Thermodynamics of random ferromagnetic-antiferromagnetic spin-1/2 chains

Using the quantum Monte Carlo loop algorithm, we calculate the temperature dependence of the uniform susceptibility, the specific heat, the correlatio n length, the generalized staggered susceptibility, and magnetization of a spin-1/2 chain with random antiferromagnetic and ferromagnetic couplings, d own to very low temperatures. Our data show a consistent scaling behavior i n all the quantities and support strongly the conjecture drawn from the app roximate real-space renormalization group treatment. A statistical analysis scheme is developed which will be useful for the search of scaling behavio r in numerical and experimental data of random spin chains. [S0163-1829(99) 11229-3].