Anisotropic superconducting gap in the spin-triplet superconductor Sr2RuO4: evidence from a Ru-NQR study

We have investigated a gap structure in the spin-triplet superconductor Sr2RuO4 through the measurement of the 101Ru nuclear spin-lattice relaxation rate (101)(1/T1) down to 0.09 K at zero magnetic field. In the superconducting state, 1/T1 in a high-quality sample with T(c) approximately 1.5 K exhibits a sharp decrease without the coherence peak, followed by a T3 behavior down to 0.15 K. This result is in marked contrast to the behavior observed below approximately 0.4 K in samples with lower T(c), where T1T is a constant. This behavior is demonstrated to be not intrinsic. We conclude that the gap structure in Sr2RuO4 is significantly anisotropic, consistent with line-node-like models.     

Nmr evidence for coexistence of superconductivity and ferromagnetic component in magnetic superconductor RuSr2YCu2O8: 99,101Ru and 63Cu NMR.

From Ru- and Cu-NMR studies, we present evidence for coexistence of superconductivity and ferromagnetism in a cuprate superconductor RuSr2YCu2O8 (RuY1212). The observation of a large enhancement of a radio-frequency field for the Ru-NMR signal at zero field reveals the existence of a ferromagnetic (FM) component in the ordered RuO2 plane below a Curie temperature of TM = 150 K. Just below the onset temperature of superconductivity T(onset)c = 45 K, a remarkable decrease of the nuclear spin-lattice relaxation rate 1/T1 was observed within the ordered RuO2 plane as well as the CuO2 plane, revealing that the superconducting gap coexists with the FM component in the RuO2 plane on a microscopic scale. In addition, from the observation of a sharp peak in 101(1/T1) at T(zero)c approximately 23 K where the resistivity becomes zero, we suggest that the motion of self-induced vortices originating from fluctuations of the FM component induces the resistivity between T(onset)c and T(zero)c in RuY1212.     

First-order reorientation transition of the flux-line lattice in CaAlSi

The flux-line lattice in CaAlSi has been studied by small-angle neutron scattering. A well-defined hexagonal flux-line lattice is seen just above H(c1) in an applied field of only 54 Oe. A 30° reorientation of this vortex lattice has been observed in a very low field of 200 Oe. This reorientation transition appears to be first-order and could be explained by nonlocal effects. The magnetic field dependence of the form factor is well-described by a single penetration depth of λ=1496(1) ? and a single coherence length of ξ=307(1) ? at 2 K. At 1.5 K, the penetration depth anisotropy is γ(λ)=2.7(1), with the field applied perpendicular to the c axis, and agrees with the coherence length anisotropy determined from critical field measurements.     

In-plane anisotropy of upper critical field in Sr2RuO4

We investigated the behavior of the spin-triplet superconductor Sr2RuO4 ( T(c) approximately 1.5 K) under the magnetic fields parallel to the quasi-two-dimensional plane. The upper critical field H(c2) exhibits a clear fourfold anisotropy of about 3% at 0.35 K. Furthermore, we detected an additional transition feature below H(c2) in both the ac susceptibility and the specific heat. These second-transition features as well as the pronounced in-plane H(c2) anisotropy disappear above 0.8 K or under intentional field misalignment of less than 1 degrees. Most of these characteristics are consistent with the predicted emergence of the second superconducting phase with a line-node gap.     

Anisotropy of magnetothermal conductivity in Sr2RuO4

The dependence of in-plane and interplane thermal conductivities of Sr2RuO4 on temperature, as well as magnetic field strength and orientation, is reported. We found no notable anisotropy in the thermal conductivity for the magnetic field rotation parallel to the conducting plane in the whole range of experimental temperatures and fields, except in the vicinity of the upper critical field H(c2), where the anisotropy of the H(c2) itself plays a dominant role. This finding imposes strong constraints on the possible models of superconductivity in Sr2RuO4 and supports the existence of a superconducting gap with a line of nodes running orthogonal to the Fermi surface cylinder.     

Specific heat and anisotropic superconducting and normal-state magnetization of HoNi2B2C

The magnetization of single-crystal HoNi₂B₂C has been measured as a function of applied field (H) and temperature in order to probe the interplay between superconductivity and magnetism in this complex layered system. The normal-state magnetic susceptibility of HoNi₂B₂C is highly anisotropic with a Curie-Weiss-like temperature dependence for H applied perpendicular to the c-axis and with a much weaker temperature dependence for H applied parallel to the c-axis, indicating that the Ho⁺³ magnetic moments lie predominately in the tetragonal a−b plane below 20 K. High-field magnetization (2000 Oe), low-field magnetization (20 Oe) and zero-field specific heat all give an antiferromagnetic ordering temperature of T_N=5.0 K. Remarkably, in 20 Oe applied field both superconductivity (T_c=8.0 K) and antiferromagnetism (T_N=5.0 K) clearly make themselves manifest in the magnetization data. From these magnetization data a phase diagram in the H−T plane was constructed for both directions of applied field. This phase diagram shows a non-monotonic temperature dependence of H_c2 with a deep minimum at T_N=5 K. The high-field magnetization data for H applied perpendicular to the c-axis also reveal a cascade of three phase transitions for T < 5 K and H < 15 000 Oe, contributing to the rich H versus T phase diagram for HoNi₂B₂C at low temperatures.     

Spin-triplet superconductivity in UNi(2)Al(3) revealed by the (27)Al Knight shift measurement

We report (27)Al Knight shift ( (27)K) measurement on a single-crystal UNi(2)Al(3) that reveals a coexistence of superconductivity and a spin-density-wave (SDW) type of magnetic ordering ( T(SDW) = 4.5 K). The spin part of (27)K, (27)K(s), does not change down to 50 mK across the superconducting (SC) transition temperature T(c) approximately 0.9 K. In contrast with the isostructural compound UPd(2)Al(3) ( T(c) approximately 2 K), which was identified to be a spin-singlet d-wave superconductor, the behavior of (27)K strongly supports that UNi(2)Al(3) , like UPt(3) and Sr(2)RuO(4), belongs to a class of spin-triplet SC pairing state superconductors.     

Magneto-optical study of the superconducting gap of MgB(2) single crystals

We present magneto-optical reflectivity results in the basal plane of the hexagonal MgB(2). The data were collected on a mosaic of MgB(2) single crystals with T(c)=38 K from the ultraviolet down to the far infrared as a function of temperature and magnetic field oriented along the c axis. In the far infrared, there is a clear signature of the superconducting gap with a gap ratio 2 Delta/k(B)T(c) approximately 1.2, well below the weak-coupling value. The gap is suppressed in an external magnetic field, which is a function of temperature. We extract the upper critical field H(c2) along the c axis. The temperature dependence of H(c2) is compatible with the Helfand-Werthamer behavior.     

Temperature dependence of ferromagnetic resonance in permalloy/NiO exchange-biased films

The temperature dependencies of the ferromagnetic resonance (FMR) linewidth and the resonance field-shift have been investigated for NiO/NiFe exchange-biased bilayers from 78 K to 450 K. A broad maximum in the linewidth of 500 Oe, solely due to the exchange-bias, is observed at ≈150 K when the magnetic field is applied along the film plane. When the magnetic field is applied perpendicular to the film plane, the maximum in the linewidth is less pronounced and amounts to 100 Oe at the same temperature. Such a behavior of the FMR linewidth is accompanied with a monotonic increase in the negative resonance field-shift with decreasing temperature. Our results are compared with the previous experimental FMR and Brillouin light scattering data for various ferromagnetic/antiferromagnetic (FM/AF) structures, and suggest that spin dynamics (spin-wave damping and anomalous resonance field-shift) in the FM/AF structures can be described in a consistent way by a single mechanism of the so-called slow-relaxation.     

Magnetic properties of Co–Cu nanowire arrays fabricated in different conditions by SC electrodeposition

Magnetic Co–Cu alloy nanowires with low Cu content were prepared by SC electrodeposition in pores of anodic aluminum oxide templates. The as-deposited Co–Cu nanowires, with a diameter of 15 nm, show distinctive magnetic anisotropy as an applied magnetic field parallel to the axis of nanowires. With increase in the molar ratio of Co and Cu, the coercivity along nanowire axis increases and reaches a maximum value of 1977.5 Oe at the Co/Cu molar ratio of 60:1, but the maximum value of coercivity increases to 1743.6 Oe with the decrease of frequency to 2 Hz.     

Weak magnetic field behavior of photon echo in a LuLiF4:Er3+ crystal

We report the first observation and study of the photon echo in Er³⁺:LuLiF₄. The energy transition is ⁴ I _15/2 → ⁴ F _9/2 (λ = 6536 Å). The density of ErF₃ is 0.025 wt %. The operation temperature is 1.9 K. Measurements were made at low (up to 1200 Oe) and even zero external magnetic fields. We studied the behavior of the photon echo intensity vs. the magnetic field magnitude and direction about the crystal axis C and vs. the laser pulse separation t ₁₂ and observed an exponential growth and then, after a certain plateau, an exponential decrease in the photon echo intensity as a function of magnetic field upon increasing the magnetic field from zero. The parameters describing the exponential growth and decrease are independent of the direction of magnetic field. The value of the magnetic field (～20–200 Oe) at which the echo intensity is maximal and the value of the maximum itself decrease with increasing pulse separation t ₁₂ and the angle Θ between the magnetic field and crystal axis. The echo intensity decreases exponentially with increasing Θ. The parameter describing the exponential decrease is independent of the magnitude of the field. The echo intensity as a function of pulse separation shows exponential decay. The phase relaxation time depends on the magnitude and direction of the magnetic field. T ₂ is equal to 202 ± 16 ns at zero magnetic field. A phenomenological formula is suggested, which qualitatively presents the mentioned dependences, and the polarization properties of the backward photon echo in this crystal are studied. Because the ion of trivalent erbium is an optimum data carrier, the above results show that fine control of the multichannel transfer of processed optical information may be achieved by weak magnetic fields.     

Polarized-neutron scattering study of the cooper-pair moment in Sr2RuO4

We report a study of the magnetization density in the mixed state of the unconventional superconductor Sr2RuO4. On entering the superconducting state we find no change in the magnitude or distribution of the induced moment for a magnetic field of 1 T applied within the Ru O2 planes. Our results are consistent with a spin-triplet Cooper pairing with spins lying in the basal plane. This is in contrast with similar experiments performed on conventional and high- T(c) superconductors.     

Superconductivity in the Einstein solid V Al(10.1)

We used magnetic susceptibility, resistivity and heat capacity measurements to characterize the superconducting state in the Einstein solid V Al(10.1). We find that V Al(10.1) is a weak-coupling, type-II superconductor with T(c)?=?1.53?K and an upper critical field of H(c2)(0)?=?800?Oe. The heat capacity data in the range 0.07?K?相似文献   

Matrix embedded cobalt-carbon nano-cluster magnets: behavior as room temperature single domain magnets

A new type of Co-C nanoparticles is synthesized from CH₂Cl₂ solution of Co₄(CO)_{1 2} by heating up to 210 ^°C in a closed vessel. Transmission electron microscope (TEM) and electron energy loss spectroscopy (EELS) observation show that the particles are embedded in amorphous carbon and their average size is 12 nm. The radial structure function obtained from the extended X-ray absorption fine structure (EXAFS) of the Co K-edge absorption of the Co-C nanoparticles provides a Co-C average distance of 2.08 Å and the Co-Co distances of 3.18 Å and 3.9 (±0.2) Å. The particles exhibit the magnetic hysteresis curve with a coercive force of 200 Oe at 20 K and 260 Oe at 300 K. The temperature dependence of the magnetic susceptibility measured under zero-field cooling and 10 Oe field cooling conditions exhibits the behavior characteristic of a set of single magnetic domain nanomagnets in an amorphous carbon matrix.     

Gd3Ga5-xAlxO12单晶低温磁性的实验研究

采用提拉法制备了Gd₃Ga_5-xAl_xO₁₂(以下简称GGAG)单晶,经X射线粉末照相证实为石榴石型单相结构。在T=1.5—77K温区里,场强从45—60000Oe范围内,沿[111]晶轴方向测量了磁化强度和磁化率,并与本文所用钇镓石榴石Gd₃Ga₅O₁₂(以下简称GGG)和文献[1,2]报道的GGG单晶数据作了比较。实验结果表明:(1)在场强H<500Oe时,磁化率X与温度T的关系遵守居里-外斯定律,其有序温度T_N远低于1.5K;(2)H>10000Oe时,该材料的磁化强度(或磁化率)比GGG高3—4％;(3)在H～30000Oe时,特定温度T_min对应的临界场H_c比GGG约高4000Oe。实验研究表明,采用离子代换法寻找新的性能良好的磁致冷剂的途径是可行的。 关键词：     

Magnetization processes in nanocrystalline gadolinium

The thermal decline in magnetization, M(T), at fixed magnetic field (H) under 'zero-field-cooled' (ZFC) and 'field-cooled' (FC) conditions, the time evolution of ZFC magnetization, M(ZFC)(t), at fixed temperature and field, M(H) hysteresis loops/isotherms, and ac susceptibility have been measured on polycrystalline Gd samples with average grain sizes of d = 12 and 18 nm. The irreversibility in magnetization, M(irr), occurring below a characteristic temperature that reduces with increasing H, is completely suppressed above a grain-size-dependent threshold field, H*. At low fields (H ≤ 100 Oe), M(irr)(T), like the coercive field, H(c)(T), exhibits a minimum at ～16 K and a broad peak at ～50 K before going to zero at T ? T(C) (Curie temperature). At fixed temperature (T < T(C)) and field (H ? H*), where M(irr) is finite, M(ZFC) has a logarithmic dependence on time. The magnetic viscosity (S) at H = 1 Oe and T ≤ 290 K is independent of the measurement time above ～2 ms but for t < 2 ms it is strongly time-dependent. S(T) peaks at T ? T(C) for H = 1 Oe. A magnetic field reduces the peak height and shifts the peak in S(T) to lower temperatures. All the above observations are put on a consistent theoretical footing within the framework of a model in which the intra-grain magnetizations overcome the energy barriers (brought about by the intra-grain and grain-boundary/interfacial magnetic anisotropies) by the thermal activation process. These field- and temperature-dependent energy barriers, that separate the high-energy metastable (ZFC) state from the stable minimum-energy (FC) state, are independent of time for t ? 2 ms and have a very broad distribution. We show that the shape anisotropy plays a decisive role in the magnetization reversal process, and that the magnetocrystalline and magnetostatic fluctuations, prevalent in the grain-boundary and interfacial regions, govern the approach-to-saturation of magnetization in nanocrystalline Gd.     

Field distribution and flux-line depinning in MgB (2)

We report the first observation of the field distribution and flux-line lattice (FLL) depinning in the vortex-state (VS) of a type-II superconductor probed by conduction electron spin resonance (CESR). CESR was performed in MgB (2) (T(c) approximately 39 K) at 4.1 GHz (1455 Oe) and 9.5 GHz (3390 Oe). The field distribution, n(H), and a standard deviation of sigma approximately 14 Oe (at 28 K/4.1 GHz and at 7 K/9.5 GHz) were inferred, respectively, from the distortion and broadening of the CESR in the VS. For both frequencies, the FLL depinning temperature was determined.     

Magnetic anisotropy-induced spin-reorientation in spinel FeCr2S4

In this paper, the low-field magnetic behavior of polycrystalline FeCr₂S₄ was investigated by thermal circling the sample from 5 to 200 K at a constant magnetic field. We show that, upon cooling the sample at 50 Oe to a temperature below the Curie temperature then warming back, the magnetization displays irreversibility between cooling and warming sequence. The irreversible behavior was suppressed gradually with increasing magnetic field. By considering spin-reorientation due to the increase of magnetic anisotropy upon cooling, the irreversible behavior has been explained qualitatively.     

Coexistence of antiferromagnetism and superconductivity near the quantum criticality of the heavy-fermion compound CeRhIn5

We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in a heavy-fermion compound CeRhIn5 under pressure P=1.75 GPa. The onset of the magnetic order is evidenced from a clear split of 115In nuclear quadrupole resonance spectrum due to the spontaneous internal field below the Néel temperature T(N)=2.5 K. Simultaneously, bulk SC below T(c)=2.0 K is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic level.     

Unusual magnetic-field-induced phase transition in the mixed state of superconducting NbSe2

The thermal conductivity kappa in the basal plane of single-crystalline hexagonal NbSe2 has been measured as a function of magnetic field H, oriented both along and perpendicular to the c axis, at several temperatures below T(c). With the magnetic field in the basal plane and oriented parallel to the heat flux we observed, in fields well below H(c2), an unexpected hysteretic behavior of kappa(H) with all the generic features of a first order phase transition. The transition is not manifest in the kappa(H) curves, if H is still in the basal plane but oriented perpendicularly to the heat-flux direction. The origin of the transition is not yet understood.