L YBCO è di MODA! U. Scotti di Uccio

Transcription

1 L YBCO è di MODA! U. Scotti di Uccio Coherentia-CNR-INFM Napoli, Italy Prof. R. Vaglio F. Miletto Granozio, M. Radovic, N. Lampis, P. Perna, A. Sambri M. Salluzzo, G. De Luca, R. Di Capua, A. Gambardella, Z. Radovic L. Parlato, G. Peluso, G. Pepe L. Marrucci, D. Paparo, A. Rubano

2 Motivation a few sentences from Nature Mat A central challenge in the physics of high-temperature superconductors is to understand superconductivity within a single copper oxide layer or bilayer Despite twenty years of research, the phase diagram of high transition temperature superconductors remains enigmatic? Carefully controlled interfaces between two materials can give rise to novel physical phenomena and functionalities not exhibited by either of the constituent materials alone Samples with unprecedented quality of bulk and surface are required

3 Coherentia Labs. Napoli UHV base P < mbar Modular system for Oxide Deposition and Analyses PLD chamber AFM STM SPA-LEED UPS-XPS RHEED

4 PLD chamber Base pressure P = 5 x 10-9 mbar Radiative heater T max = 1000 C in air Quadrupole Residual gas determination (0,-2) (0,-1) (0,0) (0,1) (0,2) High pressure RHEED Fast photography setup for analysis of the plume expansion

5 YBCO films deposition conditions This work: STO - NGO substrates Eccimer laser KrF - λ = 248 nm Target substrate 37 mm P(O 2 ) = 0.25 mbar Effective Fluency: 50 mj /2.4 mm 2 Repetition rate: 1-2 Hz Deposition temperature: 800 C Rate: 1 ML / 30 shots x10 3 rocking (005) YBCO 5.0x10 2 R [Ohm] T [K] 0.0 (001) NGO θ

6 The role of the terminating layer of the substrate ABO 3 The situation of YBCO is more complex B A B Presumably BaO CuO 2 CuO BaO Simple perovskite (i.e., LSMO) CuO 2 Y CuO 2 Y (La, Sr)O MnO 2 sub film MnO 2 (La,Sr)O SrO TiO 2? TiO 2 SrO? BaO CuO SrO TiO 2 sub film CuO 2 BaO TiO 2 SrO A site term. B site term. A site term. B site term.

7 YBCO grown on B-site terminated SrTiO 3 SrTiO 3 has good crystal quality Etching + annealing treatment results in single B-site terminated surfaces YBCO growth under tensile stress (a ~ nm) The substrate Ti O 2 Sr O (0,-2) (0,-1) (0,0) Non polar surface 0.4 nm 1 1 µm AFM image RHEED TiO 2 plane (0,2) (0,1)

8 YBCO grown on A-site terminated NdGaO 3 NdGaO 3 has very good crystal quality Annealing treatment results in single A-site terminated surfaces: 20h at 1200 in 1 atm O 2 Almost perfect lattice match with YBCO (a PC ~ nm) Slightly orthorhombic (reduces twinnig) Polar surface + - A B The substrate Nd Ga O µm AFM image RHEED crystal structure NdO plane

9 How to grow YBCO on A-site terminated SrTiO 3? STO template on NGO YBCO STO NGO STO on A side terminated (110) NGO RHEED Intensity (a.u.) STOP time (s) START STO deposition

10 A-site vs. B-site terminated SrTiO 3 Variable temperature AFM/STM Non contact UHV AFM 2 2 µm 2 Spot Profile Analysis LEED 1 1 µm 2 STO crystal TiO 2 termination (1,1) (1,0) P(2x1) STO film SrO termination (0,1) (0,0) (0,1/2)

11 Checking the termination: XPS measurements TiO 2 SrO source γ e - γ e - Normalized Intensity emission (001) STO single crystal TiO 2 terminated STO om (110) NGO Sr 3d line Binding Energy (ev) 0.0 Sample holder detector

12 The YBCO growth: RHEED oscillations RHEED STO LEED YBCO 40 RHEED Intensity (a.u.) shots RHEED YBCO

13 The YBCO growth: RHEED oscillations Background subtraction RHEED Intensity (a. u.) YBCO on (110) NGO I (a.u.) YBCO on NdO terminated NGO A Site termination shots shots RHEED Intensity (a.u.) 10 YBCO on (001) STO I (a.u.) 0 YBCO on TiO 2 terminated STO B Site termination shots shots

14 Three experiments involving YBCO

15 Grazing incidence XRD Swiss Light Source (SLS) - Paul Scherrer Institut third-generation synchrotron light source (2.4 GeV ) Philip Willmott, Christian Schlepuetz, PSI (Switzerland) MS beamline Energy range: 5 40 kev ΔE/E ~ kev DCM and mirror optics ; Focus: 0.5x0.5 mm 2 Large (2+3) circle diffractometer for heavy sample conditioning chambers Horizontal or vertical sample geometry SPEC X-ray diffraction and data acquisition

16 Recording SXRD Data (open slit) l = l = l=1.975 l = l = l = BG Signal l = l = The (21l) rod of SrTiO 3 (raw data) C.M. Schlepütz et al., Act. Cryst. A61, 418 (2005)

17 SXRD on (110) NdGaO 3 ~ images in 630 individual scans (open slit + rocking) data below recorded in open slit mode in ~ 5 shifts (40 h) results in ~ independent structure factors raw data set from on-line analysis, no correction factors

18 SXRD on (001) YBCO raw data set from on-line analysis, no correction factors

19 New materials for Superconducting Single Photon Detector L λ 2 kin 2 s l o s 2 l λ = µ d<<l o w d m c = 2 4πe n ξ Dirty limit l: length w: width d: thickness l s : electronic mean free path x o : coherence length at 0 K n s : density of superconducting electrons Previous data: Nb/NiCu 1.2 R/R Nb NiCu(6nm)/Nb NiC u(12nm )/Nb NiCu(21nm )/Nb NiCu Photoimpedance transient (mv) V (mv) NiCu 21 nm/ Nb 70 nm NiCu(21 nm)/nb(70 nm) 0.69 ns 0.69 ns V = I dl dt kin Delay Tim e (ps) -0.4 time (1 ns/div) Delay time Proximity effect phonons from recombination in Cooper pairs are lost Recovery time is lowered Proximity effect YBCO LSMO magnetic coupling Cu-Mn fundamental physics

20 LSMO/YBCO bilayer RHEED pattern P(O 2 ) > 0.1 mbar, T = 800 C a) (001) STO b) YBCO (end of deposition) c) LSMO (end of deposition) (001) STO 100 nm YBCO 10 nm LSMO STM topography SPA-LEED pattern as deposited LSMO 10, 35, 100 nm 100 nm LSMO YBCO STO

21 Preliminary results Pump-probe experiments YBCO YBCO/LSMO(10nm) YBCO/LSMO(35nm) Pump: Ti-Sapphire (λ = 800 nm, τ = 100 fs) Pump fluence: 0,05 1,5 mj Resolution delay: 100 fs R/R T = 20 K Delay time (ps)

22 Stimulated Raman Scattering Spontaneous Raman scattering ω 4 = ω1 2 ω + ( ω ) 3 Stimulated Raman scattering ω 1 ω 3 ω 2 4WM experiment

23 Motivation Raman response cubic law ω 3 0,0 0,5 1,0 1,5 Raman shift (ω/2 ) Raman response linear law Phys. Rev. B 69, (2004) 0,0 0,5 1,0 1,5 Raman shift (ω/2 ) Raman response unscreened 0,0 0,5 1,0 1,5 Raman shift (ω/2 ) χ screened depends crucially on higher vertex harmonics

24 SRS experimental setup Cooled sample

25 SRS preliminary results CARS O4 A1g O2,3 A1g Cu1 A1g O2,3 B1g O2,3 B1g Ba A1g Cu1 A1g Ba A1g O2,3 A1g O4 A1g CSRS Ba A 1g 110 cm -1 Cu1 A 1g 148 cm-1 O23 A 1g 337 cm-1 O23 B 1g 440 cm-1 O4 A 1g 505 cm-1

26 Conclusions YBCO growth: YBCO structure: full substrate control required to achieve superior quality XRD at the PSI synchrotron facility YBCO/LSMO bilayers: Stimulated Raman: non equilibrium dynamics a novel technique to provide fundamental information