B4 Research Projects 2024
(Krüger Group)Project 1: ARPES calculation and orbital tomography of small adsorbed organic molecules
Background. Angle-resolved photoemission (ARPES) is one of the most important experimental techniques to probe the electronic states of materials. In adsorbed organic molecules, the ARPES intensity pattern allows to reconstruct the probed electronic orbitals, which is known as photoemission orbital tomography (POT) [1]. The common POT theory uses the final state plane-wave approximation (FSPWA), which has many shortcomings, especially for strong interaction with the substrate. The aim of this project is to compute ARPES of small adsorbed molecules, namely C6H6/Pd(110) [2] and/or CO/Ag(111) [3] where the FSPWA is known to give very poor results. Our newly developed ARPES method will be used. It does not make the FSPWA but treats the photoemission final states with the same accuracy as the intial states [4,5]. In the first step the method will be applied to a clean surface, namely boron nitride. The energy dependence of the APRES signal is poorly understood and cannot be reproduced with the FSPWA.Tasks
1. Learn our new method [4] and compute the ARPES pattern of BN as a function of photon energy. Compare the results with the FSPWA and with experiment [6].
2. DFT calculation of CO/Ag(111) and/or benzene/Pd(110) using DFT-VASP.
3. ARPES calculation of CO/Ag(111) and/or benzene/Pd(110) with new method [4]. Compare the results with the FSPWA and with experiment.
[1] P. Puschnig et al. Reconstruction of Molecular Orbital Densities from Photoemission Data, Science 326, 702 (2009). DOI: 10.1126/science.1176105
[2] L. Egger et al. New J. Phys. 21 043003 (2019) Can photoemission tomography be useful for small, strongly-interacting adsorbate systems?
[3] Kay Walter et al. private communication
[4] M. Nozaki and P. Krüger, arXiv:2402.17199
[5] Misa Nozaki, PhD thesis, Chiba U. 2024.
[6] S. Subach, Jülich, private communication.
Project 2: Theoretical study of magnetic field-induced ferroelectricity in Ba2XGe2O7 (X=Mn,Co,Cu) using so-called p-d model
Background. A material which can be both ferroelectric and ferromagnetic (or antiferromagnetic) is called multiferroic. Multiferroic materials are promising for applications in spin-related electronics ("spintronics") because the magnetisation of the system can be switched by applying an electric field or the electric polarization can be switched by a magnetic field. Ba2XGe2O7 (X=Mn,Co,Cu) is a multiferroic compound that has a complex non-collinear magnetic order and ferroelectricity is induced when a magnetic field is applied at low temperature. The origin of this multiferroic effect is not fully understood and several different mechanisms have been proposed [1,2]. One of them, the so-called p-d model [3] was said to be unrealistic for Ba2CuGe2O7 in Ref.[4], but it appears to describe the multiferroic properties better than other models [1]. The aim of this project is to apply the p-d model to Ba2XGe2O7 and to find realistic model parameters through electronic structure (DFT) calculations.Tasks
1. Understand the theories of Refs [3,4].
2. Perform DFT calculations of Ba2XGe2O7 (X=Mn,Co,Cu) to obtain parameters of the model, esp. the X-3d - O-2p hybridization integrals.
3. Solve the p-d model on a minimum cluster (XO4) numerically and compute the electric polarization
4. Critically reexamine the claims of Ref.[4]
[1] 高橋昂大, 修士論文千葉大学, 2023.
[2] 小野凌太, 博士論文千葉大学, 2021.
[3] T. Arima, J. Phys. Soc. Jpn 76, 073702 (2007). https://journals.jps.jp/doi/10.1143/JPSJ.76.073702
[4] R. Ono et al. Phys. Rev. B 102, 064422 (2020).
Project 3: X-ray photoelectron diffraction of TaS2
Background. Tantalum sulfide (TaS2) is a layered transition metal dichalgogenide (TDMC). Layered TDMCs are considered very promising materials for future electronic devices. TaS2 has intriguing physical properties and a rich phase diagram including a Mott-insulating state and charge-density-wave (CDW) states which may be commensurate or incommensurate depending on temperature [1]. The nature of the CDW state is the subject of on-going debate [2,3]. The aim of this project is to analyze high-resolution X-ray photoelectron diffraction (XPD) data of the TaS2 surface which has recently been acquired by Prof. F. Matsui at IMS, Japan [4]. Through the XPD analysis, the surface atomic structure will be determined with high accuracy, and this will provide important insight in the origin of the CDW state.Tasks
1. Learn XPD calculations using the EDAC [5] and/or MsSpec [6] software
2. Make a surface model of 1T-TaS2
3. Optimize the surface structure which best reproduces the experimental XPD pattern
4. Perform a DFT calculation of 1T-TaS2 with the XPD optimized structure and compare the electronic state with that reported in the literature.
[1] https://en.wikipedia.org/wiki/Tantalum(IV)_sulfide
[2] J. Lee et al., Phys. Rev. Lett. 126, 196405 (2021).
[3] M. D. Johannes and I. I. Mazin, Phys. Rev. B 77, 165135 (2008)
[4] F. Matsui, private communication
[5] http://widgets.nanophotonics.es/edac/manual/edac.html
[6] https://ipr.univ-rennes.fr/en/materials-nanosciences-department/msspec
Project 4: Theory of spin-polarization in resonant photoemission of transition metals
Background. Resonant photoemission at the 2p-absorption edge of transition elements can be used as an element- and orbital-specific probe of the valence electronic structure of transition metal compounds [1]. The resonantly excited photoelectrons are often spin-polarized but the relation to the spin-polarization of the valence states is complex [2]. Strong spin-polarization of the photoelectrons may occur in RPES even for non-magnetic samples [2]. This opens the possibility of producing spin-polarized electrons without the need of a magnetic field. To this end, the appearance of spin-polarzation in RPES and its relation to the sample magnetization must be understood in a systematic way. The aim of this project is to study this problem in a ligand-field multiplet model. First, the spin-polarization of angular integrated RPES will be calculated depending on the ligand field (octahedral or tetrahedral) and the polarization of the light. In a second step, the photoelectron angular distribution will be analyzed [3].Tasks
1. Learn the RPES code developed by Krüger.
2. Compute RPES for all 3d elements in octahedral or tetrahedral ligand field and low- and high-spin ground states and various light polarizations (linear and circular).
3. Analyse the degree of spin-polarization of the photoelectrons for each case.
4a. (Application) Study the angular dependence of the spin-polarization [3].
or 4b. (Programming) Extend the RPES code by coding multiplet parameters from an atomic potential on input.
[1] P. Krüger et al. Phys. Rev. Lett. 108, 126803 (2012). (doi:10.1103/PhysRevLett.108.126803)
[2] F. Da Pieve and P. Krüger, Phys. Rev. Lett. 110, 127401 (2013) (doi: 10.1103/PhysRevLett.110.127401 )
[3] R. Sagehashi, G. Park and P. Krüger, Phys. Rev. B 107, 075407 (2023) (doi: 10.1103/PhysRevB.107.075407)
Project 5: Data-driven analysis of photoluminescence spectra of Eu3+ complexes
Background. Rare-earth complexes are much used for light-emitting devices because of their strong photoluminescence (PL) with sharp spectral lines given bright and well-defined colors. Eu3+ complexes are particularly interesting because of their strong red PL corresponding 5D0-6F2 emission line. This ``dipole-forbidden'' transition is called hyper-sensitive, because small structural changes of the Eu3+ complex lead to huge variation of the PL intensity. This phenomenon of structural hypersensitivity is important for applications but very poorly understood. The aim of this project is to understand this phenomenon and to find out what kind of structural changes have the strongest effect on the intensity of the hypersensitive line. To this end, the student will use the point-charge ligand field + multiplet model developed in our group and build a data base of spectra as a function of the structural parameters, through small displacements of the ligand atoms from their equilibrium positions and varying the charge of the ligands. First, a usual (human) analysis will be performed and conclusions about structure - PL relation will be drawn. Second, the data base will be used as training and testing data in a machine learning process. With the machine learning algorithm, chemical and structural parameters of Eu3+ ligands will be determined for optimum PL emission which will help guide the design of new complexes.Tasks
1. Learn to use the Rare-Earth PL code [1]
2. Gather literature data of PL of Eu3+ complexes [2]
3. Compute PL spectra of the chosen complexes for by varying ligand charge and positions, i.e. build the data base.
4. Analyse the results ``by hand'' and draw the physical conclusions.
5. Choose a suitable machine-learning method and apply it to the data-base.
[1] 氏家智仁, 修士論文千葉大学, 2022.
[2] 田村倫, 学士論文千葉大学, 2023.
Project 6: ARPES code development: full-potential multiple scattering
Background. Angle-resolved photoemission (ARPES) is one of the most important experimental techniques to probe the electronic states of materials. ARPES intensity calculations are mostly performed using multiple scattering (MS) theory [1]. The major shortcoming of MS theory is that it generally comes the so-called muffin-tin (MT) approximation. The MT approximation is is good for dense systems, but bad for open structures such as organic molecules. Recently we have developed a way to compute non-muffin-tin, i.e. full-potential scattering matrices and applied is successfully to X-ray absorption spectra [2]. The aim of this project is to implement the full-potential scattering calculation into the MS theory code for ARPES [1].Tasks
1. Learn C programming language
2. Learn the codes [1,2].
3. Program the interface
4. Test the new method with ARPES from graphite [3,4]
[1] P. Krüger et al. Real-space multiple scattering method for angle-resolved photoemission and valence-band photoelectron diffraction and its application to Cu(111), Phys. Rev. B 83, 115437 (2011). DOI: 10.1103/PhysRevB.83.115437
[2] 藤方悠, 修士論文千葉大学, 2022.
[3] P. Krüger and F. Matsui, J. Electron Spectrosc. Related Phenom. 258, 147219 (2022)
[4] M. Nozaki and P. Krüger, arXiv:2402.17199
Project 7: Calculation of photoluminescence spectra of praseodymium Pr3+ ion in solid laser materials
Background. Praseodymium (Pr3+) compounds are widely used for solid state lasers. Understanding the photoluminescence spectra is important for finding the best host material of the Pr3+ ion for efficient laser performance. The aim of this project is to compute the photoluminescence using the model and program that has been developed in the research group and improve the theory/model/program if necessary.Tasks
1. Compute crystal field of Pr3+:LiYF4 in point ion model [1]
2. Compute single ion Pr3+ interaction parameters
3. Compute photoluminiscence spetrum and compare with experiment and previous theory [2]
[1] 氏家智仁, 修士論文千葉大学, 2022.
[2] L. Esterowitz et al. Energy levels and line intensities of Pr3+ in LiYF4, Phys. Rev. B 19, 6442 (1979). DOI: 10.1103/PhysRevB.19.6442