867 research outputs found
Carbon doping of superconducting magnesium diboride
We present details of synthesis optimization and physical properties of
nearly single phase carbon doped MgB2 with a nominal stoichiometry of
Mg(B{0.8}C{0.2})2 synthesized from magnesium and boron carbide (B4C) as
starting materials. The superconducting transition temperature is ~ 22 K (~ 17
K lower than in pure MgB2). The temperature dependence of the upper critical
field is steeper than in pure MgB2 with Hc2(10K) ~ 9 T. Temperature dependent
specific heat data taken in different applied magnetic fields suggest that the
two-gap nature of superconductivity is still preserved for carbon doped MgB2
even with such a heavily suppressed transition temperature. In addition, the
anisotropy ratio of the upper critical field for T/Tc ~ 2/3 is gamma ~ 2. This
value is distinct from 1 (isotropic) and also distinct from 6 (the value found
for pure MgB2).Comment: 11 pages, 13 figures, submitted to Physica
Effects of stoichiometry, purity, etching and distilling on resistance of MgB2 pellets and wire segments
We present a study of the effects of non-stoichiometry, boron purity, wire
diameter and post-synthesis treatment (etching and Mg distilling) on the
temperature dependent resistance and resistivity of sintered MgB2 pellets and
wire segments. Whereas the residual resistivity ratio (RRR) varies between RRR
\~ 4 to RRR > 20 for different boron purity, it is only moderately affected by
non-stoichiometry (from 20% Mg deficiency to 20% Mg excess) and is apparently
independent of wire diameter and presence of Mg metal traces on the wire
surface. The obtained set of data indicates that RRR values in excess of 20 and
residual resistivities as low as rho{0} ~ 0.4 mu Ohm cm are intrinsic material
properties of high purity MgB2
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
Physical properties of VTiO (0 x 0.187) single crystals
Free standing, low strain, single crystals of pure and titanium doped
VO were grown out of an excess of VO using high temperature
solution growth techniques. At 340 K, pure VO exhibits a
clear first-order phase transition from a high-temperature paramagnetic
tetragonal phase (R) to a low-temperature non-magnetic monoclinic phase (M1).
With Ti doping, another monoclinic phase (M2) emerges between the R and M1
phases. The phase transition temperature between R and M2 increases with
increasing Ti doping while the transition temperature between M2 and M1
decreases.Comment: 11 pages, 8 figure
Anisotropic magnetization and resistivity of single crystalline RNi1-xBi2+-y (R = La-Nd, Sm, Gd-Dy)
We present a detailed study of RNi1-xBi2+-y (R = La-Nd, Sm, Gd-Dy) single
crystals by measurements of stoichiometry and temperature dependent magnetic
susceptibility, magnetization, and electrical resistivity. This series forms
with partial Ni occupancy as well as a variable Bi occupancy. For R = Ce-Nd,
Sm, Gd-Dy, the RNi1-xBi2+-y compounds show local-moment like behavior and order
antiferromagnetically at low temperatures. Determination of anisotropies as
well as antiferromagnetic ordering temperatures for RNi1-xBi2+-y (R = Ce-Nd,
Sm, Gd-Dy) have been made. Although crystalline samples from this family
exhibit minority, second phase superconductivity at low temperatures associated
with Ni-Bi and Bi contamination, no evidence of bulk superconductivity has been
observed
An Overview of the Basic Physical Properties of MgB
The basic physical properties of MgB have been well established over the
past two years of intensive research. At this point there is a general
consensus about the values for the isotope shift, critical fields, most of the
salient length scales, and general anisotropies. In this paper we will review
the determination of these parameters and set the stage for further, more
detailed discussions of specific aspects of the physics of MgB.Comment: accepted to Physica C, special MgB2 iss
Quantum bicriticality in the heavy-fermion metamagnet YbAgGe
Bicritical points, at which two distinct symmetry-broken phases become
simultaneously unstable, are typical for spin-flop metamagnetism.
Interestingly, the heavy-fermion compound YbAgGe also possesses such a
bicritical point (BCP) with a low temperature T_BCP ~ 0.3 K at a magnetic field
of mu_0 H_BCP ~ 4.5 T. In its vicinity, YbAgGe exhibits anomalous behavior that
we attribute to the influence of a quantum bicritical point (QBCP), that is
close in parameter space yet can be reached by tuning T_BCP further to zero.
Using high-resolution measurements of the magnetocaloric effect, we demonstrate
that the magnetic Grueneisen parameter Gamma_H indeed both changes sign and
diverges as required for quantum criticality. Moreover, Gamma_H displays a
characteristic scaling behavior but only on the low-field side, H < H_BCP,
indicating a pronounced asymmetry with respect to the critical field. We
speculate that the small value of T_BCP is related to the geometric frustration
of the Kondo-lattice of YbAgGe.Comment: submitted to PR
Effects of Boron Purity, Mg Stoichiometry and Carbon Substitution on Properties of Polycrystalline MgB
By synthesizing MgB using boron of different nominal purity we found
values of the residual resistivity ratio () from 4 to
20, which covers almost all values found in literature. To obtain high values
of , high purity reagents are necessary. With the isotopically pure boron
we obtained the highest 20 for the stoichiometric compound. We also
investigated MgB samples with 0.8 1.2. For the range
MgB up to MgB we found average values
of between 14 and 24. For smaller variations in stoichiometry () . All of our data point to the conclusion that high
() and low () are intrinsic
material properties associated with high purity MgB. In addition we have
performed initial work on optimizing the formation of carbon doped MgB
via the use of BC. Nearly single phase material can be formed by reaction
of nominal Mg(BC) for 24 hours at . The
for this composition is between and (depending on
criterion).Comment: accepted to Physica C, special MgB2 issu
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
Anisotropy parameters of superconducting MgB
Data on macroscopic superconducting anisotropy of MgB are reviewed. The
data are described within a weak coupling two-gaps anisotropic s-wave model of
superconductivity. The calculated ratio of the upper critical fields
increases with decreasing temperature in
agreement with available data, whereas the calculated ratio of London
penetration depths decreases to reach
at T=0. Possible macroscopic consequences of
are discussed.Comment: accepted to Physica C, special MgB2 issu
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