A low-temperature phase diagram H( T) of the 7.8-K superconductor HoNi~B~C (with an onset of 8.3 K) is generated through characterization of well-prepared samples by various experimental techniques including ac magnetic susceptibility, superconducting quantum interference device dc magnetic susceptibility, magnetic hysteresis, specific heat, and electrical resistivity measurements. The results yield a superconducting upper critical field H,.&(0) of 3.5 kG, a lower critical field H"(0) of 250 G, and a Ginzburg-Landau parameter a of 3.5. A nearly reentrant deep minimum at 5.2 K with very small H, z of 400 G and H, & of 5 G are observed. Two distinct magnetic transitions are observed with an incommensurate magnetic ordering temperature T of 5.7 -6 K and an antiferromagnetic Neel temperature T& of 5.2 K. The magnetic entropy A(S +Sz) estimated between 2 and 10 K is 10.4 J/molK. The efFective internal field which causes the nearly reentrant behavior is 2 kG at 5.2 K. I. INTR&DUCTIONRelatively high superconducting transition temperatures T, up to 23 K have been reported in the quaternary borocarbide RTzBzC compounds (R = Sc, Y, Th, U or a rare earth; T=Ni, Pd, or Pt). ' " The superconducting phase has been identified to be of the body-centeredtetragonal LuNizBzC type with space group I4/mmm.The structure is a three-dimensionally connected framework with LuC layers alternated with NizBz layers, where nickel is tetrahedrally coordinated by four boron atoms. 4 Among many nonmagnetic compounds in the Ni system, LuNizBzC exhibits the highest T, of 16.6 K, followed by 15 -16 K for YNizBzC and metastable ScNi~B~C, ' 7 K for ThNi~B~C, and no superconducting transition was found down to 2 K for LaNizBzC. Band-structure calculations on LuNizBzC (Refs. 12,13) indicate a high density of states X(E~) at the Fermi level near the top of the almost-filled Ni(3d) band, with only modest admixture from B and C. All characteristics are indicative of a good, three-dimensional metal. A strongcoupled phonon mechanism for the occurrence of superconductivity is deduced with a very large electronphonon coupling parameter k, which is related to an unusual combination of electronic states at the Fermi level and a substantial contribution from the vibration of the light atoms. ' For compounds containing magnetic rare earth elements such as R=Dy, Ho, Er, and Tm, lower T, values were observed due to the magnetic pair-breaking effect. In fact, HoNizBzC is the most intensively studied compound of the Ni-based system due to its nearly reentrant behavior around 5 -6 K below the superconducting transition temperature T, of 7.5 -8 K. ' ' However, the reported magnetic transition temperatures are ill defined. For example, while yielding consistently an antiferromagnetic transition temperature T& around 5 K, ' ' two neutron diffraction measurements give two different incommensurately modulated/spiral magnetic transition ternperatures T of 8 K (Ref. 15) and 6 K (Ref. 16), respectively. Meanwhile,prior specific-heat measurements show vaguely two shoulders a...

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We derive a generalized theory of a dual photoelastic modulator system designed for the measurement of the Stokes parameters of an arbitrary light beam. Based on the theory, a critique of various potential configurations that could be used to effect such measurements is presented. A calibration procedure for the entire optical system is introduced which, among other things, renders a knowledge of the frequency response of the detector system unnecessary. An analysis of the errors due to incorrect setting of optical components is presented for some of the more practical cases. This allows an assessment of the relative merit of different experimental configurations.