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We detect the persistence of the solidification and order-disorder first-order transition lines in the phase diagram of nanocrystalline Bi2Sr2CaCu2O8 vortex matter down to a system size of less than one hundred vortices. The temperature location of the vortex solidification transition line is not altered by decreasing the sample size although there is a depletion of the entropy jump at the transition with respect to macroscopic vortex matter. The solid order-disorder phase transition field moves upward on decreasing the system size due to the increase of the surface-to-volume ratio of vortices entailing a decrease on the average vortex binding energy. [ABSTRACT FROM AUTHOR]

We study the effect of quenched disorder in the thermodynamic magnitudes entailed in the first-order vortex phase transition of the extremely layered Bi $$_{2}$$ Sr $$_{2}$$ CaCu $$_{2}$$ O $$_{8 + \delta }$$ compound. We track the temperature-evolution of the enthalpy and the entropy jump at the vortex solidification transition by means of AC local magnetic measurements. Quenched disorder is introduced to the pristine samples by means of heavy-ion irradiation with Pb and Xe producing a random columnar-track pins distribution with different densities (matching field $$B_{\Phi }$$ ). In contrast with previous magneto-optical reports, we find that the first-order phase transition persists for samples with $$B_{\Phi }$$ up to 100 Gauss. For very low densities of quenched disorder (pristine samples), the evolution of the thermodynamic properties can be satisfactorily explained considering a negligible effect of pinning and only electromagnetic coupling between pancake vortices lying in adjacent CuO planes. This description is not satisfactory on increasing magnitude of quenched disorder. [ABSTRACT FROM AUTHOR]

In this work we revisit the vortex matter phase diagram in layered superconductors solving still open questions by means of ac and dc local magnetic measurements in the paradigmatic Bi2Sr2CaCu2O8 compound. We show that measuring with ac magnetic techniques is mandatory in order to probe the bulk response of vortex matter, particularly at high temperatures where surface barriers for vortex entrance dominate. From the TFOT evolution of the enthalpy and latent heat at the transition we find that, contrary to previous reports, the nature of the dominant interlayer coupling is electromagnetic in the whole temperature range. By studying the dynamic properties of the phase located at T≳TFOT, we reveal spanning in a considerable fraction of the phase diagram of a nonlinear vortex phase suggesting that bulk pinning might play a role even in the liquid vortex phase. [ABSTRACT FROM AUTHOR]

We study a series of spectra measured in the superconducting state of optimally doped Bi2Sr2Ca2Cu3O10+δ (Bi-2223) by scanning tunneling spectroscopy. Each spectrum, as well as the average of spectra presenting the same gap, is fitted using a strong-coupling model taking into account the band structure, the BCS gap, and the interaction of electrons with the spin resonance. After describing our measurements and the main characteristics of the strong-coupling model, we report the whole set of parameters determined from the fits, and we discuss trends as a function of the gap magnitude. We also simulate angle-resolved photoemission spectra, and compare with recent experimental results. [ABSTRACT FROM AUTHOR]

We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe2(As1-xPx)2, a material that qualifies as an archetypical clean superconductor, containing only sparse strong pointlike pins [ C. J. van der Beek et al. Phys. Rev. B 66 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe1-xCox)2As2 single crystals. Analysis reveals that the pinning force and energy distributions depend on the P content x. However, they are always much narrower than in Ba(Fe1-xCox)2As2, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching Tc in BaFe2(As1-xPx)2. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers L̅ ~90 nm, increasing with increasing P content x. This evolution can be understood as being the consequence of the P dependence of the London penetration depth. Further salient features are a wide vortex free "Meissner belt", observed at the edge of overdoped crystals, and characteristic chainlike vortex arrangements, observed at all levels of P substitution. [ABSTRACT FROM AUTHOR]

Abstract: The first scanning-tunneling and pseudo point-contact spectroscopy results obtained in as-grown surfaces of YBa2Cu4O8 single crystals are presented. In tunneling regime, the conductance dip observed at low-bias voltages reveals a gapped spectra, whereas in the pseudo point-contact regime zero-bias conductance peaks are detected. Conductance curves are fitted with a BTK-like model assuming a d-wave density of states and considering the same superconducting gap in both regimes, Δ =(14±2)meV. This estimation gives Δ =(2.1±0.3)kT c, well within the d-wave BCS value, but in contrast with results obtained for cuprates with an extended pseudogap phase. [Copyright &y& Elsevier]

X-ray diffraction and transport measurements on a series of La0.67Ca0.33MnO3 films grown on (110)-cut NdGaO3 substrates are presented. Contrary to widespread belief assuming strain-free growth, this work shows the presence of strain in a 42 nm film. On increasing thickness structural relaxation occurs, reaching a bulklike state for 500 nm. No evidence of coexistence of strained and relaxed regions is found. The evolution of lattice parameters toward bulk values is accompanied by an increase of the metal-to-insulator transition temperature and a decrease of the polaron activation energy. Therefore, strain effects cannot always be neglected in La0.67Ca0.33MnO3 films grown on small-mismatch NdGaO3. [ABSTRACT FROM AUTHOR]

Abstract: A study of the resistive transition into the superconducting state of the quasi-1D superconductor Tl2Mo6Se6 (T c =5K) has revealed a broadening effect under applied magnetic field similar to that seen in the high-T c cuprates. Using anisotropic Ginzburg–Landau theory we calculate a Ginzburg number G i ≈1.8×10−5, corresponding to a critical region of width ∼0.09mK, which is ∼18000 times smaller than the measured ΔT c2. This proves that fluctuations of the order parameter are not the predominant factor responsible for the broadening of the transition and suggests the possible absence of a macroscopic phase coherence over a wide region of the vortex phase diagram. [Copyright &y& Elsevier]

Abstract: We study the effect of oxygen overdoping in the vortex phase diagram of Bi2Sr2Ca2Cu3O10+δ single crystals. In the overdoped regime we do find a smooth variation of T c with the annealing pressure, indicating that Bi2Sr2Ca2Cu3O10+δ is less sensitive to oxygen-doping than the bilayer Bi2Sr2CaCu2O8+δ compound. Magnetic measurements reveal that oxygen overdoping produces a reduction of the liquid vortex region and increases the second-peak transition field. These findings imply that the coupling between CuO2 planes increases upon overdoping. We find that this anisotropy decrease is steeper in Bi2Sr2Ca2Cu3O10+δ than in Bi2Sr2CaCu2O8+δ for the same doping level. [Copyright &y& Elsevier]

X-ray diffraction and transport measurements on a series of La0.67Ca0.33MnO3 films grown on (110)-cut NdGaO3 substrates are presented. Contrary to widespread belief assuming strain-free growth, this work shows the presence of strain in a 42 nm film. On increasing thickness structural relaxation occurs, reaching a bulklike state for 500 nm. No evidence of coexistence of strained and relaxed regions is found. The evolution of lattice parameters toward bulk values is accompanied by an increase of the metal-to-insulator transition temperature and a decrease of the polaron activation energy. Therefore, strain effects cannot always be neglected in La0.67Ca0.33MnO3 films grown on small-mismatch NdGaO3. [ABSTRACT FROM AUTHOR]

Abstract: The pinning of quantized flux lines, or vortices, in the mixed state is used to quantify the effect of impurities in iron-based superconductors (IBS). Disorder at two length scales is relevant in these materials. Strong flux pinning resulting from nm-scale heterogeneity of the superconducting properties leads to the very disordered vortex ensembles observed in the IBS, and to the pronounced maximum in the critical current density j c at low magnetic fields. Disorder at the atomic scale, most likely induced by the dopant atoms, leads to “weak collective pinning” and a magnetic field-independent contribution j c coll . The latter allows one to estimate quasiparticle scattering rates. [Copyright &y& Elsevier]

Abstract: A review of our investigations on single crystals of LnFeAsO1− x F x (Ln=La, Pr, Nd, Sm, Gd) and Ba1− x Rb x Fe2As2 is presented. A high-pressure technique has been applied for the growth of LnFeAsO1− x F x crystals, while Ba1− x Rb x Fe2As2 crystals were grown using a quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show incomplete occupation of the (O,F) position in superconducting LnFeAsO1− x F x crystals. Resistivity measurements on LnFeAsO1− x F x crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba1− x Rb x Fe2As2 simply shifts to lower temperature. The critical current density for both compounds is relatively high and exceeds 2×109 A/m2 at 15K in 7T. The anisotropy of magnetic penetration depth, measured on LnFeAsO1− x F x crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba1− x Rb x Fe2As2 crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO1− x F x . Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (∼20meV). [Copyright &y& Elsevier]

We directly image individual vortex positions in nanocrystals in order to unveil the structural property that contributes to the depletion of the entropy jump entailed at the first-order transition. On reducing the nanocrystal size, the density of topological defects increases near the edges over a characteristic length. Within this "healing-length" distance from the sample edge, vortex rows tend to bend, while towards the center of the sample, the positional order of the vortex structure is what is expected for the Bragg-glass phase. This suggests that the healing length may be a key quantity to model confinement effects in the first-order transition of extremely layered vortex nanocrystals. [ABSTRACT FROM AUTHOR]

We show that the dynamical freezing of vortex structures nucleated at diluted densities in Bi2Sr2CaCu2O8 samples with a dense distribution of columnar defects, B~10−2BΦ with BΦ = 5kG, results in configurations with liquidlike correlations. We propose a freezing model considering a relaxation dynamics dominated by double-kink excitations driven by the local stresses obtained directly from experimental images. With this model we estimate the relaxation barrier and the freezing temperature. We argue that the low-field frozen vortex structures nucleated in a dense distribution of columnar defects thus correspond to an out-of-equilibrium nonentangled liquid with strongly reduced mobility rather than to a snapshot of a metastable state with divergent activation barriers as, for instance, expected for the Bose-glass phase at equilibrium. [ABSTRACT FROM AUTHOR]