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Register a new userHund excitations and the efficiency of Mott solar cells
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We study the dynamics of photo-induced charge carriers in realistic models of LaVO3 and YTiO3 polar heterostructures. It is shown that two types of impact ionization processes contribute to the carrier multiplication in these strongly correlated multi-orbital systems: The first mechanism involves local spin state transitions, while the second mechanism involves the scattering of high kinetic energy carriers. Both processes act on the 10 fs timescale and play an important role in the harvesting of high energy photons in solar cell applications. As a consequence, the optimal gap size for Mott solar cells is substantially smaller than for semiconductor devices.
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We study the photoinduced breakdown of a two-orbital Mott insulator and resulting metallic state. Using time-dependent density matrix renormalization group, we scrutinize the real-time dynamics of the half-filled two-orbital Hubbard model interacting with a resonant radiation field pulse. The breakdown, caused by production of doublon-holon pairs, is enhanced by Hunds exchange, which dynamically activates large orbital fluctuations. The melting of the Mott insulator is accompanied by a high to low spin transition with a concomitant reduction of antiferromagnetic spin fluctuations. Most notably, the overall time response is driven by the photogeneration of excitons with orbital character that are stabilized by Hunds coupling. These unconventional Hund excitons correspond to bound spin-singlet orbital-triplet doublon-holon pairs. We study exciton properties such as bandwidth, binding potential, and size within a semiclassical approach. The photometallic state results from a coexistence of Hund excitons and doublon-holon plasma.
We study the interplay between Mott physics, driven by Coulomb repulsion U, and Hund physics, driven by Hunds coupling J, for a minimal model for Hund metals, the orbital-symmetric three-band Hubbard-Hund model (3HHM) for a lattice filling of 1/3. Hund-correlated metals are characterized by spin-orbital separation (SOS), a Hunds-rule-induced two-stage Kondo-type screening process, in which spin screening occurs at much lower energy scales than orbital screening. By contrast, in Mott-correlated metals, lying close to the phase boundary of a metal-insulator transition, the SOS window becomes negligibly small and the Hubbard bands are well separated. Using dynamical mean-field theory and the numerical renormalization group as real-frequency impurity solver, we identify numerous fingerprints distinguishing Hundness from Mottness in the temperature dependence of various physical quantities. These include ARPES-type spectra, the local self-energy, static local orbital and spin susceptibilities, resistivity, thermopower, and lattice and impurity entropies. Our detailed description of the behavior of these quantities within the context of a simple model Hamiltonian will be helpful for distinguishing Hundness from Mottness in experimental and theoretical studies of real materials.
The quantum well solar cell (QWSC) has been proposed as a flexible means to ensuring current matching for tandem cells. This paper explores the further advantage afforded by the indication that QWSCs operate in the radiative limit because radiative contribution to the dark current is seen to dominate in experimental data at biases corresponding to operation under concentration. The dark currents of QWSCs are analysed in terms of a light and dark current model. The model calculates the spectral response (QE) from field bearing regions and charge neutral layers and from the quantum wells by calculating the confined densities of states and absorption coefficient, and solving transport equations analytically. The total dark current is expressed as the sum of depletion layer and charge neutral radiative and non radiative currents consistent with parameter values extracted from QE fits to data. The depletion layer dark current is a sum of Shockley-Read-Hall non radiative, and radiative contributions. The charge neutral region contribution is expressed in terms of the ideal Shockley radiative and non-radiative currents modified to include surface recombination. This analysis shows that the QWSC is inherently subject to the fundamental radiative efficiency limit at high currents where the radiative dark current dominates, whereas good homojunction cells are well described by the ideal Shockley picture where the limit is determined by radiative and non radiative recombination in the charge neutral layers of the cell.
The GaAs/AlGaAs materials system is well suited to multi-bandgap applications such as the multiple quantum well solar cell. GaAs quantum wells are inserted in the undoped AlGaAs active region of a pin structure to extend the absorption range while retaining a higher open circuit voltage than would be provided by a cell made of the well material alone. Unfortunately aluminium gallium arsenide (AlGaAs) suffers from poor transport characteristics due to DX centres and oxygen contamination during growth, which degrade the spectral response. We investigate three mechanisms for improving the spectral response of the MQW solar cell while an experimental study of the open circuit voltage examines the voltage enhancement. An optimised structure for a high efficiency GaAs/AlGaAs solar cell is proposed.
We develop a unified theoretical picture for excitations in Mott systems, portraying both the heavy quasiparticle excitations and the Hubbard bands as features of an emergent Fermi liquid state formed in an extended Hilbert space, which is non-perturbatively connected to the physical system. This observation sheds light on the fact that even the incoherent excitations in strongly correlated matter often display a well defined Bloch character, with pronounced momentum dispersion. Furthermore, it indicates that the Mott point can be viewed as a topological transition, where the number of distinct dispersing bands displays a sudden change at the critical point. Our results, obtained from an appropriate variational principle, display also remarkable quantitative accuracy. This opens an exciting avenue for fast realistic modeling of strongly correlated materials.
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