P. Hawrylak

Publications of P. Hawrylak

• Electronic properties of gated triangular graphene quantum dots: Magnetism, correlations, and geometrical effects

  Authors: P. Potasz, A. D. Guclu, A. Wojs, P. Hawrylak

  02/2012;

  We present a theory of electronic properties of gated triangular graphene quantum dots with zigzag edges as a function of size and carrier density. We focus on electronic correlations, spin andWe present a theory of electronic properties of gated triangular graphene quantum dots with zigzag edges as a function of size and carrier density. We focus on electronic correlations, spin and geometrical effects using a combination of atomistic tight-binding, Hartree-Fock and configuration interaction methods (TB+HF+CI) including long range Coulomb interactions. The single particle energy spectrum of triangular dots with zigzag edges exhibits a degenerate shell at the Fermi level with a degeneracy N_{edge} proportional to the edge size. We determine the effect of the electron-electron interactions on the ground state, the total spin and the excitation spectrum as a function of a shell filling and the degeneracy of the shell using TB+HF+CI for N_{edge} < 12 and approximate CI method for N_{edge}\geq 12. For a half-filled neutral shell we find spin polarized ground state for structures up to N=500 atoms in agreement with previous {\it ab initio} and mean-field calculations, and in agreement with Lieb's theorem for a Hubbard model on a bipartite lattice. Adding a single electron leads to the complete spin depolarization for N_{edge}\leq 9. For larger structures, the spin depolarization is shown to occur at different filling factors. Away from half-fillings excess electrons(holes) are shown to form Wigner-like spin polarized triangular molecules corresponding to large gaps in the excitation spectrum. The validity of conclusions is assessed by a comparison of results obtained from different levels of approximations. While for the charge neutral system all methods give qualitatively similar results, away from the charge neutrality an inclusion of all Coulomb scattering terms is necessary to produce results presented here.

• Quantum Interference in Exciton-Mn Spin Interactions in a CdTe Semiconductor Quantum Dot.

  Authors: A H Trojnar, M Korkusiński, E S Kadantsev, P Hawrylak, M Goryca, T Kazimierczuk, P Kossacki, P Wojnar, M Potemski

  Physical review letters. 11/2011; 107(20):207403.

  We show theoretically and experimentally the existence of a new quantum-interference effect between the electron-hole interactions and the scattering by a single Mn impurity. The theoretical model,We show theoretically and experimentally the existence of a new quantum-interference effect between the electron-hole interactions and the scattering by a single Mn impurity. The theoretical model, including electron-valence-hole correlations, the short- and long-range exchange interaction of a Mn ion with the heavy hole and with electron and anisotropy of the quantum dot, is compared with photoluminescence spectroscopy of CdTe dots with single magnetic ions. We show how the design of the electronic levels of a quantum dot enables the design of an exciton, control of the quantum interference, and hence engineering of light-Mn interaction.

• Quantum interference in exciton-Mn spin interactions in a CdTe semiconductor quantum dot

  Authors: A. Trojnar, M Korkusinski, E. Kadantsev, P. Hawrylak, M. Goryca, T. Kazimierczuk, P. Kossacki, P. Wojnar, M. Potemski

  05/2011;

  We show theoretically and experimentally the existence of a new quantum interference(QI) effect between the electron-hole interactions and the scattering by a single Mn impurity. Theoretical model,We show theoretically and experimentally the existence of a new quantum interference(QI) effect between the electron-hole interactions and the scattering by a single Mn impurity. Theoretical model, including electron-valence hole correlations, the short and long range exchange interaction of Mn ion with the heavy hole and with electron and anisotropy of the quantum dot, is compared with photoluminescence spectroscopy of CdTe dots with single magnetic ions. We show how design of the electronic levels of a quantum dot enable the design of an exciton, control of the quantum interference and hence engineering of light-Mn interaction.

• Electric-field controlled spin in bilayer triangular graphene quantum dots

  Authors: A. D. Guclu, P. Potasz, P. Hawrylak

  04/2011;

  We present theoretical results based on mean-field and exact many-body approaches showing that in bilayer triangular graphene quantum dots with zigzag edges the magnetism can be controlled by anWe present theoretical results based on mean-field and exact many-body approaches showing that in bilayer triangular graphene quantum dots with zigzag edges the magnetism can be controlled by an external vertical electric-field. We demonstrate that without electric field the spins of the two layers of the quantum dot interact ferromagnetically. At a critical value of the electric-field, the total spin of the bilayer structure can be turned off or reduced to a single localized spin, a qubit isolated from contacts and free from interaction with nuclear spins.

• Excitonic absorption in gate controlled graphene quantum dots

  Authors: A. D. Guclu, P. Potasz, P. Hawrylak

  07/2010;

  We present a theory of excitonic processes in gate controlled graphene quantum dots. The dependence of the energy gap on shape, size and edge for graphene quantum dots with up to a million atoms isWe present a theory of excitonic processes in gate controlled graphene quantum dots. The dependence of the energy gap on shape, size and edge for graphene quantum dots with up to a million atoms is predicted. Using a combination of tight-binding, Hartree-Fock and configuration interaction methods, we show that triangular graphene quantum dots with zigzag edges exhibit optical transitions simultaneously in the THz, visible and UV spectral ranges, determined by strong electron-electron and excitonic interactions. The relationship between optical properties and finite magnetic moment and charge density controlled by an external gate is predicted. Comment: ~4 pages, 4 figures

• Spin and electronic correlations in gated graphene quantum rings

  Authors: P. Potasz, A.D. Güçlü, P. Hawrylak

  05/2010;

  We present a theory of graphene quantum rings designed to produce degenerate shells of single particle states close to the Fermi level. We show that populating these shells with carriers using a gateWe present a theory of graphene quantum rings designed to produce degenerate shells of single particle states close to the Fermi level. We show that populating these shells with carriers using a gate leads to correlated ground states with finite total electronic spin. Using a combination of tight-binding and configuration interaction methods we predict ground state and total spin of the system as a function of the filling of the shell. We show that for smaller quantum rings, the spin polarization of the ground state at half filling depends strongly on the size of the system, but reaches a maximum value after reaching a critical size. Comment: 7 pages, 8 figures

• Magnetism and correlations in fractionally filled degenerate shells of graphene quantum dots.

  Authors: A D Güçlü, P Potasz, O Voznyy, M Korkusinski, P Hawrylak

  Physical review letters. 12/2009; 103(24):246805.

  We show that the ground state and magnetization of the macroscopically degenerate shell of electronic states in triangular gated graphene quantum dots depends on the filling fraction of the shell.We show that the ground state and magnetization of the macroscopically degenerate shell of electronic states in triangular gated graphene quantum dots depends on the filling fraction of the shell. The effect of degeneracy, finite size, and electron-electron interactions are treated nonperturbatively using a combination of density functional theory, tight-binding, Hartree-Fock and configuration interaction methods. We show that electronic correlations play a crucial role in determining the nature of the ground state as a function of filling fraction of the degenerate shell at the Fermi level. We find that the half-filled charge neutral shell leads to full spin polarization but this magnetic moment can be completely destroyed by adding a single electron.

• Zero-energy states in triangular and trapezoidal graphene structures

  Authors: P. Potasz, A.D. Güçlü, P. Hawrylak

  10/2009;

  We derive analytical solutions for the zero-energy states of degenerate shell obtained as a singular eigenevalue problem found in tight-binding (TB) Hamiltonian of triangular graphene quantum dotsWe derive analytical solutions for the zero-energy states of degenerate shell obtained as a singular eigenevalue problem found in tight-binding (TB) Hamiltonian of triangular graphene quantum dots with zigzag edges. These analytical solutions are in agreement with previous TB and density functional theory (DFT) results for small graphene triangles and extend to arbitrary size. We also generalize these solutions to trapezoidal structure which allow us to study bowtie graphene devices. Comment: 4 pages, 4 figures

• Electronic shells of Dirac fermions in graphene quantum rings in a magnetic field

  Authors: P. Potasz, A. D. Guclu, P. Hawrylak

  08/2009;

  We present results of tight binding calculations demonstrating existence of degenerate electronic shells of Dirac Fermions in narrow, charge neutral graphene quantum rings. We predict removal ofWe present results of tight binding calculations demonstrating existence of degenerate electronic shells of Dirac Fermions in narrow, charge neutral graphene quantum rings. We predict removal of degeneracy with finite magnetic field. We show, using a combination of tight binding and configuration interaction methods, that by filling a graphene ring with additional electrons this carbon based structure with half-filled shell acquires a finite magnetic moment. Comment: 10 pages, 4 figures

• Magnetism and correlations in fractionally filled degenerate shells of graphene quantum dots

  Authors: A. D. Guclu, P. Potasz, O. Voznyy, M Korkusinski, P. Hawrylak

  07/2009;

  When an electron is confined to a triangular atomic thick layer of graphene [1-5] with zig-zag edges, its energy spectrum collapses to a shell of degenerate states at the Fermi level (Dirac point)When an electron is confined to a triangular atomic thick layer of graphene [1-5] with zig-zag edges, its energy spectrum collapses to a shell of degenerate states at the Fermi level (Dirac point) [6-9]. The degeneracy is proportional to the edge size and can be made macroscopic. This opens up the possibility to design a strongly correlated electronic system as a function of fractional filling of the zero-energy shell, in analogy to the fractional quantum Hall effect in a quasi-two-dimensional electron gas[10], but without the need for a high magnetic field. In this work we show that electronic correlations, beyond the Hubbard model[6,7] and mean-field density functional theory (DFT) [7,8] play a crucial role in determining the nature of the ground state and the excitation spectrum of triangular graphene quantum dots as a function of dot size and filling fraction of the shell of zero-energy states. The interactions are treated by a combination of DFT, tight-binding, Hartree-Fock and configuration interaction methods (TB-HF-CI) and include all scattering and exchange terms within second nearest neighbors as well as interaction with metallic gate. We show that a half filled charge neutral shell leads to full spin polarization of the island but this magnetic moment is completely destroyed by the addition of a single electron, in analogy to the effect of skyrmions on the quantum Hall ferromagnet [11-14] and spin depolarization in electrostatically defined semiconductor quantum dots[15-18]. The depolarization of the ground state is predicted to result in blocking of current through a graphene quantum dot due to spin blockade (SB) [18]. Comment: v2: minor corrections, new format

• Multiexciton complexes in InAs self-assembled quantum dots

  Authors: M. Korkusinski, M. Zielinski, P. Hawrylak

  Journal of Applied Physics. 07/2009;

  We review our recent work on multiexciton complexes in InAs self-assembled quantum dots using a combination of effective mass, k∙p , and atomistic sp³s^*d⁵We review our recent work on multiexciton complexes in InAs self-assembled quantum dots using a combination of effective mass, k∙p , and atomistic sp³s^*d⁵ tight-binding approaches. The single-particle levels from effective mass, k∙p , and atomistic tight-binding models are used as input into configuration-interaction calculation of multiexciton spectra. We describe the principles of the atomistic approach and apply all these computational tools to illustrate the concept of hidden symmetry as underlying principle in energy levels of multiexciton complexes, optical detection of electron spin polarization, tunneling of holes in quantum dot molecules, and tuning of multiexciton spectra with lateral electric fields for entangled photon pair generation.

• Tunneling spectroscopy of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot molecule

  Authors: Y. -P. Shim, F. Delgado, P. Hawrylak

  05/2009;

  We present a theory of tunneling spectroscopy of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot molecule. The theory combines exact treatment of an isolated many-bodyWe present a theory of tunneling spectroscopy of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot molecule. The theory combines exact treatment of an isolated many-body system with the rate equation approach when the quantum dot molecule is weakly connected to the leads subject to arbitrary source-drain bias. The tunneling spectroscopy of the many-body complex is analyzed using the spectral functions of the system and applied to holes in a quantum dot molecule. Negative differential conductance is predicted and explained as a result of the redistribution of the spectral weight between transport channels. It is shown that different interference effects on singlet and triplet hole states in a magnetic field lead to spin-selective Aharonov-Bohm oscillations.

• Antibonding ground states in InAs quantum-dot molecules.

  Authors: M. F. Doty, J. I. Climente, M Korkusinski, M. Scheibner, A. S. Bracker, P. Hawrylak, D. Gammon

  Physical review letters. 02/2009; 102(4):047401.

  Coherent tunneling between two InAs quantum dots forms delocalized molecular states. Using magnetophotoluminescence spectroscopy we show that when holes tunnel through a thin barrier, the lowestCoherent tunneling between two InAs quantum dots forms delocalized molecular states. Using magnetophotoluminescence spectroscopy we show that when holes tunnel through a thin barrier, the lowest energy molecular state has bonding orbital character. However, as the thickness of the barrier increases, the molecular ground state changes character from a bonding orbital to an antibonding orbital, confirming recent theoretical predictions. We explain how the spin-orbit interaction causes this counterintuitive reversal by using a four-band k.p model and atomistic calculations that account for strain.

• Spin-selective aharonov-bohm oscillations in a lateral triple quantum dot.

  Authors: F. Delgado, Y. -P. Shim, M Korkusinski, L. Gaudreau, S. A. Studenikin, A. S. Sachrajda, P. Hawrylak

  Physical review letters. 12/2008; 101(22):226810.

  We present a theory of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot. We show that to understand the Aharonov-Bohm (AB) effect in an interacting electron system within aWe present a theory of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot. We show that to understand the Aharonov-Bohm (AB) effect in an interacting electron system within a triple quantum dot molecule (TQD) where the dots lie in a ring configuration requires one to not only consider electron charge but also spin. Using a Hubbard model supported by microscopic calculations we show that, by localizing a single electron spin in one of the dots, the current through the TQD molecule depends not only on the flux but also on the relative orientation of the spin of the incoming and localized electrons. AB oscillations are predicted only for the spin singlet electron complex resulting in a magnetic field tunable "spin valve."

• Intershell Exchange and Sequential Electrically Injected Spin Populations of InAs Quantum-Dot Shell States.

  Authors: G. Kioseoglou, M. Yasar, C. H. Li, M Korkusinski, M Diaz-Avila, A. T. Hanbicki, P. Hawrylak, A. Petrou, B. T. Jonker

  Physical review letters. 12/2008; 101(22):227203.

  We report sequential spin population of individual shell states of self-assembled InAs quantum dots controlled by a spin-polarized current from an Fe contact, and determine the s-p and p-d intershellWe report sequential spin population of individual shell states of self-assembled InAs quantum dots controlled by a spin-polarized current from an Fe contact, and determine the s-p and p-d intershell exchange energies. We resolve excitonic features in the electroluminescence (EL) spectra associated with individual quantum levels. In contrast with simple models of shell occupation, the EL circular polarization exhibits maxima shifted with respect to the intensity peaks. Calculations show that this is due to intershell exchange. Exchange energies for the s-p and p-d shells are 7+/-2 and 13.5+/-1 meV, respectively.

• Prepositioned single quantum dot in a lateral electric field

  Authors: M. E. Reimer, M. Korkusinski, D. Dalacu, J. Lefebvre, J. Lapointe, P. J. Poole, G. C. Aers, W. R. McKinnon, P. Hawrylak, R. L. Williams

  Physical Review B (Condensed Matter and Materials Physics). 11/2008; 78:195301--8.

• Antibonding ground states in semiconductor artificial molecules

  Authors: M. F. Doty, J. I. Climente, M Korkusinski, M. Scheibner, A. S. Bracker, P. Hawrylak, D. Gammon

  04/2008;

  The spin-orbit interaction is a crucial element of many semiconductor spintronic technologies. Here we report the first experimental observation, by magneto-optical spectroscopy, of a remarkableThe spin-orbit interaction is a crucial element of many semiconductor spintronic technologies. Here we report the first experimental observation, by magneto-optical spectroscopy, of a remarkable consequence of the spin-orbit interaction for holes confined in the molecular states of coupled quantum dots. As the thickness of the barrier separating two coupled quantum dots is increased, the molecular ground state changes character from a bonding orbital to an antibonding orbital. This result is counterintuitive, and antibonding molecular ground states are never observed in natural diatomic molecules. We explain the origin of the reversal using a four band k.p model that has been validated by numerical calculations that account for strain. The discovery of antibonding molecular ground states provides new opportunities for the design of artificially structured materials with complex molecular properties that cannot be achieved in natural systems. Comment: 10 pages, 4 figures, with additional 5 pages of supplementary material

• Direct observation of polarons in electron populated quantum dots by resonant Raman scattering.

  Authors: B. Aslan, H.C. Liu, M Korkusinski, P. Hawrylak, D J Lockwood

  Journal of nanoscience and nanotechnology. 03/2008; 8(2):789-94.

  The general problem of the pairing of strongly interacting elementary excitations producing new quasiparticles such as polarons arises in many areas of solid state physics. Recent interest in polaronThe general problem of the pairing of strongly interacting elementary excitations producing new quasiparticles such as polarons arises in many areas of solid state physics. Recent interest in polaron formation in semiconductor quantum dots has been motivated by the need to understand the physical nature of the carrier relaxation processes and their role in quantum-dot based devices. We report on the direct observation of polarons in InAs/GaAs self-assembled quantum dots populated by few electrons where the polarons are strongly coupled modes of quantum dot phonons and electron intersublevel transitions. The degree of coupling is varied in a systematic way in a set of samples having electron intersublevel spacing changing from larger to smaller than the longitudinal optical phonon energy. The signature of polarons is evidenced clearly by the observation of a large (12-20 meV) anticrossing for both InAs and GaAs-like quantum dot phonons using resonant Raman spectroscopy.

• Theory of electronic transport through a triple quantum dot in the presence of magnetic field

  Authors: F. Delgado, P. Hawrylak

  12/2007;

  Theory of electronic transport through a triangular triple quantum dot subject to a perpendicular magnetic field is developed using a tight binding model. We show that magnetic field allows toTheory of electronic transport through a triangular triple quantum dot subject to a perpendicular magnetic field is developed using a tight binding model. We show that magnetic field allows to engineer degeneracies in the triple quantum dot energy spectrum. The degeneracies lead to zero electronic transmission and sharp dips in the current whenever a pair of degenerate states lies between the chemical potential of the two leads. These dips can occur with a periodicity of one flux quantum if only two levels contribute to the current or with half flux quantum if the three levels of the triple dot contribute. The effect of strong bias voltage and different lead-to-dot connections on Aharonov-Bohm oscillations in the conductance is also discussed.

• Theory of spin, electronic and transport properties of the lateral triple quantum dot molecule in a magnetic field

  Authors: F. Delgado, Y. -P. Shim, M Korkusinski, P. Hawrylak

  07/2007;

  We present a theory of spin, electronic and transport properties of a few-electron lateral triangular triple quantum dot molecule in a magnetic field. Our theory is based on a generalization of aWe present a theory of spin, electronic and transport properties of a few-electron lateral triangular triple quantum dot molecule in a magnetic field. Our theory is based on a generalization of a Hubbard model and the Linear Combination of Harmonic Orbitals combined with Configuration Interaction method (LCHO-CI) for arbitrary magnetic fields. The few-particle spectra obtained as a function of the magnetic field exhibit Aharonov-Bohm oscillations. As a result, by changing the magnetic field it is possible to engineer the degeneracies of single-particle levels, and thus control the total spin of the many-electron system. For the triple dot with two and four electrons we find oscillations of total spin due to the singlet-triplet transitions occurring periodically in the magnetic field. In the three-electron system we find a transition from a magnetically frustrated to the spin-polarized state. We discuss the impact of these phase transitions on the addition spectrum and the spin blockade of the lateral triple quantum dot molecule. Comment: 30 pages (one column), 9 figures