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An optical second-harmonic generation study of a series of transition metal monopnictide Weyl semimetals reveals a giant, anisotropic nonlinear optical response in these systems. Although Weyl fermions have proven elusive in high-energy physics, their existence as emergent quasiparticles has been predicted in certain crystalline solids in which either inversion or time-reversal symmetry is broken 1 , 2 , 3 , 4 . Recently they have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention 5 , 6 , 7 . The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable electronic properties. The TMMPs are polar metals, a rare subset of inversion-breaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons 8 , 9 , 10 . Despite the absence of spontaneous polarization, polar metals can exhibit other signatures of inversion-symmetry breaking, most notably second-order nonlinear optical polarizability, χ (2) , leading to phenomena such as optical rectification and second-harmonic generation (SHG). Here we report measurements of SHG that reveal a giant, anisotropic χ (2) in the TMMPs TaAs, TaP and NbAs. With the fundamental and second-harmonic fields oriented parallel to the polar axis, the value of χ (2) is larger by almost one order of magnitude than its value in the archetypal electro-optic materials GaAs 11 and ZnTe 12 , and in fact larger than reported in any crystal to date.

Advances in controlling the correlated behaviour of transition metal dichalcogenides have opened a new frontier of many-body physics in two dimensions. A field where these materials have yet to make a deep impact is antiferromagnetic spintronics—a relatively new research direction promising technologies with fast switching times, insensitivity to magnetic perturbations and reduced cross-talk 1 – 3 . Here, we present measurements on the intercalated transition metal dichalcogenide Fe 1/3 NbS 2 that exhibits antiferromagnetic ordering below 42 K (refs. 4 , 5 ). We find that remarkably low current densities of the order of 10 4  A cm −2 can reorient the magnetic order, which can be detected through changes in the sample resistance, demonstrating its use as an electronically accessible antiferromagnetic switch. Fe 1/3 NbS 2 is part of a larger family of magnetically intercalated transition metal dichalcogenides, some of which may exhibit switching at room temperature, forming a platform from which to build tuneable antiferromagnetic spintronic devices 6 , 7 . A remarkably low critical current is found to reorient the magnetic order in a magnetically intercalated transition metal dichalcogenide, suggesting this class of materials could form a basis for antiferromagnetic spintronics.

Domain walls may play an important role in future electronic devices, given their small size as well as the fact that their location can be controlled. Here, we report the observation of room-temperature electronic conductivity at ferroelectric domain walls in the insulating multiferroic BiFeO 3 . The origin and nature of the observed conductivity are probed using a combination of conductive atomic force microscopy, high-resolution transmission electron microscopy and first-principles density functional computations. Our analyses indicate that the conductivity correlates with structurally driven changes in both the electrostatic potential and the local electronic structure, which shows a decrease in the bandgap at the domain wall. Additionally, we demonstrate the potential for device applications of such conducting nanoscale features. Domain walls may be important in future electronic devices, given their small size as well as the fact that their location can be controlled. In the case of insulating multiferroic oxides, domain walls are now discovered to be electrically conductive, suggesting their possible use in logic and memory applications.

A persistent spin helix Just as a body moving in a vacuum tends to stay in motion, the axis of a spinning electron tends to remain fixed in direction. Both phenomena are conservation laws that ultimately derive from the uniformity of empty space. By contrast, an electron moving in a semiconductor sees a lattice of charged atoms flying past at nearly 1% of light speed, causing its spin direction to fluctuate wildly. Now Koralek et al . demonstrate that the application of an external electric field to a semiconductor can precisely balance the spin-destabilizing effect of the charged lattice. The collective spin of the entire gas of electrons, rather than that of each individual particle, then emerges as a new conserved quantity — a property well suited for 'spintronics' applications. The axis of a spinning electron tends to remain fixed in direction: in contrast, an electron moving in a semiconductor sees a lattice of charged atoms flying past, causing its spin direction to fluctuate. Koralek and colleagues demonstrate that an electric field applied to the semiconductor can balance this spin-destabilizing effect; the collective spin of the entire gas of electrons is conserved, a property well-suited for 'spintronics' applications. According to Noether’s theorem 1 , for every symmetry in nature there is a corresponding conservation law. For example, invariance with respect to spatial translation corresponds to conservation of momentum. In another well-known example, invariance with respect to rotation of the electron’s spin, or SU(2) symmetry, leads to conservation of spin polarization. For electrons in a solid, this symmetry is ordinarily broken by spin–orbit coupling, allowing spin angular momentum to flow to orbital angular momentum. However, it has recently been predicted that SU(2) can be achieved in a two-dimensional electron gas, despite the presence of spin–orbit coupling 2 . The corresponding conserved quantities include the amplitude and phase of a helical spin density wave termed the ‘persistent spin helix’ 2 . SU(2) is realized, in principle, when the strengths of two dominant spin–orbit interactions, the Rashba 3 (strength parameterized by α ) and linear Dresselhaus 4 ( β 1 ) interactions, are equal. This symmetry is predicted to be robust against all forms of spin-independent scattering, including electron–electron interactions, but is broken by the cubic Dresselhaus term ( β 3 ) and spin-dependent scattering. When these terms are negligible, the distance over which spin information can propagate is predicted to diverge as α approaches β 1 . Here we report experimental observation of the emergence of the persistent spin helix in GaAs quantum wells by independently tuning α and β 1 . Using transient spin-grating spectroscopy 5 , we find a spin-lifetime enhancement of two orders of magnitude near the symmetry point. Excellent quantitative agreement with theory across a wide range of sample parameters allows us to obtain an absolute measure of all relevant spin–orbit terms, identifying β 3 as the main SU(2)-violating term in our samples. The tunable suppression of spin relaxation demonstrated in this work is well suited for application to spintronics 6 , 7 .

The high-temperature copper oxide superconductors are of fundamental and enduring interest. They not only manifest superconducting transition temperatures inconceivable 15 years ago, but also exhibit many other properties apparently incompatible with conventional metal physics. The materials expand our notions of what is possible, and compel us to develop new experimental techniques and theoretical concepts. This article provides a perspective on recent developments and their implications for our understanding of interacting electrons in metals.

In the underdoped copper-oxides, high-temperature superconductivity condenses from a nonconventional metallic ”pseudogap” phase that exhibits a variety of non-Fermi liquid properties. Recently, it has become clear that a charge density wave (CDW) phase exists within the pseudogap regime. This CDW coexists and competes with superconductivity (SC) below the transition temperature T c , suggesting that these two orders are intimately related. Here we show that the condensation of the superfluid from this unconventional precursor is reflected in deviations from the predictions of BSC theory regarding the recombination rate of quasiparticles. We report a detailed investigation of the quasiparticle (QP) recombination lifetime, τ qp , as a function of temperature and magnetic field in underdoped HgBa 2 CuO 4+ δ (Hg-1201) and YBa 2 Cu 3 O 6+ x (YBCO) single crystals by ultrafast time-resolved reflectivity. We find that τ qp ( T  ) exhibits a local maximum in a small temperature window near T c that is prominent in underdoped samples with coexisting charge order and vanishes with application of a small magnetic field. We explain this unusual, non-BCS behavior by positing that T c marks a transition from phase-fluctuating SC/CDW composite order above to a SC/CDW condensate below. Our results suggest that the superfluid in underdoped cuprates is a condensate of coherently-mixed particle-particle and particle-hole pairs.

The nature of the pseudogap phase of cuprate high-temperature superconductors is a major unsolved problem in condensed matter physics. We studied the commencement of the pseudogap state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally doped Bi2201 crystals. We observed the coincident, abrupt onset at T* of a particle-hole asymmetric antinodal gap in the electronic spectrum, a Kerr rotation in the reflected light polarization, and a change in the ultrafast relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T c ), entangled in an energy-momentum—dependent manner with the preexisting pseudogap features, ushering in a ground state with coexisting orders.

An electron propagating through a solid carries spin angular momentum in addition to its mass and charge. Of late there has been considerable interest in developing electronic devices based on the transport of spin that offer potential advantages in dissipation, size and speed over charge-based devices 1 . However, these advantages bring with them additional complexity. Because each electron carries a single, fixed value (- e ) of charge, the electrical current carried by a gas of electrons is simply proportional to its total momentum. A fundamental consequence is that the charge current is not affected by interactions that conserve total momentum, notably collisions among the electrons themselves 2 . In contrast, the electron's spin along a given spatial direction can take on two values, ± ℏ /2 (conventionally ↑,↓), so that the spin current and momentum need not be proportional. Although the transport of spin polarization is not protected by momentum conservation, it has been widely assumed that, like the charge current, spin current is unaffected by electron–electron ( e – e ) interactions. Here we demonstrate experimentally not only that this assumption is invalid, but also that over a broad range of temperature and electron density, the flow of spin polarization in a two-dimensional gas of electrons is controlled by the rate of e – e collisions.

New HIV therapies are urgently needed to address the growing problem of drug resistance. In this article, we characterize the anti-HIV drug candidate 3-O-(3′,3′-dimethylsuccinyl) betulinic acid (PA-457). We show that PA-457 potently inhibits replication of both WT and drug-resistant HIV-1 isolates and demonstrate that the compound acts by disrupting a late step in Gag processing involving conversion of the capsid precursor (p25) to mature capsid protein (p24). We find that virions from PA-457-treated cultures are noninfectious and exhibit an aberrant particle morphology characterized by a spherical, acentric core and a crescent-shaped, electron-dense shell lying just inside the viral membrane. To identify the determinants of compound activity we selected for PA-457-resistant virus in vitro. Consistent with the effect on Gag processing, we found that mutations conferring resistance to PA-457 map to the p25 to p24 cleavage site. PA-457 represents a unique class of anti-HIV compounds termed maturation inhibitors that exploit a previously unidentified viral target, providing additional opportunities for HIV drug discovery.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.An amendment to this paper has been published and can be accessed via a link at the top of the paper.