Spin Fluctuation Model Research Articles

Suppression of spin fluctuations by the magnetic field and Co concentration in amorphous alloys

Spin fluctuations manifest themselves in the temperature dependence of the spontaneous magnetization M(T,H=0) or the `in-field' magnetization M(T,H), through a contribution that varies with T as in the intermediate-temperature range and as for temperatures in the vicinity of (Curie point) but still away from criticality. Suppression of spin fluctuations by either the Co concentration in alloys or the field H for a given composition in the and alloy series is, for the first time, monitored through the decrease in the coefficients A and with increasing x for H = 0 or with H for fixed x or y. While A(0) and scale with and , respectively, A(H) and follow the relation where or , or , or and is the external field corrected for demagnetization. In contrast with the non-monotonic variation in B and n with x, varies with x as and is independent of x in the range . The spin fluctuation model explains many of, but not all, these features.

Spin fluctuations near the magnetic instability in heavy-fermion systems

In the first part of this paper, specific heat of heavy-fermion systems Ce(Cu -.2Ni 0.8) 2Ge 2 and UPt 3 around the magnetic instability are analyzed by the self-consistent renormalized (SCR) spin fluctuation model to compare with previous analysis in Ce 1- x La x Ru 2Si 2 and Ce(Cu 6- x Au x . The ordered state of UPt 3 also discussed in view point SCR. In the second part, the thermal expansion is calculated in terms of the recent renormalization group model.

Ni and Zn doping of Cu sites in superconducting Nd2-zCezCuO4, La2-βSrβCuO4, Bi2Sr2CaCu2O8, Bi1·8Pb0·2Sr2Ca2Cu3O10, YBa2Cu3O7, La0·6Ca0·4Ba1·35La0·65Cu3Ox and YBa2Cu4O8

A unified Cooper pair-breaking picture is presented for the observed degradation of the superconducting critical temperatures by Cu-site Zn and Ni impurities in Nd 2-z Ce z CuO 4 , La 2-β Sr β CuO 4 , Bi 2 Sr 2 CaCu 2 O 8 , Bi 1-8 Pb 0.2 Sr 2 Ca 2 -Cu 3 O 10 , La 0.6 Ca 0.4 Ba 1.35 La 0.65 Cu 3 O x (with x 7), YBa 2 Cu 3 O 7 and YBa 2 Cu 4 O 8 . Independent of any specific model, the data alone require physics outside the cuprate planes, and hence no two-dimensional cuprate-plane model can consistently explain all the Zn and Ni pair-breaking data. In Nd 2-z Ce z CuO 4 , Ni and Zn behave as in a BCS superconductor. Although the Ni- and Zn-doping data for YBa 2 Cu 3 O 7 have been cited as providing the so-called 'smoking gun' conclusive proof of the spin-fluctuation model, we conclude that the underlying assumptions of that data-analysis, that Ni and Zn occupy the same sites and that Ni is a weaker pair-breaker than Zn, are invalid: we argue that the exchange scattering of Cooper-pairs in YBa 2 CU 3 O x is inoperative. Thus, except for Nd 2 - z Ce z CuOB whose charge-reservoirs are adjacent to its cuprate-planes and hence are uniquely within the range of exchange scattering by cuprate-plane Cu-site Ni ions, the exchange scattering caused by the difference between cuprate-plane Ni and Zn impurities is inoperative in these high-temperature superconductors - a fact difficult and perhaps impossible to reconcile with any cuprate-plane model of superconductivity that also features a Meissner effect and Cooper pairing. We interpret the inoperative exchange scattering as evidence that the primary superconducting condensate is outside the range of the exchange interaction - and hence in the charge reservoirs, rather than in the cuprate-planes. The trends in observed pair-breaking critical compositions u c define approximately exponential functions of the distance d between the impurity sites and the charge-reservoir dopant-oxygen - also indicative of charge reservoir superconductivity. The critical tempertures T c increase (approximately linearly) with d (again, consistent with charge-reservoir superconductivity), suggesting that higher critical temperatures can be achieved in materials with increased separation between the cuprate planes and the charge reservoirs. The cuprate planes appear to be mechanically favourable, but electronically unfavourable to superconductivity. A consistent explanation of all the Ni- and Zn-doping data is obtained (i) if the superconductivity originates in the charge-resevoir or dopant-oxygen regions of each unit cell (not in the cuprate planes), (ii) if the short-ranged exchange scattering by magnetic Ni readily breaks Cooper-pairs whose holes are located on nearest-neighbour oxygen ions only, (iii) if a longer-ranged interaction is responsible for the degradation of T c by Zn and by Ni distant from the superconducting condensate, and (iv) if polarization fluctuations are responsible for the Cooper pairing. The pair-breaking data contradict not only the spin-fluctuation d-wave pairing model, but also all models with the superconductivity originating in cuprate planes.

Magnetic properties of Y(Co1−xNix)4Al compounds

Magnetic measurements were performed on Y(Co1−xNix)4Al compounds in the temperature range 4.2–750 K and fields up to 7°F. The magnetic behaviour of Ni and Co is analyzed with the spin fluctuation model.

Temperature dependent magnetic surface anisotropy in ultrathin Fe films

The temperature dependence of the uniaxial surface anisotropy, Ku(T), in ultrathin Fe films (4–10 atomic layers) grown on Cu(100) is determined by comparing the dipole and anisotropy energies at the spin reorientation temperature for films of varying thickness. It is observed that the uniaxial anisotropy has a weak temperature dependence compared to the bulk constants for Fe. The measured exponent of Γu=2.6(0.5) agrees well with the l(l+1)/2 law, which is obtained from a spin fluctuation model where l=2 at the surface. This shows that the spin reorientation transition can be understood as being driven by thermal spin fluctuations.

Magnetic properties of Y3Co11−xNixB4 compounds

Y3Co11−xNixB4 compounds are ferromagnetically ordered for × < 7. For higher nickel content an exchange enhanced paramagnetism is observed. The magnetic behaviour of cobalt is analyzed using the spin fluctuations model.

Spin fluctuations in the YNi5−xCux system

The magnetic properties of YNi5−xCux compounds were investigated in the temperature range 5–750 K and fields up to 9 T. At high temperatures the magnetic susceptibilities follow a Curie-Weiss-type dependence. The effective nickel moments are similar to that determined in nickel metal and decrease little on increasing the Cu content. The magnetic behaviour of nickel is described by the spin fluctuations model.

Quasi two-dimensional magnetism in MnGaGe

The intermetallic compound MnGaGe, which crystallizes in the Fe2As structure shows a large and positive pressure dependence of the Curie temperature. To study this behaviour we perform band structure calculations whose results enter a spin fluctuation model. It is shown that the peculiar properties of MnGaGe can be traced back to the quasi two-dimensional magnetism of the Mn layers.

First-principles calculation of the electronic and magnetic properties of Nd2Fe17−xMx (M=Si, Ga) solid solutions

We have studied the electronic and magnetic properties of the Si and Ga substituted Nd2Fe17 compounds by means of first-principles calculations. The results show that for the Nd2Fe17−xSix (x=1,2,3, and 4) series, the total Fe moments per cell decrease as x increases but the average local Fe moments actually increase up to x=3, except for the 18h site which is the most preferable site for substitution. For Nd2Fe15Ga2, the moments on every Fe site increase. These results are also compared with the earlier calculation on Nd2Fe17−xAlx (x=1,2, and 4). From the spin-polarized density of states (DOS) and local DOS for Si, Al, and Ga, it is shown that different substituting elements can change the electronic structure of Nd2Fe17 crystal substantially because of the slightly different interactions with the Fe atoms, and the change in the lattice constants. Estimation of Curie temperatures based on the spin-fluctuation model shows qualitative agreement with experiments.

Effect of magnetic field on the specific heats of intercalation compounds Mx TiS2 (M=3d transition metals)

Specific heats of single crystals of intercalation compounds MxTiS2 (M=Mn, Fe, Co, and Ni) have been measured using ac calorimetric method over the temperature range T = 0.38–14 K under magnetic fields up to 4.5 T. Considering the lattice dynamical properties of these intercalates, we have analyzed the observed specific heat data C, which can be expressed by the sum of electronic, lattice, magnetic, and anomalous terms, C = γT + βT3 + Cmag + Ca, where Ca is of a Schottky type. The temperature and magnetic field dependences of γ and Cmag are discussed from various viewpoints, such as existing spin fluctuation model, which reveals that the itinerant electron picture is appropriate to understand the electronic and magnetic properties of this material system.

Antiferromagnetic interactions and the superconducting gap function.

Spin-fluctuation-mediated superconductivity is conventionally associated with d_{x^2-y^2} pairing. We show that a generalized model of antiferromagnetic spin fluctuations in three dimensions may also yield a state with formal ``s-wave'' (A_{1g}) symmetry but with line nodes at k_z \approx \pm \pi / 2c. We study this new state within both BCS and Eliashberg theories using a realistic band structure and find that it is more stable than the d_{x^2-y^2} (B_{1g}) state over a wide range of parameters. Thus, models of spin-fluctuation-mediated superconductivity must consider both possibilities on an equal footing.

Incipient antiferromagnetism and low-energy excitations in the half-filled two-dimensional Hubbard model.

We present single-particle and thermodynamic properties of the half-filled single-band Hubbard model in 2D calculated in the self-consistent fluctuation exchange approximation. The low-energy excitations at moderate temperatures and small $U$ are quasiparticles with a short lifetime. As the temperature is lowered, coupling to evolving spin fluctuations leads to the extinction of these quasiparticles, signaled by a weak pseudogap in the density of states and by a positive slope in $\mathrm{Re}\ensuremath{\Sigma}({\mathbf{k}}_{F},\ensuremath{\varepsilon})$ and a local maximum in $|\mathrm{Im}\ensuremath{\Sigma}({\mathbf{k}}_{F},\ensuremath{\varepsilon})|$ at $\ensuremath{\varepsilon}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$. We explain these results using a simple spin-fluctuation model.

Isotope effect in spin fluctuation models

We have studied the d-wave superconducting transition temperature T c and the isotope effect in the spin fluctuation models. We found that the isotope exponent α is at least one order order of magnitude smaller than typical observed values in the cuprates with models coupling to harmonic phonon degrees of freedom and reasonable limits on coupling strength and phonon spectra. Models with coupling to non-linear tunneling modes in a strongly anharmonic multi-well lattice potential and realistic coupling strengths have effectively larger isotopic mass dependences and may therefore account for the observed magnitudes of α.

The effect of surfaces on the tunneling density of states of an anisotropically paired superconductor

We present calculations of the tunneling density of states in an anisotropically paired superconductor for two different sample geometries: a semi-infinite system with a single specular wall, and a slab of finite thickness and infinite lateral extent. In both cases we are interested in the effects of surface pair breaking on the tunneling spectrum. We take the stable bulk phase to be of $d_{x^2-y^2}$ symmetry. Our calculations are performed within two different band structure environments: an isotropic cylindrical Fermi surface with a bulk order parameter of the form $\Delta\sim k_x^2-k_y^2$, and a nontrivial tight-binding Fermi surface with the order parameter structure coming from an anti-ferromagnetic spin-fluctuation model. In each case we find additional structures in the energy spectrum coming from the surface layer. These structures are sensitive to the orientation of the surface with respect to the crystal lattice, and have their origins in the detailed form of the momentum and spatial dependence of the order parameter. By means of tunneling spectroscopy, one can obtain information on both the anisotropy of the energy gap, $|\Delta(\p)|$, as well as on the phase of the order parameter, $\Delta(\p) = |\Delta(\p)|e^{i\varphi(\p)}$.

"Chain scenario" for Josephson tunneling with pi shift in YBa2Cu3O7.

We point out that all current Josephson-junction experiments probing directly the symmetry of the superconducting state in YB${\mathrm{a}}_{2}$C${\mathrm{u}}_{3}$${\mathrm{O}}_{7}$ can be interpreted in terms of the bilayer antiferromagnetic spin fluctuation model, which renders the superconducting state with the order parameters of extended $s$ symmetry, but with the opposite signs in the bonding and antibonding Cu-O plane bands. The essential part of our interpretation includes the Cu-O chain band which would have the order parameter of the same sign as antibonding plane band. We show that in this case net Josephson currents along and perpendicular to the chains have a phase shift equal to $\ensuremath{\pi}$.

Evidence for Temperature-Dependent Antiferromagnetic Correlation Length in High-TcCu Oxides

We report the temperature( T )-dependent anisotropy r (≡ 63 W a b / 63 W c c ) of the planar 63 Cu nuclear spin-lattice relaxation rates 63 W α (α= a b , c c ) in the high- T c Cu oxides, YBa 2 Cu 4 O 8 , YBa 2 Cu 3 O 6.96 and Tl 2 Ba 2 CuO 6+δ , from around T c up to about 300 K. We discuss within the spin-fluctuation model that the anisotropy r is dependent only on the antiferromagnetic correlation length ξ and that its T -dependence for the lightly-doped material is different from that of the over-doped ones. We provide the T -dependent anisotropy r as direct evidence of the T -dependent ξ .

Raman electronic continuum in a spin-fluctuation model for superconductivity.

The Raman electronic continuum is calculated in an antiferromagnetic spin-fluctuation model of the superconducting state. The dependence of the Raman cross section on temperature and on band structure is investigated. A tight-binding model with, up to second nearest neighbors is used and in one case with effective mass anisotropy. The gap is determined from numerical solution of the BCS equations using fast Fourier transforms, with pairing through the phenomenological spin susceptibility of Millis, Monien, and Pines.

Magnetic properties of (GdxY1 − x)Co3B2 compounds

The saturation cobalt moments in (GdxY1 − x)Co3B2 system increase from zero to 0.23μB when yttrium is gradually substituted by gadolinium. The effective cobalt moments are not composition dependent. The magnetic behaviour of cobalt is analyzed in the spin fluctuations model.

Spin fluctuations in cobalt rare-earth compounds

Magnetic measurements performed on (GdxY1-x)n+1Co3n+5B2n compounds with n = 1, 2 and 3 show a great dependence of cobalt moments, determined at 4 K, on the exchange interactions. The effective cobalt moments, for a given series, are only slightly modified. The magnetic behaviour of cobalt is analyzed in the spin fluctuation model. A gradual quenching of spin fluctuations is evidenced when the exchange fields acting on cobalt increase.

Metamagnetic transition and spin fluctuations in YCo2 and LuCo2

On the spin fluctuation model for the itinerant-electron metamagnetism, the temperature dependences of the susceptibility and metamagnetic transition are discussed. Numerical calculations are carried out for actual itinerant-electron metamagnets YCo2 and LuCo2. A satisfactory agreement between the calculated and observed results is obtained.