192 research outputs found
Fluctuating Cu-O-Cu Bond model of high temperature superconductivity in cuprates
Twenty years of extensive research has yet to produce a general consensus on
the origin of high temperature superconductivity (HTS). However, several
generic characteristics of the cuprate superconductors have emerged as the
essential ingredients of and/or constraints on any viable microscopic model of
HTS. Besides a Tc of order 100K, the most prominent on the list include a
d-wave superconducting gap with Fermi liquid nodal excitations, a d-wave
pseudogap with the characteristic temperature scale T*, an anomalous
doping-dependent oxygen isotope shift, nanometer-scale gap inhomogeneity, etc..
The key role of planar oxygen vibrations implied by the isotope shift and other
evidence, in the context of CuO2 plane symmetry and charge constraints from the
strong intra-3d Coulomb repulsion U, enforces an anharmonic mechanism in which
the oxygen vibrational amplitude modulates the strength of the in-plane Cu-Cu
bond. We show, within a Fermi liquid framework, that this mechanism can lead to
strong d-wave pairing and to a natural explanation of the salient features of
HTS
Implications of the isotope effects on the magnetization, magnetic torque and susceptibility
We analyze the magnetization, magnetic torque and susceptibility data of
La2-xSrxCu(16,18)O4 and YBa2(63,65)CuO7-x near Tc in terms of the universal
3D-XY scaling relations. It is shown that the isotope effect on Tc mirrors that
on the anisotropy. Invoking the generic behavior of the anisotropy the doping
dependence of the isotope effects on the critical properties, including Tc,
correlation lengths and magnetic penetration depths are traced back to a change
of the mobile carrier concentration.Comment: 5 pages, 3 figure
Association of a genetic polymorphism (-44 C/G SNP) in the human DEFB1 gene with expression and inducibility of multiple β-defensins in gingival keratinocytes
BACKGROUND: Human β-defensins (hBDs) are antimicrobial peptides with a role in innate immune defense. Our laboratory previously showed that a single nucleotide polymorphism (SNP) in the 5' untranslated region of the hBD1 gene (DEFB1), denoted -44 (rs1800972), is correlated with protection from oral Candida. Because this SNP alters the putative mRNA structure, we hypothesized that it alters hBD1 expression. METHODS: Transfection of reporter constructs and evaluation of antimicrobial activity and mRNA expression levels in keratinocytes from multiple donors were used to evaluate the effect of this SNP on constitutive and induced levels of expression. RESULTS: Transfection of CAT reporter constructs containing the 5' untranslated region showed that the -44 G allele yielded a 2-fold increase in CAT protein compared to other common haplotypes suggesting a cis effect on transcription or translation. The constitutive hBD1 mRNA level in human oral keratinocytes was significantly greater in cells from donors with the -44 GG genotype compared to those with the common CC genotype. Surprisingly, the hBD3 mRNA level as well as antimicrobial activity of keratinocyte extracts also correlated with the -44 G allele. Induced levels of hBD1, hBD2, and hBD3 mRNA were evaluated in keratinocytes challenged with Toll-like receptor 2 and 4 ligands, interleukin-1β, TNFα, and interferon-γ (IFNγ). In contrast to constitutive expression levels, IFNγ-induced keratinocyte hBD1 and hBD3 mRNA expression was significantly greater in cells with the common CC genotype, but there was no clear correlation of genotype with hBD2 expression. CONCLUSION: The DEFB1 -44 G allele is associated with an increase in overall constitutive antimicrobial activity and expression of hBD1 and hBD3 in a manner that is consistent with protection from candidiasis, while the more common C allele is associated with IFNγ inducibility of these β-defensins and is likely to be more protective in conditions that enhance IFNγ expression such as chronic periodontitis. These results suggest a complex relationship between genetics and defensin expression that may influence periodontal health and innate immune responses
Unusual T_c variation with hole concentration in Bi_2Sr_{2-x}La_xCuO_{6+\delta}
We have investigated the variation with the hole concentration in
the La-doped Bi 2201 system, BiSrLaCuO. It is
found that the Bi 2201 system does not follow the systematics in and
observed in other high- cuprate superconductors (HTSC's). The vs
characteristics are quite similar to what observed in Zn-doped HTSC's. An
exceptionally large residual resistivity component in the inplane resistivity
indicates that strong potential scatterers of charge carriers reside in CuO
planes and are responsible for the unusual variation with , as in the
Zn-doped systems. However, contrary to the Zn-doped HTSC's, the strong scatter
in the Bi 2201 system is possibly a vacancy in the Cu site.Comment: RevTeX, 3 figures, to be published in the Physical Review
Thermopower in the strongly overdoped region of single-layer Bi2Sr2CuO6+d superconductor
The evolution of the thermoelectric power S(T) with doping, p, of
single-layer Bi2Sr2CuO6+d ceramics in the strongly overdoped region is studied
in detail. Analysis in term of drag and diffusion contributions indicates a
departure of the diffusion from the T-linear metallic behavior. This effect is
increased in the strongly overdoped range (p~0.2-0.28) and should reflect the
proximity of some topological change.Comment: 4 pages, 4 figure
Relation between the superconducting gap energy and the two-magnon Raman peak energy in Bi2Sr2Ca{1-x}YxCu2O{8+\delta}
The relation between the electronic excitation and the magnetic excitation
for the superconductivity in Bi2Sr2Ca{1-x}YxCu2O{8+\delta} was investigated by
wide-energy Raman spectroscopy. In the underdoping region the B1g scattering
intensity is depleted below the two-magnon peak energy due to the "hot spots"
effects. The depleted region decreases according to the decrease of the
two-magnon peak energy, as the carrier concentration ncreases. This two-magnon
peak energy also determines the B1g superconducting gap energy as
from under to overdoping hole concentration.Comment: 10 pages, 4 figure
Coherent quasiparticle weight and its connection to high-T_c superconductivity from angle-resolved photoemission
In conventional superconductors, the pairing energy gap (\Delta) and
superconducting phase coherence go hand-in-hand. As the temperature is lowered,
both the energy gap and phase coherence appear at the transition temperature
T_c. In contrast, in underdoped high-T_c superconductors (HTSCs), a pseudogap
appears at a much higher temperature T^*, smoothly evolving into the
superconducting gap at T_c. Phase coherence on the other hand is only
established at T_c, signaled by the appearance of a sharp quasiparticle (QP)
peak in the excitation spectrum. Another important difference between the two
types of superconductors is in the ratio of 2\Delta / T_c=R. In BCS theory,
R~3.5, is constant. In the HTSCs this ratio varies widely, continuing to
increase in the underdoped region, where the gap increases while T_c decreases.
Here we report that in HTSCs it is the ratio z_A\Delta_m/T_c which is
approximately constant, where \Delta_m is the maximum value of the d-wave gap,
and z_A is the weight of the coherent excitations in the spectral function.
This is highly unusual, since in nearly all phase transitions, T_c is
determined by an energy scale alone. We further show that in the
low-temperature limit, z_{\it A} increases monotonically with increasing doping
x. The growth is linear, i.e. z_A(x)\propto x, in the underdoped to optimally
doped regimes, and slows down in overdoped samples. The reduction of z_A with
increasing temperature resembles that of the c-axis superfluid density.Comment: 11 pages, 5 figures, revised versio
Observation of Andreev reflection in the c-axis transport of Bi_2Sr_2CaCu_2O_{8+x} single crystals near T_c and search for the preformed-pair state
We observed an enhancement of the -axis differential conductance around
the zero-bias in AuBiSrCaCuO (Bi2212) junctions near the
superconducting transition temperature . We attribute the conductance
enhancement to the Andreev reflection between the surface Cu-O bilayer with
suppressed superconductivity and the neighboring superconducting inner bilayer.
The continuous evolution from depression to an enhancement of the zero-bias
differential conductance, as the temperature approaches from below,
points to weakening of the barrier strength of the non-superconducting layer
between adjacent Cu-O bilayers. We observed that the conductance enhancement
persisted up to a few degrees above in junctions prepared on slightly
overdoped Bi2212 crystals. However, no conductance enhancement was observed
above in underdoped crystals, although recently proposed theoretical
consideration suggests an even wider temperature range of enhanced zero-bias
conductance. This seems to provide negative perspective to the existence of the
phase-incoherent preformed pairs in the pseudogap state.Comment: 17 pages including 4 figure
Anomalous electronic susceptibility in Bi2Sr2CuO6+d and comparison with other overdoped cuprates
We report magnetic susceptibility performed on overdoped Bi2Sr2CuO6+d powders
as a function of oxygen doping d and temperature T. The decrease of the spin
susceptibility with increasing T is confirmed. At sufficient high temperature,
the spin susceptibility Chi_s presents an unusual linear temperature dependence
Chi_s ~ Chi_s0 -Chi_1 T. Moreover, a linear correlation between Chi_1 and
Chi_s0 for increasing hole concentration is displayed. A temperature Tchi,
independent of hole doping characterizes this scaling. Comparison with other
cuprates of the literature(LSCO, Tl-2201 and Bi-2212), over the same overdoped
range, shows similarities with above results. These non conventional metal
features will be discussed in terms of a singular narrow-band structure.Comment: 16 pages, 4 figure
Anomalous superconducting state gap size versus Tc behavior in underdoped Bi_2Sr_2Ca_1-xDy_xCu_2O_8+d
We report angle-resolved photoemission spectroscopy measurements of the
excitation gap in underdoped superconducting thin films of
Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+d}. As Tc is reduced by a factor of 2 by
underdoping, the superconducting state gap \Delta does not fall proportionally,
but instead stays constant or increases slightly, in violation of the BCS
mean-field theory result. The different doping dependences of \Delta and kT_c
indicate that they represent different energy scales. The measurements also
show that \Delta is highly anisotropic and consistent with a d_{x^2-y^2} order
parameter, as in previous studies of samples with higher dopings. However, in
these underdoped samples, the anisotropic gap persists well above T_c. The
existence of a normal state gap is related to the failure of \Delta to scale
with T_c in theoretical models that predict pairing without phase coherence
above T_c.Comment: 10 pages, 4 postscript figures, revtex forma
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