Electron Spins in Bilayer Quantum Hall Systems Investigated by Nuclear Spin Measurements
Norio Kumada1, Koji Muraki1, and Yoshiro Hirayama2
1Physical Science Laboratory, 2Tohoku University
Two-dimensional electron system (2DES) formed in a high-mobility semiconductor heterostructures can be an ideal laboratory to study many-body phenomena in low dimensions. In a quantum Hall (QH) system, which appears by applying magnetic field perpendicular to a 2DES, the kinetic energy of electrons is quantized into Landau levels and electron-electron interactions completely dominate physics. In particular, in a bilayer system consisting of closely separated 2DESs, interplay between the spin and layer degrees of freedom leads to various broken symmetry states called QH magnets. We developed nuclear magnetic resonance (NMR) techniques for QH systems and studied static and dynamic properties of electron spin in QH magnets [1-3].
NMR is commonly used as a powerful probe of spin states in various electronic systems. For QH systems, however, a small number of nuclei in contact with the 2DES have restricted NMR measurements to multiple-layer samples. To perform NMR in a bilayer QH system, we exploit current-induced nuclear spin polarization and its resistive detection. The Knight shift of the NMR spectrum, which is proportional to the electron spin polarization, shows the existence of the canted antiferromagnetic state between the ferromagnetic and spin-singlet states with full and null spin polarizations, respectively (Fig. (a)). Furthermore, the nuclear spin relaxation rate reveals that, in the canted antiferromagnetic state, low-frequency electron spin fluctuations do not freeze out even in the low-temperature limit (Fig. (b)). The collective fluctuation at low temperature is characteristic behavior of broken symmetry states in two dimensions.
 N. Kumada, et al., Phys. Rev. Lett. 94 (2005) 096802.
 N. Kumada, K. Muraki, and Y. Hirayama,Science 313 (2006) 329.
 N. Kumada, K. Muraki, and Y. Hirayama, Phys. Rev. Lett. 99 (2007) 076805.
Fig.. (a) Electron spin polarization and (b) nuclear spin relaxation rate measured by NMR. The nuclear spin relaxation rate reflects electron spin fluctuations.
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