arXiv subjects

Mikhail Lemeshko

Publications and source records attributed to Mikhail Lemeshko.

At least 19 recordsLinked to original sources

arXiv2026

Crossing the Rotational Sound Barrier in a Quantum Solvent

Molecules embedded in superfluids provide an experimentally controllable platform for investigating impurity physics. Here, we investigate a driven molecule rotating in a superfluid environment, including helium and Bose--Einstein condensates, at rotation frequencies similar to the bath dynamics. Within the experimentally relevant platform of an optical centrifuge, we show that the rotor remains localized up to a characteristic harmonic frequency that can realistically exceed the excitation energies of the bath, enabling access to ultrafast rotating impurities. In the co-rotating frame, the bath excitations experience a rotational Doppler shift, generating angular-momentum-resolved resonances absent in equilibrium angulon theory. We identify a dissipative rotational sound barrier at which the molecule resonantly emits bath excitations and undergoes strong angular momentum exchange with the surrounding medium. Overall, we establish the dynamical phase diagram of the driven rotor in a quantum solvent and introduce a generic platform for investigating fast driven rotating impurities in quantum many-body systems.

cond-mat.quant-gas↗

arXiv2026

Optical centrifuge as a probe of strong dissipative coupling between a molecular rotor and superfluid helium

A macroscopic manifestation of superfluidity is that objects moving through liquid helium experience negligible friction below the Landau critical velocity. How this frictionless motion breaks down at the nanoscale remains an open question. Molecules embedded in helium nanodroplets represent a well-controlled system for studying this breakdown, yet none has reached the regime of strong dissipative coupling, when energy transfer from the molecule to the superfluid dominates the observed dynamics. Molecular rotation, induced by short laser pulses, offer a suitable probe to reach rotational energies in the range of the roton gap, where superfluid helium supports a large number of elementary excitations. However, the solvation shell around a rotating molecule caps the energy reachable by a free rotor after impulsive excitation well below the roton excitation energy. Here we show that continuous driving with an ultraslow optical centrifuge overcomes this limitation: the strong field dresses the molecule into pendular states whose energies fall within the spectrum of the collective excitations of the superfluid, placing the system in the strong-dissipation regime. The resulting rapid thermalization locks the molecule to the rotating field until the rotation-induced level splittings overtake the thermalization rate, beyond which the molecular alignment is progressively lost. Our approach offers a direct measurement of the molecule-bath coupling in a quantum fluid.

quant-ph↗

arXiv2026

Engineering SU($N$)-Symmetric Hubbard Models with Microwave-Shielded Dipolar Molecules

Ultracold polar molecules provide strong, long-range interactions that microwave shielding makes tunable and nearly nuclear-spin independent, giving an emergent SU($N$) symmetry. However, extended Hubbard models of polar molecules in optical lattices lack, so far, controllable finite on-site interactions, a key ingredient of strong correlated physics. We show that tuning the Rabi frequency of the microwave coupling can bring two individual molecules (monomers) on neighboring lattice sites into resonance with a field-linked dimer (doublon) on one of the sites, enabling coherent doublon--monomer-pair conversion. In this model, we characterize the key Hubbard parameters and the dimer lifetime, demonstrating that the on-site and off-site interactions can be tuned nearly independently through the microwave amplitude and orientation, respectively. Our results provide a roadmap for implementing SU($N$)-symmetric extended Hubbard models with controllable doublon fluctuations, providing access to quantum-simulation in the strongly dipolar regime.

cond-mat.quant-gas↗

arXiv2026

Collective rotational cat states of molecules in microwave cavities

We show theoretically that an ensemble of polar molecules coupled to a microwave cavity supports hybrid rotational-photonic cat states. The cavity couples to a symmetric rotor in the bright manifold of $N$ molecules with $\sqrt{N}$-enhancement. In the dispersive limit of the collective strong coupling regime, virtual multilevel transitions induce an effective Kerr nonlinearity, as confirmed by Wigner tomography and a Schrieffer-Wolff analysis, leading to parity-locked cat structure in the cavity sectors. Collective molecular rotations thus provide a new route to hybrid light-matter cat states.

quant-ph↗

arXiv2026

Supercurrents in Josephson junctions with chiral molecular potentials

The influence of chiral molecular potentials on phase-coherent transport in superconducting Josephson junctions is investigated. Within a Bogoliubov-de Gennes tight-binding framework, an SNS junction functionalized by adsorbed chiral molecules is modeled, where electrostatic gradients generated by the molecules induce spin-orbit coupling in the normal region. The equilibrium charge current-phase relation is found to remain largely insensitive to molecular chirality in symmetric, zero-field configurations. In contrast, the spin supercurrent exhibits a pronounced chirality-dependent response, with opposite enantiomers producing distinct and anisotropic spin-polarized Josephson currents. The resulting handedness contrast can be enhanced through control parameters such as molecular orientation and the strength of the induced spin-orbit coupling. The temperature dependence of these currents further shows that the chirality-dependent signatures persist across a range of temperatures well below the superconducting critical temperature. These results establish Josephson interferometry as a phase-sensitive and accessible platform for detecting molecular chirality and highlight spin-polarized superconducting transport as a promising route toward integrating chiral molecular functionality into superconducting spintronic devices.

cond-mat.supr-con↗

arXiv2026

Kerr-enhanced amplification of three-wave mixing and emergent masing regimes

Integrated optical microresonators exploiting either second-order ($\chi^{(2)}$) or third-order ($\chi^{(3)}$) nonlinearities have become key platforms for frequency conversion, low-noise microwave photonics, and quantum entanglement generation. Here, we present an analytic theory of Kerr-enhanced three-wave mixing amplification in an electro-optic microresonator with both $\chi^{(2)}$ and $\chi^{(3)}$ nonlinearities. We demonstrate that Kerr dressing hybridizes the optical sidebands, renormalizing the $\chi^{(2)}$ couplings and detunings. As a result the system exhibits gain in regions where analogous bare $\chi^{(2)}$ or $\chi^{(3)}$ amplifiers are subthreshold. Time-domain Langevin simulations confirm this threshold reduction, mapping a practical design window for experiments.

physics.optics↗

arXiv2025

Fluctuation-Induced Supersolidity at the Superfluid-Solid Interface

Supersolidity, combining superfluid and crystalline orders, has been realized in dipolar Bose-Einstein condensates by tuning interatomic interactions. Here we show that supersolidity can also emerge from mode coupling at a superfluid-solid interface, without modifying bulk interactions and for a broad class of superfluids. Using an analytical and numerical treatment of the coupled superfluid and phonon fields, we derive the criterion for a density-modulation instability driven by interfacial coupling and dependent on dimensionality. In superfluid helium, the instability first appears at the roton mode, while in a Bose-Einstein condensate with contact interactions it occurs at the lowest accessible wave vector set by the system size. Beyond the threshold, the ground state acquires an interfacial density modulation while the bulk remains superfluid, forming a hybrid superfluid-supersolid phase. Our results identify interfacial mode coupling as a promising route to supersolidity, enabling the simultaneous exploitation of interfacial supersolid and bulk superfluid quantum properties.

cond-mat.quant-gas↗

arXiv2025

The R-index: A universal metric for evaluating OAM content and mode purity in optical fields

Despite its pivotal role in optical manipulation, high-capacity communications, and quantum information, a general measure of orbital angular momentum (OAM) in structured light remains elusive. In optical fields, where multiple vortices coexist, the local nature of vortex OAM and the absence of a common rotation axis make the total OAM of the field difficult to quantify. Here, we introduce the R-index, a metric that captures the intrinsic OAM content of any structured optical field, from pure Laguerre-Gaussian modes to arbitrary multi-vortex superpositions. Not only does this metric quantify the total OAM, it also assesses field purity, providing insight into the fidelity and robustness of the OAM generation. By unifying OAM characterization into a single figure of merit, the R-index enables direct comparison across diverse beam profiles and facilitates the identification of optimal configurations for both foundational studies and applied technologies.

physics.optics↗

arXiv2025

Phenomenological model of decaying Bose polarons

Cold atom experiments show that a mobile impurity particle immersed in a Bose-Einstein condensate forms a well-defined quasiparticle (Bose polaron) for weak to moderate impurity-boson interaction strengths, whereas a significant line broadening is consistently observed for strong interactions. Motivated by this, we introduce a phenomenological theory based on the assumption that the most relevant states are characterized by the impurity correlated with at most one boson, since they have the largest overlap with the uncorrelated states to which the most common experimental probes couple. These experimentally relevant states can however decay to lower energy states characterised by correlations involving multiple bosons, and we model this using a minimal variational wave function combined with a complex impurity-boson interaction strength. We first motivate this approach by comparing to a more elaborate theory that includes correlations with up to two bosons. Our phenomenological model is shown to recover the main results of two recent experiments probing both the spectral and the non-equilibrium properties of the Bose polaron. Our work offers an intuitive framework for analyzing experimental data and highlights the importance of understanding the complicated problem of the Bose polaron decay in a many-body setting.

cond-mat.quant-gas↗

arXiv2025

Tunable Field-Linked $s$-wave Interactions in Dipolar Fermi Mixtures

Spin mixtures of degenerate fermions are a cornerstone of quantum many-body physics, enabling superfluidity, polarons, and rich spin dynamics through $s$-wave scattering resonances. Combining them with strong, long-range dipolar interactions provides highly flexible control schemes promising even more exotic quantum phases. Recently, microwave shielding gave access to spin-polarized degenerate samples of dipolar fermionic molecules, where tunable $p$-wave interactions were enabled by field-linked resonances available only by compromising the shielding. Here, we study the scattering properties of a fermionic dipolar spin mixture and show that a universal $s$-wave resonance is readily accessible without compromising the shielding. We develop a universal description of the tunable $s$-wave interaction and weakly bound tetratomic states based on the microwave-field parameters. The $s$-wave resonance paves the way to stable, controllable and strongly-interacting dipolar spin mixtures of deeply degenerate fermions and supports favorable conditions to reach this regime via evaporative cooling.

cond-mat.quant-gas↗

arXiv2025

Bottom-up Analysis of Ro-Vibrational Helical Dichroism

Helical dichroism (HD) is a proposed method for the resolution of molecular chirality, employing the orbital angular momentum (OAM) of light. Going beyond the conventional assumptions about HD, this work proposes a rigid theoretical framework for the analysis of the HD, based on molecular symmetries and rotational eigenstates. We derive the rotational selection rules, which clearly establish that HD only emerges from the spin-orbit coupling of light, even for beams without far-field OAM. Our findings refine the conditions for observing HD, shedding light on the outcome of prior experiments and guiding future designs for chiral sensing using structured light.

physics.atom-ph↗

arXiv2025

Quantum transport in the presence of a chiral molecular potential

We investigate quantum transport in a two-dimensional electron system coupled to a chiral molecular potential, demonstrating how molecular chirality and orientation affect charge and spin transport properties. We propose a minimal model for realizing true chiral symmetry breaking on a magnetized surface, with a crucial role played by the tilt angle of the molecular dipole with respect to the surface. For non-zero tilting, we show that the Hall response exhibits clear signatures of chirality-induced effects, both in charge and spin-resolved observables. Concerning the former, tilted enantiomers produce asymmetric Hall conductances and, even more remarkably, the persistence of this feature in the absence of spin-orbit coupling (SOC) signals how the enantiospecific charge response results from electron scattering off the molecular potential. Concerning spin-resolved observables where SOC plays a relevant role, we reveal that chiral symmetry breaking is crucial in enabling spin-flipping processes.

cond-mat.mes-hall↗

arXiv2025

Nonadiabatic laser-induced alignment dynamics of alkali dimers on the surface of a helium droplet

Alkali dimers, $\mathrm{Ak}_2$, located on the surface of a helium nanodroplet, are set into rotation through the polarizability interaction with a nonresonant 1-ps-long laser pulse. The time-dependent degree of alignment is recorded using femtosecond-probe-pulse-induced Coulomb explosion into a pair of $\mathrm{Ak}^+$ fragment ions. The results, obtained for $\mathrm{Na}_2$, $\mathrm{K}_2$, and $\mathrm{Rb}_2$ in both the ground state $1 ^1\Sigma_g^+$ and the lowest-lying triplet state $1 ^3\Sigma_u^+$, exhibit distinct, periodic revivals with a gradually decreasing amplitude. The dynamics differ from that expected for dimers had they behaved as free rotors. Numerically, we solve the time-dependent rotational Schr\"odinger equation, including an effective mean-field potential to describe the interaction between the dimer and the droplet. The experimental and simulated alignment dynamics agree well and their comparison enables us to determine the effective rotational constants of the alkali dimers with the exception of $\mathrm{Rb}_2$($1 ^3\Sigma_u^+$) that only exhibits a prompt alignment peak but no subsequent revivals. For $\mathrm{Na}_2$($1 ^3\Sigma_u^+$), $\mathrm{K}_2$($1 ^1\Sigma_g^+$), $\mathrm{K}_2$($1 ^3\Sigma_u^+$) and $\mathrm{Rb}_2$($1 ^1\Sigma_g^+$), the alignment dynamics are well-described by a 2D rotor model. We ascribe this to a significant confinement of the internuclear axis of these dimers, induced by the orientation-dependent droplet-dimer interaction, to the tangential plane of their residence point on the droplet.

physics.atm-clus↗

arXiv2025

Environment-limited transfer of angular momentum in Bose liquids

Impurity motion in a many-body environment has been a central issue in the field of low-temperature physics for decades. In bosonic quantum fluids, the onset of a drag force experienced by point-like objects is due to collective environment excitations, driven by the exchange of linear momentum between the impurity and the many-body bath. In this work we consider a rotating impurity, with the aim of exploring how angular momentum is exchanged with the surrounding bosonic environment. In order to elucidate this issues, we employ a quasiparticle approach based on the angulon theory, which allows us to effectively deal with the non-trivial algebra of quantized angular momentum in presence of a many-body environment. We uncover how impurity dressing by environmental excitations can establish an exchange channel, whose effectiveness crucially depends on the initial state of the impurity. Remarkably, we find that there is a critical value of initial angular momentum, above which this channel effectively freezes.

cond-mat.quant-gas↗

arXiv2024

Anomalous multi-gap topological phases in periodically driven quantum rotors

We demonstrate that periodically driven quantum rotors provide a promising and broadly applicable platform to implement multi-gap topological phases, where groups of bands can acquire topological invariants due to non-Abelian braiding of band degeneracies. By adiabatically varying the periodic kicks to the rotor we find nodal-line braiding, which causes sign flips of topological charges of band nodes and can prevent them from annihilating, indicated by non-zero values of the %non-Abelian patch Euler class. In particular, we report on the emergence of an anomalous Dirac string phase arising in the strongly driven regime, a truly out-of-equilibrium phase of the quantum rotor. This phase emanates from braiding processes involving all (quasienergy) gaps and manifests itself with edge states at zero angular momentum. Our results reveal direct applications in state-of-the-art experiments of quantum rotors, such as linear molecules driven by periodic far-off-resonant laser pulses or artificial quantum rotors in optical lattices, whose extensive versatility offers precise modification and observation of novel non-Abelian topological properties.

quant-ph↗

arXiv2024

Domain-Wall Ferroelectric Polarons in a two-dimensional Rotor Lattice Model

We demonstrate the formation of ferroelectric domain-wall polarons in a minimal two-dimensional lattice model of electrons interacting with rotating dipoles. Along the domain-wall, the rotors polarize in opposite directions, causing the electron to localize along a particular lattice direction. The rotor-electron coupling is identified as the origin of a structural instability in the crystal that leads to the domain-wall formation via a symmetry-breaking process. Our results provide the first theoretical description of ferroelectric polarons, as discussed in the context of soft semiconductors.

cond-mat.mes-hall↗

arXiv2024

Quantum rotor in a two-dimensional mesoscopic Bose gas

We investigate a molecular quantum rotor in a two-dimensional Bose-Einstein condensate. The focus is on studying the angulon quasiparticle concept in the crossover from few- to many-body physics. To this end, we formulate the problem in real space and solve it with a mean-field approach in the frame co-rotating with the impurity. We show that the system starts to feature angulon characteristics when the size of the bosonic cloud is large enough to screen the rotor. More importantly, we demonstrate the departure from the angulon picture for large system sizes or large angular momenta where the properties of the system are determined by collective excitations of the Bose gas.

cond-mat.quant-gas↗

arXiv2024

Massive Dirac-Pauli physics in lead-halide perovskites

In standard quantum electrodynamics (QED), the so-called non-minimal (Pauli) coupling is suppressed for elementary particles and has no physical implications. Here, we show that the Pauli term naturally appears in a known family of Dirac materials -- the lead-halide perovskites, suggesting a novel playground for the study of analogue QED effects. We outline measurable manifestations of the Pauli term in the phenomena pertaining to (i) relativistic corrections to bound states (ii) the Klein paradox, and (iii) spin effects in scattering. In particular, we demonstrate that (a) the binding energy of an electron in the vicinity of a positively charged defect is noticeably decreased due to the polarizability of lead ions and the appearance of a Darwin-like term, (b) strong spin-orbit coupling due to the Pauli term affects the exciton states, and (c) scattering of an electron off an energy barrier with broken mirror symmetry produces spin polarization in the outgoing current. Our study adds to understanding of quantum phenomena in lead-halide perovskites, and paves the way for tabletop simulations of analogue Dirac-Pauli equations.

cond-mat.mes-hall↗