Spin Reversal of a Quantum Hall Ferromagnet at a Landau Level Crossing

Abstract

When Landau levels (LLs) become degenerate near the Fermi energy in the quantum Hall regime, interaction effects can drastically modify the electronic ground state. We study the quantum Hall ferromagnet formed in a two-dimensional hole gas around the LL filling factor ν ¼ 1 in the vicinity of a LL crossing in the heave-hole valence band. Cavity spectroscopy in the strong-coupling regime allows us to optically extract the spin polarization of the two-dimensional hole gas. By analyzing this polarization as a function of hole density and magnetic field, we observe a spin flip of the ferromagnet. Furthermore, the depolarization away from ν ¼ 1 accelerates close to the LL crossing. This is indicative of an increase in the size of skyrmion excitations as the effective Zeeman energy vanishes at the LL crossing.

Robust Quantum Hall Ferromagnetism near a Gate-Tuned ν = 1 Landau Level Crossing

“…4 (inset) fits are indeed much larger than the slopes implied by the calculated LLs (red open circles in Fig. 4), suggesting large-size skyrmions, consistent with the conclusions reached in [40]. However, one should be cautious in determining a quantitative size for the skyrmions from our study, given the discrepancy between the observed and calculated positions of the LL crossing.…”

“…This is partly because the crossing typically occurs at small magnetic fields, implying that very dilute and yet low-disorder samples are needed for its study. Only very recently the presence of this crossing in a (001) GaAs 2DHS was demonstrated experimentally and, using optical techniques, it was shown that the 2DHS spins undergo a reversal as the LLs cross [40].…”

The ferroelectric photo ground state of SrTiO 3 : Cavity materials engineering

“…This is considered a new paradigm of material design, especially when the collective behavior of particles in quantum materials can be controlled to provide novel functionalities ( 1 , 2 ). Alternatively to the intense lasers necessary to reach such out-of-equilibrium states, one can achieve strong light–matter coupling by placing the material inside an optical cavity ( 3 – 11 ). A main advantage of this approach is that strong interaction can be achieved at equilibrium, opening up new possibilities for materials manipulation.…”

Spin-selective strong light-matter coupling in a 2D hole gas-microcavity system

“…An asymmetry in the light-matter coupling has been reported for similar systems at lower charge densities when the is in a spin-ordered state in the frame of integer or fractional quantum Hall effects [12,13,25]. In those cases, the asymmetry takes place for the specific values of B at which correlated states of matter dominate.…”

“…Recently, the possibility of coupling such 2D gases to an optical mode, forming cavity exciton-polaritons, has opened up exciting new directions. For example, this capability has been exploited to optically study integer and fractional quantum Hall states [11,12], formation of skyrmions and manipulation of the magnetic Landé g factor [13] and the enhancement of optical non-linearities, when an electron gas, in the fractional quantum Hall regime, is dressed with the cavity mode [14]. The demonstration of the mentioned effects requires that the light-matter interaction strength in the microcavity exciton-polaritons exceeds cavity and matter dissipation -a regime known as strong coupling (SC) [15].…”