ions order
anti-ferromagnetically with Neel temperatures above 450 K and the R ions'
magnetism appears to behave independently. The possible origins of this unusual
behaviour of the ordered Fe moments in this compound are discussed.Comment: 12 pages, 8 figures, accepted in J. Phys.: Cond. Matte
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