arXiv subjects

Sanjoy Datta

Publications and source records attributed to Sanjoy Datta.

At least 19 recordsLinked to original sources

arXiv2025

Topological boundaries in non-Hermitian p-wave Kitaev chains with Rashba spin-orbit coupling

In this work, we investigate the combined effects of Rashba spin-orbit coupling (RSOC) and non-Hermiticity on topological phase transitions in spinful p-wave Kitaev chains. While previous studies have separately examined non-Hermitian (NH) extensions of Kitaev chains and the effects of RSOC in Hermitian systems, the interplay between these two mechanisms remains largely unexplored. We analyze this interplay by considering two distinct types of complex on-site potentials: (i) a uniform gain/loss term and (ii) a complex quasiperiodic potential. We demonstrate that the impact of RSOC is highly model-dependent. In particular, RSOC does not affect the topological phase boundary in the Hermitian limit of the uniform gain/loss model (provided the spin-flip hopping is weaker than the pairing strength), but significantly alters the topological landscape in the NH regime. In contrast, for the quasiperiodic model, RSOC modifies the phase boundaries in both the Hermitian and non-Hermitian cases. Notably, we find that the combined interplay of non-Hermiticity and RSOC drives topological transitions at significantly lower potential strengths compared to the Hermitian limit. We derive analytical expressions for the topological phase transitions in both cases and validate our predictions through numerical calculations of energy spectra and real-space winding numbers. This work provides a comprehensive understanding of how non-Hermiticity and RSOC cooperatively reshape topological phase diagrams in one-dimensional superconducting systems.

cond-mat.dis-nn↗

arXiv2025

Emergence of non-trivial phases in interacting non-Hermitian quasiperiodic chains with power-law hopping

In the last few years, several works have identified the concurrence of the spectral, delocalization-localization and topological phase transitions in non-Hermitian quasiperiodic systems in the presence of time-reversal symmetry (TRS), with or without interaction. In this work, we investigate one-dimensional interacting non-Hermitian quasiperiodic lattices with asymmetric power-law hopping and unveil that although the Hamiltonian respects the TRS, the reality of the eigenspectrum does not necessarily indicate a topologically trivial non-Hermitian many-body localization (NHMBL) regime. In fact, we reveal the emergence of a topologically trivial intermediate regime, where the states that are primarily multifractal in nature can also possess a fully real spectrum, thereby restoring the TRS before crossing over to the NHMBL phase. Moreover, in the entire intermediate regime, the interaction completely destroys the multifractal and mobility edges observed in the non-interacting counterpart. Besides, we unveil that due to the long-range nature of the hopping, the entire topologically non-trivial ergodic regime under the periodic boundary condition does not always give rise to boundary localized skin modes under the open boundary condition. Our findings thus advances and deepens the understanding about the emergence of non-trivial phases due to the interplay of interaction and long-range hopping in non-Hermitian quasiperiodic systems.

cond-mat.dis-nn↗

arXiv2024

The fate of Wannier-Stark localization and skin effect in periodically driven non-Hermitian quasiperiodic lattices

The eigenstates of one-dimensional Hermitian and non-Hermitian tight-binding systems (in the presence/absence of quasiperiodic potential) and an external electric field undergo complete localization with equally spaced eigenenergies, known as the Wannier-Stark (WS) localization. In this work, we demonstrate that when the electric field is slowly modulated with time, new non-trivial phases with multiple mobility edges emerge in place of WS localized phase, which persists up to a certain strength of the non-Hermiticity. On the other hand, for a large driving frequency, we retrieve the usual sharp delocalization-localization transition to the usual (no WS) localized phase, similar to the static non-Hermitian Aubry-André-Harper type without any electric field. This vanishing of WS localization can be attributed solely to the time-periodic drive and occurs irrespective of the non-Hermiticity. Interestingly, under the open boundary condition (OBC), we find that contrary to the undriven systems where an external electric field destroys the SE completely, the SE appears in certain regime of the parameter space when the electric field is temporally driven. This appearance of SE is closely related to the absence of extended unitarity. In addition, in the presence of the drive, the skin states are found to be multifractal, contrary to its usual nature in such non-Hermitian systems. An in-depth understanding about the behavior of the states in the driven system is established from the long-time dynamics of an initial excitation.

cond-mat.dis-nn↗

arXiv2024

Engineering unique localization transition with coupled Hatano-Nelson chains

The Hatano-Nelson (HN) Hamiltonian has played a pivotal role in catalyzing research interest in non-Hermitian systems, primarily because it showcases unique physical phenomena that arise solely due to non-Hermiticity. The non-Hermiticity in the HN Hamiltonian, driven by asymmetric hopping amplitudes, induces a delocalization-localization (DL) transition in a one-dimensional (1D) lattice with random disorder, sharply contrasting with its Hermitian counterpart. A similar DL transition occurs in a 1D quasiperiodic HN (QHN) lattice, where a critical quasiperiodic potential strength separates metallic and insulating states, akin to the Hermitian case. In these systems, all states below the critical potential are delocalized, while those above are localized. In this study, we reveal that coupling two 1D QHN lattices can significantly alter the nature of the DL transition. We identify two critical points, $V_{c1} < V_{c2}$, when the nearest neighbors of the two 1D QHN lattices are cross-coupled with strong hopping amplitudes under periodic boundary conditions (PBC). Generally, all states are completely delocalized below $ V_{c1}$ and completely localized above $V_{c2}$, while two mobility edges symmetrically emerge about $\rm{Re[E]} = 0$ between $V_{c1}$ and $V_{c2}$. Notably, under specific asymmetric cross-hopping amplitudes, $V_{c1}$ approaches zero, resulting in localized states even for infinitesimally weak potential. Remarkably, we also find that the mobility edges precisely divide the delocalized and localized states in equal proportions. Furthermore, we observe that the conventional one-to-one correspondence between electronic states under PBC and open boundary conditions (OBC) in 1D HN lattices breaks

cond-mat.dis-nn↗

arXiv2024

Extended unitarity and absence of skin effect in periodically driven systems

One of the most striking features of non-Hermitian quasiperiodic systems with arbitrarily small asymmetry in the hopping amplitudes and open boundaries is the accumulation of all the bulk eigenstates at one of the edges of the system, termed in literature as the skin effect, below a critical strength of the potential. In this Letter, we uncover that a time-periodic drive in such systems can eliminate the SE up to a finite strength of this asymmetry. Remarkably, the critical value for the onset of SE is independent of the driving frequency and approaches to the static behavior in the thermodynamic limit. We find that the absence of SE is intricately linked to the emergence of extended unitarity in the delocalized phase, providing dynamical stability to the system. Interestingly, under periodic boundary condition, our non-Hermitian system can be mapped to a Hermitian analogue in the large driving frequency limit that leads to the extended unitarity irrespective of the hopping asymmetry and the strength of the quasiperiodic potential, in stark contrast to the static limit. Additionally, we numerically verify that this behavior persists Based on our findings, we propose a possible experimental realization of our driven system, which could be used as a switch to control the light funneling mechanism.

cond-mat.dis-nn↗

arXiv2023

Rashba splitting in polar-nonpolar sandwich heterostructure : A DFT Study

In this study, we employ density functional theory (DFT) based first-principles calculations to investigate the spin-orbit effects in the electronic structure of a polar-nonpolar sandwich heterostructure namely LAO$_{2.5}$/STO$_{5.5}$/LAO$_{2.5}$. Our focus on the Ti-3d bands reveals an inverted ordering of the STO-$\rm t_{2g}$ orbital near the n-type interface, consistent with earlier experimental work. In contrast, toward the p-type interface, the orbital ordering aligns with the natural ordering of STO orbitals, influenced by crystal field splitting. Interestingly, we have found a strong inter-orbital coupling between $t_{2g}$ and $e_g$ orbital, which has not been reported earlier in $\rm SrTiO_3$ based 2D system. Additionally, our observations highlight that the cubic Rashba splitting in this system surpasses the linear Rashba splitting, contrary to experimental findings. This comprehensive analysis contributes to a refined understanding of the role of orbital mixing in Rashba splitting in the sandwich oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2023

Evidence of linear and cubic Rashba effect in non-magnetic heterostructure

The LaAlO3/KTaO3 system serves as a prototype to study the electronic properties that emerge as a result of spin-orbit coupling. In this article, we have used first-principles calculations to systematically study two types of defect-free (0 0 1) interfaces, which are termed as Type-I and Type-II. While the Type-I heterostructure produces a two-dimensional electron gas, the Type-II heterostructure hosts an oxygen-rich two-dimensional hole gas at the interface. Furthermore, in the presence of intrinsic spin-orbit coupling, we have found evidence of both cubic and linear Rashba interactions in the conduction bands of the Type-I heterostructure. On the contrary, there is spin-splitting of both the valence and the conduction bands in the Type-II interface, which are found to be only linear Rashba type. Interestingly, the Type-II interface also harbours a potential photocurrent transition path, making it an excellent platform to study the circularly polarized photogalvanic effect.

cond-mat.mtrl-sci↗

arXiv2023

Localization transitions in non-Hermitian quasiperiodic lattice

The delocalization-localization (DL) transition in non-Hermitian systems exhibits intriguing features distinct from their Hermitian counterparts. In this study, we investigate the DL transition in a generalized non-Hermitian lattice with asymmetric hopping and complex quasi-periodic potential. Irrespective of the boundary conditions, the lattice undergoes a DL transition at a critical strength of the quasiperiodic potential with identical modulation of its real and complex parts. For periodic boundary conditions (PBC), we obtained an analytical expression that accurately predicts this critical point. Our numerical results indicate that the critical point remains the same with the open boundary condition (OBC) as well. Interestingly, we observe that a difference in the modulation of the real and the complex part of potential leads to a mixed phase that appears between the delocalized and the localized phases. Intriguingly, within the mixed state region, we observed a coexistence of skin modes and localized states in the case of OBC, while in the case of PBC, a mixed phase is created by a coexistence of delocalized and localized states. We mapped out the phase diagrams for different scenarios offering valuable insights into the role of different parameters in a wide class of non-Hermitian quasiperiodic lattices.

cond-mat.dis-nn↗

arXiv2022

Skin effect and dynamical delocalization in non-Hermitian quasicrystals with spin-orbit interaction

The investigations of the spectral and dynamical delocalization-localization (DL) transition have revealed intriguing features in a wide range of non-Hermitian systems. The present study aims at exploring the spectral and dynamical properties in a non-Hermitian quasiperiodic system with asymmetric hopping in the presence of Rashba Spin-Orbit (RSO) interaction. In particular, in such systems, we have identified that the DL transition is associated with a concurrent change in the energy spectrum, where the eigenstates always break the time-reversal symmetry for all strenghts of the quasiperiodic potential, contrary to the systems without RSO interaction. Remarkably, we find that the reality of energy spectrum under the open boundary condition that is frequently symbolised as a hallmark of the skin-effect, is a system-size dependent phenomena, and appears even when the associated energies are indeed complex. In addition, it is demonstrated that the spin-flip term in the RSO interaction in fact possesses a tendency to diminish the directionality of the skin-effect. On scrutinizing the dynamical attributes in our non-Hermitian system, we unveil that in spite of the fact that the spectral DL transition accords with the dynamical phase transition, interestingly, the system comes across hyper-diffusive and negative diffusion dynamical regimes depending upon the strength of the RSO interaction, in the spectrally localized regime.

cond-mat.dis-nn↗

arXiv2022

Effect of spin-orbit coupling in one-dimensional quasicrystals with power-law hopping

In the one-dimensional quasiperiodic Aubry-André-Harper Hamiltonian with nearest-neighbor hopping, all single-particle eigenstates undergo a phase transition from ergodic to localized states at a critical disorder strength $W_c/t = 2.0$. There is no mobility edge in this system. However, in the presence of power-law hopping having the form $1/r^a$, beyond a critical disorder strength mobility edge appears for $a > 1$, while, for $0< a\leq 1$, a multifractal edge separates the extended and the multifractal states. In both these limits, depending on the strength of the disorder, lowest $β^s L$ states are delocalized. We have found that, in the presence of the spin-orbit coupling, the critical disorder strength is always larger irrespective of the value of the parameter $a$. Furthermore, we demonstrate that for $a\leq 1$, in the presence of spin-orbit coupling, there exists multiple multifractal edges, and the energy spectrum splits up into alternative bands of delocalized and multifractal states. Moreover, the location of the multifractal edges are generally given by the fraction $(β^s \pm β^m)$. The qualitative behavior of the energy spectrum remains unaffected for $a > 1$. However, in contrast to the previously reported results, we find that in this limit, similar to the other case, multiple mobility edges can exist with or without the spin-orbit coupling.

cond-mat.dis-nn↗

arXiv2022

Half-metallicity and two-dimensional hole gas at the $\text{BiFeO}_\text{3}$(001) surface

The electronic structure and thermodynamic stability of tetragonal $\rm{BiFeO_3}$(001) surfaces have been investigated using density functional theory. In this work, four different structures having different lattice constants with two possible surface terminations have been studied. We have found that the surface electronic structure and the thermodynamic stability is quite sensitive with respect to the nature of the surface termination. The $\rm{FeO_2}$ terminated surface is found to be energetically more stable compared to $\rm{BiO}$ terminated surface in all the cases. Interestingly, we have found evidences of half-metallicity and spin-polarized two-dimensional hole gas (2DHG) at the one mono-layer thick surface in all the structures. The effect of the surface thickness have been systematically studied. It has been demonstrated that the half-metallic 2DHG survives only in one of the structures for all the thicknesses, incidentally, which is the most thermodynamically stable structure.

cond-mat.mtrl-sci↗

arXiv2021

Signature of half-metallicity in $\text{BiFeO}_\text{3}$

$\text{BiFeO}_\text{3}$ has drawn a great attention over last several decades due to its promising multiferroic character. In the ground state the bulk $\text{BiFeO}_\text{3}$ is found to be in the rhombohedral phase. However, it has been possible to stabilize $\text{BiFeO}_\text{3}$ with tetragonal structure. The importance of tetragonal phase is due to its much larger value of the electric polarization and the possible stabilization of ferromagnetism as in the rhombohedral phase. Furthermore, the tetragonal structure of $\text{BiFeO}_\text{3}$ has been reported with different $c/a$ ratio, opening up the possibility of a much richer set of electronic phases. In this work, we have used density functional theory based first-principle method to study the ferromagnetic phase of the tetragonal $\text{BiFeO}_\text{3}$ structure as a function of the $c/a$ ratio. We have found that as the $c/a$ ratio decreases from $1.264$ to $1.016$, the tetragonal $\text{BiFeO}_\text{3}$ evolve from a ferromagnetic semiconductor to a ferromagnetic metal, while passing through a \emph{half-metallic} phase. This evolution of the electronic properties becomes even more interesting when viewed with respect to the volume of each structure. The most stable half-metallic phase initially counter-intuitively evolve to the magnetic-semiconducting phase with a reduction in the volume, and after further reduction in the volume it finally becomes a metal. So far, this type of metal to insulator transition on compression was known to exist only in alkali metals, especially in Lithium, in heavy alkaline earth metals, and in some binary compound.

cond-mat.mtrl-sci↗

arXiv2021

Localization, $\mathcal{PT}$-Symmetry Breaking and Topological Transitions in non-Hermitian Quasicrystals

According to the topological band theory of a Hermitian system, the different electronic phases are classified in terms of topological invariants, wherein the transition between the two phases characterized by a different topological invariant is the primary signature of a topological phase transition. Recently, it has been argued that the delocalization-localization transition in a quasicrystal, described by the non-Hermitian $\mathcal{PT}$-symmetric extension of the Aubry-André-Harper (AAH) Hamiltonian can also be identified as a topological phase transition. Interestingly, the $\mathcal{PT}$-symmetry also breaks down at the same critical point. However, in this article, we have shown that the delocalization-localization transition and the $\mathcal{PT}$-symmetry breaking are not connected to a topological phase transition. To demonstrate this, we have studied the non-Hermitian $\mathcal{PT}$-symmetric AAH Hamiltonian in the presence of Rashba Spin-Orbit (RSO) coupling. We have obtained an analytical expression of the topological transition point and compared it with the numerically obtained critical points. We have found that, except in some special cases, the critical point and the topological transition point are not the same. In fact, the delocalization-localization transition takes place earlier than the topological transition whenever they do not coincide.

cond-mat.dis-nn↗

arXiv2021

Self-duality of One-dimensional Quasicrystals with Spin-Orbit Interaction

Non-interacting spinless electrons in one-dimensional quasicrystals, described by the Aubry-André-Harper (AAH) Hamiltonian with nearest neighbour hopping, undergoes metal to insulator transition (MIT) at a critical strength of the quasi-periodic potential. This transition is related to the self-duality of the AAH Hamiltonian. Interestingly, at the critical point, which is also known as the self-dual point, all the single particle wave functions are multifractal or non-ergodic in nature, while they are ergodic and delocalized (localized) below (above) the critical point. In this work, we have studied the one dimensional quasi-periodic AAH Hamiltonian in the presence of spin-orbit (SO) coupling of Rashba type, which introduces an additional spin conserving complex hopping and a spin-flip hopping. We have found that, although the self-dual nature of AAH Hamiltonian remains unaltered, the self-dual point gets affected significantly. Moreover, the effect of the complex and spin-flip hoppings are identical in nature. We have extended the idea of Kohn's localization tensor calculations for quasi-particles and detected the critical point very accurately. These calculations are followed by detailed multifractal analysis along with the computation of inverse participation ratio and von Neumann entropy, which clearly demonstrate that the quasi-particle eigenstates are indeed multifractal and non-ergodic at the critical point. Finally, we mapped out the phase diagram in the parameter space of quasi-periodic potential and SO coupling strength.

cond-mat.dis-nn↗

arXiv2018

Decoherence in a $\mathcal{PT}$-symmetric qubit

We investigate the decoherence in a $\mathcal{PT}$-symmetric qubit coupled with a bosonic bath. Using cannonical transformations, we map the non-Hermitian Hamiltonian representing the$\mathcal{PT}$-symmetric qubit to a spin boson model. Identifying the parameter α that demarcates the hermiticity and non-hermiticity in the model, we show that the qubit does not decohere at the transition from real eigen spectrum to complex eigen spectrum. Using a general class of spectral densities, the strong suppression of decoherence is observed due to both vaccum and thermal fluctuations of the bath, and initial correlations as we approach the transition point.

quant-ph↗

arXiv2015

Exciton-polaritons gas as a nonequilibrium coolant

Using angle-resolved Raman spectroscopy, we show that a resonantly excited ground-state exciton-polariton fluid behaves like a nonequilibrium coolant for its host solid-state semiconductor microcavity. With this optical technique, we obtain a detailed measurement of the thermal fluxes generated by the pumped polaritons. We thus find a maximum cooling power for a cryostat temperature of $50$K and below where optical cooling is usually suppressed, and we identify the participation of an ultrafast cooling mechanism. We also show that the nonequilibrium character of polaritons constitutes an unexpected resource: each scattering event can remove more heat from the solid than would be normally allowed using a thermal fluid with normal internal equilibration.

cond-mat.mes-hall↗

arXiv2013

Radio frequency spectroscopy of the attractive Hubbard model in a trap

Attractive interaction between fermions can lead to pairing and superfluidity in an optical lattice. In contrast to the `continuum', on a lattice the trap induced density variation can generate a non monotonic profile of the pairing amplitude, and completely modify the spectral signatures of any possible pseudogap phase. Using a tool that fully captures the inhomogeneity and strong thermal fluctuations, we demonstrate how the crucial radio frequency signatures of pairing are `inverted' in a trapped attractive fermion lattice compared to the traditional continuum case. These features would be central in interpreting any spectroscopic hint of fermion pairing and superfluidity.

cond-mat.str-el↗

arXiv2013

Long range correlations in a 97% excitonic one-dimensional polariton condensate

We report on the realization of an out-of-equilibrium polariton condensate under pulsed excitation in a one-dimensional geometry. We observe macroscopic occupation of a polaritonic mode with only 3% photonic fraction, and a nature strikingly close to that of a bare exciton condensate. With the help of this tiny photonic fraction, the condensate is found to display first-order coherence over distances as large as 10 microns. Based on a driven-dissipative mean field model, we find that the correlations length is limited by the effects of a shallow disorder under non-equilibrium conditions.

cond-mat.mes-hall↗