40 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
Effects of Single-site Anisotropy on Mixed Diamond Chains with Spins 1 and 1/2
Effects of single-site anisotropy on mixed diamond chains with spins 1 and
1/2 are investigated in the ground states and at finite temperatures. There are
phases where the ground state is a spin cluster solid, i.e., an array of
uncorrelated spin-1 clusters separated by singlet dimers. The ground state is
nonmagnetic for the easy-plane anisotropy, while it is paramagnetic for the
easy-axis anisotropy. Also, there are the N\'eel, Haldane, and large-
phases, where the ground state is a single spin cluster of infinite size and
the system is equivalent to the spin-1 Heisenberg chain with alternating
anisotropy. The longitudinal and transverse susceptibilities and entropy are
calculated at finite temperatures in the spin-cluster-solid phases. Their
low-temperature behaviors are sensitive to anisotropy.Comment: 8 pages, 4 figure
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
Finite Temperature Properties of the Mixed Diamond Chain with Spins 1 and 1/2
We formulate statistical mechanics for the mixed diamond chain with spins of
magnitudes 1 and 1/2. Owing to a series of conservation laws, any eigenstate of
this system is decomposed into eigenstates of finite odd-length spin-1 chains.
The ground state undergoes five quantum phase transitions with varying the
parameter controlling frustration. We obtain the values of the
residual entropy and the Curie constant which characterize each phase and phase
boundary at low temperatures. We further find various characteristic
finite-temperature properties such as the nonmonotonic temperature dependence
of the magnetic susceptibility, the multipeak structure in the
-dependence of entropy, the plateau-like temperature dependence of
entropy and the multipeak structure of specific heat.Comment: 23 pages, 10 figure
Infrared absorption by a tunneling proton in crystalline 5-bromo-9-hydroxyphenalenone
5-bromo-9-hydroxyphenalenone absorbs infrared radiation
strongly
at when cooled to 5 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 34 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