Pawel Hawrylak

Publications of Pawel Hawrylak

• Dynamical Magnetic and Nuclear Polarization in Complex Spin Systems: Semi-magnetic II-VI Quantum Dots

  Authors: Ramin M. Abolfath, Anna Trojnar, Bahman Roostaei, Thomas Brabec, Pawel Hawrylak

  02/2012;

  Dynamical magnetic and nuclear polarization in complex spin systems is discussed on the example of transfer of spin from exciton to the central spin of magnetic impurity in a quantum dot in theDynamical magnetic and nuclear polarization in complex spin systems is discussed on the example of transfer of spin from exciton to the central spin of magnetic impurity in a quantum dot in the presence of a finite number of nuclear spins. The exciton is described in terms of the electron and heavy hole spins interacting via exchange interaction with magnetic impurity, via hypeprfine interaction with a finite number of nuclear spins and via dipole interaction with photons. The time-evolution of the exciton, magnetic impurity and nuclear spins is calculated exactly between quantum jumps corresponding to exciton radiative recombination. The collapse of the wavefunction and the refilling of the quantum dot with new spin polarized exciton is shown to lead to build up of magnetization of the magnetic impurity as well as nuclear spin polarization. The competition between electron spin transfer to magnetic impurity and to nuclear spins simultaneous with the creation of dark excitons is elucidated. The technique presented here opens up the possibility of studying optically induced Dynamical Magnetic and Nuclear Polarization in Complex Spin Systems.

• Spin textures in strongly coupled electron spin and magnetic or nuclear spin systems in quantum dots

  Authors: Ramin M. Abolfath, Marek Korkusinski, Thomas Brabec, Pawel Hawrylak

  12/2011;

  Controlling electron spins strongly coupled to magnetic and nuclear spins in solid state systems is an important challenege in the field of spintronics and quantum computation. We show here thatControlling electron spins strongly coupled to magnetic and nuclear spins in solid state systems is an important challenege in the field of spintronics and quantum computation. We show here that electron droplets with no net spin in semiconductor quantum dots strongly coupled with magnetic ion/nuclear spin systems break down at low temperature and form a non-trivial anti-ferromagnetic spatially ordered spin-texture of magneto-polarons. The spatially ordered combined electron-magnetic ion spin-texture, associated with spontaneous symmetry-breaking in the parity of electronic charge density and magnetization of magnetic ions, emerge from both ab-initio density functional approach to the electronic system coupled with mean-field approximation for the magnetic/nuclear spin system and fully mircoscopic exact diagonalization of small systems. The predicted phase diagram determines the critical temperature as a function of coupling strength and identifies possible phases of the strongly coupled spin system. This prediction may arrest fluctuations in spin system and open the way to control, manipulate and prepare magnetic and nuclear spin ensembles in semiconductor nanostructures.

• Herzberg Circuit and Berry's Phase in Chirality-based Coded Qubit in a Triangular Triple Quantum Dot

  Authors: Chang-Yu Hsieh, Alexandre Rene, Pawel Hawrylak

  10/2011;

  We present a theoretical proposal for the Herzberg circuit and controlled accumulation of Berry's phase in a chirality-based coded qubit in a triangular triple quantum dot molecule with one electronWe present a theoretical proposal for the Herzberg circuit and controlled accumulation of Berry's phase in a chirality-based coded qubit in a triangular triple quantum dot molecule with one electron spin each. The qubit is encoded in the two degenerate states of a three spin complex with total spin $S=1/2$. Using a Hubbard and Heisenberg model the Herzberg circuit encircling the degeneracy point is realized by adiabatically tuning the successive on-site energies of quantum dots and tunnel couplings across a pair of neighbouring dots. It is explicitly shown that encircling the degeneracy point leads to the accumulation of the geometrical Berrys phase. We show that only triangular but not linear quantum dot molecule allows for the generation of Berry's phase and we discuss a protocol to detect this geometrical phase.

• Theory of electronic properties and quantum spin blockade in a gated linear triple quantum dot with one electron spin each

  Authors: Chang-Yu Hsieh, Yun-Pil Shim, Pawel Hawrylak

  10/2011;

  We present a theory of electronic properties and the spin blockade phenomena in a gated linear triple quantum dot. Quadruple points where four different charge configurations are on resonance,We present a theory of electronic properties and the spin blockade phenomena in a gated linear triple quantum dot. Quadruple points where four different charge configurations are on resonance, particularly involving (1,1,1) configuration, are considered. In the symmetric case, the central dot is biased to higher energy and a single electron tunnels through the device when (1,1,1) configuration is resonant with (1,0,1),(2,0,1),(1,0,2) configurations. The electronic properties of a triple quantum dot are described by a Hubbard model containing two orbitals in the two unbiased dots and a single orbital in the biased dot. The transport through the triple quantum dot molecule involves both singly and doubly occupied configurations and necessitates the description of the (1,1,1) configuration beyond the Heisenberg model. Exact eigenstates of the triple quantum dot molecule with up to three electrons are used to compute current assuming weak coupling to the leads and non-equilibrium occupation of quantum molecule states obtained from the rate equation. The intra-molecular relaxation processes due to acoustic phonons and cotunneling with the leads are included, and are shown to play a crucial role in the spin blockade effect. We find a quantum interference-based spin blockade phenomenon at low source-drain bias and a distinct spin blockade due to a trap state at higher bias. We also show that, for an asymmetric quadruple point with (0,1,1),(1,1,1,),(0,2,1),(0,1,2) configurations on resonance, the spin blockade is analogous to the spin blockade in a double quantum dot.

• Theory of highly excited semiconductor nanostructures including Auger coupling: exciton-bi-exciton mixing in CdSe nanocrystals

  Authors: Marek Korkusinski, Oleksandr Voznyy, Pawel Hawrylak

  09/2011;

  We present a theory of highly excited interacting carriers confined in a semiconductor nanostructure, incorporating Auger coupling between excited states with different number of excitations. TheWe present a theory of highly excited interacting carriers confined in a semiconductor nanostructure, incorporating Auger coupling between excited states with different number of excitations. The Coulomb matrix elements connecting exciton, bi-exciton and tri-exciton complexes are derived and an intuitive picture of breaking neutral multi-exction complexes into positively and negatively charged multi-exciton complexes is given. The general approach is illustrated by analyzing the coupling of biexciton and exciton in CdSe spherical nanocrystals. The electron and hole states are computed using atomistic $sp^3d^5s^*$ tight binding Hamiltonian including an effective crystal field splitting and surface passivation. For each number of electron-hole pairs the many-body spectrum is computed in the configuration-interaction approach. The low-energy correlated biexciton levels are broken into charged complexes: a hole and a negatively charged trion and an electron and a positively charged trion. Out of a highly excited exciton spectrum a subspace coupled to bi-exciton levels via Auger processes is identified. The interaction between correlated bi-exciton and exciton states is treated using exact diagonalization techniques. This allows to extract the spectral function of the biexciton and relate its characteristic width and amplitude to the characteristic amplitude and timescale of the coherent time evolution of the coupled system. It is shown that this process can be described by the Fermi's Golden Rule only if a fast relaxation of the excitonic subsystem is accounted for.

• Absolute deformation potentials and robust ab initio model for band shifts induced by (001) biaxial strain in group IIIA-VA semiconductors

  Authors: Eugene S. Kadantsev, Pawel Hawrylak

  Applied Physics Letters. 02/2011;

  A model for the evolution of conduction and valence bands of IIIA-VA (InAs, GaAs, and InP) semiconductors under (001) biaxial strain is developed. The model is based on the ab initio calculationsA model for the evolution of conduction and valence bands of IIIA-VA (InAs, GaAs, and InP) semiconductors under (001) biaxial strain is developed. The model is based on the ab initio calculations which take into account finite strain dependent relaxation of the reference levels. The results of ab initio full potential calculations of absolute deformation potentials (ADPs) and (001) biaxial strain-modified band edges are reported. It is shown that in type I heterostructures subjected to (001) compressive biaxial strain, the corrections due to nonzero ADP of the core reference levels reduce the strained band offset for holes.

• Greenberger-Horne-Zeilinger States in Quantum Dot Molecule

  Authors: Anand Sharma, Pawel Hawrylak

  01/2011;

  We present a microscopic theory of a lateral quantum dot molecule in a radial magnetic field with a Greenberger- Horne- Zeilinger (GHZ) maximally entangled three particle groundstate. The quantum dotWe present a microscopic theory of a lateral quantum dot molecule in a radial magnetic field with a Greenberger- Horne- Zeilinger (GHZ) maximally entangled three particle groundstate. The quantum dot molecule consists of three quantum dots with one electron spin each forming a central equilateral triangle. The anti-ferromagnetic spin-spin interaction is changed to the ferromagnetic interaction by additional doubly occupied quantum dots, one dot near each side of a triangle. The magnetic field is provided by micro-magnets. The interaction among the electrons is described within an extended Hubbard Hamiltonian and electronic states are obtained using configuration interaction approach. The set of parameters is established for which the ground state of the molecule in a radial magnetic field is well approximated by a GHZ state.

• Edge stability, reconstruction, zero-energy states and magnetism in triangular graphene quantum dots with zigzag edges

  Authors: Oleksandr Voznyy, Alev Devrim Güçlü, Pawel Potasz, Pawel Hawrylak

  11/2010;

  We present the results of ab-initio density functional theory based calculations of the stability and reconstruction of zigzag edges in triangular graphene quantum dots. We show that, while theWe present the results of ab-initio density functional theory based calculations of the stability and reconstruction of zigzag edges in triangular graphene quantum dots. We show that, while the reconstructed pentagon-heptagon zigzag edge structure is more stable in the absence of hydrogen, ideal zigzag edges are energetically favored by hydrogen passivation. Zero-energy band exists in both structures when passivated by hydrogen, however in case of pentagon-heptagon zigzag, this band is found to have stronger dispersion, leading to the loss of net magnetization. Comment: 4.2 pages, 5 figures

• Fine structure and size dependence of exciton and bi-exciton optical spectra in CdSe nanocrystals

  Authors: Marek Korkusinski, Oleksandr Voznyy, Pawel Hawrylak

  09/2010;

  Theory of electronic and optical properties of exciton and bi-exciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomisticTheory of electronic and optical properties of exciton and bi-exciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic $sp^3d^5s^*$ tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum resembles $s$ and $p$ shells as expected in a single-band effective mass approximation but the valence hole spectrum is composed of four low-lying, doubly degenerate states separated from the rest by a gap. As a result, the bi-exciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and bi-exciton spectra. The degenerate nature of the hole spectrum makes the bi-exciton energy and wave function sensitive to correlation effects. We find that the relative position of the exciton and bi-exciton peaks in the emission spectrum depends on the size of the basis, temperature, and diameter of the nanocrystal. Comment: 13 pages

• Ab initio calculation of band edges modified by (001) biaxial strain in group IIIA–VA and group IIB–VIA semiconductors: Application to quasiparticle energy levels of strained InAs/InP quantum dot

  Authors: Eugene S. Kadantsev, Michal Zielinski, Marek Korkusinski, Pawel Hawrylak

  Journal of Applied Physics. 06/2010;

  Results of first-principles full potential calculations of absolute position of valence and conduction energy bands as a function of (001) biaxial strain are reported for group IIIA–VA (InAs, GaAs,Results of first-principles full potential calculations of absolute position of valence and conduction energy bands as a function of (001) biaxial strain are reported for group IIIA–VA (InAs, GaAs, InP) and group IIB–VIA (CdTe, ZnTe) semiconductors. Our computational procedure is based on the Kohn–Sham form of density functional theory (KS DFT), local spin density approximation (LSDA), variational treatment of spin-orbital coupling, and augmented plane wave plus local orbitals method ( APW + lo ) . The band energies are evaluated at lattice constants obtained from KS DFT total energy as well as from elastic free energy. The conduction band energies are corrected with a rigid shift to account for the LSDA band gap error. The dependence of band energies on strain is fitted to polynomial of third degree and results are available for parameterization of biaxial strain coupling in empirical tight-binding models of IIIA–VA and IIB–VIA self-assembled quantum dots (SAQDs). The strain effects on the quasiparticle energy levels of InAs/InP SAQD are illustrated with empirical atomistic tight-binding calculations.

• Quantum circuits based on coded qubits encoded in chirality of electron spin complexes in triple quantum dots

  Authors: Chang-Yu Hsieh, Pawel Hawrylak

  04/2010;

  We present a theory of quantum circuits based on logical qubits encoded in chirality of electron spin complexes in lateral gated semiconductor triple quantum dot molecules with one electron spin inWe present a theory of quantum circuits based on logical qubits encoded in chirality of electron spin complexes in lateral gated semiconductor triple quantum dot molecules with one electron spin in each dot. Using microscopic Hamiltonian we show how to initialize, coherently control and measure the quantum state of a chirality based coded qubit using static in-plane magnetic field and voltage tuning of individual dots. The microscopic model of two interacting coded qubits is established and mapped to an Ising Hamiltonian, resulting in conditional two-qubit phase gate.

• Macroscopic quantum state in a semiconductor device

  Authors: Yun-Pil Shim, Anand Sharma, Chang-Yu Hsieh, Pawel Hawrylak

  03/2010;

  We show how nanostructuring of a metallic gate on a field-effect transistor (FET) can lead to a macroscopic, robust and voltage controlled quantum state in the electron channel of a FET. A chain ofWe show how nanostructuring of a metallic gate on a field-effect transistor (FET) can lead to a macroscopic, robust and voltage controlled quantum state in the electron channel of a FET. A chain of triple quantum dot molecules created by gate structure realizes a spin-half Heisenberg chain with spin-spin interactions alternating between ferromagnetic and anti-ferromagnetic. The quantum state is a semiconductor implementation of an integer spin-one antiferromagnetic Heisenberg chain with a unique correlated ground state and a finite energy gap, originally conjectured by Haldane.

• Fine structure and size dependence of exciton and biexciton optical spectra in CdSe nanocrystals

  Authors: Marek Korkusinski, Oleksandr Voznyy, Pawel Hawrylak

  Phys. Rev. B. 01/2010; 82:245304-245304.

  Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic sp3d5s∗Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic sp3d5s∗ tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum is organized in shells (s,p,…), while the valence hole spectrum is composed of four low-lying, doubly degenerate states separated from the rest by a gap. As a result, the biexciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and biexciton spectra. The quasidegenerate nature of the hole spectrum results in a correlated biexciton state, which makes it slowly convergent with basis size. We carry out a

• Theory of exciton fine structure in semiconductor quantum dots: quantum dot anisotropy and lateral electric field

  Authors: Eugene Kadantsev, Pawel Hawrylak

  10/2009;

  Theory of exciton fine structure in semiconductor quantum dots and its dependence on quantum dot anisotropy and external lateral electric field is presented. The effective exciton HamiltonianTheory of exciton fine structure in semiconductor quantum dots and its dependence on quantum dot anisotropy and external lateral electric field is presented. The effective exciton Hamiltonian including long range electron-hole exchange interaction is derived within the k*p effective mass approximation (EMA). The exchange matrix elements of the Hamiltonian are expressed explicitly in terms of electron and hole envelope functions. The matrix element responsible for the "bright" exciton splitting is identified and analyzed. An excitonic fine structure for a model quantum dot with quasi- two-dimensional anisotropic harmonic oscillator (2DLAHO) confining potential is analyzed as a function of the shape anisotropy, size and applied lateral electric field.

• Valence holes as Luttinger spinor based qubits in quantum dots

  Authors: Chang-Yu Hsieh, Ross Cheriton, Marek Korkusinski, Pawel Hawrylak

  08/2009;

  We present a theory of valence holes as Luttinger spinor based qubits in p-doped self-assembled quantum dots within the 4-band $k\cdot p$ formalism. The two qubit levels are identified with the twoWe present a theory of valence holes as Luttinger spinor based qubits in p-doped self-assembled quantum dots within the 4-band $k\cdot p$ formalism. The two qubit levels are identified with the two chiralities of the doubly degenerate ground state. We show that single qubit operations can be implemented with static magnetic field applied along the $z$ and $x$ directions, acting analogously to the $\hat{\sigma}_z$ and $\hat{\sigma}_x$ operators in the qubit subspace respectively. The coupling of two dots and hence the double qubit operations are shown to be sensitive to the orientation of the two quantum dots. For vertical qubit arrays, there exists an optimal qubit separation suitable for the voltage control of qubit-qubit interactions.

• Optical signatures of spin polarization of carriers in quantum dots.

  Authors: Marek Korkusinski, Pawel Hawrylak

  Physical review letters. 08/2008; 101(2):027205.

  We predict theoretically the optical signatures of spin polarization of carriers in self-assembled quantum dots. The emission spectra are mapped out as a function of increasing electron spinWe predict theoretically the optical signatures of spin polarization of carriers in self-assembled quantum dots. The emission spectra are mapped out as a function of increasing electron spin polarization for a fixed number of electrons and holes. The spin-polarized spectra are determined using exact diagonalization techniques for up to 12 particles, corresponding to two lowest filled shells. We predict that the spin polarization leads to photon polarization, to redshifts of emission lines due to excess exchange interactions among the spin-polarized electrons, and to a complete breakup of emission lines for spin-polarized electronic shells.

• Electronic States of Magnetic Quantum Dots

  Authors: Ramin M. Abolfath, Pawel Hawrylak, Igor Zutic

  10/2007;

  We study quantum states of electrons in magnetically doped quantum dots as a function of exchange coupling between electron and impurity spins, the strength of Coulomb interaction, confiningWe study quantum states of electrons in magnetically doped quantum dots as a function of exchange coupling between electron and impurity spins, the strength of Coulomb interaction, confining potential, and the number of electrons. The magnetic phase diagram of quantum dots, doped with a large number of magnetic Mn impurities, can be described by the energy gap in the spectrum of electrons and the mean field electron-Mn exchange coupling. A competition between these two parameters leads to a transition between spin-unpolarized and spin-polarized states, in the absence of applied magnetic field. Tuning the energy gap by electrostatic control of nonparabolicity of the confining potential can enable control of magnetization even at the fixed number of electrons. We illustrate our findings by directly comparing Mn-doped quantum dots with parabolic and Gaussian confining potential. Comment: 5 pages, 5 figures, Part of Focus on Spintronics in Reduced Dimensions

• Voltage Induced Hidden Symmetry and Photon Entanglement Generation in a Single, Site-Selected Quantum Dot

  Authors: Michael E. Reimer, Marek Korkusinski, Jacques Lefebvre, Jean Lapointe, Philip J Poole, Geof C. Aers, Dan Dalacu, W. Ross McKinnon, Simon Frederick, Pawel Hawrylak, Robin L. Williams

  07/2007;

  Present proposals for the realisation of entangled photon pair sources using the radiative decay of the biexciton in semiconductor quantum dots are limited by the need to enforce degeneracy of thePresent proposals for the realisation of entangled photon pair sources using the radiative decay of the biexciton in semiconductor quantum dots are limited by the need to enforce degeneracy of the two intermediate, single exciton states. We show how this requirement is lifted if the biexciton binding energy can be tuned to zero and we demonstrate this unbinding of the biexciton in a single, pre-positioned InAs quantum dot subject to a lateral electric field. Full Configuration-Interaction calculations are presented that reveal how the biexciton is unbound through manipulation of the electron-hole Coulomb interaction and the consequent introduction of Hidden Symmetry.

• LCHO-CI method for the voltage control of exchange interaction in gated lateral quantum dot networks

  Authors: Irene Puerto Gimenez, Marek Korkusinski, Pawel Hawrylak

  07/2007;

  We present a computational LCHO-CI approach allowing for the simulation of exchange interactions in gated lateral quantum dot networks. The approach is based on single-particle states calculatedWe present a computational LCHO-CI approach allowing for the simulation of exchange interactions in gated lateral quantum dot networks. The approach is based on single-particle states calculated using a linear combination of harmonic orbitals (LCHO) of each of the dots, and a configuration interaction (CI) approach to the interacting electron problem. The LCHO-CI method is applied to a network of three quantum dots with one electron spin per dot, and a Heisenberg spin Hamiltonian is derived. The manipulation of spin states of a three-spin molecule by applying bias to one of the dots is demonstrated and related to the bias dependence of effective exchange interaction parameters.

• Tailoring magnetism in quantum dots.

  Authors: Ramin M. Abolfath, Pawel Hawrylak, Igor Zutić

  Physical review letters. 06/2007; 98(20):207203.

  We study magnetism in magnetically doped quantum dots as a function of the confining potential, particle numbers, temperature, and strength of the Coulomb interactions. We explore the possibility ofWe study magnetism in magnetically doped quantum dots as a function of the confining potential, particle numbers, temperature, and strength of the Coulomb interactions. We explore the possibility of tailoring magnetism by controlling the nonparabolicity of the confinement potential and the electron-electron Coulomb interaction, without changing the number of particles. The interplay of strong Coulomb interactions and quantum confinement leads to enhanced inhomogeneous magnetization which persists at higher temperatures than in the noninteracting case. The temperature of the onset of magnetization can be controlled by changing the number of particles as well as by modifying the quantum confinement and the strength of the Coulomb interactions. We predict a series of electronic spin transitions which arise from the competition between the many-body gap and magnetic thermal fluctuations.