We report a combined infrared and angle-resolved photoemission study of the electronic response of Sr 3 (Ir 1-x Ru x ) 2 O 7 (x=0, 0.22, 0.34). The low-temperature optical conductivities of the three compounds exhibit the characteristic feature of the effective total angular momentum J eff =1/2 antiferromagnetic Mott state. As the temperature increases across the antiferromagnetic ordering temperature T N , the indirect gap gradually closes whereas the direct gap remains open. In the optical conductivity of Sr 3 (Ir 0.66 Ru 0.34 ) 2 O 7 which shows a thermally driven insulator-metal transition at T N , a Drude-like response from itinerant carriers is registered in the paramagnetic phase. We observe in angle-resolved photoemission data of Sr 3 (Ir 0.66 Ru 0.34 ) 2 O 7 that the valence band shifts continuously toward the Fermi energy with the weakening of the antiferromagnetic order and crosses the Fermi level in the paramagnetic phase. Our findings demonstrate that the temperature-induced metalinsulator transition of the Sr 3 (Ir 1-x Ru x ) 2 O 7 system should be attributed to a magnetically driven band shift. * These two authors contributed equally. † yeongkwan@kaist.ac.kr ‡ soonjmoon@hanyang.ac.kr A discovery of the relativistic Mott state in Sr 2 IrO 4 [1,2] suggested that the Mott physics can be applicable in 5d transition metal oxides and stimulated extensive studies on the nature of their metal-insulator transitions. While the electromagnetic properties of Sr 2 IrO 4 were successfully explained in terms of an effective total angular momentum J eff =1/2 Mott state [1-5], a number of experimental and theoretical studies suggested that its ground state should instead be envisioned as a Slater insulator or as an intermediate phase between the Mott and Slater insulators [6-9]. In the Slater picture, the metal-insulator transition occurs at antiferromagnetic ordering temperature T N via a continuous opening of the band gap due to the appearance of a magnetic supercell [10]. Pyrochlore iridates R 2 Ir 2 O 7 (R=Nd, Sm, and Eu) which have attracted much attention as potential candidates for realizing correlated topological insulators/semimetals [11-13] exhibit a continuous metal-insulator transition accompanying the onset of antiferromagnetic order [14]. A recent angle-resolved photoemission spectroscopy (ARPES) experiment on Nd 2 Ir 2 O 7 [15] observed a gap opening at T N with an energy shift of quasiparticle peaks in a fashion similar to the Slater transition. The continuous metal-insulator transitions at T N in Cd 2 Os 2 O 7 andNaOsO 3 were also attributed to the Slater transition in early studies [16][17][18][19]. Recently, however, the metalinsulator transitions of the two osmates were revisited and ascribed to the Lifshitz-type transition [20][21][22][23].Density-functional-theory calculations showed that the metal-insulator transitions of Cd 2 Os 2 O 7 [20] and NaOsO 3 [21] involved a continuous shift of the bands away from the Fermi level and the resulting vanishing of the Fermi surface with decreasing the t...
Rotation of MO6 (M = transition metal) octahedra is a key determinant of the physical properties of perovskite materials. Therefore, tuning physical properties, one of the most important goals in condensed matter research, may be accomplished by controlling octahedral rotation (OR). In this study, it is demonstrated that OR can be driven by an electric field in Sr2RuO4. Rotated octahedra in the surface layer of Sr2RuO4 are restored to the unrotated bulk structure upon dosing the surface with K. Theoretical investigation shows that OR in Sr2RuO4 originates from the surface electric field, which can be tuned via the screening effect of the overlaid K layer. This work establishes not only that variation in the OR angle can be induced by an electric field, but also provides a way to control OR, which is an important step toward in situ control of the physical properties of perovskite oxides.
Pure quantum electrons render intriguing correlated electronic phases by virtue of quantum fluctuations in addition to an exclusive electron-electron interaction. To realise such quantum electron systems, a key ingredient is dense electrons decoupled from other degrees of freedom. Here, we report the discovery of a pure quantum electron liquid, which spreads up to ~ 3 Å in the vacuum on the surface of electride crystal. An extremely high electron density and its scant hybridization with underneath atomic orbitals evidence quantum and pure nature of electrons, exhibiting polarized liquid phase demonstrated by spin-dependent measurement. Further, upon reducing the density, the dynamics of quantum electrons drastically changes to that of non-Fermi liquid along with an anomalous band deformation, manifesting a possible transition to a hexatic liquid crystalline phase. Our findings cultivate the frontier of quantum electron systems, which serve as an ideal platform for exploring the correlated electronic phases in a pure manner.
We report on systematic temperature and doping dependent measurements of the Fe 3s core level photoemission spectra in the normal state of superconducting Sr(Fe 1-x Co x) 2 As 2. The analysis of the Fe 3s spectrum provides an element-specific determination of the mean value of the magnitude of the Fe spin moment measured on the fast (10-16-10-15 s) timescale of the photoemission process. The data reveal the ubiquitous presence in the normal state of Fe spin moments with magnitude fluctuating on short time scales. The data reveal a significant reduction of the magnitude of the effective Fe spin moment on going from the parent to the optimal doped compound. The doping dependence of the magnitude of the spin moment at higher doping level is less clear, being either constant, or even non-monotonic, depending on temperature. This phenomenology indicates the importance of the interaction between spin and itinerant degrees of freedom in shaping the properties of the normal state. These findings reaffirm the complexity of the normal state of 122 Fe-pnictides, which are typically viewed as the least correlated of the high temperature unconventional superconductors.
Inverted structures of common crystal lattices, referred to as antistructures, are rare in nature due to their thermodynamic constraints imposed by the switched cation and anion positions in reference to the original structure. However, a stable antistructure formed with mixed bonding characters of constituent elements in unusual valence states can provide unexpected material properties. Here, we report a heavy-fermion behaviour of ferromagnetic gadolinium lattice in Gd3SnC antiperovskite, contradicting the common belief that ferromagnetic gadolinium cannot be a heavy-fermion system. The specific heat shows an unusually large Sommerfeld coefficient of ~ 1114 mJ⋅mol− 1⋅K− 2 with a logarithmic behaviour of non-Fermi-liquid state. We demonstrate that the heavy-fermion behaviour in the non-Fermi-liquid state appears to arise from the hybridized electronic states of gadolinium 5d-electrons participating in metallic Gd–Gd and covalent Gd–C bonds. These results accentuate unusual chemical bonds in CGd6 octahedra with the dual characters of gadolinium 5d-electrons for the emergence of heavy-fermions.
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