Spin coupling and orbital angular momentum quenching in free iron clusters

Magnetic spin and orbital moments of size-selected free iron cluster ions Fe{n}{+} (n=3-20) have been determined via x-ray magnetic circular dichroism spectroscopy. Iron atoms within the clusters exhibit ferromagnetic coupling except for Fe{13}{+}, where the central atom is coupled antiferromagnetically to the atoms in the surrounding shell. Even in very small clusters, the orbital magnetic moment is strongly quenched and reduced to 5%-25% of its atomic value while the spin magnetic moment remains at 60%-90%. This demonstrates that the formation of bonds quenches orbital angular momenta in homonuclear iron clusters already for coordination numbers much smaller than those of the bulk.     

Gate-dependent orbital magnetic moments in carbon nanotubes

We investigate how the orbital magnetic moments of electron and hole states in a carbon nanotube quantum dot depend on the number of carriers on the dot. Low temperature transport measurements are carried out in a setup where the device can be rotated in an applied magnetic field, thus enabling accurate alignment with the nanotube axis. The field dependence of the level structure is measured by excited state spectroscopy and excellent correspondence with a single-particle calculation is found. In agreement with band structure calculations we find a decrease of the orbital magnetic moment with increasing electron or hole occupation of the dot, with a scale given by the band gap of the nanotube.     

Measurement of a pauli and orbital paramagnetic state in bulk gold using x-ray magnetic circular dichroism spectroscopy

We show that bulk gold (Au) exhibits temperature-independent paramagnetism in an external magnetic field by x-ray magnetic circular dichroism spectroscopy at the Au L(2) and L(3) edges. Using the sum-rule analysis, we obtained a magnetic moment of 1.3 × 10(-4) μB/atom in an external magnetic field of 10 T and a paramagnetic susceptibility of 8.9 × 10(-6) for the 5d orbit. The induced paramagnetism in bulk Au is characterized by a large (≈ 30%) orbital contribution. This orbital component was retained even when Au atoms formed nanoparticles, playing an important role in stabilizing the spontaneous spin polarization in the Au nanoparticles.     

Kondo-transport spectroscopy of single molecule magnets

We demonstrate that in a single molecule magnet strongly coupled to electrodes the Kondo effect involves all magnetic excitations. This Kondo effect is induced by the quantum tunneling of the magnetic moment. Importantly, the Kondo temperature TK can be much larger than the magnetic splittings. We find a strong modulation of the Kondo effect as a function of the transverse anisotropy parameter or a longitudinal magnetic field. Both for integer and half-integer spin this can be used for an accurate transport spectroscopy of the magnetic states in low magnetic fields on the order of the easy-axis anisotropy parameter. We set up a relationship between the Kondo effects for successive integer and half-integer spins.     

Orbital magnetic moment of a quantum well and a quantum dot in crossed magnetic and electric fields

A new physical effect, namely, oscillations of the orbital magnetic moment with a change in the electric field strength in two types of nanostructures, has been predicted. Explicit analytical expressions for the orbital magnetic moment of a quantum well and a quantum dot in crossed magnetic and electric fields have been derived. The oscillations of the orbital magnetic moment with a change in the electric and magnetic fields have been studied. The oscillation periods in both the electric and magnetic fields have been found and the limiting cases of the strong magnetic and quantum confinement effects have been considered.     

Spin and orbital magnetic moments of free nanoparticles

The determination of spin and orbital magnetic moments from the free atom to the bulk phase is an intriguing challenge for nanoscience, in particular, since most magnetic recording materials are based on nanostructures. We present temperature-dependent x-ray magnetic circular dichroism measurements of free Co clusters (N=8-22) from which the intrinsic spin and orbital magnetic moments of noninteracting magnetic nanoparticles have been deduced. An exceptionally strong enhancement of the orbital moment is verified for free magnetic clusters which is 4-6 times larger than the bulk value. Our temperature-dependent measurements reveal that the spin orientation along the external magnetic field is nearly saturated at ~20 K and 7 T, while the orbital orientation is clearly not.     

Spin-orbit coupling and anisotropy of spin splitting in quantum dots

In lateral quantum dots, the combined effect of both Dresselhaus and Bychkov-Rashba spin-orbit coupling is equivalent to an effective magnetic field +/- B(SO) which has the opposite sign for s(z)= +/- 1/2 spin electrons. When the external magnetic field is perpendicular to the planar structure, the field B(SO) generates an additional splitting for electron states as compared to the spin splitting in the in-plane field orientation. The anisotropy of spin splitting has been measured and then analyzed in terms of spin-orbit coupling in several AlGaAs/GaAs quantum dots by means of resonant tunneling spectroscopy. From the measured values and sign of the anisotropy we are able to determine the dominating spin-orbit coupling mechanism.     

微扰法计算中等强磁场中氢原子的能级

本文在考虑氢原子轨道运动磁矩与磁场之间、自旋磁矩与磁场之间和感生磁矩与外磁场之间的相互作用的基础上,根据角动量和球谐函数的性质,应用简并态微扰方法研究了在中等强磁场中氢原子的能级,给出了计算中等强磁场中氢原子的一级近似能级的方法,具体计算了 T范围内氢原子 的各能级的数值,结果与有关文献给出的理论计算值是相近的,表明本文所给出的方法是简单的、计算结果是正确的。     

Electric dipole spin resonance for heavy holes in quantum dots

We propose and analyze a new method for manipulation of a heavy-hole spin in a quantum dot. Because of spin-orbit coupling between states with different orbital momenta and opposite spin orientations, an applied rf electric field induces transitions between spin-up and spin-down states. This scheme can be used for detection of heavy-hole spin resonance signals, for the control of the spin dynamics in two-dimensional systems, and for determining important parameters of heavy holes such as the effective g factor, mass, spin-orbit coupling constants, spin relaxation, and decoherence times.     

First-principles studies of the magnetic hyperfine field in Fe multilayers

A detailed theoretical study of the magnetic moments and magnetic hyperfine fields in several Fe multilayers (Fe fcc(001)/5X fcc(001), X=Cu and Ag, and Fe bcc(001)/5X fcc(001), X=Ag and Au) as well as in bulk Fe is presented. The calculations have been performed using the spin-polarized, relativistic linear muffin-tin orbital (SPR-LMTO) method of band structure calculation. Therefore, not only the contribution to the hyperfine fields due to the conventional Fermi contact interaction but also due to the spin dipolar and orbital contributions induced by the crystal field and by spin-orbit coupling are accounted for. To decompose the hyperfine field of non-s-electrons into these contributions it has been assumed that they are proportional to the corresponding so-called magnetic dipole moment and the orbital magnetic moment, respectively. In contrast to previous results for pure metals and alloys not only the orbital but also the spin dipolar hyperfine field was found to be non-negligible. The anisotropy of the hyperfine field determined by calculations for in-plane and perpendicular orientation of the magnetisation was found to be very pronounced and closely connected with the corresponding anisotropy of the magnetic dipole moment and the orbital moment.     

RKKY ferromagnetism with Ising-like spin states in intercalated Fe(1/4)TaS2

We investigated the magnetic nature of Fe(1/4)TaS2 using x-ray absorption spectroscopy, photoemission spectroscopy, and first principles band calculations. The results show a large unquenched orbital magnetic moment (～1.0 μ(B)/Fe) at intercalated Fe sites, resulting in a gigantic magnetic anisotropy (H(A)?60 T). The magnetic coupling is well understood in terms of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, suggesting a novel RKKY ferromagnet with Ising-type spin states. We also found that this indirect exchange coupling between the neighboring Fe spins is ferromagnetic and maximized at the Fe-Fe distance of 2×2 superstructure.     

单层正三角锯齿型石墨烯量子点的电子结构和磁性

采用基于密度泛函理论的广义梯度近似(GGA),对不同尺寸(N=2—11)的单层正三角锯齿型石墨烯量子点(Z_N -GNDs)的结构进行优化,得到与实验数据较好符合的晶格常数,进一步计算得到不同尺寸下体系的自旋多重度、磁矩、电子态密度以及自旋电子密度.结果表明:所有体系都呈现金属性,在尺寸较小的体系中量子尺寸效应对电子结构的影响比较明显;与单层石墨烯片一样,sp²杂化作用和非键态电子在量子点中仍起到非常重要的作用;费米能级上有自旋向上的电子分布,体系的 关键词： 石墨烯 量子点 电子结构 磁性     

Ni5Nd2B4的电子结构和磁性能研究

采用LSDA（Local spin-density approximation）近似及LSDA+U（在位库伦势）近似模拟金属间化合物Ni5Nd2B4的磁性能对于R-M-B合金特性的研究具有重要意义。研究结果显示,LSDA近似下,Ni5Nd2B4具备金属导体性质,晶体结构中最紧邻Ni、B原子间杂化成键,最紧邻Ni-Ni共价成键,Nd、B原子形成成键分子轨道作用,Ni原子间存在自旋消弱现象;LSDA+U近似下,Nd原子磁矩提供体系磁性来源,由于自旋排斥作用Ni原子电子与Nd原子电子自旋方向相反,体系在U值约为6.35eV的作用下能较理想的处理体系电子作用。     

Ni5Nd2B4的电子结构和磁性能研究

采用LSDA（Local spin-density approximation）近似及LSDA+U（在位库伦势）近似模拟金属间化合物Ni5Nd2B4的磁性能对于R-M-B合金特性的研究具有重要意义。研究结果显示,LSDA近似下,Ni5Nd2B4具备金属导体性质,晶体结构中最紧邻Ni、B原子间杂化成键,最紧邻Ni-Ni共价成键,Nd、B原子形成成键分子轨道作用,Ni原子间存在自旋消弱现象;LSDA+U近似下,Nd原子磁矩提供体系磁性来源,由于自旋排斥作用Ni原子电子与Nd原子电子自旋方向相反,体系在U值约为6.35eV的作用下能较理想的处理体系电子作用。     

Field dependence of spin and orbital moments of Fe in L10 FePt magnetic thin films

The field dependence of spin and orbital magnetic moments of Fe in L1₀ FePt magnetic thin films was investigated using X-ray magnetic circular dichroism (XMCD). The spin and orbital moments were calculated using the sum rules; it was found that the spin and orbital moment of Fe in L1₀ FePt films are ∼2.5 and 0.2 μ_B, respectively. The relative XMCD asymmetry at Fe L₃ peak on the dependence of applied field suggested that the majority magnetic moment of L1₀ FePt films resulted from Fe.     

Few-electron molecular states and their transitions in a single InAs quantum dot molecule

We study electronic configurations in a single pair of vertically coupled self-assembled InAs quantum dots, holding just a few electrons. By comparing the experimental data of nonlinear single-electron transport spectra in a magnetic field with many-body calculations, we identify the spin and orbital configurations to confirm the formation of molecular states by filling both the quantum mechanically coupled symmetric and antisymmetric states. Filling of the antisymmetric states is less favored with increasing magnetic field, and this leads to various magnetic field induced transitions in the molecular states.     

Evaluation of electron--electron interactions in coupled quantum dots by using far-infrared spectra

We have studied the far-infrared spectra of two-electron vertically coupled quantum dots in an axial magnetic field by exact diagonalization. The calculated results show an obvious difference in role between the interactions for spin S = 1 and for spin S = O. The results support the possibility to evaluate the interactions by far-infrared spectroscopy in vertically coupled quantum dots.     

Optical spectroscopy on semiconductor quantum dots in high magnetic fields

We review the recent literature on the use of optical spectroscopy of semiconductor quantum dots in high magnetic fields. We address both self-assembled epitaxial dots and colloidal nanocrystal quantum dots, each of which has its own characteristic optical response. Combining simple theoretical models for quantum confinement with the effect of high magnetic fields we describe the basic optically allowed transitions expected for epitaxial and colloidal quantum dots. Within these models we discuss the effects of quantum confinement and orbital and spin Zeeman effects on the optical spectra, illustrated by experimental examples. Finally, effects of electron–electron and exchange interactions are addressed.     

Excitation of the spin density and current by coherent light pulses in quantum wells

We study the orbital and spin dynamics of charge carriers induced by non-overlapping linearly polarized light pulses in semiconductor quantum wells. It is shown that such an optical excitation with coherent pulses leads to a spin orientation of photocarriers and an electric current. The effects are caused by the interference of optical transitions driven by individual pulses. The distribution of carriers in the spin and momentum spaces depends on the crystallographic orientation of quantum wells and can be efficiently controlled by the pulse polarizations, time delay and phase shift between the pulses, as well as an external magnetic field.     

Optical spin orientation of a single manganese atom in a quantum dot

The magnetic state of a single magnetic atom (Mn) embedded in an individual semiconductor quantum dot is optically probed using micro-spectroscopy. A high degree of spin polarization can be achieved for an individual Mn atom localized in a quantum dot using quasi-resonant or fully-resonant optical excitation at zero magnetic field. Optically created spin polarized carriers generate an energy splitting of the Mn spin and enable magnetic moment orientation controlled by the photon helicity and energy. The dynamics and the magnetic field dependence of the optical pumping mechanism shows that the spin lifetime of an isolated Mn atom at zero magnetic field is controlled by a magnetic anisotropy induced by the built-in strain in the quantum dots. The Mn spin distribution prepared by optical pumping is fully conserved for a few microseconds. This opens the way to full optical control of the spin state of an individual magnetic atom in a solid state environment.