526 research outputs found
Theory of the Metal-Paramagnetic Mott-Jahn-Teller Insulator Transition in A_4C_{60}
We study the unconventional insulating state in A_4C_{60} with a variety of
approaches, including density functional calculations and dynamical mean-field
theory. While the former predicts a metallic state, in disagreement with
experiment, the latter yields a (paramagnetic) Mott-Jahn-Teller insulator. In
that state, conduction between molecules is blocked by on-site Coulomb
repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and
important excitations (such as optical and spin gap) should be essentially
intra-molecular. Experimental gaps of 0.5 eV and 0.1 eV respectively compare
well with molecular ion values, in agreement with this picture.Comment: 4 pages, 2 postscript figure
Electron-phonon interaction at the Be(0001) surface
We present a first principle study of the electron-phonon (e-p) interaction
at the Be(0001) surface. The real and imaginary part of the e-p self energy are
calculated for the surface state in the binding energy range from the
point to the Fermi level. Our calculation shows an overall good
agreement with several photoemission data measured at high and low
temperatures. Additionally, we show that the energy derivative of real part of
the self-energy presents a strong temperature and energy variation close to
, making it difficult to measure its value just at .Comment: Accepted in Phys. Rev. Lett., 5 figure
Surface defreezing of glasses
A glass surface may still flow below the bulk glass transition temperature,
where the underlying bulk is frozen. Assuming the existence at T=T* of a bulk
thermodynamical glass transition, we show that the glass-vapor interface is
generally wetted by a liquid layer of thickness ~ -ln(T*-T) when T--> T*.
Contrary to standard surface melting of crystals however, the integrated value
of the diffusivity across the interface remains finite for T-->T*. Difference
in shape induced by bulk and by surface flow is discussed as a possible means
of experimental detection of surface defreezing.Comment: five pages, three figure
Disproportionation Phenomena on Free and Strained Sn/Ge(111) and Sn/Si(111) Surfaces
Distortions of the Sn/Ge(111) and Sn/Si(111) surfaces
are shown to reflect a disproportionation of an integer pseudocharge, ,
related to the surface band occupancy. A novel understanding of the
-1U (``1 up, 2 down'') and 2U (``2 up, 1 down'') distortions of
Sn/Ge(111) is obtained by a theoretical study of the phase diagram under
strain. Positive strain keeps the unstrained value Q=3 but removes distorsions.
Negative strain attracts pseudocharge from the valence band causing first a
-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a
-3U (``all up'') state with Q=6. The possibility of a
fluctuating phase in unstrained Sn/Si(111) is discussed.Comment: Revtex, 5 pages, 3 figure
Valley Jahn-Teller Effect in Twisted Bilayer Graphene
The surprising insulating and superconducting states of narrow-band graphene twisted bilayers have been mostly discussed so far in terms of strong electron correlation, with little or no attention to phonons and electron-phonon effects. We found that, among the 33 492 phonons of a fully relaxed \u3b8=1.08\ub0 twisted bilayer, there are few special, hard, and nearly dispersionless modes that resemble global vibrations of the moir\ue9 supercell, as if it were a single, ultralarge molecule. One of them, doubly degenerate at \u393 with symmetry A1+B1, couples very strongly with the valley degrees of freedom, also doubly degenerate, realizing a so-called E\ue2\u160-e Jahn-Teller (JT) coupling. The JT coupling lifts very efficiently all degeneracies which arise from the valley symmetry, and may lead, for an average atomic displacement as small as 0.5 m \uc5, to an insulating state at charge neutrality. This insulator possesses a nontrivial topology testified by the odd winding of the Wilson loop. In addition, freezing the same phonon at a zone boundary point brings about insulating states at most integer occupancies of the four ultraflat electronic bands. Following that line, we further study the properties of the superconducting state that might be stabilized by these modes. Since the JT coupling modulates the hopping between AB and BA stacked regions, pairing occurs in the spin-singlet Cooper channel at the inter-(AB-BA) scale, which may condense a superconducting order parameter in the extended s-wave and/or d\ub1id-wave symmetry
Rack-and-pinion effects in molecular rolling friction
Rolling lubrication with spherical molecules working as 'nanobearings' has
failed experimentally so far, without a full understanding of the physics
involved and of the reasons why. Past model simulations and common sense have
shown that molecules can only roll when they are not too closely packed to jam.
The same type of model simulations now shows in addition that molecular rolling
friction can develop deep minima once the molecule's peripheral 'pitch' can
match the substrate periodicity, much as ordinary cogwheels do in a
rack-and-pinion system. When the pinion-rack matching is bad, the driven
molecular rolling becomes discontinuous and noisy, whence energy is dissipated
and friction is large. This suggests experiments to be conducted by varying the
rack-and-pinion matching. That could be pursued not only by changing molecules
and substrates, but also by applying different sliding directions within the
same system, or by applying pressure, to change the effective matching.Comment: 5 figure
Impact of molecular structure on the lubricant squeeze-out between curved surfaces with long range elasticity
The properties of butane (C4H10) lubricants confined between two approaching
solids are investigated by a model that accounts for the curvature and elastic
properties of the solid surfaces. We consider the linear n-butane and the
branched iso-butane. For the linear molecule, well defined molecular layers
develop in the lubricant film when the width is of the order of a few atomic
diameters. The branched iso-butane forms more disordered structures which
permit it to stay liquid-like at smaller surface separations. During squeezing
the solvation forces show oscillations corresponding to the width of a
molecule. At low speeds (< 0.1 m/s) the last layers of iso-butane are squeezed
out before those of n-butane. Since the (interfacial) squeezing velocity in
most practical applications is very low when the lubricant layer has molecular
thickness, one expects n-butane to be a better boundary lubricant than
iso-butane. N-butane possessing a slightly lower viscosity at high pressures,
our result refutes the view that squeeze out should be harder for higher
viscosities, on the other hand our results are consistent with wear experiments
in which n-butane were shown to protect steel surfaces better than iso-butane.Comment: 7 pages, 10 figures, format revtex. Submitted to J. Chem. Phy
Static friction on the fly: velocity depinning transitions of lubricants in motion
The dragging velocity of a model solid lubricant confined between sliding
periodic substrates exhibits a phase transition between two regimes,
respectively with quantized and with continuous lubricant center-of-mass
velocity. The transition, occurring for increasing external driving force F_ext
acting on the lubricant, displays a large hysteresis, and has the features of
depinning transitions in static friction, only taking place on the fly.
Although different in nature, this phenomenon appears isomorphic to a static
Aubry depinning transition in a Frenkel-Kontorova model, the role of particles
now taken by the moving kinks of the lubricant-substrate interface. We suggest
a possible realization in 2D optical lattice experiments.Comment: 5 pages, 4 figures, revtex, in print in Phys. Rev. Let
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