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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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slide1

Oxide films and scanning probes

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)

slide2

Outline

  • A nice model system :
  • Charge order (melting) in strained thin films of Pr0.5Ca0.5MnO3
  • How STM can work (an intermezzo)
  • Melting of the vortex lattice in a superconductor (NbSe2)
  • A roadmap for SPM on oxides
  • Current status , future prospects

together with

Z.Q. Yang, A. Troyanovski, G.-J. v. Baarle Leiden

M. Y. Wu, Y. Qin, H. W. Zandbergen HREM center, Delft

slide3

1.Pr0.5Ca0.5MnO3: amodel 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)
slide4

b

Tilting due to tolerance

Mn

(RE, Ca )

t < 1 :

c

a

ABO3 structure : orthorhombic Pnma = ‘3-tilt’ ; (ap2, 2ap, ap2 )

  • Octahedra buckle, smaller Vcell
  • Decreased Mn-O-Mn bond angle, narrower eg bandwidth, less hopping, lower TIM
slide5

At Mn3+ - Mn4+ = 1 : 1

Charge + Orbital order : ‘CE’ – type, zig-zags

c

Insulating

a

could have been different :

Pr0.5Ca0.5MnO3, bulk properties

Phase diagram, Pr1-xCaxMnO3

slide6

Basic properties of Pr0.5Ca0.5MnO3

  • 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 :

Tcoenhanced by the applied distortion?

or destabilised by ‘clamping’?

Jirak et. al., PR B61 (2000)

slide7

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 ?

slide8

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)

bulk

suggests disorder at large thickness ?

pcmo on sto
PCMO on STO

b

  • 80-nm film on STO at RT:
  • clearly visible 2 ap fringes –doubling of theb-axis;
  • b-axis oriented
  • no remarkable defects/disorder

PCMO

STO

slide10

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

Transport and magnetization

R(T)

80nm

150 nm

slide11

… 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 ?

slide12

Intermezzo – why re-entrance ?

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

t co from resistance
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*
observe co and oo by hrem

[002]

[101]

[200]

[020]

[200]

Observe CO and OO by HREM

80 nm PCMO on STO

[002]

(at 95 K)

[101]

[200]

View along b-axis,

[010]-type superstructure

spot at (1/2 0 0) evidence for OO

at 300 K

View along c-axis,

[001]-type superstructure

spot at (100) evidence for CO at room temperature

t co oo vs film thickness

SrTiO3 – + 2.5%

  • NdGaO3 – + 1.3%
  • (Sr,La)GaO4 – + 0.75%
TCO,OOvs. film thickness
  • tensile strain increases TCO/OO to above room temperature
  • relaxation decreases melting fields
slide16

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

2 melting of the vortex lattice in a superconductor by stm
2. Melting of the vortex lattice in a superconductor by STM
  • 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.
slide18

vortex core:

x

l

Superconductivity elementaries

  • 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

slide19

Vortex lattice elementaries

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,

interactions small,

far from critical field Bc2.

‘decoration’ of NbSe2 at 3.6 mT and 4.2 K.

a = 0.8 m.

    • STM is the best / only probe at high
  • magnetic fields.
slide20

Include

disorderpinning glass

thermal fluctuationsmelting

Current general vortex matter (B,T) phase diagram

A-lattice

Ideal

slide21

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

slide22

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.

slide23

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

slide24

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

slide25

Image processing

Convolution with pattern of:

“single vortex”:

Unit cell

3x3:

4.30 K

1.75 T

4.44 K

4.53 K

slide27

Analysis : determine correlations in vortex motion between frames

di= ri,n -ri,n+1, dk= rk,n- rk,n+1

ri= position, n = framenumber

‘order prm’ :

Motion becomes uncorrelated at Tp1.

slide28

Above 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

slide29

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.

slide30

Au~5 nm Mo3Ge 50 nm

Si substrate

a-Mo3Ge + Au

AFM – no Au islands

Use proximity effect

signal weak,

‘spectroscopy mode’

slide32

Also for NbN, a much stronger pinner.

(NbN + a-Mo3Ge + Au)

Au~5 nm

Mo3Ge 24 nm

vortex positions are of the strongest pinner : NbN

NbN 50 nm

Si substrate

Coordination number (z):

36% has z ≠ 6

> 6

= 6

< 6

full positional disorder

slide33

Final result : triangular – to – square VL transition in a thin film sandwich La1.85Sr0.15CuO4 + MoGe + Au

LSCO-film :

Moschalkov (Leuven)

B = 0.3 T

B = 0.7 T

The transition is due to the high-Tc LSCO :

neutrons, Gilardi e.a., PRL ‘02

slide34

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

Note the differences in possible types of experiments between smooth and rough surfaces

  • 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 ?

slide35

What has been done by STM :

  • Bi2Sr2CaCu2O8-δ superconductor
  • superconducting gap, impurity resonances, stripes
  • atomic resolution, discussion aboutdisorder
  • also YBa2Cu3O7-δ , Sr2RuO4
  • La0.7Ca0.3MnO3 CMR material
  • phase separation, local spectroscopy
  • no atomic resolution
  • Bi0.24Ca0.76MnO3 Charge Order
  • atomic resolution, but not a conclusive experiment
  • A roadmap for the oxides
  • What has been done by AFM :
  • Si(111) semiconductor
  • (sub-)atomic resolution
slide36

150 Ǻ

Pan - Nature ‘00

d-wave sc; a relative success story

good metal, atomically flat surface (cleavage)

+ Zn - impurities

a. Bi2Sr2CaCu2O8-δ

  • ZB – anomaly
  • strong scattering along gap nodes

ZB map

disorder in bscco

Lang - Nature ‘02

Hoogenboom - Phys. C ‘03

Homogeneous (for optimal doping)

Different for different doping

- variations in gap spectra / gap width

Disorder in BSCCO
fourier transform sts stripes

Direct space, 7 T

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 surface

Fourier Transform STS - stripes
b la 0 7 ca 0 3 mno 3

Single Xtal STM topography

M. Fäth

Leiden

CMR

Local STM spectroscopy

MR

Different I-V characteristics

CMR and the issue of phase separation

b. La0.7Ca0.3MnO3
spectroscopy on lcmo

Small scales

topography

0 , 0.3 T

dI/dV,0 T

1 , 3 T

dI/dV,9 T

5 , 9 T

Spectroscopy on LCMO

LCMO / YBCO film, 50 K black ‘=‘ metal’

  • Surface becomes more metallic with increasing field
  • Disorder is (probably) froozen
spectroscopy on lcmo cont

LSMO thin film, T-dependence

black ‘=‘ metal’

Becker, PRL ‘02

Spectroscopy on LCMO - cont
  • 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

At 300 K, ‘some terraces’ with atomic resolution

At 146 K, doubled (a02) unit cell along [101]

Two different atomic distances

c. Bi0.24Ca0.76MnO3

Image charge order

- Renner, Nature ‘02

Bulk

TCO = 250 K

Mn3+ : Mn4+ = 1 : 3

Surface

Rotated octahedra ?

Surface reconstructs ?

Mn3+ : Mn4+ = 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

Measure Δf at constant amplitude

noise spectrum. Ampl = 1.5 pm

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)

afm sub atomic resolution

Si(111)- (7x7)

Giessibl, Science ‘00

Single adatom

Calculation for z = 285 pm

AFM – ‘sub’-atomic resolution
slide45

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
Competition between strain and disorder
  • Strain , activation energy k1 , Tco , Hc+ ;

Strain helps!

  • Strain , disorder , T , Hc+ .

Disorder weakens!

Properties of CO/OO PCMO films = Strain + disorder !