Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Research Project Sayısal çok-tanecik fiziği yöntemlerini kullanarak yüksek-sıcaklık süperiletkenleri ile manyetik yarıiletkenlerin elektronik özelliklerinin incelenmesi(TÜBİTAK - Türkiye Bilimsel ve Teknolojik Araştırma Kurumu, 2013) Bulut, Nejat; Mayda, Selma; Kandemir, ZaferKuantum çok-tanecik fiziğinin sayısal yöntemlerinin uygulandığı bu proje iki bağımsız kısımdan oluşmaktadır. Birinci kısımda d(x2 − y2)-simetrili yüksek-sıcaklık süperiletkenliği Hubbard modeli çerçevesinde kuantum Monte Carlo simülasyonları kullanarak incelenmiştir. Burada bir merdiven örgü üzerine yerleştirilmiş olan Hubbard modeli için d(x2 − y2)-simetrili süperiletkenliğin gücü incelenmiştir. Özellikle de Cooper eşleşmesini sağlayan potansiyelin sıcaklık, elektron yoğunluğu, fermiyonların momentum ve enerjisinin fonksiyonu olarak hesaplanmıştır. Bu çalışmalar Cooper eşleşmesini sağlayan potansiyelin çok kuvvetli olabileceğini, ve ayrıca bu potansiyelin kuvvetinin de model parametrelerine ve Cooper çiftinin göreceli momentum ve enerjisine hassas bir şekilde bağlı olduğunu göstermiştir. Kuantum Monte Carlo simülasyonlarında karşılaşılan "işaret problemi" de burada incelenmiş ve d(x2 − y2)-simetrili Cooper çiftini oluşturan potansiyelin en kuvvetli olduğu zaman "işaret probleminin" de en kötü olduğu bulunmuştur. Bunlar determinental kuantum Monte Carlo simülasyonlarında karşılaşılan "işaret probleminin" fermiyonlar arasındaki etkileşmenin çok kuvvetlenmesinden kaynaklanabileceğini önermektedir.Research Project Geçiş Elementi İçeren Moleküllerin Elektronik Özelliklerinin Kuantum Monte Carlo Metodu ve Yoğunluk Fonksiyoneli Kuramını Kullanarak İncelenmesi(2017) Bulut, NejatBu projede, metaloproteinlerin, metaloenzimlerin ve rutenyum atomu içeren boya moleküllerinin elektronik yapısı ve korelasyonları genişletilmiş Haldane-Anderson modeli ile incelenmiştir. Bu modelin parametreleri Hartree-Fock (HF) yaklaşımı ya da yoğunluk fonksiyoneli teoremi (DFT) yöntemleri kullanılarak hesaplanmıştır. Metaloenzimlere örnek olarak Co atomu içeren vitamin B12 molekülüne ve metaloproteinlere örnek olarak Fe atomu içeren hemoglobin molekülüne çalışılmıştır. DFT+QMC yöntemi ile vitamin B12, hemoglobin ve Ru içeren boya moleküllerinin elektronik ve manyetik özellikleri tam olarak elde edilmiştir. Sonuçlar, metaloproteinlerde ve metaloenzimlerde safsızlık bağıl durumu olarak adlandırılan yeni elektronik hallerin var olduğunu göstermiştir. Safsızlık bağıl durumlarının elektron ile dolu olup olmaması moleküllerin elektronik ve manyetik özelliklerini tamamen kontrol etmektedir.Research Project Su(2)-değişmez Hund Etkileşimi İçeren Çok-orbitalli Anderson Modeli ile Kuantum Monte Carlo Çalışması(2019) Bulut, NejatMetalopreinler geçiş-metali elementi içeren organik moleküllerdir. Metaloproteinlerin elektronik özellikleri çok-orbitalli Anderson safsızlık modelini kullanarak çalışılabilir. Çokorbitalli Anderson modeli 3d orbtiallerindeki ve aralarındaki Coulomb etkilelşmelerini de içerir. Anderson modeli Hirsch-Fye QMC algoritmasını kullanarak çözülebilir. Bugüne kadar gerçekleştirilen QMC simülasyonlarında Hund etkileşmesinin sadece doğrusal bileşeni hesaba katılmıştır. Fakat spin-çevirme ve çift-atlama terimleri hesaba katılmamıştır. Bu terimler çok-orbitalli Anderson modelini SU(2)-değişmez yapmak için gereklidir. Bu projede, spin-çevirme ve çift-atlama terimleri Anderson modeline eklenmiştir ve bu model QMC metodunu bir seri açılımı tekniği ile birleştirerek çalışılmıştır.Article Citation - WoS: 12Citation - Scopus: 15Magnetic Mechanism for the Biological Functioning of Hemoglobin(Nature Publishing Group, 2020) Mayda, Selma; Kandemir, Zafer; Bulut, Nejat; Maekawa, SadamichiThe role of magnetism in the biological functioning of hemoglobin has been debated since its discovery by Pauling and Coryell in 1936. The hemoglobin molecule contains four heme groups each having a porphyrin layer with a Fe ion at the center. Here, we present combined density-functional theory and quantum Monte Carlo calculations for an effective model of Fe in a heme cluster. In comparison with these calculations, we analyze the experimental data on human adult hemoglobin (HbA) from the magnetic susceptibility, Mossbauer and magnetic circular dichroism (MCD) measurements. In both the deoxygenated (deoxy) and the oxygenated (oxy) cases, we show that local magnetic moments develop in the porphyrin layer with antiferromagnetic coupling to the Fe moment. Our calculations reproduce the magnetic susceptibility measurements on deoxy and oxy-HbA. For deoxy-HbA, we show that the anomalous MCD signal in the UV region is an experimental evidence for the presence of antiferromagnetic Fe-porphyrin correlations. The functional properties of hemoglobin such as the binding of O-2, the Bohr effect and the cooperativity are explained based on the magnetic correlations. This analysis suggests that magnetism could be involved in the functioning of hemoglobin.Book Part Future Applications of Artificially-Synthesized Organic Molecules Containing Transition-Metal Atoms(Elsevier, 2018) Mayda, Selma; Kandemir, Zafer; Bulut, NejatArtificially-synthesized organic molecules which contain transition-metal atoms offer new possibilities for applications in the electronics, pharmaceutical, and chemical industries. Hence, developing an understanding of the electronic properties of this kind of organic molecules is important. With this purpose, here we study the electronic properties of metalloproteins, metalloenzymes, and Ru-based dye molecules as examples for this kind of organic molecules. In particular, we perform combined Hartree-Fock (HF) and quantum Monte Carlo (HF+QMC) calculations, as well as combined density functional theory (DFT) and QMC (DFT+QMC) calculations to study the electronic properties of these molecules. Our results show that new electronic states named as impurity bound states (IBS) form in metalloproteins, metalloenzymes, and Ru-based dye molecules. We show that the electron occupancy of IBS is critically important in determining the low-energy electronic properties of these molecules. In this respect, the IBS may play a central role in developing new applications based on artificially-synthesized organic molecules containing transition-metal atoms. © 2018 Elsevier Inc. All rights reserved.Article Citation - WoS: 2Citation - Scopus: 2Electronic Structure of Cyanocobalamin: Dft+qmc Study(Springer Verlag, 2017) Mayda, Selma; Kandemir, Zafer; Bulut, NejatWe study the electronic structure and the magnetic correlations of cyanocobalamin (C63H88CoN14O14P) by using the framework of the multi-orbital single-impurity Haldane-Anderson model of a transition metal impurity in a semiconductor host. Here, we first determine the parameters of the Anderson Hamiltonian by performing density functional theory (DFT) calculations. Then, we use the quantum Monte Carlo (QMC) technique to obtain the electronic structure and the magnetic correlation functions for this effective model. We find that new electronic states, which correspond to impurity bound states, form above the lowest unoccupied level of the host semiconductor. These new states derive from the atomic orbitals at the cobalt site and the rest of the molecule. We observe that magnetic moments develop at the Co(3dν) orbitals and over the surrounding sites. We also observe that antiferromagnetic correlations exist between the Co (3dν) orbitals and the surrounding atoms. These antiferromagnetic correlations depend on the filling of the impurity bound states.Article Citation - WoS: 2Citation - Scopus: 2Electronic Structure and Correlations of Vitamin B12 Studied Within the Haldane-Anderson Impurity Model(Springer Verlag, 2016) Kandemir, Zafer; Mayda, Selma; Bulut, NejatWe study the electronic structure and correlations of vitamin B12 (cyanocobalamine) by using theframework of the multi-orbital single-impurity Haldane-Anderson model of atransition-metal impurity in a semiconductor host. The parameters of the effectiveHaldane-Anderson model are obtained within the Hartree-Fock (HF) approximation. Thequantum Monte Carlo (QMC) technique is then used to calculate the one-electron andmagnetic correlation functions of this effective model. We observe that new states forminside the semiconductor gap found by HF due to the intra-orbital Coulomb interaction atthe impurity 3d orbitals. In particular, the lowest unoccupiedstates correspond to an impurity bound state, which consists of states from mainly the CNaxial ligand and the corrin ring as well as the Co eg-like orbitals. We alsoobserve that the Co (3d) orbitals can develop antiferromagneticcorrelations with the surrounding atoms depending on the filling of the impurity boundstates. In addition, we make comparisons of the HF+QMC data with the density functionaltheory calculations. We also discuss the photoabsorption spectrum of cyanocobalamine.Article Citation - WoS: 10Citation - Scopus: 10Spin-Spin Correlations of Magnetic Adatoms on Graphene(American Physical Society, 2015) Güçlü, Alev Devrim; Bulut, NejatWe study the interaction between two magnetic adatom impurities in graphene using the Anderson model. The two-impurity Anderson Hamiltonian is solved numerically by using the quantum Monte Carlo technique. We find that the interimpurity spin susceptibility is strongly enhanced at low temperatures, significantly diverging from the well-known Ruderman-Kittel-Kasuya-Yoshida result which decays as R-3.Conference Object Citation - WoS: 1A Simple Case of D (x 2- Y 2) Pairing: Hubbard Ladder(EDP Sciences, 2012) Bulut, NejatWe study the strength and the temperature scale of the d(x 2-y2) pairing correlations in the Hubbard model on a ladder lattice using Quantum Monte Carlo (QMC) simulations. In particular, we present QMC results on the particle-particle interaction and the solution of the Bethe-Salpeter equation for the d(x2-y2)-wave BCS channel. These data show that there are strong d(x2-y2) pairing correlations in the Hubbard ladder for certain values of the model parameters. © Owned by the authors, published by EDP Sciences, 2012.Article Citation - WoS: 32Citation - Scopus: 32Quantum Renormalization of the Spin Hall Effect(American Physical Society, 2010) Gu, Bo; Gan, Jing-Yu; Bulut, Nejat; Ziman, Timothy; Guo, Guang-Yu; Nagaosa, Naoto; Maekawa, SadamichiBy quantum Monte Carlo simulation of a realistic multiorbital Anderson impurity model, we study the spin-orbit interaction (SOI) of an Fe impurity in Au host metal. We show, for the first time, that the SOI is strongly renormalized by the quantum spin fluctuation. Based on this mechanism, we can explain why the gigantic spin Hall effect in Au with Fe impurities was observed in recent experiments, while it is not visible in the anomalous Hall effect. In addition, we show that the SOI is strongly renormalized by the Coulomb correlation U. Based on this picture, we can explain past discrepancies in the calculated orbital angular momenta for an Fe impurity in an Au host. © 2010 The American Physical Society.