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We study the statistical properties of charge and energy transport in electron conducting junctions with electron-phonon interactions, specifically, the thermoelectric efficiency and its fluctuations. The system comprises donor and acceptor electronic states, representing a two-site molecule or a double-quantum-dot system. Electron transfer between metals through the two molecular sites is coupled to a particular vibrational mode which is taken to be either harmonic or anharmonic, a truncated (two-state) spectrum. Considering these models we derive the cumulant generating function in steady state for charge and energy transfer, correct to second order in the electron-phonon interaction, but exact to all orders in the metal-molecule coupling strength. This is achieved by using the nonequilibrium Green's function approach (harmonic mode) and a kinetic quantum master-equation method (anharmonic mode). From the cumulant generating function we calculate the charge current and its noise and the large-deviation function for the thermoelectric efficiency. We demonstrate that at large bias the charge current, differential conductance, and the current noise can identify energetic and structural properties of the junction. We further examine the operation of the junction as a thermoelectric engine and show that while the macroscopic thermoelectric efficiency is indifferent to the nature of the mode (harmonic or anharmonic), efficiency fluctuations do reflect this property. We study the statistical properties of charge and energy transport in electron conducting junctions with electron-phonon interactions, specifically, the thermoelectric efficiency and its fluctuations. The system comprises donor and acceptor electronic states, representing a two-site molecule or a double-quantum-dot system. Electron transfer between metals through the two molecular sites is coupled to a particular vibrational mode which is taken to be either harmonic or anharmonic, a truncated (two-state) spectrum. Considering these models we derive the cumulant generating function in steady state for charge and energy transfer, correct to second order in the electron-phonon interaction, but exact to all orders in the metal-molecule coupling strength. This is achieved by using the nonequilibrium Green's function approach (harmonic mode) and a kinetic quantum master-equation method (anharmonic mode). From the cumulant generating function we calculate the charge current and its noise and the large-deviation function for the thermoelectric efficiency. We demonstrate that at large bias the charge current, differential conductance, and the current noise can identify energetic and structural properties of the junction. We further examine the operation of the junction as a thermoelectric engine and show that while the macroscopic thermoelectric efficiency is indifferent to the nature of the mode (harmonic or anharmonic), efficiency fluctuations do reflect this property. [ABSTRACT FROM AUTHOR]

We present density functional theory (DFT) + quasiparticle self-energy (G0W0)+ Bethe-Salpeter calculations ot the real and imaginary parts of the long-wavelength dielectric function of LiF between ambient pressure and P = 5 Mbars. While the optical absorption spectrum is predicted to show dramatic pressure-dependent features above the optical gap, the index of refraction well below the gap is shown to exhibit the same trends as that seen in both DFT calculations and experiment: a roughly linear increase with density. This increase does not result from a decrease in the band gap, but rather follows from the increase in oscillator strength which counteracts a smaller increase in band gap with P. Our calculations also suggest that the index of refraction (for visible and near-UV light) of the higher-T B2 phase should be quite close to that of the B1 (ambient crystalline) phase. These findings may be of interest to researchers who use LiF as a window material in dynamic compression experiments. [ABSTRACT FROM AUTHOR]

Transport and gain properties of a resonant-phonon terahertz quantum cascade laser are calculated using nonequilibrium Green's functions. Impurity scattering is shown to be responsible for contrasting nonlinear effects in the transport and the gain properties. For typical doping concentrations, the current density is found to be weakly sensitive to the impurity scattering strength. In contrast, the calculated gain is found to be very sensitive to the impurity scattering strength. This difference is attributed to the strong momentum dependence of the long-range coupling to charged impurities. Small-momentum impurity scattering is shown to be responsible for an incoherent regime of resonant tunneling processes. These insights into the crucial role of impurity scattering open a route of improvement of terahertz quantum cascade lasers by engineering of the doping profile. [ABSTRACT FROM AUTHOR]

We show that any heat definition expressed as an energy change in the reservoir energy plus any fraction of the system-reservoir interaction is not an exact differential when evaluated along reversible isothermal transformations, except when that fraction is zero. Even in that latter case the reversible heat divided by temperature, namely entropy, does not satisfy the third law of thermodynamics and diverges in the low temperature limit. These results are found within the framework of nonequilibrium Green functions (NEGF) using a single level quantum dot strongly coupled to fermionic reservoirs and subjected to a time-dependent protocol modulating the dot energy as well as the dot-reservoir coupling strength. [ABSTRACT FROM AUTHOR]

Despite the importance of Si:P δ-doped wires for modern nanoelectronics, there are currently no computational models of electron transport in these devices. In this paper we present a nonequilibrium Green's function model for electronic transport in a δ-doped wire, which is described by a tight-binding Hamiltonian matrix within a single-band effective-mass approximation. We use this transport model to calculate the current-voltage characteristics of a number of 5-doped wires, achieving good agreement with experiment. To motivate our transport model we have performed density-functional calculations for a variety of 5-doped wires, each with different donor configurations. These calculations also allow us to accurately define the electronic extent of a S-doped wire, which we find to be at least 4.6 nm. [ABSTRACT FROM AUTHOR]

The electronic structure of Pb1-xSnxTe is studied by using the relativistic Korringa-Kohn-Rostoker Green function method in the framework of density functional theory. For all concentrations x, Pb1-xSnxTe is a direct semiconductor with a narrow band gap. In contrast to pure lead telluride, tin telluride shows an inverted band characteristic close to the Fermi energy. It will be shown that this particular property can be tuned, first, by alloying PbTe and SnTe and, second, by applying hydrostatic pressure or uniaxial strain. Furthermore, the magnitude of strain needed to switch between the regular and inverted band gap can be tuned by the alloy composition. Thus there is a range of potential usage of Pb1-xSnxTe for spintronic applications. [ABSTRACT FROM AUTHOR]

We discuss the appearance of odd-frequency Cooper pairs in two-band superconductors by solving the Gor' kov equation analytically. We introduce the equal-time s-wave pair potentials as realized in MgB2 and iron pnictides. Although the order parameter symmetry is conventional, the band degree of freedom enriches the symmetry variety of pairing correlations. The hybridization and the asymmetry between the two conduction bands induce odd-frequency pairs as a subdominant pairing correlation in the uniform ground state. To study the magnetic response of odd-frequency Cooper pairs, we analyze the Meissner kernel represented by the Gor'kov Green function. In contrast to the even-frequency pairs linked to the pair potential, the induced odd-frequency Cooper pairs indicate a paramagnetic property. We also discuss the relation between the amplitude of the odd-frequency pairing correlation and the stability of superconducting states in terms of the self-consistent equation for the pair potential. [ABSTRACT FROM AUTHOR]

We theoretically demonstrate the Ruderman-Kittel-Kasuya-Yosida interaction between magnetic impurities that is mediated by the Weyl fermions embedded inside a three-dimensional Weyl semimetal (WSM). The WSM is characterized by a pair of Weyl points separated in the momentum space. Using the Green's function method and a two-band model, we show that four terms contribute to the magnetic impurity interaction in the WSM phase: the Heisenberg, Dzyaloshinsky-Moriya, spin-frustrated, and Ising terms. Except for the last term which is vanishingly small in the plane perpendicular to the line connecting two Weyl points, all the other interaction terms are finite. Furthermore, the magnetic spins of the Dzyaloshinsky-Moriya and spin-frustrated terms he in the plane perpendicular to the line connecting two Weyl points, but in this plane, the magnetic spins of the Ising term have no components. For each contribution, an analytical expression is obtained, falling off with a spatial dependence as R-5 at Weyl points and showing beating behavior that depends on the direction between two magnetic impurities. [ABSTRACT FROM AUTHOR]

The proximity-induced pair potential in a topological insulator-superconductor hybrid features an interesting superposition of a conventional spin-singlet component from the superconductor and a spin-triplet one induced by the surface state of the topological insulator. This singlet-triplet superposition can be altered by the presence of a magnetic field. We study the interplay between topological order and superconducting correlations performing a symmetry analysis of the induced pair potential, using Green functions techniques to theoretically describe ballistic junctions between superconductors and topological insulators under magnetic fields. We relate a change in the conductance from a gapped profile into one with a zero-energy peak with the transition into a topologically nontrivial regime where the odd-frequency triplet pairing becomes the dominant component in the pair potential. The nontrivial regime, which provides a signature of odd-frequency triplet superconductivity, is reached for an outof- plane effective magnetization with strength comparable to the chemical potential of the superconductor or for an in-plane one, parallel to the normal-superconductor interface, with strength of the order of the superconducting gap. Strikingly, in the latter case, a misalignment with the interface yields an asymmetry with the energy in the conductance unless the total contribution of the topological surface state is considered. [ABSTRACT FROM AUTHOR]

The phase space for graphene's minimum conductivity σmin is mapped out using Landauer theory modified for scattering using Fermi's golden rule, as well as the nonequilibrium Green's function (NEGF) simulation with a random distribution of impurity centers. The resulting "fan diagram" spans the range from ballistic to diffusive over varying aspect ratios (W/L), and bears several surprises. The device aspect ratio determines how much tunneling (between contacts) is allowed and becomes the dominant factor for the evolution of σmin from ballistic to diffusive regime. We find an increasing (for W/L > 1) or decreasing (W/L < 1) trend in σmin vs impurity density, all converging around 128q2/π3h ~ 4q2/h at the dirty limit. In the diffusive limit, the conductivity quasisaturates due to the precise cancellation between the increase in conducting modes from charge puddles vs the reduction in average transmission from scattering at the Dirac point. In the clean ballistic limit, the calculated conductivity of the lowest mode shows a surprising absence of Fabry-Pérot oscillations, unlike other materials including bilayer graphene. We argue that the lack of oscillations even at low temperature is a signature of Klein tunneling. [ABSTRACT FROM AUTHOR]

We calculate the spin transport of hydrogenated graphene using the Landauer-Büttiker formalism with a spin-dependent tight-binding Hamiltonian. Hydrogen adatoms are a common defect and they carry a finite magnetic moment, which makes it important to understand their influence on spin transport for graphene-based spin devices. Our tight-binding model accurately reproduces the density-functional theory band structure and atom-projected density of states. The advantages of using the Landauer-Büttiker formalism are that it simultaneously gives information on sheet resistance and localization length as well as spin relaxation length. Furthermore, the transport can be computed very efficiently using this method by employing the recursive Green's function technique. Here, we study hydrogen adatoms on graphene with randomly aligned magnetic moments, where interference effects are explicitly included. We show that a 5 ppm hydrogen defect density is sufficient to reduce the spin relaxation length to 2µm and that the inverse spin relaxation length and sheet resistance scale nearly linearly with the impurity concentration. Moreover, the spin relaxation mechanism in hydrogenated graphene is Markovian only near the charge neutrality point or in the highly dilute impurity limit. [ABSTRACT FROM AUTHOR]

Topological quantum phases of matter have been a topic of intense interest in contemporary condensed matter physics. Extensive efforts are devoted to investigate various exotic properties of topological matter including topological insulators, topological superconductors, and topological semimetals. For topological insulators, the dissipationless transport via gapless helical edge or surface states is supposed to play a defining role, which unfortunately has proved difficult to realize in experiments due to inevitable backscattering induced in the sample boundary. Motivated by the fundamental connection between topological invariants and the Zak phase, here, we show that the nontrivial band topologies of both two- and three-dimensional topological insulators, characterized by the Chern numbers and the Z2 invariants, respectively, are directly manifested in the winding numbers of the Wannier-Stark ladder (WSL) emerging under an electric field. We use the Floquet Green's function formalism to show that the winding number of the WSL is robust against interband interference as well as nonmagnetic impurity scattering. [ABSTRACT FROM AUTHOR]

We present the Usadel theory describing the superconducting proximity effect in heterostructures with a half-metallic layer. It is shown that the full spin polarization inside the half-metals gives rise to an additional component of the Green's function which results in the giant triplet spin-valve effect in superconductor (S)-ferromagnet (F)-half-metal (HM) trilayers and provides a natural explanation for the φ0-junction formation in the S/F/HM/F/S systems. In addition, we consider the exactly solvable model of the S/F/HM trilayers of atomic thickness and demonstrate that it reproduces the main features of the spin-valve effect found within the Usadel approach. Our results are shown to be in qualitative agreement with the recent experimental data on the spin-valve effect in MoGe/Ni/Cu/CrO2 hybrids [Singh et al., Phys. Rev. X 5, 021019 (2015)]. [ABSTRACT FROM AUTHOR]

We present a general first-principles approach to treat various linear response phenomena relevant for spintronics. It is based on a Kubo-Bastin formalism and implemented within the multiple-scattering Korringa-Kohn-Rostoker (KKR) Green's function method with the underlying electronic structure determined by density functional theory. The symmetric (e.g., longitudinal electronic transport) as well as the antisymmetric (e.g., transverse transport) parts of the response tensor are determined, including both the so-called Fermi-sea and the Fermi-surface contributions. To describe spin-orbit-induced phenomena, such as the anomalous and spin Hall effects, a fully relativistic description is employed. Exploiting the adopted Green's function method substitutional disorder in the full concentration range of alloys is treated within the coherent potential approximation, taking full account of occurring vertex corrections in the averaging procedure for the linear response quantities. Extrinsic (scattering related, e.g., side-jump and skew scattering) and intrinsic (band structure-related) contributions to the transport tensors are treated on equal footing. Other phenomena, such as Gilbert damping and spin-orbit torques, are particular cases of the general framework and their determination is briefly addressed. The versatility of the method is demonstrated by presenting results for the anomalous and spin Hall conductivities for elemental transition metals and their alloys. [ABSTRACT FROM AUTHOR]

We study the electronic transport in an infinite one-dimensional Hubbard chain, driven by a homogeneous electric field. The physical chain is coupled to fermionic bath chains in order to account for dissipation and to prevent the occurrence of Bloch oscillations. The steady-state current is computed in the frame of Keldysh Green's functions in cluster perturbation theory. The current characteristics are dominated by resonant-tunneling-like structures, which can be traced back to Wannier-Stark resonances due to antiferromagnetic correlations. The same current characteristic occurs in a noninteracting Wannier-Stark model with alternating on-site energies. Nonlocal effects of the self-energy can be accounted for the observed physical behavior. [ABSTRACT FROM AUTHOR]

A simple and efficient approximation scheme to study electronic transport characteristics of strongly correlated nanodevices, molecular junctions, or heterostructures out of equilibrium is provided by steady-state cluster perturbation theory. In this work, we improve the starting point of this perturbative, nonequilibrium Green's function based method. Specifically, we employ an improved unperturbed (so-called reference) state pS, constructed as the steady state of a quantum master equation within the Bom-Markov approximation. This resulting hybrid method inherits beneficial aspects of both the quantum master equation as well as the nonequilibrium Green's function technique. We benchmark this scheme on two experimentally relevant systems in the single-electron transistor regime: an electron-electron interaction based quantum diode and a triple quantum dot ring junction, which both feature negative differential conductance. The results of this method improve significantly with respect to the plain quantum master equation treatment at modest additional computational cost. [ABSTRACT FROM AUTHOR]

We show that the equations underlying the GW approximation have a large number of solutions. This raises the question, how can we find the physical solution? We provide two theorems which explain why the methods currently in use usually find the correct solution. These theorems are general enough to cover a large class of similar methods. An efficient algorithm for including self-consistent vertex corrections well beyond G W is described and used in a numerical validation of the two theorems. The effect of a simple mixing scheme on solutions obtained iteratively is also investigated. [ABSTRACT FROM AUTHOR]

Nonlocal pairing processes at the edge of a two-dimensional topological insulator in proximity to an 5-wave superconductor are usually suppressed by helicity. However, the additional proximity of a ferromagnetic insulator can substantially influence the helical constraint and therefore open a new conduction channel by allowing for crossed Andreev reflection (CAR) processes. We show a one-to-one correspondence between CAR and the emergence of odd-frequency triplet superconductivity. Hence, nonlocal transport experiments that identify CAR in helical liquids yield smoking-gun evidence for unconventional superconductivity. Interestingly, we identify a setup--composed of a superconductor flanked by two ferromagnetic insulators-that allows us to favor CAR over electron cotunneling, which is known to be a difficult but essential task to be able to measure CAR. [ABSTRACT FROM AUTHOR]

We derive a non-Markovian master equation for the evolution of a class of open quantum systems consisting of quadratic fermionic systems coupled to wideband reservoirs. This is done by providing an explicit correspondence between master equations and nonequilibrium Green's function approaches. Our findings permit us to address non-Markovian regimes characterized by negative decoherence rates and to characterize the dynamics with respect to a recently proposed measurement of "non-Markovianity." We study the real-time dynamics and the steady-state solution of two illustrative models: a tight-binding electronic model and XY spin chains. The rich set of nonequilibrium steady-state phases encountered extends previous results to the non-Markovian regime. [ABSTRACT FROM AUTHOR]

Transient grating spectroscopy has emerged as a useful technique to study thermal phonon transport because of its ability to perform thermal measurements over length scales comparable to phonon mean free path (MFPs). Although several prior works have performed theoretical studies of quasiballistic heat conduction in transient grating, the analysis methods are either restricted to one spatial dimension or require phenomenological fitting parameters. Here, we analyze quasiballistic transport in a two-dimensional transient grating experiment in which heat conduction can occur both in and cross plane using an analytic Green's function of the Boltzmann equation we recently reported that is free of fitting parameters. We demonstrate a method by which phonon MFPs can be extracted from these measurements, thereby extending the MFP spectroscopy technique using transient grating to opaque bulk materials. [ABSTRACT FROM AUTHOR]

We show that the local in-gap Green's function of a band insulator G0(ε,k||,r⊥=0), with r⊥ the position perpendicular to a codimension-1 or codimension-2 impurity, reveals the topological nature of the phase. For a topological insulator, the eigenvalues of this Green's function attain zeros in the gap, whereas for a trivial insulator the eigenvalues remain nonzero. This topological classification is related to the existence of in-gap bound states along codimension-1 and codimension-2 impurities. Whereas codimension-1 impurities can be viewed as soft edges, the result for codimension-2 impurities is nontrivial and allows for a direct experimental measurement of the topological nature of two-dimensional insulators. [ABSTRACT FROM AUTHOR]

We theoretically investigate the electronic properties of the interface between quantum spin Hall (QSH) and quantum anomalous Hall (QAH) insulators. A robust chiral gapless state, which substantially differs from edge states of QSH or QAH insulators, is predicted at the QSH/QAH interface using an effective Hamiltonian model. We systematically reveal distinctive properties of interface states between QSH and single-valley QAH, multivalley high-Chern-number QAH and valley-polarized QAH insulators based on tight-binding models using the interface Green's function method. As an example, first-principles calculations are conducted for the interface states between fully and semihydrogenated bismuth (111) thin films, verifying the existence of interface states in realistic material systems. Due to the physically protected junction structure, the interface state is expected to be more stable and insensitive than topological boundary states against edge defects and chemical decoration. Hence our results of the interface states provide a promising route towards enhancing the performance and stability of low-dissipation electronics in real environment. [ABSTRACT FROM AUTHOR]

Gotze and Wolfle (GW) [Phys. Rev. B 6, 1226 (1972)] wrote the conductivity in terms of a memory function M(ω) as σ (ω + iη) = (ine²/m)[ω + M(ω + η)]-1 where M(ω + η) = i /r in the Drude limit. The analytic properties of -M(ω + η) are the same as those of the self-energy Σ of a retarded Green's function. In the approximate treatment of GW, --M closely resembles a self-energy with differences, e.g., the imaginary part is twice too large. The correct relation between --M and Σ is known for the electron-phonon case and is conjectured to be similar for other perturbations. When vertex corrections are ignored there is a known relation. A derivation using Matsubara temperature Green's functions is given. [ABSTRACT FROM AUTHOR]

An approach to compute exchange parameters of the Heisenberg model in plane-wave-based methods is presented. This calculation scheme is based on the Green's function method and Wannier function projection technique. It was implemented in the framework of the pseudopotential method and tested on such materials as NiO, FeO, Li2MnO3, and KCuF3. The obtained exchange constants are in a good agreement with both the total energy calculations and experimental estimations for NiO and KCuF3. In the case of FeO our calculations explain the pressure dependence of the Neel temperature. Li2MnO3 turns out to be a Slater insulator with antiferromagnetic nearest-neighbor exchange defined by the spin splitting. The proposed approach provides a unique way to analyze magnetic interactions, since it allows one to calculate orbital contributions to the total exchange coupling and study the mechanism of the exchange coupling. [ABSTRACT FROM AUTHOR]

We theoretically study the effects of surface roughness on the magnetic response of small unconventional superconductors by solving the Eilenberger equation for the quassiclassical Green function and the Maxwell equation for the vector potential simultaneously and self-consistently. The paramagnetic phase of spin-singlet d-wave superconducting disks is greatly suppressed by the surface roughness, whereas that of spin-triplet p-wave disks is robust even in the presence of the roughness. This difference derives from the orbital symmetry of paramagnetic odd-frequency Cooper pairs appearing at the surface of the disks. The orbital part of the paramagnetic pairing correlation has p-wave symmetry in the d-wave disks, whereas it has s -wave symmetry in the p-wave disks. Calculating the free energy, we also confirm that the paramagnetic state is more stable than the normal state, which indicates a possibility of detecting the paramagnetic effect in experiments. Indeed, our results are consistent with an experimental finding on high-Tc thin films. [ABSTRACT FROM AUTHOR]

Correlated materials have been studied extensively using photoemission spectroscopy. Their optical properties are instead much less explored. Here we present calculations of the optical absorption spectrum of vanadium dioxide (VO2) in the framework of the Bethe-Salpeter equation (BSE) of many-body perturbation theory. In order to deal with localized electrons we go beyond the standard BSE implementation and extend it to correlated insulators. We show that it is not enough to describe the spectra on the basis of independent electron-hole pairs, even when the electron and hole are separately well described by state-of-the-art one-body Green's functions. Crystal local-field effects are crucial to explain the experimental findings, even qualitatively, and excitonic effects strongly modify the spectra, especially at their onset. In this context, as higheighted by the analysis of the BSE results, the quasi-one-dimensional nature of the vanadium-dimer chains plays a prominent role. [ABSTRACT FROM AUTHOR]

The Kapitza or interfacial thermal resistance at the boundary of two different insulating solids depends on the transmission of phonons across the interface and the phonon dispersion of either material. We extend the existing atomistic Green's function (AGF) method to compute the probability for individual phonon modes to be transmitted across the interface. The extended method is based on the concept of the Bloch matrix and allows us to determine the wavelength and polarization dependence of the phonon transmission as well as to analyze efficiently the contribution of individual acoustic and optical phonon modes to interfacial thermal transport. The relationship between the phonon transmission probability and dispersion is explicitly established. A detailed description of the method is given and key formulas are provided. To illustrate the role of the phonon dispersion in interfacial thermal conduction, we apply the method to study phonon transmission and thermal transport at the armchair interface between monolayer graphene and hexagonal boron nitride. We find that the phonon transmission probability is high for longitudinal (LA) and flexural (ZA) acoustic phonons at normal and oblique incidence to the interface. At room temperature, the dominant contribution to interfacial thermal transport comes from the transverse-polarized phonons in graphene (45.5%) and longitudinal-polarized phonons in boron nitride (47.4%). [ABSTRACT FROM AUTHOR]

Here we present a theoretical study of the thermoelectric transport through [2,2]paracyclophane-based singlemolecule junctions. Combining electronic and vibrational structures, obtained from density functional theory (DFT), with nonequilibrium Green's function techniques allows us to treat both electronic and phononic transport properties at a first-principles level. For the electronic part, we include an approximate self-energy correction, based on the DFT+Σ approach. This enables us to make a reliable prediction of all linear response transport coefficients entering the thermoelectric figure of merit ZT. Paracyclophane derivatives offer a great flexibility in tuning their chemical properties by attaching different functional groups. We show that, for the specific molecule, the functional groups mainly influence the thermopower, allowing us to tune its sign and absolute value. We predict that the functionalization of the bare paracyclophane leads to a largely enhanced electronic contribution ZelT to the figure of merit. Nevertheless, the high phononic contribution to the thermal conductance strongly suppresses ZT. Our work demonstrates the importance to include the phonon thermal conductance for any realistic estimate of the ZT for off-resonant molecular transport junctions. In addition, it shows the possibility of a chemical tuning of the thermoelectric properties for a series of available molecules, leading to equally performing hole- and electron-conducting junctions based on the same molecular framework. [ABSTRACT FROM AUTHOR]

We investigate the influence of low dimensionality and disorder in phonon transport in ultranarrow armchair graphene nanoribbons (GNRs) using nonequilibrium Green's function (NEGF) simulation techniques. We specifically focus on how different parts of the phonon spectrum are influenced by geometrical confinement and line edge roughness. Under ballistic conditions, phonons throughout the entire phonon energy spectrum contribute to thermal transport. With the introduction of line edge roughness, the phonon transmission is reduced, but in a manner which is significantly nonuniform throughout the spectrum. We identify four distinct behaviors within the phonon spectrum in the presence of disorder: (i) the low-energy, low-wave vector acoustic branches have very long mean-free paths and are affected the least by edge disorder, even in the case of ultranarrow W = 1 nm wide GNRs; (ii) energy regions that consist of a dense population of relatively "flat" phonon modes (including the optical branches) are also not significantly affected, except in the case of the ultranarrow W = 1 nm GNRs, in which case the transmission is reduced because of band mismatch along the phonon transport path; (iii) "quasiacoustic" bands that lie within the intermediate region of the spectrum are strongly affected by disorder as this part of the spectrum is depleted of propagating phonon modes upon both confinement and disorder [resulting in sparse E(q) phononic band structure], especially as the channel length increases; and (iv) the strongest reduction in phonon transmission is observed in energy regions that are composed of a small density of phonon modes, in which case roughness can introduce transport gaps that greatly increase with channel length. We show that in GNRs of widths as small as W = 3 nm, under moderate roughness, both the low-energy acoustic modes and dense regions of optical modes can retain semiballistic transport properties, even for channel lengths up to L = 1 gm. These modes tend to completely dominate thermal transport. Modes in the sparse regions of the spectrum, however, tend to fall into the localization regime, even for channel lengths as short as tens of nanometers, despite their relatively high phonon group velocities. [ABSTRACT FROM AUTHOR]

We have developed a new method that is able to predict the electrical properties of the source and drain contacts in realistic carbon nanotube field effect transistors (CNTFETs). It is based on large-scale ab initio calculations combined with a Green function approach. For the first time, both internal and external parts of a realistic CNT-metal contact are taken into account at the ab initio level. We have developed the procedure allowing direct calculation of the self-energy for an extended contact. Within the method, it is possible to calculate the transmission coefficient through a contact of both finite and infinite length; the local density of states can be determined in both free and embedded CNT segments. We found perfect agreement with the experimental data for Pd and A1 contacts. We have explained why CNTFETs with Pd electrodes are p-type FETs with ohmic contacts, which can carry current close to the ballistic limit (provided contact length is large enough), whereas in CNT-A1 contacts transmission is suppressed to a significant extent, especially for holes. [ABSTRACT FROM AUTHOR]

In this work, we present a nonlocal expansion scheme to study correlated electron systems aiming at a better description of its spatial fluctuations at all length scales. Taking the nonlocal coupling as a perturbation to the local degrees of freedom, we show that the nonlocality in the self-energy function can be efficiently constructed from the coupling between local fluctuations. It can provide one unified framework to incorporate nonlocality to both ordered and disordered correlated many-body fermion systems. In this application, we prove that the dual-fermion approach can be understood as a special case of this nonlocal expansion scheme. The scheme presented in this work is constructed without introducing any dual variable, in which the interacting nature and the correlated behaviors of the lattice fermions have a clear physics correspondence. Thus, in this special case, the equivalence of the dual-fermion approach to the nonlocal expansion scheme beautifully reveals the physics origin of the dual variables. We show that the noninteracting dual-fermion Green's function corresponds exactly to a nonlocal coupling of the lattice fermion renormalized by the local single-particle charge fluctuations, and the dual-fermion self-energy behaves as the one-particle fully irreducible components of the lattice Green's function. Not only limited to this specific example, the nonlocal expansion scheme presented in this work can also be applied to other problems depending on the choice of the local degrees of freedom. [ABSTRACT FROM AUTHOR]

We investigate static correlation and delocalization errors in the self-consistent GW and random-phase approximation (RPA) by studying molecular dissociation of the H2 and LiH molecules. Although both approximations contain topologically identical diagrams, the nonlocality and frequency dependence of the GW self-energy crucially influence the different energy contributions to the total energy as compared to the use of a static local potential in the RPA. The latter leads to significantly larger correlation energies, which allow for a better description of static correlation at intermediate bond distances. The substantial error found in GW is further analyzed by comparing spin-restricted and spin-unrestricted calculations. At large but finite nuclear separation, their difference gives an estimate of the so-called fractional spin error normally determined only in the dissociation limit. Furthermore, a calculation of the dipole moment of the LiH molecule at dissociation reveals a large delocalization error in GW making the fractional charge error comparable to the RPA. The analyses are supplemented by explicit formulas for the GW Green's function and total energy of a simplified two-level model providing additional insights into the dissociation limit. [ABSTRACT FROM AUTHOR]

This work provides a unified theoretical treatment of the single- and correlated double-electron emission from a general electronic system. Using Feshbach projection method, the states of interest are selected by the projection operator; the Feshbach-Schur map determines the effective Hamiltonian and the optical potential for the emitted electrons. On the other hand, the nonequilibrium Green's functions method is demonstrated to be a complementary approach, and an explicit correspondence between both methods is established. For a self-contained exposition, some results on single-electron emission are rederived using both formalisms. New insights and results are obtained for the correlated electron-pair emission: This includes the effective two-electron Hamiltonian, the explicit form of the Feshbach self-energy in terms of the many-body self-energies, and the diagrammatic expansion of the two-particle current. As an illustration of the diagrammatic technique, the process of the two-particle emission assisted by the excitation of plasmons is explicitly worked out. [ABSTRACT FROM AUTHOR]

The accuracy of the many-body perturbation theory GW formalism to calculate electron-phonon coupling matrix elements has been recently demonstrated in the case of a few important systems. However, the related computational costs are high and thus represent strong limitations to its widespread application. In the present study, we explore two less demanding alternatives for the calculation of electron-phonon coupling matrix elements on the many-body perturbation theory level. Namely, we test the accuracy of the static Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the constant screening approach, where variations of the screened Coulomb potential W upon small changes of the atomic positions along the vibrational eigenmodes are neglected. We find this latter approximation to be the most reliable, whereas the static COHSEX ansatz leads to substantial errors. Our conclusions are validated in a few paradigmatic cases: diamond, graphene, and the C60 fullerene. These findings open the way for combining the present many-body perturbation approach with efficient linear-response theories. [ABSTRACT FROM AUTHOR]

Density matrix embedding theory (DMET) [Phys. Rev. Lett. 109, 186404 (2012)], introduced an approach to quantum cluster embedding methods whereby the mapping of strongly correlated bulk problems to an impurity with finite set of bath states was rigorously formulated to exactly reproduce the entanglement of the ground state. The formalism provided similar physics to dynamical mean-field theory at a tiny fraction of the cost but was inherently limited by the construction of a bath designed to reproduce ground-state, static properties. Here, we generalize the concept of quantum embedding to dynamic properties and demonstrate accurate bulk spectral functions at similarly small computational cost. The proposed spectral DMET utilizes the Schmidt decomposition of a response vector, mapping the bulk dynamic correlation functions to that of a quantum impurity cluster coupled to a set of frequency-dependent bath states. The resultant spectral functions are obtained on the real-frequency axis, without bath discretization error, and allows for the construction of arbitrary dynamic correlation functions. We demonstrate the method on the one- (ID) and two-dimensional (2D) Hubbard model, where we obtain zero temperature and thermodynamic limit spectral functions, and show the trivial extension to two-particle Green's functions. This advance therefore extends the scope and applicability of DMET in condensed-matter problems as a computationally tractable route to correlated spectral functions of extended systems and provides a competitive alternative to dynamical mean-field theory for dynamic quantities. [ABSTRACT FROM AUTHOR]

We study the nonequilibrium Keldysh Green's function for an /V-orbital Anderson model at high bias voltages, extending a previous work which for the case only with the spin degrees of freedom N = 2 to arbitrary N. Our approach uses an effective non-Hermitian Hamiltonian that is defined with respect to a Liouville-Fock space in the context of a thermal field theory. The result correctly captures the relaxation processes at high energies, and is asymptotically exact not only in the high bias limit, but also in the high-temperature limit at thermal equilibrium. We also present an explicit continued-fraction representation of the Green's function. It clearly shows that the imaginary part is recursively determined by the decay rate of intermediate states with at most N -- 1 particle-hole-pair excitations. These high bias properties follow from the conservations of a generalized charge and current in the Liouville-Fock space. We also examine temperature dependence of the spectral function in equilibrium, comparing the exact results with the numerical finite-7 and analytical T → ∞ results of the noncrossing approximation. [ABSTRACT FROM AUTHOR]

We present a method, based on characterizing efficiency fluctuations, to assess the performance of nanoscale thermoelectric junctions. This method accounts for effects typically arising in small junctions, namely, stochasticity in the junction's performance, quantum effects, and nonequilibrium features preventing a linear response analysis. It is based on a nonequilibrium Green's function (NEGF) approach, which we use to derive the full counting statistics (FCS) for heat and work, and which in turn allows us to calculate the statistical properties of efficiency fluctuations. We simulate the latter for a variety of simple models where our method is exact. By analyzing the discrepancies with the semiclassical prediction of a quantum master equation (QME) approach, we emphasize the quantum nature of efficiency fluctuations for realistic junction parameters. We finally propose an approximate Gaussian method to express efficiency fluctuations in terms of nonequilibrium currents and noises which are experimentally measurable in molecular junctions. [ABSTRACT FROM AUTHOR]

In a recent paper [Phys. Rev. B 90, 115134 (2014)] we put forward a diagrammatic expansion for the self-energy which guarantees the positivity of the spectral function. In this work we extend the theory to the density-response function. We write the generic diagram for the density-response spectrum as the sum of "partitions." In a partition the original diagram is evaluated using time-ordered Green's functions on the left half of the diagram, antitime-ordered Green's functions on the right half of the diagram, and lesser or greater Green's function gluing the two halves. As there exists more than one way to cut a diagram in two halves, to every diagram corresponds more than one partition. We recognize that the most convenient diagrammatic objects for constructing a theory of positive spectra are the half-diagrams. Diagrammatic approximations obtained by summing the squares of half-diagrams do indeed correspond to a combination of partitions which, by construction, yield a positive spectrum. We develop the theory using bare Green's functions and subsequently extend it to dressed Green's functions. We further prove a connection between the positivity of the spectral function and the analytic properties of the polarizability. The general theory is illustrated with several examples and then applied to solve the long-standing problem of including vertex corrections without altering the positivity of the spectrum. In fact already the first-order vertex diagram, relevant to the study of gradient expansion, Friedel oscillations, etc., leads to spectra which are negative in certain frequency domain. We find that the simplest approximation to cure this deficiency is given by the sum of the zeroth-order bubble diagram, the first-order vertex diagram, and a partition of the second-order ladder diagram. We evaluate this approximation in the three-dimensional homogeneous electron gas and show the positivity of the spectrum for all frequencies and densities. [ABSTRACT FROM AUTHOR]

Electrons, holes, and photons in semiconductors are interacting fermions and bosons. In this system, a variety of ordered coherent phases can be formed through the spontaneous phase symmetry breaking because of their interactions. The Bose-Einstein condensation (BEC) of excitons and polaritons is one of such coherent phases, which can potentially cross over into the Bardeen-Cooper-Schrieffer (BCS) type ordered phase at high densities under quasiequilibrium conditions, known as the BCS-BEC crossover. In contrast, one can find the semiconductor laser, superfluorescence (SF), and superradiance as relevant phenomena under nonequilibrium conditions. In this paper, we present a comprehensive generating functional theory that yields nonequilibrium Green's functions in a rigorous way. The theory gives us a starting point to discuss these phases in a unified view with a diagrammatic technique. Comprehensible time-dependent equations are derived within the Hartree-Fock approximation, which generalize the Maxwell-semiconductor-Bloch equations under the relaxation time approximation. With the help of this formalism, we clarify the relationship among these cooperative phenomena and we show theoretically that the Fermi-edge SF is directly connected to the e-h BCS phase. We also discuss the emission spectra as well as the gain-absorption spectra. [ABSTRACT FROM AUTHOR]

Dopants positioned near edges in nanostructured graphene behave differently from bulk dopants. Most notable, the amount of charge transferred to delocalized states (i.e., doping efficiency) depends on position as well as edge chirality. We apply a self-consistent tight-binding model to analyze this problem focusing on substitutional nitrogen and boron doping. Using a Green's-function technique, very large structures can be studied, and artificial interactions between dopants in periodically repeated simulations cells are avoided. We find pronounced signatures of edges in the local impurity density of states. Importantly, the doping efficiency is found to oscillate with sublattice position, in particular, for dopants near zigzag edges. Finally, to assess the effect of electronelectron interactions, we compute the self-energy corrected Green's function [ABSTRACT FROM AUTHOR]

Conditions for the appearance of quantized anisotropic magnetoresistance (QAMR) in ferromagnetic conductors have been investigated from the viewpoint of the transmission channels calculated by method of the nonequilibrium Green's function-density functional theory. We demonstrate that the spin-orbital interaction is a precondition and the transverse size is a crucial condition for QAMR to emerge. Furthermore, we show the evolution of QAMR from a stepwise shape (corresponding to small transverse sizes) to a classical smooth cosine shape (corresponding to large transverse sizes). In addition, we prove the strong temperature dependence of QAMR, so a low temperature is necessary. Our research reconciles the different experiments and enlightens experimenters on QAMR. [ABSTRACT FROM AUTHOR]

Nonequilibrium dynamical mean-field theory (DMFT) solves correlated lattice models by obtaining their local correlation functions from an effective model consisting of a single impurity in a self-consistently determined bath. The recently developed mapping of this impurity problem from the Keldysh time contour onto a time-dependent single-impurity Anderson model (SIAM) [C. Gramsch et al., Phys. Rev. B 88, 235106 (2013)] allows one to use wave-function-based methods in the context of nonequilibrium DMFT. Within this mapping, long times in the DMFT simulation become accessible by an increasing number of bath orbitals, which requires efficient representations of the time-dependent SIAM wave function. These can be achieved by the multiconfiguration time-dependent Hartree (MCTDH) method and its multilayer extensions. We find that MCTDH outperforms exact diagonalization for large baths in which the latter approach is still within reach and allows for the calculation of SIAMs beyond the system size accessible by exact diagonalization. Moreover, we illustrate the computation of the self-consistent two-time impurity Green s function within the MCTDH second quantization representation. [ABSTRACT FROM AUTHOR]

Motivated by experimental evidence for mixed-valence correlations affecting the neutralization of strontium ions on gold surfaces, we set up an Anderson-Newns model for the Sr: Au system and calculate the neutralization probability a as a function of temperature. We employ quantum-kinetic equations for the projectile Green functions in the finite- U noncrossing approximation. Our results for a agree reasonably well with the experimental data as far as the overall order of magnitude is concerned, showing in particular the correlation-induced enhancement of a. The experimentally found nonmonotonous temperature dependence, however, could not be reproduced. Instead of an initially increasing and then decreasing a, we find over the whole temperature range only a weak negative temperature dependence. It arises, however, clearly from a mixed-valence resonance in the projectile's spectral density and thus supports qualitatively the interpretation of the experimental data in terms of a mixed-valence scenario. [ABSTRACT FROM AUTHOR]

We analytically study vacancy effects on electronic and transport properties of graphene nanoribbons and nanodots using Green's function approach. For semiconducting systems, the presence of a vacancy induces a zero-energy midgap state. The spatial pattern of the wave functions critically depends on the atomistic edge structures and can be used as an unambiguous probe of the edge structure. For metallic systems, the midgap vacancy state does not exist. In these systems, the vacancy mainly works as a source of electronic scattering and modifies electronic transmission. We derive that the electronic transmission coefficient can be written as T = cos2(a), where a denotes the phase angle of the on-site Green's function at the vacancy site of the ideal systems. At small energies, T exhibits distinctly different functional form depending on edge structures. [ABSTRACT FROM AUTHOR]

A generalized numerical model for the phonon-assisted optical interband transition based on the Green's function formalism was developed and implemented to investigate optical processes in germanium and silicon media intended for on-chip light emitter and laser applications. High-fidelity full band structures obtained from the empirical pseudopotential method, self-energies, and the corresponding spectral density functions for the phonon-perturbed electron and holes have been computed numerically as a function of strain, temperature, and doping level. Validation has been carried out by showing the model's ability to accurately reproduce the measured temperature dependent absorption coefficient data for both germanium and silicon. Absorption coefficients, radiative recombination rates of germanium and silicon active media were investigated with different biaxial tensile strain, doping concentrations and injection conditions. Furthermore, when the model is employed to compute the optical gain in strained germanium, we find that the use of tensile strain and high injection are the preferable approaches to obtain population inversion. At the same time, strong absorption from the spin-orbit to the heavy-hole band limits the maximum injection density that can be applied. Finally, when applied to study silicon, the proposed model also successfully reproduces the experimentally observed radiative recombination peak due to the two-phonon process. [ABSTRACT FROM AUTHOR]

A theoretical framework for pump-probe photoemission is presented. The approach is based on a general formulation using the Keldysh formalism for the lesser Green's function to describe the real-time evolution of the electronic degrees of freedom in the initial state after a strong pump pulse that drives the system out of equilibrium. The final state is represented by a time-reversed low-energy electron-diffraction state. Our one-step description is related as close as possible to Pendry's original formulation of the photoemission process. The formalism allows for a quantitative calculation of time-dependent photocurrent for simple metals where a picture of effectively independent electrons is assumed to be reliable. The theory is worked out for valence- and core-electron excitations. It comprises the study of different relativistic effects as a function of the pump-probe delay. [ABSTRACT FROM AUTHOR]

We study properties of a helimagnetic thin film with a quantum Heisenberg spin model by using the Green's-function method. Surface spin configuration is calculated by minimizing the spin interaction energy. It is shown that the angles between spins near the surface are strongly modified with respect to the bulk configuration. Taking into account this surface spin reconstruction, we calculate self-consistently the spin-wave spectrum and the layer magnetizations as functions of temperature up to the disordered phase. The spin-wave spectrum shows the existence of a surface-localized branch which causes a low surface magnetization. We show that quantum fluctuations give rise to a crossover between the surface magnetization and interior-layer magnetizations at low temperatures. We calculate the transition temperature and show that it depends strongly on the helical angle. Results are in agreement with existing experimental observations on the stability of helical structure in thin films and on the insensitivity of the transition temperature with the film thickness. We also study effects of various parameters such as surface exchange and anisotropy interactions. Monte Carlo simulations for the classical spin model are also carried out for comparison with the quantum theoretical result. [ABSTRACT FROM AUTHOR]

We study the spin polarized transport through Mn-decorated 8-5-5-8 topological line defects in graphene using the nonequilibrium Green's function formalism. Strong preferential bonding overcomes the high mobility of transition metal atoms on graphene and results in stable structures. Despite a large distance between the magnetic centers, we find a high magnetoresistance and attribute this unexpected property to very strong induced π magnetism, in particular for full coverage of all octagonal hollow sites by Mn atoms. In contrast to the magnetoresistance of graphene nanoribbon edges, the proposed system is well controlled and therefore suitable for applications. [ABSTRACT FROM AUTHOR]

We study the hot-electron noise properties of two-dimensional materials with a graphene-like energy dispersion under a strong applied electric field which drives the system far from equilibrium. Calculations are based on a Boltzmann–Green-function method within a two-relaxation-time approximation that allows for both inelastic scattering coming from electron-phonon scattering and elastic scattering coming from electron-impurity scattering. The steady-state distribution function is used to calculate the average current and the low-frequency spectral density for current fluctuations (noise) in the nonequilibrium steady-state. We find that as the electric field strength increases, the noise decreases from its equilibrium thermal noise value. This is in contrast with semiconductors with a quadratic energy–wave-vector dispersion where the noise increases in a constant-relaxation-time model with the square of the electric field due to the Joule heating of the electron gas by the electric field. We have also studied these properties for an electronic dispersion with a gap introduced into the Dirac spectrum. The inclusion of the gap in the electronic dispersion causes an initial increase in the noise as a function of external electric field due to the heating of the electron gas for large gap values. At high electric fields, the noise decreases with increasing electric field as in the case of gapless dispersion at higher fields. [ABSTRACT FROM AUTHOR]

We consider single-particle properties in the one-dimensional repulsive Hubbard model at commensurate fillings in the metallic phase. We determine the real-time evolution of the retarded Green's function by matrix-product state methods. We find that at sufficiently late times the numerical results are in good agreement with predictions of nonlinear Luttinger liquid theory. We argue that combining the two methods provides a way of determining the single-particle spectral function with very high frequency resolution. [ABSTRACT FROM AUTHOR]