arXiv subjects

Saurabh Basu

Publications and source records attributed to Saurabh Basu.

At least 37 records · Page 2Linked to original sources

arXiv2024

Magnons on a dice lattice: topological features and transport properties

In this paper, we study the topological properties of magnons on a dice lattice, also known as the dual of a more widely studied kagome lattice. This structure has a central atom at the center of the honeycomb lattice, which leads to the formation of a flat band. Magnetic Hamiltonians associated with the magnon bands are scarcely studied in this flat band system, which motivates us on examining an interplay of different magnetic spin interactions, such as the Heisenberg exchange, Dzyaloshinskii-Moriya interaction (DMI), pseudodipolar interaction (PDI) and magnetocrystalline anisotropies in a dice lattice. In particular, the objective is to ascertain their roles in inducing various topological phases and the phase transitions therein. The competing effects of the DMI and the PDI in inducing transitions from either topological to topological or topological to trivial phases are noted and the corresponding results are supported via the magnon band structures, presence (or absence) of edge modes in a nanoribbon geometry, and the transport characteristic, namely the discontinuities in the thermal Hall conductivities. Meanwhile, the magnetocrystalline anisotropy also plays an intriguing role, where distinct (nonuniform) values at the different sublattice sites result in a richer topological landscape with Chern numbers $C = \pm 2$, $\pm 1$, and $0$, while a uniform anisotropy yields only $C = \pm 2$ and $0$. This discrepancy arises from the broken valley symmetry in the nonuniform case. Finally, we have enriched our understanding on the role of the flat band in impacting the topological features by comparing some of the key results with that of a honeycomb structure.

cond-mat.mes-hall↗

arXiv2024

Competing topological phases in a non-Hermitian time-reversal symmetry-broken Bernevig-Hughes-Zhang model

The Bernevig-Hughes-Zhang (BHZ) model, which serves as a cornerstone in the study of the quantum spin Hall insulators, showcases robust spin-filtered helical edge states in a nanoribbon geometry. In the presence of an in-plane magnetic field, these (first-order) helical states gap out to be replaced by second-order corner states under suitable open-boundary conditions. Here, we show that the inclusion of a spin-dependent non-Hermitian balanced gain/loss potential induces a competition between these first and second-order topological phases. Surprisingly, the previously dormant first-order helical edge states in the nanoribbon resurface as the non-Hermitian effect intensifies, effectively neutralizing the role played by the magnetic field. By employing the projected spin spectra and the spin Chern number, we conclusively explain the resurgence of the first-order topological properties in the time-reversal symmetry-broken BHZ model in presence of non-Hermiticity. Finally, the biorthogonal spin-resolved Berry phase, exhibiting a non-trivial winding, definitively establishes the topological nature of these revived edge states, emphasizing the dominance of non-Hermiticity over the magnetic field.

cond-mat.mes-hall↗

arXiv2024

Quasiperiodic disorder induced critical phases in a periodically driven dimerized $p$-wave Kitaev chain

The interplay of topology and disorder in non-equilibrium quantum systems is an intriguing subject. Here, we look for a suitable platform that enables an in-depth exploration of the topic. To this end, We analyze the topological and localization properties of a dimerized one-dimensional Kitaev chain in the presence of an onsite quasiperiodic potential with its amplitude being modulated periodically in time. The topological features have been explored via computing the real-space winding numbers corresponding to both the Majorana zero and the $π$ energy modes. We enumerate the scenario at different driving frequencies. In particular, at some intermediate frequency regime, the phase diagram concerning the zero mode involves two distinct phase transitions, one from a topologically trivial to a non-trivial phase, and another from a topological phase to an Anderson localized phase. On the other hand, the study of the $π$ modes reveals the emergence of a unique topological phase, with the bulk and the edges being fully localized, which may be called as the Floquet topological Anderson phase. Furthermore, we study the localization properties of the bulk states by computing the inverse and normalized participation ratios, while the critical phase is ascertained by computing the fractal dimension. We have observed extended, critical, and localized phases at intermediate frequencies, which are further confirmed via a finite-size scaling analysis. Finally, fully extended and localized phases are respectively observed at lower and higher frequencies.

cond-mat.mes-hall↗

arXiv2024

Holstein polaron in a pseudospin-$1$ quantum spin Hall system: first and second order topological phase transitions

We theoretically propose the occurrence of a quantum spin Hall (QSH) and a second order topological phase transition (TPT) driven by electron-phonon (e-p) coupling in a pseudospin-$1$ fermionic system on an $α$-$T_3$ lattice. Our model is formulated in the spirit of the Kane-Mele model modified by the Holstein Hamiltonian. The Lang-Firsov approach is employed to describe polarons reasonably well in the anti-adiabatic (high frequency) limit and to obtain an effective electronic Hamiltonian. It is shown that the system possesses topologically nontrivial phases up to a critical e-p coupling, $λ_c$ and are characterized by the helical QSH edge states along with a non-zero $\mathbb{Z}_2$ invariant for a certain range of $α$. The topological phase vanishes beyond $λ_c$ and is accompanied by a bulk gap closing transition at $λ_c$, manifesting a TPT. We observe a more intriguing phenomenon for higher values of $α$, where the system exhibits TPTs supported by two distinct gap closing transitions at $λ_{c_1}$ and $λ_{c_2}$, while a slim region at slightly lower values hosts a semi-metallic signature below $λ_{c_1}$. Subsequently, to explore more intricate features, we introduce a time reversal symmetry breaking magnetic field to trigger the formation of a second order topological phase. The magnetic field, by construction causes a boundary dependent gapping out of the edge states, consequently giving rise to robust corner modes in a tailored open boundary conditions. We justify the formation of the higher order phase by employing an appropriate invariant, namely the projected spin Chern number. Finally, we show that the e-p coupling significantly influences the corner modes (and also the real space energy bandstructure), corroborating a higher order TPT as we tune $λ$ beyond a critical value for a given value of $α$.

cond-mat.mes-hall↗

arXiv2024

Studying magnon band topology through low-energy magnon excitations: role of anisotropic Dzyaloshinskii-Moriya interaction

In this work, we study topological properties of magnons via creating spin excitations in both ferromagnets and antiferromagnets in presence of an external magnetic field on a two-dimensional square lattice. It is known that Dzyaloshinskii-Moriya interaction (DMI) plays an important role in coupling between different particle (spin excitation) sectors, here we consider an anisotropic DMI and ascertain the role of the anisotropy parameter in inducing topological phase transitions. While the scenario, for dealing with ferromagnets, albeit with isotropic DMI is established in literature, we have developed the formalism for studying magnon band topology for the antiferromagnetic case. The calculations for the ferromagnetic case are included to facilitate a comparison between the two magnetically ordered systems. Owing to the presence of a two-sublattice structure of an antiferromagnet, a larger number of magnon bands participate in deciding upon the topological properties. However, in both the cases, an extended trivial region is observed even with the DMI to be non-zero, which is surprising since the DMI is the origin of the finite Berry curvature in presence of external magnetic field. Furthermore, in an antiferromagnet, a smaller anisotropy is capable of inducing a gap-closing transition from a topological to a trivial phase compared to that for a ferromagnet. The nature of the phases in both the cases and the phase transitions therein are characterized by the band structure, presence (or absence) of the chiral edge modes observed in a semi-infinite nano-ribbon geometry, computation of the thermal Hall effect, etc. Moreover, the strength of the magnetic field is found to play a decisive role in controlling the critical point that demarcates various topological phases.

cond-mat.mes-hall↗

arXiv2024

Depth-Resolved Characterization of Centrifugal Disk Finishing of Additively Manufactured Inconel 718

Surface characteristics are a major contributor to the in-service performance, particularly fatigue life, of additively manufactured (AM) components. Centrifugal disk finishing (CDF) is one of many rigid media, abrasive machining processes employed to smooth the surfaces and edges of AM components. Within the general family of abrasive machining processes currently applied to AM, CDF is moderate in terms of material removal rate and the inertial forces exerted. How CDF alters the underlying microstructure of the processed surface is currently unknown. Here we employ white light profilometry and high-energy X-ray diffraction to characterize surface finish, crystallographic texture, and anisotropic distributions of residual microscale strain as a function of depth in CDF-finished Inconel 718 manufactured with laser powder bed fusion. Surfaces are finished using both unimodal and bimodal finishing media size distributions. We find that CDF will remove surface crystallographic textures (here a {111} fiber texture) from AM components, but generally not alter the bulk texture (here a cube texture). CDF is also found to impart significant amounts of residual microscale strain into the first 100 $μ$m from the sample surface as evidenced by an approximately 50% increase in diffraction peak widths at 20 $μ$m from the surface in comparison to 120 $μ$m.

physics.app-ph↗

arXiv2024

Quasiperiodic potential induced corner states in a quadrupolar insulator

We systematically investigate the topological and localization properties of a quadrupolar insulator represented by the celebrated Benalcazar-Bernevig-Hughes model in presence of a quasiperiodic disorder instilled in its hopping amplitude. While disorder can be detrimental to the existence of the topological order in a system, we observe the emergence of a disorder driven topological phase where the original (clean) system demonstrates trivial behavior. This phenomenon is confirmed by the re-emergence of zero energy states in the bandstructure together with a non-zero bulk quadrupole moment, which in turn establishes the bulk boundary correspondence (BBC). Furthermore, the distribution of the excess electronic charge shows a pattern that is reminiscent of the bulk quadrupole topology. To delve into the localization properties of the mid-band states, we compute the inverse participation and normalized participation ratios. It is observed that the in-gap states become critical (multifractal) at the point that discerns a transition from a topological localized to a trivial localized phase. Finally, we carry out a similar investigation to ascertain the effect of the quasiperiodic disorder on the quadrupolar insulator when the model exhibits topological properties in the absence of disorder. Again, we note a multifractal behavior of the eigenstates in the vicinity of the transition.

cond-mat.mes-hall↗

arXiv2024

Parsing skin effect in a non-Hermitian spinless BHZ-like model

This work comprehensively investigates the non-Hermitian skin effect (NHSE) in a spinless Bernevig- Hughes-Zhang (BHZ)-like model in one dimension. It is generally believed that a system with non-reciprocal hopping amplitudes demonstrates NHSE. However, we show that there are exceptions, and more in-depth analyses are required to decode the presence of NHSE or its variants in a system. The fascinating aspects of our findings, depending on the inclusion of non-reciprocity in the inter-orbital hopping terms, concede the existence of conventional NHSE or NHSE at both edges and even a surprising absence of NHSE. The topological properties and the (bi-orthogonal) bulk-boundary correspondence, enumerated via computation of the (complex) Berry phase and spatial localization of the edge modes, highlight the topological phase transitions occurring therein. Further, to facilitate a structured discussion of the non-Hermitian model, we split the results into PT symmetric and non-PT symmetric cases with a view to comparing the two.

cond-mat.mes-hall↗

arXiv2024

Electron-phonon coupling induced topological phase transition in an $α$-$T_{3}$ Haldane-Holstein model

We present impelling evidence of topological phase transitions induced by electron-phonon (e-ph) coupling in an $α$-$T_3$ Haldane-Holstein model that presents smooth tunability between graphene ($α=0$) and a dice lattice $(α=1)$. The e-ph coupling has been incorporated via the Lang-Firsov transformation which adequately captures the polaron physics in the high frequency (anti-adiabatic) regime, and yields an effective Hamiltonian of the system through zero phonon averaging at $T=0$. While exploring the signature of the phase transition driven by polaron and its interplay with the parameter $α$, we identify two regions based on the values of $α$, namely, the low to intermediate range $(0 < α\le 0.6)$ and larger values of $α~(0.6 < α< 1)$ where the topological transitions show distinct behaviour. There exists a single critical e-ph coupling strength for the former, below which the system behaves as a topological insulator characterized by edge modes, finite Chern number, and Hall conductivity, with all of them vanishing above this value, and the system undergoes a spectral gap closing transition. Further, the critical coupling strength depends upon $α$. For the latter case $(0.6 < α< 1)$, the scenario is more interesting where there are two critical values of the e-ph coupling at which trivial-topological-trivial and topological-topological-trivial phase transitions occur for $α$ in the range $[0.6:1]$. Our studies on e-ph coupling induced phase transitions show a significant difference with regard to the well-known unique transition occurring at $α= 0.5$ (or at $0.7$) in the absence of the e-ph coupling, and thus underscore the importance of interaction effects on the topological phase transitions.

cond-mat.mes-hall↗

arXiv2024

Circuit realisation of a two-orbital non-Hermitian tight-binding chain

We examine a non-Hermitian (NH) tight-binding system comprising of two orbitals per unit cell and their electrical circuit analogues. We distinguish the PT-symmetric and non-PT symmetric cases characterised by non-reciprocal nearest neighbour couplings and onsite gain/loss terms, respectively. The localisation of the edge modes or the emergence of the topological properties are determined via the maximum inverse participation ratio, which has distinct dependencies on the parameters that define the Hamiltonian. None of the above scenarios exhibits the non-Hermitian skin effect. We investigate the boundary modes corresponding to the topological phases in a suitably designed electrical circuit by analyzing the two-port impedance and retrieve the admittance band structure of the circuit via imposing periodic boundary conditions. The obtained results are benchmarked against the Hermitian version of the two-orbital model to compare and discriminate against those obtained for the NH variants.

cond-mat.mes-hall↗

arXiv2023

Wannier charge center, spin resolved bulk polarization and corner modes in a strained quantum spin Hall insulator

Topological invariants are a significant ingredient in the study of topological phases of matter that intertwines the supposedly contradicting concepts of bulk and boundary. The nature of the invariants differ depending on the dimension of the boundary at which the topological states manifest themselves. The primary motivation of this work is to study two distinct scenarios of topological phase, differing in the dimensionality of their boundary states and study the associated bulk topological invariants that characterize them. In this regard, we study the band engineered Kane Mele model which originally is a prototypical example of a system that hosts quantum spin Hall effect on a honeycomb lattice. Under a smooth band deformation caused by varying one of the nearest neighbor hopping amplitudes (say $t_1$) as compared to the other two (say $t$), we observe that the system transits from its first order topological insulating state (or quantum spin Hall state) to a second order topological insulating (SOTI) state via a gap closing transition. This transition occurs when the system crosses a particular threshold of the deformation parameter $t_1\mathbin{/}t$ (namely $t_1\mathbin{/}t=2$). We show the presence of edge and corner modes as a signature of first and second order topology respectively. Further, we observe the evolution of the Wannier charge center (WCC), a bulk property as a function of the deformation parameter ${t_1}\mathbin{/}{t}$. We also find that, while the $\mathbb{Z}_2$ invariant successfully characterizes the QSH state, it cannot characterize higher order topology (second order here). The model being mirror invariant, we also calculate mirror winding number to show that it is rendered trivial in the SOTI phase as well, while being non-trivial in the QSH phase. Finally, spin resolved bulk polarization establishes the SOTI phase as obstructed atomic insulator.

cond-mat.mes-hall↗

arXiv2023

Ergodicity-breaking phase diagram and fractal dimensions in long-range models with generically correlated disorder

Models with correlated disorders are rather common in physics. In some of them, like the Aubry-André (AA) model, the localization phase diagram can be found from the (self)duality with respect to the Fourier transform. In the others, like the all-to-all translation-invariant Rosenzweig-Porter (TI RP) ensemble or the Hilbert-space structure of the many-body localization, one needs to develop more sophisticated and usually phenomenological methods to find the localization transition. In addition, such models contain not only localization but also the ergodicity-breaking transition, giving way to the non-ergodic extended phase of states with non-trivial fractal dimensions $D_q$. In this work, we suggest a method to calculate both the above transitions and a lower bound to the fractal dimensions $D_2$ and $D_\infty$, relevant for physical observables. In order to verify this method, we apply it to the class of long-range (self-)dual models, interpolating between AA and TI RP ones via both power-law dependences of on-site disorder correlations and hopping terms, and, thus, being out of the validity range of the previously developed methods. We show that the interplay of the correlated disorder and the power-law decaying hopping terms leads to the emergence of the two types of fractal phases in an entire range of parameters, even without having any quasiperiodicity of the AA potential. The analytical results of the above method are in full agreement with the extensive numerical calculations.

cond-mat.dis-nn↗

arXiv2023

Properties of an α-T3 Aharonov-Bohm quantum ring: Interplay of Rashba spin-orbit coupling and topological defect

In this paper we investigate the interplay of the Rashba spin-orbit coupling (RSOC) and a topological defect, such as a screw dislocation in an α-T3 Aharonov-Bohm quantum ring and scrutinized the effect of an external transverse magnetic field therein. Our study reveals that the energy spectrum follows a parabolic dependence on the Burgers vector associated with the screw dislocation. Moreover, its presence results in an effective flux, encompassing the ramifications due to both the topological flux and that due to the external magnetic field. Furthermore, we observe periodic oscillations in the persistent current in both charge and spin sectors, with a period equal to one flux quantum, which, however suffers a phase shift that is proportional to the dislocation present in the system. Such tunable oscillations of the spin persistent current highlights potential application of our system to be used as spintronic devices. Additionally, we derive and analyse the thermodynamic properties of the ring via obtaining the canonical partition function through Euler-Maclaurin formula. In particular, we compute the thermodynamic potentials, free energies, entropy, and heat capacity and found the latter to yield the expected Dulong-Petit law at large temperatures.

cond-mat.mes-hall↗

arXiv2023

Effect of magnetic field on the electronic properties of an $α$-$T_3$ ring

We consider a quantum ring of a certain radius R built from a sheet of the $α$-$T_3$ lattice and solve for its spectral properties in presence of an external magnetic field. The energy spectrum consists of a conduction band, a valence band and a zero energy flat band, all having a number of discrete levels therein which can be characterized by the angular momentum quantum number, m. The energy levels in the flat band are infinitely degenerate irrespective of the value of $α$. We reveal a two-fold degeneracy of the levels in the conduction band as well as in the valence band for $α$ = 0 and $α$ = 1. However, the m = 0 level for $α$ = 1 is an exception. Corresponding to an intermediate value of $α$, namely, 0 <$α$< 1, the energy levels become nondegenerate. The scenario remains unaltered when the ring is threaded by a magnetic flux which is an integer multiple of the flux quantum. We also calculate the persistent current which exhibits quantum oscillations as a function of the magnetic field with a period of one flux quantum at a particular Dirac point, which is often referred to as a valley. The total current oscillates with a periodicity of one flux quantum for any intermediate value of $α$. We have also explored the effect of a mass term (that breaks the sublattice symmetry) in the Hamiltonian. In the absence of a magnetic field, the energy levels in the flat band become dispersive, except for the m = 0 level in the case of $α$ = 1. In presence of the field, each of the flat band levels becomes dispersive for any $α\neq$ 0. Finally, we also see the effect of the mass term on the behaviour of the persistent current, which shows periodicity of one flux quantum, but the total current remains finite for all values of $α$.

cond-mat.mes-hall↗

arXiv2023

Tunable persistent currents in a spin-orbit coupled pseudospin-1 fermionic quantum ring

We conduct a thorough study of the persistent currents in a spin-orbit coupled $α-T_3$ pseudospin-1 fermionic quantum ring (QR) that smoothly interpolates between graphene ($α = 0$, pseudospin-1/2) and a dice lattice ($α = 1$, pseudospin-1). In particular, we have considered both intrinsic spin-orbit coupling (ISOC) and Rashba spin-orbit coupling (RSOC) in addition to an external magnetic field, and have systematically enumerated their individual and combined effects on the charge, valley and the spin-polarized persistent currents. The energy levels of the system comprise of the conduction bands, valence bands, and flat bands which show non-monotonic dependencies on the ring radius, R of the QR, in the sense that, for small R, the energy levels vary as 1/R, while the variation is linear in R for large R. The cases corresponding to zero magnetic fields are benchmarked with those for finite external fields. Further, it is noted that the flat bands demonstrate dispersive behavior, and hence can contribute to the transport properties only when ISOC is non-zero. Moreover, the RSOC yields spin-split bands, thereby contributing to the spin-resolved currents, together with distinct degeneracies for different spin branches. The persistent currents in the charge, valley, and spin sectors for each of these cases oscillate as a function of the magnetic field with a period equal to the flux quantum, as they should be, and depend upon the spin-orbit coupling terms. Further, we have explored the role played by the parameter $α$ in our entire analysis to ascertain the effect of the flat bands.

cond-mat.mes-hall↗

arXiv2023

Higher order topology in a band deformed Haldane model

Haldane model is a celebrated tight binding toy model in a 2D honeycomb lattice that exhibits quantized Hall conductance in the absence of an external magnetic field. In our work, we deform the bands of the Haldane model smoothly by varying one of its three nearest neighbour hopping amplitudes ($t_1$), while keeping the other two ($t$) fixed. This breaks the $C_3$ symmetry of the Hamiltonian, while the $M_x*T$ symmetry is preserved. The symmetry breaking causes the Dirac cones to shift from the K and the K' points in the Brillouin zone (BZ) to an intermediate M point. This is evident from the Berry curvature plots which show a similar shift in the corresponding values as a function of $\frac{t_1}{t}$. We observe two different topological phases, one being a topological insulator (TI) phase and the other is a higher order topological insulator (HOTI). The Chern number ($C$) remains perfectly quantized at a value of $C=1$ for the TI phase and goes to zero in the HOTI phase. Furthermore the evolution of the Wannier charge center (WCC) as the band is deformed shows a jump in the TI phase indicating a non-trivial bulk. We also study the HOTI phase and diagonalize the real space Hamiltonian on a rhombic supercell to show the presence of in-gap zero energy corner modes. The polarization of the system, namely $p_x$ and $p_y$, are evaluated, along the $x$ and the $y$ directions respectively. We see that both $p_x$ and $p_y$ are quantized in the HOTI phase owing to the presence of the inversion symmetry of the system.

cond-mat.mes-hall↗

arXiv2023

Band engineered bilayer Haldane model: Evidence of multiple topological phase transitions

We have studied the evolution of the topological properties of a band-engineered AB-stacked bilayer honeycomb structure in the presence of a Haldane flux. Without a Haldane flux, band engineering makes the band touching points (the so-called Dirac points) move towards each other and eventually merge into one at an intermediate $\mathbf{M}$ point in the Brillouin zone. Here the dispersion is linear along one direction and quadratic along the other. In the presence of a Haldane flux, the system acquires topological properties, and finite Chern numbers can be associated with the pairs of the conduction and the valence bands. The valence band closer to the Fermi level ($E_F$) possesses Chern numbers equal to $\pm2$ and $\pm1$, while the one further away from $E_F$ corresponds to Chern numbers $\pm1$. The conduction bands are associated with similar properties, except their signs are reversed. The Chern lobes shrink in the band-engineered model, and we find evidence of multiple topological phase transitions, where the Chern numbers discontinuously jump from $\pm2$ to $\mp2$, $\pm1$ to $\mp1$, $\pm1$ to $0$ to $\pm2$ and $\pm2$ to $\pm1$. These transitions are supported by the presence or absence of the chiral edge modes in a nanoribbon bilayer geometry and the vanishing of the plateau in the anomalous Hall conductivity. Different phases are further computed for different hopping amplitudes across the layers, which shows the shrinking of the Chern lobes for large interlayer tunneling.

cond-mat.mes-hall↗

arXiv2023

Topological properties of a periodically driven Creutz ladder

We have investigated a periodically driven Creutz ladder in presence of two different driving protocols, namely, a sinusoidal drive and a $δ$-kick imparted to the ladder at regular intervals of time. Specifically, we have studied the topological properties corresponding to the trivial and the non-trivial limits of the static (undriven) case via computing suitable topological invariants. Corresponding to the case where the chiral symmetry is intact, in addition to the zero energy modes, $π$ energy modes appear in both these cases. Further, two different frequency regimes of the driving protocol emerge, where Floquet-Magnus expansion is particularly employed to study the high frequency regime for the sinusoidal drive. Apart from the physics being identical in the high frequency and the static scenarios, the zero energy modes show distinctive features at low and high frequencies. For the sinusoidal drive, there exists a sharp frequency threshold beyond which the zero energy mode only exists in the topological limit, while in the trivial limit, it exists only upto the threshold frequency. In presence of the $δ$-kick, the Creutz ladder demonstrates higher values of the topological invariant, and as a consequence the system possesses large number of edge modes.

cond-mat.mes-hall↗