arXiv subjects

Yasuhiro Iye

Publications and source records attributed to Yasuhiro Iye.

At least 19 recordsLinked to original sources

arXiv2022

Joule heating and the thermal conductivity of a two-dimensional electron gas at cryogenic temperatures studied by modified 3$ω$ method

During the standard ac lock-in measurement of the resistance of a two-dimensional electron gas (2DEG) applying an ac current $I = \sqrt{2} I_0 \sin(ωt)$, the electron temperature $T_e$ oscillates with the angular frequency $2 ω$ due to the Joule heating $\propto I^2$. We have shown that the highest ($T_\mathrm{H}$) and the lowest ($T_\mathrm{L}$) temperatures during a cycle of the oscillations can be deduced, at cryogenic temperatures, exploiting the third-harmonic (3$ω$) component of the voltage drop generated by the ac current $I$ and employing the amplitude of the Shubnikov-de Haas oscillations as the measure of $T_e$. The temperatures $T_\mathrm{H}$ and $T_\mathrm{L}$ thus obtained allow us to roughly evaluate the thermal conductivity $κ_{xx}$ of the 2DEG via the modified 3$ω$ method, in which the method originally devised for bulk materials is modified to be applicable to a 2DEG embedded in a semiconductor wafer. The $κ_{xx}$ thus deduced is found to be consistent with the Wiedemann-Franz law. The method provides a convenient way to access $κ_{xx}$ using only a standard Hall-bar device and the simple experimental setup for the resistance measurement.

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arXiv2022

Anisotropic Behavior of the Thermoelectric Power and the Thermal Conductivity in a Unidirectional Lateral Superlattice: A Typical Anisotropic System Exhibiting Two Distinct Nernst Coefficients

We have calculated the thermoelectric conductivity tensor $\varepsilon_{ij}$ and the thermal conductivity tensor $\lambda_{ij}$ of a unidirectional lateral superlattice (ULSL) ($i,j = x,y$, with the $x$-axis aligned to the principal axis of the ULSL), %, given as the first- and the second-order moments, employing based on the asymptotic analytic formulas of the electrical conductivity tensor $\sigma_{ij}$ in the literature valid at low magnetic fields where large numbers of Landau levels are occupied. With the resulting analytic expressions, we clarify the conditions for the Mott formula (Wiedemann-Franz law) to be applicable with high precision to $\varepsilon_{ij}$ ($\lambda_{ij}$). We further present plots of the commensurability oscillations $\delta\varepsilon_{ij}$, $\delta\lambda_{ij}$, $\delta\kappa_{ij}$, and $\delta S_{ij}$ in $\varepsilon_{ij}$, $\lambda_{ij}$, (an alternative, more standard definition of) the thermal conductivity tensor $\kappa_{ij}$, and the thermopower tensor $S_{ij}$, calculated using typical parameters for a ULSL fabricated from a GaAs/AlGaAs two-dimensional electron gas (2DEG). Notable features of the $\delta S_{ij}$ are (i) anisotropic behavior ($\delta S_{xx} \ne \delta S_{yy}$) and (ii) the dominance of the $xy$ component over the other components ($|\delta S_{xy}| \gg |\delta S_{yx}|, |\delta S_{xx}|, |\delta S_{yy}|$). The latter clearly indicates that the two Nernst coefficients, $S_{xy}$ and $S_{yx}$, can be totally different from each other in an anisotropic system. Both (i) and (ii) are at variance with the previous theory and are attributable to the inclusion of a damping factor due to the small-angle scattering characteristic of GaAs/AlGaAs 2DEGs, which have not been taken into consideration in $\delta S_{ij}$ thus far.

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arXiv2021

Commensurability oscillations in the Hall resistance of unidirectional lateral superlattices

We have observed commensurability oscillations (CO) in the Hall resistance $R_{yx}$ of a unidirectional lateral superlattice (ULSL). The CO, having small amplitudes ($\sim$ 1 $Ω$) and being superposed on a roughly three-orders of magnitude larger background, are obtained by directly detecting the difference in $R_{yx}$ between the ULSL area and the adjacent unmodulated two-dimensional electron gas area, and then extracting the odd part with respect to the magnetic field. The CO thus obtained are compared with a theoretical calculation and turn out to have the amplitude much smaller than the theoretical prediction. The implication of the smaller-than-predicted CO in $R_{yx}$ on the thermoelectric power of ULSL is briefly discussed.

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arXiv2019

Spatial distribution of thermoelectric voltages in a Hall-bar shaped two-dimensional electron system under a magnetic field

We have investigated the spatial distribution of the electron temperature generated in a two-dimensional electron system (2DES) subjected to a perpendicular magnetic field. We measure thermoelectric voltages between Ohmic contacts located at the end of the voltage-probe arms of a Hall bar fabricated from a GaAs/AlGaAs 2DES wafer, immersed in the mixing chamber of a dilution refrigerator held at 20 mK. Magneto-oscillations due to the Landau quantization are examined for the thermoelectric voltages between the contact pairs straddling the main bar (arrangement to measure the transverse component $V_{yx}$), and between the pairs located along the same side of the main bar (arrangement for the longitudinal component $V_{xx}$). For the former arrangement, the oscillation amplitude diminishes with the distance from the heater. For the latter arrangement, the pair on one side exhibits much larger amplitude than the pair on the opposite side, and the relation becomes reversed by inverting the magnetic field. The behaviours of the oscillation amplitude are qualitatively explained by the spatial distribution of the electron temperature numerically calculated taking into consideration the thermal diffusion into the voltage contacts and the electron-phonon interaction. For both arrangements, the oscillations are shown to derive predominantly from the transverse (Nernst) component, $S_{yx}$, of the thermopower tensor. The calculation also reveals that the voltage probes, introducing only minor disturbance at zero magnetic field, substantially reduce the temperature once a magnetic field is applied, and the thermoelectric voltages generated at the voltage arms account for a significant part of the measured voltages.

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arXiv2018

Frequencies of the Edge-Magnetoplasmon Excitations in Gated Quantum Hall Edges

We have investigated microwave transmission through the edge of quantum Hall systems by employing a coplanar waveguide (CPW) fabricated on the surface of a GaAs/AlGaAs two-dimensional electron gas (2DEG) wafer. An edge is introduced to the slot region of the CPW by applying a negative bias $V_\mathrm{g}$ to the central electrode (CE) and depleting the 2DEG below the CE. We observe peaks attributable to the excitation of edge magnetoplasmons (EMP) at a fundamental frequency $f_0$ and at its harmonics $i f_0$ ($i$ = 2, 3,...). The frequency $f_0$ increases with decreasing $V_\mathrm{g}$, indicating that EMP propagates with higher velocity for more negative $V_\mathrm{g}$. The dependence of $f_0$ on $V_\mathrm{g}$ is interpreted in terms of the variation in the distance between the edge state and the CE, which alters the velocity by varying the capacitive coupling between them. The peaks are observed to continue, albeit with less clarity, up to the regions of $V_\mathrm{g}$ where 2DEG still remains below the CE.

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arXiv2013

Diffusion Thermopower of Quantum Hall States Measured in Corbino Geometry

We have measured the diffusion thermopower of a quantum Hall system in a Corbino setup. A concentric electron-temperature gradient is introduced by irradiating microwaves, via a coplanar waveguide, near the outer rim of a circular mesa of a two-dimensional electron gas. The resulting radial thermovoltages exhibit sawtooth-like oscillations with the magnetic field, taking large positive (negative) values just below (above) integer fillings with sign reversal at the center of the quantum Hall plateaus. The behavior is in agreement with a recent theory [Y. Barlas and K. Yang: Phys. Rev. B 85 (2012) 195107], which treats disorder within the self-consistent Born approximation.

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arXiv2012

Geometric resonances in the magnetoresistance of hexagonal lateral superlattices

We have measured magnetoresistance of hexagonal lateral superlattices. We observe three types of oscillations engendered by periodic potential modulation having hexagonal-lattice symmetry: amplitude modulation of the Shubnikov-de Haas oscillations, commensurability oscillations, and the geometric resonances of open orbits generated by Bragg reflections. The latter two reveal the presence of two characteristic periodicities, sqrt{3} a / 2 and a / 2, inherent in a hexagonal lattice with the lattice constant a. The formation of the hexagonal-superlattice minibands manifested by the observation of open orbits marks the first step toward realizing massless Dirac fermions in semiconductor 2DEGs.

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arXiv2012

Commensurability oscillations in the rf conductivity of unidirectional lateral superlattices: measurement of anisotropic conductivity by coplanar waveguide

We have measured the rf magnetoconductivity of unidirectional lateral superlattices (ULSLs) by detecting the attenuation of microwave through a coplanar waveguide placed on the surface. ULSL samples with the principal axis of the modulation perpendicular (S_perp) and parallel (S_||) to the microwave electric field are examined. For low microwave power, we observe expected anisotropic behavior of the commensurability oscillations (CO), with CO in samples S_perp and S_|| dominated by the diffusion and the collisional contributions, respectively. Amplitude modulation of the Shubnikov-de Haas oscillations is observed to be more prominent in sample S_||. The difference between the two samples is washed out with the increase of the microwave power, letting the diffusion contribution govern the CO in both samples. The failure of the intended directional selectivity in the conductivity measured with high microwave power is interpreted in terms of large-angle electron-phonon scattering.

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arXiv2011

Magnetization dependent current rectification in (Ga,Mn)As magnetic tunnel junctions

We have found that the current rectification effect in triple layer (double barrier) (Ga,Mn)As magnetic tunnel junctions strongly depends on the magnetization alignment. The direction as well as the amplitude of the rectification changes with the alignment, which can be switched by bi-directional spin-injection with very small threshold currents. A possible origin of the rectification is energy dependence of the density of states around the Fermi level. Tunneling density of states in (Ga,Mn)As shows characteristic dip around zero-bias indicating formation of correlation gap, the asymmetry of which would be a potential source of the energy dependent density of states.

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arXiv2011

Evidence of Spin-Filtering in Quantum Constrictions with Spin-Orbit Interaction

A new type of blockade effect - spin-orbit blockade (SOB) - is found in the conduction of a quantum dot (QD) made of a material with spin-orbit interaction. The blockade arises from spin-filtering effect in a quantum point contact (QPC), which is a component of the QD. Hence the appearance of the blockade itself evidences the spin-filtering effect in the QPC. The lower bound of filtering efficiency is estimated to be above 80%.

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arXiv2010

Modulation Induced Phase Transition from Fractional Quantum Hall to Stripe State at nu=5/3

We have investigated the effect of unidirectional periodic potential modulation on the fractional quantum Hall (FQH) state at filling factors nu=5/3 and 4/3. For large enough modulation amplitude, we find that the resistivity minimum at nu=5/3 gives way to a peak that grows with decreasing temperature. Density matrix renormalization group calculation reveals that phase transition from FQH state to unidirectional striped state having a period sim 4 l (with l the magnetic length) takes place at nu=1/3 (equivalent to nu=5/3 by the particle-hole symmetry) with the increase of the modulation amplitude, suggesting that the observed peak is the manifestation of the stripe phase.

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arXiv2010

Probing local electronic states in the quantum Hall regime with a side coupled quantum dot

We demonstrate a new method for locally probing the edge states in the quantum Hall regime utilizing a side coupled quantum dot positioned at an edge of a Hall bar. By measuring the tunneling of electrons from the edge states into the dot, we acquire information on the local electrochemical potential and electron temperature of the edge states. Furthermore, this method allows us to observe the spatial modulation of the electrostatic potential at the edge state due to many-body screening effect.

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arXiv2009

Effect of Oscillating Landau Bandwidth on the Integer Quantum Hall Effect in a Unidirectional Lateral Superlattice

We have measured activation gaps for odd-integer quantum Hall states in a unidirectional lateral superlattice (ULSL) -- a two-dimensional electron gas (2DEG) subjected to a unidirectional periodic modulation of the electrostatic potential. By comparing the activation gaps with those simultaneously measured in the adjacent section of the same 2DEG sample without modulation, we find that the gaps are reduced in the ULSL by an amount corresponding to the width acquired by the Landau levels through the introduction of the modulation. The decrement of the activation gap varies with the magnetic field following the variation of the Landau bandwidth due to the commensurability effect. Notably, the decrement vanishes at the flat band conditions.

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arXiv2009

Spin-Resolved Edge States around an Antidot in the Vicinity of the $ν=2$ Quantum Hall State

We have investigated spin-related effects on the electronic transport in a single antidot system in the vicinity of the $ν= 2$ quantum Hall state where two edge states with different spins of the lowest Landau level are formed around the antidot. The conductance exhibits the paired $h/2e$ Aharonov-Bohm (AB) oscillations. Using a tilted magnetic field technique, we have observed the evolution of the oscillations with the total magnetic field and a concomitant change in the source-drain bias dependence of the differential conductance. From the observation, we extract the effect of the Zeeman energy on the AB oscillations.

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arXiv2009

Detection of spin polarization with a side coupled quantum dot

We propose realistic methods to detect local spin polarization, which utilize a quantum dot side coupled to the target system. By choosing appropriate states in the dot, we can put spin selectivity to the dot and detect spins in the target with small disturbance. We also present an experiment which realizes one of the proposed spin detection schemes in magnetic fields.

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arXiv2008

Temperature-dependent screening of the edge state around antidots in the quantum Hall regime

The Aharonov-Bohm (AB) effect in a small array of antidots with large aspect ratio is investigated in the quantum Hall regime. The evolution with temperature of the AB oscillations in the magnetic field vs gate voltage (B-Vg) plane reveals the temperature dependence of the screening. The self-consistently screened potential of the compressible band surrounding an antidot is observed to gain progressively steeper slope with increasing temperature.

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arXiv2008

Fourier analyses of commensurability oscillations in Fibonacci lateral superlattices

Magnetotransport measurements have been performed on Fibonacci lateral superlattices (FLSLs) -- two-dimensional electron gases subjected to a weak potential modulation arranged in the Fibonacci sequence, LSLLSLS..., with L/S=tau (the golden ratio). Complicated commensurability oscillation (CO) is observed, which can be accounted for as a superposition of a series of COs each arising from a sinusoidal modulation representing the characteristic length scale of one of the self-similar generations in the Fibonacci sequence. Individual CO components can be separated out from the magnetoresistance trace by performing a numerical Fourier band-pass filter. From the analysis of the amplitude of a single-component CO thus extracted, the magnitude of the corresponding Fourier component in the potential modulation can be evaluated. By examining all the Fourier contents observed in the magnetoresistance trace, the profile of the modulated potential seen by the electrons can be reconstructed with some remaining ambiguity about the interrelation of the phase between different components.

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arXiv2008

Excited-state spectroscopy on a quantum dot side-coupled to a quantum wire

We report excited-state spectroscopy on a quantum dot side-coupled to a quantum wire with accurate energy estimation. Our method utilizes periodic voltage pulses on the dot, and the energy calibration is performed with reference to the bias voltage across the wire. We demonstrate the observation of the orbital excited state and the Zeeman splitting in a single dot.

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