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We present the first experimental evidence for metallicity, superconductivity (SC) and the co-existence of charge density waves (CDW) in the quasi-one-dimensional material HfTe3. The existence of such phenomena is a typical characteristic of the transition metal chalcogenides however, without the application of hydrostatic pressure/chemical doping, it is rare for a material to exhibit the co-existence of both states. Materials such as HfTe3 can therefore provide us with a unique insight into the relationship between these multiple ordered states. By improving on the original synthesis conditions, we have successfully synthesised single phase HfTe3 and confirmed the resultant structure by performing Rietveld refinement. Using low temperature resistivity measurements, we provide the first experimental evidence of SC at ~1.4 K as well as a resistive anomaly indicative of a CDW formation at ~82 K. By the application of hydrostatic-pressure, the resistivity anomaly shifts to higher temperature. The results show that HfTe3 is a promising new material to help study the relationship between SC and CDW., Comment: 9 pages, 4 figures, includes supplementary information

Superconductor-based light-emitting diode (superconductor-based LED) in strong light-confinement regime are characterized as a superconductor-based three-terminal device, and its transport properties are quantitatively investigated. In the gate-controlled region, we confirm the realization of new-type Josephson field effect transistor (JoFET) performance, where the channel cross-sectional area of the junction is directly modulated by the gate voltage. In the current-injected region, the superconducting critical current of $\mu$A order in the Josephson junction is found to be modulated by the steady current injection of pA order. This ultrahigh monitoring sensitivity of the radiative recombination process can be explained by taking into account the fact that the energy relaxation of the absorbed photons causes the conversion of superconducting pairs to quasiparticles in the active layer. Using quasiparticle density and superconducting pair density, we discuss the carrier flows together with the non-equilibrium superconductovity in the active layer and the superconducting electrodes, which take place for compensating the conversion.

Transport properties of a superconductor-semiconductor-superconductor (S-Sm-S) junction with superlattice structure are investigated. Differential resistance as a function of voltage shows oscillatory behavior under the irradiation of radio-frequency (RF) waves with the specific frequency of 1.77 GHz regardless of the superconducting materials and the junction lengths. Experimental data are quantitatively explained in terms of the coupling of superconducting quasiparticles with long-wavelength acoustic phonons indirectly excited by the RF waves. We propose that the strong coupling causes the formation of novel composite particles, Andreev polarons., Comment: 4 pages, 4 figures

This paper theoretically discusses the photon emission spectra of a superconducting pn-junction. On the basis of the second order perturbation theory for electron-photon interaction, we show that the recombination of a Cooper with two p-type carriers causes drastic enhancement of the luminescence intensity. The calculated results of photon emission spectra explain characteristic features of observed signal in an recent experiment. Our results indicate high functionalities of superconducting light-emitting devices., Comment: 4 pages, 3 figures embedded

We analyzed the Josephson bifurcation amplifier (JBA) readout process of a superconducting qubit quantum mechanically. This was achieved by employing numerical analyses of the dynamics of the density operator of a driven nonlinear oscillator and a qubit coupled system during the measurement process. In purely quantum cases, the wavefunction of the JBA is trapped in a quasienergy-state, and bifurcation is impossible. Introducing decoherence enables us to reproduce the bifurcation with a finite hysteresis. Moreover, we discuss in detail the dynamics involved when a qubit is initially in a superposition state. We have observed the qubit-probe (JBA) entangled state and it is divided into two separable states at the moment of the JBA transition begins. This corresponds to "projection". To readout the measurement result, however, we must wait until the two JBA states are macroscopically well separated. The waiting time is determined by the strength of the decoherence in the JBA., Comment: 4 pages, 5 figures

The understanding of the zero bias conductance peak (ZBCP) in the tunnelling spectra of S/N junctions involving d-wave cuprate superconductors has been important in the determination of the phase structure of the superconducting order parameter. In this context, the involvement of a p-wave superconductor such as Sr2RuO4 in tunnelling studies is indeed of great importance. We have recently succeeded in fabricating devices that enable S/N junctions forming at interfaces between Sr2RuO4 and Ru micro-inclusions in eutectic crystals to be investigated.3 We have observed a ZBCP and have interpreted it as due to the Andreev bound state, commonly seen in unconventional superconductors. Also we have proposed that the onset of the ZBCP may be used to delineate the phase boundary for the onset of a time reversal symmetry broken (TRSB) state within the superconducting state, which does not always coincide with the onset of the superconducting state. However, these measurements always involved two interfaces between Sr2RuO4 and Ru. In the present study, we have extended the previous measurements to obtain a deeper insight into the properties of a single interface between Sr2RuO4 and Ru., Comment: To appear in J. Phys. Soc. Jpn. Vol. 75 No.12 issue

The quantum state of a flux qubit was successfully pulse-controlled by using a resonant microwave. We observed Ramsey fringes by applying a pair of phase-shifted pi/2 microwave pulses without introducing detuning. With this method, the qubit state can be rotated on an arbitrary axis in the x-y plane of the Bloch sphere in a rotating frame. We obtained a qubit signal from a coherent oscillation with an angular velocity of up to 2pi*11.4 Grad/s. In combination with Rabi pulses, this method enables us to achieve full control of single qubit operation. It also offers the possibility of orders of magnitude increases in the speed of the arbitrary unitary gate operation., Comment: 3 pages, 3 figures

We study theoretically how decoherence affects superposition states composed of entangled states in inductively coupled two superconducting flux-qubits. We discover that the quantum fluctuation of an observable in a coupled flux-qubit system plays a crucial role in decoherence when the expectation value of the observable is zero. This examplifies that decoherence can be also induced through a quantum mechanically higher-order effect. We also find that there exists a decoherence free subspace for the environment coupled via a charge degree of freedom of the qubit system., Comment: 4 pages, 5 figures

Single-shot readout experiments were performed on the two lowest-energy states of a superconducting qubit with three Josephson junctions embedded in a superconducting loop. We measured the qubit state via switching current Isw of a current-biased dc-SQUID, a quantum detector surrounding the qubit loop. The qubit signals were measured in a small Isw regime of the SQUID, typically less than 100 nA, where the Isw distribution is particularly narrow. The obtained single-shot data indicate that the qubit state is readout, through the flux generated by the qubit persistent-current, as energy eigenstates rather than current eigenstates., Comment: 5 pages, 6 figures

Based on calculations using the local density approximation, we propose quantum wire networks with square and plaquette type lattice structures that form quantum dot superlattices. These artificial structures are well described by the Hubbard model. Numerical analysis reveals a superconducting ground state with transition temperatures $T_c$ of up to 90 mK for the plaquette, which is more than double the value of 40 mK for the square lattice type and is sufficiently high to allow for the experimental observation of superconductivity., Comment: 10 pages, 4 figures

Possibility of flat-band ferromagnetism in quantum dot arrays is theoretically discussed. By using a quantum dot as a building block, quantum dot superlattices are possible. We consider dot arrays on Lieb and kagome lattices known to exhibit flat band ferromagnetism. By performing an exact diagonalization of the Hubbard Hamiltonian, we calculate the energy difference between the ferromagnetic ground state and the paramagnetic excited state, and discuss the stability of the ferromagnetism against the second nearest neighbor transfer. We calculate the dot-size dependence of the energy difference in a dot model and estimate the transition temperature of the ferromagnetic-paramagnetic transition which is found to be accessible within the present fabrication technology. We point out advantages of semiconductor ferromagnets and suggest other interesting possibilities of electronic properties in quantum dot superlattices., Comment: 15 pages, 7 figures (low resolution). High-resolution figures are available at http://www.brl.ntt.co.jp/people/tamura/Research/PublicationPapers.html

Magnetic field effects on single-particle energy bands (Hofstadter butterfly), Hall conductance, flat-band ferromagnetism, and magnetoresistance of two-dimensional Kagome lattices are studied. The flat-band ferromagnetism is shown to be broken as the flat-band has finite dispersion in the magnetic field. A metal-insulator transition induced by the magnetic field (giant negative magnetoresistance) is predicted. In the half-filled flat band, the ferromagnetic-paramagnetic transition and the metal-insulator one occur simultaneously at a magnetic field for strongly interacting electrons. All of the important magnetic fields effects should be observable in mesoscopic systems such as quantum dot superlattices., Comment: 10 pages, 4 figures, and 1 table

We present the theoretical design of quantum dot (QD) artificial ferromagnetic crystals. The electronic structure calculations based on local spin density approximation (LSDA) show that our designed QD artificial crystal from a structure comprising the crossing 0.104-micrometer-wide InAs quantum wires (an effective Kagome lattice) has flat band characteristics. Our examined QD artificial crystal has the ferromagnetic ground state when the flat band is half-filled, even though it contains no magnetic elements. The ferromagnetic and the paramagnetic state can be freely switched by changing the electron filling via a gate voltage., Comment: 12 pages, 3 figures included. For related information, visit http://www.brl.ntt.co.jp/people/tamura/