Pseudogap phase
Solving the Mystery of Pseudogap: A Novel Theory Suggests Geometric Orthogonal Metals
The mysterious microscopic origins of the pseudogap phase in high-temperature superconductors may eventually be revealed by a new idea put forth by researchers, which is a big development for condensed matter physics. These complex materials have stripe and AFM phases. The study was conducted by Henning Schlömer, Annabelle Bohrdt, and Fabian Grusdt from Regensburg University, LMU München, and MCQST.
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The hole-doped cuprate pseudogap phase is a key mystery of high-temperature superconductivity. Its unusual properties, discovered decades ago, are said to be key to understanding how these materials acquire superconductivity at high temperatures. A thorough explanation for its presence and relationship to other observable phases, such antiferromagnetic and stripe orders, has been elusive despite much research.
A contradictory picture is painted by experimental observations: quantum oscillations and transport measurements (Hall, optical conductivity, magnetoresistance) show the presence of Fermi-liquid-like quasiparticles forming a small Fermi surface, while photoemission experiments point to the absence of coherent fermionic quasiparticles. The traditional view is seriously challenged by the fact that this small Fermi surface has a carrier density of δ (the hole doping) instead of the 1+δ predicted by Luttinger’s theorem.
Prior theoretical explanations for the pseudogap have suggested the creation of new fractionalised Fermi liquid (FL) and orthogonal metal (OM) phases, as well as connections to symmetry-breaking orders such as pairing or striping. The complex relationship between stripe order and the pseudogap phase has recently received support from experimental and numerical studies, despite the topological nature of FL and OM phases being proposed as compatible with a small Fermi surface without symmetry breaking a feature observed experimentally.
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The Geometric Orthogonal Metal (GOM) is an alternate and convincing scenario that is suggested by the new theory. The idea of “hidden order” is central to this theory. The spins of the material create an ordered antiferromagnetic background in this framework. Charge fluctuations stabilize fluctuating domain barriers that sit on top of this background. The underlying long-range antiferromagnetic order in actual space is disrupted and obscured by these domain barriers, giving the impression that experiments only involve short-range AFM correlations. Importantly, though, the reference frame of these background spins maintains the magnetic order.
It is proposed that the core of the pseudogap phase itself is the proliferation of these changing domain walls. The appearance of distinct fermionic quasiparticles in the form of magnetic polarons is an amazing effect of this hidden order. In the ground state, these magnetic polarons couple to Z2 topological excitations of a domain-wall string-net condensate and interact actively, making them not solitary entities. This topological order is crucial because it explains how a small Fermi surface forms alongside the short-range magnetism, which is consistent with a number of experimental findings.
These fermionic quasiparticles are “gauge-charged,” or orthogonal to actual electrons, in the GOM, a kind of orthogonal metal. They are detectable by transport and quantum oscillation measurements, which is in line with experimental observations in cuprates, even if their orthogonality prevents them from appearing coherently in ground-state Angle-Resolved Photoemission Spectroscopy (ARPES). The topological excitations absorbing momentum in Oshikawa’s flux insertion protocol account for the apparent violation of Luttinger’s theorem.
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The transition from the pseudogap phase is also clarified by the GOM framework. The researchers contend that the hidden order disappears at a particular crucial doping value. At what they refer to as a “hidden quantum critical point” (hQCP), which demonstrates distinctive quantum critical transport features, this loss propels a transition to a conventional Fermi liquid. The hQCP is “hidden” because a diverging correlation length cannot identify it because the correlations in the original lattice model stay short-ranged on both sides of the transition. Charge fluctuations that impede the effective spin system in “squeezed space” at greater doping levels are thought to be the cause of this change.
In addition to providing an explanation for the pseudogap, the GOM framework provides a deep unifying viewpoint. It reveals a strong relationship between the antiferromagnetic, stripe, and pseudogap phases, implying that they all originate from a same origin where the fluctuating domain walls “hidden” the SU(2) symmetry of the spin background, which is spontaneously shattered in the pseudogap phase.
Additionally, this theory suggests that superconductivity mechanisms may be unified across various material classes, such as heavy fermion compounds, where magnetic fluctuations are already thought to serve as the pairing mechanism for superconductivity, and both electron- and hole-doped cuprates. The GOM imagines isotropically fluctuating string-net structures and does not rely on quantum critical fluctuations of conflicting orders to explain the pseudogap, in contrast to previous “fluctuating stripe” theories that concentrated on unidirectional stripes and competing orders.
The authors use the potential of ultracold-atom simulators to support their theoretical predictions. By searching for indications of hidden order in spin- and charge-resolved snapshots, these sophisticated experimental platforms could directly verify the GOM theory and measure nonlocal correlations. Such studies are now possible with recent developments in reaching cryogenic temperatures for Fermi-Hubbard systems, which greatly increases our understanding of the microscopic genesis of the pseudogap and its crucial role in the mystery of high-temperature superconductivity.
With its sophisticated idea of fluctuating domain walls mediating concealed antiferromagnetic, this new GOM scenario offers a strong and all-encompassing framework that could help resolve a number of contradictory observations in cuprite superconductors and possibly beyond. It represents a major advancement in the quest for a thorough comprehension of one of the trickiest issues facing contemporary physics.
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