The magnetic properties of a variety of materials with promise for technological applications have been described by models in which fermions are coupled to local moment spins. Monte Carlo studies of such models usually ignore electron-electron interactions, even though the energy scale corresponding to these interactions may be comparable to or larger than other relevant energy scales. In this work we add on-site interactions between fermions to the double exchange model which we study with a Monte Carlo scheme in which temporal fluctuations of local moment spins are fully accounted for and electron-electron interactions are treated at a mean field level. We show that when the number of fermions is considerably less than the number of local moments even moderate interactions can lead to significant enhancement of ferromagnetism and the Curie temperature.
We present a theory of competing ferromagnetic and superconducting orders in twisted double bilayer graphene (TDBG). In our theory, ferromagnetism is induced by Coulomb repulsion, while superconductivity with intervalley equal-spin pairing can be mediated by electron-acoustic phonon interactions. We calculate the transition temperatures for ferromagnetism and superconductivity as a function of moiré band filling factor, and find that superconducting domes can appear on both the electron and hole sides of the ferromagnetic insulator at half filling. We show that the ferromagnetic insulating gap…
We carry out a first-principles investigation on magnetism, electronic structure, magneto-optical effects and topological property of newly grown cubic double perovskite Ba2NiOsO6 and its (111) (Ba2NiOsO6)/(BaTiO3)10 superlattice, based on the density functional theory with the generalized gradient approximation (GGA) plus onsite Couloumb interactions. Interestingly, we find that both structures are rare ferromagnetic (FM) semiconductors with estimated Curie temperatures of ~150 and 70 K, respectively. The calculated near-neighbor exchange coupling parameters reveal that the ferromagnetism is …
Canted Ferromagnetism in Double Exchange Model with on-site Coulomb Repulsion
考虑了具有在位库仑排斥的双交换模型。采用定域自旋的Schwinger-bosons表示，引入了两个自旋单态费米子算符。根据新的费米场，在位Hund相互作用是对角形式的，并确定了系统的真实磁振子。单重态费米子可以理解为被重复发射和重吸收的磁振子云装扮的电子。用施温格玻色子和自旋单重态费米子改写后的理论是U（1）规范不变的。我们表明，规范对称性的自发击穿导致\emph{\textbf{canted ferromagnetism with on-site spins of localized and delocalized electrons misaligned}}。当库仑排斥足够大时，双交换模型中出现了原位倾斜相。研究了在不同的Hund耦合和化学势参数下，库仑排斥力的变化对铁磁性和倾斜相之间的量子相变的影响。
We show that an interplay of double exchange and impurity randomness can explain the competition between metal-ferromagnetic and insulating charge ordered states in doped manganites. The double exchange is simplified in the Ising type, whereas the randomness is modeled by the Falicov-Kimball binary distribution. The combined model is considered in a framework of dynamical mean-field theory. Using the Kubo-Greenwood formalism, the transport coefficients are explicitly expressed in terms of single particle spectral functions. Dividing the system into two sublattices we have pointed out a direct …
Few-layer CrI$_3$ is the most known example among two-dimensional (2D) ferromagnets, which have attracted growing interest in recent years. Despite considerable efforts and progress in understanding the properties of 2D magnets both from theory and experiment, the mechanism behind the formation of in-plane magnetic ordering in chromium halides is still under debate. Here, we propose a microscopic orbitally-resolved description of ferromagnetism in monolayer CrI$_3$. Starting from first-principles calculations, we construct a low-energy model for the isotropic Heisenberg exchange interactions. …
The double perovskite Y_2NiMnO_6 displays ferromagnetic transition at Tc = 81 K. The ferromagnetic order at low temperature is confirmed by the saturation value of magnetization (M_s) and also, validated by the refined ordered magnetic moment values extracted from neutron powder diffraction data at 10 K. This way, the dominant Mn4+ and Ni2+ cationic ordering is confirmed. The cation-ordered P 21/n nuclear structure is revealed by neutron powder diffraction studies at 300 and 10 K. Analysis of frequency dependent dielectric constant and equivalent circuit analysis of impedance data takes into a…
We study the ground-state properties of the SU(N)-generalization of the Kondo-lattice model in one dimension when the Kondo coupling J_K (both ferromagnetic and antiferromagnetic) is sufficiently strong. Both cases can be realized using alkaline-earth-like cold gases in optical lattices. Specifically, we first carry out the strong-coupling expansion and identify two insulating phases (one of which is the SU(N)-analogue of the well-known gapped Kondo singlet phase). We then rigorously establish that the ground state in the low-density (for J_K<0) or the high-density (for J_K>0) region is ferrom…
Two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform for exploring, on the fundamental side, new microscopic mechanisms and for developing, on the technological side, ultra-compact spintronic applications. So far, bilinear spin Hamiltonians have been commonly adopted to investigate the magnetic properties of 2D magnets, neglecting higher order magnetic interactions. However, we here provide quantitative evidence of giant biquadratic exchange interactions in monolayer NiX2 (X=Cl, Br and I), by combining first-principles calculations and the newly developed machine learning…
The van der Waals ferromagnet Fe5GeTe2 has a Curie temperature TC of about 270 K, which can be raised above room temperature by tuning the Fe deficiency content. To achieve insights into its ferromagnetic exchange, we have studied the critical behavior by measuring the magnetization in bulk Fe5GeTe2 crystal around the ferromagnetic to paramagnetic phase transition. The analysis of the magnetization by employing various techniques including the modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis achieved a set of reliable critical exponents with TC = 273.7 K, beta = 0.3457, …
Recent experiments on the Co-based $d^7$ honeycomb materials BaCo$_2$(AsO$_4$)$_2$ and BaCo$_2$(PO$_4$)$_2$ have drawn renewed interest to XXZ models with competing interactions beyond the nearest neighbor exchange. While the case of dominant antiferromagnetic exchange has been extensively studied, the actual materials exhibit a ferromagnetic nearest neighbor exchange. Here we show that such a sign change of the dominant nearest neighbor coupling has substantial consequences on the phase diagram. In particular, the nature of the quantum disordered phase of interest changes from a plaquette val…
The puzzling observation of room temperature ferromagnetism in double perovskites (A$_2$BB$'$O$_6$), despite having the magnetic lattice of B-ions diluted by non-magnetic B$'$-ions, have been examined for Sr$_2$FeReO$_6$. {\it Ab-initio} spin spiral electronic structure calculations along various high symmetry directions in reciprocal space are used to determine the exchange interactions entering an extended Heisenberg model, which is then solved classically using Monte Carlo simulations to determine the ferromagnetic transition temperature T$_c$. We find that one must consider onsite Coulomb …
We derive a "Wannier-Hubbard" model consisting of an array of overlapping atomic orbitals interacting via a local Coulomb interaction. Transforming to an orthogonal Wannier basis set, the resulting Hamiltonian displays long range hopping and interactions, with new terms such as correlated hopping and ferromagnetic direct exchange, among others. We numerically study the one-dimensional version of the model at half-filling using the density matrix renormalization group (DMRG) method, unveiling a rich phase diagram as a function of the interaction $U$ and the overlap $s$ with metallic, and ferrom…
Double-exchange ferromagnetism of fermionic atoms in a $p$-orbital hexagonal lattice
Hefei National Research Center for Physical Sciences at the Microscale and School of Physical Sciences, University of Science and Technology of China（合肥微尺度物质科学国家研究中心和中国科学技术大学物理科学学院）；Department of Physics and Astronomy, George Mason University（乔治梅森大学物理与天文系）；Department of Physics and Astronomy and IQ Initiative, University of Pittsburgh（匹兹堡大学物理与天文系及IQ倡议）
We have investigated the critical phenomenon associated with the magnetic phase transition in the half-metallic full-Heusler Co$_2$TiGe. The compound undergoes a continuous ferromagnetic to paramagnetic phase transition at the Curie temperature $T_{C}$=371.5 K. The analysis of magnetization isotherms in the vicinity of $T_{c}$, following modified Arrott plot method, Kouvel-Fisher technique, and critical isotherm plot, yields the asymptotic critical exponents $β$=0.495, $γ$=1.324, and $δ$=3.67. The self-consistency and reliability of the obtained exponents are further verified by the Widom scal…
What is the Mechanism Underlying 3-D Heisenberg-like Ferromagnetism Across the Compositional Metal-Insulator Transition in La1-xCaxMnO3 (0.18 < xc < 0.22)?
Ferromagnetism in the one-dimensional Hubbard model with orbital degeneracy: From low to high electron density
用密度矩阵重整化群方法研究了双简并原子轨道的一维哈伯德模型的铁磁性，得到了基态相图。结果表明，当相互作用足够强时，铁磁性从低到高（0< n < 1.75）电子密度是稳定的。准长程序的三重态超导电性与铁磁有序共存的强Hund耦合区，其中轨道间相互作用U '-J是吸引的。在四分之一填充（n=1），绝缘铁磁态出现伴随着轨道准长程序。对于低密度（n<1），由于电子的虚跳跃引起的铁磁交换相互作用，铁磁性发生，与四分之一填充系统相同。这来自于强耦合极限中电荷和自旋-轨道自由度的分离。这种铁磁性对于能带结构的变化是脆弱的。对于高密度（n>1），铁磁相的相图类似于在无限维中获得的相图。在这种情况下，双交换机制是可操作的，以稳定的铁磁秩序和这种长程序是强大的对带色散的变化。在1.68<n<1.75的密度区出现部分极化态，在半填充区（n=2）的正下方出现相分离。
