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 light metals.

Under ambient conditions lithium is the lightest metal and also a superconductor: an ideal system to probe for lattice quantum effects!

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 under extreme conditions of pressure, temperature and magnetic field. 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.

S. Deemyad and R. Zhang (2018). "Probing quantum effects in lithium." Physica C

Abstract:In periodic table lithium is the first element immediately after helium and the lightest metal. While fascinating quantum nature of condensed helium is suppressed at high densities, lithium is expected to adapt more quantum solid behavior under compression. This is due to the presence of long range interactions in metallic systems for which an increase in the de-Boer parameter (λ/σ, where σ is the minimum interatomic distance and λ is the de-Broglie wavelength) is predicted at higher densities. Physics of dense lithium offers a rich playground to look for new emergent quantum phenomena in condensed matter and has been subject of many theoretical and experimental investigations. In this article recent progress in studying the quantum nature of dense lithium will be discussed.

Ackland, G.J., M. Dunuwille, M. Martinez-Canales, I. Loa, R. Zhang, S. Sinogeikin, W. Cai, and S. Deemyad (2017). " Quantum and isotope effects in lithium metal" Science.

Abstract:For the past 70 years, the lowest-energy crystal structure of lithium was believed to be a relatively complex one called the 9R structure. Ackland et al. show that this is incorrect. The actual lowest-energy structure for lithium is the much simpler closest-packed face-centered cubic form. In addition, 6Li and 7Li isotopes have crystal phase transitions at slightly different pressures and temperatures. This difference is chalked up to large quantum mechanical effects between the isotopes. Lithium is the only metal that shows this type of quantum effect and presents a challenge for theoreticians to explain.Science, this issue p. 1254The crystal structure of elements at zero pressure and temperature is the most fundamental information in condensed matter physics. For decades it has been believed that lithium, the simplest metallic element, has a complicated ground-state crystal structure. Using synchrotron x-ray diffraction in diamond anvil cells and multiscale simulations with density functional theory and molecular dynamics, we show that the previously accepted martensitic ground state is metastable. The actual ground state is face-centered cubic (fcc). We find that isotopes of lithium, under similar thermal paths, exhibit a considerable difference in martensitic transition temperature. Lithium exhibits nuclear quantum mechanical effects, serving as a metallic intermediate between helium, with its quantum effect–dominated structures, and the higher-mass elements. By disentangling the quantum kinetic complexities, we prove that fcc lithium is the ground state, and we synthesize it by decompression.

S. F. Elatresh, W. Cai, N. W. Ashcroft, R. Hoffmann, S. Deemyad and S. A. Bonev. (2017). "Evidence from Fermi surface analysis for the low-temperature structure of lithium"Nature Communications

Abstract:The low-temperature crystal structure of elemental lithium, the prototypical simple metal, is a several-decades-old problem. At 1 atm pressure and 298 K, Li forms a body-centered cubic lattice, which is common to all alkali metals. However, a low-temperature phase transition was experimentally detected to a structure initially identified as having the 9R stacking. This structure, proposed by Overhauser in 1984, has been questioned repeatedly but has not been confirmed. Here we present a theoretical analysis of the Fermi surface of lithium in several relevant structures. We demonstrate that experimental measurements of the Fermi surface based on the de Haas–van Alphen effect can be used as a diagnostic method to investigate the low-temperature phase diagram of lithium. This approach may overcome the limitations of X-ray and neutron diffraction techniques and makes possible, in principle, the determination of the lithium low-temperature structure (and that of other metals) at both ambient and high pressure. The theoretical results are compared with existing low-temperature ambient pressure experimental data, which are shown to be inconsistent with a 9R phase for the low-temperature structure of lithium.

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

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 .

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 .

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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