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Electron Spectroscopy Group at Physics Department, BNL / Peter Johnson, Alexei Fedorov, Tonica Valla

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Electron Spectroscopy Group at Physics Department, BNL / Peter Johnson, Alexei Fedorov, Tonica Valla Electron Spectroscopy Group at Physics Department, BNL / Peter Johnson, Alexei Fedorov, Tonica Valla/ Angle resolved photoemission: 9 High temperature superconductors /Bi2Sr2CaCu2O8 ,Sr2RuO4 / 9 Low-dimensional conductors /CDW, non-Fermi liquid behavior/ 9 Two-dimensional conductors /surface states, 2H-TaSe2 / 9 Amorphous films /search for the Coulomb gap/ Spin-polarized photoemission: 9 Micro-Mott detector connected to the Scienta analyzer /surface states in Gd(0001)/ High resolution Angle Resolved Photoemission studies of Charge Density Wave materials: quasi-one-dimensional Blue Bronze K0.3MoO3 and two-dimensional dichalcogenide 2H-TaSe2 Alexei Fedorov Physics Department, Brookhaven National Laboratory Supported by the Department of Energy DE-AC02-98CH10886 and DE-FG02-98ER24680 Physics Department, BNL: Alexei Fedorov Tonica Valla Peter Johnson V.N. Muthukumar theory Sergei Brazovskii Physics Department, Boston University: Jinyu Xue Laurent Duda Kevin Smith Chemistry Department, Rutgers University: Martha Greenblatt William McCarrol NSLS, BNL: Steven Hulbert Cornell University: F.J. DiSalvo Why are we interested in high energy and momentum resolution? What are the goals? A. Nesting properties of the Fermi surfaces /Charge density waves/ B. Photoemission spectral functions A(k,ω) /direct comparison with theoretical predictions/ Outline Experimental details: 9 Angle resolved photoemission 9 Photoelectron spectrometer K0.3MoO3 : 9 Crystal structure 9 Electronic structure 9 Structural studies /Charge Density Waves/ 9 Band structure of K0.3MoO3 9 Fermi wave vectors versus temperature 9 Incommensurate to commensurate CDW transition 9 Signatures of non-Fermi liquid behavior 2H-TaSe2: 9 CDW, Nesting: “conventional” vs. “saddle point” 9 Band mapping 9 Coupling of quasiparticles to collective excitations 9 Is 2H-TaSe2 similar to the HTSC? Angle Rsolved Photoemission /band structure mapping/ km=××××−−sinΘΦ2 η2 (hEν ) Experiment // e binding Data Energy Distribution Curves Excitation Radiation (photocurrent vs. kinetic energy) • photon energy measured at certain emission angle • polarization • angle of incidence n Photoelectrons s e • kinetic energy • emission angle p φ • polarization e hν Θ e sin gle cr hole ysta ple l sam hν Important parameters: Energy resolution (~20 meV) Angular resolution (~2º ) Electron momentum (Å-1) Surface State in Cu(011) mapped with ARPES /S. Kevan, PRB 28, 4822 (1983)/ New Instrumentation /multi-channel detection in emission angle and kinetic energy/ Wide-band Energy Analyzer 2-Dimensional (Energy and Angle) high-resolution electron detector Magnifying high-transmission Excitation imaging Electron Lens radiation hν Photoelectrons Sample Photoelectron Spectrometer SES-200: 200 millimeters hemispherical deflector capable of multichannel detection in emission angle and kinetic energy Example of angle resolved data: hν = 21.22 eV /He I radiation/ Cu(111), bulk bands and sp-surface state Emission angle 9 Energy resolution ~ 10 meV 4.0 2.0 E o F 9 Angle resolution ~ 0.2 Binding Energy (eV) 9 Base pressure ~ 2× 10-11 Torr Presently located at the undulator beamline U13UB at the National Synchrotron Light Source Why are we interested in Low-dimensional materials? • Charge Density Waves (CDW) /Peierls transitions/ • Electron correlation effects /non-Fermi liquid behavior, spin-charge separation, HTSC/ K0.3 MoO3 TCDW chains anisotropy J.-P. Pouget et al., J. Physique Lett. 44, L113 (1973) Charge Density Waves / Density Waves in Solids, G. Grüner, Addison Wesley, 1994 / Excitation gap Electronic structure of K0.3MoO3 /tight-binding calculations/ M.-H. Whangbo and L.F. Schneemeyer, Inor. Chem. 25,2424 (1986) Two bands crossing the Fermi level How many Charge Density Waves? Structural studies of CDW in K0.3MoO3 /Single Charge Density Wave/ A. Diffuse X-ray scattering B. Temperature dependent neutron scattering × /qCDW = 2kF b / /incommensurate to commensurate transition/ J.-P. Pougetet al. M.Sato, H. Fujishita and S.Hoshito, J. Phys. C: Solid State phys., 16, L877 (1983) CDW q Temperature (K) Nesting: Fermi surface of the first band is nested to the Fermi surface CDW wave vector of the second band qCDW : kF1+kF2 Temperature dependence of CDW wave vector: ◊ Thermally activated charge transfer between bands crossing the Fermi level and third band above it /Pouget et al./ ◊ Shift of the chemical potential /Pouget & Nougera, Artemenko et al./ ◊ Hidden temperature dependence of the nesting vector /Intention of the present study/ Goals of photoemission experiment: ◊ Direct monitoring kF1 and kF2 ◊ Temperature dependence of (kF1+kF2) Direct monitoring electron bands in K0.3MoO3 /3-D maps of photocurrent/ hν = 21.4 eV EF Experimental details: Samples cleaved in situ Liquid He cryostat provides 0.1 temperatures from ~20 K to ~450 K Temperature monitored with Binding Energy (eV) OMEGA CY7 sensor 0.2 0.2 0.3 0.4 Electron Momentum along ΓΧ (Å-1) Momentum Distribution Curves at EF kF2 kF1 T=300 K 200 K 180 K 150 K 120 K 100 K Intensity at the Fermi level 80 K 62 K 40 K 0.2 0.3 0.4 Electron Momentum along ΓΧ (Å-1) Incommensurate to commensurate CDW transition in K0.3MoO3 /comparing neutron scattering data with nesting vector measured in photoemission experiment/ 0.265 Present work units) × (b 0.260 CDW q Neutron scattering, ] and 1- R.M. Flemming, L.F. Schneemeyer F2 0.255 and D.E. Moncton, PRB 31,899 (1985) k + F1 k 1-[ 0.250 50 150 250 Temperature (K) Fermi surface of an array of coupled chains /tight binding calculation/ interchain t coupling Χ kF2 coupling along t// chains t reduced by 10% k coupling between F1 t⊥ chains in a unit cell Fermi surface is given by: µ = -2cos(k//) ± (t⊥ + 2t⊥ t cos(k⊥ )+t) Υ Γ Υ What are the signatures of non-Fermi liquid behavior in photoemission? Observation of two dispersing features Spin-charge separation ⇒ corresponding to the charge and spin degrees of freedom Breakdown of the ⇒ Suppression of the spectral weight quasiparticle picture at the Fermi energy k = kF ) ω A(k, ω k ω A(k,ω) ωω A(k,ω) Density ofstates k = F Spectral function in K0.3MoO3 at the Fermi wave vector T = 300 K B in d in ∆Θ = ± 0.1° g E n e r g y ( e V -1 ) Å ) tum ( omen ρ(ω) ron M Elect α ρ (ω ) ~ (ω –EF) α < 0.5 0.3 0.2 0.1 EF Binding Energy (eV) Spectral function symmetrized at kF T = 300 K Fit with Lorentzian -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 ω (eV) (a) Thermal fluctuations in Luttinger liquid (b) CDW fluctuations Suppression of spectral weight in photoemission from low-dimensional conductors: influence of momentum resolution ∆Θ = ± 0.1° ∆Θ = ± 1° k = kF 0.3 0.2 0.1 EF Binding Energy (eV) Spectral line-shapes versus electron momentum T = 300 K J. Voit /spin gap/ KΓΧ=KF N. Nakamura & Y. Suzumura /temperature effect/ KΓΧ=0 1.25 1.00 0.75 0.50 0.25 EF Binding Energy (eV) Spectral line-shapes versus temperature (a) (b) ρ σ EDC`s MDC`s o ∆ Θ = + 0.1 ∆ E = + 7 meV K = 0.83K ΓΧ F EB = 620 meV 297 K 297 K 35 K K = 0 ΓΧ 35 K 0.8 0.6 0.4 0.2 E 0.4 0.3 0.2 0.1 F -1 Binding Energy (eV) Electron Momentum KΓΧ (A ) Gap opening at the 150 K Fermi wave vector 40 20 200 K Gap (meV) 0 0.2 0.6 1.0 k = kF T/TCDW 0.2 EF Binding Energy (eV) K0.3MoO3 , Summary: Using ARP with high momentum reslution we have directly measured two Fermi wave vectors in quasi one-dimensional K0.3MoO3. By monitoring the temperature dependence of the Fermi wave vectors from 300 to 40 K, it was possible for the first time to compare the temperature dependence of the Fermi wave vectors and the CDW nesting vector. Our results show unambiguously that the temperature dependence of the CDW nesting vector observed in X-ray and neutron scattering experiments is primarily due to a change in the electronic structure. We have demonstrated that the suppression of spectral weight at EF seen in many one dimensional conductors could be an artifact of poor momentum resolution. The latter mimics effects arising from strong electron correlation. Hence, good momentum resolution is a sine qua non for studies of non Fermi liquid behavior. In the specific instance of K0.3MoO3, we did not find any suppression of intensity in our measurements of the spectral function. To our knowledge, this is the first time a Fermi edge has been observed in a quasi 1-D CDW material. Our results show that one cannot conclude about the presence of a pseudogap in this material, solely on the basis of the observed spectral intensity near EF. This issue can only be resolved by a detailed study of the photoemission lineshapes above and below the Peierls transition. 2H-TaSe2: Motivations and Questions 9 CDW coexists with superconductivity: TCDW ~ 122 K ; TSC ~ 0.15 K 9 What drives the CDW transition: “Conventional” Fermi surface nesting or “saddle point” nesting? 9 CDW does not remove the entire Fermi surface: What happens to the excitations at the Fermi energy in a presence of the CDW gap? 2H Crystal structure /D.E. Moncton, J.D. Axe, and F.J. DiSalvo, PRB 16, 801(1977)/ Neutron scattering experiment /superlattice due to the Charge Density Wave/ D.E. Moncton, J.D. Axe, and F.J. DiSalvo, PRL 34, 734 (1975) Incommensurate CDW: 122.3 K ~90 K: CDW locks to 3a superlattice CDW wave vector: qδ = 4π {1-δ(T)}/a√ 3 Nesting A. Fermi surface nesting B. “Saddle point” nesting q δ qδ J.A. Wilson, PRB 15, 5748 (1977) T.M. Rice and G.K. Scott, G. Wexler and A.M. Wooley, J. Phys.
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