| 1. |
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| 2. |
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| 3. |
- Cunningham, A. L., et al.
(författare)
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Efficacy of the Herpes Zoster Subunit Vaccine in Adults 70 Years of Age or Older
- 2016
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Ingår i: New England Journal of Medicine. - 0028-4793 .- 1533-4406. ; 375:11, s. 1019-1032
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Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND A trial involving adults 50 years of age or older (ZOE-50) showed that the herpes zoster subunit vaccine (HZ/su) containing recombinant varicella-zoster virus glycoprotein E and the AS01(B) adjuvant system was associated with a risk of herpes zoster that was 97.2% lower than that associated with placebo. A second trial was performed concurrently at the same sites and examined the safety and efficacy of HZ/su in adults 70 years of age or older (ZOE-70). METHODS This randomized, placebo-controlled, phase 3 trial was conducted in 18 countries and involved adults 70 years of age or older. Participants received two doses of HZ/su or placebo (assigned in a 1: 1 ratio) administered intramuscularly 2 months apart. Vaccine efficacy against herpes zoster and postherpetic neuralgia was assessed in participants from ZOE-70 and in participants pooled from ZOE-70 and ZOE-50. RESULTS In ZOE-70, 13,900 participants who could be evaluated (mean age, 75.6 years) received either HZ/su (6950 participants) or placebo (6950 participants). During a mean follow-up period of 3.7 years, herpes zoster occurred in 23 HZ/su recipients and in 223 placebo recipients (0.9 vs. 9.2 per 1000 person-years). Vaccine efficacy against herpes zoster was 89.8% (95% confidence interval [CI], 84.2 to 93.7; P<0.001) and was similar in participants 70 to 79 years of age (90.0%) and participants 80 years of age or older (89.1%). In pooled analyses of data from participants 70 years of age or older in ZOE-50 and ZOE-70 (16,596 participants), vaccine efficacy against herpes zoster was 91.3% (95% CI, 86.8 to 94.5; P<0.001), and vaccine efficacy against postherpetic neuralgia was 88.8% (95% CI, 68.7 to 97.1; P<0.001). Solicited reports of injection-site and systemic reactions within 7 days after injection were more frequent among HZ/su recipients than among placebo recipients (79.0% vs. 29.5%). Serious adverse events, potential immune-mediated diseases, and deaths occurred with similar frequencies in the two study groups. CONCLUSIONS In our trial, HZ/su was found to reduce the risks of herpes zoster and postherpetic neuralgia among adults 70 years of age or older. (Funded by GlaxoSmithKline Biologicals; ZOE-50 and ZOE-70 ClinicalTrials.govnumbers, NCT01165177 and NCT01165229.)
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| 4. |
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| 5. |
- Kelley, Steven P., et al.
(författare)
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Dehydration of UO2Cl2 center dot 3H(2)O and Nd(NO3)(3)center dot 6H(2)O with a Soft Donor Ligand and Comparison of Their Interactions through X-ray Diffraction and Theoretical Investigation
- 2020
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 59:5, s. 2861-2869
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Tidskriftsartikel (refereegranskat)abstract
- We investigated whether the relatively Lewis basic imidazole-2-thiones could be used to substitute water ligands bound to f-element cations and generate f-element soft donor complexes. Reactions of 1,3-diethylimidazole-2-thione (C(2)C(2)ImT) with Nd(NO3)(3)center dot 6H(2)O and UO2Cl2 center dot 3H(2)O led to the isolation of the anhydrous thione complexes Nd(NO3)(3)(C(2)C(2)ImT)(3) and UO2Cl2(C(2)C(2)ImT)(2), characterized by single crystal X-ray diffraction. Differences in the strength of metal-thione interactions have been examined by means of the crystal structure analysis and density functional theory (DFT) calculations. The C(2)C(2)ImT ligands were found to be affected by both coordination and noncovalent interactions, making it impossible to deconvolute the effects of one from the other. Calculated partial atomic charges indicated greater ligand-to-metal charge transfer in the [UO2](2+) complex, indicative of a stronger interaction. The reactivity of C(2)C(2)ImT demonstrates its usefulness in the preparation of f-element soft donor complexes from readily available hydrates that could be useful intermediates for promoting the coordination and studying the effects of soft donor anions.
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| 6. |
- López-Fauqued, M., et al.
(författare)
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Safety profile of the adjuvanted recombinant zoster vaccine : Pooled analysis of two large randomised phase 3 trials
- 2019
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Ingår i: Vaccine. - : Elsevier Ltd. - 0264-410X .- 1873-2518. ; 37:18, s. 2482-2493
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Tidskriftsartikel (refereegranskat)abstract
- Background: The ZOE-50 (NCT01165177) and ZOE-70 (NCT01165229) phase 3 clinical trials showed that the adjuvanted recombinant zoster vaccine (RZV) was ≥90% efficacious in preventing herpes zoster in adults. Here we present a comprehensive overview of the safety data from these studies. Methods: Adults aged ≥50 (ZOE-50) and ≥70 (ZOE-70) years were randomly vaccinated with RZV or placebo. Safety analyses were performed on the pooled total vaccinated cohort, consisting of participants receiving at least one dose of RZV or placebo. Solicited and unsolicited adverse events (AEs) were collected for 7 and 30 days after each vaccination, respectively. Serious AEs (SAEs) were collected from the first vaccination until 12 months post-last dose. Fatal AEs, vaccination-related SAEs, and potential immune-mediated diseases (pIMDs) were collected during the entire study period. Results: Safety was evaluated in 14,645 RZV and 14,660 placebo recipients. More RZV than placebo recipients reported unsolicited AEs (50.5% versus 32.0%); the difference was driven by transient injection site and solicited systemic reactions that were generally seen in the first week post-vaccination. The occurrence of overall SAEs (RZV: 10.1%; Placebo: 10.4%), fatal AEs (RZV: 4.3%; Placebo: 4.6%), and pIMDs (RZV: 1.2%; Placebo: 1.4%) was balanced between groups. The occurrence of possible exacerbations of pIMDs was rare and similar between groups. Overall, except for the expected local and systemic symptoms, the safety results were comparable between the RZV and Placebo groups irrespective of participant age, gender, or race. Conclusions: No safety concerns arose, supporting the favorable benefit-risk profile of RZV. © 2019 GlaxoSmithKline Biologicals SA
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| 7. |
- Mishra, Manish Kumar, et al.
(författare)
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Crystallographic evidence of Watson-Crick connectivity in the base pair of anionic adenine with thymine
- 2020
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 117:31, s. 18224-18230
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Tidskriftsartikel (refereegranskat)abstract
- Utilizing an ionic liquid strategy, we report crystal structures of salts of free anionic nucleobases and base pairs previously studied only computationally and in the gas phase. Reaction of tetrabutylammonium ([N-4444](+)) or tetrabutylphosphonium ([P-4444](+)) hydroxide with adenine (HAd) and thymine (HThy) led to hydrated salts of deprotonated adenine, [N-4444][Ad]center dot 2H(2)O, and thymine, [P4444][Thy]center dot 2H(2)O, as well as the double salt cocrystal, [P-4444](2)[Ad][Thy]center dot 3H(2)O center dot 2HThy. The cocrystal includes the anionic [Ad-(HThy)] base pair which is a stable formation in the solid state that has previously not even been suggested. It exhibits Watson-Crick connectivity as found in DNA but which is unusual for the free neutral base pairs. The stability of the observed anionic bases and their supramolecular formations and hydrates has also been examined by electronic structure calculations, contributing to more insight into how base pairs can bind when a proton is removed and highlighting mechanisms of stabilization or chemical transformation in the DNA chains.
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| 8. |
- Oostvogels, Lidia, et al.
(författare)
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Medical conditions at enrollment do not impact efficacy and safety of the adjuvanted recombinant zoster vaccine : a pooled post-hoc analysis of two parallel randomized trials
- 2019
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Ingår i: Human Vaccines & Immunotherapeutics. - : Informa UK Limited. - 2164-5515 .- 2164-554X. ; 15:12, s. 2865-2872
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Tidskriftsartikel (refereegranskat)abstract
- In two pivotal efficacy studies (ZOE-50; ZOE-70), the adjuvanted recombinant zoster vaccine (RZV) demonstrated >90% efficacy against herpes zoster (HZ). Adults aged >= 50 or >= 70 years (ZOE-50 [NCT01165177]; ZOE-70 [NCT01165229]) were randomized to receive 2 doses of RZV or placebo 2 months apart. Vaccine efficacy and safety were evaluated post-hoc in the pooled (ZOE-50/70) population according to the number and type of selected medical conditions present at enrollment. At enrollment, 82.3% of RZV and 82.7% of placebo recipients reported >= 1 of the 15 selected medical conditions. Efficacy against HZ ranged from 84.5% (95% Confidence Interval [CI]: 46.4-97.1) in participants with respiratory disorders to 97.0% (95%CI: 82.3-99.9) in those with coronary heart disease. Moreover, efficacy remained >90% irrespective of the number of selected medical conditions reported by a participant. As indicated by the similarity of the point estimates, this post-hoc analysis suggests that RZV efficacy remains high in all selected medical conditions, as well as with increasing number of medical conditions. No safety concern was identified by the type or number of medical conditions present at enrollment.
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| 9. |
- Pakhira, Santanu, et al.
(författare)
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Ferromagnetic cluster-glass phase in Ca(Co1-xIrx)(2-y)As-2 crystals
- 2020
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 102:2
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Tidskriftsartikel (refereegranskat)abstract
- Single crystals of Ca(Co1-xIrx)(2-y)As-2 with 0 <= x <= 0.35 and 0.10 <= y <= 0.14 have been grown using the self-flux technique and characterized by single-crystal x-ray diffraction (XRD), energy-dispersive x-ray spectroscopy, magnetization M, and magnetic susceptibility chi measurements versus temperature T, magnetic field H, and time t, and heat-capacity C-p(H, T) measurements. The XRD refinements reveal that all the Ir-substituted crystals crystallize in a collapsed-tetragonal structure as does the parent CaCo2-yAs2 compound. A small 3.3% Ir substitution for Co in CaCo1.86As2 drastically lowers the A-type antiferromagnetic (AFM) transition temperature T-N from 52 to 23 K with a significant enhancement of the Sommerfeld electronic heat-capacity coefficient. The A-type AFM structure consists of ab-plane layers of spins ferromagnetically aligned along the c axis with AFM alignment of the spins in adjacent layers along this axis. The positive Weiss temperatures obtained from Curie-Weiss fits to the chi (T > T-N) data indicate that the dominant magnetic interactions are ferromagnetic (FM) for all x. A magnetic phase boundary is inferred to be present between x = 0.14 and x = 0.17 from a discontinuity in the x dependencies of the effective moment and Weiss temperature in the Curie-Weiss fits. FM fluctuations that strongly increase with increasing x are also revealed from the chi (T) data. The magnetic ground state for x >= 0.17 is a spin glass as indicated by hysteresis in chi(T) between field-cooled and zero-field-cooled measurements and from the relaxation of M in a small field that exhibits a stretched-exponential time dependence. The spin glass has a small FM component to the ordering and is hence inferred to be comprised of small FM clusters. The competing AFM and FM interactions along with crystallographic disorder associated with Ir substitution are inferred to be responsible for the development of a FM cluster-glass phase. A logarithmic T dependence of C-p at low T for x = 0.14 is consistent with the presence of significant FM quantum fluctuations. This composition is near the T = 0 boundary at x approximate to 0.16 between the A-type AFM phase containing ferromagnetically-aligned layers of spins and the FM cluster-glass phase.
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| 10. |
- Pakhira, Santanu, et al.
(författare)
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Short-range ferromagnetic order due to Ir substitutions in single-crystalline Ba(Co1−xIrx)2As2 (0 ≤ x ≤ 0.25)
- 2021
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Ingår i: Journal of Physics. - 0953-8984 .- 1361-648X. ; 33:11
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Tidskriftsartikel (refereegranskat)abstract
- The ternary-arsenide compound BaCo2As2 was previously proposed to be in proximity to a quantum-critical point where long-range ferromagnetic (FM) order is suppressed by quantum fluctuations. Here we report the effect of Ir substitutions for Co on the magnetic and thermal properties of Ba(Co1-xIrx)(2)As-2 (0 <= x <= 0.25) single crystals. These compositions all crystallize in an uncollapsed body-centered-tetragonal ThCr2Si2 structure with space group I4/mmm. Magnetic susceptibility measurements reveal clear signatures of short-range FM ordering for x > 0.11 below a nearly composition-independent characteristic temperature T-cl approximate to 13 K. The small variation of T-cl with x, thermomagnetic irreversibility between zero-field-cooled and field-cooled magnetic susceptibility versus T, the occurrence of hysteresis in magnetization versus field isotherms at low field and temperature, and very small spontaneous and remanent magnetizations mu(B)/f.u. together indicate that the FM response arises from short-range FM ordering of FM spin clusters as previously inferred to occur in Ca(Co1-xIrx)(2-y)As-2. Heat-capacity C-p(T) data do not exhibit any clear feature around T-cl, consistent with the very small moments of the FM clusters. The C-p(T) in the paramagnetic temperature regime 25-300 K is well described by the sum of a Sommerfeld electronic contribution and Debye and Einstein lattice contributions where the latter lattice contribution suggests the presence of low-frequency optic modes associated with the heavy Ba atoms in the crystals.
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| 11. |
- Sangeetha, N. S., et al.
(författare)
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Antiferromagnetism in semiconducting SrMn2Sb2 and BaMn2Sb2 single crystals
- 2018
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 97:1
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Tidskriftsartikel (refereegranskat)abstract
- Crystals of SrMn2Sb2 and BaMn2Sb2 were grown using Sn flux and characterized by powder and single-crystal x-ray diffraction, respectively, and by single-crystal electrical resistivity rho, heat capacity C-p, and magnetic susceptibility chi measurements versus temperature T, and magnetization versus field M(H) isotherm measurements. SrMn2Sb2 adopts the trigonal CaAl2Si2-type structure whereas BaMn2Sb2 crystallizes in the tetragonal ThCr2Si2-type structure. The rho(T) data indicate semiconducting behaviors for both compounds with activation energies of greater than or similar to 0.35 eV for SrMn2Sb2 and 0.16 eV for BaMn2Sb2. The chi(T) and C-p(T) data reveal antiferromagnetic (AFM) ordering at T-N = 110 K for SrMn2Sb2 and 450 K for BaMn2Sb2. The anisotropic chi(T <= T-N) data also show that the ordered moments in SrMn2Sb2 are aligned in the hexagonal ab plane whereas the ordered moments in BaMn2Sb2 are aligned collinearly along the tetragonal c axis. The ab-plane M(H) data for SrMn2Sb2 exhibit a continuous metamagnetic transition at low fields 0 < H less than or similar to 1 T, whereas BaMn2Sb2 exhibits no metamagnetic transitions up to 5.5 T. The chi(T) and C-p(T) data for SrMn2Sb2 and BaMn2Sb2 indicate strong dynamic short-range AFM correlations above their respective TN up to at least 900 K within a local-moment picture, corresponding to quasi-two-dimensional magnetic behavior. The present results and a survey of the literature for Mn pnictides with the CaAl2Si2 and ThCr2Si2 crystal structures show that the T-N values for the CaAl2Si2-type compounds are much smaller than those for the ThCr2Si2-type materials.
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| 12. |
- Sangeetha, N. S., et al.
(författare)
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Enhanced moments of Eu in single crystals of the metallic helical antiferromagnet EuCo2-yAs2
- 2018
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 97:14
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Tidskriftsartikel (refereegranskat)abstract
- The compound EuCo2-yAs2 with the tetragonal ThCr2Si2 structure is known to contain Eu+2 ions with spin S = 7/2 that order below a temperature T-N approximate to 47 K into an antiferromagnetic (AFM) proper helical structure with the ordered moments aligned in the tetragonal ab plane, perpendicular to the helix axis along the c axis, with no contribution from the Co atoms. Here we carry out a detailed investigation of the properties of single crystals. We consistently find about 5% vacancies on the Co site from energy-dispersive x-ray analysis and x-ray diffraction refinements. Enhanced ordered and effective moments of the Eu spins are found in most of our crystals. Electronic structure calculations indicate that the enhanced moments arise from polarization of the d bands, as occurs in ferromagnetic Gd metal. Electrical resistivity measurements indicate metallic behavior. The low-field in-plane magnetic susceptibilities x(ab) (T < T-N) for several crystals are reported that are fitted well by unified molecular field theory (MFT), and the Eu-Eu exchange interactions J(ij) are extracted from the fits. High-field magnetization M data for magnetic fields H parallel to ab reveal what appears to be a first-order spin-flop transition followed at higher field by a second-order metamagnetic transition of unknown origin, and then by another second-order transition to the paramagnetic (PM) state. For H parallel to c, the magnetization shows only a second-order transition from the canted AFM to the PM state, as expected. The critical fields for the AFM to PM transition are in approximate agreement with the predictions of MFT. Heat capacity C-p measurements in zero and high H are reported. Phase diagrams for H parallel to c and H parallel to ab versus T are constructed from the high-field M(H, T) and C-p(H, T) measurements. The magnetic part C-mag(T, H = 0) of C-p(T, H = 0) is extracted and is fitted rather well below T-N by MFT, although dynamic short-range AFM order is apparent in Cmag(T) up to about 70 K, where the molar entropy attains its high-T limit of R ln 8.
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| 13. |
- Sangeetha, N. S., et al.
(författare)
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First-order antiferromagnetic transitions of SrMn2P2 and CaMn2P2 single crystals containing corrugated-honeycomb Mn sublattices
- 2021
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 118:44
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Tidskriftsartikel (refereegranskat)abstract
- SrMn2P2 and CaMn2P2 are insulators that adopt the trigonal CaAl2Si2-type structure containing corrugated Mn honeycomb layers. Magnetic susceptibility χ and heat capacity versus temperature T data reveal a weak first-order antiferromagnetic (AFM) transition at the Néel temperature TN=53(1) K for SrMn2P2 and a strong first-order AFM transition at TN=69.8(3) K for CaMn2P2. Both compounds exhibit isotropic and nearly T-independent χ(T≤TN), suggesting magnetic structures in which nearest-neighbor moments are aligned at ≈120° to each other. The 31P NMR measurements confirm the strong first-order transition in CaMn2P2 but show critical slowing down above TN for SrMn2P2, thus also evidencing second-order character. The 31P NMR measurements indicate that the AFM structure of CaMn2P2 is commensurate with the lattice whereas that of SrMn2P2 is incommensurate. These first-order AFM transitions are unique among the class of (Ca, Sr, Ba)Mn2 (P, As, Sb, Bi)2 compounds that otherwise exhibit second-order AFM transitions. This result challenges our understanding of the circumstances under which first-order AFM transitions occur.
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| 14. |
- Sangeetha, N. S., et al.
(författare)
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Helical antiferromagnetic ordering in EuNi1.95As2 single crystals
- 2019
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 100:9
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Tidskriftsartikel (refereegranskat)abstract
- The Eu(+2 )spins-7/2 in EuNi2As2 with the body-centered tetragonal ThCr2Si2 structure order antiferromagnetically below the Ned temperature T-N = 15 K into a helical antiferromagnetic (AFM) structure with the helix axis aligned along the tetragonal c axis and the Eu ordered moments aligned ferromagnetically within the ab plane as previously reported from neutron diffraction measurements [T. Jin et al., Phys. Rev. B 99, 014425 (2019)]. Here we study the crystallographic, magnetic, thermal, and electronic transport properties of Bi flux grown single crystals using single-crystal x-ray diffraction, anisotropic magnetic susceptibility chi, isothermal magnetization M, heat capacity C-p, and electrical resistivity rho measurements versus applied magnetic field H and temperature T. Vacancies are found on the Ni sites corresponding to the composition EuNi1.95(1)As2. A good fit of the rho(T) data by the Bloch-Grijneisen theory for metals was obtained. The chi(ab )(T) data below T-N are fitted well by molecular field theory (MFT), and the helix turn angle kd and the Eu-Eu Heisenberg exchange constants are extracted from the fit parameters. The kd value is in good agreement with the neutron-diffraction result. The magnetic contribution to the zero-field heat capacity below T-N is also fitted by MFT. The isothermal in-plane magnetization M-ab exhibits two metamagnetic transitions versus H, whereas M-c(T = 2 K) is nearly linear up to H = 14 T, both behaviors being consistent with MFT. The M-c(H, T), rho(H-c, T ), and C-p(H-c, T ) data yielded aH(c)-T phase diagram separating the AFM and paramagnetic phases in good agreement with MFT. Anisotropic chi(T) literature data for the ThCr2Si2-type helical antiferromagnet EuRh2As2 were also fitted well by MFT. A comparison is made between the crystallographic and magnetic properties of ThCr2Si2-type EuM(2)Pn(2) compounds with M = Fe, Co, Ni, Cu, or Rh, and Pn = P or As, where only ferromagnetic and c-axis helical AFM structures are found.
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| 15. |
- Sangeetha, N. S., et al.
(författare)
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Magnetic phase transitions in Eu(Co1-xNix)(2-y)As-2 single crystals
- 2020
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Ingår i: Physical Review Materials. - 2475-9953. ; 4:8
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Tidskriftsartikel (refereegranskat)abstract
- The effects of Ni doping in Eu(Co1-xNix)(2-y)As-2 single crystals with x = 0 to 1 grown out of self-flux are investigated via crystallographic, electronic transport, magnetic, and thermal measurements. All compositions adopt the body-centered-tetragonal ThCr2Si2 structure with space group I4/mmm. We also find 3%-4% of randomly distributed vacancies on the Co/Ni site. Anisotropic magnetic susceptibility chi(alpha) (alpha = ab, c) data versus temperature T show clear signatures of an antiferromagnetic (AFM) c-axis helix structure associated with the Eu+2 spins 7/2 for x = 0 and 1 as previously reported. The chi(alpha)(T) data for x = 0.03 and 0.10 suggest an anomalous 2q magnetic structure containing two helix axes along the c axis and in the ab plane, respectively, whereas for x = 0.75 and 0.82 a c-axis helix is inferred as previously found for x = 0 and 1. At intermediate compositions x = 0.2, 0.32, 0.42, 0.54, and 0.65, a magnetic structure with a large ferromagnetic (FM) c-axis component is found from magnetization versus field isotherms, suggested to be an incommensurate FM c-axis cone structure associated with the Eu spins, which consists of both AFM and FM components. In addition, the chi(T) and heat capacity C-p(T) data for x = 0.2-0.65 indicate the occurrence of itinerant FM order associated with the Co/Ni atoms with Curie temperatures from 60 to 25 K, respectively. Electrical resistivity rho(T) measurements indicate metallic character for all compositions with abrupt increases in slope on cooling below the Eu AFM transition temperatures. In addition to this panoply of magnetic transitions, Eu-151 Mossbauer measurements indicate that ordering of the Eu moments proceeds via an incommensurate sine amplitude-modulated structure with additional transition temperatures associated with this effect.
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| 16. |
- Sangeetha, N. S., et al.
(författare)
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Non-Fermi-liquid types of behavior associated with a magnetic quantum critical point in Sr(Co1-xNix)(2)As-2 single crystals
- 2019
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Ingår i: Physical Review B. - 2469-9950 .- 2469-9969. ; 100:9
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Tidskriftsartikel (refereegranskat)abstract
- The compound SrCo2As2 with the body-centered tetragonal ThCr2Si2 structure is known to remain paramagnetic down to a temperature T = 0.05 K, but inelastic neutron scattering studies have shown that both ferromagnetic (FM) and antiferromagnetic (AFM) fluctuations occur in single crystals. Thus it is of interest to study how the magnetism evolves on doping SrCo2As2. Previous work on polycrystalline samples of Sr(Co1-xNix)(2)As-2 indicated the development of AFM order for 0 < x less than or similar to 0.3. Here we studied single crystals of Sr(Co1-xNix)(2)As-2 for 0 < x <= 1 and confirmed the occurrence of AFM order which we deduce to have a c-axis helix structure. We also find evidence for an unusual composition-induced magnetic quantum critical point at x approximate to 0.3 where non-Fermi-liquid types of behavior were revealed by heat capacity and electrical resisitivity measurements at low T. Electron-doped Sr (Co1-xNix)(2)As-2 single crystals with compositions x = 0 to 0.9 were grown out of self-flux and SrNi2As2 single crystals out of Bi flux. The crystals were characterized using single-crystal x-ray diffraction (XRD) at room temperature, and magnetic susceptibility chi (H, T), isothermal magnetization M(H, T), heat capacity C-p (H, T), and electrical resistivity rho(H, T) measurements versus applied magnetic field H and T. The XRD studies show that the system undergoes a continuous structural crossover from the uncollapsed-tetragonal (ucT) structure to the collapsed tetragonal (cT) structure with increasing Ni doping. The chi (T) data show that SrCo2As2 exhibits an AFM ground state almost immediately upon Ni doping on the Co site. Ab initio electronic-structure calculations for x = 0 and 0.15 indicate that a flat band with a peak in the density of states just above the Fermi energy is responsible for this initial magnetic-ordering behavior on Ni doping. The AFM ordering is observed in the range 0.013 <= x <= 0.25 with the ordered moments aligned in the ab plane and with a maximum ordering temperature T-N = 26.5 K at x = 0.10. The Curie-Weiss-like T dependence of chi in the paramagnetic (PM) state indicates dominant FM interactions. The behavior of the anisotropic susceptibilities below T-N suggests a planar helical magnetic ground state with a composition-dependent pitch based on a local-moment molecular-field-theory model, with FM interactions in the ab plane and weaker AFM interactions along the helix c axis. However, the small ordered (saturation) moments similar to 0.1 mu(B) per transition metal atom, where mu(B) is the Bohr magneton, and the values of the Rhodes-Wohlfarth ratio indicate that the magnetism is itinerant. The high-field M(H) isotherms and the low-field chi(-1) (T > T-N) data were successfully analyzed within the framework of Takahashi's theory of FM spin fluctuations. The C-p (T) at low T exhibits Fermi-liquid behavior for 0 <= x <= 0.15, whereas an evolution to a logarithmic non-Fermi-liquid (NFL) behavior is found for x = 0.2 to 0.3. The logarithmic dependence is suppressed in an applied magnetic field. The low-T rho(H = 0, T) data show a T-2 dependence for 0 <= x <= 0.20 and a power-law dependence rho(H = 0, T) = rho(0) + AT(n) with n < 2 for x = 0.20 and 0.30. The exponent n shows a notable field dependence, suggesting both doping- and magnetic-field-tuned quantum critical phenomena. These low-T NFL types of behavior observed in the C-p and rho measurements are most evident near the quantum critical concentration x approximate to 0.3 at which a T = 0 composition-induced transition from the AFM phase to the PM phase occurs.
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| 17. |
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