A low-energy enhancement (LEE) is found in the magnetic dipole radiation of actinide nuclei for the first time
We identified a LEE in the M1 strength function of actinides, the first such observation either theoretically or experimentally in this mass region (arXiv:251111565).
The shell-model Monte Carlo is successfully applied to the heaviest nuclei thus modeled, the actinides
Previously, the shell-model Monte Carlo has been applied to nuclei as heavy as the lanthanides. In a major step forward, we used SMMC to calculate level densities of the heaviest nuclei thus modeled, the actinides, which requires many-particle space dimensions as large as 1032 (arXiv:2509.26571). Our SMMC level densities are in excellent agreement with neutron reonance data and Oslo method experiments.
March 2025: Shasta Ramachandran wins best young presenter award in NMP25
Shasta Ramachandran won the best young prsenter award in the Seventh Conference on Nuclei and Mesoscopic Physics (NMP 25) for his talk on the Fermi polaron. The conference was held at FRIB, Michigan State University. For more details, see the article in the Yale Physics News.
First precision calculations of the Fermi polaron it its strongly interacting regime
The Fermi polaron, describing the interaction of an impurity with a polarized Fermi sea is a paradiagmatic system in quantum many-body physics. Using lattice AFMC and taking the continuum limit, we performed the first controlled calculations of the thermodynamics of the Fermi polaron (PRA 2025). In particular we calculated the contact as a function of temeprature and coupling strength.
A low-energy enhancement (LEE) is identified in the magnetic dipole radiaion of lanthanide nuclei
A LEE waas observed experimentally in mid-mass nuclei and several lanthanide nuclei. Conventional CI shell-model calculations in mid-mass nuclei suggest the LEE originates in the magnetic dipole (M1) strength function, but such calculations are prohibited in heavy open-shell nuclei. Using SMMC we identified a LEE in chains of samarium and neodymium isotopes. If the LEE persists in heavy neutron-rich nuclei, it would likely have profound effects on r-process nucleosynthesis by significantly enhancing the radiative neutron-capture rates of nuclei near the neutron drip line.
A novel method to calculate gamma-ray strength functions in heavy open-shell nuclei
The gamma-ray strenngth function is among the central statistical properties of nuclei, and is important for the calculation of compound-nucleus reaction rates. Combining SMMC with other many-body methods, we developed an approach to calculate the gamma strength function from the corresponding imaginary-time response function. The analytic continuation is carried out using the maximum-entropy method with the static-path approximation plus random-phase approximaiton (SPA + RPA) (PRC Letters 2024) or the SPA alone (PRC 2024, PRC 2025) as a good choice for the prior strength function. We apply the method to calculate the magnetic dipole (M1) strength function in heavy open-shell nuclei where conventional CI shell-model calculations are intractable.
A pseudogap regime is identified in the stronlgy interacting regime of the two-dimensional interacting Fermi gas
The two-dimensional Femi gas provides a paradigm of strongly interacting Fermi superfluids in 2D. Using lattice AFMC and taking the continuum limit, we peformed the first controlled calculation of two signatures of the pseudogap regime – the spin susceptibility and a free energy pairing gap (PRL 2024, EPJ Special Topics 2025). These results serve as a benchmark for future experiments and other strong coupling theories.
Explaining the large enhancement of level densities in the crossover from closed-shell to mid-shell nuclei
The large enhancement of level densities in the crossover from spherical to defomed nuclei has been a long-standing problem in nuclear physics. Using SMMC and mean-field methods we explained this enhancement (PLB 2021). This work was published in colaboration with the experimental group of Oslo University.
Our calculation of the contact of the unitary Fermi gas is in best agreement with precision experiments
The calculation of the contact of the unitary Fermi gas has been a major challenge for more than a decade with different strong coupling theories yielding widely different results. In a major breakthrough we calculated this quantity using lattice AFMC methods in the continuum limit (PRL 2020b) and found it to be in best quantititative agreement with recent precision experiments compared with all other theoretical methods.
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. We have shown (PRL 2020) that an approximation used in these studies greatly enhanced this effect. Our quantum Monte Carlo calculations, the most accurate to date, do not show pronounced signatures of a pseudogap and will likely motivate further experiments.
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, PRC 2018). 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.
We have used a Landau-like expansion in the quadrupole invariants p to fourth order to derive the finite-temperature distribution of intrinsic deformation (PRC 2018b). 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.
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. 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 (PRC 2018). 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.
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.”
For more details, see the articles in Yale News and the Yale Daily News.
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. 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.