Spin Fluctuation Model Research Articles

Linkage between scattering rates and superconductivity in doped ferropnictides

We report an angle-resolved photoemission study of a series of hole and electron doped iron-based superconductors, their parent compound BaFe2As2, and their cousins BaCr2As2 and BaCo2As2. We focus on the energy (E) dependent scattering rate Gamma(E) as a function of the 3d count and on the renormalization function Z(E) of the inner hole pocket, which is the hot spot in these compounds. We obtain a non-Fermi-liquid-like linear in energy scattering rate Gamma(E>> kBT), independent of the dopant concentration. The main result is that the slope beta=Gamma(E >> kBT)/E, reaches its maxima near optimal doping and scales with the superconducting transition temperature. This supports the spin fluctuation model for superconductivity for these materials. In the optimally hole-doped compound, the slope of the scattering rate of the inner hole pocket is about three times bigger than the Planckian limit Gamma(E)/E~1. This result together with the energy dependence of the renormalization function Z(E) signals very incoherent charge carriers in the normal state which transform at low temperatures to a coherent unconventional superconducting state.

Spin polarization recovery and Hanle effect for charge carriers interacting with nuclear spins in semiconductors

We report on theoretical and experimental study of the spin polarization recovery and Hanle effect for the charge carriers interacting with the fluctuating nuclear spins in the semiconductor structures. We start the theoretical description from the simplest model of static and isotropic nuclear spin fluctuations. Then we describe the modification of the polarization recovery and Hanle curves due to the anisotropy of the hyperfine interaction, finite nuclear spin correlation time, and the strong pulsed spin excitation. For the latter case, we describe the resonance spin amplification effect in the Faraday geometry and discuss the manifestations of the quantum Zeno effect. The set of the experimental results for various structures and experimental conditions is chosen to highlight the specific effects predicted theoretically. We show that the spin polarization recovery is a very valuable tool for addressing carrier spin dynamics in semiconductors and their nanostructures.

Predicting the stacking fault energy of austenitic Fe-Mn-Al (Si) alloys

Aluminum and silicon are common alloying elements for tuning the stacking fault energy (SFE) of high Mn steels. Today the theoretical investigations on the Fe-Mn-Al/Si systems using Density Functional Theory (DFT) are very scarce. In the present study, we employ a state-of-the-art longitudinal spin fluctuations (LSFs) model in combination with DFT for describing the magnetic effects in Fe-Mn based alloys at finite temperature. We find that the traditional DFT-floating spin results fail to explain the experimental trends. However, the DFT-LSFs approach properly captures the Al-induced increase and Si-induced decrease of the SFE of the base alloy in line with the room-temperature observations. This finding highlights the importance of LSFs in describing the Al/Si effects on the SFE of Fe-Mn based alloys. We point out that the effects of the non-magnetic Al and Si additions on the SFE are in fact determined by the magnetic state of the host matrix. In addition, we estimate the role of carbon addition in the alloying effects of Al and Si. The present results provide a convenient pathway to access the important mechanical parameters for designing advanced high-strength alloys.

Influence of orbital two-channel Kondo effect on anomalous Hall effect in ferrimagnetic composites of LaNiO3 and CoFe2O4

In this report, we have investigated the magnetoresistance (MR) and Hall effect of the ferrimagnetic composites containing LaNiO3 and CoFe2O4 (CFO) (with CFO content 15% and 20%) which exhibit orbital two-channel Kondo (2CK) effect and therefore pronounced resistivity upturn at low temperature. Both composites manifest a negative to positive crossover in MR with increasing temperature. The MR is described by the Khosla and Fisher model of spin fluctuations scattering of conduction electrons and the two-band theory based on hybridized p-d sub-bands. The Hall resistivity of the composites consists of both ordinary and anomalous part. The negative sign of the ordinary Hall coefficient suggests electrons as the dominating charge carriers. The coefficient of anomalous Hall resistivity () follows the scaling relation () with longitudinal resistivity () at high temperature above the resistivity upturn. However, at low temperature shows non-monotonous behaviour and deviates from the scaling relation where orbital 2CK effect takes place. More detailed study below the resistivity upturn of the composite with 20% CFO reveals that this deviation occurs around the Kondo temperature. This breakdown of scaling relation around the Kondo temperature indicates the possible influence of orbital 2CK on the anomalous Hall effect.

Anisotropic hybridization in the new Kondo lattice compound CeCoInGa3

We report a detailed and comparative study of the single crystal CeCoInGa$_3$ in both experiment and theory. Resistivity measurements reveal the typical behavior of Kondo lattice with the onset temperature of coherence, $T^*\approx 50\,$K. The magnetic specific heat can be well fitted using a spin-fluctuation model at low temperatures, yielding a large Sommerfeld coefficient, $\gamma\approx172\,$mJ/mol K$^2$ at 6 K, suggesting that this is a heavy-fermion compound with a pronounced coherence effect. The magnetic susceptibility exhibits a broad field-independent peak at $T_{\chi}$ and shows an obvious anisotropy within the $bc$ plane, reflecting the anisotropy of the coherence effect at high temperatures. These are compared with strongly correlated calculations combining first-principles band structure calculations and dynamical mean-field theory. Our results confirm the onset of coherence at about 50 K and reveal a similar anisotropy in the hybridization gap, pointing to a close connection between the hybridization strength of the low-temperature Fermi-liquid state and the high-temperature coherence effect.

Suppression of spin-exciton state in hole overdoped iron-based superconductors.

The mechanism of Cooper pair formation in iron-based superconductors remains a controversial topic. The main question is whether spin or orbital fluctuations are responsible for the pairing mechanism. To solve this problem, a crucial clue can be obtained by examining the remarkable enhancement of magnetic neutron scattering signals appearing in a superconducting phase. The enhancement is called spin resonance for a spin fluctuation model, in which their energy is restricted below twice the superconducting gap value (2Δs), whereas larger energies are possible in other models such as an orbital fluctuation model. Here we report the doping dependence of low-energy magnetic excitation spectra in Ba1−xKxFe2As2 for 0.5 < x < 0.84 studied by inelastic neutron scattering. We find that the behavior of the spin resonance dramatically changes from optimum to overdoped regions. Strong resonance peaks are observed clearly below 2Δs in the optimum doping region, while they are absent in the overdoped region. Instead, there is a transfer of spectral weight from energies below 2Δs to higher energies, peaking at values of 3Δs for x = 0.84. These results suggest a reduced impact of magnetism on Cooper pair formation in the overdoped region.

Structure and magnetic transport properties of GdIn3−xMnx intermetallics

The crystal structures and magneto-transport properties of GdIn3−xMnx have been investigated using X-ray diffraction and magnetic and electric measurements. GdIn3−xMnx crystallize in cubic structure, and their lattice parameters tend to decrease with increasing Mn content due to the size effect at In site by Mn substitution for In. Mn doped GdIn3−xMnx order antiferromagnetically at low temperature. However, Mn doping into GdIn3 causes the decrease of Néel temperature due to the distortion of Gd(In,Mn)3 tetrahedron formed by Gd at corners and (In,Mn) at face centers in unit cell. The resistivities of GdIn3−xMnx are going up with increasing Mn content. The electric phase transition is associated with the magnetic transition, and the magneto-transport follows electron–magnon scattering model in low temperature region and the Stoner spin fluctuation model in high temperature region, respectively.

Calculating linear-response functions for finite temperatures on the basis of the alloy analogy model

A scheme is presented that is based on the alloy analogy model and allows one to account for thermal lattice vibrations as well as spin fluctuations when calculating response quantities in solids. Various models to deal with spin fluctuations are discussed concerning their impact on the resulting temperature-dependent magnetic moment, longitudinal conductivity, and Gilbert damping parameter. It is demonstrated that, by using the Monte Carlo (MC) spin configuration as input, the alloy analogy model is capable of reproducing the results of MC simulations on the average magnetic moment within all spin fluctuation models under discussion. On the other hand, the response quantities are much more sensitive to the spin fluctuation model. Separate calculations accounting for the thermal effect due to either lattice vibrations or spin fluctuations show that they give comparable contributions to the electrical conductivity and Gilbert damping. However, comparison to results accounting for both thermal effects demonstrates violation of Matthiessen's rule, showing the nonadditive effect of lattice vibrations and spin fluctuations. The results obtained for bcc Fe and fcc Ni are compared with the experimental data, showing rather good agreement for the temperature-dependent electrical conductivity and the Gilbert damping parameter.

Glide-plane symmetry and superconducting gap structure of iron-based superconductors.

We consider the effect of glide-plane symmetry of the Fe-pnictogen/chalcogen layer in Fe-based superconductors on pairing in spin fluctuation models. Recent theories have proposed that so-called η-pairing states with nonzero total momentum can be realized and possess exotic properties such as odd parity spin singlet symmetry and time-reversal symmetry breaking. Here we show that η pairing is inevitable when there is orbital weight at the Fermi level from orbitals with even and odd mirror reflection symmetry in z; however, by explicit calculation, we conclude that the gap function that appears in observable quantities is identical to that found in earlier, 1 Fe per unit cell pseudocrystal momentum calculations.

STM spectroscopy on deuterated κ-(ET-d[3,3])2Cu[N(Cn)2]Br

We performed the STM Spectroscopy (STS) measurement on organic superconductor deuterated κ-(ET-d[3,3])2Cu[N(CN)2]Br to clarify the mechanism of superconductivity. The mechanism of superconductivity in κ-ET2X is predicted as the spin fluctuation mechanism where the gap symmetry changes from dx2−y2 to dxy with increasing the electron correlation. In previous STS measurements, we found that d[2,2]-Br, d[0,0]-Br and Cu(NCS)2 were d-wave superconductors and the symmetry was described by the dx2−y2. The electron correlation of these salts is still weak in the frame of the spin fluctuation model. So, we investigated the d[3,3]-Br which located at stronger electron correlation region than the previously studied salts by the STS. We also investigated the STS on the cut lateral surfaces by the focused ion beam (FIB) method to measure the in-plane anisotropy of superconducting gap in more detail as well as the as-grown surfaces. It was found from the result that d[3,3]-Br was d-wave superconductor and the gap symmetry was dx2−y2 as well as the previously studied ET salts. Therefore, d[3,3]-Br is also still weak in spite of stronger electron correlation than other ET salts. On the other hand, we also observed the two-type superconducting gaps which are the nodal and anti-nodal type gaps at the same angle surface. This suggests the coexistence of dx2−y2 and dxy. It may be the evidence of the change from dx2−y2 to dxy caused by the locally stronger electron correlation around the anti-ferromagnetic insulating region.

Magnetic properties, electronic structures and pressure effects of HoxY1−xCo2 compounds

Abstract Neutron diffraction measurements were performed on HoCo2 in the pressure range p ⩽ 4.8 GPa. The evolutions with pressure of holmium and cobalt moments were correlated with those obtained from band structure calculations. Magnetic properties of HoxY1−xCo2 compounds with x ⩽ 0.3 were also investigated in a large temperature range. The magnetic behaviour of cobalt in the above system has been analysed in spin fluctuations model.

Antiferromagnetic fluctuations and proton Schottky heat capacity in doped organic conductor κ-(BEDT-TTF)4Hg2.78Cl8

Low temperature heat capacity measurements of hole-doped organic conducting compound of κ-(BEDT-TTF)4Hg2.78Cl8 were performed by the ac calorimetry and the thermal relaxation technique. It was observed that low temperature electronic heat capacity at 0T can be fitted by a kind of spin fluctuations model for two-dimensional (2D) itinerant electrons. The enhancement of electronic heat capacity is originated from these magnetic fluctuations occur at extremely low temperature region. We also observed that the low temperature heat capacity under magnetic fields show large additional contribution explained by the two-level Schottky model of protons located on the edge of BEDT-TTF molecules. The strong spin fluctuations peculiar for 2D electrons in this hole-doped organic compound influence the relaxation process of proton spins to get thermal equilibrium state. This is the first example among organic conducting systems where magnetic fluctuations of itinerant electrons affect on the relaxation of proton spins. Similar tendency was observed in another hole-doped system of κ-(BEDT-TTF)4Hg2.89Br8 of which magnitude is reduced down to 20–25% of that of κ-(BEDT-TTF)4Hg2.78Cl8.

Magnetic properties of TbCo2 compound at high pressures

The crystal structure and magnetic properties of TbCo2 compound have been analyzed by neutron diffraction, at pressures up to 4.1GPa in the temperature range 10–300K. A rhombohedral type structure of R3¯m type is shown below Curie temperature. The distortion degree of structure increases with pressure as well as when lowering temperature. The temperature dependences of terbium and cobalt moments can be described by Brillouin functions. The Curie temperatures decrease linearly with pressure with a slope of dTc/dP=−9K/GPa. The evolution with pressure of the cobalt magnetic moments and Tb5d band polarizations are theoretically analyzed. The experimental data are well described by band structure calculations. The magnetic behavior of cobalt in heavy rare-earth RCo2 compounds is analyzed in spin fluctuations model. A gradual quenching of spin fluctuations by internal field was shown.

Magnetic properties of RCo5-based systems

Band structure calculations and magnetic measurements were performed on (Gd x Y 1− x )Co 4Si as well as on YCo 4M compounds, where M = Ga, Al, and Co. The substitutions both at rare-earth and cobalt sites influence strongly the cobalt moments as well as the Curie temperatures. When Y is replaced by Gd, in Gd x Y 1− x Co 4Si system, the mean cobalt moments increase from 0.73 μ B/atom to 1.25 μ B/atom. The computed magnetic moments per formula unit agree rather well with experimental data. The reciprocal susceptibilities for ferrimagnetic compounds show non-linear temperature dependences. The effective cobalt moments are little dependent on composition. The ratio r between the mean number of cobalt spins determined from effective and saturation cobalt moments, respectively follows a dependence on Curie temperature of the form r ∝ T c − 2 / 3 . The magnetic behaviour of cobalt is analysed in spin fluctuations model.

First-Order Magnetic Phase Transitions in Anisotropic Itinerant Electron Ferromagnets

A spin fluctuation model is proposed for itinerant electron weak ferromagnets with Ising-like magnetic anisotropy. According to our theory of itinerant electron magnets, the temperature dependence of the magnetic susceptility and the spontaneous magnetic moment is studied. In contrast to those in isotropic cases, magnetic transitions in the model become of first order because of the oversuppression of thermal spin fluctuation amplitude around the critical point that hinders the spontaneous moment to diminish continuously.

Direct Observation of Paramagnons in Palladium

We report an inelastic neutron scattering study of the spin fluctuations in the nearly ferromagnetic element palladium. Dispersive over-damped collective magnetic excitations or "paramagnons" are observed up to 128 meV. We analyze our results in terms of a Moriya-Lonzarich-type spin-fluctuation model and estimate the contribution of the spin fluctuations to the low-temperature heat capacity. In spite of the paramagnon excitations being relatively strong, their relaxation rates are large. This leads to a small contribution to the low-temperature electronic specific heat.

Spin fluctuations and superconductivity in a three-dimensional tight-binding model forBaFe2As2

Despite the wealth of experimental data on the Fe-pnictide compounds of the KFe2As2-type, K = Ba, Ca, or Sr, the main theoretical work based on multiorbital tight-binding models has been restricted so far to the study of the related 1111 compounds. This can be ascribed to the more three dimensional electronic structure found by ab initio calculations for the 122 materials, making this system less amenable to model development. In addition, the more complicated Brillouin zone (BZ) of the body-centered tetragonal symmetry does not allow a straightforward unfolding of the electronic band structure into an effective 1Fe/unit cell BZ. Here we present an effective 5-orbital tight-binding fit of the full DFT band structure for BaFeAs including the kz dispersions. We compare the 5-orbital spin fluctuation model to one previously studied for LaOFeAs and calculate the RPA enhanced susceptibility. Using the fluctuation exchange approximation to determine the leading pairing instability, we then examine the differences between a strictly two dimensional model calculation over a single kz cut of the BZ and a completely three dimensional approach. We find pairing states quite similar to the 1111 materials, with generic quasi-isotropic pairing on the hole sheets and nodal states on the electron sheets at kz = 0 which however are gapped as the system is hole doped. On the other hand, a substantial kz dependence of the order parameter remains, with most of the pairing strength deriving from processes near kz = pi. These states exhibit a tendency for an enhanced anisotropy on the hole sheets and a reduced anisotropy on the electron sheets near the top of the BZ.

Antiferromagnetism and superconductivity: Determination of the Cu spin–spin relaxation time [formula omitted] and the spin–lattice relaxation time [formula omitted] in Gd 1.5Ce 0.5Sr 2Cu 2RuO 10

Gd 1.5Ce 0.5Sr 2Cu 2RuO 10 exhibits antiferromagnetic resonance at 23.9 GHz for applied fields less than 1000 Oe with a spin–spin relaxation time T 2 of approximately 0.45 ns, and with a spin–lattice relaxation time T 1 of at least 320 μs. Since in the homologue, Eu 1.5Ce0 .5Sr 2Cu 2RuO 10, the Ru atoms evidently fail to exhibit magnetic order, the antiferromagnetic resonance must arise from the cuprate planes. In other homologues, the cuprate planes are known to order ferromagnetically and are stacked in an antiferromagnetic configuration. The large value of T 1 suggests that phonon mediation plays no role in high temperature superconductivity. In addition, the presence of ferromagnetic cuprate planes is inconsistent with spin-fluctuation models of high temperature superconductivity.

Origin of gap anisotropy in spin fluctuation models of the iron pnictides

We discuss the large gap anisotropy found for the ${A}_{1g}$ ($s$ wave) state in random-phase approximation spin fluctuation and functional renormalization-group calculations and show how the simple arguments leading to isotropic sign-switched $s$-wave states in these systems need to be supplemented by a consideration of pair scattering within Fermi-surface sheets and between the individual electron sheets as well. In addition, accounting for the orbital makeup of the states on the Fermi surface is found to be crucial.

MAGNETIC PROPERTIES OF (GdxZr1-x)2CoAl3 SYSTEM

The magnetic properties of the ( Gd x Zr 1-x)2 CoAl 3 system were investigated in the 4–700 K temperature range and fields up to 70 kOe. The systems seem to be ferrimagnetically ordered. In the ordered state, the Co moments opposite to the gadolinium ones have very low values. The paramagnetic properties evidence over a characteristic temperature T*, a Curie–Weiss type behavior and the presence of the effective magnetic moments on Co atoms. The paramagnetic behavior of cobalt is analyzed in the spin fluctuations model.