Low-energy Electronic Excitations Research Articles

The third-order nonlinear optical responses of zinc porphyrin oligomers: Cycles vs linear chains

Porphyrins are widely used as potential nonlinear optical (NLO) materials because of their highly delocalized π electrons and feasible synthesis and functionalization with broad biological applications. A variety of linear and cyclic porphyrin derivatives have been synthesized, and the correlation between their structures and NLO properties awaits being disclosed. In this work, the electronic structures and third-order NLO properties of linear and cyclic butadiyne-linked zinc porphyrin oligomers have been studied by quantum chemical methods and sum-over-states model. The static second hyperpolarizability (<γ0>) increases exponentially with the number of zinc porphyrin units ([<γ0>n] = 0.67[<γ0>1]n2.63, n = 2 ∼ 6) in linear π-conjugated oligomers, and the <γ0> of the linear hexamer is about 74 times that of the monomer. Such enhancement of <γ0> in linear oligomers originates from closely-lying frontier molecular orbitals available for low energy electron excitations and strong charge transfer-based excitations across porphyrins. The <γ0>s of cyclic porphyrins are lower than that of the linear hexamer, though the interaction between the ring and the ligand enhances the <γ0> of some cyclic zinc porphyrin complexes. The large two-photon absorption cross sections confer on these zinc porphyrin derivatives excellent candidates for two-photon absorption applications.

Triply degenerate semimetal PtBi2 as van der Waals contact interlayer in two-dimensional transistor

The low-energy electronic excitations in topological semimetal yield a plethora of a range of novel physical properties. As a relatively scarce branch, the research of triple-degenerate semi-metal is mostly confined to the stage of physical properties and theoretical analysis, there are still challenges in its practical application. This research showcases the first application of the triply degenerate semimetal PtBi2 in electronic devices. Leveraging a van der Waals transfer method, PtBi2 flakes were used as interlayer contacts for metal electrodes and WS2 in transistors. The transistor achieved a switching ratio above 106 and average mobility can reach 85 cm2V−1 s−1, meeting integrated circuit requirements. Notably, the excellent air stability of PtBi2 simplifies the device preparation process and provides more stable device performance. Transfer process reduces the Schottky barrier between metal electrodes and semiconductors while avoiding Fermi pinning during metal deposition to achieve excellent contact. This groundbreaking work demonstrates the practical applicability of PtBi2 in the field of electronic devices while opening new avenues for the integration of novel materials in semiconductor technology, setting a precedent for future innovations.

Acoustic Plasmons in Nickel and Its Modification upon Hydrogen Uptake

In this work, we study, in the framework of the ab initio linear-response time-dependent density functional theory, the low-energy collective electronic excitations with characteristic sound-like dispersion, called acoustic plasmons, in bulk ferromagnetic nickel. Since the respective spatial oscillations in slow and fast charge systems involve states with different spins, excitation of such plasmons in nickel should result in the spatial variations in the spin structure as well. We extend our study to NiHx with different hydrogen concentrations x. We vary the hydrogen concentration and trace variations in the acoustic plasmons properties. Finally, at x=1 the acoustic modes disappear in paramagnetic NiH. The explanation of such evolution is based on the changes in the population of different energy bands with hydrogen content variation.

Third-order optical nonlinearity of one-dimensional Dirac fermions

The optical nonlinearity of Dirac fermions in two and three dimensions show unique properties determined by analytic expressions in the independent particle approximation, which promotes their host materials as potential candidates for providing optical nonlinear functionalities. In this work we theoretically study the third-order optical nonlinearity for one-dimensional Dirac fermions and obtain analytical expressions for third-order optical conductivities at general light frequencies. The conductivity includes many resonant peaks induced by the resonant optical transitions associated with intraband motion as well as one-, two-, and three-photon interband processes. The conductivities for field-induced second harmonic generation and third harmonic generation are discussed in detail. To connect with real materials, we choose armchair graphene nanoribbons, which host one-dimensional Dirac fermions for its low energy electronic excitation, and our analytical expressions are applied to get the third-order conductivity for harmonic generations with different ribbon width. By comparing with numerical results evaluated from a full band structure in a tight binding model, the analytic and numerical results agree pretty well for photon energy below 2.5 eV.

Nanoscale View of Engineered Massive Dirac Quasiparticles in Lithographic Superstructures

Massive Dirac fermions are low-energy electronic excitations characterized by a hyperbolic band dispersion. They play a central role in several emerging physical phenomena such as topological phase transitions, anomalous Hall effects, and superconductivity. This work demonstrates that massive Dirac fermions can be controllably induced by lithographically patterning superstructures of nanoscale holes in a graphene device. Their band dispersion is systematically visualized using angle-resolved photoemission spectroscopy with nanoscale spatial resolution. A linear scaling of effective mass with feature sizes is reported, underlining the Dirac nature of the superstructures. In situ electrostatic doping dramatically enhances the effective hole mass and leads to the direct observation of an electronic band gap that results in a peak-to-peak band separation of 0.64 ± 0.03 eV, which is shown via first-principles calculations to be strongly renormalized by carrier-induced screening. The methodology demonstrates band structure engineering guided by directly viewing structurally and electrically tunable massive Dirac quasiparticles in lithographic superstructures at the nanoscale.

Intermolecular Charge Transfer in H- and J-Aggregates of Donor–Acceptor–Donor Chromophores: The Curious Case of Bithiophene-DPP

A vibronic exciton model is developed to describe low-energy electronic excitations in slip-stack aggregates of donor–acceptor–donor (DAD) chromophores with substantial overlap between the neighboring donor and acceptor fragments. In such stacks, J- and H-aggregate behavior is driven by intermolecular charge transfer (ICT) and not Coulomb coupling as is assumed in the Kasha model. In-phase (out-of-phase) intermolecular charge transfer integrals result in J-aggregate (H-aggregate) behavior, as unambiguously determined by the vibronic spectral signatures. Interestingly, both J- and H-aggregates are red-shifted, in contrast to the predictions of the Kasha model. Simulated spectral line shapes agree well with recent experiments on two derivatives of the bithiophene diketopyrrolopyrrole chromophore, 2T-DPP-2T, with different terminal groups.

Preparation of NbAs Single Crystal by the Seed Growth Process

A Weyl semimetal is a novel crystal with low-energy electronic excitations that behave as Weyl fermions. It has received worldwide interest and was believed to have introduced the next era of condensed matter physics after graphene and three-dimensional topological insulators. However, it is not easy to obtain a single large-sized crystal because there are many nucleations in the preparation process. A bottom-seed CVT growth method is proposed in this paper, and we acquired the large-sized, high-quality NbAs single crystals up to 4 × 3 × 3 mm3 finally. X-ray diffraction and STEM confirmed that they are tetragonal NbAs, which the key is to using the seed crystal in a vertical growth furnace. Notably, the photoelectric properties of the crystal are obtained under the existing conditions, which paves the way for follow-up work.

Nature of electronic excitations in small non-stoichiometric quantum dots

Low-energy electronic excitations in non-stoichiometric quantum dots (QDs) have a unique charge transfer (CT) character: surface-to-core in anion-rich and core-to-surface in cation-rich QDs.

Evidence for strong electron correlations in a nonsymmorphic Dirac semimetal

Metallic iridium oxides (iridates) provide a fertile playground to explore new phenomena resulting from the interplay between topological protection, spin-orbit and electron-electron interactions. To date, however, few studies of the low energy electronic excitations exist due to the difficulty in synthesising crystals with sufficiently large carrier mean-free-paths. Here, we report the observation of Shubnikov-de Haas quantum oscillations in high-quality single crystals of monoclinic SrIrO3 in magnetic fields up to 35 T. Analysis of the oscillations reveals a Fermi surface comprising multiple small pockets with effective masses up to 4.5 times larger than the calculated band mass. Ab-initio calculations reveal robust linear band-crossings at the Brillouin zone boundary, due to its non-symmorphic symmetry, and overall we find good agreement between the angular dependence of the oscillations and the theoretical expectations. Further evidence of strong electron correlations is realized through the observation of signatures of non-Fermi liquid transport as well as a large Kadowaki-Woods ratio. These collective findings, coupled with knowledge of the evolution of the electronic state across the Ruddlesden-Popper iridate series, establishes monoclinic SrIrO3 as a topological semimetal on the boundary of the Mott metal-insulator transition.

Electronic properties and interfacial coupling in Pb islands on single-crystalline graphene

Introducing metal thin films on two-dimensional (2D) material may present a system to possess exotic properties due to reduced dimensionality and interfacial effects. We deposit Pb islands on single-crystalline graphene on a Ge(110) substrate and studied the nano- and atomic-scale structures and low-energy electronic excitations with scanning tunneling microscopy/spectroscopy (STM/STS). Robust quantum well states (QWSs) are observed in Pb(111) islands and their oscillation with film thickness reveals the isolation of free electrons in Pb from the graphene substrate. The spectroscopic characteristics of QWSs are consistent with the band structure of a free-standing Pb(111) film. The weak interface coupling is further evidenced by the absence of superconductivity in graphene in close proximity to the superconducting Pb islands. Accordingly, the Pb(111) islands on graphene/Ge(110) are free-standing in nature, showing very weak electronic coupling to the substrate.

Atomic Cluster Au_{10}^{+} Is a Strong Broadband Midinfrared Chromophore.

We report an intense broadband midinfrared absorption band in the Au_{10}^{+} cluster in a region in which only molecular vibrations would normally be expected. Observed in the infrared multiple photon dissociation spectra of Au_{10}Ar^{+}, Au_{10}(N_{2}O)^{+}, and Au_{10}(OCS)^{+}, the smooth feature stretches 700-3400 cm^{-1} (λ=14-2.9 μm). Calculations confirm unusually low-energy allowed electronic excitations consistent with the observed spectra. In Au_{10}(OCS)^{+}, IR absorption throughout the band drives OCS decomposition resulting in CO loss, providing an alternative method of bond activation or breaking.

Undamped plasmon modes and enhanced superconductivity in metal diborides

The anisotropic geometric and electronic structures of metal diborides (MB2) leads to many unusual optical and electronic properties. Here, we present a first-principles study on the low-energy collective electronic excitations and the superconducting performance of the MB2 (M = Be, Mg, Al, and Ca). We demonstrate the undamped cosine-like plasmon modes and the sine-like acoustic plasmons in these metal diborides. Interestingly, the energy of the acoustic plasmons shows a positive correlation with the E 2g phonon modes which are negatively correlated with the superconducting transition temperature (T c) of the MB2 materials. Moreover, hole doping can lower the energy of the acoustic plasmons and enhance the T c of CaB2 up to 48.2 K. Our work offers a theoretical guidance to regulate the plasmonic properties, as well as a new predictive factor for superconductivity.

Investigation of low-energy electronic excitations in quasi-one-dimensional Ising antiferromagnet CsFeCl3 · 2H2O via Raman spectroscopy

The frequencies of low-lying electronic excitations in the quasi-one-dimensional antiferromagnet CsFeCl3 · 2H2O were determined by Raman spectroscopy. The lowest of them (57 cm−1) was attributed to the magnon-like excitation of the chain of Ising spins of paramagnetic Fe2+ ions. And the next one (77 cm−1) can most likely be attributed to the exciton-like one, associated with the electronic transition to the lower level of the excited orbital quasi-doublet dzx, dzy split by the spin-orbit interaction. Investigations of the splittings and shifts of Raman lines in an external magnetic field allowed us to estimate the values of the magnetic moments of the sublattices in the ground state and to determine their spatial orientation relative to the crystallographic directions.

Red/green-light emission in continuous dielectric phase transition materials: [Me3NVinyl]2[MnX4] (X = Cl, Br).

The luminescence of dielectric phase transition materials is one important property for technological applications, such as low-energy electron excitation. The combination of dielectric phase transitions and luminescence within organic–inorganic hybrids would lead to a new type of luminescent dielectric phase transition multifunctional material. Here, we report two novel A2BX4 organic–inorganic hybrid complexes [Me3NVinyl]2[MnCl4] 1 and [Me3NVinyl]2[MnBr4] 2, ([Me3NVinyl] = trimethylvinyl ammonium cation). The complexes 1 and 2 were found to undergo continuous reversible phase transitions as well as switch dielectric phase transitions. Strikingly, intensive red luminescence and green luminescence were obtained under UV excitation respectively to reveal potential application of the two complexes in multi-functional materials along with dielectric switches and so on.

Completely polarized eg orbitals realized in d7−Ni3+ based heterointerface

Using parameter-free density functional approximation and maximally localized Wannier functions analysis, we demonstrate how the ${\mathrm{Ni}}^{3+}$-based heterointerface can present a complete orbital polarization of the Ni-${e}_{g}$ orbitals without introducing additional electronic correlation, which is very inspiring for superconductivity research in two-dimensional confined nickelate layers. The polarized internal electric field from the insulating $\mathrm{Dy}\mathrm{Sc}{\mathrm{O}}_{3}$ layer, spontaneous oxygen octahedral distortion, and in-plane tensile strain can fine-tune means for the generation of single occupied Ni-${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ band structures and cuperate-like Fermi surfaces. Furthermore, by including electronic correlation through on-site Hubbard $U$, we also find a stronger antiferromagnetic correlation than that in the recent synthetic infinite-layer $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_{2}$ with hole-doped ${\mathrm{Ni}}^{1+}$ superconductivity. The low-energy electronic and spin excitations in the nickelate layer resemble those of high-temperature cuprate superconductors. Our study sheds light on promising directions for the preparation of nickelate superconductors.

How silicon and boron dopants govern the cryogenic scintillation properties of N-type GaAs

This paper is the first report describing how the concentrations of silicon and boron govern the cryogenic scintillation properties of n-type GaAs. It shows that valence band holes are promptly trapped on radiative centers and then combine radiatively with silicon donor band electrons at rates that increase with the density of free carriers. It also presents the range of silicon and boron concentrations needed for efficient light emission under X-ray excitation, which along with its low band gap and apparent absence of afterglow, make scintillating GaAs suitable for the detection of rare, low-energy electronic excitations from interacting dark matter particles. A total of 29 samples from four different suppliers were studied. Luminosities and timing responses were measured for the four principal emission bands centered at 860, 930, 1070, and 1335 nm, and for the total emissions. Excitation pulses of 40 kVp X-rays were provided by a light-excited X-ray tube driven by an ultra-fast laser. Scintillation emissions from 800 to 1350 nm were measured using an InGaAs photomultiplier. Within the concentration ranges of free carriers from 2 x 1016/cm3 to 6 x 1017/cm3 and boron from 1.5 × 1018/cm3 to 6 x 1018/cm3, nine samples have luminosities > 70 photons/keV and two have luminosities > 110 photons/keV. Other samples in that range have lower luminosities due to higher concentrations of non-radiative centers. The decay times decrease by typically a factor of ten with increasing free carrier concentrations from 1017/cm3 to 2 x 1018/cm3.

Structure, reactions, and electronic spectra of the rare gas cyanohydrides and isocyanohydrides, HRgCN and HRgNC (Rg = Xe or Rn)

The low-energy electronic excitations of HRgCN and HRgNC (Rg = Xe, Rn) were computed at the TDDFT level of theory, both in the gas phase and in xenon cluster. It was found that the most prominent peak in the spectra was due to the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition (∼6 eV for HRgCN and ∼4.5 eV for HRgNC). Cluster confinement brought about bathochromic shifts in the spectra and better agreement with experiment for HXeCN. The effects of spin–orbit coupling (SOC) in the heavier Rn systems were investigated: for HRnCN, SOC red-shifted the HOMO–LUMO peak, while it blue-shifted the HOMO–LUMO peak for HRnNC. Geometry optimizations were carried out for the HRgCN and HRgNC systems to locate minima and transition states for dissociation and isomerization. Effects of isotopic substitution on reaction rates were predicted. A new model core potentials basis set was introduced and effectiveness of several pseudopotential basis sets was studied.

Control of Mooij correlations at the nanoscale in the disordered metallic Ta\u2013nanoisland FeNi multilayers

Localisation phenomena in highly disordered metals close to the extreme conditions determined by the Mott-Ioffe-Regel (MIR) limit when the electron mean free path is approximately equal to the interatomic distance is a challenging problem. Here, to shed light on these localisation phenomena, we studied the dc transport and optical conductivity properties of nanoscaled multilayered films composed of disordered metallic Ta and magnetic FeNi nanoisland layers, where ferromagnetic FeNi nanoislands have giant magnetic moments of 10^3–10^5 Bohr magnetons (mu _{mathrm{B}}). In these multilayered structures, FeNi nanoisland giant magnetic moments are interacting due to the indirect exchange forces acting via the Ta electron subsystem. We discovered that the localisation phenomena in the disordered Ta layer lead to a decrease in the Drude contribution of free charge carriers and the appearance of the low-energy electronic excitations in the 1–2 eV spectral range characteristic of electronic correlations, which may accompany the formation of electronic inhomogeneities. From the consistent results of the dc transport and optical studies we found that with an increase in the FeNi layer thickness across the percolation threshold evolution from the superferromagnetic to ferromagnetic behaviour within the FeNi layer leads to the delocalisation of Ta electrons from the associated localised electronic states. On the contrary, we discovered that when the FeNi layer is discontinuous and represented by randomly distributed superparamagnetic FeNi nanoislands, the Ta layer normalized dc conductivity falls down below the MIR limit by about 60%. The discovered effect leading to the dc conductivity fall below the MIR limit can be associated with non-ergodicity and purely quantum (many-body) localisation phenomena, which need to be challenged further.

Electron population properties with different energies in a helicon plasma source

The characteristics of electrons play a dominant role in determining the ionization and acceleration processes of plasmas. Compared with electrostatic diagnostics, the optical method is independent of the radio frequency (RF) noise, magnetic field, and electric field. In this paper, an optical emission spectroscope was used to determine the plasma emission spectra, electron excitation energy population distributions (EEEPDs), growth rates of low-energy and high-energy electrons, and their intensity jumps with input powers. The 56 emission lines with the highest signal-to-noise ratio and their corresponding electron excitation energy were used for the translation of the spectrum into EEEPD. One discrete EEEPD has two clear different regions, namely the low-energy electron excitation region (neutral lines with threshold energy of 13–15 eV) and the high-energy electron excitation region (ionic lines with threshold energy ≥19 eV). The EEEPD variations with different diameters of discharge tubes (20 mm, 40 mm, and 60 mm) and different input RF powers (200–1800 W) were investigated. By normalized intensity comparison of the ionic and neutral lines, the growth rate of the ionic population was higher than the neutral one, especially when the tube diameter was less than 40 mm and the input power was higher than 1000 W. Moreover, we found that the intensities of low-energy electrons and high-energy electrons jump at different input powers from inductively coupled (H) mode to helicon (W) mode; therefore, the determination of W mode needs to be carefully considered.

Magnetoelectric properties and low-energy excitations of multiferroic FeCr2S4

We report on the low-frequency optical excitations in the multiferroic ground state of polycrystalline FeCr2S4 in the frequency range 0.3-3~THz and their changes upon applying external magnetic fields up to 7~T. In the ground state below the orbital-ordering temperature T_OO=9K we observe the appearance of several new modes. By applying the external magnetic field parallel and perpendicular to the propagation direction of the THz radiation, we can identify the strongest absorptions to be of predominantly electric-dipole origin. We discuss these modes as the low-energy electronic excitations of the Fe^{2+} ions (3d^6, S=2) in an tetrahedral environment. The eigenfrequencies and relative intensities of these absorption lines are satisfactorily reproduced by our calculation assuming an effective exchange field of 12.8 cm-1 at the Fe^{2+}-ions sites. The direction of the exchange field is found to be slightly tilted out of the ab-plane. With our approach we can also describe previously reported results from M\"ossbauer studies and the order of magnitude of the electric polarisation induced by orbital and non-collinear spin ordering.