398 research outputs found
Topological pumping in class-D superconducting wires
We study adiabatic pumping at a normal metal/class-D superconductor hybrid
interface when superconductivity is induced through the proximity effect in a
spin-orbit coupled nanowire in the presence of a tilted Zeeman field. When the
induced order parameter in the nanowire is non-uniform, the phase diagram has
isolated trivial regions surrounded by topological ones. We show that in this
case the pumped charge is quantized in units of the elementary charge and
has a topological nature.Comment: 7 pages, 6 figures. Published versio
Nonlocal superconducting correlations in graphene in the quantum Hall regime
We study Andreev processes and nonlocal transport in a three-terminal
graphene-superconductor hybrid system under a quantizing perpendicular magnetic
field [G.-H. Lee et al., Nature Phys. 13, 693 (2017)]. We find that the
amplitude of the crossed Andreev reflection (CAR) processes crucially depends
on the orientation of the lattice. By employing Landauer-B\"{u}ttiker
scattering theory, we find that CAR is generally very small for a zigzag edge,
while for an armchair edge it can be larger than the normal transmission,
thereby resulting in a negative nonlocal resistance. In the case of an armchair
edge and with a wide superconducting region (as compared to the superconducting
coherence length), CAR exhibits large oscillations as a function of the
magnetic field due to interference effects. This results in sign changes of the
nonlocal resistance
Investigating the effect of positional isomerism on the assembly of zirconium phosphonates based on tritopic linkers
We report on the use of a novel tritopic phosphonic linker, 2,4,6-tris[3-(phosphonomethyl)phenyl]-1,3,5-triazine, for the synthesis of a layered zirconium phosphonate, named UPG-2. Comparison with the structure of the permanently porous UPG-1, based on the related linker 2,4,6-tris[4-(phosphonomethyl)phenyl]-1,3,5-triazine, reveals that positional isomerism disrupts the porous architecture in UPG-2 by preventing the formation of infinitely extended chains connected through Zr–O–P–O–Zr bonds. The presence of free, acidic P–OH groups and an extended network of hydrogen bonds makes UPG-2 a good proton conductor, reaching values as high as 5.7 × 10−4 S cm−1
Failure of conductance quantization in two-dimensional topological insulators due to non-magnetic impurities
Despite topological protection and the absence of magnetic impurities,
two-dimensional topological insulators display quantized conductance only in
surprisingly short channels, which can be as short as 100 nm for atomically
thin materials. We show that the combined action of short-range nonmagnetic
impurities located near the edges and on site electron-electron interactions
effectively creates noncollinear magnetic scatterers, and, hence, results in
strong backscattering. The mechanism causes deviations from quantization even
at zero temperature and for a modest strength of electron-electron
interactions. Our theory provides a straightforward conceptual framework to
explain experimental results, especially those in atomically thin crystals,
plagued with short-range edge disorder.Comment: 8 pages, 9 figures, 5 appendice
“Shake ‘n Bake” Route to Functionalized Zr-UiO-66 Metal–Organic Frameworks
We report a novel synthetic procedure for the high-yield synthesis of metal-organic frameworks (MOFs) with topology with a UiO-66-like structure starting from a range of commercial Zr precursors and various substituted dicarboxylic linkers. The syntheses are carried out by grinding in a ball mill the starting reagents, namely, Zr salts and the dicarboxylic linkers, in the presence of a small amount of acetic acid and water (1 mL total volume for 1 mmol of each reagent), followed by incubation at either room temperature or 120 °C. Such a simple "shake 'n bake" procedure, inspired by the solid-state reaction of inorganic materials, such as oxides, avoids the use of large amounts of solvents generally used for the syntheses of Zr-MOF. Acidity of the linkers and the amount of water are found to be crucial factors in affording materials of quality comparable to that of products obtained under solvo- or hydrothermal conditions
Local density of states in metal - topological superconductor hybrid systems
We study by means of the recursive Green's function technique the local
density-of-states of (finite and semi-infinite) multi-band spin-orbit coupled
semiconducting nanowires in proximity to an s-wave superconductor and attached
to normal-metal electrodes. When the nanowire is coupled to a normal electrode,
the zero-energy peak, corresponding to the Majorana state in the topological
phase, broadens with increasing transmission between the wire and the leads,
eventually disappearing for ideal interfaces. Interestingly, for a finite
transmission a peak is present also in the normal electrode, even though it has
a smaller amplitude and broadens more rapidly with the strength of the
coupling. Unpaired Majorana states can survive close to a topological phase
transition even when the number of open channels (defined in the absence of
superconductivity) is even. We finally study the Andreev-bound-state spectrum
in superconductor-normal metal-superconductor junctions and find that in
multi-band nanowires the distinction between topologically trivial and
non-trivial systems based on the number of zero-energy crossings is preserved.Comment: 11 pages, 12 figures, published versio
Optical and plasmonic properties of twisted bilayer graphene: Impact of interlayer tunneling asymmetry and ground-state charge inhomogeneity
We present a theoretical study of the local optical conductivity, plasmon
spectra, and thermoelectric properties of twisted bilayer graphene (TBG) at
different filling factors and twist angles . Our calculations are based
on the electronic band structures obtained from a continuum model that has two
tunable parameters, and , which parametrize the intra-sublattice
inter-layer and inter-sublattice inter-layer tunneling rate, respectively. In
this Article we focus on two key aspects: i) we study the dependence of our
results on the value of , exploring the whole range ;
ii) we take into account effects arising from the intrinsic charge density
inhomogeneity present in TBG, by calculating the band structures within the
self-consistent Hartree approximation. At zero filling factor, i.e. at the
charge neutrality point, the optical conductivity is quite sensitive to the
value of and twist angle, whereas the charge inhomogeneity brings about
only modest corrections. On the other hand, away from zero filling, static
screening dominates and the optical conductivity is appreciably affected by the
charge inhomogeneity, the largest effects being seen on the intra-band
contribution to it. These findings are also reflected by the plasmonic spectra.
We compare our results with existing ones in the literature, where effects i)
and ii) above have not been studied systematically. As natural byproducts of
our calculations, we obtain the Drude weight and Seebeck coefficient. The
former displays an enhanced particle-hole asymmetry stemming from the
inhomogeneous ground-state charge distribution. The latter is shown to display
a broad sign-changing feature even at low temperatures ()
due to the reduced slope of the bands, as compared to those of single-layer
graphene.Comment: 28 pages, 16 figures, 6 appendice
Gauge fields and interferometry in folded graphene
Folded graphene flakes are a natural byproduct of the micromechanical
exfoliation process. In this Letter we show by a combination of analytical and
numerical methods that such systems behave as intriguing interferometers due to
the interplay between an externally applied magnetic field and the gauge field
induced by the deformations in the region of the fold.Comment: 4 pages, 3 figure
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