Thickness-dependent Metal-insulator Transition Research Articles

Hard x-ray photoemission spectroscopy of LaVO3/SrTiO3 : Band alignment and electronic reconstruction

The heterostructure consisting of the Mott insulator LaVO$_3$ and the band insulator SrTiO$_3$ is considered a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of LaVO$_3$ match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the non-polar/polar interface when integrated with SrTiO$_3$, also offer remarkable advantages over conventional solar cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition is scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of soft and hard X-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the electrical properties, band bending, and band alignment of LaVO$_3$/SrTiO$_3$ heterostructures. We find a critical LaVO$_3$ thickness of five unit cells, confinement of the conducting electrons to exclusively Ti 3$d$ states at the interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as the driving mechanism for the formation of the metallic interface in LaVO$_3$/SrTiO$_3$.

Alternation of Magnetic Anisotropy Accompanied by Metal-Insulator Transition in Strained Ultrathin Manganite Heterostructures

Fundamental understanding of interfacial magnetic properties in ferromagnetic heterostructures is essential to utilize ferromagnetic materials for spintronic device applications. In this paper, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO$_3$ (LAO)/La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO)/Nb:SrTiO$_3$ (Nb:STO) heterostructures with varying LSMO-layer thickness, in which the magnetic anisotropy strongly changes depending on the LSMO thickness due to the delicate balance between the strains originating from both the Nb:STO and LAO layers, using x-ray magnetic circular dichroism (XMCD) and photoemission spectroscopy (PES). We successfully detect the clear change of the magnetic behavior of the Mn ions concomitant with the thickness-dependent metal-insulator transition (MIT). Our results suggest that double-exchange interaction induces the ferromagnetism in the metallic LSMO film under tensile strain caused by the SrTiO$_3$ substrate, while superexchange interaction determines the magnetic behavior in the insulating LSMO film under compressive strain originating from the top LAO layer. Based on those findings, the formation of a magnetic dead layer near the LAO/LSMO interface is attributed to competition between the superexchange interaction via Mn 3$d_{3z^2-r^2}$ orbitals under compressive strain and the double-exchange interaction via the 3$d_{x^2-y^2}$ orbitals.

Metal-insulator transition and interfacial thermal transport in atomic layer deposited Ru nanofilms characterized by ultrafast terahertz spectroscopy

Ruthenium nanofilms are a promising material for wide applications in nanoelectronics, such as ultrathin electrodes and metallization. However, little is known about their electrical and thermal properties. Here, we utilize ultrafast optical-pump terahertz-probe (OPTP) spectroscopy to characterize thickness- and substrate-dependent properties of ruthenium nanofilms. Atomic layer deposition produces ruthenium nanofilms with precisely controlled thicknesses and provides different morphologies of ruthenium islands on different substrates. First, we directly observe a thickness-dependent metal–insulator transition, revealed by a sign change in the OPTP signals near a critical film thickness of about 10 nm. This phase transition originates from the formation of electrical percolation networks of ruthenium islands, providing key information for film-thickness optimization in nanodevice designs. Second, OPTP reveals interfacial thermal transport from ruthenium to various substrates (sapphire, fused silica, and MgO). The thermal dissipation exhibits strong substrate dependence, attributed to the different morphologies of ruthenium islands and quality of the ruthenium-substrate interface. The observed dynamics are reproduced by simulation of spatiotemporal temperature distributions, providing the interface thermal conductance, a key design parameter in the thermal management of nanodevices. This work provides novel insight into the physical properties of ruthenium nanofilms and their dependence on the thickness and the morphology of the films.

Depth-resolved charge reconstruction at the LaNiO3/CaMnO3 interface

Rational design of low-dimensional electronic phenomena at oxide interfaces is currently considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically abrupt interface between paramagnetic $\mathrm{LaNi}{\mathrm{O}}_{3}$ and antiferromagnetic $\mathrm{CaMn}{\mathrm{O}}_{3}$ exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in $\mathrm{LaNi}{\mathrm{O}}_{3}$. Once fully understood, such emergent functionality could turn this archetypal Mott-interface system into a key building block for the above-mentioned future devices. Here, we use depth-resolved standing-wave photoemission spectroscopy in conjunction with scanning transmission electron microscopy and x-ray absorption spectroscopy, to demonstrate a depth-dependent charge reconstruction at the $\mathrm{LaNi}{\mathrm{O}}_{3}/\mathrm{CaMn}{\mathrm{O}}_{3}$ interface. Our measurements reveal an increased concentration of $\mathrm{M}{\mathrm{n}}^{3+}$ and $\mathrm{N}{\mathrm{i}}^{2+}$ cations at the interface, which create an electronic environment favorable for the emergence of interfacial ferromagnetism mediated via the $\mathrm{M}{\mathrm{n}}^{4+}\ensuremath{-}\mathrm{M}{\mathrm{n}}^{3+}$ ferromagnetic double exchange and $\mathrm{N}{\mathrm{i}}^{2+}\ensuremath{-}\mathrm{O}\ensuremath{-}\mathrm{M}{\mathrm{n}}^{4+}$ superexchange mechanisms. Our findings suggest a strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states.

In situ engineering and characterization on the artificial heterostructures of correlated materials with integrated OMBE–ARPES

The combination of oxide molecular beam epitaxy (OMBE) and angle resolved photoemission spectroscopy (ARPES) opens a new field for studying correlated electron systems. The in situ growth of single crystalline films by OMBE increases the variety of materials available for ARPES study. Investigating the electronic structure of thin films by ARPES deepens our understanding on the interaction of different degrees of freedom at thin films and interfaces. In this paper, we review the key elements of the integrated OMBE–ARPES technique and some representative works, including the in situ ARPES investigations on thin films with cubic perovskite structure, ultra-thin films with thickness dependent metal–insulator transition, the composition and strain dependence of interfacial superconductivity, and correlated behavior in superlattices. These examples illustrate the new opportunities brought by integrating OMBE and ARPES on exploring novel physical states and functional materials.

Soft X-ray Nano-spectroscopy for Electronic Structures of Transition Metal Oxide Nano-structures

In order to develop nano-devices with much lower power consumption for beyond-CMOS applications, the fundamental understanding and precise control of the electronic properties of ultrathin transition metal oxide (TMO) films are strongly required. The metal-insulator transition (MIT) is not only an important issue in solid state physics, but also a useful phenomenon for device applications like switching or memory devices. For potential use in such application, the electronic structures of MIT, observed for TMO nano-structures, have been investigated using a synchrotron radiation angle-resolved photoelectron spectroscopy system combined with a laser molecular beam epitaxy chamber and a scanning photoelectron microscopy system with 70 nm spatial resolution. In this review article, electronic structures revealed by soft X-ray nano-spectroscopy are presented for i) polarity-dependent MIT and thickness-dependent MIT of TMO ultrathin films of <TEX>$LaAlO_3/SrTiO_3$</TEX> and <TEX>$SrVO_3/SrTiO_3$</TEX>, respectively, and ii) electric field-induced MIT of TMO nano-structures showing resistance switching behaviors due to interfacial redox reactions and/or filamentary path formation. These electronic structures have been successfully correlated with the electrical properties of nano-structured films and nano-devices.

Thickness dependent metal-insulator transition and dimensional crossover for weak localization in Si0.02Zn0.98O thin films grown by pulsed laser deposition

Metal to insulator transition was observed in Si0.02Zn0.98O (SZO) films, grown by pulsed laser deposition on sapphire substrates, as the thicknesses of the films were reduced from ∼40 to 15 nm. The SZO film with thickness of ∼40 nm showed typical metallic behavior in temperature dependent resistivity measurements. On the contrary, the SZO film with thickness of ∼15 nm was found to exhibit strong localization where the transport at low temperature was dominated by variable range hopping conduction. In the intermediate thickness regime, quantum corrections were important and a dimensional crossover from 3D to 2D weak localization occurred in the SZO film with thickness of 20 nm.

Bandwidth-controlled metal-insulator transition in epitaxial PrNiO3 ultrathin films induced by dimensional crossover

The authors have investigated the thickness-dependent physical properties of PrNiO3 ultrathin films epitaxially grown on LaAlO3 substrates. The strained PrNiO3 films exhibit metallic behavior and do not show any indication of temperature-driven metal-insulator transition (MIT) in bulk form, whereas an insulating ground state is realized in a thin limit. In situ photoemission measurements reveal that the observed thickness-dependent MIT is caused by the reduction in bandwidth due to the dimensional control of the films. These results strongly suggest that the MIT in PrNiO3 films can be controlled by changing the dimensionality under epitaxial constraint.

Gradual localization of Ni3dstates in LaNiO3ultrathin films induced by dimensional crossover

$\mathit{In}$ $\mathit{situ}$ photoemission spectroscopy and x-ray absorption spectroscopy (XAS) have been performed on LaNiO${}_{3}$ (LNO) ultrathin films grown on LaAlO${}_{3}$ substrates to investigate the origin of the thickness-dependent metal-insulator transition (MIT). With decreasing film thickness, the progressive weakening of a quasiparticle peak at the Fermi level (${E}_{F}$) occurs below 10 monolayer (ML), and the further depletion of spectral weight at ${E}_{F}$ leads to pseudogap behavior at 3--6 ML. The pseudogap finally evolves into a full gap, indicating that the thickness-dependent MIT takes place at a critical film thickness of 2--3 ML. The observed spectral behavior is in line with the transport properties of LNO ultrathin films. The thickness dependence of the spectral intensity is compared with realistic multiorbital dynamical mean-field theory. The experimental spectral function was found to depend on the film thickness more strongly than the theoretical one for thinner systems, indicating that the thickness-dependent MIT in LNO is caused by the crossover from three to two dimensions, during which the spatial correlations are progressively enhanced. The XAS results suggest that a charge disproportionate state is strongly suppressed in LNO ultrathin films plausibly as a result of epitaxial strain from the substrates. These results strongly suggest that a novel insulating state is realized in LNO films at a thin limit.

Metal–insulator transition in SrIrO3 with strong spin–orbit interaction

The thickness-dependent metal–insulator transition is observed in meta-stable orthorhombic SrIrO3 thin films synthesized by pulsed laser deposition. SrIrO3 films with thicknesses less than 3 nm demonstrate insulating behaviour, whereas those thicker than 4 nm exhibit metallic conductivity at high temperature, and insulating-like behaviour at low temperature. Weak/Anderson localization is mainly responsible for the observed thickness-dependent metal–insulator transition in SrIrO3 films. Temperature-dependent resistance fitting shows that electrical-conductivity carriers are mainly scattered by the electron–boson interaction rather than the electron–electron interaction. Analysis of the magneto-conductance proves that the spin–orbit interaction plays a crucial role in the magneto-conductance property of SrIrO3.

Polar Discontinuity Doping of theLaVO3/SrTiO3Interface

We have investigated the transport properties of LaVO_{3}/SrTiO_{3} Mott-insulator-band-insulator heterointerfaces for various configurations. The (001)-oriented n-type VO_{2}/LaO/TiO_{2} polar discontinuity is conducting, exhibiting a LaVO3 thickness-dependent metal-insulator transition and low temperature anomalous Hall effect. The (001) p-type VO_{2}/SrO/TiO_{2} interface, formed by inserting a single layer of bulk metallic SrVO3 or SrO, drives the interface insulating. The (110) heterointerface is also insulating, indicating interface conduction arising from electronic reconstructions.

Thickness-dependent metal–insulator transition in V2O3 ultrathin films

In this study, V2O3 ultrathin films about 5–20 nm thick were prepared on Al2O3 (0001) substrates through a reactive evaporation process. Auger electron spectroscopy and x-ray photoelectron spectroscopy have been used in situ to characterize their compositions and chemical states. Electric resistance measurements show that V2O3 films transform from metallic to semiconducting with the decrease of film thickness, which results from the a1g level rising because the lattice mismatch between the substrate and the film expands the c/a parameter ratio. No temperature-induced metal–insulator transition (like that in bulk V2O3) was observed in V2O3 thin films at low temperature. We conclude that stress plays a major role in suppressing the temperature-induced metal–insulator transition.