J.W. Loram

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.

Twin formation and structural modulations in orthorhombic and tetragonal YBa2(Cu1-xCox)3O7-δ

Abstract The microstructure of YBa2(Cu1−xCox)3O7−δ, prepared by the standard ceramic method, shows lamellar twin structures with decreasing spacings between twin walls with increasing Co content for x≲0·02, developing into {110}-type cross-hatched ‘tweed’ modulation for x≳0·02. Several wall junctions are found for x=0·02. The structural phase transition between macroscopically orthorhombic and tetragonal material occurs at x≈0·025; structural modulations (λ≈20A) persist in the samples with high Co content (x>0·25). The modulations lead to a considerable broadening of the X-ray lines affected by orthorhombic splitting, and show maximum amplitude at the critical composition x≈0·025.

The electronic specific heat of YBa2 (Cu1−xZnx)3O7 from 1.6 K to 300 K

Abstract Using a differential technique we have determined between 1.6 K and 300 K and for 0⩽ x ⩽0.1 the difference in electronic specific heat coefficients γ ( x , T )− γ (0.07, T ) n of YBa 2 (Cu 1− x Zn x ) 3 O 7 and a YBa 2 (Cu 0.93 Zn 0.07 ) reference. The results are consistent with a superconducting Fermi liquid and cannot be readily described in terms of the condensation at T c of bosons preformed above T c . We deduce γ( x , T ) from these results, and find γ n = 1.6±0.2 mJ /( g - at . K 2 ) (density of states N ( E F ) = 8.8 states /( eV . fu )) independent of x . Superconducting anomalies are seen for all samples, with T c and the anomaly height decreasing and γ( x , 0) increasing with x consistent with magnetic pair breaking. The thermodynamic critical field H c ( x , T ) is determined for 0⩽ x ⩽0.07. We observe a (nearly) universal temperature dependence for the low temperature magnetic anomalies in Zn-doped and undoped samples with broad specific heat maxima at or above 5 K. For undoped YBa 2 Cu 3 O 7 we find θ (0) = 406 K and an electronic term γ (0, 0) = 0.15±0.15 mJ /( g - at . K 2 ) substantially smaller than the generally quoted value.

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 .

Electronic specific heat of Tl2Ba2CuO6+δ from 2 K to 300 K for 0≤δ≤0.1

Using a high-precision differential technique with a resolution of 1∶104, we have measured the heat capacity of Tl2Ba2CuO6+δ over a temperature range 2–300 K for 0≤δ≤0.1. Anomalies atTc are seen for all superconducting compositions measured, and the results are consistent with a temperature- andδ-independent normal-state electronic termγn∼0.6 mJ/g-at. K2. The samples with the largerTcs exhibit strong fluctuations in their specific heat, typical of a highly anisotropic 2D superconductor, but there is some evidence that these fluctuations become weaker atTc falls-consistent with an increase in the coherence length on hole doping. At temperatures below 5 K an upturn in the data is observed which appears to increase in magnitude asTc falls, correlating with the increase in the Curie term of published susceptibility data.

Specific heat and susceptibility determination of the pseudogap in YBCO7−δ

Abstract Based on the electronic specific heat and the bulk susceptibility data of oxygen-deficient YBa 2 Cu 3 O 7−δ , we present clear evidence of a pseudogap in the single-particle density of states (DOS) above T c in this system of oxide superconductors. Further analysis shows that the most likely shape of the gap is one in which the normal state DOS is zero at the Fermi level, then rises linearly and symmetrically to a cusp above the unmodified background. The size of the pseudogap is directly related with the number of lost carriers available for superconductivity and is proportional to δ. By comparing the measured entropy and the spin component of the susceptibility, we further conclude that spin and charge excitations do not separate in YBCO, and in the normal state, in this respect at least, they behave like an ordinary Fermi liquid.

The effect of twins on critical currents of high Tc superconductors

Abstract The effect of twin spacing on the Jc of YBaCuO is studied by doping with Co. and with Zn. The current density follows the variation of carrier density and inductive signal in both systems. The twins seem to have no effect although in the Co. doped samples their spacing varies by a factor of more than 30.

Specific heat evidence for chain superconductivity in YBa2(Cu1-xCox)3O7- delta

The authors have investigated the superconducting specific heat anomalies in YBa2(Cu1-xCox)3O7 for O .03 Tc decreases linearly with x due presumably to magnetic pair breaking with little further change in anomaly height. At low temperatures the authors find evidence of magnetic order.

Hall effect and thermoelectric power measurements on Y0.9Ca0.1Ba2Cu3O7−δ

Abstract Results of Hall effect and thermoelectric power (TEP) measurements on sintered samples of Y 0.9 Ca 0.1 Ba 2 Cu 3 O 7−δ (Ca 0.1 YBCO) are presented, and compared to previous data for sintered samples of YBa 2 Cu 3 O 7−δ (YBCO) [1] and Y x Ca 1−x Sr 2 (Tl 0.5 Pb 0.5 )Cu 2 O 7 (Tl-1212) [2] . Results indicate that calcium substitution in YBCO 7−δ does not dope holes uniformly for all values of δ.

Electronic specific heat of overdoped Tl{sub 2}Ba{sub 2}CuO{sub 6+{delta}} in a magnetic field

We have measured the magnetic field dependence of the electronic specific heat γel (=Cel/T) of polycrystalline Tl2Ba2CuO6+δ from 8 K to above Tc, for samples with Tc = 24 K to 72 K using a high resolution, differential technique. We find that in applied fields of up to 13 T the position of the specific heat jump does not shift significantly for any Tc. This is in strong disagreement with HC2(T) inferred from magnetoresistance experiments on the same compound, where the resistive transition is shifted down considerably by similar fields. However, there is a dramatic collapse of the specific heat anomaly height in field — signifying a loss in entropy around Tc — compensated by a corresponding increase in γel at lower temperatures. The superconducting anomaly is almost completely suppressed in 13 T for the lowest Tc sample, compared to a reduction of 65 % for the sample with Tc = 72 K. We find simple vertical scaling of γel(T) — γn suggesting that the field forces a proportion of the superconductor into the normal state, while the remainder in the superconducting state is unaffected.