132 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
Attenuation of polarization echoes in NMR: A study of the emergence of dynamical irreversibility in many-body quantum systems
The reversal of the time evolution of the local polarization in an
interacting spin system involves a sign change of the effective dipolar
Hamiltonian which refocuses the 'spin diffusion' process generating a
polarization echo. Here, the attenuation of these echo amplitudes as a function
of evolution time is presented for cymantrene and ferrocene polycrystalline
samples, involving one and two five spin rings per molecule respectively. We
calculate the fraction of polarization which is not refocused because only the
secular part of the dipolar Hamiltonian is inverted. The results indicate that,
as long as the spin dynamics is restricted to a single ring, the non-inverted
part of the Hamiltonian is notable by itself to explain the whole decay of the
polarization echoes. A cross over from exponential (cymantrene) to Gaussian
(ferrocene) attenuation is experimentally observed. This is attributed to an
increase of the relative importance of the spin dynamics, as compared with
irreversible interactions, which favors dynamical irreversibility.Comment: 6 pages in Revtex, 11 Postscript figures. Final versio
Hierarchy of Floquet gaps and edge states for driven honeycomb lattices
Electromagnetic driving in a honeycomb lattice can induce gaps and
topological edge states with a structure of increasing complexity as the
frequency of the driving lowers. While the high frequency case is the most
simple to analyze we focus on the multiple photon processes allowed in the low
frequency regime to unveil the hierarchy of Floquet edge-states. In the case of
low intensities an analytical approach allows us to derive effective
Hamiltonians and address the topological character of each gap in a
constructive manner. At high intensities we obtain the net number of edge
states, given by the winding number, with a numerical calculation of the Chern
numbers of each Floquet band. Using these methods, we find a hierarchy that
resembles that of a Russian nesting doll. This hierarchy classifies the gaps
and the associated edge states in different orders according to the
electron-photon coupling strength. For large driving intensities, we rely on
the numerical calculation of the winding number, illustrated in a map of
topological phase transitions. The hierarchy unveiled with the low energy
effective Hamiltonians, alongside with the map of topological phase transitions
discloses the complexity of the Floquet band structure in the low frequency
regime. The proposed method for obtaining the effective Hamiltonian can be
easily adapted to other Dirac Hamiltonians of two dimensional materials and
even the surface of a 3D topological insulator.Comment: Phys. Rev. A 91, 04362
Spin-orbit induced chirality of Andreev states in Josephson junctions
We study Josephson junctions (JJs) in which the region between the two
superconductors is a multichannel system with Rashba spin-orbit coupling (SOC)
where a barrier or a quantum point contact (QPC) is present. These systems
might present unconventional Josephson effects such as Josephson currents for
zero phase difference or critical currents that \textit{depend on} the current
direction. Here, we discuss how the spin polarizing properties of the system in
the normal state affect the spin characteristic of the Andreev bound states
inside the junction. This results in a strong correlation between the spin of
the Andreev states and the direction in which they transport Cooper pairs.
While the current-phase relation for the JJ at zero magnetic field is
qualitatively unchanged by SOC, in the presence of a weak magnetic field a
strongly anisotropic behavior and the mentioned anomalous Josephson effects
follow. We show that the situation is not restricted to barriers based on
constrictions such as QPCs and should generically arise if in the normal system
the direction of the carrier's spin is linked to its direction of motion.Comment: 19 pages, 9 figures. To appear in PR
Spin Hall effect in clean two dimensional electron gases with Rashba spin-orbit coupling
We study the spin polarization induced by a current flow in clean two
dimensional electron gases with Rashba spin-orbit coupling. This geometric
effect originates from special properties of the electron's scattering at the
edges of the sample. In wide samples, the spin polarization has it largest
value at low energies (close to the bottom of the band) and goes to zero at
higher energies. In this case, the spin polarization is dominated by the
presence of evanescent modes which have an explicit spin component outside the
plane. In quantum wires, on the other hand, the spin polarization is dominated
by interference effects induced by multiple scattering at the edges. Here, the
spin polarization is quite sensitive to the value of the Fermi energy,
especially close to the point where a new channel opens up. We analyzed
different geometries and found that the spin polarization can be strongly
enhanced.Comment: 10 pages, 13 figures, high quality figures available upon reques
Gate induced enhancement of spin-orbit coupling in dilute fluorinated graphene
We analyze the origin of spin-orbit coupling (SOC) in fluorinated graphene
using Density Functional Theory (DFT) and a tight-binding model for the
relevant orbitals. As it turns out, the dominant source of SOC is the atomic
spin-orbit of fluorine adatoms and not the impurity induced SOC based on the
distortion of the graphene plane as in hydrogenated graphene. More
interestingly, our DFT calculations show that SOC is strongly affected by both
the type and concentrations of the graphene's carriers, being enhanced by
electron doping and reduced by hole doping. This effect is due to the charge
transfer to the fluorine adatom and the consequent change in the
fluorine-carbon bonding. Our simple tight-binding model, that includes the SOC
of the orbitals of F and effective parameters based on maximally localized
Wannier functions, is able to account for the effect. The strong enhancement of
the SOC induced by graphene doping opens the possibility to tune the spin
relaxation in this material.Comment: 9 pages, 8 figure
Diffusion of fluorine adatoms on doped graphene
We calculate the diffusion barrier of fluorine adatoms on doped graphene in
the diluted limit using Density Functional Theory. We found that the barrier
strongly depends on the magnitude and character of the graphene's
doping (): it increases for hole doping () and decreases
for electron doping (). Near the neutrality point the functional
dependence can be approximately by where
meVcm. This effect leads to significant
changes of the diffusion constant with doping even at room temperature and
could also affect the low temperature diffusion dynamics due to the presence of
substrate induced charge puddles. In addition, this might open up the
possibility to engineer the F dynamics on graphene by using local gates.Comment: 4 pages, 4 figure
- …