High-resolution measurements of thermal expansion along the uniqueb-axis of squaric acid

The thermal expansion of squaric acid along the uniqueb-axis was measured between 323 K and 418 K with a resolution of 2·10^–7 in strain and 5·10^–4 K in temperature using a capacitance dilatometer. The anomaly associated with the antiferro-electric phase transition occuring atT _c=373.5 K was observed over the rangeT _c–50 K<T<T _c+7 K. The regular lattice expansion follows a generalized Grüneisen's law with a Debye-temperature _D =660 K and a uniaxial pressure-dependence of _D /p _b . The critical behaviour close toT _c was studied with a sweep rate of 36 mK/h. A hyteresis loop of width 23 mK was observed in the strain curve, indicating a 1st order transition. However, no discontinuity in dilatation was observed, and therefore the transition is close to a multicritical point. The anomalous increase in the expansion coefficient can be described with critical exponents ==0.56 in the interval 0.4 K<|T–T _c|<12 K. Closer toT _c the divergence is stronger. A generalized Pippard relation between the expansion coefficient and specific heat holds to within 1 K from the transition point. The inconsistencies apparent closer toT _c are interpreted as a consequence of a weak 1st order transition.     

Possibility of a Lifshitz phase transition in Cd observed by a singularity in the muon Knight shift

During the past much effort has been devoted to a systematic study of the muon Knight shiftK in metallic environments and its implications on the local electronic structure of hydrogen in metals [1]. These measurements in simple metals were essentially all carried out in polycrystalline samples at room temperature. The present measurements in Cd in polycrystalline and single crystal samples cover a temperature range between 20 K and the melting point of this strongly anisotropic metal (hcp crystal structure,c/a ratio 1.89 — idealc/a ratio 1.63). These measurements add qualitatively new and interesting aspects and insights on the screening of a light hydrogen isotope in a metal as well as on certain properties of the host material itself. The outstanding features of the muon Knight shift in Cd are: (i) a strong intrinsic temperature dependence with an increase ofK of more than 100% between 20 K and the melting point (T=593 K), (ii) an anomaly at 110 K in the form of a singularity in the isotropic part ofK which is interpreted as a band structure effect, (iii) an anisotropic Knight shift contribution fitting the expressionK(T,)=K _iso(T)+K _ax(T) * (3 · cos² –1)/2, where both, the isotropic and the axial contribution ofK , are strongly temperature dependent.     

Temperature dependence of the magnetic penetration depth in YBa2Cu3O6.95

We report precision transverse fieldSR measurements of the internal field distribution in the vortex state of crystalline YBa₂Cu₃O_6.95. A novel low background apparatus was used to study a mosaic sample of three high quality single crystals (T_c=93.3K). The observed frequency spectra in magnetic fields of 5kG and 15 kG applied along the c-axis have the characteristic features expected for a regular vortex lattice with some additional broadening. From a preliminary analysis we find that [(0)/(T)]² has a linear temperature coefficient forT<30 K. Such a term is inconsistent with simple s-wave pairing in the superconducting state. These results support recent microwave measurements of(T) on similar crystals in zero applied field but differ significantly from previousSR reports on sintered powders and crystals with lower T_c.This research was supported by the Natural Sciences and Engineering Research Council of Canada.     

Tunneling conductance of a Bi2Sr2CaCu2O8-SnO2 junction along the c-axis

fine structure was observed in the conductance curve of a tunneling junction composed of a single crystalline Bi2212 and an evaporated SnO₂ film. It is similar to those of Bi2212-GaAs mechanical junctions and there is a certain correspondence between the structure and the phonon density of states. Thus the previous conclusion that the structure is due to phonons has been complemented by this work. The energy gap 2 was 57 meV at 13 K and T _c was 78 K. 2(0)/k _B T _c is then 8.3. (T) showed the BCS-like temperature dependence.     

Temperature dependence of the electric field gradient in Er metal

The method of perturbed angular distributions was used to measure the temperature dependence of the electric field gradient in Er single crystal for 98 KT156 K. The I=11^– isomer in Er¹⁵⁴) was used as a probe. ₀ increases monotonically for 98 KT259 K and then decreases. A possible cause for this effect may be short range interactions between the f electrons above the Neel point.Visitor from the Weizmann Institute, Rehovoth, Israel.     

A thin film source of ultra-cold neutrons

Ultra-cold neutron (UCN) densities have been calculated for a thin film source using realistic phonon spectra of the film material (hydrogen and deuterium). The UCN's build up to a significant density inside a cryogenic container whose walls support the thin films. The UCN's density dependence on the neutron temperature,T _n, was also investigated. It was found that the density reaches a maximum value atT _n20K. A deuterium film at low temperature (5K) andT _n=30 K yields a UCN density 10^–9 ₀ where ₀ is the total flux.Work supported in part by NSF Grant # DMR-8024662     

Estimates of hypernuclear production via the exclusive (p,K+) reaction

We investigate the possibility of producing hypernuclei with proton beams via (p, K ⁺) reaction. We present differential cross sections calculations utilizing the distorted wave impulse approximation in momentum space. We consider the reactions¹²C(p,K ⁺) ¹³ and¹⁶O(p,K ⁺) ¹⁷ within the energy region 0.8 GeV1.2 GeV. We study both the case of formation of in S-state (¹²C) and P-state (¹⁶O). We take into account the contribution of both one-step and two-step processes when K pair is produced directly by incoming proton and intermediate pion, respectively. It is found that practically in all cases the two-step processes give significant contribution.Work supported by KFA Julich     

New phenomena in homoepitaxial growth of metals

The growth of Pt(111) by Pt vapour deposition is studied by He diffraction as a function of substrate temperature and deposition rate. At a deposition rate of about 2.5×10^–2 monolayers/second several growth modes are observed: layer-by-layer (2D-) growth at 450 KT _s800 K, multilayer (3D-) growth at 340 KT _s450 K and reentrant layer-by-layer (2D-) growth at T _s340 K. The observed growth modes and in particular the reentrant 2D-growth are shown to be characteristic of growing Pt(111) under clean conditions, i.e. not influenced by contaminants. The influence of the intra- and interlayer mass transport on the growth mode is discussed in the light of experimental and simulation results. The 3D-growth mode is attributed to the existence of an activation barrier which suppresses the descent of adatoms from the top of the growing adatom islands onto the lower terraces. The barrier can be overcome by thermal adatoms at T _s450 K enabling interlayer mass transport which leads to 2D-growth. The reentrant 2D-growth occurs due to a break down of this barrier for small, irregularly shaped islands.     

The decay of spin-isospin modes in a semi-infinite slab

The surface response for charge exchange (p, n) and (³He, T) reactions is studied in the-region using the semi-infinite slab model. The contribution to the total response from different decay channels, (NN, N, ), is calculated. These decay channels corresponds to the exclusive channels, (pp, p ⁺, ⁺), measured in recent (p, n) and (³He, T) experiments. The in-medium properties of the-resonance is taken into account by using microscopic calculations of the-width in nuclear matter. From the-width in nuclear matter a non-local imaginary-potential, as well as a local potential, is constructed for the semi-infinite slab model. The results in the semi-infinite slab model gives a qualitative understanding of the exclusive experiments. The exclusive (NN, N, ) channels are more sensitive, than the total response, tog-correlation parameters,-width and the absorption function used at the external vertex. Our calculations suggest low values of theg-correlation parametersg _N andg , (0.3). The results with the-width represented as a non-local or a local-potential are very similar, with only minor differences in the exclusive channels.Supported in part by the Swedish Natural Science Research Council and by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Nuclear Physics Division of the U.S. Department of Energy under Contract No. DE-ACO3-76SF00098     

Low-temperature transport properties of UCu5

The electrical resistivity (T) and the thermal conductivity (T) have been measured for UCu₅ in the temperature range between 0.02 and 20 K. Two distinct anomalies in (T) are due to previously established phase transitions at approximately 15 and 1 K, respectively. They indicate considerable changes in the electronic structure of this compound, implying sizeable truncations of the Fermi surface with decreasing temperature at both transitions. In almost the entire covered temperature range the thermal conductivity is dominated by phonon contributions. Its temperature dependence is fairly well reproduced by a calculation considering phonon scattering by electrons and by point defects. At very low temperatures, asT approaches 0 K, the Wiedemann-Franz law _e L ₀ T, where _e is the electronic part of (T) andL ₀ is the Lorenz number, is almost perfectly fulfilled.     

ИЗМЕРЕНИЯ ЭЛЕКТРОННЫХ РЕЛАКСАЦИОННЫХ ПРОЦЕССОВ В МАРГАНЦЕВЫХ ФЕРРИТАХ

The measurements of magnetic loss factor in the system Mn _x Fe_3–x O_4+y for 0.62x1.66 are described. The temperature range used was 2°K to 360°K and four fixed frequencies 75, 150, 300, and 600 kHz were employed. The results show, that three distinct processes were found with various dependences upon composition. The corresponding maxima in tg vsT curves are situated at 2°K, 10°–50°K, and 90°–350°K respectively. The connection of these processes with electron transport and/or reorientation of distortions surrounding octahedrally coordinated Mn³⁺ ions is used as a basis for an interpretation already proposed earlier.     

Thermoactivated creep and stress relaxation in Cd + 0·08 at% Sn single crystals

The activation area of plastically deformed Cd + 0·08 at% Sn single crystals was determined from differential creep test and stress relaxation at temperatures 78 K, 200 K and 295 K. The activation areaA depends on the resolved shear stress, ,A ^{* –r} wherer is dependent on temperature and the testing method used. For temperatures above 0·3T _m recovery during stress relaxation should be considered.     

Thermal expansion,sound velocities,specific heat and pressure derivative ofT c in YBa2Cu3O7

We report measurements, between helium and room temperature, of the thermal expansion (T), sound velocitiesv _L (T) andv _T (v) and specific heatC(T) of YBa₂Cu₃O₇ samples, all cut from the same sintered pellet. A linear term in (T) is resolved at low temperatures. It is compared with a corresponding term in the low temperature specific heat and, using the bulk modulus from our sound propagation measurements, a Grüneisen parameter of usual size (1.2±0.4) is derived-in conformity with the possible existence of non-superconducting carriers well belowT _c . Longtime drift effects in the length of the sample between 30 K and 60 K point to the vicinity of a structural instability. The Debye temperature derived fromC(T) amounts to about 450 K atT=T _c =91 K and forT0 approaches 350 K, in agreement with the value deduced fromv _L (T) andv _T (T). The superconducting transition is indicated by anomalies with idealized discontinuities and C. A thermodynamic relation between and C yields the relative pressure derivative ofT _c ,t _p =T _c ^–1 (T _c /p) _p0=+(0.7±0.2)·10^–7 kbar^–1.     

Electronic phase separation in La-Cuprates

The mechanisms driving the phase separation in high-T _c La cuprates is studied by magnetic susceptibility and electrical resistivity measurements. The experiments are performed under different thermal treatment. Three temperature ranges can be resolved characteristic for different physical processes: At 150T170K a space limited diffusion of superparamagnetic clusters (single hole clusters) occurs leading to the formation of a small but rather tight percolative subphase which at highT _c values becomes superconducting. Above 180 K the cluster motion becomes unrestricted in space and a more extended subphase is built up, however, with lowerT _c values. Above 230 K oxygen diffusion starts leading to an unexpected destruction of the conducting/superconducting phase.     

Possible observation of localized 4f shell breakdown in praseodymium under pressure

A phase transition occurs in Pr at a pressure of 190 kbar and 300 K as seen from a precipitous resistance drop. In the low-P phase, the temperature dependence of the electrical resistivity is highly anomalous, the following features becoming more pronounced toward the transition. The temperature coefficient is extremely small between 300 K and 100 K. Below 50 K, the resistivity drops steeply. Both these features are well known for the and phase of cerium. In contrast, the high-P phase exhibits a normal, i.e. an almost linear temperature characteristic as also observed for Ce in the collapsed phase. We suggest that the phase transition in Pr may turn out to be a counterpart of the transformation in Ce which is now understood as the delocalization of the 4f ¹ configuration into a 4f band.     

Muon diffusion in the metal hydrides β-PdHx(x=0.70 and 0.75) and LaNi5H6

We report on zero and transverse fieldSR measurements in LaNi₅H₆ and in-PdH_x (x=0.70 and 0.75) between 16 K and room temperature. The ⁺-depolarization is predominantly caused by the spread in nuclear dipole fields from the protons. Motional averaging is caused by the combined motion of protons and the ⁺. The results are quite unexpected and point to ⁺-trapping within regions of largely immobile hydrogen configurations or to a highly correlated ⁺-proton diffusion. In LaNi₅H₆ a linear change of the second moment with temperature between 20 K and 120 K is indicated.     

Detailed investigation of the “DIP” structure in the μ+-relaxation rate in Nb

The depolarization rate of positive muons implanted in a number of nominally pure, cylindrical Nb single crystals (maximal 250 ppm Ta, 100 ppm N + O) was investigated at two temperatures, viz. 14.0 and 36.8 K, in a high transverse field of 7.5 kG with the stroboscopicSR technique in order to study the nature of the dip at 22 K. To determine the sites at which the muon is trapped on both sides of this dip, the full angular dependence of the depolarization rate was measured by rotating a large single crystal around its 110 cylinder axis in a transverse magnetic field. The resulting curves for both temperatures are quite different, reflecting clearly the different environment in which the muon is trapped above and below 22 K. The trapping site at 36.8 K was identified to be of tetrahedral symmetry, located near a Ta substitutional impurity and possibly associated with an interstitial impurity. Lattice distortions due to these impurities and radial relaxation around the muon,R/R, were determined. The latter is +6.7(6)% for nearest neighbors and –6(2)% for next nearest neighbors. The 14.0 K angular dependence could not be fitted by considering distorted tetrahedral and octahedral sites and pointlike muons.     

Low temperature self-ion irradiation of pure and granular aluminium-films

Al-films, evaporated at room temperature under different oxygen partial pressures, were irradiated with self-ions (500 keV, Al⁺⁺) at low temperature (<7 K). The observed increase of the resistivity and of the superconducting transition temperatureT _c depends strongly on the oxygen contentc ₀ present in the layers. A qualitative different behaviour of the above quantities was found for different fluence ranges of the bombarding ions. For high fluences in all cases an oxygen stabilized disorder state was obtained with correspondingT _c-increases betweenT _c=0.2 K for the purest films (c ₀0.5 at %) andT _c =1.2 K for films with c ₀=40 at %. The annealing behaviour of the irradiated films is also dependent on the oxygen contentc ₀.     

On a nonstandard two-parametric quantum algebra and its connections withU p,q (gl(2)) andU p,q(gl(1/1))

A quantum algebraU _{p, q} (,H,X _±) associated with a nonstandardR-matrix with two deformation parameters (p, q) is studied and, in particular, its universal -matrix is derived using Reshetikhin's method. Explicit construction of the (p, q)-dependent nonstandardR-matrix is obtained through a coloured generalized boson realization of the universal -matrix of the standardU _{p, q}(gl(2)) corresponding to a nongeneric case. General finite dimensional coloured representation of the universal -matrix ofU _{p, q}(gl(2)) is also derived. This representation, in nongeneric cases, becomes a source for various (p, q)-dependent nonstandardR-matrices. Superization ofU _{p, q}(,H,X _±) leads to the super-Hopf algebraU _{p, q}(gl(1/1)). A contraction procedure then yields a (p, q)-deformed super-Heisenberg algebraU _{p, q}(sh(1)) and its universal -matrix.     

Gamma-ray induced reduction of W6+ into W5+ in the Na2O -WO3 glasses

The ESR spectra of the 38Na₂O · 62WO₃ glasses irradiated with the⁶⁰Co -rays of 10⁵–10⁶ Gy comprised a broad singlet peak (g=1.65, H _pp=28.6 mT), a narrow singlet (g=2.00₁, H _pp=1.36 mT), and a narrow doublet (g=2.00₁, H _pp=0.72 mT,A=4.96 mT). The broad singlet and the two narrow peaks were respectively ascribed to the W⁵⁺ and the W-OHC (oxygen hole center) produced by the -ray irradiation. The _D (360 K) and _E (180 K), obtained from the low-temperature¹¹⁹Sn-Mössbauer measurement of 38Na₂O · 61WO₃·SnO₂ glass, indicated that octahedral Sn⁴⁺ substituted for octahedral W⁶⁺ and played a role of network former (NWF). A gradual increase in the isomer shift () from –0.02 to 0.09 mm s^–1 was observed with an increasing -ray dose, due to the charge transfer from oxygen to the Sn⁴⁺.