Contribution Of Spin Fluctuations Research Articles

Quantum criticality features in the Co,Fe doped MnSi

An universal line revealing an independence of spin fluctuation contributions to the heat capacity on impurity content and its nature is discovered in the helical phase of Mn(Co,Fe)Si. This situation declares an invariance of the heat capacity of spin subsystem under doping, which probably arises as a result of relative stiffness of the helical spin structure in respect to the impurity spins. On the other hand the situation drastically changes at the helical fluctuation region when no long range spin order exists. At low temperatures the spin fluctuation contributions to the heat capacity for a whole set of compositions of Mn(Co,Fe)Si are described by a single power expression with an exponent less then unity, which implies divergence of the ratio Cp/T at T→0. The current data are revealing that a singular quantum critical point does not exist in the system under study. In its place, one can see some sort of a quantum critical cloud covering a significant range of dopant concentrations.

Weak itinerant magnetic behaviour in Al substituted Ni 92 Cr 8 alloys

In the present work we report low temperature magnetic properties of Ni92−xAlxCr8 alloys. It is found that the Curie temperature and spontaneous magnetization (MSP) get suppressed around xC ≈ 7. From the analysis of the temperature and field dependent magnetization data we have obtained high value of Rhodes–Wohlfarth ratio, which indicates the weak itinerant ferromagnetic behavior for samples below xC. This is further supported by analysing the temperature variation of MSP in the framework of spin fluctuations, SCR theory. Furthermore, the Ginzburg-Landau formalism-based on spin fluctuation theory revealed that the contribution of spin fluctuations at low temperature increases as we approach xC.

Possible Ground States and Magnetic-Field-Tuned Phase Transitions of a Geometrically Frustrated Ising Antiferromagnet on a Triangular Lattice

Possible average alignments of the spins in the ground state and the phase transitions of a geometrically frustrated Ising antiferromagnet in the presence of the magnetic field on a triangular lattice are studied in a mean field approximation. Starting from a zero-field clock phase, we can determine the phase boundaries from the curves of magnetic moments and their derivatives as functions of the field. We also analyze the behaviour of sublattice magnetic moments under the effect of the field. The experimental relevances for TmMgGaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and SrEr2O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> are discussed. Besides, using a functional integral method, we have calculated a functional for free energy to obtain the contribution of spin fluctuations. From this, we can find that the role of the quantum spin fluctuations at very low temperatures is only prominent in the vicinity of the transition points. It can therefore be seen that the results, although given in the mean field approximation, describe quite well the phase transitions and rearrangements of the magnetic moment per spin under the effect of both the transverse and longitudinal field.

CeNi4Cr compound and its thermodynamic properties

CeNi5 as a mother compound is well known as a Stoner enhanced paramagnet characterized by the spin fluctuation contribution on its transport properties. Previous work on CeNi4Cr compound showed typical features of mixed valence systems with indication of tendency to heavy fermion behavior [1]. The theoretically predicted phase transition into antiferromagnetic order wasn’t observed down to 2 K. In this work we present the effect of spin fluctuations of the CeNi4Cr compound focused on its thermodynamic properties

Selective observation of spin and charge dynamics in an organic superconductor λ−(BETS)2GaCl4 using Ga69,71 NMR measurements

For the unconventional organic superconductor $\ensuremath{\lambda}\text{\ensuremath{-}}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ [BETS = bis(ethylenedithio) tetraselenafulvalene], the importance of both spin and charge degrees of freedom has been discussed based on the broadening of the NMR linewidth due to charge disproportionation and the occurrence of a spin-density-wave phase near the superconducting phase. NMR with the nuclear spin of $1/2$, previously used for studying organic conductors, is an effective method for revealing electronic states microscopically; however, it cannot distinguish between charge and spin anomalies. To resolve this problem, in this paper, we performed $^{69,71}\mathrm{Ga}$-NMR measurements, which enabled us to study both charge and spin dynamics using different gyromagnetic ratios and quadrupole moments between two isotopes. The spin-lattice relaxation rate is dominated by electric-field gradient fluctuations originating from molecular dynamics above 150 K and, below this temperature, it is dominated by spin fluctuations derived from the $\ensuremath{\pi}$ electrons of BETS layers. This change in the relaxation mechanism is considered to be due to the development of interactions between ${\mathrm{GaCl}}_{4}$ ions and BETS layers upon freezing of the molecular motion by cooling. Below 150 K, the contribution of spin fluctuations monotonically increases, and no increase in the charge fluctuation was observed, suggesting that the spin degree of freedom plays a major role in low-temperature physical properties. Our findings will aid theoretical studies on superconducting properties.

Realistic estimates of superconducting properties for the cuprates: reciprocal-space diagrammatic expansion combined with variational approach

We propose a systematic approach to the systems of correlated electrons, the so-called -DE-GWF method, based on reciprocal-space (-resolved) diagrammatic expansion of the variational Gutzwiller-type wave function for parametrized models of correlated fermions. The present approach, in contrast to either variational Monte-Carlo (VMC), or the recently developed real-space diagrammatic expansion of the Gutzwiller-type wave function (direct-space DE-GWF technique), is applicable directly in the thermodynamic limit and thus is suitable for describing selected singular features of the wave-vector-dependent quantities. We employ the -DE-GWF method to extract the non-analytic part of the two leading moments of the fermion spectral-density function across the (two-dimensional) Brillouin zone for the Hubbard model and away from the half-filling. Those moments are used to evaluate the nodal quasiparticle velocities and their spectral weights in the correlated superconducting state. The two velocities determined in that manner exhibit scaling with the electron concentration qualitatively different from that obtained earlier for the excited states of the high-Tc cuprates within the projected quasi-particle ansatz, and the results are in a very good quantitative agreement with experimental data if interpreted as those characterizing the spectrum below and above the observed kink. We provide a detailed discussion of the two gaps and two excitation branches (two velocities) appearing naturally within our DE-GWF approach. The two separate sets of characteristics distinguish the renormalized quasiparticle states very close to the Fermi surface from the deeper correlated-state properties. Also, an enhancement of the -dependent magnetic susceptibility is shown to contain a spin-fluctuation contribution within our language. Finally, the -DE-GWF approach is compared to both the VMC and real-space DE-GWF results for the cases of Hubbard and t-J-U models.

Influence of Al doping on the magnetoresistance and transport properties of LaBaMnAlO (0 ≤ x ≤ 0.15) manganites

Abstract This report presents the structural, magnetoresistance, electrical and thermal transport properties of Aluminium substituted La 0.7 Ba 0.3 Mn 1−x Al x O 3 (0 ≤ x ≤ 0.15) compounds synthesized by solid state reaction method. To obtain crystallographic parameters, the X-ray diffraction patterns are fitted in R-3c space group with Rietveld refinement method. The resistivity and magneto-transport measurements are performed using standard four-probe assembly with and without magnetic fields. The peak resistivity ρ peak is noted at Metal-Insulator Transition temperature ( T MI ) and lowering in T MI is observed for higher concentrations of Al 3+ . The resistivity data have been analyzed in two parts. Firstly, in the metallic region below T MI the resistivity data is fitted with three degree polynomial. Secondly, in the semiconducting region above T MI data have been fitted with Variable Range Hopping (VRH) and Small Polaron Hopping (SPH) models. The Seebeck coefficients found to be positive throughout the temperature range (10 K–300 K) with holes as dominating charge carriers. Similar to the resistivity profile, in metallic region the thermo-power is explained with qualitative model based on diffusion, magnon-drag, phonon-drag and spin fluctuation contributions whereas the semiconducting region is explained with small polaron hopping model.

The dominant role of critical valence fluctuations on high Tc superconductivity in heavy fermions

Despite almost 40 years of research, the origin of heavy-fermion superconductivity is still strongly debated. Especially, the pressure-induced enhancement of superconductivity in CeCu2Si2 away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu2Si2 and several related compounds, optimal superconductivity occurs at the pressure of a valence crossover, which arises from a virtual critical end point at negative temperature Tcr. In this context, we did a meticulous analysis of a vast set of top-quality high-pressure electrical resistivity data of several Ce-based heavy fermion compounds. The key novelty is the salient correlation between the superconducting transition temperature Tc and the valence instability parameter Tcr, which is in line with theory of enhanced valence fluctuations. Moreover, it is found that, in the pressure region of superconductivity, electrical resistivity is governed by the valence crossover, which most often manifests in scaling behavior. We develop the new idea that the optimum superconducting Tc of a given sample is mainly controlled by the compound’s Tcr and limited by non-magnetic disorder. In this regard, the present study provides compelling evidence for the crucial role of critical valence fluctuations in the formation of Cooper pairs in Ce-based heavy fermion superconductors besides the contribution of spin fluctuations near magnetic quantum critical points, and corroborates a plausible superconducting mechanism in strongly correlated electron systems in general.

Free energy of quantum spin systems: Functional integral representation

In this work, we propose a method for calculating the free energy of anisotropic quantum spin systems. We use the Hubbard-Stratonovich transformation to express the partition function of a generic bilinear super-exchange Hamiltonian in terms of a functional integral over classical time-dependent fields. In the general case the result is presented as an infinite series. The series may be summed up in the case of Ising-type models. For any ordered state we derive a compact expression for the contribution of Gaussian spin fluctuations to the free energy.

Lifting mean-field degeneracies in anisotropic classical spin systems

In this work, we propose a method for calculating the free energy of anisotropic classical spin systems. We use a Hubbard-Stratonovich transformation to express the partition function of a generic bilinear superexchange Hamiltonian in terms of a functional integral over classical time-independent fields. As an example, we consider an anisotropic spin-exchange Hamiltonian on the cubic lattice as is found for compounds with strongly correlated electrons in multiorbital bands and subject to strong spin-orbit interaction. We calculate the contribution of Gaussian spin fluctuations to the free energy. While the mean-field solution of ordered states for such systems usually has full rotational symmetry, we show here that the fluctuations lead to a pinning of the spontaneous magnetization along some preferred direction of the lattice.

Origin of the tetragonal-to-orthorhombic phase transition in FeSe: a combined thermodynamic and NMR study of nematicity.

The nature of the tetragonal-to-orthorhombic structural transition at T_{s}≈90 K in single crystalline FeSe is studied using shear-modulus, heat-capacity, magnetization, and nuclear magnetic resonance measurements. The transition is shown to be accompanied by a large shear-modulus softening, which is practically identical to that of underdoped Ba(Fe,Co)_{2}As_{2}, suggesting a very similar strength of the electron-lattice coupling. On the other hand, a spin-fluctuation contribution to the spin-lattice relaxation rate is only observed below T_{s}. This indicates that the structural, or "nematic," phase transition in FeSe is not driven by magnetic fluctuations.

Electrical properties and spin fluctuations studies of Y(Co 1− xNi x) 2 compounds

Synthesis by arc melting, the structural and the electric properties of Y(Co 1− x Ni x ) 2 alloys were studied by X-ray diffraction (XRD) and four probe dc electrical measurements. XRD analysis (300 K) shows that all samples crystallize in a cubic MgCu 2-type structure. The lattice parameters linearly decrease with Ni content. Electrical resistivity for the Y(Co 1− x Ni x ) 2 intermetallic series was measured in a temperature range of 15–1100 K. The parameters involved in the dependence of resistivity on temperature were determined. Residual, phonon and spin fluctuations resistivity were separated from electrical resistivity using both the Matthiesen formula and the Bloch–Gruneisen formula. The spin fluctuations resistivity of the Y(Co 1− x Ni x ) 2 series are compared to the mean square amplitudes of spin fluctuations previously calculated by the Linear Muffin Tin Orbital-Tight Binding Approach method for these series in the literature. The contribution of spin fluctuations to total resistivity ρ sf is proportional to T 2 at low temperatures. The proportionality parameter strongly reduces across the Y(Co 1− x Ni x ) 2 series.

Static and dynamical susceptibility ofLaO1−xFxFeAs

The mechanism of superconductivity and magnetism and their possible interplay have recently been under debate in pnictides. A likely pairing mechanism includes an important role of spin fluctuations and can be expressed in terms of the magnetic susceptibility chi. The latter is therefore a key quantity in the determination of both the magnetic properties of the system in the normal state, and of the contribution of spin fluctuations to the pairing potential. A basic ingredient to obtain chi is the independent-electron susceptibility chi0. Using LaO1-xFxFeAs as a prototype material, in this report we present a detailed ab-initio study of chi0(q,omega), as a function of doping and of the internal atomic positions. The resulting static chi0(q,0) is consistent with both the observed M-point related magnetic stripe phase in the parent compound, and with the existence of incommensurate magnetic structures predicted by ab-initio calculations upon doping.

Butterflylike specific heat, magnetocaloric effect, and itinerant metamagnetism in(Er,Y)Co2compounds

The field-induced first-order phase transition in $({\text{Er}}_{1\ensuremath{-}x}{\text{Y}}_{x}){\text{Co}}_{2}$ with the yttrium concentration $x=0.45$ is observed to be accompanied by a butterflylike behavior and significant irreversibility of the specific heat. The coefficient $\ensuremath{\gamma}$ of the $T$-linear specific heat decreases by $\ensuremath{\sim}48%$ under application and removal of a magnetic field up to 20 kOe. This behavior is attributed to the itinerant electron metamagnetism of $\text{Co}\text{ }3d$ electrons. The isothermal magnetic entropy change $\ensuremath{\Delta}{S}_{m}$ in ${\text{Er}}_{0.55}{\text{Y}}_{0.45}{\text{Co}}_{2}$ includes a large contribution associated with spin fluctuations induced by the $f\text{\ensuremath{-}}d$ exchange interaction in the hybridized $3d\text{\ensuremath{-}}5d$-electron subsystem. These spin fluctuations are suggested to contribute substantially to the magnetocaloric effect of the $R{\text{Co}}_{2}$ type compounds. The maximal $\ensuremath{\Delta}{S}_{m}$ value observed for ${\text{ErCo}}_{2}$ just above the Curie temperature is ascribed to the closeness of the ${T}_{\text{C}}$ value to the spin-fluctuation temperature ${T}_{\text{sf}}$ of itinerant $\text{Co}\text{ }3d$ electrons. The nonmonotonous change in $\ensuremath{\Delta}{S}_{m}$ with the Curie temperature of ${R}_{1\ensuremath{-}x}{R}_{x}^{\ensuremath{'}}{\text{Co}}_{2}$ compounds is explained by the temperature variation in the spin-fluctuation contribution to the magnetocaloric effect.

Magnetic and transport properties of Laves phase Dy 1− xMm xCo 2 ( x = 0–0.5) alloys

The influence of Mm substitution (Mm = mischmetal) on structural, transport and magnetic properties of (Dy 1− x Mm x )Co 2 ( x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys has been investigated by means of X-ray diffraction (XRD), temperature dependent electrical resistivity ( ρ( T)), ac susceptibility ( χ( T)) and thermopower ( S( T)). XRD patterns show the formation of solid solutions crystallizing with cubic Laves (C15) type structure at room temperature. The pronounced discontinuities in the resistivity and thermopower at Curie temperature ( T C) are explained based on the suppression of the spin-fluctuation contribution. The gradual decrease in T C and sharpness of discontinuities in ρ( T) and S( T) with increasing Mm substitution has been discussed.

Thermal Signatures of the Kondo Volume Collapse in Cerium

X-ray diffraction measurements of cerium in the vicinity of the isostructural gamma-alpha transition have been performed with high precision and accuracy from room temperature to almost 800 K. The disputed location of the critical point has been found to occur at 1.5+/-0.1 GPa and 480+/-10 K. The data are well fit by the Kondo volume collapse model plus a quasiharmonic representation of the phonons. The resultant free energy is validated against data for the thermodynamic Grüneisen parameter and, beyond the dominant spin-fluctuation contribution, indicates a dramatic change in the lattice Grüneisen parameter across the transition.

Dynamic spin susceptibility of paramagnetic spinelLiV2O4

In an attempt to explain inelastic neutron scattering data for LiV2O4 the dynamical spin susceptibility \chi(Q,w) at zero temperature is calculated. Starting point is a weak coupling approach based on the LDA bandstructure for that material. It is supplemented by a RPA treatment of local on-site interactions and contains an adjustable parameter. Due to the geometrically frustrated lattice structure the magnetic response is strongly enhanced in the vicinity of a nearly spherical surface in Q-space. We compare these results with recent low-temperature neutron scattering data. The measured spin relaxation rate \Gamma is used to estimate the spin fluctuation contribution to the specific heat.

Pressure and Field Effects on Spin Fluctuations in Ce0.8RE0.2Ni5(RE = Pr, Nd)

The substitution of Ce by Pr or Nd suppresses the spin fluctuation contribution to the electrical resistivity of the Stoner enhanced paramagnet CeNi 5 . The measurements of electrical resistivity as a function of the applied magnetic field up to 12 T and pressure up to 15 kbar on Ce 0.8 RE 0.2 Ni 5 ( RE = Pr, Nd) polycrystalline samples are presented. A mostly decreasing spin fluctuation contribution to the electrical resistivity of Ce 0.8 RE 0.2 Ni 5 with both increasing magnetic field and pressure is observed.

Spin-wave and spin-fluctuation contributions to the magnetoresistance of weakitinerant-electron ferromagnets

The spin-wave and exchange-enhanced spin-density fluctuation contributions to the electrical resistivity,ρ(T),of weak itinerant-electron ferromagnets are calculated in the absence and presence of the magneticfield, H, employing the two-band (s- and d-band) model and the version of spin-fluctuation theory ford-band that makes use of the Ginzburg–Landau formalism. These self-consistent calculations(i) mark a substantial improvement over the previous theoretical treatments in that theycompletely dispense with the unrealistic electron-gas approximation, (ii) account for the effect ofH on thecontributions to ρ(T) arising from spin waves (spin-density fluctuations) at low temperatures (atintermediate temperatures and for temperatures close to the Curie point,TC), and (iii) regardless of the nature of the low-lying magnetic excitationsthat dominantly contribute to the negative magnetoresistance,Δρ/ρ, in different temperature ranges, yield analytical expressions that basically have a simplegeneral form . The expressions, so obtained, not only permit a quantitative determination ofthe suppression of spin waves and spin fluctuations by the magnetic field,H, in weakly ferromagnetic metals from the magnetoresistance data but are also capable ofyielding useful information about the band structure.

Spin-fluctuation mechanism of superconductivity in cuprates

The theory of superconductivity within the t-J model, as relevant for cuprates, is developed. It is based on the equations of motion for projected fermionic operators and the mode-coupling approximation for the self-energy matrix. The dynamical spin susceptibility at various doping is considered as an input, extracted from experiments. The analysis shows that the superconductivity onset is dominated by the spin-fluctuation contribution. We show that T_c is limited by the spin-fluctuation scale $\Gamma$ and shows a pronounced dependence on the next-nearest-neighbor hopping t'. The latter can offer an explanation for the variation of T_c among different families of cuprates.