Alkali metals are canonical free-electron systems which, according to conventional wisdom, should become even more free-electron-like under pressure. It is, therefore, remarkable that theoretical work predicts that under strong compression the nearly free electrons in the alkali and alkaline earth metals should become markedly less free and begin to couple strongly with the lattice, leading to possible transformations to lower symmetry crystal structures and/or the appearance of superconductivity. Similar results are anticipated for the relatively simple beginning transition metals Sc, Y, and Lu which, as for the alkalis Li and Cs, show anomalous superconductivity under high compression. This project will study in depth to ultrahigh pressures the superconducting phase diagram and critical magnetic field behavior of selected alkali metals, their alloys and compounds, as well as in Sc, Y, and Lu. The systematics derived from this study should shed some light on the properties of hydrogen under pressure which has been predicted to become a metal at multi-Mbar pressures with possible superconductivity near room temperature. The proposed experiments give undergraduate and graduate students an excellent opportunity to both learn and develop important laboratory techniques and collaborate with groups both in the US and abroad.
Solids which are not metallic may become metals if high pressures are applied to bring the atoms closer together, thus increasing the orbital overlap of the outer electrons. In the simplest picture, applying high pressures should turn insulators into metals or good metals into better ones. To the contrary, experiment shows that the simplest metals known, the alkali metals Li, Na, K, Rb, and Cs, become increasingly complex under very high pressures, with similar results for the alkaline earths like Ca, Sr, or Ba. Under sufficient pressure Cs even turns into a transition metal, like Ta or Nb, and becomes superconducting, as do Li and Ca at transition temperatures as high as 15-25 K. Understanding the changes under pressure in these relatively simple systems deepens our understanding of metals and superconductivity in general. These studies provide insight into a possible superconducting state near room temperature in metallic hydrogen under millions of atmospheres of pressure. The proposed experiments give undergraduate and graduate students the opportunity to learn and develop a multitude of essential experimental techniques; these students will also benefit from collaborations with groups both in the US and abroad.
Superconductivity was discovered in mercury in 1911, more than 100 years ago. Below the so-called superconducting transition temperature Tc, a superconductor possesses the magical ability to conduct electricity without loss. For nearly 50 years, no one, not even Albert Einstein, could explain where superconductivity comes from and why some metals are superconducting and others are not. Finally, in 1957 Bardeen, Cooper, and Schrieffer were able to figure it out. Their landmark achievement earned all three the 1972 Noble Prize in Physics. The reason that superconductivity is so difficult to understand is that in a superconducting metal pairs of electrons appear in which the electrons attract each other and form a bound state, even though one would think that their like (negative) charge would cause them to repel each other! This is the mystery of superconductivity; this is why it is so difficult to develop a theory which predicts whether a given metal superconducts or not and, if so, what the value of Tc is. Exposing a superconducting material to high pressures causes all its properties, including Tc, to change, thus helping one better understand what makes it tick and how to increase Tc significantly. In some cases high pressures can even transform a normal metal into a superconducting metal. Unfortunately, the values of Tc reached so far are quite low, only -269°C in mercury, the first superconductor, and -140°C in the best superconductor discovered to date, a cuprate oxide containing mercury, both Tc's being well below the coldest temperature on earth. Exposing this mercury oxide to very high pressures increases Tc by 30°C; a further increase by 200°C would yield a material which superconducts well above room temperature. This would almost certainly spark a technological revolution. Electric power could be transported over vast distances with negligible loss, energy stored in superconducting magnets indefinitely for use in electric autos and giant storage facilities, powerful dynamos constructed, and much more. In this NSF-supported project, collaborative high pressure studies were carried out on a mercury-based cuprate oxide under both hydrostatic and uniaxial pressure conditions which yielded vital information on how to increase Tc further, namely: (1) increase the separation between the cuprate planes, and (2) decrease the area of these planes. The next step is to synthesize cuprate oxides which fulfill these conditions. In related Fe-based superconductors, however, high pressure was originally reported to initially enhance Tc, however, purely hydrostatic pressure studies in this project demonstrated that this is incorrect, that Tc actually decreases. This insight may lead to a new set of synthesis criteria to further enhance Tc. One of the primarily goals in this project is to search for new superconductors by applying extreme pressures using a diamond-anvil cell. Oxygen becomes a superconducting metal if a pressure of 1 million atmospheres is applied. In fact, of the 53 known elemental solids that superconduct, 23 only do so under high pressure. The latest high-pressure superconductor was discovered in this project in europium, a rare-earth metal that is magnetic at ambient pressure. Particularly interesting is the highly anomalous superconducting state induced by pressure in the alkali metal lithium, where the compression is so great that the inner electron shells begin to touch. Similar conditions apply in the other alkali metals K, Na, Rb and Cs under extreme pressure. Such studies are part of this project, as well as our recent attempt to induce superconductivity in benzene at 2 million atmospheres, the highest pressure this group has ever reached. Under even higher pressures superconductivity near room temperature has been predicted for metallic hydrogen. In addition, some materials only form under simultaneous high pressures and high temperatures, the hope being that upon release of pressure the material retains the new structure in metastable form. An excellent example for this is the synthesis of metastable diamond from graphite. The future of high-pressure studies in superconductivity and other areas of Materials Science appears very promising.
