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Search Results: 1 - 10 of 71030 matches for " Maria Stamenova "
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Dynamical Exchange Interaction From Time-Dependent Spin Density Functional Theory
Maria Stamenova,Stefano Sanvito
Physics , 2013, DOI: 10.1103/PhysRevB.88.104423
Abstract: We report on {\it ab initio} time-dependent spin dynamics simulations for a two-center magnetic molecular complex based on time-dependent non-collinear spin density functional theory. In particular, we discuss how the dynamical behavior of the {\it ab initio} spin-density in the time-domain can be mapped onto a model Hamiltonian based on the classical Heisenberg spin-spin interaction $J\vcr{S}_1\cdot \vcr{S}_2$. By analyzing individual localized-spin trajectories, extracted from the spin-density evolution, we demonstrate a novel method for evaluating the effective Heisenberg exchange coupling constant, $J$, from first principles simulations. We find that $J$, extracted in such a new dynamical way, agrees quantitatively to that calculated by the standard density functional theory broken-symmetry scheme.
On the Newtonian origin of the spin motive force in ferromagnetic atomic wires
Maria Stamenova,Tchavdar N. Todorov,Stefano Sanvito
Physics , 2007, DOI: 10.1103/PhysRevB.77.054439
Abstract: We demonstrate numerically the existence of a spin-motive force acting on spin-carriers when moving in a time and space dependent internal field. This is the case of electrons in a one-dimensional wires with a precessing domain wall. The effect can be explained solely by considering adiabatic dynamics and it is shown to exist for both classical and quantum systems.
Electrical control of spin dynamics in finite one-dimensional systems
Anna Pertsova,Maria Stamenova,Stefano Sanvito
Physics , 2011, DOI: 10.1103/PhysRevB.84.155436
Abstract: We investigate the possibility of the electrical control of spin transfer in monoatomic chains incorporating spin-impurities. Our theoretical framework is the mixed quantum-classical (Ehrenfest) description of the spin dynamics, in the spirit of the s-d-model, where the itinerant electrons are described by a tight-binding model while localized spins are treated classically. Our main focus is on the dynamical exchange interaction between two well-separated spins. This can be quantified by the transfer of excitations in the form of transverse spin oscillations. We systematically study the effect of an electrostatic gate bias V_g on the interconnecting channel and we map out the long-range dynamical spin transfer as a function of V_g. We identify regions of V_g giving rise to significant amplification of the spin transmission at low frequencies and relate this to the electronic structure of the channel.
Current-driven magnetic rearrangements in spin-polarized point contacts
Maria Stamenova,Stefano Sanvito,Tchavdar N. Todorov
Physics , 2005, DOI: 10.1103/PhysRevB.72.134407
Abstract: A new method for investigating the dynamics of atomic magnetic moments in current-carrying magnetic point contacts under bias is presented. This combines the non-equilibrium Green's function (NEGF) method for evaluating the current and the charge density with a description of the dynamics of the magnetization in terms of quasistatic thermally-activated transitions between stationary configurations. This method is then implemented in a tight-binding (TB) model with parameters chosen to simulate the main features of the electronic structures of magnetic transition metals. We investigate the domain wall (DW) migration in magnetic monoatomic chains sandwiched between magnetic leads, and for realistic parameters find that collinear arrangement of the magnetic moments of the chain is always favorable. Several stationary magnetic configurations are identified, corresponding to a different number of Bloch walls in the chain and to a different current. The relative stability of these configurations depends on the geometrical details of the junction and on the bias, however we predict transitions between different configurations with activation barriers of the order of a few tens of meV. Since different magnetic configurations are associated to different resistances, this suggests an intrinsic random telegraph noise at microwave frequencies in the I-V characteristics of magnetic atomic point contacts at room temperature. Finally, we investigate whether or not current induced torques are conservative.
The role of spin-orbit interaction in the ultrafast demagnetization of small iron clusters
Maria Stamenova,Jacopo Simoni,Stefano Sanvito
Physics , 2015,
Abstract: The ultra-fast demagnetization of small iron clusters initiated by an intense optical excitation is studied with time-dependent spin density functional theory (TDSDFT). In particular we investigate the effect of the spin-orbit interaction on the onset of the demagnetization process. It is found that the initial rate of coherent spin loss is proportional to the square of the atomic spin-orbit coupling constant, $\lambda$. A simplified quantum spin model comprising spin-orbit interaction and a local time-dependent magnetic field is found to be the minimal model able to reproduce our {\it ab initio} results. The model predicts the $\lambda^2$ dependence of the onset rate of demagnetization when it is solved either numerically or analytically in the linear response limit. Our findings are supported by additional TDSDFT simulations of clusters made of Co and Ni.
Magneto-mechanical interplay in spin-polarized point contacts
Maria Stamenova,Sudhakar Sahoo,Cristian G. Sanchez,Tchavdar N. Todorov,Stefano Sanvito
Physics , 2006, DOI: 10.1103/PhysRevB.73.094439
Abstract: We investigate the interplay between magnetic and structural dynamics in ferromagnetic atomic point contacts. In particular, we look at the effect of the atomic relaxation on the energy barrier for magnetic domain wall migration and, reversely, at the effect of the magnetic state on the mechanical forces and structural relaxation. We observe changes of the barrier height due to the atomic relaxation up to 200%, suggesting a very strong coupling between the structural and the magnetic degrees of freedom. The reverse interplay is weak, i.e. the magnetic state has little effect on the structural relaxation at equilibrium or under non-equilibrium, current-carrying conditions.
Time-dependent electron transport through a strongly correlated quantum dot: multiple-probe open boundary conditions approach
A. Pertsova,M. Stamenova,S. Sanvito
Physics , 2012, DOI: 10.1088/0953-8984/25/10/105501
Abstract: We present a time-dependent study of electron transport through a strongly correlated quantum dot. The time-dependent current is obtained with the multiple-probe battery method, while adiabatic lattice density functional theory in the Bethe ansatz local-density approximation to the Hubbard model describes the dot electronic structure. We show that for a certain range of voltages the quantum dot can be driven into a dynamical state characterized by regular current oscillations. This is a manifestation of a recently proposed dynamical picture of Coulomb blockade. Furthermore, we investigate how the various approximations to the electron-electron interaction affect the line-shapes of the Coulomb peaks and the I-V characteristics. We show that the presence of the derivative discontinuity in the approximate exchange-correlation potential leads to significantly different results compared to those obtained at the simpler Hartree level of description. In particular, a negative differential conductance (NDC) in the I-V characteristics is observed at large bias voltages and large Coulomb interaction strengths. We demonstrate that such NDC originates from the combined effect of electron-electron interaction in the dot and the finite bandwidth of the electrodes.
Peculiarities of sulphur functionalities in the Thracian coal province, Bulgaria
Marinov S P,Stefanova M,Kostova I,Stamenova V
Bulletin of Geosciences , 2005, DOI: 10.3140/bull.geosci.2005.01.033
Abstract: The Thracian coal province in Bulgaria is comprised of three main coal basins, Maritza East, Maritza West and Elhovo, with extremely immature coals (Rr = 0.18-0.21%). These Miocene-Pliocene aged low rank coals (lignites) are characterized by high ash and sulphur contents, and low calorific values. Temperature programmed reduction/oxidation at atmospheric pressure (AP-TPR/TPO), coupled with on-line mass spectrometer (AP-TPR/TPO-MS) and with an off-line gas chromatograph-mass spectrometer (GC-MS), were applied to the qualitative and semi-quantitative analysis of sulphur functionalities in representative samples from each basin of the coal province. Carbonates and mineral sulphur were removed by preliminary treatment with diluted acids at mild conditions to prevent effects on the TPR kinetograms. This treatment mainly removed the mineral sulphur, and its effects on organic sulphur were insignificant. Most of the organic sulphur in the Thracian lignites occurs in thiophenic structures (~ 60%). The TPR-MS profiles showed the presence of alkyl and aryl thiols, dialkyl and aryl-alkyl sulphides and thiophenes. The main organic sulphur compounds registered by the off-line AP-TPR-GC-MS were highly volatile compounds (thiols, sulphides, disulphides), alkylated thiophenes, and a lesser proportion of benzothiophenes. The disulphides (-SS-) are relatively abundant in the Maritza East and Elhovo lignites. They are probably the main aliphatic sulphur species in the coal's organic matter. Aliphatic chains with sulphur bridges are also principal structural functionalities in the Maritza West organic matter. Aromatic compounds with two aromatic rings and high degrees of substitution could be key organic structures in the studied lignites.
The role of small heat-shock protein αB-crystalline (HspB5) in COPD pathogenesis
Cherneva RV, Georgiev OB, Petrova DS, Trifonova NL, Stamenova M, Ivanova V, Vlasov VI
International Journal of Chronic Obstructive Pulmonary Disease , 2012, DOI: http://dx.doi.org/10.2147/COPD.S34929
Abstract: ole of small heat-shock protein αB-crystalline (HspB5) in COPD pathogenesis Original Research (1222) Total Article Views Authors: Cherneva RV, Georgiev OB, Petrova DS, Trifonova NL, Stamenova M, Ivanova V, Vlasov VI Published Date October 2012 Volume 2012:7 Pages 633 - 640 DOI: http://dx.doi.org/10.2147/COPD.S34929 Received: 11 June 2012 Accepted: 02 August 2012 Published: 04 October 2012 Radostina V Cherneva,1 Ognian B Georgiev,1 Daniela S Petrova,1 Nedka L Trifonova,2 Maria Stamenova,2 Vesela I Ivanova,3 Veselin I Vlasov3 1Clinic of Internal Medicine, Division of Pulmonology, University Hospital Alexandrovska, Sofia, Bulgaria; 2Department of Biology, 3Department of Pathology, Medical University of Sofia, Sofia, Bulgaria Background: αB-crystallin (HspB5) is a chaperone whose role as a marker of innate immunity activation as well as its therapeutic potential have recently been investigated in several inflammatory diseases: multiple sclerosis, myocardial ischemia, and Guillain–Barré syndrome. Aim: The aim of this study is to determine the role of αB-crystallin in chronic obstructive pulmonary disease (COPD) pathogenesis and inflammation. Materials: Plasma levels of αB-crystallin were studied in 163 patients: 52 healthy non-COPD smokers; 20 COPD smokers in Global Initiative for Chronic Obstructive Lung Disease (GOLD) stages I–II; 43 COPD smokers in GOLD stages III-IV. Forty-eight patients were diagnosed with acute inflammatory respiratory disease. The plasma levels of αB-crystallin antibodies were determined by an enzyme-linked immunosorbent assay (Calbiochem), and were confirmed with Western blotting. Tissue expression of the protein was compared in three different groups of patients: COPD smokers, COPD nonsmokers, and in patients with age-related emphysema. Results: The mean level of anti-αB-crystallin antibodies in non-COPD smokers was 0.291nm. In COPD smokers it was 0.352 nm and, in patients with inflammatory lung diseases, 0.433 nm. in patients with inflammatory lung diseases. There was a statistically significant difference between COPD smokers and healthy non-COPD smokers (P = 0.010). The same could be observed comparing the group of patients with acute inflammation and non-COPD healthy smokers (P = 0.007). There was no statistically significant difference between patients with mild/moderate inflammation and those with severe COPD. Tissue detection of the protein showed that it was significantly overexpressed in COPD smokers in comparison to COPD nonsmokers and was only slightly expressed in patients with age-related emphysema. Conclusion: αB-crystallin is increased in patients with inflammatory lung diseases. Though unspecific, it could be used in a panel of markers discerning COPD smokers from healthy nonsmokers. As αB-crystallin is a regulator of innate immunity and a therapeutic anti-inflammatory agent, its exact role in COPD pathogenesis and therapy should be explored further.
型煤中硫的还原热解ms和gc/ms分析(英文)
s.p.marinov,m.stefanova,v.stamenova,l.gonsalvesh,r.carleer,j.yperman
燃料化学学报 , 2006,
Abstract: ?以煤和生物质为主要原料的商用型煤为研究对象,采用还原热解(ap-tpr)在线质谱(ms)联用,以及离线色谱/质谱(gc/ms)分析法,定性和定量研究了家用型煤释放的含硫化合物。不同组成的型煤表现出各自不同的硫释放特性。对于生物型煤来说,仅检测到少量二烷基硫醚和脂肪硫醇的存在。褐煤型煤检测到种类最多的有机硫化物。在高阶煤制成的型煤中没有检测到氧化态的硫化物,其典型特征是存在一些噻吩类硫化物。
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