Specific heat of segmented Heisenberg quantum spin chains in (Yb _1-x Lu_x) 4 As 3

We report low-temperature specific heat, C(T), measurements on (Yb1-xLux)4As3, with x=0.01 and x=0.03, where nonmagnetic Lu atoms are randomly distributed on antiferromagnetic S=1/2 Heisenberg chains with J/kB=28 K. The observed reduction of C below 15 K with increasing x is accurately described by quantum transfer matrix simulations without any adjustable parameter, implying that the system is an excellent experimental realization of segmented quantum spin chains. Finite-size effects consistent with conformal-field theory predictions are leading to the formation of an effective low-energy gap. The size of the gap increases with Lu content and accounts for the impurity-driven reduction of the specific heat. For both concentrations our results verify experimentally the low-temperature scaling behavior established theoretically and also confirm the value of J determined from pure Yb4As3.