In comparison to 3d or 4f metals, magnetism in actinides remains poorly understood due to experimental complications and the exotic behavior of the 5f states. In particular, plutonium metal is most especially vexing. Over the last five decades theories proposed the presence of either ordered or disordered local moments at low temperatures. However, experiments such as magnetic susceptibility, electrical resistivity, nuclear magnetic resonance, specific heat, and elastic and inelastic neutron scattering show no evidence for ordered or disordered magnetic moments in any of the six phases of plutonium. Beyond plutonium, the magnetic structure of other actinides is an active area of research given that temperature, pressure, and chemistry can quickly alter the magnetic structure of the 5f states. For instance, curium metal has an exceedingly large spin polarization that results in a total moment of about 8 Bohr magneton/atom, which influences the phase stability of the metal. Insight in the actinide ground state can be obtained from core-level x-ray absorption spectroscopy (XAS) and electron energy-loss spectroscopy (EELS). A sum rule relates the branching ratio of the core-level spectra measured by XAS or EELS to the expectation value of the angular part of the spin-orbit interaction.
The Ising exchange interaction is a limiting case of strong exchange anisotropy and represents a key property of many magnetic materials. Here we find necessary and sufficient conditions to achieve Ising exchange interaction for metal sites with unquenched orbital moments. Contrary to current views, the rules established here narrow much the range of lanthanide and actinide ions which can exhibit Ising exchange interaction. It is shown that the arising Ising interaction can be of two distinct types: (i) coaxial, with magnetic moments directed along the anisotropy axes on the metal sites and (i…
The full potential all electron linearized augmented plane wave plus local orbitals (FP-LAPW + lo) method, as implemented in the suite of software WIEN2K, has been used to systematically investigate the structural, electronic, and magnetic properties of the actinide compounds AnN (An = Ac, Th, Pa, U, Np, Pu, Am). The theoretical formalism used is the generalized gradient approximation to density functional theory (GGA-DFT) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Each compound has been studied at six levels of theory: non-magnetic (NM), non-magnetic with spin-orbi…
The ground state electronic structures of the actinide oxides AO, A2O3 and AO2 (A=U, Np, Pu, Am, Cm, Bk, Cf) are determined from first-principles calculations, using the self-interaction corrected local spin-density (SIC-LSD) approximation. Emphasis is put on the degree of f-electron localization, which for AO2 and A2O3 is found to follow the stoichiometry, namely corresponding to A(4+) ions in the dioxide and A(3+) ions in the sesquioxides. In contrast, the A(2+) ionic configuration is not favorable in the monoxides, which therefore become metallic. The energetics of the oxidation and reducti…
A general reformulation of the exchange energy of $5f$-shell is applied in the analysis of the magnetic structure of various actinides compounds in the framework of LDA+U method. The calculations are performed in an efficient scheme with essentially only one free parameter, the screening length. The results are analysed in terms of different polarisation channels, due to different multipoles. Generally it is found that the spin-orbital polarisation is dominating. This can be viewed as a strong enhancement of the spin-orbit coupling in these systems. This leads to a drastic decrease in spin pol…
Universal temperature-dependent electrical resistivity in actinides
M.N. Mikheev Institute of Metal Physics, Ural Branch, Russian Academy of Sciences（俄罗斯科学院乌拉尔分校 M.N. 米凯耶夫金属物理研究所）；Nanotech Centre, Ural Federal University（乌拉尔联邦大学纳米技术中心）
元素钚的不寻常性质长期以来一直是一个谜。有人认为这些性质可能与量子临界性有关[G.卓别林Smith LA Sci 26（2000）1]。在这次演讲中，我们将描述一些实验观察稀土和锕系元素材料，这表明有配对的相关性，在所有的f-电子金属，并在附近的Np/Pu/Am的元素锕系元素的异常性质，即使在高温下，是与一个临界点的变化与成对电子的密度与原子序数。
We have carried out an analysis of magnetic data in 69 uranium, 7 neptunium and 4 plutonium ferromagnets with the spin fluctuation theory developed by Takahashi (Y. Takahashi, J. Phys. Soc. Jpn. 55, 3553 (1986)). The basic and spin fluctuation parameters of the actinide ferromagnets are determined and the applicability of the spin fluctuation theory to actinide 5f system has been discussed. Itinerant ferromagnets of the 3d transition metals and their intermetallics follow a generalized Rhodes-Wohlfarth relation between p_eff/p_s and T_C/T_0, viz., p_eff/p_s ~ (T_C/T_0)^(-3/2). Here, p_s, p_eff…
The influence of electronic structure evolution upon pressure on the temperature dependencies of electrical resistivity of pure Np, Pu, Am, and Cm metals have been investigated within coherent potential approximation (CPA) for many-bands conductivity model. Electronic structure of pure actinide metals was calculated within the local density approximation with the Hubbard U and spin-orbit coupling corrections (LDA+U+SO) method in various phases at normal conditions and under pressure. They were compared with the corresponding cubic (bcc or fcc) phases. The densities of states of the latter were…
A broken symmetry ground state without any magnetic moments has been calculated by means of local-density-approximation to density functional theory plus a local exchange term, the so-called LDA+$U$ approach, for URu$_{2}$Si$_{2}$. The solution is analysed in terms of a multipole tensor expansion of the itinerant density matrix and is found to be a non-trivial magnetic multipole. Analysis and further calculations show that this type of multipole enters naturally in time reversal breaking in presence of large effective spin-orbit coupling and co-exists with magnetic moments for most magnetic ac…
We study the spin-dependent electronic structure of UTe and UT_{2}Si_{2} (T=Cu and Mn) compounds with a combination of x-ray magnetic circular dichroism measurements and first principle calculations. By exploiting the presence of sizable quadrupolar and dipolar contributions to the U L_{2,3}-edge x-ray absorption cross section we are able to provide unique information on the extent of hybridization between 5f and 6d/3d electronic states, a key parameter regulating the physical properties of all actinide materials. Since this information is hardly accessible to other probes, the new methodology…
Electronic structure of bcc Np, fcc Pu, Am, and Cm pure metals under pressure has been investigated employing the LDA+U method with spin-orbit coupling (LDA+U+SO). Magnetic state of the actinide ions was analyzed in both LS and jj coupling schemes to reveal the applicability of corresponding coupling bases. It was demonstrated that whereas Pu and Am are well described within the jj coupling scheme, Np and Cm can be described appropriately neither in {m-sigma}, nor in {jmj} basis, due to intermediate coupling scheme realizing in these metals that requires some finer treatment. The LDA+U+SO resu…
The actinide cubic Laves compounds NpAl2, NpOs2, NpFe2, and PuFe2 have been examined by X-ray magnetic circular dichroism (XMCD) at the actinide M4,5 absorption edges and Os L2,3 absorption edges. The XMCD experiments performed at the M4,5 absorption edges of Np and Pu allow us to determine the spectroscopic branching ratio, which gives information on the coupling scheme in these materials. In all materials the intermediate coupling scheme is found appropriate. Comparison with the SQUID data for NpOs2 and PuFe2 allows a determination of the individual orbital and spin magnetic moments and the …
Trivalent americium has a non-magnetic ($J$ = 0) ground state arising from the cancelation of the orbital and spin moments. However, magnetism can be induced by a large molecular field if Am$^{3+}$ is embedded in a ferromagnetic matrix. Using the technique of x-ray magnetic circular dichroism, we show that this is the case in AmFe$_2$. Since $\langle J_z \rangle$ = 0, the spin component is exactly twice as large as the orbital one, the total Am moment is opposite to that of Fe, and the magnetic dipole operator $\langle T_{z} \rangle$ can be determined directly; we discuss the progression of th…
An effective mass of charge carriers that is significantly larger that the mass of a free electron develops at low temperatures in certain lanthanide- and actinide-based metals, including those formed with plutonium, due to strong electron-electron interactions. This heavy-fermion mass is reflected in a substantially enhanced electronic coefficient of specific heat $γ$, which for elemental Pu itself is much larger than that of normal metals. By our definition, there are twelve Pu-based heavy-fermion compounds, most discovered recently, whose basic properties are known and discussed. Relative t…
Magnetic susceptibility, specific heat, and electrical resistivity measurements have been performed on single-crystal EuBe13 in the temperature range between 2 and 300 K to investigate its phonon property and the valence state of the Eu ion. The obtained temperature dependence of the magnetic susceptibility curves obey a typical Van Vleck susceptibility for Eu3+ with a nonmagnetic ground state in the entire measured temperature range. In the case of the specific heat, we observed the coexistence of Debye and Einstein phonon modes with characteristic Debye and Einstein temperatures of ~ 835 K a…
We assess the reliability of the one-crossing approximation (OCA) approach in quantitative description of the Mott transition in the framework of the dynamical mean field theory (DMFT). The OCA approach has been applied in the conjunction with DMFT to a number of heavy-fermion, actinide, transition metal compounds, and nanoscale systems. However, several recent studies in the framework of impurity models pointed out to serious deficiencies of OCA and raised questions regarding its reliability. Here we consider a single band Hubbard model on the Bethe lattice at finite temperatures and compare …
It is shown that ferromagnetic ordering in metals is associated with an opening of the energy pseudogap. This energy pseudogap belongs to a d-band type in d-metals and to an sp-band type in f-metals. A relation between the magnetic energy and the Curie temperature is obtained. Effect of magnetic ordering on the temperature dependence of the electrical resistivity is considered.
Magnetic materials are usually divided into two classes: those with localised magnetic moments, and those with itinerant charge carriers. We present a comprehensive experimental (spectroscopic ellipsomerty) and theoretical study to demonstrate that these two types of magnetism do not only coexist but complement each other in the Kondo-lattice metal, Tb2PdSi3. In this material the itinerant charge carriers interact with large localised magnetic moments of Tb(4f) states, forming complex magnetic lattices at low temperatures, which we associate with self-organisation of magnetic clusters. The for…
We study low temperature properties in the metallic magnets, considering the itinerant electron mediated ferromagnetism. Applying the Monte Carlo simulations to the extended double exchange model, we discuss reorientation phase transition and anisotropy field for the metallic magnets.
Applying time-varying magnetic fields to Weyl metals, a pair of Weyl points become oscillating. This oscillating monopole and anti-monopole pair gives rise to AC Berry magnetic fields, responsible for the emergence of Berry electric fields, which have not been discussed before at least in the context of Weyl metals. Introducing this novel information into Boltzmann transport theory, we find anomalous Hall effects beyond Berry magnetic fields as a fingerprint of Berry electric fields.
Spontaneous symmetry breakings, metal-insulator transitions, and transport properties of magnetic-domain-wall states in pyrochlore iridium oxides are studied by employing a symmetry adapted effective hamiltonian with a slab perpendicular to the (111) direction of the pyrochlore lattice. Emergent metallic domain wall, which has unconventional topological nature with a controllable and mobile metallic layer, is shown to host Fermi surfaces with modulated helical spin textures resembling Rashba metals. The helical nature of the domain-wall Fermi surfaces is experimentally detectable by anomalous …
We have explored the critical metal-insulator phenomena for pyrochlore-type $R_2$Ir$_2$O$_7$, in which electron correlation strength and magnetic configuration are systematically controlled by varying the average rare-earth ionic radius ($R$=Nd$_{1-x}$Pr$_{x}$ and Sm$_{y}$Nd$_{1-y}$), external pressure, and magnetic field. Metal-insulator transitions in bulk are caused by increasing $x$ or tuning external pressure, indicating that the effective electron correlation is responsible for the transition. The metallic state intervenes between the paramagnetic insulating and antiferromagnetically ord…
The metal--insulator phase transition is considered on the basis of Ginzburg-Landau type equations with two different order parameters. An inclusion of magnetic field in this picture is an important step for understanding of behavior of the metal--insulator phase transition kinetics. The magnetic field leads to various singularities of the surface tension and results in drastic variations of the phase transition kinetics. The strongest singularity is due to Landau diamagnetism and determines anomalous features of MI transition kinetics. This singularity supports the well known experimental fac…
