Transport Properties Of System Research Articles

Unconventional transport properties in systems with triply degenerate quadratic band crossings

Unconventional transport properties in systems with triply degenerate quadratic band crossings

Structure of the active Fokker-Planck equation.

This paper solves in one and two dimensions the steady noninteractive active Fokker-Planck (FP) equationand finds that its velocity distribution admits, under limiting cases, a dual behavior. Briefly, when the inertial relaxation time is smaller than the orientation time, the active FP equationadmits a bimodal shape, whereas the inverse condition is seen to admit a Gaussian one. Once the velocity distribution functions are available, they are used to find their effect on the system's transport properties, such as its mean-square speed. In the process, a useful mathematical identity for the first kind Bessel function as a sum of bimodal exponential functions is spotted.

Open-system spin transport and operator weight dissipation in spin chains

We use non-equilibrium steady states to study the effect of dissipation-assisted operator evolution (DAOE) on the scaling behavior of transport in one-dimensional spin chains. We consider three models in the XXZ family: the XXZ model with staggered anisotropy, which is chaotic; XXZ model with no external field and tunable interaction, which is Bethe ansatz integrable and (in the zero interaction limit) free fermion integrable; and the disordered XY model, which is free-fermion integrable and Anderson localized. We find evidence that DAOE's effect on transport is controlled by its effect on the system's conserved quantities. To the extent that DAOE preserves those symmetries, it preserves the scaling of the system's transport properties; to the extent it breaks those conserved quantities, it pushes the system towards diffusive scaling of transport.

Free Energy Sampling to Explore Ion Solvation Environments and Understand Transport and Glass Transition in Solid State Electrolytes for Battery Materials

Molecular dynamics simulations using newly developed, accurate interaction potentials have helped elucidate possible mechanisms for various transport properties in systems of lithium salts dissolved in poly(ether-acetals) ( P(nEO-mMO) where EO is ethoxyl and MO is methoxyl ) that are prime candidates for solid state polymer electrolytes. The presence of a second coordination shell around the Li ion, observed in polymers that have MO groups, seems to distort the first coordination shell, creating a more open cage structure that may help facilitate transport in these polymers. Furthermore, this distorted coordination may also assist in the formation of a lower coordination transition state while the Li ions jump from one cage to another during diffusion. Free energy calculations further elucidate the relative stability of these different Li-ether coordination environments and the possible pathways to move between them (See figure 1 for example).Studies on poly(ether-acetals) also tell us that polymers with low glass transition temperature (Tg) are not necessarily the best foundation for high transport electrolytes as addition of salt may disproportionately increase Tg and reduce transport at comparable absolute temperatures. At battery working temperatures, PEO still exhibits superior ionic conductivity as compared to other polymers in this series due a smaller increase in glass transition with salt loading. Hence it is very important for practical applications to understand how salt loading affects the glass transition temperatures in these polymers and how we can tune it to our advantage. It was observed that the softness of the polymer chain controlled by the strength of the dihedral angles is an important parameter that has a significant effect on the glass transition and hence provide insight and tune further improved potentials. We will show the associated correlations of glass transition temperature in these systems with microscopic chemical details like the polymer backbone chemistry, ion clustering and coordination by extensively studying the trends in these different systems with extensive statistics from long time molecular dynamics simulations at different temperatures and compositions and understand transport at different timescales and help design new polymers electrolytes with improved transport at battery working temperatures. Figure 1: Pathways:2D Free energy analysis of Solvation of Li cation and TFSI anion in PEO and PEOMO. Coordination w/r to oxygens of ether/acetyl and oxygens of TFSI anions are used as collective variables a)Serial Process: PEO shows distinct stepwise pathways to move from one coordination state to another (arrows for guide showing TFSI forming a bond in the first step and then the polymer bond breaking in the second step b)Concerted Process: PEOMO shows a more direct path that implies bond breaking and forming happens in a more concerted way that allows for easier cation transport from one cage to another Figure 1

Electronic Transport Inhibiting of Carbon Nanotubes by 5f Elements

AbstractBased on the combination of the non‐equilibrium Green's function and density functional theory, a theoretical method for studying the transport behavior of high angular momentum 5f electrons is developed and the transport properties of the structure for actinide atoms embedded in carbon nanotubes (An@CNTs, An = Ac, Th, Pa and U) is reported. Results show that An@CNTs have lower transmission coefficients than that of CNTs. Furthermore, electrical bias to the U@(4, 4)/(5, 5) CNTs induces an additional transition spectral peak, which demonstrates that the U@(4, 4)/(5, 5) CNTs has a lower resistance. Therefore, 5f electrons of actinide atoms inhibit the electronic transport of CNTs. These findings may provide fresh insight into the transport properties of systems having higher angular momentum electrons.

Circumventing kinetics in biogeochemical modeling

Microbial metabolism drives biogeochemical fluxes in virtually every ecosystem. Modeling these fluxes is challenged by the incredible diversity of microorganisms, whose kinetic parameters are largely unknown. In poorly mixed systems, such as stagnant water columns or sediments, however, long-term bulk microbial metabolism may become limited by physical transport rates of substrates across space. Here we mathematically show that under these conditions, biogeochemical fluxes are largely predictable based on the system's transport properties, chemical boundary conditions, and the stoichiometry of metabolic pathways, regardless of the precise kinetics of the resident microorganisms. We formalize these considerations into a predictive modeling framework and demonstrate its use for the Cariaco Basin subeuphotic zone, one of the largest anoxic marine basins worldwide. Using chemical concentration data solely from the upper boundary (depth 180 m) and lower boundary (depth 900 m), but without a priori knowledge of metabolite fluxes, chemical depth profiles, kinetic parameters, or microbial species composition, we predict the concentrations and vertical fluxes of biologically important substances, including oxygen, nitrate, hydrogen sulfide, and ammonium, across the entire considered depth range (180-900 m). Our predictions largely agree with concentration measurements over a period of 14 years ([Formula: see text] = 0.78-0.92) and become particularly accurate during a period where the system was near biogeochemical steady state (years 2007-2009, [Formula: see text] = 0.86-0.95). Our work enables geobiological predictions for a large class of ecosystems without knowledge of kinetic parameters or geochemical depth profiles. Conceptually, our work provides a possible explanation for the decoupling between microbial species composition and bulk metabolic function, observed in various ecosystems.

Tunnel magnetoresistance of a supramolecular spin valve

We theoretically study the transport properties of a supramolecular spin valve, consisting of a carbon nanotube with two attached magnetic molecules, weakly coupled to metallic contacts. The emphasis is put on analyzing the change of the system's transport properties with the application of an external magnetic field, which aligns the spins of the molecules. It is shown that magnetoresistive properties of the considered molecular junction, which are associated with changing the state of the molecules from the superparamagnetic to the ferromagnetic one, strongly depend on the applied bias voltage and the position of the nanotube's orbital levels, which can be tuned by a gate voltage. A strong dependence on the transport regime is also found in the case of the spin polarization of the current flowing through the system. The mechanisms leading to those effects are explained by invoking appropriate molecular states responsible for transport. The analysis is done with the aid of the real-time diagrammatic technique up to the second order of expansion with respect to tunneling processes.

Position-dependent mass effects in the electronic transport of two-dimensional quantum systems: Applications to nanotubes

In this work, we investigate the electronic transport properties of curved two-dimensional quantum systems with a position-dependent mass (PDM). We find the Schrödinger equation for a general surface following the da Costa approach, obtaining the geometrical potential for systems with PDM. We obtain expressions for the transmittance and reflectance for a general surface of revolution and, as a first application of the general results obtained here, we investigate the transport properties of deformed nanotubes whose variation of the effective mass with the radius has been disconsidered in previous studies. We find that the inclusion of the position-dependent mass, can induce a significant correction in the transport properties of the system. This reveals that the transport properties of two-dimensional quantum systems are sensitive to the PDM and that, when modeling electronic transport in surfaces, these effects should be considered.

Splitting efficiency and interference effects in a Cooper pair splitter based on a triple quantum dot with ferromagnetic contacts

We theoretically study the spin-resolved subgap transport properties of a Cooper pair splitter based on a triple quantum dot attached to superconducting and ferromagnetic leads. Using the Keldysh Green's function formalism, we analyze the dependence of the Andreev conductance, Cooper pair splitting efficiency, and tunnel magnetoresistance on the gate and bias voltages applied to the system. We show that the system's transport properties are strongly affected by spin dependence of tunneling processes and quantum interference between different local and nonlocal Andreev reflections. We also study the effects of finite hopping between the side quantum dots on the Andreev current. This allows for identifying the optimal conditions for enhancing the Cooper pair splitting efficiency of the device. We find that the splitting efficiency exhibits a nonmonotonic dependence on the degree of spin polarization of the leads and the magnitude and type of hopping between the dots. An almost perfect splitting efficiency is predicted in the nonlinear response regime when the energies of the side quantum dots are tuned to the energies of the corresponding Andreev bound states. In addition, we analyzed features of the tunnel magnetoresistance (TMR) for a wide range of the gate and bias voltages, as well as for different model parameters, finding the corresponding sign changes of the TMR in certain transport regimes. The mechanisms leading to these effects are thoroughly discussed.

One- and two-photon scattering by two atoms in a waveguide

We consider the interaction of one- and two-photon pulses in a waveguide with two two-level systems (TLS) that are also able to interact directly either through an exchange- or a dipole-type interaction. We focus on the system's transport properties and show how the presence of a second TLS increases the control options, especially when direct interactions are also allowed. We also obtain, within a Markov (long pulse) approximation, exact results for the nonlinear or entangled terms that arise in the two-photon case, and discuss both their potential applications and ways to minimize their effects.

Self-consistent phonon calculations of lattice dynamical properties in cubicSrTiO3with first-principles anharmonic force constants

We present an \textit{ab initio} framework to calculate anharmonic phonon frequency and phonon lifetime that is applicable to severely anharmonic systems. We employ self-consistent phonon (SCPH) theory with microscopic anharmonic force constants, which are extracted from density-functional calculations using the least absolute shrinkage and selection operator technique. We apply the method to the high-temperature phase of SrTiO$_{3}$ and obtain well-defined phonon quasiparticles that are free from imaginary frequencies. Here we show that the anharmonic phonon frequency of the antiferrodistortive mode depends significantly on the system size near the critical temperature of the cubic-to-tetragonal phase transition. By applying perturbation theory to the SCPH result, phonon lifetimes are calculated for cubic SrTiO$_{3}$, which are then employed to predict lattice thermal conductivity using the Boltzmann transport equation within the relaxation-time approximation. The presented methodology is efficient and accurate, paving the way toward a reliable description of thermodynamic, dynamic, and transport properties of systems with severe anharmonicity, including thermoelectric, ferroelectric, and superconducting materials.

Effect of edge chemistry doping on the transport and optical properties for asymmetric armchair-edge graphene nanoribbons under a uniaxial strain

The effect of edge n-type nitrogen (N) and p-type oxygen (O) doping on the transport and optical properties for asymmetric 6- and 8-armchair-edge graphene nanoribbons (AGNRs) under a uniaxial strain has been demonstrated in the method of the first-principles density functional theory combined with nonequilibrium Green’s function. The transmission spectra and current–voltage (I–V) curves of the semiconducting 6- and metallic 8-AGNRs have been found to be qualitatively unchanged as the uniaxial strain along the armchair edge (Ac-strain), while the quantitative variations come from the mutual actions of the strain and edge doping. However, the observed semiconductor–metal transition and metal–semiconductor transition in the transport properties for 6- and 8-AGNRs should be mainly from the resonance scattering of the doping edges when the external strain is applied along the zigzag edge (Zz-strain). The distinct variation trends of the system transport properties are well confirmed from the optical absorption spectra. It is believed that the obtained results are of importance in exploiting chemistry at the reactive edges of the asymmetric AGNRs and strain engineering of the nanoelectronic/optoelectronic devices based on AGNRs.

Charge, spin and thermal transport of graphene-based FNF multilayer

In this paper the impact of repetition of graphene-based ferromagnet-normal(FN) regions on the transport properties of system is studied, theoretically. Here, the purpose is to generalize the Fert–Grünberg FNF multijunction to a graphene-based one. Thus, parallel and antiparallel magnetization alignments of ferromagnetic regions are considered. The concept of quantum well is used to calculate the transmission probability using the Dirac–Bogoliubov–de Gennes equations. Charge, spin and thermal conductances are investigated for multilayer junctions in terms of magnetization alignments, strength of ferromagnets and thickness of ferromagnet regions. Finally charge, spin and thermal Giant magnetoresistances (GMR) are calculated for the above-mentioned structures. It is obtained that the spin conductance of antiparallel setups is zero, thus it can be used as a spin valve.

Transport properties for systems with deep potential wells: H + O2.

Transport properties for collisions of the oxygen molecule with hydrogen atoms are computed by means of quantum scattering calculations. Because two potential energy surfaces (PESs) arise from the interaction of H(2S) with O2(X3Σg+), namely 2A″ and 4A″, collision integrals were computed using both PESs and then averaged with weighting by their respective spin multiplicities. A PES for the 4A″ state was computed for the interaction of O2, frozen at its equilibrium internuclear separation, with a hydrogen atom, using a coupled-cluster method that includes all single and double excitations as well as perturbative contributions of connected triple excitation. A PES of similar quality was taken from Kłos et al. [J. Chem. Phys.2008, 129, 064306] for the 2A″ state. Because the 2A″ state correlates with the deep HO2(X̃2A″) well, statistical capture boundary conditions [Rackham et al., J. Chem. Phys.2003, 119, 12895] were applied to compute the S matrix, and then the transport properties, for this state.

Thermodynamic scaling and corresponding states for the self-diffusion coefficient of non-conformal soft-sphere fluids

In this work, we explore transport properties of a special type of repulsive spheres that exhibit remarkable scaling of their thermodynamic properties. In order to accomplish that we propose a new way to derive and express effective hard-sphere diameters for transport properties of simple fluids. The procedure relies on mapping the system's transport properties, in the low density limit, to the hard-sphere fluid. We have chosen a set of soft-sphere systems characterised by a well-defined variation of their softness. These systems represent an extension of the repulsive Lennard-Jones potential widely used in statistical mechanics of fluids and are an accurate representation of the effective repulsive potentials of real systems. The self-diffusion coefficient of the soft-sphere fluids is obtained by equilibrium molecular dynamics. The soft-sphere collision integrals of different systems are shown to follow quite simple relationships between each other. These collision integrals are incorporated, through the definition of the effective hard-sphere diameter, in the resulting equation for the self-diffusion coefficient. The approach followed exhibits a density rescaling that leads to a single master curve for all systems and temperatures. The scaling is carried through to the level of the mean-squared displacement.

Electronic transport for impurity-doped armchair-edge graphene nanoribbons

We investigate the electronic transport for an impurity-doped armchair-edge graphene nanoribbon (AGNR), with 7 or 8 dimer lines along zigzag direction, sandwiched between two normal leads. By using the standard nonequilibrium Green’s function technique, it is demonstrated that, the impurity influence on the transport properties for system with semiconducting 7-AGNR system is more sensitive than that for one with metallic 8-AGNR system in the vicinity of the impurity energy level. In particular, in the absence of impurity the density of states (DOS) and linear conductance G possess a small zero value interval for 7-AGNR system and a large nonzero plateau for 8-AGNR one, respectively. Interestingly, as impurity included the DOS and G show a single sharp resonant peak around the impurity energy level for 7-AGNR system due to resonant tunneling, while a small dip appears in the same position for 8-AGNR system due to the antiresonance states. Moreover, we have also inspected the behavior of the differential conductance upon varying the impurity concentration for the systems. The findings here may suggest it is more favorable to fabricate an electric switch with high on-off ratio by using an impurity-assisted semiconducting AGNR.

Tuning the magnetic and transport property of graphene with Ti atom and cluster

Atomic structure of Ti and Ti4 cluster on the surface of graphene are studied by the density functional theory and nonequilibrium Green’s function formalism. The most stable configurations for Ti and Ti4 adsorbed on the graphene surface are investigated, and the binding energies corresponding to the configurations are calculated. We found that the Ti4 clusters can strongly interact with pristine graphene surface, and the interaction can be further enhanced when structural vacancies are introduced onto the graphene surface. Electronic properties of metal/graphene composites are also presented to show their highly spin polarized behaviors. The transport properties of systems are dependent on the applied bias voltage. Our results also indicate that the Ti4/graphene composite is a simple and typical model to study the interactive effects of metal clusters absorbed on graphene. The results of this work could help to design novel devices such as nanoelectrodes and magnetic sensors based on graphene.

Emergence of Anderson localization in plasmonic waveguides

The propagation of surface plasmon polaritons in dielectric loaded waveguides with randomly placed scatterers is studied using both numerical simulations and a simplified transfer matrix framework. Despite the importance of losses in this system, we find fingerprints of the localized behavior of one-dimensional disordered systems. Furthermore, losses amplify the impact of the necklace states on the transport properties for systems not much larger than the localization length. The system presented here also offers the possibility to use localization effects for engineering purposes by means of deliberately introduced disorder.

Electronic Green's functions in a T-shaped multi-quantum dot system

We developed a set of equations to calculate the electronic Green's functions in a T-shaped multi-quantum dot system using the equation of motion method. We model the system using a generalized Anderson Hamiltonian which accounts for finite intradot on-site Coulomb interaction in all component dots as well as for the interdot electron tunneling between adjacent quantum dots. Our results are obtained within and beyond the Hartree–Fock approximation and provide a path to evaluate all the electronic correlations in the multi-quantum dot system in the Coulomb blockade regime. Both approximations provide information on the physical effects related to the finite intradot on-site Coulomb interaction. As a particular example for our generalized results, we considered the simplest T-shaped system consisting of two dots and proved that our approximation introduces important corrections in the detector and side dots Green's functions, and implicitly in the evaluation of the system's transport properties. The multi-quantum dot T-shaped setup may be of interest for the practical realization of qubit states in quantum dot systems.

Heat conduction in disordered harmonic lattices with energy-conserving noise

We study heat conduction in a harmonic crystal whose bulk dynamics is supplemented by random reversals (flips) of the velocity of each particle at a rate λ. The system is maintained in a nonequilibrium stationary state (NESS) by contacts with white-noise Langevin reservoirs at different temperatures. We show that the one-body and pair correlations in this system are the same (after an appropriate mapping of parameters) as those obtained for a model with self-consistent reservoirs. This is true both for the case of equal and random (quenched) masses. While the heat conductivity in the NESS of the ordered system is known explicitly, much less is known about the random mass case. Here we investigate the random system with velocity flips. We improve the bounds on the Green-Kubo conductivity obtained by Bernardin [J. Stat. Phys. 133, 417 (2008)]. The conductivity of the one-dimensional system is then studied both numerically and analytically. This sheds some light on the effect of noise on the transport properties of systems with localized states caused by quenched disorder.