# Robert Andrzej Żak

## Contact

Department of Physics
University of Basel
Klingelbergstrasse 82
CH-4056 Basel, Switzerland

 office: 4.9 email: view address tel: +41 61 267 37 46 (office) +41 77 448 62 15 (mobile) fax: +41 61 267 13 49

Personal webpage: http://quantumtheory.physik.unibas.ch/zak/personal/

## Short CV

 2007 - present PhD student at the University of Basel 2007 Master in Physics from the University of Warsaw

## Research interests

• Nuclear magnetism
• Strongly correlated systems
• Spintronics
• Quantum computation and quantum information theory
• Electron transport in nanostructures

## Publications

Show all abstracts.

 1. Spin susceptibility of interacting two-dimensional electrons in the presence of spin-orbit coupling Robert Andrzej Żak, Dmitrii Maslov, and Daniel Loss. Phys. Rev. B 82, 115415 (2010); arXiV:1005.1913. A long-range interaction via virtual particle-hole pairs between Fermi-liquid quasiparticles leads to a nonanalytic behavior of the spin susceptibility $\chi$ as a function of the temperature ($T$), magnetic field ($\mathbf{B}$), and wavenumber. In this paper, we study the effect of the Rashba spin-orbit interaction (SOI) on the nonanalytic behavior of $\chi$ for a two-dimensional electron liquid. Although the SOI breaks the $SU(2)$ symmetry, it does not eliminate nonanalyticity but rather makes it anisotropic: while the linear scaling of $\chi_{zz}$ with $T$ and $|\mathbf{B}|$ saturates at the energy scale set by the SOI, that of $\chi_{xx}$ ($=\chi_{yy}$) continues through this energy scale, until renormalization of the electron-electron interaction in the Cooper channel becomes important. We show that the Renormalization Group flow in the Cooper channel has a non-trivial fixed point, and study the consequences of this fixed point for the nonanalytic behavior of $\chi$. An immediate consequence of SOI-induced anisotropy in the nonanalytic behavior of $\chi$ is a possible instability of a second-order ferromagnetic quantum phase transition with respect to a first-order transition to an $XY$ ferromagnetic state. 2. Quantum computing with electron spins in quantum dots Robert Andrzej Żak, Beat Röthlisberger, Stefano Chesi, and Daniel Loss. Proceedings of the CLXXI International School of Physics 'Enrico Fermi':Quantum Coherence in Solid State Systems (Società Italiana di Fisica, 2009, 61 pages); La Rivista del Nuovo Cimento 33, 7 (2010); arXiv:0906.4045. Several topics on the implementation of spin qubits in quantum dots are reviewed. We first provide an introduction to the standard model of quantum computing and the basic criteria for its realization. Other alternative formulations such as measurement-based and adiabatic quantum computing are briefly discussed. We then focus on spin qubits in single and double GaAs electron quantum dots and review recent experimental achievements with respect to initialization, coherent manipulation and readout of the spin states. We extensively discuss the problem of decoherence in this system, with particular emphasis on its theoretical treatment and possible ways to overcome it. 3. Momentum dependence of the spin susceptibility in two dimensions: nonanalytic corrections in the Cooper channel Stefano Chesi, Robert Andrzej Żak, Pascal Simon, and Daniel Loss. Phys. Rev. B 79, 115445 (2009); arXiv:0811.0996. We consider the effect of rescattering of pairs of quasiparticles in the Cooper channel resulting in the strong renormalization of second-order corrections to the spin susceptibility in a two-dimensional electron system. We use the Fourier expansion of the scattering potential in the vicinity of the Fermi surface to find that each harmonic becomes renormalized independently. Since some of those harmonics are negative, the first derivative of the spin susceptibility is bound to be negative at small momenta, in contrast to the lowest order perturbation theory result, which predicts a positive slope. We present in detail an effective method to calculate diagrammatically corrections to the spin susceptibility to infinite order. 4. Coulomb blockade of a three-terminal quantum dot Robert Andrzej Żak and Karsten Flensberg. Phys. Rev. B 77, 045329 (2008); arXiv:0711.1787. We study an interacting single-level quantum dot weakly coupled to three electrodes. When two electrodes are biased by voltages with opposite polarities, while keeping the third lead (the stem) grounded, the current through the stem is a measure of electron-hole asymmetry of the dot. In this setup we calculate the stem current for both metallic and ferromagnetic (collinearly polarized) leads and discuss how the three-terminal device gives additional information compared to the usual two-terminal setup. We calculate both the sequential and cotunneling contribution for the currents. For the latter part we include a regularization procedure for the cotunneling current, which enables us to also describe the behavior at the charge degeneracy points. 5. Samoistne powstawanie korków (Spontaneous creation of traffic jams) Robert Andrzej Żak Delta 375, 8 (2005) Not available. 6. Piłeczka w szklance wody (Plastic ball in a glass of water) Andrzej Hryczuk and Robert Andrzej Żak. Delta 353, 10 (2003) Not available. 7. Niebieski las (Blue forest) Robert Andrzej Żak and Andrzej Hryczuk. Delta 351, 4 (2003) Not available. 8. O unoszeniu się ciał na wodzie (About floating of bodies) Andrzej Hryczuk and Robert Andrzej Żak. Delta 347, 11 (2003) Not available.