8,455 research outputs found
Structural distortion and frustrated magnetic interactions in the layered copper oxychloride [CuCl]LaNb(2)O(7)
We present a computational study of the layered copper oxychloride
[CuCl]LaNb(2)O(7) that has been recently proposed as a spin-1/2 frustrated
square lattice compound. Our results evidence an orbitally degenerate ground
state for the reported tetragonal crystal structure and reveal a
Jahn-Teller-type structural distortion. This distortion heavily changes the
local environment of copper -- CuO(2)Cl(2) plaquettes are formed instead of
CuO(2)Cl(4) octahedra -- and restores the single-orbital scenario typical for
copper oxides and oxyhalides. The calculated distortion is consistent with the
available diffraction data and the experimental results on the electric field
gradients for the Cu and Cl sites. The band structure suggests a complex
three-dimensional spin model with the interactions up to the fourth neighbors.
Despite the layered structure of (CuCl)LaNb(2)O(7), the spin system has
pronounced one-dimensional features. Yet, sizable interchain interactions lead
to the strong frustration and likely cause the spin-gap behavior. Computational
estimates of individual exchange couplings are in qualitative agreement with
the experimental data.Comment: 13 pages, 9 figures, 3 table
Ab initio modeling of Bose-Einstein condensation in Pb2V3O9
We apply density functional theory band structure calculations and quantum
Monte Carlo simulations to investigate the Bose-Einstein condensation in the
spin-1/2 quantum magnet Pb2V3O9. In contrast to previous conjectures on the
one-dimensional nature of this compound, we present a quasi-two-dimensional
model of spin dimers with ferromagnetic and antiferromagnetic interdimer
couplings. Our model is well justified microscopically and provides a
consistent description of the experimental data on the magnetic susceptibility,
high-field magnetization, and field vs. temperature phase diagram. The
Bose-Einstein condensation in the quasi-two-dimensional spin system of Pb2V3O9
is largely governed by intralayer interactions, whereas weak interlayer
couplings have a moderate effect on the ordering temperature. The proposed
computational approach is an efficient tool to analyze and predict high-field
properties of quantum magnets.Comment: 6 pages, 6 figures, 1 tabl
Extensive Air Shower Radio Detection: Recent Results and Outlook
A prototype system for detecting radio pulses associated with extensive
cosmic ray air showers is described. Sensitivity is compared with that in
previous experiments, and lessons are noted for future studies.Comment: 17 pages, LaTeX, 7 figures, invited talk presented by J. Rosner at
RADHEP-2000 Conference, UCLA, Nov. 16-18, 2000, Proceedings published by AIP.
Some figures and text changed; one reference update
Large quantum fluctuations in the strongly coupled spin-1/2 chains of green dioptase: a hidden message from birds and trees
The green mineral dioptase Cu6Si6O18(H2O)6 has been known since centuries and
plays an important role in esoteric doctrines. In particular, the green
dioptase is supposed to grant the skill to speak with trees and to understand
the language of birds. Armed with natural samples of dioptase, we were able to
unravel the magnetic nature of the mineral (presumably with hidden support from
birds and trees) and show that strong quantum fluctuations can be realized in
an essentially framework-type spin lattice of coupled chains, thus neither
frustration nor low-dimensionality are prerequisites. We present a microscopic
magnetic model for the green dioptase. Based on full-potential DFT
calculations, we find two relevant couplings in this system: an
antiferromagnetic coupling J_c, forming spiral chains along the hexagonal c
axis, and an inter-chain ferromagnetic coupling J_d within structural Cu2O6
dimers. To refine the J_c and J_d values and to confirm the proposed spin
model, we perform quantum Monte-Carlo simulations for the dioptase spin
lattice. The derived magnetic susceptibility, the magnetic ground state, and
the sublattice magnetization are in remarkably good agreement with the
experimental data. The refined model parameters are J_c = 78 K and J_d = -37 K
with J_d/J_c ~ -0.5. Despite the apparent three-dimensional features of the
spin lattice and the lack of frustration, strong quantum fluctuations in the
system are evidenced by a broad maximum in the magnetic susceptibility, a
reduced value of the Neel temperature T_N ~ 15 K >> J_c, and a low value of the
sublattice magnetization m = 0.55 Bohr magneton. All these features should be
ascribed to the low coordination number of 3 that outbalances the
three-dimensional nature of the spin lattice.Comment: Dedicated to Stefan-Ludwig Drechsler on the occasion of his 60th
birthday (9 pages, 6 figures
The Effect of Ru substitution for Ni on the superconductivity in MgCNi3-xRux
The superconductor MgCNi3 has been chemically doped by partial substitution
of Ru for Ni in the solid solution MgCNi3-xRux for 0<x<0.5. Magnetic and
specific heat measurements show that the Sommerfeld parameter (gamma_exp) and
TC decrease immediately on Ru substitution, but that a TC above 2K is
maintained even for a relatively large decrease in gamma_exp. Ferromagnetism is
not observed to develop through Ru substitution, and the normal state magnetic
susceptibility is suppressed.Comment: 18 pages, 13 figure
Microscopic model of (CuCl)LaNb2O7: coupled spin dimers replace a frustrated square lattice
We present a microscopic model of the spin-gap quantum magnet (CuCl)LaNb2O7
that was previously suggested as a realization of the spin-1/2 frustrated
square lattice. Taking advantage of the precise atomic positions from recent
crystal structure refinement, we evaluate individual exchange integrals and
construct a minimum model that naturally explains all the available
experimental data. Surprisingly, the deviation from tetragonal symmetry leads
to the formation of spin dimers between fourth neighbors due to a Cu-Cl-Cl-Cu
pathway with a leading antiferromagnetic exchange J4 ~ 25 K. The total
interdimer exchange amounts to 12 - 15 K. Our model is in agreement with
inelastic neutron scattering results and is further confirmed by quantum
Monte-Carlo simulations of the magnetic susceptibility and the high-field
magnetization. Our results establish (CuCl)LaNb2O7 as a non-frustrated system
of coupled spin dimers with predominant antiferromagnetic interactions and
provide a general perspective for related materials with unusual
low-temperature magnetic properties.Comment: 4 pages, 4 figures, 1 table + supplementar
Comparisons of rational engineering correlations of thermophoretically-augmented particle mass transfer with STAN5-predictions for developing boundary layers
Modification of the code STAN5 to properly include thermophoretic mass transport, and examination of selected test cases developing boundary layers which include variable properties, viscous dissipation, transition to turbulence and transpiration cooling. Under conditions representative of current and projected GT operation, local application of St(M)/St(M),o correlations evidently provides accurate and economical engineering design predictions, especially for suspended particles characterized by Schmidt numbers outside of the heavy vapor range
- …