arXiv subjects

Ajit Srivastava

Publications and source records attributed to Ajit Srivastava.

At least 19 recordsLinked to original sources

arXiv2026

Super-resolution Control of Two-dimensional Quantum Emitters

Localized interlayer excitons in semiconducting transition-metal dichalcogenide heterobilayers are quantum emitters with a static electric dipole moment, making them excellent nanoscale charge sensors to probe correlated quantum phases in a proximal layer. These emitters are electrically tunable and inherit spin-valley selection rules, yet their deterministic spatial control remains challenging due to subwavelength confinement. Here, we present a platform that combines cryogenic optical spectroscopy with scanning probe microscopy to investigate trapped interlayer excitons in WSe$_2$/MoSe$_2$ bilayers. By exploiting AFM-based local Stark shift, we achieve super-resolution localization of emitters separated by only a few tens of nanometers and demonstrate deterministic control of individual charge states, including trion formation, opening a path towards coherent inter-dot coupling. Time-resolved measurements reveal tip-induced modification of the electromagnetic vacuum around individual emitters, thus controlling their radiative emission. Our multi-point charge sensing platform with optical readout is particularly well-suited to study fractionalization and anyon dynamics in semiconducting FCIs.

cond-mat.mes-hall↗

arXiv2025

Correlated quantum shift vector of particle-hole excitations

Excitons are a prime example of how electron interactions affect optical response and excitation. We demonstrate that, beyond its spectra, the bound nature of an exciton's electron-hole pair produces a correlated quantum geometry: excitonic excitations possess a quantum shift vector that is independent of light polarization. We find this counterintuitive behavior has dramatic consequences for geometric response: e.g., in noncentrosymmetric but non-polar materials, vertical excitonic transitions possess vanishing shift vector zeroing their shift photocurrent; this contrasts with finite and strongly light polarization dependent shift vectors for non-interacting delocalized particle-hole excitations. This dichotomy makes shift vector a sharp diagnostic of the pair localization properties of particle-hole excitations and demonstrates the non-perturbative effects of electron interactions in excited state quantum geometric response.

cond-mat.mes-hall↗

arXiv2025

Superradiance of Strongly Interacting Dipolar Excitons in Moiré Quantum Materials

Moiré lattices created in two-dimensional heterostructures exhibit rich many-body physics of interacting electrons and excitons and, at the same time, suggest promising optoelectronic applications. Here, we study the cooperative radiance of moiré excitons that is demonstrated to emerge from the deep subwavelength nature of the moiré lattice and the strong excitonic on-site interaction. In particular, we show that the static dipole-dipole interaction between interlayer excitons can strongly affect their cooperative optical properties, suppressing superradiance of disordered states while enhancing superradiance of ordered phases of moiré excitons. Moreover, we show that doping permits direct control of optical cooperativity, e.g., by generating supperradiant dynamics of otherwise subradiant states of excitons. Our results show that interlayer moiré excitons offer a unique platform for exploring cooperative optical phenomena in strongly interacting many-body systems, thus, holding promise for applications in quantum nonlinear optics.

quant-ph↗

arXiv2025

Signatures of collective photon emission and ferroelectric ordering of excitons near their Mott insulating state in a WSe$_2$/WS$_2$ heterobilayer

Spontaneous symmetry breaking, arising from the competition of interactions and quantum fluctuations, is fundamental to understanding ordered electronic phases. Although electrically neutral, optical excitations like excitons can interact through their dipole moment, raising the possibility of optically active ordered phases. The effects of spontaneous ordering on optical properties remain largely unexplored. Recent observations of the excitonic Mott insulating state in semiconducting moiré crystals make them promising for addressing this question. Here, we present evidence for an in-plane ferroelectric phase of dipolar moiré excitons driven by strong exciton-exciton interactions. We discover a surprising speed-up of photon emission at late times and low densities in excitonic decay. This counterintuitive behavior is attributed to collective radiance, linked to the transition between disordered and symmetry-broken ferroelectric phases of moiré excitons. Our findings provide first evidence for strong dipolar inter-site interactions in moiré lattices, demonstrate collective photon emission as a probe for moiré quantum materials, and pave the way for exploring cooperative optical phenomena in strongly correlated systems.

cond-mat.mes-hall↗

arXiv2024

Many-Body Photon Blockade and Quantum Light Generation from Cavity Quantum Materials

The strong coupling regime of photons and quantum materials inside optical cavities has emerged as a promising environment for manipulating states of matter with light. Here, in turn, we show that photons bear witness to cavity quantum-electrodynamical modifications of the material, leading to profoundly non-classical properties of light passing through the cavity. By generalizing quantum-optical input-output relations to correlated quantum materials, we study the second-order photon coherence g2(t) and demonstrate that antibunching of transmitted photons serves as direct evidence of light-induced changes to the cavity-embedded material. We show that materials near a quantum critical point can realize a collective many-body photon blockade, enabling the generation of single photons or Einstein-Podolsky-Rosen pairs via leveraging strong matter fluctuations. Our findings provide new routes for interrogating and harnessing cavity-embedded quantum materials as quantum light sources, as a resource for photon-based computation and quantum sensing.

cond-mat.str-el↗

arXiv2023

Entanglement and Topology in Su-Schrieffer-Heeger Cavity Quantum Electrodynamics

Cavity materials are a frontier to investigate the role of light-matter interactions on the properties of electronic phases of matter. In this work, we raise a fundamental question: can non-local interactions mediated by cavity photons destabilize a topological electronic phase? We investigate this question by characterizing entanglement, energy spectrum and correlation functions of the topological Su-Schrieffer-Heeger (SSH) chain interacting with an optical cavity mode. Employing density-matrix renormalization group (DMRG) and exact diagonalization (ED), we demonstrate the stability of the edge state and establish an area law scaling for the ground state entanglement entropy, despite long-range correlations induced by light-matter interactions. These features are linked to gauge invariance and the scaling of virtual photon excitations entangled with matter, effectively computed in a low-dimensional Krylov subspace of the full Hilbert space. This work provides a framework for characterizing novel equilibrium phenomena in topological cavity materials.

cond-mat.str-el↗

arXiv2022

Quadrupolar excitons in a tunnel-coupled van der Waals heterotrilayer

Strongly bound excitons and many-body interactions between them determine light-matter interactions in van der Waals (vdW) heterostructures of 2D semiconductors. Unlike fundamental particles, quasiparticles in condensed matter, such as excitons, can be tailored to alter their interactions and realize emergent quantum phases. Here, using a WS$_2$/WSe$_2$/WS$_2$ heterotrilayer, we create a quantum superposition of oppositely oriented dipolar excitons - a quadrupolar exciton - wherein an electron is layer-hybridized in WS$_2$ layers while the hole localizes in WSe$_2$. In contrast to dipolar excitons, symmetric quadrupolar excitons only redshift in an out-of-plane electric field, consistent with ab initio calculations, regaining dipolar characteristics at higher fields. Electric field tunes the hybridization and allows for lifetime control through modification of the excitonic wavefunction. Lack of density-dependent blue shift of heterotrilayer excitons compared to dipolar excitons is consistent with quadrupolar interactions. Our results present vdW heterotrilayers as a field-tunable platform to engineer light-matter interactions and explore quantum phase transitions between spontaneously ordered many-exciton phases.

cond-mat.mes-hall↗

arXiv2021

Momentum-space Gravity from the Quantum Geometry and Entropy of Bloch Electrons

Quantum geometry is a key quantity that distinguishes electrons in a crystal from those in the vacuum. Its study continues to provide insights into quantum materials, uncovering new design principles for their discovery. However, unlike the Berry curvature, an intuitive understanding of the quantum metric is lacking. Here, we show that the quantum metric of Bloch electrons leads to a momentum-space gravity. In particular, by extending the semiclassical formulation of electron dynamics to second order, we find that the resulting velocity is modified by a geodesic term and becomes the momentum-space dual of the Lorentz force in curved space. We calculate this geodesic response for magic-angle twisted bilayer graphene and show that moiré systems with flat bands are ideal candidates to observe this effect. Extending this analogy with gravity further, we find that the momentum-space dual of the Einstein field equations remains sourceless for pure states while for mixed states it acquires a source term that depends on the von Neumann entropy, for small entropies. We compare this stress-energy equation with the weak-field limit of general relativity and conclude that the von Neumann entropy is the momentum-space dual of the gravitational potential. Consequently, the momentum-space geodesic equation for mixed states is modified by a term resembling an entropic force. Our results highlight connections between quantum geometry, momentum-space gravity and quantum information, prompting further exploration of this dual gravity in quantum materials.

cond-mat.mes-hall↗

arXiv2021

Local Sensing of Correlated Electrons in Dual-moiré Heterostructures using Dipolar Excitons

Moiré heterostructures are rapidly emerging as a tunable platform to study correlated electronic phenomena. Discovery of exotic quantum phases in moiré systems requires novel probes of charge and spin order. Unlike detection schemes which average over several moiré cells, local sensors can provide richer information with greater sensitivity. We study a WSe$_2$/MoSe$_2$/WSe$_2$ heterotrilayer which hosts excitons and electrons in distinct moiré lattices, and show that localized dipolar excitons are sensitive proximity charge sensors, uncovering numerous correlated electronic states at fractional fillings of the multi-orbital moiré lattice. In addition, the emission polarization can reveal the local electronic spin configuration at different fillings. Our results establish dipolar excitons as promising candidates to study emergent quantum matter and quantum magnetism in moiré crystals with higher spatial resolution.

cond-mat.mes-hall↗

arXiv2020

Second Harmonic Generation from a Single Plasmonic Nanorod Strongly Coupled to a WSe2 Monolayer

Monolayer transition metal dichalcogenides, coupled to metal plasmonic nanocavities, have recently emerged as new platforms for strong light-matter interactions. These systems are expected to have nonlinear optical properties that will enable them to be used as entangled photon sources, compact wave-mixing devices, and other elements for classical and quantum photonic technologies. Here we report the first experimental investigation of the nonlinear properties of these strongly coupled systems, by observing second harmonic generation from a WSe2 monolayer strongly coupled to a single gold nanorod. The pump frequency dependence of the second harmonic signal displays a pronounced splitting that can be explained by a coupled oscillator model with second-order nonlinearities. Rigorous numerical simulations utilizing a nonperturbative nonlinear hydrodynamic model of conduction electrons support this interpretation and reproduce experimental results. Our study thus lays the groundwork for understanding the nonlinear properties of strongly coupled nanoscale systems.

physics.optics↗

arXiv2020

Optical control of valley Zeeman effect through many-exciton interactions

Charge carriers in two-dimensional transition metal dichalcogenides (TMDs), such as WSe$_2$, have their spin and valley-pseudospin locked into an optically-addressable index that is proposed as a basis for future information processing. The manipulation of this spin-valley index requires tuning its energy, typically through external magnetic field (B), which is cumbersome. Thus, other efficient routes like all-optical control of spin-valley index are desirable. Here, we show that many-body interactions amongst interlayer excitons in WSe$_2$/MoSe$_2$ heterobilayer induce a steady-state valley Zeeman splitting corresponding to B $\sim$ 6 Tesla. This anomalous splitting, present at incident powers as low as $μ$Ws, increases with power and enhances, suppresses or even flips the sign of a B-induced splitting. Moreover, the $\it{g}$-factor of valley Zeeman splitting can be tuned by $\sim$ 30 $\%$ with incident power. In addition to valleytronics, our results are relevant for achieving optical non-reciprocity using two-dimensional materials.

cond-mat.mes-hall↗

arXiv2019

Dipolar interactions between field-tuneable, localized emitters in van der Waals heterostructures

While photons in free space barely interact, matter can mediate interactions between them resulting in optical nonlinearities. Such interactions at the single-quantum level result in an on-site photon repulsion, crucial for photon-based quantum information processing and for realizing strongly interacting many-body states of light. Here, we report repulsive dipole-dipole interactions between electric field tuneable, localized interlayer excitons in MoSe$_2$/WSe$_2$ heterobilayer. The presence of a single, localized exciton with an out-of-plane, non-oscillating dipole moment increases the energy of the second excitation by $\sim$ 2 meV -- an order of magnitude larger than the emission linewidth and corresponding to an inter-dipole distance of $\sim$ 5 nm. At higher excitation power, multi-exciton complexes appear at systematically higher energies. The magnetic field dependence of the emission polarization is consistent with spin-valley singlet nature of the dipolar molecular state. Our finding is an important step towards the creation of excitonic few- and many-body states such as dipolar crystals with spin-valley spinor in van der Waals (vdW) heterostructures.

cond-mat.mes-hall↗

arXiv2018

Optical control of a single spin-valley in charged WSe$_2$ quantum dots

Control and manipulation of single charges and their internal degrees of freedom, such as spins, is a fundamental goal of nanoscience with promising technological applications. Recently, atomically thin semiconductors such as WSe$_2$ have emerged as a platform for valleytronics, offering rich possibilities for optical, magnetic and electrical control of the valley index. While progress has been made in controlling valley index of ensemble of charge carriers, valley control of individual charges, crucial for valleytronics, remains unexplored. Here, we provide unambiguous evidence for localized holes with net spin in optically active WSe$_2$ quantum dots (QDs) and control their spin-valley state with the helicity of the excitation laser under small magnetic field. We estimate a lower bound on the valley lifetime of a single charge in QD from recombination time to be $\sim$ nanoseconds. Remarkably, neutral QDs do not exhibit such a control, demonstrating the role of excess charge in prolonging the valley lifetime. Our work extends the field of 2D valleytronics to the level of single spin-valley, relevant for quantum information and sensing applications

cond-mat.mes-hall↗

arXiv2018

Entanglement of single-photons and chiral phonons in atomically thin WSe$_2$

Quantum entanglement is a fundamental phenomenon which, on the one hand, reveals deep connections between quantum mechanics, gravity and the space-time; on the other hand, has practical applications as a key resource in quantum information processing. While it is routinely achieved in photon-atom ensembles, entanglement involving the solid-state or macroscopic objects remains challenging albeit promising for both fundamental physics and technological applications. Here, we report entanglement between collective, chiral vibrations in two-dimensional (2D) WSe$_2$ host --- chiral phonons (CPs) --- and single-photons emitted from quantum dots (QDs) present in it. CPs which carry angular momentum were recently observed in WSe$_2$ and are a distinguishing feature of the underlying honeycomb lattice. The entanglement results from a "which-way" scattering process, involving an optical excitation in a QD and doubly-degenerate CPs, which takes place via two indistinguishable paths. Our unveiling of entanglement involving a macroscopic, collective excitation together with strong interaction between CPs and QDs in 2D materials opens up ways for phonon-driven entanglement of QDs and engineering chiral or non-reciprocal interactions at the single-photon level.

cond-mat.mes-hall↗

arXiv2017

Spin transfer due to quantum fluctuations of magnetization

Spin transfer - the transfer of angular momentum from spin-polarized electrical current to magnetic materials - has been extensively researched as an efficient mechanism for the electronic manipulation of the static and dynamic states in nanomagnetic systems, advancing our understanding of nanomagnetism and electronic transport, and enabling the development of energy-efficient magnetic nanodevices. Our present understanding of spin transfer is based on the classical approximation for the magnetization, even though the spin-polarized electrons mediating spin transfer are treated quantum-mechanically. Here, we utilize a nanoscale magnetic spin-valve structure to demonstrate that quantum zero-point fluctuations of magnetization, neglected in the existing theories of spin transfer, provide the dominant contribution to this effect at cryogenic temperatures, and remain non-negligible even at room temperature. The demonstrated quantum spin transfer (QST) is distinguished by a non-smooth piecewise-linear dependence of the fluctuation intensity on current, and can be driven not only by the directional flows of electrons, but also by their thermal motion. This effect can enhance current-induced phenomena, overcoming the efficiency limitations that are presently perceived as fundamental to the spin transfer mechanism.

cond-mat.mtrl-sci↗

arXiv2017

Zeeman splitting via spin-valley-layer coupling in bilayer MoTe2

Atomically thin transition metal dichalcogenides (TMDs) possess coupling of spin and valley degrees of freedom, making them promising for spin-valleytronics. ln monolayer TMDs, the emission helicity is locked to the valleys as a consequence of spin-orbit coupling and inversion symmetry breaking, leading to a valley analog of Zeeman effect in presence of out-of-plane magnetic field. As inversion symmetry is recovered in bilayers, the emission helicity should no longer be locked to the valleys. Here we show that Zeeman splitting, however, still persists in bilayers, as a result of an additional degree of freedom viz., the layer pseudospin and spin-valley-layer locking. In contrast to monolayer, Zeeman splitting here exists without lifting valley degeneracy. The degree of circularly polarized photoluminescence can be tuned with magnetic field from $-27\%$ to $27\%$. Our results demonstrate the control of degree of freedom in bilayer with magnetic field, which, together with previous electric field control, makes bilayer a promising platform for spin-valley quantum gates based on magnetoelectric effects.

cond-mat.mtrl-sci↗

arXiv2017

Giant paramagnetism induced valley polarization of electrons in charge-tunable monolayer MoSe2

For applications exploiting the valley pseudospin degree of freedom in transition metal dichalcogenide monolayers, efficient preparation of electrons or holes in a single valley is essential. Here, we show that a magnetic field of 7 Tesla leads to a near-complete valley polarization of electrons in MoSe2 monolayer with a density 1.6x10^{12} cm^{-2}; in the absence of exchange interactions favoring single-valley occupancy, a similar degree of valley polarization would have required a pseudospin g-factor exceeding 40. To investigate the magnetic response, we use polarization resolved photoluminescence as well as resonant reflection measurements. In the latter, we observe gate voltage dependent transfer of oscillator strength from the exciton to the attractive-Fermi-polaron: stark differences in the spectrum of the two light helicities provide a confirmation of valley polarization. Our findings suggest an interaction induced giant paramagnetic response of MoSe2, which paves the way for valleytronics applications.

cond-mat.mes-hall↗

arXiv2016

Fermi polaron-polaritons in charge-tunable atomically thin semiconductors

The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking experiments with ultra-cold Fermi gases. Optical creation of an exciton or a polariton in a two-dimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity-coupling favoring ultra-low mass polariton formation and exciton-electron interactions leading to polaron or trion formation. Here, we present cavity spectroscopy of gate-tunable monolayer MoSe$_2$ exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode. As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive-exciton-polaron resonance to the lower-energy attractive-polaron manifold. Simultaneous observation of polariton formation in both attractive and repulsive branches indicate a new regime of polaron physics where the polariton impurity mass is much smaller than that of the electrons. Our findings shed new light on optical response of semiconductors in the presence of free carriers by identifying the Fermi polaron nature of excitonic resonances and constitute a first step in investigation of a new class of degenerate Bose-Fermi mixtures.

cond-mat.mes-hall↗