arXiv subjects

J. W. Loram

Publications and source records attributed to J. W. Loram.

23 records · Page 2Linked to original sources

arXiv2000

The doping dependence of T* - what is the real high-Tc phase diagram?

Underdoped high-Tc superconductors are frequently characterised by a temperature, T*, below which the normal-state pseudogap opens. Two different "phase diagrams" based on the doping (p) dependence of T* are currently considered: one where T* falls to zero at a critical doping state and the other where T* merges with Tc in the overdoped region. By examining the temperature dependence of the NMR Knight shift and relaxation rate, entropy, resistivity, infrared conductivity, Raman scattering, ARPES and tunnelling data it is concluded that the second scenario is not at all supported. Neither can one distinguish a small and a large pseudogap as is often done. T* is an energy scale which falls abruptly to zero at p=0.19.

cond-mat↗

arXiv1999

Factors affecting the Optimal Design of High-Tc Superconductors - the Pseudogap and Critical Currents

The impact of the normal-state pseudogap, present in all optimal and underdoped HTS cuprates, on critical currents and critical temperature is surveyed. With the opening of the pseudogap around a doping state of p=0.19 the condensation energy and superfluid density are rapidly suppressed due to reduction in the normal-state spectral weight. Even by optimal doping (p=0.16) these measures of the 'strength' of superconductivity are diminished by up to 40%. This results in a sharp reduction in critical currents and irreversibility field, respectively. The optimal doping state where these properties are maximised is therefore not at maximum Tc but in the lightly overdoped region where the pseudogap energy falls to zero at p=0.19. The presence of impurities and grain boundaries further heightens these effects.

cond-mat.supr-con↗

arXiv1999

Critical doping in overdoped high-Tc superconductors - a Quantum Critical Point?

Evidence is presented from the scaling of the Knight shift, entropy and transport properties together with the sharp peaking of condensation energy, critical currents, superfluid density and a variety of other physical properties for the occurrence of a common critical doping point in lightly overdoped high-Tc superconductors (HTS). This critical doping lies at the point where the doping-dependent normal-state pseudogap energy, Eg, falls to zero and bears a strong, though incomplete, resemblance to a quantum critical point (QCP). A QCP scenario could lead directly to an explanation of the non-Fermi liquid behaviour of the normal-state metallic phase and the overall generic behaviour of the HTSC.

cond-mat.supr-con↗

arXiv1999

Heat capacity and transport studies of the ferromagnetic superconductor RuSr2GdCu2O8

Resistivity, thermoelectric power and heat capacity are investigated in the ferromagnetic superconductor RuSr2GdCu2O8 with and without Zn substitution. The thermodynamic signatures of the ordering of the Ru moments and the paramagnetic Gd moments as well as the onset of superconductivity are all clearly seen and quantified. The materials are shown to exhibit bulk superconductivity which coexists with spatially-uniform ferromagnetism. They appear to be typical underdoped superconducting cuprates in which the pseudogap dominates normal-state transport, thermodynamic and substitutional properties. However, a field-induced increase in Tc could suggest some degree of triplet pairing.

cond-mat.supr-con↗

arXiv1999

Effects of carrier concentration on the superfluid density of high-T_c cuprates

The absolute values and temperature, T, dependence of the in-plane magnetic penetration depth, $λ_{ab}$, of La_{2-x}Sr_xCuO_4 and HgBa_2CuO_{4+δ} have been measured as a function of carrier concentration. We find that the superfluid density, $ρ_s$, changes substantially and systematically with doping. The values of $ρ_{s}(0)$ are closely linked to the available low energy spectral weight as determined by the electronic entropy just above T_c and the initial slope of $ρ_{s}(T)/ρ_{s}(0)$ increases rapidly with carrier concentration. The results are discussed in the context of a possible relationship between $ρ_s$ and the normal-state (or pseudo) energy gap.

cond-mat.supr-con↗