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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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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)


  • 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


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)


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


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


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)


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

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


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


… 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 ?

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


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.


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


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.


  • Include

    disorderpinning glass

    thermal fluctuationsmelting

    Current general vortex matter (B,T) phase diagram

    A-lattice

    Ideal


    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.


    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


    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


    Image processing

    Convolution with pattern of:

    “single vortex”:

    Unit cell

    3x3:

    4.30 K

    1.75 T

    4.44 K

    4.53 K



    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.


    Above framesTp1

    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


    Other superconductors - thin films ? frames

    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 frames~5 nm Mo3Ge 50 nm

    Si substrate

    a-Mo3Ge + Au

    AFM – no Au islands

    Use proximity effect

    signal weak,

    ‘spectroscopy mode’


    Optimized settings framesa-Mo2.7Ge, B = 0.8 T, d = 48 nm, 1.1 mm2

    ACF

    2D-FFT


    Also for NbN, a framesmuch 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


    frames

    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


    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 ?


    • What has been done by observed close toSTM :

    • 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


    150 observed close toǺ

    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 observed close to

    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 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 surface

    Fourier Transform STS - stripes


    B la 0 7 ca 0 3 mno 3

    Single Xtal STM topography observed close to

    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 observed close to

    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 observed close to, 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 observed close to300 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 observed close toΔf 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) observed close to

    Giessibl, Science ‘00

    Single adatom

    Calculation for z = 285 pm

    AFM – ‘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

    • 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 observed close to

    • Strain , activation energy k1 , Tco , Hc+ ;

    Strain helps!

    • Strain , disorder , T , Hc+ .

    Disorder weakens!

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


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