Superconductivity in heavy alkaline-Earth intercalated graphites

We report the discovery of superconductivity below 1.65(6) K in Sr-intercalated graphite SrC(6), by susceptibility and specific heat (C(p)) measurements. In comparison with CaC(6), we found that the anisotropy of the upper critical fields for SrC(6) is much reduced. The C(p) anomaly at T(c) is smaller than the BCS prediction, indicating an anisotropic superconducting gap for SrC6 similar to CaC6. The significantly lower T(c) of SrC(6) as compared to CaC(6) can be understood in terms of "negative" pressure effects, which decreases the electron-phonon coupling for both in-plane intercalant and the out-of-plane C phonon modes. We observed no superconductivity for BaC(6) down to 0.3 K.     

Giant nonadiabatic effects in layer metals: raman spectra of intercalated graphite explained

The occurrence of nonadiabatic effects in the vibrational properties of metals has been predicted since the 1960s, but hardly confirmed experimentally. We report the first fully ab initio calculations of nonadiabatic frequencies of a number of conventional (hcp Ti and Mg) and layered metals (MgB2, CaC6, and other intercalated graphites). Nonadiabatic effects can be spectacularly large (up to 30% of the phonon frequencies) in both cases, but they can only be experimentally observed in the Raman spectra of layered compounds. In layered metals nonadiabatic effects are crucial to explaining the observed Raman shifts and linewidths. Moreover, we show that those quantities can be used to extract the electron momentum-relaxation time.     

Electronic structure of superconducting KC8 and nonsuperconducting LiC6 graphite intercalation compounds: evidence for a graphene-sheet-driven superconducting state

We have performed photoemission studies of the electronic structure in LiC(6) and KC(8), a nonsuperconducting and a superconducting graphite intercalation compound, respectively. We have found that the charge transfer from the intercalant layers to graphene layers is larger in KC(8) than in LiC(6), opposite of what might be expected from their chemical composition. We have also measured the strength of the electron-phonon interaction on the graphene-derived Fermi surface to carbon derived phonons in both materials and found that it follows a universal trend where the coupling strength and superconductivity monotonically increase with the filling of graphene π(*) states. This correlation suggests that both graphene-derived electrons and graphene-derived phonons are crucial for superconductivity in graphite intercalation compounds.     

Specific heat of the Ca-intercalated graphite superconductor CaC6

The superconducting state of Ca-intercalated graphite CaC6 has been investigated by specific heat measurements. The characteristic anomaly at the superconducting transition (Tc = 11.4 K) indicates clearly the bulk nature of the superconductivity. The temperature and magnetic field dependence of the electronic specific heat are consistent with a fully gapped superconducting order parameter. The estimated electron-phonon coupling constant is lambda = 0.70 +/- 0.04, suggesting that the relatively high Tc of CaC6 can be explained within the intermediate coupling BCS approach.     

Evidence for gap anisotropy in CaC6 from directional point-contact spectroscopy

We present the first results of directional point-contact spectroscopy in high-quality CaC6 samples both along the ab plane and in the c-axis direction. The superconducting order parameter Delta(0), obtained by fitting the Andreev-reflection (AR) conductance curves at temperatures down to 400 mK with the single-band 3D Blonder-Tinkham-Klapwijk model, presents two different distributions in the two directions of the main current injection, peaked at 1.35 and 1.71 meV, respectively. By ab initio calculations of the AR conductance spectra, we show that the experimental results are in good agreement with the recent predictions of gap anisotropy in CaC6.     

Superconductivity of bulk CaC6

We have obtained bulk samples of the graphite intercalation compound, CaC6, by a novel method of synthesis from highly oriented pyrolytic graphite. The crystal structure has been completely determined showing that it is the only member of the MC6, metal-graphite compounds that has rhombohedral symmetry. We have clearly shown the occurrence of superconductivity in the bulk sample at 11.5 K, using magnetization measurements.     

Bulk evidence for single-Gap s-wave superconductivity in the intercalated graphite superconductor C6Yb

We report measurements of the in-plane electrical resistivity rho and thermal conductivity kappa of the intercalated graphite superconductor C6Yb down to temperatures as low as Tc/100. When a field is applied along the c axis, the residual electronic linear term kappa0/T evolves in an exponential manner for Hc1相似文献   

石墨插层化合物超导体研究进展

本文介绍了石墨层间化合物(GICs)超导体的发展及超导电性,重点结合GICs超导体的研究进展,特别是具有高超导转变温度(Tc=11.5 K)CaC6的研究,系统地分析和总结了当前GICs超导电性的研究现状,并对GICs超导体的发展进行一些展望.     

超导材料的各向异性特征与超导转变温度T_c

在本文中,以电声子机制超导电性理论为基础,用电负性均衡原理研究了由于超导材料的各向异性引起的电子态密度分布的不均匀性,晶格稳定性的差异性及超导材料中元素成键特征对超导转变温度影响的特征,提出了在各向异性的超导材料中电声子机制可以产生高的超导转变温度Tc.     

effects of rattling phonons on the dynamics of quasiparticle excitation in the beta-pyrochlore KOs(2)O(6) Superconductor

Microwave penetration depth lambda and surface resistance at 27 GHz are measured in high quality crystals of KOs(2)O(6). Firm evidence for fully gapped superconductivity is provided from lambda(T). Below the second transition at T(p) approximately 8 K, the superfluid density shows a steplike change with a suppression of effective critical temperature T(c). Concurrently, the extracted quasiparticle scattering time shows a steep enhancement, indicating a strong coupling between the anomalous rattling motion of K ions and quasiparticles. The results imply that the rattling phonons help to enhance superconductivity, and that K sites freeze to an ordered state with long quasiparticle mean free path below T(p).     

Scanning tunneling spectroscopy on the novel superconductor CaC6

We present scanning tunneling microscopy and spectroscopy of the newly discovered superconductor CaC6. The tunneling conductance spectra, measured between 3 and 15 K, show a clear superconducting gap in the quasiparticle density of states. The gap function extracted from the spectra is in good agreement with the conventional BCS theory with Delta0=1.6+/-0.2 meV. The possibility of gap anisotropy and two-gap superconductivity is also discussed. In a magnetic field, direct imaging of the vortices allows us to deduce a coherence length in the ab plane xiab approximately 33 nm.     

Enhancement of superconductivity and evidence of structural instability in intercalated graphite CaC6 under high pressure

We measured the temperature dependent resistivity, varrho(T), of the intercalated graphite superconductor CaC6 as a function of pressure up to 16 GPa. We found a large linear increase of critical temperature, Tc, from the ambient pressure value 11.5 K up to 15.1 K, the largest value for intercalated graphite, at 7.5 GPa. At approximately 8 GPa, a jump of varrho and a sudden drop of Tc down to approximately 5 K indicates the occurrence of a phase transition. Our data analysis suggests that a pressure-induced phonon softening related to an in-plane Ca phonon mode is responsible for the Tc increase and that higher pressures greater, similar8 GPa lead to a structural transition into a new phase with a low Tc less, similar3 K.     

Search for an effect of superconductivity on the phonons in Ba(Fe1−xCox)2As2

Inelastic neutron scattering was used to search for an influence of superconductivity on the phonons in optimally doped and in slightly overdoped Ba(Fe_1?xCo_x)₂As₂, x = 0.06 and x = 0.10. The study focused on phonons with energies close to the superconducting gap energy 2Δ because it is well known that such phonons will respond most strongly to the opening of the gap. We were able to obtain high quality data but nevertheless, we could not detect any influence of superconductivity on the phonons, neither on the linewidths nor on the frequencies. Our results imply that any coupling of low energy phonons to the electrons has to be very small, much smaller than observed in conventional superconductors with a high T_c. Our results are in line with the low coupling strength predicted by density functional theory for the investigated phonon branches.     

Temperature effects in two-phonon spectra

A number of vibrational spectra peculiarities in crystals are predicted in this paper, which appear at high temperatures ($\text{kT ? ?ω}$) in the summation and difference tones of vibrations. It is shown that the anharmonic interaction between the phonons grows with temperature, which leads to splitting of levels of both summation and difference biphonons from the two-particle bands. The dielectric permittivity of the crystal for the above spectral regions is calculated.     

Absence of superconductivity down to 80 mK in graphite intercalated BaC6

Following recent theoretical and experimental investigations of superconductivity in the graphite intercalated compounds CaC₆ and SrC₆, we report on a very low temperature magnetisation study on BaC₆ down to 80 mK. The data show no trace of superconductivity even at fields as low as 0.7 Oe. Using a McMillan parametrisation of the BCS parameters, we conclude that the Coulomb pseudopotential is expected to be as large as 0.19 in both BaC₆ and SrC₆, i.e. 40% larger than in CaC₆. As an alternative scenario, we argue that extrinsic effects such as intercalant disorder may depress superconductivity in both BaC₆ and SrC₆, as in the case of CaC₆.     

Electrons in cuprates: A consistent ARPES view

Angle resolved photoemission spectroscopy (ARPES) has been playing a crucial role in understanding of physics behind high-temperature superconductivity. Our ARPES investigation of superconducting cuprates, performed over a decade and accomplished by very recent results, suggests a consistent view of electronic interactions in cuprates which provides natural explanation of both the origin of the pseudogap state and the mechanism for high-temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. In view of these results and their projection to numerous other materials, two general questions are arising: is the normal state in 2D metals ever stable and how does this intrinsic instability interplay with superconductivity?     

Superconducting phase at 7.7 K in the HgxReO3 compound with a hexagonal bronze structure

We have successfully synthesized a new rhenium-based hexagonal bronze material, HgxReO3, which exhibits superconductivity with the transition temperature Tc=7.7 K at ambient pressure and 11.1 K at 4 GPa. This compound is a superconductor with the highest Tc among hexagonal bronzes. Moreover, it presents the novel crystallographic feature that (Hg2)2+ polycations, in contrast to monatomic cations in known hexagonal bronzes, are incorporated into open channels. There is evidence that conducting electrons tightly couple with Hg-related phonons. Our results inspire detailed studies on the role of the rattling phonon in the occurrence of superconductivity in the hexagonal bronzes.     

Unconventional high-temperature superconductivity from repulsive interactions: theoretical constraints

Unconventional symmetries of the order parameter allowed some researchers to maintain that a purely repulsive interaction between electrons provides superconductivity without phonons in a number of high-temperature superconductors. It is shown that the Cooper pairing in p and d states is not possible with the realistic Coulomb repulsion between fermions at relevant temperatures in any dimension.     

Interplay of superconductivity and rattling phenomena in beta-pyrochlore KOs2O6 studied by photoemission spectroscopy

The electronic structure near the Fermi level (EF) of the beta-pyrochlore superconductor KOs2O6 is studied using laser-excited ultrahigh-resolution photoemission spectroscopy. The superconducting gap clearly opens across the superconducting transition (Tc=9.6 K), with the strong electron-phonon coupling value of 2Delta(0)/k B Tc>or=4.56. A fitting analysis identifies clear anomalies at Tp=7.5 K in the temperature dependencies of the superconducting gap size and the quasiparticle relaxation lifetime. These anomalies and the fine spectral structures arising from phonons suggest that the existence of the rattling behavior of K ions significantly affects the superconductivity in KOs2O6.     

Sensitivity of the phonon in YBCO to superconductivity by infrared studies on epitaxial film

Epitaxial films of YBCO superconductors whose C axes are primarily perpendicular to the film surface, were prepared. The samples, which were treated by means of a gettered annealing technique, were investigated by the X-ray diffraction pattern method (XRD) and infrared (IR) reflectance spectra in the 140–800 cm⁻¹ spectral range. The strengths of the phonon structures are influenced greatly by the superconductivity of the sample, which can be determined by the annealing conditions. The relation between the resulting IR activity of phonons and the superconductivity is clear and the important role of phonons in superconductivity is demonstrated.