75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6% for various field H values and orientations. The sharpness of the superconducting and magnetic transitions demonstrates a homogeneity of the Co doping x better than +-0.25%. On the nanometer scale, the paramagnetic part of the NMR spectra is found very anisotropic and very narrow for H//ab which allows to rule out the interpretation of Ref.[6] in terms of strong Co induced electronic inhomogeneities. We propose that a distribution of hyperfine couplings and chemical shifts due to the Co effect on its nearest As explains the observed linewidths and relaxations. All these measurements show that Co substitution induces a very homogeneous electronic doping in BaFe2As2, from nano to micrometer lengthscales, on the contrary to the K doping.
The NMR results in iron pnictides BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 are analyzed based on the self-consistent renormalization (SCR) spin fluctuation theory. The temperature dependence of the NMR relaxation rate (T_1)^{-1} as well as the electrical resistivity is well reproduced by a SCR model where two-dimensional antiferromagnetic (AF) spin fluctuations are dominant. The successful description of the crossover feature from non-Fermi liquid to Fermi liquid behavior strongly suggests that low-lying spin fluctuations in BaFe2(As1-xPx)2 and Ba(Fe1-xCox)2As2 possess an itinerant AF nature, and …
75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6%. Nuclear Magnetic Resonance (NMR) spectra and relaxation rates allow to show that all Fe sites experience an incommensurate magnetic ordering below T=31K. Comparison with undoped compound allows to estimate a typical moment of 0.05 muB. Anisotropy of the NMR widths can be interpreted using a model of incommensurability with a wavevector (1/2-eps,0,l) with eps of the order of 0.04. Below TC=21.8K, a full volume superconductivity develops as shown by susceptibility and relaxation rate, and magnetic order remai…
We report systematic Angle Resolved Photoemission (ARPES) experiments using different photon polarizations and experimental geometries and find that the doping evolution of the normal state of Ba(Fe1-xCox)2As2 deviates significantly from the predictions of a rigid band model. The data reveal a non-monotonic dependence upon doping of key quantities such as band filling, bandwidth of the electron pocket, and quasiparticle coherence. Our analysis suggests that the observed phenomenology and the inapplicability of the rigid band model in Co-doped Ba122 are due to electronic correlations, and not t…
Recent nuclear magnetic resonance (NMR) measurements revealed the coexistence of stripe-type antiferromagnetic (AFM) and ferromagnetic (FM) spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors by a Korringa ratio analysis. Motivated by the NMR work, we investigate the possible existence of FM fluctuations in another iron pnictide superconducting family, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$. We re-analyzed our previously reported data in terms of the Korringa ratio and found clear evidence for the coexistence of stripe-type AFM and FM spin co…
An extensive calorimetric study of the normal- and superconducting-state properties of Ba(Fe1-xCox)2As2 is presented for 0 < x < 0.2. The normal-state Sommerfeld coefficient increases (decreases) with Co doping for x < 0.06 (x > 0.06), which illustrates the strong competition between magnetism and superconductivity to monopolize the Fermi surface in the underdoped region and the filling of the hole bands for overdoped Ba(Fe1-xCox)2As2. All superconducting samples exhibit a residual electronic density of states of unknown origin in the zero-temperature limit, which is minimal at optimal doping …
We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1-xCox)2As2. Unlike the HTS cuprates and 1111-phase oxypnictides, Ba(Fe1-xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. The mass anisotropy gamma = Hc2ab/Hc2c deduced from the slopes of the upper critical field dHc2ab/dT = 4.9T/K and dHc2c/dT = 2.5T/K decreases from ~2 near Tc, to ~1.5 at lower temperatures. We observed the irreversibility field close to Hc2, and a rather unusual symmetric volume pinning force…
We investigated the optical spectrum of Ba(Fe1-xCox)2As2 single crystals with various doping levels. It is found that the low-energy optical conductivity spectrum of this system can be decomposed into two components: a sharp Drude term and a broad "incoherent" term. For the compounds showing magnetic order, a gap appears predominantly in the "incoherent" component, while an s-wave like superconducting gap opens in both components for highly doped compounds. The Drude weight steadily increases as doping proceeds, consistent with electron doping in this system. On the other hand, the "incoherent…
This paper has been withdrawn by the author due to some experimental mistakes. In this paper, we reported that C66, C44 and (C11-C12)/2 show remarkable softening toward the structural transition temperature TS. The data reported in this paper were acquired using the ultrasonic frequency lower than 25 MHz. Recently, we performed high-frequency measurements for the same system. We found that the anomaly of C44 and (C11-C12)/2 tend to disappear rapidly with increasing the frequency. On the other hand, C66 anomaly is still there at high frequencies. Therefore, we concluded that the observed anomal…
We use scanning SQUID microscopy to investigate the behavior of vortices in the presence of twin boundaries in the pnictide superconductor Ba(Fe1-xCox)2As2. We show that the vortices avoid pinning on twin boundaries. Individual vortices move in a preferential way when manipulated with the SQUID: they tend to not cross a twin boundary, but rather to move parallel to it. This behavior can be explained by the observation of enhanced superfluid density on twin boundaries in Ba(Fe1-xCox)2As2. The observed repulsion from twin boundaries may be a mechanism for enhanced critical currents observed in t…
Ultrafast terahertz (THz) pump{probe spectroscopy reveals unusual out-of-equilibrium Cooper pair dynamics driven by femtosecond (fs) optical quench of superconductivity (SC) in iron pnictides. We observe a two{step quench of the SC gap, where an abnormally slow (many 100's of ps) quench process is clearly distinguished from the usual fast (sub-ps) hot{phonon{mediated scattering channel. This pair breaking dynamics depends strongly on doping, pump uence, and temperature. The above observations, together with quantum kinetic modeling of non-equilibrium SC and magnetic correlations, provide evide…
We show that despite the low anisotropy, strong vortex pinning and high irreversibility field Hirr close to the upper critical field Hc2 of Ba(Fe1-xCox)2As2, the critical current density Jgb across [001] tilt grain boundaries (GBs) of thin film Ba(Fe1-xCox)2As2 bicrystals is strongly depressed, similar to high-Tc cuprates. Our results suggest that weak-linked GBs are characteristic of both cuprates and pnictides because of competing orders, low carrier density, and unconventional pairing symmetry.
This paper reports comprehensive results on thin-film growth of 122-type iron-pnictide superconductors, AE(Fe1-xCox)2As2 (AE = Ca, Sr, and Ba, AEFe2As2:Co) by a pulsed laser deposition method using a neodymium-doped yttrium aluminum garnet laser as an excitation source. The most critical parameter to produce the SrFe2As2:Co and BaFe2As2:Co phases is the substrate temperature (Ts). It is difficult to produce highly-pure CaFe2As2:Co phase thin film at any Ts. For BaFe2As2:Co epitaxial films, controlling Ts at 800-850 °C and growth rate to 2.8-3.3 Å/s produced high-quality films with good crystal…
The biaxially textured growth of superconducting Co-doped BaFe2As2 (Ba-122) thin films has been realized on ion beam assisted deposition (IBAD) MgO coated conductor templates by employing an iron buffer architecture. The iron pnictide coated conductor showed a superconducting transition temperature of 21.5 K, which is slightly lower than that of Co-doped Ba-122 films on single crystalline MgO substrates. A self-field critical current density of over 10^5 A/cm2 has already been achieved even at 8 K. The current experiment highlights the potential of possible coated conductor applications of the…
We report muon spin relaxation/rotation (uSR) measurements of single crystal Ba(Fe1-xCox)2As2 with x=0.038 and 0.047. Zero field (ZF)-uSR and Transverse field (TF)-iSR measurements of these underdoped samples find the presence of magnetism and superconductivity. We find internal fields along the c-axis whose magnitude decreases with increasing doping. We find evidence for a low-temperature volume fraction that is only weakly magnetic, where that volume fraction increases with increasing Co doping of the sample. TF-uSR measurements show slight changes in the spectra that indicate magnetic inhom…
Superconducting and normal state transport properties in iron pnictides are sensitive to disorder and impurity scattering. By investigation of Ba(Fe1-xCox)2As2 thin films with varying Co concentration, we demonstrate that in the dirty limit the superconducting dome in the electronic phase diagram of Ba(Fe1-xCox)2As2 shifts towards lower doping concentrations, which differs significantly from observations in single crystals. We show that especially in the underdoped regime superconducting transition temperatures higher than 27 K are possible.
We discuss the irreversible magnetic properties of self-flux grown Ba(Fe1-xCox)2As2 single crystals for a wide range of concentrations covering the whole phase diagram from the underdoped to the overdoped regime, x=0.038, 0.047, 0.058, 0.071, 0.074, 0.10, 0.106 and 0.118. Samples were characterized by a magneto-optical method and show excellent spatial uniformity of the superconducting state. The overall behavior closely follows classical Bean model of the critical state. The field-dependent magnetization exhibits second peak at a temperature and doping - dependent magnetic field, Hp. The evol…
Superconductivity in Iron-Arsenic based pnictides emerges in close proximity to an antiferromagnetic (AFM)ordered parent state and the AFM phase overlaps with superconducting (SC) phase in some pnictides for certain range of doping. CaFe2-xNixAs2 belongs to this category, where both the phases overlap. Here we use scanning tunneling microscopy and spectroscopy to investigate the local electronic properties of uderdoped CaFe1.96Ni0.04As2 single crystals in the vicinity of the boundary of the two phases. Both resistivity and magnetic measurements show that a tiny portion (~ 1.2 %) of this compou…
We use NMR techniques to investigate the magnetic properties of BaCo2As2 single crystals, the non-superconducting end member of the Co-substituted iron-pnictide high Tc superconductor Ba(Fe1-xCox)2As2 with x = 1. We present 75As NMR evidence for enhancement of low frequency spin fluctuations below ~ 100 K. This enhancement is accompanied by that of static uniform spin susceptibility at the wave vector q = 0, suggesting that the primary channel of the spin correlations is ferromagnetic rather than antiferromagnetic. Comparison between the NMR Knight shift 75K and bulk susceptibility chi(bulk) d…
We report $^{75}$As nuclear magnetic resonance (NMR) measurements of single-crystalline Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$ ($x$ = 0.023, 0.028, 0.033, and 0.059) annealed at 350~$^{\circ}$C for 7 days. From the observation of a characteristic shape of $^{75}$As NMR spectra in the stripe-type antiferromagnetic (AFM) state, as in the case of $x$ = 0 ($T_{\rm N}$ = 170 K), clear evidence for the commensurate AFM phase transition with the concomitant structural phase transition is observed in $x$ = 0.023 ($T_{\rm N}$ = 106 K) and $x$ = 0.028 ($T_{\rm N}$ = 53 K). Through the temperature dependence of …
We use angle-resolved photoemission spectroscopy to study twinned and detwinned iron pnictide compound NaFeAs. Distinct signatures of electronic reconstruction are observed to occur at the structural (TS) and magnetic (TSDW) transitions. At TS, C4 rotational symmetry is broken in the form of an anisotropic shift of the orthogonal dxz and dyz bands. The magnitude of this orbital anisotropy rapidly develops to near completion upon approaching TSDW, at which temperature band folding occurs via the antiferromagnetic ordering wave vector. Interestingly, the anisotropic band shift onsetting at TS de…
Variant approaches, either based on the Fermi surface nesting or started from the proximity to a Mott-insulator, were proposed to elucidate the physics in iron pnictides, but no consensus has been reached. A fundamental problem concerns the nature of their 3d electrons. Here we report the magnetoresistivity (ρ_xx) and the Hall resistivity (ρ_xy) of Ba(Fe1-xCox)2As2 (x=0 and 0.05) in a magnetic field of up to 55T. The magnetic transition is extremely robust against magnetic field, giving strong evidence that the magnetic ordering is formed by local moments. The magnetic state is featured with a…
