National Institute Of Materials Physics - Romania

We present a simple two-dimensional model for a phase transition, then study its predictions, in particular the memory properties. The direct transformation is modeled by randomly placing small squares, "nuclei", on an initially empty surface. Then, the nuclei expand ("grow") up to finite final sizes which are randomly chosen in a given range, while keeping their square shape. An important issue is the "interaction" which forces some squares to remain at smaller sizes if the surrounding squares get in the way of their growth. Interestingly, this naturally leads to quasiequal total area covered by the squares of each size after a complete direct transformation. Next, it is shown that the system "remembers" incomplete ("arrested") reverse transformations taking place in reversed order of the squares sizes. The memory is "encrypted" in the distribution of the squares sizes after a next direct transformation and manifests as a significant imbalance between the areas covered by the "big" and "small" (relative to the arrest size) squares. We are able to also reproduce the so-called "hammer effect" and the memorizing of multiple arrest points. Our model is particularly relevant for the thermal memory effect in shape memory alloys, and we actually borrowed many features from existing thermodynamic models addressing this effect. However, here we eliminate the explicit thermodynamics and end up with a statistical geometry model, presumably easier to reproduce.

A conductance zero that results from the destructive quantum interference of the electron states in quantum transport between two given sites of a molecular system persists or disappears depending on the location of an externally applied perturbation. The a priori knowledge of the perturbation site that destroys or preserves a zero is the basis of an algorithm that outlines the creation of logic gates having external perturbations as inputs and a given conductance as output. Using a graph of the possible conductance paths between the various sites, we showcase the several different scenarios that correspond to AND/OR/XOR logical functions for a given set of contacts. This setup is shown to be independent of the strength of the coupling to the leads and magnitude of the perturbation. We illustrate this approach in the case of bipartite and nonbipartite single carbon cycle molecules (fulvene and benzene) and double carbon cycle molecules (naphthalene and biphenyl).

We use a recently proposed quantum electrodynamical density theory functional in a real-time excitation calculation for a two-dimensional electron gas in a square array of quantum dots in an external constant perpendicular magnetic field to model the influence of cavity photons on the excitation spectra of the system. The excitation is generated by a short electrical pulse. The quantum dot array is defined in an AlGaAs-GaAs heterostructure, which is in turn embedded in a parallel plate far-infrared photon microcavity. The required exchange and correlation energy functionals describing the electron-electron and electron-photon interactions have therefore been adapted for a two-dimensional electron gas in a homogeneous external magnetic field. We predict that the energies of the excitation modes activated by the pulse are generally redshifted to lower values in the presence of a cavity. The redshift can be understood in terms of the polarization of the electron charge by the cavity photons and depends on the magnetic flux, the number of electrons in a unit cell of the lattice, and the electron-photon interaction strength. We find an interesting interplay of the exchange forces in a spin-polarized two-dimensional electron gas and the square-lattice structure leading to a small but clear blueshift of the excitation mode spectra when one electron resides in each dot.

Using the master equation approach, we look for fingerprints of the interaction between the localized spin S of a nanomagnet coupled to spin-polarized leads and its quantized vibrational modes. We find that the stationary and transient currents are sensitive to vibron-assisted transitions of the molecular spin on both sides of the anisotropy barrier. Such transitions are associated with vibron-dressed states and triggered under resonant conditions. Transport calculations are presented for two antiparallel configurations of the spin-polarized electrodes. In the first configuration, and far from a resonance point, a blockade is imposed on both the electronic and molecular spins via their exchange interaction. When sweeping the magnetic field through resonance, the spin-vibron interaction removes this blockade and allows the indirect reading of resonant transitions as the molecular spin climbs the left side of the anisotropy barrier. In the second configuration, the anisotropy barrier is overcome but the vibron-assisted transitions on the right side of the anisotropy barrier "delocalize" the molecular spin and do not allow the complete current-induced magnetic switching -S & RARR; S. In both configurations, the stationary current increases on resonance, due to additional transport channels triggered by the spin-vibron coupling. Therefore, the switching of the spin-vibron coupling could be detected in future transport experiments.

Recent experiments demonstrated that single-particle quantum walks can reveal the topological properties of single-particle states. Here, we generalize this picture to the many-body realm by focusing on multiparticle quantum walks of strongly interacting fermions. After injecting N particles with multiple flavors in the interacting SU(N) Su-Schrieffer-Heeger chain, their multiparticle continuous-time quantum walk is monitored by a variety of methods. We find that the many-body Berry phase in the N-body part of the spectrum signals a topological transition upon varying the dimerization, similarly to the single-particle case. This topological transition is captured by the single- and many-body mean chiral displacement during the quantum walk and remains present for strong interaction as well as for moderate disorder. Our predictions are well within experimental reach for cold atomic gases and can be used to detect the topological properties of many-body excitations through dynamical probes.

In our work, the kinetics of martensitic transformations and the influence of thermal treatments on martensitic transformations, as well as the related magnetic properties of the Ni49Mn32Ga19 ferromagnetic shape memory melt-spun ribbons, have been investigated. Thermal treatments at 673 K for 1, 4 and 8 h can be considered an instrument for fine-tuning the performance parameters of alloys. One-hour thermal treatments promote an improvement in the crystallinity of these otherwise highly textured ribbons, reducing internal defects and stress induced by the melt-spinning technique. Longer thermal treatments induce an important magnetization rise concomitantly with a slight and continuous increase in martensitic temperatures and transformation enthalpy. The activation energy, evaluated from differential scanning calorimeter experimental data with a Friedman model, significantly increases after thermal treatments as a result of the multi-phase coexistence and stabilization of the non-modulated martensitic phase, which increases the reverse martensitic transformation hindrance.

The p-n junctions are the building blocks of nowadays electronic devices. The n- or p-type conductivity is obtained in classic semiconductors, like Si, by doping with atoms acting as donors or acceptors, respectively. Doping was used in ferroelectrics to influence the transition temperature, magnitude of some physical properties, but not necessarily conduction type. Therefore, comprehensive studies to obtain true ferroelectric p-n junctions by controlled doping are missing. Recently, it has been shown that Pb(Zr0.2Ti0.8)O-3 films doped with & AP;1% atomic Nb (n-type doping) or Fe (p-type doping) have different orientations of polarization in the as-grown state. Knowing that polarization orientation depends on doping type, the next step is to build ferroelectric p-n homojunctions and to study their properties in relation to ferroelectric polarization. p-n and n-p structures were grown for this purpose by successive deposition of Nb-doped and Fe-doped Pb(Zr,Ti)O-3 layers with different thicknesses. We find that these p-n homojunctions are ferroelectric, but the magnitude of the polarization and coercive field, as well as the dominant polarization orientation in the as-grown state, depend on the conduction type of the first grown layer. The I-V characteristics are quasi-linear, although the interfaces with the electrodes behaves as Schottky contacts. The resistance extracted from the I-V characteristics displays an exponential dependence on temperature, with an activation energy in the range of 0.14-0.17 eV. These results are explained assuming that the total current in the junction is the total of electron and hole injections at the electrode interfaces. It is shown that for relatively low doping concentrations, the current density contains a dominant term with a linear voltage dependence and an exponential temperature dependence, as observed experimentally, and a secondary (correction) term that is dependent on the free carrier density and can induce non-linear voltage dependence when this density is significant.

The influence of processing on the martensitic transformation and related magnetic properties of the Ni55Fe18Nd2Ga25 ferromagnetic shape memory alloy, as bulk and ribbons prepared by the melt spinning method and subjected to different thermal treatments, is investigated. Structural, calorimetric, and magnetic characterizations are performed. Thermal treatment at 1173 K induces a decrease in both the Curie and the martensitic transformation temperatures, while a treatment at 673 K produces the structural ordering of the ribbons, hence an increase in T-C. A maximum value of the magnetic entropy variation of -5.41 J/kgK was recorded at 310 K for the as quenched ribbons. The evaluation of the magnetoresistive effect shows a remarkable value of -13.5% at 275 K on the bulk sample, which is much higher than in the ribbons.

The orbital and spin magnetization of a cavity-embedded quantum dot array defined in a GaAs heterostruc-ture are calculated within quantum-electrodynamical density-functional theory. To this end, a gradient-based exchange-correlation functional recently employed for atomic systems is adapted to the hosting two-dimensional electron gas submitted to an external perpendicular homogeneous magnetic field. Numerical results reveal the polarizing effects of the cavity photon field on the electron charge distribution and nontrivial changes of the orbital magnetization. We discuss its intertwined dependence on the electron number in each dot, and on the electron-photon coupling strength. In particular, the calculated dispersion of the photon-dressed electron states around the Fermi energy as a function of the electron-photon coupling strength indicates the formation of magnetoplasmon-polaritons in the dots.

The ability of TiO2 to generate reactive oxygen species under UV radiation makes it an efficient candidate in antimicrobial studies. In this context, the preparation of TiO2 microparticles coated with Ca- and Cu-based composite layers over which Cu(II), Cu(I), and Cu(0) species were identified is presented here. The obtained materials were characterized by a wide range of analytical methods, such as X-ray diffraction, electron microscopy (TEM, SEM), X-ray photoelectron (XPS), and UV-VIS spectroscopy. The antimicrobial efficiency was evaluated using qualitative and quantitative standard methods and standard clinical microbial strains. A significant aspect of this composite is that the antimicrobial properties were evidenced both in the presence and absence of the light, as result of competition between photo and electrical effects. However, the antibacterial effect was similar in darkness and light for all samples. Because no photocatalytic properties were found in the absence of copper, the results sustain the antibacterial effect of the electric field (generated by the electrostatic potential of the composite layer) both under the dark and in light conditions. In this way, the composite layers supported on the TiO2 microparticles' surface can offer continuous antibacterial protection and do not require the presence of a permanent light source for activation. However, the antimicrobial effect in the dark is more significant and is considered to be the result of the electric field effect generated on the composite layer.