87 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
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
Unconventional Strong Spin-Fluctuation Effects around the Critical Pressure of the Itinerant Ising-Type Ferromagnet URhAl
Resistivity measurements were performed for the itinerant Ising-type
ferromagnet URhAl at temperatures down to 40 mK under high pressure up to 7.5
GPa, using single crystals. We found that the critical pressure of the Curie
temperature exists at around ~ 5.2 GPa. Near , the -coefficient
of the Fermi-liquid resistivity term below is largely enhanced
with a maximum around 5.2-5.5 GPa. Above , the exponent of the resistivity
deviates from 2. At , it is close to , which is
expected by the theory of three-dimensional ferromagnetic spin fluctuations for
a 2nd-order quantum-critical point (QCP). However, disappears as a
1st-order phase transition, and the critical behavior of resistivity in URhAl
cannot be explained by the theory of a 2nd-order QCP. The 1st-order nature of
the phase transition is weak, and the critical behavior is still dominated by
the spin fluctuation at low temperature. With increasing pressure, the
non-Fermi-liquid behavior is observed in higher fields. Magnetic field studies
point out a ferromagnetic wing structure with a tri-critical point (TCP) at ~
4.8-4.9 GPa in URhAl. One open possibility is that the switch from the
ferromagnetic to the paramagnetic states does not occur simply but an
intermediate state arises below the TCP as suggested theoretically recently.
Quite generally, if a drastic Fermi-surface change occurs through , the
nature of the interaction itself may change and lead to the observed
unconventional behavior.Comment: 9 pages, 9 figure
Magnetic-Field Control of Quantum Critical Points of Valence Transition
We study the mechanism how critical end points of first-order valence
transitions are controlled by a magnetic field. We show that the critical
temperature is suppressed to be a quantum critical point (QCP) by a magnetic
field and unexpectedly the QCP exhibits nonmonotonic field dependence in the
ground-state phase diagram, giving rise to emergence of metamagnetism even in
the intermediate valence-crossover regime. The driving force of the
field-induced QCP is clarified to be cooperative phenomena of Zeeman effect and
Kondo effect, which create a distinct energy scale from the Kondo temperature.
This mechanism explains peculiar magnetic response in CeIrIn5 and metamagnetic
transition in YbXCu4 for X=In as well as sharp contrast between X=Ag and Cd.Comment: 4 pages, 4 figure
Electronic nematicity in URu2Si2 revisited
The nature of the hidden-order (HO) state in URu2Si2 remains one of the major
unsolved issues in heavy-fermion physics. Recently, torque magnetometry, x-ray
diffraction and elastoresistivity data have suggested that the HO phase
transition at THO = 17.5 K is driven by electronic nematic effects. Here, we
search for thermodynamic signatures of this purported structural instability
using anisotropic thermal-expansion, Young\'s modulus, elastoresistivity and
specific-heat measurements. In contrast to the published results, we find no
evidence of a rotational symmetry-breaking in any of our data. Interestingly,
our elastoresistivity measurements, which are in full agreement with published
results, exhibit a Curie-Weiss divergence, which we however attribute to a
volume and not to a symmetry-breaking effect. Finally, clear evidence for
thermal fluctuations is observed in our heat-capacity data, from which we
estimate the HO correlation length.Comment: 4 Figures, 5 page
Thermodynamic Investigation of Metamagnetism in Pulsed High Magnetic Fields on Heavy Fermion Superconductor UTe
We investigated the thermodynamic property of the heavy fermion
superconductor UTe in pulsed high magnetic fields. The superconducting
transition in zero field was observed at =1.65 K as a sharp heat
capacity jump. Magnetocaloric effect measurements in pulsed-magnetic fields
obviously detected a thermodynamic anomaly accompanied by a first-order
metamagnetic transition at =36.0 T when the fields are
applied nearly along the hard-magnetization -axis. From the results of heat
capacity measurements in magnetic fields, we found a drastic diverging
electronic heat capacity coefficient of the normal state
with approaching . Comparing with the previous works via the
magnetic Clausius-Clapeyron relation, we unveil the thermodynamic details of
the metamagnetic transition. The enhancement of the effective mass observed as
the development of indicates that quantum fluctuation strongly
evolves around ; it assists the superconductivity emerging even in
extremely high fields.Comment: 6 pages, 6 figures, accepted for publication in JPS
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