Strongly reduced band gap in a correlated insulator in close proximity to a metal

Hesper, Ronald; Tjeng, L. H.; Sawatzky, G. A.

doi:10.1209/epl/i1997-00442-2

Cited by 133 publications

(152 citation statements)

References 42 publications

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“…The only exception is for the HOMO level, which is equal to the negative of the ionization potential. [39][40][41] It has been demonstrated experimentally [42][43][44][45][46] that the quasi-particle energy gap, E gap QP , of a molecule, defined as the difference between its ionization potential, I, and electron affinity, A, shrinks with respect to that of the gas phase by adsorbing the molecule on a polarizable substrate. Nevertheless, the electronic structure theories usually used for such calculations can only partly account for this renormalization of the molecular energy levels when the junction is formed.…”

Section: Introductionmentioning

confidence: 99%

Stretching of BDT-gold molecular junctions: thiol or thiolate termination?

et al. 2014

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It is often assumed that the hydrogen atoms in the thiol groups of a benzene-1,4-dithiol dissociate when Au-benzene-1,4-dithiol-Au junctions are formed. We demonstrate, by stability and transport property calculations, that this assumption cannot be made. We show that the dissociative adsorption of methanethiol and benzene-1,4-dithiol molecules on a flat Au(111) surface is energetically unfavorable and that the activation barrier for this reaction is as high as 1 eV. For the molecule in the junction, our results show, for all electrode geometries studied, that the thiol junctions are energetically more stable than their thiolate counterparts. Due to the fact that density functional theory (DFT) within the local density approximation (LDA) underestimates the energy difference between the lowest unoccupied molecular orbital and the highest occupied molecular orbital by several electron-volts, and that it does not capture the renormalization of the energy levels due to the image charge effect, the conductance of the Au-benzene-1,4-dithiol-Au junctions is overestimated. After taking into account corrections due to image charge effects by means of constrained-DFT calculations and electrostatic classical models, we apply a scissor operator to correct the DFT energy level positions, and calculate the transport properties of the thiol and thiolate molecular junctions as a function of the electrode separation. For the thiol junctions, we show that the conductance decreases as the electrode separation increases, whereas the opposite trend is found for the thiolate junctions. Both behaviors have been observed in experiments, therefore pointing to the possible coexistence of both thiol and thiolate junctions. Moreover, the corrected conductance values, for both thiol and thiolate, are up to two orders of magnitude smaller than those calculated with DFT-LDA.This brings the theoretical results in quantitatively good agreement with experimental data.

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Section: Introductionmentioning

confidence: 99%

Stretching of BDT-gold molecular junctions: thiol or thiolate termination?

et al. 2014

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“…The energy difference between E A and E I (the conductivity gap) is reduced by a factor of 2 when comparing the energy levels of an isolated molecule with those of a C 60 monolayer on Ag (111). Values taken from [13,21,[34][35][36][37][38][39] In the system depicted by Fig. 1c, the metal stabilizes the charged state of the C 60 -ion, since the metal effectively screens the ion.…”

Section: Introductionmentioning

confidence: 99%

“…The HOMO-LUMO gap at the metal/C 60 interface is reduced by more than 0.5 eV due to screening effects [21], and this causes the LUMO orbital to FIGURE 2 a UPS spectra of polycrystalline Ag (line) and of a severalnanometer-thick layer of C 60 deposited on polycrystalline Ag (square symbols). b Energy-level diagram of the alignment of energy levels between polycrystalline Ag and bulk C 60 deduced from both UPS spectra shown in a.…”

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On interface dipole layers between C 60 and Ag or Au

Veenstra

Heeres

Hadziioannou

et al. 2002

Applied Physics A: Materials Science & Processing

103

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C 60 layers on polycrystalline Ag and Au are studied by photoelectron spectroscopy. At these metal/C 60 interfaces an electron transfer occurs from the metal to the lowest unoccupied orbital of C 60 . We found in the case of the polycrystalline Ag/C 60 interface a dipolar layer with its associated electric field in the direction corresponding to the charge transfer, so pointing from the substrate to the adsorbent. Yet, at the Au/C 60 interface we observed an overall electric field pointing from C 60 towards the metal. We discuss our observations in terms of charge transfer, screening and hybridization effects and propose the occurrence of a hybridization mechanism similar to back-bonding at the Au/C 60 interface. We show that the alignment of energy levels at the metal/C 60 interface cannot simply be deduced using the metal workfunction and the frontier orbitals of C 60 , including screening effects, since hybridization effects may strongly alter the interfacial energy level structure. Our experimental findings on the polycrystalline metal/C 60 interfaces indicate an at-most weak dependence of the Fermi level of the C 60 overlayer on the workfunction of the polycrystalline metal substrate. These interfaces are found in donoracceptor-based organic photovoltaic devices and our results may help to understand the electrical characteristics of these devices.

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“…The basic idea is to bring the material in the close proximity of a strongly polarizable medium. The relevant exchange and superexchange interactions, and thus the related magnetic ordering temperatures, can then be substantially amplified by reducing the energies of the underlying virtual charge excitations as a result of the image-charge-like screening by the polarizable medium [12,13,14].To prove this concept we have chosen to measure the Néel temperature T N of a 3 monolayer (ML) NiO film epitaxially grown on a MgO(100) substrate and of an equally thin film on Ag(100). NiO on MgO and on Ag are ideal model systems for this study because of their simple crystal structure and well characterized growth properties.…”

mentioning

confidence: 99%

Image charge screening: A new approach to enhance magnetic ordering temperatures in ultrathin correlated oxide films

Altieri¹,

Finazzi

Hsieh

et al. 2009

Phys. Rev. B

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We have tested the concept of image charge screening as a new approach to enhance magnetic ordering temperatures and superexchange interactions in ultra thin films. Using a 3 monolayer NiO(100) film grown on Ag(100) and an identically thin film on MgO(100) as model systems, we observed that the Néel temperature of the NiO film on the highly polarizable metal substrate is 390 K while that of the film on the poorly polarizable insulator substrate is below 40 K. This demonstrates that screening by highly polarizable media may point to a practical way towards designing strongly correlated oxide nanostructures with greatly improved magnetic properties.PACS numbers: 75.30.Et, 78.70.Dm Transition metal oxides exhibit many spectacular magnetic and electrical properties including high temperature superconductivity and colossal magnetoresistance [1] making them particularly promising for nanoscience technology applications. An acute issue in the field of nanoscience, however, is the strong reduction of the relevant critical or ordering temperatures due to well known finite size effects [2,3,4,5]. If ways could be found to compensate for these reductions, one would immediately enlarge the materials basis for nano-technology. Current approaches to overcome these problems include the use of chemical doping, pressure, and strain [6,7,8,9,10,11].Here we propose to exploit image charge screening as a new method to compensate finite size phenomena and to enhance magnetic ordering temperatures well beyond the capability of conventional methods [6,7,8,9,10,11]. The basic idea is to bring the material in the close proximity of a strongly polarizable medium. The relevant exchange and superexchange interactions, and thus the related magnetic ordering temperatures, can then be substantially amplified by reducing the energies of the underlying virtual charge excitations as a result of the image-charge-like screening by the polarizable medium [12,13,14].To prove this concept we have chosen to measure the Néel temperature T N of a 3 monolayer (ML) NiO film epitaxially grown on a MgO(100) substrate and of an equally thin film on Ag(100). NiO on MgO and on Ag are ideal model systems for this study because of their simple crystal structure and well characterized growth properties. They have a rock-salt crystal structure with lattice constant a M gO = 4.212Å and a N iO = 4.176Å, respectively, corresponding to a lattice misfit of about 1%. This ensures a perfect layer-by-layer epitaxial growth of NiO(100) on MgO(100), with a NiO(100) film surface roughness of about 0.1Å [15]. Silver has a cubic fcc structure with a lattice constant a Ag = 4.086Å and a mismatch with respect to NiO of about 2%. When misfit dislocations are avoided by keeping the film thickness below the critical thickness for strain relaxation (about 30 ML for NiO/Ag [16]) as done in the present work, then NiO(100) films grow on Ag(100) in a nicely layered and coherent mode with a sharp interface. This was already demonstrated by Kado [17,18], but it has also been verified ...

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Strongly reduced band gap in a correlated insulator in close proximity to a metal

Cited by 133 publications

References 42 publications

Stretching of BDT-gold molecular junctions: thiol or thiolate termination?

Stretching of BDT-gold molecular junctions: thiol or thiolate termination?

On interface dipole layers between C 60 and Ag or Au

Image charge screening: A new approach to enhance magnetic ordering temperatures in ultrathin correlated oxide films

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