@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},
     publisher = {mathdoc},
     volume = {251},
     year = {1998},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1998_251_a3/}
}