A Supercooled Spin Liquid State in the Frustrated Pyrochlore Dy2Ti2O7

A "supercooled" liquid develops when a fluid does not crystallize upon cooling below its ordering temperature. Instead, the microscopic relaxation times diverge so rapidly that, upon further cooling, equilibration eventually becomes impossible and glass formation occurs. Classic supercooled liquids exhibit specific identifiers including microscopic relaxation times diverging on a Vogel-Tammann-Fulcher (VTF) trajectory, a Havriliak-Negami (HN) form for the dielectric function, and a general Kohlrausch-Williams-Watts (KWW) form for time-domain relaxation. Recently, the pyrochlore Dy2Ti2O7 has become of interest because its frustrated magnetic interactions may, in theory, lead to highly exotic magnetic fluids. However, its true magnetic state at low temperatures has proven very difficult to identify unambiguously. Here we introduce high-precision, boundary-free magnetization transport techniques based upon toroidal geometries and gain a fundamentally new understanding of the time- and frequency-dependent magnetization dynamics of Dy2Ti2O7. We demonstrate a virtually universal HN form for the magnetic susceptibility, a general KWW form for the real-time magnetic relaxation, and a divergence of the microscopic magnetic relaxation rates with precisely the VTF trajectory. Low temperature Dy2Ti2O7 therefore exhibits the characteristics of a supercooled magnetic liquid; the consequent implication is that this translationally invariant lattice of strongly correlated spins is evolving towards an unprecedented magnetic glass state, perhaps due to many-body localization of spin.Comment: Version 2 updates: added legend for data in Figures 4A and 4B; corrected equation reference in caption for Figure 4

Common glass-forming spin-liquid state in the pyrochlore magnets Dy2Ti2O7 and Ho2Ti2O7

Despite a well-ordered pyrochlore crystal structure and strong magnetic interactions between the Dy3+ or Ho3+ ions, no long-range magnetic order has been detected in the pyrochlore titanates Ho2Ti2O7 and Dy2Ti2O7. To explore the actual magnetic phase formed by cooling these materials, we measure their magnetization dynamics using toroidal, boundary-free magnetization transport techniques. We find that the dynamical magnetic susceptibility of both compounds has the same distinctive phenomenology, which is indistinguishable in form from that of the dielectric permittivity of dipolar glass-forming liquids. Moreover, Ho2Ti2O7 and Dy2Ti2O7 both exhibit microscopic magnetic relaxation times that increase along the super-Arrhenius trajectories analogous to those observed in glass-forming dipolar liquids. Thus, upon cooling below about 2 K, Dy2Ti2O7 and Ho2Ti2O7 both appear to enter the same magnetic state exhibiting the characteristics of a glass-forming spin liquid

Disentangling superconducting and magnetic orders in NaFe_1-xNi_xAs using muon spin rotation

Muon spin rotation and relaxation studies have been performed on a "111" family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment size and the volume fraction of the magnetically ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for higher x than 0.4 % magnetic order becomes more disordered and is completely suppressed for x = 1.5 %. The magnetic volume fraction continuously decreases with increasing x. The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below TC, but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant non-magnetic state below T_C for x = 1.3 %. The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing s-wave superconducting state, which is expected to appear on the verge of microscopic coexistence and phase separation with magnetism. We also present a universal linear relationship between the local ordered moment size and the antiferromagnetic ordering temperature TN across a variety of iron-based superconductors. We argue that this linear relationship is consistent with an itinerant-electron approach, in which Fermi surface nesting drives antiferromagnetic ordering.Comment: 20 pages, 14 figures, Correspondence should be addressed to Prof. Yasutomo Uemura: [email protected]

Supercooled spin liquid state in the frustrated pyrochlore Dy₂Ti₂O₇

A "supercooled" liquid develops when a fluid does not crystallize upon cooling below its ordering temperature. Instead, the microscopic relaxation times diverge so rapidly that, upon further cooling, equilibration eventually becomes impossible and glass formation occurs. Classic supercooled liquids exhibit specific identifiers including microscopic relaxation times diverging on a Vogel-Tammann-Fulcher (VTF) trajectory, a Havriliak-Negami (HN) form for the dielectric function epsilon(omega,T), and a general Kohlrausch-Williams-Watts (KWW) form for time-domain relaxation. Recently, the pyrochlore Dy2Ti2O7 has become of interest because its frustrated magnetic interactions may, in theory, lead to highly exotic magnetic fluids. However, its true magnetic state at low temperatures has proven very difficult to identify unambiguously. Here, we introduce high-precision, boundary-free magnetization transport techniques based upon toroidal geometries and gain an improved understanding of the time-and frequency-dependent magnetization dynamics of Dy2Ti2O7. We demonstrate a virtually universal HN form for the magnetic susceptibility chi(omega, T), a general KWW form for the real-time magnetic relaxation, and a divergence of the microscopic magnetic relaxation rates with the VTF trajectory. Low-temperature Dy2Ti2O7 therefore exhibits the characteristics of a supercooled magnetic liquid. One implication is that this translationally invariant lattice of strongly correlated spins may be evolving toward an unprecedented magnetic glass state, perhaps due to many-body localization of spin.</p

New Fluoride-arsenide Diluted Magnetic Semiconductor (Ba,K)F(Zn,Mn)As with Independent Spin and Charge Doping

We report the discovery of a new fluoride-arsenide bulk diluted magnetic semiconductor (Ba, K)F(Zn, Mn)As with the tetragonal ZrCuSiAs-type structure which is identical to that of the "1111" iron-based superconductors. The joint hole doping via (Ba, K) substitution & spin doping via (Zn, Mn) substitution results in ferromagnetic order with Curie temperature up to 30 K and demonstrates that the ferromagnetic interactions between the localized spins are mediated by the carriers. Muon spin relaxation measurements confirm the intrinsic nature of the long range magnetic order in the entire volume in the ferromagnetic phase. This is the first time that a diluted magnetic semiconductor with decoupled spin and charge doping is achieved in a fluoride compound. Comparing to the isostructure oxide counterpart of LaOZnSb, the fluoride DMS (Ba, K)F(Zn, Mn)As shows much improved semiconductive behavior that would be benefit for further application developments

Volume-wise destruction of the antiferromagnetic Mott insulating state through quantum tuning

RENiO (RE=rare-earth element) and VO are archetypal Mott insulator systems. When tuned by chemical substitution (RENiO) or pressure (VO), they exhibit a quantum phase transition (QPT) between an antiferromagnetic Mott insulating state and a paramagnetic metallic state. Because novel physics often appears near a Mott QPT, the details of this transition, such as whether it is first or second order, are important. Here, we demonstrate through muon spin relaxation/rotation (μSR) experiments that the QPT in RENiO and VO is first order: the magnetically ordered volume fraction decreases to zero at the QPT, resulting in a broad region of intrinsic phase separation, while the ordered magnetic moment retains its full value until it is suddenly destroyed at the QPT. These findings bring to light a surprising universality of the pressure-driven Mott transition, revealing the importance of phase separation and calling for further investigation into the nature of quantum fluctuations underlying the transition.YJU acknowledges support from the U.S. National Science Foundation (NSF) via Grant DMREF DMR-1436095, NSF Grant no. DMR-1105961, and the NSF PIRE programme through Grant no. OISE-0968226, with additional support from the Japan Atomic Energy Agency Reimei Project and the Friends of Todai Foundation. BAF acknowledges support from the NSF GRFP under Grant No. DGE-11-44155. SJLB acknowledges support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DOE-BES) under contract no. DE-SC00112704. Work at McMaster was supported by NSERC. JAA acknowledges financial support from MINECO (SPAIN) through the project MAT2013-41099-R. ZG acknowledges support by the Swiss National Science Foundation. Work at Kyoto University was supported by CREST. Work at the Chinese Academy of Sciences was supported by the Chinese NSF and MOst. Work at Zhejiang was supported by the Chinese NSF (No.11274268 and 11574265) and MOST (No.2016FYA0300402). GK acknowledges support from the U.S. NSF through Grant DMREF DMR-1435918. MI thanks financial support from a Grant-in-Aid for Scientific Research (No. 22104010) from MEXT, Japan, and by MEXT HPCI Strategic Programs for Innovative Research (SPIRE) (under the grant number hp130007, hp140215 and hp150211) and Computational Materials Science Initiative (CMSI). Use of the National Synchrotron Light Source II, Brookhaven National Laboratory, was supported by DOE-BES under contract No. DE-SC0012704. Use of the Spallation Neutron Source, Oak Ridge National Laboratory, was sponsored by the Scientific User Facilities Division, Office of Basic Energy Science, U.S. DOE

Quel bilan economique et social des cercles de qualite dans les entreprises industrielles francaises ?

Available at INIST (FR), Document Supply Service, under shelf-number : DO 2737 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc

Cubic Re⁶⁺ (5d¹) Double Perovskites, Ba₂MgReO₆, Ba₂ZnReO₆, and Ba₂Y_2/3ReO₆: Magnetism, Heat Capacity, μSR, and Neutron Scattering Studies and Comparison with Theory

Double perovskites (DP) of the general formula Ba₂MReO₆, where M = Mg, Zn, and Y_2/3, all based on Re⁶⁺ (5d¹, t_2g¹), were synthesized and studied using magnetization, heat capacity, muon spin relaxation, and neutron-scattering techniques. All are cubic, <i>Fm</i>3̅<i>m</i>, at ambient temperature to within the resolution of the X-ray and neutron diffraction data, although the muon data suggest the possibility of a local distortion for M = Mg. The M = Mg DP is a ferromagnet, <i>T</i>_c = 18 K, with a saturation moment ∼0.3 bohr magnetons at 3 K. There are two anomalies in the heat capacity: a sharp feature at 18 K and a broad maximum centered near 33 K. The total entropy loss below 45 K is 9.68 e.u., which approaches <i>R</i> ln 4 (11.52 e.u.) supporting a <i>j</i> = 3/2 ground state. The unit cell constants of Ba₂MgReO₆ and the isostructural, isoelectronic analogue, Ba₂LiOsO₆, differ by only 0.1%, yet the latter is an anti-ferromagnet. The M = Zn DP also appears to be a ferromagnet, <i>T</i>_c = 11 K, μ_sat(Re) = 0.1 μ_B. In this case the heat capacity shows a somewhat broad peak near 10 K and a broader maximum at ∼33 K, behavior that can be traced to a smaller particle size, ∼30 nm, for this sample. For both M = Mg and Zn, the low-temperature magnetic heat capacity follows a <i>T</i>^3/2 behavior, consistent with a ferromagnetic spin wave. An attempt to attribute the broad 33 K heat capacity anomalies to a splitting of the <i>j</i> = 3/2 state by a crystal distortion is not supported by inelastic neutron scattering, which shows no transition at the expected energy of ∼7 meV nor any transition up to 100 meV. However, the results for the two ferromagnets are compared to the theory of Chen, Pereira, and Balents, and the computed heat capacity predicts the two maxima observed experimentally. The M = Y_2/3 DP, with a significantly larger cell constant (3%) than the ferromagnets, shows predominantly anti-ferromagnetic correlations, and the ground state is complex with a spin frozen component <i>T</i>_g = 16 K from both direct current and alternating current susceptibility and μSR data but with a persistent dynamic component. The low-temperature heat capacity shows a <i>T</i>¹ power law. The unit cell constant of <i>B</i> = Y_2/3 is less than 1% larger than that of the ferromagnetic Os⁷⁺ (5d¹) DP, Ba₂NaOsO₆