Nejat Bulut
İZMİR INSTITUTE OF TECHNOLOGY, Physics, Faculty Member
Artificially-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... more
Artificially-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 thi...
The 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... more
The 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, Mössbauer 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-porph...
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ABSTRACT Motivated by the recent angle-resolved photoemission spectroscopy (ARPES) measurements on one-dimensional Mott insulators, SrCuO2 and Na0.96V2O5, we examine the single-particle spectral weight of the one-dimensional (1D) Hubbard... more
ABSTRACT Motivated by the recent angle-resolved photoemission spectroscopy (ARPES) measurements on one-dimensional Mott insulators, SrCuO2 and Na0.96V2O5, we examine the single-particle spectral weight of the one-dimensional (1D) Hubbard model at half-filling and in the doped case. We are particularly interested in the temperature dependence of the spinon and holon excitations. For this reason, we have performed dynamical density matrix renormalization group and determinantal quantum Monte Carlo (QMC) calculations for the single-particle spectral weight of the 1D Hubbard model. In the QMC data, the spinon and holon branches become observable at temperatures where the short-range antiferromagnetic correlations develop. At these temperatures, the spinon branch grows rapidly. In the light of the numerical results, we discuss the spinon and holon branches observed by the ARPES experiments on SrCuO2. These numerical results are also in agreement with the temperature dependence of the ARPES results on Na0.96V2O5. In addition, we briefly discuss the spectral weight in the doped case.
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The Knight shift data on Li and Zn substituted YBa$_2$Cu$_3$O$_{6+x}$ are analysed using an itinerant model with short-range antiferromagnetic correlations. The model parameters, which are determined by fitting the experimental data on... more
The Knight shift data on Li and Zn substituted YBa$_2$Cu$_3$O$_{6+x}$ are analysed using an itinerant model with short-range antiferromagnetic correlations. The model parameters, which are determined by fitting the experimental data on the transverse nuclear relaxation rate $T_2^{-1}$ of pure YBa$_2$Cu$_3$O$_{6+x}$, are used to calculate the Knight shifts for various nuclei around a nonmagnetic impurity located in the CuO$_2$ planes. The calculations are carried out for Li and Zn impurities substituted into optimally doped and underdoped YBa$_2$Cu$_3$O$_{6+x}$. The results are compared with the $^7$Li and $^{89}$Y Knight shift measurements on these materials.
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In this series of papers we present a detailed study of the particle-particle collective excitations of the Hubbard model, and their contribution to the density and spin excitation spectrum. In the first paper, we shall investigate the... more
In this series of papers we present a detailed study of the particle-particle collective excitations of the Hubbard model, and their contribution to the density and spin excitation spectrum. In the first paper, we shall investigate the singlet particle-particle pair with momentum (π, π), the η particle, of the negative-U Hubbard model. We review three previously obtained theorems about the η particle and develop a self-consistent linear response theory which takes into account its contribution to the density excitation spectrum in the superconducting state. We show that this self-consistent theory agrees with the exact theorems as well as the results of numerical Monte Carlo simulations.
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In this series of papers we present a detailed study of the particle-particle collective excitations of the Hubbard model, and their contribution to the density and spin excitation spectrum. In the first paper, we shall investigate the... more
In this series of papers we present a detailed study of the particle-particle collective excitations of the Hubbard model, and their contribution to the density and spin excitation spectrum. In the first paper, we shall investigate the singlet particle-particle pair with momentum (π, π), the η particle, of the negative-U Hubbard model. We review three previously obtained theorems about the η particle and develop a self-consistent linear response theory which takes into account its contribution to the density excitation spectrum in the superconducting state. We show that this self-consistent theory agrees with the exact theorems as well as the results of numerical Monte Carlo simulations.
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ABSTRACT We use quantum Monte Carlo simulations to study the electronic properties of Anderson magnetic impurities in a semiconductor host. We find that in a semiconductor the magnetic impurities exhibit ferromagnetic correlations, which... more
ABSTRACT We use quantum Monte Carlo simulations to study the electronic properties of Anderson magnetic impurities in a semiconductor host. We find that in a semiconductor the magnetic impurities exhibit ferromagnetic correlations, which can have a much longer range than in a metallic host. In particular, the range is longest when the Fermi level is located between the top of the valence band and the impurity bound state. We study the dependence of the ferromagnetic correlations on the parameters of the Anderson model, and the dimensionality and band structure of the host material. Using the tight-binding approximation for calculating the host band structure and the impurity–host hybridization, we obtain an impurity bound state, which is located at ≈100meV above the top of the valence band, which is in agreement with the transport measurements on GaAs with dilute Mn impurities.
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In this series of papers we present a detailed study of the particle--particle collective excitations of the Hubbard model, and their contribution to the density and spin excitation spectrum. In the first paper, we shall investigate the... more
In this series of papers we present a detailed study of the particle--particle collective excitations of the Hubbard model, and their contribution to the density and spin excitation spectrum. In the first paper, we shall investigate the singlet particle--particle pair with momentum $(\pi,\pi)$, the $\eta$ particle, of the negative--$U$ Hubbard model. We review three previously obtained theorems about the $\eta$ particle and develop a self-consistent linear response theory which takes into account its contribution to the density excitation spectrum in the superconducting state. We show that this self--consistent theory agrees with the exact theorems as well as the results of numerical Monte Carlo simulations.
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ABSTRACT In weak coupling, the spin gap in doped, even, n-leg periodic Hubbard ladders reflects the energy to break a pair into separate quasiparticles. Here we investigate the structure of the gap within a spin-fluctuation exchange... more
ABSTRACT In weak coupling, the spin gap in doped, even, n-leg periodic Hubbard ladders reflects the energy to break a pair into separate quasiparticles. Here we investigate the structure of the gap within a spin-fluctuation exchange approximation. We also calculate the amplitude for removing a singlet pair from two lattice sites separated by a distance (l_x,l_y), which describes the internal structure of a pair.
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ABSTRACT We study the pairing correlations in quasi one dimensional lattice models of strongly correlated systems. Examples of such systems include the coupled copper oxide chains. We are interested in understanding the effects of the... more
ABSTRACT We study the pairing correlations in quasi one dimensional lattice models of strongly correlated systems. Examples of such systems include the coupled copper oxide chains. We are interested in understanding the effects of the lattice geometries and imperfections on the leading pairing channels. The results of diagrammatic and exact numerical calculations will be discussed.
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ABSTRACT The Knight shift and the longitudinal relaxation rate T_1-1 data on Li and Zn substituted YBa_2Cu_3O_6+x are analysed using an itinerant model with short-range antiferromagnetic correlations. The model parameters, which are... more
ABSTRACT The Knight shift and the longitudinal relaxation rate T_1-1 data on Li and Zn substituted YBa_2Cu_3O_6+x are analysed using an itinerant model with short-range antiferromagnetic correlations. The model parameters, which are determined by fitting the experimental data on the transverse nuclear relaxation rate T_2-1 of pure YBa_2Cu_3O_6+x, are used to calculate the Knight shifts and the T_1-1 rates for various nuclei around a nonmagnetic impurity located in the CuO2 planes. The calculations are carried out for Li and Zn impurities substituted into optimally doped and underdoped YBa_2Cu_3O_6+x. The results are compared with the ^7Li and ^89Y measurements on these materials.
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We present a review of the Quantum Monte Carlo results on the magnetic, charge and single-particle excitations as well as the pairing correlations of the two-dimensional Hubbard model. We are particularly interested in how these... more
We present a review of the Quantum Monte Carlo results on the magnetic, charge and single-particle excitations as well as the pairing correlations of the two-dimensional Hubbard model. We are particularly interested in how these quantities are related to each other in the metallic state that forms near half-filling. These results are helpful in gaining a better understanding of the low energy excitations of the superconducting cuprates.
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An important question is, if the gap in the high-temperature cuprates has {ital d}{sub {ital x}²-{ital y}²} symmetry, what does that tell us about the underlying interaction responsible for pairing? Here we explore this by determining... more
An important question is, if the gap in the high-temperature cuprates has {ital d}{sub {ital x}²-{ital y}²} symmetry, what does that tell us about the underlying interaction responsible for pairing? Here we explore this by determining how three different types of electron-phonon interactions affect the {ital d}{sub {ital x}²-{ital y}²} pairing found within a random phase approximation treatment of the two-dimensional Hubbard model. These results imply that interactions which become more positive as the momentum transfer increases favor {ital d}{sub {ital x}²-{ital y}²} pairing in a nearly half-filled band. {copyright} {ital 1996 The American Physical Society.}
We study the singlet particle-particle excitation of the negative-U Hubbard model with momentum (π,π), the η particle. We develop a self-consistent linear response theory, which takes into account the contribution of the η particle to the... more
We study the singlet particle-particle excitation of the negative-U Hubbard model with momentum (π,π), the η particle. We develop a self-consistent linear response theory, which takes into account the contribution of the η particle to the density excitation spectrum in the superconducting state, and show that this self-consistent linear response theory agrees with the exact theorems derived from the SO Wards identities of the Hubbard model and the numerical Monte Carlo results. We find that the η particle has charge quantum number two, spin zero and is a sharp excitation of the system for a finite range of the total momentum around Q=(π,π). Its energy is U- 2 μ and it is unchanged as a function of temperature. The η particle contributes to the density spectrum only when the superconducting pairing correlations are present and the analytic calculations show that the intensity of the peak onsets as the square of the superconducting order parameter.
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We study the nature of the magnetic correlations in the two-impurity Anderson model for a semiconductor host using the quantum Monte Carlo technique and the Hartree-Fock approximation. We find that the impurity spins exhibit ferromagnetic... more
We study the nature of the magnetic correlations in the two-impurity Anderson model for a semiconductor host using the quantum Monte Carlo technique and the Hartree-Fock approximation. We find that the impurity spins exhibit ferromagnetic correlations with a range which can be much more enhanced than in a half-filled metallic band. In particular, the range is longest when the Fermi level is located above the top of the valence band and decreases as the impurity bound state becomes occupied. In addition, we investigate the magnetic correlations between the impurity moments and the host electronic spins. Comparisons with the photoemission and optical absorption experiments suggest that this model captures the basic electronic structure of Ga1-xMnxAs, the prototypical dilute magnetic semiconductor (DMS). These numerical results might also be useful for synthesizing DMS or dilute-oxide ferromagnets with higher Curie temperatures.
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The effects of frustration in spin systems have been studied for many years. Recently, the transport properties of the layered cobalt oxide NaxCoO2 and of related oxides have generated new interest in the frustrated systems. In the cobalt... more
The effects of frustration in spin systems have been studied for many years. Recently, the transport properties of the layered cobalt oxide NaxCoO2 and of related oxides have generated new interest in the frustrated systems. In the cobalt oxides, the Co ions form a triangular lattice. The hopping matrix element of electrons in the cobalt 3d orbitals is not isotropic, and we have shown [PRL91, 257003] that the triangular CoO2 lattice consists of four coupled kagom'e sublattices. For this reason, here, we examine the single-particle excitation spectrum of the Hubbard model on the kagom'e lattice, and study the motion of a carrier in this frustrated system. We use the quantum Monte Carlo and the exact-diagonalization methods. The dispersion relation of the tight-binding model on the kagom'e lattice has a flat dispersion at the top or the bottom of the energy band depending on the sign of hopping-matrix element t. This causes a two-fold degeneracy at the gamma point where th...
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ABSTRACT Monte Carlo results for a two-dimensional Hubbard model in the intermediate-coupling regime U=4t are compared with a diagrammatic spin-fluctuation approximation. Simulations on an 8×8 lattice doped away from half filling were... more
ABSTRACT Monte Carlo results for a two-dimensional Hubbard model in the intermediate-coupling regime U=4t are compared with a diagrammatic spin-fluctuation approximation. Simulations on an 8×8 lattice doped away from half filling were carried out down to temperatures of order 140 of the bandwidth. Results for the spin susceptibility chi(q,iomegam), the electron self-energy Sigma(p,iomegan), various pair-field susceptibilities, and the irreducible particle-particle scattering vertex Gamma(p',iomegan'\|p,iomegan) were obtained. A random-phase approximation for chi(q,iomegam) with a renormalized Coulomb coupling U¯ is shown to provide a fit to the Monte Carlo data. A similar approximation for the Berk-Schrieffer spin-fluctuation interaction also provides a reasonable fit to the self-energy Sigma(p,iomegan) in the region explored by the Monte Carlo data. However, a similar approximation for the irreducible particle-particle interaction failed to reproduce the Monte Carlo results. Higher-order vertex corrections were calculated, but significant discrepancies with Monte Carlo results for Gamma remain.
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We present Quantum Monte Carlo results on the antiferromagnetic correlations and the one-electron excitations of the doped two-dimensional Hubbard model. These results are helpful in interpreting the NMR, neutron scattering and... more
We present Quantum Monte Carlo results on the antiferromagnetic correlations and the one-electron excitations of the doped two-dimensional Hubbard model. These results are helpful in interpreting the NMR, neutron scattering and photoemission experiments on the layered cuprates.
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... Physical Society 748 VOLUME 73, NUMBER 5 I AUGUST 1994 VOLUME 73, NUMBER 5 PHYSICAL REVIEW LETTERS 1 AUGUST 1994 0.4 0.3 3 0.2 b 0.1 0 0 5 10 15 FIG. 1. Real part of the frequency dependent conductivity of a half-filled (n) = 1... more
... Physical Society 748 VOLUME 73, NUMBER 5 I AUGUST 1994 VOLUME 73, NUMBER 5 PHYSICAL REVIEW LETTERS 1 AUGUST 1994 0.4 0.3 3 0.2 b 0.1 0 0 5 10 15 FIG. 1. Real part of the frequency dependent conductivity of a half-filled (n) = 1 Hubbard model with U = 8t. ...
Research Interests: Monte Carlo, Physical sciences, Superconductors, Electronic properties, Electronic Structure, and 11 moreBose Hubbard Model, Mathematical Model, Frequency Dependence, Electrical Properties, Photoemission, Electric Conductivity, Physical Properties, Energy Levels, Degree of Freedom, Monte Carlo Method, and Angular Momentum
... However, for U=8t, A(p,w) is still shifting at these temperatures and it is not possible to estimate the shape of the Fermi surface. ... [91 CM Varma et al., Phys. Rev. Lett. 63, 1996 (1989). [101 E. Dagotto, A. Moreo, F. Ortolani, J.... more
... However, for U=8t, A(p,w) is still shifting at these temperatures and it is not possible to estimate the shape of the Fermi surface. ... [91 CM Varma et al., Phys. Rev. Lett. 63, 1996 (1989). [101 E. Dagotto, A. Moreo, F. Ortolani, J. Riera, and DJ Scalapino, Phys. Rev. Lett. ...
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