Spin gap, phase separation and d-wave pairing as revealed by electron spin-lattice relaxation in 123 and 124 YBaCuO systems doped with Gd3+

With an original modulation technique, the Gd³⁺ electron spin-lattice relaxation has been investigated in normal and superconducting states of YBa₂Cu₃O_6+x (123) and YBa₂Cu₄O₈ (124) compounds doped with 1% Gd. In the 123 sample withx = 0.9T _c = 90 K), theT ₁ behavior within 50 <T< 200 K reveals the [1 ? tanh²(Δ/2kT)]/T dependence typical of a spin gap opening with Δ ≈ 240 K. Below 50 K, the exponential slowing down ofT ₁ is limited by the Korringa-like behaviorT ₁ T = const); the same Korringa-like law is found in the 123 sample withx = 0.59 (T _c = 56 K) within the total 4.2–200 K temperature range. This is interpreted in terms of microscopic separation of the normal and superconducting phases allowing for the electron spin cross-relaxation between them. In the 124 sample (T _c = 82 K), the Gd³⁺ relaxation rate below 60 K is found to obey a power lawT _n with an exponentn ≈ 3. Such a behavior (previously reported for nuclear spin relaxation) is indicative of the d-wave superconducting pairing. Additional paramagnetic centers characterized by relatively slow spin-lattice relaxation are found in both 123 and 124 systems. A well-pronounced change in theT ₁ temperature dependence atTT* ≈ 180–200 K is observed for these slowly relaxing centers as well as for the conventional, fast-relaxing Gd³⁺ ions, suggesting microscopic phase separation and a change in the relaxation mechanism due to electronic crossover related with the opening of the spin gap. This hypothesis is supported by some “180 K anomalies” previously reported by other authors.     

Hebel-Slichter peak and superconducting energy gap in Rb3C60 observed by muon spin relaxation

Muon spin relaxation has been observed in both the normal and superconducting states of Rb₃C₆₀ (T _c=29.3K). The field dependence of theT ₁ spin relaxation rate is due to muonium undergoing spin-exchange scattering with conduction electrons, making this the first observation of muonium in a metal. The temperature dependence ofT ₁ ^–1 shows a Hebel-Slichter coherence peak just belowT _c which is not seen in¹³C spin relaxation. The peak can be fit assuming spin relaxation due to interaction with the quasiparticle excitations of a BCS superconductor provided the density of states is broadened relative to that of BCS. Such fits yield a value for the zero temperature energy gap, ₀/k _B , of 53(4)K, consistent with weak-coupling BCS.     

EPR observation of the spin pair correlation above Tc in lightly alkali metal-doped C60 fullerene

The existence of two temperatures: T_p-Cooper pairing and T_c-Bose–Einstein condensation in high temperature superconductors has been stipulated in a lightly potassium-doped C₆₀ by Magnetically Modulated Microwave Absorption. This doping level corresponds to the carrier density greater than the critical one: x>x₁. In case of rubidium lightly doped C₆₀, where the carrier density x was smaller than the critical one: x<x₁, anomalous EPR temperature dependence was observed. The characteristic temperature of bound electron pair formation T_p≈65 K and the energy gap 2Δ/k=30 K were estimated from the temperature dependence of the EPR signal intensity in non-superconducting state. These results suggest that the liquid fermions–liquid bosons transition can be observed as the opening of the spin gap at temperature T_p postulated in Micnas–Ranninger–Robaszkiewicz theory.     

23Na nuclear magnetic resonance investigation of the lattice distortion in α-NaxV2O5

The full temperature dependence of the electric field gradient tensor at the Na sites has been determined by nuclear magnetic resonance (NMR) in the temperature range 8–330 K in α-Na_{x 2}O₅ (x = 0.996). Above the spin-Peierls transition (T _c = 34.7 K), only a single Na site is observed in agreement with the Pmmn space group proposed to describe this compound as the first example of a 1/4-filled ladder system. Below T_c, eight distinct quadrupolar²³Na sites are observed according to the distortion wave vector k_c = (1/2, 1/2, 1/4) previously reported. In addition, the opening of a spin gap is evidenced by a rapid drop of the magnetic hyperfine shift²³K at T_c. The results are discussed in the context of a charge-order-driven spin-Peierls transition.     

Critical nuclear relaxation in cobalt metal

Nuclear magnetic resonance of cobalt metal was investigated in the paramagnetic and ferromagnetic states and in the critical region below T_c. The Knight shift and spin lattice relaxation times were measured in the paramagnetic phase in the solid and liquid states from 1578 K to 1825 K. The resonant frequency, spin-lattice and spin-spin relaxation times were measured in the ferromagnetic phase from room temperature to 1385 K. The main part of (T₁T)^-1 results from fluctuating orbital moments in both phases except near T_c where this process forms the background for critical spin relaxation. The critical exponents for T^-1₁ and for the magnetization in the ferromagnetic state were found to be n' = 0.96 ± 0.07 and β = 0.308 ± 0.012, respectively.     

Photoexcited Eu2+ spin dynamics in EuFe2As2

Employing temperature dependent time-resolved optical femtosecond spectroscopy, we investigated the quasiparticle and Eu²⁺ spin relaxation dynamics in EuFe₂As₂ (EFA). As previously reported in other undoped iron-based pnictides, we observe the quasiparticle relaxation bottleneck due to the charge gap opening in the spin density wave (SDW) state below T _SDW = 189 K. Below the Eu²⁺ antiferromagnetic (AFM) spin ordering temperature, T _AFM = 19 K, we observe another slower relaxation component, which we attribute to the Eu²⁺ AFM order dynamics. The slow dynamics of this component suggests a weak coupling between the Eu²⁺ spins and the carriers in the Fe-d derived bands.     

Nuclear spin relaxation in itinerant electron antiferromagnetic Cr1−xVx system

Nuclear spin relaxation rate T^?1₁ for ⁵¹V in an incommensurate antiferromagnetic Cr_1?xV_x system has been measured in a temperature range between 1.3 and 4.2 K and in a range of magnetic field from 0 to 13.3 kOe by using a field-cycling nuclear magnetic resonance technique. In the (T₁T)^?1 vs x curve a pronounced maximum was observed near the critical concentration (x_c～0.040). Furthermore for alloys with x = 0.038 and 0.040 a deviation from the Korringa relation, T₁T = constant, was observed. The experimental results of (T₁T)^?1 are interpreted in terms of the spin-fluctuation and d-orbital contributions.     

Magnetic behaviour of U(MnxAl1−x)2 compounds

The results of magnetic measurements performed on U(Mn_xAl_1−x)₂ compounds in the temperature range 4.2K < T < 800K are reported. In the low temperature range (T < 200K), UMn₂ shows a Pauli-type paramagnetism. Above 420K a Curie-Weiss behaviour is evidenced. The magnetic properties of U(Mn_xAl_1−x)₂ compounds were analysed assuming a superposition of a temperature dependent term on a Pauli-type contribution, χ_O. The effective moments as well as the χ_O values were determined both in the low (T < 200K) and high (T > 420K) temperature range. The experimental data were discussed considering changes in the band structure and/or quenching of spin fluctuations.     

Dynamic phenomena in layer superconducting cuprate and organic metals

Dynamic electron spin resonance (ESR) and extended x-ray absorption edge fine structure (XAFS) measurements suggest that layered organic metals and cuprate superconductors behave similarly. The response to microwave radiation in a modulated external magnetic field indicates that: (i) triplet state, T * ESR is observed below T_c for both; (ii) the condensation of free spin doublet D to T* occurs above the transition temperature to superconductivity T_c (10 ± 1 K for the organic metal (BEDT-TTF)₃Ta₂F₁₁ and 92 to 12 K for YBa₂Cu₃O_7-δ and its rare earth derivatives); (iii) antiferomagnetic (AF) resonance is detected above T_c for the organic metal. Here the exchange field between the aligned AF domains: J_AF(150 K) = 130.7 mT (153 mK) is greater than the exchange term J(150 K) ≈ 15 mT (20 mK) between free spins (S = 1/2) leading to T* states; the lifetime of AF domains τ_AF decreases below 150 K and resonance is not detected below 44 K (i.e. τ_AF < 10^-10 s) allowing a superconducting transition to appear below 10 K; (iv) the relaxation time τ₁ for the half field, triplet state ESR absorption increases fourfold near 10 K for the organic metal and, (v) the onset of superconductivity is detected in all superconductors by the appearance of an energy loss at exactly H=0 and, magnetization oscillations observed versus H below T_c when the samples are cooled in a non-zero field H. The spin-lattice relaxation time for the organic metal triplet state, half field ESR near 10 K is interpreted using the Gorter phenomenological relation τ₁ = C_H /α_H, C_H and α_H are respectively the heat capacity and the thermal contact coefficient to the lattice by the spin system, at constant field H . Complementary changes in x-ray edge widths near T_c are correlated to electron-phonon interactions.     

Proton diffusion in the superprotonic phase of [(NH4)1 − xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation

Proton diffusion in [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 < x < 1) has been studied by means of ¹H spin-lattice relaxation times, T₁. The relaxation times were measured at 200.13 MHz in the range of 296–490 K and at 19.65 MHz in the range of 300–470 K. In the high-temperature phase (phase I), translational diffusion of the acidic protons relaxes both the acidic protons and the ammonium protons. Spin diffusion averages the relaxation rate of the two kinds of protons, whereas proton exchange between them are slow. The spin-lattice relaxation times in phase I were analyzed theoretically, and parameters of proton diffusion were obtained. The mean residence time of the acidic protons increases with increase in x for [(NH₄)_1 ? xRb_x]₃H(SO₄)₂ (0 ≤ x ≤ 0.54). Rb₃H(SO₄)₂ does not obey this trend. The results of NMR well explain the macroscopic proton conductivity.     

Searching for key parameter for determining Tc in Fe-based superconductors: Study of P/As substitution in RFe(P,As)(O,F) [R=La and Nd]

We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature T_c in RFeP_1−xAs_xO_0.90F_0.10 (x=0-1.0) (R=La and Nd). In contrast to the linear change of the crystal structure with increasing x, the temperature dependence of resistivity and T_c show non-monotonous x-dependence. When the As concentration x is increased, the temperature-dependence of resistivity changes from T² to T-linear, and T_c distinctly increases in all the La compounds and the Nd ones with x<0.60. The results indicate that the substitution of As for P induces the spin fluctuation and resultantly enhances T_c. On the other hand, we could not find any relation between the temperature-dependence of resistivity and T_c in the Nd samples with x>0.60. This may suggest the existence of other parameters for determining T_c besides the antiferromagnetic correlation in this system.     

Proton diffusion in the room-temperature phase of [(NH4)1−xRbx]3H(SO4)2 as studied by 1H spin-lattice relaxation in the rotating frame

Proton diffusion in the room-temperature phase (phase II) of [(NH₄)_1?xRb_x]₃H(SO₄)₂ (0≤x≤1) has been studied by means of ¹H spin-lattice relaxation times in the rotating frame, T_1ρ. The ¹H T_1ρ values were measured at 200.13 MHz in the range of 380–490 K. The ammonium protons and the acidic protons have independent T_1ρ values in the higher temperature range of phase II, suggesting that the spin diffusion between the two species is ineffective. The translational diffusion of the acidic protons is the most dominant mechanism to relax both the ammonium protons and the acidic protons in phase II. The ¹H T_1ρ values in phase II are analyzed theoretically and the motional parameters are obtained. The results of NMR well explain the macroscopic proton conductivity.     

The classification of superconductors using muon spin rotation

In this paper we explore the role of muon spin rotation (μSR) techniques in the characterization and classification of superconducting materials. In particular we focus upon the Uemura classification scheme which considers the correlation between the superconducting transition temperature,T _c, and the effective Fermi temperature,T _F, determined from μSR measurements of the penetration depth. Within this scheme strongly correlated “exotic” superconductors, i.e, hightT _C cuprates, heavy fermions, Chevrel phases and the organic superconductors, form a common but distinct group, characterized by a universal scaling ofT _C withT _F such that 1/100 <T _C/T _F<1/10. For conventional BCS superconductorsT _C/T _F<1/1000. The results of new μSR measurements of the penetration depth in superconducting Y(Ni_1?x Co _x )₂B₂C and YB₆ are also presented. In Y(Ni_1?x Co _x )₂B₂C the decrease ofT _C with increasing Co concentration is linked to a marked decrease in the carrier density from 2.9·10²⁸ m^?3 atx=0 to 0.6·10²⁸ m^?3, atx=0.1, while the carrier mass enhancement remains almost constant at approximately 10. For YB₆ we find evidence of a modest enhancement of the carrier mass (m ^*/m=3), and a relatively low carrier density of 0.24·10²⁸ m^?3. These results are discussed within the Uemura classification scheme. It is found that neither Y(Ni_1?x Co _x )₂B₂C withT _c/T _F>>1/250 nor YB₆ withT _C/T _F>>1/340 can be definitively classified as either “exttic” or “conventional”, but instead the compounds display behavior which interpolates between the two regimes.     

Influence of annealing on magnetic properties of Co-based metallic glasses

The influence of low-temperature annealing on the magnetic hysteresis loop parameters of magnetostrictive Co-Si-B and nonmagnetostrictive Co-Fe-Si-B glasses was studied. The dependence of Curie temperature (T_C), crystallization temperature (T_x), full-stress-relaxation temperature, saturation magnetostriction constant, saturation magnetization and coercive field H_c on metalloids contents for as quenched Co_100-x(Si_0.5B_0.5)_x glasses was determined.It was found that annealing enhances remanence magnetization for all investigated Co-Si-B glasses. The coercive field of these glasses is influenced by annealing owing to: stress relaxation (resulting in an H_c decrease) and domain structure stabilization (resulting in an H_c increase). Significant H_c reduction for both magnetostrictive and non-magnetostrictive glasses was observed, after annealing above T_C. For glasses with T_C<>T_x, it was necessary to apply an external magnetic field in ord er to decrease H_c.It was found that non-magnetostrictive metallic glasses with low Curie temperatures (T_C ? 450 K) exhibit the most stable magnetic hysteresis loop parameters.     

Das kritische magnetische Verhalten von EuO

Mössbauer effect and magnetization measurements were employed in order to study the static and especially the dynamic magnetic properties of the nearly Heisenberg ferromagnet EuO near its Curie temperature,T _c=69.2 °K. The critical exponent β of the spontaneous magnetization was determined to be β=0.34±0.02. It was shown that critical slowing down of spin fluctuations takes place nearT _c with spin relaxation times between 7×10^?11 sec (T=1.01T _c) and 1.5×10^?1 sec (T=1.03T _c). The experimental values of the relaxation time were found to be in satisfactory agreement with theoretically computed ones. Just belowT _c the Mössbauer spectra exhibit relaxation effects, which are characteristic for the occurence of critical super-paramagnetism. Investigations of several samples indicated quantitatively, that critical superparamagnetism has its origin in the non ideal composition of the real crystal.     

Far-infrared probe of energy gap in LaO1−xFxFeGe compound

We report on the electrical resistivity and far-infrared reflectance measurements of LaO_1?xF_xFeGe samples. Furthermore, we introduce a new method to probe the energy gap and determine its value. The onset transition temperature was 22.8 K for x = 0.13, and a clear anomaly was observed at 90 K in the ρ(T) curve for x = 0.11 with T_c = 20.6 K. We clearly observed the phonon-suppressed feature in reflectance spectra where F-doping caused a strong suppression of a peak at 200 cm^?1. The energy gap above T_c, 2Δ = 2.10 meV, was determined from the measured spectra based on the changes in reflectivity by F-doping.     

Spin susceptibility of neutron-irradiation-disordered YBa2Cu3O6.9 in the normal and superconducting states

The spin-spin relaxation rate ⁶³ T ₂ ⁻¹ of ⁶³Cu nuclei in CuO₂ layers is measured in the normal and superconducting states of the compound YBa₂Cu₃O_6.9 (T _c ^onset =94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to T _c ^onset =68 K (Φ=7×10¹⁸ cm⁻²) and T _c ^onset <4 K (Φ=12×10¹⁸ cm⁻²). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction ⁶³ T _2G ⁻¹ , which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χ_s(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability of the x ²−y ² symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing for high-T _c cuprates. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 172–177 (10 February 1998)     

Doping dependence of the superconducting gap by Andreev reflection in Au/La2−xSrxCuO4 point-contact junctions

We studied the doping dependence of the superconducting gap in La_2−xSr_xCuO₄ (LSCO) by means of Andreev reflection measurements in Au/LSCO point-contact junctions. The Andreev reflection features were found to disappear at T_c^A close to the bulk T_c. The fit of the conductance curves with the BTK-Tanaka-Kashiwaya model gives good results if a (s+d)-wave gap symmetry is used. The low-temperature dominant isotropic gap component Δ_s follows very well the T_c vs. x curve, while the gap-like features observed by angle-resolved photoemission spectroscopy and tunneling scale with T^∗. This confirms the different origin of these two energy scales at T<T_c.     

Ferromagnetic resonance in the randomly mixed insulating ferromagnet and antiferromagnet Rb2Mn(1-x)CrxCl4

An electron spin resonance experiment has been performed on the Cr-rich concentrations of the random mixture Rb₂Mn(_1-x)Cr_xCl₄ of an insulating ferromagnet (Rb₂CrCl₄) and an insulating antiferromagnet (Rb₂MnCl₄). The resonance fields in x = 0.6, 0.7 and 0.8 samples begin to shift towards low field side at temperatures well above the Curie temperatures, when the external field is applied in the c-plane of the crystal. The shift of the resonance field with temperature of the x = 0.7 sample agreed well with that of the x = 0.8 sample, after scaling the temperature axis, while the shift in the x = 0.6 crystal did not. From this observation, we argue that the x = 0.7 and x = 0.8 samples have a long-range ordered ferromagnetic phase below T_c, and that the low temperature phase of the x = 0.6 sample is not a truely long-range ordered one. A weak resonance line was observed in the x = 0.8 sample below about 20 K. This resonance is discussed in connection with the re-entrant spin-glass behavior of this mixed system found in the ac susceptibility measurement.     

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La_1−xSr_x)₂CuO_4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature T_c ≲ 38 K and narrow transition width ΔT_c. For the sample with the maximum T_c among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔV_s(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏω_op/k_B ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high T_c of this material.