Superconducting properties of ion implantedIn Mn alloys

Annealed Indium films have been implanted with 150 keV-Mn ions at low temperatures. TheT _c -depression and the low temperature part of the resistivity exhibit the Kondo effect, the Kondo temperature being strongly influenced by lattice defects. The lattice disorder has been increased by pre-implantation of In ions. The influence of lattice disorder on the magnetic properties ofIn Mn alloys is explained by applying the strong coupling theory of superconductors.It is found that the main effect results from a shift of the Fermi energy relative to the virtual bound state, due to a variation of the effective massm ₀·(1+).     

Investigation of the hyperfine interaction of the transition element impurity193Ir in gadolinium

The combined magnetic and electric hyperfine interaction of dilute¹⁹³Ir impurities in ferromagnetic Gd has been investigated by means of the Mössbauer effect. The magnetic hyperfine field of¹⁹³Ir in Gd at 4.2 K is: |H _hf(Ir:Gd)|=624(6) kG.The electric fieldgradient at the site of Ir in Gd is:V _zz (Ir:Gd)=+19.5(5.0) × 10¹⁷ V/cm².The fieldgradient is axially asymmetric with an asymmetry parameter of 0.53(2)1.     

On the structure of non-superconducting YBa2Cu3O6+ɛ

The crystal structure of non-superconducting, tetragonal YBa₂Cu₃O₆₊ ( close to 0) (s.g.P4/mmm) was studied at room temperature by means of single crystal X-ray diffraction, electron microscopy and electron diffraction. Crystals may easily be obtained by heating YBa₂Cu₃O₇ samples under vacuum. For 0, the tetragonal single crystals are chemically and crystallographically homogeneous — without any twin domains. The only defects immediately apparent are stacking faults alongc. The structure is very closely related to that of the high-T _c superconductor YBa₂Cu₃O₇. The most important difference is, that the oxygen on theb-axis (O(1)), which together with Cu(1) forms Cu–O–Cu–O-chains alongb in YBa₂Cu₃O₇, is vacant in YBa₂Cu₃O₆. Some crystals show a superstructure with superstructure reflections which are sharp in thea ^*–a ^*-plane but form diffuse streaks alongc ^*. We propose a model which relates these superstructure reflections to a 2-dimensional ordering of residual oxygen atoms on the O(1) site (corresponding to >0 in the chemical formula YBa₂Cu₃O₆₊).     

Continued fractions in the kinetic theory of supersonic amplification in anisotropic semiconductors

The method previously developed by the author (Fiz. Tekh. Poluprovodn.,9, 360, 1975;10, 808, 1976) is applied to evaluate the electron coefficient of supersonic absorption (or of amplification) with arbitrary ql (_e) and of acoustic electric current (j ^ae ) in a single-pole one-valley semiconductor in the case of no external electric field E₀. The anisotropy of the acoustic-electronic interaction (the piezoelectrical one and the deformation potential) is taken into account as well as the anisotropy of scattering and of the effective mass. In contrast to the Zil'berman-Mishin results (Zh. Éksp. Tekh. Fiz.,65, 1473, 1973; Fiz. Tekh. Poluprovodn.,10, 1882, 1976) which were obtained for the isotropic case, compact expressions are obtained for _e and j ^ae in the form of an infinite continuous fraction of simple structure with all the spherical harmonics and all their relaxation times (_n) taken into account. The summation of the fraction in the Zil'berman-Mishkin approximation converts the latter into a finite continuous fraction yielding at once any order of this approximation which simplifies very considerably the analysis of its convergence. The taking into account of anisotropy does not really complicate the result and maintains its compact form. Another method of fraction summation enables one to estimate the contribution of individual harmonics.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 5, pp. 73–77, May, 1978.In conclusion, the author would like to express his thanks to P. E. Zil'berman for the fruitful discussions he has had with him.     

The continuity equation and the Hamiltonian formalism in quantum mechanics

The relationship between the continuity equation and the HamiltonianH of a quantum system is investigated from a nonstandard point of view. In contrast to the usual approaches, the expression of the current densityJ is givenab initio by means of a transport-velocity operatorV _T, whose existence follows from a weak formulation of the correspondence principle. Once given a Hilbert-space metricM, it is shown that the equation of motion and the continuity equation actually represent a system in theunknown operatorsH andV _T, due to the arbitrariness on the initial condition of the quantum state. The general solution is given in some cases of special interest and a straightforward application to relativistic quantum mechanics is performed.This work was partially supported by the Ministero della Pubblica Istruzione.     

Exchange interaction and crystal field splitting of (La,Gd)B6 single crystals

The ESR on single crystals of (La, Gd)B₆ (c500 ppm) is reported. The splitting of Gd is determined as 50 mK. Theg-shift g=+0.018 and thermal broadeningb=9G/K show that in LaB₆ the conduction electrons are predominantly localized at the La site.     

X-ray absorption spectroscopy of the 90K superconductor YBa2Cu3O7−δ

X-ray absorption spectroscopy has been used to probe the Cu environment of the new highT _c YBa₂Cu₃O_7– superconductor. Evidences of other valencies than two are found for copper. Besides Cu¹⁺ which is localized on Cu₁ sites when the unit cell is oxygendeficient, one invokes a ground state which merges bivalent 3d ⁹ to 3d ⁹ L configurations as proposed in a recent paper (L stands for a ligand hole in the CuO bond) very similar to that observed in many light rare earth oxides.     

Kondo effect in spin glasses: Electrical resistivity

We investigate the influence of the Kondo effect on the electrical resistivity of spin glasses. Our approximation reduces in the limit of vanishing impurity concentrations to the Suhl-Nagaoka theory for the Kondo effect. The magnetic impurity interactions are taken into account in the form of time dependent two-spin correlation functions which can be measured by neutron scattering. The dynamics of the impurity spins leads to a partial destruction of the Kondo effect. For the resistivity this can be described by a temperature dependent effective spinS _eff ² (T) withS _eff ² 0 forT0 andS _eff ² S(S+1) forT, and by a reduction of the Kondo temperatureT _K. Sufficiently strong interactions lead toT _K=0. We obtain a resistivity maximum at a temperatureT _m due to the interplay of the Kondo effect and the spin dynamics;T _m depends onT _K and on the excitation spectrum, and therefore on parameters such as impurity concentration or pressure. The ratioT _m/T_f (T _f is the freezing temperature) is calculated for a single relaxation time and for a square density of relaxation modes and is compared with experimental data forAuCr,AuMn,AgMn, andCuMn. The influence of other possible modes on various spin glass properties is discussed.SFB 125 Aachen-Jülich-Köln     

Frequency dependence of the freezing temperature in spin glasses: A comparative study of (La, Gd)B6, (Y, Gd)Al2 and (La, Gd)Al2

The frequency dependence of the freezing temperatureT _f() is determined for the dilute spin glass systems (La, Gd)B₆ and (Y, Gd)Al₂ in the frequency range 10–1,000 Hz. While for (La, Gd)B₆,T _f() is found to be weak, for (Y, Gd)Al₂ T _f() is even stronger than for the previously studied system (La, Gd)Al₂. Both, measurements of the temperature dependence of the susceptibility nearT _f and calculations of the RKKY pair interaction, suggest that this difference is correlated with a different sign of the nearest-neighbor interaction, which appears to be antiferromagnetic for (La, Gd)B₆ and ferromagnetic for (Y, Gd)Al₂ as well as (La, Gd)Al₂.     

Tunneling into lanthanum with rare earth dopings

Conductance measurements of Al–AlO _x -LaRE (RE: Pr, Tb, Ho, Gd, Y) tunnel junctions are performed in view of crystal field effects in lanthanum, doped with certain rare earth elements. The tunneling density of states ofLaPr andLaHo gives support for theoretical predictions of an inelastic pair breaking mechanism. Crystal field induced structures in the tunneling density of states, also predicted, have not been detected.Project of Sonderforschungsbereich Darmstadt/Frankfurt financed by special funds of the Deutsche Forschungsgemeinschaft     

From bunches of membranes to bundles of strings

Bunches of membranes and bundles of strings exhibit unbinding transitions from a bound state at low temperatures to an unbound state at high temperatures.N freely suspended manifolds unbind continuously at the unique unbinding temperatureT _u ^f which is independent ofN. The amplitudes of the critical singularities have a strongN-dependence, however, which implies that the critical region for the continuous transition becomes very small and the transition becomes very abrupt in the limit of largeN. IfN membranes or strings are bound to a rigid surface, they undergo a sequence of either two or ofN successive transitions. In general, the rigid surface affects the contact probabilities of the fluctuating manifolds. For effectively repulsive interactions, the contact exponent ₂ which governs the probability for local pair contacts satisfies the scaling relation ₂=d + whered and denote the dimensionality and the roughness exponent of these manifolds.Dedicated to Herbert Wagner on the occasion of his 60th birthday     

Applied field Mössbauer spectra of low-spin mononuclear and binuclear iron(III)phthalocyanines

Well characterized low-spin Fe(III)phthalocyanine complexes have recently been synthesized. Two six-coordinate examples, a binuclear -oxo bridged complex [((-picoline)FePc)₂O],1, and a mononuclear bis-azido complex (PNP)[FePc(N₃)₂],2, display typical S=1/2 Fe(III) parameters (1, =0.20 mm/s, E_Q=1.79 mm/s;2, =0.22 mm/s, E_Q=2.47 mm/s at 4.2K). As an applied longitudinal magnetic field is increased to 32kOe the corresponding splittings in1 and2 indicate very small effective fields at the Fe nuclei. This is due to an S=O ground state in the weakly antiferromagnetically coupled complex1. The small hyperfine splitting in2 contrasts with the well resolved hyperfine splitting reported for various S=1/2 Fe(III)porphyrins and Fe(III)heme proteins.     

On the slow decay ofO(2) correlations in the absence of topological excitations: Remark on the Patrascioiu-Seiler model

For spin models withO(2)-invariant ferromagnetic interactions, the Patrascioiu-Seiler constraint is |arg(S(x))–arg(S(y))|₀ for all |x–y|=1. It is shown that in two-dimensional systems of two-component spins the imposition of such contraints with ₀ small enough indeed results in the suppression of exponential clustering. More explicitly, it is shown that in such systems on every scale the spin-spin correlation function obeys S(x)·S(y)1/(2|x–y|²) at any temperature, includingT=. The derivation is along the lines proposed by A. Patrascioiu and E. Seiler, with the yet unproven conjectures invoked there replaced by another geometric argument.Dedicated to Oliver Penrose on the occasion of his 65th birthday.     

Variational Estimates for Discrete Schrödinger Operators with Potentials of Indefinite Sign

Let H be a one-dimensional discrete Schrödinger operator. We prove that if _ess(H)[–2,2], then H–H₀ is compact and _ess(H)=[–2,2]. We also prove that if has at least one bound state, then the same is true for H₀+V. Further, if has infinitely many bound states, then so does H₀+V. Consequences include the fact that for decaying potential V with , H₀+V has infinitely many bound states; the signs of V are irrelevant. Higher-dimensional analogues are also discussed. Supported in part by NSF grant DMS-0227289On leave from Department of Mathematics, University of Illinois at Urbana-Champaign, 1409 W. Green Street, Urbana, IL 61801-2975, USASupported in part by NSF grant DMS-0140592     

The magnetic moment of103Ru

The nuclear magnetic moment of¹⁰³Ru has been determined using the technique of low-temperature nuclear orientation on¹⁰³Ru as a dilute impurity in Fe. The Fe lattice was cooled to 1.7 mK using a PrNi₅ enhanced nuclear demagnetisation stage precooled with a³He–⁴He dilution refrigerator. The¹⁰³RuFe system was observed to have a long nuclear spin-lattice relaxation time, but analysis of the relaxation indicates that the¹⁰³Ru nuclei are close to thermal equilibrium with the Fe lattice at times>200 min after demagnetising. From the near thermal equilibrium anisotropy of the 497 keV -ray in¹⁰³Rh we have deduced the nuclear magnetic moment as |(¹⁰³Ru)|=0.18(2)_N. This value is almost independent of the choice of spinI=3/2 or 5/2 for the nucleus, but is only consistent with systematics ifI(¹⁰³Ru)=3/2 and <0. UsingI=3/2 we obtain the E2/M1 mixing ratio of the 497 keV -ray as =–0.42(4). The sign of the anisotropy of the 610 keV -ray from the 650 keV level in¹⁰³Rh is only consistent with an allowed -decay to the state ifI(650 keV,¹⁰³Rh)=5/2. The E2/M1 mixing ratio of the 610 keV -ray is then =+0.15(3) or +60( _–40 ⁺ ), <–80. From the initial rise in -ray anisotropy following demagnetisation we determine the Korringa constant for¹⁰³RuFe to beC=5.7(5) K·s.     

Strong probe atom dependence of the electric field gradient in Bismuth

The nuclear quadrupole interaction of the 8⁺ isomers in^{206, 208}Po was measured in the semimetallic host Bi. The comparison of the extracted electric field gradient eq for PoBi with the values for the neighboring elements Bi and Pb as probe atoms in Bi reveals a very drastic increase from Pb to Bi and to Po by an order of magnitude: eq_PbBi eq_BiBi eq_PoBi =1.64(10)4.8(3)16(2)(all values at room temperature in units of 10¹⁷ V/cm²). This behavior is discussed in terms of the changing number of outerp-electrons of the probe atoms.     

The183Re ground state dipole moment and nuclear spin-lattice relaxation of Re in Fe

The hyperfine interaction of the system¹⁸³Re(70d)Fe has been investigated with the NMR/ON technique. With the hyperfine field valueB _hf(ReFe)=–76.0(1.5) T the ground state magnetic moment was determined as: (5/2⁺,¹⁸³Re)=+3.12(6) _N. The field dependent nuclear spin-lattice relaxation time has been measured. The result for the high-field relaxation rateR _exp=1.65(5)·10^–15 T ²s K^–1 is explained in terms of indirect spin-wave interaction.     

Critical exponentβ for hyperfine fields in nickel

The magnetic hyperfine fields for⁶³Ni,⁶⁶Cu, and⁶⁷Zn nuclei in nickel metal have been measured by means of perturbed-ray angular distribution techniques at different temperatures up to 1 K below the Curie temperature,T _C . The temperature dependence of the fields can be very well fitted by (1—T/T _C ) with best values=0.322(16) for⁶³NiNi, = 0.427(42) for⁶⁶CuNi, and=0.427(14) for⁶⁷ZnNi respectively. The differences between these exponents indicate that there could be probe atom dependent deviations from proportionality between hyperfine field and bulk magnetization in the critical region.Work performed in partial fulfillment of the requirements for a doctorate in physics at the Freie Universität, Berlin     

Fundamental representations of quantum groups at roots of 1

To every finite-dimensional irreducible representation V of the quantum group U(g) where is a primitive lth root of unity (l odd) and g is a finite-dimensional complex simple Lie algebra, de Concini, Kac and Procesi have associated a conjugacy class C _V in the adjoint group G of g. We describe explicitly, when g is of type A _n , B _n , C _n , or D _n , the representations associated to the conjugacy classes of minimal positive dimension. We call such representations fundamental and prove that, for any conjugacy class, there is an associated representation which is contained in a tensor product of fundamental representations.     

The magnetic hyperfine field at Hg impurities in ferromagnetic Gd

The magnetic hyperfine field at dilute Hg impurities in Gd has been investigated by the conversion electron (e ^–)--time differential perturbed angular correlation (TDPAC) technique. The radioactivities^197m Hg and¹⁹⁹Tl were implanted into Gd foils by means of an isotope separator. TDPAC measurements were performed with the 165 keV-L-conversion electron—134 keV--cascade of¹⁹⁷Hg at different temperatures and with the 334 keV--158 keV-K-conversion electron cascade of¹⁹⁹Hg at 200 K.The regular site occupation probabilities were found to be 15(3)% for an annealed^197m HgGd sample and 29(5)% in unannealed¹⁹⁹TlGd samples.From the magnetic hyperfine interaction frequencies measured for the regular sites at 200 K the magnetic hyperfine fields |H _hf(¹⁹⁷HgGd; 200 K)|=256(13) kG and |H _hf(¹⁹⁹HgGd; 200 K)|=267(7) kG were deduced.On leave from the University of Lisboa, Portugal