Many-body Complex Research Articles

Dipolar many-body complexes and their interactions in stacked 2D heterobilayers

Dipolar many-body complexes and their interactions in stacked 2D heterobilayers

Phase Transition Study Meets Machine Learning

In recent years, machine learning (ML) techniques have emerged as powerful tools for studying many-body complex systems, and encompassing phase transitions in various domains of physics. This mini review provides a concise yet comprehensive examination of the advancements achieved in applying ML to investigate phase transitions, with a primary focus on those involved in nuclear matter studies.

Quantum speed limit for complex dynamics

Quantum speed limit focuses on the minimum time scale for a fixed mission and hence is important in quantum information where fast dynamics is usually beneficial. Most existing tools for the depiction of quantum speed limit are the lower-bound-type tools, which are in fact difficult to reveal the true minimum time, especially for many-body systems or complex dynamics. Therefore, the evaluation of this true minimum time in these scenarios is still an unsolved problem. Hereby we provide the operational definition of quantum speed limit for a general target and propose a three-step (classification-regression-calibration) methodology based on machine learning to evaluate the true minimum time in complex dynamics. Moreover, the analytical expression of the true minimum time is also provided for the time-dependent Hamiltonians with time-independent eigenstates.

Coherent Phonon-Mediated Many-Body Interaction in Monolayer WSe2

Due to the strong Coulomb interaction, the optical and electrical properties of two-dimensional transition metal dichalcogenides (TMDCs) are greatly determined by the emergence of many-body complexes such as excitons or trions. To fully realize the potential functionalities of these atomically thin materials, a comprehensive understanding of their many-body interaction mechanism is essential. Here, using the advanced femtosecond two-dimensional electronic spectroscopy technique combined with broadband transient absorption spectroscopy, a strong electron-exciton coupling effect in monolayer WSe2 following the ultrafast photoexcitation is revealed. We demonstrate that such many-body complexes can be generated effectively through the band-edge optical excitation, with a ∼1.5 ps stabilization process. The coherent optical phonon plays a dominant role in this electron-exciton interaction, and the coherence of the electron (exciton)-phonon coupling can last for ∼4.5 ps. This finding offers new insight into the formation mechanism of photoinduced many-body complexes in TMDCs.

Switching of K-Q intervalley trions fine structure and their dynamics in n-doped monolayer WS<sub>2</sub>

Monolayer group VI transition metal dichalcogenides (TMDs) have recently emerged as promising candidates for photonic and opto-valleytronic applications. The optoelectronic properties of these atomically-thin semiconducting crystals are strongly governed by the tightly bound electron-hole pairs such as excitons and trions (charged excitons). The anomalous spin and valley configurations at the conduction band edges in monolayer WS₂ give rise to even more fascinating valley many-body complexes. Here we find that the indirect Q valley in the first Brillouin zone of monolayer WS₂plays a critical role in the formation of a new excitonic state, which has not been well studied. By employing a high-quality h-BN encapsulated WS₂ field-effect transistor, we are able to switch the electron concentration within K-Q valleys at conduction band edges. Consequently, a distinct emission feature could be excited at the high electron doping region. Such feature has a competing population with the K valley trion, and experiences nonlinear power-law response and lifetime dynamics under doping. Our findings open up a new avenue for the study of valley many-body physics and quantum optics in semiconducting 2D materials, as well as provide a promising way of valley manipulation for next-generation entangled photonic devices.

Reveal flocking phase transition of self-propelled active particles by machine learning regression uncertainty

We develop the neural network based “learning from regression uncertainty” approach for the automatic detection of phases of matter in nonequilibrium active systems. Taking the flocking phase transition of self-propelled active particles described by the Vicsek model for example, we find that after training a neural network for solving the inverse statistical problem, i.e. for performing the regression task of reconstructing the noise level from given samples of such a nonequilibrium many-body complex system’s steady state configurations, the uncertainty of regression results obtained by the well-trained network can actually be utilized to reveal possible phase transitions in the system under study. The noise level dependence of regression uncertainty is assumed to be in a non-trivial M-shape, and its valley appears at the critical point of the flocking phase transition. By directly comparing this regression-based approach with the widely-used classification-based “learning by confusion” and “learning with blanking” approaches, we show that our approach has practical effectiveness, efficiency, good generality for various physical systems across interdisciplinary fields, and a greater possibility of being interpretable via conventional notions of physics. These approaches can complement each other to serve as a promising generic toolbox for investigating rich critical phenomena and providing data-driven evidence on the existence of various phase transitions, especially for those complex scenarios associated with first-order phase transitions or nonequilibrium active systems where traditional research methods in physics could face difficulties.

High-lying valley-polarized trions in 2D semiconductors

Optoelectronic functionalities of monolayer transition-metal dichalcogenide (TMDC) semiconductors are characterized by the emergence of externally tunable, correlated many-body complexes arising from strong Coulomb interactions. However, the vast majority of such states susceptible to manipulation has been limited to the region in energy around the fundamental bandgap. We report the observation of tightly bound, valley-polarized, UV-emissive trions in monolayer TMDC transistors: quasiparticles composed of an electron from a high-lying conduction band with negative effective mass, a hole from the first valence band, and an additional charge from a band-edge state. These high-lying trions have markedly different optical selection rules compared to band-edge trions and show helicity opposite to that of the excitation. An electrical gate controls both the oscillator strength and the detuning of the excitonic transitions, and therefore the Rabi frequency of the strongly driven three-level system, enabling excitonic quantum interference to be switched on and off in a deterministic fashion.

Tuning exciton complexes in twisted bilayer WSe2 at intermediate misorientation

Twist angle modifies the band alignment, screening, and interlayer (IL) coupling in twisted bilayers (tBLs) of transition metal dichalcogenides. Intermediate misorientation (twist angles > 15 degrees) bilayers (BLs) offer a unique opportunity to tune excitonic behavior within these concurrent physical mechanisms but are seldom studied. In this paper, we measure many-body excitonic complexes in monolayer (ML), natural BL, and tBL WSe2. Neutral biexciton (XX) is observed in tBL, while being undetected in nonencapsulated ML and BL, demonstrating unique effects of disorder screening in tBLs. The XX as well as charged biexciton are robust to thermal dissociation and are controllable by electrostatic doping. Vanishing of momentum-indirect IL excitons with increasing electron doping is demonstrated in tBL, resulting from the near alignment of Q-K and K-K valleys. Intermediate misorientation samples offer a high degree of control of excitonic complexes while offering possibilities for studying exciton-phonon coupling, band alignment, and screening.

Degenerated Liouvillians and steady-state reduced density matrices

Symmetries in an open quantum system lead to degenerated Liouvillians that physically imply the existence of multiple steady states. In such cases, obtaining the initial condition independent steady states is highly nontrivial since any linear combination of the true asymptotic states, which may not necessarily be a density matrix, is also a valid asymptote for the Liouvillian. Thus, in this work, we consider different approaches to obtain the true steady states of a degenerated Liouvillian. In the ideal scenario, when the open system symmetry operators are known, we show how these can be used to obtain the invariant subspaces of the Liouvillian and hence the steady states. We then discuss two other approaches that do not require any knowledge of the symmetry operators. These could be powerful numerical tools to deal with quantum many-body complex open systems. The first approach that is based on Gram–Schmidt orthonormalization of density matrices allows us to obtain all the steady states, whereas the second one based on large deviations allows us to obtain the non-degenerated maximum and minimum current carrying states. We discuss the symmetry-decomposition and the orthonormalization methods with the help of an open para-benzene ring and examine interesting scenarios such as the dynamical restoration of Hamiltonian symmetries in the long-time limit and apply the method to study the eigenspacing statistics of the nonequilibrium steady state.

Complex scaling in relativistic calculations of cross sections for electron-ion recombination processes

SynopsisWe present an approach to calculate cross sections for electron-ion recombination processes. It is based on the combination of relativistic many-body perturbation theory and complex scaling method. A preliminary comparison between measured and calculated spectra for the electron-ion recombination rate coefficients for Be-like Ca will be given.

Biexciton fine structure in monolayer transition metal dichalcogenides

The optical properties of atomically thin transition metal dichalcogenide (TMDC) semiconductors are shaped by the emergence of correlated many-body complexes due to strong Coulomb interaction. Exceptional electron-hole exchange predestines TMDCs to study fundamental and applied properties of Coulomb complexes such as valley depolarization of excitons and fine-structure splitting of trions. Biexcitons in these materials are less understood and it has been established only recently that they are spectrally located between exciton and trion. Here we show that biexcitons in monolayer TMDCs exhibit a distinct fine structure on the order of meV due to electron-hole exchange. Ultrafast pump-probe experiments on monolayer WSe$_2$ reveal decisive biexciton signatures and a fine structure in excellent agreement with a microscopic theory. We provide a pathway to access biexciton spectra with unprecedented accuracy, which is valuable beyond the class of TMDCs, and to understand even higher Coulomb complexes under the influence of electron-hole exchange.

Many-Body Complexes in 2D Semiconductors.

2D semiconductors such as transition metal dichalcogenides (TMDs) and black phosphorus (BP) are currently attracting great attention due to their intrinsic bandgaps and strong excitonic emissions, making them potential candidates for novel optoelectronic applications. Optoelectronic devices fabricated from 2D semiconductors exhibit many-body complexes (exciton, trion, biexciton, etc.) which determine the materials optical and electrical properties. Characterization and manipulation of these complexes have become a reality due to their enhanced binding energies as a direct result from reduced dielectric screening and enhanced Coulomb interactions in the 2D regime. Furthermore, the atomic thickness and extremely large surface-to-volume ratio of 2D semiconductors allow the possibility of modulating their inherent optical, electrical, and optoelectronic properties using a variety of different environmental stimuli. To fully realize the potential functionalities of these many-body complexes in optoelectronics, a comprehensive understanding of their formation mechanism is essential. A topical and concise summary of the recent frontier research progress related to many-body complexes in 2D semiconductors is provided here. Moreover, detailed discussions covering the aspects of fundamental theory, experimental investigations, modulation of properties, and optoelectronic applications are given. Lastly, personal insights into the current challenges and future outlook of many-body complexes in 2D semiconducting materials are presented.

Spontaneous synchronisation and nonequilibrium statistical mechanics of coupled phase oscillators

Spontaneous synchronisation is a remarkable collective effect observed in nature, whereby a population of oscillating units, which have diverse natural frequencies and are in weak interaction with one another, evolves to spontaneously exhibit collective oscillations at a common frequency. The Kuramoto model provides the basic analytical framework to study spontaneous synchronisation. The model comprises limit-cycle oscillators with distributed natural frequencies interacting through a mean-field coupling. Although more than forty years have passed since its introduction, the model continues to occupy the centre stage of research in the field of non-linear dynamics and is also widely applied to model diverse physical situations. In this brief review, starting with a derivation of the Kuramoto model and the synchronisation phenomenon it exhibits, we summarise recent results on the study of a generalised Kuramoto model that includes inertial effects and stochastic noise. We describe the dynamics of the generalised model from a different yet a rather useful perspective, namely, that of long-range interacting systems driven out of equilibrium by quenched disordered external torques. A system is said to be long-range interacting if the inter-particle potential decays slowly as a function of distance. Using tools of statistical physics, we highlight the equilibrium and nonequilibrium aspects of the dynamics of the generalised Kuramoto model, and uncover a rather rich and complex phase diagram that it exhibits, which underlines the basic theme of intriguing emergent phenomena that are exhibited by many-body complex systems.

Anomalous Above-Gap Photoexcitations and Optical Signatures of Localized Charge Puddles in Monolayer Molybdenum Disulfide.

Broadband optoelectronics such as artificial light harvesting technologies necessitate efficient and, ideally, tunable coupling of excited states over a wide range of energies. In monolayer MoS2, a prototypical two-dimensional layered semiconductor, the excited state manifold spans the visible electromagnetic spectrum and is comprised of an interconnected network of excitonic and free-carrier excitations. Here, photoluminescence excitation spectroscopy is used to reveal the energetic and spatial dependence of broadband excited state coupling to the ground-state luminescent excitons of monolayer MoS2. Photoexcitation of the direct band gap excitons is found to strengthen with increasing energy, demonstrating that interexcitonic coupling across the Brillouin zone is more efficient than previously reported, and thus bolstering the import and appeal of these materials for broadband optoelectronic applications. Narrow excitation resonances that are superimposed on the broadband photoexcitation spectrum are identified and coincide with the energetic positions of the higher-energy excitons and the electronic band gap as predicted by first-principles calculations. Identification of such features outlines a facile route to measure the optical and electronic band gaps and thus the exciton binding energy in the more sophisticated device architectures that are necessary for untangling the rich many-body phenomena and complex photophysics of these layered semiconductors. In as-grown materials, the excited states exhibit microscopic spatial variations that are characteristic of local carrier density fluctuations, similar to charge puddling phenomena in graphene. Such variations likely arise from substrate inhomogeneity and demonstrate the possibility to use substrate patterning to tune local carrier density and dynamically control excited states for designer optoelectronics.

Doorway Mechanism in Many-Body Systems and in Quantum Billiards

Among the numerous different ways to excite many-body and other complex quantum systems, mechanisms are often found which are clearly distinguished by a simple, typically semiclassical interpretation. In nuclei, these are the collective excitations in which all or large groups of particles move coherently. They often act as “doorways” to other excitations of single-particle character. Examples for and the limitations of the doorway mechanism are discussed. Recent results show that superscars in the barrier billiard serve as perfect object to shed light on aspects of the doorway mechanism which are not directly accessible in traditional quantum systems. To this end, two new statistical observables are employed. Some open questions are addressed.

Tunneling spectroscopy of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot molecule

We present a theory of tunneling spectroscopy of spin-selective Aharonov-Bohm oscillations in a lateral triple quantum dot molecule. The theory combines exact treatment of an isolated many-body system with the rate equation approach when the quantum dot molecule is weakly connected to the leads subject to arbitrary source-drain bias. The tunneling spectroscopy of the many-body complex is analyzed using the spectral functions of the system and applied to holes in a quantum dot molecule. Negative differential conductance is predicted and explained as a result of the redistribution of the spectral weight between transport channels. It is shown that different interference effects on singlet and triplet hole states in a magnetic field lead to spin-selective Aharonov-Bohm oscillations.

Semiclassical treatments for small-molecule dynamics in low-temperature crystals using fixed and adiabatic vibrational bases

Time-resolved coherent nonlinear optical experiments on small molecules in low-temperature host crystals are exposing valuable information on quantum mechanical dynamics in condensed media. We make use of generic features of these systems to frame two simple, comprehensive theories that will enable the efficient calculations of their ultrafast spectroscopic signals and support their interpretation in terms of the underlying chemical dynamics. Without resorting to a simple harmonic analysis, both treatments rely on the identification of normal coordinates to unambiguously partition the well-structured guest-host complex into a system and a bath. Both approaches expand the overall wave function as a sum of product states between fully anharmonic vibrational basis states for the system and approximate Gaussian wave packets for the bath degrees of freedom. The theories exploit the fact that ultrafast experiments typically drive large-amplitude motion in a few intermolecular degrees of freedom of higher frequency than the crystal phonons, while these intramolecular vibrations indirectly induce smaller-amplitude--but still perhaps coherent--motion among the lattice modes. The equations of motion for the time-dependent parameters of the bath wave packets are fairly compact in a fixed vibrational basis/Gaussian bath (FVB/GB) approach. An alternative adiabatic vibrational basis/Gaussian bath (AVB/GB) treatment leads to more complicated equations of motion involving adiabatic and nonadiabatic vector potentials. Computational demands for propagation of the parameter equations of motion appear quite manageable for tens or hundreds of atoms and scale similarly with system size in the two cases. Because of the time-scale separation between intermolecular and lattice vibrations, the AVB/GB theory may in some instances require fewer vibrational basis states than the FVB/GB approach. Either framework should enable practical first-principles calculations of nonlinear optical signals from molecules in cryogenic matrices and their semiclassical interpretation in terms of electronic and vibrational decoherence and vibrational population relaxation, all within a pure-state description of the macroscopic many-body complex.

Magnetoresistance anomaly at the transition to coherent state in SmB 6

To shed more light on the nature of puzzling “metallic” state observed in intermediate-valence compound SmB 6 below T * ∼ 5 K, precise measurements of transport and magnetic properties have been carried out on single crystals of SmB 6 in a wide range of temperatures (1.8–30 K) in magnetic fields up to 7 T. A strong enhancement of negative magnetoresistance to the maximum value of Δρ/ρ B 2∼−2.2×10 −3 T −2 at T∼T * has been detected when temperature decreases below 14 K. This effect is attributed to the features of anomalous scattering of short-range (<10 Å) exciton-polaronic complexes resulting from fast charge fluctuations on Sm-sites. The anomalies of transport and magnetic characteristics are discussed in terms of electron phase transition to the coherent state of interacting many-body complexes below T * ∼ 5 K.

Spin gap formation in SmB 6

The precise measurements of magnetic properties have been carried out on single crystals of SmB 6 for temperatures down to 4.2 K. It is shown that the non-monotonic temperature dependence of magnetic susceptibility χ( T) of SmB 6 at T>50 K can be well approximated by an activation type contribution χ a( T)∼1/ T exp(− E a / k B T) with E a / k B∼65 K. This behaviour can be attributed to a spin gap formation with thermally induced magnetic moments characterized by the value of μ eff∼1.6μ B. A correlation between the “intra-gap contribution” Δχ( T)= χ( T)− χ a( T) observed below 50 K and temperature-dependent excitations appearing in inelastic neutron and Raman scattering spectra of SmB 6 at T<70 K is discussed in terms of many-body complexes which are formed in the regime of fast charge fluctuations on Sm-sites.

An observation of electron phase transition in SmB 6 at low temperatures

In order to reveal the nature of the ground state of archetypal intermediate-valence compound SmB 6, a comprehensive study of its transport and magnetic properties was carried out on high-quality single crystals at temperatures of 1.8–300 K in magnetic fields up to 7 T. A drastic enhancement of negative magnetoresistance was observed below 14 K, with the maximum absolute value of Δ ρ/ ρB 2∼2.2×10 −3 T −2 at T≈5.2 K. This effect seems to be attributable to anomalous magnetic scattering of many-body (exciton–polaronic) complexes induced by fast valence fluctuations on Sm sites. The observed anomalies of magnetotransport, thermoelectric and magnetic characteristics are discussed in terms of electron phase transition to the coherent state of interacting many-body complexes occurring at T *∼5 K.