We investigate two-magnon Raman scattering from the $S=1/2$ Heisenberg antiferromagnet on the triangular lattice, considering both the effect of renormalization of the one-magnon spectrum by 1/S corrections and final-state magnon-magnon interactions. The bare Raman intensity displays two peaks related to one-magnon van-Hove singularities. We find that 1/S self-energy corrections to the one-magnon spectrum strongly modify this intensity profile. The central Raman-peak is significantly enhanced due to plateaus in the magnon dispersion, the high frequency peak is suppressed due to magnon damping, and the overall spectral support narrows considerably. Additionally we investigate final-state interactions by solving the Bethe-Salpeter equation to $O(1/S)$. In contrast to collinear antiferromagnets, the non-collinear nature of the magnetic ground state leads to an irreducible magnon scattering which is retarded and non-separable already to lowest order. We show that final-state interactions lead to a rather broad Raman-continuum centered around approximately twice the 'roton'-energy. We also discuss the dependence on the scattering geometry.
The zero-temperature phase diagrams of a two-dimensional frustrated quantum antiferromagnetic system, namely the Union Jack model, are studied using the coupled cluster method (CCM) for the two cases when the lattice spins have spin quantum number $s=1$ and $s=3/2$. The system is defined on a square lattice and the spins interact via isotropic Heisenberg interactions such that all nearest-neighbour (NN) exchange bonds are present with identical strength $J_{1}>0$, and only half of the next-nearest-neighbour (NNN) exchange bonds are present with identical strength $J_{2} \equiv κJ_{1} > 0$. The…
We theoretically investigate a $J_1$-$J_3$ classical Heisenberg model on the breathing pyrochlore lattice, where the nearest-neighbor (NN) exchange interactions for small and large tetrahedra, $J_1$ and $J_1'$, take different values due to the breathing bond-alternation and $J_3$ is the third NN antiferromagnetic interaction along the bond direction. It is found by means of Monte Carlo simulations that for large $J_3$, a hedgehog lattice, a three-dimensional periodic array of magnetic monopoles and antimonopoles, emerges in the form of a quadruple-${\bf Q}$ state characterized by the ordering …
We investigate the two-magnon Raman scattering from an anisotropic S=1/2 triangular Heisenberg antiferromagnet Cs2CuCl4. We find that the Raman response is very sensitive to magnon-magnon interactions and to scattering geometries, a feature that is in remarkable contrast with the polarization-independent Raman signal from the isotropic triangular Heisenberg antiferromagnet. Since a spin-liquid ground state gives rise to a similar rotationally invariant Raman response, our results on the polarization dependence of the scattering spectrum suggest that Raman spectroscopy provides a useful probe, …
We calculate the two-magnon Raman scattering spectra in antiferromagnetic phases of several frustrated spin models defined on the honeycomb lattice. These include the Néel antiferromagnetic phase of a $J_1$-$J_2$-$J_3$ model and the stripe phase of the Heisenberg-Kitaev model. We show that both the magnetic frustration and the anisotropy of interactions may significantly affect the Raman spectra. We further discuss the implications of our results to the magnetic excitations of the iron-based compound BaFe$_2$Se$_2$O and show how the magnetic interactions can be extracted from fit to the Raman …
A theory of electronic Raman scattering in nearly antiferromagnetic Fermi liquids is constructed using the phenomenological electron-electron interaction introduced by Millis, Monien, and Pines. The role of "hot spots" and their resulting signatures in the channel dependent Raman spectra is highlighted, and different scaling regimes are addressed. The theory is compared to Raman spectra taken in the normal state of overdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$, and it is shown that many features of the symmetry dependent spectra can be explained by the theory.
Using Raman scattering, the quasi-two dimensional organic superconductor k-(BEDT-TTF)2Cu[N(CN)2]Br (T_c=11.8 K) and the related antiferromagnet k-(BEDT-TTF)2Cu[N(CN)2]Cl are studied. Raman scattering provides unique spectroscopic information about magnetic degrees of freedom that has been otherwise unavailable on such organic conductors. Below T=200 K a broad band at about 500 cm-1 develops in both compounds. We identify this band with two-magnon excitation. The position and the temperature dependence of the spectral weight are similar in the antiferromagnet and in the metallic Fermi-liquid. W…
Controlled doping of SrCu$_2$(BO$_3$)$_2$, a faithful realization of the Heisenberg spin-1/2 antiferromagnet on the Shastry-Sutherland lattice, with non-magnetic impurities generates bound-states below the spin gap. These bound-states and their symmetry properties are investigated by exact diagonalisation of small clusters and within a simple effective model describing a spinon submitted to an attractive extended potential. It is shown that Raman spectroscopy is a unique technique to probe these bound-states. Quantitative theoretical Raman spectra are numerically obtained.
We show that the magnetic Raman response of a spin-liquid is independent of the polarizations of the light for triangular symmetries. In contrast, a ground-state that has a broken symmetry shows characteristic oscillations when the polarizations are rotated. This would allow to detect weak broken symmetries and emergent order-parameters. We focus on the Kagome antiferromagnet where no conventional long-range order has been found so far, and present the Raman cross-section of a spin-liquid and a valence bond crystal (VBC) using a random phase approximation.
We study the two-magnon non-resonant Raman scattering in the (pi,pi) and (pi,0) ordered antiferromagnetic phases of a J1x-J1y-J2-Jc Heisenberg model on the tetragonal lattice within the framework of the spin-wave theory. We discuss the effects of various tuning factors to the two-magnon Raman spectra. We find that both the magnetic frustration J2/J1 and the interlayer exchange coupling Jc may significantly affect the spectra in both the B1g and A1g' channels in the (pi,pi) Neel ordered phase. Moreover, we find a splitting of the two-magnon peak in the (pi,0) antiferromagnetic phase. We further…
We present a theory for Raman scattering on 2D quantum antiferromagnets. The microscopic Fleury-Loudon Hamiltonian is expressed in terms of an effective $O(3)$ - model. Well within the Néel ordered phase, the Raman spectrum contains a two-magnon and a two-Higgs contribution, which are calculated diagramatically. The vertex functions for both the Higgs and magnon contributions are determined from a numerical solution of the corresponding Bethe-Salpeter equation. Due to the momentum dependence of the Raman vertex in the relevant $B_{1g}+E_{2g}$ symmetry, the contribution from the Higgs mode is s…
We present and analyze Raman spectra of the Mott insulator Ca$_2$RuO$_4$, whose quasi-two-dimensional antiferromagnetic order has been described as a condensate of low-lying spin-orbit excitons with angular momentum $J_{eff}=1$. In the $A_g$ polarization geometry, the amplitude (Higgs) mode of the spin-orbit condensate is directly probed in the scalar channel, thus avoiding infrared-singular magnon contributions. In the $B_{1g}$ geometry, we observe a single-magnon peak as well as two-magnon and two-Higgs excitations. Model calculations using exact diagonalization quantitatively agree with the…
Motivated by various spin-1/2 compounds like Cs$_2$CuCl$_4$ or $κ$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$, we derive a Raman-scattering operator {\it à la} Shastry and Shraiman for various geometries. For T=0, the exact spectra is computed by Lanczos algorithm for finite-size clusters. We perform a systematic investigation as a function of $J_2/J_1$, the exchange constant ratio: ranging from $J_2=0$, the well known square-lattice case, to $J_2/J_1=1$ the isotropic triangular lattice. We discuss the polarization dependence of the spectra and show how it can be used to detect precursors of the instabiliti…
We study theoretically the Raman scattering spectra in the one-dimensional (1D) quantum spin-1/2 antiferromagnets. The analysis reveals that their low-energy dynamics is exquisitely sensitive to various perturbations to the Heisenberg chain with nearest-neighbor exchange interactions, such as magnetic anisotropy, longer-range exchange interactions, and bond dimerization. These weak interactions are mainly responsible for the Raman scattering and give rise to different types of spectra as functions of frequency, temperature, and external field. In contrast to the Raman spectra in higher dimensi…
A theory of electronic Raman scattering in nearly antiferromagnetic Fermi liquids is constructed using the phenomenological electron-electron interaction introduced by Millis, Monien, and Pines. The role of "hot spots" and their resulting signatures in the channel dependent Raman spectra is highlighted, and different scaling regimes are addressed. The effects of self consistency and vertex corrections are compared to a perturbative treatment of the interaction. The theory is then compared to Raman spectra taken in the normal state of cuprate superconductors, and it is shown that many features …
We present a systematic study of the interplay between lattice parameters and the energy of the optical phonons as well as the antiferromagnetic coupling strength, $J$, in the high-$T_{\text{c}}$ superconducting cuprate $R$Ba$_2$Cu$_3$O$_{6+δ}$ (\mbox{R-123}, $R =$ Y, Dy, Gd, Sm, Nd) with hole-doping $p$ ($0.00<p \lesssim 0.04$). The energy of the $B_{1g}$-mode at $ν_{B1g}\approx~335$ cm$^{-1}$ has been found to relate systematically to the inverse of the lattice parameter $a$. Our results confirm the temperature dependent phonon splitting for Nd-123 at low doping, which has been reported for …
The Raman response in the antiferromagnetic 2-leg S=1/2 Heisenberg ladder is calculated for various couplings by continuous unitary transformations. For leg couplings above 80% of the rung coupling a characteristic 2-peak structure occurs with a point of zero intensity within the continuum. Experimental data for CaV_2O_5 and La_yCa_(14-y)Cu_24O_41 are analyzed and the coupling constants are determined. Evidence is found that the Heisenberg model is not sufficient to describe cuprate ladders. We argue that a cyclic exchange term is the appropriate extension.
Although the parent iron-based pnictides and chalcogenides are itinerant antiferromagnets, the use of local moment picture to understand their magnetic properties is still widespread. We study magnetic Raman scattering from a local moment perspective for various quantum spin models proposed for this new class of superconductors. These models vary greatly in the level of magnetic frustration and show a vastly different two-magnon Raman response. Light scattering by two-magnon excitations thus provides a robust and independent measure of the underlying spin interactions. In accord with other rec…
Raman spectroscopic signature of Kitaev magnetism and complex spin-lattice coupling in S = 1/2 antiferromagnet SrLaCoNbO$_6$ double perovskite
Ames National Laboratory, U.S. Department of Energy, Iowa State University（艾姆斯国家实验室）；Department of Physics, Indian Institute of Technology Delhi（德里印度理工学院物理系）；School of Physics, The University of New South Wales (UNSW)（新南威尔士大学物理学院）
Using Raman spectroscopy, we investigate the lattice phonons, magnetic excitations, and magneto-elastic coupling in the distorted triangular-lattice Heisenberg antiferromagnet alpha-SrCr2O4, which develops helical magnetic order below 43 K. Temperature dependent phonon spectra are compared to predictions from density functional theory calculations which allows us to assign the observed modes and identify weak effects arising from coupled lattice and magnetic degrees of freedom. Raman scattering associated with two-magnon excitations is observed at 20 meV and 40 meV. These energies are in gener…
Measurement and control of magnetic order and correlations in real time is a rapidly developing scientific area relevant for magnetic memory and spintronics [1,15]. In these experiments an ultra-short laser pulse (pump) is first absorbed by excitations carrying electric dipole moment. These then give their energy to the magnetic subsystem monitored by a time-resolved probe. A lot of progress has been made in investigations of ferromagnets but antiferromagnets are more challenging. Here we introduce time-resolved two-magnon Raman scattering as a novel real time probe of magnetic correlations es…
We study Raman response of Heisenberg antiferromagnets on the $\mathbf{C}_\mathrm{5v}$ Penrose and $\mathbf{C}_\mathrm{8v}$ Ammann-Beenker lattices within and beyond the Loudon-Fleury second-order perturbation scheme intending to explore optical features peculiar to quasiperiodic magnets. Within the Loudon-Fleury mechanism, we find one and only Raman-active mode of $\mathrm{E}_2$ symmetry without any dependence on linear incident and scattered polarizations. Beyond the Loudon-Fleury mechanism, two more symmetry species $\mathrm{A}_1$ and $\mathrm{A}_2$ are activated via dynamic ring exchange a…
We apply torque equilibrium spin wave theory (TESWT) to investigate an anisotropic XXZ antiferromagnetic model with Dzyaloshinskii-Moriya (DM) interaction in a triangular lattice. Considering the quasiparticle vacuum as our reference, we provide an accurate analysis of the non-collinear ground state of a frustrated triangular lattice magnet using the TESWT formalism. We elucidate the effects of quantum fluctuations on the ordering wave vector based on model system parameters. We study the single magnon dispersion, the two-magnon continuum using the spectral function, and the Raman spectrum of …
The Raman scattering spectra from magnetic excitations in an antiferromagnetic spin-1/2 two leg ladder is investigated for weak and strong interladder coupling. In the first case, a cusp in the Raman intensityis obtained at a frequency twice the gap. In the second case, a peak at twice the gap replaces the cusp. We discuss the relevance of our calculation to recent experiments on CaV2O5 and Sr14Cu24O41.
We report magnetic susceptibility, specific heat, and Raman scattering investigations of alpha-TeVO4 containing V-O edge-sharing chains. These chains promote a system of ferromagnetic/antiferromagnetic spin-1/2 Heisenberg alternating exchange chains with pronounced spin frustration. The magnetic susceptibility and Raman scattering evidence a crossover at T* = 85 K with different slopes of the reciprocal susceptibility and a magnetic phase transition into a long-range-ordered state at Tc = 16 K. From Raman scattering data a strong mutual coupling between lattice and magnetic degrees of freedom …
Two-magnon Raman scattering provides important information about electronic correlations in the insulating parent compounds of high-$T_c$ materials. Recent experiments have shown a strong dependence of the Raman signal in $B_{1g}$ geometry on the frequency of the incoming photon. We present an analytical and numerical study of the Raman intensity in the resonant regime. It has been previously argued by one of us (A.Ch) and D. Frenkel that the most relevant contribution to the Raman vertex at resonance is given by the triple resonance diagram. We derive an expression for the Raman intensity in …
NiGa$_2$S$_4$ is a triangular lattice S=1 system with strong two-dimensionality of the lattice, actively discussed as a candidate to host spin-nematic order brought about by strong quadrupole coupling. Using Raman scattering spectroscopy we identify a phonon of E$_g$ symmetry which can modulate magnetic exchange $J_1$ and produce quadrupole coupling. Additionally, our Raman scattering results demonstrate a loss of local inversion symmetry on cooling, which we associate with sulfur vacancies. This will lead to disordered Dzyaloshinskii-Moriya interactions, which can prevent long range magnetic …
We compare the theoretical predictions of the previous article [L. Benfatto and M. B. Silva Neto, cond-mat/0602419], with Raman spectroscopy experiments in Sr(2)CuO(2)Cl(2) and untwinned La(2)CuO(4) single crystals. We construct the magnetic point group for the magnetically ordered phase of the two compounds, Sr(2)CuO(2)Cl(2) and La(2)CuO(4), and we classify all the Raman active one-magnon excitations according to the irreducible co-representations for the associated magnetic point group. We then measure the evolution of the one-magnon Raman energies and intensities for low and moderate magnet…
We report experimental and theoretical studies on the magnetoelastic interactions in MnPS$_3$. Raman scattering response measured as a function of temperature shows a blue shift of the Raman active modes at 120.2 and 155.1 cm$^{-1}$, when the temperature is raised across the antiferromagnetic-paramagnetic transition. Density functional theory (DFT) calculations have been performed to estimate the effective exchange interactions and calculate the Raman active phonon modes. The calculations lead to the conclusion that the peculiar behavior with temperature of the two low energy phonon modes can …
Recent experiments have revealed the role of staggered Dzialoshinskii-Moriya interaction in the magnetized phase of an antiferromagnetic spin 1/2 two-leg ladder compound under a uniform magnetic field. We derive a low energy effective field theory describing a magnetized two-leg ladder with a weak staggered Dzialoshinskii-Moriya interaction. This theory predicts the persistence of the spin gap in the magnetized phase, in contrast to standard two-leg ladders, and the presence of bound states in the excitation spectrum. Such bound states are observable in Raman scattering measurements. These res…
We use Raman scattering to study spin, charge, and lattice dynamics in various phases of Ca_{2-x}Sr_{x}RuO_{4}. With increasing substitution of Ca by Sr in the range 0 =< x < 0.2, we observe (1) evidence for an increase of the electron-phonon interaction strength, (2) an increased temperature-dependence of the two-magnon energy and linewidth in the antiferromagnetic insulating phase, and (3) evidence for charge gap development, and hysteresis associated with the structural phase change, both of which are indicative of a first-order metal-insulator transition (T_{MI}) and a coexistence of metal…
Perfect deporalization of the Loudon-Fleury inelastic visible-light scattering in the Kitaev honeycomb model is well known. Though it happens in Heisenberg Kagome and triangular antiferromagnets as well, yet we prove it to be of geometric origin rather than peculiar to quantum spin liquids. A Kitaev spin liquid in the square planar geometry indeed exhibits polarized Raman spectra containing defferent symmetry species, each brought by symmetry-compatible spinon geminate excitations, i.e. arising from symmetry-compatible direct-product representations made of double-valued irreducible representa…
We investigate the one-magnon Raman scattering in the layered antiferromagnetic La(2)CuO(4) compound. We find that the Raman signal is composed by two one-magnon peaks: one in the B1g channel, corresponding to the Dzyaloshinskii-Moryia (DM) mode, and another in the B3g channel, corresponding to the XY mode. Furthermore, we show that a peak corresponding to the XY mode can be induced in the planar (RR) geometry when a magnetic field is applied along the easy axis for the sublattice magnetization. The appearance of such field-induced mode (FIM) signals the existence of a new magnetic state above…
Theoretical DFT calculations using GGA+U and HSE06 frameworks enabled vibrational mode assignment and partial (atomic) phonon DOS determination in KAgF3 perovskite, a low-dimensional magnetic fluoroargentate(II). Twelve bands in the spectra of KAgF3 were assigned to either IR active or Raman active modes, reaching very good correlation with experimental values (R2>0.997). Low-temperature Raman measurements indicate that the intriguing spin-Peierls-like phase transition at 230 K is an order-disorder transition and it does not strongly impact the vibrational structure of the material.
We evaluate the Raman light scattering intensity for the square lattice Heisenberg antiferromagnet with plaquette ring exchange J_\Box. With the exchange couplings as fixed before from an accurate fit to the spin-wave dispersion in La2CuO4, leading in particular to J_\Box=0.24J, we demonstrate in a parameter free calculation that the inclusion of the plaquette exchange contribution to the dispersion and the magnon-magnon interaction vertex gives a peak position in $B_{1g}$ scattering geometry E_max = 2.71J which is in excellent agreement with the experimental data. Yet, the intrinsic width and…
We used a combination of polarized Raman spectroscopy and spin wave calculations to study magnetic excitations in the strong spin-orbit-coupled (SOC) bilayer perovskite antiferromagnet $Sr_3Ir_2O_7$. We observed two broad Raman features at ~ 800 $cm^{-1}$ and ~ 1400 $cm^{-1}$ arising from magnetic excitations. Unconventionally, the ~ 800 $cm^{-1}$ feature is fully symmetric ($A_{1g}$) with respect to the underlying tetragonal ($D_{4h}$) crystal lattice which, together with its broad line shape, definitively rules out the possibility of a single magnon excitation as its origin. In contrast, the…
We present the Raman-scattering spectra of CaV2O5 and MgV2O5. The magnetic contribution in the Raman spectra of CaV2O5 is found in the form of a strong asymmetric line, centered at 2Delta, with a tail on the high-energy side. Our analysis of its spectral shape shows that the magnetic ordering in CaV2O5 can be described using a S=1/2 two-leg ladder Heisenberg antiferromagnetic model with Jparallel/Jperp = 0.1, and a small interladder exchange. The spin gap and exchange constant are estimated to be Delta = 400 cm-1 (570 K) and Jperp = 640 K. No magnon bound states are found. In contrast to CaV2O…
We used Raman light scattering to study the current-stabilized nonequilibrium semimetallic and metallic phases in Ca$_2$RuO$_4$. By determining the local temperature through careful analysis of the Stokes and anti-Stokes intensities, we find that Joule heating can be completely avoided by supplying sufficient cooling power in a helium-flow cryostat, and that the current induces the semimetallic state without inducing any significant heating. We further investigate the current-induced semimetallic state as a function of temperature and current. We confirm the absence of long-range antiferromagn…
A formalism is developed to treat magnetic impurities in a Mott-Hubbard antiferromagnetic insulator within a representation involving multiple orbitals per site. Impurity scattering of magnons is found to be strong when the number of orbitals N' on impurity sites is different from the number N on host sites. The impurity-scattering-induced softening of magnon modes leads to enhancement in thermal excitation of magnons, and hence to a lowering of the Neel temperature in layered or three dimensional systems. Weak impurity scattering of magnons is obtained in the case N'=N, where the impurity is …
Motivated by the achievements of the $X$-ray scattering technique, we analyzed the profile of the light scattering intensity $R(q, ω)$ at a finite $q$ in a 2D Heisenberg antiferromagnet. Previous Raman scattering studies at $q=0$ identified the two-magnon peak in $B_{1g}$ scattering geometry. We found that the $B_{1g}$ peak disperses downwards at a finite $q$, and its intensity increases, reaching its maximum at $q= q_0 = (0, π)$ and symmetry related points. In addition, the intensity in the $A_{1g}$ geometry becomes non-zero at a finite $q$, and also displays a two-magnon peak which gains str…
We report a neutron and Raman scattering study of a single-crystal of La(2)CuO(4.05) prepared by high temperature electrochemical oxidation. Elastic neutron scattering measurements show the presence of two phases, corresponding to the two edges of the first miscibility gap, all the way up to 300 K. An additional oxygen redistribution, driven by electronic energies, is identified at 250 K in Raman scattering (RS) experiments by the simultaneous onset of two-phonon and two-magnon scattering, which are fingerprints of the insulating phase. Elastic neutron scattering measurements show directly an …
Over the years the research group in Firenze has produced a number of theoretical results concerning the statistical mechanics of quantum antiferromagnetic models, which range from the theory of two-magnon Raman scattering to the characterization of the phase transitions in quantum low-dimensional antiferromagnetic models. Our research activity was steadily aimed to the understanding of experimental observations.
We present a theory for site dilution in the Valence Bond Crystal Phase of the Kagome Lattice Heisenberg Model. The presence of an empty site leads to strong singlet bond across the impurity. It also creates a free spin, which delocalizes inside the unit cell. Finite concentration of quenched impurities leads to a Valence Bond Glass phase. This phase has short-range Valence Bond order, no spin-gap, large spin susceptibilities, linear specific heat due to two-level systems, as well as singlet and triplet excitations that decompose into kink-antikink pairs delocalized over a few lattice constant…
Raman scattering in cuprate oxychlorides high-temperature superconductors single-crystals
Karlsruhe Institute of Technology（卡尔斯鲁厄理工学院）；Institut NEEL CNRS/UGA UPR2940（NEEL研究所）；IMPMC-Sorbonne Universités, Université Pierre et Marie Curie, CNRS, IRD, MNHN（IMPMC-索邦大学，皮埃尔和玛丽·居里大学，CNRS，IRD，MNHN）；Synchrotron SOLEIL（SOLEIL同步辐射中心）；Department of Physics, University of Fribourg（弗里堡大学物理系）；Department of Materials Science, Graduate School of Engineering, Osaka Metropolitan University（大阪公立大学工学研究科材料科学系）
We have used polarized Raman scattering to probe lattice vibrations and charge ordering in 12 nm thick, epitaxially strained PrNiO$_3$ films, and in superlattices of PrNiO$_3$ with the band-insulator PrAlO$_3$. A carefully adjusted confocal geometry was used to eliminate the substrate contribution to the Raman spectra. In films and superlattices under tensile strain, which undergo a metal-insulator transition upon cooling, the Raman spectra reveal phonon modes characteristic of charge ordering. These anomalous phonons do not appear in compressively strained films, which remain metallic at all …
Charge-charge, current-current and Raman correlation functions are derived in a consistent way using the unified response theory. The theory is based on the improved description of the conduction electron coupling to the external electromagnetic fields, distinguishing further the direct and indirect (assisted) scattering on the quasi-static disorder. The two scattering channels are distinguished in terms of the energy and momentum conservation laws. The theory is illustrated on the Emery three-band model for the normal state of the underdoped high-$T_c$ cuprates which includes the incoherent e…
We have used Raman scattering to investigate the magnetic excitations and lattice dynamics in the prototypical spin-orbit Mott insulators Sr2IrO4 and Sr3Ir2O7. Both compounds exhibit pronounced two-magnon Raman scattering features with different energies, lineshapes, and temperature dependencies, which in part reflect the different influence of long-range frustrating exchange interactions. Additionally, we find strong Fano asymmetries in the lineshapes of low-energy phonon modes in both compounds, which disappear upon cooling below the antiferromagnetic ordering temperatures. These unusual pho…
We report results of Raman scattering experiments on twin-free BaFe$_{2}$As$_{2}$ with the main focus placed on understanding the influence of electronic and spin degrees of freedom on the lattice dynamics. In particular, we scrutinize the $E_g$ modes and the As $A_{1g}$ mode. Each of the two $E_g$ phonons in the tetragonal phase is observed to split into a $B_{2g}$ and a $B_{3g}$ mode upon entering the orthorhombic stripe-magnetic phase. The splitting amounts to approximately 10\,cm$^{-1}$ and less than 5\,cm$^{-1}$ for the low- and the high-energy $E_g$ mode, respectively. The detailed study…
The spin dynamics and magnetic excitations of the slightly distorted triangular s = 3/2 system alpha-CaCr2O4 are investigated by means of Raman spectroscopy and electron spin resonance (ESR) to elucidate its peculiar magnetic properties. Two-magnon excitations in circular RL symmetry show a multi-maximum structure with a dominant spectral weight at low energies. The temperature dependence of the ESR linewidth is described by a critical broadening DeltaHpp(T) ~ (T - T_N)^{-p} with the exponent p = 0.30(3) - 0.38(5) for temperatures above T_N = 42.6 K. The exponent is much smaller than that of o…
