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

Outline 1.A nice model system : Charge order (melting) in strained thin films of Pr 0.5 Ca 0.5 MnO 3 together with Z.Q. Yang, A. Troyanovski, G.-J. v. Baarle Leiden M. Y. Wu, Y. Qin, H. W. Zandbergen HREM center, Delft 2.How STM can work(an intermezzo) Melting of the vortex lattice in a superconductor (NbSe 2 ) 3.A roadmap for SPM on oxides Current status, future prospects

1. Pr 0.5 Ca 0.5 MnO 3 : a model 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 O 2 pressure = slow growth (  1 nm / min ) on SrTiO 3 (a 0 = nm vs nm for PCMO)

a c b (RE, Ca ) Mn ABO 3 structure : orthorhombic Pnma = ‘3-tilt’ ; (a p  2, 2a p, a p  2 ) Octahedra buckle, smaller V cell Decreased Mn-O-Mn bond angle, narrower e g bandwidth, less hopping, lower T IM Tilting due to tolerance t < 1 :

Pr 0.5 Ca 0.5 MnO 3, bulk properties Phase diagram, Pr 1-x Ca x MnO 3 Insulating Charge + Orbital order : ‘CE’ – type, zig-zags At Mn 3+ - Mn 4+ = 1 : 1 could have been different : c a

Basic properties of Pr 0.5 Ca 0.5 MnO 3 Jirak et. al., PR B61 (2000) R(T) : ‘insulating’, with small jump at T CO = T OO  (T) : peak at T CO, not at T AF lattice parameters : = nm, orthorhombic distortion at T co = T OO Staggered M : onset at T AF Question for strained films : T co enhanced by the applied distortion? or destabilised by ‘clamping’?

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

Pr 0.5 Ca 0.5 MnO 3 ( = nm) on SrTiO 3 (a 0 = nm) Growth : magnetron; no post-anneal, T s = 840 o C, 3 mbar oxygen Lattice parameter versus thickness relaxation slow ( > 150 nm) bulk suggests disorder at large thickness ?

PCMO on STO STO PCMO b 80-nm film on STO at RT: clearly visible 2 a p fringes – doubling of the b-axis; b-axis oriented no remarkable defects/disorder

Transport and magnetization 150 nm 80nm R(T) 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

… 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 T co ?

Intermezzo – why re-entrance ? Khomskii, Physica B 280, 325 (’00) 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. which is reasonable away from Mn 3+ / Mn 4+ = 1 : 1

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

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

T CO,OO vs. film thickness tensile strain increases T CO/OO to above room temperature relaxation decreases melting fields SrTiO 3 – + 2.5% NdGaO 3 – + 1.3% (Sr,La)GaO 4 – %

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

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 NbSe 2. Thin films : work in air by passivation. lattices in weakly pinning a-Mo 70 Ge 30 versus strongly pinning NbN. 2. Melting of the vortex lattice in a superconductor by STM

Superconductivity elementaries vortex core:  is ‘normal’ : no gap in DOS in radius . magnetic field distribution over radius. Type II :  << NbSe 2 8 nm265 nm a-Mo 3 Ge 5 nm750 nm YBCO 2 nm180 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  mfor B = 1 mT 49 nmfor 1 T ‘decoration’ of NbSe 2 at 3.6 mT and 4.2 K. a  = 0.8  m. Magnetic field probes (Bitter-decoration, magneto-optics, scanning SQUID / Hall ) only work well when a  < - typically mT – range, interactions small, far from critical field B c2.  STM is the best / only probe at high magnetic fields.

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

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

NbSe 2 – what can be new : vortices in the peak effect. Peak : close to B c2 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 T c / B c2 the signal disappears : B = 2 T, T = K

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

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

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

Analysis : determine correlations in vortex motion between frames ‘order prm’ : d i = r i,n -r i,n+1, d k = r k,n - r k,n+1 r i = position, n = framenumber Motion becomes uncorrelated at T p1.

Above T p1 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 T p1, 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-Mo 70 Ge 30 T c = 7 K ; can be sputtered but oxidizes; protect with Au, continuous layer.

Au ~5 nm Mo 3 Ge 50 nm Si substrate a-Mo 3 Ge + Au AFM – no Au islands Use proximity effect signal weak, ‘spectroscopy mode’

Optimized settings a-Mo 2.7 Ge, B = 0.8 T, d = 48 nm, 1.1  m 2 ACF 2D-FFT

Au ~5 nmMo 3 Ge 24 nm NbN 50 nm Si substrate Also for NbN, a much stronger pinner. (NbN + a-Mo 3 Ge + Au) vortex positions are of the strongest pinner : NbN 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 La 1.85 Sr 0.15 CuO 4 + MoGe + Au  B = 0.3 TB = 0.7 T LSCO-film : Moschalkov (Leuven) The transition is due to the high-T c LSCO : neutrons, Gilardi e.a., PRL ‘02

A solid – to – pinned – liquid transition was observed close to the upper critical field in NbSe 2. Thin films can be passivated (and structured). Disorder / defects can be studied, as shown with a-Mo 3 Ge and NbN 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 ? Note the differences in possible types of experiments between smooth and rough surfaces

3.A roadmap for the oxides What has been done by STM : a.Bi 2 Sr 2 CaCu 2 O 8-δ superconductor superconducting gap, impurity resonances, stripes atomic resolution, discussion about disorder also YBa 2 Cu 3 O 7-δ, Sr 2 RuO 4 b.La 0.7 Ca 0.3 MnO 3 CMR material phase separation, local spectroscopy no atomic resolution c.Bi 0.24 Ca 0.76 MnO 3 Charge Order atomic resolution, but not a conclusive experiment What has been done by AFM : d. Si(111) semiconductor (sub-) atomic resolution

a. Bi 2 Sr 2 CaCu 2 O 8-δ + Zn - impurities Pan - Nature ‘ Ǻ ZB – anomaly strong scattering along gap nodes d-wave sc; a relative success story good metal, atomically flat surface (cleavage) ZB map

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

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

b. La 0.7 Ca 0.3 MnO 3 CMR and the issue of phase separation CMR MR Single Xtal STM topography Local STM spectroscopy Different I-V characteristics M. Fäth Leiden

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

Spectroscopy on LCMO - cont LSMO thin film, T-dependence black ‘=‘ metal’ Becker, PRL ‘02 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

c. Bi 0.24 Ca 0.76 MnO 3 Image charge order Bulk T CO = 250 K Mn 3+ : Mn 4+ = 1 : 3 - Renner, Nature ‘02 At 300 K, ‘some terraces’ with atomic resolution At 146 K, doubled (a 0  2) unit cell along [101] Two different atomic distances Surface Rotated octahedra ? Surface reconstructs ? Mn 3+ : Mn 4+ = 1 : 1 Many insulating parts not conclusive General problem : a mixture of insulating and metallic parts makes STM difficult (… tip crashes …)

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) noise spectrum. Ampl = 1.5 pm Measure Δf at constant amplitude

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

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 k 1, Tco, H c + ; Disorder weakens! Properties of CO/OO PCMO films = Strain + disorder ! Strain, disorder,  T, H c +. Strain helps!