Neutrino induced magnetic moment and spin precession

When propagating through a dispersing medium, a massive neutrino acquires an induced magnetic moment that may give rise to a helicity flip in an external magnetic field with a larger probability than that caused by the anomalous magnetic moment. This phenomenon is investigated in the framework of relativistic quantum mechanics and of the generalized Bargmann–Michel–Telegdi equation.     

Quantum modulation effect in a graphene-based magnetic tunnel junction

The electronic (quantum) transport in a NG/F_B/FG tunnel junction (where NG, F_B and FG are a normal graphene layer, a ferromagnetic barrier connected to a gate and a ferromagnetic graphene layer, respectively) is investigated. The motions of the electrons in the graphene layers are taken to be governed by the Dirac Equation. Parallel (P) and antiparallel alignment (AP) of the magnetizations in the barrier and in the ferromagnetic graphene are considered. Our work focuses on the oscillation of the electrical conductance (Gq), of the spin conductance (Gs) and of the tunneling magneto resistance (TMR) of this magnetic tunnel junction. We find that, the quantum modulation due to the effect of the exchange field in F_B will be seen in the plots the conductance and of the TMR as functions of the thickness of ferromagnetic barrier (L). The period of two multiplied sinusoidal terms of the modulation are seen to be controlled by varying the gate potential and the exchange field of the F_B layer. The phenomenon, a quantum beating, is built up with two oscillating spin conductance components which have different periods of oscillation related to the splitting of Dirac's energies in the F_B region. The amplitudes of oscillations of Gq, Gs and TMR are not seen to decrease as the thickness increases. The decaying behaviors seen in the conventional transport through an insulator do not appear.     

Enhanced spin polarization in an asymmetric magnetic graphene superlattice

The authors investigate the spin-resolved transport through an asymmetrical magnetic graphene superlattice (MGS) consisting of the periodic barriers with abnormal one in height. To quantitatively depict the asymmetrical MGS, an asymmetry factor has been introduced to measure the height change of the abnormal barrier. It is shown that the spin filter effect is strongly enhanced by the barrier asymmetry both in the Klein and the classical tunneling regimes. In the presence of abnormal barrier, the conductance with certain spin direction is suppressed with respect to different tunneling regimes, and thus high spin polarization with opposite sign can be achieved.     

Optical studies of spin precession in magnetic two-dimensional electron gases

Magnetic two-dimensional electron gases are studied using time-resolved Kerr and Faraday rotation spectroscopy in the Voigt geometry. The data directly reveal both electron and Mn spin precession in modest transverse fields. Scattering by Mn ions dominates the electron spin relaxation processes in these materials, and prevents the electron gas from acquiring a long-lived spin polarization as observed in non-magnetic structures. Nonetheless, a persistent Mn spin polarization occurs which creates a oscillating magnetic field within the electron gas for hundreds of picoseconds.     

Thomas precession and spin

With the help of the infinitesimal covariant Lorentz transformations, it is demonstrated that the Bargmann-Michel-Telegdi classical spin equation naturally involves the Thomas precession. The same results are obtained using the alternative phenomenological approach. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 81–86, November, 2006.     

Negative muon precession in nuclei with spin

Polarized negative muons were stopped in various materials containing nuclei with nonzero spin. The TF-SR precession signal of theF ⁺ hyperfine state (frequencyv ₊ > 0) was pronounced for ^–Li and ^–Be, faint for ^–Cl (in NiCl₂), and undetectable for ^–F (in CaF₂ or NiF₂) and ^–P. TheF ^– signal (frequencyv _– < 0) was observed clearly for ^–Be, ^–Al and ^–Na, marginally for ^–K, ^–V and ^–Nb, and not at all for ^–Ga (at 4 kOe) or ^–Co (ferrromagnetic, zero field). In the heavier elements theF ^– signal is fed by transitions from theF ⁺ to theF ^– state at a rateR, as long asR(v ₊ –v _–). [See separate paper on ^–Al in these Proceedings, p. 879.]I am grateful to Alex Schenck and Bruce Patterson for the loan of equipment and samples, to Fred Gygax for helping set up the apparatus, to Jun Imazato for help with the experiment, and to Toshi Yamazaki and Tak Suzuki for valuable comments. I am also indebted to SIN for several weeks of free ^– beam time and to SIN, BOOM, and the University of Tokyo for free time on their VAX computers, which were kept busy for several months in the analysis.     

Interaction-driven spin precession in quantum-dot spin valves

We analyze spin-dependent transport through spin valves composed of an interacting quantum dot coupled to two ferromagnetic leads. The spin on the quantum dot and the linear conductance as a function of the relative angle theta of the leads' magnetization directions is derived to lowest order in the dot-lead coupling strength. Because of the applied bias voltage spin accumulates on the quantum dot, which for finite charging energy experiences a torque, resulting in spin precession. The latter leads to a nontrivial, interaction-dependent, theta dependence of the conductance. In particular, we find that the spin-valve effect is reduced for all theta not equal pi.     

半导体核磁共振显微压力的质子全自旋量子门的实现

以实现质子全自旋量子门、观察半导体核子自旋态和量子计算为目的, 依据样品的自旋-晶格弛豫时间和自旋-自旋弛豫时间,采用脉冲调制序列控制磁共振的条件和翻转旋转框架,计算了共振显微压力. 结果表明,质子全自旋量子门具有高灵敏度和高Q操控性,通过扫描片段和激光干涉可以得到磁共振压力. 共振压力兼具MRI和AFM优点,是一种强有力的通过核自旋实现量子计算获得量子信息的有效方法. 关键词： 空间分辨率 共振显微 半导体光刻 电子束刻印     

Electronic spin precession and interferometry from spin-orbital entanglement in a double quantum dot

A double quantum dot inserted in parallel between two metallic leads can entangle the electron spin with the orbital (dot index) degree of freedom. An Aharonov-Bohm orbital phase can be transferred to the spinor wave function, providing a geometrical control of the spin precession around a fixed magnetic field. A fully coherent behavior occurs in a mixed orbital-spin Kondo regime. Evidence for the spin precession can be obtained, either using spin-polarized metallic leads or by placing the double dot in one branch of a metallic loop.     

Long-lived ultrafast spin precession in manganese alloys films with a large perpendicular magnetic anisotropy

Spin precession with frequencies up to 280 GHz is observed in Mn(3-δ)Ga alloy films with a perpendicular magnetic anisotropy constant K(u)～15 M?erg/cm(3). The damping constant α, characterizing macroscopic spin relaxation and being a key factor in spin-transfer-torque systems, is not larger than 0.008 (0.015) for the δ=1.46 (0.88) film. Those are about one-tenth of α values for known materials with large K(u). First-principles calculations well describe both low α and large K(u) for these alloys.     

Correlations between electronic energy bands spin splitting and magnetic profile symmetry of magnetic superlattice

We have theoretically investigated the energy band structures of two typical magnetic superlattices formed by perpendicular or parallel magnetization ferromagnetic stripes periodically deposited on a two-dimensional electron gas (2DEG), where the magnetic profile in the perpendicular magnetization is of inversion anti-symmetry, but of inversion symmetry in parallel magnetization, respectively. We have shown that the energy bands of perpendicular magnetization display the spin-splitting and transverse wave-vector symmetry, while the energy bands of the parallel magnetization exhibit spin degeneration and transverse wave-vector asymmetry. These distinguishing spin-dependent and transverse wave-vector asymmetry features are essential for future spintronics devices applications.     

The effects of large oscillating fields on spin precession in magnetic resonance

Magnetic resonance of a spin system which is acted upon by a large near-resonance oscillating magnetic field transverse to a static field has been studied experimentally and theoretically for many years. The technique of DEMUR (Double Electron Muon Resonance) has many advantages for such studies. This paper will describe the results of an experiment to study the precession of the muonium triplet near magnetic resonance using DEMUR.     

Coherent control of spin precession motion with impulsive magnetic fields of half-cycle terahertz radiation

Coherent control of the precession motion of magnetizations in a single crystal YFeO3 with double half-cycle pulse terahertz waves was demonstrated. Quasiferromagnetic (0.299 THz) and quasiantiferromagnetic (0.527 THz) precession modes were selectively excited by choosing an appropriate interval of two pulses and were observed as free induction decay (FID) signals from the spin system. By observing the circularly polarized FID signals due to ferromagnetic resonance, we also succeeded in confirming directly the energy storage in the spin system and recovery from that to the electromagnetic radiation.     

Larmor spin precession and neutron optics

We consider the neutron-optical phenomena that emerge during the coherent interaction of a neutron with a sample when the neutron spin precesses in a magnetic field. As follows from general considerations, such an interaction gives rise to an extra precession phase, which is added to the Larmor precession phase. This phenomenon can be interpreted as a manifestation of the time delay due to a finite time of the neutron-sample interaction. The Larmor neutron spin precession with a constant frequency serves as a clock for measuring this time delay. We used such a clock to directly measure the difference between the neutron velocity in matter and its vacuum value. We also present the results of the first experiments in which Larmor clocks were used to measure the neutron tunneling time in the resonance of a quasi-bound state and the Bragg diffraction time. Prospects for further applications of the method are discussed.     

On the dispersion relation of magnetoplasmons in a planar graphene-based superlattice

The dispersion relation for magnetoplasmons in a planar superlattice with periodically alternating regions of gapless and gapped modifications of graphene has been derived within the frame of the random-phase approximation. The contribution of virtual transitions between the lower electron miniband and the upper hole miniband to the polarization operator has been taken into account in addition to the contribution of virtual intra-miniband transitions.     

Electronic surface states in superlattice with complex basis

The electronic structure of a semi-infinite superlattice (SL) with a complex basis consisting of two identical wells coupled via two different barriers is investigated. The possibility of the surface state existence for a periodic termination of the SL potential (i.e., without perturbing the SL potential at the surface) is shown for such a two-barrier basis, in contrast to the conventional one of single well and barrier. This main conclusion is based on analytical considerations, however, some numerical results for the GaAs/AlAs SL are also presented.     

On the variation with modulation wave-length of the Curie temperature of a Heisenberg magnetic superlattice

The variation with modulation wave-length of the Curie temperature of a Heisenberg magnetic superlattice is studied in the molecular field approximation for the cases of square-wave and sinusoidal modulations. The results are tentatively comparable with what are observed experimentally.     

Coherent spin precession in normal Fermi liquids

In the collisionless limit the Fermi liquid interaction gives rise to nondissipative spin currents. Due to these currents, a coherently precessing spin state may be formed in NMR-experiments with normal Fermi liquids. This state consists of two domains separated by a coherently precessing domain wall. In one of the domains the magnetization is oriented along, and in the other opposite, to the direction of the magnetic field. Such a state has been studied in NMR-experiments in liquid³He and³He-⁴He solutions. The conditions necessary for the formation of the state are considered. Possible other substances in which similar states may be observed are discussed.