Low Magnetic Field Microwave Absorption Research Articles

Comparison of Pr-doped Ca 122 and Ca 112 Pnictides by Low-field Microwave Absorption Spectroscopy

ABSTRACTPraseodymium doped CaFe2As2 (122 structure) and CaFeAs2 (112 structure) are characterized by modulated Low Magnetic Field Microwave Absorption (LFMA) spectroscopy. In both (Pr,Ca)122 and (Pr,Ca)112 structures, a strong hysteretic LFMA is found, with a TcH of ∼30 K and ∼26 K, respectively. However, in (Pr,Ca)122, measurements also show an unusual Narrow Peak (NP) LFMA signal appearing at higher temperatures, above the lower TcH superconducting state until a TcNP of 49 K. We associate this NP LFMA with interfacial superconductivity, which has been found previously by highly anisotropic magnetization measurements. Furthermore, the absence of NP in (Pr,Ca)112 correlates with the absence of an interfacial phase. These results give useful information about the microwave signature of interfacial superconductivity present in the (Pr,Ca)122 system, and may form a roadmap towards a stabilized high temperature superconducting phase in pnictides.

New superconducting sodium-nitrogen-C 60 ternary compound

We have prepared sodium-doped C 60 utilizing the thermal decomposition of sodium azide (NaN 3). The strong low-magnetic field microwave absorption with a clear hysteresis was observed in the samples of Na nC 60 ( n=3∼4), indicating the superconducting nature. By means of a SQUID measurement, the superconducting transition temperature is found to be around 12 K. Significantly, elemental analysis detected a nitrogen which was surely produced in the process of the thermal decomposition of NaN 3. We suppose that this new superconducting phase will be Na xN yC 60 where x is between 3 and 4, and y is unknown. This is the first superconducting material in Na-doped C 60 without other alkali metals.

Superconductivity in Langmuir-Blodgett multilayers of fullerene (C60) doped with potassium

We report here on the first observation of superconductivity at and below 8.1 K, as evidenced by a low magnetic field microwave absorption signal, or low-field signal (LFS), in a potassium-doped Langmuir-Blodgett (LB) 50-layer film of buckminster-fullerence, C[sub 60] on a poly(ethylene terephthalate) substrate. We address the obvious issue, which is whether LB multilayers can be made to be superconducting, in line with the result from doping of bulk C[sub 60] with potassium, rubidium, or cesium. This is of interest, because, although conducting Langmuir-Blodgett films of partially ionic salts of amphiphilic molecules have been known for some time, no superconductivity has been reported as yet in LB films. Since the C[sub 60] LB films are not perfect, and the films must have many defects, it is not surprising that we observed a lower T[sub c] than in the bulk K[sub 3]C[sub 60]. To approach the bulk T[sub c] value in LB films, there is need for further processing improvements in the compression of the Pockels-Langmuir film, the transfer mode, the aging of the undoped films, and the K-doping temperatures and times. At present we have no estimate of the superconducting fraction of the sample. Nevertheless, this is to our knowledgemore » the first time that a superconducting Langmuir-Blodgett film has been observed. 20 refs., 2 figs.« less

Measurements of low magnetic field microwave absorption in 110-K superconducting Bi-Sr-Ca-Cu-O thin films

Studies on magnetic-field dependent microwave absorption in Bi(Pb)-Sr-Ca-Cu-O thin films superconducting above 100 K are reported. The films were DC magnetron sputtered on MgO and CaNdAlO/sub 4/ single crystals from Pb-rich targets and post-annealed at high temperatures in air for less than 1 h. The measurements were performed as a function of temperature, DC magnetic field, and the film magnetic history. The results show that the microwave absorption measurements are a very sensitive tool for high-T/sub c/ thin-film characterization, since even small variations in the film phase homogeneity, granularity, or crystalline orientation are clearly visible on the modulated absorption curves. It was found that the films characterized by the lowest absorption exhibited the highest critical current densities.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

ESR and low-field microwave absorption studies of potassium doped C 70: observation of possible metallic state in K xC 70

The magnetic property of K xC 70 upon K doping process was investigated by electron spin resonance (ESR) and low-magnetic field microwave absorption (low-field signal: LFS). In the early stage of K doping (doping time t d < 40 h), ESR signal with a Curie-like behavior was observed, which means an insulating state of a doped phase in this stage. In t d > 40 h, ESR spectra broadened suddenly and the almost temperature-independent behavior of ESR intensity appeared in the high temperature region. This suggests a change from an insulating K xC 70 phase to a metallic K xC 70 phase. The chemical analysis has presented that the insulating phase corresponds to K 3C 70 and the metallic phase corresponds to K 4C 70. A very weak LFS below ca. 9 K was observed in the doping stage of t d ≥ 10 h, indicating the coexistence of a small volume of some superconducting phase. A possible origin of the superconducting phase is described.

Low magnetic field microwave absorption studies on ultra-thin superconducting BiSrCaCuO films

Non-resonant microwave absorption (NRMA) has been studied on a 350Å thick BiSrCaCuO film fabricated on an MgO substrate by MOCVD. The lower critical magnetic field H c1, absorption intensity, and the average micro-Josephson loop size have been obtained as a function of temperature. The absorption intensity reaches a maximum at around 60K, but progressively decreases below about 55K. The results for the temperature range from 50K to about 60K suggest the distinct kinds of Josephson loops in the sample.

Absolute measurements of the low magnetic field microwave absorption in YBa 2Cu 3O 7−δ ceramics

Absolute measurements of the magnetic-field-dependent nonresonant microwave absorption have been made in polycrystalline YBa 2Cu 3O 7−δ bulk samples. In low magnetic fields of several Oe the surface resistance at 9.4 GHz has the form of R= R 0+ βH 2 where β = (0.6−2) × 10 −3Ω/Oe 2 at 4.2 K and (3−8)Ω/Oe 2 at 77 K. These values agree rather well with calculations within the model of the effective Josephson medium.

Magnetic flux trapping in hollow superconductors: possible device application for optical signal detection

A hollow superconductor in its critical state with magnetic flux trapped in the hole is proposed to be used for optical signal detection and storage in a wide spectral range. The sensitivity arises from the temperature dependence of the critical magnetic field in type I superconductors or that of the volume pinning force in hard type II superconductors. With the use of the effect of low magnetic field microwave absorption we have demonstrated the flux trapping in a thin hollow disk which could serve as a basic element of planar structures for the detection of two-dimensional optical information.

Low-field microwave absorption and quantum oscillations in type-I superconductors

The effects of low-magnetic field microwave absorption (LFMA) and quantum oscillations (QO) discovered earlier in high-temperature superconductors can also be seen in conventional type-I superconductors such as lead, tin, indium and mercury. We have found that both these effects arise from surface irregularity of a sample. LFMA appears when an oxide layer of a few hundred Ångström; covers the surface and QO are observed in irregular samples with sharp boundaries.

Properties of the microwave absorption in high-temperature superconductor Y-Ba-Cu-O

It is shown that the measurement of the properties of the low-magnetic field microwave absorption using an EPR spectrometer needs to be corrected for instrumental effects, which can be accomplished by using a low-field calibration resonance from Cr3+ in ruby. The results show the existence of a hysteresis in intensity and field position, but the latter is somewhat smaller than previously reported. The temperature dependence of the intensity of the absorption is accounted for in terms of the temperature dependence of the current in a Josephson junction assuming a mean field temperature dependence for the superconducting gap.

EPR and low magnetic field microwave absorption in hafnium doped YBa2Cu3Oy

The hafnium doped Y-Ba-Cu-O system is characterised by resistivity, EPR and low magnetic field microwave absorption signal (LFMAS) studies for doping concentrations varying from 0.5% to 2%. It is found that Hf doping improves the mechanical strength of the material without changing much of its superconducting properties at least up to 2% of doping. From LFMAS studies, it is observed that the superconducting properties of these ceramics are of intergranular nature for fields ⩽ Hm (where LFMAS is maximum) and of intragranular character for fields >Hm.