Pauli Blocking Effects Research Articles

Nonlinear optical research on 2D NbSe2 nanosheets and their ultrafast photonics applications

Two-dimensional (2D) NbSe2 is a new material with a variety of excellent properties. In this article, 2D NbSe2 nanosheets are prepared using liquid phase exfoliation (LPE) and spin coating methods. At the same time, the properties of NbSe2 were calculated by using density functional theory (DFT), exploring the changes in the electronic band structure of NbSe2 with the number of layers, and studying the optical properties of NbSe2. The nonlinear optical properties caused by the Pauli blocking effect and the absorption spectra are studied through typical nonlinear testing techniques and an ultraviolet–visible-near-infrared (UV–VIS-IR) spectrophotometer. In addition, 2 µm solid-state pulse lasers have important applications in a variety of fields. For the first time, 2D NbSe2 nanosheets are prepared as saturable absorbers (SA) and applied them to solid-state lasers as nonlinear optical modulation devices, successfully achieving the generation of ultra-short pulse lasers with a pulse duration of 445.4 ps in 2 µm band. Our research results prove that 2D NbSe2 nanosheets is a promising nanomaterial, can be prepared into nonlinear optical modulation devices with excellent performance, and show great application potential as ultrafast photonic devices. It is beneficial to the miniaturization of solid-state pulse lasers in subsequent applications.

Plasma etching promoting the saturable absorption of BiOI

Materials exhibiting exceptional saturable absorption performance and etching compatibility are significant in the context of advanced optical devices. Our study demonstrates that plasma etching can effectively promote the saturable absorption of BiOI nanosheets because of defect formation. Remarkably, the BiOI nanosheet subjected to the optimized H2/Ar-plasma treatment manifests a substantial nonlinear absorption coefficient (βeff) of (−5.5 ± 0.7) × 103 cm GW−1 under laser excitation at 800 nm, while the pristine BiOI sample exhibits no discernible nonlinear optical response. The βeff of plasma-treated BiOI samples is (−4.2 ± 0.9) × 103 cm GW−1 at 515 nm, surpassing that of the untreated counterpart by more than fourfold. A comprehensive structural and spectroscopic analysis indicates that oxygen vacancies serve to increase the density of in-gap states, thereby narrowing the bandgap of the BiOI. This reduced bandgap effectively facilitates the Pauli-blocking effect and, in turn, augments the saturable absorption properties of the samples. In contrast to oxygen vacancies, iodine vacancies make a negligible contribution to the performance enhancement of BiOI. Our result would provide a promising way to improve the nonlinear optical response of materials.

15.5 MeV proton irradiation treatment of liquid phase exfoliated graphene

Exceptional Physical properties, including lightweight nature of graphene make it a highly promising material for space applications. However, the impact of proton irradiation, a common phenomenon in space environments, on the structural integrity and functional properties of graphene remains a critical area of research. This study aims to fill this gap by investigating the effects of proton irradiation on liquid-phase exfoliated graphene. We focus specifically on the alterations in infrared spectra and structural characteristics of graphene induced by irradiation with 15.5 MeV protons. Our analysis reveals that organic molecules and functional groups adsorbed on graphene during exfoliation elevate the Fermi level, suppressing graphene's infrared absorption through the Pauli blocking effect. We demonstrate that proton irradiation, at fluences up to 1 × 1015 proton/cm2, effectively removes adsorbed molecules and functional groups, thereby eliminating Pauli blocking effect. These findings contribute to a better understanding of the behavior of graphene under proton irradiation, essential for its utilization in space-centric applications such as thermal coatings, electronics, and optical devices.

Extension of Phase Variation, Higher Momentum Transfer Components, and Pauli Blocking effects on Reaction Cross Sections to Lower and Medium Energies

Extension of Phase Variation, Higher Momentum Transfer Components, and Pauli Blocking effects on Reaction Cross Sections to Lower and Medium Energies

Direct Observation of Sub-Poissonian Temporal Statistics in a Continuous Free-Electron Beam with Subpicosecond Resolution.

We present a novel method to measure the arrival time statistics of continuous electron beams with subpicosecond resolution, based on the combination of an rf deflection cavity and fast single electron imaging. We observe Poissonian statistics within time bins from 100 to 2ns and increasingly pronounced sub-Poissonian statistics as the time bin decreases from 2ps to 340fs. This 2D streak camera, in principle, enables femtosecond-level arrival time measurements, paving the way to observing Pauli blocking effects in electron beams and thus serving as an essential diagnostic tool toward degenerate electron beam sources for free-electron quantum optics.

Electronic intraband scattering in a transition-metal dichalcogenide observed by double-excitation ultrafast electron diffraction

Electronic dynamics in the excited state of transition-metal dichalcogenides (TMDs) has attracted great interest. To understand the ultrafast intraband scattering process of excited electrons in the conduction band, we demonstrated ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation and ultrafast transient reflectivity measurements of a TMD material, 2H-MoTe2. Due to the saturable absorption (or Pauli blocking) effect present in 2H-MoTe2, the system does not absorb the second excitation pulse until the excited electrons generated by the first excitation pulse with a specific fluence are scattered in the conduction band. By exploiting the Pauli blocking effect in ultrafast time-resolved electron diffraction measurements with double-optical-pulse excitation, we found that the excited electrons were scattered within 100 fs comparable to the excitation optical pulse duration. Furthermore, the excited electrons were relaxed to the lowest energy level of the conduction band (K- or Σ-valley) within 1–2 ps.

Donor-π-Acceptor Porphyrin-Assisted Bifunctional Defect Passivation for Enhanced Temporal Domain-/Wavelength-Dependent Nonlinear Absorption Properties in Perovskite Films.

Perovskite layer defects are a primary inhibiting factor for their optical nonlinearity, which restricts their use in nonlinear photonics devices. Nevertheless, due to the variety of defect types, the passivation and repair of these defects remain challenging. Herein, a novel bifunctional passivation strategy was proposed, and the porphyrin with a donor-π-acceptor structure was designed to bifunctionally repair perovskite defects by linking different types of functional groups via acetylenic π-conjugated linkage bridges on both sides, thus improving the nonlinear optical (NLO) absorption properties of porphyrin-perovskite hybrid materials. Research results indicate that the amino and carboxyl groups of porphyrins endow the ability to bifunctionally passivate charged defects via effective coordination interactions. The nonlinear absorption properties of all porphyrin-passivated MAPbI3 films were remarkably enhanced compared to that of the MAPbI3 film across multiple wavelengths and temporal domains. Particularly, the Por3-passivated perovskite film (MAPbI3/Por3) exhibited optimized strongest NLO performance, including reverse saturable absorption (RSA) under 800 nm femtosecond (fs) and 1064 nm nanosecond (ns) laser irradiations, as well as saturable absorption (SA) with 515 and 532 nm ns laser excitations. The value of the NLO absorption coefficient (β = 266.23 cm GW-1) is 1 order of magnitude higher than that of the pristine perovskite film (β = 12.93 cm GW-1), also outperforming other porphyrin-passivated perovskite films and some reported materials. The bifunctional passivation mechanism of porphyrin not only intensifies the perovskite's photoinduced ground-state dipole moment in the two-photon absorption (TPA) process and the free carrier absorption ability to deepen the RSA properties under 800 nm fs and 1064 nm ns lasers, respectively, but also enables the improvement of SA responses under 515 nm fs and 532 nm ns lasers by expediting the Pauli blocking effect of perovskite. Our study offers a viable paradigm, which aims at exploiting high-performance NLO perovskite materials across wide spectral regions and time scales.

Pauli blocking effects on pair creation in strong electric field

Pauli blocking effects on pair creation in strong electric field

Stellar cooling limits on light scalar boson revisited

We revisit the stellar cooling limits on the light scalar boson whose coupling to the Standard Model particles is described by its mixing with the Higgs boson. Strong constraints have been obtained from the electron-nucleus bremsstrahlung process (Ne→NeS) and the resonant plasma effect in the medium. We find that the bremsstrahlung contribution from the electron and nucleus scattering is of similar magnitude to the plasma mixing effect including the off-resonant mixing. The constraints on the scalar coupling are found to be about three orders of magnitude weaker than the previous evaluations because of the missing factor of the electron-nucleon mass ratio. For white dwarfs, the stellar cooling constraint is even more suppressed due to the Pauli blocking effect. We obtain limits on the Higgs-scalar mixing angle of 10−10–10−9, which are given by white dwarfs or red giants up to the luminosity limits, in the region where the scalar mass is lighter than about 10 keV.

Quarkyonic mean field theory

We discuss mean field theory of quarkyonic matter at zero temperature. We treat the nucleons with contact interactions in mean field approximation, discussing both vector and scalar mean field interactions. We treat the quarks without mean field vector interactions, but allow mass terms to be generated consistently from a scalar mean field consistent with the additive quark model for quark masses. Quarkyonic matter is composed of a shell of nucleons that under-occupy the total available phase space associated with the underlying quark degrees of freedom. The fully occupied Fermi sphere beneath this shell of nucleons at high densities is thought of as quarks, but when this fully occupied distribution of states first appears, although the phase space is filled, the matter is at low density. For the transition between this low density and high density saturated matter, we advocate a dual description of the fully filled Fermi sea in terms of hadrons, and make a phenomenological hypothesis for the equation of state of this matter. We then proceed to an example where the mean field interactions are all vector and only associated with the nucleons, ignoring the effects of mass change associated with the scalar interactions. Except for the effects of Pauli blocking, the nucleons and quarks do not interact. To get a reasonable transition to quarkyonic matter the interaction of the quarks among themselves are assumed to be nonperturbative, and a simple phenomenological relation between quark Fermi energy and density is introduced.

Calculation of tunneling current across trapezoidal potential barrier in a scanning tunneling microscope

Accurate calculation of the tunneling currents in a scanning tunneling microscope (STM) is needed for developing image processing algorithms that convert raw data of the STM into surface topographic images. In this paper, an accurate calculation of the tunneling current for several tip–sample distances, bias voltages, and tips of a hyperboloidal shape with several radii of curvature is carried out. The main features of this calculation are the following. Non-WKB exact solutions to the trapezoidal (linear) potential in the barrier region are used to calculate the tunneling probabilities. Pauli blocking effects on both forward and reverse current densities are introduced. Finite temperature (viz. 300K) calculation in which electrons belonging to a narrow band of energy about the Fermi level contribute to tunneling is carried out. Integration over a field line method is used to obtain tunneling currents for the nonplanar hyperboloidal shaped tips, using the expressions obtained in the paper, for planar model current densities. An estimate of the lateral resolution is introduced. Earlier works do not consider all these aspects together in a single calculation. Tunneling currents are found to increase rapidly with increasing bias voltage and decrease exponentially with increasing tip–sample distances. Airy function determined currents are a more accurate function of a tip–sample distance than the WKB determined currents. Pauli effects are found to not always reduce currents from their non-Pauli values. The lateral resolution is found to be degraded for blunter tips, larger bias voltages, and larger tip–sample distances.

Ultra‐High Nonlinear Saturable Absorption Responses and Ultra‐Fast Carrier Dynamics of Organic DAST

AbstractThird‐order nonlinear optical (NLO) absorption properties and ultra‐fast carrier dynamics of organic 4‐N,N‐dimethylamino‐4'‐N'‐methyl‐stilbazolium tosylate (DAST) at the wavelength of 520 nm are investigated by femtosecond open‐aperture Z‐scan technique and transient absorption spectroscopy, respectively. Z‐scan measurements indicate that DAST solution exhibits ultra‐high nonlinear saturable absorption (SA) responses with a figure of merit of 4.57 × 10−13 esu cm and a saturation strength of 3.20 GW cm−2, far superior to those of the most known 2D SA materials under similar excitations. Transient absorption results reveal that the outstanding SA performances of the solution‐phase DAST are rooted in ultra‐fast ground‐state bleaching based on the Pauli blocking effect. The subsequent excited state carrier relaxation processes are dominated by rapid intraband carrier cooling and defect‐assisted interband Auger recombination. Moreover, high‐quality DAST−polyvinyl alcohol composite films prepared by solution casting exhibit similar ultra‐high SA responses and reliable long‐term stability without any degradation of their SA responses even after exposure to air moisture for 100 days. These findings establish an experimental and theoretical foundation for the development of high‐performance ultra‐fast optoelectronic devices based on organic NLO materials in the future.

Structure and α decay for the neutron-deficient nuclei with 89≤Z≤94 in the density-dependent cluster model combined with a relativistic mean-field approach

The density-dependent cluster model combined with relativistic mean-field theory is used to explore the structure and $\ensuremath{\alpha}$ decay for the neutron-deficient nuclei with $89\ensuremath{\le}Z\ensuremath{\le}94$, including two newly discovered nuclei $^{207}\mathrm{Th}$ [Phys. Rev. C 105, L051302 (2022)] and $^{214}\mathrm{U}$ [Phys. Rev. Lett. 126, 152502 (2021)]. The effective nucleon-nucleon interactions and matter density distributions from the relativistic mean field are employed to construct the $\ensuremath{\alpha}$-daughter potential with a double-folding model. The Pauli blocking effect is considered by normalizing the strength of the $\ensuremath{\alpha}$-daughter potential with the Bohr-Sommerfeld quantization condition. The $\ensuremath{\alpha}$-preformation factor is calculated with the cluster formation model. The calculated $\ensuremath{\alpha}$-decay half-lives for the 106 observed nuclei with $89\ensuremath{\le}Z\ensuremath{\le}94$ are in excellent agreement with experimental data. Extending this model to the unknown nuclei $^{201--204}\mathrm{Ac}$, $^{205--206}\mathrm{Th}$, $^{209--210}\mathrm{Pa}$, $^{212--213,220}\mathrm{U}$, $^{215--218,221}\mathrm{Np}$, and $^{220--227}\mathrm{Pu}$, the evolution of the $N=126$ shell closure with neutron number is explored for the high-$Z$ isotopes. The available $\ensuremath{\alpha}$-decay energies, preformation factors, and $\ensuremath{\alpha}$-decay half-lives show a regular change with increasing neutron number. Especially, the robustness of the $N=126$ shell closure is shown up to the Pu isotopes.

Stability of the Repulsive Fermi Gas with Contact Interactions.

We report the creation and the study of the stability of a repulsive quasihomogeneous spin-1/2 Fermi gas with contact interactions. For the range of scattering lengths a explored, the dominant mechanism of decay is a universal three-body recombination toward a Feshbach bound state. We observe that the recombination coefficient K_{3}∝ε_{kin}a^{6}, where the first factor, the average kinetic energy per particle ε_{kin}, arises from a three-body threshold law, and the second one from the universality of recombination. Both scaling laws are consequences of Pauli blocking effects in three-body collisions involving two identical fermions. As a result of the interplay between Fermi statistics and the momentum dependence of the recombination process, the system exhibits nontrivial temperature dynamics during recombination, alternatively heating or cooling depending on its initial quantum degeneracy. The measurement of K_{3} provides an upper bound for the interaction strength achievable in equilibrium for a uniform repulsive Fermi gas.

Resonance-to-bound transition of He5 in neutron matter and its analogy with heteronuclear Feshbach molecules

We theoretically investigate the fate of a neutron-alpha $p$-wave resonance in dilute neutron matter, which may be encountered in neutron stars and supernova explosions. While $^5$He is known as a resonant state that decays to a neutron and an alpha particle in vacuum, this unstable state turns into a stable bound state in the neutron Fermi sea because the decay process is forbidden by the Pauli-blocking effect of neutrons. Such a resonance-to-bound transition assisted by the Pauli-blocking effect can be realized in cold atomic experiments for a quantum mixture near the heteronuclear Feshbach resonance.

Topological Lifshitz transition and one-dimensional Weyl mode in HfTe5.

Landau band crossings typically stem from the intra-band evolution of electronic states in magnetic fields and enhance the interaction effect in their vicinity. Here in the extreme quantum limit of topological insulator HfTe5, we report the observation of a topological Lifshitz transition from inter-band Landau level crossings using magneto-infrared spectroscopy. By tracking the Landau level transitions, we demonstrate that band inversion drives the zeroth Landau bands to cross with each other after 4.5 T and forms a one-dimensional Weyl mode with the fundamental gap persistently closed. The unusual reduction of the zeroth Landau level transition activity suggests a topological Lifshitz transition at 21 T, which shifts the Weyl mode close to the Fermi level. As a result, a broad and asymmetric absorption feature emerges due to the Pauli blocking effect in one dimension, along with a distinctive negative magneto-resistivity. Our results provide a strategy for realizing one-dimensional Weyl quasiparticles in bulk crystals.

In-medium polarization tensor in strong magnetic fields (I): Magneto-birefringence at finite temperature and density

We investigate in-medium polarization effects of the fermion and antifermion pairs at finite temperature and density in strong magnetic fields within the lowest Landau level approximation. Inspecting the integral representation of the polarization tensor by analytic and numerical methods, we provide both the real and imaginary parts of the polarization tensor obtained after delicate interplay between the vacuum and medium contributions essentially due to the Pauli-blocking effect. Especially, we provide a complete analytic form of the polarization tensor at zero temperature and finite density that exhibits an exact cancellation and associated relocation of the singular threshold behaviors for a single photon decay to a fermion and antifermion pair. As a physical application of the in-medium polarization tensor, we discuss the magneto-birefringence that is polarization-dependent dispersion relations of photons induced by the strong magnetic fields.

Optical nonlinearities in the excited carrier density of atomically thin transition metal dichalcogenides

In atomically thin semiconductors based on transition metal dichalcogenides, photoexcitation can be used to generate high densities of electron-hole pairs. Due to optical nonlinearities, which originate from Pauli blocking and many-body effects of the excited carriers, the generated carrier density will deviate from a linear increase in pump fluence. In this paper, we describe nonlinear absorption properties and excited carrier dynamics using a theoretical approach that combines results from ab initio electronic-state calculations with a many-body treatment of optical excitation. We determine the validity range of a linear approximation for the excited carrier density vs pump power and identify the role and magnitude of optical nonlinearities at elevated excitation carrier densities for ${\mathrm{MoS}}_{2}, {\mathrm{MoSe}}_{2}, {\mathrm{WS}}_{2}$, and ${\mathrm{WSe}}_{2}$ considering various excitation conditions. For photoexcitation at the bandgap, we find that the use of a linear absorption coefficient of the unexcited system can strongly underestimate the achievable carrier density in ${\mathrm{MoS}}_{2}$ due to many-body renormalizations of the two-particle density of states. The same holds for excitation of the high-energy band continuum in W-based materials.

Impact of pauli-blocking effect on optical limiting properties of WSe2 thin films

We present a detailed investigation on thickness-dependent third-order (χ(3)) nonlinear optical properties of RF-sputtered WSe2 thin-films using ultrashort pulses centered at different excitation wavelengths. The single-beam Z-scan based investigation in the visible spectrum reveals a predominant Pauli-blocking effect led saturable absorption or optical limiting in WSe2 thin-films. The study also explores the dispersion in third-order nonlinear susceptibility (χ(3)) in WSe2 thin films. Interestingly, WSe2 thin-films of any thickness exhibit a self-focusing effect depicting a positive nonlinear refractive index (n2 > 0). The frequency-dependent nonlinear absorption (β) bear distinct correlations with the bandgap of the films which is also investigated through density-functional-theory (DFT) based simulations. The alteration in bandstructure is primarily due to the Se-deficiency induced defect bands in WSe2 thin-films which has discernible impact on the optical limiting characteristics.

Parton distributions of light quarks and antiquarks in the proton

An algebraic Ansatz for the proton's Poincaré-covariant wave function, which includes both scalar and pseudovector diquark correlations, is used to calculate proton valence, sea, and glue distribution functions (DFs). Regarding contemporary data, a material pseudovector diquark component in the proton is necessary for an explanation of the neutron-proton structure function ratio; and a modest Pauli blocking effect in the gluon splitting function is sufficient to explain the proton's light-quark antimatter asymmetry. In comparison with pion DFs, the light-front momentum fractions carried by all identifiable parton classes are the same; on the other hand, the higher moments are different. Understanding these features may provide insights that explain distinctions between Nambu-Goldstone bosons and seemingly less complex hadrons.