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9

Mott-Hubbard insulator-metal transition in the VO2 thin film: A combined XAS and

resonant PES study

S.S. Majid1, K. Gautam2, A. Ahad1, F. Rahman1, R.J. Choudhary2, Frank M. F. De Groot3, D.K. Shukla2∗1 Department of Physics, Aligarh Muslim University, Aligarh-202002, India

2 UGC-DAE Consortium for Scientific Research, Indore-452001, India3 Debye Institute for Nanomaterials Science, Utrecht University-99, 3584 CG, Utrecht, The Netherlands

(Dated: July 31, 2019)

We have analyzed spectral weight changes in the conduction and the valence band across insula-tor to metal transition (IMT) in the VO2 thin film using X-ray absorption spectroscopy (XAS) andresonant photoemission spectroscopy (PES). Through temperature dependent XAS and resonantPES measurements we unveil that spectral changes in the d‖ states (V 3dx2−y2 orbitals) are di-rectly associated with temperature dependent electrical conductivity. Due to presence of the strongelectron-electron correlations among the d‖ states, across IMT, these states are found to exhibit sig-nificant intensity variation compared to insignificant changes in the π∗ and the σ∗ states (which areO 2p hybridized V 3d e

πg and e

σg states) in the conduction band. Experimentally obtained values of

the correlation parameter (Udd ∼ 5.1 eV, intra-atomic V 3d correlations) and crystal field splitting(10 Dq ∼ 2.5 eV) values are used to simulate the V L2 ,3 edge XAS spectra and an agreement be-tween simulated and experimental spectra also manifests strong correlations. These results unravelthat the IMT observed in the VO2 thin film is the Mott-Hubbard insulator-metal transition.

PACS numbers: 64.60.av, 71.23.Cq, 71.15.-m, 71.27.+a

Keywords: Insulator to metal transition, Electronic structure, X-ray absorption spectroscopy, Resonant

photoemission spectroscopy

I. INTRODUCTION

Vanadium dioxide (VO2) is an intriguing oxide ma-terial which undergoes first order transition from lowtemperature insulating monoclinic (P21 /c) structure tohigh temperature metallic rutile structure (P42 /mnm)at Tt ∼ 341 K 1,2. VO2 has attracted attention ofresearchers from last several decades because of illu-sive nature of the IMT and possible potential applica-tions such as thermochromic windows, memory devicesand ultrafast optical switches3–7. The driving mecha-nism of the IMT in the VO2 is under debate, whetherstructural driven, involving electron-phonon interactions(Peierls model), or associated with the electron-electroncorrelations (Mott-Hubbard model). Both, theoreticallyand experimentally significant efforts have been made tocompletely understand the physics of the IMT, but acomprehensive understanding about relative roles of theMott-Hubbard and the Peierls mechanism is still missing.Biermann etal .,8 concluded correlation-assisted Peierlstransition while Haverkort etal .,9 have shown orbital-assisted collaborative Mott-Peierls transition. The insu-lator to metal transition in the VO2 accompany concomi-tant spectral changes in the valence and the conductionband and are relevant for addressing the questions re-garding origin of the IMT 10–12. In this context study ofthe electronic structure and role of the orbital occupationaround the Fermi level (EF ) across the IMT in the VO2

becomes important.

The electronic structure of the VO2 can be understoodwithin a crystal field model using simple molecular or-bital theory 13. In the rutile metallic phase of the VO2,

octahedral crystal field splits the V 3d degenerate or-bitals into the doubly degenerate eσg and the triply degen-erate t2g orbitals. The small orthorhombic component ofthe crystal field associated with different equatorial andapical V–O distances further splits the triply degeneratet2g orbitals into the doubly degenerate eπg orbitals and asingle dx2−y2 (non-bonding d‖) orbital. The eπg orbitalshybridize with the O 2p orbitals to form bonding (π) andanti-bonding (π∗) molecular orbitals, while the d‖ orbitalis oriented along the c axis. We have employed XASand resonant PES techniques, both, to investigate theelectronic band structure of the unoccupied (conductionband) and the occupied (valence band) states, respec-tively. XAS is an invaluable tool to study the conductionband while the resonant PES reveals the true nature ofthe valence bands 14,15.

We have carried out temperature dependent XAS andresonant PES measurements on the VO2 thin film, grownon Silicon substrate, to unveil spectral changes in the or-bital occupations and their role in the IMT. Details ofsynthesis, structural and electrical studies of the VO2

thin film used in this study are reported elsewhere 16.Significant changes in spectral weight of the one dimen-sional (1D) charactered d‖ orbitals is observed across theIMT and is crucial for understanding the IMT in the VO2

thin film. The V L2 ,3 edge XAS spectra have been sim-ulated to match the experimental XAS spectra in the in-sulating and the metallic phases of the VO2 using chargetransfer multiplet (CTM) calculations. Findings in thisstudy confirm presence of the strong electron-electroncorrelations in the VO2 thin film.

2

II. EXPERIMENTAL DETAILS

High quality [0 0 1] oriented monoclinic phase VO2 thinfilm has been grown on cheap and commercially availableSilicon substrate using pulsed laser deposition technique16. Temperature dependent soft X-ray absorption spec-troscopy (SXAS across the V L2 ,3 and the O K edgeswere carried out in total electron yield (TEY) mode atthe beam line BL-01, Indus-2 at RRCAT, Indore, In-dia. Energy resolution during SXAS measurements atthe oxygen K edge energy was ∼ 250 meV. Bulk V2O5

was measured for reference. Photoemission spectroscopywere performed at the angle-integrated photoemissionspectroscopy (AIPES) beamline, Indus-1 synchrotron ra-diation source at RRCAT, Indore, India. In order tostudy resonance effects in the photoemission process, va-lence band spectra (VBS) were collected (at 300 K and373 K) with incident photon energies varying from 40 eVto 60 eV, to cover the V 3p → 3d transition. The samplesurface was cleaned by 500 eV Ar+ ions before measuringthe valence band spectra. The Fermi level was aligned byrecording the valence band spectra of the Ag foil, whichwas mounted along with the sample. Experimental res-olution during the VBS measurements was estimated tobe ∼ 0.25 – 0.27 eV.

III. RESULTS

Figure 1 (a) shows room temperature V L2 ,3 and OK edge X-ray absorption spectrum of the VO2 thin film.The V L2 ,3 spectrum arises due to transition from theV 2p core level (spin orbit splitted V 2p3/2 and 2p1/2 )to the unoccupied V 3d states and consists of two pro-nounced features at∼ 518.8 eV (L3) and∼ 525.5 eV (L2).Features at ∼ 516.2 eV and ∼ 522.2 eV are due to tran-sitions to crystal field splitted t2g part of the V 3d band9,17. The O K edge spectrum results from transition ofthe O 1s electrons to the O 2p states hybridized withthe V 3d orbitals. Peaks at ∼ 529.6 eV and ∼ 532.1 eVare due to transitions into the π∗ and the σ∗ hybridizedbands 13,18. The broad structures at higher energies inregion ∼ 537 eV to 550 eV are attributed to transitionsinto the V 4sp bands hybridized with the O 2p orbitals19. In order to understand the spectral changes in theconduction band across the IMT XAS spectra of metal-lic and insulating phases are overlapped as shown in theFigure 1 (b). The pre peak feature (A) at ∼ 514.1 eVin the metallic phase is assigned to the d‖ band, repre-senting the V 3d‖-V 3d‖ hybridizations. The feature Aalmost vanishes in the insulating phase of the VO2 and issignature of the IMT in the VO2 thin film. This is in ac-cordance to a recent report by Yeo etal ., 20. The featureB, designated as d‖ (O) band (d‖ (O), because of the d‖signature from the O K edge part), observed at ∼ 530.4eV in the insulating phase belongs to the unoccupied d‖band and is consistent with previous reports 10,21. The

anomalous spectral weight transfer of the d‖ states from∼ 514.1 eV to ∼ 530.4 eV is observed across the IMT inthe VO2 thin film. Such a large spectral weight trans-fer is considered as the fingerprinting of strong electroniccorrelations 20,22,23. During SXAS measurements the po-larization vector of the incident photons was perpendic-ular to the surface normal of the thin film, monoclinicaxis [0 1 1], which means that the photon polarization isparallel to the plane in which monoclinic a-axis (or rutilec-axis, as aM = 2cR) of VO2 crystallites are randomlyoriented but they all share the same surface normal [01 1] 16. So, the spectral weight of both the features Aand B is mainly due to averaged intensity of the V 3d‖states aligned along the monoclinic a-axis (or rutile c-axis) of randomly oriented VO2 crystallites sharing thesame surface normal [0 1 1].

In the metallic phase spectral weight of the featureB vanishes while negligible spectral weight changes areobserved in the π∗ and the σ∗ bands compared to theinsulating phase of the VO2 thin film. Observation ofthe hysteresis in spectral weight change of the d‖ (O)states across the IMT confirms the first-order nature ofthe electronic structure transition, see Figure 1(e). Fig-ure 1(c) shows temperature dependent behavior of theO K edge XAS spectra. Dotted line indicates transfer inspectral weight of the O K edge towards lower photonenergy with increase in temperature. With respect toroom temperature insulating phase the O K edge spec-trum in high temperature metallic phase shifts towardslower photon energy as well as shrinks.

Room temperature valence band (insulating phase, 300K) spectrum of the VO2 thin film measured with 40 eVphoton energy is shown in the Figure 2 (a). The spec-trum is constituted of mainly the V 3d band ranging fromEF to ∼ 3 eV binding energy and the O 2p bands above3 eV 15,24,25. The V 3d region is fitted with single peak Cat ∼ 1.5 eV while the O 2p region is fitted with the peaksD, E, F and G at ∼ 4.19 eV, ∼ 7.12 eV, ∼ 5.53 eV and∼ 10.13 eV, respectively. The peak C is assigned to theoccupied V 3d‖ band. Other features D to G correspond

to the V 3d–O 2p bonding orbitals25. Here, we will focusmainly on the purely V 3d‖ band, feature C. Figure 2(b) shows valence band spectra of the VO2 thin film inthe metallic phase (373 K). The V 3d band is fitted witha peak, H, at ∼ 1.2 eV and rest of the band contains fourfitted features similar to the insulating phase. In order toextract true nature of the V 3d valence bands above andbelow the IMT, measurements have been performed byrecording energy distribution curves (EDCs) for differentincident photon energies, ranging from 40 eV to 60 eV inboth the insulating (300 K)and the metallic phase (373K), shown in the Figure 2 (c). Selected range of photonenergies cover resonance of the V4+. With variation inincident photon energy changes in intensities and spec-tral shape of EDCs can be readily observed, see Figure2 (c). From resonant PES measurements, one can plotthe constant initial state (CIS) spectrum. A CIS spec-

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feature

Resistivity

528 530 532 534 536

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373 K

368 K

363 K

355 K

348 K

337 K

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332K328 K

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(a.u

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Photon energy (eV)

305 K

dII(O)

dII

L3

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(b)

B

O K

edg

e

V L3,2 edges

300 K 373 K

Nor

mal

ized

abs

orpt

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(a.u

)

Photon energy (eV)

L2

515 520 525 530 535 540 545 550

0.0

0.5

1.0

1.5

2.0

2.5

t2g

t2gV 4sp

L3

(a)

O K edge V L3,2 edges

300 K

Nor

mal

ized

abs

orpt

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(a.u

)

Photon energy (eV)

L2

FIG. 1: (a) The room temperature V L2 ,3 and O K edgeXAS spectrum of the VO2 thin film. (b) Comparison of nearedge XAS spectra of the insulating (300 K) and the metallic(373 K) phases. (c) The temperature dependent O K nearedge XAS spectra of the VO2 thin film. (d,e) Temperaturedependence of spectral weight of the features A (d‖) and B(d‖ (O)) along with resistivity curve of the VO2 thin film.All temperature dependent data shown in this figure are fromthe cooling cycle except data of the feature B which is shownin both the heating and the cooling cycles to represent thehysteresis behavior of the first order phase transition.

trum from EDCs can be obtained by plotting normalizedintensity of a particular feature versus incident photonenergies.

Resonant photoemission from the V 3d atomic statesoccur when energy of incident photon coincides with theV 3p → 3d absorption threshold 15,26,27. At resonantenergies different ionization mechanisms (direct and in-direct) leave the system in same final state. Direct pho-toemission from the V 3d band can be written as:

3p63d1 + hν → 3p63d0 + e−(ǫ),

where a photon of energy hν ionizes the V atom and anelectron with kinetic energy ǫ is emitted. Indirect processinvolves an intra-atomic excitation process:

3p63d1 + hν → [3p53d2]∗, followed by emission of a 3delectron through the super Coster Kronig decay,

[3p53d2]∗→3p63d0 + e−(ǫ).

Here final state and electron kinetic energy are same asobtained in the direct photoemission process. In this casea resonance profile is observed in photoelectron intensityversus incident photon energy.

The CIS plots for the V 3d bands, features C (insulat-ing phase) and H (metallic phase) are shown in the Fig-ure 2 (d). The resonance profile of the feature C, in theinsulating phase has onset of resonance started around∼ 44 eV and maximizes around ∼ 47 eV, which is con-sistent with the earlier reports on the single crystal VO225. The CIS curve in the metallic phase after a steep risestays nearly constant for the measured range of photonenergies. These observations are consistent with changesobserved in band structure in the VO2 across the IMT,28 as shown in the inset of the Figure 2 (b) and confirmthat V 3d peak, H, in the metallic phase is constitutedof the d‖ and the π∗ bands (inseparable due to resolu-tion limit). In the insulating phase of the VO2 thin film,resonance observed in the V 3d band is due to transitionof the V 3p electrons to the completely electron occupiedd‖ band while in the metallic phase resonance occur dueto transition of the V 3p to the overlapped partially un-occupied d‖ and π∗ bands (see inset of the Figure 2(b)).Unoccupied states allows resonance to happen, at ener-gies even above 44 eV (resonance energy of V4+) as thereare enough available vacant states in the valence band forthe V 3p → 3d transition to happen, at higher photonenergies in the metallic phase.

IV. DISCUSSION

Contribution of the V 3d bands across the EF in va-lence as well as conduction band are visualized by com-bining the V 3d bands from the VBS and the O K edgenear edge XAS spectrum, see Figure 3. Usually, PESand the inverse PES are combined for description of thevalence and the conduction band because XAS has in-volvement of the core hole effect. However, effect of thecore hole is small in case of the oxygen K edge so it can

4

FIG. 2: (a) Valence band spectrum (VBS) of the VO2 thin film collected using 40 eV incident photon energy in the insulatingphase, 300 K (a), and the metallic phase, 373 K (b). Inset (b) shows schematic representation of the energy bands in theinsulating and the metallic phases of the VO2 thin film. (c) Vanadium 3d VBS of the VO2 thin film in the insulating (featureC) and the metallic (feature H) phases collected at various photon energies. (d) Constant initial state (CIS) spectra of the V3d bands (feature C in the insulating and feature H in the metallic phase).

also be used to represent the conduction band. Fermilevel marked in Figure 3 for the VBS is obtained withthe help of Ag reference. Second derivative of the O Kedge XAS was performed to determine the correct ab-sorption onset position in XAS spectra. And to align theXAS spectra on the same axis absorption onset is takenat EF

29,30.

In valence band part, presence of finite electron densityat the Fermi level in the metallic phase compared to theinsulating phase indicates an electronic phase transitionin the VO2 thin film. The valence band in the insulatingphase is constituted of only the d‖ states while in themetallic phase the d‖ and the π∗ states both contribute28. The conduction band in the insulating phase (Figure3 (a)) is comprised of three features the π∗, the d∗‖ and

the σ∗ states. The conduction band in the metallic phase(Figure 3 (b)) is comprised of the π∗ and the σ∗ states.In the metallic phase we observe disappearance of thed∗‖ orbitals (which is marked with an arrow in the insu-

lating phase). The d‖ feature shown in the conductionband of the metallic phase is from unoccupied part ofthe non-bonding d‖ states overlapping with the π∗ state.Observation of orbital switching of the d∗‖ states across

the IMT in the VO2 thin film is consistent with previousX-ray absorption studies on the VO2 single crystals 9,30.

The behavior of temperature dependent integrated in-tensity of the d‖ states (features A and B) from XAS mea-surements are compared with resistivity curve, see theFigures 1(d,e). This clearly indicates that conductivityin the VO2 thin film is directly associated with spectralweight changes of the d‖ states. Such a large and con-comitant change in spectral weight of the d‖ states acrossthe IMT is possible only when the system is stronglycorrelated9,20,30,31. Although spectral weight change ofthe d‖ states across the IMT is also related to the V-Vdimerization along the rutile c axis, but significant spec-tral change of the d‖ states compared to almost negligi-ble changes observed in the π∗ and the σ∗ states reflect

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300 K

Dq ~ 2.5 eV

(a) Udd ~ 5.1 eV UHBLHBInsulating VO2

Conduction bandValence band

N

orm

aliz

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ity (a

.u)

dII*

dII

dII

dII*

373 K

Dq ~ 2.3 eV

Udd ~ 4.4 eV UHBLHB(b)

Conduction bandValence band

Metallic VO2

EF

Energy (eV)

FIG. 3: Approximated valence and conduction bands of theVO2 thin film (a) in the insulating phase and (b) in the metal-lic phase.

strong electron-electron correlations present in the VO2

thin film, as the π∗ and the σ∗ states are also susceptibleto the structural changes18,21,30,32,33 see Figure 3.

Further, separation between the d‖ bonding and theanti-bonding bands estimated from the energy positionsof the d‖ bonding states in the valence band and the d‖anti-bonding states in the conduction band, comes outto be ∼ 3.0 eV. According to pure one electron approxi-mation this energy separation is equal to the 2t , where tis the intradimer hopping integral 20. Local density ap-proximation (LDA) estimates value of the t as ∼ 0.7 eVand energy separation ∼ 1.4 eV, which is much smallerthan the one estimated here (∼ 3.0 eV) 8. The differenceof ∼ 1.6 eV between the calculated and the experimentalvalues is due to presence of strong electron correlationsin the VO2 thin film, as electron-electron correlations arenot considered in the LDA.

In order to further substantiate claim of presence ofthe strong electron-electron correlations in the VO2 thinfilm, we have simulated the V L2 ,3 edges XAS spec-tra of the insulating (300 K) and the metallic (371 K)phases using relevant values obtained from the exper-

FIG. 4: The experimental and simulated V L2 ,3 edge spectra(a) in the insulating and (b) in the metallic phase of the VO2

thin film. (c) Schematic illustration of the V 3d‖ (dx2−y2 ) andthe O 2p orbitals oriented along the rutile c axis. (d) The V3d valence band spectra of the insulating and the metallicphases of the VO2 thin film.

iment, see the Figure 4 (a,b). Calculations are per-formed using the Charge Transfer Multiplet Model for X-ray Absorption Spectroscopy (CTM4XAS) code 34. TheCTM4XAS code is based on three components of theo-retical calculations, the atomic multiplet, the crystal fieldand the charge transfer calculations9,34. Crystal fieldsplitting parameter, the 10 Dq, is energy between thesplitted t2g and eg states in octahedral crystal field. Thecharge transfer calculations include the intra-atomic 3d -3d Coulomb repulsion energy, Udd, which is equal to en-ergy required to transfer an electron from one vanadiumsite to another vanadium site, the core hole potential,Upd, identified as an attractive V 2p3d correlation en-ergy and the charge transfer energy, ∆ defined as energydifference between the V 3d1 and the 3d2L configura-tions, where L is a hole state in the O 2p ligand band.The V L2 ,3 edges XAS spectra is calculated assumingthe V 2p63d1 (t12g) ground state and the Slater integrals

are scaled down to 80% of their Hartee-Fock values 14,34.Values of the Udd, 10 Dq, Upd and ∆ used in calculationsare ∼ 5.1 eV, ∼ 2.5 eV, ∼ 6.5 eV and ∼ 2.5 eV for theinsulating phase and ∼ 4.4 eV, ∼ 2.3 eV, ∼ 6.5 eV and∼ 2.5 eV for the metallic phase.

The Coulomb repulsion energy, Udd is obtained fromdifference between energy positions of the lower Hubbardband, LHB (d‖ peak in the valence band) and the upperHubbard band, UHB (σ∗ peak in the conduction band)35,36, while the crystal field splitting energy, 10 Dq is ob-

6

tained from difference in energy positions of the π∗ andthe σ∗ states in the conduction band (Figure 3). Calcu-lated spectra are convoluted with a Gaussian broadeningof ∼ 0.3 eV to account for experimental resolution. Sim-ulated spectra of both, the insulating and the metallic,phases of the VO2 thin film match well with their re-spective experimental spectra apart from lower intensityof the simulated V L2 edge compared to the experimen-tally obtained L2 edge, which is attributed to absence ofexperimental background. Comparatively larger broad-ening of the high energy L2 edge feature is due to finitelife time of the core hole leading to an increase in un-certainty in its energy, consistent with the Heisenberg′sprinciple 37.

Presence of the strong electron-electron correlations inthe VO2 thin film indicate that electrical conductivitymust be explained in light of the Mott-Hubbard model.According to which electrical conductivity can be ex-plained in terms of competition between hopping of elec-trons in the V (3d‖)-V (3d‖) channels and their electronic

correlations with the O 2p and V 3d‖ electrons 20. In theinsulating phase due to high O 2p participation, evidentfrom the increased intensity of the d‖ (O) band (Figure1 (e)), correlation among electrons hopping via the V(3d‖)-V (3d‖) channel and the O 2p electrons increases.While in the metallic phase lesser O 2p participation andscreening due to the π∗ electrons reduces electronic corre-lations, so electrons can hop from the one vanadium siteto another vanadium site. Presence of the π∗ electrons inthe metallic phase can be seen from resonance behavior ofthe d‖ feature (see inset Figure 2 (d) and Figure 3 (b)).Orientation of the O 2p and the V 3d‖ orbitals shownin the Figure 4 (c) illustrates hopping of the electronsbetween the vanadium sites. Implication of hopping ofelectrons in the metallic phase can also be directly visu-alized in the band width, W , of the vanadium valence 3dband. A larger valence bandwidth in the metallic phaserepresents higher electron hopping compared to the nar-row valence band in the insulating phase, see Figure 4(d).

V. CONCLUSIONS

In summary, our results unravel that electrical con-ductivity in the VO2 thin film across the IMT is directlyrelated to spectral weight change of the d‖ states. XASresults show that intensity of the unoccupied d‖ bandsdecreases with increase in conductivity of the VO2 thinfilm. Temperature dependent resonant behavior of thed‖ states inferred from the resonant PES experimentsshow that spectral shape in the metallic phase containsthe π∗ states (together with the d‖ states) which screensthe correlation among the vanadium dimers while in theinsulating phase spectral shape is defined solely by thed‖ states. From temperature dependent SXAS, anoma-lous spectral weight transfer of the d‖ states, ∼ 16 eV,is observed across the IMT which directly signifies roleof the strong electron-electron correlations present in theVO2 thin film. A large separation between the occu-pied and unoccupied d‖ bands in the insulating phase (∼3.0 eV) also vindicates the strong electron-electron cor-relations. Considerable value of the Coulomb repulsionenergy, Udd (∼ 5.1 eV), which is experimentally obtainedand theoretically verified, further substantiates presenceof the strong electron-electron correlations in the VO2

thin film. Finally, by combining our experimental resultsand simulations the IMT observed in the VO2 thin filmis explained in context of the Mott-Hubbard model.

VI. ACKNOWLEDGMENTS

Authors are thankful to Sharad Karwal and RakeshSah for help during XPS and XAS measurements, re-spectively. D.K.S. acknowledges support from DSTand SERB, India, in form of an inspire faculty award(IFA-13/PH-52) and an early-career research award(ECR/2017/000712).

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