We present here the first-principles dynamical CPA (coherent potential approximation) combined with the tight-binding LMTO LDA+U method towards quantitative calculations of the electronic structure and magnetism at finite temperatures in transition metals and compounds. The theory takes into account the single-site dynamical charge and spin fluctuations using the functional integral technique as well as an effective medium. Numerical results for Fe, Co, and Ni show that the theory explains quantitatively the high-temperature properties such as the effective Bohr magneton numbers and the excitation spectra in the paramagnetic state, and describes the Curie temperatures semiquantitatively.
Single-crystal sample of chromium doped with ca.0.2 at.%119Sn was studied by means of a transmission Mössbauer spectroscopy in the temperature range of 310-315 K. An anomaly in the temperature behavior of the center shift was found at ca. 313 K, a temperature that coincides well with the Néel temperature of chromium. The anomaly gives evidence that the vibrations of atoms in the studied system are affected by the magnetism.
Edge magnetism in zigzag transition metal dichalcogenide nanoribbons is studied using a three-band tight-binding model with local electron-electron interactions. Both mean field theory and the unbiased, numerically exact determinant quantum Monte Carlo method are applied. Depending on the edge filling, mean field theory predicts different phases: gapped spin dimer and antiferromagnetic phases appear for two specific fillings, with a tendency towards metallic edge-ferromagnetism away from those fillings. Determinant quantum Monte Carlo simulations confirm the stability of the antiferromagnetic …
We report tests of various density functionals for ferromagnetic, Fe, Co and Ni with a focus on characterizing the behavior of the so-called strongly constrained and appropriately normed (SCAN) functional. It is found that SCAN is closer in behavior to functionals that yield localized behavior, such as hybrid functionals, than other semilocal functionals that are tested. The results are understood in terms of a tendency to differentiate orbitals, favoring integer occupation, which is necessary for a correct description of atomic systems, but inappropriate for the open shell metallic ferromagne…
Electronic structures of FeSi and Fe$_{1.02}$Si$_{0.98}$ under pressure (achieved through volume compression) have been investigated by using DFT+DMFT and KKR-CPA methods, respectively. The widening of band-gap with increasing pressure suggests that the experimentally observed insulator to metal transition temperature should shift towards the higher temperature for FeSi. KKR-CPA calculations have shown the presence of impurity states in the gapped region which predicts the half-metallic nature. The closure of gap (in one spin channel) with pressure increment appears to be responsible for exper…
RV6Sn6 (R=rare earth) compounds are appealing materials platforms for exploring the interplay between R-site magnetism and nontrivial band topology associated with the nonmagnetic vanadium-based kagome network. Here we present the synthesis and characterization of the kagome metal TbV6Sn6 via single-crystal x-ray diffraction, magnetization, transport, and heat capacity measurements. Magnetization measurements reveal strong, uniaxial magnetic anisotropy rooted in the alignment of Tb3Å moments in the interplane direction below 4.3(2) K. TbV6Sn6 exhibits multiband transport behavior with high mob…
Based on first-principles calculations, the evolution of the electronic and magnetic properties of transition metal dihalides MX$_2$ (M= V, Mn, Fe, Co, Ni; X = Cl, Br, I) is analyzed from the bulk to the monolayer limit. A variety of magnetic ground states is obtained as a result of the competition between direct exchange and superexchange. The results predict that FeX$_2$, NiX$_2$, CoCl$_2$ and CoBr$_2$ monolayers are ferromagnetic insulators with sizable magnetocrystalline anisotropies. This makes them ideal candidates for robust ferromagnetism at the single layer level. Our results also hig…
Kagome magnets are expected to feature emergent properties due to the interplays among geometry, magnetism, electronic correlation, and band topology. The magnetism and topological electronic states can be tuned via the rare earth engineering in RV6Sn6 kagome metals, where R is a rare earth element. Herein, we present the synthesis and characterization of SmV6Sn6, a metal with two-dimensional kagome nets of vanadium and frustrated triangular Sm lattice. Partial of the Sm atoms are shifted from the normal R positions by c/2 along the c axis. Magnetic measurements reveal obvious anisotropy, wher…
Coexistence of long-range magnetic order and dynamical magnetism in the V-based Kagome metals: A combined thermodynamic and $μ$SR study
Kyushu Institute of Technology（九州工业大学）；Forschungszentrum Jülich（于利希研究中心）；Technical University of Munich（慕尼黑工业大学）；National Cheng Kung University（国立成功大学）
We study the emergence of non-Hermitian band topology in a two-dimensional metal with planar spiral magnetism due to a momentum-dependent relaxation rate. A sufficiently strong momentum dependence of the relaxation rate leads to exceptional points in the Brillouin zone, where the Hamiltonian is nondiagonalizable. The exceptional points appear in pairs with opposite topological charges and are connected by arc-shaped branch cuts. We show that exceptional points inside hole and electron pockets, which are generally present in a spiral magnetic state with a small magnetic gap, can cause a drastic…
The nature of metal-insulator and magnetic transitions is still a subject under intense debate in condensed matter physics. Amongst the many possible mechanisms, the interplay between electronic correlations and spin-orbit couplings is an issue of a great deal of interest, in particular when dealing with quasi-2D compounds. In view of this, here we use a Hartree-Fock approach to investigate how the Rashba spin-orbit coupling, $V_\text{SO}$, affects the magnetic ordering provided by a Hubbard interaction, $U$, on a square lattice. At half-filling, we have found a sequence of transitions for inc…
Motivated by the recent wealth of exotic magnetic phases emerging in two-dimensional frustrated lattices, we investigate the origin of possible magnetism in the monolayer family of triangular lattice materials $MX_2$ ($M$={V, Mn, Ni}, $X$={Cl, Br, I}). We first show that consideration of general properties such as filling and hybridization enables to formulate trends for the most relevant magnetic interaction parameters. In particular, we observe that the effects of spin-orbit coupling (SOC) can be effectively tuned through the ligand elements as the considered 3$d$ transition metal ions do no…
Theory of unconventional magnetism in a Cu-based kagome metal
Universität Würzburg（维尔茨堡大学）；Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche（意大利国家研究委员会材料研究所）；ETH Zürich（苏黎世联邦理工学院）；University of Geneva（日内瓦大学）；University of Technology Sydney（悉尼科技大学）；Columbia University（哥伦比亚大学）；University of Bologna（博洛尼亚大学）
We present the results of x-ray scattering and muon-spin relaxation ($μ^{+}$SR) measurements on the iron-pnictide compound FeCrAs. Polarized non-resonant magnetic x-ray scattering results reveal the 120$^\circ$ periodicity expected from the suggested three-fold symmetric, non-collinear antiferromagnetic structure. $μ^+$SR measurements indicate a magnetically ordered phase throughout the bulk of the material below $T_\mathrm{N}$=105(5) K. There are signs of fluctuating magnetism in a narrow range of temperatures above $T_\mathrm{N}$ involving low-energy excitations, while at temperatures well b…
Two-dimensional (2D) metal-organic frameworks (MOFs) in a kagome lattice can exhibit strong electron-electron interactions, which can lead to tunable quantum phases including many exotic magnetic phases. While technological developments of 2D MOFs typically take advantage of substrates for growth, support, and electrical contacts, investigations often ignore substrates and their dramatic influence on electronic properties of MOFs. Here, we show how substrates alter the correlated magnetic phases in kagome MOFs using systematic density functional theory and mean-field Hubbard calculations. We d…
The coexistence of the strongly correlated 3$d$ transition metal (TM) atom and the strong spin-orbit coupling (SOC) of 5$d$ TM atom is potential to host exotic physical phenomena. In the present work, we have studied the magnetism resulting from such a coexistence in La$_2$CoIrO$_6$ (LCIO), a representative of bulk 3$d$-5$d$ double perovskites. In order to gain further insight into the effect of Co-$d$ states on the Ir-$d$ states, comparisons are carried out with the isostructural La$_2$ZnIrO$_6$, where nonmagnetic Zn atom replaces the Co atom. An in-depth analysis of the magnetic states in th…
Strange-metal phenomena often develop at the border of antiferromagnetic order in strongly correlated metals. It has been well established that they can originate from the fluctuations anchored by the point of continuous quantum phase transition out of the antiferromagnetic order, i.e., a quantum critical point. What has been unclear is how these phenomena can be associated with a potential new phase of matter at zero temperature. Here we show that magnetic frustration of the 4f-local moments in the distorted Kagome intermetallic compound CePdAl gives rise to such a paramagnetic quantum-critic…
Density functional theory analysis of the interplay between Jahn-Teller instability, uniaxial magnetism, spin arrangement, metal-metal interaction and spin-orbit coupling in Ca3CoMO6 (M = Co, Rh, Ir)
Kagome metals possess peculiar optical spectra consisting of contributions from free charge carriers in a Drude-type response, localized carriers seen as a strongly temperature-dependent localization peak, and, in some cases, phonons displaying strong anomalies. The rare-earth kagome metal series, $R$Mn$_6$Sn$_6$, provides a marvelous playground to study the electronic properties of kagome metals in the presence of variable magnetic order. Here, we report temperature-dependent reflectivity studies on two members of the $R$Mn$_6$Sn$_6$ family, GdMn$_6$Sn$_6$ (in-plane ferrimagnet) and TbMn$_6$S…
Based on the available literature reviewed are results pertinent to the effect of magnetism on lattice dynamics obtained by Mössbauer spectroscopy at 57Fe and 119Sn probe atoms. Presented and discussed are sigma-phase Fe-Cr, Fe-V, Fe-Cr-Ni and lambda-phase (C14 Laves) NbFe2 compounds that belong to the Frank-Kasper family of phases, metallic chromium and iron mono-arsenide, FeAs. The common feature of all these metallic systems is itinerant character of their magnetism. Two spectral parameters relevant to the lattice dynamics viz. the center shift, CS, and/or the recoil-free fraction, f, are c…
Ground state properties of the spin$-1/2$ Falicov-Kimball model on a triangular lattice in the presence of uniform external magnetic field are explored. Both the orbital and the Zeeman field-induced effects are taken into account and in each unit cell only rational flux fractions are considered. Numerical results, obtained with the help of Monte Carlo simulation algorithm, reveal that the ground state properties strongly depend on the onsite Coulomb correlation between itinerant and localized electrons, orbital magnetic field as well as the Zeeman splitting. Strikingly, for the on-site Coulomb…
Intricacies of Frustrated Magnetism in the Kondo Metal YbAgGe
PSI Center for Neutron and Muon Sciences（保罗谢勒研究所中子与μ子科学中心）；Department of Physics and Astronomy, Iowa State University（爱荷华州立大学物理与天文系）；Ames National Laboratory（艾姆斯国家实验室）；Neutron Scattering Division, Oak Ridge National Laboratory（橡树岭国家实验室中子散射部）；Dresden High Magnetic Field Laboratory (HLD-EMFL), Helmholtz-Zentrum Dresden-Rossendorf（德累斯顿强磁场实验室（HLD-EMFL），赫尔霍兹德累斯顿-罗斯多夫中心）；ISIS Facility, STFC Rutherford Appleton Laboratory（STFC卢瑟福阿普尔顿实验室ISIS设施）；Institut Laue-Langevin（朗之万研究所）
Covalent bonds against magnetism in transition metal compounds
过渡金属化合物的磁性通常从描述孤立离子开始，尽可能准确，并在稍后阶段处理它们的（交换）相互作用。我们发现，这种标准的方法可能会打破在许多情况下，特别是在$4d$和$5d$化合物。我们认为，有一个重要的intersite效应-共价金属-金属键的轨道选择性的形成，这导致了“排斥”相应的电子从磁性子系统，从而强烈影响系统的磁性。这种效应在非整数电子数时尤为突出，当它导致著名的双交换抑制时，双交换是过渡金属化合物铁磁性的主要机制。我们通过分析和数值方法研究了这种新型机制，并表明它不仅解释了几种$4d-5d$材料的磁性，包括Nb $_2$ O $_2$ F $_3$和Ba $_5$ AlIrPDMATHABMP O $_{11}$，而且还可以在$3d$过渡金属氧化物中发挥作用，例如在压力下的CrO $_2$中。我们还讨论了自旋-轨道耦合在共价性和磁性竞争中的作用。我们的研究结果表明，强的intersite耦合可能会使考虑磁性的标准单站点起点失效，并可能导致一个定性的新行为。
Spontaneous order of layer pseudospins in two-dimensional bilayers is common in quantum Hall systems, where it is responsible for hysteretic responses to gate fields in states with Ising order, and giant drag voltages in states with XY (spontaneous inter-layer phase coherence) order. In this article we predict that layer pseudospin order will also occur in double-moiré strongly correlated two-dimensional electron systems. We comment on similarities and differences in the competition between the two types of order in quantum Hall and double-moiré systems, and relate our findings to previous wor…
We employ electron paramagnetic resonance (EPR) of the spin probe Mn2+ to study the paraelectric ferroelectric transition in DMAMnF and Mn2 doped DMZnF, which are considered to be model metal organic frameworks (MOF) with a Pb free perovskite architecture. In DMAMnF, we study the variation of the Mn2+ EPR line shape and intensity at the X-band (9.4 GHz) and over 80 to 300 K, and we show the absence of magnetoelectric coupling at the ferroelectric transition. At the antiferromagnetic transition in DMMnF, we detect a magnetoelectric coupling caused by weak ferromagnetism in the AFM phase. In DMZ…
The binary intermetallic materials, $M_3$Sn$_2$ ($M$ = 3d transition metal) present a new class of strongly correlated systems that naturally allows for the interplay of magnetism and metallicity. Using first principles calculations we confirm that bulk Fe$_3$Sn$_2$ is a ferromagnetic metal, and show that $M$ = Ni and Cu are paramagnetic metals with non-trivial band structures. Focusing on Fe$_3$Sn$_2$ to understand the effect of enhanced correlations in an experimentally relevant atomistically thin single kagome-bilayer, our ab-initio results show that dimensional confinement naturally expose…
We report on the evolution of the thermal metal-insulator transition in polycrystalline samples of Nd$_2$Ir$_2$O$_7$ upon hole-doping via substitution of Ca$^{2+}$ for Nd$^{3+}$. Ca substitution mediates a filling-controlled Mott-like transition with minimal resolvable structural changes and without altering site symmetry. Local structure confirms that Ca substitution does not result in local chemical phase separation, and absorption spectroscopy establishes that Ir cations maintain a spin-orbit entangled electronic configuration. The metal-insulator transition coincides with antiferromagnetic…
Electronic properties of the single crystal of GdV6Sn6, where non-magnetic V-kagome layers are separated by magnetic Gd-triangular lattice, are investigated. GdV6Sn6 exhibits unique magnetotransport properties at low-temperature such as non-linear Hall resistivity and increase of resistance R in magnetic field H as R ~ H^0.75 up to 56 T with Shubnikov-De Haas oscillations. Investigation of the non-magnetic analogue YV6Sn6 and the first principles calculations reveal these properties are relevant to the bands arising from the V-kagome layer. A magnetic transition at 5 K in GdV6Sn6 modifies the …
Block spin magnetism and metal-insulator transition in a two-dimensional Hubbard model with perfect vacancy superstructure
本文研究了具有完全空位超结构的正方晶格哈伯德模型的相图。该模型也可以定义在一个新的二分格上，每个构建块由一个最小正方形组成。精确求解了非相互作用模型，在弱的块间跳跃区，费米能级处出现了中带隙。增加库仑相互作用将发展具有不同块自旋矩的Néel反铁磁序。在磁不稳定性$U_{\rm M}$之上发生了$U_{\rm MI}$小于无空位的金属-绝缘体转变。新兴的中间磁性金属相发展基本上在温和的块间跳跃制度。在非相互作用的带状绝缘体的边缘上的有序矩和间隙幅度的急剧增加表明磁性半导体和莫特绝缘体之间可能存在区别。这些结果对最近发现的（A，Tl）$_{y}$ Fe $_{2-x}$ Se $_2$化合物的意义进行了讨论.
Understanding the controlling principles of band gaps trends in d electron perovskites is needed both for gauging metal-insulator transitions, as well as their application in catalysis and doping. The magnitude of this band gap is rather different for different magnetic spin configurations. We find via electronic structure theory that the factors that connect gapping magnitudes to magnetism depend on the nature of the band edge orbital character (BEOC) and surprisingly scale with the number of antiferromagnetic contacts z$_i$ between neighboring transition metal ions. The dependence is weak wh…
Metallic ferromagnets with strongly interacting electrons often exhibit remarkable electronic phases such as ferromagnetic superconductivity, complex spin textures, and nontrivial topology. In this report, we discuss the synthesis of a layered magnetic metal NiTa$_4$Se$_8$ (or Ni$_{1/4}$TaSe$_{2}$) with a Curie temperature of 58 Kelvin. Magnetization data and \textit{ab initio} calculations indicate that the nickel atoms host uniaxial ferromagnetic order of about 0.7$μ_{B}$ per atom, while an even smaller moment is generated in the itinerant tantalum conduction electrons. Strong correlations a…
