Daniel Edmund Grupp

Anomalous Hopping Exponents of Ultrathin Films of Metals

and x are constantswhich depend on the disorder, the details of the inter-actions and the dimensionality of the system. Simpleactivated hopping over a constant barrier results in theArrhenius form with x = 1. For noninteracting electrons,when the average hopping distance depends on temper-ature due to the compromise between hopping to siteswhich are close in energy, but farther away, Mott variablerange hopping [1] is expected, with x = 1/(d+1), where dis the dimension. Efros and Shklovskii (ES) showed thatincluding Coulomb interactions between electrons resultsin a soft gap in the density of states at the Fermi energy,which changes the variable range hopping exponent tox = 1/2 in all dimensions [2].Hopping conduction has been investigated in a widevariety of materials, such as doped semiconductors [3,4],semiconducting heterostructures [5], amorphous metals[6–10], magnetic materials [11] and superconductors [12].Both the Mott and the ES forms of variable range hop-ping have been observed, as well as a crossover betweenthe two regimes [5,6]. It should be emphasized, how-ever, that it is often hard to distinguish between Mottand ES hopping, particularly in experiments in which theresistance changes only by one or two orders of magni-tude. The unambiguous identification of the Mott or EShopping can be further complicated by factors which areusually neglected, such as the granularity of the system,possible temperature dependence of R

Nearly symmetric field effect in ultrathin metal films

Ultrathin films of Bi and Pb have been grown incrementally under ultra-high vacuum conditions on SrTiO3 substrates held below 10K. The substrates were provided with metal gates on their rear surfaces. For films with sheet resistance in excess of the quantum resistance for pairs, the conductance was found to increase with gate voltage of either sign. This response, which was temporally stable, was largest in the most resistive films where the conductance changed by the order of 50 percent with a gate bias of 20V. The effect decreased in magnitude with decreasing sheet resistance and increasing temperature. It vanished above a characteristic temperature, and was not present in superconducting films with sheet resistances (measured at 14K) less than the quantum resistance for pairs.

Improvement of the superconducting transition and demagnetization factor in YBa2Cu3O7−δ single crystals by laser cutting

Single crystals of YBa2Cu3O7−δ have been cut into disc shapes using an excimer laser. The bulk superconducting transition was found to be sharpened in all cases measured. This is associated with the removal of material at the edges which may be depleted of oxygen or otherwise damaged. The magneto‐optical images for one crystal have been correlated with the optical image. The angular dependence of the demagnetizing factor for field applied in the plane was drastically reduced in disc‐shaped crystals relative to rectangular ones. The field of first flux entry was also increased.