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Alberto Cappellaro

Publications and source records attributed to Alberto Cappellaro.

15 recordsLinked to original sources

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

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

Boosting quantum efficiency by reducing complexity

In the context of energy storage at the nanoscale, exploring the notion of \textit{quantum advantage} implies walking on the thin line at the boundary between quantum mechanics and thermodynamics, which underpins our conventional understanding of battery devices. With no classical analogue, the Sachdev-Ye-Kitaev (SYK) model has emerged in the last years as a promising platform to boost charging and storage efficiency thanks to its highly-entangling dynamics. Here, we explore how the robustness of this setup by considering the sparse version of the SYK model, showing that, as long as chaos is not completely broken, reducing its complexity may lead to more efficient quantum batteries.

quant-ph

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

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

Achiral dipoles on a ferromagnet can affect its magnetization direction

We demonstrate the possibility of a coupling between the magnetization direction of a ferromagnet and the tilting angle of adsorbed achiral molecules. To illustrate the mechanism of the coupling, we analyze a minimal Stoner model that includes Rashba spin-orbit coupling due to the electric field on the surface of the ferromagnet. The proposed mechanism allows us to study magnetic anisotropy of the system with an extended Stoner-Wohlfarth model, and argue that adsorbed achiral molecules can change magnetocrystalline anisotropy of the substrate. Our research's aim is to motivate further experimental studies of the current-free chirality induced spin selectivity effect involving both enantiomers.

cond-mat.mes-hall

Dissipative dynamics of an impurity with spin-orbit coupling

Brownian motion of a mobile impurity in a bath is affected by spin-orbit coupling (SOC). Here, we discuss a Caldeira-Leggett-type model that can be used to propose and interpret quantum simulators of this problem in cold Bose gases. First, we derive a master equation that describes the model and explore it in a one-dimensional (1D) setting. To validate the standard assumptions needed for our derivation, we analyze available experimental data without SOC; as a byproduct, this analysis suggests that the quench dynamics of the impurity is beyond the 1D Bose-polaron approach at temperatures currently accessible in a cold-atom laboratory -- motion of the impurity is mainly driven by dissipation. For systems with SOC, we demonstrate that 1D spin-orbit coupling can be 'gauged out' even in the presence of dissipation -- the information about SOC is incorporated in the initial conditions. Observables sensitive to this information (such as spin densities) can be used to study formation of steady spin polarization domains during quench dynamics.

cond-mat.quant-gas

Shift of the critical temperature in superconductors: a self-consistent approach

Within the Ginzburg-Landau functional framework for the superconducting transition, we analyze the fluctuation-driven shift of the critical temperature. In addition to the order parameter fluctuations, we also take into account the fluctuations of the vector potential above its vacuum. We detail the approximation scheme to include the fluctuating fields contribution, based on the Hartree-Fock-Bogoliubov-Popov framework. We give explicit results for $d=2$ and $d=3$ spatial dimensions, in terms of easily accessible experimental parameters such as the Ginzburg-Levanyuk number $\text{Gi}_{(d)}$, which is related to the width of the critical region where fluctuations cannot be neglected, and the Ginzburg-Landau parameter $κ$, defined as the ratio between the magnetic penetration length and the coherence one.

cond-mat.supr-con

Effective field theory of bosons with finite-range interaction in a disordered environment

We investigate the low-temperature properties of a ultracold gas made of bosonic alkali-metal atoms with finite-range interaction under the effect of a disordered environment. The statistical characterization of the disorder is investigated within an effective-field-theory formalism for a generic spatial dimension $d$. Moving to $d=3$, where all the arising divergences are properly regularized, we focus on the depletion of both the condensate and superfluid densities. At zero temperature we obtain meaningful analytical formulas for the condensate fraction and the superfluid fraction which take into account the interplay among scattering length, effective range, and disorder strength.

cond-mat.quant-gas

Dephasing-rephasing dynamics of one-dimensional tunneling quasicondensates

We study the quantum tunneling of two one-dimensional quasi-condensates made of alkali-metal atoms, considering two different tunneling configurations: side-by-side and head-to-tail. After deriving the quasiparticle excitation spectrum, we discuss the dynamics of the relative phase following a sudden coupling of the independent subsystems. In particular, we calculate the coherence factor of the system, which, due to the nonzero tunneling amplitude, it exhibits dephasing-rephasing oscillations instead of pure dephasing. These oscillations are enhanced by a higher tunneling energy, and by higher system densities. Our predictions provide a benchmark for future experiments at temperatures below $T \lesssim 5 \, \mbox{nK}$.

cond-mat.quant-gas

Action Functional for a Particle with Damping

In this brief report we discuss the action functional of a particle with damping, showing that it can be obtained from the dissipative equation of motion through a modification which makes the new dissipative equation invariant for time reversal symmetry. This action functional is exactly the effective action of Caldeira-Leggett model but, in our approach, it is derived without the assumption that the particle is weakly coupled to a bath of infinite harmonic oscillators.

cond-mat.stat-mech

Superfluids, Fluctuations and Disorder

We present a field-theory description of ultracold bosonic atoms in presence of a disordered external potential. By means of functional integration techniques, we aim to investigate and review the interplay between disordered energy landscapes and fluctuations, both thermal and quantum ones. Within the broken-symmetry phase, up to the Gaussian level of approximation, the disorder contribution crucially modifies both the condensate depletion and the superfluid response. Remarkably, it is found that the ordered (i.e. superfluid) phase can be destroyed also in regimes where the random external potential is suitable for a perturbative analysis. We analyze the simplest case of quenched disorder and then we move to present the implementation of the replica trick for ultracold bosonic systems. In both cases, we discuss strengths and limitations of the reviewed approach, paying specific attention to possible extensions and the most recent experimental outputs.

cond-mat.quant-gas

Collective modes across the soliton-droplet crossover in binary Bose mixtures

We study the collective modes of a binary Bose mixture across the soliton to droplet crossover in a quasi one dimensional waveguide with a beyond-mean-field equation of state and a variational Gaussian ansatz for the scalar bosonic field of the corresponding effective action. We observe a sharp difference in the collective modes in the two regimes. Within the soliton regime modes vary smoothly upon the variation of particle number or interaction strength. On the droplet side collective modes are inhibited by the emission of particles. This mechanism turns out to be dominant for a wide range of particle numbers and interactions. In a small window of particle number range and for intermediate interactions we find that monopole frequency is likely to be observed. In the last part we focus on the spin-dipole modes for the case of equal intraspecies interactions and equal equilibrium particle numbers in the presence of a weak longitudinal confinement. We found that such modes might be unobservable in the real-time dynamics close to the equilibrium as their frequency is higher than the particle emission spectrum by at least one order of magnitude in the droplet phase. Our results are relevant for experiments with two-component BECs for which we provide realistic parameters.

cond-mat.quant-gas

Superfluid filaments of dipolar bosons in free space

We systematically investigate the zero temperature phase diagram of bosons interacting via dipolar interactions in three dimensions in free space via path integral Monte Carlo simulations with few hundreds of particles and periodic boundary conditions based on the worm algorithm. Upon increasing the strength of the dipolar interaction and at sufficiently high densities we find a wide region where filaments are stabilized along the direction of the external field. Most interestingly by computing the superfluid fraction we conclude that superfluidity is anisotropic and is greatly suppressed along the orthogonal plane. Finally we perform simulations at finite temperature confirming the stability of filaments against thermal fluctuations and provide an estimate of the superfluid fraction in the weak coupling limit in the framework of the Landau two-fluid model.

cond-mat.quant-gas

Equation of state and self-bound droplet in Rabi-coupled Bose mixtures

Laser induced transitions between internal states of atoms have been playing a fundamental role to manipulate atomic clouds for many decades. In absence of interactions each atom behaves independently and their coherent quantum dynamics is described by the Rabi model. Since the experimental observation of Bose condensation in dilute gases, static and dynamical properties of multicomponent quantum gases have been extensively investigated. Moreover, at very low temperatures quantum fluctuations crucially affect the equation of state of many-body systems. Here we study the effects of quantum fluctuations on a Rabi-coupled two-component Bose gas of interacting alkali atoms. The divergent zero-point energy of gapless and gapped elementary excitations of the uniform system is properly regularized obtaining a meaningful analytical expression for the beyond-mean-field equation of state. In the case of attractive inter-particle interaction we show that the quantum pressure arising from Gaussian fluctuations can prevent the collapse of the mixture with the creation of a self-bound droplet. We characterize the droplet phase and discover an energetic instability above a critical Rabi frequency provoking the evaporation of the droplet. Finally, we suggest an experiment to observe such quantum droplets using Rabi-coupled internal states of $^{39}$K atoms.

cond-mat.quant-gas