letters to nature................................................................. was found to be smaller than 1 mm in height, consistent with a fullWave–particle dualityof C60 molecules
Markus Arndt, Olaf Nairz, Julian Vos-Andreae, Claudia Keller,
Gerbrand van der Zouw & Anton Zeilinger
Institut für Experimentalphysik, Universität Wien, Boltzmanngasse 5,
A-1090 Wien, Austria.................................. ......................... ......................... ......................... ......................... ........

Quantum superposition lies at the heart of quantum mechanics
and gives rise to many of its paradoxes. Superposition of de
Broglie matter waves1 has been observed for massive particles
such as electrons2, atoms and dimers3, small van der Waals
clusters4, and neutrons5. But matter wave interferometry withlarger objects has remained experimentally challenging, despite
the development of powerful atom interferometric techniques forexperiments in fundamental quantum mechanics, metrology andlithography6. Here we report the observation of de Broglie wave
interference of C60 molecules by diffraction at a material absorption grating. This molecule is the most massive and complex
object in which wave behaviour has been observed. Of particularinterest is the fact that C60 is almost a classical body, because of its
many excited internal degrees of freedom and their possible
couplings to the environment. Such couplings are essential forthe appearance of decoherence7,8, suggesting that interference
experiments with large molecules should facilitate detailed
studies of this process.
When considering de Broglie wave phenomena of larger andmore complex objects than atoms, fullerenes come to mind as
suitable candidates. After their discovery9 and the subsequent
invention of efficient mass-production methods10, they became
easily available. In our experiment (see Fig. 1) we use commercial,
99.5% pure, C60 fullerenes (Dynamic Enterprises Ltd, Twyford, UK)
which were sublimated in an oven at temperatures between 900 and
1,000 K. The emerging molecular beam passed through twocollimation slits, each about 10 mm wide, separated by a distanceof 1.04 m. Then it traversed a free-standing nanofabricated SiNx
grating11 consisting of nominally 50-nm-wide slits with a 100-nm
period.
At a further distance of 1.25 m behind the diffraction grating, theinterference pattern was observed using a spatially resolving detector. It consisted of a beam from a visible argon-ion laser (24 W alllines), focused to a gaussian waist of 8 mm width (this is the size
required for the light intensity to drop to 1/e2 of that in the centre of
the beam). The light beam was directed vertically, parallel both tothe lines of the diffraction grating and to the collimation slits. By
using a suitable mirror assembly, the focus could be scanned withmicrometre resolution across the interference pattern. The
absorbed light then ionized the C60 fullerenes via heating andsubsequent thermal emission of electrons12. The detection region

100 nm diffraction
grating

Scanning photoionization stage

Oven

10 µm

Ion
detection
unit

10 µm

Rayleigh length of 800 mm and the strong power dependence of thisionization process. A significant advantage of the thermionicmechanism is that it does not detect any of the residual gases
present in the vacuum chamber. We could thus achieve dark countrates of less than one per second even under moderately highvacuum conditions (5 3 10 2 7 mbar). The fullerene ions werethen focused by an optimized ion lens system, and accelerated toa BeCu conversion electrode at −9 kV where they induced theemission of electrons which were subsequently amplified by aChanneltron detector.
Alignment is a crucial part of this experiment. In order to be able
to find the beam in the first place, our collimation apertures aremovable piezo slits that can be opened from 0 to 60 mm (in the caseof the first slit) and from 0 to 200 mm (for the second slit). The
vacuum chamber is rigidly mounted on an optical table together
with the ionizing laser, in order to minimize spatial drifts.
The effect of gravity also had to be considered in our set-up. For
the most probable velocity (220 m s−1), the fullerenes fall by 0.7 mm
while traversing the apparatus. This imposes a constraint on the
maximum tilt that the grating may have with respect to gravity. As atypical diffraction angle into the first-order maximum is 25 mrad,
one can tolerate a tilt angle of (at most) about one mrad before
molecules start falling from one diffraction order into the trajectoryof a neighbouring order of a different velocity class. The
experimental curves start to become asymmetric as soon as thegrating tilt deviates by more than 500 mrad from its optimum
vertical orientation.
The interference pattern of Fig. 2a clearly exhibits the central
maximum and the first-order diffraction peaks. The minima
between zeroth and first orders are well developed, and are due todestructive interference of C60 de Broglie waves passing throughneighbouring slits of the grating. For comparison, we show in Fig. 2b
the profile of the undiffracted collimated beam. The velocity
distribution has been measured independently by a time-of-flightmethod; it can be well fitted by f ðvÞ ¼ v3 expð 2 ðv 2 v0 Þ2 =v2m Þ, withv0 ¼ 166 m s 2 1 and vm ¼ 92 m s 2 1 as expected for a transition
between a maxwellian effusive beam and a supersonic beam13. The
most probable velocity was v ¼ 220 m s 2 1 , corresponding to a de
Broglie wavelength of 2.5 pm. The full-width at half-maximum was
as broad as 60%, resulting in a longitudinal coherence length ofabout 5 pm.
The essential features of the interference pattern can be understood using standard Kirchhoff diffraction theory14 for a gratingwith a period of 100 nm, by taking into account both the finite
width of the collimation and the experimentally determined velocity distribution. The parameters in the fit were the width of thecollimation, the gap width s0 of a single slit opening, the effective
beam width of the detection laser and an overall scaling factor. Thismodel, assuming all grating slits to be perfect and identical,
reproduces very well the central peak of the interference patternshown in Fig. 2a, but does not fit the ‘wings’ of this pattern.
Agreement with the experimental data, including the ‘wings’ inFigure 1 Diagram of the experimental set-up (not to scale). Hot, neutral C60 molecules
leave the oven through a nozzle of 0:33 mm 3 1:3 mm 3 0:25 mm(width 3 height 3 depth), pass through two collimating slits of 0:01 mm 3 5 mm
(width 3 height) separated by 1.04 m, traverse a SiNx grating (period 100 nm) 0.1 m after
the second slit, and are detected via thermal ionization by a laser 1.25 m behind thegrating. The ions are then accelerated and directed towards a conversion electrode. The
ejected electrons are subsequently counted by a Channeltron electron multiplier. The
laser focus can be reproducibly scanned transversely to the beam with 1-mm resolution.

Collimation slits
Laser

680

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letters to nature
Fig. 2a, can be achieved by allowing for a gaussian variation of theslit widths over the grating, with a mean open gap width centred at
s0 ¼ 38 nm with a full-width at half-maximum of 18 nm. That bestfit value for the most probable open gap width s0 is significantly
smaller than the 55 6 5 nm specified by the manufacturer (T. A.
Savas and H. Smith, personal communication). This trend isconsistent with results obtained in the diffraction of noble gases
and He clusters, where the apparently narrower slit was interpreted
as being due to the influence of the van der Waals interaction with
the SiNx grating during the passage of the molecules15. This effect is
expected to be even more pronounced for C60 molecules owing totheir larger polarizability. The width of the distribution seems also
justified in the light of previous experiments with similar gratings:
both the manufacturing process and adsorbents could accountfor this fact (ref. 16, and T. A. Savras and H. Smith, personalcommunication). Recently, we also observed interference of C70
molecules.
Observation of quantum interference with fullerenes is interesting for various reasons. First, the agreement between our measured
and calculated interference contrast suggests that not only thehighly symmetric, isotopically pure 12C60 molecules contribute to
the interference pattern but also the less symmetric isotopomericvariants 12C5913C and 12C5813C2 which occur with a total naturalabundance of about 50%. If only the isotopically pure 12C60
molecules contributed to the interference, we would observe amuch larger background.
Second, we emphasize that for calculating the de Broglie wavelength, l ¼ h=Mv, we have to use the complete mass M of the object.
Thus, each C60 molecule acts as a whole undivided particle during its
centre-of-mass propagation.
Last, the rather high temperature of the C60 molecules implies
broad distributions, both of their kinetic energy and of their internalenergies. Our good quantitative agreement between experiment andtheory indicates that the latter do not influence the observed
coherence. All these observations support the view that each C60
molecule interferes with itself only.

1,200

a

Counts in 50 s

1,000

800

600

400

200

Counts in 1 s

200

b

150

100

50

0
–100

–50

0

50

100

Position (µm)

Figure 2 Interference pattern produced by C60 molecules. a, Experimental recording(open circles) and fit using Kirchhoff diffraction theory (continuous line). The expected
zeroth and first-order maxima can be clearly seen. Details of the theory are discussed in
the text. b, The molecular beam profile without the grating in the path of the molecules.
NATURE | VOL 401 | 14 OCTOBER 1999 | www.nature.com

In quantum interference experiments, coherent superposition
only arises if no information whatsoever can be obtained, even in
principle, about which path the interfering particle took. Interaction with the environment could therefore lead to decoherence. We
now analyse why decoherence has not occurred in our experimentand how modifications of our experiment could allow studies ofdecoherence using the rich internal structure of fullerenes.
In an experiment of the kind reported here, ‘which-path’ information could be given by the molecules in scattering or emission
processes, resulting in entanglement with the environment and aloss of interference. Among all possible processes, the following are
the most relevant: decay of vibrational excitations via emission of
infrared radiation, emission or absorption of thermal blackbody
radiation over a continuous spectrum, Rayleigh scattering, andcollisions.
When considering these effects, one should keep in mind that
only those scattering processes which allow us to determine the pathof a C60 molecule will completely destroy in a single event theinterference between paths through neighbouring slits. Thisrequires l p d; that is, the wavelength l of the incident or emitted
radiation has to be smaller than the distance d between neighbouring slits, which amounts to 100 nm in our experiment. When thiscondition is not fulfilled decoherence is however also possible via
multi-photon scattering7,8,17.
At T < 900 K, as in our experiment, each C60 molecule has on
average a total vibrational energy of Ev < 7 eV (ref. 18) stored in 174
vibrational modes, four of which may emit infrared radiation at
lvib < 7–19 mm (ref. 10) each with an Einstein coefficient of
Ak < 100 s 2 1 (ref. 18). During its time of flight from the grating
towards the detector (t < 6 ms) a C60 molecule may thus emit on
average 2–3 such photons.
In addition, hot C60 has been observed19 to emit continuous
blackbody radiation, in agreement with Planck’s law, with a measured integrated emissivity of e < 4:5 ð 6 2:0Þ 3 10 2 5 (ref. 18). Fora typical value of T < 900 K, the average energy emitted during thetime of flight can then be estimated as only Ebb < 0:1 eV. This
corresponds to the emission of (for example) a single photon atl < 10 mm. Absorption of blackbody radiation has an even smaller
influence as the environment is at a lower temperature than themolecule. Finally, since the mean free path for neutral C60 exceeds
100 m in our experiment, collisions with background molecules canbe neglected.
As shown above, the wavelengths involved are too large for single
photon decoherence. Also, the scattering rates are far too small to
induce sufficient phase diffusion. This explains the decoupling ofinternal and external degrees of freedom, and the persistence ofinterference in our present experiment.
A variety of unusual decoherence experiments would be possible
in a future extension of the experiment, using a large-area interferometer. A three-grating Mach–Zehnder interferometer6 seems to
be a particularly favourable choice, since for a grating separation of
up to 1 m we will have a molecular beam separation of up to 30 mm,
much larger than the wavelength of a typical thermal photon. In thiscase, the environment obtains ‘which-path’ information eventhrough a single thermal photon, and the interference contrast
should thus be completely destroyed. The parameters that could becontrolled continuously in such an experiment would then be theinternal temperature of the fullerenes, the temperature of theenvironment, the intensity and frequency of external laser radiation,
the interferometer size, and the background pressure of various
gases.
An improved interferometer could have other applications. For
example, in contrast to previous atom-optical experiments20–22
which were limited to the interaction with only a few lines in thewhole spectrum, interferometry with fullerenes would enable us to
study these naturally occurring and ubiquitous thermal processes
and wavelength-dependent decoherence mechanisms for (we© 1999 Macmillan Magazines Ltd

681

letters to nature
believe) the first time. Another possible application of moleculeinterferometry is precision metrology; the improved interferometer
could be used to measure molecular polarizabilities6. Moreover, it
might be possible to nanofabricate SiC patterns on Si substrates
using C60 interferometry.
Furthermore, we note the fundamental difference between isotopically pure C60, which should exhibit bosonic statistics, and theisotopomers containing one 13C nucleus, which should exhibit
fermionic statistics. We intend to explore the possibility of observing this feature, for example by showing the different rotational
symmetry between the two species in an interferometer23,24.
In our experiment, the de Broglie wavelength of the interfering
fullerenes is already smaller than their diameter by a factor of almost
400. It would certainly be interesting to investigate the interferenceof objects the size of which is equal to or even bigger than thediffracting structure. Methods analogous to those used for thepresent work, probably extended to the use of optical diffraction
structures, could also be applied to study quantum interference ofeven larger macromolecules or clusters, up to small viruses25,26. M
Received 30 June; accepted 2 September 1999.
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4. Schöllkopf, W. & Toennies, J. P. Nondestructive mass selection of small van der Waals clusters. Science
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12. Ding, D., Huang, J., Compton, R. N., Klots, C. E. & Haufler, R. E. cw laser ionization of C60 and C70.
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14. Born, M. & Wolf, E. Principles of Optics (Pergamon, Oxford, 1984).
15. Grisenti, R. E., Schölkopf, W., Toennies, J. P., Hegerfeldt, G. C. & Köhler, T. Determination of atomsurface van der Waals potentials from transmission-grating diffraction intensities. Phys. Rev. Lett. 83,
1755–1758 (1999).
16. Grisenti, R. E. et al. He atom diffraction from nanostructure transmission gratings: the role ofimperfections. Phys. Rev. A (submitted).
17. Joos, E. & Zeh, H. D. The emergence of classical properties through interaction with the environment.
Z. Phys. B. 59, 223–243 (1985).
18. Kolodney, E., Budrevich, A. & Tsipinyuk, B. Unimolecular rate constants and cooling mechanisms ofsuperhot C60 molecules. Phys. Rev. Lett. 74, 510–513 (1995).
19. Mitzner, R. & Campbell, E. E. B. Optical emission studies of laser desorbed C60. J. Chem. Phys. 103,2445–2453 (1995).
20. Pfau, T., Spälter, S., Kurtsiefer, Ch., Ekstrom, C. R. & Mlynek, J. Loss of spatial coherence by a single
spontaneous emission. Phys. Rev. Lett. 73, 1223–1226 (1994).
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2430–2433 (1994).
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due to precession in a magnetic field. Phys. Rev. Lett. 35, 1053–1055 (1975).
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Acknowledgements
We thank M. Haluška, H. Kuzmany, R. Penrose, P. Scheier, J. Schmiedmayer and G. Senn
for discussions. This work was supported by the Austrian Science Foundation FWF, theAustrian Academy of Sciences, the TMR programme of the European Union, and the US
NSF.
Correspondence and requests for materials should be addressed to A.Z.
(e-mail: zeilinger-office@exp.univie.ac.at).

682.................................................................
Lanthanum-substituted bismuth
titanate for use in
non-volatile memories
B. H.Park*, B. S. Kang*, S. D. Bu*, T. W. Noh*, J. Lee† & W. Jo‡* Department of Physics and Condensed Matter Research Institute,
Seoul National University, Seoul 151-742, Korea† Department of Materials Engineering, Sung Kyun Kwan University,
Suwon 440-746, Korea‡ LG Corporate Institute of Technology, Seoul 137-140, Korea.......................................... ......................... ......................... ......................... .........................

Non-volatile memory devices are so named because they retain
information when power is interrupted; thus they are importantcomputer components. In this context, there has been considerable recent interest1,2 in developing non-volatile memories thatuse ferroelectric thin films—‘ferroelectric random access memories’, or FRAMs—in which information is stored in the polarization state of the ferroelectric material. To realize a practical
FRAM, the thin films should satisfy the following criteria: compatibility with existing dynamic random access memory technologies, large remnant polarization (Pr) and reliable polarizationcycling characteristics. Early work focused on lead zirconate
titanate (PZT) but, when films of this material were grown on
metal electrodes, they generally suffered from a reduction of Pr(‘fatigue’) with polarity switching. Strontium bismuth tantalate(SBT) and related oxides have been proposed to overcome thefatigue problem3, but such materials have other shortcomings,
such as a high deposition temperature. Here we show thatlanthanum-substituted bismuth titanate thin films provide a
promising alternative for FRAM applications. The films are
fatigue-free on metal electrodes, they can be deposited at temperatures of ,650 8C and their values of Pr are larger than those of
the SBT films.
The structure of FRAM is very similar to that of conventional
dynamic random access memory (DRAM), where memory cells arearranged in a square matrix. (Therefore, FRAM should, in principle,
have a lower power requirement, a faster access time and apotentially lower cost than many other non-volatile memory
devices1,3.) A DRAM cell usually has a capacitor, where the binary
information will be stored in terms of the signs of the stored charge.
To maintain this information, a voltage should be continuallyapplied to compensate for charge reduction due to leakage currents.
In FRAM, the dielectric material in the capacitor is replaced with aferroelectric film. Then, information can be stored in the polarization states of the ferroelectric thin film: that is, two spontaneouspolarization states under zero electric field can be utilized as ‘0’ and‘1’ digital states. It seems that the first generation of FRAM will bebased on a destructive reading scheme, where a bit is read when apositive switching voltage (that is, an applied electric field) is
applied to the memory cell2. If the cell polarization is alreadyalong the same direction, then only a linear non-switching response,
Pns, is measured. If it is in the opposite direction, a switching
response, Psw, is measured. Therefore (Psw 2 Pns ), which should benearly the same as 2Pr , is an important quantity which determines
the performance of FRAM. It should be large enough for signal
detection and should not change under repetitive read/write cycles.
The PZT and other related ferroelectric films, which are mostwidely investigated, usually have large values of (Psw 2 Pns ): depending on substituting elements and processing conditions, reported
values vary from 20 to 70 mC cm−2 (refs 4, 5). However, when acapacitor is fabricated with the PZT film on conventional platinum(Pt) electrodes, its value of (Psw 2 Pns ) is usually reduced after
repetitive read/write cycles. This fatigue problem might be induced© 1999 Macmillan Magazines Ltd

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