73 research outputs found
Strain enhancement of superconductivity in CePd2Si2 under pressure
We report resistivity and calorimetric measurements on two single crystals of
CePd2Si2 pressurized up to 7.4 GPa. A weak uniaxial stress induced in the
pressure cell demonstrates the sensitivity of the physics to anisotropy. Stress
applied along the c-axis extends the whole phase diagram to higher pressures
and enhances the superconducting phase emerging around the magnetic
instability, with a 40% increase of the maximum superconducting temperature,
Tc, and a doubled pressure range. Calorimetric measurements demonstrate the
bulk nature of the superconductivity.Comment: 4 pages, 4 figure
Specific heat of heavy fermion CePd2Si2 in high magnetic fields
We report specific heat measurements on the heavy fermion compound CePd2Si2
in magnetic fields up to 16 T and in the temperature range 1.4-16 K. A sharp
peak in the specific heat signals the antiferromagnetic transition at T_N ~ 9.3
K in zero field. The transition is found to shift to lower temperatures when a
magnetic field is applied along the crystallographic a-axis, while a field
applied parallel to the tetragonal c-axis does not affect the transition. The
magnetic contribution to the specific heat below T_N is well described by a sum
of a linear electronic term and an antiferromagnetic spin wave contribution.
Just below T_N, an additional positive curvature, especially at high fields,
arises most probably due to thermal fluctuations. The field dependence of the
coefficient of the low temperature linear term, gamma_0, extracted from the
fits shows a maximum at about 6 T, at the point where an anomaly was detected
in susceptibility measurements. The relative field dependence of both T_N and
the magnetic entropy at T_N scales as [1-(B/B_0)^2] for B // a, suggesting the
disappearance of antiferromagnetism at B_0 ~ 42 T. The expected suppression of
the antiferromagnetic transition temperature to zero makes the existence of a
magnetic quantum critical point possible.Comment: to be published in Journal of Physics: Condensed Matte
Quasi-two-dimensional Fermi surfaces of the heavy-fermion superconductor CePdIn
We report low-temperature de Haas-van Alphen (dHvA) effect measurements in
magnetic fields up to 35 T of the heavy-fermion superconductor CePdIn.
The comparison of the experimental results with band-structure calculations
implies that the 4 electrons are itinerant rather than localized. The
cyclotron masses estimated at high field are only moderately enhanced, 8 and 14
, but are substantially larger than the corresponding band masses. The
observed angular dependence of the dHvA frequencies suggests
quasi-two-dimensional Fermi surfaces in agreement with band-structure
calculations. However, the deviation from ideal two dimensionality is larger
than in CeCoIn, with which CePdIn bears a lot of similarities. This
subtle distinction accounts for the different superconducting critical
temperatures of the two compounds.Comment: accepted to Phys. Rev.
Multi-band Superconductivity in the Chevrel Phases SnMo6S8 and PbMo6S8
Sub-Kelvin scanning tunnelling spectroscopy in the Chevrel Phases SnMo6S8 and
PbMo6S8 reveals two distinct superconducting gaps with Delta_1 = 3 meV, Delta_2
~ 1.0 meV and Delta_1 = 3.1 meV, Delta_2 ~ 1.4 meV respectively. The gap
distribution is strongly anisotropic, with Delta_2 predominantly seen when
scanning across unit-cell steps on the (001) sample surface. The spectra are
well-fitted by an anisotropic two-band BCS s-wave gap function. Our
spectroscopic data are confirmed by electronic heat capacity measurements which
also provide evidence for a twin-gap scenario.Comment: 5 pages, 4 figure
Superconducting energy gap in MgCNi3 single crystals: Point-contact spectroscopy and specific-heat measurements
Specific heat has been measured down to 600 mK and up to 8 Tesla by the
highly sensitive AC microcalorimetry on the MgCNi3 single crystals with Tc ~ 7
K. Exponential decay of the electronic specific heat at low temperatures proved
that a superconducting energy gap is fully open on the whole Fermi surface, in
agreement with our previous magnetic penetration depth measurements on the same
crystals. The specific-heat data analysis shows consistently the strong
coupling strength 2D/kTc ~ 4. This scenario is supported by the direct gap
measurements via the point-contact spectroscopy. Moreover, the spectroscopy
measurements show a decrease in the critical temperature at the sample surface
accounting for the observed differences of the superfluid density deduced from
the measurements by different techniques
Calorimetric and transport investigations of CePd_{2+x}Ge_{2-x} (x=0 and 0.02) up to 22 GPa
The influence of pressure on the magnetically ordered CePd_{2.02}Ge_{1.98}
has been investigated by a combined measurement of electrical resistivity,
, and ac-calorimetry, C(T), for temperatures in the range 0.3 K<T<10 K
and pressures, p, up to 22 GPa. Simultaneously CePd_2Ge_2 has been examined by
down to 40 mK. In CePd_{2.02}Ge_{1.98} and CePd_2Ge_2 the magnetic
order is suppressed at a critical pressure p_c=11.0 GPa and p_c=13.8 GPa,
respectively. In the case of CePd_{2.02}Ge_{1.98} not only the temperature
coefficient of , A, indicates the loss of magnetic order but also the
ac-signal recorded at low temperature. The residual
resistivity is extremely pressure sensitive and passes through a maximum and
then a minimum in the vicinity of p_c. The (T,p) phase diagram and the
A(p)-dependence of both compounds can be qualitatively understood in terms of a
pressure-tuned competition between magnetic order and the Kondo effect
according to the Doniach picture. The temperature-volume (T,V) phase diagram of
CePd_2Ge_2 combined with that of CePd_2Si_2 shows that in stoichiometric
compounds mainly the change of interatomic distances influences the exchange
interaction. It will be argued that in contrast to this the much lower
p_c-value of CePd_{2.02}Ge_{1.98} is caused by an enhanced hybridization
between 4f and conduction electrons.Comment: 9 pages, 7 figure
The Fulde-Ferrell-Larkin-Ovchinnikov State in the Organic Superconductor k-(BEDT-TTF)2Cu(NCS)2 as Observed in Magnetic Torque Experiments
We present magnetic-torque experiments on the organic superconductor
k-(BEDT-TTF)2Cu(NCS)2 for magnetic fields applied parallel to the 2D
superconducting layers. The experiments show a crossover from a second-order to
a first-order transition when the upper critical field reaches 21 T. Beyond
this field, which we interpret as the Pauli limit for superconductivity, the
upper critical field line shows a pro-nounced upturn and a phase transition
line separates the superconducting state into a low- and a high-field phase. We
interpret the data in the framework of a Fulde-Ferrell-Larkin-Ovchinnikov
state.Comment: 2 pages, 1 figur
High pressure phase diagrams of CeRhIn and CeCoIn studied by ac calorimetry
The pressure-temperature phase diagrams of the heavy fermion antiferromagnet
CeRhIn and the heavy fermion superconductor CeCoIn have been studied
under hydrostatic pressure by ac calorimetry and ac susceptibility measurements
using diamond anvil cells with argon as pressure medium. In CeRhIn, the use
of a highly hydrostatic pressure transmitting medium allows for a clean
simultaneous determination by a bulk probe of the antiferromagnetic and
superconducting transitions. We compare our new phase diagram with the previous
ones, discuss the nature (first or second order) of the various lines, and the
coexistence of antiferromagnetic order and superconductivity. The link between
the collaps of the superconducting heat anomaly and the broadening of the
antiferromagnetic transition points to an inhomogeneous appearence of
superconductivity below GPa. Homogeneous bulk
superconductivity is only observed above this critical pressure. We present a
detailed analysis of the influence of pressure inomogeneities on the specific
heat anomalies which emphasizes that the observed broadening of the transitions
near is connected with the first order transition. For CeCoIn we show
that the large specific heat anomaly observed at at ambient pressure is
suppressed linearly at least up to 3 GPa
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