We have synthesized epitaxial Sr2IrO4 thin-films on various substrates and
studied their electronic structures as a function of lattice-strains. Under
tensile (compressive) strains, increased (decreased) Ir-O-Ir bond-angles are
expected to result in increased (decreased) electronic bandwidths. However, we
have observed that the two optical absorption peaks near 0.5 eV and 1.0 eV are
shifted to higher (lower) energies under tensile (compressive) strains,
indicating that the electronic-correlation energy is also affected by in-plane
lattice-strains. The effective tuning of electronic structures under
lattice-modification provides an important insight into the physics driven by
the coexisting strong spin-orbit coupling and electronic correlation.Comment: 9 pages, 5 figures, 1 tabl

Bittle, E. G.

Brill, J. W.

Cao, G.

De Long, L. E.

Korneta, O. B.

Nichols, J.

Seo, S. S. A.

Terzic, J.

English

arXiv

J. Nichols

J. Terzic

E. G. Bittle

O. B. Korneta

L. E. De Long

J. W. Brill

G. Cao

S. S. A. Seo

Crossref

Applied Physics Letters

Tuning electronic structure via epitaxial strain in Sr<sub>2</sub>IrO<sub>4</sub> thin films

We have synthesized epitaxial Sr2IrO4 thin-films on various substrates and studied their electronic structure as a function of lattice-strain. Under tensile (compressive) strain, increased (decreased) Ir-O-Ir bond-angle is expected to result in increased (decreased) electronic bandwidth. However, we have observed that the two optical absorption peaks near 0.5 eV and 1.0 eV are shifted to higher (lower) energies under tensile (compressive) strain, indicating that the electronic-correlation energy is also affected by in-plane lattice-strain. The effective tuning of electronic structure under lattice-modification provides an important insight into the physics driven by the coexisting strong spin-orbit coupling and electronic correlation

Terzic, Jsaminka

Bittle, Emily Geraldine

Korneta, Oleksandr B.

De Long, Lance E.

Brill, Joseph

Cao, Gang

Seo, Sung S. Ambrose

University of Kentucky

University of KentuckyUKnowledgePhysics and Astronomy Faculty Publications Physics and Astronomy4-11-2013Tuning Electronic Structure via Exipatial Strain inSr2IrO4 Thin FilmsJ. NicholsUniversity of KentuckyJsaminka TerzicUniversity of Kentucky, jasminka.terzic@uky.eduEmily Geraldine BittleUniversity of Kentucky, egbittle@gmail.comOleksandr B. KornetaUniversity of Kentucky, korneta@pa.uky.eduLance E. De LongUniversity of Kentucky, delong@pa.uky.eduSee next page for additional authorsRight click to open a feedback form in a new tab to let us know how this document benefits you.Follow this and additional works at: https://uknowledge.uky.edu/physastron_facpubPart of the Astrophysics and Astronomy Commons, and the Physics CommonsThis Article is brought to you for free and open access by the Physics and Astronomy at UKnowledge. It has been accepted for inclusion in Physics andAstronomy Faculty Publications by an authorized administrator of UKnowledge. For more information, please contact UKnowledge@lsv.uky.edu.Repository CitationNichols, J.; Terzic, Jsaminka; Bittle, Emily Geraldine; Korneta, Oleksandr B.; De Long, Lance E.; Brill, Joseph; Cao, Gang; and Seo,Sung S. Ambrose, "Tuning Electronic Structure via Exipatial Strain in Sr2IrO4 Thin Films" (2013). Physics and Astronomy FacultyPublications. 240.https://uknowledge.uky.edu/physastron_facpub/240AuthorsJ. Nichols, Jsaminka Terzic, Emily Geraldine Bittle, Oleksandr B. Korneta, Lance E. De Long, Joseph Brill,Gang Cao, and Sung S. Ambrose SeoTuning Electronic Structure via Exipatial Strain in Sr2IrO4 Thin FilmsNotes/Citation InformationPublished in Applied Physics Letters, v. 102, article 141908, p. 1-4.Copyright 2013 American Institute of Physics. This article may be downloaded for personal use only. Anyother use requires prior permission of the author and the American Institute of Physics.The following article appeared in Applied Physics Letters, v. 102, article 141908, p. 1-4 and may be found athttp://dx.doi.org/10.1063/1.4801877.Digital Object Identifier (DOI)http://dx.doi.org/10.1063/1.4801877This article is available at UKnowledge: https://uknowledge.uky.edu/physastron_facpub/240Tuning electronic structure via epitaxial strain in Sr2IrO4 thin filmsJ. Nichols, J. Terzic, E. G. Bittle, O. B. Korneta, L. E. De Long, J. W. Brill, G. Cao,and S. S. A. Seoa)Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, USA(Received 4 February 2013; accepted 1 April 2013; published online 11 April 2013)We have synthesized epitaxial Sr2IrO4 thin-films on various substrates and studied their electronicstructure as a function of lattice-strain. Under tensile (compressive) strain, increased (decreased) Ir-O-Irbond-angle is expected to result in increased (decreased) electronic bandwidth. However, we haveobserved that the two optical absorption peaks near 0.5 eV and 1.0 eV are shifted to higher (lower)energies under tensile (compressive) strain, indicating that the electronic-correlation energy is alsoaffected by in-plane lattice-strain. The effective tuning of electronic structure under lattice-modificationprovides an important insight into the physics driven by the coexisting strong spin-orbit coupling andelectronic correlation.VC 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4801877]Recent studies of 5d transition-metal oxides haverevealed innovative physics with strong potential for elec-tronic device applications. Although weak electronic correla-tion and paramagnetism were conventionally anticipated togovern the physics of 5d transition-metal oxides owing tothe extended nature of 5d electrons, recent research onSr2IrO4 crystals has shown that Jeff¼ 1/2 Mott states appeardue to strong spin-orbit interactions.1 The energy scale of thespin-orbit coupling is comparable to the crystal-field energyand the on-site Coulomb interaction, which creates thepotential for the emergence of unprecedented electronicstates due to the strong competition between these funda-mental interactions. Moreover, strong spin-orbit interactionscan also derive topologically protected electronic states,2 theso-called topological insulator.To understand the physics of Sr2IrO4 and to find a wayof tuning its multiple, competing interactions, experimentalstudies of chemical doping,3–9 and high pressure10 have beenrecently conducted. However, chemical doping often hasmultiple consequences, such as changes in crystal structureand the valence states of the transition metal element, whichcan make it difficult to achieve a desired state. Hydrostaticpressure techniques are not available for a few advancedspectroscopic methods such as X-ray photoemission spec-troscopy. Hence, synthesis of epitaxial thin-films under vari-ous lattice-strain conditions is a viable alternative forcreating desired modifications of the physical properties ofSr2IrO4, and there have been only a few thin-film studies.9,11In this letter, we report on the growth and optical proper-ties of Sr2IrO4 (SIO-214) thin films. The in-plane lattice mis-matches between SIO-214 and various oxide substrates canexert both tensile (þ) and compressive () strains to films,as shown in Fig. 1(a). We find that the electronic structure ofSIO-214 films is effectively altered by lattice strain, and weobserve shifted optical transitions (absorptions) between theJeff¼ 1/2 lower Hubbard band (LHB) and the Jeff¼ 1/2 upperHubbard band (UHB), and between the Jeff¼ 3/2 band andthe Jeff¼ 1/2 UHB band. Our observations strongly suggestthat not only the electronic bandwidth but also the magnitudeof the effective electronic correlation energy (Ueff), can bemanipulated by lattice strain. Our results demonstrate thatepitaxial SIO-214 thin films can be used as a model systemto study the physics of coexisting strong electron correlationand strong spin-orbit coupling under lattice modification.We have used a custom-built, pulsed laser depositionsystem equipped with in-situ reflection high-energy electrondiffraction and optical spectroscopic ellipsometry12 to growepitaxial SIO-214 thin films 10 unit-cells thick (approxi-mately 25 nm in thickness). Five different single-crystallinesubstrates are used: GdScO3 (110) (GSO), SrTiO3 (100)(STO), (LaAlO3)0.3(Sr2AlTaO6)0.7 (100) (LSAT), NdGaO3(110) (NGO), and LaAlO3 (110) (LAO). Note that the [110]surface-normal directions of orthorhombic substrates providein-plane square lattices (a[110]  a[001]), as listed in Table I.The epitaxial growth conditions are found to be the follow-ing: an oxygen partial pressure (PO2) of 10 mTorr, a substratetemperature of 700 C, and a laser (KrF excimer,k¼ 248 nm) fluence of 1.2 J/cm2. Figure 2 shows h-2h X-raydiffraction scans of the samples discussed herein. Well-defined 00l-peaks are present due to the films’ 00l-orienta-tion along the perpendicular to the substrates. The full widthsat half maximum in rocking-curve scans of the 00l peaks areall less than 0.05, which confirms the high crystallinity ofthe films. Note that the thin films’ 0012-peaks are shifted tolow angles as the substrate lattice parameters decrease (fromGSO to LAO). This behavior is consistent with the schematicdiagrams in Fig. 1(b), since elongated (contracted) out-of-plane lattice parameters are expected as compressive (ten-sile) in-plane strain is exerted on thin films.Figure 3(a) shows X-ray reciprocal space maps, whichreveal important information about both the in-plane and theout-of-plane lattice parameters of the SIO-214 thin filmsnear the 332-reflection (103-reflection) of orthorhombic(pseudo-cubic) substrates. The 1118-peaks from the thinfilms are clearly observed, and are shifted to low reciprocalwave vectors (Q?) as the lattice parameters of the substratedecrease from GSO to LAO, which is also consistent withthe h-2h scan data shown in Fig. 2. Note that the in-plane lat-tice parameters of the films are coherently strained to thesubstrates when there are small lattice-mismatches (STO andLSAT substrates), or partially relaxed when the latticemismatches are relatively large (GSO, NGO, and LAOa)E-mail: a.seo@uky.edu0003-6951/2013/102(14)/141908/4/$30.00 VC 2013 AIP Publishing LLC102, 141908-1APPLIED PHYSICS LETTERS 102, 141908 (2013) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  128.163.8.74On: Wed, 21 Oct 2015 17:40:13substrates). Figure 3(b) shows that the in-plane and out-of-plane lattice parameters change systematically as the sub-strate lattice parameters vary. From these data, we can calcu-late the in-plane lattice strain (exx), the out-of-plane strain(ezz), and the Poisson’s ratio (v) of the films, as summarizedin Table I.To investigate the electronic structure of the SIO-214 thin-films, we measured optical transmission spectra (T(x)) to obtainthe optical absorption spectra, A(x)¼ln[T(x)filmþsubstrate/T(x)substrate]/t, where t is the thickness of the thin films. Eachspectrum has been measured at room temperature using aFourier-transform infrared spectrometer in the photon energyregion of 50meV–0.6 eV and a grating-type spectrophotome-ter in the range of 0.5–6 eV. Figure 4 shows the opticalabsorption spectra of the SIO-thin films grown on GSO,STO, LSAT, and LAO substrates. Spectra of the films grownon NGO substrates cannot be obtained by our method sinceNGO substrates are opaque in this photon-energy region.The lattice-strain-dependent optical absorption spectrayield indispensable information about the electronic structureof the SIO-214 thin films. There are two optical absorptionpeaks at about 0.5 eV (a) and 1.0 eV (b), which have beeninterpreted as interband optical transitions from the LHB ofJeff¼ 1/2 and the Jeff¼ 3/2 bands to the unoccupied UHB ofthe Jeff¼ 1/2 state, respectively.13,14 The interband opticaltransition intensity, Ii!f(x0), from an initial state i to a finalstate f at x0 is described as Ii!f ðx0Þ ¼Ð jhf jMjiij2qf ðxÞqiðxx0Þdx according to Fermi’s golden rule, where M is thematrix element, and qi and qf are the densities of states for iand f, respectively. We can expect these optical peaks tobroaden (narrow) as the bond angle (Ir-O-Ir) of the in-planelattice increases (decreases), which enhances (reduces) theelectronic hopping integral and the bandwidth (W). Whentensile (compressive) strain is applied to SIO-214 thin films,FIG. 1. (a) Comparison of pseudo-cubic, in-plane lattice parameter ofSr2IrO4 with various oxide substrates. (b) Schematic diagrams of relationsbetween lattice strain and Ir-O-Ir bond-angle (h).FIG. 2. X-ray h-2h diffraction scans of SIO-214 films grown on GSO, STO,LSAT, NGO, and LAO (from top to bottom). 00l peaks from the SIO-214films (l¼ 4, 8, 12, 16, 24, 28) are clearly observed. The left panel showszoomed-in scans near the 0012-reflections of the films and the pseudo-cubic002-reflections of the substrates. The dashed line indicates the 0012-peakposition of SIO-214 single crystals. The asterisks (*) indicate peaks from thesubstrates.TABLE I. In-plane lattice mismatches and corresponding strains of Sr2IrO4 (SIO-214) films (t  25 nm).Substrate Pseudo-cubic in-plane lattice, a[110]  a[001] (Å) Lattice mismatch (%)a exx (%)b ezz (%)b Poisson’s ratiocGSO (110) 3.96 þ1.53 þ1.64 (60.44) 1.69 (60.09) 0.34 (60.08)STO (100) 3.90 þ0.45 þ0.38 (60.13) 0.37 (60.13) 0.33 (60.16)LSAT (100) 3.87 0.45 þ0.26 (60.24) þ0.00 (60.10) 0.00 (60.18)NGO (110) 3.81 2.04 1.82 (60.08) þ0.39 (60.12) 0.09 (60.02)LAO (110) 3.79 2.58 1.18 (60.37) þ0.67 (60.10) 0.22 (60.04)aValues are calculated from the pseudo-cubic lattice parameters of bulk SIO-214 (abulk) and substrates (asub) by (asub  abulk)/asub  100 (%).bValues are obtained using exx ¼ (afilm  abulk)/abulk  100 (%) and ezz ¼ (cfilm  cbulk)/cbulk  100 (%).cPoisson’s ratio, v ¼ ezz/(ezz  2 exx).141908-2 Nichols et al. Appl. Phys. Lett. 102, 141908 (2013) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  128.163.8.74On: Wed, 21 Oct 2015 17:40:13the optical peak width clearly increases (decreases), asshown in Fig. 4. This behavior is consistent with an increasedbond angle and an unaffected planar Ir-O bond length, whichis very difficult to alter, as schematically shown in Fig. 1(b).In addition, it has been recently shown that for SIO-214 thinfilms grown on STO, that in-plane tensile strain only affectsthe bond angle.11 Surprisingly, our data (Figure 5(a)) showthat not only the optical peak width but also the peak ener-gies (xa and xb) increase as tensile strain is applied to theSIO-214 thin films. Since both xa and xb vary with appliedstrain, it is unlikely that spin-orbit coupling can be solely re-sponsible for these changes in the peak energy. This meansthat Ueff should increase (decrease) as tensile-(compressive)strain is applied to these films. As a result, the SIO-214 filmsexhibit optical gap energies (0.3 eV) that are approximatelyindependent of lattice strain, as shown in Fig. 4.This behavior of the optical transitions under lattice-strain is quite unexpected. Temperature-dependent opticalmeasurements15 on bulk single-crystal Sr2IrO4 show that thewidths of the optical peaks increase as temperature increases,since the effective Ir-O-Ir bond-angle increases due to ther-mal expansion, which is qualitatively consistent with ourobservations in the SIO-214 thin films. However, thetemperature-dependent data for bulk crystal show that the op-tical peak energies decrease as temperature increases, whichis an opposite behavior to our observations in thin films, inwhich optical peak energies increase under tensile strain.Whereas the thermal effect should increase the overall latticeparameters of a sample, our thin films under biaxial in-planetensile-strain expand only the in-plane lattice parameters, butreduce the out-of-plane lattice parameters. Hence, the appa-rent inconsistency between the temperature-dependent experi-ment and our strain-dependent experiment might not be sosurprising. However, in general, theoretical models about 214structures assume ideal in-plane square lattices and neglectchanges along the out-of-plane directions. Hence, our obser-vations tell us that the change along the out-of-plane directionsuch as apical Ir-O bond-length or interlayer coupling cannotbe neglected when considering the actual electronic structureof SIO-214. Figure 5(b) describes qualitatively the change inthe electronic structure of the strongly correlated, spin-orbit-coupled bands of SIO-214 thin films under lattice strain. Inorder to further clarify this change in the electronic structureof the SIO-214 thin films, advanced spectroscopic techniques,such as angle-resolved photoemission spectroscopy and reso-nant X-ray scattering, should be pursued.In summary, epitaxial Sr2IrO4 thin films have been grownon various substrates for both tensile- and compressive-latticestrain. We have observed that optical absorption peaks ataround 0.5 eV and 1.0 eV are broadened and shifted to higherenergy as tensile strain is applied. This implies that not only isthe electronic bandwidth increased by strain but also theFIG. 3. (a) X-ray reciprocal space maps (Qk  2p/dk, Qk  2p/d?) aroundthe 332-reflection (or 103-reflection in pseudo-cubic notation) of the sub-strates, where dk and d? are in-plane and out-of-plane lattices, respectively.While the in-plane lattices of SIO-214 films are coherently strained withrespect to the STO and LSAT substrates, partial strain-relaxations are visiblefrom the films grown on the GSO, NGO, and LAO substrates. (b) In-plane(a) and out-of-plane (c) lattice parameters of the SIO-214 thin films obtainedfrom X-ray scans are plotted as a function of substrate lattice parameters.The dashed and dotted lines indicate the out-of-plane and the in-plane latticeparameters, respectively, of a SIO-214 single crystal.FIG. 4. Optical absorption spectra of SIO-214 thin films grown on GSO,STO, LSAT, and LAO substrates. The spectra are shifted for clarity. Thesolid lines are fit curves using two Lorentz oscillators for peaks a and b witha background oscillator. The dashed lines show the estimated optical gapenergies (0.3 eV) for the SIO-214 thin films.141908-3 Nichols et al. Appl. Phys. Lett. 102, 141908 (2013) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  128.163.8.74On: Wed, 21 Oct 2015 17:40:13electronic correlation is increased as well. These epitaxialSr2IrO4 thin films provide a useful two-dimensional modelsystem to study the nature of the coexistence of strong elec-tron correlation and spin-orbit interactions, and their behaviorunder lattice modifications.We thank B. I. Min, H.-Y. Kee, G. Jackeli, S. J. Moon,D. Haskel, and B. J. Kim for insightful discussions. Thisresearch was supported by the NSF through Grant No. EPS-0814194 (the Center for Advanced Materials), Grant Nos.DMR-0800367, DMR-0856234, by U.S. DoE through GrantNo. DE-FG02-97ER45653, and by the Kentucky Scienceand Engineering Foundation with the Kentucky Science andTechnology Corporation through Grant Agreement No.KSEF-148-502-12-303.1B. J. Kim, H. Jin, S. J. Moon, J. Y. Kim, B. G. Park, C. S. Leem, J. Yu, T.W. Noh, C. Kim, S. J. Oh, J. H. Park, V. Durairaj, G. Cao, and E.Rotenberg, Phys. Rev. Lett. 101, 076402 (2008).2M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).3R. J. Cava, B. Batlogg, K. Kiyono, H. Takagi, J. J. Krajewski, W. F. Peck,Jr., L. W. Rupp, Jr., and C. H. Chen, Phys. Rev. B 49, 11890 (1994).4M. De Marco, D. Graf, J. Rijssenbeek, R. J. Cava, D. Z. Wang, Y. Tu, Z.F. Ren, J. H. Wang, M. Haka, S. Toorongian, M. J. Leone, and M. J.Naughton, Phys. Rev. B 60, 7570 (1999).5C.-C. Carlos, T. Gustavo, B. Alejandro, M. Pablo de la, and E. Roberto,J. Phys.: Condens. Matter 19, 446210 (2007).6Y. Klein and I. Terasaki, J. Phys.: Condens. Matter 20, 295201 (2008).7O. B. Korneta, T. Qi, S. Chikara, S. Parkin, L. E. De Long, P.Schlottmann, and G. Cao, Phys. Rev. B 82, 115117 (2010).8T. F. Qi, O. B. Korneta, L. Li, K. Butrouna, V. S. Cao, X. Wan, P.Schlottmann, R. K. Kaul, and G. Cao, Phys. Rev. B 86, 125105 (2012).9J. S. Lee, Y. Krockenberger, K. S. Takahashi, M. Kawasaki, and Y.Tokura, Phys. Rev. B 85, 035101 (2012).10D. Haskel, G. Fabbris, M. Zhernenkov, P. P. Kong, C. Q. Jin, G. Cao, andM. van Veenendaal, Phys. Rev. Lett. 109, 027204 (2012).11C. Rayan Serrao, J. Liu, J. T. Heron, G. Singh-Bhalla, A. Yadav, S. J.Suresha, R. J. Paull, D. Yi, J. H. Chu, M. Trassin, A. Vishwanath, E.Arenholz, C. Frontera, J. Zelezny, T. Jungwirth, X. Marti, and R. Ramesh,Phys. Rev. B 87, 085121 (2013).12J. H. Gruenewald, J. Nichols, and S. S. A. Seo, Rev. Sci. Instrum. 84,043902 (2013).13S. J. Moon, H. Jin, K. W. Kim, W. S. Choi, Y. S. Lee, J. Yu, G. Cao, A.Sumi, H. Funakubo, C. Bernhard, and T. W. Noh, Phys. Rev. Lett. 101,226402 (2008).14B. H. Kim, G. Khaliullin, and B. I. Min, Phys. Rev. Lett. 109, 167205(2012).15S. J. Moon, H. Jin, W. S. Choi, J. S. Lee, S. S. A. Seo, J. Yu, G. Cao, T.W. Noh, and Y. S. Lee, Phys. Rev. B 80, 195110 (2009).FIG. 5. (a) The peak positions (xpeak)and the widths (cpeak) obtained fromLorentz oscillator fitting as a function ofthe in-plane lattice strain (exx). The val-ues at exx¼ 0 are taken from a SIO-214single crystal.13 (b) Schematic band dia-grams dependence on lattice strain sug-gested from our data.141908-4 Nichols et al. Appl. Phys. Lett. 102, 141908 (2013) This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:  128.163.8.74On: Wed, 21 Oct 2015 17:40:13

Tuning Electronic Structure via Exipatial Strain in Sr\u3csub\u3e2\u3c/sub\u3eIrO\u3csub\u3e4\u3c/sub\u3e Thin Films

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Tuning electronic structures via epitaxial strain in Sr2IrO4 thin films

Tuning electronic structures via epitaxial strain in Sr2IrO4 thin films

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