Anisotropic critical magnetic fluctuations in the ferromagnetic superconductor UCoGe

We report neutron scattering measurements of critical magnetic excitations in the weakly ferromagnetic superconductor UCoGe. The strong non-Landau damping of the excitations we observe, although unusual, has been found in another related ferromagnet, UGe(2) at zero pressure. However, we also find that there is a significant anisotropy of the magnetic correlation length in UCoGe that contrasts with an almost isotropic length for UGe(2). The values of the magnetic correlation length and damping are found to be compatible with superconductivity on small Fermi-surface pockets. The anisotropy may be important to explain why UCoGe is a superconductor at zero pressure while UGe(2) is not.     

Superconductivity induced by longitudinal ferromagnetic fluctuations in UCoGe

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)～2.5 K and T(SC)～0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.     

Extremely high upper critical magnetic field of the noncentrosymmetric heavy fermion superconductor CeRhSi3

We report an extremely high upper critical field B(c2) in the noncentrosymmetric heavy fermion superconductor CeRhSi3 for a magnetic field B along the tetragonal c axis. B(c2)(T=0) possibly reaching 30 T is much higher than B(c2)(0)=7 T for B perpendicular c and greatly exceeds the paramagnetic limiting field. The strong anisotropy of B(c2)(0) with extremely high B(c2)(0) for B || c is qualitatively explained by the paramagnetic pair-breaking mechanism specific to the noncentrosymmetric superconductor. However, an unusual B(c2)(T) curve with a positive curvature for B || c cannot be explained satisfactorily by conventional orbital pair-breaking models.     

Probing the superconducting gap symmetry of PrOs4Sb12: a penetration depth study

We report measurements of the magnetic penetration depth lambda in single crystals of PrOs4Sb12 down to 0.1 K, with the ac field applied along the a, b, and c directions. In all three field orientations, lambda approximately T2 and superfluid density rho(s) approximately T2 for T<0.3T(c). Data are best fit by the 3He A-phase-like gap with multidomains, each having two point nodes along a cube axis, and parameter Delta(0)(0)/k(B)T(c)=2.6, suggesting that PrOs4Sb12 is a strong-coupling superconductor with two point nodes on the Fermi surface. We also confirm the double transitions at 1.75 and 1.85 K seen in other measurements.     

Superconductivity on the border of weak itinerant ferromagnetism in UCoGe

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. Magnetization measurements show that UCoGe is a weak ferromagnet with a Curie temperature T(C)=3 K and a small ordered moment m(0)=0.03 micro(B). Superconductivity is observed with a resistive transition temperature T(s)=0.8 K for the best sample. Thermal-expansion and specific-heat measurements provide solid evidence for bulk magnetism and superconductivity. The proximity to a ferromagnetic instability, the defect sensitivity of T(s), and the absence of Pauli limiting, suggest triplet superconductivity mediated by critical ferromagnetic fluctuations.     

Effect of annealing on the magnetic and superconducting properties of single-crystalline UCoGe

Single-crystals of the new ferromagnetic superconductor UCoGe have been grown. The quality of as-grown samples can be significantly improved by a heat-treatment procedure, which increases the residual resistance ratio (RRR) from ∼5 to ∼30. Magnetization M(T) and resistivity ρ(T) measurements show the annealed samples have a sharp ferromagnetic transition with a Curie temperature T_C is 2.8 K. The ordered moment of 0.06 μ_B is directed along the orthorhombic c-axis. Superconductivity is found below a resistive transition temperature T_s=0.65 K.     

Anisotropic magnetic fluctuations in the ferromagnetic superconductor UCoGe studied by direction-dependent 59Co NMR measurements

We have carried out direction-dependent 59Co NMR experiments on a single crystal sample of the ferromagnetic superconductor UCoGe in order to study the magnetic properties in the normal state. The Knight-shift and nuclear spin-lattice relaxation rate measurements provide microscopic evidence that both static and dynamic susceptibilities are ferromagnetic with strong Ising anisotropy. We discuss that superconductivity induced by these magnetic fluctuations prefers spin-triplet pairing state.     

Heavy fermion superconductivity and magnetic order in noncentrosymmetric CePt3Si

CePt3Si is a novel heavy fermion superconductor, crystallizing in the CePt3B structure as a tetragonally distorted low symmetry variant of the AuCu3 structure type. CePt3Si exhibits antiferromagnetic order at T(N) approximately 2.2 K and enters into a heavy fermion superconducting state at T(c) approximately 0.75 K. Large values of H(')(c2) approximately -8.5 T/K and H(c2)(0) approximately 5 T refer to heavy quasiparticles forming Cooper pairs. Hitherto, CePt3Si is the first heavy fermion superconductor without a center of symmetry.     

Enhancement of the quantum-liquid phase by increased resistivity in thick a-MoxSi1-x films

Effects of normal-state resistivity rho(n) on the vortex phase diagram at low temperature T have been studied based on dc and ac complex resistivities for thick amorphous MoxSi(1-x) films. It is commonly observed irrespective of rho(n) that, in the limit T=0, the vortex-glass-transition line B(g)(T) is independent of T and extrapolates to a field below the T=0 upper critical field B(c2)(0), indicative of the quantum-vortex-liquid (QVL) phase in the regime B(g)(0)相似文献   

Towards the identification of a quantum critical line in the (p, B) phase diagram of CeCoIn5 with thermal-expansion measurements

The low-temperature thermal expansion of CeCoIn(5) single crystals measured parallel and perpendicular to magnetic fields B oriented along the c axis yields the volume thermal-expansion coefficient β. Considerable deviations of β(T) from Fermi-liquid behavior occur already within the superconducting region of the (B, T) phase diagram and become maximal at the upper critical field B(c2)(0). However, β(T) and the Grüneisen parameter Γ are incompatible with a quantum critical point at B(c2)(0), but allow for a quantum criticality shielded by superconductivity and extending to negative pressures for B相似文献   

Evidence for strong-coupling s-wave superconductivity in MgB2: (11)B NMR Study

We have investigated a gap structure in a newly discovered superconductor, MgB2, through measurement of the (11)B nuclear spin-lattice relaxation rate, (11)(1/T(1)). (11)(1/T(1)) is proportional to the temperature (T) in the normal state, and decreases exponentially in the superconducting (SC) state, revealing a tiny coherence peak just below T(c). The T dependence of 1/T(1) in the SC state can be accounted for by an s-wave SC model with a large gap size of 2Delta/k(B)T(c) approximately 5 which suggests it is in a strong-coupling regime.     

PdTe: a strongly coupled superconductor

We report the electrical transport, magnetic, and thermodynamic properties of polycrystalline PdTe which exhibits superconductivity below 4.5 K. Using the measured values for the lower (H(c1)) and upper (H(c2)) critical fields, and the specific heat C(p), we estimate the thermodynamic critical field H(c)(0), coherence length ξ(0), penetration depth λ(0), and the Ginzburg-Landau parameter κ. Compared with band structure calculations, the density of states at the Fermi level is enhanced due to electron-phonon coupling with λ(ep) = 1.4. Furthermore, the large values of ΔC(p)/γ(n)T(c) and 2Δ(0)/k(B)T(c) suggest that PdTe is a strongly coupled superconductor.     

Yang-Lee zeros of the antiferromagnetic Ising model

There exists the famous circle theorem on the Yang-Lee zeros of the ferromagnetic Ising model. However, the Yang-Lee zeros of the antiferromagnetic Ising model are much less well understood than those of the ferromagnetic model. The precise distribution of the Yang-Lee zeros of the antiferromagnetic Ising model only with nearest-neighbor interaction J on LxL square lattices is determined as a function of temperature a=e(2betaJ) (J<0), and its relation to the phase transitions is investigated. In the thermodynamic limit (L-->infinity), the distribution of the Yang-Lee zeros of the antiferromagnetic Ising model cuts the positive real axis in the complex x=e(-2betaH) plane, resulting in the critical magnetic field +/-H(c)(a), where H(c)>0 below the critical temperature a(c)=square root of 2-1. The results suggest that the value of the scaling exponent y(h) is 1 along the critical line for a相似文献   

Experimental evidence of s-wave superconductivity in bulk CaC6

The temperature dependence of the in-plane magnetic penetration depth, lambda(ab)(T), has been measured in a c-axis oriented polycrystalline CaC(6) bulk sample using a high-resolution mutual inductance technique. A clear exponential behavior of lambda(ab)(T) has been observed at low temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the standard BCS theory yields lambda(ab)(0) = (720 +/- 80) A and delta(0) = (1.79 +/- 0.08) meV. The ratio 2delta(0)/k(B)T(c) = (3.6 +/- 0.2) gives indication for a weakly coupled superconductor.     

半导体/磁性d波超导隧道结中的散粒噪声

考虑到磁性d波超导体中的能隙与磁交换能的依赖关系,通过求解Bogoliubov-de Gennes(BdG)方程,利用Blonder-Tinkham-Klapwijk理论方法研究半导体/磁性d波超导隧道结中的散粒噪声.计算结果表明:磁性d波超导结中的磁交换能h0可导致散粒噪声在零偏压处的双峰和能隙处的峰出现劈裂,劈裂的宽度为2h0;磁交换能h0同时对散粒噪声及噪声功率与平均电流的比值有抑制作用.     

Muon-spin-rotation measurements of the penetration depth of the infinite-layer electron-doped Sr0.9La0.1CuO2 cuprate superconductor

Muon-spin-rotation (muSR) measurements of the in-plane penetration depth lambda(ab) have been performed in the infinite-layer electron-doped Sr0.9La0.1CuO2 high-T(c) superconductor (HTS). Absence of the magnetic rare-earth ions in this compound allowed us to measure for the first time the absolute value of lambda(ab)(0) in electron-doped HTSs using muSR. We found lambda(ab)(0)=116(2) nm. The zero-temperature depolarization rate sigma(0) proportional, variant 1/lambda(2)(ab)(0)=4.6(1) micros(-1) is more than 4 times higher than expected from the Uemura line. Therefore, this electron-doped HTS does not follow the Uemura relation found for hole-doped HTSs.     

Flux flow of Abrikosov-Josephson vortices along grain boundaries in high-temperature superconductors

Low-angle grain boundaries (GBs) in superconductors exhibit intermediate Abrikosov vortices with Josephson cores, whose length l along GB is smaller than the London penetration depth, but larger than the coherence length. We found an exact solution for a periodic vortex structure moving along GBs in a magnetic field H and calculated the flux flow resistivity R(F)(H), and the nonlinear voltage-current characteristics. The predicted R(F)(H) dependence describes well our experimental data on 7 unirradiated and irradiated YBa(2)Cu(3)O(7) bicrystals, from which the core size l(T), and the intrinsic depairing density J(b)(T) on nanoscales of a few GB dislocations were measured for the first time. The observed J(b)(T) = J(b0)(1-T/T(c))(2) indicates a significant order parameter suppression on GB.     

Spin drift velocity, polarization, and current-driven domain-wall motion in (Ga,Mn)(As,P)

Current-driven domain-wall motion is studied in (Ga,Mn)(As,P) ferromagnetic semiconducting tracks with perpendicular anisotropy. A linear steady state flow regime is observed over a large temperature range of the ferromagnetic phase (0.1T(c)相似文献   

Boron isotope effect in superconducting MgB2

We report the preparation method of and boron isotope effect for MgB2, a new binary intermetallic superconductor with a remarkably high superconducting transition temperature T(c)(10B) = 40.2 K. Measurements of both temperature dependent magnetization and specific heat reveal a 1.0 K shift in T(c) between Mg11B2 and Mg10B2. Whereas such a high transition temperature might imply exotic coupling mechanisms, the boron isotope effect in MgB2 is consistent with the material being a phonon-mediated BCS superconductor.     

Proximity effect in multiterminal hybrid structures

We consider the proximity effect in multiterminal ferromagnet/superconductor (FSF) hybrid structures in which two or three electrodes are connected to a superconductor. We show that two competing effects take place in these systems: (i) pair breaking effects due to the response to the exchange field induced in the superconductor; (ii) a reduction of the superconducting order parameter at the interface that takes place already in NS junctions. We focus on this second effect that dominates if the thickness of the S layer is small enough. We consider several single-channel electrodes connected to the same site. We calculate the superconducting order parameter and the local density of state (LDOS). With two ferromagnetic electrodes connected to a superconductor we find that the superconducting order parameter in the ferromagnetic alignment is larger than the superconducting order parameter in the antiferromagnetic alignment ( > ), in agreement with [Eur. Phys. J. B 25, 373 (2002)]. If a third spin polarized electrode is connected to a superconductor we find that - can change sign as the transparency of the third electrode increases. This can be understood from the fact that the superconducting order parameter is reduced if pair correlations among the ferromagnetic electrodes increase. If the two ferromagnetic electrodes are within a finite distance we find Friedel oscillations in the Gorkov function but we still obtain > .