We study the magnetic properties of the adatom systems on a semiconductor surface Si(111):{C,Si,Sn,Pb} -(On the basis of all-electron density functional theory calculations we construct effective low-energy models taking into account spin-orbit coupling and electronic correlations. In the ground state the surface nanostructures are found to be insulators with the non-collinear 120• Néel (for C, Si, Sn monolayer coverages) and 120• row-wise (for Pb adatom) antiferromagnetic orderings. The corresponding spin Hamiltonians with anisotropic exchange interactions are derived by means of the superexchange theory and the calculated Dzyaloshinskii-Moriya interactions are revealed to be very strong and compatible with the isotropic exchange couplings in the systems with Sn and Pb adatoms. To simulate the excited magnetic states we solve the constructed spin models by means of the Monte Carlo method. At low temperatures and zero magnetic field we observe complex spin spiral patterns in Sn/Si(111) and Pb/Si(111). On this basis the formation of antiferromagnetic skyrmion lattice states in adatom sp electron systems in strong magnetic fields is discussed.

A quantitative description of the excited electronic states of point defects and impurities is crucial for understanding materials properties, and possible applications of defects in quantum technologies. This is a considerable challenge for computational methods, since Kohn-Sham density-functional theory (DFT) is inherently a ground state theory, while higher-level methods are often too computationally expensive for defect systems. Recently, embedding approaches have been applied that treat defect states with many-body methods, while using DFT to describe the bulk host material. We implement such an embedding method, based on Wannierization of defect orbitals and the constrained random-phase approximation approach, and perform systematic characterization of the method for three distinct systems with current technological relevance: a carbon dimer replacing a B and N pair in bulk hexagonal BN (CBCN), the negatively charged nitrogen-vacancy center in diamond (NV − ), and an Fe impurity on the Al site in wurtzite AlN (Fe Al ). For CBCN we show that the embedding approach gives many-body states in agreement with analytical results on the Hubbard dimer model, which allows us to elucidate the effects of the DFT functional and doublecounting correction. For the NV − center, our method demonstrates good quantitative agreement with experiments for the zero-phonon line of the triplet-triplet transition. Finally, we illustrate challenges associated with this method for determining the energies and orderings of the complex spin multiplets in Fe Al .

The pathway toward the tailored synthesis of materials starts with precise characterization of the conformational properties and dynamics of individual molecules. Electron spin resonance based scanning tunneling microscopy can potentially address molecular structure with unprecedented resolution. Here, we determine the fine structure and geometry of an individual TiH molecule, utilizing a combination of a newly developed mK ESR-STM in a vector magnetic field and ab initio approaches. We demonstrate a strikingly large anisotropy of the g-tensor unusual for a spin doublet ground state, resulting from a non-trivial orbital angular momentum. We quantify the relationship between the resultant fine structure, hindered rotational modes, and orbital excitations. Our model system provides new avenues to determine the structure and dynamics of individual molecules with unprecedented precision. Main textPrecisely determining the fine structure, dynamics, and geometry of an individual molecule, with sub-molecular resolution, is a grand challenge in numerous fields of nanoscience.Scanning probe microscopy (SPM) has emerged as a surface imaging approach capable of intramolecular resolution of individual molecules [1, 2], quantifying conformational modifications like the static Jahn-Teller distortion [3], or light-assisted conformational changes [4]. Complementary to imaging, SPM-based inelastic excitation spectroscopy (ISTS) has been successfully applied to infer the various intramolecular vibrational [5], rotational [6, 7] or hindered rotational modes [8]. However, these methods lack the precision to quantify the interplay between structure and molecular geometry like methods such as electron spin resonance (ESR) [9, 10]. These methods are also not well suited for studying low-energy dynamics, such as the quantum zero-point motion of hydrogen and other light elements that are quenched by strong tip-sample interactions. Moreover, the resolution of traditional SPM, particularly scanning tunneling microscopy (STM), is limited by both convolution [1, 11, 12] and current preamplifier related bandwidth issues that preclude insight into the structure and rotational dynamics of individual molecules. Hybrid methods have recently emerged, combining the spatial resolution of STM with temporal resolution [13, 14] driven by continuous wave excitation [15]. THz-based STM [16, 17] has been used to excite and quantify the vibrational motion of an individual phthalocyanine molecule with picosecond precision [18]. Likewise, electron paramagnetic/spin resonance has been established [15, 19, 20], based on a combination of microwave excitation of the STM junction, with the detection of spin-polarized current [21] of individual atoms. This technique, referred to as ESR-STM, has been used to quantify magnetic interactions, hyperfine couplings, and the coherent dynamics of individual magnetic impurities with unprecedented resolution [22-24]. However, in the spirit of traditional EPR/ESR, ESR-STM has yet to be applied to infer the molecular str...

Magnetic behavior of yavapaiite-type BaMoP2O8 with the spatially anisotropic triangular arrangement of the S = 1 Mo 4+ ions is explored using thermodynamic measurements, neutron diffraction, and density-functional band-structure calculations. A broad maximum in the magnetic susceptibility around 46 K is followed by the stripe antiferromagnetic order with the propagation vector k = ( 1 2 , 1 2 , 1 2 ) formed below TN 21 K. This stripe phase is triggered by a pronounced onedimensionality of the spin lattice, where one of the in-plane couplings, J2 4.6 meV, is much stronger than its J1 0.4 meV counterpart, and stabilized by the weak easy-axis anisotropy. The ordered moment of 1.42(9) µB at 1.5 K is significantly lower than the spin-only moment of 2 µB due to a combined effect of quantum fluctuations and spin-orbit coupling.

We study electronic and magnetic properties of the quasi-one-dimensional spin-1 2 magnet Ba3Cu3Sc4O12 with a distinct orthogonal connectivity of CuO4 plaquettes. An effective low-energy model taking into account spin-orbit coupling was constructed by means of first-principles calculations. On this basis a complete microscopic magnetic model of Ba3Cu3Sc4O12, including symmetric and antisymmetric anisotropic exchange interactions, is derived. The anisotropic exchanges are obtained from a distinct first-principles numerical scheme combining, on one hand, the local density approximation taking into account spin-orbit coupling, and, on the other hand, projection procedure along with the microscopic theory by Toru Moriya. The resulting tensors of the symmetric anisotropy favor collinear magnetic order along the structural chains with the leading ferromagnetic coupling J1 −9.88 meV. The interchain interactions J8 0.21 meV and J5 0.093 meV are antiferromagnetic. Quantum Monte Carlo simulations demonstrated that the proposed model reproduces the experimental Neel temperature, magnetization and magnetic susceptibility data. The modeling of neutron diffraction data reveals an important role of the covalent Cu-O bonding in Ba3Cu3Sc4O12.

Synthesis, thermodynamic properties, and microscopic magnetic model of ilinskite-type KCu5O2(SeO3)2Cl3 built by corner-sharing Cu4 tetrahedra are reported, and relevant magnetostructural correlations are discussed. Quasi-one-dimensional magnetic behavior with the short-range order around 50 K is rationalized in terms of weakly coupled spin ladders (tubes) having a complex topology formed upon fragmentation of the tetrahedral network. This fragmentation is rooted in the non-trivial effect of the SeO3 groups that render the Cu–O–Cu superexchange strongly ferromagnetic even at bridging angles exceeding 110°.

In this work, we investigate intrinsic magnetic properties of monolayer electrides LaBr2 and La2Br5, where excess electrons do not reside at any atomic orbital and act as anions located at interstitial regions. Having demonstrated that conventional first-principles approaches are incapable of treating such non-atomic magnetic orbitals largely underestimating insulating band gaps, we construct effective electronic models in the basis of Wannier functions associated with the anionic states to unveil the microscopic mechanism underlying magnetism in these systems. Being confined at zero-dimensional cavities in the crystal structure, the anionic electrons will be shown to reveal an exotic duality of strong localisation like in d-and f -electron systems and large spatial extension inherent to delocalised atomic orbitals. While the former tends to stabilise a Mott-insulating state with localised magnetic moments, the latter results in direct exchange between neighbouring anionic electrons that dominates over antiferromagnetic superexchange interactions. On the basis of the derived spin models, we argue that any long-range magnetic order is prohibited in LaBr2 by Mermin-Wagner theorem, while intersite anisotropy in La2Br5 stabilises weakly coupled ferromagnetic chains along the monoclinic b axis. Our study shows that electride materials combining peculiar features of both localised and delocalised atomic states constitute a unique class of strongly correlated materials.Introduction. Electrides are a unique class of ionic materials, where the electron density is neither localised at any atomic orbital, nor fully delocalised like in metals. Instead, their electrons occupy interstitial regions formed by cavities in the crystal structure, where they act as anions [1, 2]. Materials with anionic electrons offer versatile functionalities, such as high electrical conductivity [3], ultra-low work functions [4], and non-linear optical responses [5], ranging their applications as electron emitters [6], battery anodes [7], and agent catalysts [8].Their physical properties are in large part determined by topology of the voids confining anionic electrons. In 2003, Matsuishi et al [9] demonstrated the first inorganic material [Ca 24 Al 28 O 64 ] +4 ·4e − stable at room temperature, which shows the high density of anionic electrons trapped in the crystallographic cages, forming a quasizero-dimensional electride with high electronic conductivity due to tunnelling through the cages [10, 11]. Recently, a novel layered electride [Ca 2 N] + ·e − has been reported, where owing to a highly delocalised nature the quasi-two-dimensional anionic electrons confined in the interlayer regions display an extreme electron mobility and long mean scattering time [12, 13]. Later on, strong localisation of anionic electrons has been observed in a layered transition-metal hypocarbide [Y 2 C] 2+ ·2e − with the quasi-two-dimensional anionic electrons in the interlayer spaces [14, 15]. Finally, it was shown that Ca 2 N can be exfoliated into two-dimensional...

We present an extensive experimental and theoretical study on the low-temperature magnetic properties of the monoclinic anhydrous alum compound BaMo(PO4)2. The magnetic susceptibility reveals strong antiferromagnetic interactions θCW = −167 K and long-range magnetic order at TN = 22 K, in agreement with a recent report. Powder neutron diffraction furthermore shows that the order is collinear, with the moments near the ac plane. Neutron spectroscopy reveals a large excitation gap ∆ = 15 meV in the low-temperature ordered phase, suggesting a much larger easyaxis spin anisotropy than anticipated. However, the large anisotropy justifies the relatively high ordered moment, Néel temperature, and collinear order observed experimentally, and is furthermore reproduced in a first principles calculations using a new computational scheme. We therefore propose BaMo(PO4)2 to host S = 1 antiferromagnetic chains with large easy-axis anisotropy, which has been theoretically predicted to realize novel excitation continua. arXiv:1912.03969v1 [cond-mat.str-el]