High-entropy alloys (HEAs) are multicomponent mixtures of elements in similar concentrations, where the high entropy of mixing can stabilize disordered solid-solution phases with simple structures like a body-centered cubic or a face-centered cubic, in competition with ordered crystalline intermetallic phases. We have synthesized an HEA with the composition Ta34Nb33Hf8Zr14Ti11 (in at. %), which possesses an average body-centered cubic structure of lattice parameter a=3.36  Å. The measurements of the electrical resistivity, the magnetization and magnetic susceptibility, and the specific heat revealed that the Ta34Nb33Hf8Zr14Ti11 HEA is a type II superconductor with a transition temperature Tc≈7.3  K, an upper critical field μ0H_c2≈8.2  T, a lower critical field μ0Hc1≈32  mT, and an energy gap in the electronic density of states (DOS) at the Fermi level of 2Δ≈2.2  meV. The investigated HEA is close to a BCS-type phonon-mediated superconductor in the weak electron-phonon coupling limit, classifying it as a "dirty" superconductor. We show that the lattice degrees of freedom obey Vegard's rule of mixtures, indicating completely random mixing of the elements on the HEA lattice, whereas the electronic degrees of freedom do not obey this rule even approximately so that the electronic properties of a HEA are not a "cocktail" of properties of the constituent elements. The formation of a superconducting gap contributes to the electronic stabilization of the HEA state at low temperatures, where the entropic stabilization is ineffective, but the electronic energy gain due to the superconducting transition is too small for the global stabilization of the disordered state, which remains metastable.

The existence of a quantum spin liquid (QSL) in which quantum fluctuations of spins are sufficiently strong to preclude spin ordering down to zero temperature was originally proposed theoretically more than 40 years ago, but its experimental realisation turned out to be very elusive. Here we report on an almost ideal spin liquid state that appears to be realized by atomic-cluster spins on the triangular lattice of a charge-density wave (CDW) state of 1T-TaS 2 . In this system, the charge excitations have a well-defined gap of ∼ 0.3 eV, while nuclear magnetic quadrupole resonance and muon spin relaxation experiments reveal that the spins show gapless quantum spin liquid dynamics and no long range magnetic order down to 70 mK. Canonical T 2 power-law temperature dependence of the spin relaxation dynamics characteristic of a QSL is observed from 200 K to T f = 55 K. Below this temperature we observe a new gapless state with reduced density of spin excitations and high degree of local disorder signifying new quantum spin order emerging from the QSL.arXiv:1704.06450v1 [cond-mat.str-el]

The synthesis, structural characterization, and magnetic properties of crystalline manganese oxide nanoparticles are presented. The procedure is based on the reaction of benzyl alcohol with the two precursors: potassium permanganate KMnO 4 and manganese(II) acetylacetonate Mn(acac) 2 . Depending on the precursor used, the composition of the final product can be varied in such a way that in the case of KMnO 4 mainly Mn 3 O 4 is formed, whereas Mn(acac) 2 leads predominantly to MnO. Rietveld refinement of the XRD powder patterns, high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX) analysis, as well as electron energy loss spectroscopy (EELS) were employed for the structural characterization of the as-synthesized compounds. Especially the MnO manganosite nanocrystals exhibit some interesting features. HRTEM investigations point to the formation of a superstructure, which can be described as an ordered Mn vacancy cubic superstructure with the general formula of Mn 0.875 O x and a lattice parameter of 8.888 Å. The SQUID measurement proves a superparamagnetic behavior of the MnO nanoparticles.

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Various transition metal (TM) doped zinc oxide nanoparticles with the composition TM x Zn 1-x O (TM = V, Mn, Fe, Co, and Ni; x = 0.01-0.3) were prepared by a microwaveassisted nonaqueous sol-gel route in benzyl alcohol within a few minutes. The high doping levels in the range 20-30 atom % achieved for Co and Fe provide a promising opportunity to study the magnetic properties of such potential diluted magnetic semiconductors. However, only Fe 0.2 Zn 0.8 O was ferromagnetic at room temperature. The Co-doped sample showed Curie-Weiss behavior up to a doping level of 30 atom %. According to X-ray absorption fine structure (XAFS) measurements, at high doping levels the Fe-doped ZnO samples contain an increasing fraction of Fe 3þ ions (in addition to Fe 2þ ), whereas Co is predominantly in the oxidation state of þ2. Clustering of Fe ions into amorphous ferromagnetic Fe 3 O 4 within the ZnO host and the magnetic interactions between the Fe 3 O 4 regions is a possible explanation for the ferromagnetic properties.

We have investigated the magnetic properties of CuNCN, the first nitrogen-based analog of cupric oxide CuO. Our muon-spin relaxation, nuclear magnetic resonance, and electron-spin resonance studies reveal that classical magnetic ordering is absent down to the lowest temperatures. However, a large enhancement of spin correlations and an unexpected inhomogeneous magnetism have been observed below 80 K. We attribute this to a peculiar fragility of the electronic state against weak perturbations due to geometrical frustration, which selects between competing spin-liquid and more conventional frozen states.