Patrick Irvin

Room-temperature ferroelectricity in strained SrTiO3

Systems with a ferroelectric to paraelectric transition in the vicinity of room temperature are useful for devices. Adjusting the ferroelectric transition temperature (Tc) is traditionally accomplished by chemical substitution—as in BaxSr1-xTiO3, the material widely investigated for microwave devices in which the dielectric constant (εr) at GHz frequencies is tuned by applying a quasi-static electric field. Heterogeneity associated with chemical substitution in such films, however, can broaden this phase transition by hundreds of degrees, which is detrimental to tunability and microwave device performance. An alternative way to adjust Tc in ferroelectric films is strain. Here we show that epitaxial strain from a newly developed substrate can be harnessed to increase Tc by hundreds of degrees and produce room-temperature ferroelectricity in strontium titanate, a material that is not normally ferroelectric at any temperature. This strain-induced enhancement in Tc is the largest ever reported. Spatially resolved images of the local polarization state reveal a uniformity that far exceeds films tailored by chemical substitution. The high εr at room temperature in these films (nearly 7,000 at 10 GHz) and its sharp dependence on electric field are promising for device applications.

Rewritable nanoscale oxide photodetector

Researchers report rewritable nanoscale photodetectors that exploit 2–3 nm nanowire junctions. Large electromagnetic fields in the gap region aid the detector response, which is electric-field-tunable and spans the visible to near-infrared regime.

Electron pairing without superconductivity

Strontium titanate (SrTiO3) is the first and best known superconducting semiconductor. It exhibits an extremely low carrier density threshold for superconductivity, and possesses a phase diagram similar to that of high-temperature superconductors—two factors that suggest an unconventional pairing mechanism. Despite sustained interest for 50 years, direct experimental insight into the nature of electron pairing in SrTiO3 has remained elusive. Here we perform transport experiments with nanowire-based single-electron transistors at the interface between SrTiO3 and a thin layer of lanthanum aluminate, LaAlO3. Electrostatic gating reveals a series of two-electron conductance resonances—paired electron states—that bifurcate above a critical pairing field Bp of about 1–4 tesla, an order of magnitude larger than the superconducting critical magnetic field. For magnetic fields below Bp, these resonances are insensitive to the applied magnetic field; for fields in excess of Bp, the resonances exhibit a linear Zeeman-like energy splitting. Electron pairing is stable at temperatures as high as 900 millikelvin, well above the superconducting transition temperature (about 300 millikelvin). These experiments demonstrate the existence of a robust electronic phase in which electrons pair without forming a superconducting state. Key experimental signatures are captured by a model involving an attractive Hubbard interaction that describes real-space electron pairing as a precursor to superconductivity.

Room-temperature electronically-controlled ferromagnetism at the LaAlO3/SrTiO3 interface

Reports of emergent conductivity, superconductivity and magnetism have helped to fuel intense interest in the rich physics and technological potential of complex-oxide interfaces. Here we employ magnetic force microscopy to search for room-temperature magnetism in the well-studied LaAlO3/SrTiO3 system. Using electrical top gating to control the electron density at the oxide interface, we directly observe the emergence of an in-plane ferromagnetic phase as electrons are depleted from the interface. Itinerant electrons that are reintroduced into the interface align antiferromagnetically with the magnetization at first screening and then destabilizing it as the conductive regime is approached. Repeated cycling of the gate voltage results in new, uncorrelated magnetic patterns. This newfound control over emergent magnetism at the interface between two non-magnetic oxides portends a number of important technological applications.

Nanoscale Phenomena in Oxide Heterostructures

Recent advances in creating complex oxide heterostructures, interfaces formed between two different transition-metal oxides, have heralded a new era of materials and physics research, enabling a uniquely diverse set of coexisting physical properties to be combined with an ever-increasing degree of experimental control. These systems have exhibited varied phenomena such as superconductivity, magnetism, and ferroelasticity, all of which are gate tunable, demonstrating their promise for fundamental discovery and technological innovation. To fully exploit this richness, it is necessary to understand and control the physics on the smallest scales, making the use of nanoscale probes essential. Using the prototypical LaAlO3/SrTiO3 interface as a guide, we explore the exciting developments in the physics of oxide-based heterostructures, with a focus on nanostructures and the nanoscale probes employed to unravel their complex behavior.

Three-dimensional polarization imaging of (Ba,Sr)TiO3:MgO composites

The dielectric tuning and loss of (Ba,Sr)TiO3:MgO bulk composites depend strongly on the connectivity and interaction among the two phases. To investigate this relationship, the polar structure and dynamics of these composites are mapped as a function of space and time using a pair of three-dimensional probes: second-harmonic confocal scanning optical microscopy (SH-CSOM), which maps ferroelectric polarization in three dimensions, and time-resolved scanning optical microscopy (TR-CSOM), which maps polarization dynamics along two spatial dimensions and one time dimension. SH-CSOM measurements reveal a high degree of homogeneity within the (Ba,Sr)TiO3 regions, while TR-CSOM measurements indicate that topologically connected regions respond with a spatially uniform phase.

Anomalous High Mobility in LaAlO3/SrTiO3 Nanowires

Nanoscale control of the metal-insulator transition at the interface between LaAlO(3) and SrTiO(3) provides a pathway for reconfigurable, oxide-based nanoelectronics. Four-terminal transport measurements of LaAlO(3)/SrTiO(3) nanowires at room temperature (T = 300 K) reveal an equivalent 2D Hall mobility greatly surpassing that of bulk SrTiO(3) and approaching that of n-type Si nanowires of comparable dimensions. This large enhancement of mobility is relevant for room-temperature device applications.

Oxide-based platform for reconfigurable superconducting nanoelectronics

We report superconductivity in quasi-1D nanostructures created at the LaAlO3/SrTiO3 interface. Nanostructures having line widths w~10 nm are formed from the parent two-dimensional electron liquid using conductive atomic force microscope lithography. Nanowire cross-sections are small compared to the superconducting coherence length in LaAlO3/SrTiO3 (w<<xi~100 nm), placing them in the quasi-1D regime. Broad superconducting transitions with temperature and finite resistances in the superconducting state well below Tc~200 mK are observed. V-I curves show switching between the superconducting and normal states that are characteristic of superconducting nanowires. The four-terminal resistance in the superconducting state shows an unusual dependence on the current path, varying by as much as an order of magnitude.

Broadband Terahertz Generation and Detection at 10 nm Scale

Terahertz (0.1-30 THz) radiation reveals a wealth of information that is relevant for material, biological, and medical sciences with applications that span chemical sensing, high-speed electronics, and coherent control of semiconductor quantum bits. To date, there have been no methods capable of controlling terahertz (THz) radiation at molecular scales. Here we report both generation and detection of broadband terahertz field from 10 nm scale oxide nanojunctions. Frequency components of ultrafast optical radiation are mixed at these nanojunctions, producing broadband THz emission. These same devices detect THz electric fields with comparable spatial resolution. This unprecedented control, on a scale of 4 orders of magnitude smaller than the diffraction limit, creates a pathway toward THz-bandwidth spectroscopy and control of individual nanoparticles and molecules.

Direct imaging of LaAlO3/SrTiO3 nanostructures using piezoresponse force microscopy

The interface between LaAlO3 and TiO2-terminated SrTiO3 can be switched between metastable conductive and insulating states using a conductive atomic force microscope probe. Determination of the nanoscale dimensions has previously required a destructive readout (e.g., local restoration of an insulating state). Here it is shown that high-resolution non-destructive imaging of conductive nanostructures can be achieved using a specific piezoresponse force microscopy (PFM) technique. Images of conductive and insulating nanoscale features are achieved with feature sizes as small as 30 nm. The measured nanowire width from PFM is well correlated with those obtained from nanowire erasure.