Identification of the surface features in the electronic structure of Cr
Khadiza Ali, Shyama R. Varier, Deepnarayan Biswas, Srinivas C. Kandukuri, Kalobaran Maiti
aa r X i v : . [ c ond - m a t . m t r l - s c i ] J un Identification of the surface features in the electronic structure of Cr
Khadiza Ali, Shyama R. Varier, Deepnarayan Biswas, Srinivas C. Kandukuri, and Kalobaran Maiti Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research,Homi Bhabha Road, Colaba, Mumbai - 400 005, INDIA. (Dated: 14 August 2018)
We studied the electronic structure of high quality Cr(110) films grown on W(110) surface employing photoe-mission spectroscopy. Experiments on the differently aged samples revealed distinct signatures of the surfaceand bulk features in the electronic structure. It is observed that the adsorbed oxygens form covalent bondswith the surface Cr atoms at higher temperatures, while they remain almost unreacted at low temperatures.In addition to the spin density wave transition induced band folding across the bulk Ne´el temperature, wediscover a weakly dispersing sharp feature emerging near the Fermi level at low temperatures presumably dueto correlation induced effects.Unusual magnetism of elemental Cr led to its wideranging applications involving recording media, high den-sity storage media, magnetic sensors, giant magnetore-sistance based devices etc. Enormous effort has beenput forward to understand the exotic electronic proper-ties of this system, which is crucial to have further de-velopments in technological application of this materialas well as involved fundamental science. Bulk Cr formsin bcc structure and is a good example of Fermi sur-face nesting driven antiferromagnet exhibiting incom-mensurate spin density wave (ISDW) transition at 311K that becomes commensurate via a spin-flip transitionaround 150 K. Various studies revealed several contro-versies in its electronic properties; for example, antifer-romagnetic/ferromagnetic surface on antiferromagneticbulk , complex surface-bulk differences in the electronicstructure , orbital Kondo effect vs. shockley surfacestates , proximity to quantum criticality etc. Con-trasting scenario on the energy gap has also been re-ported such as signature of pseudogap , direct multi-ple gaps in the ISDW phase. Manifestation of suchmultifaceted exoticity in a simple elemental system is re-markable and has continued to attract much attention inthe fundamental science and technology for long.Device fabrication based on such materials requiresknowledge of the surface behavior - whether the bulkproperties survives at the surface, if differs how much isthe deviation, etc. In order to investigate the surfaceelectronic structure critically, we studied the evolution ofthe electronic structure of Cr with temperature and ag-ing. We prepared Cr(110) surface, which is antiferromag-netic and expected not to favor orbital Kondo resonance.This allows us to investigate the surface-bulk differencesin the electronic structure solely due to surface termina-tion induced effect without major change in their mag-netism. Our results reveal surface and bulk character ofvarious photoemission spectral features unambiguouslyand interesting evolution of the surface states on aging.Electron correlation is found to play important role inthe electronic properties of this system.The sample was prepared in an ultrahigh vacuumchamber (base pressure better than 1 × − Torr) byelectron beam evaporation of Cr onto a clean W(110) sur-face. The details of the sample preparation and charac- (b)
Freshly prepared
350 K 5 K B A
350 K (a)
C ADEF B I n t e n s it y ( a r b . un it s ) after 14 hrs
350 K 5 K I n t e n s it y ( a r b . un it s )
30 K
FIG. 1. (a)Aging of the normal emission spectra at 350 Kand 30 K. (b) Normal emission spectra at 350 K and 5 Kfrom freshly prepared sample (upper panel). The lower panelshows 14 hours aged spectrum at 350 K (open circles) andthen cooled the same aged sample to 5 K (line). terization is given in the supplementary materials. Angleresolved photoemission spectroscopic (ARPES) measure-ments were carried out using a Phoibos150 analyzer fromSpecs GmbH and monochromatic He I source ( hν = 21.2eV) with the momentum resolution fixed at 0.01 × π/a ,where the lattice constant of Cr, a is 2.88 ˚A. The x -rayphotoemission spectroscopy (XPS) measurements werecarried out with a monochromatic Al Kα source. Allthese experiments were carried at a base pressure ofabout 1 × − Torr. The experiment temperature wasachieved using an open cycle helium cryostat, LT-3Mfrom Advanced Research Systems, USA.First we investigate the evolution of the electronicstructure with aging. Angle integrated photoemissiontechnique was employed with an acceptance angle of ± o to enable quick collection of the data with goodsignal to noise ratio. The spectra collected at 350 K &30 K from freshly prepared sample are shown in Fig. 1(a)after normalizing by the intensity around 6 eV bindingenergy. Each spectrum exhibits several features at thebinding energies 0.2, 0.4, 0.7, 1.5, 2.4 & 3.2 eV - thefeatures are denoted by A, B, C, D, E & F in the fig-ure. The intensity between 0.3-1.2 eV binding energyrange in the 350 K spectra decreases dramatically withtime and becomes almost saturated within about 6 hours.Such spectral change with aging can be attributed to thesuppression of intensities by the impurities adsorbed onthe surface and/or bonded to the surface atoms. Theintensities near the Fermi level, ǫ F appear to remain un-influenced by aging at 350 K. Curiously, a substantial de-crease in intensity of A is observed at 30 K. Since aginginfluences the surface states most prominently, the fea-ture A must be possessing significant surface character at30 K. Such phenomena suggest a surface spectral weighttransfer from higher binding energy regime to the vicin-ity of ǫ F with the decrease in temperature and hence,the emergence of the surface character of the intensitiesaround 0.2 eV at low temperatures.Subsequent to the decrease of B and C in the 350 Kspectra, the intensities of the features, E and F increasewith aging. Employing XPS (shown in the supplemen-tary data) we observed that the intensity of the oxygen1 s signal grows with time delay as often observed by us invarious other studies . Therefore, the spectral evolutionobserved here are attributed to the influence of oxygenson the surface states. The feature, E appears due to thenon-bonding oxygen levels, and the features D & F corre-spond to the energy bands hybridized to oxygen 2 p states.The scenario at 30 K is significantly different; in additionto large decrease in intensity in the vicinity of ǫ F , inten-sity of the feature E becomes significantly intense withina very short time delay keeping the features D and F in-tensities almost unchanged. This suggests that the Cr-Obonding is less significant at low temperatures and thepresence of large amount of adsorbed oxygen that leadsto an enhancement of the non-bonding feature, E.The temperature evolution of the electronic structureacross the bulk Ne´el temperature of 311 K is investigatedin Fig. 1(b). The 350 K and 5 K spectra obtained fromthe freshly prepared sample are compared in the upperpanel of Fig. 1(b). Distinct decrease in intensity in thevicinity of ǫ F is observed in addition to a small increasein intensity of the feature D. The surface features re-main unchanged in this large temperature range due tothe fact that the surface magnetic transition occurs ata higher temperature and the changes within the mag-netically ordered phase is negligible . This is furtherverified by cooling down the 14 hours aged sample pos-sessing primarily the bulk features near ǫ F . The 350 Kand 5 K spectra exhibit significant decrease in intensitynear ǫ F as a signature of antiferromagnetic transition .Interestingly, the intensity around 0.7 eV (feature C) be- Binding energy (eV) (b) 296 K γ α S2 β S1 α ' Γ k* I n t e n s it y ( a r b . un it s ) Binding energy (eV) (a) 350 K γ β α
S2 S1 Γ N FIG. 2. ARPES data at (a) 350 K and (b) 296 K. symbolsshow the peak positions. comes stronger along with a decrease in intensity beyond1.5 eV binding energy. This corroborates the conclusionsof less Cr-O bonded objects at 5 K due to less reactivityof oxygens at lower temperatures.In order to investigate the band character of the abovefeatures, the energy band dispersions of Cr(110)/W(110)sample obtained by ARPES are shown in Figs. 2(a) and2(b). Distinct signature of three energy bands denotedby α , β and γ are observed. These three energy bandspossess behavior quite similar to the t g bands obtainedfrom band structure calculations ; the degeneracy ofthe t g bands at Γ is lifted due to the fact that He I photon energy ( hν = 21.2 eV) corresponds to a k z -value( k z ∼ × πc ; c is the lattice constant along (110) di-rection) away from the high symmetry point, Γ. Suchphoton energy dependence of the band dispersion will beabsent for the surface bands, which is the primary focusof this study. There could be additional effect due to thestrain arising from the differences in the lattice constantsof Cr films with the W(110) substrate. The energy dis-persion of the α band with d xy ( z -axis along the surfacenormal) symmetry makes an electron pocket around theΓ-point and resembles well to the theoretically calculatedresults .In addition to the bulk bands, distinct signature of aweakly dispersive band is observed around 0.7 eV bindingenergy denoted by S2 in the figure, which is within theenergy gap of the bulk bands α and β . Another almostnon-dispersive feature denoted by S1 also appears near ǫ F . These features are found to be significantly sensitiveto aging as shown in Fig. 1 (features, A and C) indi-cating their surface character. The comparison of theexperimental spectra at 296 K and 350 K in Figs. 2(a)and 2(b) reveals interesting evolution. While all the en-ergy bands remain almost similar, the α band appearsto fold back at k ∗ as shown in the figure - signature ofband folding due to incommensurate SDW transition. Inaddition, a new feature denoted by α ′ appears near ¯Γ.The temperature evolution of the features near ǫ F asa function of temperature is shown in Fig. 3. In addition I n t e n s it y ( a r b . un it s ) α Γ (a) 350 K S1 α α ' S Γ
10 K S1(c)
Binding energy (eV) k || = 0.6 10 K 296 K 350 K(g) k || = 0.0(f) I n t e n s it y ( a r b . un it s )
296 K(d)
Binding energy (eV) α ' α S2 S1(b) 296 K Γ Γ (e)10 K Binding energy (eV) I n t e n s it y ( a r b . un it s ) I n t e n s it y ( a r b . un it s ) FIG. 3. Experimental energy bands at (a) 350 K, (b) 296K and (c) 10 K. The energy distribution curves (EDC) withexpanded energy scale at 296 K and 10 K are shown in (d)and (e), respectively. EDCs at different temperatures at (f) k || = 0 . k || = 0 . to the band folding effect, the comparison of the spectraat 350 K, 296 K and 10 K exhibit emergence of a sharpfeature around 15 meV below ǫ F possessing weak disper-sion. The distinct nature of this additional intensity ismost evident in the energy distribution curves shown inFig. 3(f) and 3(g). Moreover, it exhibits different trendin the change in intensities with temperature at Γ and k ∗ ( k || = 0 .
6) - the intensity at ǫ F gradually increases withthe decrease in temperature at Γ while the intensity at296 K is the lowest at k ∗ .Detailed temperature evolution of the features in thelow temperature range (below 110 K) are shown in Fig.4, where we show the energy distribution curves at ¯Γand k || = 1.0. The features around 1.5 eV binding energyrepresent the intensities of the β & γ bands correspondingto the bulk electronic structure and exhibit enhancementin intensity with the decrease in temperature at k || =1.0, while it seems opposite at ¯Γ. Overall, the intensityincreases at 1.5 eV as observed in the angle integratedspectra shown in Fig. 1(b) suggesting an enhancement ofthe local moment of the system within the magneticallyordered phase. The intensities at ǫ F exhibit significantenhancement at low temperatures and asymmetry withrespect to ǫ F . This is shown in Figs. 4(c) and 4(d)after dividing the experimental spectra by the resolutionbroadened Fermi-distribution function.It is clear that the electronic structure of one of the k || = 1.0 Binding energy (eV) (a) S2 incidence angle = 40 o k || = 0.0 5 K15 K30 K50 K80 K Binding energy (eV)
110 K S2 incidence angle = 61 o I n t e n s it y ( a r b . un it s ) I n t e n s it y ( a r b . un it s ) (c) k || = 0.0 k || = 1.0 5 K50 K110 K Binding energy (eV) (d) k || = 0.0 k || = 1.0 5 K50 K110 K FIG. 4. Temperature evolution of EDC at (a) k || = 1 . k || = 0 .
0. (c) and (d) show the spectral density of statesnear ǫ F at different temperatures in an expanded energy scale. most used elemental metal, Cr is quite complex. The re-sults of our extensive experimental study unveil plethoraof interesting scenarios, which have significant impact onapplication of this materials for future technological ad-vancements. Firstly, we identify the surface features viareduction in intensity due to aging and observe that thedegradation of the surface features of Cr film stronglydepends on temperature. At low temperatures, we ob-serve signature of adsorbed oxygen, which reduces thesurface feature intensities. At elevated temperatures, theoxygens form bonding with the surface Cr atoms thatenhances Cr spectral contributions at higher binding en-ergies (around 3.2 eV) due to the bonding-antibondingsplitting of the electronic states. The enhancement ofthe reactivity to surface impurities at higher temperaturehappens due to the thermal energy providing suitable ac-tivation for such reactions.While the above results on aging of the Cr surface pro-vide details of the surface stability, the spectral evolutiondue to adsorption/bonding of impurities with the surfaceatoms helps to identify the surface and bulk features inthe electronic structure. There have been controversy onthe surface character of the spectral feature at 0.7 eVbinding energy denoted by S2 in Fig. 1. Substantial de-crease in intensity with aging unambiguously indicatessurface character of S2. The other important issue in-volves the observation of significant sensitivity of S1 toaging at 30 K and it’s absence at 350 K. This suggeststhat the predominant surface character around 0.2 eVbecomes evident at low temperatures presumably due tothe fact that the bulk contributions near ǫ F diminishessignificantly at low temperatures due to the formationof SDW gap making the relative surface contributionsnear ǫ F discernible at low temperatures. In addition,there could be surface spectral weight transfer with thedecrease in temperature.Photoemission measurements at room temperaturewith varied angle of incident/emitted photon beam con-cluded Shockley type surface states with d z characterof S1 . In order to learn the orbital character of thegrowing intensities at low temperatures, we compare thespectra obtained at two geometries - the incident photonbeam at 61 o ( k || =0.0) and 40 o ( k || =1.0) with respect tothe surface normal. Thus, the k || = 1.0 case becomesslightly more favorable to probe d xz d yz states and the d z states becomes significantly less sensitive due to rel-atively more in-plane alignment of the light polarizationvector. In Fig. 4(d), we observe that the intensity ofthe sharp feature at 15 meV becomes more intense in the k || =1.0 spectra suggesting d xz d yz character of the grow-ing feature over the intensities of S1.It is to note here that the studied Cr(110) surface isunlikely to exhibit orbital Kondo behavior as the surfaceis expected to be antiferromagnetic, while orbital Kondoeffect corresponds to spin-ferromagnetic order . In gen-eral, correlated electron systems such as rare-earths and transition metal oxides exhibit temperature reduc-tion induced growth of the coherent feature at ǫ F withrespect to the intensities of the incoherent feature repre-senting the correlation induced electronic states. Theemergence of the sharp feature around 15 meV bind-ing energy with the decrease in temperature observedhere presumably suggests similar scenario in this system.Clearly, more studies are necessary to understand thecomplexity of the electronic structure of this system.In summary, we have investigated the detailed elec-tronic structure of high quality Cr(110) films grown onW(110) employing angle resolved photoemission spec-troscopy. The aging of the sample surface helped toreveal the surface and bulk character of various spec-tral features. The adsorbed oxygens on the surface formbonds with the surface Cr atoms at temperatures close toroom temperature, while they behave like adsorbed gasat low temperatures. The bulk electronic structure ex-hibits signature of band folding due to bulk spin densitywave transition. There are two surface peaks around 0.2eV and 0.7 eV binding energies. The temperature varia-tion down to 5 K reveals emergence of an additional sharp feature corresponding to the surface electronic structure.These results reveal the complex surface behavior of Cr,instability of the surface states with aging and the im-portance of electron correlation induced effect in the elec-tronic structure, which are important to understand forits potential applications. I. ACKNOWLEDGEMENTS
The authors, S. R. V. and K. M. acknowledge financialsupport from the Dept. of Science and Technology, Govt.of India under the Swarnajayanti fellowship programme. E. Fawcett, Rev. Mod. Phys.
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