1. A liquid ground state for 2D helium-3?
Ashley G. Smart
Citation: Phys. Today 66(1), 16 (2013); doi: 10.1063/PT.3.1842
View online: http://dx.doi.org/10.1063/PT.3.1842
View Table of Contents: http://www.physicstoday.org/resource/1/PHTOAD/v66/i1
Published by the American Institute of Physics.
Additional resources for Physics Today
Homepage: http://www.physicstoday.org/
Information: http://www.physicstoday.org/about_us
Daily Edition: http://www.physicstoday.org/daily_edition
Downloaded 29 Jan 2013 to 14.139.59.131. Redistribution subject to AIP license or copyright; see http://www.physicstoday.org/about_us/terms

2. search and discovery
A liquid ground state for 2D helium-3?
New experiments hint at what could be the lowest-density liquid ever film used in the experiment might have
found in nature. hosted ripple-like quantum excitations,
or ripplons, which could mediate inter-
B
atomic interactions not appearing in
ecause helium-3 is so light, and be- pendent of the total number of atoms.
theoretical models.
cause, as a fermion, it can’t crowd But Bhattacharyya and Gasparini found Now, work from a University of
into low energy levels, ensembles that under certain conditions, γ grew as Tokyo team led by Hiroshi Fukuyama
of 3He atoms have unusually large ki- 3
He atoms were added. The only rea- may dispel notions that the apparent liq-
netic energy in the ground state. When sonable interpretation, they concluded, uid phase is an artifact of hydrodynamic
that energy is partitioned among two was that 3He atoms weren’t spreading or 3D effects.2 Their new experiments
dimensions instead of three, it’s thought out over the entire film surface, as a gas seem to show that the precise nature of
to be sufficient to overcome van der would, but instead were collecting into the underlying film has little bearing on
Waals attractions and prevent the sys- puddles of fixed, low density that occu- whether or not 3He condenses.
tem from condensing into a 2D liquid— pied a growing fraction of the film
even at absolute zero. surface as atoms were added. Flat liquid
It came as a surprise, then, when in The Buffalo team’s finding runs The Tokyo group’s recent work grew
a 1985 experiment1 by Bidyut Bhat- counter to nearly five decades of out of previous efforts to elucidate
tacharyya and Francis Gasparini (State theoretical and numerical work on 2D how magnetically disordered systems
University of New York at Buffalo) 3He quantum systems. And subsequent ex- known as spin liquids transform to
appeared to form a quasi-2D liquid. The periments have only muddied the ferromagnetic states. Fukuyama and
researchers had added 3He to a thin film picture: Work by Moses Chan’s group his colleagues sought to re-create such
of 4He and chilled the mixture to milli- at the Pennsylvania State University a transformation in the lab by gradually
kelvin temperatures. In such a system, seemingly corroborated the results, increasing the density of a 3He mono-
the 3He atoms strongly prefer to sit at but a later study by Hallock and layer adsorbed on graphite. Before the
the film’s surface, and because 4He be- colleagues at the University of Massa- transition occurred, however, some 3He
comes a superfluid, they can move chusetts Amherst found no signs of a atoms leapt out of the densely packed
about that surface almost as easily as liquid phase. monolayer to form a new top layer.
they would in free space. (See the article Some theorists speculate that the Those atoms seemed to form puddles
by Robert Hallock, PHYSICS TODAY, June surface of a thin film just doesn’t suit- much like the ones observed in the
1998, page 30.) ably approximate a 2D environment. Buffalo experiment.
The heat capacity of such a weakly Surely, atoms at the free surface retain Figuring that the appearance of the
interacting 2D group of fermions is some freedom to move in the normal liquid phase was probably connected
known to grow linearly with tempera- direction, and such imperfect confine- to the atoms’ out-of-plane motions,
ture. The slope, γ, can be shown to be ment should effectively reduce the Fukuyama, Tomohiro Matsui, and
proportional to the atomic mass and atoms’ ground state energy in the plane graduate students Daisuke Sato and
the occupied surface area—but inde- of interest. Furthermore, the superfluid Kimiaki Naruse decided to probe
3
He’s behavior in the three different
quasi-2D systems depicted in figure 1.
In one case (top image in figure), the
3
He layer of interest was adsorbed di-
rectly onto a graphite substrate; in a
second (center), it was deposited on a
dense monolayer of 4He; in a third
(bottom), it was deposited atop two
dense monolayers—a layer of 3He
overlying a layer of 4He. The 4He
monolayers serve to mitigate the ef-
fects of surface heterogeneities.
Each system imposes varying de-
grees of 2D confinement on the topmost
3
3 He atoms: Those adsorbed directly
He
onto graphite have the least freedom to
4
He move normal to the plane, whereas
those sitting atop two monolayers have
Graphite
the most. So if the purported liquid
phase was indeed an artifact of out-of-
plane motion, the Tokyo group should
Figure 1. Mimicking two dimensions. When chilled to millikelvin temperatures,
have seen quantitative differences in
helium-3 atoms atop a graphite substrate (top), a dense 4He monolayer (center),
the phase behavior of the three systems,
or dense 4He and 3He monolayers (bottom) closely approximate a 2D quantum
and perhaps no liquid phase at all for
fluid. In the multilayer films, the bottom monolayer of 4He serves to mitigate the 3
effects of heterogeneities in the graphite substrate. (Adapted from ref. 2.) He deposited directly on the graphite
substrate. Because the underlying
16 January 2013 Physics Today www.physicstoday.org
Downloaded 29 Jan 2013 to 14.139.59.131. Redistribution subject to AIP license or copyright; see http://www.physicstoday.org/about_us/terms

3. Figure 2. Calorimetric data for ultracold helium-3 atop a dense 4He monolayer
show signatures of a liquid phase: At low coverage densities, γ—the slope of the Your partner
heat capacity as a function of
temperature—grows linearly
with the average coverage
in innovation
density ρ. In that regime, 3He
collects into liquid puddles of
fixed, low density. Above the Supporting you all
critical density of ρ ≈ 0.6 nm−2, the way
γ (mJ/K2)
100 at which the puddles
presumably cover the entire
substrate surface area, γ
grows nonlinearly, indicating Come and visit our booth
Ideal Fermi gas strengthening atomic DPG (Regensburg), 12-14 March
interactions. Extrapolated to APS (Baltimore), 19-21 March
ρ = 0 (dot–dash line), the
nonlinear branch yields a γ
that’s roughly 25% larger Cryogenic and magnet
0 than the ideal Fermi gas experts and highly
0 1 2 3 4 value, indicated by the red technical sales engineers:
ρ (nm−2) line. (Adapted from ref. 2.) Helping you define the right tools for
your application
monolayers in both multilayer films be- sponding to an ideal, noninteracting
have as 2D solids, ripplons shouldn’t Fermi gas. The actual value, however,
factor into any of the three scenarios. is roughly 25% larger. The implication
To the team’s surprise, calorimetric is that there must be some as-yet
data obtained at temperatures of 80 mK unidentified interaction—possibly me-
and below were quite similar for all diated by quasiparticles—that drives
three systems. Figure 2 shows represen- condensation.
tative results obtained from 3He atop a Washington State University theo- Skilled manufacturing
single layer of 4He: As in the Buffalo ex- rist Michael Miller thinks there may be engineers and technicians
periment, γ grows proportionally to the a simpler answer: The Tokyo group
average areal density ρ—the number of Building reliable products
may be seeing not condensation but ag- shaped to your needs
surface atoms per unit area of sub- gregation, driven by preferential ad-
strate—at small ρ. And as in the Buffalo sorption at heterogeneities in the
experiment, that linear-growth region graphite substrate. “If it turns out that
is thought to signal puddle formation. this can’t be explained away in terms of
Notably, the linear trend can be extrap- surface inhomogeneities,” Miller com-
olated to the origin, which suggests that ments, “then something very strange
the liquid phase is stable in the infi- has to be going on.”
nitely dilute limit. A key next step for the Tokyo group
The researchers infer from their will be to determine the critical temper- Service and customer
data that the areal density inside the ature at which the 2D liquid transitions support engineers
puddles lies roughly in the range of to a pure gas. That temperature is ex-
0.6–0.9 nm−2. That would make it the Installing and
pected to lie somewhere in the range of
lowest-density liquid ever discovered supporting your
80–700 mK. To pinpoint it, Fukuyama product in your
in nature, with a mean interatomic and his colleagues will need to do some
spacing of more than a nanometer, laboratory
tinkering with their calorimeter: It cur-
more than twice that of 3D 3He. rently relies on a superconducting zinc
Implied interactions heat switch that fails at temperatures
above 80 mK.
Fukuyama isn’t yet sure how to recon-
Theorists will have plenty to mull
cile the experiments with theory, but the For further information:
over in the meantime. In the opinion of
data in figure 2 may hold an important aps.nanoscience@oxinst.com
Gasparini, “One might say the score is
clue. They show that at a critical average or visit our new website:
now three experiments in favor of a 2D
density, γ shifts from a linear function of www.oxinst.com/aps
liquid phase and one against it.”
ρ to a nonlinear one. Presumably, that
Ashley G. Smart
density marks the point at which liquid
3
He covers the entire substrate surface, References
and the nonlinear behavior reflects
1. B. K. Bhattacharyya, F. M. Gasparini,
strengthening atomic interactions as the Phys. Rev. B 31, 2719 (1985).
liquid becomes more densely packed. 2. D. Sato, K. Naruse, T. Matsui, H.
Extrapolating the nonlinear branch Fukuyama, Phys. Rev. Lett. 109, 235306
to ρ = 0 should, in theory, yield γ corre- (2012).
www.physicstoday.org January 2013 Physics Today 17
Downloaded 29 Jan 2013 to 14.139.59.131. Redistribution subject to AIP license or copyright; see http://www.physicstoday.org/about_us/terms