Magnitude Of Superconducting Gap Research Articles

Correlation-Enhanced Odd-Parity Interorbital Singlet Pairing in the Iron-Pnictide Superconductor LiFeAs

The rich variety of iron-based superconductors and their complex electronic structure lead to a wide range of possibilities for gap symmetry and pairing components. Here we solve in the two-Fe Brillouin zone the full frequency-dependent linearized Eliashberg equations to investigate spin-fluctuations mediated Cooper pairing for LiFeAs. The magnetic excitations are calculated with the random phase approximation on a correlated electronic structure obtained with density functional theory and dynamical mean field theory. The interaction between electrons through Hund's coupling promotes both the intraorbital d_{xz(yz)} and the interorbital magnetic susceptibility. As a consequence, the leading pairing channel, conventional s^{+-}, acquires sizable interorbital d_{xy}-d_{xz(yz)} singlet pairing with odd parity under glide-plane symmetry. The combination of intra- and interorbital components makes the results consistent with available experiments on the angular dependence of the gaps observed on the different Fermi surfaces.

Pair breaking caused by magnetic impurities in the high-temperature superconductor Bi2.1Sr1.9Ca(Cu1−xFex)2Oy

Conventional superconductivity is robust against the addition of impurities unless the impurities are magnetic in which case superconductivity is quickly suppressed. Here we present a study of the cuprate superconductor Bi${}_{2}$Sr${}_{2}$Ca${}_{1}$Cu${}_{2}$O${}_{8+\ensuremath{\delta}}$ that is intentionally doped with the magnetic impurity, Fe. Through the use of our tomographic density of states technique, we find that while the superconducting gap magnitude is essentially unaffected by the inclusion of iron, the onset of superconductivity, ${T}_{\mathrm{C}}$, and the pair-breaking rate are strongly dependent and correlated. These findings suggest that, in the cuprates, the pair-breaking rate is critical to the determination of ${T}_{\mathrm{C}}$ and that magnetic impurities do not disrupt the strength of pairing but rather the lifetime of the pairs.

Raman scattering study of electron-doped PrxCa1−xFe2As2superconductors

Temperature-dependent polarized Raman spectra of electron-doped superconducting Pr${}_{x}$Ca${}_{1\ensuremath{-}x}$Fe${}_{2}$As${}_{2}$ ($x\ensuremath{\approx}0.12$) single crystals are reported. All four even-parity phonons allowed by symmetry are identified. The phonon mode of B${}_{1g}$ symmetry at 222 cm${}^{\ensuremath{-}1}$, which is associated with the $c$-axis motion of Fe ions, is found to exhibit an anomalous frequency hardening at low temperatures, that signals nonvanishing electron-phonon coupling in the superconducting state and implies that the superconducting gap magnitude $2{\ensuremath{\Delta}}_{c}<27$ meV.

Theoretical Investigation of the Four-Layered Self-Doped High-TcSuperconductors: Evidence of the Pair Tunneling Effect

Based on a four-layered self-doped t-J type model and the slave-boson mean-field approach, we study theoretically the superconductivity in the electron-doped and hole-doped layers. The neighbor layers are coupled through both the single electron interlayer hopping and pair tunneling effect. The superconducting gap magnitude for the electron-doped band is nearly twice that of the hole-doped one, which contrasts with our previous understanding of the electron-hole asymmetry in high-T(c) superconductors but is consistent with recent angle-resolved photoemission spectroscopy experiments in four-layered materials Ba2Ca3Cu4O8F2. Our results propose that the pair tunneling effect is important to examine the multilayered superconducting materials.

Universality of superconducting gaps in overdoped Ba0.3K0.7Fe2As2observed by angle-resolved photoemission spectroscopy

We have performed angle-resolved photoemission spectroscopy on the overdoped Ba${}_{0.3}$K${}_{0.7}$Fe${}_{2}$As${}_{2}$ superconductor (${T}_{c}=22$ K). We demonstrate that the superconducting (SC) gap on each Fermi surface (FS) is nearly isotropic whereas the gap value varies from 4.4 to 7.9 meV on different FSs. By comparing with under- and optimally doped Ba${}_{1\ensuremath{-}x}$K${}_{x}$Fe${}_{2}$As${}_{2}$, we find that the gap value on each FS nearly scales with ${T}_{c}$ over a wide doping range ($0.25\ensuremath{\leqslant}x\ensuremath{\leqslant}0.7$). Although the FS volume and the SC gap magnitude are strongly doping dependent, the multiple nodeless gaps can be commonly fitted by a single gap function assuming pairing up to the second nearest neighbor, suggesting the universality of the short-range pairing states with the ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave symmetry.

Theoretical investigation of the superconducting gaps in the Fe-based superconductors

Based on an antiferromagnetic (AFM) spin fluctuation approximation, we study the superconducting gaps in Fe-based compound using two-band model. We find that our results are consistent with the previous work that concludes sign-reversal extended s-wave pairings between different Fermi surface sheets. The different superconducting gap magnitude around different Fermi surface sheets is probably due to the different density of states on them. This calculation can give insight to the recent angle-resolved photoemission (ARPES) experiments on these materials. To detect the phase variation of the superconducting gap over the Fermi surfaces, we propose a new method for measuring the particular wave vector phonon linewidth. In the case of the sign-reversal superconducting pairing, the linewidth shows continuities compared to the case of no phase variation.

High-resolution ARPES study of electron-doped Fe-based superconductor BaFe1.85Co0.15As2

We have performed high-resolution angle-resolved photoemission spectroscopy of an electron-doped iron-based superconductor BaFe1.85Co0.15As2, and experimentally determined the Fermi-surface, the band structure and the superconducting gap. We found an isotropic s-wave-like superconducting gap on a hole pocket centered at the Γ point and on electron pockets at the M point. The superconducting gap magnitude is significantly enhanced on the Fermi-surfaces connected by the antiferromagnetic wave vector, suggesting the importance of an interband interaction on the pairing mechanism.

Fourfold Structure of Vortex-Core States inBi2Sr2CaCu2O8+δ

We present a detailed study of vortex-core spectroscopy in slightly overdoped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ using a low-temperature scanning tunneling microscope. Inside the vortex core, we observe a fourfold symmetric modulation of the local density of states with an energy-independent period of ($4.3\ifmmode\pm\else\textpm\fi{}0.3){a}_{0}$. Furthermore, we demonstrate that this square modulation is related to the vortex-core states which are located at $\ifmmode\pm\else\textpm\fi{}6\text{ }\text{ }\mathrm{meV}$. Since the core-state energy is proportional to the superconducting gap magnitude ${\ensuremath{\Delta}}_{p}$, our results strongly suggest the existence of a direct relation between the superconducting state and the local electronic modulations in the vortex core.

Tunneling Characteristics for Bi2212 Intrinsic Josephson Junctions With Little Influence of Self-Heating Using Short Pulse Technique on 60 ns Time Scale

A small mesa structure (10 /spl mu/m /spl times/ 10 /spl mu/m, 15 nm thick) of intrinsic Josephson junctions in Bi/sub 2/Sr/sub 2/CaCu/sub 2/o/sub 8+/spl delta// is employed to observe the tunneling characteristics and the superconducting gap magnitude by short pulse technique (short pulse interlayer tunneling spectroscopy). In the measurement on 60 ns time scale, the effect of self-heating is almost negligible in the sample, whose resistivity is as small as 10 /spl Omega/cm. In slightly overdoped region, the maximum Josephson current density increases from 0.85 to 2.1 kA/cm/sup 2/ with increasing doping level. The superconducting gap magnitude 2/spl Delta/ decreases from 62 to 37 meV with increasing doping level. The value for 2/spl Delta//k/sub B/T/sub c/ decreases from 8.3 to 5.2 with increasing doping level. These values in slightly overdoped region are smaller than those in optimum and underdoped region.

Interlayer tunneling spectroscopy and doping-dependent energy-gap structure of the trilayer superconductorBi2Sr2Ca2Cu3O10+δ

The superconducting gap, the pseudogap, and their doping and temperature dependences have been measured by the short-pulse interlayer tunneling spectroscopy for the ${\mathrm{CuO}}_{2}$ triple-layer $\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}\ensuremath{-}{T}_{\mathrm{c}}$ superconducting ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{10+\ensuremath{\delta}}$ system. It is found for a nearly optimally doped sample that the superconducting gap magnitude is $\ensuremath{\approx}80\mathrm{meV}$ and the pseudogap is \ensuremath{\approx}120 meV, the values of which are slightly larger than those for ${\mathrm{CuO}}_{2}$ double-layer system. Both gap magnitudes show a clear tendency to decrease with increasing doping. In an underdoped sample, a clear dip-and-hump structure is observed, which declines with increasing doping and tends to diminish in overdoped samples. The relationship between unchanged ${T}_{\mathrm{c}}$ and decreasing superconducting gap in the overdoped region is discussed in terms of the proximity effect applied to the inequivalent doping model. We also discuss the dip-and-hump structure in comparison with other spectroscopic results. Finally, we argue an important implication of the increasing maximum Josephson current and the decreasing superconducting gap magnitude, both with increasing doping.

Tunneling Studies of Multilayered Superconducting Cuprate (Cu,C)Ba2Ca3Cu4O12+δ

Point contact tunneling data are reported in a multilayered high-T c cuprate (Cu,C)Ba 2 Ca 3 Cu 4 O 12+δ with T c = 117 K . The tunneling spectra in the superconducting state (T ≪ T c ) display spectral features such as well-defined superconducting gap peak at ±Δ as well as dip-hump structures beyond the peaks. In some cases, the spectra with two-gaps have been observed, indicating the coexistence of two inequivalent superconducting layers. The statistical distribution of superconducting gap magnitude suggests two distinct kinds of superconducting gaps that may originate from two inequivalent CuO 2 planes, a characteristics of multilayered cuprates with n ≥ 3.

Evidence for strong-coupling s-wave superconductivity in MgB 2: 11B-NMR study of MgB 2 and the related materials

We report the results of nuclear spin–lattice relaxation time, T 1 of 11B in MgB 2, Al-doped MgB 2 and NbB 2. A T 1 T=constant behavior was observed in the normal state, indicating the absence of strong magnetic correlation. In the superconducting (SC) state, 1/ T 1 shows a tiny coherence peak just below T c and decreases exponentially, demonstrating an s-wave superconductivity. The magnitude of SC gap is estimated as 2 Δ/ k B T c∼5, which is quite larger than the weak-coupling value in the BCS theory, 2 Δ/ k B T c=3.5. This rules out a possibility for the multiple SC gaps that was suggested in many literatures. The anisotropy in the upper critical field was also corroborated by the T 1 measurement on a bulk polycrystalline sample. In NbB 2 with a low value of T c=5 K, the T 1 measurement revealed the distinct coherence peak just below T c, followed by an exponential decrease with a magnitude of SC gap 2 Δ/ k B T c=3.1. These data are consistent with the weak-coupling BCS theory. As Al 3+ is substituted for Mg 2+, 1/ T 1 T, which is proportional to the square of the density of states at the Fermi level N( E F), decreases. The variation in T c against the relative changes in N( E F) deduced from 1/ T 1 T is well fitted in terms of the McMillan equation by assuming a characteristic phonon frequency ω ∼ 700 K and an electron–phonon coupling constant λ ∼ 0.87. Thus obtained values are in good agreement with the values suggested by the theoretical works. The high- T c superconductivity in MgB 2 is shown to occur through the strong coupling with high frequency phonons.

Electronic structure of the trilayer cuprate superconductor Bi(2)Sr(2)Ca(2)Cu(3)O(10+delta).

The low-energy electronic structure of the nearly optimally doped trilayer cuprate superconductor Bi(2)Sr(2)Ca(2)Cu(3)O(10+delta) is investigated by angle-resolved photoemission spectroscopy. The normal state quasiparticle dispersion and Fermi surface and the superconducting d-wave gap and coherence peak are observed and compared with those of single- and bilayer systems. We find that both the superconducting gap magnitude and the relative coherence-peak intensity scale linearly with T(c) for various optimally doped materials.

Superconducting gap in tunneling spectra and pseudogap behavior on high-Tc cuprates

Superconducting gap magnitude 2Δo is re-examined by STM/STS at T≪Tc is Bi2212. It is confirmed that 2Δo increases monotonically with lowering the doping level p and almost scales with pseudogap temperature T* over the entire p range examined; 2Δo(p)~4.3kBT*(p). Furthermore, it is shown that Tc is almost given by renormalizing 2Δo or T* with p; kBTc(p)∝ pΔo(p) or Tc(p)∝pT*(p).

Crossover behavior of in-plane and out-of-plane resistivity in La 2− xSr xCuO 4

We report that the in-plane ( ρ ab ) and out-of-plane ( ρ c ) resistivity of La214 deviate downward and upward from linear extrapolations of their high- T data below a certain temperature T*, respectively, which are very similar to the crossover behaviors of ρ ab and ρ c arising from the pseudogap formations in Y123 and Bi2212. The crossover behavior of resistivity in La214 occurs around T*∼ T co≡2 Δ o/4.3 k B as in Bi2212, where 2 Δ o is the superconducting gap magnitude at T≪ T c. These results suggest that a pseudogap may open up below T*∼ T co also in La214, although it has not been observed in microscopic experiments on La214.

Superconducting Gap in the Tunneling Spectra of High-Tc Cuprates

In Bi2212 and La214, the low-T(T ≪ T c ) superconducting gap magnitude 2Δ0 and the temperature dependence of in-plane resistivity ρ were examined over a wide carrier-doping range. In both systems, 2Δ0 increases with lowering doping level and ρ starts decreasing faster around a temperature T*, comparable to the mean-field T c or the onset temperature of a spin-channel singlet formation in the mean-field RVB thory, each of which is given by ~Δ0/2k B .

Correlation between the Doping Dependences of Superconducting Gap Magnitude2Δ0and Pseudogap TemperatureT*in High-TcCuprates

We report that in La 2- x Sr x CuO 4 (La214) and Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212), the resistivity ρ and magnetic susceptibility χ decrease more sharply below T * , corresponding to the onset temperature of a spin gap (pseudogap) at least for Bi2212. In each system, T * scales with the superconducting gap magnitude 2Δ 0 ( T * ∼2Δ 0 /4.3 k B ), which increases as the doping level is lowered. We also report that 2Δ 0 is almost proportional to T max (> T * ), where T max is the temperature exhibiting a broad peak of χ and k B T max has been considered to give the measure of effective exchange energy J eff ; the α factors in 2Δ 0 ∼α k B T max ∼α J eff are nearly one and two for La214 and Bi2212, respectively.