Specific heat of an S=1/2 Heisenberg ladder compound Cu-2(C5H12N2)(2)Cl-4 under magnetic fields

Specific heat measurements down to 0.5 K have been performed on a single cr ystal sample of a spin-ladder-like compound Cu-2(C5H12N2)(2)Cl-4 under magn etic fields up to 12 T. The temperature dependence of the observed data in a magnetic field below 6 T is well reproduced by numerical results calculat ed for the S = 1/2 two-leg ladder with J(rung)/J(leg) = 5. In the gapless r egion above 7 T (H-c1), the agreement between experiment and calculation is good above about 2 K and a sharp and a round peak were observed below 2 K in a magnetic field around 10 T, but the numerical data show only a round p eak, the magnitude of which is smaller than that of the observed one.