Quasielastic Excitations Research Articles

Majorana fermions in the Kitaev quantum spin system α-RuCl3

α-RuCl3 has recently attracted great interest as a possible experimental realization of the Kitaev model. Neutron scattering measurements of a single crystal of this material reveal signatures of Majorana excitations, consistent with Kitaev’s predictions. Geometrical constraints to the electronic degrees of freedom within condensed-matter systems often give rise to topological quantum states of matter such as fractional quantum Hall states, topological insulators, and Weyl semimetals1,2,3. In magnetism, theoretical studies predict an entangled magnetic quantum state with topological ordering and fractionalized spin excitations, the quantum spin liquid4. In particular, the so-called Kitaev spin model5, consisting of a network of spins on a honeycomb lattice, is predicted to host Majorana fermions as its excitations. By means of a combination of specific heat measurements and inelastic neutron scattering experiments, we demonstrate the emergence of Majorana fermions in single crystals of α-RuCl3, an experimental realization of the Kitaev spin lattice. The specific heat data unveils a two-stage release of magnetic entropy that is characteristic of localized and itinerant Majorana fermions. The neutron scattering results corroborate this picture by revealing quasielastic excitations at low energies around the Brillouin zone centre and an hour-glass-like magnetic continuum at high energies. Our results confirm the presence of Majorana fermions in the Kitaev quantum spin liquid and provide an opportunity to build a unified conceptual framework for investigating fractionalized excitations in condensed matter1,6,7,8.

Neutron resonance spin echo with longitudinal DC fields.

We report on the design, construction, and performance of a neutron resonance spin echo (NRSE) instrument employing radio frequency (RF) spin flippers combining RF fields with DC fields, the latter oriented parallel (longitudinal) to the neutron propagation direction (longitudinal NRSE (LNRSE)). The advantage of the longitudinal configuration is the inherent homogeneity of the effective magnetic path integrals. In the center of the RF coils, the sign of the spin precession phase is inverted by a π flip of the neutron spins, such that non-uniform spin precession at the boundaries of the RF flippers is canceled. The residual inhomogeneity can be reduced by Fresnel- or Pythagoras-coils as in the case of conventional spin echo instruments (neutron spin echo (NSE)). Due to the good intrinsic homogeneity of the B0 coils, the current densities required for the correction coils are at least a factor of three less than in conventional NSE. As the precision and the current density of the correction coils are the limiting factors for the resolution of both NSE and LNRSE, the latter has the intrinsic potential to surpass the energy resolution of present NSE instruments. Our prototype LNRSE spectrometer described here was implemented at the resonance spin echo for diverse applications (RESEDA) beamline at the MLZ in Garching, Germany. The DC fields are generated by B0 coils, based on resistive split-pair solenoids with an active shielding for low stray fields along the beam path. One pair of RF flippers at a distance of 2 m generates a field integral of ∼0.5 Tm. The LNRSE technique is a future alternative for high-resolution spectroscopy of quasi-elastic excitations. In addition, it also incorporates the MIEZE technique, which allows to achieve spin echo resolution for spin depolarizing samples and sample environments. Here we present the results of numerical optimization of the coil geometry and first data from the prototype instrument.

Electron-neutrino scattering off nuclei from two different theoretical perspectives

We analyze charged-current electron-neutrino cross sections on Carbon. We consider two different theoretical approaches, on one hand the Continuum Random Phase Approximation (CRPA) which allows a description of giant resonances and quasielastic excitations, on the other hand the RPA-based calculations which are able to describe multinucleon emission and coherent and incoherent pion production as well as quasielastic excitations. We compare the two approaches in the genuine quasielastic channel, and find a satisfactory agreement between them at large energies while at low energies the collective giant resonances show up only in the CRPA approach. We also compare electron-neutrino cross sections with the corresponding muon-neutrino ones in order to investigate the impact of the different charged-lepton masses. Finally, restricting to the RPA-based approach we compare the sum of quasielastic, multinucleon emission, coherent and incoherent one-pion production cross sections (folded with the electron-neutrino T2K flux) with the charged-current inclusive electron-neutrino differential cross sections on Carbon measured by T2K. We find a good agreement with the data. The multinucleon component is needed in order to reproduce the T2K electron-neutrino inclusive cross sections.

Muon spin rotation and neutron scattering study of the noncentrosymmetric tetragonal compoundCeAuAl3

We have investigated the noncentrosymmetric tetragonal heavy fermion compound $\mathrm{CeAuA}{\mathrm{l}}_{3}$ using muon spin rotation $(\ensuremath{\mu}\mathrm{SR})$, neutron diffraction (ND), and inelastic neutron scattering (INS) measurements. We have also revisited the magnetic, transport, and thermal properties. The magnetic susceptibility reveals an antiferromagnetic transition at 1.1 K with, possibly, another magnetic transition near 0.18 K. The heat capacity shows a sharp $\ensuremath{\lambda}$-type anomaly at 1.1 K in zero field, which broadens and moves to a higher temperature in an applied magnetic field. Our zero-field $\ensuremath{\mu}\mathrm{SR}$ and ND measurements confirm the existence of a long-range magnetic ground state below 1.2 K. Further, the ND study reveals an incommensurate magnetic order with a magnetic propagation vector $\mathbf{k}=(0,0,0.52(1))$ and a spiral structure of Ce moments coupled ferromagnetically within the $ab$ plane. Our INS study reveals the presence of two well-defined crystal electric field (CEF) excitations at 5.1 and 24.6 meV in the paramagnetic phase of $\mathrm{CeAuA}{\mathrm{l}}_{3}$ that can be explained on the basis of the CEF theory and the Kramer's theorem for a Ce ion having a $4{f}^{1}$ electronic state. Furthermore, low energy quasielastic excitations show a Gaussian line shape below 30 K compared to a Lorentzian line shape above 30 K, indicating a slowdown of spin fluctuations below 30 K. We have estimated a Kondo temperature of ${T}_{\mathrm{K}}=3.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ from the quasielastic linewidth, which is in good agreement with that estimated from the heat capacity. This study also indicates the absence of any CEF-phonon coupling unlike that observed in isostructural $\mathrm{CeCuA}{\mathrm{l}}_{3}$ The CEF parameters, energy level scheme, and their wave functions obtained from the analysis of INS data explain satisfactorily the single crystal susceptibility in the presence of two-ion anisotropic exchange interaction in $\mathrm{CeAuA}{\mathrm{l}}_{3}$.

Crystal field manifestation in inelastic neutron scattering, magnetic susceptibility and specific heat of the antiferromagnetic CeCoAl4

Inelastic and quasielastic excitations in the neutron scattering on the compounds CeCo1−xCuxAl4 (x=0, 0.05, 0.1) have been studied above and below the antiferromagnetic ordering temperature. The quasielastic contribution disappears below the Néel temperature, TN=13.5K, and its linewidth Γ/2 is below TN, therefore these results suggest that Kondo screening is not relevant in these Ce compounds. Based on the example of CeCoAl4 we show that the inelastic excitations provide crystal electric field energy levels scheme, which in consistent way describes both the magnetic susceptibility and the specific heat. The inelastic neutron scattering spectrum below TN is interpreted as the splitting of the crystal electric field levels by the internal molecular field.

Unexpected phase locking of magnetic fluctuations in the multi-kmagnet USb

The spin waves in the multi-k antiferromagnet, USb, soften and become quasielastic well below the AFM ordering temperature, T_N. This occurs without a magnetic or structural transition. It has been suggested that this change is in fact due to de-phasing of the different multi-k components: a switch from 3-k to 1-k behaviour. In this work, we use inelastic neutron scattering with tri-directional polarisation analysis to probe the quasielastic magnetic excitations and reveal that the 3-k structure does not de-phase. More surprisingly, the paramagnetic correlations also maintain the same clear phase correlations well above T_N (up to at least 1.4T_N). This precursor regime has not been observed before in a multi-k system.

Nematic spin fluid in the tetragonal phase of BaFe2As2

We use inelastic neutron scattering to study spin waves below and above ${T}_{N}$ in iron-arsenide BaFe${}_{2}$As${}_{2}$. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the antiferromagnetic $a$-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest the presence of a spin nematic fluid in the tetragonal phase of BaFe${}_{2}$As${}_{2}$, which may cause the electronic and orbital anisotropy observed in these materials.

Molecular spin-orbit excitations in theJeff=12frustrated spinel GeCo2O4

We describe powder and single-crystal inelastic neutron scattering experiments on a spinel-type antiferromagnet GeCo$_2$O$_4$, represented by an effective total angular momentum J_eff = 1/2. Several types of non-dispersive short-range magnetic excitations were discovered. The scattering intensity maps in $\vec{Q}$ space are well reproduced by dynamical structure factor analyses using molecular model Hamiltonians. The results of analyses strongly suggest that the molecular excitations below T_N arise from a hidden molecular-singlet ground state, in which ferromagnetic subunits are antiferromagnetically coupled. The quasielastic excitations above T_N are interpreted as its precursor. A combination of frustration and J_eff = 1/2 might induce these quantum phenomena.

Phase transitions in a ferromagnet with an anisotropic biquadratic exchange interaction

The phase states of a non-Heisenberg ferromagnet with anisotropy of both the Heisenberg and biquadratic interactions are investigated. Limiting cases of the model are the XY model with a biquadratic interaction and the three-component non-Heisenberg ferromagnet. The phase transitions both in respect to the material constants and in temperature are investigated for the 2D and 3D magnets. The phase diagrams of the system are constructed for various relations of the system parameters. It is shown that in the given model the soft mode is a branch of quasimagnetic and not quasielastic excitations.

μSR investigation of the quasi-elastic magnetic excitations in strongly correlated compounds

Although, according to macroscopic measurements, the intermetallics CeRu 2Si 2, UGe 2 and YbNiSn have very different electronic ground states, muon spin relaxation measurements show that these three compounds have an anomalous quasi-elastic spectral weight of magnetic excitations in the transfer energy range of 10 neV.

What does the muon spin-relaxation rate measure in 4f paramagnets with strong crystal fields and weak inter-site correlations?

We investigate the physical meaning of the longitudinal muon spin-relaxation rate measured for a paramagnet at temperatures high with respect to the magnetic phase transition temperature. The depolarization rate is shown to be a function of the relaxation rates of the quasi-elastic and inelastic magnetic excitations. Using this analysis, we consider recently published data recorded on the strongly correlated electron systems CeNiSn and .

Spectroscopic properties of Ce3Bi4Pt3

Abstract The spectrum of magnetic and electronic excitations of a theoretical model of the heavy fermion semiconducting materials is calculated. The system is modeled by the degenerate Anderson lattice Hamiltonian. The coulomb interaction between two electrons on the same f orbital is treated by projecting out the doubly occupied configurations, using the slave boson technique. In the mean-field approximation, the system shows a transition between a low-temperature insulating phase and high-temperature metallic phase. The dynamical susceptibility and optical conductivities are calculated for this system. The spectra show contributions from both quasi-elastic and inter-band excitations. The results are compared with inelastic neutron scattering experiments.