| 1. |
- Azuri, Ido, et al.
(författare)
-
Fe-porphyrin on Co(001) and Cu(001) : A Comparative Dispersion-augmented Density Functional Theory Study
- 2020
-
Ingår i: Israel Journal of Chemistry. - : Wiley. - 0021-2148 .- 1869-5868. ; 60:8-9, s. 870-875
-
Tidskriftsartikel (refereegranskat)abstract
- We present a comparative density functional theory (DFT) investigation of the interaction of the iron porphyrin (FeP) molecule with the metallic Co(001) and Cu(001) surfaces, with the aim of elucidating the effect of different choices for the treatment of dispersion. We compare a GGA+U approach, several flavors of dispersion-augmented terms, and two variants of the vdW-DF approach, which treats long-range correlation explicitly. For the Co surface, we find that all approaches predict chemisorption and a high-spin state, although vdW-DF functionals generally predict weaker bonds and weaker chemisorption. For the Cu surface, we find that the functionals augmented by pair-wise dispersion once again predict chemisorption and a preferred HS state, but the vdW-DF functionals predict physisorption and a LS state. These results demonstrate the importance of careful assessment of the level of theory at which dispersion is treated, as this may have significant quantitative and even qualitative effects on the predictions made. The results also call for additional experimental data for these systems.
|
|
| 2. |
- Brena, Barbara, et al.
(författare)
-
Valence-band electronic structure of iron phthalocyanine : An experimental and theoretical photoelectron spectroscopy study
- 2011
-
Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 134:7, s. 074312-
-
Tidskriftsartikel (refereegranskat)abstract
- The electronic structure of iron phthalocyanine (FePc) in the valence region was examined within a joint theoretical-experimental collaboration. Particular emphasis was placed on the determination of the energy position of the Fe 3d levels in proximity of the highest occupied molecular orbital (HOMO). Photoelectron spectroscopy (PES) measurements were performed on FePc in gas phase at several photon energies in the interval between 21 and 150 eV. Significant variations of the relative intensities were observed, indicating a different elemental and atomic orbital composition of the highest lying spectral features. The electronic structure of a single FePc molecule was first computed by quantum chemical calculations by means of density functional theory (DFT). The hybrid Becke 3-parameter, Lee, Yang and Parr (B3LYP) functional and the semilocal 1996 functional of Perdew, Burke and Ernzerhof (PBE) of the generalized gradient approximation (GGA-) type, exchange-correlation functionals were used. The DFT/B3LYP calculations find that the HOMO is a doubly occupied pi-type orbital formed by the carbon 2p electrons, and the HOMO-1 is a mixing of carbon 2p and iron 3d electrons. In contrast, the DFT/PBE calculations find an iron 3d contribution in the HOMO. The experimental photoelectron spectra of the valence band taken at different energies were simulated by means of the Gelius model, taking into account the atomic subshell photoionization cross sections. Moreover, calculations of the electronic structure of FePc using the GGA+U method were performed, where the strong correlations of the Fe 3d electronic states were incorporated through the Hubbard model. Through a comparison with our quantum chemical calculations we find that the best agreement with the experimental results is obtained for a U-eff value of 5 eV.
|
|
| 3. |
- Chylarecka, D., et al.
(författare)
-
Indirect Magnetic Coupling of Manganese Porphyrin to a Ferromagnetic Cobalt Substrate
- 2011
-
Ingår i: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 115:4, s. 1295-1301
-
Tidskriftsartikel (refereegranskat)abstract
- The coupling mechanism of magnetic molecules to ferromagnetic surfaces is of scientific interest to design and tune molecular spintronic interfaces utilizing their molecular and surface architecture. Indirect magnetic coupling has been proposed earlier on the basis of density functional theory +U (DFT+U) calculations, for the magnetic coupling of manganese(II) porphyrin (MnP) molecules to thin Co films. Here we provide an experimental X-ray magnetic circular dichroism (XMCD) spectroscopy and scanning tunneling microscopy (STM) study of manganese(III) tetraphenylporphyrin chloride (MnTPPCl) on rough (exhibiting a high density of monatomic steps) and smooth (exhibiting a low density of monatomic steps) thin Co films grown on a Cu(001) single crystal toward the assessment of the magnetic coupling mechanism. After deposition onto the surface, MnTPPCl molecules were found to couple ferromagnetically to both rough and smooth Co substrates. For high molecular coverage, we observed higher XMCD signals at the Mn L-edges on the smooth Co substrate than on the rough Co substrate, as expected for the proposed indirect magnetic coupling mechanism on the basis of its predominance on the flat surface areas. In particular, DFT+U calculations predict a weak ferromagnetic molecule-substrate coupling only if the chloride ion of the MnTPPCl molecule orients away (Co-Mn-Cl) from the Co surface.
|
|
| 4. |
- Dhara, Barun, et al.
(författare)
-
Possible Room-Temperature Ferromagnetism in Self-Assembled Ensembles of Paramagnetic and Diamagnetic Molecular Semiconductors
- 2016
-
Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:24, s. 4988-4995
-
Tidskriftsartikel (refereegranskat)abstract
- Owing to long spin-relaxation time and chemically customizable physical properties, molecule-based semiconductor materials like metal-phthalocyanines offer promising alternatives to conventional dilute magnetic semiconductors/oxides (DMSs/ DMOs) to achieve room-temperature (RT) ferromagnetism. However, air-stable molecule based materials exhibiting both semiconductivity and magnetic-order at RT have so far remained elusive. We present here the concept of supramolecular arrangement to accomplish possibly RT ferromagnetism. Specifically, we observe a clear hysteresis-loop (H-c approximate to 120 Oe) at 300 K in the magnetization versus field (M H) plot of the self-assembled ensembles of diamagnetic Zn-phthalocyanine having peripheral F atoms (ZnFPc; S = 0) and paramagnetic Fe-phthalocyanine having peripehral H atoms (FePc; S = 1). Tauc plot of the self-assembled FePc center dot center dot center dot ZnFPc ensembles showed an optical band gap of similar to 1.05 eV and temperature-dependent current-voltage (I-V) studies suggest semiconducting characteristics in the material. Using DFT+U quantum-chemical calculations, we reveal the origin of such unusual ferromagnetic exchange-interaction in the supramolecular FePc center dot center dot center dot ZnFPc system.
|
|
| 5. |
- Ganguly, Shreemoyee, et al.
(författare)
-
Augmented space recursion study of the effect of disorder on superconductivity
- 2009
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:22
-
Tidskriftsartikel (refereegranskat)abstract
- We present a real-space approach to study the effect of disorder on superconductivity. The method is based on augmented space formalism that goes beyond mean-field approximations for configuration averaging and effectively deals with the influence of configuration fluctuations of the neighborhood of an atom. In the regime of validity of Anderson's theorem our results for s- and d-wave dirty superconductors have excellent agreement with existing results. The formalism is extended and tested for random negative U Hubbard model. Having verified the reliability of our method we use it to study environment-dependent inhomogeneous randomness in disordered superconducting systems.
|
|
| 6. |
- Girovsky, Jan, et al.
(författare)
-
Antiferromagnetic coupling of Cr-porphyrin to a bare Co substrate
- 2014
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:22, s. 220404-
-
Tidskriftsartikel (refereegranskat)abstract
- We report the discovery of an antiferromagnetic coupling of the magnetic moment of chromium(II) tetraphenylporphyrin (CrTPP) molecules to the magnetization of the clean ferromagnetic Co(001) substrate. We assign this unusual molecule-substrate exchange coupling to the less than half-filled chromium 3d orbitals interacting with Co valence band electrons via porphyrin-ligand molecular orbitals. X-ray magnetic circular dichroism, x-ray photoelectron spectroscopy, and scanning tunneling microscopy are combined with DFT + U calculations and provide evidence for a surprising type of antiferromagnetic 90 degrees indirect magnetic exchange coupling.
|
|
| 7. |
|
|
| 8. |
- Hild, K., et al.
(författare)
-
Dominance of the first excitation step for magnetic circular dichroism in near-threshold two-photon photoemission
- 2012
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 85:1, s. 014426-
-
Tidskriftsartikel (refereegranskat)abstract
- Magnetic circular dichroism (MCD) in near-threshold photoemission is measured for a perpendicularly magnetized Cs/Co/Pt(111) film with work function adjusted by Cs adsorption. For one-photon photoemission (1PPE) the MCD asymmetry is recorded at a fixed photon energy of h nu = 3.06 eV and varying work function Phi. The asymmetry shows a nonmonotonous behavior in dependence of the excess energy h nu-Phi with a maximum value of A(1PPE) = 6.2 % at Phi = 2.45 eV. The measurement explores the first excitation step of a former two-photon photoemission (2PPE) measurement with A(2PPE) = 8.4% demonstrating that in 2PPE from Co(111) the first excitation step is the dominant asymmetry-generating process. An energy-dependent measurement in 2PPE at reduced work function (Phi approximate to 3 eV) yields a constant asymmetry of about 17% in the photon energy range between h nu = 1.53-1.66 eV. It reveals that for Co(111) the involvement of a real intermediate state is crucial for enlarged MCD asymmetries. Both results are discussed in the framework of direct interband transitions in directions deviating from the direction of normal electron emission Gamma-L. The 1PPE measurement is in reasonable agreement with calculations on the basis of this model. This reveals that an ab initio calculation considering all directions of excitation with an additional restriction in energy due to the existence of the sample work function in the photoemission process adequately describes MCD asymmetries in near-threshold photoemission.
|
|
| 9. |
- Hild, K., et al.
(författare)
-
Energy- and angle-dependent threshold photoemission magnetic circular dichroism from an ultrathin Co/Pt(111) film
- 2010
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 82:19, s. 195430-
-
Tidskriftsartikel (refereegranskat)abstract
- Threshold photoemission magnetic circular dichroism (TPMCD) in one-photon photoemission (1PPE) and two-photon photoemission (2PPE) is measured at an ultrathin Co film grown on Pt(111). Energy-dependent measurements reveal maximum asymmetries directly at the photoemission threshold (1.90% for 1PPE and 11.7% for 2PPE) which weakly decrease with increasing photon energy. The measured TPMCD asymmetries are discussed in two excitation models on the basis of spin-resolved band-structure calculations. For the model of direct band-to-band transitions in other k directions than the direction of observation (Gamma-L) ab initio calculations for 1PPE and 2PPE are performed. The theory is in reasonable agreement with the measured TPMCD responses. An explanation of the large 2PPE TPMCD signal is provided in terms of specific interband excitations within the first excitation step. In the case of 2PPE angle-dependent measurements reveal a continuous drop of the asymmetry with increasing angle of incidence that agrees perfectly well with a calculation using the Fresnel equations. For 1PPE a deviation from the Fresnel-field approximation is obvious.
|
|
| 10. |
- Kumar, Uday, et al.
(författare)
-
Experimental and theoretical study of annealed Ni-Pt alloys
- 2006
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 74:6, s. 064401-
-
Tidskriftsartikel (refereegranskat)abstract
- In this communication we present a detailed study of the magnetic properties of ordered Ni-Pt alloys. We study the alloys through field-cooled and zero-field cooled dc magnetization, nonlinear ac susceptibility and sound velocity and internal friction experiments. We also present a theoretical analysis of the magnetic moments in these alloy systems. Our analysis, both experimental and theoretical, will show that disorder has a large impact on the magnetism in the samples in the composition range of 40-60% Ni. We shall argue that any disorder still remaining in the annealed samples will crucially alter the magnetic moment in this composition range and is the most probable reason for the discrepancies between earlier experimental investigations.
|
|
