. .

310

- s&$&&!j ( Surface states >

1

2

34

5.

6.

7.8.

91011

12.

13.

14.

15.16.

17.

See also G. Binnig, H. Rohrer, Sci. Am., August 1985, p.50: J. A. Golovchenko, Science 232, 48 (I 982).D. J. Chadi, Phys. Rev. B 30, 4470 (1984). A. Baratoss,IBM J. Res. Dev., to be published. N. Garcia, C. Ocal, F.Flores, Phys. Rev. Lett. 50, 2002 (1983). W. A. Harrison,Surf. Sci. 55, 1 (1976). N. D. Lang, Phys. Rev. B 55, 23(1985). K.C. Pandey, Phys. Rev. Lett. 47, 1913 (1981).A. Selloni, P. Carnevali, E. Tosatti, C. D. Chen, Phys. Rev.B 3 1, 2602 (1985). J. Tersoff, D. R. Hamman, Phys. Rev.B 3 1, 805 (1985).G. Binnig, H. Rohrer, Sci. Am., August 1985, p. 50.J. N. Israelachvili, D. Tabor, Proc. R. Sot. London, Ser. A331, 19 (1972).E. C. Teague, F. E. Scire, S. M. Backer, S. W. Jensen, Wear83, 1 (1982). P. A. Engei, D. B. Millis, Wear 75, 423(1982).E. C. Teague, Room Temperature Gold-Vacuum-GoldTunneling Experiments, dissertation, North Texas StateUniv., Univ. Microfilms International, Ann Arbor, Mich.(1978),p. 141.J. A. Panitz, Methods Exp. Phys. 22, 349 (1985).C. A. Spindt, I. Brodie, L. Humphrey, E. R. Westerburg, J.Appl. Phys. 47, 5248 (1976).A. J. Jason, Phys. Rev. 156,266 (1967).F. Hasselback, M. Nicklaus, J. Phys. E 17, 782 (1984).C. Herring, M. H. Nichols, Rev. Mod. Phys. 21, 185(1949).Physics Survey Committee, Physics Through the 1990%National Academy P., Washington, DC (1986); for a reviewof this report, see PHYSICS TODAY, April 1986, p. 22.G. Binnig, C. F. Quate, C. Gerber, Phys. Rev. Lett. 56,930(1986).G. Binnig, H. Rohrer, C. Gerber, E. Weibel, Phys. Rev.Lett. 50, 120 (1983).N. Mott, Rep. Prog. Phys. 47,909 (1984).R. V. Coleman, B. Drake, P. K. Hansma, G. Slough, Phys.Rev. Lett. 55,394 (1985).0. C. Wells, Scanning Electron Microscopy, McGraw-Hill,New York (1974).

( f!$?#% : Cdvin F. Quate; g#ikY%@ëP

1$ Physics ToBay,26, August(l986) >

Reconstructed Silicon Surface as seenbyí Ihe scanning tunnelmg mxroscope.When silicon is heated to 900 ëC the sur-face atoms rearrange themselves Into astructure different from that of the bulkatoms. The formatIon of this new, two-dlmensional surface array is called rec-onstruction. The structure ol the recon-structed (111) surface of silicon was

known from low-energy electron-dlffractionpatterns. but the scanning tunnekng micro-scope gave the first direct images of thespahat posittons. Top: Image in the formof intensity modulation on a cathoderaytube. Bottom: The lndwldual traces foranother reconstructed Silicon surface,showing a step. (Courtesy of Sang-4 Park.)

Lab notes. At the bottom of this pagefrom Gerd Blnnigís lab notebook is his

tunneling Current between the top and the

5 January 1979 entry indicating that thesample decreases exponentially as the

Currenl from a tip 1000 A in diameter isspace between Ihe tip and the sample in-

conftned to a region only 45 A m size.creases. Later 11 was found that protrusions

The region is so small bwause theon the tip can reduce the tunnelmg region tofess than 10 A, as figure 3 indicates.

me-

Sharp tungsten tips. Naturally occurnng protuberances reduce thearea of the tunpeling current, giving the scanmng tunneling mwos-cope its atomic resolution. On tungsten needles, the protuberancesand the exponential decay of the tunnekng current as the tip-to-samplespacmg is increased combme to gwe tips that are in effect less than10 A I” diameter. The p!ezoelectnc crystal marked .? Conlrols thespacmg between the tip and the surface under study. The x and yplezoel&ctrlc devices move the tip ln a raster pattern across the surfacewhtle the tunneling current is monitored and used to create an Image.The tmage is a map of the protrusion of cha;ge density on the suriace.The pleroelectric tripod IS wsible in the phctograph on the cover of thisIssue.

314

EiEl Sharpening a tip. These field-bon-mtcroscope Images follow a (ill) tungstentip as it iS reduced I” sue throuoh fieldeVapOratlOn. In this process the tip is sub-jected to a strong electric field of 3 V/Aafter It has been annealed. Atomsevaporate from the surface, as thissequence shows (left to right), until thehnal tlP (far right) consists of but threeatoms. (Courtesy of Hans-Werner Fmk.)

__ . -

q ziGraphite surface as Seen by the scanning tunneling microscope. The bright points in thearrays indicate the positions 01 those atoms in the uppermost layer that are not dwectty aboveatoms in the tsyer betow. These atoms form a centered hexagonal array. Top. lntensity-modu,atio” image. Bottom: \ndiîidua, ,races.

3#I,’ I_

Conductivlty wrbuS tlp voltage forsillcon (a) and ntckel oxide (b) surfacesThe upper curve in a IS lor a clean (111)

surface of silicon reconstructed in the 7 x7pattern. The lower Curve in a is the result

of the same measuremenl after deposition01 a monolayer of gold on the s~kcon

surface. The conductivity peak at 1 volt isattributed to .~n electronic surface state.

The conductivity in nickel oxide (b)undergoes a three-order-of.magnitude

increase near 0.6 volts. Such a highconductwity is unlikely to be due to a

Surface state, it is attributed to conductlonthrough lhe film. (Pert a cowtesv of Frank

Salvan~ part b courtesy 01 Etinmg i

,.,_ì____.._. -. ,. y .*. .

Two surface states. On the (111) surface01 cleaved silicon there are two bands ofsurface states, separated by a gap of 0.45eV. The lower band lies near the valenceband of bulk silicon and is filled. The upperband lies near the center of the bulk gapand is empty. When the sample is biasedwith - 0.7 volls. electrons tunnel into thetip from the filled surface states near thevalence-band edge, producing the greenimage. When !he polarity is reversed.electrons tunnel lrom the tip into the emptysurface stales in the upper level. producingthe red image. The third image, asuperposition of the first two. shows boththe filled and the unfilled surface states onsikcon. The bright spots in the greenimage occur at points where the red imageis dark, and vice Versa. illustrating lhat thescanning tunnelmg microscope mapscharge density, not atomic positions.(Courlesy of Joseph Stroscio. RandallFeenstra and Aaron Fein.)

L . D I 1”

TIP VOLTAGE0 I 2

TIP VOLTAGE

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@&&@(Qusicrystals)

316

l $$&i@ : Pandora ~+--- Pandora Tf&$&$$J

1. D. S. Shechtman, I. Blech, D. Gratias, and J. W. Cahn, Phys.Rev. Lett. 53, 1951 (1984).

2. Phys. Today 38,20 (February, 1985).3. D. Levine and P. J. Steinhardt, Phys. Rev. Lett. 53, 2477

(1984).4. M. Gardner, Sci. Am. 236, 110 (January 1977).5. P. Kramer and R. Neri, Acta Crystallogr. Sect. A 40, 580

(1984); V. Elser, Bell Laboratories preprint (1985); P. A.Kalugin, A. Kitaev, and L. Levitov, JETP 4 1, 119 (1985); M.Duneau and A. Katz, Palaiseau preprint (1985).

6. News and Views, Nature 315, 178 (1985).7. D. N. Mermin and S. M. Troian, Phys. Rev. Lett. 54, 1524

(1985); P. Bak, ibid. 54, 1517 (1985); D. Levine, T. C.Lubensky, S. Ostlund, S. Ramaswamy, P. J. Steinhardt, andJ. Toner, ibid. 54, 1520 (1985); P. A. Kalugin et al., ibid.;D. R. Nelson and S. Sachdev, Harvard preprint (1985).

8. P. Bancel, P. A. Heiney, P. W. Stephens, A. I. Goldman, andP. M. Horn, Phys. Rev. Lett. 54, 2422 (1985).

(R@$$ : Paul J. Steinhardt; girjk$$;t;bzs$ Physics

T o d a y s18, January(l986))

317

1

0

-1

0.10 0.15 0 . 2 0 0 . 2 5

H ITI

1. Y. Aharonov and D. Bohm, Phys. Rev. 115,485 (1959).2. R. G. Chambers, Phys. Rev. Lett. 5, 3 (1960).3. R. A. Webb, S. Washburn, C. P. Umbach, and R.B. Laibowitz,

Phys. Rev. Lett. 54, 2696 (1985); S. Washburn, C. P. Um-bath, R. B. Laibowitz, and R. A. Webb, Phys. Rev. B (inpress).

4. C. P. Umbach, S. Washburn R. B. Laibowitz, and R. A. Webb,Phys. Rev. B 30,4048 (1984).

5. A. D. Stone, Phys. Rev. Lett. 54,2692 (1985).6. M. Buttiker, Y. Imry, R. Landauer, and S. Pinhas, Phys. Rev.

B 3 1, 6207 (1985). and references cited therein.7. M. Buttiker and Y. Imry, J. Phys. C 18, L467 (1985).8. N. B. Brandt, et al., Sov. J. Low Temp. Phys. 8,358 (1982),

and references cited therein.9. See the review by Yu. V. Sharvin, Physica B 126,288 (1984)

( Efig : R.A. Webb, S. Washburn, $U G. Present i fi@ifift

AH!f#REiPhysics Today, S-19, January

(1986) >

319