We investigated the electronic structures of the 5$d$ Ruddlesden-Popper
series Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ ($n$=1, 2, and $\infty$) using optical
spectroscopy and first-principles calculations. As 5$d$ orbitals are spatially
more extended than 3$d$ or 4$d$ orbitals, it has been widely accepted that
correlation effects are minimal in 5$d$ compounds. However, we observed a
bandwidth-controlled transition from a Mott insulator to a metal as we
increased $n$. In addition, the artificially synthesized perovskite SrIrO$_{3}$
showed a very large mass enhancement of about 6, indicating that it was in a
correlated metallic state

Bernhard, C.

Cao, G.

Choi, W. S.

Funakubo, H.

Jin, H.

Kim, K. W.

Lee, Y. S.

Moon, S. J.

Noh, T. W.

Sumi, A.

Yu, J.

Physical Review Letters

English

arXiv

We investigated the electronic structures of the 5d Ruddlesden-Popper series Srn+1IrnO3n+1 (n=1, 2, and ???) using optical spectroscopy and first-principles calculations. As 5d orbitals are spatially more extended than 3d or 4d orbitals, it has been widely accepted that correlation effects are minimal in 5d compounds. However, we observed a Mott insulator-metal transition with a change of bandwidth as we increased n. In addition, the artificially synthesized perovskite SrIrO3 showed a very large mass enhancement of about 6, indicating that it was in a correlated metallic state.open13510

Moon, SJ

Jin, Hosub

Kim, KW

Choi, WS

Lee, YS

Yu, Jaejun

Cao, G

Sumi, A

Funakubo, H

Bernhard, C

Noh, TW

ScholarWorks@UNIST

Dimensionality-Controlled Insulator-Metal Transition and Correlated Metallic Statein 5d Transition Metal Oxides Srnþ1IrnO3nþ1 (n ¼ 1, 2, and1)S. J. Moon,1 H. Jin,2 K.W. Kim,3 W. S. Choi,1 Y. S. Lee,4 J. Yu,2 G. Cao,5 A. Sumi,6H. Funakubo,6 C. Bernhard,3 and T.W. Noh1,*1ReCOE & FPRD, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea2CSCMR & FPRD, Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea3Department of Physics and Fribourg Center for Nanomaterials, University of Fribourg, CH-1700 Fribourg, Switzerland4Department of Physics, Soongsil University, Seoul 156-743, Korea5Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, USA6Department of Innovative and Engineered Materials, Interdisciplinary Graduate School of Science and Engineering,Tokyo Institute of Technology, Yokohama 226-8502, Japan(Received 14 July 2008; published 24 November 2008)We investigated the electronic structures of the 5d Ruddlesden-Popper series Srnþ1IrnO3nþ1 (n ¼ 1, 2,and 1) using optical spectroscopy and first-principles calculations. As 5d orbitals are spatially moreextended than 3d or 4d orbitals, it has been widely accepted that correlation effects are minimal in 5dcompounds. However, we observed a Mott insulator-metal transition with a change of bandwidth as weincreased n. In addition, the artificially synthesized perovskite SrIrO3 showed a very large massenhancement of about 6, indicating that it was in a correlated metallic state.DOI: 10.1103/PhysRevLett.101.226402 PACS numbers: 71.30.+h, 71.20.b, 78.20.eMott physics, driven by electron correlation, has been anessential foundation for understanding the emergent prop-erties of strongly correlated electron systems, including 3dand 4d transition metal oxides (TMOs) [1]. According tothe Hubbard model, such systems can be characterized bythe on-site Coulomb interaction U and the bandwidth W.When W  U, the system is in an insulating state, the so-called Mott insulator. When W * U, an insulator-metaltransition (IMT) occurs and the system becomes metallic.Close to the IMT, the conducting carriers have a largeeffective mass due to electron correlation. This correlatedmetallic state (CMS) has unique physical properties, whichcannot be explained by simple band theory. Therefore, theW-controlled IMT and associated CMS, especially in 3dand 4d TMOs, have been important subjects in condensedmatter physics [1–4].The 5d orbitals are spatially more extended than the 3dand 4d orbitals, so their W (U) values should be larger(smaller) than 3d and 4d orbitals. In addition, the electroncorrelation should play a much smaller role. Therefore,many 5d TMOs have metallic ground states that can bedescribed by the band theory [5,6]. However, some 5dTMOs, such as Sr2IrO4, Sr3Ir2O7, and Ba2NaOsO6, haveinsulating ground states [7–9]. There have been somerecent reports that correlation effects could be importantfor the 5d insulating TMO [9,10]. This raises a couple ofimportant questions. First, is it possible that an IMToccursin 5d TMOs by systematically varyingW? Second, can theassociated CMS be found in such a 5d TMO system?Recently, we found that the unusual insulating state ofSr2IrO4 could be explained by cooperative interactionbetween electron correlation and spin-orbit (SO) coupling[11]. The SO coupling constant of 5d TMOs is about 0.3–0.4 eV [6], which is much larger than that of 3d TMOs(20 meV) [12]. As the SO coupling constant of 5d TMOsis comparable to their U value of approximately 0.5 eV[11], it can cooperate withU. This cooperation should playan important role in their electronic structures [13]. The SOcoupling can induce the formation of Jeff;1=2 and Jeff;3=2bands, which are occupied by five electrons [14]. TheJeff;1=2 bands can be very narrow due to a decreased hop-ping integral caused by the isotropic orbital and mixed spinFIG. 1. Schematic band diagrams of 5d Srnþ1IrnO3nþ1 com-pounds, which are well described by the effective total angularmoment Jeff states due to strong spin-orbit coupling: (a) Mottinsulator Sr2IrO4, (b) barely insulator Sr3Ir2O7, and(c) correlated metal SrIrO3. EF represents the Fermi level andthe arrow indicates the direction for the bandwidth W increase.PRL 101, 226402 (2008) P HY S I CA L R EV I EW LE T T E R Sweek ending28 NOVEMBER 20080031-9007=08=101(22)=226402(4) 226402-1  2008 The American Physical Societycharacters [11]. As shown schematically in Fig. 1(a), theeffect of U can split the narrow Jeff;1=2 bands into a lowerHubbard band (LHB) and an upper Hubbard band (UHB),opening a Mott gap.The weakly correlated narrow band system is a goodstarting point from which to search for the IMT by vary-ing W and the associated CMS in 5d TMOs. There arefour neighboring Ir atoms z in Sr2IrO4. As W shouldbe proportional to z, we could increase W by increasingthe z value. Among the Ruddlesden-Popper seriesSrnþ1IrnO3nþ1 compounds, the z values for Sr3Ir2O7 andperovskite SrIrO3 are 5 and 6, respectively. In nature,however, bulk SrIrO3 has a hexagonal crystal structure atroom temperature and atmospheric pressure. It transformsto the perovskite structure only at a higher pressure andtemperature [15]. Therefore it was essential to obtain aperovskite SrIrO3 for the investigation.In this Letter, we investigated the electronic structures ofRuddlesden-Popper series Srnþ1IrnO3nþ1 (n ¼ 1, 2, and1) compounds using optical spectroscopy and first-principles calculations. We artificially fabricated the pe-rovskite SrIrO3 phase by growing an epitaxial thin film ona MgO substrate. We found that the W of Ir 5d bandsincreased with the dimensionality or the z value. As shownschematically in Figs. 1(b) and 1(c), the IMT occurredbetween Sr3Ir2O7 and SrIrO3. From the optical conductiv-ity spectrum ð!Þ, we also found that conducting carriersin the artificially fabricated SrIrO3 should have a largemass enhancement, indicating a CMS.Single crystals of Sr2IrO4 and Sr3Ir2O7 were grownusing the flux technique [7,8]. To obtain a perovskiteSrIrO3 sample, we grew epitaxial SrIrO3 thin film on cubicMgO (001) substrate. The cubic symmetry of the MgOsubstrate ensured that the SrIrO3 film grew in the perov-skite phase [16]. The insets of Fig. 2(c) show the x-raydiffraction pattern and pole figure of the SrIrO3 film. Thex-ray diffraction pattern showed that the SrIrO3 film wasgrown epitaxially in the perovskite form without any im-purity phase. The x-ray pole figure measured around the(011) reflection of the SrIrO3 film showed that it had thefourfold symmetry of a (001) oriented perovskite structure.The epitaxial stabilization of the perovskite SrIrO3 phaseenabled us to investigate the electronic structure changes ofthe iridates with W systematically.We measured ab plane reflectance spectra of Sr2IrO4and Sr3Ir2O7 single crystals at room temperature. Thecorresponding ð!Þ were obtained using the Kramers-Kronig transformation. For the SrIrO3 thin films, we mea-sured reflectance and transmittance spectra in the energyregion between 0.15 and 6 eV. By solving the Fresnelequations numerically [17], we obtained ð!Þ forSrIrO3. Between 0.01 and 0.09 eV, we used the far-infraredellipsometry technique to obtain ð!Þ directly [18].Because of the strong phonon absorption of the MgOsubstrate, we could not obtain ð!Þ between 0.09 and0.15 eV.Figure 2 shows that the Srnþ1IrnO3nþ1 compounds ex-perienced IMT when the z value, i.e., the W value, wasincreased. For the z ¼ 4 compound Sr2IrO4, ð!Þ showeda finite-sized optical gap. For the z ¼ 5 compoundSr3Ir2O7, the optical gap became almost zero. For the z ¼6 compound SrIrO3, the optical gap finally disappeared,and a Drude-like response from conducting carriers ap-peared in the low frequency region. These spectral changesindicate that an IMT occurs between Sr3Ir2O7 and SrIrO3.It should be noted that ð!Þ of Sr2IrO4 has uniquespectral features [19] that are difficult to find in most otherMott insulators. It has a double peak structure, marked asand  in Fig. 2(a). As shown in Fig. 1(a), peak  can beassigned as an optical transition from the LHB to the UHBof the Jeff;1=2 states, while peak  can be assigned as thatfrom the Jeff;3=2 bands to the UHB. Note that peak  is verynarrow with a width 3–5 times smaller than those of thecorrelation-induced peaks in other 3d and 4d Mott insu-lators. For comparison, in Fig. 2(a), we plotted the reportedð!Þ of LaTiO3 and Ca2RuO4 [20,21], which are well-known Mott insulators. The sharpness of peak  originatedfrom the narrowness of the SO coupling-triggered Jeff;1=2FIG. 2 (color online). Optical conductivity spectra ð!Þ of(a) Sr2IrO4, (b) Sr3Ir2O7, and (c) SrIrO3. In (a), ð!Þ of otherMott insulators, such as 3d LaTiO3 (Ref. [20]) and 4d Ca2RuO4(Ref. [21]), are shown for comparison. Peak  corresponds to theoptical transition from the LHB to the UHB of the Jeff;1=2 states.Peak  corresponds to the transition from the Jeff;3=2 bands to theUHB. The insets of (c) show the x-ray pole figure (left) anddiffraction pattern (right) of the perovskite SrIrO3 film. The polefigure was measured around the (011) reflection of the SrIrO3film.PRL 101, 226402 (2008) P HY S I CA L R EV I EW LE T T E R Sweek ending28 NOVEMBER 2008226402-2Hubbard bands, which could facilitate the IMT driven bythe increase of W with a small U value of a 5d TMO.To gain insight into the electronic structure changes ofSrnþ1IrnO3nþ1, we performed local density approximationðLDAÞ þU calculations with SO coupling included [22].We used a U value of 2.0 eV, which produced electronicstructures consistent with our experimental ð!Þ. Mostband structure calculations on 3d Mott insulators haveusually taken a U value of 4–7 eV [12]. Figures 3(a)–3(c) show the band dispersions of Sr2IrO4, Sr3Ir2O7, andSrIrO3, respectively. In the energy region between 2:5and 0.5 eV, the Ir 5d t2g states were the main contributors.The light and dark lines represent the Jeff;1=2 and Jeff;3=2bands, respectively. For Sr3Ir2O7 and SrIrO3, the Jeff;1=2bands split due to the increase in interlayer coupling. Whenz increased, the neighboring Ir ions along the c axis hadstronger hybridization of the d bands through the apical Oions. The hybridization split the bands into bonding andantibonding states, which resulted in an increase in W.Figures 3(d)–3(f) show the total density of states (DOS)of Sr2IrO4, Sr3Ir2O7, and SrIrO3, respectively. The Jeff;3=2states contributed strongly to the DOS between 0:5 and1:5 eV. In Figs. 3(d) and 3(e), the DOS between 0 and0.5 eV (0:5 eV) was from the UHB (LHB) of the Jeff;1=2states. In Fig. 3(f), the DOS between0:5 and 0.5 eV wasfrom the Jeff;1=2 bands. For Sr2IrO4, the separation betweenthe centers of the UHB and LHB (Jeff;3=2 bands) wasapproximately 0.5 eV (1.0 eV), consistent with the positionof peak  () in Fig. 2(a). As z increased, the Jeff;1=2 andJeff;3=2 bands clearly broadened. The W values of theJeff;1=2 bands in the DOS were estimated to be about0.48, 0.56, and 1.01 eV. As the Jeff;1=2 bands broadened,the Mott gap closed. These first-principles calculationsdemonstrate that an increase in W due to the dimension-ality increase induces the IMT in Srnþ1IrnO3nþ1.Experimental evidence for the systematic changes in Wof the Jeff;1=2 bands was obtained from ð!Þ. As shown inFig. 2, peaks  and  broadened and decreased in energywith the increase of z. For quantitative analysis, we fittedð!Þ using the Lorentz oscillator model. For the insula-tors, we used three Lorentz oscillators, which corre-sponded to peaks , , and a charge transfer excitationfrom the O 2p bands to the Ir 5d bands. For SrIrO3, weused the Drude model for metallic response and twoLorentz oscillators which corresponded to peak  andthe charge transfer excitation. From this analysis, we ob-tained the peak positions and widths of peaks  and , i.e.,!, !, , and . According to Fermi’s golden rule,ð!Þ should be proportional to a matrix element and a jointdensity of states. Therefore, the width of an absorptionpeak should reflect the W of the initial and final bands. Asshown in Fig. 4(a), both the  and  values increased asz became larger. This confirmed that the IMT inSrnþ1IrnO3nþ1 should be driven by the change in W.The Lorentz oscillator model analysis provided furtherinformation on the electronic structure changes. As shownin Fig. 4(a), both the ! and ! values decreased system-FIG. 3 (color online). Results from the LDAþU calculationsincluding spin-orbit coupling: Band structures of (a) Sr2IrO4,(b) Sr3Ir2O7, and (c) SrIrO3, and total DOS of (d) Sr2IrO4,(e) Sr3Ir2O7, and (f) SrIrO3. The positive and negative DOSvalues represent spin-up and spin-down bands, respectively. Thelight and dark lines represent the Jeff;1=2 and Jeff;3=2 bands,respectively.FIG. 4. (a) Results from Lorentz oscillator model analysis. Thesolid symbols show the positions of the  and  peaks, i.e., !and !. The open symbols indicate the widths of the  and peaks, i.e.,  and . (b) Frequency-dependent mass enhance-ment ð!Þ of correlated metal SrIrO3. The ð!Þ data of SrRuO3(our data) and Sr2RuO3 (Ref. [24]) are included for comparison.PRL 101, 226402 (2008) P HY S I CA L R EV I EW LE T T E R Sweek ending28 NOVEMBER 2008226402-3atically as z was increased. The peak shift might originatefrom the increase of the spectral weight in the gap region,which would fill the Mott gap with increasing W [4]. Forthe ! change, the electronic structure changes for bothLHB and UHB are important. However, the ! changeonly involves changes in the UHB. In Fig. 4(a), the changein ! between Sr2IrO4 and Sr3Ir2O7 was approximately0:25 0:05 eV. Conversely, the change in ! was ap-proximately 0:13 0:05 eV, which is about half of thechange in !. This suggested that the electronic structurechanges should occur symmetrically around the Fermilevel for both Hubbard bands, as shown in Fig. 1.To obtain firm evidence for Mott instability in 5dSrnþ1IrnO3nþ1, we performed extended Drude modelanalysis on metallic SrIrO3 [23,24]. From the optical spec-tra, we obtained the mass enhancement of conductingcarriers ð!Þ, which corresponded to the effective massnormalized by the band mass. Substantial enhancement ofð!Þ near the IMT is a hallmark of Mott instability and thecorresponding compound should be in CMS. As shown inFig. 4(b), the ð!Þ of SrIrO3 reached about 6 at the lowestenergy. We are not aware of any report on ð!Þ for 5dTMOs, so we included the room temperature ð!Þ of 4dcorrelated metals SrRuO3 and Sr2RuO4 in Fig. 4(b) forcomparison. The ð!Þ of 3d correlated metal ðCa;SrÞVO3was reported to be approximately 3–4 [25]. Note that theð!Þ of SrIrO3 is comparable to or larger than those of the3d and 4d TMOs. It is interesting that such a large value ofð!Þ was found for the 5d TMO compound. This largevalue implies that SrIrO3 is a correlated metal close to theMott transition.Our unique findings, the IMT with respect to change inW and the large effective mass of the resulting metalliccompound in 5d system, challenge the conventional ex-pectation that electron correlation is insignificant in 5dsystems. These results originated from the large SO cou-pling of 5d transition metal ions. Recent theoretical resultsshowed that the SO coupling in a 4d counterpart compoundcould modify the Fermi surface within the metallic phase[26,27]. However, the large SO coupling in 5d systemscould drastically enhance the effect of the electron corre-lation. This cooperative interaction drives some 5d TMOsclose to the Mott instability.In summary, we investigated the electronic struc-tures of a weakly correlated narrow band system, 5dSrnþ1IrnO3nþ1, using optical spectroscopy and first-principles calculations. We observed an insulator-metaltransition driven by the bandwidth change and the associ-ated correlated metallic state with a large effective mass.Our results clearly demonstrate that electron correlationcould play an important role even in 5d systems.This work was financially supported by the CreativeResearch Initiative Program (Functionally IntegratedOxide Heterostructure) of MOST/KOSEF, KOSEF throughthe ARP (R17-2008-033-01000-0), and SchweizerNationalfonds with Grant No. 200020-119784. Y. S. L.was supported by the Soongsil University Research Fund.The experiments at PLS were supported in part by MOSTand POSTECH.*twnoh@snu.ac.kr[1] M. Imada, A. Fujimori, and Y. Tokura, Rev. Mod. Phys.70, 1039 (1998).[2] A. Fujimori et al., Phys. 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Dimensionality-Controlled Insulator-Metal Transition and Correlated Metallic State in 5d Transition Metal Oxides Srn+1IrnO3n+1 (n=1, 2, and infinity)

S. J. Moon

H. Jin

K. W. Kim

W. S. Choi

Y. S. Lee

J. Yu

G. Cao

A. Sumi

H. Funakubo

C. Bernhard

T. W. Noh

Crossref

Dimensionality-Controlled Insulator-Metal Transition and Correlated Metallic State in
                    
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