1,113 research outputs found
Spin-Peierls and Antiferromagnetic Phases in Cu{1-x}Zn{x}GeO{3}: A Neutron Scattering Study
Comprehensive neutron scattering studies were carried out on a series of
high-quality single crystals of Cu_{1-x}Zn_xGeO_3. The Zn concentration, x, was
determined for each sample using Electron Probe Micro-Analysis. The measured Zn
concentrations were found to be 40-80% lower than the nominal values.
Nevertheless the measured concentrations cover a wide range which enables a
systematic study of the effects due to Zn-doping. We have confirmed the
coexistence of spin-Peierls (SP) and antiferromagnetic (AF) orderings at low
temperatures and the measured phase diagram is presented. Most surprisingly,
long-range AF ordering occurs even in the lowest available Zn concentration,
x=0.42%, which places important constraints on theoretical models of the AF-SP
coexistence. Magnetic excitations are also examined in detail. The AF
excitations are sharp at low energies and show no considerable broadening as x
increases indicating that the AF ordering remains long ranged for x up to 4.7%.
On the other hand, the SP phase exhibits increasing disorder as x increases, as
shown from the broadening of the SP excitations as well as the dimer reflection
peaks.Comment: 17 preprint style pages, 9 postscript files included. Submitted to
Phys. Rev. B. Also available from
http://insti.physics.sunysb.edu/~mmartin/pubs.htm
High-field Electron Spin Resonance of Cu_{1-x}Zn_{x}GeO_{3}
High-Field Electron Spin Resonance measurements were made on powder samples
of Cu_{1-x}Zn_{x}GeO_{3} (x=0.00, 0.01, 0.02, 0.03 and 0.05) at different
frequencies (95, 110, 190, 220, 330 and 440 GHz) at low temperatures. The
spectra of the doped samples show resonances whose positions are dependent on
Zn concentration, frequency and temperature. The analysis of intensity
variation of these lines with temperature allows us to identify them as
originating in transitions within states situated inside the Spin Peierls gap.
A qualitative explanation of the details of the spectra is possible if we
assume that these states in the gap are associated with "loose" spins created
near the Zn impurities, as recently theoreticaly predicted. A new phenomenon of
quenching of the ESR signal across the Dimerized to Incommensurate
phase-boundary is observed.Comment: 4 pages, 5 ps figures in the text, submitted to Phys. Rev. Let
Elementary excitations, exchange interaction and spin-Peierls transition in CuGeO
The microscopic description of the spin-Peierls transition in pure and doped
CuGeO_3 is developed taking into account realistic details of crystal
structure. It it shown that the presence of side-groups (here Ge) strongly
influences superexchange along Cu-O-Cu path, making it antiferromagnetic.
Nearest-neighbour and next-nearest neighbour exchange constants and
are calculated. Si doping effectively segments the CuO_2-chains
leading to or even slightly ferromagnetic. Strong
sensitivity of the exchange constants to Cu-O-Cu and (Cu-O-Cu)-Ge angles may be
responsible for the spin-Peierls transition itself (``bond-bending mechanism''
of the transition). The nature of excitations in the isolated and coupled
spin-Peierls chains is studied and it is shown that topological excitations
(solitons) play crucial role. Such solitons appear in particular in doped
systems (Cu_{1-x}Zn_xGeO_3, CuGe_{1-x}Si_xO_3) which can explain the
phase diagram.Comment: 7 pages, revtex, 7 Postscript figure
Direct Dynamics Simulations Using Hessian-Based Predictor-Corrector Integration Algorithms
In previous research [J. Chem. Phys.111, 3800 (1999)] a Hessian-based integration algorithm was derived for performing direct dynamics simulations. In the work presented here, improvements to this algorithm are described. The algorithm has a predictor step based on a local second-order Taylor expansion of the potential in Cartesian coordinates, within a trust radius, and a fifth-order correction to this predicted trajectory. The current algorithm determines the predicted trajectory in Cartesian coordinates, instead of the instantaneous normal mode coordinates used previously, to ensure angular momentumconservation. For the previous algorithm the corrected step was evaluated in rotated Cartesian coordinates. Since the local potential expanded in Cartesian coordinates is not invariant to rotation, the constants of motion are not necessarily conserved during the corrector step. An approximate correction to this shortcoming was made by projecting translation and rotation out of the rotated coordinates. For the current algorithm unrotated Cartesian coordinates are used for the corrected step to assure the constants of motion are conserved. An algorithm is proposed for updating the trust radius to enhance the accuracy and efficiency of the numerical integration. This modified Hessian-based integration algorithm, with its new components, has been implemented into the VENUS/NWChem software package and compared with the velocity-Verlet algorithm for the H2CO→H2+CO, O3+C3H6, and F−+CH3OOH chemical reactions
Mean-field theory of the spin-Peierls systems: Application to CuGeO3
A mean-field theory of the spin Peierls systems based on the two dimensional
dimerized Heisenberg model is proposed by introducing an alternating bond order
parameter. Improvements with respect to previous mean-field results are found
in the one-dimensional limit for the ground state and the gap energies. In two
dimensions, the analysis of the competition between antiferromagnetic long
range order and the spin-Peierls ordering is given as a function of the
coupling constants. We show that the lowest energy gap to be observed does not
have a singlet-triplet character in agreement with the low temperature
thermodynamic properties of CuGeO3.Comment: 3 Revtex pages. Submitted to Rapid Comm. Figures available upon
reques
Study of the magnetic susceptibility in the spin-Peierls system CuGeO
We study numerically, using a one-dimensional Heisenberg model, the
spin-Peierls transition in the linear Cu spin-1/2 chains in the
inorganic compound CuGeO which has been recently observed experimentally.
We suggest that the magnetic susceptibility, the temperature dependence of the
spin gap and the spin-Peierls transition temperature of this material can be
reasonably described by including nearest and next nearest neighbor
antiferromagnetic interactions along the chain. We estimate that the nearest
neighbor exchange parameter J is approximately , and that the next
nearest neighbor exchange parameter is approximately .Comment: 14 pages, Revtex v2.0, 4 figures available upon reques
Impurity-Induced Antiferromagnetic Ordering in the Spin Gap System TlCuCl_3
The magnetization measurements have been performed on the doped spin gap
system TlCu_{1-x}Mg_xCl_3 with x <= 0.025. The parent compound TlCuCl_3 is a
three-dimensional coupled spin dimer system with the excitation gap Delta/k_B =
7.7 K. The impurity-induced antiferromagnetic ordering was clearly observed.
The easy axis lies in the (0,1,0) plane. It was found that the transition
temperature increases with increasing Mg^{2+} concentration x, while the
spin-flop transition field is almost independent of x. The magnetization curve
suggests that the impurity-induced antiferromagnetic ordering coexists with the
spin gap for x <= 0.017.Comment: 5 pages, 6 figures, revtex styl
Temperature Dependence of Spin and Bond Ordering in a Spin-Peierls System
We investigate thermodynamic properties of a one-dimensional S=1/2
antiferromagnetic Heisenberg model coupled to a lattice distortion by a quantum
Monte Carlo method. In particular we study how spin and lattice dimerize as a
function of the temperature, which gives a fundamental process of the
spin-Peierls transition in higher dimensions. The degree of freedom of the
lattice is taken into account adiabatically and the thermal distribution of the
lattice distortion is obtained by the thermal bath algorithm. We find that the
dimerization develops as the temperature decreases and it converges to the
value of the dimerization of the ground state at T=0. Furthermore we find that
the coupling constants of spins fluctuate quite largly at high temperature and
there thermodynamic properties deviate from those of the uniform chain. Doping
of non-magnetic impurities causes cut of the chain into short chains with open
boundary. We investigate thermodynamic properties of open chains taking
relaxation of the lattice into consideration. We find that strong bonds locate
at the edges and a defect of the bond alternation appears in the chain with odd
number of sites, which causes enhancement of the staggered magnetic order. We
find a spreaded staggered structure which indicates that the defect moves
diffusively in the chain even at very low temperature.Comment: 7 pages, 17 figures; added comments on section 2 and 3, corrected
typo
Magnetic excitations and effects of magnetic fields on the spin-Peierls transition in CuGeO
We analyze the magnetic excitations of a spin-1/2 antiferromagnetic
Heisenberg model with alternating nearest neighbor interactions and uniform
second neighbor interactions recently proposed to describe the spin-Peierls
transition in CuGeO. We show that there is good agreement between the
calculated excitation dispersion relation and the experimental one. We have
also shown that this model reproduces satisfactorily the experimental results
for the magnetization vs. magnetic field curve and its saturation value. The
model proposed also reproduces qualitatively some features of the magnetic
phase diagram of this compound and the overall behavior of the magnetic
specific heat in the presence of applied magnetic fields.Comment: 12 pages Revtex v2.0 + 4 figures postscripts include
Phase diagram of disordered spin-Peierls systems
We study the competition between the spin-Peierls and the antiferromagnetic
ordering in disordered quasi-one-dimensional spin systems. We obtain the
temperature vs disorder-strength phase diagram, which qualitatively agrees with
recent experiments on doped CuGeO_3.Comment: 4 pages, revtex, epsf, 2 Postscript figure
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