485 research outputs found
Localized exciton states in pi-conjugated polymers with finite torsion
We investigate localized exciton states in pi-conjugated polymers with finite
torsion. The localized states are associated with a perturbed transfer integral
for which the magnitude of cosine of the torsion angle exceeds the magnitude of
the corresponding cosine of the unperturbed system. The localized state energy
is calculated as a function of the ratio of the perturbed to unperturbed
transfer integrals. Particular attention is paid to the optically active
symmetric localized states, and the effective oscillator strength, or square of
the absolute magnitude of the transition dipole moment, is calculated as a
function of the energy. The relation of the theory to recent optical studies of
poly(di-noctylfluorene)(PF8) is discussed.Comment: 11 pages pdf documen
Quantum Statistical Physics - A New Approach
The new scheme employed (throughout the thermodynamic phase space), in the
statistical thermodynamic investigation of classical systems, is extended to
quantum systems. Quantum Nearest Neighbor Probability Density Functions are
formulated (in a manner analogous to the classical case) to provide a new
quantum approach for describing structure at the microscopic level, as well as
characterize the thermodynamic properties of material systems. A major point of
this paper is that it relates the free energy of an assembly of interacting
particles to Quantum Nearest Neighbor Probability Density Functions. Also. the
methods of this paper reduces to a great extent, the degree of difficulty of
the original equilibrium quantum statistical thermodynamic problem without
compromising the accuracy of results. Application to the simple case of dilute,
weakly degenerate gases has been outlined.Comment: Submitted for publication in Physica A journa
Generalized Buneman pruning for inferring the most parsimonious multi-state phylogeny
Accurate reconstruction of phylogenies remains a key challenge in
evolutionary biology. Most biologically plausible formulations of the problem
are formally NP-hard, with no known efficient solution. The standard in
practice are fast heuristic methods that are empirically known to work very
well in general, but can yield results arbitrarily far from optimal. Practical
exact methods, which yield exponential worst-case running times but generally
much better times in practice, provide an important alternative. We report
progress in this direction by introducing a provably optimal method for the
weighted multi-state maximum parsimony phylogeny problem. The method is based
on generalizing the notion of the Buneman graph, a construction key to
efficient exact methods for binary sequences, so as to apply to sequences with
arbitrary finite numbers of states with arbitrary state transition weights. We
implement an integer linear programming (ILP) method for the multi-state
problem using this generalized Buneman graph and demonstrate that the resulting
method is able to solve data sets that are intractable by prior exact methods
in run times comparable with popular heuristics. Our work provides the first
method for provably optimal maximum parsimony phylogeny inference that is
practical for multi-state data sets of more than a few characters.Comment: 15 page
Electron Spin Resonance Above Tc In Layered Manganites
We have performed electron spin resonance (ESR) and dc magnetization measurements on single crystals of La2(1-x)Sr1+2xMn2O7 up to 800 K with special emphasis on the x = 0.4 composition. The ESR linewidth shows behavior similar to that observed in the three-dimensional perovskites and above ∼500 K can be described by a universal expression ΔHpp(T)=[C/Tχ(T)]ΔHpp (∞). The linewidth and the resonance field become anisotropic below ∼500 K. The anisotropy in the resonance field is proportional to the magnetization M, and we concluded that it is intrinsic to the system. We show that demagnetization effects can explain only part of the anisotropy. The remainder arises from short-range uniaxial terms in the Hamiltonian that are associated with the crystal field and Dzialozhinsky-Moriya interactions. The anisotropy in the linewidth is attributed to the easy-plane ferromagnetic ordering, which also arises from the short-range anisotropy.631717441311744136Ruddlesden, S.N., Popper, P., (1958) Acta Crystallogr., 11, p. 54Moritomo, Y., Asamitsu, A., Kuwahara, H., Tokura, Y., (1996) Nature (London), 380, p. 141Causa, M.T., Tovar, M., Caneiro, A., Prado, F., Ibanez, G., Ramos, C.A., Butera, A., Oseroff, S.B., (1998) Phys. Rev. B, 58, p. 3233Causa, M.T., Alejandro, G., Tovar, M., Pagliuso, P.G., Rettori, C., Oseroff, S.B., Subramanian, M.A., (1999) J. Appl. Phys., 85, p. 5408Huber, D.L., Alejandro, G., Caneiro, A., Causa, M.T., Prado, F., Tovar, M., Oseroff, S.B., (1999) Phys. Rev. B, 60, p. 12155Oseroff, S.B., Moreno, N.O., Pagliuso, P.G., Rettori, C., Huber, D.L., Gardner, J.S., Sarrao, J.L., Alascio, B.R., (2000) J. Appl. Phys., 87, p. 5810Seehra, M.S., Ibrahim, M.M., Babu, V.S., Srinivasan, G., (1996) J. Phys.: Condens. Matter, 8, p. 11283Dominguez, M., Lofland, S.E., Bhagat, S.M., Raychaudhuri, A.K., Ju, H.L., Venkates, T., Greene, R.L., (1996) Solid State Commun., 97, p. 193Lofland, S.E., Kim, P., Dahiroc, P., Bhagat, S.M., Tyagi, S.D., Karabashev, S.G., Shultyatev, D.A., Mukovskii, Y., (1997) Phys. Lett. A, 233, p. 476Kimura, T., Tomioka, Y., Kuwahara, H., Asamitsu, A., Tamura, M., Tokura, Y., (1996) Science, 274, p. 1698Perring, T.G., Aeppli, G., Moritomo, Y., Tokura, Y., (1997) Phys. Rev. Lett., 78, p. 3197Zhou, J.-S., Goodenough, J.B., Mitchell, J.F., (1998) Phys. Rev. B, 58, p. 579Zhou, J.-S., Goodenough, J.B., (1998) Phys. Rev. Lett., 80, p. 2665Kelley, T.M., Argyriou, D.N., Robinson, R.A., Nakotte, H., Mitchell, J.F., Osbron, R., Jorgensen, J.D., (1998) Physica B, 241-243, p. 439Heffner, R.H., MacLaughlin, D.E., Nieuwenhuys, G.J., Kimura, T., Luke, G.M., Tokura, Y., Uemura, Y.J., (1998) Phys. Rev. Lett., 81, p. 1706Potter, C.D., Swiatek, M., Bader, S.D., Argyriou, D.N., Mitchell, J.F., Miller, D.J., Hinks, D.G., Jorgensen, J.D., (1998) Phys. Rev. B, 57, p. 72Chauvet, O., Goglio, G., Molinie, P., Corraze, B., Brohan, L., (1998) Phys. Rev. Lett., 81, p. 1102Hirota, K., Moritomo, Y., Fujioka, H., Kubota, M., Yoshizawa, H., Endoh, Y., (1998) J. Phys. Soc. Jpn., 67, p. 3380Li, J.Q., Matsui, Y., Kimura, T., Tokura, Y., (1998) Phys. Rev. B, 57, pp. R3205Kimura, T., Kumai, R., Tokura, Y., Li, J.Q., Matsui, Y., (1998) Phys. Rev. B, 58, p. 11081Hayashi, T., Miura, N., Tokunaga, M., Kimura, T., Tokura, Y., (1998) J. Phys.: Condens. Matter, 10, p. 11525Suryanarayanan, R., Dhalenne, G., Revcolevschi, A., Prellier, W., Renard, J.P., Dupas, C., Caliebe, W., Chatterji, T., (2000) Solid State Commun., 113, p. 267Kubota, M., Fujioka, H., Ohoyama, K., Hirota, K., Moritomo, Y., Yoshizawa, H., Endoh, Y., (1999) J. Phys. Chem. Solids, 60, p. 116Bhagat, S.M., Lofland, S.E., Mitchell, J.F., (1999) Phys. Lett. A, 259, p. 326Kittel, C., (1997) Introduction to Solid State Physics, , Wiley, New YorkOkochi, M., (1970) J. Phys. Soc. Jpn., 28, p. 897Victoria, C., Barker, R.C., Yelon, A., (1967) Phys. Rev. Lett., 19, p. 792Nagata, K., (1976) J. Phys. Soc. Jpn., 40, p. 1209Nagata, K., Yamamoto, I., Takano, H., Yokozawa, Y., (1977) J. Phys. Soc. Jpn., 43, p. 857. , and references thereinHuber, D.L., Seehra, M.S., (1976) Phys. Status Solidi B, 74, p. 145Stanger, J.-L., Andre, J.-J., Turek, P., Hosokoshi, Y., Tamura, M., Kinoshita, M., Rey, P., Veciana, J., (1997) Phys. Rev. B, 55, p. 8398Van Vleck, J.H., (1950) Phys. Rev., 78, p. 266Kittel, C., (1948) Phys. Rev., 73, p. 15
The Kagome Antiferromagnet with Defects: Satisfaction, Frustration, and Spin Folding in a Random Spin System
It is shown that site disorder induces noncoplanar states, competing with the
thermal selection of coplanar states, in the nearest neighbor, classical kagome
Heisenberg antiferromagnet (AFM). For weak disorder, it is found that the
ground state energy is the sum of energies of separately satisfied triangles of
spins. This implies that disorder does not induce conventional spin glass
behavior. A transformation is presented, mapping ground state spin
configurations onto a folded triangular sheet (a new kind of ``spin origami'')
which has conformations similar to those of tethered membranes.Comment: REVTEX, 11 pages + 3 pictures upon reques
Theoretical analysis of neutron scattering results for quasi-two dimensional ferromagnets
A theoretical study has been carried out to analyse the available results
from the inelastic neutron scattering experiment performed on a quasi-two
dimensional spin-1/2 ferromagnetic material . Our formalism is based
on a conventional semi-classical like treatment involving a model of an ideal
gas of vortices/anti-vortices corresponding to an anisotropic XY Heisenberg
ferromagnet on a square lattice. The results for dynamical structure functions
for our model corresponding to spin-1/2, show occurrence of negative values in
a large range of energy transfer even encompassing the experimental range, when
convoluted with a realistic spectral window function. This result indicates
failure of the conventional theoretical framework to be applicable to the
experimental situation corresponding to low spin systems. A full quantum
formalism seems essential for treating such systems.Comment: 16 pages, 6 figures, 1 Table Submitted for publicatio
Diffusive energy transport in the S=1 Haldane chain compound AgVP2S6
We present the results of measurements of the thermal conductivity
of the spin S=1 chain compound AgVP_2S_6 in the temperature range between 2 and
300 K and with the heat flow directed either along or perpendicular to the
chain direction. The analysis of the anisotropy of the heat transport allowed
for the identification of a small but non-negligible magnon contribution
along the chains, superimposed on the dominant phonon contribution
. At temperatures above about 100 K the energy diffusion constant
D_E(T), calculated from the data, exhibits similar features as
the spin diffusion constant D_S(T), previously measured by NMR. In this regime,
the behaviour of both transport parameters is consistent with a diffusion
process that is caused by interactions inherent to one-dimensional S=1 spin
systems.Comment: 6 pages, 4 figure
Magnetic properties of the frustrated AFM spinel ZnCr_2O_4 and the spin-glass Zn_{1-x}Cd_xCr_2O_4 (x=0.05,0.10)
The -dependence (2- 400 K) of the electron paramagnetic resonance (EPR),
magnetic susceptibility, , and specific heat, , of the
antiferromagnetic (AFM) spinel ZnCrO and the spin-glass
(SG) ZnCdCrO () is reported. These
systems behave as a strongly frustrated AFM and SG with K and -400 K K. At high-
the EPR intensity follows the and the -value is -independent.
The linewidth broadens as the temperature is lowered, suggesting the existence
of short range AFM correlations in the paramagnetic phase. For
ZnCrO the EPR intensity and decreases below 90 K and 50
K, respectively. These results are discussed in terms of nearest-neighbor
Cr (S %) spin-coupled pairs with an exchange coupling of 50 K. The appearance of small resonance modes for K,
the observation of a sharp drop in and a strong peak in
at K confirms, as previously reported, the existence of long range
AFM correlations in the low- phase. A comparison with recent neutron
diffraction experiments that found a near dispersionless excitation at 4.5 meV
for and a continuous gapless spectrum for ,
is also given.Comment: 17 pages, 8 figures, 1 Table. Submitted to Physical Review
The Muonium Atom as a Probe of Physics beyond the Standard Model
The observed interactions between particles are not fully explained in the
successful theoretical description of the standard model to date. Due to the
close confinement of the bound state muonium () can be used as
an ideal probe of quantum electrodynamics and weak interaction and also for a
search for additional interactions between leptons. Of special interest is the
lepton number violating process of sponteanous conversion of muonium to
antimuonium.Comment: 15 pages,6 figure
Charge disproportionation in YNiO : ESR and susceptibility study
We present a study of the magnetic properties of YNiO in the
paramagnetic range, above and below the metal-insulator (MI) transition. The dc
susceptibility, (measured up to 1000 K) is a decreasing function of
T for 150 K (the N\'{e}el temperature) and we observe two different
Curie-Weiss regimes corresponding to the metallic and insulator phases. In the
metallic phase, this behaviour seems to be associated with the small ionic
radius of Y% . The value of the Curie constant for T T allows
us to discard the possibility of Ni localization. An electron spin
resonance (ESR) spectrum is visible in the insulator phase and only a fraction
of the Ni ions contributes to this resonance. We explain the ESR and behaviour for T T in terms of charge disproportionation of
the type 2Ni Ni+Ni that is compatible with the
previously observed structural transition across T.Comment: 10 pages, 4 figures, submitted to Phys. Rev.
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