In the framework of a holographic QCD approach we study the influence of matter on the deconfinement temperature $T_c$. We first consider the quark flavor number ($N_f$) dependence of $T_c$. We observe that $T_c$ decreases with $N_f$, which is consistent with a lattice QCD result. We also delve into how the quark number density ${\rho }_q$ affects the value of $T_c$. We find that $T_c$ drops with increasing ${\rho }_q$. In both cases, we confirm that the contributions from quarks are suppressed by $1/N_c$, as it should be, compared to the ones from a gravitational action (pure Yang-Mills theory).

The shape of a nucleus is one of the fundamental nuclear properties. We perform a systematic investigation of bubble nuclei that also exhibit shape coexistence in Hf, W, Os, Pt, and Hg even-even isotopes using the deformed relativistic Hartree-Bogoliubov theory in continuum. For a systematic study, we first consider nuclear bubble structures and shape coexistence separately. We confirm that deformations and pairing correlations hinder bubble structures by comparing our results with those from relativistic continuum Hartree-Bogoliubov theory that assumes spherical symmetry in nuclei. We then predict candidate isotopes with both bubble structure and shape coexistence. We observe that the depletion fraction factor that characterizes bubble structure is mostly smaller in oblate deformation than in prolate, while some isotopes such as ${}^{206}\mathrm{Os}$ have bubble structures both in oblate and prolate deformations. We compare the proton single-particle energy levels for the candidates of shape coexistence both with only prolate bubble structure and with prolate and oblate bubble structures.

We perform a quantitative study of the microscopic effective shell-model interactions in the valence $sd$ shell, obtained from modern nucleon-nucleon potentials, chiral N3LO, JISP16, and Daejeon16, using no-core shell-model wave functions and the Okubo-Lee-Suzuki transformation. We investigate the monopole properties of those interactions in comparison with the phenomenological universal $sd$-shell interaction, USDB. Theoretical binding energies and low-energy spectra of O isotopes and of selected $sd$-shell nuclei, are presented. We conclude that there is a noticeable improvement in the quality of the effective interaction when it is derived from the Daejeon16 potential. We show that its proton-neutron centroids are consistent with those from USDB. We then propose monopole modifications of the Daejeon16 centroids in order to provide an adjusted interaction yielding significantly improved agreement with the experiment. A spin-tensor decomposition of two-body effective interactions is applied in order to extract more information on the structure of the centroids and to understand the reason for deficiencies arising from our current theoretical approximations. The issue of the possible role of the three-nucleon forces is addressed.

We investigate spherical proton emitters in the region from I $\left(Z=53\right)$ to Bi $\left(Z=83\right)$ isotopes as a crucial application of the relativistic energy density functional PC-PK1 beyond the drip line. The continuum effect is treated carefully by using the relativistic continuum Hartree-Bogoliubov theory. We calculate the half-lives of the proton emitters using the Wentzel-Kramers-Brillouin (WKB) method and observe that the calculated values are in reasonable agreement with experimental results for most of emitters considered in this study. We also discuss a clue to improve the functional PC-PK1 in future.

We study the effects of flavor symmetry breaking on holographic dense matter and compact stars in the D4/D6 model. To this end, two light flavors and one intermediate mass flavor are considered. For two light quarks, we investigate how the strong isospin violation affects the properties of holographic dense matter and compact stars. We observe that quark-antiquark condensates are flavor dependent and show interesting behavior near the transition from dense matter with only one flavor to matter with two flavors. An intermediate mass quark is introduced to investigate the role of the third flavor. The mass-radius relations of holographic compact stars with three flavors show that the mass-radius curve changes drastically at a transition density from which the third flavor begins to appear in the matter.

The problem of the existence of a stable vacuum field in pure QCD is revised. Our approach is based on using classical stationary nonlinear wave type solutions with an intrinsic mass scale parameter. Such solutions can be treated as quantum-mechanical wave functions describing massive spinless states in quantum theory. We verify whether nonlinear wave type solutions can form a stable vacuum field background within the framework of the effective action formalism. We demonstrate that there is a special class of stationary generalized Wu-Yang monopole solutions that are stable against quantum gluon fluctuations.