[BIOMECHANICAL EVALUATION OF PRIMARY REPLACEMENT ARTHOPLASTY FOR UNSTABLE INTERTROCHANTERIC FRACTURES].

Aim - to study peculiarities of changes of stress-strain state in the proximal femur around the implanted femoral component of hip prosthesis in case of inter-trochanteric fracture on the background of involutive osteoporosis in connection with functional loading by numeral analysis on a mathematical model. Using program SolidWorks 2 variants of finite element models of inter-trochanteric fractures were built, including type 31-A2, with the implantation of the femoral component of hip prosthesis - without restoration and with the restoration of the intertrochanteric region and calcar with a ring-shaped graft. The simulated load is single-support standing and the action of the musculo-ligamentous apparatus in the vertical position of the body. Stress analysis was performed at control points in comparison with the calculated data for the model with unaltered bone tissue (without osteoporosis). In the presence of osteoporosis, the values of stresses in the bone in the places of contact with the implant increase. For models with an osteoporotic inter-trochanteric fracture and an implanted femoral component of hip prosthesis, the maximum stress values are determined in the bone tissue of the proximal metaphysis, especially in the region of the destroyed posterior and medial walls. The simulated reconstruction of the intertrochanteric region with a ring-shaped graft with the restoration of the calcar demonstrates a significant decrease in the stress state at the level of the proximal metaphysis and in the area of predominant destruction of the bone marrow canal. This can contribute to the achievement of primary stability of the femoral component of hip prosthesis.

[MATHEMATICAL MODELING OF OPTIONS FOR MOUNTING AN EXTERNAL FIXATION DEVICE ON THE TIBIA DURING ITS LENGTHENING].

The main problem in the treatment of axial deformities and limb shortening in pediatric orthopedics is the presence of incomplete growth of the patient, which gives an unfavorable basis for the achieved correction. The most advanced technology for the treatment of deformities and limb shortening is the use of external fixation devices (AVF). Target. To study the stress-strain state of the "tibia-AVF model with different variants of AVF mounting in conditions of tibia lengthening. Mathematical modeling of tibial osteosynthesis with external fixation devices in children with achondroplasia was carried out. The model of the distal end of the lower limb contained both lower leg bones and bony elements of the foot. Two variants of the AVF imposition were modeled: with the arrangement of all rods in the same plane, and according to the scheme with a V-shaped arrangement of the rods in the proximal tibia. The values of maximum stresses and values of relative deformations in the elements of the models were investigated. Sweat is a compressive load, the zone of maximum stress (66.8 MPa) in the bone tissue occurs around the upper rod. The smallest stresses (22.8 MPa) are observed around the fourth bar. On the second and third rods, the stresses are the same - 41.6 and 41.5 MPa, respectively. The V-shaped arrangement of the rods leads to a decrease in stresses around the first rod to 57.2 MPa. The stresses in the distal part of the proximal tibia fragment do not exceed 35.5 MPa. Under torsional loads in the model with parallel arrangement of rods, the zone of maximum stresses arises around the first rod (16.3 MPa). The stresses in the distal fragment of the tibia are determined in the range from 1.2 to 3.9 MPa on the lower and upper rods, respectively. The use of an AVF with a V-shaped arrangement of the rods causes stresses in the tibia at a level of 13.1 MPa on the first rod, a minimum 1.1 MPa on the fourth. When the model is compressed with a parallel arrangement of the AVF rods, the values of relative deformations in the bone regenerate are determined in the range from 62 to 85%. The use of the AVF according to the scheme with a V-shaped arrangement of the rods makes it possible to reduce the magnitude of the relative deformations to the level of 77%. In the model with a parallel arrangement of the AVF rods, the relative deformations in the bone regenerate under torsional loads are determined in the range from 3.0 to 6.0%. When using AVF with a V-shaped arrangement of rods, the level of relative deformations is determined in the range from 1.0 to 4.0%. The use of an AVF with a V-shaped arrangement of rods in the lower part of the proximal fragment of the tibia during its osteosynthesis allows reducing the level of stresses at all control points of the model for all types of loading, as compared to the model of an AVF with a parallel arrangement of rods. AVF with a V-shaped arrangement of the rods makes it possible to reduce the values of the relative deformations of the bone regenerate at all the studied control points of the models, both under compression and torsion loads.

Nonmagnetic J=0 State and Spin-Orbit Excitations in K_{2}RuCl_{6}.

Spin-orbit Mott insulators composed of t_{2g}^{4} transition metal ions may host excitonic magnetism due to the condensation of spin-orbital J=1 triplons. Prior experiments suggest that the 4d antiferromagnet Ca_{2}RuO_{4} embodies this notion, but a J=0 nonmagnetic state as a basis of the excitonic picture remains to be confirmed. We use Ru L_{3}-edge resonant inelastic x-ray scattering to reveal archetypal J multiplets with a J=0 ground state in the cubic compound K_{2}RuCl_{6}, which are well described within the LS-coupling scheme. This result highlights the critical role of unquenched orbital moments in 4d-electron compounds and calls for investigations of quantum criticality and excitonic magnetism on various crystal lattices.

Robust weak antilocalization due to spin-orbital entanglement in Dirac material Sr3SnO.

The presence of both inversion (P) and time-reversal (T) symmetries in solids leads to a double degeneracy of the electronic bands (Kramers degeneracy). By lifting the degeneracy, spin textures manifest themselves in momentum space, as in topological insulators or in strong Rashba materials. The existence of spin textures with Kramers degeneracy, however, is difficult to observe directly. Here, we use quantum interference measurements to provide evidence for the existence of hidden entanglement between spin and momentum in the antiperovskite-type Dirac material Sr3SnO. We find robust weak antilocalization (WAL) independent of the position of EF. The observed WAL is fitted using a single interference channel at low doping, which implies that the different Dirac valleys are mixed by disorder. Notably, this mixing does not suppress WAL, suggesting contrasting interference physics compared to graphene. We identify scattering among axially spin-momentum locked states as a key process that leads to a spin-orbital entanglement.

Spin waves and spin-state transitions in a ruthenate high-temperature antiferromagnet.

Ruthenium compounds serve as a platform for fundamental concepts such as spin-triplet superconductivity1, Kitaev spin liquids2-5 and solid-state analogues of the Higgs mode in particle physics6,7. However, basic questions about the electronic structure of ruthenates remain unanswered, because several key parameters (including Hund's coupling, spin-orbit coupling and exchange interactions) are comparable in magnitude and their interplay is poorly understood, partly due to difficulties in synthesizing large single crystals for spectroscopic experiments. Here we introduce a resonant inelastic X-ray scattering (RIXS)8,9 technique capable of probing collective modes in microcrystals of 4d electron materials. We observe spin waves and spin-state transitions in the honeycomb antiferromagnet SrRu2O6 (ref. 10) and use the extracted exchange interactions and measured magnon gap to explain its high Néel temperature11-16. We expect that the RIXS method presented here will enable momentum-resolved spectroscopy of a large class of 4d transition-metal compounds.

Monoclinic SrIrO3-a Dirac semimetal produced by non-symmorphic symmetry and spin-orbit coupling.

SrIrO3 crystallizes in a monoclinic structure of distorted hexagonal perovskite at ambient pressure. The transport measurements show that the monoclinic SrIrO3 is a low-carrier density semimetal, as in the orthorhombic perovskite polymorph. The electronic structure calculation indicates a semimetallic band structure with Dirac bands at two high-symmetry points of Brillouin zone only when spin-orbit coupling is incorporated, suggesting that the semimetallic state is produced by the strong spin-orbit coupling. We argue that the Dirac bands are protected by the non-symmorphic symmetry of lattice.

Linear-in-Frequency Optical Conductivity in GdPtBi due to Transitions near the Triple Points.

The complex optical conductivity of the half-Heusler compound GdPtBi is measured in a frequency range from 20 to 22 000 cm^{-1} (2.5 meV-2.73 eV) at temperatures down to 10 K in zero magnetic field. We find the real part of the conductivity, σ_{1}(ω), to be almost perfectly linear in frequency over a broad range from 50 to 800 cm^{-1} (∼6-100 meV) for T≤50 K. This linearity strongly suggests the presence of three-dimensional linear electronic bands with band crossings (nodes) near the chemical potential. Band-structure calculations show the presence of triple points, where one doubly degenerate and one nondegenerate band cross each other in close vicinity of the chemical potential. From a comparison of our data with the optical conductivity computed from the band structure, we conclude that the observed nearly linear σ_{1}(ω) originates as a cumulative effect from all the transitions near the triple points.

Observation of the weak electronic correlations in KFeCoAs2 (3d 6): an isoelectronic to the parent compounds of 122 series of iron pnictides BaFe2As2.

Using the angle-resolved photoemission spectroscopy and band structure calculations we study the electronic structure of KFeCoAs2, which is isoelectronic to the parent material of 122 series of iron-based superconductors BaFe2As2. Although band structure calculations predict nearly identical dispersions of the electronic states in both compounds, experiment reveals drastic differences in both the global renormalization and Fermi surfaces. On the basis of the comparison of electronic structures of these two isoelectronic compounds, we demonstrate local magnetic correlations as a vital role for the peculiar low-energy electron dynamics of iron-based superconductors.

Effect of nematic ordering on electronic structure of FeSe.

Electronically driven nematic order is often considered as an essential ingredient of high-temperature superconductivity. Its elusive nature in iron-based superconductors resulted in a controversy not only as regards its origin but also as to the degree of its influence on the electronic structure even in the simplest representative material FeSe. Here we utilized angle-resolved photoemission spectroscopy and density functional theory calculations to study the influence of the nematic order on the electronic structure of FeSe and determine its exact energy and momentum scales. Our results strongly suggest that the nematicity in FeSe is electronically driven, we resolve the recent controversy and provide the necessary quantitative experimental basis for a successful theory of superconductivity in iron-based materials which takes into account both, spin-orbit interaction and electronic nematicity.

Weak-coupling superconductivity in a strongly correlated iron pnictide.

Iron-based superconductors have been found to exhibit an intimate interplay of orbital, spin, and lattice degrees of freedom, dramatically affecting their low-energy electronic properties, including superconductivity. Albeit the precise pairing mechanism remains unidentified, several candidate interactions have been suggested to mediate the superconducting pairing, both in the orbital and in the spin channel. Here, we employ optical spectroscopy (OS), angle-resolved photoemission spectroscopy (ARPES), ab initio band-structure, and Eliashberg calculations to show that nearly optimally doped NaFe0.978Co0.022As exhibits some of the strongest orbitally selective electronic correlations in the family of iron pnictides. Unexpectedly, we find that the mass enhancement of itinerant charge carriers in the strongly correlated band is dramatically reduced near the Γ point and attribute this effect to orbital mixing induced by pronounced spin-orbit coupling. Embracing the true band structure allows us to describe all low-energy electronic properties obtained in our experiments with remarkable consistency and demonstrate that superconductivity in this material is rather weak and mediated by spin fluctuations.

Interaction-induced singular Fermi surface in a high-temperature oxypnictide superconductor.

In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc ≈ 55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe(0.92)Co(0.08)AsO , is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors.

Quasi one dimensional Dirac electrons on the surface of Ru2Sn3.

We present an ARPES study of the surface states of Ru2Sn3, a new type of a strong 3D topological insulator (TI). In contrast to currently known 3D TIs, which display two-dimensional Dirac cones with linear isotropic dispersions crossing through one point in the surface Brillouin Zone (SBZ), the surface states on Ru2Sn3 are highly anisotropic, displaying an almost flat dispersion along certain high-symmetry directions. This results in quasi-one dimensional (1D) Dirac electronic states throughout the SBZ that we argue are inherited from features in the bulk electronic structure of Ru2Sn3 where the bulk conduction bands are highly anisotropic. Unlike previous experimentally characterized TIs, the topological surface states of Ru2Sn3 are the result of a d-p band inversion rather than an s-p band inversion. The observed surface states are the topological equivalent to a single 2D Dirac cone at the surface Brillouin zone.

Superconductivity-induced optical anomaly in an iron arsenide.

One of the central tenets of conventional theories of superconductivity, including most models proposed for the recently discovered iron-pnictide superconductors, is the notion that only electronic excitations with energies comparable to the superconducting energy gap are affected by the transition. Here, we report the results of a comprehensive spectroscopic ellipsometry study of a high-quality crystal of superconducting Ba0.68K0.32Fe2As2 that challenges this notion. We observe a superconductivity-induced suppression of an absorption band at an energy of 2.5 eV, two orders of magnitude above the superconducting gap energy 2Δ≈20 meV. On the basis of density functional calculations, this band can be assigned to transitions from As-p to Fe-d orbitals crossing the Fermi level. We identify a related effect at the spin-density wave transition in parent compounds of the 122 family. This suggests that As-p states deep below the Fermi level contribute to the formation of the superconducting and spin-density wave states in the iron arsenides.

Superconductivity without nesting in LiFeAs.

We have studied the electronic structure of the nonmagnetic LiFeAs (T(c)∼18 K) superconductor using angle-resolved photoemission spectroscopy. We find a notable absence of the Fermi surface nesting, strong renormalization of the conduction bands by a factor of 3, high density of states at the Fermi level caused by a van Hove singularity, and no evidence for either a static or a fluctuating order except superconductivity with in-plane isotropic energy gaps. Our observations suggest that these electronic properties capture the majority of ingredients necessary for the superconductivity in iron pnictides.

Two energy gaps and Fermi-surface "arcs" in NbSe2.

Using angle-resolved photoemission spectroscopy, we report on the direct observation of the energy gap in 2H-NbSe2 caused by the charge-density waves (CDW). The gap opens in the regions of the momentum space connected by the CDW vectors, which implies a nesting mechanism of CDW formation. In remarkable analogy with the pseudogap in cuprates, the detected energy gap also exists in the normal state (T>T0) where it breaks the Fermi surface into "arcs," it is nonmonotonic as a function of temperature with a local minimum at the CDW transition temperature (T0), and it forestalls the superconducting gap by excluding the nested portions of the Fermi surface from participating in superconductivity.

(pi, pi) electronic order in iron arsenide superconductors.

The distribution of valence electrons in metals usually follows the symmetry of the underlying ionic lattice. Modulations of this distribution often occur when those electrons are not stable with respect to a new electronic order, such as spin or charge density waves. Electron density waves have been observed in many families of superconductors, and are often considered to be essential for superconductivity to exist. Recent measurements seem to show that the properties of the iron pnictides are in good agreement with band structure calculations that do not include additional ordering, implying no relation between density waves and superconductivity in these materials. Here we report that the electronic structure of Ba(1-x)K(x)Fe(2)As(2) is in sharp disagreement with those band structure calculations, and instead reveals a reconstruction characterized by a (pi, pi) wavevector. This electronic order coexists with superconductivity and persists up to room temperature (300 K).

GdI2: a new ferromagnetic excitonic solid?

The two-dimensional, colossal magnetoresistive system GdI2 develops an unusual metallic state below its ferromagnetic transition and becomes insulating at low temperatures. We argue that this geometrically frustrated, correlated poor metal is a possible candidate for a ferromagnetic excitonic liquid. The renormalized Fermi surface supports a further breaking of symmetry to a charge-ordered, excitonic solid ground state at lower temperatures via order by disorder mechanism. Several experimental predictions are made to investigate this unique orbitally correlated ground state.

Pseudogap and charge density waves in two dimensions.

Using angle-resolved photoemission spectroscopy we demonstrate that a normal-state pseudogap exists above T(N-IC) in one of the most studied two-dimensional charge-density wave (CDW) dichalcogenides 2H-TaSe(2). The initial formation of the incommensurate CDW is confirmed as being driven by a conventional nesting instability, which is marked by a pseudogap. The magnitude, character, and anisotropy of the 2D-CDW pseudogap bear considerable resemblance to those seen in superconducting cuprates.

Photoemission insight into heavy-fermion behavior in YbRh2Si2.

As shown by angle-resolved photoemission (PE), hybridization of bulk Yb 4f(2+) states with a shallow-lying valence band of the same symmetry leads in YbRh2Si2 to dispersion of a 4f PE signal in the region of the Kondo resonance with a Fermi-energy crossing close to Gamma[over ]. Additionally, renormalization of the valence state results in the formation of a heavy band that disperses parallel to the 4f originating signal. The symmetry and character of the states are probed by circular dichroism and the photon-energy dependence of the PE cross sections.

X-ray magnetic circular dichroism in UGe(2): first-principles calculations.

The electronic structure and x-ray magnetic circular dichroism (XMCD) spectra of UGe(2) at the U N(4,5), N(2,3) and Ge K and L(2,3) edges are investigated theoretically from first principles, using the fully relativistic spin-polarized Dirac linear muffin-tin orbital (LMTO) band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as the LSDA+U method. The origin of the XMCD spectra in the compound is examined.