Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures

We present a systematic study of the field and size dependencies of the static fluxon lattice configuration in Bi-2212 intrinsic Josephson junctions and investigate conditions needed for the formation of a rectangular fluxon lattice required for a high power flux-flow oscillator. We fabricate junctions of different sizes from Bi2Sr2CaCu2O8+x and Bi1.75Pb0.25Sr2CaCu2O8+x single crystals using the mesa technique and study the Fraunhofer-like modulation of the critical current with magnetic field. The modulation can be divided into three regions depending on the formed fluxon lattice. At low field, no periodic modulation and no ordered fluxon lattice is found. At intermediate fields, modulation with half-flux quantum periodicity due to a triangular lattice is seen. At high fields, the rectangular lattice gives integer flux quantum periodicity. We present these fields in dependence on the sample size and conclude that the transitions between the regions depend only on lambdaJ(Jc) and occur at about 0.4 and 1.3 fluxons per lambdaJ, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.Comment: Conference paper LT2

Persistent electrical doping of Bi2Sr2CaCu2O8+x mesa structures

Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x mesa structures leads to persistent doping of the mesas. Here we employ this effect for analysis of the doping dependence of the electronic spectra of Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are able to controllably and reversibly change the doping state of the same single crystal from underdoped to overdoped state, without changing its chemical composition. It is observed that such physical doping is affecting superconductivity in Bi-2212 similar to chemical doping by oxygen impurities: with overdoping the critical temperature and the superconducting gap decrease, with underdoping the c-axis critical current rapidly decreases due to progressively more incoherent interlayer tunneling and the pseudogap rapidly increases, indicative for the presence of the critical doping point. We distinguish two main mechanisms of persistent electric doping: (i) even in voltage contribution, attributed to a charge transfer effect, and (ii) odd in voltage contribution, attributed to reordering of oxygen impurities

Photoconductivity effects in mixed-phase BSCCO whiskers

We report on combined photoconductivity and annealing experiments in whisker-like crystals of the Bi-Sr-Ca-Cu-O (BSCCO) high-Tc superconductor. Both single-phase Bi2Sr2CaCu2O8+\delta (Bi-2212) samples and crystals of the mixed phases Bi2Sr2Ca2Cu3O10+x (Bi-2223)/Bi-2212 have been subjected to annealing treatments at 90{\deg}C in air in a few hours steps, up to a maximum total annealing time of 47 h. At every step, samples have been characterized by means of electrical resistance vs temperature (R vs T) and resistance vs time at fixed temperature (R vs t) measurements, both in the dark and under illumination with a UV-VIS halogen arc lamp. A careful comparison of the results from the two techniques has shown that, while for single-phase samples no effect is recorded, for mixed-phase samples an enhancement in the conductivity that increases with increasing the annealing time is induced by the light at the nominal temperature T = 100 K, i.e. at an intermediate temperature between the critical temperatures of the two phases. A simple pseudo-1D model based on the Kudinov's scheme [Kudinov et al., Phys. Rev. B 47, 9017-28, (1993)] has been developed to account for the observed effects, which is based on the existence of Bi-2223 filaments embedded in the Bi-2212 matrix and on the presence of electronically active defects at their interfaces. This model reproduces fairly well the photoconductive experimental results and shows that the length of the Bi-2223 filaments decreases and the number of defects increases with increasing the annealing time.Comment: 30 page

Carrier density crossover and quasiparticle mass enhancement in a doped 5dd Mott insulator

High-temperature superconductivity in cuprates emerges upon doping the parent Mott insulator. Robust signatures of the low-doped electronic state include a Hall carrier density that initially tracks the number of doped holes and the emergence of an anisotropic pseudogap; the latter characterised by disconnected Fermi arcs, closure at a critical doping level $p^* \approx 0.19$, and, in some cases, a strongly enhanced carrier effective mass. In Sr$_2$IrO$_4$, a spin-orbit-coupled Mott insulator often regarded as a 5$d$ analogue of the cuprates, surface probes have revealed the emergence of an anisotropic pseudogap and Fermi arcs under electron doping, though neither the corresponding $p^*$ nor bulk signatures of pseudogap closing have as yet been observed. Here, we report electrical transport and specific heat measurements on Sr$_{2-x}$La$_x$IrO$_4$ over an extended doping range 0 $\leq x \leq$ 0.20. The effective carrier density $n_{\rm H}$ at low temperatures exhibits a crossover from $n_{\rm H} \approx x$ to $n_{\rm H} \approx 1+x$ near $x$ = 0.16, accompanied by \textcolor{blue}{a five-orders-of-magnitude increase in conductivity} and a six-fold enhancement in the electronic specific heat. These striking parallels in the bulk pseudogap phenomenology, coupled with the absence of superconductivity in electron-doped Sr$_2$IrO$_4$, disfavour the pseudogap as a state of precursor pairing and thereby narrow the search for the key ingredient underpinning the formation of the superconducting condensate in doped Mott insulators

Bogoliubov Quasiparticle on the Gossamer Fermi Surface in Electron-Doped Cuprates

In contrast to hole-doped cuprates, electron-doped cuprates consistently exhibit strong antiferromagnetic correlations with a commensurate ({\pi}, {\pi}) ordering wave vector, leading to the prevalent belief that antiferromagnetic spin fluctuations mediate Cooper pairing in these unconventional superconductors. However, early investigations produced two paradoxical findings: while antiferromagnetic spin fluctuations create the largest pseudogap at "hot spots" in momentum space, Raman scattering and angle-resolved photoemission spectroscopy measurements using the leading-edge method seem to suggest the superconducting gap is also maximized at these locations. This presented a dilemma for spin-fluctuation-mediated pairing: Cooper pairing is strongest at momenta where normal state low energy spectral weight is most suppressed. Here we investigate this dilemma in Nd2-xCexCuO4 using angle-resolved photoemission spectroscopy under significantly improved experimental conditions. The unprecedented signal-to-noise ratio and resolution allow us to directly observe the Bogoliubov quasiparticles, demonstrating the existence and importance of two sectors of states: 1. The reconstructed main band and the states gapped by the antiferromagnetic pseudogap around the hot spots. 2. The gossamer Fermi surface states with distinct dispersion inside the pseudogap, from which Bogoliubov quasiparticle coherence peaks emerge below Tc. Supported by numerical results, we propose that the non-zero modulus of the antiferromagnetic order parameter causes the former, while fluctuations in the antiferromagnetic order parameter orientation are responsible for the latter. Our revelations of the gossamer Fermi surface reconcile the paradoxical observations, deepening our understanding of superconductivity in electron-doped cuprates in particular, and unconventional superconductivity in general.Comment: Submitted version 30 pages, 4 main figures, 8 extended data figures. Accepted version in press at Nature Physic

Vortex Properties from Resistive Transport Measurements on Extreme Type-II Superconductors

NR 2014080