5,153 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
Interplay of charge and spin correlations in nickel perovskites
Analyzing the motion of low--spin holes in a high--spin
background, we derive a sort of generalized t--J Hamiltonian for the planes of Sr--doped nickelates. In addition to the rather complex
carrier--spin and spin--spin couplings we take into account the coupling of the
doped holes to in--plane oxygen breathing modes by a Holstein--type interaction
term. Because of strong magnetic confinement effects the holes are nearly
entirely prelocalized and the electron--phonon coupling becomes much more
effective in forming polarons than in the isostructural cuprates. In the light
of recent experiments on we discuss how the variety of
the observed transport and charge/spin--ordering phenomena can be qualitatively
understood in terms of our model Hamiltonian.Comment: 2 pages, LTpaper.sty, Proc. XXI Int. Conf. on Low Temp. Phys. Prague
9
Melting of Quasi-Two-Dimensional Charge Stripes in La5/3Sr1/3NiO4
Commensurability effects for nickelates have been studied by the first
neutron experiments on La5/3Sr1/3NiO4. Upon cooling, this system undergoes
three successive phase transitions associated with quasi-two-dimensional (2D)
commensurate charge and spin stripe ordering in the NiO planes. The two
lower temperature phases (denoted as phase II and III) are stripe lattice
states with quasi-long-range in-plane charge correlation. When the lattice of
2D charge stripes melts, it goes through an intermediate glass state (phase I)
before becoming a disordered liquid state. This glass state shows short-range
charge order without spin order, and may be called a "stripe glass" which
resembles the hexatic/nematic state in 2D melting.Comment: 10 pages, RevTex, 4 figures available on request to
[email protected]
Self-vacancies in Gallium Arsenide: an ab initio calculation
We report here a reexamination of the static properties of vacancies in GaAs
by means of first-principles density-functional calculations using localized
basis sets. Our calculated formation energies yields results that are in good
agreement with recent experimental and {\it ab-initio} calculation and provide
a complete description of the relaxation geometry and energetic for various
charge state of vacancies from both sublattices. Gallium vacancies are stable
in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state
for intrinsic and n-type GaAs, confirming results from positron annihilation.
Interestingly, Arsenic vacancies show two successive negative-U transitions
making only +1, -1 and -3 charge states stable, while the intermediate defects
are metastable. The second transition (-/-3) brings a resonant bond relaxation
for V_As^-3 similar to the one identified for silicon and GaAs divacancies.Comment: 14 page
Magnetic excitations in the low-temperature ferroelectric phase of multiferroic YMn2O5 using inelastic neutron scattering
We studied magnetic excitations in a low-temperature ferroelectric phase of
the multiferroic YMn2O5 using inelastic neutron scattering (INS). We identify
low-energy magnon modes and establish a correspondence between the magnon peaks
observed by INS and electromagnon peaks observed in optical absorption [1].
Furthermore, we explain the microscopic mechanism, which results in the
lowest-energy electromagnon peak, by comparing the inelastic neutron spectral
weight with the polarization in the commensurate ferroelectric phase.Comment: 4 pages, 4 figure
Electromagnons in multiferroic YMn2O5 and TbMn2O5
Based on temperature dependent far infrared transmission spectra of YMn2O5
and TbMn2O5 single crystals, we report the observation of electric
dipole-active magnetic excitations, or electromagnons, in these multiferroics.
Electromagnons are found to be directly responsible for the step-like anomaly
of the static dielectric constant at the commensurate--incommensurate magnetic
transition and are the origin of the colossal magneto-dielectric effect
reported in these multiferroics.Comment: 4 pages, 4 figures, submitte
What Do You Know about Alternative Energy? Development and Use of a Diagnostic Instrument for Upper Secondary School Science
The need for renewable and non-fossil fuels is now recognised by nations throughout the world. Consequently, an understanding of alternative energy is needed both in schools and in everyday life-long learning situations. This study developed a two-tier instrument to diagnose students’ understanding and alternative conceptions about alternative energy in terms of: sources of alternative energy, greenhouse gas emission, as well as advantages, and disadvantages. Results obtained with Years 10 and 11 students (n = 491) using the 12-item two-tier instrument (α = 0.61) showed that students’ understanding of alternative energy was low (M = 7.03; SD = 3.90).The 23 alternative conceptions about alternative energy sources that could be identified from the instrument are reported. The implications for teaching and learning about alternative energy and suggestions for further development and improvement of the instrument are presented
- …