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Europhysics Letters (EPL)

Quantum Disordered Phase in a Doped Antiferromagnet

C. Kübert, A. Muramatsu

Published Jun 10, 1995• 4 citations
Publisher

Abstract

A quantitative description of the transition to a quantum disordered phase in a doped antiferromagnet is obtained with a U (1) gauge theory, where the gap in the spin-wave spectrum determines the strength of the gauge fields. They mediate an attractive long-range interaction whose possible bound states correspond to charge-spin separation and pairing.

DOI
10.1209/0295-5075/30/8/007

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A quantitative description of the transition to a quantum disordered phase in a doped antiferromagnet is obtained with a U (1) gauge theory, where the gap in the spin-wave spectrum determines the strength of the gauge fields. They mediate an attractive long-range interaction whose possible bound states correspond to charge-spin separation and pairing.
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A slave-fermion gauge-theory approach of the t-J model: Doping-induced complex magnetic structure and Z2_{2}2​ spin-gapped anomalous metal in an antiferromagnetic doped Mott insulator
We reinvestigate a doped antiferromagnetic Mott insulator based on the slave-fermion approach of the t-J model, where antiferromagnetic spin fluctuations and doped holes are described by bosonic spinons and fermionic holons, respectively. Earlier studies have shown that an effective field theory for the doped antiferromagnetic Mott insulator is given by a non-relativistic fermion (holon) U(1) gauge theory for charge dynamics and a relativistic boson (spinon) U(1) gauge theory for spin dynamics, thus allowing an anomalous metallic phase where bosonic spinons are gapped away from an antiferromagnetic state, analogous to the U(1) spin liquid phase in the slave-boson approach of the t-J model. We argue that the emergent U(1) gauge structure in this approach is based on a simplified picture for antiferromagnetic correlations. Considering that dynamics of doped holes frustrates a collinear antiferromagnetic spin configuration, we show that doped holes enhance ferromagnetic spin correlations and result in a complex magnetic structure. The presence of such a complex spin texture reduces the U(1) gauge structure down to Z2_{2}2​, thus a different effective field theory results for the spin-gapped non-Fermi liquid metal at low temperatures because there are no gapless U(1) gauge fluctuations.
We discuss the possible existence of a ``spin-gap'' phase in the low-doping regime of strongly correlated two-dimensional (2D) electrons within the gauge field description of the t-J model. The spin-gap phase was recently shown by \"Ubbens and Lee to be destroyed by gauge field quantum fluctuations for a single-layer 2D system in the absence of disorder and for a full gap. We show that the same conclusion applies both in the dirty limit and for the case of a gapless spinon condensate. \textcopyright{} 1996 The American Physical Society.
We discuss the universal theory of localized impurities in the paramagnetic state of 2D antiferromagnets where the spin gap is assumed to be significantly smaller than a typical exchange energy. We study the impurity spin susceptibility near the host quantum transition from a gapped paramagnet to a Néel state, and we compute the impurity-induced damping of the spin-1 mode of the gapped antiferromagnet. Under suitable conditions our results apply also to d-wave superconductors. Doped antiferromagnets (AF) have been the subject of intense studies in the context of the cuprate high-temperature superconductors and other layered transition metal compounds. We present a quantum theory of a particular class of doped AF where it is possible to neglect the coupling between the spin and charge degrees of freedom and consider a theory of the spin excitations alone. Such a theory will apply to (i) quasi-2D ‘spin gap ’ insulators like SrCu2O3 or NaV2O5 in
We calculate the excitation spectrum and the response functions of the textures induced by doping in the half-filled Hubbard model. Localized states appear within the antiferromagnetic gap. The spin-wave spectrum is distorted, and shifted towards lower energies; its total strength is reduced. The antiferromagnetic peak at (\ensuremath{\pi},\ensuremath{\pi}) is broadened and moves to higher energies. Charge excitations appear at low energies and mix with the spin waves. Their polarization induces screened interactions that can overcome the initial, repulsive, ones. The general features of the results cannot be described in conventional-Fermi-liquid or, alternatively, insulator terms.
We study the electron spectral function of various zero-temperature spin-charge separated phases in two dimensions. In these phases, the electron is not a fundamental excitation of the system, but rather ``decays'' into a spin-1/2 chargeless fermion (the spinon) and a spinless charge e boson (the chargon). Using low-energy effective theories for the spinons (d-wave pairing plus possible N\'eel order) and the chargons (condensed or quantum-disordered bosons), we explore three phases of possible relevance to the cuprate superconductors: (1) AF∗,{\mathrm{AF}}^{*},AF∗, a fractionalized antiferromagnet where the spinons are paired into a state with long-ranged N\'eel order and the chargons are 1/2-filled and (Mott) insulating; (2) the nodal liquid, a fractionalized insulator where the spinons are d-wave paired and the chargons are uncondensed; and (3) the d-wave superconductor, where the chargons are condensed and the spinons retain a d-wave gap. Working within the Z2{Z}_{2}Z2​ gauge theory of such fractionalized phases, our results should be valid at scales below the energy gap of the vison---the basic vortex excitation in the theory. However, on a phenomenological level, our results should apply to any spin-charge separated system where the excitations have these low-energy effective forms. Comparison with angle-resolved photoemission spectroscopy data in the undoped, pseudogapped, and superconducting regions is made.