Title: Anomalous thermodynamics of a quantum spin system with large residual entropy
Author(s):

	J. Richter (Institut für Physik, Otto-von-Guericke-Universität Magdeburg, P.O. Box 4120, 39016 Magdeburg, Germany; Max-Planck-Institut für Physik komplexer Systeme, Nöthnitzer Straße 38, 01187 Dresden, Germany),
	J. Schulenburg (Universitätsrechenzentrum, Universität Magdeburg, D-39016 Magdeburg, Germany),
	D.V. Dmitriev (Institute of Biochemical Physics of RAS, 4 Kosygin St., 119334 Moscow, Russia),
	V.Ya. Krivnov (Institute of Biochemical Physics of RAS, 4 Kosygin St., 119334 Moscow, Russia),
	J. Schnack (Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany)

In contrast to strongly frustrated classical systems, their quantum counterparts typically have a non-degenerate ground state. A counterexample is the celebrated Heisenberg sawtooth spin chain with ferromagnetic zigzag bonds J₁ and competing antiferromagnetic basal bonds J₂. At a quantum phase transition point |J₂/J₁|=1/2, this model exhibits a flat one-magnon excitation band leading to a massively degenerate ground-state manifold which results in a large residual entropy. Thus, for the spin-half model, the residual entropy amounts to exactly one half of its maximum value lim_{T→ ∞} S(T)/N = ln2. In the present paper we study in detail the role of the spin quantum number s and the magnetic field H in the parameter region around the transition (flat-band) point. For that we use full exact diagonalization up to N = 20 lattice sites and the finite-temperature Lanczos method up to N = 36 sites to calculate the density of states as well as the temperature dependence of the specific heat, the entropy and the susceptibility. The study of chain lengths up to N = 36 allows a careful finite-size analysis. At the flat-band point we find extremely small finite-size effects for spin s = 1/2, i.e., the numerical data virtually correspond to the thermodynamic limit. In all other cases the finite-size effects are still small and become visible at very low temperatures. In a sizeable parameter region around the flat-band point the former massively degenerate ground-state manifold acts as a large manifold of low-lying excitations leading to extraordinary thermodynamic properties at the transition point as well as in its vicinity such as an additional low-temperature maximum in the specific heat. Moreover, there is a very strong influence of the magnetic field on the low-temperature thermodynamics including an enhanced magnetocaloric effect.

Key words: quantum Heisenberg model, frustration, sawtooth chain, residual entropy

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