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Elena Gubankova

Publications and source records attributed to Elena Gubankova.

At least 19 recordsLinked to original sources
arXiv2026

Scaling limits of complex Sachdev-Ye-Kitaev models and holographic geometry

We compare different limits of the Sachdev-Ye-Kitaev model of $N$ complex fermion with $p$-fermion interactions. First, we compute the fermion Green's function and free energy in the limit of large $N$ followed subsequently by the limit of large $p$. Next, we examine the `double-scaling' limit in which the large $N,p$ limits are taken at fixed $λ= p^2/N$. Earlier results on the latter limit are resummed for small $λ$, and shown to match our results for the first limit. We also describe the holographic match of our results to two-dimensional Jackiw-Teitelboim gravity with an additional $U(1)$ gauge field.

hep-th↗
arXiv2018

On S-duality for holographic p-wave superconductors

We consider the generalization of the S-duality transformation previously investigated in the context of the FQHE and s-wave superconductivity to p-wave superconductivity in 2+1 dimensions in the framework of the AdS/CFT correspondence. The vector Cooper condensate transforms under the S-duality action to the pseudovector condensate at the dual side. The 3+1-dimensional Einstein-Yang-Mills theory, the holographic dual to p-wave superconductivity, is used to investigate the S-duality action via the AdS/CFT correspondence. It is shown that in order to implement the duality transformation, chemical potentials both on the electric and magnetic side of the duality have to be introduced. A relation for the product of the nonabelian conductivities in the dual models is derived. We also conjecture a flavor S-duality transformation in the holographic dual to 3+1-dimensional QCD low-energy QCD with non-abelian flavor gauge groups. The conjectured S-duality interchanges isospin and baryonic chemical potentials.

hep-th↗
arXiv2013

Quantum corrected phase diagram of holographic fermions

We study the phases of strongly correlated electron systems in two spatial dimensions in the framework of AdS${}_4$/CFT${}_3$ correspondence. The AdS (gravity) model consists of a Dirac fermion coupled to electromagnetic field and gravity. To classify the ground states of strongly correlated electrons on the CFT side and to construct the full phase diagram of the system, we construct a quantum many-body model of bulk fermion dynamics, based on the WKB approximation to the Dirac equation. At low temperatures, we find a quantum corrected approximation to the electron star where the edge is resolved in terms of wavefunctions extended fully through AdS. At high temperatures, the system exhibits a {\em first} order thermal phase transition to a charged AdS-RN black hole in the bulk and the emergence of local quantum criticality on the CFT side. This change from the third order transition experienced by the semi-classical electron star restores the intuition that the transition between the critical AdS-RN liquid and the finite density Fermi system is of van der Waals liquid-gas type.

hep-th↗
arXiv2010

Collective modes in asymmetric ultracold Fermi systems

We derive the low energy effective action for the collective modes in systems of fermions interacting via a short-range s-wave attraction, featuring unequal chemical potentials for the two fermionic species (asymmetric systems). As a consequence of the attractive interaction, fermions form a condensate that spontaneously breaks the U(1) symmetry associated with total number conservation. Therefore at sufficiently small temperatures and asymmetries, the system is a superfluid. We reproduce previous results for the stability conditions of the system as a function of the four-fermion coupling and asymmetry. We obtain these results analyzing the coefficients of the low energy effective Lagrangian of the modes describing fluctuations in the magnitude (Higgs mode) and in the phase (Goldstone mode) of the difermion condensate. We find that for certain values of parameters, the mass of the Higgs mode decreases with increasing mismatch between the chemical potentials of the two populations, if we keep the scattering length and the gap parameter constant. Furthermore, we find that the energy cost for creating a position dependent fluctuation of the condensate is constant in the gapped region and increases in the gapless region. These two features may lead to experimentally detectable effects. As an example, we argue that if the superfluid is put in rotation, the square of the radius of the outer core of a vortex should sharply increase on increasing the asymmetry, when we pass through the relevant region in the gapless superfluid phase. Finally, by gauging the global U(1) symmetry, we relate the coefficients of the effective Lagrangian of the Goldstone mode with the screening masses of the gauge field.

cond-mat.supr-con↗
arXiv2009

Exotic superfluidity in cold atoms

We derived the low energy effective action for the collective modes in asymmetric fermionic systems with attractive interaction. We obtained the phase diagram in terms of the chemical potentials. It features a stable gapless superfluidity with one Fermi surface on the BEC side of the resonance. Also we predict a sharp increase in outer core of a vortex, i.e. vortex size, upon entering into the gapless phase. This may serve as a signature of a gapless phase.

cond-mat.quant-gas↗
arXiv2006

Stability conditions and Fermi surface topologies in a superconductor

Candidate homogeneous, isotropic superfluid or superconducting states of paired fermion species with different chemical potentials, can lead to quasiparticle excitation energies that vanish at either zero, one, or two spheres in momentum space. With no zeroes, we have a conventional BCS superconductor. The other two cases, ``gapless'' superconductors, appear in mean field theory for sufficiently large mismatches and/or sufficiently large coupling strengths. Here we examine several stability criteria for those candidate phases. Positivity of number susceptibility appears to provide the most powerful constraint, and renders all the two-zero states that we have examined mechanically unstable. Our results should apply directly to ultracold fermionic atom systems.

cond-mat.supr-con↗
arXiv2005

Conditions for existance of neutral strange quark matter

Breached pairing solutions to the gap equation are obtained analytically in for two and three quarks and for low and high temperatures. We compare the energy of these states to that of other homogeneous states under the condition of electric neutrality. We found the two-flavor BP and the three flavor mixed BCS-BP phases, which are stable over a wide range of parameters. Both phases contain four BP modes in the quasiparticle spectrum.

hep-ph↗
arXiv2005

Stability Criteria for Breached Pair Superfluidity

We present simple, concrete, two-fermion models that exhibit thermodynamically stable isotropic translationally-invariant gapless superfluid states (breached pair superfluidity). The mass ratio between the components and the momentum structure of the interaction are crucial for determining the stability of such states: Idealized, momentum-independent (``contact'') interactions are insufficient.

hep-ph↗
arXiv2003

Breached pairing superfluidity: Possible realization in QCD

We propose a wide universality class of gapless superfluids, and analyze a limit that might be realized in quark matter at intermediate densities. In the breached pairing color superconducting phase heavy $s$-quarks, with a small Fermi surface, pair with light $u$ or $d$ quarks. The groundstate has a superfluid and a normal Fermi component simultaneously. We expect a second order phase transition, as a function of increasing density, from the breached pairing phase to the conventional color-flavor locked (CFL) phase.

hep-ph↗
arXiv2002

Solving QCD Hamiltonian for Bound States

We consider the eigenstate problem for a Hamiltonian operator of the field theory. Methods of construction the effective field theoretical Hamiltonians for which the eigenstate problem may be solved are discussed. In particular, we discuss the method of flow equations from a general perspective as well as in application to the gauge field theories. Flow equations transform the Hamiltonian to a block-diagonal form with the number of particles conserved in each block and thus reduce the original bound state problem to a set of coupled eigenstate equations with an effective Hamiltonian in each sector. Applications of flow equations to the Hamiltonians of QED and QCD in the light-front gauge and the QCD Hamiltonian in the Coulomb gauge are considered. Using flow equations, we derive the effective Hamiltonians as well as the renormalized gap equations and the Bethe-Salpeter equations for the bound states in these theories. We show that the obtained equations are finite in both UV and IR regions and are completely renormalized in UV, i.e. the corresponding solutions do not depend on the cut-off $Λ$. We calculate positronium spectrum, glueball masses, $π-ρ$ mass splitting, gluon and chiral quark condensates and compare our results with the covariant calculations and experimental results. Use of flow equations to calculate the dynamical terms is critical to achive good agreement with experimental results.

hep-ph↗
arXiv2002

Flow equations for chiral problem in QCD

We analyze the chiral symmetry breaking of QCD in the Coulomb gauge. Using flow equations, we derive the renormalized gap equation and the Bethe-Salpeter equation and show that they are finite in both UV and IR regions. No additional UV renormalization is required in the chiral limit. We take into account the hyperfine interaction as well as chiral symmetry breaking and obtain the $π-ρ$ mass splitting caused by the instantaneous and dynamical interactions.

hep-ph↗
arXiv2000

Flow equations for quark-gluon interactions in light-front QCD

The flow-equation method of continuous unitary transformations is used to eliminate the minimal quark-gluon interaction in the light-front quantized QCD Hamiltonian. The coupled differential equations in the two lowest Fock sectors correspond to the renormalization of the light-front gluon mass and the generation of an effective quark-antiquark (as well as gluon-gluon) interaction. The original gauge field coupling is completely eliminated, even in the presence of degenerate states connected by this interaction. Further, a more singular $1/q^4$ behavior for the quark and gluon effective interactions at small gluon momenta is obtained, due to the asymptotic behavior of the effective gluon mass for small cutoff. We discuss the consequences of this asymptotic behavior and possible confinement implications.

hep-ph↗
arXiv2000

Flow equations in the light-front QCD

Flow equations method of continuous unitary transformations is used to eliminate the minimal quark-gluon interaction in the light-front quantized QCD Hamiltonian. The coupled differential equations in the two lowest Fock sectors correspond to the renormalization of the light-front gluon mass and the generation of effective quark-antiquark interaction. The influence of the renormalization of the gluon effective mass on the elimination of the quark-gluon coupling and the induced quark-antiquark interaction is taken into account. Namely, the original gauge field coupling can be completely eliminated, even when the states connected by this interaction are degenerate. Furthermore, even in the case where effective interaction, obtained within perturbative schemes (bound state perturbation theory or perturbative similarity approach), is not defined, we obtain more singular behavior $1/q^4$ at small gluon momenta. This is due to asymptotic behavior of the effective gluon mass at small cutoffs. By discussing the consequences of this asymptotic behavior, it seems that our approach is superior to perturbation theory and to perturbative similarity approach.

hep-ph↗
arXiv2000

Flow Equations for Gluodynamics in the Coulomb Gauge

A systematic procedure to consistently formulate a field theoretical, QCD bound state problem with a fixed number of constituents is outlined. The approach entails applying the Hamiltonian flow equations, which are a set of continuous unitary transformations, to a QCD motivated Hamiltonian with a confining interaction. The method is developed in detail for gluodynamics in the Coulomb gauge to obtain an effective block-diagonal Hamiltonian appropriate to a reduced Fock space with fixed number of dynamical gluons. Standard many-body techniques are used to numerically diagonalize this Hamiltonian in a constituent two gluon Fock space. The calculated gluon condensates and glueball masses are in good agreement with QCD sum rule and lattice results.

hep-ph↗
arXiv1999

Flow equations in light-front QCD

Light-front QCD is studied by the method of flow equations. Dynamical gluon mass is generated, which evolves with the cut-off according to renormalization group equation. Eliminating by flow equations the quark gluon coupling with the dynamical gluon mode, one obtains an effective interaction between quark and antiquark which exibits the Coulomb and confining singularities. The scale, which regulates the light-front IR singularities in the gluon sector, defines the string tension of confining interaction. The mechanism of confinement in the light-front formalism is suggested, based on the singular nature of the light-front gauge.

hep-ph↗
arXiv1999

Hamiltonian renormalization for bound state problem in gluodynamics

The systematic approach to study bound states in gluodynamics is presented. The method utilizes flow equations together with low-energy phenomenology, that provides the perturbative renormalization scaling in conjuction with the change of the basis to constituent gluon states. The renormalized effective Hamiltonian of gluodynamics up to the second order is obtained at low energies, which provides a kind of constituent gluon model for glueball bound states. The approach allows to include perturbative QCD corrections into nonperturbative calculations of many-body techniques. The performed numerical calculations support the constituent picture of hadronic observables.

hep-ph↗
arXiv1999

Solving the QCD Hamiltonian for bound states

The systematic approach to study bound states in quantum chromodynamics is presented. The method utilizes nonperturbative flow equations in the confining background, that makes possible to perform perturbative renormalization and to bring the QCD Hamiltonian to a block-diagonal form with the number of quasiparticles conserving in each block. The effective block-diagonal Hamiltonian provides constituent description for hadron observables. The renormalized to the second order effective Hamiltonian of gluodynamics in the Coulomb gauge is obtained at low energies. The masses for scalar and pseudoscalar glueballs are predicted.

hep-ph↗