Modern day material science is based on understanding the complex interactions of atomic constituents in clusters ranging from the nanoscopic (1-100s of atoms), through to macroscopic interaction ranges. Basic physics teaches that the negatively charged electrons interacting with the positive nuclear cores is responsible for basic atomic structure. However, even within single atoms the electrons may interact strongly with one another producing phenomena like “atomic magnetism”.

Bringing together many atoms in a condensed matter environment leads to further inter-atomic interactions between electrons and nuclear cores, for example, when one electron is simultaneously attracted to several nuclear cores. This results in lowering the overall energy of the system and causes these atoms to bond together. On the other hand, electrons repel one another because they have the same electrical charge and because they are quantum objects called fermions which cannot share an identical quantum state. This quantum repulsion is ultimately responsible for both the rich electronic infrastructure of the atoms themselves and the very complex interactions that inter atomic electrons have in condensed materials.

Such complex electronic interactions go under the name “electronic correlations”, and they are a very large area in modern condensed matter science research. Often materials of interest will have many different many-body electronic phenomena potentially active at the same time and the interplay and competition among them lead to multifaceted complex behavior. This is the playground of many condensed matter physicists: sorting out and understanding the large and growing variety of cooperative phenomena which underlie the behavior of today’s sophisticated materials.

Scientists learn about such things by applying known perturbations to the microscopic electronic environment and then seeing what happens. One of the most informative perturbations is to change the average spacing between the electrons and this is done by conducting experiments under various degrees of pressure. Pressure brings all the atomic constituents closer together changing the strength of the mutual interactions. These changes can then go on to modify the interplay among these interactions which often result in dramatically different physical behavior and properties. Through observing such changes we learn more about the underlying electronic correlation mechanisms themselves.

At TRIUMF, the experimental technique used to probe electronic correlations is called MuSR, or Muon Spin Resonance Spectroscopy. Muons are short-lived (2.2µs lifetime) particles resulting from the decay of even shorter-lived (26ns lifetime) particles called pions, which are produced when the 500 MeV TRIUMF cyclotron beam breaks apart nuclei in a production target. Muons are nature’s most sensitive and exquisite magnetic probe. A muon will react to a local magnetic field and transmit the static and dynamic information about this field to the outside world via an easily detectable positron decay. The muon lifetime is ideal for many time scales of interest in condensed matter systems and they also can be implanted into anything solid. The reason that sensitivity to the subtleties of the local magnetic field is such a good probe of electronic correlations is that electrons themselves create a magnetic field and therefore contribute a unique signature to the local magnetic field to which the muon are sensitive.

The art of applying pressure in experimental physics is relatively well developed, but if the pressures required are very high, like at TRIUMF where experimentalists would like pressures of up to 30,000 Atm (440,000 PSI), very strong and thick pressure cells must be used to sustain the forces. On the other hand, one still has to get the muon beam into, and the decay positrons out of, the cell. Other considerations in designing such targets include varying the temperature (related to the average spread in energy of the mobile electrons) and placing the system in a static external magnetic field (which provides a reference frequency signal in the absence of any interactions and correlations under investigation).

So, a MuSR high pressure experiment requires five basic ingredients. i) A cyclotron, like TRIUMF, to produce the muon beam. ii) A MuSR spectrometer, which will contain a magnet, an array of particle detectors, and the environmental vessel (cryogenic or oven) to house the sample/pressure cell. iii) The cryostat/oven to be able to apply a wide temperature environment for the cell/sample. iv) The pressure cell itself. v) A practical way to deliver the cell/sample into the spectrometer and cryostat/oven to minimize time spent changing samples. The elements going into the design of a typical pressure cell are shown in Figure 1.

Fig.1.

Layout* of a high pressure “clamp style” cell for muon spin resonance spectroscopy. The sample is located in a high strength metallic cylinder within a pressure transmitting fluid. An extremely strong tungsten carbide plunger is used to compress the fluid to very high pressure and then clamped in place. The pressure is uniform within the cell, and thereby transmitted to the sample. Special sealing techniques are used to keep the cell from leaking under thermal cycling conditions. * Designed by Dr. Goko, Tokyo University of Science.

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