Abstract
Understanding and controlling the electronic structure of thin layers of quantum materials is a crucial first step towards designing heterostructures where new phases and phenomena, including the metal-insulator transition (MIT), emerge. Here, we demonstrate control of the MIT via tuning electronic bandwidth and local site environment through selection of the number of atomic layers deposited. We take CaVO3, a correlated metal in its bulk form that has only a single electron in its V4+ 3d manifold, as a representative example. We find that thick films and ultrathin films (â¤6 unit cells, u.c.) are metallic and insulating, respectively, while a 10âu.c. CaVO3 film exhibits a clear thermal MIT. Our combined X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) study reveals that the thickness-induced MIT is triggered by electronic bandwidth reduction and local moment formation from V3+ ions, that are both a consequence of the thickness confinement. The thermal MIT in our 10âu.c. CaVO3 film exhibits similar changes in the RIXS response to that of the thickness-induced MIT in terms of reduction of bandwidth and V 3dâO 2p hybridization.
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Introduction
Deposition methods with unit cell thickness precision enable new approaches to control the degree of electronic localization in quantum materials. For instance, epitaxial strain changes the lattice parameters altering the potential landscape of the electrons, dimensionality tunes the electronic bandwidth and interfacing provides a charge reservoir. Thickness-dependent metal-insulator transitions (MIT) in thin films have been widely reported in recent years in a broad swathe of correlated 3d and 5d transition metal oxides.1,2,3,4,5,6,7 Understanding and controlling the electronic structure of these quantum materials is a critical first step toward designing heterostructures where new emergent phases and phenomena, including MIT, have been widely predicted.8,9 The focus of our interest is to resolve the nature of the thickness-induced MIT, therefore, in this work we consider CaVO3 as a model system.
Bulk CaVO3 with a formal valence shell configuration of V3d1 (V4+ oxidation state) is a paramagnetic metal with enhanced Sommerfeld-Wilson and Kadowaki-Woods ratios suggesting electronic correlations are important.10 However, as is often the case for metals, neither moderate doping nor pressure induce insulating or magnetic ground states.11,12 In contrast to the bulk, enhanced electron correlation strength and a MIT have recently been reported for CaVO3 films of 20 unit cells (u.c.) or less.6,13 While these discoveries are promising for technological applications such as transparent conductors13 and MottFETs,14,15 a complete understanding of the MIT in CaVO3 has, until now, remained elusive.
Figure 1 illustrates that electronic bandwidth can control the degree of electron localization in 3d1 perovskites RXO3. In the example presented in the top panels, applicable for bulk-like materials, when the orthorhombic distortion is increased the bandwidth decreases by up to 40% and a MIT occurs.16 However, realising this MIT requires the substitution of both R and X ions making it very difficult to continuously control in practice. In our work presented here, we show that the bandwidth can be continuously controlled by up to 40% in CaVO3 simply by choosing the number of atomic layers deposited on a SrTiO3 substrate as summarized in the bottom panels of Fig. 1 and outlined in detail in the rest of this work.
Comparison of bandwidth change with cation substitution in bulk 3d1 crystals with that realized in thin films of CaVO3 by reducing film thickness. a, b The crystal structure and electronic bandwidth of different 3d1 perovskites RXO3 is presented. For Râ=âSr and Xâ=âV a cubic structure is reported. When R is substituted to Râ=âCa an orthorhombic distortion occurs changing the electronic bandwidth and local V site symmetry. This results in a more correlated metallic state. When R is substituted to Râ=âLa and X substituted to Xâ=âTi (to maintain 3d1 configuration), the distortion is further increased and a metal-insulator transition occurs (MIT). The MIT is accompanied by long range antiferromagnetic order that can be switched to ferromagnetic by substituting R to Râ=âY. c, d A comparable reduction in bandwidth is found when the MIT in CaVO3 is controlled by film thickness as described in this manuscript
Results
MIT in thin film CaVO3
Figure 2a presents electrical resistance as a function of temperature for our CaVO3 and SrVO3 thin films grown by pulsed laser deposition (PLD) on SrTiO3 (100) substrates. The film thicknesses and surface ordering were routinely monitored with single-layer accuracy by reflection high-energy electron diffraction (RHEED), see Fig. 1 in S.I. For the thinner films (15âu.c., 10âu.c., 6âu.c., 4âu.c. CaVO3) a thin 2ânm SrTiO3 overlayer was deposited to protect against environmental (in air) degradation upon removal from PLD chamber. The thick 50âu.c. films are metallic at all temperatures, like bulk CaVO3 and SrVO3.10,17 When the CaVO3 (CVO) film thickness is reduced an MIT occurs. The 15 and 10âu.c. films undergo an MIT at 160 and 220âK, respectively, while the 6 and 4âu.c. films are insulating at all temperatures (the 4âu.c. film was too insulating to be measured using our setup). The overall behaviour is similar to that previously reported for CaVO3 films.6 Comparison of RHEED patterns recorded at room temperature after deposition of 15âu.c. CVO (metallic at room temperature) and 6âu.c. CVO (insulating at room temperature) films suggests structural transitions (possibly octahedral rotation) to play a role in the thickness-induced MIT (see S.I. Figure 3). We emphasize that this structural reconstruction was only observed for the very thin CaVO3 film of 6âu.c. thickness that was capped by a 2ânm (5. u.c.) SrTiO3 over-layer (compare Figs 1 and 3 in S.I.). We ascribe an important role to the SrTiO3 over-layer in inducing and/or stabilizing this structural reconstruction.
Reduced electronic bandwidth and increased V3+ character across the metal-insulator transition in thin film CaVO3. a Resistance as a function of temperature for the samples indicated. Arrows indicate where the slope of the resistance curve reverses sign for the 15âu.c. and 10âu.c. films. b X-ray absorption (XAS) as total fluorescence yield for a CaVO3 crystal and films with thicknesses as indicated (solid lines) for Ï-polarised light at a temperature of 20âK. XAS of the 10âu.c. CaVO3 film at room temperature is shown as a dashed blue curve. XAS is also shown for a 50âu.c. SrVO3 film (dashed black) and VO2 crystal (dashed grey) and V2O3 crystal (dashed magenta)
Overview of resonant inelastic x-ray scattering (RIXS) measurements across a thickness-induced MIT in vanadate thin films. (top) X-ray absorption total fluorescence yield for samples indicated (aâd). RIXS intensity maps of energy transfer versus incident energy for different film thickness across the thickness-induced MIT. CaVO3 and SrVO3 metal films are 50 unit cells thick. e Illustrative sketch of the origin of the Raman and fluorescence peaks measured using RIXS. The sketch is drawn on the total density of states (DOS) and partial DOS for V 3d eg (red) and V 3d t2g (blue) calculated using density functional theory
X-ray absorption (XAS) at the V L2,3 edges, presented in Fig. 2b, reveals an evolution from broad d-bands and large valence fluctuations in metallic thick films of CaVO3 to a more localized 3d character in the thinner insulating films. XAS of the thick metallic 50âu.c. film (red line) is similar to that of a CaVO3 single crystal (brown), as well as a 50âu.c. SrVO3 film (dashed black). These spectra are not well reproduced by multiplet calculations because of the itinerant character of the 3d electrons.18 With reduced film thickness the XAS spectra become sharper, the L3 peak energy is reduced by ~0.5âeV, a shoulder appears on the L3 peak and a new peak is found in the dip between L3 and L2 peaks. These new peaks are commonly observed in insulating VO2 (V4+, dashed grey) and V2O3 (V3+) compounds (dashed magenta).19 In particular, XAS of the thinner most insulating CaVO3 films (6 and 4âu.c.) is similar to that reported for Y1âxCaxVO3 (xââ¥â0.6) that contains a mixture of V4+ and V3+ character.18 The increase in V3+ results from charge redistribution in the thinner films and is consistent with a reduced Madelung potential and hence reduced VâO covalency across the MIT. The sharpening of the L2,3 peaks across the MIT reflects a reduction in the bandwidth of the upper Hubbard band. XAS of the 10âu.c. film in the high temperature metallic state is presented as a dashed blue curve in Fig. 2b. A broadening of the spectral features is observed with respect to the low temperature insulating state but the overall response is still different from that of the metallic thick 50âu.c. CaVO3 film. We undertook a series of resonant inelastic X-ray scattering (RIXS) measurements to investigate further the change in the electronic excitations across the thickness-induced and thermal MIT.
Control of electronic bandwidth in CaVO3
Figure 3 presents RIXS intensity maps of energy transfer versus incident energy at Tâ=â20âK across the V L3 edge. First, we discuss the energy map for the 50âu.c. SrVO3 and CaVO3 films presented in panels 3a and b. Three fluorescence features,20 where the energy transfer disperses linearly with incident energy, are evident at higher energy transfers (marked as F1, F2 and F3 in Fig. 3c). These spectral signatures have been observed previously for single crystal CaVO3 and were assigned to, for decreasing energy transfer, a hybridized VâO band (F3), an inter-band transition involving the lower Hubbard band (F2), and an intra-band transition involving a quasiparticle-like band (F1).21 Furthermore, we observe several new Raman-like modes where the energy transfer is independent of incident energy. We assign the intense Raman mode (R1) located at ~0.3âeV (0.5âeV for SrVO3) to electron-hole pair excitations.22 The Raman-like scattering at higher energy transfers (R2) is assigned to crystal-field excitations. The assignment of fluorescence and Raman peaks is illustrated in a schematic representation of occupied and unoccupied density of states in Fig. 3e.
The RIXS response undergoes a large change as we traverse the thickness-induced MIT in CaVO3. RIXS maps for insulating 10 and 4âu.c. films are presented in Fig. 3c, d. Our first observation is that the spectral weight in the fluorescence features has decreased relative to the spectral weight in the Raman modes as the film thickness is decreased. This transfer of spectral weight from band to localised excitations is expected as we move from metallic to insulating behaviour. We also observe that the measured peaks become sharper and there is a reduction of the bandwidth of these electronic excitations. The reduced bandwidth of electronic excitations measured across the MIT in our RIXS experiment on CaVO3 is similar to the reduced bandwidth measured using angle-resolved photoemission (ARPES) across the thickness-induced MIT in thin films of another vanadate, SrVO3, that was ascribed to a dimensional crossover below 3â4âu.c.5 In the case of CaVO3 reported here the crossover occurs at a larger thickness of 10âu.c. suggesting that both strain and dimensionality influence the MIT. Indeed it has been shown that strain can substantially affect the MIT in other systems, such as NdNiO3.23
We quantify the reduction in the bandwidth of the electronic excitations by fitting the data and present the corresponding line cuts and fit results in Fig. 4. The thickness-dependence of the low energy Raman-like RIXS peak that we assign to electron-hole pair excitations20,22,24 is presented in Fig. 4a. The elastic line has been subtracted and the corresponding fits are presented in S.I. We extract from our fits the bandwidth and energy scale of the electron-hole pair excitations and present these results in Fig. 4c (bottom). The bandwidth is gradually reduced from 0.73âeV in the SrVO3 50âu.c. film to 0.63âeV in the CaVO3 50âu.c. film down to 0.38âeV in the insulating 6 and 4âu.c. CaVO3 films. The peak energy exhibits a large jump from 0.5âeV to 0.3âeV from SrVO3 50âu.c. to CaVO3 50âu.c. and then is gradually reduced to nearly 0.2âeV in the insulating 6 and 4âu.c. films.
Forty percent bandwidth reduction across the thickness-induced MIT in CaVO3 thin films. a, b Resonant inelastic x-ray scattering (RIXS) intensity as a function of energy transfer for the films indicated for an incident energy of 520âeV (in the dip between L3 and L2 peaks). SrVO3 and CaVO3 are 50âu.c. films and also shown are 15âu.c., 10âu.c., 6âu.c. and 4âu.c. CaVO3 films. All data were recorded at a temperature of 20âK, and normalized to the total integrated intensity and offset by additive constants for clarity except data in a that were multiplied by small factors away from unity for clarity. c (top) V 3d bandwidth of the two features F1 and F2 for different films indicated, as extracted from the data presented in b; fits for extracting the valence band bandwidth are shown in the supplementary information (S.I.). (bottom) Bandwidth and peak position of the electron-hole pair excitations for different films are indicated, extracted from the fits shown in S.I. Error bars are defined by the standard deviation (SD) of the fits. The empty red square, diamond, and circle represent bandwidth and peak positions extracted from measurements of the 10âu.c. film in the metallic phase at 300âK presented in Fig. 6
In Figs 4b, c (top) we present line cuts and fit results for the fluorescence features F1 and F2 that are band excitations. We illustrate the origin of the F1 and F2 features in Fig. 3 (bottom) and assign F1 to intra-band excitations involving a quasiparticle-like band and F2 to inter-band excitations involving the lower Hubbard band.21 The separation between these two RIXS peaks is ~1.8âeV, the same energy separation as the low-energy optical conductivity peaks that were also interpreted as originating from intra- and inter-band excitations. Further support for this interpretation is provided by photoemission spectra that reveal a lower Hubbard band at 1.8âeV below the Fermi level.25 The total bandwidth of our two measured RIXS peaks, determined from the first derivative of the spectra (shown in S.I.), scales with the bandwidth of the V 3d valence band.19 Fig. 4c (top) presents the results of this analysis. The total bandwidth displays a sharp drop between the metallic 50âu.c. CaVO3 film (=4âeV) and the 15âu.c. CaVO3 film (=3.5âeV) that is on the verge of being insulating (see Fig. 2a). The bandwidth continues to decrease gradually to 3.2âeV in the most insulating 4âu.c. CaVO3 film. A guide for the eye in Fig. 4b shows the reduction in the separation between intra- and inter-band peaks, consistent with the bandwidth reduction extracted from our derivative analysis. For the 4âu.c. film the relative spectral weight in the intra-band peak substantially decreases with respect to the inter-band peak (compared to the 50âu.c. CaVO3 film) as the spectral weight is transferred away from the valence band maximum. However, there is still weight in the intra-band peak even in the insulating phase indicating metallic puddles are still present even in the insulating phase.26
Local moment formation in CaVO3 films
There are several Raman-like peaks marked as R2 in Fig. 3 and we present line cuts through this region for different film thicknesses in Fig. 5 (left). In the thick metallic 50âu.c. SrVO3 film, that is nominally a perfect cubic structure with no octahedral tilting and formal valence band configuration of V 3d1, we observe a broad peak centred around 1.9âeV, a similar energy scale to the t2g-eg excitations found near 1.7âeV in the insulator YVO3 with a valence electron configuration of V 3d2.27 For the 50âu.c. CaVO3 film, that has a small orthorhombic distortion and octahedral tilting, we find a large broadening and nearly flat distribution of spectral weight. A three peaks structure is measured in the 15âu.c. CaVO3 film and these peaks continue to sharpen in the insulating 10âu.c., 6âu.c. and 4âu.c. CaVO3 films (Fig. 5 (left)). There are two possible explanations for this reconstruction: strain-induced changes in bond lengths or the formation of local magnetic moments.27 The effect of epitaxial strain is larger in the thinner films but strain effects are expected to lift the degeneracy of the t2g and eg states by only a small amount (~100âmeV)15 compared to the large effect reported here. The additional peaks are therefore a clear signature of local moment formation from V3+ ions allowing spin flips from a t2g2 high spin Sâ=â1 ground state configuration to configurations involving Sâ=â0 and the eg level.27 We already have evidence that V3+ ions are present in thinner CaVO3 films from our XAS as described earlier and thus we can attribute the additional peak near 1âeV to a local spin flipping t2g2 Sâ=â0 transition at twice the Hundâs coupling energy 2JH using the notation of ref. 27 The additional peak near 2.6âeV is also captured by several overlapping local excitations on the V3+ ion, namely by transitions to t2g2 with Sâ=â0 of higher crystal-field symmetry and to t2g1eg1 with Sâ=â1 or Sâ=â0.27 The formation of a local moment in insulating ultra-thin CaVO3 films starting from a bulk-like non-magnetic thin film is a dramatic effect, resulting from a control of electronic bandwidth and charge redistribution.
Reconstruction of crystal field levels across the thickness-induced MIT. a Resonant inelastic x-ray scattering (RIXS) intensity as a function of energy transfer for the films indicated for an incident energy of 520âeV (in the dip between L3 and L2 peaks). SrVO3 and CaVO3 are 50âu.c. films and also shown are 15âu.c., 10âu.c., 6âu.c. and 4âu.c. CaVO3 films. All data were recorded at a temperature of 20âK, and normalized to the total integrated intensity and offset by additive constants for clarity. Labels indicate the different allowed crystal field excitations for V4+ and V3+ ions. b Schematic of the change in the electronic levels of the V 3d electrons due to reconstruction of the local V site environment and valence occupation
Thermal MIT in 10âu.c. CaVO3 film and the role of VâO hybridization
Now we draw the attention to the MIT which occurs below 220âK in the 10âu.c. CaVO3 film (see Fig. 2a). We present in Fig. 6 the RIXS intensity as a function of energy transfer for the 10âu.c. film in the room temperature metallic phase (at 300âK) and at 20âK, deep in the insulating phase. The change in the RIXS response across the thermal MIT is qualitatively similar to the change for the thickness-induced MIT (see also Fig. 4c): the bandwidth of the electron-hole pair excitation decreases by ca. 15% along with a small softening of the peak position (Fig. 6a), the crystal field levels sharpen (Fig. 6b), and the total bandwidth near the valence band maximum decreases by about 5% (Fig. 6c). In addition, we can quantitatively comment on the relative strength of the high-energy V 3dâO 2p hybridization band shown in Fig. 6d: the intensity of this peak is reduced by 10% in the insulating phase. The reduced VâO hybridization across the thermal MIT is comparable to the relative suppression of weight in this hybridized band upon entering the thickness-induced insulating state as shown in Fig. 3 (also see line cuts in S.I. Fig. 7). Thus, the same trend of reduced electronic bandwidth and reduced VâO hybridization is measured in both thermal and thickness-induced MIT, with the effects on the thickness-induced MIT being a factor 2â3 larger.
Resonant inelastic x-ray scattering across the thermal metal-insulator transition in a 10âu.c. CaVO3 film. aâd Resonant inelastic X-ray scattering (RIXS) intensity as a function of energy transfer at incident photon energy of 520âeV for a 10âu.c. CaVO3 at 20âK and 300âK. (inset of a) Same with elastic line subtracted
Discussion
The large reduction of bandwidth in our CaVO3 films is comparable to that realized in the 3d1 perovskite series SrVO3-CaVO3-LaTiO3-YTiO3 as shown in Fig. 1. The MIT in the latter case is attributed to an increased orthorhombic distortion, and local moment formation, as the cation size is changed.16 Obviously control of this MIT is chemically complex as it involves substituting two elements, illustrating the usefulness of the additional parameter space enabled by thin film synthesis methods.
We can understand our results by comparing the experimental spectra for the thermal- and thickness-induced MIT. In the thermal MIT case at constant film thickness we still need to consider the epitaxial strain. Strain will induce changes in the VâO bond lengths and VâO overlap/hybridization as experimentally observed in other 3d1 perovskites.28 We thus propose that a structural and/or magnetic transition is driving the thermal MIT in our 10âu.c. CaVO3 film as observed in e.g. nickelate films such as NdNiO3.23 This proposal is supported by the data presented in Fig. 6b that shows a sharpening of crystal field excitations across the thermal MIT. For the thinner CaVO3 films, the electron-hole bandwidth (see Figs. 1 and 4c, d) is sharply reduced moving from 10âu.c. to 6âu.c. and 4âu.c. films. Additionally XAS reveals increasing V 3d2 contributions (Fig. 2b) and the crystal field excitations become much shaper for the thickness-induced MIT (Fig. 5).
The increased contribution from V3+ ions in the thinner films stems from charge redistribution and is connected to a reduced VâO covalence across the MIT. The reduction of the V 3d bandwidth is driven by a structural reconstruction accompanied by octahedral rotation, that is thereby also changing the VâO hybridization, manifested in an abrupt change of the RHEED patterns of the ultra-thin capped films. In this sense the âelectronic localization and bandwidth reductionâ as well as the âincrease of V3+ contributionsâ are a consequence of the thickness confinement. Oxygen vacancies generated during the growth of CVO films might partially contribute to V3+ states, but their amount, in principle, should scale with the film thickness. However, 50âu.c. CVO film does not show more V3+ signature in the spectra than thinner CVO films suggesting no significant influence of oxygen vacancies on the properties of the studied CVO films.
The combined effects of charge redistribution, confinement, as well as strain are important in the ultrathin CaVO3 films.29 Based on the results presented here it is difficult to distinguish which of these effects is the main player driving the thickness-induced MIT. However, we note that the MIT appears to be significantly more complex than the dimensional crossover-driven MIT reported in ultrathin SrVO3 films using ARPES spectroscopy.5
Our experiments reveal a bandwidth-driven MIT in CaVO3 films as a function of both thickness and temperature where strain, dimensionality and charge redistribution contribute. A reduced VâO hybridization in the insulating state reveals that the MIT is of Mott-Hubbard type where a dâd gap is opened by a strong Coulomb interaction U between electrons.30 The sharp thermal MIT observed in our 10âu.c. CaVO3 film should stimulate future work on this model system. Our results are promising for the future of the field of correlated electronics showing that while increased disorder can be present in ultra-thin films, it does not inhibit the Mott physics and related spin or charge ordering. However, we want to emphasise the radical changes in electronic structure that can occur in films of reduced dimensionality, and the importance of including these effects in the rational design of new quantum materials.
Methods
Experimental details
XAS and RIXS experiments were carried out at the ADRESS beamline of the Swiss Light Source at the Paul Scherrer Institut.31 All measurements were performed at grazing incidence with the X-rays incident at 15° with respect to the sample surface and with Ï or Ï polarisation. The scattering angle was fixed at 130°. We set the spectrometer32 in the high throughput configuration using the 1500 lines per mm variable line spacing spherical grating33 as well as the newly installed CCD camera that provides sub-pixel spatial resolution.34 The beamline exit slit was 20âμm. The setup yielded a total energy resolution (full width at half maximum) of around 60âmeV.
Thin films of CaVO3 and SrVO3 were prepared on SrTiO3 (100) and NdGaO3 (110) substrates respectively by PLD. More information can be found in the supplementary information.
Data availability
The datasets generated during and/or analysed during the current study are available from the corresponding authors on reasonable request.
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Acknowledgements
D.M.N. and T.S. thanks A. Georges (University of Geneva) for a helpful discussion. D.M.N. acknowledges helpful discussion and analytical tools from Eugenio Paris (PSI). The work at PSI is supported by the Swiss National Science Foundation through the NCCR MARVEL and the Sinergia network Mott Physics Beyond the Heisenberg Model (MPBH). X.L. acknowledges financial support from the European Communityâs Seventh Framework Programme (FP7/20072013) under Grant agreement No. 290605 (Cofund; PSI-Fellow). J.P. and T.S. acknowledge financial support through the Dysenos AG by Kabelwerke Brugg AG Holding, Fachhochschule Nordwestschweiz, and the Paul Scherrer Institut. J.P. acknowledges financial support by the Swiss National Science Foundation Early Postdoc Mobility fellowship Project No. P2FRP2â171824. M.D. was partially funded by the Swiss National Science Foundation within the D-A-CH programme (SNSF Research Grant 200021L 141325).
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M.R. and T.S. conceived the project. D.M.N., M.N., T.S., M.M., C.W.S., E.V.P. and M.R. grew and characterised the thin film samples. D.M.N., X.L., J.P., M.D., V.N.S., M.R. and T.S. carried out the XAS and RIXS experiments. D.M.N., M.R., and T.S. analysed the data. S.B. and C.E. carried out the DFT calculation. D.M.N., M.R., and T.S. wrote the paper with contributions from all authors.
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McNally, D.E., Lu, X., Pelliciari, J. et al. Electronic localization in CaVO3 films via bandwidth control. npj Quant Mater 4, 6 (2019). https://doi.org/10.1038/s41535-019-0146-3
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DOI: https://doi.org/10.1038/s41535-019-0146-3