Bases théoriques

Spectroscopy of electronic properties of solids

We describe here basic principles of electronic properties of solids as well as two spectroscopies we use to probe them : Scanning Tunneling Microscopy and Spectroscopy (STM/STS) and Angle-Resolved Photo-Emission Spectroscopy (ARPES). Both methods are surface-sensitive and therefore have to be used in ultrahigh vacuum to avoid contamination.

Electrons in solids : From free electrons to correlated systems Unlike free electrons moving in vacuum, the electrons in solids strongly interact with the crystal lattice and with other electrons. Their motion cannot be described classically ; it rather needs a quantum mechanical approach. By applying the quantum mechanics rules and by properly treating electron-electron and electron-lattice interactions one can understand such amazing phenomena as superconductivity, charge density waves, metal-insulator transitions among many others (to know more..)
Scanning Tunneling Microscopy and Spectroscopy (STM/STS) Invented in early 80’s by Gerd Binnig et Heinrich Rohrer (Nobel Prix 1986), the STM accesses structural properties of solids - conductors, semiconductors, semi-metals etc.- at their very surface. The STM enables visualizing the surface organisation on atomic level. In the spectroscopic mode, often referenced to as Scanning Tunneling Spectroscopy, the electronic properties are probed locally taking advantage of quantum tunneling process between a sharp metallic tip and a surface of the studied sample (to know more..)
Angle-Resolved Photo-Emission Spectroscopy (ARPES) The ARPES is a photon-in, electron-out technique performed on conducting samples with UV light. The energy of photons sent to the studied sample should be high enough to overcome the binding energy of electrons there and enable their extraction out from the studied sample. The analysis of the kinetic energy and angles (polar, azimuthal and tilt) of emission of the electrons - usually plotted on a 2D map of intensity I(k,E) - is comparable to the single-particle spectral density function A(k,w) of the occupied states, a fundamental theoretical quantity. ARPES is a surface technique because of the short mean free path of the photoelectrons in the sample (to know more..)