National Institute Of Materials Physics - Romania

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134 articles found

11. Spin-vibron coupling effects in single-molecule magnets grafted to a nanoelectromechanical system

Authors: Moldoveanu, V; Dragomir, R

Published: AUG 23 2021, PHYSICAL REVIEW B, 104, DOI: 10.1103/PhysRevB.104.075441

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.

12. Conductance zeros in complex molecules and lattices from the interference set method

Authors: Nita, M; Tolea, M; Marinescu, DC

Published: MAR 23 2021, PHYSICAL REVIEW B, 103, DOI: 10.1103/PhysRevB.103.125307

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.

13. Self-induction and magnetic effects in electron transport through a photon cavity

Authors: Gudmundsson, V; Abdullah, NR; Tang, CS; Manolescu, A; Moldoveanu, V

Published: MAR 2021, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 127, DOI: 10.1016/j.physe.2020.114544

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.

14. Spectral analysis, chiral disorder and topological edge states manifestation in open non-Hermitian Su-Schrieffer-Heeger chains

Authors: Ostahie, B; Aldea, A

Published: JAN 28 2021, PHYSICS LETTERS A, 387, DOI: 10.1016/j.physleta.2020.127030

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.

15. Magnetic and magnetostrictive properties of the ternary Fe67.5Pd30.5Ga2 ferromagnetic shape memory ribbons

Authors: Sofronie, M; Tolea, F; Tolea, M; Popescu, B; Valeanu, M

Published: JUL 2020, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 142, DOI: 10.1016/j.jpcs.2020.109446

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.

16. Robust conductance zeros in graphene quantum dots and other bipartite systems

Authors: Nita, M; Tolea, M; Marinescu, DC

Published: JUN 30 2020, PHYSICAL REVIEW B, 101, DOI: 10.1103/PhysRevB.101.235318

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.

17. Quantum turnstile regime of nanoelectromechanical systems

Authors: Dragomir, R; Moldoveanu, V; Stanciu, S; Tanatar, B

Published: APR 7 2020, PHYSICAL REVIEW B, 101, DOI: 10.1103/PhysRevB.101.165409

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.

18. Quantum dot exciton dephasing by Coulomb interaction: A fermionic analog of the independent boson model

Authors: Dinu, IV; Tolea, M; Gartner, P

Published: FEB 18 2020, PHYSICAL REVIEW B, 101, DOI: 10.1103/PhysRevB.101.085304

The time evolution of a quantum dot exciton in Coulomb interaction with wetting layer carriers is treated using an approach similar to the independent boson model. The role of the polaronic unitary transform is played by the scattering matrix, for which a diagrammatic, linked cluster expansion is available Similarities and differences to the independent boson model are discussed. A numerical example is presented.

19. Cavity-Photon-Induced High-Order Transitions between Ground States of Quantum Dots

Authors: Gudmundsson, V; Abdullah, NR; Tang, CS; Manolescu, A; Moldoveanu, V

Published: NOV 2019, ANNALEN DER PHYSIK, 531, DOI: 10.1002/andp.201900306

It is shown that quantum electromagnetic transitions to high orders are essential to describe the time-dependent path of a nanoscale electron system in a Coulomb blockade regime when coupled to external leads and placed in a 3D rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads, a slow high-order transition between the ground states of the two quantum dots is identified. The Fourier power spectrum for photon-photon correlations in the steady state shows a Fano type of resonance for the frequency of the slow transition. Full account is taken of the geometry of the multilevel electronic system, and the electron-electron Coulomb interactions together with the para- and diamagnetic electron-photon interactions are treated with step-wise exact numerical diagonalization and truncation of appropriate many-body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.

20. Internal and external surface features of newly developed porous ceramics with random interconnected 3D channels by a fibrous sacrificial porogen method

Authors: Mocanu, AC; Miculescu, M; Machedon-Pisu, T; Maidaniuc, A; Ciocoiu, RC; Ionita, M; Pasuk, J; Stan, GE; Miculescu, F

Published: SEP 30 2019, APPLIED SURFACE SCIENCE, 489, 238, DOI: 10.1016/j.apsusc.2019.05.354

The bone remodeling research field has shifted focus towards sustainable, eco-friendly and reproducible manufacturing technologies of 3D structures. It is now accepted that a suitable internal architecture and an active interface between the 3D structure and host bone-tissue constitute the two most critical traits for a successful bone tissue engineering application. A completely reproducible synthesis set-up was recently developed for calcium phosphate (CaP) bioceramics preparation from natural highly available marble and seashells. The influence of the pressing force in the fabrication process of porous 3D scaffolds derived from such CaPs by a sacrificial porogen method using natural fibers is here investigated. The fiber-ceramic based-products underwent thermal processing, followed by surface and volume features characterization. After fibers' thermal removal, interconnected 3D channels were obtained, which could allow a suitable in vivo irrigation and implant-associated negative side-effects prevention. This method provides the prospect of tunable HA/beta-TCP content in the case of both precursors' derived-scaffolds. The morphological results revealed the internal and external pores dimensions, modulated through different pressing forces that led to a controlled total porosity, evidenced by computed tomography techniques. Further, the wettability and mechanical features supported the advance of the novel porous-ceramic-structure designs as reliable bone reconstruction alternatives.

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