Helium impurity nanocluster gels in superfluid helium

Rapid cooling of a helium impurity gas mixture to the superfluid helium temperatures allows us to prepare an impurity-helium condensate with jelly-like structure (impurity gel) in superfluid He-II. It is clear that the properties of these gels should be substantially different from those of a bulk impurity substance, constituting a new class of soft matter — quantum gel, where superfluid He-II filling the nanopores between the randomly connected impurity nanoclusters serves as the dispersion medium of the gel.     

Condensed water in superfluid He-II

It was found that when ⁴He gas containing water vapor as an impurity condenses on the surface of superfluid He-II cooled to ∼ 1.4 K, semitransparent clouds (icebergs) form in the volume of a glass cell filled with He-II below the He-II surface. The form of the icebergs extracted from the superfluid liquid remains virtually unchanged on heating up to ∼ 1.8 K. In the temperature range 1.8–2.2 K the thermometers register sharp temperature jumps, which are accompanied by jumps in the gas pressure in the cell and a repeated decrease, by more than two orders of magnitude, in the total volume of the condensate, i.e., the water content in the volume of an iceberg does not exceed 10²⁰H₂O molecules per 1 cm³. It can be inferred that porous icebergs, permeated with superfluid liquid and containing cores consisting of small clusters surrounded by a layer of solidified helium, form in the volume of He-II as the gas mixture condenses. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 11, 744–748 (10 December 1999)     

Neutron diffraction study of polycrystalline 4He in a porous medium

The elastic (diffraction) component of the neutron scattering cross section, which carries information on the atomic structure of solid helium confined in silica aerogel, has been studied. Analysis of the crystalline structure of solid helium in a porous medium, which is determined from the existing neutron diffraction data, indicates that the superfluid phase is localized inside a hexagonal close-packed phase and is not present in a body-centered cubic crystal. It has also been revealed that the addition of the ³He isotope changes the structure of solid helium and hardly affects the formation of a superfluid phase.     

Filament formation by impurities embedding into superfluid helium

The hydrogen molecules embedded in superfluid helium as a gas jet are shown to form long thin filaments. These filaments survived under a helium transition to anormal phase demonstrating their conjugated entity. The concentration of an impurity in the core of the vortex may be the mechanism of the impurity coalescence providing a cotton-like structure of the condensate obtained by introducing an impurity-containing gas helium jet to He-II. The text was submitted by the authors in English.     

Electrical properties of tellurium clusters on the void sublattice of an opal crystal: The important role played by the Te-SiO2 interface

The temperature dependences of electrical resistivity and of the Hall effect of nanocluster tellurium crystals obtained by filling the voids in a dielectric (opal) matrix with a melt of pure and doped Te were studied. The Hall hole concentration p _eff was found to increase anomalously (by more than two orders of magnitude) in a sample prepared from pure Te and cooled to helium temperatures. At T=1.45 K, the hole concentration in this sample was p _eff?6×10¹⁷ cm^?3. At the same time, the Hall effect in this sample was observed to reverse sign at T?200 K from positive for T<200 K to negative at higher temperatures. This implies a low impurity concentration (N _A is less than at least 10¹⁵ cm^?3). A nanocluster crystal of doped Te does not exhibit this anomaly; here, we have p _eff?6×10¹⁷ cm^?3 throughout the temperature region covered, as in the original Te. These features are assigned to the formation of a two-dimensional conducting accumulation layer near the Te-amorphous SiO₂ (the opal material) interface at low temperatures; such a layer determines the low-temperature properties of nanocluster crystals prepared from pure Te. Actually, we obtained a model of a three-dimensional structure formed from a two-dimensional film.     

NMR on superfluid<Superscript>3</Superscript>He in aerogel

The transition to superfluidity of³He in high porosity (98.2%) acrogel has been observed by nuclear magnetic resonance techniques. The onset of the transition at all pressures above 13 bar is marked by a sharp increase in NMR frequency similar to that observed in bulk³He-A. This suggests that the aerogel/superfluid phase is highly homogeneous although both the transition temperature, T_c, and the amplitude of the order parameter are substantially suppressed. The acrogel strands are ≈ 50Å in diameter, much smaller than the superfluid coherence length. Consequently, we have attempted to interpret our observations as an impurity scattering problem. Based on our measurements of the magnetic field dependence of T_c it appears that both magnetic and potential scattering play important roles where the magnetic scattering can be associated with solid³He on the aerogel surface. This is determined by isotopic exchange with⁴He, a process which appears to stabilize a new superfluid state similar to the bulk B-phase.     

Neutron-scattering investigation of the excitation spectrum of liquid helium

The results of the investigation of the temperature evolution of the scattering law S(Q, ω) of superfluid helium measured in the wave-vector range 0.3 < Q < 0.8 Å^?1 and helium temperatures from 1.0 to 2.2 K are reported. The investigations have been performed on the high-flux reactor at the Institut Laue-Langevin (France) with the IN6 neutron inelastic-scattering spectrometer. The deviation of the experimental scattering law of liquid helium from the damping harmonic oscillator model, which was previously observed independently in the experiments with the IN6 spectrometer [1] and with the DIN-2PI spectrometer (at the IBR-2 reactor, Dubna, Russia) [2, 3] and were more recently called extrapeak, has been corroborated. The temperature dependence of the extrapeak parameters has been determined. This dependence makes it possible to propose hypotheses on the nature of the extrapeak.     

Coexistence of superfluid and solid helium in aerogel

The results of recent neutron scattering studies of solid helium in silica aerogel are discussed. Previously I.V. Kalinin et al., Pis’ma Zh. éksp. Teor. Fiz. 87 (1), 743 (2008) [JETP Lett. 87 (1), 645 (2008)], we detected the existence of a superfluid phase in solid helium at a temperature below 0.6 K and a pressure of 51 bar, although, according to the phase diagram, helium should be in the solid state under these conditions. This work is a continuation of the above studies whose main goal was to examine the detected phenomenon and to establish basic parameters of the existence of a superfluid phase. We have determined the temperature of the superfluid transition from solid to superfluid helium, T _C = 1.3 K, by analyzing experimental data. The superfluid phase excitation parameters (lifetime, intensity, and energy) have a temperature dependence similar to that of bulk helium. The superfluid phase coexists with the solid phase in the entire measured temperature range from T = 0.05 K to T _C and is a nonequilibrium one and disappears at T _C.     

Nonmagnetic impurity effects and neutron scattering spectrum in iron pnictides

We study the effect of local impurity and the neutron scattering spectrum based on the five-orbital model obtained by the first principle calculation for iron pnictides. We find that the interband impurity scattering is induced by the complex multiorbital structure. This fact means that the fully-gapped sign-reversing s-wave state, which is predicted by spin-fluctuation theories, is very fragile against impurities. The result suggests a reasonable possibility that the fully-gapped s-wave state without sign reversal (s₊₊-wave) would be realized in dirty iron pnictides. We also find that broad peak structure observed in the neutron scattering measurements can be explained by the s₊₊-wave state.     

Mesoscopic magnetic inhomogeneities in the low-temperature phase and structure of Sm1−x SrxMnO3 (x < 0.5) perovskite

Results are presented of studies of the ¹⁵⁴Sm_1?x Sr_xMnO₃ system using neutron powder diffraction and small-angle polarized neutron scattering. An analysis of the neutron diffraction spectra showed that at T < 180 K these exhibit typical Jahn-Teller distortions of the manganese-oxygen octahedrons which persist under further cooling and on transition of the sample to a metallic magnetically ordered state. The magnetic contribution to the diffraction is satisfactorily described using the (A _x (A _y )F _z ) model and is interpreted as the coexistence of ferromagnetic and antiferromagnetic phases. The exaggerated widths of the diffraction lines indicate an appreciable contribution from microdeformations evidently associated with the inhomogeneity of the system. Small-angle polarized neutron scattering showed that the Sm system for x = 0.4 and 0.25 is magnetically inhomogeneous in the low-temperature phase. Ferromagnetic correlations occur on scales of around 200 Å and having dimensions greater than 1000 Å which, combined with the temperature hysteresis of the magnetic small-angle scattering intensity observed for an x = 0.4 sample in the low-temperature phase, suggests that the transition is of a percolation nature.     

Specific features of neutron scattering upon heat treatment of the Co67Fe31V2 alloy

This paper reports on the results obtained from small-angle neutron scattering investigations during annealing of two plates of the Co₆₇Fe₃₁V₂ alloy under heating the first and second samples to 360 and 500°C, respectively, as well as in applied weak magnetic fields (H ≈ 3 Oe). To elucidate the role of the vanadium impurity, measurements have also been performed on the Co₆₈F₃₂ alloy. It has been revealed using neutron diffraction that, at temperatures below 360°C, the Fe and Co atoms begin to undergo ordering in the samples, which results in a change in the space group of the crystal structure. With an increase in the temperature, a fine-dispersed phase is formed in the Co₆₇Fe₃₁V₂ alloy, which leads to a qualitative change in the small-angle neutron scattering pattern. It has been found that, after annealing and magnetization (H > 4 kOe) of the second sample, there is a strong difference in the integrated intensities of scattered neutrons for opposite directions of their spins in the primary beam.     

Study of the heavy-fermion compound CeRu<Subscript>2</Subscript>Si<Subscript>2</Subscript> by the small-angle neutron scattering method

In order to directly observe neutron scattering by heavy fermion quasiparticles at low temperatures, a CeRu₂Si₂ single crystal has been studied by the small-angle neutron scattering method. In the experiment, neutron scattering is observed at T = 0.85 K for momentum transfers q ≤ 0.04 Å^?1, which is treated as the orbital component of magnetic scattering by heavy fermion quasiparticles. It has been found that the application of a magnetic field H = 1 T leads to both an increase in the observed scattering and its anisotropy with respect to the field direction. Moreover, measurements in the magnetic field reveal additional scattering for q > 0.04 Å^?1, which is well described by a Lorentzian and is interpreted as neutron magnetic scattering by spin-density fluctuations with a correlation radius R_c ≈ 30 Å.     

Nature of the magnetic excitation spectrum in (Sm,Y)S: CEF effects or an exciton?

The dynamic magnetic susceptibility spectrum in a single-crystal sample of the intermediate-valence compound Sm_0.67Y_0.33S is studied by inelastic neutron scattering with neutron momentum transfer and sample temperatures varying over wide ranges. Two coupled collective modes have been found in the spectrum. Unlike the higher energy mode, whose intensity approximately follows the form factor of Sm²⁺, the lower energy mode exhibits a stronger angular dependence than could be expected from the form factor for the localized f electrons. The total intensity of the inelastic component of the magnetic response decreases with increasing temperature; this is accompanied by the appearance of a broad quasi-elastic signal of a magnetic nature at significantly lower temperatures than follows from the calculated intensities of the transitions within the excited multiplet of the Sm²⁺ ion. An analysis of the observed features allows the suggestion to be made that the fine structure of the magnetic excitation spectrum in (Sm,Y)S is associated with the formation of an exciton-like intermediate-valence state on Sm ions rather than with the crystal-electric-field effects.     

Quasiparticle lifetime of electron–electron interactions for two-dimensional electron gases with magnetic field

We theoretically study the electron–electron scattering rate τ_ee⁻¹for electrons in a two-dimensional electron gas with a perpendicular magnetic field, within theGWand plasmon-pole approximations, as functions of temperatureT, impurity scattering rate Γ and magnetic fieldB. The τ_ee⁻¹increases with increasingTand increasing Γ, and shows the structure of the Landau levels asBis changed.     

Enhanced thermal conductance at a copper-liquid helium interface in the presence of acoustic streaming

In a conventional Kapitza resistance experiment involving heat transfer across a copper surface into liquid helium, an acoustic streaming velocity field (at 10 MHz) was directed transverse to the surface normal. Ultrasound had no observable effect on the heat transfer to the superfluid phase (He-II), but in the normal fluid phase (He-I) the thermal conductance increased linearly with acoustic velocity amplitude, reaching a value 2.5 times the zero sound conductance for a sound velocity amplitude of 0.8 cm s^?1.     

Observation of a superfluid phase in solid helium

Evidence of a superfluid liquid phase present in polycrystalline helium at a temperature of 0.2 K and a pressure of 51 bar has been obtained by means of inelastic neutron scattering. The superfluid component is absent at a temperature of 0.6 K and the same pressure. Thus, a “solid helium-superfluid helium” phase transition has been discovered. The sample of solid helium in a porous medium (silica aerogel) has been prepared with the use of a capillary blocking technique. The shape of the structure factor of the superfluid phase indicates the presence of clusters or the effects of a restricted geometry. The results may be used to explain the nonclassical rotational inertia phenomenon in solid helium (often referred to as supersolidity, Nature, 2004).     

Magnetic structure of a CaMnO2.75 crystal with ordered oxygen vacancies

The magnetic state of a CaMnO_{3 ? δ} crystal with ordered oxygen vacancies (for δ = 0.25, when the numbers of Mn⁴⁺ and Mn³⁺ ions in the manganite are equal to each other) is studied using neutron diffraction. Magnetic scattering in the CaMnO_2.75 crystal in the ground state is determined by the wave vector (1/2, 1/2, 1/2)2π/a _c (G-type antiferromagnetic order). In the crystal, long-range magnetic order disappears at the temperature T _N = 116 K, whereas short-range magnetic order is retained up to 240 K. It is shown that the instability of the G-type structure in the temperature range 60 K < T < T _N is associated, in many respects, with the formation of the C′ antiferromagnetic phase in the bulk of the crystal. The structure of the C′ antiferromagnetic phase involves chains with Mn³⁺-Mn⁴⁺ ferromagnetic interaction. A comparison of the results of the neutron diffraction investigations with the experimental data on the magnetic characteristics and electrical resistivity demonstrates that the specific features revealed in the spin system of the CaMnO_2.75 crystal are governed directly by the competition of the Mn³⁺-Mn⁴⁺ ferromagnetic double exchange with the antiferromagnetic superexchange between manganese ions.     

Superconductor-insulator transition in (PbzSn1−z )0.84In0.16Te

The superconductor-insulator transition that occurs at liquid helium temperatures in the (Pb_zSn_1?z )_0.84In_0.16Te semiconductor system with varying lead concentration z = 0.5–0.9 is experimentally investigated. The transition is attributed to the change in the energy characteristics of In impurity centers due to the variation in the amount of lead. The insulator state appears with the transition from the mixed band-impurity conduction, which is characterized by resonant scattering of carriers into the quasilocal indium impurity states, to the hopping conduction between indium impurity states. The sample with z = 0.8 is found to exhibit a variable range hopping conduction described by Mott’s law. Factors that lead to the hopping conduction via impurity states are considered.     

Freezing-out of nuclear polarization in radioactive core ions of microclusters, “snowballs” in superfluid helium

A novel method was tried successfully to trap ions and to freeze out their nuclear polarization inside aggregates of helium atoms, snowballs, in superfluid helium. Spin polarized¹²B (T _1/2=20.4 ms) ions were introduced into superfluid helium and snowballs were created around the impinged impurity ions. Beta-ray asymmetry was measured to obtain the nuclear polarization of decaying¹²B. The comparison with the initial value of¹²B polarization produces that no relaxation in polarization was observed throughout lifetime of¹²B.     

The special features of the ground state in CeAl3

The temperature (3–60 K) and transferred momentum (0.3–2.3 Å^?1) dependences of the intensity of quasi-elastic magnetic neutron scattering were studied for the polycrystalline heavy-fermion CeAl₃ compound to elucidate the special features of its ground state. Transferred momentum variations caused oscillations of the intensity of quasi-elastic magnetic neutron scattering, which was evidence of magnetic correlations in the f-electron subsystem occurring in a fairly wide temperature range.