W.Y. Liang

Electronic specific heat of YBa2Cu3O6+x from 1.8 to 300 K

We have determined for the first time the electronic specific heatγ(x, T) of YBa2Cu3O6+x for 0.16≤x≤0.97 from 1.8 to 300 K. Weakly superconducting behavior betweenx=0.4 and 0.8 progresses rapidly to strong coupling BCS-like superconducting and metallic normal state behavior forx> 0.9. However, the continuous development of the entropyS(x, T) withx andT across the entire series suggests a progressive modification of the low-energy spin spectrum with hole doping rather than a simple band model. Fermi statistics andk-space pairing for allx is indicated by the magnitude andT-dependence ofS(x, T). Pseudogap behavior inS(x, T) is observed over a temperature region aboveTc, which increases rapidly with oxygen depletion to around 200 K forx∼0.7. This loss of entropy reflects normal-state correlations apparently unrelated to the superconducting pairing.

The electronic specific heat of cuprate superconductors

Abstract We present the electronic specific heat coefficient γ(T) for several cuprate series determined from high resolution differential measurements. γ(T) is consistent with Fermi statistics, a low degeneracy temperature and at most a modest low temperature mass enhancement. We show the progression of γ(T) with hole doping for La 2-y Sr y CuO 4 and the overdoped system Tl 2 Ba 2 CuO 6+δ , and demonstrate that the rapid fall in condensation energy in underdoped YBa 2 Cu 3 O 6+x results from the growth of a pseudogap in the normal state quasiparticle spectrum. We discuss the effects of Zn doping on superconducting and pseudogap correlations in YBa 2 Cu 3 O 6+x .

Evidence on the pseudogap and condensate from the electronic specific heat

Abstract Evidence on the nature and hole density (p) dependence of the normal state pseudogap is obtained from the electronic specific heat of several cuprate series, including new data for Bi:2212. The pseudogap energy decreases as Eg(p)∼J(1−p/pcrit) for p pcrit, where pcrit∼0.19 is close to optimal Tc. This is correlated with an abrupt crossover from weak to strong superconductivity at pcrit. We conclude that the pseudogap is an intrinsic feature of the normal state density of states that competes with the condensate for low energy spectral weight and is not related to precursor superconducting fluctuations. The linear p-dependencies of the entropy, the superfluid density and the states-non-conserving pseudogap are all interrelated, and appear to reflect strong correlations in the doped Mott insulator.

A systematic study of the specific heat anomaly in La2-xSrxCuO4.

Abstract From differential specific heat measurements between 1.5K and 300K on La2−xSrxCuO4 for .03≤x≤.44 we observe the progression of the superconducting anomaly and determine γ(0) and the normal state value γn as functions of x. γn increases linearly with x and whilst Tc shows only a weak x dependence the superconducting fraction is sharply peaked at x=.15. Spin glass anomalies are observed for x≤.08.

The condensation energy and pseudogap energy scale of Bi:2212 from the electronic specific heat

From measurements of the electronic specific heat of three Bi2212 samples we show that the normal state pseudogap opens abruptly in the weakly overdoped region at p(crit) similar to 0.19 holes/CuO(2) and increases as Eg(p)/(k)B similar to 980(1-p/p(crit)) (K) at lower doping.. The condensation energy U(0) peaks strongly at p(crit) exhibiting strong BCS-like superconductivity (U(0)/gammaT(c)(2) similar to 0.28) for p > p(crit) and weak superconductivity below p(crit). We conclude that the pseudogap is a property of the normal state DOS and is not related to precursor superconducting fluctuations.

Growth of large and untwinned single crystals of YBCO

Abstract Large (up to 5×4×1.5 mm3) single crystals of YBa2Cu3O7−σ with perfect orthorhombic morphology were successfully grown from favourable flux composition in alumina crucibles. The number of nucleation centres was reduced by using smooth inner wall crucibles and a temperature oscillation method. The crystals could be isolated from the CuO flux by tipping the crucible over while remaining in the furnace at 970°C. By applying a uniaxial stress at the annealing temperature, both macro- and micro-twin domains in the thick crystals could be removed and a single untwinned domain was obtained. The structures of bith twin- and single-domain crystals were characterized by means of high resolution electron microscopy. We also report the magnetic property of the as-grown single crystals.

Influence of external magnetic field and its orientation on transport AC losses in Bi-2223 and Tl-1223 silver sheathed tapes

In the paper we report measurements of energy losses caused by an AC transport current in silver sheathed multifilamentary Bi-2223 and mono core Tl-1223 tapes, as a function of the magnitude and orientation of the applied external magnetic field. The external magnetic field was rotated in the range of 0 to /spl plusmn/90 degrees with respect to the plane of the tapes, and was In all cases perpendicular to the transport current flow. The transport current losses in a range of DC applied fields are consistent with the Norris equation provided the DC critical current at the same applied field is used in his expression for the loss.

Surface atomic imaging of La2O3 and La2CuO4

Surface profile images from La2O3 and La2CuO4 have been obtained using high resolution electron microscopy. A surface layer of the C-form of La2O3 about 1 nm in thickness coating the bulk of the A-form La2O3 and the movement of atomic columns within this coating were observed; these two structural forms being intergrown with the (001) plane of the A-form parallel to the (111) plane of the C-form. On the (113) surface of a La2CuO4 crystal, a single atomic layer having a different structural arrangement from that of the bulk crystal was visible, and a displacement of some of the La cations on the surface was proven by image simulation. On the (001) surface plane of La2CuO4, the movement of atomic columns in a surface re-arrangement was also observed.

Evidence for Matthiessen's rule in the normal state resistivity of zinc doped yttrium barium copper oxide

The authors have measured the electrical resistivity rho (T) for a number of sintered samples of Y1Ba2(Cu1-xZnx)3O7 with x in the range 0.01-0.10. Above Tc, d rho /dT falls smoothly with increasing bulk density and is independent of x as confirmed by measurements on alloys of equal density. Taking d rho /dT to be independent of x, they find that the extrapolated residual resistivity ( rho res) at T=O increases linearly with x in the range O<or=x<or=0.07. These facts suggest that the linear term in rho arises from an intrinsic scattering mechanism which is independent of x, and that the substituting zinc atoms act as independent scattering centres, in accordance with Matthiessens rule for normal metals.

Absence of gross inhomogeneity in HTS cuprates

Recent scanning tunnelling microscopy (STM) studies on Bi