Short-range Hopping Research Articles

Thermal transport and topological analyses of the heat-carrying modes and their relevant local structures in variously dense amorphous alumina

Engineering the thermal conductivities of amorphous materials is important for thermal management of various semiconducting devices. However, controlling the heat carriers—long-range propagating propagons and short-range hopping diffusons—in disordered lattices is difficult because the carriers are strongly correlated with lattice disorder. To clarify the relationship between lattice disorder and heat conduction, we must simultaneously investigate the important local structures hidden in a disordered system and the microscopic transport characteristics of propagons and diffusons. Here, we explore the variations in spectral thermal conductivity and the relevant local structures in amorphous alumina (a-Al2O3) at different densities by performing the spectral thermal transport and persistent homology analyses. As the density increases, the thermal conductivity of the high-frequency diffusons linearly increases but those of the propagons and low-frequency diffusons remain constant. The density increase enhances the local strain, thereby increasing the mean free paths of the high-frequency diffusons. The density of states competes with diffusivity, lowering the sensitivity of the density response to the thermal conductivity of low-frequency heat carriers. Furthermore, from the obtained topological features of the connections between the oxygen atoms, we inferred that the collapsed network of six-coordinated AlO6 octahedron clusters underlies the transport of high-frequency diffusons. Besides revealing the conductive pathways of heat-carrying modes in disordered lattices, topology-assisted spectral thermal transport analysis is useful for tailoring the thermal conductivities of amorphous materials.

Entanglement Entropy of Free Fermions with a Random Matrix as a One-Body Hamiltonian.

We consider a quantum system of large size N and its subsystem of size L, assuming that N is much larger than L, which can also be sufficiently large, i.e., 1≪L≲N. A widely accepted mathematical version of this inequality is the asymptotic regime of successive limits: first the macroscopic limit N→∞, then an asymptotic analysis of the entanglement entropy as L→∞. In this paper, we consider another version of the above inequality: the regime of asymptotically proportional L and N, i.e., the simultaneous limits L→∞,N→∞,L/N→λ>0. Specifically, we consider a system of free fermions that is in its ground state, and such that its one-body Hamiltonian is a large random matrix, which is often used to model long-range hopping. By using random matrix theory, we show that in this case, the entanglement entropy obeys the volume law known for systems with short-range hopping but described either by a mixed state or a pure strongly excited state of the Hamiltonian. We also give streamlined proof of Page's formula for the entanglement entropy of black hole radiation for a wide class of typical ground states, thereby proving the universality and the typicality of the formula.

Polaron-mediated transport and relaxation phenomena in the lead zinc niobate (PZN) modified bismuth ferrite solid solution

To investigate the dielectric and conductivity properties of PZN modified BF solid solution high temperature mixed oxide technique is employed. The material is subjected to analysis using XRD, SEM, and impedance spectroscopy. A polycrystalline PZN-BF sample with an average crystallite size of 63.3 nm has been formed. The scanning electron micrographs show that the grains and grain boundaries are distributed suitably. dielectric characteristics of the 0.1PZN-0.9BF compound are studied at frequencies ranging from 1 kHz to 1 MHz and temperatures ranging from ambient temperature to 400 °C. The confined mobility of charge carriers causes nearly continuous loss (NCL) behavior at low temperatures (<200 °C). High-temperature relaxation resulting from charge carrier hopping induces maximum loss and elevated conductivity. Rendering to the Overlapping Large Polaron Tunnelling (OLPT) model of conductivity, the transport mechanism in this modified BiFeO3 (BF) compound is facilitated by the short-range hopping of polarons. The NTCR behavior from Arrhenius plot of ac conductivity and temperature independent conductivity relaxation from loss modulus (M'') analysis are demonstrated. Sensors, actuators, and optoelectronic devices can benefit from the PZN-modified BF compound's electrical and dielectric characteristics, which have been studied in detail.

Extracting charge carrier mobility in organic solar cells through space-charge-limited current measurements

Mobility is a critical parameter influencing the overall performance of organic solar cells (OSCs). Herein, we innovatively elucidated the intricate interrelation between the photovoltaic molecular structures and the methodologies employed for the extraction of charge carrier mobility in OSCs. We proposed a simple yet effective principle to accurately extract charge carrier mobility values using the standard space-charge-limited current (SCLC) measurement, while critically assessing theoretical and experimental deficiencies through the drift-diffusion analysis. It was found that field-dependent charge transport is necessitated to describe the prominent long-range intrachain hopping carrier behavior in polymers, while short-range intermolecular hopping results in trap-involved charge transport within small molecular acceptors. Based on the above understanding, a synergetic inter/intra-molecular hopping strategy was proposed to fabricate thick-film all-polymer OSCs, and an unprecedented power conversion efficiency (PCE) of 16.61 % was achieved in the 300 nm PM6:PY-IT OSC. This work not only presents a precise and straightforward approach for measuring mobility values, but also provides a significant reference about charge carrier transport to make optimal decisions regarding photovoltaic material design and device fabrication process of high-performance OSCs.

Analysis of the microstructural and magnetodielectric behavior in multiferrioc Cr1.3Fe0.7O3 nanoparticles

Single phase Cr1.3Fe0.7O3 has been successfully prepared by wet chemical method, accompanied by the surface morphology with pure diamond shaped structure of corundum and uniform grain growth. The splitting of magnetization curves at ∼220 K during varying temperature measurement indicates the magnetocrystalline anisotropy with the coexistence of antiferromagnetic and ferromagnetic phases. The obvious dielectric relaxation behavior has been determined above ∼125 K with the activation energy of 0.0318 eV, which is attributed to the dipolar relaxation caused by short-range polaron range hopping in the magnetic grains. The arresting magnetoelectric coupling effect near room temperature is certified in Cr1.3Fe0.7O3 from ∼125 K, with the maximum coupling coefficient of 5 % at ∼212 K. These characteristics mean that the sample has potential application value in fields such as spintronics devices and magnetoelectric memories.

A correlation between microstructural and impedance properties of MnFe2-xCoxO4 nanoparticles

A comprehensive study of physical properties and impedance analysis of MnFe2-xCoxO4 (x = 0.00, 0.40, 0.80, 1.20, 1.60, 2.00) nanoparticles prepared by citrate-nitrate method has been carried out. Structural analysis by Rietveld refinement indicated the formation of pure cubic structure with the space group Fd3‾m. The chemical composition and approximate grain size of the samples were determined by energy dispersive X-ray spectroscopy and field-emission scanning electron microscopy, respectively. The presence of Co has a strong influence on space charge modulation and affect the grain-boundary resistance. The results showed that sample x = 1.20 has less mobility, less polarizability, and therefore lower dielectric constant which is preferred for high frequency or power applications to minimize electric power loss. Impedance analysis demonstrate that small polaron hopping are responsible for the short-range translation hopping in conduction process of the samples (except for x = 1.60).

Electrical conductivity and impedance spectroscopy studies of ceramic (Ba0.8Sr0.2)Ti0.95(Zn1/3Nb2/3)0.05O3 doped with Bi2O3

Polycrystalline samples of (Ba0·8Sr0.2)1–3x/2BixTi0.95(Zn1/3Nb2/3)0.05O3 (BixBSTBZN) (x = 0.00, 0.10), were prepared by the solid state reaction method. The dielectric impedance properties were studied over the range of frequency between 100 Hz and 1 MHz and in the temperature range of 420 °C–480 °C, using the modulus formalism. The impedance plot showed a first semicircle at high frequency which was assigned to the grain intrinsic effect and a second semicircle, at lower frequencies, which corresponds to grain boundary polarization (conduction phenomenon). A complex modulus spectrum was used to understand the mechanism of the electrical transport process, which indicates that a non-Debye type of multiple relaxations in the material. The values of the activation energy of the compound (calculated both from dc conductivity and the modulus spectrum) are very similar, suggesting that the relaxation process may be attributed to the same type of charge carriers. The frequency dependent conductivity plots exhibit double power law dependence suggesting three types of conduction mechanisms: low frequency conductivity owing to long range translational motion of electrons, mid-frequency conductivity due to short-range hopping, and high frequency conduction due to localized orientation hopping mechanism. Variation of ac conductivity as a function of frequency shows that the compound exhibits Arrhenius-type of electrical conductivity.

Deep understanding of structural and physical properties of BaTiO3 over a broad temperature range

BaTiO3 (BT) ceramic has been prepared by the solid-state reaction method. The Rietveld refinement and the Raman spectroscopy have been employed to characterize the structural information of the BT ceramic based on the analysis of dielectric at room temperature (RT). Detailed microstructure has been observed by scanning electron microscopy. The dielectric properties of our ceramic have been investigated over wide frequency (102-106 Hz) and temperature (−100–600 °C) ranges. At high temperature region (250–600 °C), a dielectric relaxation phenomenon has been observed. To better understand the physical mechanisms at high temperature, a macroscopic and phenomenological statistical model has been used. The calculated activation energy for relaxation and conduction was approximated to 1 eV. This suggests that relaxation in our studied sample at high temperature region is associated with the short-range hopping of ions. This is caused by oxygen vacancies in the bulk of the material. The BT ceramic demonstrated optimum electrical properties: εr = 1105, tanδ = 0.079, TC = 130 °C, εmax = 4050, Pmax = 18.35 μC/cm2, Pr = 7.93 μC/cm2, EC = 2.65 kV/cm, |Y| = 1.86*1011 N/m2, d33 = 197 pC/N, kp = 14%, and Qm = 196. In addition, the evolution of energy storage performance with an increase in the applied electric field has been investigated. The energy storage efficiently has achieved 40 % at RT, under an electrical field of 30 kV/cm.

Effects of lattice dilution on the nonequilibrium phase transition in the stochastic susceptible-infectious-recovered model.

We investigate how site dilution, as would be introduced by immunization, affects the properties of the active-to-absorbing nonequilibrium phase transition in the paradigmatic susceptible-infectious-recovered (SIR) model on regular cubic lattices. According to the Harris criterion, the critical behavior of the SIR model, which is governed by the universal scaling exponents of the dynamic isotropic percolation (DyIP) universality class, should remain unaltered after introducing impurities. However, when the SIR reactions are simulated for immobile agents on two- and three-dimensional lattices subject to quenched disorder, we observe a wide crossover region characterized by varying effective exponents. Only after a sufficient increase of the lattice sizes does it become clear that the SIR system must transition from that crossover regime before the effective critical exponents asymptotically assume the expected DyIP values. We attribute the appearance of this exceedingly long crossover to a time lag in a complete recovery of small disconnected clusters of susceptible sites, which are apt to be generated when the system is prepared with Poisson-distributed quenched disorder. Finally, we demonstrate that this transient region becomes drastically diminished when we significantly increase the value of the recovery rate or enable diffusive agent mobility through short-range hopping.

Spintronics in double stranded magnetic helix: role of non-uniform disorder

The spin dependent transport phenomena are investigated in a double stranded (ds) magnetic helix (MH) structure. Two different helical systems, short-range hopping helix and long range hopping (LRH) helix, are taken into account. We explore the role of these two kinds of geometries on spin dependent transport phenomena. Using Green’s function formalism within a tight-binding framework we compute transport quantities which include spin dependent transmission probabilities, junction currents and spin polarization (SP) coefficient. High degree of SP is obtained for the LRH MH. The SP can be tuned by changing the inter-strand hopping and the direction of magnetic moments at different lattice sites. We find atypical features when we include impurities in one strand of the MH, keeping the other strand free. Unlike uniform disordered systems, SP gets increased with impurity strength beyond a critical value. The effect of temperature on SP and experimental possibilities of our proposed quantum system are also discussed, to make the present communication a self-contained one. Our analysis may provide a new route to explore interesting spintronic properties using similar kind of fascinating helical geometries, possessing higher order electron hopping and subjected to non-uniform disorder.

Local and non-local properties of the entanglement Hamiltonian for two disjoint intervals

We consider free-fermion chains in the ground state and the entanglement Hamiltonian for a subsystem consisting of two separated intervals. In this case, one has a peculiar long-range hopping between the intervals in addition to the well-known and dominant short-range hopping. We show how the continuum expressions can be recovered from the lattice results for general filling and arbitrary intervals. We also discuss the closely related case of a single interval located at a certain distance from the end of a semi-infinite chain and the continuum limit for this problem. Finally, we show that for the double interval in the continuum a commuting operator exists which can be used to find the eigenstates.

Colossal permittivity, resistive and magnetic properties of zinc substituted manganese ferrites

Zinc substituted manganese ferrites, i.e., Mn1−xZnxFe2O4 (0.0 ≤ x ≤ 1.0) were synthesized by the citrate sol-gel route. Rietveld refinement of the X-ray diffraction patterns revealed the development of a single-phase cubic spinel structure with space group Fd3̅m. The stoichiometry of the prepared samples was confirmed by energy dispersive X-ray spectroscopy. The 57Fe Mössbauer study suggested the presence of Fe3+ and Fe2.5+ (mixed Fe3+ and Fe2+) ions at the tetrahedral and octahedral sites of the mixed spinel structure, respectively. However, the fraction of Fe2+/Fe3+ ions present at the octahedral site strongly depends on the site preference of Mn2+ and Zn2+ ions for both sites. Thus, the cation distribution among the tetrahedral and octahedral sites altered with increasing Zn2+ substitution (x). Impedance spectroscopy analysis displayed an enhancement in the resistive properties of MnFe2O4 with Zn2+ substitution, while the dielectric permittivity of the prepared samples exhibited a colossal value of the order of 104–107 at room temperature. This enhancement in the resistive and dielectric properties can be understood in terms of particle size, cation distributions, and conductivity inhomogeneity at the grain boundaries. The temperature-dependent ac conductivity analysis proposed that the short-range hopping of oxygen vacancies is responsible for the conduction process in the prepared samples. Here, our findings of high resistance (107), colossal permittivity (106), diminished tangent loss (0.018), and high magnetization (48 emu/g) for the Mn0.4Zn0.6Fe2O4 make this composition promising for radar absorbing applications.

Magnetoresistive effect in a quantum heterostructure with helical spacer: interplay between helicity and external electric field

Giant magnetoresistive effect in a multi-layered structure not only depends on the properties of magnetic systems, it also strongly depends on the type of non-magnetic spacer that is clamped between magnetic layers. In this work, we critically investigate the role of a helical spacer in presence of a transverse electric field. Two kinds of helical geometries, possessing short-range (SRH) and long-range hopping (LRH) of electrons, are taken into account mimicking single-stranded DNA and protein molecules respectively. Sandwiching the magnetic–non-magnetic–magnetic quantum heterostructure between source and drain contact electrodes, we investigate the properties of giant magnetoresistance (GMR) following the Green’s function formalism within a tight-binding framework. The interplay between SRHs and LRHs of electrons provides several nontrivial signatures in GMR, especially in the presence of transverse electric field, as it makes the system a deterministic disordered one, similar to the well-known Aubry–Andre–Harper from. The famous gapped nature of energy band structure in presence of cosine modulation leads to high degree of magnetoresistance at multiple Fermi energies, compared to the traditional spacers. The magnetoresistive effect can be monitored selectively by adjusting the electric field strength and its direction. Comparing the results between the SRH and LRH cases, we find that the later one is more superior. Finally, to make the system more realistic we include the effect of dephasing. Our analysis may provide some fundamental aspects of designing electronic and spintronic devices based on magnetoresistive effect.

Lieb–Robinson bound in one-dimensional inhomogeneous quantum systems

Lieb–Robinson bound (LRB) in one-dimensional noninteracting many-electron systems with the disordered and quasiperiodic on-site potentials is studied numerically. For the short-range hopping system, a logarithmic light cone (i.e. |x|=βlogt+x0) is found in the system with the disordered on-site potential for small time. The coefficient β decreases with the increasing strength of disordered. When time is large, the bound does not change with time (i.e. |x|=C). For the generalized Fibonacci quasiperiodic (GFQ) system, the on-site potential is taken as V or −V according to two kinds of GFQ sequences. It is found that the system has a power-law light cone (i.e. |x|∝tγ, with 0<γ<1). The exponent γ decreases with the increasing V. We also find that γ for the first class of GFQ system is larger than that for the second class of GFQ system with the same V. Finally, the effects of the long-range hopping which decays like r−α with the distance r on LRB are discussed.

Characterization of Pb(Fe1/2Nb1/2)O3 ceramic as a potential candidate for photovoltaic and magnetoelectric applications

An impurity-free Pb(Fe1/2Nb1/2)O3 (PFN) ceramic was prepared using a conventional solid-state reaction method with precisely controlled sintering temperature. The PFN ceramic exhibits monoclinic symmetry and shows a dense morphology with equiaxed grains of about 1–2 μm. It exhibits a diffused phase transition at approximately 381 K. From the data of dielectric loss tangent and the imaginary parts of impedance and electrical modulus, a non-Debye type dielectric relaxation was investigated, and it was found that the short-range hopping of oxygen vacancies contributes to the dielectric relaxation and the relaxation time distribution is temperature independent. The PFN ceramic exhibits slim ferroelectric hysteresis loops and a linear magnetic hysteresis loop, indicating that the PFN ceramic is paramagnetic; the band gap is estimated to be 2.04 eV. Photoconductivity and photovoltaic responses with a nonzero open-circuit voltage and short-circuit photocurrent were detected. Moreover, magnetoconductivity was measured. This work implies that the PFN ceramic is a potential candidate for magnetoelectric and photovoltaic applications.

Impedance Spectroscopy Analysis of PbSe Nanostructures Deposited by Aerosol Assisted Chemical Vapor Deposition Approach.

This research endeavor aimed to synthesize the lead (II) diphenyldiselenophosphinate complex and its use to obtain lead selenide nanostructured depositions and further the impedance spectroscopic analysis of these obtained PbSe nanostructures, to determine their roles in the electronics industry. The aerosol-assisted chemical vapor deposition technique was used to provide lead selenide deposition by decomposition of the complex at different temperatures using the glass substrates. The obtained films were revealed to be a pure cubic phase PbSe, as confirmed by X-ray diffraction analysis. SEM and TEM micrographs demonstrated three-dimensionally grown interlocked or aggregated nanocubes of the obtained PbSe. Characteristic dielectric measurements and the impedance spectroscopy analysis at room temperature were executed to evaluate PbSe properties over the frequency range of 100 Hz–5 MHz. The dielectric constant and dielectric loss gave similar trends, along with altering frequency, which was well explained by the Koops theory and Maxwell–Wagner theory. The effective short-range translational carrier hopping gave rise to an overdue remarkable increase in ac conductivity (σac) on the frequency increase. Fitting of a complex impedance plot was carried out with an equivalent circuit model (Rg Cg) (Rgb Qgb Cgb), which proved that grains, as well as grain boundaries, are responsible for the relaxation processes. The asymmetric depressed semicircle with the center lower to the impedance real axis provided a clear explanation of non-Debye dielectric behavior.

Structural evolution and dielectric properties of (Bi, Mg, Zr)-doped BaTiO3 ceramics for X8R-MLCC application

The (1-x) BaTiO3-x (0.5Bi2O3-0.67MgO-0.33ZrO2) ceramics (x = 5, 10, 15 and 20 mol%) are fabricated by a conventional solid-state method in a reducing atmosphere. All ceramics display a pure perovskite phase without any trace of secondary phase, and a phase transition from tetragonal phase to pseudo-cubic phase occurs between x = 0.1 and x = 0.15. The dielectric analysis manifests that both the dielectric constant and insulation resistivity at room temperature decrease with increasing dopants, which is mainly related to the reduction of grain boundaries. Meanwhile, high dielectric constant (∼2020) and excellent insulation resistivity (∼4.81 × 1013 ΩΩ ·cm) are obtained when x = 0.05. Besides, the temperature stability of ceramics in the high temperature region is improved with little BMZ-doping, and BMZ05 ceramic satisfies the EIA X8R standard. The complex impedance demonstrates that obvious heterogeneous interfaces and strong interface polarization are existed in ceramics. The fitted activation energy of grain boundary reflects that the long-range hopping polarization of oxygen vacancies dominates in the dielectric constant of ceramics when x < 0.1, while the short-range hopping polarization of defect dipoles dominates when x > 0.15.

Investigation of the role of Sm, Na in ferroelectric, piezoelectric and conduction behaviour of Strontium Bismuth Titanate ceramics

The manuscript presents a systematic study of ferroelectric, piezoelectric and conduction behaviour of Sm, Na substituted Strontium Bismuth Titanate ceramics, prepared by solid-state reaction method. The structural analysis of Bismuth layer structured ferroelectrics (BLSF) of Sr1-2xSmxNaxBi4Ti4O15 (SSNBTi) ceramics was done by X-ray Diffraction technique and it showed single-phase formation and peak shifting towards the higher angles. Scanning Electron Microscopy confirmed orthorhombic sheet-like grain structure with random orientation of grains. P-E study and Piezoelectric studies were done at room temperature (RT); improved remnant polarization (Pr = 1.95 μC/cm2), Piezoelectric charge coefficient (d33 = 21.1 pC/N) and electromechanical coupling factor (kp = 0.71) were achieved by substitution of Sm, Na (x = 0.4) in Sr of SBTi ceramics. The AC conductivity was studied from RT to 600 °C in the frequency range of 1 kHz–100 kHz. The trends shown by Activation energies obtained from Arrhenius plots suggests that the mechanism of conduction is through short-range oxygen vacancies hopping. The combination of improved ferroelectric properties and high piezoelectric constant (kp = 0.71) make the SSNBTi ceramics suitable for a wide range of sensor, actuator, and transducer applications.

Structural, electrical properties and stability in microwave dielectric properties of (1−x) MgTiO3-x SrTiO3 composite ceramics

Development of (1−x) MgTiO3 (MTO)-x SrTiO3 (STO) (for x = 0.04, 0.1–0.5) composite ceramics and tuning of temperature coefficient of resonant frequency (τf) near to zero through solid state reaction method have been reported. The MTO and STO crystals physically diffused together to form a single crystal structure without any trace of chemical interaction. A gradual improvement in dielectric constant (18.74–85.55) with an increase in STO content is obtained for the composite ceramics. Further, the dielectric constant remained almost invariant for a broad frequency range (100 Hz–100 MHz). However, temperature-dependent dielectric constant exhibited significant dispersion. The perusal of Mott’s variable range hopping 3-d model revealed the dominance of short-range hopping (<3 nm) in the composite ceramics. The activation energies for conduction are found to be decreasing (172–83 meV) with STO content which might be attributed to the increase in density of states from 1.65 × 1019 eV−1 cm−3 to 3.413 × 1020 eV−1 cm−3. The (1-x) MgTiO3-x SrTiO3 (for x = 0.04) composite ceramics exhibited best microwave dielectric properties with moderate quality factor (Q × f0) ~26,154 GHz at 8.08 GHz and better thermal stability (τf ~1.76 ppm/K). All the physical parameters followed cubic relation with STO content. The obtained microwave dielectric properties of 0.96MTO-0.04STO composite ceramics are suitable for microwave communication applications as microwave filters, resonators.

Transport and Annihilation of the Triplets in Organic Phosphorescent Systems: Kinetic Monte Carlo Simulation and Modeling

We use kinetic Monte Carlo (kMC) simulations to study transport and annihilation of the triplet excitons in organic phosphorescent host–guest systems. By assuming a short-range hopping mechanism for the triplet transport and a long-range dipolar interaction for the triplet–triplet annihilation (TTA), we examine the effect of the relevant experimental parameters on the decay kinetics of the triplets in an energy-disordered system. It is shown that for the range of parameters typical of the organic host–guest systems, transport of the triplets at room temperature is dispersive during their lifetime, with a dispersion exponent of 0.4 < γ < 0.6. We also provide a quantitative comparison between the kMC simulations and the traditional rate coefficients used to describe the TTA. We then suggest and justify a modified rate coefficient that can explain the TTA for a wide range of the system parameters. As the method of analysis, we focus on (a transformed) probability density function of the triplet decay and show that valuable insights into the statistics of the annihilation and the so-called efficiency roll-off can be obtained using this approach. We also discuss how and when describing the annihilation process in terms of an effective time-independent rate can be used to quantify the TTA in the host–guest systems. The results presented in this work are also relevant for other organic devices in which TTA plays a key role in the performance of the system.