letters to nature was found to be smaller than 1 mm in height,consistent with a full Wave-particle duality Rayleigh length of 800 um and the strong power dependence of this ionization process.A significant advantage of the thermionic of C6o molecules mechanism is that it does not detect any of the residual gases present in the vacuum chamber.We could thus achieve dark count Markus Arndt,Olaf Nairz,Julian Vos-Andreae,Claudia Keller, rates of less than one per second even under moderately high Gerbrand van der Zouw Anton Zeilinger vacuum conditions (5X 10-7mbar).The fullerene ions were then focused by an optimized ion lens system,and accelerated to a BeCu conversion electrode at -9kV where they induced the Institut fuir Experimentalphysik,Universitat Wien,Boltzmanngasse 5, A-1090 Wien,Austria emission of electrons which were subsequently amplified by a Channeltron detector. Alignment is a crucial part of this experiment.In order to be able Quantum superposition lies at the heart of quantum mechanics to find the beam in the first place,our collimation apertures are and gives rise to many of its paradoxes.Superposition of de movable piezo slits that can be opened from 0 to 60 um (in the case Broglie matter waves'has been observed for massive particles of the first slit)and from 0 to 200 jm (for the second slit).The such as electrons',atoms and dimers',small van der Waals vacuum chamber is rigidly mounted on an optical table together clusters,and neutrons'.But matter wave interferometry with with the ionizing laser,in order to minimize spatial drifts. larger objects has remained experimentally challenging,despite The effect of gravity also had to be considered in our set-up.For the development of powerful atom interferometric techniques for the most probable velocity(220 ms),the fullerenes fall by 0.7 mm experiments in fundamental quantum mechanics,metrology and while traversing the apparatus.This imposes a constraint on the lithography Here we report the observation of de Broglie wave maximum tilt that the grating may have with respect to gravity.As a interference of Coo molecules by diffraction at a material absorp- typical diffraction angle into the first-order maximum is 25 urad, tion grating.This molecule is the most massive and complex one can tolerate a tilt angle of (at most)about one mrad before object in which wave behaviour has been observed.Of particular molecules start falling from one diffraction order into the trajectory interest is the fact that Cso is almost a classical body,because of its of a neighbouring order of a different velocity class.The many excited internal degrees of freedom and their possible experimental curves start to become asymmetric as soon as the couplings to the environment.Such couplings are essential for grating tilt deviates by more than 500 prad from its optimum the appearance of decoherence2,suggesting that interference vertical orientation. experiments with large molecules should facilitate detailed The interference pattern of Fig.2a clearly exhibits the central studies of this process. maximum and the first-order diffraction peaks.The minima When considering de Broglie wave phenomena of larger and between zeroth and first orders are well developed,and are due to more complex objects than atoms,fullerenes come to mind as destructive interference of Coo de Broglie waves passing through suitable candidates.After their discovery and the subsequent neighbouring slits of the grating.For comparison,we show in Fig.2b invention of efficient mass-production methods",they became he profile of the undiffracted collimated beam.The velocity easily available.In our experiment (see Fig.1)we use commercial, distribution has been measured independently by a time-of-flight 99.5%pure,Co fullerenes(Dynamic Enterprises Ltd,Twyford,UK) method;it can be well fitted by f(v)=v'exp(-(v-vo)/v),with which were sublimated in an oven at temperatures between 900 and vo=166ms-and vm =92ms-!as expected for a transition 1,000 K.The emerging molecular beam passed through two between a maxwellian effusive beam and a supersonic beam3.The collimation slits,each about 10 um wide,separated by a distance most probable velocity was v=220ms-,corresponding to a de of 1.04 m.Then it traversed a free-standing nanofabricated SiN. Broglie wavelength of 2.5 pm.The full-width at half-maximum was grating"consisting of nominally 50-nm-wide slits with a 100-nm as broad as 60%,resulting in a longitudinal coherence length of period. about 5 pm. At a further distance of 1.25m behind the diffraction grating,the The essential features of the interference pattern can be under- interference pattern was observed using a spatially resolving detec- stood using standard Kirchhoff diffraction theory for a grating tor.It consisted of a beam from a visible argon-ion laser(24 W all with a period of 100nm,by taking into account both the finite lines),focused to a gaussian waist of 8 um width (this is the size width of the collimation and the experimentally determined veloc- required for the light intensity to drop to l/e2 of that in the centre of ity distribution.The parameters in the fit were the width of the the beam).The light beam was directed vertically,parallel both to collimation,the gap width so of a single slit opening,the effective the lines of the diffraction grating and to the collimation slits.By beam width of the detection laser and an overall scaling factor.This using a suitable mirror assembly,the focus could be scanned with model,assuming all grating slits to be perfect and identical, micrometre resolution across the interference pattern.The reproduces very well the central peak of the interference pattern absorbed light then ionized the C6o fullerenes via heating and shown in Fig.2a,but does not fit the 'wings'of this pattern. subsequent thermal emission of electrons.The detection region Agreement with the experimental data,including the 'wings'in 100 nm diffraction Scanning photo- Figure 1 Diagram of the experimental set-up(not to scale).Hot,neutral Co molecules grating ionization stade leave the oven through a nozzle of 0.33 mm x 1.3 mm X 0.25 mm (width x height x depth),pass through two collimating slits of 0.01 mm x 5mm (width x height)separated by 1.04 m,traverse a SiN,grating (period 100 nm)0.1 m after the second slit,and are detected via thermal ionization by a laser 1.25 m behind the grating.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 680 1999 Macmillan Magazines Ltd NATURE|VOL 401|14 OCTOBER 1999 www.nature.com
© 1999 Macmillan Magazines Ltd ................................................................. Wave–particle duality of C60 molecules Markus Arndt, Olaf Nairz, Julian Vos-Andreae, Claudia Keller, Gerbrand van der Zouw & Anton Zeilinger Institut fu¨r Experimentalphysik, Universita¨t 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 with larger objects has remained experimentally challenging, despite the development of powerful atom interferometric techniques for experiments in fundamental quantum mechanics, metrology and lithography6 . 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 particular interest 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 for the 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 and more 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 two collimation slits, each about 10mm wide, separated by a distance of 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, the interference pattern was observed using a spatially resolving detector. It consisted of a beam from a visible argon-ion laser (24 W all lines), focused to a gaussian waist of 8mm width (this is the size required for the light intensity to drop to 1/e 2 of that in the centre of the beam). The light beam was directed vertically, parallel both to the lines of the diffraction grating and to the collimation slits. By using a suitable mirror assembly, the focus could be scanned with micrometre resolution across the interference pattern. The absorbed light then ionized the C60 fullerenes via heating and subsequent thermal emission of electrons12. The detection region was found to be smaller than 1 mm in height, consistent with a full Rayleigh length of 800mm and the strong power dependence of this ionization process. A significant advantage of the thermionic mechanism is that it does not detect any of the residual gases present in the vacuum chamber. We could thus achieve dark count rates of less than one per second even under moderately high vacuum conditions (5 3 10 2 7 mbar). The fullerene ions were then focused by an optimized ion lens system, and accelerated to a BeCu conversion electrode at −9 kV where they induced the emission of electrons which were subsequently amplified by a Channeltron 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 are movable piezo slits that can be opened from 0 to 60 mm (in the case of the first slit) and from 0 to 200mm (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 a typical diffraction angle into the first-order maximum is 25mrad, one can tolerate a tilt angle of (at most) about one mrad before molecules start falling from one diffraction order into the trajectory of a neighbouring order of a different velocity class. The experimental curves start to become asymmetric as soon as the grating tilt deviates by more than 500mrad 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 to destructive interference of C60 de Broglie waves passing through neighbouring 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-flight method; it can be well fitted by f ðvÞ ¼ v3 expð 2 ðv 2 v0Þ 2 =v2 mÞ, with v0 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 of about 5 pm. The essential features of the interference pattern can be understood using standard Kirchhoff diffraction theory14 for a grating with 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 the collimation, the gap width s0 of a single slit opening, the effective beam width of the detection laser and an overall scaling factor. This model, assuming all grating slits to be perfect and identical, reproduces very well the central peak of the interference pattern shown in Fig. 2a, but does not fit the ‘wings’ of this pattern. Agreement with the experimental data, including the ‘wings’ in letters to nature 680 NATURE | VOL 401 | 14 OCTOBER 1999 | www.nature.com Oven Collimation slits 100 nm diffraction grating Ion detection unit 10 mµ 10 mµ Laser Scanning photoionization stage Figure 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 the grating. 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
letters to nature Fig.2a,can be achieved by allowing for a gaussian variation of the In quantum interference experiments,coherent superposition slit widths over the grating,with a mean open gap width centred at only arises if no information whatsoever can be obtained,even in 5o=38 nm with a full-width at half-maximum of 18 nm.That best- principle,about which path the interfering particle took.Interac- fit value for the most probable open gap width so is significantly tion with the environment could therefore lead to decoherence.We smaller than the 55+5nm specified by the manufacturer(T.A. now analyse why decoherence has not occurred in our experiment Savas and H.Smith,personal communication).This trend is and how modifications of our experiment could allow studies of consistent with results obtained in the diffraction of noble gases decoherence using the rich internal structure of fullerenes. and He clusters,where the apparently narrower slit was interpreted In an experiment of the kind reported here,'which-path'infor- as being due to the influence of the van der Waals interaction with mation could be given by the molecules in scattering or emission the SiN,grating during the passage of the molecules'5.This effect is processes,resulting in entanglement with the environment and a expected to be even more pronounced for Coo molecules owing to loss of interference.Among all possible processes,the following are their larger polarizability.The width of the distribution seems also the most relevant:decay of vibrational excitations via emission of justified in the light of previous experiments with similar gratings: infrared radiation,emission or absorption of thermal blackbody both the manufacturing process and adsorbents could account radiation over a continuous spectrum,Rayleigh scattering,and for this fact (ref.16,and T.A.Savras and H.Smith,personal collisions. communication).Recently,we also observed interference of C7o When considering these effects,one should keep in mind that molecules. only those scattering processes which allow us to determine the path Observation of quantum interference with fullerenes is interest- of a Coo molecule will completely destroy in a single event the ing for various reasons.First,the agreement between our measured interference between paths through neighbouring slits.This and calculated interference contrast suggests that not only the requires Ad;that is,the wavelength A of the incident or emitted highly symmetric,isotopically pure C molecules contribute to radiation has to be smaller than the distance d between neighbour- the interference pattern but also the less symmetric isotopomeric ing slits,which amounts to 100 nm in our experiment.When this variants CsC and CssC2 which occur with a total natural condition is not fulfilled decoherence is however also possible via abundance of about 50%.If only the isotopically pure ,12C60 multi-photon scattering7 molecules contributed to the interference,we would observe a At T=900K,as in our experiment,each C6o molecule has on much larger background. average a total vibrational energy of E.=7 eV(ref.18)stored in 174 Second,we emphasize that for calculating the de Broglie wave- vibrational modes,four of which may emit infrared radiation at length,A h/Mv,we have to use the complete mass M of the object. 入b≈7-l9μm(ref.l0)each with an Einstein coefficient of Thus,each C6o molecule acts as a whole undivided particle during its A=100s-(ref.18).During its time of flight from the grating centre-of-mass propagation. towards the detector (T 6 ms)a C6o molecule may thus emit on Last,the rather high temperature of the C6o molecules implies average 2-3 such photons. broad distributions,both of their kinetic energy and of their internal In addition,hot Coo has been observeds to emit continuous energies.Our good quantitative agreement between experiment and blackbody radiation,in agreement with Planck's law,with a mea- theory indicates that the latter do not influence the observed sured integrated emissivity ofe=4.5(+2.0)X 10-5(ref.18).For coherence.All these observations support the view that each C6o a typical value of T=900K,the average energy emitted during the molecule interferes with itself only. time of flight can then be estimated as only E0.1eV.This corresponds to the emission of (for example)a single photon at 入≈loμm.Absorption of blackbody radiation has an even smaller influence as the environment is at a lower temperature than the 1,200 molecule.Finally,since the mean free path for neutral C6o exceeds 100 m in our experiment,collisions with background molecules can 1.000 be neglected. As shown above,the wavelengths involved are too large for single 800 photon decoherence.Also,the scattering rates are far too small to induce sufficient phase diffusion.This explains the decoupling of 600 internal and external degrees of freedom,and the persistence of interference in our present experiment. 400 A variety of unusual decoherence experiments would be possible in a future extension of the experiment,using a large-area inter- 200 ferometer.A three-grating Mach-Zehnder interferometer seems to 200 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 pm, much larger than the wavelength of a typical thermal photon.In this 150 case,the environment obtains which-path'information even through a single thermal photon,and the interference contrast 100 should thus be completely destroyed.The parameters that could be controlled continuously in such an experiment would then be the internal temperature of the fullerenes,the temperature of the environment,the intensity and frequency ofexternal laser radiation, &8 品 the interferometer size,and the background pressure of various 0 gases. -100 -50 0 100 An improved interferometer could have other applications.For Position(um) example,incontrast to previous atom-optical experiment Figure 2 Interference patter produced by Ceo molecules.a,Experimental recording which were limited to the interaction with only a few lines in the (open circles)and fit using Kirchhoff diffraction theory (continuous line).The expected whole spectrum,interferometry with fullerenes would enable us to zeroth and first-order maxima can be clearly seen.Details of the theory are discussed in study these naturally occurring and ubiquitous thermal processes the text.b,The molecular beam profile without the grating in the path of the molecules. and wavelength-dependent decoherence mechanisms for (we NATURE|VOL 40114 OCTOBER 1999www.nature.com 1999 Macmillan Magazines Ltd 681
© 1999 Macmillan Magazines Ltd Fig. 2a, can be achieved by allowing for a gaussian variation of the slit 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 best- fit 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 is consistent 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 to their 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 account for this fact (ref. 16, and T. A. Savras and H. Smith, personal communication). 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 the highly symmetric, isotopically pure 12C60 molecules contribute to the interference pattern but also the less symmetric isotopomeric variants 12C5913C and 12C5813C2 which occur with a total natural abundance of about 50%. If only the isotopically pure 12C60 molecules contributed to the interference, we would observe a much 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 internal energies. Our good quantitative agreement between experiment and theory indicates that the latter do not influence the observed coherence. All these observations support the view that each C60 molecule interferes with itself only. 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 experiment and how modifications of our experiment could allow studies of decoherence 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 a loss 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, and collisions. When considering these effects, one should keep in mind that only those scattering processes which allow us to determine the path of a C60 molecule will completely destroy in a single event the interference between paths through neighbouring slits. This requires 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 this condition 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 < 7–19mm (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). For a typical value of T < 900 K, the average energy emitted during the time of flight can then be estimated as only Ebb < 0:1 eV. This corresponds to the emission of (for example) a single photon at l < 10mm. Absorption of blackbody radiation has an even smaller influence as the environment is at a lower temperature than the molecule. Finally, since the mean free path for neutral C60 exceeds 100 m in our experiment, collisions with background molecules can be 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 of internal and external degrees of freedom, and the persistence of interference 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 this case, the environment obtains ‘which-path’ information even through a single thermal photon, and the interference contrast should thus be completely destroyed. The parameters that could be controlled continuously in such an experiment would then be the internal temperature of the fullerenes, the temperature of the environment, 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 the whole spectrum, interferometry with fullerenes would enable us to study these naturally occurring and ubiquitous thermal processes and wavelength-dependent decoherence mechanisms for (we letters to nature NATURE | VOL 401 | 14 OCTOBER 1999 | www.nature.com 681 200 400 600 800 1,000 1,200 b a –100 –50 0 50 100 0 50 100 150 200 Counts in 1 s Counts in 50 s 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
letters to nature believe)the first time.Another possible application of molecule interferometry is precision metrology;the improved interferometer Lanthanum-substituted bismuth could be used to measure molecular polarizabilities".Moreover,it might be possible to nanofabricate SiC patterns on Si substrates titanate for use in using C6o interferometry. Furthermore,we note the fundamental difference between iso- non-volatile memories topically pure Co which should exhibit bosonic statistics,and the isotopomers containing one C nucleus,which should exhibit B.H.Park",B.S.Kang',S.D.Bu",T.W.Noh",J.Leet W.Jo fermionic statistics.We intend to explore the possibility of obser- ving this feature,for example by showing the different rotational *Department of Physics and Condensed Matter Research Institute, symmetry between the two species in an interferometer2324. Seoul National University,Seoul 151-742,Korea In our experiment,the de Broglie wavelength of the interfering Department of Materials Engineering,Sung Kyun Kwan University, fullerenes is already smaller than their diameter by a factor of almost Suwon 440-746,Korea 400.It would certainly be interesting to investigate the interference LG Corporate Institute of Technology,Seoul 137-140,Korea of objects the size of which is equal to or even bigger than the diffracting structure.Methods analogous to those used for the Non-volatile memory devices are so named because they retain present work,probably extended to the use of optical diffraction information when power is interrupted;thus they are important structures,could also be applied to study quantum interference of computer components.In this context,there has been consider- even larger macromolecules or clusters,up to small viruses5 able recent interest'2 in developing non-volatile memories that use ferroelectric thin films-ferroelectric random access mem- Received 30 June;accepted 2 September 1999. ories,or FRAMs-in which information is stored in the polariza- 1.de Broglie,L Waves and quanta.Naticre 112,540 (1923). tion state of the ferroelectric material.To realize a practical 2.Davisson,C.I&Germer,L.H.The scattering of electrons by a single crystal of nickel.119. 558-560(1927). FRAM,the thin films should satisfy the following criteria:com- Estermann,I.Stern,O.Beugung von Molekularstrahlen.Z.Phys.61,95-125(1930) patibility with existing dynamic random access memory technol Schollkopf,W.Toennies,I.P.Nondestructive mass selection of small van der Waals clusters.Science ogies,large remnant polarization(P)and reliable polarization- 266,1345-1348(199). cycling characteristics.Early work focused on lead zirconate 5.Halban,H.v.Ir Preiswerk,P.Preuve experimentale de la diffraction des neutrons.C.R.Acd.Sci. 203,73-75(19361. titanate(PZT)but,when films of this material were grown on 6.Berman.P led)Atomt (Academic.1997) metal electrodes,they generally suffered from a reduction of P 7.Zurek,W.H.Decoherence and the a ition from quantum to dassical.Phys.Today 36-44 (October ('fatigue')with polarity switching.Strontium bismuth tantalate 1991. 8. Giulini,D.et al.Dece ppe mce of the Classical World in Theory (Springer (SBT)and related oxides have been proposed to overcome the Berlin.1996) fatigue problem',but such materials have other shortcomings, Kroto,H.W.,Heath,I.B C..Curl,R.F&Smalley.R.E.C buckminsterfullerene.Nature such as a high deposition temperature.Here we show that 318.162-1661985). lanthanum-substituted bismuth titanate thin films provide a 10.Kratschmer,W.Lamb,L D. opoulos,K.Huffman,D.R.A new form of carbon.Natre 347 promising alternative for FRAM applications.The films are 354-358119901. 11.Savas,T.A..Shah,S.N.Schattenburg.M.L,Carter,I.M.&Smith,H.I.Achromatic interferometric fatigue-free on metal electrodes,they can be deposited at tem- lithography for 100-nm s and grids.I.Vac.Sci.Technol.B 13,2732-2735(1995). peratures of~650C and their values of P,are larger than those of 12.Ding,D..Huang.L.Compton,R.N..Klots,C.E Haufler,R.E cw laser ionization of C and C the SBT films. .Ra￥Lt.73.1084-1087199. 13.Scoles,G.(ed.)Atomic and Beam Methods Vol.1 (Oxford Univ.Press,1988). The structure of FRAM is very similar to that of conventional 14.Born,M.Wolf,E.Principles of Optics (Perg n.0xocd.1984】. dynamic random access memory(DRAM),where memory cells are 15.Grisenti,R.E.Scholkopf,W.Toennies.I.P.,Hegerfeldt,G.C.Kohler,T.Determination of atom. arranged in a square matrix.(Therefore,FRAM should,in principle, surface van der Waals potentials from transmission-grating diffraction intensities.Phrys Rev.Lett.83, have a lower power requirement,a faster access time and a 1755-1758（19991. 6.Grisenti,R.E.et al.He atom diffraction from nanostructure transmission gratings:the role of potentially lower cost than many other non-volatile memory imperfections.Phys Rev A (submitted). devices'3.)A DRAM cell usually has a capacitor,where the binary 17.Joos,E.Zeh,H.D.The nce of classical properties through interaction with the environn information will be stored in terms of the signs of the stored charge. ∠Phys B59.223-243（1985). 18.Kolodney.E.Budrevich,A.&Tsipinyuk.B.Unir olecular rate constants To maintain this information,a voltage should be continually and cooling mechanisms superhot Coo molecules.Plrys.Rev.Lett.74,510-513 (1995). applied to compensate for charge reduction due to leakage currents. 19.Mitzner,R.Campbell,E.E.B.Optical emission studies of laser desorbed Co.Chem.Phys.103, In FRAM,the dielectric material in the capacitor is replaced with a 2445-2453(1995). ferroelectric film.Then,information can be stored in the polariza- 20.Pfau.T.Spilter,S..Kurtsiefer,Ch,Ekstrom,C.R.&Mlynek..Loss of spatial coherence by a single spontaneous emission.Pkys Rev.Lett.73,1223-1226 (1994). tion states of the ferroelectric thin film:that is,two spontaneous 21.Clauser,I.F.Li,S."Heise ope"decoherence atom interferometry.Phys.Rev.A 50, polarization states under zero electric field can be utilized as'0'and 2430-2433(1994). 'I'digital states.It seems that the first generation of FRAM will be 22.Chapman,M.S.et al.Photon scattering from atoms in an atom interferometer:Coherence lost and based on a destructive reading scheme,where a bit is read when a 1g=ined.Pl's Rev Lett.75.3783-37871995. 23.Werner,S.A.,Colella,R..Overhauser,A.W.&Eagen.C.F Observation of the phase shif of a neutron positive switching voltage (that is,an applied electric field)is due to precession in a magnetic field.Phys.Rev.Lett.35,1053-1055(1975). applied to the memory cell.If the cell polarization is already 24.Rauch,H.et al.Verification of coherent spinor rotation of fermions.Phys.Lett A54.425-427(1975). along the same direction,then only a linear non-switching response, 25.Clauser,I.F.in Experimental Metaplysics (eds Cohen,R.S.Home,M.Stachel,1.)1-11 (Kluwer Academic,Dordrecht.1997) Pas,is measured.If it is in the opposite direction,a switching 26.Arndt,M.Nairz,O.van der Zouw,G.&Zeilinger,A.in Epistemological and Exp ental per response,Pw is measured.Therefore(Pw-P),which should be on Quantum,Phrysics (eds Greenberger,D.,Reiter,W.L&Zeilinger,A.)221-224(IVC Yearbook, nearly the same as 2P,is an important quantity which determines Kluwer Academic,Dordrecht,1999). the performance of FRAM.It should be large enough for signal detection and should not change under repetitive read/write cycles. Acknowledgements The PZT and other related ferroelectric films,which are most We thank M.Haluska,H.Kuzmany,R.Penrose,P.Scheier,J.Schmiedmayer and G.Senn widely investigated,usually have large values of(P-P):depend- for discussions.This work was supported by the Austrian Science Foundation FWF,the ing on substituting elements and processing conditions,reported Austrian Academy of Sciences,the TMR programme of the European Union,and the US values vary from 20 to 70uCcm(refs 4,5).However,when a NSE. capacitor is fabricated with the PZT film on conventional platinum Correspondence and requests for materials should be addressed to A.Z. (Pt)electrodes,its value of (P-P)is usually reduced after (e-mail:zeilinger-office@exp.univic.ac.at). repetitive read/write cycles.This fatigue problem might be induced 682 1999 Macmillan Magazines Ltd NATURE VOL 40114 OCTOBER 1999 www.nature.com
© 1999 Macmillan Magazines Ltd believe) the first time. Another possible application of molecule interferometry 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 the isotopomers 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 interference of objects the size of which is equal to or even bigger than the diffracting structure. Methods analogous to those used for the present work, probably extended to the use of optical diffraction structures, could also be applied to study quantum interference of even larger macromolecules or clusters, up to small viruses25,26. M Received 30 June; accepted 2 September 1999. 1. de Broglie, L. Waves and quanta. Nature 112, 540 (1923). 2. Davisson, C. J. & Germer, L. H. The scattering of electrons by a single crystal of nickel. Nature 119, 558–560 (1927). 3. Estermann, I. & Stern, O. Beugung von Molekularstrahlen. Z. Phys. 61, 95–125 (1930). 4. Scho¨llkopf, W. & Toennies, J. P. Nondestructive mass selection of small van der Waals clusters. Science 266, 1345–1348 (1994). 5. Halban, H. v. Jr & Preiswerk, P. Preuve expe´rimentale de la diffraction des neutrons. C.R. Acad. Sci. 203, 73–75 (1936). 6. Berman, P. (ed.) Atom Interferometry (Academic, 1997). 7. Zurek, W. H. Decoherence and the transition from quantum to classical. Phys. Today 36–44 (October 1991). 8. Giulini, D. et al. Decoherence and the Appearance of the Classical World in Quantum Theory (Springer, Berlin, 1996). 9. Kroto, H. W., Heath, J. R., O’Brien, S. C., Curl, R. F. & Smalley, R. E. C60: buckminsterfullerene. Nature 318, 162–166 (1985). 10. Kra¨tschmer, W., Lamb, L. D., Fostiropoulos, K. & Huffman, D. R. A new form of carbon. Nature 347, 354–358 (1990). 11. Savas, T. A., Shah, S. N., Schattenburg, M. L., Carter, J. M. & Smith, H. I. Achromatic interferometric lithography for 100-nm-period gratings and grids. J. Vac. Sci. Technol. B 13, 2732–2735 (1995). 12. Ding, D., Huang, J., Compton, R. N., Klots, C. E. & Haufler, R. E. cw laser ionization of C60 and C70. Phys. Rev. Lett. 73, 1084–1087 (1994). 13. Scoles, G. (ed.) Atomic and Molecular Beam Methods Vol. 1 (Oxford Univ. Press, 1988). 14. Born, M. & Wolf, E. Principles of Optics (Pergamon, Oxford, 1984). 15. Grisenti, R. E., Scho¨lkopf, W., Toennies, J. P., Hegerfeldt, G. C. & Ko¨hler, 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 of imperfections. 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 of superhot 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., Spa¨lter, S., Kurtsiefer, Ch., Ekstrom, C. R. & Mlynek, J. Loss of spatial coherence by a single spontaneous emission. Phys. Rev. Lett. 73, 1223–1226 (1994). 21. Clauser, J. F. & Li, S. ‘‘Heisenberg microscope’’ decoherence atom interferometry. Phys. Rev. A 50, 2430–2433 (1994). 22. Chapman, M. S. et al. Photon scattering from atoms in an atom interferometer: Coherence lost and regained. Phys. Rev. Lett. 75, 3783–3787 (1995). 23. Werner, S. A., Colella, R., Overhauser, A. W. & Eagen, C. F. Observation of the phase shift of a neutron due to precession in a magnetic field. Phys. Rev. Lett. 35, 1053–1055 (1975). 24. Rauch, H.et al. Verification of coherent spinor rotation of fermions. Phys. Lett. A 54; 425–427 (1975). 25. Clauser, J. F. in Experimental Metaphysics (eds Cohen, R. S., Home, M. & Stachel, J.) 1–11 (Kluwer Academic, Dordrecht, 1997). 26. Arndt, M., Nairz, O., van der Zouw, G. & Zeilinger, A. in Epistemological and Experimental Perspectives on Quantum, Physics (eds Greenberger, D., Reiter, W. L. & Zeilinger, A.) 221–224 (IVC Yearbook, Kluwer Academic, Dordrecht, 1999). Acknowledgements We thank M. Halusˇka, H. Kuzmany, R. Penrose, P. Scheier, J. Schmiedmayer and G. Senn for discussions. This work was supported by the Austrian Science Foundation FWF, the Austrian 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). letters to nature 682 NATURE | VOL 401 | 14 OCTOBER 1999 | www.nature.com ................................................................. 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 important computer components. In this context, there has been considerable recent interest1,2 in developing non-volatile memories that use 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 the fatigue problem3 , but such materials have other shortcomings, such as a high deposition temperature. Here we show that lanthanum-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 are arranged in a square matrix. (Therefore, FRAM should, in principle, have a lower power requirement, a faster access time and a potentially 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 continually applied to compensate for charge reduction due to leakage currents. In FRAM, the dielectric material in the capacitor is replaced with a ferroelectric film. Then, information can be stored in the polarization states of the ferroelectric thin film: that is, two spontaneous polarization states under zero electric field can be utilized as ‘0’ and ‘1’ digital states. It seems that the first generation of FRAM will be based on a destructive reading scheme, where a bit is read when a positive switching voltage (that is, an applied electric field) is applied to the memory cell2 . If the cell polarization is already along 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 be nearly 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 most widely investigated, usually have large values of (Psw 2 Pns): depending on substituting elements and processing conditions, reported values vary from 20 to 70mC cm−2 (refs 4, 5). However, when a capacitor 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