The energy gap of optimally doped Bi2(Sr,R)2CuOy (R=La and Eu) was probed by angle resolved photoemission spectroscopy (ARPES) using a vacuum ultraviolet laser (photon energy 6.994 eV) or He I resonance line (21.218 eV) as photon source. The results show that the gap around the node at sufficiently low temperatures can be well described by a monotonic d-wave gap function for both samples and the gap of the R=La sample is larger reflecting the higher Tc. However, an abrupt deviation from the d-wave gap function and an opposite R dependence for the gap size were observed around the antinode, which represent a clear disentanglement between the antinodal pseudogap and the nodal superconducting gap.
Within the framework of pure BCS(i.e. when the critical temperature is proportional to the Debye temperature), we show that the isotope coefficient is always less than 1/2 and could be extremely small in polyatomic superconductors depending on the chemical formula unit. This finding leads to quantitative explanation of observed values(correct orders of magnitude and sometimes exact numerical values) in optimally doped cuprates, magnesium diboride and alkali-doped fullerenes. Consequently, the smallness of the isotope coefficient is not only compatible with pure electron-phonon interaction, but…
We have grown calcium and oxygen co-doped Y1-xCaxBa2Cu3O7-δ single crystals using self-flux method. A method of fine tuning oxygen content to reach the equilibrium state through in situ monitoring of the conductivity change is established. Structural, compositional and electronic property characterizations of optimally co-doped Y1-xCaxBa2Cu3O7-δ crystals indicate that they are high quality equilibrium crystals with a sharp superconducting transition width of 0.2K.
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.
An analysis of the frequency-dependent scattering rate reveals signatures of the superconducting gap as well as the pseudogap in high-T_c cuprates. These features can be identified in the temperature-dependent spectral function W(omega), inverted from the optical data of the optimally-doped Bi2212 using an experimentally unambiguous method that shows the behavior of both the maxima and minima in the spectral function.
The calculated specific heat of optimally K-doped BaFe$_2$As$_2$ in density functional theory is about five times smaller than that found in the experiment. We report that by adjusting the potential on the iron atom to be slightly more repulsive for electrons improves the calculated heat capacity as well as the structural, magnetic, and electronic properties of Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. Applying the same correction to the antiferromagnetic state, we find that the electron-phonon coupling is strongly enhanced.
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.
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.
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 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).
Both the in-plane and out-of-plane magnetoresistivities have been measured in the normal state of an optimally doped Bi2Sr2Ca0.8Y0.2Cu2O8+d single crystal with a magnetic field applied parallel and perpendicular to the CuO2 planes. Whatever the magnetic field and the current directions are, a negative magnetoresistivity is obtained over a wide range of temperature above the critical temperature Tc. For the in-plane and out-of-plane measurements, the non-dominant orbital contribution to magnetoresistivity suggests the substantial role played by the spin degrees of freedom.
Inelastic neutron scattering was used to study the Cu-O bond-stretching vibrations in optimally doped La1.85Sr0.15CuO4 (Tc = 35 K) and in two other cuprates showing static stripe order at low temperatures, i.e. La1.48Nd0.4Sr0.12CuO4 and La1.875Ba0.125CuO4. All three compounds exhibit a very similar phonon anomaly, which is not predicted by conventional band theory. It is argued that the phonon anomaly reflects a coupling to charge inhomogeneities in the form of stripes, which remain dynamic in superconducting La1.85Sr0.15CuO4 down to the lowest temperatures. These results show that the phonon …
Low temperature specific heat has been measured in optimally doped and highly overdoped non-superconducting BaFe$_{2-x}$TM$_x$As$_{2}$ ($TM$ = Co and Ni) single crystals. By using the data of the overdoped samples, we successfully removed the phonon contribution of the optimally doped ones, and derived the electronic specific heat coefficient $γ_e$. Remarkably, we found a continuing temperature dependent $γ_e(T)$ which follows the quadratic relation $γ_e=γ_0+αT^2$ in the low temperature limit. Together with the very small residual term $γ_0$, linear magnetic field dependence of $γ_e$, it is co…
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 use femtosecond spectroscopy to investigate the quasiparticle relaxation and low-energy electronic structure in a near-optimally doped pnictide superconductor with T_c=49.5 K. Multiple relaxation processes are evident, with distinct superconducting state quasiparticle (QP) recombination dynamics exhibiting a T-dependent superconducting (SC) gap, and a clear "pseudogap" (PG) -like feature with an onset around 200K indicating the existence of a temperature-independent gap of magnitude Δ_PG= 61 +- 9 meV above T_c. Both the SC and PG components show saturation as a function of fluence with dist…
We reveal that the temperature dependence of the basal-plane normal-state electrical resistance of optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ single crystals can be with great accuracy approximated within the framework of the model of s-d electron-phonon scattering. This requires taking into account the fluctuation conductivity whose contribution exponentially increases with decreasing temperature and decreases with an increase of oxygen deficiency. Room-temperature annealing improves the sample and, thus, increases the superconducting transition temperature. The temperature of the 2D-3D crossover…
We propose a possible scenario for the new metallic conductivity of underdoped and optimally doped cuprates. Charge carriers are assumed to be large polarons which form a Fermi-liquid and Cooper-like pairs below a crossover tempurature $T^{\ast}$. We use the Boltzmann equation to calculate the conductivity of self-trapped carriers and the resistivity $ρ$ as a function of temperature and doping for different cuprates. We show that various anomalies in $ρ(T)$ below $T^{\ast}$ are caused by the competing Fermi-liquid and BCS-like precursor pairing effects. Our results for $ρ$ fit well with existi…
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 optical properties of optimally-doped Bi_2Sr_2CaCu_2O_{8+δ} (Bi2212) have been measured normal to the edge planes [ac plane, bc plane, and (1\bar{1}0) plane], for light polarized parallel to nodal and anti-nodal (gap) directions, respectively. While the superfluid contribution can be obtained from the optical conductivities in the (1\bar{1}0)-plane, it is unobservable in the ac and bc-planes. This apparent asymmetry implies that the edge region of high-T_c cuprates is unusual and further supports a d-wave symmetry of the superconducting order parameter.
Electronic heat transport in the normal state of a high-quality single crystal of optimally-doped superconductor YBa2Cu3O6.95 was studied by measurements of longitudinal and transverse transport coefficients. For the temperature range from 100 to 300 K, the Hall-Lorenz number (Lxy) depends weakly on temperature and is about two times larger than the Sommerfeld value of the Lorenz number Lo = (pi^2)/3. Our results can be interpreted using a Fermi liquid model when effects of the pseudogap that opens at the Fermi level are included. However, we find that the bipolaron model can also explain both…
We report magnetotransport measurements and its scaling analysis for the optimally electron doped Sr(Fe${_{0.88}}$Co${_{0.12}}$)${_2}$As${_2}$ system. We pbserve that both the Kohler's and modified Kohler's scalings are violated. Interestingly, the Hall angle displays a quadratic temperature dependence similar to many cuprates and heavy fermion systems. The fact that this temperature dependence is seen in spite of the violation of modified Kohler's scaling suggests that the Hall angle and the magnetoresistance are not governed by the same scattering mechanism. We also observe a linear magnetor…
The superconducting fluctuations well inside the normal state of Fe-based superconductors were experimentally studied through the in-plane paraconductivity in several high-quality optimally-doped BaFe$_{2-x}$Ni$_x$As$_2$ crystals. These measurements were performed in magnetic fields with amplitudes up to 14 T, and different orientations relative to the crystals c axis (0, 53 and 90 degrees). The results allowed a stringent check of the applicability of a recently proposed Ginzburg-Landau approach for the fluctuation electrical conductivity of 3D anisotropic materials in presence of finite appl…
The temperature dependences of the Hall-Lorenz numbers (Lxy) in a EuBa2Cu3Oy (Eu-123) single crystal before and after oxygen reduction are reported. The study is based on data on the normal state longitudinal and transversal transport coefficients. Namely, the temperature dependences of the electrical resistivity, Hall coefficient, longitudinal thermal conductivity, and transverse thermal conductivity are presented. The set of measurements was performed for an optimally doped sample (y = 7), then the oxygen content in the the same crystal was reduced to y = 6.65, and the measurements were repe…
We present measurements of the ab-plane magnetic penetration depth, λ(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6 K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities n_s(T)\propto λ^{-2}(T), high T_c's, and small transition widths are reproducible and indicative of excellent film quality. For all five films, λ^{-2}(T)/λ^{-2}(0) at low T is well fitted by an exponential temperature dependence with a gap, Δ_{min}, of 0.85 k_B T_c. This behavior is consistent with a nodeless gap and is incompatible with d-wave superconductivity.
A novel heterodyne polarimetry system determines the frequency dependence from 900 to 1100 cm^-1 and temperature dependence from 35 to 330 K of the Hall transport in single crystal, optimally doped BSCCO. The results show a significant disconnect from the behavior of the Hall angle in the existing data for YBCO in the far-infrared, which indicate a negative value for the real part of the Hall angle above 250 cm^-1, whereas that of the current work at 1000 cm^-1 is positive. The current work when analyzed using an extended Drude formalism results in a Hall mass comparable to the ARPES Fermi mas…
The temperature dependent evolution of the renormalization effect in optimally-doped Bi2212 along the nodal direction has been studied via angle-resolved photoemission spectroscopy. Fine structure is observed in the real part of the self-energy (Re$Σ$), including a subkink and maximum, suggesting that electrons couple to a spectrum of bosonic modes, instead of just one mode. Upon cooling through the superconducting phase transition, the fine structures of the extracted Re$Σ$ exhibit a two-processes evolution demonstrating an interplay between kink renormalization and superconductivity. We show…
Ac susceptibility and static magnetization measurements were performed in the optimally doped SmFeAsO$_{0.8}$F$_{0.2}$ superconductor. The field - temperature phase diagram of the superconducting state was drawn and, in particular, the features of the flux line lattice derived. The dependence of the intra-grain depinning energy on the magnetic field intensity was derived in the thermally-activated flux creep framework, enlightening a typical 1/H dependence in the high-field regime. The intra-grain critical current density was extrapolated in the zero temperature and zero magnetic field limit, …
Magnetic measurements have been carried out in the superconducting and normal states of the optimally doped nonmagnetic bismuthate superconductor Ba0.63K0.37BiO3. The magnetic data along with previous muSR, resistivity, and tunneling data consistently show that there is a large polaronic enhancement in the density of states and effective electron-phonon coupling constant. The first-principle calculation within the density-functional theory indicates a small electron-phonon coupling constant of about 0.3-0.4, which can only lead to about 1 K superconductivity within the conventional phonon-medi…
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 explore the flux front patterns during virgin penetration and after remagnetization in the iron-based superconductor Ba(Fe_{1-x}Co_{x})_{2}As_{2}. After remagnetization we observe an instability characterized by turbulent dynamics at the vortex-antivortex boundary. Associated with the turbulent flux boundary a band of excess current density is observed. The turbulence is contrasted with the instability observed in the cuprate superconductors. Our results suggest turbulent instability at the vortex-antivortex interface may be a common feature in all remagnetized type-II superconductors.
Zero field and longitudinal field muon spin relaxation measurements have been performed in optimally doped and overdoped superconductors LaFeAsO$_{1-x}$F$_x$ in order to investigate the magnetic fluctuation over a wide range of temperature and longitudinal field. We have observed no sign of magnetic fluctuation against temperature in the muons' time window (10$^{-5}\sim$ 10$^{-9}$s). Considering the current results and other results, i.e., spin fluctuation observed by neutron scattering, pseudogap-like behaviors by NMR and photoemission spectroscopy, it is suggested that not only the spin fluc…
We present the resistively-determined upper critical field H^ρ_{c2}(T) and the irreversibility lines H^ρ_{irr}(T) of various high-T_c cuprates, deduced from measurements in 61-T pulsed magnetic fields applied parallel to the c-axis. The SHAPE of both H^ρ_{c2}(T) and H^ρ_{irr}(T) depends monotonically on the anisotropy of the material and none of the samples show saturation of H^ρ(T) at low temperatures. The anomalous positive curvature, d^2 H^ρ/dT^2 > 0, is the strongest in materials with the largest normal-state anisotropy, regardless of whether anisotropy is varied by changing the carrier co…
We report on the recovery of the short-range static magnetic order and on the concomitant degradation of the superconducting state in optimally F-doped SmFe_(1-x)Ru_(x)AsO_0.85F_0.15 for 0.1< x<0.6. The two reduced order parameters coexist within nanometer-size domains in the FeAs layers and finally disappear around a common critical threshold x_c=0.6. Superconductivity and magnetism are shown to be closely related to two distinct well-defined local electronic environments of the FeAs layers. The two transition temperatures, controlled by the isoelectronic and diamagnetic Ru substitution, scal…
We report on the magnetic and superconducting properties of LaO0.5F0.5BiS2 by means of zero- (ZF) and transverse-field (TF) muon-spin spectroscopy measurements (uSR). Contrary to previous results on iron-based superconductors, measurements in zero field demonstrate the absence of magnetically ordered phases. TF-uSR data give access to the superfluid density, which shows a marked 2D character with a dominant s-wave temperature behavior. The field dependence of the magnetic penetration depth confirms this finding and further suggests the presence of an anisotropic superconducting gap.
We use inelastic neutron scattering to study the effect of an in-plane magnetic field on the magnetic resonance in optimally doped superconductors FeSe$_{0.4}$Te$_{0.6}$ ($T_c=14$ K) and BaFe$_{1.9}$Ni$_{0.1}$As$_{2}$ ($T_c=20$ K). While the magnetic field up to 14.5 Tesla does not change the energy of the resonance, it particially suppresses $T_c$ and the corresponding superconductivity-induced intensity gain of the mode. However, we find no direct evidence for the field-induced spin-1 Zeeman splitting of the resonance. Therefore, it is still unclear if the resonance is the long-sought single…
Small-angle neutron scattering on high quality single crystalline Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ reveals the transition from a low--field vortex solid phase with orientational order to a vortex polycrystal at high magnetic field. The vortex order-disorder transition is correlated with the second-peak feature in isothermal hysteresis loops, and is interpreted in terms of the generation of supplementary vortex solid dislocations. The sharp drop of the structure factor above the second peak field is explained by the dynamics of freezing of the vortex ensemble in the high field phase.
The magnitude of the powder spin susceptibility of an optimally doped superconductor HgBa$_2$CuO$_{4+δ}$ (Hg1201) in the normal state is found to be nearly the same as that of La$_{2-x}$Sr$_{x}$CuO$_{4}$ near the optimally doped level. The Stoner enhancement factor of Hg1201 is larger than that of La$_{2-x}$Sr$_{x}$CuO$_{4}$. The magnitude correlation of the Stoner enhancement factor is inconsistent with the effect of the recent theoretical Coulomb repulsion between 3$d$ electrons and that of the superexchange intereraction of a charge transfer type.
We performed combined infrared reflection and ellipsometry measurements of the in-plane optical reponse of single crystals of the pnictide high temperature superconductor BaFe$_{1.87}$Co$_{0.13}$As$_{2}$ with $T_{c}$ = 24.5 K. We observed characteristic superconductivity-induced changes which provide evidence for at least three different energy gaps. We show that a BCS-model of isotropic gaps with 2$Δ/k_{B}T_{c}$ of 3.1, 4.7, and 9.2 reproduces the experimental data rather well. We also determine the low-temperature value of the in-plane magnetic penetration depth of 270 nm.
We report the comprehensive comparative analysis of the upper critical magnetic fields $μ_{0}H_{c2}(0)$ obtained within Ginzburg-Landau and Werthamer-Helfand-Hohenberg theories for optimally-doped $YBa_2Cu_3O_{7-δ}$ thin films. For different orientations of the magnetic field, our calculations give 638 and 153 T for $μ_{0}H_{c2}(0)$, H || ab and $μ_{0}H_{c2}(0)$, H || c, respectively, when using $H_{c2}(0)$. For the first time, the temperature dependences of coherence lengths $ξ_{ab}(T)$ and $ξ_{c}(T)$ within proposed theories were determined using 50 and 90% criteria of the normal state resis…
An optimally doped iron-based superconductor LaFeAsO$_{0.92}$F$_{0.08}$ with $T_c = 29$ K has been studied by inelastic powder neutron scattering. The magnetic excitation at $Q=1.15$ Å$^{-1}$ is enhanced below $T_c$, leading to a peak at $E_{res}\sim13$ meV as the resonance mode, in addition to the formation of a gap at low energy below the crossover energy $Δ_{c}\sim10 meV$. The peak energy at $Q=1.15$ Å$^{-1}$ corresponds to $5.2 k_B T_c$ in good agreement with the other values of resonance mode observed in the various iron-based superconductors, even in the high-$T_c$ cuprates. Although the…
Magnetic excitations in Ba(Fe0.94Co0.06)2As2 are studied by polarized inelastic neutron scattering (INS) above and below the superconducting transition. In the superconducting state we find clear evidence for two resonance-like excitations. At a higher energy of about 8 meV there is an isotropic resonance mode with weak dispersion along the c-direction. In addition we find a lower excitation at 4 meV that appears only in the c-polarized channel and whose intensity strongly varies with the L-component of the scattering vector. These resonance excitations behave remarkably similar to the gap mod…
An angle-resolved photoemission study of the scattering rate in the superconducting phase of the high-temperature superconductor \LSCO\ with $x=0.145$ and $x=0.17$, as a function of binding energy and momentum, is presented. We observe that the scattering rate scales linearly with binding energy up to the high-energy scale $E_1\sim0.4$ eV. The scattering rate is found to be strongly anisotropic, with a minimum along the (0,0)-($π,π$) direction. A possible connection to a quantum-critical point is discussed.
We first present detailed measurements of the rounding behavior around the superconducting transition temperature, $T_c$, of the in-plane electrical conductivity, magnetoconductivity and magnetization, including the low and moderate magnetic field regimes, in a high-quality single crystal and a thin film of the prototypical optimally-doped YBa$_2$Cu$_3$O$_{7-δ}$ (OPT Y-123), in which the inhomogeneity effects are minimized. Then, it is presented a comparison of these experimental data with the phenomenological Ginzburg-Landau (GL) approach that takes into account the unavoidable contribution o…
We report on Cu nuclear quadrupole resonance (NQR) and NMR studies of an optimally hole-doped superconductor Ca2-xNaxCuO2Cl2 (Tc ~ 28 K for x ~ 0.2). In spite of robust oxygen composition, we observed a multiple broad NQR frequency spectrum and nonexponential Cu nuclear spin-lattice relaxation, being similar to those of La2-xSrxCuO4-d.
We present the strain and temperature dependence of an anomalous nematic phase in optimally doped BaFe$_2$(As,P)$_2$. Polarized ultrafast optical measurements reveal broken 4-fold rotational symmetry in a temperature range above $T_c$ in which bulk probes do not detect a phase transition. Using ultrafast microscopy, we find that the magnitude and sign of this nematicity vary on a ${50{-}100}~μ$m length scale, and the temperature at which it onsets ranges from 40 K near a domain boundary to 60 K deep within a domain. Scanning Laue microdiffraction maps of local strain at room temperature indica…
We present tunneling data from optimally-doped, superconducting BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor, clear coherence-like peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, Delta. The average magnitude of 2Delta is ~7.4 kBTC, which exceeds the BCS weak coupling value for either s- or d-wave superconductivity. The characteristic length scales of the deviations from the average gap value, and of an anti-correlation discovered between the gap magnitude and the zero bias conductance…
he frequency dependence in the mid IR from 900 to 1100 cm$^{-1}$ and far IR at 84 cm$^{-1}$ and temperature dependence from 35 to 330 K of the normal state Hall transport is reported in single crystal, optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. The results show a nearly Drude behavior in the Hall conductivity $σ_{xy}$, which stands in contrast to the more complex extended Drude behavior for the longitudinal conductivity $σ_{xx}$. The mid IR Hall scattering rate increases linearly with temperature and has a small, positive, projected intercept of 200 cm$^{-1}$ at T=0. In contrast, the longi…
The appearance of static magnetism, nanoscopically coexisting with superconductivity, is shown to be a general feature of optimally electron-doped LnFe(1-x)Ru(x)AsO(1-y)F(y) superconductor (Ln - lanthanide ion) upon isovalent substitution of Fe by Ru. The magnetic ordering temperature T_N and the magnitude of the internal field display a dome-like dependence on x, peaked around x=1/4, with higher T_N values for those materials characterized by a larger z cell coordinate of As. Remarkably, the latter are also those with the highest superconducting transition temperature (T_c) for x=0. The reduc…
We use angle-resolved photoemission spectroscopy (ARPES) to investigate the properties of the energy gap(s) in optimally doped (Bi,Pb)2(Sr,La)2CuO6+d (Bi2201). We find that the spectral gap has two components in the superconducting state: a superconducting gap and pseudogap. Significant differences in their momentum and temperature dependence suggest that they represent two separate energy scales. Spectra near the node reveal a sharp peak with a small gap below Tc that closes at Tc. Near the antinode, the spectra are broad with a large energy gap of ~40meV above and below Tc. The spectral shap…
We report the analysis of pseudogap Delta* derived from resistivity experiments in FeAs-based superconductor SmFeAsO(0.85), having a critical temperature T_c = 55 K. Rather specific dependence Delta*(T) with two representative temperatures followed by a minimum at about 120 K was observed. Below T_s = 147 K, corresponding to the structural transition in SmFeAsO, Delta*(T) decreases linearly down to the temperature T_AFM = 133 K. This last peculiarity can likely be attributed to the antiferromagnetic (AFM) ordering of Fe spins. It is believed that the found behavior can be explained in terms of…
Charge order in underdoped and optimally doped high-$T_\mathrm{c}$ superconductors Bi$_{2}$Sr$_{2-x}$La$_x$CuO$_{6+δ}$ (Bi2201) is investigated by Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS). We have directly observed charge density modulation in the optimally doped Bi2201 at momentum transfer $Q_{\|} \simeq 0.23$ rlu, with smaller intensity and correlation length with respect to the underdoped sample. This demonstrates that short-range charge order in Bi2201 persists up to optimal doping, as in other hole-doped cuprates. We explored the nodal (diagonal) direction and found no cha…
We report the first direct observation of the oxygen-isotope ($^{16}$O/$^{18}$O) effect on the in-plane penetration depth $λ_{ab}$ in a nearly optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ film using the novel low-energy muon-spin rotation technique. Spin polarized low energy muons are implanted in the film at a known depth $z$ beneath the surface and precess in the local magnetic field $B(z)$. This feature allows us to measure directly the profile $B(z)$ of the magnetic field inside the superconducting film in the Meissner state and to make a model independent determination of $λ_{ab}$. A substantia…
Magnetic excitations in the optimally doped high-$T_\mathrm{c}$ superconductor Bi$_{1.5}$Pb$_{0.55}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+δ}$ (OP-Bi2201, $T_\mathrm{c}\simeq 34$ K) are investigated by Cu $L_3$ edge resonant inelastic x-ray scattering (RIXS), below and above the pseudogap opening temperature. At both temperatures the broad spectral distribution disperses along the (1,0) direction up to $\sim$350~meV at zone boundary, similarly to other hole-doped cuprates. However, above $\sim$0.22 reciprocal lattice units, we observe a concurrent intensity decrease for magnetic excitations and quasi-ela…
The confrontation between the superconducting fluctuations and percolation effects as the origin of the in-plane paraconductivity in cuprate superconductors was earlier addressed at a quantitative level in the case of the optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO) compound. Using in-plane resistivity data from a high-quality YBCO thin film, we will extend these analyses to high reduced temperatures, in the case of the Gaussian-Ginzburg-Landau (GGL) approach for the conventional superconducting fluctuations, by considering the total energy cutoff. These data will also be analysed in terms of…
We report on current-voltage measurements of the zero-field normal-superconducting phase transition in thin films of Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ as a function of doping. We find that the small size of the critical regime in these materials ($\approx 25$ mK) gives rise to mean-field behavior at the phase transition with a static exponent of $ν\approx 0.5$ for all dopings (in contrast to hole-doped $\mathrm{YBa_{2}Cu_{3}O_{7-δ}}$). We also find mean-field behavior in the dynamic exponent $z$. This indicates that Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ behaves similarly to conventional superconductors in con…
We have measured the superconducting gap Ds in optimally doped samples of BSCCO, TBCCO, HBCCO and HSCCO by Andreev and tunnelling spectroscopy and by ARPES. We have found that the low-temperature value of the gap within experimental errors is linearly increasing with the number n of CuO2 layers in the unit cell of the investigated HTSC-families (n < 4)
Transport measurements along the node and anti-node directions in the a-b plane of optimally doped and epitaxial thin films of YBCO are reported. Low bias magnetoresistance measurements near and below T_c show that the flux flow resistivity along the node and anti-node directions versus magnetic field are indistinguishable. This result suggests that within the experimental error of our measurements, no correspondence is found between the flux pinning properties in YBCO and the d-wave nature of the order parameter.
By measuring the polarization changes in THz, infrared, and visible radiation over an extended energy range (3-2330 meV), we observe symmetry-breaking in cuprate high temperature superconductors over wide energy, doping, and temperature ranges. We measure the polarization rotation and ellipticity of transmitted radiation though thin films as the sample is rotated. We observe a two-fold rotational symmetry in the Faraday angle, which is associated with linear dichroism (LD) and occurs when electromagnetic radiation polarized along one direction is absorbed more strongly than radiation polarized…
Spin gap in optimally-doped YBCO was investigated by inelastic neutron scattering. We found that q-integrated spectral weight in the vicinity of the spin gap has a shape of a step function at 10K with sharp resolution-limited onset at 33 meV, i.e. there is no observable spectral weight at 31 meV but full spectral weight at 35 meV. The spin gap does not soften or "fill in" as the temperature is increased to Tc. Above Tc we find no evidence for q-dependent magnetic signal at 35 meV.
We addressed the inconsistency between the electron mass anisotropy ratios determined by the far-infrared experiments and DC conductivity measurements. By eliminating possible sources of error and increasing the sensitivity and resolution in the far-infrared reflectivity measurement on the single crystalline and on the polycrystalline La1.84Sr0.16CuO4, we have unambiguously identified that the source of the mass anisotropy problem is in the estimation of the free electron density involved in the charge transport and superconductivity. In this study we found that only 2.8 % of the total doping-…
