Ferroelectric polarization flop in a frustrated magnet MnWO4_4 induced by magnetic fields

The relationship between magnetic order and ferroelectric properties has been investigated for MnWO$_4$ with long-wavelength magnetic structure. Spontaneous electric polarization is observed in an elliptical spiral spin phase. The magnetic-field dependence of electric polarization indicates that the noncollinear spin configuration plays a key role for the appearance of ferroelectric phase. An electric polarization flop from the b direction to the a direction has been observed when a magnetic field above 10T is applied along the b axis. This result demonstrates that an electric polarization flop can be induced by a magnetic field in a simple system without rare-earth f-moments.Comment: 9 pages, 4 figure

Hall effect of quasi-hole gas in organic single-crystal transistors

Hall effect is detected in organic field-effect transistors, using appropriately shaped rubrene (C42H28) single crystals. It turned out that inverse Hall coefficient, having a positive sign, is close to the amount of electric-field induced charge upon the hole accumulation. The presence of the normal Hall effect means that the electromagnetic character of the surface charge is not of hopping carriers but resembles that of a two-dimensional hole-gas system

Quantitative analysis of electronic transport through weakly-coupled metal/organic interfaces

Using single-crystal transistors, we have performed a systematic experimental study of electronic transport through oxidized copper/rubrene interfaces as a function of temperature and bias. We find that the measurements can be reproduced quantitatively in terms of the thermionic emission theory for Schottky diodes, if the effect of the bias-induced barrier lowering is included. Our analysis emphasizes the role of the coupling between metal and molecules, which in our devices is weak due to the presence of an oxide layer at the surface of the copper electrodes.Comment: 4 pages, 3 figure

Evidence for Insulating Behavior in the Electric Conduction of (NH3_3)K3_3C60_{60} Systems

Microwave study using cavity perturbation technique revealed that the conductivity of antiferromagnet (NH$_3$)K$_{3-x}$Rb$_x$C$_{60}$ at 200K is already 3-4 orders of magnitude smaller than those of superconductors, K$_3$C$_{60}$ and (NH$_3$)$_x$NaRb$_2$C$_{60}$, and that the antiferromagnetic compounds are {\it insulators} below 250K without metal-insulator transitions. The striking difference in the magnitude of the conductivity between these materials strongly suggests that the Mott-Hubbard transition in the ammoniated alkali fullerides is driven by a reduction of lattice symmetry from face-centered-cubic to face-centered-orthorhombic, rather than by the magnetic ordering.Comment: accepted for publication in PR

Pressure screening in the interior of primary shells in double-wall carbon nanotubes

The pressure response of double-wall carbon nanotubes has been investigated by means of Raman spectroscopy up to 10 GPa. The intensity of the radial breathing modes of the outer tubes decreases rapidly but remain observable up to 9 GPa, exhibiting a behavior similar (but less pronounced) to that of single-wall carbon nanotubes, which undergo a shape distortion at higher pressures. In addition, the tangential band of the external tubes broadens and decreases in amplitude. The corresponding Raman features of the internal tubes appear to be considerably less sensitive to pressure. All findings lead to the conclusion that the outer tubes act as a protection shield for the inner tubes whereas the latter increase the structural stability of the outer tubes upon pressure application.Comment: PDF with 15 pages, 3 figures, 1 table; submitted to Physical Review

Doping Effect of Nano-Diamond on Superconductivity and Flux Pinning in MgB2

Doping effect of diamond nanoparticles on the superconducting properties of MgB2 bulk material has been studied. It is found that the superconducting transition temperature Tc of MgB2 is suppressed by the diamond-doping, however, the irreversibility field Hirr and the critical current density Jc are systematically enhanced. Microstructural analysis shows that the diamond-doped MgB2 superconductor consists of tightly-packed MgB2 nano-grains (~50-100 nm) with highly-dispersed and uniformly-distributed diamond nanoparticles (~10-20 nm) inside the grains. High density of dislocations and diamond nanoparticles may take the responsibility for the enhanced flux pinning in the diamond-doped MgB2.Comment: 16 pages, 6 figure

Structural and superconducting properties of MgB2−x_{2-x}Bex_x

We prepared MgB$_{2-x}$Be$_{x}$ ($x=0$, 0.2, 0.3, 0.4, and 0.6) samples where B is substituted with Be. MgB$_{2}$ structure is maintained up to $x=0.6$. In-plane and inter-plane lattice constants were found to decrease and increase, respectively. Superconducting transition temperature $T_{c}$ decreases with $x$. We found that the $T_{c}$ decrease is correlated with in-plane contraction but is insensitive to carrier doping, which is consistent with other substitution studies such as Mg$_{1-x}$Al$_{x}$B$_{2}$ and MgB$_{2-x}$C$_{x}$. Implication of this work is discussed in terms of the 2D nature of $\sigma$-band.Comment: 3 pages,4 figures, to be published in Phys. Rev.

Ferromagnetism and giant magnetoresistance in the rare earth fullerides Eu6-xSrxC60

We have studied crystal structure, magnetism and electric transport properties of a europium fulleride Eu6C60 and its Sr-substituted compounds, Eu6-xSrxC60. They have a bcc structure, which is an isostructure of other M6C60 (M represents an alkali atom or an alkaline earth atom). Magnetic measurements revealed that magnetic moment is ascribed to the divalent europium atom with S = 7/2 spin, and a ferromagnetic transition was observed at TC = 10 - 14 K. In Eu6C60, we also confirm the ferromagnetic transition by heat capacity measurement. The striking feature in Eu6-xSrxC60} is very large negative magnetoresistance at low temperature; the resistivity ratio \rho(H = 9 T)/\rho(H = 0 T) reaches almost 10^{-3} at 1 K in Eu6C60. Such large magnetoresistance is the manifestation of a strong pi-f interaction between conduction carriers on C60 and 4f electrons of Eu.Comment: 5 pages, 4 figure