Search

${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ is a magnetic Weyl semimetal, in which ferromagnetic ordering at 177 K is predicted to stabilize Weyl points. We perform temperature and spatial dependent angle-resolved photoemission spectroscopy measurements through the Curie temperature (${T}_{c}$), which show large band shifts and renormalization concomitant with the onset of magnetism. We argue that ${\mathrm{Co}}_{3}{\mathrm{Sn}}_{2}{\mathrm{S}}_{2}$ evolves from a Mott ferromagnet below ${T}_{c}$ to a correlated metallic state above ${T}_{c}$. To understand the magnetism, we derive a tight-binding model of Co-$3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ orbitals on the kagome lattice. At the filling obtained by first-principles calculations, this model reproduces the ferromagnetic ground state, and results in the reduction of Coulomb interactions due to cluster effects. Using a disordered local moment simulation, we show how this reduced Hubbard $U$ leads to a collapse of the bands across the magnetic transition, resulting in a correlated state, which carries associated characteristic photoemission signatures that are distinct from those of a simple lifting of exchange splitting. The behavior of topology across ${T}_{c}$ is discussed in the context of this description of the magnetism.

In two-dimensional (2D) systems, high mobility is typically achieved in low-carrier-density semiconductors and semimetals. Here, we discover that the nanobelts of Weyl semimetal NbAs maintain a high mobility even in the presence of a high sheet carrier density. We develop a growth scheme to synthesize single crystalline NbAs nanobelts with tunable Fermi levels. Owing to a large surface-to-bulk ratio, we argue that a 2D surface state gives rise to the high sheet carrier density, even though the bulk Fermi level is located near the Weyl nodes. A surface sheet conductance up to 5-100 S per □ is realized, exceeding that of conventional 2D electron gases, quasi-2D metal films, and topological insulator surface states. Corroborated by theory, we attribute the origin of the ultrahigh conductance to the disorder-tolerant Fermi arcs. The evidenced low-dissipation property of Fermi arcs has implications for both fundamental study and potential electronic applications.

Using the density-functional-based LDA+U method and linear-response theory, we study the magnetic exchange interactions of the superconductor Nd1-xSrxNiO2. Our calculated nearest-neighbor exchange constant J1=82 meV is large, weakly affected by doping, and is only slightly smaller than that found in the sister compound CaCuO2. However, we find that the hole doping significantly enhances the interlayer exchange coupling as it affects the magnetic moment of the Ni-3d3z2-r2 orbital. This can be understood in terms of the small hybridization of Ni-3d3z2-r2 within the NiO2 plane, which results in a flat band near the Fermi level, and its large overlap along the z direction. We also demonstrate that the Nd-5d states appearing at the Fermi level do not affect the magnetic exchange interactions, and thus they may not participate in the superconductivity of this compound. Whereas many previous works emphasized the importance of the Ni-3dx2-y2 and Nd-5d orbitals, we analyze instead the solution of the Ni-3dx2-y2/Ni-3d3z2-r2 minimal model using dynamical mean field theory. It reveals an underlying Mott insulating state that, depending on the precise values of the intra-atomic Hund's coupling smaller or larger than 0.83 eV, selects upon doping either S=0 or 1 two-hole states at low energies, leading to very different quasiparticle band structures. We propose that trends upon doping in the spin excitation spectrum and the quasiparticle density of states can be a way to probe the Ni 3d8 configuration. © 2021 American Physical Society. X.W. is supported by the NSFC (Grants No. 11834006, No. 11525417, No. 51721001, and No. 11790311), National Key R&D Program of China (Grants No. 2018YFA0305704 and No. 2017YFA0303203), and by 111 Project. X.W. also acknowledges the support from the Tencent Foundation through the XPLORER PRIZE. V.I., G.R., and S.Y.S. are supported by NSF DMR Grant No. 1832728. I.L. acknowledges support by the Russian Foundation for Basic Research (Project No. 18-32-20076). The DMFT electronic structure calculations were supported by the state assignment of Minobrnauki of Russia (theme “Electron” No. AAAA-A18-118020190098-5).

Non-centrosymmetric superconductors have recently attracted much attention, since the lack of inversion symmetry mixes spin singlet and triplet pairing states, which may allow the realization of topological superconductivity. In this work, we study the electronic properties of the family of inversion-broken CeTX$_3$ heavy-fermion superconductors, finding topological nodal lines as well as Dirac and Weyl points, which are renormalized closer to the Fermi energy by correlations. We find that the Weyl nodal lines have a substantial effect on the Fermi surface spin structure of the normal state, and lead to line nodes in the superconducting phase., Comment: 6 pages, 4 figures

Uranium-based ferromagnet demonstrates notable ability to convert heat to electricity., The transverse voltage generated by a temperature gradient in a perpendicularly applied magnetic field, termed the Nernst effect, has promise for thermoelectric applications and for probing electronic structure. In magnetic materials, an anomalous Nernst effect (ANE) is possible in a zero magnetic field. We report a colossal ANE in the ferromagnetic metal UCo0.8Ru0.2Al, reaching 23 microvolts per kelvin. Uranium’s 5f electrons provide strong electronic correlations that lead to narrow bands, a known route to producing a large thermoelectric response. In addition, uranium’s strong spin-orbit coupling produces an intrinsic transverse response in this material due to the Berry curvature associated with the relativistic electronic structure. Theoretical calculations show that in UCo0.8Ru0.2Al at least 148 Weyl nodes, and two nodal lines, exist within 60 millielectron volt of the Fermi level. This work demonstrates that magnetic actinide materials can host strong Nernst and Hall responses due to their combined correlated and topological nature.

Author(s): Rossi, A; Resta, G; Lee, SH; Redwing, RD; Jozwiak, C; Bostwick, A; Rotenberg, E; Savrasov, SY; Vishik, IM | Abstract: WTe2 is a multifunctional quantum material exhibiting numerous emergent phases in which tuning of the carrier density plays an important role. Here, we demonstrate two nonmonotonic changes in the electronic structure of WTe2 upon in situ electron doping. The first phase transition is interpreted in terms of a shear displacement of the top WTe2 layer, which realizes a local crystal structure not normally found in bulk WTe2. The second phase transition is associated with stronger interactions between the dopant atoms and the host, both through hybridization and electric field. These results demonstrate that electron doping can drive structural and electronics changes in bulk WTe2 with implications for realizing nontrivial band-structure changes in heterointerfaces and devices.

Although strong electronic correlations are known to be responsible for some highly unusual behaviors of solids such as metal-insulator transitions, magnetism, and even high-temperature superconductivity, their interplay with recently discovered topological states of matter awaits a full exploration. Here, we use a modern electronic structure method, combining the density functional theory of band electrons with dynamical self-energies of strongly correlated states, to predict that two well-known phases of actinide compound UNiSn, a paramagnetic semiconducting and antiferromagnetic metallic, correspond to topological insulator (TI) and Weyl semimetal (WSM) phases of topological quantum matter. Thus, the famous unconventional insulator-metal transition observed in UNiSn is also a TI-to-WSM transition. Driven by a strong hybridization between U f-electron multiplet transitions and band electrons, multiple energy gaps open up in the single-particle spectrum whose topological physics is revealed using the calculation of Z_{2} invariants in the strongly correlated regime. A simplified physical picture of these phenomena is provided based on a periodic Anderson model of strong correlations and multiple band inversions that occur in this fascinating compound. Studying the topology of interacting electrons reveals interesting opportunities for finding exotic phase transitions in strongly correlated systems.

Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in Bi2-xSbxSe3 nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons. Remarkably, these excitons can transport over hundreds of micrometers along the topological insulator (TI) nanoribbons before recombination at up to 40 K. The macroscopic transport distance, combined with short carrier lifetime obtained from transient photocurrent measurements, indicates an exciton diffusion coefficient at least 36 m2 s−1, which corresponds to a mobility of 6 × 104 m2 V−1 s−1 at 7 K and is four order of magnitude higher than the value reported for free carriers in TIs. The observation of highly dissipationless exciton transport implies the formation of superfluid-like exciton condensate at the surface of TIs., Exciton condensation may emerge at room temperature in topological materials with strong Coulomb interactions and vanishing electron effective mass. Here, Hou et al. report the formation of excitons in Bi2-xSbxSe3 nanoribbons, which can transport over hundreds of micrometres before recombination up to 40 K, further implying exciton condensation.

Although topological invariants have been introduced to classify the appearance of protected electronic states at surfaces of insulators, there are no corresponding indexes for Weyl semimetals whose nodal points may appear randomly in the bulk Brillouin Zone (BZ). Here we use a well-known result that every Weyl point acts as a Dirac monopole and generates integer Berry flux to search for the monopoles on rectangular BZ grids that are commonly employed in self-consistent electronic structure calculations. The method resembles data mining technology of computer science and is demonstrated on locating the Weyl points in known Weyl semimetals. It is subsequently used in high throughput screening several hundreds of compounds and predicting a dozen new materials hosting nodal Weyl points and/or lines., Comment: 6 pages, 4 figures, 1 table, supplementary info (1 table, 13 figures)

Topological semimetals recently stimulate intense research activities. Combining first-principles calculations and effective model analysis, we predict that CaTe is topological node-line semimetal when spin-orbit coupling (SOC) is ignored. We also obtain the nearly flat surface state which has the drumhead characteristic. When SOC is included, three node lines evolve into a pair of Dirac points along the $M-R$ line. These Dirac points are robust and protected by $C_{4}$ rotation symmetry. Once this crystal symmetry is broken, the Dirac points will be eliminated, and the system becomes a strong topological insulator., 6 pages, 3 figures

Superconductivity mediated by phonons is typically conventional, exhibiting a momentum-independent s-wave pairing function, due to the isotropic interactions between electrons and phonons along different crystalline directions. Here, by performing inelastic neutron scattering measurements on a superconducting single crystal of Sr0.1Bi2Se3, a prime candidate for realizing topological superconductivity by doping the topological insulator Bi2Se3, we find that there exist highly anisotropic phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only for those along the [001] direction. This observation indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic p-wave nematic superconducting pairing in the MxBi2Se3 (M = Cu, Sr, Nb) superconductor family. Therefore, we show these superconductors to be example systems where electron-phonon interaction can induce more exotic superconducting pairing than the s-wave, consistent with the topological superconductivity., Superconductivity mediated by phonons is usually conventional due to isotropic electron-phonon coupling. Here, Wang et al. report highly anisotropic phonons only along [001] direction in Sr0.1Bi2Se3, indicating a singular electron-phonon coupling which favors a p-wave nematic superconductivity scenario.

Applying linear response and the magnetic force theorem in correlated density functional theory, the inter-sublattice exchange constants of antiferromagnetic Eu are calculated and found to vanish near the pressure of P$_c$=82 GPa, just where magnetic order is observed experimentally to be lost. The Eu $4f^7$ moment remains unchanged at high pressure, again in agreement with spectroscopic measurements, leaving the picture of perfect frustration of interatomic Ruderman-Kittel-Kasuya-Yoshida couplings in a broad metallic background, leaving a state of electrons strongly exchange coupled to arbitrarily oriented, possibly quasistatic local moments. This strongly frustrated state gives way to superconductivity at T$_c$=1.7K, observed experimentally. These phenomena, and free energy considerations related to correlations, suggest an unusual phase of matter that is discussed within the scenarios of the Doniach Kondo lattice phase diagram, the metallic spin glass class, and itinerant spin liquid or spin gas systems., 6 pages, 4 figures

A search for new topological quantum systems is challenging due to the requirement of nontrivial band connectivity that leads to protected surface states of electrons. Progress in this field was primarily due to a realization of a band inversion mechanism between even and odd parity states that was proven to be very useful in both predicting many such systems and our understanding of their topological properties. Despite many proposed materials that assume the band inversion between $s$ and $p$ (or $p/d$, $d$/$f$) electrons, here, we explore a different mechanism where the occupied $d$ states subjected to a tetrahedral crystal field produce an active ${t}_{2g}$ manifold behaving as a state with an effective orbital momentum equal to $\ensuremath{-}1$, and pushing ${j}_{\mathrm{eff}}=1/2$ doublet at a higher energy. Via hybridization with nearest-neighbor orbitals realizable, e.g., in a zinc-blende structural environment, this allows a formation of odd parity state whose subsequent band inversion with an unoccupied $s$ band becomes possible, prompting us to look for the compounds with ${\mathrm{Cu}}^{+1}$ ionic state. Chemical valence arguments coupled to a search in the materials database of zinc-blende-like lattice space groups ${T}_{d}^{2}$ ($F\overline{4}3m$) lead us to systematically investigate electronic structures and topological properties of CuY ($Y=\text{F}$, Cl, Br, I) and $\mathrm{Cu}X\mathrm{O}$ ($X=\text{Li}$, Na, K, Rb) families of compounds. Our theoretical results show that CuF displays a behavior characteristic of an ideal Weyl semimetal with 24 Weyl nodes at the bulk Brillouin zone. We also find that other compounds, CuNaO and CuLiO, are the $s\text{\ensuremath{-}}d$ inversion-type topological insulators. Results for their electronic structures and corresponding surfaces states are presented and discussed in the context of their topological properties.

Weyl semimetals (WSMs), a new type of topological condensed matter, are currently attracting great interest due to their unusual electronic states and intriguing transport properties such as chiral anomaly induced negative magnetoresistance, a semi--quantized anomalous Hall effect and the debated chiral magnetic effect. These systems are close cousins of topological insulators (TIs) which are known for their disorder tolerant surface states. Similarly, WSMs exhibit unique topologically protected Fermi arcs surface states. Here we analyze electron--phonon scattering, a primary source of resistivity in metals at finite temperatures, as a function of the shape of the Fermi arc where we find that the impact on surface transport is significantly dependent on the arc curvature and disappears in the limit of a straight arc. Next, we discuss the effect of strong surface disorder on the resistivity by numerically simulating a tight binding model with the presence of quenched surface vacancies using the Coherent Potential Approximation (CPA) and Kubo--Greenwood formalism. We find that the limit of a straight arc geometry is remarkably disorder tolerant, producing surface conductivity that is a factor of 50 larger of a comparable set up with surface states of TI. Finally, a simulation of the effects of surface vacancies on TaAs is presented, illustrating the disorder tolerance of the topological surface states in a recently discovered WSM material., Comment: 16 pages, 10 figures

In the spirit of recently developed LDA+U and LDA+DMFT methods we implement a combination of density functional theory in its local density approximation (LDA) with a $k$- and $\omega -$dependent self-energy found from diagrammatic fluctuational exchange (FLEX) approximation. The active Hilbert space here is described by the correlated subset of electrons which allows to tremendously reduce the sizes of matrices needed to represent charge and spin susceptibilities. The method is perturbative in nature but accounts for both bubble and ladder diagrams and accumulates the physics of momentum resolved spin fluctuations missing in such popular approach as GW. As an application, we study correlation effects on band structures in V and Pd. The d-electron self-energies emergent from this calculation are found to be remarkably k-independent. However, when we compare our calculated electronic mass enhancements against LDA+DMFT, we find that for a long standing problem of spin fluctuations in Pd, LDA+FLEX delivers a better agreement with experiment, although this conclusion depends on a particular value of Hubbard $U$ used in the simulation. We also discuss outcomes of recently proposed combinations of k-dependent FLEX with DMFT., Comment: 9 pages, 3 figures

Using first-principles density functional calculations, we systematically investigate electronic structures and topological properties of layered materials $\mathrm{InNb}{X}_{2}$ ($X$=S, Se). In the absence of spin-orbit coupling (SOC), both compounds show nodal lines protected by mirror symmetry. Including SOC, the Dirac rings in ${\mathrm{InNbS}}_{2}$ split into two Weyl rings. This unique property is distinguished from other discovered nodal-line materials, which normally require the absence of SOC. On the other hand, SOC breaks the nodal lines in ${\mathrm{InNbSe}}_{2}$, and the compound becomes a type-II Weyl semimetal with 12 Weyl points in the Brillouin zone. Using a supercell slab calculation, we study the dispersion of Fermi arc surface states in ${\mathrm{InNbSe}}_{2}$; we also utilize a coherent potential approximation to probe their tolerance to the surface disorder effects. The quasi-two-dimensionality and the absence of toxic elements make these two compounds an ideal experimental platform for investigating novel properties of topological semimetals.

Z2 and Chern topological phases such as newly discovered quantum spin Hall and original quantum Hall states hardly both co–exist in a single material due to their contradictory requirement on the time–reversal symmetry (TRS). We show that although the TRS is broken in systems with a periodically driving field, an effective TRS can still be defined provided the ac–field is linearly polarized or certain other conditions are satisfied. The controllable TRS provides us a route to manipulate contradictory phases by tuning the polarization. To demonstrate the idea, we consider a tight-binding model that is relevant to several monolayered materials as a benchmark system. Our calculation shows not only topological Z2 to Chern phase transition occurs but rich Chern phases are also observed. In addition, we also discussed the realization of our proposal in real materials, such as spin-orbit coupled graphene and crystal Bismuth. This opens the possibility of manipulating various topological phases in a single material and can be a promising approach to engineer new electronic states of matter.

Multiferroics, where two or more ferroic order parameters coexist, is one of the hottest fields in condensed matter physics and materials science. To search multiferroics, currently most researches are focused on frustrated magnets, which usually have complicated magnetic structure and low magnetic ordering temperature. Here, we argue that actually simple interatomic magnetic exchange interaction already contains a driving force for ferroelectricity, thus providing a new microscopic mechanism for the coexistence and strong coupling between ferroelectricity and magnetism. We demonstrate this mechanism by showing that even the simplest antiferromagnetic insulator like MnO, could display a magnetically induced ferroelectricity under a biaxial strain. In addition, we show that such mechanism also exists in the most important single phase multiferroics, i.e. BiFeO3, suggesting that this mechanism is ubiquitous in systems with superexchange interaction.

We present a review of the basic ideas and techniques of the spectral density functional theory which are currently used in electronic structure calculations of strongly{correlated materials where the one{electron description breaks down. We illustrate the method with several examples where interactions play a dominant role: systems near metal{insulator transition, systems near volume collapse transition, and systems with local moments.

In the theoretical study of the origin of superconductivity in MgB2, the importance of the two dimensionality of the electronic structure has not been clear. Here we use the related system, and predicted superconductor, Li1−xBC to illustrate the importance the two dimensionality of the σ bands has for (1) the occurrence of high Tc, (2) the possibility of raising the critical temperature, and (3) the lattice instabilities that accompany strong electron–phonon coupling.

The search for other superconductors in the MgB2 class currently is focussed on Li{1-x}BC, which when hole-doped (concentration x) should be a metal with the potential to be a better superconductor than MgB2. Here we present the calculated phonon spectrum of the parent semiconductor LiBC. The calculated Raman-active modes are in excellent agreement with a recent observation, and comparison of calculated IR-active modes with a recent report provides a prediction of the LO--TO splitting for these four modes, which is small for the B-C bond stretching mode at ~1200 cm^{-1}, but large for clearly resolved modes at 540 cm^{-1} and 620 cm^{-1}.

The objective of the project was to develop a method for theoretical understanding of nuclear fuel materials whose physical and thermophysical properties can be predicted from first principles using a novel dynamical mean field method for electronic structure calculations. We concentrated our study on uranium, plutonium, their oxides, nitrides, carbides, as well as some rare earth materials whose 4f eletrons provide a simplified framework for understanding complex behavior of the f electrons. We addressed the issues connected to the electronic structure, lattice instabilities, phonon and magnon dynamics as well as thermal conductivity. This allowed us to evaluate characteristics of advanced nuclear fuel systems using computer based simulations and avoid costly experiments.

We introduce a theoretical framework for computaions of anisotropic multipolar exchange interactions found in many spin--orbit coupled magnetic systems and propose a method to extract these coupling constants using a density functional total energy calculation. This method is developed using a multipolar expansion of local density matrices for correlated orbitals that are responsible for magnetic degrees of freedom. Within the mean--field approximation, we show that each coupling constant can be recovered from a series of total energy calculations via what we call the ``pair--flip'' technique. This technique flips the relative phase of a pair of multipoles and computes corresponding total energy cost associated with the given exchange constant. To test it, we apply our method to Uranium Dioxide, which is a system known to have pseudospin $J=1$ superexchange induced dipolar, and superexchange plus spin--lattice induced quadrupolar orderings. Our calculation reveals that the superexchange and spin--lattice contributions to the quadrupolar exchange interactions are about the same order with ferro-- and antiferro--magnetic contributions, respectively. This highlights a competition rather than a cooperation between them. Our method could be a promising tool to explore magnetic properties of rare--earth compounds and hidden--order materials., 10 pages, 10 figures

A series of Cerium based heavy fermion materials is studied using a combination of local density functional theory and many-body Gutzwiller approximation. Computed orbial dependent electronic mass enhancements parameters are compared with available data extracted from measured values of the Sommerfeld coefficient. The Gutzwiller density functional theory is shown to remarkably follow the trends across a variety of Ce compounds, and to give important insights on orbital selective mass renormalizations that allows a better understanding of wide spread of data., 11 pages, 4 figures, 1 table

Using a theoretical approach combining the local spin density approximation (LSDA) of density functional theory and the Hubbard U term (LSDA 1 U), we analyse the connection between the experimentally observed electron energy loss spectra and elevated temperature scanning tunnelling images of surfaces of semiconducting nickel monoxide NiO and uranium dioxide UO2. We show that a combination of electron energy loss spectroscopy, atomic-resolution tunnelling imaging and first-principles ab initio calculations provides a powerful tool for studying electronic and structural properties of surfaces of transition metal and actinide oxides. q 2000 Elsevier Science Ltd. All rights reserved.

We argue that, for a square lattice and a nearly half-full band, the random-phase approximation (RPA) in general tends to give an instability towards antiferromagnetism with qAF=(π,π)/a, regardless of whether the Fermi surface (FS) is nested or not for this wave vector. Specifically, for a one-band model of YBa2Cu3O6+x, with its well-known nearly square, [10]-oriented FS, we find the real part of the Lindhard susceptibility to have a broad maximum at qAF for electron and hole-dopings up to about 10%. This hitherto overlooked result has implications for current electronic models of high-temperature superconductivity.

A variational formulation of the time--dependent linear response based on the Sternheimer method is developed in order to make practical ab initio calculations of dynamical spin susceptibilities of solids. Using gradient density functional and a muffin-tin-orbital representation, the efficiency of the approach is demonstrated by applications to selected magnetic and strongly paramagnetic metals. The results are found to be consistent with experiment and are compared with previous theoretical calculations., 11 pages, RevTex; 3 Figures, postscript, high-resolution printing (~1200dpi) is desired

We report density-functional calculations based on the local-density approximation (LDA) of the properties of doped barium bismuthates. Using the linear-response approach developed in the framework of the linear muffin-tin-orbital method the phonon spectrum of the ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{BiO}}_{3}$ system is calculated and is compared with the results of the neutron-diffraction measurements. The effect of doping in the calculation is modeled by the virtual crystal and mass approximations. The electron-phonon coupling constant $\ensuremath{\lambda}$ is then evaluated for a grid of phonon wave vectors using the change in the potential due to phonon distortion found self-consistently. A large coupling of the electrons to the bond-stretching oxygen vibrations and especially to the breathinglike vibrations is established. Also, a strongly anharmonic potential well is found for the tiltinglike motions of the oxygen octahedra. This mode is not coupled to the electrons to linear order in the displacements; therefore an anharmonic contribution to $\ensuremath{\lambda}$ is estimated using the frozen-phonon method. Our total (harmonic plus anharmonic) $\ensuremath{\lambda}$ is found to be 0.34. This is too small to explain high-temperature superconductivity in ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{BiO}}_{3}$ within the standard mechanism. Finally, based on standard LDA and LDA+$U$ like calculations, a number of properties of pure ${\mathrm{BaBiO}}_{3}$ such as tilting of the octahedra, breathing distortion, charge disproportionation, and semiconducting energy gap value is evaluated and discussed in connection with the negative-$U$ extended Hubbard model frequently applied to this compound.

Energy-dependent relaxation rates for the one-electron Green function and for the optical conductivity are calculated for a number of metals using electron-phonon spectral functions found ab initio . It is shown that the relaxation rates for these functions are very different in a broad interval of energies and temperatures. Implication of the results to the infrared spectroscopy of high- T c superconductors is discussed.

We compare experimentally observed electron energy loss spectra (EELS) of uranium dioxide UO 2 and nickel monoxide NiO with the results of ab-initio calculations carried out by using a method combining the local spin density approximation and the Hubbard U term (the LSDA + U method). We show that by taking better account of strong Coulomb correlations between electrons in the 5f shell of uranium ions in UO 2 and in the 3d shell of nickel ions in NiO it is possible to arrive at a better description of electron energy loss spectra, cohesive energies and elastic constants of both oxides compared with local spin density functional theory. For NiO we also compare the LSDA + U results and EELS spectra with a self-interaction corrected LSDA calculation.

We demonstrate how by taking better account of electron correlations in the $3d$ shell of metal ions in nickel oxide it is possible to improve the description of both electron energy loss spectra and parameters characterizing the structural stability of the material compared with local spin density functional theory.

We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum and electron--phonon coupling for newly discovered superconductor LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$. Significant similarity between LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ and LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is found, i.e. there is a strong Fermi surface nesting at ($\pi $,$\pi $,0), which results in unstable phonon branches. Combining the frozen phonon total energy calculations and an anharmonic oscillator model, we find that the quantum fluctuation prevents the appearance of static long--range order. The calculation shows that LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is highly anisotropic, and same as LaO$_{0.5}$F$_{0.5}$BiS$_{2}$, this compound is also a conventional electron-phonon coupling induced superconductor., Comment: 5 pages, 4 figures

A computer program LMTART for electronic structure calculations using full potential linear muffin-tin orbital method is described.

Understanding exotic, non s--wave--like states of Cooper pairs is important and may lead to new superconductors with higher critical temperatures and novel properties. Their existence is known to be possible but has always been thought to be associated with non--traditional mechanisms of superconductivity where electronic correlations play an important role. Here we use a first principles linear response calculation to show that in doped Bi$_{2}$Se$_{3}$ an unconventional p--wave--like state can be favored via a conventional phonon--mediated mechanism, as driven by an unusual, almost singular behavior of the electron--phonon interaction at long wavelengths. This may provide a new platform for our understanding superconductivity phenomena in doped band insulators., Published version

A new method of computing multipolar exchange interaction in spin-orbit coupled systems is developed using multipolar tensor expansion of the density matrix in LDA+U electronic structure calculation. Within mean-field approximation, exchange constants can be mapped into a series of total energy calculations by pair-flip technique. Application to Uranium dioxide shows an antiferromagnetic superexchange coupling in dipoles but ferromagnetic in quadrupoles which is very different from past studies. Further calculation of spin-lattice interaction indicates it is of the same order with superexchange and characterizes the overall behavior of quadrupolar part as a competition between them., 4 pages, 2 figures

We discuss the electronic structure, lattice dynamics, and electron-phonon interaction of the newly discovered superconductor LaO${}_{0.5}$F${}_{0.5}$BiS${}_{2}$ using density-functional-based calculations. A strong Fermi surface nesting at $\mathbf{k}=(\ensuremath{\pi},\ensuremath{\pi},0)$ suggests a proximity to charge-density-wave instability and leads to imaginary harmonic phonons at this $\mathbf{k}$ point associated with in-plane displacements of S atoms. Total energy analysis resolves only a shallow double-well potential well preventing the appearance of static long-range order. Both harmonic and anharmonic contributions to electron-phonon coupling are evaluated and give a total coupling constant $\ensuremath{\lambda}\ensuremath{\simeq}0.85$, prompting this material to be a conventional superconductor contrary to structurally similar FeAs materials.

Using density-functional linear-response theory, we calculate the electron-phonon interaction for {ital s}- and {ital d}-wave pairing in the 12{percent} hole doped infinite-layer compound CaCuO{sub 2}. We find {lambda}{sub {ital x}{sup 2}{minus}{ital y}{sup 2}}{approximately}0.3 to be positive and only slightly smaller than {lambda}{sub {ital s}}{approximately}0.4. This suggests that the electron-phonon mechanism {ital alone} is insufficient to explain the high {ital T}{sub {ital c}} but could {ital enhance} another {ital d}-wave pairing mechanism. Results of calculated lattice dynamics and transport properties are also presented and discussed. Out-of-plane distortions are found essential for the stability. {copyright} {ital 1996 The American Physical Society.}

We constructed computer-based simulations of the lattice dynamical properties of plutonium using an electronic structure method, which incorporates correlation effects among the f-shell electrons and calculates phonon spectra at arbitrary wavelengths. Our predicted spectrum for the face-centered cubic δ phase agrees well with experiments in the elastic limit and explains unusually large shear anisotropy of this material. The spectrum of the body-centered cubic phase shows an instability at zero temperature over a broad region of the wave vectors, indicating that this phase is highly anharmonic and can be stabilized at high temperatures by its phonon entropy.

A comprehensive investigation of the electronic and magnetic properties of NaOsO3 has been made using the first principle calculations, in order to understand the importance of Coulomb interaction, spin-orbit coupling and magnetic order in its temperature-induced and magnetic-related metal-insulator transition. It is found that its electronic structure near the Fermi energy is dominated by strongly hybridized Os 5d and O 2p states. Despite of the large strength of spin-orbit coupling, it has only small effect on the electronic and magnetic properties of NaOsO3. On the other hand, the on-site Coulomb repulsion affects the band structure significantly, but, a reasonable U alone cannot open a band gap. Its magnetism is itinerant, and the magnetic configuration plays an important role in determining the electronic structure. Its ground state is of a G-type antiferromagnet, and it is the combined effect of U and magnetic configuration that results in the insulating behavior of NaOsO3., accepted by PRB

We present the results of calculations for Pu and Am performed using an implementation of self-consistent relativistic GW method. The key feature of our scheme is to evaluate polarizability and self-energy in real space and Matsubara's time. We compare our GW results with the calculations using local density (LDA) and quasiparticle (QP) approximations and also with scalar-relativistic calculations. By comparing our calculated electronic structures with experimental data, we highlight the importance of both relativistic effects and effects of self-consistency in this GW calculation., Comment: 19 pages,10 figures

In 5d transition metal oxides such as the iridates, novel properties arise from the interplay of electron correlations and spin-orbit interactions. We investigate the electronic structure of the pyrochlore iridates, (such as Y$_{2}$Ir$_{2}$O$_{7}$) using density functional theory, LDA+U method, and effective low energy models. A remarkably rich phase diagram emerges on tuning the correlation strength U. The Ir magnetic moment are always found to be non-collinearly ordered. However, the ground state changes from a magnetic metal at weak U, to a Mott insulator at large U. Most interestingly, the intermediate U regime is found to be a Dirac semi-metal, with vanishing density of states at the Fermi energy. It also exhibits topological properties - manifested by special surface states in the form of Fermi arcs, that connect the bulk Dirac points. This Dirac phase, a three dimensional analog of graphene, is proposed as the ground state of Y$_{2}$Ir$_{2}$O$_{7}$ and related compounds. A narrow window of magnetic `axion' insulator, with axion parameter $\theta=\pi$, may also be present at intermediate U. An applied magnetic field induces ferromagnetic order and a metallic ground state.

Based on density functional calculation with LDA+U method, we propose that hypothetical Osmium compounds such as CaOs2O4 and SrOs2O4 can be stabilized in the geometrically frustrated spinel crystal structure. They also show some exotic electronic and magnetic properties in a reasonable range of on-site Coulomb correlation U such as ferromagnetism and orbital magnetoelectric effect characteristic to Axion electrodynamics. Other electronic phases including 3D Dirac metal and Mott insulator exist and would make perspective 5d spinels ideal for applications., 5 pages, 3 figures

We show by quantum Monte Carlo simulations of realistic Kondo lattice models derived from electronic--structure calculations that multiple quantum critical points can be realized in Plutonium--based materials. We place representative systems including PuCoGa5 on a realistic Doniach phase diagram and identify the regions where the magnetically mediated superconductivity could occur. Solution of an inverse problem to restore the quasiparticle renormalization factor for f-electrons is shown to be sufficiently good to predict the trends among Sommerfeld coefficients and magnetism. Suggestion on the possible experimental verification for this scenario is given for PuAs., 4 pages, 3 figures, 2 tables, references updated

We present realistic Kondo-lattice simulation results for the recently-discovered heavy-fermion antiferromagnet CePt2In7 comparing with its three-dimensional counterpart CeIn3 and the less two-dimensional ones, Ce-115’s. We find that the distance to the magnetic quantum critical point is the largest for CeIn3 and the smallest for Ce-115’s, and CePt2In7 falls in between. We argue that thetrend in quasi-two-dimensional materials stems from the frequency dependence of the hybridization between Cerium 4f-electrons and the conduction bands.

Given the practical importance of metallic plutonium, there is considerable interest1,2,3 in understanding its fundamental properties. Plutonium undergoes a 25 per cent increase in volume4 when transformed from its α-phase (which is stable below 400 K) to the δ-phase (stable at around 600 K), an effect that is crucial for issues of long-term storage and disposal. It has long been suspected that this unique property is a consequence of the special location of plutonium in the periodic table, on the border between the light and heavy actinides—here, electron wave–particle duality (or itinerant versus localized behaviour) is important5. This situation has resisted previous theoretical treatment. Here we report an electronic structure method, based on dynamical mean-field theory, that enables interpolation between the band-like and atomic-like behaviour of the electron. Our approach enables us to study the phase diagram of plutonium, by providing access to the energetics and one-electron spectra of strongly correlated systems. We explain the origin of the volume expansion between the α- and δ-phases, predict the existence of a strong quasiparticle peak near the Fermi level and give a new viewpoint on the physics of plutonium, in which the α- and δ-phases are on opposite sides of the interaction-driven localization–delocalization transition.