From Supersolidity to Giant Plasticity: Defects in Quantum Crystals

In 2004, Kim and Chan reported torsional
oscillator experiments on 4He crystals which showed evidence of
“non-classical rotational inertia”, the mass decoupling expected for a
long-sought “supersolid” state. It soon
became clear that this behavior is not a property of perfect crystals – defects
are involved. In 2007, we made elastic
measurements which showed, to our surprise, that the shear modulus of solid 4He
increases dramatically below 0.2 K, with the same dependence on temperature,
amplitude and 3He impurity concentration as the torsional oscillator
anomaly. These shear modulus changes are
due to dislocations and their interactions with impurities and vacancies. Our experiments raised an obvious question – could
the torsional oscillator behavior be an artifact of the elastic changes, rather
than evidence of supersolidity? During
the past two years it has become clear that the answer is “yes” and interest
has focused on the properties of dislocations in a quantum solid like 4He. By growing high quality single crystals in
optical cells, we have now been able to explore the behavior of dislocations in
4He in unprecedented detail. In some crystals the dislocations reduce the shear modulus by more than 80%, an extraordinary
effect we describe as “giant plasticity”. Solid helium has proved to be an ideal system in which to do materials
science, as well as to address fundamental questions about quantum solids.

Event Type: 
Seminar
Collection/Series: 
Scientific Area(s): 
Speaker(s): 
Event Date: 
Wednesday, October 9, 2013 - 14:00 to 15:30
Location: 
Time Room