We have fabricated single-phase samples and measured the bulk properties for one ͑Er͒ of a series... more We have fabricated single-phase samples and measured the bulk properties for one ͑Er͒ of a series of ternary, heavy rare-earth, 221 ''plumbide'' intermetallic compounds R 2 Ni 2 Pb (Rϭrare earths). These materials form in the orthorhombic ͑space group Cmmm) structure which is isostructural to Mn 2 AlB 2 compounds. Our results of susceptibility, magnetization, heat capacity, and ͑magneto͒ resistivity on Er 2 Ni 2 Pb show ͑sharp͒ multiple antiferromagnetic transitions and strong field dependences in all bulk properties for the temperature range 2-10 K. We relate this magnetic behavior to the unusual R symmetry ͑partially frustrated͒ of the highly anisotropic plumbide crystal structure.
Zeitschrift für Kristallographie - Crystalline Materials, 2020
High temperature crystal structure of UPt2Si2 determined using single-crystal neutron diffraction... more High temperature crystal structure of UPt2Si2 determined using single-crystal neutron diffraction at 400 K is reported. It is found that the crystal structure remains of the primitive tetragonal CaBe2Ge2 type with the space group P4/nmm. Anisotropic displacement factors of the Pt atoms at the 2a (3/4 1/4 0) and Si atoms at the 2c (1/4 1/4 z) Wyckoff sites are found to be anomalously large.
Motivated by experiment, we argue that the enigmatic hidden order in U Ru 2 Si 2 demands a dual d... more Motivated by experiment, we argue that the enigmatic hidden order in U Ru 2 Si 2 demands a dual description that encompasses both its itinerant and its local aspects. A combination of symmetry considerations and observation allow us to rule out a number of possibilities. The two remaining scenarios, the quadrupolar charge density wave and the orbital antiferromagnet, are discussed and experiments are suggested to select between these proposals.
We have measured Hall effect, magnetotransport and magnetostriction on the field induced phases o... more We have measured Hall effect, magnetotransport and magnetostriction on the field induced phases of single crystalline UPt2Si2 in magnetic fields up to 60 T at temperatures down to 50 mK, this way firmly establishing the phase diagram for magnetic fields B a and c axes. Moreover, for B c axis we observe strong changes in the Hall effect at the phase boundaries. From a comparison to band structure calculations utilizing the concept of a dual nature of the uranium 5f electrons, we propose that these represent field induced topological changes of the Fermi surface due to at least one Lifshitz transition. Furthermore, we find a unique history dependence of the magnetotransport and magnetostriction data, indicating that the proposed Lifshitz type transition is of a discontinuous nature, as predicted for interacting electron systems.
Cubic La(Fe"A1~)» intermetallic compounds can be stabilized with iron concentration x between 0.4... more Cubic La(Fe"A1~)» intermetallic compounds can be stabilized with iron concentration x between 0.46 and 0.92 in the NaZn»-type structure (D23) with Fm3c (O~) space-group symmetry. Here the Fe-Fe coordination number can increase up to 12. At low x values, a mictomagnetic regime occurs with distinct cusps in the ac susceptibility. With the increase of the iron concentration, a soft ferromagnetic phase is found which at lower temperatures shows anisotropy effects related to reentrant mictomagnetic behavior. Finally, for x &0.86, antiferromagnetic order appears along with a sharp metamagnetic transition in external fields of a few teslas. The saturation magnetic moment increases linearly with x from 1.4p~/Fe to 2.1p~/Fe throughout the ferromagnetic and antiferromagnetic regime. The breakdown of long-range ferromagnetic order at high x values can be explained by modifications of the iron moment and their coupling at a large Fe-Fe coordination number. However, with application of a magnetic field, the ferromagnetic state can be fully recovered. The room-temperature resistivity decreases with increasing x from 200 to 160 pQ cm. The low-temperature slope dp/dT is related to the magnetic order, being negative in the antiferromagnetic state and positive in the ferromagnetic state. The metamagnetic transition causes a decrease of the resistivity of about 20% and a sign change in dp/dT. This behavior is discussed in terms of the two-current model. The thermal expansion exhibits a strong Invar character and is described by a combined band and local-moment model which allows calculations of corresponding magnetovolume coupling constants. The metamagnetic transition causes a large magnetic striction.
We have measured the magnetization and magnetoresistance for a series of rare-earth (RϭEr,Ho,Dy) ... more We have measured the magnetization and magnetoresistance for a series of rare-earth (RϭEr,Ho,Dy) plumbide intermetallic compounds, R 2 Ni 2 Pb. These materials form in an unusual orthorhombic structure with space group Cmmm. After multiple magnetic transitions, the ground state exhibits a steplike series of largemoment metamagnetic transitions in low fields ͑1-2 T͒ concomitant with switchinglike properties of the magnetoresistance where sharp changes of up to 30% are found. We relate these properties to the layered magnetic structure of the compound.
We present a detailed study of the structural and magnetic phase transitions of the Heusler compo... more We present a detailed study of the structural and magnetic phase transitions of the Heusler compound Co 2 NbSn. This material undergoes a structural transition at T S ϭ235 K from the cubic Heusler Fm3m hightemperature phase into an orthorhombic low-temperature lattice of Pmma symmetry. Further, the system exhibits a magnetic transition at T C ϭ116 K from para-to ferromagnetism. While on a macro-and mesoscopic scale Co 2 NbSn appears to be fully Fm3m ordered, from microscopic studies we find that on an atomic scale site disorder is present. We discuss the implications of these findings for the anomalous macroscopic physical properties and domain formation in the ferromagnetic state in Co 2 NbSn.
Optical investigations are presented of the filled skutterudites AFe 4 Sb 12 with divalent cation... more Optical investigations are presented of the filled skutterudites AFe 4 Sb 12 with divalent cations A Yb; Ca; Ba. For each of these compounds a very similar pseudogap structure in the optical conductivity develops in the far-infrared spectral region at temperatures below 90 K. Highly accurate local-density approximation electronic band structure calculations can consistently explain the origin of the pseudogap structure generated largely by transition metal 3d states. In particular, a 4f-conduction electron hybridization or strong correlations can be ruled out as origin for the pseudogap.
The variation of the Curie temperature Tc for the weak ferromagnet ZrZn2 has been determined as a... more The variation of the Curie temperature Tc for the weak ferromagnet ZrZn2 has been determined as a function of hydrostatic pressure up to 25 kbar. E~i s found initially to decrease linearly with pressure at a rate 8T~/8P = (-1. 8 +0.04) &10 3'K bar ', followed by an abrupt change of slope at 7.5 kbar and a rapid fall, extrapolating to a critical pressure I' of 8.5 kbar for the destruction of ferromagnetism. The measured T, {P) curve agrees well with the relationship T, (P) = T, (0) (1-PlP~) ' which is derived from the theory of itinerant
The hyperfine coupling constants A on p. 3 of NaFe 4 Sb 12 are too large by a factor of 10. This ... more The hyperfine coupling constants A on p. 3 of NaFe 4 Sb 12 are too large by a factor of 10. This error comes from the conversion of units (kOe and T). The correct hyperfine coupling constants are determined to be A 1 ÿ1:449 kOe= B above T C and A 2 ÿ1:155 kOe= B below T C. We apologize for this error and any confusion that they may have caused. The results and conclusions of our Letter remain unaffected.
Measurements of the low-field ac susceptibility on ternary alloys of Pd+0.85 at. % Fe and a Mn co... more Measurements of the low-field ac susceptibility on ternary alloys of Pd+0.85 at. % Fe and a Mn concentration of 0 to 8 at. jp reveal, for three distinct regimes of Mn concen
The narrow gap semiconductor FeSi owes its strong paramagnetism to electron-correlation effects. ... more The narrow gap semiconductor FeSi owes its strong paramagnetism to electron-correlation effects. Partial Co substitution for Fe produces a spin-polarized doped semiconductor. The spin-polarization causes suppression of the metallic reflectivity and increased scattering of charge carriers, in contrast to what happens in other magnetic semiconductors, where magnetic order reduces the scattering. The loss of metallicity continues progressively even into the fully polarized state, and entails as much as a 25% reduction in average mean-free path. We attribute the observed effect to a deepening of the potential wells presented by the randomly distributed Co atoms to the majority spin carriers. This mechanism inverts the sequence of steps for dealing with disorder and interactions from that in the classic Al'tshuler Aronov approach-where disorder amplifies the Coulomb interaction between carriers-in that here, the Coulomb interaction leads to spin polarization which in turn amplifies the disorder-induced scattering.
The charge-density-wave ͑CDW͒ transitions in compounds R 5 Ir 4 Si 10 (Rϭrare-earth element͒ have... more The charge-density-wave ͑CDW͒ transitions in compounds R 5 Ir 4 Si 10 (Rϭrare-earth element͒ have been studied by x-ray-diffraction and electrical conductivity experiments for temperatures between 20 and 300 K. At T CDW incommensurate CDW's ͓q ជ ϭ(Ϯ0.25Ϯ␦)c ជ * with ␦Ϸ0.03] develop in compounds with RϭHo, Er, Tm, and (Lu 0.16 Er 0.84), while commensurate CDW's ͓q ជ ϭ(n/7)c ជ *͔ develop in compounds with RϭLu and (Lu 0.34 Er 0.66). T CDW varies between 83 K in RϭLu and 161.4 K in RϭHo. The compounds with an incommensurate CDW exhibit a second transition at T lock-in ϽT CDW , with T lock-in between 55 K in RϭEr and 111.5 K in RϭTm. In Ho 5 Ir 4 Si 10 and Er 5 Ir 4 Si 10 this is a pure lock-in transition at which ␦ becomes zero. In Tm 5 Ir 4 Si 10 and (Lu 0.16 Er 0.84) 5 Ir 4 Si 10 ␦ also becomes zero, but below T lock-in additional satellite reflections have been discovered, at commensurate positions (n/8)c ជ * in Tm 5 Ir 4 Si 10 and at incommensurate positions (n/8Ϯ␦ 2)c ជ * with ␦ 2 Ϸ0.01 in (Lu 0.16 Er 0.84) 5 Ir 4 Si 10. The development of this second CDW can be understood by a two-step mechanism similar to the mechanism for the development of the primary CDW in Er 5 Ir 4 Si 10 ͓Galli et al., Phys. Rev. Lett. 85, 158 ͑2000͔͒. At T lock-in the primary CDW becomes commensurate, leading to a partly restoration of the Fermi surface, as evidenced by an anomalous decrease of the electrical resistivity for T below T lock-in in Ho 5 Ir 4 Si 10 and Er 5 Ir 4 Si 10. The modified Fermi surface then provides the favorable nesting conditions for the development of a second CDW in Tm 5 Ir 4 Si 10 and (Lu 0.16 Er 0.84) 5 Ir 4 Si 10. The electronic character of this transition is suggested by the anomalous increase of the resistivity for T below T lock-in .
We performed far-infrared optical spectroscopy measurements on the heavy fermion compound URu2Si2... more We performed far-infrared optical spectroscopy measurements on the heavy fermion compound URu2Si2 as a function of temperature. The light's electric-field was applied along the a-axis or the c-axis of the tetragonal structure. We show that in addition to a pronounced anisotropy, the optical conductivity exhibits for both axis a partial suppression of spectral weight around 12 meV and below 30 K. We attribute these observations to a change in the bandstructure below 30 K. However, since these changes have no noticeable impact on the entropy nor on the DC transport properties, we suggest that this is a crossover phenomenon rather than a thermodynamic phase transition.
We present a detailed study on the (magneto)transport properties of as-cast and heat treated mate... more We present a detailed study on the (magneto)transport properties of as-cast and heat treated material UCu4Pd. We find a pronounced sample dependence of the resistivity ρ of as-cast samples, and reproduce the annealing dependence of ρ. In our study of the Hall effect we determine a metallic carrier density for all samples, and a temperature dependence of the Hall constant which is inconsistent with the Skew scattering prediction. The magnetoresistive response is very small and characteristic for spin disorder scattering, suggesting that overall the resistivity is controlled mostly by nonmagnetic scattering processes. We discuss possible sources for the temperature and field dependence of the transport properties, in particular with respect to quantum criticality and electronic localization effects.
We have examined the ground-state properties of the putative intermediate valency and/or charge d... more We have examined the ground-state properties of the putative intermediate valency and/or charge density wave ͑CDW͒ system Yb 5 Ir 4 Si 10 via resistivity, specific heat, magnetic susceptibility, and x-ray absorption spectroscopy. The bulk properties confirm a CDW-type anomaly at 48 K, but show that all the Yb ions are in an integral trivalent 4f 13 state. Additionally, our low-temperature measurements reveal two magnetic transitions at 2.3 and 1.1 K, respectively. The compound shows Kondo-lattice-type behavior that might be responsible for the weak-coupling nature of the CDW evidenced by the specific heat data.
NMR measurements on various samples of U 2 Ru 2 Sn, a tetragonal Kondo insulating system, are pre... more NMR measurements on various samples of U 2 Ru 2 Sn, a tetragonal Kondo insulating system, are presented. We observed anisotropic Knight shifts in all the samples. The anisotropy increases at low temperatures. The Knight shifts as well as the spin-lattice relaxation rates show signatures of gap opening at low temperatures. The hyperfine coupling exhibits an anomalous temperature dependence near the Kondo temperature. The results are discussed in the light of a low-temperature gap with residual density of states in this system.
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Papers by J. Mydosh