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Register a new userProbing multi-band superconductivity and magnetism in SmFeAsO$_{0.8}$F$_{0.2}$ single crystals by high-field vortex torque magnetometry
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Luis Molinuevo Balicas Dr
Publication date
2009
fields
Physics
and research's language is
English
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To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of magnetic torque $tau(theta)$ in the mixed state of SmO$_{0.8}$F$_{0.2}$FeAs single crystals as functions of temperature $T$ and high magnetic field $H$ up to 30 T. We show that the effective mass anisotropy parameter $gamma$ extracted from $tau(theta)$, can be greatly overestimated if the strong paramagnetism of Sm or Fe ions is not properly taken into account. The correctly extracted $gamma$ depends on both $T$ and $H$, saturating at $gamma simeq 9$ at lower temperatures. Neither the London penetration depth nor the superfluid density is affected by high fields fields up to the upper critical field. Our results indicate two strongly-coupled superconducting gaps of nearly equal magnitudes.
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To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of the magnetic torque $tau(theta)$ in the mixed state of LaO$_{0.9}$F$_{0.1}$FeAs single crystals as a function of temperature $T$ and magnetic fields $H$ up to 18 T. The paramagnetic contribution of the Fe ions is properly treated in order to extract the effective mass anisotropy parameter $gamma=(m_c/m_{ab})^{1/2}$ from $tau(theta)$. We show that $gamma$ depends strongly on both $T$ and $H$, reaching a maximum value of $sim$ 10 followed by a decrease towards values close to 1 as $T$ is lowered. The observed field dependencies of the London penetration depth $lambda_{ab}$ and $gamma$ suggest the onset of suppression of a superconducing gap at $H approx H_{c2}/3$.
Single crystals of SrFe2-xPtxAs2 (0 < x < 0.36) were grown using the self flux solution method and characterized using x-ray crystallography, electrical transport, magnetic susceptibility, and specific heat measurements. The magnetic/structural transition is suppressed with increasing Pt concentration, with superconductivity seen over the range 0.08 < x < 0.36 with a maximum transition temperature Tc of 16 K at x = 0.16. The shape of the phase diagram and the changes to the lattice parameters are similar to the effects of other group VIII elements Ni and Pd, however the higher transition temperature and extended range of superconductivity suggest some complexity beyond the simple electron counting picture that has been discussed thus far.
Superconducting fluctuations (SF) in SmFeAsO$_{0.8}$F$_{0.2}$ (characterized by superconducting transition temperature $T_{c} simeq 52.3$ K) are investigated by means of isothermal high-resolution dc magnetization measurements. The diamagnetic response to magnetic fields up to 1 T above $T_{c}$ is similar to what previously reported for underdoped cuprate superconductors and it can be justified in terms of metastable superconducting islands at non-zero order parameter lacking of long-range coherence because of strong phase fluctuations. In the high-field regime ($H gtrsim 1.5$ T) scaling arguments predicted on the basis of the Ginzburg-Landau theory of conventional SF are found to be applicable, at variance with what observed in the low-field regime. This fact enlightens that two different phenomena are simultaneously present in the fluctuating diamagnetism, namely the phase SF of novel character and the conventional SF. High magnetic fields (1.5 T $lesssim H ll H_{c2}$) are found to suppress the former while leaving unaltered the latter one.
The low-temperature antiferromagnetic state of the Sm-ions in both nonsuperconducting SmFeAsO and superconducting SmFeAsO$_{0.9}$F$_{0.1}$ single crystals was studied by magnetic torque, magnetization, and magnetoresistance measurements in magnetic fields up to 60~T and temperatures down to 0.6~K. We uncover in both compounds a distinct rearrangement of the antiferromagnetically ordered Sm-moments near $35-40$~T. This is seen in both, static and pulsed magnetic fields, as a sharp change in the sign of the magnetic torque, which is sensitive to the magnetic anisotropy and hence to the magnetic moment in the $ab$-plane, ({it i.e.} the FeAs-layers), and as a jump in the magnetization for magnetic fields perpendicular to the conducting planes. This rearrangement of magnetic ordering in $35-40$~T is essentially temperature independent and points towards a canted or a partially polarized magnetic state in high magnetic fields. However, the observed value for the saturation moment above this rearrangement, suggests that the complete suppression of the antiferromagnetism related to the Sm-moments would require fields in excess of 60~T. Such a large field value is particularly remarkable when compared to the relatively small N{e}el temperature $T_{rm N}simeq5$~K, suggesting very anisotropic magnetic exchange couplings. At the transition, magnetoresistivity measurements show a crossover from positive to negative field-dependence, indicating that the charge carriers in the FeAs planes are sensitive to the magnetic configuration of the rare-earth elements. This is indicates a finite magnetic/electronic coupling between the SmO and the FeAs layers which are likely to mediate the exchange interactions leading to the long range antiferromagnetic order of the Sm ions.
The electrical resistivity rho(T) and heat capacity C(T) on single crystals of SrNi2As2 and EuNi2As2 are reported. While there is no evidence for a structural transition in either compound, SrNi2As2 is found to be a bulk superconductor at T_c=0.62 K with a Sommerfeld coefficient of gamma= 8.7 mJ/mol K^2 and a small upper critical field H_{c2} sim 200 Oe. No superconductivity was found in EuNi2As2 above 0.4 K, but anomalies in rho and C reveal that magnetic order associated with the Eu^{2+} magnetic moments occurs at T_m = 14 K.
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