V.N. Narozhnyi

Upper critical field and irreversibility line in superconducting MgB2

Abstract The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between T c ∽39 K and T=3 K ∽0.1T c . The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc similar to that found for the borocarbides YNi2B2C and LuNi2B2C indicating that MgB2 is in the clean limit. The irreversibility line was consistently determined from dc magnetization measurements and from the imaginary component of ac susceptibility. The irreversibility field was found to increase up to 8.5 T at 10 K.

Ferromagnetic carbon with enhanced Curie temperature

Abstract The discovery of a ferromagnetic form of carbon (Nature 413 (2001) 716) gives a new perspective in the investigation of magnetic materials. The existence of a ferromagnetic state with the very high Curie temperature T C ≈500 K for a material with only s- and p-electrons as well as the nature of its underlying interaction are of great fundamental interest. Here we report on the observation of the ferromagnetically ordered state in a material obtained by high-pressure high-temperature treatment of the fullerene C60. It has a saturation magnetization more than four times larger than that reported previously. From our data we estimated the considerably higher value of T C ≈820 K .

The upper critical field in superconducting MgB2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between Tc∼39 K and T=3 K∼0.1Tc. The Hc2 data from ac susceptibility are consistent with resistance data and represent the upper critical field of the major fraction of the investigated sample which increases up to Hc2(0)=13 T at T=0 corresponding to a coherence length of ξo=5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by dc magnetization measurements. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc and at medium temperatures indicating that MgB2 is in the clean limit. The Hc2(T) dependence can be described within a broad temperature region 0.3Tc<T≤Tc by a simple empirical expression Hc2(T)∝(1−T/Tc)1+α, where the parameter α specifies the positive curvature of Hc2(T). This positive curvature of Hc2(T) is similar to that found for the borocarbides YNi2B2C and LuNi2B2C.

Fully dense MgB2 superconductor textured by hot deformation

Abstract Bulk textured MgB 2 material of nearly full density showing a weak c -axis alignment of the hexagonal MgB 2 grains parallel to the pressure direction was obtained by hot deformation of a stoichiometric MgB 2 pellet prepared by a gas–solid reaction. The texture of the material was verified by comparing the X-ray diffraction patterns of the hot deformed material with isotropic MgB 2 powder. A small, but distinct anisotropy of the upper critical field up to H c 2 a , b / H c 2 c ∼1.2 depending on degree of texture was found by resistance and susceptibility measurements. No anisotropy of the critical current density determined from magnetization measurements was found for the textured material.

Evidence for strong electron-phonon coupling in MgCNi3

The title compound is investigated by specific heat measurements in the normal and superconducting states supplemented by upper critical field transport, susceptibility, and magnetization measurements. From a detailed analysis including also full potential electronic structure calculations for the Fermi surface sheets, Fermi velocities, and partial densities of states the presence of both strong electron-phonon interactions and considerable pair breaking has been revealed. The specific heat and the upper critical field data can be described to a first approximation by an effective single-band model close to the clean limit derived from a strongly coupled predominant hole subsystem with small Fermi velocities. However, in order to account also for Hall-conductivity and thermopower data in the literature, an effective general two-band model is proposed. This two-band model provides a flexible enough frame to describe consistently all available data within a scenario of phonon mediated

Rare earth transition metal borocarbides (nitrides): superconducting, magnetic and normal state properties

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HALL EFFECT IN LUNI2B2C AND YNI2B2C BOROCARBIDES : A COMPARATIVE STUDY

-wave superconductivity somewhat suppressed by a sizable electron-paramagnon or electron-electron Coulomb interaction. For quantitative details the relevance of soft phonons and of a Van Hove\char21{}type singularity in the electronic density of states near the Fermi energy is suggested.

Anomalous magnetic ordering in PrBa2Cu3O7−y single crystals: evidence for magnetic coupling between the Cu and Pr sublattices

Preface. Introduction and Overview. Electronic Structure and Properties and Phonon Spectra. Magnetic Properties and CEF Effects. Interplay between Superconductivity and Magnetism. Vortex Lattice. Thin Films and Crystal Structure. Nature of the Superconducting State in Borocarbides. Summary and Outlook. Subject Index. Author Index. List of Participants.

Anomalous behavior of PrNi{sub 2}B{sub 2}C borocarbide

The Hall effect in LuNi_2B_2C and YNi_2B_2C borocarbides has been investigated in normal and superconducting mixed states. The Hall resistivity rho_{xy} for both compounds is negative in the normal as well as in the mixed state and has no sign reversal below T_c typical for high-T_c superconductors. In the mixed state the behavior of both systems is quite similar. The scaling relation rho_{xy}\sim\rho_{xx}^\beta (\rho_{xx} is the longitudinal resistivity) was found with \beta=2.0 and 2.1 for annealed Lu- and Y-based compounds, respectively. The scaling exponent \beta decreases with increasing degree of disorder and can be varied by annealing. This is attributed to a variation of the strength of flux pinning. In the normal state weakly temperature dependent Hall coefficients were observed for both compounds. A distinct nonlinearity in the \rho_{xy} dependence on field H was found for LuNi_2B_2C in the normal state below 40K, accompanied by a large magnetoresistance (MR) reaching +90% for H=160kOe at T=20K. At the same time for YNi_2B_2C only linear \rho_{xy}(H) dependences were observed in the normal state with an approximately three times lower MR value. This difference in the normal state behavior of the very similar Lu- and Y-based borocarbides seems to be connected with the difference in the topology of the Fermi surface of these compounds.

Hall-Effect study of YNi2B2C superconductor

Abstract In Al-free PrBa2Cu3O7−y single crystals the kink in the temperature dependence of magnetic susceptibility χab(T), connected with Pr antiferromagnetic (AFM) ordering, disappears after field cooling (FC) in a field H‖ab-plane. The kink in χc(T) remains unchanged after FC in H‖c-axis. As a possible explanation, freezing of the Cu magnetic moments, lying in the ab-plane, caused by FC in H‖ab, hinders their reorientation and, due to coupling between the Pr and Cu(2) sublattices, ordering of the Pr3+ moments. A field-induced phase transition and a field dependence of the Pr3+ ordering temperature have been found for both H‖c and H‖ab.