Adiabatic Hopping Research Articles

Detection of paramagnetic to charge-ordered insulator phase transitions in phase separated (La[formula omitted]Pr[formula omitted])0.67Ca0.33MnO3 thin films

Hole-doped manganese oxides (manganites) exhibit phase separation which allows for the manipulation of phase transitions with the application of external stimuli. The three competing phases of the manganite (La1−yPry)1−xCaxMnO3 (LPCMO) are paramagnetic insulating (PMI), charge-ordered insulating (COI), and ferromagnetic metallic (FMM). In LPCMO thin films, the substrate induced strain often makes it difficult to distinguish between the two insulating phases (PMI and COI) using resistivity and/or magnetization measurements. Here we report a method for determining the PMI-COI transition temperature of LPCMO thin films by fitting resistance vs. temperature (R−T) data to the Arrhenius equation. This method also yields the activation energy in the COI phase which agrees with direct measurements using scanning tunneling microscopy. Analysis of the R−T data suggests that adiabatic polaron hopping is the transport mechanism in both the PMI and COI phases while variable range hopping (VRH) is the dominant mechanism in the mixed FMM/COI state.

Nonadiabatic Dynamics in a Continuous Circularly Polarized Laser Field with Floquet Phase-Space Surface Hopping

Nonadiabatic chemical reactions involving continuous circularly polarized light (cw CPL) have not attracted as much attention as dynamics in unpolarized/linearly polarized light. However, including circularly (in contrast to linearly) polarized light allows one to effectively introduce a complex-valued time-dependent Hamiltonian, which offers a new path for control or exploration through the introduction of Berry forces. Here, we investigate several inexpensive semiclassical approaches for modeling such nonadiabatic dynamics in the presence of a time-dependent complex-valued Hamiltonian, beginning with a straightforward instantaneous adiabatic fewest-switches surface hopping (IA-FSSH) approach (where the electronic states depend on position and time), continuing to a standard Floquet fewest switches surface hopping (F-FSSH) approach (where the electronic states depend on position and frequency), and ending with an exotic Floquet phase-space surface hopping (F-PSSH) approach (where the electronic states depend on position, frequency, and momentum). Using a set of model systems with time-dependent complex-valued Hamiltonians, we show that the Floquet phase-space adiabats are the optimal choice of basis as far as accounting for Berry phase effects and delivering accuracy. Thus, the F-PSSH algorithm sets the stage for future modeling of nonadiabatic dynamics under strong externally pumped circular polarization.

Application of Vogel–Tamman–Fulcher (VTF) model to CdS–B2O3 glasses

In this paper, we report that the DC electrical conductivity is a very interesting property which provides important information about mechanism in solid. The CdS–B2O3 glasses of different composition were prepared in the laboratory. The conductivity was measure in the temperature 308–473 K. A plot of log σ versus 1/T is drawn for various compositions. From this it observed that electrical conductivity increases linearly with temperature which shows Arrhenius nature for different composition of CdS–B2O3. Activation energy for all samples is laying in the range 0.29–0.82 eV which decreases with mol% of CdS. The decrease in conductivity for greater B2O3 concentrations may be due to reduced segmental motion. The temperature dependence conductivity shows percolative behavior and a clear cross over from Vogel–Tamman–Fulcher to Arrhenius behavior. The adiabatic and non-adiabatic hopping conduction is checked by drawing a plot of log σ versus activation energy at fixed temperature.

Non-radiative luminescence decay with self-trapped hole migration in strontium titanate: Interplay between optical and transport properties

A novel model for the non-radiative decay of self-trapped excitons (STEs) in the advanced functional oxide SrTiO3 is proposed and supported by experimental observations. The study is based on the initial ionoluminescence stage for STE emission at 2.5 eV, and its dependence on temperature and electronic excitation rate under energetic heavy-ion irradiation. For all temperatures, this initial stage reaches rapidly a quasi-steady level, and then decreases as induced-structural damage increases. The quasi-steady luminescence exhibits a linear dependence on the excitation rate, suggesting a constant efficiency for STEs formation. An activation energy of 55 meV, essentially independent of the incident ion mass and energy of projectile-ion, is deduced from an Arrhenius-type relationship with irradiation temperature. This energy is in good agreement with experimental values, measured for non-radiative STE decay under ns-laser pulse excitation, and reasonably consistent with density functional theory calculations for migration of self-trapped holes (STHs) described by a small-polaron adiabatic hopping model. A new mechanism dealing with a non-radiative contribution to the STE transition is discussed, consisting of STH migration through thermally-activated hopping and annihilation with the STE-electron. Luminescence kinetics from the chromium intrinsic impurity strongly supports this model, being consistent with the annihilation of Cr3+ centers through recombination with migrating STHs.

Microstructure, optical and electrical characterizations of Mn doped ZnS nanocrystals synthesized by mechanical alloying

Mn doped ZnS nanocrystals have been successfully synthesized by mechanical alloying the stoichiometric mixture of elemental Zn, S and Mn powders at room temperature under inert atmosphere. Rietveld analysis of x-ray diffraction patterns of the samples confirms simultaneous presence of major zinc blende and minor wurtzite phases in milled samples. Phase transformation kinetics between these phases is established with milling time. Different microstructure parameters like lattice parameters, coherently diffracting domain (particle) size and r.m.s. lattice strain have been ascertained from the analysis. Distributions of crystallite size and r.m.s. lattice strain with milling time for both the phases have been revealed from Rietveld refinement. Band gap energy of milled samples with different milling time has been measured by UV–vis absorption spectra and photoluminescence emission spectra have also been analyzed. DC electrical conductivity of the semiconducting samples is measured and the conduction mechanism is explained by adiabatic polaronic hopping model.

Investigation of dielectric and electrical behaviour of nanocrystalline Zn1−xMnxO (x=0 to 0.10) semiconductors synthesized by mechanical alloying

The results on the measurement of electric and dielectric behaviour and capacitance–voltage characteristics of Zn1−xMnxO (x=0 to 0.10) nanocrystalline semiconductors are reported. Direct current conductivity increases with the increase Mn concentration and its thermal behavior can be explained by adiabatic polaronic hopping model. The alternating current conductivity obeys a power law of temperature and frequency. The temperature exponent p strongly depends on Mn concentration. The temperature dependence of frequency exponent s suggests that the overlapping large polaron conduction model is the appropriate conduction mechanism for the investigated samples. The interfacial boundaries and grain contribution to the dielectric properties can be identified by the analysis of complex impedance. Relaxation behaviour of the samples can be explained from the analysis of the electric modulus. Formation of Schottky diode can be described from capacitance–voltage characteristic of the samples and different diode parameters can be extracted from it.

Microstructure characterization and electrical transport of nanocrystalline Zn0.90Mn0.10O semiconductors synthesized by mechanical alloying

We report a comprehensive study of dc and ac conductivity, dielectric relaxation and capacitance–voltage characteristics of Mn doped ZnO nanocrystalline semiconductors prepared by mechanical alloying. Samples are characterized by HRTEM and XRD. Direct current conductivity increases with the increase in temperature and the thermal behavior of electrical dc conductivity for the investigated samples can be described by adiabatic polaronic hopping model. The frequency dependent conductivity obeys a power law σ′(f)∝fsTn. The temperature exponent n strongly depends on frequency. Analysis of the temperature dependence of frequency exponent s suggests a transformation from large polaron to small polaron conduction model with increase in temperature. Considering electric modulus the dielectric properties of the samples have been explained. Capacitance–voltage characteristic suggests the formation of Schottky diode between metallic electrode and semiconductor junction and diode parameter shows anomalous behavior with increasing milling time.

Investigation of phase coexistence and correlation in La1−xBixMnO3+δ (x = 0.4 and 0.6)

Polycrystalline samples of bismuth-doped (x = 0.4 and 0.6) have been synthesized using conventional solid state reaction methods. After characterizing the samples by x-ray diffraction, scanning electron microscope, and energy dispersive x-ray spectrometer, a systematic investigation of magnetic, electrical, and magneto-transport properties has been carried out. Our magnetic measurements indicate spin-glass magnetic behaviour accompanied by a ferromagnetic phase at low temperatures. Also, the evidence for phase coexistence, leading to overlapping of the nanoscale ferromagnetic phase in the paramagnetic host matrix is observed. The electrical resistivity measurements suggest the semiconducting nature of the samples in the measured temperature range. For both samples, the conductivity in the temperature range above the (for T > θD/2) is observed to be due to the adiabatic hopping of small polarons, whereas below the (for T < θD/2), the Shklovskii–Efros variable range hopping mechanism appears to be suitable. The dielectric permittivity as a function of temperatures and frequencies studied in the samples exhibits different relaxation processes, which are understood due to the Debye+Maxwell—Wagner relaxation mechanisms. The observed negative magneto-resistivity and the dielectric relaxation in the paramagnetic region can be attributed to the phase coexistence scenario.

Thermoelectric Properties and Conduction Mechanism of CaCu3Ti4O12 Ceramics at High Temperatures

The Seebeck coefficient and electrical conductivity of CaCu3Ti4O12 (CCTO) ceramics were measured and analyzed in the high temperature range of 300°C to 800°C, and then the electrical conduction mechanism was investigated by using a combination of experimental data fitting and first-principles calculations. The Seebeck coefficient of the CCTO ceramic sintered at 1050°C is negative with largest absolute value of ∼650 μV/K at 300°C, and the electrical conductivity is 2–3 orders greater than the value reported previously by other researchers. With increasing sintering temperature, the Seebeck coefficient decreases while the electrical conductivity increases. The temperature dependence of the electrical conductivity follows the rule of adiabatic hopping conduction of small polarons. The calculated density of states of CCTO indicates that the conduction band is mainly contributed by the antibonding states of Cu 3d electrons, therefore small-polaron hopping between CuO4 square planar clusters was proposed. Possible ways to further improve the thermoelectric properties of CCTO are also discussed.

Meyer-Neldel rule and Poole-Frenkel effect in chalcogenide glasses

A theoretical model for dc conductivity under high electric field in chalcogenide glasses is developed. This model, of correlated barrier hopping is used to treat both low field conductivity, which obeys Meyer-Neldel rule (MNR), and high field Poole-Frenkel (PF) effect. Both are incorporated in one model because the origin of electronic emission is the same: a deep well in which a polaron is trapped. We show that the characteristic temperatures associated with MNR and PF should be the same, as has been predicted using a rigorous adiabatic polaron hopping model, and previously demonstrated experimentally for fullerene films. We also predict that the extrapolated conductivity prefactors will be the same in the two cases. Experimental evidence from the literature suggests, but does not demonstrate conclusively, that the two predictions are satisfied for chalcogenide glasses. Finally, we interpret the result in terms of the multi-excitation entropy model for MNR.

Transport behavior and mechanism of conduction of simultaneously substituted Y and Fe in La 0.7Ba 0.3MnO 3 perovskite

The electrical properties and the mechanism of conduction of the simultaneously substituted La 0.7− x Y x Ba 0.3Mn 1− x Fe x O 3 perovskite (0≤ x≤0.30) have been studied. The insertion of Y 3+ and Fe 3+ ions in the parent compound La 0.7Ba 0.3MnO 3 leads to an increase of the resistivity. The undoped sample ( x=0) shows a metallic behavior, which can be fitted by the relation ρ( T)= ρ 0+ ρ 2T 2+ ρ 4.5T 4.5, indicating the importance of electron–magnon scattering effects in this material. All the other samples ( x≥0.10) are semiconductors throughout the studied temperature range (80–290 K). Several models have been used to fit their temperature-dependent resistivity: thermal activation, adiabatic nearest-neighbor hopping of small polarons (Holstein theory) and variable range hopping (VRH) models. The fits show that the electronic transport in semiconducting La 0.7− x Y x Ba 0.3Mn 1− x Fe x O 3 is well described and dominated by the VRH mechanism, for which the hopping distance ( a) grows with increasing Fe 3+ doping, thus increasing the average hopping energy W.

Dielectric Relaxation and Electrical Conduction Properties of La_2NiO_(4+δ) Ceramics

Thermoelectric power, dc conductivity, and the dielectric relaxation properties of are reported in the temperature range of 77 K - 300 K and in a frequency range of 20 Hz - 1 MHz. Thermoelectric power was positive below 300K. The measured thermoelectric power of decreased linearly with temperature. The dc conductivity showed a temperature variation consistent with the variable range hopping mechanism at low temperatures and the adiabatic polaron hopping mechanism at high temperatures. The low temperature dc conductivity mechanism in was analyzed using Mott`s approach. The temperature dependence of thermoelectric power and dc conductivity suggests that the charge carriers responsible for conduction are strongly localized. The relaxation mechanism has been discussed in the frame of the electric modulus and loss spectra. The scaling behavior of the modulus and loss tangent suggests that the relaxation describes the same mechanism at various temperatures. The logarithmic angular frequency dependence of the loss peak is found to obey the Arrhenius law with activation energy of ~ 0.106eV. At low temperature, variable range hopping and large dielectric relaxation behavior for are consistent with the polaronic nature of the charge carriers.

Small polaron hopping conduction mechanism in Ni-doped LaFeO3

The electrical transport properties of LaFe1− x Ni x O3 (0.1 ≤ x ≤ 0.6) bulk samples were investigated over a wide temperature range, i.e. 9–300 K. Powder x-ray diffraction patterns at room temperature showed that all samples were formed in a single phase. However, a structural transformation was observed from orthorhombic (Pnma) to rhombohedral crystal symmetry at x > 0.5 in Ni-doped samples, which is supported by the electrical transport analysis. Temperature-dependent resistivity data were fitted using Mott's variable-range hopping model for a limited range of temperatures to calculate the hopping distance and the density of states at Fermi level. It was found that all parameters vary systematically with an increase in Ni concentration. Moreover, the resistivity data were also fitted using the small polaron hopping (SPH) model. The non-adiabatic SPH conduction mechanism is followed up to 50% Ni concentration, whereas an adiabatic hopping conduction mechanism is active above it. Such a change in the conduction mechanism is accompanied by subtle electronically induced structural changes involving Fe3+–O–Fe3+ and Fe3+–O–Ni3+ bond angles and bond lengths. Thus, we suggest that the transport properties can be explained according to the additional delocalization of charge carriers induced by Ni doping.

Influence of sintering temperature on thermoelectric properties of La0.9Sr0.1FeO3 ceramics

Perovskite La0.9Sr0.1FeO3 ceramics have been synthesized at 1250℃，1300℃ and 1350℃ by the conventional solid-state reaction technique. From their crystal structures determined by powder X-ray diffraction，we found that the lattice volume decreases with increasing sintering temperature. The scanning electronic microscope (SEM) images of surface microstructures of the samples show that the average grain size increases with increasing sintering temperature. The electrical resistivity and Seebeck coefficient have been measured between room temperature and 800℃. At low temperatures，the electrical resistivity shows a semiconductivity-like behavior. With further increasing of temperature，the electrical resistivity slightly increases. An adiabatic hopping conduction mechanism of small-polarons is suggested from the temperature dependence of the electrical resistivity，which has different activation energies at low and high temperatures. The Seebeck coefficient rapidly decreases with increasing temperature，and reaches a saturation value about 600℃. With further increasing of temperature，the Seebeck coefficient slightly increases. With the increase of sintering temperature，the electrical resistivity decreases，while the Seebeck coefficient increases. Therefore，the power factor increases with increasing sintering temperature. The highest power factor of 90 μW/K2m was obtained at 727℃ for sample sintered at 1350℃.

Influence of Sr substitution on thermoelectric properties of La 1−xSr xFeO 3 ceramics

Perovskite La 1− x Sr x FeO 3 (0.10 ⩽ x ⩽ 0.20) ceramics have been synthesized by the conventional solid-state reaction technique. Their electrical resistivity, Seebeck coefficient and thermal conductivity have been measured. It has been found that the increase of Sr content reduces significantly both the electrical resistivity and the Seebeck coefficient, but slightly increases the high-temperature thermal conductivity. An adiabatic hopping conduction mechanism of small polaron is suggested from the analysis of the temperature dependence of the electrical resistivity. Seebeck coefficients decrease with increasing temperature, and saturate at temperature above 573 K. The saturated value of Seebeck coefficient decreases with increasing of Sr contents, from 200 μV/K for x = 0.10 to 100 μV/K for x = 0.20. All samples exhibit lower thermal conductivity with values around 2.6 W/m K. The highest dimensionless figure of merit is 0.031 at temperature 973 K in La 0.88Sr 0.12FeO 3.

Structural and magnetic ordering inLa0.5Ca0.5−xBaxMnO3 (0 < x ≤ 0.5) manganite

We report a structural, magnetic and transport study inLa0.5Ca0.5−xBaxMnO3 (0<x≤0.5) using neutron diffraction, magnetization and resistivity measurements.The samples crystallize in an orthorhombic structure withPnma space group and no structural transition is observed with Ba doping.Substituting Ca with Ba leads to an increase in average A-site ionic radii,⟨rA⟩ and cationsize disorder (σ2). Consequently, the nature of magnetic ordering undergoes changes as a function ofx. At lowtemperature for x≤0.1, the CE-type antiferromagnetic spin structure is stabilized, similar to the parent compound. Atx = 0.15, the magnetic phase is a mixture of CE-type and A-type spin structure. Forx≥0.2, the ferromagnetic phase is stabilized. The signature of orbital orderingin addition to spin and charge ordering is observed in all samples withx≤0.15.TC,TCO andTN remain nearlyconstant for x≤0.2. The transport studies reveal that for samplesx≤0.2, adiabatic hopping of small polarons governs the transport behaviour aboveTCO (∼160 K). A parallel combination of small polaron hopping and variablerange hopping mechanisms is found to describe resistivity data belowTCO. Atx = 0.2, an insulator to metal transition is observed as temperature is lowered, and atx = 0.5 the sample becomes metallic below 280 K. In the metallic regime, resistivity data followsthe Bloch–Grüneisen model together with contributions from high frequency optical phononsand magnetic scattering. The experimentally established phase diagram from the presentstudy is compared with theoretical studies in the intermediate electron–phonon interactionregime.

Structural, magnetic and electrical transport properties of V doped Bi 0.3Ca 0.7MnO 3

Structural, magnetic, and transport properties of the compounds Bi 0.3Ca 0.7Mn 1− x V x O 3 with 0 ≤ x ≤ 0.1 have been experimentally investigated. This substitution introduces non-magnetic dopant V 5+(3d 04S 0, S = 0) in the Mn–O–Mn network, which results in the variation of the Mn 3+/Mn 4+ ratio. It is suggested that Mn 4+ ions are substituted by V 5+ ions, which gives rise to the increase of Mn 3+ ions. The electronic transport and magnetization results show that the temperature of charge ordering increases with increasing V-doping content. In addition, for all the samples, the temperature of resistivity in the temperature range above the charge ordering transition can be described by the adiabatic hopping of small polaron model, whereas below the charge ordering transition, Mott's variable range hopping mechanism is seen to be valid. The enhanced charge ordering caused by V-doping is suggested to originate from the enhanced Jahn–Teller distortion of the MnO 6 octahedrons due to increased Mn 3+ ions.

Electron spin resonance study of polycrystallineLa0.75(CaxSr1−x)0.25MnO3 (x = 0,0.45, 1)

Electron spin resonance (ESR) spectra of polycrystallineLa0.75(CaxSr1−x)0.25MnO3 (x = 0, 0.45, 1) were studied within the temperature range110 K≤T≤470 K. The temperature dependence of the ESR intensity for the samples is described by athermally activated model in the paramagnetic regime. It is found that the activationenergy in the orthorhombic phase is higher than that in the rhombohedral phase forLa0.75(Ca0.45Sr0.55)0.25MnO3. It is suggested that a higher energy is required to destroy the correlated polarons due tothe fact that correlated polarons only exist in the orthorhombic phase. This proposition isconfirmed by the analysis of the ESR linewidth data, which can be well fitted by the modelof adiabatic hopping motion of small polarons. In addition, it is found that, ata fixed temperature, the linewidth decreases with increasing Sr doping, whichreveals that the structural tolerance factor has a significant effect on the linewidth.

Polaron variable range hopping in [formula omitted] thin films

The mechanisms of electrical conduction in TiO 2 - δ ( - 0.04 ⩽ δ ⩽ 0.2 ) thin films have been investigated. The samples were rf-diode reactive sputtered films deposited in O 2 + Ar gas atmospheres where changes in δ and film structure had been achieved by varying the O 2 flow rate and the substrate temperature. The electrical transport properties of these samples were investigated by measuring the conductivity as a function of temperature between 17 K and room temperature. At the temperature range between 200 and 290 K the best fit to the experimental data was obtained assuming a dependence characteristic of adiabatic variable range hopping. At lower temperature the activation energy for the conductivity tends to zero. The results suggest that the conduction mechanism is adiabatic small polaron hopping, which switches to conduction in a polaron band at low temperatures.

Structural, magnetotransport and morphological studies of Sb-doped La 2/3Ba 1/3MnO 3 ceramic perovskites

We report here the structural, magnetotransport and morphological studies of Sb-doped La 2/3Ba 1/3Mn 1− x Sb x O 3 perovskite manganites. Pristine material La 2/3Ba 1/3MnO 3 (LBMO) shows two insulator–metal (I–M) transitions in the electrical resistivity–temperature ( ρ– T) behavior. While the higher temperature transition ( T P1) at ∼340 K is reminiscent of the usual I–M transition in manganites, the lower temperature transition ( T P2) at ∼250 K has been ascribed to the grain boundary (GB) effects arising out of the ionic size mismatch between the ions present at the rare-earth site (La 3+ and Ba 2+). With Sb-doping T P1 shifts to lower temperatures while T P2 remains invariant up to 3% and shifts to lower temperature for 5%. Room temperature electrical resistivity and the peak values also increase successively with Sb-doping. Scanning electron micrographs of the samples exhibit a gradual increase in their grain sizes with Sb indicating a gradual decrease in the GB density. Shift of T P1 with doping is explained on the basis of a competition between double-exchange and super-exchange mechanisms. The overall electrical resistivity increases and the shift in the electrical resistivity hump ( T P2) with Sb-doping is found related to be gradually decreasing GB density and the ensuing lattice strain increase at the GBs. The intrinsic magnetoresistance (MR) gets suppressed and extrinsic MR gets enhanced with Sb-doping. At T> T P1, the electrical resistivity is found to follow the adiabatic polaron hopping model whereas the electron–magnon scattering is found to dominate in the metallic regime ( T< T P1).