arXiv subjects

V. A. Khodel

Publications and source records attributed to V. A. Khodel.

68 records · Page 4Linked to original sources

arXiv2003

Spin degrees of freedom and flattening of the spectra of single-particle excitations in strongly correlated Fermi systems

The impact of long-range spin-spin correlations on the structure of a flat portion in single-particle spectra $ξ(p)$, which emerges beyond the point, where the Landau state loses its stability, is studied. We supplement the well-known Nozieres model of a Fermi system with limited scalar long-range forces by a similar long-range spin-dependent term and calculate the spectra versus its strength $g$. It is found that Nozieres results hold as long as $g>0$. However, with $g$ changing its sign, the spontaneous magnetization is shown to arise at any nonzero $g$. The increase of the strength $|g|$ is demonstrated to result in shrinkage of the domain in momentum space, occupied by the flat portion of $ξ(p)$, and, eventually, in its vanishing.

cond-mat.str-el↗

arXiv2003

Divergence of the effective mass near a density wave instability in a MOSFET system

We study the renormalization of the Fermi-liquid parameters in the vicinity of a density wave quantum phase transition, which should occur in MOSFET systems at low densities. First, using a perturbative RPA treatment of fluctuations, we calculate the electronic self-energy and show that the effective mass diverges at the density wave transition point. Second, we go beyond perturbation theory, making use of the exact Pitaevskii identities. Within this exact analysis, we also find a divergence of the effective mass, which occurs at higher densities in the fluctuation region, as compared to the perturbation theory. This result signals the break-down of conventional Fermi-liquid description in the vicinity of the transition point. The divergence of the effective mass gives rise to a singular behavior of the electronic compressibility. We suggest that the experimentally observed enhancement of the effective mass is a precursor to a second order thermodynamic phase transition into a glassy density wave state.

cond-mat.str-el↗

arXiv2003

Unconventional Superconductivity in Two-Dimensional Electron Systems with Longe-Range Correlations

Properties of superfluid states of two-dimensional electron systems with critical antiferromagnetic fluctuations are investigated. These correlations are found to result in the emergence of rapidly varying in the momentum space terms in all components of the mass operator, including the gap function $Δ({\bf p})$. It is shown that a domain, where these terms reside, shrinks with the temperature, leading to a significant difference between the temperature $T_c$, at which superconductivity is terminated, and the temperature $T^*$, where the gap in the single-particle spectrum vanishes.

cond-mat.str-el↗

arXiv2003

$^3P_2$-$^3F_2$ Pairing in Dense Neutron Matter: The Spectrum of Solutions

The $^3P_2$-$^3F_2$ pairing model is generally considered to provide an adequate description of the superfluid states of neutron matter at densities some 2-3 times that of saturated symmetrical nuclear matter. The problem of solving the system of BCS gap equations expressing the $^3P_2$-$^3F_2$ model is attacked with the aid of the separation approach. This method, developed originally for quantitative study of S-wave pairing in the presence of strong short-range repulsions, serves effectively to reduce the coupled, singular, nonlinear BCS integral equations to a set of coupled algebraic equations. For the first time, sufficient precision becomes accessible to resolve small energy splittings between the different pairing states. Adopting a perturbative strategy, we are able to identify and characterize the full repertoire of real solutions of the $^3P_2$-$^3F_2$ pairing model, in the limiting regime of small tensor-coupling strength. The P-F channel coupling is seen to lift the striking parametric degeneracies revealed by a earlier separation treatment of the pure, uncoupled $^3P_2$ pairing problem. Remarkably, incisive and robust results are obtained solely on the basis of analytic arguments. Unlike the traditional Ginzburg-Landau approach, the analysis is not restricted to the immediate vicinity of the critical temperature, but is equally reliable at zero temperature. Interesting connections and contrasts are drawn between triplet pairing in dense neutron matter and triplet pairing in liquid $^3$He.

nucl-th↗

arXiv2002

Superfluid Phases of Triplet Pairing

The phase transitions in dense neutron matter with triplet pairing have been investigated. The spectrum of phases of the $^3P_2$-$^3F_2$ model, which adequately describes pairing in this system, is identified and classified with the aid of a separation method for solving BCS gap equations associated with pairing states of arbitrary angular-momentum content. In addition to usual solutions involving a single value of the magnetic quantum number (and its negative), there exist ten real multicomponent solutions. Five of the corresponding angle-dependent order parameters have nodes, and five do not. In contrast to the case of superfluid $^3$He, transitions occur between phases with nodeless order parameters. Results for the temperature dependence of the competition between the various phases have been obtained.

nucl-th↗

arXiv2001

Non-BCS pairing in anisotropic strongly correlated electron systems in solids

The problem of pairing in anisotropic electron systems possessing patches of fermion condensate in the vicinity of the van Hove points is analyzed. Attention is directed to opportunities for the occurrence of non-BCS pairing correlations between the states belonging to the fermion condensate. It is shown that the physical emergence of such pairing correlations would drastically alter the behavior of the single-particle Green function, the canonical pole of Fermi-liquid theory being replaced by a branch point.

cond-mat.str-el↗

arXiv2001

Fermi Liquid without Quasiparticles and Electron Spectral Functions of Two-Dimensional High-$T_c$ Superconductors

Properties of strongly correlated two-dimensional (2D) electron systems in solids are studied on the assumption that these systems undergo a phase transition, called fermion condensation, whose characteristic feature is flattening of the electron spectrum $ε({\pf p})$. Unlike the previous models in the present study, the decay of single-particle states is properly taken into account. Remarkably, the value of the topological charge ($N=1/2$) remains unchanged, supporting the view that systems with a fermion condensate form a separate class of Fermi liquids. Results of calculations are found to be in qualitative agreement with ARPES data.

cond-mat.str-el↗

arXiv2001

Abnormal Occupation REvisited

It is demonstrated the Fermi surface of dense neutron matter may experience a rearrangement near the onset of pion condensation, due to strong momentum dependence of the effective interaction induced by spin-isospin fluctuations. In particular, the Fermi surface may take the form of a partially hollow sphere having a spherical hole in its center (a ``Lifschitz bubble''). Thus, a second (inner) Fermi surface may form as high-momentum single-particle states are filled and low-momentum states are vacated. The influence of this phenomenon on the superfluid transition temperature of the Fermi system is characterized with the help of a separation transformation of the BCS gap equation. This work may be viewed as a revival of the search for physical realizations of abnormal occupation in infinite, homogeneous Fermi systems -- plausible instances in which the quasiparticle distribution differs from that of an ideal Fermi gas and Fermi liquid theory breaks down.

cond-mat↗

arXiv2001

Superfluid Phase Transitions in Dense Neutron Matter

The phase transitions in a realistic system with triplet pairing, dense neutron matter, have been investigated. The spectrum of phases of the $^3P_2-^3F_2$ model, which adequately describes pairing in this system, is analytically constructed with the aid of a separation method for solving BCS gap equation in states of arbitrary angular momentum. In addition to solutions involving a single value of the magnetic quantum number (and its negative), there exist ten real multicomponent solutions. Five of the corresponding angle-dependent order parameters have nodes, and five do not. In contrast to the case of superfluid $^3$He, transitions occur between phases with nodeless order parameters. The temperature dependence of the competition between the various phases is studied.

nucl-th↗

arXiv2000

Rearrangement of the Fermi Surface of Dense Neutron Matter and Direct Urca Cooling of Neutron Stars

It is proposed that a rearrangement of single-particle degrees of freedom may occur in a portion of the quantum fluid interior of a neutron star. Such a rearrangement is associated with the pronounced softening of the spin-isospin collective mode which, under increasing density, leads to pion condensation. Arguments and estimates based on fundamental relations of many-body theory show that one realization of this phenomenon could produce very rapid cooling of the star via a direct nucelon Urca process displaying a $T^5$ dependence on temperature.

astro-ph↗

arXiv2000

Triplet Pairing in Neutron Matter

The separation method developed earlier by us [Nucl. Phys. {\bf A598} 390 (1996)] to calculate and analyze solutions of the BCS gap equation for $^1$S$_0$ pairing is extended and applied to $^3$P$_2$--$^3$F$_2$ pairing in pure neutron matter. The pairing matrix elements are written as a separable part plus a remainder that vanishes when either momentum variable is on the Fermi surface. This decomposition effects a separation of the problem of determining the dependence of the gap components in a spin-angle representation on the magnitude of the momentum (described by a set of functions independent of magnetic quantum number) from the problem of determining the dependence of the gap on angle or magnetic projection. The former problem is solved through a set of nonsingular, quasilinear integral equations, providing inputs for solution of the latter problem through a coupled system of algebraic equations for a set of numerical coefficients. An incisive criterion is given for finding the upper critical density for closure of the triplet gap. The separation method and its development for triplet pairing exploit the existence of a small parameter, given by a gap-amplitude measure divided by the Fermi energy. The revised BCS equations admit analysis revealing universal properties of the full set of solutions for $^3$P$_2$ pairing in the absence of tensor coupling, referring especially to the energy degeneracy and energetic order of these solutions. The angle-average approximation introduced by Baldo et al. is illuminated in terms of the separation-transformed BCS problem and the small parameter expansion...

nucl-th↗

arXiv1999

The interplay between flattening and damping of single particle spectra in strongly correlated Fermi systems

The self-consistent theory of the fermion condensation, a specific phase transition which results in a rearrangement of the single particle degrees of freedom in strongly correlated Fermi systems is developed. Beyond the phase transition point, the single particle spectra are shown to be flat. The interplay between the flattening and the damping of the single particle spectra at $T\to 0$ is investigated. The width $γ(ε)$ of the single particle states is found to grow up linearly with $ε$ over a wide range of energy as in a marginal Fermi liquid. Our results gain insight into the success of the phenomenological theory of the normal states of high-temperature superconductors by Varma et al.

cond-mat.str-el↗

arXiv1998

Universalities of Triplet Pairing in Neutron Matter

The fundamental structure of the full set of solutions of the BCS $^3 P_2$ pairing problem in neutron matter is established. The relations between different spin-angle components in these solutions are shown to be practically independent of density, temperature, and the specific form of the pairing interaction. The spectrum of pairing energies is found to be highly degenerate.

nucl-th↗

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↗