Anderson-Mott localization behaviour of the low-temperature optical conductivity, while at high frequencies the excitation spectrum resembles that of a conventional semiconductor. The missing spectral weight below the gap energy is redistributed around the gap edge, in disagreement with previous claims based on optical measurements.
Thermal-expansion and magnetic susceptibility measurements were performed on oxygen-isotope substituted manganites ͑La 12x Ca x ͒ 12y Mn 12y O 3 with a Mn 41 concentration of ϳ33%. The linear thermal-expansion coefficient b͑T ͒ exhibits an asymmetric peak at the Curie temperature T C , indicative of a second-order ferromagnetic transition. Upon replacing 16 O with 18 O, T C is lowered by about 10 K, while the second-order jump in the thermal-expansion coefficient ͓Db͑T C ͔͒ is raised by about 20%. Such a "colossal" oxygen-isotope effect on Db͑T C ͒ is very surprising, but can be explained quantitatively on the basis of double exchange and strong polaronic effects. [S0031-9007(96)02269-7] PACS numbers: 75.70.Pa, 71.38. + i The manganese-based perovskites La 12x M x MnO 3 (M Ca, Sr, and Ba) have recently been the subject of intensive investigations because of their unusual and potentially useful magnetic properties (e.g., the colossal magnetoresistance effect [1,2]). The undoped parent compound LaMnO 3 (with Mn 31 ) is an insulating antiferromagnet [3]. When Mn 41 ions are introduced by substituting divalent ions for La 31 , the materials become ferromagnetic and metallic at low temperature for 0.2 # x # 0.5 [4]. The ferromagnetic and metallic ground state has also been observed in the cation-deficient ͑LaMn͒ 12y O 3 compound, which contains ϳ33% Mn 41 [5]. Moreover, it was shown that the Mn 41 concentration in cation-deficient materials can be adjusted by simply changing the anneal temperature [6]. This convenient feature makes it an attractive candidate for studying the effect of changing the Mn 41 concentration.The physics in manganites has primarily been described by the double-exchange model [7,8]. Millis, Littlewood, and Shraiman [9] have pointed out, however, that double exchange alone cannot fully explain the data of La 12x Sr x MnO 3 . They proposed that polaronic effects due to strong electron-phonon coupling (arising from a strong Jahn-Teller effect) should be involved. The strong coupling between the electronic and lattice subsystems has been demonstrated by the recent observation of a giant oxygen-isotope shift of the ferromagnetic transition in La 12x Ca x MnO 3 [10]. This scenario is also supported by thermal expansion [11,12] and small-angle neutron scattering experiments [13].Here we report an observation of a "colossal" oxygenisotope effect on the height of the thermal-expansion jump Db͑T C ͒ at a second-order ferromagnetic transition in the perovskite ͑La 12x Ca x ͒ 12y Mn 12y O 3 with a Mn 41 concentration of ϳ33%. The T C can be changed by varying either the Mn 41 concentration or the oxygen mass. Increasing the Mn 41 concentration does not change the thermal-expansion jump Db͑T C ͒, while increasing the oxygen mass does increase Db͑T C ͒ by ϳ20%.We propose a model to explain these unusual isotope effects.Samples of ͑LaMn͒ 12y O 3 were prepared by conventional solid state reaction using La 2 O 3 and MnO 2 . The La 2 O 3 was dried for 6 h at 900 ± C prior to weighing. The powders were mixed, g...
In vitro differentiation of pleomorphic blood-stream forms of Trypanosoma brucei to procyclic culture forms occurred rapidly and at high rates at 27 degrees C in a culture medium containing 1 mM cis-aconitate as the transformation-inducing agent. Citrate was required at a much higher concentration (10 mM) to produce a similar transformation rate. The highest percentage of transformed cells was obtained when bloodstream-form trypanosomes were treated with pronase in the absence of a feeder-cell layer. However, under these conditions, the amount of procyclic forms obtained after 72 h was lower than that obtained in the presence of cis-aconitate. Trypsin was also capable of inducing transformation in the absence of a feeder-cell layer, but this treatment again resulted in low numbers of transformed cells. Blood-stream-form trypanosomes were incapable of taking up citrate to any significant extent and the citrate content of these stages was negligible. After 72 h of exposure to citrate (3 mM), intracellular levels of this compound remained very low (< 1 nmol (10(9) cells)-1), increasing in established procyclic stages to approximately 1.7 nmol (10(9) cells)-1. These observations suggest that the tricarboxylic acid (TCA)-cycle metabolite-dependent transformation may be initiated externally to the trypanosome cell membrane. The ability of both citrate and cis-aconitate to bind calcium and, thus, to reduce the concentration of this cation in the culture medium was found not to be responsible for the triggering effect on trypanosome transformation.(ABSTRACT TRUNCATED AT 250 WORDS)
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