Santanu K. Maiti

We study spin-dependent transport in a hybrid magnetic system, where a non-magnetic (NM) wire is coupled to a side-attached antiferromagnetic mesoscopic ring, placed between two non-magnetic electrodes subject to an in-plane electric field oriented perpendicular to the NM wire. The system is described within a tight-binding framework, and transport properties are computed using the non-equilibrium Green’s function formalism. We consider two junction configurations distinguished by the ring-wire coupling: a single-coupled junction and a double-coupled junction. In the single-coupled configuration, the coupling geometry alone breaks the spin symmetry, yielding a finite spin polarization (SP) even without any external field. The in-plane electric field further enhances the symmetry breaking in both configurations, serving as an efficient tuning parameter that drives the SP nearly 100 % in the low-bias region. In the double-coupled configuration, spin symmetry is preserved in the absence of the external field, and the electric field acts as a sole source of symmetry breaking, producing a large SP. Finite temperature effects and different system sizes are examined, confirming the robustness of the observed features. To validate the findings over a wide parameter space, we considered different sets of parameters and found that the key signatures remain unchanged. Our results demonstrate that such hybrid structures are promising candidates for realizing an externally controllable spintronic device in low-dimensional systems.

We investigate a route to enhanced thermoelectric energy conversion in nanoscale systems by exploiting a correlated quasiperiodic energy landscape in a one-dimensional chain coupled to source and drain reservoirs. The considered modulation generates a highly non-uniform electronic transmission spectrum, providing favorable conditions for achieving a large thermoelectric figure of merit. By systematically tuning the incommensurability parameter, a variety of quasiperiodic configurations are explored, several of which yield high values of the figure of merit exceeding 2. Electronic transport properties are evaluated within a tight-binding framework using the non-equilibrium Green’s function formalism, while the thermoelectric coefficients, including electrical conductance, Seebeck coefficient, and electronic thermal conductance, are determined through the Landauer approach. The underlying quasiperiodic potential belongs to the Aubry–André–Harper (AAH) family and exhibits a weakly varying spatial profile, leading to transmission characteristics that are favorable for thermoelectric optimization. The influence of phonon thermal conductance on the overall energy-conversion efficiency is also analyzed in detail. For the sake of completeness, we also critically inspect the effect of conductor to electrode coupling and the coupling asymmetry on ZT. We also check the thermoelectric response of conventional AAH system, and an elaborate comparison is made with our chosen quantum system. Our findings highlight the potential for correlated quasiperiodic nanostructures as promising candidates for efficient thermoelectric applications.

In an open quantum ring (OQR), an asymmetric ring-to-electrode configuration, where the upper and lower arms have unequal lengths, generates antiresonances in the junction transmission spectrum around the doubly degenerate eigenenergies of the isolated ring Hamiltonian. The asymmetric OQR also gives rise to a net circular current transmission within the ring, specifically around these doubly degenerate eigenenergies. We investigate the system-size scaling properties of the transmission within the ring and the overall junction transmission of an OQR. Ballistic transport refers to the unhindered flow of charge carriers within a conductor, where transmission is independent of the system size. Here, we find beyond-ballistic behavior, characterized by an anomalous increase of the transmission with increasing system size, near both the degenerate and nondegenerate eigenenergies of the ring Hamiltonian, depending on the ring-to-electrode configuration. This phenomenon is unique to OQRs and is associated with the quantum interference effect between two counterpropagating electronic waves with nearly equal and opposite momenta. Consequently, there is no equivalent phenomenon in open quantum junctions with linear conductors.

We investigate circular current in both ordered and disordered Hubbard quantum rings threaded by magnetic flux, employing exact diagonalization and the Hartree–Fock mean-field approach within the tight-binding framework. The influence of on-site and extended Hubbard interactions, disorder, and electron filling on the persistent current is systematically analyzed. To construct the full many-body Hamiltonian, we introduce a linear table formalism, which, to our knowledge, has been rarely used in this context. In ordered rings, the current decreases monotonically with increasing on-site repulsion, while the impact of the extended interaction depends strongly on the filling factor. At low filling, stronger extended interaction suppresses the current, whereas near half-filling, it enhances the current up to a critical ratio, half of the on-site strength, before reducing it. Disorder significantly modifies these behaviors, notably enhancing the current at less than quarter-filling with increasing extended interaction. The localization properties of eigenstates, examined via the inverse participation ratio, further support the crucial roles of filling and the interplay between on-site and extended interactions in governing persistent current.

We present a comprehensive theoretical investigation of quantum transport, circular currents, and the resulting induced magnetic fields in Fibonacci rings, treating both Hermitian and non-Hermitian (NH) realizations with particular emphasis on parity-time ( PT )-symmetric and symmetry-broken configurations. By engineering physically balanced gain and loss distributed according to Fibonacci sequence, we construct distinct ring geometries that either preserve or explicitly violate PT symmetry, and further examine complementary scenarios obtained via gain–loss sign inversion of the on-site potentials. Employing the nonequilibrium Green’s function formalism, we systematically analyze transmission characteristics and bond-resolved current densities to quantify transport and circulating current responses. While the Hermitian limit serves as a reference exhibiting only weak current modulation in the presence of disorder, the introduction of non-Hermiticity results in a dramatic enhancement of both transport and circular currents, accompanied by a strong amplification of the induced magnetic field. Remarkably, we reveal that NH transport is highly sensitive to gain–loss sign inversion and, in PT -broken regimes, exhibits an unconventional system-size dependence governed by the parity of the Fibonacci sequence and hopping correlations. Notably, the circulating current exhibits a nonmonotonic scaling with system size, a feature entirely absent in conventional Hermitian systems. Our results establish NH Fibonacci rings as tunable platforms for amplifying transport responses through symmetry control, Fibonacci parity dependence, and gain–loss sign inversion.

The primary requirement for achieving spin-selective electron transfer in a nanojunction possessing a magnetic system with zero net magnetization is to break the symmetry between the up and down spin sub-Hamiltonians. Circumventing the available approaches, in the present work, we put forward a new mechanism for symmetry breaking by introducing a bias drop along the functional element. To demonstrate this, we consider a clean magnetic chain with antiparallel alignment of neighboring magnetic moments. The junction is modeled within a tight-binding framework, and spin-dependent transmission probabilities are evaluated using wave-guide theory. The corresponding current components are obtained through the Landauer-B"{u}ttiker formalism. Selective spin currents, exhibiting a high degree of spin polarization, are obtained over a wide bias region. Moreover, the bias-dependent transmission profile exhibits negative differential resistance (NDR), another important aspect of our study. We examine the results under three different potential profiles, one linear and two non-linear, and in each case, we observe a favorable response. This work may offer a new route for designing efficient spintronic devices based on bias-controlled magnetic systems with vanishing net magnetization.

We present an extensive theoretical analysis of transport and circular currents and the associated induced magnetic fields in Fibonacci rings, explored in both Hermitian and non-Hermitian descriptions, with particular attention to configurations preserving or breaking PT symmetry. By engineering physically balanced gain and loss following a Fibonacci sequence, we realize two distinct geometrical configurations in which the ring either preserve or explicitly break PT symmetry, and further explore complementary realizations obtained by reversing the signs of the on site potentials. Using the non equilibrium Green's function (NEGF) formalism, we analyze transmission properties and bond current densities to quantify both transport and circulating currents. A comparison with the Hermitian limit establishes a clear baseline, where the ring supports only weak responses upon introducing disorder. In sharp contrast, non-Hermiticity leads to a pronounced amplification of transport and circular currents, and hence of the induced magnetic field. We further demonstrate that non-Hermitian transport is highly sensitive to gain and loss sign reversal and, in the non-PT-symmetric case, exhibits an unconventional dependence on system size governed by the parity of the Fibonacci sequence and hopping correlations. Remarkably, the current does not decay monotonically with increasing system size, revealing a distinct scaling behavior absent in conventional Hermitian systems. Our results highlight non-Hermitian quasiperiodic rings as versatile platforms for engineering and amplifying current driven magnetic responses through symmetry, topology, and gain-loss design.