arXiv subjects

Riccardo Reho

Publications and source records attributed to Riccardo Reho.

3 recordsLinked to original sources

arXiv2026

The transverse matter Hamiltonian

Enrico Fermi in 1932 used classical Gauss equation to derive the Coulomb density--density interaction from the longitudinal electromagnetic potential. In this work we extend the Fermi procedure to the transverse component of the vector potential. By using a fully quantum canonical transformation, we replace the transverse vector potential with a current--current and current--current--density interactions. The transformed Hamiltonian is, then, projected in the fermionic space providing a matter--only, totally incoherent and gauge respecting Hamiltonian. The removal of coherence will avoid the breakdown of perturbation theory predicted by Haag's theorem, and make possible to introduce, in the transformed space, the diagrammatic approach. After discussing the implications of the transverse interactions on the current theories based on the Fermi procedure, we conclude by showing how the transverse Hamiltonian provides the quantum origin of the electromagnetic longitudinal--transverse splitting of phonons and other elemental excitations.

math-ph↗

arXiv2025

The crucial role of substrate in FeSe/STO: new insights to interface-driven superconductivity from first-principles

We investigate the superconducting properties of monolayer FeSe, both freestanding (ML FeSe) and on SrTiO$_3$ (STO), by simultaneously solving the Kohn-Sham Density Functional Theory and Bogoliubov--de Gennes equations. Our results demonstrate that the substrate profoundly alters both the normal-state and superconducting properties of FeSe. We identify proximity-induced superconductivity in the interfacial TiO$_2$ layer of STO, due to hybridization between Fe $d$ and O $p$ orbitals. This hybridization results in a fivefold increase in the superconducting gap width and confines superconducting states to the $M$ point in the Brillouin Zone. This is in contrast to ML FeSe, where superconductivity emerges at both the $\Gamma$ and $M$ points. Furthermore, the substrate modifies the orbital character of the states responsible for superconductivity, which change from Fe $d_{z^2}$ in ML FeSe to Fe $d_{xz}/d_{yz}$ in FeSe/STO. In both systems, we demonstrate an anisotropic superconducting gap with multiple coherence peaks, originating at different k-points in the Brillouin Zone. Additionally, in FeSe/STO, we identify emerging states unique to the superconducting phase arising from electron-hole hybridization at $M$, in agreement with experiments. Our findings highlight the decisive impact of substrate (hybridization, strain, charge transfer, magnetic order) on the superconducting properties of FeSe. We suggest potential pathways for engineering novel high-temperature FeSe-based superconductors by leveraging interfacial interactions in substrates with high electron affinity.

cond-mat.supr-con↗

arXiv2024

Roadmap on Quantum Magnetic Materials

Fundamental research on two-dimensional (2D) magnetic systems based on van der Waals materials has been gaining traction rapidly since their recent discovery. With the increase of recent knowledge, it has become clear that such materials have also a strong potential for applications in devices that combine magnetism with electronics, optics, and nanomechanics. Nonetheless, many challenges still lay ahead. Several fundamental aspects of 2D magnetic materials are still unknown or poorly understood, such as their often-complicated electronic structure, optical properties, and magnetization dynamics, and their magnon spectrum. To elucidate their properties and facilitate integration in devices, advanced characterization techniques and theoretical frameworks need to be developed or adapted. Moreover, developing synthesis methods which increase critical temperatures and achieve large-scale, high-quality homogeneous thin films is crucial before these materials can be used for real-world applications. Therefore, the field of 2D magnetic materials provides many challenges and opportunities for the discovery and exploration of new phenomena, as well as the development of new applications. This Roadmap presents the background, challenges, and potential research directions for various relevant topics in the field on the fundamentals, synthesis, characterization, and applications. We hope that this work can provide a strong starting point for young researchers in the field and provide a general overview of the key challenges for more experienced researchers.

cond-mat.mtrl-sci↗