Two-dimensional Fermi Gas Study Reveals Pseudogap Signatures in Strong Coupling Regimes

The behaviour of interacting quantum systems presents a fundamental challenge in physics, and understanding the transition between different states of matter is crucial for advancing our knowledge of these systems. S. Ramachandran, S. Jensen, and Y. Alhassid investigate the properties of a two-dimensional gas of interacting quantum particles, specifically a ‘Fermi gas’ where particles switch from pairing into bound states to forming a collective ‘Bose-Einstein condensate’. This research delivers precise calculations of key properties, such as the fraction of particles in the condensate and the system’s energy, by employing advanced computational techniques and carefully eliminating potential errors. The team identifies evidence of a ‘pseudogap’ regime, a state where particle pairing begins before full superfluidity emerges, offering new insights into the complex interplay of quantum interactions and providing a valuable benchmark for interpreting future experiments in this field.

Einstein condensate crossover behaviour is investigated as a function of ln(kF a), where a represents the scattering length. This work employs canonical-ensemble auxiliary-field quantum Monte Carlo methods on discrete lattices to calculate several thermodynamical quantities within the strongly interacting regime, systematically eliminating errors through extrapolation to continuous time and the continuum limit. The team presents results for the condensate fraction, spin susceptibility, contact, energy equation of state, and the free energy staggering gap, identifying signatures indicative of a pseudogap regime.

Correlated Fermi Gases and Superfluidity Studies

This research comprehensively examines the behaviour of two-dimensional Fermi gases as they transition between a state where particles form pairs, akin to superconductivity, and a state where they coalesce into composite molecules, forming a Bose-Einstein condensate. Researchers employed advanced computational techniques, specifically canonical-ensemble auxiliary-field quantum Monte Carlo simulations on discrete lattices, to investigate the system’s properties in strongly interacting conditions, carefully minimizing systematic errors through established extrapolation methods. The simulations calculated key thermodynamic quantities, including the condensate fraction, spin susceptibility, contact parameter, energy equation of state, and the free energy staggering gap, providing a detailed picture of the system’s characteristics. Results demonstrate the presence of a pseudogap regime, a state where pairing correlations persist even above the temperature at which the system becomes a superfluid. This was evidenced by signatures in both the spin susceptibility and the free energy staggering gap, indicating that particles begin to pair up at higher temperatures than previously expected.

Fermi Gas Crossover to Bose-Einstein Condensate

This work presents a detailed study of the two-dimensional Fermi gas, a system exhibiting a fascinating crossover from a state where particles bind into pairs, similar to superconductivity, to a state where they form composite molecules, a Bose-Einstein condensate. Researchers employed advanced computational methods, specifically canonical-ensemble auxiliary-field quantum Monte Carlo simulations on discrete lattices, to investigate the system’s behaviour in the strongly interacting regime, carefully eliminating systematic errors through extrapolation techniques. The simulations calculated several key thermodynamic quantities, including the condensate fraction, spin susceptibility, contact parameter, energy equation of state, and the free energy staggering gap, providing a comprehensive picture of the system’s properties. Results demonstrate the presence of a pseudogap regime, a state where pairing correlations persist even above the temperature at which the system becomes a superfluid.

This was evidenced by signatures in both the spin susceptibility and the free energy staggering gap, indicating that particles begin to pair up at higher temperatures than previously expected. Specifically, the team observed that pairing correlations remain robust even when the temperature exceeds the critical temperature for superfluidity, suggesting a complex interplay between pairing and fluctuations. The calculated energy gap at an interaction strength of approximately 1. 0 was found to be significantly larger than the energy required to break apart a single pair of particles, confirming the presence of this “many-body pairing regime” and non-trivial pseudogap phenomena.

Furthermore, the simulations provide precise measurements of the condensate fraction, revealing how the number of paired particles changes with interaction strength and temperature. The team also calculated the contact parameter, a quantity sensitive to the short-range correlations between particles, and the energy equation of state, which describes the relationship between energy, temperature, and density. These results, obtained through rigorous computational techniques, serve as a valuable benchmark for future experimental investigations of strongly correlated two-dimensional systems and provide crucial insights into the emergence of exotic quantum phases of matter.

Pseudogap and Thermodynamics of Strongly Interacting Systems

This research presents detailed calculations of the thermal properties of a two-dimensional system undergoing a transition between a BCS state and a Bose-Einstein condensate, specifically focusing on strongly interacting conditions. By employing advanced computational methods and carefully eliminating potential errors, the team obtained accurate results for quantities including the condensate fraction, energy, and a parameter known as Tan’s contact, which characterizes correlations within the system. These calculations provide a comprehensive picture of the system’s thermodynamic behaviour. A key finding is the identification of a pseudogap regime, where pairing correlations persist even above the temperature at which superfluidity emerges.

This was evidenced through analysis of the spin susceptibility and the free energy gap, revealing suppressed spin susceptibility and monotonically increasing contact and free energy gap with decreasing temperature. The team estimates a pairing temperature scale, suggesting that the influence of two-particle binding energy increases as the system approaches the Bose-Einstein condensate limit. While acknowledging that the observed pseudogap may arise from both two-particle and many-body pairing correlations, the study focused on conditions where two-particle binding is less dominant, providing insights into the many-body aspects of the phenomenon. The authors note that further research, particularly calculations of the system’s spectral function, will be valuable for direct comparison with recent experimental findings and will serve as a benchmark for future investigations.