arXiv subjects

Michael Wais

Publications and source records attributed to Michael Wais.

9 recordsLinked to original sources

arXiv2023

Transition from Diffusive to Superdiffusive Transport in Carbon Nanotube Networks via Nematic Order Control

The one-dimensional confinement of quasiparticles in individual carbon nanotubes (CNTs) leads to extremely anisotropic electronic and optical properties. In a macroscopic ensemble of randomly oriented CNTs, this anisotropy disappears together with other properties that make them attractive for certain device applications. The question however remains if not only anisotropy, but other types of behaviours are suppressed by disorder. Here, we compare the dynamics of quasiparticles under strong electric fields in aligned and random CNT networks using a combination of terahertz emission and photocurrent experiments and out-of-equilibrium numerical simulations. We find that the degree of alignment strongly influences the excited quasiparticles' dynamics, rerouting the thermalisation pathways. This is, in particular, evidenced in the high-energy, high-momentum electronic population (probed through the formation of low energy excitons via exciton impact ionization) and the transport regime evolving from diffusive to superdiffusive.

cond-mat.mtrl-sci↗

arXiv2021

Origin of the Background Absorption in Carbon Nanotubes: Phonon-Assisted Excitonic Continuum

Excitonic effects in 1D semiconductors can be qualitatively different from those in higher dimensions. In particular, the Sommerfeld factor, the ratio of the above-band-edge excitonic continuum absorption to free electron-hole pair generation, has been shown to be less than 1 (i.e., suppressed) in 1D systems while it is larger than1 (i.e., enhanced) in 2D and 3D systems. Strong continuum suppression indeed exists in semiconducting single-wall carbon nanotubes, a prototypical 1D semiconductor. However, absorption spectra for carbon nanotubes are typically fit with a combination of Lorentzians and a polynomial background baseline with little physical meaning. Here, we performed absorption measurements in aligned single-chirality (6,5) carbon nanotube films. The obtained spectra were fit with our theoretical model obtained by solving the Boltzmann scattering equation (i.e., the quantum Fokker-Planck equation), involving fifty-nine different types of transitions among three different types of quasiparticles. Specifically, we took into account microscopic interactions between photons, phonons, and excitons, including their dispersions, which unambiguously demonstrated that the background absorption is due to phonon-assisted transitions from the semiconductor vacuum to finite-momentum continuum states of excitons. The excellent agreement we obtained between experiment and theory suggests that our numerical technique can be seamlessly extended to compute strongly out-of-equilibrium many-body dynamics and time-resolved spectra in low-dimensional materials.

physics.comp-ph↗

arXiv2021

Numerical Solver for the out-of-equilibrium time dependent Boltzmann Collision operator: Application to 2D materials

The Time Dependent Boltzmann equation (TDBE) is a viable option to study strongly out-of-equilibrium thermalization dynamics which are becoming increasingly critical for many novel physical applications like Ultrafast thermalization, Terahertz radiation etc. However its applicability is greatly limited by the impractical scaling of the solution to its scattering integral term. In our previous work\cite{Michael} we had proposed a numerical solver to calculate the scattering integral term in the TDBE and then improved on it\cite{1DPaper} to include second degree momentum discretisation and adaptive time stepping. Our solver requires no close-to-equilibrium assumptions and can work with realistic band structures and scattering amplitudes. Moreover, it is numerically efficient and extremely robust against inherent numerical instabilities. While in our previous work \cite{1DPaper} we showcased the application of our solver to 1D materials, here we showcase its applications to a simple 2D system and analyse thermalisations of the introduced out-of-equilibrium excitations. The excitations added at higher energies were found to thermalise faster than those introduced at relatively lower energies. Also, we conclude that the thermalisation of the out-of-equilibrium population to equilibrium values is not a simple exponential decay but rather a non-trivial function of time. Nonetheless, by fitting a double exponential function to the decay of the out-of-equilibrium population with time we were able to generate quantitative insights into the time scales involved in the thermalisations.

physics.comp-ph↗

arXiv2020

Numerical solver for the time-dependent far-from-equilibrium Boltzmann equation

The study of strongly out-of-equilibrium states and their time evolution towards thermalization is critical to the understanding of an ever widening range of physical processes. We present a numerical method that for the first time allows for the numerical solution of the most difficult part of the time-dependent Boltzmann equation: the full scattering term. Any number of bands (and quasiparticles) with arbitrary dispersion, any number of high order scattering channels (we show here four legs scatterings: electron-electron scattering) can be treated far from equilibrium. No assumptions are done on the population and all the Pauli-blocking factors are included in the phase-space term of the scattering. The method can be straightforwardly interfaced to a deterministic solver for the transport. Finally and most critically, the method conserves to machine precision the particle number, momentum and energy for any resolution, making the computation of the time evolution till complete thermalization possible. We apply this approach to two examples, a metal and a semiconductor, undergoing thermalization from a strongly out-of-equilibrium laser excitation. These two cases, which are in literature treated hitherto under a number of approximations, can be addressed free from those approximations and straightforwardly within the same numerical method.

physics.comp-ph↗

arXiv2020

Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (SWCNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100~meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting SWCNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1~ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all our experimental data semi-quantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in SWCNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

cond-mat.mtrl-sci↗

arXiv2018

Quantum Boltzmann equation for strongly correlated systems: comparison to dynamical mean field theory

We investigate the potential of a quantum Boltzmann equation without momentum conservation for description of strongly correlated electron systems out of equilibrium. In a spirit similar to dynamical mean field theory (DMFT), the momentum conservation of the electron-electron scattering is neglected, which yields a time-dependent occupation function for the equilibrium spectral function, even in cases where well-defined quasiparticles do not exist. The main assumption of this method is that the spectral function remains sufficiently rigid under the non-equilibrium evolution. We compare the result of the quantum Boltzmann equation to non-equilibrium DMFT simulations for the case of photo-carrier relaxation in Mott insulators, where processes on very different timescales emerge, i.e., impact ionization, intra-Hubbard-band thermalization, and full thermalization. Since quantum Boltzmann simulations without momentum conservation are computationally cheaper than non-equilibrium DMFT, this method allows the simulation of more complicated systems or devices, and to access much longer times.

cond-mat.str-el↗

arXiv2018

Importance of Schottky barriers for wide-bandgap thermoelectric devices

The development of thermoelectric devices faces not only the challenge of optimizing the Seebeck coefficient, the electrical and thermal conductivity of the active material, but also further bottlenecks when going from the thermoelectric material to an actual device, e.g., the dopant diffusion at the hot contact. We show that for large bandgap thermoelectrics another aspect can dramatically reduce the efficiency of the device: the formation of Schottky barriers. Understanding the effect, it can then be fixed rather cheaply by a two-metals contact solution.

cond-mat.mtrl-sci↗

arXiv2015

Observation of molecular dipole excitations by attosecond self-streaking

We propose a protocol to probe the ultrafast evolution and dephasing of coherent electronic excitation in molecules in the time domain by the intrinsic streaking field generated by the molecule itself. Coherent electronic motion in the endohedral fullerene \Necsixty~is initiated by a moderately intense femtosecond UV-VIS pulse leading to coherent oscillations of the molecular dipole moment that persist after the end of the laser pulse. The resulting time-dependent molecular near-field is probed through the momentum modulation of photoemission from the central neon atom by a time-delayed attosecond XUV pulse. Our ab-initio time-dependent density functional theory and classical trajectory simulations predict that this self-streaking signal accurately traces the molecular dipole oscillations in real time. We discuss the underlying processes and give an analytical model that captures the essence of our ab-initio simulations.

physics.optics↗

arXiv2014

Time-resolved photoemission using attosecond streaking

We theoretically study time-resolved photoemission in atoms as probed by attosecond streaking. We review recent advances in the study of the photoelectric effect in the time domain and show that the experimentally accessible time shifts can be decomposed into distinct contributions that stem from the field-free photoionization process itself and from probe-field induced corrections. We perform accurate quantum-mechanical as well as classical simulations of attosecond streaking for effective one-electron systems and determine all relevant contributions to the time delay with attosecond precision. In particular, we investigate the properties and limitations of attosecond streaking for the transition from short-ranged potentials (photodetachment) to long-ranged Coulomb potentials (photoionization). As an example for a more complex system, we study time-resolved photoionization for endohedral fullerenes $A$@$\text{C}_{60}$ and discuss how streaking time shifts are modified due to the interaction of the $\text{C}_{60}$ cage with the probing infrared streaking field.

physics.atom-ph↗