Abstract
We report the observation of coherent surface states on cubic perovskite oxide SrVO3(001) thin films through spectroscopic-imaging scanning tunneling microscopy. A direct link between the observed quasiparticle interference patterns and the formation of a dxy-derived surface state is supported by first-principles calculations. We show that the apical oxygens on the topmost VO2 plane play a critical role in controlling the coherent surface state via modulating orbital state.
| Original language | English |
|---|---|
| Article number | 086801 |
| Journal | Physical review letters |
| Volume | 119 |
| Issue number | 8 |
| DOIs | |
| Publication status | Published - 2017 Aug 22 |
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Quasiparticle Interference on Cubic Perovskite Oxide Surfaces. / Okada, Yoshinori; Shiau, Shiue Yuan; Chang, Tay Rong; Chang, Guoqing; Kobayashi, Masaki; Shimizu, Ryota; Jeng, Horng Tay; Shiraki, Susumu; Kumigashira, Hiroshi; Bansil, Arun; Lin, Hsin; Hitosugi, Taro.
In: Physical review letters, Vol. 119, No. 8, 086801, 22.08.2017.Research output: Contribution to journal › Article › peer-review
TY - JOUR
T1 - Quasiparticle Interference on Cubic Perovskite Oxide Surfaces
AU - Okada, Yoshinori
AU - Shiau, Shiue Yuan
AU - Chang, Tay Rong
AU - Chang, Guoqing
AU - Kobayashi, Masaki
AU - Shimizu, Ryota
AU - Jeng, Horng Tay
AU - Shiraki, Susumu
AU - Kumigashira, Hiroshi
AU - Bansil, Arun
AU - Lin, Hsin
AU - Hitosugi, Taro
N1 - Funding Information: Okada Yoshinori 1 ,* Shiau Shiue-Yuan 2,3 Chang Tay-Rong 4,5 Chang Guoqing 2,3 Kobayashi Masaki 6 Shimizu Ryota 1 Jeng Horng-Tay 4,7 Shiraki Susumu 1 Kumigashira Hiroshi 6 Bansil Arun 8 Lin Hsin 2,3 Hitosugi Taro 1,9 Advanced Institute for Materials Research (AIMR), 1 Tohoku University , Sendai 980-8577, Japan Centre for Advanced 2D Materials and Graphene Research Centre, 2 National University of Singapore , Singapore 117546, Singapore Department of Physics, 3 National University of Singapore , Singapore 117542, Singapore Department of Physics, 4 National Tsing Hua University , Hsinchu 30013, Taiwan Department of Physics, 5 National Cheng Kung University , Tainan 701, Taiwan Photon Factory, Institute of Materials Structure Science, 6 High Energy Accelerator Research Organization (KEK) , 1-1 Oho, Tsukuba 305-0801, Japan 7 Institute of Physics , Academia Sinica, Taipei 11529, Taiwan Department of Physics, 8 Northeastern University , Boston, Massachusetts 02115, USA Department of Applied Chemistry, 9 Tokyo Institute of Technology , Tokyo 152-8552, Japan * yoshinori.okada.e1@tohoku.ac.jp 22 August 2017 25 August 2017 119 8 086801 25 April 2016 10 December 2016 © 2017 American Physical Society 2017 American Physical Society We report the observation of coherent surface states on cubic perovskite oxide SrVO 3 ( 001 ) thin films through spectroscopic-imaging scanning tunneling microscopy. A direct link between the observed quasiparticle interference patterns and the formation of a d x y -derived surface state is supported by first-principles calculations. We show that the apical oxygens on the topmost VO 2 plane play a critical role in controlling the coherent surface state via modulating orbital state. Japan Society for the Promotion of Science http://dx.doi.org/10.13039/501100001691 JSPS http://sws.geonames.org/1861060/ 26707016 25886004 26246022 26106502 JST-CREST JST-PRESTO 16H02115 Ministry of Education, Culture, Sports, Science and Technology http://dx.doi.org/10.13039/501100001700 MEXT http://sws.geonames.org/1861060/ MESSC-JP U.S. Department of Energy http://dx.doi.org/10.13039/100000015 DOE http://sws.geonames.org/6252001/ DOE DE-FG02-07ER46352 DE-AC02-05CH11231 DE-SC0012575 National Research Foundation Singapore http://dx.doi.org/10.13039/501100001381 National Research Foundation-Prime Minister’s office, Republic of Singapore Singapore National Research Foundation National Research Foundation of Singapore National Research Foundation, Singapore NRF http://sws.geonames.org/1880251/ NRF-NRFF2013-03 Establishing a microscopic understanding of the mechanism leading to coherent two-dimensional (2D) electronic states at surfaces or interfaces of transition metal perovskite oxides is a key step for further exploration of exotic quantum states in various systems [1–5] . Despite an extensive investigation of transition metal perovskite oxides using angle-resolved photoemission spectroscopy (ARPES) [6–10] , the underlying microscopic mechanism that leads to the formation of coherent electronic 2D states has remained elusive. In this connection, unraveling the link between the emergence of coherent surface states and atomic reconstruction constitutes an important step forward. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) allow us to observe the surface interference pattern with well-defined wave vectors [11–14] , and to reveal the microscopic mechanism giving rise to coherent electronic 2D states. However, perovskite materials are intrinsically three-dimensional (3D) crystals, and obtaining atomically well-aligned surfaces has been difficult. In this Letter, we report the first STM/STS observation of quasiparticle interference (QPI) patterns on the epitaxial film surfaces of perovskite oxide SrVO 3 ( 001 ) . The SrVO 3 bulk has simple cubic symmetry with one electron in the 3 d state, and hence it has been used as a prototype for understanding correlated transition metal perovskite oxides [15–29] . We interpret our experimental STM/STS results through parallel first-principles calculations to establish a microscopic understanding of the appearance and disappearance of the observed coherent surface states. The epitaxial SrVO 3 ( 001 ) thin films were grown on a buffered-HF-etched Nb-doped (0.05 wt. %) SrTiO 3 ( 001 ) substrate by using pulsed laser deposition (PLD). In the three samples (thicknesses: 20, 40, and 110 nm) prepared for this study [30] , the x-ray diffraction patterns showed negligible epitaxial strain. We used a scanning tunneling microscope equipped with a PLD system to transfer the as-deposited thin films to a low-temperature (4 K) STM head under ultrahigh vacuum conditions [45] . Figure 1(a) presents a STM topographic image of the SrVO 3 ( 001 ) surface showing a square lattice of protrusions along with randomly distributed defects. The periodicity of the protrusions is approximately 5.5 Å and exhibits a ( √ 2 × √ 2 ) - R 45 ° surface reconstruction everywhere on the scanned surfaces. 1 10.1103/PhysRevLett.119.086801.f1 FIG. 1. Typical topographic images of SrVO 3 ( 001 ) surface and metallic tunneling spectra (110-nm-thick film). (a),(b) Topographic images with sample bias voltage V s = − 100 mV (a); the enlarged image in (b) shows the ( √ 2 × √ 2 ) - R 45 ° structure. (c) The model used to generate the topographic image with a ( √ 2 × √ 2 ) - R 45 ° structure. (d) Spatially averaged tunneling spectra ( d I / d V ) obtained from the surface. The √ 2 × √ 2 superlattice surface structure [Fig. 1(c) ] is formed by a VO 2 -terminated layer with apical oxygen adsorption [46,47] . Naively, we would expect the topmost V atoms to have similar valence as the bulk V 4 + atoms because the bulk structure can be seen as stacked ( Sr 2 + O 2 − ) 0 and ( V 4 + O 2 − 2 ) 0 layers. However, previous emission-angle-dependent photoemission studies show more V 5 + atoms on the surface [22,48] , which means that the V atoms near the surface are more oxidized than those in the bulk [49] . This observation excludes the possibility of a bare VO 2 termination with √ 2 × √ 2 buckling near the surface since such a surface structure is unlikely to accommodate substantially oxidized V atoms. A 50% coverage of the apical oxygen sites (forming a SrVO 3.5 surface structure) is, however, allowed, which in turn would lead to the formation of the √ 2 × √ 2 superlattice. Similar √ 2 × √ 2 superstructures with 50% coverage of the apical oxygen sites have also been seen in manganite perovskites [50–52] . Figure 1(d) shows spatially averaged differential conductance ( d I / d V ) spectra. The metallic electronic properties observed on the surface are consistent with previous photoemission experiments [20,49] . The d I / d V spectra show a large spectral weight at zero sample bias voltage V s , a peak at V s = + 400 mV , and two minima at V s = − 410 and + 710 mV . Right at the peak position ( V s = + 400 mV ), a shoulder structure has also been observed in angle-integrated inverse photoemission experiments [53] . The energy at − 410 mV for the conductance minimum agrees quantitatively with the band-edge position at the Γ ¯ point in previous ARPES studies [20] . Figures 2(a) and 2(b) show the topographic and differential conductance images which were simultaneously measured at V s = − 150 mV . In order to extract the QPI pattern, which is present only in the conductance image, we compared the Fourier transforms (FTs) of the two images in Figs. 2(c) and 2(d) . While both FT images show peaks at √ 2 × √ 2 spots [green circles in Fig. 2(c) ], only the conductance FT image clearly exhibits an additional periodicity with respect to momentum [green dashed ellipses in Fig. 2(d) ]. Figure 2(e) shows the energy-dependent intensity profile of the FT images along the Γ ¯ − X ¯ direction for the three samples. The clear energy dispersion seen in Fig. 2(e) provides indisputable evidence for the existence of coherent electronic states on the SrVO 3 ( 001 ) surface. Hereafter, we will refer to this characteristic momentum dispersion along the Γ ¯ − X ¯ direction as q * . 2 10.1103/PhysRevLett.119.086801.f2 FIG. 2. Observation of quasiparticle interference pattern on the surface of SrVO 3 ( 001 ) . (a),(b) Simultaneously acquired topographic (a) and conductance (b) images at V s = − 150 mV . The inset in (b) shows the enlarged image in which we can see a wavelike pattern with approximately 1 nm periodicity. (c), (d) FTs of (a) and (b), respectively. Bragg peaks representing the √ 2 a × √ 2 a apical oxygen structure are highlighted by green circles in (c), and the wave vector q * is highlighted by green-dashed ellipses in (d). The Γ ¯ − X ¯ direction corresponds to the direction of the nearest-neighbor V atoms. The data shown in (a)–(d) are taken from a 110-nm-thick film. (e) Energy-dependent intensity profiles of the FTs along Γ ¯ − X ¯ (intensity averaged over three samples with thickness 20, 40, and 110 nm), along with the values of q * obtained from the three films. QPI patterns were independent of film thickness (Fig. S2), which is consistent with fully relaxed films. In order to understand the energy dependence of the QPI pattern, we constructed a semi-infinite slab model using Wannier functions derived from first-principles density functional theory (DFT) (see Supplemental Material for computational details [30] ). We refer to an O-adsorbed V-atom site as a “ V O site” and to a bare V-atom site without apical oxygen as a “ V b site” [Fig. 3(a) ]. Figure 3(b) shows the obtained orbital-dependent densities of states (DOS) and the total DOS averaged over V O and V b sites [54] . Using a bandwidth renormalization factor of 0.4, which is close to the value obtained from previous ARPES experiments [20] and first principles calculations [21] , the total surface DOS [bold black curve in Fig. 3(b) ] captures the overall behavior of the experimental d I / d V spectrum [Fig. 1(d) ] [55] . Such a renormalization of the DFT bands should reasonably describe the orbital information needed to model the emergent QPI pattern. 3 10.1103/PhysRevLett.119.086801.f3 FIG. 3. Simulation of the electronic states on the surface of SrVO 3 ( 001 ) . (a) Cross-sectional view of the calculated relaxed near-surface structure. V O and V b denote the V-atom sites with and without apical oxygen, respectively. (b) Calculated density of states averaged over V O and V b sites. (c) Schematic drawing of the surface Brillouin zone (BZ). Green lines mark the BZ of the ( √ 2 × √ 2 ) - R 45 ° structure. (d)–(g) Spectral weights of the d x y and d x z / d y z orbitals for V O ( V b ) are shown in (d) and (e) [(f) and (g)], respectively. The top axis of (b) and right axes of (d)–(g) represent the renormalized energy scale E re using a renormalization factor of 0.4 (i.e., E re = 0.4 E ). (h)–(j) Simulated total spectral weight at E re = − 250 meV (h) and the associated scattering probability (i). The experimental Fourier transform image of conductance mapping at V s = − 250 mV is shown in (j) for comparison with (i). (k) The orbital and site dependence of onsite energies relative to the degenerate bulk t 2 g states; see Table S2 in the Supplemental Material [30] for details. The onsite energy differences within our model mainly reflect local crystal field effects; these are not related to onsite Coulomb repulsion U . See Ref. [30] for details of our calculations. In order to establish that the QPI pattern is directly linked to the formation of the d x y -derived coherent 2D band on the surface, we project the spectral weight of d x y - and d x z / d y z - derived bands at V O and V b sites onto the surface Brillouin zone (BZ) [Fig. 3(c) ]. Results along the high-symmetry lines are shown in Figs. 3(d)–3(g) . For the V O site as well as the V b site, the d x y -derived spectral weight shows a clear energy dispersion [Figs. 3(d) and 3(f) ], indicating that the d x y -derived bands feature a strong 2D character on the (001) surface. Importantly, at low energies, this d x y -derived 2D spectral weight becomes the dominant electronic component at both V O and V b sites [see shaded region on the top horizontal axis in Fig. 3(b) ], where a clear QPI pattern is observed. Furthermore, the interlayer coupling between the in-plane extended d x y orbitals is small and their surface onsite energies are higher than the bulk [0.1 and 0.3 eV higher for V O and V b sites, respectively, see Fig. 3(k) ]. Therefore, we conclude that the emergent QPI pattern on the surface originates from d x y -derived quasiparticle surface states, which are more or less isolated not only from the surface d x z / d y z states but also from the corresponding bulk states. Our calculations also capture the appearance of the momentum q * selectively along the Γ ¯ − X ¯ direction. We computed the scattering probability I ( q ) in terms of the spectral weight A ( k ) , using I ( q ) = ∫ A ( k ) × A ( k + q ) d k . Typical A ( k ) and I ( q ) results using E re = − 250 meV are shown in Figs. 3(h) and 3(i) . Because the shape of A ( k ) deviates from a simple circle, the joint DOS is significantly enhanced along the Γ ¯ − X ¯ direction in I ( q ) . The magnitude and direction of this enhanced joint DOS [Fig. 3(i) ] is consistent with the q * value obtained from the experimental conductance image at the corresponding energy e V s = − 250 meV [Fig. 3(j) ]. As the energy increases across the Fermi level E F (here E F is set to zero), the QPI signal gradually weakens in intensity and ultimately disappears altogether [Fig. 2(e) ]. This fading of the QPI signal near E F contradicts the behavior of simple metallic surfaces in which, according to conventional Fermi liquid theory, quasiparticle states would have a longer lifetime near E F , and as a result, the QPI patterns would be more prominent [56] . We consider three possible scenarios in this connection. (i) The suppression (decoherence) of the quasiparticle interference via inelastic electron-electron, electron-phonon, and/or electron-plasmon scatterings. However, the symmetric energy dependence on both sides of E F expected for these inelastic scattering processes contradicts the monotonic fading of the QPI signal observed across E F . (ii) The effect of the shape of A ( k ) . This scenario, however, is also unlikely because the shape of the calculated A ( k ) changes from circlelike (smaller joint DOS) to squarelike (larger joint DOS) across E F , which completely fails to explain the observed QPI signal fading. (iii) The suppression of the d x y -derived spectral weight itself. Indeed, our numerical results show a suppression of the d x y -derived spectral weight, especially at the V b site [dotted square in Fig. 3(f) ]. This is driven by two mechanisms: (i) hybridization and (ii) sublattice formation. The first mechanism, which involves hybridization between the d x y - and d x z / d y z - derived states at the V b site, is illustrated schematically in Fig. 4(a) [57] . As the axially extended d x z / d y z -derived bulk bands have an intrinsically strong dispersion normal to the (001) surface, the projection of these bands onto the (001) surface BZ results in a broad spectral weight distribution in momentum space. The broad distribution of the spectral weight at the V b site [Fig. 3(g) ] indicates the existence of a strong coupling between the d x z / d y z -derived surface and bulk bands. Such a strong mixing can be naturally understood from the smaller bulk-surface energy separation ( ∼ 0.2 eV ) for d x z / d y z orbital at the V b site compared to that for the V O site ( ∼ 1.2 eV ) [Fig. 3(k) ]. An important consequence is the possible coexistence of the d x y - and d x z / d y z - derived bands at the same energy ( E ) and momentum ( k ) over a wide range across the Fermi level [dotted square in Fig. 3(f) ]. The hybridization with the d x z / d y z -derived bands then leads to (i) the suppression of the d x y -derived spectral weight through spectral weight transfer, and (ii) the enhancement of the scattering channel between the d x y - and d x z / d y z - derived states at the V b site. 4 10.1103/PhysRevLett.119.086801.f4 FIG. 4. Schematic illustration of interorbital and intraorbital spectral weight transfer mechanisms and the related appearance or disappearance of QPI patterns. (a) Spectral weight transfer from d x y orbitals (red) to d x z / d y z orbitals (blue) at the V b site. (b) Spectral weight transfer from d x y orbitals (red) at the V b site (left) to d x y orbitals (red) at the V O site (right). (a) and (b) only highlight the lower energy band branch near the X ¯ point. (c), (d) Distribution of the d x y - and d x z / d y z - derived spectral weight in real space for high (c) and low (d) energy regions. We next discuss the mechanism of sublattice formation, which involves transfer of the spectral weight from the d x y orbital at the V b site to the d x y orbital at the V O site, as shown schematically in Fig. 4(b) . The calculated on-site energies for the d x y orbital at the V O and V b sites are ∼ 0.1 and ∼ 0.3 eV , respectively. In general, a difference in on-site energies will induce a spectral weight transfer between these inequivalent sites around the zone boundary [58] . We confirm this effect by focusing on the d x y -derived lower energy-band branch near the X ¯ point [dotted squares in Figs. 3(d) and 3(f) ]. Our numerical results show that around the X ¯ point, the d x y -state spectral weight at the V b site (with higher potential) is more suppressed compared to the d x y -state spectral weight at the V O site (with lower potential) around the same ( E , k ) region. The preceding discussion of hybridization and sublattice formation mechanisms suggests that it is the nonuniformity of the d x y -derived spectral weight [Fig. 4(c) ] that principally causes the QPI signal on the ( √ 2 × √ 2 ) - R 45 ° surface to fade monotonically with energy. At low energies, we obtain a rather uniform distribution of the d x y -derived spectral weight [see Fig. 4(d) ]. In contrast, at higher energies, this spectral weight is suppressed while interorbital scatterings are enhanced at the V b site [see Fig. 4(c) ]. Both mechanisms act to disrupt the uniformity of the d x y -derived spectral weight [see also Fig. S6]. Our results provide important insights for designing 2D quasiparticle states at surfaces and interfaces of perovskite oxides. We identify the pivotal role of apical oxygens in the formation of 2D electronic states via orbital modulation. Because of the large onsite energy difference at the V O site, the negatively charged apical oxygen strongly eliminates electrons selectively from the axially extended d x z and d y z states [see Fig. 3(k) (left)]. The apical oxygen thus plays a key role in isolating the in-plane d x y and out-of-the-plane d x z / d y z orbitals on the (001) surface. Since apical oxygens are ubiquitous in perovskite oxides, these considerations would also be relevant for designing systems such as the cuprates, where the interplay between the d x 2 − y 2 and out-of-the-plane orbitals including d z 2 is important [59] , as well as for surfaces or interfaces involving sublattice formation more generally. We should keep in mind, however, that an obvious unique factor in SrVO 3 is its electron filling of one electron in the bulk 3 d system. Interestingly, our results suggest that the SrVO 3 ( 001 ) surface is similar to the 2D metallic state at SrTiO 3 ( 001 ) -based surfaces or interfaces because in both systems the degeneracy of the t 2 g orbitals is lifted and the d x y -derived band becomes dominant at low energies [60–72] . This similarity points out that the SrVO 3 ( 001 ) surface would be an interesting playground for exploring emergent quantum functionalities such as magnetism and superconductivity. We thank H. Ishida and K. Sato for helpful discussions, and D. Walkup and P. Han for critical reading of the manuscript. Y. O. acknowledges funding from the Japan Society for the Promotion of Science Grant-in-Aid No. 26707016 and No. 25886004. T. H. acknowledges funding from the Japan Society for the Promotion of Science Grant-in-Aid No. 26246022, No. 26106502, No. JST-CREST, and No. JST-PRESTO. This work was supported by the World Premier Research Center Initiative, promoted by the Ministry of Education, Culture, Sports, Science and Technology of Japan. T.-R. C. and H.-T. J acknowledge National Center for Theoretical Sciences (NCTS), Taiwan for technical support. H. K. acknowledges funding from the Japan Society for the Promotion of Science Grant-in-Aid No. 16H02115. 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PY - 2017/8/22
Y1 - 2017/8/22
N2 - We report the observation of coherent surface states on cubic perovskite oxide SrVO3(001) thin films through spectroscopic-imaging scanning tunneling microscopy. A direct link between the observed quasiparticle interference patterns and the formation of a dxy-derived surface state is supported by first-principles calculations. We show that the apical oxygens on the topmost VO2 plane play a critical role in controlling the coherent surface state via modulating orbital state.
AB - We report the observation of coherent surface states on cubic perovskite oxide SrVO3(001) thin films through spectroscopic-imaging scanning tunneling microscopy. A direct link between the observed quasiparticle interference patterns and the formation of a dxy-derived surface state is supported by first-principles calculations. We show that the apical oxygens on the topmost VO2 plane play a critical role in controlling the coherent surface state via modulating orbital state.
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U2 - 10.1103/PhysRevLett.119.086801
DO - 10.1103/PhysRevLett.119.086801
M3 - Article
C2 - 28952762
AN - SCOPUS:85029210376
VL - 119
JO - Physical Review Letters
JF - Physical Review Letters
SN - 0031-9007
IS - 8
M1 - 086801
ER -