Table of contents

We report a detailed first-principles local-density-functional investigation of the structural, electronic, dynamical and superconducting properties of MgB₂ focusing on different aspects related to this material. In particular, we examine Al doping, as well as reduced dimensionality and pressure effects on the electronic and superconducting properties of this compound. Our ab initio calculations for the case of 50% Al doping are able to correctly reproduce the measured frequencies of the E_2g phonon and explain the disappearance of superconductivity in terms of filling effects on both carrier concentration and electron–phonon coupling. The surface study shows that an enhanced density of states at the Fermi level is found in the B-terminated case. However, we find the Mg-terminated surface to be the most stable structure in the whole range admitted by the chemical potentials, in agreement with very recent experimental results. Finally, the study of the E_2g phonon frequency under pressure is able to explain the critical temperature lowering under applied pressure.

The recent discovery of a superconducting transition at 39 K in MgB₂—made of alternating Mg and graphene-like B planes—has raised great interest, for both its technological and theoretical implications. It was clear since the very beginning that the properties of this material are related to an anomalous coupling between the charge carriers in the σ bands—due to in-plane bonds between Boron atoms—and the phonon mode (E_2g) which involves in-plane vibrations of the B ions. Theoretical studies have thus been focused on the search for possible anomalies in the e–ph coupling: one of the first results was the discovery that the E_2g phonon is highly anharmonic, but the connection between anharmonicity and T_c in this material is still a controversial point. We first present a detailed first-principles study of the E_2g phonon anharmonicity in MgB₂ and analogous compounds which are not superconducting, AlB₂ and graphite, and in a hypothetical hole-doped graphite (C²⁺₂); we then introduce an analytical model which allows us to relate the onset of anharmonicity with the small Fermi energy of the carriers in σ bands. Our study suggests a possible relation between anharmonicity and non-adiabaticity; non-adiabatic effects, which can lead to a sensible increase of T_c with respect to values predicted by conventional theory, become in fact relevant when phonon frequencies are comparable to electronic energy scales.

We have performed ¹¹B NMR measurements in ¹¹B enriched MgB₂ powder sample in the normal phase. The Knight shift was accurately determined by using the magic angle spinning technique. Results for ¹¹B and ²⁷Al Knight shifts (K) and relaxation rates (1/T₁) are also reported for AlB₂. The data show a dramatic decrease of both K and 1/T₁ for ¹¹B in AlB₂ with respect to MgB₂. We compare experimental results with ab initio calculated NMR relaxation rates and Knight shifts. The experimental values for 1/T₁ and K are in most cases in good agreement with the theoretical results. We show that the decrease of K and 1/T₁ for ¹¹B is consistent with a drastic drop of the density of states at the boron site in AlB₂ with respect to MgB₂.

We measured the ²⁷Al and ¹¹B NMR spin lattice relaxation rates and the isotropic Knight shifts in powder samples of Mg_1−xAl_xB₂, as a function of the Al concentration, x. The temperature independence of the Knight shifts and the linear temperature dependence of the relaxation are verified throughout the compositions explored. The variation with x of the measured quantities is discussed in terms of the projected densities of states at the Fermi energy, finding good qualitative as well as quantitative agreement with recent band structure calculations.

We have studied the anisotropic superconductor MgB₂ using a combination of scanning electron microscopy and scanning tunnelling spectroscopy. Tunnelling spectroscopy performed on thin films and pellets reveals two distinct energy gaps at Δ₁ = 2.3 meV and Δ₂ = 7.1 meV. On different crystallites within the polycrystalline sample different spectral weights of the partial densities of states (PDOS) were observed. They reflect different tunnelling directions with respect to the crystallographic orientation of the grain in a multiband system. Indeed when tunnelling in the c-axis films only one superconducting gap is observed, which is associated with the 3D band in this system. Temperature evolution of the tunnelling spectra reveals that both gaps close simultaneously near the bulk critical temperature. Our experimental findings are consistent with the two-band superconductivity scenario in the presence of strong pair interaction between the two bands [1].

Point-contact measurements have been performed on polycrystalline MgB₂ samples using a Cu counter electrode. The current–voltage (I–V) characteristics reveal Andreev-reflection structures from the ballistic transport across the normal-metal/superconductor interface. The spectra contain two different energy gaps which close near the same bulk critical temperature. These results support the multiband model of superconductivity for MgB₂ proposed by Liu et al. Point-contact experiments in a magnetic field show that the small-gap structure disappears in fields of 1–2 T whereas the large-gap structure is suppressed in larger fields going up to around 15 T. Nonlinearities in the current–voltage characteristics contain information on the energy dependence of the electron–phonon interaction.

High-speed scanning tunnelling spectroscopy (STS) was used at low temperature to study the quasiparticle excitation spectrum on the surface of c-axis-oriented superconducting thin films of MgB₂. The tunnelling spectra measured on as-grown films were compared with those acquired on chemically etched samples. In most cases the STS reveals only one small superconducting gap to be present in the tunnelling spectra, consistent with c-axis tunnelling and the particular electronic band structure of MgB₂. We found that the etching leads to the enhancement of the gap energy by 25% from 2.2 ± 0.3 meV to 2.8 ± 0.3 meV, and to the modification of the temperature dependence of the superconducting gap which, in both cases, has clearly a non-BCS shape. We argue that the modification of the electronic structure at the surface of the material due to the etching is responsible for these changes and discuss the possible origins of the effect.

Directional point-contact spectroscopy measurements were performed for the first time in state-of-the-art MgB₂ single crystals. The selective suppression of the superconductivity in the π band by means of a suitable magnetic field allowed separation of the partial contribution of each band to the total point-contact conductance. By fitting the partial conductance curves σ_σ(V) and σ_π(V), we got an independent determination of the two gaps, Δ_σ and Δ_π, with a strong reduction of the experimental uncertainty. Their temperature dependence was found to agree well with the predictions of the two-band models for MgB₂.

We review recent measurements of upper (H_c2) and lower (H_c1) critical fields in clean single crystals of MgB₂, and their anisotropies between the two principal crystallographic directions. Such crystals are far into the 'clean limit' of type II superconductivity, and indeed for fields applied in the c-direction, the Ginzburg–Landau parameter κ is only about 3, just large enough for type II behaviour. Because μ₀H_c2 is so low, about 3 T for fields in the c-direction, MgB₂ has to be modified for it to become useful for high-current applications. It should be possible to increase H_c2 by the introduction of strong electron scattering (but because of the electronic structure and the double gap that results, the scatterers will have to be chosen carefully). In addition, pinning defects on a scale of a few nm will have to be engineered in order to enhance the critical current density at high fields.

Flux line dynamics have been investigated on MgB₂ bulk superconductors obtained by reactive liquid infiltration by measuring the ac magnetic susceptibility. The fundamental and third harmonics have been studied as a function of temperature, dc magnetic field, ac field amplitude and frequency. In order to determine the dynamical regimes governing the vortex motion, the experimental results have been compared with susceptibility curves obtained by numerical calculations of the non-linear diffusion equation for the magnetic field. The frequency behaviour of the third harmonic response, that cannot be explained by frequency dependent critical state models, has been related to the current dependence of the flux creep activation energy U(J) in the diffusion coefficient. In this way we have shown that the measured curves are correctly interpreted within the framework of a vortex glass description.

We present resistivity and thermal conductivity measurements on bulk samples, prepared either by a standard method or by a one-step technique. The latter samples, due to their high density and purity, show residual resistivity values as low as 0.5 μΩ cm and thermal conductivity values as high as 215 W mK⁻¹, higher than the single-crystal values. The thermal and electrical data of all the samples are analysed in the framework of the Bloch–Gruneisen equation, giving reliable parameter values. In particular, the temperature resistivity coefficient, obtained both from resistivity and thermal conductivity, comes out ten times larger in the dirty samples than in the clean samples. This result supports the hypothesis of Mazin et al (2002 Preprint cond-mat/0204013) that π and σ bands conduct in parallel, with prevailing π conduction in clean samples and σ conduction in dirty samples.

The upper critical field H_c2 for the fields perpendicular and parallel to Mg and B planes of the magnesium diboride is presented in the temperature range from 5.4 K up to T_c as obtained by comparison of high-field magnetotransport, ac-susceptibility and specific heat measurements. The onset of the finite resistivity yields H_c2, while the end of the resistive transition to the normal state is due to surface effects. H_c2 perpendicular to the planes reveals a conventional temperature dependence with μ₀H_c2⊥ab(0) = 3.5 T but the parallel critical field with μ₀H_c2||ab(0) = 17 T has a positive curvature at temperatures above 20 K. Consequently, the anisotropy factor Γ = H_c2||ab/H_c2⊥ab is temperature dependent with Γ equal to 4.8 at low temperatures and about 2 near T_c. The angular dependence of the upper critical field of an elliptic form is obtained at several temperatures.

Magnetic flux penetration and trapped flux patterns formed into a MgB₂ film during the increase and the subsequent decrease of the applied external magnetic field were studied using magneto-optical imaging. The film was grown by electron-beam evaporation and exhibits a granular structure. The non-homogeneous fan-like shaped penetration, already pointed out in the literature, was observed. To investigate the origin of this kind of penetration, a quantitative approach was chosen. The induction magnetic field map and the corresponding contour map of a framed zone show many isolated loops originating from the granular nature of the sample. Inside such loops, an estimation of the local current density was made through the inversion of the Biot–Savart law. All the results point towards the conclusions that the fan-like shaped patterns do mirror the percolation of dissipation paths. Along these paths, some interfaces, distributed in a hierarchical order, play the role of either pinning barriers or easy-flow channels.

The microwave surface impedance Z_s = R_s + jωμ₀λ was measured with dielectric resonator techniques for two c-axis-oriented MgB₂ thin films. The temperature dependence of the penetration depth λ measured with a sapphire resonator at 17.93 GHz can be well fitted from 5 K close to T_c by the standard BCS integral expression assuming the reduced energy gap Δ(0)/kT_c to be as low as 1.13 and 1.03 for the two samples. From these fits the penetration depth at zero temperatures was determined to be 102 nm and 107 nm, respectively. The results clearly indicate the s-wave nature of the order parameter. The temperature dependence of surface resistance R_s, measured with a rutile dielectric resonator, shows an exponential behaviour below about T_c/2 with a reduced energy gap being consistent with the one determined from the λ data. The R_s value at 4.2 K was found to be as low as 19 μΩ at 7.2 GHz, which is comparable with that of a high-quality high-temperature thin film of YBa₂Cu₃O₇. A higher-order mode at 17.9 GHz was employed to determine the frequency f dependence of R_s ∝ f^n(T). Our results revealed a decrease of n with increasing temperature ranging from n = 2 below 8 K to n = 1 from 13 to 34 K.

The new superconductors discovered after MgB₂ are discussed. Two serious claims have been made so far. The first claim concerns a graphite–sulfur composite undergoing a transition at 35 K. Since the magnetic transition clearly has the characteristics of a superconducting shielding effect, it would be worthwhile to investigate the possibility of increasing the sample volume responsible for this transition. However, there is no zero-resistance transition and the authors suggest that the superconductivity could be due to a surface effect. Carbon in this sample has the same structure as boron in MgB₂. The second claim is more modest, but it is really a superconduting transition at 1 K. The sample is a single crystal of Cd₂Re₂O₇. The Re cations could be in a mixed valence state, but the charges do not seem to localize by lowering the temperature. The importance of these results is the discovery of a new oxide structure such as the pyrochlore, which can host a superconducting state.

We discuss the important aspects of synthesis and crystal growth of MgB₂ under high pressure (P) and temperature (T) in the Mg–B–N system, including the optimization of P–T conditions for reproducible crystal growth, the role of liquid phases in this process, the temperature dependence of crystal size and the effect of growing instabilities on single-crystal morphology. Extensive experiments have been carried out on single crystals with slightly different lattice constants and defect concentrations, which have revealed the possible effects of Mg deficiency and lattice strain on the superconducting properties of MgB₂ (T_c, J_c, residual resistivity ratio, anisotropy, etc).

Single crystals of MgB₂ with a size up to 1.5 × 0.9 × 0.2 mm³ have been grown with a high pressure cubic anvil technique. The crystal growth process is very peculiar and involves an intermediate nitride, namely MgNB₉. Single crystals of BN and MgB₂ grow simultaneously by a peritectic decomposition of MgNB₉. Magnetic measurements with SQUID magnetometry in fields of 1–5 Oe show sharp transitions to the superconducting state at 37–38.6 K with a width of ∼0.5 K. The high quality of the crystals allowed the accurate determination of magnetic, transport (electric and heat) and optical properties as well as scanning tunnelling spectroscopy (STS) and decoration studies. Investigations of crystals with torque magnetometry show that H^//c_c2 for high quality crystals is very low (24 kOe at 15 K) and saturates with decreasing temperature, while H^//ab_c2 increases up to 140 kOe at 15 K. The upper critical field anisotropy γ = H^//ab_c2/H^//c_c2 was found to be temperature dependent (decreasing from γ ≅ 6 at 15 K to 2.8 at 35 K). The effective anisotropy γ_eff, as calculated from reversible torque data near T_c, is field dependent (increasing roughly linearly from γ_eff ≅ 2 in zero field to 3.7 in 10 kOe). The temperature and field dependence of the anisotropy can be related to the double gap structure of MgB₂ with a large two-dimensional gap and small three-dimensional gap, the latter of which is rapidly suppressed in a magnetic field. Torque magnetometry investigations also show a pronounced peak effect, which indicates an order–disorder phase transition of vortex matter. Decoration experiments and STS visualize a hexagonal vortex lattice. STS spectra in zero field evidence two gaps 3 meV and 6 meV with a weight depending on the tunnelling direction. Magneto-optic investigations in the far-infrared region with H//c show a clear signature of the smaller of the two superconducting gaps, completely disappearing only in fields higher than H^//c_c2.

Superconducting properties of bulk, dense, pure MgB₂ and doped (Mg_1−xA_x)B₂ samples with A = Na, Ca, Cu, Ag, Zn and Al were studied for compositional ranges 0 < x ≤ 0.20. The effects on pinning properties and critical current were investigated, particularly for A = aluminium. The samples were sintered and/or synthesized at high pressure–high temperature in a cubic multi-anvil press (typically 3.5–6 GPa, 900–1000 °C). They were characterized by x-ray diffraction, scanning electron microscopy and their superconducting properties were investigated by ac susceptibility, magnetization (VSM and SQUID) and transport measurements under a magnetic field. Only Al really substitutes on the Mg site. The other elements form secondary phases with B or Mg which do not act as pinning centres. No positive effect is observed on the superconducting properties of the bulk MgB₂ samples with these doping elements added: T_c, critical current j_c, H_irr and H_c2. For Al, the effect on H_c2 remains small, and the irreversibility line does not move, thus not improving the critical current of the Al-doped MgB₂ samples.

We have synthesized c-axis-oriented MgB₂ thin films on (1 0 2) Al₂O₃ substrates by using a pulsed laser deposition technique and have measured the longitudinal and the Hall resistivites in the ab-plane direction. The as-grown thin films show T_c of 39.2 K with a sharp transition width of ∼0.15 K. In the normal state, the Hall coefficient (R_H) behaves as R_H ∼ T with increasing temperature (T) up to 130 K and then deviates from that linear T-dependence at higher temperatures. The T² dependence of the cotangent of the Hall angle is only observed above 130 K. We estimate the absolute value of the hole density to be about two orders of magnitude higher than that of YBa₂Cu₃O₇.

High-quality MgB₂ thin films have been obtained by pulsed laser deposition both on MgO and Al₂O₃ substrates using different methods. In the standard two-step procedure, an amorphous precursor layer is deposited at room temperature starting from the stoichiometric target and from the boron target. After this first step, it is annealed in a magnesium atmosphere in order to crystallize the superconducting phase. The obtained films show a strong c-axis orientation, evidenced by XRD analysis, a critical temperature up to 38 K and very high critical fields along the basal planes, up to 22 T at 15 K. Also an in situ one step technique for the realization of superconducting MgB₂ thin films has been developed. In this case, the presence of an argon buffer gas during deposition is crucial, and we observe a strong dependence of the quality of the deposited film on the background gas pressure. The influence of the Ar atmosphere has been confirmed by time- and space-resolved spectroscopy measurements on the emission spectrum of the plume. The Ar pressure strongly modifies the plasma kinetics by promoting excitation and ionization of the plume species, especially of the most volatile Mg atoms, increasing their internal energy.

The remarkably high critical temperature at which magnesium diboride (MgB₂) undergoes transition to the superconducting state, T_c ≈ 40 K, has aroused great interest and has encouraged many groups to explore the properties and application potential of this novel superconductor. For many electronic applications and further basic studies, the availability of superconducting thin films is of great importance. Several groups have succeeded in fabricating superconducting MgB₂ films. An overview of the deposition techniques for MgB₂ thin film growth will be given, with a special focus on the in situ two-step process.

Although, meanwhile, many problems to obtain suitable films have been solved, such as oxygen impurities and magnesium volatility, the question of how single-phase epitaxial films can be grown still remains. The possibility of growing single-crystalline epitaxial films will be discussed from the deposition conditions' point of view as well as substrate choice. Necessary conditions are discussed and possible routes are reviewed.

The applicability of MgB₂ in superconducting electronic devices depends on the possibility of making well-controlled, i.e., reproducible and stable, Josephson junctions. The first attempts to make MgB₂–MgO–MgB₂ ramp-type junctions and SQUIDs from MgB₂ nanobridges are discussed.

We have recently developed a technique for fabricating SNS junctions in MgB₂ thin films using a focused ion beam. These junctions show a strong modulation of the critical current by applied magnetic field and microwaves. They also show large I_cR_N products in excess of 1 mV at 4.2 K making them attractive candidates for a range of superconducting electronics applications. We have made SQUIDs with directly coupled pick-up loops using this technique which show large voltage modulations (175 μV at 10 K) and noise comparable to HTS SQUIDs. Here we will present an overview of our devices along with our latest results.

We present a study on the temperature and field dependence of the microwave surface impedance Z_s in thin films of the superconducting MgB₂ compound. Samples were prepared by e-beam evaporation of boron on r-plane sapphire followed by an ex situ annealing in Mg vapour. Critical temperature values range between 26 and 38 K. Surface impedance measurements (Z_s = R_s + iX_s) were performed from 2 K close to T_c in the microwave region up to 20 GHz via parallel plate or dielectrically loaded resonators in 'symmetric' (two MgB₂ films) and asymmetric (an MgB₂ film and a commercial YBCO control film) configurations. At high microwave power, frequency domain measurements show a characteristic signature associated with weak links and this appears to be the limiting factor governing the performance of these films.

Mono- and multifilamentary Fe/MgB₂ tapes and wires with high transport critical current densities have been prepared by the powder-in-tube technique using fine powders. The influence of the initial MgB₂ grain size on critical current density, upper critical and irreversibility fields has been studied. After reducing the MgB₂ grains to micrometer size by ball milling, the critical current density, J_c, was enhanced, while the upper critical field, μ₀H_c2, remained unchanged. The anisotropy ratio between the upper critical fields parallel and perpendicular to the tape surface was determined to be 1.3, reflecting a deformation induced texture. A good agreement has been found between resistive and inductive J_c values, measured at various temperatures between 4.2 and 25 K. On monofilamentary tapes, J_c values close to 10⁵ A cm⁻² were measured at 25 K/1 T, while J_c values ≈10⁶ A cm⁻² were extrapolated for 4.2 K/0 T. Fe/MgB₂ tapes exhibit high exponential n factors for the resistive transition: n values of 60 and 30 were found at 4 T and 6 T, respectively. Multifilamentary wires (with seven filaments) show slightly lower J_c values, 1.1 × 10⁵ A cm⁻² at 4.2 K/2 T. The improvement of thermal and mechanical stability of MgB₂/Fe tapes and wires appears clearly as a challenge for future developments.

Nickel-sheathed MgB₂ tapes were fabricated by means of the powder-in-tube method with a monocore configuration of the superconductor. The metallic tubes were filled with commercial reacted powders and cold-worked, respectively, by groove rolling, drawing, and rolling to flat tapes of about 4 mm in width and 0.35 mm in thickness. Portions of the manufactured conductors were heat-treated in an argon atmosphere at temperatures up to 940 °C. Conductors were characterized by transport measurements to determine the magnetic field dependence of the critical current density, the irreversibility line, as well as to establish an initial relationship between the thermo-mechanical treatment and the current carrying capacity of the manufactured MgB₂ samples. Our optimized samples for the low field operation carry a critical current density of at least 4 × 10⁵ A cm⁻² at 4.2 K, 1 T. The main outcome of this study, however, is that the optimal heat treatment condition changes if the MgB₂ conductor has to be employed respectively for a low- or high-field operation. Finally, the first successful demonstration of the current carrying capability of longer MgB₂ tapes is also reported in this paper.

The capability of manufacturing long superconducting MgB₂ wires with already remarkable critical currents makes this material a very promising candidate for future applications. Tapes are prepared by the powder-in-tube technique. After the cold working procedure typically carried out by wire drawing and cold rolling, it has been found that a final sintering step carried out in argon atmosphere is a key process for further improving the superconducting properties of the conductors. To study the effect of the deformation and heat treatment processes, we performed neutron scattering experiment. Due to the high penetration depth of neutron inside matter, it was possible to analyse the MgB₂ phase still wrapped in the Ni sheath. Our studies were carried out by a full spectra refinement by the Rietveld method. In the starting superconducting powder a large Mg deficiency was observed. In the tapes we found that the large forces applied during the cold working induced a large MgB₂ lattice deformation, and that it is partly relaxed during the final sintering process. An important correlation of the residual stress with the critical temperature and the pinning properties was pointed out. We also observed the appearance of detrimental secondary phases during the sintering process. In particular, the MgB₂ phase reacted with the nickel sheath and MgB₂Ni_2.5 was formed at temperatures higher than 850 °C. These results are of basic importance for a further optimization of the transport properties at moderate fields where applications of MgB₂ tapes are already envisageable.

Instead of commercially available MgB₂ powder, we have used partially reacted powder prepared by mechanical alloying. This precursor powder consists of grains with a size of only a few nanometres and contains reacted MgB₂, and also the starting material Mg and B, and is, therefore, more reactive than fully reacted commercial powders. Using copper as a sheath material, tapes were prepared by the usual powder-in-tube process. After annealing at relatively low temperatures (770–870 K) in inert atmosphere, the tapes have good superconducting properties. Magnetically we have measured a critical current density of 400 kA cm⁻² at 4.2 K.

MgB₂ hollow wires have been produced with a new technique which uses a conventional wire manufacturing process but is applied to composite billets containing the elemental B and Mg precursors in an appropriate shape. The technique has been applied to the manufacture of both monofilamentary and multifilamentary wires of several tens of metres in length. The superconducting transport properties of the MgB₂ hollow wires have been measured in a magnetic field and in the temperature range from 4.2 to 30 K. Promising results are obtained, which indicate the possibility of the application of these wires as superconductors in the temperature range of 15–30 K and at medium-high values of magnetic field.

Composite MgB₂ wires have been made by the powder-in-tube method using commercial MgB₂ powder (Alfa Aesar) in Fe and Fe-alloy (stainless steel (SS)) sheaths. Different deforming processes have been used. Two-axial rolling (TAR) produces the highest powder density leading to the best J_c (0 T, 4.2 K) of 55000 A cm⁻² in as-deformed wire. Therefore, four-core wire has also been made by TAR. The short samples were subjected to annealing at temperatures from 850 to 1100 °C for 30 min in argon, which leads to apparently improved J_c values, 25000 A cm⁻² at 4 T and around 10⁶ A cm⁻² in the self-field (at 4.2 K). An interdiffusion layer between MgB₂ and the Fe or SS sheath was observed and analysed. The SS sheath reacts more intensely than Fe due to the presence of Ni and Cr elements. Transport currents were measured at temperatures 4.2–25 K and an external magnetic field B = 0–8 T. The engineering current density level of 10⁴ A cm⁻² is used to estimate the magnetic field possibly generated by coils wound from MgB₂ composite wires.

We discuss the results of a study of MgB₂ multifilamentary conductors and coated conductors from the point of view of their future dc and ac applications. The correlation between the slope of the irreversibility line induced by neutron irradiation defects and in situ structural imperfections and the critical temperature and critical current density is discussed with respect to the conductor performance and applicability. We debate the possible origin of the observed anomalous decrease of ac susceptibility at 50 K in copper clad in situ powder-in-tube MgB₂ wires. Different conductor preparation methods and conductor architectures, and attainable critical current densities are presented. Some numerical results on critical currents, thermal stability and ac losses of future MgB₂ multifilamentary and coated conductors with magnetic cladding of their filaments are also discussed.

Special issue on BOROMAG: The International Workshop on Superconductivity in Magnesium Diboride and Related Materials (Genoa, Italy, 17–19 June 2002)

The striking discovery of superconductivity in MgB₂ at 40K announced in January 2001 by the Japanese group of Professor Akimitsu had a huge impact on the superconductivity scene. The relevance of the news was even more astonishing considering the already broad commercial availability of this simple binary compound from a number of worldwide suppliers. As a result, just a few days after the announcement, samples were under severe analysis in many research laboratories, which immediately confirmed the importance of the discovery. This interest was soon followed by the appearance of hundreds of articles on the Web, covering fundamental, experimental and applied aspects relating to MgB₂.

The idea of organizing the BOROMAG Workshop 18 months after the public announcement by Professor Akimitsu came to us with the hope of bringing together most of the scientists that gave an important contribution in elucidating the mechanisms of superconductivity at such a high temperature in a binary compound. We also thought that it was the right time to see whether a possible future could be forecast for practical applications based on MgB₂ superconducting wires and thin films.

The reaction of the research community to this proposition was very positive, and substantial financial support for the organization of the event came from several international private and public institutions and also from a number of national companies.

More than 110 researchers attended the BOROMAG Workshop in Genoa, Italy, 17–19 June 2002, coming from 15 different countries. The presentations were subdivided into 17 invited, 22 oral, and 24 poster contributions, for a total number of 63 papers. In this special issue of Superconductor Science and Technology, a selection of papers have been included. They consist mainly of invited and oral presentations, along with a number of poster presentations that have been selected based on their relevance.

Firstly, we would like to thank the Committee that helped with the planning of the programme of the workshop. We are very grateful to the authors for being punctual with submissions and the revisions of manuscripts, and also acknowledge the help of many colleagues who reviewed the manuscripts to ensure their high quality. We would also like to take the opportunity to thank all the young researchers that have given substantial help in the organization of the meeting, and finally to The National Institute for the Physics of Matter (INFM) that kindly allowed us the use of its infrastructures.

Guest Editors: Carlo Ferdeghini Giovanni Grasso Marina Putti