SPECIFIC-HEAT OF NANOCRYSTALLINE AND COLLOIDAL NOBLE-METALS AT LOW-TEMPERATURES

We report on measurements of the specific heat C of nanocrystalline Cu and colloidal Au and Ag with an average crystallite size between 4 an d 18 nm at temperatures T between 0.06 K and 10 K and in magnetic fiel ds B up to 6 T. Above T approximate to 1 K the specific heat is field independent and strongly enhanced above the bulk specific heat. This e nhancement of C can be attributed to Einstein oscillators arising from weakly bound surface atoms. Below 1 K C is enhanced, too, but with a strong magnetic field dependence which can be described by the specifi c heat C-Sch of a Schottky anomaly. With increasing magnetic field the maximum of C-Sch is shifted to higher T. From the magnitude of the ad ditional specific heal we infer the number of excitations that contrib ute to C-Sch. A possible origin of these excitations might be an odd n umber of electrons in the small isolated particle as well as free radi cals or magnetic impurities in the colloids.