Weak Interaction Regime Research Articles

Electronic States of Single Perovskite Quantum Dots in Weak and Strong Interaction Regimes: Implications in Electrically Pumped Quantum Emitters.

We investigate the effect of Coulomb interactions on the electronic states of a single perovskite quantum dot (PQD), CsPbBr3, through scanning tunneling microscopy/spectroscopy (STM/S). Under a weak interaction regime, where the time-averaged occupation of electrons in a PQD remains zero, the peaks observed in the differential tunneling conductance (dI/dV) spectrum correspond to the single-particle density of states (DOS) without any electron-electron correlation. However, with a shorter tunnel distance between the STM tip and PQD, additional electrons are trapped in the QD, leading to a strong interaction regime with well-defined electronic fine structures due to the lifting of spin degeneracy in the conduction bands. Interestingly, we observe that the strong Coulomb interaction can modify the spin-orbit coupling (SOC) strength in the PQDs. We have concluded that the energy levels under a strong electron-electron interaction regime are of utmost importance since they will be applicable to electrically pumped PQD-based single photon quantum emitters.

CO2 transport through swelling organic-rich nanoporous media: Insights on gas permeability from coarse-grained pore-scale simulations

Sorption-induced swelling reduces porosity and permeability in porous media, impacting long-term CO2 storage injectivity. This study employs an innovative coarse-grained model to couple fluid flow with sorption-induced solid deformation at the pore-scale. Our model preserves the advantages of molecular dynamics in simulating complex fluid-solid interaction under nano-confinement while extending the time and length scales. Adjusting the gas-solid interaction energy controls solid swelling and gas slippage at pore walls. Gas permeability curves demonstrate a linear decline with the increasing gas-solid interaction energy in both swelling and non-swelling nanoporous media, with the former experiencing a greater drop. Surprisingly, in the regime of weak gas-solid interactions, swelling is not yet initiated and the porosity and permeability of flexible porous media increase, possibly due to gas flow-induced pore throat opening. Nanoporous media with lower initial porosity experience a greater permeability decline during swelling. The relationship between permeability and porosity changes shows a linear increase characterized by different slopes with varying initial porosities. These findings provide valuable insights into the complex interactions among gas transport, solid deformation, and porosity changes in nanoporous media, with implications for understanding and optimizing gas production and CO2 storage in realistic geological environments.

Out of time order correlation of the Hubbard model with random local disorder.

The out-of-time-order correlator (OTOC) serves as a powerful tool for investigating quantum information spreading and chaos in complex systems. We present a method employing non-equilibrium dynamical mean-field theory and coherent potential approximation combined with diagrammatic perturbation on the Schwinger-Keldysh contour to calculate the OTOC for correlated fermionic systems subjected to both random disorder and electron interaction. Our key finding is that random disorder enhances the OTOC decay in the Hubbard model for the metallic phase in the weakly interacting limit. However, the current limitation of our perturbative solver restricts the applicability to weak interaction regimes.

Floquet non-equilibrium Green's function and Floquet quantum master equation for electronic transport: The role of electron-electron interactions and spin current with circular light.

The non-equilibrium Green's function (NEGF) and quantum master equation (QME) are two main classes of approaches for electronic transport. We discuss various Floquet variances of these formalisms for transport properties of a quantum dot driven via interaction with an external periodic field. We first derived two versions of the Floquet NEGF. We also explore an ansatz of the Floquet NEGF formalism for the interacting systems. In addition, we derived two versions of Floquet QME in the weak interaction regime. With each method, we elaborate on the evaluation of the expectation values of the number and current operators. We examined these methods for transport through a two-level system that is subject to periodic driving. The numerical results of all four methods show good agreement for non-interacting systems in the weak regime. Furthermore, we have observed that circular light can introduce spin current. We expect these Floquet quantum transport methods to be useful in studying molecular junctions exposed to light.

Conductivity in the half-filled disordered Hubbard model: A typical medium dynamical mean-field study

The conductivity for half-filled disordered Hubbard model in different interaction regimes is studied using a typical medium dynamical mean-field theory. The Kubo formula with the geometrically averaged Green function is used to calculate the optical and dc conductivity of the system. The dependence of the conductivities in the model on the random potential and temperature in the weak and intermediate interaction regimes is investigated numerically. It is shown that in the weak interaction regime at a fixed temperature the optical conductivity at low frequencies as well as the dc conductivity get decreased as disorder strength increases. Whereas in the intermediate interaction regime, the results show a reentrant effect of the insulating phases. Our dc conductivity of a non-interacting disordered system is consistent with those known in the literature and our other results are in agreement with the phase diagram obtained by using typical medium theory with different impurity solvers.

Disassociation of a one-dimensional cold molecule via quantum scattering

Motivated by the recent experimental developments in ultracold molecules and atoms, we propose a simple theoretical model to address the disassociation, reflection, and transmission probability of a one-dimensional cold molecule via quantum scattering. First, we show the Born approximation results in the weak interaction regime. Then, by employing the Lippmann–Schwinger equation, we give the numerical solution and investigate the disassociation’s dependence on the injection momentum and the interaction strengths. We find that the maximum disassociation rate has a limit when increasing the interaction strengths and injection momentum. We expect that our model can be realized in experiments in the near future.

Topological and quantum critical properties of the interacting Majorana chain model

We study Majorana chain with the shortest possible interaction term and in the presence of hopping alternation. When formulated in terms of spins the model corresponds to the transverse field Ising model with nearest-neighbor transverse and next-nearest-neighbor longitudinal repulsion. The phase diagram obtained with extensive DMRG simulations is very rich and contains six phases. Four gapped phases include paramagnetic, period-2 with broken translation symmetry, \mathbb{Z}_2ℤ2 with broken parity symmetry and the period-2-\mathbb{Z}_2ℤ2 phase with both symmetries broken. In addition there are two floating phases: gapless and critical Luttinger liquid with incommensurate correlations, and with an additional spontaneously broken \mathbb{Z}_2ℤ2 symmetry in one of them. By analyzing an extended phase diagram we demonstrate that, in contrast with a common belief, the Luttinger liquid phase along the self-dual critical line terminates at a weaker interaction strength than the end point of the Ising critical line that we find to be in the tri-critical Ising universality class. We also show that none of these two points is a Lifshitz point terminating the incommensurability. In addition, we analyzed topological properties through Majorana zero modes emergent in the two topological phases, with and without incommensurability. In the weak interaction regime, a self-consistent mean-field treatment provides a remarkable accuracy for the description of the spectral pairing and the parity switches induced by the interaction.

Many-Body Correlations and Exciton Complexes in CsPbBr3 Quantum Dots.

All-inorganic lead-halide perovskite (LHP) (CsPbX3 , X=Cl, Br, I) quantum dots (QDs) have emerged as a competitive platform for classical light-emitting devices (in the weak light-matter interaction regime, e.g., LEDs and laser), as well as for devices exploiting strong light-matter interaction at room temperature. Many-body interactions and quantum correlations among photogenerated exciton complexes play an essential role, for example, by determining the laser threshold, the overall brightness of LEDs, and the single-photon purity in quantum light sources. Here, by combining cryogenic single-QD photoluminescence spectroscopy with configuration-interaction (CI) calculations, the size-dependent trion and biexciton binding energies are addressed. Trion binding energies increase from 7to 17meV for QD sizes decreasing from 30to 9nm, while the biexciton binding energies increase from 15to 30meV, respectively. CI calculations quantitatively corroborate the experimental results and suggest that the effective dielectric constant for biexcitons slightly deviates from the one of the single excitons, potentially as a result of coupling to the lattice in the multiexciton regime. The findings here provide a deep insight into the multiexciton properties in all-inorganic LHP QDs, essential for classical and quantum optoelectronic devices.

Thermodynamics of the inception and interactions of multiple laser-produced cavitation bubbles using the lattice Boltzmann method

Understanding the thermodynamics associated with the inception and interactions of multiple cavitation bubbles is vital in developing deeper insights into the flow field and temperature properties of cavitation bubble clouds. This paper describes a double-distribution-function thermal lattice Boltzmann method for investigating the thermodynamics of multiple-bubble interactions, and proposes a new two-dimensional method for cavitation bubble inception without the initialization of gas nuclei. This approach is validated by simulating the evolution of a laser-produced bubble. The interaction between two vapor bubbles is simulated, and the hydrodynamics and thermodynamics of cavitation bubbles are systematically investigated. Both weak and strong interactions among bubbles of equal size are accurately reproduced. The coalescence mode is further categorized according to differences in the collapse behaviors: the remaining parts collapse away from each other for coalescence in the collapse stage, but collapse toward each other for coalescence in the growth stage. Interactions among bubbles of different sizes are also explored and the different morphologies for various interaction modes are reported. In the case of small distances between bubbles under the weak interaction regime, the smaller bubble is pushed away from the symmetry axis, leading to an initial decrease in collapse intensity and then an increase as the distance between the two bubbles increases. Finally, the thermodynamics of a bubble cluster are investigated. The hydrodynamic and thermodynamic processes in different regions of the cavitation bubble cluster are determined, and the toroidal shape in the final stage is reproduced. The results demonstrate that the proposed model accurately simulates the complex interactions among multiple cavitation bubbles.

Acousto-optical modulation of airy light beams in crystals

The non-collinear diffraction of Airy light beams on ultrasound in the regime of weak and strong acousto-optical (AO) interaction is investigated. Expressions are obtained for the complex amplitudes of diffracted waves in the general case of an anisotropic medium. It is shown that with an increase in the dimensionless parameter of the Airy beam, the efficiency of Bragg diffraction decreases. At the same time, however, the shape of the modulation curve remains unchanged. It is established that the greatest efficiency of anisotropic Bragg diffraction is achieved for a case when the diffracted Airy beam propagates along the front of the ultrasonic wave. It is shown that the efficiency of AO diffraction of Airy beam on an amplitude-modulated wave is determined by the diffraction efficiencies of constant components in a diffracted beam.

Nonzero angular momentum density wave phases in SU(N) fermions with singlet-bond and triplet-current interactions

We employ the sign-problem-free projector determinant quantum Monte Carlo method to study a microscopic model of SU($N$) fermions with singlet-bond and triplet-current interactions on the square lattice. We find the gapped singlet $p_x$ and gapless triplet $d_{x^2-y^2}$ density wave states in the half-filled $N=4$ model. Specifically, the triplet $d_{x^2-y^2}$ density wave order is observed in the weak triplet-current interaction regime. As the triplet-current interaction strength is further increased, our simulations demonstrate a transition to the singlet $p_x$ density wave state, accompanied by a gapped mixed-ordered area where the two orders coexist. With increasing singlet-bond interaction strength, the triplet $d_{x^2-y^2}$-wave order persists up to a critical point after which the singlet $p_x$ density wave state is stabilized, while the ground state is disordered in between the two ordered phases. The analytical continuation is then performed to derive the single-particle spectrum. In the spectra of triplet $d_{x^2-y^2}$ and singlet $p_x$ density waves, the anisotropic Dirac cone and the parabolic shape around the Dirac point are observed, respectively. As for the mixed-ordered area, a single-particle gap opens and the velocities remain anisotropic at the Dirac point.

Multiple vapor cavitation bubble interactions with a thermal lattice Boltzmann method

The double distribution function thermal lattice Boltzmann method is applied to investigate the thermodynamics of bubble interactions. The complete growth and collapse processes during an interaction are successfully reproduced, and interaction regimes of double cavitation bubbles are systematically studied, including strong and weak interactions. The critical distances to distinguish different interaction modes between two equal-sized bubbles are also proposed. For the weak interaction regime of two unequal-sized bubbles, the pressure wave generated by the smaller cavitation bubble leads to less distortion of the larger bubble in the final collapse stage. Moreover, a modified inertia model is proposed, which can be used to predict the liquid film thickness evolution of the unequal-sized bubbles under strong interaction regimes. Unbalanced forces act on the liquid film for two unequal-sized bubbles, making the film more difficult to rupture than for equal-sized bubbles, and the coalescence regime needs a smaller initial distance between them. Finally, the “shield effect” of the outer layer cavitation bubble and the “wall effect” of inner layer cavitation bubbles have been accurately reproduced in a cavitation bubble cluster, and the toroidal bubble shape in the final collapse stage is successfully simulated.

First and Second Sound in Two-Dimensional Bosonic and Fermionic Superfluids

We review our theoretical results of the sound propagation in two-dimensional (2D) systems of ultracold fermionic and bosonic atoms. In the superfluid phase, characterized by the spontaneous symmetry breaking of the U(1) symmetry, there is the coexistence of first and second sound. In the case of weakly-interacting repulsive bosons, we model the recent measurements of the sound velocities of 39K atoms in 2D obtained in the weakly-interacting regime and around the Berezinskii–Kosterlitz–Thouless (BKT) superfluid-to-normal transition temperature. In particular, we perform a quite accurate computation of the superfluid density and show that it is reasonably consistent with the experimental results. For superfluid attractive fermions, we calculate the first and second sound velocities across the whole BCS-BEC crossover. In the low-temperature regime, we reproduce the recent measurements of first-sound speed with 6Li atoms. We also predict that there is mixing between sound modes only in the finite-temperature BEC regime.

Generation of Josephson vortices in stacked toroidal Bose-Einstein condensates

Coupled coaxially stacked toroidal condensates with persistent currents suggest an appealing physical platform for the investigation of various phenomena related to interacting superflows from Josephson effects in the regime of weak interactions to the quantum Kelvin-Helmholtz instability for merging rings. We suggest experimentally accessible methods to prepare states with different topological charges in two coupled coaxial ring-shaped atomic Bose-Einstein condensates. Our results open up the way to direct observation of rotational Josephson vortices in atomic Bose-Einstein condensates.

Manifold formation and crossings of ultracold lattice spinor atoms in the intermediate interaction regime

Ultracold spinor atoms in the weak and strong interaction regimes have been extensively investigated, while the behavior in the intermediate regime is less understood. We numerically investigate one-dimensional ultracold spinor atomic ensembles of finite size in the intermediate interaction regime and reveal the evolution of the eigenstates from the strong to the intermediate regime. In the strong interaction regime, it is well known that the eigenstates can be categorized into different manifolds, and the categorization is protected by the energy gaps between manifolds. In the intermediate interaction regime, it is found that the eigenenergy spectrum becomes gapless, while categorization of the eigenstates is still preserved even without the protection from the intermanifold gaps. The categorization in the intermediate regime is found due to the minigap induced by the finite-size effect, which prevents the intermanifold coupling. The gap vanishing in the spectrum induces both direct and avoided crossings between close-lying manifolds, of which the combined symmetries determine the type of the crossings. A modified $t\text{\ensuremath{-}}J$ model is derived to describe the low-lying eigenstates in the intermediate regime, which can capture the formation and crossings of the manifolds. State preparation through avoided crossings is also investigated.

Bogoliubov-Born-Green-Kirkwood-Yvon Hierarchy and Generalized Hydrodynamics

We consider fermions defined on a continuous one-dimensional interval and subject to weak repulsive two-body interactions. We show that it is possible to perturbatively construct an extensive number of mutually compatible conserved charges for any interaction potential. However, the contributions to the densities of these charges at second order and higher are generally nonlocal and become spatially localized only if the potential fulfils certain compatibility conditions. We prove that the only solutions to the first of these conditions are the Cheon-Shigehara potential (fermionic dual to the Lieb-Liniger model) and the Calogero-Sutherland potentials. We use our construction to show how generalized hydrodynamics emerges from the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy, and argue that generalized hydrodynamics in the weak interaction regime is robust under nonintegrable perturbations.

Increasing the decoherence rate of Rydberg polaritons due to accumulating dark Rydberg atoms

We experimentally observed an accumulative type of nonlinear attenuation and distortion of slow light, i.e., Rydberg polaritons, with the Rydberg state $|32D_{5/2}\rangle$ in the weak-interaction regime. The present effect of attenuation and distortion cannot be explained by considering only the dipole-dipole interaction (DDI) between Rydberg atoms in $|32D_{5/2}\rangle$. Our observation can be attributed to the atoms in the dark Rydberg states other than those in the bright Rydberg state, i.e., $|32D_{5/2}\rangle$, driven by the coupling field. The dark Rydberg states are all the possible states, in which the population decaying from $|32D_{5/2}\rangle$ accumulated over time, and they were not driven by the coupling field. Consequently, the DDI between the dark and bright Rydberg atoms increased the decoherence rate of the Rydberg polaritons. We performed three different experiments to verify the above hypothesis, to confirm the existence of the dark Rydberg states, and to measure the decay rate from the bright to dark Rydberg states. In the theoretical model, we included the decay process from the bright to dark Rydberg states and the DDI effect induced by both the bright and dark Rydberg atoms. All the experimental data of slow light taken at various probe Rabi frequencies were in good agreement with the theoretical predictions based on the model. This study pointed out an additional decoherence rate in the Rydberg-EIT effect, and provides a better understanding of the Rydberg-polariton system.

Universal bounds on cooling power and cooling efficiency for autonomous absorption refrigerators.

For steady-state autonomous absorption refrigerators operating in the linear response regime, we show that there exists a hierarchy between the relative fluctuation of currents for cold, hot, and work terminals. Our proof requires the Onsager reciprocity relation along with the refrigeration condition that sets the direction of the mean currents for each terminal. As a consequence, the universal bounds on the mean cooling power, obtained following the thermodynamic uncertainty relations, follow a hierarchy. Interestingly, within this hierarchy, the tightest bound is given in terms of the work current fluctuation. Furthermore, the relative uncertainty hierarchy introduces a bound on cooling efficiency that is tighter than the bound obtained from the thermodynamic uncertainty relations. Interestingly, all of these bounds saturate in the tight-coupling limit. We test the validity of our results for two paradigmatic absorption refrigerator models: (i) a four-level working fluid and (ii) a two-level working fluid, operating in the weak (additive) and strong (multiplicative) system-bath interaction regimes, respectively.

Two-fluid hydrodynamics of cold atomic bosons under the influence of quantum fluctuations at non-zero temperatures

Ultracold Bose atoms is the physical system existing at the small finite temperatures, where the quantum and nonlinear phenomena play crucial role. Bosons are considered to be composed of two different fluids: the Bose–Einstein condensate and the normal fluid (the thermal component). The extended hydrodynamic models are obtained for each fluids, where the pressure evolution equations and the pressure flux third rank tensor evolution equations are obtained along with the continuity and Euler equations. It is found that the pressure evolution equation contains zero contribution of the short-range interaction. The pressure flux evolution equation contains the interaction which simplifies to the quantum fluctuations in the zero temperature limit. The structure of the third rank tensor describing this interaction is obtained in the regime of small temperature and weak interaction. The model is derived via the straightforward calculation of evolution of macroscopic functions using the microscopic many-particle Schrodinger equation in the coordinate representation. Finally, the two-fluid hydrodynamics is constructed in form of four equations for each fluid in order to give model describing the quantum fluctuations in BEC and the thermal effects in the normal fluid.

Localization and Delocalization of Ground States of Bose--Einstein Condensates Under Disorder

This paper studies the localization behavior of Bose--Einstein condensates in disorder potentials, modeled by a Gross--Pitaevskii eigenvalue problem on a bounded interval. In the regime of weak particle interaction, we are able to quantify exponential localization of the ground state, depending on statistical parameters and the strength of the potential. Numerical studies further show delocalization if we leave the identified parameter range, which is in agreement with experimental data. These mathematical and numerical findings allow the prediction of physically relevant regimes where localization of ground states may be observed experimentally.