Fermi surface of copper

 

Strongly Correlated Electron Systems

Despite the presence of Avogadro’s number of electrons in any sizeable piece of material, interactions between electrons in normal metals appear to be very weak, resulting in the “nearly-free electron” behaviour which is well described by modern principles of condensed matter physics. This "standard model" of electrons in solids works to describe the electronic properties of simple metals such as sodium and copper. Our research is focused on fundamental properties of metallic materials in which interactions between electrons are so strong that this free “quasi-particle” picture no longer applies. Instead, extremely strong correlations between electrons give rise to exotic and anomalous states of matter, such as unconventional superconductivity, heavy-electron behaviour and quantum-critical phenomena. (See good description on "Fermi- and Non-Fermi Liquids" in report from the National Academy of Sciences.)  

When these correlations are “tuned” by changing some parameter (other than temperature itself) of the solid material - such as lattice density, external magnetic field or chemical composition – many materials are forced to switch between two or more states of matter at a specific crossing deemed a quantum critical point. Interestingly, this zero-temperature phenomenon, with experimentally observable properties reaching up to room temperatures, continues to lack theoretical explanation. It is precisely this behaviour which we study by performing detailed electrical transport, thermodynamic and electronic structure measurements in order to help elucidate the nature of strong electronic correlations and the resulting new states of matter.

 

Anisotropy in Quantum Materials

By performing condensed matter experiments at ultra-low temperatures and high magnetic fields, we are able to study materials which defy the current textbook understanding of metals, magnets and insulators. Our main focus is the measurement of transport and thermodynamic properties of strongly correlated electron systems tuned toward a phase transition at absolute zero temperature via applied pressure, magnetic field or chemical substitution. This involves:

  • testing the quasi-particle picture in the limit of strong electronic correlations
  • investigating non-Fermi liquid behavior in heavy-electron systems
  • probing the nature of unconventional superconductivity
  • searching for new physical phenomena at experimentally tunable extremes.

To elucidate the role of anisotropy in such phenomena, we are setting up a unique experimental facility which will allow systems to be studied using a number of techniques in conjunction with the precise 360 degree rotational capability of a high-field multi-axis vector magnet. Because these phenomena often involve extremely low energy scales, experiments are required to be performed down to the millikelvin temperature range, thousandths of a degree above absolute zero.

 

Crystal structure of CeCoIn5

 

Previous Studies

 

Multi-Band Superconductors: Thermal transport is an excellent probe of the anomalous properties of conventional superconductors (i.e. with phonon-mediated pairing). Because thermal transport in a superconductor’s mixed (vortex) state directly probes delocalized excitations, it is an excellent measure of the nature of the gap itself. By combining measurements of NbSe2 with comparisons to other multi-gap superconductors such as MgB2, direct, quantitative evidence has been found for multi-band superconductivity in NbSe2.1 We have also shown evidence for the first “extreme” case of multi-band superconductivity in the heavy-fermion superconductor CeCoIn5, drawing parallels to the analogous phenomenon of “unbalanced” pairing in cold fermionic gases.2

 

Multi-band superconductivity in NbSe2.

Anomalous Metallic Behaviour:  The recent discovery of “water-based” superconductivity in the NaxCoO2 system is an excellent example of the diversity of materials involved in this field of research. Our goal was to use low temperature transport measurements to identify and characterize the anomalous ground state of this material which is host to phenomena as diverse as magnetic frustration, unconventional superconductivity, and structural and magnetic instabilities. While the basic excitations of Na0.7CoO2 are indeed Landau quasiparticles (spin ½ fermions of charge ‘e’), as evidenced by observation of the Wiedemann-Franz (WF) law in the T=0 limit, they appear to scatter from each other with an unprecedented magnitude, leading to the largest Kadowaki-Woods ratio observed to date.3

 

 

Giant electron-electron scattering in NaxCoO2.

“Spectroscopy” of Spin Fluctuations:  The ratio of heat to charge conductivities in a metal, a universal quantity first identified by Wiedemann and Franz over 150 years ago, is one of the most robust manifestations of Fermi liquid theory: quasiparticles transport heat and charge equally well. At intermediate temperatures, however, deviations from the WF law expectation abound due to non-energy-conserving collision processes. It is precisely this fundamental difference between heat and charge transport quantities that can be exploited to examine the energy- and momentum-dependent aspects of processes at play. We have used this principle to extract the characteristic spin fluctuation energy scale in the antiferromagnetic metal CeRhIn5, demonstrating the spectroscopic capability of this technique and its potential for further studies.4

 

Spin fluctuation scattering in CeRhIn5.

Tunable Quantum Criticality:  One of the most fundamental questions about the nature of a quantum critical point is in regard to the extent to which Fermi liquid theory breaks down: can the basic excitations still be considered renormalized electrons? Since no previous study of the WF law had been performed continuously through a QCP, we addressed this question in CeCoIn5 by implementing a novel, low-temperature in-situ measurement of both heat and charge transport. As a result, the Landau quasiparticle description of excitations was shown to be valid and continuous for the first time at a QCP.5 This was also investigated in the metamagnetic material Sr3Ru2O7, where we verified that charge ‘e’ fermions are indeed the basic excitations of this material as well.6

 

        Field-tuned quantum criticality in CeCoIn5.

Rare Earth / f-Electron Physics:  Our most recent investigations exploit the degrees of freedom offered by the periodic table of elements in an effort to perturb the strongly correlated states of matter associated with f-electron superconductivity and the Kondo effect. By using a systematic substitution of magnetic and non-magnetic rare earth atoms into a “Kondo lattice,” we studied the properties of the superconducting transition, Kondo coherence and non-Fermi liquid behaviour found in CeCoIn5 as a function of magnetic dilution. This has uncovered a novel relationship between the infamous linear temperature dependence of resistivity found in this and other materials of interest (i.e. high-Tc superconductors) and the screening properties of the cerium-based Kondo lattice, providing new insight into the mechanisms of superconductivity, magnetic screening and quantum criticality.7

 

Rare earth impurity effects in CeCoIn5.

References

1.  E. Boaknin et al., Phys. Rev. Lett. 90, 117003 (2003).

2.  M.A. Tanatar et al., Phys. Rev. Lett. 95, 067002 (2005).

3.  S.Y. Li et al., Phys. Rev. Lett. 93, 056401 (2004).

4.  J. Paglione et al., Phys. Rev. Lett. 94, 216602 (2005).

5.  J. Paglione et al., Phys. Rev. Lett. 97, 106606 (2006).

6.  F. Ronning et al., Phys. Rev. Lett. 97, 067005 (2006).

7.  J. Paglione et al., Nature Phys. 3, 703 (2007).