Search

We report new heat capacity measurements on single crystal Ce$_2$Zr$_2$O$_7$ down to $\sim$ 0.1 K in a magnetic field along the $[1,\bar{1}, 0]$ direction. These new measurements show that the broad hump in the zero-field heat capacity moves higher in temperature with increasing field strength and is split into two humps by the $[1,\bar{1}, 0]$ field at $\sim$ 2 T. These separate features are due to the decomposition of the pyrochlore lattice into effectively decoupled chains for fields in this direction: one set of chains ($\alpha$-chains) is polarized by the field while the other ($\beta$-chains) remains free. Our theoretical modelling suggests that the $\beta$-chains are close to a critical state, with nearly-gapless excitations. We also report new elastic and inelastic neutron scattering measurements on single crystal Ce$_2$Zr$_2$O$_7$ in $[1, \bar{1}, 0]$ and $[0, 0, 1]$ magnetic fields at temperatures down to 0.03 K. The elastic scattering behaves consistently with the formation of independent chains for a $[1, \bar{1}, 0]$ field, while the $[0, 0, 1]$ field produces a single field-induced magnetic Bragg peak at $(0, 2, 0)$ and equivalent wavevectors, indicating a polarized spin ice for fields above $\sim$ 3 T. For both $[1, \bar{1}, 0]$ and $[0, 0, 1]$ fields, our inelastic neutron scattering results show an approximately-dispersionless continuum of scattering that increases in both energy and intensity with increasing field strength. By modelling the complete set of experimental data using numerical linked cluster and semiclassical molecular dynamics calculations, we demonstrate the dominantly multipolar nature of the exchange interactions in Ce$_2$Zr$_2$O$_7$ and the smallness of the parameter $\theta$ which controls the mixing between dipolar and octupolar degrees of freedom. These results support previous estimates of the microscopic exchange parameters.

A new search for two-neutrino double-beta ($2\nu\beta\beta$) decay of $^{136}\rm Xe$ to the $0^+_1$ excited state of $^{136}\rm Ba$ is performed with the full EXO-200 dataset. A deep learning-based convolutional neural network is used to discriminate signal from background events. Signal detection efficiency is increased relative to previous searches by EXO-200 by more than a factor of two. With the addition of the Phase II dataset taken with an upgraded detector, the median 90$\%$ confidence level half-life sensitivity of $2\nu\beta\beta$ decay to the $0^+_1$ state of $^{136}\rm Ba$ is $2.9 \times 10^{24}~\rm yr$ using a total $^{136}\rm Xe$ exposure of $234.1~\rm kg~yr$. No statistically significant evidence for $2\nu\beta\beta$ decay to the $0^+_1$ state is observed, leading to a lower limit of $T^{2\nu}_{1/2}(0^+ \rightarrow 0^+_1) > 1.4\times10^{24}~\rm yr$ at 90$\%$ confidence level, improved by 70$\%$ relative to the current world's best constraint., Comment: 9 pages, 7 figures, 2 tables

The dipolar-octupolar (DO) pyrochlores, R$_2$M$_2$O$_7$ (R = Ce, Sm, Nd), are key players in the search for realizable novel quantum spin liquid (QSL) states as a large parameter space within the DO pyrochlore phase diagram is theorized to host QSL states of both dipolar and octupolar nature. New single crystals and powders of Ce$_2$Sn$_2$O$_7$, synthesized by hydrothermal techniques, present an opportunity for a new characterization of the exchange parameters in Ce$_2$Sn$_2$O$_7$ using the near-neighbor $XYZ$ model Hamiltonian associated with DO pyrochlores. Utilizing quantum numerical linked cluster expansion fits to heat capacity and magnetic susceptibility measurements, and classical Monte Carlo calculations to the diffuse neutron diffraction of the new hydrothermally grown Ce$_2$Sn$_2$O$_7$ samples, we place Ce$_2$Sn$_2$O$_7$'s ground state within the ordered dipolar all-in-all-out (AIAO) N\'{e}el phase, with quantum Monte Carlo calculations showing a transition to long-range order at temperatures below those accessed experimentally. Indeed, our new neutron diffraction measurements on the hydrothermally grown Ce$_2$Sn$_2$O$_7$ powders show a broad signal at low scattering wave vectors, reminiscent of a \textit{dipolar} spin ice, in striking contrast from previous powder neutron diffraction on samples grown from solid-state synthesis, which found diffuse scattering at high scattering wave vectors associated with magnetic {\it octupoles} and suggested an octupolar quantum spin ice state. We conclude that new hydrothermally grown Ce$_2$Sn$_2$O$_7$ samples host a finite-temperature proximate dipolar spin ice phase, above the expected transition to AIAO N\'{e}el order., Comment: 16 pages, 12 figures

In his comment [arXiv:2209.03235], S. W. Lovesey argues that our analysis of neutron scattering experiments performed on Ce$_2$Zr$_2$O$_7$ is invalid. Lovesey argues that we have not properly accounted for the higher-order multipolar contributions to the magnetic scattering and that our use of pseudospin-$1/2$ operators to describe the scattering is inappropriate. In this reply, we show that the multipolar corrections discussed by Lovesey only become significant at scattering wavevectors exceeding those accessed in our experiments. This in no way contradicts or undermines our work, which never claimed a direct observation of scattering from higher-order multipoles. We further show that Lovesey's objections to our use of pseudospins are unfounded, and that the pseudospin operators are able to describe all magnetic scattering processes at the energy scale of our experiments, far below the crystal field gap. Finally, we comment on certain assumptions in Lovesey's calculations of the scattering amplitude which are inconsistent with experiment., Comment: 6 pages, 1 figure

We present a search for electron-recoil signatures from the charged-current absorption of fermionic dark matter using the EXO-200 detector. We report an average electron recoil background rate of $6.8 \times 10^{-4}\, \mathrm{cts}\,\mathrm{kg}^{-1}\mathrm{yr}^{-1}\mathrm{keV}^{-1}$ above $4\,\mathrm{MeV}$ and find no statistically significant excess over our background projection. Using a total ${}^{136}\mathrm{Xe}$ exposure of $234.1\,\mathrm{kg}\,\mathrm{yr}$ we exclude new parameter space for the charged-current absorption cross-section for dark matter masses between $m_\chi = 2.6\,\mathrm{MeV} - 11.6\,\mathrm{MeV}$ with a minimum of $6\times 10^{-51}\,\mathrm{cm}^2$ at $8.3\,\mathrm{MeV}$ at the $90\%$ confidence level., Comment: 9 pages, 6 figures, 1 table

A search for Majoron-emitting modes of the neutrinoless double-beta decay of $^{136}$Xe is performed with the full EXO-200 dataset. This dataset consists of a total $^{136}$Xe exposure of 234.1 kg$\cdot$yr, and includes data with detector upgrades that have improved the energy threshold relative to previous searches. A lower limit of T$_{1/2}^{\rm{^{136}Xe}}>$4.3$\cdot$10$^{24}$ yr at 90\% C.L. on the half-life of the spectral index $n=1$ Majoron decay was obtained, a factor of 3.6 more stringent than the previous limit from EXO-200, corresponding to a constraint on the Majoron-neutrino coupling constant of $|\langle g_{ee}^{M}\rangle|$$<(0.4$-$0.9)\cdot10^{-5}$. The lower threshold and the additional data taken resulted in a factor 8.4 improvement for the $n=7$ mode compared to the previous EXO search. This search provides the most stringent limits to-date on the Majoron-emitting decays of $^{136}$Xe with spectral indices $n=1,2,3,$ and 7.

The Ce$^{3+}$ pseudospin-$\frac{1}{2}$ degrees of freedom in the pyrochlore magnet Ce$_2$Zr$_2$O$_7$ are known to possess dipole-octupole (DO) character, making it a candidate for novel quantum spin liquid (QSL) ground states at low temperatures. We report new polarized neutron diffraction at low temperatures, as well as heat capacity ($C_p$) measurements on single crystal Ce$_2$Zr$_2$O$_7$. The former bears both similarities and differences from that measured in the canonical dipolar spin ice compound Ho$_2$Ti$_2$O$_7$, while the latter rises sharply at low temperatures, initially plateauing near 0.08 K, before falling off towards a high temperature zero beyond 3 K. Above $\sim$0.5 K, the $C_p$ data set can be fit to the results of a quantum numerical linked cluster (NLC) calculation, carried out to 4$^{\mathrm{th}}$ order, that allows estimates for the terms in the near-neighbour XYZ Hamiltonian expected for such DO pyrochlore systems. Fits of the same theory to the temperature dependence of the magnetic susceptibility and unpolarized neutron scattering complement this analysis. A comparison between the resulting best fit NLC calculation and the polarized neutron diffraction shows both agreement and discrepancies, mostly in the form of zone-boundary diffuse scattering in the non-spin flip channel, which are attributed to interactions beyond near-neighbours. The lack of an observed thermodynamic anomaly and the constraints on the near-neighbour XYZ Hamiltonian suggest that Ce$_2$Zr$_2$O$_7$ realizes a U(1)$_\pi$ QSL state at low temperatures, and one that likely resides near the boundary between dipolar and octupolar character., Comment: 11 pages, 11 figures

Reentrance, the return of a system from an ordered phase to a previously encountered less-ordered one as a controlled parameter is continuously varied, is a recurring theme found in disparate physical systems, from condensed matter to black holes. While diverse in its many incarnations and generally unsuspected, the cause of reentrance at the microscopic level is often not investigated thoroughly. Here, through detailed characterization and theoretical modeling, we uncover the microscopic mechanism behind reentrance in the strongly frustrated pyrochlore antiferromagnet Er$_2$Sn$_2$O$_7$. Taking advantage of the recent advance in rare earth stannate single crystal synthesis, we use heat capacity measurements to expose that Er$_2$Sn$_2$O$_7$ exhibits multiple instances of reentrance in its magnetic field $B$ vs. temperature $T$ phase diagram for magnetic fields along three cubic high symmetry directions. Through classical Monte Carlo simulations, mean field theory and classical linear spin-wave expansions, we argue that the origins of the multiple occurrences of reentrance observed in Er$_2$Sn$_2$O$_7$ are linked to soft modes. Depending on the field direction, these arise either from a direct $T=0$ competition between the field-evolved ground states, or from a field-induced enhancement of the competition with a distinct zero-field antiferromagnetic phase. In both scenarios, the phase competition enhances thermal fluctuations which entropically stabilize a specific ordered phase. This results in an increased transition temperature for certain field values and thus the reentrant behavior. Our work represents a detailed examination into the mechanisms responsible for reentrance in a frustrated magnet and may serve as a template for the interpretation of reentrant phenomena in other physical systems., Comment: Main text: 6 pages, 3 figures. Supplementary Material: 14 pages, 9 figures

Ising-like spin-1/2 magnetic materials are of interest for their ready connection to theory, particularly in the context of quantum critical behavior. In this work we report detailed studies of the magnetic properties of a member of the rare earth pyrosilicate family, D-Er$_{2}$Si$_{2}$O$_{7}$, which is known to display a highly anisotropic Ising-like g-tensor and effective spin-1/2 magnetic moments. We used powder neutron diffraction, powder inelastic neutron spectroscopy (INS), and single crystal AC susceptibility to characterize its magnetic properties. Neutron diffraction enabled us to determine the magnetic structure below the known transition temperature ($T_{N}$ = 1.9 K) in zero field, confirming that the magnetic state is a four-sublattice antiferromagnetic structure with two non-collinear Ising axes, as was previously hypothesized. Our powder INS data revealed a gapped excitation at zero field, consistent with anisotropic (possibly Ising) exchange. An applied field of 1 T produces a mode softening, which is consistent with a field-induced second order phase transition. To assess the relevance of D-Er$_{2}$Si$_{2}$O$_{7}$ to the transverse field Ising model, we performed AC susceptibility measurements on a single crystal with the magnetic field oriented in the direction transverse to the Ising axes. This revealed a transition at 2.65 T at 0.1 K, a field significantly higher than the mode-softening field observed by powder INS, showing that the field-induced phase transitions are highly field-direction dependent as expected. These measurements suggest that D-Er$_{2}$Si$_{2}$O$_{7}$ may be a candidate for further exploration related to the transverse field Ising model., Comment: AC suscep. data presented in the last version was incorrect. Revised with new data and conclusions

We report on a comparison between the theoretically predicted and experimentally measured spectra of the first-forbidden non-unique $\beta$-decay transition $^{137}\textrm{Xe}(7/2^-)\to\,^{137}\textrm{Cs}(7/2^+)$. The experimental data were acquired by the EXO-200 experiment during a deployment of an AmBe neutron source. The ultra-low background environment of EXO-200, together with dedicated source deployment and analysis procedures, allowed for collection of a pure sample of the decays, with an estimated signal-to-background ratio of more than 99-to-1 in the energy range from 1075 to 4175 keV. In addition to providing a rare and accurate measurement of the first-forbidden non-unique $\beta$-decay shape, this work constitutes a novel test of the calculated electron spectral shapes in the context of the reactor antineutrino anomaly and spectral bump., Comment: Version as accepted by PRL

We report inelastic neutron scattering measurements from our newly synthesized single crystals of the structurally metastable antiferromagnetic pyrochlore Yb$_{2}$Ge$_{2}$O$_{7}$. We determine the four symmetry-allowed nearest-neighbor anisotropic exchange parameters via fits to linear spin wave theory supplemented by fits of the high-temperature specific heat. The exchange parameters so-determined are strongly correlated to the values determined for the $g$-tensor components, as previously observed for the related Yb pyrochlore Yb$_{2}$Ti$_{2}$O$_{7}$. To address this issue, we directly determined the $g$-tensor from electron paramagnetic resonance of 1% Yb-doped Lu$_{2}$Ge$_{2}$O$_{7}$, thus enabling an unambiguous determination of the exchange parameters. Our results show that Yb$_{2}$Ge$_{2}$O$_{7}$ resides extremely close to the classical phase boundary between an antiferromagnetic $\Gamma_5$ phase and a splayed ferromagnet phase. By juxtaposing our results with recent ones on Yb$_{2}$Ti$_{2}$O$_{7}$, our work illustrates that the Yb pyrochlore oxides represent ideal systems for studying quantum magnets in close proximity to classical phase boundaries.

Liquid xenon (LXe) is employed in a number of current and future detectors for rare event searches. We use the EXO-200 experimental data to measure the absolute scintillation and ionization yields generated by $\gamma$ interactions from $^{228}$Th (2615~keV), $^{226}$Ra (1764~keV) and $^{60}$Co (1332~keV and 1173~keV) calibration sources, over a range of electric fields. The $W$-value that defines the recombination-independent energy scale is measured to be $11.5~\pm~0.5$~(syst.)~$\pm~0.1$~(stat.) eV. These data are also used to measure the recombination fluctuations in the number of electrons and photons produced by the calibration sources at the MeV-scale, which deviate from extrapolations of lower-energy data. Additionally, a semi-empirical model for the energy resolution of the detector is developed, which is used to constrain the recombination efficiency, i.e., the fraction of recombined electrons that result in the emission of a detectable photon. Detailed measurements of the absolute charge and light yields for MeV-scale electron recoils are important for predicting the performance of future neutrinoless double beta decay detectors.

A search for neutrinoless double-beta decay ($0\nu\beta\beta$) in $^{136}$Xe is performed with the full EXO-200 dataset using a deep neural network to discriminate between $0\nu\beta\beta$ and background events. Relative to previous analyses, the signal detection efficiency has been raised from 80.8% to 96.4$\pm$3.0% and the energy resolution of the detector at the Q-value of $^{136}$Xe $0\nu\beta\beta$ has been improved from $\sigma/E=1.23\%$ to $1.15\pm0.02\%$ with the upgraded detector. Accounting for the new data, the median 90% confidence level $0\nu\beta\beta$ half-life sensitivity for this analysis is $5.0 \cdot 10^{25}$ yr with a total $^{136}$Xe exposure of 234.1 kg$\cdot$yr. No statistically significant evidence for $0\nu\beta\beta$ is observed, leading to a lower limit on the $0\nu\beta\beta$ half-life of $3.5\cdot10^{25}$ yr at the 90% confidence level., Comment: v1, 7 pages, 5 figures; v2, fix references; v3, update to accepted version

We present a detailed magnetic study of the triangular antiferromagnet ErMgGaO$_4$. A point charge calculation under the single ion approximation reveals a crystal field ground state doublet with a strong Ising-like behavior of the Er$^{3+}$ moment along the c axis. Magnetic susceptibility and specific heat measurements indicate no presence of magnetic transitions above 0.5~K and no evidence of residual entropy as temperature approaching zero. Zero field (ZF) $\mu$SR measurements shows no sign of static uniform or random field and longitudinal field (LF) $\mu$SR measurements exhibit persistent spin fluctuations down to our lowest temperature of 25~mK. Our results provide evidence of a quantum spin liquid state in the triangular antiferromagnet ErMgGaO$_4$.

Rare earth garnets are an exciting playground for studying the exotic magnetic properties of the frustrated hyperkagome lattice. Here we present a comprehensive study of the single ion and collective magnetic properties of the garnet Er$_3$Ga$_5$O$_{12}$. Using inelastic neutron scattering, we find a crystal field ground state doublet for Er$^{3+}$ with strong Ising anisotropy along local [100] axes. Magnetic susceptibility and heat capacity measurements provide evidence for long-range magnetic ordering with $T_N$~$=$~0.8~K, and no evidence for residual entropy is found when cooling through the ordering transition. Neutron powder diffraction reveals that the ground state spin configuration corresponds to the six-sublattice, Ising antiferromagnetic state ($\Gamma_3$) common to many of the rare earth garnets. However, we also found that $\mu$SR appears to be insensitive to the ordering transition in this material, in which a low-temperature relaxation plateau was observed with no evidence of spontaneous muon precession. The combined muon and neutron results may be indicative of a dynamical ground state with a relatively long correlation time. Despite this potential complication, our work indicates that Er$_3$Ga$_5$O$_{12}$ is an excellent model system for studying the complex metamagnetism expected for a multi-axis antiferromagnet.

Neutron scattering measurements on the pyrochlore magnet Ce$_2$Zr$_2$O$_7$ reveal an unusual crystal field splitting of its lowest $J$=5/2 multiplet, such that its ground state doublet is composed of m$_J$=$\pm$3/2, giving these doublets a dipole - octupole (DO) character with local Ising anisotropy. Its magnetic susceptibility shows weak antiferromagnetic correlations with $\theta_{CW}$=-0.4(2)K, leading to a naive expectation of an All-In, All-Out ordered state at low temperatures. Instead our low energy inelastic neutron scattering measurements show a dynamic quantum spin ice state, with suppressed scattering near |$\textbf{Q}$|=0, and no long range order at low temperatures. This is consistent with recent theory predicting symmetry enriched U(1) quantum spin liquids for such DO doublets decorating the pyrochlore lattice. Finally, we show that disorder, especially oxidation of powder samples, is important in Ce$_2$Zr$_2$O$_7$ and could play an important role in the low temperature behaviour of this material., Comment: Submitted to APS journal on 11/28/2018

The quantum dimer magnet (QDM) is the canonical example of quantum magnetism. The QDM state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced triplon Bose-Einstein condensate (BEC) phase. We report on a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb$_2$Si$_2$O$_7$. Our single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet ground state at zero field with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of $H_{c1} \sim 0.4$ T and $H_{c2} \sim 1.4$ T. Using inelastic neutron scattering in an applied magnetic field we observe a Goldstone mode (gapless to within $\delta E$ = 0.037 meV) that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well-known cases of this phase, the high-field ($\mu$$_0$$H\geq1.2$T) part of the phase diagram in Yb$_2$Si$_2$O$_7$ is interrupted by an unusual regime signaled by a change in the field dependence of the ultrasound velocity and magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs., Comment: 6 pages, 4 figures, 12 pages of Supplemental Information, 15 Supplemental figures. Updated to published version

The search for neutrinoless double beta decay probes the fundamental properties of neutrinos, including whether or not the neutrino and antineutrino are distinct. Double beta detectors are large and expensive, so background reduction is essential for extracting the highest sensitivity. The identification, or 'tagging', of the $^{136}$Ba daughter atom from double beta decay of $^{136}$Xe provides a technique for eliminating backgrounds in the nEXO neutrinoless double beta decay experiment. The tagging scheme studied in this work utilizes a cryogenic probe to trap the barium atom in solid xenon, where the barium atom is tagged via fluorescence imaging in the solid xenon matrix. Here we demonstrate imaging and counting of individual atoms of barium in solid xenon by scanning a focused laser across a solid xenon matrix deposited on a sapphire window. When the laser sits on an individual atom, the fluorescence persists for $\sim$30~s before dropping abruptly to the background level, a clear confirmation of one-atom imaging. No barium fluorescence persists following evaporation of a barium deposit to a limit of $\leq$0.16\%. This is the first time that single atoms have been imaged in solid noble element. It establishes the basic principle of a barium tagging technique for nEXO.