Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2

The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from NMR to optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.Comment: 16 pages, 5 figure

Evolution of Two-Dimensional Wormlike Nanoclusters on Metal Surfaces

A pinch-off phenomenon is discovered in the evolution of 2D wormlike nanoclusters formed in homoepitaxial adlayers. This feature is shown to distinguish mass transport via periphery diffusion from other mechanisms. Continuum modeling of such evolution accurately describes experimental observations, particularly if one incorporates the anisotropy in step-edge line tension

Zero-point entropies of spin-jam and spin-glass states in a frustrated magnet

Thermodynamics of glassy states in a quasi-two-dimensional frustrated magnet Ba$_2$Sn$_2$ZnCr$_{7p}$Ga$_{10-7p}$O$_{22}$ where $p$ is the spin density are investigated experimentally. The system features a triangular network of bipyramids of spins with the quantum spin number $s = 3/2$. The DC magnetic susceptibility measurements on a series of samples with $0.44 \le p \le 0.98$ show a freezing transition with the transition temperature $T_\mathrm{f} \le 1.2$ K. $T_\mathrm{f}$ is found to decrease with decreasing $p$. The low-lying excitations in the glassy state of the system are examined via the temperature dependence of the magnetic heat capacity and are shown to consist of two components: the hydrodynamic Halperin-Saslow modes characteristic of a spin jam and the two-level systems of a spin glass. A continuous crossover between the two glassy states is observed via the varying weights of the two components as the spin density is varied. The $p$ dependence of the spin jam's zero-point entropy determined from the exotic perimeter-scaling behavior combined with the observed zero-point entropy of the samples provides the $p$ dependence of the spin glass's zero-point entropy. The obtained result shows that the correlations between orphan spins begin below $p \sim 0.8$, the limit that was also found using a neutron scattering technique in a previous report on the isostructural compound SrCr$_{9p}$Ga$_{12-9p}$O$_{19}$. The domain size of the spin-jam state estimated from the value of the zero-point entropy for the cleanest sample is approximately $4 \times 4$ bipyramids, about 2.5 times the measured spin correlation length

Probing structural and electronic properties of h-BN by HRTEM and STM

International audienceAfter the discovery of graphene and its consequences in the field of nanoscience and nanomaterials, there has been a growing interest in 2D materials and also their vertical stacking due to unique properties and potential applications.[1] For instance, it was shown the transport properties of exfoliated graphene supported by hexagonal boron nitride (h-BN) could approach the intrinsic graphene limits.[2] Nevertheless, studying the structural properties of 2D materials and 2D heterostructures is crucial to understand their physical and chemical properties. Our motivations have been to exploit state of the art aberration-corrected high resolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy (STM) to study the structure and electronic properties of graphene (G), h-BN and G/h-BN heterostructures. HRTEM analyses were conducted with a JEOL ARM microscope equipped together with a cold FEG, an aberration corrector for the objective lens and a One view camera (Gatan). Notably, we used high-speed atomic-scale imaging to study with unprecedented dynamics (up to 25 fps) the nucleation and growth mechanisms of triangular holes in h-BN under beam irradiation (Figure 1). The direct observation of B and N atom sputtering and surface reconstruction processes allow understanding how the triangular shape and orientation of holes are maintained during the growth. Interestingly, by studying the effects of the electron dose and the number of BN layers, we demonstrate that these atomic-scale processes are simultaneously driven by kinetic and thermodynamic effects. Further works are in progress to study the stability of h-BN/G stacking under electron-beam irradiation. STM analyses were carried out with low temperature STM at 4 K, on 2D heterostructures that consist in a few layers of graphene doped with nitrogen on thick exfoliated flakes of BN deposited on SiO 2. Remarkably, we show that STM allows identifying and characterizing ionization defects within the BN flakes below the graphene layers (Figure 2). This study opens new avenues to probe the electronic interactions between this two stacked materials

Dislocation Emission around Nanoindentations on a (001) fcc Metal Surface Studied by STM and Atomistic Simulations

We present a combined study by Scanning Tunneling Microscopy and atomistic simulations of the emission of dissociated dislocation loops by nanoindentation on a (001) fcc surface. The latter consist of two stacking-fault ribbons bounded by Shockley partials and a stair-rod dislocation. These dissociated loops, which intersect the surface, are shown to originate from loops of interstitial character emitted along the directions and are usually located at hundreds of angstroms away from the indentation point. Simulations reproduce the nucleation and glide of these dislocation loops.Comment: 10 pages, 4 figure

Tuning the Magnetic Anisotropy at a Molecule-Metal Interface

International audienceWe demonstrate that a C 60 overlayer enhances the perpendicular magnetic anisotropy of a Co thin film, inducing an inverse spin reorientation transition from in plane to out of plane. The driving force is the C 60 =Co interfacial magnetic anisotropy that we have measured quantitatively in situ as a function of the C 60 coverage. Comparison with state-of-the-art ab initio calculations show that this interfacial anisotropy mainly arises from the local hybridization between C 60 p z and Co d z 2 orbitals. By generalizing these arguments, we also demonstrate that the hybridization of C 60 with a Fe(110) surface decreases the perpendicular magnetic anisotropy. These results open the way to tailor the interfacial magnetic anisotropy in organic-material–ferromagnet systems

NbS3_{3}: A unique quasi one-dimensional conductor with three charge density wave transitions

Through transport, compositional and structural studies, we review the features of the charge-density wave (CDW) conductor of NbS$_{3}$ (phase II). We highlight three central results: 1) In addition to the previously reported CDW transitions at $T_{P1}$ = 360\,K and $T_{P2}$ = 150\,K, another CDW transition occurs at a much higher temperature $T_{P0}$ = 620-650\,K; evidence for the non-linear conductivity of this CDW is presented. 2) We show that CDW associated with the $T_{P2}$ - transition arises from S vacancies acting as donors. Such a CDW transition has not been observed before. 3) We show exceptional coherence of the $T_{P1}$-CDW at room-temperature. Additionally, we report on the effects of uniaxial strain on the CDW transition temperatures and transport.Comment: 16 pages, 18 figure

Near room temperature chemical vapor deposition of graphene with diluted methane and molten gallium catalyst

Direct growth of graphene integrated into electronic devices is highly desirable but difficult due to the nominal ~1000 °C chemical vapor deposition (CVD) temperature, which can seriously deteriorate the substrates. Here we report a great reduction of graphene CVD temperature, down to 50 °C on sapphire and 100 °C on polycarbonate, by using dilute methane as the source and molten gallium (Ga) as catalysts. The very low temperature graphene synthesis is made possible by carbon attachment to the island edges of pre-existing graphene nuclei islands, and causes no damages to the substrates. A key benefit of using molten Ga catalyst is the enhanced methane absorption in Ga at lower temperatures; this leads to a surprisingly low apparent reaction barrier of ~0.16 eV below 300 °C. The faster growth kinetics due to a low reaction barrier and a demonstrated low-temperature graphene nuclei transfer protocol can facilitate practical direct graphene synthesis on many kinds of substrates down to 50–100 °C. Our results represent a significant progress in reducing graphene synthesis temperature and understanding its mechanism