16,030 research outputs found
First-Principles Calculation of Electric Field Gradients and Hyperfine Couplings in YBa2Cu3O7
The local electronic structure of YBa2Cu3O7 has been calculated using
first-principles cluster methods. Several clusters embedded in an appropriate
background potential have been investigated. The electric field gradients at
the copper and oxygen sites are determined and compared to previous theoretical
calculations and experiments. Spin polarized calculations with different spin
multiplicities have enabled a detailed study of the spin density distribution
to be made and a simultaneous determination of magnetic hyperfine coupling
parameters. The contributions from on-site and transferred hyperfine fields
have been disentangled with the conclusion that the transferred spin densities
essentially are due to nearest neighbour copper ions only with marginal
influence of ions further away. This implies that the variant temperature
dependencies of the planar copper and oxygen NMR spin-lattice relaxation rates
are only compatible with commensurate antiferromagnetic correlations. The
theoretical hyperfine parameters are compared with those derived from
experimental data.Comment: 14 pages, 12 figures, accepted to appear in EPJ
Broadband Infrasound Signal of a Collapsing Hanging Glacier
A major ice collapse ((Formula presented.) 10,000 (Formula presented.)) from a hanging glacier on Mount Eiger, Switzerland was recorded by a small aperture array as a broadband (0.1–10 Hz) infrasound signal. Array analysis reveals that the high ((Formula presented.) 3 Hz) frequency signal is infrasound produced by the moving ice mass, and its back azimuth variation with time tracks the ice mass trajectory and provides a mean velocity estimate. Infrasound frequency is used to estimate a radius, that despite overestimating the volume, provides quantitative analysis in near-real time. The low ((Formula presented.) 0.1 Hz) frequency oscillation is modeled in terms of the velocity field (wind), which the moving ice mass induces on the surrounding air, producing pressure variations at the different elements. These results show how infrasound array observations may provide quantitative information of glacier collapse and ice avalanche trajectories, and possibly, volume.ISSN:0094-8276ISSN:1944-800
Spin dynamics of electrons in the first excited subband of a high-mobility low-density 2D electron system
We report on time-resolved Kerr rotation measurements of spin coherence of
electrons in the first excited subband of a high-mobility low-density
two-dimensional electron system in a GaAs/Al0.35Ga0.65As heterostructure. While
the transverse spin lifetime (T2*) of electrons decreases monotonically with
increasing magnetic field, it has a non-monotonic dependence on the
temperature, with a peak value of 596 ps at 36 K, indicating the effect of
inter-subband electron-electron scattering on the electron spin relaxation. The
spin lifetime may be long enough for potential device application with
electrons in excited subbands
First principles study of local electronic and magnetic properties in pure and electron-doped NdCuO
The local electronic structure of Nd2CuO4 is determined from ab-initio
cluster calculations in the framework of density functional theory.
Spin-polarized calculations with different multiplicities enable a detailed
study of the charge and spin density distributions, using clusters that
comprise up to 13 copper atoms in the CuO2plane. Electron doping is simulated
by two different approaches and the resulting changes in the local charge
distribution are studied in detail and compared to the corresponding changes in
hole doped La2CuO4. The electric field gradient (EFG) at the copper nucleus is
investigated in detail and good agreement is found with experimental values. In
particular the drastic reduction of the main component of the EFG in the
electron-doped material with respect to LaCuO4 is explained by a reduction of
the occupancy of the 3d3z^2-r^2 atomic orbital. Furthermore, the chemical
shieldings at the copper nucleus are determined and are compared to results
obtained from NMR measurements. The magnetic hyperfine coupling constants are
determined from the spin density distribution
Knight Field Enabled Nuclear Spin Polarization in Single Quantum Dots
We demonstrate dynamical nuclear spin polarization in the absence of an
external magnetic field, by resonant circularly polarized optical excitation of
a single electron or hole charged quantum dot. Optical pumping of the electron
spin induces an effective inhomogeneous magnetic (Knight) field that determines
the direction along which nuclear spins could polarize and enables nuclear-spin
cooling by suppressing depolarization induced by nuclear dipole-dipole
interactions. Our observations suggest a new mechanism for spin-polarization
where spin exchange with an electron reservoir plays a crucial role. These
experiments constitute a first step towards quantum measurement of the
Overhauser field.Comment: 5 pages, 3 figure
Power dependence of pure spin current injection by quantum interference
We investigate the power dependence of pure spin current injection in GaAs
bulk and quantum-well samples by a quantum interference and control technique.
Spin separation is measured as a function of the relative strength of the two
transition pathways driven by two laser pulses. By keeping the relaxation time
of the current unchanged, we are able to relate the spin separation to the
injected average velocity. We find that the average velocity is determined by
the relative strength of the two transitions in the same way as in classical
interference. Based on this, we conclude that the density of injected pure spin
current increases monotonically with the excitation laser intensities. The
experimental results are consistent with theoretical calculations based on
Fermi's golden rule.Comment: 6 pages, 4 figure
Selective Dynamic Nuclear Spin Polarization in Spin-Blocked Double-Dot
We study the mechanism of dynamical nuclear spin polarization by hyperfine
interaction in spin-blocked double quantum dot system. We calculate the
hyperfine transition rates and solve the master equations for the nuclear
spins. Specifically, we incorporate the effects of the nuclear quadrupole
coupling due to the doping-induced local lattice distortion and strain. Our
results show that nuclear quadrupole coupling induced by the 5% indium
substitution can be used to explain the recent experimental observation of
missing arsenic NMR signal in the spin-blocked double dots.Comment: 4 pages, 3 figure
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