Applied Superconductivity最新文献

The ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrathin (3.5 nm thick) NbN films deposited on sapphire by reactive magnetron sputtering into either a 5×10 μm² microbridge or 25 1 μm wide, 5 μm long strips connected in parallel. Both structures were placed at the center of a 4 mm long coplanar waveguide covered with Ti/Au. The photoresponse was studied at temperatures ranging from 2.15 K to 10 K, with the samples biased in the resistive (switched) state and illuminated with 100 fs wide laser pulses at 395 nm wavelength. At T=2.15 K, we obtained an approximately 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electron heating effect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-speed response of NbN devices makes them an excellent choice for an optoelectronic interface for superconducting digital circuits, as well as mixers for the terahertz regime. The multiple-strip detector showed a linear dependence on input optical power and a responsivity $\text{R}$=3.9 V/W.

High-T_c SQUID was applied for the detection of magnetized fine iron particles in motion. Two types of high-T_c SQUIDs with different magnetic field resolution were used. One is a large washer type and the other is a flux transformer type. It was shown that the magnetic field measured by the SQUID is proportional to the third power of diameter and is inversely proportional to the distance between the SQUID and the particle, as given by the electro-magnetic formula. As a result of the high magnetic field resolution and the quick response to transients, the SQUID detected an iron particle with 50 μm diameter at 800 m/min. This technique with high sensitive SQUID could be applicable to the in-line detection of iron particles inclusion in wires and fibers, as well as in food and medicine. This will contribute to an increased production yield and cope with the production liability.

We provide a detailed analysis of our new experimental method for the size estimate of the hotspot induced by ionizing particles in a Josephson tunnel junction. This method is based on the effect of ionizing particles on misaligned Abrikosov vortices, which are trapped in the junction by a field-cooling process. The effect of the radiation is the change of the misalignment parameter of a vortex. This, in turn, induces a change of the Josephson critical current, which can be used for an hotspot size estimate.

We measure the current-voltage (I–V) characteristics of the Bi₂Sr₂CaCu₂O_8+δ intrinsic Josephson junctions in the magnetic field parallel to the c-axis. In the liquid phase of vortices, the I–V curve reveals three stages: ohmic relation at low currents, rapid resistive transition and a quasiparticle branch. If the maximum Josephson current I_c is defined at the transition, its field dependence is in quantitative agreement with the recent measurements on the Josephson plasma frequency ω_p. This leads to the conclusion that both I_c and ω_p equivalently give the information for the interlayer phase coherence. In spite of the finite Josephson current, there appears Lorentz-force-independent dissipation at small currents, indicating that the interlayer critical current is zero in the liquid phase. We discuss the applicability of the phase-slip mechanism associated with the in-plane motion of the vortex pancakes.

A novel model is proposed to allow the exploration of the short-time (transient) dynamics of a superconductor–insulator multilayer with Josephson coupling between the layers. This model treats the charge on the layer interface planes as a dynamic variable, whose evolution is determined via the interlayer charge-current continuity equations. The high frequency current responses of both the superconducting and the insulating layers are included in the proper time domain form, derived for the general case of nonuniform layers from standard BCS theory. We present the model equations for the response of the system to an initial charge distribution, with the focus of determining the Josephson switching properties of an overdamped, nonuniform multilayer. Such structures may have potential applications in superconducting flux quantum electronics.

We have successfully fabricated a small-sized thin stack with a small number of intrinsic junctions on (Bi,Pb)₂Sr₂Ca₂Cu₃O_10+x thin films. Mesa structures with the junctions are fabricated on the surface of high-quality films prepared by rf-sputtering and subsequent heat treatment. I–V characteristics along the c-axis for the thinnest stack show a 6-branch structure which corresponds to six tunnel junction arrays and the edge structure which represents the superconductive gap. The estimated superconductive gap values strongly depend on the number of junctions in the fabricated stack. For the thin stacks with 6-junctions, their gap values are about 65 mV at 4.2 K, which is two or three times as large as those previously reported for intrinsic junctions.

We have studied the electrometric characteristics of the Bloch transistor, i.e. the structure comprising a double small-capacitance superconductor junction (where both the Josephson and the charging energies, E_J1,2∽E_C1,2≫k_BT) and an adjacent gate electrode polarizing its island. The transistor bias is realized through small-sized high-ohmic resistors (≫R_Q=h/4e²≈6.5 kΩ) to ensure a high electromagnetic impedance of the transistor environment. At low bias current, a coherent flow of single Cooper pairs occurs and the average voltage across the transistor shows a 2e-periodic dependence on the polarization charge Q₀. The sensitivity of such an electrometer has been evaluated and found to be comparable to that of the single-electron counterpart. The device has been fabricated with Al/AlO_x/Al junctions and two miniature on-chip Cr resistors (each of 80 kΩ and 10 μm long) which were located very close to the junctions. We used this device to measure 1/f noise of the background charge and found it to be about 9×10⁻⁴ e/Hz^1/2 at 10 Hz.

Compared to Stirling or Gifford-Mac Mahon crycoolers, the pulse tube refrigerator (PTR) has the advantage of having no moving parts in the cold stage. This results in lower levels of vibration, which is more suitable for the design of highly sensitive superconducting magnetometers. We describe a PTR system in which the compressor is connected to a cold part with a semi-flexible one meter long tube, under an average helium gas pressure of 3 MPa. Using a frequency of 30 Hz and a high to low pressure ratio of 1.5, the lowest temperature achieved is 56 K. The cooling power is 200 mW at 77 K with a cooling down time of about of 45 min. A bare YBaCuO dc SQUID has been successfully operated at a temperature close to 77 K. The SQUID was mounted onto a copper block, attached to the cold head of the PTR.

We present a grain microstructure for Bi(2212) consisting of only giant needle-shaped grains of around 1.5 mm length and 100 μm diameter. We study the structural and chemical changes suffered by a conventional ceramic Bi(2212) sample in the course of the thermal treatment used to obtain those giant needle-shaped grains. For that, different samples of the same batch were treated with incomplete thermal treatments, and the resulting samples were analysed by using scanning electron microscopy (SEM), optical microscopy, energy dispersed spectroscopy (EDS), inductively coupled plasma (ICP) and X-ray diffraction (XRD). To verify the superconducting nature of the needle-shaped grains, we have performed magnetization, resistivity, and critical current measurements on the original ceramic sample, and on that formed as giant needle-like grains. The critical temperature of these last grains is nearly the same as that of the ceramic sample (T_c∼90 K), which is a high value for the Bi(2212) compound. The critical current density (J_c) of the needle-shaped grains is around 2500 A/cm² at 77 K and in absence of applied magnetic field, a value comparable with that presented for the best wires and thick films. Not only are the shape and the size of these grains very suitable for making superconducting wires, but also the superconducting properties, T_c and J_c, are both high enough to be confident about the possibility of improving the actual Bi(2212) superconducting wires for high current applications.

A new design concept of the axisymmetric magnet system generating the very high pulsed magnetic field which is superimposed on the bias magnetic field of the superconducting magnet is presented. The pulsed magnet consists of two coaxial coils which are wound in opposite directions. The geometry of both pulsed coils, i.e. the working (inner) one and the compensating (outer) one is designed in such a way that the mutual coupling between the small pulsed magnet and the outer superconducting magnet is practically zero. This configuration prevents the rise of the high induced voltage on the current leads of the superconducting magnet when the pulsed magnet is being energised, hence resulting naturally in protection of the system (superconducting magnet and the current source) against possible damage. Further, it is predicted that the stray field of the pulsed magnet, which gives rise e.g. to the eddy currents in the winding of the superconducting magnet, is considerably decreased. The simple theory enabling the design of the geometry of the compensating pulsed coil is derived. The advantages of this new concept are demonstrated on the results of the theoretical analysis using, as an example, one of the pulsed coils that were designed and fabricated in the Clarendon Laboratory, in connection with the Oxford Instrument superconducting magnet (Clarendon hybrid outer) which can generate a steady magnetic field up to 10 T in a room temperature working space with a diameter of 240 mm.