Geodesic
Spectral gaps in the $XXZ$ quantum spin chain
Zapiski Nauchnykh Seminarov POMI, Questions of quantum field theory and statistical physics. Part 15, Tome 251 (1998), pp. 42-48 
H.-P. Eckle. Spectral gaps in the $XXZ$ quantum spin chain. Zapiski Nauchnykh Seminarov POMI, Questions of quantum field theory and statistical physics. Part 15, Tome 251 (1998), pp. 42-48. http://geodesic.mathdoc.fr/item/ZNSL_1998_251_a3/
@article{ZNSL_1998_251_a3,
     author = {H.-P. Eckle},
     title = {Spectral gaps in the $XXZ$ quantum spin chain},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {42--48},
     year = {1998},
     volume = {251},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1998_251_a3/}
}
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We report on the behavior of the finite-size energy spectrum of the Heisenberg–Ising ($XXZ$) quantum spin chain in the momentum $\pi$ sector. Our numerical and perturbative results for the behavior of energy gaps are explained by considering the motion of string solutions of the Bethe-Ansatz equations.