arXiv subjects

I. A. Shovkovy

Publications and source records attributed to I. A. Shovkovy.

At least 55 records · Page 3Linked to original sources

arXiv2013

Radiative corrections to chiral separation effect in QED

We calculate the leading radiative corrections to the axial current in the chiral separation effect in dense QED in a magnetic field. Contrary to the conventional wisdom suggesting that the axial current should be exactly fixed by the chiral anomaly relation and is described by the topological contribution on the lowest Landau level in the free theory, we find in fact that the axial current receives nontrivial radiative corrections. The direct calculations performed to the linear order in the external magnetic field show that the nontrivial radiative corrections to the axial current are provided by the Fermi surface singularity in the fermion propagator at nonzero fermion density.

hep-ph↗

arXiv2012

Coexistence and competition of nematic and gapped states in bilayer graphene

In bilayer graphene, the phase diagram in the plane of a strain-induced bare nematic term, ${\cal N}_{0}$, and a top-bottom gates voltage imbalance, $U_{0}$, is obtained by solving the gap equation in the random-phase approximation. At nonzero ${\cal N}_0$ and $U_0$, the phase diagram consists of two hybrid spin-valley symmetry-broken phases with both nontrivial nematic and mass-type order parameters. The corresponding phases are separated by a critical line of first- and second-order phase transitions at small and large values of ${\cal N}_0$, respectively. The existence of a critical end point, where the line of first-order phase transitions terminates, is predicted. For ${\cal N}_0=0$, a pure gapped state with a broken spin-valley symmetry is the ground state of the system. As ${\cal N}_{0}$ increases, the nematic order parameter increases, and the gap weakens in the hybrid state. For $U_{0}=0$, a quantum second-order phase transition from the hybrid state into a pure gapless nematic state occurs when the strain reaches a critical value. A nonzero $U_{0}$ suppresses the critical value of the strain. The relevance of these results to recent experiments is briefly discussed.

cond-mat.str-el↗

arXiv2012

Broken-symmetry $ν=0$ quantum Hall states in bilayer graphene: Landau level mixing and dynamical screening

For bilayer graphene in a magnetic field at the neutral point, we derive and solve a full set of gap equations including all Landau levels and taking into account the dynamically screened Coulomb interaction. There are two types of the solutions for the filling factor $ν=0$: (i) a spin-polarized type solution, which is the ground state at small values of perpendicular electric field $E_{\perp}$, and (ii) a layer-polarized solution, which is the ground state at large values of $E_{\perp}$. The critical value of $E_{\perp}$ that determines the transition point is a linear function of the magnetic field, i.e., $E_{\perp,{\rm cr}}=E_{\perp}^{\rm off}+a B$, where $E_{\perp}^{\rm off}$ is the offset electric field and $a$ is the slope. The offset electric field and energy gaps substantially increase with the inclusion of dynamical screening compared to the case of static screening. The obtained values for the offset and the energy gaps are comparable with experimental ones. The interaction with dynamical screening can be strong enough for reordering the levels in the quasiparticle spectrum (the $n=2$ Landau level sinks below the $n=0$ and $n=1$ ones).

cond-mat.mes-hall↗

arXiv2012

Coulomb interaction and magnetic catalysis in the quantum Hall effect in graphene

The dynamics of symmetry breaking responsible for lifting the degeneracy of the Landau levels in the integer quantum Hall effect in graphene is studied in a low-energy model with the Coulomb interaction. The gap equation for Dirac quasiparticles is analyzed for both the lowest and higher Landau levels, taking into account the Landau levels mixing. It is shown that the characteristic feature of the long-range Coulomb interaction is the decrease of the gap parameters with increasing the Landau level index $n$ ("running" gaps). The renormalization (running) of the Fermi velocity as a function of $n$ is also studied. The solutions of the gap equation reproduce correctly the experimentally observed integer quantum Hall plateaus in graphene in strong magnetic fields.

cond-mat.mes-hall↗

arXiv2011

Surprises in relativistic matter in a magnetic field

A short review of recent advances in understanding the dynamics of relativistic matter in a magnetic field is presented. The emphasis is on the dynamics related to the generation of the chiral shift parameter in the normal ground state. We argue that the chiral shift parameter contributes to the axial current density, but does not modify the conventional axial anomaly relation. The analysis based on gauge invariant regularization schemes in the Nambu-Jona-Lasinio model suggests that these findings should be valid also in gauge theories. It is pointed out that the chiral shift parameter can affect observable properties of compact stars and modify the key features of the chiral magnetic effect in heavy ion collisions.

hep-ph↗

arXiv2011

Normal ground state of dense relativistic matter in a magnetic field

The properties of the ground state of relativistic matter in a magnetic field are examined within the framework of a Nambu-Jona-Lasinio model. The main emphasis of this study is the normal ground state, which is realized at sufficiently high temperatures and/or sufficiently large chemical potentials. In contrast to the vacuum state, which is characterized by the magnetic catalysis of chiral symmetry breaking, the normal state is accompanied by the dynamical generation of the chiral shift parameter $Δ$. In the chiral limit, the value of $Δ$ determines a relative shift of the longitudinal momenta (along the direction of the magnetic field) in the dispersion relations of opposite chirality fermions. We argue that the chirality remains a good approximate quantum number even for massive fermions in the vicinity of the Fermi surface and, therefore, the chiral shift is expected to play an important role in many types of cold dense relativistic matter, relevant for applications in compact stars. The qualitative implications of the revealed structure of the normal ground state on the physics of protoneutron stars are discussed. A noticeable feature of the $Δ$ parameter is that it is insensitive to temperature when $T \ll μ_0$, where $μ_0$ is the chemical potential, and {\it increases} with temperature for $T > μ_0$. The latter implies that the chiral shift parameter is also generated in the regime relevant for heavy ion collisions.

hep-ph↗

arXiv2011

Chiral asymmetry and axial anomaly in magnetized relativistic matter

The induced axial current and the chiral anomaly are studied in the normal phase of magnetized relativistic matter. A special attention is paid to the role of the chiral shift parameter Delta, leading to a relative shift of the longitudinal momenta in the dispersion relations of opposite chirality fermions. In the Nambu-Jona-Lasinio model, it is shown directly from the form of the gap equation that Delta necessarily exists in the normal phase in a magnetic field. By making use of the gauge invariant point-splitting regularization, we then show that the presence of Delta essentially modifies the form of the axial current, but does not affect the conventional axial anomaly relation. By recalculating the axial current with the proper-time regularization, we conclude that the result is robust with respect to a specific regularization scheme used.

hep-ph↗

arXiv2009

Chiral asymmetry of the Fermi surface in dense relativistic matter in a magnetic field

It is revealed that in dense relativistic matter in a magnetic field, there exists a contribution to the axial current associated with a relative shift of the longitudinal momenta in the dispersion relations of opposite chirality fermions. Unlike the topological contribution in the axial current at the lowest Landau level, recently discussed in the literature, the dynamical one appears only in interacting matter and affects the fermions in all Landau levels, including those around the Fermi surface. The induced axial current and the shift of the Fermi surfaces of the left-handed and right-handed fermions are expected to play an important role in transport and emission properties of matter in various types of compact stars as well as in heavy ion collisions.

hep-ph↗

arXiv2009

Edge states on graphene ribbon in magnetic field: interplay between Dirac and ferromagnetic-like gaps

By combining analytic and numerical methods, edge states on a finite width graphene ribbon in a magnetic field are studied in the framework of low-energy effective theory that takes into account the possibility of quantum Hall ferromagnetism (QHF) gaps and dynamically generated Dirac-like masses. The analysis is done for graphene ribbons with both zigzag and armchair edges. The characteristic features of the spectrum of the edge states in both these cases are described. In particular, the conditions for the existence of the gapless edge states are established. Implications of these results for the interpretation of recent experiments are discussed.

cond-mat.mes-hall↗

arXiv2008

Dynamics in the quantum Hall effect and the phase diagram of graphene

The dynamics responsible for lifting the degeneracy of the Landau levels in the quantum Hall (QH) effect in graphene is studied by utilizing a low-energy effective model with a contact interaction. A detailed analysis of the solutions of the gap equation for Dirac quasiparticles is performed at both zero and nonzero temperatures. The characteristic feature of the solutions is that the order parameters connected with the QH ferromagnetism and magnetic catalysis scenarios necessarily coexist. The solutions reproduce correctly the experimentally observed novel QH plateaus in graphene in strong magnetic fields. The phase diagram of this system in the plane of temperature and electron chemical potential is analyzed. The phase transitions corresponding to the transitions between different QH plateaus in graphene are described.

cond-mat.mes-hall↗

arXiv2008

Edge states, mass and spin gaps, and quantum Hall effect in graphene

Motivated by recent experiments and a theoretical analysis of the gap equation for the propagator of Dirac quasiparticles, we assume that the physics underlying the recently observed removal of sublattice and spin degeneracies in graphene in a strong magnetic field is connected with the generation of both Dirac masses and spin gaps. The consequences of such a scenario for the existence of the gapless edge states with zigzag and armchair boundary conditions are discussed. In the case of graphene on a half-plane with a zigzag edge, there are gapless edge states in the spectrum only when the spin gap dominates over the mass gap. In the case of an armchair edge, however, the existence of the gapless edge states depends on the specific type of mass gaps.

cond-mat.mes-hall↗

arXiv2007

Fast Equilibration of Hadrons in an Expanding Fireball

Due to long chemical equilibration times within standard hadronic reactions during the hadron gas phase in relativistic heavy ion collisions it has been suggested that the hadrons are "born" into equilibrium after the quark gluon plasma phase. Here we develop a dynamical scheme in which possible Hagedorn states contribute to fast chemical equilibration times of baryon anti-baryon pairs (as well as kaon anti-kaon pairs) inside a hadron gas and just below the critical temperature. Within this scheme, we use master equations and derive various analytical estimates for the chemical equilibration times. Applying a Bjorken picture to the expanding fireball, the kaons and baryons as well as the bath of pions and Hagedorn resonances can indeed quickly chemically equilibrate for both an initial overpopulation or underpopulation of Hagedorn resonances. Moreover, a comparison of our results to $(B+\bar{B})/π^{+}$ and $K/π^{+}$ ratios at RHIC, indeed, shows a close match.

nucl-th↗

arXiv2007

Chemical Equilibration at the Hagedorn Temperature

One important question in relativistic heavy ion collisions is if hadrons, specifically anti-hyperons, are in equilibrium before thermal freezeout because strangeness enhancement has long been pointed to as a signature for Quark Gluon Plasma. Because anti-baryons have long equilibration times in the hadron gas phase it has been suggested that they are "born" into equilibrium. However, Hagedorn states, massive resonances, which are thought to appear near the critical temperature, contribute to fast chemical equilibration times for a hadron gas by providing extra degrees of freedom. Here we use master equations to describe the interplay between Hagedorn resonances, pions, and baryon anti-baryon pairs as they equilibrate over time and observe if the baryons and anti-baryons are fully equilibrated within the fireball.

nucl-th↗

arXiv2006

Excitonic gap, phase transition, and quantum Hall effect in graphene: strong-coupling regime

We suggest that physics underlying the recently observed removal of sublattice and spin degeneracies in graphene in a strong magnetic field describes a phase transition connected with the generation of excitonic and spin gaps. The strong-coupling regime is described using a phenomenological model with enhanced Zeeman splitting (spin gap) and excitonic gaps. The experimental form of the Hall conductivity $σ_{xy}$ with the additional $ν= 0, \pm 1$ plateaus is reproduced. The form of $σ_{xy}$ in the case of a strong-coupling regime with no enhanced Zeeman splitting is also discussed.

cond-mat.str-el↗

arXiv2006

Excitonic gap, phase transition, and quantum Hall effect in graphene

We suggest that physics underlying the recently observed removal of sublattice and spin degeneracies in graphene in a strong magnetic field describes a phase transition connected with the generation of an excitonic gap. The experimental form of the Hall conductivity is reproduced and the main characteristics of the dynamics are described. Predictions of the behavior of the gap as a function of temperature and a gate voltage are made.

cond-mat.str-el↗

arXiv2006

Gluonic phase versus LOFF phase in two-flavor quark matter

We study the gluonic phase in a two-flavor color superconductor as a function of the ratio of the gap over the chemical potential mismatch,$Δ/δμ$. We find that the gluonic phase resolves the chromomagnetic instability encountered in a two-flavor color superconductor for $Δ/δμ< \sqrt{2}$. We also calculate approximately the free energies of the gluonic phase and the single plane-wave LOFF phase and show that the former is favored over the latter for a wide range of coupling strengths.

hep-ph↗

arXiv2006

Collective excitations, instabilities, and ground state in dense quark matter

We study the spectrum of light plasmons in the (gapped and gapless) two-flavor color superconducting phases and its connection with the chromomagnetic instabilities and the structure of the ground state. It is revealed that the chromomagnetic instabilities in the 4-7th and 8th gluonic channels correspond to two very different plasmon spectra. These spectra lead us to the unequivocal conclusion about the existence of gluonic condensates (some of which can be spatially inhomogeneous) in the ground state. We also argue that spatially inhomogeneous gluonic condensates should exist in the three-flavor quark matter with the values of the mass of strange quark corresponding to the gapless color-flavor locked state.

hep-ph↗

arXiv2004

Chemical equilibration due to heavy Hagedorn states

A scenario of heavy resonances, called massive Hagedorn states, is proposed which exhibits a fast ($t\approx 1$ fm/c) chemical equilibration of (strange) baryons and anti-baryons at the QCD critical temperature $T_c$. For relativistic heavy ion collisions this scenario predicts that hadronization is followed by a brief expansion phase during which the equilibration rate is higher than the expansion rate, so that baryons and antibaryons reach chemical equilibrium before chemical freeze-out occurs.

hep-ph↗