arXiv subjects

B. Roessli

Publications and source records attributed to B. Roessli.

71 records · Page 4Linked to original sources

arXiv2004

Magnetisation dynamics in the normal and condensate phases of UPd2Al3 I: Surveys in reciprocal space using neutron inelastic scattering

This paper, I, presents new results from neutron inelastic scattering experiments on single crystals of UPd2Al3. The focus is on the experimental position whilst the sequel, II, advances theoretical perspectives. We present a detailed and complete characterisation of the wavevector- and energy-dependent magnetisation dynamics in UPd2Al3 as measured by neutron inelastic scattering primarily in the form of extensive surveys in energy-momentum space under a wide range of experimental conditions, and put our observations in context with data that has been previously published by two independent groups. In this way we emphasize the commonality and robust nature of the data which indicate the intricate nature of the dynamic magnetic susceptibility of this material. Our results yield unique insight into the low temperature ground state which exhibits a microscopic coexistence of antiferromagnetism and superconductivity making UPd2Al3 one of the most accessible heavy-fermion superconductors that can be fully characterised by neutron spectroscopy.

cond-mat.str-el↗

arXiv2004

Magnetisation dynamics in the normal and condensate phases of UPd2Al3 II: Inferences on the nodal gap symmetry

This paper provides an analysis of neutron inelastic scattering experiments on single crystals of UPd2Al3. The emphasis is on establishing robust, general, inferences on the joint antiferromagnetic-superconducting state which characterises UPd2Al3 at low temperatures. A distinction is drawn between these conclusions and various theoretical perspectives of a more model sensitive nature that have been raised in the literature.

cond-mat.str-el↗

arXiv2004

Dynamics of cubic-tetragonal phase transition in KNbO$_3$ perovskite

The low-energy part of the vibration spectrum in KNbO$_3$ was studied by cold neutron inelastic scattering in the cubic phase. In addition to acoustic phonons, we observe strong diffuse scattering, which consists of two components. The first one is quasi-static and has a temperature-independent intensity. The second component appears as quasi-elastic scattering in the neutron spectrum indicating a dynamic origin. From analysis of the inelastic data we conclude that the quasi-elastic component and the acoustic phonon are mutually coupled. The susceptibility associated with the quasi-elastic component grows as the temperature approaches T$_C$.

cond-mat.mtrl-sci↗

arXiv2004

Neutron scattering study of PbMg$_{1/3}$Ta$_{2/3}$O$_3$ and BaMg$_{1/3}$Ta$_{2/3}$O$_3$ complex perovskites

Neutron scattering investigations were carried out in PbMg$_{1/3}$Ta$_{2/3}$O$_3$ and BaMg$_{1/3}$Ta$_{2/3}$O$_3$ complex perovskites. The crystal structure of both compounds does not show any phase transition in the temperature range 1.5 -- 730 K. Whereas the temperature dependence of the lattice parameter of BaMg$_{1/3}$Ta$_{2/3}$O$_3$ follows the classical expectations, the lattice parameter of relaxor ferroelectric PbMg$_{1/3}$Ta$_{2/3}$O$_3$ exhibits anomalies. One of these anomalies is observed in the same temperature range as the peak in the dielectric susceptibility. We find that in PbMg$_{1/3}$Ta$_{2/3}$O$_3$, lead ions are displaced from the ideal positions in the perovskite structure at all temperatures. Consequently short-range order is present. This induces strong diffuse scattering with an anisotropic shape in wavevector space. The temperature dependences of the diffuse neutron scattering intensity and of the amplitude of the lead displacements are similar.

cond-mat.mtrl-sci↗

arXiv2003

Disorder and relaxation mode in the lattice dynamics of PbMg$_{1/3}$Nb$_{2/3}$O$_3$ relaxor ferroelectric

The low-energy part of vibration spectrum in PbMg$_{1/3}$Nb$_{2/3}$O$_3$ relaxor ferroelectric was studied by inelastic neutron scattering. We observed the coexistence of a resolution-limited central peak with strong quasielastic scattering. The line-width of the quasielastic component follows a $Γ_0+Dq^2$ dependence. We find that $Γ_0$ is temperature-dependent. The relaxation time follows the Arrhenius law well. The presence of a relaxation mode associated with quasi-elastic scattering in PMN indicates that order-disorder behaviour plays an important rôle in the dynamics of diffuse phase transitions.

cond-mat.mtrl-sci↗

arXiv2003

Spin waves and the origin of commensurate magnetism in Ba2CoGe2O7

The square-lattice antiferromagnet Ba2CoGe2O7 is studied by means of neutron diffraction and inelastic scattering. This material is isostructural to the well-known Dzyaloshinskii-Moriya helimagnet Ba2CuGe2O7, but exhibits commensurate long-range Neel order at low temperatures. Measurements of the spin wave dispersion relation reveal strong in-plane anisotropy that is the likely reason for the suppression of helimagnetism.

cond-mat.str-el↗

arXiv2002

Magnetic fluctuations and superconductivity in YbPd$_2$Sn

We report muon spin relaxation and inelastic neutron measurements on the Heusler system YbPd$_2$Sn. Localised anisotropic and quasi-elastic Yb magnetic fluctuations are observed below $T = 150$K. Both $μ$SR and neutron data indicate a slowing-down of the spin-fluctuations, upon lowering the temperature, similar to that observed in Kondo lattices. The temperature dependence of the quasi-elastic neutron signal is compatible with a crystal-electric field scheme having a $Γ_7^{\rm CEF}$ ground state. The muon depolarization rate exhibits an additional contribution upon decreasing the temperature below $T_{\rm c}$ suggesting a close interplay between magnetic fluctuations and the superconducting state.

cond-mat.str-el↗

arXiv2002

Chiral fluctuations in MnSi above the Curie temperature measured with polarized inelastic neutron scattering

Using polarized inelastic neutron scattering the antisymmetric part of the dynamical susceptibility in non-centrosymmetric MnSi is determined. The paramagnetic fluctuations are found to be incommensurate with the chemical lattice and to have a chiral character. We show that antisymmetric interactions must be taken into account to properly describe the critical dynamics in MnSi above the Curie temperature. The inelastic neutron data is interpreted within the framework of the SCR-theory, taking into account the Dzyaloshinskii-Moriya interaction.

cond-mat.str-el↗

arXiv2002

Spin-wave spectrum of copper metaborate in the commensurate phase 10K<T<21K

We have investigated the spin-wave spectrum of copper metaborate, CuB$_2$O$_4$, by means of inelastic neutron scattering in the commensurate magnetic phase. We have found two branches of spin-wave excitations associated with the two magnetic sublattices Cu(A) and Cu(B), respectively. In the temperature regime $10K \le T \le 21K$, where only the Cu(A) magnetic moments are ordered, the interaction between the two sublattices is found to be negligible. With this approximation we have determined the `easy plane' exchange parameters of the Cu(A) subsystem within standard spin-wave theory.

cond-mat.str-el↗

arXiv2002

Chiral fluctuations in MnSi above Tc

Polarized neutrons are used to determine the antisymmetric part of the magnetic susceptibility in non-centrosymmetric MnSi. The paramagnetic fluctuations are found to be incommensurate with the chemical lattice and to have a chiral character. We argue that antisymmetric interactions must be taken into account to properly describe the critical dynamics in MnSi above T$_C$. The possibility of directly measuring the polarization dependent part of the dynamical susceptibility in a large class of compounds through polarised inelastic neutron-scattering is outlined as it can yield direct evidence for antisymmetric interactions like spin-orbit coupling in metals as well as in insulators.

cond-mat.str-el↗

arXiv2001

Damping of spin waves and singularity of the longitudinal modes in the dipolar critical regime of the Heisenberg-ferromagnet EuS

By inelastic scattering of polarized neutrons near the (200)-Bragg reflection, the susceptibilities and linewidths of the spin waves and the longitudinal spin fluctuations were determined separately. By aligning the momentum transfers q perpendicular to both δS_sw and the spontaneous magnetization M_s, we explored the statics and dynamics of these modes with transverse polarizations with respect to q. In the dipolar critical regime, where the inverse correlation length kappa_z(T) and q are smaller than the dipolar wavenumber q_d, we observe:(i) the static susceptibility of δS_sw^T(q) displays the Goldstone divergence while for δS_z^T(q) the Ornstein-Zernicke shape fits the data with a possible indication of a thermal(mass-)renormalization at the smallest q-values, i.e. we find indications for the predicted 1/q divergence of the longitudinal susceptibility; (ii) the spin wave dispersion as predicted by the Holstein-Primakoff theory revealing q_d=0.23(1)Å^{-1}in good agreement with previous work in the paramagnetic and ferromagnetic regime of EuS; (iii) within experimental error, the (Lorentzian) linewidths of both modes turn out to be identical with respect to the q^2-variation, the temperature independence and the absolute magnitude. Due to the linear dispersion of the spin waves they remain underdamped for q =T_C. The available mode-mode coupling theory, which takes the dipolar interactions fully into account, describes the gross features of the linewidths but not all details of the T- and q-dependencies. PACS: 68.35.Rh, 75.40.Gb

cond-mat↗

arXiv2000

Polarized Neutron Scattering

The technique of polarized neutron scattering is reviewed with emphasis on applications. Many examples of the usefulness of the method in various fields of physics are given like the determination of spin density maps, measurement of complex magnetic structures with spherical neutron polarimetry, inelastic neutron scattering and separation of coherent and incoherent scattering with help of the generalized XYZ method.

cond-mat.str-el↗

arXiv2000

Spin Excitations in Localized and Itinerant Magnets

Collective excitations in magnetic materials can be investigated by means of inelastic neutron scattering. We show that this experimental method gives access to the complete spectrum of magnetic fluctuations through the energy- and momentum-dependence of the dynamical susceptibility. We focus on the dynamical properties of magnets with localized spin densities and of metals. From such studies, microscopic parameters like exchange integrals, spin-wave stiffness and relaxation times can be determined. This is of great help to test current theories in magnetism.

cond-mat.str-el↗

arXiv2000

Spin-Glass State in $\rm CuGa_2O_4$

Magnetic susceptibility, magnetization, specific heat and positive muon spin relaxation (\musr) measurements have been used to characterize the magnetic ground-state of the spinel compound $\rm CuGa_2O_4$. We observe a spin-glass transition of the S=1/2 $\rm Cu^{2+}$ spins below $\rm T_f=2.5K$ characterized by a cusp in the susceptibility curve which suppressed when a magnetic field is applied. We show that the magnetization of $\rm CuGa_2O_4$ depends on the magnetic histo Well below $\rm T_f$, the muon signal resembles the dynamical Kubo-Toyabe expression reflecting that the spin freezing process in $\rm CuGa_2O_4$ results Gaussian distribution of the magnetic moments. By means of Monte-Carlo simulati we obtain the relevant exchange integrals between the $\rm Cu^{2+}$ spins in this compound.

cond-mat.str-el↗

arXiv2000

Macroscopic and Local Magnetic Moments in Si-doped CuGeO$_3$ with Neutron and $μ$SR Studies

The temperature-concentration phase diagram of the Si-doped spin-Peierls compound CuGeO$_{3}$ is investigated by means of neutron scattering and muon spin rotation spectroscopy in order to determine the microscopic distribution of the magnetic and lattice dimerised regions as a function of doping. The analysis of the zero-field muon spectra has confirmed the spatial inhomogeneity of the staggered magnetisation that characterises the antiferromagnetic superlattice peaks observed with neutrons. In addition, the variation of the macroscopic order parameter with doping can be understood by considering the evolution of the local magnetic moment as well as of the various regions contributing to the muon signal.

cond-mat.str-el↗

arXiv2000

Magnetic Phase Transitions in the double spin-chains compound $\rm LiCu_2O_2$

We report high-resolution x-ray diffraction, muon-spin-rotation spectroscopic and specific heat measurements in the double spin-chains compound $\rm LiCu_2O_2$. The x-ray diffraction results show that the crystal structure of $\rm LiCu_2O_2$ ~is orthorhombic down to T=10K. Anisotropic line-broadening of the diffraction peaks is observed, indicating disorder along the spin chains. Muon spin relaxation and specific heat measurements show that $\rm LiCu_2O_2$ ũndergoes a phase transition to a magnetic ordered state at $\rm T_1\sim24K$. The specific heat data exhibits a second $\rm λ$-like peak at $\rm T_2\sim22.5 K$, which increases with increasing magnetic field similarly way to that found in spin-ladder compounds.

cond-mat.str-el↗

arXiv2000

Formation of a magnetic soliton lattice in copper metaborate

The magnetic ground-state of copper metaborate is investigated by means of elastic neutron scattering.The magnetic structure of CuB2O4 is incommensurate with respect to the chemical lattice at T=1.8K and undergoes a continuous phase transition to a non-collinear commensurate antiferromagnetic state which is realized at T=10K. Close to the phase transition higher-order magnetic satellites are observed. Coexistence of long-range and short range magnetic order observed in both magnetic phases. This suggests that the association of the Dzyialoshinskii-Moriya interaction, lattice symmetry and tetragonal anisotropy leads to the formation of a three-dimensional magnetic soliton lattice.

cond-mat.str-el↗