Nuclear spin relaxation in a spin-1/2 antiferromagnetic Heisenberg chain at high fields

Journal Article · Thu Jan 01 00:00:00 EST 1981 · Phys. Rev., B: Condens. Matter; (United States)

DOI:https://doi.org/10.1103/PhysRevB.23.239· OSTI ID:7012641

Lyo, S K

The proton spin relaxation rate is calculated in the one-dimensional spin-1/2 Heisenberg antiferromagnet ..cap alpha..-bis (N-methylsalicylaldiminato)-copper (II), ..cap alpha..-CuNSal by using a fermion representation for magnons above the critical field where the magnon spectrum develops a gap. The one-magnon process which is dominant below the critical field is shown to be absent in the presence of a gap in contrast to a previous theory. Instead, we find that the three-magnon rate is large enough to explain the data at low fields. The two-magnon off-resonance damping which enters the expression for the three-magnon rate is calculated by solving the two-magnon scattering exactly, leading to a much smaller value of the rate than that predicted by the Born approximation. Also, in an unsuccessful attempt to resolve the discrepancy between the recently calculated two-magnon rate (dominant at high fields) and the data of ..cap alpha..-CuNSal reported by Azevedo et al., we carry out the vertex correction for the spin-density correlation function by summing the RPA series as well as the exchange ladders for the polarization part. We find that, although the exchange enhancement is significantly large, it is nearly canceled out by the RPA correction, and the net effect of the vertex correction is small. This result agrees with the recent data of the similar spin-1/2 antiferromagnetic Heisenberg chain system CuSO/sub 4/x5H/sub 2/O reported by Groen et al. On the other hand, it disagrees with a recent calculation of the two-magnon rate based on a boson representation of spins. To resolve this discrepancy we examine the effect of the boson self-energy correction on the two-magnon rate. The boson spectral shift is found to be quite large in the region where the cited two-boson rate deviates from the two-fermion rate. As a result the two-boson rate is significantly reduced, leading to reasonable agreement with the two-fermion rate.

• Research Organization: Sandia Laboratories, Albuquerque, New Mexico 87185
• OSTI ID: 7012641
• Journal Information: Phys. Rev., B: Condens. Matter; (United States), Vol. 23:1
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
COPPER COMPOUNDS
ANTIFERROMAGNETISM
HEISENBERG MODEL
RELAXATION
NITROGEN COMPOUNDS
MAGNETIC FIELDS
MAGNONS
NUCLEAR MAGNETIC RESONANCE
ONE-DIMENSIONAL CALCULATIONS
ORGANIC COMPOUNDS
CRYSTAL MODELS
MAGNETIC RESONANCE
MAGNETISM
MATHEMATICAL MODELS
QUASI PARTICLES
RESONANCE
TRANSITION ELEMENT COMPOUNDS
360404* - Materials- Polymers & Plastics- Physical Properties- (-1987)