[Seminar] Two quantum phase transitions in nano-confined helium by Professor Keiya Shirahama

Abstract:

Superfluidity of liquid helium 4 is realized by not only quantum nature of helium atoms but also strong interatomic correlation. By confining 4He into nanoscale porous media, one can control the two properties and produce various emergent phases. We have studied 4He confined in a nanoporous glass and found a novel quantum phase transition (QPT) between superfluid and a localized Bose-Einstein Condensation (LBEC) state, in which the global phase coherence is destroyed by strong atomic correlation[1]. It is shown that the QPT is described by the four-dimensional XY universality class, in which quantum fluctuations play a crucial role[2].

In atomically thin 4He films adsorbed on the surface the glass substrate, second QPT between localized solid and superfluid is realized by changing coverage. We have found an elastic anomaly in the films, indicating that the localized state is a gapped Mott insulator or Mott glass, and the QPT occurs by closing the gap. Similar elastic anomalies seen in 3He (a fermion isotope of He), hydrogen, and neon will also be discussed.

Published Papers:

[1] K. Yamamoto et al., Phys. Rev. Lett. 100, 195301 (2008); K. 

Shirahama et al., J. Phys. Soc. Jpn. 77, 111011 (2008).

[2] Th. Eggel et al., Phys. Rev. B 84, 020515 (2011).

[3] T. Makiuchi et al., arXiv:1806.06624.