H. W. Schumacher

Single-parameter nonadiabatic quantized charge pumping

Controlled charge pumping in an AlGaAs/GaAs gated nanowire by single-parameter modulation is experimentally and theoretically studied. Transfer of integral multiples of the elementary charge per modulation cycle is clearly demonstrated. A simple theoretical model shows that such a quantized current can be generated via loading and unloading of a dynamic quasibound state. It demonstrates that nonadiabatic blockade of unwanted tunnel events can obliterate the requirement of having at least two phase-shifted periodic signals to realize quantized pumping.

Tunneling magnetothermopower in magnetic tunnel junction nanopillars.

We study tunneling magnetothermopower (TMTP) in CoFeB/MgO/CoFeB magnetic tunnel junction nanopillars. Thermal gradients across the junctions are generated by an electric heater line. Thermopower voltages up to a few tens of μV between the top and bottom contact of the nanopillars are measured which scale linearly with the applied heating power and hence the thermal gradient. The thermopower signal varies by up to 10  μV upon reversal of the relative magnetic configuration of the two CoFeB layers from parallel to antiparallel. This signal change corresponds to a large spin-dependent Seebeck coefficient of the order of 100  μV/K and a large TMTP change of the tunnel junction of up to 90%.

Fabrication of a single-electron transistor by current-controlled local oxidation of a two-dimensional electron system

The surface layers of a GaAs/AlGaAs heterostructure are locally oxidized using an atomic force microscope. The local anodic oxidation depletes the underlying two-dimensional electron gas leading to the formation of tunneling barriers. The height of the barriers is determined by measuring the thermally activated current. By varying the oxidation current, the barrier heights can be tuned between a few meV and more than 100 meV. Using these barriers as tunneling elements, a side gated single-electron transistor is fabricated.

Nanomachining of mesoscopic electronic devices using an atomic force microscope

An atomic force microscope (AFM) is used to locally deplete the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. The depletion is induced by repeated mechanical scribing of the surface layers of the heterostructure using the AFM tip. Measuring the room-temperature resistance across the scribed lines during fabrication provides in situ control of the depletion of the 2DEG. Variation of the room-temperature resistance of such lines tunes their low-temperature characteristics from tunneling up to insulating behavior. Using this technique, an in-plane-gate transistor and a single-electron transistor were fabricated.

Plasmon electron-hole resonance in epitaxial graphene.

The quasiparticle dynamics of the sheet plasmons in epitaxially grown graphene layers on SiC(0001) has been studied systematically as a function of temperature, intrinsic defects, influence of multilayers and carrier density using electron energy loss spectroscopy with high energy and momentum resolution. The opening of an inter-band decay channel appears as an anomalous kink in the plasmon dispersion which we describe as a resonance effect in the formation of electron-hole pairs. Due to the inevitable strong coupling of plasmons with single particle excitations in reduced dimensions, such signatures are generally expected.

Size determination of InAs quantum dots using magneto-tunnelling experiments

Tunnelling experiments through GaAs-AlAs-GaAs structures with InAs embedded in the AlAs barrier show steps in the current-voltage characteristics which we assign to single-electron tunnelling through self-assembled InAs quantum dots between two three-dimensional electrodes. From the magnetic field dependence of the onset of the current steps, we determine the lateral extension of the electronic wave function in the dot to 4 nm, corresponding to a dot of 14 nm in diameter. Replica of steps at higher voltages are attributed to tunnelling through charged dots. A similar structural dot size is measured independently by transmission electron microscopy on the same wafer and by atomic force microscopy on control samples with InAs dots on a GaAs or an AlAs surface, respectively.

Plasmon damping below the Landau regime: the role of defects in epitaxial graphene

The sheet plasmon in epitaxially grown graphene layers on SiC(0001) and the influence of surface roughness have been investigated in detail by means of low-energy electron diffraction (LEED) and electron energy loss spectroscopy (EELS). We show that the existence of steps or grain boundaries in this epitaxial system is a source of strong damping, while the dispersion is rather insensitive to defects. To the first order, the lifetime of the plasmons was found to be proportional to the average terrace length and to the plasmon wavelength. A possible reason for this surprisingly efficient plasmon damping may be the close coincidence of phase (and group) velocities of the plasmons (almost linear dispersion) with the Fermi velocity of the electrons. Therefore, uncorrelated defects like steps only have to act as a momentum source to effectively couple plasmons to the electron–hole continuum.

Single-parameter quantized charge pumping in high magnetic fields

We study single-parameter quantized charge pumping via a semiconductor quantum dot in high magnetic fields. The quantum dot is defined between two top gates in an AlGaAs/GaAs heterostructure. Application of an oscillating voltage to one of the gates leads to pumped current plateaus in the gate characteristic, corresponding to controlled transfer of integer multiples of electrons per cycle. In a perpendicular-to-plane magnetic field the plateaus become more pronounced indicating an improved current quantization. Current quantization is sustained up to magnetic fields where full spin polarization of the device can be expected.

Controlled mechanical AFM machining of two-dimensional electron systems: fabrication of a single-electron transistor

Abstract By mechanical scratching the surface of a GaAs/AlGaAs heterostructure with an atomic force microscope an energetic barrier for the two-dimensional electron gas is formed. The barrier formation is in-situ controlled by measuring the room-temperature resistance across the barrier. Barrier heights can be tuned from some mV up to more than 100 mV as determined by measurement of the thermally activated current. Low-resistance barriers show typical tunneling behaviour at low temperatures whereas high-resistance lines show G Ω resistances in a bias range up to some 10 V allowing their use as in-plane gates. Transport measurements of a side gated single-electron transistor fabricated this way are presented.

Magnetoresistance Anisotropy in Si/SiGe in Tilted Magnetic Fields: Experimental Evidence for a Stripe-Phase Formation

We observe pronounced transport anisotropies in magneto-transport experiments performed in the two-dimensional electron system of a Si/SiGe heterostructure. They occur when an in-plane field is used to tune two Landau levels with opposite spin to energetic coincidence. The observed anisotropies disappear drastically for temperatures above 1 K. We propose that our experimental findings may be caused by the formation of a unidirectional stripe phase oriented perpendicular to the in-plane field.