Quantum phenomena at extreme conditions
At very high pressures delocalization of electrons provides a wealth of correlated electron phenomena: e.g., insulator-metal transitions, colossal magnetoresistance, valence fluctuations, heavy fermion behavior, non-Fermi liquid behavior, superconductivity, topologic state of matter, magnetic order, quadrupolar order, etc. The occurrence of such a wide range of correlated electron phenomena arises from a delicate interplay between competing interactions that can be tuned by pressure, resulting in complex temperature T vs P phase diagrams. A particularly striking phenomenon that has been observed in many correlated electron systems, including heavy fermion f-electron compounds, high Tc superconducting cuprates, and, many Fe-based materials, is the emergence of superconductivity near the critical value of a control parameter where a magnetically ordered phase is suppressed to 0 K. For some systems, the Fermi liquid paradigm is found to be violated in the vicinity of critical value, which is manifested as weak power law and logarithmic divergences in the physical properties at low temperature. The superconductivity and the non-Fermi liquid behavior may be due to quantum fluctuations of the magnetic order parameter (OP) associated with the suppression of a second order magnetic phase transition to 0 K at critical value, where critical value is referred to as a quantum critical point (QCP). The formation of the superconducting phase appears to “protect” the QCP by removing the degeneracy associated with the OP fluctuations, and the superconducting electron pairing is apparently mediated by magnetic interactions. In contrast, magnetic interactions generally have a destructive effect on conventional BCS superconductivity. Pressure may be key tuning parameter to access and understand various quantum phenomena in novel materials.
Pressure–temperature phase diagram for carbon disulfide, showing the superconducting transition at TC, the magnetic ordering transition at TN, and local structure change from a tetrahedral to an octahedral configuration [Dias et al PNAS (2013)]
Probing room temperature superconductivity in dense hydrogen-rich materials and carbon based materials
Efforts to identify and develop new superconducting materials continue to increase rapidly, motivated by both fundamental science and the prospects for applications. High pressure plays an increasingly important role in such efforts, as at high pressures the delocalization of electrons provides a wealth of correlated electron phenomena.
The hydrogens—hydrogen and its isotopes—are the simplest and most abundant of the elements in the universe. Conceptually hydrogen, with a single proton and electron is the simplest atomic system in the periodic table of the elements, yet has exceptionally complex behavior due to its light mass and interactions with other hydrogen atoms. As a neutral electron spin-polarized gas, it does not form a liquid in the T→0 K limit; unpolarized it readily forms stable molecules that solidify at ~14 K. When pressurized to millions of bars it is predicted to dissociate to an atomic metal, predicted to have exotic properties such as high-temperature superconductivity, superfluidity, and a liquid state at megabar pressures in the low temperature limit. Understanding these quantum effects and establishing the phase diagram of the various isotopes of molecular hydrogen has been an intriguing scientific challenge for decades.
Understanding the Dynamic Response of Solid under Extreme Conditions