Ultrahigh-resolution photoemission study of superconductors and strongly correlated materials using quasi-cw VUV laser

Transcription

1 CORPES Ultrahigh-resolution photoemission study of superconductors and strongly correlated materials using quasi-cw VUV laser S. Shin, 1,2 T. Kiss, 1 S. Tsuda, 1 K. Ishizaka, 1 T. Yokoya, 1 and T. Togashi, 2 C. Chen, 3 S. Watanabe 1 1 Institute for Solid State Physics, University of Tokyo 2 RIKEN 3 Beijing Center for Crystal R&D, Chinese Academy of Science

2 Research of Superconductors by PES Resolution Temperature 100meV HgCaBaCuO 133K 10meV 1meV 100µeV 100K High T c s 10K Target of this study 1K MgB 2 39K Simple metal Metal alloy Borocarbide TMDC etc. LiTi 2 O K κ-(et) 2 Cu[N(CN) 2 ]Br 11.6K Transition metal oxide Organic molecular p, d, f - wave superconductor CeRu 2 6.0K Heavy fermions 10µeV 0.1K

3 contents 1. Development of Laser photoemission system High resolution Photoelectron analyzer Quasi-CW VUV laser Low temperature Cooling system 2. High resolution PES study on superconductors Nb ; simple metal superconductor CeRu 2 ; 4f electron superconductor with Onuki et al MgB 2 ; intermetallic compound with Tajima et al -(ET) 2 Cu[N(CN) 2 ]Br ; organic superconductor with Kanoda et al -(ET) 2 Cu (SCN) 2 3. High resolution PES study on Kondo material LiV 2 O 4 ; Kondo state in TMO with Ueda et al 4. Bulk sensitive PES in low photon energy region SrVO 3 ; comparison with DMFT with Inoue et al 5. Conclusion and Future of Laser-Photoemission

4 What limits the photoemission resolution? 1. Line width of light source 2. Electron analyzer Slit width Engineering precision Disorder of work function Instability of power source 3. Residual magnetism 4. Ripple in ground level 5. Space charge effect photoelectron Long range interaction 1/r It will be settled by developing laser They have been resolved. Recent progress is in energy region below 1 mev CW or quasi-cw light source should be necessary (= weak peak intensity, but strong average intensity) Space charge effect by pulsed light

5 Laser system 6 th harmonic (6.994eV) of Nd:YVO 4 laser using KBBF crystal T.Togashi,et al., Opt. Lett. 28, 254 (2003) Optical Contact, CaF 2 Prism-Coupled CaF 2 KBBF Window f500 CaF 2 Window To sample 6.994eV (177.3nm) f eV (354.7nm) 80MHz Quasi-CW frequency tripled Nd:YVO 4 laser (Vanguard, Spectra Physics) CaF 2 prism SHG(7.0eV) Fundamental(3.5eV) Optical crystal 3.496eV (354.7nm) HG 2.331eV (532nm) 1.165eV HG 1.165eV Nd:YVO 4 crystal 1.165eV Amplifier Oscillator (1064nm) 1.534eV 1.534eV (808nm) LD3 LD2 LD1

6 Specification of new Laser Highest photon energy quasi CW Laser Laser HeIα Photon energy 6.994eV eV FWHM 0.26meV(0.09meV with etalon) ~1meV Frequency 80MHz (quasi CW) CW Total Photon flux 2.2x10 15 photons/sec ~10 13 photons/sec polarization vertical, horizontal, circular none Size of spot 0.2µm 0.5mm 6~8mm Microscopy Bulk sensitive High energy resolution Polarization dependent Low damage of sample Photon flux of SR is weak, and becomes weak in proportion to the increase of the monochromator resolution Cf phs./sec (1 mev resolution at 100eV)

7 Laser excitation photoemission spectrometer Gammadata-R4000 analyzer Mirror Laser Lens Mirror Cryopump Mirror Lens Sample bank Sample chamber TMP Preparation chamber Cryostat SHG crystal xyz stage

8 Performance of the experimental system Intensity (arb. units) Au FD fit Resolution 1.4 mev Au T = 5.3 K He Iα (21.218eV) (SES2002) Intensity (arb.units) Au FD fit E=360µeV Au Laser hν=6.994ev (R4000) T=2.9K E F Binding energy (mev) E F Binding energy (mev)

9 contents 1. Development of Laser photoemission system High resolution Photoelectron analyzer Quasi-CW VUV laser Low temperature Cooling system 2. High resolution PES study on superconductors Nb ; simple metal superconductor CeRu 2 ; 4f electron superconductor with Onuki et al MgB 2 ; intermetallic compound with Tajima et al -(ET) 2 Cu[N(CN) 2 ]Br ; organic superconductor with Kanoda et al -(ET) 2 Cu (SCN) 2 3. High resolution PES study on Kondo material LiV 2 O 4 ; Kondo state in TMO with Ueda et al 4. Bulk sensitive PES in low photon energy SrVO 3 ; comparison with DMFT with Inoue et al 5. Conclusion and Future of Laser-Photoemission

10 Superconducting gap of Nb Intensity (arb.units) Chainani et.al., PRL, 85(2000)1966 Nb T c =9.26K He Iα (21.218eV) (SES2002) 5.8K 12K Intensity (arb. units) clean surface(scraped) scraped Nb T c =9.26K Laser (6.994eV) (R4000) 3.5K 10K 20 Binding energy (mev) E F We can measure superconducting gap without clean surface Intensity (arb. units) Nb non-scraped T c =9.26K Laser (6.994eV) 20 Binding energy (mev) (R4000) 3.5K 10K E F dirty surface(non scraped) 20 Binding energy (mev) E F

11 Another example Without surface cleaning, we can measure superconducting gaps of new materials whose crystal size is quite tiny Intensity (arb. units) KOs2O6 single crystal non-fructure(as grown) 10K 8.0K 7.0K 6.5K 6.0K 5.7K 5.3K 3.5K Hiroi Group(2003) Geometrically Frustrated Pyrochlore Unconvetional superconductor? As-grown surface! Binding Energy (mev)

12 Laser-PES is the most bulk sensitive PES New VUV laser PES Hundred SPring-8 Electron escape depth VUV laser VUV-SX SR ring He Iα Hard X-ray PES Electron kinetic energy (ev)

13 CeRu 2 1. Valence fluctuation (T K >1000K Ce valence ~ Superconductor that has the highest Tc =6.2K in Ce compounds Experiment 2 (0)/k B T C Symmetry Break-junction 1) s-wave NMR 2) 4 s-wave(anisotropic) Specific Heat 3) 3.7 s-wave NMR 4) 3.7 s-wave (large anisotropy) STS 5) s-wave Specific Heat 6) - ine line node(largest anisotropy) 1) T. Ekino et al., Phys. Rev. B 56(1997)7851 2) K. Matsuda et al., J. Phys. Soc. Jpn. 64(1995)2750 3) M. Hedo et al., J. Phys. Soc. Jpn. 67(1998)272 4) H. Mukuda et al., J. Phys. Soc. Jpn. 67(1998)2101 5) J.Mag.Mag.Mater 47-48(1985)542. Phys. Rev.B56(1997)7851. J. Phys. Soc. Jpn 69(2000)1970. Low Temp.Phys.27(2001)613. 6) J. G. Sereni et al., Mod. Phys.Lett. 3(1989)1225

14 Valence band spectrum of CeRu 2 SX Bulk Surface A. Sekiyama et al., Solid State Commun. 121(2002) eV peak Intensity (arb.units) CeRu 2 T=10K hν=6.994ev LASER Bulk sensitive Bulk sensitive in high energy region E F Binding energy (ev) Bulk sensitive in low energy region

15 Superconducting gap of CeRu 2 T c =6.2K peak CeRu 2 T c = 6.2 K (SES2002) Intensity (arb.units) 1 (R4000) T = 3.8K 8.0K gap 0 5 E F E F Binding energy (mev) Superconducting gap was clearly observed by laser-pes Kiss et al., PRL 94 (2005)57001

16 Anisotropic Superconducting gap of CeRu 2 Temperature dependence CeRu 2 T C =6.2K Intensity (arb.units) data at 3.8K Fit ( max, min, Γ ) = (1.12, 0.50, 0.07) (0.98, 0.98, 0.13) (1.18, 0.00, 0.001) / (0) Intensity (arb.units) E F Binding Energy (mev) data BCS 2 (0)/k B T C =3.67 T= 3.5K 4.0K 5.0K 6.0K 7.0K 2 1 E F Binding energy (mev) T/T C Kiss et al., PRL 94 (2005)57001

17 Superconducting gap of organic superconductor -(ET) 2 Cu[N(CN) 2 ]Br, -(ET) 2 Cu (SCN) 2 d-wave superconductor? Sekiyama et al., Phys. Rev. B 56 (1997) R. Liu et al., Phys. Rev. B 51 (1995) Previous works No intensity a E F They look like insulators All the ET-materials have the similar spectra and are inconsistent with the band calculation Why? Surface effect? or Electron correlation effect?

18 Comparison with the band calculation -(ET) 2 Cu[N(CN) 2 ]Br, -(ET) 2 Cu (SCN) 2 Intensity (arb. units) T = 3.5K Laser Laser-PES hν = 6.994eV 1.5 κ-(bedt-ttf) 2 Cu(SCN) 2 κ-(bedt-ttf) 2 Cu[N(CN) 2 ]Br Binding energy (ev) E F 1. The band calculation has been quite important for the materials design of new organic materials 2. But bulk-sensitive PES spectra are inconsistent with the band calculation in DOS Fermi edge Band calculation -(ET) 2 Cu (SCN) 2 DOS Electron correlation effect is essential, rather than the Surface effect? Energy H.Mori, Private communication

19 Fermi edge and Superconducting gap -(ET) 2 Cu[N(CN) 2 ]Br, -(ET) 2 Cu (SCN) 2 Intensity (arb. units) T = 3.5K Laser hν = 6.994eV κ-(bedt-ttf) 2 Cu(SCN) 2 κ-(bedt-ttf) 2 Cu[N(CN) 2 ]Br 1. High resolution 2. High intensity 3. Bulk sensitivity 4. Low damage by UV light Binding energy (ev) By using strong laser light First observation of Fermi edge and superconducting gap. Inconsistent with band calculation 1. Low carrier density 2. Strong correlation effect E F Intensity (arb. units) 3.5K 12K gap fitting d-wave = 1.7 mev Γ = 0.1 mev 10 κ-(bedt-ttf) 2 Cu(SCN) 2 5 E F Binding energy (mev) T c = 10.8K Laser hν = 6.994eV E = 4.5meV

20 sub mev resolution spectra on MgB 2 by laser-pes The spectrum was measured by He lamp and Laser-PES show the two gap structure for the first time Tsuda et al., PRL87,17006(2001) Tsuda et al.

21 sub mev resolution spectra on MgB 2 by laser-pes Carbon substitution effect S Mg(B 1-x C x ) 2 π band Intensity (arb. units) L σ band Mg(B 1-X C X ) 2 x = 0% x = 2% x = 5% x = 7.5% E0 F -2-4 Binding Energy (mev) Gap size (mev) Carbon concentration strong increase of the interband coupling 2 L /k B T c ~ 4.1 Tc increase by the interband coupling Carbon concentration (%) L S T c * T c (K)

22 contents 1. Development of Laser photoemission system High resolution Photoelectron analyzer Quasi-CW VUV laser Low temperature Cooling system 2. High resolution PES study on superconductors Nb ; simple metal superconductor CeRu 2 ; 4f electron superconductor with Onuki et al MgB 2 ; intermetallic compound with Tajima et al -(ET) 2 Cu[N(CN) 2 ]Br ; organic superconductor with Kanoda et al -(ET) 2 Cu (SCN) 2 3. High resolution PES study on Kondo material LiV 2 O 4 ; Kondo state in TMO with Ueda et al 4. Bulk sensitive PES in low photon energy SrVO 3 ; comparison with DMFT with Inoue et al 5. Conclusion and Future of Laser-Photoemission

23 Ultra-high resolution photoemission; Kondo peak Hüfner, Photoelectron spectroscopy The case of Ce compounds The Kondo peak becomes sharper and stronger as the temperature decreases

24 Bulk sensitive spectra using VUV laser ; Typical coherent and incoherent peaks have been observed in Sr x Ca 1-X VO 3 PES spectra DMF theory coherent incoherent coherent incoherent Inoue et al., PRL74,2539(1995) Density of states ρ( ϖ)=-im G by the d= approach at T=0 for the half-filled Hubbard model at U/t*=1, 2, 2.5, 3, and 4 from top to bottom. The calculation is done by iterative perturbation in terms of U. The bottom one (U/t*=4) is an insulator. Georges, Kotliar, Krauth, and Rozenberg, PRL1996.

25 Bulk sensitive spectra using SX Incoherent peak becomes weak in SX PES CaVO 3 and SrVO 3 spectra are similar in SX PES Bulk What happens in the bulk sensitive spectra using VUV laser? More bulk sensitive much higher resolution Photon-energy dependence of the V 3d spectral weights for Sr 1 x Ca x VO 3. The V 3d spectra are normalized by the integrated intensities of the incoherent part ranging from 0.8 to 2.6 ev. Sekiyama, PRL(2004) Surface (a) Bulk V 3d spectral functions of SrVO 3 (closed circles), Sr 0.5 Ca 0.5 VO 3 (solid line) and CaVO 3 (open squares). (b) Comparison of the experimentally obtained bulk V 3d spectral function of SrVO 3 (closed circles) to the V 3d partial density of states for SrVO 3 (dashed curve) obtained from the bandstructure calculation, which has been broadened by the experimental resolution of 140 mev. The solid curve shows the same V 3d partial density of states but the energy is scaled down by a factor of 0.6.

26 Excitation energy dependence of coherence peak SrVO 3 Surface SrVO 3 Surface Intensity (arb. units) ev 13.8 ev 9.2 ev ev CaVO 3 Bulk Surface Intensity (arb. units) ev 13.8 ev ev 13.8 ev CaVO 3 Bulk Contrary to the SX spectra, coherent peaks become weaker ev 13.8 ev 9.2 ev ev Bulk Binding energy (ev) d xz,yz surface Binding energy (ev) d xy bulk coherent part E F

27 The temperature dependence is well reproduced by the Surface and Bulk spectra Intensity (arb. units) SrVO 3 s=5a ev 13.7 ev 9.18 ev ev surface bulk red line: I surface (E) and I bulk (E) black line: I(E) I(E)=exp(-s/λ) I bulk (E)+[1-exp(-s/λ)] I surface (E) s : thickness of surface layer λ : mean free path Intensity (arb. units) Binding energy (ev) CaVO 3 s=5a ev 13.8 ev 9.2 ev ev surface bulk Binding energy (ev) parameters s : 5 E(eV) λ( ) d xy bulk d xz,yz surface E F coherent part

28 Summary Development of Quasi CW VUV-laser ; Highest hν of 7eV with 0.26meV line width Development of analyzer with high energy resolution at low temperature ; Highest energy resolution of 0.36 mev, Lowest temperature of 2.7K Laser PES is powerful for the study on low Tc superconductors and strongly correlated materials High resolution PES and Bulk sensitive Fermiology CeRu 2 ; 4f electron system, gap anisotropy MgB 2 ; multigap, interband interactions -(ET) 2 Cu (SCN) 2 ; d-wave superconductor LiV 2 O 4 ; Kondo state in TMO SrVO 3 ; comparison with DMFT(Surface and bulk) Laser-PES has solved the weaknesses of PES Low resolution High resolution Surface sensitive Bulk sensitive High temperature Low temperature Large spot size Small spot size