82 research outputs found
Gaussian to Exponential Crossover in the Attenuation of Polarization Echoes in NMR
An ingenious pulse sequence devised by S. Zhang, B. H. Meier, and R. R. Ernst
(Phys. Rev. Lett. {\bf 69}, 2149 (1992)) reverses the time evolution (``spin
diffusion'') of the local polarization in a dipolar coupled H spin
system. This refocusing originates a Polarization Echo whose amplitude
attenuates by increasing the time elapsed until the dynamics is reversed.
Different functional attenuations are found for a set of dipolar coupled
systems: ferrocene, (CH)Fe, cymantrene, (CH)Mn(CO), and
cobaltocene, (CH)Co. To control a relevant variable involved in
this attenuation a pulse sequence has been devised to progressively reduce the
dipolar dynamics. Since it reduces the evolution of the polarization echo it is
referred as REPE sequence. Two extreme behaviors were found while
characterizing the materials: In systems with a strong source of relaxation and
slow dynamics, the attenuation follows an exponential law (cymantrene). In
systems with a strong dipolar dynamics the attenuation is mainly Gaussian. By
the application of the REPE sequence the characteristic time of the Gaussian
decay is increased until the presence of an underlying dissipative mechanism is
revealed (cobaltocene). For ferrocene, however, the attenuation remains
Gaussian within the experimental time scale. These two behaviors suggest that
the many body quantum dynamics presents an extreme intrinsic instability which,
in the presence of small perturbations, leads to the onset of irreversibility.
This experimental conclusion is consistent with the tendencies displayed by the
numerical solutions of model systems.Comment: 7 pages + 7 Postscript figure
Localized Spins on Graphene
The problem of a magnetic impurity, atomic or molecular, absorbed on top of a
carbon atom in otherwise clean graphene is studied using the numerical
renormalization group. The spectral, thermodynamic, and scattering properties
of the impurity are described in detail. In the presence of a small magnetic
field, the low energy electronic features of graphene make possible to inject
spin polarized currents through the impurity using a scanning tunneling
microscope (STM). Furthermore, the impurity scattering becomes strongly spin
dependent and for a finite impurity concentration it leads to spin polarized
bulk currents and a large magnetoresistance. In gated graphene the impurity
spin is Kondo screened at low temperatures. However, at temperatures larger
than the Kondo temperature, the anomalous magnetotransport properties are
recovered.Comment: 4+ pages, 4 figures. Added reference
Partial preservation of chiral symmetry and colossal magnetoresistance in adatom doped graphene
We analyze the electronic properties of adatom doped graphene in the low
impurity concentration regime. We focus on the Anderson localized regime and
calculate the localization length () as a function of the electron doping
and an external magnetic field. The impurity states hybridize with carbon's
states and form a partially filled band close to the Dirac point. Near
the impurity band center, the chiral symmetry of the system's effective
Hamiltonian is partially preserved which leads to a large enhancement of .
The sensitivity of transport properties, namely Mott's variable range hopping
scale , to an external magnetic field perpendicular to the graphene sheet
leads to a colossal magnetoresistance effect, as observed in recent
experiments.Comment: 5 pages, 4 figs. Few comments and references added. To appear in PR
Impurities and electronic localization in graphene bilayers
We analyze the electronic properties of bilayer graphene with Bernal stacking
and a low concentration of adatoms. Assuming that the host bilayer lies on top
of a substrate, we consider the case where impurities are adsorbed only on the
upper layer. We describe non-magnetic impurities as a single orbital hybridized
with carbon's pz states. The effect of impurity doping on the local density of
states with and without a gated electric field perpendicular to the layers is
analyzed. We look for Anderson localization in the different regimes and
estimate the localization length. In the biased system, the field induced gap
is partially filled by strongly localized impurity states. Interestingly, the
structure, distribution and localization length of these states depend on the
field polarization.Comment: 7 pages, 6 figure
Transverse Electron Focusing in Systems with Spin-Orbit Coupling
We study the transverse electron focusing in a two dimensional electron gas
with Rashba spin-orbit coupling. We show that the interplay between the
external magnetic field and the spin-orbit coupling gives two branches of
states with different cyclotron radius within the same energy window. This
effect generates a splitting of the first focusing peak in two contributions.
Each one of these contributions is spin polarized. The surface reflection mixes
the two branches and the second focusing peak does not present the same effect.
While for GaAs/AlGaAs heterostructures the effect is small, in systems like
InSb/InAlSb the effect should be clearly observed.Comment: 5 pages, 5 figures, to appear in PRB(RC
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