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
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Oxide films and scanning probes J. Aarts , Kamerlingh Onnes Laboratory, Leiden University
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 - PowerPoint PPT Presentation
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Oxide films and scanning probesJ. Aarts, Kamerlingh Onnes Laboratory, Leiden University
• 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.
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 : <<
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
B 507.1 0a
1.5 m for B = 1 mT
49 nm for 1 T
‘decoration’ of NbSe2 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 Bc2.
STM is the best / only probe at high magnetic fields.
Current general vortex matter (B,T) phase diagram
Ideal
A-lattice
Include disorder pinning glassthermal fluctuations melting
Technique ( since H. Hess, 1989) : map current in the gap ( 0.5 mV).
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
NbSe2 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
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
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 10-5 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
i k
i k
d dK
d d
‘order prm’ : di= ri,n -ri,n+1, dk= rk,n- rk,n+1
ri = position, n = framenumber
Motion becomes uncorrelated at Tp1.
Above Tp1
Average 70 subsequent images in T-regime 4.50 K – 4.55 K
Brightness indicates probabilityof finding a vortex at a certain position :
Some vortices are strongly pinned
The picture : at Tp1, individual pinning wins from elasticity, mainly shear modulus :
2 2
66 ( ) (1 0.3 )(1 )cc B t b b b
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 m2
ACF2D-FFT
Au ~5 nmMo3Ge
24 nmNbN 50 nm
Si substrate
Also for NbN, a much stronger pinner.(NbN + a-Mo3Ge + 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 La1.85Sr0.15CuO4 + MoGe + Au
B = 0.3 T B = 0.7 T
LSCO-film : Moschalkov (Leuven)
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
• 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. Bi2Sr2CaCu2O8-δ superconductorsuperconducting gap, impurity resonances, stripesatomic resolution, discussion about disorder also YBa2Cu3O7-δ , Sr2RuO4
b. La0.7Ca0.3MnO3 CMR materialphase separation, local spectroscopyno atomic resolution
c. Bi0.24Ca0.76MnO3 Charge Order atomic resolution, but not a conclusive experiment