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Atomically thin van der Waals (vdW) films provide a novel material platform for epitaxial growth of quantum heterostructures. However, unlike the remote epitaxial growth of three-dimensional bulk crystals, the growth of two-dimensional (2D) material heterostructures across atomic layers has been limited due to the weak vdW interaction. Here, we report the double-sided epitaxy of vdW layered materials through atomic membranes. We grow vdW topological insulators (TIs) Sb$_2$Te$_3$ and Bi$_2$Se$_3$ by molecular beam epitaxy on both surfaces of atomically thin graphene or hBN, which serve as suspended 2D vdW "$\textit{substrate}$" layers. Both homo- and hetero- double-sided vdW TI tunnel junctions are fabricated, with the atomically thin hBN acting as a crystal-momentum-conserving tunnelling barrier with abrupt and epitaxial interface. By performing field-angle dependent magneto-tunnelling spectroscopy on these devices, we reveal the energy-momentum-spin resonant tunnelling of massless Dirac electrons between helical Landau levels developed in the topological surface states at the interface., Comment: 24 pages, 4 main figures, 7 extended data figures

Observations of emergent quantum phases in twisted bilayer graphene prompted a flurry of activities in van-der-Waals (vdW) materials beyond graphene. Most current twisted experiments use a so-called tear-and-stack method using a polymer called PPC. However, despite the clear advantage of the current PPC tear-and-stack method, there are also technical limitations, mainly a limited number of vdW materials that can be studied using this PPC-based method. This technical bottleneck has been preventing further development of the exciting field beyond a few available vdW samples. To overcome this challenge and facilitate future expansion, we developed a new tear-and-stack method using a strongly adhesive polycaprolactone (PCL). With similar angular accuracy, our technology allows fabrication without a capping layer, facilitating surface analysis and ensuring inherently clean interfaces and low operating temperatures. More importantly, it can be applied to many other vdW materials that have remained inaccessible with the PPC-based method. We present our results on twist homostructures made with a wide choice of vdW materials - from two well-studied vdW materials (graphene and MoS$_2$) to the first-ever demonstrations of other vdW materials (NbSe$_2$, NiPS$_3$, and Fe$_3$GeTe$_2$). Therefore, our new technique will help expand $moir\acute{e}$ physics beyond few selected vdW materials and open up more exciting developments., Comment: 26 pages, 4+7 figures, accepted to 2D Materials

We report the growth of high-quality GaN epitaxial thin films on graphene-coated c-sapphire substrates using pulsed-mode metalorganic vapor-phase epitaxy, together with the fabrication of freestanding GaN films by simple mechanical exfoliation for transferable light-emitting diodes (LEDs). High-quality GaN films grown on the graphene-coated sapphire substrates were easily lifted off using thermal release tape and transferred onto foreign substrates. Furthermore, we revealed that the pulsed operation of ammonia flow during GaN growth was a critical factor for the fabrication of high-quality freestanding GaN films. These films, exhibiting excellent single crystallinity, were utilized to fabricate transferable GaN LEDs by heteroepitaxially growing InxGa1-xN/GaN multiple quantum wells and a p-GaN layer on the GaN films, showing their potential application in advanced optoelectronic devices., Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters, copyright 2023 American Chemical Society after peer review. To access the final edited and published work see https://doi.org/10.1021/acs.nanolett.3c03333

Although many years have passed since the discovery of high-critical-temperature (high-$T_c$) superconducting materials, the underlying mechanism is still unknown. The B1g phonon anomaly in high-Tc cuprate superconductors has long been studied; however, the correlation between the B1g phonon anomaly and superconductivity has yet to be clarified. In the present study, we successfully reproduced the B1g phonon anomaly in YBa$_2$Cu$_3$O$_7$ (YBCO) using an ab initio molecular dynamics (AIMD) simulation and temperature-dependent effective potential (TDEP) method. The Ag phonon by Ba atoms shows a more severe anomaly than the B1g phonon at low temperatures. Our analysis of the phonon anomaly and the temperature-dependent phonon dispersion indicated that decoupling between thermal phenomena and electron transport at low temperatures leads to layer-by-layer thermal decoupling in YBCO. Electronically and thermally isolated Ba atoms in YBCO are responsible for the thermal decoupling. The analytic study of the thermal dcoupling revealed that Planckian dissipation expressing linear-T resistivity is another expression of the Fermi liquid of the CuO$_2$ plane. The Uemura plot of the relationship between Tc and the Fermi temperature, as well as the superconducting dome in YBCO, is explained rigorously and quantitatively. Our findings not only present a new paradigm for understanding high-Tc superconductivity but also imply that the spontaneous formation of low-temperature layers in materials can lead to revolutionary changes in the thermal issues of industrial fields., Comment: 21 pages, 5 figures

Hund's rule coupling ($\textit{J}$) has attracted much attention recently for its role in the description of the novel quantum phases of multi orbital materials. Depending on the orbital occupancy, $\textit{J}$ can lead to various intriguing phases. However, experimental confirmation of the orbital occupancy dependency has been difficult as controlling the orbital degrees of freedom normally accompanies chemical inhomogeneities. Here, we demonstrate a method to investigate the role of orbital occupancy in $\textit{J}$ related phenomena without inducing inhomogeneities. By growing SrRuO$_3$ monolayers on various substrates with symmetry-preserving interlayers, we gradually tune the crystal field splitting and thus the orbital degeneracy of the Ru $\textit{t$_2$$_g$}$ orbitals. It effectively varies the orbital occupancies of two-dimensional (2D) ruthenates. Via in-situ angle-resolved photoemission spectroscopy, we observe a progressive metal-insulator transition (MIT). It is found that the MIT occurs with orbital differentiation: concurrent opening of a band insulating gap in the $\textit{d$_x$$_y$}$ band and a Mott gap in the $\textit{d$_x$$_z$$_/$$_y$$_z$}$ bands. Our study provides an effective experimental method for investigation of orbital-selective phenomena in multi-orbital materials.

Functional devices with ultrathin ferroelectric layers have been attracted as a promising candidate for next-generation memory and logic device applications. Using the ultrathin ferroelectric layers, particularly approaching the two-dimensional limit, however, it is still challenging to control ferroelectric switching and to observe ferroelectricity by spectroscopic tools. In particular, conventional methods such as electrical measurements and piezoelectric response force microscopy are very limited due to leakage currents and the smallness of the ferroelectric signals. Here, we show that the ferroelectricity of ultrathin SrRuO3/BaTiO3/SrRuO3 heterostructures grown on SrTiO3(100) substrates can be measured using resonant x-ray reflectivity (RXRR). This experimental technique can provide an element-specific electronic depth profile as well as increased sensitivity to Ti off-center displacements at the Ti K pre-edge. The depth-sensitivity of RXRR selectively detects the strong polarization dependence of the Ti pre-edge features of ultrathin BaTiO3 layers while discriminating the contribution of the SrTiO3 substrate. This technique verified that the BaTiO3 layer can be ferroelectric down to the lowest experimental limit of a critical thickness of 2.5 unit cells. Our results can open a novel way to explore ultrathin ferroelectric-based nano-electronic devices., Comment: 26 pages, 5 figures

Interfaces between dissimilar correlated oxides can offer devices with versatile functionalities. In that respect, manipulating and measuring novel physical properties of oxide heterointerfaces are highly desired. Yet, despite extensive studies, obtaining direct information on their momentum-resolved electronic structure remains a great challenge. This is because most correlated interfacial phenomena appear within a few atomic layers from the interface, thus limiting the application of available experimental probes. Here, we utilize atomic-scale epitaxy and photoemission spectroscopy to demonstrate the interface control of correlated electronic phases in atomic-scale ruthenate--titanate heterostructures. While bulk SrRuO$_3$ is a ferromagnetic metal, the heterointerfaces exclusively realize three distinct correlated phases in the single-atomic-layer limit. Our theory reveals that atomic-scale structural proximity effects lead to the emergence of Fermi liquid, Hund metal, and Mott insulator phases in the quantum-confined SrRuO$_3$. These results highlight the extensive interfacial tunability of electronic phases, hitherto hidden in the atomically thin correlated heterostructure., Comment: 25 pages, 5 main figures, 16 supplementary figures

Electric fields have played a key role in discovering and controlling exotic electronic states of condensed matter. However, electric fields usually do not work in metals as free carriers tend to screen electrostatic fields. While a pseudo-electric field generated by inhomogeneous lattice strain, namely a flexoelectric field, can in principle work in all classes of materials, it remains experimentally unexplored in metals. Here, using heteroepitaxy and atomic-scale imaging, we show that flexoelectric fields can polarize a metallic oxide SrRuO3 with unexpectedly large Ru off-center displacements. We also observe that the flexoelectrically induced polar state of SrRuO3 leads to sizable lattice expansion, similar to the electrostrictive expansion caused by ionic displacements in dielectrics under an external electric field. We further suggest that flexoelectrically driven Ru off-centering promotes strong coupling between lattice and electronic degrees of freedom, possibly enhancing the ferromagnetism of SrRuO3. Beyond conventional electric fields, flexoelectric fields may universally engender novel electronic states and their control via pure atomic displacements in a nondestructive and fast manner., Comment: 33 pages, 13 figures

Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI$_{3}$, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. Here, we report the observation of an air stable and layer dependent ferromagnetic (FM) van der Waals crystal, CrPS$_{4}$, using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in the monolayer crystal. Furthermore, alternating AFM and FM properties observed in even and odd layers suggest robust antiferromagnetic exchange interactions between layers. The observed ferromagnetism in these crystals remains resilient even after the air exposure of about a day, providing possibilities for the practical applications of van der Waals spintronics., Comment: 22 pages, 5 figures

Correlated electrons in transition metal oxides (TMOs) exhibit a variety of emergent phases. When TMOs are confined to a single-atomic-layer thickness, experiments so far have shown that they usually lose diverse properties and become insulators. In an attempt to extend the range of electronic phases of the single-atomic-layer oxide, we search for a metallic phase in a monolayer-thick epitaxial SrRuO$_3$ film. Combining atomic-scale epitaxy and angle-resolved photoemission measurements, we show that the monolayer SrRuO$_3$ is a strongly correlated metal. Systematic investigation reveals that the interplay between dimensionality and electronic correlation makes the monolayer SrRuO$_3$ an incoherent metal with orbital-selective correlation. Furthermore, the unique electronic phase of the monolayer SrRuO$_3$ is found to be highly tunable, as charge modulation demonstrates an incoherent-to-coherent crossover of the two-dimensional metal. Our work emphasizes the potentially rich phases of single-atomic-layer oxides and provides a guide to the manipulation of their two-dimensional correlated electron systems.

We report the importance of anisotropic Coulomb interactions in DFT+U calculations of the electronic and magnetic properties of Mn$_3$O$_4$. The effects of anisotropic interactions in Mn$^{2+}$ and Mn$^{3+}$ are separately examined by defining two different sets of Hubbard parameters: $U^{2+}$ and $J^{2+}$ for Mn$^{2+}$ and $U^{3+}$ and $J^{3+}$ for Mn$^{3+}$. The anisotropic interactions in Mn$^{3+}$ have a significant impact on the physical properties of Mn$_3$O$_4$ including local magnetic moments, canted angle, spontaneous magnetic moment, and superexchange coupling, but those in Mn$^{2+}$ do not make any noticeable difference. Weak ferromagnetic interchain superexchange, observed in experiments, is predicted only if a sizable anisotropic interaction is considered in Mn$^{3+}$. By analyzing the eigenoccupations of the on-site Mn density matrix, we found that the spin channel involving Mn$^{3+}$ $d_{x^2-y^2}$ orbitals, which governs the 90$^\circ$ correlation superexchange, is directly controlled by the anisotropic interactions. These findings demostrate that the exchange correction $J$ for the intraorbital Coulomb potential is of critical importance for first-principles description of reduced Mn oxides containing Mn$^{3+}$ or Mn$^{4+}$.

Aurivillius ferroelectric $Bi_2WO_6$ (BWO) encompasses a broad range of functionalities, including robust fatigue-free ferroelectricity, high photocatalytic activity, and ionic conductivity. Despite these promising characteristics, an in-depth study on the growth of BWO thin films and ferroelectric characterization, especially at the atomic scale, is still lacking. Here, we report pulsed laser deposition (PLD) of BWO thin films on (001) $SrTiO_3$ substrates and characterization of ferroelectricity using the scanning transmission electron microscopy (STEM) and piezoresponse force microscopy (PFM) techniques. We show that the background oxygen gas pressure used during PLD growth mainly determines the phase stability of BWO films, whereas the influence of growth temperature is comparatively minor. Atomically resolved STEM study of a fully strained BWO film revealed collective in-plane polar off-centering displacement of W atoms. We estimated the spontaneous polarization value based on polar displacement mapping to be about 54 $\pm$ 4 ${\mu}C cm^{-2}$, which is in good agreement with the bulk polarization value. Furthermore, we found that pristine film is composed of type-I and type-II domains, with mutually orthogonal polar axes. Complementary PFM measurements further elucidated that the coexisting type-I and type-II domains formed a multidomain state that consisted of 90$\deg$ domain walls (DWs) alongside multiple head-to-head and tail-to-tail 180$\deg$ DWs. Application of an electrical bias led to in-plane 180$\deg$ polarization switching and 90$\deg$ polarization rotation, highlighting a unique aspect of domain switching, which is immune to substrate-induced strain., Comment: This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Electronic Materials, \copyright American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see: https://pubs.acs.org/doi/full/10.1021/acsaelm.1c00005 .This submission contains 34 pages

Combining various two-dimensional materials into novel van der Waals (vdW) heterostructures has been shown to lead to new emergent quantum systems. A novel heterostructure composed of a vdW topological insulator (TI) such as Bi$_{2}$Se$_{3}$ with a quantum spin liquid (QSL) such as $\alpha$-RuCl$_{3}$ is of great interest for the potential for the chiral Dirac electrons in the TI surface states to interact strongly with the fractionalized fermionic spin excitations in the QSL. We report the heteroepitaxial growth of Bi$_{2}$Se$_{3}$ thin films on $\alpha$-RuCl$_{3}$ as well as the characterization of their structural and electrical properties. Bi$_{2}$Se$_{3}$ thin films with an atomically smooth and uniform surface are grown by molecular beam epitaxy. The heterostructure exhibits a preferential epitaxial relationship corresponding to $(5 \times 5)-$Bi$_{2}$Se$_{3}/(2\sqrt{3} \times 2\sqrt{3})R30\deg-\alpha$-RuCl$_{3}$ commensurate supercells with a periodicity of 1.2 nm. The formation of the superlattice despite a lattice mismatch as large as 60% is attributed to the van der Waals heteroepitaxy. Magnetotransport measurements as a function of temperature show Bi$_{2}$Se$_{3}$ films grown on $\alpha$-RuCl$_{3}$ are heavily $n$-doped, $n_{e}$ ~10$^{14}$ cm$^{-2}$, with mobility $\mu$ ~450 cm$^{2}$ V$^{-1}$ s$^{-1}$ at low temperatures., Comment: 19 pages, 4 figures; accepted in Physical Review Materials

We investigated the effects of paramagnetic (PM) fluctuations on the thermochemistry of the MnO(100) surface in the oxygen evolution reaction (OER) using the "noncollinear magnetic sampling method \textit{plus} $U$" (NCMSM$+U$). Various physical properties, such as the electronic structure, free energy, and charge occupation, of the MnO (100) surface in the PM state with several OER intermediates, were reckoned and compared to those in the antiferromagnetic (AFM) state. We found that PM fluctuation enhances charge transfer from a surface Mn ion to each of the intermediates and strengthens the chemical bond between them, while not altering the overall features, such as the rate determining step and resting state, in reaction pathways. The enhanced charge transfer can be attributed to the delocalized nature of valence bands observed in the PM surface. In addition, it was observed that chemical-bond enhancement depends on the intermediates, resulting in significant deviations in reaction energy barriers. Our study suggests that PM fluctuations play a significant role in the thermochemistry of chemical reactions occurring on correlated oxide surfaces., Comment: Maintext: 15 pages, 3 figures 2 tables; SI: 3 pages, 2 figures

That one can stack van der Waals materials with atomically sharp interfaces has provided a new material platform of constructing heterostructures. The technical challenge of mechanical stacking is picking up the exfoliated atomically thin materials after mechanical exfoliation without chemical and mechanical degradation. Chemically inert hexagonal boron nitride (hBN) has been widely used for encapsulating and picking up vdW materials. However, due to the relatively weak adhesion of hBN, assembling vdW heterostructures based on hBN has been limited. We report a new dry transfer technique. We used two vdW semiconductors (ZnPS3 and CrPS4) to pick up and encapsulate layers for vdW heterostructures, which otherwise are known to be hard to fabricate. By combining with optimized polycaprolactone (PCL) providing strong adhesion, we demonstrated various vertical heterostructure devices, including quasi-2D superconducting NbSe2 Josephson junctions with atomically clean interface. The versatility of the PCL-based vdW stacking method provides a new route for assembling complex 2D vdW materials without interfacial degradation.

We present a new approach based on the static density functional theory (DFT) to describe paramagentic MnO, which is a representative paramagnetic Mott insulator. We appended the spin noncollinearity and the canonical ensemble to the magnetic sampling method (MSM), which is one of the supercell approaches based on disordered local moment model. The combination of the noncollinear MSM (NCMSM) with DFT$+U$ represents a highly favorable computational method called NCMSM$+U$ to accurately determine the paramagnetic properties of MnO with moderate numerical cost. The effects of electron correlations and spin noncollinearity on the properties of MnO were also investigated. We revealed that the spin noncollinearity plays an important role in determining the detailed electronic profile and precise energetics of paramagnetic MnO. Our results illustrate that the NCMSM$+U$ approach may be used as an alternative to the $\textit{ab initio}$ framework of dynamic mean field theory based on DFT in the simulation of the high-temperature properties of Mott insulators., Comment: 19 pages, 5 figures, 1 table, 1 supplementary information

Ferroic domain walls (DWs) create different symmetries and ordered states compared with those in single-domain bulk materials. In particular, the DWs of an antiferromagnet (AFM) with non-coplanar spin structure have a distinct symmetry that cannot be realized in those of their ferromagnet counterparts. In this paper, we show that an unconventional anomalous Hall effect (AHE) can arise from the DWs of a non-coplanar AFM, Nd2Ir2O7. Bulk Nd2Ir2O7 has a cubic symmetry; thus, its Hall signal should be zero without an applied magnetic field. The DWs generated in this material break the two-fold rotational symmetry, which allows for finite anomalous Hall conductivity. A strong f-d exchange interaction between the Nd and Ir magnetic moments significantly influences antiferromagnetic domain switching. Our epitaxial Nd2Ir2O7 thin film showed a large enhancement of the AHE signal when the AFM domains switched, indicating that the AHE is mainly due to DWs. Our study highlights the symmetry broken interface of AFM materials as a new means of exploring topological effects and their relevant applications.

Control of the interlayer twist angle in two-dimensional (2D) van der Waals (vdW) heterostructures enables one to engineer a quasiperiodic moir\'e superlattice of tunable length scale. In twisted bilayer graphene (TBG), the simple moir\'e superlattice band description suggests that the electronic band width can be tuned to be comparable to the vdW interlayer interaction at a 'magic angle', exhibiting strongly correlated behavior. However, the vdW interlayer interaction can also cause significant structural reconstruction at the interface by favoring interlayer commensurability, which competes with the intralayer lattice distortion. Here we report the atomic scale reconstruction in TBG and its effect on the electronic structure. We find a gradual transition from incommensurate moir\'e structure to an array of commensurate domain structures as we decrease the twist angle across the characteristic crossover angle, $\theta_c$ ~1\deg. In the twist regime smaller than $\theta_c$ where the atomic and electronic reconstruction become significant, a simple moir\'e band description breaks down. Upon applying a transverse electric field, we observe electronic transport along the network of one-dimensional (1D) topological channels that surround the alternating triangular gapped domains, providing a new pathway to engineer the system with continuous tunability.

SrRuO$_3$ (SRO) films are known to exhibit insulating behavior as their thickness approaches four unit cells. We employ electron energy$-$loss (EEL) spectroscopy to probe the spatially resolved electronic structures of both insulating and conducting SRO to correlate them with the metal$-$insulator transition (MIT). Importantly, the central layer of the ultrathin insulating film exhibits distinct features from the metallic SRO. Moreover, EEL near edge spectra adjacent to the SrTiO$_3$ (STO) substrate or to the capping layer are remarkably similar to those of STO. The site$-$projected density of states based on density functional theory (DFT) partially reflects the characteristics of the spectra of these layers. These results may provide important information on the possible influence of STO on the electronic states of ultrathin SRO., Comment: 6 pages; 5 figures

Quantum mechanical tunneling of electrons across ultrathin insulating oxide barriers has been studied extensively for decades due to its great potential in electronic device applications. In the few-nanometer-thick epitaxial oxide films, atomic-scale structural imperfections, such as the ubiquitously existed one-unit-cell-high terrace edges, can dramatically affect the tunneling probability and device performance. However, the underlying physics has not been investigated adequately. Here, taking ultrathin BaTiO3 films as a model system, we report an intrinsic tunneling conductance enhancement near the terrace edges. Scanning probe microscopy results demonstrate the existence of highly-conductive regions (tens of nanometers-wide) near the terrace edges. First-principles calculations suggest that the terrace edge geometry can trigger an electronic reconstruction, which reduces the effective tunneling barrier width locally. Furthermore, such tunneling conductance enhancement can be discovered in other transition-metal-oxides and controlled by surface termination engineering. The controllable electronic reconstruction could facilitate the implementation of oxide electronic devices and discovery of exotic low-dimensional quantum phases., Comment: 53 pages, 4 figures, and supporting information

The introduction of magnetic order on the surface of topological insulators in general breaks the two-dimensional character of topological surface state (TSS). Once the TSS disappears, it is improbable to restore the topological surface properties. In this report, we demonstrate that it is possible to restore the inherent TSS by ultra-high vacuum annealing. Starting from an antiferromagnetic Gd-doped Bi2Te3, that has surface state gap without TSS properties, after annealing we observed the gap closing as well as typical TSS features in physical properties. The microscopic mechanism of atomic migration and TSS restoration by annealing process is unraveled by the combination of scanning tunneling microscopy measurements and density functional theory calculations. This approach to control the surface of topological insulators and stabilize the TSS simply by vacuum annealing provides a new platform towards the exploitation of their topological properties., Comment: 21 pages, 4 figures, Supplementary information

With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) heterostructure as a model system, we investigated the formation of different interfacial termination sequences (BaO-RuO2 or TiO2-SrO) with oxygen partial pressure (PO2) during PLD. We found that a uniform SrO-TiO2 termination sequence at the SRO/BTO interface can be achieved by lowering the PO2 to 5 mTorr, regardless of the total background gas pressure (Ptotal), growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. This work paves the way for realizing termination engineering in functional oxide heterostructures., Comment: 27 pages, 6 figures, Supporting Information

The atomic-scale synthesis of artificial oxide heterostructures offers new opportunities to create novel states that do not occur in nature. The main challenge related to synthesizing these structures is obtaining atomically sharp interfaces with designed termination sequences. Here, we demonstrate that the oxygen pressure (PO2) during growth plays an important role in controlling the interfacial terminations of SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) ferroelectric capacitors. The SRO/BTO/SRO heterostructures were grown by the pulsed laser deposition (PLD) method. The top SRO/BTO interface grown at high PO2 (around 150 mTorr) usually exhibited a mixture of RuO2-BaO and SrO-TiO2 terminations. By reducing PO2, we obtained atomically sharp SRO/BTO top interfaces with uniform SrO-TiO2 termination. Using capacitor devices with symmetric and uniform interfacial termination, we were able to demonstrate for the first time that the ferroelectric (FE) critical thickness can reach the theoretical limit of 3.5 unit cells (u.c.)., Comment: 30 pages, 11 figures (4 for main, 7 for supporting information)