Fermi velocity spectrum and incipient magnetism inTiBe2

Abstract

We address the origin of the incipient magnetism in TiBe2 through precise first principles calculations, which overestimate the ferromagnetic tendency and therefore require correction to account for spin fluctuations. TiBe2 has sharp fine structure in its electronic density of states, with a van Hove singularity only 3 meV above the Fermi level. Similarly to the isovalent weak ferromagnet ZrZn2, it is flat bands along the K-W-U lines of hexagonal face of the fcc Brillouin zone make the system prone to magnetism, and more so if electrons are added. We find that the Moriya B coefficient (multiplying ω q in the fluctuation susceptibility ∆χ(q, ω)) is divergent when the velocity vanishes at a point on the Fermi surface, which is very close (3 meV) to occurring in TiBe2. In exploring how the FM instability (the q=0 Stoner enhancement is S ≈ 60) might be suppressed by fluctuations in TiBe2, we calculate that the Moriya A coefficient (of q 2 ) is negative, so q=0 is not the primary instability. Explicit calculation of χo(q) shows that its maximum occurs at the X point (1, 0, 0) 2π a ; TiBe2 is thus an incipient antiferromagnet rather than ferromagnet as has been supposed. We further show that simple temperature smearing of the peak accounts for most of the temperature dependence of the susceptibility, which previously had been attributed to local moments (via a Curie-Weiss fit), and that energy dependence of the density of states also strongly affects the magnetic field variation of χ.

Orbital-ordering driven structural distortion in metallicSrCrO3

“…Without any peak in the DOS, the temperature variation cannot be modeled with temperature broadening as can be done, for example, in TiBe 2 . [20] VII. ACKNOWLEDGMENTS…”

Novel metallic states at low temperatures

“…A Fermi surface such as that predicted for paramagnetic ZrZn 2 , that is characterized both by a low Fermi velocity at k-points near to van Hove singularities along <111> directions and strong nesting along <100> directions, would be consistent with the existence of enhanced ferromagnetic as well as antiferromagnetic spin fluctuations [50,51]. See for example the calculated Fermi surface of TiBe 2 in Fig.…”

“…The T 3 /2 resistivity, found to extend from p c to at least twice p c within the temperature range 1 K to 15 K [29,30], is inconsistent with the predictions of the SCR model in its conventional form. [50], which is expected to be similar to the Fermi surface of ZrZn 2 [26] in the paramagnetic state (the electron-"pseudocube" is shown only).…”

“…See for example the calculated Fermi surface of TiBe 2 in Fig. 6 [50] which is expected to be similar to the Fermi surface of ZrZn 2 in the paramagnetic phase [26,51]. These features could lead to the existence of two or more quasi-critical fields and to a [25], (ii) an example of the coexistence of superconductivity and metallic ferromagnetism (UGe 2 [53][54][55]), (iii) quantum tri-criticality in Ni 3 Ga [56], (iv) spin textured phase in MnSi [31,32,36], (v) spin-triplet superconductivity on the border of ferromagnetism (M 2 RuO 4 , where M stands for Sr [57,58] or potentially Ca at high pressures [59]) and (vi) an electronic nematic phase in Sr 3 Ru 2 O 7 [60,61] near to a quantum critical end point in high magnetic fields.…”

“…In principle, first order transitions and inhomogeneities can also arise via the effects of van Hove and nesting singularities of the Fermi surface [16,[48][49][50][51]. A Fermi surface such as that predicted for paramagnetic ZrZn 2 , that is characterized both by a low Fermi velocity at k-points near to van Hove singularities along <111> directions and strong nesting along <100> directions, would be consistent with the existence of enhanced ferromagnetic as well as antiferromagnetic spin fluctuations [50,51].…”