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We report on investigations of the quantum Hall chiral edge states using a near-field scanning optical microscope that enables us to irradiate circularly polarized light from the probe tip with spatial resolution below the diffraction limit. We have found a clear evidence for the formation of spin-split incompressible strips near the edge of a two-dimensional electron system in high magnetic fields.

A real-space mapping of photovoltage near the edge of the Hall-bar of a GaAs/AlGaAs single heterojunction has been obtained using a dilution-refrigerator-based near-field scanning optical microscope in magnetic fields. The optical probe-sample surface distance dependence of photovoltage is investigated. We obtain photovoltage profile in the vicinity of the edge, which reflects the local chemical potential of the two-dimensional electron gas determined by the distribution of the compressible and incompressible strips.

Superconducting transport is measured between two superconducting electrodes at the n-type semiconductor side of a superconductor-based LED where a Josephson junction is formed. The characteristics of the Josephson junction are found to be modulated by applying voltage to the normal electrode at the p-type semiconductor side. The Josephson junction characteristics show an extraordinary sensitivity to the radiative recombination process, which we estimate as the recombination efficiency.

We measured the photoluminescence (PL) of a GaAs quantum well (QW) with a lateral potential modulation by the front-gate bias while changing both the electron density and electric field at low temperature. Near the onset of the two-dimensional electron gas, we observed an anomalous enhancement of PL intensity of the neutral exciton X 0 accompanied by a decrease in the intensity of the charged exciton X - . The sample was a 20-nm GaAs back-gate undoped QW with semi-transparent square-mesh surface gates. By comparing the results with those for a flat transparent surface gate sample we discuss the origin of this anomalous phenomenon. We present a possible explanation for this phenomenon, which is attributed to lateral drift motion of the X 0 in a laterally modulated exciton potential.

We have experimentally investigated the optical effect on the transport properties of superconductor-semiconductorsuperconductor (S-Sm-S) junctions composed of a two-dimensional electron gas in a GaAs/AlGaAs heterostructure and NbN superconducting electrodes. Illumination at λ 800 nm onto the whole junction area increases Andreev reflection (AR) probability at S-Sm interfaces. To explore this origin, we performed scanning photovoltage measurements by using an optical microscope. The obtained image plots of the photovoltage show that the illumination brings about the photovoltage at the S-Sm interfaces. This result implies that the illumination modulates the barrier height between S and Sm, which results in a modulation of AR probability.

Observation of drastically enhanced luminescence emitted from InAs quantum dots (QDs) which were in direct contact to superconducting Niobium (Nb) is reported. Although the PL intensity was essentially temperature independent without Nb stripe, drastic temperature dependence was observed with the presence of the Nb stripe. PL intensity was substantially increased below the Nb superconducting critical temperature (Tc). About 3-times enhancement was observed below Tc. This phenomenon is attributed to the Cooper pairs penetrated from the superconducting Nb stripe to adjacent InAs QDs by the proximity effect. This is the first experimental demonstration that the Cooper pairs enhance radiative recombination through QDs. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Generation of entangled photon pairs from semiconductor quantum dots (QDs) is highly desirable for realizing practical solid-state photon sources for quantum information processing and quantum cryptography. However, the energy splitting of exciton states in QDs almost prevent the generation of entangled photon pairs. This paper discusses the new possibility with the injection of electron as well as hole Cooper pairs into QDs.

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)

We investigated the superconducting proximity effect in carbon nanotubes (CNTs). Reentrant behavior caused by the proximity effect was observed in a sandwich structure of network-like CNTs coupled to NbN electrodes. The proximity effect gives rise to enhanced magnetoconductance fluctuations that are similar to universal conductance fluctuation. The proximity effect correction to the conductance and fluctuation amplitude 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 magnetic field dependance of fluctuation amplitude. We found that a correlation magnetic field of about 3 T induces the dephasing of the interference. This high magnetic field is due to the CNTs with small diameter. We discuss this proximity effect in these quasi-one-dimensional CNTs with random network structure in experiment and theory.

The electron–hole states in the fractional quantum Hall regime is investigated with a back-gated undoped quantum well by photoluminesccence in magnetic fields. The evolution of the photoluminescence spectra is discussed depending on the electron density. We find anomalies of the photoluminescence at the integer as well as the fractional filling factors.

The electric field dependence of the electron/hole wave function and the radiation energy of an exciton in a Be-δ-doped 80 nm quantum well (QW) is studied experimentally and compared it with variational calculation. The photoluminescence (PL) spectra show Stark shifts depending on the gate electric field and PL intensity of the exciton of the first excited state has a dip in the electric-field dependence which reflects the node of the electron wave function.

Targeting highly sensitive displacement and force sensors, we fabricated a piezoresistive cantilever that integrates a superconductor–semiconductor–superconductor (S–Sm–S) junction based on an InAs/AlGaSb heterostructure. The S–Sm–S junction is composed of a submicron Nb gap patterned on the InAs thin film, and a deflection of the cantilever is detected as a resistance change at the junction. We confirmed that the resistance change caused by induced strain (i.e., piezoresistance) has a strong dependence on bias current. When the maximum Josephson current ( I c ) is biased to the junction, the resistance change is significantly enhanced by more than a factor of 10 compared to that at the bias current above I c (the resistive state). The resulting maximum resistance change is 3.9 m Ω , which is three orders of magnitude larger than that obtained for our preliminary sample. This large piezoresistance at the S–Sm–S junction will lead to highly sensitive self-detective sensors.

We have fabricated network-like single-walled carbon nanotubes with superconducting NbN electrodes. The single-wall carbon nanotubes were synthesized on SiO2 substrates by the thermal chemical vapor deposition technique using Co catalyst. We obtained sufficiently good contact characteristics between the single-walled carbon nanotubes and the NbN electrodes by infrared annealing at heater temperature 700 °C for ~15 min in vacuum. We observed multiple Andreev reflection by measuring differential resistance as a function of applied voltage below 7 K. This multiple Andreev reflection is due to the proximity effect between NbN electrodes. The superconducting energy gap of NbN, 2Δ, is about 6 meV. The multiple Andreev reflection processes occur around the dips, i.e. ±2Δ/e and ±Δ/e. On the other hand, at above 8 K, the curves of different resistance are changed from dip to peak at zero bias voltage. This behavior is similar to reentrant behavior that has a maximum conductance corresponding to the correlation energy (Thouless energy) below superconducting critical temperature Tc. This reentrant behavior has been studied in normal metal or two-dimensional gas of semiconductor heterostructures coupled to superconductor.

We measured the photoluminescence (PL) spectra of a two-dimensional electron system induced in a Be-δ-doped GaAs/AlGaAs quantum well (QW) with a back gate. The electron density is controlled by means of the back-gate voltage. We estimated the electron density using the magneto-optical method and the PL linewidth, and also by undertaking transport measurements. We show that a uniform 2DES as large as 1 mm 2 is induced by the back-gate operation from 2.5 × 10 10 cm −2 . This experiment indicates that optical measurement with a back-gated QW is advantageous for studying the low-density 2DES.

We present studies on the electric transport in a lateral GaAs/AlGaAs quantum dot defined by a patterned single connected metallic front-gate. This gate design allows to easily couple a large number of quantum dots and therefore holds high potential in the design of new materials with tailor-made band structures based on quantum dot superlattices of controlled shape. Clear Coulomb diamond structures and well pronounced tunneling peaks observed in experiment indicate that single-electron control has been achieved. However, the dependence on electron density in the heterostructure embedding the dot, which is controlled by an additional back-gate, reveals that transport characteristics are strongly influenced supposedly by potential fluctuations in the dot and lead regions.

We fabricated NbN/multi-walled carbon nanotube (MWNT)/Al junctions using MWNTs standing in nano-porous alumina membranes. We observed a reduction in the resistance within the NbN superconducting energy gap voltage caused by Andreev reflection. Moreover, we found that the conductance scales in the form of power laws with respect to temperature and bias voltage, and the power-law exponent α becomes larger when NbN becomes superconductive. These experimental results can be qualitatively explained by the theory of superconductor-Tomonaga–Luttinger liquid hybrid systems.

We report the gate-voltage dependence of the electron g factor in InGaAs/InAlAs heterostructures with double InAs-inserted wells. The g factor has been determined from a series of Shubnikov–de Haas oscillations at various angles with respect to the layers under different gate voltages. The strain in InAs layers and the penetration of the electron wave function into InGaAs and InAlAs layers are the important factors of the reduction in the | g | factor. We have also observed the changes of the | g | factor with respect to the gate voltage, though this dependence is not clear in the simple calculation.

The values of the zero-field spin-splitting energy Δ0 in InAlAs/InGaAs/InAlAs heterostructures are investigated using anti-weak-localization analysis. The obtained values for Δ0 are compared with values that were theoretically predicted assuming Rashba spin–orbit coupling (denoted by ΔR). The good agreement between Δ0 and ΔR and their dependence on quantum well asymmetry suggest that our approach provides a useful tool for designing future spintronics devices using Rashba spin–orbit coupling.

We measure the magnetic flux dependence of the supercurrent in diffusive Nb/n-InAs/Nb mesoscopic SNS Josephson junctions with junction length ranging from 220 to 720 nm. In the longer junctions, the modulation periodicity of the maximum supercurrent by magnetic flux is a quantum flux, Φ 0 = h /2 e as usual. However, we find that the magnetic flux periodicity becomes twice or 2 Φ 0 = h / e when the junction lengths are shorter than 400 nm.

We have studied the differential resistance in a gated superconductor–semiconductor junction in the ballistic transport regime. The zero-bias resistance shows a minimum as a function of the gate voltage, while the junction normal resistance measured at a high bias voltage shows a monotonic increase. This is explained as follows: the conductance enhancement due to coherent Andreev reflection is controlled by the gate voltage which changes the transparency of the gate barrier. We have also fabricated a two-gated junction with a high transparency at the superconductor–semiconductor interface and measured its resistance as functions of two gate voltages. An increase in one gate voltage results in the resistance minimum as a function of the other gate voltage. The obtained results agree well with the so-called reflectionless tunneling theory.

We report on the fabrication of InAs-inserted-channel InAlAs/InGaAs inverted HEMTs with NbN electrodes made by using a DC magnetron sputtering deposition and we describe the device characteristics we obtained. Excellent pinch-off characteristics were obtained even at /spl sim/10 K when NbN electrodes retain their superconductivity. For a 3-/spl mu/m-gate device, the maximum extrinsic transconductance at 10 K was 300 mS/mm, even at the very low drain voltage of 0.2 V. We found that the HEMTs with NbN electrodes, not only have superior characteristics at /spl sim/10 K that exceed the critical temperature of Nb, but are also able to combine with NbN Josephson junctions.

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)

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 1.5 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 .

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.

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.

We performed infrared magneto-photoluminescence (PL) spectroscopy on an InAs-inserted-channel InGaAs/InAlAs heterostructure. Series of Landau levels were clearly observed in the infrared PL spectra. From the energy separation between σ+ and σ− components of PL, the electron–hole g-factor in each Landau level were determined. We discuss the obtained Landau fan-diagram from the points that nonparabolicity of conduction band structure and penetration of electron wavefunction into InGaAs layer.

We investigate the device characteristics of InAs-inserted-channel In/sub 0.52/Al/sub 0.48/As/In/sub 0.53/Ga/sub 0.47/As inverted high electron mobility transistors (HEMTs) with superconducting Nb electrodes. In these transistors, the ohmic contact between Nb and the two-dimensional electron gas formed in the InAs layer is obtained by contact with the Nb-InAs direct, instead of with an alloyed normal metal and semiconductor as in a conventional HEMT. The contact resistance of 0.15 /spl Omega/mm between the Nb ohmic electrodes and the channel decreased by a factor of 3 compared with that when a conventional AuGe/Ni alloyed ohmic contact is used. For a 0.5-/spl mu/m-gate device, the maximum extrinsic transconductance at 4.2 K was 1 S/mm, even at a very low drain voltage of 0.2 V. These results indicate that this ohmic contact formation will allow us to utilize in this device both the improved HEMT characteristics and the superior performance of superconducting electrodes.

This paper examines a detailed analysis by Shubnikov-de Haas measurements of the effective mass of two-dimensinal electron gas (2DEG) in an In0.52Al0.48As/ In0.53Ga0.47As modulation-doped (MD) structure with an InAs quantum well inserted into the InGaAs channel (InAs-inserted channel). The measured effec-tive mass of 2DEG in the InAs-inserted-channel MD structure is in good agreement with the calculated one of the strained InAs layer on In0.53Ga0.47As. This indicates that almost all of the 2DEG forms in the strained InAs quantum well. These results show that the InAs-inserted-channel MD structure improves the electron confinement, since the 2DEG is confined in the InAs quantum well with the thickness of 4 nm.

A newly fabricated Josephson field effect transistor (JOFET) is coupled with a two-dimensional electron gas (2DEG) in a strained InAs quantum well inserted into an inverted modulation-doped structure with an HEMT-type gate. We indicate the improved characteristics of the JOFET with the HEMT-type gate, instead of the MIS-type gate. The superconducting critical current as well as the junction's normal resistance can be completely controlled via a gate voltage of about -1 V; this provides voltage gain over unity, the first time for a JOFET.

A newly fabricated three-terminal Josephson junction is coupled with a two-dimensional electron gas (2DEG) in a strained InAs quantum well inserted into an InAlAs/InGaAs modulation-doped structure. The 2DEG is confined in the InAs quantum well and has a maximum mobility of 155000 cm/sup 2//Vs at a sheet-carrier density of 1.86/spl times/10/sup 12/ cm/sup -2/ at 10 K. The supercurrent flows through the 2DEG and can be controlled by adjusting the gate voltage. The critical current and normal resistance are measured as a function of the gate voltage, and the sheet-carrier density dependence of the critical current is obtained. We also measure the temperature dependence of the critical current at different gate voltages. The results indicate that, when using this junction, the superconducting characteristics can range between the clean and dirty limits. >

The authors have investigated the kink effect In an InAs-inserted-channel InAlAs/InGaAs inverted HEMT at low temperature. The kink effect was not observed at both 77 and 300 K, but it appeared at 4.2 K. It is shown that the kink effect is caused at low drain voltages by the suppression of the drain current due to an increase in the source access resistance and at higher drain voltages by the increase in the drain current due to holes generated by impact ionization.

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.

Thin Nb films are grown by electron beam evaporation in an ultrahigh vacuum system on molecular beam epitaxy grown (001)InAs epitaxial layers. The Nb on InAs grows as a single‐crystalline deposit at a substrate temperature of 200 °C. The orientation relation is (001)Nb//(001)InAs with [110]Nb//[110]InAs, which is different from Nb growth on GaAs. The interface between Nb and InAs features a crystal‐disordered layer with a thickness of 1–2 nm, which provides relaxation from lattice mismatch. Critical current measurement of Nb/InAs/Nb junctions shows that the crystal‐disordered layer does not affect the superconducting characteristics of the junctions.

X‐ray scattering radiography has been successfully applied to an InAs lattice‐mismatched heteroepitaxial layer grown on GaAs (001) by molecular beam epitaxy. It was found that the lattice of the epitaxial layer varied the orientation over 18 min in arc around a [100] direction, whereas the substrate has a small lattice bend around [110] by less than 2 min in arc. The bending forms the layer into a concave with a valley along [100]. The independent lattice deformation in two layers comes from a complicated effect of the lattice mismatches and thermal expansion. Also local various structures were observed in the scattering radiographs.

We investigate polarization-resolved fine structure in the photoluminescence (PL) in the fractional quantum Hall effect regime at $B=4$--6 T, where small Zeeman energy allows spin-depolarized ground states. We observe up to five distinct peaks with characteristic polarization and temperature dependence in the vicinity of $\ensuremath{\nu}=1/3$ and quenching of the PL from triplet charged quasiexcitons at around $\ensuremath{\nu}=1/4$. Those findings appear to be consistent with results of exact diagonalization on a Haldane sphere including all spin configurations and are understood to be PL from fractionally charged quasiexcitons.

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.

We investigated the transport characteristics between two superconducting electrodes at the n-type semiconductor side of a superconductor-based LED where a Josephson junction is formed. The characteristics of the Josephson junction are found to be modulated by applying voltage to the normal electrode at the p-type semiconductor side. The mechanism of the modulation can be categorized into three regimes, the gate control, the radiative recombination process and the nonequilibrium carrier injection, depending on the current injection from the p-side. Among these three regimes, the Josephson junction characteristics show an extraordinary sensitivity to the radiative recombination process, which we estimate as the recombination efficiency.

We measured photoluminescence (PL) spectra from a 20-nm GaAs/AlGaAs quantum well (QW) grown on an n-type substrate by selectively exciting the GaAs at 2 K. We observed a two-stage change of PL spectra as a function of the total amount of photo-irradiation (p × t) after cooling down. This corresponds to the process of establishing the equilibrium of electrostatic potential between the sample surface and QW and between the QW and n-doped substrate.

We demonstrate that the vertical electric field and the electron density of a two-dimensional electron system (2DES) can be controlled in a double-gated GaAs/AlGaAs quantum well (QW). Photoluminescence (PL) spectra from the recombination of an electron with a hole bound to a beryllium acceptor atom are measured as functions of gate biases applied to front and back gates. By comparing the measured spectra with theoretical models, we analyze the effect of the potential gradient in the QW on the PL energy for different electron filling. While a photoexcited electron recombines with a bound hole in an empty QW, a layer of 2DES is induced when the conduction band bottom of the QW lowers below the Fermi energy by the back-gate bias. In this case, a simple capacitor model gives a good estimation of the electron density. When we increase the front-gate bias, another electron layer is formed on the other side of the QW. These two electron layers screen the electric field inside the QW where the PL energy is insens...

We have studied spin correlation in a double quantum-dot–quantum-wire coupled device revealed in low-temperature transport characteristics. We demonstrate nonlocal control of the Kondo effect in one dot by manipulating the spin states of the other. The modulation of the local density of states in the wire region due to the Fano-Kondo antiresonance and the Ruderman-Kittel-Kasuya-Yoshida exchange interaction are the two possible mechanisms underlying the observed features. When the dot states are indirectly probed in the side-coupled geometry, double suppression of the wire conductance is observed due to the Fano-Kondo antiresonance involving both dots.

We have experimentally obtained clear evidence of enhanced thermal noise in a ballistic superconductor-normal metal-superconductor junction with an InGaAs-based two-dimensional electron gas (2DEG). The thermal noise was estimated from a comparison of measured current–voltage characteristics with those obtained with an extended Ambegaokar and Halperin theory. As a consequence, we have observed enhanced thermal noise that is much larger than that expected with normal reservoirs. This enhanced thermal noise can be explained by a theory that considers both the ballistic transport of the 2DEG and the thermal fluctuation in the coherent multiple Andreev reflection regime.

We performed photoluminescence (PL) measurements of the two-dimensional electron system (2DES) in an undoped GaAs quantum well (QW) while controlling the electron density and electric field independently. We compared the PL spectrum at the same electron density but at different electric fields. The PL linewidth changes with electric field at low electron density below 4×10 10 cm −2 , but does not change with electric field at higher electron density. The sample we used is composed of a metallic mesh front gate with a 500-nm pitch. The phenomena we report here mainly originate from the longitudinal electric field bias and are commonly observed in a different-mesh-size sample.