14 research outputs found
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 -Sn thin films on In- and Sb-rich reconstructions of InSb(001)
-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 -Sn thin films. -Sn growth on InSb
often results in out-diffusion of indium, a p-type dopant. By growing
-Sn via molecular beam epitaxy on the Sb-rich c(44) surface
reconstruction of InSb(001) rather than the In-rich c(82), 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 -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
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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