7,178 research outputs found
Spin-Orbit mediated spin relaxation in monolayer MoS2
We study the intra-valley spin-orbit mediated spin relaxation in monolayers
of MoS2 within a two bands effective Hamiltonian. The intrinsic spin splitting
of the valence band as well as a Rashba-like coupling due to the breaking of
the out-of-plane inversion symmetry are considered. We show that, in the hole
doped regime, the out-of-plane spin relaxation is not very efficient since the
spin splitting of the valence band tends to stabilize the spin polarization in
this direction. We obtain spin lifetimes larger than nanoseconds, in agreement
with recent valley polarization experiments.Comment: final version, 9 pages, 5 figure
Spin relaxation in corrugated graphene
In graphene, out-of-plane (flexural) vibrations and static ripples imposed by
the substrate relax the electron spin, intrinsically protected by mirror
symmetry. We calculate the relaxation times in different scenarios, accounting
for all the possible spin-phonon couplings allowed by the hexagonal symmetry of
the lattice. Scattering by flexural phonons imposes the ultimate bound to the
spin lifetimes, in the ballpark of hundreds of nano-seconds at room
temperature. This estimate and the behavior as a function of the carrier
concentration are substantially altered by the presence of tensions or the
pinning with the substrate. Static ripples also influence the spin transport in
the diffusive regime, dominated by motional narrowing. We find that the
D'yakonov-Perel' mechanism saturates when the mean free path is comparable to
the correlation length of the heights profile. In this regime, the
spin-relaxation times are exclusively determined by the geometry of the
corrugations. Simple models for typical corrugations lead to lifetimes of the
order of tens of micro-seconds.Comment: 4 + epsilon pages; 3 figure
Elliot-Yafet mechanism in graphene
The differences between spin relaxation in graphene and in other materials
are discussed. For relaxation by scattering processes, the Elliot-Yafet
mechanism, the relation between the spin and the momentum scattering times
acquires a dependence on the carrier density, which is independent of the
scattering mechanism and the relation between mobility and carrier
concentration. This dependence puts severe restrictions on the origin of the
spin relaxation in graphene. The density dependence of the spin relaxation
allows us to distinguish between ordinary impurities and defects which modify
locally the spin-orbit interaction.Comment: 4 pages + \epsilon + S
Quark mass hierarchy in 3-3-1 models
We study the mass spectrum of the quark sector in an special type I-like
model with gauge symmetry . By
considering couplings with scalar triplets at large () and small
() scales, we obtain specific zero-texture mass matrices for the
quarks which predict three massless quarks () and three massive quarks
() at the electroweak scale ( GeV). Taking into account mixing
couplings with three heavy quarks at large scales predicted by the model, the
three massless quarks obtain masses at small order that depends on the inverse
of the large scale. Thus, masses of the form
and can be obtained naturally from the gauge structure of
the model
Interactions and magnetic moments near vacancies and resonant impurities in graphene
The effect of electronic interactions in graphene with vacancies or resonant
scatterers is investigated. We apply dynamical mean-field theory in combination
with quantum Monte Carlo simulations, which allow us to treat
non-perturbatively quantum fluctuations beyond Hartree-Fock approximations. The
interactions narrow the width of the resonance and induce a Curie magnetic
susceptibility, signaling the formation of local moments. The absence of
saturation of the susceptibility at low temperatures suggests that the coupling
between the local moment and the conduction electrons is ferromagnetic
Seeking Quantum Speedup Through Spin Glasses: The Good, the Bad, and the Ugly
There has been considerable progress in the design and construction of
quantum annealing devices. However, a conclusive detection of quantum speedup
over traditional silicon-based machines remains elusive, despite multiple
careful studies. In this work we outline strategies to design hard tunable
benchmark instances based on insights from the study of spin glasses - the
archetypal random benchmark problem for novel algorithms and optimization
devices. We propose to complement head-to-head scaling studies that compare
quantum annealing machines to state-of-the-art classical codes with an approach
that compares the performance of different algorithms and/or computing
architectures on different classes of computationally hard tunable spin-glass
instances. The advantage of such an approach lies in having to only compare the
performance hit felt by a given algorithm and/or architecture when the instance
complexity is increased. Furthermore, we propose a methodology that might not
directly translate into the detection of quantum speedup, but might elucidate
whether quantum annealing has a "`quantum advantage" over corresponding
classical algorithms like simulated annealing. Our results on a 496 qubit
D-Wave Two quantum annealing device are compared to recently-used
state-of-the-art thermal simulated annealing codes.Comment: 14 pages, 8 figures, 3 tables, way too many reference
Constraining the Noncommutative Spectral Action via Astrophysical Observations
The noncommutative spectral action extends our familiar notion of commutative
spaces, using the data encoded in a spectral triple on an almost commutative
space. Varying a rather simple action, one can derive all of the standard model
of particle physics in this setting, in addition to a modified version of
Einstein-Hilbert gravity. Thus, noncommutative geometry provides a geometric
interpretation of particle physics coupled to curvature. In this letter we use
observations of pulsar timings, assuming that no deviation from General
Relativity has been observed, to constrain the gravitational sector of this
theory. Thus, we directly constrain noncommutative geometry, a potential grand
unified theory of physics, via astrophysical observations. Whilst the bounds on
the coupling constants remain rather weak, they are comparable to existing
bounds on deviations from General Relativity in other settings and are likely
to be further constrained by future observations.Comment: 5 pages; slightly shorter version to match the one will appear in
Phys. Rev. Let
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