Magnetic Pair-breaking Effect Research Articles

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.

Andreev reflection in ballistic normal metal/graphene/superconductor junctions

We report the study of ballistic transport in normal metal/graphene/superconductor junctions in edge-contact geometry. While in the normal state, we have observed Fabry-P\'{e}rot resonances suggesting that charge carriers travel ballistically, the superconducting state shows that the Andreev reflection at the graphene/superconductor interface is affected by these interferences. Our experimental results in the superconducting state have been analyzed and explained with a modified Octavio-Tinkham-Blonder-Klapwijk model taking into account the magnetic pair-breaking effects and the two different interface transparencies, \textit{i.e.}\,between the normal metal and graphene, and between graphene and the superconductor. We show that the transparency of the normal metal/graphene interface strongly varies with doping at large scale, while it undergoes weaker changes at the graphene/superconductor interface. When a cavity is formed by the charge transfer occurring in the vicinity of the contacts, we see that the transmission probabilities follow the normal state conductance highlighting the interplay between the Andreev processes and the electronic interferometer.

Impact of electron-electron interactions on the superfluid density of dirty superconductors

Landau's theory of the Fermi liquid is adapted to analyze the impact of electron-electron ($e-e$) interactions on the deficit of the superfluid density $\rho_{s0}=\rho_s(T=0)$ in dirty superconducting electron systems in which the damping $\gamma$ of single-particle excitations exceeds the zero temperature BCS gap $\Delta_0$. In the dirty strong-coupling limit $\gamma/\Delta_0\gg 1,m^*/m_e\gg 1$, the formula derived for $\rho_{s0}$ is shown to coincide with the well-known empirical Uemura relation provided pair-breaking contributions are nonexistent. The roles of the crystal lattice and magnetic pair-breaking effects in the observed decline of the zero-temperature superfluid density $\rho_{s0}$ in overdoped LSCO compounds are also discussed. Our method is also applied to elucidation of results from the pioneering experimental studies performed recently by Bozovi\`c and collaborators in overdoped LSCO compounds.

Y89 NMR observation of ferromagnetic and antiferromagnetic spin fluctuations in the collapsed tetragonal phase of YFe2 (Ge,Si) 2

The surprising discovery of tripling the superconducting critical temperature of KFe$_2$As$_2$ at high pressures issued an intriguing question of how the superconductivity in the collapsed tetragonal phase differs from that in the non-collapsed phases of Fe-based superconductors. Here we report $^{89}$Y nuclear magnetic resonance study of YFe$_2$Ge$_{x}$Si$_{2-x}$ compounds whose electronic structure is similar to that of iron-pnictide collapsed tetragonal phases already at ambient pressure. Fe(Ge,Si) layers show strong ferromagnetic spin fluctuations whereas layers are coupled antiferromagnetically -- both positioning the studied family close to a quantum critical point. Next, localized moments attributed either to Fe interstitial or antisite defects may account for magnetic impurity pair-breaking effects thus explaining the substantial variation of superconductivity among different YFe$_2$Ge$_2$ samples.

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.

Possible unconventional superconductivity in substituted BaFe2As2 revealed by magnetic pair-breaking studies.

The possible existence of a sign-changing gap symmetry in BaFe2As2-derived superconductors (SC) has been an exciting topic of research in the last few years. To further investigate this subject we combine Electron Spin Resonance (ESR) and pressure-dependent transport measurements to investigate magnetic pair-breaking effects on BaFe1.9M0.1As2 (M = Mn, Co, Cu, and Ni) single crystals. An ESR signal, indicative of the presence of localized magnetic moments, is observed only for M = Cu and Mn compounds, which display very low SC transition temperature (Tc) and no SC, respectively. From the ESR analysis assuming the absence of bottleneck effects, the microscopic parameters are extracted to show that this reduction of Tc cannot be accounted by the Abrikosov-Gorkov pair-breaking expression for a sign-preserving gap function. Our results reveal an unconventional spin- and pressure-dependent pair-breaking effect and impose strong constraints on the pairing symmetry of these materials.

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.

Dependence of the superconducting transition temperature of the filled skutterudite compound PrPt4Ge12 on hydrostatic pressure

The temperature-dependent ac susceptibility of the filled skutterudite superconductor PrPt4Ge12 has been measured under hydrostatic He-gas pressure to 0.58GPa. The superconducting transition temperature Tc decreases linearly with pressure P from 7.91K at ambient pressure to 7.83K at 0.58GPa, giving the rate dTc/dP=−0.19±0.03K/GPa. Evidence is presented that suggests that the value of Tc in this compound is slightly reduced due to magnetic pair-breaking effects from the Pr3+ cations.

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.

Low-temperature electrical resistivity of praseodymium at pressures up to 120 GPa

Under ambient conditions, praseodymium metal possesses a localized $4{f}^{2}$-electron configuration. Near 20 GPa, the lattice volume collapses by $~$10% into the $\ensuremath{\alpha}$-uranium crystal structure, and the electrical resistivity drops dramatically. This behavior is similar to that observed in cerium metal and has been taken as evidence for $4f$ delocalization, although the precise nature of such volume collapse transitions in rare-earth metals is still a matter of debate. Since cerium metal develops superconductivity in the collapsed phase, we undertook a search for superconductivity in praseodymium metal at high pressure. Using designer diamond anvils, we measured the electrical resistivity of high purity praseodymium metal to pressures above 1 Mbar and millikelvin temperatures. No evidence for superconductivity was found in any of the measurements. The lack of superconductivity may derive from magnetic pair-breaking effects related to incomplete screening/delocalization of the $4f$-electron state.

Formation of magnetic impurities and pair-breaking effect in a superfluid Fermi gas

We theoretically investigate the introduction of magnetic impurities into a superfluid Fermi gas. In the presence of a population imbalance (${N}_{\ensuremath{\uparrow}}>{N}_{\ensuremath{\downarrow}}$, where ${N}_{\ensuremath{\sigma}}$ is the number of Fermi atoms with pseudospin $\ensuremath{\sigma}=\ensuremath{\uparrow},\ensuremath{\downarrow}$), we show that nonmagnetic potential scatterers embedded in the system are magnetized in the sense that some of the excess spin-$\ensuremath{\uparrow}$ atoms are localized around them. They destroy the superfluid order parameter around them, as in the case of the magnetic impurity effect discussed in the superconductivity literature. This pair-breaking effect naturally leads to localized excited states below the superfluid excitation gap. To confirm our idea in a simple manner, we consider an attractive Fermi-Hubbard model within the mean-field theory at $T=0$. We determine consistent superfluid properties around a nonmagnetic impurity, such as the superfluid order parameter, local population imbalance, as well as the single-particle density of states, in the presence of a population imbalance. Since competition between superconductivity and magnetism is one of the most fundamental problems in condensed-matter physics, our results would be useful for the study of this important issue in cold Fermi gases.

Suppression of Superconductivity Due to Enhanced Co Doping in Cu0.5Tl0.5Ba2Ca2Cu3−y Co y O10−δ Superconductors

Fluctuation Induced Conductivity (FIC) of Co-doped Cu0.5Tl0.5Ba2Ca2Cu3−yCoyO10−δ (y=0, 0.05, 0.1, 0.5) Superconductors have been studied as to find the possible cause of Tc suppression in these compounds. We used Aslamazov–Larkin (AL) and Lawrence–Donaich (LD) equations to analyze the FIC of our samples. In this regard, it is observed that the data seems to fit well with 3D (three-dimensional) AL equations. In addition, crossover temperature from 2D to 3D has been found. The width of the temperature window for 3D conductivity is continuously decreasing with enhanced Co concentration. The shrinking of 3D temperature region with increased Co doping can be correlated with the suppression of superconductivity. It is observed that the coherence length and th interplanar coupling strength have also been decreased with increased Co doping, consistent with the magnetic pair-breaking effect.

Josephson supercurrent in Nb/InN-nanowire/Nb junctions

We experimentally studied the Josephson supercurrent in Nb/InN-nanowire/Nb junctions. Large critical currents up to 5.7 μA have been achieved, which proves the good coupling of the nanowire to the superconductor. The effect of a magnetic field perpendicular to the plane of the Josephson junction on the critical current has been studied. The observed monotonous decrease in the critical current with magnetic field is explained by the magnetic pair-breaking effect in planar Josephson junctions of ultra-narrow width [J. C. Cuevas and F. S. Bergeret, Phys. Rev. Lett. 99, 217002 (2007)].

How the substitution of Zn for Cu destroys superconductivity in YBCO system?

Understanding the mechanism of suppression of superconductivity in perovskite cuprates is as important as to discover the cause of its evolution. Substituting Zn for Cu is known to rapidly suppress the superconductivity in these perovskites but the mechanism is still not understood. To throw some light on this, polycrystalline YBa 2(Cu 1− x Zn x ) 3O 7− δ samples ( x = 0.0–0.06) were synthesized and investigated using X-ray diffraction, titration, resistivity, magnetization, X-ray absorption and photoemission measurements. Results show that their oxygen stoichiometry ( δ) changes on Zn substitution affect their normal state resistivity as well as their T c . However, the observed changes cannot be accounted for solely on the basis of changes in δ. Zn cation is found to act as strong scattering centre in the lattice that causes local lattice distortion (LLD). It also induces local magnetic moment as evidenced from our magnetization results. It is concluded that a combination of Δ δ, LLD and magnetic pair breaking effect is responsible for the observed rapid suppression of superconductivity.

Magnetic instabilities along the superconducting phase boundary of Nb∕Ni multilayers

We report vibrating reed and superconducting quantum interference device magnetometer data that exhibit prominent dips or oscillations of the superconducting (SC) onset temperature, ΔTC(H)≈0.01–0.7K, for a [Nb(23nm)∕Ni(5nm)]5 multilayer (ML) in dc magnetic fields applied nearly parallel to the ML plane. The vibrating reed data exhibit reproducible structures below TC that may reflect multiple SC transitions, but they are sensitive to ac field amplitude and dc field orientation. This striking behavior poses challenges for theoretical and experimental investigations of interfaces between SC and ferromagnetic layers that involve magnetic pair breaking effects, “pi phase shifts” of the SC order parameter, and exotic (“LOFF”) pairing states. Alternatively, the anomalies may mark dynamical instabilities within a confined, strongly anisotropic Abrikosov vortex lattice.

Laser-photoemission study of antiferromagnetic superconductor ErNi2B2C

We have investigated the electronic structures of antiferromagnetic superconductor ErNi2B2C by using laser-photoemission spectroscopy. The observed spectra below the superconducting transition temperature (Tc=9.0K) show a very broad coherent peak and finite density of states at the Fermi level due to strong magnetic pair breaking effects. In addition, we find a dip structure in the temperature dependence of the superconducting order parameter just below the antiferromagnetic ordering temperature (TN=6.0K). This behavior can be well interpreted as a result of competition between the rapid evolution of the antiferromagnetic molecular field and the increase of the superconducting condensation energy with decreasing temperature.

Study of the superconducting and magnetic properties of niobium doped RuSr2Gd1.5Ce0.5Cu2O10−δ ruthenocuprates

Polycrystalline samples of Ru1−xNbxSr2Gd1.5Ce0.5Cu2O10−δ, 0⩽x⩽0.5, have been synthesized and studied by resistivity, magnetization and ac susceptibility measurements. It was identified a strong suppression of the spin glass (SG) transition, which was totally suppressed in samples for x⩾0.2. The hysteresis loops at low temperatures are the result of the contribution of two distinct magnetic phases: a canted AFM phase and the SG phase. More importantly, the significant changes in the magnetic response of the material affect the superconducting properties of the samples. It was found that both Tc and the superconducting fraction are reduced in samples that show the spin glass phase, possibly due to the magnetic pair breaking effect.