HTS fundamental studies

Since 1997 the Marsden Fund has supported a series of programmes on the fundamental properties of HTS^[?], which continues to attract a high international profile. Current efforts focus on the basic interactions between the electrons and the spin and charge degrees of freedom which govern the phase behaviour in both the normal and superconducting states. This behaviour is very complex but our work suggests that the key features may prove to be rather straightforward.

At bottom we wish to know what governs the pairing of electrons that lies at the heart of superconductivity and what is their effect on key superconducting properties such as the transition temperature, T_c, superfluid density, the condensation energy and ultimately practical properties like the maximum critical current. We use NMR^[?], NQR^[?], specific heat, electronic transport, and magnetic and optical properties to probe the low-energy excitations of a range of superconducting cuprates which we generally prepare and process ourselves. These techniques are combined with high pressures, high magnetic fields and the use of atomic isotopes to differentiate between the various possible contributions to electron pairing.

The onset of superconductivity results in energy gaps opening up near the Fermi level and we have accumulated much information about these gaps. One is the superconducting gap, Δ0, which is essential to superconductivity and another is the so-called pseudogap, which is present both above and below T_c and competes with superconductivity.

One recent result was to demonstrate that HTS fluctuate in and out of the superconducting state at temperatures well above T_c causing a marked reduction in T_c below the so-called mean-field value, T_c^mf, which would occur if there were no fluctuations. For the compound Bi₂Sr₂CaCu₂O_8+δ with a T_c value of 90K we find from thermodynamic measurements that T_c^mf is as high as 150K. A corollary is that if we could suppress fluctuations then these very high T_c values could be achieved. The application of high pressure seems to do this. More recently we have shown for the first time that 2Δ₀/k_BT_c^mf takes a constant value of 4.3 across the entire superconducting phase diagram. This is just the value predicted by the conventional BCS theory of superconductivity and suggests that HTS may be much simpler than we once thought.

Dr Jeff Tallon and Dr Grant Williams

The B_2g Raman gap feature (which approximates the superconducting order parameter, Δ₀') plotted as a function of hole concentration along with
2.15x k_BT_c^mf, (data points) showing that across a broad range of the superconducting phase diagram 2Δ₀'/k_BT_c^mf = 4.3, the BCS weak coupling ratio.