Atomic-scale Electronic and Structural Properties of Oxide Films and Scanning Probes

1. Oxide films and scanning probes J. Aarts, Kamerlingh Onnes Laboratory, Leiden University Wantedatomicscale electronic / structure properties (local sc gap, stripes, phase separation, charge order). Problem STM : not forinsulators ; AFM : no atomic resolution and always : clean sample surfaces …problems not solved …(today)

2. Outline • A nice model system : • Charge order (melting) in strained thin films of Pr0.5Ca0.5MnO3 • How STM can work (an intermezzo) • Melting of the vortex lattice in a superconductor (NbSe2) • A roadmap for SPM on oxides • Current status , future prospects together with Z.Q. Yang, A. Troyanovski, G.-J. v. Baarle Leiden M. Y. Wu, Y. Qin, H. W. Zandbergen HREM center, Delft

3. 1.Pr0.5Ca0.5MnO3: amodel system for charge order (melting) • Strategy • work on thin films for flexibility (and ‘applications’) • (difficulty : sample surface – no cleavage available) • use strain to vary properties • Fabrication • sputtered at 840 °C • high O2 pressure = slow growth ( 1 nm / min ) • on SrTiO3 (a0 = 0.391 nm vs. 0.382 nm for PCMO)

4. b Tilting due to tolerance Mn (RE, Ca ) t < 1 : c a ABO3 structure : orthorhombic Pnma = ‘3-tilt’ ; (ap2, 2ap, ap2 ) • Octahedra buckle, smaller Vcell • Decreased Mn-O-Mn bond angle, narrower eg bandwidth, less hopping, lower TIM

5. At Mn3+ - Mn4+ = 1 : 1 Charge + Orbital order : ‘CE’ – type, zig-zags c Insulating a could have been different : Pr0.5Ca0.5MnO3, bulk properties Phase diagram, Pr1-xCaxMnO3

6. Basic properties of Pr0.5Ca0.5MnO3 • R(T) : ‘insulating’, with small • jump at TCO= TOO • (T) : peak at TCO, not at TAF • lattice parameters : • < a0 > = 0.382 nm, • orthorhombic distortion at Tco= TOO • Staggered M : onset at TAF Question for strained films : Tcoenhanced by the applied distortion? or destabilised by ‘clamping’? Jirak et. al., PR B61 (2000)

7. Hc- Hc+ ‘Melting’ of CO by aligning Mn-core spins with a magnetic field : 1st order transition from AF-I to FM-M x = 0.5 : needs large fields, 28 T at 5 K Cax x < 0.5 : CO less stable; lower fields and ‘reentrant’. Strained films : different melting behavior ?

8. Pr0.5Ca0.5MnO3 ( <a0> = 0.382 nm) on SrTiO3 (a0 = 0.391 nm) Growth : magnetron; no post-anneal, Ts = 840 oC, 3 mbar oxygen Lattice parameter versus thickness relaxation slow ( > 150 nm) bulk suggests disorder at large thickness ?

9. PCMO on STO b • 80-nm film on STO at RT: • clearly visible 2 ap fringes –doubling of theb-axis; • b-axis oriented • no remarkable defects/disorder PCMO STO

10. R(H), 80 nm 80 nm : melting strongly hysteretic; needs 20 T at 15 K. 150 nm, melting at 15 K needs 5 T. also visible in M(H); together with FM component Transport and magnetization R(T) 80nm 150 nm

11. … which leads to the following phase diagrams • weaker CO melting with increasing thickness / relaxation • increasingly ‘reentrant’ – reminiscent of x < 0.5 •  Strain does not lead to CO-destabilization, but relaxation does but what about Tco ?

12. Intermezzo – why re-entrance ? The high-temperature phase should be the one with higher entropy (S), but it is the CO phase (lower S). Apparently : (1) the FM ground state is a Fermi liquid (S=0) and (2) the CO-state is not fully ordered. Khomskii, Physica B 280, 325 (’00) which is reasonable away from Mn3+ / Mn4+ = 1 : 1

13. TCO from resistance • no clear jump in R(T) but kink in ln(R) vs 1/T • TCO > bulk value 250 K , transition width T=TCO-T*

14. [002] [101] [200] [020] [200] Observe CO and OO by HREM 80 nm PCMO on STO [002] (at 95 K) [101] [200] View along b-axis, [010]-type superstructure spot at (1/2 0 0) evidence for OO at 300 K View along c-axis, [001]-type superstructure spot at (100) evidence for CO at room temperature

15. SrTiO3 – + 2.5% • NdGaO3 – + 1.3% • (Sr,La)GaO4 – + 0.75% TCO,OOvs. film thickness • tensile strain increases TCO/OO to above room temperature • relaxation decreases melting fields

16. PCMO thin films would be interesting for STM studies : • observe CO up to high temperatures • study melting vs. disorder in a large field range What about melting of charge order and stripes ? Formation of dislocations ? Another (model) system for STM : the vortex lattice

17. 2. Melting of the vortex lattice in a superconductor by STM • Vortex imaging : coherence length  versus penetration depth  • Vortex matter : solid – glass – liquid • related issues : elasticity, disorder, defects, vortex pinning. • dimensionality, order prm symmetry • Imaging a solid – to – (pinned) liquid transition. • the model system : single Xtal of weakly pinning NbSe2. • Thin films : work in air by passivation. • lattices in weakly pinning a-Mo70Ge30 versus strongly pinning NbN.

18. vortex core: x l Superconductivity elementaries • is ‘normal’ : • no gap in DOS in radius . • magnetic field distribution over • radius . Type II :  <<  NbSe2 8 nm 265 nm a-Mo3Ge 5 nm 750 nm YBCO 2 nm 180 nm

19. Vortex lattice elementaries A vortex contains flux 0; increasing field B leads to more vortices. Interactions then produce a triangular lattice with 1.5 m for B = 1 mT 49 nm for 1 T Magnetic field probes (Bitter-decoration, magneto-optics, scanning SQUID / Hall ) only work well when a <  - typically mT – range, interactions small, far from critical field Bc2. ‘decoration’ of NbSe2 at 3.6 mT and 4.2 K. a = 0.8 m. • STM is the best / only probe at high • magnetic fields.

20. Include disorderpinning glass thermal fluctuationsmelting Current general vortex matter (B,T) phase diagram A-lattice Ideal

21. Technique ( since H. Hess, 1989) : map current in the gap (  0.5 mV). NbSe2 is layered, passive, atomically flat (after cleaving) Ideal for constant height mode, allows fast scanning : < 1 min / frame of (1.1 m)2 NbSe2 (crystal, Tc = 7 K) STM-image, (1.1 m)2 T = 4.2 K, B = 0.9 T t = 0.6, b = 0.35 And : weakly pinning

22. NbSe2 – what can be new : vortices in the peak effect. Peak : close to Bc2 a strong peak occurs in the critical current – which indicates when vortices start to move under a driving force.

23. in 1.75 T It means that individual vortices can optimize their positions w.r.t defects, since inter-vortex elastic forces disappear – melting ? Can you ‘see’ this in the vortex lattice ? Defects ? LRO ? Not entirely trivial, close to Tc / Bc2 the signal disappears : B = 2 T, T = 4. 28 K

24. Experiment : let T drift up slowly (5 x 10-5 K/s) and measure continuously at 1 image / min (0.3 mK). Analyze the sequence of data. Typical data around T = 4.3 K, B = 1.75 T Blurring gets worse, needs data processing

25. Image processing Convolution with pattern of: “single vortex”: Unit cell 3x3: 4.30 K 1.75 T 4.44 K 4.53 K

26. A movie of the processed data. Note T 4.47K

27. Analysis : determine correlations in vortex motion between frames di= ri,n -ri,n+1, dk= rk,n- rk,n+1 ri= position, n = framenumber ‘order prm’ : Motion becomes uncorrelated at Tp1.

28. Above Tp1 Average 70 subsequent images in T-regime 4.50 K – 4.55 K Brightness indicates probability of finding a vortex at a certain position :  Some vortices are strongly pinned The picture : at Tp1, individual pinning wins from elasticity, mainly shear modulus : resulting in a pinned liquid

29. Other superconductors - thin films ? standard problem : clean and flat surface – only few crystals have been imaged; films (almost) never been used. clean : in-situ cleaning ( / cleaving) + handling in vacuum; protect with passivating layer (Au ?) . The ‘wetting’ problem. flat : after cleaving; amorphous films. • amorphous superconducting films (Nb-Ge, Mo-Ge, W-Re, V-Si, …) • are weakly pinning (no grain boundaries, precipitates … ) • have large penetration (no good with decoration) a-Mo70Ge30 Tc = 7 K ; can be sputtered but oxidizes; protect with Au, continuous layer.

30. Au~5 nm Mo3Ge 50 nm Si substrate a-Mo3Ge + Au AFM – no Au islands Use proximity effect signal weak, ‘spectroscopy mode’

31. Optimized settings a-Mo2.7Ge, B = 0.8 T, d = 48 nm, 1.1 mm2 ACF 2D-FFT

32. Also for NbN, a much stronger pinner. (NbN + a-Mo3Ge + Au) Au~5 nm Mo3Ge 24 nm vortex positions are of the strongest pinner : NbN NbN 50 nm Si substrate Coordination number (z): 36% has z ≠ 6 > 6 = 6 < 6 full positional disorder

33.   Final result : triangular – to – square VL transition in a thin film sandwich La1.85Sr0.15CuO4 + MoGe + Au LSCO-film : Moschalkov (Leuven) B = 0.3 T B = 0.7 T The transition is due to the high-Tc LSCO : neutrons, Gilardi e.a., PRL ‘02

34. A solid – to – pinned – liquid transition was observed close to • the upper critical field in NbSe2. • Thin films can be passivated (and structured). Disorder / defects • can be studied, as shown with a-Mo3Ge and NbN Note the differences in possible types of experiments between smooth and rough surfaces • STM can be an effective tool to study ordering phenomena. • Note also that for many condensed matter problems, it needs • substantial dynamic range for temperature, magnetic field and • conductance (+ bias voltage). So what about oxides ?

35. What has been done by STM : • Bi2Sr2CaCu2O8-δ superconductor • superconducting gap, impurity resonances, stripes • atomic resolution, discussion aboutdisorder • also YBa2Cu3O7-δ , Sr2RuO4 • La0.7Ca0.3MnO3 CMR material • phase separation, local spectroscopy • no atomic resolution • Bi0.24Ca0.76MnO3 Charge Order • atomic resolution, but not a conclusive experiment • A roadmap for the oxides • What has been done by AFM : • Si(111) semiconductor • (sub-)atomic resolution

36. 150 Ǻ Pan - Nature ‘00 d-wave sc; a relative success story good metal, atomically flat surface (cleavage) + Zn - impurities a. Bi2Sr2CaCu2O8-δ • ZB – anomaly • strong scattering along gap nodes ZB map

37. Lang - Nature ‘02 Hoogenboom - Phys. C ‘03 Homogeneous (for optimal doping) Different for different doping - variations in gap spectra / gap width Disorder in BSCCO

38. Direct space, 7 T FT’s at different energy Howald - PR B ‘03 Stripes through static disorder ? Hoffman - Science ‘02 Hoffman - Science ‘02 Spatial structure around cores Quasiparticle interference – maps the Fermi surface Fourier Transform STS - stripes

39. Single Xtal STM topography M. Fäth Leiden CMR Local STM spectroscopy MR Different I-V characteristics CMR and the issue of phase separation b. La0.7Ca0.3MnO3

40. Small scales topography 0 , 0.3 T dI/dV,0 T 1 , 3 T dI/dV,9 T 5 , 9 T Spectroscopy on LCMO LCMO / YBCO film, 50 K black ‘=‘ metal’ • Surface becomes more metallic with increasing field • Disorder is (probably) froozen

41. LSMO thin film, T-dependence black ‘=‘ metal’ Becker, PRL ‘02 Spectroscopy on LCMO - cont • Current picture • phase separation probably correlates with • underlying grain structure – or twin structure • no random percolation • no atomic resolution or e.g. the influence of • random scatterers such as Zn in BSCCO

42. At 300 K, ‘some terraces’ with atomic resolution At 146 K, doubled (a02) unit cell along [101] Two different atomic distances c. Bi0.24Ca0.76MnO3 Image charge order - Renner, Nature ‘02 Bulk TCO = 250 K Mn3+ : Mn4+ = 1 : 3 Surface Rotated octahedra ? Surface reconstructs ? Mn3+ : Mn4+ = 1 : 1 Many insulating parts not conclusive General problem : a mixture of insulating and metallic parts makes STM difficult (… tip crashes …)

43. Measure Δf at constant amplitude noise spectrum. Ampl = 1.5 pm d. Si(111) - a possible way out, AFM ? AFM - usually not ‘true’ atomic resolution (periodicity but not defects) new developments in frequency-modulated mode : tuning-fork AFM see : F. J. Giessibl, Rev. Mod. Phys. 75, 949 (2003)

44. Si(111)- (7x7) Giessibl, Science ‘00 Single adatom Calculation for z = 285 pm AFM – ‘sub’-atomic resolution

45. Finally, the tuning fork tip can also be used in STM-mode  Combined AFM / STM - ideal for badly conducting surfaces • In conclusion • STM has had limited success on oxide surfaces, mainly for well-behaved (super)conductors ( + cleavage surfaces) • Tuning-fork AFM / STM development is very promising

46. Competition between strain and disorder • Strain , activation energy k1 , Tco , Hc+ ; Strain helps! • Strain , disorder , T , Hc+ . Disorder weakens! Properties of CO/OO PCMO films = Strain + disorder !