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Screening limited switching performance of multilayer 2D semiconductor FETs: the case for SnS. (arXiv:1608.06501v3 [cond-mat.mes-hall] UPDATED)

March 23rd, 2017|Publications|

By Sukrit Sucharitakul, Rajesh Kumar Ulaganathan, Raman Sankar, Fang-Cheng Chou, Yit-Tsong Chen, Chuhan Wang, Cai He, Rui He, Xuan P. A. Gao

Gate tunable p-type multilayer tin mono-sulfide (SnS) field-effect transistor (FET) devices with SnS thickness between 50 and 100 nm were fabricated and studied to understand their performances. The devices showed anisotropic inplane conductance and room temperature field effect mobilities ~5 – 10 cm$^2$/Vs. However, the devices showed appreciable OFF state conductance and an ON-OFF ratio ~10 at room temperature. The weak gate tuning behavior in the depletion regime of SnS devices is explained by the finite carrier screening length effect which causes the existence of a conductive surface layer from intrinsic defects induced holes in SnS. Through etching and n-type surface doping by Cs2CO3 to reduce/compensate the not-gatable holes near SnS flake’s top surface, the devices gained an order of magnitude improvement in the ON-OFF ratio and hole Hall mobility ~ 100 cm$^2$/Vs at room temperature is observed. This work suggests that in order to obtain effective switching and low OFF state power consumption, two-dimensional (2D) semiconductor based depletion mode FETs should limit their thickness to within the Debye screening length of carriers in the semiconductor.

Published : "arXiv Mesoscale and Nanoscale Physics".

Electronic Veselago lenses in graphene: Valley polarization due to trigonal warping. (arXiv:1703.07769v1 [cond-mat.mes-hall])

March 23rd, 2017|Publications|

By K. J. A. Reijnders, M. I. Katsnelson

We study the effect of trigonal warping on the focussing of electrons by n-p junctions in graphene. We find that perfect focussing, which was predicted for massless Dirac fermions, is only preserved for one specific sample orientation. In the general case, trigonal warping leads to a different position of the focus for graphene’s two valleys. We derive these positions using the semiclassical approximation and find very good agreement with tight-binding simulations. Considering the transmission as a function of potential strength, we find that trigonal warping splits the single Dirac peak into two distinct peaks, one for each valley. This leads to valley polarization, which increases with sample length. These effects are clearly observed in the transmission curves obtained from both tight-binding simulations and a billiard model that incorporates trigonal warping. Since their origin is classical rather than quantum mechanical, they are predicted to be very robust. The general mechanism that we discuss will also play an important role in other Dirac materials, which generally exhibit stronger band bending than graphene.

Published : "arXiv Mesoscale and Nanoscale Physics".

Anisotropic plasmons, excitons and electron energy loss spectroscopy of phosphorene. (arXiv:1703.07696v1 [cond-mat.mes-hall])

March 23rd, 2017|Publications|

By Barun Ghosh, Piyush Kumar, Anmol Thakur, Yogesh Singh Chauhan, Somnath Bhowmick, Amit Agarwal

In this article, we explore the anisotropic electron energy loss spectrum (EELS) in monolayer phosphorene based on ab-initio time dependent density functional theory calculations. Similar to black phosphorous, the EELS of undoped monolayer phosphorene is characterized by anisotropic excitonic peaks for energies in vicinity of the bandgap, and by interband plasmon peaks for higher energies. On doping, an additional intraband plasmon peak also appears for energies within the bandgap. Similar to other two dimensional systems, the intraband plasmon peak disperses as $omega_{rm pl} propto sqrt{q}$ in both the zigzag and armchair directions in the long wavelength limit, and deviates for larger wavevectors. The anisotropy of the long wavelength plasmon intraband dispersion is found to be inversely proportional to the square root of the ratio of the effective masses: $omega_{rm pl}(q hat{y})/omega_{rm pl}(q hat{x}) = sqrt{m_x/m_y}$.

Published : "arXiv Mesoscale and Nanoscale Physics".

Klein tunneling in driven-dissipative photonic graphene. (arXiv:1703.07634v1 [physics.optics])

March 23rd, 2017|Publications|

By Tomoki Ozawa, Alberto Amo, Jacqueline Bloch, Iacopo Carusotto

We theoretically investigate Klein tunneling processes in photonic artificial graphene. Klein tunneling is a phenomenon in which a particle with Dirac dispersion going through a potential step shows a characteristic angle- and energy-dependent transmission. We consider a generic photonic system consisting of a honeycomb-shaped array of sites with losses, illuminated by coherent monochromatic light. We show how the transmission and reflection coefficients can be obtained from the steady-state field profile of the driven-dissipative system. Despite the presence of photonic losses, we recover the main scattering features predicted by the general theory of Klein tunneling. Signatures of negative refraction and the orientation-dependence of the inter-valley scattering are also highlighted. Our results will stimulate the experimental study of intricate transport phenomena using driven-dissipative photonic simulators.

Published : "arXiv Mesoscale and Nanoscale Physics".

Band inversion at critical magnetic fields in a silicene quantum dot. (arXiv:1703.07581v1 [cond-mat.mes-hall])

March 23rd, 2017|Publications|

By E. Romera, M. Calixto

We have found out that the band inversion in a silicene quantum dot (QD), in perpendicular magnetic $B$ and electric $Delta_z$ fields, drastically depends on the strength of the magnetic field. We study the energy spectrum of the silicene QD where the electric field provides a tunable band gap $Delta$. Boundary conditions introduce chirality, so that negative and positive angular momentum $m$ zero Landau level (ZLL) edge states show a quite different behavior regarding the band-inversion mechanism underlying the topological insulator transition. We show that, whereas some ZLLs suffer band inversion at $Delta=0$ for any $B>0$, other ZLLs only suffer band inversion above critical values of the magnetic field at nonzero values of the gap.

Published : "arXiv Mesoscale and Nanoscale Physics".

Coexistence of Interfacial Stress and Charge Transfer in Graphene Oxide based Magnetic Nanocomposites. (arXiv:1703.07545v1 [cond-mat.mes-hall])

March 23rd, 2017|Publications|

By Amodini Mishra, Vikash Kumar Singh, Tanuja Mohanty

In this paper, we establish the existence of both compressive stress and charge transfer process in hydrothermally synthesized cobalt ferrite-graphene oxide (CoFe2O4/GO) nanocomposites. Transmission electron microscopy (TEM) results reveal the decoration of CoFe2O4 nanoparticles on GO sheets. Magnetic response of nanocomposites was confirme from superconducting quantum interference device (SQUID) magnetometer measurement. Optical properties of these nanocomposites were investigated by Raman spectroscopy. Interfacial compressive stress involved in this system is evaluated from observed blue shift of characteristic G peak of graphene oxide. Increase in full width half-maximum ( FWHM) as well as up shift in D and G peaks are clear indicator of involvement of charge transfer process between GO sheets and dispersed magnetic nanoparticles. The effect of charge transfer process is quantified in terms of shifting of Fermi level of these nanocomposites. This is evaluated from variation in contact surface potential difference (CPD) using Scanning Kelvin probe microscopy (SKPM). XRD spectra of CoFe2O4/GO confirm the polycrystalline nature of CoFe2O4 nanoparticles. Lattice strain estimated from XRD peaks are correlated to the observed Raman shift.

Published : "arXiv Mesoscale and Nanoscale Physics".

Origin of layer number dependent linear and nonlinear optical properties of two-dimensional graphene-like SiC. (arXiv:1703.07430v1 [physics.optics])

March 23rd, 2017|Publications|

By You-Zhao Lan

We theoretically discuss the physical origin of the dielectric constants [{epsilon}({omega})] and second harmonic generation coefficients [{chi}(2)({omega})] of the ABA-stacked two-dimensional graphene-like silicon carbide (2D-SiC) with the number of layers up to 5. It is found that the intensities of the pronounced peaks of both {epsilon}({omega}) and {chi}(2)({omega}) exhibit a clear layer number dependence. For the light polarization parallel to the 2DSiC plane, the monolayer SiC (ML-SiC) and multilayer SiC (MuL-SiC) have very similar pronounced peak positions of {epsilon}({omega}), which are attributed to the {pi}->{pi}* and {sigma}->{sigma}* transitions. However, for the light polarization perpendicular to the 2D-SiC plane, a characteristic peak is found for the MuL-SiC at about 4.0 eV, except that the allowed {pi}->{sigma}* and {sigma}->{pi}* transition peaks are found for both ML-SiC and MuL-SiC in the high-energy region (> 8 eV). This characteristic peak is attributed to the interlayer {pi}->{pi}* transition which does not exist for the ML-SiC, and at this peak position, the ML-SiC has a weak dark exciton based on the mBJ calculation within the Bethe-Salpeter equation framework. For {chi}(2)({omega}), the single-particle transition channels based on the three-band terms dominate the second harmonic generation process of both ML-SiC and MuL-SiC and determine the size and sign of {chi}(2)({omega}). In the ultraviolet visible region, the purely interband motion and intraband motion of electrons competitively determine the size and sign of {chi}(2)({omega}). For the light polarization perpendicular to the 2D-SiC plane, the intraband motion of electrons modulated more dramatically the interband motion than for that parallel to the

Published : "arXiv Mesoscale and Nanoscale Physics".

Comparative study of the binding energy in a thin and ultra-thin organic-inorganic perovskite within dielectric mismatches effects. (arXiv:1703.07407v1 [cond-mat.mes-hall])

March 23rd, 2017|Publications|

By Haitham Zahra, Aïda Hichri, Sihem Jaziri

The multi-quantum well (MQW) organic-inorganic perovskite offer an approach of tuning the exciton binding energy based on the well-barrier dielectric mismatch effect, which called the image charge effect. The exfoliation from MQW organic-inorganic perovskite forms a twodimensional (2D) nano-sheet. As with other 2D materials, like graphene or transition metal dichalcogenides (TMDs), the ultra-thin perovskites layers are highly sensitive to the dielectric environment. We investigate the ultrathin crystalline 2D van-der-Waals (vdW) layers of organic-inorganic perovskite crystals close to a surface of the substrate. We show that binding exciton energy is strongly influenced by the surrounding dielectric environment. We find that the Keldysh model somehow estimates the strong dependence of the exciton binding energies on environmental screening. We compare our binding energies results with experimental results in the (C6H13NH3)2PbI4 perovskite, and we estimate the binding energy values of (C4H9NH3)2PbBr4.

Published : "arXiv Mesoscale and Nanoscale Physics".