Showing 1–50 of 128 results for author: Saha, K

Photoconductive nonpolar liquids based on azobenzene

Authors: Carlo Rigoni, Promeet K. Saha, Jinyu Sheng, Rafal Klajn

Abstract: The weakly conductive properties of mixtures of organic surfactants in nonpolar liquids are fundamental to many electrohydrodynamic phenomena and underpin several cutting-edge technologies, particularly the development of electrophoretic displays. To date, tuning the electrical properties of these systems has involved modifying their composition, including surfactant type and concentration, water… ▽ More The weakly conductive properties of mixtures of organic surfactants in nonpolar liquids are fundamental to many electrohydrodynamic phenomena and underpin several cutting-edge technologies, particularly the development of electrophoretic displays. To date, tuning the electrical properties of these systems has involved modifying their composition, including surfactant type and concentration, water content, and the carrier liquid, all of which influence their behavior. Here, we use photoresponsive molecules to control electric phenomena in nonpolar liquids externally with light irradiation, thereby rendering them photoconductive. In particular, we examine azobenzene solutions in toluene, whose conductivity can be adjusted by two colors of light: UV induces trans to cis isomerization, leading to an increase in conductivity, while blue light triggers cis to trans back-isomerization, decreasing conductivity. The findings of this study suggest new ways to expand the applications of weakly conductive organic fluids, such as in self-regulating devices that respond to sunlight or in externally programmable displays. △ Less

Submitted 13 July, 2026; originally announced July 2026.

Comments: Total of 42 pages and 27 figures. Main text 14 pages and 5 figures, Supplementary Information 28 pages and 22 supplementary figures

Kohn anomaly in a topological phase transition

Authors: Jingwen Li, Arpita Dutta, Kush Saha, Andrzej Szczerbakow, Tomasz Story, Manfred Fiebig, Shovon Pal

Abstract: Topological crystalline insulators extend the concept of topological insulators by hosting surface states protected by crystallographic symmetry. Their topological phase transitions arise from spin-orbit-driven band inversion in the bulk electronic structure, reshaping the low-energy electronic environment and its coupling to lattice excitations. While the electronic aspects of band topology are w… ▽ More Topological crystalline insulators extend the concept of topological insulators by hosting surface states protected by crystallographic symmetry. Their topological phase transitions arise from spin-orbit-driven band inversion in the bulk electronic structure, reshaping the low-energy electronic environment and its coupling to lattice excitations. While the electronic aspects of band topology are well established, the corresponding dynamics of lattice and electron-phonon interactions remain largely unexplored. Here, we report a pronounced softening of a low-energy surface phonon mode across the topological phase transition in Pb0.77Sn0.23Se, revealed by temperature-dependent time-domain terahertz spectroscopy. Unlike the well-known phonon softening in ferroelectrics, this effect does not signal a structural instability but instead reflects electronic reconstruction. We attribute the softening to the Kohn anomaly, indicating a strong coupling between lattice vibrations and Dirac-like surface electrons in the topological phase. Consistently, the phonon linewidth deviates from the standard anharmonic temperature dependence, further evidencing enhanced electron-phonon coupling. Our results establish phonon softening as a spectroscopic signature of topological phase transitions and provide a route to distinguish topological and trivial phases. △ Less

Submitted 24 June, 2026; originally announced June 2026.

Superconductivity from interband coupling to ferroelectric quantum critical fluctuations in two dimensions

Authors: Sudip Kumar Saha, Jonathan Ruhman, Avraham Klein

Abstract: Soft critical fluctuations associated with ferroelectric quantum phase transitions are typically transverse owing to their polar nature. This implies that the conventional density--density electron--phonon coupling to these modes is strongly suppressed, which is puzzling as a variety of materials exhibit enhanced superconductivity in the vicinity of ferroelectricity. An alternative coupling mechan… ▽ More Soft critical fluctuations associated with ferroelectric quantum phase transitions are typically transverse owing to their polar nature. This implies that the conventional density--density electron--phonon coupling to these modes is strongly suppressed, which is puzzling as a variety of materials exhibit enhanced superconductivity in the vicinity of ferroelectricity. An alternative coupling mechanism is an interband ``Stark''-like coupling that connects bands of opposite parity. In the limit where one of the bands is far in energy, these processes generate an effective quadratic (two-phonon) coupling. In contrast, when both bands lie close to the Fermi energy, the resulting interaction develops singular behavior due to the additional gapless electronic states, motivating a detailed study into the dynamics of this effective two-phonon coupling. To this end, we construct the quantum critical Eliashberg theory for a two-dimensional system across a wide range of interband gap magnitudes, near the quantum critical point. We find that the critical temperature $T_c$ is strongly enhanced relative to conventional BCS expectations. In the large-gap limit, the pairing kernel acquires higher-order logarithmic contributions, leading to a parametrically enhanced $T_c$ governed by cubic and quadratic logarithmic terms. In the small-gap regime, the pairing scale exhibits a modified BCS-like form with an enhanced dependence on the inverse square root of the dimensionless coupling constant. The enhancement is due to the dynamics of the two-phonon pairing whose infrared cutoff is set by $T_c$, resulting in a significant enhancement of superconductivity compared to three-dimensional systems, where it is set by the Fermi energy. Our results elucidate the unique dynamical properties of effective two-phonon interactions, and may be relevant to layered compounds like Td-MoTe$_2$ and doped SrTiO$_3$ membranes. △ Less

Submitted 16 June, 2026; originally announced June 2026.

Comments: 13 pages, 5 figures

Forman--Ricci Curvature for Irregular Convex Mosaics

Authors: Abhyudaya Gupta, Sayak Mukherjee, Kuldeep Saha

Abstract: Forman has defined a discrete version of the Ricci curvature on Riemannian manifolds, known as the Forman--Ricci curvature. The Forman--Ricci curvature has found significant applications in several pattern recognition problems occurring in natural sciences. Domokos and Langi, on the other hand, have defined a notion of irregularity for convex mosaics, which has also found remarkable applications t… ▽ More Forman has defined a discrete version of the Ricci curvature on Riemannian manifolds, known as the Forman--Ricci curvature. The Forman--Ricci curvature has found significant applications in several pattern recognition problems occurring in natural sciences. Domokos and Langi, on the other hand, have defined a notion of irregularity for convex mosaics, which has also found remarkable applications to the geological problem of fractures in rocks. We define a modification of the classical Forman--Ricci curvature for irregular convex mosaics and demonstrate how they can be used to distinguish between various fractures or cracking patterns appearing in nature. △ Less

Submitted 28 May, 2026; originally announced June 2026.

Comments: 12 pages, 8 figures

Information scrambling in all-to-all interacting models

Authors: Abhik Kumar Saha, Tanay Pathak, Masaki Tezuka

Abstract: Information scrambling is a hallmark of quantum chaos and thermalization in isolated quantum many-body systems. We investigate scrambling dynamics in the all-to-all interacting spin Sachdev-Ye-Kitaev (SYK)-$q$ model using both pure- and mixed-state entanglement measures. We show that von-Neumann and Rényi entropies exhibit rapid growth followed by saturation near Haar-random values, signaling effi… ▽ More Information scrambling is a hallmark of quantum chaos and thermalization in isolated quantum many-body systems. We investigate scrambling dynamics in the all-to-all interacting spin Sachdev-Ye-Kitaev (SYK)-$q$ model using both pure- and mixed-state entanglement measures. We show that von-Neumann and Rényi entropies exhibit rapid growth followed by saturation near Haar-random values, signaling efficient scrambling. The scrambling rate reveals a nontrivial dependence on the interaction order, system size, and Hamiltonian scaling. We further employ mixed-state entanglement as a powerful probe of information scrambling. We numerically find a universal relation between the Rényi-1/2 mutual information and entanglement negativity for minimal interaction order in the early growth regime. Furthermore, entanglement negativity displays a Page-curve-like behavior under unequal subsystem partitioning, characterized by the birth, spread, and eventual death of quantum correlations. Our results provide a generic description of information scrambling using entanglement dynamics in all-to-all interacting spin systems with multi-body interactions. △ Less

Submitted 1 June, 2026; originally announced June 2026.

Comments: 8+16 pages, 4+15 figures

Vibrational resonance in a one-dimensional dissipative Bose-Josephson junction

Authors: Abhik Kumar Saha

Abstract: We investigate the linear and nonlinear response of a one-dimensional dissipative Bose-Josephson junction subjected simultaneously to a weak low-frequency probe and a rapidly oscillating high-frequency external drive. Starting from the dissipative two-mode Bose-Josephson equations, we derive an effective higher-order nonlinear equation for the population imbalance by retaining the leading nonlinea… ▽ More We investigate the linear and nonlinear response of a one-dimensional dissipative Bose-Josephson junction subjected simultaneously to a weak low-frequency probe and a rapidly oscillating high-frequency external drive. Starting from the dissipative two-mode Bose-Josephson equations, we derive an effective higher-order nonlinear equation for the population imbalance by retaining the leading nonlinear correction. Using time-scale separation and perturbative analysis, we obtain analytical expressions for both the linear response at the fundamental frequency and the nonlinear response at the second harmonic. We show that the high-frequency modulation modifies the effective potential landscape and dynamically breaks the symmetry around the stationary state, giving rise to a finite second-harmonic response that is absent without the rapidly oscillating field. Both the linear and nonlinear response amplitudes exhibit resonance-like enhancement for optimal values of the high-frequency driving strength. We further analyze the dependence of the resonance characteristics on interaction strength, dissipation, and driving parameters in both the zero-phase and $π$-phase modes and compare the analytical predictions with direct numerical simulations. Our results demonstrate a controllable mechanism toward realizing linear and nonlinear vibrational resonance in a one-dimensional dissipative Bose-Josephson junction and open new possibilities for controlling collective dynamics in driven ultracold bosonic systems. △ Less

Submitted 1 June, 2026; originally announced June 2026.

Comments: 12 pages, 5 figures

Frustrated magnetic order in hybrid Kitaev spin-orbital models

Authors: Ivan Dutta, Aayush Vijayvargia, Anamitra Mukherjee, Onur Erten, Kush Saha

Abstract: Spin-orbital generalization of Kitaev model provides a robust extension to the original Kitaev model. However, real materials often exhibit competing interactions that break exact solvability which can give rise to new phases. Motivated by recent microscopic proposals of coexisting Yao-Lee and Kitaev couplings, we investigate the fate of the ground state when two independent exactly solvable spin… ▽ More Spin-orbital generalization of Kitaev model provides a robust extension to the original Kitaev model. However, real materials often exhibit competing interactions that break exact solvability which can give rise to new phases. Motivated by recent microscopic proposals of coexisting Yao-Lee and Kitaev couplings, we investigate the fate of the ground state when two independent exactly solvable spin liquid Hamiltonians each originally formulated on different lattice geometries are combined on a common lattice environment. We first focus on the hybrid Kitaev's honeycomb and square-lattice model. Using self-consistent mean-field analysis and perturbative calculation, we show that the strong-Kitaev regime yields magnetic order in the spin sector, while the orbital sector retains its topological order. We further analyze the hybridization of the Yao-Lee and square-lattice models and find that the model exhibits a rich evolution of Majorana Dirac bands and Lifshitz transitions. Remarkably, when the Yao-Lee and square-lattice couplings are equal and opposite, the model restores its exact solvability with a single itinerant Majorana flavor. These results demonstrate that hybrid spin liquid platforms may host various emergent phases beyond conventional exactly solvable limits. △ Less

Submitted 6 May, 2026; originally announced May 2026.

Comments: 12 pages, 7 figures

Size-Limited Room Temperature Single-Photon Emission from Sidewall-Treated Fractional Dimension InGaN Quantum Dots: Determined by Density-of-States-Corrected Ultrafast Carrier Dynamics and Improved Signal-to-Noise Ratio

Authors: Pratim K. Saha

Abstract: Room-temperature single-photon emission (SPE) resulting from a biexciton-exciton cascaded decay is demonstrated for the first time from chemically and photoelectrochemically etched site-controlled In0.14Ga0.86N quantum dots (QDs) embedded in vertical GaN nanowires. Diameter-dependent biexciton-exciton dynamics are analysed to determine the eligibility of QD as a single-photon emitter. The signal-t… ▽ More Room-temperature single-photon emission (SPE) resulting from a biexciton-exciton cascaded decay is demonstrated for the first time from chemically and photoelectrochemically etched site-controlled In0.14Ga0.86N quantum dots (QDs) embedded in vertical GaN nanowires. Diameter-dependent biexciton-exciton dynamics are analysed to determine the eligibility of QD as a single-photon emitter. The signal-to-noise ratio degrades with increasing QD diameter. Background noise photons pose a bottleneck to achieving SPE. This is also explained from a carrier dynamics perspective. Surface recombination contributes to inhomogeneous broadening at QD diameters larger than 35 nm. Below 35 nm, density-of-states-corrected Auger gradually becomes the principal biexciton-decay route with further reduction in QD diameter, thereby quenching the possibility of thermal broadening and setting a threshold for SPE. Below 9 nm, the Auger recombination rate becomes manyfold of other decay rates, causing multi-photon suppression via single Auger decay to form an exciton. Surface recombination probability of this exciton is minimized while biexciton state filling probability is maximized by reducing sidewall surface states through wet-treatment. These improve biexciton state preparation and enhance the single-photon purity of the exciton towards the exciton Bohr radius (3 nm) regime. Far away from this regime, higher-order autocorrelations to characterize quantum emission involving multi-photon events are discussed. This study establishes a generalized physical framework for predetermining SPE probability as a function of QD surface and geometry down to the exciton Bohr radius regime, with practical implementations. This work shows the pathway to design and develop next-generation semiconductor QDs for high-purity room-temperature SPE. △ Less

Submitted 30 April, 2026; originally announced April 2026.

Comments: 30 pages, 5 figures

Dynamics of one-dimensional Bose-Josephson Junction in a Box Trap: From Coherent Oscillations to Many-Body Dephasing and Dynamical Freezing

Authors: Abhik Kumar Saha, L. F. Calazans de Brito, Cesare Vianello, Rhombik Roy, Romain Dubessy, Barnali Chakrabarti, Arnaldo Gammal

Abstract: Understanding how coherent quantum dynamics give way to correlation-dominated behavior in low-dimensional systems remains a central challenge in quantum many-body physics. Here, we investigate a one-dimensional Bose-Josephson junction confined in a box trap using the multiconfigurational time-dependent Hartree method for bosons (MCTDHB). By varying the interaction strength and initial population i… ▽ More Understanding how coherent quantum dynamics give way to correlation-dominated behavior in low-dimensional systems remains a central challenge in quantum many-body physics. Here, we investigate a one-dimensional Bose-Josephson junction confined in a box trap using the multiconfigurational time-dependent Hartree method for bosons (MCTDHB). By varying the interaction strength and initial population imbalance, we identify distinct dynamical regimes governed by the competition between coherence and correlation-induced fragmentation. Weak interactions support coherent Josephson oscillations, whereas increasing imbalance leads to damping. At intermediate interaction strength, varying only the initial imbalance induces a crossover from nearly pure coherent oscillations to many-body dephasing with collapse-and-revival dynamics, and ultimately to equilibration accompanied by strong fragmentation and the saturation of many-body observables. In the strongly interacting regime, the system enters a dynamical freezing regime characterized by pronounced fragmentation, well-separated particle-resolved density peaks, and strongly suppressed tunneling. A systematic comparison with the Bose-Hubbard model reveals excellent agreement in the weakly interacting regime, while progressively larger deviations emerge as higher-orbital occupations beyond the two-mode approximation become significant. These results provide a unified picture of the emergence and competition of coherence, many-body dephasing, equilibration, and dynamical freezing, while delineating the regime of validity of the Bose-Hubbard description. △ Less

Submitted 6 August, 2026; v1 submitted 20 April, 2026; originally announced April 2026.

Comments: 23 pages, 21 figures, added a coauthor

Gigahertz-clocked Generation of Highly Indistinguishable Photons at C-band Wavelengths

Authors: Robert Behrends, Lucas Rickert, Nils D. Kewitz, Martin v. Helversen, Pratim K. Saha, Mareike Lach, Jochen Kaupp, Yorick Reum, Tobias-Huber-Loyola, Sven Höfling, Andreas Pfenning, Tobias Heindel

Abstract: High-performance single-photon sources at telecom C-band wavelentghs are key building blocks for applications in long-distance quantum communication. Here, we report the generation of highly indistinguishable, single photons at a clock-rate of 2.5 GHz. This is achieved by coherently driving the biexciton transition ($T_1^\mathrm{XX}=64(1)\,$ps) of a semiconductor quantum dot embedded in a microcav… ▽ More High-performance single-photon sources at telecom C-band wavelentghs are key building blocks for applications in long-distance quantum communication. Here, we report the generation of highly indistinguishable, single photons at a clock-rate of 2.5 GHz. This is achieved by coherently driving the biexciton transition ($T_1^\mathrm{XX}=64(1)\,$ps) of a semiconductor quantum dot embedded in a microcavity with strong asymmetric Purcell enhancement. Employing pulsed two-photon resonant excitation, strong multiphoton suppression with $g^{(2)}(0) < 4\%$ and high two-photon-interference visibility of $V_\mathrm{raw}> 85\%$ is observed. The observed photon indistinguishability is close to the theoretical limit expected for the photonically engineered radiative cascade and matches values obtained at lower repetition rates. Our results show a substantial advancement towards interference-based quantum information protocols at unprecedented data rates in the telecom C-Band. △ Less

Submitted 1 April, 2026; v1 submitted 27 March, 2026; originally announced March 2026.

Comments: 6 pages, 2 figures

Spin Chern phases and persistent spin texture in a quasi 2D SSH model

Authors: Hemant K Sharma, Saptarshi Mandal, Kush Saha

Abstract: We construct a quasi-two-dimensional Su Schrieffer-Heeger model (SSH) like model and uncover a rich set of topological phases with nontrivial spin textures in the presence of complex hopping and spin orbit coupling. Despite its simple structure, the combined effect of complex hopping and spin orbit interaction gives rise not only to the conventional quantum anomalous Hall insulating (QAHI) phase,… ▽ More We construct a quasi-two-dimensional Su Schrieffer-Heeger model (SSH) like model and uncover a rich set of topological phases with nontrivial spin textures in the presence of complex hopping and spin orbit coupling. Despite its simple structure, the combined effect of complex hopping and spin orbit interaction gives rise not only to the conventional quantum anomalous Hall insulating (QAHI) phase, but also to distinct combinations of spin Chern phases, namely quantum anomalous spin Hall insulating (QASHI) phase. Furthermore, we demonstrate that the bulk bands of this model can host persistent spin textures, whose formation and stability are governed by the relative strengths of nearest and next nearest neighbor complex hopping. To elucidate the underlying mechanisms, we develop a low energy continuum theory that captures the emergence of these topological phases and clarifies the origin of the persistent spin textures. Interestingly, the resulting spin textures closely resemble those typically observed in conventional semiconductor systems with topologically trivial band structures. However, in our case, they emerge within a nontrivial topological framework, enabled by carefully engineered hopping patterns that intertwine lattice geometry, complex hopping, and spin orbit coupling △ Less

Submitted 12 March, 2026; originally announced March 2026.

Probing persistent spin textures through nonlinear magnetotransport

Authors: Neelanjan Chakraborti, Akash Dey, Snehasish Nandy, Sudeep Kumar Ghosh, Kush Saha

Abstract: Persistent spin textures (PST) are special spin configurations in spin-orbit-coupled systems in which the spin polarization acquires a symmetry-enforced momentum-independent orientation, leading to exceptionally long spin lifetimes and persistent spin helices. Identifying direct experimental probes of PST, however, remains challenging because conventional quantum-geometric responses are strongly s… ▽ More Persistent spin textures (PST) are special spin configurations in spin-orbit-coupled systems in which the spin polarization acquires a symmetry-enforced momentum-independent orientation, leading to exceptionally long spin lifetimes and persistent spin helices. Identifying direct experimental probes of PST, however, remains challenging because conventional quantum-geometric responses are strongly suppressed in this regime. Here, we show that PST systems isolate spin-rotation quantum geometry, which manifests through distinctive nonlinear magnetotransport responses. Using both a fine-tuned Rashba-Dresselhaus two-dimensional electron gas and a symmetry-enforced cubic spin-splitting model realizing PST, we demonstrate that PST suppresses conventional and Zeeman quantum-geometric contributions, leaving the spin-rotation quantum geometric tensor as the sole source of nonlinear magnetic-current and spin-magnetization responses. Remarkably, the nonvanishing response components exhibit identical direction-independent behavior as a function of chemical potential, providing a distinctive signature of PST. We further show that, in the Rashba-Dresselhaus two-dimensional electron gas at the PST point, these qualitative signatures remain robust even in the presence of a cubic Dresselhaus term that breaks the exact SU(2) symmetry. Our results establish nonlinear magnetotransport as an experimentally accessible probe of PST and their underlying spin-rotation quantum geometry. △ Less

Submitted 21 May, 2026; v1 submitted 4 March, 2026; originally announced March 2026.

Comments: 7 pages and 4 figures. Comments are welcome!

Highly-Indistinguishable Single-Photons at 1550 nm from a Two-photon Resonantly Excited Purcell-enhanced Quantum Dot

Authors: Robert Behrends, Martin v. Helversen, Pratim K. Saha, Lucas Rickert, Koray Kaymazlar, Mareike Lach, Nils D. Kewitz, Jochen Kaupp, Yorick Reum, Tobias Huber-Loyola, Sven Höfling, Andreas Pfenning, Tobias Heindel

Abstract: In this work we present a cavity-enhanced InAs/$\mathrm{In_{0.53}Al_{0.23}Ga_{0.24}As}$ quantum dot (QD) single-photon source in the telecom C-band with a record-low biexciton emitter decay time of \SI{67.4(2)}{ps} under resonant two-photon excitation (TPE). We observe strong multiphoton suppression associated with $g^{(2)}_\mathrm{X}(0) = 0.006(1)$ and $g^{(2)}_\mathrm{XX}(0) = 0.007(1)$ for the… ▽ More In this work we present a cavity-enhanced InAs/$\mathrm{In_{0.53}Al_{0.23}Ga_{0.24}As}$ quantum dot (QD) single-photon source in the telecom C-band with a record-low biexciton emitter decay time of \SI{67.4(2)}{ps} under resonant two-photon excitation (TPE). We observe strong multiphoton suppression associated with $g^{(2)}_\mathrm{X}(0) = 0.006(1)$ and $g^{(2)}_\mathrm{XX}(0) = 0.007(1)$ for the exciton (X) and biexciton (XX) emission, respectively. Due to a asymmetric Purcell enhancement of the XX-X cascade, the two-photon interference (TPI) visibility of XX photons under $π$-pulse excitation of $V_{\rm{TPI}} = 90(3)\%$ reaches the theoretical limit and clearly exceeds the $\sim60\%$ expected for standard XX-X cascades without photonic engineering. Furthermore, adding a second timed laser pulse coinciding with XX emission energy, we demonstrate stimulated TPE in the telecom C-Band. The result is an improved TPI visibility of the X photons of $V_{\rm{TPI}}=0.69(3)$ compared to TPE with $V_{\rm{TPI}}=0.61(4)$, with both being reduced compared to the theoretical values due to present dephasing effects. The advances presented in this work hold important promises for the implementation of advanced schemes of quantum communication using deterministic quantum light sources. △ Less

Submitted 5 February, 2026; originally announced February 2026.

Comments: 9 pages, 3 figures

In-Substrate Imaging of Diamond hBN FET Current via Widefield Quantum Diamond Microscopy

Authors: Anuj Bathla, Subrat Kumar Pradhan, Ajit Kumar Dash, Prabhat Anand, M. Girish Chandra, Kenji Watanabe, Takashi Taniguchi, Akshay Singh, Veeresh Deshpande, Kasturi Saha

Abstract: We demonstrate widefield magnetic imaging of current flow in hydrogen terminated diamond field effect transistors (FETs) through in-substrate nitrogen vacancy (NV) centers. Hydrogen termination of the diamond surface induces a two dimensional hole gas (2DHG), while an ensemble of near surface NV centers located $ \sim 1~μm$ below the surface enables noninvasive magnetic imaging of current flow wit… ▽ More We demonstrate widefield magnetic imaging of current flow in hydrogen terminated diamond field effect transistors (FETs) through in-substrate nitrogen vacancy (NV) centers. Hydrogen termination of the diamond surface induces a two dimensional hole gas (2DHG), while an ensemble of near surface NV centers located $ \sim 1~μm$ below the surface enables noninvasive magnetic imaging of current flow with micrometer scale spatial resolution. The FETs were electrically characterized over a range of drain source biases $V_{ds}= 0$ to $-15V$ and gate voltages,$V_{gs}= +3$ to $-9V$ followed by in situ widefield NV magnetometry during device operation. Magnetic field maps and reconstructed current density distributions directly visualize current injection at the source drain contacts and transport beneath the hBN gated channel. Magnetic field maps reveal current density variations in the channel region owing to non-uniformities or defects in the gate dielectric. In addition, we observe a pronounced enhancement of the drain current ($\sim 600-900 μA$) and a shift in the apparent threshold voltage during laser illumination, reflecting photo induced changes in channel electrostatics. By correlating gate dependent magnetic images with simultaneous electrical measurements, we directly link spatial current distributions to FET transfer characteristics, providing new insight into buried interface transport and non-uniform gating effects in the transistor channel. As the methodology is compatible with top gated FETs, it can be used to map channel current distributions with micrometer resolution in emerging channel materials, such as 2D materials and wide bandgap channels, and establish widefield NV magnetometry as a powerful platform for probing charge transport in transistors and Van der Waals dielectric heterostructures. △ Less

Submitted 21 January, 2026; originally announced January 2026.

Interlayer Charge-Transfer Ferroelectric Fluctuations as a Pairing Mechanism in van der Waals Superconductors

Authors: Ankan Biswas, Jagannath Sutradhar, Sudip Kumar Saha, Avraham Klein, Jonathan Ruhman

Abstract: Signatures of unconventional superconductivity have been reported in a wide range of van der Waals (vdW) materials. However, their microscopic origin remains unclear due to competing electronic orders, strong spin-orbit coupling, and structural instabilities in the normal state. Here we investigate the role of interlayer breathing and shear modes in superconducting vdW heterostructures. Contrary t… ▽ More Signatures of unconventional superconductivity have been reported in a wide range of van der Waals (vdW) materials. However, their microscopic origin remains unclear due to competing electronic orders, strong spin-orbit coupling, and structural instabilities in the normal state. Here we investigate the role of interlayer breathing and shear modes in superconducting vdW heterostructures. Contrary to conventional wisdom -- which assumes that weak interlayer bonding and large layer separation suppress electronic coupling to these modes -- we show that the associated charge transfer can generate a substantial pairing interaction. We develop a theory of superconductivity mediated by such interlayer modes and demonstrate that proximity to a ferroelectric or antiferroelectric quantum critical point provides a strong-coupling pairing channel. Within a two-dimensional model with SU(2) symmetry and in-plane isotropy, we find an accidental degeneracy between interlayer triplet states, which can occur even for an $s$-wave in-plane gap. We further show that Josephson coupling between layers, arising from either static magnetism or induced by paramagnetic correlations, can stabilize a time-reversal-symmetry-breaking superconducting state of the $s+i\,s$ type, which couples to magnetization when at least two mirror symmetries are absent. Our results are directly applicable to candidate chiral vdW superconductors such as 4Hb-TaS$_2$ and to sliding ferroelectric metals, exemplified by bilayer MoTe$_2$. More broadly, our work identifies ferroelectric fluctuations as a promising route to unconventional pairing in vdW systems and motivates experimental searches for chiral multicomponent superconductivity. △ Less

Submitted 6 January, 2026; originally announced January 2026.

Mid-infrared intraband transitions in InAs colloidal quantum dots

Authors: Shraman Kumar Saha, Philippe Guyot-Sionnest

Abstract: III-V colloidal quantum dots are widely studied for their applications as detectors and emitters from visible to short-wave infrared. They might also be used in the mid-infrared if they can be stably n-doped to access their intraband transitions. Mid-infrared intraband transitions are therefore studied for InAs, InAs/InP, and InAs/ZnSe colloidal quantum dots with an energy gap at 1.4 micron. Using… ▽ More III-V colloidal quantum dots are widely studied for their applications as detectors and emitters from visible to short-wave infrared. They might also be used in the mid-infrared if they can be stably n-doped to access their intraband transitions. Mid-infrared intraband transitions are therefore studied for InAs, InAs/InP, and InAs/ZnSe colloidal quantum dots with an energy gap at 1.4 micron. Using electrochemistry, the quantum dot films show state-resolved mobility, state-resolved electron filling, and intraband absorption in the 3-8 micron range. The InAs/ZnSe films need a more reducing potential than InAs, but the InAs/InP films need a lower reduction potential. As a result, we found that dry films of InAs/InP dots show stable n-doping of the 1Se state, with a steady-state intraband absorption in the 3-5 micron range, and intraband luminescence at 5 micron. low-toxicity, high thermal stability, and stable n-doping, InAs quantum dots become an interesting material for mid-infrared applications. △ Less

Submitted 8 December, 2025; originally announced December 2025.

Comments: Main text: 20 pages, 5 figures excluding the TOC, SI: 15 pages

Probing Fermi surface topology by ultrafast pump pulse dynamics

Authors: Debamalya Dutta, Kush Saha

Abstract: We present a dynamical approach to detect changes in Fermi surface topology in a two-band model. Specifically, we show that the system's response to a low intensity light pulse can precisely identify topological Lifshitz transitions. At a suitable frequency, the light resonantly couples valence and conduction electrons, leading to an oscillation in the interband coherence term. This, in turn, gene… ▽ More We present a dynamical approach to detect changes in Fermi surface topology in a two-band model. Specifically, we show that the system's response to a low intensity light pulse can precisely identify topological Lifshitz transitions. At a suitable frequency, the light resonantly couples valence and conduction electrons, leading to an oscillation in the interband coherence term. This, in turn, generate a persistent oscillatory current which survives even after the end of the pulse. Notably, the relative amplitude of the oscillatory current during the pulse with that of the post-pulse reaches a minimum when the Fermi energy aligns with the saddle point, providing a robust framework for dynamically identifying Lifshitz transitions. △ Less

Submitted 9 August, 2025; originally announced August 2025.

Comments: 8 pages, 7 figures

Enhanced Andreev Reflection in Flat-Band Systems: Wave Packet Dynamics, DC Transport and the Josephson Effect

Authors: Sarbajit Mazumdar, Anamitra Mukherjee, Kush Saha, Sourin Das

Abstract: We investigate Andreev reflection (AR) in a proximity-induced normal-superconductor (NS) junction within the extended $α-\mathcal{T}_3$ lattice, emphasizing the impact of flat bands on AR. Our findings reveal that flat bands significantly enhance AR. Through wave packet dynamics, we track the real-time evolution of quasi-particle wave packets across the junction, providing deeper insight into elec… ▽ More We investigate Andreev reflection (AR) in a proximity-induced normal-superconductor (NS) junction within the extended $α-\mathcal{T}_3$ lattice, emphasizing the impact of flat bands on AR. Our findings reveal that flat bands significantly enhance AR. Through wave packet dynamics, we track the real-time evolution of quasi-particle wave packets across the junction, providing deeper insight into electron-hole conversion. Notably, the combination of band flatness and anisotropic dispersion in the $k_x-k_y$ plane induces an electronic analog of Goos-Hänchen (GH) shifts at the NS interface, exhibiting directional asymmetry along the junction. This asymmetry leads to a Hall-like response in Josephson junction in SNS geometry, where transport across the junction region is dominated by the quasi-flat bands. △ Less

Submitted 8 July, 2025; originally announced July 2025.

Comments: 13 pages, 11 figures

Journal ref: Phys. Rev. B 113, 134517 (2026)

Exactly solvable spin liquids in Kitaev bilayers and moiré superlattices

Authors: Ivan Dutta, Anamitra Mukherjee, Onur Erten, Kush Saha

Abstract: Building on the recent advancements on moiré superlattices, we propose an exactly solvable model with Kitaev-type interactions on a bilayer honeycomb lattice for both AA stacking and moiré superlattices. Using Monte Carlo simulations and variational analysis, we uncover a rich variety of phases where the intra and interlayer $\mathbb{Z}_2$ fluxes (visons) are arranged in a periodic fashion in the… ▽ More Building on the recent advancements on moiré superlattices, we propose an exactly solvable model with Kitaev-type interactions on a bilayer honeycomb lattice for both AA stacking and moiré superlattices. Using Monte Carlo simulations and variational analysis, we uncover a rich variety of phases where the intra and interlayer $\mathbb{Z}_2$ fluxes (visons) are arranged in a periodic fashion in the ground state, tuned by interlayer coupling and out-of-plane external magnetic field. We further extend our model to moiré superlattices at various commensurate twist angles around two distinct twist centers represented by $C_{3z}$ and $C_{6z}$ of the honeycomb lattice. Our simulations reveal generalized arrangements of plaquette values that correlate with the AA or AB stacking regions across the moiré unit cell. Moreover, we find that, depending on the twist angle, twist center and interlayer coupling, moiré superlattices exhibit to a variety of gapped and gapless spin liquid phases and can also host corner and edge modes. Our results highlight the rich physics in bilayer and twisted bilayer models of exactly solvable quantum spin liquids. △ Less

Submitted 9 December, 2025; v1 submitted 4 June, 2025; originally announced June 2025.

Comments: 15 pages, 12 figures

Journal ref: Phys. Rev. B 112, 245118 (2025)

Hunting for Maxwell's Demon in the Wild

Authors: Johan du Buisson, Jannik Ehrich, Matthew P. Leighton, Avijit Kundu, Tushar K. Saha, John Bechhoefer, David A. Sivak

Abstract: The paradox of Maxwell's demon motivated the development of information thermodynamics and the creation of nanoscale information engines. We now understand that machines such as the molecular motors within cells can in principle harvest fluctuations and thereby operate as a Maxwell demon -- but do they? Answering this question would seemingly require simultaneous measurement of all system degrees… ▽ More The paradox of Maxwell's demon motivated the development of information thermodynamics and the creation of nanoscale information engines. We now understand that machines such as the molecular motors within cells can in principle harvest fluctuations and thereby operate as a Maxwell demon -- but do they? Answering this question would seemingly require simultaneous measurement of all system degrees of freedom, which is generally intractable in single-molecule experiments. Here, we derive a simple statistical estimator to infer both the direction and magnitude of subsystem heat flows, and thus determine whether -- and how strongly -- a motor operates as a Maxwell demon. The estimator uses only trajectory measurements for a single degree of freedom. Simulating both colloidal information engines and kinesin molecular motors, we show that our estimator can precisely and accurately detect Maxwell-demon behavior with experimentally accessible resolution and quantities of data. Moreover, we find that kinesin transitions to a Maxwell-demon mechanism in the presence of nonequilibrium noise, with a corresponding increase in velocity consistent with experiments. These findings suggest that molecular motors may have evolved to leverage active fluctuations within cells. △ Less

Submitted 8 May, 2026; v1 submitted 15 April, 2025; originally announced April 2025.

Comments: 7 pages, 3 figures, + 8 pages SI

Non-trivial phonon dynamics and significant electron-phonon coupling of the high frequency modes in a Dirac semimetal

Authors: Debasmita Swain, Akash Dey, Anushree Roy, Kush Saha, Sitikantha D. Das

Abstract: Using finite temperature Raman spectroscopy, we investigate the electron-phonon interactions (EPI) and phonon-phonon scattering dynamics in the Dirac semimetal Cd3As2 in different fre quency regimes. Strong softening of the Raman shifts below 200 K is observed for almost all the phonon modes with a marked deviation from the standard anharmonic behavior. The experimen tally observed Raman linew… ▽ More Using finite temperature Raman spectroscopy, we investigate the electron-phonon interactions (EPI) and phonon-phonon scattering dynamics in the Dirac semimetal Cd3As2 in different fre quency regimes. Strong softening of the Raman shifts below 200 K is observed for almost all the phonon modes with a marked deviation from the standard anharmonic behavior. The experimen tally observed Raman linewidth seems to be captured well by a combination of EPI, relevant at low temperature (LT) and phonon-phonon scattering, which is predominant at high temperatures (HT), leading to an observable minima in the thermal evolution of the linewidth. While this fea ture is most prominently observed in the highest-frequency Raman mode (196 cm-1), its intensity gradually diminishes as the Raman frequency decreases. Computation of the electronic contribution to the phonon linewidth, for both the high and low frequency modes, from the phonon self-energy shows that it qualitatively mimics the experimental observations. It is found that phonon-induced interband scattering results in the presence of a maxima in phonon linewidth that crucially depends on the finiteness of the chemical potential. △ Less

Submitted 3 March, 2025; originally announced March 2025.

Journal ref: Phys. Rev. B 111, 035143 (2025)

Spin-imbalance induced buried topological edge currents in Mott \& topological insulator heterostructures

Authors: Rahul Ghosh, Subhajyoti Pal, Kush Saha, Anamitra Mukherjee

Abstract: We theoretically investigate the heterostructure between a ferrimagnetic Mott insulator and a time-reversal invariant topological band insulator on the two-dimensional Lieb lattice with periodic boundary conditions. Our Hartree-Fock and slave-rotor mean-field results incorporate long-range Coulomb interactions. We present charge and magnetic reconstructions at the two edges of the heterostructure… ▽ More We theoretically investigate the heterostructure between a ferrimagnetic Mott insulator and a time-reversal invariant topological band insulator on the two-dimensional Lieb lattice with periodic boundary conditions. Our Hartree-Fock and slave-rotor mean-field results incorporate long-range Coulomb interactions. We present charge and magnetic reconstructions at the two edges of the heterostructure and reveal how \textit{buried} topological edge modes adapt to these heterostructure edge reconstructions. In particular, we demonstrate that the interface magnetic field induces a spin imbalance in the edge modes while preserving their topological character and metallic nature. We show that this imbalance leads to topologically protected buried spin and charge currents. The inherent spin-momentum locking ensures that left and right movers contribute to the current at the two buried interfaces in opposite directions. We show that the magnitude of the spin-imbalance induced charge and spin current can be tuned by adjusting the spin-orbit coupling of the bulk topological insulator relative to the correlation strength of the bulk Mott insulator. Thus, our results demonstrate a controlled conversion of a spin Hall effect into an analog of a charge Hall effect driven by band topology and interaction effects. These topologically protected charge and spin currents pave the way for advances in low-energy electronics and spintronic devices. △ Less

Submitted 26 January, 2025; originally announced January 2025.

Journal ref: New J. Phys. 27, 113502 (2025)

Non-reciprocal interactions preserve the universality class of Potts model

Authors: Soumya K. Saha, P. K. Mohanty

Abstract: We study the $q$-state Potts model on a square lattice with directed nearest-neighbor spin-spin interactions that are inherently non-reciprocal. Both equilibrium and non-equilibrium dynamics are investigated. Analytically, we demonstrate that non-reciprocal interactions do not alter the critical exponents of the model under equilibrium dynamics. In contrast, numerical simulations with selfish non-… ▽ More We study the $q$-state Potts model on a square lattice with directed nearest-neighbor spin-spin interactions that are inherently non-reciprocal. Both equilibrium and non-equilibrium dynamics are investigated. Analytically, we demonstrate that non-reciprocal interactions do not alter the critical exponents of the model under equilibrium dynamics. In contrast, numerical simulations with selfish non-equilibrium dynamics reveal distinctive behavior. For $q=2$ (non-reciprocal non-equilibrium Ising model), the critical exponents remain consistent with those of the equilibrium Ising universality class. However, for $q=3$ and $q=4$, the critical exponents vary continuously. Remarkably, a super-universal scaling function -- Binder cumulant as a function of $ξ_2/ξ_0$, where $ξ_2$ is the second moment correlation length and $ξ_0$ its maximum value -- remains identical to that of the equilibrium $q=3,4$ Potts models. These findings indicate that non-reciprocal Potts models belong to the superuniversality class of their respective equilibrium counterparts. △ Less

Submitted 27 December, 2024; originally announced December 2024.

Comments: 6 pages, 3 Figs, Supp. Material (4 pages)

Oxygen Vacancy-Induced Monoclinic Dead Layers in Ferroelectric $Hf_xZr_{1-x}O_2$ With Metal Electrodes

Authors: Tanmoy Kumar Paul, Atanu Kumar Saha, Sumeet Kumar Gupta

Abstract: In this work, we analyze dead layer comprising non-polar monoclinic (m) phase in $Hf_xZr_{1-x}O_2$ (HZO)-based ferroelectric (FE) material using first principles analysis. We show that with widely used tungsten (W) metal electrode, the spatial distribution of the oxygen vacancy across the cross-section plays a key role in dictating the favorability of m- phase formation at the metal-HfO2 interface… ▽ More In this work, we analyze dead layer comprising non-polar monoclinic (m) phase in $Hf_xZr_{1-x}O_2$ (HZO)-based ferroelectric (FE) material using first principles analysis. We show that with widely used tungsten (W) metal electrode, the spatial distribution of the oxygen vacancy across the cross-section plays a key role in dictating the favorability of m- phase formation at the metal-HfO2 interface. The energetics are also impacted by the polarization direction as well as the depth of oxygen vacancy, i.e., position along the thickness. At the metal - $HfO_2$ interface, when polarization points towards the metal and vacancy forms at trigonally bonded O atomic site, both interfacial relaxation and m- phase formation can lead to dead layers. For vacancies at other oxygen atomic sites and polarization direction, dead layer is formed due to sole interfacial relaxation with polar phase. We also establish the relative favorability of the m-phase dead layer for different Zr concentrations (x=1 and x = 0.5) and metal electrodes. According to our analysis, 50% Zr doped $HfO_2$ exhibits less probability of m-phase dead layer formation compared to pure $HfO_2$. Moreover, with electrodes consisting of noble metal (Pt, Pd, Os, Ru, Rh), m-phase dead layer formation is less likely. Therefore, for these metals, dead layer forms mainly due to the interfacial relaxation with polar phase. △ Less

Submitted 9 December, 2024; originally announced December 2024.

Comments: 15 pages and 7 figures in main article, 3 pages of supplementary information

Strong Coupling Theory of Superconductivity and Ferroelectric Quantum Criticality in metallic SrTiO$_3$

Authors: Sudip Kumar Saha, Maria N. Gastiasoro, Jonathan Ruhman, Avraham Klein

Abstract: Superconductivity in doped SrTiO$_3$ has remained an enduring mystery for over 50 years. The material's status as a ``quantum" ferroelectric metal, characterized by a soft polar mode, suggests that quantum criticality could play a pivotal role in the emergence of its superconducting state. We show that the system is amenable to a strong coupling (Eliashberg) pairing analysis, with the dominant cou… ▽ More Superconductivity in doped SrTiO$_3$ has remained an enduring mystery for over 50 years. The material's status as a ``quantum" ferroelectric metal, characterized by a soft polar mode, suggests that quantum criticality could play a pivotal role in the emergence of its superconducting state. We show that the system is amenable to a strong coupling (Eliashberg) pairing analysis, with the dominant coupling to the soft mode being a ``dynamical'' Rashba coupling. We compute the expected $T_c$ for the entire phase diagram, all the way to the quantum critical point and beyond. We demonstrate that the linear coupling is sufficient to obtain a rough approximation of the experimentally measured phase diagram, but that nonlinear coupling terms are crucial in reproducing the finer features in the ordered phase. The primary role of nonlinear terms at the peak of the superconducting dome is to enhance the effective linear coupling induced by the broken order, shifting the dome's maximum into the ordered phase. Our theory quantitatively reproduces the three-dimensional experimental phase diagram in the space of carrier density, distance from the quantum critical point and temperature, and allows us to estimate microscopic parameters from the experimental data. △ Less

Submitted 6 December, 2024; originally announced December 2024.

Comments: 24 pages, 12 figures

Frequency-selective amplification of nonlinear response in strongly correlated bosons

Authors: Aditya Prakash, Debamalya Dutta, Arko Roy, Kush Saha

Abstract: We present a protocol to generate enhanced non-linear responses of incident pulses in the density wave phase within the extended Bose-Hubbard model using the concept of resonance-induced amplification (RIA). This method enables the selection of an incident pulse frequency to amplify the desired harmonic order. We characterize the enhancement of the non-linear harmonic spectra under various frequen… ▽ More We present a protocol to generate enhanced non-linear responses of incident pulses in the density wave phase within the extended Bose-Hubbard model using the concept of resonance-induced amplification (RIA). This method enables the selection of an incident pulse frequency to amplify the desired harmonic order. We characterize the enhancement of the non-linear harmonic spectra under various frequencies and field strengths of the incident pulses, and demonstrate that an optimal field strength is necessary to realize our protocol. △ Less

Submitted 12 September, 2024; originally announced September 2024.

Comments: 8 pages, 7 figures

Journal ref: Phys. Rev. B 112, 085152 (2025)

Current-induced spin polarisation in Rashba-Dresselhaus systems under different point groups

Authors: Akash Dey, Ashis K. Nandy, Kush Saha

Abstract: Non-magnetic materials without inversion symmetry typically exhibit strong Rashba spin-orbit coupling (SOC), enabling the well-known Rashba Edelstein effect where an external electrical current induces transverse spin polarisation. In this study, we demonstrate that electrically induced spin polarisation in non-magnetic materials, for example, electronic systems within quantum-well geometries, can… ▽ More Non-magnetic materials without inversion symmetry typically exhibit strong Rashba spin-orbit coupling (SOC), enabling the well-known Rashba Edelstein effect where an external electrical current induces transverse spin polarisation. In this study, we demonstrate that electrically induced spin polarisation in non-magnetic materials, for example, electronic systems within quantum-well geometries, can significantly be influenced by the system's point-group symmetries, such as $C_n$ and $C_{nv}$. These symmetries allow various linear and higher-order momentum, $k-$varying SOC Hamiltonian. Specifically, we show that surfaces having $C_{n}$ point-group symmetry, which permits specific linear and cubic Rashba and Dresselhaus SOC terms, can lead to both orthogonal and non-orthogonal spin polarisations with respect to the applied field. In contrast, surfaces with $C_{nv}$ symmetry exhibit only transverse spin polarisation, regardless of the linear and cubic SOC terms. We further find contrasting spin polarisation for cubic-in-$k$ SOC as compared to the linear-in-$k$ SOC when energy is varied, for example, through doping. Additionally, we show that the surfaces with $C_{n}$ symmetry may exhibit persistent spin current, depending on the relative strength between different momentum-dependent SOC terms. Our finding emphasizes the significance of crystal symmetry in understanding and manipulating induced spin polarisation in noncentrosymmetric materials, especially in surface/interface systems. △ Less

Submitted 12 December, 2024; v1 submitted 9 August, 2024; originally announced August 2024.

Comments: 27 pages, 8 figures

Journal ref: New J. Phys. 27 013024 (2025)

Disorder-induced delocalization and reentrance in a Chern-Hopf insulator

Authors: Soumya Bera, Ivan Dutta, Roderich Moessner, Kush Saha

Abstract: The Chern-Hopf insulator is an unconventional three-dimensional topological insulator with a bulk gap and gapless boundary states without protection from global discrete symmetries. This study investigates its fate in the presence of disorder. We find it stable up to moderate disorder by analyzing the surface states and the zero energy bulk density of states using large-scale numerical simulation… ▽ More The Chern-Hopf insulator is an unconventional three-dimensional topological insulator with a bulk gap and gapless boundary states without protection from global discrete symmetries. This study investigates its fate in the presence of disorder. We find it stable up to moderate disorder by analyzing the surface states and the zero energy bulk density of states using large-scale numerical simulation and the self-consistent Born approximation. The disordered Chern-Hopf insulator shows reentrant behavior: the disorder initially enhances the topological phase before driving it across an insulator-diffusive metal transition. We examine the associated critical exponents via finite-size scaling of the bulk density of states, participation entropy, and two-terminal conductance. We estimate the correlation length exponent $ν\simeq 1.0(1)$, consistent with the clean two-dimensional Chern universality and distinct from the integer quantum Hall exponent. △ Less

Submitted 7 August, 2024; originally announced August 2024.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. B 111, 184202 (2025)

Transverse resistance due to electronic inhomogeneities in superconductors

Authors: Shamashis Sengupta, Alireza Farhadizadeh, Joe Youssef, Sara Loucif, Florian Pallier, Louis Dumoulin, Kasturi Saha, Sumiran Pujari, Magnus Oden, Claire Marrache-Kikuchi, Miguel Monteverde

Abstract: Phase transitions in many-body systems are often associated with the emergence of spatial inhomogeneities. Such features may develop at microscopic lengthscales and are not necessarily evident in measurements of macroscopic quantities. In this work, we address the topic of distribution of current paths in superconducting films. Typical lengthscales associated with superconductivity are in the rang… ▽ More Phase transitions in many-body systems are often associated with the emergence of spatial inhomogeneities. Such features may develop at microscopic lengthscales and are not necessarily evident in measurements of macroscopic quantities. In this work, we address the topic of distribution of current paths in superconducting films. Typical lengthscales associated with superconductivity are in the range of nanometres. Accordingly, measurements of electrical resistance over much larger distances are supposed to be insensitive to details of spatial inhomogeneities of electronic properties. We observe that, contrary to expectations, current paths adopt a highly non-uniform distribution at the onset of the superconducting transition which is manifested in the development of a finite transverse resistance. The anisotropic distribution of current density is unrelated to the structural properties of the superconducting films, and indicates the emergence of electronic inhomogeneities perceivable over macroscopic distances. Our experiments reveal the ubiquitous nature of this phenomenon in conventional superconductors. △ Less

Submitted 23 July, 2024; originally announced July 2024.

Journal ref: Phys. Rev. B 112, 014514 (2025)

Small Signal Capacitance in Ferroelectric HZO: Mechanisms and Physical Insights

Authors: Revanth Koduru, Atanu K. Saha, Martin M. Frank, Sumeet K. Gupta

Abstract: This study presents a theoretical investigation of the physical mechanisms governing small signal capacitance in ferroelectrics, focusing on Hafnium Zirconium Oxide. Utilizing a time-dependent Ginzburg Landau formalism-based 2D multi-grain phase-field simulation framework, we simulate the capacitance of metal-ferroelectric-insulator-metal (MFIM) capacitors. Our simulation methodology closely mirro… ▽ More This study presents a theoretical investigation of the physical mechanisms governing small signal capacitance in ferroelectrics, focusing on Hafnium Zirconium Oxide. Utilizing a time-dependent Ginzburg Landau formalism-based 2D multi-grain phase-field simulation framework, we simulate the capacitance of metal-ferroelectric-insulator-metal (MFIM) capacitors. Our simulation methodology closely mirrors the experimental procedures for measuring ferroelectric small signal capacitance, and the outcomes replicate the characteristic butterfly capacitance-voltage behavior. We delve into the components of the ferroelectric capacitance associated with the dielectric response and polarization switching, discussing the primary physical mechanisms - domain bulk response and domain wall response - contributing to the butterfly characteristics. We explore their interplay and relative contributions to the capacitance and correlate them to the polarization domain characteristics. Additionally, we investigate the impact of increasing domain density with ferroelectric thickness scaling, demonstrating an enhancement in the polarization capacitance component (in addition to the dielectric component). We further analyze the relative contributions of the domain bulk and domain wall responses across different ferroelectric thicknesses. Lastly, we establish the relation of polarization capacitance components to the capacitive memory window (for memory applications) and reveal a non-monotonic dependence of the maximum memory window on HZO thickness. △ Less

Submitted 20 July, 2024; originally announced July 2024.

Site-percolation transition of run-and-tumble particles

Authors: Soumya K. Saha, Aikya Banerjee, P. K. Mohanty

Abstract: We study percolation transition of run and tumble particles (RTPs) on a two dimensional square lattice. RTPs in these models run to the nearest neighbour along their internal orientation with unit rate, and to other nearest neighbours with rates $p$. In addition, they tumble to change their internal orientation with rate $ω$. We show that for small tumble rates, RTP-clusters created by joining occ… ▽ More We study percolation transition of run and tumble particles (RTPs) on a two dimensional square lattice. RTPs in these models run to the nearest neighbour along their internal orientation with unit rate, and to other nearest neighbours with rates $p$. In addition, they tumble to change their internal orientation with rate $ω$. We show that for small tumble rates, RTP-clusters created by joining occupied nearest neighbours irrespective of their orientation form a phase separated state when the rate of positional diffusion $p$ crosses a threshold; with further increase of $p$ the clusters disintegrate and another transition to a mixed phase occurs. The critical exponents of this re-entrant site-percolation transition of RTPs vary continuously along the critical line in the $ω$-$p$ plane, but a scaling function remains invariant. This function is identical to the corresponding universal scaling function of percolation transition observed in the Ising model. We also show that the critical exponents of the underlying motility induced phase separation transition are related to corresponding percolation-critical-exponents by constant multiplicative factors known from the correspondence of magnetic and percolation critical exponents of the Ising model. △ Less

Submitted 27 December, 2024; v1 submitted 17 June, 2024; originally announced June 2024.

Comments: 8 pages, 8 figures, Supp. Material (5 pages with 9 figures)

Journal ref: Soft Matter 20, 9503(2024)

Optimized Current Density Reconstruction from Widefield Quantum Diamond Magnetic Field Maps

Authors: Siddhant Midha, Madhur Parashar, Anuj Bathla, David A. Broadway, Jean-Philippe Tetienne, Kasturi Saha

Abstract: Quantum Diamond Microscopy using Nitrogen-Vacancy (NV) defects in diamond crystals has enabled the magnetic field imaging of a wide variety of nanoscale current profiles. Intimately linked with the imaging process is the problem of reconstructing the current density, which provides critical insight into the structure under study. This manifests as a non-trivial inverse problem of current reconstru… ▽ More Quantum Diamond Microscopy using Nitrogen-Vacancy (NV) defects in diamond crystals has enabled the magnetic field imaging of a wide variety of nanoscale current profiles. Intimately linked with the imaging process is the problem of reconstructing the current density, which provides critical insight into the structure under study. This manifests as a non-trivial inverse problem of current reconstruction from noisy data, typically conducted via Fourier-based approaches. Learning algorithms and Bayesian methods have been proposed as novel alternatives for inference-based reconstructions. We study the applicability of Fourier-based and Bayesian methods for reconstructing two-dimensional current density maps from magnetic field images obtained from NV imaging. We discuss extensive numerical simulations to elucidate the performance of the reconstruction algorithms in various parameter regimes, and further validate our analysis via performing reconstructions on experimental data. Finally, we examine parameter regimes that favor specific reconstruction algorithms and provide an empirical approach for selecting regularization in Bayesian methods. △ Less

Submitted 9 November, 2024; v1 submitted 23 February, 2024; originally announced February 2024.

Comments: close to published version

Journal ref: Phys. Rev. Applied 22, 014015 (2024)

Ultrafast Green Single Photon Emission from an InGaN Quantum Dot-in-a-GaN Nanowire at Room Temperature

Authors: Swagata Bhunia, Ayan Majumder, Soumyadip Chatterjee, Ritam Sarkar, Dhiman Nag, Kasturi Saha, Suddhasatta Mahapatra, Apurba Laha

Abstract: In recent years, there has been a growing demand for room-temperature visible single-photon emission from InGaN nanowire-quantum-dots (NWQDs) due to its potential in developing quantum computing, sensing, and communication technologies. Despite various approaches explored for growing InGaN quantum dots on top of nanowires (NWs), achieving the emission of a single photon at room temperature with se… ▽ More In recent years, there has been a growing demand for room-temperature visible single-photon emission from InGaN nanowire-quantum-dots (NWQDs) due to its potential in developing quantum computing, sensing, and communication technologies. Despite various approaches explored for growing InGaN quantum dots on top of nanowires (NWs), achieving the emission of a single photon at room temperature with sensible efficiency remains a challenge. This challenge is primarily attributed to difficulties in accomplishing the radial confinement limit and the inherent giant built-in potential of the NWQD. In this report, we have employed a novel Plasma Assisted Molecular Beam Epitaxy (PAMBE) growth approach to reduce the diameter of the QD to the excitonic Bohr radius of InGaN, thereby achieving strong lateral confinement. Additionally, we have successfully suppressed the strong built-in potential by reducing the QD diameter. Toward the end of the report, we have demonstrated single-photon emission ($λ$ = 561 nm) at room-temperature from the NWQD and measured the second-order correlation function $g^{2}(0)$ as 0.11, which is notably low compared to other reported findings. Furthermore, the lifetime of carriers in the QD is determined to be 775 ps, inferring a high operational speed of the devices. △ Less

Submitted 24 November, 2023; originally announced November 2023.

Thermoelectric response in nodal-point semimetals

Authors: Ipsita Mandal, Kush Saha

Abstract: In this review, the thermoelectric properties in nodal-point semimetals with two bands are discussed. For the two-dimensional (2D) cases, it is shown that the expressions of the thermoelectric coefficients take different values depending on the nature of the scattering mechanism responsible for transport, by considering examples of short-ranged disorder potential and screened charged impurities. A… ▽ More In this review, the thermoelectric properties in nodal-point semimetals with two bands are discussed. For the two-dimensional (2D) cases, it is shown that the expressions of the thermoelectric coefficients take different values depending on the nature of the scattering mechanism responsible for transport, by considering examples of short-ranged disorder potential and screened charged impurities. An anisotropy in the energy dispersion spectrum invariably affects the thermopower quite significantly, as illustrated by the results for a node of semi-Dirac semimetal and a single valley of graphene. The scenario when a magnetic field of magnitude $B$ is applied perpendicular to the plane of the 2D semimetal is also considered. The computations for three-dimensional (3D) cases necessarily involve the inclusion of nontrivial Berry phase effects. In addition to demonstrating the expressions for the response tensors, the exotic behaviour observed in planar Hall and planar thermal Hall set-ups is also discussed. △ Less

Submitted 26 December, 2024; v1 submitted 19 September, 2023; originally announced September 2023.

Comments: typos corrected; includes the results reported in arXiv:1811.04952 and arXiv:2310.02318

Journal ref: Annalen der Physik 536 (9), 2400016 (2024)

Head-to-Head and Tail-to-Tail Domain Wall in Hafnium Zirconium Oxide: A First Principles Analysis of Domain Wall Formation and Energetics

Authors: Tanmoy K. Paul, Atanu K. Saha, Sumeet K. Gupta

Abstract: 180° domains walls (DWs) of Head-to-Head/Tail-to-Tail (H-H/T-T) type in ferroelectric (FE) materials are of immense interest for a comprehensive understanding of the FE attributes as well as harnessing them for new applications. Our first principles calculation suggests that such DW formation in Hafnium Zirconium Oxide (HZO) based FEs depends on the unique attributes of the HZO unit cell, such as… ▽ More 180° domains walls (DWs) of Head-to-Head/Tail-to-Tail (H-H/T-T) type in ferroelectric (FE) materials are of immense interest for a comprehensive understanding of the FE attributes as well as harnessing them for new applications. Our first principles calculation suggests that such DW formation in Hafnium Zirconium Oxide (HZO) based FEs depends on the unique attributes of the HZO unit cell, such as polar-spacer segmentation. Cross pattern of the polar and spacer segments in two neighboring domains along the polarization direction (where polar segment of one domain aligns with the spacer segment of another) boosts the stability of such DWs. We further show that low density of oxygen vacancies at the metal-HZO interface and high work function of metal electrodes are conducive for T-T DW formation. On the other hand, high density of oxygen vacancy and low work function of metal electrode favor H-H DW formation. Polarization bound charges at the DW get screened when band bending from depolarization field accumulates holes (electrons) in T-T (H-H) DW. For a comprehensive understanding, we also investigate their FE nature and domain growth mechanism. Our analysis suggests that a minimum thickness criterion of domains has to be satisfied for the stability of H-H/T-T DW and switching of the domains through such DW formation. △ Less

Submitted 30 November, 2023; v1 submitted 21 May, 2023; originally announced May 2023.

Comments: 10 figures

Flat Bands in Three-dimensional Lattice Models with Non-trivial Hopf Index

Authors: Ivan Dutta, Kush Saha

Abstract: We report the presence of exactly and nearly flat bands with non-trivial topology in three-dimensional (3D) lattice models. We first show that an exactly flat band can be realized in a 3D lattice model characterized by a 3D topological invariant, namely Hopf invariant. In contrast, we find another distinct 3D model, exhibiting both 2D Chern and 3D Hopf invariant, namely Hopf-Chern insulator, that… ▽ More We report the presence of exactly and nearly flat bands with non-trivial topology in three-dimensional (3D) lattice models. We first show that an exactly flat band can be realized in a 3D lattice model characterized by a 3D topological invariant, namely Hopf invariant. In contrast, we find another distinct 3D model, exhibiting both 2D Chern and 3D Hopf invariant, namely Hopf-Chern insulator, that can host nearly or perfect flat bands across different 2D planes. Such a Hopf-Chern model can be constructed by introducing specific hopping along the orthogonal direction of a simple two-orbital 2D Chern insulator in the presence of in-plane nearest-neighbor and next-nearest hopping among different orbitals. While the Chern planes host nearly perfect flat bands, the orthogonal planes can host both perfect or nearly perfect flat bands with zero Chern number at some special parameter values. Interestingly, such a 3D lattice construction from 2D allows finite Hopf invariant too. Finally, we show that higher Chern models can also be constructed in the same lattice setup with only nearest and next-nearest hopping, but the appearance of flat bands along high-symmetric path in the Brillouin zone requires longer-range hopping. We close with a discussion on possible experimental platforms to realize the models. △ Less

Submitted 6 August, 2024; v1 submitted 16 May, 2023; originally announced May 2023.

Comments: 24 pages, 11 figures

Journal ref: New J. Phys. 26 073051 (2024)

Proximate Dirac spin liquid in honeycomb lattice $J_1$-$J_3$ XXZ model: Numerical study and application to cobaltates

Authors: Anjishnu Bose, Manodip Routh, Sreekar Voleti, Sudeep Kumar Saha, Manoranjan Kumar, Tanusri Saha-Dasgupta, Arun Paramekanti

Abstract: Recent theoretical and experimental work suggest that the honeycomb cobaltates, initially proposed as candidate Kitaev quantum magnets, are in fact described by a pseudospin-$1/2$ easy-plane spin Hamiltonian with nearest neighbor ferromagnetic (FM) exchange $J_1$ being frustrated by antiferromagnetic third-neighbor exchange $J_3$ and weaker compass anisotropies. Using exact diagonalization and den… ▽ More Recent theoretical and experimental work suggest that the honeycomb cobaltates, initially proposed as candidate Kitaev quantum magnets, are in fact described by a pseudospin-$1/2$ easy-plane spin Hamiltonian with nearest neighbor ferromagnetic (FM) exchange $J_1$ being frustrated by antiferromagnetic third-neighbor exchange $J_3$ and weaker compass anisotropies. Using exact diagonalization and density-matrix renormalization group (DMRG) calculations, we show that this model exhibits FM order at small $J_3/J_1$ and zig-zag (ZZ) order at large $J_3/J_1$, separated by an intermediate phase, which we label as $\widetilde{\mathrm{SL}}$. This $\widetilde{\mathrm{SL}}$ phase is shown to exhibit spin-liquid-like correlations in DMRG, although we cannot preclude weak broken symmetries, e.g. weak Ising type Néel order, given the limits on our explored system sizes. Using a modified parton mean field theory and variational Monte Carlo on Gutzwiller projected wavefunctions, we show that the optimal FM and ZZ orders as well as the intermediate $\widetilde{\mathrm{SL}}$ state are proximate to a `parent' Dirac spin liquid (SL). This Dirac SL is shown to capture the broad continuum in the temperature and magnetic field dependent terahertz spectroscopy of BaCo$_2$(AsO$_4$)$_2$, and the reported low temperature metallic thermal conductivity in Na$_2$Co$_2$TeO$_6$ and BaCo$_2$(AsO$_4$)$_2$ upon incorporating disorder induced broadening. △ Less

Submitted 3 November, 2023; v1 submitted 26 December, 2022; originally announced December 2022.

Comments: Final PRB version - 13 pages, 11 figures. Special thanks to Federico Becca, Sasha Chernyshev, Martin Klanjsek

Journal ref: Phys. Rev. B 108, 174422 (2023)

Emergent many-body composite excitations of interacting spin-1/2 trimers

Authors: A. K. Bera, S. M. Yusuf, S. K. Saha, M. Kumar, D. Voneshen, Y. Skourski, S. A. Zvyagin

Abstract: Understanding exotic forms of magnetism in quantum spin systems is an emergent topic of modern condensed matter physics. Quantum dynamics can be described by particle-like carriers of information, known-as quasiparticles that appear from the collective behaviour of the underlying system. Spinon excitations, governing the excitations of quantum spin-systems, have been accurately calculated and prec… ▽ More Understanding exotic forms of magnetism in quantum spin systems is an emergent topic of modern condensed matter physics. Quantum dynamics can be described by particle-like carriers of information, known-as quasiparticles that appear from the collective behaviour of the underlying system. Spinon excitations, governing the excitations of quantum spin-systems, have been accurately calculated and precisely verified experimentally for the antiferromagnetic chain model. However, identification and characterization of novel quasiparticles emerging from the topological excitations of the spin system having periodic exchange interactions are yet to be obtained. Here, we report the identification of emergent composite excitations of the novel quasiparticles doublons and quartons in spin-1/2 trimer-chain antiferromagnet Na2Cu3Ge4O12 (having periodic intrachain exchange interactions J1-J1-J2) and its topologically protected quantum 1/3 magnetization-plateau state. The characteristic energies, dispersion relations, and dynamical structure factor of neutron scattering as well as macroscopic quantum 1/3 magnetization-plateau state are in good agreement with the state-of-the-art dynamical density matrix renormalization group calculations. △ Less

Submitted 21 October, 2022; originally announced October 2022.

Comments: 37 pages, 11 figures, 3 tables

Journal ref: Nature Communications 13, 6888 (2022)

Opto-thermoelectric trapping of Fluorescent Nanodiamonds on Plasmonic Nanostructures

Authors: Ashutosh Shukla, Sunny Tiwari, Ayan Majumder, Kasturi Saha, G V Pavan Kumar

Abstract: Deterministic optical manipulation of fluorescent nanodiamonds (FNDs) in fluids has emerged as an experimental challenge in multimodal biological imaging. Designing and developing nano-optical trapping strategies to serve this purpose is an important task. In this letter, we show how chemically-prepared gold nanoparticles and silver nanowires can facilitate Opto-thermoelectric force to trap indivi… ▽ More Deterministic optical manipulation of fluorescent nanodiamonds (FNDs) in fluids has emerged as an experimental challenge in multimodal biological imaging. Designing and developing nano-optical trapping strategies to serve this purpose is an important task. In this letter, we show how chemically-prepared gold nanoparticles and silver nanowires can facilitate Opto-thermoelectric force to trap individual entities of FNDs using a long working distance lens, low power-density illumination (532 nm laser, 12 $μW/μm^2$). Our trapping configuration combines the thermoplasmonic fields generated by individual plasmonic nanoparticles and the opto-thermoelectric effect facilitated by the surfactant to realise a nano-optical trap down to a single FND 120 nm in diameter. We utilise the same trapping excitation source to capture the spectral signatures of single FNDs and track their position. By tracking the FND, we observe the differences in the dynamics of FND around different plasmonic structures. We envisage that our drop-casting platform can be extrapolated to perform targeted, low-power trapping, manipulation, and multimodal imaging of FNDs inside biological systems such as cells. △ Less

Submitted 14 April, 2023; v1 submitted 6 October, 2022; originally announced October 2022.

Comments: 11 pages, 12 figures, 3 tables. Supplementary videos may be found at: https://drive.google.com/drive/folders/1gkW9g5Z7Fhl4i3ZQUOBQYuUYAPrHykzY?usp=sharing

Direction-Dependent Lateral Domain Walls in Ferroelectric Hafnium Zirconium Oxide and their Gradient Energy Coefficients: A First Principles Study

Authors: Tanmoy K. Paul, Atanu K. Saha, Sumeet K. Gupta

Abstract: To understand and harness the physical mechanisms of ferroelectric Hafnium Zirconium Oxide (HZO)-based devices, there is a need for clear understanding of domain interactions, their dynamics, negative capacitance effects, and other multi-domain characteristics. These crucial attributes depend on the coupling between neighboring domains quantified by the gradient energy coefficient (g). Furthermore… ▽ More To understand and harness the physical mechanisms of ferroelectric Hafnium Zirconium Oxide (HZO)-based devices, there is a need for clear understanding of domain interactions, their dynamics, negative capacitance effects, and other multi-domain characteristics. These crucial attributes depend on the coupling between neighboring domains quantified by the gradient energy coefficient (g). Furthermore, HZO has unique orientation-dependent lateral multidomain configurations. To develop an in-depth understanding of multi-domain effects, there is a need for thorough analysis of g. In this work, the energetics of multidomain configurations and domain growth mechanism corresponding to lateral domain walls of HZO are analyzed and gradient energy coefficients are quantified using first-principles Density Functional Theory calculations. These results indicate that one lateral direction exhibits the following characteristics: i) DW is ultra-sharp and domain growth occurs unit-cell-by-unit-cell, ii) the value of g is negative and in the order of $10^{-12} Vm^{3}C^{-1}$, and iii) g reduces (increases) with compressive (tensile) strain. In contrast, in the other lateral direction, the following attributes are observed: i) DW is gradual and domain growth occurs in quanta of half-unit-cell, ii) g is positive and in the order of $10^{-10} Vm^{3}C^{-1}$, and iii) g increases (reduces) with compressive (tensile) strain. △ Less

Submitted 4 December, 2023; v1 submitted 23 August, 2022; originally announced August 2022.

Information engine in a nonequilibrium bath

Authors: Tushar K. Saha, Jannik Ehrich, Momčilo Gavrilov, Susanne Still, David A. Sivak, John Bechhoefer

Abstract: Information engines can convert thermal fluctuations of a bath at temperature $T$ into work at rates of order $k_\mathrm{B}T$ per relaxation time of the system. We show experimentally that such engines, when in contact with a bath that is out of equilibrium, can extract much more work. We place a heavy, micron-scale bead in a harmonic potential that ratchets up to capture favorable fluctuations. A… ▽ More Information engines can convert thermal fluctuations of a bath at temperature $T$ into work at rates of order $k_\mathrm{B}T$ per relaxation time of the system. We show experimentally that such engines, when in contact with a bath that is out of equilibrium, can extract much more work. We place a heavy, micron-scale bead in a harmonic potential that ratchets up to capture favorable fluctuations. Adding a fluctuating electric field increases work extraction up to ten times, limited only by the strength of applied field. Our results connect Maxwell's demon with energy harvesting and an estimate of efficiency shows that information engines in nonequilibrium baths can greatly outperform conventional engines. △ Less

Submitted 30 July, 2022; originally announced August 2022.

Comments: 14 pages, 9 figures

Journal ref: Phys. Rev. Lett. 131, 057101 (2023)

Vector Chirality $κ$ Driven Topological Phase Transition and the Associated Anomalous Hall Conductivity Tuning in a Non-Collinear Antiferromagnet

Authors: Subhadip Pradhan, Kartik Samanta, Kush Saha, Ashis K. Nandy

Abstract: Based on the first-principles electronic structure calculations and subsequent symmetry adapted effective low-energy $\textbf{k.p}$ theory, we show the switching of the vector chirality, $κ$, in a noncollinear antiferromagnet (AFM), Mn$_3$Sn, as an unconventional route to topological phase transition from a nodal-ring to a Weyl point semimetal. Specifically, we find that the switching of $κ$ leads… ▽ More Based on the first-principles electronic structure calculations and subsequent symmetry adapted effective low-energy $\textbf{k.p}$ theory, we show the switching of the vector chirality, $κ$, in a noncollinear antiferromagnet (AFM), Mn$_3$Sn, as an unconventional route to topological phase transition from a nodal-ring to a Weyl point semimetal. Specifically, we find that the switching of $κ$ leads to gaping out an elliptic nodal-ring everywhere at the Fermi-level except for a pair of points on the ring. As a consequence, the topological phase transition switches the anomalous Hall conductivity (AHC) from zero to a giant value. Furthermore, we theoretically demonstrate how the controlled manipulation of the chiral AFM order keeping $κ$ unaltered favors unusual rotation of Weyl-points on the ring. This in turn enables us to tune in-plane components of the AHC by a collective uniform rotations of spins in the AFM unit cell. △ Less

Submitted 27 September, 2023; v1 submitted 30 June, 2022; originally announced June 2022.

Comments: 6 pages, 3 figures

Journal ref: Commun Phys 6, 272 (2023)

Spin Peierls transition of $J_{1}-J_{2}$ and extended models with ferromagnetic $J_{1}$.Sublattice dimerization and thermodynamics of zigzag chains in $β$-TeVO$_{4}$

Authors: Manodip Routh, Sudip Kumar Saha, Manoranjan Kumar, Zoltán G. Soos

Abstract: The spin$-1/2$ chain with ferromagnetic exchange $J_1 < 0$ between first neighbors and antiferromagnetic $J_2 > 0$ between second neighbors supports two spin-Peierls (SP) instabilities depending on the frustration $α= J_2/\vert J_1\vert$. Instead of chain dimerization with two spins per unit cell, $J_1-J_2$ models with $α> 0.65$ and linear spin-phonon coupling are unconditionally unstable to subla… ▽ More The spin$-1/2$ chain with ferromagnetic exchange $J_1 < 0$ between first neighbors and antiferromagnetic $J_2 > 0$ between second neighbors supports two spin-Peierls (SP) instabilities depending on the frustration $α= J_2/\vert J_1\vert$. Instead of chain dimerization with two spins per unit cell, $J_1-J_2$ models with $α> 0.65$ and linear spin-phonon coupling are unconditionally unstable to sublattice dimerization with four spins per unit cell. Unequal $J_1$ to neighbors to the right and left extends the model to gapped ($γ> 0$) chains with conditional SP transitions at $T_{SP}$ to dimerized sublattices and a weaker specific heat $C(T)$ anomaly. The spin susceptibility $χ(T)$ and $C(T)$ are obtained in the thermodynamic limit by a combination of exact diagonalization of small systems with $α> 0.65$ and density matrix renormalization group (DMRG) calculations of systems up to $N \sim 100$ spins. Both $J_1-J_2$ and $γ> 0$ models account quantitatively for $χ(T)$ and $C(T)$ in the paramagnetic phase of $β$-TeVO$_{4}$ for $T > 8$ K, but lower $T$ indicates a gapped chain instead of a $J_1-J_2$ model as previously thought. The same parameters and $T_{SP} = 4.6$ K generate a $C(T)/T$ anomaly that reproduces the anomaly at the $4.6$ K transition of $β$-TeVO$_{4}$, but not the weak $χ(T)$ signature. △ Less

Submitted 22 June, 2022; originally announced June 2022.

Comments: 12 pages, 13 figures

Journal ref: Physical Review B 105, 235109 (2022)

Non-linear response of interacting bosons in a quasiperiodic potential

Authors: Debamalya Dutta, Arko Roy, Kush Saha

Abstract: We theoretically study the electric pulse-driven non-linear response of interacting bosons loaded in an optical lattice in the presence of an incommensurate superlattice potential. In the non-interacting limit $(U=0)$, the model admits both localized and delocalized phases depending on the strength of the incommensurate potential $V_0$. We show that the particle current contains only odd harmonics… ▽ More We theoretically study the electric pulse-driven non-linear response of interacting bosons loaded in an optical lattice in the presence of an incommensurate superlattice potential. In the non-interacting limit $(U=0)$, the model admits both localized and delocalized phases depending on the strength of the incommensurate potential $V_0$. We show that the particle current contains only odd harmonics in the delocalized phase in contrast to the localised phase where both even and odd harmonics are identified. The relative magnitudes of these even and odd harmonics and sharpness of the peaks can be tuned by varying frequency and the number of cycles of the applied pulse, respectively. In the presence of repulsive interactions, the amplitudes of the even and odd harmonics further depend on the relative strengths of the interaction $U$ and the potential $V_0$. We illustrate that the disorder and interaction-induced phases can be distinguished and characterized through the particle current. Finally, we discuss the dynamics of field induced excitation responsible for exhibiting higher harmonics in the current spectrum. △ Less

Submitted 13 May, 2022; originally announced May 2022.

Comments: 11 pages, 11 figures

Journal ref: Phys. Rev. B 107, 035120(2023)

Bayesian information engine that optimally exploits noisy measurements

Authors: Tushar K. Saha, Joseph N. E. Lucero, Jannik Ehrich, David A. Sivak, John Bechhoefer

Abstract: We have experimentally realized an information engine consisting of an optically trapped, heavy bead in water. The device raises the trap center after a favorable "up" thermal fluctuation, thereby increasing the bead's average gravitational potential energy. In the presence of measurement noise, poor feedback decisions degrade its performance; below a critical signal-to-noise ratio, the engine sho… ▽ More We have experimentally realized an information engine consisting of an optically trapped, heavy bead in water. The device raises the trap center after a favorable "up" thermal fluctuation, thereby increasing the bead's average gravitational potential energy. In the presence of measurement noise, poor feedback decisions degrade its performance; below a critical signal-to-noise ratio, the engine shows a phase transition and cannot store any gravitational energy. However, using Bayesian estimates of the bead's position to make feedback decisions can extract gravitational energy at all measurement noise strengths and has maximum performance benefit at the critical signal-to-noise ratio. △ Less

Submitted 14 April, 2022; originally announced April 2022.

Comments: 14 pages, 10 figures

Journal ref: Phys. Rev. Lett. 129, 130601 (2022)

Hinge mode dynamics of periodically driven higher-order Weyl semimetals

Authors: Somsubhra Ghosh, Kush Saha, K. Sengupta

Abstract: We study the stroboscopic dynamics of hinge modes of a second-order topological material modeled by a tight-binding free fermion Hamiltonian on a cubic lattice in the intermediate drive frequency regime for both discrete (square pulse) and continuous (cosine) periodic drive protocols. We analyze the Floquet phases of this system and show that its quasienergy spectrum becomes almost gapless in the… ▽ More We study the stroboscopic dynamics of hinge modes of a second-order topological material modeled by a tight-binding free fermion Hamiltonian on a cubic lattice in the intermediate drive frequency regime for both discrete (square pulse) and continuous (cosine) periodic drive protocols. We analyze the Floquet phases of this system and show that its quasienergy spectrum becomes almost gapless in the large drive amplitude regime at special drive frequencies. Away from these frequencies, the gapped quasienergy spectrum supports weakly dispersing Floquet hinge modes. Near them, these hinge modes penetrate into the bulk and eventually become indistinguishable from the bulk modes. We provide an analytic, albeit perturbative, expression for the Floquet Hamiltonian using Floquet perturbation theory (FPT) which explains this phenomenon and leads to analytic expressions of these special frequencies. We also show that in the large drive amplitude regime, the zero energy hinge modes corresponding to the static tight-binding Hamiltonian display qualitatively different dynamics at these special frequencies. We discuss possible local density of state measurement using a scanning tunneling microscope which can test our theory. △ Less

Submitted 1 July, 2022; v1 submitted 15 February, 2022; originally announced February 2022.

Comments: Final version: 18 pages 15 figures

Journal ref: Phys. Rev. B 105 224312 (2022)

Anomalous pumping in the non-Hermitian Rice-Mele model

Authors: Abhishek Kumar, Sarbajit Mazumdar, S D Mahanti, Kush Saha

Abstract: We study topological charge pumping (TCP) in the Rice-Mele (RM) model with irreciprocal hopping. The non-Hermiticity gives rise to interesting pumping physics, owing to the presence of skin effect and exceptional points. In the static 1D RM model, we observe two independent tuning knobs that drive the topological transition, viz., non-Hermitian parameter $γ$ and system size $N$. To elucidate the s… ▽ More We study topological charge pumping (TCP) in the Rice-Mele (RM) model with irreciprocal hopping. The non-Hermiticity gives rise to interesting pumping physics, owing to the presence of skin effect and exceptional points. In the static 1D RM model, we observe two independent tuning knobs that drive the topological transition, viz., non-Hermitian parameter $γ$ and system size $N$. To elucidate the system-size dependency, we made use of the finite-size generalized Brillouin zone (GBZ) scheme. This scheme captures the state pumping of topological edge modes in the static 1D RM model and provides further insight into engineering novel gapless exceptional edge modes with the help of adiabatic drive. Finally, we apply three types of adiabatic protocols to study TCP in the 1+1D RM model. We further explain the number of pumped charges (in each period) using a non-Bloch topological invariant. This exactly explains the presence of different pumping phases in the non-Hermitian RM model as we tune the non-Hermitian parameter $γ$. We observe that in a non-Hermitian system, even a trivial adiabatic protocol can lead to pumping that has no Hermitian counterpart. △ Less

Submitted 14 November, 2023; v1 submitted 7 October, 2021; originally announced October 2021.

Comments: 16 pages, 18 figures

Journal ref: J. Phys.: Condens. Matter 37 135601 (2025)

Trajectory control using an information engine

Authors: Tushar K. Saha, John Bechhoefer

Abstract: We have built an information engine that can transport a bead in a desired direction by using favorable fluctuations from the thermal bath. However, in its original formulation, the information engine generates a fluctuating velocity and cannot control the position of the bead. Here, we introduce a feedback algorithm that can control the bead's position, to follow a desired trajectory. The bead ca… ▽ More We have built an information engine that can transport a bead in a desired direction by using favorable fluctuations from the thermal bath. However, in its original formulation, the information engine generates a fluctuating velocity and cannot control the position of the bead. Here, we introduce a feedback algorithm that can control the bead's position, to follow a desired trajectory. The bead can track the path if the maximum desired velocity is below the engine's maximum average velocity. Measuring the range of frequency that the feedback algorithm can track, we find a bandwidth that is slightly lower than the corner frequency of the bead in the trap. △ Less

Submitted 5 August, 2021; originally announced August 2021.

Comments: 9 pages, 8 figures, published in Proc. SPIE 11798, Optical Trapping and Optical Micromanipulation XVIII, 117980L (1 August 2021)

Journal ref: Proc. of SPIE (2021)

Sub-second Temporal Magnetic Field Microscopy Using Quantum Defects in Diamond

Authors: Madhur Parashar, Anuj Bathla, Dasika Shishir, Alok Gokhale, Sharba Bandyopadhyay, Kasturi Saha

Abstract: Wide field-of-view magnetic field microscopy has been realised by probing shifts in optically detected magnetic resonance (ODMR) spectrum of Nitrogen Vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require few to several minutes of acquisition to get a single magnetic field image, rendering the technique temporally static in it's current form. This limitat… ▽ More Wide field-of-view magnetic field microscopy has been realised by probing shifts in optically detected magnetic resonance (ODMR) spectrum of Nitrogen Vacancy (NV) defect centers in diamond. However, these widefield diamond NV magnetometers require few to several minutes of acquisition to get a single magnetic field image, rendering the technique temporally static in it's current form. This limitation prevents application of diamond NV magnetometers to novel imaging of dynamically varying microscale magnetic field processes. Here, we show that the magnetic field imaging frame rate can be significantly enhanced by performing lock-in detection of NV photo-luminescence (PL), simultaneously over multiple pixels of a lock-in camera. A detailed protocol for synchronization of frequency modulated PL of NV centers with fast camera frame demodulation, at few kilohertz frequencies, has been experimentally demonstrated. This experimental technique allows magnetic field imaging of sub-second varying microscale currents in planar microcoils with imaging frame rates in the range of 50 to 200 frames per second (fps). Our work demonstrates that widefield per-pixel lock-in detection of frequency modulated NV ODMR enables dynamic magnetic field microscopy. △ Less

Submitted 25 February, 2022; v1 submitted 26 July, 2021; originally announced July 2021.

Comments: 19 pages, 12 figures

Low temperature thermodynamics of the antiferromagnetic $J_1-J_2$ model: Entropy, critical points and spin gap

Authors: Sudip Kumar Saha, Manodip Routh, Manoranjan Kumar, Zoltán G. Soos

Abstract: The antiferromagnetic $J_1-J_2$ model is a spin-1/2 chain with isotropic exchange $J_1 > 0$ between first neighbors and $J_2 = αJ_1$ between second neighbors. The model supports both gapless quantum phases with nondegenerate ground states and gapped phases with $Δ(α) > 0$ and doubly degenerate ground states. Exact thermodynamics is limited to $α= 0$, the linear Heisenberg antiferromagnet (HAF). Ex… ▽ More The antiferromagnetic $J_1-J_2$ model is a spin-1/2 chain with isotropic exchange $J_1 > 0$ between first neighbors and $J_2 = αJ_1$ between second neighbors. The model supports both gapless quantum phases with nondegenerate ground states and gapped phases with $Δ(α) > 0$ and doubly degenerate ground states. Exact thermodynamics is limited to $α= 0$, the linear Heisenberg antiferromagnet (HAF). Exact diagonalization of small systems at frustration $α$ followed by density matrix renormalization group (DMRG) calculations returns the entropy density $S(T,α,N)$ and magnetic susceptibility $χ(T,α,N)$ of progressively larger systems up to $N = 96$ or 152 spins. Convergence to the thermodynamics limit, $S(T,α)$ or $χ(T,α)$, is demonstrated down to $T/J \sim 0.01$ in the sectors $α< 1$ and $α> 1$. $S(T,α)$ yields the critical points between gapless phases with $S^\prime(0,α) > 0$ and gapped phases with $S^\prime(0,α) = 0$. The $S^\prime(T,α)$ maximum at $T^*(α)$ is obtained directly in chains with large $Δ(α)$ and by extrapolation for small gaps. A phenomenological approximation for $S(T,α)$ down to $T = 0$ indicates power-law deviations $T^{-γ(α)}$ from $\exp(-Δ(α)/T)$ with exponent $γ(α)$ that increases with $α$. The $χ(T,α)$ analysis also yields power-law deviations, but with exponent $η(α)$ that decreases with $α$. $S(T,α)$ and the spin density $ρ(T,α) = 4Tχ(T,α)$ probe the thermal and magnetic fluctuations, respectively, of strongly correlated spin states. Gapless chains have constant $S(T,α)/ρ(T,α)$ for $T < 0.10$. Remarkably, the ratio decreases (increases) with $T$ in chains with large (small) $Δ(α)$. △ Less

Submitted 28 June, 2021; originally announced June 2021.

Comments: 12 pages, 14 figures

Journal ref: Phys. Rev. B 103, 245139 (2021)