Tight-binding models for the recently observed surface electronic bands of SrTiO3 and KTaO3 are analyzed with a view to bringing out the relevance of momentum-space chiral angular momentum structures of both orbital and spin origins. Depending on the strength of electric field associated with inversion symmetry breaking at the surface, the orbital and the accompanying spin angular momentum structures reveal complex linear and cubic dependencies in the momentum k (linear and cubic Rashba effects, respectively) in a band-specific manner. Analytical expressions for the cubic orbital and spin Rashba effects are derived by way of unitary transformation technique we developed, and compared to numerical calculations. Due to the C4v symmetry of the perovskite structure the cubic Rashba effect appears as in-plane modulations. The discovery of surface electronic states in strontium titanate (SrTiO 3 )[1, 2] has stirred great excitement at the time the material is being viewed as a critical component of the emerging field of oxide electronics [3]. The origin of surface states in SrTiO 3 and a related material KTaO 3 [4, 5] (STO and KTO for short, respectively) is currently under active investigation [5,6]. Both materials' surface states originate from t 2g -orbitals whose relevant tight-binding parameters for the electronic structure are largely determined, including the one pertaining to the degree of inversion symmetry breaking (ISB) at the surface [5,6].Several features make STO and KTO surface states an ideal ground for the study of Rashba-related phenomena. First is the way that Rashba effects would play out among the several observed bands of differing orbital characters. ARPES measurements up to now [1, 2, 4, 5] did not clearly resolve the Rashba-split bands, presumably due to the smallness of the predicted Rashba parameter [7]. Transport measurements do reveal the Rashba term, of cubic order in momentum, through analysis of the orientation-dependent magneto-resistance data on STO surface [8,9]. Existing theories treat Rashba effects of t 2g -derived bands phenomenologically [6, 10] and cannot, for instance, explain the complex band-specific spin and orbital angular momentum structures observed in the electronic structure calculation [7].It has recently been argued that multi-orbital bands, subject to the surface ISB electric field, must give rise to an entity called the chiral orbital angular momentum (OAM) in momentum space [11,12]. The argument remains valid as long as the crystal field splitting does not quench the multi-orbital degrees of freedom in a given band structure. Such conditions seem to be well met in both STO and KTO, leading to the term ∼ k × E · L where L is the OAM operator for t 2g -orbitals, k is the linear momentum, and E is the surface-normal electric field. The effect was dubbed "orbital Rashba effect" [11] in analogy to the similar chiral structure of spins on the surface [13]. It was shown that pre-existing chiral OAM structure implies the linear Rashba effect upon the inclusion of spin-orb...
The quantum Hall conductance in monolayer graphene on an epitaxial SrTiO3 (STO) thin film is studied to understand the role of oxygen vacancies in determining the dielectric properties of STO. As the gate voltage sweep range is gradually increased in our device, we observe systematic generation and annihilation of oxygen vacancies evidenced from the hysteretic conductance behavior in graphene. Furthermore, based on the experimentally observed linear scaling relation between the effective capacitance and the voltage sweep range, a simple model is constructed to manifest the relationship among the dielectric properties of STO with oxygen vacancies. The inherent quantum Hall conductance in graphene can be considered as a sensitive, robust, and non-invasive probe for understanding 2 the electronic and ionic phenomena in complex transition metal oxides without impairing the oxide layer underneath.Oxygen plays an indispensable role in determining the functional properties of transition metal oxides (TMOs), such as superconductivity, memristive behavior, and topotactic phase reversal. [1][2][3][4][5][6] In STO, a prototypical perovskite oxide, changes in both electronic and ionic behaviors were observed by introducing oxygen vacancies. [7] Oxygen vacancies in STO can be generated via the electric-field-induced redox process or the application of chemical pressure at high temperatures. They supply conducting electrons (3d 1 ) to normally insulating STO (3d 0 ), which can eventually form Cooper pairs at low temperatures (~1 K). [8] On the other hand, oxygen vacancies generated/redistributed via electric-field-induced redox reaction near the metal/oxide interface might further modify the metal/STO heterojunction and give rise to functional phenomena such as resistive switching. [1][2]6,[9][10][11][12][13] Modulation of the local conductivity using a biased conductive atomic force microscopy tip and observation of gas bubbles beneath the anodic metal electrodes indicate the electroforming of oxygen vacancies which affect the transport property of STO critically. [10][11] More recently, it has also been shown that oxygen vacancies can be created just by depositing the active metal, directly observed by the transmission electron microscopy (TEM). [14] However, the role of the ionic motion in modifying the fundamental dielectric properties of STO near the heterojunction is yet to be understood, despite its important implications in the electronic applications.In order to perceive the formation of oxygen vacancies and estimate their influence on the dielectric behavior of STO, we propose the 2D transport nature of graphene as a sensitive, robust, and non-invasive probe for examining the layer underneath. Among various 3 characteristics of graphene, linear dispersion relation and the gapless feature give rise to a delicate electronic response to an external electric field. In addition, a universal quantum Hall effect in graphene is exclusively dependent on its carrier concentration. Therefore, the position and shape of the char...
Most heterostructures are realized using materials within the same structural families, such as compound semiconductors, perovskite oxides, and more recently van der Waals heterostructures. [1][2][3][4] These conventional heterostructures have their advantages in the epitaxial matching of lattices with minimized structural defects. [5,6] They are also easy to apply and control the homogeneous epitaxial strain. Nevertheless, heterostructures with structurally distinct layers can also be conceived. If the interface between the dissimilar layers can be well defined, the heterostructure is equally feasible as the conventional heterostructures composed of the same structural families. This vastly expands the possibilities and potential of the heterostructures vastly as the combination of the materials is now multidimensional. Since the discovery of 2D layered materials (2DLM), including graphene, numerous studies have focused on their behavior on different substrates. Ideally, freestanding 2DLM is theoretically plausible, but they are rather difficult to achieve experimentally. In particular, homogeneous control of strain is rather challenging for freestanding 2D layers. Therefore, the choice of the substrate materials for the 2DLMs is of prime importance in discovering, studying, and utilizing novel properties. While Si-based semiconductor materials or simple metals such as Cu or Au have predominantly been utilized as the substrates for 2DLMs, [7,8] design of more exciting properties is achieved by adopting functional materials. When a 2DLM is fabricated on top of a substrate material, a heterostructure composed of two distinct materials is naturally achieved.Complex transition metal oxides (TMOs), or so-called functional oxides, foster a variety of exotic electrical and magnetic behaviors, including superconductivity, colossal magnetoresistance, 2D electron liquid, and multiferroicity. [9][10][11][12][13] The strongly correlated electronic nature originates from the strong polarizability of O ions in the interatomic scale, resulting in the versatile properties depending on the kind of transition metal element. With the recent advancement of atomic-scale epitaxy, an atomistic layer design of the physical properties of the TMOs is plausible, extending their potential with regard to "oxide electronics."In terms of the heterostructures, oxides have long served as the "gate dielectric layer" within conventional semiconductor technology. Considerable effort is directed toward the The marriage between a 2D layered material (2DLM) and a complex transition metal oxide (TMO) results in a variety of physical and chemical phenomena that cannot be achieved in either material alone. Interesting recent discoveries in systems such as graphene/SrTiO 3 , graphene/LaAlO 3 /SrTiO 3 , graphene/ferroelectric oxide, MoS 2 /SrTiO 3 , and FeSe/SrTiO 3 heterostructures include voltage scaling in field-effect transistors, charge state coupling across an interface, quantum conductance probing of the electrochemical activity, novel memory funct...
Electrical transport in monolayer graphene on SrTiO3 (STO) thin film is examined in order to promote gate-voltage scaling using a high-k dielectric material. The atomically flat surface of thin STO layer epitaxially grown on Nb-doped STO single-crystal substrate offers good adhesion between the high-k film and graphene, resulting in nonhysteretic conductance as a function of gate voltage at all temperatures down to 2 K. The two-terminal conductance quantization under magnetic fields corresponding to quantum Hall states survives up to 200 K at a magnetic field of 14 T. In addition, the substantial shift of charge neutrality point in graphene seems to correlate with the temperature-dependent dielectric constant of the STO thin film, and its effective dielectric properties could be deduced from the universality of quantum phenomena in graphene. Our experimental data prove that the operating voltage reduction can be successfully realized due to the underlying high-k STO thin film, without any noticeable degradation of graphene device performance.
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