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Clathrates of type Ba 8 TM x Si 46− x (with TM = Au, Ag, Cu) have been investigated. We report on superconductivity in Ba-inclusion silicon based clathrates when doped with transition metals. It is clarified that the transition metals are necessary in order to form the clathrates as major phases, but they destroy the superconductivity. On lowering the amount of TM, an increase in superconducting fraction has been observed, with a maximum at x ≈1. After purification a superconducting fraction of up to 50% was obtained. The critical temperatures range from 5 to 6 K depending on both, x and the type of TM.

The effect of Mn substitution, acting as a magnetic impurity for Fe, on the Dirac cone was investigated in Ba(Fe${}_{1\ensuremath{-}x}$Mn${}_{x}$As)${}_{2}$. Both magnetoresistance and Hall resistivity studies clearly indicate that the cyclotron effective mass (${m}^{*}$) of the Dirac cone is anomalously enhanced at low temperatures by the impurity, although its evolution as a function of carrier number proceeds in a conventional manner at higher temperatures. Kondo-like band renormalization induced by the magnetic impurity scattering is suggested as the most plausible explanation for this, and the anomalous mass enhancement of the Dirac fermions is discussed.

Unconventional high temperature superconductivity as well as three-dimensional bulk Dirac cone quantum states arising from the unique d-orbital topology has been a recent priority research area in physics. In iron pnictide compounds, although transport phenomena arisen from this multiple band Fermi surface are intriguing and scientifically important, they still do not give an adequate matching to neither experimental observations on the band picture nor theoretical calculations and a debate continues. Here we describe a new analytical approach of mobility spectrum, in which the carrier number is conveniently described as a function of mobility without any hypothesis about the number of carriers, on both longitudinal and transverse transport of high quality single crystal Ba(FeAs)$_2$ in a wide range of magnetic field. We show that the major numbers of carriers reside in large parabolic hole and electron pockets with very different topology as well as remarkably different mobility spectra, while the minor number of Dirac carriers resides in both hole- and electron- Dirac quantum states with the largest mobility as high as 70,000 cm$^2$(Vs)$^{-1}$., Comment: 7 pages, 3 figures

Graphene field-effect transistors with source/drain contacts made of metals that can be easily oxidized such as ferromagnetic metals often display a double dip structure in the transfer characteristics because of charge density depinning at the contacts. Generally, transfer characteristics of field-effect transistors show no dependence on the length of the source/drain contacts because charge carrier injection occurs mainly at the edges of the contact. However, the shape of the transfer characteristics of devices fabricated using Ni contacts is found to be dependent on the length of the contact. This peculiar behavior was attributed to charge carrier injection from near the center of the contacts. This is because of oxygen diffusion and the resultant formation of an interfacial oxide layer of non-uniform thickness. The observed contact length dependent transfer characteristics were reproduced using a model calculation that includes charge carrier injection from the center of the electrode and subsequent charge transport underneath the metal contact.

An SDW antiferromagnetic (SDW-AF) low temperature phase transition is generally observe and the AF spin fluctuations are considered to play an important role for the superconductivity paring mechanism in FeAs superconductors. However, a similar magnetic phase transition is not observed in FeSe superconductors, which has caused considerable discussion. We report on the intrinsic electronic states of FeSe as elucidated by transport measurements under magnetic fields using a high quality single crystal. A mobility spectrum analysis, an ab initio method that does not make assumptions on the transport parameters in a multicarrier system, provides very import and clear evidence that another hidden order, most likely the symmetry broken from the tetragonal C4 symmetry to the C2 symmetry nematicity associated with the selective d-orbital splitting, exists in the case of superconducting FeSe other than the AF magnetic order spin fluctuations. The intrinsic low temperature phase in FeSe is in the almost compensated semimetallic states but is additionally accompanied by Dirac cone like ultrafast electrons $\sim$ 10$^4$cm$^2$(VS)$^{-1}$ as minority carriers., Comment: 18pages, 4 figures, Accepted for publication in PRB

Alkali fullerides exhibit superconductivity with T c 's as high as 33 K at ambient pressure. The transition temperatures scale monotonically with unit cell size, an observation rationalized as arising from the modulation of the electronic density-of-states at the Fermi level, N ( ϵ F ) by the interfullerene spacing, a . Deviations from this relationship leading to suppressed T c 's are apparent at small interfullerene spacings for sodium fullerides, Na 2 (A,A′)C 60 and may be linked to the formation of quasi-one-dimensional bridged fulleride structures. Superconductivity is also suppressed at large interfullerene spacings for (NH 3 )K 3 C 60 whence magnetic origin effects play an important role and provide a connection with the established phenomenology in high-T c and organic superconductors.

Optoelectronic devices based on layered materials such as graphene have resulted in significant interest due to their unique properties and potential technological applications. The electric and optoelectronic properties of nano GaTe flakes as layered materials are described in this article. The transistor fabricated from multilayer GaTe shows a p-type action with a hole mobility of about 0.2 cm(2) V(-1) s(-1). The gate transistor exhibits a high photoresponsivity of 10(4) A/W, which is greatly better than that of graphene, MoS2, and other layered compounds. Meanwhile, the response speed of 6 ms is also very fast. Both the high photoresponsivity and the fast response time described in the present study strongly suggest that multilayer GaTe is a promising candidate for future optoelectronic and photosensitive device applications.

Conductivity and superconductivity of C60 fullerides are reviewed, and the specific features of these new materials are described from the viewpoint of electronic properties and structure.

We determined the isotropic {sup 13}C NMR hyperfine coupling constant as {ital a}{sub iso}/2{pi}=0.69{plus_minus}0.06 MHz for the C{sub 60}{sup 3{minus}} ion near room temperature (RT). On this basis, the spin susceptibility at RT, {chi}{sub {ital s}}(RT), was estimated in various alkali-metal doped C{sub 60} compounds {ital A}{sub 3}C{sub 60}, which is approximately proportional to the lattice constant {ital a}{sub 0}; we found {ital d}{chi}{sub {ital s}}/{ital da}{sub 0}=(0.61{plus_minus}0.06){times}10{sup {minus}3} (emu/moleC{sub 60})/A . The correlation between the superconducting transition temperature {ital T}{sub {ital c}} and {chi}{sub {ital s}}(RT) was determined. It was found that {ital T}{sub {ital c}} of Na{sub 2}RbC{sub 60}, Na{sub 2}KC{sub 60}, Li{sub 2}CsC{sub 60}, and ammoniated {ital A}{sub 3}C{sub 60} is significantly lower than that expected from the BCS relation based on {chi}{sub {ital s}}(RT). {copyright} {ital 1996 The American Physical Society.}

We have measured the low-energy inelastic neutron scattering spectra of superconducting Na 2 CsC 60 in the temperature range 100–350 K. Well-defined librational peaks are observed at 2.7(1) meV; their position and width are temperature independent. They survive well above the order-disorder transition at 299 K. A quasielastic feature with a width 1.03(6) meV at an average scattering vector of 1.09 A −1 is observed at 350 K, arising from diffusive motion of the C 60 3− ions. Evidence is found for influence of the Na + ions on the rotational motion of the C 60 3− ions.

Thin-film field-effect transistors (FETs) of azafullerene (C59N)2 are fabricated, and their properties are investigated. The (C59N)2 FET exhibits n-channel characteristics with the field-effect electron mobility of 3.8×10−4 cm2 V−1 s−1 and the on–off current ratio of 103 at room temperature. The observed differences are ascribed to the much smaller grain size and the worse crystallinity of (C59N)2 thin films, on a basis of low angle x-ray diffraction structural data. The anticipated dimer to monomer conversion with electron carrier injection is not observed. The FET characteristics are discussed from the temperature evolution of the mobilities between (C59N)2 and C60 FETs.

The structures of C60 thin films grown on Si, SiO2, and Al2O3 are studied by a low-glancing-angle x-ray diffraction method and the properties of C60 field effect transistors (FETs) are understood on a basis of this technique. Analyses using the two-dimensional Ewald circle and the Laue function give the detailed interpretation of the diffraction peaks together with information of the preferred orientation and the grain size of polycrystalline thin films. The grain size varies from 100 to 500 A upon the optimal conditions of the thin film growth and the preferred orientation of the C60 〈111〉 crystal axis with random on a substrate surface is obtained. It is clearly shown that these variations in quality of the C60 thin films exert unexpectedly little influence on the FET characteristics.

Sodium-containing alkali metal fulleride, Na 2 CsC 60 , was studied by 13 C and 23 Na NMR below and above the face-centered cubic ( f c c ) to simple cubic ( s c ) phase transition temperature, T m . It is shown that the C 60 molecules rotate faster than the rotational correlation time of 1.6×10 -5 sec above ∼140 K and 2.4×10 -9 sec around 300 K. Using 23 Na NMR spectra broadened by the second-order quadrupole interaction, the electric field gradient at Na sites, e 2 q Q / h , was estimated to be 940±20 kHz and less than 0.1 kHz below and above T m , respectively. The detailed analysis suggests that Na atoms in s c phase have a covalent character with C 60 molecules. It is also suggested that Na 2 CsC 60 is the first metallic plastic crystal.

We have measured the zero-field muon spin relaxation (ZF mu +SR) spectra of the quasi-one-dimensional fulleride conductor CsC60. Static magnetic order of a random nature develops gradually below about 30 K. The local field distribution at the muon site has a Gaussian shape with a width of 48.3(7) G at 2.8 K. Muon spin rotation (TF mu +SR) measurements at an applied transverse field of 6 kG also show the onset of magnetic order in the same temperature range.

Multiple-nuclei NMR investigations on fullerides that contain sodium, ${\mathrm{Na}}_{2}$A${\mathrm{C}}_{60}$ (A=K,Rb,Cs), with a simple-cubic structure are reported. The density of states at the Fermi level vs lattice constant correlation was determined. The superconductivity in ${\mathrm{Na}}_{2}$A${\mathrm{C}}_{60}$ connot be described by the same parameters as those of the other ${\mathit{A}}_{3}$${\mathrm{C}}_{60}$ superconductors within the Bardeen-Cooper-Schrieffer theory. We find that structural imperfection plays an important role in determining the electronic states and onset of superconductivity in ${\mathrm{Na}}_{2}$A${\mathrm{C}}_{60}$. Evidence for the orientational ordering transition was also found with $^{23}\mathrm{Na}$ NMR for A=Cs, Rb, and K.

The electronic states in metallic alkali fullerides K3C60 and Rb3C60, are studied in detail by cw- and pulsed-ESR. The ESR linewidth associated with conduction electrons varies substantially, depending sensitively on the alkali metals used. It is clearly revealed that the linewidth is homogeneous and is strongly controlled by the spin-lattice relaxation times (T1) determined via the spin-orbit interactions of the small fraction of s electrons remaining on the alkali atoms. The results indicate that the electron transfer from the alkali atoms to C60 is not perfectly complete, with the remaining fraction being much greater in the tetrahedral sites than in the octahedral sites of the face-centered cubic lattice.

A photoemission study of C60 thin films evaporated on Eu clarifies that the t1u derived states are occupied by electron transfer from Eu to C60. This result suggests that Eu forms a C60 fulleride whose electronic structure is close to the semiconducting phase like K4C60. It is also concluded that longer annealing breaks the intramolecular chemical bonds in the C60 cage.

A three-dimensional (3D) conducting phase is found upon rapid cooling below 150 K for ${\mathrm{CsC}}_{60}$ and below 125 K for ${\mathrm{RbC}}_{60}$. Electron-spin resonance, differential thermal analysis, differential scanning calorimetry, and x-ray-diffraction results suggest that the 3D conducting phase is an orientationally ordered (possibly simple-cubic) phase. We propose a complete relation for the various states for A${\mathrm{C}}_{60}$. The rapidly cooled A${\mathrm{C}}_{60}$ shows a number of phase changes as the temperature increases from 4 to 473 K.

The growth and structure of C60 and C70 thin film crystals on alkali halide and layered material substrates are described. It is demonstrated that fairly large grains can be grown with the specific orientation to the substrate lattice, especially on layered substrates. The defects in the films fabricated on alkali halides are studied by transmission electron microscopy in detail. The growth mechanism of C60 and C70 thin films on these substrates is discussed on a basis of reflection high-energy electron diffraction and atomic force microscopy observation and compared with that on other substrates. The unusual growth characteristics of C60 and C70 are discussed and compared with those encountered in thin films of conventional van der Waals crystals.

The effect of annealing on the electronic properties of nanotubes has been studied. The conduction ESR intensity is temperature-independent for both annealed and non-annealed nanotubes. However, the g-value of the nanotubes and its temperature dependence change significantly after annealing. These results suggest that some types of defects are present in the nanotubes which are then removed by the annealing process. The ESR studies of the annealed nanotubes indicate that the intrinsic electronic properties of nanotubes are quite different from those of graphite.

The silicon clathrates--materials composed of metal-doped Si(20) dodecahedra--were identified as the first superconductors based on pure silicon networks. The mechanism of superconductivity in these materials can be obtained by studying their phonon modes, as modified by isotope substitution, and specific-heat measurements. Here, we present experimental studies that provide strong evidence that superconductivity in Ba(8)Si(46) is explained in the framework of phonon-mediated Bardeen-Cooper-Schriefer theory. Analyses using the McMillan approximation of the Eliashberg equation indicate that the superconducting mechanism is in the medium coupling regime, but at the high-end limit. The large density of states at the Fermi level, which arises from hybridization of the Si(20) cluster and Ba orbitals, is responsible for the unexpectedly high superconducting temperature. The temperature evolution of the specific heat unambiguously shows that this is an s-wave symmetry superconductor.

We report electronic transport of Dirac cones when Fe is replaced by Ru, which has an isoelectronic electron configuration to Fe, using single crystals of Ba(Fe${}_{1\ensuremath{-}x}$Ru${}_{x}$As)${}_{2}$. The electronic transport of parabolic bands is shown to be suppressed by scattering due to the crystal lattice distortion and the impurity effect of Ru, while that of the Dirac cone is not significantly reduced due to the intrinsic character of Dirac cones. It is clearly shown from magnetoresistance and Hall coefficient measurements that the inverse of average mobility, proportional to cyclotron effective mass, develops as the square root of the carrier number ($n$) of the Dirac cones. This is the unique character of the Dirac cone linear dispersion relationship. Scattering of Ru on the Dirac cones is discussed in terms of the estimated mean free path using experimental parameters.

Magnetic properties of YbAl${}_{3}$C${}_{3}$ with the hexagonal ScAl${}_{3}$C${}_{3}$-type structure have been investigated by the magnetization ($M$) and specific-heat ($C$) measurements under magnetic fields ($H$). YbAl${}_{3}$C${}_{3}$ is reported to show a spin-gap state, which is considered to be ascribed to a magnetic dimer formed in the orthorhombic phase below the structural transition temperature (${T}_{\mathrm{s}}=$ 77 K). Present study has revealed history-dependent magnetic properties below ${T}_{\mathrm{s}}$ and field-induced anomalous magnetic states at low temperatures. The former is considered to be attributed to the cross correlation between the structural deformation and the magnetic field similar to those observed in multiferroic materials, although YbAl${}_{3}$C${}_{3}$ below ${T}_{\mathrm{s}}$ is not in the ferromagnetic state but certainly in the dimer state. The latter is partially similar to that observed in the field-induced ordered phase (FIOP) of $d$-electron dimer systems. The $M$ versus $H$ curve at low temperatures exhibits a kink at a certain magnetic field and then it increases in proportion to the field like that of FIOP. However, neither kink nor peak suggesting the emergence of FIOP is observed in the temperature dependence of both $M$($T$) and $C$($T$). Instead, $C/T$ shows an anomalous increase proportional to $\ensuremath{-}$ln$T$ with decreasing temperature in a finite field range, suggesting an anomalous disordered state such as a non-Fermi-liquid state in a strongly correlated $f$-electron system. These anomalous magnetic states may be partially relevant to characteristics of $f$-electron dimer system.

In order to quantitatively characterize the effect of motion of the guest atom in type-I clathrates ${X}_{8}$Ga${}_{16}$Ge${}_{30}$ ($X$ $=$ Sr, Ba) on thermal conductivity, electrostatic potential in the crystal was visualized by synchrotron radiation x-ray diffraction. The obtained electrostatic potential clearly exhibited structural aspects of the guest atom, rattling, which is considered as a cause of suppression of thermal conductivity \ensuremath{\kappa}. The parameterized degree of rattling ``rattling factor,'' ${f}_{\mathrm{R}}$, being defined as a volume ratio of the electrostatic interaction region of the guest atom to the spherical volume estimated by the covalent radius of the guest atom, was found to well classify \ensuremath{\kappa}. The \ensuremath{\kappa} monotonously decreases with increasing the ${f}_{\mathrm{R}}$. Consequently, ${f}_{\mathrm{R}}$ was judged as a better criterion to explore high-performance thermoelectric system of the clathrates.

The gate-voltage (VG) dependence of the contact resistance (RC) in graphene field-effect transistors is characterized by the transmission line model. The RC-VG characteristics of Ag, Cu, and Au contacts display a dip around the charge neutrality point, and become even negative with Ag contacts. The dip structure is well reproduced by a model calculation that considers a metal-contact-induced potential variation near the metal contact edges. The apparently negative RC originates with the carrier doping from the metal contacts to the graphene channel and appears when the doping effect is more substantial than the actual contact resistance precisely at the contacts. The negative RC can appear at the metal contacts to Dirac-cone systems such as graphene., Comment: 7 figures, 1 table

New types of carbon, C60 and tubules, as exotic materials are presented with reference to the relevant literature. Conductivity and superconductivity found for alkali C60 fullerides are described and discussed on a basis of crystal structure. The potential utility of the inner spaces of both C60 and tubules is also addressed from the nano material science point of view.

The electron spin resonance (ESR) of carbon nanotubes at various stages of purification have been measured between 4 and 296 K. The conduction electron spin resonance is identified and the results imply that metallic and/or narrow gap semiconducting nanotubes are actually present as predicted by theory. From the temperature dependences of the ESR intensity, linewidth and g -value, the electronic properties of the observed nanotubes are found to be similar to those of graphite.

The ternary system fullerides ${\mathrm{Na}}_{2}$A${\mathrm{C}}_{60}$ (A=Cs, Rb, and K) are studied by differential scanning calorimetry in the temperature range of 100 to 450 K. All show reproducible phase transitions at the transition temperatures of 299 K (\ensuremath{\Delta}H=2.5\ifmmode\pm\else\textpm\fi{}0.5 J/g) for ${\mathrm{Na}}_{2}$${\mathrm{CsC}}_{60}$, 313 K (\ensuremath{\Delta}H=2.7\ifmmode\pm\else\textpm\fi{}0.5 J/g) for ${\mathrm{Na}}_{2}$${\mathrm{RbC}}_{60}$, and 305 K (\ensuremath{\Delta}H=3.1\ifmmode\pm\else\textpm\fi{}0.5 J/g) for ${\mathrm{Na}}_{2}$${\mathrm{KC}}_{60}$. Orientational ordering of the fullerene unit is responsible for the observed phase transitions (${\mathit{T}}_{\mathrm{sc}\mathrm{\ensuremath{-}}\mathrm{fcc}}$) from a high-temperature face-centered-cubic (fcc) to a low-temperature simple-cubic (sc) phase. The observed ${\mathit{T}}_{\mathrm{sc}\mathrm{\ensuremath{-}}\mathrm{fcc}}$ and enthalpy can be compared to those for pristine ${\mathrm{C}}_{60}$ which has ${\mathit{T}}_{\mathrm{sc}\mathrm{\ensuremath{-}}\mathrm{fcc}}$=256.5 K (\ensuremath{\Delta}H=8\ifmmode\pm\else\textpm\fi{}0.5 J/g). The observed behavior is discussed in terms of the balance between the ${\mathrm{C}}_{60}^{3\mathrm{\ensuremath{-}}}$-${\mathrm{C}}_{60}^{3\mathrm{\ensuremath{-}}}$ intermolecular interaction and the ${\mathrm{C}}_{60}^{3\mathrm{\ensuremath{-}}}$-${\mathit{A}}^{+}$ Coulombic interaction as a function of lattice parameter.

A systematic 13 C NMR experiment is reported in alkali-fullerides A 3 C 60 (where A 3 is alkali-metals) between superconducting transition temperature T c and ∼100 K. Careful 13 C NMR spin-lattice relaxation time study clarified the properties of the conduction electron wave functions at Fermi surface. An upper limit of Stoner exchange parameter was estimated to be ∼3 eV/(spin-carbon), which is far from spin-wave instability criterion. Based on these studies, a variation of the density of states at Fermi level, N ( E f ), with T c was deduced. The results were analyzed in terms of McMillan formula, leading to weak or medium electron-phonon coupling constants and relevant phonon energy larger than 600 K. It was strongly suggested that the A 3 C 60 superconductor is described within a framework of the conventional phonon-mediated BCS superconductivity.

Photoexcitation of pure C60 thin films by picosecond absorption spectroscopy reveals a transient species with absorption maxima at 540 nm and 660 nm which is assigned to the exciton. Considering the dynamic behaviour of this species and luminescence properties of solid C60, it is concluded that a singlet exciton is observed both in its free (τ1/2 < 15 ps) and self-trapped (τ1/2 = 250 ns) forms.

The crystal structure of superconducting Na(2)CsC(60) was studied by high-resolution powder neutron diffraction between 1.6 and 425 K. Contrary to the literature, the structure at low temperatures is primitive cubic [See equation in the PDF file], isostructural with pristine C(60). Anticlockwise rotation of the C(60) units by 98 degrees about [111] allows simultaneous optimization of C(60)-C(60) and alkali-fulleride interactions. Optimal Na(+)-C(60)(3-) coordination is achieved with each sodium ion located above one hexagon face and three hexagon-hexagon fusions of neighboring fulleride ions (coordination number 12). Reduction of the C(60) molecule lengthens the hexagon-hexagon fusions and shortens the pentagon-hexagon fusions (to approximately 1.43 angstroms). On heating, Na(2)CsC(60) undergoes a phase transition to a face-centered-cubic [See equation in the PDF file] phase, best modeled as containing quasi-spherical C(60)(3-) ions. The modified structure and intermolecular potential provide an additional dimension to the behavior of superconducting fullerides and should sensitively affect their electronic and conducting properties.

THE discovery of fullerenes1,2 and carbon nanotubes3,4 has led to the realization that it should be possible to tailor the properties of graphitic sheets if their geometry can be controlled5–7. In exploring these possibilities, we have found that the folding and tearing of graphitic sheets follow well defined patterns which seem to be governed by the formation of sp3-like line defects in the sp2 graphitic network. Our studies with the atomic force microscope and scanning tunnelling microscope reveal that these folds and tears occur preferentially along the symmetry axes of graphite, and that 'ripples' are observed in the curved portions of the folds. We also see ripples in deformed carbon nanotubes. They lie along the directions for which sp3-like line defects can form most easily to relieve strain. Our ab initio molecular orbital calculations indicate that the ripples stabilize the π-electronic energy in the bent structures with the total energy balance being determined by the amount of nuclear repulsion. These results provide insight into the geometries that graphitic structures will preferentially accommodate, and the properties that might ensue.

We observed crystal structures of K x Rb 3− x C 60 by X-ray diffraction, and found that Rb ions preferentially occupied octahedral interstitial sites. In the case of K 2 RbC 60 , Rb and K ions are accommodated in octahedral and tetrahedral sites respectively. As for the case of KRb 2 C 60 , Rb and K ions in tetrahedral interstitial sites were found to be in a random fashion by anomalous X-ray diffraction.

The Ru doping effect on the Dirac cone states is investigated in iron pnictide superconductors Ba(Fe$_{1-x}$Ru$_x$As)$_2$ using the transverse magnetoresistance (MR) measurements as a function of temperature. The linear development of MR against magnetic field $B$ is observed for $x$ = 0 - 0.244 at low temperatures below the antiferromagnetic transition. The $B$-linear MR is interpreted in terms of the quantum limit of the Dirac cone states by using the model proposed by Abrikosov. An intriguing evidence is shown that the Dirac cone state persists on the electronic phase diagram where the antiferromagnetism and the superconductivity coexist.

Low-temperature specific heat (SH) is measured on Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ single crystals in a wide doping region under different magnetic fields. For the overdoped sample, we find the clear evidence for the presence of the ${T}^{2}$ term in the SH data, which is absent both for the underdoped and optimal doped samples, suggesting the presence of line nodes in the energy gap of the overdoped samples. Moreover, the field-induced electronic specific heat coefficient $\ensuremath{\Delta}\ensuremath{\gamma}(H)$ increases more quickly with the field for the overdoped sample than the underdoped and optimal doped ones, giving more support to our arguments. Our results suggest that the superconducting gap(s) in the present system may have different structures strongly depending on the doping regions.

Quantum transport of Dirac cone states in the iron pnictide Ba(FeAs)(2) with a d-multiband system is studied by using single crystal samples. Transverse magnetoresistance develops linearly against the magnetic field at low temperatures. The transport phenomena are interpreted in terms of the zeroth Landau level by applying the theory predicted by Abrikosov. The results of the semiclassical analyses of a two carrier system in a low magnetic field limit show that both the electron and hole reside as the high mobility states. Our results show that pairs of electron and hole Dirac cone states must be taken into account for an accurate interpretation in iron pnictides, which is in contrast with previous studies.

Low-temperature specific heat is measured on the overdoped Ba(Fe_{1-x}Co_x)_2As_2 (x = 0.13) single crystal under magnetic fields along three different directions. A clear anisotropy is observed on the field dependent electronic specific heat coefficient {\gamma}(H). The value of {\gamma}(H) is obviously larger with magnetic field along [001] (c-axis) than that within the ab-plane of the crystal lattice, which cannot be attributed to the effect by anisotropy of the upper critical field. Meanwhile, the data show a rather small difference when the direction of the field is rotated from [100] to [110] direction within the ab-plane. Our results suggest that a considerable part of the line nodes is not excited to contribute to the quasiparticle density of states by the field when the field is within the ab-plane. The constraints on the topology of the gap nodes are discussed based on our observations., Comment: 13 pages, 4 figures

We have measured the low-energy neutron inelastic-scattering (NIS) spectra of superconducting Na2RbC60 in the temperature range 50?350?K. Well-defined librational peaks are observed at 50?K at 2.83(17)?meV (FWHM?=?1.7(5)?meV). They soften and broaden with increasing temperature. Their behaviour mimics that found in solid C60 and differs markedly from K3C60. The rotational barrier for C60 reorientations in Na2RbC60 is somewhat higher than in pristine C60 and approximately half as large as in K3C60. An order-disorder transition is anticipated at a temperature higher than that found in C60.

In alkali-metal (A) doped C 60 fullerides superconductivity is found only for face-centered cubic (f.c.c.) crystal phases with the composition A 3 C 60 . The highest T c of 33 K found in these f.c.c. A 3 C 60 superconductors family is surpassed only by that of the high T c , copper oxides. We describe here the stability and dopant site selectivity of A 3 C 60 fullerides as well as the factors controlling the f.c.c. lattice parameters. Superconductivity studied by diamagnetic susceptibility measurements is described in detail, especially in the small lattice parameter region. The relationship between the f.c.c. structure and the superconductivity is described and the phonon frequency associated with electron pairing is discussed within the framework of the phonon-mediated type McMillan equation. Experimental results, which are not well understood, are also discussed with reference to the literature.

Lattice constants of alkali-metal-doped A3C60 fullerides with face-centered cubic crystal structure are reported for a wide range of lattice parameters, using Li, Na, K, Rb, Cs and their binary mixtures. It is shown that the size of the alkali-metal cations in the tetrahedral interstitial sites controls the lattice parameters. The observed lattice parameters are discussed in comparison with other body-centered cubic A6C60 and rock salt type A1C60 phases. The field repulsive interactions among the C60 molecules other than the electrostatic Madelung energy must be taken into account to understand the lattice parameters of A3C60 in the small lattice parameter region, different from standard alkali-halide ionic crystals.

We observed crystal structure of Na2RbC60 by X-ray diffraction and found that C60 molecules in Na2RbC60 superconductor are more orientationally disordered than the other alkali metal doped C60 superconductors such as Rb3C60. This disordering might be related to the superconducting transition temperature (Tc) of Na2RbC60, which is much lower than that expected from the reported simple relation between (Tc) and lattice constant.

The growth of nanotubes in carbon arc plasma is described in detail. The structure and organization of the nanotube deposits observed by SEM and AFM reveal a fractal-like pattern of growth. One of the key units of growth appears to be the micro-bundle composed of neatly packed and aligned nanotubes. The micro-bundle together with the field and current might explain the high formation yield of nanotubes.

The structure of superconducting face-centered cubic (f.c.c.) M3C60 compounds has been precisely studied for binary alkali-metal-doped C60 molecules, where M is Na, K, Rb, Cs and their binary mixtures. Refinements of the observed X-ray diffraction patterns show that MS2MLC60 compounds are stable and preferentially ordered compared with the crystal phases for MSML2C60, which are very dependent on the alkali metals used, where MS denotes the smaller alkali metal and ML the larger metal. A new relationship between the superconducting transition temperature ( T c ) and the f.c.c. lattice constant ( a 0 ) is demonstrated over a wide range of lattice parameters, using the binary alkali-metal-doped f.c.c. M3C60 superconductors. The new T c -a 0 relationship is compared with the previously reported pressure experiments and the prediction based on the simple Bardeen-Cooper-Schriefter theory.

Superconducting transition temperature (Tc) and crystal structure of binary alkali-metal doped fullerenes have been studied. A new relationships between Tc and lattice constant (a0) is presented over a wide range of a0 for superconducting face-centered cubic (fcc) M3C60 phases, using a new series of superconductors: Na2MC60 (Tc=12 K for M=rmK, Tc=2.5 K for M=Rb, Tc = 2.K for M=Cs andLi2CsC60 (Tc=12 K). The site selectivity of the doped cationic elements for the two tetrahedral and one octahedral interstitial sites per C60 in the fcc lattice is discussed in detail on a basis of refinements of the measured X-ray patterns. The departure from the previous relationship between Tc and a0 for lattice parameters smaller than that of pristine C60 and the Tc saturation tendency in the large lattice parameter region may provide new insights into what controls superconductivity in these materials.

CAPPED hollow carbon nanotubes1,2 can be modified into nanocomposite fibres by simultaneous opening of the caps (by heating in the presence of air and lead metal) and filling of the interior with an inorganic phase3. To generalize this approach, greater understanding is needed of the reaction mechanism between the tube caps and oxygen. Here we report that the oxidation of carbon nanotubes in air for short durations above about 700 °C results in the etching away of the tube caps and the thinning of tubes through layer-by-layer peeling of the outer layers, starting from the cap region. The oxidation reaction follows an Arrhenius-type relation with an activation energy barrier of about 225 kJ mol−1 in air. Heating of closed nanotubes with an oxide (Pb3O4) in an inert atmosphere lowers the activation barrier for the reaction and opening of the tubes occurs at lower temperatures. Contrary to intuition, however, open tubes are much more difficult to fill with inorganic materials than in the one-step filling of tubes reported previously3. But various other experiments might be possible in the inner nano-cavities of the open tubes such as studies of catalysis and of low-dimensional chemistry and physics.

133 Cs-NMR was observed in C 60 superconductors, Cs x Rb 3−x C 60 with x=1 and 2. It was found that Cs atoms are intercalated in octahedral interstitial sites of C 60 fcc lattice for x=1, while for x=2 in both of the octahedral and tetrahedral interstitial sites. The shift of the 133 Cs-NMR strongly suggested that the 6s electron of Cs atom is fully ionized and conduction electrons are mainly on the C 60 sites. 13 C-NMR was observed to study electronic states at C 60 sites. It showed a monotonic increase of the superconducting transition temperature, Tc , with increasing Fermi level density of states, N ( E f ), at carbon sites, where N ( E f ) −2 was assumed to be proportional to 13 C spin-lattice relaxation time T 1 times temperature T : N ( E f ) −2 ∝ T 1 T .