Search

Epitaxial FeSe0.5Te0.5 thin films with different thickness were grown by pulsed laser ablation deposition on different substrates. High purity phase and fully epitaxial growth were obtained. By varying the film thickness, superconducting transition temperatures up to 21 K were observed, significantly larger than the bulk value 16.2 K. Structural analyses indicated that the c-axis is smaller than the bulk value but it is almost independent of the film thickness and the a-axis changes significantly with the film thickness and is linearly related to the Tc. The latter result indicates the important role of the compressive strain in enhancing Tc. Tc is also related to both the Fe–(Se,Te) bond length and angle, suggesting the possibility of further enhancement.

Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

In this communication we present a transmission electron microscopy study on neutron-irradiated bulk samples. We give clear evidence that neutron irradiation creates nanometric amorphous regions within the crystal lattice. The density of these defects correctly scales with the neutron dose. These defects, whose size matches with the coherence length, act as pinning centers and are responsible for the strong increase of the critical current density at high field and for shifting the maxima of the pinning force curves toward higher fields.

In this paper, we report on field-effect experiments in ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}$ side-gate channels patterned on ultrathin epitaxial films having thickness ranging from 12 to 4 unit cells. Transport mechanisms and competition between phases, under the effect of electric and magnetic fields, as well as of other perturbations such as disorder and proximity to the interface with substrate are explored. We observe, in a 7 unit cells thick sample, a shift of the metal-insulator transition temperature as high as 43 K and a resistivity modulation up to 250% at low temperatures. In striking contrast, the 6--4 unit cells thick samples result to be insulating and almost insensitive to field-effect modulation. Such a finding indicates that for films thinner than 7 unit cells, a strong localization mechanism develops, which cannot be healed by band refilling. On the other hand, our results are compatible with a Mn ${e}_{g}$ orbital rearrangement driven by the broken translational symmetry at the surface and/or interface, which suppresses the double-exchange mechanism and localizes the carriers.

The integration of ZnO based high mobility transparent semic onductors with perovskites that exhibit a wide spectrum of physical properties (superconductivity, ferroelectricity, ferromagnetism, etc.) may lead to a wide variety of new electronic/optoelectronic devices. Here we present results ab out the deposition of high crystalline quality Al or Co doped ZnO films grown by pulsed laser deposition on 110 face of strontium titanate single crystals. Field Effect (FE) experiment, allowing to change the carrier concentration of the film by more then 4 orders of magnitude (from § 10 15 to § 10 20 e-/cm 3 , estimated by Hall effect measurements under FE), we re employed to investigat e transport mechanisms in depth. In particular we observed a crossover of low temperature magnetoresistance from a negative behaviour in the accumulation state to a positive one in the depletion state. Th e measurement of the activation energy as a function of the Gate potential allowed to get information on the density of states. Keywords: ZnO, Thin Films, Field Effect, Magnetoresistance, Dilute Magnetic Semiconductors.

Paraconductivity of MgB2 has been measured in homogeneous thin films grown by Hybrid physical chemical vapor deposition. In order to reduce the possible effects of spatial inhomogeneities, stripes of different widths were cut on the films. We measured several samples with different resistivity values; after subtracting the normal state resistivity, paraconductivity appears to be of the same order of magnitude in all the samples. The dependence on the reduced temperature epsilon = ln(T/Tc) is discussed and compared with the existing models. (c) 2007 Elsevier B.V. All rights reserved.

We measured the upper critical field H-c2 and the critical current density J(c) on neutron-irradiated MgB2 samples and studied the effects of damaging on pinning mechanisms. We observed a strong enhancement of the upper critical field and great improvement of the critical current magnetic field dependence in an intermediate range of fluence. We demonstrated that the critical current increase is not only due to the H-c2 improvement but the irradiation substantially changes the pinning force improving the critical current density at high magnetic field. (C) 2007 Elsevier B.V. All rights reserved.

The effect of disorder on normal state and super-conducting properties is studied in a series of neutron irradiated magnesium diboride thin films. Irradiation increases residual resistivity by more than two orders of magnitude and progressively decreases critical temperature down to the complete suppression of superconductivity. In this disorder-controlled set of samples, we analyze magnetoresistivity, extracting the scattering times in each band, and upper critical field. H-c2 is extremely high and only weakly depends on resistivity at low irradiation level. We propose a simple explanation for this puzzling topic, and we discuss the importance of reaching high magnetic field to correctly extrapolate H-c2 (0).

We explore the possibility of tracking magnetization orientation in artificial structures patterned in ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ epitaxial films by magnetotransport measurements, exploiting the anisotropic magnetoresistance effect. We perform resistance measurements as a function of temperature, magnetic field, and angle between the applied in-plane magnetic field and the channel or crystalline axes in micrometric channels of different widths. We analyze quantitatively our results and extract information about magnetization easy axes, crystalline anisotropy, domain wall resistance, anisotropic magnetoresistance, energy of magnetic domain pinning, and magnetic reversal mechanisms. For channel widths larger than a few micrometers, the magnetization direction at low field $(\ensuremath{\leqslant}200\phantom{\rule{0.3em}{0ex}}\mathrm{Oe})$ is determined by magnetocrystalline easy axes, whereas for channel widths of $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ or smaller, the shape anisotropy forces the magnetization to align along the channel axis. This gives a precise indication on how artificial patterning can be used to force the magnetization direction in manganite based spintronic devices. Values of magnetocrystalline constant up to $8000\phantom{\rule{0.3em}{0ex}}\mathrm{J}∕{\mathrm{m}}^{3}$ and anisotropic magnetoresistance values between 0.1% and 0.6% are found. Our data also indicate that, in the low field $(\ensuremath{\leqslant}200\phantom{\rule{0.3em}{0ex}}\mathrm{Oe})$ hysteretic regime, magnetization reversal occurs by thermal activated hopping of domain walls, with a characteristic hopping distance of $4--5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

We studied the influence of the disorder introduced in polycrystalline $\mathrm{Mg}{\mathrm{B}}_{2}$ samples by neutron irradiation. To circumvent self-shielding effects due to the strong interaction between thermal neutrons and $^{10}\mathrm{B}$ we employed isotopically enriched $^{11}\mathrm{B}$ which contains 40 times less $^{10}\mathrm{B}$ than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low $^{10}\mathrm{B}$ content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range $1--2\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behavior has been carried out to identify what scattering and pinning mechanisms come into play. Finally, the correlation between some characteristic lengths and the transition widths is analyzed.

By carrying out differential resistance measurements in oxygen deficient ${\mathrm{La}}_{0.67}{\mathrm{Ba}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films at different magnetic fields, in submicrometric constricted regions patterned by focused ion beam, we find evidence of hysteretic resistance behavior as a function of both the external magnetic field and dc bias current. The resistance curves exhibit a marked asymmetry with respect to the polarity of the current. We suggest that the spin-polarized injected current exerts a torque on magnetic domains, whose rotation accounts for the hysteretic resistance changes. The memory effect of such constrictions is potentially interesting both for studying micromagnetic effects and in view of spintronics devices applications.

We perform field-effect experiments on side-gate devices made of underdoped ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ epitaxial thin films. The resistance modulation is small due to the high carrier concentration in this compound; however, it is significantly enhanced by an external magnetic field. We discuss our results in the framework of a double-exchange mechanism and phase separation and suggest that our experimental data together with other complementary data of the literature are a straightforward consequence of the percolative nature of transport in the phase-separation regime of manganites. We demonstrate the possibility of driving the percolation process by field effect.

We studied the upper critical field in poly-crystalline MgB/sub 2/ samples in which disorder was introduced by different ways: Al substitution and neutron irradiation. We observed that by Al doping critical temperature T/sub c/ decreases, the resistivity increases but the upper critical field, H/sub c2/, decreases. This behavior can be explained considering that Al substitutions affect only /spl pi/ bands leaving /spl sigma/ bands in clean regime; the decrease of H/sub c2/ can be correlated with suppression of the larger gap in doped samples. On the other hand neutron irradiation produces an increase of H/sub c2/. Neutron irradiation was carried on up to fluences of 1.4/spl middot/10/sup 20/ n/cm/sup 2/. To obtain a disorder structure homogeneously distributed, the experiment was carried out on bulk samples prepared with the /sup 11/B isotope. Up to fluences of 10/sup 18/ n/cm/sup 2/ the critical temperature is slightly diminished (36 K) and the superconducting properties are improved; the upper critical field is significantly increased: from 13.5 T to 20.3 T at 12 K. For larger neutron fluences the critical temperature is suppressed down to 9.2 K and the superconducting properties come out strongly degraded.

We have investigated the charge transfer mechanism in artificial superlattices by Seebeck effect measurements. Such a technique allows a precise determination of the amount of charge transferred on each ${\mathrm{CuO}}_{2}$ plane. A systematic characterization of thermoelectric power in $({\mathrm{BaCuO}}_{2+\ensuremath{\delta}}{)}_{2}/({\mathrm{CaCuO}}_{2}{)}_{n}$ and $({\mathrm{Ba}}_{0.9}{\mathrm{Nd}}_{0.1}{\mathrm{CuO}}_{2+\ensuremath{\delta}}{)}_{2}/({\mathrm{CaCuO}}_{2}{)}_{n}$ superlattices demonstrates that electrical charge distributes uniformly among the ${\mathrm{CuO}}_{2}$ planes in the Ca block. The differences observed in the Seebeck effect behavior between the Nd-doped and undoped superlattices are ascribed to the different metallic character of the Ba block in the two cases. Finally, the special role of structural disorder in superlattices with $n=1$ is pointed out by such analysis.

We present thermal conductivity measurements on very pure and dense bulk samples, as indicated by residual resistivity values as low as $0.5\ensuremath{\mu}\ensuremath{\Omega}\mathrm{cm}$ and thermal conductivity values higher than 200 W/mK. In the normal state we found that the Wiedemann-Franz law, in its generalized form, works well suggesting that phonons do not contribute to the heat transport. The thermal conductivity in the superconducting state has been analyzed by using a two-gap model. Thanks to the large gap anisotropy we were able to evaluate quantitatively intraband scattering relaxation times of $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ bands, which depend on the disorder in different way; namely, as the disorder increases, it reduces more effectively the relaxation times of $\ensuremath{\pi}$ than that of $\ensuremath{\sigma}$ bands, as suggested by a recent calculation [Mazin et al., Phys. Rev. Lett. 89, 107002 (2002)].

We report a field effect device fully made of strontium titanate (STO). This perovskite-type material is very attractive for oxide electronics both for its notable dielectric properties as well as for its semiconducting properties in the doped state. We exploit both of these properties by developing a field effect device in which oxygen deficient STO acts as a conducting channel and stoichiometric STO as a dielectric barrier. Such a barrier is obtained by electrochemical oxidation of the surface of an oxygen deficient semiconducting STO film, deposited by pulsed laser ablation in ultrahigh vacuum conditions. The channel conductivity is varied by the application of an electric field between the channel itself and a metallic gate deposited onto the dielectric barrier. Modulation capability of more than 60% is achieved by applying potential lower than 1 V. Conductivity changes are due to electrostatic induced variations of the charge carrier density (n). This result is confirmed by Hall effect measurements during gate biasing. The very good agreement of the measured n with the value calculated from the device capacitance proves the electrostatic origin of the effect observed.

We show that the growth of the heterostructure LaGaO3 /SrTiO3 yields the formation of a highly conductive interface. Our samples were carefully analyzed by high resolution electron microscopy, in order to assess their crystal perfection and to evaluate the abruptness of the interface. Their carrier density and sheet resistance are compared to the case of LaAlO3 /SrTiO3 and a superconducting transition is found. The results open the route to widening the field of polar-nonpolar interfaces, pose some phenomenological constrains to their underlying physics and highlight the chance of tailoring their properties for future applications by adopting suitable polar materials.

By means of magnetocapacitance measurements, we study subband filling in field effect induced arrays of quantum wires in GaAs/AlGaAs heterostructures. The confining potential is defined by fork-shaped interdigitated metallic gates with lithographic width of ∼150 nm, realized by e-beam lithography. The capacitance allows us to investigate the density of states. Evolution of one-dimensional subband spacing and filling as a function of confinement, gate voltage and magnetic field is analyzed and quantitatively explained. Also the development of a structure related to spin splitting is studied as a function of both magnetic field and confinement. In different regimes, we find for the g factor either an enhancement up to a factor of almost 50 with respect to the "bare" value or a suppression, accounting for exchange interactions and kinetic energy of edge electrons, respectively, in agreement with theoretical models.

By applying a negative voltage to the conducting tip of an atomic force microscope, we modify on submicron-scale semiconducting oxygen deficient SrTiO3−δ thin films grown on LaAlO3 substrates. In comparison with the as-grown film, the modified regions present different electrical and structural properties, which can be exploited to realize submicrometer circuits. After a discussion on the mechanisms of the process, we report a prototype of a SrTiO3−δ-based sidegate field-effect transistor, showing a 4% modulation of channel resistivity with gate voltages up to 40 V.

The development of thin film deposition techniques allowed the growth of new artificial materials leading to the discovery of new high T/sub c/ superconducting compounds as infinite layer based superlattices [(BaCuO/sub 2/)/sub m//(CaCuO/sub 2/)/sub n/]. The order of the superlattice structure seems to be one of the crucial parameters to be improved so as to induce a superconductivity onset in these materials and to make them suitable for technological applications. In this paper we present a study on the correlation between structural disorder and transport properties of several artificial superlattices grown by pulsed laser ablation technique. Our goal was to simulate the layered structure of copper oxide superconductors, so as to create new artificial superconducting materials and study the charge transfer mechanism. We chose infinite layer phases (SrCuO/sub 2/, CaCuO/sub 2/, BaCuO/sub 3/) for the blocks containing CuO/sub 2/ planes and doped infinite layer phases (CaCu/sub 1-x/Sc/sub x/O/sub y/, SrCu/sub 1-x/Sc/sub x/O/sub y/, CaCu/sub 1-x/Ag/sub x/O/sub y/ and SrCu/sub 1-x/Ag/sub x/O/sub y/, with x ranging from 0.1 to 0.2) for the charge reservoir blocks. By means of reflection high energy electron diffraction (RHEED) and X-ray structural characterization as well as resistivity and Hall effect measurements, we perform a comparative analysis of structural and transport properties of these samples and we tried to explain them in terms of Anderson's theory of disorder.

Infinite layer based superlattices [(BaCuO/sub 2/)/sub m//(CaCuO/sub 2/)/sub n/] are a new kind of artificial superconductors with interesting properties both for fundamental comprehension purposes and for future possible applications. The artificial nature of these compounds offers the possibility of analyzing the influence of the structural properties on the superconducting behavior. For example we can study the correlation between the number of CuO/sub 2/ planes contained in the infinite layer block (CaCuO/sub 2/) and the transport properties. In this paper, we present transport properties characterization of some thin films of this family containing different numbers of CuO/sub 2/ planes. Such films have been grown by the pulsed laser deposition (PLD) technique. In order to investigate the influence of the number of CuO/sub 2/ planes and of the distance between them on critical temperature, charge concentration, and anisotropy, we perform measurements of resistivity, magnetoresistivity and Hall coefficient as functions of temperature and applied magnetic field.

The magnetic behaviour of type-II superconducting thin films subjected to a transverse external field can be explained taking demagnetising effects into account. We studied this problem following two different approaches: on the one hand, we computed flux diffusion and circulating currents, on the basis of a recent model presented in the literature; on the other hand, we carried out measurements of local magnetic field on the film surface by means of a self-made Hall probe magnetometer. We compared the measured and the computed hysteresis loops and magnetisation decays and we found good agreement. By the fit of the experimental hysteresis loops, we also obtained the dependence of the critical current density on temperature and local magnetic field in YBCO thin films.

The employment of superconducting thin films as electronic devices seems to be the most probable future application of these materials. For this application, thin films with extremely smooth surface and very good superconducting as well as structural properties are required. We deposited YBCO epitaxial thin films by pulsed laser ablation using a pulsed molecular oxygen flow synchronous with the arrival of the ablated species on the substrate. We studied the dependence of their surface morphology on the deposition pressure and we found an evident improvement in the surface quality by lowering the deposition pressure below mbar. Films grown in these conditions showed surface roughness lower than 1 ?, transition temperatures above 88 K and critical current densities of about at 4.2 K in self-field.

Structural refinement, lattice microstrain and spontaneous strain analyses have been carried out and a phase diagram has been drawn., to be published on Journal of Physics: Condensed Matter

From recent literature, it became clear that the crystallographic lattice parameters and the superconducting properties of FeSe0.5Te0.5 thin films exhibit a non-trivial dependence on the in-plane lattice constant of the substrates on which they are grown. The strain, which depends on the type of growth, can play an important role both in enhancing the critical temperature Tc and in determining the pinning mechanisms. Here, we present the effects of the substrate on the superconducting properties of FeSe0.5Te0.5 thin films. After a comprehensive overview of the different substrates used, i.e. oxides and fluorides, we compare the superconducting properties of films grown on LaAlO3(001) and SrTiO3(001). We show that the pinning properties of the two types of film are completely different: the angular dependences of the critical current density are opposite due to the presence of extrinsic pinning along the c-axis on films grown on SrTiO3. The presence of strong correlated pinning for a field perpendicular to the surface in films grown on SrTiO3 is confirmed by the analysis of the activation energy for vortex motion U0 and supported by the observation through scanning tunnelling microscopy of nanoscale threading dislocations possibly induced by the lattice mismatch with the substrate, which are not seen on the films deposited on LaAlO3.

We report on measurements of the superconducting properties of FeSe0.5Te0.5 thin films grown on lanthanum aluminate. The films have high transition temperatures (above 19 K) and sharp resistive transitions in fields up to 15 T. The temperature dependence of the upper critical field and the irreversibility lines are steep and anisotropic, as recently reported for single crystals. The critical current densities, assessed by magnetization measurements in a vector VSM, were found to be well above 109 A m-2 at low temperatures. In all samples, the critical current as a function of field orientation has a maximum, when the field is oriented parallel to the film surface. The maximum indicates the presence of correlated pinning centers. A minimum occurs in three films, when the field is applied perpendicular to the film plane. In the fourth film, instead, a local maximum caused by c-axis-correlated pinning centers was found at this orientation. The irradiation of two films with fast neutrons did not change the properties drastically, where a maximum enhancement of the critical current by a factor of two was found.

In this paper we carry out a direct comparison between transport and superconducting properties?namely resistivity, magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper critical field?of two different families of Fe-based superconductors, which can be viewed in many respects as end members: SmFeAsO1 ? xFx with the largest Tc and the largest anisotropy and Fe1 + yTe1 ? xSex, with the largest Hc2, the lowest Tc and the lowest anisotropy. In the case of the SmFeAsO1 ? xFx series, we find that a single-band description allows us to extract an approximate estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of the Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data for the Fe1 + y(Te1 ? x, Sex) series exhibit a strongly compensated behaviour, which can be described only within a multiband model. In the Fe1 + y(Te1 ? x, Sex) series, the role of the excess Fe, tuned by Se stoichiometry, is found to be twofold: on one hand it dopes electrons in the system and on the other hand it introduces localized magnetic moments, responsible for Kondo like scattering and likely pairbreaking of Cooper pairs. Hence, Fe excess also plays a crucial role in determining superconducting properties such as the Tc and the upper critical field Hc2. The huge Hc2 values of the Fe1 + yTe1 ? xSex samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAsO1 ? xFx samples. Hence, magnetic scattering by excess Fe seems to drive the system in the dirty regime, but its detrimental pairbreaking role seems not to be as severe as predicted by theory. This issue has yet to be clarified, addressing the more fundamental issue of the interplay between magnetism and superconductivity.

We study the critical current density Jc of a series of MgB2 epitaxial thin films with disorder introduced in a controlled way by neutron irradiation. We find that the in-field Jc is significantly increased for an intermediate neutron fluence of ~7.6 × 1017 cm−2, reaching ~1.8 × 105 A cm−2 at μ0H = 4 T, which is larger by more than one order of magnitude than the critical current density of the pristine sample. Moreover, in the same irradiated sample, Jc(μ0H = 9 T) is still as large as 1.1 × 104 A cm−2. By transmission electron microscopy, we find evidence of the presence of amorphous regions created by irradiation whose average size matches the coherence length ξab. Analysis of the shape of the critical current density and pinning force curves indicates that such regions with a suppressed superconducting order parameter behave as effective point-like pinning centers for magnetic flux lines.

In this letter we present the effect of neutron irradiation up to fluences of 3.9 1019 n/cm2 on the superconducting properties of MgB2. In order to obtain a disorder structure homogeneously distributed, the experiment was carried out on bulk samples prepared with the 11B isotope. Up to fluences of 1018 n/cm2 the critical temperature is slightly diminished (36 K) and the superconducting properties are significantly improved; the upper critical field is increased from 13.5 T to 20.3 T at 12 K and the irreversibility field is doubled at 5 K. For larger neutron fluences the critical temperature is suppressed down to 12 K and the superconducting properties come out strongly degraded.

High electron mobility is measured down to low temperature in epitaxial ZnO thin films deposited on (110) oriented strontium titanate substrates. Electron mobility is evaluated by both magnetoresistance and resistivity-Hall effect data. Values up to 400cm2∕Vs are found below 50 K in epitaxial thin films grown by a two-step method: first a 100-nm-thick ZnO relaxing layer is deposited on the SrTiO3 (110) substrate at relatively low temperature (550–600 °C) and then the deposition temperature is raised up to 800 °C for the growth of a second ZnO layer. Reflection high energy electron diffraction analysis during the deposition, ex situ x-ray diffraction and AFM morphology studies performed separately on each layer reveal that the first layer grows in a quasi-two-dimensional mode while the increased temperature in the second step improves the crystalline quality of the film. The integration of ZnO transparent semiconductor with high-k dielectric perovskite substrates may lead to a wide variety of new electroni...

Magnetic measurements on type-II superconducting thin films subjected to a transverse external field can be explained by a model that takes demagnetizing effects into account. We carried out measurements of magnetic flux distribution on the film surface by means of a self made apparatus, based on two Hall probes and a differential transformer and we tested the validity of a recent model presented in the literature.

Twisted and not twisted multifilamentary BSCCO (2223)-Ag tapes, prepared by the PIT technique, have been characterized by SQUID magnetometry with the magnetic field applied both parallel and perpendicular to thec-axis. From these measurements an estimation of the critical current densitiesJ ab with Hext parallel to thec-axis,J ab with Hext parallel to thea- b planes andJ c is obtained. These values are in fairly good agreement with those obtained by transport measurements. The tapes turned out to be very homogeneous and practically no difference was found in the hysteresis loops of twisted and not twisted tapes, which suggests that the mean grain alignment is the same in the two types of samples and the current path probably follows the “brick wall” model.

In this paper a short description of the problems related to the preparation of magnesium diboride samples in form of bulk, thin films and tapes is given. The principal problem is that MG is highly volatile: this fact imposes severe constraints to the sample preparatation. We briefly review the activity in this field of the INFM-LAMIA laboratory. A method has been developed to prepare very clean and dense bulk samples with good transport properties in a single step, starting from pure elements. In the tapes fabrication, very promising results are obtained with a powder in tube procedure: the low cost and the good results obtained in term of critical current density make this compound interesting for the applications. The various thin film deposition techniques developed will be also shortly reviewed with particular regard to the in situ ones.

We present a systematic study of the superconducting properties of high quality V3Si thin films, grown by high vacuum pulsed laser deposition from a stoichiometric target, a technique never reported in literature for this compound. By changing both the substrate (crystal structure and orientation) and the deposition conditions (substrate temperature, target-substrate distance, laser frequency and pulse energy) the critical temperature and the resistivity values of the samples have been finely tuned. Best results (TC = 16.1 K and Residual Resistivity Ratio RRR = 8.5) are obtained for deposition temperatures higher than 1200degC on both LaAlO3 and MgO substrates. All the samples are polycrystalline, that however does not affect the good quality of the transport properties. The possibility of multiband superconductivity in this compound is investigated, probing as a function of temperature: (i) the critical field up to 28 T using voltamperometric measurements, and (ii) the superfluid density via magnetic penetration depth measurements with a single coil inductive technique operating in the MHz region.

The voltage-spectral density SV(f) of the 2-dimensional electron gas formed at the interface of LaAlO3 /SrTiO3 has been thoroughly investigated. The low-frequency component has a clear 1/f behavior with a quadratic bias current dependence, attributed to resistance fluctuations. However, its temperature dependence is inconsistent with the classical Hooge model, based on carrier-mobility fluctuations. The experimental results are, instead, explained in terms of carrier-number fluctuations, due to an excitation-trapping mechanism of the 2-dimensional electron gas., 4 pages, 4 figures

In this work, we report resistivity measurements on MgB2 polycrystalline samples with increasing degree of doping (disorder) up to temperatures higher than 900 K. In all cases the normal state curves are linear with increasing temperatures, with no sign of flattening or downward bending (saturation) at high temperature. This behavior, in sharp contrast to what is observed in superconducting compounds such as A15 and Chevrel phases, can be explained considering the dominant pi contribution to the electric conduction, together with the smaller electron-phonon coupling of pi carriers in MgB2, as compared to characteristic coupling strength of those compounds. This indicates that in MgB2 the disorder introduced in the lattice by thermal oscillations even at 900 K can still be considered as a perturbation within the Boltzmann description of transport.