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 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 ≈ T _node, with the superconducting gap being smoothly transformed into a pseudogap. As a 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 effective masses and, intersecting near the binding energy that is on the order of the superconducting gap. It is argued that this strong change of the quasiparticle dispersion upon 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 driven by the kinetic energy.     

Quasiparticles of strongly correlated Fermi liquids at high temperatures and in high magnetic fields

Strongly correlated Fermi systems are among the most intriguing, best experimentally studied and fundamental systems in physics. There is, however, lack of theoretical understanding in this field of physics. The ideas based on the concepts like Kondo lattice and involving quantum and thermal fluctuations at a quantum critical point have been used to explain the unusual physics. Alas, being suggested to describe one property, these approaches fail to explain the others. This means a real crisis in theory suggesting that there is a hidden fundamental law of nature. It turns out that the hidden fundamental law is well forgotten old one directly related to the Landau-Migdal quasiparticles, while the basic properties and the scaling behavior of the strongly correlated systems can be described within the framework of the fermion condensation quantum phase transition (FCQPT). The phase transition comprises the extended quasiparticle paradigm that allows us to explain the non-Fermi liquid (NFL) behavior observed in these systems. In contrast to the Landau paradigm stating that the quasiparticle effective mass is a constant, the effective mass of new quasiparticles strongly depends on temperature, magnetic field, pressure, and other parameters. Our observations are in good agreement with experimental facts and show that FCQPT is responsible for the observed NFL behavior and quasiparticles survive both high temperatures and high magnetic fields.     

Phase behavior of compressible melts of multiblock polydisperse copolymers

In terms of the previously proposed model, specific features of the phase behavior of Markovian polydisperse copolymers with allowance for their compressibility have been investigated via bifurcation analysis followed by continuation with respect to a parameter that characterizes the deviation of the temperature of the system from its value on the spinodal. These features above all include competition between microphase separation and macrophase separation under conditions when the local instability of the homogeneous state appearing at the spinodal corresponds to the macrophase separation only. Nevertheless, it was shown that depending on the structural parameters, the global instability characterized by a cloud-point hypersurface can result in either macrophase or microphase separation, with the microphase separation occurring in the vicinity of the critical point. In this case, the results are consistent with the conclusions of the Landau theory of phase transitions, whose applicability limits with respect to deviation from the critical point have been evaluated in this study. Outside the range of applicability of this theory, cloud-point curves that correspond to macrophase separation and microphase separation are very similar. These conclusions remain valid over a wide range of compressibility whose influence has been assessed for the first time. It has been found that the type of copolymers under consideration has a characteristic feature that was not noticed previously: Namely, the distribution of density in the nucleus of a new phase in this case will look like a spatially localized solitonlike profile.     

Dissymmetrical tunneling in heavy-fermion metals

A tunneling conductivity between a heavy-fermion metal and a simple metallic point is considered. We show that, at low temperatures, this conductivity can be noticeably dissymmetrical with respect to the change of voltage bias. The dissymmetry can be observed in experiments on heavy-fermion metals whose electronic system has undergone the fermion-condensation quantum phase transition.     

Magnetic quantum criticality in quasi‐one‐dimensional Heisenberg antiferromagnet

We analyze exciting recent measurements [Phys. Rev. Lett. 114 (2015) 037202] of the magnetization, differential susceptibility and specific heat on one dimensional Heisenberg antiferromagnet Cu(C₄H₄N₂)(NO₃)₂ (CuPzN) subjected to strong magnetic fields. Using the mapping between magnons (bosons) in CuPzN and fermions, we demonstrate that magnetic field tunes the insulator towards quantum critical point related to so‐called fermion condensation quantum phase transition (FCQPT) at which the resulting fermion effective mass diverges kinematically. We show that the FCQPT concept permits to reveal the scaling behavior of thermodynamic characteristics, describe the experimental results quantitatively, and derive for the first time the (temperature—magnetic field) phase diagram, that contains Landau‐Fermi‐liquid, crossover and non‐Fermi liquid parts, thus resembling that of heavy‐fermion compounds.     

Flat bands and enigma of metamagnetic quantum critical regime in Sr3Ru2O7

Understanding the nature of field-tuned metamagnetic quantum criticality in the ruthenate Sr₃Ru₂O₇ has presented a significant challenge within condensed matter physics. It is known from experiments that the entropy within the ordered phase forms a peak, and is unexpectedly higher than that outside, while the magnetoresistivity experiences steep jumps near the ordered phase. We find a challenging connection between Sr₃Ru₂O₇ and heavy-fermion metals expressing universal physics that transcends microscopic details. Our construction of the T–B$T\text{–}B$ phase diagram of Sr₃Ru₂O₇ permits us to explain main features of the experimental one, and unambiguously implies an interpretation of its extraordinary low-temperature thermodynamic in terms of fermion condensation quantum phase transition leading to the formation of a flat band at the restricted range of magnetic fields B. We show that it is the flat band that generates both the entropy peak and the resistivity jumps at the QCPs.     

General properties of a magnetic-field-induced landau Fermi liquid in high-temperature superconductors and heavy fermion metals

It has been shown that the magnetic-field-induced transition from a non-Fermi-liquid state to a Fermi liquid state in the Tl₂Ba₂CuO_{6 + x} high-temperature superconductor is similar to a transition observed in heavy fermion metals. This behavior is explained in the theory of the Fermi condensate quantum-phase transition implying the existence of Landau quasiparticles. The Fermi condensate quantum-phase transition can be considered as a universal cause of the strongly correlated behavior observed in various metals and liquids such as high-temperature superconductors, heavy fermion metals, and two-dimensional Fermi systems.     

Quasiparticles in the theory of fermion condensation

It is shown that Landau’s quasiparticle formalism continues to work in systems with a fermion condensate. In the case of a finite system this formalism is suitable for describing the restructuring of states at the Fermi surface. It also works in an infinite system, and the idea of quasiparticles at low temperature as well-defined excitations at the Fermi surface remains valid. The quasiparticle lifetime is directly proportional to the temperature, and the density of states is inversely proportional to the temperature. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 9, 719–723 (10 May 1996)     

The soliton mechanism of phase separation of polydisperse multiblock copolymers

The equations of the thermodynamics of compressible polydisperse multiblock copolymers have been analytically and numerically studied. The possible density distributions of copolymer units over a wide temperature range including the cloud point and the spinodal temperature have been determined. Along with periodic structures, spatially localized soliton-type distributions have been identified in order to estimate the energy barrier that must be overcome for the occurrence of the initial stage of the phase transition.