Local Antiferromagnetic Order Research Articles

Realizing a symmetry protected topological phase through dimerized interactions

We show that in a system of one dimensional spinless fermions a topological phase and phase transition can emerge only through interaction. By allowing a dimerized or bond-alternating nearest neighbour interaction we show that the system exhibits a symmetry protected topological phase while its non-interacting limit is a gapless state. The non-trivial topological character appears due to the onset of two degenerate bond-order phases as a function of dimerized interaction which are found to be topologically distinct from each other. As a result a topological phase transition occurs between these bond order phases through a gap closing point. However, in the limit of strong interaction, the bond order phases are connected through a gapped charge density wave phase possessing local antiferromagnetic order. The topological nature is characterized by the edge states, Berry phase and non-local string order parameter. At the end we provide possible experimental signatures of the emergent symmetry protected topological phase transition in terms of Thouless charge pumping and density-density correlation.

Quantum phases and transitions of spin-1/2 quantum compass chain

Quantum phases (QPs) and quantum phase transitions (QPTs) are very important parts of the strongly correlated quantum many-body systems in condensed matter. To study the QPs and QPTs, the systems should include rich quantum phase diagram. In this sense, the corresponding quantum spin models should have strong quantum fluctuation, strong geometric frustration, complicated spin-spin exchange or orbital degrees of freedom, which induces a variety of spontaneous symmetry breaking (SSB) or hidden spontaneous symmetry breaking. The QPs induced by the SSB can be characterized by local order parameters, a concept that originates from Landau-Ginzburg-Wilson paradigm (LGW). However, there is also a novel class of topological QPs beyond LGW, which has aroused one’s great interest since the Haldane phase was found. Such QPs can be characterized only by topological long-range nonlocal string correlation order parameters instead of local order parameters. In this paper, we investigate a spin-1/2 quantum compass chain model (QCC) with orbital degrees of freedom in <i>x</i>, <i>y</i> and <i>z</i> components. The prototype of QCC is the quantum compass model including novel topological QPs beyond LGW, and consequently one can also anticipate the existence of novel topological QPs in QCC. However, very little attention has been paid to the QPs and QPTs for QCC, which deserves to be further investigated. By using the infinite time evolving block decimation in the presentation of matrix product states, we study the QPs and QPTs of QCC. To characterize QPs and QPTs of QCC, the ground state energy, local order parameter, topological long-range nonlocal string correlation order parameters, critical exponent, correlation length and central charge are calculated. The results show the phase diagram of QCC including local antiferromagnetic phase, local stripe antiferromagnetic phase, oscillatory odd Haldane phase and monotonic odd Haldane phase. The QPTs from oscillatory odd Haldane phase to local stripe antiferromagnetic phase and from local antiferromagnetic phase to monotonic odd Haldane phase are continuous; on the contrary, QPTs from local stripe antiferromagnetic phase to local antiferromagnetic phase and from oscillatory odd Haldane phase to monotonic odd Haldane phase are discontinuous. The crossing point where the line of continuous QPTs meets with the line of discontinuous QPTs is the multiple critical point. The critical exponents <i>β</i> of local antiferromagnetic order parameter, local stripe antiferromagnetic order parameter, topological long-range nonlocal oscillatory odd string correlation order parameter, and topological long-range nonlocal monotonic odd string correlation order parameter are all equal to 1/8. Moreover, <inline-formula><tex-math id="M3">\begin{document}$\beta =1/8$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211433_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211433_M3.png"/></alternatives></inline-formula> and the central charges <inline-formula><tex-math id="M4">\begin{document}$c = 1/2$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211433_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20211433_M4.png"/></alternatives></inline-formula> at the critical points show that the QPTs from local phases to nonlocal phases belong to the Ising-type universality class.

Magnetism in Au-Supported Planar Silicene.

The adsorption and substitution of transition metal atoms (Fe and Co) on Au-supported planar silicene have been studied by means of first-principles density functional theory calculations. The structural, energetic and magnetic properties have been analyzed. Both dopants favor the same atomic configurations with rather strong binding energies and noticeable charge transfer. The adsorption of Fe and Co atoms do not alter the magnetic properties of Au-supported planar silicene, unless a full layer of adsorbate is completed. In the case of substituted system only Fe is able to produce magnetic ground state. The Fe-doped Au-supported planar silicene is a ferromagnetic structure with local antiferromagnetic ordering. The present study is the very first and promising attempt towards ferromagnetic epitaxial planar silicene and points to the importance of the substrate in structural and magnetic properties of silicene.

Partial bosonization for the two-dimensional Hubbard model

Partial bosonisation of the two-dimensional Hubbard model focuses the functional renormalisation flow on channels in which interactions become strong and local order sets in. We compare the momentum structure of the four-fermion vertex, obtained on the basis of a patching approximation, to an effective bosonic description. For parameters in the antiferromagnetic phase near the onset of local antiferromagnetic order, the interaction of the electrons is indeed well described by the exchange of collective bosonic degrees of freedom. The residual four-fermion vertex after the subtraction of the bosonic exchange contribution is small. We propose that similar partial bosonisation techniques can improve the accuracy of renormalisation flow studies also for the case of competing order.

Controlled growth of boron-doped epitaxial graphene by thermal decomposition of a B4C thin film

We show that boron-doped epitaxial graphene can be successfully grown by thermal decomposition of a boron carbide thin film, which can also be epitaxially grown on a silicon carbide substrate. The interfaces of B4C on SiC and graphene on B4C had a fixed orientation relation, having a local stable structure with no dangling bonds. The first carbon layer on B4C acts as a buffer layer, and the overlaying carbon layers are graphene. Graphene on B4C was highly boron doped, and the hole concentration could be controlled over a wide range of 2 × 1013 to 2 × 1015 cm−2. Highly boron-doped graphene exhibited a spin-glass behavior, which suggests the presence of local antiferromagnetic ordering in the spin-frustration system. Thermal decomposition of carbides holds the promise of being a technique to obtain a new class of wafer-scale functional epitaxial graphene for various applications.

Role of antiferromagnetic spin axis on magnetic reconstructions at the (111)-oriented La0.7Sr0.3MnO3/LaFeO3 interface

Engineering of emergent properties at oxide interfaces is an exciting route towards realizing oxide electronics. Such properties are often the result of a balance between cooperating and competing mechanisms in the materials which are difficult to decouple. In this paper, we address the interplay between an antiferromagnetic spin axis and the occurrence of magnetic reconstructions at the (111)-oriented $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}/\mathrm{LaFe}{\mathrm{O}}_{3}$ interface. We report a critical $\mathrm{LaFe}{\mathrm{O}}_{3}$ thickness where we observed a magnetic interface reconstruction with a net switchable Fe moment only for thicknesses less than or equal to 16 ${d}_{111}$ layers of $\mathrm{LaFe}{\mathrm{O}}_{3}$. A change from an out-of-plane to in-plane antiferromagnetic spin axis is found at the same critical thickness. We ascribe the interfacial moment to a successively decreased out-of-plane canting of the antiferromagnetic spin axis towards the in-plane $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ magnetization at the interface. This points towards the importance of the local antiferromagnetic order interacting with concurrent structural reconstructions to establish a magnetically reconstructed interface.

Comparative Study of the Magnetic Structure of BaFe2As2 Doped with Co or Ni

We present a comparative study of the electronic structure of pure, Co-doped, and Ni-doped BaFe2As2. The calculations were performed by the embedded cluster method at the electron correlation level (ECM-MP2). The orbital population analysis reveals that doping with Co or Ni introduces, in both cases, a considerable transfer of charge from cations toward the anions and a transfer of spin density in the opposite direction that reflects an independence of spin and charge in non-relativistic approach. In both doping, we note the presence of spinless electron on the 3d orbitals. On the other hand, both doping cases differ in the local magnetic order: Co induces a local antiferromagnetic order along the b axis of the crystalline structure, while Ni maintains the ferromagnetic order along this axis. In the Co-doped compound, the frustration of magnetic state and formation of spinless electrons correspond to some properties of the Anderson RVB model of superconductivity.

Magnetic properties and magnetoresistance in La0.5Sr0.5Co1−xMexO3 (Me = Cr, Ga, Ti, Fe) cobaltites

Magnetic and magnetotransport properties of stoichiometric cobaltites La0.5Sr0.5Co1−xMexO3 (Me = Cr, Ga, Ti, Fe, x ≤ 0.25) have been investigated by neutron powder diffraction, magnetization and electrical measurements in fields up to 14 T. It is shown that doping with Fe up to x = 0.2 slightly increases magnetization herewith the Curie point decreases. The chromium doping leads to dramatic decrease of magnetization and the Curie point, associated coherent magnetic contribution in NPD patterns quickly decreases with doping and becomes nearly negligible for x = 0.2 compound. The substitution with diamagnetic Ga and Ti ions decreases the magnetization to a lesser extent than in the chromium doped compound. The Cr, Ti and Ga substitution leads to a strong increase in magnetoresistance at low temperature as compared with undoped cobaltite. The obtained results indicate that the magnetic interactions between Co and Fe are positive whereas those between Co and Cr ions are negative. Enhancement of magnetoresistance is attributed to the magnetic field induced transition from local antiferromagnetic order to ferromagnetic one.

Grain size and stoichiometry control over RF-sputtered multiferroic BiFeO3 thin films on silicon substrates

This work reports the morphological and chemical characterization of multiferroic BiFeO3 polycrystalline thin films grown on Si(111) by RF-sputtering. Results are shown for a large set of samples and a wide array of experimental techniques, including imaging (atomic/piezoresponse force microscopy) and spectroscopic (μ-Raman, X-ray photoemission, X-ray diffraction) probes. Through growth and post-growth annealing treatment, a fine control over stoichiometry, grain size, grain orientation, crystal order and surface roughness is achieved. In particular, the grain size can be tailored from nanocrystals to large micrometric plates as a function of the annealing temperature. For the optimal stoichiometric sample, an additional X-ray absorption and magnetic circular dichroism analysis has been carried out, which provides high quality spectra comparable with epitaxial films and further proves the expected strong local antiferromagnetic order.

Electronic and magnetic properties ofNd1−xSrxMnAsOoxyarsenides

The oxypnictides Nd1-xSrxMnAsO have been successfully synthesised with x up to 0.1. A synchrotron X-ray diffraction study demonstrates that there is no change in crystal symmetry upon doping with Sr. An expansion of the inter-layer distance between Nd-O-Nd and As-Mn-As blocks is observed with increasing x. Results from variable temperature neutron diffraction and resistivity measurements show that the local moment antiferromagnetic order of the Mn spins is preserved as the [MnAs]- layers are hole doped and the materials are driven metallic for x > 0.05. A sizeable positive magnetoresistance is observed at low temperature which demonstrates that multiple MR mechanisms are possible in LnMnAsO oxypnictides.

An EELS study of near edge structures of the oxygen K-edge in spinels

Spectra of seven aluminate, ferrite and chromate spinels were collected at the oxygen K-edge in order to examine the effect of octahedral ion composition on the near edge structures and explore whether these may shed some light on previously reported major variations in the optical absorption coefficient (e) for the spin-allowed d–d band transitions due to tetrahedrally coordinated Fe2+ in optical spectra of oxide spinels. Interpretation of the energy loss near edge structure (ELNES) spectra was aided using the multiple scattering code FEFF8.40. For the chromate samples spin polarised calculations were included. Gold’s iterative deconvolution method was utilized to improve on energy resolution, resulting in spectra equal in merit to those recorded by technically superior instruments. Results include absolute energy positions and interpretation of ELNES features in terms of transitions to available states. We conclude that the major causes of differences in the optical absorption coefficient mentioned above are hybridization between oxygen p- and octahedral metal orbitals. Our study does not support the idea presented by other authors that there exists a local antiferromagnetic ordering in MgCr2O4 at room temperature.

Persistence of local-moment antiferromagnetic order in Ba1−xKxMn2As2

BaMn2As2 is a local-moment antiferromagnetic insulator with a N\'eel temperature of 625 K and a large ordered moment of 3.9 Bohr magneton per Mn. Remarkably, this compound can be driven metallic by the substitution of as little as 1.6% K for Ba while retaining essentially the same ordered magnetic moment and N\'eel temperature, as previously reported. Here, using both powder and single crystal neutron diffraction we show that the local moment antiferromagnetic order in Ba(1-x)KxMn2As2 remains robust up to x = 0.4. The ordered moment is nearly independent of x for 0 < x < 0.4 and the N\'eel transition temperature decreases to 480 K at x = 0.4.

On the important role of the anti-Jahn-Teller effect in underdoped cuprate superconductors

In this paper it is shown that the "anti-Jahn-Teller effect" plays an essential role in giving rise to a small Fermi surface of Fermi pockets above Tc and d-wave superconductivity below Tc in underdoped cuprates. In the first part of the present paper, we review the latest developments of the model proposed by Kamimura and Suwa, which bears important characteristics born from the interplay of Jahn-Teller Physics and Mott Physics. It is shown that the feature of Fermi surfaces in underdoped LSCO is the Fermi pockets in the nodal region constructed by doped holes under the coexistence of a metallic state and of the local antiferromagnetic order. In the antinodal region in the momentum space, there are no Fermi surfaces. Then it is discussed that the phonon-involved mechanism based on the Kamimura-Suwa model leads to the d-wave superconductivity. In particular, it is shown that the origin of strong electron-phonon interactions in cuprates is due to the anti-Jahn-Teller effect. In the second part a recent theoretical result on the energy distribution curves (EDCs) of angle-resolved photoemission spectroscopy (ARPES) below Tc is discussed. It is shown that the feature of ARPES profiles of underdoped cuprates consists of a coherent peak in the nodal region and the real transitions of photoexcited electrons from occupied states below the Fermi level to a free-electron state above the vacuum level in the antinodal region, where the latter transitions form a broad hump. From this feature, the origin of the two distinct gaps observed by ARPES is elucidated without introducing the concept of the pseudogap. Finally, a remark is made on the phase diagram of underdoped cuprates.

Multiferroic properties of Co and Nd co-substituted Bi5Ti3FeO15 thin films

Co and Nd co-substituted Bi5Ti3FeO15 thin films were prepared on Pt/Ti/SiO2/Si substrates via metal organic decomposition method. The structural and multiferroic properties of the films were investigated. It was found that Co ions enter into the lattice and occupy the Fe site. The Bi4.15Nd0.85Ti3Fe0.5Co0.5O15 films simultaneously exhibit ferroelectric and ferromagnetic properties at room temperature, and its 2Pr and 2Mr are 38μC/cm2 and 3kA/m, respectively. Moreover, substitutions create local ferromagnetic order and antiferromagnetic order depending on whether the local bonding is FeOCo or FeOFe/CoOCo, respectively. The competing interaction of the ferromagnetic and antiferromagnetic phases results in an interesting magnetic behavior of the films.

On the Interplay of Jahn–Teller Physics and Mott Physics Leading to the Occurrence of Fermi Pockets Without Pseudogap Hypothesis and d-Wave High T c Superconductivity in Underdoped Cuprate Superconductors

In this Festschrift paper celebrating Prof. Jacques Friedel 90 years’ birthday, we review the latest developments of the model proposed by Kamimura and Suwa, which bears important characteristics born from the interplay of Jahn–Teller physics and Mott physics. First it is shown that the feature of Fermi surfaces is the Fermi pockets constructed by doped holes under the coexistence of a metallic state and of local antiferromagnetic order. Then the phonon-involved mechanism based on the Kamimura–Suwa model is discussed; it leads to d-wave superconductivity. Further it is shown that T c is higher in the cuprates with CuO5 pyramid than those with CuO6 octahedron. Finally a recent theoretical result on the energy distribution curves (EDCs) of angle-resolved photoemission spectroscopy (ARPES) below T c is presented, and a remark is made on the phase diagram for underdoped cuprates.

Occurrence of Fermi Pockets without Pseudogap Hypothesis and Clarification of the Energy Distribution Curves of Angle-Resolved Photoemission Spectroscopy in Underdoped Cuprate Superconductors

Central issues in the electronic structure of underdoped cuprate superconductors are to clarify the shape of the Fermi surfaces and the origin of the pseudogap. On the basis of the model proposed by Kamimura and Suwa, which bears important features originating from the interplay of Jahn-Teller physics and Mott physics, the feature of Fermi surfaces in underdoped cuprates is the presence of Fermi pockets constructed from doped holes under the coexistence of a metallic state and a local antiferromagnetic order. Below $T_{\rm c}$, the holes on Fermi pockets form Cooper pairs with d-wave symmetry in the nodal region. In the antinodal region, there are no Fermi surfaces. In this study we calculate the energy distribution curves (EDCs) of angle-resolved photoemission spectroscopy (ARPES) below $T_{\rm c}$. It is shown that the feature of ARPES profiles of underdoped cuprates consists of a coherent peak in the nodal region and real transitions of photoexcited electrons from occupied states below the Fermi level to a free-electron state above the vacuum level in the antinodal region, where the latter transitions form a broad hump. From this feature, the origin of the two distinct gaps observed by ARPES is elucidated without introducing the concept of the pseudogap. Finally, a remark is made on the phase diagram of underdoped cuprates.

Underdoped cuprates as fractionalized Fermi liquids: Transition to superconductivity

We model the underdoped cuprates using fermions moving in a background with local antiferromagnetic order. The antiferromagnetic order fluctuates in orientation, but not in magnitude, so that there is no long-range antiferromagnetism, but a 'topological' order survives. The normal state is described as a fractionalized Fermi liquid (FL*), with electron-like quasiparticles coupled to the fractionalized excitations of the fluctuating antiferromagnet. The electronic quasiparticles reside near pocket Fermi surfaces enclosing total area x (the dopant density), centered away from the magnetic Brillouin zone boundary. The violation of the conventional Luttinger theorem is linked to a 'species doubling' of these quasiparticles. We describe phenomenological theories of the pairing of these quasiparticles, and show that a large class of mean-field theories generically displays a nodal-anti-nodal `dichotomy': the interplay of local antiferromagnetism and pairing leads to a small gap near the nodes of the d-wave pairing along the Brillouin zone diagonals, and a large gap in the anti-nodal region.

Local suppression of the hidden-order phase by impurities in URu2Si2

We consider the effects of impurities on the enigmatic hidden order (HO) state of the heavy-fermion material ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$. In particular, we focus on local effects of Rh impurities as a tool to probe the suppression of the HO state. To study local properties, we introduce a lattice free energy, where the time invariant HO order parameter $\ensuremath{\Psi}$ and local antiferromagnetic (AFM) order parameter $M$ are competing orders. Near each Rh atom, the HO order parameter is suppressed, creating a hole in which local AFM order emerges as a result of competition. These local holes are created in the fabric of the HO state like in a Swiss cheese and ``filled'' with droplets of AFM order. We compare our analysis with recent NMR results on U(Rh${}_{x}$Ru${}_{1\ensuremath{-}x}$)${}_{2}$Si${}_{2}$ and find good agreement with the data.

Geometric phases and competing orders in two dimensions

We discuss the problem of characterizing ``quantum-disordered'' ground states, obtained upon loss of antiferromagnetic order on general lattices in two spatial dimensions, with arbitrary electronic band structure. A key result is the response in electron bilinears to the skyrmion density in the local antiferromagnetic order, induced by geometric phases. We also discuss the connection to topological terms obtained under situations where the electronic spectrum has a Dirac form.

Thermodynamic properties of a spin half ladder system

A complete dimerized state exists for one kind of two-leg spin half ladders, which has local antiferromagnetic ordering and frustration effect at the same time. The system’s low-lying excitations can be obtained exactly which enables us to calculate thermodynamic quantities such as specific heat and magnetic susceptibility at low temperatures. Our results also show that the subset energy spectrum is a good approximation to the whole spectrum even for the usual two-leg spin half ladder without frustration.