Coherent c-axis transport in the underdoped cuprate superconductor YBa_ {2} Cu_ {3} O_ {y}

Physical review. B, Condensed matter (Impact Factor: 3.66). 07/2011; 85(22). DOI: 10.1103/PhysRevB.85.224524
Source: arXiv

ABSTRACT The electrical resistivity ρc of the underdoped cuprate superconductor YBa2Cu3Oy was measured perpendicular to the CuO2 planes on ultrahigh quality single crystals in magnetic fields large enough to suppress superconductivity. The incoherent insulating-like behavior of ρc at high temperature, characteristic of all underdoped cuprates, is found to cross over to a coherent regime of metallic behavior at low temperature. This crossover coincides with the emergence of the small electron pocket detected in the Fermi surface of YBa2Cu3Oy via quantum oscillations, the Hall and Seebeck coefficients, and with the detection of a unidirectional modulation of the charge density as seen by high-field nuclear magnetic resonance measurements. The low coherence temperature is quantitatively consistent with the small hopping integral t⊥ inferred from the splitting of the quantum oscillation frequencies. We conclude that the Fermi-surface reconstruction in YBa2Cu3Oy at dopings from p=0.08 to at least p=0.15, attributed to stripe order, produces a metallic state with three-dimensional coherence deep in the underdoped regime.

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    ABSTRACT: We explore the evolution of superconductivity in La2−xBaxCuO4 with x=0.095 in magnetic fields of up to 35 T applied perpendicular to the CuO2 planes. Previous work on this material has shown that perpendicular fields enhance both charge- and spin-stripe order within the planes. We present measurements of the resistivity parallel and perpendicular to the planes, as well as the Hall effect. Measurements of magnetic susceptibility for fields of up to 15 T applied both parallel and perpendicular to the planes provide complementary measures of the superconductivity. We show that fields sufficient to destroy pair tunneling between the planes do not disrupt the superconducting correlations within the planes. In fact, we observe an onset of large-amplitude but phase-disordered superconductivity within the planes at approximately 30 K that is remarkably insensitive to field. With further cooling, we observe a phase-transition-like drop in the in-plane resistivity to an apparent state of superconductivity despite the lack of phase coherence between the layers. These observations raise interesting questions concerning the identification of the upper critical field, where pairing is destroyed, in underdoped cuprates.
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    ABSTRACT: The model for the cuprates based on the modified electron-phonon pairing mechanism has been tested. For this purpose, the superconductors with high value of the critical temperature have been taken into consideration. In particular: ${\rm YBa_{2}Cu_{3}O_{7-y}}$, ${\rm HgBa_{2}CuO_{4+y}}$, ${\rm HgBa_{2}Cu_{1-x}Zn_{x}O_{4+y}}$, and ${\rm HgBa_{2}Ca_{2}Cu_{3}O_{8+y}}$. It has been shown that the dependence of the ratio $R_{1}\equiv 2\Delta_{tot}^{(0)}/k_{B}T_{C}$ on the doping ($p$) can be properly predicted in the framework of the presented theory; the symbol $\Delta_{tot}^{(0)}$ denotes the energy gap amplitude at the temperature of zero Kelvin, and $T_{C}$ is the critical temperature. The numerical results have been supplemented by the formula which describes the function $R_{1}(p)$.
    Acta Physica Polonica Series a 10/2014; 126:A-92–A-96. · 0.60 Impact Factor
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    ABSTRACT: Using 63Cu NMR, we establish that the enhancement of spin order by a magnetic field H in YBa2Cu3O6.45 arises from a competition with superconductivity because the effect occurs for H perpendicular, but not parallel, to the CuO2 planes, and it persists up to field values comparable to Hc2. We also find that the spin freezing has a glassy nature and that the frozen state onsets at a temperature which is independent of the magnitude of H. These results, together with the presence of a competing charge-ordering instability at nearby doping levels, are strikingly parallel to those previously obtained in La-214. This suggests a universal interpretation of magnetic field effects in underdoped cuprates where the enhancement of spin order by the field may not be the primary phenomenon but rather a byproduct of the competition between superconductivity and charge order. We also observe that low-energy spin fluctuations are manifested up to relatively high temperatures where they partially mask the signature of the pseudogap in 1/T1 data of planar Cu sites.
    Physical Review B 07/2013; 88(1). · 3.66 Impact Factor

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