Instytut Fizyki UMK

Toruń, Poland

Instytut Fizyki UMK

Toruń, Poland

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Jask Olski W.,Instytut Fizyki UMK | Pelc M.,Instytut Fizyki UMK | Santos H.,CSIC - Institute of Materials Science | Chico L.,CSIC - Institute of Materials Science | Ayuela A.,Donostia International Physics Center
Physica Status Solidi (C) Current Topics in Solid State Physics | Year: 2010

We study the electronic band structure of several carbon nanotube superlattices built of two kinds of intermolecular junctions: (12, 0)/(6, 6) and (8, 0)/(14, 0). In particular, we focus on the energy bands originating from interface states. We find that in case of the metallic (12, 0)/(6, 6) superlattices, the interface bands change periodically their character from bonding- to antibonding-like vs. increasing length of the (6, 6) tube. We show that these changes are related to the decay of the charge density Friedel oscillations in the metallic (6, 6) tube. However, when we explore other chiralities without rotational symmetry, no changes in bondingantibonding character are observed for semiconductor superlattices, as exemplified in the case of (8, 0)/(14, 0) superlattices. Our results indicate that unless metallic tubes are employed in the junctions, the bondingantibonding crossings are not present. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


Bryant G.W.,U.S. National Institute of Standards and Technology | Zielinski M.,Instytut Fizyki UMK | Malkova N.,U.S. National Institute of Standards and Technology | Sims J.,United Information Technology | And 2 more authors.
Physical Review Letters | Year: 2010

We show how a nanomechanical strain can be used to dynamically reengineer the optics of quantum dots, giving a tool to manipulate mechanoexciton shape, orientation, fine structure splitting, and optical transitions, transfer carriers between dots, and interact qubits for quantum processing. Most importantly, a nanomechanical strain reengineers both the magnitude and phase of the exciton exchange coupling to tune exchange splittings, change the phase of spin mixing, and rotate the polarization of mechanoexcitons, providing phase and energy control of excitons. © 2010 The American Physical Society.


Chico L.,CSIC - Institute of Materials Science | Santos H.,CSIC - Institute of Materials Science | Ayuela A.,Donostia International Physics Center | Jaskolski W.,Instytut Fizyki UMK | And 2 more authors.
Acta Physica Polonica A | Year: 2010

The properties of carbon nanotubes can be dramatically altered by the presence of defects. In this work we address the properties of two different kinds of defective nanotubes: junctions of achiral tubes with topological defects and partially unzipped carbon nanotubes. In particular, we begin by focussing on the interface states in carbon nanotube junctions between achiral tubes. We show that their number and energies can be derived by applying the Born-von Karman boundary condition to an interface between armchair- and zigzag-terminated semi-infinite graphene layers. We show that these interface states, which were thought to be due to the presence of topological defects, are in fact related to the graphene zigzag edge states. Secondly, we study partially unzipped carbon nanotubes, which can be considered as the junction of a carbon nanotube and a graphene nanoribbon, which has edge features giving rise to novel properties. Carbon nanoribbons act as transparent contacts for nanotubes and viceversa, yielding a high conductance. At certain energies, nanoribbons behave as valley filters for carbon nanotubes; this holds considering electron-electron interaction effects. Furthermore, the application of a magnetic field turns the system conducting, with a 100% magnetoresistance. These novel structures may open a way for new carbon-based devices.


Bryant G.W.,U.S. National Institute of Standards and Technology | Zielinski M.,Instytut Fizyki UMK | Malkova N.,U.S. National Institute of Standards and Technology | Sims J.,United Information Technology | And 2 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We show how nanomechanical strain can be used to dynamically control the optical response of self-assembled quantum dots embedded in nanomechanical bridges, giving a tool to shift electron and hole levels, manipulate mechanoexciton shape, orientation, fine-structure splitting, and optical transitions, transfer carriers between dots, and interact qubits for quantum processing. Conversely, we show how modulation of the quantum dot optical response can be used to monitor locally an applied nanomechanical strain. Atomistic tight-binding theory is used to describe the response of electrons and holes in a self-assembled quantum dot to applied nanomechanical strain. The internal strain due to the lattice mismatch, the nanomechanical strain, and the internal atomic readjustment to minimize the applied strain must all be accounted for to model correctly the strain effects. Electrons and hole levels and charge distributions can shift together or in opposite directions depending on how the strain is applied. This gives control for tailoring band gaps and optical response. The strain can also be used to transfer electrons and holes between vertically or laterally coupled dots, giving a mechanism for manipulating transition strengths and interacting qubits for quantum information processing. Applied strain can be used to manipulate the fine-structure splitting of mechanoexcitons by distorting electron and hole charge distributions and rotating hole orientation. Most importantly, nanomechanical strain reengineers both the magnitude and phase of the exciton exchange coupling to tune exchange splittings, change the phase of spin mixing, and rotate the polarization of mechanoexcitons, providing phase and energy control of excitons. © 2011 American Physical Society.


Jaskolski W.,Instytut Fizyki UMK | Ayuela A.,Donostia International Physics Center | Pelc M.,Instytut Fizyki UMK | Santos H.,CSIC - Institute of Materials Science | Chico L.,CSIC - Institute of Materials Science
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We prescribe general rules to predict the existence of edge states and zero-energy flat bands in graphene nanoribbons and graphene edges of arbitrary shape. No calculations are needed. For the so-called minimal edges, the projection of the edge translation vector into the zigzag direction of graphene uniquely determines the edge bands. By adding nodes to minimal edges, arbitrary modified edges can be obtained; their corresponding edge bands can be found by applying hybridization rules of the extra states with those belonging to the original edge. Our prescription correctly predicts the localization and degeneracy of the zero-energy bands at one of the graphene sublattices, confirmed by tight-binding and first-principles calculations. It also allows us to qualitatively predict the existence of E 0 bands appearing in the energy gap of certain edges and nanoribbons. © 2011 American Physical Society.


Kadantsev E.S.,NRC Institute for Microstructural Sciences | Kadantsev E.S.,University of Ottawa | Zielinski M.,Instytut Fizyki UMK | Hawrylak P.,NRC Institute for Microstructural Sciences
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

Quantum dots in nanowires grow on a (111) substrate and it is expected that the modifications of the band structure due to a biaxial strain in the (111) crystallographic plane will determine the confinement of charge carriers in these systems. In this work, we develop an ab initio methodology for the determination of biaxial strain-modified band energies on an absolute energy scale due to a strain in an arbitrary crystallographic plane and apply it to calculate the evolution of band edges in group IIIA-VA zinc-blende semiconductors (InP, InAs, and GaAs) under the (111) biaxial strain. The absolute hydrostatic deformation potentials, a prerequisite for the accurate calculation of the strain-modified band energies within our scheme, are determined. The strain tensor for an InAs dot grown on a (111) GaAs substrate is calculated and the importance of the (111) biaxial strain is demonstrated. The strained band offsets in InAs under a compressive biaxial strain in the (001) and (111) crystallographic planes as well as the local band structure in a InAs/GaAs dot indicates a stronger confinement of holes in the (111) case. ©2012 American Physical Society.


Chwastyk M.,Instytut Fizyki UMK | Rozanski P.,Instytut Fizyki UMK | Zielinski M.,Instytut Fizyki UMK
Acta Physica Polonica A | Year: 2012

We report a theoretical investigation of electronic properties of semiconductor InAs and GaAs nanocrystals. Our calculation scheme starts with the single particle calculation using atomistic tight-binding model including spin-orbital interaction and d-orbitals. Then the exciton binding energies are calculated with screened Coulomb interaction. We study the role of surface passivation efects by varying value of surface passivation potential. We compare results obtained with dot center positioned on different lattice sites thus containing different number of anion and cations. We conclude that passivation of surface states affects significantly single particle energies and the value of electron-hole Coulomb attraction. Interestingly, due to limited screening, the short-range (on-site) contribution to the electron-hole Coulomb attraction plays significant role for small nanocrystals with radius smaller than 1 nm.


Zielinski M.,Instytut Fizyki UMK
Acta Physica Polonica A | Year: 2012

We demonstrate a multi-domain scheme for calculation of electronic and optical properties of semiconductor nanostructures. Three progressively smaller computational domains are used for strain simulation, single particle states calculation and computation of the Coulomb scattering matrix elements. Proposed approach offers a significant reduction of computational time and memory savings without sacrificing the accuracy of obtained spectra. We illustrate this method on the example of InAs/InP self-assembled quantum dots.


Kadantsev E.S.,NRC Institute for Microstructural Sciences | Zielinski M.,Instytut Fizyki UMK | Korkusinski M.,NRC Institute for Microstructural Sciences | Hawrylak P.,NRC Institute for Microstructural Sciences
Journal of Applied Physics | Year: 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, 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. © 2010 American Institute of Physics.


Zielinski M.,Instytut Fizyki UMK
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

A method for inclusion of strain into the tight-binding Hamiltonian is presented. This approach bridges from bulk strain to the atomistic language of bond lengths and angles, and features a diagonal parameters shift in a form suitable for atomistic calculation of million atom nanosystems with a small number of empirical parameters. I illustrate this method by calculating electronic and optical properties of self-assembled InAs/(InP,GaAs) lens-shaped quantum dots. A very different structure of confined quantum dots states is shown, depending on the matrix material and inclusion of strain effects. Results are compared with the well-established empirical pseudopotential method, and reasonable agreement is found. © 2012 American Physical Society.

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