Critical microwave-conductivity fluctuations across the phase diagram of superconductingLa2−xSrxCuO4

Abstract

We report a systematic study of the dynamic microwave conductivity near Tc for La2−xSrxCuO4 (LSCO) thin films with x=0.07 to 0.16. The strong frequency dependence of the phase stiffness together with scaling analysis of the ac fluctuating conductivity of superconductivity provide direct evidence for the 2D-XY behavior of nearly decoupled CuO2 planes in underdoped LSCO (x=0.07 and 0.12). On the other hand, the critical exponents for slightly overdoped LSCO (x=0.16) were found to agree with those for the relaxational 3D-XY model, indicating that the universality class in LSCO is changed by hole doping. The implication of these results for the phase diagram of high-Tc cuprates is discussed.One of the hallmarks of the high-T c cuprates is the large thermal fluctuation of the superconducting order enhanced by the short coherence length and the quasi two-dimensionality, which enables the exploration of the fluctuation-dominated critical regime very close to T c 1 . Although numerous measurements, such as the ac conductivity 2 , the dc magnetization 3 , the specific heat 4 , and the I-V curves 5 , have been performed to investigate the critical fluctuation, there has been no consensus among the results. This is surprising because the critical phenomena were considered to be universal, independent of the microscopic details 6 . However, if many assumptions were made implicitly to determine the universality class of the phase transition from the data, the obtained results are not convincing unless the validity of such assumptions is confirmed. Thus, one should develop a more reliable method which does not require any extra assumptions. To our knowledge, the most successful method is dynamic scaling analysis of the ac complex conductivity, σ(ω) = σ 1 (ω) − iσ 2 (ω), as will be discussed later.Another fascinating hallmark of the high-T c cuprates is that the physical properties change with hole doping. In particular, an understanding of the phase diagram as a plot of T c versus the hole concentration has been a central issue in the physics of high-T c cuprates. Interestingly, some recent models, which start from the quantum criticality for competing orders underlying the phase diagram of high-T c cuprates, provide another possible explanation for the critical fluctuations in high-T c cuprates. That is, the critical fluctuations change with hole doping, because of the existence of a quantum critical point (QCP) 7 . These models suggest that the critical dynamics should be investigated as a function of hole doping. Thus, it can be expected that such a systematic study across the phase diagram will not only resolve the disagreements among earlier studies but will also provide important information to understand the phase diagram of high-T c cuprates.In this paper, we report a systematic study of σ(ω) as a function of the swept-frequency (0.1 GHz to 12 GHz) for high-quality La 2−x Sr x CuO 4 (LSCO) thin films with a wide range of hole concentrations (x=0.07 to 0.16). For underdoped (UD) LSCO, we show clear evidence for the 2D-X...

Disorder and superfluid density in overdoped cuprate superconductors

“…19 The mean-field model considered here does not include fluctuation effects, which are known to be important in the cuprates [11][12][13][14][15][16][17][18][19][20][21][22] and are probably responsible for the downturns in the experimentally observed ρ s (T ) on the approach to T c . 64 Nevertheless, one way in which the underlying T c0 might become visible in experiments would be as rare regions in which the local disorder level is lower than average.…”

c-axis coupling in underdopedBi2Sr2CaCu2Spathis1, Colson2, Yang3 et al. 2008Phys. Rev. B4010View full textAdd to dashboardCite

“…49 As for the magnitude of the T 2 contribution to ab , a very modest change is found for the lower irradiation fluences. Only for fluences exceeding 10 19 e − cm −2 does the in-plane penetration depth increase significantly with defect density. We now switch to the central results concerning the Josephson plasma resonance.…”

“…Strong support for this scenario has come from the violation of the Glover-Tinkham-Ferrell conductivity sum rule applied to the c-axis spectral weight; 5 also, the linear relation between T c and the superfluid density 8 has been interpreted as the result of T c being determined by phase fluctuations in a KosterlitzThouless-type scenario. 9,10 A smoking gun for such a scenario would be an important reduction of the c-axis superfluid density s c , which is determined by Cooper pair and quasiparticle tunneling between adjacent strongly superonducting CuO 2 layers through the weakly superconducting rocksalt-like blocking layers, with respect to the in-plane stiffness s ab in underdoped cuprates below T c . However, apart from phase fluctuations, other mechanisms for the reduction of s c , arising from disorder in the crystalline structure of underdoped cuprates, cannot be ignored.…”

“…Crystals UD5-UD8 were irradiated to a total fluence of 0.53 ϫ 10 18 , 3ϫ 10 18 , 7.7ϫ 10 18 , and 8.8ϫ 10 19 e − cm −2 , respectively. After measurements, crystal UD5 was irradiated a second time to a total fluence of 6.0ϫ 10 19 e − cm −2 , and was henceforth labeled UD5b. The high-energy electron irradiation creates random atomic displacements in the form of Frenkel pairs, both in the CuO 2 bilayers and in the intermediate cation layers, throughout the samples.…”

Superconducting Fluctuation and Pseudogap in Disordered Short Coherence Length Superconductor

“…This may be the reason why the critical behavior in short coherence length s-wave superconductor NbN is quite different from that in the under-doped cuprates. 61 …”