Electrical Resistance and Seebeck Coefficient in PbTe Nanowires

We address a physics-based simplified analytical formulation of the diffusive electrical resistance ( (Omega)) and Seebeck coefficient () in a PbTe nanowire dominated by acoustic phonon scattering under the presence of a low static longitudinal electric field. The use of a second-order nonparabolic electron energy band structure involving a geometry-dependent band gap has been selected in principle to demonstrate that the electron mean free path (MFP) in such a system can reach as low as about 8 nm at room temperature for a 10-nm-wide PbTe nanowire. This is followed by the formulation of the carrier back-scattering coefficient for determination of (Omega) and as functions of wire dimensions, temperature, and the field, respectively. The present analytical formulation agrees well with the available experimental data and may find extensive use in determination of various electrothermal transport phenomena in PbTe-based one-dimensional electron devices.

Thermoelectricity in graphene: Effects of a gap and magnetic fields

We calculate the thermopower of monolayer graphene in various circumstances. We consider acoustic phonon scattering which might be the operative scattering mechanism in freestanding films and predict that the thermopower will be linear in any induced gap in the system. Further, the thermopower peaks at the same value of chemical potential (tunable by gate voltage) independent of the gap. We show that in the semiclassical approximation, the thermopower in a magnetic field saturates at high field to a value which can be calculated exactly and is independent of the details of the scattering. This effect might be observable experimentally. We also note that a Yukawa scattering potential can be used to fit experimental data for the thermopower for reasonable values of the screening length parameter.

High field carrier transport in graphene: Insights from fast current transient

In this work, we observe gate tunable negative differential conductance (NDC) and current saturation in single layer and bilayer graphene transistor at high source-drain field, which arise due to the interplay among (1) self-heating, (2) hot carrier injection, and (3) drain induced minority carrier injection. The magnitude of the NDC is found to be reduced for a bilayer, in agreement with its weaker carrier-optical phonon coupling and less efficient hot carrier injection. The contributions of different mechanisms to the observed results are decoupled through fast transient measurements with nanosecond resolution. The findings provide insights into high field transport in graphene. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4754103]

Hot-pressed TiB2-10 wt.% TiSi2 ceramic with extremely good thermal transport properties at elevated temperatures (up to 1273 K)

This contribution reports and analyses the high thermal transport property of hot-pressed TiB2-10 wt.% TiSi2 ceramics. Depending on the test temperature, the thermal conductivity values of the TiB2 composite (which range from 89 to 122W m(-1) K-1) are determined to be 18-25% higher than that of monolithic TiB2. The thermal transport properties are analyzed in terms of electronic and phonon contributions. The electronic contribution is the major component of the thermal conductivity of TiB2 and comparable contributions from both electronic and phonon components are observed for the TiB2-TiSi2 composite. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Layer-dependent resonant Raman scattering of a few layer MoS2

We report resonant Raman scattering of MoS2 layers comprising of single, bi, four and seven layers, showing a strong dependence on the layer thickness. Indirect band gap MoS2 in bulk becomes a direct band gap semiconductor in the monolayer form. New Raman modes are seen in the spectra of single- and few-layer MoS2 samples which are absent in the bulk. The Raman mode at similar to 230 cm(-1) appears for two, four and seven layers. This mode has been attributed to the longitudinal acoustic phonon branch at the M point (LA(M)) of the Brillouin zone. The mode at similar to 179 cm(-1) shows asymmetric character for a few-layer sample. The asymmetry is explained by the dispersion of the LA(M) branch along the G-M direction. The most intense spectral region near 455 cm(-1) shows a layer-dependent variation of peak positions and relative intensities. The high energy region between 510 and 645 cm(-1) is marked by the appearance of prominent new Raman bands, varying in intensity with layer numbers. Resonant Raman spectroscopy thus serves as a promising non invasive technique to accurately estimate the thickness of MoS2 layers down to a few atoms thick. Copyright (C) 2012 John Wiley & Sons, Ltd.

Sharp raman anomalies and broken adiabaticity at a pressure induced transition from band to topological insulator in Sb2Se3

The nontrivial electronic topology of a topological insulator is thus far known to display signatures in a robust metallic state at the surface. Here, we establish vibrational anomalies in Raman spectra of the bulk that signify changes in electronic topology: an E-g(2) phonon softens unusually and its linewidth exhibits an asymmetric peak at the pressure induced electronic topological transition (ETT) in Sb2Se3 crystal. Our first-principles calculations confirm the electronic transition from band to topological insulating state with reversal of parity of electronic bands passing through a metallic state at the ETT, but do not capture the phonon anomalies which involve breakdown of adiabatic approximation due to strongly coupled dynamics of phonons and electrons. Treating this within a four-band model of topological insulators, we elucidate how nonadiabatic renormalization of phonons constitutes readily measurable bulk signatures of an ETT, which will facilitate efforts to develop topological insulators by modifying a band insulator. DOI: 10.1103/PhysRevLett.110.107401

Structural, EPR, optical and magnetic properties of alpha-Fe2O3 nanoparticles

alpha-Fe2O3 nanoparticles were synthesized by a low temperature solution combustion method. The structural, magnetic and luminescence properties were studied. Powder X-ray diffraction (PXRD) pattern of alpha-Fe2O3 exhibits pure rhombohedral structure. SEM micrographs reveal the dumbbell shaped particles. The EPR spectrum shows an intense resonance signal at g approximate to 5.61 corresponding to isolated Fe3+ ions situated in axially distorted sites, whereas the g approximate to 2.30 is due to Fe3+ ions coupled by exchange interaction. Raman studies show A(1g) (225 cm(-1)) and E-g (293 and 409 cm(-1)) phonon modes. The absorption at 300 nm results from the ligand to metal charge transfer transitions whereas the 540 nm peak is mainly due to the (6)A(1) + (6)A(1) —> T-4(1)(4G) + T-4(1)(4G) excitation of an Fe3+-Fe3+ pair. A prominent TL glow peak was observed at 140 C at heating rate of 5 degrees C s(-1). The trapping parameters namely activation energy (E), frequency factor (s) and order of kinetics (b) were evaluated and discussed. (C) 2012 Elsevier B.V. All rights reserved.

Gap-Dependent Quasiparticle Dynamics and Coherent Acoustic Phonons in CaFe2As2 across Spin Density Wave Phase Transition

We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons in undoped CaFe2As2 iron pnictide single crystals exhibiting spin-density wave (SDW) and concurrent structural phase transition at temperature T-SDW similar to 165K using femtosecond time-resolved pump-probe spectroscopy. The contributions in transient differential reflectivity arising from exponentially decaying QP relaxation and oscillatory coherent acoustic phonon mode show large variations in the vicinity of T-SDW. From the temperature-dependence of the QP recombination dynamics in the SDW phase, we evaluate a BCS-like temperature dependent charge gap with its zero-temperature value of similar to(1.6 perpendicular to 0.2)k(B)T(SDW), whereas, much above T-SDW, an electron-phonon coupling constant of similar to 0.13 has been estimated from the linear temperature-dependence of the QP relaxation time. The long-wavelength coherent acoustic phonons with typical time-period of similar to 100 ps have been analyzed in the light of propagating strain pulse model providing important results for the optical constants, sounds velocity and the elastic modulus of the crystal in the whole temperature range of 3 to 300 K.

Pressure induced structural phase transformation in TiN: a first-principles study

Titanium nitride (TiN), which is widely used for hard coatings, reportedly undergoes a pressure-induced structural phase transformation, from a NaCl to a CsCl structure, at similar to 7 GPa. In this paper, we use first-principles calculations based on density functional theory with a generalized gradient approximation of the exchange correlation energy to determine the structural stability of this transformation. Our results show that the stress required for this structural transformation is substantially lower (by more than an order of magnitude) when it is deviatoric in nature vis-a-vis that under hydrostatic pressure. Local stability of the structure is assessed with phonon dispersion determined at different pressures, and we find that CsCl structure of TiN is expected to distort after the transformation. From the electronic structure calculations, we estimate the electrical conductivity of TiN in the CsCl structure to be about 5 times of that in NaCl structure, which should be observable experimentally. (C) 2013 American Institute of Physics. http://dx.doi.org/10.1063/1.4798591]

Spectroscopic studies of In2O3 nanostructures; photovoltaic demonstration of In2O3/p-Si heterojunction

The thermal oxidation process of the indium nitride (InN) nanorods (NRs) was studied. The SEM studies reveal that the cracked and burst mechanism for the formation of indium oxide (In2O3) nanostructures by oxidizing the InN NRs at higher temperatures. XRD results confirm the bcc crystal structure of the as prepared In2O3 nanostructures. Strong and broad photoluminescence spectrum located at the green to red region with maximum intensity at 566 nm along with a weak ultraviolet emission at 338 nm were observed due to oxygen vacancy levels and free excitonic transitions, respectively. The valence band onset energy of 2.1 eV was observed from the XPS valence band spectrum, clearly justifies the alignment of Fermi level to the donor level created due to the presence of oxygen vacancies which were observed in the PL spectrum. The elemental ratio In:O in as prepared In2O3 was found to be 42:58 which is in close agreement with the stoichiometric value of 40:60. A downward shift was observed in the Raman peak positions due to a possible phonon confinement effect in the nanoparticles formed in bursting mechanism. Such single junction devices exhibit promising photovoltaic performance with fill factor and conversion efficiency of 21% and 0.2%, respectively, under concentrated AM1.5 illumination.

Surface ferromagnetism and superconducting properties of nanocrystalline niobium nitride

Nanocrystalline delta-NbNx samples have been synthesized by reacting NbCl5 and urea at three different temperatures. A comparison of their structural, magnetic, transport and thermal properties is reported in the present study. The size of the particles and their agglomeration extent increase with increasing reaction temperature. The sample prepared at 900 degrees C showed the highest superconducting transition temperature (T-c) of 16.2 K with a transition width, similar to 1.8 K, as obtained from the resistivity measurement on cold-pressed bars. Above T-c, magnetization measurements revealed the presence of surface ferromagnetism which coexists with superconductivity below T-c. Heat capacity measurements confirm superconductivity with strong electron-phonon coupling constant. The sample prepared at 800 degrees C shows a lower T-c (10 K) while that prepared at 700 degrees C exhibit no superconductivity down to the lowest temperature (3 K) measured.

Ultrafast quasiparticle relaxation dynamics in superconducting iron pnictide Ca(Fe0.944Co0.056)(2)As-2

Nonequilibrium quasiparticle relaxation dynamics is reported in superconducting Ca(Fe0.944Co0.056)(2)As-2 single crystals by measuring transient reflectivity changes using femtosecond time-resolved pump-probe spectroscopy. Large changes in the temperature-dependent differential reflectivity values in the vicinity of the spin density wave (T-SDW) and superconducting (T-SC) transition temperatures of the sample have been inferred to have charge gap opening at those temperatures. We have estimated the zero-temperature charge gap value in the superconducting state to be similar to 1.8k(B)T(SC) and an electron-phonon coupling constant lambda of similar to 0.1 in the normal state that signifies the weak coupling in iron pnictides. From the peculiar temperature-dependence of the quasiparticle dynamics in the intermediate temperature region between T-SC and T-SDW we infer a temperature scale where the charge gap associated with the spin ordered phase is maximum and closes on either side while approaching the two phase transition temperatures.

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2-SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB2 with different amounts of B4C (0-5 wt%) and ZrB2-SiC composites (10-30 vol% SiC) were investigated experimentally over a wide range of temperature (25-1500 degrees C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB2 and ZrB2-SiC composites. At around 200 degrees C, thermal conductivity of ZrB2-SiC composites was found to be composition independent. Thermal conductivity of ZrB2-SiC composites was also correlated with theoretical predictions of the Maxwell Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB2 and ZrB2-SiC composites at room temperature were confirmed by Wiedemann Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB2 whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB2-SiC composites.

Thermoelectric performance of a single-layer graphene sheet for energy harvesting

We investigate the thermoelectric (TE) figure-of-merit of a single-layer graphene (SLG) sheet by a physics-based analytical technique. We first develop analytical models of electrical and thermal resistances and the Seebeck coefficient of SLG by considering electron interactions with the in-plane and flexural phonons. Using those models, we show that both the figure-of-merit and the TE efficiency can be substantially increased with the addition of isotope doping as it significantly reduces the phonon-dominated thermal conductivity. In addition, we report that the TE open circuit output voltage and output power depends weakly on the SLG sheet dimensions and sheet concentration in the strongly diffusive regime. Proposed models agree well with the available experimental data and demonstrate the immense potential of graphene for waste-heat recovery application.

Micro-Raman and photoluminescence studies of self-assembled quantum well microtubes on GaAs substrate

The Semiconductor Quantum Well (QW) microtubes have been fabricated by strain-induced self assembling technique. Three types of multilayer structures have consisted of GaAs/InxGa1-xAs strained layers containing with various thickness of Monolayers of (GaAs/AlGaAs) QW were grown by Varian Gen II Molecular Beam Epitaxy (MBE) on the GaAs (100) substrate. The shape of the rolled up microtubes provide a clear idea about the formation of three dimensional micro- and nanostructures. Micro-Raman and photoluminescence (PL) studies were performed to the QW microtubes and as compared with their grown area on the GaAs substrate. The results of Raman spectra show the frequency shift of phonon modes measured in tube and compared with the grown area due to residual strain. The PL peaks of the microtube were red-shifted due to the strain effect and transition of bandgap from Type-II to Type-I. (C) 2013 Elsevier B.V. All rights reserved.