We have developed a Josephson parametric amplifier, comprising a superconducting coplanar waveguide resonator terminated by a dc SQUID (superconducting quantum interference device). An external field (the pump, ∼ 20 GHz) modulates the flux threading the dc SQUID, and, thereby, the resonant frequency of the cavity field (the signal, ∼ 10 GHz), which leads to parametric signal amplification. We operated the amplifier at different band centers, and observed amplification (17 dB at maximum) and deamplification depending on the relative phase between the pump and the signal. The noise temperature is estimated to be less than 0.87 K.Degenerate parametric amplifiers are phase sensitive amplifiers, which can in principle amplify one of the two quadratures of a signal without introducing extra noise. 1,2 Parametric amplifiers based on the nonlinear inductance of a Josephson junction have been studied for a long time, 3 including one which demonstrated vacuum noise squeezing. 4 Recently, there has been a renewed interest in parametric amplifiers 5,6,7 due in part to the increasing need for quantum-limited amplification in the field of quantum information processing using superconducting circuits. 8,9 In the present work, we design a Josephson parametric amplifier, comprising a superconducting transmissionline resonator terminated by a dc SQUID (superconducting quantum interference device). Contrary to the previous works, the pump is not used to directly modulate a current through the Josephson junction, but is instead used to modulate a flux through the dc SQUID. 10 The resonant frequency of the resonator, namely, the band center of the signal, is widely controllable by a dc flux also applied to the SQUID (see also Ref. [7]). Moreover, as the pump and the signal are applied to different ports and their frequencies are twice different (see below), it is straightforward to separate the output signal from the pump. This is a unique property of the fluxpumping scheme; it arises because the finite dc flux bias allows linear coupling of the pump even in the absence of a dc current bias across the SQUID. 11 We operated such a flux-driven parametric amplifier and characterized its basic properties. Figure 1a shows a schematic diagram of the flux-driven parametric amplifier. The primary component of the amplifier is a transmission-line resonator defined by its coupling capacitance C c and a dc SQUID termination. The magnetic flux Φ penetrating the SQUID loop changes the boundary condition of the resonator at the right end (by the change of the Josephson inductance), and hence enables us to control the resonant frequency. 12,13 The resonant frequency f 0 for the first mode (λ/4 ≥ d, where λ is the wavelength and d is the cavity length) is schematically drawn as a function of Φ/Φ 0 in Fig. 1b, where Φ 0 is a flux quantum (see also Fig. 2a). We now assume the cavity resonance is set to a particular value, f 0dc , by applying a dc flux Φ dc (open circle in the figure). We then apply microwaves at a frequency 2f 0dc to the pump line wh...

Osmotic swelling and exfoliation behaviors in a lepidocrocite-type titanate H 1.07 Ti 1.73 O 4 •H 2 O were investigated upon reactions with tetramethylammonium (TMA + ) and tetrabutylammonium (TBA + ) cations. The reaction products in various physical states (suspension, wet aggregate, and deposited nanosheets) were characterized by several techniques, including X-ray diffraction under controlled humidity, small-angle X-ray scattering, particle size analysis, and atomic force microscopy. As the ratio of tetraalkylammonium ion in a solution to exchangeable proton in a solid decreased, the predominant product changed from the osmotically swollen phase, having an interlayer spacing d of several tens of nanometers, to the exfoliated nanosheets. The different behaviors of two cations in the osmotic swelling were evident from the slope and the transition point in the d versus C −1/2 plot, where C is the concentration of the cations. At a short reaction time, crystallites of a few stacks were obtained as a major product in the reaction with TMA + . On the other hand, a mixture of those crystallites and a significant portion of unilamellar nanosheets were obtained in the reaction with TBA + . In both cases, those stacks were ultimately thinned down at long reaction time to unilamellar nanosheets. The lateral size of the nanosheets could be controlled, depending on the type of the cations, the tetraalkylammonium-to-proton ratios, and the mode of the reaction (manual versus mechanical shaking). The nanosheets produced by TMA + had large lateral sizes up to tens of micrometers, and the suspension showed a distinctive silky appearance based on liquid crystallinity. Our work provides insights into the fundamentals of osmotic swelling and exfoliation, allowing a better understanding of the preparation of nanosheets, which are one of the most important building blocks in nanoarchitectonics.

A new technique has been developed to easily and rapidly arrange molecularly thin 2D materials edge by edge on various substrates.

The vortex lattices in YNi2B2C under the magnetic fields H up to 3 T applied along both the a and the c axes have been studied by scanning tunneling spectroscopy at 4.2 K. The vortex lattice transition has been found to occur in different manners for H parallela and H parallelc; in H parallela a slightly distorted hexagonal vortex lattice has been found to transform to a nearly square one above 1.0 T with increasing H, while in H parallelc the transition occurs at a much lower field around 0.1 T. The unconventional steep increase of the quasiparticle density of states outside the vortex core has also been found well below H(c2).

We studied electronic states in vortex cores of slightly overdoped Bi 2 Sr 2 CaCu 2 O x by scanning tunneling spectroscopy. We have found that they have stripe structures with a 4a 0 width extending along the Cu-O bond directions. Vortex core states are observed as two peaks at particle-hole symmetric positions in the energy gap. Along a stripe, the peak positions of vortex core states are constant and not influenced by the spatial variation of the energy gap Á. Outer stripes have a larger energy than inner stripes. A mazelike pattern in the electronic states at E ¼ AEÁ has been observed all over the surface both inside and outside the vortex core. The orientation of stripes of vortex core states was found to be related to the mazelike pattern in the vortex core region. A short-range order of the mazelike pattern spatially coexists with the superconductivity and locally breaks the symmetry of the two Cu-O bond directions. We propose that the vortex core bound states are formed by Bogoliubov quasiparticles owing to the depairing of Cooper pairs and have a local C 2 symmetry influenced by the short-range order of the mazelike pattern.KEYWORDS: Bi2212, high-T c superconductivity, vortex core, inhomogeneity, STM, STSIn vortex cores of type-II superconductors, the superconducting order parameter (pair potential) is suppressed in amplitude. Bogoliubov quasiparticles are confined in the vortex core to form Andreev bound states, reflecting the symmetry of the superconducting order parameter and the shape of the Fermi surface. Such vortex core bound states have been observed in the conventional s-wave superconductors NbSe 2 , 1) YNi 2 B 2 C, 2,3) and NbS 2 , 4) and recently in iron pnictides, 5-7) by scanning tunneling spectroscopy (STS). In d-wave superconductors, theoretical calculations based on the Bardeen-Cooper-Schrieffer theory predict that the vortex core bound states have a fourfold star-shaped spatial distribution extending in the nodal directions with a zero-bias conductance peak at the center of the vortex core. 8-10) However, STS experiments on the high-T c cuprate superconductor Bi 2 Sr 2 CaCu 2 O x (Bi2212) showed that the vortex core has characteristic states (vortex core states) at finite excitation energies E ¼ AE", and different values of " ranging from 7 to 16 meV have been reported by different research groups. 11-13) Vortex core states exhibit spatial modulations in the two Cu-O bond directions (anti-nodal directions) with a period of about 4a 0 , where a 0 is the Cu-O-Cu bond length (0.38 nm). 14,15) Our recent study has shown that the modulation of the electron-like state and that of the hole-like state are in antiphase with each other, and detected that the modulations are commensurate (4a 0 ) in one Cu-O bond direction and incommensurate (4:3a 0 ) in the other direction. 16) The latter means that the vortex core states locally have the C 2 symmetry instead of the C 4 symmetry of the CuO 2 unit cell. Many theoretical models [17][18][19][20][21][22][23][24][25][26][27][28] have been proposed to ex...

In the vortex core of slightly overdoped Bi 2 Sr 2 CaCu 2 O x , the electron-and hole-like states have been found to exhibit spatial modulations in anti-phase with each other along the Cu-O bonding direction. Some kind of one-dimensionality has been observed in the vortex core, and it is more clearly seen in differential conductance maps at lower biases below AE9 mV.

Quantized bound states at a vortex core are discretized in YNi2B2C. By using scanning tunneling spectroscopy with an unprecedented 0.1 nm spatial resolution, we find and identify the localized spectral structure, where in addition to the first main peak with a positive low energy, a second subpeak coming from the fourfold symmetric gap structure is seen inside the energy gap. Those spectral features are understood by solving the Bogoliubov-de Gennes equation for a fully three-dimensional gap structure. A particle-hole asymmetric spectrum at the core site and quantum oscillation in the spectra are clearly observed.

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