arXiv subjects

Mikio Eto

Publications and source records attributed to Mikio Eto.

33 records · Page 2Linked to original sources

arXiv2009

Enhanced spin Hall effect in semiconductor heterostructures with artificial potential

We theoretically investigate an extrinsic spin Hall effect (SHE) in semiconductor heterostructures due to the scattering by an artificial potential created by antidot, STM tip, etc. The potential is electrically tunable. First, we formulate the SHE in terms of phase shifts in the partial wave expansion for two-dimensional electron gas. The effect is significantly enhanced by the resonant scattering when the attractive potential is properly tuned. Second, we examine a three-terminal device including an antidot, which possibly produces a spin current with polarization of more than 50%.

cond-mat.mes-hall↗

arXiv2007

Spin splitting and Kondo effect in quantum dots coupled to noncollinear ferromagnetic leads

We study the Kondo effect in a quantum dot coupled to two noncollinear ferromagnetic leads. First, we study the spin splitting $δε=ε_{\downarrow}-ε_{\uparrow}$ of an energy level in the quantum dot by tunnel couplings to the ferromagnetic leads, using the Poor man's scaling method. The spin splitting takes place in an intermediate direction between magnetic moments in the two leads. $δε\propto p\sqrt{\cos^2(θ/2)+v^2\sin^2(θ/2)}$, where $p$ is the spin polarization in the leads, $θ$ is the angle between the magnetic moments, and $v$ is an asymmetric factor of tunnel barriers ($-1<v<1$). Hence the spin splitting is always maximal in the parallel alignment of two ferromagnets ($θ=0$) and minimal in the antiparallel alignment ($θ=π$). Second, we calculate the Kondo temperature $T_{\mathrm{K}}$. The scaling calculation yields an analytical expression of $T_{\mathrm{K}}$ as a function of $θ$ and $p$, $T_{\mathrm{K}}(θ, p)$, when $δε\ll T_{\mathrm{K}}$. $T_{\mathrm{K}}(θ, p)$ is a decreasing function with respect to $p\sqrt{\cos^2(θ/2)+v^2\sin^2(θ/2)}$. When $δε$ is relevant, we evaluate $T_{\mathrm{K}}(δε, θ, p)$ using the slave-boson mean-field theory. The Kondo resonance is split into two by finite $δε$, which results in the spin accumulation in the quantum dot and suppression of the Kondo effect.

cond-mat.mes-hall↗

arXiv2006

Resonant tunneling and Fano resonance in quantum dots with electron-phonon interaction

We theoretically study the resonant tunneling and Fano resonance in quantum dots with electron-phonon (e-ph) interaction. We examine the bias-voltage ($V$) dependence of the decoherence, using Keldysh Green function method and perturbation with respect to the e-ph interaction. With optical phonons of energy $ω_0$, only the elastic process takes place when $eV<ω_0$, in which electrons emit and absorb phonons virtually. The process suppresses the resonant amplitude. When $eV>ω_0$, the inelastic process is possible which is accompanied by real emission of phonons. It results in the dephasing and broadens the resonant width. The bias-voltage dependence of the decoherence cannot be obtained by the canonical transformation method to consider the e-ph interaction if its effect on the tunnel coupling is neglected. With acoustic phonons, the asymmetric shape of the Fano resonance grows like a symmetric one as the bias voltage increases, in qualitative accordance with experimental results.

cond-mat.mes-hall↗

arXiv2005

Spin Polarization at Semiconductor Point Contacts in Absence of Magnetic Field

Semiconductor point contacts can be a useful tool for producing spin-polarized currents in the presence of spin-orbit (SO) interaction. Neither magnetic fields nor magnetic materials are required. By numerical studies, we show that (i) the conductance is quantized in units of 2e^2/h unless the SO interaction is too strong, (ii) the current is spin-polarized in the transverse direction, and (iii) a spin polarization of more than 50% can be realized with experimentally accessible values of the SO interaction strength. The spin-polarization ratio is determined by the adiabaticity of the transition between subbands of different spins during the transport through the point contacts.

cond-mat.mes-hall↗

arXiv2004

Enhancement of Kondo effect in multilevel quantum dots

We theoretically study enhancement mechanisms of the Kondo effect in multilevel quantum dots. In quantum dots fabricated on semiconductors, the energy difference between discrete levels Δis tunable by applying a magnetic field. With two orbitals and spin 1/2 in the quantum dots, we evaluate the Kondo temperature T_K as a function of Δ, using the scaling method. T_K is maximal around Δ=0 and decreases with increasing |Δ|, following a power law, T_K(Δ)=T_K(0) (T_K(0)/|Δ|)^γ, which is understood as a crossover from SU(4) to SU(2) Kondo effect. The exponents on both sides of a level crossing, γ_L and γ_R, satisfy a relation of γ_L γ_R=1. We compare this enhanced Kondo effect with that by spin-singlet-triplet degeneracy for an even number of electrons, to explain recent experimental results using vertical quantum dots.

cond-mat.mes-hall↗

arXiv2003

Electronic States in Silicon Quantum Dots: Multivalley Artificial Atoms

Electronic states in silicon quantum dots are examined theoretically, taking into account a multivalley structure of the conduction band. We find that (i) exchange interaction hardly works between electrons in different valleys. In consequence electrons occupy the lowest level in different valleys in the absence of Hund's coupling when the dot size is less than 10 nm. High-spin states are easily realized by applying a small magnetic field. (ii) When the dot size is much larger, the electron-electron interaction becomes relevant in determining the electronic states. Electrons are accommodated in a valley, making the highest spin, to gain the exchange energy. (iii) In the presence of intervalley scattering, degenerate levels in different valleys are split. This could result in low-spin states. These spin states in multivalley artificial atoms can be observed by looking at the magnetic-field dependence of peak positions in the Coulomb oscillation.

cond-mat.mes-hall↗

arXiv2003

Effects of Polaron Formation in Semiconductor Quantum Dots on Transport Properties

We theoretically examine the effects of polaron formation in quantum dots on the transport properties. When a separation between two electron-levels in a quantum dot matches the energy of the longitudinal optical (LO) phonons, the polarons are strongly formed. The Rabi splitting between the levels is observable in a peak structure of the differential conductance G as a function of the bias voltage. The polaron formation suppresses the peak height of G, which is due to the competition between the resonant tunneling (resonance between a level in the dot and states in the leads) and the polaron formation (Rabi oscillation between two levels in the dot). G shows a sharp dip at the midpoint between the split peaks. This is attributable to the destructive interference between bonding and anti-bonding states in a composite system of electrons and phonons.

cond-mat.mes-hall↗

arXiv2002

Current induced entanglement of nuclear spins in quantum dots

We propose an entanglement mechanism of nuclear spins in quantum dots driven by the electric current. The current accompanied by the spin flip in quantum dots gradually increases components of larger total spin of nuclei. This entangled state drastically enhances the spin relaxation rate of electrons, which can be detected by measuring a leakage current in the spin-blockade region. This mechanism is not relevant in optical experiments which examine the spin relaxation of single excitations.

cond-mat.mes-hall↗

arXiv2002

Numerical Studies of Fano Resonance in Quantum dots Embedded in AB Rings

The Fano resonance in quantum dots embedded in Aharonov-Bohm rings is examined theoretically, using two models of non-interacting electrons. The first model yields an analytical expression for the conductance G. G is written in an extended Fano form with a complex parameter. The shape of the resonance can be asymmetric or symmetric, depending on the magnetic flux enclosed in the ring. The "phase" of the resonance is changed continuously with increasing the flux in two-terminal situations. These are in accordance with recent experimental results. In the second model, we consider the dephasing effect on the Fano resonance by numerical calculations.

cond-mat.mes-hall↗

arXiv2002

Multi-parameter scaling of the Kondo effect in quantum dots with an even number of electrons

We address a recent theoretical discrepancy concerning the Kondo effect in quantum dots with an even number of electrons where spin-singlet and -triplet states are nearly degenerate. We show that the discrepancy arises from the fact that the Kondo scaling involves many parameters, which makes the results depend on concrete microscopic models. We illustrate this by the scaling calculations of the Kondo temperature, $T_K$, as a function of the energy difference between the singlet and triplet states $Δ$. $T_K(Δ)$ decreases with increasing $Δ$, showing a crossover from a power law with a universal exponent to that with a nonuniversal exponent. The crossover depends on the initial parameters of the model.

cond-mat.mes-hall↗

arXiv2001

Kondo effect in coupled quantum dots under magnetic fields

The Kondo effect in coupled quantum dots is investigated theoretically under magnetic fields. We show that the magnetoconductance (MC) illustrates peak structures of the Kondo resonant spectra. When the dot-dot tunneling coupling $V_C$ is smaller than the dot-lead coupling $Δ$ (level broadening), the Kondo resonant levels appear at the Fermi level ($E_F$). The Zeeman splitting of the levels weakens the Kondo effect, which results in a negative MC. When $V_{C}$ is larger than $Δ$, the Kondo resonances form bonding and anti-bonding levels, located below and above $E_F$, respectively. We observe a positive MC since the Zeeman splitting increases the overlap between the levels at $E_F$. In the presence of the antiferromagnetic spin coupling between the dots, the sign of MC can change as a function of the gate voltage.

cond-mat.mes-hall↗

arXiv2001

Kondo resonant spectra in coupled quantum dots

The Kondo effect in coupled quantum dots is investigated from the viewpoint of transmission spectroscopy using the slave-boson formalism of the Anderson model. The antiferromagnetic spin-spin coupling $J$ between the dots is taken into account. Conductance $G$ through the dots connected in a series is characterized by the competition between the dot-dot tunneling coupling $V_{C}$ and the level broadening $Δ$ in the dots (dot-lead coupling). When $V_{C}/Δ< 1$, the Kondo resonance is formed between each dot and lead, which is replaced by a spin-singlet state in the dots at low gate voltages. The gate voltage dependence of $G$ has a sharp peak of $2 e^2/h$ in height in the crossover region between the Kondo and spin-singlet states. The sharp peak of $G$ survives when the energy levels are different between the dots. When $V_{C} / Δ> 1$, the "molecular levels" between the Kondo resonant states appear; the Kondo resonant peaks are located below and above the Fermi level in the leads at low gate voltages. The gate voltage dependence of $G$ has a broad peak, which is robust against $J$. The broad peak splits into two peaks when the energy levels are different, reflecting the formation of the asymmetric molecular levels between the Kondo resonant states.

cond-mat.mes-hall↗

arXiv2001

Electronic States and Transport Phenomena in Quantum Dot Systems

Electronic states and transport phenomena in semiconductor quantum dots are studied theoretically. Taking account of the electron-electron Coulomb interaction by the exact diagonalization method, the ground state and low-lying excited states are calculated as functions of magnetic field. Using the obtained many-body states, we discuss the temperature dependence of the conductance peaks in the Coulomb oscillation. In the Coulomb blockade region, elastic and inelastic cotunneling currents are evaluated under finite bias voltages. The cotunneling conductance is markedly enhanced by the Kondo effect. In coupled quantum dots, molecular orbitals and electronic correlation influence the transport properties.

cond-mat.mes-hall↗

arXiv2001

Mean Field Theory of the Kondo Effect in Quantum Dots with an Even Number of Electrons

We investigate the enhancement of the Kondo effect in quantum dots with an even number of electrons, using a scaling method and a mean field theory. We evaluate the Kondo temperature $T_K$ as a function of the energy difference between spin-singlet and triplet states in the dot, $Δ$, and the Zeeman splitting, $E_Z$. If the Zeeman splitting is small, $E_Z \ll T_K$, the competition between the singlet and triplet states enhances the Kondo effect. $T_K$ reaches its maximum around $Δ=0$ and decreases with $Δ$ obeying a power law. If the Zeeman splitting is strong, $E_Z \gg T_K$, the Kondo effect originates from the degeneracy between the singlet state and one of the components of the triplet state at $-Δ\sim E_Z$. We show that $T_K$ exhibits another power-law dependence on $E_Z$. The mean field theory provides a unified picture to illustrate the crossover between these regimes. The enhancement of the Kondo effect can be understood in terms of the overlap between the Kondo resonant states created around the Fermi level. These resonant states provide the unitary limit of the conductance $G\sim 2e^2/h$.

cond-mat.mes-hall↗

arXiv2000

Enhancement of Kondo effect in quantum dots with an even number of electrons

We investigate the Kondo effect in a quantum dot with almost degenerate spin-singlet and triplet states for an even number of electrons. We show that the Kondo temperature as a function of the energy difference between the states Delta reaches its maximum around Delta=0 and decreases with increasing Delta. The Kondo effect is thus enhanced by competition between singlet and triplet states. Our results explain recent experimental findings. We evaluate the linear conductance in the perturbative regime.

cond-mat.mes-hall↗