National Institute Of Materials Physics - Romania

The effects of severe plastic deformation (SPD) process via high-speed high-pressure torsion technique on martensitic transformation of Ni-Fe-Ga Heusler shape memory alloy are the subject of this work. The results show that moderate degrees of deformation lead to a decrease in the martensitic transformation temperatures, while the heat of reaction is enhanced only for the sample processed with the lowest degree of deformation. The results are explained by the interplay between the constituent tetragonal L10 and the cubic gamma crystal structures and the evolution of the samples morphology with the severity of deformation. The reduction in the samples granulation due to the progressive increase in the SPD is reflected by the magnetic properties of the samples with decreasing coercivity and Curie temperatures. At the highest applied degree of deformation, sample nanostructuring and a possible amorphization might explain the vanishing of MT.

Preventing and controlling the spread of multidrug-resistant (MDR) bacteria implicated in healthcare-associated infections is the greatest challenge of the health systems. In recent decades, research has shown the need for passive antibacterial protection of surfaces in order to reduce the microbial load and microbial biofilm development, frequently associated with transmission of infections. The aim of the present study is to analyze the efficiency of photocatalytic antimicrobial protection methods of surfaces using the new photocatalytic paint activated by light in the visible spectrum. The new composition is characterized by a wide range of analytical methods, such as UV-VIS spectroscopy, electron microscopy (SEM), X-ray powder diffraction (PXRD) or X-ray photoelectron spectroscopy (XPS). The photocatalytic activity in the UV-A was compared with the one in the visible light spectrum using an internal method developed on the basis of DIN 52980: 2008-10 standard and ISO 10678-2010 standard. Migration of metal ions in the composition was tested based on SR EN1186-3: 2003 standard. The new photocatalytic antimicrobial method uses a type of photocatalytic paint that is active in the visible spectral range and generates reactive oxygen species with inhibitory effect against all tested microbial strains.

The influence of the rapid solidification technique and heat treatment on the martensitic transformation, magnetic properties, thermo- and magnetic induced strain and electrical resistivity is investigated for the Cu doped NiMnGa Heusler-based ferromagnetic shape memory ribbons. The martensitic transformation temperatures are unexpectedly low (below 90 K-which can be attributed to the disordered texture as well as to the uncertainty in the elements substituted by the Cu), preceded by a premartensitic transformation (starting at around 190 K). A thermal treatment slightly increases the transformation as well as the Curie temperatures. Additionally, the thermal treatment promotes a higher magnetization value of the austenite phase and a lower one in the martensite. The shift of the martensitic transformation temperatures induced by the applied magnetic field, quantified from thermo-magnetic and thermo-magnetic induced strain measurements, is measured to have a positive value of about 1 K/T, and is then used to calculate the transformation entropy of the ribbons. The magnetostriction measurements suggest a rotational mechanism in low fields for the thermal treated samples and a saturation tendency at higher magnetic fields, except for the temperatures close to the phase transition temperatures (saturation is not reached at 5 T), where a linear volume magnetostriction cannot be ruled out. Resistivity and magnetoresistance properties have also been measured for all the samples.

We present a theoretical analysis of the interplay between the spin-vibron and electron-vibron interactions in a hybrid system made of a single-molecule magnet and a suspended conductor. The latter is coupled to particle reservoirs and supports quantized vibrational modes which, once activated, interact with the localized magnetic moment S of the nanomagnet. The dynamics of the molecular spin, the average vibron number, and the transient currents are calculated from the reduced density operator of the hybrid system. We focus on the effect of the vibron-assisted transitions from the lowest energy spin doublet S-z = +/- S to higher energy excited states. The numerical simulations performed for the simplest case S = 2 prove that the vibron-assisted spin transitions and dynamics can be described in terms of a three-level Lambda model borrowed from quantum optics. In particular we predict the existence of Rabi oscillations of the transient currents as fingerprints of the spin-vibron coupling. The role of symmetric or asymmetric bias configurations in setting different mixtures of molecular spin states in the steady-state regime is also emphasized.

Destructive quantum interference (DQI) and its effects on electron transport are studied in chemical molecules and finite physical lattices that can be described by a discrete Hamiltonian. Starting from a bipartite system whose conductance zeros are known to exist between any two points of a specially designated set, the interference set, we use the Dyson equation to develop a general algorithm for determining the zero conductance points in complex systems, which are not necessarily bipartite. We illustrate this procedure as it applies to the fulvene molecule. The stability of the conductance zeros is analyzed with respect to external perturbations.

We explore higher order dynamical effects in the transport through a two-dimensional nanoscale electron system embedded in a three-dimensional far-infrared photon cavity. The nanoscale system is considered to be a short quantum wire with a single circular quantum dot defined in a GaAs heterostructure. The whole system, the external leads and the central system are placed in a constant perpendicular magnetic field. The Coulomb interaction of the electrons, the paraand diamagnetic electron-photon interactions are all treated by a numerically exact diagonalization using step-wise truncations of the appropriate many-body Fock spaces. We focus on the difference in transport properties between a description within an electric dipole approximation and a description including all higher order terms in a single photon mode model. We find small effects mostly caused by an electrical quadrupole and a magnetic dipole terms that depend strongly on the polarization of the cavity field with respect to the transport direction and the photon energy. When the polarization is aligned along the transport direction we find indications of a weak self-induction that we analyze and compare to the classical counterpart, and the self-energy contribution of high-order interaction terms to the states the electrons cascade through on their way through the system. Like expected the electron-photon interaction is well described in the dipole approximation when it is augmented by the lowest order diamagnetic part for a nanoscale system in a cavity in an external magnetic field.

We investigate topological and disorder effects in non-Hermitian systems with chiral symmetry. The system under consideration consists in a finite Su-Schrieffer-Heeger chain to which two semi-infinite leads are attached. The system lacks the parity-time and time-reversal symmetries and is appropriate for the study of quantum transport properties. The complex energy spectrum is analyzed in terms of the chain-lead coupling and chiral disorder strength, and shows substantial differences between chains with even and odd number of sites. The mid-gap edge states acquire a finite lifetime and are both of topological origin or generated by a strong coupling to the leads. The disorder induces coalescence of the topological eigenvalues, associated with exceptional points and vanishing of the eigenfunction rigidity. The electron transmission coefficient is approached in the Landauer formalism, and an analytical expression for the transmission in the range of topological states is obtained. Notably, the chiral disorder in this non-Hermitian system induces unitary conductance enhancement in the topological phase. (C) 2020 Elsevier B.V. All rights reserved.

Specific functional characteristics of the ferromagnetic shape memory alloy Fe67.5Pd30.5Ga2 prepared as ribbons by rapid quenching technique are reported. The shift of the martensitic transformation temperature induced by the applied magnetic field was determined from the thermomagnetic measurements at various fields up to 5T being proved to be in good agreement with the result obtained from calorimetric data via the Clapeyron - Claussius relation. Magnetostriction measurements reveal a pure spin rotation mechanism under low applied magnetic fields, allowed by the reduced magnetic anisotropy. In high magnetic fields and at temperatures close to martensitic transformation, the magnetostriction has a linear increase up to the maximum considered magnetic induction of 3T. This behavior has been discussed in connection with the forced magnetostiction aspects.

Within the Landauer transport formalism we demonstrate that conductance zeros are possible in bipartite systems at half-filling when leads are contacted to different sublattice sites. In particular, we investigate the application of this theory to graphene quantum dots with leads in the armchair configuration. The obtained conductance cancellation is robust in the presence of any single-site impurity.

The effects of a turnstile operation on the current-induced vibron dynamics in nanoelectromechanical systems (NEMS) are analyzed in the framework of the generalized master equation. In our simulations each turnstile cycle allows the pumping of up to two interacting electrons across a biased mesoscopic subsystem which is electrostatically coupled to the vibrational mode of a nanoresonator. The time-dependent mean vibron number is very sensitive to the turnstile driving, rapidly increasing/decreasing along the charging/discharging sequences. This sequence of heating and cooling cycles experienced by the nanoresonator is due to specific vibron-assisted sequential tunneling processes along a turnstile period. At the end of each charging/discharging cycle the nanoresonator is described by a linear combination of vibron-dressed states s(v). associated to an electronic configuration nu. If the turnstile operation leads to complete electronic depletion the nanoresonator returns to its equilibrium position, i.e., its displacement vanishes. It turns out that a suitable bias applied on the NEMS leads to a slow but complete cooling at the end of the turnstile cycle. Our calculations show that the quantum turnstile regime switches the dynamics of the NEMS between vibron-dressed subspaces with different electronic occupation numbers. We predict that the turnstile control of the electron-vibron interaction induces measurable changes on the input and output transient currents.