Differently prepared surfaces of quasicrystalline i-Al-Pd-Mn are analyzed using angle-resolved photoemission in the x-ray andultraviolet range of photon energies. Depending on the preparation, we find both surfaces with crystalline structure and metallic character, and surfaces with quasicrystalline structural fingerprints and a suppressed density of states at the Fermi level, compatible with a pseudogap.This article is from Physical Review B 60 (1999): R16330, doi:10.1103/PhysRevB.60.R16330.</p

Aebi, P.

Beeli, C.

Delaney, Dwight

Lograsso, Thomas

Naumović, D.

Schlapbach, :L.

Digital Repository @ Iowa State University (ISU)

RAPID COMMUNICATIONSPHYSICAL REVIEW B 15 DECEMBER 1999-IIVOLUME 60, NUMBER 24Quasicrystalline nature of quasicrystal surfaces: A photoemission studyD. Naumović, P. Aebi, and L. SchlapbachInstitut de Physique, Universite´ de Fribourg, Pe´rolles, CH-1700 Fribourg, SwitzerlandC. BeeliLaboratory of Solid State Physics, ETHZ, CH-1015 Zu¨rich, SwitzerlandT. A. Lograsso and D. W. DelaneyAmes Laboratory, Iowa State University, Ames, Iowa 50011~Received 3 September 1999!Differently prepared surfaces of quasicrystalline-Al-Pd-Mn are analyzed using angle-resolved photoemis-sion in the x-rayandultraviolet range of photon energies. Depending on the preparation, we find both surfaceswith crystalline structure and metallic character, and surfaces with quasicrystalline structural fingerprints and asuppressed density of states at the Fermi level, compatible with a pseudogap.@S0163-1829~99!51048-5#n-tllolecyhe-looasifey-veelinninta,otruhedp-g-eonorurere,sur-uralis-om-isVGedtion.izedAllthealterur-as ay inSince their discovery, quasicrystals1 have attracted muchinterest. It is the fascinating structure with five-, eight-, teor 12-fold symmetry axes which brings up new challengesconventional crystallography. Even today, the crystagraphic structure is not entirely resolved. However, the etronic structure of quasicrystals is also peculiar. Quasicrtals exhibit an electrical resistivity thousands of times higthan that of their constituents.2Specific heat measurements2 indicated a significant reduction of the density of states~DOS! at the Fermi levelEF .Valence-band ultraviolet-photoemission spectroscopy~UPS!yielded conflicting results. Whereas Moriet al.3 reported asignificant depression in the DOS atEF for an i-Al-Cu-Fealloy, a very high resolution study4 on a series of icosahedraalloys claimed the samples to be metallic, based on theservation of a clearly developed sharp Fermi edge. Bstudies were done on scraped surfaces. On the other hangle-resolved UPS on a well-ordered monograini-Al-Pd-Mn surface even showed some weak bandlike disperin the valence band, together with a distinct pseudogapture at EF .5 Very recently, an UPS and photon-energdependent core-level spectroscopy study on cleamonograini-Al-Pd-Mn demonstrated that the apparent mtallicity at the surface is decreasing with increasing sampdepth.6Together with the remarkable structural and electrobulk properties we find unusually low wetting and friction otheir surfaces. These are attributes that make quasicrysalso industrially attractive.7 On a fundamental level, it istherefore, important to study differently prepared surfacesa quasicrystal to see to what extent they are related to stural and electronic properties of the bulk. In particular, tgeometrical and electronic structure has to be investigateparallel.Surface sensitivestructural techniques have only been aplied very recently.8–11 Among them are scanning tunnelinmicroscopy,8 secondary-electron imaging,9 low-energy elec-tron diffraction ~LEED!,10 and x-ray photoelectron diffraction ~XPD!.11 In structural surface studies it has been noticPRB 600163-1829/99/60~24!/16330~4!/$15.00,o--s-rb-thnd,ona-d-gclsfc-indthat different surface terminations are possible dependingthe sample preparation: heat treatment, sputtering,cleaving.8–12It is, however, not clear what are the electronic structfingerprints of such differently prepared surfaces. Therefowe present a combined geometricaland electronic structurestudy on differently preparedi-Al-Pd-Mn monograin sur-faces. The results clearly demonstrated that a sputteredface that is appropriately annealed, shows all the structfingerprints of a quasicrystalline structure together with dtinct suppression of spectral weight in the DOS nearEF . Incontrast, if the surface is disordered and the chemical cposition is slightly changed, a crystalline surface structureformed with a sharp metallic Fermi edge.The photoemission experiments were performed in aESCALAB Mk II spectrometer with a base pressure<5310211 mbar. The sample stage is modified for motorizsequential angle-scanning data acquisition over a 2p solidangle.13,14 MgKa radiation (hn51254 eV! was used forx-ray photoelectron spectroscopy~XPS! in order to check thecleanness of the sample and to determine the composiUPS measurements were performed with monochromatHe Ia radiation ~21.2 eV!.15 The energy resolution of theanalyzer for the UPS measurements was set to 30 meV.measurements were performed at room temperature.The i-Al-Pd-Mn quasicrystal ingot with its twofold axisparallel to the surface normal has been grown usingBridgeman method~Ames Laboratory!. Its stoichiometrywas determined to be Al69.8Pd20.0Mn 10.2. It was orientedwithin 0.25° and polished with diamond paste and colloidsilica.16 The sample was a 1.3-mm-thick disk with a diameof 15 mm. It was cleanedin situ by Ar1 sputtering at 1 and0.75 kV ~total time: 30 to 60 min! followed by annealing ati dicated temperatures during 5 min~an additional 10 minwere necessary to reach the desired temperature!. Tempera-ture was controlled with a pyrometer.In the present study, the geometrical structure of the sfaces has been investigated by XPD.17 Briefly, in angle-scanned XPD x-ray photoemission peaks are measuredfunction of emission angle and mapped stereographicallR16 330 ©1999 The American Physical Societytieiop,enbelloshelypahaitierasluodt-ahisn ofstioninsed.oxi-abic,a-inusendem-twoede-er-withngare:RAPID COMMUNICATIONSPRB 60 R16 331QUASICRYSTALLINE NATURE OF QUASICRYSTAL . . .a linear grayscale representation. High and low intensiare drawn in white and black, respectively. Normal emisscorresponds to the center of the plot, whereas emissionallel to the surface, i.e., the 90° polar emission angleindicated by an outer circle~Fig. 1!. XPD allows for a verysimple interpretation in the case of photoelectron kineticergies above approximately 500 eV.17 Photoelectrons leavingthe emitter atom are strongly focused in forward directionthe neighboring atoms. The measured intensities are thfore high along densely packed atomic rows and crystagraphic planes. Furthermore, in this energy regime, thecalled forward focusing is only weakly dependent on tatomic numberZ. The position of the maxima thus basicaldepends only on the local geometrical structure, and thetern represents a simple fingerprint of the crystallograparrangement near the surface.Figure 1~a! shows the angular distribution of the Pd 35/2photoemission line intensity. The surface of thei-Al-Pd-Mnmonograin cut perpendicular to a twofold symmetry axis hbeen sputtered and annealed to 650 °C. Its XPS composis found to be Al73Pd22Mn5.18 Due to the forward-focusingproperties of photoelectrons in the keV regime, the pattnicely highlights densely packed high-symmetry~two-,three- and fivefold! axes of the icosahedral point group,displayed in Fig. 1~b! ~also in stereographic projection!. Theexperiment can also be reproduced by single-scattering cter calculation using clusters as inferred from structural mFIG. 1. ~a! XPD pattern of a quasicrystalline surface ofi-Al-Pd-Mn cut perpendicularly to a twofold symmetry axis obtainafter standard preparation~sputtering and annealing at 650 °C! ~Pd3d5/2, Ekin 5 917 eV!. ~b! Stereographic projection of the icosahdral symmetry elements, i.e., axes of two-~ellipses!, three- ~tri-angles! and fivefold~pentagons! symmetry.~c! A reconstruction ofthe surface appears after Ar1 sputtering.~d! Superposition of twoexperimental XPD patterns from a Nb~110! single crystal ~Nb3d5/2, Ekin 5 1051 eV! rotated by 109° with respect to each othas indicated.snar-is-yre--o-t-icsonns--els of thei-Al-Pd-Mn phase11 giving a strong indication thathe near surface region~within the escape depth of the photoelectrons! of this surface preparation is characteristic ofquasicrystalline structure. In strong contrast to Fig. 1~a!, Fig.1~c! displays the XPD pattern of a sputtered surface. Tsurface is disordered and exhibits a changed compositioAl54Pd42Mn4. Locally, however, ordering persists, manifeas well-defined anisotropies in the XPD angular distributof Fig. 1~c!. This pattern can be understood by two domaof a cubic bcc~110!-like structure when comparing it withFig. 1~d! where two measurements taken from a Nb~110!~bcc! single crystal, rotated by 109°, are superimposTherefore, the sputtered surface corresponds to two apprmately equally populated domains of an Al-Pd alloy withbcc lattice.Annealing the sputtered surface first develops the cucrystalline structure up to;400 °C and then becomes qusicrystalline. The corresponding LEED patterns are shownFig. 2. Note that even the sputtered surface shows diffLEED spots11 which become sharper with annealing, achange into the ‘‘quasicrystalline’’ pattern~Fig. 2, left!.Figure 3 displays valence-band spectra taken at room tperature of the sputtered and annealed surfaces. We noteFIG. 2. LEED pattern of the ‘‘quasicrystalline’’ and the ‘‘crystalline’’ phase taken with electrons of 60 eV~high intensity inblack!.FIG. 3. Room-temperature valence-band spectra, takenmonochromatized He-I radiation (hn521.2 eV!, of sputtered andannealed~as labeled! surfaces. Annealing corresponds to heatithe sputtered surface~starting at room temperature! up to the la-beled temperature. XPS concentrations for the different spectraAl54Pd42Mn4 ~sputtered surface!, Al61Pd37Mn2 ~365 °C!,Al 70Pd26Mn4 ~410 °C!, Al73Pd22Mn5 ~550 °C!, Al68Pd27Mn5~700 °C!.rsdgingeVgtstaf tra°Cafoibanlithoneanacffeeeuoteape ofthegaptheal-onlyusnendisi-ap.theur-rys-re-tbe-toisoffu-aghon,is,f aowC-ce°Csearthisnt.er,F.Na-eRAPID COMMUNICATIONSR16 332 PRB 60D. NAUMOVIĆ et al.important features developing with surface treatment: fithe characteristic change of the shape of the Fermi ewhich is shown in a close-up in Fig. 4; second, the shiftand splitting of the strongest peak at approximately 4-binding energy. Along with these systematic changesstructural fingerprints of the XPD and LEED experimenSurfaces of the two topmost curves clearly show the crysline patterns shown in Figs. 1 and 2, whereas surfaces othree lowermost spectra display the quasicrystalline chateristic. LEED corresponding to the spectrum of the 410annealed surface exhibits a higher background intensity,the 365 °C annealed surface exhibits sharper spots thanrespective quasicrystalline and crystalline pattern shownFig. 2. The prominent 4-eV feature~Fig. 3! has been attrib-uted to Pd 4d electron states.19 A distinct shifting towards ahigher binding energy is observed, reaching a maximumthe 550 °C surface. The shifting and splitting can be attruted to a specific chemical environment of near surfacesurface Pd atoms. It is also interesting to note that quatively, the ratio of the 4-eV peak and the spectral weigclose to EF ~attributed to Mn 3d states4! scales with themeasured XPS composition: namely, high and low Pd ccentration for the sputtered and ‘‘quasicrystalline’’ surfacrespectively, and the highest Mn concentration for the ‘‘qusicrystalline’’ surface.Figure 4 shows the same spectra~fine black lines! as inFig. 3, zooming in closer toEF . The black dots have beeobtained by normalizing the spectra with the Fermi-Dirdistribution function,20 therefore removing the sharp cutoand creating a spectral function representing the DOS nEF within a range of approximately 4.4kBT, wherekB is theBoltzman constant.6,21 4.4 kBT represents the region wherthe Fermi-Dirac distribution function takes values betwe90% and 10%. We clearly see that the two crystalline sfaces exhibit a linear behavior over the complete rangeenergies. For the 410 °C, 550 °C, and 700 °C surfaces,shape is completely different. There is a distinct decreasDOS towardsEF which can be interpreted as a pseudogFIG. 4. Closeup of spectra of Fig. 3, displaying the nearEFregion; spectra~thin black lines! are divided by the Fermi-Diracdistribution calculated for an effective~room! temperature obtainedfrom the spectrum of the sputtered surface~s e text!; all spectra aretaken at room temperature; labels indicate the annealing tempture.t,e,o.l-hec-ndtheinr-da-t-,-arnr-fheof.Different functions have been proposed to fit this decreasDOS. Mori et al.3 used a linear function multiplied with aLorentzian subtracted from unity. Then, the amplitude ofLorentzian gives a measure for the depth of the pseudoand its width indicates the width of the pseudogap. Onother hand, Wuet al.5 used an expression of the form}(12E/EF)a connecting to the square-root behavior ofVan Hove singularity. Tunneling spectroscopy22 andphotoemission23 data have also been fitted using aAE con-tribution in connection with a scaling theory of the metainsulator transition in amorphous materials.24 However, wefind that aAE fit or even one of the forma1b(E)a is verymuch dependent on the energy range used for the fit andreasonable on a very small interval. Using the unity minLorentzian approach,3 nice agreement is obtained over ainterval of more than an eV~not shown!. The drawback ofthis procedure, however, is that parameters strongly depon the slope of the linear function used to multiply, and thslope is arbitrary. It is, therefore, very difficult to give relable numbers of size, width, and position of a pseudogDespite this difficulty, we note from the~Fermi-Dirac! nor-malized data in Fig. 4, that the DOS atEF is reduced. Thedecrease of DOS extends over several hundred meV andminimum in the DOS does not appear atEF , rather is situ-ated above, consistent with Neuholdet al.6We can thus conclude that sufficiently high annealed sfaces exhibitboth the structuralandelectronic fingerprints ofa quasicrystalline surface, and one may call them quasictalline. However, although the structural probes~LEED forthe long-range and XPD for the short-range order! are com-patible with a quasicrystalline structure and there is aduced DOS nearEF as predicted by many models, it is noclear whether quasicrystallinity is necessary to find thishavior. Similar experiments on approximants are neededgive a definite answer.Furthermore, it is not clear if the depression of DOSsufficient to explain the strongly reduced conductivityquasicrystals or whether an unusually small electron difsivity has to be considered.25 Finally, one should mentionanother possibility where photoemission may mimicpseudogap.26 There, a sharp Fermi edge is destroyed throuextrinsic, ohmic losses induced through the photoelectrleaving a poorly conducting solid. Such an explanationhowever, in contradiction with the recent observation osharp Fermi edge in high resolution photoemission at ltemperatures.4In conclusion, a sputtered as well as a slightly (365 °!annealed surface of ai-Al-Pd-Mn quasicrystal shows a crystalline and metallic behavior. In turn, if the sputtered surfais annealed to higher temperatures, (410 °C, 550 °C, 700!the structural~LEED, XPD! and electronic characteristic~depression of spectral weight nearEF) show all the finger-prints expected for a quasicrystal. It is, however, not clwhether the quasiperiodicity is a necessary condition forparticular behavior or rather a complex atomic environmeSkillful technical assistance was provided by E. MoosO. Raetzo, R. Schmid, O. Zosso, Ch. Neururer, andBourqui. This project has been supported by the Fondstional Suisse de la Recherche Scientifique.ra-h,.ndi.th ara-f theisad-RAPID COMMUNICATIONSPRB 60 R16 333QUASICRYSTALLINE NATURE OF QUASICRYSTAL . . .1D. Shechtmanet al., Phys. Rev. Lett.53, 1951~1984!.2S.J. Poon, Adv. Phys.41, 303 ~1992!.3M. Mori et al., J. Phys.: Condens. Matter3, 767 ~1991!.4Z.M. Stadniket al., Phys. Rev. Lett.77, 1777~1996!.5X. Wu et al., Phys. Rev. Lett.75, 4540~1995!.6G. Neuholdet al., Phys. Rev. B58, 734 ~1998!.7J.-M. Dubois, Phys. Scr.T49, 17 ~1993!; New Horizons in Qua-sicrystals, Research and Applications, edited by A.I. Goldmanet al. ~World Scientific, Singapore, 1997!; Quasicrystals, editedby J.-M. Duboiset al. ~Materials Research Society, Pittsburg1999!.8T.M. Schaubet al., Phys. Rev. Lett.73, 1255~1994!.9M. Erbudaket al., Phys. Rev. Lett.72, 3037~1994!.10T.M. Schaubet al., Z. Phys. B96, 93 ~1994!.11D. Naumovićet al., Surf. Sci.433-435, 302~1999!; D. Naumovićet al., in Proceedings of ICQ-6, edited by S. Takeuchi, and TFujiwara ~World Scientific, Singapore, 1998!, p. 749; D. Nau-mović, et al., in New Horizons in Quasicrystals, Research aApplications~Ref. 7!, p. 86.12Z. Shenet al., Phys. Rev. Lett.78, 1050 ~1997!; Z. Shenet al.,Phys. Rev. B58, 9961~1998!; Ph. Ebertet al., Phys. Rev. Lett.77, 3827 ~1996!; Ph. Ebert, F. Kluge, and K. Urban, Surf. Sc433-435, 312~1999!; J. Ledieuet al., ibid. 433-435, 666~1999!;F. Schmithu¨senet al., ibid. ~to be published!.13J. Osterwalderet al., Phys. Rev. B44, 13 764~1991!.14D. Naumovićet al., Phys. Rev. B47, 7462~1993!.15Th. Pillo et al., J. Electron Spectrosc. Relat. Phenom.97, 243~1998!.16C.J. Jenkset al., Appl. Surf. Sci.103, 485 ~1996!.17For a review, see, e.g., C.S. Fadley, Surf. Sci. Rep.19, 231~1993!.18Note that XPS concentrations cannot directly be compared witrue stoichiometry because they are only based on intensitytios including core-level cross sections; see Ref. 11.19G.W. Zhanget al., Phys. Rev. B50, 6696~1994!.20The parameters have been obtained by fitting the spectrum osputtered surface to a Fermi-Dirac distribution function. Thaccounts for the room temperature and the instrumental broening.21T. Greber, T.J. Kreutz, and J. Osterwalder, Phys. Rev. Lett.79,4465 ~1997!.22T. Klein et al., Phys. Rev. Lett.74, 3656~1995!.23T. Schaubet al., Phys. Rev. Lett.~to be published!.24W.L. McMillan, Phys. Rev. B24, 2739~1981!.25Z.M. Stadniket al., Phys. Rev. B55, 10 938~1997!.26R. Joynt, Science284, 777 ~1999!.

Quasicrystalline nature of quasicrystal surfaces: A photoemission study

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Quasicrystalline nature of quasicrystal surfaces: A photoemission study

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