Density of states and tunneling studies on manganites

1. Density of states and tunneling studies on manganites A. K. Raychaudhuri arup@bose.res.in S.N.Bose National Centre for Basic Sciences, Kolkata, India and Department of Physics Indian Institute of Science Bangalore, India.

2. Students who made it happen: Amlan Biswas, Mandar Paranjape, Joy Mitra, Sohini Kar Thanks to Department of Science and Technology for funding

3. 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

4. Plan:1.A brief introduction to tunneling experiments. 2. Why it is important to do tunneling experiments in manganites. What are the difficulties3.What new did we learn from tunneling experiments in manganites.

5. Electron Tunneling Spectroscopy.Primarily a technique that gives information on the density of states at the Fermi LevelN(E~EF)There are details of doing the experiment and analysis of the data

6. 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

7. Counter electrode (metal) Barrier Sample V Ohmic contact I What do we really do in tunneling spectroscopy ? 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

8. 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”

9. 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 thermalization- electron 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

10. 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

11. Tools and techniques:How to make the tunnel junction. Canonical and non-canonical methods. Wide area contacts Narrow area contacts • STM based method-vacuum as the barrier. • Use an oxidized counter-electrode like a Pb (it is soft and superconducting) • Point contact tunneling Grow a “pin hole” free oxide barrier on the sample and deposit the counter electrode • Requirements: • Absence of scattering and bulk like transport in the barrier region. • To ensure other parallel channels of transfer is much less compared to the tunneling. • A well defined counter-electrode

12. Wide area contacts 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. V- I+ Evaporate the sample at a cross geometry V+ I- How to make the canonical tunnel junction. Well defined momentum of injected electron but no spatial resolution Works well for conventional metals but not successful in oxides including HTS and CMR systems

13. Coming close to a canonical junction using an oxidized mini lead sphere (< mm) Press Oxidize Pb sphere Sample Sample The oxidized junction acts as a barrier Works well for oxides including the CMR oxides

14. Using STM to do tunneling spectroscopy

15. TunnelingSpectroscopy 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

16. Transparency of barrier-current through the junction If Ns and Nt are temperature independent T enters through f(E)

18. 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

19. 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

20. Example –1 Tunneling spectroscopy near an MI transition Opening up of a gap leading to insulating state Phys. Rev. Letts 80, 4004(1998),

21. 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

22. Example –2 Tunneling spectroscopy of epitaxial (100) oriented YBa2Cu3O7 A gap in DOS leading to superconducting state Cond-mat 0305257

23. 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

24. 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 Summing up the task in hand once we get the tunneling data- Find Ns(E) from the data It is an involved process

25. Why should we do tunneling studies in manganites? • Physics in manganites that can affect the DOS. • Insulator –metal transition at Tc. • The metallic state T < Tc-Is it a conventional metal? • 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

26. LCMO: La0.7Ca0.3MnO3 LSMO: La0.7Sr0.3MnO3 Single crystals Tc The change in resistivity at Tc, is it controlled by mobility () or carrier concentration (n)? Tc    n e  e2 N(EF)D MI transition at Tc controlled by DOS or mobility / diffusivity?

27. 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 • 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 There is a limit to the change in resistivity that mobility can contribute at Tc

28. 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.

29. 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

30. 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

31. STS on manganites – A gist of what we observe There is hard gap in the DOS polaronic state The gap “hardens” close to Tc Gap closes 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

32. Difficulties of doing tunneling experiments in manganites • Sample- single crystals and films • Difficulty in forming conventional junctions • Generic problem- phase separation

33. Our experiment STS with UHV –STM temperature variable Home made Works down to 4.2K 6T

34. STM images of CMR films LCMO films 200nm 50nm 50nm SrTiO3 substrates NdGaO3 Paranjape Phys Rev B (2003)

35. 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 No strain Uniformly strained NGO/50 STO/50 STO/200

36. STM images of CMR single crystals Mn O 0.402 nm From XRD Mn-O-Mn bond length is 0.385 nm SC -La0.6Pb0.4MnO3 Biswas etal. PRB 59 , (1999)

37. 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

38. LCMO/STO(50) CMAP- Local tunneling conductance. Phase separation leads to spatially varying tunneling conductance

40. 50 nm LCMO EPITAXIAL THINFILM ON NGO • 50 nm thin film of La0.7Ca0.3Mn03 on NGO Deposited by Pulsed Laser Deposition. • TC = 268 K • Topography shows terrace structure average step width ~ 360 nm • RMS roughness on terraces ~ 0.03 nm

41. LCMO/NGO(50)

42. STS AS A FUNCTION OF TEMPERATURE ACROSS TC

43. Gap in DOS close to Tc in single crystalline films of LCMO (x=0.3) on NdGaO3 T << Tc T ~ Tc T < Tc But finite temperature has effects T ~ Tc . J. Mitra et.al “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 (2005) T > Tc T =Tc

44. Finite temperature effects-one need to carry out a de-convolution with finite T effect

45. Variation of the gap in the DOS at EF in LCMO Transport gap ~ .07 eV 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

46. /max =  (M/Ms) Sohini Kar et.al, Solid State Communication 136,410-415 (2005) Is there any rationale in the temperature dependence of the gap ?

47. 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

48. 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)

49. 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

50. 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