Diverging thermal expansion of the spin-ladder system (C5H12N)2CuBr4.

We present high-resolution measurements of the c(*)-axis thermal expansion and magnetostriction of piperidinium copper bromide (C5H12N)2CuBr4. The experimental data at low temperatures are well accounted for by a two-leg spin-ladder Hamiltonian. The thermal expansion shows a complex behavior with various sign changes and approaches a 1/square root T divergence at the critical fields. All low-temperature features are semiquantitatively explained within a free-fermion model; full quantitative agreement is obtained with quantum Monte Carlo simulations.