Engineering magnetoresistance: a new perspective

Helical Molecule as an Efficient Rectifier: Effects of Molecular Conformation and Transverse Electric Field

The phenomenon of charge current rectification is critically investigated using a single stranded helical molecule in presence of transverse electric field. Two different helical molecules, DNA and protein, are taken into account to explore the specific roles of molecular conformation on rectification, which have not been addressed so far to the best of our concern. Sandwiching the molecular system within source and drain electrodes, we compute charge currents for two bias polarities and the degree of current rectification based on non-equilibrium Green's function formalism within a tight-binding framework. At non-zero electric field, site energies of the molecule are modulated in a cosine form, similar to the well known Aubry-André-Harper relation, resulting an atypical and fragmented energy band spectrum. The appearance of non-uniform site energies plays the central role for generating different currents in two bias polarities, and thus, the current rectification. We find that a high degree of current rectification can be established using the helical system and it becomes more effective for the protein molecule than the DNA one. At the end, the rectification operation considering a more general helical structure is discussed to make the present communication a self-contained one. Our proposition may provide a new route of getting controlled current rectification using similar kind of biological molecules and other tailor made helical geometries.

Magnetoresistive effect in a quantum heterostructure with helical spacer: interplay between helicity and external electric field

Giant magnetoresistive effect in a multi-layered structure not only depends on the properties of magnetic systems, it also strongly depends on the type of non-magnetic spacer that is clamped between magnetic layers. In this work, we critically investigate the role of a helical spacer in presence of a transverse electric field. Two kinds of helical geometries, possessing short-range (SRH) and long-range hopping (LRH) of electrons, are taken into account mimicking single-stranded DNA and protein molecules respectively. Sandwiching the magnetic-non-magnetic-magnetic quantum heterostructure between source and drain contact electrodes, we investigate the properties of giant magnetoresistance (GMR) following the Green's function formalism within a tight-binding framework. The interplay between SRHs and LRHs of electrons provides several nontrivial signatures in GMR, especially in the presence of transverse electric field, as it makes the system a deterministic disordered one, similar to the well-known Aubry-Andre-Harper from. The famous gapped nature of energy band structure in presence of cosine modulation leads to high degree of magnetoresistance at multiple Fermi energies, compared to the traditional spacers. The magnetoresistive effect can be monitored selectively by adjusting the electric field strength and its direction. Comparing the results between the SRH and LRH cases, we find that the later one is more superior. Finally, to make the system more realistic we include the effect of dephasing. Our analysis may provide some fundamental aspects of designing electronic and spintronic devices based on magnetoresistive effect.

Localization Properties of a Quasiperiodic Ladder under Physical Gain and Loss: Tuning of Critical Points, Mixed-Phase Zone and Mobility Edge

We explore the localization properties of a double-stranded ladder within a tight-binding framework where the site energies of different lattice sites are distributed in the cosine form following the Aubry-André-Harper (AAH) model. An imaginary site energy, which can be positive or negative, referred to as physical gain or loss, is included in each of these lattice sites which makes the system a non-Hermitian (NH) one. Depending on the distribution of imaginary site energies, we obtain balanced and imbalanced NH ladders of different types, and for all these cases, we critically investigate localization phenomena. Each ladder can be decoupled into two effective one-dimensional (1D) chains which exhibit two distinct critical points of transition from metallic to insulating (MI) phase. Because of the existence of two distinct critical points, a mixed-phase (MP) zone emerges which yields the possibility of getting a mobility edge (ME). The conducting behaviors of different energy eigenstates are investigated in terms of inverse participation ratio (IPR). The critical points and thus the MP window can be selectively controlled by tuning the strength of the imaginary site energies which brings a new insight into the localization aspect. A brief discussion on phase transition considering a multi-stranded ladder was also given as a general case, to make the present communication a self-contained one. Our theoretical analysis can be utilized to investigate the localization phenomena in different kinds of simple and complex quasicrystals in the presence of physical gain and/or loss.

Possible Routes to Obtain Enhanced Magnetoresistance in a Driven Quantum Heterostructure with a Quasi-Periodic Spacer

In this work, we perform a numerical study of magnetoresistance in a one-dimensional quantum heterostructure, where the change in electrical resistance is measured between parallel and antiparallel configurations of magnetic layers. This layered structure also incorporates a non-magnetic spacer, subjected to quasi-periodic potentials, which is centrally clamped between two ferromagnetic layers. The efficiency of the magnetoresistance is further tuned by injecting unpolarized light on top of the two sided magnetic layers. Modulating the characteristic properties of different layers, the value of magnetoresistance can be enhanced significantly. The site energies of the spacer is modified through the well-known Aubry-André and Harper (AAH) potential, and the hopping parameter of magnetic layers is renormalized due to light irradiation. We describe the Hamiltonian of the layered structure within a tight-binding (TB) framework and investigate the transport properties through this nanojunction following Green's function formalism. The Floquet-Bloch (FB) anstaz within the minimal coupling scheme is introduced to incorporate the effect of light irradiation in TB Hamiltonian. Several interesting features of magnetotransport properties are represented considering the interplay between cosine modulated site energies of the central region and the hopping integral of the magnetic regions that are subjected to light irradiation. Finally, the effect of temperature on magnetoresistance is also investigated to make the model more realistic and suitable for device designing. Our analysis is purely a numerical one, and it leads to some fundamental prescriptions of obtaining enhanced magnetoresistance in multilayered systems.