In this paper, we review a large set of experimental data acquired over the past decade by several groups, and demonstrate how it can be used to construct a detailed picture of the low-temperature metallic state of the unconventional superconductor Sr2RuO4. We show how the normal state properties can be consistently and quantitatively explained in terms of Landau quasi-particles moving on a quasi-two-dimensional Fermi surface. Besides presenting our full and extensive data sets, we explain the details of some novel data analysis tools that can be used within the general context of quasi-two-dimensional metals. We then use the experimental Fermi surface and band dispersion to reassess several issues relevant to the unconventional superconductivity in Sr2RuO4, such as the spin-fluctuation spectrum, quasi-particle renormalization, interlayer dispersion and pressure dependence.

Quasi-two-dimensional Fermi liquid properties of the unconventional superconductor Sr2RuO4

Simultaneous resistivity and ac specific heat measurements have been performed under pressure on single-crystalline ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ to over 6 GPa in a hydrostatic helium pressure medium. A series of anomalies was observed around the pressure coinciding with a maximum in the superconducting critical temperature, ${T}_{c}^{\mathrm{max}}.$ These anomalies can be linked with an abrupt change of the Ce valence and suggest a second quantum critical point at a pressure ${P}_{v}\ensuremath{\simeq}4.5$ GPa, where critical valence fluctuations provide the superconducting pairing mechanism, as opposed to spin fluctuations at ambient pressure. Such a valence instability---and associated superconductivity---is predicted by an extended Anderson lattice model with Coulomb repulsion between the conduction and f electrons. We explain the T-linear resistivity found at ${P}_{v}$ in this picture, while other anomalies found around ${P}_{v}$ can be qualitatively understood using the same model.

/pdf/signatures-of-valence-fluctuations-in-cecu-2-si-2-under-high-1ggdqbs0mq.pdf

Signatures of valence fluctuations in CeCu 2 Si 2 under high pressure

The Seebeck coefficient of a metal is expected to display a linear temperature dependence in the zero-temperature limit. To attain this regime, it is often necessary to cool the system well below 1 K. We put under scrutiny the magnitude of this term in different families of strongly interacting electronic systems. For a wide range of compounds (including heavy-fermion, organic and various oxide families) a remarkable correlation between this term and the electronic specific heat is found. We argue that a dimensionless ratio relating these two signatures of mass renormalization contains interesting information about the ground state of each system. The absolute value of this ratio remains close to unity in a wide range of strongly correlated electron systems.

https://iopscience.iop.org/article/10.1088/0953-8984/16/28/037/pdf

On the thermoelectricity of correlated electrons in the zero-temperature limit

Theory of superconductivity in strongly correlated electron systems

Heavy fermion compounds represent one of the most strongly correlated forms of electronic matter and give rise to low temperature states that range from small moment ordering to exotic superconductivity, both of which are often in close proximity to quantum critical points. These strong electronic correlations are associated with the transfer of entropy from the local moment degrees of freedom to the conduction electrons, and, as such, are intimately related to the low temperature degeneracy of the (originally) moment bearing ion. Here we report the discovery of six closely related Yb-based heavy fermion compounds, YbT(2)Zn(20), that are members of the larger family of dilute rare earth bearing compounds: RT(2)Zn(20) (T = Fe, Co, Ru, Rh, Os, Ir). This discovery doubles the total number of Yb-based heavy fermion materials. Given these compounds' dilute nature, systematic changes in T only weakly perturb the Yb site and allow for insight into the effects of degeneracy on the thermodynamic and transport properties of these model correlated electron systems.

https://www.pnas.org/content/pnas/104/24/9960.full.pdf

Six closely related YbT2Zn20 (T = Fe, Co, Ru, Rh, Os, Ir) heavy fermion compounds with large local moment degeneracy

Relation between resistivity and effective mass in heavy-fermion and A15 compounds

A quantum mechanical phenomenology for heavy-fermion systems is formulated. The effective Lagrangian for low-energy excitations contains a localized spin-fluctuation part which is coupled with an itinerant fermion part. Each part represents the dual nature of strongly correlated f electrons. The coupling parameter is fixed by comparison with the Fermi-liquid theory for the Anderson model. The comparison also fixes the localized spin-fluctuation spectrum. As an advantage over the Fermi-liquid description the present model takes into account the RKKY interaction and non-linear effects of spin polarization. A formula for the magnetic equation of state is derived which is applicable to the metamagnetism and weak antiferromagnetism in heavy-fermion systems.

Quantum Phenomenology for Heavy-Fermion Systems. I. Formulation of the Duality Model

A concept of duality of itenerant and localized characters of electrons in strongly correlated metals, such as heavy fermions and high- T c cuprates, is shown to be useful to understand the anomalous properties in their normal and ordered states. The systematics of metamagnetic behavior in the paramagnetic phase and a possible origin of the weak antiferromagnetism of heavy fermions are explained on the basis of the duality model, a semi-phenomenology which takes explicitly into account such dual nature of electrons. The duality model also gives us a basis for understanding some of the anomalous behaviors of high- T c cuprates in their normal state in a constant way.

Itinerant-localized duality model for heavy fermions and strongly correlated metals

Theory of metamagnetism and weak antiferromagnetism of heavy fermions based on the duality model

The problem on crossover between the Cooper-pair condensation and the Bose-Einstein condensation of di-electronic molecules in two-dimensional superconductors is discussed. A result based on the Nozieres and Schmitt-Rink formalism is reviewed and a preliminary result beyond their formalism is presented. In the latter, effect of repulsion between electron pairs due to the exchange effect among constituent electrons plays a crucial role

Kazumasa Miyake

Papers

Relation between resistivity and effective mass in heavy-fermion and A15 compounds

Quantum Phenomenology for Heavy-Fermion Systems. I. Formulation of the Duality Model

Itinerant-localized duality model for heavy fermions and strongly correlated metals

Theory of metamagnetism and weak antiferromagnetism of heavy fermions based on the duality model

Crossover between Cooper-Pair Condensation and Bose-Einstein Condensation of "Di-Electronic Molecules" in Two-Dimensional Superconductors