49 research outputs found
Unusual behaviour of the ferroelectric polarization in PbTiO/SrTiO superlattices
Artificial PbTiO/SrTiO superlattices were constructed using
off-axis RF magnetron sputtering. X-ray diffraction and piezoelectric atomic
force microscopy were used to study the evolution of the ferroelectric
polarization as the ratio of PbTiO to SrTiO was changed. For
PbTiO layer thicknesses larger than the 3-unit cells SrTiO
thickness used in the structure, the polarization is found to be reduced as the
PbTiO thickness is decreased. This observation confirms the primary role
of the depolarization field in the polarization reduction in thin films. For
the samples with ratios of PbTiO to SrTiO of less than one a
surprising recovery of ferroelectricity that cannot be explained by
electrostatic considerations was observed
Phase transitions in BaTiO from first principles
We develop a first-principles scheme to study ferroelectric phase transitions
for perovskite compounds. We obtain an effective Hamiltonian which is fully
specified by first-principles ultra-soft pseudopotential calculations. This
approach is applied to BaTiO, and the resulting Hamiltonian is studied
using Monte Carlo simulations. The calculated phase sequence, transition
temperatures, latent heats, and spontaneous polarizations are all in good
agreement with experiment. The order-disorder vs.\ displacive character of the
transitions and the roles played by different interactions are discussed.Comment: 13 page
Optimized local modes for lattice dynamical applications
We present a new scheme for the construction of highly localized lattice
Wannier functions. The approach is based on a heuristic criterion for
localization and takes the symmetry constraints into account from the start. We
compare the local modes thus obtained with those generated by other schemes and
find that they also provide a better description of the relevant vibrational
subspace.Comment: 6 pages, ReVTeX, plus four postscript files for figure
Novel A15 phase in barium-doped fullerite
A new stable compound Ba3C60 is reported in the Ba-C60 phase diagram. Rietveld refinement of x-ray powder diffraction data shows that this compound has the A15 structure with a lattice constant of 11.34. The Pm3Ě„n space group implies a perfect alternating orientational order for the C60 molecules, not previously observed in doped fullerite structures. The relative stability of the A15 phase over the fully intercalated fcc structure can be explained by a simple model involving the Madelung energy differences, orientation dependence of the C60-cation interaction energy, and distortion-induced relaxational energy gains
A tetragonal-to-monoclinic phase transition in a ferroelectric perovskite: the structure of PbZr(0.52)Ti(0.48)O3
The perovskite-like ferroelectric system PbZr(1-x)Ti(x)O3 (PZT) has a nearly
vertical morphotropic phase boundary (MPB) around x=0.45-0.50. Recent
synchrotron x-ray powder diffraction measurements by Noheda et al. [Appl. Phys.
Lett. 74, 2059 (1999)] have revealed a new monoclinic phase between the
previously-established tetragonal and rhombohedral regions. In the present work
we describe a Rietveld analysis of the detailed structure of the tetragonal and
monoclinic PZT phases on a sample with x= 0.48 for which the lattice parameters
are respectively: at= 4.044 A, ct= 4.138 A, at 325 K, and am= 5.721 A, bm=
5.708 A, cm= 4.138 A, beta= 90.496 deg., at 20K. In the tetragonal phase the
shifts of the atoms along the polar [001] direction are similar to those in
PbTiO3 but the refinement indicates that there are, in addition, local
disordered shifts of the Pb atoms of ~0.2 A perpendicular to the polar axis..
The monoclinic structure can be viewed as a condensation along one of the
directions of the local displacements present in the tetragonal phase. It
equally well corresponds to a freezing-out of the local displacements along one
of the directions recently reported by Corker et al.[J. Phys. Condens.
Matter 10, 6251 (1998)] for rhombohedral PZT. The monoclinic structure
therefore provides a microscopic picture of the MPB region in which one of the
"locally" monoclinic phases in the "average" rhombohedral or tetragonal
structures freezes out, and thus represents a bridge between these two phases.Comment: REVTeX, 7 figures. Modifications after referee's suggestion: new
figure (figure 5), comments in 2nd para. (Sect.III) and in 2nd & 3rd para.
(Sect. IV-a), in the abstract: "...of ~0.2 A perpendicular to the polar
axis.
Structure and properties of a novel fulleride Sm6C60
A novel fulleride Sm6C60 has been synthesized using high temperature solid
state reaction. The Rietveld refinement on high resolution synchrotron X-ray
powder diffraction data shows that Sm6C60 is isostructural with body-centered
cubic A6C60 (A=K, Ba). Raman spectrum of Sm6C60 is similar to that of Ba6C60,
and the frequencies of two Ag modes in Sm6C60 are nearly the same as that of
Ba6C60, suggesting that Sm is divalent and hybridization between C60 molecules
and the Sm atom could exist in Sm6C60. Resistivity measurement shows a weak
T-linear behavior above 180 K, the transport at low temperature is mainly
dominated by granular-metal theory.Comment: 9 pages, 3 figures, submitted to Phys. Rev. B (March 12, 1999
An Exact Diagonalization Demonstration of Incommensurability and Rigid Band Filling for N Holes in the t-J Model
We have calculated S(q) and the single particle distribution function
for N holes in the t - J model on a non--square sqrt{8} X sqrt{32} 16--site
lattice with periodic boundary conditions; we justify the use of this lattice
in compariosn to those of having the full square symmetry of the bulk. This new
cluster has a high density of vec k points along the diagonal of reciprocal
space, viz. along k = (k,k). The results clearly demonstrate that when the
single hole problem has a ground state with a system momentum of vec k =
(pi/2,pi/2), the resulting ground state for N holes involves a shift of the
peak of the system's structure factor away from the antiferromagnetic state.
This shift effectively increases continuously with N. When the single hole
problem has a ground state with a momentum that is not equal to k =
(pi/2,pi/2), then the above--mentioned incommensurability for N holes is not
found. The results for the incommensurate ground states can be understood in
terms of rigid--band filling: the effective occupation of the single hole k =
(pi/2,pi/2) states is demonstrated by the evaluation of the single particle
momentum distribution function . Unlike many previous studies, we show
that for the many hole ground state the occupied momentum states are indeed k =
(+/- pi/2,+/- pi/2) states.Comment: Revtex 3.0; 23 pages, 1 table, and 13 figures, all include
Maximally-localized Wannier functions for entangled energy bands
We present a method for obtaining well-localized Wannier-like functions (WFs)
for energy bands that are attached to or mixed with other bands. The present
scheme removes the limitation of the usual maximally-localized WFs method (N.
Marzari and D. Vanderbilt, Phys. Rev. B 56, 12847 (1997)) that the bands of
interest should form an isolated group, separated by gaps from higher and lower
bands everywhere in the Brillouin zone. An energy window encompassing N bands
of interest is specified by the user, and the algorithm then proceeds to
disentangle these from the remaining bands inside the window by filtering out
an optimally connected N-dimensional subspace. This is achieved by minimizing a
functional that measures the subspace dispersion across the Brillouin zone. The
maximally-localized WFs for the optimal subspace are then obtained via the
algorithm of Marzari and Vanderbilt. The method, which functions as a
postprocessing step using the output of conventional electronic-structure
codes, is applied to the s and d bands of copper, and to the valence and
low-lying conduction bands of silicon. For the low-lying nearly-free-electron
bands of copper we find WFs which are centered at the tetrahedral interstitial
sites, suggesting an alternative tight-binding parametrization.Comment: 13 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macro
Stability of the monoclinic phase in the ferroelectric perovskite PbZr(1-x)TixO3
Recent structural studies of ferroelectric PbZr(1-x)TixO3 (PZT) with x= 0.48,
have revealed a new monoclinic phase in the vicinity of the morphotropic phase
boundary (MPB), previously regarded as the the boundary separating the
rhombohedral and tetragonal regions of the PZT phase diagram. In the present
paper, the stability region of all three phases has been established from high
resolution synchrotron x-ray powder diffraction measurements on a series of
highly homogeneous samples with 0.42 <=x<= 0.52. At 20K the monoclinic phase is
stable in the range 0.46 <=x<= 0.51, and this range narrows as the temperature
is increased. A first-order phase transition from tetragonal to rhombohedral
symmetry is observed only for x= 0.45. The MPB, therefore, corresponds not to
the tetragonal-rhombohedral phase boundary, but instead to the boundary between
the tetragonal and monoclinic phases for 0.46 <=x<= 0.51. This result provides
important insight into the close relationship between the monoclinic phase and
the striking piezoelectric properties of PZT; in particular, investigations of
poled samples have shown that the monoclinic distortion is the origin of the
unusually high piezoelectric response of PZT.Comment: REVTeX file, 7 figures embedde
Strong-correlation effects in Born effective charges
Large values of Born effective charges are generally considered as reliable
indicators of the genuine tendency of an insulator towards ferroelectric
instability. However, these quantities can be very much influenced by strong
electron correlation and metallic behavior, which are not exclusive properties
of ferroelectric materials. In this paper we compare the Born effective charges
of some prototypical ferroelectrics with those of magnetic, non-ferroelectric
compounds using a novel, self-interaction free methodology that improves on the
local-density approximation description of the electronic properties. We show
that the inclusion of strong-correlation effects systermatically reduces the
size of the Born effective charges and the electron localization lengths.
Furthermore we give an interpretation of the Born effective charges in terms of
band energy structure and orbital occupations which can be used as a guideline
to rationalize their values in the general case.Comment: 10 pages, 4 postscript figure