Heavy Quasiparticles in the Ferromagnetic SuperconductorZrZn2

Abstract

We report a study of the de Haas-van Alphen effect in the normal state of the ferromagnetic superconductor ZrZn2. Our results are generally consistent with an LMTO band structure calculation which predicts four exchange-split Fermi surface sheets. Quasiparticle effective masses are enhanced by a factor of about 4.9 implying a strong coupling to magnetic excitations or phonons. Our measurements provide insight in to the mechanism for superconductivity and unusual thermodynamic properties of ZrZn2.The recent reports of the coexistence of ferromagnetism and superconductivity in A detailed knowledge of the electronic structure of ZrZn 2 is crucial to the understanding of both its normal and superconducting properties. Thus we report here a detailed study of the Fermi surface (FS) made using angle-resolved measurements of the de Haas-van Alphen effect (dHvA). Our results are compared with an ab-initio electronic structure calculation. We observe much of calculated Fermi surface which consists of four exchangesplit sheets. Our study shows that ZrZn 2 is characterized by a large quasiparticle density-of-states (DOS) at the Fermi energy in the ferromagnetic state, which arises partly from the band structure and partly from a large mass enhancement.ZrZn 2 has many unusual properties. Below T FM = 28.5 K it becomes ferromagnetic with an ordered moment of 0.17µ B per formula unit at low temperatures (T = 2 K). The intrinsic moment is unsaturated, with an applied magnetic field of 6 Tesla causing a 50% increase. Compared with other d-band metals, it has an extremely large electronic heat capacity at low temperatures C/T = 47 mJK −2 mol −1 . ZrZn 2 crystallizes in the C15 cubic Laves structure, with lattice constant a = 7.393Å, the Zr atoms forming a tetrahedrally co-ordinated diamond structure. Its magnetic properties derive from the Zr 4d orbitals, which have a significant direct overlap.Interest in ZrZn 2 has been rekindled by the discovery that it is superconducting at ambient pressure and that both the superconductivity and ferromagnetism are simultaneously destroyed by the application of hydrostatic pressure. For the samples considered here, the onset temperature for the superconductivity is T SC ≈ 0.6 K with B c2 ≈ 0.9 T. The most striking feature of ZrZn 2 , UGe 2 and URhGe is perhaps that the same electrons are thought to participate both to superconductivity and ferromagnetism, in contrast with the situation in other "magnetic" superconductors e.g. borocarbides, RuSr 2 GdCu 2 O 8 [9], where the magnetism and superconductivity occur in different parts of the unit cell. Furthermore, ZrZn 2 is a three-dimensional intermetallic compound in which the itineracy of the d-electrons is almost unquestionable, whereas some doubts remain about that of the 5f electrons and roles of the strong magnetocrystalline anisotropy and quasi-two-dimensional electronic structure in UGe 2 and URhGe.The dHvA effect is due to the quantization of the cyclotron motion of charge carriers and results in a magnetization, M osc , which oscillates ...

Probing the Fermi surface by positron annihilation and Compton scattering

“…24 and 25) caused great excitement, following so soon after the discovery of triplet superconductivity in UGe 2 . 26 The shape of the Fermi surface was the focus of much attention, 27,28 and measurements of de Haas-van Alphen oscillations in the ferromagnetic phase 29 and the paramagnetic Fermi surface by positron annihilation 24 revealed four separate sheets of Fermi surface (see Fig. 2) which were in excellent agreement with the predictions of band structure calculations.…”

Low-Energy Electrodynamics of Heavy Quasiparticles in ZrZn2

“…The calculated DOS shown in Figure 2 is similar to that previously reported. 5,23 The important feature here is that the high DOS mainly consisting of the Zr 4d states is located at E F as shown in the inset of Figure 2.…”

“…4 At low temperatures, heavy quasiparticles (m * ∼ 4.9m 0 ) were observed through a de Haasvan Alphen (dHvA) experiment. 5 The superconductivity of this compound is believed to originate from the strong electron correlation. In other words, the spin fluctuations of the Zr 4d electrons are predicted to affect the heavy quasiparticles such that they exhibit superconductivity.…”

Novel metallic states at low temperatures

“…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.…”