Observation of a Superfluid He-3<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">A</mml:mi></mml:math>-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">B</mml:mi></mml:math>Phase Transition in Silica Aerogel

Barker, Barry; Lee, Y.; Polukhina, Lidiya; Osheroff, D. D.; Hrubesh, Lawrence W.; Poco, J.F.

doi:10.1103/physrevlett.85.2148

Cited by 92 publications

(157 citation statements)

References 15 publications

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“…Two layers of 3 He are paramagnetic [9,15] and can be removed by substitution with two layers of non-magnetic 4 He reducing the addendum to less than 0.25 mJ/K. The helium isotope mixture experiments were limited to ≥ 4 mK owing to long thermal time constants in the calorimeter.…”

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confidence: 99%

Specific Heat of Disordered SuperfluidHe3

et al. 2004

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The specific heat of superfluid 3 He, disordered by a silica aerogel, is found to have a sharp discontinuity marking the thermodynamic transition to superfluidity at a temperature reduced from that of bulk 3 He. The magnitude of the discontinuity is also suppressed. This disorder effect can be understood from the Ginzburg-Landau theory which takes into account elastic quasiparticle scattering suppressing both the transition temperature and the amplitude of the order parameter. We infer that the limiting temperature dependence of the specific heat is linear at low temperatures in the disordered superfluid state, consistent with predictions of gapless excitations everywhere on the Fermi surface.PACS numbers: 67.57.Bc, 67.57.Pq Two essential characteristics of superconductors are manifest in the specific heat. First, there is a jump at the transition temperature of magnitude that depends on the strength of the electron pair interactions that lead to superconductivity. Secondly, the temperature dependence at low temperature gives a fingerprint of the energy gap structure. Consequently, the measurement of the specific heat is one of the most fundamental to understanding the nature of superconductivity. In fully gapped superconductors, such as aluminum, the specific heat decreases exponentially with decreasing temperature following an Arrhenius relation[1], thereby demonstrating unambiguously the existence of a full gap in the electron quasiparticle excitation spectrum and providing a direct measurement of its size. Both of these basic thermodynamic behaviors conform to predictions of the Bardeen, Cooper, and Schrieffer theory[2] and can be expected to hold in general for any condensate of fermion pairs into a state that is fully gapped. This is precisely what is expected [3] and observed [4] for the B-phase of superfluid 3 He, a pwave superfluid with an isotropic energy gap, similar to conventional s-wave superconductors.Shortly after the success of BCS theory it was sought to understand how perturbations, such as impurities, modify superconducting behavior. It was found [5] that magnetic scattering of quasiparticles suppresses both the transition temperature and the gap magnitude leading to 'gapless superconductivity' [2]. The same should be true for all superconductors and fermion superfluids even those that do not have s-wave states in which case all forms of elastic scattering will produce these suppression effects [6]. In particular it should be true for superfluid 3 He. In this letter we present systematic measurements of the heat capacity in the B-phase of superfluid 3 He, disordered by impurities, demonstrating suppression of the transition temperature, suppression of the order parameter, and gapless superfluid behavior.Superfluid 3 He is a unique example of a Cooper pair condensation. It was the first unconventional pairing state discovered. By unconventional we mean that there are symmetries of the normal Fermi liquid that are spontaneously broken in the superfluid phases in addition to gauge symmetry, ...

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Specific Heat of Disordered SuperfluidHe3

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“…The free energy difference between the phases, G AB , can be determined from their susceptibility difference and the field dependence of the AB-transition [18]. The susceptibility difference in the dirty superfluid has been measured [11,15] at 18 and 32 bar, and is approximately a factor of two weaker than for the pure superfluid. We have measured the field dependence of the dirty AB-transition at 25 and 33.4 bar, Fig.1 and 2.…”

Section: Fig 2 the Phase Diagrams Of The Superfluid Phases Ofmentioning

confidence: 99%

“…The magnetic field independent line marks the normal to superfluid A-phase boundary and the transition temperature from A to B-phases is quadratically suppressed by field. The first observation of an AB-transition in the dirty superfluid was reported by Barker et al [15]. We assume that the order parameter symmetry of the dirty phases corresponds to that of the pure 3 He superfluids, i.e.…”

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Nucleation and Interfacial Coupling between Pure and Dirty Superfluid Phases ofH3e

et al. 2002

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The nucleation of the first order phase transition of superfluid 3 He-B from superfluid 3 He-A is quite remarkable since it requires a seed of the order of a micron. We have studied this nucleation for 3 He confined to a very dilute silica aerogel. This dirty superfluid behaves in a manner similar to previous reports for the pure superfluid. But we have discovered a novel magnetically driven nucleation switch acting on the pure superfluid B-phase. Lastly, we find the surprising result that the proximity effect between the pure and dirty superfluids at their interface is insufficient to nucleate the B-phase in either superfluid.PACS numbers:67.57. Pq,64.60.Qb,67.57.Np The A-phase of superfluid 3 He can be extensively supercooled, far into the region at low temperatures where the B-phase is thermodynamically stable [1,2]. Leggett [3,4] pointed out that the very small difference in free energy between these phases makes homogeneous nucleation of this first order phase transition so improbable as to be unrealizable within the age of the universe. If a seed of the B-phase is to grow it must surpass a critical size of a few microns, so large as to be inaccessible by thermal fluctuations. Thus the experimental fact that the B-phase nucleates at all from the A-phase indicates the existence of some heterogeneous mechanism. Quite a number of experiments have shown that ionizing radiation [1,2], vibrations [5,6], or rough surfaces [5,7] can be, under various restrictive circumstances, active sites for this nucleation; but how the process proceeds remains a major puzzle and is actively debated [8].The discovery of a class of 3 He superfluids [9,10] constrained by silica aerogel raises new questions concerning B-phase nucleation. Does the metastability of the Aphase for these superfluids follow the same pattern as for the pure case? Is there sufficient coupling between the two superfluids across their interface for nucleating the B-phase? With an appropriate choice of experimental conditions, temperature, pressure, and magnetic field, we can arrange that the interface separates two superfluids with the same, or different, order parameter symmetry. And if they are the same, we might expect the proximity effect, well known in superconductivity, to provide a nucleation source. By this we mean that the B-phase from one side of the interface should readily nucleate the B-phase in the other. We report here that indeed the superfluid constrained in aerogel exhibits supercooling and metastability similar to that of pure superfluid 3 He in the absence of aerogel; however, contrary to our expectation, there is no evidence of nucleation from the proximity effect. In addition, we have discovered a nucleation source for the pure B-superfluid that is extremely efficient and can be switched off by applying a magnetic field.The new 'dirty' superfluids are characterized by quasiparticle scattering from a silica aerogel-matrix that reduces their transition temperature and suppresses the amplitude of the order parameter [10,11]. The aerogel...

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“…This property of ABM-order parameter is of importance in connection with the identification of the A-like phase which has been observed in liquid 3 He in aerogel just below T c [5,6].…”

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“…In case of A-like phase the competing states have to be of equal spin pairing (ESP) type. It follows from the experimental observation that magnetic susceptibility of the emerging phase coincides with that of normal and ABM-phases [5]. A general form of the order parameter for ESP-state is:…”

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Effect of a Quenched Disorder on the Order Parameter of Superfluid 3He

Фомин¹

2006

AIP Conference Proceedings

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Abstract. As a consequence of continuous degeneracy of the order parameter of the superfluid 3 He quenched disorder in a form of aerogel gives rise both to a disruption of the orientational long-range order of the condensate and to a significant change of the order parameter itself. There exist a class of quasi-isotropic order parameters which are not sensitive to the disorienting effect of aerogel. While the BW order parameter belongs to this class the ABM does not. Possible candidate for the order parameter of the observed A-like phase is discussed.Average distance ξ a between the strands of high porosity aerogels introduced as impurities in the superfluid 3 He [1, 2] does not differ much from the superfluid coherence length ξ 0 . This property is in a conflict with the condition of applicability of the conventional theory of superconducting alloys [3] nξ 3 0 ≫ 1, where n is the average density of impurities. In aerogels nξ 3 0 ∼ 1 and fluctuations of nξ 3 0 are significant. In a vicinity of the transition temperature T c effect of fluctuations can be described phenomenologically by introduction of random terms in the Ginzburg and Landau (GL) functional. For the scalar order parameter ψ such analysis has been made by Larkin and Ovchinnikov [4]. According to this analysis the most singular at T → T c corrections to the order parameter originate from the termis the density of states and η(r) -a random scalar), which describes local variations of T c . Generalization of this approach to the 3×3 complex matrix order parameter A µ j is straightforward. Additional term in the GL functional has a form:Now η jl (r) is a random tensor with zero average value. At the strength of t → −t invariance tensor η jl (r) is real and symmetrical, its isotropic part η (i) jl (r) = 1 3 η mm (r)δ jl describes local variations of T c = T c (r) due to fluctuations of the density of scatterers, it couples only to the overall amplitude of the order parameter A µl A * µl ≡ ∆ 2 . Effect of η (i) jl (r) is similar to that for the scalar case -broadening of the transition and suppression of the absolute value of the order parameter. New properties originate from the anisotropic part η (a) jl (r) ≡ η jl (r) − 1 3 η ll (r)δ jl which describes local splitting of T c for different projections of angular momenta because of breaking of spherical symmetry by the aerogel strands. Contribution of η (a) jl (r) to F η depends on the particular form of the order parameter and on its orientation with respect to the aerogel. Correspondingly η (a) jl (r) can both influence the form of order parameter and locally change its orientation. To make a qualitative guess of possible effect of η (a) jl (r) for different order parameters one can start with a uniform order parameter and substitute it into F η . No essential difference with respect to the scalar case is expected if η (a) jl A µ j A * µl = 0. This is the case for the BW-order parameter of bulk B-phase: A BW µ j = ∆e iϕ R µ j , where R µ j -a real orthogonal matrix, but not for the bulk A-phase (ABM). ...

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Observation of a Superfluid He-3A-BPhase Transition in Silica Aerogel

Cited by 92 publications

References 15 publications

Specific Heat of Disordered SuperfluidHe3

Specific Heat of Disordered SuperfluidHe3

Nucleation and Interfacial Coupling between Pure and Dirty Superfluid Phases ofH3e

Effect of a Quenched Disorder on the Order Parameter of Superfluid 3He

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