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Spin dynamics in antiferromagnets has much shorter timescales than in ferromagnets, offering attractive properties for potential applications in ultrafast devices 1 – 3 . However, spin-current generation via antiferromagnetic resonance and simultaneous electrical detection by the inverse spin Hall effect in heavy metals have not yet been explicitly demonstrated 4 – 6 . Here we report sub-terahertz spin pumping in heterostructures of a uniaxial antiferromagnetic Cr 2 O 3 crystal and a heavy metal (Pt or Ta in its β phase). At 0.240 terahertz, the antiferromagnetic resonance in Cr 2 O 3 occurs at about 2.7 tesla, which excites only right-handed magnons. In the spin-canting state, another resonance occurs at 10.5 tesla from the precession of induced magnetic moments. Both resonances generate pure spin currents in the heterostructures, which are detected by the heavy metal as peaks or dips in the open-circuit voltage. The pure-spin-current nature of the electrically detected signals is unambiguously confirmed by the reversal of the voltage polarity observed under two conditions: when switching the detector metal from Pt to Ta, reversing the sign of the spin Hall angle 7 – 9 , and when flipping the magnetic-field direction, reversing the magnon chirality 4 , 5 . The temperature dependence of the electrical signals at both resonances suggests that the spin current contains both coherent and incoherent magnon contributions, which is further confirmed by measurements of the spin Seebeck effect and is well described by a phenomenological theory. These findings reveal the unique characteristics of magnon excitations in antiferromagnets and their distinctive roles in spin–charge conversion in the high-frequency regime. Pure spin currents are simultaneously generated and detected electrically through sub-terahertz magnons in the antiferromagnetic insulator Cr 2 O 3 , demonstrating the potential of magnon excitations in antiferromagnets for high-frequency spintronic devices.

The direct measurement of Berry phases is still a great challenge in condensed-matter systems. The bottleneck has been the ability to adiabatically drive an electron coherently across a large portion of the Brillouin zone in a solid where the scattering is strong and complicated. We break through this bottleneck and show that high-order sideband generation (HSG) in semiconductors is intimately affected by Berry phases. Electron-hole recollisions and HSG occur when a near-band-gap laser beam excites a semiconductor that is driven by sufficiently strong terahertz-frequency electric fields. We carry out experimental and theoretical studies of HSG from threeGaAs/AlGaAsquantum wells. The observed HSG spectra contain sidebands up to the 90th order, to our knowledge the highest-order optical nonlinearity reported in solids. The highest-order sidebands are associated with electron-hole pairs driven coherently across roughly 10% of the Brillouin zone around theΓpoint. The principal experimental claim is a dynamical birefringence: the intensity and polarization of the sidebands depend on the relative polarization of the exciting near-infrared (NIR) and the THz electric fields, as well as on the relative orientation of the laser fields with the crystal. We explain dynamical birefringence by generalizing the three-step model for high-order harmonic generation. The hole accumulates Berry phases due to variation of its internal state as the quasimomentum changes under the THz field. Dynamical birefringence arises from quantum interference between time-reversed pairs of electron-hole recollision pathways. We propose a method to use dynamical birefringence to measure Berry curvature in solids.

In a semiconductor illuminated by a strong terahertz (THz) field, optically excited electron–hole pairs can recombine to emit light in a broad frequency comb evenly spaced by twice the THz frequency. Such high-order THz sideband generation is of interest both as an example of extreme nonlinear optics and also as a method for ultrafast electro-optical modulation. So far, this phenomenon has only been observed with large field strengths (~10 kV cm −1 ), an obstacle for technological applications. Here we predict that bi-layer graphene generates high-order sidebands at much weaker THz fields. We find that a THz field of strength 1 kV cm −1 can produce a high-sideband spectrum of about 30 THz, 100 times broader than in GaAs. The sidebands are generated despite the absence of classical collisions, with the quantum coherence of the electron–hole pairs enabling recombination. These remarkable features lower the barrier to desktop electro-optical modulation at THz frequencies, facilitating ultrafast optical communications. In terahertz sideband generation, an electron–hole pair is accelerated in a semiconductor by a terahertz field to then recombines forming a frequency comb, but so far experimental realizations have relied on the large fields of free electron lasers. Here, Crosse et al. propose bi-layer graphene for sideband generation at lower fields.

We investigate the THz intersubband absorption behavior of a single 40-nm wide GaAs/AlGaAs square quantum well (QW) using Johnson noise thermometry. In our measurements, the Johnson noise associated with intersubband absorption is measured from the in-plane conduction channel of the QW while its intersubband absorption behavior is being tuned through the independent control of the charge density and the perpendicular DC electric field. Our measurements enable the study of intersubband absorption of a small (∼20,000 and potentially fewer) number of electrons in a single mesoscopic device, as well as direct measurement of the electron heating from intersubband absorption. By measuring the Johnson noise response to monochromatic THz radiation at 2.52 THz and 4.25 THz at 20 K as a function of the DC electric field over a wide range of charge density, we show that the observed Johnson noise behavior correlates well with the expected intersubband absorption of the 40-nm QW. To explain the absorption features of the experimental results, we model the data by calculating the THz coupling efficiency based on the impedance model for intersubband absorption, which qualitatively reproduces the observed Johnson noise behavior well. Based on the temperature calibration of the Johnson noise measured at 2.52 THz, we deduce an increase in the electron temperature Δ of  K when the maximum absorption of THz power occurs in the device.

Antiferromagnetically coupled S = 1/2 spins on an isotropic triangular lattice are the paradigm of frustrated quantum magnetism, but structurally ideal realizations are rare. Here, we investigate NaYbO2, which hosts an ideal triangular lattice of effective Jeff = 1/2 moments with no inherent site disorder. No signatures of conventional magnetic order appear down to 50 mK, strongly suggesting a quantum spin liquid ground state. We observe a two-peak specific heat and a nearly quadratic temperature dependence, in agreement with expectations for a two-dimensional Dirac spin liquid. Application of a magnetic field strongly perturbs the quantum disordered ground state and induces a clear transition into a collinear ordered state, consistent with a long-predicted up–up–down structure for a triangular-lattice XXZ Hamiltonian driven by quantum fluctuations. The observation of spin liquid signatures in zero field and quantum-induced ordering in intermediate fields in the same compound demonstrates an intrinsically quantum disordered ground state. We conclude that NaYbO2 is a model, versatile platform for exploring spin liquid physics with full tunability of field and temperature.

How proteins transduce light into mechanical energy remains a central question in biology. This study tests the hypothesis that blue light activation of the LOV2 (light, oxygen, voltage sensitive) domain of phototropin 1 (AsLOV2), gives rise to concerted water movement that induces protein conformational extensions. Using electron and nuclear magnetic resonance spectroscopy, along with molecular dynamics simulations at high pressure, we find AsLOV2 activation can be initiated by blue light or high pressure, followed by selective and concerted expulsion of low-entropy, tetrahedrally coordinated \"wrap\" water from the protein hydration shell. These findings suggest that interfacial water serves as constituents to reshape the protein's free energy landscape during activation. Our study highlights hydration water as an active hydraulic fluid that can drive long-range conformational changes underlying protein mechanics upon light activation and offers a new concept for engineering externally controllable protein actuators.

Although radiation at terahertz frequencies has many uses, most sources cannot generate terahertz beams with great power. Magnetic manipulation of energetic electrons inside a particle accelerator offers a solution.

Although radiation at terahertz frequencies has many uses, most sources cannot generate terahertz beams with great power. Magnetic manipulation of energetic electrons inside a particle accelerator offers a solution.