50 research outputs found
Superconductivity in hole-doped C60 from electronic correlations
We derive a model for the highest occupied molecular orbital band of a C60
crystal which includes on-site electron-electron interactions. The form of the
interactions are based on the icosahedral symmetry of the C60 molecule together
with a perturbative treatment of an isolated C60 molecule. Using this model we
do a mean-field calculation in two dimensions on the [100] surface of the
crystal. Due to the multi-band nature we find that electron-electron
interactions can have a profound effect on the density of states as a function
of doping. The doping dependence of the transition temperature can then be
qualitatively different from that expected from simple BCS theory based on the
density of states from band structure calculations
Local Magnetic Susceptibility of the Positive Muon in the Quasi 1D S=1/2 Antiferromagnet KCuF
We report muon spin rotation measurements of the local magnetic
susceptibility around a positive muon in the paramagnetic state of the quasi
one-dimensional spin 1/2 antiferromagnet KCuF. Signals from two distinct
sites are resolved which have a temperature dependent frequency shift which is
different than the magnetic susceptibility. This difference is attributed to a
muon induced perturbation of the spin 1/2 chain.Comment: 13 pages, 4 figures, The 2002 International Conference on Muon Spin
Rotation, Relaxation and Resonance, Virginia. US
Correlations Between Charge Ordering and Local Magnetic Fields in Overdoped YBaCuO
Zero-field muon spin relaxation (ZF-SR) measurements were undertaken on
under- and overdoped samples of superconducting YBaCuO to
determine the origin of the weak static magnetism recently reported in this
system. The temperature dependence of the muon spin relaxation rate in
overdoped crystals displays an unusual behavior in the superconducting state. A
comparison to the results of NQR and lattice structure experiments on highly
doped samples provides compelling evidence for strong coupling of charge, spin
and structural inhomogeneities.Comment: 4 pages, 4 figures, new data, new figures and modified tex
Mott Transition in Degenerate Hubbard Models: Application to Doped Fullerenes
The Mott-Hubbard transition is studied for a Hubbard model with orbital
degeneracy N, using a diffusion Monte-Carlo method. Based on general arguments,
we conjecture that the Mott-Hubbard transition takes place for U/W \propto
\sqrt{N}, where U is the Coulomb interaction and W is the band width. This is
supported by exact diagonalization and Monte-Carlo calculations. Realistic
parameters for the doped fullerenes lead to the conclusion that stoichiometric
A_3 C_60 (A=K, Rb) are near the Mott-Hubbard transition, in a correlated
metallic state.Comment: 4 pages, revtex, 1 eps figure included, to be published in Phys.Rev.B
Rapid Com
Gaps and excitations in fullerides with partially filled bands : NMR study of Na2C60 and K4C60
We present an NMR study of Na2C60 and K4C60, two compounds that are related
by electron-hole symmetry in the C60 triply degenerate conduction band. In both
systems, it is known that NMR spin-lattice relaxation rate (1/T1) measurements
detect a gap in the electronic structure, most likely related to
singlet-triplet excitations of the Jahn-Teller distorted (JTD) C60^{2-} or
C60^{4-}. However, the extended temperature range of the measurements presented
here (10 K to 700 K) allows to reveal deviations with respect to this general
trend, both at high and low temperatures. Above room temperature, 1/T1 deviates
from the activated law that one would expect from the presence of the gap and
saturates. In the same temperature range, a lowering of symmetry is detected in
Na2C60 by the appearance of quadrupole effects on the 23Na spectra. In K4C60,
modifications of the 13C spectra lineshapes also indicate a structural
modification. We discuss this high temperature deviation in terms of a coupling
between JTD and local symmetry. At low temperatures, 1/TT tends to a
constant value for Na2C60, both for 13C and 23Na NMR. This indicates a residual
metallic character, which emphasizes the proximity of metallic and insulting
behaviors in alkali fullerides.Comment: 12 pages, 13 figure
The Muonium Atom as a Probe of Physics beyond the Standard Model
The observed interactions between particles are not fully explained in the
successful theoretical description of the standard model to date. Due to the
close confinement of the bound state muonium () can be used as
an ideal probe of quantum electrodynamics and weak interaction and also for a
search for additional interactions between leptons. Of special interest is the
lepton number violating process of sponteanous conversion of muonium to
antimuonium.Comment: 15 pages,6 figure
Hidden Order in the Cuprates
We propose that the enigmatic pseudogap phase of cuprate superconductors is
characterized by a hidden broken symmetry of d(x^2-y^2)-type. The transition to
this state is rounded by disorder, but in the limit that the disorder is made
sufficiently small, the pseudogap crossover should reveal itself to be such a
transition. The ordered state breaks time-reversal, translational, and
rotational symmetries, but it is invariant under the combination of any two. We
discuss these ideas in the context of ten specific experimental properties of
the cuprates, and make several predictions, including the existence of an
as-yet undetected metal-metal transition under the superconducting dome.Comment: 12 pages of RevTeX, 9 eps figure
Muon-spin-rotation study of the effect of Zn substitution on magnetism in YBa2Cu3Ox
The magnetic properties of YBa2(Cu0.96Zn0.04)3Ox were studied in detail by means of muon spin rotation and relaxation for 6.0≤x≤6.92. The complete magnetic phase diagram was mapped out and a disordered magnetic state was found to persist between x=6.4 and x≃6.7 (metallic transition), in contrast with pure YBa2Cu3Ox. The appearance of this magnetic state is attributed to the effect of Zn on the doped hole dynamics and might be associated with the freezing of local moments due to Zn (6.43≤x≤6.88), which were also detected here, in the paramagnetic state
Positive muon Knight shift and spin relaxation in heavy fermion superconductors UPt3 and UBe13
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m/sup +/SR studies on pure MnF/sub 2/ and site-diluted (Mn/sub 0. 5/Zn/sub 0. 5/)F/sub 2/
Positive muon spin rotation and relaxation measurements have been carried out on the antiferromagnets, pure MnF/sub 2/ and site-diluted (Mn/sub 0.5/Zn/sub 0.5/)F/sub 2/, above and below the Neel temperature T/sub N/ using single-crystal specimens. Two different muon signals have been found in the pure MnF/sub 2/; with the precession frequency nu/sub A/ = 1.3 GHz for the site A and nu/sub B/ = 152 MHz for the site B measured in zero external magnetic field at T = 5 K. We propose a picture that the signal from the A site represents the ''muonium'' state, and discuss the characteristic features of muonium in magnetic materials. The spin relaxation rate 1/T/sub 1/, measured in zero external field, decreases rapidly with decreasing temperature below T/sub N/. The mechanism of the spin relaxation above T/sub N/ is explained by the exchange fluctuations of the Mn moments, while below T/sub N/ by the Raman scattering of spin waves. At the same normalized temperature T/T/sub N/, 1/T/sub 1/ observed in the diluted (Mn/sub 0.5/Zn/sub 0.5/)F/sub 2/ is significantly larger than that in the pure MnF/sub 2/ below T/sub N/. The difference between the pure and diluted systems is related to the large spectral weight of low-energy magnons in (Mn/sub 0.5/Zn/sub 0.5/)F/sub 2/ found by neutron scattering