耦合于铁磁电极平行双量子点的自旋极化输运

利用双杂质的Anderson模型的哈密顿量,从理论上研究了耦合于铁磁电极的平行双量子点的自旋极化输运性质,并借助运动方程方法求解了哈密顿量.结果表明,该系统在费米能级处的Kondo共振峰与自旋极化强度和磁通量的取值有关.与此同时,在平行组态情况下,Kondo共振峰位置发生了偏移,而在反平行组态情况下,Kondo共振峰出现在相同位置处.这些现象使得这一双量子点系统的物理特性更加丰富,它们将有助于解释自旋电子学中的电子关联问题.     

与铁磁导体耦合的量子点中非线性热自旋效应

研究与两个铁磁导体耦合的单个量子点中热梯度产生的纯自旋流。发现热梯度和电子库的铁磁性会在量子点能级离开电子-空穴对称点时产生较强的自旋压。自旋压的大小和方向可以通过改变热梯度的方向来调整。当两个铁磁引线的磁化方向为相互平行时,自旋压的绝对值最小,而当两个引线中的磁矩为反平行时,自旋压的强度会显著增大。     

微波场作用下Fano效应对量子点电子态密度近藤峰的影响

讨论了一个微波场辐照下量子点电极耦合体系,当两边电极间存在非共振直接隧穿时量子．占、上电子态密度的变化情况。用非平衡格林函数方法及吴大琪假设得到了此体系能态密度在相互作用强度U有限情况下的解析表达式。数值计算的结果表明随着背景透射率及库仑相互作用能大小的变化,量子点上电子能态密度共振峰可被增强或减弱,并可能出现新的共振峰结构。     

与铁磁电极耦合的双量子点中近藤效应的研究

研究了与铁磁电极耦合的串连双量子点中的平衡和非平衡近藤效应，同时考虑了两侧电极中自旋极化的态密度为平行和反平行的情况。结果表明，每个量子点的平衡态密度在平行情况下只有一个尖峰，当有外加偏压的时候，这个尖峰将分裂为两个；在反平行情况下，每个量子点的平衡态密度有两个尖峰，分别对应与电子的两种不同自旋取向，自旋向下和向上的态密度双峰之间的距离将在外加偏压的影响下分别增大和减少。在反平行情况下，每一种自旋成份的微分电导只有一个尖峰，并且分别在费米面之上和之下；而在平行情况下，每种自旋成份的微分电导都有两个尖峰，其中自旋朝下的微分电导尖峰较高。     

交换耦合双能级量子点的全计数统计

基于粒子数分辨的量子主方程,研究了自旋交换耦合双能级量子点中的电子全计数统计,给出了平均电流、散粒噪声和偏斜度.库仑排斥和交换作用导致了平均电流中的小平台;散粒噪声和铁磁电极的极化率以及量子点与电极的不对称耦合密切相关.极化率和量子点电极不对称耦合越大,超泊松散粒噪声越容易出现.当两个自旋单态通道能级参与输运时散粒噪声会大大降低,而当两个自旋三重态通道能级同时参与输运时散粒噪声则会明显增大.     

电子有效质量随位置变化对半导体量子点杂质态结合能的影响

在连续介电模型和有效质量近似下,考虑电子有效质量随位置的变化,利用变分法从理论上研究了半导体有限高势垒球形量子点中杂质态的结合能.数值计算了AlxGa1-xAs/GaAs球形量子点杂质态基态结合能随量子点尺寸和垒材料Al组分的变化关系,讨论了有效质量随位置变化对基态结合能的影响,并与不考虑有效质量随位置变化做了比较.结果表明:当量子点半径较小时,电子有效质量随位置的变化增加了杂质态基态结合能,随量子点半径增大,杂质态基态结合能的增加幅度变小;量子点半径较大时,电子有效质量随位置变化降低了杂质态基态结合能.随着Al组分增大,杂质态基态结合能单调递增.     

尺寸和边界形貌对三角锯齿型石墨烯量子点的磁性影响

采用约束路径量子蒙特卡罗计算方法和基于密度泛函理论的第一性原理数值计算方法,研究了不同尺寸的规则三角锯齿型石墨烯量子点（ZZ）和边界重构后的三角锯齿型石墨烯量子点（ZZST）的磁学特性和电子结构特征．2种方法的计算结果均表明：在所有尺寸下的ZZ的基态均处于铁磁态;当边界重构后,除了尺寸很小的外,其他尺寸的基态均处于反铁磁态．通过Drool^3软件包计算费米能级附近的上下自旋电子态密度分布,发现ZZ在费米能级附近出现明显自旋向上和自旋向下的态密度分布的劈裂,边界重构后费米能级附近的自旋极化被弱化,表明不规则的边界对石墨烯量子点的磁性具有抑制作用．     

T型双量子点自旋二极管

用非平衡格林函数理论研究T型耦合的双量子点中自旋二极管效应。量子点与两个电子库或引线相耦合,其中的一个是正常金属,另一个为铁磁材料。由于两个引线铁磁性的不对称,流过系统的电荷流或者点中的电子占据数会在一定情况下出现二极管效应,即在正向电压时电流或占据数的自旋极化有极小值,当电压反向时,其自旋极化有极大值。这种自旋二极管效应与通常电子器件中的电荷二极管现象非常相似,在自旋电子学器件中有实际的应用价值。     

准一维有机聚合物铁磁体中的电子-电子相互作用

考虑电子-电子相互作用，对具有链间耦合的准-维有机聚合物铁磁体的电子结构和自旋结构进行了研究。结果表明，对于维持系统的铁磁态的稳定性而言，系统内的在位电子-电子Hubbard排斥相互作用与最近邻格点间的电子-电子Coulomb排斥相互作用所起的作用相反，彼此间存在着竞争；最近邻格点间的电子-电子Coulomb排斥相互作用的加强将导致主链反铁磁性自旋密度波(SDW)之振幅的减小，使得主链反铁磁性SDW的耦合传递作用减弱，进而影响到侧自由基自旋间的铁磁耦合强度，这将削弱系统铁磁态的稳定性。     

Rashba效应影响下半导体量子点中强耦合极化子的声子平均数

采用改进的线性组合算符方法，研究了Rashba效应影响下半导体量子点中强耦合极化子的光学声子平均数．导出在电子-体纵光学声子（LO）强耦合时抛物量子点中极化子的光学声子平均数、振动频率、相互作用能和有效质量随受限强度和Rashba自旋-轨道耦合常数的变化．数值计算结果表明Rashba自旋-轨道相互作用使极化子的有效质量、基态能分裂为上下两支，随耦合常数的增加极化子基态能量、有效质量表现为增加和较少两种截然相反的情形；Rashba自旋-轨道相互作用影响下强耦合极化子的光学声子平均数随量子点的受限强度、电子声子耦合强度增大而增大，极化子的相互作用能随受限强度的增加先急剧增加，当达到极值后随受限强度的增加而急剧减少．     

纳米材料锰掺杂量子点中的近藤效应

采用非平衡格林函数方法,探讨了量子点掺杂锰原子纳米材料的低温输运特性。结果表明,输运受到掺杂锰原子的强烈影响。具体来说,在微分电导中出现（2S＋1）个近藤劈裂峰,锰原子表现为一个多值量子化有效磁场,导致这种劈裂。     

Quantum phase transition in a single-molecule quantum dot

Quantum criticality is the intriguing possibility offered by the laws of quantum mechanics when the wave function of a many-particle physical system is forced to evolve continuously between two distinct, competing ground states. This phenomenon, often related to a zero-temperature magnetic phase transition, is believed to govern many of the fascinating properties of strongly correlated systems such as heavy-fermion compounds or high-temperature superconductors. In contrast to bulk materials with very complex electronic structures, artificial nanoscale devices could offer a new and simpler means of understanding quantum phase transitions. Here we demonstrate this possibility in a single-molecule quantum dot, where a gate voltage induces a crossing of two different types of electron spin state (singlet and triplet) at zero magnetic field. The quantum dot is operated in the Kondo regime, where the electron spin on the quantum dot is partially screened by metallic electrodes. This strong electronic coupling between the quantum dot and the metallic contacts provides the strong electron correlations necessary to observe quantum critical behaviour. The quantum magnetic phase transition between two different Kondo regimes is achieved by tuning gate voltages and is fundamentally different from previously observed Kondo transitions in semiconductor and nanotube quantum dots. Our work may offer new directions in terms of control and tunability for molecular spintronics.     

Quantum quench of Kondo correlations in optical absorption

The interaction between a single confined spin and the spins of an electron reservoir leads to one of the most remarkable phenomena of many-body physics--the Kondo effect. Electronic transport measurements on single artificial atoms, or quantum dots, have made it possible to study the effect in great detail. Here we report optical measurements on a single semiconductor quantum dot tunnel-coupled to a degenerate electron gas which show that absorption of a single photon leads to an abrupt change in the system Hamiltonian and a quantum quench of Kondo correlations. By inferring the characteristic power-law exponents from the experimental absorption line shapes, we find a unique signature of the quench in the form of an Anderson orthogonality catastrophe, induced by a vanishing overlap between the initial and final many-body wavefunctions. We show that the power-law exponent that determines the degree of orthogonality can be tuned using an external magnetic field, which unequivocally demonstrates that the observed absorption line shape originates from Kondo correlations. Our experiments demonstrate that optical measurements on single artificial atoms offer new perspectives on many-body phenomena previously studied using transport spectroscopy only.     

Kondo resonance in a single-molecule transistor

When an individual molecule, nanocrystal, nanotube or lithographically defined quantum dot is attached to metallic electrodes via tunnel barriers, electron transport is dominated by single-electron charging and energy-level quantization. As the coupling to the electrodes increases, higher-order tunnelling and correlated electron motion give rise to new phenomena, including the Kondo resonance. To date, all of the studies of Kondo phenomena in quantum dots have been performed on systems where precise control over the spin degrees of freedom is difficult. Molecules incorporating transition-metal atoms provide powerful new systems in this regard, because the spin and orbital degrees of freedom can be controlled through well-defined chemistry. Here we report the observation of the Kondo effect in single-molecule transistors, where an individual divanadium molecule serves as a spin impurity. We find that the Kondo resonance can be tuned reversibly using the gate voltage to alter the charge and spin state of the molecule. The resonance persists at temperatures up to 30 K and when the energy separation between the molecular state and the Fermi level of the metal exceeds 100 meV.     

Kondo physics in carbon nanotubes

The connection of electrical leads to wire-like molecules is a logical step in the development of molecular electronics, but also allows studies of fundamental physics. For example, metallic carbon nanotubes are quantum wires that have been found to act as one-dimensional quantum dots, Luttinger liquids, proximity-induced superconductors and ballistic and diffusive one-dimensional metals. Here we report that electrically contacted single-walled carbon nanotubes can serve as powerful probes of Kondo physics, demonstrating the universality of the Kondo effect. Arising in the prototypical case from the interaction between a localized impurity magnetic moment and delocalized electrons in a metallic host, the Kondo effect has been used to explain enhanced low-temperature scattering from magnetic impurities in metals, and also occurs in transport through semiconductor quantum dots. The far greater tunability of dots (in our case, nanotubes) compared with atomic impurities renders new classes of Kondo-like effects accessible. Our nanotube devices differ from previous systems in which Kondo effects have been observed, in that they are one-dimensional quantum dots with three-dimensional metal (gold) reservoirs. This allows us to observe Kondo resonances for very large electron numbers (N) in the dot, and approaching the unitary limit (where the transmission reaches its maximum possible value). Moreover, we detect a previously unobserved Kondo effect, occurring for even values of N in a magnetic field.     

Orbital Kondo effect in carbon nanotubes

Progress in the fabrication of nanometre-scale electronic devices is opening new opportunities to uncover deeper aspects of the Kondo effect--a characteristic phenomenon in the physics of strongly correlated electrons. Artificial single-impurity Kondo systems have been realized in various nanostructures, including semiconductor quantum dots, carbon nanotubes and individual molecules. The Kondo effect is usually regarded as a spin-related phenomenon, namely the coherent exchange of the spin between a localized state and a Fermi sea of delocalized electrons. In principle, however, the role of the spin could be replaced by other degrees of freedom, such as an orbital quantum number. Here we show that the unique electronic structure of carbon nanotubes enables the observation of a purely orbital Kondo effect. We use a magnetic field to tune spin-polarized states into orbital degeneracy and conclude that the orbital quantum number is conserved during tunnelling. When orbital and spin degeneracies are present simultaneously, we observe a strongly enhanced Kondo effect, with a multiple splitting of the Kondo resonance at finite field and predicted to obey a so-called SU4 symmetry.     

Kondo effect in an integer-spin quantum dot

The Kondo effect--a many-body phenomenon in condensed-matter physics involving the interaction between a localized spin and free electrons--was discovered in metals containing small amounts of magnetic impurities, although it is now recognized to be of fundamental importance in a wide class of correlated electron systems. In fabricated structures, the control of single, localized spins is of technological relevance for nanoscale electronics. Experiments have already demonstrated artificial realizations of isolated magnetic impurities at metallic surfaces, nanoscale magnets, controlled transitions between two-electron singlet and triplet states, and a tunable Kondo effect in semiconductor quantum dots. Here we report an unexpected Kondo effect in a few-electron quantum dot containing singlet and triplet spin states, whose energy difference can be tuned with a magnetic field. We observe the effect for an even number of electrons, when the singlet and triplet states are degenerate. The characteristic energy scale is much larger than in the ordinary spin-1/2 case.     

Optical pumping of a single hole spin in a quantum dot

The spin of an electron is a natural two-level system for realizing a quantum bit in the solid state. For an electron trapped in a semiconductor quantum dot, strong quantum confinement highly suppresses the detrimental effect of phonon-related spin relaxation. However, this advantage is offset by the hyperfine interaction between the electron spin and the 10(4) to 10(6) spins of the host nuclei in the quantum dot. Random fluctuations in the nuclear spin ensemble lead to fast spin decoherence in about ten nanoseconds. Spin-echo techniques have been used to mitigate the hyperfine interaction, but completely cancelling the effect is more attractive. In principle, polarizing all the nuclear spins can achieve this but is very difficult to realize in practice. Exploring materials with zero-spin nuclei is another option, and carbon nanotubes, graphene quantum dots and silicon have been proposed. An alternative is to use a semiconductor hole. Unlike an electron, a valence hole in a quantum dot has an atomic p orbital which conveniently goes to zero at the location of all the nuclei, massively suppressing the interaction with the nuclear spins. Furthermore, in a quantum dot with strong strain and strong quantization, the heavy hole with spin-3/2 behaves as a spin-1/2 system and spin decoherence mechanisms are weak. We demonstrate here high fidelity (about 99 per cent) initialization of a single hole spin confined to a self-assembled quantum dot by optical pumping. Our scheme works even at zero magnetic field, demonstrating a negligible hole spin hyperfine interaction. We determine a hole spin relaxation time at low field of about one millisecond. These results suggest a route to the realization of solid-state quantum networks that can intra-convert the spin state with the polarization of a photon.