Spin Chain in Magnetic Field: Limitations of the Large-N Mean-Field Theory

K. Wohlfelda,b, Cheng-Chien Chen c, M. van Veenendaalc,d and T.P. Devereaux a aStanford Institute for Materials and Energy Sciences, SLAC National Laboratory and Stanford University, Menlo Park, California 94025, USA bInstitute of Theoretical Physics, Faculty of Physics, University of Warsaw, Pasteura 5, PL-02093 Warszawa, Poland cAdvanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA dDepartment of Physics, Northern Illinois University, De Kalb, Illinois 60115, USA

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Motivated by the recent success in describing the spin and orbital spectrum of a spin-orbital chain using a large-N mean-field approximation, we apply the same formalism to the case of a spin chain in the external magnetic field. It occurs that in this case, which corresponds to N=2 in the approximation, the large-N mean-field theory cannot qualitatively reproduce the spin excitation spectra at high magnetic fields, which polarize more than 50% of the spins in the magnetic ground state. This, rather counterintuitively, shows that the physics of a spin chain can under some circumstances be regarded as more complex than the physics of a spin-orbital chain.

DOI: 10.12693/APhysPolA.127.201 PACS numbers: 71.10.Fd, 75.10.Jm, 75.10.Pq