Showing 1–50 of 243 results for author: Ray, S

Energy-efficient, Reconfigurable Optoelectronic Artificial Synapses Based on MoWS$_2$ Alloy for Pattern Recognition and Color Image Filtering Applications

Authors: Deepak Kumar Sahu, Santu Kumar Ghosh, Sagarneel Ghoshal, Saranya Das, Samit K. Ray

Abstract: Two-dimensional transition-metal dichalcogenide alloys are potential candidates for advanced optoelectronic and neuromorphic applications due to their strong light-matter interactions and controllable defect properties. However, large-area growth of such alloys remains challenging, while the correlation between their physical and neuromorphic properties remains largely unclear. In this work, we pr… ▽ More Two-dimensional transition-metal dichalcogenide alloys are potential candidates for advanced optoelectronic and neuromorphic applications due to their strong light-matter interactions and controllable defect properties. However, large-area growth of such alloys remains challenging, while the correlation between their physical and neuromorphic properties remains largely unclear. In this work, we present an innovative microcavity chemical vapor deposition (CVD) reactor pathway to grow uniform, and large-area MoWS$_2$ mono- and few-layer alloy films for demonstrating optoelectronic synaptic functionalities. Driven by growth-induced intrinsic sulfur vacancies, as confirmed by XPS, KPFM, and STEM measurements, our optoelectronic synaptic device (OSD) successfully emulates essential biological synaptic features, such as excitatory postsynaptic currents (EPSC), paired-pulse facilitation (PPF~170%), and stimulus-dependent short- and long-term plasticities (STP & LTP). With picojoule-order energy consumption per synaptic event and nanoampere-order dark current, the device enables low-power neuromorphic learning, including emulation of Pavlovian associative learning. Furthermore, the experimentally measured conductance weight-update characteristics enabled an artificial neural network (ANN) simulation to achieve 92.43% recognition accuracy on the MNIST handwritten digit dataset. Finally, we demonstrate advanced neuromorphic visual processing by executing color image filtering based on the device's wavelength-selective photoresponse characteristics. This simple, yet multifunctional device architecture provides a promising path toward energy-efficient, spectral-selective neuromorphic vision applications. △ Less

Submitted 18 August, 2026; originally announced August 2026.

Comments: main manuscript (5 figures, 37 pages) and supporting information (17 figures, 12 pages)

Magnetic ordering in VI3: a van der Waals material combining vastly different magnetic anisotropies

Authors: K. K. Pokhrel, S. Ray, N. Machavcova, A. Koliogiorgos, K. Carva

Abstract: Among magnetic van der Waals materials, the vanadium trihalide family exhibits unique features. In particular, VI3 contains V atoms of two different types, as two energetically close electronic occupations can coexist in real samples. These types show strikingly different magnetic anisotropy, predicted to differ by more than an order of magnitude. The combination forms a distinctive magnetic syste… ▽ More Among magnetic van der Waals materials, the vanadium trihalide family exhibits unique features. In particular, VI3 contains V atoms of two different types, as two energetically close electronic occupations can coexist in real samples. These types show strikingly different magnetic anisotropy, predicted to differ by more than an order of magnitude. The combination forms a distinctive magnetic system. VI3 also displays an unusual thickness dependence: the monolayer Curie temperature (TC) is higher than that of the bulk, contrary to the expectation that interlayer coupling reinforces magnetic order. Using atomistic spin-dynamics simulations informed by first-principles calculations, we investigate the critical temperature behavior from the combined perspective of single-ion anisotropy and exchange interactions. The strong anisotropy contrast significantly affects thermal stability: increasing the fraction of high-anisotropy sites raises the energy cost of transverse spin fluctuations and increases the ordering temperature. Furthermore, V-atom inhomogeneity makes the interlayer super-superexchange network spatially nonuniform, creating competing exchange pathways. This weakens coherent interlayer order while preserving robust intralayer ferromagnetic correlations, thus modifying the bulk-monolayer TC relation. Our model reproduces experimental TC values when the ratio of the two V types is close to 1:1, in agreement with two experimental methods. This supports the coexistence of two V configurations in VI3. The monolayer TC is reproduced with a slightly modified ratio, possibly linked to polaron concentration. The sensitivity of TC to this ratio suggests that the ordering temperature of VI3 can in principle be tuned over a broad range by controlling the relative occupation of the two vanadium configurations. △ Less

Submitted 11 August, 2026; v1 submitted 2 July, 2026; originally announced July 2026.

Geometry, elasticity, and activity in the transport of self-propelled filaments in turbulence

Authors: Kunal Kumar, Aliv Sahoo, Rahul Kumar Singh, Samriddhi Sankar Ray

Abstract: We investigate the transport of elastic active filaments in two-dimensional turbulence, focusing on how propulsion geometry and elasticity determine vortex trapping and transport. Using a bead-spring model with activity applied at the filament head, we compare propulsion that follows the instantaneous filament conformation with propulsion imposed along a fixed external direction. We find that acti… ▽ More We investigate the transport of elastic active filaments in two-dimensional turbulence, focusing on how propulsion geometry and elasticity determine vortex trapping and transport. Using a bead-spring model with activity applied at the filament head, we compare propulsion that follows the instantaneous filament conformation with propulsion imposed along a fixed external direction. We find that activity does not generically enhance transport: when propulsion remains coupled to the filament backbone, vortex trapping remains dominant and motion stays effectively diffusive, whereas fixed-direction propulsion enables persistent excursions across flow structures and leads to superdiffusive transport. In both cases, activity shifts filament conformations toward more extended states, effectively opposing elastic relaxation without eliminating preferential sampling of coherent vortical regions. At low Weissenberg number, this conformational change is amplified: activity cooperates with elasticity to enhance preferential sampling of vortical regions and strengthen vortex trapping. Transport therefore emerges from a competition between activity, elasticity, and flow-induced deformation, with elasticity determining how effectively activity-induced extensions can persist against turbulent trapping. These results establish propulsion geometry as the key control parameter for transport, with elasticity and activity acting cooperatively rather than independently to shape filament dynamics in turbulent flows. △ Less

Submitted 24 May, 2026; originally announced May 2026.

Evaluation of External Magnetic Flux Density in Piezo-Flexomagnetic Nanobeams Using a Hybrid 1D-2D Finite Element Framework

Authors: Lala Samprit Ray, Bishweshwar Babu

Abstract: This study numerically evaluates the external magnetic flux density generated in air by the bending of a piezo-flexomagnetic nanobeam. In several classes of non-contact sensors, the magnetic field induced in the surrounding medium is often more useful than the internal magnetic response. However, most theoretical studies on piezo-flexomagnetic nanostructures neglect the external magnetic domain. T… ▽ More This study numerically evaluates the external magnetic flux density generated in air by the bending of a piezo-flexomagnetic nanobeam. In several classes of non-contact sensors, the magnetic field induced in the surrounding medium is often more useful than the internal magnetic response. However, most theoretical studies on piezo-flexomagnetic nanostructures neglect the external magnetic domain. The proposed framework employs a coupled hybrid finite element formulation combining a 1D Timoshenko beam model with a 2D magnetostatic problem encompassing both the beam body and the surrounding air domain. The formulation is verified against analytical solutions of magnetically isolated piezo-flexomagnetic beams. The results demonstrate the presence of a significant external magnetic flux distribution in free-standing structures, even in the absence of piezomagnetic coupling. A systematic sensitivity analysis further identifies the material parameters most strongly influencing the external transverse magnetic flux density. These findings provide insight into the design of nanoscale non-contact magnetoelastic sensing systems. △ Less

Submitted 19 May, 2026; originally announced May 2026.

Krylov complexity and fidelity susceptibility in two-band Hamiltonians

Authors: Rishav Chaudhuri, Ayush Raj, Soham Ray, Sai Satyam Samal

Abstract: We investigate Krylov spread complexity for the ground state of two-band Hamiltonians, where the reference state is a generic state on the Bloch sphere. The spread complexity is obtained by using a purely geometric formulation in terms of Bloch sphere data without constructing the circuit Hamiltonian. For generic reference states, the derivative of the spread complexity is logarithmically divergen… ▽ More We investigate Krylov spread complexity for the ground state of two-band Hamiltonians, where the reference state is a generic state on the Bloch sphere. The spread complexity is obtained by using a purely geometric formulation in terms of Bloch sphere data without constructing the circuit Hamiltonian. For generic reference states, the derivative of the spread complexity is logarithmically divergent at the topological phase transition in the Su-Schrieffer-Heeger (SSH) model. We demonstrate that the derivative of the spread complexity is bounded by fidelity susceptibility for general two-band models, indicating the sensitivity of the spread complexity to any gap closing (topological or trivial). This is illustrated in the massive Dirac Hamiltonian with a trivial gap closing. Finally, we introduce a non-unitary duality in the SSH model between the topological and trivial phases, which manifests itself in the spread complexity and fidelity susceptibility. △ Less

Submitted 10 August, 2026; v1 submitted 18 May, 2026; originally announced May 2026.

Comments: Main text: 5 pages, 4 figures. Supplementary Materials: 25 pages, 4 figures

Mechanics of heterogeneous fiber networks

Authors: Kyu Hwan Choi, Sattvic Ray, Reef Sweeney, Zvonimir Dogic, Sho C. Takatori

Abstract: Internally generated active stresses drive soft materials into architectures inaccessible to thermal self-assembly. We use a microtubule-based active fluid to assemble and irreversibly restructure actin-fascin networks. Subsequently, we probe the mesoscale mechanics of such networks by combining active microrheology with fluorescence imaging of the strain field around the probe. Increasing motor c… ▽ More Internally generated active stresses drive soft materials into architectures inaccessible to thermal self-assembly. We use a microtubule-based active fluid to assemble and irreversibly restructure actin-fascin networks. Subsequently, we probe the mesoscale mechanics of such networks by combining active microrheology with fluorescence imaging of the strain field around the probe. Increasing motor concentration broadens the pore-size distribution and thickens load-bearing bundles, raising the mean local elastic modulus and its spatial variability. Displacement fields of actively-processed networks propagate over longer range when compared to unprocessed networks. At large strains, both networks strain soften and plastically restructure. The combined microrheology and strain-imaging approach show that tunable active stresses reprogram the structure and viscoelastic response of fiber networks at the scale of their structural heterogeneity. △ Less

Submitted 11 May, 2026; originally announced May 2026.

Comments: 7 pages, 4 figures, 4 videos

Probing the Valley-Selective Tunneling Density of States in Monolayer MoS2 based Resonant Tunneling Devices

Authors: Abir Mukherjee, Kajal Sharma, Ajit K Katiyar, Saranya Das, Samit K Ray, Samaresh Das

Abstract: The present work experimentally demonstrates the fabrication of CVD grown monolayer MoS2 ultra thin quantum well based double barrier resonant tunneling device (RTD) architecture well compatible with conventional CMOS fabrication technology. The strongly quantized electronic states from multiple valleys in the momentum space in such ultra 2D sheet along the c-axis sandwiched in between Al2O3 tunne… ▽ More The present work experimentally demonstrates the fabrication of CVD grown monolayer MoS2 ultra thin quantum well based double barrier resonant tunneling device (RTD) architecture well compatible with conventional CMOS fabrication technology. The strongly quantized electronic states from multiple valleys in the momentum space in such ultra 2D sheet along the c-axis sandwiched in between Al2O3 tunneling barriers exhibit multiple resonant tunneling peaks thereby enhancing the FWHM of the NDR region as derived from experimental I-V characteristics as well as theoretical joint invision through Density Functional Theory (DFT) and Non-Equilibrium Greens function (NEGF) visualized via Tunneling Density of States (TDOS). Understanding extended to S-vacancies not only change the bandgap, as evaluated through nanoscale Cathodoluminescence (CL) spectroscopy, but also alters the effective mass hence the mobility as investigated here within the high symmetry path in the k-space. Electrical performances of fabricated RTD, starting from cryogenic to room temperatures, show a significant milestone via exhibiting huge PVR values of 178 at 4K and 24 at RT with more possible improvement in the field of room temperature quantum technology. Momentum conserved and non conserved tunneling from highly n-doped Si through multiple valleys of 1L-MoS2 provides a tremendous opportunity in gate-induced manipulation in Spin-Valley Qubit technology operational at deep cryogenic temperatures (mK). △ Less

Submitted 4 May, 2026; originally announced May 2026.

NESSi 2.0: The Non-Equilibrium Systems Simulation package version 2.0

Authors: Fabian Künzel, Michael Schüler, Denis Golež, Yuta Murakami, Sujay Ray, Christopher Stahl, Jiajun Li, Hugo U. R. Strand, Philipp Werner, Martin Eckstein

Abstract: Nonequilibrium Green's functions provide a powerful framework for studying quantum many-body dynamics including the laser-induced dynamics in solids. The Non-Equilibrium Systems Simulation package (NESSi) offers an efficient platform for such simulations, ranging from perturbative approaches like nonequilibrium $GW$ to nonequilibrium dynamical mean-field theory. However, simulations based on noneq… ▽ More Nonequilibrium Green's functions provide a powerful framework for studying quantum many-body dynamics including the laser-induced dynamics in solids. The Non-Equilibrium Systems Simulation package (NESSi) offers an efficient platform for such simulations, ranging from perturbative approaches like nonequilibrium $GW$ to nonequilibrium dynamical mean-field theory. However, simulations based on nonequilibrium Green's functions become computationally demanding when the dynamics span a large temporal range, such as from sub-femtosecond electron dynamics to the picosecond dynamics of collective modes. Due to the memory integral in the Kadanoff-Baym equations, which serve as equations of motion for nonequilibrium Green's functions, the computational cost scales as $\mathcal{O}(N_t^3)$ with the number of timesteps $N_t$, and the memory requirement scales as $\mathcal{O}(N_t^2)$. In this work, we extend NESSi by incorporating techniques that aim to overcome this bottleneck: (i) By truncating the memory integrals in the KBE to a maximum of $N_c$ timesteps, the computational complexity is reduced to $\mathcal{O}(N_tN_c^2)$, and the memory requirement to $\mathcal{O}(N_c^2)$. Provided that the results converge with respect to the cutoff $N_c$, memory truncation allows to extend the simulations to significantly longer times. (ii) We introduce functionalities to describe nonequilibrium steady states, i.e. time-translationally invariant nonequilibrium states. Such states are relevant for transport settings, and they provide an approximate description of slowly evolving (prethermal) nonequilibrium states. △ Less

Submitted 4 May, 2026; originally announced May 2026.

Comments: 45 pages, 11 figures

Atomically-Thin Tsumoite (BiTe) based All-Photonic-Isolator, Information Converter, and Logic-Gate

Authors: Saswata Goswami, Caique Campos de Oliveira, Abhijith M. B., Varinder Pal, Vidya Kochat, Pulickel M. Ajayan, Samit K. Ray, Pedro A. S. Autreto, Chandra Sekhar Tiwary

Abstract: Two-dimensional tsumoite (BiTe), a polymorph of Bi2Te3, has emerged as a promising candidate for nonlinear photonic devices owing to its strong spin-orbit coupling, tunable bandgap, and high carrier mobility characteristics. This work presents a thorough examination of the third-order nonlinear optical response of BiTe dispersions using spatial self-phase modulation (SSPM) spectroscopy. The nonlin… ▽ More Two-dimensional tsumoite (BiTe), a polymorph of Bi2Te3, has emerged as a promising candidate for nonlinear photonic devices owing to its strong spin-orbit coupling, tunable bandgap, and high carrier mobility characteristics. This work presents a thorough examination of the third-order nonlinear optical response of BiTe dispersions using spatial self-phase modulation (SSPM) spectroscopy. The nonlinear refractive index (n2) and third-order nonlinear susceptibility are quantitatively derived from the diffraction ring patterns, demonstrating third-order nonlinear susceptibility values, similar to or surpassing those of advanced 2D materials. The temporal development and distortion of the SSPM rings are examined using the wind-chime model, and thermal factors influencing the SSPM pattern are analyzed. First-principles electronic band structure studies reveal that the elevated nonlinear susceptibility arises from band dispersion. Direct correlation between carrier transport and third-order nonlinear susceptibility is established. Utilizing these qualities, all photonic devices, including a photonic isolator based on a 2D BiTe-2D hBN heterostructure, are depicted to show asymmetric propagation. A photonic information converter and a logic gate are designed using the cross-phase modulation technique. These findings establish 2D BiTe nanostructure as a formidable nonlinear optical platform for advanced photonic signal processing and integrated photonic applications. △ Less

Submitted 13 April, 2026; originally announced April 2026.

Plasmonic Mediated Atomically Engineered 2D Aluminium Quasicrystals for Dopamine Biosensing

Authors: Saswata Goswami, Guilherme S. L. Fabris, Diganta Mondal, Raphael B. de Oliveira, Anyesha Chakraborty, Thakur Prasad Yadav, Nilay Krishna Mukhopadhyay, Samit K. Ray, Douglas S. Galvão, Chandra Sekhar Tiwary

Abstract: Dopamine levels are linked to neurological illnesses like Parkinson's and Alzheimer's. Thus, reliable and sensitive detection of dopamine is crucial for early diagnosis and surveillance of neurodegenerative diseases. Non-noble-metal-based nanomaterials are ideal for light-mediated sensing of organic molecules. Among these, 2D quasicrystal structures consisting of five elements, namely Al70Co10Fe5N… ▽ More Dopamine levels are linked to neurological illnesses like Parkinson's and Alzheimer's. Thus, reliable and sensitive detection of dopamine is crucial for early diagnosis and surveillance of neurodegenerative diseases. Non-noble-metal-based nanomaterials are ideal for light-mediated sensing of organic molecules. Among these, 2D quasicrystal structures consisting of five elements, namely Al70Co10Fe5Ni10Cu5, provide active sites due to their high surface-to-volume ratio, making them excellent for organic chemical sensing. Here, we propose a simple, label-free, spatial self-phase-modulation (SSPM)-based sensing method in liquid form. SSPM-based time evolution of the diffraction pattern for varied mixing levels of a 1100 ppb dopamine solution shows a shift in the active 2D Al QC solution. The 1100 ppb solution shows a distinct value, indicating a change in the nonlinear refractive index. Time-evolution analysis is used to calculate sensitivities to changes in the nonlinear refractive index and time constant. The SPR-activated 2D Al QC nanostructure is used to demonstrate dopamine sensing and to perform qualitative and quantitative evaluations. The SSPM-based sensing has been further compared with other optical-based sensing methods such as Raman spectroscopy, UV-Vis spectroscopy, and FTIR spectroscopy. The experimental observations are also explained using DFT-based simulations. The current SSPM method can be used for rapid, large-scale medical diagnostics. △ Less

Submitted 26 March, 2026; v1 submitted 25 March, 2026; originally announced March 2026.

Emergent thermal fluctuations and non-Hermitian phase transitions in open photon condensates

Authors: Moritz Janning, Roman Kramer, Michael Turaev, Sayak Ray, Johann Kroha

Abstract: We investigate the nonequilibrium dynamics of an open photon Bose-Einstein condensate in a dye-filled microcavity using a Lindblad master-equation approach, treating the condensate and the noncondensed fluctuations on the same footing. The driven-dissipative condensate exhibits a long-lived, metastable plateau stabilized by a ghost attractor, a fixed point that lies outside the physical domain in… ▽ More We investigate the nonequilibrium dynamics of an open photon Bose-Einstein condensate in a dye-filled microcavity using a Lindblad master-equation approach, treating the condensate and the noncondensed fluctuations on the same footing. The driven-dissipative condensate exhibits a long-lived, metastable plateau stabilized by a ghost attractor, a fixed point that lies outside the physical domain in configuration space, yet stalls the condensate dynamics for exceedingly long times before it dephases to zero [Phys. Rev. Lett. 135, 053402 (2025)]. Despite the nonequilibrium origin of this dynamical stabilization, the condensate exhibits quasithermal fluctuations in the plateau in that the relative order-parameter fluctuations scale as the inverse square root of the system size. A linear stability analysis further reveals the presence of exceptional points, resulting in multiple non-Hermitian phase transitions associated with the relaxation dynamics into and out of the metastable condensate. △ Less

Submitted 23 March, 2026; originally announced March 2026.

Comments: 8 pages, 8 figures

Unveiling the magnetic ground states in the iridate double perovskite Pr(2-x)SrxMgIrO6 (x = 0 and 0.5) series

Authors: Abhisek Bandyopadhyay, Debu Das, Dheeraj Kumar Pandey, C. Ritter, D. T. Adroja, Sugata Ray

Abstract: We report here the results of a detailed magnetic, thermodynamic, and neutron powder diffraction (NPD) studies carried out on the double perovskite iridates Pr(2-x)SrxMgIrO6 (x = 0 and 0.5). Temperature dependent bulk DC susceptibility data clearly reveals a sharp antiferromagnetic (AFM) transition at 14.5 K in Pr2MgIrO6(x = 0). Next, a weaker signature of an AFM transition at a lower temperature… ▽ More We report here the results of a detailed magnetic, thermodynamic, and neutron powder diffraction (NPD) studies carried out on the double perovskite iridates Pr(2-x)SrxMgIrO6 (x = 0 and 0.5). Temperature dependent bulk DC susceptibility data clearly reveals a sharp antiferromagnetic (AFM) transition at 14.5 K in Pr2MgIrO6(x = 0). Next, a weaker signature of an AFM transition at a lower temperature (6 K) is observed in x = 0.5 i.e., Pr1.5Sr0.5MgIrO6 (PSMIO1505). The observed magnetic transitions are further corroborated by the presence of anomalies around the same temperatures in our T-dependent specific heat results. The charge states of both Pr and Ir cations have been confirmed to be the expected ones (3+ for Pr in both the compounds, while Ir is in a pure 4+ state for x = 0 and in a mixed 4+/5+ state for x = 0.5) from the core-level x-ray photoemission spectroscopy (XPS) measurements. Using neutron powder diffraction (NPD) the magnetic ground states and the magnetic moment values were determined for both compounds. Both the Pr- and Ir-sites undergo AFM ordering below the respective transition temperatures, designated by the propagation vector k = ( 1/2 , 0, 1/2 ), in both the compounds. △ Less

Submitted 23 March, 2026; originally announced March 2026.

Reduction of Triadic Interactions Suppresses Intermittency and Anomalous Dissipation in Turbulence

Authors: Anikat Kankaria, Ritwik Mukherjee, Sugan Durai Murugan, Marco Edoardo Rosti, Samriddhi Sankar Ray

Abstract: We investigate how the defining statistical features of three-dimensional turbulence respond to systematic reductions of the Fourier-space triadic interaction network. Using direct numerical simulations of both fractally and homogeneously decimated Navier-Stokes dynamics, we show that progressive thinning of the set of active modes leads to a systematic suppression of intermittency and, most strik… ▽ More We investigate how the defining statistical features of three-dimensional turbulence respond to systematic reductions of the Fourier-space triadic interaction network. Using direct numerical simulations of both fractally and homogeneously decimated Navier-Stokes dynamics, we show that progressive thinning of the set of active modes leads to a systematic suppression of intermittency and, most strikingly, to the vanishing of the mean dissipation rate in the large-Reynolds-number limit. Structure-function exponents collapse onto their dimensional values, the multifractal singularity spectrum contracts, and the analyticity width extracted from the exponential spectral tail increases monotonically with decimation-each indicating a substantial regularization of the velocity field. Together, these results provide direct evidence that anomalous dissipation in incompressible turbulence is not a generic property of the Navier-Stokes equations, but instead requires the full combinatorial richness of their triadic nonlinear interactions. △ Less

Submitted 19 March, 2026; originally announced March 2026.

Comments: 8 pages, 6 figures

Hydrodynamics of Dense Active Fluids: Turbulence-Like States and the Role of Advected Activity

Authors: Sandip Sahoo, Siddhartha Mukherjee, Samriddhi Sankar Ray

Abstract: Dense suspensions of self-propelled bacteria and related active fluids exhibit spontaneous flow generation, vortex formation, and spatiotemporally chaotic dynamics despite operating at vanishingly small Reynolds numbers. These phenomena, commonly referred to as active turbulence, display striking visual and statistical similarities to classical inertial turbulence while arising from fundamentally… ▽ More Dense suspensions of self-propelled bacteria and related active fluids exhibit spontaneous flow generation, vortex formation, and spatiotemporally chaotic dynamics despite operating at vanishingly small Reynolds numbers. These phenomena, commonly referred to as active turbulence, display striking visual and statistical similarities to classical inertial turbulence while arising from fundamentally different nonequilibrium mechanisms. In this article, we present a combined review and theoretical study of hydrodynamic models for dense active fluids, with particular emphasis on bacterial suspensions described by the Toner--Tu--Swift--Hohenberg (TTSH) framework. We review key experimental and theoretical developments underlying the analogy between active and inertial turbulence, highlighting the emergence of multiple dynamical regimes and the conditions under which universal spectral and intermittent behavior arises in homogeneous systems. Moving beyond the conventional assumption of spatially uniform activity, we introduce a minimal model in which the activity field is heterogeneous and dynamically advected by the flow it generates. Thus treating activity as a spatiotemporally evolving field coupled to the TTSH dynamics, we investigate how advection and diffusion lead to sharp activity fronts, confinement of turbulent motion, and complex interfacial morphologies. Our numerical results demonstrate that spatial variations in activity can induce transient coexistence of distinct spectral regimes and that universality in active turbulence is inherently local and time-dependent in heterogeneous systems. These findings underscore the importance of treating activity as a dynamical field in its own right and provide a framework for studying active turbulence in more realistic, spatially structured biological and synthetic active matter systems. △ Less

Submitted 25 February, 2026; originally announced February 2026.

Comments: Mini-review and new results on heterogeneous active turbulence

Symmetry and Topology in a Non-Hermitian Kitaev chain

Authors: Ayush Raj, Soham Ray, Sai Satyam Samal

Abstract: We investigate the non-Hermitian Kitaev chain with non-reciprocal hopping amplitudes and asymmetric superconducting pairing. We work out the symmetry structure of the model and show that particle-hole symmetry (PHS) is preserved throughout the entire parameter regime. As a consequence of PHS, the topological phase transition point of a finite open chain coincides with that of the periodic (infinit… ▽ More We investigate the non-Hermitian Kitaev chain with non-reciprocal hopping amplitudes and asymmetric superconducting pairing. We work out the symmetry structure of the model and show that particle-hole symmetry (PHS) is preserved throughout the entire parameter regime. As a consequence of PHS, the topological phase transition point of a finite open chain coincides with that of the periodic (infinite) system. By explicitly constructing the zero-energy wave functions (Majorana modes), we show that Majorana modes necessarily occur as reciprocal localization pairs accumulating on opposite boundaries, whose combined probability density exhibits an exact cancellation of the non-Hermitian skin effect for the zero energy modes. Excited states, by contrast, generically display skin-effect localization, with particle and hole components accumulating at opposite ends of the system. At the level of bulk topology, we further construct a $\mathbb{Z}_2$ topological invariant in restricted parameter regimes that correctly distinguishes the topological and trivial phases. Finally, we present the topological phase diagram of the non-Hermitian Kitaev chain across a broad range of complex parameters and delineate the associated phase boundaries. △ Less

Submitted 19 May, 2026; v1 submitted 2 January, 2026; originally announced January 2026.

Comments: Main text: 6.5 papges, 5 figures, 1 table. Appendix: 6.5 pages, 1 figure, 1 table

The dynamics of thermalisation in the Galerkin-truncated, three-dimensional Euler equation

Authors: Rajarshi, Mohammad Saif Khan, Prateek Anand, Samriddhi Sankar Ray

Abstract: The inviscid, partial differential equations of hydrodynamics when projected via a Galerkin-truncation on a finite-dimensional subspace spanning wavenumbers $-{\bf K}_{\rm G} \le {\bf k} \le {\bf K}_{\rm G}$, and hence retaining a finite number of modes $N_{\rm G}$, lead to absolute equilibrium states. We review how the Galerkin-truncated, three-dimensional, incompressible Euler equation thermalis… ▽ More The inviscid, partial differential equations of hydrodynamics when projected via a Galerkin-truncation on a finite-dimensional subspace spanning wavenumbers $-{\bf K}_{\rm G} \le {\bf k} \le {\bf K}_{\rm G}$, and hence retaining a finite number of modes $N_{\rm G}$, lead to absolute equilibrium states. We review how the Galerkin-truncated, three-dimensional, incompressible Euler equation thermalises and its connection to questions in turbulence. We also discuss an emergent pseudo-dissipation range in the energy spectrum and the time-scales associated with thermalisation. △ Less

Submitted 11 December, 2025; originally announced December 2025.

Comments: A mini review and new results. 9 pages and 3 figures. Comments are welcome

Uncertainty Growth in Stably Stratified Turbulence

Authors: Mrinal Jyoti Powdel, Samriddhi Sankar Ray

Abstract: We investigate uncertainty growth and chaotic dynamics in statistically steady, stably stratified three-dimensional turbulence. Using direct numerical simulations of the Boussinesq equations, we quantify the divergence of initially infinitesimal perturbations via twin simulations and decorrelator diagnostics. At short times, perturbations exhibit exponential growth, allowing us to define a (larges… ▽ More We investigate uncertainty growth and chaotic dynamics in statistically steady, stably stratified three-dimensional turbulence. Using direct numerical simulations of the Boussinesq equations, we quantify the divergence of initially infinitesimal perturbations via twin simulations and decorrelator diagnostics. At short times, perturbations exhibit exponential growth, allowing us to define a (largest) Lyapunov exponent. We systematically examine how this exponent depends on stratification strength, quantified by the Brunt--Väisälä frequency and the Froude number, in a parameter regime relevant to oceanic flows. We find that increasing stratification leads to a monotonic reduction of the Lyapunov exponent, indicating suppressed chaoticity. Despite this reduction, uncertainty growth retains the universal temporal sequence observed in homogeneous isotropic turbulence -- initial decay, exponential growth, and saturation. The growth phase is characterized by self-similar decorrelator spectra, but exhibits strong anisotropy: uncertainty spreads much more slowly along the stratification direction than horizontally, with the disparity increasing with stratification strength. An analysis of the decorrelator evolution equation reveals that the suppression of chaos arises primarily from strain-mediated alignment dynamics rather than direct buoyancy coupling. Our results provide a quantitative characterization of predictability and uncertainty growth in stratified turbulence and highlight the utility of decorrelator-based methods for anisotropic geophysical flows. △ Less

Submitted 5 December, 2025; originally announced December 2025.

Comments: 10 pages, 5 figures

Facile Salt-Assisted Hydrothermal Synthesis of Nanodiamonds from CHO Precursors: Atomic-Scale Mechanistic Insights

Authors: Soumya Pratap Tripathy, Sayan Saha, Saurabh Kumar Gupta, Pallavee Das, Binay Priyadarsan Nayak, Anup Routray, Priya Choudhary, Srihari V, Bitop Maitra, Ashna Reyaz, Anushka Samant, Debopriya Sinha, Kritideepan Parida, Kuna Das, Abhijeet Sahoo, Kunal Pal, Sirsendu Sekhar Ray

Abstract: Hydrothermal synthesis offers an economical and scalable way to produce nanodiamonds under relatively mild, low-pressure and low-temperature conditions. However,its sustainability and the detailed mechanisms behind diamond formation in such environments are still not fully understood. In this work, we designed ten hydrothermal synthesis protocols using different CHO-based molecular precursors cont… ▽ More Hydrothermal synthesis offers an economical and scalable way to produce nanodiamonds under relatively mild, low-pressure and low-temperature conditions. However,its sustainability and the detailed mechanisms behind diamond formation in such environments are still not fully understood. In this work, we designed ten hydrothermal synthesis protocols using different CHO-based molecular precursors containing COOH and OH groups, such as organic acids, polyols, sugars, and polysaccharides.The reactions were carried out at 190 degrees Centigrade in chlorinated, strongly alkaline aqueous solutions with alkali and alkaline-earth metal ions. Using high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy, we confirmed the presence of diamond-specific lattice planes and sp3-hybridized carbon structures. Our results show that the type of precursor, its molecular size, and the ionic composition of the solution play key roles in determining the defect patterns and polymorph distribution in the resulting nanodiamonds. Atomic-scale imaging showed both coherent and incoherent transitions from graphite to diamond, along with gradual lattice compression and complex twinning patterns. These observations provide direct insight into how interfacial crystallography and defect dynamics drive diamond formation in aqueous systems. Overall, the study positions hydrothermal synthesis as a sustainable, chemistry-driven, and tunable approach for creating nanodiamonds tailored for applications in quantum technologies, biomedicine, catalysis, and advanced materials. △ Less

Submitted 9 November, 2025; originally announced November 2025.

Comments: Main Manuscript: 9 pages, 1 table, 5 figures; Supplementary Information: 18 pages, 1 table, 26 figures

Geometric Intermittency in Turbulence

Authors: Ritwik Mukherjee, Siddhartha Mukherjee, I. V. Kolokolov, V. V. Lebedev, Takeshi Matsumoto, Samriddhi Sankar Ray

Abstract: Equal-time scaling exponents in fully developed turbulence typically exhibit non anomalous scaling in the inverse cascade of two-dimensional (2D) turbulence and anomalous scaling in three dimensions. We demonstrate that multiscaling is not confined to longitudinal, scalar velocity increments, but also emerges in increments associated with the magnitude and orientation of the velocity vector. This… ▽ More Equal-time scaling exponents in fully developed turbulence typically exhibit non anomalous scaling in the inverse cascade of two-dimensional (2D) turbulence and anomalous scaling in three dimensions. We demonstrate that multiscaling is not confined to longitudinal, scalar velocity increments, but also emerges in increments associated with the magnitude and orientation of the velocity vector. This decomposition uncovers a multiscaling in the 2D inverse cascade, which remains obscured when using conventional structure functions. Our results highlight a decoupling between velocity amplitude and flow geometry, offering new insight into the statistical structure of turbulent cascades as well as showing how different classes of multiscaling emerge. △ Less

Submitted 9 November, 2025; originally announced November 2025.

First-Principles Study of Transition Metal Doped in 2D Polyaramid for Novel Material Modelling

Authors: Ravi Trivedi, Chaithanya Purushottam Bhat, Shakti S. Ray, Debashis Bandyopadhyay

Abstract: We present a first--principles density functional theory (DFT) study of transition metal (TM = Ti, Cr, Mn, Fe, Co, Ni) functionalized two--dimensional polyaramid (2DPA) to explore their structural, electronic, and magnetic properties. Mechanical parameters, such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Pugh ratio, together with phonon dispersion, confirm the mechanical… ▽ More We present a first--principles density functional theory (DFT) study of transition metal (TM = Ti, Cr, Mn, Fe, Co, Ni) functionalized two--dimensional polyaramid (2DPA) to explore their structural, electronic, and magnetic properties. Mechanical parameters, such as bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and Pugh ratio, together with phonon dispersion, confirm the mechanical and dynamic stability of all doped systems. Electronic structure analysis shows strong binding of Co, Cr, Fe, Ni, and Ti with formation energies between --1.15 eV and --2.96 eV, while Mn binds more weakly (--0.67 eV). TM doping introduces new electronic states that reduce the band gap, with Fe-doped 2DPA exhibiting the lowest value of 0.26 eV. The systems display predominantly ferromagnetic ordering, with magnetic moments of 1.14 μB (Co), 3.57 μB (Cr), 2.26 μB (Fe), 4.19 μB (Mn), and 1.62 μB (Ti). These results demonstrate that TM--doped 2DPA possesses tunable magnetic and electronic characteristics, highlighting its potential for spintronic applications. △ Less

Submitted 31 October, 2025; originally announced October 2025.

Comments: 12 pages, 7 figures, Original work

Photoinduced excitonic magnetism in a multiorbital Hubbard system

Authors: Lei Geng, Sujay Ray, Philipp Werner

Abstract: Multiorbital Hubbard models with Hund coupling and crystal-field splitting exhibit an instability toward spin-triplet excitonic order in the parameter regime characterized by strong local spin fluctuations. Upon chemical doping, two distinct types of excitonic ferromagnetism have been reported. Using steady-state nonequilibrium dynamical mean-field theory, we demonstrate that photo-doped half-fill… ▽ More Multiorbital Hubbard models with Hund coupling and crystal-field splitting exhibit an instability toward spin-triplet excitonic order in the parameter regime characterized by strong local spin fluctuations. Upon chemical doping, two distinct types of excitonic ferromagnetism have been reported. Using steady-state nonequilibrium dynamical mean-field theory, we demonstrate that photo-doped half-filled systems can host nonthermal counterparts of these excitonic phases and exhibit a rich phase diagram in the space of photo-doping and crystal field splitting. Photo-doping a spin-triplet excitonic insulator provides a route towards nonequilibrium control of magnetic order. △ Less

Submitted 31 October, 2025; originally announced October 2025.

Journal ref: Phys. Rev. B 113, 035143 (2026)

The significance of two-way coupling in two-dimensional, dusty turbulence

Authors: Harshit Joshi, Amal Manoharan, Samriddhi Sankar Ray

Abstract: The significance of small-scale forcing of particles on the carrier two-dimensional turbulent flow has been shown to influence the spectral scaling properties of the carrier fluid. We investigate possible consequences of such two-way coupling in a turbulent suspension of inertial particles through one- and two-point Eulerian and Lagrangian statistics. In particular, we find signatures of enhanced… ▽ More The significance of small-scale forcing of particles on the carrier two-dimensional turbulent flow has been shown to influence the spectral scaling properties of the carrier fluid. We investigate possible consequences of such two-way coupling in a turbulent suspension of inertial particles through one- and two-point Eulerian and Lagrangian statistics. In particular, we find signatures of enhanced intermittency in the vorticity distributions. We characterize the changes in the small-scale geometry of the flow via the Okubo-Weiss parameter. Finally, we examine the scaling properties of the second-order vorticity structure functions and find a non-trivial form of scale-invariance at finite mass loading. Motivated by these observations, we propose an effective multiscale forcing framework in which particle feedback is modeled as a spatially localized small-scale forcing. This dual-scale forcing captures the emergence of modified spectral scaling and provides a minimal Eulerian description of particle-laden turbulence that reproduces key statistical signatures of the system. △ Less

Submitted 21 April, 2026; v1 submitted 12 October, 2025; originally announced October 2025.

Comments: 10 pages, 11 figures

Non-Lagrangian phases of matter from Wilsonian renormalization of 3D Wess-Zumino-Witten theory on Stiefel manifolds

Authors: Shouryya Ray

Abstract: I study the renormalization of D-dimensional level-k Wess-Zumino-Witten theory with Stiefel-manifold target space $\mathrm{St}_{N,N-D-1} \cong \mathrm{SO}(N)/\mathrm{SO}(D+1)$, with a particular focus on $D = 3$. I investigate in particular whether such a theory admits IR-stable fixed points of the renormalization group flow. Such fixed points have been suggested to describe conformal phases of ma… ▽ More I study the renormalization of D-dimensional level-k Wess-Zumino-Witten theory with Stiefel-manifold target space $\mathrm{St}_{N,N-D-1} \cong \mathrm{SO}(N)/\mathrm{SO}(D+1)$, with a particular focus on $D = 3$. I investigate in particular whether such a theory admits IR-stable fixed points of the renormalization group flow. Such fixed points have been suggested to describe conformal phases of matter that do not have a known dual (super-)renormalizable Lagrangian for $N \geq 7$ in $D = 3$. They are hence of interest both from the point of view of quantum phases of matter as well as pure field theory. The $D$-dimensional expressions enable the computation, by analytic computation, of beta functions in $D = 2 + ε$, at least to first non-trivial order. In $D = 2$, a stable fixed point is found, serving a generalization of the famed $\mathrm{SU}(2)_k$ Wess-Zumino-Witten conformal field theory; it annihilates in $D = 2 + ε$ with an unstable fixed point which splits off from the Gaussian one for $ε> 0$. Although the story is thus qualitatively similar to that of SO(5) deconfined (pseudo-)criticality, for $N \geqslant 6$, the annihilation appears to occur only for $ε> 1$, suggesting the existence of a stable phase in $D = 3$. Comparisons of the scaling dimension of the lowest singlet operator are made with known results for $N = 6$, which is dual to QED$_3$ with $N_\mathrm{f} = 4$ fermion flavors. The predictions for the $N = 7$ Stiefel liquid represent to my knowledge the first computation of this kind for a Wess-Zumino-Witten theory without a known gauge theory dual. △ Less

Submitted 23 September, 2025; originally announced September 2025.

Comments: 5+2 pages, 2+0 figures. Comments welcome

Crystal Growth, Band Structure, Magnetism and Electrochemical Properties of Hexavalent Strontium Ruthenium Oxyhydroxide

Authors: Subham Naik, Soumili Dutta, Hiranmayee Senapati, Sweta Yadav, Subarna Ray, Jai Prakash, Rahul Sharma, Gohil S. Thakur

Abstract: Ruthenates comprise an interesting class of materials with a wide range of extremely exciting properties, and thus the discovery of new stable ruthenates remains an active area of investigation. We report the crystal growth and comprehensive studies including crystal and electronic structure, magnetic and electrochemical properties of a hexavalent ruthenium oxyhydroxide Sr3Ru2O9H2 prepared through… ▽ More Ruthenates comprise an interesting class of materials with a wide range of extremely exciting properties, and thus the discovery of new stable ruthenates remains an active area of investigation. We report the crystal growth and comprehensive studies including crystal and electronic structure, magnetic and electrochemical properties of a hexavalent ruthenium oxyhydroxide Sr3Ru2O9H2 prepared through a low-temperature hydrothermal method. Single crystals and powder samples of this phase are isolated by optimising the Sr(OH)2 to KRuO4 ratio while maintaining a high base concentration. The new structure consists of a rare five-coordinated RuVI featuring isolated trigonal prisms and crystallising in a non-centrosymmetric tetragonal system. Isolated Ru polyhedra leading to a large spatial distance ~ 50 pm between the Ru metal centres render the compound paramagnetic despite strong antiferromagnetic correlation. Band structure calculation suggests a metal-like electronic ground state with mostly Ru d and O p orbitals contributing to the Fermi surface. The electrochemical performance of Sr3Ru2O9H2, though not as impressive as RuO2, remains relevant and is on par with other reported OER catalysts. △ Less

Submitted 19 September, 2025; originally announced September 2025.

Comments: 28 pages including supplementary information

Journal ref: ACS Omega 2026

Extreme dynamics and relaxation of quantum gases: A hydrodynamic approach

Authors: Ritwik Mukherjee, Abhishek Dhar, Manas Kulkarni, Samriddhi Sankar Ray

Abstract: The evolution of quantum gases, released from traps, are studied through hydrodynamics, both analytically and numerically, in one and two dimensions. In particular, we demonstrate the existence of long time self-similar solutions of the Euler equations, for the density and velocity fields, and derive the scaling exponents as well as the scaling functions. We find that the expanding gas develops a… ▽ More The evolution of quantum gases, released from traps, are studied through hydrodynamics, both analytically and numerically, in one and two dimensions. In particular, we demonstrate the existence of long time self-similar solutions of the Euler equations, for the density and velocity fields, and derive the scaling exponents as well as the scaling functions. We find that the expanding gas develops a shock front and the size of the cloud grows in time as a powerlaw. We relate the associated exponent to that appearing in the corresponding equation of state of the quantum gas. Furthermore, we study the relaxation dynamics of a trapped quantum gas and show that the resulting steady state is in excellent agreement with that derived analytically. Our hydrodynamic approach is versatile and can be used to unravel several other far-from-equilibrium collective phenomenon of extreme nature, relevant to the growing experimental interests in quantum gases. △ Less

Submitted 30 August, 2025; originally announced September 2025.

Comments: 13 pages, 8 figures

Journal ref: Phys. Rev. A 113, 043305 (2026)

All Photonic Isolator using Atomically Thin (2D) Bismuth Telluride (Bi2Te3)

Authors: Saswata Goswami, Bruno Ipaves, Juan Gomez Quispe, Caique Campos de Oliveira, Surbhi Slathia, Abhijith M. B, Varinder Pal, Christiano J. S. de Matos, Samit K. Ray, Douglas S. Galvao, Pedro A. S. Autreto, Chandra Sekhar Tiwary

Abstract: This study demonstrates that two-dimensional (2D) Bi2Te3 exhibits strong light-matter interaction, enabling a broadband Kerr nonlinear optical response. This characteristic is advantageous for nonreciprocal light propagation in passive photonic isolators. Using Spatial Self-Phase Modulation (SSPM) spectroscopy, self-induced diffraction patterns in the far field were observed at excitation waveleng… ▽ More This study demonstrates that two-dimensional (2D) Bi2Te3 exhibits strong light-matter interaction, enabling a broadband Kerr nonlinear optical response. This characteristic is advantageous for nonreciprocal light propagation in passive photonic isolators. Using Spatial Self-Phase Modulation (SSPM) spectroscopy, self-induced diffraction patterns in the far field were observed at excitation wavelengths of 650 nm, 532 nm, and 405 nm to calculate the nonlinear refractive index (n2) and the third-order nonlinear optical susceptibility (chi^(3)) of the synthesized 2D Bi2Te3. The results show that 2D Bi2Te3 possesses a significantly higher nonlinear refractive index than graphene. The laser-induced hole coherence effect is responsible for the large magnitude of the third-order nonlinear susceptibility. Surface engineering techniques were also employed to enhance the response speed of the photonic system. Complementary ab initio simulations were performed to gain further insight into the observed nonlinear behavior. Leveraging the strong Kerr nonlinearity of 2D Bi2Te3, a nonlinear photonic isolator that breaks time-reversal symmetry and enables unidirectional light propagation was demonstrated. This work establishes Bi2Te3 as a novel 2D material for nonlinear photonics, expanding its potential applications in detectors, modulators, and optical switches. △ Less

Submitted 5 August, 2025; originally announced August 2025.

Expansion dynamics of strongly correlated lattice bosons: A selfconsistent density-matrix approach

Authors: Julian Schwingel, Michael Turaev, Johann Kroha, Sayak Ray

Abstract: We study the spatio-temporal dynamics of interacting bosons on a two-dimensional Hubbard lattice in the strongly interacting regime, taking into account the dynamics of condensate amplitude as well as the direct transport of non-condensed fluctuations. To that end we develop a selfconsistent density-matrix approach which goes beyond the standard Gutzwiller mean-field theory. Starting from the Liou… ▽ More We study the spatio-temporal dynamics of interacting bosons on a two-dimensional Hubbard lattice in the strongly interacting regime, taking into account the dynamics of condensate amplitude as well as the direct transport of non-condensed fluctuations. To that end we develop a selfconsistent density-matrix approach which goes beyond the standard Gutzwiller mean-field theory. Starting from the Liouville-von-Neumann equation we derive a quantum master equation for the time evolution of the system's local density matrix at each lattice site, with a dynamical bath that represents the rest of the system. We apply this method to the expansion dynamics of an initially prepared cloud of interacting bosons in an optical lattice. We observe a ballistic expansion of the condensate, as expected, followed by slow, diffusive transport of the normal bosons. We discuss, in particular, the robustness of the Mott insulator phase as well as its melting due to incoherent transport. The method should be applicable to various models of lattice bosons in the strongly correlated regime. △ Less

Submitted 29 October, 2025; v1 submitted 12 July, 2025; originally announced July 2025.

Journal ref: Phys. Rev. A 112, 043322 (2025)

Opposite pressure effects on magnetic phase transitions in NiBr2

Authors: Parvez Ahmed Qureshi, Krishna Kumar Pokhrel, Jiri Prchal, Subhasmita Ray, Sergiu Arapan, Karel Carva, Vladimir Sechovsky, Jiri Pospisil

Abstract: NiI2 and NiBr2 are archetypal van der Waals (vdW) triangular-lattice multiferroics that host incommensurate helimagnetic order at the lowest temperatures and undergo a transition to collinear antiferromagnetic order upon heating. Focusing on NiBr2, we reveal that both antiferromagnetic phases exhibit a pronounced sensitivity to hydrostatic pressure. The Neel temperature of the collinear phase incr… ▽ More NiI2 and NiBr2 are archetypal van der Waals (vdW) triangular-lattice multiferroics that host incommensurate helimagnetic order at the lowest temperatures and undergo a transition to collinear antiferromagnetic order upon heating. Focusing on NiBr2, we reveal that both antiferromagnetic phases exhibit a pronounced sensitivity to hydrostatic pressure. The Neel temperature of the collinear phase increases steeply at 20 K/GPa, reaching 100 K at 3 GPa without any indication of saturation, whereas the helimagnetic phase is completely suppressed only above 0.8 GPa. This behavior contrasts sharply with NiI2, in which both helical and collinear phases are strengthened until a moderate pressure of 6 GPa, above which the helical phase instantly disappears. Ab initio calculations identify the second-nearest interlayer exchange interaction (j2') as the primary driver stabilizing the collinear AFM phase in NiBr2. In addition, the in-plane exchange ratio renders the helical order in NiBr2 considerably more fragile, enabling its suppression under relatively small pressures. These results underscore the dominant role of interlayer interactions in governing the distinct pressure responses of the magnetic phases in NiBr2 and NiI2. △ Less

Submitted 11 May, 2026; v1 submitted 5 June, 2025; originally announced June 2025.

Intermittent fluctuations determine the nature of chaos in turbulence

Authors: Aikya Banerjee, Ritwik Mukherjee, Sugan Durai Murugan, Subhro Bhattacharjee, Samriddhi Sankar Ray

Abstract: We adapt recent ideas for many-body chaos in nonlinear, Hamiltonian fluids [Murugan \textit{et al.}, Phys. Rev. Lett. 127, 124501 (2021)] to revisit the question of the Reynolds number Re dependence of the Lyapunov exponent $λ\propto{\rm Re}^α$ in fully developed turbulence. The use of such decorrelators allow us to investigate the interplay of the competing effects of viscous dissipation and nonl… ▽ More We adapt recent ideas for many-body chaos in nonlinear, Hamiltonian fluids [Murugan \textit{et al.}, Phys. Rev. Lett. 127, 124501 (2021)] to revisit the question of the Reynolds number Re dependence of the Lyapunov exponent $λ\propto{\rm Re}^α$ in fully developed turbulence. The use of such decorrelators allow us to investigate the interplay of the competing effects of viscous dissipation and nonlinearity. We obtain a precise value of $α= 0.59 \pm 0.04$ and show that departure from the Kolmogorov mean field result $λ\propto \sqrt{\rm Re}$ is a consequence of the intermittent fluctuations in the velocity-gradient tensor. The robustness of our results are further confirmed in a local, dynamical systems model for turbulence. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Stabilizing open photon condensates by ghost-attractor dynamics

Authors: Aya Abouelela, Michael Turaev, Roman Kramer, Moritz Janning, Michael Kajan, Sayak Ray, Johann Kroha

Abstract: We study the temporal, driven-dissipative dynamics of open photon Bose-Einstein condensates (BEC) in a dye-filled microcavity, taking the condensate amplitude and the noncondensed fluctuations into account on the same footing by means of a cumulant expansion within the Lindblad formalism. The fluctuations fundamentally alter the dynamics in that the BEC always dephases to zero for sufficiently lon… ▽ More We study the temporal, driven-dissipative dynamics of open photon Bose-Einstein condensates (BEC) in a dye-filled microcavity, taking the condensate amplitude and the noncondensed fluctuations into account on the same footing by means of a cumulant expansion within the Lindblad formalism. The fluctuations fundamentally alter the dynamics in that the BEC always dephases to zero for sufficiently long time. However, a ghost-attractor, although it is outside of the physically accessible configuration space, attracts the dynamics and leads to a plateau-like stabilization of the BEC for an exponentially long time, consistent with experiments. We also show that the photon BEC and the lasing state are separated by a true phase transition, since they are characterized by different fixed points. The ghost-attractor nonequilibrium stabilization mechanism is alternative to prethermalization and may possibly be realized on other dynamical platforms as well. △ Less

Submitted 9 July, 2025; v1 submitted 20 March, 2025; originally announced March 2025.

Journal ref: Phys. Rev. Lett. 135, 053402 (2025)

Turbulence-Induced Fluctuating Interfaces in Heterogeneously-Active Suspensions

Authors: Siddhartha Mukherjee, Kunal Kumar, Samriddhi Sankar Ray

Abstract: We investigate the effects of heterogeneous (spatially varying) activity in a hydrodynamical model for dense bacterial suspensions, confining ourselves to experimentally realizable, simple, quenched, activity patterns. We show that the evolution of the bacterial velocity field under such activity patterning leads to the emergence of hydrodynamic interfaces separating spatially localized turbulence… ▽ More We investigate the effects of heterogeneous (spatially varying) activity in a hydrodynamical model for dense bacterial suspensions, confining ourselves to experimentally realizable, simple, quenched, activity patterns. We show that the evolution of the bacterial velocity field under such activity patterning leads to the emergence of hydrodynamic interfaces separating spatially localized turbulence from jammed frictional surroundings. We characterise the intermittent and multiscale fluctuations of this interface and also investigate how heterogeneity influences mixing via the residence times of Lagrangian tracers. This work reveals how naturally occurring heterogeneities could decisively steer active flows into more complex configurations than those typically studied, opening up parallels to droplet dynamics, front propagation and turbulent mixing layers. △ Less

Submitted 10 May, 2026; v1 submitted 23 February, 2025; originally announced February 2025.

Comments: 14 pages, 4 figures

Journal ref: PNAS Nexus 5, No. 5 (2026): pgag125

Exceptionally High Nonlinear Optical Response in Two-dimensional Type II Dirac Semimetal Nickel di-Telluride (NiTe2)

Authors: Saswata Goswami, Caique Campos de Oliveira, Bruno Ipaves, Preeti Lata Mahapatra, Varinder Pal, Suman Sarkar, Pedro A. S. Autreto, Samit K. Ray, Chandra Sekhar Tiwary

Abstract: Nickel ditelluride (NiTe2) is a newly identified Type-II Dirac semimetal, showing novel characteristics in electronic transport and optical experiments. In this study, we explored the nonlinear optical properties of two-dimensional NiTe2 using experimental and computational techniques (density functional theory-based approach). Few layered two-dimensional NiTe2 (2D-NiTe2) are synthesized using liq… ▽ More Nickel ditelluride (NiTe2) is a newly identified Type-II Dirac semimetal, showing novel characteristics in electronic transport and optical experiments. In this study, we explored the nonlinear optical properties of two-dimensional NiTe2 using experimental and computational techniques (density functional theory-based approach). Few layered two-dimensional NiTe2 (2D-NiTe2) are synthesized using liquid phase exfoliation (LPE), which is characterized using X-ray diffraction technique, transmission electron, and atomic force microscopy. The nonlinear refractive index and third-order nonlinear susceptibility of the prepared 2D-NiTe2 are determined from the self-induced diffraction pattern generated using different wavelengths ( 405, 532, and 650 nm) in the far field. In addition, the diffraction pattern generated by spatial self-phase modulation (SSPM) is further verified by varying concentration (2D-NiTe2 in the IPA solvent), wavelength (of incoming laser beams), and cuvette width (active path length). The high value of third-order nonlinear susceptibility (in order of 10-9 e.s.u.) determined using SSPM in the 2D-NiTe2 can be attributed to the laser-induced hole coherence effect. Lastly, utilizing the reverse saturable absorption property of 2D-hBN, asymmetric light propagation is also demonstrated in the 2D-NiTe2/2D-hBN heterostructure. △ Less

Submitted 15 February, 2025; originally announced February 2025.

Comments: 56 pages, 19 figures

Suppression of ferromagnetism in van der Waals insulator due to pressure-induced layer stacking variation

Authors: M. Misek, U. Dutta, P. Kral, D. Hovancik, J. Kastil, K. Pokhrel, S. Ray, J. Valenta, J. Prchal, J. Kamarad, F. Borodavka, V. Eigner, M. Dusek, V. Holy, K. Carva, S. Kamba, V. Sechovsky, J. Pospisil

Abstract: Ferromagnetism in van der Waals insulators like CrBr3 is highly sensitive to structural modifications. We explore the pressure-driven structural and magnetic transformations of the van der Waals magnet CrBr3, establishing it as a model platform for phenomena emerging in layered vdW systems. Single-crystal X-ray diffraction revealed intrinsic trimorphism with two known phases (monoclinic and rhombo… ▽ More Ferromagnetism in van der Waals insulators like CrBr3 is highly sensitive to structural modifications. We explore the pressure-driven structural and magnetic transformations of the van der Waals magnet CrBr3, establishing it as a model platform for phenomena emerging in layered vdW systems. Single-crystal X-ray diffraction revealed intrinsic trimorphism with two known phases (monoclinic and rhombohedral) and a so-far unreported trigonal phase. The paracrystal model well captures the coexistence of the rhombohedral and trigonal phases at ambient conditions. Increasing pressure drives the growth of the AA-stacked trigonal phase at the expense of the rhombohedral phase, which becomes undetectable above 8.4 GPa. Magnetization measurements provided the first direct evidence of a collapse of ferromagnetism above 5.8 GPa. This behavior is attributed to the increasing number of AFM-coupled Cr moments in AA stackings. Ab initio DFT calculations of electronic structure and atomistic simulations of finite-temperature magnetism corroborate the scenario. △ Less

Submitted 7 July, 2026; v1 submitted 23 January, 2025; originally announced January 2025.

Role of phonon coupling in driving photo-excited Mott insulators towards a transient superconducting steady state

Authors: Sujay Ray, Martin Eckstein, Philipp Werner

Abstract: Understanding light-induced hidden orders is relevant for nonequilibrium materials control and future ultrafast technologies. Hidden superconducting order, in particular, has been a focus of recent experimental and theoretical efforts. In this study, we investigate the stability of light-induced $η$ pairing. Using a memory truncated implementation of nonequilibrium dynamical mean field theory (DMF… ▽ More Understanding light-induced hidden orders is relevant for nonequilibrium materials control and future ultrafast technologies. Hidden superconducting order, in particular, has been a focus of recent experimental and theoretical efforts. In this study, we investigate the stability of light-induced $η$ pairing. Using a memory truncated implementation of nonequilibrium dynamical mean field theory (DMFT) and entropy cooling techniques, we study the long-time dynamics of the photoinduced superconducting state. In the presence of coupling to a cold phonon bath, the photodoped system reaches a quasi-steady state, which can be sustained over a long period of time in large-gap Mott insulators. We show that this long-lived prethermalized state is well described by the nonequilibrium steady state implementation of DMFT. △ Less

Submitted 26 December, 2024; originally announced December 2024.

Gross-Neveu-Yukawa SO(2) and SO(3) tensorial criticality

Authors: SangEun Han, Shouryya Ray, Igor F. Herbut

Abstract: We investigate the relativistic SO(2)- and SO(3)-invariant Gross-Neveu-Yukawa field theories for real, rank-two, symmetric, traceless tensor order parameters coupled to $N_{\text{f}}$ flavors of two-component Dirac fermions. These field theories arise as an effective description of fractionalized spin-orbital liquids. The two theories are the simplest and special cases of the more general class of… ▽ More We investigate the relativistic SO(2)- and SO(3)-invariant Gross-Neveu-Yukawa field theories for real, rank-two, symmetric, traceless tensor order parameters coupled to $N_{\text{f}}$ flavors of two-component Dirac fermions. These field theories arise as an effective description of fractionalized spin-orbital liquids. The two theories are the simplest and special cases of the more general class of field theories with SO($N$) symmetric tensor order parameter coupled to Dirac fermions, in which the symmetry is low enough to allow only one, and not the usual two quartic self-interaction terms. Using two-loop renormalization group near the upper critical dimension, we demonstrate that the theory exhibits a new critical fixed point for $N=3$ and the concomitant continuous phase transition for any value of $N_{\text{f}}$. For $N=2$ the theory is equivalent to the chiral XY model. We discuss the crucial role of the symmetry-allowed sextic self-interactions in the selection of the ground state configuration in the case of SO(3). The universal quantities such as the the anomalous dimensions of order parameters and fermions, the correlation length exponent, and the mass gap ratio between order parameter and fermion masses are computed up to $ε^{2}$ order. △ Less

Submitted 24 March, 2025; v1 submitted 25 November, 2024; originally announced November 2024.

Comments: 10 pages, 4 figures

Journal ref: Phys. Rev. B 111, 115131 (2025)

Experimental studies on collision between self-propelling liquid crystal droplets in quasi-two-dimensional geometry

Authors: Swarnak Ray, Arun Roy

Abstract: Active droplets undergoing micellar solubilization can self-propel themselves by emitting filled micelles from their surface and are by nature anti-chemotactic. These droplets get repelled by their own trail of filled micelles or by other neighbouring droplets. We experimentally study the various types of scattering events between two such active droplets. We define different collision parameters… ▽ More Active droplets undergoing micellar solubilization can self-propel themselves by emitting filled micelles from their surface and are by nature anti-chemotactic. These droplets get repelled by their own trail of filled micelles or by other neighbouring droplets. We experimentally study the various types of scattering events between two such active droplets. We define different collision parameters to characterize the scattering events and monitor the time evolution of these parameters about the closest approach between the droplets. For generic oblique collisions of the droplets, we find four major types of scattering events in our experimental studies depending on the initial angle of approach, the initial Peclet numbers, the delay time, and the initial distance between the droplets. For higher Peclet number droplets, the existence of a bound state was also found. △ Less

Submitted 18 October, 2024; originally announced October 2024.

Comments: 7 pages, 2 figures, 4 tables

Stochastic resetting prevails over sharp restart for broad target distributions

Authors: Martin R. Evans, Somrita Ray

Abstract: Resetting has been shown to reduce the completion time for a stochastic process, such as the first passage time for a diffusive searcher to find a target. The time between two consecutive resetting events is drawn from a waiting time distribution $ψ(t)$, which defines the resetting protocol. Previously, it has been shown that deterministic resetting process with a constant time period, referred to… ▽ More Resetting has been shown to reduce the completion time for a stochastic process, such as the first passage time for a diffusive searcher to find a target. The time between two consecutive resetting events is drawn from a waiting time distribution $ψ(t)$, which defines the resetting protocol. Previously, it has been shown that deterministic resetting process with a constant time period, referred to as sharp restart, can minimize the mean first passage time to a fixed target. Here we consider the more realistic problem of a target positioned at a random distance $R$ from the resetting site, selected from a given target distribution $P_T(R)$. We introduce the notion of a conjugate target distribution to a given waiting time distribution. The conjugate target distribution, $P_T^*(R)$, is that $P_T(R)$ for which $ψ(t)$ extremizes the mean time to locate the target. In the case of diffusion we derive an explicit expression for $P^*_T(R)$ conjugate to a given $ψ(t)$ which holds in arbitrary spatial dimension. Our results show that stochastic resetting prevails over sharp restart for target distributions with exponential or heavier tails. △ Less

Submitted 19 May, 2025; v1 submitted 2 October, 2024; originally announced October 2024.

Comments: 2 Figures

Journal ref: Phys. Rev. Lett. 134, 247102 (2025)

Terahertz harmonic generation across the Mott insulator-metal transition

Authors: Gulloo Lal Prajapati, Sujay Ray, Igor Ilyakov, Alexey N. Ponomaryov, Atiqa Arshad, Thales V. A. G. de Oliveira, Gaurav Dubey, Dhanvir Singh Rana, Jan-Christoph Deinert, Philipp Werner, Sergey Kovalev

Abstract: We demonstrate terahertz (THz) harmonic generation across the Mott insulator-metal transition in rare-earth nickelates (RNiO$_3$, R = rare-earth atom). The THz harmonic generation is observed in all the three different phases with distinct behaviors: the intensity of harmonics increases upon cooling in both the low-temperature antiferromagnetic (AFM) insulating and high-temperature paramagnetic (P… ▽ More We demonstrate terahertz (THz) harmonic generation across the Mott insulator-metal transition in rare-earth nickelates (RNiO$_3$, R = rare-earth atom). The THz harmonic generation is observed in all the three different phases with distinct behaviors: the intensity of harmonics increases upon cooling in both the low-temperature antiferromagnetic (AFM) insulating and high-temperature paramagnetic (PM) metallic phases, while this trend is reversed in the intermediate PM insulating phase. Using single- and two-band Hubbard models, we find different dominant origins of THz harmonics in different phases: strong spin-charge and orbital-charge couplings in the AFM insulating phase, intraband currents from renormalized quasi-particles with frequency-dependent scattering rate in the PM metallic phase, and the reduction of the charge carrier density due to the opening of the Mott gap in the PM insulating phase. Our study offers strategies for efficient THz harmonic generation from Mott and other strongly correlated systems and insights into the fundamental physics of complex materials. △ Less

Submitted 2 October, 2024; originally announced October 2024.

Comments: Submitted to Physical Review Letters. 5 pages, 3 figures

Journal ref: Phys. Rev. B 113, 045118 (2026)

Role of intercellular adhesion in modulating tissue fluidity

Authors: Soumyadipta Ray, Santidan Biswas, Dipjyoti Das

Abstract: Tuning cell rearrangements is essential in collective cell movement that underlies cancer progression, wound repair, and embryonic development. A key question is how tissue material properties and morphology emerge from cellular factors such as cell-cell adhesion. Here, we introduce a two-dimensional active force-based model of tissue monolayers that captures the liquid-to-solid transition exhibit… ▽ More Tuning cell rearrangements is essential in collective cell movement that underlies cancer progression, wound repair, and embryonic development. A key question is how tissue material properties and morphology emerge from cellular factors such as cell-cell adhesion. Here, we introduce a two-dimensional active force-based model of tissue monolayers that captures the liquid-to-solid transition exhibited by tissues. Unlike the Vertex and Voronoi models, our model shows that reducing intercellular adhesion in near-confluent tissues leads to spontaneous neighbor exchanges and fluidization. Near the liquid-solid phase boundary, we also found glassy behavior characterized by subdiffusive dynamics, swirling cell motion, and non-Gaussian exponential tails in displacement distributions. These exponential tails collapse onto a single master curve, suggesting a universal 'diffusion length' in the glassy regime. Notably, we demonstrate that structural parameters based on cell shape cannot always distinguish tissue phases due to huge cell shape fluctuations that are not observed in Vertex and Voronoi models. Our general simulation framework streamlines previous approaches by removing many arbitrary features and can reproduce known model behaviors under different conditions, offering potential applications in developmental biology and physiology. △ Less

Submitted 28 September, 2024; originally announced September 2024.

Active assembly and non-reciprocal dynamics of elastic membranes

Authors: John Berezney, Sattvic Ray, Itamar Kolvin, Fridtjof Brauns, Sihan Chen, Mark Bowick, Seth Fraden, Vincenzo Vitelli, Zvonimir Dogic

Abstract: Equilibrium self-assembly and conventional materials processing techniques fall far short of mimicking dynamic self-actuating processes that are commonplace throughout biology. To bridge the gap between living and synthetic matter, we study adhesive non-thermal fibers immersed in an active fluid. Autonomous chaotic flows power non-equilibrium fiber dynamics, inducing their collisions, generating c… ▽ More Equilibrium self-assembly and conventional materials processing techniques fall far short of mimicking dynamic self-actuating processes that are commonplace throughout biology. To bridge the gap between living and synthetic matter, we study adhesive non-thermal fibers immersed in an active fluid. Autonomous chaotic flows power non-equilibrium fiber dynamics, inducing their collisions, generating connections, and weaving a membrane-shaped elastic network. This active assembly generates a hierarchy of shapes, structures, and dynamical processes spanning nanometers to centimeters. Ultimately, it generates an active membrane that exhibits global limit cycles induced by a non-reciprocal coupling between the elastic membrane deformations and the alignment axis of the polar active fluid. Our work merges self-assembly with active matter, demonstrating self-processing materials wherein hierarchical life-like structures and dynamics emerge from an initially structureless suspension. △ Less

Submitted 4 September, 2025; v1 submitted 26 August, 2024; originally announced August 2024.

Onset of Intermittency and Multiscaling in Active Turbulence

Authors: Kolluru Venkata Kiran, Kunal Kumar, Anupam Gupta, Rahul Pandit, Samriddhi Sankar Ray

Abstract: Recent results suggest that highly active, chaotic, non-equilibrium states of living fluids might share much in common with high Reynolds number, inertial turbulence. We now show, by using a hydrodynamical model, the onset of intermittency and the consequent multiscaling of Eulerian and Lagrangian structure functions as a function of the bacterial activity. Our results bridge the worlds of low and… ▽ More Recent results suggest that highly active, chaotic, non-equilibrium states of living fluids might share much in common with high Reynolds number, inertial turbulence. We now show, by using a hydrodynamical model, the onset of intermittency and the consequent multiscaling of Eulerian and Lagrangian structure functions as a function of the bacterial activity. Our results bridge the worlds of low and high Reynolds number flows as well as open up intriguing possibilities of what makes flows intermittent. △ Less

Submitted 25 March, 2025; v1 submitted 13 August, 2024; originally announced August 2024.

Comments: A nearly identical version of this manuscript is published in Physical Review Letters ( https://doi.org/10.1103/PhysRevLett.134.088302)

Photo-induced charge, spin, and orbital order in the two-orbital extended Hubbard model

Authors: Sujay Ray, Philipp Werner

Abstract: Nonequilibrium control of electronically ordered hidden phases may lead to the development of ultrafast switches and memory devices. In this study, we demonstrate tunable hidden orders in the photo-doped two-orbital extended Hubbard model. Using steady-state nonequilibrium dynamical mean field theory, we clarify the coexistence and interplay of nonthermal charge, spin, and orbital order. The hidde… ▽ More Nonequilibrium control of electronically ordered hidden phases may lead to the development of ultrafast switches and memory devices. In this study, we demonstrate tunable hidden orders in the photo-doped two-orbital extended Hubbard model. Using steady-state nonequilibrium dynamical mean field theory, we clarify the coexistence and interplay of nonthermal charge, spin, and orbital order. The hidden state at low effective temperature and sufficiently high photo-doping is reminiscent of Kugel-Khomskii order in the two-orbital Hubbard model at $\frac{1}{4}$ and $\frac{3}{4}$ filling, but it emerges out of a nonequilibrium charge ordered state and exhibits a different magnetic structure. A low-energy effective Hamiltonian is used to analyze the exchange processes which stabilize the nonthermal order. △ Less

Submitted 12 August, 2024; originally announced August 2024.

Deep-learning Optical Flow Outperforms PIV in Obtaining Velocity Fields from Active Nematics

Authors: Phu N. Tran, Sattvic Ray, Linnea Lemma, Yunrui Li, Reef Sweeney, Aparna Baskaran, Zvonimir Dogic, Pengyu Hong, Michael F. Hagan

Abstract: Deep learning-based optical flow (DLOF) extracts features in adjacent video frames with deep convolutional neural networks. It uses those features to estimate the inter-frame motions of objects at the pixel level. In this article, we evaluate the ability of optical flow to quantify the spontaneous flows of MT-based active nematics under different labeling conditions. We compare DLOF against the co… ▽ More Deep learning-based optical flow (DLOF) extracts features in adjacent video frames with deep convolutional neural networks. It uses those features to estimate the inter-frame motions of objects at the pixel level. In this article, we evaluate the ability of optical flow to quantify the spontaneous flows of MT-based active nematics under different labeling conditions. We compare DLOF against the commonly used technique, particle imaging velocimetry (PIV). We obtain flow velocity ground truths either by performing semi-automated particle tracking on samples with sparsely labeled filaments, or from passive tracer beads. We find that DLOF produces significantly more accurate velocity fields than PIV for densely labeled samples. We show that the breakdown of PIV arises because the algorithm cannot reliably distinguish contrast variations at high densities, particularly in directions parallel to the nematic director. DLOF overcomes this limitation. For sparsely labeled samples, DLOF and PIV produce results with similar accuracy, but DLOF gives higher-resolution fields. Our work establishes DLOF as a versatile tool for measuring fluid flows in a broad class of active, soft, and biophysical systems. △ Less

Submitted 26 April, 2024; v1 submitted 23 April, 2024; originally announced April 2024.

Photo-induced Ferromagnetic and Superconducting Orders in Multi-orbital Hubbard Models

Authors: Sujay Ray, Philipp Werner

Abstract: The search for hidden orders in photoexcited lattice systems is an active research field driven by experimental reports of light-induced or light-stabilized phases. In this study, we investigate hidden electronic orders in strongly correlated two-orbital Hubbard models with orbital-dependent bandwidths. In equilibrium, the half-filled systems are antiferromagnetically ordered. Using non-equilibriu… ▽ More The search for hidden orders in photoexcited lattice systems is an active research field driven by experimental reports of light-induced or light-stabilized phases. In this study, we investigate hidden electronic orders in strongly correlated two-orbital Hubbard models with orbital-dependent bandwidths. In equilibrium, the half-filled systems are antiferromagnetically ordered. Using non-equilibrium dynamical mean field theory we demonstrate the appearance of nonthermal ferromagnetic order in the photo-doped state, if the two bandwidths are sufficiently different, and its coexistence with spin-singlet $η$-superconductivity in the high photo-doping region. Spin-triplet $η$-superconducting order appears instead if the two bandwidths are comparable. The rich nonequilibrium phasediagram uncovered in this work shows that Mott insulating multi-orbital systems provide an interesting platform for the realization of nonthermal electronic orders. △ Less

Submitted 12 March, 2024; originally announced March 2024.

The Nordic-walking mechanism and its explanation of deconfined pseudocriticality from Wess-Zumino-Witten theory

Authors: Bilal Hawashin, Astrid Eichhorn, Lukas Janssen, Michael M. Scherer, Shouryya Ray

Abstract: The understanding of phenomena falling outside the Ginzburg-Landau paradigm of phase transitions represents a key challenge in condensed matter physics. A famous class of examples is constituted by the putative deconfined quantum critical points between two symmetry-broken phases in layered quantum magnets, such as pressurised SrCu$_2$(BO$_3$)$_2$. Experiments find a weak first-order transition, w… ▽ More The understanding of phenomena falling outside the Ginzburg-Landau paradigm of phase transitions represents a key challenge in condensed matter physics. A famous class of examples is constituted by the putative deconfined quantum critical points between two symmetry-broken phases in layered quantum magnets, such as pressurised SrCu$_2$(BO$_3$)$_2$. Experiments find a weak first-order transition, which simulations of relevant microscopic models can reproduce. The origin of this behaviour has been a matter of considerable debate for several years. In this work, we demonstrate that the nature of the deconfined quantum critical point can be best understood in terms of a novel dynamical mechanism, termed Nordic walking. Nordic walking denotes a renormalisation group flow arising from a beta function that is flat over a range of couplings. This gives rise to a logarithmic flow that is faster than the well-known walking behaviour, associated with the annihilation and complexification of fixed points, but still significantly slower than the generic running of couplings. The Nordic-walking mechanism can thus explain weak first-order transitions, but may also play a role in high-energy physics, where it could solve hierarchy problems. We analyse the Wess-Zumino-Witten field theory pertinent to deconfined quantum critical points with a topological term in 2+1 dimensions. To this end, we construct an advanced functional renormalisation group approach based on higher-order regulators. We thereby calculate the beta function directly in 2+1 dimensions and provide evidence for Nordic walking. △ Less

Submitted 18 December, 2023; originally announced December 2023.

Comments: 12 + 11 pages (typeset with sn-jnl.cls), 4 + 1 figures; comments welcome

Journal ref: Nat. Commun. 16, 20 (2025)

Temporal bistability in the dissipative Dicke-Bose-Hubbard system

Authors: Tianyi Wu, Sayak Ray, Johann Kroha

Abstract: We consider a driven-dissipative system consisting of an atomic Bose-Einstein condensates loaded into a two-dimensional Hubbard lattice and coupled to a single mode of an optical cavity. Due to the interplay between strong, repulsive atomic interaction and the atom-cavity coupling, the system exhibits several phases of atoms and photons including the atomic superfluid (SF) and supersolid (SS). We… ▽ More We consider a driven-dissipative system consisting of an atomic Bose-Einstein condensates loaded into a two-dimensional Hubbard lattice and coupled to a single mode of an optical cavity. Due to the interplay between strong, repulsive atomic interaction and the atom-cavity coupling, the system exhibits several phases of atoms and photons including the atomic superfluid (SF) and supersolid (SS). We investigate the dynamical behaviour of the system, where we include dissipation by means of Lindblad master equation formalism. Due to the discontinuous nature of the Dicke transition for strong atomic repulsion, we find extended co-existence region of different phases. We investigate the resulting switching dynamics, particularly between the coexisting SF and SS phases, which eventually becomes damped by the dissipation. △ Less

Submitted 28 February, 2024; v1 submitted 22 November, 2023; originally announced November 2023.

Journal ref: Annalen der Physik, 2300505 (2024)

Quantum Materials Group Annual Report 2022

Authors: P. Kumari, S. Rani, S. Kar, T. Mukherjee, S. Majumder, K. Kumari, S. J. Ray

Abstract: The Quantum Materials group at Indian Institute of Technology Patna is working on a range of topics relating to nanoelectronics, spintronics, clean energy and memory design etc. The PI has past experiences of working extensively with superconducting systems like cuprates [1, 2], ruthanate [3], pnictide [4, 5], thin film heterostructures [6, 7] etc and magnetic recording media [8, 9] etc. In this r… ▽ More The Quantum Materials group at Indian Institute of Technology Patna is working on a range of topics relating to nanoelectronics, spintronics, clean energy and memory design etc. The PI has past experiences of working extensively with superconducting systems like cuprates [1, 2], ruthanate [3], pnictide [4, 5], thin film heterostructures [6, 7] etc and magnetic recording media [8, 9] etc. In this report, we have summarised the ongoing works in our group. We explored a range of functional materials like two-dimensional materials, oxides. topological insulators, organic materials etc. using a combination of experimnetal and computational tools. Some of the useful highlights are as follows: (a) tuning and control of the magnetic and electronic state of 2D magentic materials with rapid enhancement in the Curie temperature, (b) Design and detection of single electron transistor based nanosensors for the detection of biological species with single molecular resolution, (c) Observation of non-volatile memory behaviour in the hybrid structures made of perovskite materials and 2D hybrids. The results offer useful insight in the design of nanoelectronic architecrures for diverse applications. △ Less

Submitted 13 October, 2023; v1 submitted 30 September, 2023; originally announced October 2023.

Classical route to ergodicity and scarring in collective quantum systems

Authors: Sudip Sinha, Sayak Ray, Subhasis Sinha

Abstract: Ergodicity, a fundamental concept in statistical mechanics, is not yet a fully understood phenomena for closed quantum systems, particularly its connection with the underlying chaos. In this review, we consider a few examples of collective quantum systems to unveil the intricate relationship of ergodicity as well as its deviation due to quantum scarring phenomena with their classical counterpart.… ▽ More Ergodicity, a fundamental concept in statistical mechanics, is not yet a fully understood phenomena for closed quantum systems, particularly its connection with the underlying chaos. In this review, we consider a few examples of collective quantum systems to unveil the intricate relationship of ergodicity as well as its deviation due to quantum scarring phenomena with their classical counterpart. A comprehensive overview of classical and quantum chaos is provided, along with the tools essential for their detection. Furthermore, we survey recent theoretical and experimental advancements in the domain of ergodicity and its violations. This review aims to illuminate the classical perspective of quantum scarring phenomena in interacting quantum systems. △ Less

Submitted 19 February, 2024; v1 submitted 27 September, 2023; originally announced September 2023.

Comments: 28 pages, 15 figures, Topical Review

Journal ref: Journal of Physics: Condensed Matter 36 (16), 163001 (2024)

Resonant transport in a highly conducting single molecular junction via metal-metal covalent bond

Authors: Biswajit Pabi, Štepán Marek, Adwitiya Pal, Puja Kumari, Soumya Jyoti Ray, Arunabha Thakur, Richard Korytár, Atindra Nath Pal

Abstract: Achieving highly transmitting molecular junctions through resonant transport at low bias is key to the next-generation low-power molecular devices. Although, resonant transport in molecular junctions was observed by connecting a molecule between the metal electrodes via chemical anchors by applying a high source-drain bias (> 1V), the conductance was limited to < 0.1 G$_0$, G$_0$ being the quantum… ▽ More Achieving highly transmitting molecular junctions through resonant transport at low bias is key to the next-generation low-power molecular devices. Although, resonant transport in molecular junctions was observed by connecting a molecule between the metal electrodes via chemical anchors by applying a high source-drain bias (> 1V), the conductance was limited to < 0.1 G$_0$, G$_0$ being the quantum of conductance. Here, we report electronic transport measurements by directly connecting a Ferrocene molecule between Au electrodes at the ambient condition in a mechanically controllable break junction setup (MCBJ), revealing a conductance peak at ~ 0.2 G$_0$ in the conductance histogram. A similar experiment was repeated for Ferrocene terminated with amine (-NH2) and cyano (-CN) anchors, where conductance histograms exhibit an extended low conductance feature including the sharp high conductance peak, similar to pristine ferrocene. Statistical analysis of the data along with density functional theory-based transport calculation suggests the possible molecular conformation with a strong hybridization between the Au electrodes and Fe atom of Ferrocene molecule is responsible for a near-perfect transmission in the vicinity of the Fermi energy, leading to the resonant transport at a small applied bias (< 0.5V). Moreover, calculations including Van der Waals/dispersion corrections reveal a covalent like organometallic bonding between Au and the central Fe atom of Ferrocene, having bond energies of ~ 660 meV. Overall, our study not only demonstrates the realization of an air-stable highly transmitting molecular junction, but also provides an important insight about the nature of chemical bonding at the metal/organo-metallic interface. △ Less

Submitted 8 August, 2023; originally announced August 2023.

Comments: 23 pages, 6 figures, supplementary included

Journal ref: Nanoscale (2023)

Turbulent flows are not uniformly multifractal

Authors: Siddhartha Mukherjee, Sugan D. Murugan, Ritwik Mukherjee, Samriddhi Sankar Ray

Abstract: The Frisch-Parisi multifractal formalism remains the most compelling rationalisation for anomalous scaling in fully developed turbulence. We now show that this formalism can be adapted locally to reveal the spatial distribution of generalized dimensions and of how multifractal the energy dissipation field is. In particular, we show that most regions of the flow are close to being mono-fractal and… ▽ More The Frisch-Parisi multifractal formalism remains the most compelling rationalisation for anomalous scaling in fully developed turbulence. We now show that this formalism can be adapted locally to reveal the spatial distribution of generalized dimensions and of how multifractal the energy dissipation field is. In particular, we show that most regions of the flow are close to being mono-fractal and these are interspersed with islands of multifractality corresponding to the most singular structures in the flow. By defining a suitable measure $Φ({\bf x})$ of the spatial variation of multifractality, we show that this grows logarithmically with the extent to which the energy dissipation varies locally around ${\bf x}$. These results suggest ways to understand how singularities could arise in disparate regions of a flow and provides new directions in understanding anomalous dissipation and intermittency. We then employ the same technique to a non-intermittent, model turbulent flow to check the robustness of our conclusions. △ Less

Submitted 12 July, 2023; originally announced July 2023.

Comments: 7 Pages, 5 Figures