3,700 research outputs found
Coherent manipulation of electronic states in a double quantum dot
We investigate coherent time-evolution of charge states (pseudo-spin qubit)
in a semiconductor double quantum dot. This fully-tunable qubit is manipulated
with a high-speed voltage pulse that controls the energy and decoherence of the
system. Coherent oscillations of the qubit are observed for several
combinations of many-body ground and excited states of the quantum dots.
Possible decoherence mechanisms in the present device are also discussed.Comment: RevTe
Orbital Correlations in Doped Manganites
We review our recent x-ray scattering studies of charge and orbital order in
doped manganites, with specific emphasis on the role of orbital correlations in
Pr_1-xCa_xMnO_3. For x=0.25, we find an orbital structure indistinguishable
from the undoped structure with long range orbital order at low temperatures.
For dopings 0.3<x<0.5, we find scattering consistent with a charge and
orbitally ordered CE-type structure. While in each case the charge order peaks
are resolution limited, the orbital order exhibits only short range
correlations. We report the doping dependence of the correlation length and
discuss the connection between the orbital correlations and the finite magnetic
correlation length observed on the Mn^3+ sublattice with neutron scattering
techniques. The physical origin of these domains, which appear to be isotropic,
remains unclear. We find that weak orbital correlations persist well above the
phase transitions, with a correlation length of 1-2 lattice constants at high
temperatures. Significantly, we observe similar correlations at high
temperatures in La_0.7Ca_0.3MnO_3, which does not have an orbitally ordered
ground state, and we conclude that such correlations are robust to variations
in the relative strength of the electron-phonon coupling.Comment: 22 pagegs, 7 figure
Spin phonon coupling in frustrated magnet CdCrO
The infrared phonon spectrum of the spinel CdCr2O4 is measured as a function
temperature from 6 K to 300K. The triply degenerate Cr phonons soften in the
paramagnetic phase as temperature is lowered below 100 K and then split into a
singlet and doublet in the low T antiferromagnetic phase which is tetragonally
distorted to relieve the geometric frustration in the pyrochlore lattice of
Cr ions. The phonon splitting is inconsistent with the simple increase
(decrease) in the force constants due to deceasing (increasing) bond lengths in
the tetragonal phase. Rather they correspond to changes in the force constants
due to the magnetic order in the antiferromagnetic state. The phonon splitting
in this system is opposite of that observed earlier in ZnCr2O4 as predicted by
theory. The magnitude of the splitting gives a measure of the spin phonon
coupling strength which is smaller than in the case of ZnCr2O4.Comment: 4.2 pages, 4 figures, 1 reference added, submmite
Correlated Polarons in Dissimilar Perovskite Manganites
We report x-ray scattering studies of broad peaks located at a (0.5 0 0)/(0
0.5 0)-type wavevector in the paramagnetic insulating phases of
La_{0.7}Ca_{0.3}MnO_{3} and Pr_{0.7}Ca_{0.3}MnO_{3}. We interpret the
scattering in terms of correlated polarons and measure isotropic correlation
lengths of 1-2 lattice constants in both samples. Based on the wavevector and
correlation lengths, the correlated polarons are found to be consistent with
CE-type bipolarons. Differences in behavior between the samples arise as they
are cooled through their respective transition temperatures and become
ferromagnetic metallic (La_{0.7}Ca_{0.3}MnO_{3}) or charge and orbitally
ordered insulating (Pr_{0.7}Ca_{0.3}MnO_{3}). Since the primary difference
between the two samples is the trivalent cation size, these results illustrate
the robust nature of the correlated polarons to variations in the relative
strength of the electron-phonon coupling, and the sensitivity of the
low-temperature ground state to such variations.Comment: 13 pages, 6 figure
Temperature dependent d-d excitations in manganites probed by resonant inelastic x-ray scattering
We report the observation of temperature dependent electronic excitations in
various manganites utilizing resonant inelastic x-ray scattering (RIXS) at the
Mn K-edge. Excitations were observed between 1.5 and 16 eV with temperature
dependence found as high as 10 eV. The change in spectral weight between 1.5
and 5 eV was found to be related to the magnetic order and independent of the
conductivity. On the basis of LDA+U and Wannier function calculations, this
dependence is associated with intersite d-d excitations. Finally, the
connection between the RIXS cross-section and the loss function is addressed.Comment: 5 pages, 5 figure
Independent ferroelectric contributions and rare-earth-induced polarization reversal in multiferroic TbMn2O5
Three independent contributions to the magnetically induced spontaneous
polarization of multiferroic TbMn2O5 are uniquely separated by optical second
harmonic generation and an analysis in terms of Landau theory. Two of them are
related to the magnetic Mn3+/4+ order and are independent of applied fields of
up to 7 T. The third contribution is related to the long-range
antiferromagnetic Tb3+ order. It shows a drastic decrease upon the application
of a magnetic field and mediates the change of sign of the spontaneous electric
polarization in TbMn2O5. The close relationship between the rare-earth
long-range order and the non-linear optical properties points to isotropic
Tb-Tb exchange and oxygen spin polarization as mechanism for this rare-earth
induced ferroelectricity.Comment: 8 pages, 5 figure
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