Renormalization Effects Research Articles

Band filling and disorder effects on the normal state thermoelectric behavior in MgB2

The goal of this work is providing a comprehensive interpretation framework for the wide and varied experimental phenomenology of the Seebeck effect in MgB2 samples with different levels of doping in either π or σ bands and different levels of disorder, using a combined experimental and theoretical approach. We calculate the temperature dependent diffusive Seebeck coefficient Sdiff(T) with the Boltzmann equation resolved in relaxation time approximation, taking into account the scattering with phonons and impurities, the effect of renormalization and the effect doping in a rigid band approximation. We show that selective disorder has a sizeable effect on the Sdiff magnitude, as it tunes the relative contributions of σ and π bands. Disorder also affects the Sdiff temperature dependences, eventually yielding a linear Sdiff(T) behavior in the dirty limit. We also show that band filling has opposite effects on S, depending on which band dominates transport.In parallel, we carry out the Seebeck effect measurements on neutron-irradiated Mg11B2, and on two series of doped samples Mg1−xAlxB2 and Mg(B1−xCx)2. From comparison of calculated Sdiff(T) and experimental S(T) curves, we demonstrate that diffusive and phonon drag terms give comparable contributions in clean samples, but the phonon drag term is progressively suppressed with increasing disorder.In C and Al doped samples we observe very different experimental behaviors in terms of sign, magnitude and temperature dependence. Indeed, notwithstanding the similar electron doping introduced by both substitutions, C or Al doping yields disorder which mainly affects either σ or π bands, respectively. With the help of our theoretical approach, we are able to disentangle the various effects and prove that the Seebeck coefficient is a very sensitive probe of this kind of disorder.

Renormalization of the exciton mass in monolayer transition metal dichalcogenides

The renormalization of the total effective mass of exciton in monolayer transition metal dichalcogenides (TMDC) on different polar substrates are investigated arising from the exciton-optical phonons coupling, in which both the intrinsic longitudinal optical phonon modes and surface optical phonon modes induced by the polar substrate are taken into account. We find that the total effective mass of exciton is enhanced obviously due to these coupling, depending on the cut-off wave vector of optical phonon modes, the polarizability of material and the internal distance between the monolayer TMDC and the polar substrate. The renormalization effect on the exciton mobility and the critical temperature of quantum degeneracy of the dilute exciton gas are discussed. Our theoretical results provide insight for the analysis of the exciton mass in experiments in two-dimensional materials.

Electronic structures and optical properties of Si- and Sn-doped β-Ga2O3: A GGA+U study**Project supported by the Science and Technology Program of Guangdong Province, China (Grant No. 2015B010112002) and the Science and Technology Project of Guangzhou City, China (Grant No. 201607010250).

The electronic structures and optical properties of β-Ga2O3 and Si- and Sn-doped β-Ga2O3 are studied using the GGA + U method based on density functional theory. The calculated bandgap and Ga 3d-state peak of β-Ga2O3 are in good agreement with experimental results. Si- and Sn-doped β-Ga2O3 tend to form under O-poor conditions, and the formation energy of Si-doped β-Ga2O3 is larger than that of Sn-doped β-Ga2O3 because of the large bond length variation between Ga–O and Si–O. Si- and Sn-doped β-Ga2O3 have wider optical gaps than β-Ga2O3, due to the Burstein–Moss effect and the bandgap renormalization effect. Si-doped β-Ga2O3 shows better electron conductivity and a higher optical absorption edge than Sn-doped β-Ga2O3, so Si is more suitable as a dopant of n-type β-Ga2O3, which can be applied in deep-UV photoelectric devices.

Universal understanding of nematicity and magnetism in Fe-pnictides and FeSe

By employing the mean field and variational Monte Carlo methods, we investigate the iron-based superconductors (FeSCs) based on a realistic five-orbital Hubbard model in which an off-site Coulomb interaction is explicitly included. Our results demonstrate that plays an important role in stabilizing the nematic state in both Fe-pnictides and FeSe. Below a critical , the model is shown to lie in the striped antiferromagnetic ground state, and an increasing of leads to an energy degeneracy between different magnetic configurations. This finding provides a natural explanation for the magnetism in Fe-pnictides with relatively small , and more importantly, unveils the microscopic mechanism behind the absence of magnetic order in FeSe with larger . Simultaneously, the common anisotropy of and orbital occupations in different magnetic configurations accounts for the similar orbital ordering observed in Fe-pnictides and FeSe. In addition, the unusual smallness of the Fermi surfaces in FeSe can be obtained when the renormalization effect of on the band structure is taken into account.

Effective operators in two-nucleon systems

Effective Hamiltonians and effective electroweak operators are calculated with the Okubo-Lee-Suzuki formalism for two-nucleon systems. Working within a harmonic oscillator basis, first without and then with a confining harmonic oscillator trap, we demonstrate the effects of renormalization on observables calculated for truncated basis spaces. We illustrate the renormalization effects for the root-mean-square point-proton radius, electric quadrupole moment, magnetic dipole moment, Gamow-Teller transition and neutrinoless double-beta decay operator using nucleon-nucleon interactions from chiral Effective Field Theory. Renormalization effects tend to be larger in the weaker traps and smaller basis spaces suggesting applications to heavier nuclei with transitions dominated by weakly-bound nucleons would be subject to more significant renormalization effects within achievable basis spaces.

Anomalous dimension of subleading-power N-jet operators. Part II

We continue the investigation of the anomalous dimension of subleading-power N-jet operators. In this paper, we focus on the operators with fermion number one in each collinear direction, corresponding to quark (antiquark) initiated jets in QCD. We investigate the renormalization effects induced by the soft loop and compute the one-loop mixing of time-ordered products involving power-suppressed SCET Lagrangian insertions into N-jet currents through soft loops. We discuss fermion number conservation in collinear directions and provide explicit results for the collinear anomalous dimension matrix of the currents. The Feynman rules for the power-suppressed SCET interactions in the position-space formalism are collected in an appendix.

Conduction band electron relaxation and spin relaxation dynamics in CdZnTe alloy

Conduction band electron relaxation and spin relaxation dynamics in Cd0.96Zn0.04Te alloy are investigated using time-resolved pump-probe spectroscopy. The measured linearly polarization pump-probe spectroscopy demonstrates the presence of a fast process and a slow process, both of which indicate electron trapping and recombination at the defect/impurity level. The time constants of the fast process are within the range of 3.1 to 4.9 ps, and those of the slow process are within the range of 16.1 to 18.1 ps. During the ultrafast thermalization process in the first picosecond, an oscillating signal that results from the band gap renormalization (BGR) effect is found. The D’yakonov-Perel’ (DP) mechanism dominates the spin relaxation process, and the spin relaxation slows down with the incorporation of Zn, because of the decrease of spin-orbit splitting energy and enhancement of electron-impurity scattering.

Coupled Raman-Raman modes in the ionic Raman scattering process

We employ a lattice Hamiltonian to examine the dynamics of coupled normal modes excited through a nonlinear phononic process, extending the model beyond infrared and first-order Raman active mode couplings to include interactions between symmetry-allowed Raman active modes. We examine the strength of the interactions between two Raman active modes on the targeted driven-mode dynamics to confirm that all symmetry allowed Raman modes interact with each other. We apply the model to the correlated insulating ferromagnet YTiO3 and present the resulting renormalization effects on the driven-mode dynamics from the anharmonic interactions. Owing to the dependence of the displacive amplitude of the Raman active mode on such interactions, especially for those modes with small amplitude, we suggest that models of anharmonic phononic coupling in materials with electronic, ferroic, or superconducting properties derived from competing Raman-like distortions should include these low-order terms in the equations of motion describing the excited phonons to obtain accurate physical models.

Hot phonon and carrier relaxation in Si(100) determined by transient extreme ultraviolet spectroscopy.

The thermalization of hot carriers and phonons gives direct insight into the scattering processes that mediate electrical and thermal transport. Obtaining the scattering rates for both hot carriers and phonons currently requires multiple measurements with incommensurate timescales. Here, transient extreme-ultraviolet (XUV) spectroscopy on the silicon 2p core level at 100 eV is used to measure hot carrier and phonon thermalization in Si(100) from tens of femtoseconds to 200 ps, following photoexcitation of the indirect transition to the Δ valley at 800 nm. The ground state XUV spectrum is first theoretically predicted using a combination of a single plasmon pole model and the Bethe-Salpeter equation with density functional theory. The excited state spectrum is predicted by incorporating the electronic effects of photo-induced state-filling, broadening, and band-gap renormalization into the ground state XUV spectrum. A time-dependent lattice deformation and expansion is also required to describe the excited state spectrum. The kinetics of these structural components match the kinetics of phonons excited from the electron-phonon and phonon-phonon scattering processes following photoexcitation. Separating the contributions of electronic and structural effects on the transient XUV spectra allows the carrier population, the population of phonons involved in inter- and intra-valley electron-phonon scattering, and the population of phonons involved in phonon-phonon scattering to be quantified as a function of delay time.

Resonance Raman Spectrum of Doped Epitaxial Graphene at the Lifshitz Transition.

We employ ultra-high vacuum (UHV) Raman spectroscopy in tandem with angle-resolved photoemission (ARPES) to investigate the doping-dependent Raman spectrum of epitaxial graphene on Ir(111). The evolution of Raman spectra from pristine to heavily Cs doped graphene up to a carrier concentration of 4.4 × 1014 cm-2 is investigated. At this doping, graphene is at the onset of the Lifshitz transition and renormalization effects reduce the electronic bandwidth. The optical transition at the saddle point in the Brillouin zone then becomes experimentally accessible by ultraviolet (UV) light excitation, which achieves resonance Raman conditions in close vicinity to the van Hove singularity in the joint density of states. The position of the Raman G band of fully doped graphene/Ir(111) shifts down by ∼60 cm-1. The G band asymmetry of Cs doped epitaxial graphene assumes an unusual strong Fano asymmetry opposite to that of the G band of doped graphene on insulators. Our calculations can fully explain these observations by substrate dependent quantum interference effects in the scattering pathways for vibrational and electronic Raman scattering.

Valley Hall transport of photon-dressed quasiparticles in two-dimensional Dirac semiconductors

We present a theory of the photovoltaic valley-dependent Hall effect in a two-dimensional (2D) Dirac semiconductor subject to an intense near-resonant electromagnetic field. Our theory captures and elucidates the influence of both the field-induced resonant interband transitions and the nonequilibrium carrier kinetics on the resulting valley Hall transport in terms of photon-dressed quasiparticles (PDQs). The non-perturbative renormalization effect of the pump field manifests itself in the dynamics of the PDQs, with a quasienergy spectrum characterized by dynamical gaps δη (η is the valley index) that strongly depend on field amplitude and polarization. Nonequilibrium carrier distribution functions are determined by the pump field frequency ω as well as the ratio of intraband relaxation time τ and interband recombination time τrec. We obtain analytic results in three regimes, when (I) all relaxation processes are negligible, (II) τ ≪ τrec, and (III) τ ≫ τrec, and display corresponding asymptotic dependences on δη and ω. We then apply our theory to 2D transition-metal dichalcogenides, and find a strong enhancement of valley-dependent Hall conductivity as the pump field frequency approaches the transition energies between the pair of spin-resolved conduction and valence bands at the two valleys.

Nonadiabatic coupling effects in MgB2 reexamined

The unusual Raman spectrum of MgB$_2$ and its formidable temperature dependence are successfully reproduced by means of a parameter-free \emph{ab initio} nonadiabatic theory that accounts for the electron-hole pair scattering mechanisms with the system phonons. This example turns out to be a prototypical case where a strong nonadiabatic renormalization of the phonon frequency is partially washed out by the aforementioned scattering events, bringing along a characteristic temperature dependence. Both electron-hole pair lifetime and energy renormalization effects due to dynamical electron-phonon coupling turn out to play a crucial role. This theory could aid in comprehending other Raman spectra characterized with unconventionally strong electron-phonon interaction.

Impact of Contact in Molecular Junctions: When Physics Dictates the Chemical Properties

Molecular junctions are nanomaterials made of one molecule, or a thin organic layer, sandwiched between two metallic electrodes.[1] These systems are now widely and intensively studied due to a high potential for nanoelectronic applications. If these devices can now be reliably made and characterized, this field of research is facing a clear issue: the modification of the molecular structure offers a poor control over the device characteristics. The basic idea of the design in Molecular Electronics is that the molecular level landscape in the junction will affect the electron transport, offering control over the current/voltage characteristics.[1-3] Driven by chemical intuition, the usual design of molecular junctions relies on electron donating or accepting building blocks. In this presentation, we show and explain that this approach which focuses on the isolated molecule properties provides a very limited or even erroneous picture as the contact of a molecule to two metal electrodes leads to a completely different entity. Using first principle DFT calculations, we demonstrate that the contact of an asymmetric organic layer to metal electrodes comes with the creation of spontaneous and very intense electric fields in the junction.[4] The origin of this field are the permanent dipole moments in the junction. The built-in field completely modifies the energy landscape in the junction. We argue that only a design that takes into account such large contact renormalization effects could finally lead to the control of the electronic structure of molecular junctions and then move the molecular electronics effort in the intelligent design era. [1] Aviram and Ratner, Chemical Physics Letters, “Molecular rectifiers”, 1974 [2] Van Dyck and Ratner, Nano Letters, 10.1021/nl504091v, 2015 [3] Van Dyck and Ratner, J. Phys. Chem. C, 10.1021/acs.jpcc.6b07855, 2017 [4] Van Dyck and Bergren, Accepted in Advanced Electronic Materials, “Large Built-in Fields Control the Electronic Properties of Nanoscale Molecular Devices with Dipolar Structures”, 2018 Figure 1

Nonperturbative Renormalization of Nonlocal Quark Bilinears for Parton Quasidistribution Functions on the Lattice Using an Auxiliary Field

Parton quasidistribution functions provide a path toward an abinitio calculation of parton distribution functions (PDFs) using lattice QCD. One of the problems faced in calculations of quasi-PDFs is the renormalization of a nonlocal operator. By introducing an auxiliary field, we can replace the nonlocal operator with a pair of local operators in an extended theory. On the lattice, this is closely related to the static quark theory. In this approach, we show how to understand the pattern of mixing that is allowed by chiral symmetry breaking and obtain a master formula for renormalizing the nonlocal operator that depends on three parameters. We present an approach for nonperturbatively determining these parameters and use perturbation theory to convert to the modified minimal subtraction (MS[over ¯]) scheme. Renormalization parameters are obtained for two lattice spacings using Wilson twisted mass fermions and for different discretizations of the Wilson line in the nonlocal operator. Using these parameters, we show the effect of renormalization on nucleon matrix elements with a pion mass of approximately 370MeV and compare renormalized results for the two lattice spacings. The renormalized matrix elements are consistent among the different Wilson line discretizations and lattice spacings.

Frequency-dependent substrate screening of excitons in atomically thin transition metal dichalcogenide semiconductors

Atomically thin layers of transition metal dichalcogenides (TMDCs) exhibit exceptionally strong Coulomb interaction between charge carriers due to the two-dimensional carrier confinement in connection with weak dielectric screening. The van der Waals nature of interlayer coupling makes it easy to integrate TMDC layers into heterostructures with different dielectric or metallic substrates. This allows to tailor electronic and optical properties of these materials, as Coulomb interaction inside atomically thin layers is very susceptible to screening by the environment. Here we theoretically investigate dynamical screening effects in TMDCs due to bulk substrates doped with carriers over a large density range, thereby offering three-dimensional plasmons as tunable degree of freedom. We report a wide compensation of renormalization effects leading to a spectrally more stable exciton than predicted for static substrate screening, even if plasmons and excitons are in resonance. We also find a nontrivial dependence of the single-particle band gap on substrate doping density due to dynamical screening. Our investigation provides microscopic insight into the mechanisms that allow for manipulations of TMDC excitons by means of arbitrary plasmonic environments on the nanoscale.

Anisotropic Photoluminescence from Isotropic Optical Transition Dipoles in Semiconductor Nanoplatelets.

Many important light-matter coupling and energy-transfer processes depend critically on the dimensionality and orientation of optical transition dipoles in emitters. We investigate individual quasi-two-dimensional nanoplatelets (NPLs) using higher-order laser scanning microscopy and find that absorption dipoles in NPLs are isotropic in three dimensions at the excitation wavelength. Correlated polarization studies of the NPLs reveal that their emission polarization is strongly dependent on the aspect ratio of the lateral dimensions. Our simulations reveal that this emission anisotropy can be readily explained by the electric field renormalization effect caused by the dielectric contrast between the NPLs and the surrounding medium, and we conclude that emission dipoles in NPLs are isotropic in the plane of the NPLs. Our study presents an approach for disentangling the effects of dipole degeneracy and electric field renormalization on emission anisotropy and can be adapted for studying the intrinsic optical transition dipoles of various nanostructures.

Experimental studies of neutrino nuclear responses and nuclear structures for neutrino nuclear physics

Neutrino nuclear responses associated with astro neutrinos and double beta decays are crucial to extract neutrino properties of astro particle physics interests. The present report reviews briefly recent experimental studies of the neutrino nuclear responses. Single beta decays and electron captures, charge exchange nuclear reactions, muon and photon nuclear reactions, neutrino nuclear reactions and nucleon transfer reactions are used to study neutrino nuclear responses. Discussions are made on nucleonic and nonnucleonic spin isospin correlations and renormalization (quenching) effects on axial weak responses.

Interplay between staggered flux and d-wave superconducting orders in t-t′-J model

With cuprate superconductors in mind, we check whether the d-wave superconducting (dSC) state coexists with or excludes the staggered flux (SF) state, which was revealed to be a strong candidate for the pseudogap state. To this end, we use a variational Monte Carlo method for the square-lattice t-J model with diagonal transfer t′. We construct a trial wave function which can simultaneously represent dSC and SF orders for a continuous description of their interplay. Furthermore, a band renormalization effect is introduced into both orders independently. It is found that the SF state does not coexist with dSC and is unstable for a wide area in the parameter space. However, a strange state having both SF and antiferromagnetic orders is found to be stable for very low doping rate.

Interhole correlation and phase separation in t-J model

Interhole and interelectron correlations as well as a transition to phase separation in the t-J model near half filling are reconsidered using a variational Monte Carlo method. As a trial wave function, we use a state that can simultaneously represent a d-wave superconducting and antiferromagnetic (AF) orders and allows for nearest-neighbor hole-hole and spin-spin correlation factors and band renormalization effects. It is found that a transition occurs at J = JPS ∼ 2.5t for the doping rates δ = 0.08 and 0.20 from a seemingly uniform state to phase separation (J > JPS) of singly occupied sites and empty sites. For J < JPS, where the values used for cuprate superconductors (0.25 ≲ J/t ≲ 0.5) are included, the interhole correlation is always repulsive. This result is negative to an intuitive picture for cuprates: Two holons coherently move in the background of AF orders.

Optimized wave function by kinetic renormalization effect in strongly correlated region of the three-band Hubbard model

We investigate the ground state of the two-dimensional single-band Hubbard model and three-band Hubbard model (d-p model) by using a variational Monte Carlo method. We employ an optimization variational Monte Carlo method. In this method the many-body wave function is improved and optimized by introducing variational parameters in the form: ψ = exp(-S)ψ0 where ψ0 is a wave function, for example, the Gutzwiller function, and S is an operator used to include the many-body effect.The strong magnetic correlation is induced by the on-site Coulomb repulsive interaction U. The magnetic correlation plays a significant role in correlated electron systems as an origin of electron pairing as well as a main competitor of superconductivity. High-temperature superconductivity occurs in the strongly correlated region where U is greater than the bandwidth. We investigate the stability of antiferromagnetic state in the three-band d-p model.