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Igor F. Herbut

Publications and source records attributed to Igor F. Herbut.

At least 109 records · Page 6Linked to original sources
arXiv2006

Stable Skyrmions in spinor condensates

Globally symmetric spinor condensates in free space are argued not to support stable topological defects in either two or three dimensions. In the latter case, however, we show that a topological Skyrmion can be stabilized by forcing it to adopt certain density profiles. A sufficient condition for the existence of Skyrmion solutions in three dimensions is formulated and illustrated in simple examples. Our results pertain to Bose-Einstein condensation in Rb.

cond-mat.supr-con↗
arXiv2006

Interactions and phase transitions on graphene's honeycomb lattice

The low-energy theory of interacting electrons on graphene's two-dimensional honeycomb lattice is derived and discussed. In particular, the Hubbard model in the large-N limit is shown to have a semi-metal - antiferromagnetic insulator quantum critical point in the universality class of the Gross-Neveu model. The same equivalence is conjectured to hold in the physical case N=2, and its consequences for various physical quantities are examined. The effects of the long-range Coulomb interaction and of the magnetic field are discussed.

cond-mat.mes-hall↗
arXiv2005

Screening in anisotropic superfluids and the superfluid density in underdoped cuprates

We examine the nature of the collective excitations in a strongly anisotropic system of bosons interacting via Coulomb interaction. Such a system has often been used in the past to model the effects of quantum and classical phase fluctuations on the superfluid density of underdoped cuprates. Depending on the anisotropy and the effective strength of the interaction we find four different regimes for the temperature dependence of the superfluid density. Coulomb interaction in underdoped cuprates is argued to be effectively short-ranged, and of order of unity in appropriately defined units.

cond-mat.supr-con↗
arXiv2005

Effective theory of high-temperature superconductors

General field theory of a fluctuating d-wave superconductor is constructed and proposed as an effective description of superconducting cuprates at low energies. The theory is used to resolve a puzzle posed by recent experiments on superfluid density in severely underdoped YBCO. In particular, the overall temperature dependence of the superfluid density at low dopings is argued to be described well by the strongly anisotropic weakly interacting three-dimensional Bose gas, and thus approximately linear in temperature with an almost doping-independent slope.

cond-mat.supr-con↗
arXiv2005

Chiral symmetry breaking in ${\rm QED}_{3}$ in presence of irrelevant interactions: a renormalization group study

Motivated by recent theoretical approaches to high temperature superconductivity, we study dynamical mass generation in three dimensional quantum electrodynamics ${\rm QED}_{3}$) in presence of irrelevant four-fermion quartic terms. The problem is reformulated in terms of the renormalization group flows of certain four-fermion couplings and charge, and then studied in the limit of large number of fermion flavors $N$. We find that the critical number of fermions $N_c$ below which the mass becomes dynamically generated depends continuously on a weak chiral-symmetry-breaking interaction. One-loop calculation in our gauge-invariant approach yields $N_{c0} = 6$ in pure ${\rm QED}_3$. We also find that chiral-symmetry-preserving mass cannot become dynamically generated in pure ${\rm QED}_{3}$.

cond-mat.supr-con↗
arXiv2004

Quantum fluctuations, pseudogap, and the T=0 superfluid density in strongly correlated d-wave superconductors

I study the effect of Coulomb interaction on superconducting order in a d-wave lattice superconductor at T=0 by considering the superconducting saddle point in the two dimensional t-J-U model with an on-site repulsion U. The theory of low energy phase fluctuations around this saddle point is derived in terms of the effective hard-core bosons (representing the density of spin-up electrons and the order parameter phase), interacting with the fluctuating density of spin-down electrons. Whereas the saddle point value of the gap is found to monotonically increase towards half filling, the phase stiffness at T=0 has a maximum, and then decreases with further underdoping. Right at half filling the stiffness vanishes for large U. This argues that the pseudogap phenomenon of the type observed in cuprates is in principle possible without a development of any competing order, purely as a result of growing correlations in the superconducting state. The effects of finite temperature and disorder are discussed qualitatively.

cond-mat.supr-con↗
arXiv2004

Finite temperature superfluid density in very underdoped cuprates

The combination of a large superconducting gap, low transition temperature, and quasi two-dimensionality in strongly underdoped high temperature superconductors severely constrains the behavior of the ab-plane superfluid density ρwith temperature T. In particular, we argue that the contribution of nodal quasiparticles to ρ(T) is essential to account both for the amplitude of, and the recently observed deviations from, the Uemura scaling. A relation between T_c and ρ(0) which combines the effects of quasiparticle excitations at low temperatures and of vortex fluctuations near the critical temperature is proposed and discussed in light of recent experiments.

cond-mat.supr-con↗
arXiv2003

Self-consistent theory of compact QED3 with relativistic fermions

We consider three dimensional quantum electrodynamics (cQED3) with massless relativistic fermions coupled to a compact gauge field using a combined perturbative variational approach. Coupling to matter renders the bare interaction between magnetic monopoles logarithmic at large distances, suggesting the possibility of a confinement-deconfinement transition of the Berezinskii-Kosterlitz-Thouless type in the theory. Our self-consistent calculation suggests, however, that screening effects always destabilise the confined phase, in agreement with the previous renormalisation group study of the same model.

cond-mat.str-el↗
arXiv2003

Permanent confinement in the compact QED3 with fermionic matter

We argue that the compact three dimensional electrodynamics with massless fermions is always confining, in spite of the bare interaction between the magnetic monopoles being rendered logarithmic by fermions. The effect is caused by screening by other dipoles, which transforms the logarithmic back into the Coulomb interaction at large distances. Possible implications for the chiral symmetry breaking for fermions are discussed, and the global phase diagram of the theory is proposed.

cond-mat.str-el↗
arXiv2003

Absence of U(1) spin liquids in two dimensions

Many popular models of fractionalized spin liquids contain neutral fermionic spinon excitations on a Fermi surface, carrying unit charges under a compact U(1) gauge force. We argue that instanton effects generically render such states unstable to confinement in two spatial dimensions, so that all elementary excitations are gauge neutral, and there is no spinon Fermi surface. Similar results are expected to apply to SU(2) spin liquids. However, fractionalized states can appear when the gauge symmetry is broken down to a discrete subgroup by the Higgs mechanism. Our argument generalizes earlier results on confinement in the pure gauge theory, and on the instability of the U(1) staggered flux and algebraic spin liquids with a Dirac spectrum for fermionic spinons.

cond-mat.str-el↗
arXiv2003

Theory of strongly phase fluctuating d-wave superconductors and the spin response in underdoped cuprates

General theory of d-wave quasiparticles coupled to phase fluctuations of superconducting order parameter is discussed. In the charge sector the superfluid density is found to conform to Uemura scaling, and the charge renormalization factor to decrease with underdoping. The spin susceptibilty exhibits four distinct regimes with increasing frequency and scales with the superconducting Tc, as observed.

cond-mat.supr-con↗
arXiv2003

Theory of phase fluctuating d-wave superconductors and the spin response in underdoped cuprates

The minimal theory of spin of gapless quasiparticles coupled to fluctuating vortex defects in the phase of the d-wave superconducting order parameter at T=0 is studied. With the proliferation of the vortex loops the theory reduces to the previuosly studied QED3, with its concomitant spin-density-wave (chiral) instability. We find a single superconductor-insulator phase transition, which may be fluctuation induced first-order, at which vortices condense and the chiral symmetry for fermions dynamically breaks. We compute the spin-spin correlation function deep in the fluctuating superconducting state, and discuss some prominent trends in the neutron scattering data on underdoped cuprates in light of our results.

cond-mat.supr-con↗
arXiv2003

Theory of spin response in underdoped cuprates as strongly fluctuating d-wave superconductors

We study the spin dynamics in underdoped cuprates at low temperatures by considering them as quasi two dimensional d-wave superconductors (dSC) with strong phase fluctuations. An effective theory of spin degrees of freedom of nodal quasiparticles coupled to vortex defects in the phase of the superconducting order parameter is formulated. It represents the minimal extension of the QED3 theory of the pseudogap phase into the superconducting region. The theory predicts a single dSC-SDW transition, which may be fluctuation induced first order, without the region of coexistence between the two ordered phases. We compute the spin-spin correlator in the dSC, and explain the evolution of the spin response with frequency as observed in neutron scattering experiments on YBCO. We predict that at the frequencies much below the resonance the (weak) spin response should become narrowly peaked at four `diagonally' incommensurate wave vectors that span between the nodes of the supercopnducting order parameter. These peaks represent an inherent collective mode of the phase fluctuating dSC, the condensation of which would bring about the SDW order in the pseudogap phase. Our interpretation of the resonance suggests it should become more elusive in the superconductors with lower Tc, in accord with its' conspicuous absence in LSCO.

cond-mat.supr-con↗
arXiv2002

QED_3 theory of underdoped high temperature superconductors II: the quantum critical point

We study the effect of gapless quasiparticles in a d-wave superconductor on the T=0 end point of the Kosterlitz-Thouless transition line in underdoped high-temperature superconductors. Starting from a lattice model that has gapless fermions coupled to 3D XY phase fluctuations of the superconducting order parameter, we propose a continuum field theory to describe the quantum phase transition between the d-wave superconductor and the spin-density-wave insulator. Without fermions the theory reduces to the standard Higgs scalar electrodynamics (HSE), which is known to have the critical point in the inverted XY universality class. Extending the renormalization group calculation for the HSE to include the coupling to fermions, we find that the qualitative effect of fermions is to increase the portion of the space of coupling constants where the transition is discontinuous. The critical exponents at the stable fixed point vary continuously with the number of fermion fields $N$, and we estimate the correlation length exponent (nu = 0.65) and the vortex field anomalous dimension(eta_Phi=-0.48) at the quantum critical point for the physical case N=2. The stable critical point in the theory disappears for the number of Dirac fermions N > N_c, with N_c ~ 3.4 in our approximation. We discuss the relationship between the superconducting and the chiral (SDW) transitions, and point to some interesting parallels between our theory and the Thirring model.

cond-mat.supr-con↗
arXiv2002

QED3 theory of underdoped high temperature superconductors

Low-energy theory of d-wave quasiparticles coupled to fluctuating vortex loops that describes the loss of phase coherence in a two dimensional d-wave superconductor at T=0 is derived. The theory has the form of 2+1 dimensional quantum electrodynamics (QED3), and is proposed as an effective description of the T=0 superconductor-insulator transition in underdoped cuprates. The coupling constant ("charge") in this theory is proportional to the dual order parameter of the XY model, which is assumed to be describing the quantum fluctuations of the phase of the superconducting order parameter. The principal result is that the destruction of phase coherence in d-wave superconductors typically, and immediately, leads to antiferromagnetism. The transition can be understood in terms of the spontaneous breaking of an approximate "chiral" SU(2) symmetry, which may be discerned at low enough energies in the standard d-wave superconductor. The mechanism of the symmetry breaking is analogous to the dynamical mass generation in the QED3, with the "mass" here being proportional to staggered magnetization. Other insulating phases that break chiral symmetry include the translationally invariant "d+ip" and "d+is" insulators, and various one dimensional charge-density and spin-density waves. The theory offers an explanation for the rounded d-wave-like dispersion seen in ARPES experiments on Ca2CuO2Cl2 (F. Ronning et. al., Science 282, 2067 (1998)).

cond-mat.supr-con↗
arXiv2002

Fine structure of chiral symmetry breaking in the $QED_3$ theory of underdoped high-$T_c$ superconductors

We study the effects of the leading irrelevant perturbations on chiral symmetry breaking in the effective $QED_3$ theory of d-wave superconductor-insulator transition in underdoped cuprates. For weak symmetry breaking, the effect of a perturbation on energies of various insulating states can be classified according to its engineering dimension in the maximally symmetric theory. Considering the velocity anisotropy, repulsive interactions, and higher order derivatives we show that the insulating state with the lowest energy is the spin density wave.

cond-mat.supr-con↗
arXiv2002

The effect of velocity anisotropy on the antiferromagnetic instability of a d-wave superconductor

We consider the effective 2+1 dimensional electrodynamics (QED_3) of low-energy quasiparticles coupled to fluctuating vortex loops in the d-wave superconductor, with the velocity anisotropy: v_F not equal to v_Delta. This theory should be relevant to the quantum superconductor-insulator transition in underdoped cuprates. Working in the customary large-N approximation, we find that weak anisotropy is a marginally irrelevant perturbation to the Lorentz invariant QED_3, and that the critical number N_c of Dirac fields below which the theory suffers the antiferromagnetic (chiral) instability stays the same.

cond-mat.supr-con↗