From quantum criticality to enhanced thermopower in strongly correlated layered cobalt oxide

We report on susceptibility measurements in the strongly correlated layered cobalt oxide [BiBa0.66K0.36O2]CoO2, which demonstrate the existence of a magnetic quantum critical point (QCP) governing the electronic properties. The investigated low frequency susceptibility displays a scaling behavior with both the temperature T and the magnetic field B ranging from the high-T non-Fermi liquid down to the low-T Fermi liquid. Whereas the inferred scaling form can be discussed within the standard framework of the quantum critical phenomena, the determined critical exponents suggest an unconventional magnetic QCP of a potentially generic type. Accordingly, these quantum critical fluctuations account for the anomalous logarithmic temperature dependence of the thermopower. This result allows us to conjecture that quantum criticality can be an efficient source of enhanced thermopower

Magnetoresistance scaling in the layered cobaltate Ca3Co4O9

We investigate the low temperature magnetic field dependences of both the resistivity and the magnetization in the misfit cobaltate Ca3Co4O9 from 60 K down to 2 K. The measured negative magnetoresistance reveals a scaling behavior with the magnetization which demonstrates a spin dependent diffusion mechanism. This scaling is also found to be consistent with a shadowed metalliclike conduction over the whole temperature range. By explaining the observed transport crossover, this result shed a new light on the nature of the elementary excitations relevant to the transport

New electronic orderings observed in cobaltates under the influence of misfit periodicities

We study with ARPES the electronic structure of CoO2 slabs, stacked with rock-salt (RS) layers exhibiting a different (misfit) periodicity. Fermi Surfaces (FS) in phases with different doping and/or periodicities reveal the influence of the RS potential on the electronic structure. We show that these RS potentials are well ordered, even in incommensurate phases, where STM images reveal broad stripes with width as large as 80\AA. The anomalous evolution of the FS area at low dopings is consistent with the localization of a fraction of the electrons. We propose that this is a new form of electronic ordering, induced by the potential of the stacked layers (RS or Na in NaxCoO2) when the FS becomes smaller than the Brillouin Zone of the stacked structure

Dual electronic states in thermoelectric cobalt oxide

We investigate the low temperature magnetic field dependence of the resistivity in the thermoelectric misfit cobalt oxide [Bi1.7Ca2O4]0.59CoO2 from 60 K down to 3 K. The scaling of the negative magnetoresistance demonstrates a spin dependent transport mechanism due to a strong Hund's coupling. The inferred microscopic description implies dual electronic states which explain the coexistence between localized and itinerant electrons both contributing to the thermopower. By shedding a new light on the electronic states which lead to a high thermopower, this result likely provides a new potential way to optimize the thermoelectric properties

Experimental study of the incoherent spectral weight in the photoemission spectra of the misfit cobaltate [Bi2Ba2O4][CoO2]2

Previous ARPES experiments in NaxCoO2 reported both a strongly renormalized bandwidth near the Fermi level and moderately renormalized Fermi velocities, leaving it unclear whether the correlations are weak or strong and how they could be quantified. We explain why this situation occurs and solve the problem by extracting clearly the coherent and incoherent parts of the band crossing the Fermi level. We show that one can use their relative weight to estimate self-consistently the quasiparticle weight Z, which turns out to be very small Z=0.15 +/- 0.05. We suggest this method could be a reliable way to study the evolution of correlations in cobaltates and for comparison with other strongly correlated systems

Magnetic field dependent specific heat and enhanced Wilson ratio in strongly correlated layered cobalt oxide

International audienceWe have investigated the low-temperature specific-heat properties as a function of magnetic field in the strongly correlated layered cobalt oxide BiBa 0.66 K 0.36 O 2 CoO 2. These measurements reveal two kinds of magnetic field dependent contributions in qualitative agreement with the presence of a previously inferred magnetic quantum critical point QCP. First, the coefficient of the low-temperature T 3 behavior of the specific heat turns out to sizably decrease near a magnetic field consistent with the critical value reported in a recent paper. In addition, a moderate but significant enhancement of the Sommerfeld coefficient is found in the vicinity of the QCP suggesting a slight increase in the electronic effective mass. This result contrasts with the divergent behavior of the previously reported Pauli susceptibility. Thus, a strongly enhanced Wilson ratio is deduced, suggesting efficient ferromagnetic fluctuations in the Fermi-liquid regime which could explain the unusual magnetic field dependent specific heat. As a strong check, the high magnetic field Wilson ratio asymptotically recovers the universal limit of the local Fermi liquid against ferromagnetism. Transition-metal oxides have demonstrated over the last decades how the strong correlations could lead to unantici-pated electronic properties. Outstanding examples 1 are super-conducting cuprates, manganites with their colossal negative magnetoresistance, 2 vanadates displaying the Mott metal-insulator transition, 3 and the layered cobalt oxides which exhibit an unexpected large thermopower at room temperature. 4 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. 1,5 Belonging to this class of materials the layered cobalt oxides have revealed, besides their enhanced room-temperature thermopower, 6 a very rich phase diagram as well as striking properties 7-9 including large negative magnetoresistance in some compounds 10 or giant electron-electron scattering in Na 0.7 CoO 2. 11 Interestingly , the latter observation has already led to conjecture a possible influence of a magnetic QCP in the aforementioned compound. Density-functional calculations have also predicted at the local spin-density approximation level weak itinerant ferromagnetic state competing with weak itinerant antiferromagnetic state, favoring then quantum critical fluctuations. 12 Within this context, susceptibility measurements have recently demonstrated in the strongly correlated layered cobalt oxide BiBa 0.66 K 0.36 O 2 CoO 2 the existence of a magnetic quantum critical point QCP governing the electronic properties. 13 The investigated susceptibility has revealed a scaling behavior with both the temperature T and the magnetic field B ranging from a high-T non-Fermi liquid down to a low-T Fermi liquid. In the latter Fermi-liquid regime, the Pauli susceptibility has exhibited a divergent behavior with a power-law dependence as b −0.6 with b = B − B C which measures the distance from the QCP and the critical magnetic field B C 0.176 T. While several scenarios could explain this result, this behavior may in particular originate from either an enhancement of the electronic effective mass due to the vicinity of the QCP or because of the presence of efficient ferromagnetic fluctuations increasing the Pauli susceptibility by a Stoner factor. In order to put these scenarios under experimental test, we have investigated the low-temperature specific-heat properties as a function of magnetic field in the layered cobalt oxide BiBa 0.66 K 0.36 O 2 CoO 2 , which we report on in this paper. Similarly to Na x CoO 2 , the structure of the layered cobalt oxide BiBa 0.66 K 0.36 O 2 CoO 2 abbreviated hereafter as BBCO contains single CoO 2 layer of CdI 2 type stacked with four rocksalt-type layers, instead of a sodium deficient layer, which act as a charge reservoir. 14 The reported measurements have been performed on a single crystal with a mass of 35 mg which was grown using standard flux method. 15 The specific heat has been determined with a calo-rimeter of a Quantum Design physical properties measurement system using a relaxation method with a temperature rise on the order of 2% of the sample temperature. It is worthy to note that the calorimeter, including the thermometers , has been calibrated with each magnetic field up to 9 T. In addition, all the measurements have been duplicated without the sample for each magnetic field in order to compensate both the temperature and magnetic field dependences of the grease used to ensure a good thermal contact between the sample and the calorimeter platform. We also mention that the calorimeter's parameter which indicates the quality of the thermal contact between the sample and the platform, the so-called sample coupling, has remained between 97% and 100% during each measurement below 100 K, ensuring thus the reliability of the results. Figure 1 displays the temperature dependence of the specific heat over the range from 300 K down to 1.9 K on a double-logarithmic scale in order to show both the low-and the high-temperature behaviors. As frequently observed, the PHYSICAL REVIEW B 82, 035123 201