Electron Spin Fluctuations Research Articles

The Superconducting Mechanism in BiS2-Based Superconductors: A Comprehensive Review with Focus on Point-Contact Spectroscopy

The family of BiS2-based superconductors has attracted considerable attention since their discovery in 2012 due to the unique structural and electronic properties of these materials. Several experimental and theoretical studies have been performed to explore the basic properties and the underlying mechanism for superconductivity. In this review, we discuss the current understanding of pairing symmetry in BiS2-based superconductors and particularly the role of point-contact spectroscopy in unravelling the mechanism underlying the superconducting state. We also review experimental results obtained with different techniques including angle-resolved photoemission spectroscopy, scanning tunnelling spectroscopy, specific heat measurements, and nuclear magnetic resonance spectroscopy. The integration of experimental results and theoretical predictions sheds light on the complex interplay between electronic correlations, spin fluctuations, and Fermi surface topology in determining the coupling mechanism. Finally, we highlight recent advances and future directions in the field of BiS2-based superconductors, underlining the potential technological applications.

Anisotropic magnetization and electronic structure of the first-order ferrimagnet ErCo2 studied by polarization dependent hard X-ray photoemission spectroscopy

The first-order ferrimagnet ErCo2 attracts interest not only because of metamagnetism and magnetocaloric effect just above TC ≈ 32–34 K but also because it is closely related with the itinerant metamagnetism of YCo2 and LuCo2. We study the electronic structure of single crystals with hard X-ray photoemission spectroscopy (HAXPES). Magnetization measurements reconfirm the first-order magnetic transition, metamagnetism, and strong magnetic anisotropy. Calculated ErCo2 band structures of the ferrimagnetic and paramagnetic phases are presented in detail. In the ferrimagnetic state, the density of states just below EF is smaller than in the paramagnetic phase. Valence band spectra in the paramagnetic state show strong polarization dependence. Furthermore, the change across the first-order ferrimagnetic transition in the valence band electronic structures is observed. These experimental data are well described by the band structure calculation incorporated with the polarization dependent cross-sections of orbitals. We further discuss possible effects of electron correlation and spin fluctuation.

Information-Guided Dynamic Nuclear Polarization

A spin ensemble in thermal equilibrium continuously undergoes random fluctuations analogous to those observed in Brownian motion, a process known as spin noise. Here, we investigate the dynamics of a system comprising a spin-1 paramagnetic center and a hyperfine-coupled spin-1/2 nucleus in the vicinity of a level crossing. We theoretically show that nuclear spins polarize efficiently under the combined action of thermal fluctuations and a closed-loop feedback protocol. The latter articulates periodic observations of the electronic magnetization and radio-frequency pulses connecting hybrid states with opposite nuclear spin alignment. Since nuclear polarization emerges from electronic spin fluctuations, not spin order, this microwave-free technique generically benefits from warmer, not colder, operation temperatures. Furthermore, because the spin dynamics at play near a level crossing is rather insensitive to the absolute value of the magnetic field, our work promises opportunities for high-field dynamic nuclear polarization, which is difficult to attain through present methods.

Quantifying the role of antiferromagnetic fluctuations in the superconductivity of the doped Hubbard model

We study the contribution of the electron-spin fluctuation coupling to the superconducting state of the two dimensional Hubbard model within dynamical cluster approximation (DCA) using a numerical exact continuous time Monte Carlo solver. By analyzing the frequency dependence of the self energy, we show that only about half of the superconductivity can be attributed to a "pairing glue" arising from treating spin fluctuations as a pairing boson in the standard one-loop theory.

Monitoring electron spin fluctuations with paramagnetic relaxation enhancement

The magnetic interactions between the spin of an unpaired electron and the surrounding nuclear spins can be exploited to gain structural information, to reduce nuclear relaxation times as well as to create nuclear hyperpolarization via dynamic nuclear polarization (DNP). A central aspect that determines how these interactions manifest from the point of view of NMR is the timescale of the fluctuations of the magnetic moment of the electron spins. These fluctuations, however, are elusive, particularly when electron relaxation times are short or interactions among electronic spins are strong. Here we map the fluctuations by analyzing the ratio between longitudinal and transverse nuclear relaxation times T1/T2, a quantity which depends uniquely on the rate of the electron fluctuations and the Larmor frequency of the involved nuclei. This analysis enables rationalizing the evolution of NMR lineshapes, signal quenching as well as DNP enhancements as a function of the concentration of the paramagnetic species and the temperature, demonstrated here for LiMg1−xMnxPO4 and Fe(III) doped Li4Ti5O12, respectively. For the latter, we observe a linear dependence of the DNP enhancement and the electron relaxation time within a temperature range between 100 and 300 K.

Spin and charge fluctuations in solid solutions of titanium substitution by iron group metals

Topological electronic transitions from omega- to alpha-phase and from alpha- to beta-phase that occur with an increase in the concentration of substitution atoms in solid solutions of titanium with iron group metals are considered. It is found that due to the difference in the potentials of the Hubbard repulsion of titanium and substitution atoms, as well as temperature Bose excitations, spin and charge fluctuations of the electron density increase in the topological alpha phase. Accounting for fluctuations makes it possible to describe the concentration increase in the entropy of the phases of the titanium alloys under consideration at different temperatures. The amplification of electronic fluctuations with increasing temperature and concentration shifts the chemical potential beyond the topological area of the electronic spectrum, thereby suppressing the alpha phase and inducing an electronic transition to the beta phase. Using the example of calculations of the entropy of titanium substitution alloys with iron at different temperatures, a diagram of their structural phases is constructed, which is consistent with the available experimental data. Keywords: solid solutions, topological electronic transitions, spin and charge fluctuations, entropy, structural phases.

Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance

Underdoped PrFeAs(O,F), one of the less known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, SQUID magnetometry, nuclear magnetic resonance (NMR) measurements and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFeAs(O,F) superconductivity, shown to host at least two isotropic gaps, clearly discernible in the spectra, irrespective of the direction of current injection (i.e., along the ab planes or along the c axis). Additional features at higher energy can be interpreted as signatures of a strong electron-boson coupling, as demonstrated by a model which combines Andreev reflection with the Eliashberg theory. Magnetic resonance measurements in the normal phase indicate the lack of a magnetic order in underdoped PrFeAs(O,F), while $^{75}$As NMR spin-lattice relaxation results suggest the presence of significant electronic spin fluctuations, peaking above $T_{c}$ and expected to mediate the superconducting pairing.

Anderson transition in the cation-substituted compounds ReXMn1-XS

The electrical properties of cation-substituted ReXMn1-XS (Re = Gd, Sm, Ho) compounds are investigated in the temperature range of 77-1200 K. A change in the type of conductivity from semiconductor to “metal” in ReXMn1-XS compounds at a critical concentration of Xc with an increase in the degree of cationic substitution is detected. The metal-dielectric concentration transition in the GdXMn1-XS system is accompanied by a decrease in the value of the specific electrical resistance by 12 orders of magnitude. For Sm0.2Mn0.8S, a sharp maximum of resistance is detected at T = 100 K, which can be caused by scattering of conduction electrons on spin fluctuations of localized electrons. The metal type of conductivity was established for Sm0.25Mn0.75S. In the HoXMn1-XS system, the Anderson transition was detected for XC = 0.3 with a decrease in the value of the specific electrical resistance by 10 orders of magnitude.

Unconventional 17O and 63Cu NMR shift components in cuprate superconductors

Nuclear magnetic resonance (NMR) is a fundamental bulk probe that provides key information about the electronic properties of materials. Very recently, the analysis of all available planar copper shift as well as relaxation data proved that while the shifts cannot be understood in terms of a single temperature-dependent spin component, relaxation can be explained with one dominating Fermi liquid-like component, without enhanced electronic spin fluctuations. For the shifts, a doping-dependent isotropic term as well as doping-independent anisotropic term became obvious. Here, we focus on planar [Formula: see text]O NMR shifts and quadrupole splittings. Surprisingly, we find that they demand, independently, a similar two-component scenario and confirm most of the previous conclusions concerning the properties of the spin components, in particular that a negative spin polarization survives in the superconducting state. This should have consequences for the pairing scenario.

Slow magnetic fluctuations and critical slowing down in Sr2Ir1−xRhxO4

Hidden magnetic order of Sr$_2$Ir$_{1-x}$Rh$_x$O$_4$, $x = 0.05$ and 0.1, has been studied using muon spin relaxation spectroscopy. In zero applied field and weak longitudinal fields ($\mu_0H_L \lesssim 2$~mT), muon spin relaxation data can be well described by exponentially-damped static Lorentzian Kubo-Toyabe functions, indicating that static and dynamic local fields coexist at each muon site. For $\mu_0H_L \gtrsim 2$~mT, the static rate is completely decoupled, and the exponential decay is due to dynamic spin fluctuations. In both zero field and $\mu_0H_L = 1$--2~mT, the temperature dependencies of the exponential muon spin relaxation rate exhibit maxima at 215~K for $x = 0.05$ and 175~K for $x = 0.1$, suggesting critical slowing down of electronic spin fluctuations. The field dependencies of the dynamic spin fluctuation rates can be well described by the Redfield relation. The correlation time of this electronic spin fluctuation is in the range of~2--5~ns for Sr$_2$Ir$_{0.9}$Rh$_{0.1}$O$_4$, and shorter than 2~ns for Sr$_2$Ir$_{0.95}$Rh$_{0.05}$O$_4$. The rms fluctuating field is on the order of 1 mT, which is consistent with the polarized neutron diffraction cross-section.

Superexchange mechanism and quantum many body excitations in the archetypal di-Cu oxo-bridge

The hemocyanin protein binds and transports molecular oxygen via two copper atoms at its core. The singlet state of the {{rm{Cu}}}_{2}{{rm{O}}}_{2} core is thought to be stabilised by a superexchange pathway, but detailed in situ computational analysis is complicated by the multi-reference character of the electronic ground state. Here, electronic correlation effects in the functional site of hemocyanin are investigated using a novel approach, treating the localised copper 3d electrons with cluster dynamical mean field theory. This enables us to account for dynamical and multi-reference quantum mechanics, capturing valence and spin fluctuations of the 3d electrons. Our approach explains the stabilisation of the experimentally observed di-Cu singlet for the butterflied {{rm{Cu}}}_{2}{{rm{O}}}_{2} core, with localised charge and incoherent scattering processes across the oxo-bridge that prevent long-lived charge excitations. This suggests that the magnetic structure of hemocyanin is largely influenced by the many-body corrections.

The Characteristic Properties of Magnetostriction and Magneto-Volume Effects of Ni2MnGa-Type Ferromagnetic Heusler Alloys.

In this article, we review the magnetostriction and magneto-volume effects of Ni2MnGa-type ferromagnetic Heusler alloys at the martensitic, premartensitic, and austenitic phases. The correlations of forced magnetostriction (ΔV/V) and magnetization (M), using the self-consistent renormalization (SCR) spin fluctuation theory of an itinerant electron ferromagnet proposed by Takahashi, are evaluated for the ferromagnetic Heusler alloys. The magneto-volume effect occurs due to the interaction between the magnetism and volume change of the magnetic crystals. The magnetic field-induced strain (referred to as forced magnetostriction) and the magnetization are measured, and the correlation of magnetostriction and magnetization is evaluated. The forced volume magnetostriction ΔV/V at the Curie temperature, TC is proportional to M4, and the plots cross the origin point; that is, (M4, ΔV/V) = (0, 0). This consequence is in good agreement with the spin fluctuation theory of Takahashi. An experimental study is carried out and the results of the measurement agree with the theory. The value of forced magnetostriction is proportional to the valence electron concentration per atom (e/a). Therefore, the forced magnetostriction reflects the electronic states of the ferromagnetic alloys. The magnetostriction near the premartensitic transition temperature (TP) induces lattice softening; however, lattice softening is negligible at TC. The forced magnetostriction at TC occurs due to spin fluctuations of the itinerant electrons. In the martensitic and premartensitic phases, softening of the lattice occurs due to the shallow hollow (potential barrier) of the total energy difference between the L21 cubic and modulated 10M or 14M structures. As a result, magnetostriction is increased by the magnetic field.

Phenomenology of 63Cu Nuclear Relaxation in Cuprate Superconductors

Nuclear relaxation is an important thermodynamic probe of electronic excitations, in particular in conducting and superconducting systems. Here, an empirical phenomenology based on all available literature data for planar Cu in hole-doped cuprates is developed. It is found that most of the seemingly different relaxation rates among the systems are due to a temperature-independent anisotropy that affects mostly measured 1/T1∥, the rate with an external magnetic field along the crystal c-axis, while 1/T1⊥ is largely independent on doping and material above the critical temperature of superconductivity (Tc). This includes very strongly overdoped systems that show Fermi liquid behavior and obey the Korringa law. Below Tc, the relaxation rates are similar, as well, if plotted against the reduced temperature T/Tc. Thus, planar Cu nuclear relaxation is governed by a simple, dominant mechanism that couples the nuclei with varying anisotropy to a rather ubiquitous bath of electronic excitations that appear Fermi liquid-like irrespective of doping and family. In particular, there is no significant enhancement of the relaxation due to electronic spin fluctuations, different from earlier conclusions. Only the La2−xSrxCuO4 family appears to be an outlier as additional relaxation is present; however, the anisotropy remains temperature independent. Also systems with very low doping levels, for which there is a lack of data, may behave differently.

Properties of the Electronic Fluid of Superconducting Cuprates from 63Cu NMR Shift and Relaxation

Nuclear magnetic resonance (NMR) provides local, bulk information about the electronic properties of materials, and it has been influential for theories of high-temperature superconducting cuprates. NMR reported early that nuclear relaxation is much faster than what one expects from coupling to fermionic excitations above the critical temperature for superconductivity (Tc), i.e., what one estimates from the Knight shift with the Korringa law. As a consequence, special electronic spin fluctuations have been invoked. Here, based on literature relaxation data, it is shown that the electronic excitations, to which the nuclei couple with a material and doping-dependent anisotropy, are rather ubiquitous and Fermi liquid-like, i.e., they are only affected by Tc not the pseudogap. A suppressed NMR spin shift rather than an enhanced relaxation leads to the failure of the Korringa law for most materials. Shift and relaxation below Tc support the view of suppressed shifts, as well. A simple model of two coupled electronic spin components, one with 3d(x2 − y2) orbital symmetry and the other with an isotropic s-like interaction, can explain the data. The coupling between the two components is found to be negative, and it must be related to the pseudogap behavior of the cuprates. We can also explain the negative shift conundrum and the long-standing orbital shift discrepancy for NMR in the cuprates.

Resistivity of a 2d quantum critical metal

We calculate resistivity in the paramagnetic phase just above the curie temperature in a 2d ferromagnetic metal. The required dynamical susceptibility in the formalism of resistivity is calculated within the Random Phase Approximation (RPA). The mechanism of resistivity is magnetic scattering, in which s-band electrons are scattered off the magnetic spin fluctuations of d-band electrons. We use the s-d Hamiltonian formalism. We find that near the quantum critical point the resistivity in 2d scales as T43 and in the high temperature limit resistivity scales as proportional to T. In contrast to it, resistivity due to phonon scattering is given by T5 in low temperature limit as is well known. Our RPA result agrees with the Self-Consistence Renormalization (SCR) theory result.

Magnetic Phase Transition in MnSi Based on the First Principle Calculations of Electronic Structure and Spin Fluctuation Theory

Based on the spin-fluctuation theory and LSDA + U + SO-calculations of the electronic structure, the spin states arising in the region of the magnetic phase transition in the helicoidal ferromagnet MnSi are studied. Within the framework of the stated model of the electronic structure, a temperature dependence of the homogeneous magnetic susceptibility is obtained near the temperature of the magnetic phase transition, which is in good agreement with experiment. At the temperature of the transition to the paramagnetic state, the left chirality vanishes, and spin correlations arise with a radius equal to the length of the spin helix, which then decreases with temperature. The results were obtained within the framework of the assignment of the Ministry of Education and Science of the Russian Federation, contract 3.9521.2017 / 8.9

14N Nuclear Magnetic Resonance and Relaxation in the Paramagnetic Region of Uranium Mononitride

The spin susceptibility of a polycrystalline sample of uranium mononitride UN is studied by measuring the 14N NMR line shift, spin–lattice relaxation rates of the nuclear spin, and static magnetic susceptibility in the temperature region of 1.5TN < T < 7TN A joint analysis of the results obtained has revealed the temperature dependence of the characteristic energy of spin fluctuations of the uranium 5f electrons: Γnmr(T) ∝ T0.54(4) close to the dependence Γ(T) ∝ T0.5 characteristic of concentrated Kondo systems above the coherent state formation temperature.

Hyperfine interaction and electronic spin fluctuation study on Sr2−xLaxFeCoO6 ( x = 0, 1, 2) by high-resolution backscattering neutron spectroscopy

The study of hyperfine interaction by high-resolution inelastic neutron scattering is not very well known compared to the other competing techniques viz. NMR, M\"ossbauer, PACS etc. Also the study is limited mostly to magnetically ordered systems. Here we report such study on Sr$_{2-x}$La$_x$FeCoO$_6$ (x = 0, 1, 2) of which first (Sr$_2$FeCoO$_6$ with x = 0) has a canonical spin spin glass, the second (SrLaFeCoO$_6$ with x = 1) has a so-called magnetic glass and the third (La$_2$FeCoO$_6$ with x = 2) has a magnetically ordered ground state. Our present study revealed clear inelastic signal for SrLaFeCoO$_6$, possibly also inelastic signal for Sr$_2$FeCoO$_6$ below the spin freezing temperatures $T_{sf}$ but no inelastic signal at all for for the magnetically ordered La$_2$FeCoO$_6$ in the neutron scattering spectra. The broadened inelastic signals observed suggest hyperfine field distribution in the two disordered magnetic glassy systems and no signal for the third compound suggests no or very small hyperfine field at the Co nucleus due to Co electronic moment. For the two magnetic glassy system apart from the hyperfine signal due only to Co, we also observed electronic spin fluctuations probably from both Fe and Co electronic moments. \end{abstract}

Two-Dome Superconductivity in FeS Induced by a Lifshitz Transition.

Among iron chalcogenide superconductors, FeS can be viewed as a simple, highly compressed relative of FeSe without a nematic phase and with weaker electronic correlations. Under pressure, however, the superconductivity of stoichiometric FeS disappears and reappears, forming two domes. We perform electronic structure and spin fluctuation theory calculations for tetragonal FeS in order to analyze the nature of the superconducting order parameter. In the random phase approximation, we find a gap function with d-wave symmetry at ambient pressure, in agreement with several reports of a nodal superconducting order parameter in FeS. Our calculations show that, as a function of pressure, the superconducting pairing strength decreases until a Lifshitz transition takes place at 4.6GPa. As a hole pocket with a large density of states appears at the Lifshitz transition, the gap symmetry is altered to sign-changing s wave. At the same time, the pairing strength is severely enhanced and increases up to a new maximum at 5.5GPa. Therefore, our calculations naturally explain the occurrence of two superconducting domes in FeS.

Magnetic and transport properties driven by Sr substitution in polycrystalline Pr1-xSrxCoO3 (0.1 ≤ x ≤ 0.5) cobaltites

The effect of strontium (Sr) doping on the structural, magnetic, electrical, and thermal properties of Pr1-xSrxCoO3 (0.1 ≤ x ≤ 0.5) has been studied. The samples were synthesized by using the conventional solid-state reaction method. The Rietveld refinement of X-ray diffraction patterns confirms the single-phase composition with orthorhombic (Pbnm) perovskite symmetry. The magnetization measurements revealed the paramagnetic to ferromagnetic transition and the transition temperature (Tc) increased with increasing Sr doping. The effective magnetic moments determined by the Curie-Weiss law show an increase in the Sr concentration. The temperature dependence of electrical resistivity suppressed with increasing the Sr content. Moreover, all the compounds other than x = 0.5 show the semiconducting nature. All semiconductor compositions (x = 0.1, 0.2, 0.3, and 0.4) in the high temperature region can be explained within the framework of the small polaron hopping model and the variable range hopping model, whereas the metallic composition (x = 0.5) is explained by electron-electron, electron-phonon, and electron-spin fluctuation scattering processes. The Seebeck coefficient (S) for all the samples except x= 0.5 is found to be positive, thereby confirming the applicability of the small polaron hopping model in the high-temperature region. The sample with x = 0.5 exhibits a crossover in S from positive to negative values and again attains a positive value.