Short-range Antiferromagnetic Correlations Research Articles

Tuneable Short-Range Antiferromagnetic Correlation in Fe-Containing Entropy Stabilized Oxides.

To elucidate the impact of a high entropy elemental distribution of the lattice site on the magnetic properties in oxide compounds, a series of complex perovskites BaBO3 (B = Y, Fe, Ti, Zr, Hf, Nb, and Ta) with different Fe content ratios (0, 0.2, 0.3, and 0.4) have been synthesized and thoroughly characterized. In this complex oxide series, superconducting quantum interference device magnetometry reveals a gradual change of a well-defined magnetic phase transition and B-site magnetic moment, which correlates with the Fe content. More importantly, a comprehensive analysis of the sample with a 0.4-Fe content (40% on the B-site) including magnetization, heat capacity, neutron diffraction, and muon-spin rotation measurements suggests that in the low-temperature state, a short-range antiferromagnetic correlation may exist, which could result from the magnetic interaction of Fe ions and consequent redistribution of associated d-electrons.

Structural, optical and magnetic properties of ErxZn1-xO nanoparticles: The impact of the Er-content

In this study we report on both the successful synthesis of ErxZn1-xO nanoparticles (x = 0.000, 0.005, 0.010, 0.030, 0.050, 0.075, and 0.100) and characterization (structural, optical, and magnetic) of the as-produced nanomaterials. The nanoparticles were produced by the polymeric precursor method (Pechini’s method). X-ray diffraction and Raman spectroscopy showed formation of hexagonal wurtzite phase in all samples. Meanwhile, only samples with a high Er-content (x = 0.075 and x = 0.100) exhibited a minority (up to 7 % w/w) secondary crystalline phase (Er2O3). The Rietveld refinement method was employed to assess details of the structural parameters. It was demonstrated that the increase of the Er-content leads to systematic mean crystalline size reduction (from ∼80 to ∼13 nm) and band gap shrinking (from ∼3.17 to ∼3.00 eV). The UV-Vis spectra showed the presence of characteristic Er3+-ion absorption bands. Likewise, emission peaks associated with Er3+-ion 4S3/2 → 4I15/2 electronic transition were verified by room temperature photoluminescence (PL). Strong differences in the PL spectra profiles of the ErxZn1-xO nanoparticles with respect to the PL spectrum of a reference Er2O3 sample, suggested that the majority of Er3+-ions replace Zn2+-ions in the wurtzite hosting matrix. Magnetic measurements show a paramagnetic behavior in all doped samples. The χ×T and M×Hcurves were successfully fitted to Curie-Weiss law and Brillouin modified model, showing the existence of short-range antiferromagnetic correlation.

Instanton gas approach to the Hubbard model

In this paper, we consider a path integral formulation of the Hubbard model based on a Hubbard-Stratonovich transformation that couples the auxiliary field to the local electronic density. This decoupling is known to have a saddle-point structure that shows a remarkable regularity: The field configuration at each saddle point can be understood in terms of a set of elementary field configurations localized in space and imaginary time which we coin instantons. The interaction between instantons is short ranged. Here, we formulate a classical partition function for the instanton gas that has predictive power. For a given set of physical parameters, we can predict the distribution of instantons and show that the instanton number is sharply defined in the thermodynamic limit, thereby defining a unique dominant saddle point. Decoupling in the charge channel conserves SU(2) spin symmetry for each field configurations. Hence, the instanton approach provides an SU(2) spin-symmetric approximation to the Hubbard model. It fails, however, to capture the magnetic transition inherent to the Hubbard model on the honeycomb lattice despite being able to describe local moment formation. In fact, the instanton itself corresponds to local moment formation and concomitant short-ranged antiferromagnetic correlations. This aspect is also seen in the single particle spectral function that shows clear signs of the upper and lower Hubbard bands. Our instanton approach bears remarkable similarities to local dynamical approaches, such as dynamical mean-field theory, in the sense that it has the unique property of allowing for local moment formation without breaking the SU(2) spin symmetry. In contrast to local approaches, it captures short-ranged magnetic fluctuations. Furthermore, it also offers possibilities for systematic improvements by taking into account fluctuations around the dominant saddle point. Finally, we show that the saddle point structure depends upon the choice of lattice geometry. For the square lattice at half filling, the saddle-point structure reflects the itinerant to localized nature of the magnetism as a function of the coupling strength. The implications of our results for Lefschetz thimble approaches to alleviate the sign problem are also discussed.

Temperature Dependence of Spin and Charge Orders in the Doped Two-Dimensional Hubbard Model

Competing and intertwined orders including inhomogeneous patterns of spin and charge are observed in many correlated electron materials, such as high-temperature superconductors. Introducing a new development of the constrained-path auxiliary-field quantum Monte Carlo (AFQMC) method, we study the interplay between thermal and quantum fluctuations in the two-dimensional Hubbard model. We obtain an accurate and systematic characterization of the evolution of the spin and charge correlations as a function of temperature $T$ and how it connects to the ground state, at three representative doping levels $\delta = 1/5$, $1/8$, and $1/10$. We find increasing short-range commensurate antiferromagnetic correlations as $T$ is lowered. As the correlation length grows sufficiently large, a modulated spin-density-wave (SDW) appears. At $\delta = 1/5$, the SDW saturates and remains short-ranged as $T \rightarrow 0$. In contrast, at $\delta = 1/8$ and $1/10$ this evolves into a ground-state stripe phase. We study the relation between spin and charge orders and find that formation of charge order appears to be driven by that of the spin order. We identify a finite-temperature phase transition below which charge ordering sets in and discuss the implications of our results for the nature of this transition.

Magnetic incommensurability, short-range correlations, and properties ofHo7Rh3

The crystal structure, thermal properties, and magnetic state of the rare-earth intermetallic compound ${\mathrm{Ho}}_{7}{\mathrm{Rh}}_{3}$ was studied using the measurements of the AC and DC magnetic susceptibility, magnetization, specific heat and thermal expansion, as well as by neutron and synchrotron powder diffraction. Below the N\'eel temperature ${T}_{\mathrm{N}}=32$ K, an incommensurate antiferromagnetic (AFM) structure of the spin density wave (SDW) type was observed, described by the magnetic superspace group $Cmc{2}_{1.}{1}^{\ensuremath{'}}(00g)0sss$ and the propagation vector ${\mathbf{k}}_{\mathrm{IC}}=(0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}0.389)$. Further cooling leads to the spin reorientation transition and squaring-up transformation of the SDW magnetic structure below ${T}_{t1}=22$ K. The spin reorientation transition is accompanied by the emergence of a ferromagnetic component in the incommensurate magnetic structure upon cooling below ${T}_{t2}=9$ K. Using neutron diffraction, the existence of a short-range antiferromagnetic order was revealed up to temperatures twice as high as ${T}_{\mathrm{N}}$. These short-range AFM correlations were found to affect the magnetic susceptibility and thermal expansion, while the crystal structure retains its hexagonal symmetry below and above the N\'eel temperature.

Effect of geometry on magnetism of Hund's metals: Case study of BaRuO3

In order to explore the effects of structural geometry on the properties of correlated metals we investigate the magnetic properties of cubic ($3C$) and hexagonal ($4H$) ${\mathrm{BaRuO}}_{3}$. While the $3C$ variant of ${\mathrm{BaRuO}}_{3}$ is ferromagnetic below 60 K, the $4H$ phase does not show any long-range magnetic order, though, there is experimental evidence of short-range antiferromagnetic correlations. Employing a combination of computational tools, namely, density-functional theory and dynamical mean-field theory calculations, we probe the origin of contrasting magnetic properties of ${\mathrm{BaRuO}}_{3}$ in the $3C$ and $4H$ structures. Our study reveals that the difference in connectivity of ${\mathrm{RuO}}_{6}$ octahedra in the two phases results in different Ru-O covalency, which in turn influences substantially the strengths of screened interaction values for Hubbard $U$ and Hund's rule $J$. With estimated $U$ and $J$ values, the $3C$ phase turns out to be a ferromagnetic metal, while the $4H$ phase shows paramagnetic behavior with vanishing ordered moments. However, this paramagnetic phase bears signatures of antiferromagnetic correlations, as confirmed by a calculation of the magnetic susceptibility. We find that the $4H$ phase is found to be at the verge of antiferromagnetic long-range order, which can be stabilized upon slight changes of screened Coulomb parameters $U$ and $J$, opening up the possibility of achieving a rare example of an antiferromagnetic metal.

Real-space visualization of short-range antiferromagnetic correlations in a magnetically enhanced thermoelectric

Short-range magnetic correlations can significantly increase the thermopower of magnetic semiconductors, representing a noteworthy development in the decades-long effort to develop high-performance thermoelectric materials. Here, we reveal the nature of the thermopower-enhancing magnetic correlations in the antiferromagnetic semiconductor MnTe. Using magnetic pair distribution function analysis of neutron scattering data, we obtain a detailed, real-space view of robust, nanometer-scale, antiferromagnetic correlations that persist into the paramagnetic phase above the N\'eel temperature $T_{\mathrm{N}}$ = 307 K. The magnetic correlation length in the paramagnetic state is significantly longer along the crystallographic $c$ axis than within the $ab$ plane, pointing to anisotropic magnetic interactions. Ab initio calculations of the spin-spin correlations using density functional theory in the disordered local moment approach reproduce this result with quantitative accuracy. These findings constitute the first real-space picture of short-range spin correlations in a magnetically enhanced thermoelectric and inform future efforts to optimize thermoelectric performance by magnetic means.

Magnetic glassiness and crystal field effects on thermal and electrical properties of Er5Pd2-type compounds

Magnetic properties, thermal expansion, heat capacity, and electrical resistivity of the cluster-glass compound Er4.8Pd2 and its paramagnetic isostructural counterpart Lu4.8Pd2 have been studied in the temperature range 2–300 K and in applied magnetic fields up to 140 kOe. Short-range antiferromagnetic (AFM) correlations of localized magnetic moments of Er3+ ions were found to develop in Er4.8Pd2 on cooling below T* ≈ 40 K. A cluster-glass magnetic state was observed below freezing temperature Tf ≈ 13.3 K, while Lu4.8Pd2 showed paramagnetic behavior. The crystal electric field (CEF) was found to substantially affect both the heat capacity and the thermal expansion of Er4.8Pd2. The CEF splitting of the Er3+ ion multiplet 4I15/2 was estimated to be as high as 280 K with a doublet ground state. Both the Schottky-type anomaly and short-range antiferromagnetic order contributions were found to give rise to a large volumetric heat capacity of Er4.8Pd2 at low temperatures. High direct magnetocaloric effect with −ΔSM(T)60kOe = 18.8 J mole−1 K−1 was confirmed to exist in Er4.8Pd2 while no inverse magnetocaloric effect was observed at low temperatures. Electrical resistivity of Er4.8Pd2 revealed a non-metallic behavior at low temperatures which was ascribed to an inhomogeneous electronic state due to the formation of clusters with the AFM short-range correlations.

Spin- S impurities with XXZ anisotropy in a spin- 12 Heisenberg chain

We study the effect of anisotropy of spin-$S$ impurities on an antiferromagnetic SU(2) Heisenberg chain. The magnetic impurities are assumed to have an XXZ-type anisotropic exchange with its neighboring sites. First, using density-matrix renormalization group technique. we examine spin-spin correlation function, instability of N\'eel order, and local spin susceptibility in the presence of a single spin-$S$ impurity. Based on the results, we find that the types of spin-$S$ impurities are classified into two groups: $\rm(\hspace{.18em}i\hspace{.18em})$ nonmagnetic and $S=1$ impurities enhance only short-range antiferromagnetic correlation and $\rm(\hspace{.08em}ii\hspace{.08em})$ $S=1/2$ and $S>1$ impurities can, in contrast, stabilize a long-range N\'eel order in the disordered SU(2) Heisenberg chain. Then, we focus on the case of $S=1/2$ impurity as a representative of $\rm(\hspace{.08em}ii\hspace{.08em})$ and investigate the evolution of some experimentally observable quantities such as magnetization, specific heat, and magnetic susceptibility, as a function of concentration and XXZ anisotropy strength of the impurity. We confirm that the N\'eel order is induced in the bulk spin chain in the presence of finite amount of easy-axis XXZ $S=1/2$ impurities. Furthermore, we recover some of the aforementioned features using cluster mean-field theory, which allows us to present results on experimentally accessible quantities at finite temperatures. Interestingly, in the presence of uniform magnetic field, the total magnetization exhibits a pseudo-gap behavior for low values of applied field. We also discuss the dependence of NMR spectrum on various XXZ impurities and identify that the spin state of Co impurity in SrCu$_{0.99}$Co$_{0.01}$O$_2$ is $S = 3/2$.

Crystal-field potential and short-range order effects in inelastic neutron scattering, magnetization, and heat capacity of the cage-glass compound HoB12

The strongly correlated system Ho11B12 with boron sublattice Jahn-Teller instability and nanoscale electronic phase separation (dynamic charge stripes) was studied in detail by inelastic neutron scattering (INS), magnetometry and heat capacity measurements at temperatures in the range 3-300 K. From the analysis of registered INS spectra, we determined parameters of the cubic crystal field at holmium sites, B4=- 0.333 meV and B6= -2.003 meV (in Stevens notations), with an unconventional large ratio B6/B4 pointing on the dominant role of conduction electrons in the formation of a crystal field potential. The molecular field in the antiferromagnetic state, Bloc = (1.75+- 0.1) T has been directly determined from the INS spectra together with short-range order effects detected in the paramagnetic state. A comparison of measured magnetization in diluted Lu0.99Ho0.01B12 and concentrated HoB12 single crystals showed a strong suppression of Ho magnetic moments by antiferromagnetic exchange interactions in holmium dodecaboride. To account explicitly for the short-range antiferromagnetic correlations, a self-consistent holmium dimer model was developed that allowed us to reproduce successfully field and temperature variations of the magnetization and heat capacity in the cage-glass phase of HoB12 in external magnetic fields.

Dimerization of Many-Body Subradiant States in Waveguide Quantum Electrodynamics.

We theoretically study subradiant states in an array of atoms coupled to photons propagating in a one-dimensional waveguide focusing on the strongly interacting many-body regime with large excitation fill factor f. We introduce a generalized many-body entropy of entanglement based on exact numerical diagonalization followed by a high-order singular value decomposition. This approach has allowed us to visualize and understand the structure of a many-body quantum state. We reveal the breakdown of fermionized subradiant states with increase of f with the emergence of short-ranged dimerized antiferromagnetic correlations at the critical point f=1/2 and the complete disappearance of subradiant states at f>1/2.

Magnetic properties of honeycomb spin lattice compounds Na2M2TeO6 (M = Co, Ni) and spin dimer compound Na2Cu2TeO6 single crystals by flux-growth

We report the single crystal growth, structural and magnetic properties of honeycomb spin-lattice compounds Na 2 M 2 TeO 6 (M = Co, Ni) and spin dimer compound Na 2 Cu 2 TeO 6 . The single crystals of Na 2 M 2 TeO 6 (M = Co, Ni, Cu) have been grown by the self-flux method, which is one of the good techniques to control the magnetic impurity, different crystalline phase, crystal morphology, crystal size and the crystal surface through the selection of a suitable self-flux. Crystal structure, magnetic and thermodynamics properties are examined using X-ray diffraction (XRD), magnetic susceptibility and heat capacity measurements, respectively. The single crystal XRD studies of Na 2 M 2 TeO 6 (M = Co, Ni, Cu) show the purity and the high quality of grown single crystals. The measurements of magnetic susceptibility of Na 2 M 2 TeO 6 show the distinct anisotropic behaviour of the prepared single crystals. The anisotropic behaviour and the complicated magnetic phase transitions observed arise from the distinct honeycomb planes constituted by MO 6 octahedra in Na 2 M 2 TeO 6 (M = Co, Ni) leading to the antiferromagnetic (AFM) interaction in the honeycomb planes. In the case of Na 2 Cu 2 TeO 6 broad maximum occurring at 160 K is believed to arise from short-range one-dimensional antiferromagnetic correlations. The values of (effective magnetic moment) μ eff. observed for Na 2 Co 2 TeO 6 , Na 2 Ni 2 TeO 6 and Na 2 Cu 2 TeO 6 are 5.42 μ B , 3.17 μ B and 1.96 μ B respectively.

Stripes, Antiferromagnetism, and the Pseudogap in the Doped Hubbard Model at Finite Temperature

The interplay between thermal and quantum fluctuations controls the competition between phases of matter in strongly correlated electron systems. We study finite-temperature properties of the strongly coupled two-dimensional doped Hubbard model using the minimally-entangled typical thermal states (METTS) method on width $4$ cylinders. We discover that a phase characterized by commensurate short-range antiferromagnetic correlations and no charge ordering occurs at temperatures above the half-filled stripe phase extending to zero temperature. The transition from the antiferromagnetic phase to the stripe phase takes place at temperature $T/t \approx 0.05$ and is accompanied by a step-like feature of the specific heat. We find the single-particle gap to be smallest close to the nodal point at $\mathbf{k}=(\pi/2, \pi/2)$ and detect a maximum in the magnetic susceptibility. These features bear a strong resemblance to the pseudogap phase of high-temperature cuprate superconductors. The simulations are verified using a variety of different unbiased numerical methods in the three limiting cases of zero temperature, small lattice sizes, and half-filling. Moreover, we compare to and confirm previous determinantal quantum Monte Carlo results on incommensurate spin-density waves at finite doping and temperature.

Coupling a Mobile Hole to an Antiferromagnetic Spin Background: Transient Dynamics of a Magnetic Polaron

Understanding the interplay between charge and spin and its effects on transport is a ubiquitous challenge in quantum many-body systems. In the Fermi-Hubbard model, this interplay is thought to give rise to magnetic polarons, whose dynamics may explain emergent properties of quantum materials such as high-temperature superconductivity. In this work, we use a cold-atom quantum simulator to directly observe the formation dynamics and subsequent spreading of individual magnetic polarons. Measuring the density- and spin-resolved evolution of a single hole in a 2D Hubbard insulator with short-range antiferromagnetic correlations reveals fast initial delocalization and a dressing of the spin background, indicating polaron formation. At long times, we find that dynamics are slowed down by the spin exchange time, and they are compatible with a polaronic model with strong density and spin coupling. Our work enables the study of out-of-equilibrium emergent phenomena in the Fermi-Hubbard model, one dopant at a time.

Robust hybridization gap in the Kondo insulator YbB12 probed by femtosecond optical spectroscopy

In heavy fermions the relaxation dynamics of photoexcited carriers has been found to be governed by the low energy indirect gap, E$_{g}$, resulting from hybridization between localized moments and conduction band electrons. Here, carrier relaxation dynamics in a prototype Kondo insulator YbB${}_{12}$ is studied over large range of temperatures and over three orders of magnitude. We utilize the intrinsic non-linearity of dynamics to quantitatively determine microscopic parameters, such as electron-hole recombination rate. The extracted value reveals that hybridization is accompanied by a strong charge transfer from localized 4f-levels. The results imply the presence of a hybridization gap up to temperatures of the order of E$_{g}$/k$_{B}\approx200$ K, which is extremely robust against electronic excitation. Finally, below 20 K the data reveal changes in the low energy electronic structure, attributed to short-range antiferromagnetic correlations between the localized levels.

Experimental observation of weak antiferromagnetic correlation between blocked spins in the nanocrystalline La0.45Ca0.55MnO3 compound at low temperature

In the present study, an attempt has been taken to probe the meta-stable nature of the magnetic ground state in the nanocrystalline La0.45Ca0.55MnO3 compound at the low temperature through magnetic and magneto-transport properties and compared the results from the observation with the bulk polycrystalline form of the La0.45Ca0.55MnO3 compound. The charge-ordered antiferromagnetic ground state in the bulk compound transformed into a ferromagnetic state in the nanocrystalline sample. Additionally, in the nanocrystalline form of the compound, a short-range antiferromagnetic correlation between the blocked spin is noticed at the low temperature region (T ⩽ 50 K). Such weak magnetic interaction between magnetic spins or sublattices is probed through the magneto-transport and magnetocaloric measurements study in the nanocrystalline La0.45Ca0.55MnO3 compound.

Existence of short-range magnetic correlation and observation of large magnetocaloric effect in BiGdO3 compound

In the present study, we have investigated the detailed magnetic and magnetocaloric properties of the rare-earth-based polycrystalline BiGdO3 compound, synthesized by the solid-state reaction method. Detailed magnetic measurements show the absence of long-range magnetic ordering down-to 2 K. Zero-field heat capacity study also supports this result. However, all these magnetic and heat capacity measurements reveal the possibility of having a weak short-range antiferromagnetic correlation in this system below 40 K. Detailed analysis of the magnetic isotherms (using modified Langevin function) and field-dependent magnetic entropy change (using mean-field theory) nicely represent the existence of short-range antiferromagnetic correlation in this system. The prepared material also exhibits a large magnetic entropy change of about 25 J kg−1 K−1 at 2.6 K and a large adiabatic temperature change of 14.8 K at 12 K for the magnetic field change of 0–70 kOe. This observed large magnetocaloric effect at this low temperature region is possibly a consequence of the absence of long-range magnetic ordering and the presence of field-induced ferromagnetic behaviour in this BiGdO3 compound.

Magnetic field dependence of the Schottky anomaly in filled skutterudites Pr1−xEuxPt4Ge12

By performing a series of thermodynamic measurements in an applied magnetic field $H_{\textrm{ext}}$, we investigated the effects of Eu substitution on the Pr sites in filled skutterudite compound Pr$_{1-x}$Eu$_x$Pt$_4$Ge$_{12}$ ($ 0 \leq x \leq 1$). A heat capacity Schottky anomaly is present over the whole doping range. For the samples with $x > 0.5$, these Schottky anomaly peaks shift to lower temperature with increasing $H_{\textrm{ext}}$. We argue that this behavior reflects the antiferromagnetic (AFM) ordering of the Eu moments, as the AFM transition is suppressed by $H_{\textrm{ext}}$. The Schottky peaks in the samples with $x \leq 0.5$ shift to higher temperatures with increasing magnetic field, signaling the presence of an internal magnetic field due to short-range AFM correlations induced by magnetic moments of neighboring Eu sites. In low $H_{\textrm{ext}}$, the Schottky gaps show a non-linear relationship with $H_{\textrm{ext}}$ as the magnetic moments become weakly magnetized. In high $H_{\textrm{ext}}$, the magnetic moments of Eu sites become completely aligned with $H_{\textrm{ext}}$. Thus, increasing $H_{\textrm{ext}}$ does not further increase the magnetization, hence the Schottky gaps increase linearly with $H_{\textrm{ext}}$

Short-Range Antiferromagnetic Correlation Effect on Conduction Electrons in Two-Dimensional Strongly Correlated Electron Systems

We investigate magnetic polarons in two-dimensional strongly correlated electron systems, where conduction electrons interact with antiferromagnetically interacting localized spins. Starting from a basic model, we derive a simplified model with the help of spin Green's function and a perturbation analysis. A strong coupling analysis is applied to the model, where the sum of the scattering wave vectors is approximated to be $(\pi,\pi)$ or zero, using the equation of motion for the conduction electron Green's function, and we discuss the pseudogap like behavior associated with the suppression of the quasiparticle weights and the transition from the large magnetic polaron to the small magnetic polaron. In the antiferromagnetic long-range ordered state, the spectral weight of the conduction electrons has a form of broad humps due to Franck-Condon broadening associated with the multi-magnon scattering. The band folding feature due to the $(\pi,\pi)$ scattering disappears as we increase the number of the magnons involved in the multi-magnon scattering. It is crucial to include long-range antiferromagnetic correlations as well as dumping of magnons.

Atomic-scale fragmentation and collapse of antiferromagnetic order in a doped Mott insulator

Disentangling the relationship between the insulating state with a charge gap and the magnetic order in an antiferromagnetic (AF) Mott insulator remains difficult due to inherent phase separation as the Mott state is perturbed. Measuring magnetic and electronic properties at the atomic length scales would provide crucial insight, but this is yet to be experimentally achieved. Here we use spectroscopic-imaging spin-polarized scanning tunneling microscopy (SP-STM) to visualize periodic spin-resolved modulations originating from the AF order in a relativistic Mott insulator Sr2IrO4, and study these as a function of doping. We find that near insulator-to-metal transition (IMT), the long-range AF order melts into a fragmented state with short-range AF correlations. Crucially, we discover that the short-range AF order is locally uncorrelated with the observed spectral gap magnitude. This strongly suggests that short range AF correlations are unlikely to be the culprit behind inhomogeneous gap closing and the emergence of pseudogap regions near IMT. Our work establishes SP-STM as a powerful tool for revealing atomic-scale magnetic information in complex oxides.