18 research outputs found
Quasiparticle coherence and the nature of the metal-insulator phase transition in NaCoO
Layered cobaltates embody novel realizations of correlated quantum matter on
a spin-1/2 triangular lattice. We report a high-resolution systematic
photoemission study of the insulating cobaltates (Na1/2CoO2 and K1/2CoO2).
Observation of single-particle gap opening and band-folding provides direct
evidence of anisotropic particle-hole instability on the Fermi surface due to
its unique topology. Kinematic overlap of the measured Fermi surface is
observed with the x cobalt charge-order Brillouin zone near
x=1/3 but not at x=1/2 where insulating transition is actually observed. Unlike
conventional density-waves, charge-stripes or band insulators, the on-set of
the gap depends on the quasiparticle's quantum coherence which is found to
occur well below the disorder-order symmetry breaking temperature of the
crystal (the first known example of its kind).Comment: 4+ pages, 5 figure
Fermi surface and quasiparticle dynamics of Na(x)CoO2 {x=0.7} investigated by Angle-Resolved Photoemission Spectroscopy
We present an angle-resolved photoemission study of Na0.7CoO2, the host
cobaltate of the NaxCoO2.yH2O series. Our results show a large hexagonal-like
hole-type Fermi surface, an extremely narrow strongly renormalized
quasiparticle band and a small Fermi velocity. Along the Gamma to M high
symmetry line, the quasiparticle band crosses the Fermi level from M toward
Gamma consistent with a negative sign of effective single-particle hopping (t
): t is estimated to be about 8 meV which is on the order of exchange coupling
J in this system. This suggests that t ~ J ~ 10 meV is an important energy
scale in the system. Quasiparticles are well defined only in the T-linear
resistivity regime. Small single particle hopping and unconventional
quasiparticle dynamics may have implications for understanding the unusual
behavior of this new class of compounds.Comment: Revised text, Added Figs, Submitted to PR
Studying Nanotribological Properties of Functional Materials via Atomic Force Microscopy
Tribological Properties at 20 and 500Ā°C of TiN and CrN Cathodic ARC Coatings Deposited on Ti-6Al-4V Alloy
Tribological properties of TiN and CrN coatings deposited by cathodic arc method at three different bias potentials -50, -150 and -300V on Ti-6Al-4V alloy in pair with alumina have been investigated. X-ray diffraction analysis showed that single-phase textured cubic nitrides of TiN and CrN were formed in these coatings. It is shown that the friction coefficient of the coatings is practically equal to that established for the Ti6Al4V alloy, but the wear rate is more than an order of magnitude lower than for the titanium alloy substrate. Coatings deposited at a potential of -50 V show optimal tribological properties at temperatures 20 and 500Ā°C. Friction coefficients for TiN coatings are 0.4-0.8 at 20Ā°C and 0,75 at 500Ā°C; for CrN coatings they are 0.5 at 20Ā°C and 0,7 at 500Ā°C. Wear rates for TiN coatings are 0.86Ā·10-5 Š¼Š¼3/ŠŠ¼ at 20Ā°C and 3.56Ā·10-5 Š¼Š¼3/ŠŠ¼ at 500Ā°C; for CrN coatings they are 1.43Ā·10-5 Š¼Š¼3/ŠŠ¼ at 20Ā°C and 7.13Ā·10-5 Š¼Š¼3/ŠŠ¼ at 500Ā°C