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Register a new userMemory effects in the ion conductor Rb$_{2}$Ti$_{2}$O$_{5}$
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Recent studies on Rb2Ti2O5 crystals have demonstrated remarkable electrical properties. This material exhibits colossal electrical polarization between 200 K and 330 K. In the present work, we report on the observation of memory effects in Rb2Ti2O5 due to charge accumulation and we discuss the genuine memristive character of this material. An analytical model is proposed for the system, which takes into account the ionic diffusion and ionic migration and is in good agreement with the observed volatile memristive properties of the material.
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Electrical conductivity and high dielectric constant are in principle self-excluding, which makes the terms insulator and dielectric usually synonymous. This is certainly true when the electrical carriers are electrons, but not necessarily in a material where ions are extremely mobile, electronic conduction is negligible and the charge transfer at the interface is immaterial. Here we demonstrate in a perovskite-derived structure containing five-coordinated Ti atoms, a colossal dielectric constant (up to $mbox{10}^9$) together with very high ionic conduction $mbox{10}^{-3}mbox{S.cm}^{-1}$ at room temperature. Coupled investigations of I-V and dielectric constant behavior allow to demonstrate that, due to ion migration and accumulation, this material behaves like a giant dipole, exhibiting colossal electrical polarization (of the order of $mbox{0.1,C.cm}^{-2}$). Therefore, it may be considered as a ferro-ionet and is extremely promising in terms of applications.
The pyrochlore material $rm Ho_{2}Ti_{2}O_{7}$ has been suggested to show ``spin ice behaviour. We present neutron scattering and specific heat results that establish unambiguously that Ho$_2$Ti$_2$O$_7$ exhibits spin ice correlations at low temperature. Diffuse magnetic neutron scattering from Ho$_2$Ti$_2$O$_7$ is found to be quite well described by a near neighbour spin ice model and very accurately described by a dipolar spin ice model. The heat capacity is well accounted for by the sum of a dipolar spin ice contribution and an expected nuclear spin contribution, known to exist in other Ho$^{3+}$ salts. These results settle the question of the nature of the low temperature spin correlations in $rm Ho_{2}Ti_{2}O_{7}$ for which contradictory claims have been made.
We demonstrate that the amorphous material PAF-1, C[(C6H4)2]2, forms a continuous random network in which tetrahedral carbon sites are connected by 4,4-biphenyl linkers. Experimental neutron total scattering measurements on deuterated, hydrogenous, and null-scattering samples agree with molecular dynamics simulations based on this model. From the MD model, we are able for the first time to interrogate the atomistic structure. The small-angle scattering is consistent with Porod scattering from particle surfaces, of the form Q^{-4}, where Q is the scattering vector. We measure a distinct peak in the scattering at Q = 0.45 {AA}^{-1}, corresponding to the first sharp diffraction peak in amorphous silica, which indicates the structural analogy between these two amorphous tetrahedral networks.
Erythrosiderites with the formula A2FeX5H2O, where A = Rb, K, and (NH4) and X = Cl and Br are intriguing systems that possess various magnetic and electric phases, as well as multiferroic phases in which magnetism and ferroelectricity are coupled. In this report, we study the magnetic phase diagram of erythrosiderites as a function of superexchange interactions. To this end, we perform classical Monte Carlo simulations on magnetic Hamiltonians that contain five different superexchange interactions with single-ion anisotropies. Our phase diagram contains all magnetic ground states that have been experimentally observed in these materials. We argue that the ground states can be explained by varying the ratio of J4/J2. For J4/J2 > 0.95 a cycloidal spins structure is stabilized as observed in (NH4)2FeCl5H2O and otherwise, a collinear spin structure is stabilized as observed in (K,Rb)2FeCl5H2O. We also show that the difference in the single-ion anisotropy along a- and c- axes is essential to stabilize the intermediate state observed in (NH)2FeCl5H2O.
Neutron diffraction measurements under high magnetic fields have been performed for the multiferroic compound HoMn$_{2}$O$_{5}$. At zero field, high-temperature incommensurate magnetic (HT-ICM) -- commensurate magnetic (CM) -- low-temperature incommensurate magnetic (LT-ICM) orders occur with decreasing temperature, where ferroelectric polarization arises only in the CM phase. Upon applying a magnetic field, the LT-ICM phase completely disappears and the CM phase is induced at the lowest temperature. This field-induced CM state is completely associated with the field-induced electric polarization in this material [Higashiyama {it et al}., Phys. Rev. B {bf 72}, 064421 (2005).], strongly indicating that the commensurate spin state is essential to the ferroelectricity in the multiferroic $R$Mn$_{2}$O$_{5}$ system.
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