30 research outputs found
Two-dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2: phase transitions and AFMorder as seen by 87Rb NMR
We report the Rb nuclear magnetic resonance (NMR) results in a recently
synthesized Na5RbCu4(AsO4)Cl2. This complex novel two-dimensional (2D) cuprate
is an unique magnetic material, which contains layers of coupled Cu4O4
tetramers. In zero applied magnetic field, it orders antiferromagnetically via
a second-order low-entropy phase transition at TN = 15(1) K. We characterise
the ordered state by 87Rb NMR, and suggest for it a noncollinear rather than
collinear arrangement of spins. We discuss the properties of Rb nuclear site
and point out the new structural phase transition(s) around 74 K and 110 K.Comment: 2 pages, 2 figures, Proceedings of SCES'05, Vienna 200
Magnetic Properties of the Novel Low-Dimensional Cuprate Na5RbCu4(AsO4)4Cl2
The magnetic properties of a new compound, Na5RbCu4(AsO4)4Cl2 are reported.
The material has a layered structure comprised of square Cu4O4 tetramers. The
Cu ions are divalent and the system behaves as a low-dimensional S=1/2
antiferromagnet. Spin exchange in Na5RbCu4(AsO4)4Cl2 appears to be
quasi-two-dimensional and non-frustrated. Measurements of the bulk magnetic
susceptibility and heat capacity are consistent with low-dimensional magnetism.
The compound has an interesting, low-entropy, magnetic transition at T = 17 K.Comment: 4 pages, 5 figure
The onset of the vortex-like Nernst signal above Tc in La_{2-x}Sr_xCuO_4 and Bi_2Sr_{2-y}La_yCuO_6
The diffusion of vortices down a thermal gradient produces a Josephson signal
which is detected as the vortex Nernst effect. In a recent report, Xu et al.,
Nature 406, 486 (2000), an enhanced Nernst signal identified with vortex-like
excitations was observed in a series of La_{2-x}Sr_xCuO_4 (LSCO) crystals at
temperatures 50-100 K above T_c. To pin down the onset temperature T_{\nu} of
the vortex-like signal in the lightly doped regime (0.03 < x < 0.07), we have
re-analyzed in detail the carrier contribution to the Nernst signal. By
supplementing new Nernst measurements with thermopower and Hall-angle data, we
isolate the off-diagonal Peltier conductivity \alpha_{xy} and show that its
profile provides an objective determination of T_{\nu}. With the new results,
we revise the phase diagram for the fluctuation regime in LSCO to accomodate
the lightly doped regime. In the cuprate Bi_2Sr_{2-y}La_yCuO_6, we find that
the carrier contribution is virtually negligible for y in the range 0.4-0.6.
The evidence for an extended temperature interval with vortex-like excitations
is even stronger in this system. Finally, we discuss how T_{\nu} relates to the
pseudogap temperature T* and the implications of strong fluctuations between
the pseudogap state and the d-wave superconducting state.Comment: 10 pages, 10 figure
Combination Rules, Charge Symmetry, and Hall Effect in Cuprates
The rule relating the observed Hall coefficient to the spin and charge
responses of the uniform doped Mott insulator is derived. It is essential to
include the contribution of holon and spinon three-current correlations to the
effective action of the gauge field. In the vicinity of the Mott insulating
point the Hall coefficient is holon dominated and weakly temperature dependent.
In the vicinity of a point of charge conjugation symmetry the holon
contribution to the observed Hall coefficient is small: the Hall coefficient
follows the temperature dependence of the diamagnetic susceptibility with a
sign determined by the Fermi surface shape. NOTE: document prepared using
REVTEX. (3 Figs, not included, available on request from: [email protected])Comment: 8 page
Entropy of vortex cores on the border of the superconductor-to-insulator transition in an underdoped cuprate
We present a study of Nernst effect in underdoped in
magnetic fields as high as 28T. At high fields, a sizeable Nernst signal was
found to persist in presence of a field-induced non-metallic resistivity. By
simultaneously measuring resistivity and the Nernst coefficient, we extract the
entropy of vortex cores in the vicinity of this field-induced
superconductor-insulator transition. Moreover, the temperature dependence of
the thermo-electric Hall angle provides strong constraints on the possible
origins of the finite Nernst signal above , as recently discovered by Xu
et al.Comment: 5 Pages inculding 4 figure
INTERLAYER COUPLING AND THE METAL-INSULATOR TRANSITION IN Pr-SUBSTITUTED Bi(2)Sr(2)CaCu(2)O(8+y)
Substitution of rare-earth ions for Ca in Bi2Sr2CaCu2O8+y is known to cause a
metal-insulator transition. Using resonant photoemission we study how this
chemical substitution affects the electronic structure of the material. For the
partial Cu-density of states at E_F and in the region of the valence band we
observe no significant difference between a pure superconducting sample and an
insulating sample with 60% Pr for Ca. This suggests that the states responsible
for superconductivity are predomi- nately O-states. The partial Pr-4f density
of states was extracted utilizing the Super- Koster-Kronig Pr 4d-4f resonance.
It consists of a single peak at 1.36eV binding energy. The peak shows a
strongly assymetric Doniach-Sunjic line- shape indicating the presence of a
continuum of electronic states with sharp cut off at E_F even in this
insulating sample. This finding excludes a bandgap in the insulating sample and
supports the existance of a mobility gap caused by spatial localization of the
carriers. The presence of such carriers at the Pr-site, between the CuO_2
planes shows that the electronic structure is not purely 2-dimensional but that
there is a finite interlayer coupling. The resonance enhancement of the
photoemission cross section, at the Pr-4d threshold, was studied for the Pr-4f
and for Cu-states. Both the Pr-4f and the Cu-states show a Fano-like resonance.
This resonance of Cu-states with Pr-states is another indication of coupling
between the the Pr-states and those in the CuO_2 plane. Because of the
statistical distribution of the Pr-ions this coupling leads to a non-periodic
potential for the states in the CuO_2 plane which can lead to localization and
thus to the observed metal-insulator transition.Comment: Gziped uuencoded postscript file including 7 figures Scheduled for
publication in Physical Review B, May 1, 1995
Antiferromagnetic ordering in a 90 K copper oxide superconductor
Using elastic neutron scattering, we evidence a commensurate
antiferromagnetic Cu(2) order (AF) in the superconducting (SC) high-
cuprate (y=0.013, =93 K). As
in the Co-free system, the spin excitation spectrum is dominated by a magnetic
resonance peak at 41 meV but with a reduced spectral weight. The substitution
of Co thus leads to a state where AF and SC cohabit showing that the CuO
plane is a highly antiferromagnetically polarizable medium even for a sample
where T remains optimum.Comment: 3 figure