# Kensuke Kobayashi's research while affiliated with Osaka University and other places

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## Publications (124)

We studied flux pinning in exfoliated FeTe$_{0.6}$Se$_{0.4}$ thin-film devices with a thickness $d$ from 30 to 150 nm by measuring the critical current density $J_{\mathrm{c}}$. In bulk FeTe$_{0.6}$Se$_{0.4}$, the flux pinning has been discussed in the framework of weak collective pinning, while there is little knowledge on the pinning mechanism in...

Shot noise, originating from the discrete nature of electric charge, is generated by scattering processes. Shot-noise measurements have revealed microscopic charge dynamics in various quantum transport phenomena. In particular, beyond the single-particle picture, such measurements have proved to be powerful ways to investigate electron correlation...

We studied flux pinning in exfoliated FeTe0.6Se0.4 thin-film devices with a thickness d from 30 to 150 nm by measuring the critical current density Jc. In bulk FeTe0.6Se0.4, the flux pinning has been discussed in the framework of weak collective pinning, while there is little knowledge on the pinning mechanism in the thin-film region. From the thic...

Shot noise, originating from the discrete nature of electric charge, is generated by scattering processes. Shot-noise measurements have revealed microscopic charge dynamics in various quantum transport phenomena. In particular, beyond the single-particle picture, such measurements have proved to be powerful ways to investigate electron correlation...

The current-voltage ($IV$) characteristics beyond the linear response regime of magnetic tunnel junction (MTJ) is systematically investigated. We find a clear negative correlation between the two coefficients to characterize the linear ($I$$\propto$$V$) and the lowest-order nonlinear ($I$$\propto$$V^3$) currents, which holds regardless of the tempe...

The current-voltage (IV) characteristics beyond the linear response regime of magnetic tunnel junction (MTJ) is systematically investigated. We find a clear negative correlation between the two coefficients to characterize the linear (I∝V) and the lowest-order nonlinear (I∝V3) currents, which holds regardless of the temperature and the thickness of...

Behavior of quantum liquids is a fascinating topic in physics. Even in a strongly correlated case, the linear response of a given system to an external field is described by the fluctuation-dissipation relations based on the two-body correlations in the equilibrium. However, to explore nonlinear non-equilibrium behaviors of the system beyond this w...

Behavior of quantum liquids is a fascinating topic in physics. Even in a strongly correlated case, the linear response of a given system to an external field is described by the fluctuation-dissipation relations based on the two-body correlations in the equilibrium. However, to explore nonlinear non-equilibrium behaviors of the system beyond this w...

We show that a cryogenic amplifier composed of a homemade GaAs high-electron-mobility transistor (HEMT) is suitable for current-noise measurements in a mesoscopic device at dilution-refrigerator temperatures. The lower noise characteristics of our homemade HEMT leads to a lower noise floor in the experimental setup and enables more efficient curren...

We have performed magnetoresistance (MR) measurements on van der Waals ferromagnetic devices using quenched- (Q-) and nonquenched- (NQ-) Fe5GeTe2 crystals. A clear butterfly-shaped hysteresis has been observed for thin-film (less than 6 unit-cell layer) Q- and NQ-Fe5GeTe2 devices, but not for thicker film ones. The switching field of the butterfly-...

We show that a cryogenic amplifier composed of a homemade GaAs high-electron-mobility transistor (HEMT) is suitable for current-noise measurements in a mesoscopic device at dilution-refrigerator temperatures. The lower noise characteristics of our homemade HEMT lead to a lower noise floor in the experimental setup and enable more efficient current-...

In this study, we observe that the conductance of a quantum point contact on a GaAs/AlGaAs double quantum well depends significantly on the magnetic field perpendicular to the two-dimensional electron gas. In the presence of the magnetic field, the subband edge splitting due to the Zeeman energy reaches 0.09 meV at 0.16 T, thereby suggesting an enh...

Magnetoresistance measurements have been performed in CeTe3 thin film devices in a temperature range from 2.1 to 20 K up to 8 T. A clear Shubnikov-de-Haas oscillation was observed in the whole temperature range. The temperature dependence of the oscillation amplitude was found to deviate from the Lifshitz–Kosevich formula below the magnetic transit...

Most of the time, electronic excitations in mesoscopic conductors are well described, around equilibrium, by non-interacting Landau quasi-particles. This allows a good understanding of the transport properties in the linear regime. However, the role of interaction in the non-equilibrium properties beyond this regime has still to be established. A p...

Understanding the properties of correlated quantum liquids is a fundamental issue of condensed matter physics. Even in such a correlated case, fascinatingly, we can tell that the equilibrium fluctuations of the system govern its linear response to an external field, relying on the fluctuation dissipation relations based on the two-body correlations...

We study finite-temperature properties of the Kondo effect in a carbon nanotube (CNT) quantum dot using the Wilson numerical renormalization group (NRG). In the absence of magnetic fields, four degenerate energy levels of the CNT consisting of spin and orbital degrees of freedom give rise to the SU(4) Kondo effect. We revisit the universal scaling...

By means of spin current, the flow of spin angular momentum, we find a regime of “spin treacle” in a frustrated magnetic system. To establish its existence, we have performed spin transport measurements in nanometer-scale spin glasses. At temperatures high enough that the magnetic moments fluctuate at high frequencies, the spin Hall angle, the conv...

We report a negative resistance, namely, a voltage drop along the opposite direction of a current flow, in the superconducting gap of NbSe$_2$ thin films under the irradiation of surface acoustic waves (SAWs). The amplitude of the negative resistance becomes larger by increasing the SAW power and decreasing temperature. As one possible scenario, we...

Spintronic devices using antiferromagnets (AFMs) are promising candidates for future applications. Recently, many interesting physical properties have been reported with AFM-based devices. Here we report a butterfly-shaped magnetoresistance (MR) in a micrometer-sized triangular-lattice antiferromagnet Ag$_2$CrO$_2$. The material consists of two-dim...

By means of spin current, the flow of spin angular momentum, we find a regime of "spin treacle" in a frustrated magnetic system. To establish its existence, we have performed spin transport measurements in nanometer-scale spin glasses. At temperatures high enough that the magnetic moments fluctuate at high frequencies, the spin Hall angle, the conv...

We have performed magnetotransport measurements in CeTe3 thin films down to 0.2 K. It is known that CeTe3 has two magnetic transitions at T N 1 ≈ 3 K and T N 2 ≈ 1 K. A clear Shubnikov–de-Haas (SdH) oscillation was observed at 4 K, demonstrating the strong two-dimensional nature in this material. Below T N 2, the SdH oscillation has two frequencies...

Single electron sources have been studied as a device to establish an electric current standard for 30 years and recently as an on-demand coherent source for fermion quantum optics. In order to construct the single electron source on a GaAs/AlGaAs two-dimensional electron gas (2DEG), it is often necessary to fabricate a sub-micrometer wire by etchi...

We report a negative resistance, namely, a voltage drop along the opposite direction of a current flow, in the superconducting gap of NbSe 2 thin films under the irradiation of surface acoustic waves (SAWs). The amplitude of the negative resistance becomes larger by increasing the SAW power and decreasing temperature. As one possible scenario, we p...

We investigate the spectral characteristics of a spin torque oscillator (STO) excited by the spin Hall-induced spin current. We observe that the modest spin current injection triggers the conventional single peak oscillating behavior of the STO. As the spin current is further increased to enter the non-linear regime, we find the transition of the s...

A numerical investigation is conducted for a single spin-torque oscillator (STO) under the non-linear region. A large angle precession triggers the generation of multiple modes without any feedbacked circuits and/or magnetic couplings with neighboring oscillators. Our simulations show that a single eigenmode of a given STO can trigger up to six dis...

We have performed magnetotransport measurements in CeTe$_{3}$ thin films down to $0.2~{\rm K}$. It is known that CeTe$_{3}$ has two magnetic transitions at $T_{\rm N1} \approx 3~{\rm K}$ and $T_{\rm N2} \approx 1~{\rm K}$. A clear Shubnikov-de-Haas (SdH) oscillation was observed at $4~{\rm K}$, demonstrating the strong two-dimensional nature in thi...

Single electron sources have been studied as a device to establish an electric current standard for 30 years and recently as an on-demand coherent source for Fermion quantum optics. In order to construct the single electron source on a GaAs/AlGaAs two-dimensional electron gas (2DEG), it is often necessary to fabricate a sub-micron wire by etching....

We investigate the spectral characteristics of spin torque oscillator (STO) excited by the spin Hall-induced spin current. We observe that the modest spin current injection triggers the conventional single peak oscillating behavior of STO. As the spin current is further increased to enter the non-linear regime, we find the transition of the spectru...

A numerical investigation is conducted for a single spin-torque oscillator under the non-linear region. A large angle precession triggers the generation of multiple modes without any feedbacked circuits and/or magnetic couplings with neighboring oscillators. Our simulations show that a single eigenmode of a given spin-torque oscillator can trigger...

We study finite-temperature properties of the Kondo effect in a carbon nanotube (CNT) quantum dot using the Wilson numerical renormalization group (NRG). In the absence of magnetic fields, four degenerate energy levels of the CNT consisting of spin and orbital degrees of freedom give rise to the SU(4) Kondo effect. We revisit the universal scaling...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Spintronic devices using antiferromagnets (AFMs) are promising candidates for future applications. Recently, many interesting physical properties have been reported with AFM-based devices. Here we report a butterfly-shaped magnetoresistance (MR) in a micrometer-sized triangular-lattice antiferromagnet Ag2CrO2. The material consists of two-dimension...

Kondo correlations are responsible for the emergence of a zero-bias peak in the low temperature differential conductance of Coulomb blockaded quantum dots. In the presence of a global SU(2)⊗SU(2) symmetry, which can be realized in carbon nanotubes, they also inhibit inelastic transitions which preserve the Kramers pseudospins associated to the symm...

Most of the time, electronic excitations in mesoscopic conductors are well described, around equilibrium, by non-interacting Landau quasi-particles. This allows a good understanding of the transport properties in the linear regime. However, the role of interaction in the non-equilibrium properties beyond this regime has still to be established. A p...

Kondo correlations are responsible for the emergence of a zero-bias peak in the low temperature differential conductance of Coulomb blockaded quantum dots. In the presence of a global SU(2)$\otimes$SU(2) symmetry, which can be realized in carbon nanotubes, they also inhibit inelastic transitions which preserve the Kramers pseudospins associated to...

The effect of spin torque on magnetization is investigated in an MgO/CoFeB/W perpendicularly magnetized nanopillar with an asymmetric bistable potential landscape. While conventional pulse-current-induced magnetization switching has been implemented, a metastable switched state is observed under direct current (DC). The mean dwell time of the metas...

We have performed spin transport measurements in metallic Bi/Ni nanowires using the spin absorption method in a lateral spin valve structure. Although Bi has a strong spin-orbit interaction, the spin Hall angle obtained for Bi/Ni nanowires is about 0.008. The magnitude and sign of the spin Hall angle are much smaller than and opposite to those of C...

The sensitivity of shot noise to the interplay between Kondo correlations and superconductivity is investigated in a carbon nanotube quantum dot connected to superconducting electrodes. Depending on the gate voltage, the SU(2) and SU(4) Kondo unitary regimes can be clearly identified. We observe enhancement of the shot noise via the Fano factor in...

The Fano effect is a ubiquitous phenomenon arising from interference between a discrete energy state and an energy continuum. We explore this effect in an artificial molecule, namely, two lateral quantum dots (QDs) fabricated from a two-dimensional electron gas system and coupled in series. When the coupling between the leads and QDs is small, the...

The sensitivity of shot noise to the interplay between Kondo correlations and superconductivity is investigated in a carbon nanotube quantum dot connected to superconducting electrodes. Depending on the gate voltage, the SU(2) and SU(4) Kondo unitary regimes can be clearly identified. We observe enhancement of the shot noise via the Fano factor in...

Thin layers of Bi2Sr2CaCu2O8+δ (Bi2212) were fabricated using the mechanical exfoliation technique. Good electrical contacts to the thin Bi2212 films with low contact resistance were realized by depositing Ag and Au electrodes onto the Bi2212 films and annealing them with an oxygen flow at 350 °C for 30 min. We observed cross-section images of the...

We fabricated 100 nm thick films of two-dimensional triangular antiferromagnet Ag2CrO2 using the mechanical exfoliation technique, and performed the transport measurements down to 5 K. As in the case of polycrystalline samples, a large reduction of the resistivity due to the antiferromagnetic ordering was clearly observed at around 25 K. Surprising...

Universal properties of entangled many-body states are controlled by their symmetry and quantum fluctuations. By magnetic-field tuning of the spin-orbital degeneracy in a Kondo-correlated quantum dot, we have modified quantum fluctuations to directly measure their influence on the many-body properties along the crossover from $SU(4)$ to $SU(2)$ sym...

Pure spin current is a powerful tool for manipulating spintronic devices, and its dynamical behavior is an important issue. By using mesoscopic transport theory for electron tunneling induced by spin accumulation, we investigate the dynamics of the spin current in the high-frequency quantum regime, where the frequency is much larger than temperatur...

Mesoscopic systems-small electric circuits working in quantum regime-offer us a unique experimental stage to explorer quantum transport in a tunable and precise way. The purpose of this Review is to show how they can contribute to statistical physics. We introduce the significance of fluctuation, or equivalently noise, as noise measurement enables...

We theoretically study an emergent SU(2) symmetry which is suggested by recent magneto-transport measurements, carried out near two electrons filling of a carbon nanotube quantum dot. It emerges in the case where the spin and orbital Zeeman splittings cancel each other out for two of the one-particle dot levels among four. Using the Wilson numerica...

We report the experimental observation of Snell's law for magneto-static spin
waves in thin ferromagnetic Permalloy films by imaging incident, refracted and
reflected waves. We use a thickness step as the interface between two media
with different dispersion relation. Since the dispersion relation for
magneto-static waves in thin ferromagnetic film...

We report a precise experimental study on the shot noise of a quantum point contact (QPC) fabricated in a GaAs/AlGaAs based high-mobility two-dimensional electron gas (2DEG). The combination of unprecedented cleanliness and very high measurement accuracy has enabled us to discuss the Fano factor to characterize the shot noise with a precision of 0....

We report the observation of a magnon Hall-like effect for the scattering of magnons at the sample edge in Y3Fe5O12 crystals. The experiments are performed under an out-of-plane magnetic field at room temperature using an infrared thermometry. When the magnons are incident into the sample edge in the unsaturated magnetic structure, appreciable tran...

We investigate the noise in single layer graphene devices from equilibrium to far-from equilibrium and found that the 1/fnoise shows an anomalous dependence on the source-drain bias voltage (VSD). While the Hooge's relation is not the case around the charge neutrality point, we found that it is recovered at very low VSD region. We propose that...

We performed noise measurements for a Corbino disk in the quantum Hall effect breakdown regime. We investigated two Corbino-disk-type devices with different sizes and observed that the Fano factor increases when the length between the contacts doubles. This observation is consistent with the avalanche picture suggested by the bootstrap electron hea...

Interacting quantum many-body systems constitute a fascinating research field because they form quantum liquids with remarkable properties and universal behaviour. In fermionic systems, such quantum liquids are realized in helium-3 liquid, heavy fermion systems, neutron stars and cold gases. Their properties in the linear-response regime have been...

Massless Dirac electron systems such as graphene exhibit a distinct half-integer quantum Hall effect, and in the bipolar transport regime co-propagating edge states along the p-n junction are realized. Additionally, these edge states are uniformly mixed at the junction, which makes it a unique structure to partition electrons in these edge states....

Parity effect, which means that even-odd property of an integer physical parameter results in an essential difference, ubiquitously appears and enables us to grasp its physical essence as the microscopic mechanism is less significant in coarse graining. Here we report a new parity effect of quantum Hall edge transport in graphene antidot devices wi...

We have shown by micromagnetic simulation that the wavelength of the spin wave (SW) is modulated when the SW propagates across a step between two ferromagnetic films with different thicknesses. Since the wavelength of the SW depends on the film thickness, the thickness difference works as the different medium in Snell’s law. The wavelength modulati...

When an electric current passes across a potential barrier, the partition process of electrons at the barrier gives rise to the shot noise, reflecting the discrete nature of the electric charge. Here we report the observation of excess shot noise connected with a spin current which is induced by a nonequilibrium spin accumulation in an all-semicond...

We performed the shot noise measurement in epitaxial Fe/MgO/Fe-based magnetic tunneling junctions (MTJs) with various MgO thicknesses between 1.1 nm and 1.625 nm. While the Fano factor to characterize the shot noise is very close to 1 in MTJs with MgO barrier thicker than 1.2 nm, the magnetic configuration-dependent reduction of the Fano factor for...

When electric current passes across a potential barrier, the partition
process of electrons at the barrier gives rise to the shot noise, reflecting
the discrete nature of electric charge. Here, beyond this conventional charge
shot noise, we bring to light "spin" shot noise associated with spin current
that is induced by nonequilibrium spin-dependen...

We have measured the current noise in a device with Corbino geometry to investigate the dynamics of electrons in the breakdown regime of the integer quantum Hall effect (QHE). In the breakdown regime, the Fano factor of the current noise exceeds ${10}^{3}$, which indicates the presence of electron bunching. As super-Poissonian current noise is obse...

We report the switching of a magnetic vortex core in ferromagnetic elliptical disks induced by a nanosecond field pulse. We show that the switching probability depends on both the duration and amplitude of the field pulse. The minimum magnetic field required for the core switching depends also on the ellipticity of the disk. Micromagnetic simulatio...

We report the real-time observation of spin-wave propagation across a step inserted between two ferromagnetic films with different thicknesses. Because the dispersion relation of the spin wave depends on the thickness of the film, the step works as a junction to affect the spin wave propagation. When the spin wave transmits through the junction, th...

We study the shot noise at a quantum point contact (QPC) fabricated in an InGaAs/InGaAsP heterostructure, whose conductance can be electrically tuned by the gate voltages. Shot noise suppression is observed at the conductance plateau of
N(2e
2
/h)
(N = 4,5,and 6), which indicates the coherent quantized channel formation in the QPC. The electro...

We report the magnetoresistance fluctuation in a submicrometer-sized Hall-bar made of epitaxial
Bi
2
Se
3
thin film. We observed the magnetoresistance fluctuation below 18 K and analyzed the fluctuation as universal conductance fluctuation (UCF). According to our results, the temperature dependence is quantitatively explained by the conventio...

We report on conductance fluctuation in quasi-one-dimensional wires made of epitaxial Bi_{2}Se_{3} thin film. We found that this type of fluctuation decreases as the wire length becomes longer and that the amplitude of the fluctuation is well scaled to the coherence, thermal diffusion, and wire lengths, as predicted by conventional universal conduc...

We developed a shot noise measurement system for mesoscopic conductors (typical resistance h/2e2 = 12.9 kΩ) at very low temperature (∼20 mK). To realize required high-resolution measurement, we made a high-electron-mobility-transistor (HEMT)-based cryogenic amplifier working at a target frequency range (∼2.5 MHz), whose gain flatness and input volt...

Energy barriers in magnetization reversal dynamics have long been of interest because the barrier height determines the thermal stability of devices as well as the threshold force triggering their dynamics. Especially in memory and logic applications, there is a dilemma between the thermal stability of bit data and the operation power of devices, b...

A single vortex-core switching induced by current injection into a ferromagnetic disk is detected in real time using a three-terminal device with the tunneling magnetoresistance junction. The real time observation reveals that the electrical core switching is completed within a few 100 ps. It is also found that the core switching takes place at the...

The low-frequency and shot noises in spin-valve CoFeB/MgO/CoFeB magnetic
tunneling junctions were studied at low temperature. The measured 1/f noise
around the magnetic hysteresis loops of the free layer indicates that the main
origin of the 1/f noise is the magnetic fluctuation, which is discussed in
terms of a fluctuation-dissipation relation. Ra...

We performed noise measurements in a two-dimensional electron gas to
investigate the nonequilibrium quantum Hall effect (QHE) state. While excess
noise is perfectly suppressed around the zero-biased QHE state reflecting the
dissipationless electron transport of the QHE state, considerable finite excess
noise is observed in the breakdown regime of t...

Supplementary Figure S1, Supplementary Discussion and Supplementary Methods

The demonstration of quantized spin splitting by Stern and Gerlach is one of the most important experiments in modern physics. Their discovery was the precursor of recent developments in spin-based technologies. Although electrical spin separation of charged particles is fundamental in spintronics, in non-uniform magnetic fields it has been difficu...

We measured the shot noise in fully epitaxial Fe/MgAl2OX/Fe-based magnetic
tunneling junctions (MTJs). While the Fano factor to characterize the shot
noise is very close to unity in the antiparallel configuration, it is reduced
to 0.98 in the parallel configuration. This observation shows the
sub-Poissonian process of electron tunneling in the para...

We have measured the shot noise in a quantum point contact (QPC) fabricated
by using InGaAs/InGaAsP heterostructure, whose conductance can be electrically
tuned by the gate voltages. The reduced shot noise is observed when the QPC
conductance equals to N(2e^2/h) (N=4, 5, and 6), which is the direct
experimental evidence of the coherent quantized ch...

We propose a setup for a quantitative test of the quantum fluctuation
theorem. It consists of a quantum conductor, driven by an external voltage
source, and a classical inductor-capacitor circuit. The work done on the system
by the voltage source can be expressed by the classical degrees of freedom of
the LC circuit, which are measurable by convent...

We study the shot noise (nonequilibrium current fluctuation) associated with
dynamic nuclear polarization in a nonequilibrium quantum wire (QW) fabricated
in a two-dimensional electron gas. We observe that the spin-polarized
conductance quantization of the QW in the integer quantum Hall regime collapses
when the QW is voltage biased to be driven to...

The domain wall (DW) velocity above the Walker field drops abruptly with increasing magnetic field, because of the so-called Walker breakdown, where the DW moves with a precessional mode. On applying the higher field, the DW velocity again starts to increase gradually. We report the DW propagation around this local minimum regime in detail, investi...

In this study, we address the phase coherent transport in a
sub-micrometer-sized Hall bar made of epitaxial Bi2Se3 thin film by probing the
weak antilocalization (WAL) and the magnetoresistance fluctuation below 22 K.
The WAL effect is well described by the Hikami-Larkin-Nagaoka model, where the
temperature dependence of the coherence length indica...

The performance of spintronic devices critically depends on three material parameters, namely, the spin polarization in the current (P), the intrinsi