Quantum Criticality:
At the QCP, the system undergoes a continuous phase transition driven by quantum fluctuations, rather than thermal fluctuations as in classical critical phenomena. This quantum criticality gives rise to unusual electronic properties and scaling behaviors.
Electron Spin Fluctuations:
Quantum critical ferromagnets exhibit strong spin fluctuations due to the proximity to the magnetic instability. These spin fluctuations involve the spontaneous flipping of electron spins, leading to a reduction in the overall magnetic moment. The spin fluctuations become increasingly prominent as the system approaches the QCP.
Itinerant Electrons:
In many quantum critical ferromagnets, the electrons responsible for magnetism are itinerant, meaning they can move freely throughout the material. These itinerant electrons are strongly correlated and interact with each other through various quantum mechanical interactions, such as exchange interactions and Coulomb repulsion.
Non-Fermi Liquid Behavior:
The behavior of electrons in quantum critical ferromagnets often deviates from the conventional Fermi liquid picture, which describes electrons in metals as quasi-particles with well-defined energies and momenta. Instead, quantum critical systems exhibit non-Fermi liquid behavior, where the quasiparticle concept breaks down, and the electronic excitations have anomalous properties.
Magnetic Scaling and Universality:
Quantum critical ferromagnets often exhibit scaling behavior, where physical properties such as magnetic susceptibility, specific heat, and resistivity show power-law dependences on temperature or magnetic field. These scaling behaviors are universal, meaning they are independent of microscopic details and depend only on the dimensionality and symmetry of the system.
Quantum Critical Point:
At the QCP, the magnetic order disappears completely, and the system becomes scale-invariant. This means that the physical properties of the system are independent of the length scale, leading to self-similar behavior. The QCP is a singular point where various quantum fluctuations diverge, giving rise to critical phenomena.
Emergent Phenomena:
Quantum critical ferromagnets can host various emergent phenomena, such as unconventional superconductivity, quantum spin liquids, and topological order. These phenomena are not present in the ordered or paramagnetic phases and arise solely due to the quantum critical nature of the system.
The study of electrons in quantum critical ferromagnets is an active area of research in condensed matter physics, with implications for understanding fundamental quantum phenomena, exotic phases of matter, and the behavior of strongly correlated electron systems.
