28 Search Results

We report on the observation of a flat band situated at the Fermi level E_F along with the structural, electrical transport, and magnetic properties of BaCo₂P₂ that crystallizes in the ThCr₂Si₂-type body-center tetragonal structure. This compound has the largest inter-layer pnictide (Pn) distance d_Pn–Pn as well as the largest c/a ratio among all the known ACo₂Pn₂ (A = alkaline earth metal) compounds, where a and c are the tetragonal lattice parameters. Hence, the magnetic and electronic properties of this compound are expected to have a quasi-two-dimensional character. Despite the evidence of the presence of sizable magnetic interactions, magnetic susceptibility χ(T)more » of BaCo₂P₂ does not show magnetic ordering down to 1.8 K. The material shows good metallic conduction with a large residual resistivity ratio ρ_{300 K}/ρ_{1.8 K} ≈ 70 and a Fermi liquid behavior at low temperature. Kadowaki–Woods ratio R_KW of BaCo₂P₂ suggests the presence of sizable electronic correlations within this system. Additionally, a large many-body enhancement of 2.3 of the experimental density of states D^γ(E_F) over the band-structure D^band(E_F) is inferred to arise from sizable electron-electron and/or electron-phonon interactions leading to a substantial deviation from the free-electron behavior.« less

We report the growth of high-quality single crystals of hole-doped composition Sr_0.95K_0.05Co₂As₂ and present an investigation of its structural, electronic, thermal, and magnetic properties. Our results show that hole doping leads to a substantial suppression of ferromagnetic (FM) correlations present in SrCo₂As₂ leading to nearly isotropic magnetic susceptibilities χ_ab,c(T) of Sr_0.95K_0.05Co₂As₂. The presence of FM fluctuations is considered one of the detrimental reasons why SrCo₂As₂ does not achieve a superconducting ground state despite having a precursor for it in the form of stripe-like antiferromagnetic fluctuations. The suppression of FM fluctuations leads to qualitative similarities between the properties of hole-doped SrCo₂As₂more » and the end member of superconducting 122 iron-arsenide family KFe₂As₂. Our results also infer the presence of strongly correlated electronic states in Sr_0.95K_0.05Co₂As₂. Further, they show that the electronic structure of SrCo₂As₂ as well as this hole-doped composition is exceptionally unstable in the vicinity of Fermi level EF for small alterations in the structural parameters and sensitively depends upon the details of simulation.« less

High-quality single crystals of KCo₂As₂ with the body-centered tetragonal ThCr₂Si₂ structure were grown using KAs self flux. Structural, magnetic, thermal, and electrical transport properties were investigated. No clear evidence for any phase transitions was found in the temperature range 2–300 K. The in-plane electrical resistivity ρ versus temperature T is highly unusual, showing a T⁴ behavior below 30 K and an anomalous positive curvature up to 300 K, which is different from the linear behavior expected from the Bloch-Grüneisen theory for electron scattering by acoustic phonons. This positive curvature has been previously observed in the in-plane resistivity of high-conductivity layeredmore » delafossites such as PdCoO₂ and PtCoO₂. The in-plane ρ(T → 0) = 0.36μΩ cm of KCo₂As₂ is exceptionally small for this class of compounds. The material also exhibits a magnetoresistance at low T which attains a value of about 40% at T = 2 K and magnetic field H = 80 kOe. The magnetic susceptibility χ of KCo₂As₂ is isotropic and about an order of magnitude smaller than the values for the related compounds SrCo₂As₂ and BaCo₂As₂. The χ increases above 100 K, which is found from our first-principles calculations to arise from a sharp peak in the electronic density of states just above the Fermi energy E_F. Heat capacity C_p(T) data at low T yield an electronic density of states N(E_F) that is about 36% larger than predicted by the first-principles theory. The C_p(T) data near room temperature suggest the presence of excited optic vibration modes, which may also be the source of the positive curvature in ρ(T). Angle-resolved photoemission spectroscopy measurements are compared with the theoretical predictions of the band structure and Fermi surfaces. In conclusion, our results show that KCo₂As₂ provides a new avenue for investigating the physics of high-purity metals.« less

Here, we carried out ⁷⁵As nuclear magnetic resonance (NMR) measurements on the trigonal CaMn₂ As₂ and SrMn₂ As₂ insulators exhibiting antiferromagnetic (AFM) ordered states below Néel temperatures T_N = 62 and 120 K, respectively. In the paramagnetic state above T_N, typical quadrupolar-split ⁷⁵As NMR spectra were observed for both systems. The ⁷⁵As quadrupolar frequency ν_Q for CaMn₂ As₂ decreases with decreasing temperature, while ν_Q for SrMn₂ As₂ increases, showing an opposite temperature dependence. In the AFM state, the relatively sharp and distinct ⁷⁵As NMR lines were observed in SrMn₂ As₂ and the NMR spectra were shifted to lower fields formore » both magnetic fields H ∥ c axis and H ∥ a b plane, suggesting that the internal fields B_int at the As site produced by the Mn ordered moments are nearly perpendicular to the external magnetic field direction. No obvious distribution of B_int was observed in SrMn₂ As₂, which clearly indicates a commensurate AFM state. In sharp contrast to SrMn₂ As₂, broad and complex NMR spectra were observed in CaMn₂ As₂ in the AFM state, which clearly shows a distribution of B_int at the As site, indicating an incommensurate state. From the analysis of the characteristic shape of the observed spectra, the AFM state of CaMn₂ As₂ was determined to be a two-dimensional incommensurate state where Mn ordered moments are aligned in the ab plane. A possible origin for the different AFM states in the systems was discussed. Both CaMn₂ As₂ and SrMn₂ As₂ show very large anisotropy in the nuclear spin-lattice relaxation rate 1/T₁ in the paramagnetic state. 1/T₁ for H ∥ ab is much larger than that for H ∥ c, indicating strong anisotropic AFM spin fluctuations in both compounds.« less

In this paper, we report investigations of the structural, magnetic, electrical transport and thermal properties of five compositions of the metallic perovskite GdRh₃C_1–xB_x (0.00 ≤ x ≤ 1.00). Our results show that all the five compositions undergo magnetic ordering at low temperatures, but the nature of the ordered state is significantly different in the carbon- and the boron-rich compositions, where the former shows signatures of an amplitude-modulated magnetic structure and the latter exhibits evidences of an equal-moment incommensurate antiferromagnetic ordering. We also observe a remarkable field-dependent evolution of conduction carrier polarization in the compositionally disordered compounds. The outcomes indicate thatmore » this system is energetically situated in proximity to a magnetic instability where small variations in the control parameter(s), such as lattice constant and/or electron density, lead to considerably different ground states.« less

We report the synthesis and properties of a new layered tetragonal ternary compound CsMn4As3 (structure: KCu₄S₃-type, space group: P4/mmm, No. 123 and Z = 2). The material is a small band-gap semiconductor and exhibits an antiferromagnetic ground state associated with Mn spins. The compound exhibits a signature of a distinct magnetic moment canting event at 150(5) K with a canting angle of ≈ 0:3°. Although, some features of the magnetic characteristics of this new compound are qualitatively similar to those of the related BaMn₂As₂, the underlying Mn sublattices of the two materials are quite different. While the Mn square-lattice layersmore » in BaMn₂As₂ are equally spaced along the c-direction with the interlayer distance d_{L Ba} = 6.7341(4) Å, the Mn sublattice forms bilayers in CsMn₄As₃ with the interlayer distance within a bilayer d_{L Cs} = 3.1661(6) Å and the distance between the two adjacent bilayers d_B = 7:290(6) Å. This difference in the Mn sublattice is bound to significantly alter the energy balance between the J₁, J₂ and J_c exchange interactions within the J₁-J₂-J_c model compared to that in BaMn₂As₂ and the other related 122 compounds including the well-known iron-arsenide superconductor parent compound BaFe₂As₂. Owing to the novelty of its transition metal sublattice, this new addition to the family of tetragonal materials related to the iron-based superconductors brings prospects for doping and pressure studies in the search of new superconducting phases as well as other exciting correlated-electron properties.« less