Satoru Nakatsuji (ISSP Tokyo)

Satoru Nakatsuji (ISSP Tokyo)
"Unconventional quantum criticality, anomalous metal with strong valence/orbital fluctuations"
Blogged by Maxim Dzero

    One of the greatest challenges to Landau's Fermi liquid theory - the standard theory of metals - is presented by complex materials with strong electronic correlations. In these materials, non-Fermi liquid transport and thermodynamic properties are often explained by the presence of a continuous quantum phase transition which happens at a quantum critical point (QCP). QCP can be revealed by applying pressure, magnetic field, or changing the chemical composition. In the first part of his inspiring talk, Prof. Nakatsuji has discussed manifestation of quantum criticality in the f-orbital material beta-YbAlB4. Starting with the review of the data on the magnetization and resistivity measurements, he showed that beta-YbAlB4 displays an anomalous (non-Fermi liquid) behavior at low field and temperatures. Increase in magnetic field fully recovers conventional (metallic) Fermi-liquid properties. Prof. Nakatsuji discussed how the data analysis unambiguously points to the presence of the QCP at very small (of the order of 1G) magnetic field. This results show that beta-YbAlB4 displays quantum criticality without an apparent QCP suggesting the non-Fermi liquid metal is a phase in itself rather than is caused by system’s proximity to QCP.
    To get further insight into the physics of that system, the results of the hydrostatic and chemical pressure studies have been presented. As it turns out, pressure induces the phase transition in magnetically ordered state, while the quantum criticality remains intact. Therefore, the results of the pressure studies strongly suggest that the microscopic mechanisms, which govern quantum criticality in this material, are different from those responsible for the onset of magnetic order. In addition, measurements of the Seebeck and Zommerfeld coefficients show that quantum criticality anomalous transport and thermodynamic properties are likely governed by the quasi-particle excitations from the same heavy-Fermi surface. To explain the anomalous features in the Hall constant strongly suggest that one needs to invoke the excitations from another Fermi surface. In addition, there is an indication that there may be a line of nodes in the hybridization gap[. These results triggered the questions from the audience as to what extend the existence of the “quantum critical” Fermi surface can be reconciled with the absence of the hybridization for the specific values of momenta in the Brillouin zone. Another intriguing question is whether it would be possible to identify the Fermi surface, which develops the leading superconducting instability.
    In the second part of the talk, Prof. Nakatsuji has discussed the manifestation of an interplay between conduction and orbital degrees of freedom which under certain circumstances may lead to a phenomenon of the orbital (or quadrupolar) Kondo effect. The candidate materials, which may display this physics, are PrTr2Al20 with Tr=V, Ti. Prof. Nakatsuji has presented a set of thermodynamic and transport data consistent with the idea of quadrupolar ordering in PrTi2Al20. On the contrary, PrV2Al20 shows the dominant role of the Kondo screening processes.
    I think, the presented results mark novel and important developments in the studies of f-orbital materials.