Engineering topics

Flötotto, David

Publications and source records attributed to Flötotto, David.

8 records

DOE OSTI2022

Observation of a smoothly tunable Dirac point in Ge ( Bi x Sb 1 - x ) 2 Te 4

State-of-the-art topological devices require the use of topologically protected surface states to drive electronic transport. Here, we examine a tunable topological system, Ge(Bi x Sb 1-x ) 2 Te 4 , for a range of $\mathcal{x}$ values from 0 to 1, using a combination of Fourier transform scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. Our results show that the Dirac point shifts linearly with $\mathcal{x}$, crossing the Fermi energy near $\mathcal{x}$=0.7. This observation of a smoothly tunable, isolated Dirac point crossing through the topological transport regime and having strong linear dependence with substitution can be critical for future topological spintronics applications.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Massive Suppression of Proximity Pairing in Topological ( Bi 1 - x Sb x ) 2 Te 3 Films on Niobium

Interfacing bulk conducting topological Bi 2 Se 3 films with s-wave superconductors initiates strong superconducting order in the nontrivial surface states. Yet, bulk insulating topological (Bi 1-x Sb x ) 2 Te 3 films on bulk Nb instead exhibit a giant attenuation of surface superconductivity, even for films only two-layers thick. This massive suppression of proximity pairing is shown by ultrahigh-resolution band mappings and by contrasting quantified superconducting gaps with those of heavily n-doped topological Bi 2 Se 3 /Nb. The results underscore the limitations of using superconducting proximity effects to realize topological superconductivity in nearly intrinsic systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2018

In Situ Strain Tuning of the Dirac Surface States in Bi 2 Se 3 Films

Elastic strain has the potential for a controlled manipulation of the band gap and spin-polarized Dirac states of topological materials, which can lead to pseudomagnetic field effects, helical flat bands, and topological phase transitions. However, practical realization of these exotic phenomena is challenging and yet to be achieved. Here we show that the Dirac surface states of the topological insulator Bi 2 Se 3 can be reversibly tuned by an externally applied elastic strain. Performing in situ X-ray diffraction and in situ angle-resolved photoemission spectroscopy measurements during tensile testing of epitaxial Bi 2 Se 3 films bonded onto a flexible substrate, we demonstrate elastic strains of up to 2.1% and quantify the resulting changes in the topological surface state. Our study establishes the functional relationship between the lattice and electronic structures of Bi 2 Se 3 and, more generally, demonstrates a new route toward momentum-resolved mapping of strain-induced band structure changes.

Flötotto, David↗

DOE OSTI2018

Experimental and theoretical electronic structure and symmetry effects in ultrathin NbSe2 films

Layered quasi-two-dimensional transition-metal dichalcogenides (TMDCs), which can be readily made in ultrathin films, offer excellent opportunities for studying how dimensionality affects electronic structure and physical properties. Among all TMDCs, NbSe2 is of special interest; bulk NbSe2 hosts a charge-density-wave phase at low temperatures and has the highest known superconducting transition temperature, and these properties can be substantially modified in the ultrathin film limit. Motivated by these effects, we report herein a study of few-layer NbSe2 films, with a well-defined single-domain orientation, epitaxially grown on GaAs. Angle-resolved photoemission spectroscopy was used to determine the electronic band structure and the Fermi surface as a function of layer thickness; first-principles band-structure calculations were performed for comparison. The results show interesting changes as the film thickness increases from a monolayer (ML) to several layers. The most notable changes occur between a ML and a bilayer, where the inversion symmetry in bulk NbSe2 is preserved in the bilayer but not in the ML. Finally, the results illustrate some basic dimensional effects and provide a basis for further exploring and understanding the properties of NbSe2.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Superconducting pairing of topological surface states in bismuth selenide films on niobium

A topological insulator film coupled to a simple isotropic s-wave superconductor substrate can foster helical pairing of the Dirac fermions associated with the topological surface states. Experimental realization of such a system is exceedingly difficult, however using a novel “flip-chip” technique, we have prepared single-crystalline Bi 2 Se 3 films with predetermined thicknesses in terms of quintuple layers (QLs) on top of Nb substrates fresh from in situ cleavage. Our angle-resolved photoemission spectroscopy (ARPES) measurements of the film surface disclose superconducting gaps and coherence peaks of similar magnitude for both the topological surface states and bulk states. The ARPES spectral map as a function of temperature and film thickness up to 10 QLs reveals key characteristics relevant to the mechanism of coupling between the topological surface states and the superconducting Nb substrate; the effective coupling length is found to be much larger than the decay length of the topological surface states.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Gapped electronic structure of epitaxial stanene on InSb(111)

We report that stanene (single-layer gray tin), with an electronic structure akin to that of graphene but exhibiting a much larger spin-orbit gap, offers a promising platform for room-temperature electronics based on the quantum spin Hall (QSH) effect. This material has received much theoretical attention, but a suitable substrate for stanene growth that results in an overall gapped electronic structure has been elusive; a sizable gap is necessary for room-temperature applications. Here, we report a study of stanene, epitaxially grown on the (111)B-face of indium antimonide (InSb). Angle-resolved photoemission spectroscopy measurements reveal a gap of 0.44 eV, in agreement with our first-principles calculations. Lastly, the results indicate that stanene on InSb(111) is a strong contender for electronic QSH applications.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Interdiffusion and stress development in single-crystalline Pd/Ag bilayers

Interdiffusion and stress evolution in single-crystalline Pd/single-crystalline Ag thin films were investigated by Auger electron spectroscopy sputter-depth profiling and in-situ X-ray diffraction, respectively. The concentration-dependent chemical diffusion coefficient, as well as the impurity diffusion coefficient of Ag in Pd could be determined in the low temperature range of 356 °C–455 °C. As a consequence of the similarity of the strong concentration-dependences of the intrinsic diffusion coefficients, the chemical diffusion coefficient varies only over three orders of magnitude over the whole composition range, despite the large difference of six orders of magnitude of the self-diffusion coefficients of Ag in Ag and Pd in Pd. It is shown that the Darken-Manning treatment should be adopted for interpretation of the experimental data; the Nernst-Planck treatment yielded physically unreasonable results. Apart from the development of compressive thermal stress, the development of stress in both sublayers separately could be ascribed to compositional stress (tensile in the Ag sublayer and compressive in the Pd sublayer) and dominant relaxation processes, especially in the Ag sublayer. The effect of these internal stresses on the values determined for the diffusion coefficients is shown to be negligible.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Concentration-dependent self-diffusion coefficients in amorphous Si{sub 1−x}Ge{sub x} solid solutions: An interdiffusion study

Self-diffusion coefficients of Si and Ge in amorphous Si{sub 1−x}Ge{sub x} (a-Si{sub 1−x}Ge{sub x}) solid solutions were determined quantitatively in the temperature range of 440 °C – 460 °C by the investigation of interdiffusion in amorphous Si/Si{sub 0.52}Ge{sub 0.48} multilayers using Auger electron spectroscopy sputter-depth profiling. The determined concentration dependent self-diffusion coefficients of Si and Ge in a-Si{sub 1−x}Ge{sub x} with 0 ≤ x ≤ 0.48 at. % Ge are about ten orders of magnitude larger than in the corresponding crystalline phases, due to the inherent, excess free volume in the amorphous phase. The self-diffusion coefficient of Si (or Ge) in a-Si{sub 1−x}Ge{sub x} increases in association with a decreasing activation enthalpy with increasing Ge concentration. This concentration dependence has been related to an overall decrease of the average bond strength with increasing Ge concentration.

36 MATERIALS SCIENCE↗