Decca, R. S., H. D. Drew, B. Maiorov, J. Guimpel, and E. J. Osquiguil. "Inducing superconductivity at a nanoscale: Photodoping with a near- field scanning optical microscope." Journal of Microscopy 194, no. 2-3 (1999): 407–411.
Abstract: The local modification of an insulating GdBa2Cu3O6.5 thin film, made superconducting by illumination with a near-field scanning optical microscope (NSOM), is reported. A 100-nm aperture NSOM probe acts as a sub- wavelength light source of wavelength ?(exc)=480-650 nm, locally generating photocarriers in an otherwise insulating GdBa2-Cu3O6.5 thin film. Of the photogenerated electron-hole pairs, electrons are trapped in the crystallographic lattice, defining an electrostatic confining potential to enable the holes to move. Reflectance measurements at ? = 1.55 ?m at room temperature show that photocarriers can be induced and constrained to move on a ?? 200 nm scale for all investigated ?(exc). Photogenerated wires present a superconducting critical temperature T(c) = 12 K with a critical current density J(c) = 104 A cm-2. Exploiting the flexibility provided by photodoping through a NSOM probe, a junction was written by photodoping a wire with a narrow (? 50 nm) under-illuminated gap. The strong magnetic field modulation of the critical current provides a clear signature of the existence of a Josephson effect in the junction.
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Granada, M., B. Maiorov, M. Sirena, L. B. Steren, and J. Guimpel. "Hall effect in La0.6Sr0.4MnO3 thin films." JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS 272 (2004): 1836–1838.
Abstract: We studied the temperature and thickness dependence of the transport properties of La0.6Sr0.4MnO3 films. Hall voltage and magnetoresistance measurements on 10 and 150 nm thick films were performed with this porpose. From the ordinary Hall component, we calculated the density of carriers, which has hole-character and is systematically lower than that expected from the chemical composition of the manganite in both samples. Localization effects observed at low temperature in the resistivity of the thinner film, associated with the substrate-induced disorder, are correlated with a decrease of the density of carriers. (C) 2003 Elsevier B.V. All rights reserved.
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Bahrs, S., A. R. Goni, B. Maiorov, G. Nieva, A. Fainstein, and C. Thomsen. "Raman-study of photoinduced chain-oxygen ordering in RBa2Cu3O7-delta." IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY 13, no. 2 (2003): 3192–3195.
Abstract: We investigated Raman-forbidden signals in RBa2Cu3O7-delta with R = Y or Pr, which lose intensity under illumination at low temperatures. These defect-induced,peaks are only visible in oxygen-deficient material and for light polarized parallel to the copper-oxygen chains along the material's b-axis. In a first, two-laser based experiment we established that the bleaching effect of the light is polarization dependent as well. From this and other known properties we conclude that the signal is connected to the copper-oxygen chains, reflecting their state of disorder, and is thus closely related to aging and to the persistent photoconductivity effect in the material. In a second set of experiments we followed the temperature dependence of the Raman spectra and the intensity decrease.
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Guimpel, J., B. Maiorov, E. Osquiguil, G. Nieva, and F. Pardo. "Interrelation between persistent photoconductivity and oxygen order in GdBa2Cu3Ox thin films." Physical Review B – Condensed Matter and Materials Physics 56, no. 7 (1997): 3552–3555.
Abstract: We study the metastable states induced by photoexcitation, oxygen disorder, and both effects combined in superconducting GdBa2Cu3Ox thin films. The states are identified by the temperature dependence of the resistivity. The relaxation dynamics is characterized through the time evolution of the conductivity. The results show that photoexcitation and oxygen disorder behave as noninteracting and do not cancel each other out. However, in both cases the relaxation dynamics is related to oxygen movement .
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Decca, R. S., H. D. Drew, E. Osquiguil, B. Maiorov, and J. Guimpel. "Anomalous proximity effect in underdoped YBa2Cu3O6+x Josephson junctions." Physical Review Letters 85, no. 17 (2000): 3708–3711.
Abstract: Experiments were carried out to probe the underdoped insulating material by the Josephson effect. Junctions were fabricated by exploiting the capability of locally photodoping insulating RBa2Cu3O6+x (R = rare earth) material. The existence of an anomalously large proximity effect was confirmed. The critical current of the junctions was consisted with the conventional Josephson relationship.
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Fainstein, A., B. Maiorov, J. Guimpel, G. Nieva, and E. Osquiguil. "Annealing disorder and photoinduced order of oxygen chains in detwinned YBa2Cu3O6.65 single crystals probed by Raman scattering." Physical Review B – Condensed Matter and Materials Physics 61, no. 6 (2000): 4298–4304.
Abstract: Raman scattering in detwinned YBa2Cu3O6.65 single crystals is studied as a function of photoexcitation and annealing. Copper-oxygen chain-related forbidden Raman bands that are known to strongly bleach with illumination at low temperatures, increase their intensity with chain fragmentation induced by annealing at high temperature. This contrasting behavior proves the conjunction of short Cu-O fragments into longer chains on photoexcitation. We interpret the Raman modes as due to vibrations at the end of CuO chain fragments and Cu-O-Cu monomers, and use their evolution with illumination and annealing as anisotropic sensitive markers of oxygen reordering processes. The identification of the “forbidden” Raman bands is discussed in the context of our results and recent literature in the subject. We also present absorption measurements performed on GdBa2Cu3Ox thin films with varying oxygen content. These experiments show that the 2.2-eV absorption and the chain-related Raman peaks have different dependencies with oxygen content and illumination, ruling out an explanation that suggests that the Raman intensity reduction of these modes is due to a photobleaching of intermediate defect states. These results highlight the potentialities of Raman scattering for oxygen dynamics studies and demonstrate the presence of photoinduced oxygen ordering in these high-Tc superconductor compounds.
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Maiorov, B., G. Nieva, and E. Osquiguil. "First-order phase transition of the vortex lattice in twinned YBa2Cu3O7 single crystals in tilted magnetic fields." Physical Review B – Condensed Matter and Materials Physics 61, no. 18 (2000): 12427–12432.
Abstract: We present an exhaustive analysis of transport measurements performed in twinned YBa2Cu3O7 single crystals which establishes that the vortex solid-liquid transition is first order when the magnetic field H is applied at an angle ? away from the direction of the twin planes. We show that the resistive transitions are hysteretic and the V-I curves are nonlinear, displaying a characteristic S shape at the melting line Hm(T), which scales as ?(?)Hm(T,?), where ?(?) is the anisotropy factor. These features are gradually lost when the critical point H*(?) is approached. Above H*(?) the V-I characteristics show a linear response in the experimentally accessible V-I window, and the transition becomes reversible. Finally we show that the first-order phase transition takes place between a highly correlated vortex liquid in the field direction and a solid state of unknown symmetry. As a consequence, our data support the scenario for a vortex-line melting in twinned YBa2Cu3O7 crystals in contrast to a vortex sublimation as recently suggested for untwinned La2-xSrxCuO4, YBa2Cu3O7 and Bi2Sr2CaCu2O8 [T. Sasagawa et al., Phys. Rev. Lett. 80, 4297 (1998)].
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Bahrs, S., A. R. Goni, C. Thomsen, B. Maiorov, G. Nieva, and A. Fainstein. "Light-induced oxygen-ordering dynamics in (Y,Pr)Ba2Cu3O6.7: A Raman spectroscopy and Monte Carlo study." PHYSICAL REVIEW B 70, no. 1 (2004): 014512.
Abstract: We investigated the time and temperature dependence of photobleaching effects in RBa2Cu3O7-delta single crystals (R=Y,Pr) by Raman spectroscopy and Monte Carlo simulations based on the asymmetric next-nearest-neighbor Ising model. In a temperature range between 40 and 300 K the bleaching slows down on cooling, displaying a pronounced change in dynamics around 160 K for R=Y. To model this behavior we extended the Ising model by introducing a single energy barrier which impedes oxygen movement in the plane unless the oxygen atoms are excited by light. We obtain a time- and temperature-dependent development of superstructures under illumination with the fastest change at intermediate model temperatures. The chain-fragment development in the simulation thus matches the experimental low-temperature dynamics of Raman photobleaching, providing further support for oxygen reordering in the chain plane being at the origin of Raman photobleaching and related effects.
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Bahrs, S., A. R. Goni, C. Thomsen, B. Maiorov, G. Nieva, and A. Fainstein. "Rare-earth dependence of photoinduced chain-oxygen ordering in RBa2Cu3O7-x (x approximate to 0.3) investigated by Raman scattering." PHYSICAL REVIEW B 65, no. 2 (2002): 024522.
Abstract: We investigated a set of oxygen – deficient RBa2Cu3O7-x (xapproximate to0.3) ceramic samples with Raman spectroscopy under resonant conditions for the defect-induced Cu-O chain related modes at 230 and 600 cm(-1). Emphasis was laid on the frequency and intensity of the modes for different rare-earth atoms, and especially on their dependence under illumination. The modes display photoinduced bleaching for all R, including the nonsuperconducting R = Pr. The dynamics of their bleaching following a temperature quench indicates similar complex relaxation processes for all samples. A simple stretched exponential with a common beta = 0.36+/-0.04 and a distribution of time constants ranging from 50 to 1400 s describes our data well. We discuss our results in connection to photoinduced chain-oxygen ordering in oxygen-deficient superconductors.
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Bahrs, S., J. Guimpel, A. R. Goni, B. Maiorov, A. Fainstein, G. Nieva, and C. Thomsen. "Persistent photo-excitation in GdBa2Cu3O6.5 in a simultaneous Raman and electrical-transport experiment." Physical Review B 72, no. 14 (2005): 144501.
Abstract: We investigate the connection between persistent illumination-induced effects in underdoped RBa2Cu3O7-delta known as persistent photoconductivity and Raman bleaching. Despite the long-standing assumption that the electrical and optical properties respond to the same light-induced change in the material, they have not been directly compared until now. We present a simultaneous experiment of Raman spectroscopy and electrical transport under visible illumination at low temperatures. The time dependence of the response in the two methods differs by two orders of magnitude, showing that the effects are connected but not identical. We discuss our results within the oxygen-vacancy reordering model of photobleaching and find that different Cu-O chain lengths affect the optical and the electrical response differently. Raman bleaching and persistent photoconductivity thus provide a different perspective on the microscopic oxygen vacancy distribution.
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