78 research outputs found
Correlation between Fermi surface transformations and superconductivity in the electron-doped high- superconductor NdCeCuO
Two critical points have been revealed in the normal-state phase diagram of
the electron-doped cuprate superconductor NdCeCuO by exploring
the Fermi surface properties of high quality single crystals by high-field
magnetotransport. First, the quantitative analysis of the Shubnikov-de Haas
effect shows that the weak superlattice potential responsible for the Fermi
surface reconstruction in the overdoped regime extrapolates to zero at the
doping level corresponding to the onset of superconductivity.
Second, the high-field Hall coefficient exhibits a sharp drop right below
optimal doping where the superconducting transition
temperature is maximum. This drop is most likely caused by the onset of
long-range antiferromagnetic ordering. Thus, the superconducting dome appears
to be pinned by two critical points to the normal state phase diagram.Comment: 9 pages; 7 figures; 1 tabl
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
Magnetic oscillations in a two-dimensional network of compensated electron and hole orbits
The FS of (ET)8Hg4Cl12(C6H5Br)2 can be regarded as a 2D network of
compensated electron and hole orbits coupled by magnetic breakthrough.
Simultaneous measurements of the interlayer magnetoresistance and magnetic
torque have been performed up to 28 T. Magnetoresistance and de dHvA
oscillations spectra exhibit frequency combinations typical of such a network.
Even though some of the observed magnetoresistance oscillations cannot be
interpreted on the basis of neither conventional SdH oscillations nor quantum
interference, the temperature and magnetic field (both orientation and
magnitude) dependence of all the Fourier components of the dHvA spectra can be
consistently accounted for by the LK formula. This behaviour is at variance
with that currently reported for compounds illustrating the linear chain of
coupled orbits model.Comment: accepted for publication in europhysics Letter
Neutron Irradiation of Mg11B2 : From the Enhancement to the Suppression of Superconducting Properties
In this letter we present the effect of neutron irradiation up to fluences of
3.9 1019 n/cm2 on the superconducting properties of MgB2. In order to obtain a
disorder structure homogeneously distributed, the experiment was carried out on
bulk samples prepared with the 11B isotope. Up to fluences of 1018 n/cm2 the
critical temperature is slightly diminished (36 K) and the superconducting
properties are significantly improved; the upper critical field is increased
from 13.5 T to 20.3 T at 12 K and the irreversibility field is doubled at 5 K.
For larger neutron fluences the critical temperature is suppressed down to 12 K
and the superconducting properties come out strongly degraded.Comment: 13 pages, 4 figures. Submitted to Appl.Phys.Let
High-Field de Haas-van Alphen Effect in non-centrosymmetric CeCoGe3 and LaCoGe3
We report on de Haas-van Alphen effect measurements in the
non-centrosymmetric systems CeCoGe3 and LaCoGe3 in magnetic field up to 28
Tesla. In both compounds, two new high frequencies were observed in high
fields. The frequencies were not detected in previous lower field measurements.
The frequencies do not originate from magnetic breakdown, and, therefore, are
likely to be intrinsic features of the compounds. In CeCoGe3, the corresponding
effective masses are strongly enhanced, being of the order of 30 bare electron
masses.Comment: 3 pages, 4 figures, to be published in Proc. Int. Conf. Heavy
Electrons (ICHE2010) J. Phys. Soc. Jpn. 80 (2011
SYNTHESIS AND STRUCTURE-ACTIVITY RELATIONSHIPS OF THE NOVEL ISOTHIOBARBAMINE ANALOGUES WITH LOWERED BASICITY
This work was supported by the Russian Scientific Foundation, project № 19-13-00123
Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe
Magnetoresistivity measurements with fine tuning of the field direction on
high quality single crystals of the ferromagnetic superconductor UCoGe show
anomalous anisotropy of the upper critical field H_c2. H_c2 for H // b-axis
(H_c2^b) in the orthorhombic crystal structure is strongly enhanced with
decreasing temperature with an S-shape and reaches nearly 20 T at 0 K. The
temperature dependence of H_c2^a shows upward curvature with a low temperature
value exceeding 30 T, while H_c2^c at 0 K is very small (~ 0.6 T). Contrary to
conventional ferromagnets, the decrease of the Curie temperature with
increasing field for H // b-axis marked by an enhancement of the effective mass
of the conduction electrons appears to be the origin of the S-shaped H_c2^b
curve. These results indicate that the field-induced ferromagnetic instability
or magnetic quantum criticality reinforces superconductivity.Comment: 5 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
Spin-Dependent Mass Enhancement under Magnetic Field in the Periodic Anderson Model
In order to study the mechanism of the mass enhancement in heavy fermion
compounds in the presence of magnetic field, we study the periodic Anderson
model using the fluctuation exchange approximation. The resulting value of the
mass enhancement factor z^{-1} can become up to 10, which is significantly
larger than that in the single-band Hubbard model. We show that the difference
between the magnitude of the mass enhancement factor of up spin (minority spin)
electrons z^{-1}_up and that of down spin (majority spin) electrons z^{-1}_down
increases by the applied magnetic field B//z, which is consistent with de
Haas-van Alphen measurements for CeCoIn_5, CeRu_2Si_2 and CePd_2Si_2. We
predict that z^{-1}_up >z^{-1}_down in many Ce compounds, whereas z^{-1}_up <
z^{-1}_down in Yb compounds.Comment: 5 pages, 4 figure
Coupled CDW and SDW Fluctuations as an Origin of Anomalous Properties of Ferromagnetic Superconductor UGe_2
It is shown that anomalous properties of UGe_2 can be understood in a unified
way on the basis of a single assumption that the superconductivity is mediated
by the coupled SDW and CDW fluctuations induced by the imperfect nesting of the
Fermi surface with majority spins at T=T_x(P) deep in the ferromagnetic phase.
Excess growth of uniform magnetization is shown to develop in the temperature
range T<T_x(P) as a mode-coupling effect of coupled growth of SDW and CDW
orderings, which has been observed by two different types of experiments. The
coupled CDW and SDW fluctuations are shown to be essentially ferromagnetic spin
fluctuations which induce a spin-triplet p-wave attraction. These fluctuations
consist of two modes, spin and charge fluctuations with large momentum transfer
of the nesting vector. An anomalous temperature dependence of the upper
critical field H_c2(T) such as crossing of H_c2(T) at P=11.4 kbar and P=13.5
kbar, can be understood by the strong-coupling-superconductivity formalism.
Temperature dependence of the lattice specific heat including a large shoulder
near T_x is also explained quite well as an effect of a kind of Kohn anomaly
associated with coupled SDW-CDW transition.Comment: (12 pages, 10 eps figures) submitted to J. Phys. Soc. Jp
Point-Contact Spectroscopy in MgB_2: from Fundamental Physics to Thin-Film Characterization
In this paper we highlight the advantages of using point-contact spectroscopy
(PCS) in multigap superconductors like MgB_2, both as a fundamental research
tool and as a non-destructive diagnostic technique for the optimization of
thin-film characteristics. We first present some results of crucial fundamental
interest obtained by directional PCS in MgB_2 single crystals, for example the
temperature dependence of the gaps and of the critical fields and the effect of
a magnetic field on the gap amplitudes. Then, we show how PCS can provide
useful information about the surface properties of MgB_2 thin films (e.g. Tc,
gap amplitude(s), clean or dirty-limit conditions) in view of their
optimization for the fabrication of tunnel and Josephson junctions for
applications in superconducting electronics.Comment: 11 pages, 7 figures; Proceedings of 6th EUCAS Conference (14-18
September 2003, Sorrento - Italy
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