Shintaro Nomura

University of Tsukuba · College of Physics

Nitrogen-vacancy (NV) centers in diamond have been used as platforms for quantum information, magnetometry and imaging of microwave (MW) fields. The spatial distribution of the MW fields used to drive the electron spin of NV centers plays a key role for these applications. Here, we report a system for the control and characterization of MW magnetic...

The integer quantum Hall (QH) effects characterized by topologically quantized and nondissipative transport are caused by an electrically insulating incompressible phase that prevents backscattering between chiral metallic channels. We probed the incompressible area susceptible to the breakdown of topological protection using a scanning-gate techni...

High precision sensing by making of the unique properties of nitrogen-vacancy (NV) centers has attracted considerable attention recently. We report on our results of wide-field quantum sensing using near-surface NV centers in diamond. We describe a new method of imaging of microwave field and a frequency modulation technique for dc magnetic field s...

High precision sensing by making of the unique properties of nitrogen-vacancy (NV) centers has attracted considerable attention recently. We report on our results of wide-field quantum sensing using near-surface NV centers in diamond. We describe a new method of imaging of microwave field and a frequency modulation technique for dc magnetic field s...

In this study, we demonstrate the switching of the direction of the photocurrent in an n-type GaAs/Al0.3Ga0.7As modulation-doped quantum well using a polarization pulse-shaping apparatus containing a 4f setup. The right- and left-polarization-twisting pulses with a polarization rotation frequency in the THz-regime are incident on a modulation-doped...

Using a scanning gate technique incorporated with non-equilibrium transport, we image electrically insulating incompressible states inherent to the integer quantum Hall effect (QHE). With a reduction of the filling factor ($\nu$) to an integer $\nu$, we observe the evolution of the incompressible states from an edge state to a bulk localized state,...

Nitrogen-vacancy (NV) centers in diamond have been widely used as platforms for quantum information, magnetometry and imaging of microwave (MW) fields. High-precision spatial control of the MW field necessary to drive the electronic spin of NV centers is essential for these applications. Here, we report a controlled MW field distribution by excitat...

We report on a photodetector with encapsulated h-BN/MoS2/h-BN heterostructure as a channel of a field-effect transistor. The conductance characteristics in the linear region show the absence of hysteresis, whereas the conductance characteristics in the nonlinear region show hysteretic behaviors, which disappear under illumination. Large photorespon...

We report magnetization measurements of Sr2RuO4-Ru eutectic microplates using micro-dc-SQUIDs. Sr2RuO4 is considered as a chiral p-wave superconductor and hence Sr2RuO4-Ru eutectic becomes in an unstable state with a superconducting phase frustration between a chiral p-wave state of Sr2RuO4 and a s-wave state of Ru. To compensate the frustration, a...

We investigated the magnetic properties of a Sr2RuO4-Ru eutectic microplate containing a single Ru-inclusion using micrometer-sized DC-SQUIDs (direct-current superconducting quantum interference devices). A phase frustration at the interface between chiral p-wave superconducting Sr2RuO4 and s-wave superconducting Ru is expected to cause novel magne...

We report on the application of a frequency modulation technique to wide-field magnetic field imaging of nitrogen-vacancy centers in diamond at room temperature. We use a scientific CMOS (sCMOS) camera to collect photoluminescence images from a large number of nitrogen-vacancy center ensembles in parallel. This technique allows a significant reduct...

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 s...

We developed a two-dimensional array of superconducting quantum interference devices (SQUIDs) for investigation of fine spatial distribution of magnetization in superconducting Sr\(_{2}\)RuO\(_{4}\). Micrometer-sized SQUIDs based on homogeneously formed Al/AlO\(_{x}\)/Al tunnel-type Josephson junctions were fabricated using shadow evaporation techn...

We developed high-sensitivity magnetization measurement devices composed of micro-dc-SQUIDs and a superconducting Sr\(_2\)RuO\(_4\) microplate, aiming to investigate novel magnetic properties related to a spin-triplet chiral p-wave superconductor with a mesoscopic size. Micron-sized dc-SQUID was fabricated by thin Al electrodes, and the SQUID struc...

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...

We report on investigations of the quantum Hall chiral edge states using a near-field scanning optical microscope that enables us to irradiate circularly polarized light from the probe tip with spatial resolution below the diffraction limit. We have found a clear evidence for the formation of spin-split incompressible strips near the edge of a two-...

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 s...

Superconducting quantum interference devices (SQUIDs) with nano (micro)-meter dimensions are called nano (micro)-SQUIDs. The high sensitivity for flux and position of nano (micro)-SQUIDs can be applied to detect local magnetic fields induced by vortices and the magnetization of mesoscopic superconductors. Nano-SQUIDs based on carbon-nanotube juncti...

We report on the measurement of the magnetization of mesoscopic plates made of the chiral p-wave superconductor Sr2RuO4 by using micro-dc-superconducting quantum interference devices (SQUIDs) positioned immediately below the plates. The high sensitivity of the micro-dc-SQUIDs is expected to enable the detection of direction of the chirality. We mea...

We investigate polarization-resolved fine structure in the photoluminescence (PL) in the fractional quantum Hall effect regime at B = 4–6 T, where small Zeeman energy allows spin-depolarized ground states. We observe up to five distinct peaks with characteristic polarization and temperature dependence in the vicinity of ν = 1/3 and quenching of the...

Low-temperature photoluminescence (PL) spectra of electron-hole systems in Si nanowires (NWs) prepared by thermal oxidization of Si fin structures were studied. Mapping of PL reveals that NWs with uniform width are formed over a large area. Annealing temperature dependence of PL peak intensities was maximized at 400 °C for each NW type, which are c...

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...

We report on ultrafast transport of pulses using semi-insulating GaAs photoconductive switches by a pump-probe measurement. Oscillatory signals were observed when a pair of photoconductive switch was optically excited. The response of the photoconductive switches could be detected at the pump laser average power as small as 20 μW.

We have studied the photoluminescence (PL) spectra and PL decay profiles of electron-hole systems in high quality Si nanolayers. We have found that the PL lifetime decreases with decrease in the thickness of silicon nanolayers between 25.2 and 2.7 nm and with decrease in temperature between 5 and 70 K.

We report on measurement of the electron-hole effective g-factors (geff*) depending on electron filling factors ν from magnetophotoluminescence spectroscopy at low magnetic fields B<1 T in low electron density regime in a GaAs/Al0.33Ga0.67As-gated quantum well. Enhancement of geff* at odd ν is observed. The oscillatory behavior of geff* is compared...

By measuring the photoluminescence linewidth of a GaAs quantum well under perpendicular electric fields, we have traced the variation of the effective radius a* of a charged exciton (trion) as a function of electron density. The a* increases sharply above a critical density ns=2×1014 m−2, which is consistent with the decrease of the screening lengt...

We fabricated and examined the operation of graphene-based superconducting interference device (SQUID) consisting of two superconductor-single layer graphene-superconductor junctions connected in parallel on a superconducting loop made of aluminum. Current-voltage characteristic of the device exhibits supercurrent flowing through SGS junctions. Mea...

We investigated the dynamic response of a cascaded-ring-resonator-loaded Mach-Zehnder modulator (CRR-MZM), in which a number of cascaded ring resonators (RRs) are loaded in the interferometer as phase modulators. The analytical form is derived for the small-signal response of CRR-MZM using temporal-coupled-mode (TCM) theory, and its validity is con...

A cascaded-ring-resonator-loaded Mach–Zehnder modulator (CRR-MZM) is presented in which a number of cascaded ring resonators (RRs) are loaded in the interferometer as phase modulators. The ability of the design to provide enhanced modulation efficiency at a wide optical bandwidth is demonstrated in comparison with a conventional single-RR-type modu...

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 vicinit...

We present results of the photoluminescence (PL) measurements of silicon nanowires (NWs) and silicon nanolayers for device characterization.

We report on the mapping of quantum-Hall edge states by quasiresonant photovoltage measurements using a near-field scanning optical microscope. We have observed fine structures near sample edges that shift inward with an increase in magnetic field in accordance with the shift of the positions of the quantum-Hall edge states. We have found a transit...

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 subwavelengt...

We investigate a circularly polarized photoluminescence using a gated single GaAs quantum well sample in the fractional quantum Hall effect regime. We have found that the lower energy peak χS- is polarized to σ+ polarization. This result is explained by considering the equilibrium between fractionally charged excitons and a two-dimensional electron...

We study the temperature dependence of electron injection voltage in Si-Nano-Dot (Si-NDs) Floating Gate MOS capacitor by using the collective tunneling model, which models the tunneling between two-dimensional electron gas (2DEG) and the Si-NDs. We clarify the temperature dependence by numerical calculation, which emulate the experiment in this sys...

We develop a fabrication process of Nb-based nano-SQUIDs (superconducting quantum interference devices) containing two weak-link Josephson junctions patterned by a focused ion beam (FIB). Typical dimension of the SQUID diameter was 1 μm, and the widths of the weak links were changed in the range of 60 and 500 nm. In order to evaluate the degradatio...

We propose the collective electron tunneling model in the electron injection process between the Nano Dots (NDs) and the two-dimensional electron gas (2DEG). We report the collective motion of electrons between the 2DEG and the NDs based on the measurement of the Si-ND floating gate structure in the previous studies. However, the origin of this col...

A new tunneling model between an inversion layer and the trap sites for the charge-trap-type (CT-) nonvolatile memory (NVM) cell is proposed. By considering the geometrical mismatch between the inversion layer and the trap site of the CT-NVM cell, we can conclude that electron tunneling is induced by a rare event, which causes the localization of e...

A new tunneling model between an inversion layer and the trap sites for the charge-trap-type (CT-) nonvolatile memory (NVM) cell is proposed. By considering the geometrical mismatch between the inversion layer and the trap site of the CT-NVM cell, we can conclude that electron tunneling is induced by a rare event, which causes the localization of e...

We have investigated C-V and photoluminescence (PL) characteristics of ultra-thin silicon-on-insulator (SOI) samples. Thickness dependence of a free exciton (FE) PL and an electron-hole droplet (EHD) PL has been investigated. We have found a remarkable enhancement of an EHD PL with decrease in the thickness of SOI samples.

Ohmic contacts are crucial for both device applications and the study of fundamental physics. From the perspective of device scaling trends, nano-scale Ohmic contacts are indispensable for future LSI technologies such as metallic source and drain contacts. In this study, we investigate the I-V characteristics using a varying discrete level distribu...

We study the sweep speed dependence of electron injection voltage in Si-Nano-Dots (Si-NDs) floating gate MOS Capacitor by using our collective tunneling model, which models the tunneling between two-dimensional electron gas (2DEG) and the Si-NDs. We clarify the sweep speed dependence of electron injection energy with a numerical calculation based o...

We report the unexpected temperature dependence of electron tunneling from the two-dimensional electron gas (2DEG) to the Si-dot in a Si-dots floating gate metal-oxide-semiconductor (MOS) capacitor. We indicate that this temperature dependence of the electron tunneling cannot be explained by the conventional one-dimensional tunneling model, and sho...

Ohmic contacts are crucial for both device applications and the study of fundamental physics. In this study, we propose a new physical model for Ohmic contacts based on the detailed considerations of a metal/semiconductor interface, such as charge neutrality level concept, with which it is possible to describe the real situation precisely. Our prop...

We have revealed that the electronic states in the electrodes give a significant influence to the electron transport in nano-electronic devices. We have theoretically investigated the time-evolution of electron transport from a two-dimensional electron gas (2DEG) to a quantum dot (QD), where 2DEG represents the electrode in the nano-electronic devi...

Recently, metallic source and drain is widely discussed with LSIs scaling trend. For this technology, it is essential to fabricate low resistive Ohmic contact between electrodes and the channel materials. However, it is expected that precise Schottky barrier height control for obtaining Ohmic contact is technologically difficult. One of the main re...

We measured the photoluminescence (PL) of a GaAs quantum well (QW) with a lateral potential modulation by the front-gate bias while changing both the electron density and electric field at low temperature. Near the onset of the two-dimensional electron gas, we observed an anomalous enhancement of PL intensity of the neutral exciton X0 accompanied b...

We measured photoluminescence (PL) spectra from a 20-nm GaAs/AlGaAs quantum well (QW) grown on an n-type substrate by selectively exciting the GaAs at 2 K. We observed a two-stage change of PL spectra as a function of the total amount of photo-irradiation (p × t) after cooling down. This corresponds to the process of establishing the equilibrium of...

We present our observation of an anomalous temperature and optical excitation intensity dependence of the electron tunneling between a two-dimensional electron gas (2DEG) and Si dots in the direct tunneling mode. We find that the gate voltages required for the electron injection from the 2DEG to Si-dots become smaller with increase in the temperatu...

We constructed a dilution-refrigerator-based near-field scanning optical microscope (NSOM) which operates at temperatures down to 200 mK. Our NSOM enables us to investigate spatial properties of the electrons in semiconductor nanostructures with a subwavelength spatial resolution. We report on a result of the optical mapping of the boundary of a tw...

We have experimentally investigated the optical effect on the transport properties of superconductor-semiconductorsuperconductor (S-Sm-S) junctions composed of a two-dimensional electron gas in a GaAs/AlGaAs heterostructure and NbN superconducting electrodes. Illumination at λ800nm onto the whole junction area increases Andreev reflection (AR) prob...

Quantum mechanical electron tunneling has potential applications in both science and technology, such as flash memories in modern LSI technologies and electron transport chains in biosystems. Although it is known that one-dimensional quantum electron tunneling lacks temperature dependence, the behavior of electron tunneling between different dimens...

We investigate the two-dimensional electron gas and the discrete level under the magnetic field by numerically studying the two-dimensional tight-binding lattice and the discrete level coupled system. Focusing on the partial charge density, we show that the coupling and decoupling between the two-dimensional tight-binding lattice and the discrete l...

We studied the time evolutions of the electronic states on a quantum wire (QW) and a two‐dimensional electron gas (2DEG), which are suddenly coupled with a discrete level. We employed the tight‐binding model to represent these systems, and calculate the time evolutions of the wave functions by solving the time‐dependent (TD) Schrödinger equation...

We present results of the electron tunneling between Si-dots and the two-dimensional electron gas (2DEG) under the optical excitation at low temperatures, where modification of the 2DEG is caused by optical generation of the electron-hole pairs. We have found that the gate voltage for electron injection to Si-dots becomes remarkably smaller with in...

Fabrication of good Ohmic contacts is quite important not only for device application but also for fundamental physics. In accordance with the device scaling, it is inevitable to prepare nano-scale Ohmic contacts for future LSIs technology. In this study, we hereby propose a new electronic structure model for Ohmic contacts, with which it is possib...

Near-field photoluminescence (PL) imaging spectroscopy was used to investigate multi-exciton and charged-exciton states confined in a single GaAs interface fluctuation quantum dot. We determined the origin of peaks in the PL spectra by employing a wavefunction mapping technique. We observed distortion of the exciton wavefunction due to the electric...

We investigate the electron states of the coupled quantum dot system with a two-dimensional electron gas by numerically studying the two-dimensional tight-binding lattice in the magnetic fields. In this system, it is expected that the electron states are remarkably changed by the magnetic fields. We show coupling and decoupling between the two-dime...

We theoretically study the time evolution of the electronic state on a quantum wire (QW) coupled with the quantum dot (QD). We find that the characteristic of the electron density on the QW sensitively depends on the energy levels of the QDs. We systematically discuss the mechanism of these characteristic modulations by the projection analysis, whe...

We study the optical effects on superconductor-normal metal superconductor (S-Sm-S) junctions composed of two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure and NbN superconducting electrodes. When the whole junction area was illuminated at lambda = 800 nm, we observe a reduction in the normal resistance due to an increase in the...

We investigated the transport properties of an S-Sm-S junction with a two-dimensional electron gas (2DEG) in an In0.52Al0.48As/In0.7Ga0.3As heterostructure when exposed to light from infrared laser diodes. When the sample was illuminated with lambda = 1.3 mum, we observed a reduction in the junction resistance as well as an enhancement of Andreev r...

We report on the temperature dependence of the capacitance of Si-quantum dots (Si-QDs) floating gate MOS capacitor samples both in dark and under laser excitation at low temperatures. The capacitance increases with increase in temperature from 6 to 40 K and saturates at temperature higher than 40 K in dark. The capacitance under laser excitation at...

We have theoretically investigated the time-evolution of electron transport from a two-dimensional electron system (2DES) to a quantum dot (QD). We clearly showed that the coherent electron transport is remarkably modified depending on the initial electronic state in the 2DES. The electron transport from the 2DES to the QD is strongly enhanced when...

We have theoretically investigated the time-evolution of electron wave function in tunneling from a two-dimensional electron gas (2DEG) to a quantum dot (QD). We have revealed that the electronic state in the electron gas significantly influences the electron tunneling. We clearly showed that the electron tunneling is modified depending on the init...

We report that gate voltages required for electron injection to quantum dots (QDs) from electron gas and for emission from QDs strongly depend on temperature. For this experiment, Si-QDs floating gate MOS capacitors were designed and fabricated. Displacement current (I) was measured as a function of the gate voltage (V). Peaks in I-V curves appear...

Physics of nano-contact in metal-oxide-semiconductor field- effect transistors has been attracting interests of many researchers recently. Contacts are conventionally treated as a reservoir in thermal equilibrium, but behavior of contacts in ultra-small devices is still open questions. In this paper, we investigate electron tunneling between a two-...

The electron density and electric field in a GaAs quantum well can be controlled independently using front and back gates. We observed Landau level splitting of the photoluminescence (PL) spectrum of a two-dimensional electron system with holes at the delta-doped acceptors with changing the electron density and the PL energy shift due to the gate e...

The electron density dependent effective mass of an electron in a 2-dimensional electron system is reported in a back-gated undoped GaAs/AlGaAs quantum well. The enhancement of the effective masses is observed with decrease in the electron density by the transport and the photoluminescence measurements.

We theoretically study the time evolution of the electron density distribution on a two-dimensional electron gas, which suddenly couples with a discrete level. Depending on the relative position between a discrete level and the Fermi level (Ef), a “dip” or a “peak” of electron density appears after a discrete level couples with the two-dimensional...

Imaging spectroscopy of a single GaAs interface fluctuation quantum dot (IFQD) was performed using a near-field scanning optical microscope (NSOM) with a spatial resolution of 40 nm. A difference in the emission profiles of an exciton and a biexciton was found for several IFQDs. By comparing with a numerical simulation based on the finite-differenc...

We present an optical evidence for Aharonov–Bohm (AB) effect in InAs quantum tubes fabricated by a selective area-metal organic vapor phase epitaxy. The oscillations of the photoluminescence peak energies are observed in quantum tubes depending on the magnetic flux through the tube. These oscillations in energies are shown to be due to the AB effec...

We report on results of determination of the electron effective mass and the reduced mass as functions of the electron density (ns) at about 100mK in perpendicular magnetic fields in the range between 1.5rs6 by magneto-photoluminescence measurements. We find that the obtained effective masses increase with decrease in ns at ns1×1011cm-2.

Real-space mappings of a Hall photovoltage are performed by local laser excitation using optical microscope setup with a spatial resolution of about 2μm. The obtained images of broad and narrow strips presumably reflect the diffusion of the optically created electrons in the bulk and the edge states. The image near a current contact is considered t...

We investigated the transport of a two-dimensional electron gas (2DEG) in an In0.52Al0.48As/In0.7Ga0.3As heterostructure when exposed to light from infrared laser diodes (λ=0.78, 1.3μm) by means of both Shubnikov–de Haas and Hall-effect measurements. We observed negative photoconductivity due to a reduction in the number of electrons in the 2DEG wh...

Both the electron and the optically created hole effective masses are found to be density dependent in a two-dimensional electron system of a GaAs/Al(0.33)Ga(0.67)As back-gated quantum well by magnetophotoluminescence spectroscopy. We show that the density-dependent electron effective mass increases with a decrease in the electron density (n(s)) to...

A linear scale method for calculating electronic properties of large and complex systems is introduced within a local density approximation. The method is based on the Chebyshev polynomial expansion and the time-dependent method, which is tested on the calculation of the electronic structure of a model n-type GaAs quantum dot.

The oscillation of photoluminescence peak energies is observed in InAs quantum tubes depending on the magnetic flux through the tube. The oscillation is shown to be due to the Aharonov-Bohm effect of a charged exciton in a quantum tube. No quadratic shift in photoluminescence peak energies is observed, which is a characteristic feature of a thin qu...

We report systematic measurements of the electron effective mass (me*) and the electron-hole reduced mass (mu*) depending on the electron density in a high quality back-gated undoped GaAs/AlGaAs quantum well by transport and photoluminescence measurements in magnetic fields. Both me* and mu* increase with decrease in the electron density indicating...

We present a novel method for calculating electronic properties of large and complex systems based on a local density approximation by using a combination of Chebyshev polynomial expansion and time-dependent method. The electron density is obtained without calculating eigenenergies and eigenstates with the computational time which scales as O(N). T...

The optical transition energies of neutral and charged excitons in a quantum tube are calculated as a function of the Aharonov–Bohm magnetic flux U. The oscillation amplitude of the ground state energy of the electron–hole relative motion is shown to be larger in a quantum tube than a quantum ring with strong confinement in the axis direction. We f...

We have developed a new method of fabricating nanoparticles utilizing a mechanical rubbing process. Highly ordered arrays of Au nanoparticles with a diameter of 60 nm have been fabricated on a porous anodic alumina template. Pressure measurements during the mechanical rubbing process confirmed that these nanoparticles were formed only by a mechanic...

The electron–hole states in the fractional quantum Hall regime is investigated with a back-gated undoped quantum well by photoluminesccence in magnetic fields. The evolution of the photoluminescence spectra is discussed depending on the electron density. We find anomalies of the photoluminescence at the integer as well as the fractional filling fac...

The electric field dependence of the electron/hole wave function and the radiation energy of an exciton in a Be-delta-doped 80 nm quantum well (QW) is studied experimentally and compared it with variational calculation. The photoluminescence (PL) spectra show Stark shifts depending on the gate electric field and PL intensity of the exciton of the f...

We performed infrared magneto-photoluminescence (PL) spectroscopy on an InAs-inserted-channel InGaAs/InAlAs heterostructure. Series of Landau levels were clearly observed in the infrared PL spectra. From the energy separation between sigma+ and sigma- components of PL, the electron-hole g-factor in each Landau level were determined. We discuss the...

We report the magneto-optical measurement of GaAs quantum wire lattices of 0.7 micrometer period grown on GaAs(111)B substrates by using the selective-area MOVPE technique. We measure the photoluminescence (PL) spectra at 60 mK from the triangular and the Kagome lattice patterns as well as from the unpatterned single quantum well (SQW) as a referen...