900 research outputs found
Toward Perfection: Kapellasite, Cu3Zn(OH)6Cl2, a New Model S = 1/2 Kagome Antiferromagnet
The search for the resonating valence bond (RVB) state continues to underpin
many areas of condensed matter research. The RVB is made from the dimerisation
of spins on different sites into fluctuating singlets, and was proposed by
Anderson to be the reference state from which the transition to BCS
superconductivity occurs. Little is known about the state experimentally, due
to the scarcity of model materials. Theoretical work has put forward the S =
1/2 kagome antiferromagnet (KAFM) as a good candidate for the realization of
the RVB state. In this paper we introduce a new model system, the S = 1/2 KAFM
Kapellasite, Cu3Zn(OH)6Cl2. We show that its crystal structure is a good
approximation to a 2-dimensional kagome antiferromagnet and that susceptibility
data indicate a collapse of the magnetic moment below T = 25 K that is
compatible with the spins condensing into the non-magnetic RVB state.Comment: Communication, 3 pages, 3 figure
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Screening of alternative technologies to incineration for treatment of chemical-agent-contaminated soil
As part of the Rocky Mountain Arsenal (RMA) Remedial Investigation/Feasibility Study, RMA has contracted Argonne National Laboratory to investigate potential remedial alternatives for the cleanup of agent-contaminated soils. The chemical agents of concern include levinstein mustard, lewisite, sarin, and VX. This investigation has been initially divided into three phases: (1) a literature search to determine what, if any, previous studies have been conducted; (2) a technologies-screening critique of remedial technologies as alternatives to incineration; and (3) an investigation of promising alternatives on RMA soil at the laboratory and bench-scale levels. This paper summarizes the document produced as a result of the technologies screening. The purpose of the document was to determine the applicability of 25 technologies to remediation of agent-contaminated soil for a general site. Technologies were critiqued on the basis of applicability to soil type, applicability to the agents of concern at RMA, applicability to other types of contaminants, cost of the treatment, current status of the technology, and residuals produced
Hole-doping effects on a frustrated spin ladder
Hole-doping effects are investigated on the {\it t-J} ladder model with the
linked-tetrahedra structure. We discuss how a metal-insulator transition occurs
upon hole doping with particular emphasis on the effects of geometrical
frustration. By computing the electron density and the spin correlation
function by the density matrix renormalization group, we show that strong
frustration triggers a first-order transition to a metallic phase, when holes
are doped into the plaquette-singlet phase. By examining spin excitations in a
metallic case in detail, we discuss whether the spin-gap phase persists upon
hole doping according to the strength of frustration. It is further shown that
the lowest excited state in a spin-gap metallic phase can be described in two
independent quasiparticles.Comment: 7 pages, 9 figure
Spectral functions in itinerant electron systems with geometrical frustration
The Hubbard model with geometrical frustration is investigated in a metallic
phase close to half-filling. We calculate the single particle spectral function
for the triangular lattice within dynamical cluster approximation, which is
further combined with non-crossing approximation and fluctuation exchange
approximation to treat the resulting cluster Anderson model. It is shown that
frustration due to non-local correlations suppresses short-range
antiferromagnetic fluctuations and thereby assists the formation of heavy
quasi-particles near half-filling.Comment: 4 pages, 5 eps figure
Hidden magnetic frustration by quantum relaxation in anisotropic Nd-langasite
The static and dynamic magnetic properties of the NdGaSiO
compound, which appears as the first materialization of a rare-earth
kagome-type lattice, were re-examined, owing to contradictory results in the
previous studies. Neutron scattering, magnetization and specific heat
measurements were performed and analyzed, in particular by fully taking account
of the crystal electric field effects on the Nd ions. One of the novel
findings is that the peculiar temperature independent spin dynamics observed
below 10 K expresses single-ion quantum processes. This would short-circuit the
frustration induced cooperative dynamics, which would emerge only at very low
temperature
Pyrochlore Antiferromagnet: A Three-Dimensional Quantum Spin Liquid
The quantum pyrochlore antiferromagnet is studied by perturbative expansions
and exact diagonalization of small clusters. We find that the ground state is a
spin-liquid state: The spin-spin correlation functions decay exponentially with
distance and the correlation length never exceeds the interatomic distance. The
calculated magnetic neutron diffraction cross section is in very good agreement
with experiments performed on Y(Sc)Mn2. The low energy excitations are
singlet-singlet ones, with a finite spin gap.Comment: 4 pages, 4 figure
Electron impact excitation cross sections for allowed transitions in atoms
We present a semiempirical Gaunt factor for widely used Van Regemorter
formula [Astrophys. J. 136, 906 (1962)] for the case of allowed transitions in
atoms with the LS coupling scheme. Cross sections calculated using this Gaunt
factor agree with measured cross sections to within the experimental error.Comment: RevTeX, 3 pages, 10 PS figures, 2 PS tables, submitted to Phys. Rev.
Thermodynamic Study of Excitations in a 3D Spin Liquid
In order to characterize thermal excitations in a frustrated spin liquid, we
have examined the magnetothermodynamics of a model geometrically frustrated
magnet. Our data demonstrate a crossover in the nature of the spin excitations
between the spin liquid phase and the high-temperature paramagnetic state. The
temperature dependence of both the specific heat and magnetization in the spin
liquid phase can be fit within a simple model which assumes that the spin
excitations have a gapped quadratic dispersion relation.Comment: 5 figure
Magnetic susceptibility of diluted pyrochlore and SCGO antiferromagnets
We investigate the magnetic susceptibility of the classical Heisenberg
antiferromagnet with nearest-neighbour interactions on the geometrically
frustrated pyrochlore lattice, for a pure system and in the presence of
dilution with nonmagnetic ions. Using the fact that the correlation length in
this system for small dilution is always short, we obtain an approximate but
accurate expression for the magnetic susceptibility at all temperatures. We
extend this theory to the compound SrCr_{9-9x}Ga_{3+9x}O_{19} (SCGO) and
provide an explanation of the phenomenological model recently proposed by
Schiffer and Daruka [Phys. Rev. B56, 13712 (1997)].Comment: 4 pages, Latex, 4 postscript figures automatically include
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