The mechanism of accelerated phase formation of MgB2 by Cu-doping during low-temperature sintering

Thermal analysis, phase identification and microstructure observation were adopted to explore the accelerating sintering effect of Cu-doping on the low-temperature formation of MgB₂ below Mg melting. It is found that the Cu-doping sintering of MgB₂ bulk followed an activated sintering mechanism and thus obviously promoted the reaction between Mg and B forming bulk MgB₂. Accordingly, dense MgB₂ bulks with excellent T_c and J_c were successfully synthesized by the Cu-doping sintering at 575 °C for only 5 h. Further, a sintering model was proposed to illuminate the Cu-doping activated sintering mechanism. It is found that during the Cu-doping activated sintering process, the local Mg-Cu liquid generates firstly and then segregates to the interface between Mg and B particles, which can wet Mg particles and provide a rapid transport for the diffusion of neighboring Mg atoms into B particles resulting in the accelerated formation of MgB₂ phase.     

Electronic and Magnetic Properties in the Novel Skutterudite ThPt4Ge12

Theoretical calculations on ThPt₄Ge₁₂ filled skutterudite were performed. The calculated energy bands yielded indication of metallic behavior, while the projected density of states provided indication of a hybridization formed by Th f-, Pt d- and Ge p-states. The absence of a detected minigap provides support that this material is not a good thermoelectric material. Furthermore, Crystal Orbital Overlap Population yielded indication of the absence of a ferromagnetic instability.      

Physics of Silicene Stripes

Silicene, a monolayer of silicon atoms tightly packed into a two-dimensional honeycomb lattice, is the challenging hypothetical reflection in the silicon realm of graphene, a one-atom thick graphite sheet, presently the hottest material in condensed matter physics. If existing, it would also reveal a cornucopia of new physics and potential applications. Here, we reveal the epitaxial growth of silicene stripes self-aligned in a massively parallel array on the anisotropic silver (110) surface. This crucial step in the silicene “gold rush” could give a new kick to silicon on the electronics road-map and open the most promising route towards wide-ranging applications. A hint of superconductivity in these silicene stripes poses intriguing questions related to the delicate interplay between paired correlated fermions, massless Dirac fermions and bosonic quasiparticles in low dimensions.     

Electronic and phononic friction of solids at low temperatures

It has recently been shown both experimentally and theoretically that there is no static friction in a contact of atomically flat crystalline solids provided the periods of their lattices are incommensurate and their interaction does not exceed some critical value. The only mechanisms of friction in this case are phonon generation and excitation of conducting electrons. It is shown that, at low temperatures, the phonon contribution to the coefficient of friction can be very small by virtue of the quantum mechanical nature of the elementary excitations in a solid. Incommensurate dielectric crystals could therefore slip at low temperatures practically without friction. In metals, on the contrary, the excitation of electrons leads to a finite dynamic friction force at any temperature. Presently, both phonon and electron contributions to the friction force are estimated (the latter both in the normal and the superconducting state of metal).     

Calculated Nb superconducting transition temperature under hydrostatic pressure

A full-potential linear muffin-tin orbital method (FP-LMTO) based linear-response approach is used to calculate the electron–phonon coupling in Nb under hydrostatic pressure. The superconducting transition temperature T_c is calculated using the Eliashberg equation. The calculated T_c agrees nicely with the experimental result at ambient pressure, but the agreement is only fair at high pressures. The T_c measured anomaly at 60–70 GPa is understood in terms of the 2.5-order Lifshitz transition and its origin is traced back to the qualitative changes in the Fermi surface topology.     

About Two-Component Physics of HTSC

We present a first-principles, microscopic calculation of the ground state of cuprates indicating the presence of two groups of charge carriers in these compounds associated with free and localized states. The localized component arises due to a charge density wave instability in the free component. The instability occurs in underdoped cuprates where the attractive Coulomb interaction between dopant impurities and charge carriers becomes strongly over-screened at low carrier densities and divides charge carriers into two orthogonal states. Within this new two-component state a novel quasi-particle entity, a microscopic Coulomb clump (CC), emerges. Our results are completely consistent with the analysis of the Hall effect and the ARPES spectra made earlier by Gorkov and Teitelbaum (GT) (Phys. Rev. Lett., 97:247003, 2006, and J. Phys.: Conf. Ser., 108:012009, 2008) that includes also available neutron scattering, NMR and μSR data. The density of localized component is temperature dependent, which is due to thermal activation of bound holes from Coulomb clumps. The clumps also induce nanoscale superstructures observed in Scanning Tunneling Microscope (STM) experiments (Pan, et al. Nature, 413:282–285, 2001; Dubi, et al. Nature, 449:876–879, 2007; Gomes, et al. Nature, 447:569, 2007; Lee, et al. Nature, 442:546, 2006; McElroy, et al. Science, 309:1048, 2005; Zhu, et al. Phys. Rev. Lett., 97:177001, 2006) and are responsible for the pseudogap and Nernst effect in HTSC. Following GT we stress the importance of these findings for the pseudogap physics. We present a first-principles, microscopic calculation of the ground state of cuprates indicating the presence of two groups of charge carriers in these compounds associated with free and localized states. The localized component arises due to a charge density wave instability in the free component. The instability occurs in underdoped cuprates where the attractive Coulomb interaction between dopant impurities and charge carriers becomes strongly over-screened at low carrier densities and divides charge carriers into two orthogonal states. Within this new two-component state a novel quasi-particle entity, a microscopic Coulomb clump (CC), emerges.     

Intrinsic excess noise in a transition edge sensor

The noise in the measurement of the resistance of a transition edge sensor slightly above the zero resistance state contains a noise component associated with fluctuations in the superconducting order parameter. This noise has been calculated by Nagaev a dozen years ago in the context of the formation of fluctuating Cooper pairs in the normal state slightly above the transition. With reasonable assumptions concerning the properties of TESs it is found that this noise is comparable to Johnson noise only when the temperature is very close to the transition. We discuss the noise from pair fluctuations and methods to decrease its magnitude by pair breaking mechanisms should it be a problem.     

Photoinduced enhancement of superconductivity

The photoinduced enhancement of superconductivity in RBa₂Cu₃O _x (R=rare earth or yttrium) and Pr _y R_1–y Ba₂Cu₃O _x was explored through temperature-dependent resistivity, Hall coefficient and mobility, and x-ray diffraction measurements. The increases inT _c are enhanced near the metal-insulator transition, although photoinduced changes always exist in oxygendeficient samples. Several explanations, including intergrain Josephson coupling, photoassisted oxygen ordering, and the trapping of photogenerated electrons in oxygen vacancies, are discussed.     

Electronic Structure of New Superconducting Perovskite MgCNi3

The electronic structures of new superconducting perovskite MgCNi3 and related compounds MgCNi2T (T=Co, Fe, and Cu) have been studied using MS-Xa method. In MgCNi3, the main peak of density of states is located below the Fermi level and dominated by Ni d. From the results of total energy calculations, it was found that the number of Ni valence electron decreases faster for the Fe-doped case than that for the Co-doped case. The valence state of Ni changes from +1.43 in MgCNi2Co to +3.02 in MgCNi2Fe. It was confirmed that Co and Fe dopants in MgCNi3 behave as a source of d-band holes and the suppression of superconductivity occurs faster for the Fe-doped case than that for the Co-doped case. In order to explain the fact that Co and Fe dopants in MgCNi3 behave as a source of d-band holes rather than magnetic scattering centers that quench superconductivity, we have also investigated the effects of electron (Cu) doping on the superconductivity and found that both electron (Cu) doping and hole (Co, Fe) dopin