Haruyoshi Aoki

Discovery of superconductivity in KTaO3 by electrostatic carrier doping

Superconductivity at interfaces has been investigated since the first demonstration of electric-field-tunable superconductivity in ultrathin films in 1960(1). So far, research on interface superconductivity has focused on materials that are known to be superconductors in bulk. Here, we show that electrostatic carrier doping can induce superconductivity in KTaO(3), a material in which superconductivity has not been observed before. Taking advantage of the large capacitance of the self-organized electric double layer that forms at the interface between an ionic liquid and KTaO(3) (ref. 12), we achieve a charge carrier density that is an order of magnitude larger than the density that can be achieved with conventional chemical doping. Superconductivity emerges in KTaO(3) at 50 mK for two-dimensional carrier densities in the range 2.3 × 10(14) to 3.7 × 10(14) cm(-2). The present result clearly shows that electrostatic carrier doping can lead to new states of matter at nanoscale interfaces.

dHvA Effect Study of Metamagnetic Transition in CeRu 2Si 2 II - The State above the Metamagnetic Transition

We have performed an extensive dHvA effect study of CeRu 2 Si 2 to gain more insight into the nature of the metamagnetic transition. The angular dependence of the dHvA frequencies confirms the transition of the Fermi surfaces from those of the itinerant f electron below the metamagnetic transition field ( H m ) to those of the localized f electron above H m . The field dependence of the frequencies and effective masses are measured for all the frequencies observed with fields parallel to [001]. A waveform analysis suggests that the Fermi surfaces of the up and down spin electrons are similar in size, whereas the effective masses are significantly different. The amplitudes of the dHvA oscillations and their field dependence are anomalous above H m , implying that the dHvA oscillations above H m may possibly come from a state which is not well developed near H m .

Normal and Superconducting Properties of Noncentrosymmetric Heavy Fermion CeRhSi3

We review the normal and superconducting properties in the noncentrosymmetric heavy-fermion CeRhSi 3 . In the normal state, CeRhSi 3 exhibits the antiferromagnetic order at low temperatures (1.6 K at ambient pressure) although its Kondo temperature is much higher than the ordering temperature T N . With applying pressure P , T N initially increases and subsequently decreases. The superconductivity arises at the pressures where the antiferromagnetic transition occurs. T N does not seem to fall to zero but becomes nearly constant with further application of pressure, and then T N vs P merges with the superconducting transition temperature T c vs P at 26 kbar where T c reaches the maximum. Nearly perfect magnetic-shielding associated with the superconductivity is observed below 26 kbar, suggesting that the bulk superconductivity is realized below T N . We observe an anomaly below T c in the resistivity as well as in the ac-susceptibility. The origin of the both anomalies seem to be the same but have not been...

De Hass - van Alphen oscillations in the normal and superconducting states of the boro-carbide superconductor YNi2B2C

Abstract The de Hass - van Alphen (dHvA) measurements in the normal and mixed states of the boro-carbide superconductor YNi2B2C are reported. Five dHvA branches are observed in the normal state with magnetic fields in the (010) plane. They can be accounted for by the electronic band-structure calculation by Lee et al. A clear dHvA oscillation is also observed in the mixed state. The quasi-particle scattering rate in the mixed state is discussed in connection with the superconducting gap.

dHvA Effect Study of Metamagnetic Transition in CeRu2Si2

The de Haas-van Alphen (dHvA) effect measurements have been performed for both the field ranges above and below the metamagnetic transition field ( H m ) in the (010) plane in CeRu 2 Si 2 . The experimental frequencies and effective masses below H m agree well with the predictions of the first principle or the renormalized band structure calculations based on the itinerant 4f electron model. On the other hand, those above H m can be explained well by the localized 4f electron model. The one to one correspondence between the dHvA oscillations below and above H m can be made. The frequencies are found to change abruptly around H m . The effective masses decrease considerably around H m and continue to decrease with increasing field. These observations indicate that the 4f electron nature changes from itinerant to localized around H m .

Valence State of Cu in Nd2-xCexCuO4 System

We present XPS studies of semiconducting Nd2-xCexCuO4 (0x0.20) and superconducting Nd1.85Ce0.15CuO4-y to clarify the location of the electron carriers. Examination of the Ce3d spectra shows that the Ce ions are donors of electron carriers. From the analysis of the Cu2p spectra, the Cu+ concentration is found to increase with the Ce concentration x. A definite increase in the Cu+ concentration due to the reduction procedure is found for the superconducting sample. The present results suggest that the conduction of the electron carriers takes place in the CuO4 square plane in this system.

Observation of heavy electrons in CeRu2Si2 via the dHvA effect

We have observed a new de Haas-van Alphen (dHvA) oscillation in CeRu 2 Si 2 of frequency 5.36×10 7 Oe and cyclotron effective mass 120 m 0 for magnetic fields along the [100] direction. The new dHvA frequency is thought to arise from the heavy electrons of the large hole surface centered at the Z point which is predicted by the band structure calculations.

Observation of a Main Fermi Surface in UPt3

We have performed the de Haas-van Alphen (dHvA) experiments in UPt 3 . A new branch has been observed in a relatively wide angle region. It has a large cyclotron effective mass of 80 m 0 and the dHvA frequency is 5.66 ×10 7 Oe for the field along the [0001] direction. The new branch most likely corresponds to the main electron Fermi surface in the 37th band of the 5 f -itinerant band model.

Ferromagnetism vs paramagnetism and false quantum oscillations in lanthanum-doped CaB6

Magnetization and resistivity were measured as functions of temperature and magnetic field on two single crystals of CaB 6 and three of (Ca 0.995 La 0.005 )B 6 . One of the doped crystals exhibited...

Magnetic breakdown in the organic conductor (BEDT-TTF)2KHg(SCN)4

Abstract We have carried out the Shubnikov-de Haas (SdH) measurements of the quasi-two-dimensional (2D) conductor (BEDT-TTF)2KHg(SCN)4. A new SdH oscillation with the frequency of 4260 T for Hb ∗ -axis is observed above 10 T. The oscillation is found to come from the magnetic breakdown (MB) orbit. The observation of the MB indicates that the nesting over the large portion of the 1D Fermi surface is not present under magnetic fields above 10 T. The magnetoresistance maximum behavior and its anisotropy are consistently explained in terms of the MB effect.