101 research outputs found
The magnetic and electronic structure of vanadyl pyrophosphate from density functional theory
We have studied the magnetic structure of the high
symmetry vanadyl pyrophosphate ((VO)_(2)P_(2)O)7, VOPO), focusing on the spin exchange couplings, using density functional theory (B3LYP) with the full three-dimensional periodicity. VOPO involves four distinct spin couplings: two larger couplings exist along the chain direction (a-axis), which we predict to be antiferromagnetic, J_(OPO) = ā156.8 K and J_O = ā68.6 K, and two weaker couplings appear along the c (between two layers) and b directions (between two chains in the same layer), which we calculate to be ferromagnetic, J_layer = 19.2 K and J_chain = 2.8 K. Based on the local density of states and the response of spin couplings to varying the cell parameter a, we found that J_(OPO) originates from a super-exchange interaction through the bridging āOāPāOā unit. In contrast, J_O results from a direct overlap of 3d_(x^2 ā y^2) orbitals on two vanadium atoms in the same V_(2)O_8 motif, making it very sensitive to structural fluctuations. Based on the variations in VāO bond length as a function of strain along a, we found that the VāO bonds of Vā(OPO)_(2)āV are covalent and rigid, whereas the bonds of Vā(O)_(2)āV are fragile and dative. These distinctions suggest that compression along the a-axis would have a dramatic impact on J_O, changing the magnetic structure and spin gap of VOPO. This result also suggests that assuming J_O to be a constant over the range of 2ā300 K whilst fitting couplings to the experimental magnetic susceptibility is an invalid method. Regarding its role as a catalyst, the bonding pattern suggests that O_2 can penetrate beyond the top layers of the VOPO surface, converting multiple V atoms from the +4 to +5 oxidation state, which seems crucial to explain the deep oxidation of n-butane to maleic anhydride
Ab Initio Evidence for the Formation of Impurity d(3z^2-r^2) Holes in Doped La_{2-x}Sr_xCuO_4
Using the spin unrestricted Becke-3-Lee-Yang-Parr density functional, we
computed the electronic structure of explicitly doped La_{2-x}Sr_xCuO_4 (x =
0.125, 0.25, and 0.5). At each doping level, an impurity hole band is formed
within the undoped insulating gap. This band is well-localized to CuO_6
octahedra adjacent to the Sr impurities. The nature of the impurity hole is
A_{1g} in symmetry, formed primarily from the z^2 orbital on the Cu and p_z
orbitals on the apical O's. There is a strong triplet coupling of this hole
with the intrinsic B_{1g} Cu x^2-y^2/O1 p_{sigma} hole on the same site.
Optimization of the c coordinate of the apical O's in the doped CuO_6
octahedron lead to an asymmetric anti-Jahn-Teller distortion of the O2 atoms
toward the central Cu. In particular, the O2 atom between the Cu and Sr is
displaced 0.26 A while the O2 atom between the Cu and La is displaced 0.10 A.
Contrary to expectations, investigation of a 0.1 A enhanced Jahn-Teller
distortion of this octahedron does not force formation of an x^2-y^2 hole, but
instead leads to migration of the z^2 hole to the four other CuO_6 octahedra
surrounding the Sr impurity. This latter observation offers a simple
explanation for the bifurcation of the Sr-O2 distance revealed in x-ray
absorption fine structure data.Comment: Submitted to Phys. Rev. B. See http://www.firstprinciples.com for
more informatio
Electron Correlation and the c-axis Dispersion of Cu d_z^2: a New Band Structure for High Temperature Superconductors
Previously we showed the major effect of electron correlation in the cuprate
superconductors is to lower the energy of the Cu d_x^2-y^2/O p_sigma (x^2-y^2)
band with respect to the Cu d_z^2/O' p_z (z^2) band. In our 2D Hubbard model
for La_1.85Sr_0.15CuO_4 (LaSCO), the z^2 band is narrow and crosses the
standard x^2-y^2 band just below the Fermi level. In this work, we introduce
c-axis dispersion to the model and find the z^2 band to have considerable
anisotropic 3D character. An additional hole-like surface opens up in the z^2
band at (0,0,2pi/c) which expands with doping. At sufficient doping levels, a
symmetry allowed x^2-y^2/z^2 band crossing along the (0,0)-(pi,pi) direction of
the Brillouin zone appears at the Fermi level. At this point, Cooper pairs
between the two bands (e.g. (k uparrow x^2-y^2/k downarrow z^2)) can form,
providing the basis for the Interband Pairing Theory of superconductivity in
these materials.Comment: submitted to Phys. Rev. Lett. Related publications: Phys. Rev. B 58,
12303 (1998); Phys. Rev. B 58, 12323 (1998); cond-mat/9903088;
cond-mat/990310
Diffusion on a stepped substrate
We present results for collective diffusion of adatoms on a stepped substrate
with a submonolayer coverage. We study the combined effect of the additional
binding at step edge, the Schwoebel barrier, the enhanced diffusion along step
edges, and the finite coverage on diffusion as a function of step density. In
particular, we examine the crossover from step--dominated diffusion at high
step density to terrace-dominated behavior at low step density in a lattice-gas
model using analytical Green's function techniques and Monte Carlo simulations.
The influence of steps on diffusion is shown to be more pronounced than
previously anticipated.Comment: 4 pages, RevTeX, 3 Postscript figure
The NMR of High Temperature Superconductors without Anti-Ferromagnetic Spin Fluctuations
A microscopic theory for the NMR anomalies of the planar Cu and O sites in
superconducting La_1.85Sr_0.15CuO_4 is presented that quantitatively explains
the observations without the need to invoke anit-ferromagnetic spin
fluctuations on the planar Cu sites and its significant discrepancy with the
observed incommensurate neutron spin fluctuations. The theory is derived from
the recently published ab-initio band structure calculations that correct LDA
computations tendency to overestimate the self-coulomb repulsion for the
half-filled Cu d_x2-y2 orbital for these ionic systems. The new band structure
leads to two bands at the Fermi level with holes in the Cu d_z2 and apical O
p_z orbitals in addition to the standard Cu d_x2-y2 and planar O p_sigma
orbitals. This band structure is part of a new theory for the cuprates that
explains a broad range of experiments and is based upon the formation of Cooper
pairs comprised of a k up spin electron from one band and a -k down spin
electron from another band (Interband Pairing Model).Comment: In Press, Journal of Physical Chemistry. See also
http://www.firstprinciples.com. Minor changes to references and figure
readabilit
Origin of the Pseudogap in High-Temperature Cuprate Superconductors
Cuprate high-temperature superconductors exhibit a pseudogap in the normal
state that decreases monotonically with increasing hole doping and closes at x
\approx 0.19 holes per planar CuO2 while the superconducting doping range is
0.05 < x < 0.27 with optimal Tc at x \approx 0.16. Using ab initio quantum
calculations at the level that leads to accurate band gaps, we found that
four-Cu-site plaquettes are created in the vicinity of dopants. At x \approx
0.05 the plaquettes percolate, so that the Cu dx2y2/O p{\sigma} orbitals inside
the plaquettes now form a band of states along the percolating swath. This
leads to metallic conductivity and below Tc to superconductivity. Plaquettes
disconnected from the percolating swath are found to have degenerate states at
the Fermi level that split and lead to the pseudogap. The pseudogap can be
calculated by simply counting the spatial distribution of isolated plaquettes,
leading to an excellent fit to experiment. This provides strong evidence in
favor of inhomogeneous plaquettes in cuprates.Comment: 24 pages (4 pages main text plus 20 pages supplement
The Importance of Static Correlation in the Band Structure of High Temperature Superconductors
Recently we presented a new band structure for La(2-x)Sr(x)CuO(4) and other
high temperature superconductors in which a second narrow band was seen to
cross the primary band at the Fermi level. The existence of this second Fermi
level band is in complete disagreement with the commonly accepted LDA band
structure. Yet it provided a crucial piece of physics which led to an
explanation for superconductivity and other unusual phenomena in these
materials. In this work we present details as to the nature of the failure of
conventional methods in deriving the band structure of the cuprates. In
particular, we use a number of chemical analogues to describe the problem of
static correlation in the band structure calculations and show how this can be
corrected with the predictable outcome of a Fermi level band crossing.Comment: The Journal of Physical Chemistry, in press. References and figures
updated. See www.firstprinciples.com for more information related to this
wor
The Antiferromagnetic Band Structure of La2CuO4 Revisited
Using the Becke-3-LYP functional, we have performed band structure
calculations on the high temperature superconductor parent compound, La2CuO4.
Under the restricted spin formalism (rho(alpha) equal to rho(beta)), the
R-B3LYP band structure agrees well with the standard LDA band structure. It is
metallic with a single Cu x2-y2/O p(sigma) band crossing the Fermi level. Under
the unrestricted spin formalism (rho(alpha) not equal to rho(beta)), the UB3LYP
band structure has a spin polarized antiferromagnetic solution with a band gap
of 2.0 eV, agreeing well with experiment. This state is 1.0 eV (per formula
unit) lower than that calculated from the R-B3LYP. The apparent high energy of
the spin restricted state is attributed to an overestimate of on-site Coulomb
repulsion which is corrected in the unrestricted spin calculations. The
stabilization of the total energy with spin polarization arises primarily from
the stabilization of the x2-y2 band, such that the character of the eigenstates
at the top of the valence band in the antiferromagnetic state becomes a strong
mixture of Cu x2-y2/O p(sigma) and Cu z2/O' p(z). Since the Hohenberg-Kohn
theorem requires the spin restricted and spin unrestricted calculations give
exactly the same ground state energy and total density for the exact
functionals, this large disparity in energy reflects the inadequacy of current
functionals for describing the cuprates. This calls into question the use of
band structures based on current restricted spin density functionals (including
LDA) as a basis for single band theories of superconductivity in these
materials.Comment: 13 pages, 8 figures, to appear in Phys. Rev. B, for more information
see http://www.firstprinciples.co
Chiral Plaquette Polaron Theory of Cuprate Superconductivity
Ab-initio density functional calculations on explicitly doped
La(2-x)Sr(x)CuO4 find doping creates localized holes in out-of-plane orbitals.
A model for superconductivity is developed based on the assumption that doping
leads to the formation of holes on a four-site Cu plaquette composed of the
out-of-plane A1 orbitals apical O pz, planar Cu dz2, and planar O psigma. This
is in contrast to the assumption of hole doping into planar Cu dx2-y2 and O
psigma orbitals as in the t-J model. Interaction of holes with the d9 spin
background leads to chiral polarons with either a clockwise or anti-clockwise
charge current. When the polaron plaquettes percolate through the crystal at
x~0.05 for LaSrCuO, a Cu dx2-y2 and planar O psigma band is formed. Spin
exchange Coulomb repulsion with chiral polarons leads to D-wave
superconductivity. The equivalent of the Debye energy in phonon
superconductivity is the maximum energy separation between a chiral polaron and
its time-reversed partner. An additive skew-scattering contribution to the Hall
effect is induced by chiral polarons and leads to a temperature dependent Hall
effect that fits the measured values for LaSrCuO. The integrated imaginary
susceptibility satisfies omega/T scaling due to chirality and spin-flip
scattering of polarons along with a uniform distribution of polaron energy
splittings. The derived functional form is compatible with experiments. The
static spin structure factor is computed and is incommensurate with a
separation distance from (pi,pi) given by ~(2pi)x. Coulomb scattering of the
x2-y2 band with polarons leads to linear resistivity. Coupling of the x2-y2
band to the undoped Cu d9 spins leads to the ARPES pseudogap and its doping and
temperature dependence.Comment: 32 pages, 17 figure
The Design of the W. M. Keck Observatory Archive
The Michelson Science Center (MSC) and the W. M. Keck Observatory are building an archive that will serve data obtained at the Keck Observatory. The archive has begun operations and is ingesting Level 0 (uncalibrated) observations made with the recently upgraded High Resolution Echelle Spectrometer (HIRES); these observations will be publicly accessible after expiration of a proprietary period. Observatory staff have begun using the archived data to determine the long-term performance of the HIRES instrument. The archive is housed at the Michelson Science Center (MSC) and employs a modular design with the following components: (1) Data Evaluation and Preparation: images from the telescope are evaluated and native FITS headers are converted to metadata that will support archiving; (2) Trans Pacific Data Transfer: metadata are sent daily by e-mail and ingested into the archive in a highly fault tolerant fashion, and FITS images are written to DVDs and sent to MSC each week; (3) Science Information System: inherited from the NASA/IPAC Infrared Science Archive, it provides all the functionality needed to support database inquiries and processing of requests; and a Web-based (4) User Interface, a thin layer above the information system that accepts user requests and returns results. The design offers two major cost-saving benefits: it overcomes the geographical separation between the telescope and the archive and enables development at Keck and at MSC to proceed independently; and it permits direct inheritance of the IRSA architecture
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