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We report a study of the superconducting-insulator (S-I) transition in epitaxial untwinned Nd1.2Ba1.8Cu3O7+x films using Cu-L and O-K edge polarized X-ray absorption spectroscopy (XAS). The S-I transition in realized by reducing the films thickness from 9 to 6 u.c. Due to strain, thin NdBCO films are characterized by structurally identical CuO2 planes. The major differences in the XAS spectra are not associated to states within the CuO2 units in the ab-plane, but to the unoccupied 3d(3z(2)-r(2)) states of copper at the Cu(1) and Cu(2) sites hybridized with the apical 2p(z) O(4) state, and to the unoccupied 2p(z) O(4) states themselves. The experimental results show that the S-I transition induced by change of thickness in Nd1.2Ba1.8Cu3O7+x films is correlated to holes localization at the copper sites of the Cu(1)O charge reservoir layer. The results open new perspectives for the interpretation of the superconducting to insulating transition in the HTS compounds. (c) 2007 Elsevier B.V. All rights reserved.

The low-temperature $(l60\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ transport properties of as-grown ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ ultrathin films, deposited by the molecular beam epitaxy, have been investigated as a function of the sample thickness (from $40\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}3.5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$) and in the presence of an external magnetic field. With decreasing thickness, a clear low-temperature resistivity minimum slightly affected by the application of the magnetic field has been observed, and its presence has been possibly interpreted in terms of quantum interference effects. As a function of the thickness, a crossover from a three-dimensional (3D) to a two-dimensional (2D) behavior of the system takes place below $20\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. A re-entrant 3D behavior is induced in ultrathin films by the application of large $(g20\phantom{\rule{0.3em}{0ex}}\mathrm{kOe})$ magnetic fields. Negative values of the magnetoresistance have been observed in all of the investigated samples for all of the measured magnetic fields.

To analyze the conducting and magnetic properties near the film and substrate interface in manganites, ultrathin films $(\mathrm{thickness}\ensuremath{\le}100\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$ of ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ were epitaxially grown by molecular-beam epitaxy on single-crystal (001) ${\mathrm{LaAlO}}_{3}$, (110) ${\mathrm{NdGaO}}_{3}$, and (001) ${\mathrm{SrTiO}}_{3}$ substrates. Structural, magnetic, and magnetoresistive properties were investigated. All samples exhibit a substrate-independent decrease of the $c$-lattice parameter for thinnest films. Highly anisotropic behavior in both transport and magnetic properties were measured along the in-plane directions parallel to the substrate crystallographic axes. In particular, for the thinnest films $(60\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}})$, the negative magnetoresistance at about $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with the average current along one of the crystallographic directions, is larger than the room-temperature colossal value. In the same low-temperature range, with the current along the other in-plane crystallographic direction, the magnetoresistance changes sign (resulting to be magnetic field independent for $T\ensuremath{\sim}150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$). Such an in-plane anisotropy of transport and magnetic properties is investigated with respect to possible intrinsic and extrinsic physical mechanisms.

We have studied the effect of thickness on the structure and superconductivity of ${\mathrm{Nd}}_{1.2}{\mathrm{Ba}}_{1.8}{\mathrm{Cu}}_{3}{\mathrm{O}}_{z}$ (NdBCO) epitaxial films deposited on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ (100) substrates. High-quality NdBCO films were deposited by high oxygen pressure sputtering. The samples have been carefully characterized by transport measurements and x-ray diffraction using conventional and synchrotron radiation sources. Two structural transformations are observed. The first one is a crossover from the orthorhombic and twinned structure, typical of thick samples, to a pseudotetragonal phase at thickness lower than 60 unit cells. The pseudotetragonal phase exhibits peculiar x-ray diffuse scattering around the Bragg reflections, associated with nanostructured twinned domains. The second one appears at a thickness lower than 20 unit cells and is assigned to a pseudotetragonal-tetragonal transition. The transport properties are not modified at the first crossover, while a substantial decrease in ${T}_{c}$ and in the number of holes is observed at the pseudotetragonal-tetragonal transformation. Our results suggest that the structural rearrangement and the transport properties are explained by the ordering of Cu(1)O chains. A simple model, able to explain the experimental results and capturing the essential features of the Cu(1)O chains nucleation process in the ${\mathrm{R}}_{1+x}{\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ ($\mathrm{R}=\mathrm{Y}$ and rare-earth elements) compounds, is presented.

High quality, very flat Nd1.2Ba1.8Cu3Oy thin films have been grown by sputtering on TiO2 terminated SrTiO3 (100) single crystals and their surface structure have been determined by using Grazing Incidence X-ray Diffraction (GIXD) technique, employing synchrotron radiation. A 52 unit cells film presents a double terminated surface composed by a complete and ordered Cu(1)–O plane and a disordered Cu(1)–O plane partially covered by a BaO layer, in full agreement with scanning tunneling microscopy data. A preliminary comparison between the structure of 8 and 52 u.c. NdBCO films will be presented.

Lithium intercalate thin films have been obtained from MnPS3 starting material and characterized by the x-ray photoelectron spectroscopy technique. The regions of Mn 2p and 3p, P and S 2p, and Li 1s core levels have been investigated. The analysis of the spectra has revealed the lack of nonequivalent atoms of Mn, P and S, and Li. The presence of shake-up-type satellites at the Mn 2p and 3p core levels suggests that Li2xMn1−xPS3 is a large-gap insulating Mn compound analogously to the parent MnPS3. After lithium intercalation no remarkable shift has been observed in the binding energy positions of the investigated core levels in comparison with the corresponding MnPS3 pure compound. On these bases, we assume that only an ion transfer accompanies the lithium intercalation process.

We report a study on the transport properties of ultrathin ${\mathrm{Nd}}_{1.2}{\mathrm{Ba}}_{1.8}{\mathrm{Cu}}_{3}{\mathrm{O}}_{z}$ (NdBCO) films by using field effect devices. Very high quality NdBCO films, having thickness ranging between 5 and 130 nm, have been prepared using diode high oxygen pressure sputtering. The temperature dependence of the resistivity has been studied as a function of the number of layers and of doping induced by field effect. An insulating-superconducting transition is observed in these films when the thickness is increased above 9 unit cells (u.c.). Below 9 u.c. the resistivity follows a 2D Mott variable range hopping temperature dependence and the localization length, estimated from a fit, is found to increase when holes are injected in the sample by field effect. A similar trend is observed when the number of layers in the film increases as a result of the changes of doping. The analysis suggests that hole density plays a major role in the transport properties of NdBCO ultrathin films.

We report a study on the transport properties of ultrathin Nd1.2Ba1.8Cu3Oz (NdBCO) films by using field effect devices. Very high quality NdBCO films, having thickness ranging between 5 and 130 nm, have been prepared using diode high oxygen pressure sputtering. The temperature dependence of the resistivity has been studied as a function of the number of layers and of doping induced by field effect. An nsulatingsuperconducting transition is observed in these films when the thickness is increased above 9 unit cells (u.c.). Below 9 u.c. the resistivity follows a 2D Mott variable range hopping temperature dependence and the localization length, estimated from a fit, is found to increase when holes are injected in the sample by field effect. A similar trend is observed when the number of layers in the film increases as a result of the changes of doping. The analysis suggests that hole density plays a major role in the transport properties of NdBCO ultrathin films.

Using a field effect device we modified the number of holes in the surface layers of 4 to 10 unit cell Nd1.2Ba1.8Cu3Oy (NBCO) epitaxial films grown on (100) SrTiO3 substrates. The results obtained on a set of 12 devices demonstrate that it is possible to induce reversible changes of the hole density of NBCO films by field effect. It is found that the field effect becomes less pronounced increasing the film thickness. Insulating–superconducting transition was observed in one 8 unit cell NBCO field effect device. © 2004 American Institute of Physics.

Using a field effect device we modified the number of holes in the surface layers of 4 to 10 unit cell Nd1.2Ba1.8Cu3Oy (NBCO) epitaxial films grown on (100) SrTiO3 substrates. The results obtained on a set of 12 devices demonstrate that it is possible to induce reversible changes of the hole density of NBCO films by field effect. It is found that the field effect becomes less pronounced increasing the film thickness. Insulating–superconducting transition was observed in one 8 unit cell NBCO field effect device.

We have studied the effect of thickness on the structure and superconductivity of Nd1.2Ba1.8Cu3O7+x (NdBCO) epitaxial films deposited on SrTiO3 (100) substrates. High quality NdBCO films were deposited by high oxygen pressure sputtering. The samples have been carefully characterized by transport measurements and X-ray diffraction using conventional and synchrotron radiation sources. Two structural transformations are observed. The first one is a crossover from the orthorhombic and twinned structure, typical of thick samples, to a pseudotetragonal phase at thickness lower than 60 unit cells. The pseudotetragonal phase exhibits peculiar X-ray diffuse scattering around the Bragg reflections, associated to nanostructured twinned domains. The second one appears at thickness lower than 20 unit cells and is assigned to a pseudotetragonal-tetragonal transition. The transport properties are not modified at the first crossover, while a substantial decrease of T-c and of the number of holes is observed at the pseudotetragonal-tetragonal transformation. Our results suggest that the structural rearrangement and the transport properties are explained by the ordering of Cu(I)O chains. (c) 2007 Elsevier B.V. All rights reserved.

We studied the ferroelectric and ferromagnetic properties of compressive strained and unstrained BiMnO3 thin films grown by rf-magnetron sputtering. BiMnO3 samples exhibit a two-dimensional cube-on-cube growth mode and a pseudo-cubic structure up to a thickness of 15 nm and of 25 nm when deposited on (001) SrTiO3 and (110) DyScO3, respectively. Above these thicknesses, we observe a switching to a three-dimensional island growth mode and a simultaneous structural change to a (00l) oriented monoclinic unit cell. While ferromagnetism is observed below a T-C approximate to 100K for all samples, signatures of room temperature ferroelectricity were found only in the pseudo-cubic ultra-thin films, indicating a correlation between electronic and structural orders.

The temperature dependence of the in plane magnetic penetration depth λ ab ( T ) of high quality c -axis epitaxial Nd-rich R(Nd x Ba 2− x )Cu 3 O 7 (R=Y, Nd) films has been measured at 4 MHz, 1.4 and 19.8 GHz with different techniques. While for stoichiometric films λ ab ( T )∝ T , the Nd-rich films exhibit a quadratic temperature dependence of λ ab ( T ) with a slope ∂ λ ab ( T )/∂ T 2 that decreases with the increasing frequency. The experimental results are discussed in the framework of the d-wave dirty model.

The physics of high critical temperature superconductors (HTS) remains a fascinating but undisclosed issue in condensed matter. One of the most interesting topics is the transition from the insulating phase of the parent compound, having long range antiferromagnetic order, to the superconducting phase. A method to investigate in detail the superconducting to insulating (SIT) transition in HTS is to control the doping of the CuO(2) planes in a fine way. Here, by using the electric field effect on thin Nd(1)Ba(2)Cu(3)O(7) films, we present a study of the HTS phase diagram close to the SIT with unprecedented detail. By virtue of these data, we will show that doping of holes in samples located at the boundary separating the superconducting and insulating regions produces changes in the transport characteristic consistent with an electronic phase separation scenario. Some consequences of these data are the failure of standard 2D quantum scaling theory and the possible coexistence of superconducting and weakly insulating phases in this region of the phase diagram. A continuous transition between the two competing phases as a function of doping place evident constraints on the mechanism of superconductivity.

Cesium ions have been intercalated by ion exchange into layered manganese thiophosphate to produce Cs2xMn1-xPS3. Investigations have been conducted by x-ray diffraction analysis, IR absorption spectroscopy, x-ray photoemission spectroscopy (XPS), and dielectric spectroscopy. The XPS spectra, recorded at room temperature and at the Mn, P, and S 2p, Mn 3p, and Cs 3d and 4d core-level regions, indicate that no charge transfer occurs from guest species (Cs+) to pure host lattice (MnPS3). The presence of shake-up satellites at the Mn 2p and 3p core levels suggests that the cesium intercalation does not alter the nature of the Mn–S bond which is mostly ionic. Dielectric measurements, carried out between 260 and 375 K and in the 20Hz–1MHz frequency range, classify Cs2xMn1-xPS3 as hopping charge carrier systems. The nature of such carriers can be hypothesized on the basis of both the intercalation process by cationic substitution and the comparison with the observed dielectric response of the pure MnPS3.

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