87 research outputs found
Negative thermal expansion of MgB in the superconducting state and anomalous behavior of the bulk Gr\"uneisen function
The thermal expansion coefficient of MgB is revealed to change
from positive to negative on cooling through the superconducting transition
temperature . The Gr\"uneisen function also becomes negative at
followed by a dramatic increase to large positive values at low temperature.
The results suggest anomalous coupling between superconducting electrons and
low-energy phonons.Comment: 5 figures. submitted to Phys. Rev. Let
Influence of Rb, Cs and Ba on Superconductivity of Magnesium Diboride
Magnesium diboride has been thermally treated in the presence of Rb, Cs, and
Ba. Magnetic susceptibility shows onsets of superconductivity in the resulting
samples at 52K (Rb), 58K (Cs) and 45K (Ba). Room-temperature 11B NMR indicates
to cubic symmetry of the electric field gradient at boron site for the samples
reacted with Rb and Cs, in contrast to the axial symmetry in the initial MgB2
and in the sample treated with Ba.Comment: 3 pages (twocolumn), 2 figure
Quantum and thermal spin relaxation in diluted spin ice: Dy(2-x)MxTi2O7 (M = Lu, Y)
We have studied the low temperature a.c. magnetic susceptibility of the
diluted spin ice compound Dy(2-x)MxTi2O7, where the magnetic Dy ions on the
frustrated pyrochlore lattice have been replaced with non-magnetic ions, M = Y
or Lu. We examine a broad range of dilutions, 0 <= x <= 1.98, and we find that
the T ~ 16 K freezing is suppressed for low levels of dilution but re-emerges
for x > 0.4 and persists to x = 1.98. This behavior can be understood as a
non-monotonic dependence of the quantum spin relaxation time with dilution. The
results suggest that the observed spin freezing is fundamentally a single spin
process which is affected by the local environment, rather than the development
of spin-spin correlations as earlier data suggested.Comment: 26 pages, 9 figure
Quantum-Classical Reentrant Relaxation Crossover in Dy2Ti2O7 Spin-Ice
We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through
measurements of the a.c. magnetic susceptibility. While the characteristic spin
relaxation time is thermally activated at high temperatures, it becomes almost
temperature independent below Tcross ~ 13 K, suggesting that quantum tunneling
dominates the relaxation process below that temperature. As the low-entropy
spin ice state develops below Tice ~ 4 K, the spin relaxation time increases
sharply with decreasing temperature, suggesting the emergence of a collective
degree of freedom for which thermal relaxation processes again become important
as the spins become highly correlated
Giant anharmonicity and non-linear electron-phonon coupling in MgB; A combined first-principles calculations and neutron scattering study
We report first-principles calculations of the electronic band structure and
lattice dynamics for the new superconductor MgB. The excellent agreement
between theory and our inelastic neutron scattering measurements of the phonon
density of states gives confidence that the calculations provide a sound
description of the physical properties of the system. The numerical results
reveal that the in-plane boron phonons (with E symmetry) near the
zone-center are very anharmonic, and are strongly coupled to the partially
occupied planar B bands near the Fermi level. This giant anharmonicity
and non-linear electron-phonon coupling is key to explaining the observed high
T and boron isotope effect in MgBComment: In this revised version (to appear in PRL) we also discuss the boron
isotope effect. Please visit http://www.ncnr.nist.gov/staff/taner/mgb2 for
detail
Structural and superconducting properties of MgBBe
We prepared MgBBe (, 0.2, 0.3, 0.4, and 0.6) samples where
B is substituted with Be. MgB structure is maintained up to .
In-plane and inter-plane lattice constants were found to decrease and increase,
respectively. Superconducting transition temperature decreases with
. We found that the decrease is correlated with in-plane contraction
but is insensitive to carrier doping, which is consistent with other
substitution studies such as MgAlB and MgBC.
Implication of this work is discussed in terms of the 2D nature of -band.Comment: 3 pages,4 figures, to be published in Phys. Rev.
Scanning Tunneling Spectroscopy in MgB2
We present scanning tunneling microscopy measurements of the surface of
superconducting MgB2 with a critical temperature of 39K. In zero magnetic field
the conductance spectra can be analyzed in terms of the standard BCS theory
with a smearing parameter Gamma. The value of the superconducting gap is 5.2
meV at 4.2 K, with no experimentally significant variation across the surface
of the sample. The temperature dependence of the gap follows the BCS form,
fully consistent with phonon-mediated superconductivity in this novel
superconductor. The application of a magnetic field induces strong
pair-breaking as seen in the conductance spectra in fields up to 6 T.Comment: 4 pages, 4 figure
Thermoelectric power of MgBBe
We investigated thermoelectric power of MgBBe (,
0.2, 0.3, 0.4, and 0.6). decreases systematically with , suggesting
that the hole density increases. Our band calculation shows that the increase
occurs in the -band. With the hole-doping, decreases.
Implication of this phenomenon is discussed within the BCS framework. While the
Mott formula explains only the linear part of at low temperature,
incorporation of electron-phonon interaction enables us to explain over
wide temperature range including the anomalous behavior at high temperature.Comment: 4 pages, 4 figure
Low Temperature Spin Freezing in Dy2Ti2O7 Spin Ice
We report a study of the low temperature bulk magnetic properties of the spin
ice compound Dy2Ti2O7 with particular attention to the (T < 4 K) spin freezing
transition. While this transition is superficially similar to that in a spin
glass, there are important qualitative differences from spin glass behavior:
the freezing temperature increases slightly with applied magnetic field, and
the distribution of spin relaxation times remains extremely narrow down to the
lowest temperatures. Furthermore, the characteristic spin relaxation time
increases faster than exponentially down to the lowest temperatures studied.
These results indicate that spin-freezing in spin ice materials represents a
novel form of magnetic glassiness associated with the unusual nature of
geometrical frustration in these materials.Comment: 24 pages, 8 figure
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