Electronic Degeneracy and Intrinsic Magnetic Properties of Epitaxial$\mathrm{Nb}:\text{ }{\mathrm{SrTiO}}_{3}$ Thin Films Controlled by Defects

Electronic Degeneracy and Intrinsic Magnetic Properties of Epitaxial $Nb : {SrTiO}_{3}$ Thin Films Controlled by Defects

A. Sarantopoulos, E. Ferreiro-Vila, V. Pardo, C. Magén, M. H. Aguirre, and F. Rivadulla

Phys. Rev. Lett. 115, 166801 – Published 14 October 2015

Abstract

We report thermoelectric power experiments in $e$ -doped thin films of ${SrTiO}_{3}$ (STO) which demonstrate that the electronic band degeneracy can be lifted through defect management during growth. We show that even small amounts of cationic vacancies, combined with epitaxial stress, produce a homogeneous tetragonal distortion of the films, resulting in a Kondo-like resistance upturn at low temperature, large anisotropic magnetoresistance, and nonlinear Hall effect. Ab initio calculations confirm a different occupation of each band depending on the degree of tetragonal distortion. The phenomenology reported in this Letter for tetragonally distorted $e$ -doped STO thin films, is similar to that observed in ${LaAlO}_{3} / STO$ interfaces and magnetic STO quantum wells.

Received 8 April 2015

DOI:https://doi.org/10.1103/PhysRevLett.115.166801

Authors & Affiliations

A. Sarantopoulos¹, E. Ferreiro-Vila¹, V. Pardo^2,3, C. Magén^4,5,6, M. H. Aguirre^4,5, and F. Rivadulla^1,*

¹Centro de Investigación en Química Biológica y Materiales Moleculares (CIQUS), Universidad de Santiago de Compostela, 15782-Santiago de Compostela, Spain
²Departamento de Física Aplicada, Universidade de Santiago de Compostela, Santiago de Compostela, Spain
³Instituto de Investigacións Tecnolóxicas, Universidade de Santiago de Compostela, Santiago de Compostela, Spain
⁴Laboratorio de Microscopás Avanzadas, Instituto de Nanociencia de Aragón, Universidad de Zaragoza, 50018 Zaragoza, Spain
⁵Departamento de Física de la Materia Condensada, Universidad de Zaragoza, 50009 Zaragoza, Spain
⁶Fundación ARAID, 50018 Zaragoza, Spain

^*f.rivadulla@usc.es

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Vol. 115, Iss. 16 — 16 October 2015

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Figure 1
(a) XRD patterns of 25(1) nm thick Nb:STO films grown on (001) STO, at different oxygen pressures. Numbers indicate the percentage of tetragonal distortion $Δ t$ . (b) High resolution reciprocal space map around the ( $- 103$ ) reflection, for one film deposited at 1 mTorr. (c) Pressure dependence of the tetragonal distortion for thin films deposited on STO and LSAT. (d) Room temperature carrier density as a function of the tetragonal distortion, determined from Hall effect for samples deposited on STO. The cross corresponds to the theoretical value expected for the nominal Nb doping.
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Figure 2
HAADF-STEM image of Nb:STO thin films deposited on STO (e) and LSAT (a). GPA analysis showing the in-plane $ϵ_{x x}$ (b, f) and out of plane $ϵ_{z z}$ (c, g) elongation, with respect to the substrate. A profile along the film thickness is shown in panels (d) and (h). The analysis indicates a homogeneous tetragonal distortion $Δ t \approx 1.5 %$ for STO and $\approx 2.5 %$ for LSAT.
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Figure 3
(a) Band structure of bulk STO under a tetragonal distortion $Δ t = 2 %$ with the upper band split by spin-orbit coupling. The low-lying doublet is split as a result of the tetragonal distortion. (b) Total number of electrons with $d_{x y}$ symmetry as a function of the total number of $d$ electrons, which decreases as the tetragonal distortion increases. (c) Temperature dependence of the Seebeck coefficient for thin films deposited on STO and (d) LSAT, for different $Δ t$ . The lines are calculations from Eq. (1).
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Figure 4
(a) Hall resistivity at different temperatures. The nonlinear behavior at high field disappears above $T^{*}$ . (b) The resistivity for two samples grown at the same oxygen pressure on different substrates, indicating the $T^{*}$ and the corresponding $Δ t$ . The solid line represents the fitting to the Kondo equation (see Supplemental Material [19]). (c) Temperature dependence of the resistivity for Nb:STO thin films deposited on STO substrates at different oxygen pressures (resulting in a different tetragonal distortion). (d) Variation of $T^{*}$ , as a function of the tetragonal distortion $Δ t$ .
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Figure 5
(a) Anisotropic magnetoresistance, for a 50 nm thick film deposited on STO at 10 mTorr ( $Δ t \approx 0.68 %$ ). The magnetic field is applied in the plane of the film, perpendicular to the current. (b) Magnetoresistance of the same film measured at 2 K, with the magnetic field applied perpendicular to the surface of the film.
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Physical Review Letters

Electronic Degeneracy and Intrinsic Magnetic Properties of EpitaxialNb: SrTiO3 Thin Films Controlled by Defects

A. Sarantopoulos, E. Ferreiro-Vila, V. Pardo, C. Magén, M. H. Aguirre, and F. Rivadulla

Phys. Rev. Lett. 115, 166801 – Published 14 October 2015

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Electronic Degeneracy and Intrinsic Magnetic Properties of Epitaxial $Nb : {SrTiO}_{3}$ Thin Films Controlled by Defects