Quantum Solids

Quantum theory predicts a ground state or zero-point energy with a non-zero dynamic part for every bound system of particles. This minimum energy is larger for lighter particles causing large lattice dynamics. Large zero point effects lead to exotic states such as liquid ground state of helium and predicted to prompt novel quantum states in compressed light metallic systems, such as pressure-induced melting at T~0K and two-component superconductivity and superfluidity. At sufficiently low temperatures, where thermal energy is less dominant, observations of lattice quantum dynamics are possible by studying isotope effects in lithium; the lightest metal.

In our lab we extensively study properties of lithium in unexplored regions of its phase diagram and looking for deviations from semi-classical models and new exciting physical phenomena. Indeed we find physics of lithium always full of new surprises!

The following video summarizes the result of one of our studies where we find an anomalous isotope effect in lithium.

Below you can read more about some of our findings in this area.

Schaeffer A. M, Cai. W., Olejnik E. † , Molaison J. J., Sinogeikin S., dos Santos A. M., Deemyad S. (2015). "New boundaries for martensitic transition of 7Li under pressure." Nature Communications 2015. 6.

Abstract: Physical properties of lithium under extreme pressures continuously reveal unexpected features. These include a sequence of structural transitions to lower symmetry phases, metal-insulator-metal transition, superconductivity with one of the highest elemental transition temperature and a maximum followed by a minimum in its melting line. The instability of lithium’s bcc structure, is well established by the presence of a temperature-driven martensitic phase transition. The boundaries of this phase, however, have not been previously explored above 3 GPa. All higher pressure phase boundaries are either extrapolations or inferred based on indirect evidence. Here, we explore the pressure dependence of the martensitic transition of lithium up to 7 GPa using a combination of neutron and X-ray scattering. We find a rather unexpected deviation from the extrapolated boundaries of hR3 phase of lithium. Furthermore, there is evidence that, above ~3 GPa, once in fcc phase, lithium does not undergo a martensitic transition.

Schaeffer, A. M., Temple S. R., Bishop J.K. and Deemyad S.(2015). "High-pressure superconducting phase diagram of 6Li: Isotope effects in dense lithium." Proceedings of the National Academy of Sciences 112(1): 60-64.

Abstract: We measured the superconducting transition temperature of 6Li between 16 and 26 GPa, and report the lightest system to exhibit superconductivity to date. The superconducting phase diagram of 6Li is compared with that of 7Li through simultaneous measurement in a diamond anvil cell (DAC). Below 21 GPa, Li exhibits a direct (the superconducting coefficient, α, Tc∝ M−α, is positive), but unusually large isotope effect, whereas between 21 and 26 GPa, lithium shows an inverse superconducting isotope effect. The unusual dependence of the superconducting phase diagram of lithium on its atomic mass opens up the question of whether the lattice quantum dynamic effects dominate the low-temperature properties of dense lithium.

Schaeffer, A. M.,Talmadge W. B., Temple S. R. and Deemyad S. (2012). "High Pressure Melting of Lithium." Physical Review Letters 109(18): 185702.

Abstract: The melting curve of lithium between ambient pressure and 64 GPa is measured by detection of an abrupt change in its electrical resistivity at melting and by visual observation. Here we have used a quasi-four-point resistance measurement in a diamond anvil cell and measured the resistance of lithium as it goes through melting. The resistivity near melting exhibits a well documented sharp increase which allowed us to pinpoint the melting transition from ambient pressure to 64 GPa. Our data show that lithium melts clearly above 300 K in all pressure regions and its melting behavior adheres to the classical model. Moreover, we observed an abrupt increase in the slope of the melting curve around 10 GPa. The onset of this increase fits well to the linear extrapolation of the lower temperature bcc-fcc phase boundary.

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