43 research outputs found
From spinons to magnons in explicit and spontaneously dimerized antiferromagnetic chains
We reconsider the excitation spectra of a dimerized and frustrated
antiferromagnetic Heisenberg chain. This model is taken as the simpler example
of compiting spontaneous and explicit dimerization relevant for Spin-Peierls
compounds. The bosonized theory is a two frequency Sine-Gordon field theory. We
analize the excitation spectrum by semiclassical methods. The elementary
triplet excitation corresponds to an extended magnon whose radius diverge for
vanishing dimerization. The internal oscilations of the magnon give rise to a
series of excited state until another magnon is emited and a two magnon
continuum is reached. We discuss, for weak dimerization, in which way the
magnon forms as a result of a spinon-spinon interaction potential.Comment: 5 pages, latex, 3 figures embedded in the tex
Domain excitations in spin-Peierls systems
We study a model of a Spin-Peierls material consisting of a set of
antiferromagnetic Heisenberg chains coupled with phonons and interacting among
them via an inter-chain elastic coupling. The excitation spectrum is analyzed
by bosonization techniques and the self-harmonic approximation. The elementary
excitation is the creation of a localized domain structure where the dimerized
order is the opposite to the one of the surroundings. It is a triplet
excitation whose formation energy is smaller than the magnon gap. Magnetic
internal excitations of the domain are possible and give the further
excitations of the system. We discuss these results in the context of recent
experimental measurements on the inorganic Spin-Peierls compound CuGeOComment: 5 pages, 2 figures, corrected version to appear in Phys. Rev.
Submillimeter Wave ESR Study of Spin Gap Excitations in CuGeO3
Transitions between the ground singlet state to the excited triplet state has
been observed in CuGeO3 by means of submillimeter wave electron spin resonance.
The strong absorption intensity shows the break down of the selection rule. The
energy gap at zero field is evaluated to be 570 GHz(2.36 meV) and this value is
nearly identical to the gap at the zone center observed by inelastic neutron
scattering. The absorption intensity shows strong field orientation dependence
but shows no significant dependence on magnetic field intensity. These features
have been explained by considering the existence of Dzyaloshinsky-Moriya (DM)
antisymmetric exchange interaction. The doping effect on this singlet-triplet
excitation has been also studied. A drastic broadening of the absorption line
is observed by the doping of only 0.5 % of Si.Comment: 6 pages, 8figures submitted to J. Phys. Soc. Jp
Infrared signatures of the spin-Peierls transition in CuGeO3
We investigated the infrared reflectivity of several Mg- and Si-substituted
CuGeO3 single crystals. The temperature dependent b-axis and c-axis optical
response is reported. For T<Tsp we detected the activation of zone-boundary
phonons along the b axis of the crystal on the pure sample and for 1% Mg and
0.7% Si concentrations. From a detailed analysis of the phonon parameters the
redshift of the B2u mode at 48 cm^-1 is observed and discussed in relation to
the soft mode expected to drive the spin-Peierls phase transition in CuGeO3.
Moreover, the polarization dependence of a magnetic excitation measured in
transmission at 44 cm^-1 has been investigated.Comment: Revtex, 3 pages, 5 postscript pictures, submitted to PRB Rapid
Communication
Nonadiabatic Approach to Spin-Peierls Transitions via Flow Equations
The validity of the adiabatic approach to spin-Peierls transitions is
assessed. An alternative approach is developed which maps the initial
magneto-elastic problem to an effective magnetic problem only. Thus the
equivalence of magneto-elastic solitons and magnetic spinons is shown. No soft
phonon is required for the transition. Temperature dependent couplings are
predicted in accordance with the analysis of experimental data.Comment: Latex, 4 pages, Phys. Rev. B, Rap. Comm. in press final version
containing some clarification
Dynamical structure factors of the magnetization-plateau state in the bond-alternating spin chain with a next-nearest-neighbor interaction
We calculate the dynamical structure factors of the magnetization-plateau
state in the bond-alternating spin chain with a next-nearest-neighbor
interaction. The results show characteristic behaviors depending on the
next-nearest-neighbor interaction and the bond-alternation .
We discuss the lower excited states in comparison with the exact excitation
spectrums of an effective Hamiltonian. From the finite size effects,
characteristics of the lowest excited states are investigated. The
dispersionless mode of the lowest excitation appears in adequate sets of
and , indicating that the lowest excitation is localized
spatially and forms an isolated mode below the excitation continuum. We further
calculate the static structure factors. The largest intensity is located at
for small in fixed . With increasing , the
wavenumber of the largest intensity shifts towards , taking the
incommensurate value.Comment: to appear in Phys. Rev. B (2001
Far-Infrared Spectroscopy in Spin-Peierls Compound CuGeO_3 under High Magnetic Fields
Polarized far-infrared (FIR) spectroscopic measurements and FIR
magneto-optical studies were performed on the inorganic spin-Peierls compound
CuGeO_3. An absorption line, which was found at 98 cm in the dimerized
phase (D phase), was assigned to a folded phonon mode of B symmetry. The
splitting of the folded mode into two components in the incommensurate phase
(IC phase) has been observed for the first time. A new broad absorption
centered at 63 cm was observed only in the axis
polarization, which was assigned to a magnetic excitation from singlet ground
state to a continuum state.Comment: 9 pages multicolREVTeX, 10 figure
A Study of the S=1/2 Alternating Chain using Multiprecision Methods
In this paper we present results for the ground state and low-lying
excitations of the alternating Heisenberg antiferromagnetic chain. Our
more conventional techniques include perturbation theory about the dimer limit
and numerical diagonalization of systems of up to 28 spins. A novel application
of multiple precision numerical diagonalization allows us to determine
analytical perturbation series to high order; the results found using this
approach include ninth-order perturbation series for the ground state energy
and one magnon gap, which were previously known only to third order. We also
give the fifth-order dispersion relation and third-order exclusive neutron
scattering structure factor for one-magnon modes and numerical and analytical
binding energies of S=0 and S=1 two-magnon bound states.Comment: 16 pages, 9 figures. for submission to Phys.Rev.B. PICT files of figs
available at http://csep2.phy.ornl.gov/theory_group/people/barnes/barnes.htm
Spin Dynamics of the One-Dimensional J-J' Model and Spin-Peierls Transition in CuGeO_3
Spin dynamics as well as static properties of the one-dimensional J-J' model
(S=1/2, J>0 and 0\le \alpha=J'/J\le 0.5) are studied by the exact
diagonalization and the recursion method of finite systems up to 26 sites.
Especially, the dynamical structure factor S(q,\omega) is investigated
carefully for various values of \alpha. As \alpha increases beyond the
gapless-gapful critical value \alpha_c=0.2411, there appear features definitely
different from the Heisenberg model but the same with the Majumdar-Ghosh model.
Some of these features depend only on the value of \alpha and not on \delta: a
parameter introduced for the coupling alternation. By comparing these results
with a recent inelastic neutron scattering spectrum of an inorganic
spin-Peierls compound CuGeO_3 [M. Arai et al.: Phys. Rev. Lett. 77 (1996)
3649], it is found that the frustration by J' in CuGeO_3 is unexpectedly strong
(\alpha=0.4-0.45), and at least \alpha must be larger than \alpha_c to some
extent. The value of J is evaluated at \sim 180K consistent with other
estimations. The coupling alternation is extremely small. This large
frustration is a primary origin of the various anomalous properties CuGeO_3
possesses. For comparison we refer also to \alpha'-NaV_2O_5.Comment: 14 pages. A hard copy of 20 figures is available on request. To be
published in J. Phys. Soc. Jpn. Vol. 66 No. 11 (1997
Carbon Abundances of Three Carbon-Enhanced Metal-Poor Stars from High-Resolution Gemini-S/bHROS Spectra of the 8727A [C I] Line
We present the results from an analysis of the 8727ang forbidden [C I] line
in high-resolution Gemini-S/bHROS spectra of three CEMP stars. We find the
[C/Fe] ratios based on the [C I] abundances of the two most Fe-rich stars in
our sample (HIP 0507-1653: [Fe/H] = -1.42 and HIP 0054-2542: [Fe/H] = -2.66) to
be in good agreement with previously determined CH and C_2 line-based values.
For the most Fe-deficient star in our sample (HIP 1005-1439: [Fe/H] = -3.08),
however, the [C/Fe] ratio is found to be 0.34 dex lower than the published
molecular-based value. We have carried out 3D local thermodynamic equilibrium
(LTE) calculations for [C I], and the resulting corrections are found to be
modest for all three stars, suggesting that the discrepancy between the [C I]
and molecular-based C abundances of HIP 1005-1439 is due to more severe 3D
effects on the molecular lines. Carbon abundances are also derived from C I
high-excitation lines and are found to be 0.45-0.64 dex higher than the [C
I]-based abundances. Previously published non-LTE C I abundance corrections
bring the [C I] and C I abundances into better agreement; however, targeted
NLTE calculations for CEMP stars are clearly needed. We have also derived the
abundances of N, K, and Fe for each star. The Fe abundances agree well with
previously derived values, and the K abundances are similar to those of
C-normal metal-poor stars. Nitrogen abundances have been derived from resolved
lines of the CN red system. The abundances are found to be approximately 0.44
dex larger than literature values, which have been derived from CN blue bands
near 3880 and 4215 ang. We discuss evidence that suggests that analyses of the
CN blue system bands underestimate the N abundances of metal-poor giants.Comment: Accepted for publication in AJ; 42 pages, 6 figures, 7 table