Marzena Szymanska | University College London (original) (raw)
Papers by Marzena Szymanska
The second part of this thesis is concerned with properties of excitons and modelling of excitoni... more The second part of this thesis is concerned with properties of excitons and modelling of excitonic lasing in quasi-one-dimensional quantum wires. I develop a very general numerical method of calculating the properties of wires with different shapes and materials. Using this method I study the properties of very wide range of T-shaped quantum wires.
Physical Review Letters, 2008
Fast sweep projection onto Feshbach molecules has been widely used as a probe of fermionic conden... more Fast sweep projection onto Feshbach molecules has been widely used as a probe of fermionic condensates. By determining the exact dynamics of a pair of atoms in time varying magnetic fields, we calculate the number of condensed and noncondensed molecules created after fast magnetic field sweeps from the BCS to the BEC side of the resonances in 40^{40}40K and 6^{6}6Li, for different sweep rates and a range of initial and final fields. We discuss the relation between the initial fermionic condensate fraction and the molecular condensate fraction measured after the sweep.
We use the standard fermionic and boson-fermion Hamiltonians to study the BCS-BEC cross-over near... more We use the standard fermionic and boson-fermion Hamiltonians to study the BCS-BEC cross-over near the 202 G resonance in a two-component mixture of fermionic 40 K atoms employed in the experiment of C. A. Regal et al. [Phys. Rev. Lett. 92, 040403 (2004)]. Our mean-field analysis of many-body equilibrium quantities shows virtually no differences between the predictions of the two approaches, provided they are both implemented in a manner that properly includes the effect of the highest excited bound state of the background scattering potential, rather than just the magnetic-field dependence of the scattering length. Consequently, we rule out the macroscopic occupation of the molecular field as a mechanism behind the fermionic pair condensation and show that the BCS-BEC cross-over in ultra-cold 40 K gases can be analysed and understood on the same basis as in the conventional systems of solid state physics.
Physical Review A, 2005
We use the standard fermionic and boson-fermion Hamiltonians to study the BCS-BEC cross-over near... more We use the standard fermionic and boson-fermion Hamiltonians to study the BCS-BEC cross-over near the 202 G resonance in a two-component mixture of fermionic 40K atoms employed in the experiment of C.A. Regal et al., Phys. Rev. Lett. 92, 040403 (2004). Our mean-field analysis of many-body equilibrium quantities shows virtually no differences between the predictions of the two approaches, provided they are both implemented in a manner that properly includes the effect of the highest excited bound state of the background scattering potential, rather than just the magnetic-field dependence of the scattering length. Consequently, we rule out the macroscopic occupation of the molecular field as a mechanism behind the fermionic pair condensation and show that the BCS-BEC cross-over in ultra-cold 40K gases can be analysed and understood on the same basis as in the conventional systems of solid state physics.
Applied Physics Letters, 2003
We measured polarization-dependent photoluminescence-excitation spectra of highly uniform T-shape... more We measured polarization-dependent photoluminescence-excitation spectra of highly uniform T-shaped quantum wires at 5 K. We attribute one peak to the 1D-exciton ground state and the continuous absorption band to 1D continuum states. These had similar polarization-dependences. We also observed some other peaks above the 1D-exciton ground state and attribute them to exciton states consisting of excited hole subbands. These results show good agreement with a model calculation of a single electron-hole pair in T-shaped geometry with exact diagonalizations of the Coulomb interaction.
We study a system of ultra-cold, dilute, fermionic atoms close to a Feshbach resonance. By using ... more We study a system of ultra-cold, dilute, fermionic atoms close to a Feshbach resonance. By using a variational approach, we avoid making the approximation of preformed molecular pairs. This enables us to follow the crossover from BCS to Blatt-Schafroth pairs.
Physical Review Letters, 2006
We study spontaneous quantum coherence in an out of equilibrium system, coupled to multiple baths... more We study spontaneous quantum coherence in an out of equilibrium system, coupled to multiple baths describing pumping and decay. For a range of parameters describing coupling to, and occupation of the baths, a stable steady-state condensed solution exists. The presence of pumping and decay significantly modifies the spectra of phase fluctuations, leading to correlation functions that differ both from an isolated condensate and from a laser.
Solid state quantum condensates often differ from previous examples of condensates (such as Heliu... more Solid state quantum condensates often differ from previous examples of condensates (such as Helium, ultra-cold atomic gases, and superconductors) in that the quasiparticles condensing have relatively short lifetimes, and so as for lasers, external pumping is required to maintain a steady state. On the other hand, compared to lasers, the quasiparticles are generally more strongly interacting, and therefore better able to thermalise. This leads to questions of how to describe such non-equilibrium condensates, and their relation to equilibrium condensates and lasers. This chapter discusses in detail how the non-equilibrium Green's function approach can be applied to the description of such a non-equilibrium condensate, in particular, a system of microcavity polaritons, driven out of equilibrium by coupling to multiple baths. By considering the steady states, and fluctuations about them, it is possible to provide a description that relates both to equilibrium condensation and to lasing, while at the same time, making clear the differences from simple lasers.
Solid State Communications, 2007
We analyse the spatial and temporal coherence properties of a two-dimensional and finite sized po... more We analyse the spatial and temporal coherence properties of a two-dimensional and finite sized polariton condensate with parameters tailored to the recent experiments which have shown spontaneous and thermal equilibrium polariton condensation in a CdTe microcavity [J. Kasprzak, M. Richard, S. Kundermann, A. Baas, P. Jeambrun, J.M.J. Keeling, F.M. Marchetti, M.H. Szymanska, R. Andre, J.L. Staehli, et al., Nature 443 (7110) (2006) 409]. We obtain a theoretical estimate of the thermal length, the lengthscale over which full coherence effectively exists (and beyond which power-law decay of correlations in a two-dimensional condensate occurs), of the order of 5 μm. In addition, the exponential decay of temporal coherence predicted for a finite size system is consistent with that found in the experiment. From our analysis of the luminescence spectra of the polariton condensate, taking into account pumping and decay, we obtain a dispersionless region at small momenta of the order of 4 degrees. In addition, we determine the polariton linewidth as a function of the pump power. Finally, we discuss how, by increasing the exciton photon detuning, it is in principle possible to move the threshold for condensation from a region of the phase diagram where polaritons can be described as a weakly interacting Bose gas to a region where instead the composite nature of polaritons becomes important.
Physical Review A, 2003
We explore the behaviour of a system which consists of a photon mode dipole coupled to a medium o... more We explore the behaviour of a system which consists of a photon mode dipole coupled to a medium of two-level oscillators in a microcavity in the presence of decoherence. We consider two types of decoherence processes which are analogous to magnetic and non-magnetic impurities in superconductors. We study different phases of this system as the decoherence strength and the excitation density is changed. For a low decoherence we obtain a polariton condensate with comparable excitonic and photonic parts at low densities and a BCS-like state with bigger photon component due to the fermionic phase space filling effect at high densities. In both cases there is a large gap in the density of states. As the decoherence is increased the gap is broadened and suppressed, resulting in a gapless condensate and finally a suppression of the coherence in a low density regime and a laser at high density limit. A crossover between these regimes is studied in a self-consistent way analogous to the Abrikosov and Gor'kov theory of gapless superconductivity.
Journal of Physics-condensed Matter, 2007
Recent years have witnessed novel and exciting advances on the subject of optical coherence and c... more Recent years have witnessed novel and exciting advances on the subject of optical coherence and collective phenomena in nanostructures. This volume overviews the forefront progress in this area, collecting nine reviews and ten new contributions by leading experts in the field. The subfields included in this volume span from two-dimensional electron gases, semiconductor excitons, coupled quantum wells, microcavity polaritons, quantum dots and quantum wires. One of the most exciting directions in coupled quantum wells is the possibility to explore novel quantum fluid phases of indirect excitons and the formation of spontaneous coherence. Strong light-matter interaction in semiconductor microcavities has lead to the ability of controlling, manipulating and detecting the matter properties by all optical means. Structures with reduced dimensionality, such as quantum dots and quantum wires, offer the possibility to explore novel physics and new applications for nanoscience technology. Finally, recent advances in probing and controlling spin and charge dynamics in two-dimensional electron gases open new perspectives towards spintronics. The intellectual and applied links between all these problems offer fascinating opportunities for further advances in this field. The editors would like to acknowledge the support of the EU Network `Photon mediated phenomena in semiconductor nanostructures' HPRN-CT-2002-00298 in the preparation of this volume.
Physical Review Letters, 2006
We study the thermodynamic condensation of microcavity polaritons using a realistic model of diso... more We study the thermodynamic condensation of microcavity polaritons using a realistic model of disorder in semiconductor quantum wells. This approach correctly describes the polariton inhomogeneous broadening in the low density limit, and treats scattering by disorder to all orders in the condensed regime. While the weak disorder changes the thermodynamic properties of the transition little, the effects of disorder in the condensed state are prominent in the excitations and can be seen in resonant Rayleigh scattering.
Journal of Physics-condensed Matter, 2004
That excitons in solids might condense into a phase-coherent ground state was proposed about 40 y... more That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focussing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers.
Solid State Communications, 2003
We connect three phenomena in which a coherent electromagnetic field could be generated: polarito... more We connect three phenomena in which a coherent electromagnetic field could be generated: polariton condensation, phase-locking in arrays of underdamped Josephson junctions, and lasing. All these phenomena have been described using Dicke-type models of spins coupled to a single photon mode. These descriptions may be distinguished by whether the spins are quantum or classical, and whether they are strongly or weakly damped.
Solid State Communications, 2001
We report that excitonic lasing gain coexists with spontaneous optical emission characteristic of... more We report that excitonic lasing gain coexists with spontaneous optical emission characteristic of an electron±hole plasma in highly photoexcited one-dimensional semiconductors. The experiments probe quantum T-wire laser structures optimized for high photoexcitation. Evidence of dense electron±hole plasma is clearly seen in the spontaneous recombination measured when lasing emission displays distinct excitonic character. These ®ndings differ strikingly from those in higher dimentional semiconductors, and offer insights on optical processes considered by recent theories of dense electron±hole plasmas. q 2001 Published by
Nature Physics, 2010
Semiconductor microcavity polaritons in the optical parametric scattering regime have been recent... more Semiconductor microcavity polaritons in the optical parametric scattering regime have been recently demonstrated to display a new variety of dissipationless superfluid behaviour. We report the first observation in resonantly pumped exciton polaritons of a metastable persistent superflow carrying quantum of angular momentum, m. The quantised vortex, excited by a weak 2 ps pulsed probe, is shown to last for at least 80 ps, limited only by the leaking outside the cavity. The polariton circulating superfluid persists in the absence of the driving rotating probe with no apparent dissipation. In addition, for a moving superfluid, we show the coherent splitting of a quantised double vortex, with charge m=2, into two singly quantised vortices of m=1. Remarkably, we observe the m=2 vortex to be stable when they are at rest. The experimental results are compared with a theoretical analysis, obtained describing the triggered parametric scattering regime of polaritons via a two-component Gross-Pitaevskii equation, including pump and decay processes.
Physical Review B, 2001
We calculate energies, oscillator strengths for radiative recombination, and two-particle wave fu... more We calculate energies, oscillator strengths for radiative recombination, and two-particle wave functions for the ground state exciton and around 100 excited states in a T-shaped quantum wire. We include the single-particle potential and the Coulomb interaction between the electron and hole on an equal footing, and perform exact diagonalisation of the two-particle problem within a finite basis set. We calculate spectra for all of the experimentally studied cases of T-shaped wires including symmetric and asymmetric GaAs/AlxGa1−xAs and InyGa1−yAs/AlxGa1−xAs structures. We study in detail the shape of the wave functions to gain insight into the nature of the various states for selected symmetric and asymmetric wires in which laser emission has been experimentally observed. We also calculate the binding energy of the ground state exciton and the confinement energy of the 1D quantum-wire-exciton state with respect to the 2D quantum-well exciton for a wide range of structures, varying the well width and the Al molar fraction x. We find that the largest binding energy of any wire constructed to date is 16.5 meV. We also notice that in asymmetric structures, the confinement energy is enhanced with respect to the symmetric forms with comparable parameters but the binding energy of the exciton is then lower than in the symmetric structures. For GaAs/AlxGa1−xAs wires we obtain an upper limit for the binding energy of around 25 meV in a 10Å wide GaAs/AlAs structure which suggests that other materials must be explored in order to achieve room temperature applications. There are some indications that InyGa1−yAs/AlxGa1−xAs might be a good candidate.
Journal of Physics-condensed Matter, 2007
Physica Status Solidi B-basic Solid State Physics, 2002
ABSTRACT
The second part of this thesis is concerned with properties of excitons and modelling of excitoni... more The second part of this thesis is concerned with properties of excitons and modelling of excitonic lasing in quasi-one-dimensional quantum wires. I develop a very general numerical method of calculating the properties of wires with different shapes and materials. Using this method I study the properties of very wide range of T-shaped quantum wires.
Physical Review Letters, 2008
Fast sweep projection onto Feshbach molecules has been widely used as a probe of fermionic conden... more Fast sweep projection onto Feshbach molecules has been widely used as a probe of fermionic condensates. By determining the exact dynamics of a pair of atoms in time varying magnetic fields, we calculate the number of condensed and noncondensed molecules created after fast magnetic field sweeps from the BCS to the BEC side of the resonances in 40^{40}40K and 6^{6}6Li, for different sweep rates and a range of initial and final fields. We discuss the relation between the initial fermionic condensate fraction and the molecular condensate fraction measured after the sweep.
We use the standard fermionic and boson-fermion Hamiltonians to study the BCS-BEC cross-over near... more We use the standard fermionic and boson-fermion Hamiltonians to study the BCS-BEC cross-over near the 202 G resonance in a two-component mixture of fermionic 40 K atoms employed in the experiment of C. A. Regal et al. [Phys. Rev. Lett. 92, 040403 (2004)]. Our mean-field analysis of many-body equilibrium quantities shows virtually no differences between the predictions of the two approaches, provided they are both implemented in a manner that properly includes the effect of the highest excited bound state of the background scattering potential, rather than just the magnetic-field dependence of the scattering length. Consequently, we rule out the macroscopic occupation of the molecular field as a mechanism behind the fermionic pair condensation and show that the BCS-BEC cross-over in ultra-cold 40 K gases can be analysed and understood on the same basis as in the conventional systems of solid state physics.
Physical Review A, 2005
We use the standard fermionic and boson-fermion Hamiltonians to study the BCS-BEC cross-over near... more We use the standard fermionic and boson-fermion Hamiltonians to study the BCS-BEC cross-over near the 202 G resonance in a two-component mixture of fermionic 40K atoms employed in the experiment of C.A. Regal et al., Phys. Rev. Lett. 92, 040403 (2004). Our mean-field analysis of many-body equilibrium quantities shows virtually no differences between the predictions of the two approaches, provided they are both implemented in a manner that properly includes the effect of the highest excited bound state of the background scattering potential, rather than just the magnetic-field dependence of the scattering length. Consequently, we rule out the macroscopic occupation of the molecular field as a mechanism behind the fermionic pair condensation and show that the BCS-BEC cross-over in ultra-cold 40K gases can be analysed and understood on the same basis as in the conventional systems of solid state physics.
Applied Physics Letters, 2003
We measured polarization-dependent photoluminescence-excitation spectra of highly uniform T-shape... more We measured polarization-dependent photoluminescence-excitation spectra of highly uniform T-shaped quantum wires at 5 K. We attribute one peak to the 1D-exciton ground state and the continuous absorption band to 1D continuum states. These had similar polarization-dependences. We also observed some other peaks above the 1D-exciton ground state and attribute them to exciton states consisting of excited hole subbands. These results show good agreement with a model calculation of a single electron-hole pair in T-shaped geometry with exact diagonalizations of the Coulomb interaction.
We study a system of ultra-cold, dilute, fermionic atoms close to a Feshbach resonance. By using ... more We study a system of ultra-cold, dilute, fermionic atoms close to a Feshbach resonance. By using a variational approach, we avoid making the approximation of preformed molecular pairs. This enables us to follow the crossover from BCS to Blatt-Schafroth pairs.
Physical Review Letters, 2006
We study spontaneous quantum coherence in an out of equilibrium system, coupled to multiple baths... more We study spontaneous quantum coherence in an out of equilibrium system, coupled to multiple baths describing pumping and decay. For a range of parameters describing coupling to, and occupation of the baths, a stable steady-state condensed solution exists. The presence of pumping and decay significantly modifies the spectra of phase fluctuations, leading to correlation functions that differ both from an isolated condensate and from a laser.
Solid state quantum condensates often differ from previous examples of condensates (such as Heliu... more Solid state quantum condensates often differ from previous examples of condensates (such as Helium, ultra-cold atomic gases, and superconductors) in that the quasiparticles condensing have relatively short lifetimes, and so as for lasers, external pumping is required to maintain a steady state. On the other hand, compared to lasers, the quasiparticles are generally more strongly interacting, and therefore better able to thermalise. This leads to questions of how to describe such non-equilibrium condensates, and their relation to equilibrium condensates and lasers. This chapter discusses in detail how the non-equilibrium Green's function approach can be applied to the description of such a non-equilibrium condensate, in particular, a system of microcavity polaritons, driven out of equilibrium by coupling to multiple baths. By considering the steady states, and fluctuations about them, it is possible to provide a description that relates both to equilibrium condensation and to lasing, while at the same time, making clear the differences from simple lasers.
Solid State Communications, 2007
We analyse the spatial and temporal coherence properties of a two-dimensional and finite sized po... more We analyse the spatial and temporal coherence properties of a two-dimensional and finite sized polariton condensate with parameters tailored to the recent experiments which have shown spontaneous and thermal equilibrium polariton condensation in a CdTe microcavity [J. Kasprzak, M. Richard, S. Kundermann, A. Baas, P. Jeambrun, J.M.J. Keeling, F.M. Marchetti, M.H. Szymanska, R. Andre, J.L. Staehli, et al., Nature 443 (7110) (2006) 409]. We obtain a theoretical estimate of the thermal length, the lengthscale over which full coherence effectively exists (and beyond which power-law decay of correlations in a two-dimensional condensate occurs), of the order of 5 μm. In addition, the exponential decay of temporal coherence predicted for a finite size system is consistent with that found in the experiment. From our analysis of the luminescence spectra of the polariton condensate, taking into account pumping and decay, we obtain a dispersionless region at small momenta of the order of 4 degrees. In addition, we determine the polariton linewidth as a function of the pump power. Finally, we discuss how, by increasing the exciton photon detuning, it is in principle possible to move the threshold for condensation from a region of the phase diagram where polaritons can be described as a weakly interacting Bose gas to a region where instead the composite nature of polaritons becomes important.
Physical Review A, 2003
We explore the behaviour of a system which consists of a photon mode dipole coupled to a medium o... more We explore the behaviour of a system which consists of a photon mode dipole coupled to a medium of two-level oscillators in a microcavity in the presence of decoherence. We consider two types of decoherence processes which are analogous to magnetic and non-magnetic impurities in superconductors. We study different phases of this system as the decoherence strength and the excitation density is changed. For a low decoherence we obtain a polariton condensate with comparable excitonic and photonic parts at low densities and a BCS-like state with bigger photon component due to the fermionic phase space filling effect at high densities. In both cases there is a large gap in the density of states. As the decoherence is increased the gap is broadened and suppressed, resulting in a gapless condensate and finally a suppression of the coherence in a low density regime and a laser at high density limit. A crossover between these regimes is studied in a self-consistent way analogous to the Abrikosov and Gor'kov theory of gapless superconductivity.
Journal of Physics-condensed Matter, 2007
Recent years have witnessed novel and exciting advances on the subject of optical coherence and c... more Recent years have witnessed novel and exciting advances on the subject of optical coherence and collective phenomena in nanostructures. This volume overviews the forefront progress in this area, collecting nine reviews and ten new contributions by leading experts in the field. The subfields included in this volume span from two-dimensional electron gases, semiconductor excitons, coupled quantum wells, microcavity polaritons, quantum dots and quantum wires. One of the most exciting directions in coupled quantum wells is the possibility to explore novel quantum fluid phases of indirect excitons and the formation of spontaneous coherence. Strong light-matter interaction in semiconductor microcavities has lead to the ability of controlling, manipulating and detecting the matter properties by all optical means. Structures with reduced dimensionality, such as quantum dots and quantum wires, offer the possibility to explore novel physics and new applications for nanoscience technology. Finally, recent advances in probing and controlling spin and charge dynamics in two-dimensional electron gases open new perspectives towards spintronics. The intellectual and applied links between all these problems offer fascinating opportunities for further advances in this field. The editors would like to acknowledge the support of the EU Network `Photon mediated phenomena in semiconductor nanostructures' HPRN-CT-2002-00298 in the preparation of this volume.
Physical Review Letters, 2006
We study the thermodynamic condensation of microcavity polaritons using a realistic model of diso... more We study the thermodynamic condensation of microcavity polaritons using a realistic model of disorder in semiconductor quantum wells. This approach correctly describes the polariton inhomogeneous broadening in the low density limit, and treats scattering by disorder to all orders in the condensed regime. While the weak disorder changes the thermodynamic properties of the transition little, the effects of disorder in the condensed state are prominent in the excitations and can be seen in resonant Rayleigh scattering.
Journal of Physics-condensed Matter, 2004
That excitons in solids might condense into a phase-coherent ground state was proposed about 40 y... more That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focussing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers.
Solid State Communications, 2003
We connect three phenomena in which a coherent electromagnetic field could be generated: polarito... more We connect three phenomena in which a coherent electromagnetic field could be generated: polariton condensation, phase-locking in arrays of underdamped Josephson junctions, and lasing. All these phenomena have been described using Dicke-type models of spins coupled to a single photon mode. These descriptions may be distinguished by whether the spins are quantum or classical, and whether they are strongly or weakly damped.
Solid State Communications, 2001
We report that excitonic lasing gain coexists with spontaneous optical emission characteristic of... more We report that excitonic lasing gain coexists with spontaneous optical emission characteristic of an electron±hole plasma in highly photoexcited one-dimensional semiconductors. The experiments probe quantum T-wire laser structures optimized for high photoexcitation. Evidence of dense electron±hole plasma is clearly seen in the spontaneous recombination measured when lasing emission displays distinct excitonic character. These ®ndings differ strikingly from those in higher dimentional semiconductors, and offer insights on optical processes considered by recent theories of dense electron±hole plasmas. q 2001 Published by
Nature Physics, 2010
Semiconductor microcavity polaritons in the optical parametric scattering regime have been recent... more Semiconductor microcavity polaritons in the optical parametric scattering regime have been recently demonstrated to display a new variety of dissipationless superfluid behaviour. We report the first observation in resonantly pumped exciton polaritons of a metastable persistent superflow carrying quantum of angular momentum, m. The quantised vortex, excited by a weak 2 ps pulsed probe, is shown to last for at least 80 ps, limited only by the leaking outside the cavity. The polariton circulating superfluid persists in the absence of the driving rotating probe with no apparent dissipation. In addition, for a moving superfluid, we show the coherent splitting of a quantised double vortex, with charge m=2, into two singly quantised vortices of m=1. Remarkably, we observe the m=2 vortex to be stable when they are at rest. The experimental results are compared with a theoretical analysis, obtained describing the triggered parametric scattering regime of polaritons via a two-component Gross-Pitaevskii equation, including pump and decay processes.
Physical Review B, 2001
We calculate energies, oscillator strengths for radiative recombination, and two-particle wave fu... more We calculate energies, oscillator strengths for radiative recombination, and two-particle wave functions for the ground state exciton and around 100 excited states in a T-shaped quantum wire. We include the single-particle potential and the Coulomb interaction between the electron and hole on an equal footing, and perform exact diagonalisation of the two-particle problem within a finite basis set. We calculate spectra for all of the experimentally studied cases of T-shaped wires including symmetric and asymmetric GaAs/AlxGa1−xAs and InyGa1−yAs/AlxGa1−xAs structures. We study in detail the shape of the wave functions to gain insight into the nature of the various states for selected symmetric and asymmetric wires in which laser emission has been experimentally observed. We also calculate the binding energy of the ground state exciton and the confinement energy of the 1D quantum-wire-exciton state with respect to the 2D quantum-well exciton for a wide range of structures, varying the well width and the Al molar fraction x. We find that the largest binding energy of any wire constructed to date is 16.5 meV. We also notice that in asymmetric structures, the confinement energy is enhanced with respect to the symmetric forms with comparable parameters but the binding energy of the exciton is then lower than in the symmetric structures. For GaAs/AlxGa1−xAs wires we obtain an upper limit for the binding energy of around 25 meV in a 10Å wide GaAs/AlAs structure which suggests that other materials must be explored in order to achieve room temperature applications. There are some indications that InyGa1−yAs/AlxGa1−xAs might be a good candidate.
Journal of Physics-condensed Matter, 2007
Physica Status Solidi B-basic Solid State Physics, 2002
ABSTRACT