A Raman spectroscopic study of the high-frequency optical phonons in single crystals of the multiferroic system RMn 2 O 5 ͑R = Bi, Eu, Dy͒ was performed. All studied materials show anomalous phonon shifts, below a new characteristic temperature for these materials, T * ϳ 60-65 K. The sign and magnitude of such shifts appear to be correlated with the ionic radius of R, envolving from softenings for R = Bi to hardenings for R = Dy and showing an intermediary behavior for R = Eu. Additional phonon anomalies were identified below ϳT N ϳ 40-43 K, reflecting the onset of long-range ferroelectric and/or magnetic order of the Mn sublattice. Complementary dc-magnetic susceptibility ͓͑T͔͒ measurements for BiMn 2 O 5 up to 800 K yield a Curie-Weiss temperature CW = −253͑3͒ K, revealing a fairly large frustration ratio ͉͑ CW ͉ / T N = 6.3͒. Deviations of ͑T͒ from a Curie-Weiss paramagnetic behavior due to magnetic correlations were observed below temperatures of the order of ͉ CW ͉, with the inverse susceptibility showing inflection points at ϳ160 K and ϳT *. Supported by ͑T͒ data, the anomalous Raman phonon shifts below T * are interpreted in terms of the spinphonon coupling, in a scenario of strong magnetic correlations. Overall, these results support significant magnetic frustration, introduce a new characteristic temperature ͑T * ͒, and suggest a surprisingly rich behavior for the magnetic correlations in the paramagnetic phase of this system.

The occurrence at low temperatures of an ultrasharp field-induced transition in phase separated manganites is analyzed. Experimental results show that magnetization and specific heat step-like transitions below 5 K are correlated with an abrupt change of the sample temperature, which happens at a certain critical field. This temperature rise, a magnetocaloric effect, is interpreted as produced by the released energy at the transition point, and is the key to understand the existence of the abrupt field-induced transition. A qualitative analysis of the results suggests the existence of a critical growing rate of the ferromagnetic phase, beyond which an avalanche effect is triggered. PACS numbers: 75.30.Kz, 75.30.Sg, 75.30.Vn Mixed valent manganites show a great deal of fascinating properties, arising from the strong interplay between spin, charge, orbital, and lattice degrees of freedom [1]. The most intriguing one is the existence of a phase separated state, the simultaneous coexistence of submicrometer ferromagnetic (FM) metallic and charge ordered (CO) insulating regions [2]. The phase separation scenario has its origin in the unusual proximity of the free energies of these very distinct FM and CO states, and in the fact that the competition between both phases is resolved in mesoscopic length scales, giving rise to real space inhomogeneities in the material.Yet another surprising result more recently found in manganites is the appearance of ultrasharp magnetization steps at low temperatures (below ∼ 5 K) in the isothermal magnetization M (H) curves [3,4,5,6,7]. This effect, the field induced transition of the entire compound from one phase to the other of the coexisting states, is included in the category of metamagnetic transitions [8]. However, unlike the broad continuous transitions expected for inhomogeneous granular systems, in this case it occurs in an extremely narrow window of magnetic fields. These ultrasharp steps were observed in both single crystals and polycrystalline samples, indicating that it is not related to a particular micro-structure of the material.The actual existence of a phase separated state was recognized as a key parameter for the observation of these magnetization jumps [7]. The effect was first reported in manganites doped at the Mn site, and the disorder in the spin lattice was thought to play a relevant role [4]. However, a similar behavior was also found in Pr 0.6 Ca 0.4 MnO 3 , and the qualitative interpretation of the phenomenon shifted to the martensitic character of the phase separated state [6]. Accommodation strains were shown to be relevant in the stabilization of phase separation [9, 10], but their role in the magnetization steps is not clear, since it is expected that grain boundaries would act as a sort of "firewall" for the movement of the domain walls, stopping the avalanche process. Additionally, despite its intrinsic first-order character, the martensitic transformation is spread over a large range of the external parameter driving the transition, the magnetic ...

FT-Raman spectroscopy was employed to study normal human colorectal tissues in vitro with the aim of evaluating the spectral differences of the complex colon mucous in order to establish a characteristic Raman spectrum. The samples were collected from 39 patients, providing 144 spectra for the statistical analysis. The results enable one to establish three well-defined spectroscopic groups of non-altered colorectal tissues that were consistently checked by statistical (clustering) and biological (histopathology) analyses: group 1 is represented by samples with the presence of epithelial layer, connective tissue papillae, and smooth muscle tissue; group 2 comprises tissues with epithelial layer and connective tissue papillae; group 3 presented mostly fatty and slack conjunctive tissue. The study reveals the existence of an intrinsic spectral variability for each patient that must be considered when sampling tissues fragments to build a spectral database. This is the first step for future studies and applications of Raman spectroscopy to optical biopsy and diagnosis of colorectal cancer.

Neutron-diffraction measurements in LaCrSb3 show a coexistence of ferromagnetic and antiferromagnetic sublattices below T(C)=126 K, with ordered moments of 1.65(4) and 0.49(4)mu(B)/formula unit, respectively (T=10 K), and a spin-reorientation transition at approximately 95 K. No clear peak or step was observed in the specific heat at T(C). Coexisting localized and itinerant spins are suggested.

The T dependence (2Ϫ400 K) of the electron paramagnetic resonance ͑EPR͒, magnetic susceptibility (T), and specific heat C v (T) of the normal antiferromagnetic ͑AFM͒ spinel ZnCr 2 O 4 and the spin-glass ͑SG͒ Zn 1Ϫx Cd x Cr 2 O 4 (xϭ0.05,0.10) are reported. These systems behave as a strongly frustrated AFM and SG with T N ϷT G Ϸ12 K and Ϫ400 Kտ⌰ CW տϪ500 K. At high-T the EPR intensity follows the (T) and the g value is T independent. The linewidth broadens as the temperature is lowered, suggesting the existence of short range AFM correlations in the paramagnetic phase. For ZnCr 2 O 4 the EPR intensity and (T) decreases below 90 and 50 K, respectively. These results are discussed in terms of both nearest-neighbor Cr 3ϩ (Sϭ3/2) spin-coupled pairs and spin-coupled tetrahedral clusters with an exchange coupling of ͉J/k͉Ϸ35Ϫ45 K. The appearance of small resonance modes for TՇ17 K, the observation of a sharp drop in (T) and a strong peak in C v (T) at T N ϭ12 K confirms, as previously reported, the existence of long range AFM correlations in the low-T phase. A comparison with recent neutron diffraction experiments, that found a near dispersionless excitation at 4.5 meV for TՇT N and a continuous gapless spectrum for TտT N , is also given.

The exchange interactions in polycrystalline samples of Ca1-xLaxMnO3 (0.00< or =x< or =0.05) are studied by means of Raman scattering and electron paramagnetic resonance. Dramatic reductions in the spin-phonon interactions and magnetic correlations are observed for La doping levels as small as approximately 2%-3%. These results show that the charge carriers play an important role in the overall exchange coupling in the electron-doped manganites, even at very low doping levels.

Temperature-dependent Raman scattering studies in polycrystalline MgB2 (10 < T < 300 K) reveal that the E2g phonon does not experience any self-energy renormalization effect across the superconducting critical temperature TC ≈ 39 K, in contrast with most of the current theoretical models. In the presence of our results, those models must be reviewed. The analysis of the temperature dependence of the E2g phonon frequency yields an isobaric Grüneisen parameter of | γE 2g | 1, smaller than the value of 3.9 obtained from isothermal Raman experiments under pressure. It is suggested that this apparent disagreement can be explained in terms of pressureinduced changes of the topology of the Fermi surface. MgB 2 has attracted much recent interest due to its remarkable physical properties such as: i) relatively high superconducting transition, T C = 39 K, for a binary compound with a simple crystal structure, ii) large and anisotropic coherence lengths, critical fields and current densities, and iii) critical currents that are not limited by grain boundaries (absence of weak link effects). [1] MgB 2 forms in a hexagonal structure with space group P6/mmm (D 1 6h ). The B atoms are located on a primitive honeycomb lattice consisting of graphite-type sheets. The B 2 -layers are intercalated with Mg-layers that also form a honeycomb lattice with a Mg atom in the center. For this space group, factor-group analysis predicts four modes at the Γ point: E u + A 2u + E 2g + B 1g , where only the E 2g mode is Raman-active and the B 1g mode is silent. The E 2g phonon is a doubly degenerate in-plane B-B bond-stretching mode[2] with nonvanishing Raman tensor elements (α xx − α yy ) and α xy . First principles lattice dynamics calculations indicate that these modes would be observed at 327 cm −1 (E u ), 405 cm −1 (A 2u ), 572 cm −1 (E 2g ) and 702 cm −1 (B 1g ). [3] In analogy with high T C superconductors (HTS), Hall effect measurements [4] indicate that the charge carriers in MgB 2 are holes with a hole density at 300 K of 1.7 − 2.8×10 23 holes/cm 3 . In fact, hole-mediated su- * Electronic address: hercules@ifi.unicamp.br; URL: http://www.ifi.unicamp.br/gpoms perconductivity has been proposed by An and Pickett for MgB 2 .[5] These authors attribute the relatively high value of T C to the strong coupling between holes and the in-plane boron phonon, E 2g modes. According to this model, the holes originate in the σ (sp 2 orbitals) bands due to charge transfer from the σ bands to the π (p z orbitals) bands. Based on this model, several papers have discussed the possibility of E 2g phonon being a frozen-in mode, strongly coupled to the σ electronic bands near the Fermi level. [2,3,6,7] It is claimed that the E 2g phonon, due to its rather strong coupling to the σ electronic bands, would be highly anharmonic, presenting a very large linewidth. However, Boeri et al [8] pointed out that neither the presence at the Fermi level of the σ bands nor their strong coupling to the E 2g phonon are sufficient to induce such anharmonic effects, and the...

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