We propose an explanation for the appearance of superconductivity at the interfaces of graphite with Bernal stacking order. A network of line defects with flat bands appears at the interfaces between two slightly twisted graphite structures. Due to the flat band the probability to find high temperature superconductivity at these quasi one-dimensional corridors is strongly enhanced. When the network of superconducting lines is dense it becomes effectively two-dimensional. The model provides an explanation for several reports on the observation of superconductivity up to room temperature in different oriented graphite samples, graphite powders as well as graphite-composite samples published in the past.Comment: 4 pages, two figures, JETP Letters (September 2014, in press

The magnetic properties of CeRu 2 Si 2 at microkelvin temperatures ͑down to 170 K) and ultrasmall magnetic fields (0.02ϳ6.21 mT) are investigated experimentally. The simultaneously measured ac susceptibility and static magnetization show neither evidence of the magnetic ordering, superconductivity down to the lowest temperatures nor conventional Landau Fermi-Liquid behavior. The results imply the magnetic transition temperature in undoped CeRu 2 Si 2 is very close to absolute 0 K. The possibility for proximity of CeRu 2 Si 2 to the quantum critical point without any doping is discussed. DOI: 10.1103/PhysRevB.67.180407 PACS number͑s͒: 75.30.Cr, 71.10.Hf, 71.27.ϩa The unusual properties of heavy fermion ͑HF͒ systems are determined by the competition between intersite spin couplings, Ruderman-Kittel-Kasuya-Yosida interaction, and intrasite Kondo interaction. 1 In a system dominated by the Kondo effect, the Pauli paramagnetic ͑PP͒ state with massive quasiparticles is achieved through screening of the f electron's magnetic moments by conduction electrons below the characteristic temperature T K . The physical properties of the HF compounds below T K are well understood within the framework of the Landau Fermi-liquid ͑LFL͒ theory.Recently, however, non-Fermi-liquid ͑NFL͒ behavior was observed in a large class of HF compounds near the quantum critical point ͑QCP͒. 2,3 NFL systems exhibit anomalous temperature dependence of the physical quantities in contrast to the LFL theory, such as specific heat ⌬C/TϰϪln T, resistivity ⌬ ϰT ⑀ (1р⑀Ͻ2), and magnetic susceptibility ⌬ ϰ either 1ϪͱT or Ϫln T. In general, the quantum ͑zero-temperature͒ phase transition is driven by a control parameter other than temperature, for example, composition, pressure, or magnetic field, and is accompanied by a qualitative change in the correlations in the ground state. The second order quantum phase transitions and QCPs in HF systems can be classified into two types. ͑i͒ The longwavelength fluctuations of the order parameter are the only critical degrees of freedom and the quantum criticality is developed as spin-density wave instability, 4,5 here the zerotemperature spin fluctuations are given by the Gaussian fluctuations of the order parameter. ͑ii͒ Local critical modes coexist with long-wavelength fluctuations of the order parameter and there is non-Gaussian distribution of the fluctuations. 6 These are the so-called locally critical phase transitions where the quantum criticality of CeCu (6Ϫx) Au x ͑Ref. 7͒ and YbRh 2 Si 2 ͑Ref. 8͒ are regarded as type-͑ii͒ QCP. 6 CeRu 2 Si 2 with a ThCr 2 Si 2 -type crystal structure is well known to be a typical HF compound with an electronic specific-heat coefficient ␥ϭ350 mJ/K 2 mol below T K ϭ20 K. 9,10 This compound exhibits the pseudometamagnetic transition into the ferromagnetically ordered state induced by the magnetic field at H M ϭ7.8 T below 10 K. 11-15 The neutron-scattering measurements note short-range antiferromagnetic ͑AFM͒ correlations in CeRu 2 Si 2 even below T K . These time-fluctuati...

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Stabilizing nanoparticles on surfaces, such as graphene, is a growing field of research. Thereby, iron particle stabilization on carbon materials is attractive and finds applications in charge-storage devices, catalysis, and others. In this work, we describe the discovery of iron nanoparticles with the face-centered cubic structure that was postulated not to exist at ambient conditions. In bulk, the γ-iron phase is formed only above 917 °C, and transforms back to the thermodynamically favored α-phase upon cooling. Here, with X-ray diffraction and Mössbauer spectroscopy we unambiguously demonstrate the unexpected room-temperature stability of the γ-phase of iron in the form of the austenitic nanoparticles with low carbon content from 0.60% through 0.93%. The nanoparticles have controllable diameter range from 30 nm through 200 nm. They are stabilized by a layer of Fe/C solid solution on the surface, serving as the buffer controlling carbon content in the core, and by a few-layer graphene as an outermost shell.

The role of electronic correlation effects for a realistic description of the electronic properties of [Formula: see text]/[Formula: see text] heterostructures as covered by the on-site Coulomb repulsion within the GGA+U approach is investigated. Performing a systematic variation of the values of the Coulomb parameters applied to the Ti 3d and La 4f orbitals we put previous suggestions to include a large value for the La 4f states into perspective. Furthermore, our calculations provide deeper insight into the band gap landscape in the space spanned by these Coulomb parameters and the resulting complex interference effects. In addition, we identify important correlations between the local Coulomb interaction within the La 4f shell, the band gap, and the atomic displacements at the interface. In particular, these on-site Coulomb interactions influence buckling within the LaO interface layer, which via its strong coupling to the electrostatic potential in the LAO overlayer causes considerable shifts of the electronic states at the surface and eventually controls the band gap.

The FeTe parent compound for iron-superconductor chalcogenides was studied applying Mössbauer spectroscopy accompanied by ab initio calculations of electric field gradients at the iron nuclei. Room-temperature (RT) Mössbauer spectra of single crystals have shown asymmetric doublet structure commonly ascribed to contributions of overstoichiometric iron or impurity phases. Low-temperature Mössbauer spectra of the magnetically ordered compound could be well described by four hyperfine-split sextets, although no other foreign phases different from Fe 1.05 Te were detected by XRD and microanalysis within the sensitivity limits of the equipment. Density functional ab initio calculations have shown that over-stoichiometric iron atoms significantly affect electron charge and spin density up to the second coordination sphere of the iron sub-lattice, and, as a result, four non-equivalent groups of iron atoms are formed by their local environment. The resulting four-group model consistently describes the angular dependence of the single crystals Mössbauer spectra as well as intensity asymmetry of the doublet absorption lines in powdered samples at RT. We suppose that our approach could be extended to the entire class of Fe 1+y Se 1−x Te x compounds, which contain excess iron atoms.

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We investigate the effect of oxygen vacancies and hydrogen dopants at the surface and inside slabs of LaAlO3, SrTiO3, and LaAlO3/SrTiO3 heterostructures on the electronic properties by means of electronic structure calculations as based on density functional theory. Depending on the concentration, the presence of these defects in a LaAlO3 slab can suppress the surface conductivity. In contrast, in insulating SrTiO3 slabs already very small concentrations of oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the conduction band. Surface defects in insulating LaAlO3/SrTiO3 heterostructure slabs with three LaAlO3 overlayers lead to the emergence of interface conductivity. Calculated defect formation energies reveal strong preference of hydrogen dopant atoms for surface sites for all structures and concentrations considered. Strong decrease of the defect formation energy of hydrogen adatoms with increasing thickness of the LaAlO3 overlayer and crossover from positive to negative values, taken together with the metallic conductivity induced by hydrogen adatoms, seamlessly explains the semiconductor-metal transition observed for these heterostructures as a function of the overlayer thickness. Moreover, we show that the potential drop and concomitant shift of (layer resolved) band edges is suppressed for the metallic configuration. Finally, magnetism with stable local moments, which form atomically thin magnetic layers at the interface, is generated by oxygen vacancies either at the surface or the interface, or by hydrogen atoms buried at the interface. In particular, oxygen vacancies in the TiO2 interface layer cause drastic downshift of the 3d eg states of the Ti atoms neighboring the vacancies, giving rise to strongly localized magnetic moments, which add to the two-dimensional background magnetization.