Ying Jia

New Fe-based superconductors: properties relevant for applications

Less than two years after the discovery of high temperature superconductivity in oxypnictide LaFeAs(O, F) several families of superconductors based on Fe layers (1111, 122, 11, 111) are available. They share several characteristics with cuprate superconductors that compromise easy applications, such as the layered structure, the small coherence length and unconventional pairing. On the other hand, the Fe-based superconductors have metallic parent compounds and their electronic anisotropy is generally smaller and does not strongly depend on the level of doping, and the supposed order parameter symmetry is s-wave, thus in principle not so detrimental to current transmission across grain boundaries. From the application point of view, the main efforts are still devoted to investigate the superconducting properties, to distinguish intrinsic from extrinsic behaviors and to compare the different families in order to identify which one is the fittest for the quest for better and more practical superconductors. The 1111 family shows the highest Tc, huge but also the most anisotropic upper critical field and in-field, fan-shaped resistive transitions reminiscent of those of cuprates. On the other hand, the 122 family is much less anisotropic with sharper resistive transitions as in low temperature superconductors, but with about half the Tc of the 1111 compounds. An overview of the main superconducting properties relevant to applications will be presented. Upper critical field, electronic anisotropy parameter, and intragranular and intergranular critical current density will be discussed and compared, where possible, across the Fe-based superconductor families.

Critical fields and anisotropy of NdFeAsO0.82F0.18 single crystals

The newly discovered iron-based superconductors have stimulated enormous interests in the field of superconductivity. Since the new superconductor is a layered system, the anisotropy is a parameter with the first priority to know. Meanwhile any relevant message about the critical fields (upper critical field and irreversibility line) are essentially important. By using flux method, we have successfully grown the single crystals NdO0.82F0.18FeAs at ambient pressure. Resistive measurements reveal a surprising discovery that the anisotropy \Gamma = (mc/mab)^{1/2} is below 5, which is much smaller than the theoretically calculated results. The data measured up to 400 K show a continuing curved feature which prevents a conjectured linear behavior for an unconventional metal. The upper critical fields determined based on the Werthamer-Helfand-Hohenberg formula are H_{c2}^{H||ab}(T=0 K) = 304 T and H_{c2}^{H||c}(T=0 K)=62-70 T, indicating a very encouraging application of the new superconductors.

Roles of multiband effects and electron-hole asymmetry in the superconductivity and normal-state properties of Ba(Fe(1-x)Cox)(2)As-2

We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe1-xCox)(2)As-2 over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons in excess of one order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between antiferromagnetism and superconductivity, while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing but also in the normal transport.

Ultrahigh current-carrying capability in clean MgB2 films

We have studied the current-carrying capability of high quality epitaxial MgB2 films synthesized using the hybrid physical-chemical vapor deposition technique by both transport measurement in nanobridge constrictions and magnetization measurement. An extremely high self-field critical current density Jc(0)>108A∕cm2, approaching the theoretical depairing current of MgB2, was observed on a 150nm bridge, indicating an excellent current-carrying capability in these films. The magnetization measurement also showed very high Jc.

Suppression of the critical temperature of superconducting NdFeAs(OF) single crystals by Kondo-like defect sites induced by alpha-particle irradiation.

We report the effect of alpha-particle irradiation on the reduction of the critical temperature T{c} of a NdFeAs(OF) single crystal. Our data indicate that irradiation defects cause both nonmagnetic and magnetic scattering, resulting in the Kondo-like excess resistance Delta rho(T) proportional to lnT over 2 decades in temperatures above T{c}. The critical density of magnetic irradiation defects which suppresses T{c} is found to be much higher than those for cuprates and multiband BCS superconductors. We suggest that such anomalously weak pair breaking by irradiation defects indicates that magnetic scattering in pnictides is coupled with pairing interactions mediated by spin fluctuations.

Hall effect and magnetoresistance in single crystals of NdFeAsO(1-x)F(x) (x=0 and 0.18)

Hall effect and magnetoresistance have been measured on single crystals of NdFeAsO(1-x)F(x) with x=0 (T(c)=0 K) and x=0.18 (T(c)=50 K). For the undoped samples, strong Hall effect and magnetoresistance with strong temperature dependence were found below about 150 K. The magnetoresistance was found to be as large as 30% at 15 K at a magnetic field of 9 T. From the transport data we found that the transition near 155 K was accomplished in two steps: first one occurs at 155 K which may be associated with the structural transition, the second one takes place at about 140 K which may correspond to the spin-density-wave-like transition. In the superconducting sample with T(c)=50 K, it is found that the Hall coefficient also reveals a strong temperature dependence with a negative sign. But the magnetoresistance becomes very weak and does not satisfy Kohlers scaling law. These dilemmatic results (strong Hall effect and very weak magnetoresistance) prevent understanding of the normal-state electric conduction by a simple multi-band model by taking into account the electron and hole pockets. Detailed analysis further indicates that the strong temperature dependence of R(H) cannot be easily understood with the simple multi-band model either. A picture concerning a suppression to the density of states at the Fermi energy in lowering temperature is more reasonable. A comparison between the Hall coefficient of the undoped sample and the superconducting sample suggests that the doping may remove the nesting condition for the formation of the spin-density wave order, since both samples have very similar temperature dependence above 175 K.

Superconductivity and phase diagrams of the 4d-and 5d-metal-doped iron arsenides SrFe2-xMxAs2 (M=Rh,Ir,Pd)

By substituting the Fe with the 5d-transition metal Ir in SrFe2As2, we have successfully synthesized the superconductor SrFe2-xIrxAs2 with Tc = 22.3 K at x = 0.5. X-ray diffraction indicates that the material has formed the ThCr2Si2-type structure with a space group I4/mmm. The temperature dependence of resistivity and dc magnetization both reveal sharp superconducting transitions at around 22 K. An estimate on the diamagnetization signal reveals a high Meissner shielding volume. Interestingly, the normal state resistivity exhibits a roughly linear behavior up to 300 K. The superconducting transitions at different magnetic fields were also measured yielding a slope of -dHc2/dT = 3.8 T/K near Tc. Using the Werthamer-Helfand-Hohenberg (WHH) formula, the upper critical field at zero K is found to be about 58 T. Counting the possible number of electrons doped into the system in SrFe2-xIrxAs2, we argue that the superconductivity in the Ir-doped system is different from the Co-doped case, which should add more ingredients to the underlying physics of the iron pnictide superconductors.By doping Rh to the Fe sites in SrFe2As2, superconductivity is induced when the antiferromagnetic (AF) order is suppressed. The maximum superconducting transition temperature was found at about 22 K with the doping level of x = 0.25. It is found that the resistivity anomaly associating with the AF order reveals a sharp drop for the parent phase and the samples with low doping (x = 0.05), while it evolves into an uprising at a higher doping level (x = 0.1). A general phase diagram is depicted which exhibits the similarity with that of Co doping. Regarding the close maximum superconducting transition temperatures in doping Co, Rh and Ir, we argue that the superconductivity is intimately related to the suppression of the AF order, rather than the electron-phonon coupling.By using solid state reaction method, we have synthesized the Pd-doped superconductor SrFe2-xPdxAs2. The systematic evolution of the lattice constants indicated that the Fe ions were successfully replaced by the Pd. By increasing the doping content of Pd, the antiferromagnetic state of the parent phase is suppressed and superconductivity is induced at a doping level of about x=0.05. Superconductivity with a maximum transition temperature Tc of about 8.7 K was achieved at the doping level of x = 0.15. The general phase diagram of Tc versus x was obtained and found to be similar to the case of Ni and Co doping to the Fe sites. Our results suggest that superconductivity can be easily induced in the FeAs family by adding electrons into the system, regardless with the transition metals of 3d or higher d-orbital electrons.By substituting the Fe with the 4d- and 5d-transition metals Rh, Ir, and Pd in SrFe2As2, we have successfully synthesized a series of superconductors SrFe2-xMxAs2 (M=Rh, Ir, and Pd) and explored the phase diagrams of them. The systematic evolution of the lattice constants indicated that part of the Fe ions were successfully replaced by the transition metals Rh, Ir, and Pd. By increasing the doping content of Rh, Ir, and Pd, the antiferromagnetic (AF) state of the parent phase is suppressed progressively and superconductivity is induced. The general phase diagrams were obtained and found to be similar to the case of doping Co and Ni to the Fe sites. However, the detailed structure of the phase diagram, in terms of how fast to suppress the antiferromagnetic order and induce the superconductivity, varies from one kind of doped element to another. Regarding the close values of the maximum superconducting transition temperatures in doping Co, Rh, and Ir which locate actually in the same column in the periodic table of elements but have very different masses, we argue that the superconductivity is intimately related to the suppression of the AF order, rather than the electron-phonon coupling.

Fully Band-Resolved Scattering Rate in MgB2 Revealed by the Nonlinear Hall Effect and Magnetoresistance Measurements

We have measured the normal state temperature dependence of the Hall effect and magnetoresistance in epitaxial MgB2 thin films with variable disorders characterized by the residual resistance ratio RRR ranging from 4.0 to 33.3. A strong nonlinearity of the Hall effect and magnetoresistance have been found in clean samples, and they decrease gradually with the increase of disorders or temperature. By fitting the data to the theoretical model based on the Boltzmann equation and ab initio calculations for a four-band system, for the first time, we derived the scattering rates of these four bands at different temperatures and magnitude of disorders. Our method provides a unique way to derive these important parameters in multiband systems.

I-V characteristics of the vortex state in MgB(2) thin films

The current-voltage (I-V) characteristics of various MgB(2) films have been studied at different magnetic fields parallel to the c axis. At fields mu(0)H between 0 and 5 T, vortex liquid-glass transitions were found in the I-V isotherms. Consistently, the I-V curves measured at different temperatures show a scaling behavior in the framework of quasi-two-dimension (quasi-2D) vortex-glass theory. However, at mu(0)H >= 5 T, a finite dissipation was observed down to the lowest temperature here, T=1.7 K, and the I-V isotherms did not scale in terms of any known scaling law, of any dimensionality. We suggest that this may be caused by a mixture of sigma band vortices and pi band quasiparticles. Interestingly, the I-V curves at zero magnetic field can still be scaled according to the quasi-2D vortex-glass formalism, indicating an equivalent effect of self-field due to persistent current and applied magnetic field.

Superconductivity at 22.3 K in SrFe2-xIrxAs2

By substituting the Fe with the 5d-transition metal Ir in SrFe2As2, we have successfully synthesized the superconductor SrFe2-xIrxAs2 with Tc = 22.3 K at x = 0.5. X-ray diffraction indicates that the material has formed the ThCr2Si2-type structure with a space group I4/mmm. The temperature dependence of resistivity and dc magnetization both reveal sharp superconducting transitions at around 22 K. An estimate on the diamagnetization signal reveals a high Meissner shielding volume. Interestingly, the normal state resistivity exhibits a roughly linear behavior up to 300 K. The superconducting transitions at different magnetic fields were also measured yielding a slope of -dHc2/dT = 3.8 T/K near Tc. Using the Werthamer-Helfand-Hohenberg (WHH) formula, the upper critical field at zero K is found to be about 58 T. Counting the possible number of electrons doped into the system in SrFe2-xIrxAs2, we argue that the superconductivity in the Ir-doped system is different from the Co-doped case, which should add more ingredients to the underlying physics of the iron pnictide superconductors.By doping Rh to the Fe sites in SrFe2As2, superconductivity is induced when the antiferromagnetic (AF) order is suppressed. The maximum superconducting transition temperature was found at about 22 K with the doping level of x = 0.25. It is found that the resistivity anomaly associating with the AF order reveals a sharp drop for the parent phase and the samples with low doping (x = 0.05), while it evolves into an uprising at a higher doping level (x = 0.1). A general phase diagram is depicted which exhibits the similarity with that of Co doping. Regarding the close maximum superconducting transition temperatures in doping Co, Rh and Ir, we argue that the superconductivity is intimately related to the suppression of the AF order, rather than the electron-phonon coupling.By using solid state reaction method, we have synthesized the Pd-doped superconductor SrFe2-xPdxAs2. The systematic evolution of the lattice constants indicated that the Fe ions were successfully replaced by the Pd. By increasing the doping content of Pd, the antiferromagnetic state of the parent phase is suppressed and superconductivity is induced at a doping level of about x=0.05. Superconductivity with a maximum transition temperature Tc of about 8.7 K was achieved at the doping level of x = 0.15. The general phase diagram of Tc versus x was obtained and found to be similar to the case of Ni and Co doping to the Fe sites. Our results suggest that superconductivity can be easily induced in the FeAs family by adding electrons into the system, regardless with the transition metals of 3d or higher d-orbital electrons.By substituting the Fe with the 4d- and 5d-transition metals Rh, Ir, and Pd in SrFe2As2, we have successfully synthesized a series of superconductors SrFe2-xMxAs2 (M=Rh, Ir, and Pd) and explored the phase diagrams of them. The systematic evolution of the lattice constants indicated that part of the Fe ions were successfully replaced by the transition metals Rh, Ir, and Pd. By increasing the doping content of Rh, Ir, and Pd, the antiferromagnetic (AF) state of the parent phase is suppressed progressively and superconductivity is induced. The general phase diagrams were obtained and found to be similar to the case of doping Co and Ni to the Fe sites. However, the detailed structure of the phase diagram, in terms of how fast to suppress the antiferromagnetic order and induce the superconductivity, varies from one kind of doped element to another. Regarding the close values of the maximum superconducting transition temperatures in doping Co, Rh, and Ir which locate actually in the same column in the periodic table of elements but have very different masses, we argue that the superconductivity is intimately related to the suppression of the AF order, rather than the electron-phonon coupling.