213 research outputs found
Quantum solitons in the sawtooth lattice
We study the sawtooth lattice of a coupled spin ½ Heisenberg system, a variant of the railroad-trestle lattice. The ground state of this system is twofold degenerate with periodic boundary conditions and supports kink-antikink excitations, which are distinct in this case, unlike the railroad-trestle lattice. The resulting low-temperature thermodynamics is compared with the recently discovered delafossites YCuO2.5
Normal State Magnetic Properties of Ni and Zn Substituted in YBa_{2}Cu_{3} O_{6+x}: Hole-Doping Dependence
We present SQUID susceptibility data on Zn and Ni substituted
YBa_{2}Cu_{3}O_{6+x}. Cross-checks with NMR yield an unprecedented accuracy in
the estimate of the magnetic susceptibility associated with the substituants,
from the underdoped to the lightly overdoped case. This allows us to determine
the Weiss temperature \theta for YBCO: its value is very small for all hole
dopings n_h. Since in conventional metals, the Kondo temperature,
, magnetic screening effects would not be expected for ; in contrast, increasing n_h produces a reduction of the small moment
induced by Zn^{2+} and a nearly constant effective moment for Ni^{2+}
corresponding to a spin 1/2 rather than to a spin 1.Comment: 4 pages, 5 figures, to be published in Europhysics Letter
A pseudogap term in the magnetic response of the cuprate superconductors
We combine neutron scattering (INS) and NMR/NQR nuclear spin lattice
relaxation rate data to deduce the existence of a new contribution to the
magnetic response (dynamic susceptibility) in cuprate superconductors. This
contribution, which has yet to be observed with INS, is shown to embody the
magnetic pseudogap effects. As such, it explains the long-standing puzzle of
pseudogap effects missing from cuprate INS data, dominated by stripe
fluctuations, for the dynamic susceptibility at low energies. For
La_(1.86)Sr_(0.14)CuO_(4) and YBa_(2)Ba_(3)O_(6.5), the new term is the chief
contributor to nuclear spin lattice relaxation at T >> T_(c).Comment: 11 pages, 2 figures. Augmented version to be published in Physical
Review
The NMR of High Temperature Superconductors without Anti-Ferromagnetic Spin Fluctuations
A microscopic theory for the NMR anomalies of the planar Cu and O sites in
superconducting La_1.85Sr_0.15CuO_4 is presented that quantitatively explains
the observations without the need to invoke anit-ferromagnetic spin
fluctuations on the planar Cu sites and its significant discrepancy with the
observed incommensurate neutron spin fluctuations. The theory is derived from
the recently published ab-initio band structure calculations that correct LDA
computations tendency to overestimate the self-coulomb repulsion for the
half-filled Cu d_x2-y2 orbital for these ionic systems. The new band structure
leads to two bands at the Fermi level with holes in the Cu d_z2 and apical O
p_z orbitals in addition to the standard Cu d_x2-y2 and planar O p_sigma
orbitals. This band structure is part of a new theory for the cuprates that
explains a broad range of experiments and is based upon the formation of Cooper
pairs comprised of a k up spin electron from one band and a -k down spin
electron from another band (Interband Pairing Model).Comment: In Press, Journal of Physical Chemistry. See also
http://www.firstprinciples.com. Minor changes to references and figure
readabilit
CeRuSn: heavy fermions emerging from a Kondo-insulating state
The combination of low-temperature specific-heat and
nuclear-magnetic-resonance (NMR) measurements reveals important information of
the ground-state properties of CeRuSn, which has been proposed as a
rare example of a tetragonal Kondo-insulator (KI). The NMR
spin-latticerelaxation rate deviates from the Korringa law below 100 K
signaling the onset of an energy gap K. This gap is
stable against magnetic fields up to 10 T. Below 10 K, however, unusual
low-energy excitations of in-gap states are observed, which depend strongly on
the field H. The specific heat C detects these excitations in the form of an
enhanced Sommerfeld coefficient : In zero field,
increases steeply below 5 K, reaching a maximum at 0.1 K, and then saturates at
J/molK. This maximum is shifted to higher temperatures with
increasing field suggesting a residual density of states at the Fermi level
developing a spin gap . A simple model, based on two narrow
quasiparticle bands located at the Fermi level - which cross the Fermi level in
zero field at 0.022 states/meV f.u. - can account qualitatively as well as
quantitatively for the measured observables. In particular, it is demonstrated
that fitting our data of both specific heat and NMR to the model, incorporating
a Ce magnetic moment of , leads to
the prediction of the field dependence of the gap. Our measurements rule out
the presence of a quantum critical point as the origin for the enhanced
in CeRuSn and suggest that this arises rather from correlated,
residual in-gap states at the Fermi level. This work provides a fundamental
route for future investigations into the phenomenon of narrow-gap formation in
the strongly correlated class of systemComment: 11 pages, 13 figure
Comment on "Localized behavior near the Zn impurity in YBa2Cu4O8 as measured by nuclear quadrupole resonance"
Williams and Kramer [Phys. Rev. B {\bf 64}, 104506 (2001)] have recently
argued against the existence of staggered magnetic moments residing on several
lattice sites around Zn impurities in YBCO superconductors. This claim, which
is in line with an earlier publication by Williams, Tallon and Dupree [Phys.
Rev. B {\bf 61}, 4319 (2000)], is however in contradiction with a large body of
experimental data from different NMR groups. On the contrary, the authors argue
in favor of a very localized spin and charge density on Cu sites first
neighbors to Zn. We show that the conclusions of Williams and Kramer arise from
erroneous interpretations of NMR and NQR data.Comment: 4 page
Planar CuO_2 hole density estimation in multilayered high-T_c cuprates
We report that planar CuO_2 hole densities in high-T_c cuprates are
consistently determined by the Cu-NMR Knight shift. In single- and bi-layered
cuprates, it is demonstrated that the spin part of the Knight shift K_s(300 K)
at room temperature monotonically increases with the hole density from
underdoped to overdoped regions, suggesting that the relationship of K_s(300 K)
vs. p is a reliable measure to determine p. The validity of this K_s(300 K)-p
relationship is confirmed by the investigation of the p-dependencies of
hyperfine magnetic fields and of spin susceptibility for single- and bi-layered
cuprates with tetragonal symmetry. Moreover, the analyses are compared with the
NMR data on three-layered Ba_2Ca_2Cu_3O_6(F,O)_2, HgBa_2Ca_2Cu_3O_{8+delta},
and five-layered HgBa_2Ca_4Cu_5O_{12+delta}, which suggests the general
applicability of the K_s(300 K)-p relationship to multilayered compounds with
more than three CuO_2 planes. We remark that the measurement of K_s(300 K)
enables us to separately estimate p for each CuO_2 plane in multilayered
compounds, where doped hole carriers are inequivalent between outer CuO_2
planes and inner CuO_2 planes.Comment: 7 pages, 5 figures, 2 Tables, to be published in Physical Review
NMR, Neutron scattering, and the one-band model of La<SUB>2-x</SUB>Sr<SUB>x</SUB>CuO<SUB>4</SUB>
NMR data on 17O and 63Cu in La1.85SrxCuO4 are presented and agree well with published work. Relaxation curves for 63Cu and for both apical and planar 17O sites are compared with calculations based on parametrization of recent absolute neutron susceptibilities X"(q,ω). A reasonable accord is found with the 63Cu data. However, a significant discrepancy with the observed planar 17O relaxation behavior poses an apparent contradiction with the widely accepted one-band dynamical model for this system
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