Increasing Coulomb Repulsion Research Articles

Optical signatures of type I–type II band alignment transition in Cd(Se,Te)/ZnTe self-assembled quantum dots

Self-assembled Cd(Se,Te) quantum dots with various Se compositions embedded in the ZnTe matrix are grown by molecular beam epitaxy. A huge redshift of the near band edge emission, from 2.1 eV to 1.5 eV, with an increasing Se content in the dots is observed. It is accompanied by an increase in the excitonic lifetime by the factor of 10. We associate these effects with a gradual change from the direct type I confinement character in CdTe/ZnTe quantum dots to the staggered type II band alignment in the case of Cd(Se,Te)/ZnTe dots. This interpretation is consistent with the micro-photoluminescence study of several individual quantum dots, which reveals a gradual decrease in the biexciton–exciton energy difference with the increasing content of Se in the dots, which leads ultimately to the change from the binding to antibinding character of biexcitons. The latter effect originates, most likely, from the increasing Coulomb repulsion between excitons forming dipoles at the dot/barrier interface.

Surface-state Coulomb repulsion accelerates a metal-insulator transition in topological semimetal nanofilms.

The emergence of quantization at the nanoscale, the quantum size effect (QSE), allows flexible control of matter and is a rich source of advanced functionalities. A QSE-induced transition into an insulating phase in semimetallic nanofilms was predicted for bismuth a half-century ago and has regained new interest with regard to its surface states exhibiting nontrivial electronic topology. Here, we reveal an unexpected mechanism of the transition by high-resolution angle-resolved photoelectron spectroscopy combined with theoretical calculations. Anomalous evolution and degeneracy of quantized energy levels indicate that increased Coulomb repulsion from the surface states deforms a quantum confinement potential with decreasing thickness. The potential deformation strongly modulates spatial distributions of quantized wave functions, which leads to acceleration of the transition beyond the original QSE picture. This discovery establishes a complete picture of the long-discussed transition and highlights a new class of size effects dominating nanoscale transport in systems with metallic surface states.

Quantum anomalous Hall effect and topological phase transition in two-dimensional antiferromagnetic Chern insulator NiOsCl6

By doing calculations based on density functional theory, we predict that the two-dimensional anti-ferromagnetic (AFM) NiOsCl6 as a Chern insulator can realize the quantum anomalous Hall (QAH) effect. We investigate the magnetocrystalline anisotropy energies in different magnetic configurations and the Néel AFM configuration is proved to be ground state. When considering spin–orbit coupling (SOC), this layered material with spins perpendicular to the plane shows properties as a Chern insulator characterized by an inversion band structure and a nonzero Chern number. The nontrivial band gap is 37 meV and the Chern number C = −1, which are induced by a strong SOC and AFM order. With strong SOC, the NiOsCl6 system performs a continuous topological phase transition from the Chern insulator to the trivial insulator upon the increasing Coulomb repulsion U. The critical Uc is indicated as 0.23 eV, at which the system is in a metallic phase with . Upon increasing U, the Eg reduces linearly with C = −1 for 0 < U < Uc and increases linearly with C = 0 for U > Uc. At last we analysis the QAH properties and this continuous topological phase transition theoretically in a two-band model. This AFM Chern insulator NiOsCl6 proposes not only a promising way to realize the QAH effect, but also a new material to study the continuous topological phase transition.

Understanding the charge transport properties of redox active metal-organic conjugated wires.

Layer-by-layer assembly of the dirhodium complex [Rh2(O2CCH3)4] (Rh2) with linear N,N'-bidentate ligands pyrazine (LS) or 1,2-bis(4-pyridyl)ethene (LL) on a gold substrate has developed two series of redox active molecular wires, (Rh2LS) n @Au and (Rh2LL) n @Au (n = 1-6). By controlling the number of assembling cycles, the molecular wires in the two series vary systematically in length, as characterized by UV-vis spectroscopy, cyclic voltammetry and atomic force microscopy. The current-voltage characteristics recorded by conductive probe atomic force microscopy indicate a mechanistic transition for charge transport from voltage-driven to electrical field-driven in wires with n = 4, irrespective of the nature and length of the wires. Whilst weak length dependence of electrical resistance is observed for both series, (Rh2LL) n @Au wires exhibit smaller distance attenuation factors (β) in both the tunneling (β = 0.044 Å-1) and hopping (β = 0.003 Å-1) regimes, although in (Rh2LS) n @Au the electronic coupling between the adjacent Rh2 centers is stronger. DFT calculations reveal that these wires have a π-conjugated molecular backbone established through π(Rh2)-π(L) orbital interactions, and (Rh2LL) n @Au has a smaller energy gap between the filled π*(Rh2) and the empty π*(L) orbitals. Thus, for (Rh2LL) n @Au, electron hopping across the bridge is facilitated by the decreased metal to ligand charge transfer gap, while in (Rh2LS) n @Au the hopping pathway is disfavored likely due to the increased Coulomb repulsion. On this basis, we propose that the super-exchange tunneling and the underlying incoherent hopping are the dominant charge transport mechanisms for shorter (n ≤ 4) and longer (n > 4) wires, respectively, and the Rh2L subunits in mixed-valence states alternately arranged along the wire serve as the hopping sites.

What's Wrong with the Hodgkin-Huxley Model? An Exercise in Critical Thinking

Critical thinking is fundamental to science: let us examine an assumption of the Hodgkin-Huxley model of excitable membranes. The circuit elements of the HH model are a capacitance C and parallel ionic conductances g in series with their electromotive forces. HH assumes that C is constant while the gs vary with voltage and time. This assumption, adapted to voltage-sensitive ion channels, places the entire burden of explaining channel function on the gs. While the assumption of constant C—and its dimensionless coefficient, dielectric permittivity ε—is a good approximation for a membrane patch, its application at the macromolecular nanoscale is dubious, considering the ubiquitous presence of branched-chain amino acids—isoleucine, leucine and valine—in the channels. The sidechains of these amino acids (and no others) strongly affect ε in certain molecules with structures resembling channel segments. These ferroelectric liquid crystals, such as 4'-3M2CPOOB, exhibit ε values as high as 3000 in their ferroelectric state. Changes in channel ε affect all electrostatic interactions, including the mutual repulsions between the positively charged residues in S4 segments, key components of voltage sensors. The Channel Activation by Electrostatic Repulsion (CAbER) hypothesis assumes the resting channel, at a high electric field, is in a polar, far-from-equilibrium state with high ε, and that threshold depolarization converts it to a nonpolar state with much lower ε. The consequently increased Coulomb repulsion between arginines and lysines explains the observed outward motion of the S4 segments, which then drives the opening of the pore domain to stochastic ion currents. Does critical thinking support constant-C or CAbER? Problem: Two proton charges on a helix of radius 0.80 nm are 0.45 nm apart axially, with an angular separation of 300°. What is their repulsion energy (kJ/mol) when ε is 500? When it is 4? Further reading: HR Leuchtag, Voltage-Sensitive Ion Channels, Springer, 2008.

Pseudogap and singlet formation in organic and cuprate superconductors

The pseudogap phase occurring in cuprate and organic superconductors is analyzed based on the dynamical cluster approximation approach to the Hubbard model. In this method a cluster embedded in a self-consistent bath is studied. With increasing Coulomb repulsion, $U$, the antinodal point [$\mathbf{k}=(\ensuremath{\pi},0)$] displays a gradual suppression of spectral density of states around the Fermi energy which is not observed at the nodal point [$\mathbf{k}=(\ensuremath{\pi}/2,\ensuremath{\pi}/2)$]. The opening of the antinodal pseudogap is found to be related to the internal structure of the cluster and the much weaker bath-cluster couplings at the antinodal than nodal point. The role played by internal cluster correlations is elucidated from a simple four-level model. For small $U$, the cluster levels form Kondo singlets with their baths leading to a peak in the spectral density. As $U$ is increased a localized state is formed localizing the electrons in the cluster. If this cluster localized state is nondegenerate, the Kondo effect is destroyed and a pseudogap opens up in the spectra at the antinodal point. The pseudogap can be understood in terms of destructive interference between different paths for electrons hopping between the cluster and the bath. However, electrons at the nodal points remain in Kondo states up to larger $U$ since they are more strongly coupled to the bath. The strong correlation between the $(\ensuremath{\pi},0)$ and the $(0,\ensuremath{\pi})$ cluster levels in the localized state leads to a large correlation energy gain, which is important for localizing electrons and opening up a pseudogap at the antinodal point. Such a scenario is in contrast with two independent Mott transitions found in two-band systems with different bandwidths in which the localized cluster electron does not correlate strongly with any other cluster electron for intermediate $U$. The important intracluster sector correlations are associated with the resonating valence bond character of the cluster ground state containing $d$-wave singlet pairs. The low-energy excitations determining the pseudogap have suppressed $d$-wave pairing, indicating that the pseudogap can be related to breaking very short-range $d$-wave pairs. Geometrical frustration on the anisotropic triangular lattice relevant to $\ensuremath{\kappa}$-(BEDT-TTF)${}_{2}X$ leads to a switch in the character of the ground state of the cluster at intermediate hopping ratios ${t}^{\ensuremath{'}}/t\ensuremath{\sim}0.7$. Electron doping of the frustrated square lattice destroys the pseudogap, in agreement with photoemission experiments on cuprates, due to a larger Schrieffer-Wolff exchange coupling, ${J}_{K}$, and a stronger cluster-bath coupling for the antinodal point.

Supercharging with Trivalent Metal Ions in Native Mass Spectrometry

Addition of 1.0 mM LaCl(3) to aqueous ammonium acetate solutions containing proteins in their folded native forms can result in a significant increase in the molecular ion charging obtained with electrospray ionization as a result of cation adduction. In combination with m-nitrobenzyl alcohol, molecular ion charge states that are greater than the number of basic sites in the protein can be produced from these native solutions, even for lysozyme, which is conformationally constrained by four intramolecular disulfide bonds. Circular dichroism spectroscopy indicates that the conformation of ubiquitin is not measurably affected with up to 1.0 M LaCl(3), but ion mobility data indicate that the high charge states that are formed when 1.0 mM LaCl(3) is present are more unfolded than the low charge states formed without this reagent. These and other results indicate that the increased charging is a result of La(3+) preferentially adducting onto compact or more native-like conformers during ESI and the gas-phase ions subsequently unfolding as a result of increased Coulomb repulsion. Electron capture dissociation of these high charge-state ions formed from these native solutions results in comparable sequence coverage to that obtained for ions formed from denaturing solutions without supercharging reagents, making this method a potentially powerful tool for obtaining structural information in native mass spectrometry.

Artificial quantum-dot helium molecules: Electronic spectra, spin structures, and Heisenberg clusters

Energy spectra and spin configurations of a system of $N=4$ electrons in lateral double quantum dots (quantum dot helium molecules) are investigated using exact diagonalization (EXD), as a function of interdot separation, applied magnetic field $(B)$, and strength of interelectron repulsion. As a function of the magnetic field, the energy spectra exhibit a low-energy band consisting of a group of six states, with the number six being a consequence of the conservation of the total spin and the ensuing spin degeneracies for four electrons. The energies of the six states appear to cross at a single value of the magnetic field, and with increasing Coulomb repulsion they tend to become degenerate, with a well-defined energy gap separating them from the higher-in-energy excited states. The appearance of the low-energy band is a consequence of the formation of a Wigner supermolecule, with the four electrons (two in each dot) being localized at the vertices of a rectangle. Using spin-resolved pair-correlation distributions, a method for mapping the complicated EXD many-body wave functions onto simpler spin functions associated with a system of four localized spins is introduced. Detailed interpretation of the EXD spin functions and EXD spectra associated with the low-energy band via a four-site Heisenberg cluster (with $B$-dependent exchange integrals) is demonstrated. Aspects of spin entanglement, referring to the well-known $N$-qubit Dicke states, are also discussed.

Teleportation on a quantum dot array.

We present a model of quantum teleportation protocol based on a double quantum dot array. The unknown qubit is encoded using a pair of quantum dots, with one excess electron, coupled by tunneling. It is shown how to create a maximally entangled state using an adiabatically increasing Coulomb repulsion between different dot pairs. This entangled state is exploited to perform teleportation again using an adiabatic coupling between itself and the incoming unknown state. Finally, a sudden separation of Bob's qubit allows a time evolution of Alice's, which amounts to a modified version of standard Bell measurement. A transmission over a long distance could be obtained by considering the entangled state of a chain of N coupled double quantum dots. The system is shown to be increasingly robust with N against decoherence due to phonons.

Exciton Excitations in a One-Dimensional Band Insulator withHubbard Interactions

We study the properties of exciton excitations in theHubbard model with alternating potential and dimerized hoppingterms at half-filling. With increasing Coulomb repulsion, we findthat a spin triplet exciton band develops below the band gap. At acertain critical interaction, the excitation gap of this excitonband vanishes and a new phase with a dimerized ground stateemerges. The value of this critical Coulomb interaction isdetermined perturbatively.

On the nature of the magnetic transition in a Mott insulator

Using a combination of exact enumeration and the dynamical mean-field theory (DMFT) we study the drastic change of the spectral properties, obtained in the half-filled two-dimensional Hubbard model at a transition from an antiferromagnetic to a paramagnetic Mott insulator, and compare it with the results obtained using the quantum Monte Carlo method. The coherent hole (electron) quasiparticle spin-polaron subbands are gradually smeared out when the AF order disappears, either for increasing Coulomb repulsion U at fixed temperature T, or for increasing T at fixed U. Within the DMFT we present numerical evidence (a continuous disappearence of the order parameter) suggesting that the above magnetic transition is second order both in two and in three dimensions.

Andreev tunneling, Coulomb blockade, and resonant transport of nonlocal spin-entangled electrons

We propose and analyze a spin-entangler for electrons based on an s-wave superconductor coupled to two quantum dots each of which is tunnel-coupled to normal Fermi leads. We show that in the presence of a voltage bias and in the Coulomb blockade regime two correlated electrons provided by the Andreev process can coherently tunnel from the superconductor via different dots into different leads. The spin-singlet coming from the Cooper pair remains preserved in this process, and the setup provides a source of mobile and nonlocal spin-entangled electrons. The transport current is calculated and shown to be dominated by a two-particle Breit-Wigner resonance which allows the injection of two spin-entangled electrons into different leads at exactly the same orbital energy, which is a crucial requirement for the detection of spin entanglement via noise measurements. The coherent tunneling of both electrons into the same lead is suppressed by the on-site Coulomb repulsion and/or the superconducting gap, while the tunneling into different leads is suppressed through the initial separation of the tunneling electrons. In the regime of interest the particle-hole excitations of the leads are shown to be negligible. The Aharonov-Bohm oscillations in the current are shown to contain single- and two-electron periods with amplitudes that both vanish with increasing Coulomb repulsion albeit differently fast.

Density-matrix renormalization study of the Hubbard model [4]on a Bethe lattice

The half-filled Hubbard model on the Bethe lattice with coordination number $z=3$ is studied using the density-matrix renormalization group (DMRG) method. Ground-state properties such as the energy $E$, average local magnetization $<\hat S_z>$, its fluctuations $<\hat S_z^2 > - < \hat S_z>^2$ and various spin correlation functions $<\hat S_z(i) \hat S_z(j) > - < S_z(i)> < S_z(j)>$ are determined as a function of the Coulomb interaction strength $U/t$. The calculated local magnetic moments $<\hat S_z(i)>$ increase monotonically with increasing Coulomb repulsion $U/t$ forming an antiferromagnetic spin-density-wave state which matches the two sublattices of the bipartite Bethe lattice. At large $U/t$, $<\hat S_z(i)>$ is strongly reduced with respect to the saturation value 1/2 due to exchange fluctuations between nearest neighbors (NN) spins ($|< S_z(i)>|\simeq 0.35$ for $U/t\to +\infty$). $<S_z(i)^2> - < S_z(i)>^2$ shows a maximum for $U/t=2.4$--2.9 which results from the interplay between the usual increase of $<S_z(i)^2>$ with increasing $U/t$ and the formation of important permanent moments $<S_z(i)>$ at large $U/t$. NN sites show antiferromagnetic spin correlations which increase with increasing Coulomb repulsion. In contrast next NN sites are very weakly correlated over the whole range of $U/t$. The accuracy of the DMRG results is discussed by comparison with tight-binding exact results, independent DMRG calculations for the Heisenberg model and simple first-order perturbation estimates.

Photoelectron Spectroscopy of Doubly Charged Anions: Intramolecular Coulomb Repulsion and Solvent Stabilization

Photoelectron spectroscopy has been combined with an electrospray ionization technique to study gas-phase multiply charged anions and their intramolecular Coulomb repulsion and solvation stabilization. We report an initial study on a series of linear dicarboxylate dianions (DC2-), -O2C(CH2)nCO2- (n = 3−6), solvated by one and two waters. The second electron binding energy of DC2- decreases with decreasing aliphatic chain length owing to the increasing Coulomb repulsion between the two charges; the dianion with n = 2 has a binding energy close to zero and is not stable in the gas phase. We found that this dianion can be stabilized by one water, and in general the first water stabilizes the electron binding energy of DC2- by ∼0.3 eV. The second water, however, was observed to show a much stronger stabilization effect (∼0.6 eV), suggesting that the two waters solvate the two carboxylate groups separately.

On the dissociation and conformation of gas-phase methonium ions

The dissociation pathways of both doubly and singly charged methonium ions, (CH 3) 3N + -(CH 2) n - +N(CH 3) 3·X − ( n = 6, 10; X = Br, I, and OAc), are measured using blackbody infrared radiative dissociation (BIRD) and SORI-CAD in a Fourier transform mass spectrometer. SORI-CAD of the doubly charged decamethonium ions results primarily in the formation of even electron ions by hydrogen rearrangements. In contrast, homolytic bond cleavage to form two odd electron ions is highly favored in the hexamethonium ion, presumably due to increased Coulomb repulsion in this ion. For BIRD of the singly charged salts, ions are mass selected and dissociated by heating the vacuum chamber to elevated temperatures. Under the low pressure conditions of our experiment, energy is transferred from the chamber walls to the ions by the absorption of blackbody radiation. From the temperature dependence of the unimolecular rate constants for dissociation, Arrhenius activation energies in the zero-pressure limit are obtained. The primary dissociation pathways correspond to counterion substitution reactions which result in loss of N(CH 3) 3 and CH 3X. For hexamethonium and decamethonium with X = Br or I, the branching ratios for these pathways differ dramatically; the ratio of loss of N(CH 3) 3 and CH 3Br is 3.8 and 0.4 for hexamethonium and decamethonium bromide, respectively. The hexamethonium acetate salt has a branching ratio of 0.1. The Arrhenius activation energies for hexamethonium (Br or I) and decamethonium (Br or I) are 0.9 and 1.0 eV, respectively. This value for hexamethonium acetate is 0.6 eV. Molecular dynamics simulations and Monte Carlo conformation searching are used to obtain the lowest energy structures of hexamethonium and decamethonium bromide. These calculations indicate that the methonium ion folds around the counterion to form a cyclic salt-bridge structure in which both quaternary nitrogens interact with the oppositely charged counterion. The significantly different branching ratios observed for these ions is attributed to the large change in orientation of the counterion with respect to the ammonium centers as the number of methylene groups in these ions increases. Similar ion conformational differences appear to explain the fragmentation for the OAc counter ion as well.

Isotope-shift exponent in the polaronic model of superconductors

We have studied the isotope-shift exponent (α) within the polaronic theory of superconductivity as functions of electron-phonon coupling and carrier concentration. α is found to be consistently negative for all concentrations and electron-phonon couplings in contrast to experimental reports. Increasing Coulomb repulsion or use of a singular density of states pushes α to more negative values.

Experimental and Theoretical Studies on the Redox Properties of Conducting Polymers

AbstractVoltammetric measurements on selected polypyrrole (PPy) films and defined soluble oligomers belonging to the oligo‐p‐phenylene and the oligo‐p‐phenylenevinylene series provide new insights into the charging/discharging properties of conducting polymers. It turns out that the current response in the whole accessible potential range of conventional conducting polymers is mainly caused by faradaic charge transfer reactions rather than capacitive effects. This is supported by digital simulations which are based on a model of n‐fold electron transfers within a finite layer. In addition, from the data of defined oligomers it becomes clear that the number and energies of accessible redox states in such materials depend on the extension of conjugation in the monomeric unit and on the chain length of the polymer. In general, in systems such as PPy up to a charging level of approximately 20 – 25 mol% all redox states are degenerated. Then, the successive redox states are gradually shifted to higher energies in accordance with increasing Coulomb repulsion between the charges. The results show that the bipolaron model must be amended. Nevertheless, experimental data reveal that the charging steps in conducting polymers are coupled with structural relaxation processes; but they occur, in contrast to the theoretical predictions, already on the polaron state.

Thermodynamics of superconductors with a disorder induced increased coulomb repulsion

We show that thermodynamic properties can be used to determine whether, the degradation in T c with increasing disorder observed in some A-15 compounds, is due to a reduction in electronic density of states and/or electron-phonon interaction or is due instead to an increase in Coulomb repulsion μ ∗ coming from increased localization as suggested by Anderson et al.

Relationship between the Spin Susceptibility Enhancement and the Superconducting Transition Temperature

The observed relationship between the magnetic spin susceptibility and the superconducting transition temperature for the fcc transition metals is analyzed in terms of the recent theory of Berk and Schrieffer for the effect of spin fluctuations on the pairing interaction. The results indicate that the decrease in the transition temperature and the increase in the susceptibility are intimately connected and are both basically caused by an increasing Coulomb repulsion. The band density of states is found to decrease on going from iridium to rhodium, platinum, and palladium in qualitative agreement with recent x-ray emission spectroscopy results of Claus and Ulmer. The theory allows one to estimate the size of the spin susceptibility in the superconducting alloys to within about 5%, from which one concludes that the decrease in the transition temperature past iridium should be even more rapid that is actually observed.