Shift Of Leading Edge Research Articles

Temperature dependence of the superconducting gap of single-layer FeSe/SrTiO3 : Direct comparison between transport and spectroscopic measurements

We studied the superconducting properties of the one-unit cell FeSe grown on high-doped ${\mathrm{SrTiO}}_{3}$(001) (STO) substrate by in situ transport and angle-resolved photoemission spectroscopy (ARPES) measurements. By comparing with previous data taken for the low-doped STO substrate [Phys. Rev. Lett. 124, 227002 (2020)], it was revealed that the onset of superconductivity occurs at $\ensuremath{\sim}40$ K irrespective of the STO doping level. Furthermore, from the temperature dependence of the critical current, we were able to deduce the superconducting gap that is relevant in the transport phenomena, which gradually increases by cooling the sample down from 40 K. In contrast, the coherent peak emerges at 60 K that likely corresponds to the Cooper pair formation, while the leading edge shift near the Fermi level is clearly observed at 40 K in the ARPES spectra. We speculate that the STO surface presumably acts as an effective ``disorder'' in this system and the Cooper pairs are preformed well above the onset of the resistance drop.

Sheet Dependence on Superconducting Gap in Oxygen-Deficient Iron-Based Oxypnictide Superconductors NdFeAsO0.85

Photoemission spectroscopy with low-energy tunable photons on oxygen-deficient iron-based oxypnictide superconductors NdFeAsO 0.85 ( T c =52 K) reveals a distinct photon-energy dependence of the electronic structure near the Fermi level ( E F ). A clear shift of the leading-edge can be observed in the superconducting states with 9.5 eV photons, while a clear Fermi cutoff with little leading-edge shift can be observed with 6.0 eV photons. The results are indicative of the superconducting gap opening not on the hole-like ones around Γ(0,0) point but on the electron-like sheets around M(π,π) point.

Pseudogap and Superconducting Gap in Sm FeAs(O1-xFx) Superconductor from Photoemission Spectroscopy

High resolution photoemission measurements are carried out on non-superconducting SmOFeAs parent compound and superconducting SmFeAs(O1-xFx) (x = 0.12, and 0.15) compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state. A robust feature at 13meV is identified in all these samples. Spectral weight suppression near Ef with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds.

Magnetic properties of cuprate superconductors based on a phase separation theory

There is a great debate concerning the hole of the inhomogeneities in high critical temperature superconductors (HTS). Several experiments indicate a possible electronic phase separation (PS). However, there is not a method to quantify how such transition occurs and how it develops. Here we show that the Cahn–Hilliard (CH) theory of phase separation provides a way to trace the phase separation process as a function of temperature. We connect these calculations with the Bogoliubov–deGennes (BdG) approach to an inhomogeneous superconductor and derive many HTS properties of the La 2− x SrCuO 4 (LSCO) system. The results yield: an onset of superconductivity that follows close the Nernst signal, the leading edge shift is close to the zero temperature average gap, and the superconducting phase is achieved by percolation. Our approach reproduces also the experimental measurements of the H c2 field.

Anomalous superconducting-gap symmetry of noncentrosymmetricLa2C3observed by ultrahigh-resolution photoemission spectroscopy

We have performed ultrahigh-resolution photoemission spectroscopy on noncentrosymmetric superconductor ${\mathrm{La}}_{2}{\mathrm{C}}_{3}$ to study the electronic structure near the Fermi level $({E}_{F})$ and the superconducting-gap symmetry. We clearly observed the opening of the superconducting gap, as evidenced by the emergence of a sharp quasiparticle peak just below ${E}_{F}$ together with the leading-edge shift toward higher binding energy. Quantitative analysis of the gap function has revealed a marked deviation from the single $s$- or $d$-wave symmetry, suggesting the existence of multiple superconducting gaps.

Laser-excited ultrahigh-resolution photoemission spectroscopy of superconducting Na0.35CoO2·1.3H2O

Abstract We have studied the electronic structure near the Fermi level of the layered cobaltate superconductor Na 0.35 CoO 2 ·1.3H 2 O using laser-excited ultrahigh-resolution photoemission spectroscopy (PES). In the normal state, we observed the formation of a pseudogap with an energy scale of ∼20 meV. Across the superconducting transition, we observed the leading-edge shift at 3.5 K compared to that at 10 K, indicating superconducting gap (SC gap) opening having a gap value Δ of 0.5–0.7 meV. The energy scale of the SC gap is ∼20 times smaller than that of the pseudogap, suggesting the existence of competing order parameters at low temperatures.

Gap Observation in the Photoemission Spectra of (Y,Ca)Ba2Cu3O7−δ with Little Surface Effect

We measured angle-resolved photoemission spectra of untwinned Ca substituted YBa2Cu3O7-δ (YBCO) in the overdoped region, and compared them to those of optimally doped Ca-free YBCO. In contrast to the spectra of Ca-free YBCO, where a gap feature is not clearly seen due to the surface-derived states, the spectra of Ca substituted YBCO disclose bulk features with little effect of the surface states. At (π, 0), a superconducting peak and a leading edge shift are clearly observed, while, at (0, π), a strong peak remains at the Fermi energy both above and below Tc, unaccompanied by a shift due to superconductivity.

Doping-dependent evolution of the electronic structure ofLa2−xSrxCuO4in the superconducting and metallic phases

The electronic structure of the La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) system has been studied by angle-resolved photoemission spectroscopy (ARPES). We report on the evolution of the Fermi surface, the superconducting gap and the band dispersion around the extended saddle point $k=(\pi,0)$ with hole doping in the superconducting and metallic phases. As hole concentration $x$ decreases, the flat band at $(\pi,0)$ moves from above the Fermi level ($E_F$) for $x>0.2$ to below $E_F$ for $x<0.2$, and is further lowered down to $x=0.05$. From the leading-edge shift of ARPES spectra, the magnitude of the superconducting gap around $(\pi,0)$ is found to monotonically increase as $x$ decreases from $x=0.30$ down to $x=0.05$ even though $T_c$ decreases in the underdoped region, and the superconducting gap appears to smoothly evolve into the normal-state gap at $x=0.05$. It is shown that the energy scales characterizing these low-energy structures have similar doping dependences. For the heavily overdoped sample ($x=0.30$), the band dispersion and the ARPES spectral lineshape are analyzed using a simple phenomenological self-energy form, and the electronic effective mass enhancement factor $m^*/m_b \simeq 2$ has been found. As the hole concentration decreases, an incoherent component that cannot be described within the simple self-energy analysis grows intense in the high-energy tail of the ARPES peak. Some unusual features of the electronic structure observed for the underdoped region ($x \lesssim 0.10$) are consistent with the numerical works on the stripe model.

Ultra-high resolution photoemission spectroscopy of simple metals: direct observation of superconducting gap and phonon-induced fine structures

Abstract We have performed ultra-high resolution photoemission spectroscopy of simple metals, Nb ( T c =9.26 K) and Pb ( T c =7.19 K) in order to study the changes in the density of states across the transition temperatures. The spectra of both compounds measured at 5.3 K show an opening of the superconducting gap accompanied by a leading-edge shift and an appearance of a sharp peak just below the Fermi level. Furthermore, the Pb spectrum clearly shows peak-dip-hump structures due to strong electron–phonon coupling, as seen in the high- T c cuprates. This makes it a characteristic of strong-coupling superconductivity.

GAP AND PSEUDOGAP EVOLUTION IN UNDERDOPED CUPRATES

We describe the spectral properties of underdoped cuprates as resulting from a momentum-dependent pseudogap in the normal-state spectrum. Such a model accounts, within a BCS approach, for the doping dependence of the critical temperature and of the leading-edge shift observed in the cuprates. We also reproduce the Tc-T* bifurcation near optimum doping.

Gap and pseudogap evolution within the charge-ordering scenario for superconducting cuprates

We describe the spectral properties of underdoped cuprates as resulting from a momentum-dependent pseudogap in the normal state spectrum. Such a model accounts, within a BCS approach, for the doping dependence of the critical temperature and for the two-parameter leading-edge shift observed in the cuprates. By introducing a phenomenological temperature dependence of the pseudogap, which finds a natural interpretation within the stripe quantum-critical-point scenario for high-T_c superconductors, we reproduce also the T_c-T^* bifurcation near optimum doping. Finally, we briefly discuss the different role of the gap and the pseudogap in determining the spectral and thermodynamical properties of the model at low temperatures.

Superconducting-gap anisotropy inYBa2Cu3O7−δ:mPhotoemission results on untwinned crystals

Polarization-dependent angle-resolved photoemission studies of untwinned single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.95}$ (Y123) reveal modifications of the near-Fermi-edge spectral weight which are indicative of an anisotropic superconducting gap. The measured magnitude of the leading edge shift increases monotonically from the \ensuremath{\Gamma}-S line toward the zone boundary, reaching a maximum value of approximately 25 meV, similar to the behavior seen in ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ (Bi2212). Within experimental uncertainty, we find variation which is consistent with that expected for a ${\mathrm{d}}_{{\mathrm{x}}^{2}\mathrm{\ensuremath{-}}{\mathrm{y}}^{2}}$ order parameter, providing additional evidence in support of the d-wave pairing hypothesis.

SECOND-ORDER EFFECTS IN A LAMINAR AXISYMMETRIC PLUME SUBJECTED TO AN EXTERNAL COFLOWING UNIFORM STREAM

Asymptotic solutions are found for the steady laminar mixed convective flow of a Boussinesq fluid above a point heat source sited in an aligned uniform coflowing stream. The equations are first developed in vorticity-temperature form and then analysed in inner and outer regions using matched asymptotic expansions. Attention is restricted to the case of unit Prandt1 number and consideration is given to the effects of varying the strength of the external stream. When this is zero, analytical solutions are available for the zeroth-order shear-layer flow thus providing useful checks on the numerical procedure employed for non-zero strengths. The higher-order correction, which is of higher order than for the two-dimensional line-source problem, is found up to arbitrary multiples of the leading-edge shift eigensolution. The analysis facilitates the investigation of the non-parallel flow stability of this problem and this will be reported elsewhere.