Effect of Lifshitz Quantum Phase Transitions on the Normal and Superconducting States in Cuprates

We study the doping evolution of the electronic structure in the normal phase of high-Tc cuprates. The electronic structure and the Fermi surface of cuprates with a single CuO2 layer in the unit cell like La2-xSrxCuO4 have been calculated by the LDA+GTB method in the regime of strong electron correlations (SEC) and compared to ARPES and quantum oscillations data. We have found two critical concentrations, xc1 and xc2, where the Fermi surface topology changes. Following I.M. Lifshitz’s ideas of the quantum phase transitions (QPT) of the 2:5-order, we discuss the concentration dependence of the low-temperature thermodynamics. The behavior of the electronic specific heat δ(C/T ) ~ (x - xc)^½ is similar to the Loram and Cooper experimental data in the vicinity of xc1 ≈ 0.15. In the superconducting state of cuprates, we consider both magnetic and phonon contributions to the d-wave pairing and found that there is no dominant mechanism of superconductivity. Magnetic and phonon contributions to the critical temperature are of the same order.У цiй работi обговорено змiни електронної структури в нормальнiй фазi високотемпературних надпровiдникiв – шаруватих купратiв. Результати розрахункiв електронної структури та поверхнi Фермi одношарових купратiв методом LDA+GTB iз врахуванням сильних кореляцiй порiвнюються з даними ARPES та квантових осциляцiй. Виявлено двi критичнi точки xc1 та xc2, в яких вiдбувається перебудова поверхнi Фермi. В околi критичних точок у межах iдеологiї I.М. Лiфшица про квантовi фазовi переходи 2,5 роду знайдено змiни термодинамiчних властивостей за низьких температур. Особливiсть електронної теплоємностi δ(C/T ) ~ (x - xc)^½ достатньо добре узгоджується з вiдомими експериментальними даними в околi xc1 ≈ 0.15. Якiсно обговорюються змiни знака константи Холла з допуванням. Також розглянуто надпровiдний стан з урахуванням магнiтного i фононного механiзмiв спарювання

Electron structure and electron–phonon interaction in the strongly correlated electron system of cuprates

The generalized tight-binding method presents a practical realization of the scheme that describes quasiparticles in strongly correlated electron system and consists of exact intra-cell diagonalization of the model Hamiltonian and perturbative treatment of the inter-cell hoppings. In present paper this method and its ab initio modification applied to undoped and weakly doped HTSC cuprates. Results are in very good agreement with the experimental ARPES data on various compounds. Starting with multiband p—d model the realistic effective low-energy Hamiltonian of strongly correlated electrons interacting with spin fluctuations and phonons is derived both for hole and electron doped systems. Without electron—phonon interaction the pure magnetic mechanism of pairing does not provide the correct value of Tc even for single-layer La₂₋xSrxCuO₄ and Nd₂₋xCexCuO₄

Magnetooptics in Gold and Silver NanoSizes Low-Dimensional Objects

The spectra of optical absorption and of magnetic circular dichroism (MCD) have been measured in the 350–1150 nm wavelength range for a set of colloidal solutions containing Au and Ag nanoparticles. The average size of Au nanoparticles was 6 nm and having thiolate coatings with different degrees of chirality. (The average size of Ag nanoparticles was 14 nm and having citrate coatings) The form of absorption and MCD spectra suggests the dipole character of interband transitions involving the 5d–6(sp) for Au orbitals and 4d–5(sp) for Ag orbitals. The absence (within the experimental error) of the MCD spectra dependence on the coating type rules out the hypothesis on the orbital nature of the observed magnetism. We argue that the spin polarization plays the dominant role in the magnetism both for Au and Ag nanoparticles. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3534

Spin state crossover in Co3BO5

We have investigated the spin and oxidation states of Co in Co3BO5 using x-ray magnetic circular dichroism (XMCD) and dc magnetic susceptibility measurements. At low temperatures, XMCD experiments have been performed at the Co K-edge in Co3BO5 and Co2FeBO5 single crystals in the fully ferrimagnetically ordered phase. The Co (K-edge) XMCD signal is found to be related to the Co2+ magnetic sublattices in both compounds, providing strong experimental support for the low-spin (LS) Co3+ scenario. The paramagnetic susceptibility is highly anisotropic. An estimation of the effective magnetic moment in the temperature range 100-250 K correlates well with two Co2+ ions in the high-spin (HS) state and some orbital contribution, while Co3+ remains in the LS state. The crystal structure of the Co3BO5 single crystal has been solved in detail at the T range 296-703 K. The unit cell parameters and volume show anomalies at 500 and 700 K. The octahedral environment of the Co4 site strongly changes with heating. The generalized gradient approximation with Hubbard U correction calculations have revealed that, at low-temperatures, the system is insulating with a band gap of 1.4 eV, and the Co2+ ions are in the HS state, while Co3+ are in the LS state. At high temperatures (T > 700 K), the charge ordering disappears, and the system becomes metallic with all Co ions in 3d7 electronic configuration and HS state. © 2021 American Physical Society

In Situ Spectral Magnetoellipsometry for Structural, Magnetic and Optical Properties of Me/Si (Me Mn, Fe) Nanolayers

In our work we present in-situ spectral magnetoellipsometer is equipped with sapphire manipulator. which allows us to carry out in-situ and in-time optical and magnetooptical measurements in the range from 10 K to 1500 K in spectral range 1.5 eV-4.0 eV (830 nm-300 nm), the range of magnetic fields is +/-0.4 T. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3555

Decay constants, light quark masses and quark mass bounds from light quark pseudoscalar sum rules

The flavor $ud$ and $us$ pseudoscalar correlators are investigated using families of finite energy sum rules (FESR's) known to be very accurately satisfied in the isovector vector channel. It is shown that the combination of constraints provided by the full set of these sum rules is sufficiently strong to allow determination of both the light quark mass combinations $m_u+m_d$, $m_s+m_u$ and the decay constants of the first excited pseudoscalar mesons in these channels. The resulting masses and decay constants are also shown to produce well-satisfied Borel transformed sum rules, thus providing non-trivial constraints on the treatment of direct instanton effects in the FESR analysis. The values of $m_u+m_d$ and $m_s+m_u$ obtained are in good agreement with the values implied by recent hadronic $\tau$ decay analyses and the ratios obtained from ChPT. New light quark mass bounds based on FESR's involving weight functions which strongly suppress spectral contributions from the excited resonance region are also presented.Comment: 28 pages, 10 figure

Geometric phases and quantum phase transitions in open systems

The relationship is established between quantum phase transitions and complex geometric phases for open quantum systems governed by a non-Hermitian effective Hamiltonian with accidental crossing of the eigenvalues. In particular, the geometric phase associated with the ground state of the one-dimensional dissipative Ising model in a transverse magnetic field is evaluated, and it is demonstrated that the related quantum phase transition is of the first order. © 2008 The American Physical Society

Stability of nonlinear teleoperators using PD controllers without velocity measurements

The relationship is established between the Berry phase and spin crossover in condensed matter physics induced by high pressure. It is shown that the geometric phase has topological origin and can be considered as the order parameter for such transition. " Pleiades Publishing, Ltd., 2009.",,,,,,"10.1134/S0021364009190072",,,"http://hdl.handle.net/20.500.12104/44688","http://www.scopus.com/inward/record.url?eid=2-s2.0-71949123093&partnerID=40&md5=8972a1a0aa5de476dd032b3db6cbb214",,,,,,"7",,"JETP Letters",,"53

On generalized synchronization of different-order chaotic systems: A submanifold approach

The relationship is established between quantum phase transitions and complex geometric phases for open quantum systems governed by a non-Hermitian effective Hamiltonian with accidental crossing of the eigenvalues. In particular, the geometric phase associated with the ground state of the one-dimensional dissipative Ising model in a transverse magnetic field is evaluated, and it is demonstrated that the related quantum phase transition is of the first order. " 2008 The American Physical Society.",,,,,,"10.1103/PhysRevE.78.015202",,,"http://hdl.handle.net/20.500.12104/41756","http://www.scopus.com/inward/record.url?eid=2-s2.0-48349138629&partnerID=40&md5=089ea86e4fd9201d1eb494584dc72b8

The Strong Electron Correlation Effects in XAFS Spectra of HTSC Cuprates

Theoretical XAFS spectra of highly correlated copper oxides have been presented as a product of a single-election part obtained by the SCF Xα- SW method, and a multi-electron part obtained by the multi-band multi-electron p-d model. Trie main line of CuK-spectram of La2CuO4 was aligned to the Cud10 L - configuration and the single satellite line was aligned to the Cud9. Comparison of the theoretical data with the experimental ones shows that the ground state of the two-holes in the CuO4 doped cell is triplet. The last conclusion has been confirmed by analysis of CuL2,3- spectra of two-hole impurity states of doped CuO4 cell. Another interesting future of CuL2,3- spectra - a peak 2.8 eV higher the threshold was aligned to the transitions into quasi-stationary states due to existence of a high barrier in the Cud- state potential