Electron-hole Quantum Research Articles

Design and Optimization of High-Performance Novel RbPbBr3-Based Solar Cells with Wide-Band-Gap S-Chalcogenide Electron Transport Layers (ETLs).

Inorganic cubic rubidium-lead-halide perovskites have attracted considerable attention owing to their structural, electronic, and unique optical properties. In this study, novel rubidium-lead-bromide (RbPbBr3)-based hybrid perovskite solar cells (HPSCs) with several high-band-gap chalcogenide electron transport layers (ETLs) of In2S3, WS2, and SnS2 were studied by density functional theory (DFT) and using the SCAPS-1D simulator. Initially, the band gap and optical performance were computed using DFT, and these results were utilized for the first time in the SCAPS-1D simulator. Furthermore, the impact of different major influencing parameters, that is, the thickness of the layer, bulk defect density, doping concentration, and defect density of interfaces, including the working temperature, were also investigated and unveiled. Further, a study on an optimized device with the most potential ETL (SnS2) layer was performed systematically. Finally, a comparative study of different reported heterostructures was performed to explore the benchmark of the most recent efficient RbPbBr3-based photovoltaics. The highest power conversion efficiency (PCE) was 29.75% for the SnS2 ETL with Voc of 0.9789 V, Jsc of 34.57863 mA cm-2, and fill factor (FF) of 87.91%, while the PCEs of 21.15 and 24.57% were obtained for In2S3 and WS2 ETLs, respectively. The electron-hole generation, recombination rates, and quantum efficiency (QE) characteristics were also investigated in detail. Thus, the SnS2 ETL shows strong potential for use in RbPbBr3-based hybrid perovskite high-performance photovoltaic devices.

Amplified Spontaneous Emission from Electron-Hole Quantum Droplets in Colloidal CdSe Nanoplatelets.

Two-dimensional cadmium selenide nanoplatelets (NPLs) exhibit large absorption cross sections and homogeneously broadened band-edge transitions that offer utility in wide-ranging optoelectronic applications. Here, we examine the temperature-dependence of amplified spontaneous emission (ASE) in 4- and 5-monolayer thick NPLs and show that the threshold for close-packed (neat) films decreases with decreasing temperature by a factor of 2-10 relative to ambient temperature owing to extrinsic (trapping) and intrinsic (phonon-derived line width) factors. Interestingly, for pump intensities that exceed the ASE threshold, we find development of intense emission to lower energy in particular provided that the film temperature is ≤200 K. For NPLs diluted in an inert polymer, both biexcitonic ASE and low-energy emission are suppressed, suggesting that described neat-film observables rely upon high chromophore density and rapid, collective processes. Transient emission spectra reveal ultrafast red-shifting with the time of the lower energy emission. Taken together, these findings indicate a previously unreported process of amplified stimulated emission from polyexciton states that is consistent with quantum droplets and constitutes a form of exciton condensate. For studied samples, quantum droplets form provided that roughly 17 meV or less of thermal energy is available, which we hypothesize relates to polyexciton binding energy. Polyexciton ASE can produce pump-fluence-tunable red-shifted ASE even 120 meV lower in energy than biexciton ASE. Our findings convey the importance of biexciton and polyexciton populations in nanoplatelets and show that quantum droplets can exhibit light amplification at significantly lower photon energies than biexcitonic ASE.

On some structural phase transitions in coupled quantum wires at finite temperature

In this paper, we explore some structural phase transitions in GaAs-based coupled electron-electron (e-e) and electron-hole (e-h) quantum wires at finite temperature. To this endeavour, the intra- and inter-wire static-structure factors, pair-correlation functions and static (charge) density susceptibilities are calculated over a wide range of temperature T, particle number density parameter r se and some selected values of inter-wire spacing d. The particle exchange-correlations (xc) are included using the dynamic version of self-consistent mean-field theory of Singwi et al (the qSTLS theory), and the results have been compared with the static STLS model. It has been found that in the e-h system, the inclusion of dynamic nature of xc leads to the formation of Wigner crystal (WC) state in the close proximity of two wires at wave-vector q ∼ 3.5k Fe . However, a charge-density-wave (CDW) instability is observed at q ∼ 2k Fe when the xc are treated statically (k Fe being the electron’s Fermi wave vector). On the other hand, the e-e system shows comparatively small signatures of the WC phase when wires are kept sufficiently far apart, but, a long-wavelength instability is encountered in close vicinity of the wires. Interestingly, the CDW phase is completely missing in the e-e system at the investigated parameters. Expectedly, the quantum phase transitions are predicted to occur in the strongly correlated regime i.e. at sufficiently small T and high r se .

Induced Wigner crystallization in a coupled electron-hole quantum wire system at finite-temperature

We explore the possibility of induced Wigner crystallization in a coupled electron-hole (e-h) quantum wire system at finite-temperature T by calculating the intra- and inter-wire static structure factors in the dynamic mean-field approximation of Singwi et al (the qSTLS theory). We find that owing to the dynamics of severely correlated particles, the e-h system may favour an induced phase transition from quantum liquid to the Wigner crystal state at a non-zero T and sufficiently low particle (electron/hole) number density in the close proximity of two wires. On the other hand, the static STLS approximation does not predict such phase transition due to complete neglect of the dynamics of particle correlations.

Phase Diagram of High-Temperature Electron-Hole Quantum Droplet in Two-Dimensional Semiconductors.

Quantum liquids, systems exhibiting effects of quantum mechanics and quantum statistics at macroscopic levels, represent one of the most exciting research frontiers of modern physical science and engineering. Notable examples include Bose-Einstein condensation (BEC), superconductivity, quantum entanglement, and a quantum liquid. However, quantum liquids are usually only stable at cryogenic temperatures, significantly limiting fundamental studies and device development. Here we demonstrate the formation of stable electron-hole liquid (EHL) with the quantum statistic nature at temperatures as high as 700 K in monolayer MoS2 and elucidate that the high-temperature EHL exists as droplets in sizes of around 100-160 nm. We also develop a thermodynamic model of high-temperature EHL and, based on the model, compile an exciton phase diagram, revealing that the ionized photocarrier drives the gas-liquid transition, which is subsequently validated with experimental results. The high-temperature EHL provides a model system to enable opportunities for studies in the pursuit of other high-temperature quantum liquids. The results can also allow for the development of quantum liquid devices with practical applications in quantum information processing, optoelectronics, and optical interconnections.

Particle–hole symmetry protects spin-valley blockade in graphene quantum dots

Particle-hole symmetry plays an important role in the characterization of topological phases in solid-state systems1. It is found, for example, in free-fermion systems at half filling and it is closely related to the notion of antiparticles in relativistic field theories2. In the low-energy limit, graphene is a prime example of a gapless particle-hole symmetric system described by an effective Dirac equation3,4 in which topological phases can be understood by studying ways to open a gap by preserving (or breaking) symmetries5,6. An important example is the intrinsic Kane-Mele spin-orbit gap of graphene, which leads to a lifting of thespin-valley degeneracy and renders graphene a topological insulator in a quantumspin Hall phase7 while preserving particle-hole symmetry. Here we show that bilayer graphene allows the realization of electron-hole double quantum dots that exhibit near-perfect particle-hole symmetry, in which transport occurs via the creation and annihilation of single electron-hole pairs with opposite quantum numbers. Moreover, we show that particle-hole symmetric spin and valley textures lead to a protected single-particle spin-valley blockade. The latter will allow robust spin-to-charge and valley-to-charge conversion, which are essential for the operation of spin and valley qubits.

Emergence and Relaxation of an e–h Quantum Liquid Phase in Photoexcited MoS2 Nanoparticles at Room Temperature

AbstractLow‐dimensional transition metal dichalcogenide (TMDC) materials are heralding a new era in optoelectronics and valleytronics owing to their unique properties. Photo‐induced dynamics in these systems is mostly studied from the perspective of individual quasi‐particles—excitons, bi‐excitons, or, even, trions—their formation, evolution, and decay. The role of multi‐body and exciton dynamics, the associated collective behavior, condensation, and inter‐excitonic interactions remain intriguing and seek attention, especially in room‐temperature scenarios that are relevant for device applications. In this work, the formation and decay of an unexpected electron–hole quantum liquid phase at room‐temperature on ultrafast timescales in multi‐layer MoS2 nanoparticles is evidenced through femtosecond broadband transient absorption spectroscopy. The studies presented here reveal the complete dynamical picture: the initial electron–hole plasma (EHP) condenses into a quantum electron–hole liquid (EHL) phase that typically lasts as long as 10 ps, revealing its robustness, whereafter the system decays through phonons. The authors employ a successful physical model using a set of coupled nonlinear rate equations governing the individual populations of these constituent phases to extract their contributions to bandgap renormalization (BGR). Beyond the observation of the electron–hole liquid‐like state at room temperature, this work reveals the ultrafast dynamics of photo‐excited low‐dimensional systems arising out of collective many‐particle behavior and correlations.

Interaction of laser pulse with a quantum electron-hole semiconductor plasma

Interaction of laser pulse with a quantum electron-hole semiconductor plasma

A Pair of 2D Quantum Liquids: Investigating the Phase Behavior of Indirect Excitons.

Long-lived indirect excitons (IXs) exhibit a rich phase diagram, including a Bose–Einstein condensate (BEC), a Wigner crystal, and other exotic phases. Recent experiments have hinted at a “classical” liquid of IXs above the BEC transition. To uncover the nature of this phase, we use a broad range of theoretical tools and find no evidence of a driving force toward classical condensation. Instead, we attribute the condensed phase to a quantum electron–hole liquid (EHL), first proposed by Keldysh for direct excitons. Taking into account the association of free carriers into bound excitons, we study the phase equilibrium between a gas of excitons, a gas of free carriers, and an EHL for a wide range of electron–hole separations, temperatures, densities, and mass ratios. Our results agree reasonably well with recent measurements of GaAs/AlGaAs coupled quantum wells.

Exchange-correlation effects in coupled quantum wire systems at finite temperature

We investigate the exchange-correlation effects in coupled quantum wire systems at finite-temperature within the self-consistent mean-field approximation of Singwi et al by assuming the charge carriers to be electrons in one wire and electrons or heavier holes in the other. Numerical results are presented for the intra- and inter-wire static structure factors, pair-correlation functions and the static charge density susceptibility over a wide range of system parameters (viz. temperature T, particle number density and inter-wire spacing) at equal and fixed transverse width of both the wires. We find for the first time that the coupled electron-hole (e-h) quantum wire system may favor a charge-density-wave (CDW) instability at sufficiently low T and carrier density in the close proximity of the wires, where as no such phase transition is observed in the electron-electron (e-e) quantum wire system at any non-zero T. The intra-wire contact pair-correlation functions of both the systems show a non-monotonous behavior with increasing (decreasing) T (carrier number density), and increase consistently with decrease in inter-wire spacing. On the other hand, the corresponding inter-wire contact pair-correlation functions show a non-monotonous T-dependence and consistent increase with decrease in carrier number density and/or inter-wire separation. Results of free exchange-correlation energy for both the e-h and e-e coupled systems are also reported which are found to have a noticeable dependence upon T. To highlight the effect of exchange-correlations, our results have been compared with the predictions of the random-phase approximation (RPA).

Analytical WKB theory for high-harmonic generation and its application to massive Dirac electrons

We propose an analytical approach to high-harmonic generation (HHG) for nonperturbative low-frequency and high-intensity fields based on the (Jeffreys-)Wentzel-Kramers-Brillouin (WKB) approximation. By properly taking into account Stokes phenomena of WKB solutions, we obtain wave functions that systematically include the repetitive dynamics of production and acceleration of electron-hole pairs and quantum interference due to phase accumulation between different pair production times (St\"uckelberg phase). Using the obtained wave functions without relying on any phenomenological assumptions, we explicitly compute electric current (including intra- and interband contributions) as the source of HHG for a massive Dirac system in $(1+1)$ dimensions under an ac electric field. We demonstrate that the WKB approximation agrees well with numerical results obtained by solving the time-dependent Schr\"odinger equation and point out that the quantum interference is important in HHG. We also predict in the deep nonperturbative regime that (1) harmonic intensities oscillate with respect to electric-field amplitude ${E}_{0}$ and frequency $\mathrm{\ensuremath{\Omega}}$, with a period determined by the St\"uckelberg phase, (2) the cutoff order of HHG is determined by $2e{E}_{0}/\ensuremath{\hbar}{\mathrm{\ensuremath{\Omega}}}^{2}$, with $e$ being the electron charge, and that (3) noninteger harmonics, controlled by the St\"uckelberg phase, appear as a transient effect. Our WKB theory is particularly suited for a parameter regime, where the Keldysh parameter $\ensuremath{\gamma}=(\mathrm{\ensuremath{\Delta}}/2)\mathrm{\ensuremath{\Omega}}/e{E}_{0}$, with $\mathrm{\ensuremath{\Delta}}$ being the gap size, is small. This parameter regime corresponds to intense lasers in the terahertz regime for realistic massive Dirac materials. Our analysis implies that the so-called HHG plateau can be observed at the terahertz frequency within the current technology.

(Invited) Experimental Quantum Transport in Strained Graphene and SWCNTs - Towards Noems

We measure ballistic charge conductivity in strained suspended graphene and observe the theoretically predicted [1] strain-induced scalar and vector potentials. To do so, we built an experimental platform for in-situ quantum transport strain engineering in 2D materials. This instrumentation also permits low temperature (0.3 K) transport in 0 to 9 Tesla magnetic fields. The tunable uniaxial strain (up to 3%) is completely decoupled from the gate-tunable charge density, permitting quantitative understanding of strain effects. We show slippage-free mechanical clamping of high aspect-ratio graphene crystals, where atomically ordered edges are unnecessary for quantitative straintronics. We study in detail transport in a ballistic graphene channel whose length is 90 nm and width is 600 nm. By applying strain up to 0.6 % in this device, we observe that the strain-induced scalar potential shifts its low energy band structure downward by up to 30 meV. We show precise control of the gauge vector potential which reversibly suppresses the conductance. We discuss our progress towards strain engineering in SWCNTs, and calculations showing its potential in valley filtering and tuning electron-hole asymmetric quantum transistors.We conclude with an overview of another project to integrate suspended bilayer graphene devices in planar optical cavities towards achieving fully tunable nano-opto-electro-mechanical (NOEMS) carbon systems. To do so, we discuss a nitrocellulose-based dry-stamping method to manipulate and suspend individual 2D materials [2]. Using this method we assembled optical cavities able to a widely tune Raman scattering and absorption in graphene. [1] A. C. McRae, G. Wei, and A. R. Champagne, Phys. Rev. Applied 11, 054019 (2019). [2] I. G. Rebollo, F. C. Rodrigues-Machado, W. Wright, G. J. Melin, A. R. Champagne, ArXiv:2011.14166 Figure 1

Effect of Mismatched Electron-Hole Effective Masses on Superfluidity in Double Layer Solid-State Systems

Superfluidity has been predicted and now observed in a number of different electron-hole double-layer semiconductor heterostructures. In some of the heterostructures, such as GaAs and Ge-Si electron-hole double quantum wells, there is a strong mismatch between the electron and hole effective masses. We systematically investigate the sensitivity to unequal masses of the superfluid properties and the self-consistent screening of the electron-hole pairing interaction. We find that the superfluid properties are insensitive to mass imbalance in the low density BEC regime of strongly-coupled boson-like electron-hole pairs. At higher densities, in the BEC-BCS crossover regime of fermionic pairs, we find that mass imbalance between electrons and holes weakens the superfluidity and expands the density range for the BEC-BCS crossover regime. This permits screening to kill the superfluid at a lower density than for equal masses.

Impact of an External Magnetic Field on Solitary Waves in Quantum Electron–Hole Plasmas of Semiconductors

The propagation of solitary acoustic pulses in magnetized quantum electron–hole plasmas of semiconductors has been studied. The effects of an external magnetic field and quantum terms, including the electrons and holes quantum recoil effects, exchange-correlation force, and the degenerate pressure of charge carriers are considered. Starting from quantum hydrodynamic equations, by using the reductive perturbation method, the Zakharov–Kuznetsov equation is derived. The theoretical model is applied to three types of semiconductors, namely GaAs, GaSb, and InP. It is clear that basic features of solitary acoustic waves are modified significantly by plasma parameters, especially number densities of semiconductors and magnetic field strength. The value of the amplitude and width of solitary waves strongly depends on the type of semiconductors and the propagation angle in a magnetized quantum electron-hole semiconductor plasma. Numerical results reveal that the width of the solitons gets reduced significantly with the increase of the magnetic field intensity while there is no effect on the amplitude of solitary waves. The weakest magnetic field that sets the pulse width to zero belongs to GaAs. Finally, the stability of the pulse soliton solution of the Zakharov–Kuznetsov equation has been discussed, analytically and numerically. The instability growth rate depends on plasma parameters and magnetic field intensity. The present paper can be meaningful in studying non-linear phenomena and instability behaviors in magnetized quantum electron–hole plasmas of semiconductors and the particle and energy transport mechanism in plasma-assisted quantum semiconductor nanodevices.

Nature of the Pseudogap Phase of HTSC Cuprates

The pseudogap phase of HTSC cuprates is associated with the formation of a system of quantum electron–hole (EH) dimers similar to the Anderson RVB phase. A specific role of the electron–lattice relaxation in the formation of metastable EH dimers in cuprates with the T and T ' structures is considered. An effective spin–pseudospin Hamiltonian of the CuO2 plane of cuprate is introduced in the model of charge triplets and S = 1 the pseudospin formalism. In the molecular-field approximation (MFA), for the coordinate representation, the main MFA phases have been found: antiferromagnetic insulator, charge density wave, boson superconductor with the d-symmetry of the order parameter, and two metallic Fermi-phases that form the “strange”-metal phase. MFA is shown to enable, as a whole, a proper description of the features of the phase diagrams typical of cuprates. As in the case of typical s = 1/2 quantum antiferromagnet, the actually observed cuprate phases such as the charge ordering and the superconductivity reflect a “physical” ground state close to MFA phases but with strongly reduced values of local order parameters.

Criteria of the electron pumping in electron-hole quantum plasma

The degradation of quantum semiconductor plasma during electron pumping is an important problem since it has several applications in the technological industry. Moreover, the identification of such a problem cannot be accomplished by experiment alone and a further theoretical investigation is required. Owing to the energy gap of semiconductors, electrons should acquire energy in order to move from the valence band to the conduction band. Therefore, to employ the quantum fluid model that includes the exchange-correlation potentials, Bohm potential, and the degenerate pressure, the electron-hole lifetime should be greater than the plasma oscillation time. If the latter condition is not satisfied, the quantum fluid model failed to describe the system and it is convenient to use the density functional theory. As a result of that, we apply the quantum fluid model to GaN where the plasma oscillation has a time of 0.002 ps while the electron-hole lifetime is 1 ps. The modified Kadomtsev-Petviashvili (mKP) equation had been obtained using a new set of stretched coordinates as the Kadomtsev-Petviashvili (KP) equation had been derived in (Afify et al 2019 Chaos Soliton Fract 124, 18–25). Our findings determine whether the quantum fluid model is valid or not in the case of semiconductor plasma. Studying exchange effects in a dynamic context has certain limitations. Thus, we stressed that the more accurate approaches introduced in the literature are hard to apply for nonlinear issues. Further, the applicability of our results is discussed, but not this much. Moreover, the numerical results showed that the electron beam parameters and quantum effects have significant effects on the growth of the solitary waves.

Природа псевдощелевой фазы ВТСП купратов

The pseudogap phase of HTSC cuprates is associated with the formation of a system of quantum electron-hole (EH) dimers similar to the Anderson RVB-phase. We considered the specific role of electron-lattice relaxation in the formation of metastable EH dimers in cuprates with T- and T′-structures. In the model of charge triplets and S = 1 pseudospin formalism, the effective spin-pseudospin Hamiltonian of the cuprate CuO2 plane is introduced. In the framework of the molecular field approximation (MFA) for the coordinate representation, the main MFA phases were found: an antiferromagnetic insulator, a charge density wave, a bosonic superconductor with d-symmetry of the order parameter, and two metal Fermi-phases forming the phase of the "strange" metal. We argue that the MFA can correctly reproduce all the features of the typical cuprate phase diagrams. As for typical s = 1/2 quantum antiferromagnet the actually observed cuprate phases such as charge order and superconductivity reflect "physical" ground state, which is close to MFA-phases but with strongly reduced magnitudes of the local order parameters.

Dispersion relation properties of quantum electron–hole semiconductor plasmas in the presence of an external magnetic field

In this paper, a derivation of the general dielectric tensor and the dispersion equation in a magnetized semiconductor plasma containing electrons and holes is carried out by employing the quantum hydrodynamic model, under non-relativistic and degenerate charged species assumptions. Dispersion curves are shown for different types of semiconductors. The effects of an external magnetic field and quantum terms, including electrons’ and holes’ quantum recoil effects, exchange-correlation potentials, and the degenerate pressures of species on dispersion relations are considered. When the angle of the wave vector with respect to the z axis is zero (), two well-separated Langmuir and acoustic modes are depicted and when , three branches are revealed. The growth rate of streaming instability is numerically presented for GaN. This study would be beneficial to understand the linear phenomena and instability behaviors of a magnetized quantum electron–hole semiconductor plasma.

Correlation functions in electron-electron and electron-hole double quantum wells: Temperature, density, and barrier-width dependence

The classical-map hyper-netted-chain (CHNC) scheme, developed for treating fermion fluids at strong coupling and at finite temperatures, is applied to electron-electron and electron-hole double quantum wells. The pair distribution functions and the local field factors needed in linear response theory are determined for a range of temperatures, carrier densities, and barrier widths typical for experimental double quantum well systems in GaAs-GaAlAs. For electron-hole double quantum wells, a large enhancement in the pair distribution functions is found for small carrier separations. The CHNC equations for electron-hole systems no longer hold at low densities where bound-state formation occurs.

Electrostatic Surface Waves on Semi-Bounded Quantum Electron-Hole Semiconductor Plasmas

The electrostatic surface waves on semi-bounded quantum electron-hole semiconductor plasmas are studied within the framework of the quantum hydrodynamic model, including the electrons and holes quantum recoil effects, quantum statistical pressures of the plasma species, as well as exchange and correlation effects. The dispersion characteristics of surface electrostatic oscillations are investigated by using the typical values of GaAs, GaSb and GaN semiconductors. Numerical results show the existence of one low-frequency branch due to the mass difference between the electrons and holes in addition to one high-frequency branch due to charge-separation effects.