We report the discovery of a strong and tunable spin-lifetime anisotropy with excellent out-of-plane spin lifetimes up to 7.8 ns at 100 K in dual-gated bilayer graphene. Remarkably, this realizes the manipulation of spins in graphene by electrically controlled spin-orbit fields, which is unexpected due to graphene’s weak intrinsic spin-orbit coupling ($\sim 12\mu \mathrm{eV}$). We utilize both the in-plane magnetic field Hanle precession and oblique Hanle precession measurements to directly compare the lifetimes of out-of-plane vs in-plane spins. We find that near the charge neutrality point, the application of a perpendicular electric field opens a band gap and generates an out-of-plane spin-orbit field that stabilizes out-of-plane spins against spin relaxation, leading to a large spin-lifetime anisotropy (defined as the ratio between out-of-plane and in-plane spin lifetime) up to $\sim 12$ at 100 K. This intriguing behavior occurs because of the unique spin-valley coupled band structure of bilayer graphene. Our results demonstrate the potential for highly tunable spintronic devices based on dual-gated 2D materials.

Topological phases, such as Chern insulators, are defined in terms of additive indices that are stable against the addition of trivial degrees of freedom. Such topology presents an obstruction to any Wannier representation, namely, the representation of the electronic states in terms of symmetric, exponentially localized Wannier functions. Here, we address the converse question: Do obstructions to Wannier representation imply stable band topology? We answer this in the negative, pointing out that some bands can also display a distinct type of “fragile topology.” Bands with fragile topology do not admit any Wannier representation by themselves, but such a representation becomes possible once certain additional trivial degrees of freedom are supplied. We construct a physical model of fragile topology on the honeycomb lattice that also helps resolve a recent puzzle in band theory. This model provides a counterexample to the assumption that splitting of an “elementary band representation” introduced in [B. Bradlyn et al., Topological quantum chemistry, Nature (London) 547, 298 (2017)] leads to bands that are individually topological. Instead, half of the split bands of our model realize a trivial band with exponentially localized symmetric Wannier functions, whereas the second half possess fragile topology. Our work highlights an important and previously overlooked connection between band structure and Wannier functions, and is expected to have far-reaching consequences given the central role played by Wannier functions in the modeling of real materials.

We formulate an energy-storage concept based on the free energy associated with metastable magnetic configurations. Despite the active magnetic region of the battery being electrically insulating, it can sustain effective hydrodynamics of spin textures, whose conservation law is governed by topology. To illustrate the key physics and potential functionality, we focus here on the simplest quasi-one-dimensional case of planar winding of the magnetic order parameter. The energy is stored in the metastable winding number, which can be injected electrically by an appropriately tailored spin torque. Because of the nonvolatility and the endurance of magnetic systems, the injected energy can be stored essentially indefinitely, with the topological charge cycles that do not degrade over time.

We study the dynamics of knotted deformable closed chains sedimenting in a viscous fluid. We show experimentally that trefoil and other torus knots often attain a remarkably regular horizontal toroidal structure while sedimenting, with a number of intertwined loops, oscillating periodically around each other. We then recover this motion numerically and find out that it is accompanied by a very slow rotation around the vertical symmetry axis. We analyze the dependence of the characteristic timescales on the chain flexibility and aspect ratio. It is observed in the experiments that this oscillating mode of the dynamics can spontaneously form even when starting from a qualitatively different initial configuration. In numerical simulations, the oscillating modes are usually present as transients or final stages of the evolution, depending on chain aspect ratio and flexibility, and the number of loops.

The observation of the standard model (SM) Higgs boson decay to a pair of bottom quarks is presented. The main contribution to this result is from processes in which Higgs bosons are produced in association with a $W$ or $Z$ boson ($VH$), and are searched for in final states including 0, 1, or 2 charged leptons and two identified bottom quark jets. The results from the measurement of these processes in a data sample recorded by the CMS experiment in 2017, comprising $41.3{\mathrm{fb}}^{-1}$ of proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$, are described. When combined with previous $VH$ measurements using data collected at $\sqrt{s}=7$, 8, and 13 TeV, an excess of events is observed at $m_H=125\mathrm{GeV}$ with a significance of 4.8 standard deviations, where the expectation for the SM Higgs boson is 4.9. The corresponding measured signal strength is $1.01\pm 0.22$. The combination of this result with searches by the CMS experiment for $H\to b\overline{b}$ in other production processes yields an observed (expected) significance of 5.6 (5.5) standard deviations and a signal strength of $1.04\pm 0.20$.

Despite the rapid progress in the field of the quantum spin Hall (QSH) effect, most of the QSH systems studied up to now are based on crystalline materials. Here we propose that the QSH effect can be realized in quasicrystal lattices (QLs). We show that the electronic topology of aperiodic and amorphous insulators can be characterized by a spin Bott index $B_s$. The nontrivial QSH state in a QL is identified by a nonzero spin Bott index $B_s=1$, associated with robust edge states and quantized conductance. We also map out a topological phase diagram in which the QSH state lies in between a normal insulator and a weak metal phase due to the unique wave functions of QLs. Our findings not only provide a better understanding of electronic properties of quasicrystals but also extend the search of the QSH phase to aperiodic and amorphous materials that are experimentally feasible.

We argue that a correlated fluid of electrons and holes can exhibit a fractional quantum Hall effect at zero magnetic field analogous to the Laughlin state at filling $1/m$. We introduce a variant of the Laughlin wave function for electrons and holes and show that for $m=1$ it is the exact ground state of a free fermion model that describes $p_x+i{p}_y$ excitonic pairing. For $m>1$ we develop a simple composite fermion mean field theory, and we present evidence that our wave function correctly describes this phase. We derive an interacting Hamiltonian for which our wave function is the exact ground state, and we present physical arguments that the $m=3$ state can be realized in a system in which energy bands with angular momentum that differ by 3 cross at the Fermi energy. This leads to a gapless state with $(p_x+i{p}_y{)}^3$ excitonic pairing, which we argue is conducive to forming the fractional excitonic insulator in the presence of interactions. Prospects for numerics on model systems and band structure engineering to realize this phase in real materials are discussed.

Two-dimensional (2D) materials consisting of heavy atoms with particular arrangements may host exotic quantum properties. Here, we report a unique 2D semiconducting binary compound, a ${\mathrm{Sn}}_2\mathrm{Bi}$ atomic layer on Si(111), in which hexagons are formed by bonding Bi with a triangular network of Sn. Because of the unique honeycomb configuration, the heavy elements, and the energy-dependent hybridization between Sn and Bi, 2D ${\mathrm{Sn}}_2\mathrm{Bi}$ not only shows strong spin-orbit coupling effects but also exhibits high electron-hole asymmetry: Nearly free hole bands and dispersionless flat electron bands coexist in the same system. By tuning the Fermi level, it is possible to preserve both nearly free and strongly localized charge carriers in the same 2D material, which provides an ideal platform for the studies of strongly correlated phenomena and possible applications in nanodevices.

The mechanism of high superconducting transition temperatures ($T_c$) in bismuthates remains under debate despite more than 30 years of extensive research. Our angle-resolved photoemission spectroscopy studies on ${\mathrm{Ba}}_{0.51}{\mathrm{K}}_{0.49}{\mathrm{BiO}}_3$ reveal an unexpectedly 34% larger bandwidth than in conventional density functional theory calculations. This can be reproduced by calculations that fully account for long-range Coulomb interactions—the first direct demonstration of bandwidth expansion due to the Fock exchange term, a long-accepted and yet uncorroborated fundamental effect in many body physics.Furthermore, we observe an isotropic superconducting gap with $2{\mathrm{\Delta }}_0/k_B{T}_c=3.51\pm 0.05$, and strong electron-phonon interactions with a coupling constant $\lambda \sim 1.3\pm 0.2$. These findings solve a long-standing mystery—${\mathrm{Ba}}_{0.51}{\mathrm{K}}_{0.49}{\mathrm{BiO}}_3$ is an extraordinary Bardeen-Cooper-Schrieffer superconductor, where long-range Coulomb interactions expand the bandwidth, enhance electron-phonon coupling, and generate the high $T_c$. Such effects will also be critical for finding new superconductors.

We report on a search for weakly interacting massive particles (WIMPs) using 278.8 days of data collected with the XENON1T experiment at LNGS. XENON1T utilizes a liquid xenon time projection chamber with a fiducial mass of $(1.30\pm 0.01)\mathrm{t}\mathrm{o}\mathrm{n}$, resulting in a 1.0 ton yr exposure. The energy region of interest, $[1.4,10.6]{\mathrm{keV}}_{\mathrm{ee}}$ ($[4.9,40.9]{\mathrm{keV}}_{\mathrm{nr}}$), exhibits an ultralow electron recoil background rate of $[{82}_{-3}^{+5}(\mathrm{syst})\pm 3(\mathrm{stat})]\mathrm{events}/(\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{yr}{\mathrm{keV}}_{\mathrm{ee}})$. No significant excess over background is found, and a profile likelihood analysis parametrized in spatial and energy dimensions excludes new parameter space for the WIMP-nucleon spin-independent elastic scatter cross section for WIMP masses above $6\mathrm{GeV}/c^2$, with a minimum of $4.1\times {10}^{-47}{\mathrm{cm}}^2$ at $30\mathrm{GeV}/c^2$ and a 90% confidence level.

We present powerful new analysis techniques to constrain effective field theories at the LHC. By leveraging the structure of particle physics processes, we extract extra information from Monte Carlo simulations, which can be used to train neural network models that estimate the likelihood ratio. These methods scale well to processes with many observables and theory parameters, do not require any approximations of the parton shower or detector response, and can be evaluated in microseconds. We show that they allow us to put significantly stronger bounds on dimension-six operators than existing methods, demonstrating their potential to improve the precision of the LHC legacy constraints.

The “trilobite”-type of molecule, predicted in 2000 and observed experimentally in 2015, arises when a Rydberg electron exerts a weak attractive force on a neutral ground state atom. Such molecules have bond lengths exceeding 100 nm. The ultralong-range chemical bond between the two atoms is a nonperturbative linear combination of the many degenerate electronic states associated with high principal quantum numbers, and the resulting electron probability distribution closely resembles a fossil trilobite from antiquity. We show how to coherently engineer this same long-range orbital through a sequence of electric and magnetic field pulses even when the ground-state atom is not present and propose several methods to observe the resulting orbital. The existence of such a ghost chemical bond in which an electron reaches out from one atom to a nonexistent second atom is a consequence of the high level degeneracy.

Measuring inelastic rates with partial-wave resolution requires temperatures close to a Kelvin or below, even for the lightest molecule. In a recent experiment, Perreault, Mukherjee, and Zare [Nat. Chem. 10, 561 (2018).] studied collisional relaxation of excited HD molecules in the $v=1$, $j=2$ state by para- and ortho-${\mathrm{H}}_2$ at a temperature of about 1 K, extracting the angular distribution of scattered HD in the $v=1$, $j=0$ state. By state preparation of the HD molecules, control of the angular distribution of scattered HD was demonstrated. Here, we report a first-principles simulation of that experiment which enables us to attribute the main features of the observed angular distribution to a single $L=2$ partial-wave shape resonance. Our results demonstrate important stereodynamical insights that can be gained when numerically exact quantum scattering calculations are combined with experimental results in the few-partial-wave regime.

In this Letter we report a demonstration of electron ghost imaging. A digital micromirror device directly modulates the photocathode drive laser to control the transverse distribution of a relativistic electron beam incident on a sample. Correlating the structured illumination pattern to the total sample transmission then retrieves the target image, avoiding the need for a pixelated detector. In our example, we use a compressed sensing framework to improve the reconstruction quality and reduce the number of shots compared to raster scanning a small beam across the target. Compressed electron ghost imaging can reduce both acquisition time and sample damage in experiments for which spatially resolved detectors are unavailable (e.g., spectroscopy) or in which the experimental architecture precludes full frame direct imaging.

Glasses at low temperature fluctuate around their inherent states; glassy anomalies reflect the structure of these states. Recently, there have been numerous observations of long-range stress correlations in glassy materials, from supercooled liquids to colloids and granular materials, but without a common explanation. Herein it is shown, using a field theory of inherent states, that long-range stress correlations follow from mechanical equilibrium alone, with explicit predictions for stress correlations in two and three dimensions. “Equations of state” relating fluctuations to imposed stresses are derived, as well as field equations that fix the spatial structure of stresses in arbitrary geometries. Finally, a new holographic quantity in 3D amorphous systems is identified.

Einstein-Podolsky-Rosen steering is a quantum phenomenon wherein one party influences, or steers, the state of a distant party’s particle beyond what could be achieved with a separable state, by making measurements on one-half of an entangled state. This type of quantum nonlocality stands out through its asymmetric setting and even allows for cases where one party can steer the other but where the reverse is not true. A series of experiments have demonstrated one-way steering in the past, but all were based on significant limiting assumptions. These consisted either of restrictions on the type of allowed measurements or of assumptions about the quantum state at hand, by mapping to a specific family of states and analyzing the ideal target state rather than the real experimental state. Here, we present the first experimental demonstration of one-way steering free of such assumptions. We achieve this using a new sufficient condition for nonsteerability and, although not required by our analysis, using a novel source of extremely high-quality photonic Werner states.

By introducing concepts from transformation optics to the manipulation of water waves, we design and experimentally demonstrate two annular devices for concentrating waves, which employ gradient depth profiles based on Fabry-Pérot resonances. Our measurements and numerical simulations confirm the concentrating effect of the annular devices and show that they are effectively invisible to the water waves. We show that transformation optics is thus an effective framework for designing devices to improve the efficiency of wave energy collection, and we expect potential applications in coastline ocean engineering.

Time-reversal symmetry suppresses electron backscattering in a quantum-spin-Hall edge, yielding quantized conductance at zero temperature. Understanding the dominant corrections in finite-temperature experiments remains an unsettled issue. We study a novel mechanism for conductance suppression: backscattering caused by incoherent electromagnetic noise. Specifically, we show that an electric potential fluctuating randomly in time can backscatter electrons inelastically without constraints faced by electron-electron interactions. We quantify noise-induced corrections to the dc conductance in various regimes and propose an experiment to test this scenario.

The existence of Wigner crystallization, one of the most significant hallmarks of strong electron correlations, has to date only been definitively observed in two-dimensional systems. In one-dimensional (1D) quantum wires Wigner crystals correspond to regularly spaced electrons; however, weakening the confinement and allowing the electrons to relax in a second dimension is predicted to lead to the formation of a new ground state constituting a zigzag chain with nontrivial spin phases and properties. Here we report the observation of such zigzag Wigner crystals by use of on-chip charge and spin detectors employing electron focusing to image the charge density distribution and probe their spin properties. This experiment demonstrates both the structural and spin phase diagrams of the 1D Wigner crystallization. The existence of zigzag spin chains and phases which can be electrically controlled in semiconductor systems may open avenues for experimental studies of Wigner crystals and their technological applications in spintronics and quantum information.

We propose that the noncentrosymmetric LiGaGe-type hexagonal $ABC$ crystal SrHgPb realizes a new type of topological semimetal that hosts both Dirac and Weyl points in momentum space. The symmetry-protected Dirac points arise due to a band inversion and are located on the sixfold rotation $z$ axis, whereas the six pairs of Weyl points related by sixfold symmetry are located on the perpendicular $k_z=0$ plane. By studying the electronic structure as a function of the buckling of the HgPb layer, which is the origin of inversion symmetry breaking, we establish that the coexistence of Dirac and Weyl fermions defines a phase separating two topologically distinct Dirac semimetals. These two Dirac semimetals are distinguished by the ${\mathbb{Z}}_2$ index of the $k_z=0$ plane and the corresponding presence or absence of 2D Dirac fermions on side surfaces. We formalize our first-principles calculations by deriving and studying a low-energy model Hamiltonian describing the Dirac-Weyl semimetal phase. We conclude by proposing several other materials in the noncentrosymmetric $ABC$ material class, in particular SrHgSn and CaHgSn, as candidates for realizing the Dirac-Weyl semimetal.