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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-Buttiker 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.

We examine the current–voltage (I–V) characteristics of different polyacenes, such as anthracene, tetracene, pentacene, etc., under the influence of an arbitrarily polarized light.

Two dimensional (2D) Dirac materials, such as graphene, hold promise of being useful in energy storage, and thus have merged as candidates that are worth exploring through the last couple of decades. In this chapter, we mainly focus on three aspects of these materials, namely, the electronic properties, via computing the band structure, the topological properties through the topological invariants, and the prospects of these 2D materials for spintronic applications, via studying the spin polarized transport. All of these properties are correlated, and hence warrant a thorough discussion. Further, in order to ascertain whether a band deformation induces noticeable effects on the electronic, topological and spintronic properties, we have considered a 2D semi-Dirac system, that does not have Dirac cones, however the conduction and the valence bands touch at an intermediate to the Dirac points in the Brillouin zone. From our studies, we infer that the behaviour of these semi-Dirac systems is quite distinct from their Dirac counterpart. Finally, in order to have noticeable spin polarized transport, we use heavy adatoms (such as, Au) on the graphene matrix which enhances the spin–orbit coupling, and thereby propose a mechanism that will ramify on the spintronic applications.

The environmental influence is inevitable but often ignored in the study of electronic transport properties of small-scale systems. Such an environment-mediated interaction can generally be described by a parity-time symmetric non-Hermitian system with a balanced distribution of physical gain and loss. It is quite known in the literature that along with the conventional junction current, another current called bias-driven circular current can be established in a loop geometry depending upon the junction configuration. This current, further, induces a strong magnetic field that can even reach to few Tesla. What will happen to these quantities when the system interacts with its surrounding environment? Would it exhibit a detrimental response? We address such issues considering a two-terminal ring geometry where the junction setup is described within a tight-binding framework. All the transport quantities are evaluated using the standard Green's function formalism based on the Landauer-B\"{u}ttiker approach., Comment: 12 pages, 9 figures

A possible route is given to achieve enhanced thermoelectric (TE) performance in a spin-orbit (SO) coupled graphene nanoribbon (GNR). The GNR is irradiated with a suitable polarized light so that the transmission function can have asymmetry around the chemical potential, which is the key requirement to enhance the TE performance. The interplay between SO coupling and irradiation on the nature of transmission function is critically investigated. Astounding TE results are obtained in the presence of light irradiation without modifying the other physical parameters of the system. The thermopower is enhanced significantly and at the same time, a considerable suppression in the thermal conductance is observed which yields favorable energy conversion. We discuss thermoelectric properties for the unpassivated and passivated GNRs as well. Both zigzag and armchair GNRs are taken into account, and finally, the results are studied by varying their widths in a wide range. Enhancing the thermoelectric performance by means of irradiation in a SO coupled system is completely new and has not been explored so far to the best of our concern, and might be applicable as well in other such topological systems.

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 discussed. 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., Comment: 10 pages, 8 figures

Noncolinear 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 noncolinear 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-B\"{u}ttiker prescription. The present work brings forth new insights into spintronic properties of noncolinear antiferromagnetic SPG and should entice the AFM spintronic community to explore other fractal structures with the possibility of unconventional features., Comment: 12 pages, 10 figures

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.

We propose a new route of getting controlled electron transmission through a molecular wire having a single loop geometry, by irradiating the loop with an arbitrarily polarized light. Along with conventional junction current, a new current called bias driven circular current can be established in the loop under certain conditions depending on the junction configuration. This current, on the other hand, induces a strong magnetic field that can even reach to few tesla. All the physical phenomena can be regulated selectively by adjusting the irradiation parameters. In addition, we put forward another new route of regulating transport behavior by introducing a new path due to the proximity of the contact electrodes for a typical junction configuration. Employing a tight-binding framework, we include the effect of light irradiation within a minimal coupling scheme following the well known Floquet ansatz. Using the wave-guide theory we compute two-terminal transmission probability, and the currents are determined through the Landauer–Büttiker formalism. The present analysis may be utilized to investigate transport phenomena in any other molecular wires as well as tailor-made geometries having simple and/or complex loop sub-structures.

We study the charge and spin transport in two and four terminal graphene nanoribbons (GNR) decorated with random distribution of magnetic adatoms. The inclusion of the magnetic adatoms generates only the $z$-component of the spin polarized conductance via an exchange bias in the absence of Rashba spin-orbit interaction (SOI), while in presence of Rashba SOI, one is able to create all the three ($x$, $y$ and $z$) components. This has important consequences for possible spintronic applications. The charge conductance shows interesting behaviour near the zero of the Fermi energy. Where in presence of magnetic adatoms the familiar plateau at $2e^2/h$ vanishes, thereby transforming a quantum spin Hall insulating phase to an ordinary insulator. The local charge current and the local spin current provide an intuitive idea on the conductance features of the system. We found that, the local charge current is independent of Rashba SOI, while the three components of the local spin currents are sensitive to Rashba SOI. Moreover the fluctuations of the spin polarized conductance are found to be useful quantities as they show specific trends, that is, they enhance with increasing adatom densities. A two terminal GNR device seems to be better suited for possible spintronic applications., Comment: 11 pages, 16 figures. arXiv admin note: text overlap with arXiv:1704.06532

To manufacture an efficient thermoelectric device at the nanoscale and to get its controlled performance, we suggest a finite-width diamond ribbon in the presence of a perpendicular magnetic field. Contemplating the asymmetry in transmission function in a narrow conducting channel, the possibility of a gap around the energy band center in the presence of a magnetic field is discussed here. The eigenvalue spectrum and band structures show several interesting features. The different thermoelectric quantities are evaluated following the Landauer prescription. We show that the gap is tunable by controlling the strength of the magnetic field. Near the edge of the gap, our results exhibit a favorable thermoelectric response. We believe that our analysis of the proposed model can be experimentally tested in the laboratory and hope our study helps to construct an efficient thermoelectric device at the mesoscopic level.

We study the behavior of charge and spin transport properties in the presence of magnetic field and Rashba spin-orbit interaction in a six-terminal graphene nanoribbon using the multi-terminal Land...

Several proposals are available to get selective spin transmission through different nano-junctions and in all the cases the regulation is done either by applying a magnetic field or by tuning spin–orbit (SO) coupling. In the present work, we explore a separate scheme where the spin-dependent transport is regulated externally by irradiating a quantum ring that bridges the contact electrodes. This is a new proposal of generating spin selective transmission through a nano-junction, to the best of our knowledge. A high degree of spin polarization along with its phase alteration can be achieved by suitably adjusting the irradiation, circumventing the regulation of magnetic field and/or SO coupling. The effect of irradiation is included through the well-known Floquet-Bloch ansatz, where all the spin-dependent transport phenomena are worked out using Green’s function formalism following the Landauer–Buttiker prescription within a tight-binding framework. Precise dependencies of light irradiation, SO coupling, magnetic flux threaded by the ring, interface sensitivity, system temperature, and impurities on spin polarization are critically investigated. Our analysis may give a new platform for spin selective electron transmission and make it applicable to other complex nano-structured materials also. We strongly believe that the present proposal can be examined in a suitable laboratory.

The key requirement for an enhanced thermoelectric (TE) performance is the presence of asymmetry in transmission function. Focussing on this issue, we propose a unique idea to enhance TE performance in a graphene nanoribbon (GNR) that has not been explored so far to the best of our concern. In the present work, one part of the GNR is considered as a disordered region while the rest of the system is clean. Such an ordered-disordered separated structure yields more asymmetric transmission function over the conventional uniform disordered one. Finally, we include the effect of electron–electron (e–e) interaction to check whether it brings any non-trivial signature on TE performance. The e–e interaction is taken in the form of an on-site Hubbard model and we compute our results within a Hartree–Fock mean field approach. The results obtained in the present work exhibit quite remarkable TE performance along with some non-trivial features.

Generally, pristine graphene nanoribbons show poor thermoelectric (TE) performance. Several proposals are available to enhance the TE performance, which however requires some highly structural modifications of the system. In the present work, we explore a new prescription where a favorable TE response can be achieved by irradiating the ribbon. The electronic conductance, thermopower and thermal conductance due to electron are found to be highly sensitive to the irradiation. We obtain significantly enhanced thermopower along with highly suppressed thermal conductance for typical sets of irradiation parameters. As a result of this higher figure of merit is found. Therefore, we can suggest that a graphene nanoribbon can be used as an efficient energy conversion device in the presence of irradiation and hope that it will bear a significant impact in the energy market soon.

We investigate a novel way to manipulate spin transport properties in two-terminal zigzag graphene nanoribbons with a square-shaped and a triangular-shaped antidots in presence of Rashba spin-orbit coupling. The geometric structure of the antidot shows interesting feature in the spintronic properties of ZGNR. The square-shaped antidote can generate only the y-component of the spin-polarized transmission coefficient, while the triangular-shaped antidote is able to generate all the three components (Px, Py, Pz) of the spin-polarized transmission coefficient. Our results can be generalized in different topological systems and may be tested experimentally.We investigate a novel way to manipulate spin transport properties in two-terminal zigzag graphene nanoribbons with a square-shaped and a triangular-shaped antidots in presence of Rashba spin-orbit coupling. The geometric structure of the antidot shows interesting feature in the spintronic properties of ZGNR. The square-shaped antidote can generate only the y-component of the spin-polarized transmission coefficient, while the triangular-shaped antidote is able to generate all the three components (Px, Py, Pz) of the spin-polarized transmission coefficient. Our results can be generalized in different topological systems and may be tested experimentally.

We compute analytic expressions for the edge states in a zigzag Kane-Mele nanoribbon (KMNR) by solving the eigenvalue equations in presence of intrinsic and Rashba spin-orbit couplings. Owing to the P-T symmetry of the Hamiltonian the edge states are protected by topological invariance and hence are found to be robust. We have done a systematic study for each of the above cases, for example, a pristine graphene, graphene with an intrinsic spin-orbit coupling, graphene with a Rashba spin-orbit coupling, a Kane-Mele nanoribbon and supported our results on the robustness of the edge states by analytic computation of the electronic probability amplitudes, the local density of states (LDOS), band structures and the conductance spectra., 14 pages, 13 figures

Spin dependent transport in a three-terminal graphene nanoribbon (GNR) is investigated in presence of Rashba spin-orbit interaction. Such a three-terminal structure is shown to be highly effective in filtering electron spins from an unpolarized source simultaneously into two output leads and thus can be used as an efficient spin filter device compared to a two-terminal one. The study of sensitivity of the spin-polarized transmission on the location of the outgoing leads results in interesting consequences and is explored in details. There exist certain symmetry relations between the two outgoing leads with regard to spin-polarized transport, especially when they are connected to the system in a particular manner. We believe that the prototype presented here can be realized experimentally and hence the results can also be verified.

The thermoelectric (TE) properties of graphene nanoribbon (GNR) with various regularly structured nanopores (NPs) are studied in detail. Describing the system within a tight-binding (TB) framework we compute all the TE quantities using the Landauer integrals. Results show a significant reduction in thermal conductance along with higher thermopower on the incorporation of most of the NPs, considered in this work, which enhances the TE performance. The different NP structures, such as the circle, rectangle, particular configuration of an isosceles triangle, etc., are found to have a higher figure of merit (FOM). A strategy is also proposed to enhance FOM by increasing the height of NPs across the junction. In light of the simplicity of our proposed models with such promising prospects, we strongly believe that NP-GNR system can be highly effective as an efficient TE device in the commercial market in the near future.

Electronic transport with a line (or a few lines) of Anderson type disorder in a zigzag graphene nanoribbon is investigated in presence of Rashba spin-orbit interaction. Such line disorders give rise to peculiar behavior in both charge as well as spin-polarized transmission in the following sense. In the weak disorder regime, the charge transport data show Anderson localization up to a certain disorder strength, beyond which the extended states emerge and start dominating over the localized states. These results are the hallmark signature of a selectively disordered (as opposed to bulk disorder) graphene nanoribbon. However, the spin-polarized transport shows a completely contradicting behavior. Further, the structural symmetries are shown to have an important role in the spintronic properties of the nanoribbons. Moreover, the edge-disorder scenario (disorder selectively placed at the edges) seems to hold promise for the spin-filter and switching device applications., 7 pages, 7 figures (Accepted for Publication in Europhysics Letters)

We investigate a novel way to manipulate the spin polarized transmission in a two terminal zigzag graphene nanoribbon in presence of Rashba spin-orbit (SO) interaction with circular shaped cavity engraved into it. A usual technique to control the spin polarized transport behaviour of a nanoribbon can be achieved by tuning the strength of the SO coupling, while we show that an efficient engineering of the spin polarized transport properties can also be done via cavities of different radii engraved in the nanoribbon. Simplicity of the technique in creating such cavities in the experiments renders an additional handle to explore transport properties as a function of the location of the cavity in the nanoribbon. Further, a systematic assessment of the interplay of the Rashba interaction and the dimensions of the nanoribbon is presented. These results should provide useful input to the spintronic behaviour of such devices. In addition to the spin polarization, we have also included an interesting discussion on the charge transmission properties of the nanoribbon, where, in absence of any SO interaction a metal-insulator transition induced by the presence of a cavity is observed., 7 pages, 9 figures (Accepted in Europhysics Letters)

We study spin Hall effect in a three terminal Y-shaped device in presence of tunable spin-orbit (SO) interactions via Landauer-Buttiker formalism. We have evolved a fabrication technique for creating different angular separation between the two arms of the Y-shaped device so as to investigate the effect of angular width on the spin Hall conductance (SHC). A smaller angular separation yields a larger conductance. Also arbitrary orientation of the spin quantization axes yields interesting three dimensional contour maps for the SHC corresponding to different angular separation of the Y-shaped device. In addition to the GSH demonstrating bounded behaviour for different angular separations, there are distinct symmetry axes about which SHC demonstrates reflection symmetry. The results explicitly show breaking of the spin rotational symmetry. Further a systematic study is carried out to compare and contrast between the different SO terms, such as Rashba and Dresselhaus SO interactions and the interplay of the angular separation therein.

We have compared and contrasted between the diagonal and off-diagonal disorder with regard to the conductance of a two dimensional conducting system. The diagonal (onsite) disorder is found to be more predictable in the sense that the conductivity gradually vanishes as the disorder strength is enhanced. A few of the physical quantities such as the participation ratio and the correlation length, etc. directly correlate with the localization length of a strongly disordered conductor. Such an intuitive picture is found to be absent for the off diagonal (hopping) disorder. The hopping disorder strongly enhances the conductance properties even in the weak localization regime and surprisingly yields metallic conductivity at large disorder values.

We study the spin Hall effect in Y-shaped junction devices in presence of Rashba spin orbit coupling (RSOC). The voltage and the net spin current registered at one of the arms of the Y-junction are seen to increases although the spin Hall conductance (SHC) diminishes as the strength of the RSOC is increased. This implies the voltage increases further than that of the current, thereby causing a loss of the RSOC. Various other characteristic features obtained from our study include, a perfectly antisymmetric behaviour of the spin current and the SHC with respect to the zero bias, while the voltage shows a symmetric character. Finally a large RSOC completely destroys the SHC, owing to a complete disappearance of the local density of states, thereby reinforcing our earlier claim that RSOC emulates the effect of disorder on the quantum conductance of junction device.

In a system consisting of a metal-(s-wave) superconductor junction, we study the conductance characteristics in presence of Rashba spin orbit coupling (RSOC) and an external magnetic field applied along the plane of the sample. With a selective inclusion of the Rashba coupling either in the metallic or in both we note that there is a distinct effect with regard to the magnitude of the Andreev peak that occurs at a biasing voltage lower than the superconducting gap energy. The height of the peak is sensitive to the RSOC (increases with increase in RSOC) for RSOC to be present only in the metallic region, (same is true when RSOC is present throughout the junction), while the peak height is fairly independent when RSOC is solely present in the superconducting region. The in-plane magnetic field has very interesting effects which show up in the form of having a conductance peak at zero bias, thereby making it possible to realize a Majorana bound state.

Using the multi-probe Landauer-BUttiker formula and Green's function approach, we calculate the longitudinal conductance (LC) and spin Hall conductance (SHC) numerically in a two-dimensional junction system with the Rashba and Dresselhaus spin orbit coupling (SOC) and spin dependent disorder (SDD) in presence of both random onsite and hopping disorder strengths. It has been found that when the strengths of the RSOC and DSOC are same, the SHC vanishes. Further in presence of random onsite or hopping disorder, the SHC is still zero when the strengths of the two types of SOC, that is Rashba and Dressselhaus are the same. This indicates that the cancellation of SHC is robust even in the presence of random disorder. Only with the inclusion of SDD (onsite or hopping), a non-zero SHC is found and it increases as the strength of SDD increases. The physical implication of the existence of a non-zero SHC has been explored in this work. Finally, we have compared the effect of onsite SDD and hopping SDD on both longitudinal and spin Hall conductances.