Many-body Process Research Articles

Slower Auger Recombination in 12-Faceted Dodecahedron CsPbBr3 Nanocrystals.

Over the past two decades, intensive research efforts have been devoted to suppressions of Auger recombination in metal-chalcogenide and perovskite nanocrystals (PNCs) for the application of photovoltaics and light emitting devices (LEDs). Here, we have explored dodecahedron cesium lead bromide perovskite nanocrystals (DNCs), which show slower Auger recombination time compared to hexahedron nanocrystals (HNCs). We investigate many-body interactions that are manifested under high excitation flux density in both NCs using ultrafast spectroscopic pump-probe measurements. We demonstrate that the Auger recombination rate due to multiexciton recombinations are lower in DNCs than in HNCs. At low and intermediate excitation density, the majority of carriers recombine through biexcitonic recombination. However, at high excitation density (>1018 cm-3) a higher number of many-body Auger process dominates over biexcitonic recombination. Compared to HNCs, high PLQY and slower Auger recombinations in DNCs are likely to be significant for the fabrication of highly efficient perovskite-based photonics and LEDs.

Non-equilibrium quantum many-body physics with ultracold atoms

Combining quantum many-body physics and nonequilibrium physics is an important opportunity and challenge for current physics research. Nonequilibrium quantum many-body physics is not only a subject of common interest to many branches of physics but also an indispensable theoretical foundation for developing emergent quantum technologies. Cold atom system provides an ideal platform for studying nonequilibrium quantum many-body physics. The advantages of cold atom system, as well as other synthetic quantum systems, are reflected in studying various nonequilibrium processes such as the thermalization of isolated system, dissipation induced by coupling to the environment, ramping, quench, or periodically driving physical parameters of a system. In this work, three examples from our research are discussed to show how the study of nonequilibrium quantum many-body physics with cold atoms can help us go beyond the existing framework of topological physics, uncover new methods of detecting quantum many-body correlations, and enrich the physical content of gauge theory. Such a research concerns the fundamental properties of quantum many-body system, such as topology and correlation, utilizes the advantages of cold atomic system to achieve a quantitative comparison between theory and experiment, and aims at discovering universal physical rules for nonequilibrium quantum many-body process, which can be extended to condensed matter and nuclear matter systems.

Many-Body Radiative Decay in Strongly Interacting Rydberg Ensembles.

When atoms are excited to high-lying Rydberg states they interact strongly with dipolar forces. The resulting state-dependent level shifts allow us to study many-body systems displaying intriguing nonequilibrium phenomena, such as constrained spin systems, and are at the heart of numerous technological applications, e.g., in quantum simulation and computation platforms. Here, we show that these interactions also have a significant impact on dissipative effects caused by the inevitable coupling of Rydberg atoms to the surrounding electromagnetic field. We demonstrate that their presence modifies the frequency of the photons emitted from the Rydberg atoms, making it dependent on the local neighborhood of the emitting atom. Interactions among Rydberg atoms thus turn spontaneous emission into a many-body process which manifests, in a thermodynamically consistent Markovian setting, in the emergence of collective jump operators in the quantum master equation governing the dynamics. We discuss how this collective dissipation-stemming from a mechanism different from the much studied superradiance and subradiance-accelerates decoherence and affects dissipative phase transitions in Rydberg ensembles.

Model for the dynamics of carrier injection in a band with polaronic states: Application to exciton dissociation in organic solar cells

We develop a quantum model for the dynamics of carrier injection in a band that presents a strong carrier-vibration coupling. This coupling modifies the spectral density of the band and can even create pseudogaps that signal the onset of polaronic states. The injection of a carrier that interacts with many vibration modes is a complex many-body process that is treated by combining the quantum scattering theory and the dynamical mean-field theory (DMFT). For the model analyzed here, which is adapted to compact phases, the number $Z$ of neighbors of a given site is large and in this limit the DMFT becomes exact. The model is applied to the excitonic dissociation at the donor-acceptor interface for organic solar cells. The main ingredients are the electron-hole Coulomb interaction, the recombination process, and the existence of polaronic states in the acceptor band. Using parameters extracted from ab initio calculations we analyze the spectral density on the charge transfer state (CTS), the average energy transferred to phonons on the CTS, and the quantum yield of the injection process. We find in particular that even with a strong electron-vibration coupling, one can get a vibrationally cold charge transfer state with a high injection yield as often observed experimentally.

Spectral form factor in a minimal bosonic model of many-body quantum chaos.

We study spectral form factor in periodically kicked bosonic chains. We consider a family of models where a Hamiltonian with the terms diagonal in the Fock space basis, including random chemical potentials and pairwise interactions, is kicked periodically by another Hamiltonian with nearest-neighbor hopping and pairing terms. We show that, for intermediate-range interactions, the random phase approximation can be used to rewrite the spectral form factor in terms of a bistochastic many-body process generated by an effective bosonic Hamiltonian. In the particle-number conserving case, i.e., when pairing terms are absent, the effective Hamiltonian has a non-Abelian SU(1,1) symmetry, resulting in universal quadratic scaling of the Thouless time with the system size, irrespective of the particle number. This is a consequence of degenerate symmetry multiplets of the subleading eigenvalue of the effective Hamiltonian and is broken by the pairing terms. In such a case, we numerically find a nontrivial systematic system-size dependence of the Thouless time, in contrast to a related recent study for kicked fermionic chains.

Acceleration of enzymatic catalysis by active hydrodynamic fluctuations

The cellular milieu is teeming with biochemical nano-machines whose activity is a strong source of correlated non-thermal fluctuations termed active noise. Essential elements of this circuitry are enzymes, catalysts that speed up the rate of metabolic reactions by orders of magnitude, thereby making life possible. Here, we examine the possibility that active noise in the cell, or in vitro, affects enzymatic catalytic rate by accelerating or decelerating the crossing rate of energy barriers during the reaction. Considering hydrodynamic perturbations induced by biochemical activity as a source of active noise, we evaluate their impact on the enzymatic cycle using a combination of analytic and numerical methods. Our estimates show that the fast component of the active noise spectrum may significantly enhance the turnover rate of enzymes, while reactions remain practically unaffected by the slow noise spectrum. Revisiting the physics of barrier crossing under the influence of active hydrodynamic fluctuations suggests that the biochemical activity of macromolecules such as enzymes is coupled to active noise. Thus, we propose that enzymatic catalysis is a collective, many-body process in which enzymes may affect each other’s activity via long-range hydrodynamic interaction, with potential impact on biochemical networks in living and artificial systems alike.

Revealing the interrelation between C- and A-exciton dynamics in monolayer WS2 via transient absorption spectroscopy

Two-dimensional transition metal dichalcogenides (TMDs) are emerging as a promising complement for traditional semiconductor materials in ultrathin optoelectronic device fields. Developing a better understanding of high-energy C-exciton dynamics is essential for efficiently extracting hot carriers and building high-performance TMD-based light-harnessing devices; however, insight into the C-exciton dynamics remains scarce. To further understand the C-exciton dynamics, here, we have unraveled the interrelation between C-exciton and band edge A-exciton dynamics in monolayer WS2 by transient absorption spectroscopy. It is found that the band edge A-excitons could effectively generate high-energy C-excitons via the many-body process, and, in turn, the hot carriers relaxing from C-excitons to band edge states could compensate and slow the decay of the A-excitons. The comprehensive understanding of the interrelation between C-exciton and A-exciton dynamics in monolayer TMDs may trigger the potential applications for future TMD-based light-harvesting devices.

A new one-site coarse-grained model for water: Bottom-up many-body projected water (BUMPer). II. Temperature transferability and structural properties at low temperature.

A number of studies have constructed coarse-grained (CG) models of water to understand its anomalous properties. Most of these properties emerge at low temperatures, and an accurate CG model needs to be applicable to these low-temperature ranges. However, direct use of CG models parameterized from other temperatures, e.g., room temperature, encounters a problem known as transferability, as the CG potential essentially follows the form of the many-body CG free energy function. Therefore, temperature-dependent changes to CG interactions must be accounted for. The collective behavior of water at low temperature is generally a many-body process, which often motivates the use of expensive many-body terms in the CG interactions. To surmount the aforementioned problems, we apply the Bottom-Up Many-Body Projected Water (BUMPer) CG model constructed from Paper I to study the low-temperature behavior of water. We report for the first time that the embedded three-body interaction enables BUMPer, despite its pairwise form, to capture the growth of ice at the ice/water interface with corroborating many-body correlations during the crystal growth. Furthermore, we propose temperature transferable BUMPer models that are indirectly constructed from the free energy decomposition scheme. Changes in CG interactions and corresponding structures are faithfully recapitulated by this framework. We further extend BUMPer to examine its ability to predict the structure, density, and diffusion anomalies by employing an alternative analysis based on structural correlations and pairwise potential forms to predict such anomalies. The presented analysis highlights the existence of these anomalies in the low-temperature regime and overcomes potential transferability problems.

Many-body echo

In this letter we propose a protocol to reverse a quantum many-body dynamical process. We name it "many-body echo" because the underlying physics is closely related to the spin echo effect in nuclear magnetic resonance systems. We consider a periodical modulation of the interaction strength in a weakly interacting Bose condensate, which resonantly excites quasi-particles from the condensate. A dramatic phenomenon is that, after pausing the interaction modulation for half a period and then continuing on with the same modulation, nearly all the excited quasi-particles in the resonance modes will be absorbed back into the condensate. During the intermediate half period, the free evolution introduces a $\pi$ phase, which plays a role reminiscent of that played by the $\pi$-pulse in the spin echo. Comparing our protocol with another one implemented by the Chicago group in a recent experiment, we find that ours is more effective at reversing the many-body process. The difference between these two schemes manifests the physical effect of the micro-motion in the Floquet theory. Our scheme can be generalised to other periodically driven many-body systems.

Defect Assisted Carrier Multiplication in Amorphous Silicon

Recent studies show disordered materials such as amorphous silicon (a-Si), in spite of their low mobility, can efficiently amplify photocurrent via carrier multiplication process. Detectors with a thin (~40nm) a-Si multiplication layer has demonstrated single photon sensitivity at high speed under room temperature. It is believed that the abundance of bandtail states that cause low mobility of the amorphous materials actually contribute to the detector’s superior performance because carrier impact ionization involving these states, which can be modeled by donor-acceptor pairs (DAPs), relax the k-selection rule of the many-body carrier multiplication process. Our paper presents a theoretical framework to calculate the carrier multiplication process in a-Si or other disordered materials involving DAPs. Our analysis also establishes the relations between detector characteristics and key parameters such as the density of band tail states, layer thickness, and applied electric field. DAP assisted carrier multiplication rate is computed first. Carrier multiplication coefficients for electrons and holes under given applied field are then calculated using a trial distribution function that satisfies both the continuity equation and the energy balance equation. Our analysis shows that thinner a-Si gives rise to higher multiplication efficiency than thicker a-Si because of its higher carrier kinetic energy when the layer thickness is shorter than the length of energy relaxation by phonon scattering. Using the calculated carrier multiplication coefficients, voltage dependent gain of the device is computed and the results agree well with the measured results of a-Si cycling excitation process (CEP) detectors.

Ultrafast Auger process in few-layer PtSe2.

Enhanced many-body interactions due to strong Coulomb interactions and quantum confinement are one of the most prominent features of two-dimensional systems. The Auger process is a representative many-body interaction typically observed in two-dimensional semiconductors, determining important physical properties of materials, such as carrier lifetime, photoconductivity, and emission quantum yield. Recently, platinum dichalcogenides, represented by PtSe2 and PtS2, have attracted great attention due to their superior air stability, thickness-dependent semimetal-to-semiconductor transition, and exotic magnetic characteristics. However, the Auger process in platinum dichalcogenides has not been investigated to date. Here, we utilized ultrafast optical-pump terahertz-probe spectroscopy to explore carrier dynamics in few-layer semiconducting PtSe2. Most of the excited carriers are trapped by defects within ∼10 ps after excitation due to high defect density. We overcome this challenge by raising the excitation intensity to saturate trap sites with carriers, and observed a many-body process involving the carriers that survived the rapid trapping. This process is not band-to-band Auger recombination, but rather defect-assisted Auger recombination in which free carriers interact with trapped carriers at defects. Theoretical simulations show that this three-body Auger process can be approximated as bimolecular recombination at the rate of ∼3.3 × 10-3 cm2 s-1. This work provides insights into the interplay between ultrafast many-body processes and defects in two-dimensional semiconductors.

Origin of diverse nematic orders in Fe-based superconductors: 45∘ rotated nematicity in AFe2As2(A=Cs,Rb)

The origin of diverse nematicity and their order parameters in Fe-based superconductors have been attracting increasing attention. Recently, a new type of nematic order has been discovered in heavily hole-doped ($n_d=5.5$) compound AFe$_2$As$_2$ (A=Cs, Rb). The discovered nematicity has $B_{2g}$ (=$d_{xy}$) symmetry, rotated by $45^\circ$ from the $B_{1g}$ (=$d_{x^2-y^2}$) nematicity in usual compounds with $n_d\approx6$. We predict that the "nematic bond order", which is the symmetry-breaking of the correlated hopping, is responsible for the $B_{2g}$ nematic order in AFe$_2$As$_2$. The Dirac pockets in AFe$_2$As$_2$ is essential to stabilize the $B_{2g}$ bond order. Both $B_{1g}$ and $B_{2g}$ nematicity in A$_{1-x}$Ba$_x$Fe$_2$As$_2$ are naturally induced by the Aslamazov-Larkin many-body process, which describes the spin-fluctuation-driven charge instability. The present study gives a great hint to control the nature of charge nematicity by modifying the orbital character and the topology of the Fermi surface.

Hierarchical top-down bottom-up calibration with consideration for uncertainty and inter-scale discrepancy of Peierls stress of bcc Fe

A hierarchical multiscale model of plasticity in single crystal bcc Fe, framed at the mesoscale pertaining to a statistically representative set of dislocations for each slip system, relies on specification of a set of parameters which are informed using both bottom-up (BU) (atomistic) and top-down (TD) (experimental) information. We term this specification process a ‘connection’ between BU and TD pathways to inform the mesoscale model. The connection is considered in the presence of error, uncertainty, and discrepancy between the models. We expand upon a previously developed reconciled TD and BU calibration method to account for anticipated discrepancy between information from different length scales. The results of a previously formulated likelihood-based connection test of the multiscale model suggest a ‘missing link’ may be responsible for part of the inter-scale discrepancy. In this case, this link is assumed to be a relation between the unit-process (single dislocation line) of coordinated kink-pair nucleation on a screw dislocation segment and the many-body dislocation process which manifests in the onset of experimentally observed plastic deformation in a single crystal. A physics-informed discrepancy layer is formulated to improve the connection in the presence of additional persistent uncertainties. This physics-informed hypothesis testing is demonstrated as an alternative to fully data-driven search methods, particularly applicable to datasets of limited size that are often encountered in such multiscale material modeling applications. Discrete dislocation dynamics simulations are used to investigate the effect of the Peierls stress on straight screw dislocations and in the operation of Frank–Read sources. Discussion concerns the critical role of uncertainty quantification in providing a basis for this form of incremental hypothesis testing.

Molecular ionization of chloromethane in strong fields: nearest neighbor gateway to highly charged ions

The strong and ultrastrong field-molecule interaction is a complex, many-body process involving multiple ionization processes. We present ion yields and molecular fragment energies for the ionization of chloromethane (CH3Cl) in a laser field with intensities spanning from 1014 to 1017 W cm−2. As the laser intensity increases, ionization of CH3Cl is observed to pass from molecular tunneling, to enhanced ionization (EI), to an atomic-like response. The energy spectra of the ions show no dependence on the intensity and has its source in dissociative molecular ionization. A classical model of an aligned C–Cl ion is used to model the interaction. Following an initial molecular ionization process, our results show EI is a driving influence in the formation of low charge states until ionization become atomic-like and involves tightly bound ion states whose ionization is unaffected by nearest neighbor ions of similar ion charge.

Modified dipole-dipole interaction and dissipation in an atomic ensemble near surfaces

We study how the radiative properties of a dense ensemble of atoms can be modified when they are placed near or between metallic or dielectric surfaces. If the average separation between the atoms is comparable or smaller than the wavelength of the scattered photons, the coupling to the radiation field induces long-range coherent interactions based on the interatomic exchange of virtual photons. Moreover, the incoherent scattering of photons back to the electromagnetic field is known to be a many-body process, characterized by the appearance of superradiant and subradiant emission modes. By changing the radiation field properties, in this case by considering a layered medium where the atoms are near metallic or dielectric surfaces, these scattering properties can be dramatically modified. We perform a detailed study of these effects, with focus on experimentally relevant parameter regimes. We finish with a specific application in the context of quantum information storage, where the presence of a nearby surface is shown to increase the storage time of an atomic excitation that is transported across a one-dimensional chain.

Glass transition: from interaction potential changes to the corresponding structural and relaxation responses along the path of reaching the final equilibrium

Glass transition involves a many-body interaction and relaxation process. The split of relaxation spectrum and the extremely slow dynamics bring into considerations of non-linearity and non-equilibrium. Some of our recent findings in two measurable colloidal systems are reviewed, one with a simple attractive interparticle potential and the other with a competitive (repulsive vs. attractive) inter-particle interaction. With an approach from interaction potential changes to the corresponding structural and relaxation responses, along the path of reaching the final equilibrium, we illustrate some interesting physics in glass formation process. Also, some reviews on the popular glass transition theories are made to remind readers to avoid artifacts and misinterpretations.

A microscopic army

Ensembles of magnetic colloids can undergo an instability triggering the formation of clusters that move faster than the particles themselves. The many-body process relies on hydrodynamics alone and may prove useful for load delivery in fluidics.

Many-body interferometry of a Rydberg-dressed spin lattice

Ultracold atoms are an ideal platform to study strongly correlated phases of matter in and out of equilibrium. Much of the experimental progress in this field crucially relies on the control of the contact interaction between two atoms. Control of strong long-range interactions between distant ground state atoms has remained a long standing goal, opening the path towards the study of fundamentally new quantum many-body systems including frustrated or topological magnets and supersolids. Optical dressing of ground state atoms by near-resonant laser coupling to Rydberg states has been proposed as a versatile method to engineer such interactions. However, up to now the great potential of this approach for interaction control in a many-body setting has eluded experimental confirmation. Here we report the realisation of coherent Rydberg-dressing in an ultracold atomic lattice gas and directly probe the induced interaction potential using an interferometric technique with single atom sensitivity. We use this approach to implement a two-dimensional synthetic spin lattice and demonstrate its versatility by tuning the range and anisotropy of the effective spin interactions. Our measurements are in remarkable agreement with exact solutions of the many-body dynamics, providing further evidence for the high degree of accurate interaction control in these systems. Finally, we identify a collective many-body decay process, and discuss possible routes to overcome this current limitation of coherence times. Our work marks the first step towards the use of laser-controlled Rydberg interactions for the study of exotic quantum magnets in optical lattices.

Variable-Range Hopping through Marginally Localized Phonons.

We investigate the effect of coupling Anderson localized particles in one dimension to a system of marginally localized phonons having a symmetry protected delocalized mode at zero frequency. This situation is naturally realized for electrons coupled to phonons in a disordered nanowire as well as for ultracold fermions coupled to phonons of a superfluid in a one-dimensional disordered trap. To determine if the coupled system can be many-body localized we analyze the phonon-mediated hopping transport for both the weak and strong coupling regimes. We show that the usual variable-range hopping mechanism involving a low-order phonon process is ineffective at low temperature due to discreteness of the bath at the required energy. Instead, the system thermalizes through a many-body process involving exchange of a diverging number n∝-logT of phonons in the low temperature limit. This effect leads to a highly singular prefactor to Mott's well-known formula and strongly suppresses the variable range hopping rate. Finally, we comment on possible implications of this physics in higher dimensional electron-phonon coupled systems.

Breakdown of Anderson localization in the transport of Bose-Einstein condensates through one-dimensional disordered potentials

We study the transport of an interacting Bose--Einstein condensate through a 1D correlated disorder potential. We use for this purpose the truncated Wigner method, which is, as we show, corresponding to the diagonal approximation of a semiclassical van Vleck-Gutzwiller representation of this many-body transport process. We also argue that semiclassical corrections beyond this diagonal approximation are vanishing under disorder average, thus confirming the validity of the truncated Wigner method in this context. Numerical calculations show that, while for weak atom-atom interaction strength Anderson localization is preserved with a slight modification of the localization length, for larger interaction strenghts a crossover to a delocalized regime exists due to inelastic scattering. In this case, the transport is fully incoherent.