Electron-electron Interaction Effects Research Articles

Exploring the exemplary electronic and optical nature in NaInX2 (X = S, Se and Te) ternary type chalcogenides materials: A GGA+U and hybrid functionals study

In this work, a systematic study is carried about electronic and optical properties of Indium-based ternary chalcogenides materials by using the well-known density functional theory. We employed the GGA ​+ ​U and the hybrid functionals approach to consider the effects of on-site electron-electron interactions. A clear direct band gap nature was observed located at Γ-point showing all the three materials possess a direct band gap semiconductors nature. The In-s state signify smaller energy gap closer to high Γ-point symmetry. The Sodium ions in the materials are predominantly bonded ionically and there exists a covalent nature in the corresponding In–X (X ​= ​S, Se, Te) pairs. The optical parameters, were computed and are discussed in detail up to photon energy range of 14 ​eV. The first critical point related to the threshold energy in dielectric functions exist around the band gap, is due to the splitting which stretch threshold in term of direct optical transition between the valence band maxima and the conduction band minima. A sharp cut off response in absorption spectra at high energy value, confirms these materials as potential candidates in optoelectronic devices mainly operational in the UV region. The reflectivity spectra specify anisotropy and maximum peaks of about 70% at around 13.4 ​eV energy value making them promising shielding type material against the ultraviolet radiations. A substantial variation exists in the optical behavior of these studied materials which predicts that these calculations can lead to understand future theoretical and experimental investigations depending on the basic material properties of our studied materials as well as their applications specifically in optoelectronic devices.

Strong electron-electron interactions in Si/SiGe quantum dots

The authors implement a nonperturbative method for calculating the effects of strong electron-electron interactions, nearly degenerate electronic valleys, and interface disorder in multielectron silicon quantum dots. They show that all three ingredients play an important role in qubit experiments, and that Wigner-molecule physics in silicon can be substantially different from that in materials without valley degeneracy. The results provide key insights for the design and implementation of quantum dot qubits in silicon.

Zero-field spin resonance in graphene with proximity-induced spin-orbit coupling

We investigate collective spin excitations in graphene with proximity-induced spin-orbit coupling (SOC) of the Rashba and valley-Zeeman types, as it is the case, e.g., for graphene on transition- metal-dichalcogenide substrates. It is shown that, even in the absence of an external magnetic field, such a system supports collective modes, which correspond to coupled oscillations of the uniform and valley-staggered magnetizations. These modes can be detected via both zero-field electron spin resonance (ESR) and zero-field electric-dipole spin resonance (EDSR), with EDSR response coming solely from Rashba SOC. We analyze the effect of electron-electron interaction within the Fermi- liquid kinetic equation and show that the interaction splits both the ESR and EDSR peaks into two. The magnitude of splitting and the relative weights of the resonances can be used to extract the spin-orbit coupling constants and many-body interaction parameters that may not be accessible by other methods.

Flat bands, electron interactions, and magnetic order in magic-angle mono-trilayer graphene

Starting with twisted bilayer graphene, graphene-based moir\'e materials have recently been established as a new platform for studying strong electron correlations. In this paper, we study twisted graphene monolayers on trilayer graphene and demonstrate that this system can host flat bands when the twist angle is close to the magic angle of $1.{16}^{\ensuremath{\circ}}$. When monolayer graphene is twisted on ABA trilayer graphene, the flat bands are not isolated, but are intersected by a Dirac cone with a large Fermi velocity. In contrast, graphene twisted on ABC trilayer graphene (denoted AtABC) exhibits a gap between flat and remote bands. Since ABC trilayer graphene and twisted bilayer graphene are known to host broken-symmetry phases, we further investigate the ostensibly similar magic-angle AtABC system. We study the effect of electron-electron interactions in AtABC using both Hartree theory and an atomic Hubbard theory to calculate the magnetic phase diagram as a function of doping, twist angle, and perpendicular electric field. Our analysis reveals a rich variety of magnetic orderings, including ferromagnetism and ferrimagnetism, and demonstrates that a perpendicular electric field makes AtABC more susceptible to magnetic ordering.

Dirac node engineering and flat bands in doped Dirac materials

We suggest the tried approach of impurity band engineering to produce flat bands and additional nodes in Dirac materials. We show that surface impurities give rise to nearly flat impurity bands close to the Dirac point. The hybridization of the Dirac nodal state induces the splitting of the surface Dirac nodes and the appearance of new nodes at high-symmetry points of the Brillouin zone. The results are robust and not model dependent: our tight-binding calculations are supported by a low-energy effective model of a topological insulator surface state hybridized with an impurity band. Finally, we address the effects of electron-electron interactions between localized electrons on the impurity site. We confirm that the correlation effects, while producing band hybridization and the Kondo effect, keep the hybridized band flat. Our findings open up prospects for impurity band engineering of nodal structures and flat-band correlated phases in doped Dirac materials.

Effect of lattice strain on the electronic structure and magnetic correlations in infinite-layer (Nd,Sr)NiO2

We report a theoretical study of the effect of electron-electron interactions and Sr doping on the electronic structure of infinite-layer (Nd,Sr)NiO2 using the DFT+dynamical mean-field theory approach (DFT+DMFT). Here, we explore the effect of lattice strain that experience (Nd,Sr)NiO2 films upon growing on the SrTiO3 substrate on the electronic properties, magnetic correlations, and exchange couplings of (Nd,Sr)NiO2. For both strained and unstrained Sr-doped NdNiO2 our results reveal the crucial importance of orbital-dependent correlation effects in the Ni 3d shell. Upon doping with Sr, it undergoes a Lifshitz transition which is accompanied by a reconstruction of magnetic correlations. For Sr x < 0.2 (Nd,Sr)NiO2 adopts the Néel (111) antiferromagnetic (AFM) order, while for x > 0.3 the C-type (110) AFM sets in the unstrained (Nd,Sr)NiO2, with a highly frustrated region at x ≃ 0.2, all within DFT+DMFT at T = 290 K. Our results for the Néel AFM at Sr x = 0 suggest that AFM NdNiO2 appears at the verge of a Mott-Hubbard transition, providing a plausible explanation for the experimentally observed weakly insulating behavior of NdNiO2 for Sr x < 0.1. We observe that the Lifshitz transition makes a change of the band structure character from electron- to hole-like with Sr x, in agreement with recent experiments. We conclude that the in-plane strain adjusts a bandwidth of the Ni x2−y2 band, i.e., controls the effect of electron correlations in the Ni x2−y2 orbitals. It leads to a suppression of the static C-type (110) ordering in (Nd,Sr)NiO2 for Sr x > 0.3. Our results for the electronic structure and magnetic correlations of (Nd,Sr)NiO2 reveal an anomalous sensitivity upon a change of the crystal structure parameters.

Studying the lifetime of charge and heat carriers due to intrinsic scattering mechanisms in FeVSb half-Heusler thermoelectric

This work, presents a study of lifetime of carriers due to intrinsic scattering mechanisms viz. electron–electron interaction (EEI), electron–phonon interaction (EPI) and phonon–phonon interaction (PPI) in a promising half-Heusler thermoelectric FeVSb. Using the full-GW method, the effect of EEI and temperature on the valence and conduction band extrema and band gap are studied. The lifetime of carriers with temperature are estimated at these band extrema. At 300 K, estimated value of lifetime at VBM (CBM) is ∼1.91 × 10−14 s (∼2.05 × 10−14 s). The estimated ground state band gap considering EEI is ∼378 meV. Next, the effect of EPI on the lifetime of electrons and phonons with temperature are discussed. The comparison of two electron lifetimes suggests that EEI should be considered in transport calculations along with EPI. The average acoustic, optical and overall phonon lifetimes due to EPI are studied with temperature. Further, the effect of PPI is studied by computing average phonon lifetime for acoustic and optical phonon branches. The lifetime of the acoustic phonons are higher compared to optical phonons which indicates acoustic phonons contribute more to lattice thermal conductivity (κ ph). The comparison of phonon lifetime due to EPI and PPI suggests that, above 500 K EPI is the dominant phonon scattering mechanism and cannot be ignored in κ ph calculations. Lastly, a prediction of the power factor and figure of merit of n-type and p-type FeVSb is made by considering the temperature dependent carrier lifetime for the electronic transport terms. This study shows the importance of considering EEI in electronic transport calculations and EPI in phonon transport calculations in FeVSb. Our study is expected to provide results to further explore the thermoelectric transport in this material.

Weak Kondo effect in the monocrystalline transition metal dichalcogenide ZrTe2

Zr-Te compounds are an ideal platform to investigate novel electrical transport properties. Here we report the Kondo effect in single-crystalline ${\mathrm{ZrTe}}_{2}$. When $T&lt;8$ K, ${\mathrm{ZrTe}}_{2}$ exhibits a $\mathrm{log}T$ dependence of resistivity, which may derive from three different mechanisms, including electron-electron interaction, weak localization, and Kondo effect. By measuring transport properties with magnetic field along different directions, the former two mechanisms were excluded. Isotropic negative magnetoresistance was extracted by subtracting different quadratic background signals caused by Lorentz force, which reveals the existence of a weak Kondo effect in this material.

Oxygen vacancies in strontium titanate: A DFT+DMFT study

We address the long-standing question of the nature of oxygen vacancies in strontium titanate, using a combination of density functional theory and dynamical mean-field theory (DFT+DMFT) to investigate in particular the effect of vacancy-site correlations on the electronic properties. Our approach uses a minimal low-energy electronic subspace including the Ti-$t_{2g}$ orbitals plus an additional vacancy-centered Wannier function, and provides an intuitive and physically transparent framework to study the effect of the local electron-electron interactions on the excess charge introduced by the oxygen vacancies. We estimate the strength of the screened interaction parameters using the constrained random phase approximation and find a sizeable Hubbard $U$ parameter for the vacancy orbital. Our main finding, which reconciles previous experimental and computational results, is that the ground state is either a state with double occupation of the localized defect state or a state with a singly-occupied vacancy and one electron transferred to the conduction band. The balance between these two competing states is determined by the strength of the interaction both on the vacancy and the Ti sites, and on the Ti-Ti distance across the vacancy. Finally, we contrast the case of vacancy doping in SrTiO$_3$ with doping via La substitution, and show that the latter is well described by a simple rigid-band picture.

Orbital-selective coherence-incoherence crossover and metal-insulator transition in Cu-doped NaFeAs

We study the effects of electron-electron interactions and hole doping on the electronic structure of Cu-doped NaFeAs using the density functional theory plus dynamical mean-field theory (DFT+DMFT) method. In particular, we employ an effective multi-orbital Hubbard model with a realistic bandstructure of NaFeAs in which Cu-doping was modeled within a rigid band approximation and compute the evolution of the spectral properties, orbital-dependent electronic mass renormalizations, and magnetic properties of NaFeAs upon doping with Cu. In addition, we perform fully charge self-consistent DFT+DMFT calculations for the long-range antiferromagnetically ordered Na(Fe,Cu)As with Cu $x=0.5$ with a real-space ordering of Fe and Cu ions. Our results reveal a crucial importance of strong electron-electron correlations and local potential difference between the Cu and Fe ions for understanding the \textbf{k}-resolved spectra of Na(Fe,Cu)As. Upon Cu-doping, we observe a strong orbital-dependent localization of the Fe $3d$ states accompanied by a large renormalization of the Fe $xy$ and $xz$/$yz$ orbitals. Na(Fe,Cu)As exhibits bad metal behavior associated with a coherence-to-incoherence crossover of the Fe $3d$ electronic states and local moments formation near a Mott metal-insulator transition (MIT). For heavily doped NaFeAs with Cu $x \sim 0.5$ we obtain a Mott insulator with a band gap of $\sim$0.3 eV characterized by divergence of the quasiparticle effective mass of the Fe $xy$ states. In contrast to this, the quasiparticle weights of the Fe $xz$/$yz$ and $e_g$ states remain finite at the MIT. The MIT occurs via an orbital-selective Mott phase to appear at Cu $x\simeq0.375$ with the Fe $xy$ states being Mott localized. We propose the possible importance of Fe/Cu disorder to explain the magnetic properties of Cu-doped NaFeAs.

Accurate Measurement of the Gap of Graphene/h-BN Moiré Superlattice through Photocurrent Spectroscopy.

Monolayer graphene aligned with hexagonal boron nitride (h-BN) develops a gap at the charge neutrality point (CNP). This gap has previously been extensively studied by electrical transport through thermal activation measurements. Here, we report the determination of the gap size at the CNP of graphene/h-BN superlattice through photocurrent spectroscopy study. We demonstrate two distinct measurement approaches to extract the gap size. A maximum of ∼14 meV gap is observed for devices with a twist angle of less than 1°. This value is significantly smaller than that obtained from thermal activation measurements, yet larger than the theoretically predicted single-particle gap. Our results suggest that lattice relaxation and moderate electron-electron interaction effects may enhance the CNP gap in graphene/h-BN superlattice.

Effects of screened Coulomb interaction on spin transfer torque

In a magnetic multilayer, magnetizations can be manipulated by spin transfer torque. Both spin transfer torque and its reciprocal effect, spin pumping, are governed by spin mixing conductance. The magnitude of spin mixing conductance at the interface of nearly magnetic metal has been theoretically shown to be enhanced by electron-electron interaction. However, experiments show both increasing and decreasing values of spin mixing conductance for metals with larger electron-electron interaction. Here we take into account the effect of electron-electron interaction on the screening of the Coulomb interaction at the magnetic interface to correctly describe the experiment.

Effect of Electron-Electron Interactions on Metallic State in Quasicrystals

We theoretically study the effect of electron-electron interactions on the metallic state of quasicrystals. To address the problem, we introduce the extended Hubbard model on the Ammann-Beenker tiling as a simple theoretical model. The model is numerically solved within an inhomogeneous mean-field theory. Because of the lack of periodicity, the metallic state is nonuniform in the electron density even in the noninteracting limit. We clarify how this charge distribution pattern changes with electron-electron interactions. We find that the intersite interactions change the distribution substantially and in an electron-hole asymmetric way. We clarify the origin of these changes through the analyses in the real and perpendicular spaces. Our results offer a fundamental basis to understand the electronic states in quasicrystalline metals.

Localization and interference induced quantum effects at low magnetic fields in InGaAs/GaAs structures

The longitudinal ρxx(B, T) and Hall ρxy(B, T) resistances are experimentally investigated in n-InGaAs/GaAs nanostructures with a single and double quantum wells in the magnetic field range B = 0–2.5 T and temperatures T = 1.8–20 K. It is shown that the origin of the temperature-independent point located at ωcτ≅1 on the ρxx(B, T) curves is due to the combined action of the classical cyclotron motion and the quantum interference effects of weak localization and electron-electron interaction. The results obtained indicate that the transition from the dielectric phase to the phase of the quantum Hall effect is a crossover from weak localization (quantum interference effects in a weak magnetic field) to strong localization in quantizing magnetic fields in the quantum Hall effect regime.

Temperature and Intensity Dependence of the Limiting Efficiency of Silicon Solar Cells

The temperature and intensity dependence of the limiting efficiencies of monofacial and bifacial silicon solar cells are calculated from the physical properties of silicon assuming light trapping by Lambertian scattering from rough surfaces. The maximum efficiency of a bifacial cell (28.92%) is lower than the efficiency of a monofacial cell (29.46%) at room temperature and Air Mass 1.5 Global illumination. The effects of electron-electron interactions on the band gap, radiative recombination rate, and optical absorption are included self-consistently. The temperature coefficient of the output power is -0.23%/°C for the optimum thickness monofacial cell at room temperature. The optimum thickness of silicon solar cells decreases strongly with temperature following a power law T <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> and thin cells have a lower temperature coefficient than thick cells. A surface recombination velocity of 1 cm/s is found to be a turning point below which surface recombination has a small effect on the efficiency.

Effect of electron-electron interactions on high-order harmonic generation in crystals

As a potential high-efficiency XUV light source, high-order harmonic generation (HHG) in solids contains the dynamic information of electrons. The interactions among many particles in solids are ubiquitous and, to some extent, modulate the motion of electrons. However, the role of electron-electron interaction (EEI) effect in solid HHG has not been well understood. In this paper, we study the laser-intensity-dependent EEI effect under the given laser pulse durations. By comparing the finite chain model with the infinite extension model, we find that, if the electric field is strong enough, the induced electric field caused by EEI is nontrivial for the finite chain model. We also find that EEI only can enhance HHG spectra in a limited frequency range under some specific intensities of the laser field in both models, and the EEI effect is related to the configuration of the system. This study can provide a reference for whether EEI can be ignored when simulating the interaction between one-dimensional solids and lasers.

Fermi-surface topology and renormalization of bare ellipticity in an interacting anisotropic electron gas

We investigate effects of electron-electron interactions on the shape of the Fermi surface in an anisotropic two-dimensional electron gas using the `RPA-GW' self-energy approximation. We find that the interacting Fermi surface deviates from an ellipse, but not in an arbitrary way. The interacting Fermi surface has only two qualitatively distinct shapes for most values of $r_s$. The Fermi surface undergoes two distinct transitions between these two shapes as $r_s$ increases. For larger $r_s$, the degree of the deviation from an ellipse rapidly increases, but, in general, our theory provides a justification for the widely used elliptical Fermi surface approximation even for the interacting system since the non-elliptic corrections are quantitatively rather small except for very large $r_s$.

Excitons and plasmons of graphene nanoribbons in infrared frequencies in an effective-mass approximation

Effects of electron-electron interactions on the optical response of graphene nanoribbons are theoretically investigated in an effective-mass approximation in a comprehensive manner. In optical absorption spectra of armchair and zigzag nanoribbons without and with doping, excitons, which are bound states of electrons and holes, and plasmons manifest themselves as various prominent peaks. For light polarized parallel to nanoribbons, exciton peaks at high energies split because of interactions with dark excitons, to which optical transition is prohibited. For light polarized perpendicular to nanoribbons, in nondoped semiconducting armchair nanoribbons, moderate exciton peaks appear while in doped armchair and zigzag nanoribbons, only when the Fermi energy crosses at least two energy bands, large plasmon peaks occur because of a nature of Dirac electrons. These peaks can be assigned to specific optical transitions in energy bands. The optical absorption peaks arising from the excitons and plasmons in a wide range of categories of nanoribbons approximately correspond to those of carbon nanotubes by appropriate scaling of energy and the wave vector.

Many-body exchange-correlation effects in MoS2 monolayer: The key role of nonlocal dielectric screening

We calculate the quasiparticle properties of $\rm{MoS_2}$ monolayer at $T=0$ considering the dynamical electron-electron interaction effect within random-phase-approximation (RPA). The calculations are carried out for an electron-doped slab of $\rm{MoS_2}$ monolayer using a minimal massive Dirac Hamiltonian and the quasi-two-dimensional nature of the Coulomb interaction in this system is taken into account considering a modified interaction of Keldysh type. Having calculated the real and imaginary parts of the retarded self-energy, we find the spectral function and discuss the impact of extrinsic variables such as the dielectric medium and the charge carrier density on the appearance and position of the quasiparticle peaks. We also report the results of the renormalization constant and the effective Fermi velocity calculations in a broad range of the coupling constant and carrier density. We show that the effective Fermi velocity obtained solving the self-consistent Dyson equation has an absolutely different behavior from the one found from the on-shell approximation. Our results show that the nonlocal dielectric screening of the monolayer tends to stabilize the Fermi liquid picture in $\rm{MoS_2}$ monolayer and that the interaction strength parameter of this system is a multivariable function of the coupling constant, carrier density, and also the screening length.

Ultrafast Electron Correlations and Memory Effects at Work: Femtosecond Demagnetization in Ni.

Experimental observations of the ultrafast (less than 50fs) demagnetization of Ni have so far defied theoretical explanations particularly since its spin-flipping time is much less than that resulting from spin-orbit and electron-lattice interactions. Through the application of an approach that benefits from spin-flip time-dependent density-functional theory and dynamical mean-field theory, we show that proper inclusion of electron correlations and memory (time dependence of electron-electron interaction) effects leads to demagnetization at the femtosecond scale, in good agreement with experimental observations. Furthermore, our calculations reveal that this ultrafast demagnetization results mainly from spin-flip transitions from occupied to unoccupied orbitals implying a dynamical reduction of exchange splitting. These conclusions are found to be valid for a wide range of laser pulse amplitudes. They also pave the way for abinitio investigations of ultrafast charge and spin dynamics in a variety of quantum materials in which electron correlations may play a definitive role.