Oxide films and scanning probes J. Aarts , Kamerlingh Onnes Laboratory, Leiden University
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Transcript of Oxide films and scanning probes J. Aarts , Kamerlingh Onnes Laboratory, Leiden University
Oxide films and scanning probesJ. 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 Pr0.5Ca0.5MnO3
together with
Z.Q. Yang, A. Troyanovski, G.-J. v. Baarle LeidenM. 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 (NbSe2)
3. A roadmap for SPM on oxidesCurrent status , future prospects
1. Pr0.5Ca0.5MnO3 : 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 O2 pressure = slow growth ( 1 nm / min )• on SrTiO3 (a0 = 0.391 nm vs. 0.382 nm for PCMO)
a
c
b
(RE, Ca )Mn
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
)(2 OrBr
OrAr
t
Tilting due to tolerance
t < 1 :
Pr0.5Ca0.5MnO3, bulk properties
Phase diagram, Pr1-xCaxMnO3
Insulating
Charge + Orbital order : ‘CE’ – type, zig-zags
At Mn3+ - Mn4+ = 1 : 1
could have been different :
c
a
Basic properties of Pr0.5Ca0.5MnO3
Jirak et. al., PR B61 (2000)
• 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 : Tco enhanced by the applied distortion?or destabilised by ‘clamping’?
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
0 40 80 1200.376
0.380
0.384
0.388 in-plane a, c
out-of-plane b
La
ttic
e p
ara
me
ter
( n
m )
Thickness ( nm )
Lattice parameter versus thickness
relaxation slow ( > 150 nm)
bulk
suggests disorder at large thickness ?
PCMO on STO
STO
PCMOb 80-nm film on STO at RT:
• clearly visible 2 ap fringes – doubling of the b-axis;
• b-axis oriented
• no remarkable defects/disorder
Transport and magnetization
102
104
106
108
1, 3, 5, 7, 9T
0 50 100 150 200 250 300 35010
1
103
105
107
109
9 T7 T
5 T0 T
T ( K )
0 100 200 300-0.5
-0.4
-0.3
-0.2
-0.1
0.0
250 K
H = 3 kOe
M (
10
-3
em
u )
T ( K )
150 nm
80nm
R(T)
0 5 10 15 20
0.0
5.0x106
1.0x107
1.5x107
2.0x107
2.5x107
80 nm
15 K
41 K
61 K
83 K
R (
)
B ( 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.
-6 -4 -2 0 2 4 6
-0.5
0.0
0.5 a
150nm
100K
1.8 T
M (
10
-3 e
mu
)
B ( 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 Tco ?
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 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*
0 100 200 300102
104
106
108
0.004 0.006 0.0086
8
10
12
14
Tco
=285K
80 nm PCMO/STOR
(
)
T ( K )
LnR
80 nm PCMO/STO
Tco
=285 K
T*=208 K
T -1 (K-1)
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
TCO,OO vs. film thickness
• tensile strain increases TCO/OO to above room temperature
• relaxation decreases melting fields
• SrTiO3 – + 2.5%• NdGaO3 – + 1.3%• (Sr,La)GaO4 – + 0.75%
0 40 80 120 160
200
220
240
260
280
300
320 TCO
from R(T): Open symbolsT
OO from HREM: Closed
as-grown on STO Relaxed on STO Annealed on STO On less tensile NGO, On matched SLGO as-grown on STO, Relaxed on STO
TC
O,O
O (
K)
Thickness (nm)
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 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
What has been done by AFM :
d. Si(111) semiconductor (sub-) atomic resolution
a. Bi2Sr2CaCu2O8-δ
+ Zn - impurities
Pan - Nature ‘00
150 Ǻ
• ZB – anomaly
• strong scattering along gap nodes
d-wave sc; a relative success storygood 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 ‘02Stripes through static disorder ?
Howald - PR B ‘03
b. La0.7Ca0.3MnO3CMR 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. Bi0.24Ca0.76MnO3 Image charge order
BulkTCO = 250 K
Mn3+ : Mn4+ = 1 : 3
- Renner, Nature ‘02
At 300 K, ‘some terraces’ with atomic
resolution
At 146 K, doubled (a02) unit cell along [101]
Two different atomic distances
SurfaceRotated octahedra ?Surface reconstructs ?Mn3+ : Mn4+ = 1 : 1
Many insulating partsnot 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 pmMeasure 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
Thickness(nm) k1(K) TCO(K) T(K) Hc+(T)
12 >325 >2025 1889 307 67 >18
Strained 50 1342 290 64 >15Relaxed 50 1190 277 124 4Strained 80 1331 285 77 15Relaxed 80 1095 259 79 2
120 1269 284 74 9150 1114 260 72 4150 1058 250 70 <4
• Strain , activation energy k1 , Tco , Hc+ ;
Disorder weakens!
Properties of CO/OO PCMO films = Strain + disorder !
• Strain , disorder , T , Hc+ .
Strain helps!