Magnetic Pair Breaking Research Articles

Superconductivity with large upper critical field in noncentrosymmetric Cr-bearing high-entropy alloys

A series of new Cr5+xMo35−xW12Re35Ru13C20 high-entropy alloys (HEAs) have been synthesized and characterized by x-ray diffraction, scanning electron microscopy, electrical resistivity, magnetic susceptibility and specific heat measurements. It is found that the HEAs adopt a noncentrosymmetric cubic β-Mn type structure and exhibit bulk superconductivity for 0 ≤x≤ 9. With increasing x, the cubic lattice parameter decreases from 6.7940(3) Å to 6.7516(3) Å. Meanwhile, the superconducting transition temperature Tc is suppressed from 5.49 K to 3.35 K due to the magnetic pair breaking caused by Cr moments. For all these noncentrosymmetric HEAs, the zero-temperature upper critical field Bc2(0) is comparable to the Pauli paramagnetic limit BP(0) = 1.86Tc. In particular, the Bc2(0)/BP(0) ratio reaches a maximum of ∼1.03 at x = 6, which is among the highest for β-Mn type superconductors.

Pressure-Induced Superconductivity up to 9 K in the Quasi-One-Dimensional KMn6Bi5

The Mn-based superconductor is rare owing to the strong magnetic pair-breaking effect. Here we report on the discovery of pressure-induced superconductivity in KMn6Bi5, which becomes the first ternary Mn-based superconductor. At ambient pressure, the quasi-one-dimensional KMn6Bi5 is an antiferromagnetic metal with TN = 75 K. By measuring resistivity and ac magnetic susceptibility under hydrostatic pressures up to 14.2 GPa in a cubic anvil cell apparatus, we find that its antiferromagnetic transition can be suppressed completely at a critical pressure of Pc = 13 GPa, around which bulk superconductivity emerges and displays a superconducting dome with the maximal Tconset = 9.3 K achieved at about 14.2 GPa. The close proximity of superconductivity to a magnetic instability in the temperature-pressure phase diagram of KMn6Bi5 and an unusually large {\mu}0Hc2(0) = 18.9 T exceeding the Pauli limit suggests an unconventional magnetism-mediated paring mechanism. In contrast to the binary MnP, the flexibility of the crystal structure and chemical compositions in the ternary AMn6Bi5 (A = alkali metal) can open a new avenue for finding more Mn-based superconductors.

Magnetic disorder and gap symmetry in the optimally electron-doped Sr(Fe,Co)2As2 superconductor

We investigate the magnetic pair-breaking due to Mn impurities in the optimally electron doped Sr(Fe$_{0.88} $Co$_{0.12} $)$_{2} $As$_2 $ superconductor to deduce the symmetry of the superconducting order parameter. Experiments on the as-grown crystals reveal a T$_c $ suppression rate of $ \sim $30 mK/$\mu \Omega cm$, which is in close agreement with similarly slower values of T$_c $ suppression rates reported previously for various transition metal impurities, both, magnetic and non-magnetic, in several structurally analogous iron-based superconductors. However, careful annealing of these crystals at low temperature for longer durations reveals new information crucial to the determination of the pairing symmetry. We found that the crystallographic defects are a significant source of pair-breaking in the as-grown crystals. We first establish that these defects are point-like by showing that their sole effect on electrical transport is to add a temperature independent scattering term that shifts the whole $ \rho $ vs. T curves rigidly up. The T$_c $ suppression rate due to these point-like defects is slow, $ \le$ 35 mK/$\mu \Omega cm$. On the other hand, T$_c $ suppression rate due to magnetic pair-breaking is estimated to be faster than 325 mK/$\mu \Omega cm$. A slower pair-breaking rate (measured in mK/$\mu \Omega cm$) than expected due to non-magnetic crystallographic defects, together with a faster pair-breaking rate due to magnetic impurities disfavors a sign-changing $s_{+-}$-wave and argues in the favor of a non-sign-changing $s_{++}$-wave state in the optimally electron doped SrFe$_2 $As$_2 $ superconductor.

Combined external pressure and Cu-substitution studies on BaFe2As2 single crystals

We report a combined study of external pressure and Cu-substitution on BaFe2As2 single crystals grown by the in-flux technique. At ambient pressure, the Cu-substitution is known to suppress the spin density wave (SDW) phase in pure BaFe2As2(TSDW ≈ 140 K) and to induce a superconducting (SC) dome with a maximum transition temperature . This is much lower than the Tc ∼ 15–28 K achieved in the case of Ru, Ni and Co substitutions. Such a lower Tc is attributed to a Cu2+ magnetic pair-breaking effect. The latter is strongly suppressed by applied pressure, as shown herein, Tc can be significantly enhanced by applying high pressures. In this work, we investigated the pressure effects on Cu2+ magnetic pair-breaking in the BaFe2−xCuxAs2 series. Around the optimal concentration (xopd = 0.11), all samples showed a substantial increase of Tc as a function of pressure. Yet for those samples with a slightly higher doping level (over-doped regime), Tc presented a dome-like shape with maximum Tc ≃ 8 K. Remarkably interesting, the under-doped samples, e.g. x = 0.02 display a maximum pressure induced Tc ≃ 30 K which is comparable to the maximum Tc's found for the pure compound under external pressures. Furthermore, the magnetoresistance effect as a function of pressure in the normal state of the x = 0.02 sample also presented an evolution consistent with the screening of the Cu2+ local moments. These findings demonstrate that the Cu2+ magnetic pair-breaking effect is completely suppressed by applying pressure in the low concentration regime of Cu2+ substituted BaFe2As2.

Transport critical current measurements on a Cu-substituted BaFe2As2 superconductor

The critical current density Jc is a crucial parameter to establish the actual technological potential of a superconducting (SC) material. Furthermore, being proportional to the SC gap parameter, it can reveal important information about the microscopic nature of the SC state in a given material. The FeAs-based class of SC materials has been a focus of intense scientific investigation lately, but direct investigation of Jc by transport measurements is rather scarce in literature. For these materials, it is very interesting to map Jc as a function of their distinct SC tuning parameters such as applied pressure and chemical substitution. In this work, detailed investigation of the field, temperature, and pressure dependences of transport critical current density Jc for Cu-substituted BaFe2As2 single crystals is reported. In this particular material, Cu-substitution has a strong magnetic pair breaking effect. However, with increasing pressure, this sample shows an almost twofold increase of Tc, from 3.2 K to 6.9 K, which is followed by an increase in Jc. These observations are discussed considering the presence of magnetic pinning centers in the Fe-As plane, which, in principle, could suggest effective routes to increase Jc in the this class of materials.

Pressure effects on magnetic pair-breaking in Mn- and Eu-substituted BaFe2As2

We report a combined study of hydrostatic pressure (P ≤ 25 kbar) and chemical substitution on the magnetic pair-breaking effect in Eu- and Mn-substituted BaFe2As2 single crystals. At ambient pressure, both substitutions suppress the superconducting (SC) transition temperature (Tc) of BaFe2–xCoxAs2 samples slightly under the optimally doped region, indicating the presence of a pair-breaking effect. At low pressures, an increase of Tc is observed for all studied compounds followed by an expected decrease at higher pressures. However, in the Eu dilute system, Tc further increases at higher pressure along with a narrowing of the SC transition, suggesting that a pair-breaking mechanism reminiscent of the Eu Kondo single impurity regime is being suppressed by pressure. Furthermore, Electron Spin Resonance (ESR) measurements indicate the presence of Mn2+ and Eu2+ local moments and the microscopic parameters extracted from the ESR analysis reveal that the Abrikosov–Gor'kov expression for magnetic pair-breaking in a conventional sign-preserving superconducting state cannot describe the observed reduction of Tc.

Magnetic scattering and electron pair breaking by rare-earth-ion substitution in BaFe2As2 epitaxial films

The effect of electron doping by trivalent charge state rare-earth-ion (RE = La, Ce, Pr and Nd) substitutions on the superconductivity in BaFe2As2 was examined using epitaxial films. Each of the RE substitutions suppressed the resistivity anomaly associated with magnetic/structural phase transitions, leading to resistivity drops and superconductivity transitions. Bulk superconductivity was observed at the maximum onset critical temperature (Tconset) of 22.4 K for La doping and 13.4 K for Ce doping, while only broad resistivity drops were observed at 6.2 K for Pr doping and 5.8 K for Nd doping although zero resistivity and the distinct Meissner effect were not observed at least down to 2 K. The decrease in Tconset with increasing the number of RE 4f electrons cannot be explained in terms of the crystalline quality or crystallographic structure parameters of BaFe2As2 films. It was clarified, based on resistivity–temperature analyses, that magnetic scattering became increasingly significant in the above order of RE dopants. The negative magnetoresistance was enhanced by Ce and Pr doping, implying that the decrease in Tc originates from magnetic pair breaking by interaction of the localized 4f orbitals in the RE dopants with the itinerant Fe 3d orbitals.

Local Lattice Distortion in Superconducting Cuprates Studied by XAS

In-plane local distortion in cuprates characterized by strong doping and temperature dependence is a signature of polaron (bipolaron). The nature of dynamic lattice response in relation to the onset of superconductivity was probed by polarized x-ray absorption spectroscopy (XAS), for pure and magnetic impurity-doped MxLa1.85Sr0.15Cu1-xO4 (M=Mn, Ni, Co, x<0.05) single crystals grown by a TSFZ method. The results confirm that the dynamical anti Jahn-Teller (AJT) distorted domains maximize at Tdmax~Tconset, which is described by the disappearance of the Cu-O(1) bond alternation upon the completion of phase coherence (onset of superconductivity). In contrast to the Ni and Co dopings which quickly suppress superconductivity by magnetic pair breaking, the Mn doping shows sustainable superconductivity i.e., a constant Tdmax and Tconset upto x=0.03 with the presence of dynamical AJT distortion. The results suggest that the dynamical AJT type distortion with either Q2 or pseudo JT symmetry extending over the basal plane could be a prerequisite for high temperature superconductivity.

Crystallographic &amp; Electrical Properties of Pr Substituted Gd-123 Nanometre Sized High Temperature Superconductors

The samples of Gd1-xPrxBa2Cu3O7-δ with compositions x = 0.0, 0.1 &amp; 0.3 were prepared by ceramic route. The XRD study of all samples is carried out. The XRD studies of the samples prepared by ceramic route show GdPr-123 system have an orthorhombic pervoskite structure. The lattice parameters, oxygen content, volume of unit cell, orthorhombic distortion, hole concentration in CuO2 plane and charge on Cu-O plane were evaluated using XRD data. It is found that the lattice parameters, volume of unit cell increase while oxygen content orthorhombic distortion, hole concentration (Psh) and average charge (p) decrease as doping percentage of Pr increases. The resistivity measurement shows the transition temperature decreases while transition width increases with increasing Pr concentration. The suppression of superconductivity may be due to reduction in hole concentration or magnetic pair breaking. We report such qualitative results of Pr substituted Gd-123 in this paper quantitatively.

STUDY OF SUBSTITUTION OF Zn FOR Cu IN YBa2Cu3O7 SYSTEM

The polycrystalline YBa2(Cu1-XZnX)3O7-δ samples (x=0.0 to 0.06) were synthesized and studied using X Ray diffraction, titration, resistivity, magnetization and X ray photoelectron spectroscopy (XPS). Results show that O2 stoichiometry (δ) changes on Zn substitution which affects their normal state resistivity as well as the TC. Zn also induces local magnetic moment as evidenced from magnetization results. A combination of change in O2 stoichiometry and magnetic pair breaking is found to be responsible for a rapid suppression of superconductivity.

Synthesis and characterization of Pr substituted Gd-123 high-Tc superconductors

High Tc superconducting materials with chemical formula Gd1-xPr xBa2Cu3O7-δ, where x = 0.0, 0.1, 0.2 and 0.3 were prepared by ceramic route. XRD studies confirm the orthorhombic pervoskite structure. Electrical resistivity measurement shows that Tc decreases with increasing Pr concentration. Copper valency and oxygen content decreasing with increasing Pr content. Value of n(CuO2) decreases with increasing x and oxygen deficiency (δ) has a linear dependence with (2x+δ). SQUID investigation reveals that Bohr magneton number and Curie constant are decreasing with increasing Pr concentration. Annihilation of superconductivity may be owing to reduction in CuO2 charge carriers or magnetic pair breaking.

Field-dependent anomalies in the thermodynamic and transport properties of RuSr2GdCu2O8—Evidence for a spin-flop transition

Thermopower and magnetization studies of epitaxial thin films and sintered polycrystalline ceramics of RuSr2GdCu2O8 show evidence for the occurrence of a spin-flop transition, consistent with earlier neutron diffraction data. Realignment of spins in a magnetic field enhances Tc by up to 4K, presumably due to reduced magnetic pair-breaking. Specific heat and powder AC susceptibility measurements give strong evidence for bulk superconductivity. High temperature magnetization measurements indicate in-plane ferromagnetic correlations which remain to be reconciled with low-temperature neutron diffraction evidence for conventional G-type antiferromagnetism.

Superconducting and magnetic anisotropy ofLnFePO (Ln=La,Pr, and Nd) single crystals

The superconducting and magnetic properties of PrFePO and NdFePO are reported. Measurements of the dc and ac magnetic susceptibility reveal that, in comparison to the analogous compound without $f$ electrons, LaFePO, the superconducting transition temperatures ${T}_{c}$ are only moderately affected by the lanthanide 4$f$-electron magnetic moments. These results indicate that either the exchange interaction that leads to magnetic pair breaking is extremely weak or the superconducting state is unconventional. Measurements of the upper-critical-field curves and the magnetic susceptibility also reveal that the magnetic-field-induced suppression of the superconducting state is not strongly correlated with spin polarization of the magnetic lanthanide ions.

Contrasting effects of magnetic ions on the superconductivity in Tl0.4K0.4Fe2−y−xMxSe2 (M = Mn and Ni)

The transport and magnetic properties together with the superconductivity of Tl0.4K0.4Fe2−y−xMxSe2 (M = Mn and Ni) single crystals are studied. On the transport side, both Mn doping and Ni doping lead to a decrease of charge carrier density. However, contrasting effects on superconductivity are found, i.e. substitution of Fe by Ni results in a drastic depression of the onset superconducting transition temperature (Tonsetc), while the Tonsetc value remains nearly unchanged in Mn-doped samples. A remarkable ferromagnetic interaction is found in Ni-doped samples but is absent in Mn-doped samples. The present results suggest that the magnetic pair-breaking effect alone cannot explain the different behavior of depression of superconductivity in Mn-doped and Ni-doped samples. The occurrence of ferromagnetic interaction is responsible for the rapid decrease of Tonsetc in Ni-doped samples.

The upper critical field of CeCoIn5

We present a detailed analysis of the upper critical field for CeCoIn5 under high pressure. We show that, consistent with other measurements, this system shows a decoupling between the maximum of the superconducting transition temperature Tc and the maximum pairing strength. We propose a model in which, in order to account for the discrepancy in pressure between the maximum of the upper critical field and the maximum of Tc, we introduce magnetic pair-breaking effects, already widely suggested by other measurements. We found that within the Eliashberg frame work, the unusual shape of Hc2(T) can be completely reproduced when magnetic pair breaking is taken into account. Surprisingly, we found that the maximum of pair breaking and of pair coupling coincide in pressure, suggesting that both mechanisms originate from quantum criticality. Our model implies that CeCoIn5 is the first compound of its family that shows clear decoupling between the maximum of Tc and quantum criticality.

Different response of superconductivity to the transition-metal impurities in K0.8Fe2−y−xMxSe2(M=Cr, Mn, Co, Zn)

In this paper, we perform Fe-site substitution experiments on K${}_{0.8}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ superconductor by choosing Cr, Mn, Co, and Zn as dopants. It is found that the doping of Cr, Co, and Zn leads to a drastic depression of superconductivity, while the introduction of Mn does not decrease the superconducting transition temperature (${T}_{c}$) when $x\ensuremath{\leqslant}0.067$. Electron-spin-resonance measurements reveal the existence of strong magnetic fluctuation in both the parent compound and the doped samples. In the Cr-, Co-, and Zn-doped samples, the magnetic pair-breaking effect severely affects the superconductivity, resulting in drastic depression of superconductivity. On the other hand, in the Mn-doped case, it does not break the cooper pairs and thus hardly affects the ${T}_{c}$ value.

Superconducting state of very thin Pd films deposited on a diluted insulating EuxSr1−xS ferromagnet

The electronic transport in very thin Pd films deposited on insulating Eu${}_{x}$Sr${}_{1\ensuremath{-}x}$S ($0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$) is investigated. The temperature dependent resistance $R(T)$ of films with ${d}_{\mathrm{Pd}}\ensuremath{\geqslant}14$ nm shows metallic behavior and a logarithmic increase toward low temperatures characteristic of (anti-)localization and electron-electron interaction effects also observed for Pd films on Si(111). Films with ${d}_{\mathrm{Pd}}=7$ nm are superconducting below a transition temperature ${T}_{c}$ which decreases with increasing Eu concentration from ${T}_{c}=0.9$ K for $x=0$ to ${T}_{c}l50$ mK for $x=1$. The origin of superconductivity in the thin Pd films is presumably due to the formation of an interfacial Pd-S alloy and/or to a charge transfer at the Pd/Eu${}_{x}$Sr${}_{1\ensuremath{-}x}$S interface. The decrease of ${T}_{c}$ vs $x$ is attributed to the magnetic pair breaking of the ferromagnetic Eu${}_{x}$Sr${}_{1\ensuremath{-}x}$S underneath. The $R(T)$ behavior of films with ${d}_{\mathrm{Pd}}\ensuremath{\geqslant}10$ nm suggests that these are not superconducting or have a strongly reduced ${T}_{c}$ presumably due to the proximity effect of the nonsuperconducting S-free upper part of the Pd film.

Structural, Superconducting and Magnetic Properties of La&lt;sub&gt;3-X&lt;/sub&gt;&lt;i&gt;R&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt;Ni&lt;sub&gt;2&lt;/sub&gt;B&lt;sub&gt;2&lt;/sub&gt;N&lt;sub&gt;3-δ&lt;/sub&gt; with &lt;i&gt;R&lt;/i&gt; = Ce, Pr, Nd

We report on structural and superconducting properties of La3-xRxNi2B2N3- where La is substituted by the magnetic rare-earth elements Ce, Pr, Nd. The compounds Pr3Ni2B2N3- and Nd3Ni2B2N3- are characterized for the first time. Powder X-ray diffraction confirmed all samples R3Ni2B2N3- with R = La, Ce, Pr, Nd and their solid solutions to crystallize in the body centered tetragonal La3Ni2B2N3 structure type. Superconducting and magnetic properties of La3-xRxNi2B2N3- were studied by resistivity, specific heat and susceptibility measurements. While La3Ni2B2N3- has a superconducting transition temperature Tc ~ 14 K, substitution of La by Ce, Pr, and Nd leads to magnetic pair breaking and, thus, to a gradual suppression of superconductivity. Pr3Ni2B2N3- exibits no long range magnetic order down to 2 K, Nd3Ni2B2N3- shows ferrimagnetic ordering below TC =17 K and a spin reorientation transition to a nearly antiferromagnetic state at 10 K.

TRANSPORT ANISOTROPY IN ErNi2B2C ALONG c-AXIS

The resistivities along c-axis ρc(H, T) of ErNi 2 B 2 C have been measured with H⊥ and H‖c-axis for 2 &lt; T &lt; 300 K and the superconducting upper critical field Hc2(T) curves of ErNi 2 B 2 C were constructed for each magnetic fields. Our Hc2(T) curves have been compared and discussed with those from ρab(H, T) measurements which explain the anisotropy and its temperature dependence of Hc2(T) are thought to arise from magnetic pair breaking and the anisotropic field dependence of Néel temperature TN originated from Er +3 sublattice.

Anomalous Meissner effect in pnictide superconductors

The Meissner effect has been studied in Ba(Fe0.926Co0.074)2As2 and Ba0.6K0.4Fe2As2 single crystals and compared to well known, type-II superconductors LuNi2B2C and V3Si. Whereas flux penetration is mostly determined by the bulk pinning (and, perhaps, surface barrier) resulting in a large negative magnetization, the flux expulsion upon cooling in a magnetic field is very small, which could also be due to pinning and/or surface barrier effects. However, in stark contrast with the expected behavior, the amount of the expelled flux increases almost linearly with the applied magnetic field, at least up to our maximum field of 5.5 T, which far exceeds the upper limit for the surface barrier. One interpretation of the observed behavior is that there is a field-driven suppression of magnetic pair-breaking.