Transcript AKraychaudhury - SN Bose National Centre for Basic Sciences

Density of states and tunneling studies on
manganites
A. K. Raychaudhuri
[email protected]
S.N.Bose National Centre for Basic Sciences,
Kolkata, India
and
Department of Physics
Indian Institute of Science
Bangalore, India.
Students who made it happen:
Amlan Biswas,
Mandar Paranjape,
Joy Mitra,
Sohini Kar
Thanks to
Department of Science and Technology for funding
Important publications:
1.A. Biswas, S. Elizabeth, A.K. Raychaudhuri and H.L. Bhat
(1999) “ The density of states of hole-doped manganites : A scanning tunneling
microscopy/spectroscopy study” Phys. Rev. B 59 , 5368
2.Mandar Paranjape, A.K. Raychaudhuri, N.Mathur and M.Blamaire
(2003) “Effect of strain and microstructure on the electrical conduction in epitaxial
films of La0.7Ca0.3MnO3 “. Phys Rev. B 67 , 214415
3.J. Mitra and A. K. Raychaudhuri, Ya. M. Mukovskii and D.Shulyatev
(2003) ” Depletion of density of states at the Fermi level in metallic colossal
magnetoresistive Manganites” Phys Rev B 68, 134428
4. J. Mitra, Mandar Paranjape , A. K. Raychaudhuri, N. D. Mathur and M. G. Blamire
(2005) “Temperature dependence of the density of states near Fermi level in a strain
free epitaxial film of hole doped manganite La0.7 Ca 0.3MnO3.” Phys. Rev B 71 , 094426
5. Sohini Kar, J. Mitra and A. K. Raychaudhuri
(2005) “Temperature dependence of the gap in the density of states near the Fermi level
in a hole doped manganite “ Solid State Comm. 136,410-415
6. . Sohini Kar, Jaynta Sarkar, Barnali Ghosh and A. K. Raychaudhuri
(2006) “Spatially resolved study of electronic transport through grain boundaries in
nanostructered films of colossal magnetoresistive (CMR) manganites.” Phys. Rev. B
74, 085412
Plan:
1.A brief introduction to tunneling experiments.
2. Why it is important to do tunneling
experiments in manganites. What are the
difficulties
3.What new did we learn from tunneling
experiments in manganites.
Electron Tunneling Spectroscopy.
Primarily a technique that gives
information on the density of states at
the Fermi Level
N(E~EF)
There are details of doing the
experiment and analysis of the data
What are the techniques that give the density of
states (DOS) near the Fermi level N() ?
•
In a metallic solid the DOS at EF (N(EF)) can be found
from the specific heat linear term   N(EF)
•
Tunneling spectroscopy allows determination of DOS
for  < 1eV. Has very high low energy resolution (V).
Can do spin polarized tunneling.
•
•
Frequency dependent Optical conductivity
Photo –electron spectroscopy (in its various forms)
can be used to find N(). Complementary to Tunneling
spectroscopy
What do we really do in tunneling spectroscopy ?
Counter electrode (metal)
Barrier
Sample
Ohmic
contact
V
I
1.Measure I vs V and obtain
G(V)=dI/dV vs V or measure
dI/dV vs V directly.
2. I-V will not be linear and
G(V) will have voltage
dependence.
3. The information on the
DOS (N())
is obtained from the
“tunneling conductance”
dI/dV (=G) vs V where
 = eV.
Challenge is to obtain information on N() from the
measured G(V) by deconvoluting the barrier
What do we really do in tunneling spectroscopy ?
We measure the junction conductance G=dI/dV as a
function of V
V-the junction bias gives the energy of the electron
Measure “transparency of junction as a function of
electron energy”
Barrier
Counter
electrode
For any electron spectroscopy like tunneling we need to prepare
the electrons in one electrode and inject them to the other
without scattering or thermalizationelectron energy > kBT
For tunneling (as the process of charge transport across the barrier )
to occur the bias applied should be small compared to the barrier
otherwise processes like field emission will take over.
Tunneling is forbidden classically
Basic physics of electron tunneling
Electrons will go from filled state of
sample to empty state of counter
electrode-filled state spectroscopy
Electrons will go from filled state of
counter-electrode to empty state of
samples-empty state spectroscopy
Tools and techniques:How to make the tunnel junction.
Canonical and non-canonical methods.
Wide area
contacts
Grow a “pin hole” free
oxide barrier on the sample
and deposit the counter
electrode
Narrow area
contacts
1.
STM based method-vacuum as the barrier.
2.
Use an oxidized counter-electrode like a Pb (it is
soft and superconducting)
3.
Point contact tunneling
Requirements:
1.
Absence of scattering and bulk like transport in the barrier region.
2. To ensure other parallel channels of transfer is much less compared
to the tunneling.
3. A well defined counter-electrode
Wide area
contacts
How to make the canonical tunnel junction.
Well defined momentum of injected electron but no
spatial resolution
Evaporate counter electrode(e.g, Pb or Al)
on a smooth substrate like sapphire
Oxidize the counter electrode. If it is Pb or Al
they can be oxidized or a thin layer of Al is
evaporated and it is oxidized to Al2O3.
VI+
V+
I-
Evaporate the sample at a cross
geometry
Works well for conventional metals but not
successful in oxides including HTS and
CMR systems
Coming close to a canonical junction using an oxidized mini lead
sphere (< mm)
Press
Oxidize Pb sphere
Sample
Sample
Works well for oxides including the CMR oxides
The oxidized junction
acts as a barrier
Using STM to do tunneling spectroscopy
•Tunneling Spectroscopy with STM popularly known as Scanning
Tunneling Spectroscopy (STS) gives spatial resolution.
•Vacuum is the best tunneling barrier and the barrier width can be
changed at will by changing the tip-sample distance
Transparency of barrier-current through the junction
If Ns and Nt are temperature independent T
enters through f(E)
Tunneling conductance of conventional metal- Platinum
Pt-Rh tip and Pt film data taken in an STM
Parabolic curve for
Platinum
The parabolic nature is
due to the nature o
barrier
Pt has a flat DOS
Basic physics of electron tunneling
For an insulator the Ns(E) is zero over the gap
E-/2 to E+/2. There will be no current in the
junction and G will be zero for | eV | /2,
Here  is the gap. Current is strictly zero at
T=0
Caution : charging effect and Coulomb blockade
Example –1
Tunneling
spectroscopy near
an MI transition
Opening up of a
gap leading to
insulating state
Phys. Rev. Letts 80, 4004(1998),
Basic physics of electron tunneling
For a superconductor also the Ns(E) is zero
over the gap E-/2 to E+/2. There will be no
current in the junction and G will be zero for |
eV | /2, Here  is the gap. Current is strictly
zero at T=0
Example –2
Tunneling spectroscopy of epitaxial (100) oriented
YBa2Cu3O7
A gap in DOS leading to superconducting state
Cond-mat 0305257
Doing tunneling spectroscopy on
oxides with conventional
electrodes
Testing the junctions – is it tunneling ?
Look for the gap of the counter
electrode if it is superconducting
Using BTK model (1982)
Z as a parameter to measure quality
of junction
Summing up the task in hand once we get the
tunneling data- Find Ns(E) from the data
Use a tip that has nearly flat Nt
•How to fix the barrier parameter?-Assume a model
•If it is a an STM junction we can vary the tip sample
distance and thus can vary the barrier parameters
•De-convolute the data taken with different tip sample
distance
It is an involved process
Why should we do tunneling studies in
manganites?
Physics in manganites that can affect the DOS.
1. Insulator –metal transition at Tc.
2. The metallic state T < Tc-Is it a conventional
metal?
3. In manganites like many oxides the region near
the EF has very low DOS even if the DOS
away from the EF can have large spectral
weight.-Small perturbations can change the
DOS at EF
LCMO: La0.7Ca0.3MnO3
LSMO: La0.7Sr0.3MnO3
Single
crystals
Tc
Tc
The change in resistivity
at Tc, is it controlled by
mobility () or carrier
concentration (n)?
   n e  e2 N(EF)D
MI transition at Tc controlled by DOS or mobility /
diffusivity?
Spin Scattering and negative MR
in ferromagnets neat Tc
Enhancement of mobility  in
magnetic field
Increase of Zeeman
splitting in magnetic
field Suppression of
Spin-flip scattering
There is a limit to the change in
resistivity that mobility can
contribute at Tc
•Potential scattering modified by
the spin
term  ± sJ
• Suppression of Spin disorder
scattering
m = (42mcm/ne2h)N(EF)2J(J+I)
• /  - 4 (/V)2 <JZ> 2  M2
Typical example of a mobility driven resistivity change
at Tc –Example of high purity Ni wire
Expect a gap for T > Tc and also
expect it to close at T<Tc
LSMO may have some
similarity but definitely not
LCMO that has a polaronic
insulating paramagnetic
state.
Tunneling spectroscopy probes this region in the
manganites and a good deal of physics is determined by
what happens here. The separation as well as the width of
the bands
The physics of manganites like other
transition metal oxides occur close to the
metal –insulator transition
The density of states near the Fermi-level are
affected by correlations and disorder
In particular the approach to insulating state
introduces a dip in the DOS at the Fermi level
Becomes apparent at low T
STS on manganites – A gist of what we observe
Gap closes
The gap
“hardens”
close to Tc
There is hard
gap in the
DOS
polaronic state
T/Tc
0
0.5
1
1.5
Dip developing in
the DOS very
close to EF
The tunneling curve has significant temperature
dependence and we find transfer of spectral weight as
a function of temperature
Difficulties of doing tunneling experiments in
manganites
1. Sample- single crystals and films
2. Difficulty in forming conventional junctions
3. Generic problem- phase separation
Our experiment
STS with UHV –STM temperature variable
Home made
Works down to 4.2K
6T
STM images of CMR
films
LCMO films
200nm
50nm
SrTiO3 substrates
Paranjape Phys Rev B (2003)
50nm
NdGaO3
La0.7Ca0.3MnO3 (LCMO) films grown on
different substrates-same chemistry , same method of preparation
Paranjape et al
Phys. Rev B 67, 214415 (2003)
Strain
relaxed
STO/200
Uniformly
strained
STO/50
No strain
NGO/50
STM images of CMR single crystals
Mn
O
From XRD Mn-O-Mn
bond length is
0.402 nm
0.385 nm
SC -La0.6Pb0.4MnO3
Biswas etal. PRB 59 , (1999)
3D view of STM images
LCMO/NGO(50)
LCMO/STO(50)
Large temperature
dependent phase separation
Negligible phase separation
1.2 m  1.2 m  0.5 nm
0.5 m  0.5 m  0.6 nm
LCMO/STO(50)
CMAP- Local tunneling
conductance.
Phase separation leads to spatially
varying tunneling conductance
50 nm LCMO EPITAXIAL THINFILM ON NGO
25
90
80
20
70
15
50
40
10
30
5
0
50
100
150
200
T (K)
250
300
• 50 nm thin film of La0.7Ca0.3Mn03 on
NGO Deposited by Pulsed Laser
Deposition.
20
• TC = 268 K
10
• Topography shows terrace structure
average step width ~ 360 nm
0
0
- M.R.%
 (mW-cm)
60
• RMS roughness on terraces ~ 0.03 nm
LCMO/NGO(50)
STS AS A FUNCTION OF TEMPERATURE
ACROSS TC
0.50
60.0n
40.0n
IT (nA)
0.25
0.00
-0.25
IT (A)
20.0n
200 K
260 K
275 K
-0.50
-0.30
-0.15
0.00
0.15
0.30
Bias (V)
0.0
-20.0n
LCMO/NGO
50nm thinfilm
Tc = 268 K
-40.0n
320 K
310 K
300 K
290 K
280 K
275 K
270 K
260 K
250 K
245 K
230 K
220 K
210 K
200 K
190 K
180 K
170 K
-60.0n
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Sample Bias (V)
0.4
0.6
0.8
1.0
T << Tc
T ~ Tc
Gap in DOS close to
Tc in single
crystalline films of
LCMO (x=0.3) on
NdGaO3
T < Tc
But finite temperature
has effects
T ~ Tc
. J. Mitra et.al “Temperature
T > Tc
T =Tc
dependence of the density of
states near Fermi level in a
strain free epitaxial film of
hole doped manganite La0.7
Ca 0.3MnO3.” Phys. Rev B
71 , 094426 (2005)
Finite temperature effects-one need to carry out a
de-convolution with finite T effect
Variation of the gap in
the DOS at EF in LCMO
25
90
80
20
70
15
50
40
10
30
Transport gap ~ .07 eV
20
5
10
There is indeed a gap in the DOS ,
that is present for T>Tc, that hardens
close to Tc and then closes by
T/Tc~.085
0
0
0
50
100
150
200
T (K)
250
300
- M.R.%
 (mW-cm)
60
Is there any rationale in the
temperature dependence of the
gap ?
/max =  (M/Ms)
Sohini Kar et.al, Solid State
Communication 136,410415 (2005)
STS on manganites –Some observations
The gap that opens in the DOS close to Tc depends on
the material
The gap () decreases as Tc (which is a measure of the
bandwidth) increases
The gap in DOS at EF at T=Tc
EF
As the Tc increases the gap that opens up near Tc becomes
smaller. In materials like LSMO the gap does not exist.
(Tc is a measure of the band width)
LCMO : Gap in DOS
and the transport
behavior is governed
by the gap
LSMO: No gap in the
DOS , transport
behavior controlled by
the mobility
STS on manganites –Some observations
The temperature dependence of the DOS is
perceptible even away from EF
There are large density of states sitting close to EF
even though a gap opens up at EF
There is large rearrangement of spectral weight close
to EF as the temperature is changed through Tc
La0.7Pb0.3MnO3,
Tc= 338K
La0.6Pb0.4MnO3,
Tc= 401K
Large Change in the DOS even
at E > 0.2 eV as the sample
passes through the transition.
The DOS at EF builds up and
the gap closes by contribution
from higher energy states.
(NdLa)0.73Pb0.27MnO3,
Tc= 275K
Biswas etal. PRB 59 ,
(1999)
STS on manganites –Some observations
At T<< Tc a dip opens up in the DOS close to
EF
Result obtained with a conventional tunnel
junction with Pb counter electrode
Tunneling conductance in CMR system
Temperature dependence of the Pb-La0.7Ca0.3MnO3 junctions
Mitra et.al, Phys. Rev B (2003)
Tunneling conductance in CMR systems at T=4.2K
Investigation using barrier type junctions
Pb-La0.7Ca0.3MnO3 and Pb-La0.75Sr0.25MnO3
Superconducting
gap of Pb
n
V
G(V)  G O [1    ]
 
ln (G/G0-1) vs V
Mitra et.al, ” Phys
Rev B 68, 134428
(2003)
The Density of States calculated for
La0.7Ca0.3MnO3 and La0.75Sr0.25MnO3 from the tunnel junctions.
The DOS is temperature
dependent unlike a
conventional metal and at
low T a dip develops at
low E
Similar to seen in a
number of correlated
oxides
The solid lines show the fit to the calculated DOS
to a equation N() = N(F) + (d)2
AKR, Advances in
Physics 44, 21
(1995)
STS on manganites– A summary
The gap
in DOS
hardens
close to Tc
Gap closes
(LCMO)
T/Tc
0
0.5
Dip developing in
the DOS close to
EF
1
Metallic
1.5
Polaronic Insulator
with a gap in DOS
The tunneling curve has large temperature dependence
And in LSMO it appears there is
no gap in DOS
We are trying to sort out
what happens in a magnetic
field
Thank you