Tuning the Electronic Structure of Sr ₂ IrO ₄ Thin Films by Bulk Electronic Doping Using Molecular Beam Epitaxy

Abstract

By means of oxide molecular beam epitaxy with shutter-growth mode, we have fabricated a series of electrondoped (Sr 1−x La x ) 2 IrO 4 (001) (x = 0, 0.05, 0.1 and 0.15) single crystalline thin films and then investigated the doping dependence of electronic structure utilizing in-situ angle-resolved photoemission spectroscopy. We find that with increasing doping proportion, the Fermi levels of samples progressively shift upward. Prominently, an extra electron pocket crossing the Fermi level around the M point has been evidently observed in 15% nominal doping sample. Moreover, bulk-sensitive transport measurements confirm that doping effectively suppresses the insulating state with respect to the as-grown Sr 2 IrO 4 , though doped samples still remain insulating at low temperatures due to the localization effect possibly stemming from disorders including oxygen deficiencies. Our work provides another feasible doping method to tune electronic structure of Sr 2 IrO 4 . 71.30.+h, 73.20.r, 77.55.Px 5d transition-metal oxides (TMOs) have recently drawn a lot of attention. In these compounds, the complicated interplay between spin-orbit coupling (SOC) and electron correlations has been suggested to host multiple novel quantum states, including topological Mott insulators 1 , Weyl semimetals 2,3 , axion insulators 4,5 , and spin liquids 6-8 . These studies were mostly initialized by the pioneering experiments on the prototype layered perovskite Sr 2 IrO 4 , in which the strong SOC was revealed to lift the orbital degeneracy and then result in a narrow half-filled J e f f = 1/2 band that even the relatively weak Coulomb repulsion of 5d electrons could induce a Mott metal-insulator transition (MIT) therein 9,10 . Scattering experiments discovered that, this Mott insulator, showing an effective pseudospin 1/2 antiferromagnetic (AFM) order at low temperature, can be well described by the Heisenberg model with an exchange coupling of 60 to 100 meV 11,12 . Such findings indicate that the low-energy behavior of this single-layer iridate rather resembles that of cuprates. Consequently, it is tempting to investigate the carrier doping of Sr 2 IrO 4 , which might pave the way to discover a new family of unconventional superconductors. Indeed, a recent theoretical work has predicted the unconventional superconductivity in the electron-doped Sr 2 IrO 4 13 .In this context, various experimental attempts have been performed to achieve the effective carrier doping of Sr 2 IrO 4 14-20 . So far, one of the most promising progresses is the try of surface electron doping of Sr 2 IrO 4 via in-situ potassium deposition, in which Fermi arcs and pseudogap behavior have been reported 21 ; nevertheless, this finding is not consistent with the reports on the sibling La-doped Sr 3 Ir 2 O 7 , in which only small Fermi pockets rather than Fermi arcs were revealed 22,23 . This controversy naturally raises one open question, namely, whether the existence of Fermi arcs is an universal property of the electron-doped iridates irrespective of the specifi...

Electron-DopedSr2IrO4: An Analogue of Hole-Doped Cuprate Superconductors Demonstrated by Scanning Tunneling Microscopy

“…The gap magnitude changes with K coverage (25-30 meV for 0.5 ML and 0.6 ML, 10 meV for 0.7 ML). Hereafter, it is called the low-energy gap or LEG, to distinguish it from the high-energy pseudogap previously found in ARPES [14][15][16]. For all the observed LEG's, about 95% of DOS vanishes near E F .…”

“…Experimentally, electron doping was realized in Sr 2 IrO 4 by La substitution, oxygen deficiency, or surface K dosing [4,[14][15][16], while hole doping was realized by Rh substitution of Ir [17]. A unique electronic state with nodal quasiparticles and an antinodal pseudogap was found in the electron-underdoped regime by angle-resolved photoemission spectroscopy (ARPES) [14][15][16], resembling the underdoped cuprates. However, no experimental evidence of superconductivity has been found up to now.…”

“…Recently, the 5d transition metal oxide Sr 2 IrO 4 has attracted much attention because it possesses several distinct characteristics that are considered to be important for the high-temperature superconductivity [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Sr 2 IrO 4 is isostructural to La 2 CuO 4 , which adopts the same quasitwo-dimensional (2D) layered perovskite structure of K 2 NiF 4 [2][3][4].…”

Collapse of the Mott Gap and Emergence of a Nodal Liquid in Lightly DopedSr2IrO4

“…Consistent with an earlier report on undoped Sr 2 IrO 4 [13], we find the top of the lower Hubbard band (LHB) at the ðπ; 0Þ point. Yet, already for x ¼ 0.01, the low-energy spectral weight shifts to ðπ=2; π=2Þ [23,24]. On increasing the doping to x ¼ 0.05, coherent excitations emerge along arcs stretching out from the nodal direction, while the spectral weight along the Ir-Ir nearest neighbor direction remains weak and is devoid of sharp features, reminiscent of the nodal-antinodal dichotomy in underdoped cuprates [3,25].…”

Interface-engineered hole doping in Sr2IrO4/LaNiO3 heterostructure

“…Recent measurements have provided many evidences for the possible superconducting hidden phase in Sr 2 IrO 4 [14][15][16][17][18]. Despite the early encouraging progress, to date, there is no report on superconducting behavior in doped Sr 2 IrO 4 [19][20][21][22]. Naturally, one of the possible reasons could be that the degree of doping has not reached the critical value from which the superconducting state can emerge.…”