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Swapan K. Pati

Publications and source records attributed to Swapan K. Pati.

At least 19 recordsLinked to original sources
arXiv2025

Localization transitions in an open quasiperiodic ladder

We investigate localization transition in an open quasiperiodic ladder where the quasiperiodicity is described by the Aubry-André-Harper model. While previous studies have shown that higher-order hopping or constrained quasiperiodic potentials can induce a mixed-phase zone in one dimension, we demonstrate that the dissipation can induce mixed phase zone in a one dimensional nearest-neighbor system without imposing any explicit constraints on the quasiperiodic potential or hopping parameter. Our approach exploits an exact correspondence between the eigenspectrum of the Liouvillian superoperator and that of the non-Hermitian Hamiltonian, valid for quadratic fermionic systems under linear dissipation. Using third quantization approach within Majorana fermionic representation, we analyze two dissipation configurations: alternating gain and loss at every site, and at alternate sites under balanced and imbalanced conditions. By computing the inverse and normalized participation ratios, we show that dissipation can drive the system into three distinct phases: delocalizd, mixed, and localized. Notably, the mixed-phase zone is absent for balanced dissipation at every site but emerges upon introducing imbalance, while for alternate site dissipation it appears in both balanced and imbalanced cases. Furthermore, the critical points and the width of the mixed-phase window can be selectively tuned by varying the dissipation strength. These findings reveal that the dissipation plays a decisive role in reshaping localization transitions in quasiperiodic systems, offering new insight into the interplay between non-Hermitian effects and quasiperiodic order.

cond-mat.mes-hall↗
arXiv2025

Phases and phase transtions in one-dimensional alternating mixed spin (1/2-1) chain: effects of frustration and anisotropy

We investigate the phases and phase-transitions in one-dimensional alternating mixed-spin (1/2-1) chain in the presence of both frustration and anisotropy. Frustration is introduced via next-nearest-neighbor interactions, while single-ion anisotropy is incorporated at each lattice site. Our results show that moderate frustration can drive a phase transition from a ferrimagnetic state to an anti-ferromagnetic ground state. Remarkably, the presence of a weak easy-plane anisotropy destabilizes the ferrimagnetic order, also leading to the emergence of an antiferromagnetic phase. Interestingly, under strong frustration and anisotropy, the system exhibits signatures of a novel phase with spin density wave (SDW)-like modulation . We explore these anomalous phase transitions by employing exact diagonalization (ED) for small system sizes and the density matrix renormalization group (DMRG) method to characterize ground state properties for larger system sizes. We also investigate the finite-temperature behavior across various phases using the ancilla-based time-evolving block decimation (TEBD) approach. The primary objective of this work is to elucidate the phase structure of alternating mixed-spin chains under the combined effects of frustration and anisotropy. The primary objective of this work is to elucidate the intricate interplay between frustration and anisotropy in identifying the exotic phases and phase-transitions in alternating mixed-spin chains. Our findings contribute to a deeper understanding of mixed-spin quantum systems and may offer insights for future theoretical and experimental studies.

cond-mat.str-el↗
arXiv2025

Enhancement of persistent current in a non-Hermitian disordered ring

We have studied the Aharonov-Bohm flux-induced magnetic response of a disordered non-Hermitian ring. The disorder is introduced through an on-site quasiperiodic potential described by the Aubry-André-Harper (AAH) model, incorporating a complex phase that renders the model non-Hermitian. Our findings reveal that this form of non-Hermiticity enhances the persistent current, without requiring hopping dimerization. We explore both non-interacting and interacting scenarios. In the former, we examine spinless fermions, while in the latter, we consider fermions with Hubbard interactions. The Non-Hermitian phase induces both the real and imaginary components of the current. We thoroughly analyze the energy eigenspectrum, ground state energy, and persistent current in both real and imaginary spaces for various system parameters. Our primary goal is to investigate the combined effects of non-Hermiticity and disorder strength on persistent currents. We find an enhancement in both the real and imaginary components of the persistent current with increasing disorder strength, as well as the non-Hermiticity, up to a critical value. Furthermore, we observe an enhancement in persistent current in the presence of Hubbard correlation. Our findings may provide a new route to get nontrivial characteristics in persistent current for a special type of non-Hermitian systems.

cond-mat.mes-hall↗
arXiv2020

Quench dynamics of two component dipolar fermions subject to a quasiperiodic potential

Motivated by recent experiments in fermionic polar gases, we study the non-equilibrium dynamics of two-component dipolar fermions subject to a quasiperiodic potential. We investigate the localization of charge and spin degrees of freedom time evolving with a long-range spin-SU(2) symmetric fermionic Hamiltonian, by calculating experimentally accessible dynamical observables. To study the non-equilibrium dynamics, we start the time evolution with two initial states at half-filling: (i) product state with doublons $|\uparrow \downarrow 0 \uparrow \downarrow 0 \uparrow \downarrow 0 \uparrow \downarrow 0 \uparrow \downarrow \rangle$ and (ii) product state with singlons $|\uparrow \ \downarrow \ \uparrow \ \downarrow \ \uparrow \ \downarrow \ \uparrow \ \downarrow \ \uparrow \ \downarrow \ \rangle$. We carried out the real-time evolution via the fermionic Hamiltonian using exact diagonalization(ED) and the time-dependent variational principle (TDVP) for finite Matrix product states(MPSs), within experimentally relevant time scales. For the product state with doublons, we observe a delocalized to localized phase transition varying disorder strengths, by monitoring the decay of charge imbalance with time. For the long-range interacting Hamiltonian of our focus, and in the presence of strong enough disorder, starting the time evolution with singlons we find a strong reduction in the spin delocalization, contrary to results of previous studies using the disordered short-range (on-site) Hubbard model with SU(2) symmetry. Our predictions for localization of both charge and spin should be observable in ultra-cold experiments with fermionic dipolar atoms subject to a quasiperiodic potential.

cond-mat.quant-gas↗
arXiv2020

Signatures of nonlinear magnetoelectricity in second harmonic spectra of SU(2) symmetry broken quantum many-body systems

Quantum mechanical perturbative expressions for second order dynamical magnetoelectric (ME) susceptibilities have been derived and calculated for a small molecular system using the Hubbard Hamiltonian with SU(2) symmetry breaking in the form of spin-orbit coupling (SOC) or spin-phonon coupling. These susceptibilities will have signatures in second harmonic generation spectra. We show that SU(2) symmetry breaking is the key to generate these susceptibilities. We have calculated these ME coefficients by solving the Hamiltonian for low lying excited states using Lanczos method. Varying the Hubbard term along with SOC strength, we find spin and charge and both spin-charge dominated spectra of dynamical ME coefficients. We have shown that intensities of the peaks in the spectra are highest when the magnitudes of Hubbard term and SOC coupling term are in similar range.

cond-mat.mtrl-sci↗
arXiv2020

Small Heterocyclic Molecule as Multistate Transistor: A Quantum Many-body Approach

Weakly coupled molecular junctions are an active and important field of research as they exhibit various non-linear transport phenomena. We have investigated the carrier transport through weakly coupled B2C2N2H6 molecules using quantum many-body approach coupled with kinetic (master) equations. Interestingly, various types of non-linear current-voltage characteristics, such as, negative differential conductance (NDC), rectifications, Coulomb staircase, which is the hallmark of multistate transport devices, have been obtained. The source-drain voltage induced change in the occupation probabilities of low-lying many-body states which are different in nature towards carrier transport, directly control the net current flowing through the molecular junctions. We further investigate the effect of different kinds of perturbations such as gate voltage and perpendicular magnetic field, over carrier-flow through this molecular bridge. Interestingly, we find that depending on the strength of the applied perturbating field, several phenomena, such as switching off of current, suppression of NDC appears in the devices. Fundamentally, this applied perturbations modifies both the site charge density as well as occupation probabilities of transport active channels, resulting in a significant alteration in transport behavior of this molecular junction.

cond-mat.mes-hall↗
arXiv2017

Generalized Charge Energy Rate for Organic Solids and Biomolecular Aggregates Through Drift-Diffusion and Hopping Transport Equations: A Unified Theory

We derive generalized charge energy rate equations for organic solids and biomolecular aggregates, even when these are dynamically disordered. These equations suggest that the transport in such cases rely on both drift and diffusion phenomena. The presence of disorder and field effects makes the equations nonlinear and together with cooperativity, these enhance the charge and energy transport. The generalized drift diffusion expression connects the adiabatic band and nonadiabatic hopping transport mechanisms, well suited for any complex organic semiconductors or assemblies of bio molecular systems. Here we have proposed donor-acceptor (DA) reaction state model, which examines the probability of charge transfer and the rate between two distinct transition state identities. From our analytical equations, we suggest that charge and energy transport property in DA states can be tuned by only a single parameter, i.e., the chemical potential. Importantly, we find the non-equilibrium assisted drift-diffusion transport at non- steady state regime in 2D and 3D semiconducting devices. The numerical results clearly support our unified analytical equations, which goes beyond Einstein's diffusion law even in quasi- equilibrium cases.

cond-mat.mes-hall↗
arXiv2017

Unified Quantum Classical Theory of Einstein Diffusion-Mobility Relationship for Ordered and Disordered Semiconductors

We propose a unified diffusion-mobility relation which quantifies both quantum and classical levels of understanding on electron dynamics in ordered and disordered materials. This attempt overcomes the inability of classical Einstein relation (diffusion-mobility ratio) to explain the quantum behaviors, conceptually well-settles the dimensional effect, phase transition and nonlinear behavior of electronic transport. Our proposed theory relies on the chemical potential which provides the coupling mechanism of charge-heat current, due to electron-phonon coupling. We have derived expressions which explain charge transport in both degenerate and nondegenerate materials, and also provide the linear and nonlinear relationship between the charge density and chemical potential. Theoretically, we find that the symmetrical nature of electron-hole transport in strongly correlated two-dimensional semiconductors indicates linear dispersion. We also observe the broken symmetry in the nonlinear regime. This generalized diffusion-mobility relation explains both the strongly and weakly correlated systems from low temperature to high temperature, in both the relativistic as well as nonrelativistic domains. In vanishing charge density limit of nondegenerate cases, the nonlinear transport reduces to linear like transport, which is the classical Einstein relation.

cond-mat.other↗
arXiv2017

Triplet Superfluidity on a triangular ladder with dipolar fermions

Motivated by recent experimental progress in the field of dipolar-Fermi gases, we investigate the quantum phases of dipolar fermions, on a triangular ladder at half filling. Using density matrix renormalization group method, in presence of onsite repulsion and intersite attractive interaction, we find exotic spin-triplet superfluid phase in addition to the usual spin-density and charge-density waves. We examine the stability of spin-triplet superfluid phase by varying hopping along the rungs of the triangle. Possibility of fermionic supersolidity has also been discussed, by considering three- body interaction in the Hamiltonian. We also study the effect of spin-dependent hopping on the stability of spin-triplet superfluid phase.

cond-mat.str-el↗
arXiv2016

Breakdown of electron-pairs in the presence of an electric field of a superconducting ring

The quantum dynamics of quasi-one-dimensional ring with varying electron filling factor is investigated in presence of external electric field. The system is modeled within Hubbard Hamiltonian with attractive Coulomb correlation, which results in superconducting ground state when away from half-filling. The electric field is induced by applying time-dependent Aharonov-Bohm flux in the perpendicular direction. To explore the non-equilibrium phenomena arising from the field, we adopt exact diagonalization and Crank-Nicolson numerical method. With increase in electric field strength, the electron pairs, a signature of superconducting phase, start breaking and the system enters into a metallic phase. However, the strength of the electric field for this quantum phase transition depends on the electronic correlation. This phenomenon has been confirmed by flux-quantization of time-dependent current and pair correlation functions

cond-mat.supr-con↗
arXiv2015

Quantum phases of hardcore bosons in two coupled chains: A density matrix renormalization group study

We consider hardcore bosons in two coupled chain of one dimensional lattices at half filling with repulsive intra-chain interaction and inter-chain attraction. This can be mapped on to a coupled chain of spin-1/2 XXZ model with inter chain ferromagnetic coupling. We investigate various phases of hardcore bosons (and related spin model) at zero temperature by density matrix renormalization group method. Apart from the usual superfluid and density wave phases, pairing of inter chain bosons leads to the formation of novel phases like pair-superfluid and density wave of strongly bound pairs. We discuss the possible experimental realization of such correlated phases in the context of cold dipolar gas.

cond-mat.str-el↗
arXiv2015

Linear and Nonlinear Optical Properties of Graphene Quantum Dots: A Computational Study

Due to the advantage of tunability via size, shape, doping and relatively low level of loss and high extent of spatial confinement, graphene quantum dots (GQDs) are emerging as an effective way to control light by molecular engineering. The collective excitation in GQDs shows both high energy plasmon frequency along with frequencies in the terahertz (THz) region making these systems powerful materials for photonic technologies. Here, we report a systematic study of the linear and nonlinear optical properties of large varieties of GQDs (400 systems) in size and topology utilizing the strengths of both semiempirical and first-principles methods. Our detailed study shows how the spectral shift and trends in the optical nonlinearity of GQDs depends on their structure, size and shape. Among the circular, triangular, stripe, and random shaped GQDs, we find that GQDs with inequivalent sublattice atoms always possess lower HOMO-LUMO gap, broadband absorption and high nonlinear optical coefficients. Also, we find that for majority of the GQDs with interesting linear and nonlinear optical properties have zigzag edges, although reverse is not always true. We strongly believe that our findings can act as guidelines to design GQDs in optical parametric oscillators, higher harmonic generators and optical modulators.

cond-mat.mtrl-sci↗
arXiv2012

Negative Differential Conductance in Nano-junctions: A Current Constrained Approach

A current constrained approach is proposed to calculate negative differential conductance in molecular nano-junctions. A four-site junction is considered where a steady-state current is forced by inserting only the two central sites within the circuit. The two lateral sites (representing e.g. dangling molecular groups) do not actively participate in transport, but exchange electrons with the two main sites. These auxiliary sites allow for a variable number of electrons within the junction, while, as required by the current constrained approach, the total number of electrons in the system is kept constant. We discuss the conditions for negative differential conductance in terms of cooperativity, variability of the number of electrons in the junction, and electron correlations.

cond-mat.mes-hall↗
arXiv2011

One-Dimensional Organometallic V-Anthracene Wire and Its B-N Analogue: Efficient Half-Metallic Spin Filters

Using density functional theory, we have investigated the structural, electronic and magnetic properties of infinitely periodic organometallic vanadium-anthracene ($[V_2Ant]_\infinity)$ and $[V_4(BNAnt)_2]_\infinity$(where BNAnt is B-N analogue of anthracene) for their possible application in spintronics. From our calculations, we find that one-dimensional $[V_2Ant]_\infinity$ and $[V_4(BNAnt)_2]_\infinity$ wires exhibit robust ferromagnetic half-metallic and metallic behavior, respectively. The finite sized $V_6Ant_2$ and $V_6(BNAnt)_2$ clusters are also found to exhibit efficient spin filter properties when coupled to graphene electrodes on either side.

cond-mat.mes-hall↗
arXiv2009

Edge reconstruction induces magnetic and metallic behavior in zigzag graphene nanoribbons

The edge reconstruction of zigzag graphene nanoribbons to a stable line of alternatively fused seven and five membered rings with hydrogen passivation has been studied within density functional theory with both localized and extended basis approximations. Reconstruction of both edges results in a nonmagnetic metallic ground state, whereas the one edge reconstruction stabilizes the system in a ferromagnetic metallic ground state. The reconstructed edge suppresses the local spin density of atoms and contributes finite density of states at Fermi energy. Our study paves a new way to fabricate the metallic electrodes for semiconducting graphene devices with full control over the magnetic behavior without any lattice mismatch between leads and the channel.

cond-mat.mtrl-sci↗