Insulator-to-Metal Transition at Oxide Interfaces Induced by WO3

Subscriber access provided by READING UNIV

Article

Insulator to metal transition at oxide interfaces induced by WO3 overlayers Giordano Mattoni, David J Baek, Nicola Manca, Nils Verhagen, Dirk Groenendijk, Lena F. Kourkoutis, Alessio Filippetti, and Andrea D. Caviglia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13202 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Insulator to metal transition at oxide interfaces induced by WO3 overlayers Giordano Mattoni,∗,† David J. Baek,‡ Nicola Manca,† Nils Verhagen,† Dirk J. Groenendijk,† Lena F. Kourkoutis,¶ Alessio Filippetti,§ and Andrea D. Caviglia† †Kavli Institute of Nanoscience, Delft University of Technology, 2628 CJ Delft, Netherlands ‡School of Electrical and Computer Engineering, Cornell University, Ithaca, New York 14853, USA ¶School of Applied and Engineering Physics and Kavli Institute at Cornell for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA §Dipartimento di Fisica, Università di Cagliari, and CNR-IOM, Istituto Officina dei Materiali, Cittadella Universitaria, Cagliari, Monserrato 09042-I, Italy E-mail: [email protected]

Abstract Interfaces between complex oxides constitute a unique playground for two-dimensional electron systems (2DES), where superconductivity and magnetism can arise from combinations of bulk insulators. The 2DES at the LaAlO3 /SrTiO3 interface is one of the most studied in this regard, and its origin is determined by the polar field in LaAlO3 as well as by the presence of point defects, like oxygen vacancies and intermixed cations. These defects usually reside in the conduction channel and are responsible for a decrease of the electronic mobility. In this work, we use an amorphous WO3 overlayer to obtain a high mobility 2DES in WO3 /LaAlO3 /SrTiO3 heterostructures. The studied system shows a sharp insulator-to-metal transition as a function of both LaAlO3

1 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and WO3 layer thickness. Low-temperature magnetotransport reveals a strong magnetoresistance reaching 900% at 10 T and 1.5 K, the presence of multiple conduction channels with carrier mobility up to 80 000 cm2 V−1 s−1 and quantum oscillations of conductance.

KEYWORDS WO3 overlayers, LaAlO3 /SrTiO3 interface, metal–insulator, high mobility, strong classical magnetoresistance, two-dimensional electron systems, quantum oscillations

1 INTRODUCTION The formation of a two-dimensional electron system (2DES) at the interface between band insulators SrTiO3 (STO) and LaAlO3 (LAO) is among the most intriguing effects studied in oxide electronics. 1 Gate-tunable superconductivity, 2,3 strong spin-orbit coupling 4,5 and magnetism 6,7 are some of the many phenomena observed. The origin of this 2DES is a long-standing question and recent results indicate that a consistent picture should take into account both the built-in polar field and the presence of point defects. 8–10 Among these, oxygen vacancies and cation off-stoichiometry in STO are capable of inducing a 2DES. 11,12 However, while defects can contribute to the conductivity, they also act as scattering centres within the potential well, lowering the electronic mobility. 13 In order to promote high electron mobility, it is desirable to spatially separate the donor sites from the conducting plane, while allowing 2DES formation in the STO top layers. There have been various approaches to control the defect concentration profile and enhance the mobility which involve the use of crystalline insulating overlayers, 14,15 adsorbates, 16 amorphous materials 17 and even thin metallic layers. 18,19 The potential of these overlayers for defect management is determined by their reactivity and capacity to host defects such as oxygen vacancies. A promising material 2 ACS Paragon Plus Environment

Page 2 of 25

Page 3 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in this respect is the transition metal oxide WO3 . Both crystalline and amorphous WO3 can host vacancies and interstitial atoms, thus allowing cation accommodation and diffusion, with a tendency to form compounds such as tungsten bronzes. 20,21 In addition, the several possible oxidations states of tungsten make WO3 particularly active in undergoing redox reactions and, for this reason, this material is often utilized in electrochemical applications and electrochromic devices. 22–24 In this work, we combine the use of a crystalline LAO/STO interface with the high reactivity of amorphous WO3 to realise a high-mobility 2DES in WO3 /LAO/STO heterostructures. The WO3 overlayers reduce the critical LAO thickness required for the formation of a 2DES. We characterise the transport properties of this system as a function of WO3 and LAO thickness and find multiple conduction channels with high electron mobility up to 80 000 cm2 V−1 s−1 . The multi-channel conduction leads to a remarkably strong classical magnetoresistance, which reaches 900% at 10 T and 1.5 K. In the low-temperature regime, Shubnikov-de Haas oscillations indicate strong two-dimensional confinement of the charge carriers. Our results underscore WO3 as an efficient extrinsic dopant for oxide interfaces that can be used to engineer novel conductive electron systems.

2 RESULTS AND DISCUSSION 2.1 Structural characterisation Heterostructures of amorphous WO3 and crystalline LAO are grown on TiO2 -terminated STO (001) substrates by pulsed laser deposition (details on the growth and X-rays diffraction analysis are provided in Figures S1 and S2). We use the notation (m, n) to denote the heterostructures consisting of m unit cells (uc) of WO3 and n uc of LAO. Since WO3 is amorphous, this corresponds to the crystalline equivalent number of unit cells. To inves-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

Page 4 of 25

Amorphous WO3 (4 uc) LaAlO3 (2 uc)

SrTiO3 substrate

10 nm

c

b

La

1 nm

Ti

1 nm

d

1 nm

e

La+Ti

f

La

g

Ti

1 nm

0 I/Imax

400 nm

1

Figure 1: Structural characterisation of WO3 /LAO/STO heterostructures. (a) Lower magnification and (b) close-up HAADF-STEM image from a (4, 2) heterostructure along the (001) direction. (c) EELS elemental map showing normalized core-loss signals for La-M4,5 , (d) Ti-L2,3 edges and (e) combined signal. (f) Normalised EELS intensity profile averaged along the direction perpendicular to the interface. (g) Surface topography by AFM. tigate the atomic structure of the WO3 /LAO/STO system, we perform high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The HAADF-STEM images in Figures 1a and 1b acquired on a (4, 2) heterostructure show a uniform layer of amorphous WO3 corresponding to 4 uc in thickness, on top of 2 uc crystalline LAO. Due to the difference in atomic number of La and Sr, the HAADF signal from the LAO is more intense than that of the underlying STO. To further confirm the crystallinity and conformity of the LAO layers, electron energy loss spectroscopy (EELS) is performed. With an energy dispersion of 0.25 eV/channel, the Ti-L2,3 and La-M4,5 edges are recorded simultaneously, providing atomic-resolution Ti and La elemental maps as presented in Figures 1c to 1e. By averaging the La map parallel to the interface in Figure 1f, two clear peaks are found for La, consistent with the growth of 2 LAO layers in our heterostructure. The measurements remark a crystalline LAO/STO interface, with small amount of La interdiffusion into the substrate. However, we observe significant diffusion of La into the WO3 layer. The sur4 ACS Paragon Plus Environment

Page 5 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


face of all heterostructures is additionally measured by atomic force microscopy (Figure 1g), revealing the steps and terraces of the underlying STO substrate, indicating uniform film growth.

2.2 Insulator-to-metal transition The series of resistance versus temperature curves of (m, n) heterostructures in Figure 2a shows a sharp thickness dependent insulator-to-metal transition. The transport measurements are performed in a Van der Pauw configuration (see methods for details). For different (m, n) combinations the samples show either insulating (orange curves) or metallic (blue curves) character, with a sharp transition between the two regimes as function of layer thickness. This is particularly evident when comparing the (4, 1) and (4, 2) curves, where a variation of a single uc of the LAO interlayer determines a three orders of magnitude difference in room temperature resistivity, which diverges upon cooling. The onset of the metallic state corresponds to the sheet resistance value h/e2 (dotted line in Figure 2a) which is the quantum limit for metallicity in 2D, 25–27 providing strong evidence for the two-dimensional nature of this electronic system. The interplay between WO3 and LAO thicknesses is summarised in the phase diagram of Figure 2b, where we indicate the (m, n) combinations resulting in insulating samples with a shaded orange background. For LAO-only films (0, n) we reproduce the critical thickness for metallicity of 4 uc in crystalline LAO/STO interfaces, while samples with only WO3 (m, 0) are always insulating. Heterostructures with 1 uc of LAO (m, 1) are insulating independent of the WO3 layer thickness. When n = 2 the insulating state persists for WO3 thickness m ≤ 2 only, above which a metallic state is induced. With 3 cells of LAO a single layer of WO3 is enough to trigger the metallic state. The main difference between our metallic WO3 /LAO/STO heterostructures compared to typical LAO/STO interfaces manifests itself below 30 K. While the resistivity curves ap-



a

108

ρxx (Ω/square)

107

Measurement Limit

105 10

(8,1)

(1,2)

h/e 2

4

103 102 101

b

(4,1)

(2,2)

106

n LaAlO3 (unit cells)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 25

(0,4)

(m, n)

WO3

(8,2) (3,2) (4,2) 0

50

SrTiO3

100 150 Temperature (K)

200

LaAlO3

250

300

Insulating Metallic

4 3

10

100

ρ300K/ρ1.5K

2

1000

1 0 0

1

c

2

3 4 5 m WO3 (unit cells)

6

7

8

E 5.6 eV 2.6 eV 3.2 eV STO

2 uc LAO

WO3

Figure 2: Insulator-to-metal transition in WO3 /LAO/STO heterostructures. (a) Resistance versus temperature for different (m, n) thickness combinations and (b) transport phase diagram showing insulating (orange crosses) and metallic (stars with colour scale) heterostructures. The colour scale indicates the residual-resistance ratio for the metallic samples and the orange shaded area marks the (m, n) combinations constituting insulating heterostructures. (c) Schematic of the energy levels alignment for a (m ≥ 3, 2) heterostructure. Valence band (solid colours), conduction band (shaded colours) and amorphous WO3 energy levels. Charge transfer from the oxygen vacancy levels, residing just below WO3 conduction states, to STO is indicated by the yellow arrow. proximately overlap above 30 K, the resistivity of the WO3 -doped samples decreases sharply at lower temperatures. To highlight this trend, we compare the metallicity of the conducting heterostructures by evaluating their residual resistivity ratio defined as RRR = ρxx (300 K)/ρxx (1.5 K). Higher RRR values indicate more pronounced metallic behaviour


Page 7 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and are represented by the colour map in Figure 2b. Our reference LAO/STO heterostructure (0, 4) has RRR = 110, similarly to previous reports. 12,28 In the WO3 /LAO/STO system we find higher values for decreasing thickness of the LAO interlayer. As an example, the (4, 2) combination shows RRR = 700. Based on our experimental observations, we propose the mechanism sketched in Figure 2c for the 2DES formation. The non-polar STO and the amorphous WO3 have flat energy levels along the direction perpendicular to the interface, while the polar field in LAO determines a potential rise of about 1 eV per uc. According to our self-interaction-free density functional calculations 29 in Figure S3 and the following paragraph, oxygen vacancies form donor levels in WO3 that reside about 0.1 eV below its conduction states. Above a critical LAO thickness, a charge transfer to STO conduction band occurs. Our data shows that the minimum LAO thickness required to observe metallicity is 2 uc and, as a general trend above this value, thinner LAO determines higher RRR. This phenomenon is the balance of two competing effects. On the one hand, a sufficiently thick LAO interlayer is required to provide the electric field necessary for driving charge carriers at the LAO/STO interface. 30 On the other hand, the doping mechanism is enhanced if the WO3 overlayer is closer to the STO. Within this picture, the optimal LAO thickness for the formation of a conductive state with high RRR is found to be 2 uc. This is in agreement with previous works on STO-based systems, where a polar interlayer was shown to effectively separate the conductive interface from its source of doping, enhancing its metallicity. 15 Insights into the doping mechanism involving oxygen vacancies can be obtained by varying the WO3 growth conditions. Conductive WO3 /LAO/STO heterostructures are obtained only if WO3 is deposited at sufficiently low oxygen pressure (pO2 < 4 × 10−2 mbar), determining oxygen-deficient films. 31,32 Furthermore, a post-annealing treatment in O2 atmosphere performed on conductive samples turns them insulating (Figure S4). This is a strong indication that oxygen vacancies in the top layers play an important role in the insulator-to-metal transition at WO3 /LAO/STO heterostructures, where the dopants are in a thermodynam-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 25

ically quenched state. 33 We also note that, although the WO3 overlayers are amorphous, if their growth is performed at room temperature the resulting heterostructures are insulating. This indicates that the doping mechanism is a thermally-activated process happening during the high-temperature growth of WO3 . In contrast to other STO-based 2DES where the dopant layer lies directly on top of the conductive interface, 30,34 here the crystalline LAO interlayer effectively separates it from the STO, determining a 2DES with high-mobility carriers, as discussed below. With this approach we are thus able to combine the doping provided by the amorphous overlayer with the advantage of a crystalline conductive interface.

2.3 Magnetotransport The characteristics of the metallic state in WO3 /LAO/STO are investigated by performing magnetotransport measurements on a (4, 2) heterostructure, which has a high RRR value. In Figure 3a we present its magnetoresistance (MR) defined as MR =

ρxx (B)−ρ0 , ρ0

where ρ0 is

the sheet resistance at B = 0 and the magnetic field is applied perpendicular to the interface plane. At 1.5 K the MR is positive and reaches 900% at 10 T, corresponding to one order of magnitude increase in sheet resistance. This differs greatly from what is usually observed in LAO/STO heterostructures (0, n) as can be seen from the comparison with a (0, 4) sample in Figure 3a. The LAO/STO, in fact, shows a positive MR in the order of 10 % at 10 T, consistently with previous reports. 4,35,36 The Hall resistance of the (4, 2) heterostructure (Figure 3b) is negative, indicating electron transport, with a kink at about 1 T. A non linear component in the Hall effect of STO-based systems is typically related to multiple conduction channels contributing to the transport. 17,37–39 In the two-channel system, the classical magnetoresistance (ρxx ) and the Hall resistance (ρxy ) are given by

ρxx =

(nI µI + nII µII ) + (nI µII + nII µI )µI µII B 2 1 · , (nI µI + nII µII )2 + (nµI µII B)2 e


(1a)

Page 9 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ρxy =

(±nI µ2I ± nII µ2II ) + n(µI µII B)2 B · , (nI µI + nII µII )2 + (nµI µII B)2 e

(1b)

where ni , µi are the carrier density and mobility of the i-th channel, n = (±nI ± nII ) with the ± sign indicating hole or electron carriers, respectively.

The nonlinearity in

the Hall effect is well captured after fitting the data to Eq. (1b) (dashed line in Figure 3b). The extracted transport parameters are presented in Table 1 (see methods for details). The (4, 2) heterostructure presents two channels of electrons: one with lowermobility µI = 3600 cm2 V−1 s−1 , nI = 1.7 × 1013 cm−2 and one with higher mobility µII = 80 000 cm2 V−1 s−1 , nII = 9.3 × 1012 cm−2 . Higher mobility values are observed for lower carrier densities, consistently with previous studies of STO-based 2DES. 38,39 We note that the sheet resistance of the higher-mobility channel ρII is one order of magnitude smaller than ρI , indicating that it dominates the low-temperature transport. The (4, 2) mobility is about two orders of magnitude higher than what observed in the reference (0, 4) sample (µI = 840 cm2 V−1 s−1 ), which is consistent with the higher resistivity at 1.5 K and the lower RRR value usually found in LAO/STO heterostructures. Using ni , µi extracted from the Hall effect we calculate with Eq. (1a) the classical twochannel MR (dashed line in Figure 3a). The resulting curve has the same order of magnitude of the measured signal, signalling that the classical component is the dominant MR contribution, in particular for small magnetic fields. The residual MR is much larger than the typical magnitude of quantum correction effects, which are thus negligible in this case. The additional MR signal can be explained considering Boltzmann transport contributions for mixed orbital states, 5 or disorder, which can lead to a linear high MR 40,41 (Figure S6).

2.4 Temperature-dependent magnetotransport Additional insight into the effects of these parallel conduction channels is obtained by tracking them as a function of temperature. The measurements are performed on a (4, 2) heterostructure, in a 100 µm × 500 µm Hall bar geometry (inset of Figure 4c), as described in 9 ACS Paragon Plus Environment


a

1000 WO3/LAO/STO LAO/STO

MR (%)

800 600

(4,2)

400

10 5

200

b

0 −10

(0,4)

0 200

0 −100 −200

−10

0

10

0

1

40

100 ρxy (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 25

(4, 2) (0, 4)

ρ0 ρI

0 −40

−1

ρII −5

0 B (T)

5

10

Figure 3: Comparison of WO3 /LAO/STO and LAO/STO magnetotransport. (a) Magnetoresistance and (b) Hall effect measured at 1.5 K in Van der Pauw geometry, with respective zoom-in (insets). The Hall data is fitted (dashed lines) with Eq. (1b) and used to extract the values in Table 1. The classical MR (dashed line in (a)) is calculated from these value using Eq. (1a). the methods. We observe a nonlinear Hall effect below 30 K, while it is linear at higher temperatures (Figure S7). The temperature dependence of mobility and carrier density for the two channels are extracted in Figures 4a and 4b. For the higher mobility channel we find that both µII , nII decrease and then completely disappear above 30 K. The lower mobility Channel I shows an almost temperature-independent carrier density and a mobility decreasing several orders of magnitude upon warming, similar to what previously reported for conventional LAO/STO heterostructures. 42 Interestingly, above 30 K, where the highTable 1: Transport parameters. Mobility, carrier density, sheet resistance and mean free path of the conductive channels extracted from the fits in Figure 3b.

(4, 2)II (4, 2)I (0, 4)I

µ (cm2 V−1 s−1 ) 80 000 3600 840

n2D (cm−2 ) ρ0 (Ω) λ (nm) 9.3 × 1012 8 4100 13 1.7 × 10 100 250 13 2.6 × 10 290 70 10

ACS Paragon Plus Environment

Page 11 of 25

c

μII

104

ρxx (Ω/square)

250 μm

nI

1013

2DES Al2O3 mask

103

μI

μII

ρI B=0T

101

1


300K


< 30K

1

LAO STO

B = 12 T 102

nII 1012

Oxygen vacancy

WO3

μI

102

b n2D (cm−2)

d 104

Low ε

μ (cm2/V s)

a

High ε

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4: Temperature dependence of the magnetotransport. (a) Mobility and (b) carrier density as a function of temperature for a (4, 2) WO3 /LAO/STO heterostructure. The parameters for the two channels (squares and circles) are extracted by fitting the Hall effect with Eq. (1b). (c) Resistance versus temperature with applied perpendicular magnetic field B = 0 and B = 12 T compared to the resistivity of the lower-mobility channel (circles). The inset shows an optical image of a 100 µm × 500 µm Hall bar used for electrical characterisation. (d) Proposed mechanism for the formation of two parallel conduction channels in STO below 30 K. The lower-mobility Channel I is closer to the defect-rich interface region, while Channel II extends deep in the less-defected material. The temperature-dependent STO dielectric constant ε changes the electron charge distribution (indicated in yellow), determining a crossover from multi-channel (T < 30 K) to single channel transport. mobility electrons disappear, the resistivity curves in Figure 2a overlap with LAO/STO, indicating that Channel II is a peculiar characteristic of WO3 /LAO/STO heterostructures. The presence of two distinct conduction channels can be explained considering a different spatial confinement of the charge carriers in the STO (Figure 4d). The electrons of Channel I, closer to the LAO layer, experience a stronger polar electric field and a higher concentration of defects, resulting in lower mobility. Channel II, instead, arises from electrons that extend deeper into the substrate, where a less-defected STO determines a higher mobility. In this scenario, the spatial extent of the conductive channels is related to the STO dielectric constant, which drops sharply upon warming, determining the depopulation of Channel II 43 (see also Figure S8). A similar mechanism and temperature dependence have been previously reported for other STO-based heterostructures. 38,39,44,45 This interpretation is further supported by field-effect measurements performed in a back-gate configuration (Figure S9).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 25

We now study the effect of a magnetic field on the higher mobility Channel II, showing that it dominates the transport only at small values of B. In Figure 4c we compare the resistivity versus temperature curve measured with B = 0 T and B = 12 T applied perpendicular to the interface plane. At 1.5 K the curves are well separated, determining a strong positive MR of 200 %. On warming, the MR decreases and the curves overlap. The strong MR in our system can be explained by considering the high mobility of Channel II and the large resistivity ratio of the two channels. In general, classical MR predicts a strong resistivity increase with applied magnetic field whenever the charge carriers possess high mobility. In systems with multiple channels high MR is observed only if the high mobility channel is dominant in the electronic conduction (i.e. ρII /ρI 1). Both conditions are met in WO3 /LAO/STO, where we find a direct correlation between the ratio ρII /ρI and the MR magnitude at 10 T: with ρII /ρI ∼ 10−1 in Figure 3 we measure MR ∼ 900%, and with ρII /ρI ∼ 1 in Figure 4 we have a lower MR ∼ 200%. A confirmation of this behaviour is given by considering that ρI values in Figure 4c well represent the resistivity versus temperature curve at B = 12 T. This indicates that the transport response is dominated by the low-mobility channel at high magnetic field.

2.5 Quantum oscillations of conductance The electronic state confined in our (4, 2) WO3 /LAO/STO heterostructures shows Shubnikovde Haas (SdH) oscillations superimposed on the background of strong positive MR. The SdH as a function of temperature are shown in Figure 5a, where their signal was extracted by fitting the background with a 3rd order polynomial (dashed line in Figure 5b). The oscillations disappear when the magnetic field is applied parallel to the interface plane, as expected for a two-dimensional system. SdH oscillations in 2DES can be modelled by

∆ρxx = 4ρc e−αTD

ω αT SdH sin 2π , sinh(αT ) B


(2)

1 T (mK)

0

1900 1100

100

580

d

340 μII, Hall = 17, 700 cm2 V−1 s−1

220

μI, Hall = 2, 140 cm2 V−1 s−1

180 140

−4

Δρxx(T)/Δρxx(41 mK)

380 270

−3

nII, Hall = 6.5 × 10

110

nI, Hall = 2.9 × 10

41

0.06

ρ0 = 35 Ω/square

8

450

−2

K

m 41

90

680

0.08

0.10 B −1 (T−1)

0.12

12

13

cm

cm

−2

−2

0.14

10 B (T)

0.0 0.0

12

14

B = 11.85 T

1.0

0.5

K

0m

0

19

95

750

−1

c

105

FT Amplitude (a.u.)

b

2

ρxx (Ω/square)

a

Δρxx (Ω/square)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


m * = 5.6 me

0.5

1.0 T (K)

1.5

2.0

1.25

nSdH = 2.5 × 1012 cm−2

1.00 0.75 0.50 0.25 0.00

e

log(Δρxxsinh(αT)/αT)

Page 13 of 25

2

0

100 200 ωSdH (T)

300

T = 41 mK

1 0 −1 −2

TD = 0.45 K τ = 2.7 ps μSdH = 850 cm2 V−1 s−1 ρC = 14 Ω/square

0.08 0.10 B −1 (T−1)

0.12

Figure 5: Quantum oscillations of conductance. (a) Temperature dependence of the SdH oscillations after the removal of a 3rd order polynomial background for a (4, 2) WO3 /LAO/STO heterostructure. The dashed and dotted lines indicate the data in (d) and (e), respectively. (b) Raw ρxx data at two different temperatures showing the fit of the polynomial background (dashed lines). (c) Fourier spectra of the oscillations in the range 7 T to 14 T. (d) Temperature dependence of the oscillations amplitude at B = 11.85 T and fit to Eq. (2) (dashed line) from which the carrier effective mass m∗ is extracted. (e) Dingle plot of the SdH oscillations minima at T = 41 mK and fit to Eq. (2) (dotted line) from which the Dingle temperature TD , elastic scattering time τ , mobility µSdH and the classical sheet resistance ρc are extracted. where ρc is the classical sheet resistance in zero magnetic field, α = 2π 2 kB /~ωc with cyclotron frequency ωc = eB/m∗ , Boltzmann’s constant kB , reduced Planck’s constant ~, carrier effective mass m∗ and Dingle temperature TD . 46 Fourier analysis in Figure 5c reveals that the oscillations are periodic in B −1 , with a main frequency peak at ωSdH = 50 T. Assuming a 2DES with circular sections of the Fermi surface, we can estimate the carrier density as nSdH =

νs e ω , h SdH

where νs indicates the spin degeneracy. By considering νs = 2 we find

nSdH = 2.5 × 1012 cm−2 . To extract the mass of the electrons showing the SdH effect, in Figure 5d we track the oscillation amplitude at B = 11.85 T as a function of temperature (similar results are obtained using different values of B). Fitting the trend with Eq. (2), we find m∗ = 5.6 me . From the Dingle plot in Figure 5e we extract TD = 0.45 K. This value points to an ordered electronic



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

system with sharp Landau levels, considering that their energy smearing kB TD ∼ 40 µeV is much smaller than their spacing ~ωC ∼ 250 µeV. The extracted value ρC = 14 Ω/square is in good agreement with ρ0 = 35 Ω/square, corroborating the performed analysis. Using TD = ~/2πkB τ and τ = m∗ µSdH /e we calculate the elastic scattering time τ = 2.7 ps and the quantum mobility µSdH = 850 cm2 V−1 s−1 . It is worth to compare the results of the SdH analysis with the Hall effect measurements (values for this sample in Figure 5a). While the Hall effect shows the presence of two conduction channels, a single oscillation frequency dominates the SdH signal. We identify the lower-mobility Channel I as the source of the observed SdH. This conclusion stems from two considerations: in STO-based materials nSdH is usually about an order of magnitude smaller than nHall , 47–49 and, similarly, quantum mobility µSdH is a factor 2-5 smaller than µHall . 17,50 The properties of Channel I fulfil these conditions. This naturally raises the question of why the higher-mobility channel is not visible in the SdH signal. We show in a Supplementary note, Figure S10 and Table S1 that the quantum oscillations of Channel II have low visibility due to its high classical MR, and that this is well-explained within the two-channel model. Furthermore, this observation is in agreement with Figure 4c, where we showed that transport at high magnetic field is dominated by the lower-mobility channel. Quite unusual is the effective mass value m∗ = 5.6 me , which is two to three times larger than what typically observed in LAO/STO heterostructures. 17,36 Possible explanations for this electron mass renormalization are the modified defect profile with respect to conventional LAO/STO interfaces, 51 large phonon-drag due to tight spatial electron confinement, 52,53 or strong polaronic effects which have been reported in both LAO/STO interfaces and amorphous WO3 thin films. 54,55 Further studies are required to reveal the origin of this phenomenon.


Page 14 of 25

Page 15 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 CONCLUSIONS To conclude, we have demonstrated that amorphous WO3 is an effective overlayer to induce an insulator-to-metal transition at WO3 /LAO/STO heterostructures. Reducing the crystalline LAO critical thickness from 4 to 2 unit cells, the WO3 overlayer determined a metallic system with large RRR and high electron mobility. We ascribed the appearance of a strong classical magnetoresistance to the large resistivity ratio and high mobility of the multiple conduction channels observed in the system. Quantum oscillations of conductance confirmed the realisation of high-quality WO3 /LAO/STO heterostructures, where a strong two-dimensional confinement of carriers is observed. All these results are obtained using an amorphous WO3 overlayer, which does not require crystal matching and could thus be exploited to induce 2DES in other oxide materials.

4 METHODS 4.1 Samples growth WO3 /LaAlO3 /SrTiO3 heterostructures were grown by pulsed laser deposition on commercially available 5 mm × 5 mm SrTiO3 (001) substrates, with TiO2 surface termination. The laser ablation was performed using a KrF excimer laser (Coherent COMPexPro 205, λ = 248 nm) with a 1 Hz repetition rate and 1 J cm−2 fluence. The target-substrate distance was fixed at 55 mm. For the LaAlO3 thin films a crystalline target was employed and the deposition performed at 800 ◦C substrate temperature and 3 × 10−5 mbar oxygen pressure. LaAlO3 film thickness was monitored in-situ during growth by intensity oscillations of reflection high-energy electron diffraction (RHEED). The samples were annealed for 1 h at 600 ◦C in 300 mbar of O2 atmosphere to compensate for the possible formation of oxygen vacancies. The amorphous WO3 thin films were deposited from a WO3 sintered



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

target at 500 ◦C substrate temperature and 5 × 10−3 mbar oxygen pressure. WO3 film thickness was calibrated by depositing crystalline WO3 on SrTiO3 and monitoring the growth by RHEED. The thickness value was then confirmed by X-ray diffraction and transmission electron microscopy measurements (further details in 32 ). At the end of the growth the heterostructures were cooled down to ambient temperature in 5 × 10−3 mbar oxygen pressure (further details in Figure S1).

4.2 Hall bar geometry fabrication SrTiO3 substrates were patterned prior to WO3 /LaAlO3 thin films deposition with standard e-beam lithography followed by the evaporation of an insulating Al2 O3 mask. The mask was deposited at room temperature by RF sputtering in a 5 µbar Ar atmosphere, resulting in amorphous alumina.

4.3 Electrical measurements The measurements in Figures 2 and 3 were carried out in van der Pauw configuration, while for the ones in Figures 4 and 5 a Hall bar geometry was used. In both measurement configurations the metallic interface was directly contacted by ultrasonically wire-bonded Al.

4.4 Nonlinear Hall effect fits The fits are performed with the least squares method using data in the magnetic field range −4 T to 4 T. The constraint 1/ρ0 = 1/ρI + 1/ρII is applied to the fitting parameters, and ρ0 is extracted from the ρxx (B) measurement. With the assumption 1/ρi = ni eµi , only three free parameters among ρi , ni , µi , with i = I, II, are varied in the fitting procedure.


Page 16 of 25

Page 17 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 ASSOCIATED CONTENT 5.1 Supporting Information Growth conditions, X-ray diffraction characterisation, band structure of WO3 with oxygen vacancies, effect of different oxygen pressure during growth, additional magnetotransport measurements and fits, back-gate experiments, and calculation of Shubnikov de Haas with two-channel magnetoresistance.

6 ACKNOWLEDGMENTS We thank P. Zubko for valuable feedback and for performing XRD measurements; Y. M. Blanter and A. Fête for fruitful discussions. This work was supported by The Netherlands Organisation for Scientific Research (NWO/OCW) as part of the Frontiers of Nanoscience program (NanoFront), the Dutch Foundation for Fundamental Research on Matter (FOM), the European Research Council under the European Union’s H2020 programme/ ERC GrantAgreement n. [677458] and the Cornell Center for Materials Research with funding from the NSF MRSEC program (DMR-1120296). The FEI Titan Themis 300 TEM was acquired through NSF-MRI-1429155, with additional support from Cornell University, the Weill Institute and the Kavli Institute at Cornell. A. F. thanks TU Delft and Kavli Institute for the access to Computing Center resources, and computational support from the CRS4 Computing Center (Piscina Manna, Pula, Italy).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7 COMPETING INTERESTS The authors declare no competing interests.

References (1) Ohtomo, A.; Hwang, H. A High-mobility Electron Gas at the LaAlO3 /SrTiO3 Heterointerface. Nature 2004, 427, 423–426. (2) Reyren, N.; Thiel, S.; Caviglia, A.; Kourkoutis, L. F.; Hammerl, G.; Richter, C. et al. Superconducting Interfaces Between Insulating Oxides. Science 2007, 317, 1196–1199. (3) Caviglia, A.; Gariglio, S.; Reyren, N.; Jaccard, D.; Schneider, T.; Gabay, M. et al. Electric Field Control of the LaAlO3 /SrTiO3 Interface Ground State. Nature 2008, 456, 624–627. (4) Caviglia, A.; Gabay, M.; Gariglio, S.; Reyren, N.; Cancellieri, C.; Triscone, J.-M. Tunable Rashba Spin-Orbit Interaction at Oxide Interfaces. Physical Review Letters 2010, 104, 126803. (5) Diez, M.; Monteiro, A.; Mattoni, G.; Cobanera, E.; Hyart, T.; Mulazimoglu, E. et al. Giant Negative Magnetoresistance Driven by Spin-Orbit Coupling at the LaAlO3 /SrTiO3 Interface. Physical Review Letters 2015, 115, 016803. (6) Bert, J. A.; Kalisky, B.; Bell, C.; Kim, M.; Hikita, Y.; Hwang, H. Y. et al. Direct Imaging of the Coexistence of Ferromagnetism and Superconductivity at the LaAlO3 /SrTiO3 Interface. Nature Physics 2011, 7, 767–771. (7) Li, L.; Richter, C.; Mannhart, J.; Ashoori, R. Coexistence of Magnetic Order and Twodimensional Superconductivity at LaAlO3 /SrTiO3 Interfaces. Nature Physics 2011, 7, 762–766. 18 ACS Paragon Plus Environment

Page 18 of 25

Page 19 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Nakagawa, N.; Hwang, H. Y.; Muller, D. A. Why Some Interfaces Cannot Be Sharp. Nature Materials 2006, 5, 204–209. (9) Gunkel, F.; Brinks, P.; Hoffmann-Eifert, S.; Dittmann, R.; Huijben, M.; Kleibeuker, J. et al. Influence of Charge Compensation Mechanisms on the Sheet Electron Density at Conducting LaAlO3 /SrTiO3 Interfaces. Applied Physics Letters 2012, 100, 052103. (10) Yu, L.; Zunger, A. A Polarity-Induced Defect Mechanism for Conductivity and Magnetism at Polar–Nonpolar Oxide Interfaces. Nature Communications 2014, 5 . (11) Kalabukhov, A.; Gunnarsson, R.; Börjesson, J.; Olsson, E.; Claeson, T.; Winkler, D. Effect of Oxygen Vacancies in the SrTiO3 Substrate on the Electrical Properties of the LaAlO3/SrTiO3 Interface. Physical Review B 2007, 75, 121404. (12) Warusawithana, M.; Richter, C.; Mundy, J.; Roy, P.; Ludwig, J.; Paetel, S. et al. LaAlO3 Stoichiometry is Key to Electron Liquid Formation at LaAlO3 /SrTiO3 Interfaces. Nature Communications 2013, 4 . (13) Bristowe, N.; Littlewood, P.; Artacho, E. Surface Defects and Conduction in Polar Oxide Heterostructures. Physical Review B 2011, 83, 205405. (14) Huijben, M.; Koster, G.; Kruize, M. K.; Wenderich, S.; Verbeeck, J.; Bals, S. et al. Defect Engineering in Oxide Heterostructures by Enhanced Oxygen Surface Exchange. Advanced Functional Materials 2013, 23, 5240–5248. (15) Chen, Y.; Trier, F.; Wijnands, T.; Green, R.; Gauquelin, N.; Egoavil, R. et al. Extreme Mobility Enhancement of Two-dimensional Electron Gases at Oxide Interfaces by Charge-transfer-Induced Modulation Doping. Nature Materials 2015, 14, 801–806. (16) Xie, Y.; Bell, C.; Hikita, Y.; Harashima, S.; Hwang, H. Y. Enhancing Electron Mobility at the LaAlO3 /SrTiO3 Interface by Surface Control. Advanced Materials 2013, 25, 4735–4738. 19 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(17) Chen, Y.; Bovet, N.; Trier, F.; Christensen, D.; Qu, F.; Andersen, N. H. et al. A High-mobility Two-dimensional Electron Gas at the Spinel/Perovskite Interface of γAl2 O3 /SrTiO3 . Nature Communications 2013, 4, 1371. (18) Wu, S.; Luo, X.; Turner, S.; Peng, H.; Lin, W.; Ding, J. et al. Nonvolatile Resistive Switching in Pt/LaAlO3 /SrTiO3 Heterostructures. Physical Review X 2013, 3, 041027. (19) Lesne, E.; Reyren, N.; Doennig, D.; Mattana, R.; Jaffrès, H.; Cros, V. et al. Suppression of the Critical Thickness Threshold for Conductivity at the LaAlO3 /SrTiO3 Interface. Nature Communications 2014, 5 . (20) Arab, M.; Lopes-Moriyama, A. L.; dos Santos, T. R.; de Souza, C. P.; Gavarri, J. R.; Leroux, C. Strontium and Cerium Tungstate Materials SrWO4 and Ce2 (WO4 )3 : Methane Oxidation and Mixed Conduction. Catalysis Today 2013, 208, 35–41. (21) He, Y.; Gu, M.; Xiao, H.; Luo, L.; Shao, Y.; Gao, F. et al. Atomistic Conversion Reaction Mechanism of WO3 in Secondary Ion Batteries of Li, Na, and Ca. Angewandte Chemie International Edition 2016, 55, 6244–6247. (22) Deb, S. K. Opportunities and Challenges in Science and Technology of WO3 for Electrochromic and Related Applications. Solar Energy Materials and Solar Cells 2008, 92, 245–258. (23) Meng, X.; Quenneville, F.; Venne, F.; Di Mauro, E.; I¸sık, D.; Barbosa, M. et al. Electrolyte-Gated WO3 Transistors: Electrochemistry, Structure, and Device Performance. The Journal of Physical Chemistry C 2015, 119, 21732–21738. (24) Cong, S.; Geng, F.; Zhao, Z. Tungsten Oxide Materials for Optoelectronic Applications. Advanced Materials 2016, 28, 10518–10528. (25) Mott, N. Conduction in Non-crystalline Systems IX. The Minimum Metallic Conductivity. Philosophical Magazine 1972, 26, 1015–1026. 20 ACS Paragon Plus Environment

Page 20 of 25

Page 21 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Licciardello, D.; Thouless, D. Constancy of Minimum Metallic Conductivity in Two Dimensions. Physical Review Letters 1975, 35, 1475. (27) Kravchenko, S.; Sarachik, M. Metal–Insulator Transition in Two-dimensional Electron Systems. Reports on Progress in Physics 2003, 67, 1. (28) Gariglio, S.; Reyren, N.; Caviglia, A.; Triscone, J. Superconductivity at the LaAlO3 /SrTiO3 Interface. Journal of Physics: Condensed Matter 2009, 21, 164213. (29) Filippetti, A.; Pemmaraju, C. D.; Sanvito, S.; Delugas, P.; Puggioni, D.; Fiorentini, V. Variational Pseudo-Self-Interaction-Corrected Density Functional Approach to the Ab Initio Description of Correlated Solids and Molecules. Physical Review B 2011, 84, 195127. (30) Vaz, D. C.; Lesne, E.; Sander, A.; Naganuma, H.; Jacquet, E.; Santamaria, J. et al. Tuning Up or Down the Critical Thickness in LaAlO3 /SrTiO3 through In Situ Deposition of Metal Overlayers. Advanced Materials 2017, (31) Kalhori, H.; Porter, S. B.; Esmaeily, A. S.; Coey, M.; Ranjbar, M.; Salamati, H. Morphology and Structural Studies of WO3 Films Deposited on SrTiO3 by Pulsed Laser Deposition. Applied Surface Science 2016, 390, 43–49. (32) Mattoni, G. Growth and Oxygen–Doping of WO3 Thin Films. To be published 2017, (33) Gunkel, F.; Hoffmann-Eifert, S.; Heinen, R. A.; Christensen, D. V.; Chen, Y.; Pryds, N. et al. Thermodynamic Ground States of Complex Oxide Heterointerfaces. ACS Applied Materials & Interfaces 2016, 9, 1086–1092. (34) Chen, Y.; Pryds, N.; Kleibeuker, J. E.; Koster, G.; Sun, J.; Stamate, E. et al. Metallic and Insulating Interfaces of Amorphous SrTiO3 -Based Oxide Heterostructures. Nano Letters 2011, 11, 3774–3778.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(35) Joshua, A.; Pecker, S.; Ruhman, J.; Altman, E.; Ilani, S. A Universal Critical Density Underlying the Physics of Electrons at the LaAlO3 /SrTiO3 Interface. Nature Communications 2012, 3, 1129–1129. (36) McCollam, A.; Wenderich, S.; Kruize, M.; Guduru, V.; Molegraaf, H.; Huijben, M. et al. Quantum Oscillations and Subband Properties of the Two-dimensional Electron Gas at the LaAlO3 /SrTiO3 Interface. APL Materials 2014, 2, 022102. (37) Pentcheva, R.; Huijben, M.; Otte, K.; Pickett, W. E.; Kleibeuker, J.; Huijben, J. et al. Parallel Electron-Hole Bilayer Conductivity from Electronic Interface Reconstruction. Physical Review Letters 2010, 104, 166804. (38) Gunkel, F.; Bell, C.; Inoue, H.; Kim, B.; Swart, A. G.; Merz, T. A. et al. Defect Control of Conventional and Anomalous Electron Transport at Complex Oxide Interfaces. Physical Review X 2016, 6, 031035. (39) Kim, J. S.; Seo, S. S. A.; Chisholm, M. F.; Kremer, R.; Habermeier, H.-U.; Keimer, B. et al. Nonlinear Hall Effect and Multichannel Conduction in LaTiO3 /SrTiO3 Superlattices. Physical Review B 2010, 82, 201407. (40) Mirlin, A.; Wilke, J.; Evers, F.; Polyakov, D.; Wölfle, P. Strong Magnetoresistance Induced by Long-range Disorder. Physical Review Letters 1999, 83, 2801. (41) Parish, M. M.; Littlewood, P. B. Non-Saturating Magnetoresistance in Heavily Disordered Semiconductors. Nature 2003, 426, 162–165. (42) Fête, A.; Cancellieri, C.; Li, D.; Stornaiuolo, D.; Caviglia, A.; Gariglio, S. et al. Growth-Induced Electron Mobility Enhancement at the LaAlO3 /SrTiO3 interface. Applied Physics Letters 2015, 106, 051604. (43) Sakudo, T.; Unoki, H. Dielectric Properties of SrTiO3 at Low Temperatures. Physical Review Letters 1971, 26, 851. 22 ACS Paragon Plus Environment

Page 22 of 25

Page 23 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44) Delugas, P.; Filippetti, A.; Fiorentini, V.; Bilc, D. I.; Fontaine, D.; Ghosez, P. Spontaneous 2-dimensional Carrier Confinement at the n-type SrTiO3 /LaAlO3 Interface. Physical Review Letters 2011, 106, 166807. (45) Sch¨ utz, P.; Christensen, D.; Borisov, V.; Pfaff, F.; Scheiderer, P.; Dudy, L. et al. Microscopic Origin of the Mobility Enhancement at a Spinel/Perovskite Oxide Heterointerface Revealed by Photoemission Spectroscopy. Physical Review B 2017, 96, 161409. (46) Shoenberg, D. Magnetic Oscillations in Metals. Press, Cambridge 1984, (47) Son, J.; Moetakef, P.; Jalan, B.; Bierwagen, O.; Wright, N. J.; Engel-Herbert, R. et al. Epitaxial SrTiO3 Films with Electron Mobilities Exceeding 30 000 cm2 V−1 s−1 . Nature Materials 2010, 9, 482–484. (48) Caviglia, A.; Gariglio, S.; Cancellieri, C.; Sacepe, B.; Fête, A.; Reyren, N. et al. Twodimensional Quantum Oscillations of the Conductance at LaAlO3 /SrTiO3 Interfaces. Physical Review Letters 2010, 105, 236802. (49) Shalom, M. B.; Ron, A.; Palevski, A.; Dagan, Y. Shubnikov–de Haas Oscillations in SrTiO3 /LaAlO3 Interface. Physical Review Letters 2010, 105, 206401. (50) Kozuka, Y.; Kim, M.; Bell, C.; Kim, B. G.; Hikita, Y.; Hwang, H. Two-dimensional Normal-state Quantum Oscillations in a Superconducting Heterostructure. Nature 2009, 462, 487. (51) Wunderlich, W.; Ohta, H.; Koumoto, K. Enhanced Effective Mass in Doped SrTiO3 and Related Perovskites. Physica B: Condensed Matter 2009, 404, 2202–2212. (52) Pallecchi, I.; Telesio, F.; Li, D.; Fête, A.; Gariglio, S.; Triscone, J.-M. et al. Giant Oscillating Thermopower at Oxide Interfaces. Nature Communications 2015, 6 . (53) Pallecchi, I.; Telesio, F.; Marré, D.; Li, D.; Gariglio, S.; Triscone, J.-M. et al.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Large Phonon-drag Enhancement Induced by Narrow Quantum Confinement at the LaAlO3 /SrTiO3 Interface. Physical Review B 2016, 93, 195309. (54) Cancellieri, C.; Mishchenko, A.; Aschauer, U.; Filippetti, A.; Faber, C.; Bariˇsić, O. et al. Polaronic Metal State at the LaAlO3 /SrTiO3 Interface. Nature Communications 2016, 7 . (55) Berggren, L.; Azens, A.; Niklasson, G. A. Polaron Absorption in Amorphous Tungsten Oxide Films. Journal of Applied Physics 2001, 90, 1860–1863.


Page 24 of 25

Page 25 of 25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


WO3 LAO STO

Table of Contents (TOC) Graphic.


Insulator-to-Metal Transition at Oxide Interfaces Induced by WO3

Recommend Documents