| I will discuss the re-entrant quantum phase transitions in quantum spin chains when the XXZ anisotropy parameter is run. In some cases, for example, we can see the succesive transitions such as TL-Neel-TL-Ferri, where TL means the Tomonaga-Luttinger spin-fluid state. This phenomenon can be found in S=1/2 spin chains having three key words: XXZ anisotropy, trimer nature and frustration. We also find that four Berezinski-Kosterlitz-Thouless quantum phase transition lines meet together at one point in the phase diagram on the plane of quantum parameters. |
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