Control of the responsivity and the detectivity of superconductive edge-transition YBa2Cu3O7-x bolometers through substrate properties

Cataloged from PDF version of article.The detectivity D* limits of YBa2Cu3O7-x bolometer on 0.05-cm-thick crystalline substrates are investigated, and a method to increase D* to greater than 10(9) (cm Hz(1/2))/W at a 20-mu m wavelength is proposed. Because the response increases proportionally with the bias current I-b, whereas the noise near T-c (the transition or critical temperature) of our MgO and SrTiO3 substrate samples does not, an increase in D* of these samples is obtained by an increase in I-b. Another limiting factor is the de thermal conductance G(0) of the device, which, although controlled by the substrate-holder thermal boundary resistance for our samples, can be changed by means of thinning the substrate to increase D*. The optimal amount of thinning depends on the substrate's thermal parameters and the radiation modulation frequency. D* in our samples is also found to follow the spectral-radiation absorption of the substrate material. (C) 1999 Optical Society of America

Analytic modeling of patterned high-Tc superconductive bolometers: Film and substrate interface effects

Superconducting film and substrate interface effects on the response of superconductive edge-transition bolometers are modeled with a one dimensional thermal model in closed form, for samples with large area patterns compared to the substrate thickness. The results from the model agree with experimental results on samples made of meander line patterned granular YBCO films on crystalline substrates, in both the magnitude and phase of the response versus modulation frequency up to about 100 KHz, the limit of the characterization setup. Using the fit of the calculated frequency response curves obtained from the model to the measured ones, values of the film-substrate and substrate-holder thermal boundary resistance, heat capacity of the superconducting film, and the thermal parameters of the substrate materials could be investigated. While the calculated magnitude and phase of the response of the SrTiO3 substrate samples obtained from the model is in a very good agreement with the measured values, the calculated response of the LaAlO3 and MgO substrate samples deviate slightly from the measured values at very low frequencies, increasing with an increase in the thermal conductivity of the substrate material. Using the fit of the calculated response to the measured values, film-substrate thermal boundary resistances in the range of 4.4* 10-3 to 4.4* 10-2 K-cm2-w-1 are obtained for different substrate materials. The effect of substrate optical absorption in the response of the samples is also investigated

A superconducting-nanowire 3-terminal electronic device

In existing superconducting electronic systems, Josephson junctions play a central role in processing and transmitting small-amplitude electrical signals. However, Josephson-junction-based devices have a number of limitations including: (1) sensitivity to magnetic fields, (2) limited gain, (3) inability to drive large impedances, and (4) difficulty in controlling the junction critical current (which depends sensitively on sub-Angstrom-scale thickness variation of the tunneling barrier). Here we present a nanowire-based superconducting electronic device, which we call the nanocryotron (nTron), that does not rely on Josephson junctions and can be patterned from a single thin film of superconducting material with conventional electron-beam lithography. The nTron is a 3-terminal, T-shaped planar device with a gain of ~20 that is capable of driving impedances of more than 100 k{\Omega}, and operates in typical ambient magnetic fields at temperatures of 4.2K. The device uses a localized, Joule-heated hotspot formed in the gate to modulate current flow in a perpendicular superconducting channel. We have characterized the nTron, matched it to a theoretical framework, and applied it both as a digital logic element in a half-adder circuit, and as a digital amplifier for superconducting nanowire single-photon detectors pulses. The nTron has immediate applications in classical and quantum communications, photon sensing and astronomy, and its performance characteristics make it compatible with existing superconducting technologies. Furthermore, because the hotspot effect occurs in all known superconductors, we expect the design to be extensible to other materials, providing a path to digital logic, switching, and amplification in high-temperature superconductors

Relaxation Dynamics of Photoinduced Changes in the Superfluid Weight of High-Tc Superconductors

In the transient state of d-wave superconductors, we investigate the temporal variation of photoinduced changes in the superfluid weight. We derive the formula that relates the nonlinear response function to the nonequilibrium distribution function. The latter qunatity is obtained by solving the kinetic equation with the electron-electron and the electron-phonon interaction included. By numerical calculations, a nonexponential decay is found at low temperatures in contrast to the usual exponential decay at high temperatures. The nonexponential decay originates from the nonmonotonous temporal variation of the nonequilibrium distribution function at low energies. The main physical process that causes this behavior is not the recombination of quasiparticles as previous phenomenological studies suggested, but the absorption of phonons.Comment: 18 pages, 12 figures; to be published in J. Phys. Soc. Jpn. Vol. 80, No.

Vortex avalanches and magnetic flux fragmentation in superconductors

We report results of numerical simulations of non isothermal dendritic flux penetration in type-II superconductors. We propose a generic mechanism of dynamic branching of a propagating hotspot of a flux flow/normal state triggered by a local heat pulse. The branching occurs when the flux hotspot reflects from inhomogeneities or the boundary on which magnetization currents either vanish, or change direction. Then the hotspot undergoes a cascade of successive splittings, giving rise to a dissipative dendritic-type flux structure. This dynamic state eventually cools down, turning into a frozen multi-filamentary pattern of magnetization currents.Comment: 4 pages, 4 figures, accepted to Phys. Rev. Let

Non-equilibrium Superconductivity and Quasiparticle Dynamics studied by Photo Induced Activation of Mm-Wave Absorption (PIAMA)

We present a study of non-equilibrium superconductivity in DyBa2Cu3O7-d using photo induced activation of mm-wave absorption (PIAMA). We monitor the time evolution of the thin film transmissivity at 5 cm-1 subject to pulsed infrared radiation. In addition to a positive bolometric signal we observe a second, faster, decay with a sign opposite to the bolometric signal for T>40 K. We attribute this to the unusual properties of quasi-particles residing near the nodes of an unconventional superconductor, resulting in a strong enhancement of the recombination time.Comment: 4 pages, REVTeX, Submitted to Phys. Rev. Letter

Calculation of magnetic anisotropy energy in SmCo5

SmCo5 is an important hard magnetic material, due to its large magnetic anisotropy energy (MAE). We have studied the magnetic properties of SmCo5 using density functional theory (DFT) calculations where the Sm f-bands, which are difficult to include in DFT calculations, have been treated within the LDA+U formalism. The large MAE comes mostly from the Sm f-shell anisotropy, stemming from an interplay between the crystal field and the spin-orbit coupling. We found that both are of similar strengths, unlike some other Sm compounds, leading to a partial quenching of the orbital moment (f-states cannot be described as either pure lattice harmonics or pure complex harmonics), an optimal situation for enhanced MAE. A smaller portion of the MAE can be associated with the Co-d band anisotropy, related to the peak in the density of states at the Fermi energy. Our result for the MAE of SmCo5, 21.6 meV/f.u., agrees reasonably with the experimental value of 13-16 meV/f.u., and the calculated magnetic moment (including the orbital component) of 9.4 mu_B agrees with the experimental value of 8.9 mu_B.Comment: Submitted to Phys. Rev.

Role of deep levels and interface states in the capacitance characteristics of all‐sputtered CuInSe2/CdS solar cell heterojunctions

All‐sputtered CuInSe2/CdS solar cellheterojunctions have been analyzed by means of capacitance‐frequency (C‐F) and capacitance‐bias voltage (C‐V) measurements. Depending on the CuInSe2 layer composition, two kinds of heterojunctions were analyzed: type 1 heterojunctions (based on stoichiometric or slightly In‐rich CuInSe2 layers) and type 2 heterojunctions (based on Cu‐rich CuInSe2 layers). In type 1 heterojunctions, a 80‐meV donor level has been found. Densities of interface states in the range 101 0–101 1 cm2 eV− 1 (type 1) and in the range 101 2–101 3 cm− 2 eV− 1 (type 2) have been deduced. On the other hand, doping concentrations of 1.6×101 6 cm− 3 for stoichiometric CuInSe2 (type 1 heterojunction) and 8×101 7 cm− 3 for the CdS (type 2 heterojunction) have been deduced from C‐Vmeasurements

NEMO oligomerization and its ubiquitin-binding properties

The IKK [IκB (inhibitory κB) kinase] complex is a key regulatory component of NF-κB (nuclear factor κB) activation and is responsible for mediating the degradation of IκB, thereby allowing nuclear translocation of NF-κB and transcription of target genes. NEMO (NF-κB essential modulator), the regulatory subunit of the IKK complex, plays a pivotal role in this process by integrating upstream signals, in particular the recognition of polyubiquitin chains, and relaying these to the activation of IKKα and IKKβ, the catalytic subunits of the IKK complex. The oligomeric state of NEMO is controversial and the mechanism by which it regulates activation of the IKK complex is poorly understood. Using a combination of hydrodynamic techniques we now show that apo-NEMO is a highly elongated, dimeric protein that is in weak equilibrium with a tetrameric assembly. Interaction with peptides derived from IKKβ disrupts formation of the tetrameric NEMO complex, indicating that interaction with IKKα and IKKβ and tetramerization are mutually exclusive. Furthermore, we show that NEMO binds to linear di-ubiquitin with a stoichiometry of one molecule of di-ubiquitin per NEMO dimer. This stoichiometry is preserved in a construct comprising the second coiled-coil region and the leucine zipper and in one that essentially spans the full-length protein. However, our data show that at high di-ubiquitin concentrations a second weaker binding site becomes apparent, implying that two different NEMO–di-ubiquitin complexes are formed during the IKK activation process. We propose that the role of these two complexes is to provide a threshold for activation, thereby ensuring sufficient specificity during NF-κB signalling

Gap-dependent quasiparticle dynamics and coherent acoustic phonons in parent iron pnictide CaFe2As2 across the spin density wave phase transition

We report ultrafast quasiparticle (QP) dynamics and coherent acoustic phonons in undoped CaFe_2As_2 iron pnictide single crystals exhibiting spin-density wave (SDW) and concurrent structural phase transition at temperature TSDW ~ 165 K using femtosecond time-resolved pump-probe spectroscopy. The contributions in transient differential reflectivity arising from exponentially decaying QP relaxation and oscillatory coherent acoustic phonon mode show large variations in the vicinity of T_SDW. From the temperature-dependence of the QP recombination dynamics in the SDW phase, we evaluate a BCS-like temperature dependent charge gap with its zero-temperature value of ~(1.6+/-0.2)k_BT_SDW, whereas, much above T_SDW, an electron-phonon coupling constant of ~0.13 has been estimated from the linear temperature-dependence of the QP relaxation time. The long-wavelength coherent acoustic phonons with typical time-period of ~100 ps have been analyzed in the light of propagating strain pulse model providing important results for the optical constants, sounds velocity and the elastic modulus of the crystal in the whole temperature range of 3 K to 300 K.Comment: Revised version (to appear as Full Paper in Journal of Physical Society of Japan (2013)); http://jpsj.ipap.jp/link?JPSJ/82/044715