Experimental signature of the attractive Coulomb force between positive and negative magnetic monopoles in spin ice

A non-Ohmic current that grows exponentially with the square root of applied electric field is well known from thermionic field emission (the Schottky effect)1, electrolytes (the second Wien effect)2 and semiconductors (the Poole–Frenkel effect)3. It is a universal signature of the attractive Coulomb force between positive and negative electrical charges, which is revealed as the charges are driven in opposite directions by the force of an applied electric field. Here we apply thermal quenches4 to spin ice5,6,7,8,9,10,11 to prepare metastable populations of bound pairs of positive and negative emergent magnetic monopoles12,13,14,15,16 at millikelvin temperatures. We find that the application of a magnetic field results in a universal exponential-root field growth of magnetic current, thus confirming the microscopic Coulomb force between the magnetic monopole quasiparticles and establishing a magnetic analogue of the Poole–Frenkel effect. At temperatures above 300 mK, gradual restoration of kinetic monopole equilibria causes the non-Ohmic current to smoothly evolve into the high-field Wien effect2 for magnetic monopoles, as confirmed by comparison to a recent and rigorous theory of the Wien effect in spin ice17,18. Our results extend the universality of the exponential-root field form into magnetism and illustrate the power of emergent particle kinetics to describe far-from-equilibrium response in complex systems

Far-from-equilibrium monopole dynamics in spin ice

Condensed matter in the low-temperature limit reveals exotic physics associated with unusual orders and excitations, with examples ranging from helium superfluidity1 to magnetic monopoles in spin ice2, 3. The far-from-equilibrium physics of such low-temperature states may be even more exotic, yet to access it in the laboratory remains a challenge. Here we demonstrate a simple and robust technique—the ‘magnetothermal avalanche quench’—and its use in the controlled creation of non-equilibrium populations of magnetic monopoles in spin ice at millikelvin temperatures. These populations are found to exhibit spontaneous dynamical effects that typify far-from-equilibrium systems and yet are captured by simple models. Our method thus opens new directions in the study of far-from-equilibrium states in spin ice and other exotic magnets

Gapped and gapless short range ordered magnetic states with oneandhalf,1 2,1 2 wave vectors in the pyrochlore magnet Tb2 xTi2 xO7 delta

Recent low temperature heat capacity CP measurements on polycrystalline samples of the pyrochlore antiferromagnet Tb2 xTi2 xO7 delta; have shown a strong sensitivity to the precise Tb concentration x, with a large anomaly exhibited for x 0.005 at TC 0.5 K and no such anomaly and corresponding phase transition for x lt;0. We have grown single crystal samples of Tb2 xTi2 xO7 delta;, with approximate composition x 0.001, 0.0042, and 0.0147, where the x 0.0042 single crystal exhibits a large CP anomaly at TC 0.45 K, but neither the x 0.001 nor the x 0.0147 single crystals display any such anomaly. We present new time of flight neutron scattering measurements on the x 0.001 and the x 0.0147 samples which show strong 1 2,1 2,1 2 quasi Bragg peaks at low temperatures characteristic of short range antiferromagnetic spin ice AFSI order at zero magnetic field but only under field cooled conditions, as was previously observed in our x 0.0042 single crystal. Furthermore, the frozen AFSI state displays a gapped spin excitation spectrum around 1 2,1 2,1 2 , with a gap of 0.1 meV, again similar to previous observations on the x 0.0042 single crystal. These results show that the strong 1 2,1 2,1 2 quasi Bragg peaks and gapped AFSI state at low temperatures under field cooled conditions are robust features of Tb2Ti2O7, and are not correlated with the presence or absence of the CP anomaly and phase transition at low temperatures. Further, these results show that the ordered state giving rise to the CP anomaly is confined to 0 lt;x lt;0.01 for Tb2 xTi2 xO7 delta, and is not obviously connected with conventional order of magnetic dipole degrees of freedo

Atypical flavobacteria recovered from diseased fish in the Western United States.

Flavobacterial diseases, caused by bacteria in the order Flavobacteriales, are responsible for devastating losses in farmed and wild fish populations worldwide. The genera Flavobacterium (Family Flavobacteriaceae) and Chryseobacterium (Weeksellaceae) encompass the most well-known agents of fish disease in the order, but the full extent of piscine-pathogenic species within these diverse groups is unresolved, and likely underappreciated. To identify emerging agents of flavobacterial disease in US aquaculture, 183 presumptive Flavobacterium and Chryseobacterium isolates were collected from clinically affected fish representing 19 host types, from across six western states. Isolates were characterized by 16S rRNA gene sequencing and phylogenetic analysis using the gyrB gene. Antimicrobial susceptibility profiles were compared between representatives from each major phylogenetic clade. Of the isolates, 52 were identified as Chryseobacterium species and 131 as Flavobacterium. The majority of Chryseobacterium isolates fell into six clades (A-F) consisting of ≥ 5 fish isolates with ≥ 70% bootstrap support, and Flavobacterium into nine (A-I). Phylogenetic clades showed distinct patterns in antimicrobial susceptibility. Two Chryseobacterium clades (F & G), and four Flavobacterium clades (B, G-I) had comparably high minimal inhibitory concentrations (MICs) for 11/18 antimicrobials tested. Multiple clades in both genera exhibited MICs surpassing the established F. psychrophilum breakpoints for oxytetracycline and florfenicol, indicating potential resistance to two of the three antimicrobials approved for use in finfish aquaculture. Further work to investigate the virulence and antigenic diversity of these genetic groups will improve our understanding of flavobacterial disease, with applications for treatment and vaccination strategies