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The properties of kagome metals are governed by the interdependence of band topology and electronic correlations resulting in remarkably rich phase diagrams. Here, we study the temperature evolution of the bulk electronic structure of the antiferromagnetic kagome metal FeGe using infrared spectroscopy. We uncover drastic changes in the low-energy interband absorption at the 100 K structural phase transition that has been linked to a charge-density-wave (CDW) instability. We explain this effect by the minuscule Fe displacement in the kagome plane, which results in parallel bands in the vicinity of the Fermi level. In contrast to conventional CDW materials, however, the spectral weight shifts to low energies, ruling out the opening of a CDW gap in FeGe.

Plasmon polaritons, or plasmons, are coupled oscillations of electrons and electromagnetic fields that can confine the latter into deeply subwavelength scales, enabling novel polaritonic devices. While plasmons have been extensively studied in normal metals or semimetals, they remain largely unexplored in correlated materials. In this paper, we report infrared (IR) nano-imaging of thin flakes of CsV 3 Sb 5 , a prototypical layered Kagome metal. We observe propagating plasmon waves in real-space with wavelengths tunable by the flake thickness. From their frequency-momentum dispersion, we infer the out-of-plane dielectric function ϵ c that is generally difficult to obtain in conventional far-field optics, and elucidate signatures of electronic correlations when compared to density functional theory (DFT). We propose correlation effects might have switched the real part of ϵ c from negative to positive values over a wide range of middle-IR frequencies, transforming the surface plasmons into hyperbolic bulk plasmons, and have dramatically suppressed their dissipation., (© 2024. The Author(s).)

Materials properties are determined by their compositions and structures. In ABO 3 oxides different cation orderings lead to mainly perovskite- or corundum like derivatives with exciting physical properties. Sometimes, a material can be stabilized in more than one structural modification, providing a unique opportunity to explore structure-properties relationship. Here, CoVO 3 obtained in both ilmenite-(CoVO 3 -I) and LiNbO 3 -type (CoVO 3 -II) polymorphs at moderate (8-12 GPa) and high pressures (22 GPa), respectively are presented. Their distinctive cation distributions affect drastically the magnetic properties as CoVO 3 -II shows a cluster-glass behavior while CoVO 3 -I hosts a honeycomb zigzag magnetic structure in the cobalt network. First principles calculations show that the influence of vanadium is crucial for CoVO 3 -I, although it is previously considered as non-magnetic in a dimerized spin-singlet state. Contrarily, CoVO 3 -II shows two independent interpenetrating antiferromagnetic Co- and ferromagnetic V-hcp sublattices, which intrinsically frustrate any possible magnetic order. CoVO 3 -II is also remarkable as the first oxide crystallizing with the LiNbO 3 -type structure where both metals contain free d electrons. CoVO 3 polymorphs pinpoint therefore as well to a much broader phase field of high-pressure A-site Cobaltites., (© 2023 The Authors. Advanced Science published by Wiley-VCH GmbH.)

A new guanidinium-templated hydrated iron sulfate, [CN 3 H 6 ][Fe II Fe III (SO 4 ) 3 (H 2 O) 3 ] ( 1 ), was prepared from strongly acidic aqueous solutions. Its crystal structure is comprised from Fe III O 6 and Fe II O 3 (H 2 O) 3 octahedra linked by sulfate bridges forming a [Fe II Fe II (SO 4 ) 3 (H 2 O) 3 ] - 3 D framework with a layer-by-layer ordering of ferric and ferrous cations. The structural topology of the framework is related to the anhydrous rhombohedral mikasaite Fe 2 (SO 4 ) 3 . The removal of part of the sulfate tetrahedra and the partial replacement of the Fe 3+ cations in the [Fe 3+ 2 (SO 4 ) 3 ] 0 framework by Fe 2+ provide a negative charge and allow the incorporation of the protonated organic species in the voids. The compound 1 has been characterized by single-crystal X-ray diffraction, TG and DSC analyses, UV-vis-NIR spectroscopy, magnetic susceptibility, Mössbauer spectroscopy, IR and Raman spectroscopy, and density functional band-structure calculations. The magnetic behavior of 1 shows an interplay of Fe II ( S = 2) and Fe III ( S = 5/2) sublattices that exhibit different types of antiferromagnetic couplings, one Fe III -Fe III ( J 1 ∼ 6.1 K) and two Fe II -Fe III couplings ( J 2 ∼ 1 K, J 3 ∼ 5.9 K) within corrugated honeycomb layers. These ferrimagnetic layers are coupled antiparallel to each other, resulting in an overall antiferromagnetic order below T N = 31 K.

The 1111 compounds with an alternating sequence of fluorite and antifluorite layers serve as structural hosts for the vast family of Fe-based superconductors. Here, we use neutron powder diffraction and density-functional-theory (DFT) band-structure calculations to study magnetic order of Eu 2+ in the [EuF] + fluorite layers depending on the nature of the [TAs] - antifluorite layer that can be non-magnetic semiconducting (T = Zn), magnetic semiconducting (T = Mn), or magnetic metallic (T = Fe). Antiferromagnetic transitions at T N ∼ 2.4-3 K due to an ordering of the Eu 2+ magnetic moments were confirmed in all three EuTAsF compounds. Whereas in EuTAsF (T = Zn and Mn), the commensurate k 1 = (½ ½ 0) stripe order pattern with magnetic moments within the a - b plane is observed, the order in EuFeAsF is incommensurate with k = (0 0.961(1) ½) and represents a cycloid of Eu 2+ magnetic moments confined within the bc -plane. Additionally, the Mn 2+ sublattice in EuMnAsF features a robust G-type antiferromagnetic order that persists at least up to room temperature, with magnetic moments along the c -direction. Although DFT calculations suggest stripe antiferromagnetic order in the Fe-sublattice of EuFeAsF as the ground state, neutron diffraction reveals no evidence of long-range magnetic order associated with Fe. We show that the frustrating interplane interaction J 3 between the adjacent [EuF] + layers is comparable with in-plane J 1 - J 2 interactions already in the case of semiconducting fluorite layers [TAs] - (T = Zn and Mn) and becomes dominant in the case of the metallic [FeAs] - ones. The latter, along with a slight orthorhombic distortion, is proposed to be the origin of the incommensurate magnetic structure observed in EuFeAsF.

A new copper vanadyl arsenate, Cu(VO) 2 (AsO 4 ) 2 , was synthesized via the chemical vapor transport method. Cu(VO) 2 (AsO 4 ) 2 adopts an original structure type. It is characterized by layers formed by edge-sharing and corner-sharing V-centered octahedra resulting in a unique topology that was hitherto not reported for vanadates. Single CuO 6 octahedra connect vanadate layers into a rigid framework. The thermal expansion of the framework studied by the single-crystal HT X-ray diffraction is reported. The magnetic behavior of Cu(VO) 2 (AsO 4 ) 2 shows an interplay of ferromagnetic V 4+ -V 4+ and antiferromagnetic Cu 2+ -V 4+ interactions that result in a ferrimagnetic long-range order below T C = 66 K.

Polymorphism is observed in the Y 3+ x Rh 4 Ge 13- x series. The decrease of Y-content leads to the transformation of the primitive cubic Y 3.6 Rh 4 Ge 12.4 [ x = 0.6, space group Pm 3̄ n , a = 8.96095(9) Å], revealing a strongly disordered structure of the Yb 3 Rh 4 Sn 13 Remeika prototype, into a body-centred cubic structure [La 3 Rh 4 Sn 13 structure type, space group I 4 1 32, a = 17.90876(6) Å] for x = 0.4 and further into a tetragonal arrangement (Lu 3 Ir 4 Ge 13 structure type, space group I 4 1 / amd , a = 17.86453(4) Å, a = 17.91076(6) Å) for the stoichiometric ( i.e. x = 0) Y 3 Rh 4 Ge 13 . Analogous symmetry lowering is found within the Y 3+ x Ir 4 Ge 13- x series, where the compound with Y-content x = 0.6 is crystallizing with La 3 Rh 4 Sn 13 structure type [ a = 17.90833(8) Å] and the stoichiometric Y 3 Ir 4 Ge 13 is isostructural with the Rh-analogue [ a = 17.89411(9) Å, a = 17.9353(1) Å]. The structural relationships of these derivatives of the Remeika prototype are discussed. Compounds from the Y 3+ x Rh 4 Ge 13- x series are found to be weakly-coupled BCS-like superconductors with T c = 1.25, 0.43 and 0.6, for x = 0.6, 0.4 and 0, respectively. They also reveal low thermal conductivity (<1.5 W K -1 m -1 in the temperature range 1.8-350 K) and small Seebeck coefficients. The latter are common for metallic systems. Y 3 Rh 4 Ge 13 undergoes a first-order phase transition at T f = 177 K, with signatures compatible to a charge density wave scenario. The electronic structure calculations confirm the instability of the idealized Yb 3 Rh 4 Sn 13 -like structural arrangements for Y 3 Rh 4 Ge 13 and Y 3 Ir 4 Ge 13 .

Narrow-gap semiconductors are very rare among intermetallic compounds. They appear only when two factors come together: strong hybridization of valence orbitals in the vicinity of the Fermi level and an appropriate number of valence electrons. Surprisingly, the IrIn 3 family of intermetallics contains a number of semiconductors, including 17 e - FeGa 3 , RuGa 3 , OsGa 3 , and RuIn 3 , for which the d-p hybridization gap opens at the Fermi energy. We present comprehensive total energy electronic-structure calculations and crystal orbital Hamilton population analysis of the stable IrIn 3 -type compounds with semiconducting and metallic properties. The calculated electronic structures possess two pseudogaps and one real gap at the magic valence electron count of 15, 17, and 18 e - per formula unit. When the Fermi level is located in these gaps, the antibonding states are minimized. Total energies calculated for the isomorphous compounds suggest that the metallic state with 18 e - leads to a comparable or even higher thermodynamic stability than the semiconducting state with 17 e - .

(Na,K) 2 Cu 3 O(SO 4 ) 3 compounds form structural chains of Cu 6 hexameric units with nominal S = 1 spins due to the interplay between inner strong antiferromagnetic and ferromagnetic exchanges. We show here that the lattice relaxation after the replacement of alkali by larger Rb and Cs ones is accompanied by the insertion of neutral CuO species into (Rb,Cs) 2 Cu 3 O(CuO) x (SO 4 ) 3 phases. Structurally, interstitial CuO links the next two Cu 6 units in longer Cu 14 tetradecameric ones. For A = Cs ( x = 0.5), the cationic ordering is perfect inside a double-cell superstructure. Magnetically, the original Cu 14 units consist of frustrated fragments of an S = 1 / 2 spin ladder, with ferromagnetic rung-like but antiferromagnetic leg-like and next-nearest neighbor couplings. It returns S = 1 Cu 14 spin clusters, effective around 100 K. Our density functional theory calculations and susceptibility fits also show that at low temperatures they interact in two-dimensional lattices, despite the existence of short inter-Cu-Cu distances between the next two clusters along pseudo-one-dimensional chains.

In contrast to the corundum-type A 2 X 3 structure, which has only one crystallographic site available for trivalent cations (e.g., in hematite), the closely related ABX 3 ilmenite-type structure comprises two different octahedrally coordinated positions that are usually filled with differently charged ions (e.g., in Fe 2+ Ti 4+ O 3 ilmenite). Here, we report a synthesis of the first binary ilmenite-type compound fabricated from a simple transition-metal oxide (Mn 2 O 3 ) at high-pressure high-temperature (HP-HT) conditions. We experimentally established that, at normal conditions, the ilmenite-type Mn 2+ Mn 4+ O 3 (ε-Mn 2 O 3 ) is an n-type semiconductor with an indirect narrow band gap of E g = 0.55 eV. Comparative investigations of the electronic properties of ε-Mn 2 O 3 and previously discovered quadruple perovskite ζ-Mn 2 O 3 phase were performed using X-ray absorption near edge spectroscopy. Magnetic susceptibility measurements reveal an antiferromagnetic ordering in ε-Mn 2 O 3 below 210 K. The synthesis of ε-Mn 2 O 3 indicates that HP-HT conditions can induce a charge disproportionation in simple transition-metal oxides A 2 O 3 , and potentially various mixed-valence polymorphs of these oxides, for example, with ilmenite-type, LiNbO 3 -type, perovskite-type, and other structures, could be stabilized at HP-HT conditions.

We synthesized single crystals of marokite (CaMn 2 O 4 )-type orthorhombic manganese (II,III) oxide, γ-Mn 3 O 4 , in a multianvil apparatus at pressures of 10-24 GPa. The magnetic, electronic, and optical properties of the crystals were investigated at ambient pressure. It was found that γ-Mn 3 O 4 is a semiconductor with an indirect band gap E g of 0.96 eV and two antiferromagnetic transitions ( T N ) at ∼200 and ∼55 K. The phase stability of the γ-Mn 3 O 4 crystals was examined in the pressure range of 0-60 GPa using single-crystal X-ray diffraction and Raman spectroscopy. A bulk modulus of γ-Mn 3 O 4 was determined to be B 0 = 235.3(2) GPa with B ' = 2.6(6). The γ-Mn 3 O 4 phase persisted over the whole pressure range studied and did not transform or decompose upon laser heating of the sample to ∼3500 K at 60 GPa. This result seems surprising, given the high-pressure structural diversity of iron oxides with similar stoichiometries. With an increase in pressure, the degree of distortion of MnO 6 polyhedra decreased. Furthermore, there are signs indicating a limited charge transfer between the Mn 3+ ions in the octahedra and the Mn 2+ ions in the trigonal prisms. Our results demonstrate that the high-pressure behavior of the structural, electronic, and chemical properties of manganese oxides strongly differs from that of iron oxides with similar stoichiometries.

Mixed-valent transition-metal compounds display complex structural, electronic and magnetic properties, which often intricately coexist. Here, we report the new ternary oxide GaV 4 O 8 , a structural sibling of skyrmion-hosting lacunar spinels. GaV 4 O 8 contains a vanadium trimer and an original spin-orbital-charge texture that forms upon the structural phase transition at T S = 68 K followed by the magnetic transition at T N = 35 K. The texture arises from the coexistence of orbital molecules on the vanadium trimers and localized electrons on the remaining vanadium atoms. Such hybrid electrons create opportunities for novel types of spin, charge, and orbital order in mixed-valent transition-metal compounds.

We report the observation of an antipolar phase in cubic GaNb&#95;{4}S&#95;{8} driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb&#95;{4}S&#95;{4} molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at T&#95;{JT}=31  K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above T&#95;{JT}. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb&#95;{4}S&#95;{8} may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferroelectric.

Fe 32+δ Ge 33 As 2 and Fe 32+δ' Ge 35-x P x are quasi-binary intermetallic compounds that possess a rare variant of intergrowth-type crystal structure, which is a combination of the column shaped Co 2 Al 5 and MgFe 6 Ge 6 structure type blocks. The compounds are antiferromagnets with the Néel temperatures around 125 K. Neutron powder diffraction experiments on the samples with δ≈ 0.1, δ'≈ 0.5 and x≈ 3 reveal commensurate magnetic ordering of low symmetry in both compounds and a non-monotonic change in the intensities of magnetic reflections. On the other hand, temperature dependence of the hyperfine fields obtained from 57 Fe Mössbauer spectroscopy indicates a gradual, monotonic increase in local magnetic fields upon cooling. We interpret these results as a spin reorientation within the Co 2 Al 5 -type block of the crystal structure, with the possible formation of a non-collinear magnetic order at low temperatures. Between the compounds, the reorientation occurs at significantly different temperatures, however the resulting magnetic structures themselves are similar as well as the average values of the magnetic moments and the hyperfine fields.

Poly- and single-crystalline samples of In0.67□0.33In2S4 thiospinel were obtained by various powder metallurgical and chemical vapor transport methods, respectively. All synthesized samples contained β-In0.67□0.33In2S4 modification only, independent of the synthesis procedure. High-resolution powder X-ray diffraction (PXRD) experiments at 80 K enabled the observation of split tetragonal reflections (completely overlapped at room temperature), which prove the correctness of the crystal structure model accepted for the β-polymorph. Combining single-crystal XRD, transmission electron microscopy and selected-area electron diffraction studies, the presence of three twin domains in the as-grown crystals was confirmed. A high temperature PXRD study revealed both abrupt (in full widths at half maxima of main reflections and in unit-cell volume) and gradual (in intensity of satellites and c/a ratio) changes in the vicinity of the α-β phase transition. These observations, together with a clear endothermic peak in the heat capacity, the magnitude of enthalpy/entropy change and the temperature dependence of electrical resistivity (associated with hysteresis), hinted towards the 1st order type of transition. Three scenarios, based on Rietveld refinement analysis, were considered for the description of the crystal structure evolution from β- to α-modification, including the (3+3)D-modulated cubic structure at 693 K as an intermediate state during the β-α transformation. The Seebeck coefficient, electrical resistivity and thermal conductivity were not only influenced by phase transition, but also by annealing conditions (S-poor or S-rich atmosphere). Density functional theory calculations predicted semiconducting behavior of In0.67□0.33In2S4, as well as instability of the fictitious InIn2S4 thiospinel.

Measurements of the magnetic Grüneisen parameter (Γ&#95;{B}) and specific heat on the Kitaev material candidate α-RuCl&#95;{3} are used to access in-plane field and temperature dependence of the entropy up to 12 T and down to 1 K. No signatures corresponding to phase transitions are detected beyond the boundary of the magnetically ordered region, but only a shoulderlike anomaly in Γ&#95;{B}, involving an entropy increment as small as 10^{-5}Rlog2. These observations put into question the presence of a phase transition between the purported quantum spin liquid and the field-polarized state of α-RuCl&#95;{3}. We show theoretically that at low temperatures Γ&#95;{B} is sensitive to crossings in the lowest excitations within gapped phases, and identify the measured shoulderlike anomaly as being of such origin. Exact diagonalization calculations demonstrate that the shoulderlike anomaly can be reproduced in extended Kitaev models that gain proximity to an additional phase at finite field without entering it. We discuss manifestations of this proximity in other measurements.

A new lithium copper diselenite chloride hydrate, Li 2 Se 2 O 5 (H 2 O) 1.5 ·CuCl 2 , was prepared from aqueous solution. Its unique 2D structure can be interpreted as Li 2 Se 2 O 5 (H 2 O) 1.5 layers with CuCl 2 hosts embedded in the interlayer space, and be considered halfway between the salt-inclusion and host-guest structures. The CuO 2 Cl 4 octahedra form chains similar to those in CuCl 2 ·2H 2 O, yet water molecules are coordinated exclusively to Li + while Se 2 O 5 2- and Cl - bridge Li + and Cu 2+ . The complexity of the structure is likely responsible for both very long pre-crystallization time and non-existence of the corresponding bromide analogue. Despite the relatively long CuCu separations, the compound exhibits weak ferromagnetic interactions along the chains of CuO 2 Cl 4 octahedra.

Systematic studies of the ZrCuSiAs (also LaOAgS or 1111) structure type resulted in the synthesis of two new fluoride chalcogenides, EuFAgSe and EuFAg1-δTe, whereas their sulfide analog, EuFAgS, could not be obtained. Both new compounds are tetragonal, P4/nmm, with cell parameters a = 4.1542(1) Å, c = 9.2182(1) Å for the selenide and a = 4.3255(1) Å, c = 9.5486(1) Å for the telluride. Rietveld refinement reveals a significant silver deficiency in the telluride (δ = 0.05), while the selenide is nearly stoichiometric. Both compounds are semiconductors as shown by diffuse reflectance spectroscopy and confirmed by density-functional calculations of the band structure. Magnetism of both compounds is predominantly driven by Eu2+, as indicated by magnetic susceptibility measurements and corroborated by 151Eu Mössbauer spectroscopy. EuFAg1-δTe and EuFAgSe are paramagnetic down to 1.8 K.

The cradle of quantum spin liquids, triangular antiferromagnets show strong proclivity to magnetic order and require deliberate tuning to stabilize a spin-liquid state. In this brief review, we juxtapose recent theoretical developments that trace the parameter regime of the spin-liquid phase, with experimental results for Co-based and Yb-based triangular antiferromagnets. Unconventional spin dynamics arising from both ordered and disordered ground states are discussed, and the notion of a geometrically perfect triangular system is scrutinized to demonstrate non-trivial imperfections that may assist magnetic frustration in stabilizing dynamic spin states with peculiar excitations.

Employing high-pressure infrared spectroscopy we unveil the Weyl semimetal phase of elemental Te and its topological properties. The linear frequency dependence of the optical conductivity provides clear evidence for metallization of trigonal tellurium (Te-I) and the linear band dispersion above 3.0 GPa. This semimetallic Weyl phase can be tuned by increasing pressure further: a kink separates two linear regimes in the optical conductivity (at 3.7 GPa), a signature proposed for Type-II Weyl semimetals with tilted cones; this however reveals a different origin in trigonal tellurium. Our density-functional calculations do not reveal any significant tilting and suggest that Te-I remains in the Type-I Weyl phase, but with two valence bands in the vicinity of the Fermi level. Their interplay gives rise to the peculiar optical conductivity behavior with more than one linear regime. Pressure above 4.3 GPa stabilizes the more complex Te-II and Te-III polymorphs, which are robust metals.

Functional oxides whose physicochemical properties may be reversibly changed at standard conditions are potential candidates for the use in next-generation nanoelectronic devices. To date, vanadium dioxide (VO 2 ) is the only known simple transition-metal oxide that demonstrates a near-room-temperature metal-insulator transition that may be used in such appliances. In this work, we synthesized and investigated the crystals of a novel mixed-valent iron oxide with an unconventional Fe 5 O 6 stoichiometry. Near 275 K, Fe 5 O 6 undergoes a Verwey-type charge-ordering transition that is concurrent with a dimerization in the iron chains and a following formation of new Fe-Fe chemical bonds. This unique feature highlights Fe 5 O 6 as a promising candidate for the use in innovative applications. We established that the minimal Fe-Fe distance in the octahedral chains is a key parameter that determines the type and temperature of charge ordering. This model provides new insights into charge-ordering phenomena in transition-metal oxides in general., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

A new spin- 1 / 2 frustrated antiferromagnet, Cu 9 O 2 (VO 4 ) 4 Cl 2 , was synthesized via chemical vapor transport method that emulates mineral formation in volcanic fumaroles. Cu 9 O 2 (VO 4 ) 4 Cl 2 is the first copper oxychloride vanadate obtained in the ternary CuO-V 2 O 5 -CuCl 2 anhydrous system. Copper ions constitute a three-dimensional complex framework with a topological structure novel for synthetic compounds but similar to that in the fumarolic mineral yaroshevskite. All of the oxygen atoms except for the O7 site are strongly bonded in the VO 4 tetrahedra. The O7 site belongs to an additional oxygen atom (O a ) being tetrahedrally coordinated by four Cu atoms, thus forming the OCu 4 tetrahedra. The structural formula can be represented as Cu 3 [Cu 6 O 2 ](VO 4 ) 4 Cl 2 highlighting oxocentered units in the structure. IR spectra reveal several absorption bands at 526, 578, and 601 cm -1 interpreted as a characteristic feature of the OCu 4 tetrahedra. Cu 9 O 2 (VO 4 ) 4 Cl 2 reveals ferrimagnetic behavior with the Curie temperature T C = 24 K and the uncompensated moment of M r ∼ 1.9 μ B /f.u.

Endohedral Ga cluster compounds feature nontrivial superconducting states including the two-gap superconductivity similar in nature to MgB 2 . We use the joint flux synthetic technique to introduce Sn into the Ga matrix and tune the valence electron count in the two new endohedral cluster superconductors Mo 8 Ga 41- x Sn x and Mo 4 Ga 21- x -δ Sn x with critical temperatures of T c = 8.7 and 5.85 K, respectively. While the former compound is a derivative of the previously known Mo 8 Ga 41 superconductor, where Sn atoms are enclosed inside the Sn@Ga 6 octahedral clusters, the latter is a new architecture built upon Mo@Ga 9 Sn clusters, Ga@Ga 12 cuboctahedra, and Sn 4 squares. We show that this novel Mo 4 Ga 21- x -δ Sn x superconductor features strong electron-phonon coupling with the large ratio of 2Δ(0)/( k B T c ) = 4.1 similar to that of the Mo 8 Ga 41 superconductor with the closely related crystal structure.

Two new ternary Pt-rich phosphides, EuPt6P2 and EuPt3P, have been prepared via a two-step solid state reaction. Their crystal structures have been determined from powder XRD data. EuPt6P2 is isostructural to SrPt6P2 (cubic, Pa3[combining macron], a = 8.4603(1) Å); its crystal structure comprises corner-sharing Pt6P trigonal prisms hosting Eu2+ cations in the cuboctahedral voids of the framework. EuPt3P is isostructural to the SrPt3P anti-perovskite (P4/nmm, a = 5.7452(1) Å and c = 5.4212(1) Å). Magnetization measurements reveal the magnetic response caused by the Eu2+(4f7) cations. EuPt6P2 is paramagnetic exhibiting no phase transitions down to 1.8 K, whereas EuPt3P orders ferromagnetically below 19 K. Similar to SrPt6P2 and SrPt3P, the new compounds are metallic with states near the Fermi level predominantly formed by the 5d orbitals of Pt.

Using the crystal-growth joint flux technique based on the combination of two aliovalent low-melt metals, gallium and zinc, we adjust the gross valence electron count in the Mo-Ga-Zn system and produce the Mo 8 Ga 41-x Zn x and Mo 7 Ga 52-x Zn x intermetallic compounds. Gradual reduction in the valence electron count first leads to the Zn for Ga substitution in the Mo 8 Ga 41 endohedral cluster superconductor, accompanied by the formation of Zn-containing clusters in the crystal structure and by the gradual suppression of superconductivity. Mo 8 Ga 41-x Zn x with x = 7.2(2) exhibits superconducting properties below T C = 4 K, whereas there is no superconducting transition at temperatures above 2 K for the limiting composition of x = 11.3(2). Further, the Mo 7 Ga 52-x Zn x phase is formed from the flux with a higher content of Zn. Mo 7 Ga 52-x Zn x crystallizes in the Mo 7 Sn 12 Zn 40 structure type with a narrow homogeneity range and exhibits metallic behavior with no sign of superconductivity down to at least 1.8 K. Its experimental valence electron count of 2.9 e per atom is below that of endohedral gallium cluster superconductors. Electronic structures of Mo 8 Ga 41-x Zn x and Mo 7 Ga 52-x Zn x feature the opening of a pseudogap slightly below the Fermi level indicating the specific stability of these structure types at the valence electron count of 3.2 e per atom and 2.7 e per atom, respectively.

dc-magnetization data measured down to 40 mK speak against conventional freezing and reinstate YbMgGaO&#95;{4} as a triangular spin-liquid candidate. Magnetic susceptibility measured parallel and perpendicular to the c axis reaches constant values below 0.1 and 0.2 K, respectively, thus indicating the presence of gapless low-energy spin excitations. We elucidate their nature in the triple-axis inelastic neutron scattering experiment that pinpoints the low-energy (E≤J&#95;{0}∼0.2  meV) part of the excitation continuum present at low temperatures (TJ&#95;{0} that is rooted in the breaking of nearest-neighbor valence bonds and persists to temperatures well above J&#95;{0}/k&#95;{B}, the low-energy one originates from the rearrangement of the valence bonds and thus from the propagation of unpaired spins. We further extend this picture to herbertsmithite, the spin-liquid candidate on the kagome lattice, and argue that such a hierarchy of magnetic excitations may be a universal feature of quantum spin liquids.

In this study, we modify the flux-growth method for the purpose of exploratory synthesis of ternary intermetallic compounds. Our concept is based on the assumption that valence electron count plays a crucial role in the stability of polar intermetallic compounds of different structure types. Control of the valence electron count parameter is made possible through the use of an excess of two metals having a different number of valence electrons. By gradually changing the ratio between these metals in the joint flux, we scan the gross number of valence electrons and explore the crystallization of new compounds. In the ternary system Re-Ga-Zn, we detect compounds belonging to three structure types, ReGa 5 , PtHg 4 , and V 8 Ga 41 , while gradually increasing the content of Zn metal in the flux. Two new compounds, ReGa 3 Zn and Re 8 Ga 41- x Zn x with x = 21.2(5), are obtained in the form of high-quality single crystals, and the former compound shows the narrow-gap semiconducting behavior favorable for high thermoelectric performance.

Two new sodium nickel phosphates, Na 5 Ni 2 (PO 4 ) 3 ·H 2 O (I) and Na 6 Ni 2 (PO 4 ) 3 OH (II), have been synthesized hydrothermally and characterized by synchrotron X-ray diffraction, electron diffraction, low-temperature thermodynamic and magnetic measurements, and ab initio calculations. Unlike the majority of Ni 2+ compounds, I and II show predominant ferromagnetic exchange couplings. I crystallizes in the monoclinic space group P2 1 / n ( a = 14.0395(4) Å, b = 5.1847(14) Å, c = 16.4739(4) Å, β = 110.4186(14)°) and features chains of ferromagnetically coupled Ni 2+ ions. In II with the orthorhombic space group Pcmb ( a = 7.5007(15) Å, b = 21.4661(4) Å, c = 7.1732(15) Å), the ferromagnetically coupled Ni 2+ ions form dimers arranged on a spin ladder. Both compounds represent rare examples of quasi-one-dimensional ferromagnets. Structural features behind this unusual magnetic behavior are discussed.

Two new structural forms of Na x Co 0.5 Ti 0.5 O 2 , the layered O3- and P3-forms, were synthesized and comprehensively characterized. Both materials show electrochemical activity as electrodes in Na-ion batteries. During cell charging (desodiation of the Na x Co 0.5 Ti 0.5 O 2 cathode), we observed a structural phase transformation of O3-Na 0.95 Co 0.5 Ti 0.5 O 2 into P3-Na x Co 0.5 Ti 0.5 O 2 , whereas no changes other than conventional unit cell volume shrinkage were detected for P3-Na 0.65 Co 0.5 Ti 0.5 O 2 . During Na insertion (cell discharging), the reconversion of the P3-form into O3-Na x Co 0.5 Ti 0.5 O 2 was impeded for both materials and occurs well below 1 V versus Na + /Na only. The reconversion is hindered by the charge and spin transfers of Co (LS-Co 3+ → HS-Co 2+ ) and by a significant unit cell volume expansion at the P3 → O3 transformation, as revealed from the magnetization, crystallographic, and spectroscopic studies. As the kinetics of such transformations depend on numerous parameters such as time, temperature, and particle size, a large cell overpotential ensues. An extended cutoff voltage at 0.2 V versus Na + /Na during discharging allows to complete the P3 → O3 transformation and increases the specific discharging capacity to 200 mA h g -1 . Moreover, a quasi-symmetrical full cell, based on the O3- and P3-forms, was designed, eliminating safety concerns associated with sodium anodes and delivering a discharge capacity of 130 mA h g -1 .

A Verwey-type charge-ordering transition in magnetite at 120 K leads to the formation of linear units of three iron ions with one shared electron, called trimerons. The recently-discovered iron pentoxide (Fe 4 O 5 ) comprising mixed-valent iron cations at octahedral chains, demonstrates another unusual charge-ordering transition at 150 K involving competing formation of iron trimerons and dimerons. Here, we experimentally show that applied pressure can tune the charge-ordering pattern in Fe 4 O 5 and strongly affect the ordering temperature. We report two charge-ordered phases, the first of which may comprise both dimeron and trimeron units, whereas, the second exhibits an overall dimerization involving both the octahedral and trigonal-prismatic chains of iron in the crystal structure. We link the dramatic change in the charge-ordering pattern in the second phase to redistribution of electrons between the octahedral and prismatic iron chains, and propose that the average oxidation state of the iron cations can pre-determine a charge-ordering pattern.

High-pressure single-crystal X-ray diffraction at ambient temperature and high-pressure SQUID measurements down to 2 K were performed up to ∼2.5 GPa on ammonium metal formates, [NH 4 ][M(HCOO) 3 ] where M = Mn 2+ , Fe 2+ , and Ni 2+ , in order to correlate structural variations to magnetic behaviour. Similar structural distortions and phase transitions were observed for all compounds, although the transition pressures varied with the size of the metal cation. The antiferromagnetic ordering in [NH 4 ][M(HCOO) 3 ] compounds was maintained as a function of pressure, and the magnetic ordering transition temperature changed within a few kelvins depending on the structural distortion and the metal cation involved. These compounds, in particular [NH 4 ][Fe(HCOO) 3 ], showed greatest sensitivity to the degree of spin canting upon compression, clearly visible from the twenty-fold increase in the low-temperature magnetisation for [NH 4 ][Fe(HCOO) 3 ] at 1.4 GPa, and the change from purely antiferromagnetic to weakly ferromagnetic ordering in [NH 4 ][Mn(HCOO) 3 ] at 1 GPa. The variation in the exchange couplings and spin canting was checked with density-functional calculations that reproduce well the increase in canted moment within [NH 4 ][Fe(HCOO) 3 ] upon compression, and suggest that the Dzyaloshinskii-Moriya (DM) interaction is evolving as a function of pressure. The pressure dependence of spin canting is found to be highly dependent on the metal cation, as magnetisation magnitudes did not change significantly for when M = Ni 2+ or Mn 2+ . These results demonstrate that the overall magnetic behaviour of each phase upon compression was not only dependent on the structural distortions but also on the electronic configuration of the metal cation.

The ABO 3 perovskite oxides exhibit a wide range of interesting physical phenomena remaining in the focus of extensive scientific investigations and various industrial applications. In order to form a perovskite structure, the cations occupying the A and B positions in the lattice, as a rule, should be different. Nevertheless, the unique binary perovskite manganite Mn 2 O 3 containing the same element in both A and B positions can be synthesized under high-pressure high-temperature conditions. Here, we show that this material exhibits magnetically driven ferroelectricity and a pronounced magnetoelectric effect at low temperatures. Neutron powder diffraction revealed two intricate antiferromagnetic structures below 100 K, driven by a strong interplay between spin, charge, and orbital degrees of freedom. The peculiar multiferroicity in the Mn 2 O 3 perovskite is ascribed to a combined effect involving several mechanisms. Our work demonstrates the potential of binary perovskite oxides for creating materials with highly promising electric and magnetic properties.

Temperature- and field-dependent 1 H-, 19 F-, and 79,81 Br-NMR measurements together with zero - field 79,81 Br-NQR measurements on polycrystalline samples of barlowite, Cu 4 (OH) 6 FBr are conducted to study the magnetism and possible structural distortions on a microscopic level. The temperature dependence of the 79,81 Br-NMR spin-lattice relaxation rates 1/T 1 indicate a phase transition at T N  [Formula: see text] 15 K which is of magnetic origin, but with an unusually weak slowing down of fluctuations below T N . Moreover, 1/T 1 T scales linear with the bulk susceptibility which indicates persisting spin fluctuations down to 2 K. Quadupolare resonance (NQR) studies reveal a pair of zero-field NQR- lines associated with the two isotopes of Br with the nuclear spins of I = 3/2. Quadrupole coupling constants of v Q  ≃ 28.5 MHz and 24.7 MHz for 79 Br- and 81 Br-nuclei are determined from Br-NMR and the asymmetry parameter of the electric field gradient was estimated to η ≃ 0.2. The Br-NQR lines are consistent with our findings from Br-NMR and they are relatively broad, even above T N . This broadening and the relative large η value suggests a symmetry reduction at the Br- site reflecting the presence of a local distortion in the lattice. Our density-functional calculations show that the displacements of Cu2 atoms located between the kagome planes do not account for this relatively large η. On the other hand, full structural relaxation, including the deformation of kagome planes, leads to a better agreement with the experiment.

Temperature-pressure phase diagram of the Kitaev hyperhoneycomb iridate β-Li&#95;{2}IrO&#95;{3} is explored using magnetization, thermal expansion, magnetostriction, and muon spin rotation measurements, as well as single-crystal x-ray diffraction under pressure and ab initio calculations. The Néel temperature of β-Li&#95;{2}IrO&#95;{3} increases with the slope of 0.9  K/GPa upon initial compression, but the reduction in the polarization field H&#95;{c} reflects a growing instability of the incommensurate order. At 1.4 GPa, the ordered state breaks down upon a first-order transition, giving way to a new ground state marked by the coexistence of dynamically correlated and frozen spins. This partial freezing in the absence of any conspicuous structural defects may indicate the classical nature of the resulting pressure-induced spin liquid, an observation paralleled to the increase in the nearest-neighbor off-diagonal exchange Γ under pressure.

BiCu 2 PO 6 is a unique example of a S = 1 / 2 ladder where the magnetic exchanges are mainly confined in 1D ∞ [BiCu 2 O 2 ] 3+ cationic ribbons, although the shortest Cu-Cu separation between them exists. Its original magnetic topology gives the most representative example of a frustrated quantum ladder to investigate the complex physics behind it. Herein, we report the synthesis and characterization of one high-pressure polymorph. In this new phase, the preservation of 1D ∞ [BiCu 2 O 2 ] 3+ units somewhat restacked leads to the preservation of its gapped magnetic ground state and ladder topology. The comparison of both compounds highlights the start of a thermodynamic conjuncture, where both the stable ambient-pressure (AP) and metastable high-pressure (HP) forms display the same equilibrium volume and superposed volume dependence of the energy, leading to a first-order AP → HP transition undetected by differential thermal analysis.

Synthesis, thermodynamic properties, and microscopic magnetic model of ilinskite-type KCu 5 O 2 (SeO 3 ) 2 Cl 3 built by corner-sharing Cu 4 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 SeO 3 groups that render the Cu-O-Cu superexchange strongly ferromagnetic even at bridging angles exceeding 110°.

Correction for 'Composition-dependent charge transfer and phase separation in the V 1-x Re x O 2 solid solution' by D. Mikhailova, et al., Dalton Trans., 2017, 46, 1606-1617.

The exactly solvable Kitaev model on the honeycomb lattice has recently received enormous attention linked to the hope of achieving novel spin-liquid states with fractionalized Majorana-like excitations. In this review, we analyze the mechanism proposed by Jackeli and Khaliullin to identify Kitaev materials based on spin-orbital dependent bond interactions and provide a comprehensive overview of its implications in real materials. We set the focus on experimental results and current theoretical understanding of planar honeycomb systems (Na 2 IrO 3 , α-Li 2 IrO 3 , and α-RuCl 3 ), three-dimensional Kitaev materials (β- and γ-Li 2 IrO 3 ), and other potential candidates, completing the review with the list of open questions awaiting new insights.

A rapid and anisotropic modification of the Fermi-surface shape can be associated with abrupt changes in crystalline lattice geometry or in the magnetic state of a material. We show that such an electronic topological transition is at the basis of the formation of an unusual pressure-induced tetragonal ferromagnetic phase in Fe&#95;{1.08}Te. Around 2 GPa, the orthorhombic and incommensurate antiferromagnetic ground state of Fe&#95;{1.08}Te is transformed upon increasing pressure into a tetragonal ferromagnetic state via a conventional first-order transition. On the other hand, an isostructural transition takes place from the paramagnetic high-temperature state into the ferromagnetic phase as a rare case of a "type-0" transformation with anisotropic properties. Electronic-structure calculations in combination with electrical resistivity, magnetization, and x-ray diffraction experiments show that the electronic system of Fe&#95;{1.08}Te is instable with respect to profound topological transitions that can drive fundamental changes of the lattice anisotropy and the associated magnetic order.

Since its proposal by Anderson, resonating valence bonds (RVB) formed by a superposition of fluctuating singlet pairs have been a paradigmatic concept in understanding quantum spin liquids. Here, we show that excitations related to singlet breaking on nearest-neighbour bonds describe the high-energy part of the excitation spectrum in YbMgGaO 4 , the effective spin-1/2 frustrated antiferromagnet on the triangular lattice, as originally considered by Anderson. By a thorough single-crystal inelastic neutron scattering study, we demonstrate that nearest-neighbour RVB excitations account for the bulk of the spectral weight above 0.5 meV. This renders YbMgGaO 4 the first experimental system where putative RVB correlations restricted to nearest neighbours are observed, and poses a fundamental question of how complex interactions on the triangular lattice conspire to form this unique many-body state.

We investigated the structural, vibrational, magnetic, and electronic properties of the recently synthesized CaCo 3 V 4 O 12 double perovskite with the high-spin (HS) Co 2+ ions in a square-planar oxygen coordination at extreme conditions of high pressures and low temperatures. The single-crystal X-ray diffraction and Raman spectroscopy studies up to 60 GPa showed a conservation of its cubic crystal structure but indicated a crossover near 30 GPa. Above 30 GPa, we observed both an abnormally high "compressibility" of the Co-O bonds in the square-planar oxygen coordination and a huge anisotropic displacement of HS-Co 2+ ions in the direction perpendicular to the oxygen planes. Although this effect is reminiscent of a continuous HS → LS transformation of the Co 2+ ions, it did not result in the anticipated shrinkage of the cell volume because of a certain "stiffing" of the bonds of the Ca and V cations. We verified that the oxidation states of all the cations did not change across this crossover, and hence, no charge-transfer effects were involved. Consequently, we proposed that CaCo 3 V 4 O 12 could undergo a phase transition at which the large HS-Co 2+ ions were pushed out of the oxygen planes because of lattice compression. The antiferromagnetic transition in CaCo 3 V 4 O 12 at 100 K was investigated by neutron powder diffraction at ambient pressure. We established that the magnetic moments of the Co 2+ ions were aligned along one of the cubic axes, and the magnetic structure had a 2-fold periodicity along this axis, compared to the crystallographic one.

We apply moderate-high-energy inelastic neutron scattering (INS) measurements to investigate Yb^{3+} crystalline electric field (CEF) levels in the triangular spin-liquid candidate YbMgGaO&#95;{4}. Three CEF excitations from the ground-state Kramers doublet are centered at the energies ℏω=39, 61, and 97 meV in agreement with the effective spin-1/2 g factors and experimental heat capacity, but reveal sizable broadening. We argue that this broadening originates from the site mixing between Mg^{2+} and Ga^{3+} giving rise to a distribution of Yb-O distances and orientations and, thus, of CEF parameters that account for the peculiar energy profile of the CEF excitations. The CEF randomness gives rise to a distribution of the effective spin-1/2 g factors and explains the unprecedented broadening of low-energy magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO&#95;{4}, although a distribution of magnetic couplings due to the Mg/Ga disorder may be important as well.

The novel borophosphate Rb 2.3 (H 2 O) 0.8 Mn 3 [B 4 P 6 O 24 (O,OH) 2 ] was prepared under hydrothermal conditions at 553 K. Its crystal structure was determined using single-crystal X-ray diffraction data obtained from a non-merohedral twin and refined against F 2 to R = 0.057. The compound crystallizes in the orthorhombic space group Pbcn, with unit-cell parameters a = 20.076(2) Å, b = 9.151(1) Å, c = 12.257(1) Å, V = 2251.8(2) Å 3 , and Z = 4. The title compound is the first example of a borophosphate with manganese ions adopting both octahedral and tetrahedral coordinations. Its unique crystal structure is formed by borophosphate slabs and chains of Mn 2+ -centered polyhedra sharing edges and vertices. These 2D and 1D fragments interconnect into a framework with open channels that accommodate Rb + cations and water molecules. Topological relationships between borophosphates built from three-membered rings of two borate and one phosphate tetrahedra sharing oxygen vertices, amended by additional PO 4 and HPO 4 tetrahedra, are discussed. The temperature dependence of the magnetic susceptibility of Rb 2.3 (H 2 O) 0.8 Mn 3 [B 4 P 6 O 24 (O,OH) 2 ] reveals predominant antiferromagnetic exchange interactions and the high-temperature effective magnetic moment corresponding to the high-spin S = 5/2 state of Mn 2+ ions. At 12.5 K, a magnetic transition is evidenced by ac-susceptibility and specific heat measurements. A spin-trimer model with the leading exchange interaction J ∼ 3.2 K is derived from density-functional band-structure calculations and accounts for all experimental observations.

The substitution of vanadium in vanadium dioxide VO 2 influences the critical temperatures of structural and metal-to-insulator transitions in different ways depending on the valence of the dopant. Rhenium adopts valence states between +4 and +7 in an octahedral oxygen surrounding and is particularly interesting in this context. Structural investigation of V 1-x Re x O 2 solid solutions (0.01 ≤ x ≤ 0.30) between 80 and 1200 K using synchrotron X-ray powder diffraction revealed only two polymorphs that resemble VO 2 : the low-temperature monoclinic MoO 2 -type form (space group P2 1 /c), and the tetragonal rutile-like form (space group P4 2 /mnm). However, for compositions with 0.03 < x ≤ 0.15 a phase separation in the solid solution was observed below 1000 K upon cooling down from 1200 K, giving rise to two isostructural phases with slightly different lattice parameters. This is reflected in the appearance of two metal-to-insulator transition temperatures detected by magnetization and specific heat measurements. Comprehensive X-ray photoelectron spectroscopy studies showed that an increased amount of Re leads to a change in the Re valence state from solely Re 6+ at a low doping level (≤3 at% Re) via mixed-valence states Re 4+ /Re 6+ for at least 0.03 < x ≤ 0.10, up to nearly pure Re 4+ in V 0.70 Re 0.30 O 2 . Thus, compositions V 1-x Re x O 2 with only one valence state of Re in the material (Re 6+ or Re 4+ ) can be obtained as a single phase, while intermediate compositions are subjected to a phase separation, presumably due to different valence states of Re.

A new phase Fe 32+δ Ge 33 As 2 (δ ≤ 0.136) was obtained by two-step synthesis from the elements. Fe 32+δ Ge 33 As 2 crystallizes in its own structure type (space group P6/mmm, Z = 1, a = 11.919(3) Å, c = 7.558(4) Å) that can be described as a recurrent two-dimensional intergrowth of two intermetallic structure types, MgFe 6 Ge 6 and Co 2 Al 5 . Their blocks are represented by infinite columns in the structure. No visible structural changes were observed in the temperature range from 10 to 300 K. At 125 K, Fe 32+δ Ge 33 As 2 undergoes an antiferromagnetic-like transition, while above 150 K it shows a typical Curie-Weiss paramagnetic behavior. Below the transition temperature, a peculiar field-dependent magnetic susceptibility, that shows a significant increase of the susceptibility upon increasing the magnetic field, and a change in transport properties have been observed. Above 140 K, Fe 32+δ Ge 33 As 2 reveals a metallic behavior, in agreement with electronic structure calculation, while below this point the resistivity nonmonotonically increases upon cooling. The Seebeck coefficient is positive, indicating that holes are the major charge carriers, and shows a broad maximum around 57 K.

Two new compounds with the LaOAgS structure, EuFZnAs (1) and EuFZnSb (2), were obtained via solid state reaction. Both compounds are tetragonal (P4/nmm) with the cell parameters a = 4.1000(1) Å and c = 9.0811(1) Å for 1 and a = 4.2852(1)Å and c = 9.4238(1)Å for 2. The absence of their phosphide analog can be explained based on crystal chemical considerations as well as on quantum-chemical estimates of their thermodynamic stability with respect to EuF 2 and EuZn 2 Pn 2 . The magnetic response of 1 and 2 is ascribed to the presence of Eu 2+ ions. Both compounds are paramagnetic down to low temperatures, where they order antiferromagnetically at ∼5 K and ∼3 K, respectively. They are narrow-gap semiconductors, and EuFZnSb demonstrates a relatively high value of the Seebeck coefficient.

A new ternary telluride, Fe 3-δ As 1-y Te 2 , was synthesized from elements at 600 °C. It crystallizes in the hexagonal P6 3 /mmc space group with the unit cell parameters a = 3.85091(9) Å and c = 17.1367(4) Å for δ = 0.3 and y = 0.04. Its layered crystal structure contains partially occupied intralayer and interlayer Fe positions, which give rise to significant nonstoichiometry: Fe 3-δ As 1-y Te 2 was found to possess the homogeneity range of 0.25 < δ < 0.45 and y = 0.04. Regions of local vacancy ordering alternate with regions of randomly distributed vacancies, so that the ordering of Fe atoms and vacancies is not complete in the average structure. Clear evidence of the magnetic phase transition is obtained by thermodynamic measurements, Mössbauer spectroscopy, and neutron powder diffraction. Magnetic susceptibility measurements reveal weak ferromagnetism below T C = 123 K with a net moment of M S ∼ 0.1μ B /Fe at T = 2 K. This transition is confirmed by differential scanning calorimetry. Additionally, neutron powder diffraction indicates the onset of a complex AFM-like magnetic ordering below 100 K.

Muon spin relaxation (μSR) experiments on single crystals of the structurally perfect triangular antiferromagnet YbMgGaO&#95;{4} indicate the absence of both static long-range magnetic order and spin freezing down to 0.048 K in a zero field. Below 0.4 K, the μ^{+} spin relaxation rates, which are proportional to the dynamic correlation function of the Yb^{3+} spins, exhibit temperature-independent plateaus. All these μSR results unequivocally support the formation of a gapless U(1) quantum spin liquid ground state in the triangular antiferromagnet YbMgGaO&#95;{4}.