CASTILLO-GARZA ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX

www.acsnano.org

A

CXXXX American Chemical Society

Nanoscale Imaging of Neutral Atomswith a Pulsed Magnetic LensRodrigo Castillo-Garza,* Jamie Gardner, Sagi Zisman, and Mark G. Raizen

Center for Nonlinear Dynamics and Department of Physics, The University of Texas at Austin, Austin, Texas 78712, United States

Great advances in nanolithographyand nanofabrication over the yearshave been constantly pushing the

limits of resolution. Despite these impres-sive results, it is clear that in order to reachthe few nanometer level, a new approach isrequired. Parallel, top-down approachesbased on photolithography are limited bythe wavelength of the light that is used forthese processes.State-of-the-art technology can now

achieve a feature size of 20�100 nm.1,2

While other top-down approaches basedon electron-beam lithography can achievenanometer scale structures, they are limitedby the thickness of the resist and are inher-ently very slow.3,4 Alternate methods thatrely on bottom-up techniques are based onmolecular self-assembly.5 These can achievenanometer scale structures, but they are cur-rently limited by random thermal processes.6

The possibility of using neutral atoms in abeam to reach the nanometer scale resolu-tion stimulated a great deal of work, startingover 20 years ago.7�11 This field, known asatom lithography, mainly utilized standingwaves of light to focus the atoms.7,9 Whilethe basic method was successful, it waslimited to parallel lines and a few simplespecific arrays12 due to the periodic nature

of a standingwave of light. The resolution ofatom lithography was also limited by severeaberrations of the standing wave, a funda-mental problem that has stopped furtherprogress.12

An alternative approach is to use themagnetic moment of an atom, which is anearly universal property of most elementsin the periodic table. In previous work,permanent magnets have been used tofocus atoms.10,13 The refractive power ofthese magnetic lenses is a function of thegradient of the magnetic field and themagnetic moment to mass ratio of theatom. The method has been limited so farby the low magnetic field gradients avail-able, which result in a large focal length, aswell as by the fringe fields that the atomsexperience as they enter and leave themagnets. Clark et al.14 recently proposed aparallel method to achieve focal spots withdimensions of sub-10 nm; however, thatmethod does not enable true imaging andhence has limited applicability.Here we propose an efficient method to

control the structure of materials at thenanometer scale. In this method, a smallbunch of fast and cold atoms producedwitha pulsed supersonic source is divided intoseveral bunches of atoms with a perforated

* Address correspondence torcastillogarza@chaos.utexas.edu.

Received for review February 21, 2013and accepted April 1, 2013.

Published online10.1021/nn400896y

ABSTRACT We present a scheme for imaging of neutral atoms to the

nanoscale with a pulsed magnetic lens and show its viability through

numerical calculations. This scheme achieves focal lengths on the order of

several centimeters and focal spots of less than 10 nm. With these results, it

is possible to create sub-10 nm structures on surfaces in a parallel and time-

efficient manner. When used with metastable noble gas atoms, and in

combination with electron spectroscopy, this scheme can create a chemically

sensitive microscope which can probe surfaces on the nanometer scale.

KEYWORDS: nanolithography . pulsed magnetic lens . nanocharacterization . atom microscope . sub-10 nm . tapered hexapole .chemically sensitive probe

ARTIC

LE

CASTILLO-GARZA ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX

www.acsnano.org

B

pattern on a transmission mask. The pattern is thenimaged to a much smaller size by a pulsed magneticlens. A schematic diagram of an experimental setup forthis method is shown in Figure 1. It contains an Even-Lavie pulsed supersonic source (Figure 1a), a magneticmirror (Figure 1d) to select the magnetic state of theatoms that enter the lens, and a pulsed magnetic lens(Figure 1f). Due to the geometry and extent of the lens,this method can enable parallel nanofabrication ofstructures. In addition, this lens is aberration-corrected,and it is effective on all atoms that are paramagnetic,which encompass most elements in the periodic table.The choice of a high-flux pulsed supersonic source

(Figure 1a) is a critical starting point for this work, as itproduces a fast and a highly monoenergetic beam,Δv )/v )∼ 0.01, with an intensity of 1024 atoms/(sr s).15 Itcan produce pulsewidths as lowas 10μs at a frequencyof up to 1 kHz.15 Furthermore, this source can be usedin conjunction with seeding techniques, such as laserablation, to produce a supersonic beamof any elementin the periodic table and of many molecules.16 In theimportant case of noble gas atoms that are nonmag-netic in their ground state, such as heliumor neon, theycan be excited to a long-lived metastable state byemploying a pulsed discharge near the nozzle17

(Figure 1a). Metastable atoms are particularly useful,as will be described below.Once these atoms exit the source, they form a bullet

(Figure 1b) that will be optically manipulated to en-hance the image contrast and to allow the lens tocorrect for aberrations (Figure 1c). For the former, wewill increase the beam brightness with an optical 2Dmolasses setup, as this technique has been successfulin achieving high brightness values.18,19 For the latter,we will reduce the longitudinal velocity spread (Δv )) inthe bullet's frame of reference and optically tag the

bullet (Figure 1c) as it comes out of the source. Toreduce Δv ), we plan to laser cool the atoms in thelongitudinal direction but without changing their ini-tial average velocity (v )). To tag the bullet, we wouldoptically pump a thin (100 μm) slice of the bullet to acertain magnetic state and pump the rest to a differentmagnetic state.The desired magnetic state of the atoms will be

selected with a magnetic mirror in a planar Halbachconfiguration.20,21 Thismirror consists of a collection ofconstant amplitude permanent magnets, each ar-ranged according to the direction of their individualmagnetization. Each magnet's magnetization is ro-tated 90� with respect to its nearest neighbors andfollows a sense of rotation (Figure 1d). This periodicconfiguration produces, on one side only, an inhomo-geneous magnetic field (B) that decays exponentiallyin the normal direction to themirror. This field interactswith the magnetic dipole of the atoms in the beam.Their interaction energy is

U ¼ mFgFμBB (1)

where mF is the magnetic quantum number of theparticle, gF is the Landé g-factor and μB is the Bohrmagneton. Low-field-seeking atoms with mFgF > 0 arepushed away from themirror, while atoms withmFgF < 0are attracted to it. The mirror is positioned at an anglewith respect to the beamdirection such that the thin sliceof the bullet is reflected and directed toward the trans-missionmask (Figure 1e). For instance, using commercialneodymiummagnets (remanence∼ 1.3 T) in the mirror,this angle is ∼2.5� for metastable neon at 300 K in the33P2 mF =þ2 state. This selection method allows for thefocal plane to be out of sight of the source and forthe beam divergence to be unaltered. More impor-tantly, as will be shown below, this selection of a sliceallows the lens to correct for aberrations.Since supersonic sources produce fast atoms, a lens

with high refractive power is required to achievepractical focal lengths, on the order of centimeters.Current permanent magnet technology has not yetoffered a viable solution.22 Instead, we propose using alensbasedonapulsed, coreless electromagnet (Figure 2).An electromagnet generates an inhomogeneous mag-netic field and provides complete control of the field'smagnitude through the modification of its current. Itsrefractive power is a function of the same parametersas that of the permanent magnets, as the interactionenergy between a magnetic dipole of an atom and thefield is that of eq 1. To achieve the desired refractivepower, the electromagnet we are proposing hasto generate a magnetic field gradient of 1�2 T/mm.A practically advantageous method to accomplish thisis using a pulsed electromagnet with small dimensions,∼O(mm). This is a common technique that has beensuccessfully used to generate the magnetic field gra-dients we require.15

Figure 1. Schematic diagram of the proposed experimentalsetup. (a) Even-Lavie pulsed supersonic nozzle with a high-voltage pulsed discharge. (b) Initial spatial extent of thebullet of atoms. (c) Brightening of the beam, reduction ofbeam's dispersion, and optical tagging of the bullet. (d)Magnetic mirror. (e) Transmission mask with a circularaperture (the object plane). (f) Pulsed magnetic lens. (g)Image plane. The lines represent the trajectories of para-magnetic atoms. The angle between the supersonic beamand the mirror selects the magnetic species that enter themagnetic lens. In this configuration, atoms of a particularlow-field-seeking state are selected and the rest of them arerejected.

ARTIC

LE

CASTILLO-GARZA ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX

www.acsnano.org

C

The electromagnetic lens we are proposing isaberration-corrected. It consists of six wire bundlesalong the longitudinal direction of the atomic beam,each at the vertex of an imaginary hexagon. Thebundles are slanted, while keeping the hexapole crosssection. They linearly decrease their distance to thecenter of the hexagon as seen by the particles travers-ing this wire configuration. Figure 2a shows a diagramof the tapered electromagnetic lens. The configurationof the electric current running through the bundles ischosen to produce a hexapole field. Figure 2b depicts aschematic diagram of the cross section of this mag-netic field and the wire bundle configuration.Through the magnetic interaction energy of eq 1, a

hexapole field exerts on each atom a radial force that isproportional to its distance from the center. Taperingoff the hexapole results in a radial force, Fr, and alongitudinal force, Fz. Both forces depend on thedistance of the atoms from the center and their long-itudinal position in the lens:

Fr ¼ �f2μBgFjmFjg

� 1 � ri � r0r0L0

� �(z � L0)

� ��2

� B0r

r20(2)

and

Fz ¼ �f2μBgFjmFjg � ri � r0r0L0

� �

� 1 � ri � r0r0L0

� �(z � L0)

� ��3

� B0r2

r20(3)

L0 ∼ Æv )æΔt is the effective lens width. Along thisdistance, the atoms interact with the tapered hexapolefor a timeΔt, which is the electromagnet pulsing time.B0 is the magnetic field at r = r0, the smallest innerradius of the lens, and ri is the largest inner radius. Thisgeometry results in the following tapering angle (R):

R ¼ tan�1 r0 � riL0

� �(4)

The force in eq 2 can focus a beam of low-field-seeking particles correcting for chromatic aberrationand the effects of the spatial extent of the bullet as itexits the source. For this to be successful, the long-itudinal position of the atoms in the bullet when insidethe lens has to be correlated with their initial long-itudinal velocity. This allows for Fr to be larger for fasteratoms than for slower atoms, therefore reducing thelens' focal length for faster atoms and increasing it forthe slower ones. We achieve this correlation throughthe selection, as described above, of a thin slice of thebullet of atoms at the source, where their longitudinalvelocity andposition are uncorrelated. If the hexapole'stapering angle is correctly selected, the focal lengthassociated with each atom will be forced to convergeto the same value, therefore effectively reducing chro-matic aberration and aberration effects due to thespatial extent of the bullet at the source.The longitudinal force of eq 3 resembles spherical

aberration from optical lenses, as it is stronger near theedges of the lens. Its effects are minimal in our setupsince the atomic beamwidth in the lens is smaller thanthe lens aperture by at least 1 mm. Furthermore, theabsolute value of this force is much smaller than theradial force of eq 2 for the effective part of the lens (L0),allowing us to neglect its effects. Its effects wereconfirmed in our numerical calculations.Additional aberrations arise due to deviations from

the hexapole field. This occurs close to the wire bun-dles due to its geometry and due to dispersion forces.Their effects are minimized by keeping the atomicbeam width small and far from the bundles wheninside the lens. An additional source of aberration isdue to the finite size of the lens in the longitudinal axis.The ends of the lens produce fringe fields that deviatefrom the magnetic field of a hexapole. We minimizedthe effect of these fields on the atom trajectories usingthe pulsing feature of the electromagnet. That is, thetiming of the current pulse generating the magneticfield is synchronized with the pulsed source so thatonly a hexapole-like field interacts with the atomicbeam bunch.To show the performance of this imaging method,

we use finite element analysis and numerical integra-tion to simulate the path of metastable helium (He*)atoms as they traverse the elements of this imagingmethod. We evaluate the performance of this methodin terms of the image that themagnetic lens produces.For this simulation, we use a pulsed beam of He* in the23S1 state and with mF = þ1. The spatial extent of thebullet's thin slice is 70 μm (FWHM) which, as wasdescribed above, would have been selected whencombining optical tagging (Figure 1c) and a planarHalbach array (Figure 1d). The source size for this beamis 600 μm, its temperature is 20 K, and it is 1.4 m awayfrom the transmission mask. The source produces apulsed beam of He* atoms with an average longitudinal

Figure 2. Electromagnetic Lens. (a) Tapered hexapole con-figuration. Wire configuration tapers off toward the centerof an imaginary hexagon as particles traverse through it,where t2 = (ri � r0)

2 þ L02. (b) Cross section of the magnetic

lens while switched on. Magnetic field is calculated withfinite element methods. The color scale represents themagnitude of the magnetic field: The field in the light-bluezones is greater than thefield in thedark-blue zones, the latterbeing the lowest field. The thin light-blue contour lines showwhere the magnetic field is constant. The black line circlessurrounding the hexapole field represent the wires.

ARTIC

LE

CASTILLO-GARZA ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX

www.acsnano.org

D

speed of v )= 455.77m/s and a relative beamdispersionof Δv )/v ) ∼ 2e�5. The latter is assuming that we havelaser-cooled the He* atoms from the beam in thelongitudinal direction and that they have reached therecoil cooling limit. The atomic beam is coaxial withthe perforated mask and the axis of the taperedhexapole lens. The distance between the mask andthe lens (analogous to the object distance in optics) is1.75m. Once the beam is in the lens, themagnetic fieldis switched on for 16 μs. The lens dimensions are r0 =1.5 mm and ri = 2.155 mm. No fringe fields areconsidered, as the atomic beam pulse is mostly insidethe lens. With a current pulse of I = 900 A, the lensproduces a magnetic field gradient of ∼1 T/mm andresults in a focal length of f ∼ 15.4 mm, as measuredfrom the beginning of the lens.

RESULTS AND DISCUSSION

Two image planes are examined: In the first case, weshow the image produced by 1000 He* atoms traver-sing a 400 nm circular aperture on the mask. In thesecond case, we show the image produced by 1000He* atoms traversing through an array of four 400 nmcircular apertures. In the first case, the aperture iscentered with the hexapole field center and an imageis produced at ∼1.78 m away from the mask. Thiscorresponds to an image distance of ∼15.5 mm, asmeasured from the beginning of the lens. Figure 3bshows the image plane and Figure 3a the object plane.This former figure shows the position of the He* atomsat the image plane in a 2D histogram, where each binhas dimensions of 0.5 nm� 0.5 nm. The gray scale bars

indicate the color code for the number of atoms. Whilethe atoms are spread on the image plane, most binscontain either zero or a few He* atoms. This is not thecase for the bins at the center of the image, where adark spot of about 4 nm � 4 nm contains most of theHe* atoms from the beambunch. These statements aremore evident in Figure 3c, where the density of atomsis plotted as a function of the radial position of theatoms from the center of the image. Note that theeffective demagnification factor of the lens is 100�,which is in agreement with the image distance toobject distance ratio used in light optics.The diffraction limit of this lens setup is about 6 nm.

However, the atom spot size is not diffraction-limited.Shorter focal lengths can be attained with highercurrents through the lens and larger numerical aper-tures. Particularly for this setup, the latter can beachieved by reducing the initial beam collimationand allowing a larger size beam to enter the lens. Aminimum diffraction limit of about 1 nm could beachieved.In the second case we simulated, imaging four

circular apertures on the transmission mask, we showthe parallel capability of the imaging method de-scribed in this article. All of the apertures on the mask(the object plane) have a 400 nm diameter and a pitchof 20 μm. Furthermore, passing through each aperture,there are a total of 250 He* atoms. In Figure 4, we show2D histograms of the position of the atoms on the (a)object plane and on a (b) close-up of the image plane.Figure 4c shows a closer look at one of the features inFigure 4b. This feature is indicated by a black arrow in

Figure 3. Imaging a beam of atoms through a 400 nm circular aperture. Two-dimensional histograms of the position of theatomson the (a) object plane and a (b) close-upof the center of the imageplane. Thegray scale bars indicate the color code forthe number of atoms. (c) Density of He* atoms as a function of their radial position on the image plane.

ARTIC

LE

CASTILLO-GARZA ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX

www.acsnano.org

E

Figure 4a,b. Similarly to Figure 3, the image plane ofFigure 4b has bins with few He* atoms and bins withlarge numbers of them, except that they are in separateand in specific sites. These sites have a pitch of about200 nm, and each of them extends for about 15 nm �15 nm. Note that in this figure and in Figure 4c the bindimensions are 3 nm� 3 nm, andmost of the particlesin each of these sites are contained in an area of 6 nm�6 nm (Figure 4c). In fact, Figure 4c shows that morethan 50% of the particles going through each apertureare in this area. As was found in the first case, the lensdemagnification factor is about 100�.Considering the parameters used in the simulation

of the beam passing through a single 400 nm apertureand the techniques we mentioned to prepare thebeam, we have estimated that ∼10�100 He* atomswill go through the aperture with each nozzle pulse.While this value may be sufficient for our immediatepurposes, this value is not yet limited. It can beincreased by at least 100 if sub-Doppler techniquesof laser cooling are used to increase the beam's bright-ness. Alternatively, we can increase the contrast of theimage by increasing the pulse frequency of the nozzleand magnet lens.Incorporating laser cooling and optical pumping

techniques to this imaging method reduces the num-ber of atomic species that can be utilized. Nevertheless,their combination can be successfully used forpatterning, as was the case for previous focusingschemes usingmolecular resists based on self-assembled

monolayers.23 In contrast with those schemes, themethod described here will be able to pattern surfaceswith higher spatial resolution, in less time, and withreduced complexity. The extension to other atomicspecies requires a new brightening method otherthan laser cooling, as proposed by M. G. Raizen andE. Narevicius (personal communications).

CONCLUSIONS

The method we are proposing is not only useful forparallel manufacturing of sub-10 nm structures, but itcan also be used as an atommicroscope. In analogy toelectron-beam lithography and a scanning electronmicroscope, a focused beam of metastable atoms canbe used to pattern as well as to probe a substrate. Inthis method, metastable atoms have thermal kineticenergies of∼O(10�2 eV) and carry an excitation energywith respect to their ground state of ∼ O(101 eV). Thislatter energy is released when in contact or close tocontact with a surface. When metastable atoms areused for patterning, this energy breaks the molecularbonds of a resist. When they are used to probe surfaces,this energy releases electrons from a substrate. In fact,electron spectroscopy of surfaces using metastableatoms has been a powerful technique in surfacescience,24,25 especially because it probes the outer-most layer of a surface without inflicting significantdamage, as compared to scanning electron micros-copy, and has a spatial resolution of approximatelymicrometers.26 Since the kinetic energy of the released

Figure 4. Parallel capability of the imaging method presented. Two-dimensional histograms of the position of the atoms (a)on the object plane, (b) on a close-up of the image plane, and (c) on a zoom-in of one of the features on the image plane(marked with an arrow in panels a,b). The (a) object plane has four black 500 nm� 500 nm bins that represent the number ofatoms traversing through the 400 nm apertures on the transmissionmask. The gray scale bars indicate the color code for thenumber of atoms.

ARTIC

LE

CASTILLO-GARZA ET AL. VOL. XXX ’ NO. XX ’ 000–000 ’ XXXX

www.acsnano.org

F

electrons depends on the work function and theelectronic properties of the bombarded material, thecombination of this technique with a focused beam ofmetastable atoms can be a chemically sensitive nano-probe. This technique can be used to probe and

understand materials in the forefront of science andtechnology, such as high-Tc superconductors, dopedsemiconductors, and topological insulators, in whichdesirable electronic properties may rely on the chemi-cal composition at the nanoscale.27�29

METHODSWe use finite element analysis to obtain the magnetic field in

the lens and numerical integration to simulate the trajectoriesof He* atoms as they traverse the elements of this imagingscheme. The parameters used for this simulation can be foundabove.

Conflict of Interest: The authors declare no competingfinancial interest.

Acknowledgment. We are pleased to acknowledge BruceKlappauf and Edvardas Narevicius for helpful discussions andthe careful reading of this manuscript. M.G.R. acknowledgessupport from the Sid W. Richardson Foundation and the R. A.Welch Foundation.

Note Added after ASAP Publication. This paper was publishedon the Web on April 9, 2013. A change was made to a value inthe last paragraph of the first section. The revised version wasreposted on April 11, 2013.

REFERENCES AND NOTES1. Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.;

Whitesides, G. M. New Approaches to Nanofabrication:Molding, Printing, and Other Techniques. Chem. Rev.2005, 105, 1171–1196.

2. Pease, R. F.; Chou, S. Y. Lithography and Other PatterningTechniques for Future Electronics. Proc. IEEE 2008, 96,248–270.

3. Chou, S. Y.; Krauss, P. R.; Zhang, W.; Guo, L.; Zhuang, L. InSub-10 nm Imprint Lithography and Applications, Papersfrom the 41st international conference on electron, ion,and photon beam technology and nanofabrication, DanaPoint, California, AVS: Dana Point, California, 1997; pp2897�2904.

4. Wei, C.; Ahmed, H. Fabrication of 5�7 nm Wide EtchedLines in Silicon Using 100 keV Electron-Beam Lithographyand Polymethylmethacrylate Resist. Appl. Phys. Lett. 1993,62, 1499.

5. Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Molecular Self-Assembly and Nanochemistry: A Chemical Strategy for theSynthesis of Nanostructures. Science 1991, 254, 1312–1319.

6. Stangl, J.; Hol�y, V.; Bauer, G. Structural Properties of Self-Organized Semiconductor Nanostructures. Rev. Mod. Phys.2004, 76, 725–783.

7. Timp, G.; Behringer, R. E.; Tennant, D. M.; Cunningham, J. E.;Prentiss, M.; Berggren, K. K. Using Light as a Lens forSubmicron, Neutral-Atom Lithography. Phys. Rev. Lett.1992, 69, 1636–1639.

8. Berggren, K. K.; Bard, A.; Wilbur, J. L.; Gillaspy, J. D.; Helg,A. G.; McClelland, J. J.; Rolston, S. L.; Phillips, W. D.; Prentiss,M.; Whitesides, G. M. Microlithography by Using NeutralMetastable Atoms and Self-Assembled Monolayers.Science 1995, 269, 1255–1257.

9. McGowan, R. W.; Giltner, D. M.; Lee, S. A. Light ForceCooling, Focusing, and Nanometer-Scale Deposition ofAluminum Atoms. Opt. Lett. 1995, 20, 2535–2537.

10. Kaenders, W. G.; Lison, F.; Richter, A.; Wynands, R.;Meschede, D. Imaging with an Atomic Beam. Nature1995, 375, 214–216.

11. Drodofsky, U.; Stuhler, J.; Schulze, T.; Drewsen, M.; Brezger,B.; Pfau, T.; Mlynek, J. Hexagonal Nanostructures

Generated by Light Masks for Neutral Atoms. Appl. Phys.B: Laser Opt. 1997, 65, 755–759.

12. Meschede, D.; Metcalf, H. Atomic Nanofabrication: AtomicDeposition and Lithography by Laser and MagneticForces. J. Phys. D: Appl. Phys. 2003, 36, R17.

13. Beardmore, J. P.; Palmer, A. J.; Kuiper, K. C.; Sang, R. T. AHexapole Magnetic Guide for Neutral Atomic Beams. Rev.Sci. Instrum. 2009, 80, 073105.

14. Clark, R. J.; Mazur, T. R.; Libson, A.; Raizen, M. G. Nanofab-rication by Magnetic Focusing of Supersonic Beams. Appl.Phys. B: Laser Opt. 2011, 103, 547–551.

15. Narevicius, E.; Libson, A.; Parthey, C. G.; Chavez, I.; Narevicius,J.; Even, U.; Raizen, M. G. Stopping Supersonic Beams with aSeries of Pulsed Electromagnetic Coils: An Atomic Coilgun.Phys. Rev. Lett. 2008, 100, 093003.

16. Smalley, R. E. Laser Studies of Metal Cluster Beams. LaserChem. 1983, 2, 167–184.

17. Luria, K.; Lavie, N.; Even, U. Dielectric Barrier DischargeSource for Supersonic Beams. Rev. Sci. Instrum. 2009, 80,104102-4.

18. Hoogerland, M. D.; Driessen, J. P. J.; Vredenbregt, E. J. D.;Megens, H. J. L.; Schuwer, M. P.; Beijerinck, H. C. W.; vanLeeuwen, K. A. H. In A Factor 1600 Increase in NeutralAtomic Beam Intensity Using Laser Cooling, FrequencyControl Symposium. 48th Proceedings of the 1994 IEEEInternational Conference, 1�3 Jun 1994; pp 651�654.

19. Rooijakkers, W.; Hogervorst, W.; Vassen, W. An IntenseCollimated Beam of Metastable Helium Atoms by Two-Dimensional Laser Cooling.Opt. Commun. 1996, 123, 321–330.

20. Hinds, E. A.; Hughes, I. G. Magnetic Atom Optics: Mirrors,Guides, Traps, and Chips for Atoms. J. Phys. D: Appl. Phys.1999, 32, R119.

21. West, A. D.; Weatherill, K. J.; Hayward, T. J.; Fry, P.W.; Schrefl,T.; Gibbs, M. R. J.; Adams, C. S.; Allwood, D. A.; Hughes, I. G.Realization of the Manipulation of Ultracold Atoms with aReconfigurable Nanomagnetic System of Domain Walls.Nano Lett. 2012, 12, 4065–4069.

22. Kaenders, W. G.; Lison, F.; Müller, I.; Richter, A.; Wynands, R.;Meschede, D. Refractive Components for Magnetic AtomOptics. Phys. Rev. A: At., Mol., Opt. Phys. 1996, 54, 5067–5075.

23. Baldwin, K. Metastable Helium: Atom Optics with Nano-Grenades. Contemp. Phys. 2005, 46, 105–120.

24. Morgner, H. The Quantitative Characterization of Liquidand Solid Surfaces with Metastable Helium Atoms. AIPConf. Proc. 2000, 500, 687–698.

25. Harada, Y.; Masuda, S.; Ozaki, H. Electron SpectroscopyUsing Metastable Atoms as Probes for Solid Surfaces.Chem. Rev. 1997, 97, 1897–1952.

26. Harada, Y.; Yamamoto, S.; Aoki, M.; Masuda, S.; Ichinokawa,T.; Kato, M.; Sakai, Y. Surface Spectroscopy with HighSpatial Resolution Using Metastable Atoms. Nature1994, 372, 657–659.

27. Billinge, S. J. L.; Levin, I. The Problem with DeterminingAtomic Structure at the Nanoscale. Science 2007, 316,561–565.

28. Dagotto, E. Complexity in Strongly Correlated ElectronicSystems. Science 2005, 309, 257–262.

29. Hasan, M. Z.; Kane, C. L. Colloquium: Topological Insula-tors. Rev. Mod. Phys. 2010, 82, 3045–3067.

ARTIC

LE