A Brillouin study of the temperature-dependence of the acoustic modes across the insulator-metal transitions in V2O3 and Cr-doped V2O3

Brillouin scattering studies have been carried out on high-quality single crystals of undoped and 0.9% Cr-doped V₂O₃. The observed modes in both the samples at ∼12 and ∼60 GHz are associated with the surface Rayleigh wave (SRW) and bulk acoustic wave (BAW), respectively. In the undoped sample, the mode frequencies of the SRW and BAW modes decrease as the temperature is lowered from room temperature to the insulator-metal transition temperature (T_IM=T_N=∼130 K). Below the transition, the modes show hardening. In the doped sample, the SRW mode shows a similar temperature-dependence as the undoped one, but the BAW mode shows hardening from room temperature down to the lowest temperature (50 K). This is the first measurement of the sound velocity below T_IM in the V₂O₃ system. The softening of the SRW frequency from 330 K to T_IM can be qualitatively understood on the basis of the temperature-dependence of C₄₄, which, in turn, is related to the orbital fluctuations in the paramagnetic metallic phase. The hardening of the mode frequencies below T_IM suggests that C₄₄ must increase in the antiferromagnetic insulating phase, possibly due to the orbital ordering.     

ENHANCEMENT OF FERROMAGNETIC CLUSTER INDUCED BY MAGNETIC FIELD IN THE PHASE-SEPARATED La0.5Ca0.5MnO3

Magnetic and transport properties of La_0.5Ca_0.5MnO₃ have been investigated by measuring the magnetization and resistance in zero-field-cooled (ZFC) and field-cooled (FC) modes. Conspicuously irreversible behaviors of magnetization/resistance in the two different modes were observed below the charge ordering transition temperature (T_CO). The ZFC and FC magnetizations at 5K, as functions of the magnetic field, coincide for μ₀H≤1T. Afterwards, the ZFC magnetization tends to an approximate constant, but the FC one increases linearly with increasing field. There exists an excellent correspondence between magnetization and resistance below T_CO. All the results suggest that the ferromagnetic clusters embed in the charge-ordered matrix. The phenomenon of ferromagnetic clusters growing up easily in the FC procedure has been interpreted according to the model of thermally activated two-level system.     

Sound velocity in La0.5Ca0.5MnO3

In this Letter the microscopic theory of the relative change in velocity of sound with temperature of La_0.5Ca_0.5MnO₃ is reported. The phonon Green function is calculated using the Green function technique of Zubarev in the limit of zero wave vector and low temperature. The lattice model electronic Hamiltonian in the presence of the phonon interaction with hybridization between the conduction electrons and the l-electrons is used. The relative change in velocity of sound at various temperatures is studied for different model parameters namely the position of the l-level, the effective phonon coupling strength and hybridization strength. The phonon anomalies observed experimentally at different temperatures are explained theoretically. An abrupt change in velocity at Neel temperature (TN) is observed clearly. It is observed that different parameters influence the velocity of sound.     

Electron-phonon coupling and longitudinal sound velocity of La0.5Ca0.5MnO3

In this paper, the microscopic theory of the relative change in velocity of sound with temperature of La_0.5Ca_0.5MnO₃ is reported. The phonon Green function is calculated using the Green function technique of Zubarev in the limit of zero wave vector and low temperature. The lattice model electronic Hamiltonian in the presence of the phonon interaction with hybridization between the conduction electrons and the l-electrons is used. The relative change in velocity of sound at various temperatures is studied for different model parameters, namely the position of the l-level, the effective phonon coupling strength and hybridization strength. The phonon anomalies observed experimentally at different temperatures are explained theoretically. An abrupt change in velocity at Neel temperature (T_N) is observed clearly. It is observed that different parameters influence the velocity of sound.     

Intrinsic inhomogeneity and non-linear conduction in charge-ordered Bi0.5Ca0.4Sr0.1MnO3

Systematic studies of X-ray, magnetic, electronic transport, and elastic properties have been performed on polycrystalline Bi_0.5Ca_0.4Sr_0.1MnO₃ sample. The sample exhibits charge ordering (CO) state below T_CO (=304 K), accompanied by a distinct maximum in magnetization. The softening of Young's modulus in the vicinity of T_CO indicates that there is a strong coupling of electron-phonon due to Jahn-Teller (JT) effect. The dynamic ferromagnetic spin correlations are observed at high temperatures above T_CO, which are replaced by antiferromagnetic (AFM) spin fluctuations at a concomitant CO transition. Below 32 K, a spin-glass (SG) state dominates at low temperatures. The voltage-current (V-I) characteristics measurement results indicate that the non-linear conduction starts above a threshold current, giving rise to a region of negative differential resistance (NDR). The origin of the non-linear conduction is discussed in view of current induced collapse of CO state associated with phase-separation mechanism.     

(Hf0.5Zr0.5V2)Hx的声子谱与超导性的关系

本文报道了用Be过滤探测器谱仪对C-15相的(Hf_0.5Zr_0.5V₂₎H_x等五种样品的声子谱的测量。观察到了它们的声子谱随含氢量而变化以及声学支有规则的软化现象。表明(Hf_0.5Zr_0.5V₂₎H_x的T_c随x的变化,主要是声学支的贡献,声学文软化有利于T_c的提高,而光学支“软化”可能会抑制超导性。 关键词：     

Charge and lattice dynamics of ordered state in La1/2Ca1/2MnO3: infrared reflection spectroscopy study

We report an infrared reflection spectroscopy study of La_1/2Ca_1/2MnO₃ over a broad frequency range and temperature interval which covers the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. The structural phase transition, accompanied by a ferromagnetic ordering at T_C=234 K, leads to enrichment of the phonon spectrum. A charge ordered antiferromagnetic insulating ground state develops below the Néel transition temperature T_N=163 K. This is evidenced by the formation of charge density waves and opening of a gap with the magnitude of 2Δ₀=(320±15) cm⁻¹ in the excitation spectrum. Several of the infrared active phonons are found to exhibit anomalous frequency softening. The experimental data suggest coexistence of ferromagnetic and antiferromangetic phases at low temperatures.     

Charge order melting and magnetic transition in Nd0.5(1+x)Ca0.5(1−x)Mn(1−x)CrxO3 system

The magnetic property of double doped manganite Nd_0.5(1+x)Ca_0.5(1−x)Mn_(1−x)Cr_xO₃ with a fixed ratio of Mn³⁺:Mn⁴⁺=1:1 has been investigated. For the undoped sample, it undergoes one transition from charge disordering to charge ordering (CO) associated with paramagnetic (PM)-antiferromagnetic (AFM) phase transition at T<250 K. The long range AFM ordering seems to form at 35 K, rather than previously reported 150 K. At low temperature, an asymmetrical M-H hysteresis loop occurs due to weak AFM coupling. For the doped samples, the substitution of Cr³⁺ for Mn³⁺ ions causes the increase of magnetization and the rise of T_c. As the Cr³⁺ concentration increases, the CO domain gradually becomes smaller and the CO melting process emerges. At low temperature, the FM superexchange interaction between Mn³⁺ and Cr³⁺ ions causes a magnetic upturn, namely, the second FM phase transition.     

Nd0.5Ca0.5Mn0.97Co0.03O3体系的超声研究

系统研究了Nd0.5Ca0.5Mn0.97Co0.03O3单相多晶样品在低温下的电磁性质和超声特性.超声测量结果表明,体系在低温下超声声速出现异常,超声声速从室温开始随着温度的降低逐渐减小,并在220K附近声速达到最小,其相对变化(ΔV/V)超过2%.之后,随着温度的进一步降低,声速急剧硬化,其相对变化达到14.5%.分析表明这是由于体系中电-声子相互作用导致的电荷有序态转变的结果,该电声子耦合来源于Mn3 的JahnTeller效应.同时,该体系在100K左右出现了金属绝缘体转变同时伴随着铁磁相变,分析表明在低温下Co离子与Mn离子之间存在着铁磁交换作用,因此微量的Co掺杂会在反铁磁区域内形成部分铁磁小团簇,当铁磁团簇连通后,体系由反铁磁电荷有序态(AFMCO)向铁磁金属态(FMM)转变.研究结果表明了样品在电荷有序相变温度TCO与金属绝缘体转变温度TMI之间(TMI相似文献   

Brillouin scattering in KH3 (SeO3)2 above the transition temperature

The Brillouin scattering measurement in KH₃(SeO₃)₂ above the transition temperature shows a large anomaly in hypersonic velocity of the transverse wave with the wave vector q = [010] and the polarization ξ = [001], which indicates acoustic phonon softening.     

Influence of magnetoelastic effects on lattice vibrations in the ferromagnetic invar alloy Fe0.65Ni0.35

Neutron scattering measurements have been made of phonons in a Fe₆₅Ni₃₅ crystal at several temperatures. Marked softening of the [110] acoustic shear modes and a dip in the dispersion relation are found at low temperatures. Above the magnetic ordering temperature T_c, the frequency shift is removed completely, which suggests phonon perturbation by magnetoelastic coupling.     

Raman spectroscopy of RuSr2(Eu1.5Ce0.5)Cu2O10 magneto-superconductor

Raman spectroscopy studies are reported for the RuSr₂Eu_1.5Ce_0.5Cu₂O₁₀ (Ru-1222) compound at various temperatures of 300, 250, 200 and 90 K. Three distinct vibrational bands: the first at 110, 140, and 160 cm⁻¹, the second at 295 and 347 cm⁻¹, and third one at 651 cm⁻¹ are seen in Raman spectra of the compound at room temperature. These bands are attached to the Cu atoms’ c-direction, the Ru atoms’ ab-plane stretching and Ru atoms’ c-direction anti-stretching modes. Below 200 K, an extra vibrational mode is also seen at 260 cm⁻¹. Also, with a decrease in temperature, though the Cu vibrational modes remain intact, the Ru atoms’ ab-plane stretching (295 cm⁻¹) and c-direction anti-stretching (651 cm⁻¹) modes shift gradually to higher wave number positions. The frequencies of modes at 260 and 651 cm⁻¹ showed anomalous softening and line-width broadening below 100 K that corroborates well with the spin ordering seen in susceptibility studies. The studied compound is a ferromagnetic superconductor with magnetic ordering of the Ru spins at 200 K and superconductivity below 30 K. A magnetic and electrical transport characterization of the compound is also presented briefly.     

Metamagnetic phase transitions induced by static and pulsed fields in (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics

It has been found that the magnetic susceptibility of (Sm_0.5Gd_0.5)_0.55Sr_0.45MnO₃ ceramic samples in zero external magnetic field exhibits a sharp peak near the temperature of 48.5 K with a small temperature hysteresis that does not depend on the frequency of measurements and is characteristic of the phase transition to an antiferromagnetic state with a long-range charge orbital ordering, which is accompanied by an increase in the magnetic susceptibility with a decrease in the temperature. The magnetization isotherms in static and pulsed magnetic fields at temperatures below 60 K demonstrate the occurrence of an irreversible metamagnetic transition to a homogeneous ferromagnetic state with a critical transition field independent of the measurement temperature, which, apparently, is associated with the destruction of the insulating state with a long-range charge ordering. In the temperature range 60 K ?? T ?? 150 K, the ceramic samples undergo a magnetic-field-induced reversible phase transition to the ferromagnetic state, which is similar to the metamagnetic transition in the low-temperature phase and is caused by the destruction of local charge/orbital correlations. With an increase in the temperature, the critical transition fields increase almost linearly and the field hysteresis disappears. Near the critical fields of magnetic phase transitions, small ultra-narrow magnetization steps have been revealed in pulsed fields with a high rate of change in the magnetic field of ??400 kOe/??s.     

Structural phase transitions and sodium ordering in Na0.5CoO2: a combined electron diffraction and Raman spectroscopy study

The nonstoichiometric Na_xCoO₂ system exhibits extraordinary physical properties that correlate with temperature and Na concentration in its layered lattice without evident long-range structure modification when conventional crystallographic techniques are applied. For instance, Na_0.7CoO₂, a thermodynamically stable phase, shows large thermoelectric power; water-intercalated Na_0.33CoO₂·1.3H₂O is a newly discovered superconductor with T_c∼4 K, and Na_0.5CoO₂ exhibits an unexpected charge ordering transition at around T_co∼55 K. Recent studies suggest that the transport and magnetic properties in the Na_xCoO₂ system strongly depend on the charge carrier density and local structural properties. Here we report a combined variable temperature transmission electron microscopy and Raman scattering investigation on structural transformations in Na_0.5CoO₂ single crystals. A series of structural phase transitions in the temperature range from 80 to 1000 K are directly identified and the observed superstructures and modulated phases can be interpreted by Na-ordering. The Raman scattering measurements reveal phase separation and a systematic evolution of active modes along with phase transitions. Our work demonstrates that the high mobility and ordering of sodium cations among the CoO₂ layers are a key factor for the presence of complex structural properties in Na_xCoO₂ materials, and also demonstrate that the combination of electron diffraction and Raman spectroscopy measurements is an efficient way for studying the cation ordering and phase transitions in related systems.     

Acousto-induced structural change in amorphous As2S3

The acoustic flux injection method was applied to amorphous As₂S₃. The velocity and the attenuation constant for shear wave at ～ 100MHz were measured through the observation of the Brillouin scattering of the light beam from a He-Ne laser. The optical transmission around absorption edge was found to change after the injection of acoustic fluxes. It was also found that the sound velocity decreased with increasing injection time of acoustic fluxes and the amount of the change of the sound velocity reached ～ 20% after 10⁵ injections. The changes in the optical transmission and the sound velocity tended to be erased by annealing below the glass transition temperature. These phenomena are considered to be due to structural changes caused by the injection of strong acoustic fluxes.     

Effect of Cr doping on the magnetic and electrical transport properties of charge-ordered Bi0.5Ca0.5MnO3

An experimental study on the magnetic and electrical transport properties of the manganites Bi_0.5Ca_0.5Mn_1−xCr_xO₃ (BCMCO) (0≤x≤0.12) is carried out. The results show that Cr doping can suppress the charge-ordering transition, favoring the ferromagnetic clusters. For x=0.12, the charge-ordering transition disappears but a very broad paramagnetic-ferromagnetic-like transition is detected at the Curie temperature T_C=72.6 K. It is caused by phase separation or coexistence of the charge-ordering and ferromagnetic phase. Moreover, the critical Cr content to destroy charge ordering phase in BCMCO does not match the general monotonous tendencies shown by Cr-doped Re_0.5Ca_0.5MnO₃ (Re-rare-earth). These differences are ascribed to the fact that the ground state in BCMCO differs markedly from the ferromagnetic metallic phase in Cr-doped Re_0.5Ca_0.5MnO₃ compounds.     

Acoustic properties of multiferroic PbFe1/2Ta1/2O3

The longitudinal acoustic wave velocity and attenuation in PbFe_1/2Ta_1/2O₃ ceramics have been measured by pulse-echo technique in the temperature range from 4.2 to 530 K. The anomalies observed in the sound velocity and attenuation behavior versus temperature were correlated with Burns temperature, temperature range of the coexistence of relaxor ferroelectric and antiferromagnetic states, and a suggested second antiferromagnetic phase transition at low temperatures.     

Behaviour of electrical resistivity and elastic constants at the phase transition in Ge0.08Sn0.92Te

Electrical resistivity and ultrasonic wave velocity measurements made through the phase transition in Ge_0.08Sn_0.92Te crystals provide further evidence that the acoustic anomaly is due to interaction between the softening optic phonon mode and the acoustic modes at the zone centre.     

Nd0.5Ca0.5Mn1-xAlxO3 (x=0,0.03)体系中电荷有序态的超声研究

系统研究了Nd0．5Ca0．5Mn1-xAlxO3(x=0,0．03)单相多晶样品在低温下的电磁性质和超声特性．电阻和磁化率测量表明,Nd0．5Ca0.5O3体系在TCO-257 K处发生了电荷有序相变．超声声速从室温开始随着温度的降低逐渐减小,并在TCO附近达到最小,之后,随着温度的进一步降低,声速急刷增加,同时伴随着一个尖锐的超声衰减峰出现.TCO附近的超声异常表明体系中存在着强烈的电-声子相互作用,该电-声子耦合来源于Mn3 的Jahn-Teller效应．在低温下,出现了另一个超声衰减峰,它的出现归结为反铁磁相与顺磁相之间的相分离现象.随着Al在Mn位的掺入,超声声速的最低点和衰减峰向低温移动,表明体系中的电荷有序态和反铁磁相均被部分抑制,     

Anomalous reduced sound velocity of multiferroic TbMn2O5

The anomalous reduced sound velocity of multiferroic TbMn₂O₅ (TMO) has been studied using Green's function technique. To achieve this aim, the anharmonic phonon-phonon interaction and the spin-phonon interaction were used. It was shown that the reduced velocity of sound of TMO exhibits a kink at the ferroelectric phase transition temperature T_C. This can be explained as an effect of vanishing ferroelectric ordering above T_C. It was found that the reduced sound velocity increases with increasing V⁽³⁾ (the third-order atomic force constants of the anharmonic phonons) in the interval T?<?T_C, whereas the reduced sound velocity remains unchanged in the interval T_C?<?T?<?T_N. It was also found that the reduced sound velocity increases with the increase of V⁽⁴⁾ (the fourth-order atomic force constants of the anharmonic phonons) in the interval T?<?T_N. In addition, the ferroelectric phase transition temperature T_C decreases when V⁽⁴⁾ increases in the interval T?<?T_N. Those theoretical results are in agreement with the experimental data.