Glasses and other non-crystalline solids exhibit thermal and acousticproperties at low temperatures anomalously different from those found incrystalline solids, and with a remarkable degree of universality. Below a fewK, these universal properties have been successfully interpreted using theTunneling Model, which has enjoyed (almost) unanimous recognition for decades.Here we present low-temperature specific-heat measurements of ultrastableglasses of indomethacin that clearly show the disappearance of the ubiquitouslinear contribution traditionally ascribed to the existence of tunnelingtwo-level systems (TLS). When the ultrastable thin-film sample is thermallyconverted into a conventional glass, the material recovers a typical amount ofTLS. This remarkable suppression of the TLS found in ultrastable glasses ofindomethacin is argued to be due to their particular anisotropic and layeredcharacter, which strongly influences the dynamical network and may hinderisotropic interactions among low-energy defects, rather than to thethermodynamic stabilization itself. This explanation may lend support to thecriticisms by Leggett and others to the standard Tunneling Model, although moreexperiments in different kinds of ultrastable glasses are needed to ascertainthis hypothesis.
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