Generation and manipulation of spin current via a hybrid four-terminal single-molecule junction

We present a new device which consists of a molecular quantum dot (MQD) attached to a normal-metal, two ferromagnetic (FM), and a superconducting leads. The spin-related Andreev reflection (AR) current and the spin-dependent single-particle tunneling current through the normal-metal terminal are obtained, and it is found that the spin current exhibits the transistor-like behavior. The joint effects of the coherent spin flip and the angle between magnetic moments of the two FM leads on the spin current are also studied, these results provide the possibility to manipulate the spin current with the system parameters.     

Influence of dephasing on the entanglement teleportation via a two-qubit Heisenberg XYZ system

We study the entanglement dynamics of an anisotropic two-qubit Heisenberg XYZ system in the presence of intrinsic decoherence. The usefulness of such a system for performance of the quantum teleportation protocol T₀\mathcal{T}_0 and entanglement teleportation protocol T₁\mathcal{T}_1 is also investigated. The results depend on the initial conditions and the parameters of the system. The roles of system parameters such as the inhomogeneity of the magnetic field b and the spin-orbit interaction parameter D, in entanglement dynamics and fidelity of teleportation, are studied for both product and maximally entangled initial states of the resource. We show that for the product and maximally entangled initial states, increasing D amplifies the effects of dephasing and hence decreases the asymptotic entanglement and fidelity of the teleportation. For a product initial state and specific interval of the magnetic field B, the asymptotic entanglement and hence the fidelity of teleportation can be improved by increasing B. The XY and XYZ Heisenberg systems provide a minimal resource entanglement, required for realizing efficient teleportation. Also, in the absence of the magnetic field, the degree of entanglement is preserved for the maximally entangled initial states $\left| {\psi \left. {\left( 0 \right)} \right\rangle = \frac{1} {{\sqrt 2 }}\left( {\left| {\left. {00} \right\rangle \pm } \right|\left. {11} \right\rangle } \right)} \right.$\left| {\psi \left. {\left( 0 \right)} \right\rangle = \frac{1} {{\sqrt 2 }}\left( {\left| {\left. {00} \right\rangle \pm } \right|\left. {11} \right\rangle } \right)} \right.. The same is true for the maximally entangled initial states $\left| {\psi \left. {\left( 0 \right)} \right\rangle = \frac{1} {{\sqrt 2 }}\left( {\left| {\left. {01} \right\rangle \pm } \right|\left. {10} \right\rangle } \right)} \right.$\left| {\psi \left. {\left( 0 \right)} \right\rangle = \frac{1} {{\sqrt 2 }}\left( {\left| {\left. {01} \right\rangle \pm } \right|\left. {10} \right\rangle } \right)} \right., in the absence of spin-orbit interaction D and the inhomogeneity parameter b. Therefore, it is possible to perform quantum teleportation protocol T₀\mathcal{T}_0 and entanglement teleportation T₁\mathcal{T}_1, with perfect quality, by choosing a proper set of parameters and employing one of these maximally entangled robust states as the initial state of the resource.     

Effect of the ac field on a single-molecule magnet bridged between conducting leads

We study quantum spin-rotation effects for a single-molecule magnet bridged between two conducting leads in the ac and dc magnetic fields. The Landau-Zener dynamics induced by the magnetic field generates mechanical torque, making the molecule to oscillate. This mechanical motion of the molecule exhibits unique features that can be detected by measuring the electronic tunneling current through the molecule.     

Mesoscopic transport through toroidal carbon nanotubes threaded with a THz magnetic flux

We have investigated the quantum transport through mesoscopic systems with a toroidal carbon nanotube coupled with two metal leads (N-TCN-N) threaded with an ac magnetic flux. The energy shifting takes place by applying the magnetic flux, and this shifting arises from both the dc and ac components of magnetic flux. The dc magnetic flux induces the periodic variation of energy gap E _g of the TCN, and the ac magnetic flux component always increases the energy gap. As the photon energy is larger than the energy gap , the electrons in the valence band can jump to the conductance band at zero temperature, and the tunneling current appears for , ( ). The differential conductance and tunneling current display clear effect of ac flux by modifying the current oscillation structures. The photon-assisted tunneling current exhibits stair-like I-V characteristics, and it shows different behaviors for different TCN systems. The magnitude of the current is suppressed by the applied ac flux. We also present the time-dependent current evolution, which is contributed by the oscillating current components.Received: 31 May 2004, Published online: 3 August 2004PACS: 73.40.-c Electronic transport in interface structures - 73.63.Fg Nanotubes - 73.61.Wp Fullerenes and related materials - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals     

Spin-flip assisted tunneling through quantum dot based magnetic tunnel junctions

We theoretically study the spin-polarized transport through double barrier magnetic tunnel junction (DBMTJ) consisting of the quantum dot sandwiched by two ferromagnetic (FM) leads. The tunneling current through the DBMTJ is evaluated based on the Keldysh nonequilibrium Green’s function approach. The self-energy and Green’s function of the dot are analytically obtained via the equation of motion method, by systematically incorporating two spin-flip phenomena, namely, intra-dot spin-flip, and spin-flip coupling between the lead and the central dot region. The effects of both spin-flip processes on the spectral functions, tunneling current and tunnel magnetoresistance (TMR) are analyzed. The spin-flip effects result in spin mixing, thus contributing to the spectral function of the off-diagonal Green’s function components ( G_s[`(s)] ^r )\left( {G_{\sigma \bar \sigma }^r } \right). Interestingly, the spin-flip coupling between the lead and dot enhances both the tunneling current and the TMR for applied bias above the threshold voltage V _th . On the other hand, the intra-dot spin-flip results in an additional step in the I-V characteristics near V _th . Additionally, it suppresses the tunneling current but enhances the TMR. The opposing effects of the two types of spin-flip on the tunneling current means that one spin-flip mechanism can be engineered to counteract the other, so as to maintain the tunneling current without reducing the TMR. Their additive effect on the TMR enables the DBMTJ to attain a large tunneling current and high TMR for above threshold bias values.     

Spin-dependent conductance in FM/quantum-dot/FM tunneling system

We present a theoretical study of the spin-dependent conductance spectra in a FM/semiconductor quantum-dot (QD)/FM system. Both the Rashba spin-orbit (SO) coupling in the QD and spin-flip scattering caused by magnetic barrier impurities are taken into account. It is found that in the single-level QD system with parallel magnetic moments in the two FM leads, due to the interference between different tunneling paths through the spin-degenerate level, a dip or a narrow resonant peak can appear in the conductance spectra, which depends on the property of the spin-flip scattering. When the magnetizations of the two FM leads are noncollinear, the resonant peak can be transformed into a dip. The Rashba SO coupling manifests itself by a Rashba phase factor, which changes the phase information of every tunneling path and can greatly modulate the conductance. When the QD has multiple levels, the Rashba interlevel spin-flip effect appears, which changes the topological property of the structure. Its interplay with the Rashba phase can directly tune the coupling strengths between dot and leads, and can result in switching from resonance into antiresonance in the conductance spectra.     

Mechanism of low-voltage field emission from nanocarbon materials

Field emission from nanostructured carbon materials is analyzed by applying the model of emission center in which the emitting surface contains two phases of carbon having substantially different electronic properties. In accordance with this model, the proposed mechanism involves electron tunneling through two potential barriers. The calculated probability of tunneling through two potential barriers implies that the low-voltage field emission observed experimentally can be attributed to the existence of resonant surface states. Numerical estimates suggest that the emission current can increase by at least four orders of magnitude owing to resonant tunneling through two potential barriers.     

Inelastic tunneling of electrons through a quantum dot with an embedded single molecular magnet

We report a theoretical analysis of electron transport through a quantum dot with an embedded biaxial single-molecule magnet (SMM) based on mapping of the many-body interaction-system onto a one-body problem by means of the non-equilibrium Green function technique. It is found that the conducting current exhibits a stepwise behavior and the nonlinear differential conductance displays additional peaks with variation of the sweeping speed and the magnitude of magnetic field. This observation can be interpreted by the interaction of electron-spin with the SMM and the quantum tunneling of magnetization. The inelastic conductance and the corresponding tunneling processes are investigated with normal as well as ferromagnetic electrodes. In the case of ferromagnetic configuration, the coupling to the SMM leads to an asymmetric tunneling magnetoresistance (TMR), which can be enhanced or suppressed greatly in certain regions. Moreover, a sudden TMR-switch with the variation of magnetic field is observed, which is seen to be caused by the inelastic tunneling.     

Manipulating magnetic anisotropy and ultrafast spin dynamics of magnetic nanostructures

We present our extensive research into magnetic anisotropy. We tuned the terrace width of Si(111) substrate by a novel method: varying the direction of heating current and consequently manipulating the magnetic anisotropy of magnetic structures on the stepped substrate by decorating its atomic steps. Laser-induced ultrafast demagnetization of a Co Fe B/Mg O/Co Fe B magnetic tunneling junction was explored by the time-resolved magneto-optical Kerr effect(TRMOKE) for both the parallel state(P state) and the antiparallel state(AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two Co Fe B layers via the tunneling of hot electrons through the Mg O barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electron tunneling current. This opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions. Furthermore, an all-optical TR-MOKE technique provides the flexibility for exploring the nonlinear magnetization dynamics in ferromagnetic materials, especially with metallic materials.     

伊辛模型中量子的热纠缠和保真度

研究了伊辛模型中两个粒子在均匀和非均匀磁场中的热纠缠以及利用它作为量子信息传输的通道的传输保真度.计算出纠缠度的度量Concurrence,以及在不同种情况下呈现出来的纠缠度的表现形为.与均匀磁场相反,我们发现在非均匀磁场中传输的保真度能够得到增强,同时我们发现保真度还与耦合系数和温度有关.我们通过图形清楚地表示它们的性质,从图形中我们得出,第一:在其它条件相同的情况下,无论磁场方向是相同还是相反,它们的纠缠度都是相同的;但是当磁场方向相反时,平均保真度比均匀磁场具有更大的值.第二:为了提高纠缠和平均保真度我们可以通过选择适当的磁场强度、耦合系数和降低温度来实现.     

Thermal Entanglement in a Two-Qubit Heisenberg XXZ Spin Chain Under a Single Pulse Magnetic Field

The thermal entanglement in a two-qubit Heisenberg XXZ spin chain is investigated under a single pulse magnetic field Bsinθ. The paper shows that the greater the contribution of the inhomogeneity on the exchange interaction, the higher thermal entanglement will be attained at the fixed temperature except the case that $\theta=2k\pi+\frac{3}{2}\pi$ . J _z  significantly disturb the thermal entanglement in uniform case. When the structure of the spin system are given, changing the external magnetic field B can induce controllable entanglement. Our study may provide a useful tool to change the entanglement of spin chain system.     

Entanglement via tunable Fano-type interference in asymmetric semiconductor quantum wells

Entanglement is realized in asymmetric coupled double quantum wells (DQWs) trapped in a doubly resonant cavity by means of Fano-type interference through a tunneling barrier, which is different from the previous studies on entanglement induced by strong external driven fields in atomic media. We investigate the generation and evolution of entanglement and show that the strength of Fano interference can influence effectively the degree of the entanglement between two cavity modes and the enhanced entanglement can be generated in this DQW system. The present investigation may provide research opportunities in quantum entangled experiments in the DQW solid-state nanostructures and may result in a substantial impact on the technology for entanglement engineering in quantum information processing.     

Entangled electron current through finite size normal-superconductor tunneling structures

We investigate theoretically the simultaneous tunneling of two electrons from a superconductor into a normal metal at low temperatures and voltages. Such an emission process is shown to be equivalent to the Andreev reflection of an incident hole. We obtain a local tunneling Hamiltonian that permits to investigate transport through interfaces of arbitrary geometry and potential barrier shapes. We prove that the bilinear momentum dependence of the low-energy tunneling matrix element translates into a real space Hamiltonian involving the normal derivatives of the electron fields in each electrode. The angular distribution of the electron current as it is emitted into the normal metal is analyzed for various experimental setups. We show that, in a full three-dimensional problem, the neglect of the momentum dependence of tunneling causes a violation of unitarity and leads to the wrong thermodynamic (broad interface) limit. More importantly for current research on quantum information devices, in the case of an interface made of two narrow tunneling contacts separated by a distance r, the assumption of momentum-independent hopping yields a nonlocally entangled electron current that decays with a prefactor proportional to r ^-2 instead of the correct r ^-4.Received: 14 June 2004, Published online: 24 September 2004PACS: 74.45. + c Proximity effects; Andreev effect; SN and SNS junctions - 74.50. + r Tunneling phenomena; point contacts, weak links, Josephson effects     

Manipulating the spin states in a double molecular magnets tunneling junction

We theoretically explore the spin transport through nano-structures consisting of two serially coupled single-molecular magnets (SMM) sandwiched between two nonmagnetic electrodes. We find that the magnetization of SMM can be controlled by the spin transfer torque with respect to the bias voltage direction, and the electron current can be switched on/off in different magnetic structures. Such a manipulation is performed by full electrical manner, and needs neither external magnetic field nor ferromagnetic electrodes in the tunneling junction. The proposal device scheme can be realized with the use of the present technology [6] and has potential applications in molecular spintronics or quantum information processing.     

Entanglement and coherence of a three-level atom in Λ configuration interacting with two fields

We investigate the entanglement of a three-level atom in λ configuration interacting with two quantized field modes by using logarithmic negativity. Then, we study the relationship of the atomic coherence and the entanglement between two fields which are initially prepared in vacuum or thermal states. We find that if the two fields are prepared in thermal states, the atomic coherence can induce the entanglement between two thermal fields. However, there is no coherence-induced entanglement between two vacuum fields.     

Spin-polarized transport through a coupled double-dot

We investigate the quantum transport through a mesoscopic device consisting of an open, lateral double-quantum-dot coupled by time oscillating and spin-polarization dependent tunneling which results from a static magnetic field applied in the tunneling junction. In the presence of a non-vanishing bias voltage applied to two attached macroscopic leads both spin and charge currents are driven through the device. We demonstrate that the spin and charge currents are controllable by adjusting the gate voltage, the frequency of driving field and the magnitude of the magnetic field as well. An interesting resonance phenomenon is observed.     

Spin-Polarized Electron Tunneling TD:\pdf\.PDFhrough a Quantum Dot

Nonequilibrium Green's function is uscd to study spin-polarized electron tunneling through a quantum dot connected to two ferromagnetic electrodes with different orientations via two insulating barriers (FM/I/QD/I/FA.f). Intra-level Coulomb interaction in the dot is considered. General formula of tunneling current which can be used for arbitrary angle between the two electrodes' magnetizations is derived for both the weak and strong intra-dot interactions.We find that the transport current can be divided into two parts: the current with the spin-flip and the current without the spin-flip, which critically depend on the linewidth function near the Fermi level of the ferromagnetic electrodes. If a magnetic field is applied in the quantum dot, different behaviors will be found for weak and strong interactions.     

Lower bound for electron spin entanglement from beam splitter current correlations

We determine a lower bound for the entanglement of formation of pairs of electron spins injected into a mesoscopic conductor. The bound can be expressed in terms of experimentally accessible quantities, the zero-frequency current correlators (shot noise power or cross correlators) after transmission through an electronic beam splitter and can be used to gain information about the entanglement from experiment. Spin relaxation (T1 processes) and decoherence (T2) during the ballistic coherent transmission of carriers are taken into account within Bloch theory. A variable inhomogeneous magnetic field gives rise to a useful lower bound for the entanglement of arbitrary states. The decrease in entanglement due to thermally mixed states is studied. Both the entanglement of the output of a source (entangler) and T(1,2) can be determined from current correlators.