Longitudinal Magnetic Excitations Research Articles

Total Moment Sum Rule for Magnets in the Vicinity of Quantum Critical Point

It is known that the longitudinal and transverse excitation modes can exist in the vicinity of a quantum critical point in the ordered phase of quantum magnetic systems. The total moment sum rule for such systems is derived on the basis of the extended spin-wave theory, where both longitudinal and transverse magnetic excitations are taken into account. The sum rule is resolved into elastic, one-magnon, and two-magnon components. The formulation is applicable to spin systems with the longitudinal mode, such as $S=1$ systems with single-ion anisotropy of easy-plane type and spin dimer systems. The result helps us analyze and understand measured data of inelastic neutron scattering.

Resonant and anti‐resonant modes of the dilute, spin‐inbalanced, two‐dimensional electron liquid including correlations

A spin‐sensitive linear response theory is presented that includes correlations beyond the well‐known random phase approximation. Especially for very dilute systems, such correlations play an important role. The response functions obtained give insight into both charge and longitudinal magnetic excitations. In addition to the spin‐plasmon, we propose a new regime where no magnetic excitation is possible, namely the magnetic anti‐resonance. Both effects lie in experimentally accessible ranges.

Anisotropic resonance modes emerging in an antiferromagnetic superconducting state

Two strong arguments in favor of magnetically driven unconventional superconductivity arise from the coexistence and closeness of superconducting and magnetically ordered phases on the one hand, and from the emergence of magnetic spin-resonance modes at the superconducting transition on the other hand. Combining these two arguments one may ask about the nature of superconducting spin-resonance modes occurring in an antiferromagnetic state. This problem can be studied in underdoped BaFe2 As2, for which the local coexistence of large moment antiferromagnetism and superconductivity is well established by local probes. However, polarized neutron scattering experiments are required to identify the nature of the resonance modes. In the normal state of Co underdoped BaFe2 As2 the antiferromagnetic order results in broad magnetic gaps opening in all three spin directions that are reminiscent of the magnetic response in the parent compound. In the superconducting state two distinct anisotropic resonance excitations emerge, but in contrast to numerous studies on optimum and over-doped BaFe2 As2 there is no isotropic resonance excitation. The two anisotropic resonance modes appearing within the antiferromagnetic phase are attributed to a band selective superconducting state, in which longitudinal magnetic excitations are gapped by antiferromagnetic order with sizable moment.

Collective dynamics in atomistic models with coupled translational and spin degrees of freedom

Using an atomistic model that simultaneously treats the dynamics of translational and spin degrees of freedom, we perform combined molecular and spin dynamics simulations to investigate the mutual influence of the phonons and magnons on their respective frequency spectra and lifetimes in ferromagnetic bcc iron. By calculating the Fourier transforms of the space- and time-displaced correlation functions, the characteristic frequencies and the linewidths of the vibrational and magnetic excitation modes were determined. Comparison of the results with that of the stand-alone molecular dynamics and spin dynamics simulations reveals that the dynamic interplay between the phonons and magnons leads to a shift in the respective frequency spectra and a decrease in the lifetimes. Moreover, in the presence of lattice vibrations, additional longitudinal magnetic excitations were observed with the same frequencies as the longitudinal phonons.

Longitudinal magnetic excitation in KCuCl3 studied by Raman scattering under hydrostatic pressures

We measure Raman scattering in an interacting spin-dimer system KCuCl3 under hydrostatic pressures up to 5 GPa mediated by He gas. In the pressure-induced quantum phase, we observe a one-magnon Raman peak, which originates from the longitudinal magnetic excitation and is observable through the second-order exchange interaction Raman process. We report the pressure dependence of the frequency, halfwidth and Raman intensity of this mode.

Longitudinal magnetic fluctuations in Langevin spin dynamics

We develop a generalized Langevin spin dynamics (GLSD) algorithm where both the longitudinal and transverse (rotational) degrees of freedom of atomic magnetic moments are treated as dynamic variables. This removes the fundamental limitation associated with the use of stochastic Landau-Lifshitz (sLL) equations, in which the magnitude of magnetic moments is assumed constant. A generalized Langevin spin equation of motion is shown to be equivalent to the sLL equation if the dynamics of an atomic moment vector is constrained to the surface of a sphere. A fluctuation-dissipation relation for GLSD and an expression for the dynamic spin temperature are derived using the Fokker-Planck equation. Numerical simulations, performed using ferromagnetic iron as an example, illustrate the fundamental difference between the two- and three-dimensional dynamic evolution of interacting moments, where the three-dimensional GLSD includes the treatment of both transverse and longitudinal magnetic excitations.

Magnetic properties of URu2Si2under uniaxial stress by neutron scattering

The aim of this study is to compare the magnetic behavior of URu2Si2 under uniaxial stress along the a-axis with the behavior under hydrostatic pressure. Both are very similar, but uniaxial stress presents a critical stress {\sigma}xa smaller (0.33(5)GPa) than the hydrostatic critical pressure px =0.5 GPa where the ground state switches from HO (hidden order) to AF (antiferromagnetic) ground state. From these critical values and from Larmor neutron diffraction (LND), we conclude that the magnetic properties are governed by the shortest U-U distance in the plane (a lattice parameter). Under stress, the orthorhombic unit cell stays centered. A key point shown by this study is the presence of a threshold for the uniaxial stress along the a-axis before the appearance of the large AF moment which indicates no-mixture of order parameter (OP) between the HO ground state and the AF one as under hydrostatic pressure. The two most intense longitudinal magnetic excitations at Q0=(1,0,0) and Q1=(0.6,0,0) were measured in the HO state: the excitation at Q0 decreases in energy while the excitation at Q1 increases in energy with the uniaxial stress along the a-axis. The decrease of the energy of the excitation at Q0 seems to indicate a critical energy gap value of 1.2(1) meV at {\sigma}xa. A similar value was derived from studies under hydrostatic pressure at px.

Width of the longitudinal magnon in the vicinity of the O(3) quantum critical point

We consider a three-dimensional quantum antiferromagnet in the vicinity of a quantum critical point separating the magnetically ordered and the magnetically disordered phases. A specific example is TlCuCl${}_{3}$ where the quantum phase transition can be driven by hydrostatic pressure and/or by external magnetic field. As expected, two transverse and one longitudinal magnetic excitations have been observed in the pressure-driven magnetically ordered phase. According to the experimental data, the longitudinal magnon has a substantial width, which has not been understood and has remained a puzzle. In the present work, we explain the mechanism for the width, calculate the width and relate the value of the width with the parameters of the Bose condensate of magnons observed in the same compound. The method of an effective quantum field theory is employed in the work.

Optical spectra of the heavy fermion uniaxial ferromagnetUGe2

We report a detailed study of UGe$_{2}$ single crystals using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low frequency interband transition and a narrow free-carrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp changes in the low frequency mass and scattering rate below the upper ferromagnetic transition $T_C = 53 K$. The characteristic changes are exhibited most strongly at an energy scale of around 12 meV (100 cm$^{-1}$). They recover their unrenormalized value above $T_C$ and for $\omega >$ 40 meV. In contrast no sign of an anomaly is seen at the lower transition temperature of unknown nature $T_x \sim$ 30 K, observed in transport and thermodynamic experiments. In the ferromagnetic state we find signatures of a strong coupling to the longitudinal magnetic excitations that have been proposed to mediate unconventional superconductivity in this compound.

Temperature-induced longitudinal spin fluctuations in Fe and Ni

We have developed an ab initio framework for calculating parameters of a high-temperature magnetic Hamiltonian. In the adiabatic approximation, this includes transverse and longitudinal magnetic excitation spectra on equal footing. The exchange interaction parameters of the Hamiltonian for bcc Fe and fcc Ni are determined from constrained local spin-density approximation calculations. Finite temperature magnetic properties of the resulting model Hamiltonian are then investigated by a Monte Carlo simulation technique. The calculated Curie temperatures and paramagnetic susceptibilities are in good agreement with experimental data for both metals. We demonstrate that the temperature-induced longitudinal spin fluctuations are important for high temperature properties such as susceptibility and magnetic specific heat.

Dynamics of PrNi 2Si 2 in its modulated magnetic phase

Results of inelastic neutron scattering in the amplitude-modulated magnetic phase of PrNi 2Si 2 are presented. The main, lowest longitudinal branch of the dispersion curve exhibits a shape much more dispersive than that present above T N = 20 K. Close to the incommensurate vector Q = (0, 0, 0.875) , a critical behaviour occurs, characterized by the appearance of a new longitudinal magnetic excitation at low energy, showing a nearly complete softening for q = Q . Both low-energy excitations could be interpreted as phasons and amplitudons. Results are analyzed by a generalized susceptibility formalism.

Bulk and surface longitudinal magnetic central mode and Van Vleck excitations in cerium compounds

Low-frequency longitudinal magnetic modes using Mori's projection formalism are analyzed and the surface and the bulk central modes are discussed. The conditions for the occurence of bulk and surface Curie temperatures are also indicated. “Surface” and “bulk” Van Vleck longitudinal magnetic excitations in the paramagnetic phase are also considered. Our final conclusions indicate an absolute necessity of the Γ 7-Γ 8 exchange parameter for the occurence of the “bulk” and “surface” VV modes, whereas the Γ 7-Γ 7 and Γ 8-Γ 8 parameters seem to be of less importance.

On the apparent spin wave stiffness of amorphous ferromagnets

The authors show that longitudinal magnetic excitations are responsible for an additional T3/2 temperature dependent term in the decrease of the magnetisation of amorphous ferromagnets at low temperatures. This provides a possible explanation for the discrepancy on the values for the spin wave stiffness as measured by magnetisation and neutron scattering experiments. The discrepancy is compared to the anomalous temperature dependence of the resistivity, another effect of these longitudinal magnetic excitations.

Influence of the crystal-field level-ordering on the temperature-renormalised spectrum of magnetic excitations in Kramers light-rare-earth compounds

The temperature-renormalised longitudinal magnetic excitation spectrum in Kramers light-rare-earth ferromagnetic compounds is studied, and found to consist of two modes. The temperature dependence, as well as the dispersion formula, is obtained in hybrid random phase/molecular field approximation. The results are expressed in terms of the parameters of the Hamiltonian, comprising the single-ion crystal-field operator and the isotropic two-ion effective exchange interaction. The conclusions cover a wide range of the ratio of the molecular and crystal fields. Within the framework of the model the authors also discuss the softening of the longitudinal magnetic modes at the magnetic phase transition. At T=0K, the result is in agreement with that derived in the low-temperature boson approach. Numerical calculations are performed for cubic ferromagnetic cerium compounds.