Electron spin dynamics in a hexaboride superconductor YB6 probed by 89Y and 11B NMR

Vyaselev, O. M.; Sluchanko, Nikolai; Bogach, A. V.; Shitsevalova, N. Yu.; Филипов, В. Б.; Gippius, A. A.

doi:10.1016/j.jallcom.2022.165627

Cited by 7 publications

(6 citation statements)

References 45 publications

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“…The calculations definitely showed the covalent bonding within the octahedral boron framework with a larger ELF value for interoctahedral B–B out bonds, i.e., 0.95 versus 0.81 for intraoctahedral B–B in bonds (Figure a). The maxima of ELF, visualized by the isosurface (at 0.8 level) (Figure b) are located at the [B 6 ]–[B 6 ] bond midpoints and above the octahedral triangular faces and represent a typical picture for boron bonding in hexaborides. ,,− Besides those, another ELF maximum is found in the (110) plane around the Y atom site, further followed by the almost empty region along the Y–B interatomic line (blue area, ELF value less than 0.07); the empty regions in the ELF map denote the transfer of electrons from yttrium to boron, indicating the dominating ionic character of this interaction. In accordance with a substantial charge transfer from yttrium to the boron framework, the analysis of Bader charges within the quantum theory of atoms in molecules (QTAIM) approach , (Table ) reveals a positive charge of +1.842 e – for yttrium and −0.307 e – for boron in excellent agreement with literature data. − …”

Section: Resultsmentioning

confidence: 99%

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Electronic and Structural Properties of MPt_xB_6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

Salamakha,

Sologub,

Stöger

et al. 2023

Inorg. Chem.

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Two new ternary platinum borides, YPt x B6–2x and YbPt x B6–2x , were obtained by argon-arc melting of the elements followed by annealing at 780 °C (750 °C). The structures of these compounds combine the fragments of CaB6- and AuCu3-type structures [space group Pm3̅m; x = 1.15, a = 4.0550(4) Å and x = 1.34, a = 4.0449(2) Å for YPt x B6–2x and YbPt x B6–2x , respectively; single-crystal X-ray diffraction]. Two possible variants of B/Pt ordering (space group P4/mmm) were created via a group-subgroup approach targeting the derived stoichiometry. The architecture of the type-I YPt x B6–2x structure model (a′ = a, b′ = b, c′ = c) combines the 4.82 boron nets alternating with the layers of Y and Pt; the type-II YPt x B6–2x structure model (a′ = 2a, b′ = 2b, c′ = c) exhibits columns of linked [B24] truncated cubes filled with Y running along the c axis. The striking features of both structural models are [B4Pt2] octahedra. The structural similarities with hitherto reported structures (YB2C2, M2Ni21B20, MNi21B20, and ErNiB4) were drawn supporting the verity of these models. A chemical bonding analysis for type-I and type-II YPt x B6–2x based on electron localization function distribution revealed a two-center interaction forming the 4.82 boron nets for type-I YPt x B6–2x and a covalent bonding within [B4Pt2] octahedra as well as a two-center interaction for B–B intraoctahedral bonds for type-II YPt x B6–2x . Analysis of Bader charges revealed the cationic character of the yttrium atoms. The interactions for nondistorted areas of the structures agree well with the bonding picture calculated for constituent building structures, YB6 and YPt3. Electronic structure calculations predict YPt x B6–2x to be a metal with the density of states of around N(E F) = 1 states eV–1 f.u.–1. The exploration of the Y–Pt–B system in the relevant concentration range elucidated the homogeneity field of YPt x B6–2x (0.90 ≤ x ≤ 1.40) and revealed the existence of three more ternary phases at 780 °C: YPt2B (space group P6222), YPt3B (space group P4mm), and YPt5B2 (space group C2/m).

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Section: Resultsmentioning

confidence: 99%

“…In accordance with a substantial charge transfer from yttrium to the boron framework, the analysis of Bader charges within the quantum theory of atoms in molecules (QTAIM) approach 96 , 97 ( Table 3 ) reveals a positive charge of +1.842 e – for yttrium and −0.307 e – for boron in excellent agreement with literature data. 92 − 95 …”

Section: Resultsmentioning

confidence: 99%

Electronic and Structural Properties of MPt_xB_6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

Salamakha,

Sologub,

Stöger

et al. 2023

Inorg. Chem.

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“…Figure 1c shows a visualization of each of these vibrations (labeled i, ii, and iii) while Figures 1d and 1e show how each breaks electronic degeneracy in YB6 and LaB6. The other, higher-energy group of phonons which can be responsible for Cooper pairs are at 35-40 THz for YB6 and 32-36 THz for LaB6 (phonons [19][20][21]. Although the BCS model of superconductivity allows virtual phonons (i.e.…”

Section: Resultsmentioning

confidence: 99%

“…[2,[9][10][11][12][13][14][15] Lanthanum hexaboride, though primarily exploited in hot cathodes for its high electron emissivity, is also known to be a low-temperature, Bardeen-Cooper-Schrieffer (BCS, i.e phonon-mediated) superconductor at temperatures below 0.45 K. [16] Isostructural yttrium hexaboride, possessing a near-identical phonon spectrum, a similar lattice constant (4.155 Å for LaB6, 4.098 Å for YB6), and yttrium being one row directly above lanthanum in the periodic table, is a significantly better BCS superconductor at a critical temperature (Tc) of up to 8.4 K. [17][18][19] This notably makes it the only reported superconducting hexaboride with a Tc reachable with 4 He cryogenics. [20,21] It also possesses the second highest superconducting Tc of any metal boride [22] , the highest being that of the unusual [23] and thoroughly studied MgB2 [24][25][26][27][28] , the highest-Tc "conventional" (i.e. phonon-mediated) superconductor.…”

Section: Introductionmentioning

confidence: 99%

Why Yttrium Hexaboride Exhibits a Much Higher Superconducting Tc than Near-Identical Lanthanum Hexaboride

Lavroff,

Munarriz,

Dickerson

et al. 2023

Preprint

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Though YB6 and LaB6 share the same crystal structure, atomic valence electron configuration, and phonon modes, they exhibit drastically different phonon-mediated superconductivity. YB6 superconducts below 8.4 K, giving it the second-highest critical temperature of known borides. LaB6 does not superconduct until near-absolute zero temperatures, however. Though previous studies have quantified the canonical superconductivity descriptors of YB6’s greater Fermi-level (Ef) density of states and higher electron-phonon coupling (EPC), the root of this difference has not been assessed with full detail of the electronic structure. Through chemical bonding, we determine low-lying, unoccupied 4f atomic orbitals in lanthanum to be the key difference between these superconductors. These orbitals, which are not accessible in YB6, hybridize with pi B-B bonds and bring this pi-system lower in energy than the sigma B-B bonds otherwise at Ef. This inversion of bands is crucial: the phonon modes we show responsible for superconductivity cause the sigma-orbitals of YB6 to change drastically in overlap, but couple weakly to the pi-orbitals of LaB6. These phonons in YB6 even access an electronic-state crossing, indicating strong EPC. No such crossing in LaB6 is observed. Finally, a supercell (the M k-point) is shown to undergo a Peierls-like effect in YB6, introducing additional EPC.

show abstract

Section: Introductionmentioning

confidence: 99%

Why Yttrium Hexaboride Exhibits a Much Higher Superconducting Tc than Near-Identical Lanthanum Hexaboride

Lavroff,

Munarriz,

Dickerson

et al. 2023

Preprint

View full text Add to dashboard Cite

Though YB6 and LaB6 share the same crystal structure, atomic valence electron configuration, and phonon modes, they exhibit drastically different phonon-mediated superconductivity. YB6 superconducts below 8.4 K, giving it the second-highest critical temperature of known borides. LaB6 does not superconduct until near-absolute zero temperatures, however. Though previous studies have quantified the canonical superconductivity descriptors of YB6’s greater Fermi-level (Ef) density of states and higher electron-phonon coupling (EPC), the root of this difference has not been assessed with full detail of the electronic structure. Through chemical bonding, we determine low-lying, unoccupied 4f atomic orbitals in lanthanum to be the key difference between these superconductors. These orbitals, which are not accessible in YB6, hybridize with sigma B-B bonds and bring this pi-system lower in energy than the B-B bonds otherwise at Ef. This inversion of bands is crucial: the phonon modes we show responsible for superconductivity cause the sigma-orbitals of YB6 to change drastically in overlap, but couple weakly to the pi-orbitals of LaB6. These phonons in YB6 even access an electronic-state crossing, indicating strong EPC. No such crossing in LaB6 is observed. Finally, a supercell (the M k-point) is shown to undergo a Peierls-like effect in YB6, introducing additional EPC.

show abstract

Electron spin dynamics in a hexaboride superconductor YB6 probed by 89Y and 11B NMR

Cited by 7 publications

References 45 publications

Electronic and Structural Properties of MPt_xB_6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

Electronic and Structural Properties of MPt_xB_6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

Why Yttrium Hexaboride Exhibits a Much Higher Superconducting Tc than Near-Identical Lanthanum Hexaboride

Why Yttrium Hexaboride Exhibits a Much Higher Superconducting Tc than Near-Identical Lanthanum Hexaboride

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Electron spin dynamics in a hexaboride superconductor YB6 probed by 89Y and 11B NMR

Cited by 7 publications

References 45 publications

Electronic and Structural Properties of MPtxB6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

Electronic and Structural Properties of MPtxB6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

Why Yttrium Hexaboride Exhibits a Much Higher Superconducting Tc than Near-Identical Lanthanum Hexaboride

Why Yttrium Hexaboride Exhibits a Much Higher Superconducting Tc than Near-Identical Lanthanum Hexaboride

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Electronic and Structural Properties of MPt_xB_6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework

Electronic and Structural Properties of MPt_xB_6–2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework