Upon closely examining the Seebeck behaviour of Hg-1201 we have discovered that the system, whether stoichiometric or mercury deficient, appears to behave in similar fashion to the LBCO and YBCO-123 systems in that a dip appears in Tc ( p ) at p ~1/8. We have earlier indicated that this concentration of holes marks the point at which stripe phase formation in the organization of charge and spin in the two-subsystem, mixed-valent, HTSC materials passes from one charge wall loading scheme to another. We observe a strong difference in doping characteristics between the mercury-deficient and mercury-stoichiometric systems, and assemble here a plausible scenario of what is involved. While HgBa2 CuO4+ indeed does offer very considerable potential advantages as an archetype to the cuprate HTSC phenomenon, excess anion systems like this clearly introduce materials complexities. These will always demand careful attention and perhaps not endear the system to many accustomed to the relative simplicities of a cation-doped system.
We have synthesized single crystals of Yb 2 Pt 2 Pb, which crystallize in the layered U 2 Pt 2 Sn-type structure, where planes of Yb ions lie on a triangular network. Here, we report the results of magnetization, specific heat, and electrical resistivity experiments. The lattice constants and high temperature magnetic susceptibility indicate that the Yb ions are trivalent, while the Schottky peaks in the specific heat show that the ground state is a well isolated doublet. A significant magnetic anisotropy is observed, with the ratio of susceptibilities perpendicular and parallel to the magnetic planes differing by as much as a factor of 30 at the lowest temperatures. Antiferromagnetic order occurs at a Néel temperature T N = 2.07 K. Evidence of short range magnetic fluctuations is found in the magnetic susceptibility and electrical resistivity, which have broad peaks above T N , and in the slow development of the magnetic entropy at T N . Our experiments indicate that Yb 2 Pt 2 Pb is a quasi-twodimensional and localized moment system, where strong magnetic frustration may arise from the geometry of the underlying Shastry-Sutherland lattice.
We report the synthesis and basic properties of single crystals of a new binary compound, Yb3Pt4. The Yb ions in this compound are fully trivalent, and heat capacity measurements show that the crystal field scheme involves a doublet ground state, well separated from the excited states, which are fully occupied above ∼ 150 K. The heat capacity displays a large, weakly first order anomaly at 2.4 K, where a cusp is observed in the magnetic susceptibility signalling the onset of antiferromagnetic order. The entropy associated with this order is the full Rln2 of the doublet ground state, however the magnetic susceptibility in the ordered phase is dominated by a large and temperature independent component below the Neel temperature. The heat capacity in the ordered state originates with ferromagnetic spin waves, giving evidence for the inherently local moment character of the ordered state. The electrical resistivity is unusually large, and becomes quadratic in temperature exactly at the Neel temperature. The absence of analogous Fermi liquid behavior in the heat capacity and the magnetic susceptibility implies that Yb3Pt4 is a low electron density system, where the Fermi surface is further gapped by the onset of magnetic order.
PET image reconstruction using a SM made from an accurately characterized PSF that accounts for r and d dependencies results in improved spatial resolution and contrast-noise relations, which may aid in lesion boundary detection for treatment planning or quantitative assessment of treatment response.
We present measurements of the specific heat, magnetization, magnetocaloric effect and magnetic neutron diffraction carried out on single crystals of antiferromagnetic Yb3Pt4, where highly localized Yb moments order at TN = 2.4 K in zero field. The antiferromagnetic order was suppressed to TN → 0 by applying a field of 1.85 T in the ab plane. Magnetocaloric effect measurements show that the antiferromagnetic phase transition is always continuous for TN > 0, although a pronounced step in the magnetization is observed at the critical field in both neutron diffraction and magnetization measurements. These steps sharpen with decreasing temperature, but the related divergences in the magnetic susceptibility are cut off at the lowest temperatures, where the phase line itself becomes vertical in the field-temperature plane. As TN → 0, the antiferromagnetic transition is increasingly influenced by a quantum critical endpoint, where TN ultimately vanishes in a first order phase transition.
We present the results of electrical resistivity and Hall effect measurements on single crystals of HfNiSn, TiPtSn, and TiNiSn. Semiconducting behavior is observed in each case, involving the transport of a small number of highly compensated carriers. Magnetization measurements suggest that impurities and site disorder create both localized magnetic moments and extended paramagnetic states, with the susceptibility of the latter increasing strongly with reduced temperature.The magnetoresistance is sublinear or linear in fields ranging from 0.01 -9 Tesla at the lowest temperatures. As the temperature increases, the normal quadratic magnetoresistance is regained, initially at low fields, and at the highest temperatures extending over the complete range of fields.The origin of the vanishingly small field scale implied by these measurements remains unknown, presenting a challenge to existing classical and quantum mechanical theories of magnetoresistance.
We have combined the results of magnetization and Hall effect measurements to conclude that the ferromagnetic moments of lightly doped CaB 6 samples display no systematic variation with electron doping level. Removal of the surface with acid etching substantially reduces the measured moment, although the Hall constant and resistivity are unaffected, indicating that the ferromagnetism largely resides on and near the sample surface. Electron microprobe experiments reveal that Fe and Ni are found at the edges of facets and growth steps, and on other surface features introduced during growth. Our results indicate that the weak ferromagnetism previously reported in undoped CaB 6 is most likely of extrinsic origin.
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