Coexistence of Superconductivity and Ferromagnetism in Electron and Nuclear Magnets

A phenomenological model uniting the London model of superconductivity and the micromagnetic approach to ferromagnetism is applied to new examples of coexistence of ferromagnetism and superconductivity revealed in recent experiments: (i) Type-I superconductivity in a nuclear ferromagnet AuIn ₂ ; (ii) Type-II high-Tc superconductivity in a compound R _1.5 Ce _0.5 RuSr ₂ Cu ₂ O ₁₀ which is weakly ferromagnetic.     

Ferromagnetic Superconductors

The coexistence of itinerant electron ferromagnetism and superconductivity in UGe ₂ and ZrZn ₂ raises the question if itinerant ferromagnetism is a universal precondition for certain forms of superconductivity. The d.c. magnetization of both compounds provides evidence of nearly critical, longitudinal spin fluctuations in the ferromagnetically ordered regime where superconductivity is observed. When taken together with key characteristics of the superconductivity, this is consistent with magnetically mediated pairing in these materials.     

Superconductivity and Ferromagnetism of the Ru-1232 Compounds in the (Ru1?xNbx)Sr2(GdCe1.8Sr0.2)Cu2Oz System

The Ru-1232 compounds have been synthesized in the (Ru_1–xNb _x )Sr₂(GdCe_1.8Sr_0.2)Cu₂O _z system, and effects of Nb substitution for Ru on superconductivity and ferromagnetism of the Ru-1232 compounds have been investigated. First, X-ray powder diffraction study shows that nearly the single 1232 phase samples can be obtained in the x composition range from 0.0 to 0.3. Then, from the electrical resistivity study, it is found that each of the samples shows resistivity dropping phenomenon at two temperatures of T _c ^l and T _c ^h, which originates from superconductivity of the Ru-1232 phase and the Ru-1222 one, respectively. Both of the starting temperatures are lowering with increasing Nb content x. Lastly, from the magnetic susceptibility study, it is found that superconducting transition temperature T _c is 20 K for the Ru-1232 sample with x = 0.0 and the ferromagnetic transition temperature T _m is about 90 K. This study also shows that both of the values of T _c and T _m become low with increasing x from 0.0 to 0.3.     

Theoretical Study of Magnetic Behavior in Ru-based Ferromagnetic Superconductors

A microscopic theory of interplay of superconductivity and ferromagnetism in hybrid rutheno-cuprate superconductors RuSr₂RECu₂O₈ (Ru-1212) (RE=Gd, Eu) is developed from first principles considering s–f model. Self-consistent equations for superconducting order parameter Δ and ferromagnetic order parameter 〈S ^Z 〉 are obtained using Green’s function technique and equation of motion method. The theory is applied to explain the experimental results for RuSr₂RECu₂O₈ system. This model clearly predicts the coexistence of superconductivity and spatially uniform ferromagnetism in RuSr₂RECu₂O₈. The theory shows that it is possible to become superconducting if the system is already ferromagnetic. Study of specific heat, density of states, free energy, critical field, and ac spin susceptibility is also presented. Agreement between experimental results and theory is quite convincing.     

Thermodynamic, transport and magnetic properties of the ferromagnetic superconductor RuSr2GdCu2O8 and related compounds

The transport, magnetic and thermodynamic properties of RuSr ₂ GdCu ₂ O ₈ and related compounds are reviewed. These indicate uniform coexistence of superconductivity (T _c =46K) and ferromagnetism (T _M =132K) in which the former arises in the CuO ₂ planes and the latter in the RuO ₂ planes. The earliest data indicated a typical underdoped cuprate with local Ru moment ferromagnetism, the absence of pairbreaking and transport properties dominated by the CuO ₂ planes. However, recent results suggest a more complex scenario of itinerant ferromagnetism and exchange coupling between the CuO ₂ and RuO ₂ layers.     

Critical Temperatures in Ferromagnetic-Superconducting All-Oxide Superlattices

Ferromagnetic/superconducting superlattices represent a new class of materials with the simultaneous occurrence of superconductivity and ferromagnetism. The mutual interaction of these antagonistic ordering phenomena is of vital fundamental interest and opens novel possibilities for spin-injection devices. Therefore, we systematically studied YBa₂Cu₃O₇-La_2/3Ca_1/3MnO₃ superlattices of different modulation lengths especially with respect to the reduced phase transition temperatures to ferromagnetism and superconductivity, respectively. Conventional models to explain the reduction of T _C and T _Curie fail and novel concepts giving rise to a long-range proximity effect have to be introduced. Furthermore, it is suggested that the pseudogap opening of the YBa₂Cu₃O₇ weakens the innerlayer ferromagnetic coupling of the La_2/3Ca_1/3MnO₃ layers, thus contributing to the reduction of T _Curie.     

Coexistence of Superconductivity and Ferromagnetism

An overview of the problem of the coexistence of superconductivity and ferromagnetism in uranium-based superconductors such as UGe₂, URhGe, UCoGe, UIr and ZrZn₂ will be briefly presented. Starting with a pairing Hamiltonian with equal spin superconducting triplet pairs and using quantum field theory Green function formalism, the coexistent ferromagnetic order with superconductivity will be shown to be a very distinct possibility. The singlet superconductivity coexisting with ferromagnetism is against the experimental observations. It will be argued that the superconductivity can be destroyed at two points as a result of magnetic fields both intrinsic and or extrinsic, and thus establishing reentrant superconductivity as observed most recently in URhGe. In the absence of consensus on the mechanism, the same is not highlighted although the magnetic quantum fluctuations emanating from quantum critical point could provide the pairing. The limitations of the model will also be discussed.     

Superconductivity of FeSr2YCu2O6+δ

Resistivity measurements of polycrystalline FeSr₂YCu₂O_6+δ under magnetic fields up to 160 kOe were made to study the superconductivity of FeSr₂YCu₂O_6+δ in detail. The resistivity began to decrease at 64 K and dropped to zero at 38 K under zero magnetic field. The superconductivity in inter grain was affected by the magnetic field and zero resistivity was observed below 12 K under H=10 kOe. Above 20 kOe, the superconductivity in the grain began to be affected. Even with increasing magnetic field up to 160 kOe, zero resistivity was persisted below 10 K.     

Structural and Electrical Transport Properties of Al1?xMxB2 (M = Li, Vacancy)

AlB₂ and MgB₂ are neighboring compounds and have similar crystal structures. However, their superconducting properties are very different because the holes which play a key role in the superconductivity of MgB₂ do not exist in AlB₂. In this paper we reported our attempts to dope holes into AlB₂ by substituting Li and vacancy for Al, and these holes were expected to induce superconductivity in the system. However, experimental results showed that Li was hard to substitute Al in AlB₂. The variation of resistivity with temperature and x for samples with the nominal composition of Al_1–xLi _x B₂ was analyzed by considering the phase variation of the samples with x. The temperature dependence of resistivity for Al_1–xB₂ was also studied and orders of magnitude change in resistivity was observed when x was up to 0.4 and 0.5, similar to that observed in Mg_1–xB₂ system by Sharma et al. (A. Sharma, N. Hur, Y. Horibe, C. H. Chen, B. G. Kim, S. Guha, Marta Z. Cieplak, and S. W. Cheong, Phys. Rev. Lett. 89, 167003 (2002)). It is proposed that phase separation may exist in Al_1–xB₂, leading to the dramatic change of resistivity because of percolation.     

Effect of Y Doping on Superconductivity and Spin-Density-Wave States in Sm1?xYxFeAsO0.8F0.2 and Sm1?xYxFeAsO

Y doping effects on crystal structure, resistivity, superconductivity, and spin-density-wave states in the parent and F-doped SmFeAsO compounds have been studied. Y doping leads to shrinkage of the lattice parameters in both systems. The superconductivity of Sm_1−x Y _x FeAsO_0.8F_0.2 is suppressed by Y doping with onset T _c decreasing monotonically from 52.6 K at x=0 to 25 K at x=0.5. A weak resistivity anomalies around 120 K ascribed to the spin-density-wave instability is observed in the sample of x=0.5. In parent SmFeAsO compound, the resistivity anomaly temperature was detected to be around 150 K and shifted towards lower temperatures with the Y doping level increased. It is concluded that a negative pressure effect takes responsibilities for the decrease of T _c in Sm_1−x Y _x FeAsO_0.8F_0.2.     

Neodymium-Doping Induced Superconductivity in 1111-SrFeAsF Iron-Pnictide System

Polycrystalline Sr_1−x Nd _x FeAsF samples at various Nd-doping levels have been prepared by conventional solid-state synthesis close to ambient pressure. Susceptibility and resistivity were studied down to 4 K showing superconductivity competing with a low-temperature magnetic ordering and probably the coexistence of both, well into the superconducting region. The temperature dependence of the resistivity of all the Nd-doped samples clearly show the presence of this transition below 15 K, most likely originating from the magnetic ordering of Nd moments. This transition is gradually attenuated but not completely suppressed even at optimum-doping region at x≈0.54. The spin-density wave anomaly at 175 K survives up to 0.35 Nd-doping, while superconductivity occurs only above x=0.40. The maximum onset transition temperature, T _c , reaches as high as 54.3 K for x=0.70, but superconductivity disappears beyond that. The estimated upper critical fields, H _c2, are high, with values exceeding 98 Tesla obtained from two samples of different doping level.     

Enhanced Superconductivity of the Pb-Based 1212 Cuprates in the (Pb0.75S0.25)(Sr2?xBax)(Y0.4Ca0.6)Cu2Oz System by High-Pressure O2 Annealing

We have already reported on substitution effects of Ba for Sr in the sulfur contained Pb-based 1212 compound with the composition of (Pb_0.75S_0.25)(Sr_2−x Ba _x ) (Y_0.4Ca_0.6)Cu₂O _z . It was found that each sample with x=0.3–0.6 showed a resistivity drop originating from superconductivity, and the highest onset temperature of the drop was about 32 K for a sample with x=0.5, but the volume fraction was very small. More recently, we have investigated effect of high-pressure O₂ annealing on superconductivity of the Ba-substituted samples with x=0.4–0.6. As a result, the sample with x=0.5 is found to show an onset of the resistivity drop at the highest temperature of about 41.7 K and it shows an onset of the diamagnetic signal at about 42.0 K. These transition temperatures are higher by ∼10 K than those of the previously reported sample with x=0.5. Moreover, the superconducting volume fraction is increasing more by the high-pressure O₂ annealing. These results indicate that the high-pressure O₂ annealing effectively promotes superconductivity of the present (Pb_0.75S_0.25)(Sr_2−x Ba _x )(Y_0.4Ca_0.6)Cu₂O _z system.     

Controlled Creation of Structural Defects in the Heavy Fermion Superconductor UPt3 and Its Influence on the Superconducting Properties

The superconducting properties of the heavy fermion UPt ₃ have been changed by irradiation with high energy electrons which creates point defects in a reproducible and controled way. Measurements of the residual resistivity, critical temperature, upper critical field and thermal conductivity have been realized on these irradiated samples. The strong suppresion of superconductivity with increasing defect concentration is in agreement with the theory of unconventional superconductivity. However, our thermal conductivity data contradicts the simple predictions derived from the most popular modelsE _1g and E _2u) of the superconducting order parameter in UPt ₃ .     

New Pb-based 1212 Cuprate Superconductors Containing Sulfur, (Pb0.75S0.25)Sr2 (Y1?xCax)Cu2Oz

New Pb-based 1212 layered cuprates containing sulfur have been synthesized in the (Pb_0.75S_0.25) Sr₂ (Y_1−x Ca _x ) Cu₂O _z system. X-ray diffraction analysis shows that the almost single-phase samples are obtained within a region of 0.0≤x≤0.5. The crystal structure of each sample has a tetragonal symmetry with the typical lattice constants a=0.3837 nm and c=1.186 nm. As Ca content x is increasing, the semiconductor-like behavior is suppressed. But after only annealing under ambient O₂ pressure, none of the samples show any trace of superconductivity. On the other hand, when the samples are annealed under high O₂ pressure of about 13.6 MPa, they show resistivity dropping phenomenon in a region of 0.5≤x≤0.7. Among them, the lowest resistivity sample with x=0.6 has an onset temperature of the resistivity dropping at about 22.5 K. Moreover, this sample shows a diamagnetic signal at about 21.5 K. These phenomena are attributed to superconductivity.     

Possibility of Superconductivity of the 2212 Phase in the Bi(Pb)-Sr-Ce-Cu-O System

Synthesis of the Bi-2212 compound in the Bi-Sr-Ce-Cu-O system has been already known. But, there has been no report on superconductivity of the compound yet. We have prepared many Bi-2212 samples partially substituted by Pb for Bi in the Bi-Sr-Ce-Cu-O system. The nominal composition is (Bi_2−y Pb _y )Sr₂(Sr_1−x Ce _x )Cu₂O _z . Then, we have investigated possibility of superconductivity for the samples. As a result, we find that a sample with nominal composition ofx=0.23 andy=0.1, which is of almost the single 2212 phase, shows an anomaly at about 70 K in addition to temperature dependence of the resistivity like a semiconductor. Furthermore, the sample also shows a decrease of magnetic susceptibility starting at about 70 K with decreasing temperature. These experimental results can be considered to result from superconductivity of the 2212 phase in the Bi(Pb)-Sr-Ce-Cu-O system.     

Magnetoresistance of Sr1−x K x BiO3: a second-family of bismuth-oxide-based superconductors

The superconducting Sr _1–x K _x BiO ₃ samples with x = 0.45 – 0.6 were synthesized by the high-pressure-high-temperature technique in a belt type apparatus (2 GPa, 700 °C, 1 h, Pt capsules) with stoichiometric mixtures of Sr ₂ Bi ₂ O ₅ , Bi ₂ O ₃ and KO ₂ as described earlier. ¹ The X-ray diffraction results appear as a single perovskite-like phase. The superconductivity occurs at T _c 12 K in the A. C. susceptibility measurement. The onset (zero resistivity) temperature of superconductivity in resistivity measurement of the investigated sample was . The transition temperature region was a little bit broad and a shoulder was present about 11.3 K indicating probably the existence of crystallites of different K content. The particularly interesting point is that the resistance begins to reappear at T < 6 K at zero magnetic field. As the external magnetic field is applied, the reentrant resistance disappears and superconductivity is recovered until the applied magnetic field becomes higher than 0.65 Tesla. The superconductivity for T < 6 K is destroyed for the higher magnetic field. The T_c onset decreases as the magnetic field increases like in the BCS type superconductors. The transition region becomes broader under the magnetic field, which indicate a kind of vortex transition as in the case of high T _c cuprates.     

Possibility of Superconductivity of the 2212 Phase in the Bi(Pb)-Sr-Ce-Cu-O System

Synthesis of the Bi-2212 compound in the Bi-Sr-Ce-Cu-O system has been already known. But, there has been no report on superconductivity of the compound yet. We have prepared many Bi-2212 samples partially substituted by Pb for Bi in the Bi-Sr-Ce-Cu-O system. The nominal composition is (Bi_{2− y} Pb _y )Sr₂(Sr_{1− x} Ce _x )Cu₂O _z . Then, we have investigated possibility of superconductivity for the samples. As a result, we find that a sample with nominal composition of x=0.23 and y=0.1, which is of almost the single 2212 phase, shows an anomaly at about 70 K in addition to temperature dependence of the resistivity like a semiconductor. Furthermore, the sample also shows a decrease of magnetic susceptibility starting at about 70 K with decreasing temperature. These experimental results can be considered to result from superconductivity of the 2212 phase in the Bi(Pb)-Sr-Ce-Cu-O system.     

Magnetic-Field Induced Superconductor–Antiferromagnet Transition in Lightly Doped RBa2Cu3O6+x (R = Lu, Y) Crystals

The remarkable sensitivity of the c-axis resistivity and magnetoresistance in cuprates to the spin ordering is used to clarify the doping-induced transformation from an antiferromagnetic (AF) insulator to a superconducting (SC) metal in RBa₂Cu₃O_6+x (R = Lu, Y) single crystals. The established phase diagram demonstrates that the AF and SC regions apparently overlap: The superconductivity in RBa₂Cu₃O_6+x , in contrast to La_2−x Sr _x CuO₄, sets in before the long-range AF order is completely destroyed by hole doping. Magnetoresistance measurements of superconducting crystals with low T _c ≤15–20 K give a clear view of the magnetic-field induced superconductivity suppression and recovery of the long-range AF state. What still remains to be understood is whether the AF order actually persists in the SC state or just revives when the superconductivity is suppressed, and in the former case, whether the antiferromagnetism and superconductivity reside in nanoscopically separated phases or coexist on an atomic scale.     

Superconducting properties of the Bi-Ca-Sr-Cu-O system

The electrical properties of unquenched and rapidly quenched bulk samples of granular Bi₂-Ca₁-Sr₂-Cu₂-O _x system have been studied. Electrical resistivity measurements show superconductivity transition temperature (R=0) at 72 K, 80 K and 90·5 K. X-ray crystallographic studies show that most samples were of single-phase.     

Effect of Pressure on Transport Properties of Heavy Fermion CePtSi<Subscript>2</Subscript>

We measured the resistivity of heavy fermion CePtSi₂ under pressure. At ambient pressure, CePtSi₂ shows an antiferromagnetic (AF) transition at 2 K and a Fermi liquid like T ² dependence in resistivity below 1.5 K. With increasing pressure, the AF phase and T ² dependence are suppressed. Above 1.4 GPa, a T linear dependence and pressure-induced superconductivity were found with the maximum T _c=0.14 K at 1.7 GPa. Above 2 GPa, the T ² dependence recovers just above T _c.