Magnetic Pair Breaking Research Articles

?Giant? normal state magnetoresistances of Bi $\mathsf{_{2}}$ Sr $\mathsf{_{2}}$ CaCu $\mathsf{_{2}}$ O $\mathsf{_{8 + \delta}}$

Magnetoresistance (MR) of Bi-2212 single crystals with T$_{c}$ $\approx 87-92 K$ is studied in pulsed magnetic fields up to 50T along the c-axis in a wide temperature range. The negative out-of-plane and the positive in-plane MRs are measured in the normal state. Both MRs have similar magnitudes, exceeding any orbital contribution by two orders in magnitude. These are explained as a result of the magnetic pair-breaking of preformed pairs. Resistive upper critical fields H$_{c2}$(T) determined from the in-- and out-of-plane MRs are about the same. They show non-BCS temperature dependences compatible with the Bose-Einstein condensation field of preformed charged bosons.

Crystal Field Potential ofPrOs4Sb12: Consequences for Superconductivity

The results of inelastic neutron scattering provide a solution for the crystal field level scheme in PrOs4Sb12 in which the ground state in the cubic crystal field potential of Th symmetry is a Gamma_1 singlet. The conduction electron mass enhancement is consistent with inelastic exchange scattering, and we propose that inelastic quadrupolar, or aspherical Coulomb, scattering is responsible for enhancing the superconducting transition temperature. PrOs4Sb12 appears to be the first compound in which aspherical Coulomb scattering is strong enough to overcome magnetic pair-breaking and increase Tc.

Andreev reflection spectroscopy of MgB 2 in the vortex state

Spectroscopy based on the Andreev reflection (AR) process at the interface between the normal metal tip and the superconductor has become one of the very successful methods for studies in novel exotic superconductors. The method is capable to address the size, symmetry as well as multiplicity of the superconducting order parameter. The method provided one of the first evidences of the two-gap superconductivity in MgB 2 with a detailed temperature dependence of the both gaps. A theory treating the Andreev reflection spectroscopy in the mixed state is missing. We analyse the AR spectra of MgB 2 in the mixed state via modelling the magnetic pair-breaking by the increasing spectral broadening parameter Γ. As a result a non-trivial pair breaking effect in the π-band is found.

Spin injection in High-Tc cuprates: An STM spectroscopy study

To study how the high-Tc order parameter (OP) evolves under the injection of spin-polarized quasiparticles, STM spectroscopy has been performed on superconductor/ferromagnet thin-film heterostructures comprising YBa2Cu3O7−x (YBCO) and La0.7Ca0.3MnO3 (LCMO) at 4.2 K. Quasiparticle-tunneling and Andreev-reflection characteristics measured on the YBCO under spin-injection from the LCMO were analyzed with the d-wave Blonder-Tinkham-Klapwijk theory, to reveal a spectral evolution which provides direct evidence for dynamic magnetic pair-breaking. The spectral analysis also shows the d-wave OP to remain time-reversal invariant as it is suppressed by the spin-injection. These results are discussed in terms of the general search for quantum-critical points in the high-Tc phase diagram.

Magnetic pair breaking and weak ferromagnetism in single crystals ofEr1−xTbxNi2B2C

The magnetic and superconducting properties in the single crystals of Er 1 - x Tb x Ni 2 B 2 C compounds are investigated by temperature-dependent magnetization M(T) and in-plane resistivity ρ(T). It is found that the superconducting transition temperature is suppressed by adding Tb in the sites of Er and it follows well de Gennes scaling for both T c > T N and T c < T N . From the T c variation, the magnetic pair breaking strength of Tb is estimated to be similar to the Er. The antiferromagnetic and weak ferromagnetic transition temperatures show distinct behaviors in terms of the de Gennes factor in the Er-rich (x≤0.4) and Tb-rich (x≥0.6) side. These observations will be discussed under the consideration of magnetic ground states of ErNi 2 B 2 C and TbNi 2 B 2 C and their interactions.

Nonmagnetic ground state of the superconductorPrPt2B2Cand magnetic pairbreaking inLa0.9R0.1Pt2B2C(R=Ce,Pr, Nd)

${\mathrm{PrPt}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ is an unusual superconducting member ${(T}_{c}\ensuremath{\sim}6 \mathrm{K})$ of quaternary borocarbide family. At high temperatures, Pr ions are found to carry magnetic moment, but so far no magnetic order is reported. We have investigated the nature of the electronic ground state of this material (nominal composition ${\mathrm{PrPt}}_{2.1}{\mathrm{B}}_{2.4}{\mathrm{C}}_{1.2})$ through magnetic and heat capacity studies. We confirm that ${\mathrm{PrPt}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ exhibits bulk superconductivity below 6 K. Our studies show that (non-Kramers) Pr ions at low temperatures are in the crystal field split singlet ground state (nonmagnetic) and there is no indication of an induced magnetic moment. In the nonmagnetic analogue superconductor ${\mathrm{LaPt}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ ${(T}_{c}\ensuremath{\sim}10.5 \mathrm{K}),$ we have studied magnetic pairbreaking by magnetic ions Ce, Pr, and Nd. We find severe pair-breaking caused by Ce ions suggesting an enhanced Ce-$4f$-conduction electron hybridization.

Depression of superconducting transition temperature due to Pr in the (La2.5−xPrxNd0.5)CaBa3Cu7Oz system

Structural and superconducting properties of (La2.5−xPrxNd0.5)CaBa3Cu7Oz (0⩽x⩽0.7) compounds have been investigated. Neutron diffraction studies reveal that these crystallize in the triple-perovskite structure (space group P4/mmm) with an oxygen content of 16.6 per formula unit nearly independent of the Pr content. The superconducting transition temperature, Tc, of x=0 compound is ∼79K and decreases almost linearly with increasing x to ∼42K for x=0.7. In order to see if the depression of Tc due to Pr can be compensated by Ca, we examined the compounds (La2.5−x−yPrxCayNd0.5)CaBa3Cu7Oz with x=0.7 and y=0 and 0.4. The observed Tc in these compound is almost independent of Ca content suggesting the absence of a contribution from hole filling in the depression of Tc by Pr. The observed depression of Tc with increasing Pr content in the title compound is analyzed on the basis of magnetic pair breaking by the Pr-4f electrons.

Strong magnetic pair breaking and weak magnetic exchange interaction amongRions in(Y,R)Pd2B2C

In multiphase material ${\mathrm{YPd}}_{5}{\mathrm{B}}_{3}{\mathrm{C}}_{0.35}$ of the Y-Pd-B-C system, ${\mathrm{YPd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ (space group I4/mmm) is believed to be the superconducting phase ${(T}_{c}\ensuremath{\approx}23 \mathrm{K}).$ Despite the nonavailability of the superconducting phase ${\mathrm{YPd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ in pure form, we have been able to obtain crucial information on the magnetic pair breaking and magnetic exchange interaction among R ions in the $R{\mathrm{Pd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ $(R=\mathrm{Y},$ rare-earth elements) system by studying multiphase $\mathrm{Y}(R){\mathrm{Pd}}_{5}{\mathrm{B}}_{3}{\mathrm{C}}_{0.35}$ materials. Strong pair-breaking effects are observed in ${\mathrm{Y}}_{0.9}{R}_{0.1}{\mathrm{Pd}}_{5}{\mathrm{B}}_{3}{\mathrm{C}}_{0.35};$ for instance, $\ensuremath{\Delta}{T}_{c}$---the depression of the superconducting transition temperature ${T}_{c}$---is $\ensuremath{\approx}8 \mathrm{K}$ for $R=\mathrm{Gd}.$ $\ensuremath{\Delta}{T}_{c}(R),$ except for $R=\mathrm{Ce},$ Eu, and Yb, nearly follows the de Gennes scaling $\ensuremath{\Delta}{T}_{c}\ensuremath{\propto}{(g}_{J}\ensuremath{-}{1)}^{2}J(J+1),$ both for light as well as heavy rare-earth atoms. $\ensuremath{\Delta}{T}_{c}$ $(\ensuremath{\approx}8.5 \mathrm{K})$ is anomalously large for $R=\mathrm{Ce}.$ Magnetic exchange interaction among R ions is rather weak in $R{\mathrm{Pd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ as is inferred from our studies in ${\mathrm{DyPd}}_{5}{\mathrm{B}}_{3}{\mathrm{C}}_{0.35},$ which does not exhibit a magnetic transition down to 1.7 K. Our present investigations bring out striking dissimilarities in the superconducting and magnetic properties of the materials of $R{\mathrm{Pd}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ vis-\`a-vis those of $R{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}.$

Suppression of superconductivity in R xY 1− xNi 2B 2C compounds by paramagnetic impurities

The suppression of the superconducting transition temperature T c of pseudo-quaternary R x Y 1− x Ni 2B 2C compounds due to paramagnetic impurities R=Tm, Er, Ho, Dy, Tb, Gd, Sm, Nd and Pr was investigated in the concentration range x<0.1 by susceptibility measurements. The suppression of T c was found to be mainly caused by magnetic pair-breaking in the case of the heavy rare-earth elements (R=Tm, Er, Ho, Dy, Tb, Gd) and by lattice distortions in the case of the light rare-earth elements (R=Sm, Pr, Nd) resulting from the large difference in the ionic radii between the yttrium host and the R 3+ ions.

Interplay between magnetism and superconductivity in Nb/Co multilayers

We have used torque magnetometry to investigate the influence of magnetic spacer layers on the superconductivity in Nb/Co multilayers. Compared to similar systems which have nonmagnetic spacer layers, the samples exhibit two dimensional superconductivity for relatively thin magnetic layer thicknesses ${d}_{\mathrm{Co}},$ reflecting the effects of magnetic pair breaking. Moreover, the transition temperature to the superconducting state ${T}_{c}$ shows a nonmonatonic dependence on ${d}_{\mathrm{Co}},$ which we examine within the framework of recent theories.

Magnetic pair breaking in superconductingBa1−xKxBiO3investigated by magnetotunneling

The de Gennes and Maki theory of gapless superconductivity for dirty superconductors is used to interpret the tunneling measurements on the strongly type-II high-Tc oxide-superconductor Ba1-xKxBiO3 in high magnetic fields up to 30 Tesla. We show that this theory is applicable at all temperatures and in a wide range of magnetic fields starting from 50 percent of the upper critical field Bc2. In this magnetic field range the measured superconducting density of states (DOS) has the simple energy dependence as predicted by de Gennes from which the temperature dependence of the pair-breaking parameter alpha(T), or Bc2(T), has been obtained. The deduced temperature dependence of Bc2(T) follows the Werthamer-Helfand-Hohenberg prediction for classical type-II superconductors in agreement with our previous direct determination. The amplitudes of the deviations in the DOS depend on the magnetic field via the spatially averaged superconducting order parameter which has a square-root dependence on the magnetic field. Finally, the second Ginzburg-Landau parameter kappa2(T) has been determined from the experimental data.

Magnetic pair breaking by the Pr ions in the two “RE-123” EuBa 2Cu 3O 7− δ and ErBa 2Cu 3O 7− δ HTSCs codoped with Ca

The suppression of superconductivity by the equal substitution of Pr and Ca into the Eu and Er “123” HTSCs are studied. The decreases in the T cs of Eu 1−2 x Pr x Ca x Ba 2Cu 3O 7− δ and Er 1−2 x Pr x Ca x Ba 2Cu 3O 7− δ are observed to be linear as x increases from 0.0 to 0.010. The rate of suppression d T c/d x for the “Eu-(Pr,Ca)-123” HTCSs is larger than that for the “Er-(Pr,Ca)-123” HTSCs. It is observed that in the specimens studied, the Cu valencies (reflecting the hole concentration) for each of the doped specimen in the two series remain almost constant. This points to the suppression of superconductivity in the present “123” HTSCs by Pr substitution being due to magnetic pair breaking by the Pr ions. The relative magnitudes of the rates of suppression of the two systems and the of the T cs of the host superconductors are taken to be manifestation of an rare earth size effect.

Magnetotunneling and magnetic pair-breaking in superconducting Ba 1− xK xBiO 3

The gapless superconductivity theory for dirty superconductors has been used to interpret the tunneling measurements on the strongly type-II high- T c oxide-superconductor Ba 1− x K x BiO 3 in high magnetic fields up to 30 T . This theory describes the tunneling curves at all temperatures below T c not only near the upper critical field H c2 but in a wide range of magnetic fields H>0.5 H c2, yielding an estimation of the pair-breaking parameter α. This allows us to determine the temperature dependence of the upper critical field H c2( T) which is in agreement with the predictions of the classical Werthamer–Helfand–Hohenberg theory. Our results are comparable with H c2 data directly obtained from the tunneling experiment.

Specific heat anomalies in the magnetic borocarbide superconductorsY1−xRxNi2B2C(R=Gd,Dy, Ho, and Er)

The specific heat anomalies associated with the superconducting transition of ${\mathrm{Y}}_{1\ensuremath{-}x}{R}_{x}{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ $(R=\mathrm{Gd},$ Dy, Ho, and Er) show a common correlation between the specific heat jump $\ensuremath{\Delta}C$ and the transition temperature ${T}_{c}$ with an almost quadratic relation $\ensuremath{\Delta}C\ensuremath{\propto}{T}_{c}^{2}$ in all these solid solutions. The antiferromagnetic superconductor ${\mathrm{DyNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ with ${T}_{N}g{T}_{c}$ as well as Ho and ${\mathrm{ErNi}}_{2}{\mathrm{B}}_{2}\mathrm{C},$ which are paramagnetic at ${T}_{c},$ fit into the same scheme of $\ensuremath{\Delta}C$ versus ${T}_{c}$ as the pseudoquaternaries. A simple description which combines the weak-coupling results of magnetic pair breaking theory with strong-coupling corrections explains reasonably well the quadratic relation between $\ensuremath{\Delta}C$ and ${T}_{c}.$ The overall agreement between this model and our experimental results indicates that the Sommerfeld value $\ensuremath{\gamma}$ and hence the electronic density of states are almost constant within these heavy rare-earth solid solutions and their boundary compounds.

Surface resistance of epitaxialYBa2Cu3O7−xfilms on various substrates: Effects of pair condensation and quasiparticle scattering

The temperature dependent surface resistance ${R}_{s}(T)$ of eight high-quality epitaxial ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ films on ${\mathrm{LaAlO}}_{3}(\mathrm{Y}/\mathrm{L}),$ MgO (Y/M), and ${\mathrm{CeO}}_{2}$-buffered sapphire (Y/S) substrates was investigated at 19 GHz with a resolution of 20 \ensuremath{\mu}\ensuremath{\Omega}. The residual level ${R}_{s}(\stackrel{\ensuremath{\rightarrow}}{T}0)$ was (90--180) \ensuremath{\mu}\ensuremath{\Omega} for all films. The slope $\ensuremath{\partial}{R}_{s}(T)/\ensuremath{\partial}T$ in the range (4--20) K decreased from (6--8) \ensuremath{\mu}\ensuremath{\Omega}/K for Y/L to (0--2) \ensuremath{\mu}\ensuremath{\Omega}/K for Y/M, and to slightly negative values for Y/S. This slope correlated with the transition temperature ${T}_{c},$ including published data of about (10--20) \ensuremath{\mu}\ensuremath{\Omega}/K for high-purity ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ single crystals. The increase of ${R}_{s}$ from 4.2 showed power-law behavior $\ensuremath{\Delta}{R}_{s}(T)\ensuremath{\propto}{T}^{a}$ up to 40 K with $a\ensuremath{\approx}1.3$ for Y/L and Y/M. However, it was exponential for Y/S, $\ensuremath{\Delta}{R}_{s}(T)\ensuremath{\propto}\mathrm{exp}(\ensuremath{-}\ensuremath{\delta}\ifmmode\times\else\texttimes\fi{}{T}_{c}/T)$ with $\ensuremath{\delta}\ensuremath{\approx}0.8.$ The results can be described with the two-fluid model in terms of the quasiparticle density ${n}_{N}(T)$ and the scattering time $\ensuremath{\tau}(T).$ The high and reproducible residual resistance implies (magnetic) impurity scattering at a rate ${\ensuremath{\tau}}^{\ensuremath{-}1}(0)\ensuremath{\propto}{n}_{N}(0),$ i.e., proportional to the density of quasiparticles. In crystals and unstrained films, the pair condensation is gapless, and the power-law temperature dependences of \ensuremath{\tau} and ${n}_{N}$ are reflected in that of ${R}_{s}.$ In contrast, in strained films, ${n}_{N}(T)$ displays activated behavior, and the low-temperature behavior of ${R}_{s}$ changes to exponential. The formation of an energy gap is attributed to the interaction between the Cu-O planes and the chains, which is affected by strain. The existence of an energy range with zero density of states limits possible interpretations of the order parameter of YBaCuO. A two-band scenario with magnetic pairbreaking and two different order parameters $(2\ensuremath{\Delta}{/kT}_{c}=6--8$ for the planes and \ensuremath{\geqslant}0 for the chains) with s-wave symmetry, at least in the chains, appears to be an adequate explanation.

Influence of hole filling and pair breaking by Nd and hole doping by Ca on the superconductivity of NdBa2Cu3Oz

The structural and superconducting properties of Nd- and Ca-substituted (Nd1-xCax)(Ba2-yNdy)Cu3Oz samples prepared under identical conditions have been investigated by x-ray diffraction, resistivity, ac susceptibility and oxygen content measurements. The effect of increasing Nd concentration in NdBa2-yNdyCu3Oz lowers the carrier concentration and decreases Tc, which is attributed to magnetic pair breaking by local moments and hole filling by Nd. This suppression in Tc has been compensated by appropriate hole doping with Ca in (Nd1-xCax)(Ba2-yNdy)Cu3Oz, which shows that for a fixed Nd content at the Ba site Tc increases dramatically as the Ca content increases. For instance, in the oxide (Nd1-xCax)(Baz-yNdy)Cu3Oz, the Tc increases from 0.0 K (x = 0.0, non-superconductor) to 77.5 K (x = 0.4, compensated oxide) as the magnetic pair breaking and hole filling by Nd is nearly balanced by hole doping from Ca.

Superconducting properties of the Fe-substituted La 2.5Y 0.5CaBa 3(Cu 1− xFe x) 7O z perovskite

The structural and superconducting properties of single-phase Fe-substituted La 2.5Y 0.5CaBa 3(Cu 1− x Fe x ) 7O z [LYCBCuFe] with 0≤ x≤0.15 compounds having triple-perovskite structure are investigated using X-ray diffraction, resistivity, AC Susceptibility, oxygen content and Mössbauer effect measurements. The observed reduction of T c with increasing x in LYCBCuFe supports the view that magnetic pair breaking by local moments contributes predominantly to the suppression of superconductivity by Fe. The observed Fe-dopant site occupancies and the Cu(2) plane to the Cu(1) chain site ratio in the LYCBCuFe obtained from Mössbauer spectral analysis of x=0.02 sample are quite different from those of the usual Fe-doped YBa 2Cu 3O z .

Hole distribution andTcsuppression inY1−xPrxBa2Cu3O7

A model Hamiltonian, which includes the components of the ${\mathrm{CuO}}_{2}$ planes, ${\mathrm{CuO}}_{3}$ chains, and the Pr-O hybridization, is considered to describe the electronic structure of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}.$ We make the coherent-potential approximation calculation for the Hamiltonian to present the variation of the distribution of the holes against the Pr concentration. The results show that the hole transfer is mainly from the $\mathrm{pd}\ensuremath{\sigma}$ band to the $p\ensuremath{-}f$ hybridization band, and a small reduction of the holes residing on the ${\mathrm{CuO}}_{3}$ chains is caused by the Pr doping. A metal-nonmetal transition around $x=0.5$ can be obtained according to our calculation. We present a quantitative interpretation for the ${T}_{c}$ suppression in terms of the combination of hole-transfer and magnetic scattering effects. Moreover, the enhancement of the magnetic pair breaking due to the presence of a normal-state pseudogap is also considered. Consequently, we give a satisfactory explanation of ${T}_{c}$ suppression for the whole Pr-doping region.

Diluted and concentrated isoelectronic substitutional effects in superconducting R{sub x}Y{sub 1{minus}x}Ni{sub 2}B{sub 2}C compounds

The suppression of superconductivity of pseudo-quaternary R{sub x}Y{sub 1{minus}x}Ni{sub 2}B{sub 2}C compounds in the dilute limit was found to be mainly caused by magnetic pair-breaking in the case of the heavy rare-earth elements (R = Tm, Er, Ho, Dy, Tb, Gd) and by lattice distortions in the case of the light rare-earth elements (R = Pr, Nd) resulting from the large difference of the ionic radii between the Y host ions and the R impurity ones.

A magnetic pair-breaking effect in rare earth-doped manganites

The magnetic properties of La 0.60R 0.07Di 0.33MnO 3 ferromagnetic manganites (Di = Sr, Ba) are studied, where La is partly replaced by magnetic rare earths R. It is shown that (i) there is a ferromagnetic coupling between Mn and R spins, (ii) the Curie temperature is lowered compared to the parent La compound and (iii) its depression is correlated with the effective moment of the rare earth ion. This last relation is tentatively explained by a magnetic pair-breaking effect, where fluctuating R moments lower the double-exchange coupling between Mn atoms.