763 research outputs found
Ferroelectric Phase Transitions from First Principles
An effective Hamiltonian for the ferroelectric transition in is
constructed from first-principles density-functional-theory total-energy and
linear-response calculations through the use of a localized, symmetrized basis
set of ``lattice Wannier functions.'' Preliminary results of Monte Carlo
simulations for this system show a first-order cubic-tetragonal transition at
660 K. The involvement of the Pb atom in the lattice instability and the
coupling of local distortions to strain are found to be particularly important
in producing the behavior characteristic of the transition. A
tentative explanation for the presence of local distortions experimentally
observed above is suggested. Further applications of this method to a
variety of systems and structures are proposed for first-principles study of
finite-temperature structural properties in individual materials.Comment: 14 pages, harvmac, 4 uuencoded figure
Magnetic-induced phonon anisotropy in ZnCrO from first principles
We have studied the influence of magnetic order on the optical phonons of the
geometrically frustrated spinel ZnCrO from first-principles. By mapping
the first-principles phonon calculations onto a Heisenberg-like model, we
developed a method to calculate exchange derivatives and subsequently the
spin-phonon couping parameter from first-principles. All calculations were
performed within LSDA+U
Dynamic Behavior in Piezoresponse Force Microscopy
Frequency dependent dynamic behavior in Piezoresponse Force Microscopy (PFM)
implemented on a beam-deflection atomic force microscope (AFM) is analyzed
using a combination of modeling and experimental measurements. The PFM signal
comprises contributions from local electrostatic forces acting on the tip,
distributed forces acting on the cantilever, and three components of the
electromechanical response vector. These interactions result in the bending and
torsion of the cantilever, detected as vertical and lateral PFM signals. The
relative magnitudes of these contributions depend on geometric parameters of
the system, the stiffness and frictional forces of tip-surface junction, and
operation frequencies. The dynamic signal formation mechanism in PFM is
analyzed and conditions for optimal PFM imaging are formulated. The
experimental approach for probing cantilever dynamics using frequency-bias
spectroscopy and deconvolution of electromechanical and electrostatic contrast
is implemented.Comment: 65 pages, 15 figures, high quality version available upon reques
Materials Characterization Using High-Frequency Atomic Force Microscopy and Friction Force Microscopy
During the last decade, Atomic Force Microscopy (AFM) has been widely used to image the topography of various surfaces with corrugations down to the atomic scale [1,2]. Since then, development of new techniques based on AFM has been conducted to evaluate physical, chemical or mechanical surface properties [3]. We describe the use of near-field acoustic microscopy, based on AFM and hereafter referred to as Acoustic Microscopy by Atomic Force Microscopy (AFAM), as it has been developed earlier [4]. The relevance of this new scanning probe microscopy for high-resolution nondestructive testing and evaluation purposes is pointed out. It is shown that AFAM is capable of measuring elasticity on surfaces with a spatial resolution of less than 100 nm. Subsurface elastic properties and subsurface microdefect characterization can be performed by this technique. The high frequency Friction Force Microscopy (FFM) image, hereafter called Acoustic Friction Force Micropscopy (AFFM), reveals information different from the conventionally taken friction force image. We describe experimental and theoretical aspects of high-frequency atomic force and friction force microscopy
Localized Basis for Effective Lattice Hamiltonians: Lattice Wannier Functions
A systematic method is presented for constructing effective Hamiltonians for
general phonon-related structural transitions. The key feature is the
application of group theoretical methods to identify the subspace in which the
effective Hamiltonian acts and construct for it localized basis vectors, which
are the analogue of electronic Wannier functions. The results of the symmetry
analysis for the perovskite, rocksalt, fluorite and A15 structures and the
forms of effective Hamiltonians for the ferroelectric transition in
and , the oxygen-octahedron rotation transition in , the
Jahn-Teller instability in and the
antiferroelectric transition in are discussed. For the oxygen-
octahedron rotation transition in , this method provides an
alternative to the rotational variable approach which is well behaved
throughout the Brillouin zone. The parameters appearing in the Wannier basis
vectors and in the effective Hamiltonian, given by the corresponding invariant
energy expansion, can be obtained for individual materials using first-
principles density-functional-theory total energy and linear response
techniques, or any technique that can reliably calculate force constants and
distortion energies. A practical approach to the determination of these
parameters is presented and the application to ferroelectric
discussed.Comment: extensive revisions in presentation, 32 pages, Revtex, 7 Postscript
figure
Deep Learning for Temporal Logics
Temporal logics are a well established formal specification paradigm to specify the behavior of systems, and serve as inputs to industrial-strength verification tools. We report on current advances in applying deep learning to temporal logical reasoning tasks, showing that models can even solve instances where competitive classical algorithms timed out
- …