Thermodynamics of the S=1/2 hyperkagome-lattice Heisenberg antiferromagnet

The S = 1/2 hyperkagome-lattice Heisenberg antiferromagnet allows us to study the interplay of geometrical frustration and quantum as well as thermal fluctuations in three dimensions. We use 16 terms of a hightemperature series expansion complemented by the entropy-method interpolation to examine the specific heat and the uniform susceptibility of this model. We obtain thermodynamic quantities for several possible scenarios determined by the behavior of the specific heat as T -> 0: A power-law decay with the exponent alpha = 1, 2, and also 3 (gapless energy spectrum) or an exponential decay (gapped energy spectrum). All scenarios give rise to a low-temperature peak in c(T) (almost a shoulder for alpha = 1) at T < 0.05, i.e., well below the main high-temperature peak. The functional form of the uniform susceptibility chi(T) below about T = 0.5 depends strongly not only on the chosen scenario but also on an input parameter chi(0) = chi(T = 0). An estimate for the ground-state energy e(0) depends on the adopted specific scenario but is expected to lie between -0.441 and -0.435. In addition to the entropy-method interpolation we use the finite-temperature Lanczos method to calculate c(T) and chi(T) for finite lattices of N = 24 and 36 sites. A combined view on both methods leads us to favor the gapless scenario with alpha = 2 (but alpha = 1 cannot be excluded) and finite chi(0) around 0.1.