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Fermi liquid (FL) theory explains how in crystals, despite interactions with each other and the crystal lattice, conduction electrons still behave as though they are free (non-interacting) particles, although quantities like their effective mass are rescaled. Experimentally, the temperature and time dependences of physical observables correspond to predictions
for non-interacting particles. However, in the vicinity of a quantum critical point (QCP), the physical properties no longer conform to FL theory and are said to display non-Fermi liquid (NFL) behavior. QCPs are values of chemical
concentration, pressure, or magnetic field where a second-order phase transition has been suppressed to 0 K. At low temperatures, the NFL characteristics in the physical properties take the form of weak power-law or logarithmic
divergences in their temperature dependence. There are other scenarios that can also lead to NFL behavior, without proximity to a QCP, which may suggest that violations of FL theory are a general phenomenon.
Examples of materials exhibiting NFL behavior:
- U1-xMxPd2Al3 (M = La,Y,Th)
- URu2-xRexSi2
- Ce1-xYbxRhIn5
- M1-xUxPd3 (M = Sc,Y)
- CeRu4Sb12
- UCu5-xPdx
- Yb2Fe12P7
- CeRu4As12
Heavy fermion superconductivity has been observed in a variety of materials, including UPt3, UBe13, and several of the In-5 compounds and skutterudites. Even though the electrons behave like heavy
quasiparticles, in general they obey Fermi-liquid theory. However, their superconducting state is rather unconventional.
Experimental evidence indicates that their superconductivity cannot be explained by the standard BCS theory. Instead, it is
characterized by nodes or lines in the superconducting energy gap and is probably not mediated by phonons, but by magnetic interactions. In addition, recent studies of PrOs4Sb12 suggest that other (eg antiferroquadrupolar) interactions may also mediate superconductivity.
Examples of materials exhibiting heavy fermion superconductivity:
Magnetism in materials can be very complex, involving interactions between the magnetic moments of electrons in localized atomic orbitals and those of conduction electrons. Generally, these two species can also hybridize.
One well-known example of such behavior is the Kondo effect, where conduction electrons interact with isolated magnetic impurities in a metal. Even in systems where the effects of local moments dominate, interactions can be highly anisotropic and/or sensitive to chemical disorder. Due to the variety of ways in which magnetic systems can order, including ferromagnetism, antiferromagnetism and spin-density wave, magnetic interactions are still not well understood. However, due to the role they play in materials like the high Tc and HF superconductors, their study is relevant to all of our work.
Examples of materials exhibiting unconventional magnetism:
Ferromagnetism recently has been observed to coexist with superconductivity.
Examples of materials exhibiting coexisting ferromagnetism and superconductivity:
- UGe2
- UCo1-xFexGe
- UCo1-xNixGe
Superconductors with transition temperatures (Tc) in excess of 77 K, the boiling point of liquid
nitrogen, have many important technological applications. However, the mechanism responsible for their superconductivity is still not well understood. In recent years, a 'generic' high Tc phase diagram has emerged, suggesting that despite compositional differences, the physical interactions in these materials are similar. The investigation of these materials is complicated by the possible existence of a QCP in the phase diagram, uncertainty in the role magnetic interactions play and the unknown nature of the pseudogap phase. The majority of cuprates studied are hole-doped via oxygen depletion. One focus of our research has been characterization of electron-doped cuprates, which behave differently than the hole-doped varieties. We also study effects such as flux motion/pinning and variations in critical current densities. Thin film growth techniques allow investigation of the effects of film thickness on SC properties. Non-cuprate superconductors with uncommonly high Tc, such as MgB2, are also studied.
Examples of materials exhibiting high Tc superconductivity:
- YBa2Cu3O7-δ
- Oxypnictides
- Sm2-xCexCuO4-γ
- MgB2
- Y1-xPrxBa2Cu3O7-δ
- Nd1.85Ce0.15CuO4-δ
We study other strongly correlated f-electron phenomena such as valence fluctuations, Kondo
insulator behavior, magnetic order, and quadrupolar order in f-electron materials of interest, especially the filled skutterudites. In addition, we continue to search for new f-electron materials that exhibit interesting correlated electron phenomena.
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