Fermi- to non-Fermi-liquid crossover and Kondo behavior in two-dimensional (Cu2/3V1/3)V2S4.

By means of a specific heat (C) and electrical resistivity ([Formula: see text]) study, we give evidence of a pronounced Fermi liquid (FL) behavior with sizable mass renormalization, [Formula: see text], up to unusually high temperatures ∼70 K in the layered system (Cu2/3V1/3)V2S4. At low temperature, a marked upturn of both C and [Formula: see text] is suppressed by magnetic field, which suggests a picture of Kondo coupling between conduction electrons in the VS2 layers and impurity spins of the V3+ ions located between layers. This picture opens the possibility of controlling electronic correlations and the FL to non-FL crossover in simple layered materials. For instance, we envisage that the coupling between layers provided by the impurity spins may realize a two-channel Kondo state.

Weak localization and Mott state in two-dimensional Sr3V5S11.

We report on the high-pressure synthesis, structural study and physical properties of a new layered compound, Sr3V5S11. Single-crystals of ~0.3 mm size were synthesized at the optimized growth conditions of 6 GPa and 1600 ° C. The refinement of x-ray diffraction data indicates that the crystal structure is monoclinic (space group C2/c), with cell parameters a = 8.7165(7) Å, b = 15.1096(13) Å, c = 23.111(2) Å, and ß = 98.734(9). The structure consists of a stacking of VS2 layers with a CdI2-type structure within the ab-plane connected by trimers of face-sharing VS6 octahedra oriented along the out-of-plane direction. Salient features are a 4 + valence of the V ions in the planes and a 3 + valence in the trimers and a large stripe-like modulation of the V-V distances in the planes leading to quasi-one-dimensional properties. The magnetic susceptibility displays a large temperature-independent contribution, χ0, in addition to a moderate Curie-Weiss term. In the 300-120 K range, the electrical resistivity is described well by a semiconducting-like behaviour with a room-temperature value of ~1.2-10 mΩ cm and a modest activation energy of ~13.5 meV. At lower temperatures, a crossover to a one-dimensional variable range hopping regime is observed, supporting a scenario of a correlated 1D system.

Anomalous metallic state in the vicinity of metal to valence-bond solid insulator transition in LiVS2.

We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2 known for the formation of a valence-bond solid (VBS) state out of an S=1 frustrated magnet. LiVS2, which is located at the border between a metal and a correlated insulator, shows a first order transition from a paramagnetic metal to a VBS insulator at Tc approximately 305 K upon cooling. The presence of a VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of a VBS state. We argue that the high temperature metallic phase of LiVS2 has a pseudogap, likely originating from the VBS fluctuation. LiVSe2 was found to be a paramagnetic metal down to 2 K.

Enhancement of superconductivity and evidence of structural instability in intercalated graphite CaC6 under high pressure.

We measured the temperature dependent resistivity, varrho(T), of the intercalated graphite superconductor CaC6 as a function of pressure up to 16 GPa. We found a large linear increase of critical temperature, Tc, from the ambient pressure value 11.5 K up to 15.1 K, the largest value for intercalated graphite, at 7.5 GPa. At approximately 8 GPa, a jump of varrho and a sudden drop of Tc down to approximately 5 K indicates the occurrence of a phase transition. Our data analysis suggests that a pressure-induced phonon softening related to an in-plane Ca phonon mode is responsible for the Tc increase and that higher pressures greater, similar8 GPa lead to a structural transition into a new phase with a low Tc less, similar3 K.

Charge, orbital and spin ordering phenomena in the mixed valence manganite (NaMn3+(3))(Mn3+(2)Mn4+(2))O12.

Mixed-valence manganites with the ABO3 perovskite structure display a variety of magnetic and structural transitions, dramatic changes of electrical conductivity and magnetoresistance effects. The physical properties vary with the relative concentration of Mn3+ and Mn4+ in the octahedral corner-sharing network, and the proportion of these two cations is usually changed by doping the trivalent large A cation (for example, La3+) with divalent cations. As the dopant and the original cation have, in general, different sizes, and as they are distributed randomly in the structure, such systems are characterized by local distortions that make it difficult to obtain direct information about their crystallographic and physical properties. On the other hand, the double oxides of formula AA'3Mn4O12 contain a perovskite-like network of oxygen octahedra centred on the Mn cations, coupled with an ordered arrangement of the A and A' cations, whose valences control the proportion of Mn3+ and Mn4+ in the structure. The compound investigated in this work, (NaMn3+(3))(Mn3+(2)Mn4+(2))O12, contains an equal number of Mn3+ and Mn4+ in the octahedral sites. We show that the absence of disorder enables the unambiguous determination of symmetry, the direct observation of full, or nearly full, charge ordering of Mn3+ and Mn4+ in distinct crystallographic sites, and a nearly perfect orbital ordering of the Mn3+ octahedra.