Abstract-A systematic design of practicable media suitable for re-writeable, ultra-high density (> 1Tbit/sq.in.), high data rate (> 1Mbit/s/tip) scanning probe phase-change memories is presented. The basic design requirements were met by a Si/TiN/GST/DLC structure, with properly tailored electrical and thermal conductivities. Various alternatives for providing re-writeability were investigated. In the first case amorphous marks were written into a crystalline starting phase and subsequently erased by re-crystallization, as in other already-established phasechange memory technologies. Results imply that this approach is also appropriate for probe-based memories. However, experimentally the successful writing of amorphous bits using scanning electrical probes has not been widely reported. In light of this a second approach has been studied, that of writing crystalline bits in an amorphous starting matrix, with subsequent erasure by re-amorphization. With conventional phase-change materials, such as continuous films of Ge 2 Sb 2 Te 5 , this approach invariably leads to the formation of a crystalline 'halo' surrounding the erased (re-amorphized) region, with severe adverse consequences on the achievable density. Suppression of the 'halo' was achieved using patterned media or slow-growth phase-change media, with the latter seemingly more viable.

^{75}As, ^{87}Rb, and ^{85}Rb nuclear quadrupole resonance (NQR) and ^{87}Rb nuclear magnetic resonance measurements in a RbFe_{2}As_{2} iron-based superconductor are presented. We observe a marked broadening of the ^{75}As NQR spectrum below T_{0}≃140  K which is associated with the onset of a charge order in the FeAs planes. Below T_{0} we observe a power-law decrease in the ^{75}As nuclear spin-lattice relaxation rate down to T^{*}≃20  K. Below T^{*} the nuclei start to probe different dynamics owing to the different local electronic configurations induced by the charge order. A fraction of the nuclei probes spin dynamics associated with electrons approaching a localization while another fraction probes activated dynamics possibly associated with a pseudogap. These different trends are discussed in light of an orbital selective behavior expected for the electronic correlations.

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75 As Nuclear Magnetic (NMR) and Quadrupolar (NQR) Resonance were used, together with Mössbauer spectroscopy, to investigate the magnetic state induced by Mn for Fe substitutions in F-doped LaFe1−xMnxAsO superconductors. The results show that 0.5% of Mn doping is enough to suppress the superconducting transition temperature Tc from 27 K to zero and to recover the magnetic structure observed in the parent undoped LaFeAsO. Also the tetragonal to orthorhombic transition of the parent compound is recovered by introducing Mn, as evidenced by a sharp drop of the NQR frequency. The NQR spectra also show that a charge localization process is at play in the system. Theoretical calculations using a realistic five-band model show that correlation-enhanced RKKY exchange interactions between nearby Mn ions stabilize the observed stripe magnetic order. These results give compelling evidence that F-doped LaFeAsO is a strongly correlated electron system at the verge of an electronic instability.PACS numbers: 74.70. Xa, 76.75.+i, 74.40.Kb, 74.20.Mn The interplay between impurity induced disorder and electronic correlations often gives rise to complex phase diagrams in condensed matter [1, 2]. The electronic correlations drive a system towards a quantum phase transition, as it is typically found in the fullerides [3] and in heavy-fermion compounds [4, 5], with an enhancement of the local susceptibility and, hence, a small perturbation, as the one associated with a tiny amount of impurities, can significantly affect the electronic ground-state [6][7][8][9]. In the cuprates and in the electron-doped iron-based superconductors (IBS) the strength of the electronic correlations can be tuned either by charge doping or by applying an external or a chemical pressure. [10][11][12][13][14] In particular, upon increasing the charge doping, the strength of the electronic correlations tend to decrease [15][16][17][18][19][20][21][22] and a metallic Fermi liquid (FL) ground state is usually restored [23][24][25][26][27]. However significant electronic correlations may still be present even close to the charge doping levels yielding the maximum superconducting transition temperature T c and a convenient method to test their magnitude is to perturb the system with impurities.The introduction of Mn impurities at the Fe sites was reported to strongly suppress T c in several IBS, both of the BaFe 2 As 2 [28-30] and of the LnFeAsO (Ln1111, Ln=Lanthanides) [31] families. Within the Ln1111 family the effect of impurities is particularly significant in La1111 [27,32]. In fact, while in most the IBS compounds the T c suppression rate (dT c /dx) is well below 10 K/% Mn, in LaFeAsO 0.89 F 0.11 just 0.2 -0.3% of Mn impurities suppress superconductivity from the optimal T c 27 K (dT c /dx ∼ 110 K/% Mn) and then, at higher Mn doping levels, a magnetic order develops (see Fig. 1b) [32][33][34]. The understanding of why such a dramatic effect is present, what type of magnetic order is developing and how to describe these materials at the microscopic level ...