Dense, thick films of aligned single wall carbon nanotubes and nanotube ropes have been produced by filtration/deposition from suspension in strong magnetic fields. Electrical resistivity exhibits moderate anisotropy with respect to the alignment axis, while the thermopower is the same when measured parallel or perpendicular to this axis. Both parameters have identical temperature dependencies in the two orientations. Thermal conductivity in the parallel direction exceeds 200 W/mK, within a decade of graphite.

The electronic properties of p-doped single-walled carbon nanotube ͑SWNT͒ bulk samples were studied by temperature-dependent resistivity and thermopower, optical reflectivity, and Raman spectroscopy. These all give consistent results for the Fermi level downshift ͑⌬E F ͒ induced by doping. We find ⌬E F Ϸ 0.35 eV and 0.50 eV for concentrated nitric and sulfuric acid doping respectively. With these values, the evolution of Raman spectra can be explained by variations in the resonance condition as E F moves down into the valence band. Furthermore, we find no evidence for diameter-selective doping, nor any distinction between doping responses of metallic and semiconducting tubes. DOI: 10.1103/PhysRevB.71.205423 PACS number͑s͒: 71.23.Ϫk, 61.48.ϩc, 73.63.Fg, 81.07.De The electronic spectra of single-wall carbon nanotubes ͑SWNTs͒ are dominated by van Hove singularities, manifestations of the 1-D structure. The location of the Fermi energy E F with respect to these singularities can be tuned by chemical ͑alkali metals, acids, halogens, …͒ 1 or electrochemical doping. 2 Doping response in bulk samples is complicated by the presence of metallic and semiconducting tubes and by diameter and chirality dispersion, both of which imply a distribution of initial work functions. 3,4 Further complications arise from tube-tube interactions in bundles or ropes. 5,6 Here we report a systematic study of chemically p-doped SWNTs combining resistivity, thermopower, reflectivity, and Raman spectroscopy. Experimental results from each of the above techniques have been reported before, but quantitative analysis of the Fermi level shift has not been routinely performed. Also, the consistency of experimental results from different techniques has never been carefully addressed. In this work, we compare data obtained for relatively weak and strong protonic acids, HNO 3 and H 2 SO 4 , respectively, in order to test consistency of results from different measurements. We discuss the results in terms of a rigid band model 7 whereby doping shifts E F without affecting the band structure. We assume all tubes in the undoped bulk sample have the same work function, and that E F is initially near the middle of the gap or pseudogap of semiconducting or metallic tubes, respectively. We also assume that doping is spatially uniform, with no energy barriers between metallic and semiconducting tubes. We find that this simplest of models gives consistent results for ⌬E F , the Fermi level shift upon doping. Using the experimentally determined ⌬E F values as input, the evolution of Raman spectra with doping can be simply explained by the variation of resonance conditions with E F , with no evidence for diameter-selective doping as recently proposed. [8][9][10] Samples were prepared from pulsed laser vaporization ͑PLV͒ 11 and HiPco SWNT. 12 The former have a narrow distribution of relatively large diameters, 13 1.36± 0.09 nm, while HiPco tubes have smaller average diameters extending over a broad range, 14 0.8 to 1.4 nm. Starting materials for the doping ex...

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The structure and electronic properties of potassium-doped single-wall carbon nanotubes have been studied by conduction electron spin resonance, conductivity (), and x-ray diffraction ͑XRD͒, using in situ electrochemical methods. The spin susceptibility P of the K-saturated phase is independent of temperature; a lower bound is 5ϫ10 Ϫ8 emu/g. At 300 K both and P increase monotonically and reversibly with K/C. The spin relaxation rate and g factor do not change with doping, and XRD reveals an irreversible loss of crystallinity upon doping. We propose an inhomogeneous doping model to explain these results. RAPID COMMUNICATIONS R4846PRB 62 CLAYE, NEMES, JÁ NOSSY, AND FISCHER RAPID COMMUNICATIONS R4848PRB 62 CLAYE, NEMES, JÁ NOSSY, AND FISCHER

Direct evidence for charge leakage at the interface of epitaxial SrTiO3/LaMnO3 superlattices with atomically sharp interfaces is provided. The direction of charge leakage can be reversed by changing the LMO/STO thickness ratio. This result will be important for the understanding of some of the reported limitations of oxide devices involving manganite/titanate interfaces.

The thermal properties of carbon nanotubes are strongly dependent on their unique structure and size, and show promise as an ideal material for thermal management on the micro- and macro-scale. The specific heat of nanotubes is similar to that of two-dimensional graphene at high temperatures, but is sensitive to the effects of rolling the the graphene sheet into a small cylinder at low temperatures. Specifically, the acoustic phonon modes are stiffened due to the cylindrical geometry, and the phonon spectrum is quantized due to the small diameter of the tube. In bundles of single-walled nanotubes, the specific heat is a sensitive probe of inter-tube mechanical coupling. Measurements of the specific heat show that inter-tube coupling is relatively weak, and show direct evidence for quantum effects. The thermal conductivity of nanotubes should reflect the on-tube phonon structure. Aligned bundles of SWNTs show a high thermal conductivity (>200 W/m-K at room temperature), and possible quantization effects at low temperature.

The detection of true magnetocapacitance (MC) as a manifestation of magnetoelectric coupling (MEC) in multiferroic materials is a nontrivial task, because pure magnetoresistance (MR) of an extrinsic MaxwellWagner-type dielectric relaxation can lead to changes in capacitance [G. Catalan, Appl. Phys. Lett. 88, 102902 (2006)]. In order to clarify such difficulties involved with dielectric spectroscopy on multiferroic materials, we have simulated the dielectric permittivity ε of two dielectric relaxations in terms of a series of one intrinsic film-type and one extrinsic Maxwell-Wagner-type relaxation. Such a series of two relaxations was represented in the frequency-(f -) and temperature-(T -) dependent notations ε vs f and ε vs T by a circuit model consisting in a series of two ideal resistor-capacitor (RC) elements. Such simulations enabled rationalizing experimental f -, T-, and magnetic field-(H -) dependent dielectric spectroscopy data from multiferroic epitaxial thin films of BiMnO 3 (BMO) and BiFeO 3 (BFO) grown on Nb-doped SrTiO 3 . Concomitantly, the deconvolution of intrinsic film and extrinsic Maxwell-Wagner relaxations in BMO and BFO films was achieved by fitting f -dependent dielectric data to an adequate equivalent circuit model. Analysis of the H -dependent data in the form of determining the H -dependent values of the equivalent circuit resistors and capacitors then yielded the deconvoluted MC and MR values for the separated intrinsic dielectric relaxations in BMO and BFO thin films. Substantial intrinsic MR effects up to 65% in BMO films below the magnetic transition (T C ≈ 100 K) and perceptible intrinsic MEC up to − 1.5% near T C were identified unambiguously.

SnSe has been prepared by arc-melting, as mechanically robust pellets, consisting of highly oriented polycrystals. This material has been characterized by neutron powder diffraction (NPD), scanning electron microscopy, and transport measurements. A microscopic analysis from NPD data demonstrates a quite perfect stoichiometry SnSe0.98(2) and a fair amount of anharmonicity of the chemical bonds. The Seebeck coefficient reaches a record maximum value of 668 μV K−1 at 380 K; simultaneously, this highly oriented sample exhibits an extremely low thermal conductivity lower than 0.1 W m−1 K−1 around room temperature, which are two of the main ingredients of good thermoelectric materials. These excellent features exceed the reported values for this semiconducting compound in single crystalline form in the moderate-temperatures region and highlight its possibilities as a potential thermoelectric material.