650 research outputs found
Classical Heisenberg antiferromagnet on a garnet lattice: a Monte Carlo simulation
We have studied a classical antiferromagnet on a garnet lattice by means of
Monte Carlo simulations in an attempt to examine the role of geometrical
frustration in Gadolinium Gallium Garnet, Gd3Ga5O12 (GGG). Low-temperature
specific heat, magnetisation, susceptibility, the autocorrelation function A(t)
and the neutron scattering function S(Q) have been calculated for several
models including different types of magnetic interactions and with the presence
of an external magnetic field applied along the principal symmetry axes. A
model, which includes only nearest-neighbour exchange, J1, neither orders down
to the lowest temperature nor does it show any tendency towards forming a
short-range coplanar spin structure. This model, however, does demonstrate a
magnetic field induced ordering below T ~ 0.01 J1. In order to reproduce the
experimentally observed properties of GGG, the simulated model must include
nearest neighbour exchange interactions and also dipolar forces. The presence
of weak next-to-nearest exchange interactions is found to be insignificant. In
zero field S(Q) exhibits diffuse magnetic scattering around positions in
reciprocal space where antiferromagnetic Bragg peaks appear in an applied
magnetic field.Comment: 8 pages, 8 figures, to appear in PRB (JAN 2001
Paramagnetic magnetization signals and curious metastable behaviour in field-cooled magnetization of a single crystal of superconductor 2H-NbSe2
We present here some newer characteristics pertaining to paramagnetic
Meissner effect like response in a single crystal of the low Tc superconducting
compound 2H-NbSe2 via a detailed study of effects of perturbation on the
field-cooled magnetization response. In the temperature range, where an
anomalous paramagnetic magnetization occurs, the field-cooled magnetization
response is found to be highly metastable: it displays a curious tendency to
switch randomly from a given paramagnetic value to a diamagnetic or to a
different paramagnetic value, when the system is perturbed by an impulse of an
externally applied ac field. The new facets revealed in a single crystal of
2H-NbSe2 surprisingly bear a marked resemblance with the characteristics of
magnetization behaviour anticipated for the giant vortex states with multiple
flux quanta predicted to occur in mesoscopic-sized superconducting specimen and
possible transitions amongst such states.Comment: 12 pages, 9 figures, submitted to Journal of Physics: Condensed
Matte
Crystal growth and properties of the non-centrosymmetric superconductor, Ru7B3
We describe the crystal growth of high quality single crystals of the
non-centrosymmetric superconductor, Ru7B3 by the floating zone technique, using
an optical furnace equipped with xenon arc lamps. The crystals obtained are
large and suitable for detailed measurements, and have been examined using
x-ray Laue patterns. The superconducting properties of the crystals obtained
have been investigated by magnetisation and resistivity measurements. Crystals
have also been grown starting with enriched 11B isotope, making them suitable
for neutron scattering experiments.Comment: 4 pages, 5 figures. Accepted for publication in Journal of Crystal
Growt
Probing the superconducting ground state of the rare-earth ternary boride superconductors RuB ( = Lu,Y) using muon-spin rotation and relaxation
The superconductivity in the rare-earth transition metal ternary borides
RuB (where = Lu and Y) has been investigated using muon-spin
rotation and relaxation. Measurements made in zero-field suggest that
time-reversal symmetry is preserved upon entering the superconducting state in
both materials; a small difference in depolarization is observed above and
below the superconducting transition in both compounds, however this has been
attributed to quasistatic magnetic fluctuations. Transverse-field measurements
of the flux-line lattice indicate that the superconductivity in both materials
is fully gapped, with a conventional s-wave pairing symmetry and BCS-like
magnitudes for the zero-temperature gap energies. The electronic properties of
the charge carriers in the superconducting state have been calculated, with
effective masses and in the Lu
and Y compounds, respectively, with superconducting carrier densities
() m and ()
m. The materials have been classified according to the
Uemura scheme for superconductivity, with values for
of and , implying that
the superconductivity may not be entirely conventional in nature.Comment: 8 pages, 8 figure
Time-reversal symmetry breaking in noncentrosymmetric superconductor Re6Hf:further evidence for unconventional behaviour in the alpha-Mn family of materials
The discovery of new families of unconventional superconductors is important
both experimentally and theoretically, especially if it challenges current
models and thinking. By using muon spin relaxation in zero-field, time-reversal
symmetry breaking has been observed in Re6Hf. Moreover, the temperature
dependence of the superfluid density exhibits s-wave superconductivity with an
enhanced electron-phonon coupling. This, coupled with the results from
isostructural Re6Zr, shows that the Re6X family are indeed a new and important
group of unconventional superconductors.Comment: 5 pages, 2 figures Accepted Physical Review B, Rapid Communicatio
Superconducting and normal-state properties of the noncentrosymmetric superconductor Re6Zr
We systematically investigate the normal and superconducting properties of
non-centrosymmetric ReZr using magnetization, heat capacity, and
electrical resistivity measurements. Resistivity measurements indicate
ReZr has poor metallic behavior and is dominated by disorder. ReZr
undergoes a superconducting transition at K. Magnetization measurements give a lower critical
field, mT. The
Werthamer-Helfand-Hohenberg model is used to approximate the upper critical
field T which is close to
the Pauli limiting field of 12.35 T and which could indicate singlet-triplet
mixing. However, low-temperature specific-heat data suggest that ReZr is
an isotropic, fully gapped s-wave superconductor with enhanced electron-phonon
coupling. Unusual flux pinning resulting in a peak effect is observed in the
magnetization data, indicating an unconventional vortex state.Comment: 11 pages, 7 figures, 2 table
Magnetic phase diagram of the antiferromagnetic pyrochlore Gd2Ti2O7
Gd2Ti2O7 is a highly frustrated antiferromagnet on a pyrochlore lattice,
where apart from the Heisenberg exchange the spins also interact via
dipole-dipole forces. We report on low-temperature specific heat measurements
performed on single crystals of Gd2Ti2O7 for three different directions of an
applied magnetic field. The measurements reveal the strongly anisotropic
behaviour of Gd2Ti2O7 in a magnetic field despite the apparent absence of a
significant single-ion anisotropy for Gd3+. The H-T phase diagrams are
constructed for H//111], H//[110] and H//[112]. The results indicate that
further theoretical work beyond a simple mean-field model is required.Comment: 4 figure
Probing the superconducting ground state of the noncentrosymmetric superconductors CaTSi3 (T = Ir, Pt) using muon-spin relaxation and rotation
The superconducting properties of CaTSi3 (where T = Pt and Ir) have been
investigated using muon spectroscopy. Our muon-spin relaxation results suggest
that in both these superconductors time-reversal symmetry is preserved, while
muon-spin rotation data show that the temperature dependence of the superfluid
density is consistent with an isotropic s-wave gap. The magnetic penetration
depths and upper critical fields determined from our transverse-field muon-spin
rotation spectra are found to be 448(6) and 170(6) nm, and 3800(500) and
1700(300) G, for CaPtSi3 and CaIrSi3 respectively. The superconducting
coherence lengths of the two materials have also been determined and are 29(2)
nm for CaPtSi3 and 44(4) nm for CaIrSi3.Comment: 6 pages, 7 figure
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