Modern Physics Letters B

It is important to understand the phase diagram of electronic states in the CuO₂ plane to clarify the mechanism of high-temperature superconductivity. We investigate the ground state of electronic models with strong correlation by employing the optimization variational Monte Carlo method. We consider the two-dimensional Hubbard model as well as the three-band $d$–$p$ model. We use the improved wave function that takes account of inter-site electron correlation to go beyond the Gutzwiller wave function. The ground state energy is lowered considerably, which now gives the best estimate of the ground state energy for the two-dimensional Hubbard model.

The many-body effect plays an important role as an origin of spin correlation and superconductivity in correlated electron systems. We investigate the competition between the antiferromagnetic state and superconducting state by varying the Coulomb repulsion $U$, the band parameter $t^{\prime }$ and the electron density $n_e$ for the Hubbard model. We show phase diagrams that include superconducting and antiferromagnetic phases. We expect that high-temperature superconductivity occurs near the boundary between antiferromagnetic phase and superconducting one. Since the three-band $d$–$p$ model contains many-band parameters, high-temperature superconductivity may be more likely to occur in the $d$–$p$ model than in single-band models.

Nuclear magnetic resonance (NMR) is a fundamental bulk probe that provides key information about the electronic properties of materials. Very recently, the analysis of all available planar copper shift as well as relaxation data proved that while the shifts cannot be understood in terms of a single temperature-dependent spin component, relaxation can be explained with one dominating Fermi liquid-like component, without enhanced electronic spin fluctuations. For the shifts, a doping-dependent isotropic term as well as doping-independent anisotropic term became obvious. Here, we focus on planar ${}^{17}$O NMR shifts and quadrupole splittings. Surprisingly, we find that they demand, independently, a similar two-component scenario and confirm most of the previous conclusions concerning the properties of the spin components, in particular that a negative spin polarization survives in the superconducting state. This should have consequences for the pairing scenario.

We performed ${}^{77}\mathrm{\text{Se}}$-nuclear magnetic resonance measurements on ${\mathrm{\text{FeSe}}}_{1-x}{\mathrm{\text{S}}}_x$ ($x$ = 0.12 and 0.23). We found from the relaxation time $T_1$ that nematicity and antiferromagnetic fluctuations show cooperative coupling.

The variational quantum eigensolver has been proposed as a low-depth quantum circuit that can be employed to examine strongly correlated systems on today’s noisy intermediate-scale quantum computers. We examine details associated with the factorized form of the unitary coupled-cluster variant of this algorithm. We apply it to a simple strongly correlated condensed-matter system with nontrivial behavior — the four-site Hubbard model at half-filling. This work show some of the subtle issues one needs to take into account when applying this algorithm in practice, especially to condensed-matter systems.

The paramagnetic phase of the extended attractive Hubbard model on the cubic lattice is studied within the spin rotation invariant Kotliar-Ruckenstein slave boson representation at zero temperature. It is obtained that the quasiparticle residue of the Fermi liquid phase vanishes for all densities at an interaction strength slightly smaller than $U_c$ that signals the Brinkman–Rice transition, and that it weakly depends on density. While for vanishing non-local interaction parameters, homogeneous static charge instabilities are found in a rather narrow window centered around quarter filling and $U\simeq 0.8U_c$, increasing them to $V=-0.2U$ results into a severe narrowing of this window. On the contrary, when all interaction parameters are attractive, for example for $V=0.2U$, a large parameter range in which homogeneous static charge instabilities is found. Yet, this systematically happens inside the Fermi liquid phase.

Among the Fe-based superconductors, ${\mathrm{\text{Fe}}}_{1+y}{\mathrm{\text{Te}}}_{1-x}{\mathrm{\text{Se}}}_x$ is unique in that its crystal structure is the simplest and the electron correlation level is the strongest, and thus it is important to investigate the doping $(x)$-temperature $(T)$ phase diagram of this system. However, inevitably incorporated excess Fe currently prevents the establishment of the true phase diagram. We overcome the aforementioned significant problem via developing a new annealing method termed as “Te-annealing” wherein single crystals are annealed under Te vapor. Specifically, we conducted various magnetotransport measurements on Te-annealed superconducting ${\mathrm{\text{Fe}}}_{1+y}{\mathrm{\text{Te}}}_{1-x}{\mathrm{\text{Se}}}_x$. We observed that crossover from the incoherent to the coherent electronic state and opening of the pseudogap occurs at high temperatures ($\approx$150 K for $x=0.2$). This is accompanied by a more substantial pseudogap and the emergence of a phase with a multi-band nature at lower temperatures (below $\approx 50\mathrm{\text{K}}$ for $x=0.2$) before superconductivity sets in. Based on the results, the third type electronic phase diagram in Fe-based high-$T_c$ superconductors is revealed.

We provide results for the leading superconducting instabilities for a model pertaining to ${\mathrm{\text{Sr}}}_2{\mathrm{\text{RuO}}}_4$ obtained within spin-fluctuation mediated superconductivity in the very weak-coupling limit. The theory incorporates spin–orbit coupling (SOC) effects both in the band structure and in the pairing kernel in the form of associated magnetic anisotropies. The leading superconducting phase is found to be $d_{x^2-y^2}$ and a nodal $s$-wave state. However, the odd-parity helical solution can become leading either for small SOC and Hund’s coupling $J$ in the weak $U$-limit, or in the opposite limit with large SOC and $J$ at larger values of the Hubbard-$U$. The odd-parity chiral solution is never found to be leading. Finally we discuss the form of the resulting superconducting spectral gaps in the different explored parameter regimes.

Within the framework of the kinetic energy-driven superconductivity, the complicated line shape in the electron quasiparticle excitation spectrum of the electron-doped cuprate superconductors is studied. It is shown that as in the hole-doped counterparts, the momentum and energy dependence of the quasiparticle scattering rate in the electron-doped cuprate superconductors has a well-pronounced peak structure at around the antinodal and nodal regions. However, this peak structure is absent from the hot spots. This special momentum and energy dependence of the quasiparticle scattering rate therefore generates a remarkable peak-dip-hump structure in the electron quasiparticle excitation spectrum of the electron-doped cuprate superconductors at around the antinodal and nodal regions except for at around the hot spots, where the peak-dip-hump structure is absent. The theory also indicates that there is a common physical origin for the peak-dip-hump structure both in the hole- and electron-doped cuprate superconductors.

Certain significant fallacies are involved in discussions of the high-$T_c$ mechanism unsolved for over 30 years in cuprate superconductors. These fallacies are explored with the aim of unraveling this mechanism. Moreover, using polarized electronic Raman scattering in inhomogeneous underdoped cuprate superconductors, the intrinsic nonlinear Raman spectrum is obtained by subtracting the pseudogap characteristic of a nonlinear from the linear Raman spectrum measured in the ${\mathrm{\text{B}}}_{2\mathrm{\text{g}}}$ mode of the node area below the critical temperature. The intrinsic nonlinear behavior implies the existence of the nodal superconducting gap denying $d_{x^2-y^2}$-wave pairing symmetry. An origin of the nodal superconducting gap is discussed.

Ultrafast laser pulses can redistribute charges in Mott insulators on extremely short time scales, leading to the fast generation of photocarriers. It has recently been demonstrated that these photocarriers can form a novel $\eta$-paired condensate at low temperatures, featuring a staggered superconducting pairing field. In this conference paper, we discuss the origin of the $\eta$-paired hidden phase and its optical response, which may be detected in a pump-probe experiment. The hidden phase may be relevant for possible light-induced superconductivity in Mott insulators.

A model of twisted bilayer graphene has been offered on the base of developed quasi-relativistic approach with high energy $k\cdot p$-Hamiltonian. Monolayer-graphene twist is accounted as a perturbation of monolayer-graphene Hamiltonian in such a way that at a given point of the Brillouin zone there exists an external non-Abelian gauge field of another monolayer. Majorana-like resonances have been revealed in the band structure of model at a magic rotation angle ${𝜃}_M=1.05^{\circ }$. The simulations have also shown that a superlattice energy gap existing at a rotation angle $1.08^{\circ }$ vanishes at a rotation angle $1.0^{\circ }$.

The structural stability and electronic properties of ${\mathrm{\text{Ca}}}_{2.75}{\mathrm{\text{C}}}_{60}$ are probed by means of high-pressure Raman spectroscopy using two different pressure transmitting media that lead to practically identical results. Although ${\mathrm{\text{Ca}}}_{2.75}{\mathrm{\text{C}}}_{60}$ is isostructural with the rare-earth metal fulleride, ${\mathrm{\text{Sm}}}_{2.75}{\mathrm{\text{C}}}_{60}$ of the same stoichiometry, the pressure coefficients of characteristic intramolecular ${\mathrm{\text{C}}}_{60}$ modes are larger than those in the ${\mathrm{\text{Sm}}}_{2.75}{\mathrm{\text{C}}}_{60}$ counterpart but similar to those in the pristine ${\mathrm{\text{C}}}_{60}$ solid. Since the reduced pressure coefficients in the Sm fulleride have been attributed to strong $\mathrm{\text{Sm}}\text{-}{\mathrm{\text{C}}}_{60}$ coupling, our findings for the isostructural Ca fulleride are compatible with a reduced orbital mixing and $\mathrm{\text{Ca}}\text{-}{\mathrm{\text{C}}}_{60}$ coupling.

We carried out ab initio calculations for technologically important ABO₃ perovskite, such as, SrTiO₃, BaTiO₃, SrZrO₃ and PbZrO₃ (001), (011) and (111) nano-surfaces, interfaces and bulk $F$-centers. For SrTiO₃, BaTiO₃, SrZrO₃ and PbZrO₃ (001) nano-surfaces, as a rule, all first surface layer atoms relax inward, whereas all second layer atoms relax outward, and, typically, all third surface layer atoms, again, relax inward. Calculated (001) surface energies for SrTiO₃, BaTiO₃, SrZrO₃ and PbZrO₃ perovskites are almost equal for both BO₂ and AO-terminations, and always smaller than the (011) and (111) surface energies. We discussed the results of our ab initio calculations dealing with BaTiO₃/SrTiO₃ and SrZrO₃/PbZrO₃ (001) interfaces. We analyzed systematic trends in ABO₃ perovskite bulk $F$-center ab initio calculations.

The spin-sensitive charge oscillation, controlled by an external magnetic field, was recently proposed as a mechanism of transformations of qubits, defined as two-electron spin-charge Wannier molecules in a square quantum dot.¹ The paper expands this idea by including the effects of Rashba-type spin-orbit coupling. The problem is studied theoretically by mapping the system to an analytic effective Hamiltonian for 8 low-energy states, comprising singlet and triplet on each dot diagonal. The validity of mapping is confirmed by comparing the energy and spin of full and mapped system, and also by the reproduction of charge-oscillation dynamics in the presence of magnetic flux. The newly introduced Rashba coupling significantly enriches the system dynamics, affecting the magnitude of charge oscillations and allowing the controlled transitions between singlet and triplet states due to the spin rotations, induced by spin-orbit coupling. The results indicate the possibility for use of the studied system for quantum information processing, while possible extensions of the system to serve as a qubit in a universal quantum computer, fulfilling all five Di Vincenzo criteria, is also discussed.

The appearance of electron correlations–induced glassy state of low-energy phonons in the non-dimeric organic charge transfer salt in $𝜃$-(BEDT-TTF)₂CsZn(SCN)₄ is considered as a strong evidence of charge lattice coupling in molecular charge transfer salts. We discuss the temperature and the magnetic field dependences of the heat capacity of this salt in terms of the soft potential model to describe the thermodynamic properties of enhanced phonons that occur in molecular glasses. The evaluated $\beta$ term of $𝜃$-(BEDT-TTF)₂CsZn(SCN)₄ is about 30 mJ K${}^{-4}$ mol${}^{-1}$, which is much larger than other charge transfer salts described by the low-temperature approximation of the Debye model. The magnetic fields dependence of the boson peak is almost negligible, but the low-energy term, for example, in the temperature-linear term of heat capacity shows a slight change, probably due to the small amount of localized spin moments. The comparison with other systems is also performed.

We have grown, by Pulsed Laser Deposition (PLD), a large number of YBa₂Cu₃O₇ films with artificial pinning centers, with various impurities, various architectures and thickness, and various techniques of nano-scale pinning engineering: substrate decoration, BaZrO₃ (BZO) nano-inclusions, and (quasi) multilayer architecture. Here we will present the results regarding vortex matter, dynamics, and pinning, in some of our best samples. Magnetic relaxation studies with magnetic field perpendicular to the film show that in both cases the splayed defects and/or nanoscale inclusions help reduce the dissipation in certain field-temperature range, by inhibiting the detrimental double vortex-kink formation. The response of the vortex system to AC excitation is rather complex but extremely useful for the characterization of the vortex dynamics. We have measured AC susceptibility in various DC fields, with various AC fields and frequencies and found that the effective vortex activation energy $U_{\mathrm{\text{eff}}}$ has a logarithmic dependence on the AC-field-induced current density $J$. Synergetic pinning centers assured a low anisotropy of the critical current for various field orientations, a property desired in coil/solenoid applications.

The main goal of our research is to find the connection between micro particles and microorganisms motion in the Nature, considered as Brownian’s Motion within the fractal’s nature. For ceramics and generally material science it is important to clarify the particles motion and other phenomena, especially for grains and pores. Our idea is to establish control over the relation order–disorder on particle motion and their collision effects by Brownian motion phenomena in the frame of fractal nature matter. We performed some experiments and got interesting results based on microorganism motion initiated by different outer energetic impulses. This is practically the idea of biomimetic correlation between particles and microorganisms Worlds, what is very original and leads towards biunivocal different phenomena’s understanding. Another idea is to establish some controlling effects for electro ceramic particle motion in chemical-materials sciences consolidation by some phenomena in the nature. These important research directions open new frontiers with very specific reflections for future of microelectronics materials.

The automatic frequency tuning method in the high-pressure ac calorimetry system constructed to measure heat capacity for molecules-based compounds with CuBe+NiCrAl cramp-type pressure cell is reported. This development is performed for increasing resolution and temperature ranges of the heat capacity measurements under external pressure up to 2.0 GPa. The system can check the appropriate conditions by tracing frequency dependence of $\omega T_{\mathrm{\text{ac}}}$ to determine the oscillation frequency at the center of the plateau region of this value. The experiments using the powder samples of metal complexes clarified that the appropriate frequency changes sensitively depending on the difference of temperature and that of external pressures, especially at low temperature region. It decreases with increasing temperature and this relation was found to be almost linear with temperature in ambient pressure and under pressure conditions. The change of thermal diffusion from the sample part to the heat bath should be treated carefully in order to get enough resolution in high pressure AC heat capacity measurements of molecule-based compounds.

The role of the gate voltage in the regulation of electroluminescence (EL) of a molecular photodiode with asymmetric localization of electron density on the frontier highest occupied and lowest unoccupied molecular orbitals of the photochromic molecule is considered. It is shown that the gate voltage can have a significant effect on the formation of EL in devices where one of the orbital energy levels are outside the gap between the biased Fermi levels of the electrodes. The role of the gate voltage consists in shifting the position of the orbital energy levels until both frontier levels fall into the gap and thereby provide a resonant mechanism for the formation of EL. This leads to the inclusion of EL at a lower bias voltage than that which includes EL at zero gate voltage. In addition, the shift in energy levels caused by the gate voltage explains the mechanism for controlling the kinetics of bipolarity formation. The effect is carried out by turning on and off the resonant hopping’s of the electron between the conducting states of the electrodes and molecular orbitals localized at different distances from the electrode surfaces.

Complex drugs based on C${}_{60}$ fullerence and pyrrole derivatives are promising for the development of antitumor and anti-inflammatory agents for use in targeted therapy, but have little efficiency in the synthesis of such complexes and have virtually no stable synthesis products with fullerene. The stability of the nanocomplex based on [60] PCBA and the intermediate compound based on “$1H$-pyrrole-2,5-dione” effective for the synthesis of targeted therapeutic agents was investigated by means of numerical calculations. The application of quantum-chemical methods in the Gaussian package establishes the stability of the nanocomplex at human body temperatures, the possibility of using such complexes for further study of the therapeutic properties of the individual components of the decomposition products of the nanocomplex.

This paper presents a new method of describing electronic spectrum, thermodynamic potential, and electrical conductivity of disordered crystals based on the Hamiltonian of multi-electron system and diagram method for Green’s functions finding. Electronic states of a system were described by multi-band tight-binding model. The Hamiltonian of a system is defined on the basis of the wave functions of electron in the atom nucleus field. Electrons scattering on the oscillations of the crystal lattice are taken into account. The proposed method includes long-range Coulomb interaction of electrons at different sites of the lattice. Precise expressions for Green’s functions, thermodynamic potential and conductivity tensor are derived using diagram method. Cluster expansion is obtained for density of states, free energy, and electrical conductivity of disordered systems. We show that contribution of the electron scattering processes to clusters is decreasing along with increasing number of sites in the cluster, which depends on small parameter. The computation accuracy is determined by renormalization precision of the vertex parts of the mass operators of electron-electron and electron-phonon interactions. This accuracy also can be determined by small parameter of cluster expansion for Green’s functions of electrons and phonons.

We consider the influence of the fractality of a medium on the dynamics of charged particles in the external magnetic field and show a significant increase in the sensitivity to weak effects with the fractality of a medium. The dynamical processes in the nonequilibrium nonconservative medium are analyzed in the framework of the Tsallis nonextensive thermodynamics with the use of Jackson’s derivatives.