58 research outputs found
Orbital Currents in Underdoped Cuprates
Orbital currents, either fluctuating or static, have emerged as promising
candidates for a description of the pseudogap state in underdoped cuprates. I
shall review the evolution of these ideas and describe some experiments which
have been proposed to test the existence of orbital currents.Comment: 6 pages; Proceedings of the SNS 2001 Conferenc
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]
Bond operator theory of doped antiferromagnets: from Mott insulators with bond-centered charge order, to superconductors with nodal fermions
The ground states and excitations of two-dimensional insulating and doped
Mott insulators are described by a bond operator formalism. While the method
represents the degrees of freedom of an arbitrary antiferromagnet exactly, it
is especially suited to systems in which there is a natural pairing of sites
into bonds, as in states with spontaneous or explicit spin-Peierls order (or
bond-centered charge order). In the undoped insulator, as discussed previously,
we obtain both paramagnetic and magnetically-ordered states. We describe the
evolution of superconducting order in the ground state with increasing
doping--at low doping, the superconductivity is weak, can co-exist with
magnetic order, and there are no gapless spin 1/2 fermionic excitations; at
high doping, the magnetic order is absent and we obtain a BCS d-wave
superconductor with gapless spin 1/2, nodal fermions. We present the critical
theory describing the onset of these nodal fermionic excitations. We discuss
the evolution of the spin spectrum, and obtain regimes where a spin 1 exciton
contributes a sharp resonance in the dynamic spin susceptiblity. We also
discuss the experimental consequences of low-energy, dynamically fluctuating,
spin-Peierls order in an isotropic CuO_2 plane--we compute consequences for the
damping and dispersion of an optical phonon involving primarily the O ions, and
compare the results with recent neutron scattering measurements of phonon
spectra.Comment: 16 pages + 14 pages of appendices, 18 figures; (v3) expanded
discussion of theory and experimental implications; (v4) Removed some
introductory review discussion and moved it to cond-mat/010823
Developments in the negative-U modelling of the cuprate HTSC systems
The paper deals with the many stands that go into creating the unique and
complex nature of the HTSC cuprates above Tc as below. Like its predecessors it
treats charge, not spin or lattice, as prime mover, but thus taken in the
context of the chemical bonding relevant to these copper oxides. The crucial
shell filling, negative-U, double-loading fluctuations possible there require
accessing at high valent local environment as prevails within the mixed valent,
inhomogeneous two sub-system circumstance of the HTSC materials. Close
attention is paid to the recent results from Corson, Demsar, Li, Johnson,
Norman, Varma, Gyorffy and colleagues.Comment: 44 pages:200+ references. Submitted to J.Phys.:Condensed Matter, Sept
7 200
The pseudogap: friend or foe of high Tc?
Although nineteen years have passed since the discovery of high temperature
superconductivity, there is still no consensus on its physical origin. This is
in large part because of a lack of understanding of the state of matter out of
which the superconductivity arises. In optimally and underdoped materials, this
state exhibits a pseudogap at temperatures large compared to the
superconducting transition temperature. Although discovered only three years
after the pioneering work of Bednorz and Muller, the physical origin of this
pseudogap behavior and whether it constitutes a distinct phase of matter is
still shrouded in mystery. In the summer of 2004, a band of physicists gathered
for five weeks at the Aspen Center for Physics to discuss the pseudogap. In
this perspective, we would like to summarize some of the results presented
there and discuss its importance in the context of strongly correlated electron
systems.Comment: expanded version, 20 pages, 11 figures, to be published, Advances in
Physic
Quantum impurity dynamics in two-dimensional antiferromagnets and superconductors
We present the universal theory of arbitrary, localized impurities in a
confining paramagnetic state of two-dimensional antiferromagnets with global
SU(2) spin symmetry. The energy gap of the host antiferromagnet to spin-1
excitations, \Delta, is assumed to be significantly smaller than a typical
nearest neighbor exchange. In the absence of impurities, it was argued in
earlier work (Chubukov et al. cond-mat/9304046) that the low-temperature
quantum dynamics is universally and completely determined by the values of
\Delta and a spin-wave velocity c. Here we establish the remarkable fact that
no additional parameters are necessary for an antiferromagnet with a dilute
concentration of impurities, n_{imp} - each impurity is completely
characterized by a integer/half-odd-integer valued spin, S, which measures the
net uncompensated Berry phase due to spin precession in its vicinity. We
compute the impurity-induced damping of the spin-1 collective mode of the
antiferromagnet: the damping occurs on an energy scale \Gamma= n_{imp} (\hbar
c)^2/\Delta, and we predict a universal, asymmetric lineshape for the
collective mode peak. We argue that, under suitable conditions, our results
apply unchanged (or in some cases, with minor modifications) to d-wave
superconductors, and compare them to recent neutron scattering experiments on
YBCO by Fong et al. (cond-mat/9812047). We also describe the universal
evolution of numerous measurable correlations as the host antiferromagnet
undergoes a quantum phase transition to a Neel ordered state.Comment: 36 pages, 12 figures; added reference
How to detect fluctuating order in the high-temperature superconductors
We discuss fluctuating order in a quantum disordered phase proximate to a
quantum critical point, with particular emphasis on fluctuating stripe order.
Optimal strategies for extracting information concerning such local order from
experiments are derived with emphasis on neutron scattering and scanning
tunneling microscopy. These ideas are tested by application to two model
systems - the exactly solvable one dimensional electron gas with an impurity,
and a weakly-interacting 2D electron gas. We extensively review experiments on
the cuprate high-temperature superconductors which can be analyzed using these
strategies. We adduce evidence that stripe correlations are widespread in the
cuprates. Finally, we compare and contrast the advantages of two limiting
perspectives on the high-temperature superconductor: weak coupling, in which
correlation effects are treated as a perturbation on an underlying metallic
(although renormalized) Fermi liquid state, and strong coupling, in which the
magnetism is associated with well defined localized spins, and stripes are
viewed as a form of micro-phase separation. We present quantitative indicators
that the latter view better accounts for the observed stripe phenomena in the
cuprates.Comment: 43 pages, 11 figures, submitted to RMP; extensively revised and
greatly improved text; one new figure, one new section, two new appendices
and more reference
Inhibitory effect of 4-O-methylhonokiol on lipopolysaccharide-induced neuroinflammation, amyloidogenesis and memory impairment via inhibition of nuclear factor-kappaB in vitro and in vivo models
<p>Abstract</p> <p>Background</p> <p>Neuroinflammation is important in the pathogenesis and progression of Alzheimer disease (AD). Previously, we demonstrated that lipopolysaccharide (LPS)-induced neuroinflammation caused memory impairments. In the present study, we investigated the possible preventive effects of 4-<it>O</it>-methylhonokiol, a constituent of <it>Magnolia officinalis</it>, on memory deficiency caused by LPS, along with the underlying mechanisms.</p> <p>Methods</p> <p>We investigated whether 4-<it>O</it>-methylhonokiol (0.5 and 1 mg/kg in 0.05% ethanol) prevents memory dysfunction and amyloidogenesis on AD model mice by intraperitoneal LPS (250 Ī¼g/kg daily 7 times) injection. In addition, LPS-treated cultured astrocytes and microglial BV-2 cells were investigated for anti-neuroinflammatory and anti-amyloidogenic effect of 4-<it>O</it>-methylhonkiol (0.5, 1 and 2 Ī¼M).</p> <p>Results</p> <p>Oral administration of 4-<it>O</it>-methylhonokiol ameliorated LPS-induced memory impairment in a dose-dependent manner. In addition, 4-<it>O</it>-methylhonokiol prevented the LPS-induced expression of inflammatory proteins; inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) as well as activation of astrocytes (expression of glial fibrillary acidic protein; GFAP) in the brain. In <it>in vitro </it>study, we also found that 4-<it>O</it>-methylhonokiol suppressed the expression of iNOS and COX-2 as well as the production of reactive oxygen species, nitric oxide, prostaglandin E<sub>2</sub>, tumor necrosis factor-Ī±, and interleukin-1Ī² in the LPS-stimulated cultured astrocytes. 4-<it>O</it>-methylhonokiol also inhibited transcriptional and DNA binding activity of NF-ĪŗB via inhibition of IĪŗB degradation as well as p50 and p65 translocation into nucleus of the brain and cultured astrocytes. Consistent with the inhibitory effect on neuroinflammation, 4-<it>O</it>-methylhonokiol inhibited LPS-induced AĪ²<sub>1-42 </sub>generation, Ī²- and Ī³-secretase activities, and expression of amyloid precursor protein (APP), BACE1 and C99 as well as activation of astrocytes and neuronal cell death in the brain, in cultured astrocytes and in microglial BV-2 cells.</p> <p>Conclusion</p> <p>These results suggest that 4-<it>O</it>-methylhonokiol inhibits LPS-induced amyloidogenesis via anti-inflammatory mechanisms. Thus, 4-<it>O</it>-methylhonokiol can be a useful agent against neuroinflammation-associated development or the progression of AD.</p
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