Nanotransformation and current fluctuations in exciton condensate junctions

We analyze the nonlinear transport properties of a bilayer exciton condensate that is contacted by four metallic leads by calculating the full counting statistics of electron transport for arbitrary system parameters. Despite its formal similarity to a superconductor the transport properties of the exciton condensate turn out to be completely different. We recover the generic features of exciton condensates such as counterpropagating currents driven by excitonic Andreev reflections and make predictions for nonlinear transconductance between the layers as well as for the current (cross)correlations and generalized Johnson-Nyquist relationships. Finally, we explore the possibility of connecting another mesoscopic system (in our case a quantum point contact) to the bottom layer of the exciton condensate and show how the excitonic Andreev reflections can be used for transforming voltage at the nanoscale.     

Simulation of exciton formation and transport in electrically driven polariton laser structures

A model for exciton formation, dissociation and transport is proposed for the simulation of an electrically pumped polariton laser with a geometry similar to that of a VCSEL and resonant cavity LEDs. We demonstrate how the strain effects and the geometry of the device influence the exciton distribution for a GaN/InGaN laser structure.     

The exciton dead layer revisited

Although the exciton in a quantum well is not a rigid ball but distords when its center of mass gets close to a surface, it is mathematically possible to write the exciton energy change from its bulk value as an effective decrease of the well width in which the center of mass would freely move. In the large well limit, the exciton dead layer defined this way is related to the third order term in the expansion of the exciton energy as a function of the inverse well width. A quite precise calculation of this exciton energy is thus necessary to obtain this dead layer. We present a new calculation which relies on a Born-Oppenheimer procedure to decouple the relative motion of the e-h pair from its center of mass motion. This is associated to a quite precise calculation of the relative motion energy, based on our recent work on the exact envelope function for confined motion. We predict that the dead layer increases with the exciton total mass in contradiction with previous results.     

Theoretical study on the exciton dynamics of coherent excitation energy transfer in the phycoerythrin 545 light-harvesting complex

The experimental observation of long-lived quantum coherence in the excitation energy transfer(EET)process of the several photosynthetic light-harvesting complexes at low and room temperatures has aroused hot debate.It challenges the common perception in the field of complicated pigment molecular systems and evokes considerable theoretical efforts to seek reasonable explanations.In this work,we investigate the coherent exciton dynamics of the phycoerythrin 545(PE545)complex.We use the dissipation equation of motion to theoretically investigate the effect of the local pigment vibrations on the population transfer process.The result indicates that the realistic local pigment vibrations do assist the energy transmission.We demonstrate the coherence between different pigment molecules in the PE545 system is an essential ingredient in the EET process among various sites.The coherence makes the excitation energy delocalized,which leads to the redistribution of the excitation among all the chromophores in the steady state.Furthermore,we investigate the effects of the complex high-frequency spectral density function on the exciton dynamics and find that the high-frequency Brownian oscillator model contributes most to the exciton dynamic process.The discussions on the local pigment vibrations of the Brownian oscillator model suggest that the local heterogeneous protein environments and the effects of active vibration modes play a significant role in coherent energy transport.     

Thermal effects in exciton harvesting in biased one-dimensional systems

The study of energy harvesting in chain-like structures is important due to its relevance to a variety of interesting physical systems. Harvesting is understood as the combination of exciton transport through intra-band exciton relaxation (via scattering on phonon modes) and subsequent quenching by a trap. Recently, we have shown that in the low temperature limit different harvesting scenarios as a function of the applied bias strength (magnitude of the energy gradient towards the trap) are possible [S.M. Vlaming, V.A. Malyshev, J. Knoester, J. Chem. Phys. 127 (2007) 154719]. This paper generalizes the results for both homogeneous and disordered chains to nonzero temperatures. We show that thermal effects are appreciable only for low bias strengths, particularly so in disordered systems, and lead to faster harvesting.     

Dependence of resonance energy transfer on exciton dimensionality

We investigate the dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality. We exploit the excitonic potential disorder in a single quantum well to tune the balance between localized and free excitons by scaling the Boltzmann distribution of excitons through temperature. Theoretical calculations predict the experimentally observed temperature dependence of resonance energy transfer and allow us to quantify the contribution of localized and free excitons. We show that free excitons can undergo resonance energy transfer with an order of magnitude higher rate compared to localized excitons, emphasizing the potential of hybrid optoelectronic devices utilizing resonance energy transfer as a means to overcome charge transfer related limitations.     

Direct resonant interband tunneling into exciton states in multiquantum well structures

Resonant Zener tunneling is investigated in a multiquantum-well p-i-n diode at cryogenic temperatures. Current-voltage characterization in presence of high magnetic fields demonstrates the dominant role of the electron–hole Coulomb interaction in driving this process. After a theoretical modelization in a variational scheme we conclude that tunneling electrons are directly injected into exciton states. We also suggest that similar transport experiments can be an alternative exciton spectroscopic technique with respect to the more conventional optical ones.     

Quantum effects in the interaction of the exciton with a leaky quasi-mode cavity

We have studied quantum effects in the interaction of the exciton with a leaky quasi-mode cavity field. When the exciton is initially prepared in a superposition state which exhibits holes in its photon-number distribution, whereas the cavity field initially is in the vacuum state, it is found that there exists an energy exchange between the exciton and the cavity field. The exciton and the cavity field may exhibit sub-Poissonian distributions and quadrature squeezings. It is shown that there does not exist a violation of the Cauchy-Schwartz inequality, which means that the correlation between the exciton and the cavity field is classical. Received 25 November 2000 and Received in final form 1st January 2001     

Electron-hole diagrams versus exciton diagrams: the simplest problem on interacting excitons

We study the interaction of an exciton with a distant metal, which is the simplest problem on interacting excitons: The semiconductor and metal electrons being “different” species, we do not have to worry about the tricky consequences of Pauli exclusion between identical carriers, which appear in any other problem on interacting excitons. We show how the exciton absorption, in the presence of semiconductor-metal interaction, can be derived in a very simple and transparent way from an exciton diagram procedure, provided that we use the appropriate exciton-metal interaction vertex, which contains the scattering from an exciton state to another exciton state under a Coulomb excitation. We also show that the resolution of this problem using standard electron-hole diagrams is dreadfully complicated at the lowest order in the semiconductor-metal interaction already, preventing a full calculation of the exciton-metal coupling from this usual technique. Received 26 February 2001     

Tuning of exciton states in a magnetic quantum ring

The exciton states in a CdTe quantum ring subjected to an external magnetic field containing a single magnetic impurity are investigated. We have used the multiband approximation which includes the heavy hole–light hole coupling effects. The electron–hole spin interactions and the s, p–d interactions between the electron, the hole and the magnetic impurity are also included. The exciton energy levels and optical transitions are evaluated using the exact diagonalization scheme. We show that due to the spin interactions it is possible to change the bright exciton state into the dark state and vice versa with the help of a magnetic field. We propose a new route to experimentally estimate the s, p–d spin interaction constants.     

Effect of disorder on exciton dissociation in conjugated polymers

By using a multi-configurational time-dependent Hartree–Fock(MCTDHF) method for the time-dependent Schr ?dinger equation and a Newtonian equation of motion for lattice, we investigate the disorder effects on the dissociation process of excitons in conjugated polymer chains. The simulations are performed within the framework of an extended version of the Su–Schrieffer–Heeger model modified to include on-site disorder, off-diagonal, electron–electron interaction, and an external electric field. Our results show that Coulomb correlation effects play an important role in determining the exciton dissociation process. The electric field required to dissociate an exciton can practically impossibly occur in a pure polymer chain, especially in the case of triplet exciton. However, when the on-site disorder effects are taken into account, this leads to a reduction in mean dissociation electric fields. As the disorder strength increases, the dissociation field decreases effectively. On the contrary, the effects of off-diagonal disorder are negative in most cases. Moreover, the dependence of exciton dissociation on the conjugated length is also discussed.     

Polaronic exciton in quantum wells wires and nanotubes

We investigate the effect of the longitudinal-optical phonon field on the binding energies of excitons in quantum wells, well-wires and nanotubes based on ionic semiconductors. We take into account the exciton-phonon interaction by using the Aldrich-Bajaj effective potential for Wannier excitons in a polarizable medium. We extend the fractional-dimensional method developed previously for neutral and negatively charged donors to calculate the exciton binding energies in these heterostructures. In this method, the exciton wave function is taken as a product of the ground state functions of the electron polaron and hole polaron with a correlation function that depends only on the electron-hole separation. Starting from the variational principle we derive a one-dimensional differential equation, which is solved numerically by using the trigonometric sweep method. We find that the potential that takes into account polaronic effects always give rise to larger exciton binding energies than those obtained using a Coulomb potential screened by a static dielectric constant. This enhancement of the binding energy is more considerable in quantum wires and nanotubes than in quantum wells. Our results for quantum wells are in a good agreement with previous variational calculations. Also, we present novel curves of the exciton binding energies as a function of the wire and nanotubes radii for different models of the confinement potential.     

Exact evaluation of the kernel of the generalized master equation for the coupled coherent and incoherent exciton motion

The kernel of the Nakajima-Zwanzig generalized master equation is calculated exactly for the coupled coherent and incoherent exciton motion according to the Haken-Strobl model. As a simple application the resulting master equation is used for the determination of the mean square displacement of exciton transport.     

Investigations of the exciton condensate

Recent experiments on quantum Hall bilayers in the vicinity of total filling factor 1 (ν_T=1) have revealed many exciting observations characteristic of a superfluidic exciton condensate. We report on our experimental work involving the ν_T=1 exciton condensate in independently contacted bilayer two-dimensional electron systems. We observe previously reported phenomena as a zero-bias resonant tunneling peak, a quantized Hall drag resistivity, and in counter-flow configuration, the near vanishing of both ρ_xx and ρ_xy resistivity components. At balanced electron densities in the layers, we find for both drag and counter-flow current configurations, thermally activated transport with a monotonic increase of the activation energy for d/ℓ_B<1.65 with activation energies up to 0.4 K. In the imbalanced system the activation energies show a striking asymmetry around the balance point, implying that the gap to charge excitations is considerably different in the separate layers that form the bilayer condensate. This indicates that the measured activation energy is neither the binding energy of the excitons, nor their condensation energy.     

Density of exciton states in one-dimensional disordered molecular crystals

We investigate the effects of structural disorder on the exciton density of states of molecular crystals, utilizing an exact soluble model which involves correlated Lorentzian distributions of both the site-excitation energies and the transfer integrals. Off-diagonal disorder results in asymmetric density of states functions, the asymmetry being prominent for one-dimensional exciton band structure.     

Dielectric enhancement of the exciton energies in laser-dressed near-surface quantum wells

A theoretical study of the intense high-frequency laser field effect on the interband transitions and on the ground (1S-like) and excited (2S-like) exciton states in InGaAs/GaAs near-surface quantum wells is performed within the effective mass approximation. The carrier confinement potentials and image charge contributions to the Coulomb interaction can significantly be modified and controlled by the capped layer thickness and laser field intensity. We found that: (i) the interband and exciton transition energies monotonically enhance with the laser amplitude; (ii) for small capped layers the splitting between the 2S and 1S exciton lines are more sensitive to the dressing laser parameter, and (iii) for high enough laser intensities the dressing effects on both confining potential and Coulomb interactions can yield entirely different exciton emission spectra depending on the cap layer thickness. Our results are compared with the theoretical and experimental data obtained in the absence of the laser field and a good agreement is reached.     

Formation mechanism and low-temperature instability of exciton rings

The macroscopic rings observed in the photoluminescence patterns of excitons in coupled quantum wells are explained by a mechanism of carrier imbalance, transport, and recombination. The rings originate from the spatial separation of p and n carriers, and occur at the interface of the p and n domains, where excitons are generated. We explore the states of excitons in the ring over a range of temperatures down to 380 mK and report a transition of the ring into a periodic array of aggregates, a new low-temperature ordered exciton state.     

Magnetic-field effects on exciton–polaron in a CdSe quantum disk

We study theoretically the interaction between excitons and longitudinal optical (LO) phonons in a cylindrical disk-like semiconductor quantum dot under an applied magnetic field. Due to the intensity of the interaction in the strong coupling regime, a composite quasi-particle called exciton–polaron is formed. We focus on the effect of the disk size and an external magnetic field on the exciton–phonon interaction energy and the exciton–polaron modes. The numerical computation for a CdSe quantum disk have shown that the exciton–phonon interaction energy is very significant and is even dominant when the disk height is small, which leads to a large Rabi splitting between the exciton–polaron modes. We investigate also the effect of the temperature on the integrated photoluminescence (PL) intensity, and show that at relatively high temperature the LO phonons have a noticeable effect on it. This physical parameter also shows a great dependence on quantum disk size and on magnetic field.     

The trion as an exciton interacting with a carrier

The X⁻ trion is essentially an electron bound to an exciton. However, due to the composite nature of the exciton, there is no way to write an exciton-electron interaction potential. We can overcome this difficulty by using a commutation technique similar to the one we introduced for excitons interacting with excitons, which allows to take exactly into account the close-to-boson character of the excitons. From it, we can obtain the X⁻ trion creation operator in terms of excitons and electrons. We can also derive the X⁻ trion ladder diagram between an exciton and an electron. These are the basic tools for future works on many-body effects involving trions.     

The free exciton in KI and RbI: Emissions related to the triplet B exciton

We put in evidence the existence of the triplet exciton B by its emissions as associated with LO phonons in KI and RbI. We explain the possibility of such emissions by the process of interband transition coupling the B exciton with the C polariton (low branch). The exchange interaction and spin-orbit interaction values are calculated.