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Dec 2010

Volume 36, Issue 12, pp. 1023-1108

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Complex conductivity of 2D electron crystals over liquid helium in the dynamic transition region

V. E. Syvokon and K. A. Nasyedkin

Low Temp. Phys. 36, 1023 (2010); http://dx.doi.org/10.1063/1.3530189 (7 pages) | Cited 1 time

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The complex conductivity of electron crystals with surface densities of 3.2×108–12.6×108 cm−2 is studied under dynamic transition conditions. Measurements at temperatures of 70–90 mK, well below the thermodynamic equilibrium melting point, show that a transition is observed as the driving electric field in the plane of the electron layer is increased. This is accompanied by jumps in both components of the inverse conductivity of the layer when the critical field is reached. The dependence of the critical field on the surface density of the electron layer is determined.
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73.25.+i Surface conductivity and carrier phenomena
72.20.Fr Low-field transport and mobility; piezoresistance
64.70.dj Melting of specific substances
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Quasiparticle current in ballistic F/ScN junctions

I. V. Kulagina and V. N. Krivoruchko

Low Temp. Phys. 36, 1030 (2010); http://dx.doi.org/10.1063/1.3530190 (6 pages)

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The current-voltage (I-V) characteristics of ballistic F/ScN junctions, consisting of a hybrid F/S bilayer of ferromagnetic (F) and superconducting (S) metals and a normal (N) metal, are studied. It is assumed that the size of the constriction, c, is much smaller than the electron mean free path and the superconducting coherence length, and that the transparency is rather high. The amplitudes of Andreev and normal reflection of electrons with specified spin orientations from the constriction are calculated. It is shown that the exchange field (hex) induced in the S metal causes spin splitting of the features of the differential conductivity of the junction. Measurements of the spin splitting of the junction conductivity, which is proportional to hex, can be used to study the induced magnetism of superconductors in hybrid S/F structures with the aid of the measured I-V characteristics of F/ScN junctions.
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74.45.+c Proximity effects; Andreev reflection; SN and SNS junctions
74.25.fc Electric and thermal conductivity
74.25.Ha Magnetic properties including vortex structures and related phenomena
73.40.-c Electronic transport in interface structures

Superconductivity of bulk molybdenum samples with carbidized surfaces

M. O. Dzyuba, Yu. N. Chiang, O. G. Shevchenko, A. V. Semenov, and V. Ph. Khirnyi

Low Temp. Phys. 36, 1036 (2010); http://dx.doi.org/10.1063/1.3530191 (6 pages)

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Results are presented from a study of the superconducting properties of molybdenum carbide synthesized in the surface layer of bulk molybdenum using an original technology: ion-plasma bombardment of metallic molybdenum with carbon and silicon ions to implant carbon in the surface layer of the molybdenum. The temperature evolution of the superconducting resistive transition of samples with a carbidized surface is studied as a function of the thickness and continuity of the carbide layer. A generalization of the parameters of the superconducting transition for all the samples indicates that the carbide layers produced by this technology have a mosaic-island structure and a nonuniform superconductivity associated with this structure. The main contribution to the smearing out of the superconducting transition under these conditions may come from pulling of the potential at the N-S boundaries between normal molybdenum and carbide clusters.
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74.62.-c Transition temperature variations, phase diagrams
61.72.U- Doping and impurity implantation
74.70.Wz Carbon-based superconductors

Vortex resistivity owing to the magnetic field of the transport current in wide superconducting films

I. V. Zolochevskii

Low Temp. Phys. 36, 1042 (2010); http://dx.doi.org/10.1063/1.3530192 (3 pages)

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The resistive properties of vortex superconducting films are studied experimentally over a wide temperature range. It is shown that in superconducting films whose width w is an order of magnitude greater than the penetration depth λ⊥(T) of the magnetic field (w/λ⊥(T)>10), the differential resistance of the linear vortex part of the current-voltage characteristic is independent of temperature, in accordance with the Aslamazov-Lempitskii model for the vortex resistive state. Immediately adjacent to Tc, in the region where the effective sizes of the vortices and antivortices which produce the film current resistance in chains exceed half the width of the film, i.e., 4<w/λ⊥(T)<8, a sharp increase in the differential resistance of the vortex segment of the current-voltage characteristic is observed as T→Tc that is not explained by existing theories.
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74.78.-w Superconducting films and low-dimensional structures
74.25.fc Electric and thermal conductivity
74.25.Wx Vortex pinning (includes mechanisms and flux creep)
74.25.Ha Magnetic properties including vortex structures and related phenomena

Effect of deformation and heat treatment with equal-channel, multiple-angle pressing on the superconducting properties of NbTi alloy

V. A. Beloshenko, V. V. Chishko, N. I. Matrosov, V. P. Dyakonov, R. Szymczak, J. Piętoza, R. Puźniak, H. Szymczak, D. Gajda, and A. Zaleski

Low Temp. Phys. 36, 1045 (2010); http://dx.doi.org/10.1063/1.3533234 (4 pages)

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The effect of equal-channel, multiple-angle pressing combined with hydroextrusion and drawing, on the critical current density, pinning force, and superconducting transition temperature of bimetallic Nb+60 at.% Ti wires is examined.
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74.25.Ld Mechanical and acoustical properties, elasticity, and ultrasonic attenuation
74.62.Yb Other effects
74.25.Sv Critical currents
74.70.Ad Metals; alloys and binary compounds (including A15, MgB2, etc.)
81.40.Gh Other heat and thermomechanical treatments
81.40.Lm Deformation, plasticity, and creep

Charge density waves in d-wave superconductors

A. I. Voitenko and A. M. Gabovich

Low Temp. Phys. 36, 1049 (2010); http://dx.doi.org/10.1063/1.3533237 (9 pages) | Cited 1 time

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A self-consistent theory of charge density waves in partially dielectrically gapped superconductors with d-pairing is proposed. The dependences of the dielectric (Σ) and superconducting (Δ) order parameters on the temperature and other parameters of the problem is examined. The corresponding angular diagrams for the gap distribution over the Fermi surface are constructed for the first time. The theory is used to explain the properties of high-temperature oxides. The effect of the displacement angle between the lobes of the order parameters Σ and Δ on the gap distribution in momentum space and on the reentrance phenomenon for Σ with respect to temperature is analyzed.
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71.45.Lr Charge-density-wave systems
74.20.Rp Pairing symmetries (other than s-wave)
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor

Superconductivity of 80NbN–20SiO2 granular films

O. I. Yuzephovich, S. V. Bengus, B. Kościelska, and A. Witkowska

Low Temp. Phys. 36, 1058 (2010); http://dx.doi.org/10.1063/1.3533238 (6 pages)

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The feasibility of creating granular superconducting films of NbN–SiO2 with a controlled grain size by a sol-gel method is demonstrated. A comprehensive study is made of the structural and transport properties of granular films of 80%NbN–20%SiO2 with different thicknesses. It is found that production of a complete superconducting transition requires that the samples have thicknesses greater than 750 nm. The critical temperatures for a superconducting transition and the upper critical magnetic fields are roughly constant for films with different thicknesses and equal 4.5 K and 4.4 T, respectively. A crossover from 2D to 3D behavior is discovered in the temperature variation of the upper parallel critical field. It is shown that at low magnetic fields the resistive transitions obey an Arrhenius law. The mechanism for broadening of resistive transitions in magnetic fields is most likely magnetic flux creep. The magnetic field dependence of the activation energy is obtained. Typical initial signs of a magnetically induced superconductor-insulator transition are observed at high magnetic fields.
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74.78.-w Superconducting films and low-dimensional structures
74.81.Bd Granular, melt-textured, amorphous, and composite superconductors
74.62.Yb Other effects
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
74.25.F- Transport properties
74.25.Wx Vortex pinning (includes mechanisms and flux creep)
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Magnetic order-order phase transitions in magnets with collective electrons: MnCoSi

V. I. Valkov, A. V. Golovchan, E. A. Dvornikov, and B. M. Todris

Low Temp. Phys. 36, 1064 (2010); http://dx.doi.org/10.1063/1.3533253 (7 pages)

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First principles calculations of the electronic structure of MnCoSi and a spiral state model are used to analyze magnetic-field induced magnetic order-order phase transitions in MnCoSi at pressures up to 2 kbar. This shows that the magnetic moments of Mn and Co in MnCoSi are created by collective d-electrons and that pressure stimulation of induced order-order transitions in this material can be a consequence of structural changes in the density of d-electron states as the pressure is raised. Induced order-order transitions are described by a single-band model for the spiral state in which the band filling and the nonmagnetic density of electron states are based on first principles calculations of the electronic structure of MnCoSi. This model shows that the low-temperature states induced by a magnetic field are not collinear ferromagnetic structures, but may represent the coexistence of a homogeneous ferromagnetic component and a periodic spiral component of the total magnetic moment of the d-band. A pure ferromagnetic state at low temperatures can be regarded as metastable.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Cc Other ferromagnetic metals and alloys
75.30.Cr Saturation moments and magnetic susceptibilities
71.20.Be Transition metals and alloys
71.15.-m Methods of electronic structure calculations
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Spin polarization oscillations in a magnetically inhomogeneous conducting ring

P. V. Pyshkin

Low Temp. Phys. 36, 1071 (2010); http://dx.doi.org/10.1063/1.3536341 (5 pages)

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The real part of the conductance of a 1D conducting ring with magnetic properties that vary along the conductor is examined. The possibility of exciting a new type of spin polarization oscillations in the ring with an external emf is analyzed.
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75.76.+j Spin transport effects
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
72.15.Nj Collective modes (e.g., in one-dimensional conductors)
72.25.-b Spin polarized transport

Positive quasiclassical magnetoresistance and quantum effects in germanium quantum wells

I. B. Berkutov, V. V. Andrievskii, Yu. F. Komnik, and O. A. Mironov

Low Temp. Phys. 36, 1076 (2010); http://dx.doi.org/10.1063/1.3536348 (10 pages)

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Changes in the conductivity of p-type quantum-well heterostructures of Si0.05Ge0.95 alloy are studied at temperatures ranging from 0.352–7.1 K and magnetic fields of up to 11 T. The distinctive feature of the sample was asymmetric doping, with layers of Si0.4Ge0.6 with boron impurity concentrations of 2⋅1018 and 8⋅1018 cm−3 positioned on opposite sides of the quantum well. Shubnikov–de Haas oscillations were observed clearly against the background of a high quasiclassical positive magnetoresistance. The field dependence of the magnetoresistance is well described by a function of the form ρxx(B)/ρxx(0)∝B12/7, as predicted by a theory including the combined effect of both short- and long-range disorder. The contribution to the temperature and magnetic field dependences of the resistance owing to quantum corrections associated with weak localization and charge carrier interactions is determined. Strong spin-orbital scattering of holes on the quantum well is revealed by analyzing these corrections. A study of the variations in the amplitude of the Shubnikov–de Haas oscillations with temperature and magnetic field (including the monotonic behavior of the resistance with changing magnetic field) makes it possible to determine the effective mass of the charge carriers, m* = 0.17m0 The temperature dependence of the hole-phonon relaxation time was found by studying the overheating of charge carriers by an electric field.
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73.63.Hs Quantum wells
61.72.sd Impurity concentration
73.21.Fg Quantum wells
73.20.Fz Weak or Anderson localization
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
72.20.My Galvanomagnetic and other magnetotransport effects
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Structure and magnetic properties of multi-walled carbon nanotubes modified with iron

G. E. Grechnev, V. A. Desnenko, A. V. Fedorchenko, A. S. Panfilov, Yu. A. Kolesnichenko, L. Yu. Matzui, M. I. Grybova, Yu. I. Prylutskyy, U. Ritter, and P. Scharff

Low Temp. Phys. 36, 1086 (2010); http://dx.doi.org/10.1063/1.3530422 (5 pages)

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Magnetic properties of multi-walled carbon nanotubes modified with iron (MWCNT+Fe) are studied in detail in the temperature range 4.2–300 K. Carbon encapsulated Fe nanoparticles were produced by chemical vapor deposition. Low-temperature SQUID magnetization measurements are supplemented by structural studies employing thermogravimetric (TG) analysis, transmission electron microscopy (TEM), x-ray diffraction spectroscopy (XRD), and scanning electron microscopy (SEM). The magnetic susceptibility of MWCNT+Fe was also studied above room temperature to provide a complete picture of its magnetic phase transitions.
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61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
75.30.Cr Saturation moments and magnetic susceptibilities
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Kinetics of 4He gas sorption by fullerite C60. Quantum effects

A. V. Dolbin, V. B. Esel’son, V. G. Gavrilko, V. G. Manzhelii, N. A. Vinnikov, and S. N. Popov

Low Temp. Phys. 36, 1091 (2010); http://dx.doi.org/10.1063/1.3530423 (3 pages)

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The kinetics of helium gas sorption by C60 powder and subsequent desorption of the 4He impurity from the saturated powder is studied in the temperature interval 2–292 K. Evidence is obtained supporting the existence of two stages in the temperature dependences of sorption and desorption. These stages originate in different times taken by helium to occupy octahedral and tetrahedral interstices in the C60 lattice. The characteristic times of sorption and desorption coincide. It is found that the temperature dependences of the characteristic times for occupying the octahedral and tetrahedral interstices are nonmonotonic. When the temperature is lowered from 292 to 79.3 K, the characteristic times increase, which indicates thermally activated diffusion of helium in C60. With a further reduction to T = 10 K, the characteristic times decrease by more than an order of magnitude. Below 8 K the characteristic times of sorption and desorption are temperature-independent. This suggests tunnel diffusion of 4He in C60.
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68.43.Nr Desorption kinetics
66.30.Xj Thermal diffusivity
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Orientational order parameter in CO2–Kr solid solutions

V. V. Danchuk, M. A. Strizhemechny, and A. A. Solodovnik

Low Temp. Phys. 36, 1094 (2010); http://dx.doi.org/10.1063/1.3538923 (6 pages)

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Orientational order in binary CO2–Kr cryoalloys is studied at a fixed temperature of 30 K as a function of the krypton content x in the deposited gas mixture ranging from 0 to 59 mol.% by transmission electron diffraction. The integrated intensities of two regular reflections, (111) and (200), and two superlattice reflections, (210) and (211), are determined. The absolute values of the orientational order parameter η in the CO2 subsystem is recovered for all x using a previously proposed method modified for binary solid substitution solutions with randomly distributed components. For small x ⩽ 0.05, where the predominant clusters are isolated krypton atoms, η decreases linearly with x and for 0.05<x ⩽ 0.25 the orientational order parameter is constant to within the measurement error. For larger x, η manifests an unphysical increase. This last point means that for x ≥ 0.25 the assumption of a random distribution of the components is not valid. An analysis of these data shows that the first signs of a deviation from a random distribution (an indication of the onset of a phase transition) appear at x ≃ 0.258. The maximum concentration of krypton in a regular solution is about 33 mol.%, rather than the 38 mol.% derived from an analysis of the dependence of the lattice parameter on x. Values of the rank-4 orientational order parameter η4 are recovered. Peculiarities in the behavior of η4 for x>0.25 confirm the results of this analysis of η(x).
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61.66.Fn Inorganic compounds
82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
64.70.K- Solid-solid transitions
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling
68.65.Cd Superlattices
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Low-temperature plastic deformation of AZ31 magnesium alloy with different microstructures

Yu. Z. Estrin, P. A. Zabrodin, I. S. Braude, T. V. Grigorova, N. V. Isaev, V. V. Pustovalov, V. S. Fomenko, and S. E. Shumilin

Low Temp. Phys. 36, 1100 (2010); http://dx.doi.org/10.1063/1.3539781 (7 pages) | Cited 1 time

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The plastic deformation of AZ31 magnesium alloy under tension at temperatures of 4.2–295 K is studied as a function of its microstructure following squeeze casting (SC) and after severe plastic deformation (SPD) by hot rolling and equal-channel angular pressing. SPD reduces the average grain size and creates a texture that favors basal-plane dislocation glide. It is found that plastic deformation becomes unstable (serrated) at temperatures of 4.2–25 K and more stress jerks occur in the SPD polycrystal than in the SC alloy. The temperature dependence of the yield stress of the alloy is typical of thermally activated unpinning of dislocations from short-range barriers. The ratio of the yield stresses for the SPD and SC alloys at a given temperature is explained by hardening owing to a reduction in grain size and softening owing to a favorable texture. As the grain size is reduced, the rate of strain hardening of the alloy falls off, but its ductility (strain to fracture) increases because of the texture. The strain rate sensitivity of the alloy for T ⩽ 100 K is independent of microstructure and is determined by intersections with forest dislocations. As the temperature is raised over 150–295 K the strain rate sensitivity becomes greater owing to activation of dynamic recovery and an enhanced contribution from diffusion processes during plastic deformation of micrograined materials.
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81.40.Lm Deformation, plasticity, and creep
62.20.fk Ductility, malleability
62.20.fg Shape-memory effect; yield stress; superelasticity
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.mm Fracture
66.30.Fq Self-diffusion in metals, semimetals, and alloys
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Response to comments on the article “Electric properties of metallic nanowires obtained in quantum vortices of superfluid helium,” by E. B. Gordon, A. V. Karabulin, V. I. Matyushenko, V. D. Sizov, and I. I. Khodos, Fiz. Nizk. Temp. 36, 740 (2010) [ Low Temp. Phys. 36, 590 (2010) ]

E. B. Gordon, A. V. Karabulin, V. I. Matyushenko, V. D. Sizov, and I. I. Khodos

Low Temp. Phys. 36, 1108 (2010); http://dx.doi.org/10.1063/1.3539783 (1 page)

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Abstract Unavailable
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73.63.Nm Quantum wires
79.70.+q Field emission, ionization, evaporation, and desorption
67.25.dk Vortices and turbulence
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