青木 伸之

We present evidence for a re-entrant metal-insulator transition that arises in quantum dot arrays as the gate voltage is used to sweep their density of states past the Fermi level. The form of the temperature variation of the conductance observed in these arrays can be accounted for using a functional form derived from studies of the metal-insulator transition in two dimensions, although the values obtained for the fit parameters suggest that the behavior we observe here may be quite distinct to that found in two dimensions.

Magnetic characterization of a quantum dot array was carried out to extract the phase-breaking time as a function of temperature and current. These measurements indicate a saturation of the phase-breaking time at low temperature and low current. The phase-breaking time varies as 1/T for temperatures above 1 K, which can be attributed to carrier-carrier scattering processes. Variation of the phase-breaking time with current yields an estimate for the electron temperature.

In recent studies of disordered quantum dot arrays, it was found that a re-entrant metal-insulator transition occurs in these structures as the gate voltage is varied, even though there is no change in carrier density or disorder. Here, we study resonances that appear with the quantum Hall effect in these dot arrays. Several resonances are observed on the side of the plateaus, which may be due either to charging of isolated islands or to transmission resonances in the quantum point contacts. Heating of the carriers causes these peaks to decrease with an energy relaxation time comparable to that of the dots themselves.

We have measured the low temperature resistance in amorphous SixGe1-x alloy films. Fitting to the theoretical results, we found two kinds of transport regime in the low temperature transport, variable range hopping type (T-1/4) and Coulomb gap type (T-1/2), and can determine the crossover temperature. The crossover temperature clearly depends on the ratio of the alloy composition.

Clear (Formula presented) conductance steps are observed in a quantum point contact (QPC) with a ferromagnetic (Ni) dot embedded in its center, which is made on the surface of a (Formula presented) heterojunction. Those steps are observed under no applied magnetic field. The interface between the embedded Ni dot and the heterojunction seems to become (Formula presented) alloy due to the strong reactivity of Ni forming an adiabatic semiconductor-metal-semiconductor transient region, which has Ohmic and possibly ferromagnetic natures. The main origin of the (Formula presented) steps thus could be a quasiballistic transport through the Ni dot and/or the transient region, which acts as a local spin filter at the QPC center. © 2000 The American Physical Society.

We describe the observation of novel localization in mesoscopic quantum dots and quantum dot arrays, which are realized in high mobility GaAs/AlGaAs heterojunctions using the split-gate technique. With a sufficient gate voltage applied to form the devices, their resistance diverges as the temperature is lowered below a degree Kelvin, behavior which we attribute to localization. Evidence for the localization is found over the entire range of gate voltage for which the dots are defined, persisting to conductances higher than 50e2/h.

A review of the earlier studies of coherent electron transport in open quantum dots and quantum dot arrays was presented. By modifying the details of the coupling between the dot and its quantum mechanical leads, it was possible to select different wavefunction states. Open dot arrays, which revealed evidence for collective transport signatures were also discussed briefly.

We discuss the observation of an unusual type of localization in split-gate quantum dots and quantum-dot arrays. While no evidence for its existence is found prior to biasing the gates, the localization persists to conductance values as high as 50 e(2)/h and is not destroyed by the application of a weak magnetic field. The carrier density in the dots remains constant over the range of gate bias studied and these characteristics suggest that the localization is quite distinct to that studied previously in two-dimensional semiconductors. We suggest that a confinement-induced enhancement of the electron-electron interaction may be responsible for the localization and propose a simple functional form which allows us to account for its variation as a function of either temperature or source-drain voltage. [S0163-1829(99)52848-8].

We present evidence for gate voltage-induced localization, and a possibly re-entrant metal-insulator transition, in linear quantum dot arrays that are realized using the split-gate technique. No evidence for the localization is observed prior to biasing the gates of the array, and the details of this behavior are found to be strongly device dependent. The metal-insulator transition is thought to arise as the gate voltage sweeps the discrete level spectrum of the array past the Fermi surface, mapping out regions of localized and extended states. In the metallic regime, the resistance varies logarithmically with temperature, behavior which is not seen in the underlying two-dimensional electron gas, but which is consistent with recent predictions for a correlated electron liquid. [S0163-1829(99)15747-3].

The nonlinear conductivity of an array of quantum dots, formed by split-gates on a GaAs/AlGaAs heterostructure, is studied. Previously, this array was shown to exhibit a gate bias-induced localization behavior at low temperatures, similar to the insulating state found in 2D systems. Here, we study the effect of source-drain bias potential on the heating of the electron system in the dots. We find that the scaling of conductance with this bias voltage can be described by an equation similar to that for temperature. In addition, we determine the effective temperature and energy-relaxation time. This relaxation time appears to decay as T-3/2 at higher temperatures, but shows a saturation at low temperatures.

We observed cross-sectional transmission electron microscope images of small structures on a GaAs substrate made by the scanning tunneling microscope (STM) field-induced fabrication method. A cross-sectional image of a GaAs dot, fabricated by applying a voltage pulse to a W tip, was 400 nm wide and had a highly symmetric double ditch structure. The inside of the dot consisted of GaAs polycrystal and the boundary was clearly limited by specific crystal planes. If the fabrication mechanism is considered to be field-induced evaporation in the STM regime, the anisotropy would have arisen due to a difference in work function between each plane. We also observed a Ni dot fabricated using a Ni-coated tip. The dot was a spherical with about a 110 nm diameter and it consisted of Ni polycrystal. Using the tip, we could obtain only one or two Ni dots, suggesting it behaves like a solid source rather than a liquid ion source.

We discuss the observation of an unusual type of localization in split-gate quantum dots and quantum-dot arrays. While no evidence for its existence is found prior to biasing the gates, the localization persists to conductance values as high as 50e2/h and is not destroyed by the application of a weak magnetic field. The carrier density in the dots remains constant over the range of gate bias studied and these characteristics suggest that the localization is quite distinct to that studied previously in two-dimensional semiconductors. We suggest that a confinement-induced enhancement of the electron-electron interaction may be responsible for the localization and propose a simple functional form which allows us to account for its variation as a function of either temperature or source-drain voltage. © 1999 American Physical Society.

We discuss the observation of an unusual type of localization in split-gate quantum dots and quantum-dot arrays. While no evidence for its existence is found prior to biasing the gates, the localization persists to conductance values as high as 50e2/h and is not destroyed by the application of a weak magnetic field. The carrier density in the dots remains constant over the range of gate bias studied and these characteristics suggest that the localization is quite distinct to that studied previously in two-dimensional semiconductors. We suggest that a confinement-induced enhancement of the electron-electron interaction may be responsible for the localization and propose a simple functional form which allows us to account for its variation as a function of either temperature or source-drain voltage. ? 1999 American Physical Society.

We have made a ferromagnetic-(Ni) dot structure embedded in split-gate quantum wires on AlGaAs/InGaAs/GaAs pseudmorphic high electron mobility transistor (PM-HEMT) by two-step surface modification with scanning tunneling microscope (STM). In low-temperature transport measurements, we observed interesting two magnetization-dependent features. First, aperiodic conductance oscillations were observed against gate voltage before magnetization, while periodic and reproducible oscillations were recorded after magnetization. Second, in the current-voltage characteristic, we observed a variation of the Coulomb gap width depending on whether the magnetic field was applied or not. The gap width with ± 5000 G application decreases down to less than half width 0 G. To consider the origin of those transport features, magnetic force microscopy (MFM) measurements have been performed for the embedded Ni dot samples. In some cases, we found the domain wall formation inside the dot before magnetization. It disappeared after magnetization, which implies a single-domain formation in the entire dot. The domain wall formation in the dot are closely related to the transport results. © 1998 Elsevier Science B.V. All rights reserved.

Two unique quantum transport features, a clear appearance of Coulomb oscillations and a narrowing of Coulomb gap, are observed in an embedded ferromagnet (Ni) dot in a semiconductor wire after magnetic field application. Magnetic force microscopy analysis suggests the existence of a domain wall inside the dot before the field application, but it disappears after the field application forming a single domain in the dot. If we establish a transport model by assuming that the domain wall plays the role of a “resistive barrier,” we can explain those unique transport features in terms of “Coulomb blockade effects modified by domain dynamics.”. © 1998 The American Physical Society.

We have made ferromagnetic (Ni) dot structure embedded in semiconductor quantum wires by two-step surface modifications with STM. The wire was a kind of split-gate type made on pseudomorphic (PM) HEMT surface and a hole into which the Ni was buried has a size of 200 nm upper diameter and 100 nm depth reaching two-dimensional electron gas (2DEG) plane located at 60 nm depth from the surface. To investigate the magnetic properties of the dot, which is tightly related to those of transport, magnetic force microscopy (MFM) measurements were carried out at ambient conditions. The observations suggest a domain wall formation inside the dot before magnetization. It disappeared after magnetization, which implies a single domain formation in the entire dot. In low temperature transport measurements, we observed two distinct magnetization-dependent features: before magnetization, observed were aperiodic conductance oscillations against gate voltage sweep, while periodic and reproducible oscillations were recorded after magnetization. Moreover, in the current-voltage characteristics, Coulomb blockade-like features were observed and the Coulomb gap reduced to about a half by applying an external magnetic fields of ±5000 G, which seems to mean the dot size expansion. On the basis of those results, we propose a domain related transport model which essentially interprets the observed transport properties. © 1998 Published by Elsevier Science Ltd. All rights reserved.

We have made a ferromagnetic (Ni) dot structure buried in semiconductor quantum wires by two-step surface modifications with STM. The wire was a type of split-gate, and the hole in which the Ni was buried was in the form of a small reversed cone (200 nm upper diameter and 100 nm depth). This size was selected in order to observe strong spin-selected transports, because we adopted a pseudomorphic heterojunctionwhich has a two-dimensional electron gas layer at 60 nm depth from the surface. In low-temperature tranport measurements, we observed two distinct magnetization-dependent features. Before magnetization, aperiodic conductance oscillations against gate voltage sweep were observed: after magnetization, periodic and reproducible oscillations were recorded. In studying the current-voltage characteristics, we observed Coulomb blockade-like features. The Coulomb gap is reduced to about a half when externalmagnetic fields of ±5000 Gs are applied, which seems to indicate expansion of the dot size. To study the origin of those transport features, magnetic force microscopy (MFM) measurements have been made for similar Ni dot samples at ambient conditions. MFM observation suggests preliminary domain wall formation inside the dot before magnetization. This then disappears after magnetization, implying a single domain formation for the entire dot. On the basis of those results, we propose a domainrelated transport model which essentially interprets the observed transport properties. © 1998 Springer-Verlag.

We fabricated a new class of quantum structure which has a buried superconductor dot between the split-gate wire using a new scanning tunneling microscope (STM) fabrication technique. In order to fabricate a buried dot at a desired surface position, we employed a scanning electron microscope (SEM)/STM combined system and performed a two-step fabrication process. In the first step, we used a tungsten (W) tip and placed it close to the surface between the split-gate. When a voltage pulse was applied, a small hole (80 nm deep and 150 nm in diameter at half-depth) was created by electric evaporation. Such a hole means that a pin-point deep etching for the two-dimensional electron gas (2DEG) plane can be located at a depth of 60 nm from the surface. In the second step, we changed the W tip to an indium (In) tip and placed it close to the bottom of the hole by observing the SEM image. By applying a voltage pulse, In was evaporated from the end of the tip, electrically or thermally, onto the hole structure. Such a structure is expected to form a normal (2DEG)-super (In dot)-normal (2DEG) double tunneling junction when the wire is properly squeezed at low temperature. © 1997 Academic Press Limited.

The fabrication and low temperature transport properties of a new class of quantum structures which have a buried superconductor dot in a split-gate wire are described. To create such a structure, we employed a combined scanning electron microscope (SEM)/scanning tunneling microscope (STM) and performed a two-step fabrication process using a tip-induced field and thermal evaporation. We chose indium as a superconductor material and the dot size at the two dimensional electron gas (2DEG) plane was about 100 nm. Such a structure is expected to form a normal (2DEG)-super (In dot)-normal (2DEG) double tunneling junction when the wire is properly squeezed. In transport experiments at 0.3 K, different conductance step behavior and different nonlinear current-voltage characteristics were observed in the systems with and without an In dot.

Recently scanning tunneling microscopes (STMs) have become important tools for nanometer scale fabrication. In order to make a nanoscale structure on a semiconductor surface, we attempted to lithograph very fine lines (possibly less than 10 nm) by using a resist process with an STM as an electron beam source. A diluted positive electron beam resist, poly-methyl methacrylate (PMMA), was first coated on a S-doped GaAs substrate. Using the STM in air, we approached a W-tip to the surface at a bias voltage of 10 V and a tunneling current of 0.5 nA, and made a line using by scanning the tip with various scan times. We then analyzed those lines made by the STM tip using an atomic force microscope (AFM) in air. After these processes, we deposited a Ti membrane on the sample surface by vacuum evaporation, then lifted it off. As a result, we confirmed that it was possible to make fine Ti lines as narrow as 50 nm by this process.

We have fabricated split-gate quantum wires having a buried ferromagnetic dot, by successively utilizing electron-beam (EB) and two-step scanning tunneling microscope (STM) fabrication. For STM fabrication, we used an STM/scanning electron microscope (SEM) combined with a system operated in high vacuum. The fabrication method is a kind of electrical evaporation with a tungsten (W) tip (top curvature is less than 50 nm). In the first step, a W tip was brought between the split-gate, and then a hole was fabricated by applying a pulse voltage between the W tip and the sample surface. In the second step, a W tip coated with nickel (Ni) was brought near the fabricated hole. Then by applying a pulse voltage between the Ni-coated W tip and sample surface, electrically evaporated Ni from the tip is buried into the hole. In a preliminary measurement at 0.3 K, we obtained the following unique transport properties. In a 4-terminal conductance (G4t) as a function of gate voltage (Vg), we observed a clear 'kink' (an abrupt change of dG4t/dVg and step structures) before full pinch-off of the wire. In both regions of G4t, that is, when Vgk < Vg (before the kink appears) and when Vg < Vgk (after the kink appears) (Vgk is the gate voltage at which the 'kink' appears), some step structures are seen. The step difference (ΔG4t) is, however, different between the two regions. That is, ΔG4t = 2 - 4 × (2e2/h) before the kink appears, while ΔG4t = (1/8) - ( 1/4 ) × (2e2/h) after the kink appears.

Fabrication of a new class of quantum structure which has a buried nickel (Ni) dot in a split-gate quantum wire using a scanning tunneling microscope (STM) is described. In order to fabricate small structures at the desired wire surface position, we employ a combined STM/scanning electron microscope (SEM) system in high vacuum. The fabrication methods are those based on simple electrical evaporation with a tungsten (W) tip. On the free surface far from the split-gate electrodes, the structure produced after applying a single voltage pulse is a small mesa (150 nm diameter, 20 nm high). However near the gates, large holes (150 nm diameter at half-depth, 85 nm deep) are created. Such large holes act as the pinpoint antidot for the two-dimensional electron gas (2DEG) lying at a depth of 60 nm from the wafer surface. As a metallic material, we adopted a Ni. For burying Ni into the hole, we moved the Ni-coated W tip to the hole bottom by observing the SEM image and created a Ni dot in the hole by applying a single voltage pulse.

To investigate the magnetic mesoscopic quantum effects on the conductivity in low temperature, the magnetic metal quantum dots and mesoscopic magnetic circuits with micron size ferromagnets on the HEMT substrate have been prepared. The ESR signal of magnetic quantum dot and the data of the threshold on the magnetoresistance in the mesoscopic circuit are analyzed.

DC and AC magnetization for H t c-axis of an air-annealed Bi2Sr2CaCu2O8 single crystal have been investigated in detail. In a DC magnetization (M-H) curve above 30 K a distinct kink has been observed at Hk close to the irreversibility field (Hirr), where a distinct change also in AC loss (χ″) has been observed. Below 40 K a prominent second peak has been exhibited in the M-H curve. It has been shown that the field (Hm) at the maximum | dM / dH|, where the second grows rapidly, is almost temperature independent. The χ″ versus HDC curve has exhibited a sharp dip near the Hm. Also has been observed another characteristics field Ht, where a crossover in the magnetic relaxation occurs. These characteristics fields (Hk, Hm and Ht) give a well defined vortex phase diagram, which is discussed in relation to a flux-melting and a possible vortex/Bose glass transition. © 1995.

We have measured electrical resistivity, AC and DC magnetization of a Bi2Sr2CaCu2O8+δ single crystal in as-grown and air-annealed state. From these measurements we have observed changes in the superconducting transition temperature and flux-pinning behavior reflected on the magnetization. In addition we have observed a new sharp kink in the out-of-phase signal of AC susceptibility, which corresponds to the magnetic field of a maximum slope at the second peak in DC magnetization.