Andrew P. Iyengar

Magnetic Field Effects in the Pseudogap Phase: A Competing Energy Gap Scenario for Precursor Superconductivity

We study the sensitivity of T_c and T^* to low fields, H, within the pseudogap state using a BCS-based approach extended to arbitrary coupling. We find that T^* and T_c, which are of the same superconducting origin, have very different H dependences. This is due to the pseudogap, \Delta_{pg}, which is present at the latter, but not former temperature. Our results for the coherence length \xi fit well with existing experiments.We predict that very near the insulator \xi will rapidly increase.

The origin of the pseudogap phase: precursor superconductivity versus a competing energy gap scenario

Abstract In the last few years evidence has been accumulating that there are a multiplicity of energy scales which characterize superconductivity in the underdoped cuprates. In contrast to the situation in BCS superconductors, the phase coherence temperature T c is different from the energy gap onset temperature T ∗ . In addition, thermodynamic and tunneling spectroscopies have led to the inference that the order parameter Δ sc is to be distinguished from the excitation gap Δ ; in this way, pseudogap effects persist below T c . It has been argued by many in the community that the presence of these distinct energy scales demonstrates that the pseudogap is unrelated to superconductivity. In this paper, we show that this inference is incorrect. We demonstrate that the difference between the order parameter and excitation gap and the contrasting dependences of T ∗ and T c on hole concentration x and magnetic field H follow from a natural generalization of BCS theory. This simple generalized form is based on a BCS-like ground state, but with self-consistently determined chemical potential in the presence of arbitrary attractive coupling g . We have applied this mean field theory with some success to tunneling, transport, thermodynamics, and magnetic field effects. We contrast the present approach with the phase fluctuation scenario and discuss key features which might distinguish our precursor superconductivity picture from that involving a competing order parameter.

A precursor superconductivity approach to magnetic field effects in the pseudogap phase

Abstract We demonstrate that the observed dependences of Tc and T∗ on small magnetic fields can be readily understood in a precursor superconductivity approach to the pseudogap phase. In this approach, the presence of a pseudogap at Tc (but not at T∗) and the associated suppression of the density of states lead to very different sensitivities to pair-breaking perturbations for the two temperatures. Our semi-quantitative results address the puzzling experimental observation that the coherence length ξ is weakly dependent on hole concentration x throughout most of the phase diagram. We present our results in a form which can be compared with the recent experiments of Shibauchi et al. and argue that orbital effects contribute in an important way to the H dependence of T∗.We demonstrate that the observed dependences of Tc and T ∗ on small magnetic fields can be readily understood in a precursor superconductivity approach to the pseudogap phase. In this approach, the presence of a pseudogap at Tc (but not at T ∗) and the associated suppression of the density of states lead to very different sensitivities to pair-breaking perturbations for the two temperatures. Our semi-quantitative results address the puzzling experimental observation that the coherence length ξ is weakly dependent on hole concentration x throughout most of the phase diagram. We present our results in a form which can be compared with the recent experiments of Shibauchi et al, and argue that orbital effects contribute in an important way to the H dependence of T ∗.

MAGNETIC FIELD EFFECTS ON TC AND THE PSEUDOGAP ONSET TEMPERATURE IN CUPRATE SUPERCONDUCTORS

We study the sensitivity of Tc and the pseudogap onset temperature, T*, to low fields, H, for cuprate superconductors, using a BCS-based approach extended to arbitrary coupling. We find that T* and Tc, which are of the same superconducting origin, have very different H dependences. The small coherence length makes T* rather insensitive to the field. However, the presence of the pseudogap at Tc makes Tc more sensitive to H. Our results for the coherence length ξ fit well with existing experiments. We predict that very near the insulator ξ will rapidly increase.