Metals Get an Awkward Cousin

Abstract

We present a fractionalized metallic phase which is indistinguishable from the Fermi liquid in conductivity and thermodynamics, but is sharply distinct in one-electron properties, such as the electron spectral function. We dub this phase the "orthogonal metal." The orthogonal metal and the transition to it from the Fermi liquid are naturally described using a slave-particle representation wherein the electron is expressed as a product of a fermion and a slave Ising spin. We emphasize that when the slave spins are disordered, the result is not a Mott insulator (as erroneously assumed in the prior literature), but rather the orthogonal metal. We construct prototypical ground-state wave functions for the orthogonal metal by modifying the Jastrow factor of Slater-Jastrow wave functions that describe ordinary Fermi liquids. We further demonstrate that the transition from the Fermi liquid to the orthogonal metal can, in some circumstances, provide a simple example of a continuous destruction of a Fermi surface with a critical Fermi surface appearing right at the critical point. We present exactly soluble models that realize an orthogonal metal phase, and the phase transition to the Fermi liquid. These models thus provide valuable solvable examples for phase transitions associated with the death of a Fermi surface. Despite tremendous effort in the last two decades, our theoretical understanding of non-Fermi liquid phases of metallic matter in spatial dimension d > 1 is still in its infancy. Important examples of such non-Fermi liquids are provided by phases where the electron is fractionalized into multiple parts. Here, we present a particularly simple example of a fractionalized non-Fermi liquid phase, which behaves in every respect like a Fermi liquid, except that the charge carriers are orthogonal to the underlying electrons. In some circumstances, this phase is accessed via a direct second-order phase transition from a Fermi liquid, which is marked by a sharp change in the electron spectral function. The transition proceeds via a critical point that is characterized by a sharp critical Fermi surface where the Landau quasiparticle is critically destroyed. 1 To study this fractionalized non-Fermi liquid phase and the associated phase transition to a Fermi liquid, we employ a slave-particle representation where the electron is expressed as a product of a slave Ising spin and a fermion. Unlike the more traditional "slave-boson" representations, which have a U(1) gauge redundancy, and hence require introducing a compact U(1) gauge field, 2 the slave-spin representation has only a Z 2 gauge redundancy. A "slave-spin representation" of this type was first introduced to study the multiorbital Hubbard model 3-6 and the possibility of orbital selective Mott transitions in such models. The slave-spin formulation has gained in popularity over the last few years, and has been employed to describe correlation effects in multiband metals such as the iron pnictides, and also to investigate nonequilibrium physics in quantum quenche...

Metal to Orthogonal Metal Transition

“…The phase with Z 2 deconfinement as well as the vanishing of quasiparticle of composite fermions and their FS is a new state of quantum metal [1][2][3]30] (following the literatures, we denote it as orthogonal metal, or OM in short), and the continuous quantum phase transition from it to the normal metal, or NM in short, of composite fermion carrying the flavor of Higgs transition of Ising gauge field [31,32]. In the NM phase, a gauge-neutral string-order in the Ising matter field is developed and the coherent fermionic quasiparticles reappear.…”

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Orthogonal metal [1][2][3] is a new quantum metallic state that conducts electricity but acquires no Fermi surface (FS) or quasiparticles, and hence orthogonal to the established paradigm of Landau's Fermi-liquid (FL). Such state might hold the key of understanding the perplexing experimental observations of quantum metals that are beyond FL -dubbed non-Fermi-liquid (nFL) -ranging from the Cu-and Fe-based oxides [4-7], heavy fermion compounds [8][9][10][11] to the recently discovered twisted graphene heterostructures [12][13][14][15].

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Semiconductor of spinons: from Ising band insulator to orthogonal band insulator

“…Upon further increase in the interaction strength beyond U c the system becomes a Mott insulator [28,29]. OM is an interesting -and perhaps the simplest -non-FL state for U ⊥ < U < U Mott that separates a FL from a Mott insulator.…”

“…Recently, Nandkishore and coworkers have argued that starting from a metallic state, as one increases the Hubbard U beyond U ⊥ the Fermi liquid (FL) undergoes a phase transition to an exotic non-FL state termed orthogonal metal (OM). Upon a further increase in the interaction strength beyond U Mott , the system is expected to become a Mott insulator [34,35]. OM is an interesting-and perhaps the simplest-non-FL state for U ⊥ < U < U Mott that separates an FL from a Mott insulator.…”

Metal to Orthogonal Metal Transition*

“…The phase with Z 2 deconfinement as well as the vanishing of quasiparticle of composite fermions and their FS is a new state of quantum metal [24,25,28,29] (following the literatures, we denote it as orthogonal metal, or OM in short), and the continuous quantum phase transition from it to the normal metal, or NM in short, of composite fermion carrying the flavor of Higgs transition of Ising gauge field. [30,31] In the NM phase, a gauge-neutral string-order in the Ising matter field is developed and the coherent fermionic quasiparticles reappear.…”