2,720 research outputs found
Effective Hamiltonians and dilution effects in kagome and related antiferromagnets
What is the zero-temperature ordering pattern of a Heisenberg antiferromagnet
with large spin length (and possibly small dilution), on the kagome
lattice, or others built from corner-sharing triangles and tetrahedra? First, I
summarize the uses of effective Hamiltonians to resolve the large ground-state
degeneracy, leading to long-range order of the usual kind. Secondly, I discuss
the effects of dilution, in particular to {\it non}-frustration of classical
ground states, in that every simplex of spins is optimally satisfied. Of three
explanations for this, the most complete is Moessner-Chalker
constraint-counting. Quantum zero-point energy may compete with classical
exchange energy in a diluted system, creating frustration and enabling a
spin-glass state. I suggest that the regime of over 97% occupation is
qualitatively different from the more strongly diluted regime.Comment: 11 pages; invited talk at "HFM 2000" (Waterloo, June 2000); submitted
to Can. J. Phy
Boson features in STM spectra of cuprate superconductors: Weak-coupling phenomenology
We derive the shape of the high-energy features due to a weakly coupled boson
in cuprate superconductors, as seen experimentally in Bi_2 Sr_2 Ca_1 Cu_2 O_8+x
(BSCCO) by Lee et al. [Nature (London) 442, 546 (2006)]. A simplified model is
used of d-wave Bogoliubov quasiparticles coupled to Einstein oscillators with a
momentum-independent electron-boson coupling and an analytic fitting form is
derived, which allows us (a) to extract the boson mode's frequency and (b) to
estimate the electron-boson coupling strength. We further calculate the maximum
possible superconducting gap due to an Einstein oscillator with the extracted
electron-boson coupling strength, which is found to be less than 0.2 times of
the observed gap indicating at the observed boson's non-dominant role in the
superconductivity's mechanism. The extracted momentum-independent
electron-boson coupling parameter (that we show a posteriori to indeed be in
the weak-coupling regime) is then to be interpreted as an (band-structure
detail dependent weighted) average over the Brillouin zone of the actual
momentum-dependent electron-boson coupling in BSCCO.Comment: 6.5 page
Hole on a stripe in a spinless fermion model
In the spinless fermion model on a square lattice with infinite
nearest-neighbor repulsion, holes doped into the half-filled ordered state form
stripes which, at low doping, are stable against phase separation into an
ordered state and a hole-rich metal. Here we consider transport of additional
holes along these stripes. The motion of a single hole on a stripe is mapped to
a one-dimensional problem, a variational wavefunction is constructed and the
energy spectrum is calculated and compared to energies obtained by exact
diagonalization.Comment: 7 Pages, 6 figures; Revised version accepted for publication in
Europhysics Letter
Temperature-dependent "phason" elasticity in a random tiling quasicrystal
Both ``phason'' elastic constants have been measured from Monte Carlo
simulations of a random-tiling icosahedral quasicrystal model with a
Hamiltonian. The low-temperature limit approximates the ``canonical-cell''
tiling used to describe several real quasicrystals. The elastic constant K2
changes sign from positive to negative with decreasing temperature; in the
``canonical-cell'' limit, K2/K1 appears to approach -0.7, about the critical
value for a phason-mode modulation instability. We compare to the experiments
on i-AlPdMn and i-AlCuFe.Comment: 5 pages, 2 Postscript figures, LaTeX, uses revtex4, submitted to PR
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