33 research outputs found
Phase fluctuations and the pseudogap in YBa2Cu3Ox
The thermodynamics of the superconducting transition is studied as a function
of doping using high-resolution expansivity data of YBa2Cu3Ox single crystals
and Monte-Carlo simulations of the anisotropic 3D-XY model. We directly show
that Tc of underdoped YBa2Cu3Ox is strongly suppressed from its mean-field
value (Tc-MF) by phase fluctuations of the superconducting order parameter. For
overdoped YBa2Cu3Ox fluctuation effects are greatly reduced and Tc ~ Tc-MF . We
find that Tc-MF exhibits a similar doping dependence as the pseudogap energy,
naturally suggesting that the pseudogap arises from phase-incoherent Cooper
pairing.Comment: 9 pages, 3 Figure
Anomalously large oxygen-ordering contribution to the thermal expansion of untwinned YBa2Cu3O6.95 single crystals: a glass-like transition near room temperature
We present high-resolution capacitance dilatometry studies from 5 - 500 K of
untwinned YBa2Cu3Ox (Y123) single crystals for x ~ 6.95 and x = 7.0. Large
contributions to the thermal expansivities due to O-ordering are found for x ~
6.95, which disappear below a kinetic glass-like transition near room
temperature. The kinetics at this glass transition is governed by an energy
barrier of 0.98 +- 0.07 eV, in very good agreement with other O-ordering
studies. Using thermodynamic arguments, we show that O-ordering in the Y123
system is particularly sensitive to uniaxial pressure (stress) along the chain
axis and that the lack of well-ordered chains in Nd123 and La123 is most likely
a consequence of a chemical-pressure effect.Comment: 4 pages, 3 figures, submitted to PR
X-ray diffraction measurements of the c-axis Debye-Waller factors of YBa2Cu3O7 and HgBa2CaCu2O6
We report the first application of x-rays to the measurement of the
temperature dependent Bragg peak intensities to obtain Debye-Waller factors on
high-temperature superconductors. Intensities of (0,0,l) peaks of YBa2Cu3O7 and
HgBa2CaCu2O6 thin films are measured to obtain the c-axis Debye-Waller factors.
While lattice constant and some Debye-Waller factor measurements on high Tc
superconductors show anomalies at the transition temperature, our measurements
by x-ray diffraction show a smooth transition of the c-axis Debye-Waller
factors through T. This suggests that the dynamic displacements of the
heavy elements along the c-axis direction in these compounds do not have
anomalies at Tc. This method in combination with measurements by other
techniques will give more details concerning dynamics of the lattice.Comment: 4 pages, 2 figures. To be published in Physical Review B (Brief
Report
Quantum oscillations from Fermi arcs
When a metal is subjected to strong magnetic field B nearly all measurable
quantities exhibit oscillations periodic in 1/B. Such quantum oscillations
represent a canonical probe of the defining aspect of a metal, its Fermi
surface (FS). In this study we establish a new mechanism for quantum
oscillations which requires only finite segments of a FS to exist. Oscillations
periodic in 1/B occur if the FS segments are terminated by a pairing gap. Our
results reconcile the recent breakthrough experiments showing quantum
oscillations in a cuprate superconductor YBCO, with a well-established result
of many angle resolved photoemission (ARPES) studies which consistently
indicate "Fermi arcs" -- truncated segments of a Fermi surface -- in the normal
state of the cuprates.Comment: 8 pages, 5 figure
Absolute values of the London penetration depth in YBa2Cu3O6+y measured by zero field ESR spectroscopy on Gd doped single crystals
Zero-field electron spin resonance (ESR) of dilute Gd ions substituted for Y
in the cuprate superconductor YBaCuO is used as a novel
technique for measuring the absolute value of the low temperature magnetic
penetration depth . The Gd ESR spectrum of samples with
substitution was obtained with a broadband microwave technique
that measures power absorption bolometrically from 0.5 GHz to 21 GHz. This ESR
spectrum is determined by the crystal field that lifts the level degeneracy of
the spin 7/2 Gd ion and details of this spectrum provide information
concerning oxygen ordering in the samples. The magnetic penetration depth is
obtained by relating the number of Gd ions exposed to the microwave magnetic
field to the frequency-integrated intensity of the observed ESR transitions.
This technique has allowed us to determine precise values of for
screening currents flowing in the three crystallographic orientations (, and ) in samples of GdYBaCuO of three different oxygen contents ( K), ( K) and
( K). The in-plane values are found to depart substantially from the
widely reported relation .Comment: 14 pages, 12 figures; version to appear in PR
Flux Phase as a Dynamic Jahn-Teller Phase: Berryonic Matter in the Cuprates?
There is considerable evidence for some form of charge ordering on the
hole-doped stripes in the cuprates, mainly associated with the low-temperature
tetragonal phase, but with some evidence for either charge density waves or a
flux phase, which is a form of dynamic charge-density wave. These three states
form a pseudospin triplet, demonstrating a close connection with the E X e
dynamic Jahn-Teller effect, suggesting that the cuprates constitute a form of
Berryonic matter. This in turn suggests a new model for the dynamic Jahn-Teller
effect as a form of flux phase. A simple model of the Cu-O bond stretching
phonons allows an estimate of electron-phonon coupling for these modes,
explaining why the half breathing mode softens so much more than the full
oxygen breathing mode. The anomalous properties of provide a coupling
(correlated hopping) which acts to stabilize density wave phases.Comment: Major Revisions: includes comparisons with specific cuprate phonon
modes, 16 eps figures, revte
Effect of Thermal Phase Fluctuations on the Inductances of Josephson Junctions, Arrays of Junctions, and Superconducting Films
We calculate the factor by which thermal phase fluctuations, as distinct from
phase-slip fluctuations, increase the inductance, LJ, of a resistively-shunted
Josephson junction (JJ) above its mean-field value, L0. We find that quantum
mechanics suppresses fluctuations when T drops below a temperature, TQ =
h/kBGL0, where G is the shunt conductance. Examination of the calculated sheet
inductance, LA(T)/L0(T), of arrays of JJ's reveals that 2-D interconnections
halve fluctuation effects, while reducing phase-slip effects by a much larger
factor. Guided by these results, we calculate the sheet inductance,
LF(T)/L0(T), of 2-D films by treating each plasma oscillation mode as an
overdamped JJ. In disordered s-wave superconductors, quantum suppression is
important for LF(0)/LF(T) > 0.14, (or, T/TC0 < 0.94). In optimally doped YBCO
and BSCCO quantum suppression is important for l2(0)/l2(T) > 0.25, where l is
the penetration depth.Comment: 15 pages; 4 figures. Submitted to Physical Review B, May 199
Superconducting Fluctuation investigated by THz Conductivity of LaSrCuO Thin Films
Frequency-dependent terahertz conductivities of LaSrCuO thin
films with various carrier concentrations were investigated. The imaginary part
of the complex conductivity considerably increased from far above a
zero-resistance superconducting transition temperature,
, because of the existence of the fluctuating
superfluid density with a short lifetime. The onset temperature of the
superconducting fluctuation is at most for
underdoped samples, which is consistent with the previously reported analysis
of microwave conductivity. The superconducting fluctuation was not enhanced
under a 0.5 T magnetic field. We also found that the temperature dependence of
the superconducting fluctuation was sensitive to the carrier concentration of
LaSrCuO, which reflects the difference in the nature of the
critical dynamics near the superconducting transition temperature. Our results
suggest that the onset temperature of the Nernst signal is not related to the
superconducting fluctuation we argued in this paper.Comment: J. Phys. Soc. Jpn. in pres
Distinct Expression/Function of Potassium and Chloride Channels Contributes to the Diverse Volume Regulation in Cortical Astrocytes of GFAP/EGFP Mice
Recently, we have identified two astrocytic subpopulations in the cortex of GFAP-EGFP mice, in which the astrocytes are visualized by the enhanced green–fluorescent protein (EGFP) under the control of the human glial fibrillary acidic protein (GFAP) promotor. These astrocytic subpopulations, termed high response- (HR-) and low response- (LR-) astrocytes, differed in the extent of their swelling during oxygen-glucose deprivation (OGD). In the present study we focused on identifying the ion channels or transporters that might underlie the different capabilities of these two astrocytic subpopulations to regulate their volume during OGD. Using three-dimensional confocal morphometry, which enables quantification of the total astrocytic volume, the effects of selected inhibitors of K+ and Cl− channels/transporters or glutamate transporters on astrocyte volume changes were determined during 20 minute-OGD in situ. The inhibition of volume regulated anion channels (VRACs) and two-pore domain potassium channels (K2P) highlighted their distinct contributions to volume regulation in HR-/LR-astrocytes. While the inhibition of VRACs or K2P channels revealed their contribution to the swelling of HR-astrocytes, in LR-astrocytes they were both involved in anion/K+ effluxes. Additionally, the inhibition of Na+-K+-Cl− co-transporters in HR-astrocytes led to a reduction of cell swelling, but it had no effect on LR-astrocyte volume. Moreover, employing real-time single-cell quantitative polymerase chain reaction (PCR), we characterized the expression profiles of EGFP-positive astrocytes with a focus on those ion channels and transporters participating in astrocyte swelling and volume regulation. The PCR data revealed the existence of two astrocytic subpopulations markedly differing in their gene expression levels for inwardly rectifying K+ channels (Kir4.1), K2P channels (TREK-1 and TWIK-1) and Cl− channels (ClC2). Thus, we propose that the diverse volume changes displayed by cortical astrocytes during OGD mainly result from their distinct expression patterns of ClC2 and K2P channels