Low Temperature/Solid State Physics

Prof. Joe Amato studies the electrical properties of normal and superconducting thin films in high magnetic fields at temperatures approaching absolute zero.

The electrical resistance of normal conducting thin films approaches a near-constant value as the temperature falls below 20 Kelvin.  Nevertheless, high resolution resistance measurements reveal a small but significant change in the resistance as the temperature is lowered further or as a magnetic field is applied.  This behavior is caused by the interference of deBroglie waves that describe the motion of conduction electrons within the thin films.  Careful analysis of the resistance measurements reveal details about the frequency and type of collisions experienced by the electrons as they move through the films.

 Thin films are made at Colgate by either evaporation or sputtering within a high vacuum chamber.  Microscopic patterns with features as small as 2 µm are defined using photolithography. After establishing electrical contact to the samples, they are mounted in a "dipper" cryostat and cooled.  Using a dilution refrigerator, the minimum accessible temperature is 0.06K.  A superconducting magnet is used to provide magnetic fields as high as 6 Tesla.

The electrical resistance of a superconducting thin film in zero magnetic field is zero.  But if a magnetic field is present (electric current generates magnetic field), the thin film is pierced by bundles of magnetic flux called vortices.  An applied current produces a sideways force on the vortices.  If the vortices move in response to this force, a voltage is created and the resistance of the film is no longer zero.  (This is the reason that superconducting materials have not yet found wide-scale application in modern technology.)  Vortex motion is studied by measuring the resistance of thin film samples while varying the temperature, magnetic field, and the applied current.  In particular, we inhibit vortex motion by introducing periodic arrays of pinning sites, i.e., tiny regions where vortices prefer to be located.  This is done using an atomic force microscope to oxidize nanometer-size regions of the film.  By measuring the effectiveness of our pinning arrays, we hope to obtain a more detailed understanding of vortex behavior. 

Colleen Campbell '01 getting ready to prepare a thin-film sample.

A recent sample consisted of square arrays on sputtered MoGe thin films using the contact mode atomic force microscope. The pinning sites are oxidized regions of the film 150 nm in diameter, spaced 400 nm apart. Vortex motion was monitored by measurement of differential resistance. Measurements of R_ac vs. applied field B display distinct changes in slope at the first (130 G) and second matching fields for temperatures T > 0.94 T_c.

This research is funded by Research Corporation.

  Recent Publications:

"Low Temperature Dimensionality of Tantalum Thin Films", J.C. Amato, S.A.FitzGerald, J. Holman, M. Cashen, T. Forstner, J. Haeni, Bull. Am. Phys. Soc. 41, 166 (1996).

"Dimensionality of Tantalum Thin Films Determined by Magnetoresistance", J.C. Amato, W.J. Beckler, J.H. Haeni, L. Northrop, D.A. Shapiro, Bull. Am. Phys. Soc. 43, 431 (1998).

"Electron Phase Coherence and the Electron-Electron Interaction in Tantalum Thin Films", J.C. Amato, W.J. Beckler, J.H. Haeni, D.A. Shapiro, and L.A. Northrop, Bull. Am. Phys. Soc. 45, 314 (2000)

[The background  is an image of an integrated circuit taken by John Bonifacio '98 with our Atomic Force Microscope; bright objects are individual transistors.]

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