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Oxide films and scanning probes J. Aarts , Kamerlingh Onnes Laboratory, Leiden University. Wanted atomic scale electronic / structure properties ( local sc gap, stripes, phase separation, charge order). Problem STM : not for insulators ; AFM : no atomic resolution
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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)
Z.Q. Yang, A. Troyanovski, G.-J. v. Baarle Leiden
M. Y. Wu, Y. Qin, H. W. Zandbergen HREM center, Delft
Tilting due to tolerance
(RE, Ca )
t < 1 :
ABO3 structure : orthorhombic Pnma = ‘3-tilt’ ; (ap2, 2ap, ap2 )
Charge + Orbital order : ‘CE’ – type, zig-zags
could have been different :
Pr0.5Ca0.5MnO3, bulk properties
Phase diagram, Pr1-xCaxMnO3
Question for strained films :
Tcoenhanced by the applied distortion?
or destabilised by ‘clamping’?
Jirak et. al., PR B61 (2000)
‘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
x < 0.5 : CO less stable; lower fields and ‘reentrant’.
Strained films : different melting behavior ?
Growth : magnetron; no post-anneal, Ts = 840 oC, 3 mbar oxygen
Lattice parameter versus thickness
relaxation slow ( > 150 nm)
suggests disorder at large thickness ?
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
but what about Tco ?
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
What about melting of charge order and stripes ?
Formation of dislocations ?
Another (model) system for STM : the vortex lattice
Type II : <<
NbSe2 8 nm 265 nm
a-Mo3Ge 5 nm 750 nm
YBCO 2 nm 180 nm
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,
far from critical field Bc2.
‘decoration’ of NbSe2 at 3.6 mT and 4.2 K.
a = 0.8 m.
Current general vortex matter (B,T) phase diagram
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
Peak : close to Bc2 a strong peak occurs in the critical current – which indicates when vortices start to move under a driving force.
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
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
Convolution with pattern of:
di= ri,n -ri,n+1, dk= rk,n- rk,n+1
ri= position, n = framenumber
‘order prm’ :
Motion becomes uncorrelated at 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
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.
a-Mo70Ge30 Tc = 7 K ; can be sputtered but oxidizes; protect with Au, continuous layer.
a-Mo3Ge + Au
AFM – no Au islands
Use proximity effect
(NbN + a-Mo3Ge + Au)
Mo3Ge 24 nm
vortex positions are of the strongest pinner : NbN
NbN 50 nm
Coordination number (z):
36% has z ≠ 6
full positional disorder
Final result : triangular – to – square VL transition in a thin film sandwich La1.85Sr0.15CuO4 + MoGe + Au
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
Note the differences in possible types of experiments between smooth and rough surfaces
So what about oxides ?
Pan - Nature ‘00
d-wave sc; a relative success story
good metal, atomically flat surface (cleavage)
+ Zn - impurities
At 146 K, doubled (a02) unit cell along 
Two different atomic distancesc. Bi0.24Ca0.76MnO3
Image charge order
- Renner, Nature ‘02
TCO = 250 K
Mn3+ : Mn4+ = 1 : 3
Rotated octahedra ?
Surface reconstructs ?
Mn3+ : Mn4+ = 1 : 1
Many insulating parts
General problem : a mixture of insulating and metallic parts makes STM difficult (… tip crashes …)
Combined AFM / STM - ideal for badly conducting surfaces
Properties of CO/OO PCMO films = Strain + disorder !