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Register a new userLarge Anisotropic Thermal Expansion Anomaly near the Superconducting Transition Temperature in MgB2
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An anisotropic lattice anomaly near the superconducting transition temperature, Tc, was observed in MgB2 by high-resolution neutron powder diffraction. The a-axis thermal expansion becomes negative near Tc, while the c-axis thermal expansion is unaffected. This is qualitatively consistent with a depletion of the boron-boron s-band as the superconducting gap opens, resulting in weaker bonding. However, the observed anomaly is much larger than predicted by the Ehrenfest relation, strongly suggesting that the phonon thermal expansion also changes sign, as commonly observed in hexagonal layered crystals. These two effects may be connected through subtle changes in the phonon spectrum at Tc.
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Choi et al. [Phys. Rev. B 66, 020513 (2002)] recently presented first principles calculations of the electron-phonon coupling and superconductivity in MgB2, emphasizing the importance of anisotropy and anharmonicity. We point out that (1) variation of the superconducting gap inside the sigma- or the pi-bands can hardly be observed in real samples, and (2) taking the anisotropy of the Coulomb repulsion into account influences the size of the small gap, Delta_pi.
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.
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.
Heterostructures of superconducting and ferromagnetic materials are of fundamental interest because of the mutual interaction of antagonistic kinds of ordering at the S-F interface. Normally, the superconducting transition temperature Tc should be strongly suppressed at the S-F interface owing to the penetration of Cooper pairs into the ferromagnetic side. Nevertheless, constructive interactions between S and F orders have been suggested to occur via the modification of ferromagnetic order by the superconducting state. This may induce an inhomogeneous magnetic state, often called a cryptoferromagnetic state, and the relevant domain wall effect, which will lead to a local decrease of the pair-breaking parameter. However, the domain wall effect, even if it exists, is quite subtle from the experimental view point and is normally difficult to observe. Here we show that the defect-related d0 ferromagnetism in MgO and the superconductivity in MgB2 do not antagonize, but rather enhance the superconducting transition temperature Tc to any significant degree. We found in superconducting MgB2-d0 ferromagnetic MgO composites that the superconducting transition proceeds in two steps. The first at the S-F interface, between 110-120 K, then in the rest of the bulk at 39 K, which is the Tc of single phase MgB2 superconductor. Moreover, the additional transition emerges at 60 K at the S-F interface especially in the ferromagnetic side, showing a spin-glass-like magnetic state. Our findings reveal that the proximity effect in the superconductor-d0 ferromagnet heterostructures will provide the knowledge and basis to enhance the Tc value of the existing superconductors.
We report measurements of the coefficient of linear thermal expansion, $alpha (T)$, of the superconducting ferromagnet UCoGe. The data taken on a single-crystalline sample along the orthorhombic crystal axes reveal a pronounced anisotropy, with the largest length changes along the $b$ axis. The large values of the step sizes $Delta alpha$ at the magnetic and superconducting phase transitions provide solid evidence for bulk magnetism and superconductivity. Specific-heat measurements corroborate bulk superconductivity. Thermal-expansion measurements in magnetic fields $B parallel a,b$ show $Delta alpha$ at $T_C$ grows rapidly, which indicates the character of the ferromagnetic transition becomes first-order-like.
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