30 research outputs found
Berry phase jumps and giant nonreciprocity in Dirac quantum dots
We predict that a strong nonreciprocity in the resonance spectra of Dirac
quantum dots can be induced by the Berry phase. The nonreciprocity arises in
relatively weak magnetic fields and is manifest in anomalously large
field-induced splittings of quantum dot resonances which are degenerate at
due to time-reversal symmetry. This exotic behavior, which is governed by
field-induced jumps in the Berry phase of confined electronic states, is unique
to quantum dots in Dirac materials and is absent in conventional quantum dots.
The effect is strong for gapless Dirac particles and can overwhelm the
-induced orbital and Zeeman splittings. A finite Dirac mass suppresses the
effect. The nonreciprocity, predicted for generic two-dimensional Dirac
materials, is accessible through Faraday and Kerr optical rotation measurements
and scanning tunneling spectroscopy.Comment: 6 pages, 6 figure
Resonant Tunneling and Intrinsic Bistability in Twisted Graphene Structures
We predict that vertical transport in heterostructures formed by twisted
graphene layers can exhibit a unique bistability mechanism. Intrinsically
bistable - characteristics arise from resonant tunneling and interlayer
charge coupling, enabling multiple stable states in the sequential tunneling
regime. We consider a simple trilayer architecture, with the outer layers
acting as the source and drain and the middle layer floating. Under bias, the
middle layer can be either resonant or non-resonant with the source and drain
layers. The bistability is controlled by geometric device parameters easily
tunable in experiments. The nanoscale architecture can enable uniquely fast
switching times.Comment: 7 pages, 4 figure
Effects of isotope doping on the phonon modes in graphene
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 41-46).Carbon related systems have attracted a large amount of attention of the science and technology community during the last few decades. In particular, graphene and carbon nanotubes have remarkable properties that have inspired applications in several fields of science and engineering. Despite these properties, creating structurally perfect samples is a difficult objective to achieve. Defects are usually seen as imperfections that degrade the properties of materials. However, defects can also be exploited to create novel materials and devices. The main topic of this thesis is studying the effect of isotope doping on the phonon properties of graphene. The advantage of the isotope enrichment technique is that only phonon frequencies or thermal properties can be modified without changing the electrical or chemical properties. We calculated the values of the phonon lifetimes due to isotope impurity scattering for all values of isotopic fractions, isotopic masses and for all wave-vectors using second order perturbation theory. We found that for natural concentrations of 13C, the contribution of isotopic scattering of optical modes is negligible when compared to the contribution from the electron-phonon interaction. Nevertheless, for atomic concentrations of 13C as high as [rho] = 0.5 both the isotopic and electron-phonon contributions become comparable. Our results are compared with recent experimental results and we find good agreement both in the 13C atomic density dependence of the lifetime as well as in the calculated spectral width of the G-band. Due to phonon scattering by 13C isotopes, some graphene phonon wave-functions become localized in real space. Numerical calculations show that phonon localized states exist in the high-energy optical phonon modes and in regions of flat phonon dispersion. In particular, for the case of in-plane optical phonon modes, a typical localization length is on the order of 3 nm for 13C atomic concentrations of [rho] ~~ 0.5. Optical excitation of phonon modes may provide a way to experimentally observe localization effects for phonons in graphene.by Joaquin F. Rodriguez-Nieva.S.M
Enhanced thermionic-dominated photoresponse in graphene Schottky junctions
Vertical heterostructures of van der Waals materials enable new pathways to
tune charge and energy transport characteristics in nanoscale systems. We
propose that graphene Schottky junctions can host a special kind of
photoresponse which is characterized by strongly coupled heat and charge flows
that run vertically out of the graphene plane. This regime can be accessed when
vertical energy transport mediated by thermionic emission of hot carriers
overwhelms electron-lattice cooling as well as lateral diffusive energy
transport. As such, the power pumped into the system is efficiently extracted
across the entire graphene active area via thermionic emission of hot carriers
into a semiconductor material. Experimental signatures of this regime include a
large and tunable internal responsivity with a non-monotonic
temperature dependence. In particular, peaks at electronic
temperatures on the order of the Schottky potential and has a large
upper limit ( when ). Our proposal opens up new approaches for engineering the
photoresponse in optically-active graphene heterostructures.Comment: 6 pages, 2 figure
Goldstone bosons and fluctuating hydrodynamics with dipole and momentum conservation
We develop a Schwinger-Keldysh effective field theory describing the
hydrodynamics of a fluid with conserved charge and dipole moments, together
with conserved momentum. The resulting hydrodynamic modes are highly unusual,
including sound waves with quadratic (magnon-like) dispersion relation and
subdiffusive decay rate. Hydrodynamics itself is unstable below four spatial
dimensions. We show that the momentum density is, at leading order, the
Goldstone boson for a dipole symmetry which appears spontaneously broken at
finite charge density. Unlike an ordinary fluid, the presence or absence of
energy conservation qualitatively changes the decay rates of the hydrodynamic
modes. This effective field theory naturally couples to curved spacetime and
background gauge fields; in the flat spacetime limit, we reproduce the "mixed
rank tensor fields" previously coupled to fracton matter.Comment: 20+10 pages. v2, v3: minor edit