We report on the measurement of the ^{7}Be(n,p)^{7}Li cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n_TOF facility at CERN. This reaction plays a key role in the lithium yield of the big bang nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and they showed a large discrepancy between each other. The measurement was performed with a Si telescope and a high-purity sample produced by implantation of a ^{7}Be ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low energy, relative to current evaluations, in the region of BBN interest, the present results are consistent with the values inferred from the time-reversal ^{7}Li(p,n)^{7}Be reaction, thus yielding only a relatively minor improvement on the so-called cosmological lithium problem. The relevance of these results on the near-threshold neutron production in the p+^{7}Li reaction is also discussed.

In this Letter a detailed study of the electric field gradient (EFG) across the Pr 1ÿx Ca x MnO 3 phase diagram and its temperature dependence is given. Clearly, distinct EFG behavior for samples outside or inside the charge order (CO) region are observed. The EFG temperature dependence evidences a new phase transition occurring over the broad CO region of the phase diagram. This transition is discontinuous and occurs at temperatures between the charge ordering and the Néel temperatures. The prominent features observed in the EFG are associated with polar atomic vibrations which eventually lead to a spontaneous local electric polarization below CO transition. DOI: 10.1103/PhysRevLett.100.155702 PACS numbers: 64.60.ÿi, 75.47.Lx, 76.80.+y, 77.80.Bh The exquisite coupling between lattice, spin, charge, and orbital degrees of freedom, that led to renowned phenomena like high-T c superconductivity, colossal magnetoresistance, and multiferroic behavior, still challenges our understanding of transition metal oxides [1]. In Mn 3 =Mn 4 mixed valence manganites this subtle entanglement of the several degrees of freedom brings about competing orbital, magnetic, and dielectric orders depending on the doping, temperature, and external stimulation. In particular, much attention has been devoted to the charge ordered (CO) and orbital ordered (OO) phases, i.e., a realspace ordering of charge and orbitals due to the electronphonon and long-range Coulomb interactions. The classic CO picture with a Mn 3 -Mn 4 checkerboard pattern [2] has been questioned [3,4] since the work of Daoud-Aladine et al. [5]. These authors proposed an electronic ground state where one e g electron is shared by two Mn 3 ions, the so-called bond-centered Zener polaron picture. Subsequently, Efremov et al. [6] proposed a new scenario where the bond-centered (Mn 3 -O ÿ -Mn 3 dimmers) and the site-centered CO pictures coexist and the result breaks the inversion symmetry, leading to the appearance of a spontaneous electric polarization. More recently, it has been demonstrated that a commensurate spin-densitywave ordering with a phase dislocation can also give rise to a polar ferroelectric distortion in rare-earth manganites [7]. In a different context, a frustrated CO state was also shown to lead to an electrical polarization in LuFe 2 O 4 [8]. Although the CO state in Pr 1ÿx Ca x MnO 3 is currently referred to as a new paradigm for ferroelectrics [9][10][11], it has been very hard to prove that electric polarization exists in CO Pr 1ÿx Ca x MnO 3 and in similar CO manganites [9,10,12]. This is connected to the relatively high conductivity of these materials, and to the possibility that the suspected electric dipole order may only occur within nanoscopic regions. However, a very recent work of Jooss et al. [13] provides, by refinements of electron diffraction microscopy data, indirect evidence for canted antiferroelectricity in Pr 0:68 Ca 0:32 MnO 3 .The measurement of the electric field gradient tensor (EFG) via hyperfine techniques offers a very sensitiv...

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We report on the lattice location of Er in Si using the emission channeling technique. The angular distribution of conversion electrons emitted by the decay chain 167 Tm ͑t 1͞2 9.25 d͒ ! 167m Er ͑2.27 s͒ was monitored with a position-sensitive detector following room temperature implantation and annealing up to 950 ± C. Our experiments give direct evidence that Er is stable on tetrahedral interstitial sites in float-zone Si. We also confirm that rare earth atoms strongly interact with oxygen, which finally leads to their incorporation on low-symmetry lattice sites in Czochralski Si. [S0031-9007(97) Rare earth doping of Si is known to result in the formation of luminescent centers and is considered as a possible way to manufacture Si-based optoelectronic devices [1]. Among the various rare earth elements, Er is of special interest since its atomic transition at 1.54 mm matches the absorption minimum of SiO 2 , a highly desirable feature both for signal transmission through glass fiber cables and optical on-chip communication. Luminescence at this wavelength from Er-implanted Si was already established several years ago [2]. Meanwhile Er-based light-emitting diodes operating at room temperature have been reported [3]. The basic understanding of Er luminescence in Si, however, is far from complete. This concerns both the lattice sites of Er and the role of codopants such as O, N, or F, which were found to have a beneficial influence on luminescence yield. Photoluminescence (PL) spectroscopy studies have identified a number of Er-related centers with different crystal surroundings in Si [4]. The most intense PL yield was due to two centers having cubic and axial symmetry, respectively. While the cubic center occurred in both float-zone (FZ) and Czochralski (CZ) Si and was attributed to tetrahedral ͑T ͒ interstitial Er, the center with axial symmetry was observed only in CZ Si and ascribed to Er-O complexes. The existence of tetrahedral interstitial Er would be also in agreement with theoretical studies, which predict that T sites are the most stable sites for all oxidation states of isolated Er atoms in Si [5]. Direct lattice location using the Rutherford backscattering (RBS) channeling technique only suggested substitutional [6] or hexagonal ͑H͒ interstitial Er [7,8]. The reasons for these discrepancies, however, are unclear.To study the lattice sites and damage recovery after rare earth implantation, we have applied conversion electron emission channeling [9] combined with position sensitive detection. Emission channeling makes use of the fact that charged particles emitted from radioactive isotopes in single crystals experience channeling or blocking effects along low-index crystal directions. This leads to an anisotropic particle emission yield from the crystal surface which depends in a characteristic way on the lattice sites occupied by the emitter atoms. While this technique as such is not new and, in case of rare earths, was already used once for the lattice location of 175 Yb in Si [10], we have for the fi...