arXiv subjects

V. R. Shaginyan

Publications and source records attributed to V. R. Shaginyan.

106 records · Page 6Linked to original sources

arXiv2002

Relationships between the superconducting gap, pseudogap and transition temperature in High-T_c superconductors

The crossover from superconducting gap to pseudogap is considered. We show that the superconductivity is destroyed at the temperature $T_c$, with the superconducting gap being smoothly transformed into the pseudogap. Relations, which are of general interest, between the maximum values of the gap $Δ_1$, $T_c$ and $T^*$ are established. We presents arguments that $T^*$ as a function of the doping level $x$ is approximately a straight line that crosses the abscissa at some point $x_{FC}$. In the vicinity of this point $T^*$ coincides with $T_c$, and at $x=x_{FC}$ the fermion condensation quantum phase transition takes place.

cond-mat.str-el↗

arXiv2002

Coulomb Energy of Nuclei

The density functional determining the Coulomb energy of nuclei is calculated to the first order in $e^2$. It is shown that the Coulomb energy includes three terms: the Hartree energy; the Fock energy; and the correlation Coulomb energy (CCE), which contributes considerably to the surface energy, the mass difference between mirror nuclei, and the single-particle spectrum. A CCE-based mechanism of a systematic shift of the single-particle spectrum is proposed. A dominant contribution to the CCE is shown to come from the surface region of nuclei. The CCE effect on the calculated proton drip line is examined, and the maximum charge $Z$ of nuclei near this line is found to decrease by 2 or 3 units. The effect of Coulomb interaction on the effective proton mass is analyzed.

nucl-th↗

arXiv2001

Trapped Atomic Fermi Gases

A many-body system of fermion atoms with a model interaction characterized by the scattering length $a$ is considered. We treat both $a$ and the density as parameters assuming that the system can be created artificially in a trap. If $a$ is negative the system becomes strongly correlated at densities $ρ\sim |a|^{-3}$, provided the scattering length is the dominant parameter of the problem. It means that we consider $|a|$ to be much bigger than the radius of the interaction or any other relevant parameter of the system. The density $ρ_{c1}$ at which the compressibility vanishes is defined by $ρ_{c1}\sim |a|^{-3}$. Thus, a system composed of fermion atoms with the scattering length $a\to -\infty$ is completely unstable at low densities, inevitably collapsing until the repulsive core stops the density growth. As a result, any Fermi system possesses the equilibrium density and energy if the bare particle-particle interaction is sufficiently strong to make $a$ negative and to be the dominant parameter. This behavior can be realized in a trap. Our results show that a low density neutron matter can have the equilibrium density.

cond-mat.stat-mech↗

arXiv2001

Theory of high-T_c superconductivity based on the fermion-condensation quantum phase transition

A theory of high temperature superconductivity based on the combination of the fermion-condensation quantum phase transition and the conventional theory of superconductivity is presented. This theory describes maximum values of the superconducting gap which can be as big as $Δ_1\sim 0.1ε_F$, with $ε_F$ being the Fermi level. We show that the critical temperature $2T_c\simeqΔ_1$. If there exists the pseudogap above $T_c$ then $2T^*\simeqΔ_1$, and $T^*$ is the temperature at which the pseudogap vanishes. A discontinuity in the specific heat at $T_c$ is calculated. The transition from conventional superconductors to high-$T_c$ ones as a function of the doping level is investigated. The single-particle excitations and their lineshape are also considered.

cond-mat.str-el↗

arXiv2001

Fermion-condensation quantum phase transition in high temperature superconductors

The effect of a quantum phase transition associated with the appearance of the fermion condensate in an electron liquid on the properties of superconductors is considered. It is shown that the electron system in both superconducting and normal states exhibits characteristic features of a quantum protectorate after the point of this fermion-condensation quantum phase transition. The single-particle spectrum of a superconductor can be represented by two straight lines corresponding to two effective masses $M^*_{FC}$ and $M^*_{L}$. The $M^*_{FC}$ mass characterizes the spectrum up to the binding energy $E_0$ and $M^*_L$ determines the spectrum at higher binding energies. Both effective masses are retained in the normal state. These results are used to explain the lineshape of single-particle excitations and other properties of high-$T_c$ superconductors and are in a good agreement with recent experimental data.

cond-mat.str-el↗

arXiv2001

Fermion-Condensation Quantum Phase Transition in High-T_c Superconductors

The effect of a quantum phase transition associated with the appearance of the fermion condensate in an electron liquid on the properties of superconductors is considered. It is shown that the electron system in both superconducting and normal states exhibits characteristic features of a quantum protectorate after the point of this fermion-condensation quantum phase transition. The single-particle spectrum of a superconductor can be represented by two straight lines corresponding to two effective masses $M^*_{FC}$ and $M^*_{L}$. The $M^*_{FC}$ mass characterizes the spectrum up to the binding energy $E_0$, which is of the order of the superconducting gap in magnitude, and $M^*_{L}$ determines the spectrum at higher binding energies. Both effective masses are retained in the normal state; however, $E_0=4T$. These results are used to explain the lineshape of single-particle excitations and other remarkable properties of high-$T_c$ superconductors and are in a good agreement with recent experimental data.

cond-mat.str-el↗

arXiv2001

Comment on "Evidence for Coexistence of the Gap and the Pseudogap in Bi-2212 from Intrinsic Tunneling Spectroscopy"

It is argued that the superconducting and pseudogaps represent different coexisting phenomena, and this observation speaks against the precursor-superconductivity scenario of the pseudogap; see, V.M. Krasnov et al., Phys. Rev. Lett. 84, 5860 (2000). We present brief comments showing that the experimental facts of this paper do not contradict to the observations that the pseudogap in underdoped Bi-2212 smoothly evolves into a pseudogap above critical temperature $T_c$, and the large energy gap below $T_c$ is mainly of superconducting origin. We show that appearance of this pseudogap is closely related to a new energy scale observed recently.

cond-mat.str-el↗

arXiv2001

Quasiparticle picture of high temperature superconductors in the frame of Fermi liquid with the fermion condensate

A model of a Fermi liquid with the fermion condensate (FC) is applied to the consideration of quasiparticle excitations in high temperature superconductors, in their superconducting and normal states. Within our model the appearance of the fermion condensate presents a quantum phase transition, that separates the regions of normal and strongly correlated electron liquids. Beyond the phase transition point the quasiparticle system is divided into two subsystems, one containing normal quasiparticles and the other --- fermion condensate localized at the Fermi surface and characterized by almost dispersionless single-particle excitations. In the superconducting state the quasiparticle dispersion in systems with FC can be presented by two straight lines, characterized by effective masses $M^*_{FC}$ and $M^*_L$, respectively, and intersecting near the binding energy which is of the order of the superconducting gap. This same quasiparticle picture persists in the normal state, thus manifesting itself over a wide range of temperatures as new energy scales. Arguments are presented that fermion systems with FC have features of a quantum protectorate.

cond-mat.str-el↗

arXiv2000

Quasiparticles in a strongly correlated liquid with the fermion condensate: applications to high-temperature superconductors

A model of a strongly correlated electron liquid based on the fermion condensation (FC) is extended to high-temperature superconductors. Within our model, the appearance of FC presents a boundary separating the region of a strongly interacting electron liquid from the region of a strongly correlated electron liquid. We study the superconductivity of a strongly correlated liquid and show that under certain conditions, the superconductivity vanishes at temperatures $T>T_c\simeq T_{node}$, with the superconducting gap being smoothly transformed into a pseudogap. As the result, the pseudogap occupies only a part of the Fermi surface. The gapped area shrinks with increasing the temperature and vanishes at $T=T^*$. The single-particle excitation width is also studied. The quasiparticle dispersion in systems with FC can be represented by two straight lines characterized by the respective effective masses $M^*_{FC}$ and $M^*_L$, and intersecting near the binding energy that is of the order of the superconducting gap. It is argued that this strong change of the quasiparticle dispersion at the binding can be enhanced in underdoped samples because of strengthening the FC influence. The FC phase transition in the presence of the superconductivity is examined, and it is shown that this phase transition can be considered as kinetic energy driven.

cond-mat.str-el↗

arXiv2000

Quasiparticle dispersion and lineshape in a strongly correlated liquid with the fermion condensate

A model of a strongly correlated electron liquid based on the fermion condensation (FC) is applied to the consideration of high temperature superconductors in its superconducting and normal states. Within our model the appearance of FC presents a boundary, separating the region of strongly interacting electron liquid from the region of strongly correlated electron liquid. We show that at temperatures $T<T_c$ the quasiparticle dispersion in systems with FC can be presented by two straight lines, characterized by effective masses $M^*_{FC}$ and $M^*_L$, respectively, and intersecting near the binding energy which is of the order of the superconducting gap. This same picture persists in the normal state being modified only by the presence of the quasiparticle width. We argue that this strong change of the quasiparticle dispersion can be enhanced in underdoped samples due to strengthening the FC influence. The single particle excitation width and lineshape are also studied.

cond-mat.supr-con↗

arXiv2000

On the compatibility of causality and symmetry (Comments on "Analysis of causality in time-dependent density functional theory")

It is argued that there exists the only one inverse of the linear response function $χ$, i.e. $χ^{-1}$, which depends symmetrically of its spatial-times variables, see M.K. Harbola, and A. Banerjee, Phys. Rev. A {\bf 60}, 5101 (1999). Some brief comments on this consideration are presented. We show instead, that it is possible to construct the causal inverse also. At the same time we confirm the main statement of M.K. Harbola and A. Banerjee that in fact there is no contradiction between the symmetry and causality.

cond-mat.dis-nn↗

arXiv2000

Calculations of single particle spectra in density functional theory

In the context of the density functional theory we consider the single particle excitation spectra of electron systems. As a result, we have related the single particle excitations with the eigenvalues of the corresponding Kohn-Sham equations. In a case when the exchange correlation functional is approximated by the exchange functional the coupling equations are very simple, while the single particle spectra do not coincide neither with the eigenvalues of the Kohn-Sham equations nor with the ones of the Hartree-Fock equations.

cond-mat↗

arXiv2000

Neutron matter with a model interaction

An infinite system of neutrons interacting by a model pair potential is considered. We investigate a case when this potential is sufficiently strong attractive, so that its scattering length tends to infinity. It appeared, that if the structure of the potential is simple enough, including no finite parameters, reliable evidences can be presented that such a system is completely unstable at any finite density. The incompressibility as a function of the density is negative, reaching zero value when the density tends to zero. If the potential contains a sufficiently strong repulsive core then the system possesses an equilibrium density. The main features of a theory describing such systems are considered.

nucl-th↗

arXiv2000

Description of Dynamic Properties of Finite Electron Systems in Density Functional Theory

The self consistent version of the density functional theory (DFT) is presented, which allows to calculate the ground state and dynamic properties of finite multi-electron systems such as atoms, molecules and clusters. The exact functional equation for the effective interaction, using which one can construct the action functional, density functional, the response functions, and excitation spectra of the considered systems, are discussed. We have also related the eigenvalues of the single-particle Kohn-Sham equations to the real single-particle spectra. We begin with the standard action functional and show that it is useful in calculating the linear response functions via which all physical characteristics of a many-body system can be expressed. It is shown, that function can be causal (retarded), noncausal and advanced ones. This resolves the well known paradox related to the causality and symmetry properties of the response functions and the effective interaction.

cond-mat↗

arXiv1998

Ground state instability in systems of strongly interacting fermions

We analyze stability of a fermion system with model repulsive pair interaction potential. The possibility for different types of restructuring of the Fermi ground state (at sufficiently great coupling constant) is related to the analytic properties of such potential. In particular, for the screened Coulomb law it is shown that the restructuring cannot be of the Fermi condensation type, known earlier for some exactly solvable models, and instead it belongs to the class of topological transitions (TT). For this model, a phase diagram has been built in the variables "screening parameter - coupling constant" which displays two kinds of TT: a 5/2-kind similar to the known Lifshitz transitions in metals, and a 2-kind characteristic for a uniform strongly interacting system.

cond-mat.str-el↗

arXiv1996

Fermion Condensation and Non Fermi Liquid Behavior in a Model with Long Range Forces

The phenomenon of the so called Fermion condensation, a phase transition analogous to Bose condensation but for Fermions, postulated in the past to occur in systems with strong momentum dependent forces, is reanalysed in a model with infinite range interactions. The strongly non Fermi Liquid behavior of this system is demonstrated analytically at $T=0$ and at $T\neq 0$ in the superconducting and normal phases. The validity of the quasiparticle picture is investigated and seems to hold true for temperatures less than the characteristic temperature $T_f$ of the Fermion condensation.

cond-mat↗