This document summarizes several experiments measuring out-of-equilibrium transport in oxide materials. It focuses on measurements using conducting atomic force microscopy (AFM) and electrostatic force microscopy (EFM) to study the metal-insulator transition in vanadium dioxide (VO2) thin films at the nanoscale. In conducting AFM mode, current switching was observed in local I-V curves, indicating a transition between metallic and insulating states. EFM detected random telegraphic noise in VO2 at room temperature, suggesting many-body effects are involved in the transition absent Joule heating effects.

1. Local probe of metal-insulator transitions

3. Out of equilibrium transport Collective tuneling E field Oka, T. & Aoki, H Physical Review Letters 95 , 137601 (2005). Correlated Insultors Band Insultors

4. Measurements Out of equilibrium transport in VO 2

5. VO 2 : from metal to Mott insulator Two problems: - Joule heating - inhomogeous sample E = 3  10 6 V/m M. M. Qazilbash, et. al. Science Vol. 318 no. 5857 pp. 1750-1753 (2007). 300K 5  m VO 2

6. Microscope AFM/STM/EFM C:sersXWuesktopork-amplify3.jpg tip etching f 0 30 000Hz 3 modes T~400-30K H=9T Conducting AFM STM EFM (Electrostatic force microscopy) D:士毕业论文写作hesis_BXWu010-05-04igureshapter-5etup.jpg

7. VO 2 : Conducting AFM mode Local I(V) curves of the metal/insultor transition 100nm Substrat VO 2 1  m E = 5  10 7 V/m After I(V) curve C:sersabUseresktoputputfter_4-ZAbsolue_IN_2_SFSxM_Image_file215.bmp C:sersabUseresktoputputO2 thin films topoZAbsolue_IN_2_SFSxM_Image_file91.bmp

8. Electrostatic force microscopy (EFM) mode U électrostatic = F électrostatic = SAMPLE f 0 V tip/sampe -  U électrostatic  f   F/  z At the frequency 2  (600Hz) F. J. Giessibl, Reviews of Modern Physics 75, 949 (2003). 2 2 1 CV

9. ≠ λ D Why do we expect a capacitive modulation in a metal to insulator transition? C I ∝ 1/(d+ λ D ) d C M ∝ 1/d Tip/Metal sample capacitance Tip/Insulator sample capacitance

10. EFM Mode in VO 2 Random telegraphic noise (RTN) appears at 300K for VO 2 only. VO 2

11. Consistency checks RTN only appears at: - low tip/sample dinstance < 90nm - High voltage >6V

12. Tunnel mode