The interface between lanthanum aluminate (LaAlO3) and strontium titanate (SrTiO3) is a notable materials interface because it exhibits properties not found in its constituent materials. Individually, LaAlO3 and SrTiO3 are non-magnetic insulators, yet LaAlO3/SrTiO3 interfaces can exhibit electrical metallic conductivity, [1] superconductivity, [2] ferromagnetism, [3] large negative in-plane magnetoresistance, [4] and giant persistent photoconductivity. [5] The study of how these properties emerge at the LaAlO3/SrTiO3 interface is a growing area of research in condensed matter physics.
Under the right conditions, the LaAlO3/SrTiO3 interface is electrically conductive, like a metal. The angular dependence of Shubnikov–de Haas oscillations indicates that the conductivity is two-dimensional, [6] leading many researchers to refer to it as a two-dimensional electron gas (2DEG). Two-dimensional does not mean that the conductivity has zero thickness, but rather that the electrons are confined to only move in two directions. It is also sometimes called a two-dimensional electron liquid (2DEL) to emphasize the importance of inter-electron interactions. [7]
Not all LaAlO3/SrTiO3 interfaces are conductive. Typically, conductivity is achieved only when:
Conductivity can also be achieved when the SrTiO3 is doped with oxygen vacancies; however, in that case, the interface is technically LaAlO3/SrTiO3−x instead of LaAlO3/SrTiO3.
The source of conductivity at the LaAlO3/SrTiO3 interface has been debated for years. SrTiO3 is a wide-band gap semiconductor that can be doped n-type in a variety of ways. Clarifying the mechanism behind the conductivity is a major goal of current research. Four leading hypotheses are:
Polar gating was the first mechanism used to explain the conductivity at LaAlO3/SrTiO3 interfaces. [1] It postulates that the LaAlO3, which is polar in the 001 direction (with alternating sheets of positive and negative charge), acts as an electrostatic gate on the semiconducting SrTiO3. [1] When the LaAlO3 layer grows thicker than three unit cells, its valence band energy rises above the Fermi level, causing holes (or positively charged oxygen vacancies [9] ) to form on the outer surface of the LaAlO3. The positive charge on the surface of the LaAlO3 attracts negative charge to nearby available states. In the case of the LaAlO3/SrTiO3 interface, this means electrons accumulate in the surface of the SrTiO3, in the Ti d bands.
The strengths of the polar gating hypothesis are that it explains why conductivity requires a critical thickness of four unit cells of LaAlO3 and that it explains why conductivity requires the SrTiO3 to be TiO2-terminated. The polar gating hypothesis also explains why alloying the LaAlO3 increases the critical thickness for conductivity. [10]
One weakness of the hypothesis is that it predicts that the LaAlO3 films should exhibit a built-in electric field; so far, x-ray photoemission experiments [11] [12] [13] [14] and other experiments [15] [16] [17] have shown little to no built-in field in the LaAlO3 films. The polar gating hypothesis also cannot explain why Ti3+ is detected when the LaAlO3 films are thinner than the critical thickness for conductivity. [12]
The polar gating hypothesis is sometimes called the polar catastrophe hypothesis, [18] alluding to the counterfactual scenario where electrons don't accumulate at the interface and instead voltage in the LaAlO3 builds up forever. The hypothesis has also been called the electronic reconstruction hypothesis, [18] highlighting the fact that electrons, not ions, move to compensate the building voltage.
Another hypothesis is that the conductivity comes from free electrons left by oxygen vacancies in the SrTiO3. [19] SrTiO3 is known to be easily doped by oxygen vacancies, so this was initially considered a promising hypothesis. However, electron energy loss spectroscopy measurements have bounded the density of oxygen vacancies well below the density necessary to supply the measured free electron densities. [20] Another proposed possibility is that oxygen vacancies in the surface of the LaAlO3 are remotely doping the SrTiO3. [12] Under generic growth conditions, multiple mechanisms can coexist. A systematic study [21] across a wide growth parameter space demonstrated different roles played by oxygen vacancy formation and the polar gating at different interfaces. An obvious difference between oxygen vacancies and polar gating in creating the interface conductivity is that the carriers from oxygen vacancies are thermally activated as the donor level of oxygen vacancies is usually separated from the SrTiO3 conduction band, consequently exhibiting the carrier freeze-out effect [22] at low temperatures; in contrast, the carriers originating from the polar gating are transferred into the SrTiO3 conduction band (Ti 3d orbitals) and are therefore degenerate. [21]
Lanthanum is a known dopant in SrTiO3, [23] so it has been suggested that La from the LaAlO3 mixes into the SrTiO3 and dopes it n-type. Multiple studies have shown that intermixing takes place at the interface; [24] however, it is not clear whether there is enough intermixing to provide all of the free carriers. For example, a flipped interface between a SrTiO3 film and a LaAlO3 substrate is insulating. [25]
A fourth hypothesis is that the LaAlO3 crystal structure undergoes octahedral rotations in response to the strain from the SrTiO3. These octahedral rotations in the LaAlO3 induce octahedral rotations in the SrTiO3, increasing the Ti d-band width enough so that electrons are no longer localized. [26]
Superconductivity was first observed in LaAlO3/SrTiO3 interfaces in 2007, with a critical temperature of ~200 mK. [27] Like the conductivity, the superconductivity appears to be two-dimensional. [2]
Hints of ferromagnetism in LaAlO3/SrTiO3 were first seen in 2007, when Dutch researchers observed hysteresis in the magnetoresistance of LaAlO3/SrTiO3. [28] Follow up measurements with torque magnetometry indicated that the magnetism in LaAlO3/SrTiO3 persisted all the way to room temperature. [29] In 2011, researchers at Stanford University used a scanning SQUID to directly image the ferromagnetism, and found that it occurred in heterogeneous patches. [3] Like the conductivity in LaAlO3/SrTiO3, the magnetism only appeared when the LaAlO3 films were thicker than a few unit cells. [30] However, unlike conductivity, magnetism was seen at SrO-terminated surfaces as well as TiO2-terminated surfaces. [30]
The discovery of ferromagnetism in a materials system that also superconducts spurred a flurry of research and debate, because ferromagnetism and superconductivity almost never coexist together. [3] Ferromagnetism requires electron spins to align, while superconductivity typically requires electron spins to anti-align.
Magnetoresistance measurements are a major experimental tool used to understand the electronic properties of materials. The magnetoresistance of LaAlO3/SrTiO3 interfaces has been used to reveal the 2D nature of conduction, carrier concentrations (through the hall effect), electron mobilities, and more. [6]
At low magnetic field, the magnetoresistance of LaAlO3/SrTiO3 is parabolic versus field, as expected for an ordinary metal. [31] However, at higher fields, the magnetoresistance appears to become linear versus field. [31] Linear magnetoresistance can have many causes, but so far there is no scientific consensus on the cause of linear magnetoresistance in LaAlO3/SrTiO3 interfaces. [31] Linear magnetoresistance has also been measured in pure SrTiO3 crystals, [32] so it may be unrelated to the emergent properties of the interface.
At low temperature (T < 30 K), the LaAlO3/SrTiO3 interface exhibits negative in-plane magnetoresistance, [31] sometimes as large as -90%. [4] The large negative in-plane magnetoresistance has been ascribed to the interface's enhanced spin-orbit interaction. [4] [33]
Experimentally, the charge density profile of the electron gas at the LaAlO3/SrTiO3 interface has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm. [34] [35] A wide variety of theoretical calculations support this result. Importantly, to get electron distribution one have to take into account field-dependent dielectric constant of SrTiO3. [36] [37] [38]
The 2D electron gas that arises at the LaAlO3/SrTiO3 interface is notable for two main reasons. First, it has very high carrier concentration, on the order of 1013 cm−2. Second, if the polar gating hypothesis is true, the 2D electron gas has the potential to be totally free of disorder, unlike other 2D electron gases that require doping or gating to form. However, so far researchers have been unable to synthesize interfaces that realize the promise of low disorder.
Most LaAlO3/SrTiO3 interfaces are synthesized using pulsed laser deposition. A high-power laser ablates a LaAlO3 target, and the plume of ejected material is deposited onto a heated SrTiO3 substrate. Typical conditions used are:
Some LaAlO3/SrTiO3 interfaces have also been synthesized by molecular beam epitaxy, sputtering, and atomic layer deposition. [40]
To better understand in the LaAlO3/SrTiO3 interface, researchers have synthesized a number of analogous interfaces between other polar perovskite films and SrTiO3. Some of these analogues have properties similar to LaAlO3/SrTiO3, but some do not.
As of 2015, there are no commercial applications of the LaAlO3/SrTiO3 interface. However, speculative applications have been suggested, including field-effect devices, sensors, photodetectors, and thermoelectrics; [53] related LaVO3/SrTiO3 is a functional solar cell [54] albeit hitherto with a low efficiency. [55]
Spintronics, also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices. The field of spintronics concerns spin-charge coupling in metallic systems; the analogous effects in insulators fall into the field of multiferroics.
A polaron is a quasiparticle used in condensed matter physics to understand the interactions between electrons and atoms in a solid material. The polaron concept was proposed by Lev Landau in 1933 and Solomon Pekar in 1946 to describe an electron moving in a dielectric crystal where the atoms displace from their equilibrium positions to effectively screen the charge of an electron, known as a phonon cloud. This lowers the electron mobility and increases the electron's effective mass.
Tunnel magnetoresistance (TMR) is a magnetoresistive effect that occurs in a magnetic tunnel junction (MTJ), which is a component consisting of two ferromagnets separated by a thin insulator. If the insulating layer is thin enough, electrons can tunnel from one ferromagnet into the other. Since this process is forbidden in classical physics, the tunnel magnetoresistance is a strictly quantum mechanical phenomenon.
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A two-dimensional electron gas (2DEG) is a scientific model in solid-state physics. It is an electron gas that is free to move in two dimensions, but tightly confined in the third. This tight confinement leads to quantized energy levels for motion in the third direction, which can then be ignored for most problems. Thus the electrons appear to be a 2D sheet embedded in a 3D world. The analogous construct of holes is called a two-dimensional hole gas (2DHG), and such systems have many useful and interesting properties.
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Lanthanum strontium manganite (LSM or LSMO) is an oxide ceramic material with the general formula La1−xSrxMnO3, where x describes the doping level.
Gallium manganese arsenide, chemical formula (Ga,Mn)As is a magnetic semiconductor. It is based on the world's second most commonly used semiconductor, gallium arsenide,, and readily compatible with existing semiconductor technologies. Differently from other dilute magnetic semiconductors, such as the majority of those based on II-VI semiconductors, it is not paramagnetic but ferromagnetic, and hence exhibits hysteretic magnetization behavior. This memory effect is of importance for the creation of persistent devices. In (Ga,Mn)As, the manganese atoms provide a magnetic moment, and each also acts as an acceptor, making it a p-type material. The presence of carriers allows the material to be used for spin-polarized currents. In contrast, many other ferromagnetic magnetic semiconductors are strongly insulating and so do not possess free carriers. (Ga,Mn)As is therefore a candidate as a spintronic material.
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Samarium hexaboride (SmB6) is an intermediate-valence compound where samarium is present both as Sm2+ and Sm3+ ions at the ratio 3:7. It is a Kondo insulator having a metallic surface state.
Kathryn Ann Moler is an American physicist, and current Dean of Research at Stanford University. She received her BSc (1988) and Ph.D. (1995) from Stanford University. After working as a visiting scientist at IBM T.J. Watson Research Center in 1995, she held a postdoctoral position at Princeton University from 1995 to 1998. She joined the faculty of Stanford University in 1998, and became an Associate in CIFAR's Superconductivity Program in 2000. She became an Associate Professor at Stanford in 2002 and is currently a professor of Applied Physics and of Physics at Stanford. She currently works in the Geballe Laboratory for Advanced Materials (GLAM), and is the Director of the Center for Probing the Nanoscale (CPN), a National Science Foundation-funded center where Stanford and IBM scientists continue to improve scanning probe methods for measuring, imaging, and controlling nanoscale phenomena. She lists her scientific interests and main areas of research and experimentation as:
Lanthanum aluminate is an inorganic compound with the formula LaAlO3, often abbreviated as LAO. It is an optically transparent ceramic oxide with a distorted perovskite structure.
A polar metal, metallic ferroelectric, or ferroelectric metal is a metal that contains an electric dipole moment. Its components have an ordered electric dipole. Such metals should be unexpected, because the charge should conduct by way of the free electrons in the metal and neutralize the polarized charge. However they do exist. Probably the first report of a polar metal was in single crystals of the cuprate superconductors YBa2Cu3O7−δ,. A polarization was observed along one (001) axis by pyroelectric effect measurements, and the sign of the polarization was shown to be reversible, while its magnitude could be increased by poling with an electric field. The polarization was found to disappear in the superconducting state. The lattice distortions responsible were considered to be a result of oxygen ion displacements induced by doped charges that break inversion symmetry. The effect was utilized for fabrication of pyroelectric detectors for space applications, having the advantage of large pyroelectric coefficient and low intrinsic resistance. Another substance family that can produce a polar metal is the nickelate perovskites. One example interpreted to show polar metallic behavior is lanthanum nickelate, LaNiO3. A thin film of LaNiO3 grown on the (111) crystal face of lanthanum aluminate, (LaAlO3) was interpreted to be both conductor and a polar material at room temperature. The resistivity of this system, however, shows an upturn with decreasing temperature, hence does not strictly adhere to the definition of a metal. Also, when grown 3 or 4 unit cells thick (1-2 nm) on the (100) crystal face of LaAlO3, the LaNiO3 can be a polar insulator or polar metal depending on the atomic termination of the surface. Lithium osmate, LiOsO3 also undergoes a ferrorelectric transition when it is cooled below 140K. The point group changes from R3c to R3c losing its centrosymmetry. At room temperature and below, lithium osmate is an electric conductor, in single crystal, polycrystalline or powder forms, and the ferroelectric form only appears below 140K. Above 140K the material behaves like a normal metal. Artificial two-dimensional polar metal by charge transfer to a ferroelectric insulator has been realized in LaAlO3/Ba0.8Sr0.2TiO3/SrTiO3 complex oxide heterostructures.
Spin Hall magnetoresistance (SMR) is a transport phenomenon that is found in some electrical conductors that have at least one surface in direct contact with another magnetic material due to changes in the spin current that are present in metals and semiconductors with a large spin Hall angle. It is most easily detected when the magnetic material is an insulator which eliminates other magnetically sensitive transport effects arising from conduction in the magnetic material.
John F. Mitchell is an American chemist and researcher. He is the deputy director of the materials science division at the U.S. Department of Energy's (DOE) Argonne National Laboratory and leads Argonne's Emerging Materials Group.
Jeremy Levy is an American physicist who is a Distinguished Professor of Physics at the University of Pittsburgh.
Elbio Rubén Dagotto is an Argentinian-American theoretical physicist and academic. He is a Distinguished Professor in the Department of Physics and Astronomy at the University of Tennessee, Knoxville, and Distinguished Scientist in the Materials Science and Technology Division at the Oak Ridge National Laboratory.