Esra Yüksel - Research

Research overview

My research develops microscopic theories of atomic nuclei and applies them to questions in nuclear structure, nuclear astrophysics and fundamental interactions. The principal theoretical framework is nuclear energy density functional theory in relativistic and non-relativistic formulations. I also develop and apply random-phase approximation and quasiparticle random-phase approximation methods, including their finite-temperature and charge-exchange extensions. These approaches make it possible to study nuclear systems under conditions that are difficult or impossible to reproduce directly in laboratories.

Nuclear structure at zero and finite temperature

Atomic nuclei behave differently when they are heated to the temperatures found in stellar environments. Pairing correlations weaken, new excitation channels become accessible and the boundaries of nuclear stability may shift. My research investigates:

Ground-state properties of atomic nuclei
Pairing correlations at finite temperature
Nuclear deformation and clustering
Changes in the neutron and proton drip lines
Nuclear stability in hot stellar environments

This work has shown that the limits of nuclear existence are not fixed and can expand as temperature increases.

Collective excitations and electromagnetic response

Collective excitations provide important information about the internal structure of nuclei and the nuclear interaction. I calculate electric and magnetic multipole responses using microscopic energy density functional methods. Topics include:

Giant resonances; Electric dipole excitations, Magnetic dipole transitions, Pygmy dipole strength, Quadrupole and octupole collectivity
Gamma-ray strength functions
Temperature-dependent electromagnetic response

These calculations help interpret experimental measurements and provide nuclear-physics inputs for astrophysical models.

Weak interactions in stellar environments

Weak-interaction processes strongly influence the evolution of massive stars, supernovae and nucleosynthesis. I develop finite-temperature charge-exchange models to calculate:

Gamow–Teller transitions
Electron-capture cross sections and rates
Beta-decay half-lives
Temperature-dependent weak-interaction strength

The aim is to provide microscopic weak-interaction rates for astrophysical simulations under realistic stellar conditions.

Nuclear energy density functionals

A central part of my research concerns the development and constraint of nuclear energy density functionals. I use nuclear ground-state properties together with collective excitation data and parity-violating electron-scattering measurements to improve the predictive power of nuclear models. This work contributed to the development of the DD-PCX relativistic point-coupling interaction. The interaction was constrained using both ground-state and excitation properties, providing a framework for studying finite nuclei and nuclear matter.

From finite nuclei to neutron stars

The properties of neutron-rich atomic nuclei are closely connected to the physics of neutron stars. My research studies relationships between:

Dipole polarizability, Neutron-skin thickness, Nuclear symmetry energy
Neutron-star radius, Neutron-star compactness, Tidal deformability

These connections provide a bridge between laboratory measurements, microscopic nuclear theory and multimessenger astronomy. Recent work has examined whether experimentally measurable properties of finite nuclei can serve as probes of neutron-star structure.

Machine learning for nuclear physics

Experimental nuclear data become increasingly limited for nuclei far from stability. Machine-learning methods can help identify patterns and make predictions in these unexplored regions. My work applies machine-learning models to:

Nuclear mass predictions, binding energies
Physics-informed feature engineering
Extrapolation beyond measured nuclei
Uncertainty-aware nuclear modelling

The objective is to combine physical knowledge with data-driven methods rather than treating machine learning as a replacement for microscopic theory.

03/09/2024

Using machine learning to predict nuclear properties

Hi Reader,

We are pleased to present our new paper on nuclear mass predictions using machine learning models. In our study, we used two different machine learning (ML) techniques: Gaussian Process Regression and Support Vector Regression, to predict nuclear mass.

These models perform well in reproducing available experimental data and also have the capability to make extrapolations. Although machine learning models can still be considered a black box, they offer innovative approaches to predicting nuclear properties. The future is bright for ML.

For more information, please have a look to our paper

Esra Yüksel, Derya Soydaner, and Hüseyin Bahtiyar, Phys. Rev. C 109, 064322 (2024).

Figure: The evolution of two-neutron and two-proton drip lines, with the corresponding systematic uncertainty bands, for even-even nuclei between 8 ≤ Z ≤ 104 at T = 0, 0.5, 1.0 and 2.0 MeV. Black squares denote the experimental data for even-even stable nuclei at zero temperature. The changes in the drip lines can be seen with increasing temperature.

30/08/2023

The limits of nuclear landscape at finite temperatures

Hi Reader,

I have some exciting news – our article on 'Expanding the limits of nuclear stability at finite temperature' has been published in Nature Communications! :D

In this work, we conducted large-scale calculations to study the impact of temperature on nuclear properties and drip lines. As shown in the figure provided on the left, the two-neutron drip lines display sensitivity to the increase in temperature; the number of bound nuclei changes with increasing temperature.

These findings can impact the calculations in hot stellar environments in which the precise knowledge of nuclear input is required.

Ante Ravlić, Esra Yüksel, Tamara Nikšić & Nils Paar, Nature Communications volume 14, Article number: 4834 (2023) .

01/01/2023

Implications of parity-violating electron scattering experiments on 48Ca (CREX) and 208Pb (PREX-II) for nuclear energy density functionals

Hello Reader :D

Here is the first post of the new year :D

Recent precise parity-violating electron scattering experiments on 48Ca (CREX) and 208Pb (PREX-II) provide new results on the neutron-skin thicknesses of the relevant nuclei. The neutron skin thickness measurements are especially critical to constrain the isovector sector of the nuclear energy density functionals (EDF), hence, these measurements have a critical importance for improving the EDFs.

In this study, we investigate the implications of CREX and PREX-II data on nuclear matter symmetry energy and isovector properties of finite nuclei: neutron skin thickness and dipole polarizability.

Interestingly, the CREX and PREX-II experiments could not provide consistent constraints for the isovector sector of the EDFs! Our results show that the symmetry energy values predicted using the PREX-II and CREX experimental data are quite different, and it seems that we have to work more on the EDfs.

If you want to more about our work, please check the following paper!.

E. Yüksel and N. Paar, Physics Letters B, 836, 13762, (2023).

03/12/2021

β-decay half-lives in stellar environments

Nowadays, Large-scale calculations are my favorite :D

Recently, we extended our finite temperature calculations to calculate nuclear weak-interaction processes in stellar environments.

Our calculations indicate a significant decrease in the beta-decay half-lives of nuclei near the stability line with increasing temperature. Although it is less compared to the nuclei near the stability line, the beta-decay half-lives of drip-line nuclei also decrease.

In our work, we also discuss the effects of the first forbidden transitions and de-excitations at finite temperatures. Please check the following paper if you are interested :D

Reference: A. Ravlić, E. Yüksel, Y. F. Niu, and N. Paar, Phys. Rev. C 104, 054318 – 2021.

24/02/2021

Performance of the artificial neural networks on the nuclear binding energy predictions

Here we are again :D

Recently, we explored the predictive power of the artificial neural networks for the total binding energies of nuclei. Although we use a quite simple model, the results seem quite promising, and the root-mean-square deviation of our results compared to the experimental data is obtained as 1.84 MeV.

It seems that we have a new promising research area :D Our group is working diligently on this topic and trying to find better results by using more sophisticated methods.

Reference: Esra Yuksel, Derya Soydaner, Huseyin Bahtiyar, International Journal of Modern Physics E (2021).

Electron capture rates λe for 56Fe with respect to temperature T for different densities. The results for the FT-PNRRPA (red solid line) are shown together with the FT-PNRQRPA calculations for different strengths of isoscalar pairing. The EC rates using the TQRPA (blue squares), the shell-model (SM) with GXPF1J interaction (black dots) and LSSM calculations (purple triangles) are also shown for comparison.

02/01/2021

Electron capture rates at finite temperatures

Keeping this web page up-to-date is a bit challenging :D

Here is another work, which is related to the temperature effects on the electron capture rates of nuclei.

It is well-known that one of the essential ingredients of the electron capture calculations is the Gamow-Teller (GT) excitations. Therefore, the changes in the GT excitations can affect the electron-capture rates considerably.

In the presupernova collapse, electron capture occurs at finite temperatures and high densities. Therefore, one should also implement these conditions to the calculations in order to see their effect on the nuclear excitations and electron capture rates. This was the aim of the present work and we studied the temperature effects on the GT excitations and electron capture rates of nuclei. We show that, at low temperatures, pairing plays a crucial role in the determination of the electron capture rates and it has to be taken into account properly.

Of course, we need to do more sophisticated models and calculations (including deformation effects, particle-vibration techniques, removing the sharp pairing phase transition at high temperatures, etc.) to get better results. However, this kind of calculation is not feasible yet. Nonetheless, our results seem interesting :D

Reference: A. Ravlić et. al., Phys. Rev. C 102, 065804 (2020).

08/11/2020

Gamow-Teller excitations at finite temperature

After a long term "run" to complete finite-temperature codes and calculations, our results are ready to be presented. <----

In this work, we studied the effect of the temperature on the Gamow-Teller excitations of nuclei. Similar works had also been published about the effects of temperature on different excitation modes of nuclei. However, most of the calculations were done without the inclusion of the pairing effects, which is very crucial for a proper description of open-shell nuclei.

Using the Finite temperature QRPA, we investigated the behavior of the Gamow-Teller excitations with increasing temperature. For this purpose, we choose 42Ca, which is quite sensitive to the changes in the pairing values, especially in the low-energy region. The competition between the decreasing effects of the isoscalar and isovector pairing and increasing effect of the temperature on the excited states were discussed in detail.

Reference: E. Yüksel, N. Paar, G. Colò, E. Khan, and Y. F. Niu, Phys. Rev. C 101, 044305 (2020).

05/06/2020

Constraining the relativistic energy density functional with nuclear ground state and collective excitation properties

Today, I would like to present you our work about the optimization of a new relativistic functional called DD-PCX.

In this work, we introduce a new relativistic energy density functional DD-PCX. This functional is optimized by the ground-state properties of atomic nuclei along with the isoscalar giant monopole resonance energy and dipole polarizability in 208Pb.

The relativistic point-coupling interaction DD-PCX introduced in this work, represent the first effective interaction that is optimized using the ISGMR energy and dipole polarizability in the χ2 minimization.

As can be seen from the figures, the DD-PCX results for the neutron skin thickness of 208Pb lies within the experimental results. In addition, the dipole polarizability results are in good agreement with the experimental data.

Reference: E. Yüksel, T. Marketin, and N. Paar, Phys. Rev. C 99, 034318 (2019).