34 research outputs found
Ground states with cluster structures in a frustrated Heisenberg chain
We examine the ground state of a Heisenberg model with arbitrary spin S on a
one-dimensional lattice composed of diamond-shaped units. A unit includes two
types of antiferromagnetic exchange interactions which frustrate each other.
The system undergoes phase changes when the ratio between the
exchange parameters varies. In some phases, strong frustration leads to larger
local structures or clusters of spins than a dimer. We prove for arbitrary S
that there exists a phase with four-spin cluster states, which was previously
found numerically for a special value of in the S=1/2 case. For S=1/2
we show that there are three ground state phases and determine their
boundaries.Comment: 4 pages, uses revtex.sty, 2 figures available on request from
[email protected], to be published in J. Phys.: Cond. Mat
Bosonic representation of one-dimensional Heisenberg ferrimagnets
The energy structure and the thermodynamics of ferrimagnetic Heisenberg
chains of alternating spins S and s are described in terms of the Schwinger
bosons and modified spin waves. In the Schwinger representation, we average the
local constraints on the bosons and diagonalize the Hamiltonian at the
Hartree-Fock level. In the Holstein-Primakoff representation, we optimize the
free energy in two different ways introducing an additional constraint on the
staggered magnetization. A new modified spin-wave scheme, which employs a
Lagrange multiplier keeping the native energy structure free from temperature
and thus differs from the original Takahashi Scheme, is particularly stressed
as an excellent language to interpret one-dimensional quantum ferrimagnetism.
Other types of one-dimensional ferrimagnets and the antiferromagnetic limit S=s
are also mentioned.Comment: to be published in Phys. Rev. B 69, No. 6, 0644XX (2004
Infrared absorption by a tunneling proton in crystalline 5-bromo-9-hydroxyphenalenone
5-bromo-9-hydroxyphenalenone absorbs infrared radiation
strongly
at when cooled to 5Â K. The absorption is attributed to a
transition
from the ground state to the first excited state of a tunneling proton in
a
double-well potential of the intramolecular hydrogen bond. The temperature
dependence of the spectrum was studied in detail. The linewidth was
exceptionally large. Two mechanisms of the broadening are discussed. The
atomic motion in a deuterated crystal corresponding to the tunneling
freezes at 34Â K in a deuteration-induced phase transition. This is the
first spectroscopic evidence for the absorption of radiation by a
tunneling
proton in a neat molecular crystal