The theory of electron correlation in semiconductor quantum dots is reviewed with emphasis on the physics of dots in strong magnetic fields. A brief survey of dot fabrication and experimental results is given, the quantum mechanics of small numbers of interacting electrons in a dot is discussed and the special values of angular momentum quantum number that the ground state is allowed to have, or magic numbers, are introduced. These numbers are selected because of the symmetry properties of the ground state and the symmetry is particularly evident in the limit of strong magnetic field if the system is examined in a moving reference frame. Physically, the system in this limit can be pictured as an electron molecule that rotates and vibrates in the dot, and this is the quantum dot analogue of a Wigner crystal. This is illustrated with a detailed treatment of a two-electron dot which can be studied without resorting to any special concepts of molecular physics. Next, the molecular physics concepts, such as the Eckart reference frame, needed to deal with rotational-vibrational motion of larger numbers of electrons are introduced. The physics of dots with more than two electrons is then described, including the evolution of magic numbers with electron number and the implications of symmetry. Finally, the extension of these ideas to larger systems and coupled dots is briefly discussed. Quantum dots in strong magnetic fields provide a unique opportunity to realize what could be called electron molecular physics, and some possible ways of probing the system experimentally are also proposed.
We theoretically study the spin-dependent transport in a ferromagnet/super-conductor/ferromagnet double tunnel junction. The tunneling current in the antiferromagnetic alignment of the magnetizations gives rise to a spin imbalance in the superconductor. The resulting nonequilibrium spin density strongly suppresses the superconductivity with increase of bias voltage and destroys it at a critical voltage Vc. The results provide a new method not only for measuring the spin polarization of ferromagnets but also for controlling superconductivity and tunnel magnetoresistance (TMR) by applying the bias voltage.Since the early experiments demonstrated the spinpolarized tunneling of electrons from ferromagnetic metals (FM) into superconductors (SC) in FM/SC junctions [1], the concept of the spin-polarized transport has been of vital importance in magnetic junctions and multilayers. Firstly the tunneling currents strongly depends on the relative orientation of magnetizations in FM/FM tunnel junctions; the tunnel resistance decreases when the magnetizations are aligned in a magnetic field, causing tunnel magnetoresistance (TMR) [2][3][4][5]. Secondly the spin-polarized current driven from a FM into a normal metal (N) or superconductor (SC) gives rise to a nonequilibrium spin density in N or SC [6][7][8]. In a FM/N/FM double junction the TMR effect is brought about by accumulation of spin-polarized electrons in N [9]. In a FM/SC/FM double junction [8], on the other hand, we expect the strong competition between superconductivity and magnetism induced by the spin polarization in SC. Of particular interest in the FM/SC/FM structure is not only to find novel magnetoresistive effects due to the competition but also application to magnetoelectronics.In this Letter we show that a FM/SC/FM double tunnel junction is a new magnetoresistive device to control superconductivity by applying the bias voltage (or current). In the antiferromagnetic (A) alignment of magnetizations (see Fig. 1), a nonequilibrium spin density is induced in SC due to the imbalance in the tunneling currents carried by the spin-up and spin-down electrons, so that the superconducting gap ∆ is reduced with increasing bias voltage and vanishes at a critical voltage V c . In the ferromagnetic (F) alignment, however, there is no spin-density in SC. Consequently, TMR has a strong voltage dependence around V c ; TMR is enhanced compared with that in the normal state above V c , while it changes sign to show an inverse TMR effect for some voltage range below V c . It is shown that V c is inversely proportional to the spin polarization P of FM (V c ∝ 1/P ), which provides a new method for determining P of FM.We consider a FM/SC/FM double tunnel junction as shown in Fig. 1. The left and right electrodes are made of the same FM and the central one is a thin film SC. The magnetization of the left FM is chosen to point up and that of the right FM is either up or down. The voltages −V 1 and V 2 (= V − V 1 ) are applied to the left and right electrodes, respectively. We assume...
We fabricated a spin-torque oscillator (STO) having a nanopillar-shaped magnetic tunnel junction with perpendicularly magnetized FeB free and in-plane magnetized CoFeB reference layers. The perpendicular magnetization of the FeB was stabilized by strong perpendicular magnetic anisotropy induced at both the MgO tunnel barrier/FeB and FeB/MgO cap interfaces. Under a perpendicular field (3 kOe), the STO exhibited a large emission power (0.55 W), a high frequency (6.3 GHz) and a high Q factor (135) simultaneously, all of which are the largest to date among nanopillar-shaped STOs. The bias voltage dependence of the oscillation property was well explained by the macrospin model.
Highly sensitive microwave devices that are operational at room temperature are important for high-speed multiplex telecommunications. Quantum devices such as superconducting bolometers possess high performance but work only at low temperature. On the other hand, semiconductor devices, although enabling high-speed operation at room temperature, have poor signal-to-noise ratios. In this regard, the demonstration of a diode based on spin-torque-induced ferromagnetic resonance between nanomagnets represented a promising development, even though the rectification output was too small for applications (1.4 mV mW(-1)). Here we show that by applying d.c. bias currents to nanomagnets while precisely controlling their magnetization-potential profiles, a much greater radiofrequency detection sensitivity of 12,000 mV mW(-1) is achievable at room temperature, exceeding that of semiconductor diode detectors (3,800 mV mW(-1)). Theoretical analysis reveals essential roles for nonlinear ferromagnetic resonance, which enhances the signal-to-noise ratio even at room temperature as the size of the magnets decreases.
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.
The oscillation properties of a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer are studied based on an analysis of the energy balance between spin torque and damping. The critical value of an external magnetic field applied normal to the film plane is found, below which the controllable range of the oscillation frequency by the current is suppressed. The value of the critical field depends on the magnetic anisotropy, the saturation magnetization, and the spin torque parameter.
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.
Generation of practical random numbers (RNs) by a spintronics-based, scalable truly RN generator called “spin dice” was demonstrated. The generator utilizes the stochastic nature of spin-torque switching in a magnetic tunnel junction (MTJ) to generate RNs. We fabricated eight perpendicularly magnetized MTJs on a single-board circuit and generated eight sequences of RNs simultaneously. The sequences of RNs of different MTJs were not correlated with each other, and performing an exclusive OR (XOR) operation among them improved the quality of the RNs. The RNs obtained by performing a nested XOR operation passed the statistical test of NIST SP-800 with the appropriate pass rate.
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