Feshbach Shape Resonance in Multiband Superconductivity in Heterostructures

We report experimental data showing the Feshbach shape resonance in the electron doped MgB₂ where the chemical potential is tuned by Al, Sc, and C substitutions. The scaling of the critical temperature T _c as a function of the Lifshitz parameter z = E _Γ−E _F, where E _F is the chemical potential and E _Γ is the energy of the Γ critical point where the σ Fermi surface changes from the 3D to a 2D topology, is reported. The resonant amplification of T _c(z) driven by the interband pairing is assigned to a Feshbach shape resonance characterized by quantum superposition of pairs in states corresponding to different spatial location and different parity. It is centered at z = 0 where the chemical potential is tuned to a Van Hove-Lifshits feature for the change of Fermi surface dimensionality in the electronic energy spectrum in one of the subbands. In this heterostructure at atomic limit the multiband superconductivity is in the clean limit because the disparity and negligible overlap between electron wavefunctions in different subbands suppresses the single electron interband impurity scattering rate. The emerging scenario from these experimental data suggests that the Feshbach shape resonance could be the mechanism for high T _c in particular nanostructured architectures.     

Progress in standard classification in Russia

A revised classification is considered for standards and metrological terms. There is a brief survey of the history of terms.Translated from Izmeritel'naya Tekhnika, No. 10, pp. 3–5, October, 1994.     

Vortices in superconductors

In type II superconductors where the London penetration depthλ is larger than the coherence lengthξ, there is a possibility of flux penetration inside the sample for magnetic field greater than \(H_{0_1 } \left( { = \frac{{\phi _0 }}{{4\pi \lambda ^2 }}ln \lambda /\xi , \phi _0 = \frac{{hc}}{{2e}}} \right).\) The flux penetrates in the form of vortices with core of sizeξ. However these vortices differ from those in superfluid He⁴ in variation of currentj(r) circulating around them. For superconductorsj(r) ～ 1/r only up to a distanceλ and then it falls exponentially whilev(r) ～ 1/r for all distances in superfluids. The reason is that in superconductors vortex carries a magnetic flux which is screened by conduction electrons. This coupling of order parameter field (the pairing wavefunction) with the gauge field has many interesting implications for superconductors and for non-Abelian gauge theories. Some examples are as follows:

1. The energy of the vortices is reduced. The energy of vortex of lengthL (ind = 3 sample) is of orderL lnL for a superfluid, is of orderL in a superconductor, and (in ad = 2 sample) the energy of a vortex point which diverges like lnR (whereR is the size of the sample) in a superfluid becomes finite in a superconductor.
2. The superconducting-normal transition in three dimension is very weak first order, because the fluctuations of the gauge field, when summed over, add to Ginzburg Landau free energy a term proportional to |ψ|³, whereψ is the order parameter.
3. Because of the lnr behaviour of interaction energy of vortices, a two-dimensional superfluid sample can exhibit a Kosterlitz-Thouless type transition whereas a strictlyd = 2 superconductors should not have any. However for dirty superconducting films whereλ is large vortex binding-unbinding transition can be observed with quite a rich phase diagram.

The paper presented at the discussion meeting discusses the above in detail. Here we give only a brief summary of results and some relevant references.     

Aging in velocity autocorrelations in granular gas of viscoelastic particles in two dimensions

We use large-scale molecular dynamics simulations to study aging of the velocity autocorrelation function of a force-free granular gas consisting of viscoelastic particles. We study the velocity autocorrelation function for a simplified model where the coefficient of restitution is constant for all collisions, but depends on current temperature of granular gas, it is called quasi-constant coefficient of restitution ε_eff. From our simulation results, it is observed that A(τ_w, τ) depends independently on both τ and τ_w. Initially, A(τ_w, τ) decays exponentially but later as τ_w increases, A(τ_w, τ) decays slowly due to emergence of correlations in velocity field. The explicit dependence of A(τ_w, τ) on τ_w implies that the system exhibits aging property.     

An experiment in genetic optimization in lens design

Abstract

A genetic algorithm and its application in lens design is studied. A combined algorithm is also presented which includes both a genetic algorithm (GA) and a damped least squares (DLS) method. The convergence property of a GA in lens design is demonstrated: first of all, a GA-generated design is obtained without artificial intervention, then, the aberration correction is completed by DLS, and the result is close to the GA-generated design.     

Bose-Einstein Condensation in Liquid 4He in Vycor

We present neutron scattering measurements of Bose-Einstein condensation (BEC) in liquid ⁴He confined in Vycor. The data show clear evidence of a condensate in Vycor with a condensate fraction comparable to that of bulk superfluid ⁴He, approximately 7.5% at low temperature and SVP. The temperature dependence of n₀(T) is also similar to that in the bulk with critical temperature for BEC, T_BEC, in the range 1.80BEC<2.05 K. The data are not accurate enough to show whether T_BEC for Vycor is the same or greater than the depressed critical temperature for superfluidity, T_c=1.95 K.     

Cracking resistance in welded joints in tubular specimens

Summary An apparatus is described for automatic welding of ring joints in tubular specimens, which is based on an A-825M welding apparatus working in a protective gas.Twelve forms of welded joints in tubular specimens have been tested, which have been made from steels 40Kh and 30KhGSA after quenching and high annealing and from steels 35 and St3 after normalization. K_c in the joint is less by about a factor three than the resistance in the initial metal. The wide spread in the values (K_c = 19.3–47.4 MPa · m) is due to the poor weldability of the inhomogeneous materials, in which there are defects in the weld structure (unwelded spots, inclusions, pores, cracks, and so on). The viability in welded constructions must be forecast from the cracking resistance in the joints, not in the initial material.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 4, pp. 108–109, July–August, 1990.     

Penny-shaped crack in a sphere embedded in an infinite medium

We consider the problem of determining the stress intensity factor and the crack energy in an Isotropie, homogeneous elastic sphere embedded in an infinite Isotropie, homogeneous elastic medium when there is a diametrical crack in the sphere. We assume that the crack is opened by an internal pressure and the sphere is bonded to the surrounding material. The problem is reduced to the solution of a Fredholm integral equation of the second kind in the auxiliary function φ($\text{t}$). Expressions for the stress intensity factor and the crack energy are obtained in terms of φ($\text{t}$). The integral equation is solved numerically and the numerical values of the stress intensity factor and the crack energy are graphed.     

Elementary processes in prebreakdown phenomena in the atmosphere

The effect of negative oxygen ion destruction upon breakdown conditions in atmospheric air is analyzed. It is shown that ozone accumulation due to plasmochemical reactions occurring in ionized air produces a reduction in the breakdown voltage, related to negative O^– ion destruction upon collision with ozone molecules under realistic conditions. A relationship is derived for electric field breakdown intensity and ozone molecule lifetime for the real atmosphere.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 62, No. 5 pp. 661–664, May, 1992.     

Losses in diamond in the millimeter range

A theoretical and experimental investigation is made of the magnitude and nature of the dielectric losses in weakly absorbing synthetic diamonds in the wavelength range 1.75–6.8 mm at temperatures T=20–500 °C. Some samples exhibited extremely low losses (tan δ <10⁻⁵) which makes plasma-chemically deposited diamond wafers suitable for fabricating windows for megawatt continuous gyrotrons. It is shown that in principle, a further substantial reduction in losses can be achieved. Pis’ma Zh. Tekh. Fiz. 25, 85–89 (April 12, 1999)     

Vortices in 3He-A in a weak magnetic field

The structure and the energy of vortices in rotating ³He-A are considered in the presence of a weak magnetic field. It is shown, using the logarithmic approximation for the free energy, that there will be a sequence of textural transitions when the magnetic field is increased. The transition from a nonsingular texture to a singular texture is studied in detail. It is shown that the l vector is almost uniform outside the cores of vortices. To verify these results, measurements by NMR and by ultrasound are suggested.This work has been financially supported by the Academy of Science of the USSR, by the Academy of Finland, and by the Committee of Scientific and Technical Cooperation between Finland and the USSR.     

Hiemenz flow in hydromagnetics

Summary The laminar flow of an incompressible, viscous, electrically conducting fluid impinging normal to a plane in the presence of a transverse magnetic field is investigated. Using finite-differences and quasilinearization, an exact numerical solution is presented which takes into account the asymptotic boundary condition. It is demonstrated that iff denotes the dimensionless stream function, the value off(0) increases monotonically withM, the Hartmann number, where a prime denotes the derivative normal to the plane.This conclusion is supported by deriving a perturbation solution valid for smallM. Also, an analytical solution is obtained valid for largeM. Finally, an approximate solution is given which is simple and sufficiently accurate for the entire range of values ofM.     

Phonon drag in graphite

This paper is principally concerned with the electron-phonon interaction near the Kohn condition (q = 2k _F ) in crystals such as graphite, where the anisotropy of both the Fermi surface and the vibration spectrum is extremely large. By neglecting the trigonal warping of the Fermi surface of graphite, the inverse phonon lifetime is evaluated and is found to have a sharp maximum for the phonons in the Kohn regime. With increasing temperature, the thermal smearing of the Fermi surface leads to a lowering of the magnitude of this peak which decreases approximately asT ^–1. The previous conclusions seem to remain valid when the trigonal warping is taken into account. By arguing that the effectively dragged phonons are just the phonons within the Kohn regime we have found that the phonon-drag component of the thermoelectric power reflects this strong coupling. In spite of numerous approximations in the calculations, a one-parameter fit to the experimental results seems to give satisfactory agreement. The model furthermore predicts the right negative sign of the thermoelectric anomaly, provided the new carrier assignment of Schroederet al. is used.     

Second-viscosity phenomena in dilute solutions of3He in superfluid4He

A theory of second-viscosity phenomena in dilute solutions of³He in superfluid⁴He is presented. The theory considers only phonon and³He quasiparticle excitations and is therefore valid at temperatures below about 0.6 K. It is shown, by an exact calculation, that within the framework of the Landau-Pomeranchuck model for the³He quasiparticle excitation energy, the four second-viscosity coefficients are related to one another and that only one of them is actually an independent kinetic coefficient. The relations between the second-viscosity coefficients are applied to analyze the expressions for the dissipative function and the first- and the second-sound attenuation coefficients. It is shown that the second-viscosity contribution to the second-sound attenuation is smaller by an order of magnitude than its contribution to the first-sound attenuation.     

On the possibility of exciton formation in semimetals in extremely high magnetic fields. II

The semimetal model ¹ is discussed for the case of extremely high magnetic fields, such that ln(p ₀/x) >> 1, wherep ₀ is the Fermi momentum and x is the reciprocal of the Debye radius. It is shown that a symmetrical vertex has only one pole in this case. This pole is governed by the interaction constant, which includes the Coulomb logarithm, ln(p ₀/x).     

An end-notched beam test for specific fracture energy in mixed mode in timber

A simple method for achieving stable crack propagation in a beam notched at a support is presented. The method allows the measurement of fracture energy in mixed modeG _f,mix. Results from a small number of laminated veneer lumber specimens suggest that there is a relationship betweenG _f,mix and density and that the ratioG _f,mix/(G _f,I+G _f,II) is about 0.35. Calculations of fracture energy in mode IG _f,I and mode, IIG _f,II did not coincide with values from mode-specific tests, indicating that an adjustment is necessary to take account of the interaction between the modes.     

Slip planes in orthogonally anisotropic flat bars in uniaxial stress tests

Summary The inclination and the instability of the Lüders band on an orthogonally anisotropic flat bar is theoretically analysed. The Lüders band is assumed to be a strip flow region confined by two parallel slip planes. The stress and the plastic flow on a slip plane are derived from Hill's anisotropic yield criterion and V. Mises' concept of plastic potential. The relation between the inclination angle of the slip line and the angle of principal axis of anisotropy from the uniaxial loading direction is derived. The obtained relation is identical with that given by the method of characteristics. It is proved that the Lüders band is unstable for tension test and stable for compression test.

---

Gleitebenen des orthogonal-anisotropen ebenen Stabes in eindimensionalen Spannungsversuchen

Zusammenfassung Neigungswinkel und Instabilität der Lüderschen Streifen des orthogonal-anisotropen ebenen Stabes werden theoretisch untersucht. Der Lüdersche Streifen wird dabei als ein durch zwei Gleitebenen begrenzter dünner Fließbereich angenommen. Spannung und plastisches Fließen auf einer Gleitebene werden, ausgehend von der Hillschen anisotropen Fließbedingung und vom v. Misesschen Begriff des plastischen Potentials hergeleitet. Die Beziehung zwischen dem Neigungswinkel der Gleitebene, dem Winkel der Anisotropie zur Richtung der eindimensionalen Belastung wird abgeleitet. Diese Beziehung erweist sich als identisch zu der mit Hilfe der Charakteristikenmethode hergeleiteten. Es wird gezeigt, daß der Lüdersche Streifen instabil bei Zug und stabil bei Druck ist.

---

---

With 1 Figure.     

The motion of negative ions in liquid helium in strong electric fields

The motion of negative ions in liquid helium at temperatures above 0.6 K is governed principally by the scattering of rotons. The mobility is defined as the ratio of the drift velocity,v to the electric fieldE for small electric fields. In stronger electric fields the ratioE/v usually increases as the velocity increases. However, it has been found recently that for negative ions under high pressures the ratioE/v decreases asv increases. This apparently anomalous behavior is explained by extending a theory of the low-field ionic mobility to higher drift velocities corresponding to stronger electric fields. Good quantitative agreement is then found with the recent experiments of McClintock and co-workers for drift velocities up to about3 m/sec.     

Stability of cracks in elastic and in elastic plastic materials

The theory of the stability of cracks in an elastic material is summarized. The theory is extended to the case of a non-work-hardening elastic plastic material and is applied to a number of crack geometries and loadings.

---

Zusammenfassung Die Theorie der Risstabilitat irn elastischen Material wird zusammengefasst. Anschliessend wird these Theorie ausgedehnt auf durch Behandlung nicht härtbare elastisch-plastische Materialien Bowie auf verschiedenartige Rissformen and Belastungen angewandt.

---

Résumé On a resumé la théorie de la stabilité des fissures dans un material elastíque. Cette théorie est étendue au cas de materiaux elasto-plastique non sujets au durcissement par écrouissage, est appliquée a plusieurs types de géometries de fissures et de conditions de sollicitations.

     

Chryseobacterium meningosepticum in a parapneumonic effusion in a chronic kidney disease patient on hemodialysis

Chryseobacterium meningosepticum is an uncommon human pathogen, which is an inhabitant of soil and water. It should be included in the list of suspected nosocomial infections, especially in patients with immunocompromised status. C. meningosepticum infections are not common but are clinically important because the organism is naturally resistant to multiple antibiotics. We report a case where the bacterium was isolated from the pleural fluid from a chronic kidney disease patient on hemodialysis, who developed pneumonia with complicated parapneumonic effusion. To the best of our knowedge, this is the first case where C. meningosepticum is isolated from the pleural effusion, from India.