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1. Valence transition theory of the pressure-induced dimensionality crossover in superconducting Sr14−xCaxCu24O41

   https://doi.org/10.1103/PhysRevB.108.134510  

   Song, Jeong-Pil ; Clay, R. Torsten ; Mazumdar, Sumit ( October 2023 , Physical Review B)

   Stephen E. Nagler (Ed.)

   One of the strongest justifications for the continued search for superconductivity within the single-band Hubbard Hamiltonian originates from the apparent success of single-band ladder-based theories in predicting the occurrence of superconductivity in the cuprate coupled-ladder compound Sr{14−x}Ca{x}Cu{24}O{41}. Recent theoretical works have, however, shown the complete absence of quasi-long-range superconducting correlations within the hole-doped multiband ladder Hamiltonian including realistic Coulomb repulsion between holes on oxygen sites and oxygen-oxygen hole hopping. Experimentally, superconductivity in Sr{14−x}Ca{x}Cu{24}O{41} occurs only under pressure and is preceded by dramatic transition from one to two dimensions that remains not understood. We show that understanding the dimensional crossover requires adopting a valence transition model within which there occurs transition in Cu-ion ionicity from +2 to +1 , with transfer of holes from Cu to O ions [S. Mazumdar, Phys. Rev. B 98, 205153 (2018)]. The driving force behind the valence transition is the closed-shell electron configuration of Cu^{1+} , a feature shared by cations of all oxides with a negative charge-transfer gap. We make a falsifiable experimental prediction for Sr{14−x}Ca{x}Cu{24}O{41} and discuss the implications of our results for layered two-dimensional cuprates. 

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2. Emergent charge order near the doping-induced Mott-insulating quantum phase transition in Sr3Ru2O7

   https://doi.org/10.1038/s42005-019-0138-4  

   Leshen, Justin ; Kavai, Mariam ; Giannakis, Ioannis ; Kaneko, Yoshio ; Tokura, Yoshi ; Mukherjee, Shantanu ; Lee, Wei-Cheng ; Aynajian, Pegor ( March 2019 , Communications Physics)

   Search for novel electronically ordered states of matter emerging near quantum phase transitions is an intriguing frontier of condensed matter physics. In ruthenates, the interplay between Coulomb correlations among the 4delectronic states and their spin-orbit interactions, lead to complex forms of electronic phenomena. Here we investigate the double layered Sr₃(Ru_1−xMn_x)₂O₇and its doping-induced quantum phase transition from a metal to an antiferromagnetic Mott insulator. Using spectroscopic imaging with the scanning tunneling microscope, we visualize the evolution of the electronic states in real- and momentum-space. We find a partial-gap at the Fermi energy that develops with doping to form a weak Mott insulating state. Near the quantum phase transition, we discover a spatial electronic reorganization into a commensurate checkerboard charge order. These findings bear a resemblance to the universal charge order in the pseudogap phase of cuprates and demonstrate the ubiquity of charge order that emanates from doped Mott insulators.

    

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3. Hole antidoping of oxides

   Malyi, Oleksandr I ; Zunger, Alex ( January 2020 , Physical review)

   In standard doping, adding charge carrier to a compound results in a shift of the Fermi level towards the conduction band for electron doping and towards the valence band for hole doping. We discuss the curious case of antidoping, where the direction of band movements in response to doping is reversed. Specifically, p-type antidoping moves the previously occupied bands to the principal conduction band resulting in an increase of band gap energy and reduction of electronic conductivity. We find that this is a generic behavior for a class of materials: early transition and rare earth metal (e.g., Ti, Ce) oxides where the sum of composition-weighed formal oxidation states is positive; such compounds tend to form the well-known electron-trapped intermediate bands localized on the reduced cation orbitals. What is less known is that doping by a hole annihilates a single trapped electron on a cation. The latter thus becomes electronically inequivalent with respect to the normal cation in the undoped lattice, thus representing a symmetry-breaking effect. We give specific theoretical predictions for target compounds where hole antidoping might be observed experimentally: Magnéli-like phases (i.e., CeO2-x and TiO2-x) and ternary compounds (i.e., Ba2Ti6O13 and Ba4Ti12O27), and note that this unique behavior opens the possibility of unconventional control of materials conductivity by doping. 

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4. Ground-state phase diagram of the t-t ′ -J model

   https://doi.org/10.1073/pnas.2109978118  

   Jiang, Shengtao ; Scalapino, Douglas J. ; White, Steven R. ( October 2021 , Proceedings of the National Academy of Sciences)

   We report results of large-scale ground-state density matrix renormalization group (DMRG) calculations on t-$t^{\prime }$-J cylinders with circumferences 6 and 8. We determine a rough phase diagram that appears to approximate the two-dimensional (2D) system. While for many properties, positive and negative$t^{\prime }$values ($t^{\prime }/t=\pm 0.2$) appear to correspond to electron- and hole-doped cuprate systems, respectively, the behavior of superconductivity itself shows an inconsistency between the model and the materials. The$t^{\prime }<0$(hole-doped) region shows antiferromagnetism limited to very low doping, stripes more generally, and the familiar Fermi surface of the hole-doped cuprates. However, we find$t^{\prime }<0$strongly suppresses superconductivity. The$t^{\prime }>0$(electron-doped) region shows the expected circular Fermi pocket of holes around the$\left(\pi ,\pi \right)$point and a broad low-doped region of coexisting antiferromagnetism and d-wave pairing with a triplet p component at wavevector$\left(\pi ,\pi \right)$induced by the antiferromagnetism and d-wave pairing. The pairing for the electron low-doped system with$t^{\prime }>0$is strong and unambiguous in the DMRG simulations. At larger doping another broad region with stripes in addition to weaker d-wave pairing and striped p-wave pairing appears. In a small doping region near$x=0.08$for$t^{\prime }\sim -0.2$, we find an unconventional type of stripe involving unpaired holes located predominantly on chains spaced three lattice spacings apart. The undoped two-leg ladder regions in between mimic the short-ranged spin correlations seen in two-leg Heisenberg ladders.

    

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5. Doping induced Mott collapse and possible density wave instabilities in (Sr1−xLax)3Ir2O7

   https://doi.org/10.1038/s41535-019-0183-y  

   Wang, Zhenyu ; Walkup, Daniel ; Maximenko, Yulia ; Zhou, Wenwen ; Hogan, Tom ; Wang, Ziqiang ; Wilson, Stephen D. ; Madhavan, Vidya ( August 2019 , npj Quantum Materials)

   The path from a Mott insulating phase to high temperature superconductivity encounters a rich set of unconventional phenomena involving the insulator-to-metal transition (IMT), such as emergent electronic orders and pseudogaps, that ultimately affect the condensation of Cooper pairs. A huge hindrance to understanding the origin of these phenomena is the difficulty in accessing doping levels near the parent state. TheJ_eff = 1/2 Mott state of the perovskite strontium iridates has revealed intriguing parallels to the cuprates, with the advantage that it provides unique access to the Mott transition. Here, we exploit this accessibility to study the IMT and the possible nearby electronic orders in the electron-doped bilayer iridate (Sr_1 − xLa_x)₃Ir₂O₇. Using spectroscopic imaging scanning tunneling microscopy, we image the La dopants in the top as well as the interlayer SrO planes. Surprisingly, we find a disproportionate distribution of La between these layers with the interlayer La being primarily responsible for the IMT. This reveals the distinct site-dependent effects of dopants on the electronic properties of bilayer systems. Electron doping also results in charge reordering. We find unidirectional electronic order concomitant with the structural distortion known to exist in this system. Intriguingly, similar to the single layer iridate, we also find local resonant states forming a checkerboard-like pattern trapped by La. This suggests that multiple charge orders may exist simultaneously in Mott systems, even with only one band crossing the Fermi energy.

    

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