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what a PRL typically looks like (4 pages)PRL 99, 187001 (2007)PHYSICAL REVIEW LETTERS week ending2 NOVEMBER 2007PRL 99, 187001 (2007)PHYSICAL REVIEW LETTERS week ending2 NOVEMBER 2007Evolution of the Fermi Surface and Quasiparticle Renormalizationthrough a van Hove Singularity in Sr 2 y La y RuO 4K. M. Shen, 1,2, * N. Kikugawa, 3,† C. Bergemann, 4 L. Balicas, 5 F. Baumberger, 1,3 W. Meevasana, 1 N. J. C. Ingle, 1,2Y. Maeno, 6,7 Z.-X. Shen, 1 and A. P. Mackenzie 31 Departments of Applied Physics, Physics, and Stanford Synchrotron Radiation Laboratory, Stanford University,Stanford, California 94305, USA2 Department of Physics and Astronomy, University of British Columbia, Vancouver, B.C., V6T 1Z4, Canada3 School of Physics & Astronomy and Scottish Universities Physics Alliance, University of St. Andrews,North Haugh, St. Andrews KY16 9SS, United Kingdom4 Cavendish Laboratory, University of Cambridge, J. J. Thomson Avenue, Cambridge CB3 OHE, United Kingdom5 National High Magnetic Field Laboratory, Florida State University, Tallahassee Florida 32306, USA6 International Innovation Center, Kyoto University, Kyoto 606-8501, Japan7 Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan(Received 26 March 2007; published 29 October 2007)We employ a combination of chemical substitution and angle resolved photoemission spectroscopy toprove that the Fermi level in the band of Sr 2 y La y RuO 4 can be made to traverse a van Hove singularity.Remarkably, the large mass renormalization has little dependence on either k or doping. By combiningthe results from photoemission with thermodynamic measurements on the same batches of crystals, wededuce a parametrization of the full many-body quasiparticle dispersion in Sr 2 RuO 4 which extends fromthe Fermi level to approximately 20 meV above it.DOI: 10.1103/PhysRevLett.99.187001 PACS numbers: 74.20.Rp, 74.25.Jb, 74.72. h, 79.60. iThe layered perovskite ruthenate Sr 2 RuO 4 has attractedconsiderable attention and interest, stimulated initially byits unconventional superconductivity [1,2]. There is now aconsiderable body of evidence that it is a spin tripletsuperconductor with a time-reversal symmetry breakingground state [2–5] and that the metal from which thesuperconductivity condenses is a nearly two-dimensionalFermi liquid with a substantial mass enhancement [6]. Theelectronic structure near the Fermi level consists of twoFermi surface sheets ( and ) having strong Ru d xz;yzcharacter and a third, , based on the in-plane Ru d xyorbital.The comprehensive knowledge that has been acquiredabout the Fermi liquid properties of Sr 2 RuO 4 is unprecedentedfor a transition metal oxide, making it an idealmaterial in which to address a deeper layer of importantquestions, for example the origin of the mass enhancementand its relationship to the microscopic mechanism of thesuperconductivity. Recently, a new route to probing thiswas opened by the observation that the heterovalent substitutionof La 3 for Sr 2 to form Sr 2 y La y RuO 4 producesa rigid-band shift of the Fermi level for low y [7]. The limiton those experiments was the window for observing ade Haas–van Alphen (dHvA) signal, which is exponentiallysuppressed by disorder scattering, and not the solubilitylimit of La 3 in Sr 2 RuO 4 . In fact, much highersubstitution levels of up to y 0:27 have been achieved,raising the possibility that the Fermi level can be forced totraverse a van Hove singularity (vHS) in the band. Thisissue is of great interest, since strong electronic renormalizationand scattering of quasiparticles could occur nearsuch a singularity in the density of states. For instance, ithas even been postulated that some of the unusual physicalproperties observed in the cuprate superconductors mightbe as a result of the vicinity of a vHS to E F at k ; 0 .In this Letter we report the results of a careful set ofexperiments which prove conclusively that the mass renormalizationremains remarkably independent of both k anddoping as the vHS traverses E F .In principle, angle resolved photoemission spectroscopy(ARPES) is ideally suited to this experiment, since it is notsubject to the same purity restrictions as dHvA. However,since it is highly surface-sensitive there is always theconcern that some of the information that it yields is aproperty of the surface rather than the bulk electronicstructure. This issue has been of particular significance toARPES studies of Sr 2 RuO 4 , as discussed in Refs. [2,8,9].For this reason we set ourselves the more ambitious goal ofperforming a detailed comparison of ARPES with bulkmeasurements of the Fermi surface topography and excitationspectrum. The ARPES work was performed with aGammadata VUV5000 He-discharge lamp, monochromator,and a SES2002 electron analyzer, at a base temperatureof 10 K and a pressure of better than 4 10 11 torr.All data shown in this Letter were collected using He IIphotons (40.81 eV) on samples cleaved at 200 K, in orderto suppress the surface-related features discussed inRefs. [8,9]. A new generation of dHvA measurementswas also performed in the world’s highest steady magneticfield of 45 T using using a Cu-Be torque magnetometerwith capacitive readout, at temperatures between 0.04 and0.9 K.0031-9007=07=99(18)=187001(4) 187001-1 © 2007 The American Physical SocietyEnergy (meV)0-50-100a)0.8y = 010 K1.0also verify that these probes yield an electronic structureconsistent with one expected from measuring bulk thermodynamicproperties (specific heat). In Fig. 3(a) we presentthe direct comparison of Fermi surface topography measurementsby dHvA and ARPES across a range of cationsubstituted single crystals. By using the 45 T hybrid magnetat the U. S. National High Magnetic Field Laboratory inTallahassee, we were able to measure all three main dHvAfrequencies out to y 0:06, and the smallest frequencyall the way to y 0:1.Our findings have significant implications for the physicsof many-body renormalization in Sr 2 RuO 4 . If thisrenormalization is dominated by coupling to a sharpmode, it results in a kink in the dispersion at a characteristicenergy associated with that mode. If, on the otherhand, it is dominated by coupling to a continuum ofelectron-hole excitations, renormalization of the wholeband is possible. For this to occur, the source of therenormalization must be local in real space since its effectsare spread out over all k. A notable example is the simplifiedon-site repulsion U employed to partially accountfor correlations in modern electronic structure calculations.The extent of renormalization can also be linked tothe magnitude of the bare density of states, so naively onemight expect a feedback effect in which the correlationinducedenhancement factor is increased near the vHS.Several features of our data suggest that, perhaps surprisingly,the renormalization of the band is dominated by aglobal band renormalization. The data from across therange of La dopings that we have employed fall onto asingle cosine, even on the most crucial -M- cut throughthe vHS, and has a total bandwidth approximately a factorof 6 smaller than the LDA prediction, in agreement, withinexperimental error, with the dHvA-measured mass renormalizationat y 0 of m =m band 5:5 [2]. It can also betracked over a wide range of k (at least 20% of theBrillouin zone dimension), confirming that the source ofthe renormalization is highly local in real space.1.2b)0.8y = 0100 K1.01.2c)0.8y = 0.1810 K1.01.2k x(π/a)The data in Fig. 2(e) are from a restricted cut in theBrillouin zone, and do not, in isolation, allow us to drawfirm conclusions about the renormalized band shape for theremainder of the band or that of the and bands.Indeed, attempting to do this using ARPES would be rathercomplex [15]. However, if local correlations dominate therenormalization in the band, it is highly plausible that thesame considerations apply elsewhere in the Brillouin zoneas well.We are able to investigate this issue by assuming that thewhole-bandwidth renormalization applies throughout theCarrier number (per Ru)0.84.44.3 a)4.24.14.03.9 Total electron~ ~1.41.21.00.80.60.4y = 0.2710 K(electron)(electron)ARPESdHvATight-Bindinge)0.8(hole)0.2(hole)00 0.1 0.2Additional electron y0.3y = 0y = 0 (100 K)y = 0.18y = 0.20y = 0.27FIG. 2 (color online). Scans along the -M- line in the Brillouin zone for samples with y 0, y 0:18 and y 0:27 at 10 K andE