39 research outputs found
Tuning of the Thermoelectric Figure of Merit of CHNHMI (M=Pb,Sn) Photovoltaic Perovskites
The hybrid halide perovskites, the very performant compounds in photovoltaic
applications, possess large Seebeck coefficient and low thermal conductivity
making them potentially interesting high figure of merit () materials. For
this purpose one needs to tune the electrical conductivity of these
semiconductors to higher values. We have studied the CHNHMI
(M=Pb,Sn) samples in pristine form showing very low values for both
materials; however, photoinduced doping (in M=Pb) and chemical doping (in M=Sn)
indicate that, by further doping optimization, can be enhanced toward
unity and reach the performance level of the presently most efficient
thermoelectric materials.Comment: 9 pages, 2 figures + toc figur
Edge channel confinement in a bilayer graphene -- quantum dot
We combine electrostatic and magnetic confinement to define a quantum dot in
bilayer graphene. The employed geometry couples -doped reservoirs to a
-doped dot. At magnetic field values around T, Coulomb blockade is
observed. This demonstrates that the coupling of the co-propagating modes at
the - interface is weak enough to form a tunnel barrier, facilitating
transport of single charge carriers onto the dot. This result may be of use for
quantum Hall interferometry experiments
Interactions and magnetotransport through spin-valley coupled Landau levels in monolayer MoS
The strong spin-orbit coupling and the broken inversion symmetry in monolayer
transition metal dichalcogenides (TMDs) results in spin-valley coupled band
structures. Such a band structure leads to novel applications in the fields of
electronics and optoelectronics. Density functional theory calculations as well
as optical experiments have focused on spin-valley coupling in the valence
band. Here we present magnetotransport experiments on high-quality n-type
monolayer molybdenum disulphide (MoS) samples, displaying highly resolved
Shubnikov-de Haas oscillations at magnetic fields as low as . We find the
effective mass , about twice as large as theoretically predicted and
almost independent of magnetic field and carrier density. We further detect the
occupation of the second spin-orbit split band at an energy of about ,
i.e. about a factor larger than predicted. In addition, we demonstrate an
intricate Landau level spectrum arising from a complex interplay between a
density-dependent Zeeman splitting and spin and valley-split Landau levels.
These observations, enabled by the high electronic quality of our samples,
testify to the importance of interaction effects in the conduction band of
monolayer MoS.Comment: Phys.Rev.Lett. (2018
Spin and Valley States in Gate-defined Bilayer Graphene Quantum Dots
In bilayer graphene, electrostatic confinement can be realized by a suitable
design of top and back gate electrodes. We measure electronic transport through
a bilayer graphene quantum dot, which is laterally confined by gapped regions
and connected to the leads via p-n junctions. Single electron and hole
occupancy is realized and charge carriers can be filled
successively into the quantum system with charging energies exceeding $10 \
\mathrm{meV}g_{s}\approx 2$. In the low
field-limit, the valley splitting depends linearly on the perpendicular
magnetic field and is in qualitative agreement with calculations.Comment: 7 pages, 4 figure
The electronic thickness of graphene
When two dimensional crystals are atomically close, their finite thickness becomes relevant. Using transport measurements, we investigate the electrostatics of two graphene layers, twisted by θ = 22° such that the layers are decoupled by the huge momentum mismatch between the K and K′ points of the two layers. We observe a splitting of the zero-density lines of the two layers with increasing interlayer energy difference. This splitting is given by the ratio of single-layer quantum capacitance over interlayer capacitance Cm and is therefore suited to extract Cm. We explain the large observed value of Cm by considering the finite dielectric thickness dg of each graphene layer and determine dg ≈ 2.6 Å. In a second experiment, we map out the entire density range with a Fabry-Pérot resonator. We can precisely measure the Fermi wavelength λ in each layer, showing that the layers are decoupled. Our findings are reproduced using tight-binding calculations