Solid-State Quantum Transport Group

Today’s electronics are based on the progress of semiconductor devices. Modern nanotechnology pushes the minimum line width of semiconductor structures to less than 10 nm. These semiconductors are very important in engineering, but they also provide an ideal stage to study physics. A structural size smaller than or nearly equal to the electron wavelength results in prominent quantum effects and novel physical phenomena.

We are studying these semiconductor quantum structures with an emphasis on the concept of carrier interaction. We can utilize quantum transport phenomena in two-dimensional electron systems through the interplay between spin, orbital, and valley degrees of freedom. The quantum wire and quantum point contact are fundamental semiconductor nanostructures, and we are studying manipulation and detection of electron spin and nuclear spin features in these structures.

We have verified a strong interaction between electrons and nuclear spins in certain conditions. We have successfully applied this phenomenon to novel highly-sensitive resistively-detected NMR (Nuclear Magnetic Resonance). Such studies are connected to the study of electron spin features by using nuclear spin detector, and high-precision coherent control of nuclear spins in semiconductor nanodevice. We are leading the world in these topics. We are also interested in developing an interface between nuclear spins and photons via optical means in addition to electrical ones. NMR and MRI (Magnetic Resonance Image) have been widely used in chemical and medical analysis, and the extension of the powerful nature of nuclear-spin-based measurements to solid-state systems will establish a novel spintronics. We have also succeeded to extend high-sensitive resistively detected NMR to InSb two-dimensional system, typical narrow-gap quantum system.

In order to elucidate these physical properties, it is essential to understand them on a nanoscale, in addition to macroscopic measurements such as resistance. We are focusing on direct observation of the local properties of semiconductor quantum structures using nanoprobes operating at dilution refrigerator temperature. A semiconductor quantum structure version of MRI has been demonstrated recently.

Keywords

Two-dimensional electron system

When electrons are confined within a membrane which is thinner than the electron wavelength, they lose their freedom of movement normal to the plane of the membrane but maintain freedom to move within it. Under this condition, we say the electrons constitute a two-dimensional electron system. The quantum Hall effect occurs when an external magnetic field is introduced perpendicular to the plane of the 2D electrons and causes Landau quantization. In a clean system, it is possible to obtain a single flat sheet of electrons, which is a favorable geometry for experiments on Coulomb and spin-based interactions between electrons.

Valley degree of freedom

In some semiconductors the bottom of the conduction band is located at the Γ point, e.g. GaAs, whereas in others it is located elsewhere, e.g. Si and graphene. In the latter case, electrons are granted a degree of freedom in which conduction band minimum they occupy. This degree of freedom exists in addition to the normal spin degree of freedom of electrons, and is termed the “valley degree of freedom”.

GaAs (gallium arsenide)-based semiconductors

GaAs and related semicobductors are expensive compared to silicon, but because of its high quality it is used in devices like mobile phones and lasers where added quality is necessary. Currently the cleanest 2D systems are made using GaAs. The thin layer containing the electrons (quantum well) is created by sandwiching GaAs, which has a small bandgap, between AlGaAs, which has a large bandgap. This type of layered structure is called a double heterostructure, and is fabricated using molecular beam epitaxy (MBE).

Quantum wire (quantum point contact)

A quantum wire is the thinnest possible electrical wire, so to speak. If electrons are made to flow within a region with a width comparable to their wavelength, a few tens of nanometers (1 nanometer is 10-9 meters), Ohm’s Law no longer applies as normal, and instead quantum behavior begins to emerge. The conductance through the wire takes on specific quantized values.

NMR (nuclear magnetic resonance)

The detection of the spins of atomic nuclei by NMR provides an analytical method for identifying materials. This is used widely in the fields of chemistry and biology.

High-sensitive resistively detected NMR

Unlike conventional NMR, this NMR technique probes nuclear spin information via an electrical resistance reading. It uses a special quantum state, but because the resistance measurement can be conducted in thin films and nanostructures, this implementation of NMR constitutes a dramatic improvement in sensitivity over conventional NMR. Up until now this high-sensitive resistively detected NMR has been limited to GaAs semiconductor systems; however, recently it has successfully extended to an InSb and other quantum systems.

MRI (magnetic resonance imaging)

Using the nuclear magnetic resonance of atoms like hydrogen and carbon, it is possible to produce images of the interiors of organisms. MRI machines are well-known for their uses in medical screenings and the tomographic imaging of the brain.

Integer and fractional quantum Hall effects

When a strong magnetic field is applied to the 2D electrons residing in a semiconductor heterostructure, plateaus form at fixed values in the Hall resistance. At the same time the magnetoresistance in the longitudinal direction becomes zero. This is the quantum Hall effect.
The values of the Hall resistance plateaus are expressed in terms of only the values ν, Plank’s constant h and the elementary charge e, and are fixed without any dependence on the size or characteristics of the sample. It was at first discovered that ν is an integer (integer quantum Hall effect), but with subsequent improvements in sample quality it was found that ν can take fractional values. This effect is known as the fractional quantum Hall effect. In particular, it has been pointed out that the even denominator state has a mechanism different from the normal fractional quantum Hall effect, connecting to an error-tolerant quantum computer.

Nanoprobe

This is a general term for an instrument which images the surface of semiconductors and other materials at the nanoscale by scanning with a sharp needle at a nearly atomic-scale precision. STM (scanning tunneling microscopy) and AFM (atomic force microscopy) are two representative probing techniques. In another important nanoprobe technique, called scanning gate microscopy, the tip of the needle is utilized as a microscopic gate.

Quantum computer

Even the most advanced supercomputers have bits, their fundamental unit of information, which are based on classical principles (classical bits). Overlapping with classical computers in many ways, a quantum computer would be a new class of computer which implements quantum bits which are based on the principles of quantum mechanics. If this were actualized, it would become possible to solve several difficult problems that have not been accessible to classical computers (supercomputers included) up until now. However, a purely quantum regime is difficult to maintain, making the creation of a quantum computer non-trivial. Currently, qubits have been realized using atoms, ions, electron spins, nuclear spins, etc., which have quantum mechanical properties. Quantum simulators consisting of thousands of qubits have also been realized using superconductors, but the favorite technology for full-spec quantum computers that can correct errors and integrate them has not yet been determined.

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