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1
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Current Fluctuations in Ferromagnetic Tunnel Junctions

100%
Bułka B., Martinek J., Michałek G., Barnaś J.
Acta Physica Polonica A
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2000
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vol. 97
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issue 3
423-426
EN
Current shot noise in a double junction consisting of a small metallic grain connected to two ferromagnetic electrodes is analysed in the limit of sequential tunneling. We show that, apart from charge fluctuations, there are strong spin fluctuations. We also show that two distinct relaxation processes can be distinguished in the frequency dependent current noise: one in a low frequency range and corresponding to spin fluctuations and another one in a high frequency range corresponding to charge fluctuations.
2
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Correlation Histograms in Conductance Measurements of Nanowires Formed at Semiconductor Interfaces

100%
Wawrzyniak M., Martinek J., Susła B., Ilnicki G.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
384-386
EN
We demonstrate experimentally that conductance steps can occur in nanowires formed at metal-semiconductor junctions, between a cobalt tip and a germanium surface revealing long-duration plateaus at reproducible levels. The high reproducibility of the conductance traces obtained leads to very sharp peaks in the conductance histogram suggesting formation of stable atomic configurations. We develop a new type of correlation analysis of the preferred conductance values that provide new type of information on a few-atomic-nanocontact formation dynamics.
3
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Entanglement Detection by Current Measurements in Double Quantum Dot System

100%
Busz P., Rożek P., Tomaszewski D., Martinek J.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
490-492
EN
The development of quantum computing in quantum dots systems requires highly efficient and continuous solid-state source of spatially separated spin-entangled electrons. One of the approaches is a use of double quantum dot system connected to superconducting lead, where Cooper pairs provide a source of naturally entangled electrons. Apart from the source, an useful tool for detection of quantum entanglement is needed. We present entanglement detection by the ferromagnetic electrodes using entanglement witness operator method and direct measurement of spin polarized current in the system. We investigate requirements that have to be fulfilled by ferromagnetic detectors.
4
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Cotunneling Transport at Singlet-Triplet Transition in Carbon Nanotube Quantum Dots

88%
Ilnicki G., Moriyama S., Martinek J., Fuse T., Ishibashi K.
Acta Physica Polonica A
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2009
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vol. 115
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issue 1
299-302
EN
Electronic transport through a quantum dot in the Coulomb-blockade cotunelling regime is investigated both experimentally and theoretically, where a single-wall carbon-nanotube weakly coupled to metallic leads plays a role of the dot. We observe a pronounced current peak for the singlet-triplet transition in a half-shell filling regime due to inelastic cotunneling processes. Using the second order perturbation theory we explain physical mechanisms determining the details of signal.
5
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Zero-Bias Anomaly in Magnetic Tunnel Junctions

88%
Yang H., Yang S., Ilnicki G., Martinek J., Parkin S.
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issue 2
316-318
EN
We present experimental results, which may indicate the possibility of the coexistence of the Kondo effect and ferromagnetism in macroscopic planar magnetic tunnel junctions with a layer of nanodots inside tunnel barriers. A conductance double peak structure was observed. Magnetic field dependence of the splitting of a conductance peak, and temperature evolution of the conductance curves are well explained from the theoretical point of view according to the predictions of the Kondo physics and cotunneling in the Anderson quantum dot coupled to ferromagnetic leads.
6
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Current-Voltage Characteristics of Nanowires Formed at the Co-Ge_{99.99}Ga_{0.01} Interface

88%
Wawrzyniak M., Maćkowski M., Śniadecki Z., Idzikowski B., Martinek J.
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issue 2
375-378
EN
We present a method of measurement of the current-voltage (I-V) and conductance-voltage (G-V) characteristics of nanowires with quantum point contact formed at the Co-Ge_{99.99}Ga_{0.01} interface. The effect of the Fermi level pinning leads to the formation of an ohmic contact between Co and Ge_{99.99}Ga_{0.01}. On the measured characteristics, above the threshold value of voltage an exponential current growth is observed. Such effect could be useful in the production of the electronic nanodevices.
7
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Spin Polarized Transport through Quantum Dots: Coulomb Blockade and Kondo Effect

76%
Barnaś J., Świrkowicz R., Wilczyński M., Weymann I., Martinek J., Dugaev V.
Acta Physica Polonica A
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2003
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vol. 104
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issue 2
165-177
EN
Spin related effects in electronic transport through quantum dots, coupled via tunneling barriers to two metallic leads, are discussed from the point of view of fundamental physics and possible applications in spin electronics. The effects follow either from long spin relaxation time in the dots or from spin dependent tunneling through the barriers when the external leads are ferromagnetic. In the former case large nonequilibrium spin fluctuations in the dot can be induced by flowing current. These fluctuations modify transport characteristics, particularly the shape of the Coulomb steps. In the latter case electric current depends on magnetic configuration of the system, and tunnel magnetoresistance effect due to magnetization rotation can occur. Transport properties in the weak coupling regime are described perturbatively in the first (sequential) and second (cotunneling) orders. In the strong coupling regime, on the other hand, the equation of motion for nonequilibrium Green functions is used to calculate electric current at low temperatures, where the Kondo peak in conductance is formed in the zero bias regime. In symmetrical systems the Kondo peak is split in the parallel magnetic configuration, whereas no splitting occurs for the antiparallel alignment. Theoretical results are discussed in view of available experimental data.
8
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Entanglement Detection with Non-Ideal Ferromagnetic Detectors

64%
Rożek P., Busz P., Kłobus W., Tomaszewski D., Grudka A., Baumgartner A., Schönenberger C., Martinek J.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
493-495
EN
Entangled states are essential in basics quantum communication protocols and quantum cryptography. Ferromagnetic contacts can work as a spin detector, giving possibility of converting information about electron spin to the electric charge, and therefore, detection of entangled states with the electric current measurements is possible. Method of confirming entanglement with non-ideal detectors is presented, the impact of decoherence and noise on states and quality of entanglement is discussed. Entanglement witness (EW) operator method is compared with the CHSH inequalities approach. Required spin polarization for the EW is lower than for the CHSH inequalities. System with asymmetric spin polarizations of detectors was analyzed, including the CHSH inequalities and the EW method.
9
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Magnetic Properties of Epitaxial Fe/(Ga,Mn)As Hybrids

52%
Kalbarczyk K., Dziatkowski K., Szczytko J., Gosk J., Tokarczyk M., Kowalski G., Twardowski A., Bednarski W., Ostrowski A., Waplak S., Martinek J., Liu X., Furdyna J.
Acta Physica Polonica A
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2013
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vol. 124
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issue 5
873-876
EN
Thin-film structures composed of two kinds of ferromagnetic material - metallic Fe and semiconducting (Ga,Mn)As - were investigated by means of SQUID magnetometry and ferromagnetic resonance spectroscopy. Dependence of remnant magnetic moment on temperature showed unexpected anisotropic features when recorded along two orthogonal in-plane directions. For one of these orientations, the change in sign of the slope of m(T) curve at the Curie point of (Ga,Mn)As was observed, while for the other, an analogous m(T) curve retained monotonic character. Based on the comparison with ferromagnetic resonance data, the apparent non-monotonicity was attributed to the temperature-induced change of balance between the external magnetic field and uniaxial magnetic anisotropy in the plane of Fe layer.
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