Electron correlation effects in Co nanoscale islands on a nitrogen-covered Cu(001) surface

Kan Nakatsuji,* Yoshihide Yoshimoto, Daiichiro Sekiba,† Shunsuke Doi, Takushi Iimori, Kazuma Yagyu, Yasumasa Takagi,‡

Shin-ya Ohno,§ Hideharu Miyaoka, Masamichi Yamada, and Fumio Komori�

Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwashi, Chiba 277-8581, Japan

Kenta Amemiya,¶ Daiju Matsumura,** and Toshiaki Ohta††

Department of Chemistry, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan�Received 10 August 2007; revised manuscript received 2 June 2008; published 25 June 2008�

The electronic states of cobalt nanoislands on a nitrogen-saturated Cu�001� c�2�2�-N surface have beeninvestigated by means of x-ray absorption spectroscopy. In the case of low coverages, when the surface iscovered by small Co islands, a shoulder structure in the Co L-edge absorption spectrum is observable at anenergy about 3 eV higher than the absorption edge. By considering the unoccupied Co 3d state calculated byfirst-principles method, x-ray photoelectron spectra and the multiple scattering during the absorption process,the shoulder is attributed to the correlation among 3d electrons. The satellite intensity decreases with theincrease in the Co island size. The perimeter Co atoms of the island are assigned as the origins of thecorrelation satellite by comparing its intensity with the morphology of the islands observed using scanningtunneling microscopy. The polarization dependent satellite intensity suggests that the electron correlation in thein-plane 3d orbital of the perimeter atoms is larger than that in the out-of-plane one.

DOI: 10.1103/PhysRevB.77.235436 PACS number�s�: 73.20.�r, 78.70.Dm, 68.37.Ef, 79.60.Jv

I. INTRODUCTION

The electron correlation is known to be one of the impor-tant issues for electronic properties of transition metals andtheir compounds, which have localized 3d electrons. It de-pends on the hybridization of 3d electrons with the neighbor-ing atoms and on the screening induced by the conductionelectrons as well as by the intra-atomic Coulombinteractions.1 The correlation effect of the 3d elements inmetallic nanoparticles and at metal surfaces would be, thus,enhanced because of the reduction in the coordination num-ber and the poor screening effect.

Experimentally, L-edge x-ray absorption spectroscopy�XAS� and x-ray photoelectron spectroscopy �XPS� are wellestablished techniques for the study of electron correlations.A well-known example is the Ni metal, in which x-ray pho-toelectron spectrum of the 2p core has a satellite structure ata binding energy of 6 eV higher than the main peak due tothe electron correlation.1 This is well explained by a configu-ration interaction �CI� model considering the electron corre-lation between the 2p core hole and the 3d valence state andthat among the 3d electrons.2,3 In this model, the Ni 2p mainpeak and the satellite are attributed to the d10 and d9 finalstates, respectively. The satellites have been also found in theL-edge x-ray absorption spectrum at energies 3 and 6 eVhigher than the main peak and were explained by the CI; forinstance, the 3-eV satellite originates from the ground state,which has 15%–20% of the d8 configuration other than d9.4

So far, however, no clear enhancement in the correlation sat-ellites has been reported for Ni thin films and nanoparticles.

In the case of Co metal, a satellite in the bulk was re-ported at an energy 3–4 eV higher than the Co 2p XPS mainpeak and was attributed to the correlation satellite althoughits intensity is much weaker than in the case of Ni �Ref. 5�.For the Co thin films that are less than three monoatomiclayer �ML� thick on the clean Cu�001� surface, its intensitywas enhanced.6,7 This is attributed to the localization of the

3d electrons and the reduction in the screening due to thelow dimensionality; either two-dimensional or zero-dimensional states. Thus, we can expect further increase inthe satellite signal due to the correlation in well-controlledCo nanostructures.

For Co films on the clean Cu, the interface Co 3d-Cu 4shybridization affects the electron correlation in the film. Itsgrowth mode at the initial stage at room temperature �RT� isknown to trace the so-called bimodal growth up to 0.25 MLof Co on average.8 The deposited Co atoms exchange withthe substrate Cu and the expelled Cu atoms make small is-lands, which are surrounded by Co atoms. Consequently,their interface is disordered, and the electronic state is spa-tially inhomogeneous. This makes it difficult to discuss theexperimental data quantitatively on the electron correlation,which depends on its shape and interface.

One way to achieve the well-defined Co-Cu interfacewould be the use of segregation of light elements such asoxygen and nitrogen as surfactants.6,9,10 Recently, initialgrowth of Co nano-islands on a nitrogen-saturated Cu�001�c�2�2�-N surface was studied by scanning tunneling mi-croscopy �STM� and XPS. The results indicate that the nitro-gen atom segregates on top of the Co islands and formCo-c�2�2�N structure.11 The nitrogen atom acts as a surfac-tant and leave the spatially uniform Co 3d-Cu 4s hybridiza-tion at the interface. For iron on N/Cu�001�, an extendedx-ray absorption fine structure study provided the surfaceatomic structure consistent with this N segregation model.12

In the present paper, we show a clear shoulder structure at�3 eV higher than the main peak in Co L-edge x-ray ab-sorption spectra of the Co film deposited on the nitrogen-saturated Cu�001� surface. Its intensity largely depends onthe average Co coverage, i.e., the Co island size. We ob-served the surface morphology as a function of the averageCo coverage, and we calculated Co 3d partial density ofstates �PDOS� by the first-principles method to discuss the

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origin of the shoulder structure. It is finally attributed to thecorrelation satellite.

II. METHODS

The absorption spectra were measured in an ultrahighvacuum �UHV�-XAS chamber installed at BL-7A or 11A ofthe Photon Factory at the Institute of Materials StructureScience, High Energy Accelerator Research Organization�KEK-PF�. The chamber was equipped with multichannelplates as a detector for emitted electrons from the sample anda low-energy electron diffraction �LEED�-Auger optics �OCIBDL800�13 with the base pressure below 5�10−8 Pa. Thespectra were acquired by so-called partial electron yieldmode to enhance the surface sensitivity with retarding volt-ages of −300 and −500 V in the cases of N K- andCo L-edge measurements, respectively. The x-ray incidenceangle was either normal �90°� or grazing �30°� to the surface,which is referred to as “NI” and “GI” geometries. The polar-ization of the light was in-plane and out-of-plane for NI andGI geometries, respectively. The photoelectron spectra wererecorded in a separated UHV chamber with the base pressureof 1�10−8 Pa, equipped with a hemispherical analyzer �VGScienta SES100� as the photoelectron detector and a LEED-Auger optics �OCI BDL600�. A conventional Mg K� twinanode �VSW TA10� was used as an x-ray source. The inci-dence angle of the x-ray was 15° and the detection angle ofthe photoelectron was 60° from the surface normal. The sur-face morphology was studied using STM in another UHVsystem, which consists of surface preparation and measure-ment chambers equipped with a microscope �Omicronmicro-STM� with a tungsten tip and LEED-Auger optics�OCI BDL600�. The base pressure of these chambers wasbelow 1�10−8 Pa. All the measurements had been per-formed at RT.

A Cu�001� substrate was cleaned by repeated cycles ofAr+ ion sputtering at 1 keV and subsequent annealing to 870K. The structural order and the cleanliness of the surfacewere confirmed by a sharp 1�1 LEED pattern and the lackof the contaminants in the Auger signal and/or the STM im-age. The nitrogen-saturated surface was obtained by nitrogenion exposure at 500 eV and subsequent annealing to 670 K.We confirmed the formation of the saturated surface by ob-serving clear c�2�2� LEED pattern with sharp streaks along�110� direction around the integer spots.14 In the XAS andXPS chambers, pure �5N� cobalt was deposited on the sur-face at RT by electron bombardment of a cobalt rod at therate of 0.2–0.3 ML/min. Here, the deposition rate was cali-brated by Cu LMM �920 eV� /Co LMM �716 eV� Auger in-tensity ratio on a Co/Cu�001� surface on which the relationbetween the average thickness and the Cu/Co Auger intensityratio has been already known.15 In the STM chamber, cobaltwas also deposited at RT from an alumina crucible. Thedeposition rate was 0.04 ML/min, which was calibrated fromthe STM image. Here, the ML is defined as the same numberdensity of atoms as that on the clean Cu�001� 1�1 surface.

We have investigated the most stable atomic structure andthe PDOS of the in-plane and out-of-plane 3d states for asingle-atomic layer of Co on the Cu�001� surface, and single-

and double-atomic layers of Co on the Cu�001� surface withadsorbed nitrogen atoms on top of them in c�2�2� period-icity on the basis of first-principles calculations. The Cofilms were simulated using a symmetric slab model of9-atomic-layer thick including Co layer�s�. The vacuum re-gion was 9-atomic-layer thick. With this slab model, we per-formed a standard density-functional plane-wave-pseudopotential calculation. To stabilize the calculation,Fermi surface was smeared by 3�103 Hartree ��0.1 eV�using Methfessel-Paxton algorithm.16 The Perdew-Burke-Ernzerhof type exchange-correlation potential17 was used.Cu, N, and Co atoms were simulated by the ultrasoftpseudopotentials.18 The cutoff energy of the plane-wave ba-sis set was 49 Ry. All of the symmetry was utilized and thenumber of the total sampled k-points for 1�1 and c�2�2�structures in the first Brillouin zone were 16�16 and 12�12, respectively. The atomic structures were optimized sothat the maximal force acting on an atom becomes�10.3 Hartree /a .u.. As for PDOS, electronic states wereprojected on the atomic orbital of a Co atom calculated withthe �Ar��3d�7 �4s�2 configuration. In addition, this calculationdid not consider its spin polarization. Our calculation wasperformed with an extended version of TAPP �Tokyo ab-initioprogram package�.19

III. RESULTS AND DISCUSSION

A. Growth process of Co islands

1. STM observations

Figures 1�a�–1�c� show STM images of the nitrogen-saturated Cu�001� surface with Co islands. Here, the averagecoverages of Co are 0.2, 0.8, and 1.2 ML. At 0.2 ML, smallCo islands, of about 10 nm2 in area, randomly distribute onthe N/Cu substrate. Not only single-atomic layer islands butdouble-atomic layer islands have already started to grow atthis deposition rate, 0.04 ML/min. With increasing Co cov-erage, the island size increases, and some islands start tocoalesce. The second layer grows on top of the large single-atomic layer islands. A small number of the islands with thethird layer are seen in Figs. 1�b� and 1�c�. The cross sectionalong the line A-B in Fig. 1�c� is shown in Fig. 1�d� forclarity. The morphology of the surface is homogeneousthroughout the substrate. Above 2 ML of Co on average,most of the substrate would be covered with the double-atomic layer islands.

The Co island surface is covered with nitrogen atoms.11

When a single-atomic layer Co island grows on the substrate,the N atoms segregate on top of the island and form ac�2�2� N/Co layer on Cu substrate. �We call this a single-layer N/Co island, hereafter.� After subsequent growth of thesecond layer Co, the N atoms segregate to the topmost layerand form the c�2�2� N/Co layer on the interfacial Co layer.�We call this a double-layer N/Co island, hereafter.� Sche-matic cross sections of single- and double-layer Co islandsare shown in Fig. 1�e�. The Co atoms on the surface with 0.2ML of Co are, thus, classified into three categories; those inthe single-layer N/Co island, in the N/Co layer and in theinterfacial layer of the double-layer N/Co island.

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2. N K-edge XAS

We investigated the unoccupied electronic states at N at-oms on the Co deposited N/Cu�001� using N K-edge absorp-tion spectra to confirm the structure model and hybridizationof N and Co. The absorption spectra observed for 2.4 ML ofCo on average are presented in Fig. 2�a� showing the inci-dent light polarization dependence in NI and GI geometries.The spectrum for 0 ML case in NI geometry is also shown inthe figure. On the nitrogen-saturated surface without Co at-oms, the overall spectral features are consistent with the pre-vious report.20 The Fermi level �EF� locates at 396.3 eV,which corresponds to the binding energy of N 1s core levelmeasured by XPS.11 At 2.4 ML of Co on average, the inten-sity just above EF grows, at which most of the Cu substrate iscovered with the double-layer N/Co as already mentioned inthe previous section.

Judging from the selection rule for the optical transition,the spectra in NI and GI geometries are related to N 2px,yand 2pz unoccupied states, respectively. The observed spec-tra at Co coverage of 2.4 ML depend on the polarization verysimilarly to those observed on the Ni�100� p4g�2�2�-Nsurface21 in which N atoms locate at the fourfold hollow siteas in the cases of the N/Cu surface and the N/Co islands.Figure 2�b� illustrates a ball model of a double-layer N/Coisland made on the N/Cu substrate.

According to the interpretations for the Ni surface,21 thethree peaks for NI geometry in Fig. 2�a�, A, B, and C, can beassigned to N 2p—Co 3d hybridization, N np—Co 4sphybridization,22 and N 2p—Co 4sp antibonding state, re-spectively. Here, the 2px,y states of N would hybridize withthe surrounding Co 3d states, which have the same geometrywith Cu 3d states on the N/Cu substrate as shown in Fig.2�b�. In the case of N/Cu�001�, the N 2px,y states bond tosurrounding Cu 3d states, which contribute a little to the un-occupied state.20 Whereas on N/Co, the hybridized stateswould have significant contribution to the unoccupied statebecause the 3d density of states �DOS� of Co above EF is

N/Co in double-layer

N/Co insingle-layer interfacial Co

N/Cu

Cu(001)

(e)

(d)

A

B

single

double

N/Cu

triple

(a)

0.2ML

single-layer

double-layertrench

5 nm

(b)

0.8ML

triple-layer

5 nm

(c)

1.2ML

A

B

5 nm

FIG. 1. �a�–�c� STM images of the Co islands on the N-saturatedCu�001� surface for �a� 0.2, �b� 0.8, and �c� 1.2 ML of Co onaverage. The size of the images is 25�25 nm2. The darkest area isthe N/Cu substrate with trenches as black lines. The single- anddouble-layer islands are identified in the images. A few triple-layerislands are seen in �b� and �c� �the brightest area�. The sample biasvoltage was +2.0 V and the tunneling current was 0.5 nA. �d� Crosssection of the line A-B in �c�. �e� A schematic cross section of thesingle- and double-layer Co islands. The Co atoms are classifiedinto three categories; those in the single-layer N/Co island, in theN/Co layer, and in the interfacial layer of the double-layer N/Coisland.

[110]

[100]

N

Co (2nd)

Co (1st)

Cu

β

α

(b)N K-edge

Co: 2.4ML

NI (p )x,y

GI (p )z

B C

NI

Co: 0 ML

A

(a)

FIG. 2. �a� N K-edge absorption spectra observed in NI geometry at the average Co coverage of 0 and 2.4 ML. The spectrum at 2.4 MLin GI geometry is also depicted. The intensity of the spectra is normalized at 445 eV. The Fermi level locates at 396.3 and 397.16 eV for 0and 2.4 ML cases �dashed line� judging from the N 1s core level measured by XPS �Ref. 11�. �b� A ball model of the double-layer N/Coisland on N/Cu substrate. The black and gray balls are the Co atoms in the second and first layers, respectively. N atoms at the fourfoldhollow site ��� segregate on top of the Co island and sit at the same site ���.

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larger than that of Cu. The appearance of the peak A in Fig.2�a� is consistent with this model. The N segregation and thehybridization with Co are thus confirmed by the XAS inves-tigations.

B. Satellite in Co L-edge XAS

We examined the coverage dependence of the Co L-edgeabsorption spectra. The results are shown in Fig. 3�a�. The

spectra were measured in NI and GI geometries drawn bysolid and dashed lines, respectively.

In NI geometry, especially at low Co coverage, a shoulderstructure appears at an energy �3 eV higher than the mainpeak both for the L3 and L2 edges. They are denoted byarrows in Fig. 3�a�. The intensity ratio of the shoulder struc-ture to the main peak at L3 edge is plotted in Fig. 3�b� as afunction of the average Co coverage. It is largest at 0.2 MLand gradually decreases with the increase in the coverageuntil 2 ML. The ratio at L2 edge showed the same trend. Notethat the intensity of the main peak at L3 edge is almost con-stant or even increases by a few percent with the increase inthe coverage. In contrast to the spectra in NI geometry, thereis no such shoulder structure in those in GI geometry even at0.2 ML as in Fig. 3�a�. In other words, the shoulder structureis strong for the incident light with in-plane polarization. Wenoticed that the intensity of the L3 edge continuously de-creases in GI geometry with increasing Co coverage.

The simplest explanation for the origin of the shoulderstructure could be a peak in the 3d DOS. We investigated thispossibility by calculating the layer-resolved PDOS for each3d atomic orbital on the basis of first-principles calculations.Before obtaining the PDOS, we optimized the atomic struc-ture of the Co films. The optimized model and parameters forthe PDOS calculation are given in Fig. 4 and Table I. ThePDOS results for the single- and double-layer N/Co films,and the single-layer Co film are summarized in Fig. 5. Weassume that each state has a Lorentzian shape with 0.1 eV inwidth. Here, the x and y axes are along �100� and �010�directions, respectively. In the calculation for Co atoms inthe N/Co layer, the nearest-neighbor N atoms are located inthe y direction, resulting in different PDOS of dyz and dzx.

In the PDOS of the unoccupied states higher than 2 eVabove EF, only the in-plane 3dx2−y2 state, which has the lobestoward the N atoms, has a finite DOS both for the single- anddouble-layer N/Co films. There is, however, no distinct peakof DOS, which would generate the shoulder structure in theabsorption spectra.

So far, shoulder structures observed in the Ni and Co 2pphotoelectron spectra have been explained as correlation sat-

Co L-edge NIGI

(a)

Co L -edge (NI)

shoulder / mainintensity ratio

3

(b)

FIG. 3. �a� Co coverage dependence of Co L-edge absorptionspectra on the N/Cu substrate. The geometries were NI �solid lines�and GI �dashed lines�. The intensity of the spectra is normalized at830 eV. �3 eV shoulder structures of both L3 and L2 edges for theNI geometry are identified by arrows in the spectrum for 0.2 ML.�b� Co coverage dependence of the intensity ratio of the shoulderstructure to the main peak in the Co L3 edge spectra measured in NIgeometry.

N

Co

Co or Cu

Cu

[001]h h

h1

h32a 2b

FIG. 4. A ball model showing the structural parameters. Notethat h2a �h2b� represent the distance between Co and substrate Culayers for a single-layer N/Co film, whereas between the second andthe first Co layers for a double-layer film. h2a is the distance justbelow the N atom while h2b is another.

TABLE I. Optimized structural parameters used in the PDOScalculation for single- and double-layer N/Co films schematicallyshown in Fig. 4. The unit of the parameters is Å.

h1 h2a h2b h3

Single-layer N/Co 0.291 1.942 1.889 3.745

Double-layer N/Co 0.363 1.776 1.796 3.624

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ellites, as well as those in Ni L-edge absorption spectra. Theground state of Ni is considered as a mixture of d8, d9, andd10 configurations in the CI model.4 For Co, the �3 eVsatellite is enhanced when a small amount of Co was depos-ited on the clean Cu�001� surface. In this case, the depositedCo atoms exchange with the substrate Cu atoms,8 and thesatellite is attributed to the interfacial Co �Ref. 7�. For thissystem with Co below �0.2 ML, we have found a �3 eVshoulder structure in the L-edge absorption spectra as shownin Fig. 6. The shoulder structure appears both in NI and GIgeometries at both L3 and L2 edges in the 0.05 ML case.Judging from these corresponding shoulders in both absorp-tion and photoelectron spectra, it is natural to interpret the3-eV shoulder structure in the Co absorption spectrum in Fig.6 as the correlation satellite just like in the case of bulk Ni.

For the N/Co surface, the Co 2p photoelectron spectra forCo below 3 ML on average showed a �3 eV shoulder struc-ture for both the 2p3/2 and 2p1/2 main peaks as in Fig. 7.These should originate from the electron correlation simi-larly to Co on the clean Cu surface. This interpretation issupported by the fact that the Co 3d band width of thesingle-layer N/Co film is narrower than that of the single-layer Co film on the Cu�001� surface in the total PDOS cal-

N

Binding Energy (eV)

Partialdensityofstate(eV

)-1

0 -2 -42468 0 -2 -42468 0 -2 -42468

1.2

0

0.4

0.8

0

0.4

0.8

0

1

2

3

0

0.4

0.8

0

0.4

0.8

1.2

0

0.4

0.8

Co

Cu

NCo

CuCo

NCo

CuCo

total

dxy

dyz

dzx

d3z -r 22

dx -y2 2

total

dxy

dyz

dzx

d3z -r 22

dx -y2 2

total

dxy

dyz

dzx

d3z -r 22

dx -y2 2

FIG. 5. The calculated PDOS of each 3d atomic orbital and their total. In the left column, the solid and dashed lines are those for thesingle-layer N/Co and single-layer Co films on the Cu�001� surface, respectively. Other columns show the PDOS for the double-layer N/Cofilm. The solid and dashed lines in the right column are those of the interfacial Co under the N atom and another, respectively.

NI

GI

Co/Cu Co L-edge

GI

0.05ML

0.05ML

2.7ML

FIG. 6. Co L-edge absorption spectra for 0.05 ML of Co onaverage on the clean Cu surface in NI and GI geometries and for 2.7ML in GI geometry. The intensity of the spectra is normalized at830 eV. The �3 eV shoulder structures of both L3 and L2 edges areidentified by arrows in the spectra for 0.05 ML.

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culation as in Fig. 5, suggesting the localization of the 3dstates in the N/Co film. Consequently, the 3-eV shoulderstructure in the absorption spectra in Fig. 3 is most likely tobe the correlation satellite.

The 3d hole number can be estimated by assuming that itis proportional to the integrated white line intensity of theabsorption spectrum after subtraction of the backgrounds. Inthe case of Co on the clean Cu surface, it decreases with thedecrease of Co coverage as in Fig. 6. This behavior is con-sistent with a previous report23 and is attributed to the chargetransfer from Cu to Co at the interface. On the contrary in thecase of N/Co, it increases in both NI and GI geometries withthe decrease in Co coverage. This is consistent with thePDOS calculation in Fig. 5, in which the unoccupied DOS ofthe single-layer N/Co film is larger than that of the double-layer N/Co film both in in-plane and out-of-plane 3d orbitals.The 3d hole number of the N/Co film would be determinedby the balance of charge transfer from Co to N and that fromthe substrate to Co. One possible reason for the increasedhole number of the single-layer N/Co film is the increase inthe charge transfer to N atom due to the decrease in N-Codistance as in Table I.

For the 0.4 ML of Co in NI geometry in Fig. 3�a�, thein-plane hole number is about 1.2 times larger than that of2.0 ML. This is caused by the change at the satellite shoul-der. On the contrary, the out-of-plane hole number changes atthe L3-edge main peak in GI geometry. Clear difference ofthe hole distribution is detected with decreasing Co coveragepossibly due to the anisotropy of the electron correlation.

We can exclude the possibility of the core-level shift ofthe Co 2p state as the origin of the shoulder structure byconsidering the XPS result in Fig. 7. The intensity ratio ofthe shoulder structure to the main peak is significantlysmaller than that in the corresponding absorption spectrum.If the origin of the shoulder structure in the absorption spec-tra was the core-level shift, both the intensity ratios shouldbe at the same level. Therefore, the 3-eV shoulder structure,both in the photoelectron and absorption spectra, is not dueto the core-level shift.

Another possible origin of the shoulder structure is themultiple scattering �MS� in the absorption process. Recent

depth-resolved XAS study on a Ni thin film revealed that the6-eV satellite at the L-edge absorption spectrum originatesfrom the MS effect.24 We have simulated the Co L3-edgeabsorption spectra using FEFF8.2 code25 to verify the contri-bution of the MS to the absorption process as previouslydiscussed for the Ni thin film. The simulation was carried outonly for L3-edge for single- and double-layer N/Co films in a5�5 surface unit cell of the substrate Cu�001�. The sameoptimized atomic structure for the surface two layers wasused as in the PDOS calculation.

In Fig. 8, the simulated spectra are shown for the single-layer film in NI and GI geometries and the double-layer filmin NI geometry. For the double-layer film, the contributionfrom the interfacial Co layer is included considering themean-free path of the emitted electrons. All the simulatedspectra including the GI geometry show a shoulder structureat an energy around 3 eV higher than the main peak. Thisqualitatively differs from the experimental results shown inFig. 3�a� while the position of the shoulder coincides withthe observation in the NI geometry. The calculated intensityratio of the shoulder to the main peak for the single-layerfilm is smaller than that for the double-layer film. This is anopposite trend to the observation shown in Fig. 3�b�. Conse-quently, at least below 2 ML on average, the 3-eV shoulderstructure is attributed to the correlation satellite rather thanthe MS effect.

In the L-edge absorption spectra in Fig. 3�a�, the intensityof the shoulder structure still remains even above 2 ML com-paring to that on the Co film on the clean Cu�001� surfacewith the same thickness as shown in Fig. 6. In this thicknessregion, the MS might contribute to the small shoulder inten-sity.

In the last, we note that the atomic multiplet is not theorigin of the observed 3-eV shoulder. For a small amount��0.1 ML� of Co on an alkali-metal surface,26 the observedshoulder structure in the L3-edge absorption spectrum has

2p3/2

1/22p

Co 2p XPS

FIG. 7. Co 2p photoelectron spectra for 0.25 and 3 ML of Co onaverage on N/Cu substrate. The �3 eV shoulder structures are seenboth for the 2p3/2 and 2p1/2 main peaks in the spectrum for 0.25 MLas indicated by the arrows.

FIG. 8. Simulated Co L3 edge spectra including multiple-scattering effects. The structural parameters are the same as those inthe PDOS calculation in Fig. 5. Black and gray solid lines are thespectra from the single-layer N/Co film in NI and GI geometries,respectively. The dashed line is the spectrum from the double-layerN/Co film in NI geometry.

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been attributed to the atomic multiplet with d8 configurationas the ground state by comparing it with the theoreticalcalculations.27 In this case, there is no splitting in the L2 edgein both the experimental results and the calculation withoutthe crystal field. In the present case, the 3-eV shoulder isobserved in both edges even in the case of Co on the cleanCu surface, in which the crystal field is not necessary to beconsidered. Moreover, the density of Co in our case is morethan 0.2 ML, which is much larger than the Co on the alkalimetal.

C. Origin of the 3 eV correlation satellite

In this subsection, we discuss which part of the Co atomson the surface shows the 3 eV correlation satellite, whichbecomes weaker with increasing Co coverage. Figure 9�a�shows the fraction of perimeter Co atoms in single-layerN/Co islands and in the surface N/Co layer of double-layerN/Co islands deduced from the STM observation. It de-creases with the coverage monotonically until 1.5 ML of Coon average. This suggests that the island perimeter atoms ofthe N/Co layer generate the 3-eV satellite. The coordinationnumber of the perimeter atoms is smaller than that of theinternal atoms of the island, resulting in further localizationof the Co 3d states, i.e., further reduction in the 3d bandwidth. This leads the electron correlation more effective atthe perimeter.

Another possible origin of the satellite would be the Coatoms in the single-layer N/Co island since the bandwidth ofCo 3d state is narrow in the single-layer N/Co film as calcu-lated in Fig. 5. However, the STM observations suggest thatthey are not the origin because they keep their fraction al-most constant by increasing the average coverage as shownin Fig. 9�b�. Similarly, Co atoms in the N/Co layer and in theinterfacial layer, of the double-layer N/Co island, are not theorigin of the satellite judging from their constant fraction asin Fig. 9�b�.

The 3-eV satellite is strong for the incident light within-plane polarization as shown in Fig. 3�a�. The electron cor-relation increases in the in-plane 3d orbital of the perimeter

Co atom. This is in contrast to the isotropic generation of the3-eV satellite in the case of small amount of Co on the cleanCu�001� surface where the satellite is observed in both NIand GI geometries as in Fig. 6. Such a difference can beattributed to anisotropic hybridization of Co 3d states withsurrounding p or s states at the perimeter of the N/Co layer.The p-d hybridization between N and Co atoms significantlychanges the PDOS of the N/Co layers of the single- anddouble-layer N/Co films from that of the single-layer Cofilm, strongly depending on each 3d orbital as calculated inFig. 5. This will result in the modification of anisotropy ofp-d or s-d hybridization between Co and surrounding atoms.

The calculated bandwidth of the single-layer N/Co film is,however, larger in the in-plane 3d orbitals than that in theout-of-plane ones. This is inconsistent with the observed an-isotropy of the electron correlation. To further understand thecorrelated electronic states in the present system, band cal-culation and CI calculation are necessary considering the re-alistic surface perimeter structure based on the first-principles calculations.

IV. SUMMARY

In summary, we have investigated the electronic statesof the Co nano-islands on a nitrogen-saturated Cu�001�c�2�2�-N surface by XAS and discussed the electron corre-lation in Co 3d states considering the calculated PDOS,x-ray photoelectron spectra, and surface morphology studiedby STM.

In the Co L-edge absorption spectrum, a �3 eV shoulderstructure was found both at L3 and L2 absorption edges onlyin the case of in-plane polarization of the incident light. Itsrelative intensity to the main peak decreases with increasingCo coverage below 2 ML on average. On the surface, a mix-ture of single- and double-layer N/Co islands is formed fromthe initial stage of the growth.

The origin of this 3-eV shoulder was discussed consider-ing the unoccupied PDOS, XPS results on the same surfaceand multiple-scattering effect in the absorption process. Thelocalization of Co 3d states and the increase in 3d hole num-

perimeter of N/Co layer

(a) (b)

N/Co in single-layer

N/Co in double-layerinterfacial CoN/Co in triple-layer

FIG. 9. �a� Evolution of the fraction of the perimeter atoms in the N/Co layer in single- and double-layer N/Co islands as a function ofthe average Co coverage. �b� Evolution of the fraction of three kinds of Co atoms included in the Co islands schematically shown in Fig.1�e�. A small fraction of atoms in the third layer is also plotted. The data are deduced from the STM observations in Fig. 1.

ELECTRON CORRELATION EFFECTS IN Co NANOSCALE… PHYSICAL REVIEW B 77, 235436 �2008�

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ber was revealed by the photoelectron and absorption spec-tra. We finally attributed the shoulder to the correlation sat-ellite.

Comparing with the STM observation, the satellite inten-sity is associated with the fraction of the perimeter Co atomsof the N/Co islands. The perimeter atoms should have local-ized 3d states due to reduced coordination number, andshould have contributed to the satellite.

Nitrogen K-edge absorption spectrum showed increase inthe absorption intensity just above the Fermi level with theCo adsorption. This suggests the N 2p—Co 3d hybridizationand confirms the nitrogen segregation onto the Co islands.The in-plane large correlation satellite intensity would beexplained by anisotropic p-d or s-d hybridization of the pe-rimeter Co with surrounding atoms caused by theN 2p—Co 3d hybridization.

ACKNOWLEDGMENTS

The authors greatly acknowledge Akio Kimura, ShinImada, and Akira Ishii for fruitful discussion about the cor-relation satellites and Toshihiko Yokoyama for the kind ar-rangement of XAS experiments. The present work was per-formed under the approval of the Photon Factory ProgramAdvisory Committee �PF-PAC No. 2001G006 and No.2004G009�. The theoretical calculation was partly supportedby the Next Generation Super Computing Project, Nano-science Program, MEXT, Japan, and a Grant-in-Aid for Sci-entific Research in Priority Areas “Development of NewQuantum Simulators and Quantum Design” �Grant No.17064004�, MEXT, Japan. The computation in this work hasbeen done using the facilities of the Supercomputer Center,Institute for Solid State Physics, University of Tokyo.

*nakatuji@issp.u-tokyo.ac.jp†Present address: Tandem Accelerator Complex, Research Facility

Center for Science and Technology, University of Tsukuba,Tsukuba, Ibaraki 305-8577, Japan.

‡Present address: Institute for Molecular Science, Okazaki, Aichi444-8585, Japan.

§Present address: Faculty of Engineering, Yokohama National Uni-versity, Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan.

�komori@issp.u-tokyo.ac.jp¶Present address: Institute of Materials Structure Science, High En-

ergy Accelerator Research Organization �KEK�, Oho, Tsukuba,Ibaraki 305-0801, Japan.

**Present address: Kansai Photon Science Institute, Japan AtomicEnergy Agency, Sayo, Hyogo 679-5148, Japan.

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