Nature of the Mott Transition inCa2RuO4

Abstract

We study the origin of the temperature-induced Mott transition in Ca2RuO4. As a method we use the local-density approximation+dynamical mean-field theory. We show the following. (i) The Mott transition is driven by the change in structure from long to short c-axis layered perovskite (L-Pbca → S-Pbca); it occurs together with orbital order, which follows, rather than produces, the structural transition. (ii) In the metallic L-Pbca phase the orbital polarization is ∼ 0. (iii) In the insulating S-Pbca phase the lower energy orbital, ∼ xy, is full. (iv) The spin-flip and pair-hopping Coulomb terms reduce the effective masses in the metallic phase. Our results indicate that a similar scenario applies to Ca2−xSrxRuO4 (x ≤ 0.2). In the metallic x ≤ 0.5 structures electrons are progressively transferred to the xz/yz bands with increasing x, however we find no orbital-selective Mott transition down to ∼ 300 K. ) undergoes a paramagnetic metal-paramagnetic insulator transition (MIT) at T MIT ∼ 360 K [1]. A similar insulator-to-metal transition happens also by application of a modest (∼ 0.5 GPa) pressure [2] and finally when Ca is partially substituted by Sr (Ca 2−x Sr x RuO 4 , x ≤ 0.2) [3,4]. The nature of these transitions, in particular across x = 0.2, has been debated for a decade [5][6][7][8][9][10][11][12][13]. While it is clear that a Mott-type mechanism makes the 2/3-filled t 2g bands insulating, two opposite scenarios, with different orbital occupations n = (n xy , n xz + n yz ) and polarizations p ≡ n xy − (n xz + n yz )/2, have been suggested. In the first, only the xy band becomes metallic, i.e. the transition is orbital-selective (OSMT) [5]; n and p jump from (2, 2) and 1 in the insulator to (1, 3) and −1/2 in the metal. In the second, there is a single Mott transition, assisted by the crystal-field splitting ∆ = ǫ xz/yz −ǫ xy > 0 [13], similar to the case of 3d 1 perovskites [14]; p > 0 in all phases. To date the issue remains open. Recently, for x = 0.2 a novel (xy insulating, n xy = 1.5 and p = 1/4) OSMT was inferred from angle-resolved photoemission (ARPES) experiments [7], but other ARPES data show three metallic bands and no OSMT [8].Ca 2 RuO 4 is made of RuO 2 layers built up of cornersharing RuO 6 octhahedra (space group Pbca [3,15]). This structure (Fig. 1) combines a rotation of the octahedra around the c axis with a tilt around the b axis. It is similar to that of the tetragonal unconventional superconductor Sr 2 RuO 4 ; the corresponding pseudo-tetragonal axes x, y and z are shown in Fig. 1. The structure of Ca 2 RuO 4 is characterized by a long c axis (L-Pbca) above T S ∼ 356 K and by a short one (S-Pbca) below T S . The L-and S-Pbca phases are also observed in Ca 2−x Sr x RuO 4 for all x ≤ 0.2, but T S decreases with increasing x; for x > 0.2 the system becomes tetragonal (for x < 1.5: I4 1 /acd, c-axis rotations only).Because of the layered structure, the ∼ xz, yz band- width, W xz/yz , is about one half of the ∼ xy bandwidth, W xy . Due to the structural distortions, the t 2g manifold splits i...

Mott transition, spin-orbit effects, and magnetism in Ca2RuO4

“…Ca 2 RuO 4 itself exhibits a peculiar paramagnetic metal-insulator transition [6] (MIT) at T MIT = 360 K, basically concurrent with the change from L-Pbca (long c axis) to S-Pbca (short c axis) structure [8,9] at T S = 356 K. Similar transitions have been reported when Ca is partially replaced by Sr (x 0.2) or under pressure [10,11]. The origin of the MIT has been intensively investigated, both experimentally [5][6][7][8][9][10][11][12][13][14][15] and theoretically [16][17][18][19][20][21][22][23][24][25][26]. Electronically, Ca 2 RuO 4 is characterized by 2/3-filled t 2g bands (t 4 2g e 0 g electronic configuration).…”

“…The first, U = 3.1 eV and J = 0.7 eV, has been obtained for Sr 2 RuO 4 , the sister compound of Ca 2 RuO 4 , via the constrained local-density approximation (cLDA) [41]. These parameters have been already successfully used [23] to describe the metal-insulator transition in Ca 2 RuO 4 with the LDA+DMFT approach. The second set of parameters are the constrained random-phase approximation (cRPA) values [42], U = 2.3 eV and J = 0.4 eV.…”

“…From angle-resolved photoemission (ARPES) data, for x = 0.2 a new type of OSMT (p = 1/4,n xy = 1.5) was inferred [13]; other ARPES experiments reported, however, three metallic bands [14] and no OSMT. Later, accurate local-density approximation + dynamical mean-field theory (LDA+DMFT) calculations [23] showed that the change in crystal structure from L-Pbca to S-Pbca is a decisive factor, leading to a reduction of the bandwidth ratio R and to an enhancement of the crystal-field splitting ε CF ; in the metallic L-Pbca phase, the orbital polarization is p ∼ 0 (no orbital order) and three metallic bands are obtained, in line with ARPES results from Ref. [14].…”

Magnetic excitations in the metallic single-layer ruthenates Ca2−xSrxRuOSteffens1, Friedt2, Sidis3 et al. 2011Phys. Rev. B

“…The large width and the significant overlap of the single contributions do not allow us to resolve if there are even more structures intrinsic to these fluctuations or not. At least an additional ferromagnetic component, which is discussed in detail in the next section, still plays some role at this energy and is the likely origin of the less well-pronounced minimum at (1,0,0) and the slightly lower q 1 in comparison to Ca 1.8 33,34,36,37 Qualitatively this picture of orbital order seems to be valid for all Ca 2−x Sr x RuO 4 undergoing the metal-insulator transition, i.e., for x < 0.2. The electronic structure of the metallic samples with a slightly larger Sr content studied here remains a matter of controversy both in experiment and in theory.…”

Higgs mode and its decay in a two-dimensional antiferromagnet

“…1). Because the local symmetry around the Ru(IV) ion is very low 8,9 (having only inversion symmetry), it is widely believed that the orbital moment is completely quenched by the crystalline electric field [10][11][12][13] , which is dominated by the compressive distortion of the RuO 6 octahedra along the c-axis (Fig. 1).…”