B. Fåk

Crystal-field ground state of the noncentrosymmetric superconductor CePt3Si: A combined polarized soft x-ray absorption and polarized neutron study

We determined the crystal-field split Hunds rule ground state of the non-centrosymmetric heavy fermion superconductor CePt3Si with polarization dependent soft X-ray absorption spectroscopy (XAS) and polarized neutron scattering. We are also able to give the sequence of the crystal-field states from the temperature evolution of the linear dichroic signal in the XAS. The quantitative analysis of the XAS temperature dependence together with the neutron transition energies complete the identification of the crystal-field level scheme.

Pressure dependence of the magnetic structure of the itinerant electron magnet MnSi

Elastic neutron scattering measurements have been performed under hydrostatic pressure on single-crystalline MnSi. The scaling between the staggered moment and the transition temperature of the helical magnetic structure does not follow predictions from spin-fluctuation theories. For pressures below –13 kbar, where the transition is second order, the length of the helix decreases with increasing temperature. Such a behaviour is not expected for a ferromagnetic Dzyaloshinsky–Moriya instability. With increasing pressure, the length of the helix also decreases, but there is a lock-in to an incommensurate temperature-independent value above p*, where the phase transition is first order.

From heavy fermion antiferromagnetism to localized ferromagnetism: competition of two ground states in CeRu2Ge2 on cooling

Resistivity, specific heat and neutron scattering measurements performed on single crystals of CeRu2Ge2 are reported. The intrinsic behaviour of this compound is to exhibit a double transition from a paramagnetic to an antiferromagnetic and then a ferromagnetic state on lowering the temperature with TN = 8.3 K and respectively TC = 7.5 K. For the first time the antiferromagnetic order is characterized by neutron scattering. The propagation vector of the sine-wave modulated structure is k = (0.31,0,0) as in the Si doped compounds. This phase is found to be easily weakened by disorder, but an antiferromagnetic moment as large as 1.5 µB develops between TN and TC for the best crystal.

Magnetic correlations in single-crystalline CeNi2Ge2

The antiferromagnetic correlations in the non-Fermi-liquid compound CeNi2Ge2 have been measured on single crystals using inelastic neutron scattering. The spin fluctuations with a characteristic energy of 4 meV are centred at the incommensurate wave vector (0.23, 0.23, 0.5), which is similar to where magnetic order develops in the related compound CeCu2Ge2. The magnetic correlations are anisotropic, with a quasi-two-dimensional extension along the [110] direction, implying that the moments are mostly correlated in the [110] planes. With increasing temperature, the correlated signal decreases, and above T* ≈ 30 K the magnetic scattering appears to be wave-vector independent.

Neutron-diffraction study of the evolution of antiferromagnetic order in UPt3 doped with Pd

Neutron-diffraction experiments have been carried out on U(Pt1-xPdx)3 single crystals (x = 0.01 a large moment antiferromagnetic phase (LMAF) is found. For this phase at optimum doping (x= 0.05) TN attains a maximum value of 5.8 K and the ordered moment equals 0.63(5) mB/U-atom. TN(x) for the LMAF follows a Doniach-type phase diagram. We infer that the antiferromagnetic instability in U(Pt1-xPdx)3 is located in the range 0.5-1 at.% Pd.

Low-energy magnetic response of the noncentrosymmetric heavy-fermion superconductor CePt 3 Si studied via inelastic neutron scattering

The low-energy magnetic excitations of the noncentrosymmetric heavy-fermion superconductor CePt3Si have been measured with inelastic neutron scattering on a single crystal. Kondo-type spin fluctuations with an anisotropic wave vector dependence are observed in the paramagnetic state. These fluctuations do not survive in the antiferromagnetically ordered state below TN=2.2 K but are replaced by damped spin waves, whose dispersion is much stronger along the c-axis than in other directions. No change is observed in the excitation spectrum or the magnetic order as the system enters the superconducting state below Tc=0.7 K.

Neutron-diffraction study of antiferromagnetic order in U(Pt, Pd)3

Abstract Neutron-diffraction experiments have been carried out on single-crystalline samples of the heavy-fermion pseudobinaries U(Pt1−xPdx)3 (x ⩽ 0.05) in order to investigate the evolution of magnetic order. At low Pd contents (x = 0.002 and 0.005), small-moment magnetic order is observed below ∼ 6 K, just like in pure UPt3. For x = 0.02 and 0.05, the ordered moment is about one order of magnitude larger compared to UPt3. The compound with x = 0.01 represents a most intriguing intermediate case as it exhibits a cross-over behaviour from small-moment to large-moment antiferromagnetic order.

Excitations of liquid 4He in porous media

Abstract We report on neutron inelastic scattering measurements of the elementary excitations in liquid 4 He in porous aerogel and Vycor glass. The measured dynamical structure factor is very similar to bulk 4 He, with a similar temperature dependence. The small changes in the energy and width of the phonon–roton excitations and their temperature dependences reported earlier in aerogel are critically examined and re-evaluated. In Vycor, the most important difference with respect to bulk 4 He is the existence of additional low-energy excitations, which we tentatively identify with so-called layer modes, i.e. excitations that propagate in the first few layers of the liquid close to the substrate. The importance of multiple scattering for the interpretation of the data is discussed.

Magnetic order of the heavy-fermion URu2Si2 in a field of 12 T

Abstract We have performed elastic and inelastic neutron-scattering measurements on the heavy-fermion superconductor URu2Si2 in a magnetic field of 12 T along the c-axis of the tetragonal crystal structure. The small ordered moment and the energy gap in the magnetic excitation spectrum show a comparable transition temperature in a magnetic field of 12 T. This is in disagreement with recent predictions of a decoupling of the ordered moment and the energy gap in high magnetic fields.

Magnetic excitations in CePd2Si2

Abstract We have studied the magnetic excitations of single-crystalline CePd2Si2 by inelastic neutron scattering measurements. In the paramagnetic phase a quasi-elastic magnetic contribution, characteristic for Kondo fluctuations, is observed. Below TN=9 K an additional spin-wave type magnetic excitation with a minimum energy of 0.7 meV is observed, which is strongly dispersive around the magnetic zone center k =( 1 2, 1 2 , 0) .