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Register a new userThe influence of defects on magnetic properties of fcc-Pu
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The influence of vacancies and interstitial atoms on magnetism in Pu has been considered in frames of the Density Functional Theory (DFT). The relaxation of crystal structure arising due to different types of defects was calculated using the molecular dynamic method with modified embedded atom model (MEAM). The LDA+U+SO (Local Density Approximation with explicit inclusion of Coulomb and spin-orbital interactions) method in matrix invariant form was applied to describe correlation effects in Pu with these types of defects. The calculations show that both vacancies and interstitials give rise to local moments in $f$-shell of Pu in good agreement with experimental data for annealed Pu. Magnetism appears due to destroying of delicate balance between spin-orbital and exchange interactions.
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We present measurements of the magnetic susceptibility, heat capacity and electrical resistivity of Pu$_{1-x}$Lu$_x$Pd$_3$, with $x$=0, 0.1, 0.2, 0.5, 0.8 and 1. PuPd$_3$ is an antiferromagnetic heavy fermion compound with $T_N=24$~K. With increasing Lu doping, both the Kondo and RKKY interaction strengths fall, as judged by the Sommerfeld coefficient $gamma$ and Neel temperature $T_N$. Fits to a crystal field model of the resistivity also support these conclusions. The paramagnetic effective moment $mu_{mathrm{eff}}$ increases with Lu dilution, indicating a decrease in the Kondo screening. In the highly dilute limit, $mu_{mathrm{eff}}$ approaches the value predicted by intermediate coupling calculations. In conjunction with an observed Schottky peak at $sim$60~K in the magnetic heat capacity, corresponding to a crystal field splitting of $sim$12~meV, a mean-field intermediate coupling model with nearest neighbour interactions has been developed.
Spectral properties of fcc-Ce have been calculated in frames of modern DFT+DMFT method with Hybridization expansion CT-QMC solver. The influence of Hunds exchange and spin-orbit coupling (SOC) on spectral properties of Ce were investigated. SOC is responsible for the shape of spectra near the Fermi level and Hunds exchange interaction doesnt change the obtained spectra and can be neglected.
We have synthesized and characterized the four different stable phases of Na ordered Na$_{x}$CoO$_{2}$, for $0.65<xlesssim 0.75$. Above 100K they display similar Curie-Weiss spin susceptibilities as well as ferromagnetic $q=0$ spin fluctuations in the CoO$_{2}$ planes revealed respectively by $^{23}$Na NMR shift and spin lattice $T_{1}$ data. The Co disproportionate already above 300K into Co$^{3+}$ and $approx $Co$^{3.5+}$ in all phases, which allows us to understand that magnetism is favoured. Below 100K the paramagnetic properties become quite distinct, and a 3D magnetic order sets in only for $x=0.75$, so that charge order has a subtle incidence on the low $T$ energy scales and transverse magnetic couplings.
Magnetic oxyselenides have been the topic of research for several decades being first of interest in the context of photoconductivity and thermoelectricity owing to their intrinsic semiconducting properties and ability to tune the energy gap through metal ion substitution. More recently, interest in the oxyselenides has experienced a resurgence owing to the possible relation to strongly correlated phenomena given the fact that many oxyslenides share a similar structure to unconventional superconducting pnictides and chalcogenides. The two dimensional nature of many oxyselenide systems also draws an analogy to cuprate physics where a strong interplay between unconventional electronic phases and localised magnetism has been studied for several decades. It is therefore timely to review the physics of the oxyselenides in the context of the broader field of strongly correlated magnetism and electronic phenomena. Here we review the current status and progress in this area of research with the focus on the influence of lanthanides and transition metal ions on the intertwined magnetic and electronic properties of oxyselenides. The emphasis of the review is on the magnetic properties and comparisons are made with iron based pnictide and chalcogenide systems.
The specific heat, magnetic susceptibility and ESR signals of a Na-deficient vanadate Na_xV_2O_5 (x=1.00 - 0.90) were studied in the temperature range 0.07 - 10 K, well below the transition point to a spin-gap state. The contribution of defects provided by sodium vacancies to the specific heat was observed. It has a low temperature part which does not tend to zero till at least 0.3 K and a high temperature power-like tail appears above 2 K. Such dependence may correspond to the existence of local modes and correlations between defects in V-O layers. The magnetic measurements and ESR data reveal S=1/2 degrees of freedom for the defects, with their effective number increasing in temperature and under magnetic field. The latter results in the nonsaturating magnetization at low temperature. No long-range magnetic ordering in the system of defects was found. A model for the defects based on electron jumps near vacancies is proposed to explain the observed effects. The concept of a frustrated two-dimensional correlated magnet induced by the defects is considered to be responsible for the absence of magnetic ordering.
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