Weak Coulomb Research Articles

Conduction mechanism and shallow donor defects in Nb-doped β-Ga2O3 single crystals

Electrical properties, electronic defects, and photoluminescence (PL) of Nb-doped β-Ga2O3 crystals grown by the floating zone method have been studied in the temperature range from 10 to 350 K. The activation energies of shallow and deep traps were obtained and compared with their counterparts. The 0.25-Nb-doped Ga2O3 crystal has a metallic behavior at room temperature and a semiconducting behavior at low temperatures. This behavior was interpreted by the quantum corrections to the electrical resistivity resulting from the weak localization and Coulomb interaction. In addition, the 0.25-Nb-doped Ga2O3 crystal has a shallow donor ionization energy of 0.03 eV, indicating that it requires less energy to enable n-type conductivity. Hence, Nb is a good choice for improving the electrical conductivity of the β-Ga2O3 crystal. Three defect trap states were found at EC-0.81 eV, EC-0.49 eV, and EC-0.12 eV below the conduction band edge. The PL spectra of the undoped sample show two characteristic bands in the UV and blue regions under the excitation wavelength of 256 nm. On the other hand, in the Nb-doped β-Ga2O3 sample, a novel UV emission band with a shorter wavelength was observed.

Decoupling of Majorana bound states in T-shaped double-quantum-dot structure with ferromagnetic leads

The significant potential applications of Majorana bound state (MBS) in topological quantum computing manifest the importance and necessity of relevant in-depth research. To understand the physical properties of MBS, the most practical approach is to integrate it to a mesoscopic circuit and then investigate its quantum transport behaviors. In this work, we investigate the transport properties in the systems with MBS, and provide theoretical support for its further understanding and detection, by utilizing the nonequilibrium Green’s function method and scattering matrix theory. Specifically, we investigate theoretically the transport properties in a T-shaped double-quantum dot structure, by considering MBS to be coupled to the dot in the main channel, which shows that in the linear transmission region, when the level of side-coupled dot is tuned to the Fermi energy level, the contribution of MBS to the conductance is eliminated under weak and strong Coulomb interaction. The side-coupled dot is far away from the Fermi energy level, leading to different results. When Majorana zero mode is added, the linear conductance is independent of the level of the side-coupled quantum dot, and the conductance plateau appears. However, with coupling between the MBSs, the linear conductance is the same as that without coupling between the MBSs. The decoupling phenomenon of the MBS remains strong. Therefore, the signature of the MBS can be eliminated by adjusting the level of the side-coupled quantum dot or the inter-MBS coupling. When ferromagnetic leads are introduced, the appearance or disappearance of the conductance plateau is clearly dependent on the difference between the magnetic field direction and the lead polarization direction in the system, whereas the decoupling behavior of the MBS is still existent. This work contributes to further explaining the decoupling phenomenon of MBSs in a T-shaped double-quantum-dot system, and presents a theoretical approach to more in-depth understanding and detection of the MBS.

Design of Donor–Acceptor Polymer Semiconductors for Optimizing Combinations with Dopants to Maximize Thermoelectric Performance

Several conjugated polymers have been designed for applications in thermoelectrics (TEs) to improve doping efficiency and charge transport. However, optimizing the compatibility of the polymer with molecular dopants through judicious design to achieve high TE performance remains a challenge. By designing a novel donor–acceptor (D-A)-based conjugated polymer, poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)-co-(4,7-bis(4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-2-yl)-5,6-difluorobenzo[c][1,2,5] thiadiazole) (PBDTC2FBT), compatibility with molecular dopants was maximized, resulting in enhanced TE properties. PBDTC2FBT was designed to include two types of electron donor building blocks with one acceptor in a repeat unit; this structure formed local short-range ordered crystals. Two molecular doping processes employing FeCl3 and tris(pentafluorophenyl)borane (BCF)–water complexes generated polaronic charge carriers in PBDTC2FBT while minimizing crystal deterioration. The transport behavior of the charge carriers in PBDTC2FBT differed depending on the dopant. Studies using the temperature-dependent electrical conductivity and Kang–Snyder charge transport models indicated that the weak Coulomb binding energy with the dopant counterion in BCF-doped PBDTC2FBT facilitated charge transport compared with that in the FeCl3-doped sample at the low doping region. Finally, BCF-doped PBDTC2FBT exhibited an optimized power factor of 35.1 μW m–1 K–2, which is extremely close to the theoretical maximum (36.3 μW m–1 K–2) that can be achieved with PBDTC2FBT. This study provides useful guidelines for maximizing the TE performance of D-A conjugated polymers through judicious design.

First-principles ultrafast exciton dynamics and time-domain spectroscopies: Dark-exciton mediated valley depolarization in monolayer WSe2

Calculations combining first-principles electron-phonon ($e$-ph) interactions with the Boltzmann equation enable studies of ultrafast carrier and phonon dynamics. However, in materials with weak Coulomb screening, electrons and holes form bound excitons so their scattering processes become correlated, posing additional challenges for modeling nonequilibrium physics. Here we show calculations of ultrafast exciton dynamics and related time-domain spectroscopies using ab initio exciton-phonon (ex-ph) interactions together with an excitonic Boltzmann equation. Starting from the nonequilibrium exciton populations, we develop simulations of time-domain absorption and photoemission spectra that take into account electron-hole correlations. We use this method to study monolayer ${\mathrm{WSe}}_{2}$, where our calculations predict subpicosecond timescales for exciton relaxation and valley depolarization and reveal the key role of intermediate dark excitons. The approach introduced in this paper enables a quantitative description of nonequilibrium dynamics and ultrafast spectroscopies in materials with strongly bound excitons.

Weak coulomb interaction between anions and Na+ during solvation enabling desirable solid electrolyte interphase and superior kinetics for HC-based sodium ion batteries

Electrolyte plays an important role in conducting sodium ions and carrying out chemical/electrochemical reactions. However, the impact mechanism of microscopic solvation structure remains vague but is significant for the electrolyte design. Here we reveal that the inner relationship between solvation structure and electrochemical reaction is manifested intuitively by manipulating the anions in the solvated shell, which determines Na storage kinetics and SEI evolution process. The result elucidates the weak coulomb interaction between PF6− and Na+ could promote Na+ transport number in bulk liquid and Na storage kinetics. And it could induce PF6− to reduce preferentially and facilitate additive to enter solvation structure, thereby forming a thin&compact&protective SEI with layered morphology. The excellent SEI would protect anode effectively, inhibiting decomposition of solvents and improving battery performances. As expected, 5 Ah Na3V2(PO4)3-hard carbon pouch-type batteries with well-matched PF6−-based electrolyte display a high specific energy of 129 Wh kg−1, a superior rate capability with capacity retention of 90 % at 5 C and cycling stability with retention of 86.82 % at 1 C after 700 cycles.

Destructive interference and Fano antiresonance in T-shaped double quantum dots coupled to Luttinger liquid leads

We investigate the interplay of quantum interference effects and the electronic correlation on the transport through T-shaped spinless double quantum dots coupled to Luttinger liquid leads by using the nonequilibrium Green's function method. We find that the weak intralead Coulomb interaction leads to a zero-bias antiresonance dip in the differential conductance for small interdot tunneling coupling, which is distinct destructive interference characteristic. In particular, an intriguing Fano antiresonance profile for asymmetrically coupling to the Luttinger liquid leads is exhibited, consistent with the experiment findings but different physics screening origin. The strong intralead Coulomb interaction or large interdot coupling tunneling quenches the quantum interference effects, resulting in the disappearance of Fano antiresonance. The scaling behavior of differential conductance on temperature is presented for different intralead interactions. The relationship between destructive interference and two-channel Kondo effect is given. Our results provide a way of adjusting in and out of antiresonance to achieve quantum interference transistors and of switching by tuning the gate voltage controlling the interdot tunneling. • The interplay of interference and intralead Coulomb interaction on transport of T-shaped quantum dots is studied. • Weak intralead interaction leads to zero-bias dip in differential conductance for small interdot tunneling. • New physical mechanism is presented to explain for the antiresonance dip and Fano effect. • Scaling behavior of differential conductance on temperature is presented for different intralead interactions.

Vibronic recovering of functionality of quantum cellular automata based on bi-dimeric square cells with violated condition of strong Coulomb repulsion.

Strong Coulomb repulsion between the two charges in a square planar mixed-valence cell in quantum cellular automata (QCA) allows us to encode the binary information in the two energetically beneficial diagonal distributions of the electronic density. In this article, we pose a question: to what extent is this condition obligatory for the design of the molecular cell? To answer this question, we examine the ability to use a square-planar cell composed of one-electron mixed valence dimers to function in QCA in a general case when the intracell Coulomb interaction U is not supposed to be extremely strong, which means that it is comparable with the characteristic electron transfer energy (violated strong U limit). Using the two-mode vibronic model treated within the semiclassical (adiabatic) and quantum-mechanical approaches, we demonstrate that strong vibronic coupling is able to create a considerable barrier between the two diagonal-type charge configurations, thus ensuring bistability and polarizability of the cells even if the Coulomb barrier is not sufficient. The cases of weak and moderate Coulomb repulsion and strong vibronic coupling are exemplified by consideration of the cation radicals of the two polycyclic derivatives of norbornadiene [C12H12]+ and [C17H16]+ with the terminal C=C chromophores playing the role of redox sites. By using the detailed ab initio data, we reveal the main characteristics of the bi-dimeric cells composed of these molecules and illustrate the pronounced effect of the vibronic recovery clearly manifesting itself in the shape of the cell-cell response function. Revealing such "vibronic recovery" of strong localization when the strong U limit is violated suggests a way to a significant expansion of the class of molecular systems suitable as QCA cells.

Phase diagram of graphene in the presence of spin-orbit coupling and electron-electron interaction

We theoretically investigate in this work the interplay between the intrinsic spin-orbit coupling (SOC) and electron-electron interaction on the magnetism in the graphene honeycomb lattice. Particularly, a phase diagram is here explored and studied by Kane-Mele-Hubbard (KMH) model combined with mean-field theory and self-consistent algorithm. Results show that the graphene undergoes a phase transition from the gapless semi-metal to topological band insulator (TBI) at a finite SOC and weak Coulomb energy U, and another transition from the TBI to antiferromagnetic ordered insulator at stronger Coulomb energy U observed as well. Moreover, our calculations also point out the prior developing orientation of magnetic moments in the in-plane direction driven by the SOC rather than that in the out-of-plane direction without the SOC.

Nonadiabatic Exchange-Correlation Potential for Strongly Correlated Materials in the Weak and Strong Interaction Limits

In this work, nonadiabatic exchange-correlation (XC) potentials for time-dependent density-functional theory (TDDFT) for strongly correlated materials are derived in the limits of strong and weak correlations. After summarizing some essentials of the available dynamical mean-field theory (DMFT) XC potentials valid for these systems, we present details of the Sham–Schluter equation approach that we use to obtain, in principle, an exact XC potential from a many-body theory solution for the nonequilibrium electron self-energy. We derive the XC potentials for the one-band Hubbard model in the limits of weak and strong on-site Coulomb repulsion. To test the accuracy of the obtained potentials, we compare the TDDFT results obtained with these potentials with the corresponding nonequilibrium DMFT solution for the one-band Hubbard model and find that the agreement between the solutions is rather good. We also discuss possible directions to obtain a universal XC potential that would be appropriate for the case of intermediate interaction strengths, i.e., a nonadiabatic potential that can be used to perform TDDFT analysis of nonequilibrium phenomena, such as transport and other ultrafast properties of materials with any strength of electron correlation at any value in the applied perturbing field.

Teleportation of Werner state via graphene-based quantum channels under dephasing environment

The teleportation of Werner state in the graphene-based quantum channels under the dephasing environment is studied through the effective low-energy theory in this paper. The results show that the output entanglement normally reaches a higher level as the input entanglement increases, while the performance of the corresponding fidelity is opposite. Given the input state, the greater entanglement in the quantum channel can provide the higher-quality output state. For graphene-based quantum channels, the low temperature and weak Coulomb repulsive potential can decelerate the attenuation of entanglement resources in the dephasing environment. Moreover, when the temperature is lower than 40 K and the coulomb repulsive potential between electrons is less than 6 eV, the average fidelity of the output state reaches more than 80%. These results indicate that graphene has potential applications in quantum information.

Weak Coulomb Interaction between Anions and Na+ During Solvation Enabling Desirable Solid Electrolyte Interphase and Superior Kinetics for Hc-Based Sodium Ion Batteries

Weak Coulomb Interaction between Anions and Na+ During Solvation Enabling Desirable Solid Electrolyte Interphase and Superior Kinetics for Hc-Based Sodium Ion Batteries

Effect of electronic concentration on the self-trapping transition of polaron for the weakly correlated system in the adiabatic limit

The self-trapping transition of the polaron is investigated for the two-dimensional extended Holstein-Hubbard model. A series of canonical transformations are performed on the Hamiltonian followed by a many phonon averaging, to eliminate the phonon degrees of freedom from the system. The mean field Hartree-Fock calculation has been done to solve the effective electronic Hamiltonian and the ground state energy is obtained variationally. The effect of electronic concentration on the polaronic hopping parameter is studied with respect to the electron–phonon interaction strengths in the weak Coulomb correlation limit. The transition from a delocalized mobile polaron to a localized self-trapped polaron has been found to be discontinuous for the adiabatic motion of the polaron and the self-trapping transition needs stronger electron–phonon interaction strength as the electronic concentration minimizes.

Effect of the electronic charge gap on LO bond-stretching phonons in undoped La2CuO4 calculated using LDA+U

Typical density-functional theory calculations that wrongly predict undoped cuprates to be metallic also predict Cu-O half- and full-breathing phonon energies that are significantly softer than observed, presumably because of weak on-site Coulomb repulsion on the Cu 3d orbitals. We used DFT+U calculations with antiferromagnetic supercells of La$_2$CuO$_4$ to establish correlation between the on-site repulsion strength, tuned via adjusting the value of U, and phonon dispersions. We find that breathing and half-breathing phonons reach experimental values when U is tuned to obtain the correct optical gap and magnetic moments. We demonstrate that using distorted supercells within DFT+U is a promising framework to model phonons in undoped cuprates and other perovskite oxides with complex, interrelated structural and electronic degrees of freedom.

Shortening X-Ray Pulse Duration via Saturable Absorption.

To shorten the duration of x-ray pulses, we present a nonlinear optical technique using atoms with core-hole vacancies (core-hole atoms) generated by inner-shell photoionization. The weak Coulomb screening in the core-hole atoms results in decreased absorption at photon energies immediately above the absorption edge. By employing this phenomenon, referred to as saturable absorption, we successfully reduce the duration of x-ray free-electron laser pulses (photon energy: 9.000keV, duration: 6-7fs, fluence: 2.0-3.5×10^{5} J/cm^{2}) by ∼35%. This finding that core-hole atoms are applicable to nonlinear x-ray optics is an essential stepping stone for extending nonlinear technologies commonplace at optical wavelengths to the hard x-ray region.

Instantons in the out-of-equilibrium Coulomb blockade

Physical properties of single-electron devices in the weak Coulomb blockade regime are significantly dependent on non-perturbative effects. They arise as instanton solutions of equations of motion for the corresponding Ambegaokar-Eckern-Schön action. In equilibrium those solutions are known as Korshunov instantons. In this paper we study non-equilibrium Ambegaokar-Eckern-Schön action using Keldysh technique. We found instantons for the most general stationary out-of-equilibrium state. We also found that action saddle-point value assumes a universal value irrespective of the stationary non-equilibrium state.

Superconducting insulators and localization of Cooper pairs

Rapid miniaturization of electronic devices and circuits demands profound understanding of fluctuation phenomena at the nanoscale. Superconducting nanowires – serving as important building blocks for such devices – may seriously suffer from fluctuations which tend to destroy long-range order and suppress superconductivity. In particular, quantum phase slips (QPS) proliferating at low temperatures may turn a quasi-one-dimensional superconductor into a resistor or an insulator. Here, we introduce a physical concept of QPS-controlled localization of Cooper pairs that may occur even in uniform nanowires without any dielectric barriers being a fundamental manifestation of the flux-charge duality in superconductors. We demonstrate – both experimentally and theoretically – that deep in the “insulating” state such nanowires actually exhibit non-trivial superposition of superconductivity and weak Coulomb blockade of Cooper pairs generated by quantum tunneling of magnetic fluxons across the wire.

Theoretical study on structural, excitonic and electronic properties of Nb2O5 with five crystal structures

Using the first-principles density functional theory, we have studied the structural, excitonic and electronic properties of Nb2O5 with five different crystal structures. In this paper, the structural characteristics of monoclinic, orthorhombic and tetragonal systems have been studied. Different from the classical layered structure, Nb2O5 is a layered shear structure composed of a certain number of NbO6 octahedrons. Exciton effect plays an important role in the physical process of semiconductor. The absorption and recombination of excitons directly affect the light absorption and luminescence of semiconductors. In order to explore the strength of exciton effect, the binding energy of exciton is calculated. By calculating the exciton binding energies, we found that the exciton binding energy for monoclinic Nb2O5 (the space group is C2/c) and orthorhombic Nb2O5 were as high as 69.611 and 75.548 meV. The high exciton binding energy is mainly due to the weak dielectric shielding ability and effective electron hole coulomb interaction. The microcosmic reasons for the difference of exciton binding energies in different crystal structures were analyzed by electrical properties. By analyzing the energy band diagram, it is found that the band gap is closely related to the excitonic binding energy. In general, the position of the atom, the shape of the energy band and the strength of the bond are all factors that contribute to the differences in exciton binding energy.

Coulomb mechanism of Raman radiation in graphene

The phenomena of single-layer graphene resonant photoluminescence and Raman radiation are discussed taking into account the photo-generated electron–hole Coulomb interaction. On the base of general principles of a many-particle interactions and the interband resonance optical transitions a photon radiation new mechanism (Coulomb mechanism) is proposed. Through Stokes 2D'-mode particular case analysis has shown that the graphene photoluminescence and the resonant Raman radiation are characterized by the same frequency shifts. Probabilities of resonance photo-radiation processes have been presented where the electron–hole Coulomb attraction has been taken into account. The probabilities are the same fourth-order small values. The weak photo-radiation Coulomb mechanism has a common character. It is applicable to both zero and nonzero band gap crystals.

Ultrabosonic behavior in photoexcited one-dimensional Mott insulators in the region of weak intersite Coulomb interaction

This paper reports a unique behavior expected for the optical excitations in one-dimensional Mott insulators. When the intersite Coulombic attraction does not work between two elementary charge excitations, i.e., a doublon and a holon, the lowest optical excitations are described as an unbound pair of them, still having finite mutual correlation due to the forbidden double occupancy on the same site. We find that, in such a situation, the pairs obey a particle statistics that exceeds that of ordinary bosons. This feature appears most directly in the enhancement factor of the matrix element for the pair photocreation. We discuss the possible mechanism for this ultrabosonic behavior and also demonstrate that this behavior is maintained even if we include a small amount of interiste Coulomibic interaction. Lastly, this enhancement factor almost coincides with that for ordinary bosons when the degree of the intersite interaction exceeds a certain value that switches on the formation of a bound doublon-holon pair.

Magnetism and topological phases in an interacting decorated honeycomb lattice with spin-orbit coupling

We study the interplay between spin-orbit coupling (SOC) and Coulomb repulsion in a Hubbard model on a decorated honeycomb lattice which leads to a plethora of phases. While a quantum spin hall insulator is stable at weak Coulomb repulsion and moderate SOC, a semimetallic phase emerges at large SOC in a broad range of Coulomb repulsion. This semimetallic phase has topological properties not observed in conventional metals such as a finite, non-quantized spin Hall conductivity. At large Coulomb repulsion and negligible spin-orbit coupling, electronic correlations stabilize a resonance valence bond (RVB) spin liquid state in contrast to the classical antiferromagnetic state predicted by mean-field theory. Under sufficiently strong SOC, such RVB state is transformed into a magnetic insulator consisting on S~3/2 localized moments on a honeycomb lattice with antiferromagnetic order and topological features.