Dhara Trivedi

Global Flux Surface Hopping Approach for Mixed Quantum-Classical Dynamics

A novel global flux surface hopping (GFSH) approach is proposed. In this method, the surface hopping probabilities rely on the gross population flow between states, rather than the state-to-state flux as in the standard fewest switches surface hopping (FSSH). GFSH captures the superexchange mechanism of population transfer, while FSSH lacks this capability. In other aspects, including minimization of the number of hops, internal consistency, velocity rescaling, and detailed balance, the GFSH algorithm is similar to FSSH. The advantages of GFSH are demonstrated with a model 3-level system and an Auger process in a semiconductor quantum dot. Current studies indicate that GFSH can replace FSSH, but further tests are needed.

Nonadiabatic Molecular Dynamics for Thousand Atom Systems: A Tight-Binding Approach toward PYXAID

Excited state dynamics at the nanoscale requires treatment of systems involving hundreds and thousands of atoms. In the majority of cases, depending on the process under investigation, the electronic structure component of the calculation constitutes the computation bottleneck. We developed an efficient approach for simulating nonadiabatic molecular dynamics (NA-MD) of large systems in the framework of the self-consistent charge density functional tight binding (SCC-DFTB) method. SCC-DFTB is combined with the fewest switches surface hopping (FSSH) and decoherence induced surface hopping (DISH) techniques for NA-MD. The approach is implemented within the Python extension for the ab initio dynamics (PYXAID) simulation package, which is an open source NA-MD program designed to handle nanoscale materials. The accuracy of the developed approach is tested with ab initio DFT and experimental data, by considering intraband electron and hole relaxation, and nonradiative electron-hole recombination in a CdSe quantum dot and the (10,5) semiconducting carbon nanotube. The technique is capable of treating accurately and efficiently excitation dynamics in large, realistic nanoscale materials, employing modest computational resources.

Auger-Mediated Electron Relaxation Is Robust to Deep Hole Traps: Time-Domain Ab Initio Study of CdSe Quantum Dots

By slowing down electron-phonon relaxation in nanoscale materials, one can increase efficiencies of solar energy conversion via hot electron extraction, multiple exciton generation, and elimination of exciton trapping. The elusive phonon bottleneck is hard to achieve, in particular, due to Auger-type energy exchange between electrons and holes. The Auger channel can be suppressed by hole trapping. Using time-domain ab initio simulation, we show that deep hole traps cannot fully eliminate the Auger channel. The simulations show that the hole-mediated electron relaxation is slowed down only by about 30%, which is in agreement with the recent experiments. The Auger energy exchange and hole relaxation to the trap state occur on similar time scales. Hole trapping is slow, because holes themselves experience a weak bottleneck effect. The study establishes the fundamental mechanisms of the electron and hole relaxation processes with and without hole traps. It shows that more sophisticated hole trapping strategies, for example, involving shell layers, are required in order to achieve the phonon bottleneck and to reduce electronic energy losses.

Optical spin injection and spin lifetime in Ge heterostructures

We studied spin properties of Ge heterostructures by optical orientation and Hanle measurements. The circular polarization of the direct gap photoluminescence is shown to exceed the theoretical bulk limit, yielding about 37% and 85% for transitions with heavy and light holes, respectively. The energetic proximity of Γ and L valleys and ultrafast scattering of electrons from Γ to L states allowed us to resolve the spin dynamics of holes and to observe the polarization of electrons after scattering to L valleys. The spin relaxation analysis indicates that the spin lifetime of electrons exceeds 5 ns below 150 K, whereas it is in the 500 ps range for holes.

Observation of an Excitonic Quantum Coherence in CdSe Nanocrystals.

Recent observations of excitonic coherences within photosynthetic complexes suggest that quantum coherences could enhance biological light harvesting efficiencies. Here, we employ optical pump-probe spectroscopy with few-femtosecond pulses to observe an excitonic quantum coherence in CdSe nanocrystals, a prototypical artificial light harvesting system. This coherence, which encodes the high-speed migration of charge over nanometer length scales, is also found to markedly alter the displacement amplitudes of phonons, signaling dynamics in the non-Born-Oppenheimer regime.

Analysis of the Trajectory Surface Hopping Method from the Markov State Model Perspective

We analyze the applicability of the seminal fewest switches surface hopping (FSSH) method of Tully to modeling quantum transitions between electronic states that are not coupled directly, in the processes such as Auger recombination. We address the known deficiency of the method to describe such transitions by introducing an alternative definition for the surface hopping probabilities, as derived from the Markov state model perspective. We show that the resulting transition probabilities simplify to the quantum state populations derived from the time-dependent Schrodinger equation, reducing to the rapidly switching surface hopping approach of Tully and Preston. The resulting surface hopping scheme is simple and appeals to the fundamentals of quantum mechanics. The computational approach is similar to the FSSH method of Tully, yet it leads to a notably different performance. We demonstrate that the method is particularly accurate when applied to superexchange modeling. We further show improved accuracy of ...

Spin-dependent optical properties in strained silicon and germanium

We present a comprehensive theory of the circularly polarized luminescence and its dependence on strain in spin-polarized Si and Ge. Symmetries of wavefunctions and interactions are used to derive concise ratios between intensities of the right and left circularly polarized luminescence for each of the dominant phonon-assisted optical transitions. These ratios are then used to explain the circular polarization degrees of the luminescence peaks in the spectra of biaxially-strained Si and Ge, and of relaxed

Decoherence Allows Model Reduction in Nonadiabatic Dynamics Simulations

{\rm{Si}}_{1-x}{\rm{Ge}}_{x}

Observation of an Excitonic Quantum Coherence in CdSe Nanocrystals

alloys. The spectra are numerically calculated by a combination of an empirical pseudopotential method, an adiabatic bond-charge model and a rigid-ion model.

Time-Domain Ab Initio Modeling of Charge and Exciton Dynamics in Nanomaterials

A nonadiabatic (NA) molecular dynamics (MD) simulation requires calculation of NA coupling matrix elements, the number of which scales as a square of the number of basis states. The basis size can be huge in studies of nanoscale materials, and calculation of the NA couplings can present a significant bottleneck. A quantum-classical approximation, NAMD overestimates coherence in the quantum, electronic subsystem, requiring decoherence correction. Generally, decoherence times decrease with increasing energy separation between pairs of states forming coherent superpositions. Since rapid decoherence stops quantum dynamics, one expects that decoherence-corrected NAMD can eliminate the need for calculation of NA couplings between energetically distant states, notably reducing the computational cost. Considering several types of dynamics in a semiconductor quantum dot, we demonstrate that indeed, decoherence allows one to reduce the number of needed NA coupling matrix elements. If the energy levels are spaced closer than 0.1 eV, one obtains good results while including only three nearest-neighbor couplings, and in some cases even with just the first nearest-neighbor coupling scheme. If the energy levels are spaced by about 0.4 eV, the nearest-neighbor model fails, while three or more nearest-neighbor schemes also provide good results. In comparison, the results of NAMD simulation without decoherence vary continuously with changes in the number of NA couplings. Thus, decoherence effects induced by coupling to a quantum-mechanical environment not only provide the physical mechanism for NAMD trajectory branding and improve the accuracy of NAMD simulations, but also afford significant computational savings.