Tatsushi Akazaki

Improved InAlAs/InGaAs HEMT characteristics by inserting an InAs layer into the InGaAs channel

An InAlAs/InGaAs HEMT with a thin InAs layer inserted into the InGaAs channel is proposed and its electron transport properties and device performances have been investigated. By optimizing the thickness and the exact point of insertion in the InAs layer, the mobility and electron velocity at 300 K have been increased by 30% and 15%, respectively, compared to the conventional heterostructure. In addition, a maximum intrinsic transconductance of 970 mS/mm and a maximum current gain cutoff frequency of 58.1 GHz have been attained by a 0.6 mu m-gate-length device.<<ETX>>

A Josephson field effect transistor using an InAs‐inserted‐channel In0.52Al0.48As/In0.53Ga0.47As inverted modulation‐doped structure

A Josephson field effect transistor (JOFET) was coupled with a two‐dimensional electron gas in a strained InAs quantum well inserted into an In0.52Al0.48As/In0.53Ga0.47As inverted modulation‐doped structure. The characteristics of this JOFET are much improved over previous devices by using a high electron mobility transistor (HEMT)‐type gate instead of the usual metal‐insulator‐ semiconductor (MIS)‐type gate. The superconducting critical current as well as the junction normal resistance are completely controlled via a gate voltage of about −1 V; this provides voltage gain over 1 for a JOFET.

Single-photon detection using magnesium diboride superconducting nanowires

We fabricated 10 nm thick MgB2 nanowires with a width down to 100 nm using the liftoff process. The I-V characteristics of the nanowire show hysteresis and a sharp voltage jump at Ic. Though a 150 nm wide nanowire exhibits the capacity for detecting a single photon at 405 nm wavelength, the nanowire is too wide to detect a single photon at 1560 nm. A 100 nm wide nanowire exhibits the capacity for detecting single photons in the 405–1560 nm wavelength range. This indicates a possible application of MgB2 as a high-performance superconducting nanowire single-photon detector.

Fano-Kondo interplay in a side-coupled double quantum dot.

We investigate low-temperature transport characteristics of a side-coupled double quantum dot where only one of the dots is directly connected to the leads. We observe Fano resonances, which arise from interference between discrete levels in one dot and the Kondo effect, or cotunneling in general, in the other dot, playing the role of a continuum. The Kondo resonance is partially suppressed by destructive Fano interference, reflecting novel Fano-Kondo competition. We also present a theoretical calculation based on the tight-binding model with the slave boson mean field approximation, which qualitatively reproduces the experimental findings.

Improving the mobility of an In0.52Al0.48As/In0.53Ga0.47As inverted modulation‐doped structure by inserting a strained InAs quantum well

The mobility of two‐dimensional electrons in an In0.52Al0.48As/In0.53Ga0.47As inverted modulation‐doped structure improved by inserting an InAs quantum well into the InGaAs channel. This letter addresses the main cause of this mobility improvement. By optimizing the thickness of the InAs quantum well, its distance from the underlying InAlAs spacer layer, and the InAlAs spacer‐layer thickness, maximum mobilities of 16 500 cm2/V s at 300 K and 155 000 cm2/V s at 10 K are attained. The improvement in mobility is attributed to a decrease in scattering caused by ionized impurities, interface‐roughness, and trap impurities. This decrease is a result of the superior confinement of two‐dimensional electron gas in the InAs quantum well.

Collapse of thermal activation in moderately damped Josephson junctions

We study switching current statistics in different moderately damped Josephson junctions: a paradoxical collapse of the thermal activation with increasing temperature is reported and explained by interplay of two conflicting consequences of thermal fluctuations, which can both assist in premature escape and help in retrapping back into the stationary state. We analyze the influence of dissipation on the thermal escape by tuning the damping parameter with a gate voltage, magnetic field, temperature and an in-situ capacitor.

SEMICONDUCTOR-COUPLED SUPERCONDUCTING JUNCTIONS USING NBN ELECTRODES WITH HIGH HC2 AND TC

Abstract We investigated normal transport in a superconductor (S)–two-dimensional electron gas (2DEG)–superconductor(S) junction in the quantum Hall regime. Andreev reflection takes place between the two S–2DEG interfaces through the edge states. The differential resistance-bias voltage characteristics of the junction are strongly dependent on the Hall conductance regimes, that is, the plateau or the threshold regimes.

Superconductor-Based Quantum-Dot Light-Emitting Diodes: Role of Cooper Pairs in Generating Entangled Photon Pairs

The realization of solid-state photon sources that are capable of on-demand generation of an entangled single-photon pair at a time is highly desired for quantum information processing and communication. A new method of generating an entangled single-photon pair at a time is proposed employing Cooper-pair-related radiative recombination in a quantum dot (QD). Cooper pairs are bosons and the control of their number states is difficult. Paulis exclusion principle on quasiparticles in a discrete state of a QD regulates the number state of the generated photon pairs in this scheme. The fundamental heterostructures for constructing superconductor-based quantum-dot light-emitting diodes (SQ-LEDs) and the fundamental operation conditions of SQ-LED will be discussed. The experimental studies on Cooper-pair injection into the related semiconductor structures will be also discussed.

Superconductor-based Light Emitting Diode : Demonstration of Role of Cooper Pairs in Radiative Recombination Processes

A light emitting diode with superconducting Nb electrodes was fabricated to investigate the contribution of cooper pairs to radiative recombination in a semiconductor. Electroluminescence observed from the active layer in which electron cooper pairs and normal holes are injected was drastically enhanced at the temperature lower than the superconducting transition temperature of the Nb electrodes. This is the first experimental evidence that cooper pairs enhance radiative recombinations by the superradiance effect.

Gate control of spin–orbit interaction in an InAs-inserted In0.53Ga0.47As/In0.52Al0.48As heterostructure

Abstract We have confirmed by analyzing beating patterns of Shubnikov–de Haas oscillations that a spin–orbit interaction of the conduction band in an InAs-inserted In0.53Ga0.47As/In0.52Al0.48As heterostructure can be controlled by applying the gate voltage. The change in the spin–orbit interaction can be attributed to the Rashba term. Comparison with an In0.53Ga0.47As/In0.52Al0.48As heterostructure reveals that the spin–orbit interaction in the InAs-inserted system is more sensitive to the carrier concentration.