Shintaro Nakamura

Electric-field-induced superconductivity in an insulator

Electric field control of charge carrier density has long been a key technology to tune the physical properties of condensed matter, exploring the modern semiconductor industry. One of the big challenges is to increase the maximum attainable carrier density so that we can induce superconductivity in field-effect-transistor geometry. However, such experiments have so far been limited to modulation of the critical temperature in originally conducting samples because of dielectric breakdown. Here we report electric-field-induced superconductivity in an insulator by using an electric-double-layer gating in an organic electrolyte. Sheet carrier density was enhanced from zero to 10(14) cm(-2) by applying a gate voltage of up to 3.5 V to a pristine SrTiO(3) single-crystal channel. A two-dimensional superconducting state emerged below a critical temperature of 0.4 K, comparable to the maximum value for chemically doped bulk crystals, indicating this method as promising for searching for unprecedented superconducting states.

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.

Quadrupole-Strain Interaction in Rare Earth Hexaborides

Temperature dependence of the elastic constants in a series of rare earth hexaborides has been investigated by means of the ultrasonic method. The remarkable softening of the transverse C 44 mode in PrB 6 , NdB 6 , DyB 6 and HoB 6 has been discussed in connection with the cooperative Jahn-Teller effect due to the CEF ground state degenerated by the quadrupolar moment. The low temperature behavior of C 44 in Ce x La 1- x B 6 compounds is presented to show the Kondo effect on the quadrupolar moment of Γ 8 ground state. The anomaly of the bulk modulus C B in SmB 6 is related to the valence fluctuation state in the system.

Magnetic Phase Diagrams of Kondo Compounds Ce 0.75La 0.25B 6 and Ce 0.6La 0.4B 6

We measured the elastic and thermal properties to examine the competing effect between the intersite interactions and the Kondo coupling in Ce 0.75 La 0.25 B 6 and Ce 0.6 La 0.4 B 6 with the Γ 8 ground state. The huge softening of the transverse C 44 mode of both compounds at low temperatures indicates a remarkable fluctuation of the quadrupolar moments O y z , O z x and O x y with Γ 5 symmetry. In the magnetic phase diagram of Ce 0.75 La 0.25 B 6 , we found a new ordered phase IV in addition to the antiferroquadrupolar phase II and the antiferromagnetic phase III. The compound Ce 0.6 La 0.4 B 6 , however, shows the Fermi liquid state without the long-range order at low fields and transits into the ordered phase II or III in the magnetic field above 16 kOe.

Magnetic Phase Diagrams of the Dense Kondo Compounds CeB6 and Ce0.5La0.5B6

We have studied the magnetic phase diagrams of the dense Kondo compounds CeB 6 and Ce 0.5 La 0.5 B 6 by use of low temperature ultrasonic measurements. A transition associated with a quadrupolar reorientation has clearly been observed around 1 kOe in the antiferroquadrupolar phase II of CeB 6 . In the paramagnetic phase I of Ce 0.5 La 0.5 B 6 a crossover from the ordinaly Γ 8 ground state to the Kondo singlet state has been found. The field induced transition from the Kondo singlet state to the antiferromagnetic phase III or the phase II has been found at a critical field 17 kOe. The field angle dependence of the phase diagram of Ce 0.5 La 0.5 B 6 is also presented.

Trigonal Lattice Distortion and Ferro-Quadrupole Ordering in Phase IV of CexLa1-xB6 (x = 0.75 and 0.70)

The low-temperature lattice effects of the Kondo compounds Ce x La 1- x B 6 ( x = 0.75 and 0.70) with a Γ 8 ground state have been investigated by dilatometric measurements. In an ordered phase IV below T C = 1.6 K (1.4 K) for x = 0.75 (0.70), the lattice length along the [001] direction reveals an increase, while the length along the [111] direction shows an appreciable decrease. This lattice distortion is described in terms of the trigonal strain = = ≠0 with Γ 5 symmetry in addition to a volume expansion e B = e x x + e y y + e z z with full symmetry Γ 1 . The trigonal strain in phase IV indicates a ferro-quadrupole ordering of = = ≠0.

Observation of Low-Temperature Elastic Softening due to Vacancy in Crystalline Silicon

In challenging a direct observation of the vacancy in crystalline silicon, we have carried out low-temperature ultrasonic measurements down to 20 mK. The longitudinal elastic constants of non-doped and B-doped crystalline silicons, which were grown by a floating zone (FZ) method in commercial base, reveal the elastic softening proportional to the reciprocal temperature below 20 K. The applied magnetic fields turn the elastic softening of the B-doped FZ silicon to a temperature-independent behavior, while the fields up to 16 T do not affect the elastic softening of the non-doped FZ silicon. We present a plausible scenario for this result. Namely the vacancy with the non-magnetic charge state V 0 in the non-doped silicon and the magnetic V + in the B-doped silicon is responsible for the low-temperature softening of the shear elastic constants ( C 11 - C 12 )/2 and C 44 , which can be described in terms of the quadrupole susceptibility due to the Jahn–Teller effect.

Electron-Strain Interaction in ValenceFluctuation Compound SmB6

Temperature dependence of the elastic constants in the valence fluctuation compound of SmB 6 and the reference one of LaB 6 has been investigated by an ultrasonic method. The DC sputtering technique of a piezoelectric ZnO film on SmB 6 has been employed for a reliable ultrasonic measurement free from bond breaks. A remarkable softening of the bulk modulus C B and an anomaly of the C 44 mode in SmB 6 are responsible for the interaction between the elastic strain and the narrow 4f-band state with the energy gap of 160 K around the Fermi level.

Magnetic Field-Induced Insulator-Semimetal Transition in a Pyrochlore Nd2Ir2O7.

We investigate magnetotransport properties in a single crystal of pyrochore-type Nd2Ir2O7. The metallic conduction is observed on the antiferromagnetic domain walls of the all-in-all-out-type Ir 5d moment ordered insulating bulk state that can be finely controlled by an external magnetic field along [111]. On the other hand, an applied field along [001] induces the bulk phase transition from insulator to semimetal as a consequence of the field-induced modification of the Nd 4f and Ir 5d moment configurations. A theoretical calculation consistently describing the experimentally observed features suggests a variety of exotic topological states as functions of electron correlation and Ir 5d moment orders, which can be finely tuned by the choice of rare-earth ion and magnetic field, respectively.

Low-temperature properties of the dense Kondo system Ce0.5La0.5B6

Low-temperature specific heat and electric resistivity of the dense Kondo compound Ce 0.5 La 0.5 B 6 have been measured. The specific heat for H =0 shows a broad peak at around 0.9 K corresponding to the Kondo effect. No indication of magnetic or quadrupolar phase transition is observed down to 100 mK for H =0. Very large specific heat coefficients of γ=1.8 J/K 2 mol and β=2.9 J/K 4 mol for H =0 indicate the Kondo singlet ground state without long-range ordering.