Cotunneling current through a two-level quantum dot coupled to magnetic leads: the role of exchange interaction

The cotunneling current through a two-level quantum dot weakly coupled to ferromagnetic leads is studied in the Coulomb blockade regime. The cotunneling current is calculated analytically under simple but realistic assumptions as follows: (i)?the quantum dot is described by the universal Hamiltonian, (ii)?it is doubly occupied, and (iii)?it displays a fast spin relaxation. We find that the dependence of the differential conductance on the bias voltage is significantly affected by the exchange interaction on the quantum dot. In particular, for antiparallel magnetic configurations in the leads, the exchange interaction results in the appearance of interference-type contributions from the inelastic processes to the cotunneling current. Such dependence of the cotunneling current on the tunneling amplitude phases should also occur in multi-level quantum dots weakly coupled to ferromagnetic leads near the mesoscopic Stoner instabilities.     

Scanning tunneling microscopy observation of copper-phthalocyanine molecules on Si(100) and Si(111) surfaces

Molecules of Cu-phthalocyanine (CuPc) deposited on Si(100) and Si(111) surfaces have been observed by an ultra high vacuum field ion scanning tunneling microscope (FI-STM). On a Si(100) surface, STM images with four-fold symmetry are observed, which reflect the shape of the CuPc molecule. The STM pictures show that CuPc molecules are deposited with the molecular plane parallel to the substrate surface and have three kinds of adsorption configurations on the dimer-row of Si(100). The images of the CuPc are modified by the electronic state of the Si(100) surface. This behavior suggests strong interaction between the molecule and the substrate. The molecular images on the Si(111) surface have a unique bias-voltage dependence. At a sample bias of 1.6 V, the molecule looks transparent by STM, and becomes dark like a vacancy at 1.2 V. From the bias dependence, the electronic interaction between the CuPc molecule and the Si surface is discussed.     

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.     

Phase coherence of the electron and hole in a ferromagnetic film in proximity with a superconductor

The scattering matrix approach between the clean and dirty limits is developed for the study of tunneling spectra in a ferromagnetic film in proximity to a superconductor. The minigap and the damped oscillation from ``0" to ``π" state in tunneling conductance are attributed to the phase coherence of the electrons and the corresponding Andreev-reflected holes in the ferromagnetic film. The calculated results provide a reasonable explanation for the behavior observed in recent experiments.     

Absence of zero-bias anomaly in spin-polarized vacuum tunneling in Co(0001)

In a joint experimental and theoretical study, we investigate the bias-voltage dependence of the tunnel magnetoresistance (TMR) through a vacuum barrier. The TMR observed by spin-polarized scanning tunneling microscopy between an amorphous magnetic tip and a Co(0001) sample is almost independent of the bias voltage at large tip-sample separations. Whereas qualitative understanding is achieved by means of the electronic surface structure of Co, the experimental findings are compared quantitatively with bias-voltage dependent first-principles calculations for ballistic tunneling. At small tip-sample separations, a pronounced minimum in the experimental TMR was found at +200 mV bias.     

Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.     

Ac-cotunneling through an interacting quantum dot in a magnetic field

We analyze inelastic cotunneling through an interacting quantum dot subject to an ambient magnetic field in the weak tunneling regime under a non-adiabatic time-dependent bias-voltage. Our results clearly exhibit photon-assisted satellites and an overall suppression of differential conductance with increasing driving amplitude, which is consistent with experiments. We also predict a zero-anomaly in differential conductance under an appropriate driving frequency.     

Surface magnetism of Fe(110) from polarized electron scattering

Exchange- and spin—orbit-induced scattering asymmetry spectra of polarized slow electrons from the ferromagnetic Fe(110) surface have been calculated by dynamical theory and found to agree with recent experimental data taken at room temperature. Comparison of exchange asymmetry spectra, obtained for various interaction and layer-dependent magnetization models, with the data implies firstly an enhancement of the surface magnetization by about 30% with respect to the bulk, and secondly the importance of spin-dependent localized inelastic electron—electron scattering processes.     

One-dimensional diffusion of vacancies on Sr/Si(100)-c(2×4) surface

An Sr/Si(100)-c(2×4) surface is investigated by high-resolution scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The semiconductor property of this surface is confirmed by STS. The STM images of this surface shows that it is bias-voltage dependent and an atomic resolution image can be obtained at an empty state under a bias voltage of 1.5 V. Furthermore, one-dimensional (1D) diffusion of vacancies can be found in the room-temperature STM images. Sr vacancies diffuse along the valley channels, which are constructed by silicon dimers in the surface. Weak interaction between Sr and silicon dimers, low metal coverage, surface vacancy, and energy of thermal fluctuation at room temperature all contribute to this 1D diffusion.     

Spin-dependent transport through quantum dot with inelastic interactions

Using the Keldysh nonequilibrium Green function method, we theoretically investigate the electron transport properties of a quantum dot coupled to two ferromagnetic electrodes, with inelastic electron-phonon interaction and spin flip scattering present in the quantum dot. It is found that the electron-phonon interaction reduces the current, induces new satellite polaronic peaks in the differential conductance spectrum, and at the same time leads to oscillatory tunneling magnetoresistance effect. Spin flip scattering suppresses the zero-bias conductance peak and splits it into two, with different behaviors for parallel and anti-parallel magnetic configuration of the two electrodes. Consequently, a negative tunneling magnetoresistance effect may occur in the resonant tunneling region, with increasing spin flip scattering rate.     

金属电极对有机分子非弹性电子隧穿谱的影响

利用第一性原理计算金属电极下1,6-己二硫醇和1,4-二巯基苯分子结的非弹性电子隧穿谱,发现非弹性电子隧穿谱对金属电极的变化十分灵敏,并且非弹性电子隧穿谱的振动峰位置和强度与硫原子和金属电极表面的距离密切相关.结果表明电极材料和分子与金属成键的情况是影响分子结的非弹性电子输运的重要因素.理论分析进一步表明不同金属电极和有机分子的耦合能不同导致了谱峰强弱的调整.     

Tip and surface determination from experiments and simulations of scanning tunneling microscopy and spectroscopy

We present a very efficient and accurate method to simulate scanning tunneling microscopy images and spectra from first-principles density functional calculations. The wave functions of the tip and sample are calculated separately on the same footing and propagated far from the surface using the vacuum Green function. This allows us to express the Bardeen matrix elements in terms of convolutions and to obtain the tunneling current at all tip positions and bias voltages in a single calculation. The efficiency of the method opens the door to real time determination of both tip and surface composition and structure, by comparing experiments to simulated images for a variety of precomputed tips. Comparison with the experimental topography and spectra of the Si111-(7 x 7) surface shows a much better agreement with Si than with W tips, implying that the metallic tip is terminated by silicon.     

Role of inelastic tunneling through the insulating barrier in scanning-tunneling-microscope experiments on cuprate superconductors

The tunneling path between the CuO2 layers in cuprate superconductors and a scanning-tunneling-microscope tip passes through a barrier made from other oxide layers. This opens up the possibility that inelastic processes in the barrier contribute to the tunneling spectra. Such processes cause one or possibly more peaks in the second derivative current-voltage spectra displaced by phonon energies from the density of states singularity associated with superconductivity. Calculations of inelastic processes generated by apical O phonons show good qualitative agreement with recent experiments reported by Lee et al. Further tests to discriminate between these inelastic processes and coupling to planar phonons are proposed.     

Inelastic tunneling in a double quantum dot coupled to a bosonic environment

Coupling a quantum system to a bosonic environment always give rise to inelastic processes, which reduce the coherency of the system. We measure energy-dependent rates for inelastic tunneling processes in a fully controllable two-level system of a double quantum dot. The emission and absorption rates are well reproduced by Einstein's coefficients, which relate to the spontaneous emission rate. The inelastic tunneling rate can be comparable to the elastic tunneling rate if the boson occupation number becomes large. In the specific semiconductor double dot, the energy dependence of the inelastic rate suggests that acoustic phonons are coupled to the double dot piezoelectrically.     

The exchange interaction effect in the scanning tunneling spectroscopy of a two-orbital Anderson impurity on metallic surface

We investigate the scanning tunneling spectroscopy (STS) of a two-orbital Anderson impurity adsorbed on a metallic surface by using the numerical renormalization group (NRG) method. The density of state of magnetic impurity and the local conduction electron are calculated. We obtain the Fano resonance line shape in the STM conductance at zero temperature. For the impurity atom with antiferromagnetic inter-orbital exchange interaction and a spin singlet ground state, we show that a dip in the STM spectra around zero bias voltage regime and side peaks of spin excitation can be observed. The spin excitation energy is proportional to the exchange interaction strength. As the exchange interaction is ferromagnetic, the underscreened Kondo effect dominates the low energy properties of this system, and it gives rise to drastically different STM spectra as compared with the spin singlet case.     

Theory of single molecule vibrational spectroscopy and microscopy

We have carried out a density functional study of vibrationally inelastic tunneling in the scanning tunneling microscope of acetylene on copper. Our approach is based on a many-body generalization of the Tersoff-Hamann theory. We explain why only the carbon-hydrogen stretch modes are observed in terms of inelastic and elastic contributions to the tunneling conductance. The inelastic tunneling is found to be efficient and highly localized in space without any resonant interaction and to be governed by a vibration-induced change in tunneling amplitude.     

Effect of localized charge states on the low-frequency part of the tunneling current spectrum (1/<Emphasis Type="Italic">f</Emphasis><Superscript><Emphasis Type="Italic">α</Emphasis></Superscript>)

The dependence of the low-frequency part of the tunneling current spectra (1/f ^α ) above a clean surface and above isolated impurity atoms on the InAs(110) cleaved surface has been investigated by scanning tunneling microscopy/spectroscopy in high vacuum. A theoretical model is proposed to explain the experimental results, which takes into account the many-body interaction of conduction electrons (with suddenly switched on and off Coulomb interaction on an impurity atom and on the scanning tunneling microscope tip) with the continuous-spectrum states in the tunneling contact leads.     

From high resolution spectroscopy to the modeling of potential energy surfaces

Methods and recipes used to establish potential energy surfaces in condensed molecular phases are discussed. The reliability of calculations is tested by confrontation with spectroscopic measurements in crystals. Optical spectroscopy, in particular, hole burning as a line-narrowing technique, as well as high resolution inelastic neutron scattering (INS), are used to resolve tunneling level structures corresponding to large-amplitude atomic and molecular motions. Rotational tunneling of methyl groups is discussed, and new measurements by INS are presented for crystals that are proposed as suitable candidates for optical studies. Translational tunneling in benzoic acid crystals and the role of promoting modes are reviewed, and new measurements of vibrational spectra by inelastic x-ray scattering are compared with INS and Raman spectra.     

Impurity states and interlayer tunneling in high temperature superconductors

We argue that the scanning tunneling microscope (STM) images of resonant states generated by doping Zn or Ni impurities into Cu-O planes of BSCCO are the result of quantum interference of the impurity signal coming from several distinct paths. The impurity image seen on the surface is greatly affected by interlayer tunneling matrix elements. We find that the optimal tunneling path between the STM tip and the metal (Cu, Zn, or Ni) d(x(2)-y(2)) orbitals in the Cu-O plane involves intermediate excited states. This tunneling path leads to the fourfold nonlocal filter of the impurity state in Cu-O plane that explains the experimental impurity spectra. Applications of the tunneling filter to the Cu vacancy defects and "direct" tunneling into Cu-O planes are also discussed.