arXiv subjects

Dennis M. Nenno

Publications and source records attributed to Dennis M. Nenno.

4 recordsLinked to original sources

arXiv2020

Anisotropic phonon-mediated electronic transport in chiral Weyl semimetals

Discovery and observations of exotic, quantized optical and electrical responses have sparked renewed interest in nonmagnetic chiral crystals. Within this class of materials, six group V transition metal ditetrelides, that is, XY$_2$ (X = V, Nb, Ta and Y = Si, Ge), host composite Weyl nodes on high-symmetry lines, with Kramers-Weyl fermions at time-reversal invariant momenta. In addition, at least two of these materials, NbGe$_2$ and NbSi$_2$, exhibit superconducting transitions at low temperatures. The interplay of strong electron-phonon interaction and complex Fermi surface topology present an opportunity to study both superconductivity and hydrodynamic electron transport in these systems. Towards this broader question, we present an ab initio theoretical study of the electronic transport and electron-phonon scattering in this family of materials, with a particular focus on NbGe$_2$ vs. NbSi$_2$, and the other group V ditetrelides. We shed light on the microscopic origin of NbGe$_2$'s large and anisotropic room temperature resistivity and contextualize its strong electron-phonon scattering with a presentation of other relevant scattering lifetimes, both momentum-relaxing and momentum-conserving. Our work explores the intriguing possibility of observing hydrodynamic electron transport in these chiral Weyl semimetals.

cond-mat.supr-con↗

arXiv2019

Modification of spintronic terahertz emitter performance through defect engineering

Spintronic ferromagnetic/non-magnetic heterostructures are novel sources for the generation of THz radiation based on spin-to-charge conversion in the layers. The key technological and scientific challenge of THz spintronic emitters is to increase their intensity and frequency bandwidth. Our work reveals the factors to engineer spintronic Terahertz generation by introducing the scattering lifetime and the interface transmission for spin polarized, non-equilibrium electrons. We clarify the influence of the electron-defect scattering lifetime on the spectral shape and the interface transmission on the THz amplitude, and how this is linked to structural defects of bilayer emitters. The results of our study define a roadmap of the properties of emitted as well as detected THz-pulse shapes and spectra that is essential for future applications of metallic spintronic THz emitters.

cond-mat.mes-hall↗

arXiv2018

Simulation of Hot-Carrier Dynamics and Terahertz Emission in Laser-Excited Metallic Bilayers

Metallic bilayer structures have been shown to emit strong terahertz (THz) pulses. We present a predictive multiscale mode that simulates optically induced spin-currents in a Fe/Pt-heterostructure and the emitted electric field. Electronic effects are treated on the nano scale using the Boltzmann transport equation for the dynamics of out-of-equilibrium charge carriers, numerically solved with a particle-in-cell code. The optical effects are simulated with a formalism that bridges the nanometer scale of the structure to the micrometer scale of the emitted waves. The approach helps to understand recent experimental findings on the basis of microscopic scattering effects and transport phenomena. Our theory's versatility allows it to be readily adapted to a wide spectrum of spintronic THz emitter designs. As an example, we show how the THz generation efficiency, defined as output to input power ratio, can be improved and optimized using serially stacked layers in conjunction with THz anti-reflection coatings.

physics.app-ph↗

arXiv2018

Particle-in-cell simulation of ultrafast hot-carrier transport in Fe/Au-heterostructures

We describe a theoretical approach for spin-polarized hot-electron transport, as it occurs after excitation by ultrafast optical pulses in heterostructures formed by ferromagnetic and normal metals. We formulate a spin-dependent particle-in-cell model that solves the Boltzmann equation for excited electrons. It includes lifetimes and transmission coefficients as parameters, which can be taken from ab-initio calculations or experiment, and can be easily extended to multilayer systems. This approach is capable of describing electron transport in the ballistic, super-diffusive and diffusive regime including secondary-carrier generation. We apply the model to optically excited carriers in Fe/Au bilayers and Fe/Au/Fe spin-valve structures. We gain microscopic insight into the hot-electron transport dynamics probed in recent experiments on spin-valves. We find contributions to the demagnetization dynamics induced in Fe/Au/Fe trilayers regardless of the parallel or antiparallel magnetic alignment of the Fe layers.

cond-mat.mtrl-sci↗