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1. A model of d -wave superconductivity, antiferromagnetism, and charge order on the square lattice

   https://doi.org/10.1073/pnas.2302701120  

   Christos, Maine; Luo, Zhu-Xi; Shackleton, Henry; Zhang, Ya-Hui; Scheurer, Mathias_S; Sachdev, Subir (May 2023, Proceedings of the National Academy of Sciences)

   We describe the confining instabilities of a proposed quantum spin liquid underlying the pseudogap metal state of the hole-doped cuprates. The spin liquid can be described by a SU(2) gauge theory ofN_f= 2 massless Dirac fermions carrying fundamental gauge charges—this is the low-energy theory of a mean-field state of fermionic spinons moving on the square lattice withπ-flux per plaquette in the ℤ₂center of SU(2). This theory has an emergent SO(5)_fglobal symmetry and is presumed to confine at low energies to the Néel state. At nonzero doping (or smaller Hubbard repulsionUat half-filling), we argue that confinement occurs via the Higgs condensation of bosonic chargons carrying fundamental SU(2) gauge charges also moving inπℤ₂-flux. At half-filling, the low-energy theory of the Higgs sector hasN_b= 2 relativistic bosons with a possible emergent SO(5)_bglobal symmetry describing rotations between ad-wave superconductor, period-2 charge stripes, and the time-reversal breaking “d-density wave” state. We propose a conformal SU(2) gauge theory withN_f= 2 fundamental fermions,N_b= 2 fundamental bosons, and a SO(5)_f×SO(5)_bglobal symmetry, which describes a deconfined quantum critical point between a confining state which breaks SO(5)_fand a confining state which breaks SO(5)_b. The pattern of symmetry breaking within both SO(5)s is determined by terms likely irrelevant at the critical point, which can be chosen to obtain a transition between Néel order andd-wave superconductivity. A similar theory applies at nonzero doping and largeU, with longer-range couplings of the chargons leading to charge order with longer periods. 

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2. Observation of odd-parity superconductivity in UTe2

   https://doi.org/10.1073/pnas.2419734122  

   Li, Zixuan; Moir, Camilla M; McKee, Nathan J; Lee-Wong, Eric; Baumbach, Ryan E; Maple, M Brian; Liu, Ying (February 2025, Proceedings of the National Academy of Sciences)

   Symmetry properties of the order parameter are among the most fundamental characteristics of a superconductor. UTe₂, which was found to feature an exceedingly large upper critical field and striking reentrant behavior at low temperatures, is widely believed to possess a spin-triplet pairing symmetry. However, unambiguous evidence for such a pairing symmetry is still lacking, especially at zero and low magnetic fields. The presence of an inversion crystalline symmetry in UTe₂requires that, if it is indeed a spin-triplet superconductor, the order parameter must be of odd parity. We report here phase-sensitive measurements of the symmetry of the orbital part of the order parameter using the Josephson effect. The selection rule in the orientation dependence of the Josephson coupling between In, ans-wave superconductor, and UTe₂suggests strongly that UTe₂possesses the odd-parity pairing state of B₁usymmetry near zero magnetic field, making it a spin-triplet superconductor. We also report the apparent formation of Andreev surface bound states on the (1−10) surface of UTe₂. 

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3. Connection between f-electron correlations and magnetic excitations in UTe2

   https://doi.org/10.1038/s41535-024-00720-9  

   Halloran, Thomas; Czajka, Peter; Saucedo_Salas, Gicela; Frank, Corey_E; Kang, Chang-Jong; Rodriguez-Rivera, J_A; Lass, Jakob; Mazzone, Daniel_G; Janoschek, Marc; Kotliar, Gabriel; et al (January 2025, npj Quantum Materials)

   Abstract The detailed anisotropic dispersion of the low-temperature, low-energy magnetic excitations of the candidate spin-triplet superconductor UTe₂is revealed using inelastic neutron scattering. The magnetic excitations emerge from the Brillouin zone boundary at the high symmetryYandTpoints and disperse along the crystallographic$$\hat{b}$$ $\hat{b}$-axis. In applied magnetic fields to at leastμ₀H= 11 T along the$$\hat{c}-{\rm{axis}}$$ $\hat{c}-\mathrm{axis}$, the magnetism is found to be field-independent in the (hk0) plane. The scattering intensity is consistent with that expected from U³⁺/U⁴⁺ f-electron spins with preferential orientation along the crystallographic$$\hat{a}$$ $\hat{a}$-axis, and a fluctuating magnetic moment ofμ_eff=1.7(5)μ_B. We propose interband spin excitons arising fromf-electron hybridization as a possible origin of the magnetic excitations in UTe₂. 

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4. Abrupt shift of El Niño periodicity under CO ₂ mitigation

   https://doi.org/10.1073/pnas.2426048122  

   Iwakiri, Tomoki; Kug, Jong-Seong; Jin, Fei-Fei; Zhao, Sen; An, Soon-Il; Kim, Geon-Il; Park, Dongkyu (June 2025, Proceedings of the National Academy of Sciences)

   Removing CO₂from the atmosphere is emerging as a viable strategy to mitigate global warming, yet the responses of the climate system to CO₂reduction remain uncertain. One of the most uncertain aspects of El Niño behavior is the change in periodicity in response to CO₂forcing [O. Alizadeh,Earth-Sci. Rev.235, 104246 (2022)]. In this study, we show that climate models consistently project an abrupt shortening of El Niño periodicity once CO₂reductions commence in ramp-up and ramp-down CO₂experiments. Besides the contribution of slow mean state changes, this phenomenon is shown to be driven by a southward shift of the Intertropical Convergence Zone (ITCZ) [J.-S. Kug,et al.,Nat. Clim. Chang.12, 47–53 (2022)] and the consequent narrowing of El Niño’s spatial pattern, which enhances the effectiveness of ocean heat recharge/discharge processes, thereby shortening its periodicity. This suggests that the abrupt shift in El Niño periodicity results from a cascading reaction involving ITCZ dynamics and El Niño’s spatial configuration. These findings highlight the critical role of the global energy balance in shaping El Niño characteristics. 

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5. HfO ₂ -based platform for high-index-contrast visible and UV integrated photonics

   https://doi.org/10.1364/OL.553552  

   Jaramillo, Oscar; Natarajan, Vighnesh; Mahmud_Rivy, Hamim; Tensuan, Joshua; Massai, Leonardo; Mehta, Karan K (January 2025, Optics Letters)

   Ultraviolet and visible integrated photonics enable applications in quantum information, sensing, and spectroscopy, among others. Few materials support low-loss photonics into the UV, and the relatively low refractive index of known depositable materials limits the achievable functionality. Here, we present a high-index integrated photonics platform based on HfO₂and Al₂O₃composites deposited via atomic layer deposition (ALD) with low loss in the visible and near UV. We show that Al₂O₃incorporation dramatically decreases bulk loss compared to pure HfO₂, consistent with inhibited crystallization due to the admixture of Al₂O₃. Composites exhibit refractive indexnfollowing the average of that of HfO₂and Al₂O₃, weighted by the HfO₂fractional compositionx. Atλ = 375 nm, composites withx = 0.67 exhibitn = 2.01, preserving most of HfO₂’s significantly higher index, and 3.8(7) dB/cm material loss. We further present fully etched and cladded waveguides, grating couplers, and ring resonators, realizing a single-mode waveguide loss of 0.25(2) dB/cm inferred from resonators of 2.6 million intrinsic quality factor atλ = 729 nm, 2.6(2) dB/cm atλ = 405 nm, and 7.7(6) dB/cm atλ = 375 nm. We measure the composite’s thermo-optic coefficient (TOC) to be 2.44(3) × 10⁻⁵RIU/°C nearλ = 397 nm. This work establishes (HfO₂)_x(Al₂O₃)_1−xcomposites as a platform amenable to integration for low-loss, high-index photonics spanning the UV to NIR. 

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