[Paper Review] Cooling by photo-doping $--$ Light-induced symmetry breaking in the Hubbard model

Philipp Werner, Martin Eckstein+2 more|arXiv (Cornell University)|Apr 1, 2019

Physics of Superconductivity and Magnetism37 references11 citations

TL;DR

This paper proposes a light-induced cooling mechanism in the Hubbard model via photo-doping, where holes are transferred from a doped Mott insulator to a filled, narrow band, enabling isentropic cooling. Using nonequilibrium dynamical mean-field theory, it demonstrates that chirped laser pulses can cool the system below the Néel temperature, inducing antiferromagnetic order in a state initially above the critical temperature.

ABSTRACT

An elusive goal in the field of driven quantum matter is the induction of long-range order. Here, we demonstrate a mechanism based on light-induced evaporative cooling of holes in a correlated electron system. Since the entropy of a filled narrow band grows rapidly with hole doping, the isentropic transfer of holes from a doped Mott insulator to such a band results in a drop of temperature. Strongly correlated Fermi liquids and symmetry-broken states could thus be produced by dipolar excitations. Using nonequilibrium dynamical mean field theory, we show that suitably designed chirped pulses allow to realize this cooling effect. In particular, we demonstrate the emergence of antiferromagnetic order in a system which is initially in a weakly correlated state above the maximum Néel temperature. Our work suggests a general strategy for inducing strong correlation phenomena and electronic orders in light-driven materials or periodically modulated atomic gases in optical lattice potentials.

Motivation & Objective

• To address the challenge of inducing long-range electronic order in driven quantum matter, particularly in the absence of thermalization and heating effects.
• To explore whether photo-doping can enable effective cooling in strongly correlated systems by leveraging entropy transfer to a filled band.
• To demonstrate that symmetry-broken states like antiferromagnetism can emerge in a system initially above the Néel temperature through controlled optical excitation.
• To develop a realistic protocol using chirped pulses to achieve isentropic cooling and electronic ordering in correlated electron systems.

Proposed method

• Uses nonequilibrium dynamical mean-field theory (DMFT) with the non-perturbative NCA+DMFT solver to simulate the driven system.
• Models the system as a doped Hubbard band (the 'system') coupled to a fully filled, narrow band (the 'core') via dipolar, time-periodic tunneling.
• Applies a time-dependent hybridization function $ v_{\text{system-core}}(t) = a(t)f(t-t_p) $, where $ a(t) = a_{\text{max}} \sin(\Omega t) $, to simulate laser-induced transitions.
• Employs a linearly chirped pulse to dynamically tune the driving frequency $ \Omega $ to match the evolving chemical potential difference between the bands.
• Calculates time-dependent spectral functions and occupation distributions via retarded and lesser Green's functions to track the distribution function and cooling.
• Implements antiferromagnetic order via a two-sublattice DMFT approach with spin-flipped hybridization in the self-consistency loop.

Experimental results

Research questions

• RQ1 Can photo-doping induce effective cooling in a strongly correlated electron system by transferring entropy to a filled band?
• RQ2 Can a system initially above the Néel temperature be cooled below it via light-induced hole evaporation?
• RQ3 Is antiferromagnetic order achievable in a nonequilibrium state driven by a chirped laser pulse?
• RQ4 How does the choice of driving protocol (e.g., constant vs. chirped frequency) affect the cooling efficiency and emergence of order?
• RQ5 What is the role of the core band's bandwidth and initial chemical potential in enabling isentropic cooling?

Key findings

• A linearly chirped laser pulse enables efficient hole transfer from the Hubbard band to the core band, resulting in a significant drop in effective temperature.
• The system cools below the Néel temperature of the Hubbard model, even though it was initially prepared above it, indicating the emergence of antiferromagnetic order.
• The cooling mechanism relies on isentropic transfer: entropy per hole is high in the filled narrow band, so hole ejection reduces system entropy and temperature.
• The time-dependent distribution function evolves toward a colder Fermi-like distribution, indicating effective cooling despite nonequilibrium driving.
• Simulations show that the optical conductivity and spectral functions evolve to reflect a colder, more coherent state with enhanced antiferromagnetic correlations.
• The protocol remains effective even with realistic parameters: a core band of width 0.4 and a driving pulse of duration ~30–120 in units of the hopping, with minimal double occupancy.