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Quantum Electrodynamics in 2+1 Dimensions, Confinement, and the Stability of U(1) Spin Liquids

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Quantum Electrodynamics in 2+1 Dimensions, Confinement, and the Stability of U(1) Spin Liquids Quantum electrodynamics in 2+1 dimensions, confinement, and the stability of U(1) spin liquids Flavio S. Nogueira and Hagen Kleinert Institut f¨ur Theoretische Physik, Freie Universit¨at Berlin, Arnimallee 14, D-14195 Berlin, Germany (Dated: Received October 23, 2005) Compact quantum electrodynamics in 2 + 1 dimensions often arises as an effective theory for a Mott insulator, with the Dirac fermions representing the low-energy spinons. An important and controversial issue in this context is whether a deconfinement transition takes place. We perform a 3 renormalization group analysis to show that deconfinement occurs when N > Nc = 36/π ≈ 1.161, where N is the number of fermion replica. For N < Nc, however, there are two stable fixed points separated by a line containing a unstable non-trivial fixed point: a fixed point corresponding to the scaling limit of the non-compact theory, and another one governing the scaling behavior of the compact theory. The string tension associated to the confining interspinon potential is shown to − exhibit a universal jump as N → Nc . Our results imply the stability of a spin liquid at the physical value N = 2 for Mott insulators. PACS numbers: 11.10.Kk, 71.10.Hf, 11.15.Ha An important topic currently under discussion in con- fact, it is possible to introduce two different QED3s, one densed matter physics community is the emergence of which conserves parity and one which does not. The lat- deconfined quantum critical points in gauge theories of ter theory involves two-component spinors, and allows Mott insulators in 2 + 1 dimensions [1, 2]. A closely re- for a chirally-invariant mass term which is not parity- lated problem concerns the stability of U(1) spin liquids invariant. In such a QED3 theory a Chern-Simons term in 2 + 1 dimensions [3, 4]. In either case, models which [12] is generated by fluctuations [13]. The QED3 the- are often considered as toy models in the high-energy ory relevant to Mott insulators and d-wave supercon- physics literature are supposed to describe the low-energy ductors involves four-component spinors, and does pos- properties of real systems in condensed matter physics. sess chiral symmetry [5, 6]. In such a model, the chiral For instance, a model that frequently appears in the symmetry can be spontaneously broken through the dy- condensed matter literature is the (2 + 1)-dimensional namical generation of a fermion mass. In the context quantum electrodynamics (QED3) [5, 6]. It emerges, for of Mott insulators, the chiral symmetry breaking corre- instance, as an effective theory for Mott insulators [7– sponds to the development of N´eel order [9]. Indeed, the 9]. Let us briefly recall how it arises in this context. nonzero condensate ψψ¯ corresponds to the staggered The Hamiltonian of a SU(N) Heisenberg antiferromag- magnetization. Sinceh in condensedi matter physics parity- net is written in a slave-fermion representation as H = conserving QED3 is not just a toy model, and that the (J/N) f † f f † f , where the local constraint four-component Dirac spinors represent physical excita- − hi,ji iα jα jβ iβ † tions — the low-energy spinons, we may call this theory fiαfiα =PN/2 holds. A Hubbard-Stratonovich transfor- † quantum spinodynamics. mation introduces the auxiliary field χij = fiαfjα [7]. The resulting effective theory can be treatedh as a latticei An important feature of QED3 for Mott insulators is gauge theory, where the gauge field A emerges as the ij that the U(1) gauge group is compact. The compactness phase of χ , i.e., χ = χ eiAij , where χ is determined ij ij 0 0 causes important changes in the physical properties of from mean-field theory. The (2 + 1)-dimensional low- the theory. It allows for quantum excitation of magnetic energy effective Lagrangian in imaginary time has the monopoles which play an important role in determining form [4, 7–9] the phase structure of the theory. This has been known N for a long time. In particular, Polyakov [14] has shown 1 = F 2 + ψ¯ γ (∂ + iA )ψ , (1) that compact Maxwell theory in 2 + 1 dimensions con- L 4e2 µν a µ µ µ a 0 a=1 fines permanently electric test charges. The electrostatic X potential has the form V (R) R, instead of the usual ∼ where each ψa is a four-component Dirac spinor and two-dimensional Coulomb potential V (R) ln R of the ∼ Fµν = ∂µAν ∂ν Aµ is the usual field strength tensor. non-compact Maxwell theory in 2 + 1 dimensions. Since − A rough estimate of the bare gauge coupling is given by V (R) R holds for all values of the gauge coupling, e2 χ4a3, where a is the lattice spacing. the compact∼ (2 + 1)-dimensional Maxwell theory does 0 ∼ 0 An anisotropic version of QED3 has also been stud- not exhibit any phase transition, i.e., the confinement ied in the context of phase fluctuations in d-wave super- is permanent. This theory is equivalent to a Coulomb conductors [10, 11]. A key feature of the QED3 the- gas of magnetic monopoles in three dimensions and it is ory of Mott insulators is its parity conservation. In well known that such a gas does not undergo any phase 2 transition. However, when matter fields are included the This is also the field theory of a Coulomb gas of situation changes, and a deconfinement transition may monopoles, with z0 being the bare fugacity of the gas. occur. Indeed, matter fields induce shape fluctuations of The RG equations for a Coulomb gas in d = 3 were the electric flux tube, leading to a correction term to the obtained a long time ago by Kosterlitz [20]. His re- linearly confining potential. Using a string model for the sults can be used here to obtain the RG equation for electric flux tube, L¨uscher [15] found the gauge coupling in the absence of fermionic matter. 2 2 By introducing the dimensionless couplings f e (l)/e0 (d 2)π 2 2 3 2 ≡ V (R)= σR − + (1/R ), (2) and y z(l)/(e0) , where l = ln(e0r) is a logarithmic − 24R O length≡ scale, we obtain where σ is the string tension. At the deconfining critical df point, the string tension vanishes and only the L¨uscher = 4π2y2 + f, (7) dl term remains at long distances. It represents the “black- dy π3 body” energy of the (d 2) transverse fluctuations of the = 3 y. (8) − dl − f two-dimensional worldsheet of the string [16]. Interestingly, by studying QCD near four dimensions, The above equations imply that there is no fixed point Peskin [17] found that, at the critical point, the in- for the gauge coupling. Therefore, the compact three- terquark potential does have the 1/R-behavior for all dimensional Maxwell theory does not undergo any phase d (4, 4+ ), and argued that this should also be valid 2 2 2 ∈ transition. The photon mass M = 8π z/e is always outside this small dimension interval. Recently we [18] non-zero and the theory confines permanently the electric have found that such a behavior is also realized in an charges. It can be seen from Eq. (7) that a kind of anti- Abelian U(1)-gauge theory for d (2, 4), provided that ∈ screening happens in this theory, which is responsible for this is coupled to matter fields. confinement. Indeed, we can rewrite Eq. (7) as df/dl = In order to better illustrate this mechanism, we shall 2 2 (1 γˆA)f, withγ ˆA = 4π y /f. The negative sign of explicitly perform the calculation of the interspinon po- − − γˆA is actually a remarkable example of the intimate link tential to one-loop order in arbitrary space-time dimen- between asymptotic freedom and confinement [21]. sion 2 < d 4, going to the case of interest d =3 at the ≤ Next we obtain the modification to Eqs. (7) and end [19]. The potential is defined by (8) due to the coupling with the matter fields. In or- dd−1q eiq·R der to derive the RG equations including matter we V (R)= e2 , (3) have employed a formalism similar to the one devel- − 0 (2π)d−1 q2[1 + Π(q)] Z oped by Young [22] in the case of the two-dimensional where e0 is the bare electric charge, and Π(q) is the Coulomb gas. This formalism is based on a mean-field vacuum polarization. At one-loop order, the vaccum self-consistent approximation and applies very well to the 2 d−4 polarization is given by Π(q)=8A(d)Ne0 q , with d > 2 case, since d = 2 is the upper critical dimension A(d) = Γ(2 d/2)Γ2(d/2)/[(4π)d/2Γ(d)]. At| | large dis- for the Coulomb gas. The needed modification comes tances the vacuum− polarization gives the more relevant from the extra renormalization of the gauge coupling due contribution to the scaling behavior if 2 <d< 4, and the to the vacuum polarization. This leads to an effective 2 l 2 l 2 interspinon potential is given by charge e (l)= ε(e /e0)ZA(l)e e0, where ε(r) is the scale- dependent “dielectric” constant of the Coulomb gas of 1 1 V (R)= . (4) magnetic monopoles, and ZA(l) is the gauge field wave −2d+1π(d−2)/2Γ(d/2 1)A(d)N R − function renormalization. In terms of dimensionless cou- For d = 3 the above potential becomes simply V (R) = plings this leads to 4/(πNR). Interestingly, by expanding (4) near d = 2, df − =4π2y2 + (1 γ )f, (9) we obtain at lowest order dl − A (d 2)π 2 V (R) − , (5) where γA d ln ZA/dl.
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