arXiv subjects

Samuel Linser

Publications and source records attributed to Samuel Linser.

4 recordsLinked to original sources

arXiv2021

Evidence for Topological Band-to-Band Transitions in a Type-II Weyl Semimetal

The ternary van der Waals material NbIrTe$_4$ is a Type-II Weyl semimetal. We use a tunable circularly polarized mid-infrared laser to investigate the existence of band-to-band excitations using transient reflectivity in an exfoliated nanoflake and photothermoelectric and photogalvanic signals in a device. Unpolarized photothermoelectric spectroscopy shows that the absorption in the Weyl semimetal increases rapidly above 0.3 eV as expected from the increase in the density of states from DFT calculations. However, the reflectivity shows a sign change in the circular dichroism of the reflected light which is dominated by $σ^-$ light for energies below 0.5 eV and $σ^+$ light for energies above that energy. Using an intense pulse to perturb the electrons near the Fermi level, we show that this 0.5 eV energy is associated with a band-to-band transition from a band slightly below the Fermi energy to a higher lying empty band. We use spectroscopy of the circular photogalvanic effect (CPGE) from 0.3 to 1 eV to show a strong peak near 0.5 eV which lies on top of a two orders-of-magnitude increase at the lowest energies as the Weyl points are approached. We conclude that this band-to-band optical transition enhances the Berry Curvature responsible for the CPGE, and may involve the Weyl points just below the Fermi energy.

cond-mat.mtrl-sci↗

arXiv2020

Exploring the Band Structure of Wurtzite InAs Nanowires Using Photocurrent Spectroscopy

We use polarized photocurrent spectroscopy in a nanowire device to investigate the band structure of hexagonal Wurtzite InAs. Signatures of optical transitions between four valence bands and two conduction bands are observed which are consistent with the symmetries expected from group theory. The ground state transition energy identified from photocurrent spectra is seen to be consistent with photoluminescence emitted from a cluster of nanowires from the same growth substrate. From the energies of the observed bands we determine the spin orbit and crystal field energies in Wurtzite InAs. This information is essential to the development of crystal phase engineering of this important III-V semiconductor.

cond-mat.mes-hall↗

arXiv2019

Strong Hot Carrier Effects Observed in a Single Nanowire Heterostructure

We use Transient Rayleigh Scattering to study the thermalization of hot photoexcited carriers in single GaAsSb/InP nanowire heterostructures. By comparing the energy loss rate in single bare GaAsSb nanowires which do not show substantial hot carrier effects with the core-shell nanowires, we show that the presence of an InP shell substantially suppresses the LO phonon emission rate at low temperatures leading to strong hot carrier effects.

cond-mat.mes-hall↗

arXiv2018

Revealing Optical Transitions and Carrier Recombination Dynamics within the Bulk Band Structure of Bi2Se3

Bismuth selenide (Bi2Se3) is a prototypical three-dimensional topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements in a thick exfoliated nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3-1.2 eV). Theoretical modeling of the reflectivity spectral lineshapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding of how such states interact dynamically with the topologically protected Dirac surface states.

cond-mat.mes-hall↗