arXiv subjects

Hiroshi Imamura

Publications and source records attributed to Hiroshi Imamura.

At least 73 records · Page 4Linked to original sources

arXiv2008

Effective Resistance Mismatch and Magnetoresistance of a CPP-GMR system with Current-Confined-Paths

We theoretically study the magnetoresistance of a CPP-GMR system with current confined paths (CCP) in the framework of Valet-Fert theory. The continuity equations for charge and spin currents are numerically solved with the three-dimensional CCP geometry by use of finite element method. It is confirmed that the MR ratio is enhanced by the CCP structure, which is consistent with the experimental results. Moreover, we find that there exists a certain contact width which maximize the MR ratio. We show that the contact width which maximize the MR ratio is well described by the effective resistance matching.

cond-mat.mes-hall↗

arXiv2008

Penetration Depth of Transverse Spin Current in Ferromagnetic Metals

The line width of the ferromagnetic resonance (FMR) spectrum of Cu/CoFeB/Cu/Co/Cu is studied. Analyzing the FMR spectrum by the theory of spin pumping, we determined the penetration depth of the transverse spin current in the Co layer. The obtained penetration depth of Co is 1.7 nm.

cond-mat.mes-hall↗

arXiv2008

Non-equilibrium thermodynamic study of magnetization dynamics in the presence of spin-transfer torque

The dynamics of magnetization in the presence of spin-transfer torque was studied. We derived the equation for the motion of magnetization in the presence of a spin current by using the local equilibrium assumption in non-equilibrium thermodynamics. We show that, in the resultant equation, the ratio of the Gilbert damping constant, $α$, and the coefficient, $β$, of the current-induced torque, called non-adiabatic torque, depends on the relaxation time of the fluctuating field $τ_{c}$. The equality $α=β$ holds when $τ_c$ is very short compared to the time scale of magnetization dynamics. We apply our theory to current-induced magnetization reversal in magnetic multilayers and show that the switching time is a decreasing function of $τ_{c}$.

cond-mat.mes-hall↗

arXiv2008

Determination of Penetration Depth of Transverse Spin Current in Ferromagnetic Metals by Spin Pumping

Spin pumping in nonmagnetic/ferromagnetic metal multilayers is studied both theoretically and experimentally. We show that the line widths of the ferromagnetic resonance (FMR) spectrum depend on the thickness of the ferromagnetic metal layers, which must not be in resonance with the oscillating magnetic field. We also show that the penetration depths of the transverse spin current in ferromagnetic metals can be determined by analyzing the line widths of their FMR spectra. The obtained penetration depths in NiFe, CoFe and CoFeB were 3.7 [nm], 2.5 [nm] and 12.0 [nm], respectively.

cond-mat.mes-hall↗

arXiv2007

Coherent transfer of light polarization to electron spins in a semiconductor

We demonstrate that the superposition of light polarization states is coherently transferred to electron spins in a semiconductor quantum well. By using time-resolved Kerr rotation we observe the initial phase of Larmor precession of electron spins whose coherence is transferred from light. To break the electron-hole spin entanglement, we utilized the big discrepancy between the transverse g-factors of electrons and light holes. The result encourages us to make a quantum media converter between flying photon qubits and stationary electron spin qubits in semiconductors.

quant-ph↗

arXiv2007

Enhancement of the Gilbert damping constant due to spin pumping in noncollinear ferromagnet/nonmagnet/ferromagnet trilayer systems

We analyzed the enhancement of the Gilbert damping constant due to spin pumping in non-collinear ferromagnet / non-magnet / ferromagnet trilayer systems. We show that the Gilbert damping constant depends both on the precession angle of the magnetization of the free layer and on the direction of the magntization of the fixed layer. We find the condition to be satisfied to realize strong enhancement of the Gilbert damping constant.

cond-mat.mes-hall↗

arXiv2006

Effect of exchange interaction on fidelity of quantum state transfer from a photon qubit to an electron-spin qubit

We analyzed the fidelity of the quantum state transfer (QST) from a photon-polarization qubit to an electron-spin-polarization qubit in a semiconductor quantum dot, with special attention to the exchange interaction between the electron and the simultaneously created hole. In order to realize a high-fidelity QST we had to separate the electron and hole as soon as possible, since the electron-hole exchange interaction modifies the orientation of the electron spin. Thus, we propose a double-dot structure to separate the electron and hole quickly, and show that the fidelity of the QST can reach as high as 0.996 if the resonant tunneling condition is satisfied.

quant-ph↗

arXiv2005

Decoherence of localized spins interacting via RKKY interaction

We theoretically study decoherence of two localized spins interacting via the RKKY interaction in one-, two-, and three-dimensional electron gas. We derive the kinetic equation for the reduced density matrix of the localized spins and show that energy relaxation caused by singlet-triplet transition is suppressed when the RKKY interaction is ferromagnetic. We also estimate the decoherence time of the system consisting of two quantum dots embedded in a two dimensional electron gas.

cond-mat.mes-hall↗

arXiv2005

Nonequilibrium Kondo Effect in a Quantum Dot Coupled to Ferromagnetic Leads

We study the Kondo effect in the electron transport through a quantum dot coupled to ferromagnetic leads, using a real-time diagrammatic technique which provides a systematic description of the nonequilibrium dynamics of a system with strong local electron correlations. We evaluate the theory in an extension of the `resonant tunneling approximation', introduced earlier, by introducing the self-energy of the off-diagonal component of the reduced propagator in spin space. In this way we develop a charge and spin conserving approximation that accounts not only for Kondo correlations but also for the spin splitting and spin accumulation out of equilibrium. We show that the Kondo resonances, split by the applied bias voltage, may be spin polarized. A left-right asymmetry in the coupling strength and/or spin polarization of the electrodes significantly affects both the spin accumulation and the weight of the split Kondo resonances out of equilibrium. The effects are observable in the nonlinear differential conductance. We also discuss the influence of decoherence on the Kondo resonance in the frame of the real-time formulation.

cond-mat.mes-hall↗

arXiv2003

Twisted exchange interaction between localized spins embedded in a one- or two-dimensional electron gas with Rashba spin-orbit coupling

We study theoretically the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in one- and two-dimensions in presence of a Rashba spin-orbit (SO) coupling. We show that rotation of the spin of conduction electrons due to SO coupling causes a twisted RKKY interaction between localized spins which consists of three different terms: Heisenberg, Dzyaloshinsky-Moriya, and Ising interactions. We also show that the effective spin Hamiltonian reduces to the usual RKKY interaction Hamiltonian in the twisted spin space where the spin quantization axis of one localized spin is rotated.

cond-mat.mes-hall↗

arXiv2003

Indirect Exchange Interaction between two Quantum Dots in an Aharonov-Bohm Ring

We investigate the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two spins located at two quantum dots embedded in an Aharonov-Bohm (AB) ring. In such a system the RKKY interaction, which oscillates as a function of the distance between two local spins, is affected by the flux. For the case of the ferromagnetic RKKY interaction, we find that the amplitude of AB oscillations is enhanced by the Kondo correlations and an additional maximum appears at half flux, where the interaction is switched off. For the case of the antiferromagnetic RKKY interaction, we find that the phase of AB oscillations is shifted by pi, which is attributed to the formation of a singlet state between two spins for the flux value close to integer value of flux.

cond-mat.mes-hall↗

arXiv2002

Twisted Singlet in Semiconductor Artificial Molecules

The spin-configuration of semiconductor artificial molecules consisting of quantum dots and spin field effect transistors is studied theoretically. We find that the antiferromagnetic spin configuration can be changed to the ferromagnetic one by applying a gate voltage to the spin field effect transistors. We show that the square-norm of the total spin of an artificial molecule oscillates with the twist angle $θ$ and has maxima at $θ=$ an odd integer times $π$. We also show that the square-norm of the total spin of the ring-shaped artificial molecule changes drastically at certain values of $θ$ where the lowest two energy levels cross each other.

cond-mat.mes-hall↗

arXiv2002

The effect of large quantum fluctuation on the noise of a single-electron transistor

We theoretically investigate the noise of a single-electron transistor in the regime of large quantum fluctuation of charge out of equilibrium. We show that the charge noise is suppressed due to the charge renormalization caused by the quantum fluctuation. However the fluctuation is not strong enough to wash out the charge quantization. We find that the renormalization effect reduces the performance of a single-electron electrometer.

cond-mat.mes-hall↗

arXiv2002

Noise of a single-electron transistor in the regime of large quantum fluctuations of island charge out of equilibrium

By using the drone-fermion representation and the Schwinger-Keldysh approach, we calculate the current noise and the charge noise for a single-electron transistor in the non-equilibrium state in the presence of large quantum fluctuation of island charge. Our result interpolates between those of the "orthodox theory" and the "co-tunneling theory". We find the following effects which are not treated by previous theories: (i) At zero temperature T=0 and at finite applied bias voltage $|eV| \gg T_{K}$, where $T_{K}$ is the "Kondo temperature", we find the Fano factor is suppressed more than the suppression caused by Coulomb correlation both in the Coulomb blockade regime and in the sequential tunneling regime. (ii) For $T \gg |eV|/2 \gg T_{K}$, the current noise in the presence of large charge fluctuation is modified and deviates from the prediction of the orthodox theory. However, the Fano factor coincides with that of the orthodox theory and is proportional to the temperature. (iii) For $eV, T \lesssim T_{K}$, the charge noise is suppressed due to the renormalization of system parameters caused by quantum fluctuation of charge. We interpret it in terms of the modification of the "unit" of the island charge.

cond-mat.mes-hall↗

arXiv2002

Andreev Reflection in Ferromagnet/Superconductor/Ferromagnet Double Junction Systems

We present a theory of Andreev reflection in a ferromagnet/superconductor/ferromagnet double junction system. The spin polarized quasiparticles penetrate to the superconductor in the range of penetration depth from the interface by the Andreev reflection. When the thickness of the superconductor is comparable to or smaller than the penetration depth, the spin polarized quasiparticles pass through the superconductor and therefore the electric current depends on the relative orientation of magnetizations of the ferromagnets. The dependences of the magnetoresistance on the thickness of the superconductor, temperature, the exchange field of the ferromagnets and the height of the interfacial barriers are analyzed. Our theory explains recent experimental results well.

cond-mat.mes-hall↗

arXiv2002

The current and the charge noise of a single-electron transistor in the regime of large charge fluctuations out of equilibrium

By using the Schwinger-Keldysh approach, we evaluate the current noise and the charge noise of the single-electron transistor (SET) in the regime of large charge fluctuations caused by large tunneling conductance. Our result interpolates between previous theories; the "orthodox" theory and the "co-tunneling theory". We find that the life-time broadening effect suppresses the Fano factor below the value estimated by the previous theories. We also show that the large tunnel conductance does not reduce the energy sensitivity so much. Our results demonstrate quantitatively that SET electrometer can be used as the high-sensitivity and high-speed device for quantum measurements.

cond-mat.mes-hall↗

arXiv2002

Parity Effect and Tunnel Magnetoresistance of Ferromagnet / Superconductor / Ferromagnet Single-Electron Tunneling Transistors

We theoretically study the tunnel magnetoresistance(TMR) of ferromagnet / superconductor / ferromagnet single-electron tunneling transistors with a special attention to the parity effect. It is shown that in the plateau region, there is no spin accumulation in the island even at finite bias voltage. However, the information of the injected spin is carried by the excess electron and thus the TMR exists. The spin relaxation rate of the excess electron can be estimated from the TMR. We also show that the TMR increases with decreasing the size of the superconducting island.

cond-mat.supr-con↗

arXiv2002

Charge fluctuation between even and odd states of a superconducting island

We theoretically investigate effects of quantum fluctuation between even and odd charge states on the transport properties of a normal-superconducting-normal single-electron tunneling transistor. The charge fluctuation is discussed beyond the orthodox theory. We find that the charging energy renormalization enhances the parity effect: The Coulomb blockade regime for odd state is reduced and that for even state is widened. We show that the renormalization factor can be obtained by measuring the peak position of the conductance. The renormalization factor reduces with increasing the applied bias voltage.

cond-mat.mes-hall↗