Neutron Scattering Studies of spin excitations in hole-doped Ba0.67K0.33Fe2As2 superconductor
Chenglin Zhang, Meng Wang, Huiqian Luo, Miaoyin Wang, Mengshu Liu, Jun Zhao, D. L. Abernathy, T. A. Maier, Karol Marty, M. D. Lumsden, Songxue Chi, Sung Chang, … 展开作者
Chenglin Zhang, Meng Wang, Huiqian Luo, Miaoyin Wang, Mengshu Liu, Jun Zhao, D. L. Abernathy, T. A. Maier, Karol Marty, M. D. Lumsden, Songxue Chi, Sung Chang, Jose A. Rodriguez-Rivera, J. W. Lynn, Tao Xiang, Jiangping Hu, Pengcheng Dai
We report inelastic neutron scattering experiments on single crystals of superconducting Ba0.67K0.33Fe2As2 (Tc = 38 K). In addition to confirming the resonance previously found in powder samples, we find that spin excitations in the normal state form longitudinally elongated ellipses along the QAFM direction in momentum space, consistent with density functional theory predictions. On cooling below Tc, while the resonance preserves its momentum anisotropy as expected, spin excitations at energies below the resonance become essentially isotropic in the in-plane momentum space and dramatically increase their correlation length. These results suggest that the superconducting gap structures in Ba0.67Ka0.33Fe2As2 are more complicated than those suggested from angle resolved photoemission experiments.
Within the framework of the kinetic energy driven superconducting mechanism, the doping dependence of the electromagnetic response in the electron-doped cuprate superconductors is studied. It is shown that although there is an electron-hole asymmetry in the phase diagram, the electromagnetic response in the electron-doped cuprate superconductors is similar to that observed in the hole-doped cuprate superconductors. The superfluid density depends linearly on temperature, except for the strong deviation from the linear characteristics at the extremely low temperatures.
The electronic Raman response in the electron-doped cuprate superconductors is studied based on the $t$-$t'$-$J$ model. It is shown that although the domelike shape of the doping dependent peak energy in the $B_{2g}$ symmetry is a common feature for both electron-doped and hole-doped cuprate superconductors, there are pronounced deviations from a cubic response in the $B_{1g}$ channel and a linear response in the $B_{2g}$ channel for the electron-doped case in the low energy limit. It is also shown that these pronounced deviations are mainly caused by a nonmonotonic d-wave gap in the electron-…
Within the framework of kinetic-energy-driven superconductivity, the asymmetric doping dependence of superconductivity between the hole- and electron-doped triangular-lattice superconductors has been studied. It is shown that although the superconducting transition temperature has a dome-shaped doping dependence for both the hole- and electron-doped triangular-lattice superconductors, superconductivity appears over a wide doping of range in the hole-doped case, while it only exists in a narrow range of the doping in the electron-doped side. Moreover, the maximum superconducting transition temp…
The effect of pressure on the magnetic penetration depth $λ$ was tested in the hole-doped superconductor YBa2Cu3O(7-d) and in the electron-doped one Sr0.9La0.1CuO2 by means of magnetization measurements. Whereas a large change of $λ$ was found in YBa2Cu3O(7-d), confirming the non-adiabatic character of the electron-phonon coupling in hole-doped superconductors, the same quantity is not affected by pressure in the electron-doped Sr0.9La0.1CuO2, suggesting a close similarity of the latter to conventional adiabatic BCS superconductors. The present results imply a remarkable difference between the…
We report on the effect of substitution for Cu on Tc of electron-doped infinite-layer superconductors Sr0.9La0.1Cu1-xRxO2, R = Zn and Ni. We found that Tc was nearly constant until x = 0.03 for R = Zn, while superconductivity was nearly suppressed for x = 0.02 with dTc/dx = 20 K/% for R = Ni. This behavior is very similar to that of conventional superconductors. These findings are discussed in terms of the superconducting gap symmetry in the cuprate superconductors including another electron-doped superconductor, (Nd,Ce)2CuO4-y.
Since their experimental discovery in 1989, the electron-doped cuprate superconductors have presented both a major challenge and a major opportunity. The major challenge has been to determine whether these materials are fundamentally different from or essentially similar to their hole-doped counterparts; a major opportunity because answering this question would strongly constrain the possible explanations for what is the essential physics that leads to high temperature superconductivity in the cuprates, which is still not agreed upon. Here we argue that experimental results over the past 30 ye…
We have calculated the superconducting critical temperature $T_c$ of hole-doped delafossite CuAlO$_2$ based on the first-principles calculations. According our calculation, $0.2\sim0.3$ hole-doped CuAlO$_2$ can become a phonon-mediated high-$T_c$ superconductor with $T_c\sim50$ K. In the hole-doped CuAlO$_2$, the A$_1$L$_1$ phonon mode that stretches O-Cu-O dumbbell has a strong electron-phonon interaction with electrons of the flat band in Cu 3d$_{3z^2-r^2}$ and the O 2p$_z$ anti-bonding $π$-band.
We performed high-resolution angle-resolved photoemission spectroscopy on electron-doped high-Tc superconductor Pr0.89LaCe0.11CuO4 to study the anisotropy of the superconducting gap. The observed momentum dependence is basically consistent with the dx2-y2-wave symmetry, but obviously deviates from the monotonic dx2-y2 gap function. The maximum gap is observed not at the zone boundary, but at the hot spot where the antiferromagnetic spin fluctuation strongly couples to the electrons on the Fermi surface. The present experimental results unambiguously indicate the spin-mediated pairing mechanism…
Impurity doping like Zn atoms in cuprates were systematically studied to provide important information on the pseudogap phase because this process substantially reduces $T_c$ without effect $T^*$. Despite many important results and advances, the normal phase of these superconductors is still subject of a great debate. We show that the observed Zn-doped data can be reproduced by constructing a nanoscale granular superconductor whose resistivity transition is achieved by Josephson coupling, what provides also a simple interpretation to the pseudogap phase.
We argue that parametrically strong enhancement of a thermoelectric current can be observed in conventional superconductors doped by magnetic impurities. This effect is caused by violation of the symmetry between electron-like and hole-like excitations due to formation of subgap Andreev bound states in the vicinity of magnetic impurities. We develop a quantitative theory of this effect and demonstrate that it can be detected in modern experiments.
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 of that of the hole-doped one, which contrasts to our previous understanding of the electron-hole asymmetry in high-T$_c$ superconductors but consistent with recent angle-resolved-photoemission-spectroscopy experiments in four-l…
The spin dynamics of an optimally doped YBa2Cu3O7 (Tc = 93 K) crystal array have been investigated in a wide range of momentum and energy (Q - E) space using the time-of-flight neutron scattering method. Incommensurate spin modulation in Q is a characteristic feature, as it is in the under-doped YBa2Cu3O6.7 with a different incommensurability. A linear relationship between the incommensurability and Tc is proposed. Along with the discovery of the same incommensurability in under-doped La2-ySryCuO4, it may be a generic characteristic of the high-Tc oxide superconductor.
Following the recent NMR experiments by Gorny et al., we discuss the effect of a magnetic field on the superconducting T_c and the spin pseudo-gap T^*. As a testable prediction, we argue that a spin pseudo-gap should also be observed in over-doped samples in a magnetic fied (while normally there is no pseudo-gap above T_c). We find that different theoretical approaches have marked differences in their predictions for over-doped HTc cuprates.
We use inelastic neutron scattering to probe magnetic excitations of an optimally electron-doped superconductor Nd$_{1.85}$Ce$_{0.15}$CuO$_{4-δ}$ above and below its superconducting transition temperature $T_c=25$ K. In addition to gradually opening a spin pseudo gap at the antiferromagnetic ordering wavevector ${\bf Q}=(1/2,1/2,0)$, the effect of superconductivity is to form a resonance centered also at ${\bf Q}=(1/2,1/2,0)$ but at energies above the spin pseudo gap. The intensity of the resonance develops like a superconducting order parameter, similar to those for hole-doped superconductors…
The nematic susceptibility of hole-doped Ba1-xKxFe2As2 and electron-doped Ba(Fe1-xCox)2As2 iron-based superconductors is obtained from measurements of the elastic shear modulus using a three-point bending setup in a capacitance dilatometer. Nematic fluctuations, although weakened by doping, extend over the whole superconducting dome in both systems, suggesting their close tie to superconductivity. Evidence for quantum critical behavior of the nematic susceptibility is, surprisingly, only found for Ba(Fe1-xCox)2As2, the system with the lower maximal Tc value.
We propose a weakly coupled two-band model with $d_{x^2-y^2}$ pairing symmetry to account for the anomalous temperature dependence of superfluid density $ρ_s$ in electron-doped cuprate superconductors. This model gives a unified explanation to the presence of a upward curvature in $ρ_s$ near $T_c$ and a weak temperature dependence of $ρ_s$ in low temperatures. Our work resolves a discrepancy in the interpretation of different experimental measurements and suggests that the pairing in electron-doped cuprates has predominately $d_{x^2-y^2}$ symmetry in the whole doping range.
We report the iron (Fe) isotope effect on the transition temperature (Tc) in the oxygen-deficient SmFeAsO_{1-y}, a 50 K-class Fe-based superconductor. For the optimally-doped samples with Tc = 54 K, change of the Fe average atomic mass (MFe) causes a negligibly small shift in Tc, with the Fe isotope coefficient (alphaFe) as small as -0.024 \pm 0.015. This result contrasts with the finite, inverse isotope shift observed in optimally-doped (Ba,K)Fe2As2, indicating that the contribution of the electron-phonon interaction markedly differs between these two Fe-based high-Tc superconductors.
We have studied the electronic structure of electron-doped cuprate superconductors via measurements of high-field Shubnikov-de Haas oscillations in thin films. In optimally doped Pr$_{2-x}$Ce$_{x}$CuO$_{4\pmδ}$ and La$_{2-x}$Ce$_{x}$CuO$_{4\pmδ}$, quantum oscillations indicate the presence of a small Fermi surface, demonstrating that electronic reconstruction is a general feature of the electron-doped cuprates, despite the location of the superconducting dome at very different doping levels. Negative high-field magnetoresistance is correlated with an anomalous low-temperature change in scatter…
We present phase-sensitive evidence that the electron-doped cuprates Nd1.85Ce0.15CuO4-y (NCCO) and Pr1.85Ce0.15CuO4-y (PCCO) have d-wave pairing symmetry. This evidence was obtained by observing the half-flux quantum effect, using a scanning SQUID microscope, in c-axis oriented films of NCCO or PCCO epitaxially grown on tricrystal [100] SrTiO3 substrates designed to be frustrated for a d(x2-y2) order parameter. Samples with two other configurations, designed to b unfrustrated for a d-wave superconductor, do not show the half-flux quantum effect.
To explain the high-temperature superconductivity of hole-doped copper-oxide high-temperature superconductors (HDCO-HTSCs), Anderson proposed a theory: (A) the pseudogap state is a resonating valence-bond (RVB) state below T* and (B) the RVB state translates itself into high-temperature superconducting state below Tc. In this paper we abandon Anderson theory B but still retain Anderson theory A and add three new hypotheses. Jointed three hypotheses with Anderson theory A, we construct an effective Hamiltonian of HDCO-HTSCs and explain why Tc-line is a dome in phase diagram and why HDCO-HTSCs h…
Muon spin rotation (mSR) measurements of the in-plane penetration depth lambda_ab have been performed in the electron-doped infinite layer high-Tc superconductor (HTS) Sr0.9La0.1CuO2. Absence of the magnetic rare-earth ions in this compound allowed to measure for the first time the absolute value of lambda_ab(0) in electron-doped HTS using mSR. We found lambda_ab(0)=116(2) nm. The zero-temperature depolarization rate sigma(0)?1/lambda_ab(0)^2=4.6(1) MHz is more than four times higher than expected from the Uemura line. Therefore this electron-doped HTS does not follow the Uemura relation found…
We have analyzed scanning tunneling spectra of two electron-doped cuprates Pr0.88LaCe0.12CuO4 (Tc = 21 K and 24 K) and compared them with tunneling spectrum of hole-doped La1.84Sr0.16CuO4 and effective electron-boson spectral function of hole-doped La1.97Sr0.03CuO4 (extracted from angle-resolved photoemission spectrum). We have also analyzed tunneling spectra and angle-resolved photoemission spectra for hole-doped Bi2Sr2CaCu2O8. These results unambiguously rule out magnetic pairing mechanism in both electron- and hole-doped cuprates and support polaronic/bipolaronic superconductivity in hole-d…
The aim of this work is to investigate the temperature dependencies both in CuO2-plane and out-of plane resistivities in electron-doped Nd2-xCexCuO4 for x from 0.135 up to 0.15 in order to analyze the anisotropy of the electrical transport in the process of the evolution from antiferromagnetic (AF) order in underdoped region to superconducting (SC) order in optimally doped region.
We use in-plane tunneling spectroscopy to study the temperature dependence of the local superconducting gap $Δ(T)$ in electron-doped copper oxides with various $T_c$'s and Ce-doping concentrations. We show that the temperature dependence of $Δ(T)$ follows the expectation of the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity, where $Δ(0)/k_{B}T_{c}\approx1.72\pm 0.15$ and $Δ(0)$ is the average superconducting gap across the Fermi surface, for all the doping levels investigated. These results suggest that the electron-doped superconducting copper oxides are weak coupling BCS superco…
The angle-resolved photoemission spectroscopy (ARPES) autocorrelation in the electron-doped cuprate superconductors is studied based on the kinetic-energy driven superconducting (SC) mechanism. It is shown that the strong electron correlation induces the electron Fermi surface (EFS) reconstruction, where the most of the quasiparticles locate at around the hot spots on EFS, and then these hot spots connected by the scattering wave vectors ${\bf q}_{i}$ construct an {\it octet} scattering model. In a striking analogy to the hole-doped case, the sharp ARPES autocorrelation peaks are directly corr…
The recent experiments revealed a remarkable possibility for the absence of the disparity between the phase diagrams of the electron- and hole-doped cuprate superconductors, while such an aspect should be also reflected in the dressing of the electrons. Here the phase diagram of the electron-doped cuprate superconductors and the related exotic features of the anisotropic dressing of the electrons are studied based on the kinetic-energy driven superconductivity. It is shown that although the optimized Tc in the electron-doped side is much smaller than that in the hole-doped case, the electron- …
Iron-based superconductors have grown to be a new continent of high Tc superconductors comparable to cuprates. The optimal critical temperature (Tc) of 56 K in electron-doped 1111-type iron oxypnictides attracts considerable attention of physicists and chemists. Carrier doping is not only essential to induce superconductivity but also is a critical parameter that governs the electronic, magnetic, and crystallographic properties of ground states in high-Tc superconductors. Hydride ion (H-) which is an anionic state of hydrogen acts as an efficient electron donor in the 1111-type, leading to sev…
We studied the transport properties of meso-junctions of semiconducting (Sm) highly doped Si with different superconductors (Sc) through point contact Andreev reflection (PCAR) spectroscopy. Spectra of low transparency point contacts between Si and In showed an enhancement in the superconducting energy gap of In. This was due to the effect of an additional gap arising from the Schottky barrier at the Sm-Sc interface. For higher transparency Si-Nb and Si-Pb point contacts, no gap enhancement was observed though there were weak sub gap features. These were due to proximity induced interface supe…
We have synthesized a Ba-doped FeSe superconductor using liquid ammonia, and investigated in detail the structure and composition of the Ba-FeSe phase. In the superconducting phase, both the Tc and the shielding fraction are almost unchanged even if the nominal composition of Ba is varied in the region from 0.3 to 0.7. The optimized Tc is 38.8 K. We have determined the structure of the superconducting phase with X-ray powder diffraction (using synchrotron radiation) and here describe the co-intercalation of Ba and NH3 molecules between the FeSe layers.
Measurements of longitudinal and transverse transport coefficients were performed in the normal state of a high-quality single crystal of optimally-doped superconductor YBa2Cu3O6.95. We found the Hall-Lorenz number (Lxy) depends only weakly on temperature, thus no appreciable violation of the Wiedemann-Franz law has been observed. However, Lxy is about two times larger than the Sommerfeld's value of the Lorenz number. Our results can be interpreted consistently if we assume that the electronic system of YBa2Cu3O6.95 behaves like a Fermi-liquid with a pseudo-gap that opens at the Fermi level.
We apply the density functional theory for superconductors (SCDFT) based on the local-density approximation (LDA) to alkali-doped fullerides A3C60 with the face-centered cubic structure. We evaluate the superconducting transition temperature (Tc) from first principles considering energy dependence of electron-phonon coupling, the mass renormalization, and the retardation effect. The calculated Tc=7.5, 9.0 and 15.7 K for A=K, Rb, Cs are approximately 60 % smaller than the experimentally observed values. Our results strongly suggest necessity to go beyond the framework of the Migdal-Eliashberg t…
The superconducting sesquicarbides R2C3 have noncentrosymmetric point group symmetry Td. Spin-orbit coupling lifts the spin degeneracy of electronic bands in the most of the Brillouin zone. Nevertheless, due to high symmetry, there are a few directions along which the Fermi surfaces must touch. This leads to the two-band effects in the superconducting state.
We report on measurements of the in-plane magnetic penetration depth λ_{ab} in the infinite-layer electron-doped high-temperature cuprate superconductor Sr_0.9La_0.1CuO_2 by means of muon-spin rotation. The observed temperature and magnetic field dependences of λ_{ab} are consistent with the presence of a substantial s-wave component in the superconducting order parameter in good agreement with the results of tunneling, specific heat, and small-angle neutron scattering experiments.
The renormalization of the electrons in cuprate superconductors is characterized by the kink in the quasiparticle dispersion. Here the bilayer coupling effect on the quasiparticle dispersion kink in the electron-doped bilayer cuprate superconductors is studied based on the kinetic-energy-driven superconductivity. It is shown that the kink in the quasiparticle dispersion is present all around the electron Fermi surface, as the quasiparticle dispersion kink in the single-layer case. However, in comparison with the corresponding single-layer case, the kink effect in the quasiparticle dispersion a…
We investigate the electronic structure of a recently discovered, heavily electron-doped iron based superconductor LaFe$_2$As$_2$. Although first principles calculation shows apparent absence of hole Fermi surfaces around the $Γ$ point, we reveal, by hypothetically removing the La $d$ orbital contribution, that a hole Fermi surface around the $Γ$ point is essentially present. In the collapsed phase of LaFe$_2$As$_2$, which is non-superconducting, the hole Fermi surface is found to be absent, and the difference from the uncollapsed superconducting phase can be naturally understood within the sp…
Thin film junctions and bilayers of the doped topological insulator $\rm Bi_2Se_3$ and the s-wave superconductor NbN were found to exhibit conductance spectra with prominent zero bias and coherence peaks. Various tunneling models with different pair potentials have failed to fit our data, except for the triplet $p_x+ip_y$ pair potential, which breaks time reversal symmetry, but yielded reasonably good fits. This provides evidence for proximity induced triplet superconductivity in the $\rm Bi_2Se_3$ layer near the interface with the NbN film.
We investigate the phase diagram of the electron-doped systems in high-Tc cuprates. We calculate the superconducting transition temperature Tc, the antiferromagnetic transition temperature TN, the NMR relaxation rate 1/T1 with the antiferromagnetic fluctuations in the fluctuation-exchange (FLEX) approximation and with the superconducting fluctuations in the self-consistent t-matrix approximation. Obtained phase diagram has common features as those in the hole-doped systems, including the antiferromagnetic state, the superconducting state and the spin gap phenomenon. Doping-dependences of TN, T…
Based on a phenomenological model with $s_{\pm}$ or s-wave pairing symmetry, the mixed-state effect on the low-energy spin dynamics in optimally-doped iron pnictide superconductors is studied by solving Bogoliubov-de Gennes equations. Our results of the spin susceptibility at $\mathbf{q}=\mathbf{Q}$ in the normal, superconducting and mixed states agree qualitatively with recent neutron scattering experiments. We also propose that the field-induced intensity change shows different behaviors between the $s_{\pm}$ and s-wave symmetries in both momentum and real space, thus it can be used to disti…
We have computed complete electronic structure of new Pt-doped superconductor CaFe1-xPtxAs2. Our findings for the parent compound of this superconductor agree well with experimental predictions.
We have measured the low-energy quasiparticle excitation spectrum of the electron doped high-temperature superconductors (HTS) Nd(1.85)Ce(0.15)CuO(4-y) and Pr(1.85)Ce(0.15)CuO(4-y) as a function of temperature and applied magnetic field using tunneling spectroscopy. At zero magnetic field, for these optimum doped samples no excitation gap is observed in the tunneling spectra above the transition temperature Tc. In contrast, below Tc for applied magnetic fields well above the resistively determined upper critical field, a clear excitation gap at the Fermi level is found which is comparable to t…
Epitaxial growth of Fe-based superconductors such as Co-doped SrFe$_2$As$_2$ (SrFe$_2$As$_2$:Co) was reported recently, but has still insufficient properties for device application because they have rough surfaces and are decomposed by reactions with water vapor in an ambient atmosphere. This letter reports that epitaxial films of Co-doped BaFe$_2$As$_2$ grown at 700 oC show the onset superconducting transition tempearture of 20 K. The transition is sharper than those observed on the SrFe$_2$As$_2$:Co films, which would originate from their improved crystallinity. These films also have atomica…
Using high-energy diffraction we show that a 4-unit-cell superstructure, q0=(1/4,0,0), along the shorter Cu-Cu bonds coexists with superconductivity in optimally doped YBCO. A complex set of anisotropic atomic displacements on neighboring CuO chain planes, BaO planes, and CuO2 planes, respectively, correlated over ~3-6 unit cells gives rise to diffuse superlattice peaks. Our observations are consistent with the presence of Ortho-IV nanodomains containing these displacements.
We report c-axis resistivity and longitudinal magnetoresistance measurements of superconducting Pr1.85Ce0.15CuO4 single crystals. In the temperature range 13K<T<32K, a negative magnetoresistance is observed at fields just above Hc2. Our studies suggest that this negative magnetoresistance is caused by superconducting fluctuations. At lower temperatures (T<13K), a different magnetoresistance behavior and a resistivity upturn are observed, whose origin is still unknown.
Temperature-dependent polarized Raman spectra of electron-doped superconducting Pr$_x$Ca$_{1-x}$Fe$_2$As$_2$ ($x \approx 0.12$) single crystals are reported. All four allowed by symmetry even-parity phonons are identified. Phonon mode of B$_{1g}$ symmetry at 222 cm$^{-1}$, which is associated with the c-axis motion of Fe ions, is found to exhibit an anomalous frequency hardening at low temperatures, that signals non-vanishing electron-phonon coupling in the superconducting state and implies that the superconducting gap magnitude $2Δ_c < 27$meV.
We report superconducting anisotropy measurements in the electron-doped high-$T_{c}$ superconductors (HTSCs) Pr$_{2-x}$Ce$_{x}$CuO$_{4-y}$ (PCCO, $x$ = 0.15 and 0.17) with applied magnetic field ($H_{0}$) up to 28 T. Our results show that the upper critical field [$H_{c2}(T)$] is highly anisotropic, and as temperature $T$ $\rightarrow$ 0 the value of it at $H_{0}$ $\parallel$ $c$ [$H_{c2,\parallel c}$(0)] is far less than the Pauli limit, which is very different from that at $H_{0}$ $\perp$ $c$. The $H_{c2}(0)$ character along with the evaluated zero $T$ coherence length $[ξ_{ab(c)}(0)]$ and p…
We present resistivity measurements of the upper critical field (H$_{c2}$) phase diagram as a function of temperature (T) for Nd$_{1.85}$Ce$_{0.15}$CuO$_{4+δ}$/SrTiO$_3$ single crystal films with different degree of disorder ($δ$) in magnetic fields up to 90 kOe at temperatures down to 0.4 K. The data are well described by a two-band/two-gap model for a superconductor in the dirty limit.
We theoretically study the spin fluctuation and superconductivity in La1111 and Sm1111 iron-based superconductors for a wide range of electron doping. When we take into account the band structure variation by electron doping, the hole Fermi surface originating from the $d_{X^2-Y^2}$ orbital turns out to be robust against electron doping, and this gives rise to large spin fluctuations and consequently $s\pm$ pairing even in the heavily doped regime. The stable hole Fermi surface is larger for Sm1111 than for La1111, which can be considered as the origin of the apparent difference in the phase d…
Measurements of the superconducting transition temperatures for Al-doped, C-doped and neutron-damaged-annealed MgB2 samples under pressure up to ~8 kbar are presented. The dT_c/dP values change systematically with the decrease of the ambient pressure T_c in a regular fashion. The evolution of the pressure derivatives can be understood assuming that the change in phonon spectrum is a dominant contribution to dT_c/dP.
We investigate the competition between the electron-vibron interaction (interaction with the Jahn-Teller phonons) and the Coulomb repulsion in a system with local pairing of electrons on the triply degenerate lowest unoccupied molecular orbital (LUMO). The electron-vibron interaction radically changes conductivity and magnetic properties of alkali-doped fullerides AnC60, which should be antiferromagnetic Mott insulators: we have found that materials with n=1,2 and A=K,Rb are conductors but not superconductors; n=3 and A=K,Rb are conductors (superconductors at low temperatures), but with A=Cs a…
We have observed a Fermi-surface (FS) induced lattice modulation in a YBCO superconductor with a wavevector along CuO chains, {\it i.e.} ${\bf q}_1$=(0,$δ$,0). The value of $δ\sim0.21$ is twice the Fermi wavevector ($2{\bf k}_F$) along {\bf b*} connecting nearly nested FS `ridges'. The ${\bf q}_1$ modulation exists only within O-vacancy-ordered islands (characterized by ${\bf q}_0$=$(\frac14,0,0))$ and persists well above and below $T_c$. Our results are consistent with the presence of a FS-induced charge-density wave.
We present results of neutron scattering experiments on YBa2Cu3O6.95 (Tc=93K). Our results indicate that magnetic collective modes due to correlated local moments are present both above and below Tc in optimally doped YBCO. The magnon-like modes are robust and not overdamped by itinerant particle-hole excitations, which may point at a substantial static or slowly fluctuating charge inhomogeneity. We compare the experimental results to predictions of the Fermi liquid (FL) theory in the Random Phase Approximation (RPA).
When electron doped, the layered transition metal nitrides ${\cal T}$NCl (${\cal T}$ = group IVB transition metal ion) become impressive superconductors with critical temperature T$_c$= 15-26K. Here we take the most studied member, ZrNCl, as a representative and calculate the dielectric response $ε(ω)$ versus frequency and concentration of doped electronic carriers. The static dielectric constant $ε_{\infty}$=5 is reproduced extremely well. We establish that the differences between rigid band modeling and virtual crystal treatment are small, and compare also with actual lithium doping using su…
We use neutron polarization analysis to study spin excitation anisotropy in the optimal-isovalent-doped superconductor BaFe2(As0.7P0.3)2 (Tc = 30 K). Different from optimally hole and electron-doped BaFe2As2, where there is a clear spin excitation anisotropy in the paramagnetic tetragonal state well above Tc, we find no spin excitation anisotropy for energies above 2 meV in the normal state of BaFe2(As0.7P0.3)2. Upon entering the superconducting state, significant spin excitation anisotropy develops at the antiferromagnetic (AF) zone center QAF = (1, 0, L = odd), while magnetic spectrum is iso…
The carrier transport and the motion of a vortex system in the electron-doped high-temperature superconductors Nd2-xCexCuO4 in underdoped and optimally doped (x = 0135, 0.145, 0.15) regions, in the area of the evolution from antiferromagnetic to superconducting order were investigated. To study the anisotropy of the transport properties of highly layered NdCeCuO system we have synthesized Nd2-xCexCuO4/SrTiO3 epitaxial films of three types with different orientations of the c-axis and conductive CuO2 layers relative to the substrate. Such a set of samples allowed us to study the processes of bo…
We analyze optical spectroscopy data of the electron-doped superconductor (Pr$_{2-x}$Ce$_x$)CuO$_4$ (PCCO) to investigate the coupling of the charge carriers to bosonic modes. The method of analysis is the inversion of the optical scattering rate $τ^{-1}_{\rm op}(ω,T)$ at different temperatures $T$ by means of maximum entropy technique combined with Eliashberg theory. We find that in the superconducting state the charge carriers couple to two dominant modes one at $\sim 10 $meV and a second one at $\sim 45 $meV. The low energy mode shows a strong temperature dependence and disappears at or sli…
Effects of magnetic fields (applied along the c-axis) on static spin correlation were studied for the electron-doped superconductors Pr$_{1-x}$LaCe$_{x}$CuO$_4$ with $x$=0.11 ($T_c$=26 K) and $x$=0.15 ($T_c$=16 K) by neutron scattering measurements. In the $x$=0.11 sample located near the antiferromagnetic(AF) and superconducting phase boundary, the field dependence of both the magnetic intensity at T=3 K and the onset temperature of the magnetic order exhibits a peak at $\sim$5 T. In contrast, in the overdoped $x$=0.15 sample a static AF order is neither observed at zero-field nor induced by …
Recent experiments on Zn-substituted 122-type iron-based superconductors (FeSCs) at electron- and hole- doped region provide us with a testing ground for understanding the effect of Zn impurities in these systems. Our first-principle calculations of the electronic structure reveal that the Zn 3d orbitals are far below the Fermi level and chemically inactive, while the Zn 4s-orbital is partially occupied and its wave function overlapping with those 3d-orbitals of neighboring Fe-ions. This suggests that the impurity effect is originating in the Zn 4s-orbital, not its 3d-orbitals. Employing a phe…
Based on a phenomenological model with competing spin-density-wave (SDW) and extended $s-$wave superconductivity, the vortex states in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap.
The evolution of the superconducting properties of the carbon-doped MgB2 superconductors, MgB(2-x)Cx (x= 0.02, 0.04, 0.06) have been investigated by the transverse-field muon spin rotation (TF-muSR) technique. The low-temperature depolarisation rate, sigma(0) at 0.6 T which is proportional to the second moment of the field distribution of the vortex lattice decreases monotonically with increasing electron doping and decreasing Tc. In addition, the temperature dependence of sigma(T) has been analysed in terms of a two-gap model. The size of the two superconducting gaps decreases linearly as the…
