P.H. Frings

Magnetic properties of UxPty compounds

Abstract Magnetization, magnetic susceptibility and specific heat studies have been performed on a series of uranium-platinum compounds. The magnetic ordr of UPt is confirmed, whereas in UPt 2 and UPt 5 no magnetic order is observed. An antiferromagnetic type of order, suggested for UPt 3 by a peak in the susceptibility near 19 K and by the magnetic transition in the magnetization curve at 4.2 K near 19 T is not supported by the specific heat data in which no anomaly near 19 K was found. Additional magnetization and susceptibility measurements on a single-crystalline sample exclude a simple type of antiferromagnetic order. The low-temperature specific heat of UPt 3 is rather similar to that of the “spin-fluctuation” compound UAl 2 .

Excitons in carbon nanotubes with broken time-reversal symmetry.

Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields (>55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or dark).

Electron−phonon coupling in uranium compounds

Abstract We present results of elastic measurements for UN, UPd 3 , UBe 13 and UPt 3 . Exchange striction effects are seen for UN and crystal field anomalies for UPd 3 . Polycrystalline UBe 13 exhibits an anomalously low Poisson ratio. UPt 3 shows low temperature anomalies in various elastic constants and strong anharmonic effects for the c 44 mode.

Magnetic order in UPt3 with 5% Pd

Abstract The compound U(Pt 0.95 Pd 0.05 ) 3 is shown to order antiferromagnetically with q = ( 1 2, 0, 0 ) and the moments parallel to this direction. The calculated value of (0.6 ± 0.2)μ B for the moment per uranium atom is in agreement with magnetisation data. The very similar magnetisation curve in the basal plane of Upt 3 could be caused by strong antiferromagnetic correlations as well.

Magnetic and superconducting properties of UPt3

Abstract Recent results of experimental studies on the thermodynamic and transport properties of the heavy-fermion superconductor UPt 3 are reviewed. These studies include high-magnetic-field and high-pressure experiments in the normal and superconducting states as well as alloying effects on the normal- and superconducting-state properties. The low-temperature properties of UPt 3 are unusual and no satisfactory description in terms of crystal-field effects, spin-fluctuation phenomena, antiferromagnetic order, singlet or triplet superconductivity can be offered.

Magnetic properties of UPt

Abstract Recent results regarding the magnetic and electrical properties of UPt under high pressures are presented, together with the results of neutron diffraction and neutron depolarisation studies. The experiments point to two different magnetic phases in UPt with ordering temperatures of 19 and 27 K, respectively. Internal stresses and oxide content determine the amount in which each of these two phases is present in a particular sample. The magnetic phase with the ordering temperature of 27 K exhibits a ferro- to antiferromagnetic transition at 4 kbar. This antiferromagnetic phase is characterised at 4.2 K by a metamagnetic transition at a field of 3 T.

Strength versus conductivity

Abstract In the development of conductors for high-field coils one has to find a compromise between high strenght (to withstand the Lorentz force) and high conductivity (longer pulse time, minimal power, energy and volume required). The design of coils is a non-linear process that is partly based on educated guesses guided by trial and error. Based on the concept of the “current carrying capability” a simple scheme for optimal choice of wire for the relatively simple cases of energy limited coils (or the innermost parts of power limited coils) is developed. On basis of this criterion, the optimal direction of research on new wires or materials is determined.

Superconductivity, spin fluctuations and magnetic order in uranium compounds under high pressure

Abstract The systematics in the occurrence of superconductivity and magnetic order in the actinide compounds is usually represented by a critical value for the nearest neighbour distance between the actinide ions. On the basis of these systematics a suppression of magnetism and an enhancement of superconductivity with pressure is expected. High-pressure results are reviewed for a superconducting system (U6Co), a spin-fluctuating compound (UPt3) and for a series of (nearly) ferromagnetic compounds (UX2 with X = Fe, Co, Ni, Pt, Ga and Ge).

Magnetic properties of single-crystalline UNi2

Abstract Magnetisation measurements on single-crystalline samples of the hexagonal Laves-phase compound UNi 2 reveal a spontaneous magnetic moment along an easy (basal plane) direction of 0.081 μ B per formula unit at 4.2 K. The anisotropy field as determined from the magnetisation curves along the easy and hard directions at 4.2 K is estimated to be much larger than the maximal field applied in these experiments (35 T). Specific-heat measurements yield a value for the coefficient of the electronic term of 65 mJ/K 2 mol f.u. and a magnetic entropy, one order of magnitude smaller than that associated with a doublet.

Anti-ferromagnetic correlations in UPt3

Abstract We report on polarised as well as unpolarised neutron scattering experiments on two single-crystalline samples of UPt3 from different sources. Previously reported magnetic intensity, probably due to anti-ferromagnetic correlations with relatively high characteristic energies (of the order of 5–8 meV) along the c-axis of the hexagonal structure, was observed in both samples. However, the ratio of the magnetic intensity to the phonon intensity was found to be a factor of five less than suggested by other authors. The magnetic character of the observed scattering was unambiguously proved by means of polarised neutron scattering. Furthermore, we observed in both samples a temperature-dependent quasi-elastic response for Q-values in the neighbourhood of the magnetic satellite positions for the anti-ferromagnetically ordered compounds U0.95Th0.05Pt3 and U(Pt0.95Pd0.05)3. This indicates anti-ferromagnetic correlations in the basal plane along the a∗-axis in pure UPt3. Moreover, we observed in only one of the samples long-range anti-ferromagnetic order with an ordered moment of the order of 0.01μB and a Neel temperature of about 5 K.