Y. Takagaki

Nonlocal quantum transport in narrow multibranched electron wave guide of GaAs-AlGaAs

Abstract Quantum transport of electrons through a narrow multibranched electron wave guide is investigated. The electron wave guides are fabricated from high and low mobility GaAsAlGaAs wafers. The background average resistance revealed quite different behavior in both samples. It is found in a high mobility (ballistic) sample that the resistance is negative and with applying magnetic field it approaches to zero at B∼0.2T, where the diameter of the cyclotron orbit is comparable to the channel width W∼0.3μm. In a low mobility (diffusive) sample, the average resistance is positive and non-zero in the entire field range. The positive resistance of the diffusive sample is comparable to a starting film resistivity, while the negative resistance originates from a bend of the current carrying channel and is attributed to the ballistic electron transmission.

Superhigh-frequency surface-acoustic-wave transducers using AlN layers grown on SiC substrates

We demonstrate the operation of surface-acoustic-wave (SAW) transducers fabricated on AlN/SiC structures at frequencies as high as 19 GHz. The high SAW velocity of the AlN film is enhanced by the even higher sound velocity of the SiC substrate, enabling us to achieve these frequencies with a SAW wavelength of 400 nm.

Nonlocal voltage fluctuations in a quasi ballistic electron waveguide

Abstract The nonlocal voltage fluctuations in a high mobility quasi ballistic electron waveguide are measured as a function of magnetic field. Although voltage probes are 1.2μm apart from the closest current path, nonlocal fluctuations are observed below 0.2T where the diameter of a cyclotron orbit is larger than the channel width. The behaviors are quite different from those in a diffusive sample and are considered to originate from a ballistic nature of the transport.

Overshoot of four-terminal magnetoresistance at GaAsAlGaAs narrow wire junctions

Abstract The magnetic field and temperature dependences of a four-terminal negative resistance are studied. The magnetic field dependence shows an overshooting behavior. The negative resistance decays with increasing magnetic fields, and exceeds the background resistance. Further increase of the field returns the overshooting contribution to zero. A possible explanation of this behavior is given by making use of the Landauer-Buttiker resistance formula. At low temperatures, the magnitude of the negative resistance is almost independent of temperature. This is also explained by the above formulation.

Surface-acoustic-wave transducers for the extremely-high-frequency range using AlN/SiC(0001)

The fast sound propagation in AlN layers is demonstrated to enable generation of surface acoustic waves (SAWs) by interdigital transducers at a frequency beyond 30 GHz. While scaling down the wavelength of the transducers fabricated on AlN/SiC structures and the thickness of the AlN top layer to raise the operation frequency, the excitation of SAWs becomes intricate due to the weak electromechanical coupling in SiC. We examine the dependence of the feasibility of SAW generation on the AlN layer thickness.

Fabrication of Ballistic Quantum Wires and Their Transport Properties

Quantum wires with width less than the elastic mean free path and comparable to the Fermi wavelength are fabricated using high resolution electron beam lithography and ion beam etching. The low temperature magnetotransport properties of the quasi-ballistic channels reveal novel phenomena resulting from the ballistic motion of electrons. We propose new side-gate transistors. The two dimensional electron gas remaining at both sides of the channel is utilized to narrow the conduction width. This technique may open the way to control electrostatically an Aharonov-Bohm phase shift in the GaAs-AlGaAs loop.

Observation of Aharonov-Bohm magnetoresistance oscillations in selectively doped GaAs−AlGaAs submicron structures

Abstract Aharonov-Bohm magnetoresistance oscillations are investigated in a selectively doped GaAs-AlGaAs submicron ring with a width of 0.35μm and a diameter of 1μm. The ring is fabricated from a MBE grown film making use of electron beam lithography and dry etching methods. A wire with submicron width and several micron length, which is fabricated for comparison, shows only aperiodic fluctuations with an average period much larger than h/e. However, the ring shows the MR oscillations with the period of h/e due to AB effect, superimposed on aperiodic fluctuations.

Commensurate classical orbits on triangular lattices of anti-dots

Abstract A novel low-field magnetoresistance oscillation is observed in a high- mobility GaAs-AlGaAs heterojunction, where an array of scatterers is introduced artificially by etching. The period of the oscillation is governed by the ratio of the cyclotron diameter at the Fermi energy to the period of the anti-dots. The effect arises from the commensurate cyclotron orbits on the triangular lattice of anti-dots.

Shubnikov-de Haas effect of multi-terminal GaAs/AlGaAs quantum wire in non-local geometry

Abstract The magnetoresistance and Hall resistance in a quasi-ballistic multi-terminal quantum wire of GaAs/AlGaAs heterostructure have been investigated. The Shubnikov-de Haas (SdH.) oscillations in the non-local geometry, where the current probes and voltage probes are separated, are observed at high magnetic fields, in addition to the observation of the last Hall plateau, the negative bend and transfer resistance at low magnetic fields. The amplitudes of SdH. oscillations decrease with increasing nominal separation length between the current and voltage probes. The decay length (Ls) decreases from 3.5μm at B=1T to 2.0μm at B=5T with increasing magnetic field. In all cases, Ls is longer than the ballistic mean free path of electrons. Possible origins about this magnetic field dependence of Ls are discussed in connection with the edge states of the quantum wire.

Quantized resistance in in‐plane gated narrow constriction fabricated by wet etching

A narrow in‐plane gated constriction is defined by technique employing electron beam lithography and wet chemical etching. Two‐dimensional electron gas beside the narrow channel is used to control the number of occupied subbands in the constriction. A ballistic transport through the point contact is manifested by the observation of successive resistance steps when the subbands are depopulated.