Spin jam induced by quantum fluctuations in a frustrated magnet

Since the discovery of spin glasses in dilute magnetic systems, their study has been largely focused on understanding randomness and defects as the driving mechanism. The same paradigm has also been applied to explain glassy states found in dense frustrated systems. Recently, however, it has been theoretically suggested that different mechanisms, such as quantum fluctuations and topological features, may induce glassy states in defect-free spin systems, far from the conventional dilute limit. Here we report experimental evidence for existence of a glassy state, which we call a spin jam, in the vicinity of the clean limit of a frustrated magnet, which is insensitive to a low concentration of defects. We have studied the effect of impurities on SrCr_9pGa_12-9pO₁₉ [SCGO(p)], a highly frustrated magnet, in which the magnetic Cr³⁺ (s = 3/2) ions form a quasi-2D triangular system of bipyramids. Our experimental data show that as the nonmagnetic Ga³⁺ impurity concentration is changed, there are two distinct phases of glassiness: an exotic glassy state, which we call a spin jam, for the high magnetic concentration region (p > 0.8) and a cluster spin glass for lower magnetic concentration (p < 0.8). This observation indicates that a spin jam is a unique vantage point from which the class of glassy states of dense frustrated magnets can be understood.Understanding glassy states found in dense frustrated magnets has been an intellectual challenge since peculiar low-temperature glassy behaviors were observed experimentally in the quasi-2D SrCr_9pGa_12-9pO₁₉ (SCGO) (1–3) and in the 3D pyrochlore Y₂Mo₂O₇ (4). Immediately following, theoretical investigations (5–9) were performed to see if an intrinsic spin freezing transition is possible in a defect-free situation, aided by quantum fluctuations, as in the order-by-fluctuations phenomenon (10, 11). Quantum fluctuations at T = 0 were shown to select a long-range ordered state in the 2D kagome isotropic antiferromagnet (AFM) (5, 6), later expanded to the isotropic pyroclore and SCGO (9). Anisotropic interactions were also considered as a possible origin of the glassy kagome AFM (7). For an XY pyrochlore AFM, thermal fluctuations were found to induce a conventional Neel order (8). Experimental works were also performed to investigate if the glassy states are extrinsic due to site defects or random couplings or intrinsic to the magnetic lattice (12, 13). The consensus is that the low-temperature spin freezing transitions in SCGO(p) near the clean limit (p ≈ 1) is not driven by site defects (13).The nature of the frozen state in SCGO has been investigated by numerous experimental techniques, including bulk susceptibility (1–3), specific heat (2, 14), muon spin relaxation (μSR) (15), nuclear magnetic resonance (NMR) (13, 16), and elastic and inelastic neutron scattering (17). Observed are spin glassy behaviors, such as field-cooled and zero-field-cooled (FC/ZFC) hysteresis in bulk susceptibility (3), as well as non-spin-glassy behaviors, such as a quadratic behavior of specific heat at low T, C_v ∝ T² (14), linear dependence of the imaginary part of the dynamic susceptibility at low energies, χ″(ω) ∝ ω (17), and a broad but prominent momentum dependence of the elastic neutron scattering intensity (17). The interpretation of the frozen state below T_f is still controversial. One possibility suggested was a spin liquid with unconfined spinons or resonating valence bond state, based on NMR and μSR studies (15, 16). Many-body singlet excitations were also suggested to be responsible for the C_v ∝ T² behavior (14).Recently, some of us presented an alternative scenario involving a spin jam state by considering the effects of quantum fluctuations in the disorder-free quasi-2D ideal SCGO lattice with a simple nearest neighbor (NN) spin interaction Hamiltonian ? = J∑_NNS_i ? S_j (18, 19). The spin jam framework provided a qualitatively coherent understanding of all of the low-temperature behaviors such as that a complex energy landscape is responsible for the frozen state without long-range order (18), and Halperin−Saslow (HS)-like modes for the C_v ∝ T² and χ″(ω) ∝ ω behaviors (5, 18). In this system, which we refer to as the ideal SCGO model (iSCGO), semiclassical magnetic moments (or spins) are arranged in a triangular network of bipyramids and interact uniformly with their NN (18, 19). The microscopic mechanism for the spin jam state is purely quantum mechanical. The system has a continuous and flat manifold of ground states at the mean field level, including locally collinear, coplanar, and noncoplanar spin arrangements. Quantum fluctuations lift the classical ground state degeneracy (order by fluctuations), resulting in a complex rugged energy landscape that has a plethora of local minima consisting of the locally collinear states separated from each other by potential barriers (18). Although the work of ref. 18 dealt with a similar phase space constriction by quantum fluctuations as the aforementioned other theoretical works did, we would like to stress here the difference between the two: Whereas the other works mainly focused on the selection of the long-range-ordered (LRO) energetic ground state, the work of ref. 18 showed that the short-range-ordered (SRO) states that exist at higher energies are long-lived, dominate entropically over the LRO states, and govern the low-T physics.The introduction of nonmagnetic impurities into a topological spin jam state breaks some of the constraints in the system, and possibly allows local transitions between minima, with a time scale dependent on the density of impurities. At a sufficiently high vacancy concentration, the system exits the spin jam state and becomes either paramagnetic or an ordinary spin glass at lower temperatures. Here we try to identify and explore the spin jam regime in an experimentally accessible system. The three most important signatures we seek for the existence of a spin jam state, different from conventional spin glass states, are (i) a linear dependence of the imaginary part of the dynamic susceptibility at low energies, χ″(ω) ∝ ω, (ii) intrinsic short range static spin correlations, and (iii) insensitivity of its physics to nonmagnetic doping near the clean limit. In the rest of the paper, we provide experimental demonstration of these properties.Experimentally, there are, so far, two materials, SrCr_9pGa_12-9pO₁₉ [SCGO(p)] (1–3, 13–17, 20) and qs-ferrites like Ba₂Sn₂ZnGa₃Cr₇O₂₂ (BSZGCO) (21), in which the magnetic Cr³⁺ (3d³) ion surrounded by six oxygen octahedrally, form distorted quasi-2D triangular lattice of bipyramids (20, 21) as shown in Fig. 1A, and thus may realize a spin jam state. We would like to emphasize that these systems are very good insulators (resistivity ρ > ?10¹³?Ω ? cm at 300 K) and the Cr³⁺ (t2g3) ion has no orbital degree of freedom. Furthermore, the neighboring Cr ions share one edge of oxygen octahedral, and thus the direct overlap of the t2g3 orbitals of the neighboring Cr³⁺ ions make the AFM NN Heisenberg exchange interactions dominant and further neighbor interactions negligible (22, 23), as found in Cr₂O₃ (24) and ZnCr₂O₄ (25).Open in a separate windowFig. 1.(A) In SrCr_9pGa_12-9pO₁₉ [SCGO(p)], the magnetic Cr³⁺ (3d³, s = 3/2) ions form the kagome−triangular−kagome trilayer (Top). The blue and red spheres represent kagome and triangular sites, respectively. When viewed from the top of the layers, they form the triangular network of bipyramids (Bottom). (B) The p−T phase diagram of SCGO(p) constructed by bulk susceptibility and elastic neutron scattering measurements on powder samples with various p values. The freezing temperatures, T_f, marked with blue square and black circle symbols are obtained by bulk susceptibility and elastic neutron scattering measurements, respectively. Note that the values of T_f are much lower than the Curie–Weiss temperatures (see Fig. S1). Filled blue squares represent the data obtained from samples whose crystal structural parameters including the Cr/Ga concentrations were refined by neutron diffraction measurements (see Fig. S2 and Fig. S3).     

Controlling caries in exposed root surfaces with silver diamine fluoride: A systematic review with meta-analysis

Background

In this systematic review, the authors aim to assess the effect of silver diamine fluoride (SDF) in preventing and arresting caries in exposed root surfaces of adults.

Patient,physician, and family member understanding of living wills

This study examines understanding of living wills by patients, family members, and physicians. Questionnaires were used to examine whether each cohort understood patients' living wills regarding endotracheal intubation and cardiopulmonary rescuscitation (CPR). Of 4,800 patients admitted during the study period, 206 reported having living wills, all of which precluded intubation and CPR for "terminal conditions." Of 140 admitted to the general hospital wards, 17 (12%) wanted their living wills to preclude intubation/mechanical ventilation and 12 (8.6%) did not want resuscitation under any circumstances. Seven of 120 (6%) physicians and 4 of 108 family members would not intubate or perform CPR even if there was a chance of recovery. Of 88 patients with complete data (including physicians and family members), 29 (33%) wanted their living wills to block intubation/mechanical ventilation only if they were deemed terminal and 46 (52%) wanted the living will to block intubation even if there was a 10% chance of recovery. Thirteen (15%) wanted to block intubation even if the chance of recovery was > or = 50. Results were similar for wishes regarding CPR. These data suggest substantial differences of patient, physician, and family member understanding of living wills. Living wills did not reflect fully patients' expectations of receiving (or not receiving) life-sustaining modalities.     

Primary hepatic carcinoid tumours

BACKGROUND: Carcinoid tumours of the liver are predominantly metastases from the gastrointestinal tract. Primary hepatic carcinoids are extremely rare. DISCUSSION: In contrast to metastases, primary hepatic carcinoids are usually solitary and resectable. It is important that these tumours are differentiated from metastases. Complete surgical resection should be contemplated and is generally curative.     

Human islet mass,morphology, and survival after cryopreservation using the Edmonton protocol

The aim of this study was to assess recovery, cell death, and cell composition of post-thaw cultured human islets. Cryopreserved islets were provided by the Clinical Islet Transplant Program, Edmonton, Canada. Islets were processed using media prepared in accordance with Pre-Edmonton and Edmonton protocols. Cryopreserved islets were rapidly thawed and cultured for 24 h, 3 d, 5 d, and 7 d, following which they were processed for histology. Islet quantification, integrity, morphology and tissue turnover were studied via hematoxylin and eosin stained sections. Ultrastructure was studied by electron microscopy and endocrine cell composition by immunohistochemistry. Using the Pre-Edmonton protocol, islet recovery was 50.1% and islet survival was 50% at 24 h while for the Edmonton protocol, the islet recovery was 69.4% (p < 0.001) and islet survival, 50% at ≈2.5 d. With an increasing culture duration although the physical integrity was retained there was an increasing loss of cohesivity both at light microscopic and at ultrastructure level regardless of the protocols used. Percentage islet survival and tissue turnover correlated negatively with culture duration in both protocols. The Edmonton protocol appears to preserve the islets better. However, culture duration adversely affects islet survival and quality, indicating the need for more optimal cryopreservation and culture techniques.