1,498 research outputs found
Use of Otolith Strontium:Calcium Ratios for Hindcasting Larval Cod Gadus Morhua Distributions Relative to Water Masses on Georges Bank
The concentration ratios of strontium to calcium in laboratory-reared larval cod otoliths are shown to be related to the water temperature (T) at the time of otolith precipitation. This relationship is curvilinear, and is best described by a simple exponential equation of the form (Sr/Ca x 1000 = a exp(-T/b). We show that when Sr/Ca elemental analyses are related to the daily growth increments in the larval otoliths, relative temperature histories of individual field-caught larvae can be reconstructed from the egg stage to the time of capture. We present preliminary examples of how such reconstructed temperature histories of Atlantic cod Gadus morhua larvae, collected on Georges Bank during April and May 1993, may be interpreted in relation to the broad-scale larval distributions and the hydrography of the Bank
Theory of the c-Axis Penetration Depth in the Cuprates
Recent measurements of the London penetration depth tensor in the cuprates
find a weak temperature dependence along the c-direction which is seemingly
inconsistent with evidence for d-wave pairing deduced from in-plane
measurements. We demonstrate in this paper that these disparate results are not
in contradiction, but can be explained within a theory based on incoherent
quasiparticle hopping between the CuO2 layers. By relating the calculated
temperature dependence of the penetration depth \lambda_c(T) to the c-axis
resistivity, we show how the measured ratio \lambda_c^2(0) / \lambda_c^2(T) can
provide insight into the behavior of c-axis transport below Tc and the related
issue of ``confinement.''Comment: 4 pages, REVTEX with psfig, 3 PostScript figures included in
compressed for
Combined effect of nonmagnetic and magnetic scatterers on critical temperatures of superconductors with different gap anisotropy
The combined effect of nonmagnetic and magnetic defects and impurities on
critical temperatures of superconductors with different gap anisotropy is
studied theoretically within the weak coupling limit of the BCS model. An
expression is derived which relates the critical temperature to relaxation
rates of charge carriers by nonmagnetic and magnetic scatterers, as well as to
the coefficient of anisotropy of the superconducting order parameter on the
Fermi surface. Particular cases of d-wave, (s+d)-wave, and anisotropic s-wave
superconductors are briefly discussed.Comment: 5 pages, Te
Pseudogap in the microwave response of YBa_2Cu_3O_{7-x}
The in-plane and out-of-plane surface impedance and microwave conductivity
components of one and the same YBa_2Cu_3O_{7-x} (0.07\le x\le 0.47) single
crystal are determined in the wide ranges of temperature T and carrier
concentration p in CuO_2 planes. The following features of the superfluid
density n_s(T,p)\propto\lambda_{ab}^{-2}(T,p) are observed at T<Tc/2 and
0.078\le p\le 0.16: (i) n_s(0,p) depends linearly on p, (ii) the derivative
|dn_s(T,p)/dT|_{T\to 0} depends on p slightly in the optimally and moderately
doped regions (0.10<p\le 0.16); however, it rapidly increases with p further
lowering and (iii) the latter finding is accompanied by the linear
low-temperature dependence \Delta n_s(T)\propto(-T) changing to \Delta
n_s(T)\propto(-\sqrt{T}). For optimum oxygen content the temperature dependence
of the normalized imaginary part of the c-axis conductivity
\lambda_c^2(0)/\lambda_c^2(T) is found to be strikingly similar to that of
\lambda_{ab}^2(0)/\lambda_{ab}^2(T) and becomes more convex with p lowering.
\lambda_c^{-2}(0,p) values are roughly proportional to the normal state
conductivities \sigma_c(T_c,p) along the c-axis. All these properties can be
treated in the framework of d-density wave order of pseudogap.Comment: 7 pages, 9 figures, presented at EUCAS 2003 (September 14-18),
submitted to SUS
Exchange Instabilities in Semiconductor Double Quantum Well Systems
We consider various exchange-driven electronic instabilities in semiconductor
double-layer systems in the absence of any external magnetic field. We
establish that there is no exchange-driven bilayer to monolayer charge transfer
instability in the double-layer systems. We show that, within the unrestricted
Hartree-Fock approximation, the low density stable phase (even in the absence
of any interlayer tunneling) is a quantum ``pseudospin rotated'' spontaneous
interlayer phase coherent spin-polarized symmetric state rather than the
classical Ising-like charge-transfer phase. The U(1) symmetry of the double
quantum well system is broken spontaneously at this low density quantum phase
transition, and the layer density develops quantum fluctuations even in the
absence of any interlayer tunneling. The phase diagram for the double quantum
well system is calculated in the carrier density--layer separation space, and
the possibility of experimentally observing various quantum phases is
discussed. The situation in the presence of an external electric field is
investigated in some detail using the
spin-polarized-local-density-approximation-based self-consistent technique and
good agreement with existing experimental results is obtained.Comment: 24 pages, figures included. Also available at
http://www-cmg.physics.umd.edu/~lzheng/preprint/ct.uu/ . Revised final
version to appear in PR
Disorder Effects in Superconductors with Anisotropic Pairing: From Cooper Pairs to Compact Bosons
In the weak coupling BCS-approximation normal impurities do not influence
superconducting T_{c} in significant manner in case of isotropic s-wave
pairing. However, in case of d-wave pairing these are strongly pair-breaking.
This fact is in rather strong contradiction with many experiments on disordered
high-T_{c} superconductors assuming the d-wave nature of pairing in these
systems. With the growth of electron attraction within the Cooper pair the
system smoothly crosses over from BCS-pairs to compact Boson picture of
superconductivity. As pairing strength grows and pairs become compact
significant deviations from universal Abrikosov-Gorkov dependence of T_{c} on
disorder appear in case of d-wave pairing with superconducting state becoming
more stable than in the weak coupling case. As high-T_{c} superconductors are
actually in the intermediate region with Cooper pairs size of the order of few
interatomic lengths, these results can explain the relative stability of d-wave
pairing under rather strong disordering.Comment: 8 pages, 3 figures, RevTeX 3.0, 1 Postscript figure attached,
submitted to JETP Letter
Effects of an in-plane magnetic field on c-axis sum rule and superfluid density in high- cuprates
In layered cuprates, the application of an in-plane magnetic field changes the c-axis optical sum rule and superfluid density . For
pure incoherent c-axis coupling, has no effect on either quantities
but it does if an additional coherent component is present. For the coherent
contribution, different characteristic variations on and on
temperature result from the constant part of the hopping matrix
element and from the part which has zero on the diagonal of the
Brillouin zone. Only the constant part leads to a dependence on
the direction of as well as on its magnitude.Comment: 3 figure
Role of anisotropic impurity scattering in anisotropic superconductors
A theory of nonmagnetic impurities in an anisotropic superconductor including
the effect of anisotropic (momentum-dependent) impurity scattering is given. It
is shown that for a strongly anisotropic scattering the reduction of the
pair-breaking effect of the impurities is large. For a significant overlap
between the anisotropy functions of the scattering potential and that of the
pair potential and for a large amount of anisotropic scattering rate in
impurity potential the superconductivity becomes robust vis a vis impurity
concentration. The implications of our result for YBCO high-temperature
superconductor are discussed.Comment: 14 pages, RevTeX, 5 PostScript figures, to be published in Phys. Rev.
B (December 1, 1996
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