Cooper quartet correlations in infinite symmetric nuclear matter

Biexciton-like quartet condensates in an electron-hole liquid

“…With the consideration of the coherent state for the four-body sector, the trial wave function is adopted as [10,15]…”

“…While it is known that superconductors and fermionic superfluids are triggered by the formation of two-body loosely-bound states called Cooper pairs as a result of the Fermi-surface instability in the presence of twobody attractions [2], it is an interesting problem to explore condensation phenomena accompanying more than two-body bound states. While spin-1/2 fermions with s-wave interaction tend to form two-body Cooper pairs because of its spin degree of freedom and Pauli's exclusion principles, multi-body counterparts such as Cooper triples [3][4][5][6] and quartets [7][8][9][10][11][12][13][14][15] can be formed in the presence of larger degrees of freedom for fermions (e.g., isospin, color, atomic hyperfine states).…”

“…Nuclear equation of state and their droplet properties are associated with strongly attractive nuclear forces leading to the formation of bound states such as deuteron, alpha particle, and heavier nuclei in the low-density region [42], and the Fermi degenerate pressure of nucleons and multi-body forces in the high-density region [43]. Since alpha particle consisting of two neutrons and two protons is a stable cluster state with a large binding energy, the socalled alpha-particle condensation has been extensively studied in the context of Cooper quartets [7][8][9][10][11][12][13][14][15]. Note that fluctuation-driven quartet formations have also been investigated in unconventional superconductors [44,45].…”

“…In this paper, we theoretically investigate thermodynamic properties in an electron-hole system at zero temperature within the quartet BCS framework, which uses the extended BCS variational wave function involving Cooper pairing and quarteting in the momentum space at the thermodynamic limit [15]. A special attention is paid to the biexciton-like condensates, that is, the Cooper quartets consisting of two electrons and two holes as a result of the Cooper instability of Fermi seas.…”

“…(Note that we call it "biexciton-like" since a Cooper quartet considered here is a loosely-bound quantum state, unlike usual point-like bound states.) Recently, such a framework has been employed to study pair and quartet correlations in nuclear systems [10,[13][14][15]. Effects of Fermi degenerate pressure are automatically considered in this framework as in the usual BCS theory.…”