Electron Dispersion Relation Research Articles

Nonlinear Thermoplasmonics in Graphene Nanostructures.

The linear electronic dispersion relation of graphene endows the atomically thin carbon layer with a large intrinsic optical nonlinearity, with regard to both parametric and photothermal processes. While plasmons in graphene nanostructures can further enhance nonlinear optical phenomena, boosting resonances to the technologically relevant mid- and near-infrared (IR) spectral regimes necessitates patterning on ∼10 nm length scales, for which quantum finite-size effects play a crucial role. Here we show that thermoplasmons in narrow graphene nanoribbons can be activated at mid- and near-IR frequencies with moderate absorbed energy density, and furthermore can drive substantial third-harmonic generation and optical Kerr nonlinearities. Our findings suggest that photothermal excitation by ultrashort optical pulses offers a promising approach to enable nonlinear plasmonic phenomena in nanostructured graphene that avoids potentially invasive electrical gating schemes and excessive charge carrier doping levels.

Hints of granular superconductivity in natural graphite verified by trapped flux transport measurements

This study describes electrical transport measurements as a function of magnetic field and temperature that provide hints of the existence of superconductivity with a critical temperature of K in a natural graphite sample. The measurements were done in a restricted temperature 300 K K and magnetic field mT range. Electrical resistance measurements at remanence (zero field), after the application of a magnetic field, indicate the existence of trapped flux, which remains nearly unchanged within min but it vanishes at a temperature of K. The apparent transition is accompanied by a clear enhancement of the magnetoresistance at . Raman measurements on the bulk sample reveal the existence of the rhombohedral stacking order, which interfaces with the usual Bernal phase were predicted to lead to high temperature superconductivity due to the formation of robust flat bands in the electron dispersion relation.

Weyl–Kondo semimetal behavior in the chiral structure phase of Ce3Rh4Sn13

The spin dynamics, crystalline-electric-field (CEF) level scheme, specific heat, and x-ray photoemission spectra (XPS) of ${\mathrm{Ce}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ were investigated, which exhibits anomalous semimetal transport in the chiral crystallographic phase. CEF excitations observed at approximately 7 and 39 meV are consistent with the two inequivalent Ce-ion cites in the chiral structure. Because of broader CEF excitations and a strong $4{f}^{1}$ peak at the Fermi level in the Ce $4f$ on-resonance spectrum, the hybridized Ce $4f$ electrons contribute to the semimetal carriers. In addition, the spin fluctuation associated with the Kramers doublet ground state is characterized by the peak located at 0.15 meV. The electronic state involving the spin fluctuation causes the ${T}^{3}$ behavior of specific heat below 0.6 K, which is attributed to linear dispersion relations of electrons of the Weyl--Kondo semimetal in the chiral-lattice symmetry.

Pressure-dependent physical properties of cesium-niobium oxide: a comprehensive study.

Perovskites, an important class of materials, are mostly utilized in memory and spintronic devices. The thermoelectric response calculations for some perovskite oxides have been reported, but their attributes under pressure have rarely been explored. In this current study, the effects of high pressure on various properties of CsNbO3 perovskite oxides in the cubic phase were investigated using the pseudopotential approach and Boltzmann transport theory. Specifically, the structural electronic dispersion relations, density of states, phonon properties, elasto-mechanical properties, optical constants, and thermoelectric performance of the material were analyzed. CsNbO3 was reported to be dynamically stable through the optimization of energy against volume under ambient pressure conditions. The phonon dispersion curves of CsNbO3 were computed at pressures ranging from 60 to 100 GPa to demonstrate its stability under these pressures. At ambient pressure, CsNbO3 is a semiconductor with a wide direct band gap of 1.95 eV. With the increase in pressure, the band gap starts decreasing. An analysis of the imaginary part of the dielectric constant suggests that this material may be useful for sensors and optoelectronic devices. Various thermoelectric response parameters were tested for CsNbO3 at temperatures from 50 K to 800 K, with a step size of 50 K, and pressures of 60-100 GPa. Based on the calculated power factor values and optical parameters, CsNbO3 proved to be a potential candidate for energy harvesting applications.

Effect of pressure on structural, electronic dispersion relations, optical and thermoelectric properties of CsNbO3 perovskite for photovoltaic and energy applications

The perovskites have not only been recognized for memory devices and spintronic applications but are equally important for the thermoelectric response calculations. In this research work, the effect of pressure on structural, electronic dispersion curves, optical and thermoelectric parameters of CsNbO3 cubic perovskite have been investigated by using the pseudopotential approach alongside the Boltzmann transport theory. The CsNbO3 was reported stable in simple cubic structure by means of energy-volume optimization and phonon dispersion curves by B. Sabir et. al. CsNbO3 is reported as the direct band gap semiconductor with band gap around 1.95 eV at ambient pressure. Interestingly the band gap decrease with the increase in the pressure. The absence of negative phonon frequency shows the dynamical stability of this material under pressure. The analysis of imaginary part of dielectric response function predicts the utility of this perovskite in high frequency microwave sensors as well as the optoelectronic devices like optical sensors and interconnects. Further, the thermoelectric parameters such as Seebeck coefficient (S), electronic part of thermal conductivity (κe/τ), electrical conductivity (σ/τ) and power factor (PF) have been discussed within a specific temperature and pressure range (300–1200 K) and (10–50 GPa) respectively. The calculated values of power factor and optical constants prove it a potential candidate for thermoelectric and photovoltaic applications.

Optical absorption of conduction electrons in semiconductor nanowires in the presence of the electron–phonon interaction

We calculate the energy states and the optical absorption coefficient for electrons in a nanowire in the presence of the Rashba and the longitudinal-optical phonon interactions. The interplay of those interactions results in a splitting in the electron dispersion relation at zero wavevector that grows linearly for the ground state as the strength of the lateral quantum confinement increases. For higher states, the energy splitting increases more rapidly till the state of resonant polaron is reached, then it increases slowly due to the pinning effect. The frequency separation between the well-resolved absorption peaks and their number are greatly influenced by the state of the polarons.

Scanning tunneling spectroscopy of two-dimensional Dirac materials on substrates with a band gap

In this paper we show that conventional current-voltage spectroscopy can provide quantitative information on the dispersion relation of the low-energy electrons of monoelemental group-IV two-dimensional (2D) Dirac materials provided that (1) the 2D material is placed on a substrate with a band gap and (2) the density of states of the scanning tunneling microscopy tip is constant. We have derived an expression for the differential conductivity of a monoelemental group-IV 2D Dirac material that is placed on a substrate with a band gap. The differential conductivity scales as $|E\ensuremath{-}{E}_{D}|{e}^{b|E\ensuremath{-}{E}_{D}|},$ rather than the commonly assumed $|E\ensuremath{-}{E}_{D}|$ scaling, where $E$ and ${E}_{D}$ refer to the energy of the electrons and the Dirac point, respectively. The parameter $b$ is inversely proportional to the Fermi velocity.

그래핀에서의 스핀-전하 결합 수송 연구 동향

그래핀(graphene)은 독특한 물리적 성질로 인하여 나노전자소자에 응용에 최적의 2차원 소재로 인식 되어왔다. 그래핀의 전자 분산관계(electron dispersion relation)를 스핀에 의존하게 제어하는 것은 스핀기반 로직소자(spin-based logic device) 개발에 있어서 매우 중요하다. 특히, 그래핀에서 스핀-궤도 상호작용(spin-orbit interaction)을 강화 시킨다면 강자성체(ferromagnet) 없이 스핀 편향(spin polarization)을 생산하거나 검출하는 것이 가능하게 될 것이다. 최근 그래핀에 스핀-궤도 상호작용을 인가시키고 그로 인한 수송특성을 다양한 형태의 소자에서 연구한 결과들이 활발히 발표되고 있다. 본 리뷰에서는 그래핀 스핀트로닉스 연구에 대한 최근의 발전을 스핀-궤도 상호작용과 그에 따른 수송특성에 중점을 두어 논하였다.

First-principles calculations to investigate elasto-mechanical and opto-electronic properties of pyrochlore oxides X2Zr2O7 (X=La, Nd)

This paper contains the comparative study of elasto-mechanical, electronic and optical properties of pyrochlores. These structures are first optimized and second order-independent cubic elastic constants C11, C12 and C44 are computed using the optimized data. These computed cubic elastic constants are further used to check the mechanical stability, Poisson's ratio, Pugh's ratio and Cauchy pressure which depicts the flexible and ductile nature of both materials. Kleinman ratio is more significant for La2Zr2O7, so it has excellent bond stretching minimization phenomena. Young's modulus and lame's constants describe that Nd2Zr2O7 is stiffer and difficult to compress than La2Zr2O7. Debye temperature and sound velocities are also calculated for both oxides. Due to the high melting temperature, Nd2Zr2O7 is suitable for high-temperature environments and is used in aerospace and construction industries. Band structures reveal that these oxides are metallic as there appears no gap in electronic dispersion relations. Analysis of the density of states reveals the magnetic nature of these oxides as the density of states is not the perfect image of each other in both spin channels. The imaginary part of the dielectric function also shows the optically metallic nature and suitability for sensors and opto-electronic devices.

Testing Lorentz invariance of electrons with LHAASO observations of PeV gamma-rays from the Crab Nebula

The Large High Altitude Air Shower Observatory (LHAASO) recently reported the detection of gamma-ray emissions with energies up to 1.1PeV from the Crab Nebula. Using the absence of vacuum Cherenkov effect by inverse-Compton electrons, we improve previous bounds to linear-order Lorentz invariance violation (LV) in the dispersion relations of electrons by 104 times. We show that the LV effect on electrons is severely constrained, compatible with certain type of LV as expected by some models of quantum gravity (QG), such as the string/D-brane inspired space-time foam. We argue that such models are supported by the Crab Nebula constraints from the LHAASO observations, as well as various LV phenomenologies for photons to date.

Ultrafast Sub-100 fs All-Optical Modulation and Efficient Third-Harmonic Generation in Weyl Semimetal Niobium Phosphide Thin Films.

Since their experimental discovery in 2015, Weyl semimetals have generated a large amount of attention due their intriguing physical properties that arise from their linear electron dispersion relation and topological surface states. In particular, in the field of nonlinear (NL) optics and light harvesting, Weyl semimetals have shown outstanding performances and achieved record NL conversion coefficients. In this context, the first steps toward Weyl semimetal nanophotonics are performed here by thoroughly characterizing the linear and NL optical behavior of epitaxially grown niobium phosphide (NbP) thin films, covering the visible to the near-infrared regime of the electromagnetic spectrum. Despite the measured high linear absorption, third-harmonic generation studies demonstrate high conversion efficiencies up to 10-4 % that can be attributed to the topological electron states at the surface of the material. Furthermore, nondegenerate pump-probe measurements with sub-10 fs pulses reveal a maximum modulation depth of ≈1%, completely decaying within 100 fs and therefore suggesting the possibility of developing all-optical switching devices based on NbP. Altogether, this work reveals the promising NL optical properties of Weyl semimetal thin films, which outperform bulk crystals of the same material, laying the grounds for nanoscale applications, enabled by top-down nanostructuring, such as light-harvesting, on-chip frequency conversion, and all-optical processing.

Energy spectrum of semimetallic HgTe quantum wells

Quantum wells (QWs) based on mercury telluride (HgTe) thin films provide a large scale of unusual physical properties starting from an insulator via a two-dimensional Dirac semimetal to a three-dimensional topological insulator. These properties result from the dramatic change of the QW band structure with the HgTe film thickness. Although being a key property, these energy dispersion relations cannot be reflected in experiments due to the lack of appropriate tools. Here we report an experimental and theoretical study of two HgTe quantum wells with inverted energy spectrum in which two-dimensional semimetallic states are realized. Using magneto-optical spectroscopy at sub-THz frequencies we were able to obtain information about electron and hole cyclotron masses at all relevant Fermi level positions and different charge densities. The outcome is also supported by a Shubnikov-de Haas analysis of capacitance measurements, which allows obtaining information about the degeneracy of the active modes. From these data, it is possible to reconstruct electron and hole dispersion relations. Detailed comparative analysis of the energy dispersion relations with theoretical calculations demonstrates a good agreement, reflecting even several subtle features like band splitting, the second conduction band, and the overlaps between the first conduction and first valence band. Our study demonstrates that the cyclotron resonance experiments can be efficiently used to directly obtain the band structures of semimetallic 2D materials.

Superatomic Signature and Reactivity of Silver Clusters with Oxygen: Double Magic Ag 17 – with Geometric and Electronic Shell Closure

Understanding the stability and reactivity of silver clusters toward oxygen provides insights to design new materials of coinage metals with atomic precision. Herein, we report a systematic study o...

Anomalous Thermodiffusion of Electrons in Graphene

We reveal a dramatic departure of electron thermodiffusion in solids relative to the commonly accepted picture of the ideal free-electron gas model. In particular, we show that the interaction with the lattice and impurities, combined with a strong material dependence of the electron dispersion relation, leads to counterintuitive diffusion behavior, which we identify by comparing a two-dimensional electron gas (2DEG) and single-layer graphene. When subject to a temperature gradient ∇T, thermodiffusion of massless Dirac fermions in graphene exhibits an anomalous behavior with electrons moving along ∇T and accumulating in hot regions, in contrast to normal electron diffusion in a 2DEG with parabolic dispersion, where net motion against ∇T is observed, accompanied by electron depletion in hot regions. These findings bear fundamental importance for the understanding of the spatial electron dynamics in emerging materials, establishing close relations with other branches of physics dealing with electron systems under nonuniform temperature conditions.

Trion ground-state energy: Simple results

We investigate the trion binding energy in a three-dimensional semiconductor, with bare Coulomb interaction between charges, and effective mass approximation for the electron and hole dispersion relations. This is done by making use of a previously proposed exact method for the three-body problem. The calculations cover the complete range of electron-to-hole mass ratio. We find a perfect agreement with existing variational calculations. Investigating the small and large mass ratio regimes, we build a three parameters interpolating formula for the trion binding energy $E_b(r)$ in terms of the exciton binding energy, where $r$ is the electron to exciton mass ratio. This formula $E_b(r)=0.71347-0.11527 \,r -0.18580\, \sqrt{1-r}\,$, in atomic units, is in full agreement, within our precision, with our numerical results over the complete range of mass ratio.

Nonlinear Graphene Nanoplasmonics.

Nonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, such as in nanophotonic structures, one can alternatively use localized optical resonances with high quality factors to increase light-matter interaction times, although this approach is limited by inelastic losses partly associated with the nonlinear response. Plasmons-the collective oscillations of electrons in conducting media-offer the means to concentrate light into nanometric volumes, well below the light-wavelength-scale limit imposed by diffraction, amplifying the electromagnetic fields upon which nonlinear optical phenomena depend. Due to their abundant supply of free electrons, noble metals are the traditional material platform for plasmonics and have thus dominated research in nanophotonics over the past several decades, despite exhibiting large ohmic losses and inherent difficulties to actively modulate plasmon resonances, which are primarily determined by size, composition, and morphology. Highly doped graphene has recently emerged as an appealing platform for plasmonics due to its unique optoelectronic properties, which give rise to relatively long-lived, highly confined, and actively tunable plasmon resonances that mainly appear in the infrared and terahertz frequency regimes. Efforts to extend graphene plasmonics to the near-infrared and visible ranges involve patterning of graphene into nanostructured elements, thus facilitating the optical excitation of localized resonances that can be blue-shifted through geometrical confinement while maintaining electrical tunability. Besides these appealing plasmonic attributes, the conical electronic dispersion relation of graphene renders its charge carrier motion in response to light intrinsically anharmonic, resulting in a comparatively intense nonlinear optical response. The combined synergy of extreme plasmonic field enhancement and large intrinsic optical nonlinearity are now motivating intensive research efforts in nonlinear graphene plasmonics, the recent progress of which we discuss in this Account. We start with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene, its enhancement by resonant coupling to its highly confined plasmons to yield intense high harmonic generation and Kerr nonlinearities, the extraordinary thermo-optical capabilities of this material enabling large nonlinear optical switching down to the single-photon level, and its strong interaction with quantum emitters.

Rate of dark photon emission from electron positron annihilation in massive stars

We calculate the rate of production of dark photons from electron-positron pair annihilation in hot and dense matter characteristic of supernova progenitors. Given the non-linear dependence of the emission rate on the dark photon mass and current astrophysical constraints on the dark photon parameter space, we focus on the mass range of 1--10 MeV. For the conditions under consideration both mixing with the in-medium photon and plasma effects on the electron dispersion relation are non-negligible and are explored in detail. We perform our calculations to the leading order in the fine-structure constant. Transverse and longitudinal photon modes are treated separately given their different dispersion relations. We consider the implications for the evolution of massive stars when dark photons decay either into particles of the standard model or of the dark sector.

Feasible model for photoinduced interband pairing

Recently, it has been proposed that a mechanism for the appearance of non-equilibrium superconductivity in a resonantly driven semiconductor with repulsive interband interactions exists.~\cite{Goldstein_PRB15} The underlying microscopic model relies on the appearance of a specific fermionic dissipation mechanism and the careful simultaneous tailoring of the electronic dispersion relation and electron-electron interactions. We, instead, show that the phenomenon is rather general and does not need a special fine tuning of parameters. By considering a pair of bands with locally the same sign of concavity, we demonstrate that interband pairing arises under the natural assumption of the presence of phononic baths and radiative recombination. In light of these findings, we demonstrate how the emergence of superconductivity can be understood in terms of standard equilibrium interband BCS theory.

Thermionic emission laws for general electron dispersion relations and band structure data

In this article, a thermionic electron emission theory for general electron dispersion relations E(k) is presented, relating electron energy E to wave-vector magnitude k = |k|. This theory does not require the construction of a model Hamiltonian for the electrode's materials, like Dirac or Weyl Hamiltonians. Instead, use is made of the material's band structure data, e.g., the parabolic E(k) approximation for the Richardson–Dushman equation and linear E(k), as used for graphene and 3D Dirac semimetals. This new theory confirms previous findings on parabolic E(k), e.g., that the emission current is independent of effective electron mass in the material as long as it is larger than real electron mass m0. For effective mass lower than m0, the emission is reduced and tends to zero for vanishing effective mass. For materials with negative electron affinity, additional terms arise in the emission current equation. It turns out that the linear E(k) dispersion, e.g., for Dirac semimetals, does not have the potential to surpass the Richardson emission in materials with the same work function. In addition, a more rigorous electron emission theory is established by utilizing real anisotropic band structure data En(k) for electrode materials. For collimated electron emission normal to the surface, the transverse electron velocities tend to zero, i.e., the transverse derivatives of En(k) have to be comparatively small. If stable electrode materials of this kind can be realized, a considerable increase of electron emission by a factor 100 or more can be achieved, compared to the Richardson–Dushman theory, especially for small lattice constants perpendicular to the emission direction.

Ultralow thermal conductivity of BaAg2SnSe4 and the effect of doping by Ga and In

We report on investigations of transport properties of BaAg2SnSe4 and the effect of Ga and In doping in the structure. The experimental work is supported by calculations of the electronic band structure and phonon dispersion relations. The thermal conductivity of BaAg2SnSe4 is exceptionally low with a value of 0.50 Wm−1K−1 at 300 K and decreasing to 0.26 Wm−1K−1 at 723 K. Phonon spectral calculations and low-temperature heat capacity reveal enhanced atomic displacement parameters (rattling behavior) of Ag and Se atoms in the structure. Acoustic phonons tend to flatten rather rapidly and, at 1 THz, are cut off by low-lying optic phonon modes, resulting in a suppression of the maximum Debye frequency over the first Brillouin zone. Weak chemical bonding, particularly of Ag and Se, gives rise to softened phonon modes and low speed of sound. Band structure calculations indicate that the conduction band edge is formed mainly by contributions of Se p orbitals and Sn s orbitals, while the valence band edge is dominated by contributions of Ag d orbitals and Se p orbitals. Doping with Ga or In on the site of Sn increases the room temperature carrier concentration from 4.9 × 1016 cm−3 for BaAg2SnSe4 to 7.8 × 1017 cm−3 for BaAg2Ga0.009Sn0.991Se4. The power factor is greatly improved from 35 μW m−1K−2 for pristine BaAg2SnSe4 to 121.1 μW m−1K−2 at 623 K for BaAg2Ga0.009Sn0.991Se4 sample. A combination of the intrinsically ultralow thermal conductivity and the substantially enhanced power factor results in a peak ZT = 0.25 at 673 K for BaAg2Ga0.002Sn0.998Se4. Although this value represents a 2.5 times improvement over the figure of merit of pristine BaAg2SnSe4, it remains rather low on account of very low carrier concentration observed even in doped samples.