The longitudinal in-plane magnetoresistance (LMR) has been measured in different Ba(Fe1-xCox)(2)As-2 single crystals and in LiFeAs. For all these compounds, we find a negative LMR in the paramagnetic phase whose magnitude increases as H-2. We show that this negative LMR can be readily explained in terms of suppression of the spin fluctuations by the magnetic field. In the Co-doped samples, the absolute value of the LMR coefficient is found to decrease with doping content in the paramagnetic phase. The analysis of its T dependence in an itinerant nearly antiferromagnetic Fermi-liquid model evidences that the LMR displays a qualitative change of T variation with increasing Co content. The latter occurs at optimal doping for which the antiferromagnetic ground state is suppressed. The same type of analysis for the negative LMR measured in LiFeAs suggests that this compound is on the verge of magnetism.

The pinning of quantized flux lines, or vortices, in the mixed state is used to quantify the effect of impurities in iron-based superconductors (IBS). Disorder at two length scales is relevant in these materials. Strong flux pinning resulting from nm-scale heterogeneity of the superconducting properties leads to the very disordered vortex ensembles observed in the IBS, and to the pronounced maximum in the critical current density jc at low magnetic fields. Disorder at the atomic scale, most likely induced by the dopant atoms, leads to "weak collective pinning" and a magnetic field-independent contribution jcoll. The latter allows one to estimate quasi-particle scattering rates.

The resistivity measurements of bulk super-conducting oxides La2−xSrxCuO4−x/2+ 〈0.15〈x〈0.2) irradiated by 2.5 Mev-electrons and YBa2Cu3O7-δ 〈0〈δ〈0.15) irradiated by 2.9 GeU-krypton ions have shown a regular and constant decrease of the critical temperature Tc as a function of the fluence. On the contrary, an increase of Tc has been observed durino irradiation of super-conduct ino La2CuO4 compound by 2.9 GeV-krypton ions, which saturates at Tc/Tco = 0.1 for fluences hioher than 3.1012 Kr.cm−. The evolution of the R(T) curves during irradiation is very similar to the one observed under Dressure. A comparison of the variations of Tc as a function of the number of di solacements oer atom for various particles (n. e−. He+. O. Kr) suggests that the hijjh electronic stoppino power of the heavy ions could be the main factor of damage.

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 reference [J. Phys. Soc. Jpn 78, 013711 (2009); Phys. Rev. B 79, R 140506 (2009)] 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.

A Reply to the Comment by L. A. Openov.Received 12 December 2003DOI:https://doi.org/10.1103/PhysRevLett.93.129702©2004 American Physical Society

We present transport and specific heat measurements on high quality single crystals of UPt3 before and after irradiation by high energy electrons. The induced changes of the superconducting critical temperature are found to depend strongly on the distance from the irradiated surface. The dramatic effects of the irradiation on the specific heat in the superconducting state can then be simply explained in terms of an inhomogeneous distribution of superconducting transition temperatures. The question of the failure of the “universal limit” in the heat transport of UPt3 is reexamined, and the conditions for a clean experimental test are established.

Spinless point defects in the CuO 2 planes of the cuprates have been shown for long to induce a magnetic response on the neighboring copper sites which behaves as a nearly free local moment in the under-doped case.1,2 In the case of Li + substitution on the Cu site, accurate 7 Li NMR shift data allow to evidence that the local moment susceptibility displays a ( T + T K ) -1 dependence, with a Kondo like temperature T K which increases abruptly near optimal doping.3 While a Curie paramagnetic response can be understood on general theoretical grounds in the case of undoped quantum spin systems, the Kondo screening of the moment might directly reflect the influence of charge carriers. This Kondo like behavior is also corroborated by measurements of the fluctuation time of the local moment deduced from 7 Li NMR spin lattice relaxation time.4 The local moment is found to survive in the superconducting state although a marked reduction of the screening is observed when T K > T c (see Ref. [5]), as might be expected from theoretical considerations. Correlatively it will be shown that the scattering of charge carriers by spinless defects can be studied accurately in single crystals in which such defects are created by electron irradiation. The low T upturns of the resistivity can be associated with a Kondo-like T dependence of the scattering in the under-doped case. In the over-doped case, T K being very high, the scattering becomes T-independent and 2D weak localization effects dominate.6

Substitutional impurities in the CuO2 planes of the cuprates allow us to probe the electronic properties of the host material. The pseudo;gap in the underdoped regime is unmodified far from the impurities even though T-c is greatly reduced. In the overdoped regime, it is not detected above T-c and the data in presence of impurities seems to imply that the pseudo-gap and T-c lines cross each other. The spin polarisation induced by magnetic impurities has an oscillatory staggered behaviour reflecting the existing AF correlations between the Cu spins. Its influence on the NMR spectra opens a way to determine the q dependence of the static spin susceptibility and the T dependence of the AF correlation length. NMR measurements demonstrate that non-magnetic impurities such as Zn or Li induce a local moment behaviour which results from staggered magnetism on the near neighbour copper sites. Li NMR shift data allow to measure accurately the susceptibility associated with these local moments. It is found to display a Kondo like (T + Theta)(-1) dependence, with a Kondo Theta which increases abruptly near optimal doping. The magnetism revealed by spin-less sites can be understood on theoretical grounds in the case of undoped quantum spin systems. The present results reflect the influence of oxygen hole carriers on the AF correlations between hole spins in the cuprates. Another manifestation of this coupling is the large scattering of the carriers by spin-less impurities detected in transport properties. Resistivity measurements on single crystals with point defects created by electron irradiation allow to demonstrate that the residual resistivity and the reduction of T-c due to these defects are in agreement with scattering in a d-wave superconductor, and that the concentration of carriers remains the concentration n(h) of doped holes even in the overdoped regime. The large scattering due to spin-less defects should be related to the Kondo like resonance peak detected with local magnetic probes.

We will review the experimental results which have led to and supported the “hybrid gap picture” in the heavy-fermion superconductor UPt3: power-law temperature dependence of the thermal conductivity along the c-axis and in the basal plane, scaling laws under magnetic field. Experiments on samples with sizeable amounts of point defects (created by high-energy electron irradiation) seem to challenge these results. We conclude on the actual scenarios for the phase diagram of UPt3 in conjunction with the results from the NMR Knight-shift measurements.

Resistivity damage rates, determined during low-temperature electron irradiations in the energy range 0.3–2.5 MeV, were used for evaluating displacement threshold energies of titanium in high purity hcp titanium, and of titanium and aluminium in γ-TiAl intermetallic compounds. These parameters were deduced from a comparison of experimental displacement cross-section variations as a function of electron energy, with theoretical curves based on a displacement model for diatomic materials. The displacement energy of titanium in hcp titanium appears to depend on the electron energy. A threshold value of 21±1 eV was obtained in the range 0.3–0.5 MeV, and a larger value of 30±2 eV is determined in the range 0.5–2.5 MeV. In γ-TiAl, aluminium atoms are displaced first, with a threshold displacement energy (34±2 eV) larger than the one of titanium atoms, and much higher than the value in pure aluminium. The displacement energy of Ti atoms is 28±2 eV, close to the one obtained in pure titanium under similar conditions. These results were used for re-evaluating the Frenkel-pair resistivity of the stoichiometric TiAl compound.

The superconducting properties of the heavy fermion UPt3have been changed by irradiation with high energy electrons which creates point defects in a reproducible and controled way. Measurements of the residual resistivity, critical temperature, upper critical field and thermal conductivity have been realized on these irradiated samples. The strong suppresion of superconductivity with increasing defect concentration is in agreement with the theory of unconventional superconductivity. However, our thermal conductivity data contradicts the simple predictions derived from the most popular modelsE1gand E2u) of the superconducting order parameter in UPt3.

The superconducting properties of the heavy fermion UPt 3 have been changed by irradiation with high energy electrons which creates point defects in a reproducible and controled way. Measurements of the residual resistivity, critical temperature, upper critical field and thermal conductivity have been realized on these irradiated samples. The strong suppresion of superconductivity with increasing defect concentration is in agreement with the theory of unconventional superconductivity. However, our thermal conductivity data contradicts the simple predictions derived from the most popular models E 1 g and E 2 u ) of the superconducting order parameter in UPt 3 .

We have investigated the effects of electron irradiation on various single crystals, i.e. YBa2Cu3O6+x (YBCO6+x) with x = 0.6, 0.8 and ≈1 and HgBa2Ca2Cu3O8+δ (Hg-1223). It appears that the decrease of the critical temperature (Tc) and the increase of the in-plane resistivity under irradiation have the same origin in all samples, namely defects created in the CuO2 planes. For YBCO, we found similarities between electron irradiation and Zn doping effects. Indeed, the decrease rate of Tc as a function of the fluence of irradiation is roughly two times higher for underdoped compounds than for the optimally doped one.

We studied the Hall effect in a ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CaCu}}_{2}{O}_{8}$ epitaxial film and in an ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ single crystal in the mixed state before and after irradiation with high energy Pb ions. Pinning enhancement due to irradiation-induced columnar defects leads to the decrease in magnitude of the longitudinal and Hall resistivities, but does not modify the behavior of the Hall conductivity. This result is valid independent of the sign of the Hall effect in the pinned region (positive for ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CaCu}}_{2}{O}_{8}$ and negative for ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$). The present work proves straightforwardly that the mixed-state Hall conductivity does not depend on the pinning strength, in agreement with theory. This result is a dual analog of the behavior of 2D electronic systems where the Hall resistivity remains unaffected by disorder.

The effect of electron-irradiation induced defects on the superconducting transition temperature (Tc), on the normal-state resistivity and the stability of defects on thermal cycling were studied in Bi2Sr2CaCu2O8 and Bi2Sr2CuO6 superconductors. It was found that Tc/Tc0 is about three times less sensitive to irradiation-induced defects in the former than in the latter compound. We show that the ab plane resistivity increase in both compounds is due to the decrease of the mean free path caused by defects created in the CuO2 planes. The defects start to anneal above 100 K and, at 300 K; only 50% of the low-temperature defect concentration is left.

(1993). Irradiation effects of high energy ions in superconductor YBa2Cu3O7. Radiation Effects and Defects in Solids: Vol. 126, No. 1-4, pp. 155-158.

We have studied the transport properties of twinned ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ single crystals in the presence of linear defects induced by 5.6-GeV Pb ion irradiation. Our results give clear evidence of pinning enhancement revealed by a significant increase in the depth of the potential well in the thermally assisted flux-flow regime. Furthermore, the scaling law E\ensuremath{\propto}${\mathit{J}}^{\ensuremath{\gamma}}$, relating the electric field to the current density at the transition associated with the irreversibility line, holds after irradiation from a qualitative point of view. Nevertheless a significant increase and a strong field dependence of \ensuremath{\gamma} are observed for the irradiated samples. This strongly supports the fact that the irreversibility line as determined here is not associated with a second-order phase transition.