168 research outputs found
Bi_2 Sr_2 CaCu_2 O_{8+delta} Bicrystal c-Axis Twist Josephson Junctions: A New Phase-Sensitive Test of Order Parameter Symmetry
Li {\it et al.} [Phys. Rev. Lett. {\bf 83}, 4160 (1999)] prepared atomically
clean BiSrCaCuO (BSCCO) Josephson junctions between
identical single crystal cleaves stacked and twisted an angle about
the c axis. For each bicrystal, the ratio of the c-axis twist
junction critical current density to that across either single crystal part is
unity, independent of and the ratio of junction areas. From
extensive theoretical studies involving a variety of tunneling and
superconducting order parameter (OP) forms, we conclude that the results
provide strong evidence for incoherent c-axis tunneling and that the dominant
OP is s-wave for . Recently, Takano {\it et al.} [Phys. Rev. B {\bf
65}, 140315(R) (2002)] obtained results from BSCCO whisker twist junctions
which also rule out a pure d-wave OP, but which are surprisingly suggestive of
coherent c-axis tunneling form small Fermi surface hot spots.Comment: 5 pages, 11 figures, invited talk at LT23, to be published in Physica
Radiation from a Josephson STAR-emitter
We calculate the angular dependence of the radiation-zone output power and
electric polarization of stimulated terahertz amplified radiation (STAR)
emitted from a voltage applied across cylindrical and rectangular stacks
of intrinsic Josephson junctions. During coherent emission, a spatially uniform
Josephson current density in the stack acts as a surface electric current
density antenna source, leading to an harmonic radiation frequency spectrum, as
in experiment, but absent in all cavity modesl of cylindrical mesas. Spatial
fluctuations of the Josephson current cause its fundamental mode to lock
onto the lowest finite energy cylindrical cavity mode, causing it to resonate,
leading to a non-uniform magnetic surface current density radiation source, and
a non-trivial combined fundamental frequency output power with linear
polarization We also present a model of the superconducting substrate, and
present results for rectangular mesas.Comment: 18 pages, 26 figures, submitted to PR
Zero-field Time Correlation Functions of Four Classical Heisenberg Spins on a Ring
A model relevant for the study of certain molecular magnets is the ring of
N=4 classical spins with equal near-neighbor isotropic Heisenberg exchange
interactions. Assuming classical Heisenberg spin dynamics, we solve explicitly
for the time evolution of each of the spins. Exact triple integral
representations are derived for the auto, near-neighbor, and
next-nearest-neighbor time correlation functions for any temperature. At
infinite temperature, the correlation functions are reduced to quadrature. We
then evaluate the Fourier transforms of these functions in closed form, which
are double integrals. At low temperatures, the Fourier transform functions
explicitly demonstrate the presence of magnons. Our exact results for the
infinite temperature correlation functions in the long-time asymptotic limit
differ qualitatively from those obtained assuming diffusive spin dynamics.
Whether such explicitly non-hydrodynamic behavior would be maintained for
large-N rings is discussed.Comment: 18 pages, 21 figure
Enhancement of ferromagnetism by p-wave Cooper pairing in superconducting ferromagnets
In superconducting ferromagnets for which the Curie temperature
exceeds the superconducting transition temperature , it was suggested
that ferromagnetic spin fluctuations could lead to superconductivity with
p-wave spin triplet Cooper pairing. Using the Stoner model of itinerant
ferromagnetism, we study the feedback effect of the p-wave superconductivity on
the ferromagnetism. Below , the ferromagnetism is enhanced by the p-wave
superconductivity. At zero temperature, the critical Stoner value for itinerant
ferromagnetism is reduced by the strength of the p-wave pairing potential, and
the magnetization increases correspondingly. More important, our results
suggest that once Stoner ferromagnetism is established, is unlikely to
ever be below . For strong and weak ferromagnetism, three and two peaks in
the temperature dependence of the specific heat are respectively predicted, the
upper peak in the latter case corresponding to a first-order transition.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
Is the anisotropy of the upper critical field of SrRuO consistent with a helical -wave state?
We calculate the angular and temperature dependencies of the upper
critical field for the point group helical
-wave states, assuming a single uniaxial ellipsoidal Fermi surface, Pauli
limiting, and strong spin-orbit coupling that locks the spin-triplet -vectors onto the layers. Good fits to the SrRuO
data of Kittaka {\it et al.} [Phys. Rev. B {\bf 80},
174514 (2009)] are obtained. Helical states with and (or
and ) produce
that greatly exceed (or do not exhibit) the
four-fold azimuthal anisotropy magnitudes observed in SrRuO by Kittaka
{\it et al.} and by Mao {\it et al.} [Phys. Rev. Lett. {\bf 84}, 991 (2000)],
respectively.Comment: 5+ pages, 4 figures, submitted as a Fast Track Communication to J.
Phys. Condens. Matte
Single-ion and exchange anisotropy effects and multiferroic behavior in high-symmetry tetramer single molecule magnets
We study single-ion and exchange anisotropy effects in equal-spin
tetramer single molecule magnets exhibiting , , ,
, , or ionic point group symmetry. We first write the
group-invariant quadratic single-ion and symmetric anisotropic exchange
Hamiltonians in the appropriate local coordinates. We then rewrite these local
Hamiltonians in the molecular or laboratory representation, along with the
Dzyaloshinskii-Moriay (DM) and isotropic Heisenberg, biquadratic, and
three-center quartic Hamiltonians. Using our exact, compact forms for the
single-ion spin matrix elements, we evaluate the eigenstate energies
analytically to first order in the microscopic anisotropy interactions,
corresponding to the strong exchange limit, and provide tables of simple
formulas for the energies of the lowest four eigenstate manifolds of
ferromagnetic (FM) and anitiferromagnetic (AFM) tetramers with arbitrary .
For AFM tetramers, we illustrate the first-order level-crossing inductions for
, and obtain a preliminary estimate of the microscopic
parameters in a Ni from a fit to magnetization data.
Accurate analytic expressions for the thermodynamics, electron paramagnetic
resonance absorption and inelastic neutron scattering cross-section are given,
allowing for a determination of three of the microscopic anisotropy
interactions from the second excited state manifold of FM tetramers. We also
predict that tetramers with symmetries and should exhibit both
DM interactions and multiferroic states, and illustrate our predictions for
.Comment: 30 pages, 14 figures, submitted to Phys. Rev.
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