Phenomenon of multiple reentrant localization in a double-stranded helix with transverse electric field.

The present work explores the potential for observing multiple reentrant localization behavior in a double-stranded helical (DSH) system, extending beyond the conventional nearest-neighbor hopping (NNH) interaction. The DSH system is considered to have hopping dimerization in each strand, while also being subjected to a transverse electric field. The inclusion of an electric field serves the dual purpose of inducing quasi-periodic disorder and strand-wise staggered site energies. Two reentrant localization regions are identified: one exhibiting true extended behavior in the thermodynamic limit, while the second region shows quasi-extended characteristics with partial spreading within the helix. The DSH system exhibits three distinct single-particle mobility edges linked to localization transitions present in the system. The analysis in this study involves examining various parameters such as the single-particle energy spectrum, inverse participation ratio, local probability amplitude, and more. Our proposal, combining achievable hopping dimerization and induced correlated disorder, presents a unique opportunity to study phenomenon of reentrant localization, generating significant research interest.

Electrical analogue of one-dimensional and quasi-one-dimensional Aubry-André-Harper lattices.

This work explores the potential for achieving correlated disorder in electrical circuits by utilizing reactive elements. By establishing a direct correspondence between the tight-binding Hamiltonian and the admittance matrix of the circuit, a novel approach is presented. The localization phenomena within the circuit are investigated through the analysis of the two-port impedance. To introduce correlated disorder, the Aubry-André-Harper (AAH) model is employed. Both one-dimensional and quasi-one-dimensional AAH structures are examined and effectively mapped to their tight-binding counterparts. Notably, transitions from a high-conducting phase to a low-conducting phase are observed in these circuits, highlighting the impact of correlated disorder.

Comparative study between charge and spin thermoelectric figure of merits in an antiferromagnetic ring.

Considering the unique and diverse characteristic features of antiferromagnetic (AFM) systems, here in our work, we explore spin-dependent thermoelectric behavior in an AFM ring geometry. A reasonably large (≫1) spin figure of merit, referred to asZST, is obtained under suitable input conditions. Two important prerequisites are (i) breaking the symmetry among up and down spin sub-Hamiltonians and (ii) generating different asymmetric transmission line shapes across a Fermi energy for two opposite spin electrons. Describing the physical system within a tight-binding framework, where spin-dependent scattering occurs due to the interaction of itinerant electrons with local magnetic moments via the usual spin-moment exchange interaction, we compute all the thermoelectric quantities based on Landauer integrals following the Green's function technique. The behavior of charge figure of merit, denoted asZCT, is also discussed along withZST. ThoughZCTreaches above unity, it is much smaller compared toZST. This is the key finding of our investigation. To make the present communication a self-contained one, we compare the results with another arrangement of magnetic moments in a ring-like geometry and in a chain-like one. Our analysis gives a suitable hint that in the presence of spin-dependent scattering, much more favorable energy conversion can be substantiated.

Circular current in a one-dimensional Hubbard quasi-periodic Su-Schrieffer-Heeger ring.

In this work, we investigate the behavior of interacting electrons in a Su-Schrieffer-Heeger quantum ring, threaded by an Aharonov-Bohm (AB) fluxφ, within a tight-binding framework. The site energies of the ring follow the Aubry-Andre-Harper (AAH) pattern, and, depending on the specific arrangement of neighboring site energies two different configurations, namely, non-staggered and staggered, are taken into account. The electron-electron (e-e) interaction is incorporated through the well-known Hubbard form and the results are computed within the mean-field (MF) approximation. Due to AB fluxφ, a non-decaying charge current is established in the ring, and its characteristics are critically studied in terms of the Hubbard interaction, AAH modulation, and hopping dimerization. Several unusual phenomena are observed under different input conditions, that might be useful to analyze the properties of interacting electrons in similar kinds of other fascinating quasi-crystals in the presence of additional correlation in hopping integrals. A comparison between exact and MF results is given, for the sake of completeness of our analysis.

Generation and manipulation of pure spin current in a conducting loop coupled to an Aharonov-Bohm ring.

In this work we put forward a new prescription for the generation and manipulation of non-decaying pure spin current (SC) in a Rashba spin-orbit (SO) coupled conducting loop which is attached to an Aharonov-Bohm (AB) ring. In presence of a single link between the rings, a SC is established in the flux-free ring, without accompanying any charge current (CC). The magnitude and direction of this SC are controlled by means of the AB flux, without tuning the SO coupling, which is the central aspect of our study. Employing a tight-binding framework we describe the two-ring quantum system, where the effect of magnetic flux is incorporated through Peierls phase factor. The specific roles of AB flux, SO coupling and the connectivity among the rings are critically investigated which yield several non-trivial signatures in energy band spectrum and pure SC. Along with SC, the phenomenon of flux-driven CC is also discussed, and at the end, different other effects like electron filling, system size and disorder are analyzed to make the present communication a self contained one. Our detailed investigation may provide some key aspects of designing efficient spintronic devices where SC can be guided in an other way.

Interplay between hopping dimerization and quasi-periodicity on flux-driven circular current in an incommensurate Su-Schrieffer-Heeger ring.

We report for the first time the phenomenon of flux-driven circular current in an isolated Su-Schrieffer-Heeger (SSH) quantum ring in presence of cosine modulation in the form of the Aubry-André-Harper (AAH) model. The quantum ring is described within a tight-binding framework, where the effect of magnetic flux is incorporated through Peierls substitution. Depending on the arrangements of AAH site potentials we have two different kinds of ring systems that are referred to as staggered and non-staggered AAH SSH rings. The interplay between the hopping dimerization and quasiperiodic modulation leads to several new features in the energy band spectrum and persistent current which we investigate critically. An atypical enhancement of current with increasing AAH modulation strength is obtained that gives a clear signature of transition from a low conducting phase to a high conducting one. The specific roles of AAH phase, magnetic flux, electron filling, intra- and inter-cell hopping integrals, and ring size are discussed thoroughly. We also study the effect of random disorder on persistent current with hopping dimerization to compare the results with the uncorrelated ones. Our analysis can be extended further in studying magnetic responses of similar kinds of other hybrid systems in presence of magnetic flux.

Efficient current rectification in driven acenes.

We examine the current-voltage (I-V) characteristics of different polyacenes, such as anthracene, tetracene, pentacene, etc., under the influence of arbitrarily polarized light. The irradiation effect produces an anisotropy in the system and acenes may therefore be employed as molecular rectifiers. We find that the rectification efficiency can be more than 90% with a specific set of light parameters. The phase of rectification (positive or negative) can suitably be engineered by controlling the light conditions. The effect of light irradiation is incorporated through the Floquet-Bloch ansatz with the minimal coupling scheme. The transport properties are calculated using Green's function technique following the Landauer-Büttiker formalism. Given the promising rectification results, the present prescription may be useful in designing functional elements, employing several other single/complex molecular structures in digital circuit design with the possibility of immense applications.

Strain-induced thermoelectricity in pentacene.

The present work discusses a non-synthetic strategy to achieve a favorable thermoelectric response in pentacene via strain. It is found that a uni-axial strain is capable of inducing spatial anisotropy in the molecule. As a result, the transmission spectrum becomes highly asymmetric under a particular strained scenario, which is the primary requirement to get a favorable thermoelectric response. Different thermoelectric quantities are computed for the strain-induced pentacene using Green's function formalism following the Landauer-Büttiker prescription. Various scenarios are considered to make the present work more realistic, such as the effects of substrate, coupling strength between the molecule and electrodes, dangling bonds, etc. Such a scheme to enhance the thermoelectric performance in pentacene is technologically intriguing and completely new to the best of our knowledge.

Spintronics in double stranded magnetic helix: role of non-uniform disorder.

The spin dependent transport phenomena are investigated in a double stranded (ds) magnetic helix (MH) structure. Two different helical systems, short-range hopping helix and long range hopping (LRH) helix, are taken into account. We explore the role of these two kinds of geometries on spin dependent transport phenomena. Using Green's function formalism within a tight-binding framework we compute transport quantities which include spin dependent transmission probabilities, junction currents and spin polarization (SP) coefficient. High degree of SP is obtained for the LRH MH. The SP can be tuned by changing the inter-strand hopping and the direction of magnetic moments at different lattice sites. We find atypical features when we include impurities in one strand of the MH, keeping the other strand free. Unlike uniform disordered systems, SP gets increased with impurity strength beyond a critical value. The effect of temperature on SP and experimental possibilities of our proposed quantum system are also discussed, to make the present communication a self-contained one. Our analysis may provide a new route to explore interesting spintronic properties using similar kind of fascinating helical geometries, possessing higher order electron hopping and subjected to non-uniform disorder.

Role of inter-electrode coupling on thermoelectricity in an interferometric geometry: a new proposition.

Efficient thermoelectric (TE) energy conversion is one of the most desirable solutions of our current day energy crisis. Exploiting the effect of quantum interference among electronic waves, in this work we propose a prescription of getting high TE efficiency, the so-calledfigure of merit(ZT), considering an interferometric geometry where a loop conductor is clamped between two heat baths. Unlike conventional junction configurations, we introduce an additional path for electron transfer directly from source to drain, due to their close proximity. The interplay between different paths leads to an enhancedZT(ZT > 1). Moreover, the efficiency can be further regulated by tuning the inter-electrode coupling. The effects of magnetic flux threaded by the ring and disorder are also discussed. Our proposed prescription may lead to a new route of designing tunable TE devices at nanoscale level.

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.

Spin-dependent transport in a driven non-collinear antiferromagnetic fractal network.

Non-collinear magnetic texture breaks the spin-sublattice symmetry which gives rise to a spin-splitting effect. Inspired by this, we study the spin-dependent transport properties in a non-collinear antiferromagnetic fractal structure, namely, the Sierpinski Gasket (SPG) triangle. We find that though the spin-up and spin-down currents are different, the degree of spin polarization is too weak. Finally, we come up with a proposal, where the degree of spin polarization can be enhanced significantly in the presence of a time-periodic driving field. Such a prescription of getting spin-filtering effect from an unpolarized source in a fractal network is completely new to the best of our knowledge. Starting from a higher generation of SPG to smaller ones, the precise dependencies of driving field parameters, spin-dependent scattering strength, interface sensitivity on spin polarization are critically investigated. The spatial distribution of spin-resolved bond current density is also explored. Interestingly, our proposed setup exhibits finite spin polarization for different spin-quantization axes. Arbitrarily polarized light is considered and its effect is incorporated through Floquet-Bloch ansatz. All the spin-resolved transport quantities are computed using Green's function formalism following the Landauer-Büttiker prescription. In light of the experimental feasibility of such fractal structures and manipulation of magnetic textures, the present work brings forth new insights into spintronic properties of non-collinear antiferromagnetic SPG. This should also entice the AFM spintronic community to explore other fractal structures with the possibility of unconventional features.

Enhanced current rectification in graphene nanoribbons: effects of geometries and orientations of nanopores.

We discuss the possibility of getting rectification operation in graphene nanoribbon (GNR). For a system to be a rectifier, it must be physically asymmetric and we induce the asymmetry in GNR by introducing nanopores. The rectification properties are discussed for differently structured nanopores. We find that shape and orientation of the nanopores are critical and sensitive to the degree of current rectification. As the choice of Fermi energy is crucial for obtaining significant current rectification, explicit dependence of Fermi energy on the degree of current rectification is also studied for a particular shape of the nanopore. Finally, the role of nanopore size and different spatial distributions of the electrostatic potential profile across the GNR are explored. The stability of the nanopores is also discussed with a possible solution. Given the simplicity of the proposed method and promising results, the present proposition may lead to a new route of getting current rectification in different kinds of materials where nanopores can be formed selectively.

Localization phenomena and electronic transport in irradiated Aubry-André-Harper systems.

The role of light irradiation on electronic localization is critically investigated for the first time in a tight-binding lattice where site energies are modulated in the cosine form following the Aubry-André-Harper (AAH) model. The critical point of transition from delocalized-to-localized phase can be monitored selectively by regulating the light parameters that is extremely useful to have controlled electron transmission across the system. Starting with a strictly one-dimensional (1D) AAH chain, we extend our analysis considering a two-stranded ladder model which brings peculiar signatures in presence of irradiation. Unlike 1D system, AAH ladder exhibits a mixed phase (MP) zone where both extended and localized energy eigenstates co-exist. This is the fundamental requirement to have mobility edge in energy band spectrum. A mathematical description is given for decoupling the irradiated ladder into two effective 1D AAH chains. The underlying mechanism of getting a MP zone relies on the availability of two distinct critical points (CPs) of the decoupled chains, in presence of second-neighbor hopping between the two strands. Using a minimal coupling scheme the effect of light irradiation is incorporated following the Floquet-Bloch ansatz. The localization behaviors of different energy eigenstates are studied by calculating inverse participation ratio, and, are further explained in a more compact way by calculating two-terminal transmission probabilities together with average density of states. Finally, the decoupling procedure is extended for a more general multi-stranded AAH ladders where multiple CPs and thus multiple mobility edges are found. Our analysis may provide a new route of engineering localization properties in similar kind of other fascinating quasiperiodic systems.

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.

Generation of circular spin current in an AB magnetic ring with vanishing net magnetization: a new prescription.

In this work we report for the first time the appearance of non-decaying circular spin current in a magnetic ring with vanishing net magnetization, even in absence of any spin chirality. Breaking the symmetry in hopping integrals we can misalign up and down spin electronic energy levels which yields a net spin current in the magnetic quantum ring, threaded by an Aharonov-Bohm flux. Along with spin current, a net charge current also appears, and we compute both these currents using the second quantized approach. A tight-binding framework is employed to describe the magnetic ring where each site of the ring contains a finite magnetic moment. Itinerant electrons get scattered from the localized magnetic moments at different lattice sites, and the moments are arranged in such a way that the net magnetization vanishes. The interplay between magnetic moments and asymmetric hopping integrals leads to several atypical features in energy spectra, especially the existence of vanishing current carrying energy eigenstates together with the current carrying ones. The formation of such states those do not contribute any current is the artifact of different kinds of on-site energies and/or hopping integrals in different segments of the magnetic ring. The atypical signatures of energy levels are directly reflected into the charge and spin currents, and here we critically investigate the behaviors of circular currents as functions of electron filling, hopping integrals, strength of spin-moment interaction and ring size. Finally, we discuss briefly the possible experimental realization to implement our proposed magnetic system. The present analysis may provide a new route of generating persistent spin current in magnetic quantum rings with vanishing net magnetization, circumventing the use of spin-orbit coupled systems.

Possible route to efficient thermoelectric applications in a driven fractal network.

An essential attribute of many fractal structures is self-similarity. A Sierpinski gasket (SPG) triangle is a promising example of a fractal lattice that exhibits localized energy eigenstates. In the present work, for the first time we establish that a mixture of both extended and localized energy eigenstates can be generated yeilding mobility edges at multiple energies in presence of a time-periodic driving field. We obtain several compelling features by studying the transmission and energy eigenvalue spectra. As a possible application of our new findings, different thermoelectric properties are discussed, such as electrical conductance, thermopower, thermal conductance due to electrons and phonons. We show that our proposed method indeed exhibits highly favorable thermoelectric performance. The time-periodic driving field is assumed through an arbitrarily polarized light, and its effect is incorporated via Floquet-Bloch ansatz. All transport phenomena are worked out using Green's function formalism following the Landauer-Büttiker prescription.

A new prescription to achieve a high degree of spin polarization in a spin-orbit coupled quantum ring: efficient engineering by irradiation.

The present work discusses the possibility to achieve a high degree of spin polarization in a three-terminal quantum system. Irradiating the system, subjected to Rashba spin-orbit (SO) interaction, we find high degree of spin polarization under a suitable input condition along with different magnitudes and phases at the two output leads. The system is described within a tight-binding (TB) framework and the effect of irradiation is incorporated following the Floquet-Bloch (FB) ansatz. All the spin-dependent transmission probabilities are evaluated through Green's function technique using Landauer-Büttiker formalism. Several possible aspects are included to make the system more realistic and examined rigorously in the present work. To name a few, the effects of irradiation, SO interaction, interface sensitivity, system size, system temperature are investigated, and finally, the role of correlated impurities are studied. Despite having numerous proposals available to generate and manipulate spin-selective transmissions, such a prescription exploiting the irradiation effect is relatively new to the best of our concern.