Research

RESEARCH PROJECTS

Neutral edge trasport

The quantum Hall effect takes place in a two-dimensional electron gas under a strong magnetic field and involves current flow along the edges of the sample. For some particle-hole conjugate states of the fractional regime, early predictions suggested the presence of counter-propagating edge currents in addition to the expected ones. When this did not agree with the measured conductance, it was suggested that disorder and interactions will lead to counter-propagating modes that carry only energy-the so called neutral modes. In addition, a neutral upstream mode (the Majorana mode) was expected for selected wavefunctions proposed for the even-denominator filling 5/2. We observed counter-propagating neutral modes for fillings of 2/3, 3/5 and 5/2. Now we are in position to investigate these new currents further.

High mobility two dimensional electron gas

One of the most important ingredients of mesoscopic structures is the quality of the 2DEG. We have an extremely pure MBE system, and developed unique grwth processes for obtaining low disorder 2DEG. Record low temperature mobility of 36×106cm2/V-s was measured in the dark. The material exhibits long mean free path of the electrons and a rich structure of fractional states in the fractional quantum Hall regime.

Interference effects in mesoscopic systems

In mesoscopic systems, with size smaller than the dephasing length of the electrons, phase is an important parameter. Building interferometers that function under weak and strong magnetic fields allows studying coherence of electrons and quasi-particles under different conditions. Aharonov-Bohm rings; two-path interferometers; electronic Mach-Zehnder and Fabry-Perot interferometers are being now routinely used.

Nanowire Superconductor Devices

Heterostructures of high quality MBE grown nanowires, combined with superconductors form interesting playgrounds. Understanding the way current flows in these devices can shine light on many interesting questions in superconductivity and quantum mechanics, which are of growing interest nowadays. Studies of splitting Cooper pairs to form entangled electrons when exiting a Y-junction (superconductor contact on top of nanowire), or forming Majorana fermions,which are manifested by a p-wave superconductor being induced in the nanowires, are being perused. Working together with theory teams in our department we try to address these exciting questions.

Studies of fractionally charges quasi-particles in the FQHE regime

A sophisticated measurement system, which is able to measure weak signals of noise (on a large background) was developed. It is composed mainly of a low noise cold preamplifier located inside the dilution refrigerator. Measring shot noise aloowed the determination of the fractional charge of the quasi-particles in the FQHE regime. Earlier measurements determined the so called Laughlin’s quasi-particles, e/3, e/5 and e/7, while recent measurements determined the e/4 fractional charges at the (hopefully) non-abelian 5/2 fractional state. A variet of experiments were performed to show the transport properties of these quasi-particles, such as their transport through barriers and their bunching properties. Experiments are now underway to measure the statistics of such fractionally charged quasi-particles. While fractional statsitics is expected for the Laughlin’s quasi-particles, a non-abelian one is expected for the e/4 quasi-particles.

Measuring phase evolution in quantum dots

Utilizing interference the phase of the transmission and reflection amplitudes can be measured. Measurements are done by itroducing the system under study in one arm of a two-path interferometer, while measuring the phase of the interference fringes as function of energy. Detailed studies are performed with quantum dots in the Coulomb Blockade and the Kondo correlated regimes. While the transmission coefficient is commonly understood, the phase seems to present complex behavior, which tends to be very sensitive to detailes and minute correlation effects.

Controlled dephasing and Bohr’s complementarity principle

With an added ‘which path’ detector in close proximity to the interferometer, the path the electron chooses can be determined with some certainty. The accuracy of path detection determines the visibility (contrast) of the interference. Utilizing a variety of detectors, such as current and phase detectors, the visibility of the interference fringes could be totally quenched. Moreover, it was demonstrated that it in order to dephase the interferometer, it suffices that the detector should be able to detect the path ‘in principle’, without an actual measurement taking place.

Phase recovery and entanglement

When complete dephasing, due to a nearby path detector is taking place, one can, under certain conditions, recover the ‘lost interference’ by ‘post selction’ type mesurements. Performing cross correlation measurements between current fluctuations in the dephased interferometer and the path detector may exhibit the interfernce pattern. Such experiments prove that the total system: interferometer + detector are a single, quantum system, with no loss of information.

Growth and characterization of semiconductor nanowires

Recently we lounged a new research activity which is at the cutting edge of the nanoscience – study of the growth and structural properties as well as electrical and optical characteristics of III-V nanowires, in particular GaAs and InAs. The main idea of the project is to develop a novel mesoscopic devices based on high-purity, high-aspect ratio single crystal semiconductor devices. Reduced lateral dimensionality of the nanowires (10 – 100 nm) provides an opportunity for studying new phenomina in 1D charge transport, photonics and nanosize mechanics. The main focus of the research is on the quantum transport phenomena in nanowires, also a significant part of the activity is directed at understanding nanowires growth and material science related issues, such as facilitating the growth of axial and lateral heterostructures. To achieve highest purity and crystalline perfection the nanowires are grown in the dedicated high-purity MBE machine equipped with a treatment chamber which enables in situ application of the gold catalyst which assists the vapor-liquid-solid growth procedure.

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