We report the results of a low-temperature (300K-15K) high-pressure (up to
22GPa) Raman study of the Verwey transition in magnetite (Fe3O4). We use
additional Raman modes observed below the Verwey transition to determine how
the transition temperature changes with the quasihydrostatic pressure. Increase
of the pressure results in the linear decrease of the Verwey transition
temperature, with no discontinuity. The corresponding pressure coefficient
dTV/dP is found to be ~ -5.2 K/GPa. Such a decrease is substantially larger
than the one predicted by the mean-field Coulomb interaction model of the
transition
PACS. 71.20.Be -Transition metals and alloys. PACS. 73.43.Nq -Quantum phase transitions. PACS. 75.10.Jm -Quantized spin models.Abstract. -The study of quantum phase transitions [1], which are zero-temperature phase transitions between distinct states of matter, is of current interest in research since it allows for a description of low-temperature properties based on universal relations. Here we show that the crystal green dioptase Cu6Si6O18·6H2O, known to the ancient Roman as the gem of Venus, has a magnetic crystal structure, formed by the Cu(II) ions, which allows for a quantum phase transition between an antiferromagnetically ordered state and a quantum spin liquid.c EDP Sciences
We report on polarized Raman scattering of single crystals of Nd 1−x Sr x MnO 3 (x = 0.3, 0.5). Raman spectra of Nd 0.7 Sr 0.3 MnO 3 show a significant change through the metal-insulator transition. In the ferromagnetic metallic phase phonon modes grow in intensity and number while the electronic continuum becomes more pronounced. We suggest that these effects are due to the strong competition between the localization and the delocalization of carriers which is the origin of the largest colossal magnetoresistance effect ever reported for the manganites. Raman spectra of Nd 0.5 Sr 0.5 MnO 3 , upon cooling through the charge-ordering temperature T C O = 148 K, exhibit several new lines which undergo a substantial hardening. This hardening is interpreted as a freezing of the Jahn-Teller distortions with a gradual decrease of a fraction of the ferromagnetic phase in the CE-type charge/orbital ordered state.
We report high-field magnetization, high-frequency electron spin resonance (ESR), and 77 Se nuclear magnetic resonance (NMR) measurements on the linear spin tetramer system CuSeO 3 , consisting of strongly interacting Cu(1) dimers and weakly coupled Cu(2) spins. The magnetization exhibits anisotropic half-step magnetization plateaux at µ 0 H = 45 T, depending on a crystallographic orientation. A temperature dependence of the ESR linewidth ∆H pp in a paramagnetic phase points towards the significance of anisotropic exchange interactions. Below T N = 9 − 10 K long-range magnetic order is evidenced by the observation of a critical divergence of both ∆H pp (T ) and the nuclear spin-lattice relaxation rate 1/T 1 . In addition, we identify a magnetic anomaly at T * = 6.0(5) K below T N , which is caused by a spin reorientation. The nuclear spin-spin relaxation rate 1/T 2 unveils the development of site-specific spin correlations. The intriguing magnetism of CuSeO 3 is discussed in terms of the energy hierarchy of Cu(1) and Cu(2) spins in concert with additional intertetramer interactions.
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