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
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
1.Pr0.5Ca0.5MnO3: amodel system for charge order (melting)
Tilting due to tolerance
(RE, Ca )
t < 1 :
ABO3 structure : orthorhombic Pnma = ‘3-tilt’ ; (ap2, 2ap, ap2 )
At Mn3+ - Mn4+ = 1 : 1
Charge + Orbital order : ‘CE’ – type, zig-zags
could have been different :
Pr0.5Ca0.5MnO3, bulk properties
Phase diagram, Pr1-xCaxMnO3
Basic properties of Pr0.5Ca0.5MnO3
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 ?
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)
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
Observe CO and OO by HREM
80 nm PCMO on STO
(at 95 K)
View along b-axis,
spot at (1/2 0 0) evidence for OO
at 300 K
View along c-axis,
spot at (100) evidence for CO at room temperature
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
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.
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
Convolution with pattern of:
A movie of the processed data. Note T 4.47K
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.
Au frames~5 nm Mo3Ge 50 nm
a-Mo3Ge + Au
AFM – no Au islands
Use proximity effect
Optimized settings framesa-Mo2.7Ge, B = 0.8 T, d = 48 nm, 1.1 mm2
Also for NbN, a framesmuch stronger pinner.
(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
Note the differences in possible types of experiments between smooth and rough surfaces
So what about oxides ?
150 observed close toǺ
Pan - Nature ‘00
d-wave sc; a relative success story
good metal, atomically flat surface (cleavage)
+ Zn - impurities
Lang - Nature ‘02 observed close to
Hoogenboom - Phys. C ‘03
Homogeneous (for optimal doping)
Different for different doping
- variations in gap spectra / gap widthDisorder in BSCCO
Direct space, 7 T observed close to
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 surfaceFourier Transform STS - stripes
Single Xtal STM topography observed close to
Local STM spectroscopy
Different I-V characteristics
CMR and the issue of phase separationb. La0.7Ca0.3MnO3
Small scales observed close to
0 , 0.3 T
1 , 3 T
5 , 9 TSpectroscopy on LCMO
LCMO / YBCO film, 50 K black ‘=‘ metal’
LSMO thin film observed close to, T-dependence
black ‘=‘ metal’
Becker, PRL ‘02Spectroscopy on LCMO - cont
At observed close to300 K, ‘some terraces’ with atomic resolution
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 …)
Measure observed close toΔf at constant amplitude
noise spectrum. Ampl = 1.5 pmd. 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) observed close to
Giessibl, Science ‘00
Calculation for z = 285 pmAFM – ‘sub’-atomic resolution
Finally, the tuning fork tip can also be used in STM-mode observed close to
Combined AFM / STM - ideal for badly conducting surfaces
Properties of CO/OO PCMO films = Strain + disorder !