Presence Of Spin Fluctuations Research Articles

Superconducting critical temperature and isotope effect in the presence of electron-phonon,electron-paramagnon and electron-electron interactions in rhodium

An explicit expression for the superconducting critical temperature, TC, and the exponent of vibrating ionic mass, α, has been derived in the simultaneous presence of attractive electron-phonon,repulsive electron-paramagnon and repulsive electron-electron interactions. The dependence of TC, on these interactions is quite involved,but the observed value of TC in transition element Rhodium can be explained using realistic physical parameters.Variation of TC with λe−s, electron-paramagnon coupling constant and Ee−s, electron-paramagnon energy, has also been studied. Rhodium shows inverse isotope effect in the presence of spin fluctuations.

Effects of magnetic dilution in the ferrimagnetic columnar ordered Sm2MnMnMn4−xTixO12 perovskites

Powder neutron diffraction experiments have been employed to establish the effects of site-selective magnetic dilution in the Sm2MnMnMn4-x Tix O12 A-site columnar ordered quadruple perovskite manganites (x = 1, x = 2 and x = 3). We show that in all three compositions the Mn ions adopt a collinear ferrimagnetic structure below 27 K, 62 K and 34 K, respectively. An unexpected increase in the ordering temperature was observed between the x = 1 and x = 2 samples, which indicates a considerable departure from mean field behaviour. This result is corroborated by large reductions in the theoretical ground state magnetic moments observed across the series, which indicate the presence of spin fluctuations and or disorder. We show that long range magnetic order in the x = 3 sample, which occurs below the percolation threshold for B-B exchange, can only be understood to arise if magnetic order in Sm2MnMnMn4-xTixO12 is mediated via both A-B and B-B exchange, hence confirming the importance of A-B exchange interactions in these materials. Finally we show that site-selective magnetic dilution enables the tuning of a ferrimagnetic compensation point and the introduction of temperature-induced magnetization reversal.

Multigap Superconductivity in RbCa2Fe4As4F2 Investigated Using μSR Measurements

The superconducting properties of the recently discovered double Fe$_2$As$_2$ layered high-$T_c$ superconductor RbCa$_2$Fe$_4$As$_4$F$_2$ with $T_c\approx$ 30~K have been investigated using magnetization, heat capacity, transverse-field (TF) and zero-field (ZF) muon-spin rotation/relaxation ($\mu$SR) measurements. Our low field magnetization measurements and heat capacity (C$_p$) reveal an onset of bulk superconductivity with $T_{\bf c}\sim$ 30.0(4) K. Furthermore, the heat capacity exhibits a jump at $T_{\bf c}$ of $\Delta$C$_p$/$T_{\bf c}$=94.6 (mJ/mole-K$^2$) and no clear effect of applied magnetic fields was observed on C$_p$(T) up to 9 T between 2 K and 5 K. Our analysis of the TF-$\mu$SR results shows that the temperature dependence of the magnetic penetration depth is better described by a two-gap model, either isotropic $s$+$s$-wave or $s$+$d$-wave than a single gap isotropic $s$-wave or $d$-wave model for the superconducting gap. The presence of two superconducting gaps in RbCa$_2$Fe$_4$As$_4$F$_2$ suggests a multiband nature of the superconductivity, which is consistent with the multigap superconductivity observed in other Fe-based superconductors, including ACa$_2$Fe$_4$As$_4$F$_2$ (A=K and Cs). Furthermore, from our TF-$\mu$SR study we have estimated an in-plane penetration depth $\lambda_{\mathrm{ab}}$$(0)$ =231.5(3) nm, superconducting carrier density $n_s = 7.45 \times 10^{26}~ $m$^{-3}$, and carrier's effective-mass $m^*$ = 2.45\textit{m}$_{e}$. Our ZF $\mu$SR measurements do not reveal a clear sign of time reversal symmetry breaking at $T_{\bf c}$, but the temperature dependent relaxation between 150 K and 1.2 K might indicate the presence of spin-fluctuations. The results of our present study have been compared with those reported for other Fe pnictide superconductors.

A brief review on μSR studies of unconventional Fe- and Cr-based superconductors

Muon spin relaxation/rotation (muSR) is a vital technique for probing the superconducting gap structure, pairing symmetry and time reversal symmetry breaking, enabling an understanding of the mechanisms behind the unconventional superconductivity of cuprates and Fe-based high-temperature superconductors, which remain a puzzle. Very recently double layered Fe-based super-conductors having quasi-2D crystal structures and Cr-based superconductors with a quasi-1D structure have drawn considerable attention. Here we present a brief review of the characteristics of a few selected Fe- and Cr-based superconducting materials and highlight some of the major outstanding problems, with an emphasis on the superconducting pairing symmetries of these materials. We focus on muSR studies of the newly discovered superconductors ACa2Fe4As4F2(A = K, Rb, and Cs), ThFeAsN, and A2Cr3As3(A = K, Cs), which were used to determine the superconducting gap structures, the presence of spin fluctuations, and to search for time reversal symmetry breaking in the superconducting states. We also briefly discuss the results of muSR investigations of the superconductivity in hole and electron doped BaFe2As2.

Magnetocaloric and magneto-transport properties of Gd10Co20Si70 alloy

We report the results of magnetic, thermodynamic, transport and magnetocaloric effect (MCE) studies of newly synthesized Gd10Co20Si70 alloy. These measurements confirm an antiferromagnetic transition at TN = 9 K. Both MCE and magnetoresistance (MR) show quadratic dependence on the applied magnetic field, indicating the presence of spin fluctuations in the alloy. The maximum values of the magnetic entropy change determined from the isothermal magnetization data for magnetic field change of 7 and 9 T are found to be 10.5 and 15.6 J/kg·K, respectively. As a consequence of the spin fluctuations effect, the MCE peaks are pulled towards high temperature side as asymmetrically broadened peak. The MR attains a large positive value of 73% at 2 K in 8 T. The large MR and reversible MCE make this alloy an attractive multifunctional magnetic material.

Spin fluctuations, Fermi surface hotspots and nesting in PuCoGa5

ABSTRACTSurprisingly little is known about the mechanism and symmetry of superconducting pairing in PuCoGa5. A common thread with other unconventional superconductors is the presence of spin fluctuations in the normal state, which in this particular case is controlled by strong spin–orbit coupling split bands. The many and anisotropic Fermi surfaces make the guessing of the potential spin-fluctuation nesting vector and resulting symmetry of the pairing function a nontrivial task. To provide much needed guidance for the identification of the pairing symmetry in this multiband superconductor, we perform first-principles based magnetic spin susceptibility calculations to identify the dominant nesting vectors that potentially give rise to interband pairing with nodal d- or s±-wave gap functions.

Spin-fluctuations in Ti0.6V0.4 alloy and its influence on the superconductivity

We report experimental studies on the temperature and magnetic field dependence of resistivity, dc magnetic susceptibility and heat capacity in Ti0.6V0.4 alloy. The temperature dependence of the normal state dc magnetic susceptibility in this Ti0.6V0.4 alloy shows T2lnT behavior. The temperature dependence of resistivity follows a T2 behavior in the range 20–50K whereas a term T3lnT is needed in addition to the electronic and lattice terms to explain the temperature dependence of heat capacity in this region. These observations indicate the presence of spin-fluctuations in the system. Further experimental evidence for the spin fluctuations is obtained in the form of a negative value of T5 term in the temperature dependence of resistivity. We show that the spin fluctuations present in Ti0.6V0.4 alloy are itinerant in nature. The influence of spin-fluctuations on the superconducting properties of Ti0.6V0.4 is discussed in detail.

The magnetotransport properties of the intermetallic compound GdCu6

We have studied the temperature and magnetic field dependence of the electrical resistivity of GdCu6 and have co-related the results with the temperature dependence of heat capacity and magnetization. The magnetoresistance of GdCu6 is found to be positive both in the paramagnetic and antiferromagnetic regimes. Within the antiferromagnetic regime, the magnetoresistance is very high and increases to still higher values both with increasing field and decreasing temperature. In the paramagnetic regime the magnetoresistance continues to exhibit a finite positive value up to temperatures much higher than that corresponding to the antiferromagnetic to paramagnetic phase transition. We have shown through quantitative analysis that both the temperature dependences of resistivity and heat capacity indicate the presence of spin fluctuations within the paramagnetic regime of GdCu6. The field dependence of electrical resistivity indicates that the positive magnetoresistance in the paramagnetic phase is not related to the orbital motion of the conduction electrons in a magnetic field (the Kohler rule). In contrast, our analysis indicates that these spin fluctuations are responsible for the positive magnetoresistance observed within this paramagnetic regime. The nature of the field dependence of electrical resistivity is found to be qualitatively similar both in the antiferromagnetic and paramagnetic regimes, which probably indicates that spin fluctuations in the paramagnetic regime are of the antiferromagnetic type.

A comparative study of the magnetocaloric effect in Gd 3Co and Gd 3Ni

Magnetic and magnetocaloric properties of polycrystalline samples of Gd 3Co and Gd 3Ni have been studied. Both these compounds are antiferromagnets and undergo metamagnetic transitions in the antiferromagnetic phase. The Neel temperatures are found to be 128 and 99 K, respectively for Gd 3Co and Gd 3Ni. Though both these compounds have the same crystal structure, their magnetic structures seem to be different. It is found that Gd 3Ni possesses larger magnetic anisotropy compared to Gd 3Co. The maximum values of isothermal magnetic entropy change are 11 and 18.5 J Kg −1 K −1 for Gd 3Co and Gd 3Ni respectively, while their refrigerant capacities are 2.6 J cm −3 for both of them. Magnetic entropy change in the paramagnetic region shows a quadratic dependence on the magnetic field in the case of Gd 3Ni, indicating the presence of spin fluctuations above the Neel temperature.

Thermal and magnetic properties ofCe1−xLaxRhSn near the critical concentration

The main aim of this work is to investigate the electronic structure, specific heat and magnetic propertiesof Ce1−xLaxRhSn compounds near the critical concentration . Near x0 the magneticproperties for x>x0 correspond to spin fluctuations, while for the Ce-rich samples(x<x0) the mictomagnetic-like behaviour indicates the formation of magnetic clusters.Both the magnetic susceptibility and electrical resistivity data obtained for theCe0.55La0.45RhSn,Ce0.5La0.5RhSn and Ce0.45La0.55RhSn compounds obey a non-Fermi liquid temperature dependences due to an atomic disorderand/or the presence of spin fluctuations.

Magnetism, spin fluctuations, and non-Fermi-liquid behavior in(UxLa1−x)2Zn17

We present results of the low-temperature specific heat C of samples of the series $({\mathrm{U}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{)}_{2}{\mathrm{Zn}}_{17},$ combined with measurements of the low-temperature magnetic susceptibility (\ensuremath{\chi}) and resistivity (\ensuremath{\rho}). For $xg0.8$ we find antiferromagnetic order in coexistence with heavy-fermion behavior. An extrapolation of ${T}_{N}$ as a function of the uranium concentration implies that ${T}_{N}$ vanishes for $x=0.8;$ at $x=0.8,$ no magnetic order is detected experimentally at temperatures above 0.06 K. The non-Fermi-liquid (NFL) behavior predicted at such a point in the magnetic phase diagram may be observed, but not as clearly as in other systems; some of the behavior is more consistent with spin fluctuations. As the uranium concentration is lowered below $x=0.8,$ C continues to rise in the low-temperature limit, while $\ensuremath{\chi}\ensuremath{\propto}{\ensuremath{\chi}}_{0}\ensuremath{-}{\mathrm{aT}}^{0.5},$ but C seems to tend towards the behavior of a Fermi liquid with spin fluctuations at the lowest temperatures $(Tl0.25\mathrm{K}).$ First at $x=0.3$ the temperature dependence of $C/T$ is found to be contrary to Fermi-liquid behavior, while $\ensuremath{\chi}\ensuremath{\propto}{\ensuremath{\chi}}_{0}\ensuremath{-}a\mathrm{log}T.$ Thus non-Fermi-liquid behavior is not found so unambiguously at the concentration where ${T}_{N}$ vanishes as expected by a quantum critical point theory, but rather at lower uranium concentrations. This presents the possibility that NFL behavior in $({\mathrm{U}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{)}_{2}{\mathrm{Zn}}_{17}$ is not due to nearness to a quantum critical point, but rather to disorder or the presence of spin fluctuations.

Magnetic study of the U 1− xLa xAl 4 system

The magnetic properties of the U 1− x La x Al 4 system were investigated in the 4–600 K temperature range and fields up to 7 T. The UAl 4 compound shows a magnetic behaviour typical for a spin fluctuations system. The U 1− x La x Al 4 systems present Curie–Weiss type behaviour for x≤0.6 and above a certain temperature T*. The negative paramagnetic Curie temperatures and the effective magnetic moment values indicate the presence of spin fluctuations. For x≥0.8 a temperature-independent susceptibility is observed. As the lanthanum content increases from x=0 to 0.6 the effective magnetic moment per uranium atom remains constant.

Enhanced induced magnetization in coupled magnetic trilayers in the presence of spin fluctuations

Motivated by recent experiments, the effect of the interlayer exchange interaction $J_{inter}$ on the magnetic properties of coupled Co/Cu/Ni trilayers is studied theoretically. Here the Ni film has a lower Curie temperature $T_{C,\rm Ni}$ than the Co film in case of decoupled layers. We show that by taking into account magnetic fluctuations the interlayer coupling induces a strong magnetization for $T\gtsim T_{C,\rm Ni}$ in the Ni film. For an increasing $J_{inter}$ the resonance-like peak of the longitudinal Ni susceptibility is shifted to larger temperatures, whereas its maximum value decreases strongly. A decreasing Ni film thickness enhances the induced Ni magnetization for $T\gtsim T_{C,\rm Ni}$. The measurements cannot be explained properly by a mean field estimate, which yields a ten times smaller effect. Thus, the observed magnetic properties indicate the strong effect of 2D magnetic fluctuations in these layered magnetic systems. The calculations are performed with the help of a Heisenberg Hamiltonian and a Green's function approach.

Spin dependent interface scattering, low temperature resistivity and magnetoresistance in NiFeCo/Cu superlattices

The temperature dependence of the resistivity and magnetoresistance of the NiFeCo (15 Å)/Cu superlattices with and 21-22 Å, characterized by different correlated interface roughness, is found to depend strongly on the nature of the dominant electron scattering centres in the interface zone, and is determined by that of the spin dependent interface resistivity. This explains why the magnetoresistance changes in a quite different manner with temperature from specimen to specimen. We propose, within the frame of a simplified two-current model, that the presence of spin fluctuations and/or spin glass states in the interface zone is responsible for the temperature dependent behaviour of the spin dependent interface resistivity. The main features of the derived interface resistivity data, such as a maximum at for the specimen with Å and a or T dependence for those with Å, are explained satisfactorily in a unified picture of scattering of the electrons by spin fluctuations and/or spin glass states. The latter are in turn mediated appreciably by magnetic impurity concentration in the interface zone. The magnetoresistance at finite temperatures manifests itself to increase whenever the spin asymmetry is large, but to decrease with increasing magnetic impurity concentration in the interface zone.

Emission Mössbauer study of CMR manganite La0.8Ca0.2MnO3. I. Anomalous ferromagnetism

Using Co57 emission Mössbauer technique, we present clear evidence that in Ca-doped manganite, the magnetic and paramagnetic phases coexist below TC, with the abundance of the latter increasing with temperature. In contrast with the regular ferromagnetic materials, the variation of the hyperfine internal magnetic field Hint with temperature deviates considerably from the Brillouin relation, and exhibits an abrupt drop at TC. These features characterize the magnetic transition as a first-order transition. The non-Brillouin behavior of Hint(T) and the temperature dependence of the shape of the magnetically split sextet indicate the presence of spin fluctuations in this material well below TC.

Spin fluctuations in the heavy-fermion paramagnetic U(In0.5Sn0.5)3

Abstract 119 Sn Mossbauer spectra of the heavy-fermion intermetallic system U(In 0.5 Sn 0.5 ) 3 have been measured in the temperature range 4.2–300 K. The room temperature spectrum is characterized by a quadrupole doublet with a splitting value of 1.45 mm/s. With decreasing temperature the spectral line width increases and a hyperfine magnetic structure is developed at helium temperatures. This hyperfine field may be interpreted as being transferred via the conduction-electron polarization from the U to the Sn nucleus. A detailed analysis of the spectral evolution with temperature proves the presence of spin fluctuations in U(In 0.5 Sn 0.5 ) 3 near the transition from local-moment magnetism to itinerant f-electron behavior. The temperature dependence of the spin relaxation rate has been determined.

A neutron diffraction study of the phase transition in Pd2TiIn

Powder neutron diffraction measurements have been undertaken on Pd2TiIn over the temperature range 4-300 K. Previous measurements of the bulk magnetic properties indicated a magnetic transition in the neighbourhood of 110 K. The neutron measurements reveal a structural phase transition at 92 K. The transition appeared to be of first order, and no evidence was found of any long-range magnetic order below 92 K. However, the existence of ferrimagnetic order, involving the Pd and Ti atoms, could not be ruled out. The results are discussed in terms of the stability of the magnetic moment and the presence of spin fluctuations.

Disorder effects in transport properties of weak itinerant-electron ferromagnets.

The electric and thermal conductivity of weak itinerant-electron ferromagnets in the presence of spin fluctuations and disorder is calculated and the validity of the Wiedemann-Franz law in these systems is discussed. We used the Keldysh-diagram technique to obtain the expression for the conductivities of the clean system as well as the correction due to disorder. The electric conductivity shows a maximum at a low temperature, which characterizes the crossover behavior from the disorder-dominated region to the spin-fluctuation region. The thermal conductivity (κ/T) also has a maximum at the crossover temperature. The contribution due to spin fluctuations and the quantum correction due to disorder violate the Wiedemann-Franz law while the Drude term restores it as disorder increases

Ising mean field model for antiferromagnets in the presence of spin fluctuations. Scaling approach for the non-mean field case and role of defects

Starting from the case of a single impurity we built a very simple phenomenological model for Ising antiferromagnets in the presence of spin fluctuations at zero kelvin and at finite temperatures. A qualitative scaling of critical athermal (quantum) fluctuations is enough to understand many features of the low temperature behaviour of systems such as heavy fermions, 2d antiferromagnets, unstable transition metal magnetism. In particular, although several energy scales are considered in these systems (fluctuation temperature ( T f), exchange energy between different sublattices, etc.) we show that only a single scale is relevant which is given by T f− T c f (critical energy scale).

Determination of magnetic penetration depth from saddle-point field analysis in Tl2Ba2Ca2Cu3O10+ delta.

$^{205}\mathrm{Tl}$ NMR spectra and spin-lattice relaxation in the normal and superconducting states were measured on an aligned sample of ${\mathrm{Tl}}_{2}$${\mathrm{Ba}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10+\mathrm{\ensuremath{\delta}}}$. The spin-lattice relaxation time, ${\mathit{T}}_{1}$, was found to be isotropic and, in the normal state, 1/${\mathit{T}}_{1}$T to be temperature dependent, indicating the presence of spin fluctuations in the thallium-oxygen plane similar to that found in the copper-oxygen plane of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. $ sup 205---Tl NMR spectra at 5 K were studied and the angular dependence of the field shift at the saddle points of the magnetic-field distribution was found to be consistent with the anisotropic London theory. The average penetration depth was inferred to be 7040\ifmmode\pm\else\textpm\fi{}350 \AA{}. Our approach to determine the penetration depth, the saddle-point field analysis, is insensitive to distortion of the flux lattice associated with pinning. This is in contrast to the more commonly used second-moment method.