This textbook explores foundational and cutting-edge topics in condensed matter physics. Inspired by the influential work of Prof. Yuri Kagan, the book spans key areas such as Bose-Einstein condensation, superfluid helium, crystal lattice dynamics, and phonon physics. It also addresses electronic excitations in metals and superconductors, including the BCS theory, electron-phonon interactions, and metallic hydrogen. Professor Maxim Kagan extends this work with advanced lectures on strongly correlated electron systems, quantum hydrodynamics, and mechanisms of unconventional superconductivity. The book concludes with a focus on groundbreaking research by Prof. Yuri Kagan, covering topics such as quantum gases, metallic hydrogen, and high-temperature superconductivity. This textbook serves as both a tribute to Prof. Kagan's legacy and a comprehensive resource for students and researchers in theoretical condensed matter physics.
