G. Michałek

Spin effects in ferromagnetic single-electron transistors

Electron tunneling in ferromagnetic single-electron transistors is considered theoretically in the sequential tunneling regime. A formalism is developed, which operates in a two-dimensional space of states, instead of one-dimensional space used in the spinless case. It is shown that spin fluctuations can be significantly larger than the charge fluctuations. The influence of discrete energy spectrum of a small central electrode on tunneling current, charge and spin accumulation, charge and spin fluctuations, and on tunnel magnetoresistance is analyzed in detail. Two different scales are found in the bias dependence of the basic transport characteristics; the shorter one originates from the discrete energy spectrum and the longer one from discrete charging of the central electrode. The features due to discrete spectrum and discrete charging disappear at high temperatures.

SHOT NOISE IN FERROMAGNETIC SINGLE-ELECTRON TUNNELING DEVICES

Frequency-dependent current noise in ferromagnetic double junctions with a Coulomb blockade is studied theoretically in the limit of sequential tunneling. Two different relaxation processes are found in the correlations between spin-polarized tunneling currents: low-frequency spin fluctuations and high-frequency charge fluctuations. Spin accumulation in strongly asymmetric junctions is shown to lead to a negative differential resistance. We also show that large spin noise activated in the range of negative differential resistance gives rise to a significant enhancement of the current noise.

Interplay between direct and crossed Andreev reflections in hybrid nanostructures

The interplay between various many body effects in a quantum dot attached to two normal and one superconducting lead is considered in the limit of large superconducting gap. By the proximity effect the superconducting lead induces pairing correlations on the quantum dot. In the subgap region one observes the anomalous tunneling via direct and crossed Andreev scattering, whereas the usual single particle electronic transfer is suppressed. The interactions of electrons on the dot leading to such phenomena as the Coulomb blockade and the Kondo effect severely modify the currents flowing in the system. In particular: (i) they prevent the existence of the negative differential conductance observed for non-interacting quantum dot over the whole range of voltages, (ii) affect the distribution of the currents as function of the applied voltage and (iii) lead to the appearance of additional low bias feature due to the formation of the Abrikosov-Suhl resonance. The non-local correlations in the Coulomb blockade regime are most pronounced for the particle-hole symmetric dot and thus can be easily tuned by means of gate voltage. They are observed even in the Kondo regime and dominate the behavior close to the Abrikosov-Suhl resonance showing convincingly that Kondo correlations do not destroy subtle entanglement between electrons.

Dynamical correlations in electronic transport through a system of coupled quantum dots

Current auto- and cross-correlations are studied in a system of two capacitively coupled quantum dots. We are interested in a role of Coulomb interaction in dynamical correlations, which occur outside the Coulomb blockade region (for high bias). After decomposition of the current correlation functions into contributions between individual tunneling events, we can show which of them are relevant and lead to sub-/supper-Poissonian shot noise and negative/positive cross-correlations. The results are differentiated for a weak and strong inter-dot coupling. Interesting results are for the strong coupling case when electron transfer in one of the channel is strongly correlated with charge drag in the second channel. We show that cross-correlations are non-monotonic functions of bias voltage and they are in general negative (except some cases with asymmetric tunnel resistances). This is effect of local potential fluctuations correlated by Coulomb interaction, which mimics the Pauli exclusion principle.

Cooper Pair Splitting Efficiency in the Hybrid Three-Terminal Quantum Dot

Nanodevices consisting of a quantum dot tunnel coupled to one superconducting and two normal electrodes may serve as a source of entangled electrons. As a result of crossed Andreev reflection the Cooper pair of s-wave character may be split into two electrons and each of them goes into a distinct normal electrode, preserving entanglement. Efficiency of the process depends on the specific system and is tunable by electric means. Our calculations show that in the studied device this efficiency may attain values as large as 80 %.

Novel non-local effects in three-terminal hybrid devices with quantum dot.

We predict non-local effect in the three-terminal hybrid device consisting of the quantum dot (QD) tunnel coupled to two normal and one superconducting reservoirs. It manifests itself as the negative non-local resistance and results from the competition between the ballistic electron transfer (ET) and the crossed Andreev scattering (CAR). The effect is robust both in the linear and non-linear regimes. In the latter case the screening of charges and the long-range interactions play significant role. We show that sign change of the non-local conductance depends on the subgap Shiba/Andreev states, and it takes place even in absence of the Coulomb interactions. The effect is large and can be experimentally verified using the four probe setup. Since the induced non-local voltage changes sign and magnitude upon varying the gate potential and/or coupling of the quantum dot to the superconducting lead, such measurement could hence provide a controlled and precise method to determine the positions of the Shiba/Andreev states. Our predictions ought to be contrasted with non-local effects observed hitherto in the three-terminal planar junctions where the residual negative non-local conductance has been observed at large voltages, related to the Thouless energy of quasiparticles tunneling through the superconducting slab.

Current and Shot Noise in a Ferromagnetic Double Tunnel Junction with an Atomic Size Spacer

Recent interest in electronic transport in magnetic systems of a nanometer size is stimulated by expected applications in electronic devices. There are also interesting physical phenomena due to the interplay of spin and charge degrees of freedom as well as due to correlation between conducting channels for electrons with opposite spin orientations [1]. We have studied a ferromagnetic single electron transistor (fSET) [2], which consists of a metallic grain connected through tunnel barriers to two ferromagnetic electrodes. The grain size is of the order of one nanometer, so the single electron charging energy E c = e 2/2C and the interlevel spacing ΔE are larger than the thermal energy k B T. We show that Coulomb correlations on the grain can lead to new effects, for instance to negative differential resistance (NDR). Correlations between currents of electrons with different spin orientations have a significant influence on the shot noise. We show that apart from a typical reduction of the zero-frequency power spectrum below the Poissonian value S I 0 = 2 eI