Jun Ishihara

Direct imaging of gate-controlled persistent spin helix state in a modulation-doped GaAs/AlGaAs quantum well

Using time- and spatially-resolved Kerr microscopy, we directly measure the spatiotemporal evolution of photoexcited local spins of a two-dimensional electron gas in a modulation-doped GaAs/AlGaAs quantum well with a top gate electrode. The spatial pattern of spins after diffusion is controlled by a gate voltage that changes the strength of the spin–orbit interaction (SOI) field. By measuring the time dependence of spin distribution with an external magnetic field, we successfully observe a persistent spin helix state by tuning the gate voltage, and we obtain both Rashba and Dresselhauss SOI parameters separately.

A strong anisotropy of spin dephasing time of quasi-one dimensional electron gas in modulation-doped GaAs/AlGaAs wires

We investigated the wire orientation dependence of ensemble electron spin dynamics by using a time-resolved Kerr rotation technique in wires made from a modulation-doped GaAs/AlGaAs quantum well. We observed a strong anisotropy of the electron spin dephasing time of as large as 40 depending on the wire orientation at 5 K by tuning the spin-orbit interaction. The wire width dependence of the spin dynamics is reproduced in a Monte Carlo simulation. In addition, two characteristic spin decay rates were observed in wires whose widths were close to the spin precession length. The dependence of the magnetic field direction on the spin dynamics of the wires was also studied to determine the difference from that of two dimensional electron gas.

Coherent Manipulation of Nuclear Spins in Semiconductors with an Electric Field

Electrical coherent manipulation of a quadrupolar-split nuclear spin ensemble in a GaAs quantum well (QW) with an rf electric field is demonstrated by the optical time-resolved Kerr rotation technique. Rabi oscillations and phase control between two spin-3/2 states are demonstrated by nuclear electric resonance in a gated GaAs QW device. This enables us to achieve coherent control of geometrically specified nuclear spins.

Direct mapping of photoexcited local spins in a modulation-doped GaAs/AlGaAs wires

We directly measured the spatiotemporal evolution of photoexcited local spins in wires made from a modulation-doped GaAs/AlGaAs quantum well by using a time- and spatially resolved Kerr microscopy. We observed the spatial pattern of spin-up and -down near the case of persistent spin helix (PSH) in wires and the retention of the initial spin state in [110] wires which were designed such that the Dresselhaus and Rashba spin–orbit interaction are nearly equal to each other in magnitude.

Nuclear spin coherence time in a strained GaAs quantum well

We investigated the quadrupolar splitting dependence of the nuclear spin coherence time in an n‐(110)GaAs/AlGaAs quantum well by time‐resolved Kerr‐rotation measurements combined with pulsed‐rf nuclear magnetic resonance. In GaAs, all the constituent nuclei have quadrupole moment. Therefore the quadrupolar splitting is induced by electric field gradient, which can be controlled by varying strain. We found that the nuclear spin coherence time is extended as the quadrupolar splitting increases by applying the strain. This suggests that the influences of the other transitions are suppressed with increase of the quadrupolar splitting.

Magnetic Field Dependence of Quadrupolar Splitting and Nuclear Spin Coherence Time in a Strained (110) GaAs Quantum Well

We investigated the magnetic field dependence of quadrupolar splitting and phase relaxation time of nuclear spins in an n-GaAs/AlGaAs (110) quantum well (QW) by optically detected nuclear magnetic resonance (NMR). The NMR spectra show large quadrupolar splitting induced by an internal field gradient in the QW epilayer. At lower magnetic fields, the quadrupolar splitting become much enhanced, resulting in the appearance of the second order magnetic field dependence of the quadrupole interaction in the NMR spectra. It is also shown that the intrinsic coherence time and the effective coherence time for resonant transitions become shorter as a result that the energy splitting between the two levels becomes small and incoherent transition is occurred by irradiation of an rf magnetic field.

Strain dependence of nuclear spin coherence in a (110)GaAs/AlGaAs quantum well

We have investigated effects of an applied uniaxial strain on nuclear spin coherence in an n-(110) GaAs/AlGaAs quantum well by optically detected nuclear magnetic resonance (ODNMR). The ODNMR line widths associated with the quadrupolar splitting increase with the applied external strain because of the increase of inhomogeneous broadening. It is found that the phase decoherence of nuclear spin ensemble for the transition between quadrupolar-split two levels |?1/2? and |+1/2? is suppressed with increasing the quadrupolar splitting.

Mapping of photoexcited local spins in a modulation-doped GaAs/AlGaAs wires

We directly measured the evolution of photoexcited local spins in wires made from a modulation-doped GaAs/AlGaAs quantum well by using a timeand spatially resolved Kerr microscopy measurement. We observed the spatial precession of spins in [1 _ 10] wires and the retention of a spin up state in [110] wires which was designed that the Dresselhaus and Rashba spin orbit interaction are nearly equal to each other in magnitude.