Quasiparticle Effects Research Articles

Skyrmion Hall Effect in Altermagnets.

It is widely believed that the skyrmion Hall effect is absent in antiferromagnets because of the vanishing topological charge. However, the Aharonov-Casher theory indicates the possibility of topological effects for neutral particles. In this Letter, we predict the skyrmion Hall effect in emerging altermagnets with zero net magnetization and zero skyrmion charge. We first show that the neutral skyrmion manifests as a magnetic quadrupole in altermagnets. We reveal a hidden gauge field from the magnetic quadrupole, which induces the skyrmion Hall effect when driven by spin transfer torque. Interestingly, we identify a sign change of the Hall angle when one swaps the anisotropic exchange couplings in altermagnets. Furthermore, we demonstrate that both the velocity and Hall angle of altermagnetic skyrmions sensitively depend on the current direction. Our findings real the critical role of magnetic quadrupole in driving the skyrmion Hall effect with vanishing charge, and pave the way to discovering new Hall effect of neutral quasiparticles beyond magnetic skyrmions.

Beyond the random phase approximation (RPA): First principles calculation of the valence EELS spectrum for KBr including local field, quasiparticle, excitonic and spin orbit coupling effects

The low energy region (< 50 eV) of the electron energy loss spectrum (EELS) can contain a great deal of spectral detail associated with excitations of the valence electrons. Calculation of the spectra from first principles can assist with interpretation and the most widely used method is the random phase approximation (RPA), usually neglecting local field effects (LFE). For KBr this approach is insufficient due to the importance of quasiparticle and excitonic effects. Calculations including these multi-electron effects are shown to give much improved agreement with the experimental spectra, and the inclusion of spin-orbit coupling (SOC) reproduces the excitonic doublet just above band-edge onset. A review of the complex theory behind these methods is given along with practical guidance on performing these calculations.

Quasiparticle effects in magnetic-field-resilient three-dimensional transmons

Quasiparticle effects in magnetic-field-resilient three-dimensional transmons

Tuning the optical absorption and exciton bound states of germanene by chemical functionalization

We present a comprehensive study of buckled honeycomb germanene functionalized with alternately bonded side groups hydroxyl (–H), methyl (–CH3) and trifluoro methyl (–CF3). By means of most modern theoretical and computational methods we determine the atomic geometries versus the functionalizing groups. The quasiparticle excitation effects on the electronic structure are taken into account by means of exchange-correlation treatment within the GW framework. The Bethe–Salpeter equation is solved ab initio to derive optical spectra including excitonic and quasiparticle effects. Band edge excitons are investigated in detail. The binding properties are compared with those resulting from model studies. The functionalization leads to significantly modified band structures compared with pristine germanene. The Dirac bands near the K point are destroyed and direct gaps appear at the \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Gamma$$\\end{document} point. Together with the many-body effects, quasiparticle gaps of 2.3, 1.8 or 1.0 eV result for –H, –CH3 and –CF3 functionalization. Totally different absorption spectra are found for in-plane and out-of-plane light polarization. Strongly bound excitons are visible below the quasiparticle band edge with binding energies of about 0.5, 0.4 or 0.3 eV. The nature of these band-gap excitons is investigated via their wave function, the contribution of various interband combinations and the dipole selection rules.

Excitonic properties and solar harvesting performance of Cs2ZnY2X2 as quasi-2D mixed-halide perovskites

Due to their low toxicity and high-temperature stability, lead-free quasi-2D metal halide perovskites (MHPs) are promising materials for optoelectronic applications. However, understanding their structural and optoelectronic properties, especially excitonic effects (electron-hole pair, e-h), is challenging due to their quantum well topology. We propose an efficient protocol for band gap correction of Cs2ZnX (X = Cl4, Br2Cl2, and I2Cl2) and Cs2PbI2Cl2 (used as a benchmark) quasi-2D MHPs. This protocol is based on a relativistic quasiparticle approach (DFT-1/2) using density functional theory (DFT). We first examined the hybrid contributions of the halide alloys, leading to excitonic characterization using the Bethe–Salpeter equation within the maximally localized Wannier functions tight-binding method. This method provides a cost-effective approach for describing e-h quasiparticle effects. Additionally, we analyzed the crystal structure, thermodynamic stability, and optoelectronic properties of the newly synthesized Zn-based systems, focusing on their structural stability mechanisms. We found a correlation between the allocation of Cs+ cations in the crystal structure and the formation of two distinct stabilization patterns, influenced by the radii of the metal (Zn or Pb) and halide components. The relativistic correction of the band gap energies yielded results within 10 % of the experimental values. Furthermore, incorporating e-h quasiparticle effects for Cs2PbI2Cl2 resulted in an exciton binding energy of 0.160 eV, in excellent agreement with experimental data. This work represents a significant advance in the optoelectronic characterization of Cs2ZnX systems, previously unexplored for their excitonic properties.

Signature of excitonic insulators in phosphorene nanoribbons

Phosphorene is a recently developed two-dimensional (2D) material that has attracted tremendous attention because of its unique anisotropic optical properties and quasi-one-dimensional (1D) excitons. We use first-principles calculations combined with the maximally localized Wannier function tight binding Hamiltonian and Bethe–Salpeter equation (BSE) formalism to investigate quasiparticle effects of 2D and quasi-1D blue and black phosphorene nanoribbons. Our electronic structure calculations shows that both blue and black monolayered phases are semiconductors. On the other hand black phosphorene zigzag nanoribbons are metallic. Similar behavior is found for very thin blue phosphorene zig-zag and armchair nanoribbon. As a general behavior, the exciton binding energy decreases as the ribbon width increases, which highlights the importance of quantum confinement effects. The solution of the BSE shows that the blue phosphorene monolayer has an exciton binding energy four times higher than that of the black phosphorene counterpart. Furthermore, both monolayers show a different linear optical response with respect to light polarization, as black phosphorene is highly anisotropic. We find a similar, but less pronounced, optical anisotropy for blue phosphorene monolayer, caused exclusively by the quasi-particle effects. Finally, we show that some of the investigated nanoribbons show a spin-triplet excitonic insulator behavior, thus revealing exciting features of these nanoribbons and therefore provides important advances in the understanding of quasi-one dimensional phosphorus-based materials.

Unveiling excitons in two-dimensional -pnictogens.

In this paper, we investigate the optical, electronic, vibrational, and excitonic properties of four two-dimensional -pnictogen materials-nitrogenene, phosphorene, arsenene, and antimonene-via density functional theory calculations and the Bethe-Salpeter equation. These materials possess indirect gaps with significant exciton binding energies, demonstrating isotropic behavior under circular light polarization and anisotropic behavior under linear polarization by absorbing light within the visible solar spectrum (except for nitrogenene). Furthermore, we observed that Raman frequencies red-shift in heavier pnictogen atoms aligning with experimental observations; simultaneously, quasi-particle effects notably influence the linear optical response intensively. These monolayers' excitonic effects lead to optical band gaps optimized for solar energy harvesting, positioning them as promising candidates for advanced optoelectronic device applications.

Correlation-induced sensitivity and non-Hermitian skin effect of quasiparticles

Correlation-induced sensitivity and non-Hermitian skin effect of quasiparticles

Detection of two-dimensional small polarons at oxide interfaces by optical spectroscopy

Two-dimensional (2D) perovskite oxide interfaces are ideal systems to uncover diverse emergent properties, such as the arising polaronic properties from short-range charge–lattice interactions. Thus, a technique to detect this quasiparticle phenomenon at the buried interface is highly coveted. Here, we report the observation of 2D small-polarons at the LaAlO3/SrTiO3 conducting interface using high-resolution spectroscopic ellipsometry. First-principles investigations show that interfacial electron–lattice coupling mediated by the longitudinal phonon mode facilitates the formation of these polarons. This study resolves the long-standing question by attributing the formation of interfacial 2D small polarons to the significant mismatch between experimentally measured interfacial carrier density and theoretical values. Our study sheds light on the complexity of broken periodic lattice-induced quasi-particle effects and its relationship with exotic phenomena at complex oxide interfaces. Meanwhile, this work establishes spectroscopic ellipsometry as a useful technique to detect and locate optical evidence of polaronic states and other emerging quantum properties at the buried interface.

Z2 non-Hermitian skin effect in equilibrium heavy-fermion systems

We demonstrate that a correlated equilibrium $f$-electron system with time-reversal symmetry can exhibit a ${\mathbb{Z}}_{2}$ non-Hermitian skin effect of quasiparticles. In particular, we analyze a two-dimensional periodic Anderson model with spin-orbit coupling by combining the dynamical mean-field theory (DMFT) and the numerical renormalization group. We prove the existence of the ${\mathbb{Z}}_{2}$ skin effect by explicitly calculating the topological invariant and show that spin-orbit interaction is essential to this effect. Our DMFT analysis demonstrates that the ${\mathbb{Z}}_{2}$ skin effect of quasiparticles is reflected on the pseudospectrum. Furthermore, we analyze temperature effects on this skin effect using the generalized Brillouin zone technique, which clarifies that the skin modes are strongly localized above the Kondo temperature.

Entanglement of stationary states in the presence of unstable quasiparticles

The effect of unstable quasiparticles in the out-of-equilibrium dynamics of certain integrable systems has been the subject of several recent studies. In this paper we focus on the stationary value of the entanglement entropy density, its growth rate, and related functions, after a quantum quench. We consider several quenches, each of which is characterised by a corresponding squeezed coherent state. In the quench action approach, the coherent state amplitudes K(θ) become input data that fully characterise the large-time stationary state, thus also the corresponding Yang-Yang entropy. We find that, as function of the mass of the unstable particle, the entropy growth rate has a global minimum signalling the depletion of entropy that accompanies a slowdown of stable quasiparticles at the threshold for the formation of an unstable excitation. We also observe a separation of scales governed by the interplay between the mass of the unstable particle and the quench parameter, separating a non-interacting regime described by free fermions from an interacting regime where the unstable particle is present. This separation of scales leads to a double-plateau structure of many functions, where the relative height of the plateaux is related to the ratio of central charges of the UV fixed points associated with the two regimes, in full agreement with conformal field theory predictions. The properties of several other functions of the entropy and its growth rate are also studied in detail, both for fixed quench parameter and varying unstable particle mass and viceversa.

First-Principles Study of the Quasi-Particle and Excitonic Effect in o-BC2N: The GW + BSE Study

Ternary boron-carbon-nitride compounds are the hardest, chemically stable, and most applicable semiconductors in optoelectronic devices. We investigate the quasi-particle and excitonic properties of type II o-BC2N using many-body perturbation theory (MBPT). The state-of-the-art GW and BSE methods were used to determine the accurate band gap and excited-state characteristics of this material. We simulate the convergence test and structural optimization in DFT, which is the starting point for the GW calculation. We also compute the convergence test of the parameters in GW and BSE. As a result, the bandgap of our system is found to be 2.31 eV and 1.95 eV using the GW approximation and DFT-PBE, respectively. Since the valence and conduction band edges are located at different Brillouin zones, we decide that o-BC2N is an indirect bandgap semiconductor. In addition, by applying the scissor operator, we corrected the quasi-particle bandgap, which shows almost the same result as the GW approximation. Furthermore, using the BSE algorithm, we calculate the optical bandgap of type II o-BC2N to be 4.0 eV with the excitonic effect and 4.4 eV without the excitonic effect. The highest peaks of the imaginary dielectric function with the excitonic effect shift to a lower energy level at 11 eV than without the excitonic effect at 13.5 eV. The electron charge distribution is computed by fixing the hole position. Finally, we suggest that type II o-BC2N is promising for the application of optoelectronic semiconductors.

Ab-initio investigation of the structural stability, electronic and optical properties of the LiBO2 compound by using the G0W0+BSE approach

This article investigates the structural stability, and electronic and optical properties of the LiBO2 compound using density functional theory (DFT) and solving the many-body Bethe-Salpeter equation within the G0W0+BSE approach. The structural properties showed that the monoclinic structure of LiBO2 is thermodynamically and dynamically stable within both LDA and GGA approximations. The band structure results indicated that the direct band gap of LiBO2 at the Γ point is 6.05 eV, which increased to 8.95 eV by considering the quasi-particle effects in G0W0@LDA calculations. The maximum reflectance in the x, y, and z directions is 0.48, 0.86, and 0.54, respectively, with the corresponding static refractive index values of 1.52, 1.61, and 1.54. The direct optical band gap obtained from the imaginary part of the dielectric function is 5.95 eV, 5.95 eV, and 6.10 eV for the various directions x, y, and z respectively. Considering the electron-hole effects in the G0W0+BSE approach, the optical gap showed more accurate results, and the value of 7.54 eV was obtained in the x and z-directions and 8.10 eV in the y-direction. Therefore, excitonic effects within the G0W0+BSE approximation, in comparison to G0W0+RPA, leads to the redshift of the optical absorption spectrum. The calculated exciton binding energy is 1.41 eV for the x and z-directions and 0.85 eV for the y-direction. Finally, it can be stated that LiBO2 is a wide-gap insulator with a gap greater than 7 eV in the monoclinic structure. This indicates the high transparency of the LiBO2 crystal, with the prospect of application in optical instruments.

Quasiparticles in Superconducting Qubits with Asymmetric Junctions

Designing the spatial profile of the superconducting gap—gap engineering—has long been recognized as an effective way of controlling quasiparticles in superconducting devices. In aluminum films, their thickness modulates the gap; therefore, standard fabrication of Al/AlOx/Al Josephson junctions, which relies on overlapping a thicker film on top of a thinner one, always results in gap-engineered devices. Here, we reconsider quasiparticle effects in superconducting qubits to explicitly account for the unavoidable asymmetry in the gap on the two sides of a Josephson junction. We find that different regimes can be encountered in which the quasiparticles have either similar densities in the two junction leads or are largely confined to the lower-gap lead. Qualitatively, for similar densities the qubit excited-state population is lower but its relaxation rate is higher than when the quasiparticles are confined; therefore, there is a potential trade-off between two desirable properties in a qubit.3 MoreReceived 7 June 2022Revised 1 September 2022Accepted 23 November 2022DOI:https://doi.org/10.1103/PRXQuantum.3.040338Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasQuantum information with solid state qubitsPhysical SystemsJosephson junctionsSuperconducting qubitsThin filmsTechniquesBCS theoryCondensed Matter, Materials & Applied Physics

An investigation of dynamical stability, electronic, and optical properties of sodium metaborate (NaBO2): A many-body G0W0+BSE approach

This study investigated the electronic, optical, and phonon properties of bulk sodium metaborate (NaBO2) using density functional theory (DFT), and the many-body G0W0 plus Bethe-Salpeter equation (BSE) approach. The investigation of the phonon dispersion curve indicates the dynamic stability of the NaBO2 compound in the rhombohedral phase, which is in good agreement with experimental results. Also, cohesive energy calculation shows the thermodynamic stability of this material. The band structure results indicated that the direct band gap of NaBO2 at the Γ point is 3.81 eV/4.16 eV within LDA/PBE approximation, which increased to 6.78 eV/6.57 eV by considering the quasi-particle effects in G0W0@LDA/PBE calculations. Considering the electron-hole effects in the G0W0+BSE approach, the optical gap showed more accurate results, and the value of 5.83 eV/5.71 eV was obtained for the LDA/PBE starting point. Therefore, excitonic effects within the G0W0+BSE approximation, in comparison to G0W0+RPA, leads to the redshift of the optical absorption spectrum. The optical results estimate the exciton binding energy of 0.95 eV/0.86 eV within LDA/PBE approximation. The investigation of the optical absorption spectrum introduces sodium metaborate as a transparent material with the prospect of use in optical devices.

Superconductivity and Weak Anti-localization at KTaO3 (111) Interfaces

The intersection of two-dimensional superconductivity and topologically nontrivial states hosts a wide range of quantum phenomena, including Majorana fermions. We report on the observation of two-dimensional superconductivity and weak anti-localization at the TiOx/KTaO3 (111) interfaces. A remnant, saturating resistance persists below the transition temperature as superconducting puddles fail to reach phase coherence. Signatures of weak anti-localization are observed near the superconducting transition, suggesting the coexistence of superconducting fluctuations and quantum coherent quasiparticle effects. The superconducting interfaces show roughly one order of magnitude larger weak anti-localization correction, compared to non-superconducting interfaces, alluding to a relatively large coherence length in these interfaces.

One-dimensional excitons in long phosphorene atomic chains

The electronic structure of phosphorene atomic chains (PACs) embedded in various dielectric environments is studied theoretically by using a configuration interaction approach beyond the conventional double-excitation scheme. While the nominal single-particle gap of the PACs is shown to roughly obey an expected scaling law of ${L}^{\ensuremath{-}2}$, where $L$ measures the length of PACs, the quasiparticle shift is found to gradually converge to a value that is insensitive to the dielectric environment as $L$ increases. In the meantime, exciton binding energies are revealed to obey a simple scaling law of $0.96/(\ensuremath{\kappa}L)\ensuremath{-}0.02$, with $\ensuremath{\kappa}$ being the effective dielectric constant. As $L$ goes to infinity, the quasiparticle and excitonic effect are both found to be greatly suppressed as if the long-range electron-electron interactions are quenched in long PACs.

Nonlinear optical response of ferroelectric oxides: First-principles calculations within the time and frequency domains

The second- and third-order nonlinear susceptibilities of the ferroelectric oxides ${\mathrm{LiNbO}}_{3}$, ${\mathrm{LiTaO}}_{3}$, and ${\mathrm{KNbO}}_{3}$ are calculated from first principles. Two distinct methodologies are compared: one approach is based on a perturbative approach within the frequency domain, another on the time evolution of the electric polarization. The frequency dependence of the second harmonic coefficients of the ferroelectric phase of ${\mathrm{LiNbO}}_{3}$ calculated within the two approaches is in excellent agreement. This is further validated by experimental data for ${\mathrm{LiNbO}}_{3}$ and ${\mathrm{LiTaO}}_{3}$, measured for an incident range of photon energies between 0.78 and 1.6 eV. The real-time-based approach is furthermore employed to estimate the third-order nonlinear susceptibilities of all investigated ferroelectric oxides. We show that the quasiparticle effects, considered by means of a scissors shift in combination with the computationally efficient independent particle approximation, result in a shift of all spectral features towards higher energies, and decrease the magnitude of the optical nonlinearities. The energy of the main resonances in the hyperpolarizabilities suggests that the spectra can be understood by multiphoton absorption within the fundamental band gap for all investigated materials.

Bismuth Vacancy-Mediated Quantum Dot Precipitation to Trigger Efficient Piezocatalytic Activity of Bi2WO6 Nanosheets.

Point defects in piezoelectric semiconductors play a significant role in regulating the piezocatalytic performance. However, the role of metal vacancies in piezocatalysis has been less explored than that of oxygen vacancies. Herein, Bi2WO6 (BWO) nanosheets with tunable Bi defects were synthesized using an ion exchange method. High-resolution transmission electron microscopy directly revealed the existence of Bi vacancies in the lattice of BWO nanosheets and the precipitation of Bi quasiparticles. The BWO nanosheets with the highest concentration of Bi vacancies exhibited an excellent decomposition efficiency (7.83 × 10-2 min-1) over rhodamine B under ultrasound. The phenomenon is mainly attributed to the increased charge carrier concentration as a consequence of defect energy levels. In addition, the significant enhancement of light absorption capacity caused by the surface plasmon resonance effect of quasiparticles indicates that Bi ions escape from the lattice and combine with free electrons around BWO to form Bi quantum dots, which function as electron traps to facilitate the separation of charge carriers during the piezocatalytic process. This work systematically reveals the essential affiliation of metal vacancies and surface metal clusters in piezocatalysts and verifies the significance of vacancy engineering in piezocatalytic application.

Operating a passive on-chip superconducting circulator: Device control and quasiparticle effects

Microwave circulators play an important role in quantum technology based on superconducting circuits. The conventional circulator design, which employs ferrite materials, is bulky and involves strong magnetic fields, rendering it unsuitable for integration on superconducting chips. One promising design for an on-chip superconducting circulator is based on a passive Josephson-junction ring. In this paper, we consider two operational issues for such a device: circuit tuning and the effects of quasiparticle tunneling. We compute the scattering matrix using adiabatic elimination and derive the parameter constraints to achieve optimal circulation. We then numerically optimize the circulator performance over the full set of external control parameters, including gate voltages and flux bias, to demonstrate that this multi-dimensional optimization converges quickly to find optimal working points. We also consider the possibility of quasiparticle tunneling in the circulator ring and how it affects signal circulation. Our results form the basis for practical operation of a passive on-chip superconducting circulator made from a ring of Josephson junctions.