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Keywords:

  • Quantum spin chains;
  • Matrix-product ground states;
  • Variational treatment

Abstract

We study the phase diagram of the bilinear-biquadratic spin-1 chain. Using so-called matrix-product (MP) states we obtain upper bounds for the groundstate energy. The advantage of these MP states is their simplicity. Moreover they give an accurate description of the physics, especially in the Haldane phase.