Showing papers by "Philipp Gegenwart published in 2001"

Staggered field effect on the one-dimensional S = 1/2 antiferromagnet Yb4As3.

Abstract: Inelastic neutron scattering measurements of magnetic excitations in the charge-ordered state of ${\mathrm{Yb}}_{4}{\mathrm{As}}_{3}$ have been performed under magnetic field up to about 6 T. By applying a magnetic field, the spectrum at the one-dimensional wave vector $q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ [ $\ensuremath{\pi}/d$] changes drastically from a broad one corresponding to the spinon excitation continuum of the one-dimensional $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\frac{1}{2}$ spin system to a sharp one at a finite energy, indicating the opening of an energy gap in the system. The magnetic field dependence of the gap is well fitted by the power law ${H}^{2/3}$. The experimental result gives strong evidence for the existence of a staggered field alternating along ${\mathrm{Yb}}^{3+}$ chains induced by the Dzyaloshinsky-Moriya interaction.

Low-energy excitations of the semimetallic one-dimensional S=1/2 antiferromagnet Yb4As3

Abstract: We investigate a new route to quasi-one-dimensional spin-chain systems which originates from the charge-ordering transition out of a homogeneous mixed-valence state. We present evidence that Yb4As3 and P, Sb doped mixed crystals are well described by this mechanism. Many thermodynamic and low-temperature transport heavy-fermion-like properties can be explained by the existence of low-lying quasi-one-dimensional spin excitations in the Yb3+-chains. The observation of soliton excitations in a transverse external field gives further support to the existence of spin chains in Yb4As3. We also present recent results on spin-glass behavior at very low temperature caused by the interchain coupling and disorder. In addition, the yet unexplained magnetotransport effects are discussed.

Evidence for low-dimensional magnetic behaviour in CePt5Ge3

Abstract: We present the results of magnetic susceptibility, magnetization, heat capacity, resistivity and magnetoresistance measurements for the low-dimensional Ce-based intermetallic compounds CePd5Ge3 and CePt5Ge3, which crystallize in the YNi5Si3-type structure. Magnetic order is manifested by a peak in both the magnetic susceptibility and the specific heat at 1.9 K and 1.1 K for CePd5Ge3 and CePt5Ge3, respectively. For CePt5Ge3 a significant tail in the specific heat from TN up to 5 K indicates pronounced antiferromagnetic short-range correlation of low-dimensional character above the 3D long-range order. Both compounds exhibit large negative magnetoresistance at low temperature.

Influence of disorder on superconductivity in rare-earth nickel borocarbides

Abstract: The superconducting properties of pseudo-quaternary Hox R 1_xNi2B2C compounds with R = Y and Lu and of nonmagnetic YxLu1-xNi2B2C compounds have been investigated. The superconducting transition temperature T c of the Ho-based compounds is mainly controlled by magnetic pair-breaking and, for R = Lu, additionally influenced by local lattice distortions due to the large difference of the ionic radii between the Lu and the Ho ion. The effect of disorder-induced local lattice distortions is studied in nonmagnetic YxLu1-xNi2B2C compounds. Disorder is found to reduce several relevant quantities as T c , the upper critical field H c2 , a characteristic positive curvature of H c2 (T) observed for these compounds, the Sommerfeld parameter y N in the normal state as well as the curvature of γ (H) with γ as the electronic specific heat coefficient in the mixed state. These quantities have their highest values for the pure compounds and show a minimum for the most disordered compounds near x = 0.5. The results for H c2 (T) are explained within a two-band model for clean limit superconductors. It is shown that growing degree of substitutional disorder results in a reduction of the relevant properties. The case of dirty limit is not yet reached even for the maximum disordered samples, whereas disorder on the transition metal site in Y(Ni0.8Pt0.2)2B2C reported in literature points to a dirty limit superconductor.

Specific heat and disorder in the mixed state of non-magnetic borocarbides

Abstract: The temperature and magnetic field dependence of the specific heat cp(T,H) in the superconducting mixed state as well as the upper critical field Hc2(T) have been measured for polycrystalline Y_xLu_{1-x}Ni_2B_2C and Y(Ni_{1-y}Pt_y)_2B_2C samples. The linear-in-T electronic specific heat contribution gamma(H)T exhibits significant deviations from the usual linear-in-H law for all x and y the transition metal site (T) resulting in a disorder dependent negative curvature of gamma(H). The deviations from that linear behaviour of our unsubstituted samples are the largest reported so far for any superconductor. The H_c2(T) data point to the quasi-clean limit for (Y,Lu)-substitutions and to a transition to the quasi-dirty limit for (Ni,Pt)-substitutions. The gamma(H) dependence is discussed in the unitary d-wave as well as in the quasi-clean s-wave limits. From a consideration of gamma(H) data only, d-wave pairing cannot be ruled out.

SPECIFIC HEAT OF YxLu1-xNi2B2C IN THE MIXED STATE

Abstract: The temperature and magnetic field dependences of the electronic specific heat c p in the superconducting mixed state as well as the upper critical field H C2 (T) have been measured on a series of polycrystalline Y x Lu1-x Ni2B2C samples with the aim to study the influence of disorder. The electronic specific heat contribution c esm = γ(H) · T in the mixed (vortex) state exhibits significant deviations from the usual γ(H) α H-law for conventional superconductors for all compositions resulting in a disorder dependent negative curvature of γ α H 1- β(x) For the samples with sub-stitutional disorder at the rare earth site β and the positive curvature exponent α of H C2 (T) are reduced but α, β > 0 holds for all x, as distinct from the case of much stronger disorder due to iso-electronic partial Pt substitutions at the Ni site where α and β rapidly vanish (Nohara et al. (1999) J. Phys. Soc. Jpn. 68 1078). β exhibits a clear minimum near x = 0.5. The magnitudes of H C2 (T) and T c , the curvatures of H C2 (T)and γ(H), as well as the Sommerfeld constant βN in the normal state are reduced in a similar fashion for intermediate composition.