Chemically-Mediated quantum criticality in NbFe_2

Laves-phase Nb{1+c}Fe_{2-c} is a rare itinerant intermetallic compound exhibiting magnetic quantum criticality at c_{cr}=1.5%Nb excess; its origin, and how alloying mediates it, remains an enigma. For NbFe_2, we show that an unconventional band critical point (uBCP) above the Fermi level E_F explains most observations, and that chemical alloying mediates access to this uBCP by an increase in E_F with decreasing electrons (increasing %Nb), counter to rigid-band concepts. We calculate that E_F enters the uBCP region for c_{cr} > 1.5%Nb and by 1.74%Nb there is no Nb site-occupation preference between symmetry-distinct Fe sites, i.e., no electron-hopping disorder, making resistivity near constant as observed. At larger Nb (Fe) excess, the ferromagnetic Stoner criterion is satisfied.Comment: 4 pages, 5 figure

Unified multi-tupled fixed point theorems involving mixed monotone property in ordered metric spaces

In the present article, we introduce a unified notion of multi-tupled fixed points and utilize the same to prove some existence and uniqueness unified multi-tupled fixed point theorems for Boyd-Wong type nonlinear contractions satisfying generalized mixed monotone property in ordered metric spaces. Our results unify several classical and well-known n-tupled (including coupled, tripled and quadrupled ones) fixed point results existing in the literature.Comment: arXiv admin note: substantial text overlap with arXiv: 1601.0251

Fermi surfaces and Phase Stability of Ba(Fe1−x_{1-x}Mx_x)2_2As2_2 (M=Co, Ni, Cu, Zn)

BaFe$_2$As$_2$ with transition-metal doping exhibits a variety of rich phenomenon from coupling of structure, magnetism, and superconductivity. Using density functional theory, we systematically compare the Fermi surfaces (FS), formation energies ($\Delta E_f$), and density of states (DOS) of electron-doped Ba(Fe$_{1-x}$M$_x$)$_2$As$_2$ with M={Co, Ni, Cu, Zn} in tetragonal (I$4/mmm$) and orthorhombic (F$mmm$) structures in nonmagnetic (NM), antiferromagnetic (AFM), and paramagnetic (PM, disordered local moment) states. We explain changes to phase stability ($\Delta E_f$) and Fermi surfaces (and nesting) due to chemical and magnetic disorder, and compare to observed/assessed properties and contrast alloy theory with that expected from rigid-band model. With alloying, the DOS changes from common-band (Co,Ni) to split-band (Cu,Zn), which dictates $\Delta E_f$ and can overwhelm FS-nesting instabilities, as for Cu,Zn cases