1,612 research outputs found
Muon Spin Relaxation and Susceptibility Studies of Pure and Doped Spin 1/2 Kagom\'{e}-like system (CuZn)VO(OH) 2HO
Muon spin relaxation (SR) and magnetic susceptibility measurements have
been performed on the pure and diluted spin 1/2 kagom\'{e} system
(CuZn)VO(OH) 2HO. In the pure
system we found a slowing down of Cu spin fluctuations with decreasing
temperature towards K, followed by slow and nearly
temperature-independent spin fluctuations persisting down to = 50 mK,
indicative of quantum fluctuations. No indication of static spin freezing was
detected in either of the pure (=1.0) or diluted samples. The observed
magnitude of fluctuating fields indicates that the slow spin fluctuations
represent an intrinsic property of kagom\'e network rather than impurity spins.Comment: 4 pges, 4 color figures, Phys. Rev. Lett. in pres
Muon spin relaxation studies of incommensurate magnetism and superconductivity in stage-4 LaCuO and LaSrCuO
This paper reports muon spin relaxation (MuSR) measurements of two single
crystals of the title high-Tc cuprate systems where static incommensurate
magnetism and superconductivity coexist. By zero-field MuSR measurements and
subsequent analyses with simulations, we show that (1) the maximum ordered Cu
moment size (0.36 Bohr magneton) and local spin structure are identical to
those in prototypical stripe spin systems with the 1/8 hole concentration; (2)
the static magnetism is confined to less than a half of the volume of the
sample, and (3) regions with static magnetism form nano-scale islands with the
size comparable to the in-plane superconducting coherence length. By
transverse-field MuSR measurements, we show that Tc of these systems is related
to the superfluid density, in the same way as observed in cuprate systems
without static magnetism. We discuss a heuristic model involving percolation of
these nanoscale islands with static magnetism as a possible picture to
reconcile heterogeneity found by the present MuSR study and long-range spin
correlations found by neutron scattering.Comment: 19 pages, 15 figures, submitted to Phys. Rev. B. E-mail:
[email protected]
Enhancement of electronic anomalies in iron-substituted La_2-x_Sr_x_Cu_1-y_Fe_y_O_4_ around x=0.22
We have measured the temperature dependences of Rho and Chi for
Fe-substituted La_2-x_Sr_x_Cu_1-y_Fe_y_O_4_ in the overdoped regime, in order
to investigate Fe-substitution effects on electronic properties around x=0.22.
From the Rho measurements, it has been found around x=0.22 that the values of
Rho are large at room temperature and that Rho exhibits a pronounced upturn at
low temperatures. Moreover, from the Rho and Chi measurements, it has been
found that T_c_ is anomalously depressed around x=0.22. These results indicate
that the electronic anomalies around x=0.22 are enhanced by Fe substitution,
which might be related to the development of stripe correlations by Fe
substitution.Comment: 7 pages, 3 figure
Rapid Oscillations in Cataclysmic Variables. XVI. DW Cancri
We report photometry and spectroscopy of the novalike variable DW Cancri. The
spectra show the usual broad H and He emission lines, with an excitation and
continuum slope characteristic of a moderately high accretion rate. A
radial-velocity search yields strong detections at two periods, 86.1015(3) min
and 38.58377(6) min. We interpret these as respectively the orbital period
P_orb of the binary, and the spin period P_spin of a magnetic white dwarf. The
light curve also shows the spin period, plus an additional strong signal at
69.9133(10) min, which coincides with the difference frequency
1/P_spin-1/P_orb. These periods are stable over the 1 year baseline of
measurement.
This triply-periodic structure mimics the behavior of several
well-credentialed members of the "DQ Herculis" (intermediate polar) class of
cataclysmic variables. DQ Her membership is also suggested by the mysteriously
strong sideband signal (at nu_spin-nu_orb), attesting to a strong pulsed flux
at X-ray/EUV/UV wavelengths. DW Cnc is a new member of this class, and would be
an excellent target for extended observation at these wavelengths.Comment: PDF, 28 pages, 6 tables, 9 figures; accepted, in press, to appear
June 2004, PASP; more info at http://cba.phys.columbia.edu
Site-Dilution in quasi one-dimensional antiferromagnet Sr2(Cu1-xPdx)O3: reduction of Neel Temperature and spatial distribution of ordered moment sizes
We investigate the Neel temperature of Sr2CuO3 as a function of the site
dilution at the Cu (S=1/2) sites with Pd (S=0), utilizing the muon spin
relaxation (muSR) technique. The Neel temperature, which is Tn=5.4K for the
undoped system, becomes significantly reduced for less than one percent of
doping Pd, giving a support for the previous proposal for the good
one-dimensionality. The Pd concentration dependence of the Neel temperature is
compared with a recent theoretical study (S. Eggert, I. Affleck and M.D.P.
Horton, Phys. Rev. Lett. 89, 47202 (2002)) of weakly coupled one-dimensional
antiferromagnetic chains of S=1/2 spins, and a quantitative agreement is found.
The inhomogeneity of the ordered moment sizes is characterized by the muSR time
spectra. We propose a model that the ordered moment size recovers away from the
dopant S=0 sites with a recovery length of \xi = 150-200 sites. The origin of
the finite recovery length \xi for the gapless S=1/2 antiferromagnetic chain is
compared to the estimate based on the effective staggered magnetic field from
the neighboring chains.Comment: 10 pages, 9 figures, submitted to PR
Compensation driven superconductor-insulator transition
The superconductor-insulator transition in the presence of strong
compensation of dopants was recently realized in La doped YBCO. The
compensation of acceptors by donors makes it possible to change independently
the concentration of holes n and the total concentration of charged impurities
N. We propose a theory of the superconductor-insulator phase diagram in the
(N,n) plane. It exhibits interesting new features in the case of strong
coupling superconductivity, where Cooper pairs are compact, non-overlapping
bosons. For compact Cooper pairs the transition occurs at a significantly
higher density than in the case of spatially overlapping pairs. We establish
the superconductor-insulator phase diagram by studying how the potential of
randomly positioned charged impurities is screened by holes or by strongly
bound Cooper pairs, both in isotropic and layered superconductors. In the
resulting self-consistent potential the carriers are either delocalized or
localized, which corresponds to the superconducting or insulating phase,
respectively
BCS-Bose Crossover in Color Superconductivity
It is shown that the onset of the color superconducting phase occurs in the
BCS-BE crossover region.Comment: 5 pages, LaTeX, references adde
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
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