Y. Shibata

Imaging of current density distributions with a Nb weak-link scanning nano-SQUID microscope

Superconducting quantum interference devices (SQUIDs) are accepted as one of the highest magnetic field sensitive probes. There are increasing demands to image local magnetic fields to explore spin properties and current density distributions in a two-dimensional layer of semiconductors or superconductors. Nano-SQUIDs have recently attracting much interest for high spatial resolution measurements in nanometer-scale samples. Whereas weak-link Dayem Josephson junction nano-SQUIDs are suitable to miniaturization, hysteresis in current-voltage (I-V) characteristics that is often observed in Dayem Josephson junction is not desirable for a scanning microscope. Here we report on our development of a weak-link nano-SQUIDs scanning microscope with small hysteresis in I-V curve and on reconstructions of two-dimensional current density vector in two-dimensional electron gas from measured magnetic field.

Circularly polarized near-field optical mapping of spin-resolved quantum Hall chiral edge states.

We have successfully developed a circularly polarized near-field scanning optical microscope (NSOM) that enables us to irradiate circularly polarized light with spatial resolution below the diffraction limit. As a demonstration, we perform real-space mapping of the quantum Hall chiral edge states near the edge of a Hall-bar structure by injecting spin polarized electrons optically at low temperature. The obtained real-space mappings show that spin-polarized electrons are injected optically to the two-dimensional electron layer. Our general method to locally inject spins using a circularly polarized NSOM should be broadly applicable to characterize a variety of nanomaterials and nanostructures.

Magnetic field imaging of a tungsten carbide film by scanning nano-SQUID microscope

We present the results of magnetic field imaging by scanning nano-superconducting quantum interference device (SQUID) microscopy on a tungsten carbide (W-C) film fabricated using focused-ion-beam chemical vapor deposition. We have investigated magnetic field change by a W-C film in an external magnetic field using a scanning nano-SQUID microscope system. We have found that the reduction of the magnetic field above the W-C film was 0.9%, indicating the penetration of vortices in the W-C at an external magnetic field of 0.171 mT.

Development of Scanning dc-SQUID system for local magnetic imaging

where I0 is a critical current of a Josephson junction and Φ0 is magnetic flux quantum. A SQUID is used as local probe of magnetic flux by reducing the radius of a SQUID loop. In order to obtain high spatial resolution, it is equally important to fabricate SQUID loop close to the edge of the probe. Weak-link Josephson junctions have advantages that fabrication by milling without lithography process and observation of the resultant loop may be performed in situ by a focused ion beam (FIB) [2]. This process is thus suitable for preparing small loop size of ~1 μm at the edge of substrate for a SQUID probe. In this paper, we describe fabrication process of SQUID probe with weak-link Josephson junctions and our newly built scanning SQUID microscope system. We demonstrate successful mapping of magnetic flux induced by the current distribution in a GaAs/AlGaAs Hall-bar structure.

Real-space Mapping of Spin-resolved Quantum Hall Chiral Edge States by Near-field Scanning Optical Microscopy

The quantum Hall edge states are formed in the vicinity of the edge of a sample that hosts two-dimensional electron system (2DES) in a perpendicular magnetic field at low temperature. The bulk 2DES states rise up due to the confinement potential near the edge and intersect with the Fermi-level, forming current carrying compressible states. The compressible states are separated by insulating incompressible states. The distribution of the compressible/ incompressible states has been investigated by various scanning probe microscopy, however, there have been few reports on real-space mapping of spin-resolved quantum Hall edge states. Here we report on the first observation of spin-resolved chiral quantum Hall edge states by a low-temperature near-field scanning optical microscope.

Imaging of quantum Hall edge states under quasiresonant excitation by a near-field scanning optical microscope

A high resolution mapping of quantum Hall edge states has been performed by locally creating electrons with small excess energies with a near-field scanning optical microscope in a dilution refrigerator. We have observed fine structures parallel to the edge in photovoltage signals, which appear only at low temperature. The observed fine structures near sample edges have been seen to shift inward with increase in magnetic field in accordance with Chklovskii Shklovskii, and Glazman model.

Real‐space mapping of compressible and incompressible strips by a near‐field scanning optical microscope

Compressible and incompressible strips formed near the boundary of a two‐dimensional electron system were mapped out by near‐field scanning optical microscope at 230 mK. Dilution‐refrigerator based near‐field scanning optical microscope enables us to investigate spatial properties of the electrons in semiconductor nanostructures with a subwavelength spatial resolution. We obtain real‐space mapping of photovoltage in the vicinity of the edge of Hall‐bar, which reflects the local chemical potential determined by the distribution of the compressible and incompressible strips.

MAPPING OF QUANTUM-HALL EDGE CHANNELS BY A DILUTION-REFRIGERATOR BASED NEAR-FIELD SCANNING OPTICAL MICROSCOPE

A real-space mapping of photovoltage near the edge of the Hall-bar of a GaAs/AlGaAs single heterojunction has been obtained using a dilution-refrigerator-based near-field scanning optical microscope in magnetic fields. The optical probe-sample surface distance dependence of photovoltage is investigated. We obtain photovoltage profile in the vicinity of the edge, which reflects the local chemical potential of the two-dimensional electron gas determined by the distribution of the compressible and incompressible strips.