Researchers
Dr. Peter Wahl (recently left), Mohammad Hamidian

Introduction
Heavy fermion physics has been a major subject of investigation in the condensed matter community for the past 30 years. The term heavy fermion refers to materials which have low temperature specific heats 2-3 orders of magnitude greater than those of ordinary metals this effect being attributed to an equally augmented electronic effective mass. Even after three decades of study cracking the physics responsible for the various behaviors of these compounds has been elusive. From understanding the electronic structure and heavy band formation to possible¡± hidden orders¡± and the emergence of low temperature superconductivity, no cohesive picture has emerged.

In the J.C. Davis laboratory, we have recently undertaken STS studies of URu2Si2 to probe heavy fermion physics. Our home built SI-STM is capable of atomic scale resolution spectroscopy down to milliKelvin temperatures. Our goals are to use QPI to infer the band structure while also using the asymmetric tunneling spectra to better understand the hybridization interaction between the localized and itinerant electronic states. This lies at the heart of the heavy fermion problem.

Instrument - Millikelvin 9-Tesla Spectroscopic Imaging STM (SI-STM):s
The system consists of a low circulation Kelvinox 400 dilution refrigerator retrofitted with ultra low vibration ⁴He-pot and for very high vacuum capabilities, surrounded by a persistent American Magnetics magnet. It is suspended in a very low boilrate Dewar from a massive low vibration cryostat. The cryostat is housed inside an acoustic/RF shield room (shown left), itself supported on a 25 ton inertial block on vibration isolators. This whole assembly is installed in an underground acoustic/vibration isolation vault. The pump set is remote and highly vibration isolated and the control room is remote. The SI-STM head is at the center of the magnet suspended below the refrigerator. We use this system to study Bi₂Sr₂CuO_6+¥ä, NbSe₂, YBa₂Cu₃O_6+¥ä, Sr₂RuO₄, GaAs, GaSb, and Ca_2-xNa_xCuO₂Cl₂. Sample exchange from room temperature to 4K takes ~6 hours and to 10mK takes ~9 hours.

See BNL SI-STM program to see more images and descriptions.

Results

Collaborators
Alexander V. Balatsky - Los Alamos National Lab
G. Luke - McMaster University