N. E. Staley

Electrostatic tuning of the superconductor-insulator transition in two dimensions.

Superconductivity has been induced in insulating ultrathin films of amorphous bismuth using the electric field effect. The screening of the electron-electron interaction was found to increase with electron concentration in a manner correlated with the tendency towards superconductivity. This does not preclude an increase in the density of states being important in the development of superconductivity. The superconductor-insulator transition appears to belong to the universality class of the three dimensional XY model.

Micromachined SrTiO3 single crystals as dielectrics for electrostatic doping of thin films

Single crystal substrates of SrTiO3 have been micromachined locally on their back faces to thicknesses between 10 and 100μm using a mechanical abrasive technique. Subsequently, the growth surface has been treated to obtain atomically smooth unit cell terraces, suitable for high quality interfaces. Using a capacitor geometry, surface charge densities suitable for electrostatic doping of films have been obtained at the lowest temperatures, and the off-diagonal component of strain due to the applied electric field has been measured.

Low-temperature glassy response of ultrathin La0.8Ca0.2MnO3 films to electric and magnetic fields

The glassy response of thin films of La0.8Ca0.2MnO3 to external magnetic and gated electrostatic fields in a field-effect geometry has been studied at low temperatures. A hierarchical response with irreversible memory effects, non-ergodic time evolution, aging and annealing behavior of the resistance suggest that the dynamics are governed by strain relaxation for both electronic and magnetic perturbations. Cross-coupling of charge, spin, and strain have been exploited to tune the coercivity of an ultrathin manganite film by electrostatic gating.

Tuning the 2D Superconductor‐Insulator Transition by Use of the Electric Field Effect

Some investigations of the superconductor‐insulator (SI) transitions in two dimensions have been hindered by aspects of the intrinsic disorder of the studied systems. As a solution to this problem, we have induced superconductivity in insulating, ultrathin films of amorphous bismuth by utilization of the electric field effect. This method of tuning the SI transition does not alter the intrinsic disorder. Analysis of the response to transferred charge density has revealed that screening and the density of states are both involved. This SI transition has been analyzed as a quantum phase transition using a finite size scaling analysis with electron concentration as a tuning parameter, yielding a critical exponent product vz = 0.7 ± 0.05. If z = 1 as expected, this product is consistent with the universality classes of the (2D+1) XY model and the 2D Boson Hubbard model in the absence of disorder.

Low‐Temperature Response of Ultrathin Manganite Films in a Field‐Effect Geometry

Manganites, with their strong cross‐couplings between spin, charge, orbital, and lattice degrees of freedom, possess rich phase diagrams as a function of chemical doping. Under certain circumstances, manganites at a particular doping will have an admixture of phases of very different electronic, magnetic, and structural properties, but with nearly equal free energies. The properties of these systems are susceptible to external perturbations that lead phase conversion within the admixture. We have investigated the low‐temperature response of ultrathin La0.8Ca0.2MnO3 films in a field‐effect geometry to magnetic and gate electric fields. The composition is close to the phase boundary between a ferromagnetic metal at higher Ca doping and a ferromagnetic charge ordered insulator at lower Ca doping. An applied gate voltage produces a large ambipolar decrease in resistance at low temperatures, and the magnetization responds to the application of a gate voltage. The films also exhibit a hierarchical and glass‐lik...