New quantum Monte Carlo calculations for the unitary Fermi gas show no clear signatures of a pseudogap
The unitary Fermi gas, a strongly-interacting quantum many-body system, has been studied extensively both theoretically and experimentally, and has connections to many areas of physics. A major topic of interest and debate is the existence of a so-called pseudogap regime (known from high-Tc superconductors) above the critical temperature for superfluidity, in which pairing correlations exist even though a superfluid condensate is not present. Previous Monte Carlo simulations by other groups claimed to demonstrate a pronounced pseudogap effect. In a recent paper (arXiv:1801.06163), we have shown that an approximation used in these studies greatly enhanced this effect. Our quantum Monte Carlo calculations, the most accurate to date, do not show clear signatures of a pseudogap and will likely motivate further experiments.
October 17, 2017: Yoram Alhassid is named the Frederick Phineas Rose Professor of Physics
President Peter Salovey announced today that the Yale Corporation voted to appoint Yoram Alhassid as the Frederick Phineas Rose Professor of Physics.
Endowed chairs at Yale are awarded to those whose scholarship has brought distinction to the University. Yoram’s outstanding research program is in nuclear physics and in many-body theory applied to a variety of other systems including cold atoms and quantum dots.
Novel reaction model clarifies a puzzle in the statistical theory of compound nucleus reactions
Statistical reaction theory is widely used in nuclear physics, atomic and mesoscopic physics. Recently observed deviations from its theoretical predictions in neutron scattering off Pt isotopes have generated widespread interest, and several explanations have been proposed. However, it has been unclear to what extent these explanations hold in the corresponding physical application. In a recent paper (arXiv:1710.00792), Paul Fanto and Yoram Alhassid, together with George Bertsch of the Institute for Nuclear Theory, have studied neutron scattering off 194Pt using for the first time a model that combines a realistic description of the neutron channel with the usual statistical description of the compound nucleus. The work confirms the prediction of the statistical theory for a large range of the model parameters. In addition, the results indicate a range of model parameters where the usual experimental analysis would yield apparent deviations from theory.
Novel method to extract deformation without invoking an intrinsic frame or a mean-field approximation
Deformation is a key concept in understanding the physics of heavy nuclei, but it is introduced in the framework of a mean-field approximation the breaks the rotational invariance of the underlying Hamiltonian. Yoram Alhassid, in collaboration with Christopher Gilbreth and George Bertsch of the Institute for Nuclear Theory, have developed a formulation that enables the study of lab-frame and intrinsic deformation in the rotationally invariant framework of the configuration-interaction shell model without invoking a mean-field approximation (PRL 2014, arXiv:1710.00072). The method is based on quantum Monte Carlo calculations of the lab-frame shape distribution and the use of rotational invariants constructed from the relevant deformation parameters.
Together with Mika Mustonen, they have used a Landau-like expansion to derive the finite-temperature distribution of intrinsic deformation (see in arXiv:1801.06175). This allows the calculation of statistical properties of nuclei as a function of intrinsic deformation, an important input for models of shape dynamics such as fission.
New method for restoring particle-number conservation in mean-field theories
Finite-temperature mean-field theories are efficient and widely used to calculate statistical properties of nuclei. However, they are derived in the grand-canonical ensemble where particle number fluctuates. Since the nucleus is a finite-size system, it is necessary to restore good particle number to make accurate predictions. In a recent paper that benchmarked the accuracy of mean-field theories (PRC 2016), we have tested several approximate particle-number projection formulas. However, in mean-field theories that account for pairing correlations, particle-number symmetry is inherently broken, introducing a phase ambiguity that limits the applicability of known projection formulas. In a recent work, Paul Fanto, Yoram Alhassid, and George Bertsch of the INT have devised a formula to circumvent this ambiguity in cases where the mean-field Hamiltonian preserves time-reversal symmetry (PRC 2017). In another paper (PRC 2017), Paul Fanto has extended this method for the restoration of general broken symmetries without assuming time-reversal invariance of the mean-field Hamiltonian.
March 22, 2017: Paul Fanto is awarded DOE NNSA Stewardship Science Graduate Fellowship
Graduate student Paul Fanto is one of five students selected to receive the Department of Energy National Nuclear Security Administration Stewardship Science Graduate Fellowship (SSGF). This fellowship, awarded to exceptional students in fields that “solve complex science and engineering problems critical to stewardship science,” provides up to four years of financial to support. In addition, the fellowship provides the opportunity to for a three month “practicum” at a NNSA national laboratory.