Applying the Briggs-Bers “pole-pinch” criterion to the exact transcendental dispersion relation of a dielectric traveling wave tube (TWT), we find that there is no absolute instability regardless of the beam current. We extend this analysis to the circuit band edges of a linear beam TWT by approximating the circuit mode as a hyperbola in the frequency-wave-number ($\omega \text{−}k$) plane and consider the weak coupling limit. For an operating mode whose group velocity is in the same direction as the beam mode, we find that the lower band edge is not subjected to absolute instability. At the upper band edge, we find a threshold beam current beyond which absolute instability is excited. The nonexistence of absolute instability in a linear beam TWT and the existence in a gyrotron TWT, both at the lower band edge, is contrasted. The general study given here is applicable to some contemporary TWTs such as metamaterial-based and advanced Smith-Purcell TWTs.

Shock waves exist throughout the Universe and are fundamental to understanding the nature of collisionless plasmas. Reformation is a process, driven by microphysics, which typically occurs at high Mach number supercritical shocks. While ongoing studies have investigated this process extensively both theoretically and via simulations, their observations remain few and far between. In this Letter we present a study of very high Mach number shocks in a parameter space that has been poorly explored and we identify reformation using in situ magnetic field observations from the Cassini spacecraft at 10 AU. This has given us an insight into quasiperpendicular shocks across 2 orders of magnitude in Alfvén Mach number ($M_A$) which could potentially bridge the gap between modest terrestrial shocks and more exotic astrophysical shocks. For the first time, we show evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted time scale of $\sim 0.3{\tau }_c$, where ${\tau }_c$ is the ion gyroperiod. In addition, we experimentally reveal the relationship between reformation and $M_A$ and focus on the magnetic structure of such shocks to further show that for the same $M_A$, a reforming shock exhibits stronger magnetic field amplification than a shock that is not reforming.

The Harper-Hofstadter model provides a fractal spectrum containing topological bands of any integer Chern number $C$. We study the many-body physics that is realized by interacting particles occupying Harper-Hofstadter bands with $|C|>1$. We formulate the predictions of Chern-Simons or composite fermion theory in terms of the filling factor $\nu$, defined as the ratio of particle density to the number of single-particle states per unit area. We show that this theory predicts a series of fractional quantum Hall states with filling factors $\nu =r/(r|C|+1)$ for bosons, or $\nu =r/(2r|C|+1)$ for fermions. This series includes a bosonic integer quantum Hall state in $|C|=2$ bands. We construct specific cases where a single band of the Harper-Hofstadter model is occupied. For these cases, we provide numerical evidence that several states in this series are realized as incompressible quantum liquids for bosons with contact interactions.

When the constituent spins have an energetic preference to lie along an easy axis, triangular and kagome lattice antiferromagnets often develop long-range order that distinguishes the three sublattices of the underlying triangular Bravais lattice. In zero magnetic field, this three-sublattice order melts either in a two-step manner, i.e., via an intermediate phase with power-law three-sublattice order controlled by a temperature-dependent exponent $\eta (T)\in (\frac{1}{9},\frac{1}{4})$, or via a transition in the three-state Potts universality class. Here, I predict that the uniform susceptibility to a small easy-axis field $B$ diverges as $\chi (B)\sim |B{|}^{-[(4-18\eta )/(4-9\eta )]}$ in a large part of the intermediate power-law ordered phase [corresponding to $\eta (T)\in (\frac{1}{9},\frac{2}{9})$], providing an easy-to-measure thermodynamic signature of two-step melting. I also show that these two melting scenarios can be generically connected via an intervening multicritical point and obtain numerical estimates of multicritical exponents.

We investigate the dynamics of kinetically frozen block copolymer micelles of different softness across a wide range of particle concentrations, from the fluid to the onset of glassy behavior, through a combination of rheology, dynamic light scattering, and pulsed field gradient NMR spectroscopy. We additionally perform Brownian dynamics simulations based on an ultrasoft coarse-grained potential, which are found to be in quantitative agreement with experiments, capturing even the very details of dynamic structure factors $S(Q,t)$ on approaching the glass transition. We provide evidence that for these systems the Stokes-Einstein relation holds up to the glass transition; given that it is violated for dense suspensions of hard colloids, our findings suggest that its validity is an intriguing signature of ultrasoft interactions.

Postselected weak measurement is a useful protocol for amplifying weak physical effects. However, there has recently been controversy over whether it gives any advantage in precision. While it is now clear that retaining failed postselections can yield more Fisher information than discarding them, the advantage of postselection measurement itself still remains to be clarified. In this Letter, we address this problem by studying two widely used estimation strategies: averaging measurement results, and maximum likelihood estimation, respectively. For the first strategy, we find a surprising result that squeezed coherent states of the pointer can give postselected weak measurements a higher signal-to-noise ratio than standard ones while all standard coherent states cannot, which suggests that raising the precision of weak measurements by postselection calls for the presence of “nonclassicality” in the pointer states. For the second strategy, we show that the quantum Fisher information of postselected weak measurements is generally larger than that of standard weak measurements, even without using the failed postselection events, but the gap can be closed with a proper choice of system state.

We investigate the self-correcting properties of a network of Majorana wires, in the form of a trijunction, in contact with a parity-preserving thermal environment. As opposed to the case where Majorana bound states are immobile, braiding Majorana bound states within a trijunction introduces dangerous error processes that we identify. Such errors prevent the lifetime of the memory from increasing with the size of the system. We confirm our predictions with Monte Carlo simulations. Our findings put a restriction on the degree of self-correction of this specific quantum computing architecture.

The ultrahigh-energy cosmic rays observed on the Earth are most likely accelerated in extra-Galactic sources. For the typical luminosities invoked for such sources, the electric current associated to the flux of cosmic rays that leave them is large. The associated plasma instabilities create magnetic fluctuations that can efficiently scatter particles. We argue that this phenomenon forces cosmic rays to be self-confined in the source proximity for energies $E<E_{\mathrm{cut}}$, where $E_{\mathrm{cut}}\approx {10}^7{L}_{44}^{2/3}\mathrm{GeV}$ for low background magnetic fields ($B_0\ll \mathrm{nG}$). For larger values of $B_0$, cosmic rays are confined close to their sources for energies $E<E_{\mathrm{cut}}\approx 2\times {10}^8{\lambda }_{10}{L}_{44}^{1/4}{B}_{-10}^{1/2}\mathrm{GeV}$, where $B_{-10}$ is the field in units of 0.1 nG, ${\lambda }_{10}$ is its coherence lengths in units of 10 Mpc, and $L_{44}$ is the source luminosity in units of ${10}^{44}\mathrm{erg}/\mathrm{s}$.

We propose a new exact quantization condition for a class of quantum mechanical systems derived from local toric Calabi-Yau threefolds. Our proposal includes all contributions to the energy spectrum which are nonperturbative in the Planck constant, and is much simpler than the available quantization condition in the literature. We check that our proposal is consistent with previous works and implies nontrivial relations among the topological Gopakumar-Vafa invariants of the toric Calabi-Yau geometries. Together with the recent developments, our proposal opens a new avenue in the long investigations at the interface of geometry, topology and quantum mechanics.

The sum-energy spectrum of two correlated electrons emitted in nonsequential strong-field double ionization (SFDI) of Ar was studied for intensities of 0.3 to $2\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. We find the mean sum energy, the maximum of the distributions as well as the high-energy tail of the scaled (to the ponderomotive energy) spectra increase with decreasing intensity below the recollision threshold (BRT). At higher intensities the spectra collapse into a single distribution. This behavior can be well explained within a semiclassical model providing clear evidence of the importance of multiple recollisions in the BRT regime. Here, ultrafast thermalization between both electrons is found occurring within three optical cycles only and leaving its clear footprint in the sum-energy spectra.

The temporal magnetic correlations of the triangular-lattice antiferromagnet ${\mathrm{NiGa}}_2{\mathrm{S}}_4$ are examined through 13 decades (${10}^{-13}-1\sec$) using ultrahigh-resolution inelastic neutron scattering, muon spin relaxation, and ac and nonlinear susceptibility measurements. Unlike the short-ranged spatial correlations, the temperature dependence of the temporal correlations show distinct anomalies. The spin fluctuation rate decreases precipitously upon cooling towards $T^{*}=8.5\mathrm{K}$, but fluctuations on the microsecond time scale then persist in an anomalous dynamical regime for $4\mathrm{K}<T\le T^{*}$. As this time scale exceeds that of single-site dynamics by 6 orders of magnitude, these fluctuations bear evidence of emergent degrees of freedom within the short-range correlated incommensurate state of ${\mathrm{NiGa}}_2{\mathrm{S}}_4$.

Multiferroics permit the magnetic control of the electric polarization and the electric control of the magnetization. These static magnetoelectric (ME) effects are of enormous interest: The ability to read and write a magnetic state current-free by an electric voltage would provide a huge technological advantage. Dynamic or optical ME effects are equally interesting, because they give rise to unidirectional light propagation as recently observed in low-temperature multiferroics. This phenomenon, if realized at room temperature, would allow the development of optical diodes which transmit unpolarized light in one, but not in the opposite, direction. Here, we report strong unidirectional transmission in the room-temperature multiferroic ${\mathrm{BiFeO}}_3$ over the gigahertz-terahertz frequency range. The supporting theory attributes the observed unidirectional transmission to the spin-current-driven dynamic ME effect. These findings are an important step toward the realization of optical diodes, supplemented by the ability to switch the transmission direction with a magnetic or electric field.

Using a high-speed camera, we follow the propagation of the detachment front during the peeling of an adhesive tape from a flat surface. In a given range of peeling velocity, this front displays a multiscale unstable dynamics, entangling two well-separated spatiotemporal scales, which correspond to microscopic and macroscopic dynamical stick-slip instabilities. While the periodic release of the stretch energy of the whole peeled ribbon drives the classical macro-stick-slip, we show that the micro-stick-slip, due to the regular propagation of transverse dynamic fractures discovered by Thoroddsen et al. [Phys. Rev. E 82, 046107 (2010)], is related to a high-frequency periodic release of the elastic bending energy of the adhesive ribbon concentrated in the vicinity of the peeling front.

We predict several new exotic doubly heavy hadronic resonances, inferring from the observed exotic bottomoniumlike and charmoniumlike narrow states $X(3872)$, $Z_b(10610)$, $Z_b(10650)$, $Z_c(3900)$, and $Z_c(4020/4025)$. We interpret the binding mechanism as mostly molecularlike isospin-exchange attraction between two heavy-light mesons in a relative $S$-wave state. We then generalize it to other systems containing two heavy hadrons which can couple through isospin exchange. The new predicted states include resonances in meson-meson, meson-baryon, baryon-baryon, and baryon-antibaryon channels. These include those giving rise to final states involving a heavy quark $Q=c,b$ and antiquark ${\overline{Q}}^{\prime }=\overline{c},\overline{b}$, namely, $D{\overline{D}}^{*}$, $D^{*}{\overline{D}}^{*}$, $D^{*}{B}^{*}$, $\overline{B}{B}^{*}$, ${\overline{B}}^{*}{B}^{*}$, ${\mathrm{\Sigma }}_c{\overline{D}}^{*}$, ${\mathrm{\Sigma }}_c{B}^{*}$, ${\mathrm{\Sigma }}_b{\overline{D}}^{*}$, ${\mathrm{\Sigma }}_b{B}^{*}$, ${\mathrm{\Sigma }}_c{\overline{\mathrm{\Sigma }}}_c$, ${\mathrm{\Sigma }}_c{\overline{\mathrm{\Lambda }}}_c$, ${\mathrm{\Sigma }}_c{\overline{\mathrm{\Lambda }}}_b$, ${\mathrm{\Sigma }}_b{\overline{\mathrm{\Sigma }}}_b$, ${\mathrm{\Sigma }}_b{\overline{\mathrm{\Lambda }}}_b$, and ${\mathrm{\Sigma }}_b{\overline{\mathrm{\Lambda }}}_c$, as well as corresponding $S$-wave states giving rise to $Q{Q}^{\prime }$ or $\overline{Q}{\overline{Q}}^{\prime }$.

The collective excitations in ensembles of dissipative, laser driven ultracold atoms exhibit crystal-like patterns, a many-body effect of the Rydberg blockade mechanism. These crystalline structures are revealed in an experiment from a postselection of configurations with fixed numbers of excitations. Here, we show that these subensembles can be well represented by ensembles of effective particles that interact via logarithmic pair potentials. This allows one to study the emergent patterns with a small number of effective particles to determine the phases of Rydberg crystals and to systematically study contributions from $N$-body terms.