Pair Breaking Research Articles

Quantum criticality in the (Cr98.4Al1.6)100-yMoy alloy system

Indications of quantum critical behaviour (QCB), driven by electron-hole pair breaking effects through Mo doping of Cr98.4Al1.6 are reported. Previous studies on the electrical resistivity (ρ), Seebeck coefficient (S), Sommerfeld electronic specific heat coefficient (γ) and magnetic susceptibility (χ) as a function of temperature on the (Cr98.4Al1.6)100-yMoy alloy system point towards the existence of a quantum critical point (QCP) at yc ≈ 4.5. In the present study the Hall coefficient RH measurements and an analysis of the previous results are used to substantiate the presence of a QCP in the alloy system. The charge carrier density at 2 K, (qRH(2K))−1, of the alloy systems increases continuously with y from the antiferromagnetic (AFM) phase through yc ≈ 4.5 and into the paramagnetic (P) phase. Scaling relationships between the magnetic contribution to the electrical resistivity, Δρ2K, the magnetic contribution to the Sommerfeld electronic specific heat coefficient, Δγ, and (qRH(2K))−1, are furthermore used to support the presence of a QCP in the alloy system. dS/dT in the temperature limit T→ 2 K depicts an anomalous behaviour in the form of an upturn on decreasing y through the QCP, yc ≈ 4.5. The critical exponents in the vicinity of the QCP were found to be β = 0.5 ± 0.1 for the Δρ2K/ρ2K(y) curve and γ = 0.4 ± 0.1 for the TN(y) curve. The nature of the QCP in the present alloy system is classified as an incommensurate (I) spin-density-wave (SDW) type, driven mainly by electron-hole pair breaking effects.

Divorce in Canada Geese (<i>Branta canadensis</i>): frequency, causes, and consequences

Most Canada Geese (Branta canadensis) form lifelong pair bonds (same-mate geese), but some pairs break apart and the geese mate with new partners while their former mates are still alive (divorcees). Over 25 years, we assessed lifelong reproduction of 160 collared Canada Geese that nested for multiple years in New Haven County, Connecticut. We examined whether same-mate geese and divorcee geese differed from each other prior to or after the divorce. Fifteen percent of females and 18% of males divorced during their lifetimes. Divorces were more frequent in pairs that produced fewer hatchlings during their prior nesting year. Most divorcees that nested again did so on their former nesting territories. Replacement partners of divorcees averaged younger and had fewer years of nesting experience than the divorcees’ prior mate. Usually after a divorce, one divorcee of each former pair nested immediately while the other skipped one or more years before nesting again. Under such circumstances, the partner able to nest immediately can increase its direct fitness by finding a new partner and nesting rather than foregoing the opportunity to nest that year. During their first nesting year after the divorce, the reproductive success of divorcees and same-mate geese were similar.

Ultrafast nonthermal terahertz electrodynamics and possible quantum energy transfer in the Nb3Sn superconductor

We report the low-energy electrodynamics of a moderately clean A15 superconductor (SC) following ultrafast excitation to understand and manipulate terahertz (THz) quasi--particle (QP) transport by tuning pump photoexcitation of from competing orders. Using 35-fs optical pulses, we observe a non-thermal enhancement in the low frequency conductivity, opposite to that observed for THz pump, which persists up to an additional critical temperature, above the SC one, from an electronic order in the Martensitic normal state. In the SC state, the fluence dependence of pair breaking kinetics together with an analytic model provides evidence for a `one photon-to-one pair' non-resonant energy transfer during the laser pulse. Such initial transfer of photon energy $\hbar\omega$ to QPs at the {\em quantum} limit, set by $2\Delta_{SC}/\hbar\omega$=0.33$\%$, is more than one order of magnitude smaller than in previously studied BCS SCs, which we attribute to strong electron--phonon coupling and possible influence of phonon condensation in A15 SCs.

Giant proximity effect in single-crystalline MgB2 bilayers

Although giant proximity effect (GPE) can shed important information on understanding superconducting pairing mechanisms and superconducting electronics, reports on the GPE are few because the fabrication of the junctions with GPE is technologically difficult. Here, we report a GPE in the single-crystalline MgB2 bilayers (S′/S), where the S′ is the damaged MgB2 layer by cobalt (Co)-ion irradiation and the S is the undamaged MgB2 layer. Superconducting properties of the S′ is remarkably degraded by the irradiation, whereas those of the S is uninfluenced by the irradiation. The degraded superconductivity in the S′ is fully recovered by increasing the thickness of undamaged MgB2 layer S despite almost ten times larger thickness ~ 95 nm of S′ than the superconducting coherence length ξab(0) ~ 8.5 nm of the S, indicating a presence of GPE in the S′/S MgB2 bilayers. A diffusion of electrons in the S′ into the S can reduce a pair breaking scattering in the S′, and the similar electronic structures of S′ and S layers and a finite attractive electron-electron interaction in the S′ are thought to be origins of unpredicted GPE between the same superconducting materials. Both upper critical field (μ0Hc2) and in-field critical current density (Jc) of S′/S bilayers show a significant enhancement, representing a strong correlation between S′ and S. These discoveries provide the blue print to the design of the superconducting multilayers for fundamental researches on the mechanism of the GPE as well as their technological applications.

Enhanced magnetic ordering in Sm metal under extreme pressure

The dependence of the magnetic ordering temperature To of Sm metal was determined through four-point electrical resistivity measurements to pressures as high as 150 GPa. A strong increase in To with pressure is observed above 85 GPa. In this pressure range Sm ions alloyed in dilute concentration with superconducting Y exhibit giant Kondo pair breaking. Taken together, these results suggest that for pressures above 85 GPa Sm is in a highly correlated electron state, like a Kondo lattice, with an unusually high value of To. A detailed comparison is made with similar results obtained earlier on Nd, Tb and Dy and their dilute magnetic alloys with superconducting Y.

Analysis of superconducting response and flux pinning ability of (Mg0.8Zn0.2Fe2O4)x/CuTl-1223 composites

The superconducting properties of (Mg0.8Zn0.2Fe2O4)x/Cu0.5Tl0.5–1223 matrix for concentrations (x = 0.0, 0.5, 1.0, 1.5 wt%) were synthesized by two-step solid state reaction method and analyzed by different characterization techniques. The magnetic Mg0.8Zn0.2Fe2O4 nanoparticles were synthesized by sol-gel technique separately and later inserted in CuTl-1223 superconducting matrix to obtain final product. The Mg0.8Zn0.2Fe2O4 nanoparticles were separately characterized by various techniques such as x-ray Diffraction, Scanning Electron Microscopy (SEM), Energy Dispersive x-ray Spectroscopy (EDX) and Vibrating Sample Magnetometer (VSM). The superconducting (Mg0.8Zn0.2Fe2O4)x/CuTl-1223 composite samples (x = 0 ∼ 1.5 wt%) were also characterized by various available characterization techniques. The XRD analysis showed spinel cubic structures of magnetic nanoparticles and tetragonal structure of superconducting composites. The XRD spectra revealed that the tetragonal structure of CuTl-1223 superconducting phase was not disturbed after the inclusion of magnetic Mg0.8Zn0.2Fe2O4 nanoparticles. The behavior of Mg0.8Zn0.2Fe2O4 was found ferromagnetic in nature during an applied magnetic field. The FTIR analysis exhibited results that Mg0.8Zn0.2Fe2O4 nanoparticles are settled at grain-boundaries and there is no major variation in oxygen modes. The value of critical temperature Tc(0) reduce with increase in magnetic nanoparticles content. The reduction in activation energy is also noticed with greater addition of magnetic nanoparticles. The overall suppression of critical parameters and activation energy values is due to the net spin of ferromagnetic natured Mg0.8Zn0.2Fe2O4 nanoparticles, which may scatter charge carriers and increase dissipation energy and ultimately result in pair breaking and resistive broadening to deteriorate flux pinning ability of the superconducting composite matrices.

Mitigation of cosmic ray effect on microwave kinetic inductance detector arrays

For space observatories, the glitches caused by high energy phonons created by the interaction of cosmic ray particles with a detector substrate lead to dead time during observation. Mitigating the impact of cosmic rays is therefore an important requirement for detectors to be used in future space missions. In order to investigate possible solutions, we carry out a systematic study by testing four large arrays of Microwave Kinetic Inductance Detectors (MKIDs), each consisting of ∼960 pixels and fabricated on monolithic 55 mm × 55 mm × 0.35 mm Si substrates. We compare the response to cosmic ray interactions in our laboratory for different detector arrays: A standard array with only the MKID array as reference, an array with a low Tc superconducting film as a phonon absorber on the opposite side of the substrate, and arrays with MKIDs on membranes. The idea is that the low Tc layer down converts the phonon energy to values below the pair breaking threshold of the MKIDs, and the membranes isolate the sensitive part of the MKIDs from phonons created in the substrate. We find that the dead time can be reduced up to a factor of 40 when compared to the reference array. Simulations show that the dead time can be reduced to below 1% for the tested detector arrays when operated in a spacecraft in an L2 or a similar far-Earth orbit. The technique described here is also applicable and important for large superconducting qubit arrays for future quantum computers.

S-shaped heat capacity in an odd–odd deformed nucleus

We examine the thermodynamic properties of mass A∼200 nuclei utilizing angular momentum (J) gated nuclear level densities (NLDs) extracted in the excitation energy range of 2–15 MeV. Interestingly, the experimental NLDs are in good agreement with the results of a microscopic approach, which is derived based on the exact pairing plus the independent-particle model at finite temperature (EP+IPM), whereas the conventional Hartree–Fock BCS (HFBCS) and Hartree–Fock–Bogoliubov plus combinatorial method (HFBC) fail to describe these data. Consequently, the thermodynamic properties of those nuclei at finite angular momentum have been extracted using the EP+IPM NLDs. While the heat capacities of 200Tl, 211Po and 212At (near spherical nuclei) follow the trend as expected in odd–odd and even–odd masses, surprisingly an S-shaped heat capacity is found in odd–odd deformed nucleus 184Re. It has been shown that this S-shaped heat capacity observed in 184Re is caused by not only the breaking of nucleon Cooper pairs but also the change of pairing induced by deformation.

Geometric quenching of orbital pair breaking in a single crystalline superconducting nanomesh network

In a superconductor Cooper pairs condense into a single state and in so doing support dissipation free charge flow and perfect diamagnetism. In a magnetic field the minimum kinetic energy of the Cooper pairs increases, producing an orbital pair breaking effect. We show that it is possible to significantly quench the orbital pair breaking effect for both parallel and perpendicular magnetic fields in a thin film superconductor with lateral nanostructure on a length scale smaller than the magnetic length. By growing an ultra-thin (2 nm thick) single crystalline Pb nanowire network, we establish nm scale lateral structure without introducing weak links. Our network suppresses orbital pair breaking for both perpendicular and in-plane fields with a negligible reduction in zero-field resistive critical temperatures. Our study opens a frontier in nanoscale superconductivity by providing a strategy for maintaining pairing in strong field environments in all directions with important technological implications.

The Impact of the Logistic Sector on Competitiveness in the Presence of Structural Breaks: A Study on Turkey

The main aim of this paper, based on the importance of the logistics sector for economic growth and, accordingly, competitiveness, is to explain the impact of the logistics sector on competitiveness by testing its relationship with macroeconomic factors. In accordance with this aim, principally, some research literature has been given a place. A cointegration model with structural breaks has been used to analyze the effects of the logistics sector, exports, imports, the industry production index and oil prices on GDP, which is an indication of competitiveness. For two different pairs of structural breaks and the two cointegration relationships there is a mutual positive relationship between the logistics sector and the GDP. Along with this, while export, import and industrial production index affect the GDP, the finding that oil prices reduce the performance of the logistics sector has been reached.

Magnetotransport properties of Ba(Fe1−xNix)2As2 thin films grown by PLD method

In this paper we report on the structural and superconducting properties of epitaxial Ba(Fe1−xNix)2As2 thin films with x = 0.035, 0.05, and 0.08 grown by pulsed laser deposition on CaF2. The superconducting transition temperatures are 20.9 K, 21.3 K, and 10.8 K for the nominal nickel concentrations x = 0.035, 0.05, and 0.08, respectively. A large contribution of the electron-electron interaction to the scattering processes is observed. The critical fields for Ba(Fe1−xNix)2As2 with x = 0.035 and 0.05 are higher than for single crystals and there is no noticeable contribution from the spin–orbit interaction to the pair breaking.

Micron-scale measurements of low anisotropic strain response of local Tc in Sr2RuO4

Topological superconductivity is of both fundamental and technological interest as a novel state of matter with potential applications in quantum information processing. Among the most promising materials for exhibiting intrinsic topological superconductivity is Sr${}_{2}$RuO${}_{4}$, which has shown evidence of triplet pairing and time-reversal symmetry breaking. Here, the authors use a scanning SQUID microscope to measure the local change in the critical temperature under application of anisotropic strain to test for a linear cusp at small strains, a key prediction of existing models for the superconducting state. The reported absence of such a cusp constrains future descriptions of this enigmatic material.

Quasiparticle scattering in 3 MeV proton irradiated BaFe2(As0.67P0.33)2

Effects of 3 MeV ${\mathrm{H}}^{+}$ (proton) irradiation on superconductivity in ${\mathrm{BaFe}}_{2}{({\mathrm{As}}_{0.67}{\mathrm{P}}_{0.33})}_{2}$ was investigated through in situ resistivity measurements and magnetotransport measurements. The suppression of ${T}_{\mathrm{c}}$ by ${\mathrm{H}}^{+}$ irradiation is more moderate in comparison with electron irradiation, yet more effective than that of irradiation of heavier ${\mathrm{He}}^{+}$ ions. Surprisingly, the $B$-linear magnetoresistance (MR) in the pristine crystal crosses over to an anomalous negative MR after the irradiation. We discuss that the negative MR is most likely to emanate from sparse magnetic impurities. The appearance of a paramagnetic signal in magnetization measurements supports the existence of magnetic impurities in the sample. Although the estimated concentration of local magnetic moments suggests that the concentration of magnetic impurities is dilute, thereby signifying that irradiation-induced defects are dominantly nonmagnetic, the role of magnetic scattering cannot be completely neglected. Hence, ${\mathrm{H}}^{+}$ irradiation in this system contains magnetic-scattering centers, which are pair breakers for both the ${s}_{++}$- and the ${s}_{\ifmmode\pm\else\textpm\fi{}}$-gap structures, indicating that the suppression of ${T}_{\mathrm{c}}$ is a consequence of both magnetic and nonmagnetic scatterings. This result opens up the possibility for the scenario of the slow but steady suppression of ${T}_{\mathrm{c}}$ emanating from the ${s}_{++}$-gap symmetry with pair breaking due to dilute magnetic scatterers.

Spontaneous Helical Order of a Chiral p-Wave Superfluid Confined in Nanoscale Channels.

Strong interactions that favor chiral p-wave pairing, combined with strong pair breaking by confining boundaries, are shown to lead to new equilibrium states with different broken symmetries. Based on a strong-coupling extension of the Ginzburg-Landau theory that accurately accounts for the thermodynamics and phase diagram of the bulk phases of superfluid ^{3}He, we predict new phases of superfluid ^{3}He for confined geometries that spontaneously break rotational and translational symmetry in combination with parity and time-reversal symmetry. One of the newly predicted phases exhibits a unique combination of chiral and helical order that is energetically stable in cylindrical channels of radius approaching the Cooper pair coherence length, e.g., R∼100 nm. Precise numerical minimization of the free energy yields a broad region of stability of the helical phase as a function of pressure and temperature, in addition to three translationally invariant phases with distinct broken spin and orbital rotation symmetries. The helical phase is stable at both high and low pressures and favored by boundaries with strong pair breaking. We present calculations of transverse NMR frequency shifts as functions of rf pulse tipping angle, magnetic field orientation, and temperature as signatures of these broken symmetry phases.

Site-specific perspective on interactions in polyelectrolyte complexes: Toward quantitative understanding.

The composition and properties of hydrated polyelectrolyte complexes, PECs, depend strongly on the salt concentration of solutions in which they are immersed. This fascinating and polyelectrolyte-specific behavior is often treated with extensions of theory developed for single-component polyelectrolyte solutions. As an alternative, the response of PECs to salt (i.e., small ions) may be treated as a competition between the pairing of positive, Pol +, and negative, Pol -, repeat units and their salt counterions. Simple equilibrium expressions provide the degree of reversible Pol + Pol - pair breaking as more salt is added. This work summarizes the site-specific ion pairing view of PECs.

Temperature and field dependence of the intrinsic tunnelling structure in overdoped Bi2Sr2CaCu2O8+δ

We report intrinsic tunnelling data for mesa structures fabricated on three over- and optimally-doped $\rm{Bi_{2.15}Sr_{1.85}CaCu_{2}O_{8+\delta}}$ crystals with transition temperatures of 86-78~K and 0.16-0.19~holes per CuO$_2$ unit, for a wide range of temperature ($T$) and applied magnetic field ($H$), primarily focusing on one over-doped crystal(OD80). The differential conductance above the gap edge shows clear dip structure which is highly suggestive of strong coupling to a narrow boson mode. Data below the gap edge suggest that tunnelling is weaker near the nodes of the d-wave gap and give clear evidence for strong $T$-dependent pair breaking. These findings could help theorists make a detailed Eliashberg analysis and thereby contribute towards understanding the pairing mechanism. We show that for our OD80 crystal the gap above $T_c$ although large, is reasonably consistent with the theory of superconducting fluctuations.

On the origin of in-gap states in homogeneously disordered ultrathin films. MoC case

Many disordered superconducting films exhibit smeared tunneling spectra with evident in-gap states. We have found that the tunneling density of states in ultrathin MoC films is gapless and can be described by the Dynes version of the BCS density of states with a strong broadening parameter Γ accounting for the suppression of coherence peaks and increased in-gap states. The thinner the film, the lower the Tc and the superconducting energy gap Δ and the larger the Γ. MoC films of 3 nm thickness deposited simultaneously on silicon and sapphire substrates reveal very similar scalar disorder, evidenced by the equal sheet resistance, but exhibit different superconducting characteristics of Tc, Δ and Γ, suggesting that pair breaking responsible for the dissipation channel and the suppression of superconductivity originates on the film-substrate interface. It indicates that sapphire is a stronger pair breaker. Interface pair breaking can be operative in other cases as well.

Unusual evolution from a superconducting to an antiferromagnetic ground state in Y1−xGdxPb3 (0 ≤ x ≤ 1)

We have studied the evolution of ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}{\mathrm{Pb}}_{3}$ ($0\ensuremath{\le}x\ensuremath{\le}1$) from its superconducting ($x=0$) to antiferromagnetic ($x=1$) states with critical temperatures ${T}_{c}=4.6(2)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{N}=15.7(2)\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively. At relatively low Gd concentrations ($x\ensuremath{\le}0.03$) ${T}_{c}$ presents a weak but linear suppression with increasing $x$, as expected from the Abrikosov-Gorkov (AG) theory describing the Cooper pair breaking due to the exchange interaction between the impurity localized magnetic moment and the conduction electrons ($ce$). This linear ${T}_{c}$ suppression rate leads to an effective exchange parameter of ${\ensuremath{\langle}{J}^{2}(\mathbf{q})\ensuremath{\rangle}}_{AG}^{1/2}=0.20(7)\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ between the $4f$ localized electrons and the $ce$. For intermediate Gd concentrations ($0.03\ensuremath{\le}x\ensuremath{\le}0.15$), although no Gd clustering is observed, there is an unusual inflection and leveling off trend in ${T}_{c}$, indicating that the superconducting phase persists until the highest Gd concentration in this region where magnetic interactions are evidenced by increasingly negative values of the Curie-Weiss temperature ${\mathrm{\ensuremath{\Theta}}}_{C}$. Analysis of low-$T\phantom{\rule{4pt}{0ex}}{\mathrm{Gd}}^{3+}$ electron spin resonance (ESR) experiments and their ESR line-shape simulations, in conjunction with the trend in ${T}_{c}$, leads us to suggest that an exchange bottleneck mechanism between the ${\mathrm{Gd}}^{+3}$ localized magnetic moment and the $ce$ may be the reason behind the inhibition of the Cooper pair-breaking mechanism, in favor of a magnetic interaction via $ce$ polarization. Therefore, our superconducting and ESR results suggest a scenario where superconducting and magnetic phases coexist extensively in the ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}{\mathrm{Pb}}_{3}$ system, mediated by different types of $ce$.

Proximity-reduced range of internal phase differences in double Josephson junctions with closely spaced interfaces

A substantial influence of the proximity and pair breaking effects on the range of internal phase differences is shown to take place in symmetric double Josephson junctions with closely spaced interfaces and to affect the evolution of the supercurrent $j$ with the changing central lead's length $L$. If the phase difference $\ensuremath{\phi}$ between the external leads is controlled and $L$ exceeds a few coherence lengths, the regime of interchanging modes is established. The range of the phase differences across the two individual interfaces is reduced with decreasing $L$, and the states of the higher energy mode are gradually eliminated. With a further decrease of $L$ the regime of interchanging modes is destroyed along with the asymmetric mode. The conventional single junction current-phase relation $j(\ensuremath{\phi})$ is eventually established and the condensate states' doubling is fully removed at very small $L$.

Effect of external turbulence on the short-wavelength instability of a counter-rotating vortex pair

The effect of background turbulence on the evolution and instability of a counter-rotating vortex pair is investigated via the use of direct numerical simulation at a circulation Reynolds number of 2400. The ambient turbulence is modelled as decaying homogeneous isotropic turbulence with the initial levels of turbulence intensity (Tu) varied from 0.0% to 10.0%. The major effect of the external turbulence during the early-time development is that it reduces the vortex separation distance, leading to the mutual interaction of the two vortices. The onset to transition is also found to occur earlier. For a low-level turbulence intensity (Tu ≤ 6.25%), the vortex pair breaks down to turbulence due to the short-wavelength elliptic instability, whose characteristic is qualitatively similar to that in a quiescent background despite exhibiting a shorter wavelength. Increasing the turbulence intensity results in a decrease in the growth rate of the elliptic mode. The long-wavelength Crow mode dominates the breakdown process of the flow once Tu ≥ 7.5%. The Crow instability gives rise to the vortex disconnection and reconnection, resulting in the transformation of the vortex pair into a vortex ring.