49 Search Results

We show that a simple coupling between dark energy and dark matter can simultaneously address two distinct hints at new physics coming from cosmological observations. The first is the recent evidence from the DESI project and supernovae observations that the dark energy equation of state w is evolving over cosmic time from an earlier value that is <-1 to a present-day value >-1. The second observation is the so-called S8 tension, describing the suppression of the growth of matter overdensities compared to that expected in the ΛCDM model. Here, we propose a stable, technically natural particle physics implementation of thismore » idea, in which dark matter consists of dark baryons in a strongly coupled hidden sector, and the dark energy field is the associated dark axion. The time variation of the dark matter mass results in an effective dark energy equation of state that exhibits a phantom crossing behavior consistent with recent results. It also results in a slight delay in matter-radiation equality, which suppresses the overall growth of density perturbations.« less

Since discoveries of protected conducting surface states, topological superconducting qubits have enchanted quantum science as prime elements in future fault-tolerant devices, particularly those based on Josephson junctions containing topological insulators. Still, Josephson coupling is often eclipsed by other proximity effects that can dilute topological superconducting pairing at the nontrivial insulator’s boundaries. Here, however, using an ultra-low-temperature scanning tunneling microscope, we detect Josephson physics in topological Bi₂Te₃ films on superconducting Nb. At low temperatures, a previously undetected proximity gap varies little with Bi₂Te₃ thickness and the density of states exhibits normal and superconducting components. Such observations are rationalized via Josephson pairmore » tunneling through the (nearly) insulating Bi₂Te₃ bulk, creating a rare, pure topological superconducting sheet. Our findings establish routes toward accessible topological superconducting states in qubits.« less

Astrometry, the precise measurement of stellar positions and velocities, offers a promising approach to probing the low-frequency stochastic gravitational wave background (SGWB). Notably, astrometric vector sky maps are sensitive to parity-violating SGWB signals, which cannot be distinguished using pulsar timing array observations in an isotropic SGWB. We present the first astrometric constraints on parity-violating SGWB using quasar catalogs from Gaia DR3 and VLBA data. By analyzing the EB correlation in the two-point correlation function of the proper motions of the quasars, we find 2σ constraints on the parity-violating SGWB amplitude h₇₀²Ω_V = -0.020 ± 0.025 from Gaia DR3 and h₇₀²Ω_Vmore » = -0.004 ± 0.010 from VLBA. These constraints are valid in the frequency range 4.2 × 10^-18 Hz < f < 1.1 × 10^-8 Hz. Although not currently a tight constraint on theoretical models, this first attempt lays the groundwork for future investigations using more precise astrometric data.« less

The quasi-one-dimensional charge density wave (CDW) material (TaSe₄)₂I has been recently predicted to host Kramers-Weyl (KW) fermions which should exist in the vicinity of high symmetry points in the Brillouin zone in chiral materials with strong spin-orbit coupling. However, direct spectroscopic evidence of KW fermions is limited. Here we use helicity-dependent laser-based angle-resolved photoemission spectroscopy (ARPES) in conjunction with tight-binding and first-principles calculations to identify KW fermions in (TaSe₄)₂I. We find that topological and symmetry considerations place distinct constraints on the (pseudo-) spin texture and the observed spectra around a KW node. Our findings highlight the unique topological nature ofmore » (TaSe₄)₂I and provide a pathway for identifying KW fermions in other chiral materials.« less

Spin-helical Dirac Fermions at a doped topological insulator’s boundaries can support Majorana quasiparticles when coupled with s-wave superconductors, but in n-doped systems, the requisite induced Cooper pairing in topological states is often buried at heterointerfaces or complicated by degenerate coupling with bulk conduction carriers. Rarely probed are p-doped topological structures with nondegenerate Dirac and bulk valence bands at the Fermi level, which may foster long-range superconductivity without sacrificing Majorana physics. Using ultrahigh-resolution photoemission, we report proximity pairing with a large decay length in p-doped topological Sb₂Te₃ on superconducting Nb. Despite no momentum-space degeneracy, the topological and bulk states of Sb₂Te₃/Nbmore » exhibit the same isotropic superconducting gaps at low temperatures. Furthermore, our results unify principles for realizing accessible pairing in Dirac Fermions relevant to topological superconductivity.« less

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Illustration of hot hole transfer through interfacial S–Pb interactions, and a peak efficiency of 27.30% was achieved under 5.9 suns via ultrafast hot hole extraction.

We consider the effects of relaxing the assumption that gravitational waves composing the stochastic gravitational wave background (SGWB) are uncorrelated between frequencies in analyses of the data from Pulsar Timing Arrays (PTAs). While individual monochromatic plane waves are often a good approximation, a background composed of astrophysical sources cannot be monochromatic since an infinite plane wave carries no signal. We consider how relaxing this assumption allows us to extract potential information about modified dispersion relations and other fundamental physics questions, as both the group and phase velocity of waves become relevant. After developing the formalism we carry out simple Gaussianmore » wavepacket examples and then consider more realistic waveforms, such as that from binary inspirals. When the frequency evolves only slowly across the PTA temporal baseline, the monochromatic assumption at an effective mean frequency remains a good approximation and we provide scaling relations that characterize its accuracy.« less