EPR linewidths in an unusual copper chloride chain system

The EPR linewidths in [(CH₃)₂CHNH₃]₂CuCl₄ have been measured at 78 K. The salt contains two types of structurally and magnetically inequivalent chains. Each chain can be visuallized as a narrow ribbon cut from the two dimensional (RNH₃)₂CuCl₄ structures. The EPR g values are consistent with the structural characteristics of each chain. The linewidths are influenced by anisotropic and antisymmetric exchange, and also exhibit low dimensional spin diffusion behavior. The symmetry axis of the diffusional behavior is normal to the plane of the ribbon, implying that the spin correlations in the ribbons are two dimensional in nature.     

Tunneling and EPR linewidths due to dislocations in Mn acetate

We compute the width and shape of the EPR and tunneling resonances due to dislocations in Mn₁₂ acetate crystals. Uncorrelated dislocations produce the Gaussian shape of resonances while dislocations bound in pairs produce the Lorentzian shape. We stress that the uniaxial spin Hamiltonian together with crystal defects can explain the totality of experimental data on Mn₁₂. Received 2 August 2001 / Received in final form 15 March 2002 Published online 17 September 2002     

An explanation of the anomalous temperature dependence of optical linewidths in glasses

Recently photochemical holeburning experiments have been carried out in organic glasses down to 0.4 K. Here we give a theoretical explanation of theT ^1.3-dependence of the holewidth. It is based on the strain field coupling between impurity molecules and two-level tunneling states leading to a kind of spectral diffusion of the optical excitation energy.     

Fluorescence linewidths in glasses

The homogeneous optical linewidth of an impurity ion in a glass is described in terms of a process which arises from thermal transitions between the states of the two level systems of a glass; these give shifts in the electronic energy levels via the strain field coupling. With available values of the parameters, this contribution gives reasonably good agreement for the overall magnitude as well as for the temperature and time dependences observed recently for the “anomalous” fluorescence linewidth of Eu³⁺ in silicate glass for temperatures up to $\text{? 100}\text{K}$.     

The influence of Landau level broadening on temperature dependent cyclotron resonance linewidths in semiconductors

The effects of Landau level broadening, due to impurity fluctuations, on cyclotron resonance linewidths in semiconductors are reported. Allowing for level broadening, a calculation of the linewidth Γ shows a rapid low temperature increase, in contrast to earlier (non-broadening) calculations which give $\text{Γ ∞ T}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$. At low temperatures the broadening of the density of states causes a strong narrowing of the linewidth. These results are in good agreement with the experimental data reported for III-V and II-VI semiconductors. The effects of electron localisation are also considered.     

EPR investigation of nanosized La0.67Ca0.33MnO3−δ manganites

We report an electron paramagnetic resonance (EPR) investigation of the spin dynamics in the paramagnetic regime of nanosized La_0.67Ca_0.33MnO_3?δ manganites. The temperature dependences of the EPR line width and integral intensity have been analyzed in terms of the bottlenecked spin relaxation and small polaron hopping scenarios. The exchange coupling integral between Mn³⁺ and Mn⁴⁺ ions and the polaron activation energy decrease with the reduction of grain size. A discussion is given concerning the factors which could explain the observed changes.     

EPR Evidence for Premonitory Charge-Ordering Fluctuations in Nanomanganite Pr0.57Ca0.41Ba0.02MnO3

Electron paramagnetic resonance (EPR) and magnetic properties of nanocrystalline Pr_0.57Ca_0.41Ba_0.02MnO₃ are studied and compared with those of the bulk material. The nanoscale samples prepared by the sol–gel method were annealed at different temperatures to obtain particles of different sizes. The crystallite and particle sizes were estimated by X-ray diffraction and transmission electron microscopy and found to be approximately equal to 30, 60 and 100 nm. Magnetization studies on the 60 and 100 nm particles show charge-ordering at 230 K similar to the bulk sample prepared by solid-state reaction, while in the 30 nm particles the charge-ordering signature is absent. At low temperatures all the three nanosamples show ferromagnetic transitions. This result is confirmed by the EPR intensity behavior as well. However, the EPR line width, which is reflective of the spin dynamics, shows a shallow minimum for the 30 nm particles at the temperature of 230 K, corresponding to the charge-ordering transition. We interpret this result as an indication of the presence of charge-ordering fluctuations in the nanoparticles of 30 nm size even though the static charge order is absent, thus heralding the occurrence of charge order in the larger particles. Authors' address: Subray V. Bhat, Department of Physics, Indian Institute of Science, Bangalore 560012, India     

The temperature dependence of the elastic constants of MnO

We have measured the temperature dependence of the elastic constants of a monodomain single crystal of MnO, obtained by applying a unixial stress, from 4.2 to 300 K. Measurements were made by standard ultrasonic techniques and the acoustic waves were generated and detected by CdS transducers. Problems associated with the coupling between transducer and sample encountered by previous workers were therefore overcome. For propagation down the [111] direction, only the slow shear mode [$\text{1}\text{2}$(c₁₁?c₁₂)] was observable below the Néel temperature (T_N. The fast shear and longitudinal modes were heavily attenuated. Complex behaviour in the region of T_N and a further anomaly at 42 K were observed.     

Low temperature charge and orbital textures in La0.875Sr0.125MnO3

By using nuclear magnetic resonance techniques we show that for T<30 K the La0.875Sr0.125MnO3 compound displays a nonuniform charge distribution, comprised of two interconnected Mn ion subsystems with different spin, orbital, and charge couplings. The NMR results agree very well with the two spin wave stiffness constants observed at small q values in the spin wave dispersion curves [Phys. Rev. B 67, 214430 (2003)]. This picture is probably related to a yet undetermined charge and orbital superstructure occurring in the ferromagnetic insulating state of the La0.875Sr0.125MnO3 compound.     

Angular variation of the EPR linewidths of ions in tetragonal symmetries; Mn2+IN CaWO4

The angular dependence of the EPR linewidths of Mn²⁺ in tetragonal crystals is calculated under the assumption of a random distribution of lattice strains. A change in this angular dependence for different tetragonal groups is found. Data obtained for Mn²⁺ in CaWO₄ support the theory.     

EPR evidence of the low temperature phase transition in C60

A standard EPR technique has been applied to perform temperature studies of polycrystalline samples of C₆₀ fullerene. A comparison of the three main parameters of EPR spectra obtained for a C₆₀ sample before and after (solvent free) purification has been made in temperatures from 4 to 300 K. When compared to the spectrum of the as-obtained (crude) sample the spectrum of the purified sample revealed considerable changes i.e. an increase in the EPR signal intensity and evidence of a phase transition at about 90 K. This transition is proved to be related to the two processes: freezing of the orientational motion of C₆₀ molecules and redistribution of the positive and negative charge observed as paramagnetic centres localized on C₆₀ molecules.     

Pressure and temperature dependence of optical phonons in La0.75Ca0.25MnO3

We present the results of extensive studies on Raman and infrared active phonons in the La_0.75Ca_0.25MnO₃ manganite over a wide temperature (100–320 K) and pressure (0–14 GPa) range. The analysis of the temperature dependent data allows to identify a clear spectroscopic signature of the insulator to metal transition. Indeed the abrupt reduction of the effective electron–phonon interaction on entering the metallic phase determines a change in slope in the temperature dependence of the Jahn–Teller phonon line width. The analysis of the pressure dependent data shows that the octahedral Jahn–Teller distortion, and consequently the electron–phonon interaction, is strongly reduced only in the low-pressure regime. At very high pressure, the onset of a pressure-activated localizing mechanism efficiently contrasts the natural delocalizing tendency of pressure. We finally guess that this effect could be attributed to charge-localizing antiferromagnetic interactions activated by the strong lattice compression.     

Dynamic response in La0.55Ca0.45MnO3: temperature and field dependence

We report AC susceptibility measurements on bulk La_0.55Ca_0.45MnO₃ as functions of temperature, DC field and frequency ( f<1500 Hz). Both the in-phase (χ′) and out-of-phase (χ″) parts of the response are studied. It is observed that the in-phase part goes through a maximum that is removed on the application of moderate DC fields. The temperature and field dependence of χ″ are consistent with the picture of an inhomogeneous transition proceeding through a growth of clusters. DC fields suppress both the components with the strongest effects being at or close to T_c. The out of phase response is completely suppressed by fields of the order of a few hundred Gauss. The effect of the DC field is discussed in terms of the suppression of spin fluctuations close to T_c, and to the changes in coercivity for lower temperatures.     

Measurements of far-infrared intersubband absorption linewidths in GaAs/AlGaAs quantum wells as a function of temperature and charge density

Intersubband transitions in doped quantum wells are of fundamental scientific interest, as well as technological interest for potential devices. One of the important characteristics of the transitions is their linewidth. We present the results of linear absorption spectroscopy on a coupled double asymmetric QW structure. Using a backgated sample, we can independently vary the charge density and DC field at the well. The absorption lines appear to be homogeneously broadened. The lines appear to become lifetime broadened for temperatures above 70 K.     

Estimation of T1 for Co2+ ions from temperature variation of the EPR linewidths for Gd3+ in Ce2Co3(NO3)12.24H2O single crystals

EPR of Gd³⁺ doped in Ce₂M″₃(NO₃)₁₂.24H₂O (M″ = Mg, Zn, Co) single crystals has been studied at various temperatures from room temperature to 77 K using ∼ 9.45 GHz EPR spectrometer. The observation of resolved Gd³⁺ spectra at room temperature in Ce₃Co₂(NO₃)₁₂.24H₂O has been interpreted in terms of a random modulation of the interaction between the Gd³⁺ and the Co²⁺ ions by the rapid spin-lattice relaxation of Co²⁺ ions. It is found that the effective spin-lattice relaxation time T₁ ∝ T⁻ⁿ where n = 1.85 (B∥z axis) and n = 1.75 (B⊥z axis) if 103 < T < 283 K.