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The realization of remarkable thermoelectric (TE) properties in a novel single-crystalline quantum material is a topic of prime interest in the field of thermoelectricity. It necessitates a proper understanding of transport properties under magnetic field and magnetic properties at low field. We report polarized Raman spectroscopic study, TE properties, and magneto-resistance (MR) along with magnetic characterization of single-crystalline Bi2Se3. Polarized Raman spectrum confirms the strong polarization effect of A 1 g 1 and A 1 g 2 phonon modes, which verifies the anisotropic nature of the Bi2Se3 single crystal. Magnetization measurement along the in-plane direction of single crystal divulges a cusp-like paramagnetic response in susceptibility plot, indicating the presence of topological surface states (TSSs) in the material. In-depth MR studies performed in different configurations also confirm the presence of anisotropy in the single-crystalline Bi2Se3 sample. A sharp rise in MR value near zero magnetic field and low-temperature regime manifests a weak anti-localization (WAL) effect, depicting the quantum origin of the conductivity behavior at low temperature. Moreover, in-plane magneto-conductivity data at low-temperature (up to 5 K) and low-field region (≤15 kOe) confirm the dominance of the WAL effect (due to TSS) with a negligible bulk contribution. Quantum oscillation (SdH) in magneto-transport data also exhibits the signature of TSS. Additionally, an exceptional TE power factor of ∼950 μW m−1 K−2 at 300 K is achieved, which is one of the highest values reported for pristine Bi2Se3. Our findings pave the way for designing single crystals, which give dual advantages of being a good TE material along with a topological insulator bearing potential application. [ABSTRACT FROM AUTHOR]

A study of Nd based cuprate francisite (NdCufr) is fascinating due to the presence of competing magnetic interactions in this geometrically frustrated layered compound, and Ni doping helps in tailoring these interactions. Here, Ni doped NdCufr is synthesized to compare its magnetic properties with pristine and Co doped NdCufr. Structural (x-ray diffraction, XRD), electronic (x-ray absorption near edge spectroscopy, XANES, and extended x-ray absorption fine structure, EXAFS), and magnetic (temperature dependent magnetization, M–T; field dependent magnetization, M–B) properties are investigated. The XRD, XANES, and EXAFS measurement data show a perfect crystal structure and confirm the merit of doping. A comprehensive comparison for examining the role of Co and Ni in modulation with ferromagnetic–antiferromagnetic interaction strength is presented. A crucial interplay of intra- and interlayer couplings, along with f–d interaction, effectively describes the tuning of Neel temperature (TN) and M–B. Detailed processes of field induced metamagnetic transition around the spin-flip ordering field (BSF), which is equal to that of the critical field (BC) and doping induced ferromagnetism, are addressed considering the relative spin orientations of magnetic elements and energy level splitting schemes, respectively. It is found that the studied compound is magnetically superior to the compared compounds. [ABSTRACT FROM AUTHOR]

Improvement in thermoelectric parameters is reported with graphite incorporation in n-type Bi2Te3/graphite nanocomposite systems. In-depth thermoelectric properties of nanostructured Bi2Te3/graphite composites are probed both microscopically and macroscopically using x-ray diffraction, Raman spectroscopy, inelastic neutron scattering, and measurement of the temperature dependence of thermal conductivity κ, Seebeck coefficient S, resistivity ρ, and carrier concentration nH. Raman spectroscopic analysis confirms that graphite introduces defects and disorder in the system. Graphite addition induces a large (∼17%) decrease of κ, originating from a strong phonon scattering effect. A low lattice thermal conductivity, κL, value of 0.77 W m−1 K−1, approaching the κmin value, estimated using the Cahill–Pohl model, is reported for Bi2Te3 + 1.0 wt. % graphite sample. Graphite dispersion alters the low-energy inelastic neutron scattering spectrum providing evidence for modification of the Bi2Te3 phonon density of states. Improvement in other thermoelectric parameters, viz., Seebeck coefficient and resistivity, is also reported. Theoretical modeling of electrical and thermal transport parameters is carried out and a plausible explanation of the underlying transport mechanism is provided assuming a simple model of ballistic electron transport in 1D contact channels with two different energies. [ABSTRACT FROM AUTHOR]

Thermoelectric properties of nanostructured Sb2Te3/graphite composites are investigated both experimentally and theoretically and the contribution of graphite to heat transport mechanism is addressed. XRD results indicate the solid state insolubility of the graphite phase in Sb2Te3 as no shift is observed in the diffraction peaks corresponding to Sb2Te3 in the composite samples. Raman spectroscopic analysis confirms the presence of graphite in the nanostructured composite samples. Temperature dependence of carrier concentration nH, thermal conductivity κ, Seebeck coefficient S, resistivity ρ, and, hence, the thermoelectric figure of merit ZT is reported. Graphite incorporation leads to around 40% decrease of κ, essentially due to the decrease of lattice thermal conductivity, κL. A low κL value of 0.8 Wm−1 K−1 is reported, which mostly arises due to enhanced phonon scattering at the heterointerfaces created by the addition of graphite. S increases in Sb2Te3/graphite nanocomposites. Calculation reveals that reported large S value is related with the low hole concentration. Theoretical simulation in the frame of Boltzmann equation approach shows satisfactory agreement of nH, S, and ρ with experimental data, and based on the modification of the density of states and its derivative near Fermi energy with graphite addition, a plausible explanation is provided. [ABSTRACT FROM AUTHOR]

Structural and thermoelectric properties of metallic and semiconducting Sb2Te3 are reported. X-Ray diffraction and Raman spectroscopy studies reveal that semiconducting sample has higher defect density. Nature and origin of possible defects are highlighted. Semiconducting Sb2Te3 hosts larger numbers of defects, which act as scattering center and give rise to the increased value of resistivity, thermopower, and power factor. Thermopower data indicate p-type nature of the synthesized samples. It is evidenced that the surface states are often mixed with the bulk state, giving rise to metallicity in Sb2Te3. Role of different scattering mechanism on the thermoelectric property of Sb2Te3 is discussed. [ABSTRACT FROM AUTHOR]

Polycrystalline Bi1-xSbx alloys have been synthesized over a wide range of antimony concentration (0.08 ≤ x ≤ 0.20) by solid state reaction method. In depth structural analysis using x-ray diffraction (XRD) and temperature dependent resistivity (ρ) measurement of synthesized samples have been performed. XRD data confirmed single phase nature of polycrystalline samples and revealed that complete solid solution is formed between bismuth and antimony. Rietveld refinement technique, utilizing maud software, has been used to perform detailed structural analysis of the samples and lattice parameters of polycrystalline Bi1-xSbx alloys have been estimated. Lattice parameter and unit cell volume decreases monotonically with increasing antimony content. The variation of lattice parameters with antimony concentration depicts a distinct slope change at x = 0.12. Band gap (Eg) has been estimated from the thermal variation of resistivity data, with the 12% Sb content sample showing maximum Eg. It has been observed that with increasing antimony concentration the transition from direct to indirect gap semiconductor is intimately related to the variation of the estimated lattice parameters. Band diagram for the polycrystalline Bi1-xSbx alloy system has also been proposed. [ABSTRACT FROM AUTHOR]

Magnetization and transport properties of La[sub 0.7]Pb[sub 0.3]MnO[sub 3]+x Ag (x=0-20 wt %) nanocomposites have been reported. In this Ag-containing colossal magnetoresistive (CMR) La[sub 0.7]Pb[sub 0.3]MnO[sub 3] composite (referred to as CMR-Ag), conductivity (σ) and metal–insulator transition temperature (T[sub p]) increase with increasing Ag. Electron microscopy and elemental mapping indicated a uniform distribution of Ag nanoparticles/clusters. The enhancement of T[sub p] is accompanied by a reduction of the c-axis lattice constant. Coexistence of interfacial tunneling with intrinsic transport behavior has been observed at the grain boundaries in the samples with a higher (>=10 wt %) Ag content. Due to the presence of nonmagnetic Ag in the ferromagnetic La[sub 0.7]Pb[sub 0.3]MnO[sub 3] material, dc magnetization decreases but the corresponding T[sub p] increases. In the low-temperature (TT[sub p]), conductivity data follow the adiabatic polaron hopping conduction mechanism. Unlike the Ag free sample, the variable-range-hopping model is found to be inapplicable for the present Ag containing nanocomposites. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Pulsed magnetic field (0-4.4 T) was used to study the magnetic field dependent resistivity (12-350 K) and thermoelectric power (0-1.5 T) of Na and K-doped La[SUB0.7]Ca[SUB0.7-y]A[SUBy]MnO[SUB3] (0.0≤y≤0.3, A=Na, K) system showing semiconducting to metallic transitions around temperature T[SUBp]. Na/K-doping increases both conductivity and T[SUBp]. In La[SUB1-x]Ca[SUBx]MnO[SUB3], an increase of T[SUBp] and conductivity with an increase of Ca (for x≤0.33) are small and the small polaron coupling constant (γ) and hence the electron-lattice (phonon) interaction is strong. But in the Na/K doped system, γ is small and for y≥0.05, Motts' condition of strong el-ph interaction breaks down in the high temperature (T>T[SUBp]) phase. Increase of conductivity in the Na/K doped system is caused by the)decrease of γ, binding energy (W[SUBp]), hopping energy (W[SUBH]), and effective mass (m[SUBp]) of the polarons leading to the melting (we call it) of polarons in the T>T[SUBp] phase. This melting results in an increase)of exchange coupling constant between spins. Field dependent thermoelectric power (TEP) of the samples (measured between 80-300 K) also supports the small polaron hopping conduction. The resistivity data are well fitted with the variable range hopping model for a limited range of temperature (T[SUBp]θ[SUBD]/2. With the application of a magnetic field, the density of states at the Fermi level increases. The TEP data indicate the importance of electron-magnon contribution in the low temperature (TT[SUBp]. [ABSTRACT FROM AUTHOR]

Magnetic-field (B=0-1.5T) dependent thermoelectric power (TEP) of the Na-doped La[sub 0.7]Ca[sub 0.3-y]Na[sub y]MnO[sub 3] (0.0≤y≤0.3) system has been studied in the temperature range 80-300 K. X-ray diffraction studies indicate a rhombohedrally distorted perovskite structure of the samples. The observed sharp peak in the resistivity (ρ) versus temperature (T) curve falls and shifts to higher temperature with increasing Na concentration (y). In the low-temperature ferromagnetic (FM) regime, thermopower (Seebeck coefficient, S) obeys the expression S=S[sub 0] + S[sub 1.5]T[sup 1.5] + S[sub 4]T[sup 4] over the entire range of y. Electron-magnon scattering is found to dominate the low-temperature resistivity and TEP data. High-temperature TEP data can be well fitted with Mott's small polaron hopping model. The activation energy (E[sub S]) and polaron hopping energy (W[sub H]) decrease with increasing Na content. Both E[sub S] and W[sub H] decrease while polaron radius (r[sub p]) increase with the application of a magnetic field. Field-dependent TEP data also indicate the suppression of spin fluctuations in the presence of a magnetic field. In the low-temperature FM region, both magnon drag and phonon drag effects coexist. [ABSTRACT FROM AUTHOR]

The effect of particle size on the transport properties (resistivity and thermopower) of La[sub 0.5]Pb[sub 0.5]MnO[sub 3] has been investigated both in the presence and in the absence of magnetic field B=0.0–1.5 T (maximum). Grain size, dc conductivity; and the metal–insulator transition temperature T[sub p] of the sample increase with increasing annealing time. Grain size has, however, comparatively little effect on the Seebeck coefficient S. Magnetoresistance is higher for the samples with smaller grain sizes. dc magnetic susceptibility also increases with increasing grain size. High temperature (T>θ[sub D]/2) resistivity data well fit the small polaron hopping model. Polaron hopping energy W[sub H] decreases but polaron radius r[sub p] increases with the increase of grain size. In the metallic regime (for TT>θ[sub D]/2), it is hard to justify this model from the temperature dependent thermopower data. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two different phases of LaMnO[sub 3+δ] [one showing a metal-insulator transition (MIT), referred to as LaMn-C, and the other not showing a MIT, referred to as LaMn-S] have been clearly observed to follow two different conduction mechanisms. Interestingly, small polaron hopping models of Mott, Schnakenberg, and Emin are found to fit the conductivity data of all the samples above the corresponding MIT temperature. The conductivity data of the insulating (semiconducting) LaMn-S followed a nonadiabatic hopping conduction mechanism while LaMn-C and the Pb doped samples viz. La[sub 1-x]Pb[sub x]MnO[sub 3] (x=0.05-0.5) showed a similar type of MIT and followed an adiabatic small polaron hopping conduction mechanism in the high temperature paramagnetic phase (above the respective MIT temperature). Activation energy (W), density of states at the Fermi level N(E[sub F]), Debye temperature (θ[sub D]), electron-phonon interaction constant (γ[sub P]), etc. of LaMn-S showed appreciable differences from those of LaMn-C and La[sub 1-x]Pb[sub x]MnO[sub 3], which show a MIT. Polaron hopping conduction is also supported by thermoelectric power (TEP) measurements. An observed small but appreciable magnetic field dependence of the TEP data (measured at B=1.5 T) is considered to be associated with magnetic polarons. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Anharmonic lattice vibration in metallic and semiconducting Sb2Te3 is investigated using temperature dependent Raman spectroscopic studies, synchrotron powder diffraction, and heat-capacity measurements. Thermal variation of structural parameters divulges temperature dependence of structural anisotropy in both the samples. It is revealed that semiconducting Sb2Te3, having higher defect density and larger phonon anharmonicity, possesses lower thermal conductivity. As compared to its metallic counterpart, significant increase in thermoelectric figure of merit, ZT is reported for the semiconducting Sb2Te3. Correlation among structural anisotropy, phonon anharmonicity and ZT is established. [ABSTRACT FROM AUTHOR]

We report magneto-resistive properties of direct and indirect band gap Bismuth-Antimony (Bi-Sb) alloys. Band gap increases with magnetic field. Large positive magnetoresistance (MR) approaching to 400% is observed. Low field MR experiences quadratic growth and at high field it follows a nearly linear behavior without sign of saturation. Carrier mobility extracted from low field MR data depicts remarkable high value of around 5 m²V-1s-1. Correlation between MR and mobility is revealed. We demonstrate that the strong nearly linear MR at high field can be well understood by classical method, co-build by Parish and Littlewood, Nature 426, 162 (2003) and Phys. Rev. B 72, 094417 (2005). [ABSTRACT FROM AUTHOR]

Thermal variation of structural property, linear thermal expansion coefficient (α), resistivity (ρ), thermopower (S), and power factor (PF) of polycrystalline Bi1-xSbx (x = 0.10 and 0.14) samples are reported. Temperature-dependent powder diffraction experiments indicate that samples do not undergo any structural phase transition. Rietveld refinement technique has been used to perform detailed structural analysis. Temperature dependence of α is found to be stronger for Bi0.90Sb0.10. Also, PF for direct band gap Bi0.90Sb0.10 is higher as compared to that for indirect band gap Bi0.86Sb0.14. Role of electron-electron and electron-phonon scattering on ρ, S, and PF has been discussed. [ABSTRACT FROM AUTHOR]

The conductivity and magnetoresistance of La[sub 0.5]Pb[sub 0.5]Mn[sub 1-x]Cr[sub x]O[sub 3] (0.0≤x≤0.45) measured at 0.0 and 1.5 T magnetic field have been reported. All the oxide samples except x=0.45, showed metal insulator transition (MIT) between 158–276 K, depending on x. In contrast to the behavior of a similar sample La[sub 0.7]Ca[sub 0.3]Mn[sub 1-x]Cr[sub x]O[sub 3] showing no (MIT) for x>=0.3, the Pb doped samples showed MIT even with x=0.35. The MIT peak temperature (T[sub p]) shifts towards lower temperature with increasing x while magnetic field shifts T[sub p] to the high temperature regime. The metallic (ferromagnetic) part of the temperature dependent resistivity (ρ) curve (below T[sub p]) is well fitted with ρ(T)=ρ[sub 0]+ρ[sub 2.5]T[sup 2.5] indicating the importance of electron–magnon interaction (second term). We have successfully fitted the high temperature (T>θ[sub D]/2, θ[sub D] is Debye temperature) conductivity data, both in presence and in absence of magnetic field, with small polaron hopping conduction mechanism. Adiabatic small polaron hopping conduction mechanism is followed by the samples showing MIT while nonadiabatic hopping conduction mechanism is obeyed by the samples showing no MIT. The lower temperature (between T[sub p] and θ[sub D]/2) conductivity data of all the samples can be well fitted to the variable range hopping (VRH) model similar to the case of many semiconducting transition metal oxides. Temperature dependent Seebeck coefficient data also support the small polaron hopping conduction mechanism above T[sub p]. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]