32,135 research outputs found
Bending modes, elastic constants and mechanical stability of graphitic systems
The thermodynamic and mechanical properties of graphitic systems are strongly
dependent on the shear elastic constant C44. Using state-of-the-art density
functional calculations, we provide the first complete determination of their
elastic constants and exfoliation energies. We show that stacking
misorientations lead to a severe lowering of C44 of at least one order of
magnitude. The lower exfoliation energy and the lower C44 (more bending modes)
suggest that flakes with random stacking should be easier to exfoliate than the
ones with perfect or rhombohedral stacking. We also predict ultralow friction
behaviour in turbostratic graphitic systems.Comment: 7 pages, 6 figure
Elastic Properties and Magnetic Phase Diagrams of Dense Kondo Compound Ce0.75La0.25B6
We have investigated the elastic properties of the cubic dense Kondo compound
Ce0.75La0.25B6 by means of ultrasonic measurements. We have obtained magnetic
fields vs temperatures (H-T) phase diagrams under magnetic fields along the
crystallographic [001], [110] and [111] axes. An ordered phase IV showing the
elastic softening of c44 locates in low temperature region between 1.6 and 1.1
K below 0.7 T in all field directions. The phase IV shows an isotropic nature
with regard to the field directions, while the antiferro-magnetic phase III
shows an anisotropic character. A remarkable softening of c44 and a spontaneous
trigonal distortion εyz+εzx+εxy recently reported by Akatsu et
al. [J. Phys. Soc. Jpn. 72 (2003) 205] in the phase IV favor a ferro-quadrupole
(FQ) moment of Oyz+Ozx+Oxy induced by an octupole ordering.Comment: 9 figures, Strongly Correlated Electron
Nonlinear Hydrodynamics of Disentangled Flux-Line Liquids
In this paper we use non-Gaussian hydrodynamics to study the magnetic
response of a flux-line liquid in the mixed state of a type-II superconductor.
Both the derivation of our model, which goes beyond conventional Gaussian flux
liquid hydrodynamics, and its relationship to other approaches used in the
literature are discussed. We focus on the response to a transverse tilting
field which is controlled by the tilt modulus, c44, of the flux array. We show
that interaction effects can enhance c44 even in infinitely thick clean
materials. This enhancement can be interpreted as the appearance of a
disentangled flux-liquid fraction. In contrast to earlier work, our theory
incorporates the nonlocality of the intervortex interaction in the field
direction. This nonlocality is crucial for obtaining a nonvanishing
renormalization of the tilt modulus in the thermodynamic limit of thick
samples.Comment: 20 pages, 3 figures (submitted to PRB
Dinamika rešetke binarnih legura sličnih atoma
In the present article, the calculations of the lattice dynamical properties of four equiatomic binary alloys viz. Na0.5Li0.5, Na0.5K0.5, Na0.5Rb0.5 and Na0.5Cs0.5 to second order in local model potential is discussed in terms of real-space sum of Born von Karman central force constants. Well known Ashcroft’s empty core (EMC) model potential has been used to study the lattice dynamical properties. Instead of the average of the force constants of metallic Li, Na, K, Rb and Cs, the pseudo-alloyatom (PAA) is adopted to compute directly the force constants of four equiatomic sodium-based binary alloys. The exchange and correlation functions due to Hartree (H) and Ichimaru-Utsumi (IU) are used to investigate influence of screening effects. Results for the lattice constants i.e. C11, C12, C44, C12 − C44, C12/C44 and bulk modulus (B) obtained using the Hartree screening function have higher values in comparison with the results obtained for the same properties using IchimaruUtsumi (IU) screening function. The results for the shear modulus (C ′ ), deviation from Cauchy’s relation (C12/C44), Poisson’s ratio (σ), Young modulus (Y ), propagation velocity of elastic waves, phonon dispersion curves and degree of anisotropy (A) are encouraging for the four equiatomic Na-based binary alloys.Polazeći od Born von Karmanovih konstanti za središnju silu i zbrajanja u realnom prostoru, proveli smo račune za dinamička svojstva rešetke četiri binarne legure od sličnih atoma, Na0.5Li0.5, Na0.5K0.5, Na0.5Rb0.5 i Na0.5Cs0.5, do drugog reda u modelnom lokalnom potencijalu. Primijenili smo poznat Ashcroftov potencijal s praznom sredicom za račune dinamičkih svojstava rešetke. Umjesto prosjeka konstanti sila metalnih Li, Na, K, Rb i Cs, pretpostavili smo pseudo-atom legure (PAA) za izravno računanje konstanti sila četiri slično-atomne legure na bazi natrija. Uzeli smo Hartree-eve (H) i Ichimaru-Utsumi-eve (IU) funkcije izmjene i korelacija za istraživanje učinaka zasjenjenja. Ishodi za konstante rešetke, tj. C11, C12, C44, C12 − C44 i C12/C44, te volumnog modula (B) postignute s Hartree-evom funkcijom zasjenjenja veće su od ishoda tih veličina izračunatih Ichimaru-Utsumi-evom (IU) funkcijom zasjenjenja. Ishodi za modul smika (C ′ ), odstupanje od Cauchy-eve relacije (C12/C44), Poissonov omjer (σ), Youngov modul (Y ), brzine elastičnih valova, fononske disperzijske krivulje stupanj anizotropije (A) važni su podaci za ove četiri binarne legure sličnih atoma na bazi natrija
Structure, energetics, and mechanical stability of Fe-Cu bcc alloys from first-principles calculations
Atomic volumes, magnetic moments, mixing energies, and the elastic properties of bcc Fe1–xCux solid solutions are studied by ab initio calculations based on the cluster expansion framework. For the calculation of concentration-dependent elastic moduli in disordered solid solutions, we introduce a generalization of the cluster expansion technique that is designed to handle tensorial quantities in high-symmetry phases. Calculated mixing energies, atomic volumes, and magnetic moments are found to be in good agreement with available measurements for metastable alloys prepared through nonequilibrium processing techniques. Additionally, the predicted variations of the bulk modulus and shear moduli C44 and C[prime] with respect to copper concentration are calculated for the disordered bcc phase. While the bulk modulus and C44 are positive for all concentrations, C[prime] is predicted to be positive only for Cu concentration less than 50 atomic %, and negative otherwise. Our results thus indicate that the mechanical instability of bcc Cu persists over a wide range of compositions. The implications of the present results are discussed in relation to the observed metastability of bcc Fe-Cu alloys, and the strengthening mechanism of nanoscale bcc precipitates in an alpha-Fe matrix
Tunneling and rattling in clathrate crystal
We present tunneling and rattling motions of an off-center guest atom in cage
referring to a clathrate crystal La_3Pd_{20}Ge_6. The elastic constant C_{44}
of La_3Pd_{20}Ge_6 shows a Debye-type dispersion around 20 K obeying a
relaxation time tau = tau_0exp(E/k_{B}T) with an attempt time tau_0 =
2.0*10^{-12} sec and an activation energy E = 197 K. At low temperatures below
3 K down to 20 mK, the C_{44} shows a softening of C_{44} =
C_{44}^0(T-T_C^0)/(T-Theta) with T_C^0 = -337.970 mK and Theta = -338.044 mK.
These facts are attributed to two different types of the off-center motions
with Gamma_5 symmetry in 4a-site cage of La_3Pd_{20}Ge_6, a thermally activated
rattling motion over the potential hill and a tunneling motion through the
potential hill at low temperatures.Comment: 5 pages, 4 figures, to be published Phys. Rev.
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