Tomonori Arakawa

Weak antilocalization and conductance fluctuation in a submicrometer-sized wire of epitaxial Bi2Se3

In this paper, we address the phase-coherent transport in a submicrometer-sized Hall bar made of epitaxial Bi2Se3 thin film by probing the weak antilocalization (WAL) and the magnetoresistance fluctuation below 22 K. The WAL effect is well described by the Hikami-Larkin-Nagaoka model, where the temperature dependence of the coherence length indicates that electron conduction occurs quasi-one-dimensionally in the narrow Hall bar. The temperature-dependent magnetoresistance fluctuation is analyzed in terms of the universal conductance fluctuation, which gives a coherence length consistent with that derived from the WAL effect.

Evolution of the Kondo effect in a quantum dot probed by shot noise

We measure the current and shot noise in a quantum dot in the Kondo regime to address the nonequilibrium properties of the Kondo effect. By systematically tuning the temperature and gate voltages to define the level positions in the quantum dot, we observe an enhancement of the shot noise as temperature decreases below the Kondo temperature, which indicates that the two-particle scattering process grows as the Kondo state evolves. Below the Kondo temperature, the Fano factor defined at finite temperature is found to exceed the expected value of unity from the noninteracting model, reaching 1.8±0.2.

Observation of full shot noise in CoFeB/MgO/CoFeB-based magnetic tunneling junctions

The electron transport through the CoFeB/MgO/CoFeB-based magnetic tunneling junction (MTJ) was studied by the shot noise measurement. The obtained Fano factor to characterize the shot noise is very close to unity, indicating the full shot noise, namely, the shot noise in the Schottky limit, both in the parallel and antiparallel magnetization configurations. This means the Poissonian process of the electron tunneling and the absence of the electron–electron correlation in the low bias regime. The shot noise measurements will be a good guideline to make up tunneling criteria for designing MTJ-based spin devices.

Shot Noise Induced by Nonequilibrium Spin Accumulation

When an electric current passes across a potential barrier, the partition process of electrons at the barrier gives rise to the shot noise, reflecting the discrete nature of the electric charge. Here we report the observation of excess shot noise connected with a spin current which is induced by a nonequilibrium spin accumulation in an all-semiconductor lateral spin-valve device. We find that this excess shot noise is proportional to the spin current. Additionally, we determine quantitatively the spin-injection-induced electron temperature by measuring the current noise. Our experiments show that spin accumulation driven shot noise provides a novel means of investigating nonequilibrium spin transport.

Sub-Poissonian shot noise in CoFeB/MgO/CoFeB-based magnetic tunneling junctions

We measured the shot noise in the CoFeB/MgO/CoFeB-based magnetic tunneling junctions with a high tunneling magnetoresistance ratio (over 200% at 3 K). Although the Fano factor in the antiparallel configuration is close to unity, it is observed to be typically 0.91±0.01 in the parallel configuration. It indicates the sub-Poissonian process of the electron tunneling in the parallel configuration due to the relevance of the spin-dependent coherent transport in the low bias regime.

Dynamic nuclear spin polarization in an all-semiconductor spin injection device with (Ga,Mn)As/n-GaAs spin Esaki diode

We investigate the dynamic nuclear spinpolarization in an n-GaAs lateral channel induced by electrical spin injection from a (Ga,Mn)As/n-GaAs spin Esaki diode. Signatures of nuclear spinpolarization are studied in both three-terminal and non-local voltage signals, where a strong electron spin depolarization feature is observed close to zero magnetic field. This is due to the large nuclear field induced in the channel through hyperfine interaction between injected electron spins and localized nuclear spins. We study the time evolution of the dynamic nuclear spinpolarization and evaluate polarization and relaxation times of nuclear spins in the channel.

Quantum Fluctuations along Symmetry Crossover in a Kondo-Correlated Quantum Dot

Universal properties of entangled many-body states are controlled by their symmetry and quantum fluctuations. By the magnetic-field tuning of the spin-orbital degeneracy in a Kondo-correlated quantum dot, we have modified quantum fluctuations to directly measure their influence on the many-body properties along the crossover from SU(4) to SU(2) symmetry of the ground state. High-sensitive current noise measurements combined with the nonequilibrium Fermi liquid theory clarify that the Kondo resonance and electron correlations are enhanced as the fluctuations, measured by the Wilson ratio, increase along the symmetry crossover. Our achievement demonstrates that nonlinear noise constitutes a measure of quantum fluctuations that can be used to tackle quantum phase transitions.

Cryogenic amplifier for shot noise measurement at 20 mK

We developed a shot noise measurement system for mesoscopic conductors (typical resistance h/2e2 = 12.9 kΩ) at very low temperature (∼20 mK). To realize required high-resolution measurement, we made a high-electron-mobility-transistor (HEMT)-based cryogenic amplifier working at a target frequency range (∼2.5 MHz), whose gain flatness and input voltage noise were carefully tuned. We can suppress the 1/f noise of the amplifier by using two HEMTs in parallel. The performance of the noise measurement system at 20 mK was demonstrated for a quantum point contact with high experimental accuracy below 10−29 A2/Hz.

Signature of Coherent Transport in Epitaxial Spinel-Based Magnetic Tunnel Junctions Probed by Shot Noise Measurement

We measured the shot noise in fully epitaxial Fe/MgAl2OX/Fe-based magnetic tunneling junctions (MTJs). While the Fano factor to characterize the shot noise is very close to unity in the antiparallel configuration, it is reduced to 0.98 in the parallel configuration. This observation shows the sub-Poissonian process of electron tunneling in the parallel configuration, indicating the coherent tunneling through the spinel-based tunneling barrier of the MTJs.

Anomalous behavior of 1/f noise in graphene near the charge neutrality point

We investigate the noise in single layer graphene devices from equilibrium to far-from equilibrium and found that the 1/f noise shows an anomalous dependence on the source-drain bias voltage (VSD). While the Hooges relation is not the case around the charge neutrality point, we found that it is recovered at very low VSD region. We propose that the depinning of the electron-hole puddles is induced at finite VSD, which may explain this anomalous noise behavior.