Monthly Archives: October 2015

The Shortcomings of Our Experimental Probes

Most experimental probes in condensed matter suffer from some inherent limitations. Though these limitations can be understood by reading reviews, I find that the most fruitful approach is to speak to someone working in the area. This is because new experimental probes are always cropping up and it is difficult to read up on a probe when there are not a lot of articles available.

Here are some relatively common probes and their corresponding limitations:

  1. Scanning Tunneling Microscopy/Spectroscopy (STM): Surface sensitive; energy gaps can sometimes depend on sample-to-tip distance; the density of states of the tip material can affect measurements and is not often measured
  2. Angle-Resolved Photoemission Spectroscopy (ARPES): Surface sensitive; p_z not resolvable as a result; needs a freshly cleaved surface; material must be a reasonable metal so that it doesn’t charge during photoemission process
  3. Inelastic Neutron Scattering (INS): Large single crystals needed; can’t measure collective modes like plasmons, as neutrons are not charged particles
  4. Inelastic X-ray Scattering (IXS): Particularly useful for smaller samples where INS cannot be used; even though IXS is sensitive to the electron density, most electrons are “core” electrons, meaning that phonons are more prominent in IXS spectra than electronic excitations, especially for materials containing high atomic numbers
  5. Infrared/Optics: To obtain most quantities of interest, such as conductivity or loss function, a Kramers-Kronig analysis is needed — this can affect the spectra depending on the energy range of data taken. Ellipsometry obviates the need for a K-K analysis, however; optics also cannot probe excitations away from the \Gamma point; for reflectivity measurements, a reference sample, such as gold, is needed
  6. Electron Energy Loss Spectroscopy (EELS): Often suffers from multiple scattering problems so that response is not perfectly linear; energy resolution is not great for transmission EELS (~80 meV), while reflection EELS is usually surface sensitive (probe depth ~5-20 Angstroms).
  7. NMR Knight Shift: the orbital magnetization is usually subtracted — how this is done is important.

Transport and thermodynamic measurements suffer from fewer shortcomings than most of the spectroscopic tools that I’ve mentioned above. I’m far from an expert in those measurements, so I’d be interested to hear more in the comments section about the inherent limitations of those experimental probes.

I do feel that it is not only experimentalists that need to have a good grasp of the limitations of experimental probes, but also theorists. A deeper understanding of experimental methods will ultimately result in a more nuanced view of data. This point is especially important when it comes to a topic like high-Tc superconductivity where it seems like there are new probes being used every day.

If anyone wants to add to the list, please feel free to do so in the comments. If there are any glaring ones that I’ve missed, please let me know!

Some Questions that Arose During My Graduate Years





