311 research outputs found
Spin rotational symmetry breaking by orbital current patterns in two-leg ladders.
We investigate the physical consequences of orbital current patterns (OCP) in
doped two-leg Cu-O Hubbard ladders. The internal symmetry of the pattern, in
the case of the ladder structure, differs slightly from that suggested so far
for cuprates. We focus on this OCP and look for measurable signatures of its
existence. We compute the magnetic field produced by the OCP at each lattice
site, and estimate its value in view of a possible experimental detection.
Using a renormalization group (RG) analysis, we determine the changes that are
caused by the SU(2) spin-rotational symmetry breaking which occurs when the OCP
is present in the ground state phase diagram. The most signifcant one is an
in-plane SDW gap opening in an otherwise critical phase, at intermediate
dopings. We estimate the value of this gap, give an analytic expression for the
correlation functions and examine some of the magnetic properties of this new
phase which can be revealed in measurements. We compute the conductance in the
presence of a single impurity, using an RG analysis. A discussion of the
various sources of SU(2) symmetry breaking underscores the specificity of the
OCP induced effects.Comment: 12 pages, 3 figures, submitted to PR
NMR and NQR study of pressure-induced superconductivity and the origin of critical-temperature enhancement in the spin-ladder cuprate SrCaCuO
Pressure-induced superconductivity was studied for a spin-ladder cuprate
SrCaCuO using nuclear magnetic resonance (NMR) under
pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a
transitional change from a spin-gapped state to a Fermi-liquid state at
temperatures higher than . The relaxation rate shows
activated-type behavior at an onset pressure, whereas Korringa-like behavior
becomes predominant at the optimal pressure, suggesting that an increase in the
density of states (DOS) at the Fermi energy leads to enhancement of .
Nuclear quadrupole resonance (NQR) spectra suggest that pressure application
causes transfer of holes from the chain to the ladder sites. The transfer of
holes increases DOS below the optimal pressure. A dome-shaped versus
pressure curve arises from naive balance between the transfer of holes and
broadening of the band width
First principles evaluation on photocatalytic suitability of 2H structured and [0001] oriented WS2 nanosheets and nanotubes
This study was supported by the EC ERA.Net RUS Plus Project No. 237 WATERSPLIT. R.E. acknowledges the financial support provided by the Russian Foundation for Basic Research (grant N 17-03-00130a) and High Performance Computer Center of St. Petersburg University for the assistance. The authors are indebted to D. Bocharov, O. Lisovski and E. Spohr for stimulating discussions.Pristine WS2 multilayer nanosheets (NSs), which thickness h NS varies from 1 to 12 monolayers (MLs), as well as single- and multi-walled nanotubes (SW and MW NTs) of different chirality, which diameter d NT exceeds 1.9 nm, display photocatalytic suitability to split H2O molecules. Obviously, such a phenomenon can occur since the band gap of these nanostructures corresponds to the energy range of visible light between the red and violet edges of spectrum (1.55 eV < Δϵgap < 2.65 eV). For all the studied WS2 NSs and NTs, the levels of the top of the valence band and the bottom of the conduction band must be properly aligned relatively to H2O oxidation and reduction potentials separated by 1.23 eV: ϵ VB < ϵO2/H2O < ϵH+/H2 < ϵ CB. The values of Δϵgap decrease with growth of h NS and increase with enlargement of dNT. 1 ML nanosheet can be considered as a limit of infinite SW NT thickness growth (d NT→∞), which band gap increases up to ∼2.65 eV. First principles calculations have been performed using the hybrid DFT-HF method (HSE06 Hamiltonian) adapted for 2H WS2 bulk. The highest solar energy conversion efficiency (15-18%) expected at Δϵgap = 2.0-2.2 eV (yellow-green range) has been found for 2 ML thick (stoichiometric) WS2 (0001) NS as well as WS2 NTs with diameters 2.7-3.2 nm (irrespectively on morphology and chirality indices n of nanotubes). Moreover, unlike discrete variation of hNS magnitudes, tuning of d NT values provides much higher energy resolution.Russian Foundation for Basic Research N 17-03-00130a; European Commission EC 237 WATERSPLIT; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART
(Sr/Ca)_{14}Cu_{24}O_{41} spin ladders studied by NMR under pressure
(63)Cu-NMR measurements have been performed on two-leg hole-doped spin
ladders Sr_{14-x}Ca_{x}Cu_{24}O_{41} single crystals (0-x-12) at several
pressures up to the pressure domain where the stabilization of a
superconducting ground state can be achieved. The data reveal marked decrease
of the spin gap derived from Knight shift measurements upon Ca substitution and
also under pressure and confirm the onset of low lying spin excitations around
P_{c} as previously reported. The spin gap in Sr_{2}Ca_{12}Cu_{24}O_{41} is
strongly reduced above 20 kbar. However, the data of an experiment performed at
P=36 kbar where superconductivity has been detected at 6.7K by an inductive
technique have shown that a significant amount of spin excitations remains
gapped at 80K when superconductivity sets in. The standard relaxation model
with two and three-magnon modes explains fairly well the activated relaxation
data in the intermediate temperature regime corresponding to gapped spin
excitations using the spin gap data derived from Knight shift experiments.The
data of Gaussian relaxation rates of heavily doped samples support the
limitation of the coherence lenght at low temperature by the average distance
between doped holes. We discuss the interplay between superconductivity and the
spin gap and suggest that these new results support the exciting prospect of
superconductivity induced by the interladder tunnelling of preformed pairs as
long as the pressure remains lower than the pressure corresponding to the
maximum of the superconducting critical temperature.Comment: 15 pages Latex, 13 figures. to be published in Eur.Phys.Jour.B,200
Influence of non-magnetic impurities on hole doped two-leg Cu-O Hubbard ladders
We study the influence of non magnetic impurities on the phase diagram of
doped two-leg Hubbard Cu-O ladders. In the absence of impurities this system
posseses d-wave superconducting states and orbital current states depending on
the doping. A single, strong, scatterer modifies its environment locally and
this effect is assessed using a renormalization group analysis. At high doping,
disorder causes intraband instabilities and at low doping it promotes interband
instabilities. In the former case, we extend the boundary conformal field
theory method, developed in the context of single chains, to handle the ladder
problem, and we find exact closed-form analytical expressions for the
correlation functions. This allows us to compute experimentally measurable
local quantities such as the nuclear magnetic resonance line broadenings and
scanning tunnelling microscope profiles. We also discuss the low doping regime
where Kondo physics is at play, making qualitative predictions about its
nature. Insight into collective effects is also given in the many weak
impurities case, based on an RG approach. In this regime, one sees the
interplay between interactions and disorder. We emphasize the influence of the
O atoms on disorder effects both for the single- and for the many-defect
situations.Comment: accepted to be published in NJP special editio
Risk-oriented internal control: The essence, management methods at small enterprises
The research topic relevance is inspired by necessity to develop theoretical and methodical provisions on the internal control system, risk-based management at small enterprises and to prove application feasibility, using economic-mathematical methods its implementation. The purpose of this research is to develop theoretical and methodical approaches to internal control system formation in small businesses, generating reliable and relevant information on the commercial organization activities, enabling risks identification. The leading approach to study this problem is situational and systematic in the frame of theory and methodology internal control at small businesses in the risk-based management system, allowing to analyze the impact of various risks on small enterprises activity and to systematize obtained results. According to the study results, there were proved the implementing internal control feasibility at small businesses, presenting the author's systematization and classification of internal and external risks in small companies, identifying organizational and methodological approaches to risk-based internal control development; implemented the adaptation of existing economic-mathematical methods within risk-based internal control at small businesses. The article can be useful for practical and scientific workers in the field of company’s internal control, teachers, postgraduates, undergraduates and students, studying Economics and Management at higher educational institutions. © 2016 Piskunov et al
Superconductivity of the spin ladder system: Are the superconducting pairing and the spin-gap formation of the same origin?
Pressure-induced superconductivity in a spin-ladder cuprate
SrCaCuO has not been studied on a microscopic level so
far although the superconductivity was already discovered in 1996. We have
improved high-pressure technique with using a large high-quality crystal, and
succeeded in studying the superconductivity using Cu nuclear magnetic
resonance (NMR). We found that anomalous metallic state reflecting the
spin-ladder structure is realized and the superconductivity possesses a
s-wavelike character in the meaning that a finite gap exists in the
quasi-particle excitation: At pressure of 3.5GPa we observed two excitation
modes in the normal state from the relaxation rate . One gives rise
to an activation-type component in , and the other -linear
component linking directly with the superconductivity. This gapless mode likely
arises from free motion of holon-spinon bound states appearing by hole doping,
and the pairing of them likely causes the superconductivity.Comment: to be published in Phys. Rev. Let
First HARPSpol discoveries of magnetic fields in massive stars
In the framework of the Magnetism in Massive Stars (MiMeS) project, a
HARPSpol Large Program at the 3.6m-ESO telescope has recently started to
collect high-resolution spectropolarimetric data of a large number of Southern
massive OB stars in the field of the Galaxy and in many young clusters and
associations. In this Letter, we report on the first discoveries of magnetic
fields in two massive stars with HARPSpol - HD 130807 and HD 122451, and
confirm the presence of a magnetic field at the surface of HD 105382 that was
previously observed with a low spectral resolution device. The longitudinal
magnetic field measurements are strongly varying for HD 130807 from -100
G to 700 G. Those of HD 122451 and HD 105382 are less variable with
values ranging from -40 to -80 G, and from -300 to -600 G,
respectively. The discovery and confirmation of three new magnetic massive
stars, including at least two He-weak stars, is an important contribution to
one of the MiMeS objectives: the understanding of origin of magnetic fields in
massive stars and their impacts on stellar structure and evolution.Comment: 4 pages, 2 figures, accepted for publication in A&A Lette
Diamagnetism above Tc in underdoped Bi2.2Sr1.8Ca2Cu3O10+d
Single crystals of (Bi2223) with were grown by a traveling solvent floating
zone method in order to investigate the superconducting properties of highly
underdoped Bi2223.Grown crystals were characterized by X-ray diffraction, DC
susceptibility and resistivity measurements, confirming Bi2223 to be the main
phase.The crystals were annealed under various oxygen partial pressures to
adjust their carrier densities from optimally doped to highly underdoped.The
fluctuation diamagnetic component above the superconducting transition
temperature extracted from the anisotropic normal state
susceptibilities () and ()
was found to increase with underdoping, suggesting a decrease in the
superconducting dimensionality and/or increase in the fluctuating vortex liquid
region.Comment: 6 pages, 7 figures, corrected fig.4 and references, published in J.
Phys. Soc. Jpn. 79, 114711 (2010
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