Fraction Of Spins Research Articles

Emerging Anomalous Hall Effect in (111)‐Oriented Artificial Iridate Honeycomb Lattices

AbstractComplex Ir oxides provide a promising platform for investigating cooperativity and competition between Mott physics and spin‐orbit coupling in condensed matter physics. These materials have the potential to reveal intriguing phases, such as topological phases and the Kitaev spin liquid state, an exactly solvable S = 1/2 spin model describing Ir4+ ions on a 2D honeycomb lattice. However, experimental confirmation of such phases in iridate films has been hindered by the difficulty of preparing (111)‐oriented samples of the perovskite phase. Herein, by constructing SrIrO3/SrTiO3 superlattices on SrTiO3 (111) substrates, (111)‐oriented perovskite phase SrIrO3 films are achieved with the unprecedentedly large thickness of 6 unit cells. Electrical and magnetic characterizations reveal that these films exhibit anomalous Hall insulating behavior, significant charge fluctuations, and long‐range antiferromagnetic order. Moreover, these properties can be tuned by varying the thickness of the SrIrO₃ layer. These emergent phenomena underscore the complex interactions among spin‐orbit coupling, electron–electron correlations, and crystal structure in (111)‐oriented artificial iridate honeycomb lattices. Further, spin fractionalization and topological quantum spin liquid may be achieved by tuning the spin–orbit coupling and crystal field intensity from interface engineering.

Magnetocaloric effect of topological excitations in Kitaev magnets

Traditional magnetic sub-Kelvin cooling relies on the nearly free local moments in hydrate paramagnetic salts, whose utility is hampered by the dilute magnetic ions and low thermal conductivity. Here we propose to use instead fractional excitations inherent to quantum spin liquids (QSLs) as an alternative, which are sensitive to external fields and can induce a very distinctive magnetocaloric effect. With state-of-the-art tensor-network approach, we compute low-temperature properties of Kitaev honeycomb model. For the ferromagnetic case, strong demagnetization cooling effect is observed due to the nearly free Z2 vortices via spin fractionalization, described by a paramagnetic equation of state with a renormalized Curie constant. For the antiferromagnetic Kitaev case, we uncover an intermediate-field gapless QSL phase with very large spin entropy, possibly due to the emergence of spinon Fermi surface and gauge field. Potential realization of topological excitation magnetocalorics in Kitaev materials is also discussed, which may offer a promising pathway to circumvent existing limitations in the paramagnetic hydrates.

Transition metal centers with anharmonic elastic interactions in one-dimensional spin-crossover solids

An exact examination of a one-dimensional (1D) anharmonic elastic model describing the thermodynamic properties of spin-crossover (SCO) solids is presented. This elastic model incorporates both intra-chain and mean field inter-chain interactions, providing a more understanding of 1D SCO materials. Numerical findings for a linear chain of spins unveil intriguing features, such as a two-step-like transition. Extending the model to interacting chains leads to an additional term in the effective ligand field of the system’s Hamiltonian, thereby enhancing its predictive power by accounting for more elastic constants of interactions. This augmentation reinforces the model’s validity in capturing the spin-crossover phenomenon. Numerical results in this extended framework elucidate clearer two-step-like spin crossover transitions, with the HS_LS fraction of spins approaching maximum values at equilibrium temperature Teq.

Probing spin fractionalization with electron spin resonance based on scanning tunneling microscopy

Probing spin fractionalization with electron spin resonance based on scanning tunneling microscopy

Matrix product study of spin fractionalization in the one-dimensional Kondo insulator

The Kondo lattice is one of the classic examples of strongly correlated electronic systems. We conduct a controlled study of the Kondo lattice in one dimension, highlighting the role of excitations created by the composite fermion operator. Using time-dependent matrix product state methods, we compute various correlation functions and contrast them with both large-N mean-field theory and the strong-coupling expansion. We show that the composite fermion operator creates long-lived, charge-e and spin-1/2 excitations, which cover the low-lying single-particle excitation spectrum of the system. Furthermore, spin excitations can be thought to be composed of such fractionalized quasiparticles with a residual interaction which tend to disappear at weak Kondo coupling. Published by the American Physical Society 2024

The role of anharmonicity in single-molecule spin-crossover

We exploit the system-bath paradigm to investigate vibrational-anharmonicity effects on spin-crossover in a single molecule. Focusing on weak coupling, we use the linear response approximation to deal with the nonlinear vibrational bath and propagate the Redfield master equation to obtain the equilibrium high spin fraction. We take both the anharmonicity in the bath potentials and the nonlinearity in the spin-vibration coupling into account and find a strong interplay between these two effects. Further, we show that the spin-crossover in a single molecule is always a gradual transition and the anharmonicity-induced phonon drag greatly affects the transition behavior.

Proximate spin liquid and fractionalization in the triangular antiferromagnet KYbSe2

Proximate spin liquid and fractionalization in the triangular antiferromagnet KYbSe2

Spin Polarization in Transport Studies of Chirality-Induced Spin Selectivity.

Chirality-induced spin selectivity (CISS) is a recently discovered effect in which structural chirality can result in different conductivities for electrons with opposite spins. In the CISS community, the degree of spin polarization is commonly used to describe the efficiency of the spin filtering/polarizing process, as it represents the fraction of spins aligned along the chiral axis of chiral materials originating from non-spin-polarized currents. However, the methods of defining, calculating, and analyzing spin polarization have been inconsistent across various studies, hindering advances in this field. In this Perspective, we connect the relevant background and the definition of spin polarization, discuss its calculation in different contexts in the CISS, and propose a practical and meaningful figure of merit by quantitative analysis of magnetoresistance in CISS transport studies.

Majorana quasiparticles emergent in Kitaev spin liquid

Abstract This paper reviews the nature of two quasiparticles, the Majorana fermion and vison, emergent in the Kitaev model, mainly from the theoretical point of view. We demonstrate how the fractionalization of a quantum spin into these two quasiparticles occurs and clarify their properties in the presence and absence of magnetic fields. In addition to discussions on the itinerant nature inherent to Majorana fermions, the quantum dynamics of visons induced by an applied magnetic field is revealed. Fractional quasiparticles manifest themselves in temperature evolutions of thermodynamic quantities, excitation spectra, and transport properties. The formulations for them and calculation results are shown in detail. Based on the results, we present how the signatures of fractional quasiparticles appear in physical quantities. The paper also mentions the recent developments of the experimental and theoretical works of Kitaev-related systems and presents outlooks of studies on these systems.

Remanence Increase in SrFe12O19/Fe Exchange-Decoupled Hard-Soft Composite Magnets Owing to Dipolar Interactions.

In the search for improved permanent magnets, fueled by the geostrategic and environmental issues associated with rare-earth-based magnets, magnetically hard (high anisotropy)-soft (high magnetization) composite magnets hold promise as alternative magnets that could replace modern permanent magnets, such as rare-earth-based and ceramic magnets, in certain applications. However, so far, the magnetic properties reported for hard-soft composites have been underwhelming. Here, an attempt to further understand the correlation between magnetic and microstructural properties in strontium ferrite-based composites, hard SrFe12O19 (SFO) ceramics with different contents of Fe particles as soft phase, both in powder and in dense injection molded magnets, is presented. In addition, the influence of soft phase particle dimension, in the nano- and micron-sized regimes, on these properties is studied. While Fe and SFO are not exchange-coupled in our magnets, a remanence that is higher than expected is measured. In fact, in composite injection molded anisotropic (magnetically oriented) magnets, remanence is improved by 2.4% with respect to a pure ferrite identical magnet. The analysis of the experimental results in combination with micromagnetic simulations allows us to establish that the type of interaction between hard and soft phases is of a dipolar nature, and is responsible for the alignment of a fraction of the soft spins with the magnetization of the hard. The mechanism unraveled in this work has implications for the development of novel hard-soft permanent magnets.

Double-replica theory for evolution of genotype-phenotype interrelationship

The relationship between genotype and phenotype plays a crucial role in determining the function and robustness of biological systems. Here the evolution progresses through the change in genotype, whereas the selection is based on the phenotype, and the genotype-phenotype relation also evolves. The theory for such phenotypic evolution remains poorly developed, in contrast to evolution under the fitness landscape determined by genotypes. Here we provide a statistical-physics formulation of this problem by introducing replicas for genotype and phenotype. We apply it to an evolution model in which phenotypes are given by spin configurations; genotypes are an interaction matrix for spins to give the Hamiltonian, and the fitness depends only on the configuration of a subset of spins called the target. We describe the interplay between the genetic variations and phenotypic variances by noise in this model by our approach that extends the replica theory for spin glasses to include a spin replica for phenotypes and a coupling replica for genotypes. Within this framework we obtain a phase diagram of the evolved phenotypes against the noise and selection pressure, where each phase is distinguished by the fitness and overlaps for genotypes and phenotypes. Among the phases, a robust fitted phase, relevant to biological evolution, is achieved under the intermediate level of noise (temperature), where robustnesses to noise and to genetic mutation are correlated, as a result of replica symmetry. We also find a tradeoff between maintaining a high fitness level of phenotype and acquiring a robust pattern of genes as well as the dependence of this tradeoff on the ratio between the size of the functional (target) part to that of the remaining nonfunctional (non-target) one. The selection pressure needed to achieve high fitness increases with the fraction of target spins.

Dynamical Mechanical Analysis and Micromechanics Simulations of Spin-Crossover@Polymer Particulate Composites: Toward Soft Actuator Devices

We investigated the interplay between the thermomechanical and spin-crossover (SCO) properties of a series of stimuli-responsive polymer composites consisting of [Fe(NH2trz)3]SO4 and [Fe(Htrz)(trz)2]BF4 particles (trz = 1,2,4-triazolato) embedded in thermoplastic polyurethane, TPU, and poly(vinylidene fluoride–trifluoroethylene), P(VDF-TrFE), matrices. The effective thermoelastic coefficients and transformation stress and strain in the composites were assessed utilizing dynamical mechanical analysis (DMA), differential scanning calorimetry (DSC), thermal expansion, and thermal stress measurements. Remarkably, the composites display a characteristic elastic softening and increased mechanical damping around the spin transition temperature, which arise from the significant spin state–volume strain coupling in the particles and scale semiquantitatively with the pressure derivative of the low spin fraction (∂n∂P)T. Crucially, for a given particle volume fraction, the transformation strain (respectively stress) substantially increases (respectively decreases) in soft matrices, which was rationalized by micromechanical simulations. The results provide a fundamental understanding and a quantitative guideline for the design of SCO@polymer composites for applications in mechanical transducers.

Fluctuating fractionalized spins in quasi-two-dimensional magnetic V0.85PS3

Quantum spin liquid (QSL), a state characterized by exotic low energy fractionalized excitations and statistics, is still elusive experimentally and may be gauged via indirect experimental signatures. A remnant of the QSL phase may reflect in the spin dynamics as well as quanta of lattice vibrations, i.e., phonons, via the strong coupling of phonons with underlying fractionalized excitations. Inelastic light scattering (Raman) studies on ${\mathrm{V}}_{1--x}\mathrm{P}{\mathrm{S}}_{3}$ single crystals evidence the spin fractionalization into Majorana fermions deep into the paramagnetic phase reflected in the emergence of a low frequency quasielastic response, along with a broad magnetic continuum marked by a crossover temperature ${T}^{*}\ensuremath{\sim}200$ K from a pure paramagnetic state to a fractionalized spin regime qualitatively gauged via dynamic Raman susceptibility. We found further evidence of anomalies in the phonons' self-energy parameters, in particular, phonon line broadening and line asymmetry evolution at this crossover temperature, attributed to the decaying of phonons into itinerant Majorana fermions. This anomalous scattering response is thus indicative of fluctuating fractionalized spins suggesting a phase proximate to the quantum spin liquid state in this quasi-two-dimensional magnetic system.

Quark orbital angular momentum of ground-state octet baryons

Here we study the quark orbital angular momentum of the ground octet baryons employing an extended chiral constituent quark model, within which the baryon wave functions are taken to be superposition of the traditional $qqq$ and the $qqqq\overline{q}$ higher Fock components. Coupling between the two configurations is estimated using the $^{3}P_{0}$ quark-antiquark creation mechanism, and the corresponding coupling strength is determined by fitting the sea flavor asymmetry of the nucleon. The obtained numerical results show that the quark angular momentum of the nucleon, $\mathrm{\ensuremath{\Sigma}}, \mathrm{\ensuremath{\Lambda}}$, and $\mathrm{\ensuremath{\Xi}}$ hyperons, are in the range 0.10--0.30. In addition, the quark angular momentum of all the hyperons are a little bit smaller than that of the nucleon. And the octet baryons spin fractions taken by the intrinsic quark orbital angular momentum could be up to $60%$ in present model.

Probing details of spin-orbit coupling through Pauli spin blockade

Spin-orbit interaction (SOI) plays a fundamental role in many low-dimensional semiconductor and hybrid quantum devices. In the rapidly evolving field of semiconductor spin qubits, SOI is an essential ingredient that can allow for ultrafast qubit control. The exact manifestation of SOI in a given device is, however, often both hard to predict theoretically and probe experimentally. Here, we develop a detailed theoretical connection between the leakage current through a double quantum dot in Pauli spin blockade and the underlying SOI in the system. We present a general analytic expression for the leakage current, which allows to connect experimentally observable features to both the magnitude and orientation of an effective spin-orbit field acting on the moving carriers. Motivated by the large recent interest in hole-based quantum devices, we further zoom in on the case of Pauli blockade of hole spins, assuming a strong transverse confinement potential. In this limit we also find an analytic expression for the current at low external magnetic field, that includes the effect of hyperfine coupling of the hole spins to randomly fluctuating nuclear spin baths. This result can be used to extract information about both hyperfine and spin-orbit coupling parameters for hole spins in devices with a significant fraction of nonzero nuclear spins.

Phase diagrams of Kitaev models for arbitrary magnetic field orientations

The Kitaev model is an exactly solvable quantum spin model within the language of constrained real fermions. In spite of numerous studies for magnetic fields along special orientations, there is a limited amount of knowledge on the complete field-angle characterization, which can provide valuable information on the existence of fractionalized excitations. For this purpose, we first study the pure ferromagnetic and antiferromagnetic Kitaev models for arbitrary external magnetic field directions via a mean-field theory, showing that there are many topological phases with different (or zero) Chern numbers, depending on the magnetic field strength and orientations. However, a realistic description of the candidate Kitaev materials, within the edge-sharing octahedra paradigm, requires additional coupling terms, including a large off-diagonal term $\mathrm{\ensuremath{\Gamma}}$ along with possible anisotropic corrections ${\mathrm{\ensuremath{\Gamma}}}_{p}$. It is therefore not sufficient to rely on the topological properties of the bare Kitaev model as the basis for the observed thermal Hall-conductivity signals, and an understanding of these extended Kitaev models with a complete field response is demanded. Starting from the zero-field phase diagram of $\mathrm{K}\text{\ensuremath{-}}\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}{\mathrm{\ensuremath{\Gamma}}}_{p}$ models, we identify, besides the Kitaev spin liquid phase, antiferromagnetic zigzag, ferromagnetic phases, as well as an unusual Kitaev(-$\mathrm{\ensuremath{\Gamma}}$) spin liquid phase. The magnetic field response of these phases for arbitrary field orientations provides a remarkably rich phase diagram. For an extended parameter range and just above the critical field where the zigzag phase is suppressed, there is an intermediate phase region with suppressed energy gaps and substantial spin fractionalization. To comply our findings with experiments, we also reproduce a large asymmetry in the extent of this intermediate phases specifically for the two different field directions $\ensuremath{\theta}=\ifmmode\pm\else\textpm\fi{}{60}^{o}$ with respect to the normal to the plane of the honeycomb lattice.

Accurate determination of the electron spin polarization in magnetized iron and nickel foils for Møller polarimetry

The Møller polarimeter in Hall A at Jefferson Lab in Newport News, VA, has provided reliable measurements of electron beam polarization for the past two decades. Past experiments have typically required polarimetry at the 1% level of absolute uncertainty which the Møller polarimeter has delivered. However, the upcoming proposed experimental program including MOLLER and SoLID have stringent requirements on beam polarimetry precision at the level of 0.4% (The MOLLER Collaboration, 2014; The SoLID collaboration, 2019), requiring a systematic re-examination of all the contributing uncertainties.Møller polarimetry uses the double polarized scattering asymmetry of a polarized electron beam on a target with polarized atomic electrons. The target is a ferromagnetic material magnetized to align the spins in a given direction. In Hall A, the target is a pure iron foil aligned perpendicular to the beam and magnetized out of plane parallel or antiparallel to the beam direction. The acceptance of the detector is engineered to collect scattered electrons close to 90° in the center of mass frame where the analyzing power is a maximum (-7/9).One of the leading systematic errors comes from determination of the target foil polarization. Polarization of a magnetically saturated target foil requires knowledge of both the saturation magnetization and g′, the electron g-factor which includes components from both spin and orbital angular momentum from which the spin fraction of magnetization is determined. Target foil polarization has been previously addressed in a 1997 publication “A precise target for Møller polarimetry” by de Bever et al. (1997) at a level of precision sufficient for experiments up to this point. Several shortcomings with the previous published value require revisiting the result prior to MOLLER. This paper utilizes the existing world data to provide a best estimate for target polarization for both nickel and iron foils including uncertainties in magnetization, high-field and temperature dependence, and fractional contribution to magnetization from orbital effects. We determine the foil electron spin polarization at 294 K to be 0.08020 ± 0.00018 (@4 T applied field) for iron and 0.018845±0.000053 (@2 T applied field) for nickel. We conclude with a brief discussion of additional systematic uncertainties to Møller polarimetry using this technique.

Proton momentum and angular momentum decompositions with overlap fermions

We present a calculation of the proton momentum and angular momentum decompositions using overlap fermions on a $2+1$-flavor RBC/UKQCD domain-wall lattice at 0.143 fm with a pion mass of 171 MeV which is close to the physical one. A complete determination of the momentum and angular momentum fractions carried by up, down, strange, and glue inside the proton has been done with valence pion masses varying from 171 to 391 MeV. We have utilized fast Fourier transform on the stochastic-sandwich method for connected-insertion parts and the cluster-decomposition error reduction technique technique for disconnected-insertion parts has been used to reduce statistical errors. The full nonperturbative renormalization and mixing between the quark and glue operators are carried out. The final results are normalized with the momentum and angular momentum sum rules and reported at the physical valence pion mass at $\overline{\mathrm{MS}}(\ensuremath{\mu}=2\text{ }\text{ }\mathrm{GeV})$. The renormalized momentum fractions for the quarks and glue are $⟨x{⟩}^{q}=0.491(20)(23)$ and $⟨x{⟩}^{g}=0.509(20)(23)$, respectively, and the renormalized total angular momentum fractions for quarks and glue are $2{J}^{q}=0.539(22)(44)$ and $2{J}^{g}=0.461(22)(44)$, respectively. The quark spin fraction is $\mathrm{\ensuremath{\Sigma}}=0.405(25)(37)$ from our previous work and the quark orbital angular momentum fraction is deduced from $2{L}^{q}=2{J}^{q}\ensuremath{-}\mathrm{\ensuremath{\Sigma}}$ to be 0.134(22)(44).

Unconventional dual 1D\u20132D quantum spin liquid revealed by ab initio studies on organic solids family

Organic solids host various electronic phases. Especially, a milestone compound of organic solid, beta ^{prime}-X[Pd(dmit)2]2 with X=EtMe3Sb shows quantum spin-liquid (QSL) properties suggesting a novel state of matter. However, the nature of the QSL has been largely unknown. Here, we computationally study five compounds comprehensively with different X using 2D ab initio Hamiltonians and correctly reproduce the experimental phase diagram with antiferromagnetic order for X=Me4P, Me4As, Me4Sb, Et2Me2As and a QSL for X=EtMe3Sb without adjustable parameters. We find that the QSL for X=EtMe3Sb exhibits 1D nature characterized by algebraic decay of spin correlation along one direction, while exponential decay in the other direction, indicating dimensional reduction from 2D to 1D. The 1D nature indeed accounts for the experimental specific heat, thermal conductivity and magnetic susceptibility. The identified QSL, however, preserves 2D nature as well consistently with spin fractionalization into spinon with Dirac-like gapless excitations and reveals duality bridging the 1D and 2D QSLs.

Probing signatures of fractionalization in the candidate quantum spin liquid Cu2IrO3 via anomalous Raman scattering

Long-range entanglement in quantum spin liquids (QSLs) leads to novel low-energy excitations with fractionalized quantum numbers and (in two dimensions) statistics. Experimental detection and manipulation of these excitations present a challenge particularly in view of diverse candidate magnets. A promising probe of fractionalization is their coupling to phonons. Here, we present Raman scattering results for the $S=1/2$ honeycomb iridate ${\mathrm{Cu}}_{2}{\mathrm{IrO}}_{3}$, a candidate Kitaev QSL with fractionalized Majorana fermions and Ising flux excitations. We observe anomalous low-temperature frequency shift and linewidth broadening of the Raman intensities in addition to a broad magnetic continuum, both of which, as we derive, are naturally attributed to the phonon decaying into itinerant Majoranas. The dynamic Raman susceptibility marks a crossover from the QSL to a thermal paramagnet at $\ensuremath{\sim}120$ K. The phonon anomalies below this temperature demonstrate a strong phonon-Majorana coupling. These results provide evidence of spin fractionalization in ${\mathrm{Cu}}_{2}{\mathrm{IrO}}_{3}$.