Charge-localized States Research Articles

The influence of localized states charging on [formula omitted] tunneling current noise spectrum

We report the results of theoretical investigations of low frequency tunneling current noise spectra component ( 1 / f α ). Localized states of individual impurity atoms play a key role in low frequency tunneling current noise formation. It is found that switching “on” and “off” of Coulomb interaction of conduction electrons with one or two charged localized states results in power law singularity of low-frequency tunneling current noise spectrum 1 / f α . Power law exponent in different low frequency ranges depends on the relative values of Coulomb interaction of conduction electrons with different charged impurities.

Can the second order multireference perturbation theory be considered a reliable tool to study mixed-valence compounds?

In this paper, the problem of the calculation of the electronic structure of mixed-valence compounds is addressed in the frame of multireference perturbation theory (MRPT). Using a simple mixed-valence compound (the 5,5(') (4H,4H('))-spirobi[ciclopenta[c]pyrrole] 2,2('),6,6(') tetrahydro cation), and the n-electron valence state perturbation theory (NEVPT2) and CASPT2 approaches, it is shown that the ground state (GS) energy curve presents an unphysical "well" for nuclear coordinates close to the symmetric case, where a maximum is expected. For NEVPT, the correct shape of the energy curve is retrieved by applying the MPRT at the (computationally expensive) third order. This behavior is rationalized using a simple model (the ionized GS of two weakly interacting identical systems, each neutral system being described by two electrons in two orbitals), showing that the unphysical well is due to the canonical orbital energies which at the symmetric (delocalized) conformation lead to a sudden modification of the denominators in the perturbation expansion. In this model, the bias introduced in the second order correction to the energy is almost entirely removed going to the third order. With the results of the model in mind, one can predict that all MRPT methods in which the zero order Hamiltonian is based on canonical orbital energies are prone to present unreasonable energy profiles close to the symmetric situation. However, the model allows a strategy to be devised which can give a correct behavior even at the second order, by simply averaging the orbital energies of the two charge-localized electronic states. Such a strategy is adopted in a NEVPT2 scheme obtaining a good agreement with the third order results based on the canonical orbital energies. The answer to the question reported in the title (is this theoretical approach a reliable tool for a correct description of these systems?) is therefore positive, but care must be exercised, either in defining the orbital energies or by resorting to the third order using for them the standard definition.

Optical Control in Coupled Two-Electron Quantum Dots

The dynamics of two electrons in a 2-dimensional quantum dot molecule in the presence of a time-dependent electromagnetic field is calculated from first principles. We show that carefully selected microwave pulses can exclusively populate a single state of the first excitation band and that the transition time can be further decreased by optimal pulse control. Finally we demonstrate that an oscillating charge localized state may be created by multiple transitions using a sequence of pulses.

Manipulation of localized charge states in n-MOSFETs with microwave irradiation

We demonstrate how the charge state of a trap at the Si/SiO 2 interface in a MOSFET can be controlled by microwave irradiation. The device is immersed in a static magnetic field at 300 mK and operates at small bias. Under such experimental conditions the electron spins are almost fully polarized. The electron occupancy of the trap is reversibily raised from one to two electrons by turning on a microwave field of less than 10 μW. Such contactless method controls the charge bound to defects close to the channel and it enables the fast initialization of the charge and spin state of the trapped electrons. This is particularly important in those spin resonance quantum computation schemes where a channel current senses the charge state, to improve the switching clock and to eliminate the related noise.

Muon spin spectroscopy evidence of a charge density wave in magnetite below the Verwey transition

We present new muon spectroscopy data on a Fe$_3$O$_4$ single crystal, revealing different spin precession patterns in five distinct temperature ranges. A careful analysis of the local field and its straightforward modeling obtains surprisingly good agreement with experiments only if a very specific model of localized charges violating Anderson condition, and a correlated muon local dynamics are implemented. Muon evidence for fluctuations just above the Verwey temperature, precursor of the low temperature charge localized state, is provided.

Configuration interaction based on constrained density functional theory: A multireference method

Existing density functional theory (DFT) methods are typically very effective in capturing dynamic correlation, but run into difficulty treating near-degenerate systems where static correlation becomes important. In this work, we propose a configuration interaction (CI) method that allows one to use a multireference approach to treat static correlation but incorporates DFT's efficacy for the dynamic part as well. The new technique uses localized charge or spin states built by a constrained DFT approach to construct an active space in which the effective Hamiltonian matrix is built. These local configurations have significantly less static correlation compared to their delocalized counterparts and possess an essentially constant amount of self-interaction error. Thus their energies can be reliably calculated by DFT with existing functionals. Using a small number of local configurations as different references in the active space, a simple CI step is then able to recover the static correlation missing from the localized states. Practical issues of choosing configurations and adjusting constraint values are discussed, employing as examples the ground state dissociation curves of H(2) (+), H(2), and LiF. Excellent results are obtained for these curves at all interatomic distances, which is a strong indication that this method can be used to accurately describe bond breaking and forming processes.

Nonadiabatic quantum dynamics based on a hierarchical electron-phonon model: Exciton dissociation in semiconducting polymers

A hierarchical electron-phonon coupling model is applied to describe the ultrafast decay of a photogenerated exciton at a donor-acceptor polymer heterojunction, via a vibronic coupling mechanism by which a charge-localized interfacial state is created. Expanding upon an earlier Communication [H. Tamura et al., J. Chem. Phys. 126, 021103 (2007)], we present a quantum dynamical analysis based on a two-state linear vibronic coupling model, which accounts for a two-band phonon bath including high-frequency C[Double Bond]C stretch modes and low-frequency ring torsional modes. Building upon this model, an analysis in terms of a hierarchical chain of effective modes is carried out, whose construction is detailed in the present paper. Truncation of this chain at the order n (i.e., 3n+3 modes) conserves the Hamiltonian moments (cumulants) up to the (2n+3)rd order. The effective-mode analysis highlights (i) the dominance of the high-frequency modes in the coupling to the electronic subsystem and (ii) the key role of the low-frequency modes in the intramolecular vibrational redistribution process that is essential in mediating the decay to the charge-localized state. Due to this dynamical interplay, the effective-mode hierarchy has to be carried beyond the first order in order to obtain a qualitatively correct picture of the nonadiabatic process. A reduced model of the dynamics, including a Markovian closure of the hierarchy, is presented. Dynamical calculations were carried out using the multiconfiguration time-dependent Hartree method.

Extracting electron transfer coupling elements from constrained density functional theory

Constrained density functional theory (DFT) is a useful tool for studying electron transfer (ET) reactions. It can straightforwardly construct the charge-localized diabatic states and give a direct measure of the inner-sphere reorganization energy. In this work, a method is presented for calculating the electronic coupling matrix element (Hab) based on constrained DFT. This method completely avoids the use of ground-state DFT energies because they are known to irrationally predict fractional electron transfer in many cases. Instead it makes use of the constrained DFT energies and the Kohn-Sham wave functions for the diabatic states in a careful way. Test calculations on the Zn2+ and the benzene-Cl atom systems show that the new prescription yields reasonable agreement with the standard generalized Mulliken-Hush method. We then proceed to produce the diabatic and adiabatic potential energy curves along the reaction pathway for intervalence ET in the tetrathiafulvalene-diquinone (Q-TTF-Q) anion. While the unconstrained DFT curve has no reaction barrier and gives Hab approximately 17 kcal/mol, which qualitatively disagrees with experimental results, the Hab calculated from constrained DFT is about 3 kcalmol and the generated ground state has a barrier height of 1.70 kcal/mol, successfully predicting (Q-TTF-Q)- to be a class II mixed-valence compound.

Influence of crystallite-boundary potential barriers on the thermopower and peltier effect in polycrystalline ferroelectric semiconductors

The influence of local charged states at crystallite boundaries on the thermoelectric effects in polycrystalline ferroelectrics is considered. It is shown that the differential thermopower and the Peltier coefficient depend on the height of the crystallite-boundary potential barriers. The possible manifestation of an anomalous behavior of the thermopower in the vicinity of the Curie ferroelectric point is demonstrated.

Ion Core Structures in Anthracene Anion Tetramers

Anthracene is a typical example of organic molecular crystals for the study of charge carrier states. The weakness of van der Waals intermolecular interaction makes the molecular properties dominant over the crystalline ones, which leads to localized charge states in molecular crystals. The localized charge carrier polarizes the subsystems of the surrounding lattice, where a polaron-like particle emerges due to many-electron interactions in the crystals. As it provides a unique environment to investigate the physics of polaron, the organic molecular crystal has been a subject of extensive studies. In particular, the study of anthracene anion clusters, as a microscopic model of charge localization, will contribute to understand the nature of polaron in organic molecular crystals. Despite numerous studies for the core structures of aromatic cation and neutral clusters, little has been investigated for those of anion clusters. Recently, the multiple ion cores and switching of ion cores in the anthracene anion clusters, Ann − (n = 1-16), were investigated by anion photoelectron spectroscopy and theoretical calculations. The study revealed that a few different ion cores exist depending on the cluster size. The character of ion core changes from monomeric to dimeric to trimeric, as the size increases from n = 1 to 3. For n = 4, coexistence of two ion cores was proposed to account for the anomalous shape of the photoelectron spectrum, which were tentatively assigned as dimeric and trimeric ion cores. Due to lack of further information, the explanation could be mainly deduced from the optimized geometries of neutral clusters. The ion core becomes monomeric again at n = 5 with the completion of half-filled solvation shell. It remains still unclear what ion cores represent the unexpected photoelectron spectrum at n = 4. In this Note, we revisit the ion core structures at n = 4, and report a few ion core structures found in the stable geometries at n = 4. Figure 1 shows the photoelectron spectra of Ann − (n = 15), which are fitted with Gaussian profiles representing the vibration progression of the v6 mode of An molecule. The vertical detachment energies (VDEs) obtained from the fitting are presented as solid bars. As previously reported, the spectral shape of An4 − is distinctly different from those of the others. The anomalous shape is deconvoluted into two separate vibrational progressions with different Figure 1. Photoelectron spectra of anthracene cluster anions, Ann −

NMR Study of the Magnetic and Metal-Insulator Transitions inNa0.5CoO2: A Nesting Scenario

Co and Na NMR are used to probe the local susceptibility and charge state of the two Co sites of the Na-ordered orthorhombic Na0.5CoO2. Above T_N=86K, both sites display a similar T-dependence of the spin shift, suggesting that there is no charge segregation into Co3+ and Co4+ sites. Below T_N, the magnetic long range commensurate order found is only slightly affected by the metal-insulator transition (MIT) at T_MIT=51K. Furthermore, the electric field gradient at the Co site does not change at these transitions, indicating the absence of charge ordering. All these observations can be explained by successive SDW induced by two nestings of the Fermi Surface specific to the x=0.5 Na-ordering.

Charge shelving and bias spectroscopy for the readout of a charge qubit on the basis of superposition states

Charge-based qubits have been proposed as fundamental elements for quantum computers. One commonly proposed readout device is the single-electron transistor (SET). SETs can distinguish between localized charge states, but lack the sensitivity to directly distinguish superposition states, which have greatly enhanced coherence times compared with position states. We propose introducing a third dot, and exploiting energy dependent tunneling from the qubit into this dot (bias spectroscopy) for pseudo-spin to charge conversion and superposition basis readout. We introduce an adiabatic fast passage-style charge pumping technique which enables efficient and robust readout via charge shelving, avoiding problems due to finite SET measurement time.

Valence Change of Cations in Ceria-Zirconia Solid Solution Associated with Redox Reactions Studied with Electron Energy-Loss Spectroscopy

It has been known that the ceria-zirconia solid solutions (Ce2Zr2O7+x, 0 ≤ x ≤ 1) have an excellent ability for oxygen absorption/release. The local charge states of cations in the CeO2-ZrO2 solid solutions have been studied by means of electron energy-loss spectroscopy (EELS) attached to a transmission electron microscope. The relative intensity of cerium-M4,5 white-line peaks from Ce2Zr2O7, Ce2Zr2O7.5 and Ce2Zr2O8 showed a systematic change, which corresponded to the valence change of cerium; Ce3+ in Ce2Zr2O7, Ce4+ in Ce2Zr2O8. By contrast, the valence of the zirconium ions, which are mainly at the nearest neighbor sites of absorbed or emitted oxygen atoms, remains to be quaternary irrespective of oxygen contents. It was found that the energy-loss near edge structure (ELNES) of the cerium-M4,5 edge in Ce2Zr2O7.5 was well reproduced by the sum of the ELNES profiles of Ce2Zr2O7 and Ce2Zr2O8 with an equal weight, the phenomenon of which is known as the ‘valence fluctuation,’ or ‘mixed valence state,’ often observed in rare earth metal compounds.

Coulomb gap and variable range hopping in a pinned Wigner crystal

It is shown that pinning of an electron Wigner crystal by a small concentration of charged impurities creates the finite density of charged localized states near the Fermi level. In the case of residual impurities in the spacer this density of states is related to nonlinear screening of a close acceptor by a Wigner crystal vacancy. On the other hand, intentional doping by a remote layer of donors is a source of a long range potential, which generates dislocations in Wigner crystal. Dislocations in turn create charged localized states near the Fermi level. In both cases Coulomb interaction of localized states leads to the soft Coulomb gap and ES variable range hopping at low enough temperatures.

Charge density and electric charge in quantum electrodynamics

The convergence of integrals over charge densities is discussed in relation with the problem of electric charge and (nonlocal) charged states in quantum electrodynamics. Delicate points like the domain dependence of local charges as quadratic forms and the class of time smearing ensuring strong convergence of integrals of charge densities are analyzed and shown to be crucial in QED, also for the control of vacuum polarization effects leading to time dependence of the charge (Swieca phenomenon). The possibility of constructing physical charged states in the Feynman–Gupta–Bleuler gauge as limits of local state vectors is discussed, compatibly with the vanishing of the Gauss charge on local states. A modification of the Dirac exponential factor which yields the physical Coulomb fields from the Feynman–Gupta–Bleuler fields is shown to remove the infrared divergence of scalar products of local and physical charged states, allowing for a construction of physical charged fields with well-defined correlation functions with local fields.

ESR Investigation of Charge Localized States in (TMTTF) 2X

ESR investigations were performed for a series of organic conductors, (TMTTF) 2 X (X=SbF 6 , AsF 6 , PF 6 , ReO 4 , ClO 4 , SCN, Br). The ESR linewidth shows abrupt jumps or humps in the paramagnetic insulating region. The (TMTTF) 2 X compounds are roughly divided into three groups in the aspect of the anisotropy of the ESR linewidth at low-temperatures. We discuss the low-temperature charge distributed pattern from the microscopic point of view.

On transverse cooling of channeled ions by electron capture and loss

To study the charge exchange cooling of channeled ions, we have made simulations of a single collision with an atomic string for 1.5 MeV/u Al ions in (1 0 0) Si. The relative magnitude of the probabilities for capture and loss varies with the distance from the string, and the irreversibility leading to cooling is associated with an asymmetry in time of the ion charge state during the collision. Combining the simulations with simple analytical calculations, we investigate the transition from the perturbation limit with very few charge exchanges to the adiabatic limit with a local charge state equilibrium. In this limit, the symmetry in time is restored and the cooling is strongly suppressed. The simulations illustrate the interplay of charge exchange and multiple scattering due to thermal displacements of atoms, and cooling by charge exchange can be included in the diffusion equation for dechanneling. We use the results of the simulations to obtain an estimate of the flux enhancement in an axial direction from an initially isotropic angular distribution. The conclusion is that the observed magnitude of this enhancement can be explained by charge exchange cooling, with plausible assumption about the impact parameter dependencies of electron capture and loss.

NMR study of charge localized states of (TMTTF) 2Br

13C NMR study was performed for a quasi-one-dimensional organic conductor (TMTTF) 2Br, using a single crystal in which two central carbon sites of TMTTF molecules were labeled with 13C. To investigate the relation between the charge localization around 100 K and the magnetic ground state from the microscopic point of view, we measured the NMR spectra and nuclear relaxation rate. We found slight broadening of the spectra above the magnetic phase transition temperature (T N ∼15 K). We discuss this anomaly with the newly measured data of uniform susceptibility and ESR.

The localization/delocalization dilemma in mixed-valence systems: a theoretical investigation on the application of the photon echo technique

The photon echo (PE) signal from a symmetric mixed-valence system modeled with two electronic states and a single vibrational coordinate is investigated. The two charge-localized diabatic curves have different equilibrium position and the electronic coupling or hopping term (th) is assumed to be a constant (PKS model). The solvent effect is simulated by a static random perturbation removing the degeneracy between the left and right charge-localized states. A set of calculations is performed, keeping fixed the parameters of the model and varying only th, to study the transition from a localized to a delocalized regime. The PE signals (both time resolved and time integrated) show clearly the signature of such a transition.

Superexchange, Localized, and Domain-Localized Charge States for Intramolecular Electron Transfer in Large Molecules and in Arrays of Quantum Dots

Superexchange is a longer-range electron-transfer mediated by a nonresonant bridge between the donating and accepting states. We discuss a coupled set of donor/acceptor levels that are not resonant, with special reference to coupling of intermediate strengths. Examples of such systems are peptide cations or arrays of quantum dots. If the coupling is strong enough to overcome the gaps, charge can migrate. If the coupling is too weak, the charge remains localized. In the intermediate case, the charge is shown to be localized over a finite, connected, subset of sites. Degenerate perturbation theory provides a suitable zero-order basis for this intermediate regime. In a time dependent language, in the domain-localized regime, the charge migrates over a limited range of states. Also discussed is an effect of electron correlation, the so-called Coulomb blockade, on charge localization with computational examples. The experimental probing of the domain-localized regime is considered. Probes of the energy depende...