Hopping Regime Research Articles

A tunable palladium nanoparticle film-based strain sensor in a Mott variable-range hopping regime

A tunable palladium nanoparticle film-based strain sensor in a Mott variable-range hopping regime

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.

Особенности частотной зависимости проводимости неупорядоченных полупроводников в области перехода к режиму с постоянной длиной прыжка

AbstractThe effect of hybridization of electron states on the high-frequency conductivity of disordered semiconductors is studied. It is shown that the dependence of the pre-exponential factor of the resonance integral on the intercenter separation in a pair determines the abruptness of the change in conductivity mechanisms near the transition of the frequency dependence of the real part of the conductivity σ_1(ω) from sublinear to quadratic. The abruptness of the change of the conductivity regimes is associated with a rapid decrease in hopping distance with increasing frequency near the transition, which leads to a substantial relative decrease in the contribution from the phononless conductivity component in the variable-range hopping regime with increasing frequency and transition to the fixed-range hopping conductivity regime.

Localized diabatization applied to excitons in molecular crystals.

Traditional ab initio electronic structure calculations of periodic systems yield delocalized eigenstates that should be understood as adiabatic states. For example, excitons are bands of extended states which superimpose localized excitations on every lattice site. However, in general, in order to study the effects of nuclear motion on exciton transport, it is standard to work with a localized description of excitons, especially in a hopping regime; even in a band regime, a localized description can be helpful. To extract localized excitons from a band requires essentially a diabatization procedure. In this paper, three distinct methods are proposed for such localized diabatization: (i) a simple projection method, (ii) a more general Pipek-Mezey localization scheme, and (iii) a variant of Boys diabatization. Approaches (i) and (ii) require localized, single-particle Wannier orbitals, while approach (iii) has no such dependence. These methods should be very useful for studying energy transfer through solids with ab initio calculations.

The crossover between tunnel and hopping conductivity in granulated films of noble metals

The conductivity of thin films composed by clusters of gold and silver nanoparticles has been studies in a wide range of temperatures. The switch from a temperature independence to an exponential thermal dependence of the conductivity manifests the crossover between the tunnel and thermally activated hopping regimes of the electronic transport at the temperature of 60 °C. The characteristic thermal activation energy that governs hopping of electrons between nanoparticles is estimated as 1.3 eV. We have achieved a good control of the composition and thicknesses of nano-cluster films by use of the laser ablation method in colloidal solutions.

Cotunneling and polaronic effect in granular systems

We theoretically study the conductivity in arrays of metallic grains due to the variable-range multiple cotunneling of electrons with short-range (screened) Coulomb interaction. The system is supposed to be coupled to random stray charges in the dielectric matrix that are only loosely bounded to their spatial positions by elastic forces. The flexibility of the stray charges gives rise to a polaronic effect, which leads to the onset of Arrhenius-like conductivity behavior at low temperatures, replacing conventional Mott variable-range hopping. The effective activation energy logarithmically depends on temperature due to fluctuations of the polaron barrier heights. We present the unified theory that covers both weak and strong polaron effect regimes of hopping in granular metals and describes the crossover from elastic to inelastic cotunneling.

Quantum dynamics of nuclear spins and spin relaxation in organic semiconductors

We investigate the role of the nuclear spin quantum dynamics in hyperfine-induced spin relaxation of hopping carriers in organic semiconductors. The fast hopping regime with a small carrier spin precession during a waiting time between hops is typical for organic semiconductors possessing long spin coherence times. We consider this regime and focus on a carrier random walk diffusion in one dimension, where the effect of the nuclear spin dynamics is expected to be the strongest. Exact numerical simulations of spin systems with up to 25 nuclear spins are performed using the Suzuki-Trotter decomposition of evolution operator. Larger nuclear spin systems are modeled utilizing the spin-coherent state $P$-representation approach developed earlier. We find that the nuclear spin dynamics strongly influences the carrier spin relaxation at long times. If the random walk is restricted to a small area, it leads to the quenching of carrier spin polarization at a non-zero value at long times. If the random walk is unrestricted, the carrier spin polarization acquires a long-time tail, decaying as $ 1/\sqrt{t}$. Based on the numerical results, we devise a simple formula describing the effect quantitatively.

Profiling of Hop-Derived Flavan-3-ols from Lager Beer in Relation to Hopping Technology

Little is understood regarding the fate of polyphenols during beer aging. Lager beers were produced with varying hopping regimes to investigate hop product contribution to beer polyphenol and flavanol contents. Finished beers underwent forced aging (30°C) and were monitored for changes in flavan-3-ol profiles. Sephadex LH20 resin was used to produce polyphenol-rich isolates from the finished and aged beers. European Brewery Convention standard spectrophotometric methods and reversed-phase HPLC coupled with diode array detection and electrospray ionization mass spectrometry (RP-HPLC-DAD-ESI(–)-MS) were used to measure polyphenol and total flavanoid contents of the lager beers and the polyphenolic-rich isolates. Phloroglucinolysis was used in conjunction with RP-HPLC-ESI(–)-MS to reveal flavan-3-ol subunit composition and to estimate changes in proanthocyanidin relative mean degree of polymerization. The beers produced with varying hopping regimes were different in total polyphenols and flavonoids by spectrophotometery. Six major phloroglucinolysis flavan-3-ol products were observed; however, galloylated flavanols were not detected in any of the beers. The predominant subunits by molar ratio were (+)-catechin followed by (–)-epigallocatechin, except for beers brewed solely with spent hops, which were also high in (–)-epicatechin by molar ratio. The major extension subunit was (+)-catechin for all treatments. Total flavanoid and proanthocyanidin contents of the beers increased initially during storage, with eventual decreases occurring after 6 weeks of storage.

Contributions of Select Hopping Regimes to the Terpenoid Content and Hop Aroma Profile of Ale and Lager Beers

Recently, brewers have dramatically increased the complexity and intensity of aromas in hop-forward beers by using diverse hopping regimes. For this study, the terpenoid content and sensory attributes of beers made using different hop additions were measured. Beers were brewed while varying two factors: hop cultivar (Simcoe and Hallertau Mittelfrüh) and timing of hop addition (60 min boil, 25 min whirlpool, or 48 h dry hopping). Additionally, the impact of yeast strain on treatment was investigated. Each treatment was compared with an unhopped control using stir-bar sorptive extraction GC-MS and descriptive sensory analysis. Multivariate statistical analysis showed relationships between instrumental and sensory techniques. Whirlpool additions produced beers with the highest concentrations of geraniol, linalool, and β-citronellol; beers brewed with highly aromatic Simcoe hops produced more intense and individually distinct aromas for each hopping regime compared with the Hallertau Mittelfrüh hopped beers. Conversely, beers brewed with Hallertau Mittelfrüh hops showed less intense aromas with less distinction between hopping regimes, except for the dry-hopped treatment, which was characterized by a more floral type of aroma than the other Hallertau Mittelfrüh treatments. This research shows that despite the popularity of dry hopping as an aroma hopping method, whirlpool additions can produce more intensely aromatic beers.

Electrical Spin Orientation, Spin-Galvanic, and Spin-Hall Effects in Disordered Two-Dimensional Systems.

In disordered systems, the hopping conductivity regime is usually realized at low temperatures where spin-related phenomena differ strongly from the cases of delocalized carriers. We develop the unified microscopic theory of current-induced spin orientation, spin-galvanic, and spin-Hall effects for the two-dimensional hopping regime. We show that the corresponding susceptibilities are proportional to each other and determined by the interplay between the drift and the diffusion spin currents. Estimations are made for realistic semiconductor heterostructures using the percolation theory. We show that the electrical spin polarization in the hopping regime increases exponentially with the increase of the concentration of localization sites and may reach a few percent at the crossover from the hopping to the diffusion conductivity regime.

Dependence of conductivity on thickness within the variable-range hopping regime for Coulomb glasses

In this paper, we provide some computational evidence concerning the dependence of conductivity on the system thickness for Coulomb glasses. We also verify the Efros–Shklovskii law and deal with the calculation of its characteristic parameter as a function of the thickness. Our results strengthen the link between theoretical and experimental fields.

Quantum chemical calculation of hole transport properties in crystalline polyethylene

Hole mobility in crystalline polyethylene (PE) is evaluated from an atomistic point of view, with the combination of quantum chemical calculation and kinetic Monte Carlo simulations. Hole hopping rate between PE chains are computed by Fermi’s golden rule and Marcus rate expression. It turns out that hole transfer in PE occurs in a hopping regime rather than in a band regime even for crystalline structure without any inherent structural disorders. The results indicate that in crystalline PE, hole hops between localized states that are created by polaronic effect. The hole mobility is strongly dependent on the chain length, and increases with increasing chain-length. When the chain length was comparable to the thickness of lamellar crystals, i.e., the characteristic length scale of crystal in the direction of c axis, hole mobility and activation energy for hole transfer were in reasonable agreement with experimental values. Simulated results support the experimental prediction that fast and slow mobility are due to charge transfer in the crystalline region and amorphous region, respectively. Our findings show that high hole mobility observed in crystalline PE, regardless of the small inter-molecular electronic couplings, is due to the small reorganization energy, which is realized through the intra-molecular hole delocalization, and due to quantum tunneling effects owing to the small energetic disorder and high-frequency intra-molecular phonon modes.

Toward the Ultimate Limit of Connectivity in Quantum Dots with High Mobility and Clean Gaps.

Colloidal quantum dots (CQDs) are highly versatile nanoscale optoelectronic building blocks, but despite their materials engineering flexibility, there is a considerable lack of fundamental understanding of their electronic structure as they couple within thin films. By employing a joint experimental-theoretical study, we reveal the impact of connectivity in CQD assemblies, going beyond the single CQD picture. High-resolution transmission electron microscopy (HR-TEM) demonstrates connectivity motifs across different CQD sizes and length scales and provides the necessary perspective to build robust computational models to systematically study the achievable degree of connectivity in these materials. We focused on state-of-the-art surface ligand treatments, taking into account both the degree of connectivity and nanocrystal orientation, and performed ab initio simulations within the phonon-assisted hopping regime. Importantly, both the TEM studies and our simulation results revealed morphological and electronic defects that could dramatically reduce optoelectronic performance, and yet would not have been captured within a single CQD model that neglects connectivity. We calculate carrier mobility in the presence of such defect states and conclude that the best-achievable CQD assemblies for optoelectronics will require a modest degree of fusing via the {001} facet, followed by atomic ligand passivation to generate a clean band gap and unprecedentedly high charge transport.

Self-injection locking of the DFB laser through an external ring fiber cavity: Polarization behavior

We study stability of self-injection locking realized with DFB laser coupled with an external fiber optic ring cavity. Polarization behavior of the radiation circulating in the feedback loop is reported. Two regimes of mode hopping have been observed; one of them is accompanied by polarization bistability involving two orthogonal polarization states.

Polaronic conduction and Anderson localization in reduced strontium barium niobate

Electron transport mechanisms in reduced Sr0.5Ba0.5Nb2O6 (SBN50) are investigated from ∼100 to 955 K through an analysis of the electrical conductivity (σ) and the Seebeck coefficient (S) with respect to temperature (T). Notably, experimental evidence is presented that supports a scenario of Anderson localization below 600 K and carrier excitation across a mobility edge at higher temperature. As a relaxor ferroelectric, stoichiometric SBN has intrinsic disorder associated with both the distribution of Sr/Ba vacancies and the formation of polarized nanoregions. The removal of oxygen through reduction generates conduction electrons in SBN. At the lowest temperatures measured (100–155 K), the electrical conductivity exhibits a temperature dependence characteristic of variable range hopping, followed by a transition to small polaron hopping at intermediate temperatures (250–545 K). In both the variable range and small polaron hopping regimes, a semiconductor-like temperature dependence of the electrical conductivity (dσ/dT > 0) was observed. However, above 615 K, dσ/dT decreases dramatically and eventually becomes metal-like (dσ/dT < 0). Concomitantly, the Seebeck coefficient exhibits a linear dependence on lnT from 615 to 955 K with the same slope (∼104 μ V/K) for both polycrystalline SBN50 and single crystalline SBN61 (both reduced), indicating a similar, constant density of states near the Fermi level for both compositions. The application of Seebeck coefficient theory to this inherently disordered material reveals that the excitation of carriers across a mobility edge is likely responsible for the change in dσ/dT at high temperature. Such findings may have a significant impact in the field of conductive ferroelectrics.

Poole-Frenkel effect and variable-range hopping conduction in metal/YBCO resistive switching devices

Current-voltage (IV) characteristics and the temperature dependence of the contact resistance [R(T)] of Au/YBa2Cu3O7−δ (optimally doped YBCO) interfaces have been studied at different resistance states. These states were produced by resistive switching after accumulating cyclic electrical pulses of increasing number and voltage amplitude. The IV characteristics and the R(T) dependence of the different states are consistent with a Poole-Frenkel (P-F) emission mechanism with trapping-energy levels Et in the 0.06–0.11 eV range. Et remains constant up to a number-of-pulses-dependent critical voltage and increases linearly with a further increase in the voltage amplitude of the pulses. The observation of a P-F mechanism reveals the existence of an oxygen-depleted layer of YBCO near the interface. A simple electrical transport scenario is discussed, where the degree of disorder, the trap energy level, and the temperature range determine an electrical conduction dominated by non-linear effects, either in a P-F emission or in a variable-range hopping regime.

Coherent quantum transport in disordered systems: A unified polaron treatment of hopping and band-like transport.

Quantum transport in disordered systems is studied using a polaron-based master equation. The polaron approach is capable of bridging the results from the coherent band-like transport regime governed by the Redfield equation to incoherent hopping transport in the classical regime. A non-monotonic dependence of the diffusion coefficient is observed both as a function of temperature and system-phonon coupling strength. In the band-like transport regime, the diffusion coefficient is shown to be linearly proportional to the system-phonon coupling strength and vanishes at zero coupling due to Anderson localization. In the opposite classical hopping regime, we correctly recover the dynamics described by the Fermi's Golden Rule and establish that the scaling of the diffusion coefficient depends on the phonon bath relaxation time. In both the hopping and band-like transport regimes, it is demonstrated that at low temperature, the zero-point fluctuations of the bath lead to non-zero transport rates and hence a finite diffusion constant. Application to rubrene and other organic semiconductor materials shows a good agreement with experimental mobility data.

Mode-coupling mechanisms in nanocontact spin-torque oscillators

Spin-torque oscillators (STOs) are devices that allow for the excitation of a variety of magnetodynamical modes at the nanoscale. Depending on both external conditions and intrinsic magnetic properties, STOs can exhibit regimes of mode hopping and even mode coexistence. Whereas mode hopping has been extensively studied in STOs patterned as nanopillars, coexistence has been only recently observed for localized modes in nanocontact STOs (NC-STOs), where the current is confined to flow through a NC fabricated on an extended pseudo spin valve. By means of electrical characterization and a multimode STO theory, we investigate the physical origin of the mode-coupling mechanisms favoring coexistence. Two coupling mechanisms are identified: (i) magnon-mediated scattering and (ii) intermode interactions. These mechanisms can be physically disentangled by fabricating devices where the NCs have an elliptical cross section. The generation power and linewidth from such devices are found to be in good qualitative agreement with the theoretical predictions, as well as provide evidence of the dominant mode-coupling mechanisms.

Possibilities to improve the antioxidative capacity of beer by optimized hopping regimes

Different hopping regimes were evaluated to investigate the effect on the oxidative stability of wort and beer. Compared with a single hop dosage at the beginning of wort boil, it was possible to increase the concentration of α-acids in pitching wort and beer by applying incremental hop dosage, dry hopping or the use of a pre-isomerized hop product in combination with an α-acid extract, which concomitantly resulted in lower iron concentrations and an enhanced flavour stability as indicated by standard wort and beer analyses, atomic absorption spectroscopy, electron spin resonance spectroscopy and sensory analysis of fresh and force-aged beers. The functional principle of hop dosage variations is explained by saving of α-acids throughout the wort production process, which yields an increased formation and precipitation of pro-oxidative acting transition metal ions (e.g. Fe) in α-acid-complexes during the whirlpool rest and fermentation. Consequently, fewer reactive oxygen species are generated. Additional laboratory trials simulating wort cooling and beer storage in buffered model solutions proved that un-isomerized α-acids are strong iron chelators and confirmed the functional principle of the applied hopping regimes. Negative effects of higher α-acid contents on fermentation performance and depletion of the zinc concentration, which is an essential nutrient for yeast, could be excluded. Copyright © 2014 The Institute of Brewing & Distilling

Large positive magnetoresistance effects in the dilute magnetic semiconductor (Zn,Mn)Se in the regime of electron hopping

Magnetoresistance in dilute magnetic semiconductors is studied in the hopping transport regime. Measurements performed on Cl-doped Zn1–xMnxSe with x < 8% are compared with simulation results obtained by a hopping transport model. The energy levels of the Cl donors are affected by the magnetization of Mn atoms in their vicinity via the s-d exchange interaction. Compositional disorder, in particular, the random distribution of magnetic atoms, leads to a magnetic-field induced broadening of the donor energy distribution. As the energy distribution broadens, the electron transport is hindered and a large positive contribution to the magnetoresistance arises. This broadening of the donor energy distribution is largely sufficient to account for the experimentally observed magnetoresistance effects in n-type (Zn,Mn)Se with donor concentrations below the metal–insulator transition.