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This research focuses on exploring atomic-scale electronic and structural properties of oxide films using scanning probe microscopy techniques. It examines charge order melting in strained thin films and the melting of the vortex lattice in superconductors. The goal is to understand the behavior and potential applications of these materials.
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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)
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
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)
b Tilting due to tolerance Mn (RE, Ca ) t < 1 : c a ABO3 structure : orthorhombic Pnma = ‘3-tilt’ ; (ap2, 2ap, ap2 ) • Octahedra buckle, smaller Vcell • Decreased Mn-O-Mn bond angle, narrower eg bandwidth, less hopping, lower TIM
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
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)
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 ?
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 ?
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
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
… 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 ?
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
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*
[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
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
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
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.
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
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.
Include disorderpinning glass thermal fluctuationsmelting Current general vortex matter (B,T) phase diagram A-lattice Ideal
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
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.
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
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
Image processing Convolution with pattern of: “single vortex”: Unit cell 3x3: 4.30 K 1.75 T 4.44 K 4.53 K
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.
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
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.
Au~5 nm Mo3Ge 50 nm Si substrate a-Mo3Ge + Au AFM – no Au islands Use proximity effect signal weak, ‘spectroscopy mode’
Optimized settings a-Mo2.7Ge, B = 0.8 T, d = 48 nm, 1.1 mm2 ACF 2D-FFT
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
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
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 ?
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
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
Lang - Nature ‘02 Hoogenboom - Phys. C ‘03 Homogeneous (for optimal doping) Different for different doping - variations in gap spectra / gap width Disorder in BSCCO
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
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
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
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
At 300 K, ‘some terraces’ with atomic resolution At 146 K, doubled (a02) 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 …)
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)
Si(111)- (7x7) Giessibl, Science ‘00 Single adatom Calculation for z = 285 pm AFM – ‘sub’-atomic resolution
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
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 !