Mariusz Ciorga

Addition spectrum of a lateral dot from Coulomb and spin-blockade spectroscopy

Transport measurements are presented on a class of electrostatically defined lateral dots within a high mobility two dimensional electron gas (2DEG). The new design allows Coulomb Blockade(CB) measurements to be performed on a single lateral dot containing 0, 1 to over 50 electrons. The CB measurements are enhanced by the spin polarized injection from and into 2DEG magnetic edge states. This combines the measurement of charge with the measurement of spin through spin blockade spectroscopy. The results of Coulomb and spin blockade spectroscopy for first 45 electrons enable us to construct the addition spectrum of a lateral device. We also demonstrate that a lateral dot containing a single electron is an effective local probe of a 2DEG edge.

Electrical spin injection and detection in lateral all-semiconductor devices

Both electrical injection and detection of spin-polarized electrons are demonstrated in a single wafer all-semiconductor GaAs-based lateral spintronic device, employing p+-(Ga,Mn)As/n+-GaAs ferromagnetic Esaki diodes as spin aligning contacts. Spin-dependent phenomena, such as spin precession and spin-valve effect, are observed in nonlocal signal and the measurements reveal the unusual origin of the latter in the investigated devices. The conversion of spin-polarized holes into spin-polarized electrons via Esaki tunneling leaves its mark in a bias dependence of the spin-injection efficiency, which at maximum reaches the value of 50%.

Collapse of the spin-singlet phase in quantum dots

We present experimental and theoretical results on a new regime in quantum dots in which the filling factor two-singlet state is replaced by new spin polarized phases. We make use of spin blockade spectroscopy to identify the transition to this new regime as a function of the number of electrons. The key experimental observation is a reversal of the phase in the systematic oscillation of the amplitude of Coulomb blockade peaks as the number of electrons is increased above a critical number. It is found theoretically that correlations are crucial to the existence of the new phases.

Voltage-tunable singlet-triplet transition in lateral quantum dots

(Received 30 November 2001; published 18 July 2002) Results of calculations and high source-drain transport measurements are presented, which demonstrate voltage-tunable entanglement of electron pairs in lateral quantum dots. At a fixed magnetic field, the application of a judiciously chosen gate voltage alters the ground state of an electron pair from an entagled spin singlet to a spin triplet.

Tunable negative differential resistance controlled by spin blockade in single-electron transistors

The single-electron transistors containing a few electrons and spin polarized source and drain contacts were formed in GaAs/GaAlAs heterojunctions using metallic gates. Coulomb blockade measurements reveal well-defined regimes where a decrease in the current is observed with increasing bias. We establish that the origin of the negative regime is the spin-polarized detection of electrons combined with a long spin relaxation time in the dot.

Demonstration of the spin solar cell and spin photodiode effect

Spin injection and extraction are at the core of semiconductor spintronics. Electrical injection is one method of choice for the creation of a sizeable spin polarization in a semiconductor, requiring especially tailored tunnel or Schottky barriers. Alternatively, optical orientation can be used to generate spins in semiconductors with significant spin-orbit interaction, if optical selection rules are obeyed, typically by using circularly polarized light at a well-defined wavelength. Here we introduce a novel concept for spin injection/extraction that combines the principle of a solar cell with the creation of spin accumulation. We demonstrate that efficient optical spin injection can be achieved with unpolarized light by illuminating a p-n junction where the p-type region consists of a ferromagnet. The discovered mechanism opens the window for the optical generation of a sizeable spin accumulation also in semiconductors without direct band gap such as Si or Ge.

Readout of a single electron spin based quantum bit by current detection

Abstract We present experimental and theoretical results demonstrating the readout of a single spin based quantum bit by current detection. The quantum bit has been built by isolating a single electron spin in a lateral dot with a tunable number of electrons. The spin state of the dot was read from the current amplitude resulting from the injection of spin polarized electrons. The current amplitude showed a systematic dependence on the spin state of the dot controlled by the number of electrons.

Annealing-induced magnetic moments detected by spin precession measurements in epitaxial graphene on SiC

We present results of nonlocal and three-terminal (3T) spin precession measurements on spin injection devices fabricated on epitaxial graphene on SiC. The measurements were performed before and after an annealing step at 150 ∘C for 15 minutes in vacuum. The values of spin relaxation length Ls and spin relaxation time τs obtained after annealing are reduced by a factor 2 and 4, respectively, compared to those before annealing. An apparent discrepancy between spin diffusion constant Ds and charge diffusion constant Dc can be resolved by investigating the temperature dependence of the g factor, which is consistent with a model for paramagnetic magnetic moments.

Shot Noise Induced by Nonequilibrium Spin Accumulation

When an electric current passes across a potential barrier, the partition process of electrons at the barrier gives rise to the shot noise, reflecting the discrete nature of the electric charge. Here we report the observation of excess shot noise connected with a spin current which is induced by a nonequilibrium spin accumulation in an all-semiconductor lateral spin-valve device. We find that this excess shot noise is proportional to the spin current. Additionally, we determine quantitatively the spin-injection-induced electron temperature by measuring the current noise. Our experiments show that spin accumulation driven shot noise provides a novel means of investigating nonequilibrium spin transport.

TAMR effect in (Ga,Mn)As-based tunnel structures

We discuss the results of our experiments on tunnel devices based on (Ga,Mn)As structures. Those include p+-(Ga, Mn)As/n+-GaAs Esaki diodes and laterally defined narrow nanoconstrictions in (Ga,Mn)As epilayers. We found in those structures strong anisotropic magnetoresistance behaviour with features that could be attributed to the novel tunnelling anisotropic magnetoresistance effect. We argue however, that in case of nanoconstricted (Ga,Mn)As wires, some other physics has to be additionally employed to fully explain the observed effects.