86 research outputs found
Harmonic Maa{\ss}-Jacobi forms of degree 1 with higher rank indices
We define and investigate real analytic weak Jacobi forms of degree 1 and
arbitrary rank. En route we calculate the Casimir operator associated to the
maximal central extension of the real Jacobi group, which for rank exceeding 1
is of order 4. In ranks exceeding 1, the notions of H-harmonicity and
semi-holomorphicity are the same.Comment: 28 page
Effective critical behaviour of diluted Heisenberg-like magnets
In agreement with the Harris criterion, asymptotic critical exponents of
three-dimensional (3d) Heisenberg-like magnets are not influenced by weak
quenched dilution of non-magnetic component. However, often in the experimental
studies of corresponding systems concentration- and temperature-dependent
exponents are found with values differing from those of the 3d Heisenberg
model.
In our study, we use the field--theoretical renormalization group approach to
explain this observation and to calculate the effective critical exponents of
weakly diluted quenched Heisenberg-like magnet. Being non-universal, these
exponents change with distance to the critical point as observed
experimentally. In the asymptotic limit (at ) they equal to the critical
exponents of the pure 3d Heisenberg magnet as predicted by the Harris
criterion.Comment: 15 pages, 4 figure
Experimental Observation of the Inverse Proximity Effect in Superconductor/Ferromagnet Layered Structures
We have studied the nuclear magnetic resonance (NMR) of 51V nuclei in the
superconductor/ferromagnet thin film heterostructures Ni/V/Ni and
Pd{1-x}Fe{x}/V/Pd{1-x}Fe{x} in the normaland superconducting state. Whereas the
position and shape of the NMR line in the normal state for the trilayers is
identical to that observed in a single V-layer, in the superconducting state
the line shape definitely changes, developing a systematic distortion of the
high-field wing of the resonance line. We consider this as the first
experimental evidence for the penetration of ferromagnetism into the
superconducting layer, a phenomenon which has been theoretically predicted
recently and dubbed the inverse proximity effect.Comment: about 5 pages, 3 figures, 1 tabl
Spin screening effect in superconductor/ferromagnet thin film heterostructures studied using nuclear magnetic resonance
Using NMR spectroscopy of the V 51 nuclei in the superconducting state of Ni/V/Ni and Pd1-x Fex /V/ Pd1-x Fex trilayers we reported in a recent letter an experimental observation of the spin screening effect. This effect, which designates the formation of a spin polarization in the superconducting state, was predicted previously by Bergeret. Here, we extend our earlier experiments by varying the thickness of the superconducting V layer and by applying the magnetic field not only perpendicular to the film plane as in the previous experiments, but also in the parallel direction. For the latter geometry, which for experimental reasons is difficult to realize, the film is in the vortex-free state. This allows a direct quantitative comparison of the experimental screening effect as derived from a characteristic distortion of the high-field wing of the resonance line in the superconducting state and the theoretical model calculations. We derive a reasonable agreement between theory and experiment, confirming the spin screening effect in the superconductor. © 2009 The American Physical Society
Experimental observation of the spin screening effect in superconductor/ferromagnet thin film heterostructures
We have studied the nuclear magnetic resonance (NMR) of V51 nuclei in the superconductor/ferromagnet thin film heterostructures Pd1-xFex/V/Pd1-xFex and Ni/V/Ni in the normal and superconducting state. Whereas the position and shape of the NMR line in the normal state for the trilayers is identical to that observed in a single V layer, in the superconducting state the line shape definitely changes, developing a systematic distortion of the high-field wing of the resonance line. We consider this as the first experimental evidence for the penetration of ferromagnetism into the superconducting layer, a phenomenon which has been theoretically predicted recently and dubbed the spin screening effect. © 2009 The American Physical Society
Intrinsic room temperature ferromagnetism in Co-implanted ZnO
We report on the structural and magnetic properties of a cobalt-implanted ZnO film grown on a sapphire substrate. X-ray diffraction and transmission electron microscopy reveal the presence of a ( )-oriented hexagonal Co phase in the Al2O3 sapphire substrate, but not in the ZnO film. Co clusters, with a diameter of about 5-6 nm, form a Co rich layer in the substrate close to the ZnO/Al2O3 interface. Magnetization measurements indicate that there exist two different magnetic phases in the implanted region. One originates from the Co clusters in Al2O 3, the other one belongs to a homogeneous ferromagnetic phase with a ferromagnetic Curie temperature far above room temperature. In the latter case, the ferromagnetism can be attributed to Co substitution on Zn sites in the ZnO layer. We have observed magnetic dichroism at the Co L2,3 and O K edges at room temperature as well as the multiplet structure in x-ray absorption spectra around the Co L3 edge, supporting the intrinsic nature of the observed ferromagnetism in a Co-implanted ZnO film. The magnetic moment per substituted cobalt is found to be about 2.81 νB, which is very close to the theoretical expected value of 3 νB/Co for Co 2+ in its high spin state. © 2008 IOP Publishing Ltd
Effect of anisotropic impurity scattering in superconductors
We discuss the weak-coupling BCS theory of a superconductor with the
impurities, accounting for their anisotropic momentum-dependent potential. The
impurity scattering process is considered in the t-matrix approximation and its
influence on the superconducting critical temperature is studied in the Born
and unitary limit for a d- and (d+s)-wave superconductors. We observe a
significant dependence of the pair-breaking strength on the symmetry of the
scattering potential and classify the impurity potentials according to their
ability to alter T_c. A good agreement with the experimental data for Zn doping
and oxygen irradiation in the overdoped cuprates is found.Comment: 31 pages, RevTex, 15 PostScript figure
Coherent Potential Approximation for `d - wave' Superconductivity in Disordered Systems
A Coherent Potential Approximation is developed for s-wave and d-wave
superconductivity in disordered systems. We show that the CPA formalism
reproduces the standard pair-breaking formula, the self-consistent Born
Approximation and the self-consistent T-matrix approximation in the appropriate
limits. We implement the theory and compute T_c for s-wave and d-wave pairing
using an attractive nearest neighbor Hubbard model featuring both binary alloy
disorder and a uniform distribution of scattering site potentials. We determine
the density of states and examine its consequences for low temperature heat
capacity. We find that our results are in qualitative agreement with
measurements on Zn doped YBCO superconductors.Comment: 35 pages, 23 figures, submitted to Phys Rev.
suppression in co-doped striped cuprates
We propose a model that explains the reduction of due to the pinning of
stripes by planar impurity co-doping in cuprates. A geometrical argument about
the planar fraction of carriers affected by stripe pinning leads to a a linear
suppression as a function of impurity concentration . The critical
value for the vanishing of superconductivity is shown to scale like
in the under-doped regime and becomes universal in the optimally- and
over-doped regimes. Our theory agrees very well with the experimental data in
single- and bi-layer cuprates co-doped with Zn, Li, Co, etc...Comment: 4 pages, 4 figure
High-temperature ferromagnetism in Co-implanted TiO2 rutile
We report on structural, magnetic and electronic properties of Co-implanted TiO2(1 0 0) rutile single crystals for different implantation doses. Strong ferromagnetism at room temperature and above is observed in TiO 2 rutile plates after cobalt ion implantation, with magnetic parameters depending on the cobalt implantation dose. While the structural data indicate the presence of metallic cobalt clusters, the multiplet structure of the Co L3 edge in the XAS spectra provides evidence that a sizeable portion of the dopants occupy substitutional Co2+ sites. The detailed analysis of the structural and magnetic properties indicates that there are two magnetic phases in Co-implanted TiO2 plates. One is a ferromagnetic phase due to the formation of long range ferromagnetic ordering between implanted magnetic cobalt ions in the rutile phase, and the second one is a superparamagnetic phase which originates from the formation of metallic cobalt clusters in the implanted region. Using x-ray resonant magnetic scattering, the element specific magnetizations of cobalt, oxygen and titanium in Co-implanted TiO2 single crystals are investigated. Magnetic dichroism was observed at the Co L2,3 edges as well as at the O K edge. Anomalous Hall effect measurement indicates n-type carriers in Co-implanted TiO 2 rutile. The interaction mechanism, which leads to ferromagnetic ordering of substituted cobalt ions in the host matrix, is also discussed. © 2009 IOP Publishing Ltd
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