Superconducting Magnesium Diboride Research Articles

Synthesis of MgB2 films on Hastelloy-C276 tape with Al2O3/Y2O3/MgO/LaMnO3 buffer layers by magnetron sputtering in co-evaporation mode

This study presents the initial results of developing a technology for synthesizing a flexible superconducting magnesium diboride composite on a Hastelloy®-C276TM substrate coated with Al2O3/Y2O3/MgO/LaMnO3 buffer layers. The superconducting composite was deposited by magnetron sputtering from two Mg and B targets, followed by vacuum annealing at various substrate temperatures ranging from 400 °C to 700 °C. The superconducting transition temperature T c ≈ 22 K, with a transition width ΔT ≈ 1 K, and critical current J c ≈ 500 kA cm−2 (T= 5 K, H = 2 T) and J c ≈ 11 kA cm−2 (T = 15 K, H = 2 T) at the optimal annealing temperature of 400 °C.

Effect of nano-Ag addition on the superconducting properties of MWCNT doped magnesium diboride superconductor

Bulk MgB2 superconductor added with nano silver (Ag), multiwalled carbon nanotube (MWCNT) and both the added (i.e., nano-Ag and MWCNT) samples were synthesized. The influence of nano-Ag and MWCNT addition on the structural and superconducting properties were studied. The XRD analysis confirms the formation of hexagonal MgB2 superconductor. Microstructural studies of all the synthesized samples are carried out by field emission scanning electron microscope (FE-SEM) and transmission electron microscope (TEM). The superconducting properties such as critical temperature (Tc), magnetic critical current density (Jc), irreversibility field (Birr) and flux pinning properties of all the synthesized samples were investigated at 10 K and field up to 7 T. Nano Ag added MWCNT doped MgB2 shows maximum enhancement in the critical current density i.e., 5.83 × 104 A/cm2 at 5 T and 10 K.

Features of the Transport Properties of Superconducting Magnesium Diboride Samples

Features of the Transport Properties of Superconducting Magnesium Diboride Samples

Theoretical study of I-V characteristics in a coupled long Josephson junctions based on magnesium diboride superconductor

In the present work, the current-voltage (I-V) characteristics in a coupled long Josephson junction based on magnesium diboride are studied by establishing a system of equations of phase differences of various inter- and intra-band channels starting from the microscopic Hamiltonian of the junction system and simplifying it through the phenomenological procedures such as action, partition function, Hubbard-Stratonovich transformation (bosonization), Grassmann integral, saddle-point approximation, Goldstone mode, phase dependent effective Lagrangian and, finally, Euler-Lagrange equation of motion. The system of equations are solved using finite difference approximation for which the solution of unperturbed sine-Gordon equation is taken as the initial condition. Neumann boundary condition is maintained at both the ends so that the fluxon is capable of reflecting from the end of the system. The phase dependent current is calculated for different tunnel voltage and averaged out over space and time. The current-voltage characteristics are almost linear at low voltage and non-linear at higher voltage which indicates that the more complicated physical phenomena at this situation may occur. At some region of the characteristics, there exist a negative resistance which means that the junction system can be used in specific electronic devices such as oscillators, switches, memories etc. The non-linearity is also sensitive to the layer as well as to the junction thicknesses. Non-linearity occurs for lower voltage and for higher junction and layer thicknesses.

Microscopic Study of Two Band Superconductivity in Magnesium Diboride Superconductor (MgB<sub>2</sub>)

We formulate a Model Hamiltonian of two band superconductivity for Magnesium Diboride superconductors (MgB₂). It is a conventional BCS type metallic superconductor which has the highest critical temperature T_c=39K. It is assumed that the superconductivity in MgB₂ arises due to metallic nature of the 2D sheets. From band structure calculations, it is observed that two types of bands i.e. σ and π bands are located at Fermi surface. Here, we consider phonon mediated superconductivity in which σ band is dominant over π band i.e. σ band is more coupled to a superconductor with much higher coupling. We consider a model Hamiltonian with mean field approach and solve this by calculating equations of motion of Green functions for a single particle. We determine the quasi-particle energy from the poles of the Green functions. We derive the single particle correlation functions and determine the two SC order parameters for both σ and π band. Here, the two SC order parameters for the bands are solved self- consistently and numerically. The conduction bandwidth (W) is considered as W=8t₀, where t₀ is the hopping integral. To make all the physical quantities dimensionless, we divide 2t₀ in each of the physical quantities. We then calculate the gap ratio 2∆(0)/K<SUB>B</SUB>T_c for both the bands. It is seen form our theoretical model that the two bands of MgB₂ superconductors have two different SC gaps with the same critical temperature. We also observe the variation of dispersion curves of quasi-particles for different temperature parameters for both σ and π band.

Strong-coupling character of superconducting phase in compressed selenium hydride

At present, metal hydrides are considered highly promising materials for phonon-mediated superconductors that exhibit high values of the critical temperature. In the present study, the superconducting properties of the compressed selenium hydride in its simplest form (HSe) are analyzed, toward quantitative characterization of this phase. By using the state-of-art Migdal-Eliashberg formalism, it is shown that the critical temperature in this material is relatively high ([Formula: see text][Formula: see text]K) and surpasses the level of magnesium diboride superconductor, assuming that the Coulomb pseudopotential takes value of [Formula: see text]. Moreover, the employed theoretical model allows us to characterize other pivotal thermodynamic properties such as the superconducting band gap, the free energy, the specific heat, and the critical magnetic field. In what follows, it is shown that the characteristic thermodynamic ratios for the aforementioned parameters differ from the predictions of the Bardeen-Cooper-Schrieffer theory. As a result, we argue that strong-coupling and retardation effects play important role in the discussed superconducting state, which cannot be described within the weak-coupling regime.

Superconducting magnesium diboride films for levitation of laser targets

Inertial confinement fusion targets would benefit from being levitated inside hohlraums to avoid capsule support-related implosion perturbations. Levitation inside a magnetic trap requires coating the capsule with a thin film that is superconducting at 20 K. Such non-epitaxial film growth on non-planar substrates is challenging. Here, we study Mg vapor annealing and solid-phase reactive inter-diffusion methods to form superconducting magnesium diboride (MgB2) films on different planar and spherical carbon substrates, evaluating glassy carbon, polycrystalline diamond made by chemical vapor deposition, and carbon deposited by magnetron sputtering of graphite targets. Thin films of B and Mg are produced by magnetron sputtering onto stationary planar or rolling spherical substrates and annealed at either 600 or 850 ∘C in Mg vapor. The films are characterized by a combination of high-energy ion scattering, electron microscopy, and magnetometry. Results show that the critical superconducting temperature of resultant films depends on film microstructure and oxygen impurity content. The formation of MgB2 films is also strongly substrate dependent, even in this case of non-epitaxial growth. Important factors to consider are oxygen outgassing of the substrate during thermal processing, substrate surface roughness, and the matching of thermal expansion coefficients of different layers in the multilayer structure.

Aluminum Ion Implantation in MgB2 Thin Films

Superconducting magnesium diboride is a possible candidate for replacing Nb in superconducting radio frequency (SRF) cavities because of its higher T c and potentially higher breakdown field. However, the existence of two energy gaps, the σ gap at 7.2 eV and the π gap at 2.2 eV, could impact the RF properties of MgB 2 . It has been shown that chemical doping and disorder can increase the interband scattering, thus modifying the two gaps. In this work, we performed Al ion implantation in MgB 2 thin films to introduce Al doping in the films. The Al ion implanted films showed decreases in transition temperature, from 40.4 K in the pristine films to non-superconducting above the dose of 2 × 10 14 atoms/cm 2 . An expansion in the c-axis lattice parameter with increasing dose was also observed. These results suggest that the degradation of superconductivity in the Al ion implanted films is mainly due to the ion beam damage rather than Al doping.

Design and Comparison Analysis of 3 T Superconducting Magnets Using MgB2 and 2G HTS Wires for DC Induction Heaters

Recently, certain superconducting wires, such as second-generation high-temperature superconducting (2G HTS) and low-temperature superconducting magnesium diboride (MgB2) wires, have been used in dc induction heaters. Therefore, it is necessary to analyze and compare which wire type is better for the magnet design of a superconducting dc induction heater. This paper presents the design process and comparative analysis results of 3 T superconducting magnets for a 300-kW-class dc induction heater using 2G HTS yttrium barium copper oxide (YBCO) and MgB2 wires. The specifications of the YBCO wire and two types of MgB2 wires were investigated first. The magnets were then designed so that the maximum magnetic flux density was 3 T, and the magnetic field distributions were analyzed using a three-dimensional finite element method program. Finally, the magnet designs were compared in terms of size, magnetic flux density, operating current and temperature, winding method, required length, and wire cost. This study result will be effectively applied to a commercial 300-kW-class superconducting dc induction heater.

Spontaneous magnetic field and odd-frequency pairing state in multiband superconductors

We investigate the interband coupling induced odd-frequency pairing state by solving the microscopic Ginzburg-Landau model for the two band superconductor magnesium diboride (MgB&lt;sub&gt;2&lt;/sub&gt;). It is found that the interband coupling can induce a new domain structure and a heliacal spontaneous magnetic vortex-antivortex pair around the cyclical domain wall, which breaks down spin-rotational symmetry and supports a time-reversal violating bound state, allowing the coexistence of spin-singlet and spin-triplet state close to the spontaneous vortex core. The odd-frequency spin-triplet even parity pairing state occurs since a successive operation in the orbital parity (P) and the time-reversal (T) obeys PT = + 1(–1) for spin-singlet (spin-triplet) pairing amplitude. A general phase diagram is presented.

Proximity two bands Eliashberg theory of electrostatic field-effect doping in a superconducting film of MgB2

A key aspect of field effect experiments is the possibility to induce charges on the first layers of a sample as a function of an applied gate voltage. It is therefore possible to study correlated phases of matter as a function of the induced charge density and the applied electric field. Moreover, resulting charge modulation along the direction of the applied electric field gives rise to junctions between perturbed and uneffected regions of the sample. In the framework of proximity effect Eliashberg theory, we investigate the consequence of an applied static electric field on the transition temperature of a two-band s-wave superconductor magnesium diboride. In most cases the only free parameter in the theory is the penetration depth of the applied electric field, whereas there is no freedom when the static perturbation is sufficiently weak. We come to the conclusion that the optimal way to enhance the critical temperature is to have a very thin film of magnesium diboride, otherwise the external electric field would not have substantial effect on superconductivity in this material.

Preparation and Properties of MgB2 Thin Films

The superconducting transition temperature of 140 nm ± 10 nm thin films on sapphire substrates, which were deposited by magnetron sputtering, was around 36 K. Using a magnetization technique, the film's critical current density was estimated as J C = 1.8 × 107 А/cm2 at 10 K, J C = 8 × 106 А/m 2 at 20 K in a zero magnetic field B, and J C = 3 × 106 А/m2 at 10 K and B = 5 T. The values of the upper critical magnetic field B c2 and the irreversibility field B irr estimated using the four-probe technique were B C2(22 K) = 15 T when H||film surface, 11 T when H⊥film surface, and B irr(22 K) = 11 T when H||film surface. The X-ray study showed that the microstructure of the film contains only MgB2 and MgO (in minority). The SEM and EPXMA study and quantitative Auger spectroscopy analysis revealed periodical variations of the film composition on the nanolevel and the presence of (mainly) two intercalated Mg-B-O-C phases of slightly different, especially in oxygen content, and thus with different conductivity and, possibly, with different T C. The characteristics of superconducting magnesium diboride films make them promising for application in electronic devices, e.g., as high-pass filters.

Magnesium diboride on inner wall of copper tube: A test case for superconducting radio frequency cavities

Superconductor magnesium diboride is considered one of the viable materials to substitute bulk niobium for superconducting radio frequency cavities. Utilizing a ${\mathrm{MgB}}_{2}$ coating on the inner wall of a copper cavity will allow operation at higher temperatures (20--25 K) than Nb cavities due to the high transition temperature of ${\mathrm{MgB}}_{2}$ (39 K) and the high thermal conductivity of Cu. In this paper, we present results of ${\mathrm{MgB}}_{2}$ coating on Cu tubes with similar dimensions to a 3 GHz cavity, as the first step towards coating the actual cavity, using the hybrid physical chemical vapor deposition technique. The results show successful coating of a uniform ${\mathrm{MgB}}_{2}$ layer on the inner wall of the Cu tubes with ${T}_{c}$ as high as 37 K.

Measurement of mass attenuation coefficients for undoped and boron nitride–doped magnesium diboride superconductors in the x-ray region 8.04–59.5 keV

ABSTRACTIn this study, mass attenuation coefficients of the undoped and 2% boron nitride–doped magnesium diboride superconductor samples were investigated. Mass attenuation coefficients were measured at 8.04–59.5 keV x-ray energies by using a high-purity germanium detector with a resolution of 182 eV at 5.9 keV. It is observed that mass attenuation coefficients in undoped and doped magnesium diboride samples decrease with increasing photon energy, and doping with the boron nitride leads to increase the absorption of the electromagnetic radiation.

The Influence of Stearic Acid Addition on the Superconducting Properties of MgB2

Magnesium diboride superconductors are important materials used for practical applications around the world, due to the properties and to the low price of the precursor elements, as well as the possibility to apply the material in magnets and electronic devices, operating with cryocoolers. In the present work, a methodology to optimize the properties of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> bulk superconductors is described. The method uses the addition of an organic carbon source (stearic acid, C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">18</sub> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">36</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), due to the homogeneous distribution of the doping material in the matrix, before final chemical reaction. The procedure was developed using high-energy ball milling to mix the precursor powders and performing heat treatment in a hot isostatic press, to improve the densification and grain connectivity. As a result, the critical current density enhanced around 600 times at 3T, when compared to the pure MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> samples produced using the same approach.

海上輸送のための外部加熱型MgB&lt;sub&gt;2&lt;/sub&gt;液面センサーによる液体水素タンク内部のスロッシングの基礎研究

We aim to provide basic technology for the marine transportation of liquid hydrogen (LH2). As one of the basic technologies, we are currently developing an external-heating-type superconducting magnesium diboride (MgB2) level sensor for large liquid hydrogen tanks. The optimization of sensor performance and the effect of sensor length on thermal response were previously reported. Sloshing inside a liquid hydrogen tank, which is predicted to be more complex than that of liquefied natural gas (LNG), has not yet been clarified. In this study, we examined sloshing in a small liquid hydrogen tank using multiple external-heating-type MgB2 sensors since these sensors exhibit good level-detection characteristics. As a result, the liquid level detection error from these sensors was up to about 4 mm, thereby showing good characteristics of dynamic level detection. Moreover, sloshing measurements with five level sensors were carried out successfully.

Dynamic Level-Detecting Characteristics of External-Heating-Type MgB2 Liquid Hydrogen Level Sensors under Liquid Level Oscillation and Its Application to Sloshing Measurement

To establish the worldwide storage and marine transportation of hydrogen, it is important to develop liquid hydrogen tanks/carriers as well as a long level sensor such as a superconducting magnesium diboride (MgB 2 ) level sensor. An external-heating-type MgB 2 level sensor is expected to be an excellent choice for liquid hydrogen because of its high linearity, resolution, and reproducibility. The dynamic level-detecting characteristics of three 500-mm-long MgB 2 level sensors have been evaluated under conditions of oscillating liquid level using an optical cryostat and a high-speed microscope. The response time to variations of the liquid hydrogen level is about 0.1 s, and the difference between the level read optically and that detected by the MgB 2 level sensors is about 5 mm under these conditions. Thus, the MgB2 level sensors have superior response performance for the sloshing measurement of liquid hydrogen.

Superconducting characteristics of short MgB2 wires of long level sensor for liquid hydrogen

To establish the worldwide storage and marine transport of hydrogen, it is important to develop a high-precision and long level sensor, such as a superconducting magnesium diboride (MgB2) level sensor for large liquid hydrogen (LH2) tanks on board ships. Three 1.7- m-long MgB2 wires were fabricated by an in situ method, and the superconducting characteristics of twenty-four 20-mm-long MgB2 wires on the 1.7-m-long wires were studied. In addition, the static level-detecting characteristics of five 500-mm-long MgB2 level sensors were evaluated under atmospheric pressure.

Effect of interband interactions on the pressure dependence on transition temperature of MgB2

A two-band BCS model with interactions, both phonon and non-phonon induced interactions, were employed to investigate the pressure dependence on superconducting transition temperature of two-band superconductor. We derived the transition temperature and its pressure dependence within Bogoliubov--Valatin formalism for magnesium diboride superconductor. We examined the influence of interband interactions on transition temperature at varying pressure and analyzed the relevance of this calculation in magnesium diboride, MgB2.

Fundamental Study of Tank with MgB2 Level Sensor for Transportation of Liquid Hydrogen

We are currently developing an external-heating-type superconducting magnesium diboride (MgB2) level sensor for a liquid hydrogen (LH2) tank. The aim of this study is to investigate the measuring current dependence of the level-detecting characteristics of the MgB2 level sensor for LH2 under a static condition which has not yet been clarified. It was found that the linear correlation coefficient was 0.99 or more, indicating high linearity, regardless of the measuring current at heater inputs of 3W and 6W. Moreover, there was no effect of self-heating by the measuring current and it was found that a current of up to 100mA can be used.