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:
- 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
- Angle-Resolved Photoemission Spectroscopy (ARPES): Surface sensitive; 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
- Inelastic Neutron Scattering (INS): Large single crystals needed; can’t measure collective modes like plasmons, as neutrons are not charged particles
- 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
- 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 point; for reflectivity measurements, a reference sample, such as gold, is needed
- 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).
- 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!
Below are questions and concerns that arose during my graduate school career, many of which I still cannot answer. Most of these are specific to an experimentalist, but there are some in there that apply to theorists as well. Here is a list of some of these points:
- If I spend my time developing a new instrument and learning how to build a lab, knowing that my publication record will be delayed/suffer as a result, should I take on such a long-term project? Keep in mind that taking on a long-term project will instill me with the necessary skills to construct and design other experiments in the future.
- Related to 1: Are experimentalists who prioritize technique development (which may take years) over churning out papers with a few standard experimental techniques necessarily going to suffer as a result? Should the physics community necessarily prioritize one over the other?
- How much time should I spend trying to gain a broader knowledge of different subjects in my field and outside my field, knowing that while this may be a fruitful long-term strategy, I am likely to suffer in the short term?
- When is a good time to graduate? Is this dictated by the number of papers that I have published or when I feel like I am no longer growing a physicist in my current climate? Should I stay just so that the papers that are in the works get published?
- How much time should I devote to theory? Is it worth the time to learn quantum field theoretical methods and the like as an experimentalist? For a theorist: How much time should I spend getting accustomed to the various experimental techniques?
- How do I manage the two-body problem in the modern academic climate if my spouse is likely to get a job at an institution where I’m unlikely to receive an offer (or vice versa)?
- Is it possible nowadays to make a departure into the industrial sector and make my way back into academia at some point?
- If there is a prominent physicist I would like to postdoc with at an institution that does not have the “brand name”/reputation of a premier university, should I still work with him/her? What if the physicist is not particularly well-known, but is prominent in an non-mainstream sub-field?
- Are the papers one has published (number, prominence of journals, etc.) the only metric by which to measure the contribution of a graduate student? Have recommendations become more empty?
- Should one work in a field that is trendy/popular? Or should one search for physics in places that are less explored, realizing that one will necessarily receive a lower number of citations?
I suspect that many experimentalists (and maybe some theorists) have questions out there that are similar to these. On most of these questions, I think the safest bet is to take the middle path, but one’s natural inclination may be to lean one way or the other. For instance, I lean towards developing new spectroscopic methods (which I find creatively fulfilling), but this can take years, and publication output is not high during such an undertaking. In some sense, it is “safer” career-wise to just perform tried and true experiments on new materials.
I will try in the near future to answer some of these questions, but it should be said that I am far from being an authority of any kind on these topics. These are just some of the questions that inevitably arise in everyone’s mind during their time in graduate school in condensed matter physics, and I thought sharing my thoughts may help foster some conversation.
In some sense, it is a shame that questions like these have to arise in one’s mind at all during graduate school. Ideally, one would like to concentrate on one’s work and scientific output — however, the modern climate is such that these questions become unavoidable especially when one nears the end of a graduate school career.
Please feel free to share more questions/concerns in the comments.