Probing a quantum Hall pseudospin ferromagnet by resistively detected nuclear magnetic resonance

Abstract

Resistively Detected Nuclear Magnetic Resonance (RD-NMR) has been used to investigate a twosubband electron system in a regime where quantum Hall pseudo-spin ferromagnetic (QHPF) states are prominently developed. It reveals that the easy-axis QHPF state around the total filling factor ν = 4 can be detected by the RD-NMR measurement. Approaching one of the Landau level (LL) crossing points, the RD-NMR signal strength and the nuclear spin relaxation rate 1/T1 enhance significantly, a signature of low energy spin excitations. However, the RD-NMR signal at another identical LL crossing point is surprisingly missing which presents a puzzle.PACS numbers: 73.43. Nq, 71.30.+h, 72.20.My The multi-component electron systems have been continuously drawing intensive research interest because of its novel ground states and excitations [1]. In experimental systems, different Landau levels (LLs) can be tuned to cross by varying gate voltage, charge density, magnetic field or the magnetic field tilted angle to the sample. Electron-electron correlations become particularly prominent when two or more sets of LLs with different layer, subband, valley, spin, or Landau level indices are brought into degeneracy [1,2,3,4,5,6,7,8]. Recent experiments in single quantum well with two subbands occupied systems [5,6], showed evidence of the formation of quantum Hall pseudospin ferromagnets (QHPFs) due to the interactions of the two subbands (termed as pseudospins) around the LLs crossing point. The QHPFs taking place at total filling factor ν = 3, 5 and ν = 4 are easy-plane or easy-axis QHPFs respectively, depending on the details of the two subbands configurations. In spite of various theoretical models [9,10,11] motivated by these findings, a comprehensive understanding is not yet achieved. Thus far, experimental and theoretical studies all focused on the pseudospin freedom. However, in this work we would address the unique spin excitations in the QHPF states.To address the question whether spin states in twosubband systems in nature, measurements other than the conventional transport and optical means are needed. Since the Zeeman energy of nuclear spin is about 3 orders of magnitude smaller than that of electron spin, exchange of spin angular momentum between the electron and nuclear spin is allowed only when the electron system supports spin excitations with low energy. The nuclear spin relaxation rate 1/T 1 thus probes the density of states at low energy of the electron spin system that cannot be accessed by other means. The resistively detected NMR technique has recently emerged as an effective method to probe collective spin states in the fractional quantum Hall regime [12,13], the Skyrmion spin texture close to the filling factor 1 [14,15], the role of electron spin polarization in the phase transition of a bilayer system [16,17], and the ferromagnetic state accompanied by collective spin excitations of a two-subband system [18]. Here we use this technique to study spin freedom and its relation with pseudospin in the vicinity of...

Pump-probe nuclear spin relaxation study of the quantum Hall ferromagnet at filling factor ν = 2

“…The T 1 measurement of the ν=2/3 QHF gives a current-dependent relaxation time constant between 180 s and 1350 s [20], while that of the ν=2 QHF is relatively short (∼45 s) and current-independent. Furthermore, T 1 of the two-subband QHF is filling dependent [22] as contrasted with the ν-independent T 1 of the ν=2 QHF. Note here that the magnetotransport data are indicative of thermally activated transport in both the ν=2/3 and two-subband QHFs of the GaAs 2DEG [20,22].…”

“…Furthermore, T 1 of the two-subband QHF is filling dependent [22] as contrasted with the ν-independent T 1 of the ν=2 QHF. Note here that the magnetotransport data are indicative of thermally activated transport in both the ν=2/3 and two-subband QHFs of the GaAs 2DEG [20,22]. A sharp reduction in the activation energy gap at the phase transition suggests the presence of skyrmion excitations in the DW that dominate the nuclear spin relaxation and thus T 1 [20].…”

“…Furthermore, this highlysensitive technique combined with the nuclear spin-lattice relaxation time T 1 measurement [7,8] as a unique probe of low-frequency spin fluctuations can be applied to investigate the domain-wall (DW) structures of the QHF that are still poorly characterized, which will lead to a more comprehensive understanding of the QHF. Up to now, the T 1 results of the QHF formed at ν=2/3 [20] and at integers ν in a two-subband 2DEG (hereafter called two-subband QHF) [21,22] are suggestive of low energy DW excitations but exhibit different properties. Note that intricate CF-CF interactions at fractional ν (different from electron-electron interactions at integer ν) [23] and an additional degree of freedom associated with the subband index [24] may complicate the interpretation of these differences.…”

Ferromagnetismo no regime Hall quântico inteiro via teoria do funcional de densidade

“…Resultados experimentais recentes obtidos pelo grupo do prof. Jiang da UCLA, Refs. (36)(37)(38)(39)(40), exibem características interessantes destas instabilidades nas medidas de ρ xx . Estes resultados experimentais são usados como referência nesta tese.…”

“…Tal efeito pode ser visto como um fator giromagnético efetivo, induzindo o cruzamento de níveis de Landau de spins opostos e mesmoíndice de Landau e subbanda. (iii) Considerando-se sistemas com duas subbandas ocupadas (36)(37)(38)(39)(40)(41)(42)(43)(44)(45), nos quais cada subbanda possui uma sequência de níveis de Landau, tal que aumentando-se a densidade de cargas ocorrem cruzamentos de níveis de Landau de subbandas distintas, tanto de mesmo spin quanto de spins opostos.…”