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1.
We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature Tc in RFeP1−xAsxO0.90F0.10 (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 Tc show non-monotonous x-dependence. When the As concentration x is increased, the temperature-dependence of resistivity changes from T2 to T-linear, and Tc 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 Tc. On the other hand, we could not find any relation between the temperature-dependence of resistivity and Tc in the Nd samples with x>0.60. This may suggest the existence of other parameters for determining Tc besides the antiferromagnetic correlation in this system.  相似文献   

2.
We report the 11B and 195Pt NMR measurements in non-centrosymmetric superconductors Li2(Pd1−xPtx)3B (x = 0.0, 0.2, 0.5, 1.0). From the measurements of spin–lattice relaxation time (T1), we found that there was a coherence peak (CP) just below superconducting transition temperature (Tc) for x = 0–0.5 but no CP in x = 1. We demonstrated that the system for x = 0–0.5 were BCS superconductors but there existed line node in the superconducting gap for x = 1.0. The 195Pt Knight Shift in x = 0.2 decreased below Tc, indicating spin-singlet state. The results showed that BCS superconducting state evolves into an exotic state with line-nodes in the gap function when x is increased, as the spin–orbit coupling is enhanced.  相似文献   

3.
We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La2−2xSr1+2xMn2O7 single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, Tc, increases under pressure (dTc/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of Tc is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dTc/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below Tc at atmospheric pressure. Both peak structure in χc and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La1.25Sr1.75Mn2O7 single crystal.  相似文献   

4.
Polycrystalline Sr and Co co-doped superconducting oxypnictides Pr1−ySryFe1−xCoxAsO were synthesized by solid state reaction method, and superconductivity was investigated by measuring resistivity, magnetic susceptibility and thermopower. In the PrFe1−xCoxAsO samples with only Co doping, Tc reaches a maximum of 16 K at x=0.075-0.1, and the variation of Tc with Co content (x) is dome-shaped, consistent with the other Co-doped 1 1 1 1 phase systems such as LaFe1−xCoxAsO and SmFe1−xCoxAsO. In the Co and Sr co-doped Pr0.8Sr0.2Fe1−xCoxAsO system, Tc reaches a maximum of about 16 K at x=0.15, and the Tcx dome shifts to higher Co content in the phase diagram of Tc versus Co content (x). Our result indicates that Sr doping effectively induces hole-type charge carriers in the PrFeAsO system and the disorder caused by Sr and Co dopants has little effect on Tc. The thermopower data confirms that Sr dopant is hole-type and Co dopant is electron-type. The shift of Tcx dome to higher Co content caused by Sr doping implies that the Sr dopant can compensate for the Co doping effect. The results suggest that there could be a universal dependence of Tc on charge carrier density in both hole-type and electron-type oxypnictide superconductors.  相似文献   

5.
SrBi2−xPrxNb2O9 (x=0, 0.04 and 0.2) ceramics were prepared by a solid state reaction method. X-ray diffraction analysis indicated that single-phase layered perovskite structure ferroelectrics were obtained. A relaxor behavior of frequency dispersion was observed among Pr-doped SrBi2Nb2O9. The degree of frequency dispersion ΔT increased from 0 for x=0-7 °C for x=0.2, and the extent of relaxor behavior γ increased from 0.94 for x=0-1.45 for x=0.2. The substitution of Pr ions for Bi3+ ions in the Bi2O2 layers resulted in a shift of the Curie point to lower temperatures and a decrease in remanent polarization. In addition, the coercive field 2Ec reduced from 110 kV/cm for an undoped specimen to 90 kV/cm for x=0.2.  相似文献   

6.
(Tl0.5Pb0.5)Sr2Ca(Cu2−xMx)O7 (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028-3.8040 Å and c=12.0748-12.1558 Å. In (Tl0.5Pb0.5)Sr2Ca(Cu2−xMx)O7 system (M=Co or Ni), the superconducting critical temperature Tc decreases linearly with both Co and Ni concentrations and the rate of Tc decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl0.5Pb0.5)Sr2Ca (Cu2−xZnx)O7 system, the dependence of Tc on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses Tc much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in Tc in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of Tc in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu dx2y2 state due to the full d band of Zn ions.  相似文献   

7.
The intermetallic compound, YRhAl, has been prepared and is found to be isomorphic with RRhAl (R=Pr, Nd, Gd, Ho and Tm) compounds crystallizing in the orthorhombic TiNiSi-type structure (space group Pnma). Heat capacity and electrical resistivity measurements in the He-3 temperature range reveal that this compound is superconducting with a transition temperature, Tc, of 0.9 K. The electronic specific heat coefficient, γ, and the Debye temperature are found to be 6.1 mJ/mol K and 197 K, respectively. The specific heat jump at the superconducting transition is found to be consistent with the BCS weak-coupling limit. This combined with the earlier observation of superconductivity in LaRhAl (Tc=2.4 K) having a different structure than that of YRhAl, suggests that the underlying structure is not very crucial for the occurrence of superconductivity in RRhAl series of compounds.  相似文献   

8.
La0.85−xSmxAg0.15MnO3 (x=0−0.2) ceramics were prepared using the conventional solid-state synthesis method to investigate the effect of Sm3+ substitution on magnetic and electrical transport properties. Magnetic susceptibility versus temperature measurements showed all samples exhibit ferromagnetic to paramagnetic transition with Curie temperature, Tc decreasing from 283 K (x=0) to 164 K (x=0.2) with increasing Sm3+. The observed slope in susceptibility, χ′ versus temperature curves below Tc indicates the possible presence of FM and AFM phases in the metallic region. In addition, a deviation from the Curie-Weiss law above Tc in 1/χ′ versus T curves indicates the existence of a Griffith's phase in the x=0.05−0.2 samples due to the Sm3+ ion substitution. The Griffith temperature, TG was found to decrease from 295 K (x=0.05) to 229 K (x=0.2). Electrical resistivity measurements of the samples in zero field showed transition from metallic behavior to insulating behavior as the temperature was increased. For x=0, two metal-insulator, MI transition peaks were observed at Tp1=282 K and at Tp2=250 K. Both peaks shifted to lower temperatures with the increase in Sm3+. The relative resistivity of the first peak to the second peak decreases with increasing Sm3+ for x>0.05 while at x=0.2 the Tp1 peak was strongly suppressed. Magnetoresistance, MR was observed to weaken with Sm3+ substitution. The metallic region of the ρ(T) curve of the x=0−0.15 samples was fitted to the model of electron-electron and electron-magnon scattering while the insulating region was fitted to the variable range hopping, VRH model. The resistivity behavior indicated that the substitution of Sm3+ weakened the double exchange process and enhanced the Jahn-Teller effect. Our results indicated that the Tp1 peak is strongly related to the double-exchange mechanism while the Tp2 peak is suggested to originate from magnetic inhomogeneity.  相似文献   

9.
IrO2 thin films were prepared on Si(1 0 0) substrates by laser ablation. The effect of substrate temperature (Tsub) on the structure (crystal orientation and surface morphology) and property (electrical resistivity) of the laser-ablated IrO2 thin films was investigated. Well crystallized and single-phase IrO2 thin films were obtained at Tsub = 573-773 K in an oxygen partial pressure of 20 Pa. The preferred orientation of the laser-ablated IrO2 thin films changed from (2 0 0) to (1 1 0) and (1 0 1) depending on Tsub. With the increasing of Tsub, both the surface roughness and crystallite size increased. The room-temperature electrical resistivity of IrO2 thin films decreased with increasing Tsub, showing a low value of (42 ± 6) × 10−8 Ω m at Tsub = 773 K.  相似文献   

10.
With a view to understand the structural, magnetic and electrical properties of La1−xAgxMnO3 (x=0.05-0.3), a series of samples were prepared by polyvinyl alcohol (PVA) gel route. It has been found that both the metal-insulator and ferro- to paramagnetic transition temperatures after increasing up to the composition x=0.20, are found to remain constant thereafter. The electrical resistivity vs. temperature plot of the sample x=0.10 is found to exhibit an insulating behavior below 36 K, while the sample, x=0.20 exhibits two peaks, and the observed behavior is explained on the basis of the phase separation model. The low-temperature (T<TP), electrical resistivity data were analyzed by a theoretical model, ρ=ρ0+ρ2T2+ρ4.5T4.5, indicating the importance of grain/domain boundary effects, electron-electron and two-magnon scattering processes. The low-temperature resistivity data (T<50 K) were fitted to an equation, which is based on the combined effect of weak localization, electron-electron and electron-phonon scattering.  相似文献   

11.
The temperature dependences of DC electrical resistivity for perovskite-type oxides Y1−xCaxCoO3 (0?x?0.1), prepared by sol-gel process, were investigated in the temperature range from 20 K up to 305 K. The results indicated that with increase of doping content of Ca the resistivity of Y1−xCaxCoO3 decreased remarkably, which was found to be caused mainly by increase of carrier (hole) concentration. In the whole temperature range investigated the temperature dependence of resistivity ρ(T) for the un-doped (x=0) sample decreased exponentially with decreasing temperature (i.e. ln ρ∝1/T), with a conduction activation energy ; the resisitivity of lightly doped oxide (x=0.01) possessed a similar temperature behavior but has a reduced Ea (0.155 eV). Moreover, experiments showed that the relationship ln ρ∝1/T existed only in high-temperature regime for the heavily doped samples (T?82 and ∼89 K for x=0.05 and 0.1, respectively); at low temperatures Mott's ln ρT−1/4 law was observed, indicating that heavy doping produced strong random potential, which led to formation of considerable localized states. By fitting of the experimental data to Mott's T−1/4 law, we estimated the density of localized states N(EF) at the Fermi level, which was found to increase with increasing doping content.  相似文献   

12.
The structures and magnetocaloric effects of (Gd1−xTbx)Co2 (x=0, 0.25, 0.4, 0.5, 0.6, 0.7, 0.8, and 1) pseudobinary compounds were investigated by X-ray powder diffraction and magnetic properties measurement. The results show that the Tc of the alloy is near room temperature when X=0.6. The magnetic entropy changes of the compounds increase from 1.7 to 3.6 J/kg K with increasing the content of Tb under an applied field up to 2 T. All the compounds exhibit second order magnetic change. As a result, the values of their ΔSM are lower than that of some large magnetocaloric effect materials.  相似文献   

13.
Fine powders of NiCuZn ferrite with composition Ni(0.7−x)CuxZn0.3Fe2O4 (where x=0, 0.2, 0.4 and 0.6) were prepared by the citrate precursor method. X-ray diffraction measurements confirm the formation of single-phase cubic spinel structure. The grain size was estimated by SEM micrograph which increases with Cu content. Dielectric constant (?) and loss tangent (tan δ) were measured as a function of frequency. The ? and tan δ show a decreasing trend with increase of frequency for all the samples. The DC resistivity was measured as a function of temperature. The temperature-dependent DC resistivity measurements show that the room-temperature DC resistivity of NiCuZn ferrite with x=0.2 is of the order of 109 Ω cm. The AC conductivity (σAC) was studied as a function of frequency. The hysteresis data indicate that the maximum saturation magnetization of 38.66 emu/g is obtained for the composition with x=0.2.  相似文献   

14.
We report on experimental studies of the Kondo physics and the development of non-Fermi-liquid scaling in UCu4+xAl8−x family. We studied 7 different compounds with compositions between x=0 and 2. We measured electrical transport (down to 65 mK) and thermoelectric power (down to 1.8 K) as a function of temperature, hydrostatic pressure, and/or magnetic field.Compounds with Cu content below x=1.25 exhibit long-range antiferromagnetic order at low temperatures. Magnetic order is suppressed with increasing Cu content and our data indicate a possible quantum critical point at xcr≈1.15. For compounds with higher Cu content, non-Fermi-liquid behavior is observed. Non-Fermi-liquid scaling is inferred from electrical resistivity results for the x=1.25 and 1.5 compounds. For compounds with even higher Cu content, a sharp kink occurs in the resistivity data at low temperatures, and this may be indicative of another quantum critical point that occurs at higher Cu compositions.For the magnetically ordered compounds, hydrostatic pressure is found to increase the Néel temperature, which can be understood in terms of the Kondo physics. For the non-magnetic compounds, application of a magnetic field promotes a tendency toward Fermi-liquid behavior. Thermoelectric power was analyzed using a two-band Lorentzian model, and the results indicate one fairly narrow band (10 meV and below) and a second broad band (around hundred meV). The results imply that there are two relevant energy scales that need to be considered for the physics in this family of compounds.  相似文献   

15.
A series of the SmFeAsO1−xFx and GdFeAsO1−xFx (x=0.05, 0.1, 0.15, 0.2, 0.25) samples have been prepared using nano-scaled ReF3 as the fluorine resource at a relatively low temperature. The samples have been sintered at 1100 and 1120 °C for SmFeAsO1−xFx and GdFeAsO1−xFx, respectively. These temperatures are at least 50-60° lower than other previous reports. All of the so-prepared samples possess a tetragonal ZrCuSiAs-type structure. Dramatically supression of the lattice parameters and increase in Tc proved that this low temperature process was more effective to introduce fluorine into REFeAsO. Superconducting transition appeared at 39.5 K for SmFeAsO1−xFx with x=0.05 and at 22 K for GdFeAsO1−xFx with x=0.1. The highest Tc was detected to be 54 K in SmFeAsO0.8F0.2 and 40.2 K in GdFeAsO0.75F0.25. The use of the nano-scaled ReF3 compounds has improved the efficiency of the present low temperature method in synthesizing the fluorine-doped iron-based superconductors.  相似文献   

16.
Manoranjan Kar  S Ravi 《Pramana》2002,58(5-6):1009-1012
Electron-doped (Ba1−x La x )MnO3 compounds were prepared for x=0−0.5. Measurements of X-ray diffraction (XRD) at room temperature and temperature variation of dc electrical resistivity down to 20 K were carried out. Samples with x=0.2–0.5 exhibit metal-insulator (M-I) transition. The maximum M-I transition temperature (T c) of 289 K was observed for 30% of La doping (x=0.3). XRD patterns of these samples (x=0.2−0.5) were analyzed using Rietveld refinement. These samples are found to be mostly in single-phase form with orthorhombic symmetry (space group Pbnm). We have found strong correlation between Mn-O-Mn bond angles and T c of M-I transition. The resistivity data below T c could be fitted to the expression ρ=ρ 1+ρ 2 T 2 and this shows that double exchange interaction plays a major role even in Mn4+-rich compound. Above T c the resistivity data were fitted to variable range hopping and small polaron models.  相似文献   

17.
Samples with nominal compositions MoSnxSr2YCu2O8−δ (0 ? x ? 0.075) were synthesized and their superconducting and magnetotransport properties were investigated. It was established that the optimum Sn-doping (x = 0.02 and 0.03) increases the Tc of the undoped MoSr2YCu2O8−δ. The upper critical fields of the samples were determined using magnetization, susceptibility and resistivity measurements. It was established that the Sn-doping weakly affects the first and the intragrain second critical field of Mo-1212 but enhances the extrapolated to T = 0 (i.e. the intergrain) upper critical field. The observed phenomena were discussed on the basis of the Josephson-junction-arrays model. A phenomenological expression describing the temperature dependency of the susceptibility of Mo-1212 was found.  相似文献   

18.
We have successfully synthesized the α-FeSex binary tetragonal superconductors with nominal composition of FeSex (x=0.6-1.0) via conventional solid state reactions between Fe and Se sealed in quartz tubes. Fe and β-FeSe are the most commonly seen impurities in this binary system. A low-temperature annealing at 400 °C is found to be crucial to remove β-FeSe, which is the thermodynamic stable phase with hexagonal symmetry. For all the samples of FeSex, superconductivity is confirmed by magnetic measurements as well as resistivity measurements with their Tc at around 8 K. We noticed that their Tc does not vary with the different nominal Se amount. High-resolution synchrotron X-ray diffraction analysis revealed that the unit cell parameters of all these samples do not change within the error range, and their structure only tolerate the same very small amount of Se deficiency. Based on this study, we concluded that the α-FeSex superconductor only exist in a very narrow deficiency range.  相似文献   

19.
The temperature-dependent resistivity and thermoelectric power of monovalent (K) doped La1−xKxMnO3 polycrystalline pellets (x=0.05, 0.10 and 0.15) between 50 and 300 K are reported. K substitution enhances the conductivity of this system. Curie temperature (TC) also increases from 260 to 309 K with increasing K content. In the paramagnetic region (T>TC), the electrical resistivity is well represented by adiabatic polaron hopping, while in the ferromagnetic region (T<TC), the resistivity data show a nearly perfect fit for all the samples to an expression containing, the residual resistivity, spin-wave and two-magnon scattering and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Small polaron hopping mechanism is found to fit well to the thermoelectric power (S) data for T>TC whereas at low temperatures (T<TC) in ferromagnetic region (SFM), SFM is well explained with the spin-wave fluctuation and electron–magnon scattering. Both, resistivity and thermopower data over the entire temperature range (50–300 K) are also examined in light of a two-phase model based on an effective medium approximation.  相似文献   

20.
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 TC is 2.8 K. The ordered moment of 0.06 μB is directed along the orthorhombic c-axis. Superconductivity is found below a resistive transition temperature Ts=0.65 K.  相似文献   

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