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Electrostatic carrier doping of GdTiO3/SrTiO3 interfaces  [PDF]
Pouya Moetakef,Tyler A. Cain,Daniel G. Ouellette,Jack Y. Zhang,Dmitri O. Klenov,Anderson Janotti,Chris G. Van de Walle,Siddharth Rajan,S. James Allen,Susanne Stemmer
Physics , 2011, DOI: 10.1063/1.3669402
Abstract: Heterostructures and superlattices consisting of a prototype Mott insulator, GdTiO3, and the band insulator SrTiO3 are grown by molecular beam epitaxy and show intrinsic electronic reconstruction, approximately 1/2 electron per surface unit cell at each GdTiO3/SrTiO3 interface. The sheet carrier densities in all structures containing more than one unit cell of SrTiO3 are independent of layer thicknesses and growth sequences, indicating that the mobile carriers are in a high concentration, two-dimensional electron gas bound to the interface. These carrier densities closely meet the electrostatic requirements for compensating the fixed charge at these polar interfaces. Based on the experimental results, insights into interfacial band alignments, charge distribution and the influence of different electrostatic boundary conditions are obtained.
Low temperature metallic state induced by electrostatic carrier doping of SrTiO$_3$  [PDF]
H. Nakamura,I. H. Inoue,Y. Takahashi,T. Hasegawa,Y. Tokura,H. Takagi
Physics , 2006, DOI: 10.1063/1.2357850
Abstract: Transport properties of SrTiO$_3$-channel field-effect transistors with parylene organic gate insulator have been investigated. By applying gate voltage, the sheet resistance falls below $R_{\Box}$ $\sim$ 10 k$\Omega$ at low temperatures, with carrier mobility exceeding 1000 cm$^2$/Vs. The temperature dependence of the sheet resistance taken under constant gate voltage exhibits metallic behavior ($dR$/$dT$ $>$ 0). Our results demonstrate an insulator to metal transition in SrTiO$_3$ driven by electrostatic carrier density control.
Electrostatic charge accumulation versus electrochemical doping in SrTiO3 electric double layer transistors  [PDF]
Kazunori Ueno,Hidekazu Shimotani,Yoshihiro Iwasa,Masashi Kawasaki
Physics , 2010, DOI: 10.1063/1.3457785
Abstract: In electric double layer transistors with SrTiO3 single crystals, we found distinct differences between electrostatic charge accumulation and electrochemical reaction depending on bias voltages. In contrast to the reversible electrostatic process below 3.7 V with a maximum sheet charge carrier density, nS, of 1014 cm-2, the electrochemical process causes persistent conduction even after removal of the gate bias above 3.75 V. nS reached 1015 cm-2 at 5 V, and the electron mobility at 2 K was as large as 104 cm2/Vs. This persistent conduction originates from defect formation within a few micrometers depth of SrTiO3.
Two-dimensional superconductivity induced by high-mobility carrier doping in LaTiO3/SrTiO3 hetero-structures  [PDF]
J. Biscaras,N. Bergeal,S. Hurand,C. Grossetete,A. Rastogi,R. C. Budhani,D. LeBoeuf,C. Proust,J. Lesueur
Physics , 2011,
Abstract: In this letter, we show that a superconducting two-dimensional electron gas is formed at the LaTiO3/SrTiO3 interface whose transition temperature can be modulated by a back-gate voltage. The gas consists of two types of carriers : a majority of low-mobility carriers always present, and a few high-mobility ones that can be injected by electrostatic doping. The calculation of the electrons spatial distribution in the confinement potential shows that the high-mobility electrons responsible for superconductivity set at the edge of the gas whose extension can be tuned by field effect.
Carrier density independent scattering rate in SrTiO3-based electron liquids  [PDF]
Evgeny Mikheev,Santosh Raghavan,Jack Y. Zhang,Patrick B. Marshall,Adam P. Kajdos,Leon Balents,Susanne Stemmer
Physics , 2015,
Abstract: We examine the carrier density dependence of the scattering rate in two- and three-dimensional electron liquids in SrTiO3 in the regime where it scales with T^n (T is the temperature and n <= 2) in the cases when it is varied by electrostatic control and chemical doping, respectively. It is shown that the scattering rate is independent of the carrier density. This is contrary to the expectations from Landau Fermi liquid theory, where the scattering rate scales inversely with the Fermi energy (E_F). We discuss that the behavior is very similar to systems traditionally identified as non-Fermi liquids (n < 2). This includes the cuprates and other transition metal oxide perovskites, where strikingly similar density-independent scattering rates have been observed. The results indicate that the applicability of Fermi liquid theory should be questioned for a much broader range of correlated materials and point to the need for a unified theory.
Carrier-controlled ferromagnetism in SrTiO3  [PDF]
Pouya Moetakef,James R. Williams,Daniel G. Ouellette,Adam Kajdos,David Goldhaber-Gordon,S. James Allen,Susanne Stemmer
Physics , 2012, DOI: 10.1103/PhysRevX.2.021014
Abstract: Magnetotransport and superconducting properties are investigated for uniformly La-doped SrTiO3 films and GdTiO3/SrTiO3 heterostructures, respectively. GdTiO3/SrTiO3 interfaces exhibit a high-density two-dimensional electron gas on the SrTiO3-side of the interface, while for the SrTiO3 films carriers are provided by the dopant atoms. Both types of samples exhibit ferromagnetism at low temperatures, as evidenced by a hysteresis in the magnetoresistance. For the uniformly doped SrTiO3 films, the Curie temperature is found to increase with doping and to coexist with superconductivity for carrier concentrations on the high-density side of the superconducting dome. The Curie temperature of the GdTiO3/SrTiO3 heterostructures scales with the thickness of the SrTiO3 quantum well. The results are used to construct a stability diagram for the ferromagnetic and superconducting phases of SrTiO3.
Tuning of metal-insulator transition of two-dimensional electrons at parylene/SrTiO$_3$ interface by electric field  [PDF]
H. Nakamura,H. Tomita,H. Akimoto,R. Matsumura,I. H. Inoue,T. Hasegawa,K. Kono,Y. Tokura,H. Takagi
Physics , 2008, DOI: 10.1143/JPSJ.78.083713
Abstract: Electrostatic carrier doping using a field-effect-transistor structure is an intriguing approach to explore electronic phases by critical control of carrier concentration. We demonstrate the reversible control of the insulator-metal transition (IMT) in a two dimensional (2D) electron gas at the interface of insulating SrTiO$_3$ single crystals. Superconductivity was observed in a limited number of devices doped far beyond the IMT, which may imply the presence of 2D metal-superconductor transition. This realization of a two-dimensional metallic state on the most widely-used perovskite oxide is the best manifestation of the potential of oxide electronics.
Oxygen-vacancy-induced charge carrier in n-type interface of LaAlO3 overlayer on SrTiO3 (001): interface vs bulk doping carrier  [PDF]
Yun Li,S. Na Phattalung,S. Limpijumnong,Jaejun Yu
Physics , 2009,
Abstract: We investigated the role of oxygen vacancy in n-type interface of LaAlO3 (LAO) overlayer on SrTiO3 (STO) (001) by carrying out density-functional-theory calculations. Comparing the total energies of the configurations with one vacancy in varying locations we found that oxygen vacancies favor to appear first in LAO surface. These oxygen vacancies in the surface generate a two-dimensional distribution of carriers at the interface, resulting in band bending at the interface in STO side. Dependent on the concentration of oxygen vacancies in LAO surface, the induced carrier charge at the interface partially or completely compensates the polar electric field in LAO. Moreover, the electronic properties of oxygen vacancies in STO are also presented. Every oxygen vacancy in STO generates two electron carriers, but this carrier charge has no effect on screening polar field in LAO. Band structures at the interface dependent on the concentrations of oxygen vacancies are presented and compared with experimental results.
Enhanced spin-orbit coupling and charge carrier density suppression in LaAl1-xCrxO3/SrTiO3 heterointerfaces  [PDF]
Pramod Kumar,Anjana Dogra,P. P. S. Bhadauria,Anurag Gupta,K. K. Maurya,R. C. Budhani
Physics , 2014, DOI: 10.1088/0953-8984/27/12/125007
Abstract: We report a gradual suppression of the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface on substitution of chromium at the Al sites. The sheet carrier density at the interface (ns) drops monotonically from 2.2x10(14) cm-2 to 2.5x10(13) cm-2 on replacing nearly 60 percent of Al sites by Cr and the sheet resistance (Rs) exceeds the quantum limit for localization (h/2e2) in the concentrating range 40 to 60 percent of Cr.
Electric Field Tuned Dimensional Crossover in Ar-Irradiated SrTiO3  [PDF]
J. H. Ngai,Y. Segal,F. J. Walker,S. Ismail-Beigi,K. Le Hur,C. H. Ahn
Physics , 2009,
Abstract: We present low temperature magnetoresistance measurements of Ar-irradiated SrTiO3 under an applied electrostatic field. The electric field, applied through a back gate bias, modulates both the mobility and sheet density, with a greater effect on the former. For high mobilities, 3-dimensional orbital magnetoresistance is observed. For low mobilities, negative magnetoresistance that is consistent with the suppression of 2-dimensional weak-localization is observed. The crossover from 3 to 2-dimensional transport arises from a modulation in the carrier confinement, which is enhanced by the electric field dependent dielectric constant of SrTiO3. The implications of our results on the development of oxide electronic devices will be discussed.
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