Mutual influence of structural distortion and superconductivity in systems with degenerate bands

The interplay between the band Jahn-Teller distortion and the superconductivity is studied for the system whose Fermi level lies in two-fold degenerate band. Assuming that the lattice distortion is coupled to the orbital electron density and the superconductivity arises due to BCS pairing mechanism between the electrons, the phase diagram is obtained for different doping with respect to half-filled band situation. The coexistence phase of superconductivity and distortion occurs within limited range of doping and the distortion lowers the superconducting transition temperature $T_c$. In presence of strong electron-lattice interaction the lattice strain is found to be maximum at half-filling and superconductivity does not appear for low doping. The maximum value of $T_c$ obtainable for an optimum doping is limited by the structural transition temperature $T_s$. The growth of distortion is arrested with the onset of superconductivity and the distortion is found to disappear at lower temperature for some hole density. Such arresting of the growth of distortion at $T_c$ produces discontinuous jump in thermal expansion coefficient. The variation of strain with temperature as well as with doping, thermal expansion coefficient, the $T_c$ vs $\delta$ behaviour are in qualitative agreement with recent experimental observations on interplay of distortion and superconductivity in cuprates.Comment: 15 pages Revtex style, 9 figures available on request to first Autho

Ultrasound measurement of the fetal cerebral ventricles: a prospective, consecutive study

Real-time ultrasound was used in 654 consecutive pregnancies to obtain standard growth parameters for the fetal brain. Measurement of the width of the lateral ventricle (LVW) and hemisphere and their relationships to menstrual age, biparietal diameter, and birth weight were determined. The growth of the LVW was to a great extent independent of birth weight but dependent on menstrual age. Thirteen fetuses with a single ventricular width measurements exceeding +2 SD from the mean were separately evaluated, and all but one case were found to be normal