We perform a comprehensive analysis of the spectral statistics of the molecular resonances in $^{166}$Er and $^{168}$Er observed in recent ultracold collision experiments [Frisch et al., Nature {bf 507}, 475 (2014)] with the aim of determining the ch
aoticity of this system. We calculate different independent statistical properties to check their degree of agreement with random matrix theory (RMT), and analyze if they are consistent with the possibility of having missing resonances. The analysis of the short-range fluctuations as a function of the magnetic field points to a steady increase of chaoticity until $B sim 30$ G. The repulsion parameter decreases for higher magnetic fields, an effect that can be interpreted as due to missing resonances. The analysis of long-range fluctuations allows us to be more quantitative and estimate a $20-25%$ fraction of missing levels. Finally, a study of the distribution of resonance widths provides additional evidence supporting missing resonances of small width compared with the experimental magnetic field resolution. We conclude that further measurements with increased resolution will be necessary to give a final answer to the problem of missing resonances and the agreement with RMT.
We have studied the spin correlations with $bf{k}$= ($frac12$, $frac12$, $frac12$) propagation vector which appear below 0.4, K in tbti spin liquid by combining powder neutron diffraction and specific heat on Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ samples wit
h $x$=0, 0.01, -0.01. The $bf{k}$= ($frac12$, $frac12$, $frac12$) order clearly appears on all neutron patterns by subtracting a pattern at 1.2(1),K. Refining the subtracted patterns at 0.07,K yields two possible spin structures, with spin-ice-like and monopole-like correlations respectively. Mesoscopic correlations involve Tb moments of 1 to 2 mub ordered on a length scale of about 20 AA. In addition, long range order involving a small spin component of 0.1 to 0.2 mub is detected for the $x$= 0 and 0.01 samples showing a peak in the specific heat. Comparison with previous single crystals data suggests that the ($frac12$, $frac12$, $frac12$) order settles in through nanometric spin textures with dominant spin ice character and correlated orientations, analogous to nanomagnetic twins.
In this work, we show that the zero field excitation spectra in the quantum spin ice candidate pyrochlore compound ybti is a continuum characterized by a very broad and almost flat dynamical response which extends up to $1-1.5$ meV, coexisting or not
with a quasi-elastic response depending on the wave-vector. The spectra do not evolve between 50 mK and 2 K, indicating that the spin dynamics is only little affected by the temperature in both the short-range correlated and ordered regimes. Although classical spin dynamics simulations qualitatively capture some of the experimental observations, we show that they fail to reproduce this broad continuum. In particular, the simulations predict an energy scale twice smaller than the experimental observations. This analysis is based on a careful determination of the exchange couplings, able to reproduce both the zero field diffuse scattering and the spin wave spectrum rising in the field polarized state. According to this analysis, ybti lies at the border between a ferro and an antiferromagnetic phase. These results suggest that the unconventional ground state of ybti is governed by strong quantum fluctuations arising from the competition between those phases. The observed spectra may correspond to a continuum of deconfined spinons as expected in quantum spin liquids.
Examples of materials where an order by disorder mechanism is at play to select a particular ground state are scarce. It has recently been proposed, however, that the antiferromagnetic XY pyrochlore Er2Ti2O7, reveals a most convincing case of this me
chanism. Observation of a spin gap at zone centers has recently been interpreted as a corroboration of this physics. In this paper, we argue, however, that the anisotropy generated by the interaction-induced admixing between the crystal-field ground and excited levels provides for an alternative mechanism. It especially predicts the opening of a spin gap of about 15 micro-eV, which is of the same order of magnitude as the one observed experimentally. We report new high resolution inelastic neutron scattering data which can be well understood within this scenario.
We present a distinct mechanism for the formation of bound states in the continuum (BICs). In chiral quantum systems there appear zero-energy states in which the wave function has finite amplitude only in one of the subsystems defined by the chiral s
ymmetry. When the system is coupled to leads with a continuum energy band, part of these states remain bound. We derive some algebraic rules for the number of these states depending on the dimensionality and rank of the total Hamiltonian. We examine the transport properties of such systems including the appearance of Fano resonances in some limiting cases. Finally, we discuss experimental setups based on microwave dielectric resonators and atoms in optical lattices where these predictions can be tested.
ersn, is considered, together with erti, as a realization of the XY antiferromagnet on the pyrochlore lattice. We present magnetization measurements confirming that ersn, does not order down to 100 mK but exhibits a freezing below 200 mK. Our neutron
scattering experiments evidence the strong XY character of the er moment and point out the existence of short range correlations in which the magnetic moments are in peculiar configurations, the Palmer-Chalker states, predicted theoretically for an XY pyrochlore antiferromagnet with dipolar interactions. Our estimation of the ersn, parameters confirm the role of the latter interactions on top of relatively weak and isotropic exchange couplings.
We study a simple model consisting of an atomic ion and a polar molecule trapped in a single setup, taking into consideration their electrostatic interaction. We determine analytically their collective modes of excitation as a function of their masse
s, trapping frequencies, distance, and the molecules electric dipole moment. We then discuss the application of these collective excitations to cool molecules, to entangle molecules and ions, and to realize two-qubit gates between them. We finally present a numerical analysis of the possibility of applying these tools to study magnetically ordered phases of two-dimensional arrays of polar molecules, a setup proposed to quantum-simulate some strongly-correlated models of condensed matter.
The dynamical magnetic correlations in tbti, have been investigated using polarized inelastic neutron scattering. Dispersive excitations are observed, emerging from pinch points in reciprocal space and characterized by an anisotropic spectral weight.
Anomalies in the crystal field and phonon excitation spectrum at Brillouin zone centers are also reported. These findings suggest that Coulomb phases, although they present a disordered ground state with dipolar correlations, allow the propagation of collective excitations. They also point out a strong spin-lattice coupling, which likely drives effective interactions between the $4f$ quadrupolar moments.
The B0.2 V magnetic star tau Sco stands out from the larger population of massive OB stars due to its high X-ray activity, peculiar wind diagnostics and complex magnetic field. Recently, Petit et al. 2011 presented the discovery of the first two tau
Sco analogues -- HD 66665 and HD 63425, identified by the striking similarity of their UV spectra to that of tau Sco. ESPaDOnS and Narval spectropolarimetric observations were obtained by the Magnetism in Massive Stars CFHT and TBL Large Programs, in order to characterize the stellar and magnetic properties of these stars. A magnetic field of similar surface strength was found on both stars, reinforcing the connection between the presence of a magnetic field and wind peculiarities. We present additional phase-resolved observations secured by the MiMeS collaboration for HD 66665 in order to measure its magnetic geometry, and correlate that geometry with diagnostics of mass-loss.
The B0.2 V magnetic star tau Sco stands out from the larger population of massive OB stars due to its high X-ray activity, peculiar wind diagnostics and highly complex magnetic field. This paper presents the discovery of the first two tau Sco analogu
es - HD 66665 and HD 63425, identified by the striking similarity of their UV spectra to that of tau Sco. ESPaDOnS spectropolarimetric observations were secured by the Magnetism in Massive Stars CFHT Large Program, in order to characterize the stellar and magnetic properties of these stars. CMFGEN modelling of optical ESPaDOnS spectra and archived IUE UV spectra showed that these stars have stellar parameters similar to those of tau Sco. A magnetic field of similar surface strength is found on both stars, reinforcing the connection between the presence of a magnetic field and wind peculiarities. However, additional phase-resolved observations will be required in order to assess the potential complexity of the magnetic fields, and verify if the wind anomalies are linked to this property.
