Raman evidence for the superconducting gap and spin–phonon coupling in the superconductor Ca(Fe 0.95 Co 0.05 ) 2 As 2 (original) (raw)
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Applied Physics Letters, 2012
Raman studies on Ca 4 Al 2 O 5.7 Fe 2 As 2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature T c ~ 28.3 K, reveal that the Raman mode at ~ 230 cm -1 shows a sharp jump in frequency by ~ 2 % and linewidth increases by ~ 175 % at T o ~ 60 K. Below T o, anomalous softening of the mode frequency and a large decrease by ~ 10 cm -1 in the linewidth is observed. These precursor effects at T 0 ( ~ 2T c ) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weaked coupling between the well separated (~ 15 Å ) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K to 60 K ( ~ 7 % ) indicates strong spin-phonon coupling in this superconductor.
Enhanced electron–phonon coupling and its irrelevence to high Tc superconductivity
Solid State Communications, 1998
It is argued that the origin of the buckling of the CuO2 planes in certain cuprates as well as the strong electron-phonon coupling of the B1g phonon is due to the electric field across the planes induced by atoms with different valence above and below. The magnitude of the electric field is deduced from new Raman results on YBa2Cu3O6+x and Bi2Sr2(Ca1−xYx)Cu2O8 with different O and Y doping, respectively. In the latter case it is shown that the symmetry breaking by replacing Ca partially by Y enhances the coupling by an order of magnitude, while the superconducting Tc drops to about two third of its original value.
2014
We report inelastic light scattering experiments on superconductor Ce 0.6 Y 0.4 FeAsO 0.8 F 0.2 from 4K to 300K covering the superconducting transition temperature T c ~ 48.6K. A strong evidence of the superconductivity induced phonon renormalization for the A 1g phonon mode near 150 cm -1 associated with the Ce/Y vibrations is observed as reflected in the anomalous red-shift and decrease in the linewidth below T c. Invoking the coupling of this mode with the superconducting gap, the superconducting gap (2∆ ) at zero temperature is estimated to be ~ 20 meV i.e the ratio 2 (0) / B c T κ ∆ is ~ 5, suggesting Ce 0.6 Y 0.4 FeAsO 0.8 F 0.2 to belong to the class of strong coupling superconductors. In addition, the mode near 430 cm -1 associated with Ce 3+ crystal field excitation also shows anomalous increase in its linewidth below T c suggesting strong coupling between crystal field excitation and the superconducting quasi-particles. Our observations of two high frequency modes (S9 and S10) evidence the non-degenerate nature of Fe 2+ d xz/yz orbitals suggesting the electronic nematicity in these systems.
Superconductivity-induced phonon renormalization onNaFe1−xCoxAs
Physical Review B, 2014
We report a study of the lattice dynamics in superconducting NaFeAs (Tc = 8 K) and doped NaFe0.97Co0.03As (Tc = 20 K) using Raman light scattering. Five of the six phonon modes expected from group theory are observed. In contrast with results obtained on iso-structural and iso-electronic LiFeAs, anomalous broadening of Eg(As) and A1g(Na) modes upon cooling is observed in both samples. In addition, in the Co-doped sample, a superconductivity-induced renormalization of the frequency and linewidth of the B1g(Fe) vibration is observed. This renormalization can not be understood within a single band and simple multi-band approaches. A theoretical model that includes the effects of SDW correlations along with sign-changing s-wave pairing state and interband scattering has been developed to explain the observed behavior of the B1g(Fe) mode.
Superconductivity and Phonon Softening
Physical Review Letters, 1972
A new formula is derived which relates changes in electron-phonon coupling strength A, to shifts in phonon frequencies. Using this formula, the superconducting transition temperature T~is discussed for bce transition metals and rock-salt-structure transition-metal carbides. Among superconducting elements and compounds, it is common to find pairs of metals with the same crystal structure, made of elements in neighboring columns of the periodic table, in which the transition temperature T, differs greatly. Good examples are the rock-salt-structure materials HfC (with T, & 0.015'K) and TaC (with T,-11'K). Recently Smith and 615,ser' and Smith' have re
Unconventional electron-phonon interactions in high-temperature superconductors
Physical Review B, 1991
The infrared absorption of the 155-cm c-axis mode of YBa2Cu307 is calculated on the basis of an anharmonic-electron-phonon-interaction model and a large enhancement of its intensity is obtained. A double-well potential in the electron-phonon interaction gives the right order of magnitude for shifts in the bridging O(4) position in agreement with recent extended x-ray-absorption fine-structure data. Electron-density-two-phonon-interaction terms are derived which represent a violation of the Migdal theorem and a BCS-type superconducting state with nonlinearly enhanced electron-phonon coupling is expected together with an anisotropy of the superconducting energy gap.
The superconducting gap in pure crystals of YBa2Cu3O7 from phonon Raman spectra
Solid State Communications, 1991
The temperature dependence of the linewidths and frequencies of the Raman active phonons have been Teasured in high quality single crystals of YBa.$u 0 phonon at 340 cm st7. Below T, the sharpens and the phonons at 435 and 500 cm-are observed to broaden. From the measured linewidth changes for T < Tc the superconducting gap energy is estimated to have an energy greater than 340 cm-', a value which is significantly larger than that obtained from ceramic samples by the same procedure. This difference cannot be attributed to the presence of impurities in the single crystals used in this work.
Electron Phonon Interactions and Superconductivity
Electron–Phonon Interactions and Superconductivity in (Cu 0.5 Tl 0.5)Ba 2 Ca 3 (Cu 4−y Ti y)O 12−δ (y = 0, 0.25, 0.50, 0.75, 1.0) Abstract We have used two-step solid state reaction method for the synthesis of (Cu 0.5 Tl 0.5)Ba 2 Ca 3 (Cu 4−y Ti y) O 12−δ (y = 0, 0.25, 0.50, 0.75, 1.0) superconductors at 880 • C. The oxygen contents in the samples were optimized by carrying out self-doping which is accomplished by post-annealed in flowing oxygen environment at 500 • C for approximately 5 h. The superconducting properties of all the samples were improved, after the self-doping of the carriers in the conducting planes. In the x-ray diffraction scans of the samples, the a-axis length of tetragonal unit cell increases whereas the c-axis decreases with increased Ti doping in the final compound. The Fourier transform infrared spectrometer (FTIR) absorption measurements of these samples have shown that the apical oxygen mode at 548 cm −1 and the pla-nar oxygen mode at 596 cm −1 are softened with increased Ti doping. The origin of softening of planar oxygen mode lies in increase bond lengths of apical oxygen atoms promoted by larger covalent radius of Ti (1.32Å) atoms relative to Cu (1.17Å) atoms that in turn promotes the softening of the apical oxygen modes. Doped Ti (47.90 amu) atoms at the Cu (63.54 amu) sites initiate the an-harmonic oscillations resulting into the suppression of density of phonon modes. The suppression in the values of superconductiv-ity parameters with Ti doping at the Cu sites shows the essential role of phonon in mechanism of high T c supercon-ductivity and hence the electron–phonons interactions. The excess conductivity analyses (FIC) of conductivity data of oxygen-post-annealed samples have shown decrease in the mean field critical temperature, coherence length along the c-axis, interlayer coupling, and Fermi velocity with increase Ti doping. However, the values of B c , B c1 , and J c(0) increase with Ti doping, showing increase in the population of the pinning centers.