Dynamical typicality for initial states with a preset measurement statistics of several commuting observables

We consider all pure or mixed states of a quantum many-body system which exhibit the same, arbitrary but fixed measurement outcome statistics for several commuting observables. Taking those states as initial conditions, which are then propagated by the pertinent Schr\"odinger or von Neumann equation up to some later time point, and invoking a few additional, fairly weak and realistic assumptions, we show that most of them still entail very similar expectation values for any given observable. This so-called dynamical typicality property thus corroborates the widespread observation that a few macroscopic features are sufficient to ensure the reproducibility of experimental measurements despite many unknown and uncontrollable microscopic details of the system. We also discuss and exemplify the usefulness of our general analytical result as a powerful numerical tool

Spin dynamics of the ordered dipolar octupolar pseudospin 1 2 pyrochlore Nd2Zr2O7 probed by muon spin relaxation

We present a muon spin relaxation study on the Ising pyrochlore Nd2Zr2O7 which develops an all in all out magnetic order below 0.4 K. At 20 mK, far below the ordering transition temperature, the zero field muon spin relaxation spectra showno static features and can be well described by a dynamical Gaussian broadened Gaussian Kubo Toyabe function indicating strong fluctuations of the ordered state. The spectra of the paramagnetic state below 4.2 K reveal anomalously slow paramagnetic spin dynamics and show only a small difference with the spectra of the ordered state.We find that the fluctuation rate decreases with decreasing temperature and becomes nearly temperature independent below the transition temperature, indicating persistent slow spin dynamics in the ground state. The field distribution width shows a small but sudden increase at the transition temperature and then becomes almost constant. The spectra in applied longitudinal fields are well fitted by the conventional dynamical Gaussian Kubo Toyabe function, which further supports the dynamical nature of the ground state. The fluctuation rate shows a peak as a function of external field which is associated with a field induced spin flip transition. The strong dynamics in the ordered state is attributed to the transverse coupling of the Ising spins introduced by the multipole interaction

Continuous monitoring reveals multiple controls on ecosystem metabolism in a suburban stream

1. Primary production and respiration in streams, collectively referred to as stream ecosystem metabolism, are fundamental processes that determine trophic structure, biomass and nutrient cycling. Few studies have used high‐frequency measurements of gross primary production (GPP) and ecosystem respiration (ER) over extended periods to characterise the factors that control stream ecosystem metabolism at hourly, daily, seasonal and annual scales. 2. We measured ecosystem metabolism at 5‐min intervals for 23 months in Shepherd Creek, a small suburban stream in Cincinnati, Ohio (U.S.A.). 3. Daily GPP was best predicted by a model containing light and its synergistic interaction with water temperature. Water temperature alone was not significantly related to daily GPP, rather high temperatures enhanced the capacity of autotrophs to use available light. 4. The relationship between GPP and light was further explored using photosynthesis–irradiance curves (P–I curves). Light saturation of GPP was evident throughout the winter and spring and the P–I curve frequently exhibited strong counterclockwise hysteresis. Hysteresis occurred when water temperatures were greater in the afternoon than in the morning, although light was similar, further suggesting that light availability interacts synergistically with water temperature. 5. Storm flows strongly depressed GPP in the spring while desiccation arrested aquatic GPP and ER in late summer and autumn. 6. Ecosystem respiration was best predicted by GPP, water temperature and the rate of water exchange between the surface channel and transient storage zones. We estimate that c. 70% of newly fixed carbon was immediately respired by autotrophs and closely associated heterotrophs. 7. Interannual, seasonal, daily and hourly variability in ecosystem metabolism was attributable to a combination of light availability, water temperature, storm flow dynamics and desiccation. Human activities affect all these factors in urban and suburban streams, suggesting stream ecosystem processes are likely to respond in complex ways to changing land use and climate

Magnetic ground state of the Kitaev Na2_2Co2_2TeO6_6 spin liquid candidate

As a candidate Kitaev material, Na$_2$Co$_2$TeO$_6$ exhibits intriguing magnetism on a honeycomb lattice that is believed to be $C_3$-symmetric. Here we report a neutron diffraction study of high quality single crystals under $a$-axis magnetic fields. Our data support the less common notion of a magnetic ground state that corresponds to a triple-$\mathbf{q}$ magnetic structure with $C_3$ symmetry, rather than the multi-domain zigzag structure typically assumed in prototype Kitaev spin liquid candidates. In particular, we find that the field is unable to repopulate the supposed zigzag domains, where the only alternative explanation is that the domains are strongly pinned by hitherto unidentified structural reasons. If the triple-$\mathbf{q}$ structure is correct then this requires reevaluation of many candidate Kitaev materials. We also find that fields beyond about 10 Tesla suppress the long range antiferromagnetic order, allowing new magnetic behavior to emerge different from that expected for a spin liquid.Comment: 4 pages, 4 figures, plus Supplemental Materia