74 research outputs found
Doping a semiconductor to create an unconventional metal
Landau Fermi liquid theory, with its pivotal assertion that electrons in
metals can be simply understood as independent particles with effective masses
replacing the free electron mass, has been astonishingly successful. This is
true despite the Coulomb interactions an electron experiences from the host
crystal lattice, its defects, and the other ~1022/cm3 electrons. An important
extension to the theory accounts for the behaviour of doped semiconductors1,2.
Because little in the vast literature on materials contradicts Fermi liquid
theory and its extensions, exceptions have attracted great attention, and they
include the high temperature superconductors3, silicon-based field effect
transistors which host two-dimensional metals4, and certain rare earth
compounds at the threshold of magnetism5-8. The origin of the non-Fermi liquid
behaviour in all of these systems remains controversial. Here we report that an
entirely different and exceedingly simple class of materials - doped small gap
semiconductors near a metal-insulator transition - can also display a non-Fermi
liquid state. Remarkably, a modest magnetic field functions as a switch which
restores the ordinary disordered Fermi liquid. Our data suggest that we have
finally found a physical realization of the only mathematically rigourous route
to a non-Fermi liquid, namely the 'undercompensated Kondo effect', where there
are too few mobile electrons to compensate for the spins of unpaired electrons
localized on impurity atoms9-12.Comment: 17 pages 4 figures supplemental information included with 2 figure
Large Anomalous Hall effect in a silicon-based magnetic semiconductor
Magnetic semiconductors are attracting high interest because of their
potential use for spintronics, a new technology which merges electronics and
manipulation of conduction electron spins. (GaMn)As and (GaMn)N have recently
emerged as the most popular materials for this new technology. While Curie
temperatures are rising towards room temperature, these materials can only be
fabricated in thin film form, are heavily defective, and are not obviously
compatible with Si. We show here that it is productive to consider transition
metal monosilicides as potential alternatives. In particular, we report the
discovery that the bulk metallic magnets derived from doping the narrow gap
insulator FeSi with Co share the very high anomalous Hall conductance of
(GaMn)As, while displaying Curie temperatures as high as 53 K. Our work opens
up a new arena for spintronics, involving a bulk material based only on
transition metals and Si, and which we have proven to display a variety of
large magnetic field effects on easily measured electrical properties.Comment: 19 pages with 5 figure
Electron quantum metamaterials in van der Waals heterostructures
In recent decades, scientists have developed the means to engineer synthetic
periodic arrays with feature sizes below the wavelength of light. When such
features are appropriately structured, electromagnetic radiation can be
manipulated in unusual ways, resulting in optical metamaterials whose function
is directly controlled through nanoscale structure. Nature, too, has adopted
such techniques -- for example in the unique coloring of butterfly wings -- to
manipulate photons as they propagate through nanoscale periodic assemblies. In
this Perspective, we highlight the intriguing potential of designer
sub-electron wavelength (as well as wavelength-scale) structuring of electronic
matter, which affords a new range of synthetic quantum metamaterials with
unconventional responses. Driven by experimental developments in stacking
atomically layered heterostructures -- e.g., mechanical pick-up/transfer
assembly -- atomic scale registrations and structures can be readily tuned over
distances smaller than characteristic electronic length-scales (such as
electron wavelength, screening length, and electron mean free path). Yet
electronic metamaterials promise far richer categories of behavior than those
found in conventional optical metamaterial technologies. This is because unlike
photons that scarcely interact with each other, electrons in subwavelength
structured metamaterials are charged, and strongly interact. As a result, an
enormous variety of emergent phenomena can be expected, and radically new
classes of interacting quantum metamaterials designed
Directed emission of CdSe nanoplatelets originating from strongly anisotropic 2D electronic structure
ntrinsically directional light emitters are potentially important for applications in photonics including lasing and energy-efficient display technology. Here, we propose a new route to overcome intrinsic efficiency limitations in light-emitting devices by studying a CdSe nanoplatelets monolayer that exhibits strongly anisotropic, directed photoluminescence. Analysis of the two-dimensional k-space distribution reveals the underlying internal transition dipole distribution. The observed directed emission is related to the anisotropy of the electronic Bloch states governing the exciton transition dipole moment and forming a bright plane. The strongly directed emission perpendicular to the platelet is further enhanced by the optical local density of states and local fields. In contrast to the emission directionality, the off-resonant absorption into the energetically higher 2D-continuum of states is isotropic. These contrasting optical properties make the oriented CdSe nanoplatelets, or superstructures of parallel-oriented platelets, an interesting and potentially useful class of semiconductor-based emitters
Electronic properties of single-layer and multilayer transition metal dichalcogenides ( Mo, W and S, Se)
Single- and few-layer transition metal dichalcogenides have recently emerged
as a new family of layered crystals with great interest, not only from the
fundamental point of view, but also because of their potential application in
ultrathin devices. Here we review the electronic properties of semiconducting
, where Mo or W and S or Se. Based on of density functional
theory calculations, which include the effect of spin-orbit interaction, we
discuss the band structure of single-layer, bilayer and bulk compounds. The
band structure of these compounds is highly sensitive to elastic deformations,
and we review how strain engineering can be used to manipulate and tune the
electronic and optical properties of those materials. We further discuss the
effect of disorder and imperfections in the lattice structure and their effect
on the optical and transport properties of . The superconducting
transition in these compounds, which has been observed experimentally, is
analyzed, as well as the different mechanisms proposed so far to explain the
pairing. Finally, we include a discussion on the excitonic effects which are
present in these systems.Comment: 9 pages, 4 figures. Short review article for special issue of Ann.
Phys. on "Two-dimensional materials
The electrodynamic response of the icosahedral quasicrystal Al/sub 70/Mn/sub 9/Pd/sub 21/
Dynamic Microstructural Evolution of Graphite under Displacing Irradiation
Graphitic materials and graphite composites experience dimensional change when exposed to
radiation-induced atomic displacements. This has major implications for current and future
technological ranging from nuclear fission reactors to the processing of graphene-silicon
hybrid devices. Dimensional change in nuclear graphites is a complex problem involving the
filler, binder, porosity, cracks and atomic-level effects all interacting within the polygranular
structure. An improved understanding of the atomistic mechanisms which drive dimensional
change within individual graphitic crystals is required to feed into the multiscale modelling of
this system.
In this study, micromechanically exfoliated samples of highly oriented pyrolytic graphite
have been ion irradiated and studied in situ using transmission electron microscopy (TEM) in
order to gain insights into the response of single graphitic crystals to displacing radiation.
Under continuous ion bombardment, a complex dynamic sequence of deformation evolves
featuring several distinct stages from the inducement of strain, the creation of dislocations
leading to dislocation arrays, the formation of kink band networks and localised doming of the sample. Observing these ion irradiation-induced processes using in situ TEM reveals
previously unknown details of the sequence of microstructural developments and physics
driving these phenomena. A mechanistic model consistent with the microstructural changes
observed is presented
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