Helium in Aerogel

Chan, Moses H. W.; Mulders, N.; Reppy, J. D.

doi:10.1063/1.881509

Cited by 90 publications

(90 citation statements)

References 22 publications

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“…There is increasing interest in the dynamical properties of systems that are characterized by prominent quantum statistical effects, such as the spectroscopy of impurity atoms or molecules embedded in superfluid 4 He nanodroplets (38)(39)(40), the manifestation of superfluidity and BEC in confined geometries such as on surfaces and in porous media (41), and the recently reported observation of ''supersolidity'' (42). The combined quantum-semiclassical methodology used in the calculations presented in this article will help deepen the microscopic understanding of the intriguing dynamical behaviors of such systems.…”

Section: Resultsmentioning

confidence: 99%

Simulation of dynamical properties of normal and superfluid helium

Nakayama

Makri

2005

Proc. Natl. Acad. Sci. U.S.A.

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The formation of a superfluid when 4 He is cooled below the characteristic lambda transition temperature is accompanied by intricate quantum mechanical phenomena, including the emergence of a Bose condensate. A combination of path integral and semiclassical techniques is used to calculate the single-particle velocity autocorrelation function across the normal-to-superfluid transition. We find that the inclusion of particle exchange alters qualitatively the shape of the correlation function below the characteristic transition temperature but has no noticeable effect on the dynamics in the normal phase. The incoherent structure factor extracted from the velocity autocorrelation function is in very good agreement with neutron scattering data, reproducing the width, height, frequency shift, and asymmetry of the curves, as well as the observed increase in peak height characteristic of the superfluid phase. Our simulation demonstrates that the peak enhancement observed in the neutron scattering experiments below the transition temperature arises exclusively from particle exchange, illuminating the role of Bose-statistical effects on the dynamics of the quantum liquid.semiclassical dynamics ͉ forward-backward ͉ Bose-Einstein condensation ͉ time correlation function ͉ incoherent structure factor T he remarkable properties of superfluids, in particular the observed frictionless flow and heat conduction without a temperature gradient, have fascinated scientists for several decades (1-5). Liquid 4 He has for long served as the paradigm of superfluidity, and the unusual properties of this system around and below the characteristic lambda transition temperature have been the subject of persistent experimental and theoretical investigations. Superfluidity is intimately connected with Bose-Einstein condensation (6, 7) (BEC), but many questions still surround their relationship. A number of experimental properties of superfluid 4 He are consistent with theoretical descriptions that assume the presence of BEC. However, direct observation of BEC in this system has been elusive, owing primarily to strong repulsive interactions associated with the closed-shell electronic structure of 4 He.Theoretical and computational studies have yielded a wealth of information regarding the momentum distribution of superfluid 4 He, from which one can extract the fraction of particles in the zero momentum state (8, 9). These works, the majority of which are based on quantum mechanical simulations such as quantum Monte Carlo (10-14) and path integral Monte Carlo (15) methods, estimate the condensate fraction to be Ϸ7% at saturated vapor pressure (SVP) (12)(13)(14)(15). Experimental studies of these properties are largely based on inelastic neutron scattering measurements (16). At small values of momentum transfer, the differential cross section obtained from these experiments displays characteristics associated with collective response (with a sharp peak that follows the well known phonon-maxon-roton dispersion curve) (3-5, 17, 18). On the other ha...

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Section: Resultsmentioning

confidence: 99%

Simulation of dynamical properties of normal and superfluid helium

Nakayama

Makri

2005

Proc. Natl. Acad. Sci. U.S.A.

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“…For 3 He- 4 He mixtures, the presence of 2% or even 0.5% silica (i.e. aerogels with porosity of 98% or 99.5%) has dramatic effects 5,6,7 on the entire phase diagram, causing the phase separation curve to detach from the lambda line and stabilizing a region of dilute 4 He superfluid inaccessible in bulk mixtures.…”

Section: Introductionmentioning

confidence: 99%

Helium adsorption in silica aerogel near the liquid-vapor critical point

2005

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We have investigated the adsorption and desorption of helium near its liquid-vapor critical point in silica aerogels with porosities between 95% and 98%. We used a capacitive measurement technique which allowed us to probe the helium density inside the aerogel directly, even though the samples were surrounded by bulk helium. The aerogel's very low thermal conductivity resulted in long equilibration times so we monitored the pressure and the helium density, both inside the aerogel and in the surrounding bulk, and waited at each point until all had stabilized. Our measurements were made at temperatures far from the critical point, where a well defined liquid-vapor interface exists, and at temperatures up to the bulk critical point. Hysteresis between adsorption and desorption isotherms persisted to temperatures close to the liquid-vapor critical point and there was no sign of an equilibrium liquid-vapor transition once the hysteresis disappeared. Many features of our isotherms can be described in terms of capillary condensation, although this picture becomes less applicable as the liquid-vapor critical point is approached and it is unclear how it can be applied to aerogels, whose tenuous structure includes a wide range of length scales.

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“…The experimental efforts have concentrated on idealized model systems in the hopes of isolating the essential features of quenched random disorder. They include the still enigmatic superfluid transition of 4 He in aerogels and porous glasses, the superfluid transition and phase separation of 4 He- 3 He mixtures in silica aerogels [1] and doped magnet systems [2]. Relatively recent efforts with liquid crystal (LC) -silica composites [3,4,5,6,7,8,9] have demonstrated that these are especially interesting model systems.…”

Section: Introductionmentioning

confidence: 99%

Smectic ordering in liquid-crystal–aerosil dispersions. II. Scaling analysis

et al. 2003

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Liquid crystals offer many unique opportunities to study various phase transitions with continuous symmetry in the presence of quenched random disorder (QRD). The QRD arises from the presence of porous solids in the form of a random gel network. Experimental and theoretical work support the view that for fixed (static) inclusions, quasi-long-range smectic order is destroyed for arbitrarily small volume fractions of the solid. However, the presence of porous solids indicates that finite-size effects could play some role in limiting long-range order. In an earlier work, the nematic -smectic-A transition region of octylcyanobiphenyl (8CB) and silica aerosils was investigated calorimetrically. A detailed x-ray study of this system is presented in the preceding Paper I, which indicates that pseudo-critical scaling behavior is observed. In the present paper, the role of finite-size scaling and two-scale universality aspects of the 8CB+aerosil system are presented and the dependence of the QRD strength on the aerosil density is discussed.

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Helium in Aerogel

Cited by 90 publications

References 22 publications

Simulation of dynamical properties of normal and superfluid helium

Simulation of dynamical properties of normal and superfluid helium

Helium adsorption in silica aerogel near the liquid-vapor critical point

Smectic ordering in liquid-crystal–aerosil dispersions. II. Scaling analysis

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