We present the first observation of a prominent quasi-particle peak in the
photoemission spectrum of the metallic phase of V_2O_3 and report new spectral
calculations that combine the local density approximation with the dynamical
mean-field theory (using quantum Monte Carlo simulations) to show the
development of such a distinct peak with decreasing temperature. The
experimental peak width and weight are significantly larger than in the theory.Comment: 4 pages, 3 figures, supercedes cond-mat/010804

Allen, J. W.

Anisimov, V. I.

Denlinger, J. D.

Eyert, V.

Held, K.

Kadono, K.

Keller, G.

Kim, H. -D.

Metcalf, P.

Mo, S. -K.

Muro, T.

Park, J. -H.

Saitoh, Y.

Sekiyama, A.

Suga, S.

Vollhardt, D.

Yamasaki, A.

English

arXiv

Mo, S.-K.

Kim, H.-D.

Park, J.-H.

Keller, Georg

Held, Karsten

Eyert, Volker

Vollhardt, Dieter

OPUS Augsburg

P H Y S I C A L R E V I E W L E T T E R S week ending9 MAY 2003VOLUME 90, NUMBER 18Prominent Quasiparticle Peak in the Photoemission Spectrum of the Metallic Phase of V2O3S.-K. Mo,1 J. D. Denlinger,2 H.-D. Kim,3 J.-H. Park,4 J.W. Allen,1 A. Sekiyama,5 A. Yamasaki,5 K. Kadono,5 S. Suga,5Y. Saitoh,6 T. Muro,7 P. Metcalf,8 G. Keller,9 K. Held,10 V. Eyert,11 V. I. Anisimov,12 and D. Vollhardt91Randall Laboratory of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA2Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA3Pohang Accelerator Laboratory, Pohang 790-784, Korea4Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea5Department of Material Physics, Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama,Toyonaka, Osaka 560-8531, Japan6Department of Synchrotron Radiation Research, Japan Atomic Energy Research Institute, SPring-8, Sayo,Hyogo 679-5143, Japan7Japan Synchrotron Radiation Research Institute, SPring-8, Sayo, Hyogo 679-5143, Japan8Department of Physics, Purdue University, West Lafayette, Indiana 47907, USA9Theoretical Physics III, Center for Electronic Correlations and Magnetism, University of Augsburg, 86135 Augsburg, Germany10Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany11Theoretical Physics II, University of Augsburg, 86135 Augsburg, Germany12Institute of Metal Physics, Ekaterinburg GSP-170, Russia(Received 24 November 2002; published 8 May 2003)186403-1We present the first observation of a prominent quasiparticle peak in the photoemission spectrum ofthe metallic phase of V2O3 and report new spectral calculations that combine the local-densityapproximation with the dynamical mean-field theory (using quantum Monte Carlo simulations) toshow the development of such a distinct peak with decreasing temperature. The experimental peakwidth and weight are significantly larger than in the theory.DOI: 10.1103/PhysRevLett.90.186403 PACS numbers: 71.10.–w, 71.30.+h, 79.60.–iupper and lower Hubbard bands, there remains at EF adistinctive quasiparticle (QP) peak, not accidentallyimportant to note that numerical complexities con-strained the calculation to be performed at a temperatureV1xCrx2O3 displays a complex phase diagram withparamagnetic metal (PM), paramagnetic insulator (PI),and antiferromagnetic insulator (AFI) regions. The PM toPI transition serves as the paradigm of the Mott-Hubbard(MH) metal-insulator transition (MIT) [1]. The MH sce-nario for V1xCrx2O3 was put forth originally in thecontext of the half-filled one-band Hubbard model inwhich the tendency of the on-site Coulomb repulsion‘‘U’’ to make a correlation gap insulator competes withthe tendency of site to site hopping to make a broad bandmetal of bandwidth ‘‘B.’’ A coherent thermodynamicallyconsistent description of the MIT became possible withthe development of the dynamical mean-field theory(DMFT) [2]. DMFT describes the strongly interactingmetal in terms of Fermi liquid quasiparticles, i.e., singleparticle excitations near the Fermi energy EF whichremain well defined as in a noninteracting system buthave a self-energy correction that increases their effectivemass and reduces their spectral weight. In application tothe Hubbard model, DMFT is significant as the bestdescription that can be made by using a local (i.e., inde-pendent of momentum k) self-energy. It may be formu-lated as a mapping of the lattice problem onto an effectiveAnderson impurity model coupled self-consistently to aneffective conduction band bath [3]. In the metallic phase,although the large U value acts to separate much of theband’s spectral weight away from EF into the so-called0031-9007=03=90(18)=186403(4)$20.00 reminiscent of the Kondo/Suhl-Abrikosov resonance [4]of the Anderson impurity model. The weight of theQP-peak decreases with increasing U=B and goes tozero at a critical value of U=B, which thus marks the MIT.Such a distinctive peak could in principle be seen inphotoemission spectroscopy (PES). However, in spite ofcontinuing efforts for over 20 years, literature [5–10] V3d PES spectra for the PM phase of V1xCrx2O3 haveshown at most a near EF feature that is the smallest partof the spectrum. One could hypothesize that the distinc-tive central peak of the half-filled one-band model isobscured and washed out by the multiband complexityof the actual electronic structure of V1xCrx2O3 inwhich the two 3d electrons of the V3 ion must bedistributed among singly degenerate a1g and doubly de-generate eg orbitals derived from a small trigonal crystalfield splitting of the cubic t2g manifold of the V 3d states.Indeed, recent experiment [11] and theory [12] establish-ing an S  1 state for the V3 ions show that such amultiband description is essential. Further, a recent studythat included these material specific aspects by combin-ing the local-density approximations with DMFT (usingquantum Monte Carlo simulations) [LDA DMFT(QMC)] [13] found for the PM phase a weak EF peak ingood agreement with that of a well resolved and highquality PM phase spectrum by Schramme et al. [14],taken at a photon energy (h) of 60 eV. However, it is2003 The American Physical Society 186403-1FIG. 1. h  700 eV and k-averaged PES spectra of V2O3.The inset shows the k space covered by the detector angularacceptance for various h.P H Y S I C A L R E V I E W L E T T E R S week ending9 MAY 2003VOLUME 90, NUMBER 18T of 1160 K, whereas the photoemission spectrum wasobtained at a much lower temperature of 300 K, and thatfor the photon energy used, recent PES studies of othervanadium oxides have been found [15,16] to yield spectranot characteristic of the bulk, but rather of the surfaceatoms whose lower coordination number reduces B pro-portionately and thus can render the surface layer muchmore strongly correlated or even insulating. Both circum-stances go in the direction of a greatly reduced QP peak.In this Letter we report angle-integrated PES spectrafor the metallic phase of V2O3 that are different from allprevious spectra in showing an EF peak whose amplitudeexceeds that of the rest of the V 3d spectrum by a factorapproaching 2. These results were achieved by measuringat high photon energies in order to increase the bulksensitivity but with much better resolution [17] than inprevious [10,18] studies at these energies. Equally im-portant is the use of a small photon spot ( 100 mdiameter) that we have found [19] to be essential forminimizing the effects of steps and other roughnessthat can be seen in high resolution optical micrographsof typical cleaved single crystal surfaces of this material.Doing so is very important because atoms with the edgeand corner (i.e., acute) geometries of steps and generalroughness have coordinations that are reduced from thoseof the planar surface atoms. Concomitantly, as is knownfrom past theory [20] and experiment [21] on correlatedelectron materials, they have even larger differences fromthe electronic structure of the bulk than occurs for theplanar surface atoms. The large amplitude of the newlyobserved peak disagrees with the theory of Ref. [13].However, we also report new LDA DMFT (QMC)calculations for the same electronic structure and Uvalue as previously [13], showing that with temperaturedecreasing to 300 K such a prominent central peak devel-ops. Nonetheless, the quantitative comparison enabled bythe new experiment and new theory reveals importantdifferences.PES using circularly polarized photons with h be-tween 310 and 700 eV was performed at the twin-helicalundulator beam line BL25SU [22] of SPring-8 equippedwith a SCIENTA SES200 analyzer and giving a photonspot size of diameter 100 m. The Fermi level and over-all energy resolution ( 90 to 170 meV over the hrange) were determined from the Fermi-edge spectrumof a Pd metal reference. Well annealed oriented single-crystalline samples of V2O3 were cleaved to expose ahexagonal 1012 plane in a vacuum of 2 1010 Torr.Using a closed-cycle He cryostat and an embedded resis-tive heater, the sample temperature was held at T 175 K, somewhat above the PM/AFI transition. Surfaceintegrity was well maintained under the vacuum andphoton exposure, as shown by the repeatability of thespectra for at least 10 hours after cleaving, after whichthe PM/AFI transition could still be observed in thespectra at the temperature appropriate for stoichiometricV2O3. The inset of Fig. 1 shows a cross section of the186403-2k-space Brillouin zone stacking normal to the cleavageplane and also the spherical surfaces [23] traversed as thedetector angles are varied about the normal for variousfixed h between 310 eVand 700 V. Radial lines show thek range corresponding to the analyzer acceptance angleof about 6	. This range is 1:2 A1 for a photoelectronwith 500 eV kinetic energy and covers more than oneBrillouin zone for any h arc.The h  700 eV spectrum of Fig. 1 shows the generalcharacter of the data. The V 3demission is well separatedfrom the O 2p emission, and the newly observed promi-nent EF peak is very obvious.We are interested here in theh dependence of the spectral shape of theV 3demission.Although there is a huge 3d enhancement over the range510 to 560 eV, due to a 3d cross-section resonance at theV 2p edge [10,18], we have deliberately avoided theresonance region because we have found [24] in thisrange incoherent Auger emission that distorts the spec-tral shape. Because we make comparisons to a k-summedtheory, Fig. 1 also shows the result of k averaging across afull Brillouin zone by using spectra taken in 10 eV stepsfrom 380 to 500 eV. The EF peak in the k-averagedspectrum is somewhat smaller than that in the 700 eVspectrum. This is, however, due to the reduced bulk sen-sitivity of the lower photon energies rather than the kselectivity of the single photon energy, as discussed next.Figure 2 shows our V 3d spectra for h from 310 to700 eValong with the h  60 eV spectrum [14] that wasfound to compare favorably to the 1160 K theory spec-trum in Ref. [13]. A Shirley-type inelastic backgroundhas been removed in an identical way [25] for eachspectrum. The spectra are normalized over the range186403-2Intensity (arb. unit)-3 -2 -1 0E-EF(eV) U=5.0eV, T=1160K U=5.0eV, T=700K U=5.0eV, T=300K hν=700eV, T=175K Schramme et al. [14]          hν=60eV, T=300KFIG. 3. LDA DMFT (QMC) calculations made at threetemperatures for U  5:0 eV, the h  60 eV PES spectrum[14] compared to theory in Ref. [13], and our new h  700 eVbulk sensitive spectrum.Intensity (arb. unit)-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5E-EF(eV)abcdea: hν=700eVb: hν=500eVc: k-averagedd: hν=310eVe: Schramme et al. [14]    hν=60eVFIG. 2. PES spectra taken with various h, the largest ofwhich yields the greatest bulk sensitivity.P H Y S I C A L R E V I E W L E T T E R S week ending9 MAY 2003VOLUME 90, NUMBER 18below 1 eV for ease of comparing their EF peaks.Relative to the spectrum below 1 eV, the EF peaks inthe spectra of Fig. 2 increase monotonically with increas-ing h. Over the range for the k-averaged spectrum thepeak is never larger than at 500 eV. Although some non-monotonic variation over this range and also below 300 eV(not shown) could signal k dependence, we conclude thatabove 300 eV the change of probe depth dominatesk-dependent effects in the data. This conclusion is re-inforced by the observation at 500 eV that rotating thedetection angle away from the normal in steps of 15	 outto 60	, which monotonically decreases the effective probedepth, monotonically decreases the EF peak to be com-parable to the EF feature that is seen in the 60 eV data.Reliably estimating a bulk spectrum from the h andangle dependences using various phenomenological mod-els is not simple. We will present such estimates in a moredetailed paper. However, the results do not differ from the700 eV spectrum itself in any way that is significant forthe comparison to new theory that we make next.On the theoretical side, new, very CPU-intensiveLDA DMFT (QMC) calculations have been made atconsiderably lower temperatures than previously [13],i.e., T  700 and 300 K, in exactly the same manner[26] as before. The LDA density of states of the threet2g bands at the Fermi energy was identified with a one-particle Hamiltonian which was supplemented by a localCoulomb interaction U  5 eV (only U values close to5 eV generate the experimentally observed MIT upon Crdoping [13]) and a Hund’s rule coupling J  0:93 eV(according to constrained LDA calculations [27]). Thisthree-band many-body problem was solved by DMFTusing QMC simulations.Figure 3 shows our theory spectra for three tempera-tures, compared to the experimental h  60 eV spec-186403-3trum (300 K) and the 700 eV spectrum (175 K) of Fig. 2,all scaled to have equal areas. The theory curves includethe Fermi function for the appropriate temperature andGaussian broadening of 90 meV to simulate experimentalresolution. Qualitatively similar to the behavior of theimpurity model’s Kondo resonance [4], at higher tempera-tures the QP peak is greatly broadened by thermal occu-pation of states bearing local magnetic moments. As thesystem settles with decreasing temperature toward itsnonmagnetic ground state, the peak sharpens, its ampli-tude increases strongly, and it becomes clearly separatedfrom the lower Hubbard band at about 1:25 eV. Thesetemperature-induced changes strongly enhance the dif-ferences between theory and experiment that were al-ready present but small in Ref. [13]; i.e., the amplitudeof the QP peak is much larger in the theory than in theh  60 eV experimental spectrum. Qualitatively, thecomparison to the new bulk sensitive spectrum is muchbetter, with the prominent QP peak and the lowerHubbard band of the theory being very similar, respec-tively, to the newly observed EF peak and the broad humpcentered at 1:25 eV. Strikingly different, however, arethat the experimental EF peak is broader and also hasmore spectral weight. These differences will persist fortheory at the slightly lower temperature of the data. Thewidth difference could increase somewhat. The weightdifference would decrease but probably not greatly.From 1160 to 300 K the theory peak’s integrated spectralweight, taken as that above 0:63 eV, increases byonly 11%.Within the DMFT scheme the larger experimentalwidth and weight implies weaker correlation that couldbe described by using a reduced U value. However, in the186403-3P H Y S I C A L R E V I E W L E T T E R S week ending9 MAY 2003VOLUME 90, NUMBER 18present theory [13] the MIT under Cr doping would notoccur for such a reduced U value unless U then increaseswith Cr doping or through the MIT transition. Eithermight be expected to occur through reduction in variouseffective screening processes, some of which are excludedfrom the present theory by its restriction to three bands.Another possible origin for the increased width of theexperimental peak is the k dependence of the singleparticle self-energy. This is beyond our current calcula-tions, but may partly be included by an improved DMFTcalculation which uses a pair of V ions [28] instead of asingle site.In summary, we have discovered a prominent EF peakin the PES spectrum of V2O3 in the PM phase, at atemperature somewhat above that of the AFI phase tran-sition. New LDA DMFT (QMC) theory for a compa-rable temperature also finds such a prominent QP peak,generic to the DMFT theory of the Hubbard model nearthe MIT transition. The quantitative comparison that isenabled by the two new results reveals significant differ-ences that must be addressed in future work. Nonetheless,considering the various approximations of the presenttheory, the comparison is very encouraging as to theoccurrence of the basic QP peak that is central to theDMFT description.This work was supported by the U.S. NSF at the Uni-versity of Michigan (UM) (Grant No. DMR-99-71611),by the U.S. DoE at UM (Contract No. DE-FG-02-90ER45416) and at the ALS (Contract No. DE-AC03-76SF00098), by Grant-in-Aid for COE Research(10CE2004) of MEXT, Japan by JASRI (No. 2000B0335-NS-np), by KOSEF through eSSC at POSTECH, by theDeutsche Forschungsgemeinschaft (DFG) through SFB484, by the Emmy-Noether program of the DFG, by theRussian Foundation for Basic Research Grant No. RFFI-01-17063, by the Alexander von Humboldt-Foundation,and by the Leibniz-Rechenzentrum, München.18640[1] N. F. Mott, Rev. Mod. Phys. 40, 677 (1968); Metal-Insulator Transitions (Taylor & Francis, London,1990); F. Gebhard, The Mott Metal-Insulator Transition(Springer, Berlin, 1997).[2] W. Metzner and D. Vollhardt, Phys. Rev. Lett. 62, 324(1989); A. Georges et al., Rev. Mod. Phys. 68, 13 (1996).[3] A. Georges and G. Kotliar, Phys. Rev. B 45, 6479 (1992);M. Jarrell, Phys. Rev. Lett. 69, 168 (1992).[4] O. Gunnarsson and K. Schönhammer, in Handbook onthe Physics and Chemistry of Rare Earths, edited by3-4K. A. Gschneidner, L. Eyring, and S. Hüfner (North-Holland, Amsterdam, 1987), Vol. 10, p. 103.[5] G. A. Sawatzky and D. Post, Phys. Rev. B 20, 1546(1979).[6] K. E. Smith and V. E. Henrich, Phys. Rev. B 38, 5965(1988).[7] K. E. Smith and V. E. Henrich, Phys. Rev. B 50, 1382(1994).[8] S. Shin et al., J. Phys. Soc. Jpn. 64, 1230 (1995).[9] H.-D. Kim et al., Phys. Rev. B 57, 1316 (1998).[10] R. Zimmermann et al., J. Phys. Condens. Matter 10, 5697(1998).[11] J.-H. Park et al., Phys. Rev. B 61, 11 506 (2000).[12] S.Yu. Ezhov et al., Phys. Rev. Lett. 83, 4136 (1999).[13] K. Held et al., Phys. Rev. Lett. 86, 5345 (2001).[14] M. Schramme, Ph.D. thesis, Universität Augsburg, 2000;M. Schramme et al. (unpublished).[15] K. Maiti, P. Mahadevan, and D. D. Sarma, Phys. Rev.Lett. 80, 2885 (1998).[16] A. Sekiyama et al., cond-mat/0206471.[17] A. Sekiyama et al., Nature (London) 403, 396(2000).[18] J.-H. Park, Ph.D. thesis, University of Michigan, 1994.[19] Our initial high h work, unpublished, but reportedin cond-mat/0108044, employed a photon spot size of 1 mm diameter. The importance of a small spot sizewas discovered in subsequent exploratory studies madewith lower energy resolution on the spectromicroscopybeamline 7.0 at the ALS.[20] B. Johansson, Phys. Rev. B 19, 6615 (1979).[21] M. Domke et al., Phys. Rev. Lett. 56, 1287 (1986).[22] Y. Saitoh et al., Rev. Sci. Instrum. 71, 3254 (2000).[23] Following Ref. [6] the figure is drawn for an inner po-tential of 21.76 eV, but, in fact, there is negligible depen-dence on the inner potential value for these large h.[24] The overlapping Auger emission was discovered afterour initial (cond-mat/0108044) high h work. Althoughnot intense, it nonetheless distorts the line shape to peaksomewhat below EF.[25] The background brings the spectra to zero at the mini-mum between the V 3d and O 2p regions, implicitlymaking a separation between the two.[26] More details and a pedagogical introduction are given inK. Held et al., in Quantum Simulations of ComplexMany-Body Systems: From Theory to Algorithms, editedby J. Grotendorst, D. Marks, and A. Muramatsu, NICSeries Vol. 10 (John von Neumann Institute forComputing, Julich, Germany, 2002), pp. 175–209.[27] I. Solovyev et al., Phys. Rev. B 53, 7158 (1996).[28] Past and recent discussion of pair models is given in J.W.Allen, Phys. Rev. Lett. 36, 1249 (1976); F. Mila et al.,Phys. Rev. Lett. 85, 1714 (2000); S. di Matteo et al., Phys.Rev. B 65, 054413 (2002).186403-4

Prominent quasi-particle peak in the photoemission spectrum of the metallic phase of V2O3

S.-K. Mo

J. D. Denlinger

H.-D. Kim

J.-H. Park

J. W. Allen

A. Sekiyama

A. Yamasaki

K. Kadono

S. Suga

Y. Saitoh

T. Muro

P. Metcalf

G. Keller

K. Held

V. Eyert

V. I. Anisimov

D. Vollhardt

Crossref

Prominent Quasiparticle Peak in the Photoemission Spectrum of the Metallic Phase ofV2O3

We present the first observation of a prominent quasiparticle peak in the photoemission spectrum of the metallic phase of V2O3 and report new spectral calculations that combine the local-density approximation with the dynamical mean-field theory (using quantum Monte Carlo simulations) to show the development of such a distinct peak with decreasing temperature. The experimental peak width and weight are significantly larger than in the theory.open11127sciescopu

Mo, SK

Denlinger, JD

Kim, HD

Park, JH

Allen, JW

Sekiyama, A

Yamasaki, A

Kadono, K

Suga, S

Saitoh, Y

Muro, T

Metcalf, P

Keller, G

Held, K

Eyert, V

Anisimov, VI

Vollhardt, D

포항공과대학교

Physical Review Letters

Prominent quasiparticle peak in the photoemission spectrum of the metallic phase of V2O3

Online-Publikationserver Augsburg

Mo, S. K.

Kim, H. D.

Park, J. H.

Max Planck Society eDoc Server

Prominent Quasiparticle Peak in the Photoemission Spectrum of the Metallic Phase of V2O3

Prominent quasi-particle peak in the photoemission spectrum of the
  metallic phase of V_2O_3

Prominent quasi-particle peak in the photoemission spectrum of the metallic phase of V_2O_3

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OPUS Augsburg

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포항공과대학교

Online-Publikationserver Augsburg

Max Planck Society eDoc Server