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Register a new userCalorimetric study of the superconducting and normal state properties of Ca(Fe1-xCox)2As2
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Added by
Mahmoud Abdel-Hafiez
Publication date
2011
fields
Physics
and research's language is
English
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We present a calorimetric study on single crystals of Ca(Fe1-xCox)2As2 (x = 0, 0.032, 0.051, 0.056, 0.063, and 0.146). The combined first order spin-density wave/structural transition occurs in the parent CaFe2As2 compound at 168 K and gradually shifts to lower temperature for low doping levels (x = 0.032 and x = 0.051). It is completely suppressed upon higher doping x = 0.056. Simultaneously, superconductivity appears at lower temperature with a transition temperature around Tc = 14.1 K for Ca(Fe0.937Co0.063)2As2. The phase diagram of Ca(Fe0.937Co0.063)2As2 has been derived and the upper critical field is found to be H(c) c2 = 11.5
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We use polarised neutron diffraction to study the induced magnetization density of near optimally doped Ba(Fe0.935Co0.065)2As2 (T_C=24 K) as a function of magnetic field (1<H<9 T) and temperature (2<T<300 K). The T-dependence of the induced moment in the superconducting state is consistent with the Yosida function, characteristic of spin-singlet pairing. The induced moment is proportional to applied field for H < 9 T ~ Hc2/6. In addition to the Yosida spin-susceptibility, our results reveal a large zero-field contribution M (H=>0,T=>0)/H ~ 2/3 chi_{normal} which does not scale with the field or number of vortices and is most likely due to the van Vleck susceptibility. Magnetic structure factors derived from the polarization dependence of 15 Bragg reflections were used to make a maximum entropy reconstruction of the induced magnetization distribution in real space. The magnetization is confined to the Fe atoms and the measured density distribution is in good agreement with LAPW band structure calculations which suggest that the relevant bands near the Fermi energy are of the d_{xz/yz} and d_{xy} type.
Platelet-like single crystals of the Ca(Fe1-xCox)2As2 series having lateral dimensions up to 15 mm and thickness up to 0.5 mm were obtained from the high temperature solution growth technique using Sn flux. Upon Co doping, the c-axis of the tetragonal unit cell decreases, while the a-axis shows a less significant variation. Pristine CaFe2As2 shows a combined spin-density-wave and structural transition near T = 166 K which gradually shifts to lower temperatures and splits with increasing Co-doping. Both transitions terminate abruptly at a critical Co-concentration of xc = 0.075. For x geq 0.05, superconductivity appears at low temperatures with a maximum transition temperature TC of around 20 K. The superconducting volume fraction increases with Co concentration up to x = 0.09 followed by a gradual decrease with further increase of the doping level. The electronic phase diagram of Ca(Fe1-xCox)2As2 (0 leq x leq 0.2) series is constructed from the magnetization and electric resistivity data. We show that the low-temperature superconducting properties of Co-doped CaFe2As2 differ considerably from those of BaFe2As2 reported previously. These differences seem to be related to the extreme pressure sensitivity of CaFe2As2 relative to its Ba counterpart.
A simple two-band model is used to describe the magnitude and temperature dependence of the magnetic susceptibility, Hall coefficient and Seebeck data from undoped and Co doped BaFe2As2. Overlapping, rigid parabolic electron and hole bands are considered as a model of the electronic structure of the FeAs-based semimetals. The model has only three parameters: the electron and hole effective masses and the position of the valence band maximum with respect to the conduction band minimum. The model is able to reproduce in a semiquantitative fashion the magnitude and temperature dependence of many of the normal state magnetic and transport data from the FeAs-type materials, including the ubiquitous increase in the magnetic susceptibility with increasing temperature.
Inelastic neutron scattering measurements have been performed on underdoped Ba(Fe1-xCox)2As2 (x = 4.7%) where superconductivity and long-range antiferromagnetic (AFM) order coexist. The broad magnetic spectrum found in the normal state develops into a magnetic resonance feature below TC that has appreciable dispersion along c-axis with a bandwidth of 3-4 meV. This is in contrast to the optimally doped x = 8.0% composition, with no long-range AFM order, where the resonance exhibits a much weaker dispersion [see Lumsden et al. Phys. Rev. Lett. 102, 107005 (2009)]. The results suggest that the resonance dispersion arises from interlayer spin correlations present in the AFM ordered state.
A simple two-band model is used to describe the magnitude and temperature dependence of the magnetic susceptibility, Hall coefficient and Seebeck data from undoped and Co doped BaFe2As2. Overlapping rigid parabolic electron and hole bands are considered as a model of the electronic structure of the FeAs-based semimetals. The model has only three parameters: the electron and hole effective masses and the position of the valence band maximum with respect to the conduction band minimum. The model is able to reproduce in a semiquantitative fashion the magnitude and temperature dependence of many of the normal state magnetic and transport data from the FeAs-type materials, including the ubiquitous increase in the magnetic susceptibility with increasing temperature.
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