Structure-property relations in pnictides and chalcogenides of Eu2+

In this thesis, the structure-properties relations in pnictides and chalcogenides of divalent Eu were elaborated. Interest to its compounds was due to the high magnetic moment of Eu2+ cations, owing to a half-filled 4f shell. It was expected to induce unusual magnetic states and to have a substantial impact on the physics of the matrixes, in which it is nested. The highly-localized nature of the 4f electrons precludes substantial direct magnetic exchange or superexchange, but interactions through the conducting frameworks can be sizable and lead to non-trivial magnetic states. The crystallographic similarity of Eu2+ and Sr2+ was a tool for predicting the new materials. The synthetic work resulted in the preparation of more than ten new compounds, including rare fluorides belonging to the 1111 structure type and the first magnetic tetrel-free clathrate EuNi2P4. Crystallographic and synthetic work were accompanied by physical properties measurements – temperature-dependent magnetization, heat capacity, and resistivity. Neutron powder diffraction was instrumental for establishing the natures of the low-temperature magnetically ordered phases despite the strong neutron absorption of natural Eu

Ferromagnetic quantum critical point in a locally noncentrosymmetric and nonsymmorphic Kondo metal

Quantum critical points (QCPs), zero-temperature phase transitions, are windows to fundamental quantum-mechanical phenomena associated with universal behaviour and can provide parallels to the physics of black holes. Magnetic QCPs have been extensively investigated in the vicinity of antiferromagnetic order. However, QCPs are rare in metallic ferromagnets due to the coupling of the order parameter to electronic soft modes [1,2]. Recently, antisymmetric spin-orbit coupling in noncentrosymmetric systems was suggested to protect ferromagnetic QCPs [3]. Nonetheless, multiple centrosymmetric materials host FM QCPs, suggesting a more general mechanism behind their protection. In this context, CeSi$_{2-\delta}$, a dense Kondo lattice crystallising in a centrosymmetric structure, exhibits ferromagnetic order when Si is replaced with Ag. We report that the Ag-substitution controls the strength of the Kondo coupling, leading to a transition between paramagnetic and ferromagnetic Kondo phases. Remarkably, a ferromagnetic QCP accompanied by concurrent strange-metal behaviour emerges. Herein, we suggest that, despite the centrosymmetric structure, spin-orbit coupling arising from the local noncentrosymmetric structure, in combination with nonsymmorphic symmetry, can protect ferromagnetic QCPs. Our findings present a unique example of Kondo coupling-driven ferromagnetic QCP through chemical doping and offer a general guideline for discovering new ferromagnetic QCPs.Comment: Main text with 3 figures and 1 table, and supplementary informatio

On the magnetic structures of 1:1:1 stoichiometric topological phases LnSbTe (Ln = Pr, Nd, Dy and Er)

LnSbTe (Ln - lanthanide) group of materials, belonging to ZrSiS/PbFCl (P4/nmm) structure type, is a platform to study the phenomena originating from the interplay between the electronic correlations, magnetism, structural instabilities and topological electronic structure. Here we report a systematic study of magnetic properties and magnetic structures of LnSbTe materials. The studied materials undergo antiferromagnetic ordering at TN = 2.1 K (Ln = Er), 6.7 K (Ln = Dy), 3.1 K (Ln = Nd). Neutron powder diffraction reveals ordering with k1 = (1/2 + d 0 0) in ErSbTe, k2 = (1/2 0 1/4) in NdSbTe. DySbTe features two propagation vectors k2 and k4 = (0 0 1/2). No long-range magnetic order is observed in PrSbTe down to 1.8 K. We propose the most probable models of magnetic structures, discuss their symmetry and possible relation between the electronic structure and magnetic ordering.Comment: 21 pages, 10 figure

Magnetic and crystal structure of the antiferromagnetic skyrmion candidate GdSb0.71Te1.22

GdSb0.46Te1.48, a nonsymmorphic Dirac semimetal with Dirac nodes at the Fermi level, has a rich magnetic phase diagram with one of the phases predicted to be an antiferromagnetic skyrmion state. In the current work, we investigate GdSb0.71Te1.22 through bulk magnetization measurements, single-crystal, and powder synchrotron X-ray diffraction, as well as single-crystal hot-neutron diffraction. We resolve a weak orthorhombic distortion with respect to the tetragonal structure and charge density wave (CDW) satellites due to incommensurate modulations of the crystal structure. At 2 K the magnetic structure is modulated with two propagation vectors, kI = (0.45 0 0.45) and kII = (0.4 0 0), with all their arms visible. While kI persists up to the transition to the paramagnetic state at TN = 11.9 K, kII disappears above an intermediate magnetic transition at T1 = 5 K. Whereas magnetic field applied along the c-axis has only a weak effect on the intensity of antiferromagnetic reflections, it is effective in inducing an additional ferromagnetic component on Gd atoms. We refine possible magnetic structures of GdSb0.71Te1.22 and discuss the possibility of hosting magnetic textures with non-trivial 3D+ 2 topologies in the GdSb1−xTe1+x series.ISSN:0925-8388ISSN:1873-466

New layered compounds BaFMgPn (Pn= P, As, Sb and Bi), transition-metal-free representatives of the LaOAgS structure

Four new transition metal-free pnictide representatives of the LaOAgS structure type were predicted by DFT calculations and found in the BaFMgPn (Pn = P, As, Sb and Bi) family. The compounds adopt the tetragonal space group P4/nmm with the unit cell parameters a/c 4.3097(1) angstrom/9.5032(1) angstrom, 4.3855(1) angstrom/9.5918(1) angstrom, 4.5733(1) angstrom/9.8184(1) angstrom, and 4.6359(1) angstrom/9.8599(1) angstrom, respectively. According to the DFT calculations, these new compounds are semiconductors with band gaps steadily decreasing from Pn = P (ca. 2 eV) to Pn = Bi (ca. 1 eV). The corresponding strontium fluoride and rare-earth oxide analogs are unlikely to exist and have not been observed yet. The trends of the stability within 1111 and structurally and/or chemically related compounds based on a combined consideration of geometry and DFT calculations are discussed

Phonon promoted charge density wave in topological kagome metal ScV6_{6}Sn6_{6}

Charge density wave (CDW) orders in vanadium-based kagome metals have recently received tremendous attention due to their unique properties and intricate interplay with exotic correlated phenomena, topological and symmetry-breaking states. However, the origin of the CDW order remains a topic of debate. The discovery of ScV$_{6}$Sn$_{6}$, a vanadium-based bilayer kagome metal exhibiting an in-plane $\sqrt{3}$ x $\sqrt{3}$ $\textit{R}$30${\deg}$ CDW order with time-reversal symmetry breaking, provides a novel platform to explore the underlying mechanism behind the unconventional CDW. Here, we combine high-resolution angle-resolved photoemission spectroscopy, Raman scattering measurements and density functional theory to investigate the electronic structures and phonon modes of ScV$_{6}$Sn$_{6}$ and their evolution with temperature. We identify topologically nontrivial Dirac surface states and multiple van Hove singularities (VHSs) in the vicinity of the Fermi level, with one VHS near the K point exhibiting nesting wave vectors in proximity to the $\sqrt{3}$ x $\sqrt{3}$ $\textit{R}$30${\deg}$ CDW wave vector. Additionally, Raman measurements indicate a strong intrinsic electron-phonon coupling in ScV$_{6}$Sn$_{6}$, as evidenced by the presence of a two-phonon mode and a large frequency amplitude mode. Our findings highlight the fundamental role of lattice degrees of freedom in promoting the CDW in ScV$_{6}$Sn$_{6}$ and provide important insights into the fascinating correlation phenomena observed in kagome metals

Litharge-derived compounds structurally based on layers of Cl− and Br−-centered tetrahedra: Synthesis and structures of the new representatives of MX(ReO4) family (M=Ba, Pb; X=Cl, Br)

International audienc

Competing magnetic phases in LnSbTe (Ln = Ho and Tb)

The interplay between topological electronic structure and magnetism may result in intricate physics. In this work, we describe a case of rather peculiar coexistence or competition of several magnetic phases below a seemingly single antiferromagnetic transition in LnSbTe (Ln = Ho and Tb) topological semimetals, the magnetic members of the ZrSiS/PbFCl structure type (space group P4/nmm). Neutron diffraction experiments reveal a complex multi-step order below TN = 3.8 K (Ln = Ho) and TN = 6.4 K (Ln = Tb). Magnetic phases can be described using four propagation vectors: k1 = (1/2 0 0) and k2 = (1/2 0 1/4) at the base temperature of 1.7 K, which transform into incommensurate vectors k1' = (1/2 - d 0 0), k3 = (1/2 - d 0 1/2) at elevated temperatures in both compounds. Together with the refined models of magnetic structures, we present the group-theoretical analysis of the magnetic symmetry of the proposed solutions. These results prompt further investigations of the relation between the electronic structure of those semimetals and the determined antiferromagnetic ordering existing therein

Expanding Family of Litharge-Derived Sulfate Minerals and Synthetic Compounds: Preparation and Crystal Structures of [Bi2CuO3]SO4 and [Ln2O2]SO4 (Ln = Dy and Ho)

During the last decades, layered structures have attracted particular and increasing interest due to the multitude of outstanding properties exhibited by their representatives. Particularly common among their archetypes, with a significant number of mineral and synthetic species structural derivatives, is that of litharge. In the current paper, we report the structural studies of two later rare-earth oxysulfates, [Ln(2)O(2)]SO4 (Ln = Dy, Ho), which belong indeed to the grandreefite family, and a novel compound [Bi2CuO3]SO4, which belongs to a new structure type and demonstrates the second example of Cu2+ incorporation into litharge-type slabs. Crystals of [Bi2CuO3]SO4 were obtained under high-pressure/high-temperature (HP/HT) conditions, whereas polycrystalline samples of [Ln(2)O(2)]SO4 (Ln = Dy, Ho) compounds were prepared via an exchange solid-state reaction. The crystal structure of [Bi2CuO3]SO4 is based on alternation of continuous [Bi2CuO3](2+) layers of edge-sharing OBi2Cu2 and OBi3Cu tetrahedra and sheets of sulfate groups. Cu2+ cations are in cis position in O5Bi(2)Cu(2) and O6Bi(2)Cu(2) oxocentered tetrahedra in litharge slab. The crystal structure of [Ln(2)O(2)]SO4 (Ln = Dy, Ho) is completely analogous to those of grandreefite and oxysulfates of La, Sm, Eu, and Bi