Fe-vacancy order and superconductivity in tetragonal β-Fe1-xSe

Several superconducting transition temperatures in the range of 30–46 K were reported in the recently discovered intercalated FeSe system (A_1-xFe_2-ySe₂, A = K, Rb, Cs, Tl). Although the superconducting phases were not yet conclusively decided, more than one magnetic phase with particular orders of iron vacancy and/or potassium vacancy were identified, and some were argued to be the parent phase. Here we show the discovery of the presence and ordering of iron vacancy in nonintercalated FeSe (PbO-type tetragonal β-Fe_1-xSe). Three types of iron-vacancy order were found through analytical electron microscopy, and one was identified to be nonsuperconducting and magnetic at low temperature. This discovery suggests that the rich-phases found in A_1-xFe_2-ySe₂ are not exclusive in Fe-Se and related superconductors. In addition, the magnetic β-Fe_1-xSe phases with particular iron-vacancy orders are more likely to be the parent phase of the FeSe superconducting system instead of the previously assigned β-Fe_1+δTe.The iron pnictide superconductors have opened the door to a new way to obtain superconductivity at very high temperatures. β-Fe_1+δSe is remarkable among those superconductors in that it contains the essential electronic and structural constituents required for superconductivity without the conceptual complexity seen in other systems (1). Previous studies showed that the superconducting property of β-Fe_1+δSe made under high-temperature thermodynamic conditions is very sensitive to stoichiometry (1, 2). In the Fe-Se binary phase diagram (2–4), the PbO-type tetragonal structure (the β phase) only stabilized at the Fe-rich side (δ = 0.01–0.04), whereas bulk superconductivity was observed in samples with δ close to 0.01 (5). McQueen et al. showed no superconductivity for samples with δ = 0.03 (5). On the other hand, the fact that only one superconducting phase has been reported in FeSe, unlike the other Fe-As–based superconductors that exhibit clear doping dependence of superconductivity and the absence of superconductivity in FeTe, led to the suggestion that FeTe is the nonsuperconducting parent compound of FeSe (6). Thus, the phase diagram derived from this picture shows very different features compared with other Fe-As–based superconductors (6, 7). In this work, we use low-temperature synthesis methods to prepare β-Fe_1-xSe for a wide range of compositions, which allows for the determination for the composition-dependent electronic behavior for this important superconducting system.The recent discovered alkali/alkaline-intercalated iron selenide (A_1-xFe_2-ySe₂) superconductors with rich superconducting phases, where A = K, Rb, Cs, Tl, attracted great attention not only due to its high superconducting transition temperature (T_c, up to 46 K) (8), but also because of their dissimilar characteristics compared with other iron-based superconductors, especially its seemingly intrinsic multiphase nature and the presence of iron vacancies and orders in the nonsuperconducting regime (9–13). The most frequently observed Fe-vacancy order in A_1-xFe_2-ySe₂ is the × × 1 superstructure, which yields a phase of A_0.8Fe_1.6Se₂ (or A₂Fe₄Se₅). Scanning tunneling microscopy (STM) (11, 14, 15) and transport studies (12, 13, 16, 17) showed that A₂Fe₄Se₅ is an antiferromagnetic (AFM) insulator. Neutron scattering measurements (9) revealed a blocked checkerboard AFM with magnetic moments along the c axis for A₂Fe₄Se₅, ordered at a temperature as high as >500 K, with an unexpected large ordered magnetic moment of ∼3.3 μ_B/Fe at 10 K. Experiments have further shown that the type of vacancy and magnetic orders is highly sensitive to the stoichiometry (x and y) of A_1-xFe_2-ySe₂. Reports have shown the existence of other Fe-vacancy order with the forms × × 1 (10), × 2 × 1 (13, 18), and × × 1 (19). However, the magnetic properties such as the type and transition temperature of the magnetic order are far less studied compared with that of the K₂Fe₄Se₅ phase. In addition, there were also results showing in K_1-xFe_2-ySe₂ samples with a typical T_c = 31 K and additional superconducting phase with T_c = 44 K (20), whereas no clear identification of the new phases was available.The complexity of phases and phase separation during crystal preparation in A_1-xFe_2-ySe₂ make it difficult to conclusively verify the phase-property relationship, even for the superconducting phases. β-Fe_1+δSe, on the other hand, has the simplest structure among all iron-based superconductor families. Several surprising results related to the Fe-Se system appeared in the literature during the last few years, including the enhancement of T_c to about 40 K under high pressure (21–23) and the intriguing extremely high T_c (with a superconducting energy gap of ∼20 meV) in molecular beam epitaxy (MBE)-grown single-layer FeSe (24–26). We also demonstrated the presence of a superconducting-like feature with T_c close to 40 K in samples of nano-dimensional form (27). Therefore, it is quite natural to ask whether the presence of the complex phases observed in A_1-xFe_2-ySe₂ compounds and Fe-vacancy order exist in samples without alkaline metals. Here we present the first discovery of iron vacancies and three types of vacancy orders in tetragonal β-Fe_1-xSe, characterized by analytical transmission electron microscopy (TEM). Our observations imply that an unprecedented phase diagram should be considered in the Fe-Se superconductors.     

Polytypism,polymorphism, and superconductivity in TaSe2?xTex

Polymorphism in materials often leads to significantly different physical properties—the rutile and anatase polymorphs of TiO₂ are a prime example. Polytypism is a special type of polymorphism, occurring in layered materials when the geometry of a repeating structural layer is maintained but the layer-stacking sequence of the overall crystal structure can be varied; SiC is an example of a material with many polytypes. Although polymorphs can have radically different physical properties, it is much rarer for polytypism to impact physical properties in a dramatic fashion. Here we study the effects of polytypism and polymorphism on the superconductivity of TaSe₂, one of the archetypal members of the large family of layered dichalcogenides. We show that it is possible to access two stable polytypes and two stable polymorphs in the TaSe_2−xTe_x solid solution and find that the 3R polytype shows a superconducting transition temperature that is between 6 and 17 times higher than that of the much more commonly found 2H polytype. The reason for this dramatic change is not apparent, but we propose that it arises either from a remarkable dependence of T_c on subtle differences in the characteristics of the single layers present or from a surprising effect of the layer-stacking sequence on electronic properties that are typically expected to be dominated by the properties of a single layer in materials of this kind.The MX₂ layered transition-metal dichalcogenides (TMDCs, M = Mo, W, V, Nb, Ta, Ti, Zr, Hf, or Re and X = Se, S, or Te), have long been of interest due to the rich electronic properties that emerge due to their low dimensionality (1–9). Structurally, these compounds can be regarded as having strongly bonded (2D) X–M–X layers, with M in either trigonal prismatic or octahedral coordination with X, and weak interlayer X–X bonding of the van der Waals type. Many of these materials manifest charge-density waves and competition between charge-density waves (CDWs) and superconductivity, e.g., refs. 5–9. Among the TMDCs, the 2H (H: hexagonal) polytype of tantalum diselenide (2H-TaSe₂) is considered one of the foundational materials (8–18), showing a transition from a metallic phase to an incommensurate charge-density wave (ICDW) phase at 123 K, followed by a “lock-in” transition to a commensurate charge-density wave (CCDW) phase at 90 K. It finally becomes a superconductor with a rather low transition temperature (T_c) of 0.15 K. Although detailed studies have been performed on the physics of CDWs and superconductivity in 2H-TaSe₂ (16–18), a comparative study of the superconductivity of the polytypes and polymorphs of TaSe₂ from the chemical perspective has not been done.TaSe₂ is highly polymorphic, possibly the most polymorphic of the TMDCs (19). In some of its forms, notably the 2H and 3R (R: rhombohedral) polytypes (Fig. 1A), Ta is found in trigonal prismatic coordination in Se-Ta-Se layers that are stacked along the c axis of the hexagonal (or rhombohedral) cell. The 2H and 3R polytypes differ only in their stacking periodicity—the structure repeats after two layers in the 2H form and three layers in the 3R form (20–22). The 3R form can be synthesized, but it is not the stable variant (the 2H form is) and so has been the subject of little study. In one of the other polymorphs, the 1T (T: trigonal) type, Ta is found in octahedral coordination in the Se-Ta-Se layers and the layer stacking along the c axis of the trigonal cell such that the structure repeats after only one layer (23) (Fig. 1A). Again, the 1T form has not been the subject of much study. Here we show that the 3R and 1T polymorphs are both quite stable in the TaSe_2−xTe_x system and that they are both superconducting. For pure TaTe₂, the monoclinic structure is 1T based (Fig. 1A), but is distorted such that there are two nonequivalent Ta and three nonequivalent Te positions in the unit cell (24); we find TaSe_2−xTe_x in this polymorph to be nonsuperconducting down to 0.4 K.Open in a separate windowFig. 1.Structural characterization and analysis of the polytypes and polymorphs of TaSe_2−xTe_x. (A) The crystal structures of 2H-TaSe₂, 3R-TaSe_1.65Te_0.35, 1T-TaSeTe, and monoclinic TaTe₂. (B) Powder X-ray diffraction pattern for 3R-TaSe_1.65Te_0.35. Inset shows the reduced lattice parameter ratio, (c/n)/a, where n = number of layers per cell, for 2H-TaSe₂ (38) and 3R-TaSe_2−xTe_x. (C) Powder X-ray diffraction pattern for 1T-TaTeSe. Inset shows the reduced lattice parameter ratio, (c/n)/a, for 1T-TaSe_2−xTe_x. (D) The variation of in-plane lattice parameter, a, with x for 2H-, 3R-, and 1T-TaSe_2−xTe_x. (E) The variation of reduced stacking-direction lattice parameter, c/n, with x for 2H-, 3R-, and 1T-TaSe_2−xTe_x. (F) The variation of the TaX₂ slab thickness, ((c·(∆z)), with x for 2H-, 3R-, and 1T-TaSe_2−xTe_x. (G) The variation of the van der Waals gap (vdWG) thickness with x for 2H-, 3R-, and 1T-TaSe_2−xTe_x.We report the structures and superconducting properties of TaSe_2−xTe_x for 0 ≤ x ≤ 2. The 2H, 3R, 1T, and monoclinic distorted 1T-structure forms were successfully synthesized. Only a small amount of Te doping (x = 0.02) changes 2H-TaSe₂ into the 3R polytype. Within the 3R polytype, TaSe_2−xTe_x shows the coexistence of a CDW and superconductivity above 0.4 K for 0.1 ≤ x ≤ 0.35. The Te-rich limit of the 3R-TaSe_1.65Te_0.35 polytype shows the highest T_c in the system, 2.4 K, which is 17 times higher than that of 2H-TaSe₂. For 0.8 ≤ x ≤ 1.3, 1T-type TaSe_2−xTe_x emerges and shows a lower T_c, of 0.5–0.7 K. At higher Te substitutions (1.8 ≤ x ≤ 2), TaSe_2−xTe_x changes again, into the monoclinic polymorph, and shows normal metallic behavior to 0.4 K. We argue that the isovalent Te/Se substitution acts to tune the anisotropy of the layers, inducing the 3R to 1T transition, consistent with what has been proposed previously (25). The driving force for the 2H to 3R transition currently remains obscure.     

Interplay Between SAFE and RISK Pathways in Sphingosine-1-Phosphate–Induced Cardioprotection

Purpose

We studied the role of two powerful molecular signalling mechanisms involved in the cardioprotective effect of sphingosine-1-phosphate (S1P), a major component of high density lipoprotein (HDL) against myocardial ischaemic-reperfusion injury, namely the RISK pathway (Akt/Erk), including its downstream target FOXO-1 and, the SAFE pathway (TNF/STAT-3).

Methods

Control hearts from wildtype, TNF deficient (TNF^?/?) or cardiomyocyte STAT-3 deficient (STAT-3^?/?) male mice were perfused on a Langendorff apparatus (35?min global ischaemia and 45?min reperfusion). S1P (10 nM) was given at the onset of reperfusion for the first 7?min, with/without STAT-3 or Akt inhibitors, AG490 and wortmannin (W), respectively.

Results

S1P reduced myocardial infarct size in wildtype hearts (39.3?±?4.4% in control vs 17.3?±?3.1% in S1P-treated hearts; n????6; p??/? or TNF^?/? mice (34.2?±?4.3% in STAT-3^?/? and 34.1?±?2.0% in TNF^?/? mice; n????6; p?=?ns vs. their respective control). Both STAT-3 and Akt inhibitors abolished the protective effects of S1P (33.7?±?3.3% in S1P?+?AG490 and 36.6?±?4.9% in S1P?+?W; n?=?6; p?=?ns vs. their respective control). Increased nuclear levels of phosphorylated STAT-3 (pSTAT-3), Akt and FOXO-1 were observed at 15?min reperfusion in wildtype mice with Western Blot analysis (53% STAT-3, 47% Akt, 41% FOXO-1; p?Conclusions In conclusion, S1P activates both the SAFE and RISK pathways, therefore suggesting a dual protective signalling in S1P-induced cardioprotection.     

GLUT-1 overexpression: Link between hemodynamic and metabolic factors in glomerular injury?

Mesangial matrix deposition is the hallmark of hypertensive and diabetic glomerulopathy. At similar levels of systemic hypertension, Dahl salt-sensitive but not spontaneously hypertensive rats (SHR) develop glomerular hypertension, which is accompanied by upregulation of transforming growth factor beta1 (TGF-beta1), mesangial matrix expansion, and sclerosis. GLUT-1 is ubiquitously expressed and is the predominant glucose transporter in mesangial cells. In mesangial cells in vitro, GLUT-1 overexpression increases basal glucose transport, resulting in excess fibronectin and collagen production. TGF-beta1 has been shown to upregulate GLUT-1 expression. We demonstrated that in hypertensive Dahl salt-sensitive (S) rats fed 4% NaCl (systolic blood pressure [SBP]: 236+/-9 mm Hg), but not in similarly hypertensive SHR (SBP: 230+/-10 mm Hg) or their normotensive counterparts (Dahl S fed 0.5% NaCl, SBP: 145+/-5 mm Hg; and Wistar-Kyoto, SBP: 137+/-3 mm Hg), there was an 80% upregulation of glomerular GLUT-1 protein expression (P< or =0.03). This was accompanied by a 2.7-fold upregulation of TGF-beta1 protein expression in glomeruli of DSH compared with DSN rats (P=0.02). TGF-beta1 expression was not upregulated and did not differ in the glomeruli of Wistar-Kyoto and SHR rats. As an in vitro surrogate of the in vivo hemodynamic stress imposed by glomerular hypertension, we used mechanical stretching of human and rat mesangial cells. We found that after 33 hours of stretching, mesangial cells overexpressed GLUT-1 (40%) and showed an increase in basal glucose transport of similar magnitude (both P< or =0.01), which could be blocked with an anti TGF-beta1-neutralizing antibody. These studies suggest a novel link between hemodynamic and metabolic factors that may cooperate in inducing progressive glomerular injury in conditions characterized by glomerular hypertension.     

Interplay between the renin-angiotensin system,the canonical WNT/β-catenin pathway and PPARγ in hypertension

Purpose of Review

Heterogeneous causes can determinate hypertension.

Recent Findings

The renin-angiotensin system (RAS) has a major role in the pathophysiology of blood pressure. Angiotensin II and aldosterone are overexpressed during hypertension and lead to hypertension development and its cardiovascular complications. In several tissues, the overactivation of the canonical WNT/β-catenin pathway leads to inactivation of peroxisome proliferator-activated receptor gamma (PPARγ), while PPARγ stimulation induces a decrease of the canonical WNT/β-catenin pathway. In hypertension, the WNT/β-catenin pathway is upregulated, whereas PPARγ is decreased. The WNT/β-catenin pathway and RAS regulate positively each other during hypertension, whereas PPARγ agonists can decrease the expression of both the WNT/β-catenin pathway and RAS.

Summary

We focus this review on the hypothesis of an opposite interplay between PPARγ and both the canonical WNT/β-catenin pathway and RAS in regulating the molecular mechanism underlying hypertension. The interactions between PPARγ and the canonical WNT/β-catenin pathway through the regulation of the renin-angiotensin system in hypertension may be an interesting way to better understand the actions and the effects of PPARγ agonists as antihypertensive drugs.

     

Interplay between FAK, PKCδ, and p190RhoGAP in the regulation of endothelial barrier function

Disruption of either intercellular or extracellular junctions involved in maintaining endothelial barrier function can result in increased endothelial permeability. Increased endothelial permeability, in turn, allows for the unregulated movement of fluid and solutes out of the vasculature and into the surrounding connective tissue, contributing to a number of disease states, including stroke and pulmonary edema (Ermert et al., 1995; Lee and Slutsky, 2010; van Hinsbergh, 1997; Waller et al., 1996; Warboys et al., 2010). Thus, a better understanding of the molecular mechanisms by which endothelial cell junction integrity is controlled is necessary for development of therapies aimed at treating such conditions. In this review, we will discuss the functions of three signaling molecules known to be involved in regulation of endothelial permeability: focal adhesion kinase (FAK), protein kinase C delta (PKCδ), and p190RhoGAP (p190). We will discuss the independent functions of each protein, as well as the interplay that exists between them and the effects of such interactions on endothelial function.     

Relationship between Egr—1 gene expression and apoptosis in esophageal carcinoma and precancerous lesions

AIM: To study the expression of early growth response gene-1 (Egr-1 gene) and Bcl-X/(L) protein and its relationship with the cell apoptosis in human esophageal carcinoma (EC) and precancerous lesions. METHODS: In situ hybridization(ISH), immunohistochemistry (IHC) and TUNEL method were used respectively to detect Egr-1mRNA, Egr-1 protein, apoptosis related-protein Bcl-X/(L) and cell apoptosis in situ from 66 cases of esophageal squamous cell carcinoma and their upper cut edge and paracancerous mucosa. RESULTS: Egr-1 gene in situ hybridization, Bcl-X/(L) immunohistochemistry positive products were located in the cytoplasm, while Egr-1 immunohistochemistry and TUNEL positive signal were located in the nuclei. The apoptosis index(AI) and the frequency of apoptosis occurrence were increased gradually from precancerous lesion to cancer (P<0.01) and the expression of Egr-1mRNA and Egr-1 protein in dysplasia was the highest among all specimens (P<0.01). The AI of Egr-1 positive cancer tissues was much higher than that of Egr-1 negative cancer tissues (P<0.01), while the AI of Bcl-X/(L) positive cancer tissues was much lower than that of Bcl-X/(L) negative cancer tissues (P<0.01). The AI and Egr-1 expression were not correlated with invasiveness and lymphatic metastasis in EC. CONCLUSION: Cell apoptosis was present through esophageal carcinogenesis. The expression of Egr-1 mRNA and Egr-1 protein were high in precancerous lesion of esophagus. The AI was increased significantly in Egr-1 positive squamous cell carcinoma. Egr-1 might promote apoptotic effect. Egr-1 expression and cell apoptosis may have an important biological significance in esophageal carcinogenesis.     

Is impairment similar between arm and leg cranking exercise in COPD patients?

Impaired skeletal muscle function has been reported in patients with chronic obstructive disease (COPD), but such impairment is not homogenous and its distribution between the upper and the lower limbs is still unclear. The present study was designed to assess and compare upper and lower limb capacities in patients with moderate to severe COPD during incremental and constant-load exercises. Thirteen COPD patients of similar age with moderate to severe air flow limitation (FEV(1): 35%+/-5% predicted) and 19 healthy subjects were studied. Four sessions were organized: two incremental and two constant-load cycling exercises with arm or leg in randomized order. As observed in a previous study involving incremental and constant tests, power, VO(2), RER, VE, and HR were all significantly lower in the upper and lower limbs of patients with COPD than in healthy controls. In the healthy population, aerobic capacity and mechanical efficiency (ME) were lower in the course of arm exercises than in leg exercises. For the same relative workload, dyspnea and blood lactate production were higher during arm exercise. In contrast, no significant difference was observed between arm and leg capacities for any of these parameters in COPD patients. Conclusion: Although aerobic capacity is impaired in COPD patients, arm aerobic capacity is relatively preserved. Given the lack of significant difference between arm and leg capacities in COPD, we hypothesize that upper limb muscles are less compromised than lower limb muscles in this patient population.     

Is there a difference in adenoma detection rates between gastroenterologists and surgeons?

AIM To compare the adenoma detection rate(ADR) between gastroenterologists and colorectal surgeons at Box Hill Hospital, Melbourne, Australia.METHODS A total of 300 colonoscopies performed by gastroenterologists and colorectal surgeons at Box Hill Hospital were retrospectively reviewed from May 2016 to June 2017. Exclusion criteria were: Patients ≤ 50 years old, colonoscopies with failure of caecal intubation, patients who previously had colon cancer and/or a colonic resection, history of polyposis syndromes or inflammatory bowel disease, or a colonoscopy within the last 10 years. Patient demographics, indications, symptoms and procedural-related outcomes were measured.RESULTS The ADR was not significantly different between gastroenterologists and colorectal surgeons(34% vs 34.67%; P = 0.90). The adjusted odds ratio correcting for gender, age, 1~(st) degree relative with colorectal cancer, previous colonoscopy, trainee involvement and caecal or terminal ileum intubation rate was 1.19(0.69-2.05).CONCLUSION Both specialties at our institution exceed benchmark standards suggested by published Australian and American guidelines. An association between endoscopist specialty and ADR was not observed.     

Is there any association between inhaled ipratropium and mortality in patients with COPD and asthma?

BACKGROUND: To determine whether ipratropium was associated with premature death in patients with asthma and chronic obstructive pulmonary disease (COPD). METHODS: A longitudinal cohort of 827 patients with COPD and 273 with asthma who were evaluated for compensation by two specialists in pulmonary medicine. RESULTS: By June 1999, 538 of the patients with COPD and 77 of those with asthma had died. Atthe consultation, 77% ofthe COPD patients and 8.1% of the asthmatic patients were treated with inhaled ipratropium. Ipratropium was associated with mortality risk ratio (RR) of 2.0 (95% confidence interval: 1.5-2.6) for COPD and 3.6 (1.8-7.1) for asthma patients. After adjustment for confounding factors [forced expiratory volume 1 s (FEV1), smoking habits, asthma medication, and presence of cor pulmonale] the RR for COPD was 1.6 (1.2-2.1) and for asthma 24 (1.2-5.0). CONCLUSIONS: The increased risk of premature death associated with ipratropium is of concern and necessitates further evaluation, e.g., in a randomised study.     

Is there any link between insulin resistance and inflammation in established preeclampsia?

Both insulin resistance and inflammation may contribute to the onset of preeclampsia. They also could be interrelated. We studied the relationship between inflammatory cytokines and markers of insulin resistance. During their third trimester, 22 proteinuric preeclamptic women and 16 normotensive controls underwent intravenous glucose tolerance test (minimal model). Preeclamptic women were more insulin-resistant (P = .009), and they had higher levels of serum soluble tumor necrosis alpha receptor II (TNFalpha RII) (P = .002), triglycerides (P = .006), uric acid (P = .001), and leptin (P = .002) than did the controls. However, the study groups did not differ in serum TNFalpha, C-reactive protein (CRP), interleukin-6 (IL-6), sex hormone-binding globulin (SHBG), and high-density lipoprotein-2 (HDL(2))-cholesterol. In multiple regression analysis only SHBG (P = .01) and triglycerides (P = .0036) were associated with insulin sensitivity independently of body mass index (BMI), weight gain, HDL(2)-cholesterol, CRP, TNFalpha, and TNFalpha RII, IL-6, and leptin. We conclude that insulin resistance and the inflammatory markers studied were not associated in established preeclampsia.