More Like this

1. Electronic reconstruction in confined ${\mathrm{SrRuO}}_3$ monolayers

   https://doi.org/10.1103/PhysRevB.110.235104  

   Lamichhane, U; Sankhi, B; Kundu, N; Fabbris, G; Choi, Y; Haskel, D; McChesney, J L; Cao, Yue; Li, J; Bisogni, V; et al (December 2024, Physical Review B)

   We report the observation of an electronic reconstruction in dimensionally controlled ruthenate heterostructures synthesized by pulsed laser deposition. High structural and electronic quality of superlattices comprised of a single SrRuO3 layer inter-spaced with varying thicknesses of insulating SrTiO3 layers was verified by reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, reciprocal space mapping, and x-ray absorption spectroscopy. X-ray absorption spectroscopy evidences a confinement-driven evolution of the Ru electronic configuration from the d5L to the d4 state. Significant increases of the spin-orbit coupling are observed in connection with the configuration changes supporting recent works identifying large enhancement of the magnetic anisotropy. The growth of high quality two-dimensional confined ruthenate layers under precisely controlled environments highlights the potential to directly manipulate interlayer coupling and selectively perturb the electronic state in ruthenates in analogy to superconducting Sr2RuO4. 

   more » « less
2. Valence, charge transfer, and orbital-dependent correlation in bilayer nickelates ${\mathrm{Nd}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$

   https://doi.org/10.1103/PhysRevB.111.165101  

   Takegami, Daisuke; Okauchi, Takaki; Morales, Edgar Abarca; Fujinuma, Koto; Furo, Mizuki; Yoshimura, Masato; Tsuei, Ku-Ding; Pan, Grace A; Segedin, Dan Ferenc; Song, Qi; et al (April 2025, Physical Review B)

   We examine the bulk electronic structure of ${\mathrm{Nd}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$using Ni $2p$core-level hard x-ray photoemission spectroscopy combined with density functional theory $+$dynamical mean-field theory. Our results reveal a large deviation of the Ni $3d$occupation from the formal ${\mathrm{Ni}}^{2.5+}$valency, highlighting the importance of the charge transfer from oxygen ligands. We find that the dominant $d^8$configuration is accompanied by nearly equal contributions from $d^7$and $d^9$states, exhibiting an unusual valence state among Ni-based oxides. Finally, we discuss the Ni $d_{x^2-y^2}$and $d_{z^2}$orbital-dependent hybridization, correlation and local spin dynamics. Published by the American Physical Society2025 

   more » « less
3. Anomalous Landau Level Gaps Near Magnetic Transitions in Monolayer ${\mathrm{WSe}}_2$

   https://doi.org/10.1103/PhysRevX.14.031018  

   Foutty, Benjamin A; Calvera, Vladimir; Han, Zhaoyu; Kometter, Carlos R; Liu, Song; Watanabe, Kenji; Taniguchi, Takashi; Hone, James C; Kivelson, Steven A; Feldman, Benjamin E (August 2024, Physical Review X)

   First-order phase transitions produce abrupt changes to the character of both ground and excited electronic states. Here we conduct electronic compressibility measurements to map the spin phase diagram and Landau level (LL) energies of monolayer ${\mathrm{WSe}}_2$in a magnetic field. We resolve a sequence of first-order phase transitions between completely spin-polarized LLs and states with LLs of both spins. Unexpectedly, the LL gaps are roughly constant over a wide range of magnetic fields below the transitions, which we show reflects spin-polarized ground states with opposite spin excitations. These transitions also extend into compressible regimes, with a sawtooth boundary between full and partial spin polarization. We link these observations to the important influence of LL filling on the exchange energy beyond a smooth density-dependent contribution. Our results show that ${\mathrm{WSe}}_2$realizes a unique hierarchy of energy scales where such effects induce reentrant magnetic phase transitions tuned by density and magnetic field. Published by the American Physical Society2024 

   more » « less
4. Structure and equation of state of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_{n-1}{\mathrm{Cu}}_n{\mathrm{O}}_{2n+4+\delta }$ from x-ray diffraction to megabar pressures

   https://doi.org/10.1103/PhysRevMaterials.7.064803  

   Mark, Alexander C.; Ahart, Muhtar; Kumar, Ravhi; Park, Changyong; Meng, Yue; Popov, Dmitry; Deng, Liangzi; Chu, Ching-Wu; Campuzano, Juan Carlos; Hemley, Russell J. (June 2023, Physical Review Materials)

   - (Ed.)

   Pressure is a unique tuning parameter for probing the properties of materials, and it has been particularly useful for studies of electronic materials such as high-temperature cuprate superconductors. Here we report the effects of quasihydrostatic compression produced by a neon pressure medium on the structures of bismuth-based high-Tc cuprate superconductors with the nominal composition Bi2Sr2Can−1CunO2n+4+δ (n = 1, 2, 3) up to 155 GPa. The structures of all three compositions obtained by synchrotron x-ray diffraction can be described as pseudotetragonal over the entire pressure range studied. We show that previously reported pressure-induced distortions and structural changes arise from the large strains that can be induced in these layered materials by nonhydrostatic stresses. The pressure-volume equations of state (EOS) measured under these quasihydrostatic conditions cannot be fit to single phenomenological formulation over the pressure ranges studied, starting below 20 GPa. This intrinsic anomalous compression as well as the sensitivity of Bi2Sr2Can−1CunO2n+4+δ to deviatoric stresses provide explanations for the numerous inconsistencies in reported EOS parameters for these materials. We conclude that the anomalous compressional behavior of all three compositions is a manifestation of the changes in electronic properties that are also responsible for the remarkable nonmonotonic dependence of Tc with pressure, including the increase in Tc at the highest pressures studied so far for each. Transport and spectroscopic measurements up to megabar pressures are needed to fully characterize these cuprates and explore higher possible critical temperatures in these materials. 

   more » « less
5. Valence transition theory of the pressure-induced dimensionality crossover in superconducting ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$

   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. 

   more » « less