Applying pressure in a non-destructive manner is often easier said than done. It is often a fine line that is walked between applying just enough force to do the job and breaking something important. As a part of an exploratory training project, a part of my studies in the CDT-CMP at the Universities of Bristol and Bath, I have learned to better navigate this line and gain first-hand experience of some of the methods used within condensed matter physics. Here, I’ll discuss some of the methods and considerations when exploring pressure, and some current, active areas of research.
Pressure is an important tuning parameter. When compared with magnetic fields, for example, the accessible energy scales are enormous. A quick estimate reveals that even a relatively modest pressure of 10 GPa corresponds to an energy density of 0.1 eV, three orders of magnitude greater than that of a 10 T magnetic field. The energies associated with such pressures quickly become comparable to chemical bond energies and thus, one can expect substantial physical changes. Notable examples are the onset of superconductivity in solid oxygen at around 100 GPa, or more recently, the discovery of superconductivity at a record high temperature of 203 K in H2S when pressurised to 150 GPa.
An additional advantage of the study of materials at high pressures is the relative simplicity with which pressure can be modelled theoretically. As an illustration of the strength of such theoretical approaches, a recent density-functional-theory study of iron oxides involved the computational searching of tens of thousands of possible structures and revealed new stable arrangements of a number of iron oxides at pressures up to 500 GPa. Such studies have the potential to uncover a plethora of new systems in which novel states of matter may reside.
The practical realisation of pressures of the aforementioned orders of magnitude was previously only possible using dynamic methods involving the generation of shockwaves. Originally generated by detonating explosives, shockwaves only produce high pressures for limited time intervals and with only an approximate degree of control. Despite these limitations, high-pressure phase transitions in ionic solids, the metallisation of silicon and the halides was successfully observed in studies utilising these methods, as shown in this review article. More recently, the generation of shockwaves has been achieved using pulsed lasers. Notably, the National Ignition Facility, USA has achieved pressures up to 5 TPa in this fashion.
However, these dynamic methods intrinsically involve substantial heating and are not well suited for the study of typically low-temperature, electronic phenomena within condensed matter physics. The use of static methods allows for the prolonged measurement of optical and transport properties under applied magnetic fields, at low temperatures and does not necessitate the destruction of the sample.
Typically, piston cylinder cells, capable of achieving pressures up to 10 GPa and diamond anvil cells, which have proved to be capable of up to hundreds of gigapascals are used. Piston cylinder cells are well-suited for studying larger samples. By nature, they are opaque, making them suitable mainly for transport and neutron scattering measurements. In contrast, diamond anvil cells usually have a much smaller sample-space but, being transparent, are ideally-suited for optical and x-ray studies as well.
It is not typical to apply pressure directly on a sample, but rather through a medium in which a sample is held. A good choice of pressure medium will ensure that pressure is applied isotropically. Condensed, inert gases, oils and soft solids are commonly used. Both soft and hard solids are convenient for heating as they are typically good thermal insulators. Hard solids, however, are nonhydrostatic media. Consequently, they are suited primarily for preferentially applying uniaxial pressure.
Another important consideration is the choice of pressure gauge. For use with diamond anvil cells, the fluorescence of ruby makes for a convenient, well-calibrated choice. Otherwise, it is common to measure a property of a material that has a close-to-linear dependence on pressure and calibrate it against known fixed points. Such a parameter might be the resistance of Manganin and the fixed points could be well-resolved superconducting or crystallographic transitions.
High-pressure condensed matter physics is a growing field. There is a large amount of interest surrounding superconductivity. Numerous studies have found that
superconductivity can be both enhanced and induced by pressure and the application of pressure appears to be instrumental to the search for higher temperature superconductors.
Another active area of research is the study of quantum critical phenomena. Pressure is an ideal non-thermal parameter that can be tuned in the vicinity of quantum critical points. Quantum critical behaviour has been observed in a wide range of systems including heavy fermion metals.
In summary, pressure is capable of revealing a wide variety of novel phenomena. Although theory is still able to predict properties at pressures exceeding those that are experimentally realisable, the discrepancy is closing. New methods and techniques are capable of exploring condensed matter physics at ever increasing pressures.
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