Liquid–Solid Transition and Phase Diagram of⁴He Confined in Nanoporous Glass

Abstract

We have studied the liquid-solid (L-S) phase transition of 4 He confined in nanoporous Gelsil glass that has interconnected nanopores of 2.5-nm diameter. The L-S boundary is determined by a series of measurements of pressure versus temperature along isochores and the rate of temperature change during slow cooling and warming in a semi-adiabatic condition. Below 1 K, the freezing pressure is elevated by 1.2 MPa from the bulk freezing pressure, and appears to be independent of temperature. The Tindependent freezing pressure strongly suggests the existence of the localized Bose-Einstein condensation state.KEYWORDS: superfluidity, Bose-Einstein condensation, quantum phase transition, porous media DOI: 10.1143/JPSJ.77.013601Quantum phase transition (QPT) has attracted considerable attention in condensed matter physics.1) Bose systems in periodic or random potential offer a typical example of QPT.2) In ultracold atoms with periodic potential, a QPT between the superfluid and Mott insulator has been reported.3) Thin superconducting films 4) and Josephson junction arrays 5) have also been extensively studied as disordered Bose systems exhibiting QPTs.4 He in porous media is an important model system as strongly correlated Bosons in an external potential. One can freely control many properties such as the system dimensionality, topology, and disorder by changing the pore size and the pore structure. For the last three decades, a number of experimental studies have been carried out.6-10) The effects of the quenched disorder on the critical phenomena of superfluid 4 He were investigated using Aerogel. 7) In porous Vycor glass, a dilute Bose gas state was demonstrated.9) Recently, we studied the superfluid transition of 4 He confined in a nanoporous Gelsil glass of 2.5 nm pore diameter using a torsional oscillator (TO).11) The Gelsil glass has three-dimensionally interconnected nanopores and the pore structure is random. We summarize the results in Fig. 1 together with the result of the present work. The TO study revealed that superfluid transition temperature T c near 0 MPa is 1.4 K, suppressed to 2/3 of the bulk point, 2.17 K. T c decreases progressively as pressure increases and then approaches 0 K at a critical pressure P c ¼ 3:4 MPa. This is strikingly different from the behavior of bulk 4 He, namely the line, which terminates at the freezing curve. This behavior clearly shows that the confined 4 He undergoes a QPT at the quantum critical pressure P c .Our finding of the quantum critical pressure raises an interesting question. The nonsuperfluid (NSF) state may exist even at 0 K. What is the nature of the NSF state? It must be entirely different from the usual normal fluid of bulk 4 He. There should be the liquid-solid (L-S) phase transition, and the determination of the L-S boundary on the P-T phase diagram provides thermodynamic information on the NSF state. To reveal the L-S boundary, we measured pressure versus temperature PðTÞ along isochores of 4 He in Gelsil. 12)However, the temperature regulation in the PðTÞ me...

4He glass phase: A model for liquid elements

“…Their mean value of 0.0932 is in good agreement with the theoretical value of 0.0925. Other measurements indicate hysteresis between the melting temperature window observed by heating and that of freezing observed by cooling [42,43], confirming the first-order character of the glass transition. The radius reduction of nano-pores decreases the freezing onset temperature [43].…”

Localized Bose–Einstein Condensation of 4He Confined in Nanoporous Media

“…We study superfluid and thermodynamic properties and the P-T phase diagram of 4 He confined in Gelsil by making full use of torsional oscillator, pressure and heat capacity measurements. [8][9][10][11][12][13][14][15][16] In this paper we review the discovered phenomena, that is, superfluidity and BEC occur at different temperatures in the confined 4 He. This difference is attributed to the formation of localized Bose-Einstein condensates (LBECs) by the strong confinement of 4 He in the nanopores or by the random potential provided by the porous structure.…”

Formation of superfluid liquid pocket in aerogel and its solidification by cooling

“…Properties of superfluid 4 He in porous materials have been investigated extensively and variations in the static properties, such as reduction in the superfluid transition temperatures and elevation of the crystallization pressure have been revealed [1][2][3][4][5][6][7][8][9]. However, not much is known about the dynamical process of superfluid 4 He in porous materials due to the lack of an appropriate method to tackle the physics.…”