Electron Spectral Density Research Articles

Ultra-Low Noise Level Infrared Quantum Dot Photodiodes with Self-Screenable Polymeric Optical Window.

Quantum dot photodiodes (QPDs) have garnered significant attention because of their unparalleled near-infrared (NIR) detection capabilities, primarily attributable to their size-dependent bandgap tunability. Nevertheless, the broadband absorption spectrum of QPD engenders substantial noise floor within superfluous visible light regions, notably hindering their use in several emerging applications necessitating the detection of faint micro-light signals. To overcome these hurdles, a self-screenable NIR QPD featuring an internal optical filter with a thick polymeric interlayer to reduce electronic noise is demonstrated. This effectively screens out undesirable visible light regions while reducing the ionized defect owing to decreased density of state, yielding an extremely low dark current (≈1010 A cm-2 at V = -1V). Consequently, the electronic noise spectral density is attained at levels below ≈10-27 -10-28 A2 Hz-1 , and responsivity (R) dropped to 92% within the visible light spectrum.

Electron-to-nuclear spectral mapping via dynamic nuclear polarization.

We report on a strategy to indirectly read out the spectrum of an electronic spin via polarization transfer to nuclear spins in its local environment. The nuclear spins are far more abundant and have longer lifetimes, allowing for repeated polarization accumulation in them. Subsequent nuclear interrogation can reveal information about the electronic spectral density of states. We experimentally demonstrate the method by reading out the ESR spectrum of nitrogen vacancy center electrons in diamond via readout of lattice 13C nuclei. Spin-lock control on the 13C nuclei yields a significantly enhanced signal-to-noise ratio for the nuclear readout. Spectrally mapped readout presents operational advantages in being background-free and immune to crystal orientation and optical scattering. We harness these advantages to demonstrate applications in underwater magnetometry. The physical basis for the "one-to-many" spectral map is itself intriguing. To uncover its origin, we develop a theoretical model that maps the system dynamics, involving traversal of a cascaded structure of Landau-Zener anti-crossings, to the operation of a tilted "Galton board." This work points to new opportunities for "ESR-via-NMR" in dilute electronic systems and in hybrid electron-nuclear quantum memories and sensors.

Electron Pumping and Spectral Density Dynamics in Energy-Gapped Topological Chains

Electron pumping through energy-gapped systems is restricted for vanishing local density of states at the Fermi level. In this paper, we propose a topological Su–Schrieffer–Heeger (SSH) chain between unbiased leads as an effective electron pump. We analyze the electron transport properties of topologically trivial and nontrivial systems in the presence of external time-dependent forces in the form of one-Gaussian or two-Gaussian perturbations (train impulses). We have found that the topologically trivial chain stands for much better charge pump than other normal or nontrivial chains. It is important that, during the perturbation, electrons are pumped through the mid-gap temporary states or through the induced sidebands states outside the energy gap. We also analyze the local density of states dynamics during the quench transition between different topological phases of the SSH chain. It turns out that after the quench, the edge topological states migrate through other sites and can temporarily exist in a topologically trivial part of the system. The tight-binding Hamiltonian and the evolution operator technique are used in our calculations.

Reproducing the low-temperature excitation energy transfer dynamics of phycoerythrin 545 light-harvesting complex with a structure-based model Hamiltonian

To elucidate the energy transfer mechanism of the PE545 light-harvesting complex, an exciton model is constructed with the full Hamiltonian obtained from structure-based calculations. The electronic couplings and spectral densities are evaluated on the basis of the site energies and transition dipole moments obtained from our recent Molecular Dynamics-Quantum Mechanical/Molecular Mechanical (MD-QM/MM) study [Tong et al., J. Phys. Chem. B 123, 2040-2049 (2019)]. The polarized protein-specific charge model is employed both in the MD simulation and in the QM/MM calculations to account for the environmental fluctuation of the protein scaffold. The energy transfer pathways are, thus, derived, which agree well with the phenomenological models based on the spatial organization of the chromophores and the experimental observations. Moreover, the simulated linear absorption spectra using the dissipaton equation of motion approach agree well with the experimental ones, and the resulting population dynamics indicates that an optimal energy transfer efficiency is reproduced.

Manipulating Majorana zero modes in double quantum dots

Majorana zero modes (MZMs) emerging at the edges of topological superconducting wires have been proposed as the building blocks of novel, fault-tolerant quantum computation protocols. Coherent detection and manipulation of such states in scalable devices are, therefore, essential in these applications. Recent detection proposals include semiconductor quantum dots (QDs) coupled to the end of these wires, as changes in the QD electronic spectral density due to the MZM coupling could be detected in transport experiments. Here, we propose that multi-QD systems can also be used to manipulate MZMs through precise control over the QDs' parameters. The simplest case where Majorana manipulation is possible is in a double quantum dot (DQD) geometry. By using exact analytical methods and numerical renormalization-group calculations, we show that the QDs' spectral functions can be used to characterize the presence or not of MZMs "leaking" into the DQD. More importantly, we find that these signatures respond to changes in the DQD parameters such as gate-voltages and couplings in a consistent fashion. Additionally, we show that different MZM-DQD coupling geometries ("symmetric" , "in-series" and "T-shaped" junctions) offer distinct ways in which MZMs can be switched from dot to dot. These results highlight the interesting possibilities that DQDs offer for all-electrical MZM control in scalable devices.

Probing the interaction between 2,2'-bithiophene-5-carboxylic acid and TiO2 by photoelectron spectroscopy: A joint experimental and theoretical study.

The interaction between 2,2'-bithiophene-5-carboxylic acid (PT2) sublimed under ultra-high vacuum conditions and anatase (101) and rutile (110) TiO2 single crystal surfaces is investigated by studying the electronic spectral density near the Fermi level with synchrotron-based spectroscopy. The experimental results are compared to density functional theory calculations of the isolated PT2 molecule and of the molecule adsorbed on an anatase TiO2 (101) cluster. The relative concentrations of Ti, C, and S atoms indicate that the adsorbed molecule remains intact upon deposition, which is typical of a Stranski-Krastanov growth mode. The analysis of the O1s spectrum suggests a predominant bidentate geometry of the adsorption with both rutile and anatase surfaces, as supported by previous theoretical simulations. It is also theoretically and experimentally demonstrated that the PT2 adsorption causes the appearance of new electronic states in the gap near the TiO2 valence band. A pinning effect of the LUMO level of the dye is also theoretically predicted.

Spectral density of Cooper pairs in two level quantum dot–superconductors Josephson junction

In the present paper, we report the role of quantum dot energy levels on the electronic spectral density for a two level quantum dot coupled to s-wave superconducting leads. The theoretical arguments in this work are based on the Anderson model so that it necessarily includes dot energies, single particle tunneling and superconducting order parameter for BCS superconductors. The expression for single particle spectral function is obtained by using the Green's function equation of motion technique. On the basis of numerical computation of spectral function of superconducting leads, it has been found that the charge transfer across such junctions can be controlled by the positions and availability of the dot levels.

Van Hove singularities and spectral smearing in high-temperature superconductingH3S

The superconducting phase of hydrogen sulfide at Tc=200 K observed by Drozdov and collaborators at pressures around 200 GPa is simple bcc Im-3m H3S, predicted beforehand by Duan {\it et al.}, has experimental confirmation. The various "extremes" that are involved -- pressure, implying extreme reduction of volume, extremely high H phonon energy scale around 1500K, extremely high temperature for a superconductor -- necessitates a close look at new issues raised by these characteristics in relation to high Tc. We use first principles methods to analyze the H3S electronic structure, particularly the van Hove singularities (vHs) and the effect of sulfur. Focusing on the two closely spaced vHs near the Fermi level that give rise to the impressively sharp peak in the density of states, the implications of strong coupling Migdal-Eliashberg theory are assessed. The electron spectral density smearing due to virtual phonon emission and absorption needs to be included explicitly to obtain accurate theoretical predictions and current understanding. Means for increasing Tc in H3S-like materials are addressed.

Isotope effect on the superconducting critical temperature of cuprates in the presence of charge order

Using the large-N limit of the t–J model and also allowing for phonons and the electron–phonon interaction, we study the isotope effect α for coupling constants appropriate for YB2C3Oy. We find that α has a minimum at optimal doping and increases strongly (slightly) towards the underdoped (overdoped) region. Using values for the electron–phonon interaction from the local density approximation we get good agreement for α as a function of Tc and doping δ with recent experimental data in YB2C3Oy. Our results strongly suggest that the large increase of α in the underdoped region is (a) caused by the shift of electronic spectral density from low to high energies associated with a competing phase (in our case a charge density wave) and the formation of a gap, and (b) compatible with the small electron–phonon coupling constants obtained from the local density approximation. We propose a similar explanation for the anomalous behavior of α in Sr-doped La2CuO4 near the doping 1/8.

SYNCHROTRON AND COMPTON SPECTRA FROM A STEADY-STATE ELECTRON DISTRIBUTION

Energy densities of relativistic electrons and protons in extended galactic and intracluster regions are commonly determined from spectral radio and (rarely) $\gamma$-ray measurements. The time-independent particle spectral density distributions are commonly assumed to have a power-law (PL) form over the relevant energy range. A theoretical relation between energy densities of electrons and protons is usually adopted, and energy equipartition is invoked to determine the mean magnetic field strength in the emitting region. We show that for typical conditions, in both star-forming and starburst galaxies, these estimates need to be scaled down substantially due to significant energy losses that (effectively) flatten the electron spectral density distribution, resulting in a much lower energy density than deduced when the distribution is assumed to have a PL form. The steady-state electron distribution in the nuclear regions of starburst galaxies is calculated by accounting for Coulomb, bremsstrahlung, Compton, and synchrotron losses; the corresponding emission spectra of the latter two processes are calculated and compared to the respective PL spectra. We also determine the proton steady-state distribution by taking into account Coulomb and pion production losses, and briefly discuss implications of our steady-state particle spectra for estimates of proton energy densities and magnetic fields.

Spin-orbit effects in two-electron emission from ferromagnetic surfaces

In previous experiments on electron-induced two-electron emission from the ferromagnetic surface system Co/W(110), spin-orbit coupling effects have been observed, which are comparable in size to the magnetic exchange effects. The present theoretical work aims at a detailed understanding of such effects, in particular their relation to the spin-dependent electronic structure and to collision dynamics. To this end, we have developed a formalism, which is based on a Dirac equation with an effective magnetic field. Magnetic exchange and spin-orbit coupling are thus incorporated simultaneously. Typical numerical two-electron emission results are presented for W(110) and for ultrathin Co films on W(110) together with the underlying spin- and layer-resolved valence electron spectral density. More detailed insight is provided by calculations, in which SOC was selectively switched off for the valence electron and for the primary and emitted electrons. Our theoretical results are in overall agreement with the experimental data. Furthermore, we predict sizable magnetic dichroism.

Interface superconductor with gap behaviour like a high-temperature superconductor

The physics of the superconducting state in two-dimensional (2D) electron systems is relevant to understanding the high-transition-temperature copper oxide superconductors and for the development of future superconductors based on interface electron systems. But it is not yet understood how fundamental superconducting parameters, such as the spectral density of states, change when these superconducting electron systems are depleted of charge carriers. Here we use tunnel spectroscopy with planar junctions to measure the behaviour of the electronic spectral density of states as a function of carrier density, clarifying this issue experimentally. We chose the conducting LaAlO3-SrTiO3 interface as the 2D superconductor, because this electron system can be tuned continuously with an electric gate field. We observed an energy gap of the order of 40 microelectronvolts in the density of states, whose shape is well described by the Bardeen-Cooper-Schrieffer superconducting gap function. In contrast to the dome-shaped dependence of the critical temperature, the gap increases with charge carrier depletion in both the underdoped region and the overdoped region. These results are analogous to the pseudogap behaviour of the high-transition-temperature copper oxide superconductors and imply that the smooth continuation of the superconducting gap into pseudogap-like behaviour could be a general property of 2D superconductivity.

Magnetic coupling of Fe-porphyrin molecules adsorbed on clean andc(2×2) oxygen-reconstructed Co(100) investigated by spin-polarized photoemission spectroscopy

The spin-polarized electronic structure of iron octaethylporphyrin (FeOEP) molecules adsorbed on a pristine and on a $c$($2\ifmmode\times\else\texttimes\fi{}2$) oxygen-reconstructed Co(100) surface has been analyzed by means of spin-polarized photoemission spectroscopy (SPPES) and first-principles density functional theory with the on-site Coulomb repulsion $U$ term ($\mathrm{DFT}+U$) calculations with and without Van der Waals corrections. The aim is to examine the magnetic exchange mechanism between the FeOEP molecules and the Co(100) substrate in the presence or absence of the oxygen mediator. The results demonstrate that the magnetic coupling from the ferromagnetic substrate to the adsorbed FeOEP molecules is ferromagnetic, whereas, the coupling is antiferromagnetic for the FeOEP on the $c$($2\ifmmode\times\else\texttimes\fi{}2$)O/Co(100) system. Spin-resolved partial densities of states extracted from ab initio $\mathrm{DFT}+U$ modeling are in fairly good comparison with the electronic spectral densities seen in angle-integrated SPPES energy dispersion curves for submonolayer coverages of FeOEP. Through combined analysis of these spectra and theoretical results, we determine that hybridization of 2$p$ orbitals of N and O with Co 3$d$ orbitals facilitates indirect magnetic exchange interactions between Fe and Co, whereas, a direct Fe-Co interaction involving the Fe ${d}_{{z}^{2}}$ orbital is also found for FeOEP on Co. It is observed through SPPES that the spin polarization of the photoemission-visible molecular overlayers decreases to zero as coverage is increased beyond the submonolayer regime, indicating that only interfacial magnetic coupling is at work. Microspot low-energy electron diffraction and low-energy electron microscopy were performed to characterize the physical order of the molecular coverage, revealing that FeOEP structural domains are orders of magnitude greater in size on $c$($2\ifmmode\times\else\texttimes\fi{}2$)O/Co(100) than on clean Co(100), which coincides with reduced scattering from the disorder and sharper features seen in SPPES.

GINZBURG–LANDAU LIKE THEORY OF HIGH TEMPERATURE SUPERCONDUCTIVITY IN THE CUPRATES: EMERGENT d-WAVE ORDER

High temperature superconductivity in the cuprates remains one of the most widely investigated, constantly surprising and poorly understood phenomena in physics. Here, we describe briefly a new phenomenological theory inspired by the celebrated description of superconductivity due to Ginzburg and Landau and believed to describe its essence. This posits a free energy functional for the superconductor in terms of a complex order parameter characterizing it. We propose that there is, for superconducting cuprates, a similar functional of the complex, in plane, nearest neighbor spin singlet bond (or Cooper) pair amplitude ψij. Further, we suggest that a crucial part of it is a (short range) positive interaction between nearest neighbor bond pairs, of strength J′. Such an interaction leads to nonzero long wavelength phase stiffness or superconductive long range order, with the observed d-wave symmetry, below a temperature Tc~z J′ where z is the number of nearest neighbors; d-wave superconductivity is thus an emergent, collective consequence. Using the functional, we calculate a large range of properties, e.g., the pseudogap transition temperature T* as a function of hole doping x, the transition curve Tc(x), the superfluid stiffness ρs(x, T), the specific heat (without and with a magnetic field) due to the fluctuating pair degrees of freedom and the zero temperature vortex structure. We find remarkable agreement with experiment. We also calculate the self-energy of electrons hopping on the square cuprate lattice and coupled to electrons of nearly opposite momenta via inevitable long wavelength Cooper pair fluctuations formed of these electrons. The ensuing results for electron spectral density are successfully compared with recent experimental results for angle resolved photo emission spectroscopy (ARPES), and comprehensively explain strange features such as temperature dependent Fermi arcs above Tc and the "bending" of the superconducting gap below Tc.

Distinguishing Coulomb and electron-phonon interactions for massless Dirac fermions

While many physical properties of graphene can be understood qualitatively on the basis of bare Dirac bands, there is specific evidence that electron-electron (EE) and electron-phonon (EP) interactions can also play an important role. We discuss strategies for extracting separate images of the EE and EP interactions as they present themselves in the electron spectral density and related self-energies. While for momentum, $k$, equal to its Fermi value, ${k}_{F}$, a composite structure is obtained that can be difficult to separate into its two constituent parts, at smaller values of $k$ the spectral function shows distinct incoherent sidebands on the left and right of the main quasiparticle line. These image, respectively, the EE and EP interactions, each being most prominent in its own energy window. We employ a maximum entropy inversion technique on the self-energy to reveal the electron-phonon spectral density separate from the excitation spectrum due to Coulomb correlations. Our calculations show that this technique can provide important new insights into inelastic-scattering processes in graphene.

Phenomenological Ginzburg-Landau-like theory for superconductivity in the cuprates

We propose a phenomenological Ginzburg-Landau-like theory of cuprate superconductivity. The free energy is expressed as a functional F of the spin-singlet pair amplitude psi_ij=psi_m=Delta_m exp(i phi_m); i and j are nearest-neighbor sites of the Cu lattice in which the superconductivity is believed to primarily reside and m labels the site at the center of the bond between i and j. The system is modeled as a weakly coupled stack of such planes. We hypothesize a simple form, F[Delta,phi]=sum_m (A Delta_m^2+ B Delta_m^4/2)+C sum_<mn> Delta_m Delta_n cos(phi_m-phi_n), for the functional. The coefficients A, B and C are determined from comparison with experiments. We work out a number of consequences of the proposed functional for specific choices of A, B and C as functions of hole density x and temperature T. There can be a rapid crossover of <Delta_m> from small to large values as A changes sign on lowering T and the crossover temperatures is identified with the observed pseudogap temperature. The superconducting phase-coherence transition occurs at a different temperature T_c, and describes superconductivity with d-wave symmetry for C>0. We calculate T_c(x) which has the observed parabolic shape, being strongly influenced by the coupling between Delta_m and phi_m present in F. The superfluid density, the local gap magnitude, the specific heat (with and without a magnetic field) and vortex properties are obtained using F. We compare our results successfully with experiments. We also obtain the electron spectral density as influenced by the coupling between the electrons and the pair correlation function calculated from F. Features such as temperature dependent Fermi arcs, antinodal pseudogap filling temperature, pseudogapped density of states in different momentum regions of the Fermi surface and `bending' of the energy gap versus momentum on the Fermi surface emerge from the theory.

Insulating state of electron-doped Cu-phthalocyanine layers

A copper-phthalocyanine (CuPc) thin film and a highly ordered CuPc single layer prepared on the Au(110) surface are electron doped by exposition to alkali metal. The CuPc thin film remains insulating when the donated electrons flow into the unoccupied molecular orbitals. The CuPc/Au interface is metallic in the absence of doping, and transition to an insulating state is observed by the filling up of the former empty states. In particular, two occupied states are generated by filling the lowest unoccupied molecular orbitals with the doping electrons. Sweeping of the former empty states through the Fermi energy does not induce any enhancing of the electronic spectral density, as detected by high-resolution photoelectron spectroscopy. Electron correlation and Jahn-Teller effects act to determine the insulating response of electron-doped CuPc.

Non-delta-function electronic spectral densities in individual quantum dots

Using a simplified model approach we estimate the optical line shape of the transition lines observable in photoluminescence experiments on quantum dots. We use the theory based on the interaction of electrons with the longitudinal optical phonons only. This theory gives, in the self-consistent Born approximation, the lowest-energy excited state line shape in the form of a very narrow peak with a shoulder on the low-energy side. We turn the attention to a comparison with experiments which appear to support this theoretical conclusion. This agreement emphasizes the role of the electronic multiple scattering on optical phonons in quantum dots. It is demonstrated that the optical line shape can give an information about the quantum dot system.

Electronic spectral densities in quantum dots

The spectral line profiles of the luminescence transitions in individual quantum dots have been studied both theoretically and experimentally in the recent years. In these works the spectral line profiles have been shown to be clearly different from the simple form of a broadened delta function of frequency variable. In the present work we turn attention to these earlier measurements and calculations and compare them with the electronic spectral line profiles in individual quantum dots calculated under the assumption of quantum dot electrons multiple scattering on the longitudinal optical phonons. The calculated luminescence spectral line profiles of the optical transition will be expressed with help of the electronic spectral densities dependent on the electronic statistical distribution, as it develops in the course of the relaxation processes or other kinetic processes in quantum dots.

Low-energy properties of the ferromagnetic metallic phase in manganites: Slave-fermion approach to the quantum double-exchange model

We study the low-energy properties of the one-orbital quantum double-exchange model by using the slave fermion formulation. We construct a mean-field theory which gives a simple explanation for the magnetic and thermodynamic properties of the ferromagnetic metallic phase in manganites at low energy. The resulting electron spectral function and tunneling density of states show an incoherent asymmetric peak with weak temperature dependence, in addition to a quasiparticle peak. We also show that the gauge fluctuations in the ferromagnetic metallic phase are completely screened due to the Anderson-Higgs mechanism. Therefore, the mean-field state is robust against gauge fluctuations and exhibits spin-charge separation at low energy.