Quantum theory of radiation-pressure induced electrical currents: I. Free electrons

The maximum superconducting transition temperatures,T _c , of noncrystalline (Ti, Zr and Hf)-(3d metal) alloys are reported. The alloys have been produced by low temperature implantation of the 3d metals. The systematic behavior ofT _c as a function of 3d element content is discussed in comparison with metallic glasses and vapor-quenched films. A correlation between the recently observedd-band splitting in metallic glasses and superconducting behavior is pointed out.     

Surface states ond-band metals

Electron surface states ond-band metals are investigated by the method of matching the crystal wave function to the outside solution at the surface. On a (100) surface of anfcc structure such a state is found near to the crossover of thes-band and thed-band of the same symmetry. In Ni this state lies 4–5 eV below the Fermi energy, for Cu 5–6 V beloweE _F. It explains the density of states anomaly seen in photoemission.     

Structural instability and superconductivity in niobium-titanium alloys

Niobium-titanium is the most widely used technical superconductor. Titaniumrich transition metal alloys, quenched from high temperatures, can generally be retained in the bccβ phase. This phase is metastable and the instability is relieved by a variety of low temperature structural transformations. This aspect has been investigated using x-ray, TEM, low temperature resistivity,T _c and dH _c2/dT studies, in a series of Nb-Ti alloys. The instability has been characterized by the normal state resistivityρ _n and dρ/dT. The commercially used Nb-Ti alloys are Ti rich per atom-wise. This stems basically from the anomalous increase in the normal state resistivityρ _n as the Ti concentration is increased. This is a consequence of a dynamical process through which theβ phase instability tends to be relieved leading to athermal ω precipitation. The resulting anomalous resistivity behaviour can be understood in terms of a ‘two-level system’ model generally invoked for amorphous materials. It has also been possible to induce instability towards athermal ω precipitation in a system spontaneously undergoing a martensitic transformation to become stable. Thus in an alloy of Nb-83 at % Ti, addition of 1% nitrogen has suppressed the martensitic transformation, giving a three-fold increase inρ _n (about 150µΘ cm), the highest known in Nb-Ti so far. The increase in the normal state resistivity has beneficial effects on the upper critical field. From studies on several Nb-Ti alloys, it is inferred that a peak inH _c2(0) occurs at 17–18 tesla at aρ _n value of 100µΘ cm. It is pointed out that in the present commercial alloys, the sequence of thermo-mechanical treatments given to optimizeJ _c, restrictsρ _n, perhaps owing to the partial relieving of the metastability of theβ phase. They are therefore non-optimized with respect toH _c2.     

Paraelastische Defektelektronen-Zentren in Alkalihalogeniden

We have investigated the stress-induced alignment of Cl₂ ^? and Br₂ ^? molecular ions substituting for Cl^? and Br^? in KCl and KBr, respectively (so calledH-centers). By means of the dichroism induced in the optical absorption (H-band) by uniaxial stress we determined the static coupling to the lattice and the kinetics of the reorientation. Contrary to the O₂ ^?-center, which also substitutes for an anion, the reorientation-process of theH-center can be described by an Arrhenius-relation. TheH-centers freeze in between 10 and 13°K. In KBr a new center was found which is closely related to theH-center, but differs in the kinetics of the reorientation. It is presumably anH-center associated with another lattice defect. Further we have studied the paraelastic properties of the defects which in KCl and KCl:NaCl give rise to theV ₁-band. We found that depending upon the doping of the crystal different defects produce theV ₁-band.     

Rotational band crossings in the actinides

In the actinides bothi _13/2 protons andj _15/2 neutrons are close to the Fermi surface. At rapid rotation these high-j particles will unpair and align their orbital angular momentum along the axis of rotation giving rise tos-bands that cross the ground-state band. Coulomb excitation of the odd nuclei ₂₃₇ ⁹³ Np (established up to the 45/2⁺ state) and ₂₃₅ ⁹² U (established up to the 51/2^? state) provides specific information about these band crossings: From the saturating alignment of the odd high-j particle in both nuclei at intermediate rotational frequencies we find the aligned angular momentum of thei _13/2 protons-band to be 6.6? while the corresponding value for thej _15/2 neutrons-band is 5.5?. At more rapid rotation above ?ω=0.18 MeV we observe additional alignment in²³⁵U. This is ascribed to the interaction of the protons-band. From the gradual onset of the additional alignment we deduce that forZ=92 the protons-band interacts strongly with the ground-state band and from a comparison of the actual amount of alignment with the full value of 6.6? we estimate the crossing to occur around ?gw _c ^p =0.25 MeV.     

On the electronic density of states of the transition metals

XIS measurements of the elements Ir, Pt and Au are reported. High precision and reproducibility is obtained by the application of a “π/2-method” which approximates a Bragg angle of 90° at the dispersing crystal. Theoretical densities of states exist for Pt and Au. The agreement with the measured isochromats is good. A rigid-band model for Ir, Pt, and Au is ruled out by the measurements. Rather they suggest (combined with results of photoemission experiments) a narrowing of thed-bands from Ir through Pt to Au. Moreover, thed-band of Ir lies relative to thesp-band at a lower energy than thed-bands of Pt and Au. A fitting parameter concerning the experimental resolution is explained and considered as a possible indication of localization for XIS.     

Electrical resistivity of iron-vanadium alloys between 78 and 1200 K

Electrical resistivity (?) of FeV alloys containing 0.5, 0.9, 2.7, and 6.1 at% V has been measured as a function of temperature (T) between 78 and 1200 K. The ? vs. T curves exhibit a change in the slope at the ferromagnetic Currie temperature (T_c). The d?/dT vs. T curves in the neibhorhood of T_c are similar to the corresponding plot for pure Fe. Our studies confirm the previously observed anomalous effect of V on T_c of Fe, i.e., that T_c increase with small additions of V to Fe. The critical index λ⁺ associated with the power law of d?/dT just above T_c has been determined as a function of V concentration.     

Supraleitung von Pb- und Sn-Schichten nach Implantation von Mn-Ionen

Ion-implantation is a useful method when preparing dilute alloys, especially of elements with very low solubility like 3d-transition-elements in most non-transition superconductors. A strong decrease inT _c and an increase in the residual resistivity were observed in Pb- and Sn-films doped with Mn-ions. Nonlinearity in theT _c -dependence which was found at very low concentrations (0-100 ppm) is caused by lattice defects. They cannot be avoided if implantation method is used. This was proved by irradiation with nonmagnetic ions of comparable mass (such as Cu or Zn). The linear part in theT _c -depression is compared with results obtained with the quenched film technique by Barth (Pb-Mn) and Schertel (Sn-Mn). A discrepancy to these measurements is found in the case of Sn-Mn. The increase in film resistivity is due to lattice defects as well as magnetic impurities. The magnetic contribution (p/c) _m =(4.5 ± 0.2) μΩ cm/at-% agrees well with a theoretical calculation. Heavily doped Sn-films show a small Kondo-resistance-minimum at 7 K with a depth of 2.5‰ of the residual resistivity.     

3-Band theory of Fe pnictide superconductors

For the Fe pnictide superconductors, band features important for superconductivity (SC) in the neighborhood of ε_F are mimicked by tight-binding bands with bases of Fe d_xz, d_yz and d_xy orbitals. In terms of this 3-band model, effective pair transfer processes originating from Coulombic integrals are shown to be largely enhanced due to the nesting between electron and hole pockets by explicitly summing up ladder diagrams. Obtained coupled gap equations lead to s^±-type SC. This framework are argued to provide a scheme for discussing important elements working for the high T_c SC, although its validity is restricted to the case where Dirac cones are away from ε_F. Experimental results on Fe pnictides are discussed in terms of the present results. The pair transfer process explicitly due to the exchange-like integral is suggested to be considerably important for the high T_c in MFeAsO_1−xF_x to occur.     

Superconductivity in amorphous Cr films

Superconducting transition has been observed for the first time in amorphous Cr films prepared by co-sputtering of Cr and BN. Superconducting transition temperatures (Tc) up to 1.14 K have been observed with an orbital contribution (i.e.(dH_c2⊥/dT)_Tc) as large as 49 kOe K^-1. The electronic specific heat coefficient (γ) has been estimated to be about 1.6 times as large as that of amorphous Mo-BN (or Mo-metalloid) alloys. This finding, i.e. the low Tc with the large γ for amorphous Cr is inconsistent with data reported for amorphous alloys of 4d and 5d transition elements. This inconsistency implies a formation of local magnetic moments or spin fluctuation in the amorphous Cr film.     

Effect of hydrostatic pressure on the Curie temperature of the Heusler alloys Ni2MnZ(Z = Al,Ga, In,Sn and Sb)

The pressure derivative of the Curie temperature dT_c/dp of the Heusler alloys Ni₂MnZ(Z = Al, Ga, In, Sn and Sb) has been obtained from the results of temperature dependence of initial permeability under pressure up to about 6 kbar. For all alloys the Curie temperatures increase linearly with increasing pressure at the rate of dT_c/d_p: +0.7 K/kbar for Ni₂MnAl, +1.0 K/kbar for Ni₂MnGa, +0.9 K/kbar for Ni₂MnIn, +1.4 K/kbar for Ni₂MnSn and +4.1 K/kbar for Ni₂MnSb. On the basis of these results, the interatomic dependence of the exchange interaction for Heusler alloys is discussed. The magnetic susceptibilities of those alloys are also reported.     

Magnetic properties and hydrostatic pressure effect on the curie temperature of (Co,Mn)2B amorphous alloys

The temperature dependence of magnetization and magnetic susceptibility and hydrostatic pressure effect on the Surie temperature (dT_c/dP) are measured for (Co_1-xMn_x)₂B (0?x?0.4) amorphous alloys and the results are compared with those of crystalline compounds with the same composition. The Curie temperature decreases linearly with an increasing Mn content but magnetization shows a maximum around x=0.15. The reciprocal magnetic susceptibility of all the prepared alloys obeys the Curie-Weiss law above T_c. The magnitude of the negative value of dT_c/dP decreases linearly with increasing x from about 1.1 K/kbar (x=0) to zero (x=0.4), the composition dependence of which is opposite to that of the crystalline compound. The composition dependence of the average magnetic moment per transition metal atom and the Curie temperature and dT_c/dP are analysed on the basis of the local environment and the pair order interaction mode, respectively.     

Role of p-d hybridization in the Tc-variation of Y(Rh1−xRux)4B4 superconductors

Auger and photoemission studies for Y(Rh_1?xRu_x)₄B₄ superconductors elucidate the mechanism for the abrupt drop in T_c near x=0.5. The B $\text{p}$-states admix more strongly with Ru than Rh $\text{d}$-states, thereby yielding low-T_c values for Ru-rich compounds due to broadening of the $\text{d}$-band feutures. These effects underlie a previous suggestion that the preservation of Rh₄ tetrahedral units is essential for high-T_c values in ternary boride compounds.     

Finite width effect on the upper critical field Hc2 of superconducting networks

A recent prediction for the continuum limit of the upper critical field of infinite superconducting networks H_c2(T)= dH^(bulk)_c2 (d=spatial dimension of the network) is checked experimentally. A direct comparison of the measured critical fields on both samples: film and infinite 2D regular networks, made of the same material, support strongly this theoretical prediction. The deviation of the proportionality coefficient from d=2 is attributed to the finite width of wires. Wires of finite width are shown to be responsible for a renormalization of the diffusion coefficient of Cooper pairs. The expression so obtained for H_c2 as a function of the filling factor is consistent with the measured value.     

Observation of the “bottleneck effect” for a non S-state ion in a d-band intermetallic : LuRh2 : Nd

We demonstrate the existence of bottleneck effect in the relaxation mechanism of Nd³⁺ localized moment in the d-band LuRh₂ intermetallic compound. The ESR data are analyzed using a phenomenological theory for the dynamic susceptibility. The resonance properties yield information about the spin-flip relaxation of the conduction electrons to the lattice δ_eL. The values of δ_eL are smaller than those observed previously for the analogous s-band compounds.     

Structural optimization andd-band holes in Cu monolayers

The equilibrium lattice parameter and bandstructure of copper monolayers, both in the square (100) and hexagonal (111) symmetry, have been determined using self-consistent full-potential local density approximation (LDA) calculations. Two quite different procedures have been employed: FILMS, a linear-combination-of-gaussian-type-orbitals method, and a full-potential linearized augmented plane-wave (F-LAPW) method. The copper monolayer is bound with respect to the atomic LSDA ground state in the configurationd ¹⁰ s ¹. Nearest-neighbor distancesa _nn are determined as 4.25 a.u. in the square geometry and 4.42 a.u. in hexagonal geometry, the latter being favored in energy by 0.33 eV/atom. Both monolayers thus exhibit a nearest-neighbor distance substantially shorter than that found in bulk copper,a _nn=4.8238 a.u. Excellent agreement between the two methods is obtained for the bandstructure, with no indication of ad-band hole at theM point (corner) of the Brillouin zone, in contrast to some other recent self-consistent calculations. Combined use of the von Barth-Hedin LDA and scalar-relativistic corrections produces the smallest gap at theM point, 0.15 eV, at the Hedin-Lundqvist equilibrium geometry. This may be suggestive evidence for the origin ofd-band holes when combined with further approximations in the representation of the one-electron orbitals and the charge density.     

Core hole satellites in NixPd1-x alloys

The two and three hole satellites accompanying the main Ni 2p and Pd 3d XPS core lines in Ni_xPd_1-x alloys have been measured as function of the alloy concentration. The changes in satellite distance with concentration cannot be explained by the thermochemical approach, which has recently been shown to describe successfully the one hole core level binding energy shifts (main line) in alloys. The reason is probably, that in this approach, which uses thermochemical data of Z + 1, Z + 2 and Z + 3 elements - where Z stands for Ni or Pd and Z + n is the nth element following them in the periodic table - d-band contributions to the cohesive energies of the final d-hole states are neglected.     

Theory of solid solubility for rare earth metal based alloys

Both parabola and ellipse separating schemes are used to study the solid solubilities for the binary alloy systems based on the 13 rare earth metals. It has been found that the soluble elements can be separated from the insoluble ones by a parabolyy ₁=a?bx ₁ ² or an ellipse(x ₂?m)²/c²+(y₂?n)²/d²=1. The results show that the overall reliabilities of the solid solubilities for the 897 binary alloys based on the rare earth metals are 89.2% and 92.8% for the parabola and ellipse regularities respectively. The constants a and b in the parabola equation, andm, n, c andd in the ellipse equation are discussed, which can be related to some appropriate parameters for each host metal respectively.     

A CEMS study of ac-axis oriented thin film of YBa2(Cu0.97 57Fe0.03)3O7

⁵⁷Fe Mössbauer spectra of a fullyc-axis oriented epitaxial thin film of YBa₂(Cu_0.97 ⁵⁷Fe_0.03)₃O₇, recorded by CEMS, are reported. Spectra taken at different angles between the normal of the film (i.e. thec-axis) and the gamma ray direction show that for two of the three observed quadrupole doublets the main component of the EFG lies in thea?b-plane, for the third doublet it is parallel to thec-axis. The EFG of all three doublets has approximately axial symmetry.