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Register a new userGrowth, structure analysis and anisotropic superconducting properties of MgB2 single crystals
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Here we report the growth of sub-millimeter MgB2 single crystals of various shapes under high pressure in Mg-B-N system. Structure refinement using a single-crystal X-ray diffraction analysis gives lattice parameters a=3.0851(5) A and c=3.5201(5) A with small reliability factors (Rw =0.025, R=0.018), which enables us to analyze the Fourier and Fourier difference maps. The maps clearly show the B sp2 orbitals and covalency of the B-B bonds. The sharp superconducting transitions at Tc =38.1-38.3K were obtained in both magnetization (DTc =0.6K) and resistivity (DTc <0.3K) measurements. Resistivity measurements with magnetic fields applied parallel and perpendicular to the Mg and B sheets reveal the anisotropic nature of this compound, with upper critical field anisotropy ratio of about 2.7.
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In-plane electrical transport properties of MgB2 single crystals grown under high pressure of 4-6 GPa and temperature of 1400-1700oC in Mg-B-N system have been measured. For all specimens we found sharp superconducting transition around 38.1-38.3K with transition width within 0.2-0.3K. Estimated resistivity value at 40K is about 1 mkOhmcm and resistivity ratio R(273K)/R(40K) of about 4.9. Results of measurements in magnetic field up to 5.5T perpendicular to Mg and B planes and up to 9T in parallel orientation show temperature dependent anisotropy of the upper critical field with anisotropy ratio increasing from 2.2 close to Tc up to about 3 below 30K. Strong deviation of the angular dependence of Hc2 from anisotropic mass model has been also found.
We discuss the important aspects of synthesis and crystal growth of MgB2 under high pressure (P) and temperature (T) in Mg-B-N system, including the optimisation of P-T conditions for reproducible crystal growth, the role of liquid phases in this process, the temperature dependence of crystal size and the effect of growing instabilities on single crystals morphology. Extensive experiments have been carried out on single crystals with slightly different lattice constants and defects concentration, which revealed and possible effects of Mg-deficiency and lattice strain on the superconducting properties of MgB2 (Tc, Jc, residual resistivity ratio, anisotropy etc.).
The discovery of superconductor in magnesium diboride MgB2 with high Tc (39 K) has raised some challenging issues; whether this new superconductor resembles a high temperature cuprate superconductor(HTS) or a low temperature metallic superconductor; which superconducting mechanism, a phonon- mediated BCS or a hole superconducting mechanism or other new exotic mechanism may account for this superconductivity; and how about its future for applications. In order to clarify the above questions, experiments using the single crystal sample are urgently required. Here we have first succeeded in obtaining the single crystal of this new MgB2 superconductivity, and performed its electrical resistance and magnetization measurements. Their experiments show that the electronic and magnetic properties depend on the crystallographic direction. Our results indicate that the single crystal MgB2 superconductor shows anisotropic superconducting properties and thus can provide scientific basis for the research of its superconducting mechanism and its applications.
A review of our investigations on single crystals of LnFeAsO1-xFx (Ln=La, Pr, Nd, Sm, Gd) and Ba1-xRbxFe2As2 is presented. A high pressure technique has been applied for the growth of LnFeAsO1-xFx crystals, while Ba1-xRbxFe2As2 crystals were grown using quartz ampoule method. Single crystals were used for electrical transport, structure, magnetic torque and spectroscopic studies. Investigations of the crystal structure confirmed high structural perfection and show less than full occupation of the (O, F) position in superconducting LnFeAsO1-xFx crystals. Resistivity measurements on LnFeAsO1-xFx crystals show a significant broadening of the transition in high magnetic fields, whereas the resistive transition in Ba1 xRbxFe2As2 simply shifts to lower temperature. Critical current density for both compounds is relatively high and exceeds 2x109 A/m2 at 15 K in 7 T. The anisotropy of magnetic penetration depth, measured on LnFeAsO1-xFx crystals by torque magnetometry is temperature dependent and apparently larger than the anisotropy of the upper critical field. Ba1-xRbxFe2As2 crystals are electronically significantly less anisotropic. Point-Contact Andreev-Reflection spectroscopy indicates the existence of two energy gaps in LnFeAsO1-xFx. Scanning Tunneling Spectroscopy reveals in addition to a superconducting gap, also some feature at high energy (~20 meV).
We report flux free growth of superconducting FeSe single crystals by an easy and versatile high temperature melt and slow cooling method for first time. The room temperature XRD on the surface of the piece of such obtained crystals showed single 101 plane of Beta-FeSe tetragonal phase. The bulk powder XRD, being obtained by crushing the part of crystal chunk showed majority tetragonal and minority FeSe hexagonal crystalline phases. Detailed HRTEM images along with SAED (selected area electron diffraction) showed the abundance of both majority and minority FeSe phases. Both transport (RT) and magnetization (MT) exhibited superconductivity at below around 10K. Interestingly, the magnetization signal of these crystals is dominated by the magnetism of minority magnetic phase, and hence the isothermal magnetization (MH) at 4K was seen to be ferromagnetic (FM) like. Transport (R-T) measurements under magnetic field showed superconductivity onset at below 12K, and R = 0 (Tc) at 9K. Superconducting transition temperature (Tc) decreases with applied field to around 6K at 7Tesla, with dTc/dH of 0.4K/Tesla, giving rise to an Hc2 value of around 50 Tesla, 30 Tesla and 20 Tesla for Rn = 90, 50 and 10 percent respectively. FeSe single crystal activation energy is calculated from Thermally Activated Flux Flow (TAFF) model which is found to decreases with field.
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