César González Pascual

PhD-Physics: Postdoctoral researcher

  • Dr. César Gonzalez Pascual cesar.gonzalez@uam.es Phone number: +34 914972665

    STM code

    I am the main developer of the code STM2010 devoted to the simulation of the electronic current in a Scanning Tunneling Microscope (STM). A metallic or semiconducting substrate is scanned by a metallic at a close distance (less than 1 nm). An small bias is applied between both subsystems and, due to the quantum tunneling property, an electronic current is established. As a result, an atomic resolution can be obtained.


    The simulation of the STM current is based on the non-equilibrium Green function formalism. Within this methodology, the problem (our hamiltonian) can be separated in three different contributions: H=HS+HT+HI

    ˇThe first one is the sample hamiltonian (HS). In our methodology, it is calculated with FIREBALL code.

    ˇSecondly, the tip hamiltonian (HT) is calculated using a cluster of 105 W atoms (or 104 if we want to simulate an FCC metal such as Au. Cu or Pd). Again, we will use the FIREBALL code in order to simulate the tip. In the STM code, we will use only the density of states (DOS) of the last atoms..

    ˇFinally, the interactions hamiltonian (HI) couples the tip and the sample through a probability of hopping of one electron in an orbital from the tip to an orbital at the sample. These values are estimated for a great variety of distances with the FIREBALL code, using a dimer formed by all the possible combinations of elements in the tip and in the sample.

    After several pages of equations, the expression for the current is obtained, for a bias V, at 0K temperature:

    If you are interested in the simulation of the STM image for any new system, please contact me at: cesar.gonzalez@uam.es

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