712 research outputs found
Triangular Mott-Hubbard Insulator Phases of Sn/Si(111) and Sn/Ge(111) Surfaces
The ground state of Sn/Si(111) and Sn/Ge(111) surface -phases is
reexamined theoretically, based on calculations where correlations
are approximately included through the orbital dependence of the Coulomb
interaction (in the local density + Hubbard U approximation). The effect of
correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make
the surface magnetic, with a metal-insulator transition for both systems. This
signals the onset of a stable narrow gap Mott-Hubbard insulating state, in
agreement with very recent experiments. Antiferromagnetic exchange is proposed
to be responsible for the observed -point photoemission intensity, as
well asfor the partial metallization observed above above 60 K in Sn/Si(111).
Extrinsic metallization of Sn/Si(111) by, alkali doping, could lead to
a novel 2D triangular superconducting state of this and similar surfaces.Comment: 4 pages, 4 figure
Connection Between Magnetism and Structure in Fe Double Chains on the Ir(100) Surface
The magnetic ground state of nanosized systems such as Fe double chains,
recently shown to form in the early stages of Fe deposition on Ir(100), is
generally nontrivial. Using ab initio density functional theory we find that
the straight ferromagnetic (FM) state typical of bulk Fe as well as of isolated
Fe chains and double chains is disfavored after deposition on Ir(100) for all
the experimentally relevant double chain structures considered. So long as
spin-orbit coupling (SOC) is neglected, the double chain lowest energy state is
generally antiferromagnetic (AFM), a state which appears to prevail over the FM
state due to Fe-Ir hybridization. Successive inclusion of SOC adds two further
elements, namely a magnetocrystalline anisotropy, and a Dzyaloshinskii-Moriya
(DM) spin-spin interaction, the former stabilizing the collinear AFM state, the
second favoring a long-period spin modulation. We find that anisotropy is most
important when the double chain is adsorbed on the partially deconstructed
Ir(100) -- a state which we find to be substantially lower in energy than any
reconstructed structure -- so that in this case the Fe double chain should
remain collinear AFM. Alternatively, when the same Fe double chain is adsorbed
in a metastable state onto the (5x1) fully reconstructed Ir(100) surface, the
FM-AFM energy difference is very much reduced and the DM interaction is
expected to prevail, probably yielding a helical spin structure.Comment: to appear on PR
Spectroscopic fingerprints of a surface Mott-Hubbard insulator: the case of SiC(0001)
We discuss the spectroscopic fingerprints that a surface Mott-Hubbard
insulator should show at the intra-atomic level. The test case considered is
that of the Si-terminated SiC(0001) sqrt{3}xsqrt{3} surface, which is known
experimentally to be insulating. We argue that, due to the Mott-Hubbard
phenomenon, spin unpaired electrons in the Si adatom dangling bonds are
expected to give rise to a Si-2p core level spectrum with a characteristic
three-peaked structure, as seen experimentally. This structure results from the
joint effect of intra-atomic exchange, spatial anisotropy, and spin-orbit
coupling. Auger intensities are also discussed.Comment: 4 pages, 2 figures, ECOSS-18 conferenc
Atomic spin sensitive dissipation on magnetic surfaces
We identify the mechanism of energy dissipation relevant to spin-sensitive
nanomechanics including the recently introduced magnetic exchange force
microscopy, where oscillating magnetic tips approach surface atomic spins. The
tip-surface exchange couples spin and atom coordinates, leading to a
spin-phonon problem with Caldeira-Leggett type dissipation. In the overdamped
regime, that can lead to a hysteretic flip of the local spin with a large
spin-dependent dissipation, even down to the very low experimental tip
oscillation frequencies, describing recent observations for Fe tips on NiO. A
phase transition to an underdamped regime with dramatic drop of magnetic tip
dissipation should in principle be possible by tuning tip-surface distance.Comment: 4 pages, 4 figure
Ballistic conductance of magnetic Co and Ni nanowires with ultrasoft pseudo-potentials
The scattering-based approach for calculating the ballistic conductance of
open quantum systems is generalized to deal with magnetic transition metals as
described by ultrasoft pseudo-potentials. As an application we present
quantum-mechanical conductance calculations for monatomic Co and Ni nanowires
with a magnetization reversal. We find that in both Co and Ni nanowires, at the
Fermi energy, the conductance of electrons is blocked by a magnetization
reversal, while the states (one per spin) are perfectly transmitted.
electrons have a non-vanishing transmission in a small energy window below the
Fermi level. Here, transmission is larger in Ni than in Co.Comment: 9 pages, 6 figures, to appear in PR
Phonons Softening in Tip-Stretched Monatomic Nanowires
It has been shown in recent experiments that electronic transport through a
gold monatomic nanowire is dissipative above a threshold voltage due to
excitation of phonons via the electron-phonon interaction. We address that data
by computing, via density functional theory, the zone boundary longitudinal
phonon frequency of a perfect monatomic nanowire during its mechanical
elongation. The theoretical frequency that we find for an ideally strained
nanowire is not compatible with experiment if a uniformly distributed stretch
is assumed. With the help of a semi-empirical Au-Au potential, we model the
realistic nanowire stretching as exerted by two tips. In this model we see that
strain tends to concentrate in the junctions, so that the mean strain of the
nanowire is roughly one half of the ideal value. With this reduced strain, the
calculated phonon softening is in much better agreement with experiment.Comment: 9 pages,3 figures, Surface Science, in pres
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