Theory of Quantum Materials

Origin of the Drude peak shift

We have explained the origin of the displaced Drude peak in optical conductivity. Quantum acoustics field is emerging! Quantum acoustics—a recently developed framework parallel to quantum optics—establishes a nonperturbative and coherent treatment of the electron-phonon interaction in real space. The quantum-acoustical representation reveals a displaced Drude peak hiding in plain sight within the venerable Fröhlich model: the optical conductivity exhibits a finite frequency maximum in the far-infrared range and the dc conductivity is suppressed. Our results elucidate the origin of the high-temperature absorption peaks in strange or bad metals, revealing that dynamical lattice disorder steers the system towards a non-Drude behavior.

Read more:

Phys. Rev. Lett. 132, 186303, (2024).

Planckian resistivity in strange metals

Strange metals (high-temperature superconducting materials such as cuprates and heavy Fermion materials) exhibit universal linear-in-temperature resistivity described by the Planckian scattering rate, the origin of which was not understood. It was thought that phonons were disqualified as a prime agent, in spite of their predominance in many other resistivity contexts. The problem seems to have been not with phonons, but how they were treated. We showed that Planckian resistivity can in fact emerge from the nonperturbative scattering dynamics of thermal lattice vibrations and coherent charge carriers by a wave-on-wave approach. This becomes apparent when describing the lattice vibrations in the coherent state representation, and treating charge carriers as quantum wave packets negotiating the resulting dynamic disorder field formed by lattice vibrations, the deformation potential. Under this coherent and nonperturbative scattering dynamics, we find that carrier velocities are suppressed, the quasiparticle picture breaks down and a new phase called the vibronic fluid emerges. A competition between the static and dynamic aspects of the random acoustic deformation potential is set up, leading to a previously conjectured quantum bound of diffusion with universal properties. We successfully obtained the T-linear resistivity of three different strange metals using their experimental parameters in our numerical simulations. We explained the underlying mechanism for the Planckian scattering based on the phenomenological model of Thouless scaling theory. 

Read more:

PNAS, 121, 28, e2404853121, (2024).

Universal resistivity

Coherent state representation of lattice vibrations

Usage of coherent states to describe the electromagnetic field paved the way for comprehensive understanding of coherence in quantum optics. Following the same route in condensed matter physics, we provided a new description of lattice vibrations in terms of coherent states. When lattice vibrations are treated as coherent states, the deformation potential becomes a real field acting on electrons, making the electron-phonon interaction inherently non-perturbative. In our approach, the lattice vibrations create a disordered landscape where conduction electrons can quasi-elastically scatter, which preserves electron coherence beyond single collision events. This allows us to take the quantum coherence of electrons into account. Coherent state picture of the lattice has a potential to shed light on the unexplained properties of quantum materials.

Read more:

Phys. Rev. B, 106, 054311, (2022)