Magnetic Excitation Spectrum Research Articles

On the Magnetic Excitation Spectra of Ba(Fe0.9Co0.1)2As2in the Superconducting State

Magnetic excitation spectra of Ba(Fe 0.9 Co 0.1 ) 2 As 2 obtained for aligned crystals with the superconducting transition temperature T c ∼23 K are presented after an overview of results of our preceding studies on the symmetry of the order parameter Δ. Although many researchers have reported that their experimental results are consistently explained by the so-called S ± symmetry, and believe that the spin-fluctuation is relevant to the superconducting mechanism of Fe pnictide superconductors. However, the situation is not so simple, because almost all these data can be understood by the S ++ -symmetry, too. Furthermore, as we first pointed out and discussed in detail, the rates of the T c suppression by nonmagnetic impurities can hardly be explained by the S ± symmetry. Here, we show our own data of the magnetic excitation spectra of Ba(Fe 0.9 Co 0.1 ) 2 As 2 , and discuss which one of the S ± and S ++ -symmetries can better describe the characteristics of the peak observed there. These processes are im...

Magnetically driven superconductivity in CeCu2Si2

The origin of unconventional superconductivity, including high-temperature and heavy-fermion superconductivity, is still a matter of controversy. Spin excitations instead of phonons are thought to be responsible for the formation of Cooper pairs. Using inelastic neutron scattering, we present the first in-depth study of the magnetic excitation spectrum in momentum and energy space in the superconducting and the normal states of CeCu2Si2. A clear spin excitation gap is observed in the superconducting state. We determine a lowering of the magnetic exchange energy in the superconducting state, in an amount considerably larger than the superconducting condensation energy. Our findings identify the antiferromagnetic excitations as the main driving force for superconducting pairing in this prototypical heavy-fermion compound located near an antiferromagnetic quantum critical point.

Magnetic spectrum of the two-dimensional antiferromagnetLa2CoO4studied by inelastic neutron scattering

We report measurements of the magnetic excitation spectrum of the layered antiferromagnet ${\text{La}}_{2}{\text{CoO}}_{4}$ by time-of-flight neutron inelastic scattering. In the energy range probed in our experiments (0--250 meV) the magnetic spectrum consists of spin-wave modes with strong in-plane dispersion extending up to 60 meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a small $(\ensuremath{\sim}1\text{ }\text{meV})$ dispersion along the magnetic zone boundary. We show that the magnetic spectrum can be described very well by a model of a Heisenberg antiferromagnet that includes the full spin and orbital degrees of freedom of ${\text{Co}}^{2+}$ in an axially distorted crystal field. The collective magnetic dynamics are found to be controlled by dominant nearest-neighbor exchange interactions, strong XY-like single-ion anisotropy and a substantial unquenched orbital angular momentum.

Band Calculations for Ce Compounds with AuCu3-type Crystal Structure on the basis of Dynamical Mean Field Theory: I. CePd3 and CeRh3

Band calculations for Ce compounds with the AuCu 3 -type crystal structure were carried out on the basis of dynamical mean field theory (DMFT). The auxiliary impurity problem was solved by a method named NCA f 2 vc (noncrossing approximation including the f 2 state as a vertex correction). The calculations take into account the crystal-field splitting, the spin–orbit interaction, and the correct exchange process of the f 1 → f 0 , f 2 virtual excitation. These are necessary features in the quantitative band theory for Ce compounds and in the calculation of their excitation spectra. The results of applying the calculation to CePd 3 and CeRh 3 are presented as the first in a series of papers. The experimental results of the photoemission spectrum (PES), the inverse PES, the angle-resolved PES, and the magnetic excitation spectra were reasonably reproduced by the first-principles DMFT band calculation. At low temperatures, the Fermi surface (FS) structure of CePd 3 is similar to that of the band obtained by the local density approximation. It gradually changes into a form that is similar to the FS of LaPd 3 as the temperature increases, since the 4 f band shifts to the high-energy side and the lifetime broadening becomes large.

Spin fluctuation spectrum of electron-doped high-Tc superconductors

The magnetic excitation spectrum of electron-doped copper oxide superconductors is calculated using the Hubbard model on a square lattice. First, the on-site repulsion is treated with the random phase approximation. The spectrum of electron-doped systems in the superconducting state is compared with that of hole-doped systems, and the relationship between the frequency at which a peak grows in the spectrum and the superconducting energy gap at a hot spot (an intersection of the Fermi surface and the magnetic Brillouin zone boundary) is investigated. As compared with the hole-doped systems, the resonance condition is difficult to be satisfied in the electron-doped systems because of the small density of states around the hot spot. Moreover, the correlation effect in the Hubbard model is treated by the fluctuation-exchange approximation (FLEX), and the spin fluctuation spectra in the superconducting state in a wide region of the wave vector and frequency are calculated. We have found that the intensity of the magnetic spectrum at incommensurate wave vectors obtained with the FLEX is considerably weaker than that obtained with the RPA. The validity of the Fermi-liquid approach is also discussed.

Evidence for site-centered stripes from magnetic excitations in cuprate superconductors

The success of models of coupled two-leg spin ladders in describing the magnetic excitation spectrum of ${\text{La}}_{2\ensuremath{-}x}{\text{Ba}}_{x}{\text{CuO}}_{4}$ has been widely interpreted as evidence for bond-centered stripes. Here, we determine the magnetic coupling induced by the charge stripes between bond- or site-centered spin stripe modeled by two- or three-leg ladder, respectively. We find that only the site-centered models order. We further report excellent agreement of a fully consistent analysis of coupled three-leg ladders using a spin-wave theory of bond operators with the experiment.

Interplay of thermal and quantum spin fluctuations in the kagome lattice compound herbertsmithite

We present a Raman spectroscopic investigation of the herbertsmithite ${\text{ZnCu}}_{3}{(\text{OH})}_{6}{\text{Cl}}_{2}$, a realization of a Heisenberg $s=1/2$ antiferromagnet on a perfect kagome lattice. Focusing on its magnetic excitation spectrum we find two components, a high-temperature quasielastic signal and a low temperature, broad maximum. The latter has a linear low-energy slope and extends to high energy. We have investigated the temperature dependence and symmetry properties of both signals. Our data agree with previous calculations and point to a gapless spin liquid ground state.

Magnetic excitations in theXY-pyrochlore antiferromagnetEr2Ti2O7

The XY-pyrochlore antiferromagnet ${\text{Er}}_{2}{\text{Ti}}_{2}{\text{O}}_{7}$ is studied by heat-capacity measurements and electron-spin resonance spectroscopy performed on single-crystal samples. The magnetic phase diagrams are established for two directions of applied field, $H\ensuremath{\parallel}[100]$ and $H\ensuremath{\parallel}[111]$. In the magnetically ordered phase observed below ${T}_{N}=1.2\text{ }\text{K}$, the magnetic excitation spectrum consists of a Goldstone mode acquiring an isotropic gap in an applied field, and another mode with a gap softening in the vicinity of a field-induced phase transition. This second-order transition takes place at a critical field ${H}_{c}$ above which the magnetization process is accompanied by a canting of the magnetic moments off their local ``easy planes.'' The specific-heat curves for $H\ensuremath{\parallel}[100]$ $(H⪢{H}_{c})$ are well described by a model presuming a single dispersionless excitation mode with the energy gap obtained from the spectroscopic measurements.

Low-energy magnetic excitation spectrum of the unconventional ferromagnetCeRh3B2

The magnetic excitation spectrum of the unconventional ferromagnet CeRh$_{3}$B$_{2}$ was measured by inelastic neutron scattering on single crystal sample in the magnetically ordered and paramagnetic phases. The spin-wave excitation spectrum evidences high exchange interaction along the c-axis about two orders of magnitude higher than the ones in the basal plane of the hexagonal structure. Both strong out of plane and small in plane anisotropies are found. This latter point confirms that considering the $J$=5/2 multiplet alone is not adequate for describing the ground state of CeRh$_{3}$B$_{2}$. Quasielastic scattering measured above $T_{Curie}$ is also strongly anisotropic between the basal plane and the c-axis and suggests localized magnetism.

Magnetic excitations in systems with a nonmagnetic ground state and valence fluctuations

In this work, we report and analyze the results of a cycle of the neutron investigations of a number of rare� earth compounds based on SmB6 , SmS, and YbB 12 . These systems are usually classified as Kondo insula� tors (1) (for the case of SmS, at a pressure of higher than 0.6 GPa) with various valence instability degrees of a rareearth ion. This degree is sufficiently high for SmB6 and its derivatives, varies in a wide range for "chemically compressed" Sm(Y)S (simulating the intermediatevalence state of SmS under compres� sion), and is slightly different from an integer for YbB12, as in heavyfermion systems. All of these com� pounds are characterized by the presence of a gap (or pseudogap) in the electron density of states near the Fermi energy and the nonmagnetic ground state, which are formed due to strong electron correlations at temperatures below a certain characteristic value of about several tens of Kelvin. In the diverse neutron experiments, the energy structure of the magneticexcitation spectrum, the characteristic features of the momentum dependence of the energy (dispersion relation), and the intensities of these excitations have been determined. The tem� perature dependence of the spectral function and the effect of the introduction of the defects into the mag� netic sublattice, i.e., the role of the cooperative effects on the formation of the ground state have been ana� lyzed. Specific excitations have been revealed in the energy range of 10-20 meV; the main properties of these excitations are similar to those of the socalled "resonance" mode, which is studied and actively dis� cussed in works devoted to the hightemperature superconductor cuprates (see (2, 3) and references therein).

Orbital ordering and unfrustrated(π,0)magnetism from degenerate double exchange in the iron pnictides

The magnetic excitations of the iron pnictides are explained within a degenerate double-exchange model. The local-moment spins are coupled by superexchanges ${J}_{1}$ and ${J}_{2}$ between nearest and next-nearest neighbors, respectively, and interact with the itinerant electrons of the degenerate ${d}_{xz}$ and ${d}_{yz}$ orbitals via a ferromagnetic Hund exchange. The latter stabilizes $(\ensuremath{\pi},0)$ stripe antiferromagnetism due to emergent ferro-orbital order and the resulting kinetic-energy gain by hopping preferably along the ferromagnetic spin direction. Taking the quantum nature of the spins into account, we calculate the magnetic excitation spectra in the presence of both, superexchange and double exchange. A dramatic increase in the spin-wave energies at the competing N\'eel ordering wave vector is found, in agreement with recent neutron-scattering data. The spectra are fitted to a spin-only model with a strong spatial anisotropy and additional longer-ranged couplings along the ferromagnetic chains. Over a realistic parameter range, the effective couplings along the chains are negative corresponding to unfrustrated stripe antiferromagnetism.

Magnetic excitations in ferropnictide materials controlled by a quantum critical point into hidden order

The two-orbital ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ model that describes a square lattice of frustrated spin-1 iron atoms is analyzed within the linear spin-wave approximation and by exact diagonalization over a $4\ifmmode\times\else\texttimes\fi{}4$ cluster. A quantum critical point (QCP) is identified that separates hidden magnetic order at weak Hund's rule coupling from a commensurate spin-density wave (cSDW) at strong Hund's rule coupling. Although the moment for cSDW order is small at the QCP, the critical linear spin-wave spectrum shows strong low-energy excitations centered at the wave numbers that correspond to cSDW order. These disperse anisotropically. A fit to the magnetic excitation spectrum of ferropnictide materials obtained recently by inelastic neutron-scattering measurements notably accounts for the absence of softening at the wave number that corresponds to N\'eel order.

On the weak confinement of kinks in the one-dimensional quantum ferromagnetCoNb2O6

In a recent paper Coldea et al (2010 Science 327 177) report observationsof the weak confinement of kinks in the Ising spin chain ferromagnetCoNb2O6 at low temperatures. To interpret the entire spectra of magnetic excitations measured vianeutron scattering, they introduce a phenomenological model, which takes into accountonly the two-kink configurations of the spin chain. We present the exact solution of thismodel. The explicit expressions for the two-kink bound-state energy spectra and for therelative intensities of neutron scattering on these magnetic modes are obtained in terms ofthe Bessel function.

Brillouin light scattering studies of planar metallic magnonic crystals

The application of Brillouin light scattering to the study of the spin-wave spectrum of one- and two-dimensional planar magnonic crystals consisting of arrays of interacting stripes, dots and antidots is reviewed. It is shown that the discrete set of allowed frequencies of an isolated nanoelement becomes a finite-width frequency band for an array of identical interacting elements. It is possible to tune the permitted and forbidden frequency bands, modifying the geometrical or the material magnetic parameters, as well as the external magnetic field. From a technological point of view, the accurate fabrication of planar magnonic crystals and a proper understanding of their magnetic excitation spectrum in the gigahertz range is oriented to the design of filters and waveguides for microwave communication systems.

Dynamical properties of a three-dimensional diluted Heisenberg model

We study the magnetic excitation spectrum in three-dimensional diluted ferromagnetic nearest-neighbor systems down to the percolation threshold. The disorder effects resulting from the dilution are handled accurately within self-consistent local random phase approximation approach. The calculations are performed using relatively large systems containing typically 20 000 localized spins, a systematic average over many configurations of disorder is performed. We analyze in details the change in the magnon spectrum and magnon density of states as we increase the dilution. The zone of stability of the well-defined magnon modes is shown to shrink drastically as we approach the percolation threshold. We also calculate the spin stiffness which appears to vanish at the percolation threshold exactly. A comparison with available data, based on a different theoretical approach, is also provided. We hope that this study will motivate new experimental studies based on inelastic neutron-scattering measurements.

Spin dynamics in Yb- and Sm-based systems with the nonmagnetic ground state

The nature and formation conditions of the ground state and the dynamic magnetic response (the magnetic excitation spectrum) of the YbB12 and Sm(Y)S systems with the nonmagnetic ground state have been studied using inelastic neutron scattering.

Dynamical Spin Susceptibility of Electron-Doped High-Tc Cuprates –Comparison with Hole-Doped Systems–

The magnetic excitation spectrum of electron-doped copper oxide superconductors is studied by calculating the dynamical spin susceptibility of the two-dimensional Hubbard model in which a d x 2 - y 2 -wave superconducting order parameter is assumed. The spectrum of electron-doped systems is compared with that of hole-doped systems, and the relationship between the frequency at which a peak grows in the spectrum and the superconducting energy gap at a hot spot is investigated. A peak may be observed even when the magnetic resonance condition is not exactly satisfied. We find that, in the electron-doped systems, the resonance condition is less likely to be satisfied than in the hole-doped systems because of the small density of states around the hot spots, and the peak frequency is close to twice the gap magnitude at the hot spots.

Spin Modes in Elliptical Nanorings in the Vortex State: Two-Dimensional Mapping by Micro-Focused Brillouin Light Scattering

Micro-focused and conventional Brillouin light scattering techniques have been exploited to investigate the spectrum of magnetic excitations (eigenmodes) of elliptical permalloy nanorings in the vortex ground state. The interpretation of the experimental data has been achieved using the dynamical matrix method. A careful comparison of the calculated frequency and profiles of magnetic modes with the experimental data, allowed us to identify the character of the different modes in terms of spatial symmetry and localization. In particular, the spatial extent of each mode was directly visualized by measurement of two-dimensional maps of the mode intensity over the ring surface.

Electrical Manipulation of Spin States in a Single Electrostatically Gated Transition-Metal Complex

We demonstrate an electrically controlled high-spin (S = 5/2) to low-spin (S = 1/2) transition in a three-terminal device incorporating a single Mn(2+) ion coordinated by two terpyridine ligands. By adjusting the gate-voltage we reduce the terpyridine moiety and thereby strengthen the ligand-field on the Mn-atom. Adding a single electron thus stabilizes the low-spin configuration and the corresponding sequential tunnelling current is suppressed by spin-blockade. From low-temperature inelastic cotunneling spectroscopy, we infer the magnetic excitation spectrum of the molecule and uncover also a strongly gate-dependent singlet-triplet splitting on the low-spin side. The measured bias-spectroscopy is shown to be consistent with an exact diagonalization of the Mn-complex, and an interpretation of the data is given in terms of a simplified effective model.

Evolution of the Spin Resonance in CeCoIn5 under Magnetic Field

The evolution of the magnetic excitation spectrum of the unconventional superconductor CeCoIn 5 was studied by inelastic neutron scattering as a function of magnetic field applied in the [1,-1,0] direction. The spin resonance characteristic of the superconducting state was observed up to about H c2 /2: it is still centered at (1/2,1/2,1/2) when the magnetic field increases while its characteristic energy decreases and its lineshape substantially broadens.