5,328 research outputs found
A deterministic alternative to the full configuration interaction quantum Monte Carlo method
Development of exponentially scaling methods has seen great progress in
tackling larger systems than previously thought possible. One such technique,
full configuration interaction quantum Monte Carlo, is a useful algorithm that
allows exact diagonalization through stochastically sampling determinants. The
method derives its utility from the information in the matrix elements of the
Hamiltonian, along with a stochastic projected wave function, to find the
important parts of Hilbert space. However, the stochastic representation of the
wave function is not required to search Hilbert space efficiently, and here we
describe a highly efficient deterministic method to achieve chemical accuracy
for a wide range of systems, including the difficult Cr dimer. In
addition our method also allows efficient calculation of excited state
energies, for which we illustrate with benchmark results for the excited states
of C.Comment: 4 pages, 2 figure
Magnetic ground state of pyrochlore oxides close to metal-insulator boundary probed by muon spin rotation
Magnetism of ruthernium pyrochlore oxides A2Ru2O7 (A = Hg, Cd, Ca), whose
electronic properties within a localized ion picture are characterized by
non-degenerate t2g orbitals (Ru5+, 4d3) and thereby subject to geometrical
frustration, has been investigated by muon spin rotation/relaxation (muSR)
technique. The A cation (mostly divalent) was varied to examine the effect of
covalency (Hg > Cd > Ca) on their electronic property. In a sample with A = Hg
that exhibits a clear metal-insulator (MI) transition below >> 100 K (which is
associated with a weak structural transition), a nearly commensurate magnetic
order is observed to develop in accordance with the MI transition. Meanwhile,
in the case of A = Cd where the MI transition is suppressed to the level of
small anomaly in the resistivity, the local field distribution probed by muon
indicates emergence of a certain magnetic inhomogeneity below {\guillemotright}
30 K. Moreover, in Ca2Ru2O7 that remains metallic, we find a highly
inhomogeneous local magnetism below >>25 K that comes from randomly oriented Ru
moments and thus described as a "frozen spin liquid" state. The systematic
trend of increasing randomness and itinerant character with decreasing
covalency suggests close relationship between these two characters. As a
reference for the effect of orbital degeneracy and associated Jahn-Teller
instability, we examine a tetravalent ruthernium pyrochlore, Tl2Ru2O7 (Ru4+,
4d4), where the result of muSR indicates a non-magnetic ground state that is
consistent with the formation of the Haldane chains suggested by neutron
diffraction experiment.Comment: 12 pages, 13 figure
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