Showing papers by "Tatsushi Akazaki published in 2008"
TL;DR: In this paper, a 10-nm-thick MgB 2 thin film with the superconducting transition temperature T c ǫ = 21 K was synthesized by using molecular-beam epitaxy.
Abstract: We report the fabrication of MgB 2 nanowire and its optical response. A 10-nm-thick MgB 2 thin film with the superconducting transition temperature T c = 21 K was synthesized by using molecular-beam epitaxy. The film was processed into a nanowire with a width of 300 nm and a length of 10 μm using e-beam lithography and Ar-ion milling. With a dc bias current applied close to the critical current ( I c ), the nanowire showed an electrical signal when illuminated by a laser pulse at the telecommunications wavelength.
37 citations
TL;DR: In this article, a light emitting diode with superconducting Nb electrodes was fabricated to investigate the contribution of cooper pairs to radiative recombination in a semiconductor, and it was shown that electron cooper pairs and normal holes were significantly enhanced at the temperature lower than the super-conducting transition temperature of the Nb electrode.
Abstract: 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.
27 citations
TL;DR: It is clear that single-electron-localization, nonlinear screening (NLS), and linear screening (LS) regimes of gate induced electrons in a GaAs quantum well from photoluminescence spectra and intergate capacitance are identified.
Abstract: We clearly identify single-electron-localization (SEL), nonlinear screening (NLS), and linear screening (LS) regimes of gate induced electrons in a GaAs quantum well from photoluminescence spectra and intergate capacitance. Neutral and charged excitons observed in the SEL regime rapidly lose their oscillator strength when electron puddles are formed, which mark the onset of NLS. A further increase in the density of the electrons induces the transition from the NLS to LS, where the emission of a charged exciton changes to the recombination of two-dimensional electron gas and a hole.
10 citations
TL;DR: In this article, the authors investigated the transport of a two-dimensional electron gas (2DEG) in an In0.52Al0.48As/In0.7Ga0.3As heterostructure when exposed to light from infrared laser diodes (λ=0.78, 1.3μm).
Abstract: We investigated the transport of a two-dimensional electron gas (2DEG) in an In0.52Al0.48As/In0.7Ga0.3As heterostructure when exposed to light from infrared laser diodes (λ=0.78, 1.3 μm) by means of both Shubnikov–de Haas and Hall-effect measurements. We observed negative photoconductivity due to a reduction in the number of electrons in the 2DEG when they were illuminated by photons at λ=1.3 μm. We speculate that the negative photoconductivity originates from the diffusion and trapping of photo-induced hot electrons at deep impurity levels in the InAlAs barrier layer near the InAlAs/InGaAs interface.
7 citations
TL;DR: In this article, a junction field effect transistor (FET) with a Nb/n-InGaAs/Nb junction was fabricated and micro-photoluminescence was observed from a 110nm wide gap between the source and drain Nb electrodes.
Abstract: To investigate the contribution of Cooper pairs to radiative recombination in a semiconductor, a junction fieldeffect transistor (FET) with a Nb/n-InGaAs/Nb junction was fabricated. Electroluminescence was observed from a 110nm-wide gap between the source and drain Nb electrodes in addition to the confirmation of the FET operation. Micro-photoluminescence observed from the gap in the Nb electrodes was drastically enhanced at the temperature lower than the superconducting critical value of ∼ 8K. This new finding is discussed based on the proximity effect at the super-normal junction. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
5 citations
TL;DR: This paper discusses the new possibility with the injection of electron as well as hole Cooper pairs into QDs for generation of entangled photon pairs from semiconductor quantum dots.
5 citations
TL;DR: In this paper, real-space mappings of a Hall photovoltage are performed by local laser excitation using optical microscope setup with a spatial resolution of about 2μm.
Abstract: Real-space mappings of a Hall photovoltage are performed by local laser excitation using optical microscope setup with a spatial resolution of about 2μm. The obtained images of broad and narrow strips presumably reflect the diffusion of the optically created electrons in the bulk and the edge states. The image near a current contact is considered to reflect the potential gradient near the contact.
3 citations
01 Feb 2008
TL;DR: In this article, a photon generation process from a single quantum dot was studied for the control of the quantum states of generated photons, and an approach to open new paradigm was discussed with preliminary results.
Abstract: Quantum information processing and quantum cryptography are expected to realize highly secure future information networks. How to control photon qubits will be one of the major issues for this direction, but the technologies related to generations and detections of individual photons are still being developed. Semiconductor quantum dots (QDs) have been expected to play major role for on-demand generations of single photons as well as entangled photon pairs. In this talk, photon generation processes from a single QD will be studied for the control of the quantum states of generated photons. Generation of entangled photon pairs from QDs based on biexciton-exciton cascade recombination processes is presently in a difficult situation due to exciton states energy splitting related to growth-related issues. An approach to open new paradigm will be discussed with preliminary results.
3 citations
01 Mar 2008
TL;DR: In this paper, the transport properties of a superconductor-semiconductor junction were measured and differential resistance as a function of bias voltage of the junction showed oscillation with a microwave irradiation of specific frequencies (1.715 GHz).
Abstract: Transport properties of a superconductor-semiconductor junction are measured. Differential resistance as a function of bias voltage of the junction showed oscillation with a microwave irradiation of specific frequencies (1.715 GHz), which indicate the modulation of Andreev reflection probability by microwave irradiation.
3 citations
TL;DR: In this paper, the superconducting proximity effect in a sandwich structure of network-like carbon nanotubes coupled to NbN electrodes was investigated, which gives rise to multiple Andreev reflection processes and enhanced magnetoconductance fluctuations.
Abstract: We investigated the superconducting proximity effect in a sandwich structure of network-like carbon nanotubes coupled to NbN electrodes. The proximity effect gives rise to multiple Andreev reflection processes and enhanced magnetoconductance fluctuations that are similar to universal conductance fluctuation. Reentrant behavior caused by the proximity effect was observed. The proximity effect correction to the conductance disappears at low temperature and reaches a maximum value at about 8 K, which corresponds to Thouless energy. This reentrant behavior was also observed in the temperature dependance of fluctuation amplitude. These results are compared with theory.
2 citations
23 Jun 2008
TL;DR: In this paper, a photon-emitting LED combined with superconducting electrodes is proposed to enhance the oscillator strength of the radiative recombination processes by the Cooper-pair superradiance effect.
Abstract: Application fields of light emitting diodes (LEDs) are expanding in various fields. Development of LED-based single photon sources is expected to open a new possibility to expand the applications to quantum information communication and processing. The authors have proposed a photon-emitting LED combined with superconducting electrodes, which is expected to be an on-demand entangled photon pair source. The main mechanism is based on the coherent spatial extension of the Cooper-pair states to the photon emitting layer, which is expected to enhance the oscillator strength of the radiative recombination processes by the Cooper-pair superradiance effect. The preliminary operation was demonstrated with InGaAs quantum well (QW) LEDs and about 20-times enhancement of the electroluminescence (EL) was observed under the low-injection current regime. This is the demonstration of the improved internal quantum efficiency (QE) under the low internal QE operation of the LED. In this paper, the enhancement of the radiative recombination processes with the superconducting (SC) effect is demonstrated by the lifetime measurements under the operation with high (~100%) internal quantum efficiency (QE).
TL;DR: In this paper, the authors studied low temperature transport characteristics of an independently contacted double quantum dot-quantum wire coupled system, where each quantum dot is connected to three leads; source, drain and wire.
Abstract: We have studied low temperature transport characteristics of an independently contacted double quantum dot–quantum wire coupled system. Each quantum dot is connected to three leads; source, drain and wire. Fano resonances associated with one dot appear in the conductance of the other dot, which are gradually suppressed as the inter-dot tunnel coupling via the wire is reduced. When the dot-wire tunnel coupling is lost, conductance modulation similar to the Fano resonances reappear, which is ascribed to purely inter-dot electrostatic coupling effect.
25 May 2008
Abstract: To demonstrate the contribution of Cooper pairs to radiative recombination in a semiconductor, a n-InGaAs/p-InP light emitting diode with superconducting Nb electrodes was fabricated. Electroluminescence as well as photoluminescence from the n-InGaAs into which electron Cooper pairs were expected to penetrate from the superconducting Nb electrodes by the proximity effect was drastically enhanced at the temperature lower than the superconducting transition temperature. This is the experimental evidence that Cooper pairs enhance radiative recombinations by the superradiance effect.
TL;DR: In this paper, the electron effective mass and reduced mass as functions of the electron density (n s ) at about 100mK in perpendicular magnetic fields in the range between 15 r s 6 by magneto-photoluminescence measurements were determined.
Abstract: We report on results of determination of the electron effective mass and the reduced mass as functions of the electron density ( n s ) at about 100 mK in perpendicular magnetic fields in the range between 15 r s 6 by magneto-photoluminescence measurements We find that the obtained effective masses increase with decrease in n s at n s 1 × 10 11 cm - 2
TL;DR: In this article, the authors present measurements on the energy distribution profile of the ballistic hot carriers emitted from a quantum point contact using magnetic focusing technique and analyzed by measuring differential conductance of the quantum dot.
Abstract: We present measurements on the energy distribution profile of the ballistic hot carriers emitted from a quantum point contact. The hot carriers were injected to a quantum dot using magnetic focusing technique and analyzed by measuring differential conductance of the quantum dot. We found a thermally broadened energy distribution of the hot carriers near the bias voltage applied to the quantum point contact in an otherwise uniform distribution down to the equilibrium Fermi energy. We also measured an occupation ratio of available states by the injected hot carriers, and fully characterized their energy distribution profile. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)