Short-range Spin Research Articles

Free Energy Difference Fluctuations in Short-Range Spin Glasses

Free Energy Difference Fluctuations in Short-Range Spin Glasses

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.

Syntheses, structures, and magnetic properties of new [Co9] cluster

A new spin-cluster compound K6BaCo9(PO4H)3(PO4)6Cl2 was synthesized by the traditional solid-phase method. The fundamental magnetic unit of the compound is a coupled [Co9] cluster, which is composed of three non-coplanar triangles. These [Co9] clusters are interconnected by phosphate groups, constructing a two-dimensional framework in the ab plane. The crystal shows the clear easy-axis anisotropy. No long-range magnetic ordering was observed down to 2 K, but there is short-range spin correlation below T = 2.45 K. The Curie-Weiss temperature is about θCW = −19.38 K for the powder sample, which indicates that the dominant exchange interaction is antiferromagnetic with strong frustration (f = |θCW|/T ≈ 10). The magnetization curve presents a likely magnetic field induced transition at μ0Hc = 1.85 T.

Dynamic and magnetic properties of excited magnons in tetragonal fcc [formula omitted]/SiO multilayer alloys

In this paper, we investigate by numerical lattice simulations the magnetic and dynamic properties of excited magnons in d transition metal alloys FexPt1−x/SiO using Green functions and second quantification techniques. The sandwich structure consists of magnetic (FePt) and non-magnetic (SiO) layers. The corresponding quantum Heisenberg Hamiltonian includes exchange interactions, magnetocrystalline anisotropy, surface anisotropy and magnetic field. The existence of the surface magnons is demonstrated by the surface creation factor η, which is in good agreement with the measured lifetime data. Surface magnons are characterized by their excitation spectrum, susceptibility of creation, creation gap, permitted band for creation, and lifetime, which are different from volume magnons properties. The susceptibility of creation analysis is compatible with experimental values, and the calculated lifetimes are in the order of femtoseconds (fs) reproducing the analytical behavior of the measured values along the lattice symmetry axis. The surface magnons are found to be more confined in time and space than those excited in volume, indicating the presence of short-range spin correlations. Magnetization per spin versus temperature is also calculated for different iron concentrations x and found to be in excellent agreement with the measured values. Also, the response of the system to an external magnetic field is shown for xFe=0.29 with various temperature values, and it agrees well with the experience.

Changing the order of a dynamical phase transition through fluctuations in a quantum p-spin model

We study the non-equilibrium phase diagram of a fully-connected Ising p-spin model, for generic p > 2, and investigate its robustness with respect to the inclusion of spin-wave fluctuations, resulting from a ferromagnetic, short-range spin interaction. In particular, we investigate the dynamics of the mean-field model after a quantum quench: we observe a new dynamical phase transition which is either first or second order depending on the even or odd parity of p, in stark contrast with its thermal counterpart which is first order for all p. The dynamical phase diagram is qualitatively modified by the fluctuations introduced by a short-range interaction which drive the system always towards various prethermal paramagnetic phases determined by the strength of time dependent fluctuations of the magnetization.

Overlap structure and free energy fluctuations in short-range spin glasses

We investigate scenarios in which the low-temperature phase of short-range spin glasses comprises thermodynamic states which are nontrivial mixtures of multiple incongruent pure state pairs. We construct a new kind of metastate supported on Gibbs states whose edge overlap values with a reference state fall within a specified range. Using this metastate we show that, in any dimension, the variance of free energy difference fluctuations between pure states within a single mixed Gibbs state with multiple edge overlap values diverges linearly with the volume. We discuss some implications of these results.

Griffiths-Type Theorems for Short-Range Spin Glass Models

We establish relations between different characterizations of order in spin glass models. We first prove that the broadening of the replica overlap distribution indicated by a nonzero standard deviation of the replica overlap R1,2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$R^{1,2}$$\\end{document} implies the non-differentiability of the two-replica free energy with respect to the replica coupling parameter λ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\lambda $$\\end{document}. In Z2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\mathbb {Z}}_2$$\\end{document} invariant models such as the standard Edwards–Anderson model, the non-differentiability is equivalent to the spin glass order characterized by a nonzero Edwards–Anderson order parameter. This generalization of Griffiths’ theorem is proved for any short-range spin glass models with classical bounded spins. We also prove that the non-differentiability of the two-replica free energy mentioned above implies replica symmetry breaking in the literal sense, i.e., a spontaneous breakdown of the permutation symmetry in the model with three replicas. This is a general result that applies to a large class of random spin models, including long-range models such as the Sherrington-Kirkpatrick model and the random energy model. There is a 25-minute video that explains the main results of the present work:https://youtu.be/BF3hJiY1xvI

Magnetic properties of S=12 distorted J1−J2 honeycomb lattice compound NaCuIn(PO4)2

The subtle interplay between low-dimensionality and spin correlations can lead to exotic ground states with unconventional excitations in two-dimensional honeycomb-lattice-based quantum magnets. Herein, we present the structural, magnetic, and heat capacity measurements; density functional theory $+$ Hubbard $U$ ($\mathrm{DFT}+U$) based electronic structure calculations; and quantum Monte Carlo simulations for $\mathrm{NaCuIn}{({\mathrm{PO}}_{4})}_{2}$. The structure of $\mathrm{NaCuIn}{({\mathrm{PO}}_{4})}_{2}$ consists of a well-separated, $\mathit{S}=\frac{1}{2}$ distorted ${\mathit{J}}_{1}\text{\ensuremath{-}}{\mathit{J}}_{2}$ honeycomb layer which is a combination of the magnetic couplings ${\mathit{J}}_{1}$ (forming spin dimers) and ${\mathit{J}}_{2}$ (constituting spin chains). At high temperatures, the magnetic susceptibility $\ensuremath{\chi}(T)$ follows paramagnetic behavior with a Curie-Weiss temperature ${\ensuremath{\theta}}_{\mathrm{CW}}\ensuremath{\approx}\ensuremath{-}16$ K, implying the presence of antiferromagnetic interactions. A broad maximum is observed at about $13\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in $\ensuremath{\chi}(T)$, indicating the presence of short-range spin correlations. The quantum Monte Carlo simulations using the $\mathit{S}=\frac{1}{2}\phantom{\rule{4pt}{0ex}}{\mathit{J}}_{1}\text{\ensuremath{-}}{\mathit{J}}_{2}$ Heisenberg model on a distorted honeycomb lattice are in good agreement with the measured magnetic susceptibility data. The obtained ratio of the exchange couplings ($\frac{{\mathit{J}}_{2}}{{\mathit{J}}_{1}}$) is 2.63, which is consistent with the value obtained from our $\mathrm{DFT}+U$ calculations. The title material undergoes a magnetic long-range order at 0.4 K in the heat capacity, which is suppressed with an applied magnetic field of 10 kOe. The magnetic heat capacity data follow a linear temperature-dependent behavior well above the transition temperature, suggesting the presence of gapless excitations. The observed behavior can be attributed to the presence of low connectivity and weak magnetic frustration in this two-dimensional distorted honeycomb lattice.

Superconductivity in doped triangular Mott insulators: The roles of parent spin backgrounds and charge kinetic energy

We study the prerequisites for realizing superconductivity in doped triangular-lattice Mott insulators by considering three distinct parent spin backgrounds, i.e., ${120}^{\ensuremath{\circ}}$ antiferromagnets, quantum spin liquid, and stripy antiferromagnets, and all possible sign combinations $({\ensuremath{\tau}}_{1},{\ensuremath{\tau}}_{2})$ of nearest-neighbor hopping and next-nearest-neighbor hopping $({t}_{1},{t}_{2})$. Based on density matrix renormalization group calculations, we find that, with finite ${t}_{2}$ and specific sign combinations $({\ensuremath{\tau}}_{1},{\ensuremath{\tau}}_{2})$, the quasi-long-range superconductivity order can always be achieved, regardless of the nature of the parent spin backgrounds. Besides specific hopping signs $({\ensuremath{\tau}}_{1},{\ensuremath{\tau}}_{2})$, these superconductivity phases in triangular lattices are commonly characterized by short-ranged spin correlations and two charges per stripe. In the robust superconductivity phase realized at larger ${t}_{2}/{t}_{1}$, flipping the signs ${\ensuremath{\tau}}_{2}$ and ${\ensuremath{\tau}}_{1}$ gives rise to the stripe phase without strong pairing and a pseudogaplike phase without Cooper-pair phase coherence, respectively. Interestingly, the roles of the two hopping signs are switched at smaller ${t}_{2}/{t}_{1}$. Moreover, different sign combinations $({\ensuremath{\tau}}_{1},{\ensuremath{\tau}}_{2})$ would stabilize distinct phases including superconductivity, charge density waves, spin density waves, and pseudogaplike phases accordingly. Our findings suggest the important role of charge kinetic energy in realizing superconductivity in doped triangular-lattice Mott insulators.

Crystal structure and physical properties of layered Na[formula omitted]Fe[formula omitted]S[formula omitted

Layered iron oxysulfide Na2Fe2S2O serves as a cathode material for Na-ion batteries as well as a possible parent compound of novel superconductors. Here we report the crystal structure and physical properties on the basis of our successful synthesis of the high-purity sample. Na2Fe2S2O crystallizes in an anti-K2NiF4-type structure, which gives rise to the characteristic [Fe2S2O]2− block layers. The physical property measurements indicate that it is an antiferromagnetic insulator with a Néel temperature of TN=91.5 K and an energy gap no less than 0.43 eV. Evidence of two-dimensional short-range spin correlations above TN is given by the magnetic and specific-heat measurements. Our structural correlation analysis suggests that the Fe–S bond distance dominantly determines TN.

A comparison of modified extremal optimization algorithm vs quenching for electron glass system optimization

Coulomb glass or electron glass refers to a system of localized electrons interacting via unscreened Coulomb interactions. The electrons are localized on a square lattice site, and the site's energies are random. This model was mainly used to represent amorphous and compensated semiconductors at high disorders and low temperatures. Recently, the model has been used to model granular systems, an array of nanocrystals, and graphene. In this work, we have obtained the ground state of an electron glass model in two dimensions using a modified extremal optimization. The work is done for disorders less than the critical disorder and at half-filling. For this case, the ground state has anti-ferromagnetic ordering, unlike the high disorder case. This allows us to check the efficiency of our new algorithm.The results are compared with quenching using Monte Carlo simulation. In the electron glass model, the number of electrons is conserved, and the interaction is long-ranged. Extremal optimization was developed to obtain the ground state of short-ranged spin systems with a non-conserved magnetization. The algorithm was extended to find the ground state of a long ranged anti-ferromagnet with conserved magnetization. We show that the efficiency of extremal optimization is comparable to the quenching method. The efficiency for both ways decreases as the disorder is increased. The minimum energy states obtained consist of mainly two large domains when the ground state was not achieved. These meta-stable states are stable, and increasing the run time increases the efficiency only by a small amount. The stability of the domains is analyzed using an activity graph. A comparison of the activity graphs for the same disorder configuration for the two algorithms shows that the formation of a meta-stable state is a general feature independent of the algorithm used. The formation of domain states is more prevalent for specific disorder configurations. These results can be explained in terms of domain formation theory.

Ferromagnetic single-atom spin catalyst for boosting water splitting.

Heterogeneous single-atom spin catalysts combined with magnetic fields provide a powerful means for accelerating chemical reactions with enhanced metal utilization and reaction efficiency. However, designing these catalysts remains challenging due to the need for a high density of atomically dispersed active sites with a short-range quantum spin exchange interaction and long-range ferromagnetic ordering. Here, we devised a scalable hydrothermal approach involving an operando acidic environment for synthesizing various single-atom spin catalysts with widely tunable substitutional magnetic atoms (M1) in a MoS2 host. Among all the M1/MoS2 species, Ni1/MoS2 adopts a distorted tetragonal structure that prompts both ferromagnetic coupling to nearby S atoms as well as adjacent Ni1 sites, resulting in global room-temperature ferromagnetism. Such coupling benefits spin-selective charge transfer in oxygen evolution reactions to produce triplet O2. Furthermore, a mild magnetic field of ~0.5 T enhances the oxygen evolution reaction magnetocurrent by ~2,880% over Ni1/MoS2, leading to excellent activity and stability in both seawater and pure water splitting cells. As supported by operando characterizations and theoretical calculations, a great magnetic-field-enhanced oxygen evolution reaction performance over Ni1/MoS2 is attributed to a field-induced spin alignment and spin density optimization over S active sites arising from field-regulated S(p)-Ni(d) hybridization, which in turn optimizes the adsorption energies for radical intermediates to reduce overall reaction barriers.

Unusual dynamics of spin-[formula omitted] antiferromagnets on the triangular lattice in magnetic field

We theoretically discuss dynamical properties of spin-12 Heisenberg antiferromagnet on the triangular lattice in magnetic field H. We use the recently proposed bond-operator theory which operates with quantum states of the whole magnetic unit cell containing three spins. This technique describes accurately short-range spin correlations and provides a quantitative description of elementary excitations which appear in other approaches as bound states of conventional low-energy quasiparticles (e.g., magnons). In quantitative agreement with previous numerical and analytical findings, we observe four phases with coplanar spin arrangements upon the field increasing: the three-sublattice Y-phase, the collinear “up-up-down” (UUD) state, the non-collinear V-phase, and the collinear fully polarized (FP) state. We demonstrate that apart from magnons (spin-1 quasiparticles) there are spin-0 elementary excitations in the UUD state one of which is long lived and its spectrum lies below magnon branches. This mode originates from a high-energy quasiparticle at H=0 and it produces anomalies only in the longitudinal spin correlator because longitudinal and transverse channels are separated in collinear states. All other spin-0 excitations have finite lifetime and produce visibly anomalies in the dynamical structure factor. In the V-phase, we obtain multiple short-wavelength spin excitations which have no counterparts in the semiclassical spin-wave theory. Besides, we demonstrate a highly nontrivial field evolution of quasiparticles spectra on the way from one collinear state (UUD) to another one (FP) via the non-collinear V-phase (in which the longitudinal and the transverse channels are mixed). In particular, some parts of the spin-0 branch in the UUD state become parts of the spin-1 (magnon) branch in the FP phase whereas some parts of one magnon branch turn into parts of spin-2 branch. Such evolution would be very difficult to find by any conventional analytical approach. Our results are in good agreement with neutron experimental data obtained recently in Ba3CoSb2O9, KYbSe2, and CsYbSe2.

Theoretical study on the magnetic properties of cathode materials in the lithium-ion battery.

The layered LiMO2 (M = Co, Ni, and Mn) materials are commonly used as the cathode materials in the lithium-ion battery due to the distinctive layer structure for lithium extraction and insertion. Although their electrochemical properties have been extensively studied, the structural and magnetic properties of LiNiO2 are still under considerable debate, and the magnetic properties of monoclinic LiMnO2 are seldom reported. In this work, a detailed study of LiNiO2, LiMnO2, and a half-doped material LiNi0.5Mn0.5O2 is performed via both first-principles calculations and Monte Carlo simulations based on the effective spin Hamiltonian model. Through considering different structures, it is verified that a structure with a zigzag-type pattern is the most stable one of LiNiO2. Moreover, in order to figure out the magnetic properties, the spin exchange interactions are calculated, and then magnetic ground states are predicted in these three systems. The results show that LiNiO2 forms a spiral order that is caused by the competition from both the short-range and long-range spin exchange interactions, whereas the magnetic ground state of LiMnO2 is collinearly antiferromagnetic due to its nearest and next-nearest neighbor antiferromagnetic spin exchange interactions. However, LiNi0.5Mn0.5O2 is collinearly ferrimagnetic because of the ferromagnetic nearest neighbor Ni-Ni and Mn-Mn exchange interactions. Our work demonstrates the competition between the different exchange interactions in these cathode materials, which may be relevant to the performance of the lithium-ion battery.

Proof of Single-Replica Equivalence in Short-Range Spin Glasses.

We consider short-range Ising spin glasses in equilibrium at infinite system size, and prove that, for fixed bond realization and a given Gibbs state drawn from a suitable metastate, each translation and locally invariant function (for example, self-overlaps) of a single pure state in the decomposition of the Gibbs state takes the same value for all the pure states in that Gibbs state. We describe several significant applications to spin glasses.

Short-range spin order in paramagnetic AgCrSe2

Muon spin rotation (μSR) experiments were performed to directly investigate the temperature dependence of the short-range order or correlations of chromium spins in the paramagnetic phase of AgCrSe2. The detailed investigation of the μSR spectra over a wide temperature range from low to high temperatures revealed that the short-range spin correlations developed below about 184 K. It also confirmed no short-range correlation at high temperatures, which had been controversial in previous quasi-elastic neutron scattering studies. In addition, the muon spin relaxation was qualitatively changed and showed large missing asymmetry below about 89 K, where the correlation clearly implied short-range spin order. There could be a preparative phenomenon of the phase transition or an anisotropic short-range spin correlation.

Magnetic short-range order in polycrystalline SrGd2O4 and SrNd2O4 studied by reverse Monte Carlo simulations and magnetic pair-distribution function analysis

We present a study combining total-scattering powder neutron diffraction, reverse Monte Carlo simulations, and magnetic pair-distribution function analysis to deduce both the static and dynamic short-range magnetic spin correlations in two compounds of the rare-earth strontium oxides: ${\mathrm{SrGd}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{SrNd}}_{2}{\mathrm{O}}_{4}$. Both compounds exhibit a distorted honeycomb lattice which forms a set of zigzag ladders along the crystallographic $c$ axis of space group $Pnam$. Each set consists of two one-dimensional chains separated by diagonal rungs forming triangles, thus inducing a large degree of geometrical frustration due to the antiferromagnetic exchange between the magnetic ions. Significant magnetic diffuse scattering was observed well above the respective N\'eel temperatures and analyzed with reverse Monte Carlo techniques in reciprocal space. As a complementary analysis in real space we have derived the magnetic pair-distribution function for both compounds as a function of temperature. Our results clearly indicate that the individual zigzag ladders begin ordering well above ${T}_{\mathrm{N}}$ due to the dominating nearest and next-nearest interactions, long before interladder spin correlations become significant only slightly above ${T}_{\mathrm{N}}$. Additionally, for ${\mathrm{SrNd}}_{2}{\mathrm{O}}_{4}$ we find some qualitative differences between the short-range spin correlations above ${T}_{\mathrm{N}}$ and those observed in the ordered state.

Spin–Spin Interaction in ε-VOPO4 through Doping Light Elements

Nonmagnetic transition-metal oxides are usually magnetized by doping magnetic metallic elements. Here, we propose an alternative route to achieving magnetization by doping nonmagnetic H and Li atoms. Using vanadium oxide (VOPO4) as an example, we find that the doping of H and Li can effectively localize the excess electrons on V-d orbitals. As such, nonmagnetic ε-VOPO4 is predicted to possess short-range spin–spin interaction. When the doped H2 molecules are bonded with the axial oxygen, we observe a notable charge transfer from oxygen to vanadium. The transferred electrons are localized on two adjacent VO6 octahedrons and induce the spin–spin interaction. We further show that H2 dopants in ε-VOPO4 tend to be dispersed with a homogeneous magnetization. Our results suggest that H2-doped ε-VOPO4 can be utilized in the field of spintronics.

Frustrated mixed-spin ladders: Evidence for a bond-order wave phase between rung-singlet and Haldane phases

In frustrated spin ladders the interplay of frustration and correlations leads to the familiar Haldane (H) and rung-singlet (RS) phases. The nature of the transition between these two phases is still under debate. In this paper we tackle this issue using tools of quantum information theory. We consider frustrated mixed-spin-(1, 1/2) ladders with antiferromagnetic leg, rung and diagonal couplings, and calculate various quantities, such as the entanglement entropy (EE), the Schmidt gap, and the level degeneracy of the entanglement spectrum (ES). We use two numerical techniques, the infinite time-evolving block decimation (iTEBD) and the density matrix renormalization group (DMRG). We demonstrate that there exists an intermediate phase in which the ES levels do not exhibit the characteristic degeneracies of the H and RS phases. To understand the underlying physics in this phase, we investigate short-range spin correlations along legs, rungs and diagonals and show that in this intermediate phase long-wavelength modulations occur, akin to bond order waves.

Synthesis, structure, and magnetism in the ferromagnet La3MnAs5 : Well-separated spin chains coupled via itinerant electrons

In this work, we systematically report the synthesis, structure, and magnetism of a compound of filled anti-${\mathrm{Mn}}_{3}{\mathrm{Si}}_{5}$ type ${\mathrm{La}}_{3}{\mathrm{MnAs}}_{5}$. It crystallizes in a hexagonal structure with the space group of $P{6}_{3}/mcm$ (193). The structure consists of face-sharing ${\mathrm{MnAs}}_{6}$ octahedral chains along the $c$ axis, which are well separated by a large distance of 8.$9913\phantom{\rule{0.28em}{0ex}}\AA{}$, demonstrating a strong one-dimensional (1D) structural character. Physical property measurements indicate that ${\mathrm{La}}_{3}{\mathrm{MnAs}}_{5}$ is a ferromagnetic metal with ${T}_{\mathrm{C}}\ensuremath{\sim}112\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Due to the short-range intrachain spin coupling, the susceptibility deviates from the Curie-Weiss behavior in a wide temperature window and the magnetic entropy corresponding to the ferromagnetic transition is significantly lower than that expected from the fully saturated state. The magnetic critical behavior studies show that ${\mathrm{La}}_{3}{\mathrm{MnAs}}_{5}$ can be described by the three-dimensional Heisenberg model. The orbital hybridization between the 1D ${\mathrm{MnAs}}_{6}$ chain and intermediate La atom near the Fermi level reveals that the itinerant electrons play a key role in transmitting spin interaction among the ${\mathrm{MnAs}}_{6}$ spin chains. Our results indicate that ${\mathrm{La}}_{3}{\mathrm{MnAs}}_{5}$ is a rare ferromagnetic metal with well-separated spin chains, which provides a good opportunity to study the mechanism of interchain spin coupling via itinerant electrons.