Quasiparticle interference imaging
Quasiparticle interference (QPI) imaging is a technique used in condensed matter physics that allows a scanning tunneling microscope to image the electronic structure of a material and infer information about the momentum space electronic structure from imaging the density of states in real space. In a scanning tunneling microscope, a very sharp metal tip is brought within a few angstrom of a sample. When a voltage is applied between the two and the tip is sufficiently close, a tunneling current between the two can be measured and used, for example, to record atomically resolved images of the surface. Keeping the position of the tip constant and changing the bias voltage allows acquisition of tunneling spectra.
While scanning tunneling microscopy and spectroscopy of a perfect crystal would show the same tunneling spectrum at each point on the surface of the crystal due to translational invariance, if there is a defect, the density of states acquires a spatial dependence with modulated patterns that reflect the characteristic wavelength of the electrons in the material. These spatial modulations are effectively Friedel oscillations, except that Friedel oscillations describe the modulation in the charge density rather than the density of states.
Quasiparticle interference imaging has been applied to the study of a range of quantum materials and low-energy electronic structures. While angle-resolved photoemission spectroscopy (ARPES) is a more direct technique to study the electronic structure of a material, QPI differs from ARPES that its energy resolution is only limited by the temperature of the experiment. QPI measures both occupied and unoccupied states in the same measurement, and it can be measured in a magnetic field.