Gapless Phases in an s=1/2 Quantum Spin Chain with Bond Alternation

The $S=1/2$ XXZ spin chain with the staggered XY anisotropy $$H = J \sum_{n}^{N} (S^{x}_{n} S^{x}_{n+1} + S^{y}_{n} S^{y}_{n+1} + \Delta S^{z}_{n} S^{z}_{n+1}) - \delta \sum_{n}^{N} (-1)^{n} (S^{x}_{n} S^{x}_{n+1} - S^{y}_{n} S^{y}_{n+1})$$ is shown to possess gapless, Luttinger-liquid-like phases in a wide range of its parameters: the XY-like phase and spin nematic phases, the latter characterized by a two-spin order parameter breaking translational and spin rotation symmetries. In the simplest, exactly solvable case $\Delta = 0$, the spectrum remains gapless at arbitrary $J$ and $\delta$ and is described by two massless Majorana (real) fermions with different velocities $v_{\pm} = |J \pm \delta|$. At $|\delta| < J$ the staggered XY anisotropy does not influence the ground state of the system (XY phase). At $|\delta| > J$, due to level crossing, a spin nematic state is realized, with $\uparrow \uparrow \downarrow \downarrow$ and $\uparrow \downarrow \downarrow \uparrow$ local symmetry of the $xx$ and $yy$ spin correlations. The spin correlation functions are calculated and the effect of thermally induced spin nematic ordering in the XY phase ("order from disorder") is discussed. The role of a finite $\Delta$ is studied in the limiting cases $|\delta| \ll J$Comment: 25 pages, REVTEX; (to appear in Phys.Rev.B), ITP-CTH 9437

Phase diagram of the S=1/2 quantum spin chain with bond alternation

We study the ground state properties of the bond alternating $S=1/2$ quantum spin chain whose Hamiltonian is H=\sum_j (S_{2j}^x S_{2j+1}^x +S_{2j}^y S_{2j+1}^y +\lambda S_{2j}^z S_{2j+1}^z ) +\beta \sum_j {\bf S}_{2j-1} \cdot {\bf S}_{2j} . When $\beta=0$, the ground state is a collection of local singlets with a finite excitation gap. In the limit of strong ferromagnetic coupling $\beta \to - \infty$, this is equivalent to the $S=1 \ XXZ$ Hamiltonian. It has several ground state phases in the $\lambda$-$\beta$ plane including the gapful Haldane phase. They are characterized by a full breakdown, partial breakdowns and a non-breakdown of the hidden discrete $Z_2 \times Z_2$ symmetry. The ground state phase diagram is obtained by series expansions.Comment: 25 pages, RevTex 2.0, 9 Figures available on request, Tec.rep. of ISSP No.A265

Thermodynamics of Spin S = 1/2 Antiferromagnetic Uniform and Alternating-Exchange Heisenberg Chains

The magnetic susceptibility chi and specific heat C versus temperature T of the spin-1/2 antiferromagnetic alternating-exchange (J1 and J2) Heisenberg chain are studied for the entire range 0 \leq alpha \leq 1 of the alternation parameter alpha = J2/J1. For the uniform chain (alpha = 1), detailed comparisons of the high-accuracy chi(T) and C(T) Bethe ansatz data of Kluemper and Johnston are made with the asymptotically exact low-T field theory predictions of Lukyanov. QMC simulations and TMRG calculations of chi(alpha,T) are presented. From the low-T TMRG data, the spin gap Delta(alpha)/J1 is extracted for 0.8 \leq alpha \leq 0.995. High accuracy fits to all of the above numerical data are obtained. We examine in detail the theoretical predictions of Bulaevskii for chi(alpha,T) and compare them with our results. Our experimental chi(T) and C(T) data for NaV2O5 single crystals are modeled in detail. The chi(T) data above the spin dimerization temperature Tc = 34 K are not in agreement with the prediction for the uniform Heisenberg chain, but can be explained if there is a moderate ferromagnetic interchain coupling and/or if J changes with T. By fitting the chi(T) data, we obtain Delta(T = 0) = 103(2) K, alternation parameter delta(0) = (1 - alpha)/(1 + alpha) = 0.034(6) and average exchange constant J(0) = 640(80) K. The delta(T) and Delta(T) are derived from the data. A spin pseudogap with a large magnitude \approx 0.4 Delta(0) is consistently found just above Tc, which decreases with increasing T. Analysis of our C(T) data indicates that at Tc, at least 77% of the entropy change due to the transition at Tc and associated order parameter fluctuations arise from the lattice and/or charge degrees of freedom and less than 23% from the spin degrees of freedom.Comment: 53 two-column REVTeX pages, 50 embedded figures, 7 tables. Revisions required due to incorrect Eq. (39) in Ref. 51 which gives the low-T approximation for the specific heat of a S = 1/2 1D system with a spin gap; no conclusions were changed. Additional minor revisions made. Phys. Rev. B (in press

Cryptosporidium Priming Is More Effective than Vaccine for Protection against Cryptosporidiosis in a Murine Protein Malnutrition Model

Cryptosporidium is a major cause of severe diarrhea, especially in malnourished children. Using a murine model of C. parvum oocyst challenge that recapitulates clinical features of severe cryptosporidiosis during malnutrition, we interrogated the effect of protein malnutrition (PM) on primary and secondary responses to C. parvum challenge, and tested the differential ability of mucosal priming strategies to overcome the PM-induced susceptibility. We determined that while PM fundamentally alters systemic and mucosal primary immune responses to Cryptosporidium, priming with C. parvum (106 oocysts) provides robust protective immunity against re-challenge despite ongoing PM. C. parvum priming restores mucosal Th1-type effectors (CD3+CD8+CD103+ T-cells) and cytokines (IFNγ, and IL12p40) that otherwise decrease with ongoing PM. Vaccination strategies with Cryptosporidium antigens expressed in the S. Typhi vector 908htr, however, do not enhance Th1-type responses to C. parvum challenge during PM, even though vaccination strongly boosts immunity in challenged fully nourished hosts. Remote non-specific exposures to the attenuated S. Typhi vector alone or the TLR9 agonist CpG ODN-1668 can partially attenuate C. parvum severity during PM, but neither as effectively as viable C. parvum priming. We conclude that although PM interferes with basal and vaccine-boosted immune responses to C. parvum, sustained reductions in disease severity are possible through mucosal activators of host defenses, and specifically C. parvum priming can elicit impressively robust Th1-type protective immunity despite ongoing protein malnutrition. These findings add insight into potential correlates of Cryptosporidium immunity and future vaccine strategies in malnourished children

Climate fluctuations of tropical coupled system: The role of ocean dynamics

The tropical oceans have long been recognized as the most important region for large-scale ocean–atmosphere interactions, giving rise to coupled climate variations on several time scales. During the Tropical Ocean Global Atmosphere (TOGA) decade, the focus of much tropical ocean research was on understanding El Niño–related processes and on development of tropical ocean models capable of simulating and predicting El Niño. These studies led to an appreciation of the vital role the ocean plays in providing the memory for predicting El Niño and thus making seasonal climate prediction feasible. With the end of TOGA and the beginning of Climate Variability and Prediction (CLIVAR), the scope of climate variability and predictability studies has expanded from the tropical Pacific and ENSO-centric basis to the global domain. In this paper the progress that has been made in tropical ocean climate studies during the early years of CLIVAR is discussed. The discussion is divided geographically into three tropical ocean basins with an emphasis on the dynamical processes that are most relevant to the coupling between the atmosphere and oceans. For the tropical Pacific, the continuing effort to improve understanding of large- and small-scale dynamics for the purpose of extending the skill of ENSO prediction is assessed. This paper then goes beyond the time and space scales of El Niño and discusses recent research activities on the fundamental issue of the processes maintaining the tropical thermocline. This includes the study of subtropical cells (STCs) and ventilated thermocline processes, which are potentially important to the understanding of the low-frequency modulation of El Niño. For the tropical Atlantic, the dominant oceanic processes that interact with regional atmospheric feedbacks are examined as well as the remote influence from both the Pacific El Niño and extratropical climate fluctuations giving rise to multiple patterns of variability distinguished by season and location. The potential impact of Atlantic thermohaline circulation on tropical Atlantic variability (TAV) is also discussed. For the tropical Indian Ocean, local and remote mechanisms governing low-frequency sea surface temperature variations are examined. After reviewing the recent rapid progress in the understanding of coupled dynamics in the region, this study focuses on the active role of ocean dynamics in a seasonally locked east–west internal mode of variability, known as the Indian Ocean dipole (IOD). Influences of the IOD on climatic conditions in Asia, Australia, East Africa, and Europe are discussed. While the attempt throughout is to give a comprehensive overview of what is known about the role of the tropical oceans in climate, the fact of the matter is that much remains to be understood and explained. The complex nature of the tropical coupled phenomena and the interaction among them argue strongly for coordinated and sustained observations, as well as additional careful modeling investigations in order to further advance the current understanding of the role of tropical oceans in climate

Evaluation of telemental health services for people with intellectual and developmental disabilities: protocol for a randomized non-inferiority trial

Abstract Background Roughly 40% of those with intellectual/developmental disabilities (IDD) have mental health needs, twice the national average. Unfortunately, outpatient mental health services are often inaccessible, increasing reliance on hospital-based services. While telemental health services hold potential to address this gap, little is known about the effectiveness of telemental health for the diversity of persons with IDD, especially as it relates to crisis prevention and intervention services. Accordingly, the aims of this study are to: (1) compare telemental health versus in-person crisis prevention and intervention services among people with IDD; and (2) understand if outcomes vary across subpopulations, in order to identify potential disparities. Methods This study will take place within START (Systemic, Therapeutic, Assessment, Resources, and Treatment), a national evidence-based model of mental health crisis prevention and intervention for people with IDD. A total of 500 youth and adults, located across nine states, will be randomized 1:1 to telemental health vs. in-person. Participant inclusion criteria are ages 12–45 years, living in a family setting, and newly enrolled (within 90 days) to START. Outcomes will be assessed, using a non-inferiority design, for up to 1 year or until discharge. The intervention is comprised of four components: (1) outreach; (2) consultation/coping skills; (3) intake/assessment; and, (4) 24-hour crisis response. The in-person condition will deliver all components in-person. The telemental health condition will deliver components 1 & 2, via telephonic or other communication technology, and components 3 & 4 in-person. Outcomes include mental health crisis contacts, mental health symptoms, emergency psychiatric service use, perceived quality of mental healthcare, and time to discharge. Discussion To our knowledge, this will be the first trial of a telemental health crisis program for the IDD population. The study will be executed by an interdisciplinary team of experts that includes persons with lived experience of disability. Understanding the benefits of specific telemental health methods has important implications to the design of interventions. This telemental health study offers promise to address disparities in access to mental health care for people with IDD across diverse racial, ethnic, linguistic, and cultural groups. Trial Registration Clinicaltrials.gov ( #NCT05336955 ; Registration Date: 4/20/2022)