Scanning Tunnelling Microscope (STM)

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Discussion Overview

The discussion centers around the functioning and interpretation of the Scanning Tunnelling Microscope (STM), particularly its role in probing the density of states of materials. Participants explore the implications of tunnelling current, the relationship between current and bias potential, and the concept of density of states in both atomic and solid contexts.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant states that the STM probes the density of states of a material, prompting questions about its meaning.
  • Another participant explains that the STM signal is proportional to the tunnelling current, which is influenced by the density of states at a specific bias.
  • A different participant elaborates that the STM performs spectroscopy by scanning an "I vs. V" curve, where the density of states is related to the derivative of this curve.
  • One participant seeks clarification on whether the density of states refers to energy levels in an atom or in the material as a whole, particularly in relation to the energy gap between the valence and conduction bands.
  • Another participant agrees that the density of states pertains to how closely packed energy levels are in a quantum mechanical system.
  • A further contribution emphasizes that the density of states from an STM of a solid provides a continuous result, contrasting with the discrete energy levels of isolated atoms.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of density of states, particularly regarding its application to atomic versus solid-state physics. There is no consensus on the definitions and implications of density of states in this context.

Contextual Notes

Participants highlight the complexity of applying atomic-level concepts to solid-state systems, indicating that assumptions about energy levels may not hold in the same way for solids as they do for isolated atoms.

EIRE2003
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''The Scanning Tunnelling Microscope probes the density of states of a material''...

What does this actually mean?
 
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The signal from an STM is proportional to the tunnelling current between the tip of the probe and the surface that is being scanned.
Now, tunnelling current is of course proportional to the tunnelling propability which in turn is determined by -among other things- the number of states that are allowed to tunnell for a specfic bias, i.e. it is proportional to the density of states.
 
EIRE2003 said:
''The Scanning Tunnelling Microscope probes the density of states of a material''...

What does this actually mean?

It means that the STM is doing "spectroscopy" rather than imaging. It scans something resembling an "I vs. V", at a particular location of a material, where I is the current, and V is the bias potential between the STM tip and the material. The density of states (assuming you know what this is), is proportional to the the derivative of this curve, i.e. dI/dV vs. V.

Zz.
 
Ok, am I right in suggesting that the 'density of states' is, how closely packed the energy levels in an atom are, or is it how closely packed the energy levels in the material as a whole? That is, the energy gap between the valence band and the conduction band?
 
Your first definition of density of states is the more correct one. The density of states determines how closely packed (in energy space) the energy levels in a quantum mechanical system are.
 
EIRE2003 said:
Ok, am I right in suggesting that the 'density of states' is, how closely packed the energy levels in an atom are, or is it how closely packed the energy levels in the material as a whole? That is, the energy gap between the valence band and the conduction band?

Er... DOS is dN/dE, where N is the number of states per unit volume.

You can't really consider the "energy levels in an atom" when you're doing a STM on a solid. I'm sure I've mentioned this many times. A solid doesn't behave the same way as isolated atoms. That's why "solid state physics" isn't the same field of study as "atomic/molecular physics". An atom's "DOS" is meaningless because an atom has discrete energy levels, and thus a dN/dE operation would be strange. The DOS you get from an STM of a solid, such as a superconductor, gives a continuous result!

Zz.
 
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