Rabani, E. ; Baer, R. Distribution of multiexciton generation rates in CdSe and InAs nanocrystals.
Nano Lett. 2008,
8 4488–4492.
AbstractThe distribution of rates of multiexciton generation following photon absorption is calculated for semiconductor nanocrystals (NCs). The rates of biexciton generation are calculated using Fermi’s golden rule with all relevant Coulomb matrix elements, taking into account proper selection rules within a screened semiempirical pseudopotential approach. In CdSe and InAs NCs, we find a broad distribution of biexciton generation rates depending strongly on the exciton energy and size of the NC. Multiexciton generation becomes inefficient for NCs exceeding 3 nm in diameter in the photon energy range of 2-3 times the band gap.
rabani2008distribution.pdf Kurzweil, Y. ; Baer, R. Adapting approximate-memory potentials for time-dependent density functional theory.
Phys. Rev. B 2008,
77, 085121.
AbstractFrequency dependent exchange-correlation kernels for time-dependent density functional theory can be used to construct approximate exchange-correlation potentials. The resulting potentials are usually not translationally covariant nor do they obey the so-called zero-force condition. These two basic symmetry requirements are essential for using the potentials in actual applications (even in the linear regime). We provide two pragmatic methods for fully imposing these conditions for both linear and nonlinear regimes. As an example, we take the Gross and Kohn frequency dependent XC functional [Phys. Rev. Lett. 55, 2850 (1985)], correct it, and numerically test it on a sodium metal cluster. Violation of the basic symmetries causes instabilities or spurious low frequency modes.
kurzweil2008adapting.pdf Livshits, E. ; Baer, R. A Density Functional Theory for Symmetric Radical Cations from Bonding to Dissociation.
J. Phys. Chem. A 2008,
112, 12789–12791.
Publisher's VersionAbstractIt has been known for quite some time that approximate density functional (ADF) theories fail disastrously when describing the dissociative symmetric radical cations R2+. By considering this dissociation limit, previous work has shown that Hartree-Fock (HF) theory favors the R+1-R-0 charge distribution, whereas DF approximations favor the R+(0.5)-R+0.5. Yet, general quantum mechanical principles indicate that both these (as well as all intermediate) average charge distributions are asymptotically energy degenerate. Thus, HF and ADF theories mistakenly break the symmetry but in a contradicting way. In this letter, we show how to construct system-dependent long-range corrected (LC) density functionals that can successfully treat this class of molecules, avoiding the spurious symmetry breaking. Examples and comparisons to experimental data is given for R = H, He, and Ne, and it is shown that the new LC theory improves considerably the theoretical description of the R-2(+) bond properties, the long-range form of the asymptotic potential curve, and the atomic polarizability. The broader impact of this finding is discussed as well, and it is argued that the widespread semiempirical approach which advocates treating the LC parameter as a system-independent parameter is in fact inappropriate under general circumstances.
livshits2008.pdf Hod, O. ; Baer, R. ; Rabani, E. Magnetoresistance of nanoscale molecular devices based on Aharonov-Bohm interferometry.
J. Phys. C 2008,
20, 383201.
AbstractControl of conductance in molecular junctions is of key importance in the growing field of molecular electronics. The current in these junctions is often controlled by an electric gate designed to shift conductance peaks into the low bias regime. Magnetic fields, on the other hand, have rarely been used due to the small magnetic flux captured by molecular conductors ( an exception is the Kondo effect in single-molecule transistors). This is in contrast to a related field, electronic transport through mesoscopic devices, where considerable activity with magnetic fields has led to a rich description of transport. The scarcity of experimental activity is due to the belief that significant magnetic response is obtained only when the magnetic flux is of the order of the quantum flux, while attaining such a flux for molecular and nanoscale devices requires unrealistic magnetic fields. Here we review recent theoretical work regarding the essential physical requirements necessary for the construction of nanometer-scale magnetoresistance devices based on an Aharonov-Bohm molecular interferometer. We show that control of the conductance properties using small fractions of a magnetic flux can be achieved by carefully adjusting the lifetime of the conducting electrons through a pre-selected single state that is well separated from other states due to quantum confinement effects. Using a simple analytical model and more elaborate atomistic calculations we demonstrate that magnetic fields which give rise to a magnetic flux comparable to 10(-3) of the quantum flux can be used to switch a class of different molecular and nanometer rings, ranging from quantum corrals, carbon nanotubes and even a molecular ring composed of polyconjugated aromatic materials. The unique characteristics of the magnetic field as a gate is further discussed and demonstrated in two different directions. First, a three-terminal molecular router devices that can function as a parallel logic gate, processing two logic operations simultaneously, is presented. Second, the role of inelastic effects arising from electron-phonon couplings on the magnetoresistance properties is analyzed. We show that a remarkable difference between electric and magnetic gating is also revealed when inelastic effects become significant. The inelastic broadening of response curves to electric gates is replaced by a narrowing of magnetoconductance peaks, thereby enhancing the sensitivity of the device.
hod2006.pdf