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Novel Heavy Electron Scattering in URu2Si2

Principal Investigators

Graeme Luke & Tom Timusk, McMaster University
J. C. Seamus Davis, Cornell University

Introduction

The behaviour of simple metals such as sodium, copper and aluminum is well understood using traditional theories of solid state physics. Magnetic ordering, such as ferromagnetism in iron or more complicated forms such as the spiral state in chromium are also well understood. Superconductivity in conventional metals was understood theoretically by the 1960’s following the work of Bardeen Cooper and Schrieffer. Metals where localized magnetic moments interact with delocalized conduction electrons are less well understood.

We have a significant effort in the group of Prof. Graeme Luke in the BIMR to understand the nature of URu2Si2.

So-called heavy fermion metals generally possess highly localized f-electrons embedded in a sea of conduction electrons with which they interact magnetically via the competing Kondo and RKKY interactions. As these two different electron systems interact, they hybridize to form new states with a high density at the Fermi level resulting in a large effective mass, seen in thermodynamic and transport measurements.

URu2Si2 is one of the most enigmatic heavy fermion systems. There is a broad peak in its magnetic susceptibility around 70K, which has been taken as the onset of hybridization. There is a prominent phase transition at 17.5K seen in a variety of techniques but whose microscopic nature has not been elucidated despite over 30 years of study. At even lower temperatures (1.5K) it becomes superconducting (likely in an unconventional state).

We have grown a series of high quality single crystals using BIMR’s tri-arc crystal growth facility using high purity electro-refined depleted uranium from Ames and Los Alamos National Laboratories . These crystals have been shared with world-leading collaborators in various experimental techniques from around the world.

In 2010, the group of J.C. Davis at Cornell performed the first scanning tunneling microscopy study [1] of a heavy fermion state and directly imaged the hybridization of the localized and itinerant electron states as the heavy fermion state was formed. In a subsequent paper[2], the same group imaged the electronic structure of a hole in the heavy fermion lattice: this hole was created when we grew a crystal in the BIMR substituting a small amount of non-magnetic thorium on the magnetic uranium site.

Optical spectroscopy is also sensitive to the nature of the electronic states and the hybridization process.

In 2012, 3 optical studies were performed on single crystals of URu2Si2 grown in the BIMR. Guo et al.[3] characterized gaps in the conductivity spectra and determined that the gap associated with the hybridization process and that of the hidden order transition were distinct, occurring at different energy scales. BIMR member Prof. Tom Timusk’s group studied the optical conductivity for different orientations of the URu2Si2 crystal and found highly anisotropic behaviour with a single hidden order gap in the ab plane and a double gap structure for the c-axis conductivity [4].

Fermi-liquid theory, which is thought to describe most metals, makes numerous predictions for their properties.

Among these are that the low temperature resistivity should follow a T2 temperature dependence and that the optical conductivity should simultaneously exhibit an ω2 frequency dependence. Furthermore, the ratio between the strengths of these dependences is predicted.

Timusk’s group measured [5] the optical conductivity, finding this ratio was 1, rather than the Fermi-liquid prediction of 4 (in appropriate units). This result indicates that a new electron scattering mechanism is at play in this material (and likely in a broad range of systems) which has been overlooked to date.

Figure 1: Artist’s view of hybridization between localized f-electron states (blue and yellow) and itinerant conduction states (silver) in URu2Si2.
Figure courtesy of M. Hamidian.

[1] A.R. Schmidt et al., Nature 465, 570 (2010).
[2] M.H. Hamidian et al., Proceedings of the National Academy of Science 108, 18233 (2011).
[3] W.T. Guo et al., Phys. Rev. B 85, 195105 (2012).
[4] J.S. Hall et al., Phys. Rev. B 86, 035132 (2012).
[5] U. Nagel et al., Proceedings of the National Academy of Science 109, 19161 (2012).