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T. Takenobu

Publications and source records attributed to T. Takenobu.

13 recordsLinked to original sources
arXiv2016

Gate-tunable spin-charge conversion and a role of spin-orbit interaction in graphene

The small spin-orbit interaction of carbon atoms in graphene promises a long spin diffusion length and potential to create a spin field-effect transistor. However, for this reason, graphene was largely overlooked as a possible spin-charge conversion material. We report electric gate tuning of the spin-charge conversion voltage signal in a single-layer graphene. Using spin pumping from yttrium iron garnet ferrimagnetic insulator and ionic liquid top gate we determined that the inverse spin Hall effect is the dominant spin-charge conversion mechanism in a single-layer graphene. From the gate dependence of the electromotive force we showed dominance of the intrinsic over Rashba spin-orbit interaction: a long-standing question in graphene research.

cond-mat.mtrl-sci↗
arXiv2008

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.

cond-mat.mtrl-sci↗
arXiv2006

Ferroelectric polarization flop in a frustrated magnet MnWO$_4$ 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.

cond-mat.str-el↗
arXiv2005

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.

cond-mat.mtrl-sci↗
arXiv2005

Regularity in the intratube interaction in double-wall carbon nanotubes

High pressure Raman experiments were performed on bundled double wall carbon nanotubes (DWCNTs). We observe peculiar regularity in the strength of the inner-outer tube (intratube) interaction, which is attributed to the interplay between the tube size and the intratube spacing. Moreover, a pressure-induced red shift of the band gap energies of the probed inner tubes is deduced from the Raman spectra. These experimental findings reveal that the interaction between primary and secondary shells, although generally assumed to be weak, determines the dynamics in the interior of the DWCNTs.

cond-mat.mtrl-sci↗
arXiv2004

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.

cond-mat.mtrl-sci↗
arXiv2004

Effects of polarized organosilane self-assembled monolayers on organic single-crystal field-effect transistors

The surface conductivity is measured by a four-probe technique for pentacene and rubrene single-crystals laminated on polarized and nearly unpolarized molecular monolayers with application of perpendicular electric fields. The polarization of the self-assembled monolayers (SAMs) shifts the threshold gate voltage, while maintaining a very low subthreshold swing of the single-crystal devices (0.11 V/decade). The results, excluding influences of parasitic contacts and grain boundaries, demonstrate SAM-induced nanoscale charge injection up to ~10^12 cm^-2 at the surface of the organic single crystals.

cond-mat.mtrl-sci↗
arXiv2004

$^{11}$B NMR study of pure and lightly carbon doped MgB$_2$ superconductors

We report a $^{11}$B NMR line shape and spin-lattice relaxation rate ($1/(T_1T)$) study of pure and lightly carbon doped MgB$_{2-x}$C$_{x}$ for $x=0$, 0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB$_2$ exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase of $H_{c2}^c$ with carbon doping with respect to pure MgB$_2$. The spin-lattice relaxation rate $1/(T_1T)$ demonstrates clearly the presence of a coherence peak right below $T_c$ in pure MgB$_2$, followed by a typical BCS decrease on cooling. However, at temperatures lower than $\approx 10$K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon doped systems both the coherence peak and the BCS temperature dependence of $1/(T_1T)$ weaken, an effect attributed to the gradual shrinking of the $σ$ hole cylinders of the Fermi surface with electron doping.

cond-mat.supr-con↗
arXiv2002

muSR study of carbon-doped MgB2 superconductors

The evolution of the superconducting properties of the carbon-doped MgB2 superconductors, MgB(2-x)Cx (x= 0.02, 0.04, 0.06) have been investigated by the transverse-field muon spin rotation (TF-muSR) technique. The low-temperature depolarisation rate, sigma(0) at 0.6 T which is proportional to the second moment of the field distribution of the vortex lattice decreases monotonically with increasing electron doping and decreasing Tc. In addition, the temperature dependence of sigma(T) has been analysed in terms of a two-gap model. The size of the two superconducting gaps decreases linearly as the carbon content increases, while the doping effect is more pronounced for the smaller gap related to the 3D pi-sheets of the Fermi surface.

cond-mat.supr-con↗
arXiv2002

Evidence for Insulating Behavior in the Electric Conduction of (NH$_3$)K$_3$C$_{60}$ 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.

cond-mat.supr-con↗
arXiv2001

Intralayer Carbon Substitution in the MgB2 Superconductor

We report that the ternary MgB2-xCx compounds adopt an isostructural AlB2-type hexagonal structure in a relatively small range of nominal carbon concentration, x<0.1. The lattice parameter a decreases almost linearly with increasing carbon content x, while the c parameter remains unchanged, indicating that carbon is exclusively substituted in the boron honeycomb layer without affecting the interlayer interactions. The superconducting transition temperature Tc, determined by magnetometry experiments, also decreases quasilinearly as a function of the carbon concentration. The structural and electronic behavior of MgB2-xCx displays a remarkable similarity with the isoelectronic Mg1-xAlxB2 despite the different substitution sites.

cond-mat.supr-con↗
arXiv1999

Anomalous Raman shift in the ternary fullerides with t1g states

Raman shift dependence of charge transfer is studied in the two fulleride families of AxBa3C60 (x=0,3; A=K,Rb) and KxSm2.75C60 (x=0,3.25). It is found that the Raman spectra are essentially identical to each other for all ternary fullerides, The results show a crossover point at the boundary of t1u and t1g bands for the Raman shift dependence of charge transfer. For the t1u fullerides, the Ag(2) pinch mode follows the characteristic shift of ~6.3cm-1 per elementary charge widely observed in KxC60. The two Ag modes in t1g fullerides show a complicated behavior. An anomalous Raman shift is observed in the t1g fullerides. This anomalous Raman shift of the two Ag modes could not be explained only by charge transfer.

cond-mat.supr-con↗