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Search Results: 1 - 10 of 239392 matches for " N. Y. Shitsevalova "
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Characterization of the electronic properties of YB$_4$ and YB$_6$ using $^{11}$B NMR and first-principles calculations
B. J?ger,S. Paluch,W. Wolf,P. Herzig,O. J. ?oga?,N. Shitsevalova,Y. Paderno
Physics , 2003,
Abstract: Two compounds, tetragonal YB$_4$ and cubic YB$_6$, have been investigated by electric-field gradient (EFG) and Knight shift measurements at the boron sites using the $^{11}$B nuclear magnetic resonance (NMR) technique and by performing first-principles calculations. In YB$_6$ $^{11}$B ($I=3/2$) NMR spectra reveal patterns typical for an axially symmetric field gradient with a quadrupole coupling frequency of $\nu_Q=600\pm15$ kHz. In the second boride (YB$_4$) three different EFGs were observed corresponding to the three inequivalent crystallographic sites for the boron atoms ($4h$, $4e$, and $8j$). They correspond to: $\nu_Q(4h)=700\pm30$ kHz with an asymmetry parameter $\eta=0.02\pm0.02$, $\nu_Q(4e)=515\pm30$ kHz, $\eta=0.00+0.02/-0.00$, and $\nu_Q(8j)=515\pm40$ kHz, $\eta=0.46\pm0.08$. The Knight shifts measured by Magic-Angle Spinning (MAS) NMR at room temperature are very small being $0.6\pm8$ ppm and $-1\pm8$ ppm for YB$_4$ and YB$_6$, respectively. For the theoretical calculations structure optimizations were performed as a first step. For the obtained structural parameters the EFGs were computed within the local-density approximation. Very satisfactory agreement between experimental and theoretical results is obtained both for the structural parameters and the B EFGs thus confirming the underlying structural models. In addition to the EFGs, band structures, densities of states, and valence-electron densities are presented and the bonding situation in the two yttrium borides is discussed. The band-structure results are compatible with the very low values for the Knight shifts mentioned above.
Neutron scattering on HoB$_{12}$ - short-range correlations above T$_{N}$
K. Siemensmeyer,K. Flachbart,S. Gabani,S. Matas,Y. Paderno,N. Shitsevalova
Physics , 2006,
Abstract: Neutron scattering experiments on frustrated fcc - antiferromagnet HoB$_{12}$ have been performed above T$_{N}$ = 7.4 K. Diffuse scattering patterns indicate that above Neel temperature correlations between neighboring magnetic moments of Ho - ions appear, similar to the appearance of coupled spin-pairs (dimers) in low dimensional magnets. Moreover, the diffuse patterns show a symmetry reduction from fcc to simple cubic. Analogous behavior in 3D systems is not known, although it was predicted by theory. The role of various interactions leading to this behavior is being analyzed and discussed.
Surface properties of SmB6 from x-ray photoelectron spectroscopy
N. Heming,U. Treske,M. Knupfer,B. Büchner,D. S. Inosov,N. Y. Shitsevalova,V. B. Filipov,S. Krause,A. Koitzsch
Physics , 2015, DOI: 10.1103/PhysRevB.90.195128
Abstract: We have investigated the properties of cleaved SmB$_6$ single crystals by x-ray photoelectron spectroscopy. At low temperatures and freshly cleaved samples a surface core level shift is observed which vanishes when the temperature is increased. A Sm valence between 2.5 - 2.6 is derived from the relative intensities of the Sm$^{2+}$ and Sm$^{3+}$ multiplets. The B/Sm intensity ratio obtained from the core levels is always larger than the stoichiometric value. Possible reasons for this deviation are discussed. The B $1s$ signal shows an unexpected complexity: an anomalous low energy component appears with increasing temperature and is assigned to the formation of a suboxide at the surface. While several interesting intrinsic and extrinsic properties of the SmB$_6$ surface are elucidated in this manuscript no clear indication of a trivial mechanism for the prominent surface conductivity is found.
Superconductivity mediated by a soft phonon mode: specific heat, resistivity, thermal expansion and magnetization of YB6
R. Lortz,Y. Wang,U. Tutsch,S. Abe,C. Meingast,P. Popovich,W. Knafo,N. Shitsevalova,Yu. B. Paderno,A. Junod
Physics , 2005, DOI: 10.1103/PhysRevB.73.024512
Abstract: The superconductor YB6 has the second highest critical temperature Tc among the boride family MBn. We report measurements of the specific heat, resistivity, magnetic susceptibility and thermal expansion from 2 to 300 K, using a single crystal with Tc = 7.2 K. The superconducting gap is characteristic of medium-strong coupling. The specific heat, resistivity and expansivity curves are deconvolved to yield approximations of the phonon density of states, the spectral electron-phonon scattering function and the phonon density of states weighted by the frequency-dependent Grueneisen parameter respectively. Lattice vibrations extend to high frequencies >100 meV, but a dominant Einstein-like mode at ~8 meV, associated with the vibrations of yttrium ions in oversized boron cages, appears to provide most of the superconducting coupling and gives rise to an unusual temperature behavior of several observable quantities. A surface critical field Hc3 is also observed.
Magnetic-field and doping dependence of low-energy spin fluctuations in the antiferroquadrupolar compound Ce(1-x)La(x)B(6)
G. Friemel,H. Jang,A. Schneidewind,A. Ivanov,A. V. Dukhnenko,N. Y. Shitsevalova,V. B. Filipov,B. Keimer,D. S. Inosov
Physics , 2015, DOI: 10.1103/PhysRevB.92.014410
Abstract: CeB(6) is a model compound exhibiting antiferroquadrupolar (AFQ) order, its magnetic properties being typically interpreted within localized models. More recently, the observation of strong and sharp magnetic exciton modes forming in its antiferromagnetic (AFM) state at both ferromagnetic and AFQ wave vectors suggested a significant contribution of itinerant electrons to the spin dynamics. Here we investigate the evolution of the AFQ excitation upon the application of an external magnetic field and the substitution of Ce with non-magnetic La, both parameters known to suppress the AFM phase. We find that the exciton energy decreases proportionally to T_N upon doping. In field, its intensity is suppressed, while its energy remains constant. Its disappearance above the critical field of the AFM phase is preceded by the formation of two modes, whose energies grow linearly with magnetic field upon entering the AFQ phase. These findings suggest a crossover from itinerant to localized spin dynamics between the two phases, the coupling to heavy-fermion quasiparticles being crucial for a comprehensive description of the magnon spectrum.
Samarium hexaboride: A trivial surface conductor
P. Hlawenka,K. Siemensmeyer,E. Weschke,A. Varykhalov,J. Sánchez-Barriga,N. Y. Shitsevalova,A. V. Dukhnenko,V. B. Filipov,S. Gabáni,K. Flachbart,O. Rader,E. D. L. Rienks
Physics , 2015,
Abstract: Recent theoretical and experimental studies suggest that SmB$_6$ is the first topological Kondo insulator: A material in which the interaction between localized and itinerant electrons renders the bulk insulating at low temperature, while topological surface states leave the surface metallic. While this would elegantly explain the material's puzzling conductivity, we find the experimentally observed candidates for both predicted topological surface states to be of trivial character instead: The surface state at $\bar{\Gamma}$ is very heavy and shallow with a mere $\sim 2$ meV binding energy. It exhibits large Rashba splitting which excludes a topological nature. We further demonstrate that the other metallic surface state, located at $\bar{X}$, is not an independent in-gap state as supposed previously, but part of a massive band with much higher binding energy (1.7 eV). We show that it remains metallic down to 1 K due to reduced hybridization with the energy-shifted surface 4$f$ level.
Magnetic structure and phase diagram of TmB4
S. Gabani,S. Matas,P. Priputen,K. Flachbart,K. Siemensmeyer,E. Wulf,A. Evdokimova,N. Shitsevalova
Physics , 2007,
Abstract: Magnetic structure of single crystalline TmB4 has been studied by magnetization, magnetoresistivity and specific heat measurements. A complex phase diagram with different antiferromagnetic (AF) phases was observed below TN1 = 11.7 K. Besides the plateau at half-saturated magnetization (1/2 MS), also plateaus at 1/9, 1/8 and 1/7 of MS were observed as function of applied magnetic field B//c. From additional neutron scattering experiments on TmB4, we suppose that those plateaus arise from a stripe structure which appears to be coherent domain boundaries between AF ordered blocks of 7 or 9 lattice constants. The received results suggest that the frustration among the Tm3+ magnetic ions, which maps to a geometrically frustrated Shastry-Sutherland lattice lead to strong competition between AF and ferromagnetic (FM) order. Thus, stripe structures in intermediate field appear to be the best way to minimize the magnetostatic energy against other magnetic interactions between the Tm ions combined with very strong Ising anisotropy.
On the importance of the electron-phonon coupling function on the superconducting transition temperature in dodecaboride superconductors: A comparison of LuB12 with ZrB12
J. Teyssier,R. Lortz,A. Petrovic,D. van der Marel,V. Filippov,N. Shitsevalova
Physics , 2008, DOI: 10.1103/PhysRevB.78.134504
Abstract: We report a detailed study of specific heat, electrical resistivity and optical spectroscopy in the superconducting boride LuB12 (Tc = 0.4 K) and compare it to the higher Tc compound ZrB12 (Tc = 6 K). Both compounds have the same structure based on enclosed metallic Lu or Zr ions in oversized boron cages. The infrared reflectivity and ellipsometry in the visible range allow us to extract the optical conductivity from 6 meV to 4 eV in the normal state from 20 to 280 K. By extracting the superconducting properties, phonon density of states and electron-phonon coupling function from these measurements we discuss the important factors governing Tc and explain the difference between the two compounds. The phonon density of states seems to be insignificantly modified by substitution of Zr with Lu. However, the soft vibrations of the metal ions in boron cages, responsible for the relatively high Tc in ZrB12, have almost no contribution to the electron-phonon coupling in LuB12.
Surface superconducting states in yttrium hexaboride single crystal in a tilted magnetic field
M. I. Tsindlekht,V. M. Genkin,G. I. Leviev,N. Yu. Shitsevalova
Physics , 2009, DOI: 10.1088/0953-8984/22/9/095701
Abstract: We present the results of an experimental study of the nucleation of superconductivity at the surface of a single crystal YB$_6$ in a tilted dc magnetic field. A recently developed experimental technique allowed us to determine $H_{c3}$ at each side of the sample as a function of the angle between the dc magnetic field and the surface. Experiment shows that the ratio $H_{c3}/ H_{c2}\approx 1.28 $ in the direction perpendicular to the surface dc field while according to the theory this ratio should be equal to 1. This sharp distinction cannot be ascribed to the surface roughness.
Pressure induced softening of YB_6: pressure effect on the Ginzburg-Landau parameter κ=λ/ξ
R. Khasanov,P. S. H?fliger,N. Shitsevalova,A. Dukhnenko,H. Keller
Physics , 2006, DOI: 10.1103/PhysRevLett.97.157002
Abstract: Measurements of the transition temperature T_c, the second critical filed H_{c2} and the magnetic penetration depth \lambda under hydrostatic pressure (up to 9.2 kbar) in the YB_6 superconductor were carried out. A pronounced and {\it negative} pressure effects (PE) on T_c and H_{c2} with dT_c/dp=-0.0547(4) K/kbar and \mu_0dH_{c2}(0)/dp =-4.84(20) mT/kbar, and zero PE on \lambda(0) were observed. The PE on the coherence length d\xi(0)/dp=0.28(2) nm/kbar was calculated from the measured pressure dependence of H_{c2}(0). Together with the zero PE on the magnetic penetration depth \lambda(0), our results imply that the Ginzburg-Landau parameter \kappa(0)=\xi(0)/\lambda(0) depends on pressure and that pressure "softens" YB_6, e.g. moves it to the type-I direction.
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