Using a laterally-fabricated quantum-dot (QD) spin-valve device, we experimentally study the Kondo effect in the electron transport through a semiconductor QD with an odd number of electrons (N). In a parallel magnetic configuration of the ferromagnetic electrodes, the Kondo resonance at N = 3 splits clearly without external magnetic fields. With applying magnetic fields (B), the splitting is gradually reduced, and then the Kondo effect is almost restored at B = 1.2 T. This means that, in the Kondo regime, an inverse effective magnetic field of B ~ 1.2 T can be applied to the QD in the parallel magnetic configuration of the ferromagnetic electrodes.
We have fabricated single electron tunneling structures by forming nanogap metallic electrodes directly upon single self-assembled InAs quantum dots (QDs). The fabricated samples exhibited clear Coulomb blockade effects. Furthermore, a clear Kondo effect was observed when strong coupling between the electrodes and the QDs was realized using a large QD with a diameter of ~ 100 nm. From the temperature dependence of the linear conductance at the Kondo valley, the Kondo temperature TK was determined to be ~ 81 K. This is the highest TK ever reported for artificial quantum nanostructures.
We study the nonlocal spin transport in a lateral spin valve with multiple ferromagnetic (FM) electrodes. When two current-injecting and two spin current-detecting electrodes are all ferromagnetic, the number of possible nonlocal spin signal states is four at maximum. In reality, this number is reduced, depending on the inter-probe distance and the relative magnitudes of the spin resistances. Our theoretical results are in agreement with recent experiments of spin injection into an Al island, a carbon nanotube, and graphene.
The relative contribution of spin pumping and spin rectification from the ferromagnetic resonance of CoFeB/non-magnetic bilayers was investigated as a function of non-magnetic electrode resistance. Samples with highly resistive electrodes of Ta or Ti exhibit a stronger spin rectification signal, which may result in over-(or under-)estimation of the spin Hall angle of the materials, while those with low resistive electrodes of Pt or Pd show the domination of the inverse spin Hall effect from spin pumping. By comparison with samples of single FM layer and an inverted structure, we provide a prop…
We consider the transport properties in an applied magnetic field of the spin S=1/2 Kondo model with ferromagnetic exchange coupling to electronic reservoirs, a description relevant for the strong coupling limit of underscreened spin S=1 Kondo impurities. Because the ferromagnetic Kondo interaction is marginally irrelevant, perturbative methods should prove accurate down to low energies. For the purpose of this study, we use a combination of Majorana diagrammatic theory with Density Matrix Numerical Renormalization Group simulations. In the standard case of antiferromagnetic Kondo exchange, we…
Effects of impurity on the spin-dependent transport in a single wall carbon nanotube spin-valve, as ferromagnetic electrode/carbon nanotube/ferromagnetic electrode model junction is numerically investigated. Using a generalized Green's function method and the Landauer-Buttiker formalism, the impurity conditions are determined by randomly substitution of carbon atoms in the honeycomb carbon nanotube lattice by nitrogen and boron atoms. We have found that transport characteristics, including the spin-dependent current and tunnel magnetoresistance are strongly influenced by the impurity effects. …
Recently, it has been shown that magnetic tunnel junctions with thin MgO tunnel barriers exhibit extraordinarily high tunneling magnetoresistance (TMR) values at room temperature1, 2. However, the physics of spin dependent tunneling through MgO barriers is only beginning to be unravelled. Using planar magnetic tunnel junctions in which ultra-thin layers of magnetic metals are deposited in the middle of a MgO tunnel barrier here we demonstrate that the TMR is strongly modified when these layers are discontinuous and composed of small pancake shaped nanodots. At low temperatures, in the Coulomb …
We investigate the electronic transport through a molecule in the Kondo regime. The tunneling between the electrode and the molecule is asymmetrically modulated by the oscillations of the molecule, i.e., if the molecule gets closer to one of the electrodes the tunneling to that electrode will increase while for the other electrode it will decrease. The system is described by a two-channel Anderson model with phonon-assisted hybridization, which is solved with the Wilson numerical renormalization group method. The results for several functional forms of tunneling modulation are presented. For a…
We investigate the many-body effects of a magnetic adatom in ferromagnetic graphene by using the numerical renormalization group method. The nontrivial band dispersion of ferromagnetic graphene gives rise to interesting Kondo physics different from that in conventional ferromagnetic materials. For a half-filled impurity in undoped graphene, the presence of ferromagnetism can bring forth Kondo correlations, yielding two kink structures in the local spectral function near the Fermi energy. When the spin splitting of local occupations is compensated by an external magnetic field, the two Kondo ki…
We report a first principles study of spin-transport under finite bias through a graphene-ferromagnet (FM) interface, where FM=Co(111), Ni(111). The use of Co and Ni electrodes achieves spin efficiencies reaching 80% and 60%, respectively. This large spin filtering results from the materials specific interaction between graphene and the FM which destroys the linear dispersion relation of the graphene bands and leads to an opening of spin-dependent energy gaps of roughly 0.4-0.5 eV at the K points. The minority spin band gap resides higher in energy than the majority spin band gap located near …
We study electron transport through C60 molecules in the Kondo regime using a mechanically controllable break junction. By varying the electrode spacing, we are able to change both the width and height of the Kondo resonance, indicating modification of the Kondo temperature and the relative strength of coupling to the two electrodes. The linear conductance as a function of T/T_K agrees with the scaling function expected for the spin-1/2 Kondo problem. We are also able to tune finite-bias Kondo features which appear at the energy of the first C60 intracage vibrational mode.
We investigate the effects induced by spin polarization in the contacts attached to a serial double quantum dot. The polarization generates effective magnetic fields and suppresses the Kondo effect in each dot. The super-exchange interaction ($J_{\mathrm{AFM}}$), tuned by the inter-dot tunnelling rate $t$, can be used to compensate the effective fields and restore the Kondo resonance when the contact polarizations are aligned. As a consequence, the direction of the spin conductance can be controlled and even reversed using electrostatic gates alone. Furthermore, we study the associated two-imp…
Spin-polarized transport through a quantum dot strongly coupled to ferromagnetic electrodes with non-collinear magnetic moments is analyzed theoretically in terms of the non-equilibrium Green function formalism. Electrons in the dot are assumed to be coupled to a phonon bath. The influence of electron-phonon coupling on tunnelling current, linear and nonlinear conductance, and on tunnel magnetoresistance is studied in detail. Variation of the main Kondo peaks and phonon satellites with the angle between magnetic moments of the leads is analyzed.
We investigate the effects of spin-polarized leads on the Kondo physics of a quantum dot using the numerical renormalization group method. Our study demonstrates in an unambiguous way that the Kondo effect is not necessarily suppressed by the lead polarization: While the Kondo effect is quenched for the asymmetric Anderson model, it survives even for finite polarizations in the regime where charge fluctuations are negligible. We propose the linear tunneling magnetoresistance as an experimental signature of these behaviors. We also report on the influence of spin-flip processes.
We investigate quantum dots in clean single-wall carbon nanotubes with ferromagnetic PdNi-leads in the Kondo regime. In most odd Coulomb valleys the Kondo resonance exhibits a pronounced splitting, which depends on the tunnel coupling to the leads and an external magnetic field $B$, and only weakly on gate voltage. Using numerical renormalization group calculations, we demonstrate that all salient features of the data can be understood using a simple model for the magnetic properties of the leads. The magnetoconductance at zero bias and low temperature depends in a universal way on $g μ_B (B-B…
Non-equilibrium Green's function technique has been used to calculate spin-dependent electronic transport through a quantum dot in the Kondo regime. The dot is described by the Anderson Hamiltonian and is coupled either symmetrically or asymmetrically to ferromagnetic leads, whose magnetic moments are noncollinear. It is shown that the splitting of the zero bias Kondo anomaly in differential conductance decreases monotonically with increasing angle between magnetizations, and for antiparallel configuration it vanishes in the symmetrical case while remains finite in the asymmetrical one.
Nonequilibrium electronic transport through a quantum dot coupled to ferromagnetic leads (electrodes) is studied theoretically by the nonequilibrium Green function technique. The system is described by the Anderson model with arbitrary correlation parameter $U$. Exchange interaction between the dot and ferromagnetic electrodes is taken into account {\it via} an effective molecular field. The following situations are analyzed numerically: (i) the dot is symmetrically coupled to two ferromagnetic leads, (ii) one of the two ferromagnetic leads is half-metallic with almost total spin polarization …
We study the spin-resolved transport through single-level quantum dots strongly coupled to ferromagnetic leads in the Kondo regime, with a focus on contact and material asymmetry-related effects. By using the numerical renormalization group method, we analyze the dependence of relevant spectral functions, linear conductance and tunnel magnetoresistance on the system asymmetry parameters. In the parallel magnetic configuration of the device the Kondo effect is generally suppressed due to the presence of exchange field, irrespective of system's asymmetry. In the antiparallel configuration, on th…
We propose an unconventional type of Hall effect in a topological Dirac semimetal with ferromagnetic electrodes. The topological Dirac semimetal itself has time-reversal symmetry, whereas attached ferromagnetic electrodes break it, causing the large Hall response. This induced Hall effect is a characteristic of the helical surface/edge states that arise in topological materials, such as topological Dirac semimetals or quantum spin Hall insulators. We compute the Hall conductance/resistance and the Hall angle by using a lattice model with four-terminal geometry. For topological Dirac semimetals…
Atomic contacts made of ferromagnetic metals present zero-bias anomalies in the differential conductance due to the Kondo effect. These systems provide a unique opportunity to perform a statistical analysis of the Kondo parameters in nanostructures since a large number of contacts can be easily fabricated using break-junction techniques. The details of the atomic structure differ from one contact to another so a large number of different configurations can be statistically analyzed. Here we present such a statistical analysis of the Kondo effect in atomic contacts made from the ferromagnetic t…
We study the Kondo effect in a quantum dot which is coupled to ferromagnetic leads and analyse its properties as a function of the spin polarization of the leads. Based on a scaling approach we predict that for parallel alignment of the magnetizations in the leads the strong-coupling limit of the Kondo effect is reached at a finite value of the magnetic field. Using an equation-of-motion technique we study nonlinear transport through the dot. For parallel alignment the zero-bias anomaly may be split even in the absence of an external magnetic field. For antiparallel spin alignment and symmetri…
We investigate the spin-resolved transport properties, such as the linear conductance and the tunnel magnetoresistance, of a double quantum dot device attached to ferromagnetic leads and look for signatures of SU(4) symmetry in the Kondo regime. We show that the transport behavior greatly depends on the magnetic configuration of the device, and the spin-SU(2) as well as the orbital and spin-SU(4) Kondo effects become generally suppressed when the magnetic configuration of the leads varies from the antiparallel to the parallel one. Furthermore, a finite spin polarization of the leads lifts the …
Equilibrium transport properties of a single-level quantum dot tunnel-coupled to ferromagnetic leads and exchange-coupled to a side nonmagnetic reservoir are analyzed theoretically in the Kondo regime. The equilibrium spectral functions and conductance through the dot are calculated using the numerical renormalization group (NRG) method. It is shown that in the antiparallel magnetic configuration, the system undergoes a quantum phase transition with increasing exchange coupling $J$, where the conductance drops from its maximum value to zero. In the parallel configuration, on the other hand, th…
We analyze the equilibrium transport properties of underscreened Kondo effect in the case of a two-level quantum dot coupled to ferromagnetic leads. Using the numerical renormalization group (NRG) method, we have determined the gate voltage dependence of the dot's spin and level-resolved spectral functions. We have shown that the polarization of the dot is very susceptible to spin imbalance in the leads and changes sign in the middle of the S=1 Coulomb valley. Furthermore, we have also found that by fine-tuning an external magnetic field one can compensate for the presence of ferromagnetic lea…
We investigate the linear and nonlinear dc transport through an interacting quantum dot connected to two ferromagnetic electrodes around Kondo regime with spin-flip scattering in the dot. Using a slave-boson mean field approach for the Anderson Hamiltonian having finite on-site Coulomb repulsion, we find that a spin-flip scattering always depresses the Kondo correlation at arbitrary polarization strength in both parallel and antiparallel alignment of the lead magnetization and that it effectively reinforces the tunneling related conductance in the antiparallel configuration. For systems deep i…
We propose a new approximation scheme within equation of motion approach (EOM) to spin polarized transport through a quantum dot coupled to ferromagnetic leads. It has some advantages over a widely used in the literature standard EOM technique, in particular when we are interested in spin polarized quantities. Namely, it gives the values of the dot spin polarization which are closer to the ones obtained within numerical renormalization group (NRG), than the standard EOM approach. While restoring the Kondo effect, the spin polarization vanishes and the transport becomes unpolarized, in agreemen…
The interplay between the Kondo effect and the inter-dot magnetic interaction in a coupled-dot system is studied. An exact result for the transport properties at zero temperature is obtained by diagonalizing a cluster, composed by the double-dot and its vicinity, which is connected to leads. It is shown that the system goes continuously from the Kondo regime to an anti-ferromagnetic state as the inter-dot interaction is increased. The conductance, the charge at the dots and the spin-spin correlation are obtained as a function of the gate potential.
The Kondo effect of a Co atom on Cu(100) was investigated with a low-temperature scanning tunneling microscope using a monoatomically sharp nickel tip. Upon a tip-Co contact, the differential conductance spectra exhibit a spin-split asymmetric Kondo resonance. The computed ab initio value of the exchange coupling is too small to suppress the Kondo effect, but sufficiently large to produce the splitting observed. A quantitative analysis of the line shape using the numerical renormalization group technique indicates that the junction spin polarization is weak.
An analytical description of non-equilibrium phenomena in interacting quantum systems is rarely possible. Here we present one example where such a description can be achieved, namely the ferromagnetic Kondo model. In equilibrium, this model is tractable via perturbative renormalization-group techniques. We employ a recently developed extension of the flow-equation method to calculate the non-equilibrium decay of the local magnetization at zero temperature. The flow equations admit analytical solutions which become exact at short and long times, in the latter case revealing that the system alwa…
Motivated by recent experiments on molecular quantum dots we investigate the relaxation of pure spin states when coupled to metallic leads. Under suitable conditions these systems are well described by a ferromagnetic Kondo model. Using two recently developed theoretical approaches, the time-dependent numerical renormalization group and an extended ow equation method, we calculate the real-time evolution of a Kondo spin into its partially screened steady state. We obtain exact analytical results which agree well with numerical implementations of both methods. Analytical expressions for the ste…
In this study, based on the self-energy method and the total energy calculation, the indirect exchange coupling between two semi-infinite ferromagnetic strips (FM electrodes) separated by metallic graphene nanoribbons (GNRs) is investigated. In order to form a FM/GNR/FM junction, a graphitic region of finite length is coupled to the FM electrodes along graphitic zigzag or armchair interfaces of width $N$. The numerical results show that, the exchange coupling strength which can be obtained from the difference between the total energies of electrons in the ferromagnetic and antiferromagnetic co…
We study the Kondo effect in a quantum dot coupled to two noncollinear ferromagnetic leads. First, we study the spin splitting $δε=ε_{\downarrow}-ε_{\uparrow}$ of an energy level in the quantum dot by tunnel couplings to the ferromagnetic leads, using the Poor man's scaling method. The spin splitting takes place in an intermediate direction between magnetic moments in the two leads. $δε\propto p\sqrt{\cos^2(θ/2)+v^2\sin^2(θ/2)}$, where $p$ is the spin polarization in the leads, $θ$ is the angle between the magnetic moments, and $v$ is an asymmetric factor of tunnel barriers ($-1<v<1$). Hence t…
We analyze the low energy properties of a device with $N+1$ quantum dots in a star configuration. A central quantum dot is tunnel coupled to source and drain electrodes and to $N$ quantum dots. Extending previous results for the $N=2$ case we show that, in the appropriate parameter regime, the low energy Hamiltonian of the system is a ferromagnetic Kondo model for a $S=(N-1)/2$ impurity spin. For small enough interdot tunnel coupling, however, a two-stage Kondo effect takes place as the temperature is decreased. The spin $1/2$ in the central quantum dot is Kondo screened first and at lower tem…
We study the Kondo effect in the electron transport through a quantum dot coupled to ferromagnetic leads, using a real-time diagrammatic technique which provides a systematic description of the nonequilibrium dynamics of a system with strong local electron correlations. We evaluate the theory in an extension of the `resonant tunneling approximation', introduced earlier, by introducing the self-energy of the off-diagonal component of the reduced propagator in spin space. In this way we develop a charge and spin conserving approximation that accounts not only for Kondo correlations but also for …
We have contacted single wall carbon nanotubes grown by chemical vapor deposition to superconducting Ti/Al/Ti electrodes. The device, we here report on is in the Kondo regime exhibiting a four-fold shell structure, where a clear signature of the superconducting electrodes is observed below the critical temperature. Multiple Andreev reflections are revealed by sub-gap structure and a narrow peak in the differential conductance around zero bias is seen depending on the shell filling. We interpret the peak as a proximity induced supercurrent and examine its interplay with Kondo resonances.
The recently suggested mechanism [Y. Song and H. Dery, Phys. Rev. Lett. 113, 047205 (2014)] of the three-terminal spin transport is based on the resonant tunneling of electrons between ferromagnetic and normal electrodes via an impurity. The sensitivity of current to a weak external magnetic field stems from a spin blockade, which, in turn, is enabled by strong on-site repulsion. We demonstrate that this sensitivity exists even in the absence of repulsion when a single-particle description applies. Within this description, we calculate exactly the resonant-tunneling current between the electro…
In this paper, we study the zero temperature persistent current in a ferromagnetic Kondo lattice model in the strong coupling limit. In this model, there are spontaneous spin textures at some values of the external magnetic flux. These spin textures contribute a geometric flux, which can induce an additional spontaneous persistent current. Since this spin texture changes with the external magnetic flux, we find that there is an anomalous persistent current in some region of magnetic flux: near Phi/Phi_0=0 for an even number of electrons and Phi/Phi_0=1/2 for an odd number of electrons.
Empa – Swiss Federal Laboratories for Materials Science and Technology（瑞士联邦材料科技研究院）；Delft University of Technology（代尔夫特理工大学）；Kavli Institute of Nanoscience, Delft University of Technology（代尔夫特理工大学卡弗里纳米科学研究所）；Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院）；University of Bern（伯尔尼大学）；Universidad de Alicante（阿利坎特大学）
We propose an efficient mechanism for the operation of writing spin in a quantum dot, which is an ideal candidate for qubit. The idea is based on the Andreev reflection induced spin polarization (ARISP) in a ferromagnetic / quantum-dot / superconductor system. We find that on the resonance of Andreev reflection, the spin polarization of quantum dot strongly denpends on the magnetization of ferromagnetic electrode, and the sign of the spin polarization is controllable by bias voltage. In the presence of intradot Coulomb interaction, we show that ARISP effect can still survive as long as the cha…
Strong electron correlations and interference effects are discussed in parallel-coupled single-level and orbitally doubly degenerate quantum dots. The finite-U mean-field slave boson approach is used to study many-body effects. The analysis is carried out in a wide range of parameter space including both atomic-like and molecular-like Kondo regimes and taking into account various perturbations, like interdot tunneling, interdot interaction, mixing of the electrode channels and exchange interaction. We also discuss the influence of singularities of electronic structure and the impact of polariz…
We consider a non-local spin valve in a Van der Pauw cross geometry with four ferromagnetic electrodes. Two antiparallel ferromagnets are used as (charge) source and drain while the detector circuit involves measuring the voltage between two collinear ferromagnets with parallel or antiparallel magnetizations. We find a potentially large increase of the non-local spin voltage. The setup displays several additional interesting properties: (i) infinite GMR for the non-local resistance (if a symmetry requirement for the device is met); (ii) ON-OFF switch effect, when the injector electrodes are pa…
We theoretically analyzed a magnetic wall confined in a nano-contact spin valve paying special attention to the penetration of the magnetic wall into thin ferromagnetic electrodes. We showed that, compared with the Bloch wall, the penetration of the Neel wall is suppressed by increases of the demagnetization energy. We found the optimal conditions of the radius and height of the nano-contact to maximize the power of the current-induced oscillation of the magnetic wall. We also found that the thermal stability of the Bloch wall increases when the nano-contact's radius increases or height decrea…
Spin polarized transport through a quantum dot coupled to ferromagnetic electrodes with noncollinear magnetizations is discussed in terms of nonequilibrium Green functions formalism in the finite-U slave boson mean field approximation. The difference of orientations of the magnetizations of electrodes opens off-diagonal spin-orbital transmission and apart from spin currents of longitudinal polarization also spin-flip currents appear. We also study equilibrium pure spin current at zero bias and discuss its dependence on magnetization orientation, spin-orbit coupling strength and gate voltage. I…
Controlling the magnetization reversal process of magnetic elements is important for a wide range of applications that make use of magnetoresistive effects, but is difficult to achieve for devices that require adjacent thin film structures capable of contacting an individual molecule or quantum dot. We report on the fabrication and measurement of ferromagnetic break junction devices with planar, elliptical leads to address the particular challenge of controlling the relative magnetization alignment between neighboring electrodes. Low temperature transport measurements, supported by finite-elem…
Spintronics devices rely on spin-dependent transport behavior evoked by the presence of spin-polarized electrons. Transport through nanostructures, on the other hand, is dominated by strong Coulomb interaction. We study a model system in the intersection of both fields, a quantum dot attached to ferromagnetic leads. The combination of spin-polarization in the leads and strong Coulomb interaction in the quantum dot gives rise to an exchange field acting on electron spins in the dot. Depending on the parameter regime, this exchange field is visible in the transport either via a precession of an …
We theoretically study the spin-polarized transport through a single-molecule magnet, which is weakly coupled to ferromagnetic leads, by means of the rate-equation approach. We consider both the ferromagnetic and antiferromagnetic exchange-couplings between the molecular magnet and transported electron-spin in the nonlinear tunneling regime. For the ferromagnetic exchangecoupling, spin current exhibits step- and basin-like behaviors in the parallel and antiparallel configurations respectively. An interesting observation is that the polarization reversal of spin-current can be realized and mani…
We study the nonequilibrium spin transport through a quantum dot containing two spin levels coupled to the magnetic electrodes. A formula for the spin-dependent current is obtained and is applied to discuss the linear conductance and magnetoresistance in the interacting regime, where the so-called Kondo effect arises. We show that the Kondo resonance and the correlation-induced spin splitting of the dot levels may be systematically controlled by internal magnetization in the electrodes. As a result, when the electrodes are in parallel magnetic configuration, the linear conductance is character…
Iron, cobalt and nickel are archetypal ferromagnetic metals. In bulk, electronic conduction in these materials takes place mainly through the $s$ and $p$ electrons, whereas the magnetic moments are mostly in the narrow $d$-electron bands, where they tend to align. This general picture may change at the nanoscale because electrons at the surfaces of materials experience interactions that differ from those in the bulk. Here we show direct evidence for such changes: electronic transport in atomic-scale contacts of pure ferromagnets (iron, cobalt and nickel), despite their strong bulk ferromagneti…
We investigate the equilibrium and out-of-equilibrium Kondo effects in a single-level interacting quantum dot connected to two ferromagnetic leads. Within the non-crossing approximation, we calculate the total density of states (DOS), the linear conductance, and the nonlinear differential conductance for both the parallel and the anti-parallel alignments of the spin polarization orientation in the leads, followed by a brief discussion regarding the validity of this approach. Numerical calculations show that for the anti-parallel alignment, a single Kondo peak always appears in the equilibrium …
We have observed the Kondo effect in strongly coupled semiconducting nanowire quantum dots. The devices are made from indium arsenide nanowires, grown by molecular beam epitaxy, and contacted by titanium leads. The device transparency can be tuned by changing the potential on a gate electrode, and for increasing transparencies the effects dominating the transport changes from Coulomb Blockade to Universal Conductance Fluctuations with Kondo physics appearing in the intermediate region.
We systematically study the influence of ferromagnetic leads on the Kondo resonance in a quantum dot tuned to the local moment regime. We employ Wilson's numerical renormalization group method, extended to handle leads with a spin asymmetric density of states, to identify the effects of (i) a finite spin polarization in the leads (at the Fermi-surface), (ii) a Stoner splitting in the bands (governed by the band edges) and (iii) an arbitrary shape of the leads density of states. For a generic lead density of states the quantum dot favors being occupied by a particular spin-species due to exchan…
We investigate electronic transport through parallel double quantum dot(DQD) system with strong on-site Coulomb interaction and capacitive interdot coupling. By applying numerical renormalization group(NRG) method, the ground state of the system and the transmission probability at zero temperature have been obtained. For a system of quantum dots with degenerate energy levels and small interdot tunnel coupling, the spin correlations between the DQDs is ferromagnetic, and the ground state of the system is a spin 1 triplet state. The linear conductance will reach the unitary limit ($2e^2/h$) due …
The mechanism of the magneto-Coulomb oscillation in ferromagnetic single electron transistors (SET's) is theoretically considered. Variations in the chemical potentials of the conduction electrons in the ferromagnetic island electrode and the ferromagnetic lead electrodes in magnetic fields cause changes in the free energy of the island electrode of the SET. Experimental results of the magneto-Coulomb oscillation in a Ni/Co/Ni ferromagnetic SET are presented and discussed. Possible applications of this phenomenon are also discussed.
We study the spin-dependent transport through a quantum dot coupled to two ferromagnetic electrodes using the equation of motion method for the nonequilibrium Green's functions. Our results show that the conductance and the density of states (DOS) are strongly dependent on the configurations of the magnetic electrodes. In parallel configuration of magnetic electrodes the conductance is affected by the spin-flip process and the Coulomb repulsion on the dot. The Kondo peak for spin-dependent transport is splitted into two peaks by the spin-flip process. The locations of the two peaks are symmetr…
It is demonstrated that non local Cooper pairs can propagate in ferromagnetic electrodes having an opposite spin orientation. In the presence of such crossed correlations, the superconducting gap is found to depend explicitly on the relative orientation of the ferromagnetic electrodes. Non local Cooper pairs can in principle be probed with dc-transport. With two ferromagnetic electrodes, we propose a ``quantum switch'' that can be used to detect correlated pairs of electrons. With three or more ferromagnetic electrodes, the Cooper pair-like state is a linear superposition of Cooper pairs which…
We study spin-resolved noise in Coulomb blockaded double quantum dots coupled to ferromagnetic electrodes. The modulation of the interdot coupling and spin polarization in the electrodes gives rise to an intriguing dynamical spin $\uparrow$-$\uparrow$ ($\downarrow$-$\downarrow$) blockade mechanism: Bunching of up (down) spins due to dynamical blockade of an up (down) spin. In contrast to the conventional dynamical spin $\uparrow$-$\downarrow$ bunching (bunching of up spins entailed by dynamical blockade of a down spin), this new bunching behavior is found to be intimately associated with the s…
We report on the giant magnetoresistance (GMR) of multiwall carbon nanotubes with ultra small diameters. In particular, we consider the effect of the inter-wall interactions and the lead/nanotube coupling. Comparative studies have been performed to show that in the case when all walls are well coupled to the electrodes, the so-called inverse GMR can appear. The tendency towards a negative GMR depends on the inter-wall interaction and on the nanotube le ngth. If, however, the inner nanotubes are out of contact with one of the electrodes, the GMR remains positive even for relatively strong inter…
We have investigated break junctions of normal non-magnetic metals as well as ferromagnets at low temperatures. The point contacts with radii 0.15 - 15 nm showed zero-bias anomalies which can be attributed to Kondo scattering at a single Kondo impurity at the contact or to the switching of a single conducting channel. The Kondo temperatures derived from the width of the anomalies varied between 10 and 1000 K. These results agree well with literature data on atomic-size contacts of the ferromagnets as well as with spear-anvil type contacts on a wide variety of metals.
We show how the charge input signal applied to the gate electrode in double quantum dot may be converted to a pulse in the Kondo cotunneling current as a spin response of a nano-device in a strong Coulomb blockade regime. The stochastic component of the input signal comes as the infrared cutoff of Kondo transmission.
We propose to study the transport through tunneling-coupled double quantum dots (DQDs) connected in series to leads, using the finite-$U$ slave-boson mean field approach developed initially by Kotliar and Ruckenstein [Phys. Rev. Lett. {\bf 57}, 1362 (1986)]. This approach treats the dot-lead coupling and the inter-dot tunnelling $t$ nonperturbatively at arbitrary Coulomb correlation $U$, thus allows the anti-ferromagnetic exchange coupling parameter $J=4t^2/U$ to appear naturally. We find that, with increasing the inter-dot hopping, the DQDs manifest three distinct physical scenarios: the Kond…
We study thermoelectric effects in Kondo correlated quantum dot coupled to ferromagnetic electrodes by calculating thermopower S in the Kondo regime as function of on-dot energy level and temperature. The system is represented by the Anderson model and the results agree well with those recently measured for a quantum dot coupled to nonmagnetic leads. For magnetic electrodes one observes marked dependence of S on the degree of their polarization.
