Masayuki Morino

Simultaneous excitation of spins and pseudospins in the bilayer ν=1 quantum Hall state

The tilting angular dependence of the energy gap was measured in the bilayer quantum Hall state at the Landau level filling v = 1 by changing the density imbalance between the two layers. The observed gap behavior shows acontinuous transformation from the bilayer balanced density state to the monolayer state. Even a sample with 33 K tunneling gap shows the same activation energy anomaly reported by Murphy et al. (Phys. Rev. Lett. 72 (1994) 728). We discuss a possible relation between our experimental results and the quantum Hall ferromagnet of spins and pseudospins.

Continuous transformation from spin- to pseudospin-type excitation

Abstract We measured the activation energy of bilayer ν=1 quantum Hall states. By changing the density difference between layers, the tilting behavior of the pseudospin(P)-type activation energy at the equal density point gradually transforms into the spin(S)-type one at the monolayer density point. At the intermediate density difference, by increasing the tilting angle the activation energy starts to decrease as the P-type excitation gap and then increases as the S-type excitation gap. It is impossible to explain this behavior by the level crossing of the P- and S-type excitations. The result of the overall behavior indicates the excitation in a bilayer system is the simultaneous flip of spin and pseudospin.

Anisotropy of Current‐pumped Nuclear Spin Polarization in the ν = 2/3 Quantum Hall State

We investigate the anisotropy of the current‐pumped nuclear spin polarization by changing an angle φ between the current I and the in‐plane magnetic field B∥. The nuclear spin polarization develops at the spin transition point of the quantum Hall state at the Landau level filling factor ν = 2/3. When large I is driven, the nuclear spin polarization appears independently of φ. However, as I is decreased, the polarization disappears when B∥ is parallel to I, while it remains when B∥ is perpendicular to I. The anisotropy indicates the difference of the nuclear spin pumping rate about the direction of B∥.

Anisotropic Behavior of Hysteretic Transport Phenomena in the ν = 2/3 Quantum Hall State in the Tilted Magnetic Field

We study an anisotropic transport around the spin transition point at the Landau‐level filling factor ν = 2/3 in tilted magnetic fields. When the direction of the in‐plane component of the magnetic field (B∥) is normal to the current I, a strong hysteretic transport due to the domain structure of different spin configurations occurs. As the angle φ between B∥ and I is decreased, the hysteresis becomes small, and almost disappears when the direction of B∥ is along I. We suggest that domain walls are aligned parallel to B∥, and the number of electrons scattered by domain walls increases with φ.

ANISOTROPIC TRANSPORT ON THE ν=1 BILAYER QUANTUM HALL SYSTEM UNDER TILTED MAGNETIC FIELD

We measured an anisotropic magnetotransport that depends on directions of an in-plane field in a bilayer quantum Hall system. We found that the longitudinal resistance near the bilayer ν=1 quantum Hall state strongly varies when we changed an relative angle between the current and the in-plane field. It is a phenomenon peculiar to the bilayer ν=1 quantum Hall state, because this phenomenon does not appear in a monolayer system.

Anisotropic nuclear spin relaxation and dynamic polarization rates in the ν = 2/3 quantum Hall states

We present an anisotropy of the hysteretic transport due to the dynamic nuclear polarization (DNP) around the spin transition point at Landau level filling factor ν = 2/3 in tilted magnetic field. When the direction of the in-plane component of the magnetic field B|| is normal to the probe current J, a strong hysteresis related with DNP occurs. On the other hand, as B|| is parallel to I, the hysteresis almost disappears. To clear the origin of the anisotropy of DNP, we study the nuclear spin-lattice relaxation and nuclear spin polarization rates at the transition point and find out that they are anisotropic for a relative angle between B|| and I.

Anisotropy of Magnetoresistance Hysteresis around the ν=2/3 Quantum Hall State in Tilted Magnetic Field

We present an anisotropy of the hysteretic transport around the spin transition point at the Landau level filling factor ν=2/3 in a tilted magnetic field. When the direction of the in-plane component of the magnetic field B || is normal to the probe current I , a strong hysteretic transport due to the current-induced nuclear spin polarization occurs. When B || is parallel to I , on the other hand, the hysteresis almost disappears. We also demonstrate that the nuclear spin–lattice relaxation rate T 1 -1 at the transition point increases with decreasing angle between the directions of B || and I . These results suggest that the morphology of electron spin domains around ν=2/3 is affected by the current direction.Graduate School of Science, Kyoto University, Kyoto, 606-8502 Graduate School of Science, Tohoku University, Sendai, 980-8578 Department of Physics, Hyogo College of Medicine, Nishinomiya 663-8501 NTT Basic Research Laboratories, NTT Corporation, Atsugi, 243-0198 Theoretical Physics Laboratory, RIKEN, Saitama, 351-0198 Research Center for Low Temperature and Materials Sciences, Kyoto University, Kyoto, 606-8501

Activation study of the pseudospin soliton in the v = 1 bilayer quantum Hall effect

We study thermal excitations of the pseudospin soliton phase in the bilayer quantum Hall (QH) state at total Landau level filling factor v = 1. Near the commensurate-incommensurate phase transition point in the bilayer v = 1 QH state, we have found anomalous magnetotransport phenomena, such as the magnetoresistance peak and its anisotropic transport, which are ascribed to the formation of the pseudospin soliton phase (A. Fukuda et al., Phys. Rev. Lett. 100, 016801 (2008)). In this work, we elaborately measured activation energy gap Δ of the bilayer v = 1 QH state. Δ shows a broad minimum in the pseudospin soliton phase between the commensurate and incommensurate phase. We suggest that the minimum in Δ is due to collective modes of the pseudospin solitons forming a lattice.

Anisotropic Magnetotransport near the Bilayer ν = 2/3 Quantum Hall State

We study a magnetotransport anisotropy of a bilayer electron system at the Landau level filling factor ν = 2/3 under tilted magnetic fields. When the direction of the applied in‐plane component of magnetic fields with respect to the current is altered, an anisotropy appears only when the interlayer tunneling is allowed. Therefore the phenomenon suggests that the interlayer tunneling process is affected by the direction of the in‐plane field.

In‐plane field induced anisotropy of the longitudinal resistance in a bilayer quantum Hall system

We examine an anisotropic transport in a bilayer system around the quantum Hall state at the total Landau level filling factor v = 1. We find that the anisotropy becomes remarkable at the phase transition point between the commensurate and incommensurate states. This result suggests that a unidirectional state appears around the transition.