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

Abstract : We report on susceptibility measurements in the strongly correlated layered cobalt oxide BiBa 0.66 K 0.36 O 2 CoO 2 , 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. The strongly correlated quantum matter exhibits various outstanding and often puzzling properties related to quantum criticality. 1 Providing a route toward non-Fermi Liquid behavior or unconventional superconductivity, these quantum fluctuations originate from a so-called quantum critical point QCP. Located at zero temperature, a QCP results from competing interactions which can be tuned by an appropriate nonthermal control parameter, namely, such as pressure, doping or magnetic field. 2 While heavy Fermions metals have early emerged as prototypical materials to investigate QCPs, 3-5 some transition-metal oxides as the ruthenates 6 and the cuprate high-Tc superconductors 7 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. 8 Belonging to this class of materials, the layered cobalt oxides have revealed, besides their enhanced room temperature thermopower, 9,10 striking properties 11 including superconductivity, 12 large negative magnetoresistance in some compounds, 13 or giant electron-electron scattering in Na 0.7 CoO 2. 14 The latter observation has already led to conjecture a possible influence of a magnetic QCP. Interestingly, density functional calculations 15 have predicted at the local spin-density approximation level weak itinerant ferromag-netic state competing with weak itinerant antiferromagnetic state in electron doped Na x CoO 2. Due to these competing interactions, quantum critical fluctuations have been speculated and even a possible triplet superconductivity. Here we report on the low frequency susceptibility measurements which allow us to identify a magnetic quantum critical point in the layered cobalt oxide BiBa 0.66 K 0.36 O 2 CoO 2 leading to anomalous thermoelectric behavior. Similarly to Na x CoO 2 , the structure of the layered cobalt oxide BiBa 0.66 K 0.36 O 2 CoO 2 contains single CoO 2 layer of CdI 2 type stacked with four rocksalt-type layers as depicted in Fig. 1, instead of a sodium deficient layer, which act as a charge reservoir. 16 The reported measurements have been performed on single crystals, which were grown using standard flux method 17 and were chemically characterized from energy dispersive spectroscopy. The low frequency susceptibility has been determined with an ac field modulation up to 1.7 mT at a frequency of 1 kHz superposed to a constant field B using an ac magnetometer of a Quantum Design physical properties measurement system. A stacking of few single crystals, all oriented with c axis parallel to the magnetic field, has been used to reach a mass of 20 mg for these measurements. Figure 2 displays the temperature dependences of the susceptibility for selected values of magnetic field, which basically span three regimes with specific behaviors as highlighted by the areas. In particular, a magnetic field induced Fermi liquid regime characterized by a temperature independent susceptibility is revealed at the lowest temperatures blue or gray area. This behavior is followed by a power law T-dependence at higher temperatures yellow or light gray area. At very low fields, an anomaly can be seen around 6 K indicating a possible enhancement of the magnetic correlations. These anomalies disappear above nearly 0.2 T as exemplified in Fig. 3a by delimitating the third regime in Fig. 2 red or dark gray area. Contrasting FIG. 1. Color online Schematic picture of the crystal structure of the layered cobalt oxide BiBa 0.66 K 0.36 O 2 CoO 2. It displays from the top to the bottom a CoO 2 plane, a Ba 0.66 K 0.36 O plane, a BiO plane followed by symmetrical layers. PHYSICAL REVIEW B 81, 115113 2010
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P. Limelette, W. Saulquin, H. Muguerra, D. Grebille. From quantum criticality to enhanced thermopower in strongly correlated layered cobalt oxide. Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2010, 81 (11), ⟨10.1103/physrevb.81.115113⟩. ⟨hal-01870094⟩



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