Scanning Tunneling Microscopy & Spectroscopy: A tool for probing electronic inhomogeneities in correlated systems

Transcription

1 Scanning Tunneling Microscopy & Spectroscopy: A tool for probing electronic inhomogeneities in correlated systems Anjan K. Gupta Physics Department, I. I. T Kanpur ICTS-GJ, IITK, Feb 2010

2 Acknowledgements Manganites: 1. Udai Raj Singh A. Datta S. Chaudhuri R. C. Budhani 2. Goutam Sheeth V. Chandrasekhar (NW Univ, USA). 3. H. W. Jang C. B. Eom (Univ. of Wisconsin-Madison, USA). Graphene and Graphite: 1. Shyam K. Choudhary 2. Mandar Deshmukh (TIFR, Mumbai) 3. Rajeev Gupta (IITK). Funding: 1. IIT Kanpur 2. MHRD 3. DST

3 Quantum Tunneling and STM I( x, y) + = G0 M ( ε, V ) 2 [ f ( ε ev ) f ( ε )] N ( x, y, ε ev ) N ( ε ) dε sam tip ~ exp( kd) Topography : k ~ 1 Å -1 Spectroscopy : di dv ( V ) N sam ( ε ) f ( ε ev ) dε ε + ~ N sam (ev) (T ~ 0)

4 STS: Vortex Imaging 2H-NbSe 2 at T = 1.8K and 1 Tesla, di/dv at 1.3mV 200G, 350nm T c =7.2K, Δ=1meV, ξ =8nm, κ =30 Ref.: H. F. Hess et.al., Phys. Rev. Lett. 62, (1989)

5 STM/S on Bi-2212 Topography LDOS Image 35 Å, -9 mv Surface inhomogeniety & defect scattering Magnetic Ni-impurities In Bi-2212 d-wave nature 35 Å, +9 mv E. W. Hudson et.al., Nature 411, 920 S. H. Pan et. al., Nature 413, 282 Further Reading: Nature 422, 592 (2003) (Quasiparticle interference)

6 Our STM Design Gupta and Ng, Rev. Sci. Instrum.72, 3552 (2001)

7 Our STM Systems

8 STM / STS Sample Tip Amp Lock-In A + v 0 sin ωt Z-Feedback Topography Cond-map STM: Constant current or Constant height STS: ac-modulation I di dv 2 ( V + v sinωt) = I( V ) + v sinωt + v ( 1 cos 2ωt ) + L 0 V d I 2 4 dv V 0

9 Atomic Resolution (HOPG) Bi 2 Te 3 at 77K Lithography Low Temp (liq. He) CDW in 2H-NbSe 2 Spectroscopy (CDW gap) HOPG, 139nm

10 Perovskite Manganites RE 1-x AE x MnO 3 :RE = La, Nd, Bi, etc. AE = Ca, Sr, Pb Mn 3+ :Mn 4+ = (1-x):x RE/AE Mn Double Exchange O 2- Ref. Zener (1951); Anderson and Hasegawa (1955)

11 Effect of doping (Mn 3+ : Mn 4+ ): P. Schiffer et. al., Phys. Rev. Lett. 75, 3336 (1995)

12 Bandwidth & Tolerance Factor Tolerance Factor for AMnO 3 : f t = 2 r A ( r + r ) Mn + r O O A: RE / AE, B: Mn f t =1 for perfectly cubic perovskite f t controls hopping (t ij ) f t < 1 for LaMnO 3 structure Change in Mn-O-Mn bond angle leads to orthorhombic structure O A Mn

13 Effect of Bandwidth Ref: Hwang et. al., PRL 75, p914 (1995)

14 La Pr Ca MnO 3 Thin Films ( Narrow bandwidth) Strain free (on NdGaO 3 substrates by PLD) Hysteresis: phase separation? Activation gap, ΔE = 0.12 ev in high temperature paramagnetic regime T IM = 170 K (low T c, small banwidth)

15 STM Topography (La Pr Ca MnO 3 ) Singh, et. al., Phys. Rev. B 77, (2008)

16 Conductance imaging below T IM At 140K (below T IM =170K), at 1.0 V / 0.1nA Bright Spots (Insulating) possible that these belong to parent compounds. Film Surface Electronically almost homogeneous ( Variation in gap ~0.1 ev) a) nm 2 (c) nm 2 Dark Regions ~ Conducting Bright Regions ~ Insulating

17 M ( ε, V Voltage dependent tunneling matrix? ) Const. for large V di/dv (na/v) dlni/dlnv Bias (V) Bias (V) However, at V=0 dlni/dlnv=1, by definition DOS at E F? Feenstra, Surf. Sci. 299, p 965 (1994)

18 T- dependent DOS Spectra Singh, et. al., Phys. Rev. B 77, (2008)

19 Thermally smeared gap at high-t. ZBC activation energy ~ 0.08eV for 180 K T 330 K. From resistivity, ΔE = 0.12 ev. Zero Bias Conductance (ZBC) Both measurements are in agreement for T > 200K. Conclusion 1. Film s surface nearly homogeneous (electronically) at all temperatures. 2. Spectra are gapped at low temperatures. 3. The presence of energy gap and the absence of phase separation on the surface contradicts the resistivity below T IM = 170 K. 4. Formation of CO near the surface masking the phase separation in the bulk. Singh, et. al., Phys. Rev. B 77, (2008)

20 STM/S of La0.7Sr0.3MnO3 films 50 nm PLD LSMO on (001) LSAT 524 nm, 1V / 0.1nA

21 LaSrMnO3 on LSAT substrate T=295K, 0.6V/0.1nA, 90nm T=118K, 1V/0.1nA, 70nm

22 DOS: T- dependence (La 0.7 Sr 0.3 MnO 3 ) (a) V shaped spectra at higher temperatures and a pseudogap at low T. (b) The ZBC value (DOS at E F ) is slightly increases with cooling, with a pronounced gap-like spectra. Singh et. al., Appl. Phys. Lett. 93, (2008)

23 STM/STS of La Ca MnO 3 Thin Films 100nm On NGO by PLD T IM = 250K E A = 0.12 ev 304 nm78 K

24 DOS: T- dependence Singh et. al., J. Phys.: Cond. Mat. 21, (2009)

25 STM/STS of Pr Ca MnO 3 Thin Films (Narrow bandwidth) Surface topography: Dislocation Line Annealed at C As Grown Area μm 2, 1.0 V bias and 0.2 na tunnel current. Area μm 2, 1.0 V bias and 0.12 na tunnel current.

26 Pr Ca MnO 3 Thin Films Resistivity measurement: The CO Transition temperature T CO = 230 K k B Δ( T ) = d ln ρ( T ) d(1/ T ) ρ (Ω.-cm) Conductance imaging at 190 K: x dρ /dt (Ω-cm/K) 0-2 (a) Δ (ev) T CO = 230 K T (K) T(K) T CO = 230 K T (K) (b) Scan area = nm 2, 1.0 V/ 0.1 na, Homogeneous film s surface (electronically)

27 DOS: T- dependent (spectroscopy) Zero Bias Conductance Pr Ca MnO 3, 1.0 V/ 0.1 na Pr Ca MnO 3, 1.0 V/ 0.1 na The appearance of dip of certain in low temperature spectra is indicating the CO transition to be around 230 K. There is a non-zero DOS at E F in the antiferromagnetic CO phase at low temperatures.

28 Conclusions Gap feature: more robust at low-t States inside the gap at low-t Two types of carriers (localized polarons and delocalized) Surface effect? Bilayer ARPES: QP pockets due to polaronic coherence Polaronic Coherence: Is it more common than just bilayers?

29 Coherent Polarons in bilayer manganites: ARPES The low energy quasiparticle peak in small k-pockets appear. Coherent polarons in FM state? [N. Mannella et al. Nature (2005)] [N. Mannella et al. PRB (2007)] A. Chikamatsu et al. PRB (2008)

30 STM/S on Graphene

31 Bilayer Graphene FET Intensity (10 3 count W -1 s -1 ) G 1579 cm cm cm -1 2D Raman Shift (10 3 cm -1 ) Intensity (count W -1 s -1 ) Raman Shift (cm -1 )

32 FET Transport 0.60 Conductivity (m ohm -1 ) Min at 22.4 V Gate Voltage (V)

33 Coarse xy Positioning in STM Ref: Gupta et.al. Rev. Sci. Instr., 79, (2008).

34 STM Topography 2.0 Height (nm) Distance (nm)

35 Local Spectra V di/dv (a.u.) Bias (V) 0 V - 60 V

36 E F shift vs Gate Voltage ΔE = α F V g 2 2 h v π ε α = F K 0 d exp t et λ t d = 0.4eV, λ = 1.2nm, = 0.7nm α = e E F (ev) Slope: 1.21 x 10-3 e V G (V) Ref.: A. H. Castro Neto et. al. Rev. Mod. Phy. 81, 109 (2009 ); H. Miyazaki et. al., Appl. Phys. Express 1,

37 Thank You