Observing the onset of effective mass

Area of Science:

• Atomic physics
• Condensed matter physics
• Quantum mechanics

Background:

• Particles in periodic potentials are often described by effective mass.
• Effective mass models interactions between particles and their surrounding potential.
• Short-time dynamics of particle response remain less understood.

Purpose of the Study:

• To experimentally investigate the initial response of particles in a periodic potential.
• To determine when effective mass accurately describes particle behavior.
• To elucidate the role of bare mass versus effective mass on short time scales.

Main Methods:

• Utilized a Bose-Einstein condensate of Rubidium-87 atoms.
• Initial state prepared in the ground band of an optical lattice.
• Applied an external force to observe particle response dynamics.

Main Results:

• The initial particle response was characterized by the bare mass, not the effective mass.
• Observed rapid oscillations in particle response over time.
• Effective mass description became accurate only at longer time scales, beyond interband dynamics.

Conclusions:

• The bare mass governs the immediate response of particles to forces in periodic potentials.
• Effective mass is an appropriate description only for long-time dynamics.
• Findings are relevant for understanding light-matter interactions, such as few-cycle laser pulses with materials.