arXiv subjects

Kristen Kaasbjerg

Publications and source records attributed to Kristen Kaasbjerg.

24 records · Page 2Linked to original sources

arXiv2013

Induced spin filtering in electron transmission through chiral molecular layers adsorbed on metals with strong spin-orbit coupling

Recent observations of considerable spin polarization in photoemission from metal surfaces through monolayers of chiral molecules were followed by several efforts to rationalize the results as the effect of spin-orbit interaction that accompanies electronic motion on helical, or more generally strongly curved, potential surfaces. In this paper we (a) argue, using simple models, that motion in curved force-fields with the typical energies used and the characteristic geometry of DNA cannot account for such observations; (b) introduce the concept of induced spin filtering, whereupon selectivity in the transmission of the electron orbital angular momentum can induce spin selectivity in the transmission process provided there is strong spin-orbit coupling in the substrate; and (c) show that the spin polarizability in the tunneling current as well as the photoemission current from gold covered by helical adsorbates can be of the observed order of magnitude. Our results can account for most of the published observations that involved gold and silver substrates, however recent results obtained with an aluminum substrate can be rationalized within the present model only if strong spin-orbit coupling is caused by the built-in electric field at the molecule-metal interface.

cond-mat.mes-hall↗

arXiv2013

Acoustic phonon limited mobility in two-dimensional semiconductors: Deformation potential and piezoelectric scattering in monolayer MoS2 from first principles

We theoretically study the acoustic phonon limited mobility in n-doped two-dimensional MoS2 for temperatures T < 100 K and high carrier densities using the Boltzmann equation and first-principles calculations of the acoustic electron-phonon (el-ph) interaction. In combination with a continuum elastic model, analytic expressions and the coupling strengths for the deformation potential and piezoelectric interactions are established. We furthermore show that the deformation potential interaction has contributions from both normal and umklapp processes and that the latter contribution is only weakly affected by carrier screening. Consequently, the calculated mobilities show a transition from a high-temperature μT^{-1} behavior to a stronger μT^{-4} behavior in the low-temperature Bloch-Gruneisen regime characteristic of unscreened deformation potential scattering. Intrinsic mobilities in excess of 10^5 cm^2 V^{-1} s^{-1} are predicted at T < 10 K and high carrier densities (n > 10^{11} cm^{-2}). At 100 K, the mobility does not exceed ~7 x 10^3 cm2 V^{-1} s^{-1}. Our findings provide new and important understanding of the acoustic el-ph interaction and its screening by free carriers, and is of high relevance for the understanding of acoustic phonon limited mobilities in general.

cond-mat.mes-hall↗

arXiv2012

First-principles study of the phonon-limited mobility in n-type single-layer MoS2

In the present work we calculate the phonon-limited mobility in intrinsic n-type single-layer MoS2 as a function of carrier density and temperature for T > 100 K. Using a first-principles approach for the calculation of the electron-phonon interaction, the deformation potentials and Fröhlich interaction in the isolated MoS2 layer are determined. We find that the calculated room-temperature mobility of ~410 cm^2 V^-1 s^-1 is dominated by optical phonon scattering via deformation potential couplings and the Fröhlich interaction with the deformation potentials to the intravalley homopolar and intervalley longitudinal optical phonons given by 4.1 x 10^8 eV/cm and 2.6 x 10^8 eV/cm, respectively. The mobility is weakly dependent on the carrier density and follows a μ~ T^-1 temperature dependence with γ= 1.69 at room temperature. It is shown that a quenching of the characteristic homopolar mode which is likely to occur in top-gated samples, boosts the mobility with 70 cm^2 V^-1 s^-1 and can be observed as a decrease in the exponent to γ= 1.52. Our findings indicate that the intrinsic phonon-limited mobility is approached in samples where a high-kappa dielectric that effectively screens charge impurities is used as gate oxide.

cond-mat.mtrl-sci↗

arXiv2012

Unraveling the acoustic electron-phonon interaction in graphene

Using a first-principles approach we calculate the acoustic electron-phonon couplings in graphene for the transverse (TA) and longitudinal (LA) acoustic phonons. Analytic forms of the coupling matrix elements valid in the long-wavelength limit are found to give an almost quantitative description of the first-principles based matrix elements even at shorter wavelengths. Using the analytic forms of the coupling matrix elements, we study the acoustic phonon-limited carrier mobility for temperatures 0-200 K and high carrier densities of 10^{12}-10^{13} cm^{-2}. We find that the intrinsic effective acoustic deformation potential of graphene is Ξ_eff = 6.8 eV and that the temperature dependence of the mobility μ~ T^{-α} increases beyond an α= 4 dependence even in the absence of screening when the full coupling matrix elements are considered. The large disagreement between our calculated deformation potential and those extracted from experimental measurements (18-29 eV) indicates that additional or modified acoustic phonon-scattering mechanisms are at play in experimental situations.

cond-mat.mtrl-sci↗

arXiv2010

Semi-Empirical Model for Nano-Scale Device Simulations

We present a new semi-empirical model for calculating electron transport in atomic-scale devices. The model is an extension of the Extended Hückel method with a self-consistent Hartree potential. This potential models the effect of an external bias and corresponding charge re-arrangements in the device. It is also possible to include the effect of external gate potentials and continuum dielectric regions in the device. The model is used to study the electron transport through an organic molecule between gold surfaces, and it is demonstrated that the results are in closer agreement with experiments than ab initio approaches provide. In another example, we study the transition from tunneling to thermionic emission in a transistor structure based on graphene nanoribbons.

cond-mat.mes-hall↗

arXiv2008

Strong polarization-induced reduction of addition energies in single-molecule nanojunctions

We address polarization-induced renormalization of molecular levels in solid-state based single-molecule transistors and focus on an organic conjugate molecule where a surprisingly large reduction of the addition energy has been observed. We have developed a scheme that combines a self-consistent solution of a quantum chemical calculation with a realistic description of the screening environment. Our results indeed show a large reduction, and we explain this to be a consequence of both (a) a reduction of the electrostatic molecular charging energy and (b) polarization induced level shifts of the HOMO and LUMO levels. Finally, we calculate the charge stability diagram and explain at a qualitative level general features observed experimentally.

cond-mat.mes-hall↗