Abstracts

Kagome bilayer inter-metallic systems as a platform for realising novel electronic phases.

Presenter: Aabhaas Vineet Mallik

Bar Ilan University, Israel

Inspired by the first principles results for a bilayer Fe3Sn2 [PRL 125, 026401 (2020)] we obtain low energy tight binding models for this class of materials and show that symmetry considerations generally allow for a low energy isolated band which may acquire a non-zero Chern number. We then introduce short ranged interactions in these models and use exact diagonalization and field theoretic arguments to demonstrate the possibility of realizing exotic phases of electrons like the fractional Chern insulator and spin liquid phases. (This work is done in collaboration with Adhip Agarwala, Tanusri Saha-Dasgupta and Subhro Bhattacharjee.)

A Yu-Shiba-Rusinov qubit

Presenter: Archana Mishra

Institute of Physics, Polish Academy of Sciences, Warsaw, Poland.

Magnetic impurities in s-wave superconductors lead to spin-polarized Yu-Shiba-Rusinov (YSR) in-gap states. Chains of magnetic impurities offer one of the most viable routes for the realization of Majorana bound states which hold a promise for topological quantum computing. However, this ambitious goal looks distant since no quantum coherent degrees of freedom have yet been identified in these systems. To fill this gap we propose an effective two-level system, a YSR qubit, stemming from two nearby impurities. Using a time-dependent wave-function approach, we derive an effective Hamiltonian describing the YSR qubit evolution as a function of distance between the impurity spins, their relative orientations, and their dynamics. We show that the YSR qubit can be controlled and read out using state-of-the-art experimental techniques for manipulation of the spins. Finally, we address the effect of spin noise on the coherence properties of the YSR qubit, and show a robust behavior for a wide range of experimentally relevant parameters. Looking forward, the YSR qubit could facilitate the implementation of a universal set of quantum gates in hybrid systems where they are coupled to topological Majorana qubits.

Spin dynamics in low-dimensional quantum gases

Presenter: Arko Roy

INO-CNR BEC Center at Trento, Italy

Cooled to nano-Kelvin temperatures, ultracold dilute atomic gases manifest as macroscopic quantum systems. Observable in experiments, atomic quantum gases give direct access to the physics of low-dimensional quantum phenomena. Be it the simulators of condensed matter systems or components of novel quantum technologies, it is of immense value to know the transport properties of ultracold atomic quantum gases to a fine detail. In this talk, we shall discuss that thermal fluctuations in a two-dimensional spinor system, in particular binary Bose-Bose mixtures, are much more important than in three dimensions. Not only they inhibit true Bose-Einstein condensation, as already well known, but they also smoothen all sharp features in the spin properties at finite temperature.

Fate of metal insulator transition in an interacting kagome lattice under periodic drive.

Presenter: Ganesh Paul

Technische Universität Braunschweig, Germany

We theoretically study the interplay between electron-electron interaction and external periodic drive on a kagome lattice with drive frequency being much larger than t and U. For U=0, using Brillouin-Wigner perturbation theory for obtaining effective hamiltonian in the high frequency limit, we see that the external drive modulates the bare hopping and generates emergent nearest neighbour and next-nearest neighbour spin orbit coupling terms which in turn induces topological phase transition that are characterized by change in band Chern numbers. Drive strength gives a control over the position of the flat bands in the system. Within a slave rotor mean field theory, it is shown that in the presence of the drive, and small U, the system exhibits repeated metal-insulator transitions as a function of the drive amplitude A. The charge gap between the low energy bands oscillates periodically with A and leads to a semi-metallic phase at specific values of A where the band gap becomes zero.

Quantum phases of ferromagnetically coupled Shastry-Sutherland model

Presenter: Santanu Pal

S. N. Bose National Centre for Basic Sciences, Block-JD, Sector-III, Salt Lake, Kolkata 700106, India

We study a ferromagnetically coupled Shastry-Sutherland model with the state-of-the-art numerical technique density matrix renormalization group (DMRG). The coupling constants along the x-axis (J2 ) and y-axis (J1 ) are ferromagnetic (FM) and the coupling constant for the diagonal bonds (J) is antiferromagnetic. For J = 1, the quantum phase diagram in the J1 - J2 parameter space consists of six phases. For small values of either J1 or J2 , a stripe phase with ordering wave vector (0, π) or (π, 0) with) or (π) or (π, 0) with, 0) with quasi-long-range order is found. The system shows a perfect dimer phase for J1 - J2 line and extended up to up to |J1|,|J2|<J. In a moderate coupling strength of J1 and J2 we found an incommensurate non- collinear spin ordering, the spiral phase. Depending on the relative coupling strength of J1 and J2, the incommensurate ordering wave vector modulates along the length (x-axis) or width (y-axis) and the corresponding phases are known as X-spiral and Y-spiral respectively. The spin-spin correlation in the spiral phase is short-range and the pitch angles vary between 0 and π). The spin-spin correlation calculated from the DMRG calculation confirms the existence of the spiral phases. When J1 and J2 are both in the strong coupling limit a long-range FM magnetic order phase occurs.

Nonreciprocal Entanglement in Cavity Magnomechanical Systems

Subhadeep Chakraborty

Centre for Quantum Engineering Research and Education,

TCG Centres for Research and Education in Science and Technology, Sector V, Salt Lake, Kolkata 70091, India

We propose a scheme to create nonreciprocal entanglement in magnon-photon-phonon modes in a spinning microwave resonator. The microwave photons and the magnons couple via the magnetic dipole interaction, while the magnons and the phonons couples via the magnetostrictive interaction. By spinning the resonator, the counter-propagating modes experience an opposite Fizeau drag. We show that the magnon-photon-phonon modes get entangled only when the resonator is driven along a particular direction, while they remain fully uncorrelated in the other. The entanglement is also found to be stationary and robust against the cavity dissipation and environmental temperature. Our findings could provide a new route towards chiral quantum technologies in cavity based magnomechanical systems.

Pairing symmetries in the Zeeman-coupled extended attractive Hubbard model.

Presenter: Swagatam Nayak,

National Institute of Science Education and Research

In a conventional s-wave superconductor a phonon-mediated effective attraction causes the electrons to form spin-singlet (with total spin S = 0) Cooper pairs with an isotropic s-wave orbital order parameter (OP) symmetry. On the contrary, the general consensus about the novel high TC cuprate superconductors is that they are identified as strong candidates for unconventional d-wave superconductors, which support the formation of spin-singlet Cooper pairs with an anisotropic d-wave orbital OP symmetry. In general, Cooper pairs can also be formed in the spin-triplet state, with total spin S = 1 and anisotropic orbital OP. Recent studies revealed that a two-dimensional chiral p-wave spin-triplet superconductor is a candidate to host Majorana fermions. In the case of broken inversion symmetry, there is also the possibility of forming Cooper pairs with mixed-parity superconducting (SC) states. Most interestingly, the response of a superconducting state to an external magnetic field can reveal significant details about the pairing state of the Cooper pairs. By introducing the possibility of equal-and opposite-spin pairings concurrently, we show that the ground state of the extended attractive Hubbard model (EAHM) exhibits rich phase diagrams with a variety of singlet, triplet, and mixed parity superconducting orders. We study the competition between these superconducting pairing symmetries invoking an unrestricted Hartree-Fock-Bogoliubov-de Gennes (HFBdG) mean-field approach, and we use the d-vector formalism to characterize the nature of the stabilized superconducting orders. We discover that, while all other types of orders are suppressed, a non-unitary triplet order dominates the phase space in the presence of an in-plane external magnetic field. We also find a transition between a non-unitary to unitary superconducting phase driven by the change in average electron density. Our results serve as a reference for identifying and understanding the nature of superconductivity based on the symmetries of the pairing correlations. The results further highlight that EAHM is a suitable effective model for describing most of the pairing symmetries discovered in different materials.

Ultrafast terahertz nutation dynamics in magnets

Presenter: Ritwik Mondal

Department of Mechanical Engineering, Indian Institute of Science, Bangalore, India

While semi-analytical solutions of the frequency-dependent phonon Boltzmann equation exist in the literature to describe thermal transport in simple semiconductor geometries, such solutions are challenging to obtain for complex nanostructures with multiple interfaces and boundaries. Here we present transient and steady-state numerical solutions of the linearized phonon Boltzmann equation in complex nanoscale semiconductor devices using a Monte-Carlo integration technique. In our calculations, we accurately represent various scattering processes that the phonons undergo in the nanosctructure - such as the intrinsic phonon-phonon scattering and the geometric specular and diffuse boundary scattering, and also consider realistic complex heating profiles that the nanoscale devices encounter in real applications. We demonstrate that our computational scheme reproduces the experimental results on the thermal transport in nanoscale devices accurately, while retaining the low computational cost of the semi-analytical solutions reported in the literature for simple geometries.

Supersymmetry, topology, and entanglement

Presenter: Krishanu Roychowdhury

Stockholm University

Supersymmetry (SUSY) posits an equivalence between two fundamental degrees of freedom: fermions and bosons. Using this, a variety of phonon and magnon models can be identified which have topologically nontrivial free fermion models as superpartners. At the single-particle level, the bosonic and the fermionic models that are generated by SUSY are isospectral except for zero modes such as flat bands. The existence of such zero modes is intricately related to the Witten index of the SUSY theory. Interestingly, a Hermitian form of the supercharge operator can itself be identified as a hopping Hamiltonian on a bipartite lattice in two and three dimensions, which further enables us to identify a wide class of pairs of lattices where the hopping Hamiltonians are superpartners of each other and also possess the same band topology. Extending to real degrees of freedom, we can systematically construct topological mechanical systems by an exact SUSY. As examples, we discuss mechanical analogs of the Kitaev honeycomb model and of a second-order topological insulator with floppy corner modes. Our SUSY construction naturally defines hitherto unexplored topological invariants for bosonic (mechanical) systems, such as bosonic Wilson loop operators that are formulated in terms of SUSY-related fermionic Berry curvature. This would be useful to identify topological bosonic models whose fermionic partner represents some symmetry-protected topological phase. We will conclude by demonstrating how the locality and entanglement content of a Gaussian state are influenced by SUSY.

Edge Reconstruction and Neutral Modes in Integer and Laughlin-like Quantum Hall Phases

Presenter: Udit Khanna

Bar Ilan University, Israel

Chiral gapless boundary modes are characteristic of quantum Hall (QH) states. Orthodox models of the integer QH state at filling factor unity, as well as the Laughlin phases anticipate a single chiral edge mode. Here we show that, in the presence of a smooth confining potential, edge reconstruction leads to the emergence of counter-propagating modes, which, by way of mode renormalization, may give rise to non-topological upstream neutral modes. This may explain the experimental observation of ubiquitous neutral modes, and the overwhelming suppression of anyonic interference in Mach-Zehnder interferometry platforms. We also point out other signatures of such edge reconstruction.

Ph.D. Students: (Poster)

Hierarchy of higher-order topological superconductors in three dimensions

Presenter: Arnob Kumar Ghosh

Institute of Physics, Bhubaneswar

After exploring much on two-dimensional higher-order topological superconductors (HOTSCs) hosting Majorana corner modes (MCMs) only, we propose a simple fermionic model based on a three-dimensional topological insulator proximized with s-wave superconductor to realize Majorana hinge modes (MHMs) followed by MCMs under the application of appropriate Wilson-Dirac perturbations. We interestingly find that the second-order topological superconductor, hosting MHMs, appears above a threshold value of the first type perturbation while the third-order topological superconducting phase, supporting MCMs, immediately arises incorporating infinitesimal perturbation of the second kind. Thus, a hierarchy of HOTSC phases can be realized in a single three-dimensional model. Additionally, the application of bulk magnetic field is found to be instrumental in manipulating the number of MHMs, leaving the number for MCMs unaltered. We analytically understand these above-mentioned numerical findings by resorting to the low-energy model. We further characterize these topological phases with a distinct structure of the Wannier spectra. From the practical point of view, we manifest quantized transport signatures of these higher-order modes. Finally, we construct Floquet engineering to generate the hierarchy of HOTSC phases by kicking the same perturbations as considered in their static counterpart.

Ref.: Arnob Kumar Ghosh, Tanay Nag, and Arijit Saha, “Hierarchy of higher-order topological superconductors in three dimensions,” Phys. Rev. B 104, 134508 (2021).

Emergence of multiple localization transitions in a one-dimensional quasiperiodic lattice

Presenter: Ashirbad Padhan

IIT Guwahati

Low dimensional quasiperiodic systems exhibit localization transitions by turning all quantum states localized after a critical quasidisorder. While certain systems with modified or constrained quasiperiodic potential undergo multiple localization transitions in one dimension, we predict an emergence of multiple localization transitions without directly imposing any constraints on the quasiperiodic potential. By considering a one-dimensional system described by the Aubry-Andre (AA) model, we show that an additional staggered onsite potential can drive the system through a series of localization transitions as a function of the staggered potential. Interestingly, we find that the number of localization transitions strongly depends on the strength of the quasiperiodic potential. Moreover, we obtain the signatures of these localization transitions in the expansion dynamics and propose an experimental scheme for their detection in the quantum gas experiment. Source: arXiv:2109.09621v2

Single particle Green's functions across the many-body localization transition

Presenter: Atanu Jana

National Institute of Science Education and Research

We present an analysis of the many body localisation transition from the point of view of eigenstate Green's function quantities, namely the local density of states and the scattering rates. Using a complete diagonalisation analysis of a disordered fermionic chain with nearest neighbour interactions, we find that the delocalisation to many body localisation transition can be tracked using the typical values of these quantities which are closely tied to the single particle excitations in the system. The typical values which are of the same order of the average values in the delocalised phase become vanishingly small in the localised regime. The transition point thus obtained is seen to be very close to the point obtained using the level spacing statistics and so is the approximate location of mobility edges.

Reference: A. Jana, V. R. Chandra and A. Garg Phys. Rev. B. 104 L140201 (2021)

Long-range Spectral Statistics in a Variety of Correlated Spin Models: A Comparison with Random Matrix Theory

Presenter: Debojyoti Kundu

Department of Physics, School of Natural Sciences, Shiv Nadar University, NCR, India

We investigate two correlated Heisenberg spin-chain systems, one, in the presence of random magnetic fields and the 3-site Scalar Chiral interaction, and the other with Kramers Degeneracy, in the presence of random-Ising and Dzyaloshinskii-Moriya (DM) interaction terms. We have carried out a detailed study of the long-range spectral correlations like Spectral Rigidity and Number Variance and the spectral-crossovers between Poissonian and different random matrix Gaussian (Orthogonal/Unitary/Symplectic) Ensembles (GOE/GUE/GSE). In the process, we shed light on the extent of agreement between physical spin systems and RMT predictions. Also, we compare the long-range crossover conditions with the corresponding short-range spectral fluctuation results.

Emergent half metal at finite temperatures in a Mott insulator

Presenter: Gour Jana

National Institute of Science Education and Research

Fermi Hubbard model is one of the simplest model for studying strongly correlated systems that can capture phenomena like magnetic order, exotic metals, and high-Tc superconductivity. We study the model on square lattice at half-filling in the presence of staggered onsite 'ionic' potentials and second nearest neighbour hopping (t') which acts as frustration in the system, using the recently developed semi-classical Monte Carlo approach (s-MC). The method allows to extract an effective Hamiltonian with free fermions coupled to classical fields from the fully interacting problem. The approximation reduces computational complexity of the problem drastically and allows to access the physics in the non-perturbative regimes on large lattices over a wide temperature (T) range. In the study, we investigate the impact of 'ionic' potentials in the frustrated Hubbard model called 'ionic Hubbard model' with correlation strength U to study half metallicity -- a metal with 100% spin polarization. We will first show that a low-temperature insulator with an unequal charge gap for the two spin channels can arise from a competition between Mott and band insulating tendencies. Then we will show that thermal fluctuations can drive this insulator to a half-metal transition by closing the charge gap for one spin channel. This metal has 100% spin polarization at the onset temperature of metallization. By constructing a U-T phase diagram, we will discuss that by varying the strength of electron repulsion, one can enhance the onset temperature while preserving spin polarization. We will also discuss the possible experimental scenarios for realizing this tunable finite temperature half-metal.

Non-equilibrium scalar field dynamics starting from Fock states: Absence of thermalization in one-dimensional phonons coupled to fermions

Presenter: Md Mursalin Islam

Tata Institute of Fundamental Research, Mumbai

We propose a new method to study non-equilibrium dynamics of scalar fields starting from non-Gaussian initial conditions using Keldysh field theory. We use it to study dynamics of phonons coupled to bosonic and fermionic baths, starting from initial Fock states. We find that in one dimension long wavelength phonons coupled to fermionic baths do not thermalize both at low and high bath temperatures. At low temperature, constraints from energy-momentum conservation lead to a narrow bandwidth of particle-hole excitations and the phonons effectively do not feel the effects of this bath. On the other hand, the strong band edge divergence of particle-hole density of states leads to an undamped ``polariton-like'' mode of the dressed phonons above the band edge of the particle-hole excitations. These undamped modes contribute to the lack of thermalization of long wavelength phonons at high temperatures. In higher dimensions, these constraints and the divergence of density of states are weakened and leads to thermalization at all wavelengths.

Two component quantum walk in one-dimensional lattice with hopping imbalance

Presenter: Mrinal Kanti Giri

IIT Guwahati

We investigate the two-component quantum walk in a one-dimensional lattice. We show that the inter-component interaction strength together with the hopping imbalance between the components exhibits distinct features in the quantum walk for different initial states. When the walkers are initially on the same site, both the slow and fast particles perform independent particle quantum walks when the interaction between them is weak. However, stronger inter-particle interactions result in quantum walks by the repulsively bound pair formed between the two particles. For different initial states when the walkers are on different sites initially, the quantum walk performed by the slow particle is almost independent of that of the fast particle, which exhibits reflected and transmitted components across the particle with large hopping strength for weak interactions. Beyond a critical value of the interaction strength, the wave function of the fast particle ceases to penetrate through the slow particle signalling a spatial phase separation. However, when the two particles are initially at the two opposite edges of the lattice, then the interaction facilitates the complete reflection of both of them from each other. We analyze the above-mentioned features by examining various physical quantities such as the on-site density evolution, two-particle correlation functions, and transmission coefficients.

References: M. K. Giri, S. Mondal, B.P Das, and T. Mishra, "Quantum walk of two interacting particles with mass imbalance," (2021), arXiv:2008.11142. (To be published Scientific Reports)

Phonon transport in ultrahigh thermal conductivity materials beyond the relaxation time approximation

Presenter: Nikhil Malviya

Department of Mechanical Engineering, IISc Bangalore

In electrical insulators, heat is carried by the quantized collective lattice vibrations called phonons. Resistance to heat flow in these materials is caused by phonon scattering processes. Thermalphonon transport in these materials is governed by the semi-classical Boltzmann Transport Equation (BTE). Solutions of the BTE are commonly derived assuming the validity of relaxation time approximation (RTA), where all phonon scattering events are assumed to be momentum-dissipative in nature. While the RTA-based BTE solution describes the heat flow in several materials reasonably well, it fails to capture the ultrahigh thermal conductivity and the exceptional phonon transport properties of materials like diamond and boron nitride. Here we present the solutions of the BTE without the RTA for phonon transport through these ultrahigh thermal conductivity materials and demonstrate that accurately distinguishing momentum-conserving (Normal) and momentum-dissipative (Umklapp) scattering events in our formulation is crucial to correctly predict their thermal transport properties.

This work is supported by the DST-SERB Core Research Grant (CRG/2020/006166).

Fock Space Recursive Green’s functions for bound complexes in partially filled bands

Presenter: Prabhakar

National Institute of Science Education and Research

The Greens function of a system directly allows us to calculate the local density of states for the system, without using exact diagonalization (ED) methods. In order to calculate the Green’s function one needs to invert the (zI − H) matrix, H is the Hubbard Hamiltonian of the system. But the direct inversion is limited for small lattices because the dimension of Hilbert space grows exponentially with system size. Here we present a mapping between the Hilbert space of a L size periodic chain of spinless fermions to an abstract one-dimensional Fock-space lattice. It allows for a novel recursive algorithm with a O(1/L) suppression at half filling in memory requirements for the exact computation of many-body correlations in vacuum, as compared to brute force methods. Further, we have derived the formula to calculate any P-particle Green's function from many body Green's functions. We have applied our method to study how interaction and charge transfer energy in a one dimensional lattice can stabilize local two-hole bound states at various fillings.

Thermoelectric properties of inversion symmetry broken Weyl semimetal-Insulator-Superconductor hybrid junction.

Presenter: Pritam Chatterjee

Institute of Physics, Bhubaneswar

Owing to its non-trivial topology, Weyl semimetals (WSMs) exhibit many interesting physical effects such as the quantum anomalous Hall effect, the chiral magnetic effect, negative magneto-resistance and unusual surface states called Fermi arcs.Intense theoretical and experimental research works have been carried out in both time-reversal and inversion symmetry broken WSMs.Although electronic properties of WSMs (both in bulk and hetero-junctions) have been extensively studied, there is less information available about its thermoelectric properties in hybrid setups. In particular, we are interested to study the thermoelectric properties of heterostructures consisting of WSMs with superconductors

which form the foundation of applications in electronics and spintronics.

Orbital selective superconductivity in infinite layer Nickelates

Presenter: Priyo Adhikary

Department of physics, Indian Institute of Science, Bangalore, 560012, India

Recently discovered superconductivity in doped infinite layer nickelates has created a lot of research interests in the condensed matter field. Understanding the superconducting gap symmetry is a crucial step towards unravelling the mechanism of superconductivity and its applications. Starting with the two-band Hubbard model, we predict a novel orbital-selective two-gap superconductivity.[1] One of the gaps is a 3D dz2 symmetry, which is fully-gapped, while the other gap is of 2D dx2-y2 nature and contains nodal quasiparticles. Remarkably, a recent STM study confirms our prediction. In various ways, we demonstrate that nickelates superconductors are characteristically different from the cuprates and pnictides families, and provide a novel route to superconductivity.

[1] Priyo Adhikary, Subhadeep Bandyopadhyay, Tanmoy Das, Indra Dasgupta, Tanusri Saha-

Dasgupta, Orbital selective superconductivity in a two-band model of infinite-layer nickelates,

Phys. Rev. B 102,100501(R) (2020).

[2] Subhadeep Bandyopadhyay, Priyo Adhikary, Tanmoy Das, Indra Dasgupta, Tanusri Saha-

Dasgupta, Superconductivity in Infinite-layer Nickelates: Role of Non-zero f-ness, arXiv:2010.02527

Geometric quench as a probe to detect localization-delocalization transition

Presenter: Ravi Kumar

IIT-BHU, Varanasi, India

While global quantum quench has been extensively used in the literature to understand the localization-delocalization transition for the 1D quantum spin chain, the effect of geometric quench (which corresponds to a sudden change of the geometry of the chain) in the context of such transitions is yet to be understood. In this work, we investigate the effect of geometric quench in the Aubrey-Andre model, which supports localization-delocalization transition even in 1D. We study the spreading of the entanglement and the site-occupation with time and find many interesting features that can be used to characterize localization-delocalization transition. Interestingly these features also persist even in the presence of interactions.

Ref: Ravi Kumar and Ranjan Modak, arXiv: 2110.XXXX (2021)

Spin Orbit Coupling driven Novel Quantum Magnetism in Iridate Double Perovskites.

Presenter: Roumita Roy,

Indian Institute of Technology Goa.

Double Perovskites has been an area of interest owing to the rich physics it promises.The interplay amongst lattice, charge and spin degrees of freedom,lays the foundation for exhibiting a wide range of exotic phenomena. 3d-5d based Double Perovskite is ideal to understand the interplay of Spin Orbit Coupling with other intrinsic energy scales of the system. With this motivation,electronic structure calculation using state of the art density functional theory was performed on a series of compounds with the general formula (Sr_xCa_1-x)₂FeIrO₆ with x ranging from 0 to 1. The idea was to look for the change in magnetic properties of the system due to the influence of structural distortion introduced due to the difference in size between Strontium and Calcium atoms. It was found that the basic interactions remained unchanged,however there were changes in the Crystal field effect and

bandwidth.The emergence of novel insulating phases was studied from the perspective of the influence of Spin Orbit Coupling in transition metal oxides. With an AFM ground state,these materials lie intermediate between conventional Mott Insulators and SOC driven Mott Insulators. Further calculations showed the evolution of spin and orbital magnetic moments across the series along with the magnetisation density.

Entanglement Entropy of Fermionic Open Quantum Systems from Wigner Characteristics

Presenter: Saranyo Moitra

Tata Institute of Fundamental Research, Mumbai

We formulate a new "Wigner characteristics'' based method to calculate entanglement entropies of subsystems of Fermions using Keldysh field theory. This bypasses the requirements of working with complicated manifolds to calculate Rényi entropies for many body systems. We provide an exact analytic formula for Rényi and von-Neumann entanglement entropies in non-interacting open quantum systems, which are initialised in arbitrary Fock states. We use this formalism to look at entanglement entropies of momentum Fock states of one-dimensional Fermions. We show that the entanglement entropy of a Fock state can scale either logarithmically or linearly with subsystem size, depending on whether the number of discontinuities in the momentum distribution is smaller or larger than the subsystem size. We also use this formalism to describe entanglement dynamics of an open quantum system starting with a single domain wall at the center of the system. Using entanglement entropy and mutual information, we understand the dynamics in terms of coherent motion of the domain wall wavefronts, creation and annihilation of domain walls, and incoherent exchange of particles with the bath.

Principal Components Analysis in a frustrated spin-1/2 chain system with nearest neighbour (J1) and next nearest neighbour (J2) exchange interactions

Presenter: Sk Saniur Rahaman

S.N.B.N.C.B.S Kolkata

From a multivariate data table (so called features), Principal Components Analysis (PCA) reduces down highly correlated data into some less number of significant components in another subspace, which in fact is a projection of the real feature space. Principal Components are reported as a standard alternative to order parameters in detecting thermal phase transitions for the spin systems. We use PCA in analysing quantum phase transition for a spin-1/2 J1-J2 model. By using the exact diagonalisation method, we choose the most probable states in the ground state and 1st excitation energy level for each J2/J1 value and then we perform PCA within a particular range of J2/J1. We observe that 1st principal component shows a phase transition around J2/J1=0.241 for system size 24. This critical value of J2/J1 is consistent with the earlier reported result, indicating a quantum phase transition from gaples to gaped phase.

Kane-Mele-Hubbard model revisited : a slave-spin theory approach

Presenter: Subhajyoti Pal

National Institute of Science Education and Research

Kane Mele (KM) with onsite Hubbard interaction is a prototypical model to study metal insulator transition, magnetism and its interplay with topological properties of matter. Due to this, the model has been investigated by a multitude of many body techniques. We will discuss the theoretical investigation on the Kane-Mele model with intrinsic spin-orbit coupling and onsite Hubbard interaction term employing the slave-spin method, a simple version of the slave-particle method. In the slave-spin approach, the electronic operator is decomposed into two parts - an auxiliary fermion (spinon) assimilating the spin and a U(1) slave-spin operator encapsulating spin-dependent charge/number fluctuations. We show that quasiparticle weight remains finite till a critical interaction strength, and afterward, the system becomes a Mott insulator. In the Mott phase, the system develops anti-ferromagnetic order, and is captured by a magnetic structure-factor. In ribbon geometry, we will show that there exists an electronic edge state even at finite interaction, and deep inside the Mott phase, the edge state disappears. However, the auxiliary fermonic edge state still survives even in the Mott phase.

Eightfold quantum Hall phases in a time reversal symmetry broken tight binding model

Presenter: Sudarshan Saha

Institute of Physics, Bhubaneswar

We consider a time reversal symmetry (TRS) broken Kane-Mele model superimposed with a Haldane model and chart out the phase diagram using spin Chern number to investigate the fate of the quantum anomalous Hall insulator (QAHI) and quantum spin Hall insulator (QSHI) phases. Interestingly, in addition to the QSHI and QAHI phase, the phase diagram unveils a quantum anomalous spin Hall insulator (QASHI) phase where only one spin sector is topological. We also find multicritical points where three or four topological phase boundaries coalesce. These topological phases are protected by an effective TRS and a composite antiunitary particle-hole symmetry leading to remarkable properties of edge modes. We find spin-selective, spin-polarized, and spin-neutral edge transport in the QASHI, QSHI, and QAHI phases, respectively. Our study indicates that the robustness of the topological phase mainly depends on the spin gap which does not necessarily vanish at the Dirac points across a topological phase transition. We believe that our proposals can be tested in the near future using recent experimental advancements in solid state and cold atomic systems.

Pressure driven topological phase transition in chalcopyrite ZnGeSb2

Presenter: Surasree Sadhukhan

School of Physical Sciences, Indian Institute of Technology Goa

Few topological non-trivial phases in chalcopyrites class have been predicted by studying their electronic band structures and calculating topological invariants. Here we have found ZnGeSb2 having a nontrivial topological phase in its ambient structure, and a small uniform hydrostatic pressure ( 7 GPa) can drive the system to a topologically trivial phase of matter. We have confirmed our results by studying the primary signature of non-trivial phases like band inversion, Z2 number calculation, spin texture, spin momentum locking in momentum space, the existence of an odd number of Dirac cones in spectral functions, and Berry phase calculations for ambient and high-pressure structures. We have found that the tetragonal distortion of chalcopyrites may be responsible for such topological quantum phase transition.

Reference : W. Feng et. al., Phys. Rev. Lett. 106 016402 (2011)

Probing helical vs chiral character of topological superconductors via non-local Hanbury-Brown and Twiss correlations

Presenter: Tusaradri Mohapatra

National Institute of Science Education and Research

Topological superconductors host surface Andreev bound states which can be classified as gapful chiral, gapful helical, and nodal. In 2D chiral superconductors belong to the Z symmetry class which breaks time-reversal symmetry (TRS) while helical superconductors belong to the Z2 symmetry class which preserves TRS. In this talk using Hanbury Brown and Twiss (HBT) shot noise correlations and the non-local conductance, we probe metal/unconventional superconductor/metal junctions in order to better understand the pairing symmetry of unconventional superconductors, gapful (helical or chiral) versus nodal (chiral or helical) in topological superconductors. We also compare these to non-topological superconductors. Topological superconductors are carriers of Majorana fermions which are important for topological quantum computation. By distinguishing chiral superconductors, wherein Majoran surface modes (MSM) persist even in presence of a magnetic field, from helical superconductors where MSM are fragile, our study will help in the search for stable Majorana fermions which will be useful for topological quantum computation.

The Metallic Exciton States and Non-Fermi Liquid Behaviour in Twisted Double Bilayer Graphene

Presenter: Unmesh Ghorai

Dept. of Theoretical Physics, TIFR

In recent years, small-angle twisted systems have shown a plethora of exotic quantum phases like unconventional superconductor, ferromagnetism, metal to insulator transition, topological insulating states, etc. close to the integer band fillings. There have been plenty of theoretical studies as well to explain the unusual features of experimental data and to predict new phenomenons in these Moire materials. Among this family of materials, twisted double bilayer graphene is known to host coexisting electron-hole pockets at the Fermi surface, making it a compensated semi-metal. In this work, we have shown the plausible presence of “excitons'' coming from this energy overlap of valence and conduction bands. Starting from a detailed mean-field study of the system Hamiltonian, we predicted some very interesting electric transport features of the fermionic excitations of these exciton states. Random Phase Approximation (RPA) based static screening calculation was used to estimate the modified Coulomb interaction strength due to the presence of substrate and huge electron density. We have also found evidence of Landau damping of the excitonic fluctuations in this system through the dependence of Lindhard polarizability function on momentum and frequency. This indicates probable non-Fermi liquid behaviour and subsequently the possibility of finding non-standard temperature dependence of the resistivity in the system.

Simulating non-Hermitian dynamics of a multi-spin quantum system and an emergent central spin model.

Presenter: Anant Vijay Varma

IISER Kolkata

In recent times there has been much discussion of non-Hermitian quantum systems in the context of many-body systems owing to the exotic manifestations like a violation of Lieb-Robinson bound (PRL 124,136802 (2020)), non-Hermitian skin effect (PRL 121, 086803 (2018)), suppression of defect production in Kibbel-Zurek mechanism (Nature Comm. 10, 2254 (2019)) and correspondence between (d+1) dimensional gapped Hermitian systems and d-dimensional point-gapped non-Hermitian systems (PRL 123, 206404 (2019)). Hence, the possibility of simulating such a system that will directly observe such phenomena could be of great importance. It is known that the dynamics of a single spin-1/2 PT-symmetric system can be simulated by conveniently embedding it into a subspace of a larger Hilbert space with unitary dynamics. In the context of many-body physics, what would be the consequence of the complexity of such ideas of embedding non-Hermitian many-body systems in unknown. We show that such an embedding leads to non-trivial Hamiltonian which has complex interactions. We consider a simple example of N free PT-symmetric spin-1/2s to obtain the resulting many-body interacting Hamiltonian of N+1 spin-1/2s. We can visualize it as a strongly correlated central spin model with the additional spin-1/2 playing the role of central spin. We would show that due to the orthogonality catastrophe, even a vanishing small exchange field applied along the anisotropy axis of the central spin leads to a strong suppression of its decoherence arising from spin-flipping perturbations.

This poster would be based on the following paper: Anant V. Varma, and Sourin Das, Simulating many-body non-Hermitian PT-symmetric spin dynamics” (Phys. Rev. B 104, 035153 (2021)).

Spin Berry phase in a helical edge state: Sz nonconservation and transport signatures

Presenter:Vivekananda Adak

IISER Kolkata

The topological protection of edge states in quantum spin Hall systems relies only on time-reversal symmetry, hence, Sz conservation on the edge can be relaxed which can have an interferometric manifestation in terms of spin Berry phase arising from a closed-loop dynamics of electrons. In this presentation, I will provide a minimal framework to generate and detect such effects via resonances or anti-resonances in the two-terminal conductance. Numerical results of a device set-up (using the KWANT package) will be in order to put our theoretical predictions on a firm footing.

Ref: PHYSICAL REVIEW B 102, 035423 (2020)

Enhancement in tunneling density of states in a Luttinger liquid: Role of non-local interactions.

Presenter: Amulya Ratnakar

IISER Kolkata

One of the defining features of Luttinger liquid (LL) is the suppression of local single-electron tunneling density of states (TDOS) in the zero-bias limit, which is due to the fact that the low- energy excitation spectrum of a LL is devoid of electrons like quasiparticles. Previous studies have shown that the an exception to this is possible if we have Andreev like process at the junction. This can be achieved for a LL in proximity to a superconductor (SC) or due to strong correlations localised at the junction of multiple LLs which can drive the system to a fixed point giving rise to a possibility of hole current being reflected back from the junction. In general, the fixed points showing enhancement in TDOS are found to be unstable against perturbations that can be switched on at the junction. Here we report the possibility for a junction of two LLs with nonlocal density-density interaction, with current conserving boundary condition as fixed point, to show simultaneous TDOS enhancement (even in the absence of Andreev like process) and stability at the junction in the renormalization group (RG) sense. For a junction of LL with SC, we reaffirm the fact that enhancement in the density of states (DOS) cannot be explained as the mere consequence of pair correlation induced due to proximity effect. The junction is overall unstable in the RG sense for the set of interaction parameters it shows enhancement in the TDOS. We also establish the duality condition between the current conserving (normal junction) and current non-conserving (superconducting junction) sectors of the theory in the presence of non-local interactions. Such a model, with prospect of having non-local interaction, can be realized on the edge states of a bilayer quantum Hall system with tunnel coupling (or cooper pair coupling in the case of junction with SC) at the apex. Intralayer and interlayer density-density interactions between the two layers mimics the role of local and nonlocal interactions, respectively.

Thermal Signature of Majorana Fermion in a Josephson Junction

Presenter: Aabir Mukhopadhyay

IISER Kolkata

Experimental attempts for detection of Majorana bound state (MBS) are primarily based on two theoretical predictions: (1) 2𝑒^2/h resonant conductance peak at zero bias and (2) the 4𝜋 Josephson effect. Both of these strategies rely heavily on the subgap physics of topological superconductors. In a complementary approach, we look for non-trivial signatures of MBS in heat transport across a Josephson Junction. The heat current is dominated by the above-the-gap transport of Bogoliubov quasiparticles. Specifically, we consider a thermally biased Josephson junction hosting a pair of MBS in a helical edge state of a 2D topological insulator. We show that the presence of Majorana end states in a three-terminal Josephson junction setup results in two sets of testable relations:

(i) The ratios between the various multiterminal thermal conductances are independent of the Josephson phase bias.

(ii) The ratios of ‘ratios of the phase derivative of multiterminal thermal conductance and the corresponding Josephson current’ are also independent of the Josephson phase bias.

These results owe their existence to the fact that hybridization of an odd number of Majoranas always leaves a zero mode in the subgap spectrum of the junction. We establish that these results are unique to the presence of Majorana fermions and cannot be mimicked in a Josephson junction setup composed of a 1D topological superconductor hosting “Andreev type end states” (for example, the kind of end-state as observed by P. Marra and M. Nitta, Phys. Rev. B 100, 220502(R)). We also show that the thermal noise expected in our proposed setup will not overwhelm our proposed measurements within realistic values of the parameters obtained from existing experiments.

We have recently extended our work to the thermoelectric conductance of a Josephson junction, which has a very long history dated to 1944 in a work by Ginzberg.

Ref: Phys. Rev. B 103, 144502

Electrically strained graphene: A route to controlled cleavage and switchable band-gap

Presenter: Sujith N. S.

National Institute of Science Education and Research

We demonstrate computationally primarily from first-principles, a route to strain graphene in-homogeneously yet controllably in any predetermined pattern leading to switchable variation in band-gap, and magnetism through patterned application of biasing. We find that the in-plane electric field due to variation of the applied bias causes systematic strain leading to regions of stretched and contracted segments of graphene, with the degree of strain controlled by the applied bias as well as the size and shape of the biased regions. In particular we have considered stripes of biased regions whose orientation with respect to that of graphene crucially determine the effect on the band-structure.

Self-energy corrected tight binding parameters in hybridized atomic orbital basis from first principles.

Presenter: Manoar Hossain

National Institute of Science Education and Research

We present self-energy corrected tight-binding(TB) parameters in the basis of directed hybridised atomic orbitals constructed from the first principles. We show results for nano-diamond clusters as well as bulk diamond and zinc-blende structures made of elements of group 13, 14 and 15 in the 2p, 3p and 4p blocks. With increasing principal quantum number of frontier orbitals, the lowering of self-energy corrections (SEC) to the band-gap and consequently to the nearest neighbor TB parameters, is much faster in bulk than in nano-diamonds and hence not transferable from bulks to nano-diamonds. However, the TB parameters transferred from smaller nano-diamonds to much larger ones exclusively through mapping of neighborhoods of atoms not limited to the nearest neighbors, are found to reproduce the HOMO-LUMO gaps of larger nano-diamonds having hundreds of atoms with explicitly calculated values at the DFT and DFT+G0W0 levels. Both TB and SEC-TB parameters are found to vary significantly from 2p to 3p but negligibly from 3p to 4p block and vary rather slowly within each block, implying the possibility of transfer of SEC across blocks. The demonstrated easy transferability of the self-energy corrected TB parameters in the directed hybrid orbital basis thus promises computationally inexpensive estimation of quasi-particle electronic structure of large finite systems with thousands of atoms.

Fermi Arc Reconstruction at the Interface of Twisted Weyl Semimetals

Presenter: Faruk Abdulla

Harish-Chandra Research Institute, Allahabad

Three-dimensional Weyl semimetals have pairs of topologically protected Weyl nodes, whose projections onto the surface Brillouin zone are the end points of zero energy surface states called Fermi arcs. At the endpoints of the Fermi arcs, surface states extend into and are hybridized with the bulk. Here, we consider a two-dimensional junction of two identical Weyl semimetals whose surfaces are twisted with respect to each other and tunnel-coupled. Confining ourselves to commensurate angles (such that a larger unit cell preserves a reduced translation symmetry at the interface) enables us to analyze arbitrary strengths of the tunnel-coupling. We study the evolution of the Fermi arcs at the interface, in detail, as a function of the twisting angle and the strength of the tunnel-coupling. We show unambiguously that in certain parameter regimes, all surface states decay exponentially into the bulk, and the Fermi arcs become Fermi loops without endpoints. We study the evolution of the

‘Fermi surfaces’ of these surface states as the tunnel-coupling strengths vary. We show that changes in the connectivity of the Fermi arcs/loops have interesting signatures in the optical conductivity in the presence of a magnetic field perpendicular to the surface.