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|Date and time:||30th January (Thu.) 16:00〜|
|Place:||SPring-8, "HOUKOUKAN" seminar room|
|Title:||Possible phases and phase transitions in quantum spin ladders|
|講演者:||藤 陽平 氏 （東京大学物性研究所）|
|Speaker:||Mr. Yohei Fuji (ISSP, University of Tokyo)|
Quantum spin ladders, which consist of coupled spin chains, have been extensively studied both analytically and numerically, in order to investigate the cuprate superconductors, dimensional crossover, and effects of geometrical frustration. According to the Lieb-Schultz-Mattis theorem, these systems exhibit quite different properties, depending on the parities of spin and the number of chains. For example, two-leg ladders should have a unique gapped ground state (GS), while three-leg ladders should have either a gapless GS or a degenerate gapped GS.
In this seminar, after an instructive and historical review on spin ladders, we present two of our recent results: (i) The gapped-gapless phase transition under a continuous deformation from the three-leg ladder to the three-leg tube is studied from the strong-coupling limit where spin and (emergent) chirality cooperate each other.
(ii) The fact that unique gapped phases in spin ladders are protected by a certain set of symmetries is reproduced within the effective field theory first developed by Schulz.
This provides an alternative way to see the symmetry protected topological nature [3,4] of such phases.
 YF, S. Nishimoto, H. Nakada, and M. Oshikawa, arXiv:1311.3041.
 H. J. Schulz, Phys. Rev. B 34, 6372 (1986).
 F. Pollmann, A. M. Turner, E. Berg, and M. Oshikawa, Phys. Rev. B 81, 064439 (2010).
 X. Chen, Z.-C. Gu, and X.-G. Wen, Phys. Rev. B 83, 035107 (2011).