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Electric and thermoelectric transport properties are mutually intertangled in diffusive transport equations. In particular, in high mobility multiband systems an anomalous behavior may occur, which can be tracked down to the properties of the individual bands. Here, we present magnetoelectric and magnetothermoelectric transport properties of a ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ high-quality single crystal, for different magnetic field directions up to 30 T. We detect a giant Nernst effect and an anomalous field dependence of the Seebeck coefficient. The extraction of the Peltier tensor coefficients ${\ensuremath{\alpha}}_{xx}, {\ensuremath{\alpha}}_{xy}$, and ${\ensuremath{\alpha}}_{xz}$ allows us to disentangle the main transport mechanisms in play. The large ${\ensuremath{\alpha}}_{xy}$ and ${\ensuremath{\alpha}}_{xz}$ values and their field dependence provide evidence of the presence of a high mobility band, compatible with a Dirac dispersion band, crossing the Fermi level, and suggest a possible three-dimensional nature of the Dirac fermions.

The thermoelectric power of the two-dimensional electron system (2DES) at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface is explored below room temperature, in comparison with that of Nb-doped ${\mathrm{SrTiO}}_{3}$ single crystals. For the interface, we find a region below $T=50$ K where thermopower is dominated by phonon drag, whose amplitude is hugely amplified with respect to the corresponding bulk value, reaching values $\ensuremath{\sim}\mathrm{mV}$/K and above. The phonon-drag enhancement at the interface is traced back to the tight carrier confinement of the 2DES, and represents a sharp signature of strong electron-acoustic phonon coupling at the interface.

Investigations of $\mathrm{Mg}{\mathrm{B}}_{2}$ and Fe-based superconductors in recent years have revealed many unusual effects of multiband superconductivity, but manifestations of anisotropic multiband effects in the critical current density ${J}_{c}$ have not been addressed experimentally, mostly because of the difficulties in measuring ${J}_{c}$ along the $c$ axis. To investigate the effect of very different intrinsic anisotropies of \ensuremath{\sigma} and \ensuremath{\pi} electron bands in $\mathrm{Mg}{\mathrm{B}}_{2}$ on current transport, we grew epitaxial films with a tilted $c$ axis (\ensuremath{\theta} \ensuremath{\sim} 19.5\ifmmode^\circ\else\textdegree\fi{}), which enabled us to measure the components of ${J}_{c}$ along both the $ab$ plane and the $c$ axis using magneto-optical and transport techniques. These measurements were combined with scanning and transmission electron microscopy, which revealed terraced steps on the surface of the tilted-$c$-axis films. The measured field and temperature dependencies of the anisotropic ${J}_{c}(H)$ show that ${J}_{c,\mathrm{L}}$ parallel to the terraced steps is higher than ${J}_{c,\mathrm{T}}$ perpendicular to the terraced steps, and ${J}_{c}$ of thinner films (50 nm) obtained from transport experiments at 0.1 T reaches \ensuremath{\sim}10% of the depairing current density ${J}_{d}$ in the $ab$ plane, while magneto-optical imaging revealed much higher ${J}_{c}$ at lower fields. To analyze the experimental data we developed a model of anisotropic vortex pinning which accounts for the observed behavior of ${J}_{c}$ in the tilted-$c$-axis films and suggests that the apparent anisotropy of ${J}_{c}$ is affected by current pairbreaking effects in the weaker \ensuremath{\pi} band. Our results indicate that the out-of-plane current transport mediated by the \ensuremath{\pi} band could set the ultimate limit of ${J}_{c}$ in $\mathrm{Mg}{\mathrm{B}}_{2}$ polycrystals.

We investigate Seebeck effect in REFeAsO (RE=rare earth)compounds as a function of temperature and magnetic field up to 30T. The Seebeck curves are characterized by a broad negative bump around 50K, which is sample dependent and strongly enhanced by the application of a magnetic field. A model for the temperature and field dependence of the magnon drag contribution to the Seebeck effect by antiferromagnetic (AFM) spin fluctuation is developed. It accounts for the magnitude and scaling properties of such bump feature in our experimental data. This analysis allows to extract precious information on the coupling between electrons and AFM spin fluctuations in these parent compound systems, with implications on the pairing mechanism of the related superconducting compounds.

Using advanced first-principles calculations we predict that the non-polar SrTiO$_3$/SrZrO$_3$ (001) interface, designed as either thin SrZrO$_3$ film deposited on SrTiO$_3$ or short-period (SrTiO$_3$)$_m$/(SrZrO$_3$)$_n$ superlattice, host a 2-dimensionally confined electron gas. Mobile electron charge due to native impurities, field-effect, or modulation doping remains tightly trapped at the interface. Key ingredients for this occurrence are a) the peculiar chemistry of 3d orbitals, b) the large band offset at titanate-zirconate interface.

In this work we investigate the Ru substituted LaFeAsO compound, by studying the magnetotransport behaviour and its relationship with the band structure, in different regimes of temperature, magnetic field and Ru content. In particular we analyse the magnetoresistance of LaFe1-xRuxAsO (0, 18 pages, 12 figures, 1 table

We report on measurements of electric transport and voltage noise in FeTe${}_{0.5}$Se${}_{0.5}$ epitaxial thin films. Detailed structural investigations demonstrate the high quality of the samples, also testified by a superconducting transition temperature of $18$ K, larger than the value observed for bulk compounds. An unusual dependence of the 1/$f$ noise is found by varying the applied voltage and the temperature. Above a threshold voltage the 1/$f$ noise shows a nonquadratic dependence on applied voltage with a temperature-dependent exponent. Conversely, at low voltages the 1/$f$ noise amplitude follows always a quadratic law. In the superconductive transition region, a simple percolative model allows one to estimate the dimensionality of the system, which is found to be two-dimensional, with a critical exponent similar to values reported for other high-${T}_{c}$ superconductors. The experimental findings on noise properties give evidence of unusual transport processes, occurring in this new superconducting material, and can provide useful information on possible conduction mechanisms.

In this work we present a systematic experimental and theoretical study of the structural, transport, and superconducting properties of $\text{Sm}({\text{Fe}}_{1\ensuremath{-}x}{\text{Ru}}_{x})\text{As}({\text{O}}_{0.85}{\text{F}}_{0.15})$ polycrystalline samples as a function of Ru content $(x)$ ranging from 0 to 1. The choice of Ru as isoelectronic substitution at Fe site of F-doped compounds allows to better clarify the role of structural disorder in modifying the normal and superconducting properties of these recently discovered multiband superconductors. Two different regions are identified: the Fe-rich phase $(xl0.5)$ where superconducting and normal-state properties are strongly affected by disorder induced by Ru substitution; the Ru-rich phase $(xg0.5)$ where the system is metallic and strongly compensated and the presence of Ru frustrates the magnetic moment on Fe ions. Here the lack of magnetic features and related spin fluctuations may be the cause for the suppression of superconductivity.

We probe spin transport in ${\text{Cu}}_{2}\text{O}$ by measuring spin-valve effect in ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}/{\text{Cu}}_{2}\text{O}/\text{Co}$ and ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}/{\text{Cu}}_{2}\text{O}/{\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}$ epitaxial heterostructures. In ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}/{\text{Cu}}_{2}\text{O}/\text{Co}$ systems, we find that a fraction of out-of-equilibrium spin-polarized carrier actually travel across the ${\text{Cu}}_{2}\text{O}$ layer up to distances of almost 100 nm at low temperature. The corresponding spin-diffusion length ${d}_{\text{spin}}$ is estimated around 40 nm. Furthermore, we find that the insertion of a ${\text{SrTiO}}_{3}$ tunneling barrier does not improve spin injection, likely due to the matching of resistances at the interfaces. Our result on ${d}_{\text{spin}}$ may be likely improved, both in terms of ${\text{Cu}}_{2}\text{O}$ crystalline quality and submicrometric morphology and in terms of device geometry, indicating that ${\text{Cu}}_{2}\text{O}$ is a potential material for efficient spin transport in devices based on crystalline oxides.

The observation of metallic behavior at the interface between insulating oxides has triggered worldwide efforts to shed light on the physics of these systems and clarify some still open issues, among which the dimensional character of the conducting system. In order to address this issue, we measure electrical transport (Seebeck effect, Hall effect, and conductivity) in ${\text{LaAlO}}_{3}/{\text{SrTiO}}_{3}$ interfaces and, for comparison, in a doped ${\text{SrTiO}}_{3}$ bulk single crystal. In these experiments, the carrier concentration is tuned, using the field effect in a back-gate geometry. The combined analysis of all experimental data at 77 K indicates that the thickness of the conducting layer is $\ensuremath{\sim}7\text{ }\text{nm}$ and that the Seebeck-effect data are well described by a two-dimensional density of states. We find that the back-gate voltage is effective in varying not only the charge density but also the thickness of the conducting layer, which is found to change by a factor of $\ensuremath{\sim}2$, using an electric field between $\ensuremath{-}4$ and $+4\text{ }\text{MV}/\text{m}$ at 77 K. No enhancement of the Seebeck effect due to the electronic confinement and no evidence for two-dimensional quantization steps are observed at the interfaces.

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.

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 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)].

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.