Dynamics of onedimensional Heisenberg antiferromagnets
Numerous finite temperature properties of quantum spin chains were computed,
and many of these have since been quantitatively tested in a number of
experiments.The dynamics of halfinteger spin chains can be probed by NMR
experiments, and predictions for NMR relaxation rates were made in paper
2; these have been tested in experiments of Takigawa et al (Physical
Review Letters 76, 4612 (1996)).
Integer spin chains and
evenleg spin ladders, which have an energy gap, were considered in papers
4 and 5. It was argued that their low temperature dynamics permits a quasiclassical
description in a "beads on a string" model sketched below.
Quasiclassical model for the real time dynamics of gapped spin chains and ladders
at low temperatures. The horizontal axis represents spatial position and the vertical axis
is time. The lines are the trajectories of thermally excited spin 1 "excitons", and the
three colors represent the S_{z} quantum number. Notice that there is perfect
reflection of the S_{z} quantum number at each collision.
This model allowed computation of the low temperature values of the spin1 exciton lifetime and
the spin diffusivity (these results are believed to be exact
in the limit of vanishing temperature), which led to predictions for NMR
and neutron scattering experiments. The results imply that the ratio of
the "dynamical energy gap" (measured by the nuclear relaxation rate)
to the "static energy gap" (measured by the magnetic suscepibility) is
exactly 3/2 at low temperatures: this is consistent with tends noted by
Itoh and Yasuoka (Journal of the Physical Society of Japan 66, 334
(1997)) across a number of observations on gapped spin chains as shown below.
Experimental values of "dynamical" and "static" energy gaps of a number of
spin gap compounds. The blue triangles are onedimensional systems, and cluster
around the theoretically predicted slope of 1.5. The red diamonds are isolated spin dimer
compounds and, as expected, these do not show the onedimensional anomaly..
The predictions
for the field and temperature dependence of the NMR relaxation rate explain
well the observations of Takigawa et al (Physical Review Letters
76, 2173 (1996)).
The lifetime of the spin 1 excitons can be
measured in neutron scattering experiments. Such experiments have been carried
out by M. Kenzelmann et al, Physical Review B
63, 134417 (2001) and Physical Review B
66, 174412 (2001), and the results are in good quantitative agreement with our predictions, which had
no free parameters.
There is also good agreement
with the neutron scattering measurements of Xu, Broholm et al (Science 317, 1049 (2007)).
Paper 5 also predicted the existence of certain two magnon bound states
in the twoleg ladder, and these appear to have been observed by Windt et al. (Physical
Review Letters 87, 127002 (2001)) and Grueninger et al.,
condmat/0109524.
PAPERS

Finite temperature properties of quantum antiferromagnets
in a uniform magnetic field in one and two dimensions, S. Sachdev,
T. Senthil, and R. Shankar, Physical Review B 50, 258 (1994); condmat/9401040.

NMR relaxation in halfinteger spin chains, S. Sachdev,
Physical Review B 50, 13006 (1994); condmat/9411079.

Multicritical crossovers near the dilute Bose gas quantum
critical point, K. Damle and S. Sachdev, Physical Review Letters 76,
4412 (1996); condmat/9602073.

Low temperature spin diffusion in the onedimensional
quantum O(3) nonlinear sigma model, S. Sachdev
and K. Damle, Physical Review Letters 78, 943 (1997); condmat/9610115.

Spin dynamics and transport in gapped onedimensional
Heisenberg antiferromagnets ar nonzero temperatures, K. Damle and S.
Sachdev, Physical Review B 57, 8307 (1998); condmat/9711014.

Intermediate temperature dynamics of onedimensional
Heisenberg antiferromagnets, C. Buragohain and S. Sachdev, Physical
Review B 59, 9285 (1999); condmat/9811083.

Dynamics and transport near quantumcritical points,
S. Sachdev, Dynamical properties of unconventional magnetic systems,
A. Skjeltorp and D. Sherrington eds., NATO ASI Series E: Applied Sciences,
vol 349, Kluwer Academic, Dordrecht (1997); condmat/9705266.

Comment on "Spin Transport of the quantum onedimensional
nonlinear sigma model", S. Sachdev and K. Damle, Journal of the Physical
Society of Japan 69, 2712 (2000); condmat/9906054.

Universal relaxational dynamics of gapped one dimensional models in the quantum sineGordon universality class, K. Damle and S. Sachdev, Phys. Rev. Lett. 95, 187201 (2005); condmat/0507380.
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