Unusual Mott transition in multiferroic PbCrO3

The Mott insulator in correlated electron systems arises from classical Coulomb repulsion between carriers to provide a powerful force for electron localization. Turning such an insulator into a metal, the so-called Mott transition, is commonly achieved by “bandwidth” control or “band filling.” However, both mechanisms deviate from the original concept of Mott, which attributes such a transition to the screening of Coulomb potential and associated lattice contraction. Here, we report a pressure-induced isostructural Mott transition in cubic perovskite PbCrO₃. At the transition pressure of ∼3 GPa, PbCrO₃ exhibits significant collapse in both lattice volume and Coulomb potential. Concurrent with the collapse, it transforms from a hybrid multiferroic insulator to a metal. For the first time to our knowledge, these findings validate the scenario conceived by Mott. Close to the Mott criticality at ∼300 K, fluctuations of the lattice and charge give rise to elastic anomalies and Laudau critical behaviors resembling the classic liquid–gas transition. The anomalously large lattice volume and Coulomb potential in the low-pressure insulating phase are largely associated with the ferroelectric distortion, which is substantially suppressed at high pressures, leading to the first-order phase transition without symmetry breaking.Early transition-metal (TM) oxides with partially filled d electrons are strongly correlated (1, 2). Such correlated systems often present exciting new physics and technologically useful electronic and magnetic properties. Mott transition, characterized by delocalization of d electrons, is an attractive phenomenon for exploring the correlated nature of electrons (2, 3). Since the early failure of band theory in the 1930s, the Coulomb repulsion (U) has been proposed to be a strong force that causes electron localization (4, 5). In such electrostatic interaction, the repulsion energy decreases with the compressed lattice because of the screening effect (5–7). Consequently, as originally predicted by Mott (5), the Mott transition is controlled by U at pressures (P).Despite several decades of intensive study, it is still challenging to experimentally validate this view of Mott transition, because U is experimentally difficult to determine, and for most correlated materials it is independent of the pressure. For the known Mott systems, they are found to be controlled by either the bandwidth [e.g., the organic compound κ-Cl (8–10) and Cr-doped V₂O₃ (11, 12)] or band filling (i.e., doping of charge carriers into the parent insulator) (2). Recently, electronic transitions have frequently been reported in late 3d TM oxides (e.g., MnO) (13–16), which are theoretically attributed to bandwidth control (15) or crystal-field splitting (17). For those oxides, a U-controlled mechanism has also been proposed by Gavriliuk et al. (14) and Gavriliuk and coworkers (18); however, the spin cross-over, instead of the screening effect, is believed to contribute to the decreased U (14, 18). Complicating matter further is that the U of (Mg_1–xFe_x)O was computed to increase with pressures (13).TM oxides with a perovskite structure (ABO₃) often exhibit intriguing structural, magnetic, and electronic properties for the study of correlated systems. Among them, PbCrO₃ is such a material that can only be synthesized at high pressures. At ambient pressure, it adopts a paramagnetic (PM), cubic structure at room temperature (T) with an anomalously large unit-cell volume and transforms to an antiferromagnetic (AFM) ground state at low temperatures (19, 20). The magnetic properties arise from unpaired 3d electrons in Cr (i.e., nominally 3d²) with a large U value of 8.28 eV (19–21). Under high pressures, an isostructural phase transition (i.e., no symmetry breaking) has recently been reported in PbCrO₃ with ∼9.8% volume reduction at ∼1.6 GPa; it is the largest volume reduction known in transition-metal oxides (22). Compared with the low-P phase, the high-P phase possesses a more “normal” unit-cell volume (see refs. 21 and 22) and a moderate U of ∼3 eV (23), suggesting a collapse of Coulomb repulsion energy at the phase transition. Because of the reduced U, the mobility of 3d electrons at high pressures is energetically more favorable, which would lead to d-electron delocalization. Apparently, this is a U-driven Mott transition in PbCrO₃ as conceived by Mott. However, to date, the electronic properties of both PbCrO₃ phases have only poorly been explored. In particular, controversial electronic states, including semiconductor (24, 25), half-metal (21), or insulator (20), have been reported for the low-P phase. Besides, the crystal structure and elastic and magnetic properties, as well as the underlying mechanism for the isostructural transition, are still unsettled issues, calling for rigorous investigation into this material.With these aims, in this work we present a comprehensive study on PbCrO₃ with a focus on the P-induced electronic transition. Our findings unveil a unique Mott transition in this perovskite and a new mechanism underlying the isostructural transition.     

Organogenesis in deep time: A problem in genomics,development, and paleontology

The fossil record is a unique repository of information on major morphological transitions. Increasingly, developmental, embryological, and functional genomic approaches have also conspired to reveal evolutionary trajectory of phenotypic shifts. Here, we use the vertebrate appendage to demonstrate how these disciplines can mutually reinforce each other to facilitate the generation and testing of hypotheses of morphological evolution. We discuss classical theories on the origins of paired fins, recent data on regulatory modulations of fish fins and tetrapod limbs, and case studies exploring the mechanisms of digit loss in tetrapods. We envision an era of research in which the deep history of morphological evolution can be revealed by integrating fossils of transitional forms with direct experimentation in the laboratory via genome manipulation, thereby shedding light on the relationship between genes, developmental processes, and the evolving phenotype.Paleontologists in recent decades have discovered a host of new taxa that reveal transitional stages in the evolution of birds, whales, mammals, tetrapods, frogs, salamanders, and arthropods (1–9). This pulse of discovery is not an accident, but the result of an elaboration of our ability to identify likely sites for fossil recovery by using increasingly refined phylogenies, stratigraphic maps, and geological records. Likewise, imaging techniques, such as high-energy CT, have opened up old and understudied fossil collections as new vehicles for discovery. With advances in both fieldwork and imaging, the discovery of the phenotypic basis for morphological innovation is at a critical moment in its long history: Novel perspectives on classical questions of anatomical evolution are within our reach.Fossils, when placed in a phylogenetic context, can reveal taxa with novel combinations of characters that could not be predicted by studying extant creatures alone. If we lacked fossil evidence of mammal-like reptiles, for example, then the physiological and morphological similarities of birds and mammals would likely be interpreted as homologies rather than examples of parallel evolution (e.g., the discredited “Haemothermia” clade) (10, 11). In addition to identifying solid taxonomic groupings, these same fossils reveal transitional series in the origin of the mammalian dentition, ear, and cranium (3). Our understanding of numerous other transformations, from the origin of birds to the origin of tetrapods, is seriously limited without the knowledge of extinct stem taxa.A rich fossil record permits us to document robustly supported transformation series in the evolution of an anatomical feature, organ system, or body plan. However, to understand the pattern and process of evolutionary transitions, paleontologists have increasingly turned their attention to development. In recent years, the combination of technologies from developmental biology and abundant genomic resources for a multitude of model and nonmodel organisms has greatly enriched our understanding of the genetic and developmental processes underlying organogenesis. This broad set of tools provides a new framework for testing hypotheses derived from paleontological findings, thereby forming an interdisciplinary research program with comparative genomics as well as genetic manipulation of embryonic development (12–15).Here, we use the evolution and diversification of the vertebrate limb as an exemplar to reveal how discoveries in paleontology can leverage experimental and comparative work in molecular biology, genomics, and embryology. First, we review how fossil analyses of early gnathostomes, coupled with embryological studies, offer the foundation for hypotheses on the origin of paired appendages. Then, we discuss current research on model and nonmodel species that shed light on the origin of digits by comparing gene expression and regulatory mechanisms underlying fin and limb development. Next, we examine recent studies that identify the genetic and developmental basis for digit reduction in tetrapods. Finally, we highlight novel technologies that are enabling biologists to solve century-old evolutionary puzzles with state-of-the-art molecular approaches. The synthesis of modern technology with paleontological findings has been an ongoing topic of interest (16–18). Continued advances in technology now give morphologists an ever-expanding toolkit to test genome function and, ultimately, manipulate genomes in a phylogenetic framework. When these new technologies are coupled with paleontological discovery, new insights into classical questions in evolutionary morphology lie in the offing.     

Evaluating the validity of model for end‐stage liver disease exception points for hepatocellular carcinoma patients with multiple nodules <2 cm

Liver transplant allocation policy does not give model for end‐stage liver disease (MELD) exception points for patients with a single hepatocellular carcinoma (HCC) <2 cm in size, but does give points to patients with multiple small nodules. Because standard‐of‐care imaging for HCC struggles to differentiate HCC from other nodules, it is possible that a subset of patients receiving liver transplant for multiple nodules <2 cm in size does not have HCC. We evaluate risk of post‐transplant HCC recurrence and wait‐list dropout for patients with multiple small nodules using competing risks regression based on the Fine and Gray model. We identified 5002 adult HCC patients in the OPTN/UNOS dataset diagnosed and transplanted between January 2006 and September 2010. Compared to patients with >1 tumor <2 cm, risk of developing recurrence was significantly higher in patients with one or more tumors with only one tumor ≥2 cm (SHR 1.63, p = 0.009), as well as in patients with 2–3 tumors ≥2 cm (SHR 1.84, p = 0.02). Dropout risk was not significantly different among size categories. HCC recurrence risk was significantly lower in patients with multiple nodules <2 cm in size than in those with larger tumors, supporting the possibility that some patients received unnecessary transplants. The priority given to these patients must be re‐examined.     

Autistic Adults May Be Erroneously Perceived as Deceptive and Lacking Credibility

Journal of Autism and Developmental Disorders - We hypothesized that autistic adults may be erroneously judged as deceptive or lacking credibility due to demonstrating unexpected and atypical...     

Composite score for prediction of 30-day orthopedic surgery outcomes

The LACE+ (Length of stay, Acuity of admission, Charlson Comorbidity Index score, and Emergency department visits in the past 6 months) risk-prediction tool has never been tested in an orthopedic surgery population. LACE+ may help physicians more effectively identify and support high-risk orthopedics patients after hospital discharge. LACE+ scores were retrospectively calculated for all consecutive orthopedic surgery patients (n = 18 893) at a multi-center health system over 3 years (2016-2018). Coarsened exact matching was employed to create “matched” study groups with different LACE+ score quartiles (Q1, Q2, Q3, Q4). Outcomes were compared between quartiles. In all, 1444 patients were matched between Q1 and Q4 (n = 2888); 2079 patients between Q2 and Q4 (n = 4158); 3032 patients between Q3 and Q4 (n = 6064). Higher LACE+ scores significantly predicted 30D readmission risk for Q4 vs Q1 and Q4 vs Q3 (P < .001). Larger LACE+ scores also significantly predicted 30D risk of ED visits for Q4 vs Q1, Q4 vs Q2, and Q4 vs Q3 (P < .001). Increased LACE+ score also significantly predicted 30D risk of reoperation for Q4 vs Q1 (P = .018), Q4 vs Q2 (P < .001), and Q4 vs Q3 (P < .001).