The Seebeck and Nernst coefficients S and ν of the cuprate superconductor YBa2Cu3Oy (YBCO) were measured in a single crystal with doping p = 0.12 in magnetic fields up to H = 28 T. Down to T = 9 K, ν becomes independent of field by H ≃ 30 T, showing that superconducting fluctuations have become negligible. In this field-induced normal state, S/T and ν/T are both large and negative in the T → 0 limit, with the magnitude and sign of S/T consistent with the small electron-like Fermi surface pocket detected previously by quantum oscillations and the Hall effect. The change of sign in S(T ) at T ≃ 50 K is remarkably similar to that observed in La2−xBaxCuO4, La2−x−yNdySrxCuO4 and La2−x−yEuySrxCuO4, where it is clearly associated with the onset of stripe order. We propose that a similar density-wave mechanism causes the Fermi surface reconstruction in YBCO.PACS numbers: 74.25.Fy A major hurdle in understanding high-temperature superconductivity is the nature of the pseudogap phase. No consensus has yet been reached on whether this enigmatic phase is a precursor of superconductivity or a second ordered phase [1]. One way to shed light on this question is to study the ground state of the pseudogap phase in the absence of superconductivity, achieved by applying a strong enough magnetic field. This approach has recently revealed a qualitative change in the Fermi surface of cuprates measured via quantum oscillations, from a large hole-like cylinder in the overdoped regime outside the pseudogap phase [2] to a Fermi surface containing small electron-like pockets [3] in the underdoped regime inside the pseudogap phase [3][4][5][6][7]. Because the presence of an electron pocket in the Fermi surface of hole-doped cuprates almost certainly implies that the lattice translational symmetry is broken by some density-wave order [8], it is important to confirm the electron-like nature of the Fermi pocket detected in YBa 2 Cu 3 O y (YBCO), and elucidate the mechanism that causes it to emerge.In this Letter, we show that: 1) the low-temperature Nernst coefficient of YBCO at p = 0.12 is independent of field by H ≃ 30 T, proof that the vortex contribution is negligible by then, and the normal state has been reached; 2) the magnitude and negative sign of the thermopower at low temperature are consistent with the frequency and cyclotron mass of quantum oscillations only if these come from orbits around an electron pocket. From the fact that both the thermopower and the Hall coefficient of YBCO are very similar to those of three cuprate materials exhibiting 'stripe' order, a form of spin/charge density wave, we infer that the Fermi surface of underdoped YBCO also undergoes a reconstruction due to a similar form of spin and/or charge ordering.When a temperature difference ∆T is applied along the x-axis of a metallic sample, a longitudinal voltage V x develops across a length L, and the Seebeck coefficient (or thermopower) is defined as S ≡ V x /∆T . In the presence of a perpendicular magnetic field H (along the z-axis), a transverse voltage V ...
IntroductionThere is a substantial amount of evidence from animal models that early brain injury (EBI) may play an important role for secondary brain injury after aneurysmal subarachnoid hemorrhage (aSAH). Cerebral microdialysis (CMD) allows online measurement of brain metabolites, including the pro-inflammatory cytokine interleukin-6 (IL-6) and matrix metalloproteinase-9 (MMP-9), which is indicative for disruption of the blood-brain barrier.MethodsTwenty-six consecutive poor-grade aSAH patients with multimodal neuromonitoring were analyzed for brain hemodynamic and metabolic changes, including CMD-IL-6 and CMD-MMP-9 levels. Statistical analysis was performed by using a generalized estimating equation with an autoregressive function.ResultsThe baseline cerebral metabolic profile revealed brain metabolic distress and an excitatory response which improved over the following 5 days (P <0.001). Brain tissue hypoxia (brain tissue oxygen tension of less than 20 mm Hg) was common (more than 60% of patients) in the first 24 hours of neuromonitoring and improved thereafter (P <0.05). Baseline CMD-IL-6 and CMD-MMP-9 levels were elevated in all patients (median = 4,059 pg/mL, interquartile range (IQR) = 1,316 to 12,456 pg/mL and median = 851 pg/mL, IQR = 98 to 25,860 pg/mL) and significantly decreased over days (P <0.05). A higher pro-inflammatory response was associated with the development of delayed cerebral ischemia (P = 0.04), whereas admission disease severity and early brain tissue hypoxia were associated with higher CMD-MMP-9 levels (P <0.03). Brain metabolic distress and increased IL-6 levels were associated with poor functional outcome (modified Rankin Scale of more than 3, P ≤0.01). All models were adjusted for probe location, aneurysm securing procedure, and disease severity as appropriate.ConclusionsMultimodal neuromonitoring techniques allow insight into pathophysiologic changes in the early phase after aSAH. The results may be used as endpoints for future interventions targeting EBI in poor-grade aSAH patients.
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