321 research outputs found
Atomic-scale compensation phenomena at polar interfaces
The interfacial screening charge that arises to compensate electric fields of
dielectric or ferroelectric thin films is now recognized as the most important
factor in determining the capacitance or polarization of ultrathin
ferroelectrics. Here we investigate using aberration-corrected electron
microscopy and density functional theory how interfaces cope with the need to
terminate ferroelectric polarization. In one case, we show evidence for ionic
screening, which has been predicted by theory but never observed. For a
ferroelectric film on an insulating substrate, we found that compensation can
be mediated by interfacial charge generated, for example, by oxygen vacancies.Comment: 3 figure
Spin-orbit interaction from low-symmetry localized defects in semiconductors
The presence of low-symmetry impurities or defect complexes in the
zinc-blende direct-gap semiconductors (e.g. interstitials, DX-centers) results
in a novel spin-orbit term in the effective Hamiltonian for the conduction
band. The new extrinsic spin-orbit interaction is proportional to the matrix
element of the defect potential between the conduction and the valence bands.
Because this interaction arises already in the first order of the expansion of
the effective Hamiltonian in powers of Uext/Eg << 1 (where Uext is the
pseudopotential of an interstitial atom, and Eg is the band gap), its
contribution to the spin relaxation rate may exceed that of the previously
studied extrinsic contributions, even for moderate concentrations of
impurities.Comment: extended version, 5+ page
Structural “δ doping” to control local magnetization in isovalent oxide heterostructures
Modulation and
δ
-doping strategies, in which atomically thin layers of charged dopants are precisely deposited within a heterostructure, have played enabling roles in the discovery of new physical behavior in electronic materials. Here, we demonstrate a purely structural “
δ
-doping” strategy in complex oxide heterostructures, in which atomically thin manganite layers are inserted into an isovalent manganite host, thereby modifying the local rotations of corner-connected
MnO
6
octahedra. Combining scanning transmission electron microscopy, polarized neutron reflectometry, and density functional theory, we reveal how local magnetic exchange interactions are enhanced within the spatially confined regions of suppressed octahedral rotations. The combined experimental and theoretical results illustrate the potential to utilize noncharge-based approaches to “doping” in order to enhance or suppress functional properties within spatially confined regions of oxide heterostructures
Coupled-barrier diffusion: the case of oxygen in silicon
Oxygen migration in silicon corresponds to an apparently simple jump between
neighboring bridge sites. Yet, extensive theoretical calculations have so far
produced conflicting results and have failed to provide a satisfactory account
of the observed eV activation energy. We report a comprehensive set of
first-principles calculations that demonstrate that the seemingly simple oxygen
jump is actually a complex process involving coupled barriers and can be
properly described quantitatively in terms of an energy hypersurface with a
``saddle ridge'' and an activation energy of eV. Earlier
calculations correspond to different points or lines on this hypersurface.Comment: 4 Figures available upon request. Accepted for publication in Phys.
Rev. Let
Thermodynamic Behavior of a Model Covalent Material Described by the Environment-Dependent Interatomic Potential
Using molecular dynamics simulations we study the thermodynamic behavior of a
single-component covalent material described by the recently proposed
Environment-Dependent Interatomic Potential (EDIP). The parameterization of
EDIP for silicon exhibits a range of unusual properties typically found in more
complex materials, such as the existence of two structurally distinct
disordered phases, a density decrease upon melting of the low-temperature
amorphous phase, and negative thermal expansion coefficients for both the
crystal (at high temperatures) and the amorphous phase (at all temperatures).
Structural differences between the two disordered phases also lead to a
first-order transition between them, which suggests the existence of a second
critical point, as is believed to exist for amorphous forms of frozen water.
For EDIP-Si, however, the unusual behavior is associated not only with the open
nature of tetrahedral bonding but also with a competition between four-fold
(covalent) and five-fold (metallic) coordination. The unusual behavior of the
model and its unique ability to simulation the liquid/amorphous transition on
molecular-dynamics time scales make it a suitable prototype for fundamental
studies of anomalous thermodynamics in disordeered systems.Comment: 48 pages (double-spaced), 13 figure
Optical to UV spectra and birefringence of SiO and TiO: First-principles calculations with excitonic effects
A first principles approach is presented for calculations of optical --
ultraviolet (UV) spectra including excitonic effects. The approach is based on
Bethe-Salpeter equation calculations using the \textsc{NBSE} code combined with
ground-state density-functional theory calculations from the electronic
structure code \textsc{ABINIT}. Test calculations for bulk Si are presented,
and the approach is illustrated with calculations of the optical spectra and
birefringence of -phase SiO and the rutile and anatase phases of
TiO. An interpretation of the strong birefringence in TiO is presented.Comment: 8 figure
Auxiliary-level-assisted operations with charge qubits in semiconductors
We present a new scheme for rotations of a charge qubit associated with a
singly ionized pair of donor atoms in a semiconductor host. The logical states
of such a qubit proposed recently by Hollenberg et al. are defined by the
lowest two energy states of the remaining valence electron localized around one
or another donor. We show that an electron located initially at one donor site
can be transferred to another donor site via an auxiliary molecular level
formed upon the hybridization of the excited states of two donors. The electron
transfer is driven by a single resonant microwave pulse in the case that the
energies of the lowest donor states coincide or two resonant pulses in the case
that they differ from each other. Depending on the pulse parameters, various
one-qubit operations, including the phase gate, the NOT gate, and the Hadamard
gate, can be realized in short times. Decoherence of an electron due to the
interaction with acoustic phonons is analyzed and shown to be weak enough for
coherent qubit manipulation being possible, at least in the proof-of-principle
experiments on one-qubit devices.Comment: Extended version of cond-mat/0411605 with detailed discussion of
phonon-induced decoherence including dephasing and relaxation; to be
published in JET
Emerging Diluted Ferromagnetism in High-T-c Superconductors Driven by Point Defect Clusters
Defects in ceramic materials are generally seen as detrimental to their functionality and applicability. Yet, in some complex oxides, defects present an opportunity to enhance some of their properties or even lead to the discovery of exciting physics, particularly in the presence of strong correlations. A paradigmatic case is the high-temperature superconductor YBa2Cu3O7-delta(Y123), in which nanoscale defects play an important role as they can immobilize quantized magnetic flux vortices. Here previously unforeseen point defects buried in Y123 thin films that lead to the formation of ferromagnetic clusters embedded within the superconductor are unveiled. Aberration-corrected scanning transmission microscopy has been used for exploring, on a single unit-cell level, the structure and chemistry resulting from these complex point defects, along with density functional theory calculations, for providing new insights about their nature including an unexpected defect-driven ferromagnetism, and X-ray magnetic circular dichroism for bearing evidence of Cu magnetic moments that align ferromagnetically even below the superconducting critical temperature to form a dilute system of magnetic clusters associated with the point defects
Measuring the decoherence rate in a semiconductor charge qubit
We describe a method by which the decoherence time of a solid state qubit may
be measured. The qubit is coded in the orbital degree of freedom of a single
electron bound to a pair of donor impurities in a semiconductor host. The qubit
is manipulated by adiabatically varying an external electric field. We show
that, by measuring the total probability of a successful qubit rotation as a
function of the control field parameters, the decoherence rate may be
determined. We estimate various system parameters, including the decoherence
rates due to electromagnetic fluctuations and acoustic phonons. We find that,
for reasonable physical parameters, the experiment is possible with existing
technology. In particular, the use of adiabatic control fields implies that the
experiment can be performed with control electronics with a time resolution of
tens of nanoseconds.Comment: 9 pages, 6 figures, revtex
- …