If one adds sufficient holes, the system changes its ground state from an insulator to a superconductor.
In the normal state of these superconducting materials, the itinerant, but nearly localized, Cu2+ spins form an unconventional Fermi liquid, with its quasi-particle spins displaying strong AF correlations even for systems at doping levels greater than those at which Tc is maximum—so-called “overdoped” materials.
Measurements of HTS electronic properties in magnetic fields will be the key component of the experimental work that must be done before a full understanding of the HTS phenomenon will be attained. The main issues that can be addressed this way include these:
Effects dependent on the quantization of electron states in high magnetic fields and so-called quantum oscillations (QO), of which two kinds can be distinguished: (1) oscillations of magnetic susceptibility (de Haas-van Alphen effect (dHvA)) and (2) oscillations of conductivity (Shubnikov-de Haas effect) and
Cyclotron resonance (CR).
For CR effects to be observed, ωcτ >> 1, where ωc is the cyclotron frequency and τ is the electron relaxation time. The maximum value of τ at Tc in optimally doped YBaCuO (Tc ≈ 92 K) is 10−13 s, and it drops by factor of 2-3 at room temperature. In order to observe CR or QO, the material must be in the normal state; for a typical ωc of ~1014 s−1, one needs T > 90 K and H > 500 T. Even higher fields may be necessary if the effective mass of the carriers in the material is greater than that of a free electron, and this explains why preliminary dHvA measurements made on optimally doped YBaCuO at H < 250 T did not give conclusive results.
Superfluid density, ρs, in HTS materials is a key quantity that speaks to the nature of high-temperature superconductivity, and it can be studied using high magnetic fields. At zero temperature, ρs is a direct measure of the electronic states that participate in superconductivity, and at low temperatures, it probes low-lying, quasi-particle states, reflecting the symmetry of the order parameter. At intermediate temperatures, the superfluid density follows the temperature behavior of the order parameter and can reveal normal-state anomalies below Tc. Superfluid densities are obtained from measurements of the London penetration depth. Optical experiments combined with DC transport, AC penetration depth, and tunneling measurements will be instrumental in answering these long-standing questions about the nature of high-temperature superconductivity.