The Interplay of Electronic and Nuclear Magnetism in PtFe x at Milli-, Micro-, and Nanokelvin Temperatures

We have measured ac susceptibility, nuclear magnetic resonance, and nuclear heat capacity of two PtFe _x samples with concentrations of magnetic impurities x = 11 ppm and 41 ppm at magnetic fields (0 ± 0.05) mTB248 mT. The susceptibility data have been measured at temperatures of 0.3 KT100 mK, no hint for nuclear magnetic ordering could be detected to a temperature of 0.3 K. The nuclear heat capacity data taken at 1.4 KT10 mK show enhanced values which scale with x at low polarization. This effect is described by a model assuming an internal magnetic field caused by the impurities. No indication for nuclear magnetic ordering could be detected to 1.4 K. The nuclear magnetic resonance experiments have been performed on these samples at 0.8 KT0.5 mK and 2.5 mTB22.8 mT as well as on three other samples with x = 5, 10, 31 ppm in a different setup at 40 KT0.5 mK and at 5.4 mTB200 mT. Spin-lattice and effective spin-spin relaxation times ₁and ₂ ^* of ¹⁹⁵ Pt strongly depend on x and on the external magnetic field. No temperature dependence of ₁and ₂ ^* could be detected and the NMR data, too, give no hint for nuclear magnetic ordering to 0.8 K.     

Investigation of the gapless state in bismuth-antimony alloys

The results are presented from an experimental study of the gapless state produced in semiconducting alloysBi _1–x Sb _x by pressure-induced band inversion. The magnetoresistance properties of the alloys have been investigated both in weak magnetic fields (H « 1) and in strong fields (H75 kOe) at liquidhelium temperatures in the Sb concentration interval 0.06x0.15 and pressure interval 1 bar p<20 kbar. At pressuresp close to the pressurep _k at which the gapless state is realized a semiconductor-semimetal-semiconductor transition is detected inBi _1–x Sb _x alloys withx=0.070 and 0.071. The rates of change of the gap _gL before and after inversion are determined: –(2.5±0.5)×10^–6 eV/bar and (1.5±0.5)×10^–6 eV/bar, respectively. A reduction in the carrier effective mass as _gL 0 is observed down to values of 10^–4 m ₀. It is shown that as _gL 0 the carrier mobilities in the alloys increase abruptly, the effect being a maximum in the purest alloys, where forT=4.2 K the mobility along the binary axis attains the record-high value of 3×10⁸ cm²/V · sec.     

Effect of mechanical pressure and sintering temperature on the electrical properties of the rare earth ferrite Li0.6Co0.1Dy x Fe2.3−x O4; 0.0 ≤ x ≤ 0.2

Polycrystalline samples of mixed ferrite Li_0.6Co_0.1Dy _x Fe_2.3–x O₄; 0.0 x 0.2 were prepared by the standard ceramic technique. X-ray diffraction analysis was carried out to assure the formation of the samples. The resistance was measured at different temperatures (300–800 K) as a function of frequency (200 kHz–5 MHz) for the investigated samples prepared at different sintering temperatures (1100°C T _s 1250°C). It is found that this type of rare earth ferrite gives a distinguishable behavior at T _s = 1200°C where the resistance of the samples at room temperature reduced to 90% of their values of the others sintering temperatures. The data showed also that the resistance of these samples sintered at 1100°C is changed at 100 kHz from 12 M without applying pressure to 19 k with 0.8 MPa. This change occurs in a reversible manner (i.e., after removing the mechanical pressure effect the resistance of the samples well return to their initial values again immediately). Thus this type of rare earth ferrite could be more applicable in several advanced electronic devices can be used as a sensor for the mechanical pressure.     

Anomalous normal state and high temperature superconductivity in the cuprates

It is generally believed that a high electronic density-of-states n(E_F), and therefore a low Fermi velocity v_F, are required to obtain a high transition temperature, since T_{c ph} exp(–1/) and = n(E_F)V. However, V = < I² >/M _ph ² , and Bardeen showed that I E_Fk_F = (1/2)k _F ² v_F. Thus one may expect that should increase with v_F. While it may not be feasable to increase the one-electron velocity v_F signifcantly, the velocity may increase greatly as a result of renormalization by electron-electron interactions. Such a renormalization exists in Hartree-Fock theory for an unscreened electron-gas. We found that for a medium-density electron-gas (r_s 10–20) imbedded in a background with a dielectric constant () such that / > 10, there is a significant increase of v_F by renormalization, even when screening is taken into consideration. The peak of v(k) at k = k_F is very narrow, the half width being somewhat less than the frequency ₀ at which () falls by a factor of 2. When () is due to ionic polarization, _o is a typical phonon frequency. The height of the peak v_F/v _F ⁰ is of order E_F/₀, and the width in units of momentum is: k/k_F (₀/E_F)². This velocity peak is associated with a minimum in the screening constant at E_p. Its sharpness causes the normal state properties to be highly anomalous; namely the conductivity is exceptionally high, with an anomalous temperature dependence; the conductivity anisotropy, thermoelectric power, and Hall constant are anomalous. Direct determination of v(k) in YBCO by several methods indeed suggests a large, sharp peak, the width being of order 10 me V. This peak manifests itself in the tunneling and point-contact spectroscopy I–V curves. We suggest that the reduced screening at the Fermi level, associated with this velocity peak, is responsible for the high T_c of the cuprates and several other exotic superconductors. Thus, the high T_c is a reflection of the anomalous normal-state properties.     

Interplay of Nuclear Magnetism and Superconductivity

The ratio of nuclear saturation magnetization and superconducting critical field, ₀ M _sat / B _S0*, classifies the strength of mutual influence of nuclear magnetism and superconductivity. In order to investigate the interplay of both phenomena for the three distinct cases 1, 1, and 1 we have measured the ac susceptibility of Al, of the intermetallic compound AuIn ₂ , and of the metal hydride TiH _2.07 at ultralow temperatures, 17 K T 1 K, as function of static field 0 B 15 mT. For Al, the interplay enables an absolute measurement of the nuclear magnetization. For AuIn ₂ , we get a steep decrease of B _S (T) and a broadening of the superconducting transition in its nuclear ferromagnetic phase. Surprisingly, the nuclear ferromagnetic state coexists with type-I superconductivity in AuIn ₂. The metal hydride TiH _2.07 , which is under present investigation, is a good candidate to show reentrant superconductivity.     

Isochoric Heat Capacity of Heavy Water at Subcritical and Supercritical Conditions

The heat capacity of heavy water was measured in the temperature range from 294 to 746 K and at densities between 52 and 1105 kg·m^–3 using a high-temperature, high-pressure adiabatic calorimeter. The measurements were performed at 14 liquid and 9 vapor densities between 52 and 1105 kg·m^–3. Uncertainties of the measurements are estimated to be within 3% for vapor isochores and 1.5% for the liquid isochores. In the region of the immediate vicinity of the critical point (0.97T/T _c1.03 and 0.75/_c1.25), the uncertainty is 4.5%. The original C _V data were corrected and converted to the new ITS-90 temperature scale. A parametric crossover equation of state was used to represent the isochoric heat capacity measurements of heavy water in the extended critical region, 0.8T/T _c1.5 and 0.35/_c1.65. The liquid and vapor one- and two-phase isochoric heat capacities, temperatures, and saturation densities were extracted from experimental data for each measured isochore. Most of the experimental data are compared with the Hill equation of state, and the overall statistics of deviations between experimental data and the equation of state are given.     

Spatial structure of the superconducting phase in heavy-fermion systems and anomalies in the excitation spectrum

A degenerate superconducting phase in a metal with strong spin-orbital coupling may have a domain structure at low temperatures. The domain walls (of width =v_F/₀) carry a finite density of states N(0)mk _f/² per unit volume, which in turn results in the presence of a linear term T in the electronic specific heatC _e (T). With the help of the Atiyah-Singer theorem, this can be proved for a wide class of domain structures.     

Localized single-particle excitations and the density of states for superconducting composites

The problem of localized single-particle excitations and the density of states (DOS) for an inhomogeneous system consisting of a spherical superconductor (with radius a and order parameter ₁) embedded in another superconductor (order parameter ₂) of infinite size is considered. With the assumption of constant values of ₁ and ₂, the Bogoliubov equations are solved for general values of l (the orbital angular momentum quantum number). For a fixed value of ₁/₂ and different values of ₂/E _F, the dependence of the excitation energy (l=0)/₂ on the particle sizek _F a is shown (k _F is the Fermi wave vector andE _F is the Fermi energy). Fork _F a=300, 450, and 800 and a fixed value of ₂/E _F, the variations in the DOS by changing ₁/₂ are also shown.     

Asymptotic behavior of the vector potential and the order parameter for an isolated vortex

Eilenberger's formulation of the theory of inhomogeneous superconductors is used to study an isolated vortex in a type-II superconductor. Exact integral expressions for the vector potential and the order parameter are obtained and used to determine the asymptotic behavior of these quantities. Far from the axis of the vortex, the vector potential approaches its BCS value in an approximately exponential fashion, the decay constant being equal to the quantity obtained by Eilenberger and Büttner in the local case, and equal to {2[(k _BT)²+ _BCS ² ]^1/2/v_F+(1/v _F)}^–1 in the case where nonlocal effects dominate. The order parameter also approaches its BCS value approximately exponentially, the decay constant being equal to the quantity of Eilenberger and Büttner when <2, and equal to 1/2 when >2.Supported in part by the National Research Council of Canada.Sole affiliation is now the University of Toronto.     

Variation of the Kondo temperature with the nature and concentration of the solute metal

From the available data on the physical properties of dilute magnetic alloys, Daybell and Steyert obtain V-shaped semilog plots ofT _K vs. N (whereT _K is the Kondo temperature andN is the serial number of the magnetic solute in the 3d series). This requires a large variation in the rather small exchange interaction parameterJ, while the Kondos–d exchange model is based on a smallJE _F (whereE _F is the Fermi energy). Taking the ordinary scattering into account,T _K =T _F exp((1– cos ²)/(JD cos²)), whereJ<0, 1, is the phase shift for ordinary scattering, andD is the density of states of one spin direction per atom. It has been shown by us recently, from the concentration dependences of the Kondo slopes and the extremum values of the thermoelectric power of dilute magnetic alloys, that 0<d shells, and 3/4<< for those having less-than-half filled 3d shells. We show, on this basis, that the range of variation ofJ is now reduced to more acceptable values, as the solute metal traverses the 3d series. Again, based on our inference that increases with the increase of the solute concentrationc, we show that an increase of the more-than-half filled solute concentration decreasesT _K , while the increase of the less-than-half filled solute concentration increasesT _K . The fact that the increase of cobalt content in copper or gold favors magnetism while the increase of vanadium content in gold favors nonmagnetism is understandable on this basis.     

Normal-State Gap, Supercarrier Mass Anisotropy and Isotope Effects in La2?xSrxCuO4

Magnetization measurements have been performed on the oxygen-isotopeexchanged samples (¹⁶O and ¹⁸O) of the one-layer cuprate superconductorsLa_2–x Sr _x CuO₄ (0.06x0.20). We find that there is an oxygen-isotopeeffect on the effective supercarrier mass m**, which is huge for x=0.06,and reduced to a smaller value for x=0.15. We also find that there coexistsmall (bi)polarons and free carriers for x0.09, but only small (bi)polaronsare present for x0.09. The normal-state gap is related to the bipolaronbinding energy which is inversely proportional to x for 0.05x0.15. Theobserved isotope effects, supercarrier mass anisotropy, normal-state gap,and mid-infrared spectra for x0.09 can be quantitatively explained by the small(bi)polaron theory of superconductivity proposed by Alexandrov, Kabanov, andMott (AKM).     

Magnetic properties of highly dilutedPdFex andPtFex-alloys. Part II. Susceptibility at micro- and milli-kelvin temperatures

We have measured the 16 Hz susceptibility of the giant magnetic moments induced by Fe impurities in highly dilutedPdFe_x andPtFe_x samples with 2.5 ppm x 75 ppm in a wide temperature range, 30 K T 300 mK, and at static magnetic fields 0,01 mT B 25 mT. We find spin glass freezing at Tf(X)/X0,19mK/ppm Fe forPdFe_x and the larger value 0.26 mK/ppm Fe forPtFe_x. This is the first observation of spin glass freezing inPtFe_x. In the low-temperature range T 0.5T_f(x), the susceptibilities follow — ₀ T with small zero-temperature ₀ values forPdFe_X and vanishing ₀ values forPtFe_x. In the paramagnetic high-temperature range, we find (T — )^it-1 at T 10 mK independent of x forPdFe_x, and at T 2Tf(x) dependent of x forPtFe_x with vanishing values for both systems. The data compare well to the predictions of the Thouless-Anderson-Palmer TAP approach of the Sherrington-Kirkpatrick SK model for spin glasses.     

Theory of Josephson flow oscillations in superfluid3He-B

Avenel and Varoquaux have proposed an experiment for observing pressure-induced Josephson flow oscillations across an orifice connecting two reservoirs of³He-B. The aim of the present work is to specify the conditions for the observability of this effect. First the six coupled Ginzburg-Landau (GL) equations for the order parameter components within the orifice are solved, subject to rigid boundary conditions at the ends (fixed phase difference between the bulk order parameters) and specular reflection boundary conditions at the walls. The longitudinal component is depressed and one of the transverse components is enhanced toward the middle of the orifice. From the order parameter functions the supercurrent through the orifice I(), and the GL energyF() are calculated for 0 2 and for different lengths and cross sections of the orifice. A new feature of the functionI() in comparison to the current-phase relationship for superconducting microbridges is its multivaluedness and the occurrence of two nontrivial zeros ofI(). The functionF() exhibits two local maxima besides the absolute maximum at =. The optimal length of the orifice for the observability of the ac Josephson effect turns out to be about seven GL coherence lengths . Then the frequency of the Josephson flow oscillations, which follow the sudden application of a force F on a diaphragm, depends on F in a way similar to that in which the frequency of the parallel ringing experiment in³He-A depends on the field step H.     

The effective surface energy of brittle materials

The effective surface energy of four brittle materials, alumina, poly(methylmethacrylate), glass, and graphite, is calculated from load/deflection curves of notched bars deformed in three-point bending. Two of the methods, which are commonly used in fracture mechanics studies,viz the modified Griffith treatment and the compliance analysis method, are concerned with the effective surface energy at the initiation of fracture, _I . The third method, the work of fracture test, is concerned with the mean effective surface energy over the whole fracture process, _F . The two estimates of _I give consistent values, and there is no systematic variation of _I with notch depth. Values of _F decrease with increasing notch depth as the fracture process becomes more controlled. For alumina _{I F} . For PMMA and glass _I > _F because of a multiplicity of crack sources during fracture initiation. For graphite _I < _F because of subsidiary cracking as fracture proceeds.     

Simultaneous Measurement of the Density and Viscosity of Compressed Liquid Toluene

A vibrating-wire densimeter described previously has been used to perform simultaneous measurements of the density and viscosity of toluene at temperatures from 222 to 348 K and pressures up to 80 MPa. The density measurements are essentially based on the hydrostatic weighing principle, using a vibrating-wire device operated in forced mode of oscillation, as a sensor of the apparent weight of a cylindrical sinker immersed in the test fluid. The resonance characteristics for the transverse oscillations of the wire, which is also immersed in the fluid, are described by a rigorous theoretical model, which includes both the buoyancy and the hydrodynamic effects, owing to the presence of the fluid, on the wire motion. It is thus possible, from the working equations, to determine simultaneously, both the density and the viscosity of the fluid from the analysis of the resonance curve of the wire oscillation, the density being related essentially to the position of the maximum and the viscosity to its width. New results of measurements of the density and viscosity of toluene in the compressed liquid region are presented, and compared with literature data. The density results extend over a temperature range 222 KT348 K, and pressures up to 80 MPa. The viscosity results cover a temperature range of 248 KT348 K and pressures up to 80 MPa. The uncertainty of the present density data is estimated to be within ±0.1% at temperatures 298 KT350 K, and ±0.15% at 222 KT273 K. The corresponding overall uncertainty of the viscosity measurements is estimated to be ±2% for temperatures 298 KT350 K, and ±3% for 248 KT273 K.     

Specific heat measurements of intercalation compounds M x TiS2 (M=3d transition metals) using ac calorimetry technique

Specific heats of 3d transition metal intercalates of 1T-CdI₂-type TiS₂, M _x TiS₂ (M=V, Cr, Mn, Fe, Co, and Ni; 0x1), have been measured in the temperature range 1.6–300 K using an ac calorimetry technique. The electronic specific heat coefficient (2–100 mJ/mole K²) and the Debye temperature _D (240–430 K) are found to depend on the guest 3d metals and their concentrations. All the intercalates show anomalous specific heat at low temperatures following an – lnT dependence ( and are constants), as found in dilute alloys.     

Electronic specific heat of La2−xSrxCu1−yMyO4(M=Ga,Zn): Impurity effects on a D-wave superconductor

The electronic specific heat C_el was studied in Ga- and Zn-doped La_2–xSr_xCuO₄ (0.16x0.22) at T10K. Partial substitution of Ga or Zn for Cu suppresses T_c and revives the T-linear electronic specific heat, T, markedly. The (y)/n vs T_c/T_c0 relation for Zn-doped samples with x0.2 is in good agreement with the theoretical one for resonant impurity scattering in a d-wave superconductor, while those for Ga-doped samples and for Zn-doped samples with x 0.2 deviate slightly from the theoretical curve. The deviation will be discussed in relation to changes in the magnetic properties of 3d electrons.     

Temperature dependence of surface impedance Z(T, ω) and mean free paths I(T) of YBCO-superconductors

The surface impedance Z(T,) at 10 and 145 GHz and between 4 and 300 K is obtained experimentally. Z(T_aTT_c) is quantitatively fitted by the BCS theory with a mean free path I(T) increasing rapidely below T_c. This I-increase in the frame of the BCS-theory is limited at T_a by inelastic surface scattering at weak or strong links, e.g., by twin boundaries in distances a_TW which dominates scattering for a_Tw2I(T_a). Below T_a the enforced energy transfer from YBCO-crystallites to weak links may enhance Z(Ta) until at T T* the weak link surface impedance dominates Z_res(T相似文献   

Superconductivity in bulk A15 Nb3Si under pressure

We have determined the effect of hydrostatic pressureP on the superconducting transition temperatureT _c of bulk, A15 Nb₃Si. For 0P20 kbar (2 GPa),T _c decrease linearly with increasing pressure at a rate T _c/P=–2.67×10^–5 K/bar. From an estimate of T _c/P obtained using recent band structure calculations for the density-of-electronic-states change as a function of lattice parameter in Nb₃Si, we conclude that the pressure dependence of the electron-phonon interaction primarily determines T _c/P.Work performed under the auspices of the U.S. DOE.     

Marginal behavior of a Fermi system interacting with bosonic excitations

We calculated the self-energy of a Fermi system interacting with bosonic excitation. If the bosonic system is near instability, the imaginary part of the self-energy which measures the lifetime of quasiparticles is of the order of ((p)- _F ) which is typical for a marginal Fermi liquid. Such a behavior can appear for three- and two-dimensional Fermi systems.