The superconducting and magnetic properties of HoNi2B2C single crystals are investigated through transport, magnetometry and small-angle neutron scattering (SANS) measurements. In the magnetic phases that enter below the superconducting critical temperature, the small-angle neutron scattering data uncover networks of magnetic surfaces. These likely originate from uncompensated moments, e.g., at domain walls pinned to crystallo-graphic grain boundaries. The field and temperature dependent behavior of SANS appears consistent with the metamagnetic transitions reported in earlier works.

We report on the measurements of the superconducting order parameter in the nonmagnetic borocarbides LuNi${}_{2}$B${}_{2}$C and YNi${}_{2}$B${}_{2}$C. Andreev conductance spectra are obtained from nanoscale metallic junctions on single crystal surfaces prepared along three major crystallographic orientations: [001], [110], and [100]. The gap values extracted by the single-gap Blonder-Tinkham-Klapwijk model follow the theoretical predictions as a function of temperature and magnetic field and exhibit a small anisotropy with no indication of proposed gap nodes along the [100] and [010] directions. These observations are robust and reproducible among all the measurements on two different sets of LuNi${}_{2}$B${}_{2}$C crystals and one set of YNi${}_{2}$B${}_{2}$C crystals. We suggest that the possible gap nodes in the [100] direction may be masked by two effects: different gap anisotropy across multiple Fermi surfaces, as reported in the recent photoemission spectroscopy, and the large tunneling cone. Our results provide a consistent picture of the superconducting gap structure in these materials, addressing the controversy particularly in the reported results of point-contact Andreev reflection spectroscopy.

We report on the measurements of the superconducting order parameter in the nonmagnetic borocarbides LuNi2B2C and YNi2B2C. Andreev conductance spectra are obtained from nanoscale metallic junctions on single crystal surfaces prepared along three major crystallographic orientations: [001], [110], and [100]. The gap values extracted by the single-gap Blonder-Tinkham-Klapwijk model follow the theoretical predictions as a function of temperature and magnetic field and exhibit a small anisotropy with no indication of proposed gap nodes along the [100] and [010] directions. These observations are robust and reproducible among all the measurements on two different sets of LuNi2B2C crystals and one set of YNi2B2C crystals. We suggest that the possible gap nodes in the [100] direction may be masked by two effects: different gap anisotropy across multiple Fermi surfaces, as reported in the recent photoemission spectroscopy, and the large tunneling cone. Our results provide a consistent picture of the superconducting gap structure in these materials, addressing the controversy particularly in the reported results of point-contact Andreev reflection spectroscopy.

The effect of irradiation by 50MeV Li3+ and 200MeV Ag15+ ions on single crystals of Tl2Ca2Ba2Cu3O10 (Tl2223) superconductor has been investigated at different fluences. Isothermal magnetization hysteresis loops have been recorded at different temperatures using a SQUID magnetometer and the effect of irradiation on the critical current density, irreversible field, second magnetization peak and pinning force has been studied. Irradiation by 200MeV Ag15+ ions resulted in increased hysteresis and irreversibility field while no change in second magnetization peak position and critical temperature was observed. A broadening in the hysteresis loop before the second magnetization peak was also observed for the crystals irradiated by Li3+ ions. Annealing of irradiated crystals at 500°C resulted in reduction of point defects created by Li3+ ions.

The low-energy excitations of cuprate superconductors exhibit various characteristics that differ from those of simple Bogoliubov quasiparticles for pure d_{x^2-y^2}-wave superconductors. Here we report experimental studies of spatially resolved quasiparticle tunnelling spectra of hole- and electron-type cuprate superconductors that manifest direct evidences for the presence of competing orders (COs) in the cuprates. In contrast to conventional type-II superconductors that exhibit enhanced local density of states (LDOS) peaking at zero energy near the centre of field-induced vortices, the vortex-state LDOS of YBa_2Cu_3O_{7-\delta} (Y-123) and La_{0.1}Sr_{0.9}CuO_2 (La-112) remains suppressed inside the vortex core, with pseudogap (PG)-like features at an energy larger (smaller) than the superconducting (SC) gap \Delta_{SC} in Y-123 (La-112). Energy histograms of the SC and PG features reveal steady spectral shifts from SC to PG with increasing magnetic field H. These findings may be explained by coexisting COs and SC: For hole-type cuprates with PG above T_c, the primary CO gap (V_{CO}) is larger than \Delta_{SC} and the corresponding COs are charge/pair-density waves with wave-vectors parallel to (\pi,0)/(0,\pi). For electron-type cuprates without PG above T_c, V_{CO} is smaller than \Delta_{SC} and the CO wave-vector is along (\pi,\pi). This CO scenario may be extended to the ARPES data to consistently account for the presence (absence) of Fermi arcs in hole- (electron)-type cuprates. Fourier transformation of the vortex-state LDOS in Y-123 further reveals multiple sets of energy-independent wave-vectors due to field-enhanced pair- and spin-density waves. These results imply important interplay of SC with low-energy collective excitations.

Impurity-substituted Kondo semiconductors Yb1−xLuxB12 and Sm1−yEuyB12 have been studied by the highly bulk-sensitive hard x-ray (hv ~ 8 keV) photoelectron spectroscopy. The valence-band spectra for Yb1−xLuxB12 have been analyzed by the single-impurity Anderson model (SIAM). The temperature dependence of the bulk Yb 4/ peak positions for x = 0 cannot be interpreted within the SIAM, whereas that for x = 1/8 could be understood by the SIAM. This suggests that the Kondo lattice coherence, which is essential for pure YbB12, collapses due to the substitution of x = 1/8. For Smi1−yEuyB6, the y dependence of the Sm and Eu 3d core-level spectra has been systematically examined. We have found that the Sm valence increases with y due to the reduction of the hybridization between the Sm 4f and valence-bands.

The flux pinning mechanisms of nano-Si-doped MgB2 are reported in this work. The field dependence of the critical current density, J(c)(B), was analyzed within the collective pinning model. We found that the mechanisms for both delta l pinning, i.e., pinning associated with charge-carrier mean free path fluctuations, and delta T-c pinning, which is associated with spatial fluctuations of the transition temperature, coexist in the nano-Si-doped MgB2 samples, while H-c2 increases greatly with increasing nano-Si doping level. However, their contributions are strongly temperature dependent. The delta l pinning is dominant at low temperatures, decreases with increasing temperature, and is suppressed completely at temperatures close to the critical temperature, T-c. However, the delta T-c pinning mechanism shows opposite trends.

The magnetic penetration depth lambda has been measured in MgCNi3 single crystals using both a high-precision tunnel diode oscillator (TDO) technique and Hall probe magnetization (HPM). In striking contrast to previous measurements in powders, delta lambda(T) deduced from TDO measurements increases exponentially at low temperature, clearly showing that the superconducting gap is fully open over the whole Fermi surface. An absolute value at zero temperature lambda(0)=230 nm is found from the lower critical field measured by HPM. We also discuss the observed difference of the superfluid density deduced from both techniques. A possible explanation could be due to a systematic decrease in the critical temperature at the sample surface.

We have studied the order–disorder transition in high quality MgB2 single crystals, using a torque magnetometry combined with a ‘vortex shaking’ technique. In the wide range of temperature T, field H and the H direction, we succeed in obtaining reversible magnetization curves Mrev(T, H) by shaking the pinned vortices. Especially at low temperatures below 25K and high fields, where the irreversible magnetization curve exhibits the peak effect due to the order–disorder transition, it is found that the peak is transformed into the clear step in Mrev(H). Similar step-like behavior is also observed in the temperature dependence of magnetization Mrev(T). These results give direct evidence that the order–disorder transition, which is hidden by the large hysteresis of magnetization, has the nature of first-order transition.

Measurements of reversible magnetization have been performed on high quality MgB2 single crystals using torque magnetometry combined with the 'vortex shaking' technique. At high temperatures above 26 K, a step like decrease in the magnetization curve is observed at H-m with and without shaking. This anomaly can be ascribed to the first order vortex lattice melting transition. At low temperature below 25 K, where the irreversible magnetization curve reveals the peak effect, we succeed in suppressing the hysteresis completely by shaking and observing the clear step in the reversible magnetization curve at the field H* for the peak effect. The entropy change, estimated from the step of the reversible magnetization at H-m is 2-3 times larger than that at H*.