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We establish the presence of topologically protected edge states on the (001) surface of HgS in the zinc-blende structure using density-functional electronic structure calculations. The Dirac point of the edge state cone is very close to the bulk valence band maximum. The Dirac cone is extremely anisotropic with a very large electron velocity along one diagonal of the surface elementary cell x' and a nearly flat dispersion in the perpendicular direction y'. The strong anisotropy originates from a broken fourfold rotoinversion symmetry at the surface., Comment: Phys. Rev. Lett., in press

The maximum isothermal entropy change in a magnetic refrigerant with a second-order phase transition is shown to depend on applied magnetic field $H$ as follows: $(\ensuremath{-}\ensuremath{\Delta}S){}_{\mathrm{max}}$ $=$ $A$($H$ $+$ ${H}_{0}$)${}^{2/3}$ -- ${\mathit{AH}}_{0}^{2/3}$ $+$ $\mathit{BH}$${}^{4/3}$. Here $A$ and $B$ are intrinsic parameters of the cooling material and ${H}_{0}$ is an extrinsic parameter determined by the purity and homogeneity of the sample. This theoretical prediction is confirmed by measurements on variously pure poly- and single-crystalline samples of Gd. The Curie point of pure Gd is found to be 295(1) K; however, the maximum of $\ensuremath{-}\ensuremath{\Delta}{S}_{\mathrm{M}}$ is attained at a lower temperature: The higher the quality of the sample, the closer the peak position to 295 K. Further tests are reported for a series of melt-spun LaFe${}_{13\ensuremath{-}x}$Si${}_{x}$ alloys. These are found to follow the same field dependence, despite the fact that for certain compositions ($x$ 1.8) they experience a phase transition of first, rather than second, order.

The charge distribution of RCo5 is calculated in the local spin density approximation (LSDA), and used to evaluate crystal field (CF) parameters for the 4f states at the rare-earth (R) site. The calculations are based on an optimized LCAO scheme for the conduction electron magnetism and on an 'open 4f core shell' treatment of the localized R 4f moments. The results are compared with the available experimental values and with data from independent density functional calculations. The LSDA calculations are found to yield the correct. sign for all CF parameters, but to overestimate the magnitude of the most essential second order parameter [Formula: see text]. It is shown, that neither a restriction on the lattice terms (as, e.g., in a point. charge calculation) nor an exclusion of these contributions is justified a priori.

The itinerant-state magnetocrystalline anisotropy energies (MAE) of RCo 5 (R=Y, La, Pr, Nd, Sm, Gd) have been determined by relativistic density-functional calculations in local spin density approximation, with additionally taking into account orbital polarization. The calculated MAEs are found to be strongly affected by changes of the lattice geometry ( c / a ratio and volume) resulting from (a) uniaxial strain in YCo 5 and (b) the lanthanide contraction along the RCo 5 series.

The band structure theory of magnetism in 3d-4f compounds is reviewed. Among the open-shell electrons, a hierarchy of interactions is present which governs the intrinsic magnetic properties of these materials. Density functional theory (DFT) is an appropriate tool to describe and quantitatively investigate both ground state properties and model interaction parameters, which are necessary to calculate the temperature-dependent behaviour. The cornerstones of the DFT are explicitly sketched, as well as recent developments needed to cope with the treatment of localized 4f and itinerant 3d magnetism in one and the same framework. This includes the open-core scheme, self-interaction corrected DFT, relativistic DFT, and orbital polarization. On this basis, the exchange coupling among itinerant and localized states can be understood, together with the size of Curie temperature and ground state spin and orbital magnetic moments. Finally, the problem of magnetocrystalline anisotropy is addressed, concerning both the band and the 4f crystal field contribution.

Density functional (DF) electronic structure calculations for UFeX (X=Si, Ge) compounds were performed using fully relativistic optimized linear combination of atomic orbitals (OLCAO), relativistic general potential linear augmented plane wave (LAPW) and scalar relativistic augmented sphere wave (ASW) methods. The 5f-states of uranium were assumed itinerant and treated as band states. We confirm the experimental observation that UFeGe is a paramagnet. On the other hand, our extensive DF calculations including the fixed-spin-moment method predict an instability of the paramagnetic state of UFeSi in the orthorhombic TiNiSi structure. Since both UFeSi and UFeGe are isoelectronic with respect to the number of valence electrons the actual ground states result from a subtle interplay between the band filling of the free electron background (s-, p-, and U-d electrons) and the iron bonding bands on one side and the covalency (hybridization) between bonding predominantly 3d states of iron and antibonding predominantly 5f-states of uranium on the other side.

We report the successful stabilization of a thick bulk-like, distorted $\alpha$-Mn film with (110) orientation on a W(110) substrate. The observed $(3\times3)$ overstructure for the Mn film with respect to the original W(110) low-energy electron diffraction pattern is consistent with the presented structure model. The possibility to stabilize such a pseudomorphic Mn film is supported by density functional total energy calculations. Angle-resolved photoemission spectra of the stabilized $\alpha$-Mn(110) film show weak dispersions of the valence band electronic states in accordance with the large unit cell., Comment: 11 pages; 5 figures

The Fermi surface response to external stress is studied by magnetoelastic quantum oscillations at single Pd crystals. The oscillatory magnetization and the magnetostriction of two crystals were measured by capacitive dilatometry at different low temperatures up to 32 T in static field. The experiment confirms the extremal cross sections of the Fermi surface for the L- and X-pockets. The combination of both methods allows to obtain the uniaxial stress dependence of the Fermi surface (using Shoenberg’s formalism). The procedure yields deformation response values in the range from 10 (TPa)−1 to 100 (TPa)−1, for stress along [100] and [111]. The new values for the [111] direction seem to have a stronger temperature dependence compared to other directions.

We report a density-functional study of the heteronuclear CoIr dimer adsorbed on benzene or graphene. In either case, CoIr prefers an upright position above the center of a carbon hexagon with the Co atom next to it. The Ir atom stays away from the carbon ring and thus preserves its free-atom-like properties. This results in a very large magnetic anisotropy of more than 0.2 eV per dimer. So high a value should suffice for long-term data storage at the temperature of liquid nitrogen.

We have undertaken theoretical studies of spin and orbital magnetic moments as well as magnetic anisotropy energies for M13 (M=Fe,Co,Ni) and M13Ptn(n=3,4,5,20) clusters including the spin-orbit coupling in the framework of density functional theory. For all M13 clusters considered we find tendencies for small structural distortions which can be characterized by either Jahn-Teller (JT) or Mackay transformations (MT). The magnetic anisotropy energy (MAE) along with the spin and orbital moments are calculated for M13 icosahedral clusters and the angle-dependent energy differences are modeled using a Néel model with local anisotropies. From our studies, the MAE for JT-distorted M13 clusters are found to be larger relative to the MT clusters and more than two orders of magnitude larger compared to the corresponding bcc or fcc bulk values. In addition, we demonstrate for one example that Pt capping may further enhance the MAE compared to the uncapped JT- and the Mackay-distorted Fe13 cluster., This work was supported by SFB 445 and SPP 1145.

Strained coherent film growth is commonly either limited to ultrathin films or low strains. Here, we present an approach to achieve high strains in thicker films, by using materials with inherent structural instabilities. As an example, 50 nm thick epitaxial films of the ${\mathrm{Fe}}_{70}{\mathrm{Pd}}_{30}$ magnetic shape memory alloy are examined. Strained coherent growth on various substrates allows us to adjust the tetragonal distortion from $c/{a}_{\mathrm{bct}}=1.09$ to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. Magnetometry and x-ray circular dichroism measurements show that the Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy change over broad ranges.

It is demonstrated by means of density functional and ab-initio quantum chemical calculations, that transition metal - carbon systems have the potential to enhance the presently achievable area density of magnetic recording by three orders of magnitude. As a model system, Co_2-benzene with a diameter of 0.5 nm is investigated. It shows a magnetic anisotropy in the order of 0.1 eV per molecule, large enough to store permanently one bit of information at temperatures considerably larger than 4 K. A similar performance can be expected, if cobalt dimers are deposited on graphene or on graphite. It is suggested that the subnanometer bits can be written by simultaneous application of a moderate magnetic and a strong electric field., 13 pages, 4 figures

The high-spin low-spin transition under pressure in CoO is investigated by the local spin-density approximation plus onsite Coulomb correlation $(\text{LSDA}+U)$ approach. The magnetic moment collapse is found to be caused by a competition between the ligand field and the intra-atomic exchange. The interplay of the interactions leads to a reduction in the symmetry below that of the antiferromagnetic order. This symmetry breaking causes a considerable reduction in the predicted moment-collapse transition pressure. For the multivalley functional of the $\text{LSDA}+U$ approach it is crucial to consider initial conditions of self-consistency of sufficiently low symmetry. In particular, if an imposed symmetry would enforce a metallic solution for $U$ of the order of the bandwidth, the commonly used symmetry breaking just by initial spin polarization may not be sufficient.

We have investigated the low-energy electronic structure of the heavy-fermion superconductor ${\text{CeCoIn}}_{5}$ by angle-resolved photoemission and band-structure calculations. We measured the Fermi surface and energy distribution maps along the high-symmetry directions at $h\ensuremath{\nu}=100\text{ }\text{eV}$ and $T=25\text{ }\text{K}$. The compound has quasi-two-dimensional Fermi-surface sheets centered at the $M\text{\ensuremath{-}}A$ line of the Brillouin zone. The band-structure calculations have been carried out within the local-density approximation where the $4f$ electrons have been treated either localized or itinerant. We discuss the comparison to the experimental data and the implications for the nature of the $4f$ electrons at the given temperature.

Ferromagnetic rare-earth-transition-metal (RE-TM) compounds are of particular interest in view of possible applications as permanent magnetic materials. The intrinsic magnetic properties (magnetic moment, magnetocrystalline anisotropy, and Curie temperature) of such materials are more or less accessible to ab initio studies. We argue that density functional calculations at the present state of the art should treat the RE 4f electrons as “open core shell”. In this frame, we calculated the ground state spin and orbital magnetic moments of several Sm-TM compounds in the CaCu5 structure. Our results agree well with available experimental and theoretical data and - if these are not available - with common experience.

Crystal field parameters of RFe 12 and RFe 8 M 4 have been computed on the basis of the charge density obtained from self-consistent spin-polarised electronic structure calculations using the scalar-relativistic version of the basis-optimised LCAO method, with the effective potential corrected for self-interaction.

The magnetic properties of Fe-Pd magnetic shape memory alloys with different Pd contents are investigated in epitaxial films and by means of density functional calculations. The magnetocrystalline anisotropy energy (MAE) of ordered and disordered Fe-Pd alloys is calculated as a function of $c/a$ ratio and Pd concentration. Disorder is found to be essential for the experimentally observed easy $a$-axis anisotropy in fct-${\text{Fe}}_{70}{\text{Pd}}_{30}$. The MAE of disordered fct-Fe-Pd not only increases with decreasing $c/a$ but also increases with increasing Pd concentration for fixed $c/a$. Epitaxial Fe-Pd films deposited on MgO (100) exhibit (with increasing Pd concentration from 25 to $37\text{ }\text{at}\text{.}\text{ }%$ Pd) bcc, bct, and fct structures at room temperature. Temperature dependent magnetic properties are studied by using hysteresis, saturation magnetization, and susceptibility measurements. The coercivity and MAE at room temperature in the fct films is higher than in bct and/or bcc films. Increasing MAE is observed with increasing Pd concentration within the fct phase. Due to the nanocscale microstructure, the magnetocrystalline anisotropy energy in the fct films is reduced compared to bulk. Furthermore, it is observed that the fct Fe-Pd films with $\text{Pd}l33\text{ }\text{at}\text{.}\text{ }%$ continuously transform to the bct phase during cooling. In contrast to bulk materials, this transformation is found to be reversible in thin films.

We report density functional calculations of the electronic structure, Fermi surface, and de Haas\char21{}van Alphen (dHvA) quantities of the $\mathrm{Pr}M{\mathrm{In}}_{5}$ ($M=\mathrm{Co}$, Rh, and Ir) compounds. Our investigation is carried out within the framework of the local density approximation, using a relativistic, full-potential band-structure method. A critical analysis of the electronic structures and the de Haas\char21{}van Alphen quantities is performed, which shows that good agreement with recent measurements is obtained when we assume the $\mathrm{Pr}\phantom{\rule{0.2em}{0ex}}4f$ states to be localized. The topology of the Fermi surface is calculated to be similar to that of non-$4f$ reference compounds, e.g., $\mathrm{La}\mathrm{Rh}{\mathrm{In}}_{5}$. The similarities of the Fermi surfaces and the dHvA extremal orbits among the compounds in the series are discussed. We, furthermore, compare our calculated effective masses with experimental measurements and discuss the differences between them.

High hydrostatic pressure destroys the strong ferromagnetism in $\mathrm{Y}{\mathrm{Co}}_{5}$. The transformation proceeds in a stepwise fashion, as a first-order phase transition $({P}_{\mathrm{crit}}=18\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$, and is accompanied by an isomorphic (without change of symmetry) lattice collapse and a topological change of the Fermi surface. Accurate full-potential density-functional calculations enable us to ascribe these phenomena to a quasi-one-dimensional $3d$-like band, whose top under ambient pressure is situated $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the Fermi level. Lattice compression drives the top of the nondispersive band---and the associated peak in the majority density of states---upward in energy, until they reach the Fermi level and the system becomes unstable. A similar transition at a somewhat higher pressure $(23\phantom{\rule{0.3em}{0ex}}\mathrm{GPa})$ is predicted for the isomorphous compound $\mathrm{La}{\mathrm{Co}}_{5}$.

Magnetic shape memory (MSM) alloys, which transform martensitically below the Curie temperature in the ferromagnetic (FM) state, represent a new class of actuators. In Ni2MnGa, unusually large magnetic field-induced strains of about 10% have been observed. This effect is related to a high mobility of martensitic twin boundaries in connection with a large magneto-crystalline anisotropy. MSM materials exist in a variety of different martensitic structures depending on temperature and compositions. We investigate the energetics of L10 phase twin boundary motion quasi-statically with ab initio methods and relate the results to calculations of the magneto-crystalline anisotropy energy. Our results indicate that for the L10 structure the energy needed for a coherent shift of a twin boundary may be too large to be overcome solely by magnetic field-induced strains.

The electronic structure and magnetic properties of disordered Fe$_{x}$Pd$_{100-x}$ alloys $(50 < x < 85)$ are investigated in the framework of density functional theory using the full potential local orbital method (FPLO). Disorder is treated in the coherent potential approximation (CPA). Our calculations explain the experimental magnetization data. The origin of the tetragonal distortion in the Fe-Pd magnetic shape memory alloys is found to be a Jahn-Teller like effect which allows the system to reduce its band energy in a narrow composition range. Prospects for an optimization of the alloys' properties by adding third elements are discussed.

Electronic structure calculations reveal the presence of several shallow minima and maxima in the energy-vs-magnetization curves, which otherwise are surprisingly flat. The main implication---a fast magnetization and/or demagnetization process with little hysteresis---is of primary importance for the performance of ${\mathrm{LaFe}}_{13\ensuremath{-}x}{\mathrm{Si}}_{x}$ in magnetic cooling devices.

Topological changes of the Fermi surface under pressure may cause anomalies in the low-temperature elastic properties. Our density functional calculations for elemental Osmium evidence that this metal undergoes three such Lifshitz transitions in the pressure range between 70 GPa and 130 GPa. The related elastic anomalies are, however, invisibly weak. The critical pressures considerably exceed the values for recently measured and calculated anomalies in the pressure (P) dependence of the hexagonal c/a lattice parameter ratio close to 25 GPa. We demonstrate that the latter anomalies are statistically not significant and that (c/a)(P) can be fitted equally well by a smooth dependence., 8 pages, 8 figures

Angle-resolved photoemission (PE) study of thin films of Cr grown on Fe(100) reveals thickness-dependent short-period oscillations of the PE intensity close to the Fermi energy at ${\mathbf{k}}_{\ensuremath{\Vert}}\ensuremath{\sim}0$. The oscillations are assigned to quantum-well states (QWS) caused by the nesting between the Fermi-surface sheets around the $\ensuremath{\Gamma}$ and the $X$ points in the Brillouin zone of antiferromagnetic Cr. The experimental data are confirmed by density-functional calculations applying a screened Korringa-Kohn-Rostoker Green's function method. The period of the experimentally observed QWS oscillations amounts to about 2.6 monolayers and is larger than the fundamental 2-monolayer period of antiferromagnetic coupling in Cr.

An upturn in the high-field magnetization curve of a ${\mathrm{GdCo}}_{5}$ single crystal is observed at about 46 T, which is interpreted as the onset of a process of spin reorientation from the ferrimagnetic towards ferromagnetic structure via an intermediate canted phase. This enables us to evaluate the Co-Gd exchange field in ${\mathrm{GdCo}}_{5}$ as ${B}_{TR}=233\ifmmode\pm\else\textpm\fi{}12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. This value is very close to the total exchange field on Gd, ${B}_{R}=236\ifmmode\pm\else\textpm\fi{}8\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, known from earlier inelastic neutron experiments. The difference ${B}_{RR}={B}_{R}\ensuremath{-}{B}_{TR}$ is attributed to the Gd-Gd exchange interaction. Its smallness provides solid experimental justification for neglecting the $4f\text{\ensuremath{-}}4f$ exchange in $3d\text{\ensuremath{-}}4f$ hard magnetic materials. A full-potential density-functional calculation of ${B}_{TR}$ has been carried out using an approach within the fixed-spin-moment (FSM) technique. The calculated value ${B}_{TR}=258\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is 11% too high, most likely because the FSM formalism disregards orbital magnetic moments.

We have performed scanning tunneling spectroscopy (STS) investigations on clean metallic uranium epitaxially grown on the (110) surface of a tungsten single crystal at room temperature. This system is known to exhibit an hcp phase with the band structure close to the Fermi level dominated by $5f$ states. The STS spectra are found to be in good agreement with the results of relativistic band structure calculations based on the local density approximation. The calculated density of states of the Uranium film indicates that the features in the STS spectra are related to $5f$-states.

Analyzing the phenomenon of folded bands in photoemission spectra of solids, we have disclosed the coexistence of two different representations of two-dimensional (2D) Brillouin zones for occupied graphite- and intercalant-derived electronic states in a La-graphite intercalation compound. These two representations are characterized by separated 2D Fermi-surface areas, which are related to the graphene and intercalant layer, respectively. Each individual area shows symmetry properties of the respective layer. Our fundamental finding is of high importance for deeper understanding of quasi-two-dimensional electronic properties in layered structures.

Magnetic and specific‐heat studies of U2T2X compounds show a frequent occurrence of the γ enhancement in conjunction with the onset of antiferromagnetic ordering. The largest value of 830 mJ/mol K2 was observed in U2Pt2In, which is nonmagnetic down to 1.2 K. Variations of electronic structure are documented by optimized relativistic LCAO calculation.

Coherent epitaxial growth allows to produce crystalline films with strained structures which are unstable in the bulk. Thereby, the relationship between the lattice parameters of the overlayer in the interface plane, $(a,b)$, and its minimum-energy out-of-plane lattice parameter, $c_{\text{min}}(a,b)$, need not be continuous. This general statement is proven by examples of total energy calculations. As a consequence, $c_{\text{min}}$, which is determined by the choice of the substrate, and $c_{\text{min}}$-dependent intrinsic properties of the overlayer cannot always be tuned in a continuous way as one may aim to do by means of strained epitaxy. Employing the model of the epitaxial Bain path we predict that such discontinuities occur in films of the elements V, Nb, Ru, La, Os, and Ir. The abrupt change of $c_{\text{min}}$ can be exploited to switch properties specific to the overlayer material. This is demonstrated for the example of the superconducting critical temperature of a V film which we predict to jump by 20% at a discontinuity of $c_{\text{min}}$

A technique of determining the exchange field ${B}_{\mathrm{ex}}$ on the $4f$ shell of Sm atoms in Sm-based magnets is proposed. It makes use of the $4f$ intermultiplet transition in Sm, observed in inelastic neutron scattering (INS) experiments. The method is used to analyze previously published data for a number of Sm-Fe and Sm-Co intermetallics, for all of which ${B}_{\mathrm{ex}}$ is determined. Additional information on intramultiplet transitions in ${\mathrm{SmCo}}_{5}$ and ${\mathrm{Sm}}_{2}{\mathrm{Co}}_{17}$ makes it possible to obtain more accurate ${B}_{\mathrm{ex}}$ values as well as to estimate the leading crystal field parameter (CFP) ${A}_{2}^{0}$ for these compounds. For the same systems an independent determination of ${A}_{2}^{0}$ is carried out using published magnetization curves and the ${B}_{\mathrm{ex}}$ values found from the INS spectra. The two ``experimental'' values of ${A}_{2}^{0}$ (INS and magnetization) agree well. For comparison, theoretical Sm-Co exchange fields and CFP for ${\mathrm{SmCo}}_{5}$ and ${\mathrm{Sm}}_{2}{\mathrm{Co}}_{17}$ are obtained from full-potential density-functional calculations. The theoretical ${A}_{2}^{0}〈{r}^{2}〉$ are shifted toward more negative values with respect to their experimental counterparts by a few millielectronvolts. The calculated Sm-Co exchange fields are in fair agreement with the experimentally determined values of the total exchange field on Sm, ${B}_{\mathrm{ex}},$ the weak Sm-Sm exchange interaction being accountable for the remaining small discrepancies.

A peak is detected in the high-energy inelastic neutron scattering spectra of ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ and ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}{\mathrm{N}}_{3},$ which is associated with the intermultiplet transition in the $4f$ shell of samarium. The peak in the nitride ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}{\mathrm{N}}_{3}$ is situated at a lower energy (163 meV) as compared with the parent compound ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ (177 meV). The peak position provides direct information on the strength of the exchange field on Sm in both compounds: ${B}_{\mathrm{ex}}=380$ and 270 T in ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ and ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}{\mathrm{N}}_{3},$ respectively. The 30% reduction in ${B}_{\mathrm{ex}}$ as a result of nitrogenating ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}$ is consistent with the earlier discovered similar effect in ${\mathrm{Gd}}_{2}{\mathrm{Fe}}_{17}.$ High-field $(Bl~50\mathrm{T})$ magnetization curves have been measured on an oriented powder sample of ${\mathrm{Sm}}_{2}{\mathrm{Fe}}_{17}{\mathrm{N}}_{3}$ and analyzed using the newly determined ${B}_{\mathrm{ex}}.$ The leading crystal field parameter is thus found to be ${A}_{2}^{0}〈{r}^{2}〉=\ensuremath{-}28\mathrm{meV}.$

The charge distribution of rare-earth transition-metal (RE-TM) compounds is calculated in the local-spin-density approximation (LSDA) and used to analyze the various contributions to crystal field (CF) parameters for the 4f states at the RE site. The LSDA calculations are found to yield the correct sign for all CF parameters but to overestimate the magnitude of the most essential second-order parameter A02. It is shown that the charge distribution up to a distance of 100 a.u. from the RE atom has to be taken into consideration for calculating A02 and that neither a restriction on the lattice terms (as e.g. in a point charge calculation) nor an exclusion of these contributions is justified a priori. Furthermore, the value of A02 is extremely sensitive to changes in occupation among the Re 5d orbitals.

We present an angle-resolved photoemission (PE) and angle-resolved resonant PE study of a single crystalline U-graphite intercalation compound (U-GIC). Stage-2 U-GIC samples were prepared in situ, by deposition of U metal onto clean graphite (0001) substrates and subsequent annealing. As follows from comparison with the results of local-density approximation (LDA)-linear combination of atomic orbitals (LCAO) band-structure calculations, the electronic structure of the grown GIC may be understood by the filling of unoccupied graphite ${\ensuremath{\pi}}_{0}^{*}$ bands by additional electrons contributed by U. This charge transfer is monitored by a shift to higher binding energies of all graphite-derived PE features and the appearance of an additional rather narrow feature at the Fermi energy $({\mathrm{E}}_{F}),$ upon intercalation. This Fermi-level peak is superimposed by a sharp U $5f$ signal, that is located directly at ${\mathrm{E}}_{F}$ and reveals no trace of multiplet splitting or dispersion. Results of band-structure calculations indicate a weak hybridization of the U $5f$ states and their occupation of about 2. A description of the observed $5f$-related spectra in the framework of a single-impurity Anderson model is possible.

A polycrystalline sample of the MgB_2 superconductor was investigated by measurements of the electrical resistivity, the thermopower and the thermal conductivity in the temperature range between 1.8K and 300K in zero magnetic field. The electrical resistivity shows a superconducting transition at T_c=38.7K and, similarly to borocarbides, a T^2.4 behaviour up to 200K. The electron diffusion thermopower and its bandstructure-derived value indicate the dominant hole character of the charge carriers. The total thermopower can be explained by the diffusion term renormalized by a significant electron-phonon interaction and a phonon drag term. In the thermal conductivity, for decreasing temperature, a significant decrease below T_c is observed resulting in a T^3 behaviour below 7K. The reduced Lorenz number exhibits values smaller than 1 and a characteristic minimum which resembles the behaviour of non-magnetic borocarbides., 7 pages, 5 figures; added references and minor changes; accepted for publication in Physica C

The contribution of the itinerant states to the magnetocrystalline anisotropy (MA) energy of ${\mathrm{YCo}}_{5}$ and isostructural compounds has been calculated using a fully relativistic optimized LCAO band-structure scheme within the framework of density-functional theory in local spin density approximation (LSDA), and its dependence on lattice geometry and Fe substitution has been investigated. Additionally taking into account orbital polarization, a correction to LSDA accounting for Hund's second rule, enhances the calculated orbital moments, orbital moment anisotropies and MA energies, and leads to good agreement with available experimental data for ${\mathrm{YCo}}_{5}.$ The MA energies are found to be strongly affected by changes of the lattice geometry $(c/a$ ratio and volume) resulting from (i) uniaxial strain in ${\mathrm{YCo}}_{5}$ and (ii) the lanthanide contraction along the $R{\mathrm{Co}}_{5}$ $(R=\mathrm{Y},$ La, Pr, Nd, Sm, Gd) series, because of the sensitivity of the MA energy to changes of the band structure. We obtain a large variation of the MA energy of $R{\mathrm{Co}}_{5}$ along the R series which is shown to be predominantly a lattice geometry effect. It is in contrast to the commonly assumed independence of the transition-metal sublattice MA on the R constituent. The calculated band-filling dependence of the MA energies of ordered $\mathrm{Y}({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{)}_{5}$ compounds $(x=0,0.4,0.6,1.0)$ qualitatively explains the experimentally observed concentration dependence of the MA energy in $\mathrm{Y}({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}{)}_{5}$ pseudobinaries at low Fe concentrations.

Purpose: The EndoPearl (Linvatec, Largo, FL), a biodegradable device to augment the femoral interference screw fixation of hamstring tendon grafts has been developed. The first objective of this study was to compare the initial fixation strength of quadrupled hamstring tendons and biodegradable interference screw fixation with and without the application of the EndoPearl device. The second objective was to determine the influence of the EndoPearl device on the fatigue behavior under incremental cyclic loading conditions in a simulation of critical fixation conditions. Type of Study: Biomechanical study. Methods: Fresh human hamstring tendons were harvested and grafts were fixed with biodegradable poly-L-lactide interference screws. Twenty proximal calf tibias were used to compare the initial fixation strength of the study and the control group. In the study group, the EndoPearl device was secured to the graft using two No. 5 Ethibond sutures (Ethicon, Somerville, NJ). Specimens were loaded until failure in a materials testing machine. For cyclic testing, human hamstring tendons and 20 distal porcine femurs were used. Critical graft fixation conditions were simulated by increasing tunnel diameter 2 mm over the graft diameter. Grafts were loaded progressively in increments of 100 N until failure; 100 cycles were applied per load increment. Results: Graft fixation with the additional EndoPearl device had a significantly higher maximum load to failure (658.9 ± 118.1 N v 385.9 ± 185.6 N, P =.003) and stiffness (41.7 ± 11 N/mm v 25.7 ± 8.5 N/mm). Graft fixation with the EndoPearl device sustained a significant higher total number of cycles (388.5 ± 125.6) compared with the control group (152.8 ± 144.9, P =.002). Conclusions: We demonstrated that the augmentation of a hamstring tendon graft with the EndoPearl device increases interference screw fixation strength significantly. Under dynamic loading conditions, specimens of the study group sustained substantially higher loads and a larger number of cycles, which indicates a greater resistance to graft slippage. The application of the EndoPearl device may also allow for a secure soft-tissue graft fixation with interference screws in cases of critical fixation conditions. Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 17, No 4 (April), 2001: pp 353–359

Thin films of UGa2 and UGa3 were prepared by sputter deposition on a Si substrate and studied by photoemission spectroscopy (XPS and UPS) and by ion scattering spectroscopy (ISS). The films were prepared from very small samples (less than 100 mg) and are practically oxygen-free up to high temperatures. The study of the electronic structure of clean stoichiometric films revealed itinerant character of the 5f electronic states in both compounds. By varying the deposition temperature, we could modify the surface composition in the case of UGa3. At high temperatures, the surface was entirely covered by a mostly Ga overlayer.

Publisher Summary Metallic systems containing rare earth or actinide elements exhibit a particularly rich variety of magnetic properties and phenomena that are caused by the incompletely filled 4f or 5f shells, respectively. The elemental metal dysprosium possesses the largest saturation magnetic moment per unit mass among all elements, being about 60% higher than that of iron. On the other hand, it is possible to obtain a ferromagnetic system with vanishing magnetization, if the compound SmA1 2 is doped with small amounts of Gd. Special attention is paid to new developments in the field, but the presentation is based on a broad tutorial discussion. In particular, density functional theory (DFT) is introduced on a level that will allow the reader to judge the most important details of the presented calculations.

It is shown that valence-band photoemission (PE) spectra of epitaxial hcp films of U metal may be understood within a one-step model of PE based on an itinerant description of the U $5f$ states. Particularly, the PE intensities of the $f$-derived bands are correctly reproduced for photon energies between 60 and 103 eV. It is found that a cross-section minimum of the Fermi-energy feature at $h\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}90--94\mathrm{eV}$, which previously was assigned to a Fano antiresonance at the $5d\ensuremath{\rightarrow}5f$ excitation threshold, is mainly caused by a Cooper minimum. Analogous $5f$ cross-section variations in the region of the $5d\ensuremath{\rightarrow}5f$ resonances were obtained for $\mathrm{UPd}{}_{3}$ and Pu metal.

An angle-resolved photoemission (PE) study is performed on graphite intercalation compounds (GIC's) with Eu and Yb prepared in situ by deposition of the pure metals onto single-crystalline graphite(0001) surfaces and subsequent annealing. The low-energy-electron-diffraction patterns reveal a sharp (\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3)R30\ifmmode^\circ\else\textdegree\fi{} overstructure as observed for other rare-earth (RE) and alkali GIC's. The PE data are in excellent agreement with local-density-approximation band-structure calculations for stage-2 GIC's that indicate charge transfer of about 0.5 electrons per RE atom into graphite-derived states of \ensuremath{\pi} symmetry. This charge transfer is caused by an energetical lowering of the graphite-derived \ensuremath{\pi} bands and leads to the appearance of an intense Fermi-level structure in the PE spectra taken around the K(${\mathrm{\ensuremath{\Gamma}}}^{\ensuremath{'}}$) point of the Brillouin zone. Admixtures of rare-earth-derivedd states are found close to the Fermi energy in the region of the \ensuremath{\Gamma} and the ${\mathit{M}}^{\ensuremath{'}}$ points. \textcopyright{} 1996 The American Physical Society.

The compound LaCrSb3 is a non-collinear ferromagnet with a spin reorientation at T sr ≈95 K . Coexistence of itinerant and localized spins at one and the same Cr site was suggested to explain the experimental findings. Here, arguments against this model are provided on the basis of electronic structure calculations and re-considered experimental information. An alternative model is proposed.

We have investigated the spontaneous anisotropic magnetostriction of element hcp Gd metal in two different approximations to density functional theory, the local spin density approximation (LSDA) and the so-called LSDA+ U . As a rough model for the paramagnetic state with large persisting 4f-moments several static AFM arrangements were used. The obtained value of spontaneous volume magnetostriction is reasonable, but the sign of the anisotropic magnetostriction is different in LSDA and LSDA+ U .

The electronic structure of thin Cr films on Fe/W(1 0 0) and W(1 0 0) has been studied by angle-resolved photoemission (PE). The data have been compared with results of density functional supercell calculations. A thickness-dependent PE intensity variation at the Fermi level is observed, which is assigned to Cr quantum-well-like states.