Suppression Of Superconductivity Research Articles

Structural phase transition in the mixed state of superconducting films in a parallel magnetic field

A thin film of a second-kind superconductor in a magnetic field parallel to the surface of the film is considered in the London approximation. It has been shown that if bulk pinning is absent and the suppression of super-conductivity by the magnetic field is negligible, the splitting of a vortex chain in the film occurs as a structural phase transition either of the first or second order, depending on the ratio of the thickness of the film d to the penetration depth of the magnetic field λ. The ratio d/λ, and thereby the character of the transition in the vortex lattice, can be changed by varying the temperature. The corresponding critical thicknesses of films and field ranges in which this effect can be observed experimentally have been calculated.

The suppression of superconductivity by the localization of oxygen holes in the double-doped La 1.85−1.5xSr 0.15+1.5xCu 1−xMn xO 4 system

A series of double-doped La 1.85−1.5 x Sr 0.15+1.5 x Cu 1− x Mn x O 4 ( 0 ⩽ x ⩽ 0.2 ) samples were synthesized by the solid-state reaction method. The structural, transport and magnetic properties were studied. The superconductivity can even survive till Mn doping level to x = 0.15. It is noteworthy that the sample with x = 0.06 undergoes two superconducting transitions in the M– T curves. It is suggested that the Cu–O–Cu pdσ hybridization interaction is weakened by the superexchange interaction between the doped Mn ions and the O ions in the Cu–O–Cu paths neighboring Mn, which leads to the suppression of superconductivity.

Spin-vortices and Spin-vortex-induced Loop Currents in the Pseudogap Phase of Cuprates

We explain several anomalous phenomena observed in the pseudogap phase of hole-doped cuprates based on the recently proposed spin-vortex superconductivity theory. In this theory, doped-holes become almost immobile small polarons, and spin-vortices are formed with those small polarons as their centers. A Hartree–Fock field for conduction electrons that is optimized for the interaction energy of local moments is derived; it contains a fictitious magnetic field arising from spin-vortices, and yields current carrying states. The obtained currents are loop currents around spin-vortices, i.e., the spin-vortex-induced loop currents (SVILCs), and a collection of them produces a macroscopic current. The SVILC explains (1) nonzero Kerr rotation in zero-magnetic field after exposed in a strong magnetic field; (2) the change of the sign of the Hall coefficient with temperature change; (3) the suppression of superconductivity in the x=1/8 static-stripe ordered sample; and (4) a large anomalous Nernst signal, including its sign-change with temperature change. We show that the hourglass-shaped magnetic excitation spectrum is the evidence for the existence of spin-vortices. We further argue that the “Fermi-arc” in the ARPES is a support for the presence of localized moments in the bulk; a disconnected arc-shaped Fermi surface is obtained by assuming an antiferromagnetic interaction between the localized moments in the bulk and itinerant electrons in the surface region.

Suppression of superconductivity in Zn-doped SmFeAsO0.8F0.2 system

A series of SmFe1−xZnxAsO0.8F0.2 samples with x=0, 0.05, 0.1, 0.2 and 0.4 have been successfully synthesized using a solid state method. The lattice parameters are found to increase with increasing Zn doping content. The superconductivity has been definitely suppressed by Zn doping at Fe site with the transition temperature Tc being reduced from 52.5K to 23.3K for the sample of x=0.05, and to 18.2K for the sample of x=0.1. For the samples with x>0.1, the superconducting transition vanishes, and, at the meantime, the spin-density-wave anomaly recovers at 140K. The metal to semiconductor transition is also observed in the SmFe1−xZnxAsO0.8F0.2 system. The behavior of SmFe1−xZnxAsO0.8F0.2 is very different from that of REFeAsO (RE=rare earth metal), which reveals a very strong electron correlation in SmFe1−xZnxAsO0.8F0.2.

Nonmagnetic pair-breaking effect in La(Fe1−xZnx)AsO0.85studied byAs75andLa139NMR and NQR

$^{75}$As and $^{139}$La NMR and nuclear quadrupole resonance (NQR) studies on Zn-substituted LaFeAsO$_{0.85}$ have been performed to investigate the Zn-impurity effects microscopically. Although superconductivity in LaFeAsO$_{0.85}$ disappears by 3% Zn substitution, we found that NMR/NQR spectra and NMR physical quantities in the normal state are hardly changed, indicating that the crystal structure and electronic states are not modified by Zn substitution. Our results suggest that the suppression of superconductivity by Zn substitution is not due to the change of the normal-state properties, but due to strong non-magnetic pair-breaking effect to superconductivity.

Domain wall modulated superconductivity in Nb/Y3Fe5O12 hybrids

In superconductor/ferromagnet hybrids, the stray field and inhomogeneous exchange field coming from magnetic domain walls have opposite influence on the superconductivity if the ferromagnet displays in-plane anisotropy. In this paper, we investigate modulation of superconductivity by the stray field of Bloch walls in Nb/Y3Fe5O12 hybrids where the proximity effect is excluded. By applying in-plane magnetic field, we show that the resistance as a function of magnetic field displays two dips precisely at the saturation field Hs of Y3Fe5O12. The superconducting transition temperature Tc at Hs is higher than that at lower fields, suggesting the suppression of superconductivity by the stray fields of Bloch walls. By effective controlling of the domain walls, the superconductivity can be switched either on or off.

Suppression of superconductivity by interparticle interactions in Al nanoparticle assembly

We report on the observations of enhanced superconductivity in a very loosely packed 6.8 nm Al nanoparticle assembly and the suppression of superconductivity by interparticle interactions, through ac magnetic susceptibility and magnetization measurements. TC and HC of the 6.8 nm Al particles are, respectively, 1.9 and 77 times higher than that of bulk Al. M(Ha) curves taken below and above TC can all be described by a Langevin profile, showing the existence of a spontaneous magnetic component in superconducting phase. Superconductivity is found to be gradually suppressed by interparticle interactions.

Structural, Superconducting and Magnetic Properties of La<sub>3-X</sub><i>R</i><sub>x</sub>Ni<sub>2</sub>B<sub>2</sub>N<sub>3-δ</sub> with <i>R</i> = 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.

Magnetism and superconductivity in MxFe1+yTe1–zSez (M = Cr, Mn, Co, Ni, Cu, and Zn) single crystals

High-quality single crystals with nominal composition M0.05Fe0.95Te0.8Se0.2 (M = Cr, Mn, Co, Ni, Cu, and Zn) have been grown, through which the doping effect on magnetism and superconductivity is studied. Elementary analysis reveals that Cu, Co, and Ni, with smaller ionic radii for valence state 2+, can substitute effectively for Fe with doping levels near 5%. In contrast, the solid solution of Cr, Mn, and Zn in the host system is low. Magnetic and electronic investigations show that the substitution of Co, Ni, or Cu for Fe leads to the formation of spin-glass state and suppression of superconductivity. The superconductivity is partly suppressed by Co doping, while completely destroyed by Ni and Cu doping. Compared with Cu- and Ni-doped samples, the Co-doped sample has the smallest lattice constant, indicating that the superconductivity might be also modulated by the changes of microstructure.

Intermittent superconductivity in three-dimensional mesoscopic rings

Three-dimensional mesoscopic rings are investigated by the phenomenological Ginzburg–Landau theory in the presence of an externally applied magnetic-field. By solving an eigenvalue problem and minimizing systematic free energy, we find that the superconducting and normal states cyclically appear, and that between the state L and the state L + 1 the field extent of existing normal state increases with increase of vorticity L. The dimensional phase diagram with the maximum vorticity and existing normal state between two superconducting states is given. The normal state can be controlled by the medium with surface enhancement or suppression of superconductivity.

Fluctuations in a superconducting quantum critical point of multi-band metals

In multi-band metals quasi-particles arising from different atomic orbitals coexist at acommon Fermi surface. Superconductivity in these materials may appear due tointeractions within a band (intra-band) or among the distinct metallic bands (inter-band).Here we consider the suppression of superconductivity in the intra-band case due tohybridization. The fluctuations at the superconducting quantum critical point (SQCP) areobtained by calculating the response of the system to a fictitious space- and time-dependentfield, which couples to the superconducting order parameter. The appearance ofsuperconductivity is related to the divergence of a generalized susceptibility. For asingle-band superconductor this coincides with the Thouless criterion. For fixed chemicalpotential and large hybridization, the superconducting state has many features in commonwith breached pair superconductivity with unpaired electrons at the Fermi surface. TheT = 0 phase transition from the superconductor to the normal state is in the universality class ofthe density-driven Bose–Einstein condensation. For a fixed number of particles and in thestrong coupling limit, the system still has an instability to the normal state with increasinghybridization.

Suppression of superconductivity in epitaxial NbN ultrathin films

This paper studies the suppression of superconducting transition temperature (Tc) of ultrathin NbN film. We fabricated epitaxial NbN superconducting thin films of thicknesses ranging from 2.5 to 100 nm on single crystal MgO (100) substrates by dc magnetron sputtering. We performed structure analyses and measured their electric and far infrared properties. The experimental results were compared with several mechanisms of the suppression of superconductivity proposed in the literature, including the weak localization effect, the proximity effect, and quantum size effect (electron wave leakage model). We found that the electron wave leakage model matches best to the experimental data.

Magnetic Excitation Spectra from Spin-Vortices and Spin-Vortex-Induced Loop Currents in Hole-Doped Cuprates

We argue that spin-vortices and loop currents induced by them ( spin-vortex-induced loop currents ) are crucial ingredients for superconductivity in hole-doped cuprates. The existence of the former is indicated by the hourglass-shaped magnetic excitation spectrum obtained by the inelastic neutron scattering experiments. The existence of the latter is indicated by the Kerr effect measurement and the recently observed hidden magnetic excitation spectrum. The existence of both spin-vortices and spin-vortex-induced currents points to the occurrence of the spin-vortex superconductivity in the cuprates. The suppression of superconductivity in the x =1/8 stripe-ordered sample is also explained by this theory.

Superconducting condensation energy in a diluted Kondo lattice

Heavy fermion compounds are host to many interesting but poorly understood strong correlation effects, leading in exceptional cases to unconventional superconductivity. CeCoIn5 is such an example of an ambient pressure heavy fermion superconductor with a Tc of 2.3K. Such compounds are best described as a Kondo lattice, where Ce, Yb or U ions form a periodic lattice of magnetic moments coupled to the conduction electrons via antiferromagnetic exchange interaction. Dilution, or substitution, of the Kondo ion with a lanthanide, actinide or alkali-earth element that does not participate in the Kondo lattice, results in the so-called "Kondo hole". The electron scattering off the Kondo hole leads to significant changes in both the normal and superconducting properties. We have investigated the suppression of superconductivity in CeCoIn5 diluted with La, Yb, Y, Ca, Th, Gd, Er, Eu and Lu via specific heat measurements on well characterized single crystals. The suppression of the superconducting condensation energy exhibits a remarkable departure from the Abrikosov-Gorkov impurity pair breaking picture, emphasizing that the Kondo holes are no ordinary pair-breakers.

Resistive transition in disordered superconductors with varying intergrain coupling

The effect of disorder is investigated in granular superconductive materials withstrong- and weak-links. The transition is controlled by the interplay of the tunnelingg andintragrain gintr conductances, which depend on the strength of the intergrain coupling. For , the transition first involves the grain boundary, while forg ∼ gintr the transition occurs into the whole grain. The different intergrain couplings are consideredby modeling the superconducting material as a disordered network of Josephson junctions.Numerical simulations show that on increasing the disorder, the resistive transition occursfor lower temperatures and the curve broadens. These features are enhanced indisordered superconductors with strong-links. The different behavior is further checkedby estimating the average network resistance for weak- and strong-links in theframework of the effective medium approximation theory. These results may shedlight on long standing puzzles such as: (i) enhancement of the superconductingtransition temperature of many metals in the granular states; (ii) suppressionof superconductivity in homogeneously disordered films compared to standardgranular systems close to the metal–insulator transition; (iii) enhanced degradation ofsuperconductivity by doping and impurities in strongly linked materials, such asmagnesium diboride, compared to weakly linked superconductors, such as cuprates.

Possible magnetic order and suppression of superconductivity by V doping inSr2VO3FeAs

Superconductivity at 33 K in Sr2VO3FeAs is completely suppressed by small amounts of V-doping in Sr2VO3[Fe0.93(+/-0.01)V0.07(+/-0.01)]As. The crystal structures and exact stoichiometries are determined by combined neutron- and x-ray powder diffraction. Sr2VO3FeAs is shown to be very sensitive to Fe/V mixing, which interferes with or even suppresses superconductivity. This inhomogeneity may be intrinsic and explains scattered reports regarding Tc and reduced superconducting phase fractions in Sr2VO3FeAs. Neutron diffraction data collected at 4 K indicates incommensurate mag- netic ordering of the V-sublattice with a propagation vector q = (0,0,0.306). This suggests strongly correlated vanadium, which does not contribute significantly to the Fermi surface of Sr2VO3FeAs.

Three-dimensional simulation of quasi-particle structures in nano-scaled superconductors

We have developed numerical simulation method for quasi-particle structures in the three-dimensional nano-sized superconductors, using the three-dimensional finite element method and the Bogoliubov-de Gennes equation. Using this method, we analyzed the superconducting state in the nano-sized cubic superconductors. We found the spatial oscillations of order parameter because of the confinement of superconducting electrons, and also we found the quasi-particle bound states at the corners of the cubic superconductors because of suppression of superconductivity at the corners.

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.

Interband and intraband effects in the upper critical field of disorderedMgB2

In this work the superconducting properties of disordered ${\text{MgB}}_{2}$ in applied magnetic field are studied within the ${\ensuremath{\lambda}}^{\ensuremath{\theta}\ensuremath{\theta}}$ model, by taking into account the presence of both interband and intraband scattering with impurities. This approach allows to extract the suppression of the critical temperature ${T}_{c}$ and the enhancement of the upper critical field ${H}_{{c}_{2}}$, as a consequence of the introduction of impurities in the samples. We analyze the dependence of ${H}_{{c}_{2}}$ on temperature, anisotropy of the electronic structure, and intraband $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ band scattering rates. Comparing our numerical calculations with experimental data on irradiated samples, we find that irradiation defects mainly affect the mobility of $\ensuremath{\sigma}$ carriers. These results rationalize why the ${H}_{{c}_{2}}$ anisotropy of irradiated samples is quickly reduced with increasing doses and full suppression of superconductivity occurs at rather low-resistivity values. Moreover, our calculations point out that disorder in the $\ensuremath{\pi}$ bands affects only weakly the coupling constants and thus it could yield a significant enhancement of ${H}_{{c}_{2}}$ without severe suppression of ${T}_{c}$.

Observation of Higher Suppression of Superconducting Transition Temperature in YBa2Cu3O7−δ /La0.5Sr0.5CoO3 as Compared to YBa2Cu3O7−δ /La0.7Sr0.3MnO3 Bilayers

We report the experimental investigations of the suppression of superconductivity due to the pair breaking effect induced by ferromagnetic layer in YBa2Cu3O7−δ /La0.5Sr0.5CoO3 and YBa2Cu3O7−δ /La0.7Sr0.3MnO3 bi-layers fabricated by pulsed laser deposition. The current dependent electrical transport studies in the bilayers exhibit a significant reduction in the superconducting transition temperature in accord to I 2/3 law as compared to single YBa2Cu3O7−δ layer, and moreover the superconducting transition temperature in YBa2Cu3O7−δ /La0.7Sr0.3MnO3 bilayer is surprisingly found to be much larger than the YBa2Cu3O7−δ /La0.5Sr0.5CoO3. It appears that the current driven from a low spin polarization (−11%) material like La0.5Sr0.5CoO3 can also suppress the superconductivity to a larger extent. This indicates that the degree of spin polarization of the ferromagnetic electrode is not the only criteria to determine the suppression of superconductivity by pair breaking effect in superconductor/ferromagnet hybrid structures; rather the transparency of the interface for the spin polarization, the formation of vortex state due to the stray field of ferromagnetic layer and the ferromagnetic domain patterns might play significant roles to determine such effect.