Electric Field Effects in High-T _c Superconductors

Abstract

The concentration of mobile carriers (electrons and holes) governs the electron transport of any substance. In semiconductors essential electric field effects are prevalent. Only little influence of the electrostatic field $\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}} {E}$ on the properties of conventional metals and superconductors has been known. For the first time the electric field shift of T_c (∼ 10^-4 K) was discovered in low-T_c, superconductors in [1]. The electric field effects in high-T_c superconductors are subject of relatively active research [2–11]. Electrostatic screening, as described in the Thomas-Fermi model, counteracts the penetration of the electric field into a superconducting film and thus reduces the field ef-fects. The Thomas-Fermi charge screening length equals $\lambda = {\left( {\frac{{{h^2}\pi {c_s}}}{{4{m_e}{k_F}{e^2}}}} \right)^{1/2}}$ where ∈ _s is the static dielectric constant of the superconductor. For YBCO, the carrier concentration n = 5 ·10²¹ cm^-3, ∈ _s = 26 [3]. and thus λ = 5Å [3]. In [8] estimates of λ = 5 10Å have been obtained. In superconducting films of conventional metals λ =1Å and therefore $\vartriangle {T_c}\left( {\vec E} \right)$ in these metals are small. In high-T _c , superconductors the low density of charge carriers is advantageous, leading to relatively large screening lengths Correspondingly the superconducting order pa.rameter(s) is allowed to change quickly enough (the small correlation length) to probe the field-penes rated area. An additional reduction in screening, attained e.g. by using samples into which weak links have been incorporated, would help achieving even larger field effects.