511 keV γ\gamma-ray emission from the galactic bulge by MeV millicharged dark matter

We propose a possible explanation for the recently observed anomalous 511 keV line with a new "millicharged" fermion. This new fermion is light [${\cal O}({\rm MeV})$]. Nevertheless, it has never been observed by any collider experiments by virtue of its tiny electromagnetic charge $\epsilon e$. In particular, we constrain parameters of this millicharged particle if the 511 keV cosmic $\gamma$-ray emission from the galactic bulge is due to positron production from this new particle.Comment: 3 pages, 1 figure, A talk given by J.C.Park at the 16th International Conference on Supersymmetry and the Unification of Fundamental Interactions (SUSY08), Seoul, Korea, June 16-21, 200

Mott transition and abnormal instability of electronic structure in FeSe

FeSe has been extensively explored as a quantum material, primarily due to the observed highest superconducting transition temperature among Fe-based unconventional superconductors. Nonetheless, the electronic structure and the electron correlations responsible for the remarkable diversity of physical properties in FeSe remain elusive. We undertook a comprehensive investigation of the electronic structure of FeSe, known as a Hund metal, and found that it is not uniquely defined. Through accounting for all two-particle irreducible diagrams constructed from electron Green's function $G$ and screened Coulomb interaction $W$ in a self-consistent manner, a Mott-insulator phase of FeSe is unveiled. The metal-insulator transition is driven by the strong on-site Coulomb interaction in its paramagnetic phase, accompanied by the weakening of both local and non-local screening effects on the Fe-3$d$ orbitals. Our results suggest that Mott physics may play a pivotal role in shaping the electronic, optical, and superconducting properties of monolayer or nanostructured FeSe

Impact of F-D Kondo Cloud on Superconductivity of Nickelates

The Discovery of Superconducting Nickelates Reignited Hope for Elucidating the High-Tc Superconductivity Mechanism in Isostructural Cuprates. While the Superconducting Gap Opens Up on a Single Band of the Quasi-2D Fermi Surface in the Cuprates, the Nickelates Are Known to Have a 3D Nature of an Electronic Structure with a Multi-Band. This Raises a Serious Question About the Role of the 2D Nature for the High-Tc Superconductivity. Here, Employing GW + Dynamical Mean Field Theory (DMFT), We Report the Kondo Effect Driven by the Strong Correlation of Nd-4f and Ni-3d Electrons Emerging at Low Temperature. the Kondo Effect Modifies the Topology of the Fermi Surface, Leading to a 3D Multi-Band Nature. Remarkably, the Kondo Effect is Easily Destroyed by Lattice Modulation, Leading to the Quasi-2D Nature. Our Findings Could Provide a New Perspective for Explaining the Inconsistent Occurrence of Superconductivity and Distinct Electrical Resistivity Behavior between NdNiO2 Bulk and Films, Calling for an Experimental Measure of the Fermi Surface of Bulk NdNiO2

Topological acoustic triple point

Acoustic phonon in a crystalline solid is a well-known and ubiquitous example of elementary excitation with a triple degeneracy in the band structure. Because of the Nambu-Goldstone theorem, this triple degeneracy is always present in the phonon band structure. Here, we show that the triple degeneracy of acoustic phonons can be characterized by a topological charge $\mathfrak{q}$ that is a property of three-band systems with $\mathcal{PT}$ symmetry, where $\mathcal{P}$ and $\mathcal{T}$ are the inversion and the time-reversal symmetries, respectively. We therefore call triple points with nontrivial $\mathfrak{q}$ the topological acoustic triple point (TATP). The topological charge $\mathfrak{q}$ can equivalently be characterized by the skyrmion number of the longitudinal mode, or by the Euler number of the transverse modes, and this strongly constrains the nodal structure around the TATP. The TATP can also be symmetry-protected at high-symmetry momenta in the band structure of phonons and spinless electrons by the $O_h$ and the $T_h$ groups. The nontrivial wavefunction texture around the TATP can induce anomalous thermal transport in phononic systems and orbital Hall effect in electronic systems. Our theory demonstrates that the gapless points associated with the Nambu-Goldstone theorem are an avenue for discovering new classes of degeneracy points with distinct topological characteristics.Comment: 7+15 pages, 5+6 figure