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Logarithmic Amplifier

Logarithmic Amplifier. Serial number: N9503A Pin to the right hand side !. General Remarks: 1.) The switch-pin on the amplifier should always be kept to the right when viewed from front If the pin is to the left only very small currents up to ~ 10nA can be measured

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Logarithmic Amplifier

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  1. Logarithmic Amplifier Serial number: N9503A Pin to the right hand side !

  2. General Remarks: 1.) The switch-pin on the amplifier should always be kept to the right when viewed from front If the pin is to the left only very small currents up to ~ 10nA can be measured 2.) The set-point current should be usually kept at 1nA Higher set-point currents have to be calculated from the equations given on the following pages A nominal set point current of 2.5 nA for example corresponds to A real current of ~ 20 mA !!!! 3.) Break-Junction experiments should not be performed at voltages larger than 0.4 V 4.) Do not try to resolve the A Group with it 5.) This head is configured for the old (basement) STM The voltage offset is - 29 mV. To measure at a real tip-voltage of + 0.1 V you choose a voltage of + 0.071 V To measure at a real tip-voltage of – 0.1 V you choose a voltage of - 0.129 V.

  3. Calibration Curves using 5 different Resistors I = abs(Uout) + 10^(abs(Uout)*4.121 - 5.98)

  4. But only if Uout values are spaced equidistant

  5. The output signal (Uout) which appears as current in the output file is logarithmic in the range from 10 – 10000 nA where: For Positive tip Voltages: I[nA] = - 10(4.05 (Uout) - 5.83) = - invlog(4.05 (Uout) - 5.83) where Uout is negative (‘nA’); For example Uout = -2 V → log(I) = 2.27 → I = 186 nA

  6. To calculate the output signal for a given conductance (sM in nS and U in V): Uout = (log(sM× U) + 5.83) / 4.05

  7. All Current Results BJ -2 nm to 2 nm; 1s; + 0.2 V; 1 nA

  8. Resolution is not very good – C and B group cannot be clearly resolved

  9. Linear Amplifier: C4DT

  10. SMC of HS-(CH2)4-SH Break Junction Method – Linear Amplifier Observed Current Window: 10 – 100 nA B1 A1 Observed Current Window: 5 – 25 nA C1 Observed Current Window: 0 – 10 nA It is necessary to focus on the relevant current range to resolve the different conductance groups

  11. At high I (1mA) and high U (1 V), the gap heats up, measurements are impossible due to current oscillation. 200 nA 1 mA Shortcut ~ 100 mA

  12. At U = 1 V measurements can be performed up to ~ 10 micro-Ampere Conclusion: For Break-Junction Experiments at high Voltages, a current limiting resistor may help to facilitate measurements. Critical Current

  13. General Remarks

  14. Tip-Sample Distance (s) under various conditions

  15. Voltage drop due to Amplifier impedance at close proximity

  16. Simmons: f = (b(U)/(a × 10.25))2 + U/2(f in eV; b in nm-1) For a =1 and U = 0 V the above equation is identical with: f = (dlnI/ds)2 * 9.526 meV where dlnI/ds = -2k = -2(2mf/hbar2)

  17. Different Representations of the Simmons Fit to the ODT I(V) curve Not useful Sinc I(V) is linear For small V

  18. Transition Voltage Spectroscopy on Simmons Curves a = 0.5 a = 1

  19. Dependence of I on a

  20. No a and hence no m* used: Area dependent Transmission

  21. b(V) = 10.25 * a(f – (U/2))1/2 (result in nm-1) bN(V) = 10.25 * a(f – (U/2))1/2 * 0.153 (result per CH2) 0.153 = dC-C in ODT f = (b(V) /(a*10.25))2 + U/2(result in eV; b in nm-1) f = (bN(V) /(a*10.25*0.153))2 + U/2(result in eV; bN per CH2)

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