Weak antilocalization in quasi-two-dimensional electronic states of epitaxial LuSb thin films

Observation of large non-saturating magnetoresistance in rare-earth monopnictides has raised enormous interest in understanding the role of its electronic structure. Here, by a combination of molecular-beam epitaxy, low-temperature transport, angle-resolved photoemssion spectroscopy, and hybrid density functional theory we have unveiled the bandstructure of LuSb, where electron-hole compensation is identified as a mechanism responsible for large magnetoresistance in this topologically trivial compound. In contrast to bulk single crystal analogues, quasi-two-dimensional behavior is observed in our thin films for both electron and holelike carriers, indicative of dimensional confinement of the electronic states. Introduction of defects through growth parameter tuning results in the appearance of quantum interference effects at low temperatures, which has allowed us to identify the dominant inelastic scattering processes and elucidate the role of spin-orbit coupling. Our findings open up new possibilities of band structure engineering and control of transport properties in rare-earth monopnictides via epitaxial synthesis.Comment: 20 pages, 12 figures; includes supplementary informatio

Why Virtue Ethics?

Contemporary virtue ethics, an agent-centred ethical theory, has been presented as a response to inadequacies in more traditional act-centred theories. In this paper, I argue that such a response is insufοcient: contemporary virtue ethics fails to avoid the inadequacies that it purports to avoid, and brings with it problems of its own. This paper is divided into 5 sections, in the οrst of which I introduce contemporary virtue ethics as an agent-centred and pluralistic ethical theory. In section 2, I present inadequacies that virtue ethics claims to avoid: being too reductive, too algorithmic, too abstract, self-effacing, and self-other asymmetric. In section 3, I consider and analyse virtue ethics’ account of right action and of motives in order to argue in section 4 that, if these inadequacies are indeed problems affecting traditional ethical theories, virtue ethics does not avoid these problems either— particularly because of its basis in the concept of virtues and its heavy reliance on phronesis. I show that another ethical theory, limited moral pluralism, has the same advantages of not being overly reductive, algorithmic, or abstract, and being self-other symmetric, and that virtue ethics does not avoid self-effacement as it claims to. I also question here whether self-effacement and self-other asymmetry should be considered problems when evaluating moral theories. Finally, I suggest in section 5 that virtue ethics is open to further criticisms of indeterminacy and lack of explanatory power

Growth and characterization of α\alpha-Sn thin films on In- and Sb-rich reconstructions of InSb(001)

$\alpha$-Sn thin films can exhibit a variety of topologically non-trivial phases. Both studying the transitions between these phases and making use of these phases in eventual applications requires good control over the electronic and structural quality of $\alpha$-Sn thin films. $\alpha$-Sn growth on InSb often results in out-diffusion of indium, a p-type dopant. By growing $\alpha$-Sn via molecular beam epitaxy on the Sb-rich c(4$\times$4) surface reconstruction of InSb(001) rather than the In-rich c(8$\times$2), we demonstrate a route to substantially decrease and minimize this indium incorporation. The reduction in indium concentration allows for the study of the surface and bulk Dirac nodes in $\alpha$-Sn via angle-resolved photoelectron spectroscopy without the common approaches of bulk doping or surface dosing, simplifying topological phase identification. The lack of indium incorporation is verified in angle-resolved and -integrated ultraviolet photoelectron spectroscopy as well as in clear changes in the Hall response

Epitaxial growth, magnetoresistance, and electronic band structure of GdSb magnetic semimetal films

Motivated by observations of extreme magnetoresistance (XMR) in bulk crystals of rare-earth monopnictide (RE-V) compounds and emerging applications in novel spintronic and plasmonic devices based on thin-film semimetals, we have investigated the electronic band structure and transport behavior of epitaxial GdSb thin films grown on III-V semiconductor surfaces. The Gd3+ ion in GdSb has a high spin S=7/2 and no orbital angular momentum, serving as a model system for studying the effects of antiferromagnetic order and strong exchange coupling on the resulting Fermi surface and magnetotransport properties of RE-Vs. We present a surface and structural characterization study mapping the optimal synthesis window of thin epitaxial GdSb films grown on III-V lattice-matched buffer layers via molecular beam epitaxy. To determine the factors limiting XMR in RE-V thin films and provide a benchmark for band structure predictions of topological phases of RE-Vs, the electronic band structure of GdSb thin films is studied, comparing carrier densities extracted from magnetotransport, angle-resolved photoemission spectroscopy (ARPES), and density functional theory (DFT) calculations. ARPES shows hole-carrier rich topologically-trivial semi-metallic band structure close to complete electron-hole compensation, with quantum confinement effects in the thin films observed through the presence of quantum well states. DFT predicted Fermi wavevectors are in excellent agreement with values obtained from quantum oscillations observed in magnetic field-dependent resistivity measurements. An electron-rich Hall coefficient is measured despite the higher hole carrier density, attributed to the higher electron Hall mobility. The carrier mobilities are limited by surface and interface scattering, resulting in lower magnetoresistance than that measured for bulk crystals

Tuning the Band Topology of GdSb by Epitaxial Strain

Rare-earth monopnictide (RE-V) semimetal crystals subjected to hydrostatic pressure have shown interesting trends in magnetoresistance, magnetic ordering, and superconductivity, with theory predicting pressure-induced band inversion. Yet, thus far, there have been no direct experimental reports of interchanged band order in RE-Vs due to strain. This work studies the evolution of band topology in biaxially strained GdSb (001) epitaxial films using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). We find that biaxial strain continuously tunes the electronic structure from topologically trivial to nontrivial, reducing the gap between the hole and the electron bands dispersing along the [001] direction. The conduction and valence band shifts seen in DFT and ARPES measurements are explained by a tight-binding model that accounts for the orbital symmetry of each band. Finally, we discuss the effect of biaxial strain on carrier compensation and magnetic ordering temperature

Revealing quantum Hall states in epitaxial topological half-Heusler semimetal

Prediction of topological surface states (TSS) in half-Heusler compounds raises exciting possibilities to realize exotic electronic states and novel devices by exploiting their multifunctional nature. However, an important prerequisite is identification of macroscopic physical observables of the TSS, which has been difficult in these semi-metallic systems due to prohibitively large number of bulk carriers. Here, we introduce compensation alloying in epitaxial thin films as an effective route to tune the chemical potential and simultaneously reduce the bulk carrier concentration by more than two orders of magnitude compared to the parent compound. Linear magnetoresistance is shown to appear as a precursor phase that transmutes into a TSS induced quantum Hall phase on further reduction of the coupling between the surface states and the bulk carriers. Our approach paves the way to reveal and manipulate exotic properties of topological phases in Heusler compounds.Comment: 8 pages, 4 figures. Supplementary Infromation contains 7 sections and 17 figure

Tuning the band topology of GdSb by epitaxial strain

Rare-earth monopnictide (RE-V) semimetal crystals subjected to hydrostatic pressure have shown interesting trends in magnetoresistance, magnetic ordering, and superconductivity, with theory predicting pressure-induced band inversion. Yet, thus far, there have been no direct experimental reports of interchanged band order in RE-Vs due to strain. This work studies the evolution of band topology in biaxially strained GdSb(001) epitaxial films using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). As biaxial strain is tuned from tensile to compressive strain, the gap between the hole and the electron bands dispersed along [001] decreases. The conduction and valence band shifts seen in DFT and ARPES measurements are explained by a tight-binding model that accounts for the orbital symmetry of each band. Finally, we discuss the effect of biaxial strain on carrier compensation and magnetic ordering temperature

Band Engineering of Epitaxial Semimetal Films

This dissertation explores epitaxial growth and modifications to the electronic band structure of topological semimetal materials through heteroepitaxy, biaxial strain, and reduced dimensionality. High-quality thin films are grown via molecular beam epitaxy (MBE) and studied using a combination of angle-resolved photoemission spectroscopy (ARPES), density functional theory (DFT), and low-temperature magnetotransport.Recent predictions of topological phases and observations of extremely large magnetoresistance in the class of rare-earth monopnictides, and specifically GdSb, have opened up a new research front aimed at studying the interplay between magnetoresistance, topology, and magnetic ordering. The first part of the dissertation focuses on magnetoresistance and band topology evolution in lattice-matched and biaxially strained GdSb (001) thin films. Lattice-matched GdSb films show a mobility and carrier concentration imbalance, deviating from the commonly assumed compensated charge carrier densities seen in bulk rare-earth monopnictides. Next, we established a clear connection between biaxial strain in GdSb films and the affected band dispersions based on their orbital composition. As biaxial strain is tuned from tensile to compressive strain, the gap between the hole and the electron bands dispersed along [001] decreases.The second part of this dissertation reports the first ARPES investigation conclusively assigning the topological character of bismuth as a trivial ℤ2 state by studying ultrathin Bi (111) films grown on InSb (111)B. Bismuth films hold promise for potential applications in spintronic devices and topological one-dimensional edge transport. Yet synthesizing high-quality, wafer-scale ultrathin bismuth films on non-metallic substrates remains challenging. We achieved large-area Bi (111) films with a single epitaxial domain orientation and mapped the dispersion of surface states and quantum well states. Strong film-substrate interactions were found to promote epitaxial stabilization of the (111) orientation and lead to inversion symmetry breaking. Our results demonstrate that interfacial bonds prevent the semimetal-to-semiconductor transition predicted for freestanding bismuth layers, highlighting the importance of controlled functionalization and surface passivation in two-dimensional materials.Finally, the growth parameters of biaxially strained LuPtBi films on InSb were explored. The half-Heusler compound LuPtBi belongs to a unique group of superconductors with a topologically nontrivial band structure, and very low carrier densities (<10^20 cm^-3). A suitable growth window could not be identified for (111) oriented LuPtBi. Room-temperature nucleation of LuPtBi on LuPtSb/InSb (001) allowed higher incorporation of Bi in LuPtBi and produced smooth, coherently strained films