SLIDE 1
Origin of charge density wave formation in insulators from a high - - PowerPoint PPT Presentation
Origin of charge density wave formation in insulators from a high - - PowerPoint PPT Presentation
Origin of charge density wave formation in insulators from a high resolution photoemission study of BaIrO 3 Kalobaran Maiti Department of Condensed Matter Physics and Materials Science Tata Institute of Fundamental Research Homi Bhabha Road,
SLIDE 2
SLIDE 3
d-band in transition metal
- xides
- A. Fujimori et al. PRL 69, 1796 (1992).
SLIDE 4
DMFT results
U/D = 1 U/D = 2 U/D = 2.5 U/D = 3 U/D = 4
- A. Georges et al. Rev. Mod. Phys. 68, 13 (1996).
SLIDE 5
Energy (eV) Intensity (arb. Units) DOS
Coherent feature Incoherent feature
Ca1-xSrxVO3
He I spectra I.H. Inoue et al. PRL 74, 2539 (1995) M.J. Rozenberg et al. PRL 76, 4781 (1996)
- A. Georges et al. Rev. Mod. Phys. 68 13 (1996)
SLIDE 6
CaVO3
Paramagnetic metal V 4+ à 3d 1
SrVO3
Paramagnetic metal V 4+ à 3d 1 x y z a = 5.318 Å ; / V-O-V = 154.3 o b = 5.343 Å (in xy-plane) c = 7.543 Å ; / V-O-V = 171 o a ~ 5.43 Å / V-O-V = 180 o
SLIDE 7
All the spectra are dominated by the incoherent feature intensity à energy scale ~ eV Experimental specific heats are close to the LDA values à Energy scale ~ meV CaVO3 mJ/(mole.K2) SrVO3 mJ/(mole.K2) LDA 4.85 4.13 Experimental 7.3 6.4
Estimated values from photoemission ~ 10 times higher than the experimental values
PHYSICS AT HIGH AND LOW SCALES MAY BE DIFFERENT
SLIDE 8
Spectra exhibit strong dependence on the surface sensitivity of the technique.
Sr0.7Ca0.3VO3
SLIDE 9
CaVO3 SrVO3
SLIDE 10
Fit the surface and bulk spectra within Hubbard model using dynamical mean field theory (DMFT). The DMFT equations are solved using iterated perturbation theory (IPT). v Surface spectral function, ρ s(ε): U/W = 1.5 for SrVO3 = 2.0 for CaVO3 v Bulk spectral function, ρ b(ε): U/W = 0.67 for SrVO3 = 0.83 for CaVO3
SLIDE 11
CaVO3 mJ/(mole.K2) SrVO3 mJ/(mole.K2) LDA 4.85 4.13 Experimental 7.3 6.4 DMFT 5.5 3.7 The above results are obtained for the case of a non-degenerate half filled band. Small underestimation of the specific heat parameters may be related to the neglect of the d-band degeneracy.
SLIDE 12
Correlation in 4d oxides
SLIDE 13
4d orbitals in 4d TMO are more extended that 3d orbitals in 3d TMO. Expectations: Correlation effects à less important Ab initio approaches à more
- successful.
3d 4d
SLIDE 14
CaRuO3
Paramagnetic / anti-ferromagnetic /
spin-glass metal
Ru 4+ à 4d 4
SrRuO3
Ferromagnetic metal Ru 4+ à 4d 4 x z y a = 5.519 Å; Ru-O-Ru = 150 o b = 7.665 Å c = 5.364 Å a = 5.574 Å; Ru-O1-Ru = 167.6 o b = 7.852 Å (in xy-plane) c = 5.538 Å ; Ru-O2-Ru = 159.7 o Ru
SLIDE 15
Coherent feature Incoherent feature Dominant incoherent feature à U/W large XPS spectra: Lineshape in XPS is different
Dominant peak at ~ 0.6 eV Surface might have different
electronic structure t2g peak appears between 0 – 1 eV
SLIDE 16
SLIDE 17
Conclusion from 3d and 4d systems
Surface and bulk electronic structure in transition metal
- xides can be significantly different
Electron correlation strength reduces significantly while going down the series
SLIDE 18
5d oxides
Expected behavior Correlation effect is significantly weak Ab initio approaches may be applicable Electron-lattice interaction will increase
SLIDE 19
Structure
Orthorhombic BaIrO3: Monoclinic
SLIDE 20
Structure
SLIDE 21
Current-voltage
Non-linear Anisotropy
- G. Cao et al. Solid State
- Commun. 113, 657 (2000)
SLIDE 22
Resistivity (single crystals)
- G. Cao et al.
Solid State Commun. 113, 657 (2000)
Resistivity is highly anisotropic à Quasi-one-dimensional structure Insulator to insulator transition at 175 K Insulator to metal transition at 80 K and metal to insulator transition at 26K
vs 1/T à λ-type peak
Ferromagnetic transition at 175 K
SLIDE 23
Periodic lattice Half filled band Metal Insulator a 2a CDW CDW
SLIDE 24
BaIrO3
Insulator and exhibit Charge Density Wave transition at 175 K It also exhibit ferromagnetic transition at the same temperature
SLIDE 25
Specific heat (polycrystals)
Anomaly is stronger (ΔC = 2J/molK) compared to single single crystal data (ΔC = 0.7 J/moleK)
N.S. Kini et al. Physica B (2005)
Polycrystals seem to be better than single crystals
SLIDE 26
Resistivity (polycrystals) d(lnρ’) /d(1/T)
111 K 185 K 250 K
185 K Higher resistivity than single crystal data Transition temperature is higher Polycrystals seem to be better than single crystals
SLIDE 27
Valence band
SLIDE 28
Band structure results
SLIDE 29
Surface appears to be similar to the bulk bandwidth is similar to the calculated results There is some spectral redistribution !!
Ir 5d
SLIDE 30
Temperature dependence
SLIDE 31
SLIDE 32
SLIDE 33
At high energy resolution, the intensity close to the Fermi level can be expressed as g(ε) à Spectral DOS
SLIDE 34
Pseudo gap
SLIDE 35
Elector-magnon coupling !!
SLIDE 36
Ba 3d
Core levels are shifted across 250K and no effect below 250 K
SLIDE 37
Ba 4d
Ba Core levels are shifted across 250K and no effect below 250 K BUT not Ir core levels
SLIDE 38
Signature of multiple different Ba sites A B 78% 22% 78% 22% C D A,B,C,D are 25K spectra
SLIDE 39
Conclusions
The evolution of charge density wave due to localized electronic states directly demonstrated. We observe opening of a soft gap across the CDW transition. The spectral density of states reveals the role of magnetism on the electronic structure. à an intimate relationship between ferromagnetism and charge density wave in this system. The profound changes in the Ba-O covalency prior to the formation of charge density wave poses a new question with respect to the role of precursor effects.
SLIDE 40
Collaborators:
Ravi Shankar Singh, V.R.R. Medicherla,
- S. Rayaprol,
E.V. Sampathkumaran D.D. Sarma I.H. Inoue M.J. Rozenberg
SLIDE 41