The origin of the microbranching instability is a long-standing unresolved issue in the fracture of brittle amorphous materials. We investigate the onset of this instability by measuring the real-time dynamics and symmetries of the strain fields produced by rapid tensile cracks in brittle gels. We find that once a simple tensile crack is subjected to shear perturbations, cracks undergo the microbranching instability above a finite velocity-dependent threshold. We further reveal a distinct relation between the microbranching and the oscillatory instabilities of rapid cracks.

An atomistic description of shear stress relaxation in a viscoelastic liquid is developed from first principles through accurate molecular dynamic simulations in a model Yukawa system. It is shown that the relaxation time ${\tau }_M^{\mathrm{ex}}$ of the excess part of the shear stress autocorrelation function provides a correct measure of the relaxation process. Below a certain critical value ${\mathrm{\Gamma }}_c$ of the Coulomb coupling strength, the lifetime of local atomic connectivity ${\tau }_{\mathrm{LC}}$ converges to ${\tau }_M^{\mathrm{ex}}$ and is the microscopic origin of the relaxation. At $\mathrm{\Gamma }\gg {\mathrm{\Gamma }}_c$, i.e., in the potential energy dominated regime, ${\tau }_M^{\mathrm{ex}}\to {\tau }_M$ (the Maxwell relaxation time) and can, therefore, fully account for the elastic or “solidlike” behavior. Our results can help provide a better fundamental understanding of viscoelastic behavior in a variety of strongly coupled systems such as dusty plasmas, colloids, and non-Newtonian fluids.

We report that $R$ martensite isothermally forms with time in a solution-treated ${\mathrm{Ti}}_{48.7}{\mathrm{Ni}}_{51.3}$ single crystal. This abnormal formation originates from the growth of a short-range ordered $R$ phase with time, i.e., the “crystallization” of strain glass. The established time-composition-temperature Ti-Ni diagram shows a time evolution of the $R$ phase and composition-temperature phase diagram. The presence or absence of the $R$ phase in this new diagram, as well as in other conditions (like doping Fe or aging), is explained in a unified framework of free-energy landscape. Our finding suggests a new mechanism for the isothermal martensite formation, which could be applied to other metal and ceramic martensitic systems to find new phases and novel properties.

We present a novel generic framework to approximate the nonequilibrium steady states of dissipative quantum many-body systems. It is based on the variational minimization of a suitable norm of the quantum master equation describing the dynamics. We show how to apply this approach to different classes of variational quantum states and demonstrate its successful application to a dissipative extension of the Ising model, which is of importance to ongoing experiments on ultracold Rydberg atoms, as well as to a driven-dissipative variant of the Bose-Hubbard model. Finally, we identify several advantages of the variational approach over previously employed mean-field-like methods.

The time-dependent $CP$ asymmetry in $B_s^0\to J/\psi K^{+}{K}^{-}$ decays is measured using $pp$ collision data, corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV. In a sample of 96 000 $B_s^0\to J/\psi K^{+}{K}^{-}$ decays, the $CP$-violating phase ${\varphi }_s$ is measured, as well as the decay widths ${\mathrm{\Gamma }}_{\mathrm{L}}$ and ${\mathrm{\Gamma }}_{\mathrm{H}}$ of the light and heavy mass eigenstates of the $B_s^0–{\overline{B}}_s^0$ system. The values obtained are ${\varphi }_s=-0.058\pm 0.049\pm 0.006\mathrm{rad}$, ${\mathrm{\Gamma }}_s\equiv ({\mathrm{\Gamma }}_{\mathrm{L}}+{\mathrm{\Gamma }}_{\mathrm{H}})/2=0.6603\pm 0.0027\pm 0.0015{\mathrm{ps}}^{-1}$, and $\mathrm{\Delta }{\mathrm{\Gamma }}_s\equiv {\mathrm{\Gamma }}_{\mathrm{L}}-{\mathrm{\Gamma }}_{\mathrm{H}}=0.0805\pm 0.0091\pm 0.0032{\mathrm{ps}}^{-1}$, where the first uncertainty is statistical and the second, systematic. These are the most precise single measurements of those quantities to date. A combined analysis with $B_s^0\to J/\psi {\pi }^{+}{\pi }^{-}$ decays gives ${\varphi }_s=-0.010\pm 0.039\mathrm{rad}$. All measurements are in agreement with the standard model predictions. For the first time, the phase ${\varphi }_s$ is measured independently for each polarization state of the $K^{+}{K}^{-}$ system and shows no evidence for polarization dependence.

The experimental detection of resonances has played a vital role in the development of subatomic physics. The overwhelming multijet backgrounds at the Large Hadron Collider (LHC) necessitate the invention of new techniques to identify resonances decaying into a pair of partons. In this Letter we introduce an observable that achieves a significant improvement in several key measurements at the LHC: the Higgs boson decay to a pair of $b$ quarks; $W^{\pm }/Z^0$ vector-boson hadronic decay; and extensions of the standard model (SM) with a new hadronic resonance. Measuring the Higgs decay to $b$ quarks is a central test of the fermion mass generation mechanism in the SM, whereas the $W^{\pm }/Z^0$ production rates are important observables of the electroweak sector. Our technique is effective in large parts of phase space where the resonance is mildly boosted and is particularly well suited for experimental searches dominated by systematic uncertainties, which is true of many analyses in the high-luminosity running of the LHC.

We present a unified approach to the theory of multimodal laser cavities including a variable amount of structural disorder. A general mean-field theory is studied for waves in media with variable nonlinearity and randomness. Phase diagrams are reported in terms of optical power, degree of disorder, and degree of nonlinearity, tuning between closed and open cavity scenarios. In the thermodynamic limit of infinitely many modes, the theory predicts four distinct regimes: a continuous wave behavior for low power, a standard mode-locking laser regime for high power and weak disorder, a random laser for high pumped power and large disorder, and a novel intermediate regime of phase locking occurring in the presence of disorder but below the lasing threshold.

We experimentally explore the use of a novel device where lateral electric fields can be applied to break the translational symmetry within the isotropic plane and hence change the selection rules to allow normally forbidden transverse acoustic (TA) phonon generations. The ultrafast screening of the lateral electric field by the photocarriers relieves shear strain in the structure and switches on the propagating TA waves. The amplitude and on-state time of the TA mode can be modulated by the external field strength and size of the laterally biased region. The observed frequency shift with an external bias as well as the strong geometrical dependence confirm the role of the asymmetric potential distribution in electrically manipulating the crystal symmetry to control modal behavior of acoustic phonons.

We have produced and characterized spin-squeezed states at a temperature of $26°\mathrm{C}$ in a nuclear magnetic resonance quadrupolar system. The experiment was carried out on ${}^{133}\mathrm{Cs}$ nuclei of spin $I=7/2$ in a sample of lyotropic liquid crystal. The source of spin squeezing was identified as the interaction between the quadrupole moment of the nuclei and the electric field gradients present within the molecules. We use the spin angular momentum representation to describe formally the nonlinear operators that produce the spin squeezing on a Hilbert space of dimension $2I+1=8$. The quantitative and qualitative characterization of this spin-squeezing phenomenon is expressed by a squeezing parameter and squeezing angle developed for the two-mode Bose-Einstein condensate system, as well as by the Wigner quasiprobability distribution function. The generality of the present experimental scheme points to potential applications in solid-state physics.

We present a formalism for simulating quantum dynamics of lattice spin-1 systems by first introducing local hidden variables and then doing semiclassical (truncated Wigner) approximation in the extended phase space. In this way, we exactly take into account the local on-site Hamiltonian and approximately treat spin-spin interactions. In particular, we represent each spin with eight classical $SU(3)$ variables. Three of them represent the usual spin components and five others are hidden variables representing local spin-spin correlations. We compare our formalism with exact quantum dynamics of fully connected spin systems and find very good agreement. As an application, we discuss quench dynamics of a Bose-Hubbard model near the superfluid-insulator transition for a 3D lattice system consisting of 1000 sites.

Electric-field control of spin-dependent properties has become one of the most attractive phenomena in modern materials research due to the promise of new device functionalities. One of the paradigms in this approach is to electrically toggle the spin polarization of carriers injected into a semiconductor using ferroelectric polarization as a control parameter. Using first-principles density-functional calculations, we explore the effect of ferroelectric polarization of electron-doped ${\mathrm{BaTiO}}_3$ ($n{\text{−}\mathrm{BaTiO}}_3$) on the spin-polarized transmission across the ${\mathrm{SrRuO}}_3/n{\text{−}\mathrm{BaTiO}}_3(001)$ interface. Our study reveals that, in this system, the interface transmission is negatively spin polarized and that ferroelectric polarization reversal leads to a change in the transport spin polarization from $-65\%$ to $-98\%$. Analytical model calculations demonstrate that this is a general effect for ferromagnetic-metal–ferroelectric-semiconductor systems and, furthermore, that ferroelectric modulation can even reverse the sign of spin polarization. The predicted effect provides a nonvolatile mechanism to electrically control spin injection in semiconductor-based spintronics devices.

The electronic structure of epitaxial single-layer ${\mathrm{MoS}}_2$ on Au(111) is investigated by angle-resolved photoemission spectroscopy. Pristine and potassium-doped layers are studied in order to gain access to the conduction band. The potassium-doped layer is found to have a $(1.39\pm 0.05)\mathrm{eV}$ direct band gap at $\overline{K}$ with the valence band top at $\overline{\mathrm{\Gamma }}$ having a significantly higher binding energy than at $\overline{K}$. The moiré superstructure of the epitaxial system does not lead to the presence of observable replica bands or minigaps. The degeneracy of the upper valence band at $\overline{K}$ is found to be lifted by the spin-orbit interaction, leading to a splitting of $(145\pm 4)\mathrm{meV}$. This splitting is anisotropic and in excellent agreement with recent calculations. Finally, it is shown that the potassium doping does not only give rise to a rigid shift of the band structure but also to a distortion, leading to the possibility of band structure engineering in single-layers of transition metal dichalcogenides.

Inelastic tunneling spectroscopy of Pb islands on Cu(111) obtained by scanning tunneling microscopy below 1 K provides a direct access to the local Eliashberg function of the islands with high energy resolution. The Eliashberg function describes the electron-phonon interaction causing conventional superconductivity. The measured Eliashberg function strongly depends on the local thickness of the Pb nanostructures and shows a sharp maximum when quantum well states of the Pb islands come close to the Fermi energy. Ab initio calculations reveal that this is related to enhanced electron-phonon coupling at these thicknesses.

The semileptonic $CP$ asymmetry in $B^0$-${\overline{B}}^0$ mixing, $a_{\text{sl}}^d$, is measured in proton-proton collision data, corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, recorded by the LHCb experiment. Semileptonic $B^0$ decays are reconstructed in the inclusive final states $D^{-}{\mu }^{+}$ and $D^{*-}{\mu }^{+}$, where the $D^{-}$ meson decays into the $K^{+}{\pi }^{-}{\pi }^{-}$ final state and the $D^{*-}$ meson into the ${\overline{D}}^0(\to K^{+}{\pi }^{-}){\pi }^{-}$ final state. The asymmetry between the numbers of $D^{(*)-}{\mu }^{+}$ and $D^{(*)+}{\mu }^{-}$ decays is measured as a function of the decay time of the $B^0$ mesons. The $CP$ asymmetry is measured to be $a_{\text{sl}}^d=(-0.02\pm 0.19\pm 0.30)\%$, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of $a_{\text{sl}}^d$ to date and is consistent with the prediction from the standard model.

A magnetic impurity in a fermionic superfluid hosts bound quasiparticle states known as Yu-Shiba-Rusinov states. We argue here that, if the impurity is mobile (i.e., has a finite mass), the impurity and its bound Yu-Shiba-Rusinov quasiparticle move together as a midgap molecule, which has an unusual “Mexican-hat” dispersion that is tunable via the fermion density. We map out the impurity dispersion, which consists of an “atomic” branch (in which the impurity is dressed by quasiparticle pairs) and a “molecular” branch (in which the impurity binds a quasiparticle). We discuss the experimental realization and detection of midgap Shiba molecules, focusing on Li-Cs mixtures, and comment on the prospects they offer for realizing exotic many-body states.

Weyl superconductivity or superfluidity, a fascinating topological state of matter, features novel phenomena such as emergent Weyl fermionic excitations and anomalies. Here we report that an anisotropic Weyl superfluid state can arise as a low temperature stable phase in a 3D dipolar Fermi gas. A crucial ingredient of our model is a direction-dependent two-body effective attraction generated by a rotating external field. Experimental signatures are predicted for cold gases in radio-frequency spectroscopy. The finite temperature phase diagram of this system is studied and the transition temperature of the Weyl superfluidity is found to be within the experimental scope for atomic dipolar Fermi gases.