Giant Piezoelectric Effect in Strontium Titanate at Cryogenic Temperatures

Slow Magnetic Relaxation of Dy Adatoms with In-Plane Magnetic Anisotropy on a Two-Dimensional Electron Gas

We report on the magnetic properties of Dy atoms adsorbed on the (001) surface of SrTiO₃. X-ray magnetic circular dichroism reveals slow relaxation of the Dy magnetization on a time scale of about 800 s at 2.5 K, unusually associated with an easy-plane magnetic anisotropy. We attribute these properties to Dy atoms occupying hollow adsorption sites on the TiO₂-terminated surface. Conversely, Ho atoms adsorbed on the same surface show paramagnetic behavior down to 2.5 K. With the help of atomic multiplet simulations and first-principles calculations, we establish that Dy populates also the top-O and bridge sites on the coexisting SrO-terminated surface. A simple magnetization relaxation model predicts these two sites to have an even longer magnetization lifetime than the hollow site. Moreover, the adsorption of Dy on the insulating SrTiO₃ crystal leads, regardless of the surface termination, to the formation of a spin-polarized two-dimensional electron gas of Ti 3d_xy character, together with an antiferromagnetic Dy-Ti coupling. Our findings support the feasibility of tuning the magnetic properties of the rare-earth atoms by acting on the substrate electronic gas with electric fields.

Ionic Liquid Gating of SrTiO3 Lamellas Fabricated with a Focused Ion Beam

In this work, we combine two previously incompatible techniques for defining electronic devices: shaping three-dimensional crystals by focused ion beam (FIB), and two-dimensional electrostatic accumulation of charge carriers. The principal challenge for this integration is nanometer-scale surface damage inherent to any FIB-based fabrication. We address this by using a sacrificial protective layer to preserve a selected pristine surface. The test case presented here is accumulation of 2D carriers by ionic liquid gating at the surface of a micron-scale SrTiO₃ lamella. Preservation of surface quality is reflected in superconductivity of the accumulated carriers. This technique opens new avenues for realizing electrostatic charge tuning in materials that are not available as large or exfoliatable single crystals, and for patterning the geometry of the accumulated carriers.

On the thermoelectric properties of Nb-doped SrTiO3 epitaxial thin films

The exploration for thermoelectric thin films of complex oxides such as SrTiO₃-based oxides is driven by the need for miniaturized harvesting devices for powering the Internet of Things (IoT). However, there is still not a clear consensus in the literature for the underlying influence of film thickness on thermoelectric properties. Here, we report the fabrication of epitaxial thin films of 6% Nb-doped SrTiO₃ on (001) (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7 (LSAT) single crystal using pulsed laser deposition (PLD) where the film thickness was varied from 2 nm to 68 nm. The thickness dependence shows a subtle increase of tetragonality of the thin film lattice and a gradual drop of the electrical conductivity, the density of charge carriers, and the thermoelectric Seebeck coefficient as the film thickness decreases. DFT-based calculations show that ∼2.8% increase in tetragonality results in an increased splitting between t_2g and e_g orbitals to ∼42.3 meV. However, experimentally observed tetragonality for films between 68 to 13 nm is only 0.06%. Hence, the effect of thickness on tetragonality is neglected. We have discussed the decrease of conductivity and the Seebeck coefficient based on the decrease of carriers and change in the scattering mechanism, respectively.

Large Current Driven Domain Wall Mobility and Gate Tuning of Coercivity in Ferrimagnetic Mn4N Thin Films

Spintronics, which is the basis of a low-power, beyond-CMOS technology for computational and memory devices, remains up to now entirely based on critical materials such as Co, heavy metals and rare-earths. Here, we show that Mn₄N, a rare-earth free ferrimagnet made of abundant elements, is an exciting candidate for the development of sustainable spintronics devices. Mn₄N thin films grown epitaxially on SrTiO₃ substrates possess remarkable properties, such as a perpendicular magnetization, a very high extraordinary Hall angle (2%) and smooth domain walls at the millimeter scale. Moreover, domain walls can be moved at record speeds by spin-polarized currents, in absence of spin-orbit torques. This can be explained by the large efficiency of the adiabatic spin transfer torque, due to the conjunction of a reduced magnetization and a large spin polarization. Finally, we show that the application of gate voltages through the SrTiO₃ substrates allows modulating the Mn₄N coercive field with a large efficiency.

Three-dimensional printing of piezoelectric materials with designed anisotropy and directional response

Piezoelectric coefficients are constrained by the intrinsic crystal structure of the constituent material. Here we describe design and manufacturing routes to previously inaccessible classes of piezoelectric materials that have arbitrary piezoelectric coefficient tensors. Our scheme is based on the manipulation of electric displacement maps from families of structural cell patterns. We implement our designs by additively manufacturing free-form, perovskite-based piezoelectric nanocomposites with complex three-dimensional architectures. The resulting voltage response of the activated piezoelectric metamaterials at a given mode can be selectively suppressed, reversed or enhanced with applied stress. Additionally, these electromechanical metamaterials achieve high specific piezoelectric constants and tailorable flexibility using only a fraction of their parent materials. This strategy may be applied to create the next generation of intelligent infrastructure, able to perform a variety of structural and functional tasks, including simultaneous impact absorption and monitoring, three-dimensional pressure mapping and directionality detection.

Ultrathin Films of Superconducting Metals as a Platform for Topological Superconductivity

The ingredients normally required to achieve topological superconductivity (TSC) are Cooper pairing, broken inversion symmetry, and broken time-reversal symmetry. We present a theoretical exploration of the possibility of using ultrathin films of superconducting metals as a platform for TSC. Because they necessarily break inversion symmetry when prepared on a substrate and have intrinsic Cooper pairing, they can be TSCs when time-reversal symmetry is broken by an external magnetic field. Using microscopic density functional theory calculations we show that, for ultrathin Pb and β-Sn superconductors, the position of the Fermi level can be tuned to quasi-2D band extrema energies using strain, and that the g factors of states at time-reversal invariant momenta can be extremely large, enhancing the influence of external magnetic fields.

Thickness-Dependent Double-Epitaxial Growth in Strained SrTi0.7Co0.3O3-δ Films

Perovskite-structured SrTi_0.7Co_0.3O_3-δ (STCo) films of varying thicknesses were grown on SrTiO₃(001) substrates using pulsed laser deposition. Thin films grow with a cube-on-cube epitaxy, but for films exceeding a critical thickness of about 120 nm, a double-epitaxial microstructure was observed, in which (110)-oriented crystals nucleated within the (001)-oriented STCo matrix, both orientations being epitaxial with the substrate. The crystal structure, strain state, and magnetic properties are described as a function of film thickness. Both the magnetic moment and the coercivity show maxima at the critical thickness. The formation of a double-epitaxial microstructure provides a mechanism for strain relief in epitaxially mismatched films.

Prospects and applications near ferroelectric quantum phase transitions: a key issues review

The emergence of complex and fascinating states of quantum matter in the neighborhood of zero temperature phase transitions suggests that such quantum phenomena should be studied in a variety of settings. Advanced technologies of the future may be fabricated from materials where the cooperative behavior of charge, spin and current can be manipulated at cryogenic temperatures. The progagating lattice dynamics of displacive ferroelectrics make them appealing for the study of quantum critical phenomena that is characterized by both space- and time-dependent quantities. In this key issues article we aim to provide a self-contained overview of ferroelectrics near quantum phase transitions. Unlike most magnetic cases, the ferroelectric quantum critical point can be tuned experimentally to reside at, above or below its upper critical dimension; this feature allows for detailed interplay between experiment and theory using both scaling and self-consistent field models. Empirically the sensitivity of the ferroelectric T _c's to external and to chemical pressure gives practical access to a broad range of temperature behavior over several hundreds of Kelvin. Additional degrees of freedom like charge and spin can be added and characterized systematically. Satellite memories, electrocaloric cooling and low-loss phased-array radar are among possible applications of low-temperature ferroelectrics. We end with open questions for future research that include textured polarization states and unusual forms of superconductivity that remain to be understood theoretically.

Advantages and Challenges of Relaxor-PbTiO3 Ferroelectric Crystals for Electroacoustic Transducers- A Review

Relaxor-PbTiO3 (PT) based ferroelectric crystals with the perovskite structure have been investigated over the last few decades due to their ultrahigh piezoelectric coefficients (d33 > 1500 pC/N) and electromechanical coupling factors (k33 > 90%), far outperforming state-of-the-art ferroelectric polycrystalline Pb(Zr,Ti)O3 ceramics, and are at the forefront of advanced electroacoustic applications. In this review, the performance merits of relaxor-PT crystals in various electroacoustic devices are presented from a piezoelectric material viewpoint. Opportunities come from not only the ultrahigh properties, specifically coupling and piezoelectric coefficients, but through novel vibration modes and crystallographic/domain engineering. Figure of merits (FOMs) of crystals with various compositions and phases were established for various applications, including medical ultrasonic transducers, underwater transducers, acoustic sensors and tweezers. For each device application, recent developments in relaxor-PT ferroelectric crystals were surveyed and compared with state-of-the-art polycrystalline piezoelectrics, with an emphasis on their strong anisotropic features and crystallographic uniqueness, including engineered domain - property relationships. This review starts with an introduction on electroacoustic transducers and the history of piezoelectric materials. The development of the high performance relaxor-PT single crystals, with a focus on their uniqueness in transducer applications, is then discussed. In the third part, various FOMs of piezoelectric materials for a wide range of ultrasound applications, including diagnostic ultrasound, therapeutic ultrasound, underwater acoustic and passive sensors, tactile sensors and acoustic tweezers, are evaluated to provide a thorough understanding of the materials' behavior under operational conditions. Structure-property-performance relationships are then established. Finally, the impacts and challenges of relaxor-PT crystals are summarized to guide on-going and future research in the development of relaxor-PT crystals for the next generation electroacoustic transducers.

Local electrostatic imaging of striped domain order in LaAlO3/SrTiO3

The emerging field of complex oxide interfaces is generically built on one of the most celebrated substrates--strontium titanate (SrTiO3). This material hosts a range of phenomena, including ferroelasticity, incipient ferroelectricity, and most puzzlingly, contested giant piezoelectricity. Although these properties may markedly influence the oxide interfaces, especially on microscopic length scales, the lack of local probes capable of studying such buried systems has left their effects largely unexplored. Here we use a scanning charge detector--a nanotube single-electron transistor--to non-invasively image the electrostatic landscape and local mechanical response in the prototypical LaAlO3/SrTiO3 system with unprecedented sensitivity. Our measurements reveal that on microscopic scales SrTiO3 exhibits large anomalous piezoelectricity with curious spatial dependence. Through electrostatic imaging we unravel the microscopic origin for this extrinsic piezoelectricity, demonstrating its direct, quantitative connection to the motion of locally ordered tetragonal domains under applied gate voltage. These domains create striped potential modulations that can markedly influence the two-dimensional electron system at the conducting interface. Our results have broad implications to all complex oxide interfaces built on SrTiO3 and demonstrate the importance of microscopic structure to the physics of electrons at the LaAlO3/SrTiO3 interface.

Combinatorial pulsed laser deposition of Fe, Cr, Mn, and Ni-substituted SrTiO3 films on Si substrates

Combinatorial pulsed laser deposition (CPLD) using two targets was used to produce a range of transition metal-substituted perovskite-structured Sr(Ti(1-x)M(x))O(3-δ) films on buffered silicon substrates, where M = Fe, Cr, Ni and Mn and x = 0.05-0.5. CPLD produced samples whose composition vs distance fitted a linear combination of the compositions of the two targets. Sr(Ti(1-x)Fe(x))O(3-δ) films produced from a pair of perovskite targets (SrTiO(3) and SrFeO(3) or SrTiO(3) and SrTi0(0.575)Fe(0.425)O(3)) had properties similar to those of films produced from single targets, showing a single phase microstructure, a saturation magnetization of 0.5 μ(B)/Fe, and a strong out-of-plane magnetoelastic anisotropy at room temperature. Films produced from an SrTiO(3) and a metal oxide target consisted of majority perovskite phases with additional metal oxide (or metal in the case of Ni) phases. Films made from SrTiO(3) and Fe(2)O(3) targets retained the high magnetic anisotropy of Sr(Ti(1-x)Fe(x))O(3-δ), but had a much higher saturation magnetization than single-target films, reaching for example an out-of-plane coercivity of >2 kOe and a saturation magnetization of 125 emu/cm(3) at 24%Fe. This was attributed to the presence of maghemite or magnetite exchange-coupled to the Sr(Ti(1-x)Fe(x))O(3-δ). Films of Sr(Ti(1-x)Cr(x))O(3-δ) and Sr(Ti(1-x)Mn(x))O(3-δ) showed no room temperature ferromagnetism, but Sr(Ti(1-x)Ni(x))O(3-δ) did show a high anisotropy and magnetization attributed mainly to the perovskite phase. Combinatorial synthesis is shown to be an efficient process for enabling evaluation of the properties of epitaxial substituted perovskite films as well as multiphase films which have potential for a wide range of electronic, magnetic, optical, and catalytic applications.

Luminescence properties of K1/2Bi1/2TiO3:Pr3+ and Na1/2Bi1/2TiO3:Pr3+

The luminescence properties of K(1/2)Bi(1/2)TiO(3):Pr(3+) and Na(1/2)Bi(1/2)TiO(3):Pr(3+) powders are investigated in the temperature range 10-600 K. The experimental data are interpreted on the basis of metal-to-metal charge transfer processes and by considering Bi(3+)-to-Pr(3+) sensitization effects.

Facile synthesis of SrTiO3 hollow microspheres built as assembly of nanocubes and their associated photocatalytic activity

SrTiO(3) hollow microspheres built by regular nanocubes were synthesized by a general and facile hydrothermal method. The resulting samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) measurements. Owing to the special hollow structure, the synthesized SrTiO(3) microspheres exhibit a superior photocatalytic activity in photoreduction of Cr(VI). As anatase and titanate contain common structural features, it makes this in situ phase transition from anatase to titanate relatively easy. Therefore, this method is rather convenient for controlling the morphology of the products by varying the presynthesized anatase TiO(2) precursor.

Elastic relaxations associated with the Pm3m-R3c transition in LaAlO(3): IV. An incipient instability below room temperature

Resonant ultrasound spectroscopy has been used to characterize elastic softening and acoustic dissipation behaviour in single crystal and ceramic samples of LaAlO(3) between 10 and 300 K. For the twinned R3c single crystals, average values of the cubic elastic moduli (1/2)(C(11) - C(12)) and C(44) were followed while the ceramic sample provided data for the bulk and shear moduli. A Debye-like dissipation peak occurs in the vicinity of 250 K, from which an activation energy of 43 ± 6 kJ mol(-1) has been obtained. The mechanism for this is not known, but it is associated with C(44) and therefore could be related in some way to the cubic <--> rhombohedral transition at ∼817 K. Slight softening in the temperature interval ~220 --> 70 K of resonance peaks determined by shear elastic moduli hints at an incipient E(g) ferroelastic instability in LaAlO(3). The softening interval ends with a further dissipation peak at ∼60 K, the origin of which is discussed in terms of freezing of atomic motions of La and/or Al away from their high symmetry positions in the R3c structure. LaAlO(3) thus shows evidence of incipient structural instability at low temperatures which is potentially analogous with the phenomenologically rich behaviour of SrTiO(3).

Competition between quantum fluctuations and antiferroelectric order in Ru-doped Sr(0.8)Ca(0.2)Ti(1-x)Ru(x)O(3)

The competition between quantum fluctuations and the antiferroelectric state in Sr(0.8)Ca(0.2)Ti(1-x)Ru(x)O(3) is investigated by measuring the low-temperature dielectric permittivity and by Raman spectroscopy. We demonstrate the significant impact of quantum fluctuations on the stability of the antiferroelectric polar order. It is revealed that the structural phase transitions can be modified by the quantum fluctuations, enhancing the stability of the high-symmetry phase and suppressing the antiferroelectric transitions. More importantly, a quantum antiferroelectric state, exhibiting similar behavior as the quantum ferroelectric state in terms of dielectric response, is identified. In addition, the effect of quantum fluctuations on the increasing permittivity at low temperature is also discussed.

Elastic anomalies associated with transformation sequences in perovskites: II. The strontium zirconate-titanate Sr(Zr,Ti)O(3) solid solution series

The sequence of phase transitions due to octahedral tilting across the Sr(Zr,Ti)O(3) solid solution series has been investigated by resonant ultrasound spectroscopy at high and low temperatures using ceramic samples. The elastic behaviour associated with phase transitions as a function of composition in Sr(Zr,Ti)O(3) at room temperature is proposed to be analogous to that as a function of temperature in SrZrO(3), with the [Formula: see text] transition at SrZr(0.57)Ti(0.43)O(3), [Formula: see text] at SrZr(0.35)Ti(0.65)O(3), and [Formula: see text] at SrZr(0.05)Ti(0.95)O(3). Changes in elastic constants and acoustic dissipation with temperature have been analysed for samples across the compositional range. The intermediate phases, I4/mcm and what is assumed to be Imma, appear to have stability fields across the full compositional range and both show large dissipation effects, most probably due to twin wall mobility. In contrast, the Zr-rich Pnma phase, which should contain transformation twin walls, is an unexpectedly stiff and non-dissipating material, similar to the high temperature and/or Ti-rich [Formula: see text] phase. In the case of Pnma, this is attributed to coupling between the two order parameters, which could impede relaxation responses to an applied stress. The [Formula: see text] structure is a classically stiff cubic perovskite and no transformation-related dissipation processes are expected.

Ultrafast Optical Excitation of a Combined Coherent-Squeezed Phonon field in SrTiO3

We have simultaneously excited a coherent and a squeezed phonon field in SrTiO3 using femtosecond laser pulses and stimulated Raman scattering. The frequency of the coherent state (a 1.3 THz) is that of the A1g-component of the soft mode responsible for the cubic-tetragonal phase transformation at approximately 110 K. The squeezed field involves a continuum of transverse acoustic phonons dominated by a narrow peak in the density of states at a 6.9 THz.