Type of Document Dissertation Author De Lia, Anthony F. URN etd-09212003-193752 Title Dynamical Studies Of Antiferromagnetic Exchange Interactions In Low Dimensional Quantum Spin Systems Degree Doctor of Philosophy Department Physics, Department of Advisory Committee
Advisor Name Title Elbio Dagotto Committee Chair Adriana Moreo Committee Member Jorge Piekarewicz Committee Member Mark Riley Committee Member Wolfgang Heil Committee Member Keywords
- Antiferromagnetic Exchange Interactions
Date of Defense 2003-07-02 Availability unrestricted AbstractVarious forms of antiferromagnetic exchange interaction among quantized spins in onedimensional and quasi-one-dimensional lattices are examined. Primary results are reported
in two real compounds, the sodium vanadate NaV2O5 and the copper germanate CuGeO3, and in a class of compounds of real materials modeled as dimerized antiferromagnetically coupled spin-1/2 chains susceptible to spin-1 impurity doping and that include such examples as the strontium cuprate Sr14Cu24O41 and vanadium pyrophosphate (VO)2P2O7.
Sodium vanadate, NaV2O5, a quarter-ﬁlled two-leg ladder compound that was originally thought to be composed of magnetic legs and nonmagnetic legs was subsequently shown to be a charge ordered system below room temperature. Initial models of the material as isolated antiferromagnetically coupled spin-1/2 chains can be mapped to the subsequently resolved magnetic system of the charge ordered state with the spins on molecular orbitals of V-O-V rungs antiferromagentically coupled between neighboring rungs either on the same ladder or on neighboring ladders. Comparing the model¡¯s dynamic structure factor to inelastic
neutron scattering data could distinguish between the proposed spin coupling schemes and
consequently reveal the details of the interaction between the lattice and the ¡°zig-zag¡± charge density as well as the magnetic stabilization of the low temperature phase.
Copper germanate, the ﬁrst and only known inorganic spin-Peierls compound, exhibits a strong temperature dependent exchange coupling pattern among the spin-1/2 Cu2+ chains. Above the spin-Peierls transition temperature an unmodulated antiferromagnetic nearest neighbor exchange interaction J1 competes with an antiferromagnetic next nearest neighbor interaction J2. The next nearest neighbor exchange represents an effective coupling
equivalent to the net effect of all longer range exchange interactions. This frustration to the
nearest neighbor spin exchange produces a distinctive magnetic susceptibility ¦Ö(T) much
different from the Bonner-Fisher susceptibility of the spin-1/2 Heisenberg chain with nearest
neighbor antiferromagnetic exchange only. The ratio of the second to ﬁrst nearest neighbor
couplings ¦Á = J2/J1 ¡Ö 0.36 is suﬃcient to open a spontaneous gap in the spin-wave excitation
spectrum at low temperatures. Below TSP = 14K, the dynamical structure factor is used to ﬁt the dimerization ¦Ä and the exchange interactions J1 and J2 to the inelastic neutron scattering data of CuGeO3 at T = 10K. It is found that both ¦Ä and ¦Á increase signiﬁcantly at lower temperatures, relative to the values obtained in the high temperature phase and at the onset of dimeration at 14 K. Static structure factor calculations show scattering inconsistent with the ¦Ä−J1−J2 model and can be attributed primarily to the phonon degrees of freedom but possibly also to the couplings between chains in the b and a directions of the crystal lattice.
Structurally dimerized compounds of antiferromagnetic spin-1/2 chains possess dynamic
structure factor and magnetic susceptibility features that are very sensitive to doping with
magnetic and nonmagnetic impurities. It is shown that the effects of spin-1 impurities are
very similar to those of nonmagnetic (S = 0) impurities if the coupling between impurity
and native spin is about the same magnitude as or larger than between native S = 1/2 spins. The microscopic origins of the similarity can be appreciated with the consideration that neighboring spins to the impurity ﬁnd it more energetically favorable to couple to the
impurity than the other spins of the lattice. Thus the spin-1 impurity and its neighboring
S = 1/2 spins decouple from the lattice, creating a nonmagnetic break in the chain. The spins that now observe a nonmagnetic cluster on one side can couple to the S = 1/2 spins on the other side with a higher AF correlation than in the pure compound since they do not have to have a ﬂuctuating dimer resonating between two neighbors. Such enhancements to the local correlations can stabilize the global AF order in one dimensional compounds as shown in theoretical and experimental studies of CuGeO3. ED calculations of the dynamic structure factor show that S(q, ¦Ø) developes states with ¦Ø = 0 in the gap at q = ¦Ð. Magnetic susceptibilites in the presense of spin-1 impurities were obtained for 80 site chains by QMC simulations and for 16 site chains by ED calculations. An impurity concentration-dependent
second peak in the low temperature region reveals an increasing second maximum with T as observed in doped copper germanates and suggests that the results of the study are applicable even to phonon mediated dimerization. This is not immediately apparent because spin-phonon coupling could distort the dimerization patterns. However, experimental and theoretical results show that elastic coupling between chains can stabilize the dimerization pattern due to the impurities and the results will still apply.
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