Critical fluctuations and pseudogap observed in the microwave conductivity ofBi2Sr2CaCu

Abstract

Critical fluctuations have been studied in the microwave conductivity of Bi2Sr2CaCu2O 8+δ , Bi2Sr2Ca2Cu3O 10+δ , and Y Ba2Cu3O 7−δ thin films above Tc. It is found that a consistent analysis of the real and imaginary parts of the fluctuation conductivity can be achieved only if an appropriate wavevector or energy cutoff in the fluctuation spectrum is taken into account. In all of the three underdoped superconducting films one observes strong fluctuations extending far above Tc. The coherence length inferred from the imaginary part of the conductivity exhibits the static critical exponent ν = 1 very close to Tc, and a crossover to the region with ν = 2/3 at higher temperatures. In parallel, our analysis reveals the absence of the normal conductivity near Tc, i. e. fully opened pseudogap. Following the crossover to the region with ν = 2/3, the normal conductivity is gradually recovered, i. e. the closing of the pseudogap is monitored.

Temperature range of superconducting fluctuations aboveTcin YBa2CuGrbić1, Požek2, Paar3 et al. 2011Phys. Rev. BSelf Cite9216922View full textAdd to dashboardCite

“…2,3 Indeed, critical fluctuations have been observed in HTSC by a number of experimental techniques, including penetration depth measurements, 4 thermal expansivity 5,6 , two-coil inductive measurements, 7 and microwave measurements. [8][9][10][11][12] The experimental evidence was consistent with the three-dimensional (3D) XY universality class. The 3D character implies that the fluctuation correlations extend over many atomic layers along the c axis.…”

“…(4) can be determined by evaluating. σ 2ab at T c and equating the result to the experimental value 12,45 In the case of our UD87 sample, we have obtained ξ c0 = 0.08 nm. The only free parameters are the cutoff parameter in Eq.…”

Universal critical behavior in single crystals and films ofYBa2Cu3O7−δ

“…[15][16][17][18][19][20][21] ac measurements can determine both the real and imaginary parts of the fluctuation conductivity, providing another probe of critical dynamics. 1,18,22,23 Measurements over a broad frequency range allow one to probe the dynamical behavior of the system and directly measure the fluctuation lifetime. 18 These experiments are difficult and seldom done, and the available results are inconsistent, with values of z ranging from 2 to 5.6.…”

Effects of self field and low magnetic fields on the normal-superconducting phase transition

“…11 Transport measurements in thin-film YBCO have yielded exponents similar to those predicted by 3D-XY theory when extrapolating from high fields to zero field, 12 while measurements in low fields yield exponents larger than those expected from 3D-XY theory (ν ≈ 1.1, z ≈ 8.3). 13 Measurements on Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO), a similar hole-doped cuprate superconductor, yield similarly con-flicting results in zero field: in crystals, there has been reported a 2D to 3D crossover 14 as well as a critical regime with multiple exponents; 15 this multiple critical regime has also been observed in thin films, 16 while other measurements on films claim to see 3D diffusive dynamics 17 and still others see a 2D Kosterlitz-Thouless transition in this material. 18 Our work on this complex topic has called into question the method most researchers previously used to analyze the data and has suggested a criterion for determining the existence of a phase transition and also for determining the critical parameters.…”