3,671 research outputs found
Fermi Surface Instabilities in Ferromagnetic Superconductor URhGe
The field-reentrant (field-reinforced) superconductivity on ferromagnetic
superconductors is one of the most interesting topics in unconventional
superconductivity. The enhancement of effective mass and the induced
ferromagnetic fluctuations play key roles for reentrant superconductivity.
However, the associated change of the Fermi surface, which is often observed at
(pseudo-) metamagnetic transition, can also be a key ingredient. In order to
study the Fermi surface instability, we performed Hall effect measurements in
the ferromagnetic superconductor URhGe. The Hall effect of URhGe is well
explained by two contributions, namely by the normal Hall effect and by the
large anomalous Hall effect due to skew scattering. The large change in the
Hall coefficient is observed at low fields between the paramagnetic and
ferromagnetic states for H // c-axis (easy-magnetization axis) in the
orthorhombic structure, indicating that the Fermi surface is reconstructed in
the ferromagnetic state below the Curie temperature (T_Curie=9.5K). At low
temperatures (T << T_Curie), when the field is applied along the b-axis, the
reentrant superconductivity was observed in both the Hall resistivity and the
magnetoresistance below 0.4K. Above 0.4K, a large jump with the first-order
nature was detected in the Hall resistivity at a spin-reorientation field H_R ~
12.5T, demonstrating that the marked change of the Fermi surface occurs between
the ferromagnetic state and the polarized state above H_R. The results can be
understood by the Lifshitz-type transition, induced by the magnetic field or by
the change of the effective magnetic field.Comment: 7 pages, 6 figures, accepted for publication in J. Phys. Soc. Jp
Molten Salt Flux Liquid Transport Method for Ultra Clean Single Crystals UTe2
Various single crystal growth techniques are presented for the unconventional
superconductor UTe2. The molten salt flux liquid transport (MSFLT) method is
employed to grow high-quality and large single crystals, exhibiting a high
residual resistivity ratio (RRR = 200-800). On the other hand, the Te self-flux
and chemical vapor transport (CVT) method produces samples of lower quality.
The MSFLT method is a hybrid approach that combines the molten salt flux (MSF)
and CVT methods. One significant advantage is that the materials gradually
crystallize at the relatively low temperature which is fixed during the main
process. This might be crucial for preventing U deficiency and obtaining
high-quality and large single crystals of UTe2. Many different single crystals
obtained by different technique were characterized by resistivity, specific
heat measurements. The superconducting transition temperature decreases with
the residual resistivity, followed by the Abrikosov-Gor'kov pair breaking
theory. The highest quality sample reaches Tc=2.1K. The residual gamma-value of
specific heat for the highest quality sample is only 3 percents of the normal
state gamma-value. The specific heat jump, Delta C/(gamma Tc) reaches about 2.7
for high quality samples, indicating a strong coupling superconductor.
Furthermore, the magnetic susceptibility for the field along a-axis in a high
quality single crystal does not show an up-turn behavior on cooling, which is
consistent with the results of NMR Knight shift and muSR experiments.Comment: 6 pages, 6 figures, accepted for publication in J. Phys. Soc. Jp
Collapse of ferromagnetism and Fermi surface instability near reentrant superconductivity of URhGe
We present thermoelectric power and resistivity measurements in the
ferromagnetic superconductor URhGe for magnetic field applied along the hard
magnetization b axis of the orthorhombic crystal. Reentrant superconductivity
is observed near the the spin reorientation transition at =12.75 T,
where a first order transition from the ferromagnetic to the polarized
paramagnetic state occurs. Special focus is given to the longitudinal
configuration, where both electric and heat current are parallel to the applied
field. The validity of the Fermi-liquid dependence of the resistivity
through demonstrates clearly that no quantum critical point occurs at
. Thus the ferromagnetic transition line at becomes first order
implying the existence of a tricritical point at finite temperature. The
enhancement of magnetic fluctuations in the vicinity of the tricritical point
stimulates the reentrance of superconductivity. The abrupt sign change observed
in the thermoelectric power with the thermal gradient applied along the b axis
together with the strong anomalies in the other directions is a definitive
macroscopic evidence that in addition a significant change of the Fermi surface
appears through .Comment: 6 pages, 5 figure
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