Manuel Otero

We present a measure of quantum evolution based on the degree of average distinguishability between the states visited by a quantum system over a given time interval. This measure generalizes the concept of "amount of evolution" previously introduced for pure states, to arbitrary (mixed) states, thereby enabling the study of non-unitary dynamics in open quantum systems. For qubit systems, we establish fundamental relationships between the proposed evolution measure and state purity, deriving rigorous bounds for the evolution quantity. Furthermore, we demonstrate that the proposed measure admits a natural interpretation in terms of the entanglement between the system and a purifying environment, in such a way that the evolution measure globally reflects the degree of variation of the system-enviroment entanglement. Finally, we illustrate the behavior of the evolution measure in qubit systems interacting with an environment under paradigmatic decoherence dynamics.

The impact of dendrite geometry on the ⁷Li NMR spectrum was explored through numerical calculations of the magnetic field. Dendrites were modeled as cylinders with diverse heights, diameters, densities, and orientations relative to the static field.

Silicon anodes hold great promise for next-generation Li-ion batteries. The main obstacle to exploiting their high performance is the challenge of linking experimental observations to atomic structures due to the amorphous nature of Li-Si alloys. We unveil the atomistic-scale structures of amorphous Li-Si using our recently developed density functional tight-binding model. Our claim is supported by the successful reproduction of experimental x-ray pair distribution functions, NMR, and Mössbauer spectra using simple nearest-neighbor models. The predicted structures are publicly available.

The predictive power of molecular dynamic simulations is mainly restricted by the time scale and model accuracy. Many systems of current relevance are of such complexity that requires addressing both issues simultaneously. This is the case of silicon electrodes in Li-ion batteries, where different Li$_x$Si alloys are formed during charge/discharge cycles. While first-principles treatments for this system are seriously limited by the computational cost of exploring its large conformational space, classical force-fields are not transferable enough to represent it accurately. Density Functional Tight-Binding (DFTB) is an intermediate complexity approach capable of capturing the electronic nature of different environments with a relatively low computational cost. In this work, we present a new set of DFTB parameters suited to model amorphous Li$_x$Si alloys. The latter is the usual finding upon cycling the Si electrodes in the presence of Li-ions. The model parameters are constructed with a particular emphasis on their transferability for the entire Li$_x$Si composition range. This is achieved by introducing a new optimization procedure that weights stoichiometries differently to improve the prediction of their formation energies. The resulting model is shown to be robust for predicting crystal and amorphous structures for the different compositions, giving excellent agreement with DFT calculations and outperforming state-of-the-art ReaxFF potentials.