Robustness of magnons near the quantum critical point in the heavy fermion superconductor CeCu2Si2

Paramagnons are supposed to provide the pairing glue for unconventional superconductors. For the heavy fermion superconductor CeCu2Si2, there is indeed good evidence from inelastic neutron scattering INS that spin fluctuations drive the superconductivity. Here, we present the INS measurement of the inelastic response of the antiferromagnetic parent compound, A type CeCu2Si2, to probe the relation to the excitations of the superconducting S type sample. We find that the dispersion is very similar in the antiferromagnetic state and in the normal state of the superconducting sample. Pronounced differences to the response in the superconducting state exist at low energies around the zone centre. These findings are in line with observations of other unconventional superconductor

Observations of the effect of strong Pauli paramagnetism on the vortex lattice in superconducting CeCu2Si2

We present the results of a study of the vortex lattice in the heavy fermion superconductor CeCu2Si2, using small angle neutron scattering SANS . In this material at temperatures well below Tc amp; 8764;0.6 K, the value of the upper critical field Bc2 amp; 8764;2.2 T is strongly limited by the Pauli paramagnetism of the heavy fermions. In this temperature region, our SANS data show an increase in the magnetization of the flux line cores with field, followed by a rapid fall near Bc2. This behavior is the effect of Pauli paramagnetism and we present a theory based model, which can be used to describe this effect in a range of materials. The pairing in CeCu2Si2 appears to arise from the effect of magnetic fluctuations, but the evidence for a d wave order parameter is rather weak. We find that the vortex lattice structure in CeCu2Si2 is close to regular hexagonal. There are no phase transitions to square or rhombic structures; such transitions are expected for d wave superconductors and observed in CeCoIn5; however, the temperature dependence of the SANS intensity indicates that both large and small gap values are present, most likely due to multiband s wave superconductivity, rather than a nodal gap structur

Field-Angle-Resolved Magnetic Excitations as a Probe of Hidden-Order Symmetry in CeB6

In contrast to magnetic order formed by electrons' dipolar moments, ordering phenomena associated with higher-order multipoles (quadrupoles, octupoles, etc.) are more difficult to characterize because of the limited choice of experimental probes that can distinguish different multipolar moments. The heavy-fermion compound CeB6 and its La-diluted alloys are among the best-studied realizations of the long-range-ordered multipolar phases, often referred to as "hidden order."Previously, the hidden order in phase II was identified as primary antiferroquadrupolar and field-induced octupolar order. Here, we present a combined experimental and theoretical investigation of collective excitations in phase II of CeB6. Inelastic neutron scattering (INS) in fields up to 16.5 T reveals a new high-energy mode above 14 T in addition to the low-energy magnetic excitations. The experimental dependence of their energy on the magnitude and angle of the applied magnetic field is compared to the results of a multipolar interaction model. The magnetic excitation spectrum in a rotating field is calculated within a localized approach using the pseudospin representation for the Γ8 states. We show that the rotating-field technique at fixed momentum can complement conventional INS measurements of the dispersion at a constant field and holds great promise for identifying the symmetry of multipolar order parameters and the details of intermultipolar interactions that stabilize hidden-order phases. © 2020 authors. Published by the American Physical Society

Dispersion corrected DFT calculations for the adsorption of N₂O on MgO

We have calculated adsorption energies for N2O on the MgO (001) surface using periodic DFT calculations with the B3LYP functional and subsequent dispersion correction. Additionally a wave function-based correlation treatment at the MP2 level was performed. Whilst the B3LYP calculation failed to find a bond state, both the dispersion corrections and the MP2 treatment result in a significantly better description. The best agreement with experiment is obtained with a dispersion correction via the D3 scheme. The calculated binding energies are very similar for adsorption with the nitrogen or the oxygen end towards the surface, whilst calculated vibrational frequencies of adsorbed N2O match the experimental values better when assuming an O-down adsorption structure. (C) 2014 Elsevier B.V. All rights reserved

Characterising MgF₂ surfaces with CO adsorption calculations

We have characterised the Lewis acidity of unsaturated surface cations of MgF2 crystals using periodic calculations at the B3LYP level. The relative importance of low index surfaces was determined by calculating surface energies, and these surfaces were then probed by CO adsorption. We found that a MgF2 microcrystal should expose mainly (110), (100) and (101) surfaces in which the undercoordinated cations are fivefold coordinated. The adsorption energies and CO stretching frequencies are discussed with respect to the coordination number of surface cations and the stability of the surfaces and are compared to IR spectra from the literature. (C) 2012 Elsevier B.V. All rights reserved

Ferromagnetic fluctuations in YbNi₄P₂ measured by inelastic neutron scattering

YbNi4P2 is one of the very few heavy-fermion systems which allow the study of ferromagnetic quantum criticality. The Curie temperature T-c=0.17 K can be suppressed by substituting arsenic on the phosphorus site, without changing the ferromagnetic nature of the ordered state. The ordered moment, even of the unsubstituted compound, is only around 0.05,LB, which hinders elastic neutron scattering experiments. To gain microscopic insight into the nature of the interactions, we have studied the magnetic excitations of polycrystalline YbNi4P2 by time-of-flight neutron spectroscopy. For momentum transfers larger than about 0.6 A-1 we find a quasi-elastic response whose width at low temperatures is limited by the Kondo effect. In contrast, the low-energy magnetic response is distinctly different for Q approaching zero: At low temperatures, but still in the paramagnetic phase, susceptibility and lifetime of the spin fluctuations are strongly enhanced, indicating the proximity of ferromagnetism

Analysis of the crystal electric field parameters of YbNi₄P₂

The crystal electric field (CEF) scheme ofYbNi(4)P(2) is determined, based on experimental data from inelastic neutron scattering, heat capacity, susceptibility and NMR measurements. Despite the tetragonal crystal structure, 9 parameters are needed to describe the crystal field in YbNi4P2 due to the orthorhombic site symmetry of the Yb ion. A large basal plane anisotropy is detected by the local probe NMR. Our analysis yields CEF excitation energies of 8.5, 12.5 and roughly 30 meV and a ground state wave function that is dominated by the 5/2 state. Furthermore, we present an analysis of the CEF scheme based on density functional theory calculations, which confirms the large basal plane anisotropy

Analysis of the crystal electric field parameters of YbNi4P2

The crystal electric field CEF scheme of YbNi4P2 is determined, based on experimental data from inelastic neutron scattering, heat capacity, susceptibility and NMR measurements. Despite the tetragonal crystal structure, 9 parameters are needed to describe the crystal field in YbNi4P2 due to the orthorhombic site symmetry of the Yb ion. A large basal plane anisotropy is detected by the local probe NMR. Our analysis yields CEF excitation energies of 8.5 meV, 12.5 meV and roughly 30meV and a ground state wave function that is dominated by the 5 2 state. Furthermore, we present an analysis of the CEF scheme based on DFT calculations, which confirms the large basal plane anisotrop