Short-range Spin Fluctuations Research Articles

Unconventional insulator-to-metal phase transition in Mn3Si2Te6

The nodal-line semiconductor Mn3Si2Te6 is generating enormous excitment due to the recent discovery of a field-driven insulator-to-metal transition and associated colossal magnetoresistance as well as evidence for a new type of quantum state involving chiral orbital currents. Strikingly, these qualities persist even in the absence of traditional Jahn-Teller distortions and double-exchange mechanisms, raising questions about exactly how and why magnetoresistance occurs along with conjecture as to the likely signatures of loop currents. Here, we measured the infrared response of Mn3Si2Te6 across the magnetic ordering and field-induced insulator-to-metal transitions in order to explore colossal magnetoresistance in the absence of Jahn-Teller and double-exchange interactions. Rather than a traditional metal with screened phonons, the field-driven insulator-to-metal transition leads to a weakly metallic state with localized carriers. Our spectral data are fit by a percolation model, providing evidence for electronic inhomogeneity and phase separation. Modeling also reveals a frequency-dependent threshold field for carriers contributing to colossal magnetoresistance which we discuss in terms of polaron formation, chiral orbital currents, and short-range spin fluctuations. These findings enhance the understanding of insulator-to-metal transitions in new settings and open the door to the design of unconventional colossal magnetoresistant materials.

Hund-Heisenberg model in superconducting infinite-layer nickelates

We theoretically investigate the unconventional superconductivity in the newly discovered infinite-layer nickelates Nd$_{1-x}$Sr$_{x}$NiO$_{2}$ based on a two-band model. By analyzing the transport experiments, we propose that the doped holes dominantly enter the Ni $d_{xy}$ or/and $d_{3z^{2}-r^{2}}$ orbitals as charged carriers, and form a conducting band. Via the onsite Hund coupling, the doped holes are coupled to the Ni localized holes in the $d_{x^{2}-y^{2}}$ orbital band. We demonstrate that this two-band model could be further reduced to a Hund-Heisenberg model. Using the reduced model, we show the non-Fermi liquid state above the critical $T_{c}$ could stem from the carriers coupled to the spin fluctuations of the localized holes. In the superconducting phase, the short-range spin fluctuations mediate the carriers into Cooper pairs and establish $d_{x^{2}-y^{2}}$-wave superconductivity. We further predict that the doped holes ferromagnetically coupled with the local magnetic moments remain itinerant even at very low temperature, and thus the pseudogap hardly emerges in nickelates. Our work provides a new superconductivity mechanism for strongly correlated multi-orbital systems and paves a distinct way to exploring new superconductors in transition or rare-earth metal oxides.

Antiferromagnetism and the emergence of frustration in the sawtooth lattice chalcogenide olivines Mn2SiS4−xSex ( x=0–4 )

The magnetism in the sawtooth lattice of Mn in the olivine chalcogenides, ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$ ($x=1--4$), is studied in detail by analyzing their magnetization, specific heat, and thermal conductivity properties and complemented with density functional theory calculations. The air-stable chalcogenides are antiferromagnets and show a linear trend in the transition temperature ${T}_{N}$ as a function of Se content ($x$), which shows a decrease from ${T}_{N}\ensuremath{\approx}86\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4}$ to 66 K for ${\mathrm{Mn}}_{2}{\mathrm{SiSe}}_{4}$. Additional magnetic anomalies are revealed at low temperatures for all the compositions. Magnetization irreversibilities are observed as a function of $x$. The specific heat and the magnetic entropy indicate the presence of short-range spin fluctuations in ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$. A spin-flop antiferromagnetic phase transition in the presence of applied magnetic field is present in ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$, where the critical field for the spin flop increases from $x=0$ towards 4 in a nonlinear fashion. Density functional theory calculations show that an overall antiferromagnetic structure with ferromagnetic coupling of the spins in the $ab$ plane minimizes the total energy. The band structures calculated for ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4}$ and ${\mathrm{Mn}}_{2}{\mathrm{SiSe}}_{4}$ reveal features near the band edges similar to those reported for Fe-based olivines suggested as thermoelectrics; however the experimentally determined thermal transport data do not support superior thermoelectric features. The transition from long-range magnetic order in ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4}$ to short-range order and spin fluctuations in ${\mathrm{Mn}}_{2}{\mathrm{SiSe}}_{4}$ is explained using the variation of the Mn-Mn distances in the triangle units that constitutes the sawtooth lattice upon progressive replacement of sulfur with selenium. Overall, the results presented here point towards the role played by magnetic anisotropy and geometric frustration in the antiferromagnetic state of the sawtooth olivines.

Phase Competition in the Palmer-Chalker XY Pyrochlore Er_{2}Pt_{2}O_{7}.

We report neutron scattering measurements on Er_{2}Pt_{2}O_{7}, a new addition to the XY family of frustrated pyrochlore magnets. Symmetry analysis of our elastic scattering data shows that Er_{2}Pt_{2}O_{7} orders into the k=0, Γ_{7} magnetic structure (the Palmer-Chalker state), at T_{N}=0.38 K. This contrasts with its sister XY pyrochlore antiferromagnets Er_{2}Ti_{2}O_{7} and Er_{2}Ge_{2}O_{7}, both of which order into Γ_{5} magnetic structures at much higher temperatures, T_{N}=1.2 and 1.4K, respectively. In this temperature range, the magnetic heat capacity of Er_{2}Pt_{2}O_{7} contains a broad anomaly centered at T^{*}=1.5 K. Our inelastic neutron scattering measurements reveal that this broad heat capacity anomaly sets the temperature scale for strong short-range spin fluctuations. Below T_{N}=0.38 K, Er_{2}Pt_{2}O_{7} displays a gapped spin-wave spectrum with an intense, flat band of excitations at lower energy and a weak, diffusive band of excitations at higher energy. The flat band is well described by classical spin-wave calculations, but these calculations also predict sharp dispersive branches at higher energy, a striking discrepancy with the experimental data. This, in concert with the strong suppression of T_{N}, is attributable to enhanced quantum fluctuations due to phase competition between the Γ_{7} and Γ_{5} states that border each other within a classically predicted phase diagram.

Phase separation in Ca1−xLaxFe2As2 superconductors: a 57Fe Mössbauer study

We report a detailed 57Fe Mössbauer study of lanthanum doped CaFe2As2 superconductors. The quadrupole splitting distribution (QSD) method was adopted to analyze the Mössbauer spectra of Ca1−xLaxFe2As2 (x = 0.2, 0.3) single crystals. For both compounds we observed two QSD contributions centered at 0.31 mm s−1 and −0.32 mm s−1 at room temperature. The first principles calculations of the electronic structures and the electric field gradient (EFG) of Ca1−yLayFe2As2 model systems reveal that the EFG changes from positive to negative with increasing dopant concentration, indicating that the La atoms distribute heterogeneously in the compounds. The two QSD components behave differently with decreasing temperature. The minority La-rich phase undergoes superconducting transition, while short range spin fluctuations and/or spin-phonon coupling appear in the majority La-poor phase. Our experiments provide new evidence of the phase separation picture at low temperatures in Ca1−xLaxFe2As2 superconductors.

Orbital-dependent electron correlation in LiFeAs revealed by angle-resolved photoemission spectroscopy

We report on the electronic structure of the 111-type iron pnictide superconductor LiFeAs as a function of temperature using angle-resolved photoemission spectroscopy. Below approximately 50 K, both the ${d}_{yz}$ hole band at the $Z$ point and the ${d}_{xz/yz}$ electron band at the $A$ point shift to a higher binding energy side. However, at the high-symmetry points $Z,\phantom{\rule{0.16em}{0ex}}A,\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Gamma}}$, and $M$, the remaining bands are almost independent of temperature. One of the possible scenarios for these observations is that a strong, three-dimensional orbital-dependent correlation exists in the normal state of LiFeAs in relation to short-range spin fluctuations.

Pseudogap behavior in the emery model for the electron-doped superconductor Nd2 − x Ce x CuO4: Multiband LDA + DMFT + Σk approach

We propose a generalization of the LDA + DMFT + Σk approach to the multiband case, in which correlated and uncorrelated states are present in the model simultaneously. Using the multiband version of the LDA + DMFT + Σk approach, we calculate the density of states and spectral functions for the Emery model in a wide energy interval around the Fermi level. We also obtain the Fermi surfaces for the electron-doped high-temperature superconductor Nd2 − xCexCuO4 in the pseudogap phase. The self-energy part Σk introduced additionally to take into account pseudogap fluctuations describes the nonlocal interaction of correlated electrons with collective Heisenberg short-range spin fluctuations. To solve the effective impurity model, the numerical renorm-group (NRG) method is used for the DMFT equations. Good qualitative agreement of the Fermi surfaces calculated using the LDA + DMFT + Σk approach and experimental angle-resolved photoemission spectroscopic data is attained. The stability of the dielectric solution with charge transfer in the Emery model with correction for double counting is analyzed in the Appendix.

Observation of Partial-Disorder-Type Spin Fluctuations in Frustrated Mn3Pt

The spin-frustrated conductor Mn$_3$Pt exhibits a characteristic magnetic structure called partial disorder in which some spin sites can form magnetic order through the generation of non-ordered sites that locally relieve the frustration. Here we report the results of a single-crystal inelastic neutron scattering study of this compound. The measured momentum $\vec{Q}$ correlations of diffusive magnetic scattering reveal that the paramagnetic phase exhibits short-range spin fluctuations with the same type of partial disorder. Its relation to conductivity is also discussed.

Spectral analysis for the iron-based superconductors: Anisotropic spin fluctuations and fully gappeds±-wave superconductivity

Spin fluctuations are considered to be one of the candidates that drive a sign-reversed ${s}^{\ifmmode\pm\else\textpm\fi{}}$ superconducting state in the iron pnictides. In the magnetic scenario, whether the spin-fluctuation spectrum exhibits certain unique fine structures is an interesting aspect for theoretical study in order to understand experimental observations. We investigate the detailed momentum dependence of the short-range spin fluctuations using a two-orbital model in the self-consistent fluctuation exchange approximation and find that a common feature of those fluctuations that are capable of inducing a fully gapped ${s}^{\ifmmode\pm\else\textpm\fi{}}$ state is the momentum anisotropy with lengthened span along the direction transverse to the antiferromagnetic momentum transfer. Performing a qualitative analysis based on the orbital character and the deviation from perfect nesting of the electronic structure for the two-orbital and a more complete five-orbital model, we gain the insight that this type of anisotropic spin fluctuations favor superconductivity due to their enhancement of intraorbital, but interband, pair scattering processes. The momentum anisotropy leads to elliptically shaped magnetic responses which have been observed in inelastic neutron-scattering measurements. Meanwhile, our detailed study on the magnetic and the electronic spectrum shows that the dispersion of the magnetic resonance mode in the nearly isotropic ${s}^{\ifmmode\pm\else\textpm\fi{}}$ superconducting state exhibits anisotropic propagating behavior in an upward pattern and the coupling of the resonance mode to fermions leads to a dip feature in the spectral function.

Pairing dynamics in strongly correlated superconductivity

Confirmation of the phononic origin of Cooper pair formation in superconductors came with the demonstration that the interaction was retarded and that the corresponding energy scales were associated with phonons. Using cellular dynamical mean-field theory for the two-dimensional Hubbard model, we identify such retardation effects in $d$-wave pairing and associate the corresponding energy scales with short-range spin fluctuations. We find which frequencies are relevant for pairing as a function of interaction strength and doping and show that the disappearance of superconductivity on the overdoped side coincides with the disappearance of the low-energy feature in the antiferromagnetic fluctuations, as observed in neutron-scattering experiments.

Phonon spectra inCaFe2As2andCa0.6Na0.4Fe2As2: Measurement of the pressure and temperature dependence and comparison withab initioand shell model calculations

We report the pressure and temperature dependence of the phonon density-of-states in superconducting Ca0.6Na0.4Fe2As2 (Tc=21 K) and the parent compound CaFe2As2, using inelastic neutron scattering. We observe no significant change in the phonon spectrum for Ca0.6Na0.4Fe2As2 at 295 K up to pressures of 5 kbar. The phonon spectrum for CaFe2As2 shows softening of the low-energy modes by about 1 meV when decreasing the temperature from 300 K to 180 K. There is no appreciable change in the phonon density of states across the structural and anti-ferromagnetic phase transition at 172 K. These results, combined with our earlier temperature dependent phonon density of states measurements for Ca0.6Na0.4Fe2As2, indicate that the softening of low-energy phonon modes in these compounds may be due to the interaction of phonons with electron or short-range spin fluctuations in the normal state of the superconducting compound as well as in the parent compound. The phonon spectra are analyzed with ab-initio and empirical model calculations giving partial densities of states and dispersion relations.

Anisotropic itinerant magnetism and spin fluctuations inBaFe2As2: A neutron scattering study

Neutron scattering measurements were performed to investigate magnetic excitations in a single-crystal sample of the ternary iron arsenide ${\text{BaFe}}_{2}{\text{As}}_{2}$, a parent compound of a recently discovered family of Fe-based superconductors. In the ordered state, we observe low energy spin-wave excitations with a gap energy $\ensuremath{\Delta}=9.8(4)\text{ }\text{meV}$. The in-plane spin-wave velocity ${v}_{ab}$ and out-of-plane spin-wave velocity ${v}_{c}$ measured at 12 meV are 280(150) and $57(7)\text{ }\text{meV}\text{ }\text{\AA{}}$, respectively. At high energy, we observe anisotropic scattering centered at the antiferromagnetic wave vectors. This scattering indicates two-dimensional spin dynamics, which possibly exist inside the Stoner continuum. At ${T}_{N}=136(1)\text{ }\text{K}$, the gap closes and quasielastic scattering is observed above ${T}_{N}$, indicative of short-range spin fluctuations. In the paramagnetic state, the scattering intensity along the $L$ direction becomes ``rodlike,'' characteristic of uncorrelated out-of-plane spins, attesting to the two-dimensionality of the system.

Thermal destruction of chiral order in a two-dimensional model of coupled trihedra

We introduce a minimal model describing the physics of classical two-dimensional (2D) frustrated Heisenberg systems, where spins order in a nonplanar way at $T=0$. This model, consisting of coupled trihedra (or Ising-$\mathbb{R}{P}^{3}$ model), encompasses Ising (chiral) degrees of freedom, spin-wave excitations, and ${\mathbb{Z}}_{2}$ vortices. Extensive Monte Carlo simulations show that the $T=0$ chiral order disappears at finite temperature in a continuous phase transition in the 2D Ising universality class, despite misleading intermediate-size effects observed at the transition. The analysis of configurations reveals that short-range spin fluctuations and ${\mathbb{Z}}_{2}$ vortices proliferate near the chiral domain walls, explaining the strong renormalization of the transition temperature. Chiral domain walls can themselves carry an unlocalized ${\mathbb{Z}}_{2}$ topological charge, and vortices are then preferentially paired with charged walls. Further, we conjecture that the anomalous size effects suggest the proximity of the present model to a tricritical point. A body of results is presented, which all support this claim: (i) first-order transitions obtained by Monte Carlo simulations on several related models, (ii) approximate mapping between the Ising-$\mathbb{R}{P}^{3}$ model and a dilute Ising model (exhibiting a tricritical point), and, finally, (iii) mean-field results obtained for Ising-multispin Hamiltonians, derived from the high-temperature expansion for the vector spins of the Ising-$\mathbb{R}{P}^{3}$ model.

Large-scale simulation of the spin-lattice dynamics in ferromagnetic iron

combined application of the Langevin spin dynamics and the fluctuation-dissipation theorem. We investigate several applications of the method, performing microcanonical ensemble simulations of adiabatic spin-lattice relaxation of periodic arrays of 180° domain walls, and isothermal-isobaric ensemble dynamical simulations of thermally equilibrated homogeneous systems at various temperatures. The predicted isothermal magnetization curve agrees well with the experimental data for a broad range of temperatures. The equilibrium as well as time-correlation functions of spin orientations exhibit the presence of short-range magnetic order above the Curie temperature. Furthermore, short-range order spin fluctuations are shown to contribute to the thermal expansion of the material. Our analysis illustrates the significant part played by the spin degrees of freedom in the dynamics of motion of atoms in magnetic iron and iron-based alloys. It also shows that the spin-lattice dynamics algorithm developed in this paper offers a viable way of performing large-scale dynamical atomistic simulations of magnetic materials.

Pseudogap behavior in Bi2Ca2SrCu2O8: Results of the generalized dynamical mean-field approach

Pseudogap phenomena are observed for the normal underdoped phase of different high-T c cuprates. Among others, the Bi2Sr2CaCu2O8 − δ (Bi2212) compound is one of the most studied experimentally. To describe the pseudogap regime in Bi2212, we use a novel generalized ab initio LDA + DMFT + Σk hybrid scheme. This scheme is based on the strategy of one of the most powerful computational tools for real correlated materials: the local density approximation (LDA) + dynamical mean-field theory (DMFT). Conventional LDA + DMFT equations are here supplied with an additional (momentum-dependent) self-energy Σk in the spirit of our recently proposed DMFT + Σk approach taking into account pseudogap fluctuations. In the present model, Σk describes nonlocal correlations induced by short-range collective Heisenberg-like antiferromagnetic spin fluctuations. The effective single-impurity problem of the DMFT is solved by the numerical renormalization group (NRG) method. Material-specific model parameters for the effective x 2 − y 2 orbital of Cu-3d shell of the Bi2212 compound, e.g., the values of intra-and interlayer hopping integrals between different Cu sites, the local Coulomb interaction U, and the pseudogap potential Δ were obtained within the LDA and LDA + DMFT schemes. Here, we report on the theoretical LDA + DMFT + Σk quasiparticle band dispersion and damping, Fermi surface renormalization, momentum anisotropy of (quasi)static scattering, densities of states, spectral densities, and angular-resolved photoemission (ARPES) spectra, taking into account pseudogap and bilayer splitting effects for normal (slightly) underdoped Bi2212 (δ = 0.15). We show that LDA + DMFT + Σk successfully describes strong (pseudogap) scattering close to Brillouin zone boundaries. Our calculated LDA + DMFT + Σk Fermi surfaces and ARPES spectra in the presence of pseudogap fluctuations are almost insensitive to the bilayer splitting strength. However, our LDA-calculated value of bilayer splitting is rather small to describe the experimentally observed peak-dip-hump structure. The results obtained are in good semiquantitative agreement with various recent ARPES experiments.

Irreversibility of the magnetic state ofTm1−xTbxCo2revealed by specific heat, electrical resistivity, and neutron diffraction measurements

The substitution of Tb for Tm in the Laves phase compound $\mathrm{Tm}{\mathrm{Co}}_{2}$ leads to appearance of a magnetic moment on the Co atoms through the metamagnetic transition in the itinerant $d$-electron subsystem and gives rise to long-range ferrimagnetic order in ${\mathrm{Tm}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}{\mathrm{Co}}_{2}$ at $x\ensuremath{\geqslant}0.15$. The magnetic state of the compound ${\mathrm{Tm}}_{0.9}{\mathrm{Tb}}_{0.1}{\mathrm{Co}}_{2}$, i.e., just below the critical Tb concentration, is characterized by the presence of large regions with short-range magnetic order and localized spin fluctuations (LSFs) induced in the Co $3d$-electron subsystem by the fluctuating $f\text{\ensuremath{-}}d$ exchange due to the Tm-Tb substitution. The peculiar magnetic state of this compound is strongly influenced by an external magnetic field which produces a first-order magnetic phase transition to a long-range ferrimagnetic state with the magnetic moment on the Co atoms up to $(0.7--0.8){\ensuremath{\mu}}_{B}$. This field-induced transition in ${\mathrm{Tm}}_{0.9}{\mathrm{Tb}}_{0.1}{\mathrm{Co}}_{2}$ is found to be irreversible. It is accompanied by a giant and irreversible reduction of the electrical resistivity $(\ensuremath{\Delta}\ensuremath{\rho}∕\ensuremath{\rho}\ensuremath{\sim}\ensuremath{-}45%)$, specific heat (by about 3.7 times at $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$), and intensity of magnetic neutron scattering. Such behavior is associated with the field-induced metamagnetic transition in the itinerant $d$-electron subsystem mediated by the $f\text{\ensuremath{-}}d$ exchange. Significantly enhanced values of the residual resistivity and the coefficient $\ensuremath{\gamma}$ of the $T$-linear contribution to the specific heat in the compound with $x=0.1$ as well as their unusual behavior with temperature and under application of the magnetic field is ascribed to the presence of LSF.

Quantized massive collective excitations, short-range spin fluctuations and high-Tcsuperconductivity

We have succeeded in quantizing massive collective gauge fields around the doped hole by using the theoretical formula, which is based on the gauge-invariant effective Lagrangian density, in underdoped cuprates. Quantized massive gauge fields, which are induced around the doped hole, explain naturally the broad spectra of angle-resolved photoemission around the hot spot, short-range spin fluctuations and anomalous transport properties in underdoped cuprates.

Model for Unconventional Superconductivity ofSr2RuO4: Effect of Impurity Scattering on Time-Reversal Breaking Triplet Pairing with a Tiny Gap

A model for unconventional superconductivity of ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ is presented to resolve its puzzle. It is shown that the short-range ferromagnetic spin fluctuations give rise to the triplet pairing with $p$-like symmetry which is breaking the time-reversal symmetry and has a tiny gap due to the salient shape of the Fermi surface characteristic to ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$. The effect of nonmagnetic-impurity scattering in the unitarity limit is shown to fill up easily the tiny gap giving rise to an appreciable residual density of states, which explains consistently the puzzling properties observed so far.

Pressure dependence of the SDW transition in (MDT-TTF) 2Au(CN) 2

The pressure dependence of the SDW transition in the title material has been investigated by the measurements of 1H-NMR relaxation rate, T 1 −1. In the metallic state, 1H- T 1 −1 follows a Korringa relation. The pressure dependence of T 1 −1 is large compared with other organic conductors. The large peak of 1H- T 1 −1 observed in the SDW state disappears at 5kbar, in agreement with the resistivity measurement: The SDW order is completely suppressed at this pressure. There is still observed an appreciable enhancement of ( T 1 T) −1 below 8K, which should be attributed to the short-range spin fluctuations near the SDW ordering. The temperature dependence of T 1 −1 and NMR linewidth was quite unusual for the standard SDW state, possible reasons of which are discussed.

Raman scattering from spin fluctuations in Pr2-xCexCuO4-y, Nd2-xCexCuO4-y, and Sm2-xCexCuO4-y.

Antiferromagnetic excitations in single crystals of Pr2−xCexCuO4−y, Nd2xCexCuO4−y and Sm2−xCexCuO4−y were investigated as a function of Ce concentration using Raman scattering at room temperature. The introduction of electrons by Ce doping significantly broadens the two-magnon peak in the B1g mode, indicating that zone-boundary magnons become damped with increasing Ce concentration. The existence of broad spin-pair-excitation spectra in the B1g and B2g modes at x=0.15 suggests that short-range spin fluctuations persist in these electron-doped superconductors. The result for the spin-pair-excitation peak in Pr2−xCexCuO4−y up to x=0.2 demonstrates the absence of softening of the short-wavelength magnons.