156 research outputs found
Pressure-induced Superconductivity in a Ferromagnet UGe -- Resistivity Measurements in Magnetic Field --
The electrical resistivity measurements in the magnetic field are carried out
on the pressure-induced superconductor UGe. The superconductivity is
observed from 1.06 to 1.44 GPa. The upper critical field of is
anisotropic where exhibits positive curvature for and
-axis. The characteristic enhancement of is reconfirmed for
-axis. In the temperature and field dependence of resistivity at where the ferromagnetic ordering disappears, it is observed that the
application of the external field along the {\it a}-axis increases the
coefficient of Fermi liquid behavior correspondingly to the
metamagnetic transition.Comment: To be published in the proceeding of the International Conference on
High Pressure Science and Technology(AIRAPT-18),Beijing,China,23-27 July 200
Pressure-temperature Phase Diagram of Polycrystalline UCoGe Studied by Resistivity Measurement
Recently, coexistence of ferromagnetism (T_Curie = 2.8K) and
superconductivity (T_sc = 0.8K) has been reported in UCoGe, a compound close to
a ferromagnetic instability at ambient pressure P. Here we present resistivity
measurements under pressure on a UCoGe polycrystal. The phase diagram obtained
from resistivity measurements on a polycrystalline sample is found to be
qualitatively different to those of all other ferromagnetic superconductors. By
applying high pressure, ferromagnetism is suppressed at a rate of 1.4 K/GPa. No
indication of ferromagnetic order has been observed above P ~ 1GPa. The
resistive superconducting transition is, however, quite stable in temperature
and persists up to the highest measured pressure of about 2.4GPa.
Superconductivity would therefore appear also in the paramagnetic phase.
However, the appearance of superconductivity seems to change at a
characteristic pressure P* ~ 0.8GPa. Close to a ferromagnetic instability, the
homogeneity of the sample can influence strongly the electronic and magnetic
properties and therefore bulk phase transitions may differ from the
determination by resistivity measurements.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
Tomonaga-Luttinger Liquid in a Quasi-One-Dimensional S=1 Antiferromagnet Observed by the Specific Heat
Specific heat experiments on single crystals of the S=1 quasi-one-dimensional
bond-alternating antiferromagnet Ni(C_9H_24N_4)(NO_2)ClO_4, alias NTENP, have
been performed in magnetic fields applied both parallel and perpendicular to
the spin chains. We have found for the parallel field configuration that the
magnetic specific heat (C_mag) is proportional to temperature (T) above a
critical field H_c, at which the energy gap vanishes, in a temperature region
above that of the long-range ordered state. The ratio C_mag/T increases as the
magnetic field approaches H_c from above. The data are in good quantitative
agreement with the prediction of the c=1 conformal field theory in conjunction
with the velocity of the excitations calculated by a numerical diagonalization,
providing a conclusive evidence for a Tomonaga-Luttinger liquid.Comment: 4 pages, 4 postscript figure
Ferromagnetic phases in spin-Fermion systems
Spin-Fermion systems which obtain their magnetic properties from a system of
localized magnetic moments being coupled to conducting electrons are
considered. The dynamical degrees of freedom are spin- operators of
localized spins and spin-1/2 Fermi operators of itinerant electrons.
Renormalized spin-wave theory, which accounts for the magnon-magnon
interaction, and its extension are developed to describe the two ferrimagnetic
phases in the system: low temperature phase , where all electrons
contribute the ordered ferromagnetic moment, and high temperature phase
, where only localized spins form magnetic moment. The
magnetization as a function of temperature is calculated. The theoretical
predictions are utilize to interpret the experimentally measured
magnetization-temperature curves of ..Comment: 9 pages, 5 figure
Specific Heat Study of an S=1/2 Alternating Heisenberg Chain System F_5PNN Under Magnetic Field
We have measured the specific heat of an S=1/2 antiferromagnetic alternating
Heisenberg chain pentafulorophenyl nitronyl nitroxide under magnetic fields up
to H>H_C2. This compound has the field-induced magnetic ordered (FIMO) phase
between H_C1 and H_C2. Characteristic behaviors are observed depending on the
magnetic field up to above H_C2 outside of the H-T boundary for the FIMO.
Temperature and field dependence of the specific heat are qualitatively in good
agreement with the theoretical calculation on an S=1/2 two-leg ladder. [Wang et
al. Phys. Rev. Lett 84 5399 (2000)] This agreement suggests that the observed
behaviors are related with the low-energy excitation in the Tomonaga-Luttinger
liquid.Comment: 4pages, 4figures, replaced with revised version accepted to Physical
Review Letter
The Magnetic Phase Diagram and the Pressure and Field Dependence of the Fermi Surface in UGe
The ac susceptibility and de Haas-van Alphen (dHvA) effect in UGe are
measured at pressures {\it P} up to 17.7 kbar for the magnetic field {\it B}
parallel to the {\it a} axis, which is the easy axis of magnetization. Two
anomalies are observed at {\it B}({\it P}) and {\it B}({\it P}) ({\it
B} {\it B} at any {\it P}), and the {\it P}-{\it B} phase diagram
is presented. The Fermi surface and quasiparticle mass are found to vary
smoothly with pressure up to 17.7 kbar unless the phase boundary {\it
B}({\it P}) is crossed. The observed dHvA frequencies may be grouped into
three according to their pressure dependences, which are largely positive,
nearly constant or negative. It is suggested that the quasiparticle mass
moderately increases as the boundary {\it B}({\it P}) is approached. DHvA
effect measurements are also performed across the boundary at 16.8 kbar.Comment: to be published in Phys. Rev.
Specific heat of the spin-dimer antiferromagnet BaMnO in high magnetic fields
We have measured the specific heat of the coupled spin-dimer antiferromagnet
BaMnO to 50 mK in temperature and to 29 T in the magnetic field.
The experiment extends to the midpoint of the field region (25.9 T 32.3 T) of the magnetization plateau at 1/2 of the saturation
magnetization, and reveals the presence of three ordered phases in the field
region between that of the magnetization plateau and the low-field spin-liquid
region. The exponent of the phase boundary with the thermally disordered region
is smaller than the theoretical value based on the Bose-Einstein condensation
of spin triplets. At zero field and 29 T, the specific-heat data show gapped
behaviors characteristic of spin liquids. The zero-field data indicate that the
gapped triplet excitations form two levels whose energies differ by nearly a
factor of two. At least the lower level is well localized. The data at 29 T
reveal that the low-lying excitations at the magnetization plateau are weakly
delocalized.Comment: 6 pages, 5 figures, revised versio
Magnetic Properties of a Pressure-induced Superconductor UGe
We performed the DC-magnetization and neutron scattering experiments under
pressure {\it P} for a pressure-induced superconductor UGe. We found that
the magnetic moment is enhanced at a characteristic temperature {\it T}
in the ferromagnetic state, where {\it T} is smaller than a Curie
temperature {\it T}. This enhancement becomes remarkable in the
vicinity of {\it P} = 1.20 GPa, where {\it T} becomes 0 K
and the superconducting transition temperature {\it T} shows a
maximum. The characteristic temperature {\it T}, which decreases with
increasing pressure, also depends on the magnetic field.Comment: To be published in J.Phys.Soc.Jp
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