Hans Rudolf Ott

Magnetism and heavy fermion-like behavior in the RBiPt series

Members of the RBiPt (R=Ce–Lu with the exceptions of Pm and Eu) series have been grown as single crystals. Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heat measurements have been performed on representatives. The high temperature resistance uniformly changes from that of a small‐gap semiconductor or semimetal seen in NdBiPt to that of a heavy‐fermion metal seen in YbBiPt, which shows a linear coefficient of specific heat at low temperatures of 8 J/K2u2009mole. Further, the lighter rare earth members show an unusually sharp increase in their resistance associated with antiferromagnetic ordering at low temperatures.

The electrodynamic response of the icosahedral quasicrystal Al70Mn9Pd21

Abstract We report an optical investigation of single-phase icosahedral Al70Mn9Pd21 in a very broad frequency range between 14 and 105 cm-1. The electrodynamic response of this material is characterized by a very low optical conductivity in the infrared and a prominent absorption in the visible frequency range. These unusual optical properties are compatible with the presence of pseudogaps in the electronic density of states of quasi-crystalline materials. The implication of the results of our ac-response experiments with respect to the thermodynamic and dc-transport results, obtained on the same specimen, are also discussed.

Distribution of NMR Relaxations in a Random Heisenberg Chain

NMR measurements of the (29)Si spin-lattice relaxation time T(1) were used to probe the spin-1/2 random Heisenberg chain compound BaCu(2)(Si(1-x)Ge(x))(2)O(7). Remarkable differences between the pure (x=0) and the fully random (x=0.5) cases are observed, indicating that randomness generates a distribution of local magnetic relaxations. This distribution, which is reflected in a stretched exponential NMR relaxation, exhibits a progressive broadening with decreasing temperature, caused by a growing inequivalence of magnetic sites. Compelling independent evidence for the influence of randomness is also obtained from magnetization data and Monte Carlo calculations. These results suggest the formation of random-singlet states in this class of materials, as previously predicted by theory.

High-Tc Superconductivity

The unexpected and really amazing discovery of J.G. Bednorz and K.A. Muller[1], suggesting that certain copper-oxide compounds enter a superconducting state at temperatures exceeding 30 K, provoked an avalanche of research activities worldwide. Early confirmations of the conjecture [2,4], as well as rapid progress in further enhancing the critical temperatures T c in related materials, i.e., other types of copper oxides, to temperatures above the boiling point of nitrogen [5,6], pushed research in superconductivity into the limelight of science and public interest. This unprecedented development in the field of superconductivity is best illustrated by plotting the critical temperature T c as a function of time (see Fig.6.1), exemplifying the role of the cuprate materials.

Field-Induced Quantum Soliton Lattice in a Frustrated Two-Leg Spin-1/2 Ladder

Based on high-field (31)P nuclear magnetic resonance experiments and accompanying numerical calculations, it is argued that in the frustrated S=1/2 ladder compound BiCu(2)PO(6) a field-induced soliton lattice develops above a critical field of μ(0)H(c1)=20.96(7) T. Solitons result from the fractionalization of the S=1, bosonlike triplet excitations, which in other quantum antiferromagnets are commonly known to experience Bose-Einstein condensation or to crystallize in a superstructure. Unlike in spin-Peierls systems, these field-induced quantum domain walls do not arise from a state with broken translational symmetry and are triggered exclusively by magnetic frustration. Our model predicts yet another second-order phase transition at H(c2)>H(c1), driven by soliton-soliton interactions, most likely corresponding to the one observed in recent magnetocaloric and other bulk measurements.

Optical Probing of the Antiferromagnetic Phase Transitions in the Heavy-Electron Compounds U2Zn17 and UCu5

We have investigated the complete electrodynamic response of the heavy-electron compounds U2Zn17 and UCu5. Particular emphasis has been devoted to the optical evidence of the antiferromagnetic phase transitions at 9.7 K and 15 K for U2Zn17 and UCu5, respectively. In UCu5, we found an absorption in the far infrared, which is ascribed to excitations across a spin-density-wave-type gap. This feature is absent in U2Zn17. We argue that UCu5 belongs to a characteristically different class of antiferromagnets than U2Zn17 which represents the class of heavy-electron compounds with localized magnetic moments.

Optical evidence for a spin-filter effect in the charge transport of Eu0.6Ca0.4B6.

We have measured the optical reflectivity R(omega) of Eu0.6Ca0.4B6 as a function of temperature (T) between 1.5 and 300 K and in external magnetic fields (H) up to 7 T. R(omega) increases with decreasing T and increasing H field, but the plasma edge feature does not exhibit the sharp onset and steep slope that is observed in EuB6. The analysis of the H-field dependence of the low-T optical conductivity confirms the previously observed exponential decrease of the electrical resistivity upon increasing bulk magnetization at constant T. The individual exponential magnetization dependences of the plasma frequency and scattering rate are also extracted from the optical data.

Pairing of weakly correlated electrons in the platinum-based centrosymmetric superconductor SrPt3P

We report a study of the normal- and superconducting-state electronic properties of the centrosymmetric compound SrPt3 Pv ia 31 P nuclear-magnetic-resonance (NMR) and magnetometry investigations. Essential features such as a sharp drop of the Knight shift at T< T c and an exponential decrease of the NMR spin-lattice relaxation ratio 1/(T1T )b elowTc are consistent with an s-wave electron pairing in SrPt3P, although a direct confirmation in the form of a Hebel-Slichter-type peak is lacking. Normal-state NMR data at T< 50 K indicate conventional features of the conduction electrons, typical of simple metals such as lithium or silver. Our data are finally compared with available NMR results for the noncentrosymmetric superconductors LaPt3Si and CePt3Si, which adopt similar crystal structures.

Magneto-optical behavior of EuIn2 P2

We report results of a magneto-optical investigation of the Zintl-phase compound EuIn2 P2. The compound orders magnetically at TC =24 K and exhibits concomitant large magnetoresistance effects. For T≤50 K and increasing magnetic fields we observe a transfer of spectral weight in σ1 (ω) from energies above 1 eV into the low-energy metallic component as well as into a midinfrared signal centered at about 600 cm-1. This latter absorption is reminiscent to what has been seen in a large variety of so-called Kondo materials and ascribed to excitations across the hybridization gap. The observed gain of Drude weight upon increasing magnetic field suggests an enhancement

Connections between magnetism and superconductivity in UBe13 doped with thorium or boron (invited)

Magnetism and superconductivity appear to be intimately connected in the heavy electron (HE) superconductors. For example, it has been conjectured but not proven that the exchange of antiferromagnetic spin fluctuations are responsible for pairing in HE superconductors. In this paper we review recent results in U1−xThxBe13, where specific heat, lower critical field, and zero‐field muon‐spin‐resonance (μSR) measurements reveal another second‐order phase transition (below the superconducting transition) to a state which possesses small‐moment magnetic correlations for 0.019≤x≤0.043. We present a new phase diagram for (U,Th)Be13 which indicates that the superconducting and magnetic order parameters are closely coupled. A discussion of the nature of the lower phase is presented, including the consideration of a possible magnetic (time‐reversal‐violating) superconducting state. When UBe13 is doped with B (UBe12.97B0.03) the Kondo temperature is decreased and the specific heat jump at the superconducting transit...