Quantum Critical phenomena in doped Mott insulators – QCM

The main scientific purpose of the QCM project is to improve our knowledge on quantum critical phenomena and their connection to unconventional metallic states of matter. In particular, we want to characterize the electronic properties of such systems in order to be able to identify non-Fermi-liquid phases in strongly correlated systems.
While preliminary measurements have already revealed quantum critical behaviours as well as non-Fermi liquid features in the cobalt oxide (BiBa0.66K0.36O2)CoO2, we propose to investigate more deeply both the transport and the specific heat properties as a function of magnetic field and at very low temperatures, namely below 4 K. Indeed, such measurements will allow to precisely determine crossover lines between Fermi liquid and non-Fermi liquid, power law dependences characterizing the transport and the Sommerfeld coefficients, including their exponents. In addition, important quantities such as the Kadowaki-Woods ratio will be analyzed, especially within the quantum critical regime, as well as the scaling properties.
From an experimental point of view, the second part of the QCM project is probably the most ambitious. This indeed requires to synthesize new compositions in the titanium oxides family. Actually, while the perovskite-like system R1-xAxTiO3 is among the most famous one studied undergoing a filling controlled MIT, it appears that only the undoped solid solution (R1-yR’y)TiO3 has been investigated. Therefore, doping such a system will allow us to explore new regions of the phase diagram by systematically characterizing the electrical, thermal and thermodynamical properties as a function of temperature, magnetic field and doping. This could at least bring new insights into the effects of the competition between ferromagnetism and antiferromagnetism in doped Mott insulators, shade new light on quantum criticality and may be, even reveal unusual metallic phases.

We have investigated infrared spectroscopic properties of the strongly correlated layered cobalt oxide [BiBa0.66K0.36O2]CoO2. These measurements, performed on single crystals, allow us to determine the optical conductivity as a function of the temperature. In addition to a large temperature-dependent transfer of spectral weight, an unconventional low-energy mode is found. We show that both its frequency and its damping scale as the temperature itself. In fact, a basic analysis demonstrates that this mode fully scales onto a function of ?/T up to room temperature. This behavior suggests low-energy excitations of non-Fermi liquid type originating from quantum criticality.

We are now investigating the synthesis of the undoped solid solution (R1-yR’y)TiO3 as well as the related electronic properties focussing first on the magnetic properties in order to clarify their magnetic phase diagram.

«omega/T scaling of the optical conductivity in strongly correlated layered cobalt oxide «; Limelette, P.; Phuoc, V. Ta; Gervais, F.; et al.
PHYSICAL REVIEW B 87, Issue: 3, 035102 (2013 ).

The strongly correlated quantum matter exhibits various outstanding and often puzzling properties related to quantum criticality. Providing a route toward non-Fermi Liquid behaviour or unconventional superconductivity, these quantum fluctuations originate from a so-called quantum critical point. Located at zero temperature, a quantum critical point results from competing interactions which can be tuned by an appropriate nonthermal control parameter, namely, such as pressure, doping or magnetic field. While heavy Fermions (HF) metals have early emerged as prototypical materials to investigate these phenomena, some transition-metal oxides as the ruthenates and the cuprate high-Tc superconductors have also revealed amazing properties related to quantum criticality. In addition, most of these oxides share in common that they are doped Mott insulator, i.e., their metallicity originates from the introduction of charge carriers by doping, otherwise the strong Coulomb repulsion would localize electrons to form a Mott insulating state.
Within this context, the QCM project is a basic research program in the main field of the condensed matter physics. It deals with the “Quantum Critical phenomena in doped Mott insulators” in order to identify and to characterize unusual and potentially new metallic phases, namely beyond the Fermi liquid paradigm.
More precisely, we propose to investigate Quantum Critical phenomena and electronic correlations in two kinds of doped Mott insulators including first, a layered cobalt oxide. Besides their enhanced room temperature thermopower, the cobaltates AxCoO2 have revealed a very rich phase diagram as well as striking properties which have already led to conjecture a possible influence of a quantum critical point both theoretically and experimentally. Also, recent results have displayed magnetic field induced quantum criticality in the metallic compound [BiBa0.66K0.36O2]CoO2 in a close analogy with the HF metal YbRh2[Si0.95Ge0.05]2. Then, we propose to investigate both the transport and the specific heat properties as a function of magnetic field down to 0.4 K in the former compound. This will provide in particular a precise determination of the critical exponents allowing to discuss their universality and then the nature of this unconventional quantum criticality (local, multicritical …).
On the other hand, we would like to synthesize titanates with the generic formula (R1-yR’y)1-xAxTiO3, with the trivalent rare-earth ions (R,R’) and the divalent alkaline-earth ion A, in order to characterize their electronic properties. These compositions are indeed of particular interest since it has been shown that the ions (R,R’) can be chosen such as R (La,Sm…) favours low temperature antiferromagnetic (AF) interactions whereas R’(Y,Gd…) favours low temperature ferromagnetic (F) interactions. As a consequence, one might expect by fine-tuning the ratio (R,R’) to observe quantum critical behaviours as a function of the doping x and/or the magnetic field. It must be emphasized that the latter part of the QCM project is probably the most ambitious. This indeed requires to synthesize new compositions in the titanium oxides family. Actually, while the perovskite-like system R1-xAxTiO3 is among the most famous one which undergoes a filling controlled Mott metal insulator transition, it appears that only the undoped solid solution (R1-yR’y)TiO3 has been investigated. Therefore, doping such a system will allow us to explore new regions of their electronic phase diagram by systematically characterizing the electrical, thermal and thermodynamical properties as a function of temperature, magnetic field and doping.