Recent observation of a "kink" in single-particle dispersion in photoemission experiments on cuprate superconductors has re-initiated a heated debate over the issue of a boson that "mediates" the pairing in cuprates. If the "kink" is indeed caused by interaction with a bosonic excitation, then there are two possible candidates: phonons and spin fluctuations. Here, the role of anti-ferromagnetic spin fluctuations in shaping the phase diagram of cuprate superconductors will be discussed. By using the local (momentum-integrated) dynamic spin susceptibility, recently measured in neutron scattering experiments to high energies, the electronic self-energies are calculated that agree in many aspects with those measured directly in angular resolved photoemission and optical spectroscopies. The spin fluctuations therefore seem to play a role typically played by phonons in renormalizing single particles. The key question emerging from this picture is whether the coupling detected in angle-resolved photoemission spectroscopy (ARPES) reflects the mediating boson, i.e. whether the spin fluctuations may be responsible for superconducting pairing.

We systematically study the structural and electronic properties of very thin cuprate films. Our direct angle resolved photoemission spectroscopy (ARPES) measurements on the low binding energy electronic structure of La_2-xSr_xCuO₄ (LSCO) films confirmed that the Fermi surface evolves with doping, but changes even more significantly with growth-induced compressive strain. For a given doping, the in-plane compressive strain enhances T_C's and modifies the 2-dimensional hole-like Fermi surface as to appear more electron-like. In contrast, the in-plane tensile strain reduces T_C (suppressing superconductivity for huge tensile strain) and shows 3-dimensional ARPES dispersion with a corresponding 3-dimensional Fermi surface. To account for these striking changes in electronic structure and superconductivity, the out-of-plane states should be taken into account, as well as some subtle changes in the associated atomic distances.

We argue that the extension of the BCS theory to the strong-coupling regime describes the high-temperature superconductivity of cuprates and the colossal magnetoresistance (CMR) of ferromagnetic oxides if the phonon dressing of carriers and strong attractive correlations are taken into account. The attraction between carriers, which is prerequisite to high-temperature superconductivity, is caused by an almost unretarted electron-phonon interaction sufficient to overcome the direct Coulomb repulsion in the strong-coupling limit, where electrons become polarons and bipolarons (real-space electron or hole pairs dressed by phonons). The long-range Froehlich electron-phonon interaction has been identified as the most essential in cuprates providing "superlight" lattice polarons and bipolarons. A number of key observations have been predicted and/or explained with polarons and bipolarons including unusual isotope effects, normal state (pseudo)gaps, upper critical fields, etc. Here some kinetic, magnetic, and more recent thermomagnetic normal state measurements are interpreted in the framework of the strong-coupling theory, including the Nernst effect and normal state diamagnetism. Remarkably, a similar strong-coupling approach offers a simple explanation of CMR in ferromagnetic oxides, while the conventional double-exchange (DEX) model, proposed half a century ago and generalised more recently to include the electronphonon interaction, is in conflict with a number of modern experiments. Among these experiments are site-selective spectroscopies, which have shown that oxygen p-holes are current carriers rather than d-electrons in ferromagnetic manganites (and in cuprates) ruling out DEX mechanism of CMR. Also some samples of ferromagnetic manganites manifest an insulating-like optical conductivity at all temperatures contradicting the DEX notion that their ferromagnetic phase is metallic. On the other hand, the pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair-breaking in the ferromagnetic phase account for the first-order ferromagnetic phase transition, CMR, isotope effects, and pseudogaps in doped manganites. Here we propose an explanation of the phase coexistence and describe the shape of resistivity of manganites near the transition in the framework of the strong-coupling approach.

High temperature superconductivity (HTSC) in copper oxides appears upon doping an antiferromagnetic Mott-Hubbard insulator. While at high temperatures the dopants are randomly distributed over the host lattice, at the pseudo-gap temperature T* dynamic patterning in terms of stripe segments is observed. In this regime charge rich and charge poor regions coexist and interact dynamically with each other. It is shown here that this form of heterogeneity leads to multicomponent superconductivity with largely enhanced values of the superconducting transition temperature T_c. The special role played by the lattice is addressed and it is shown that intermediate sized polarons are formed which are the origin of unconventional isotope and strain effects.

One type of order that has been observed to compete with superconductivity in cuprates involves alternating charge and antiferromagnetic stripes. Recent neutron scattering studies indicate that the magnetic excitation spectrum of a stripe-ordered sample is very similar to that observed in superconducting samples. In fact, it now appears that there may be a universal magnetic spectrum for the cuprates. One likely implication of this universal spectrum is that stripes of a dynamic form are present in the superconducting samples. On cooling through the superconducting transition temperature, a gap opens in the magnetic spectrum, and the weight lost at low energy piles up above the gap; the transition temperature is correlated with the size of the spin gap. Depending on the magnitude of the spin gap with respect to the magnetic spectrum, the enhanced magnetic scattering at low temperature can be either commensurate or incommensurate. Connections between stripe correlations and superconductivity are discussed.

The competition of antiferromagnetic and d-wave superconductivity order parameters in cuprates is studied within a phenomenological model. An unbiased numerical analysis is carried out. The results suggest that the transition from the antiferromagnetic to the superconducting region is not universal. When disorder is present, a glassy state forms leading to the possibility of "colossal" effects in some cuprates, analog of those in other transition metal oxides, in particular manganites. Non-superconducting Cu-oxides could rapidly become superconducting by the influence of weak perturbations. Consequences of this mechanism for thin-film and angle-resolved photoemission experiments are discussed. In addition, a recent study of the strong-coupling region in d-wave superconductors with a numerically exact technique is briefly reviewed.

Normal and superconducting state spectral properties of cuprates are theoretically described within the extended t-J model. The method is based on the equations of motion for projected fermionic operators and the mode-coupling approximation for the self-energy matrix. The dynamical spin susceptibility at various doping is considered as an input, extracted from experiments. The analysis shows that the onset of superconductivity is dominated by the spin-fluctuation contribution. The coupling to spin fluctuations directly involves the next-nearest-neighbor hopping t', hence T_c shows a pronounced dependence on t'. The latter can offer an explanation for the variation of T_c among different families of hole-doped cuprates. A formula for maximum T_c is given and it is shown that optimum doping, where maximum T_c is reached, is with increasing -t' progresively increased.

A detailed theoretical description is provided of the narrow low-energy peak in the ARPES response of superconducting optimally doped and weakly underdoped BSCCO near the van Hove point. The pseudogap is taken to be due to electron-paramagnon scattering. The narrow peak is antiadiabatic: it consists of electrons which are so slow that the scattering is not effective in suppressing their spectral strength. We find two temperature regimes for the pseudogap. The low-temperature one is relevant for experiment in BSCCO, where the paramagnon band-edge is much higher than the temperature. The high-temperature regime occurs when the band-edge is lower than the temperature. It is characterized by hot spots when the band-edge is finite, and develops a macroscopic antiferromagnetic potential when it vanishes. We argue that it is relevant for the electron-doped high-T_c compounds. Our work gives a connection between the simultaneous appearance of a magnetic resonance and a narrow low-energy feature in ARPES at the superconducting transition in BSCCO. In the model, both can be obtained by switching the paramagnon damping from supercritical to subcritical, without even including the superconducting correlations explicitly. The leading edge scale of the narrow peak is controlled by the chemical potential and is incidental to the pseudogap mechanism, whose physical scale is given by the high-energy 'hump.'

Optical spectroscopy and angle resolved photoemission have replaced tunneling spectroscopy as techniques that yield the best data on the self energy of the charge carriers in new superconductors. We review the most recent results of self-energy spectroscopy for several cuprate superconductors. We show that the self energy is determined by two channels of bosonic excitations, a sharp peak and a continuous background. Both channels appear to be of magnetic origin, the sharp mode is associated with the so-called 41 meV neutron resonance that appears only at low temperatures, a few tens of degrees above the superconducting transition temperature in underdoped materials, while the continuous background has a temperature independent spectral function.

A scaling relation N_c or ρ_s ασ_dcΤ_c has been observed in the copper-oxide superconductors, where ρ_s is the superconducting condensate and the spectral weight is N_c=ρ_s/8, T_c is the critical temperature, and σ_dc is the normal-state dc conductivity close to T_c. This scaling relation is examined within the context of clean and dirty-limit BCS superconductors. It is shown that the scaling relation N_c ≃ 4.4σ_dcT_c, which follows directly from the Ferrell-Glover-Tinkham sum rule, is the hallmark of a BCS system in the dirty-limit. The scaling relation implies that the copper-oxide superconductors are likely to be in the dirty limit, and that as a result the energy scale associated with the formation of the condensate scales linearly with T_c. The a-b planes and the c axis also follow the same scaling relation. It is observed that the scaling behavior for the dirty limit and the Josephson effect (assuming a BCS formalism) are essentially identical, suggesting that in some regime these two pictures may be viewed as equivalent.

We consider the Josephson vortex flow in layered superconductors in the presence of the transport DC current perpendicular to the layers and high magnetic field parallel to the layers. Moving vortex lattice induces the radiation from the edge of the crystal into the dielectric. We derive radiation power for moving rectangular and triangular lattice. We estimate corresponding radiation power for Bi-based cuprate superconductor when the magnetic field of order of one tesla is applied and the radiation frequency is in THz interval. We show qualitatively that radiation power for lattice disordered along the c-axis is weaker than that for regular lattice. We estimate the heat flow which in cuprate crystals is determined mainly by the nodal quasiparticles.

We study the Josephson effect in short ballistic SINIS and SIFIS double-barrier junctions, consisting of clean superconductors (S), a normal metal (N) or ferromagnet (F), and insulating interfaces (I). For SINIS double-tunnel junctions, the critical Josephson current as a function of the junction width shows sharp peaks because of resonant amplification of the Andreev process when the quasi-bound states of the normal interlayer enter the superconducting gap and morph into phase-sensitive bound states. For SIFIS double-tunnel junctions the corresponding quasi-bound states are spin-split, they amplify the supercurrent less efficiently, and trigger transitions between 0 and π states of the junction. In contrast to SINIS junctions where the critical current reaches a peak value when the Andreev bound states cross the Fermi surface, here a narrow dip opens up exactly at the peak due to compensation of partial currents flowing in opposite directions. With increased barrier transparency, the described mechanism is modified by the broadening and overlap of quasi-bound states. Temperature-induced transitions both from 0 to π and from π to 0 states are studied by computing the phase diagram (with temperature and junction width as the variables) for different interfacial transparencies varying from transparent metallic to the tunnel limit.

Magnetic properties of SFS and SF ramp-type junctions with YBa₂Cu₃O_7-δ (YBCO) electrodes (S), and the itinerant ferromagnet SrRuO₃ (SRO - F), were investigated. We looked for a crossed Andreev reflection effect (CARE) in which an electron from one magnetic domain in F is Andreev reflected as a hole into an adjacent, oppositely polarized, domain while a pair is transmitted into S. CARE is possible in SRO since the width of its domain walls is of the order of the YBCO coherence length (2-3 nm). Our junctions behave as typical magnetic tunneling junctions, as the conductance spectra were always asymmetric, and a few showed bound state peaks at finite bias which shifted with field according to the classical Tedrow and Meservey theory. In many of our SFS junctions with a barrier thickness of 10-20 nm, a prominent zero bias conductance peak (ZBCP) has been observed. This peak was found to decrease linearly with magnetic field, as expected for Andreev and CARE scattering. In contrast, in SF junctions, the observed ZBCP was found to decrease versus field almost exponentially, in agreement with the Anderson-Appelbaum theory of scattering by magnetic states in F. Thus, transport in our SFS and SF junctions depends strongly on the size of the F layer. We also found that in both cases, the ZBCP height at zero field decreased with increasing magnetic order of the domains in F, in agreement with the CARE mechanism.

The significance of a negative dielectric constant has long been recognized. We report here the observation of a field-induced large negative dielectric constant of aggregates of oxide nano-particles at frequencies below ~ 1 Hz at room temperature. The accompanying induced charge detected opposes the electric field applied in the field-induced negative dielectric constant state. A possible collective effect in the nano-particle aggregates is proposed to account for the observations. Materials with a negative dielectric constant are expected to provide an attraction between similar charges and unusual scattering to electromagnetic waves with possible profound implications for high temperature superconductivity and communications.

Using combinatorial molecular beam epitaxy (COMBE), we have deposited a one-dimensional (1D) combinatorial library of La_2-xSr_xCuO₄ (LSCO) single-crystal thin films with different Sr content above the optimum doping level. A study of this LSCO library allows a detailed evaluation of the COMBE method. We have also developed and tested a custom-made multiple-probe transport measurement set-up that allowed us to measure the R(T) curves from more than 2,000 different samples (pixels in the 1D combinatorial library of LSCO) within one week. We also studied in detail the dependence of the crystal structure (specifically, the c-axis lattice constant) on the Sr content and on the type of epitaxial strain (compressive or tensile). For the films grown on LSAO substrates, we found that the c-axis lattice constant of LSCO films decreased as the Sr content was increased. This we attribute to the reduction in epitaxial strain that occurs because of Sr-doping-induced decrease of in-plane lattice constant of LSCO. Next, we have detected a small deviation of the beam profile from the linear dependence, noticeable for the deposition area larger that 1". If an array of substrates is used whereby some substrates are separated by more than 1" and if the stoichiometry is optimized at or close to the center of the array, in the films positioned at the outer edges of the array this effect causes slight off-stoichiometry and generation of secondary-phase defects.

We present a new class of nanoscale experiments on cuprate superconductors in the current-carrying state. These experiments are aimed at studying how the high-T_c order parameter responds, in both real and reciprocal space, when driven by a sizable phase gradient. First, scanning tunneling spectroscopy was performed on current-carrying YBa₂Cu₃O_6+x (YBCO) thin-film strips, to reveal a remarkable suppression of the low-energy Andreev states indicating an increase in the local phase fluctuations. Second, transport measurements were made on optimally-doped YBCO nanostructures, to show anomalous current-voltage nonlinearities consistent with the formation of current-driven phase slip lines. These results are discussed in the general context of non-rigidity of the high-T_c order parameter under electrodynamic perturbation.

Perovskite-related layered multicomponent oxide films with strongly correlated electrons such as (La, Sr)MnO₃ and HTS are promissing candidates for advanced MRAM, Josephson devices and others. These devices have usually sandwich-type structure with an ultra-thin intermediate layer. Formation of the impurity-precipitates on the surface during the growth of the multicomponent oxide films is a fatal problem working against high-performance of devices. In this study, high quality and surface-clean thin films of multicomponent oxides have been grown by MOCVD on substrates with artificial steps of predefined height and width. The surface of the films grown on the steps having width equal to the 'double of the migration length' of the atomic species depositing on the substrate is totally free of precipitates: precipitates are gathered at the step edges where the free energy is lowest. The method has several advantages: it is simple, universal (it is independent of the materials, substrates, deposition technique or application) and allows control of precipitates segregates so that the quality and growth conditions of the films are the same as for the films grown on conventional substrates. The method is expected to result in new opportunities for the device fabrication, integration, design and performance. As an example we present successful fabrication of a mesa structure showing intrinsic Josephson effect. We have used completely precipitate-free thin films of Bi-2212/Bi-2223 superstructure grown on (001) SrTiO₃ single crystal substrates with artificial steps.

The effect of competing orders and quantum criticality on the macroscopic vortex dynamics and microscopic low-energy excitations of cuprate superconductors is investigated using high-field magnetic measurements and low-temperature scanning tunneling spectroscopy. Our experimental results suggest that significant field-induced quantum fluctuations at low temperatures are present in all cuprates investigated, suggesting that cuprate superconductors are in close proximity to a quantum critical point (QCP) that separates a pure superconducting phase (SC) from a phase consisting of coexisting SC and a competing order. The proximity of a cuprate to the QCP can be determined from a normalized characteristic field in the zero-temperature limit, and the characteristic field correlates well with the quasiparticle tunneling spectra, showing increasing spectral deviation from the mean-field behavior for samples of closer proximity to the QCP. Macroscopically, the presence of competing order can induce strong fluctuation effects in the cuprate superconductors, which is consistent with the extreme type-II nature of the cuprates. The relevant competing orders in different cuprates are examined by comparing theory with experimental data, and the physics implications of these studies are discussed.

Since the ferromagnetic side of a superconductor-ferromagnet junction is spin polarized, Andreev reflections are suppressed. Consequently, the induced superconductor order parameter in the ferromagnet is expected to decay rapidly, on the order of a few nm. Our scanning tunneling spectroscopy measurements on thin epitaxial (100)YBa₂Cu₃O_7-δ-SrRuO₃ (YBCO-SRO) bilayers, where SRO is a ferromagnet, indeed show that on most of the junction area the superconductor order parameter vanishes in the SRO over a distance less than 8 nm. However, we find localized regions, arranged along narrow (<10 nm) stripes, where the order parameter (superconductor-like gap structure) penetrates the ferromagnet more than 20 nm. This is attributed to "crossed Andreev reflections", taking place at domain boundaries, where an electron from one magnetic domain is retro reflected as a hole with opposite spin in an adjacent domain. This phenomenon, directly observed here for the first time, may account for the (not abundant) cases where a long-range proximity effect was found in superconductor-ferromagnet proximity systems.

Electronic phase behavior in correlated-electron systems is a fundamental problem of condensed matter physics. We argue here that the change in the phase behavior near the surface and interface, i.e., electronic reconstruction, is the fundamental issue of the correlated-electron surface or interface science. Beyond its importance to basic science, understanding of this behavior is crucial for potential devices exploiting the novel properties of the correlated systems. We present a general overview of the field, and then illustrate the general concepts by theoretical studies of the model heterostructures comprised of a Mott-insulator and a band-insulator, which show that spin (and orbital) orderings in thin heterostructures are generically different from the bulk and that the interface region, about three-unit-cell wide, is always metallic, demonstrating that electronic reconstruction generally occurs. Predictions for photoemission experiments are made to show how the electronic properties change as a function of position, and the magnetic phase diagram is determined as a function of temperature, number of layers, and interaction strength. Future directions for research are also discussed.

We revisit the interlayer tunneling theory of high temperature superconductors and formulate it as a mechanism by which the striking systematics of the transition temperature within a given homologous series can be understood. We pay attention not only to the enhancement of pairing, as was originally suggested, but also to the role of competing order parameters that tend to suppress superconductivity, and to the charge imbalance between inequivalent outer and inner CuO₂ planes in a unit cell. Calculations based on a generalized Ginzburg-Landau theory yield results that bear robust and remarkable resemblance to experimental observations.

A systematic study of the geometry dependent nucleation of superconductivity in nanoscaled superconductors is presented in this paper. The experimental Τ_c(Η) phase boundary is compared to theoretical calculations obtained in the framework of the linearized Ginzburg-Landau theory for different geometries (square, triangle, disk). The influence of the transformation of a square into a rectangle on the Τ_c(Η) phase boundary is analyzed. In elongated rectangles, a crossover from a linear to a parabolic field dependence of Τ_c has been observed. The evolution of the superconducting state is studied in a perforated disk by varying the size of the hole. A transition from a one-dimensional to a two-dimensional regime is seen when increasing the magnetic field for disks with small holes.

Two magnetic transitions at T_M~160 K and at T_M2~80-100 K, are observed in the magneto-superconducting RuEu_2-xCe_xSr₂Cu₂O_10-δ (T_C = 32-50 K). Below T_M2, the Ru⁵⁺ moments are weakly-ferromagnetically ordered and wide ferromagnetic hysteresis loops are observed, they become narrow and disappear at ~ 60-70 K. Dc magnetic studies on c-axis oriented epitaxial thin films of Ru-1222 on (100) SrTiO₃ wafers show that the easy axis of the magnetization is in the basal plane and the Ru⁵⁺ ions are in their high-spin state. Above T_M2, (i) small antiferromagnetic-like hysteresis loops reappear with a peak in the coercive fields around 120 K. (ii) A small peak at ~120 K is also observed in the dc and ac susceptibility curves. The two phenomena are absent in the non-superconducting RuEuCeSr₂Cu₂O_10-δ (x=1) samples. Two scenarios are suggested for the phenomena at T_M2M. (i) For x<1, the decrease of the Ce⁴⁺ content, is compensated by non-homogeneous oxygen depletion, which may induce a reduction of Ru⁵⁺ ions to Ru⁴⁺. The higher ordering temperature, T_M, which does not change with x, may result from Ru⁴⁺ nano-size rich clusters, in which the Ru⁴⁺-Ru⁴⁺ exchange interactions are stronger than the Ru⁵⁺-Ru⁵⁺ interactions. Or alternatively,(ii) The presence of nanoparticles of a minor foreign extra Ru⁴⁺ magnetic phase, such as Sr-Cu-Ru-O₃, in which Cu is distributed inhomogeneously in both the Ru and Sr sites.

We present here our research on time-resolved carrier, phonon, and spin dynamics in the diluted-magnetic semiconductor Cd_1-xMn_xTe [Cd(Mn)Te] system. Our test samples were the high-quality single crystals with the Mn doping concentrations ranging from 9% to 12%, grown by a modified Bridgeman method. Femtosecond optical pump-probe spectroscopy experiments allowed us to study time-resolved dynamics of both the excited carriers and coherent acoustic phonons. Using pump photons with the energy just exceeding the Cd(Mn)Te energy gap, we observed the bleaching effect as excited carriers occupied essentially all available states at the bottom of the conduction band. With the increase of the pump photon energy, the normalized differential reflectivity (ΔR/R) signal changed sign to positive, being dominated by the electron-phonon relaxation process. All our ΔR/R traces, on the delay-time scale well above 100 ps, exhibited very regular oscillations, which were identified, as the signature of coherent acoustic phonons, generated by an electronic and thermal stress introduced at the sample surface by the pump photons. We have also excited our samples with sub-picosecond magnetic transients, generated by a low-temperature-grown GaAs photoconductive switch, and observed the sub-picosecond magneto-optic (Faraday) effect (Mn-ion spin dynamics). The sub-picosecond Faraday response makes this semimagnetic semiconductor an excellent candidate for practical applications in magneto-optics, such as in time-resolved magneto-optical sampling and imaging techniques, or in ultrafast magneto-optical transducers and modulators. In addition, Cd(Mn)Te is a very promising material for ultrafast spintronic and magnetic memory-type devices.

Nb-doped SrTiO₃ epitaxial thin films have been prepared on (001) SrTiO₃ substrates using pulsed laser deposition. A high substrate temperature (>1000°C) was found to be necessary to achieve 2-dimensional growth. Atomic force microscopy reveals atomically flat surfaces with 3.9 Å steps. The films show a metallic behavior, residual resistivity ratios between 10 and 100, and low residual resistivity of the order of 10^-4Ωcm. At 0.3 K, a sharp superconducting transition, reaching zero resistance, is observed.

Materials exhibiting a coupled response to both applied electric and magnetic fields offer potential for novel devices. These materials are often complex, and thus present challenges for materials design. We address the role of the sign of the microscopic magnetic interaction for coupling to ferroelectric fluctuations and illustrate the surrounding issues with results in three different model systems, (Se,Te)CuO₃, DyMnO₃, and Ba_0.5Sr_1.5Zn₂Fe₁₂O₂₂. We show from phase space considerations that antiferromagnetism exhibits a much larger coupling to the dielectric constant, and therefore to ferroelectricity, than ferromagnetism. We show in several materials how this symmetry requirement becomes a design principle. Specifically, an antiferromagnet can generate both the magnetoelectric coupling needed for ferroelectricity as well as a Zeeman coupling needed for field control. The latter arises from a parasitic canted FM state, opening up the possibility of magnetoelectric effects in broader classes of materials.

In order to probe the order parameter symmetry of the heavy-fermion superconductor (HFS) CeCoIn₅, we employ point-contact spectroscopy, where dynamic conductance spectra are taken from a nano-scale junction between a normal-metal (N) Au tip and a single crystal of CeCoIn₅. The point-contact junction (PCJ) is formed on a single crystal surface with two crystallographic orientations, (001) and (110). Our conductance spectra, reproducibly obtained over wide ranges of temperature, constitute the cleanest data sets ever reported for HFSs. The point contacts are shown to be in the Sharvin limit, ensuring spectroscopic nature of the measured data. A signature for the emerging heavy-fermion liquid is evidenced by the development of the asymmetry in the background conductance, starting at T* (~ 45 K) and increasing with decreasing temperature down to T_c (2.3 K). Below T_c, an enhancement of the sub-gap conductance arising from Andreev reflection is observed, with the magnitude of ~ 13.3% and ~ 11.8% for the (001) and the (110) PCJ, respectively. These values are an order of magnitude smaller than those observed in conventional superconductors, but consistent with those in other HFSs. Our zero-bias conductance data for the (001) PCJ are best fit with the extended Blonder-Tinkham-Klapwijk model using the d-wave order parameter. The fit to the full conductance curve of the (001) PCJ at 400 mK indicates the strong coupling nature (2Δ/k_BT_c = 4.64). However, our observed suppression of both the Andreev reflection signal and the energy gap indicates the failure of existing models. We provide possible directions for theoretical formulations of the electronic transport across an N/HFS interface in general, and the Au/CeCoIn₅ interface in particular. Several qualitative features observed in the (110) PCJ provide the first clear spectroscopic evidence for the d_x2_-y2 symmetry of the superconducting order parameter in CeCoIn₅.

Recent experimental evidence, based on STM studies of BSCCO crystal surface, revealed the presence of nanostructures (nanodomains or stripe-like features) in this material. This raises two important questions: Are the nanostructures universal in high temperature superconductors (HTSC), i.e. are they also present in other HTSC compounds like YBCO and TBCCO? Are the nanostructures present in the bulk of a superconductor, or only on its surface? The presence of nanostructures in a superconductor implies an intrinsic phase separation and consequently a filamentary (percolative) flow of transport current. A superconductor could then behave like a glass with its properties governed by a 2D network of superconducting and normal filaments. We tested these ideas by investigating transport properties of HTSC (YBCO and TBCCO) in the normal and the superconducting states as a function of temperature, and their dependence on the annealing time.

The search for superconductivity in new and unexpected structures has been ongoing since the initial discovery in Leiden over 9 decades ago. Though the successes are few the rewards are great. Our meeting here today is a direct result of Bednorz and Mueller's discovery of cuprate superconductivity [1]. The questions which have arisen as a result of this single discovery have uncovered inadequacies of theory and stimulated new ways of thinking. Understanding the mechanism(s) of high temperature superconductivity is among the foremost challenges of theoretical and experimental research today [2]. Searching for new superconductors has always been a fruitful research enterprise, and as we see, there are new opportunities for doing so today. For more than 4 decades after the initial discovery there was no microscopic theory (the most outstanding theorists from Heisenberg down tried and failed to come up with a satisfactory theory) and the experimental basis for understanding the underlying mechanisms was inadequate. It must have been a surprise for Kamerlingh Onnes, after taking care to use the purest Hg he could obtain in the investigation that led to the discovery of superconductivity, to find that ordinary solder was also superconducting. In 1932 Meissner discovered barely metallic copper sulfide was superconducting, while high conductivity copper itself was not superconducting. These puzzles and others like it suggested that a comprehensive search for new superconductors might reveal a pattern of occurrence that would reveal clues, and prompted John Hulm and Bernd Matthias, with encouragement from Enrico Fermi [3] in 1951 to undertake a full-scale effort to find new superconductors. This was a propitious time for such an undertaking for a number of reasons. Today parallel reasons exist.

We review magnetic and transport properties of FeSb₂ and Fe_0.75Co_0.25Sb₂. Single crystals of pure and Co-substituted FeSb₂ have been grown using molten metallic fluxes. Synchrotron powder x-ray diffraction confirms phase purity and orthorhombic Pnnm space group. Cobalt substitution drives system from temperature independent diamagnet to a ferromagnet at T=0 with very small ordered moment. Application of H=70kOe enhances resistivity [ρ(H)-ρ(0)]/ρ (0) more than two orders of magnitude at T=2K. Underlying physics and possible mechanisms for the colossal response of resistivity to magnetic field will be discussed.

We report results of low temperature thermodynamic and transport measurements of Pb_1-xTl_xTe single crystals for Tl concentrations up to the solubility limit of approximately 1.5 %. The material superconducts for x > 0.3 %, with a maximum T_c of 1.5 K for the highest Tl concentrations. All superconducting samples exhibit an anomalous resistivity upturn at low temperatures, whereas non-superconducting samples (x < 0.3%) do not. The temperature and field dependence of this resistivity upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature T_K ~ 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high T_c values.

Similar to superconductivity, Quantum Hall Effects^1,2 (QHE) are macroscopic quantum phenomenon that arise from electronic condensation into a new form of strongly correlated quantum liquid. Typically QHE are observed in very high mobility, two-dimensional, electron (hole)-gas or (TDEG) systems under high magnetic fields and at low temperatures (T), i.e., in the extreme quantum limit. Quantum Hall effect is applied as calibration benchmark, international resistance standard, and a characterization technique for semiconductor heterostructures. Applications can be widespread if the devices and the operating conditions were more accessible. Here we report results of magneto resistance measurements in a novel bulk porous semiconducting structure, carbon replica opal. We show universality of QHE in TDEG, in porous semiconductor and bulk semiconductors. The data were analyzed to provide evidence of both fractional and integer quantum hall effects (2/3, 4/5, 1, etc) in our porous system at a remarkably high temperature T~40K and in a very soft quantum limit.

The lattice effects on the magnetic and transport properties in La_0.67-xGd_xSr_0.33CoO₃ series are studied. The introduction of smaller Gd³⁺ ions leads to an enhanced mismatch between the La-O layer and the CoO₂ layer and a decrease of the tolerance factor t. The spin-state of trivalent Co ion transits to low-spin state with the decrease of Co-O bond length. The doping of Gd³⁺ drives the system from the cluster-glass state to the spin-glass state and progressively decreases the Curie temperature. At high Gd³⁺ doping content, an interesting negative magnetoresistance occurs at low temperature.

We discuss recent soft x-ray resonant diffraction studies of magnetic and structural correlations in manganites and cobaltates. In half-doped manganites, the resonant enhancement of super-lattice diffraction peaks resulting from orbital and magnetic order is utilized to make a direct comparison of orbital and spin correlations. The main finding is that the correlation length associated with magnetic order exceeds that of the orbital order by of order a factor of two--a result which appears at odds with orbitally driven magnetic exchange pathways. Similar resonant diffraction measurements at the Co L-edge were performed to study the oxygen-doped cobaltate Bi₂Sr₂CoO_6+δ, in which we find a surprising incommensurate antiferromagnetic order.

Electric field induced resistance switching has been investigated for perovskite-oxide heterojunctions consisting of various metal electrodes and p-type or n-type semiconducting perovskite oxides such as Pr_0.7Ca_0.3MnO₃ or Nb-doped SrTiO₃, respectively. The metal/perovskite-oxide heterojunction devices show either ohmic or rectifying I-V characteristics similar to those of a Schottky junction, depending on the work function of the metals. In addition, the rectifying I-V characteristics have large hysteresis. Corresponding to the hysteresis directions, the junction devices show reversible resistance switching upon voltage pulse applications. On the basis of the experimental results, we propose that the resistance switching and memory effect are originated by a charging effect in the trapping states at the Schottky-like metal/perovskite-oxide interfaces.

The magnetic phase diagram of high-temperature superconductors can contain many exotic vortex phases not observed in conventional superconducting materials. For example, the familiar vortex lattice may melt at high temperatures into a vortex liquid. The influence of defects, which pin the vortices, is of particular interest from both a theoretical and an experimental point of view. We have used a combination of small angle neutron scattering (SANS) and muon-spin rotation to probe the order of the vortex system on a microscopic scale and have succeeded, for the first time, to measure a well-ordered vortex lattice (VL) structure at all doping regimes of LSCO. In the optimally to overdoped regime a field-induced transition from hexagonal to square coordination is reported. The possible connections of our neutron results to photoemission data, as well as the implications for various competing theoretical models will be discussed. In the underdoped regime we observe, as a function of applied magnetic field, a transition from an ordered vortex state to a vortex glass phase that results from the presence of random pinning. Finally, recent measurements of the VL on electron doped high-temperature superconductors are presented.

We have investigated non-linear electrical characteristics in low bandwidth manganites R_1-xCa_xMnO₃ with R=Pr, Nd, Ho, Er and x=0.3-0.5. In all these materials we observe strong nonlinear I-V characteristic that is manifested in negative differential resistance (NDR) and breakdown voltages V_br as low as few volts. These effects are accompanied by intense Joule heating that seems to be inseparable from the effect itself. We present different types of measurements with the aim to resolve the origin of the phenomena. Melting of insulating state is observed regardless of its origin - antiferromagnetic, charge ordered or paramagnetic. This nonlinearity is found to have microscopic origin, indicating that it is some sort of percolation effect. Hysteretic effects also indicate that the heating is not the only cause of NDR. Theory of phase separation gives a plausible explanation for these nonlinearities, although it appears to be diffcult to prove. Pulsed and relaxation measurements show that the effect is time-dependent, similarly to nonlinearities induced by magnetic field.

We have investigated the charge ordering phenomenon from the temperature dependence of inverse susceptibility, resistivity, and thermoelectric power (TEP) for Bi_1-xSr_xMnO₃ (BSMO) from 300 K to 700 K. At high temperatures, susceptibility follows Curie-Weiss law. The resistivity data indicate insulating behavior of BSMO. TEP (S(T)) value is negative and weakly temperature-dependent in the high temperature regime. The slope of TEP changes dramatically near the charge ordering temperature (Τ_CO), indicating an increase of energy gap due to the charge ordering. In the vicinity of Τ_CO, thermal hysteresis is observed in TEP data as well as in the resistivity data, which is consistent with the nature of the martensitic transition of the charge ordering phenomena. From this hysteretic behavior, we estimated Τ_CO. As Sr concentration increases, Τ_CO shifts to lower temperature from Τ_CO ~ 490 K for x = 0.45 to Τ_CO ~ 435 K for x = 0.8, and the thermal hysteretic behavior becomes less pronounced. The electrical transport properties have been discussed in terms of carrier localization due to charge ordering transition accompanied by the local lattice distortions.

The pressure dependence of the phonon spectrum of three La_{1 - x}Ca_xMnO_{3 - δ} manganites have been studied by means of Far-IR absorption spectroscopy coupled with a Diamond Anvil Cell. The effect of the applied pressure on the charge delocalization have been investigated defining the nature of the metallization process in the low-frequency domain.