A talk by P A Lee on SC and FM coexisting in oxide interface

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More on the LAO/STO interface

This interface is really booming interests in similar structures ! See a summary by A.J.Millis of what might be expected in this system.

Electronic phase separation at the LaAlO(3)/SrTiO(3) interface
Authors: Ariando et. al.
Nature Communications 2 Article 188 (2011).
Coexistence of Superconductivity and Ferromagnetism in Two Dimensions
Authors: D. A. Dikin, M. Mehta, C. W. Bark, C. M. Folkman, C. B. Eom, and V. Chandrasekhar.
Phys. Rev. Lett. 107 056802 (2011).
Coexistence of magnetic order and two-dimensional superconductivity at LaAlO3/SrTiO3
interfaces
Authors: Lu Li, C. Richter, J. Mannhart and R. C. Ashoori
Nature Physics, doi 10.1038/nphys2080.
Direct Imaging of the coexistence of ferromagnetism and superconductivity at the
LaAlO3/SrTiO3 interface
Authors: J. A. Bert, B. Kalisky, C. Bell. M. Kim, Y. Hikita, H. Y. Hwang and K. Moler
Nature Physics, doi 10.1038/nphys2079

More on This LAO/STO layer

2D electron gas was observed at the interface between LAO and STO. This 2DEG displays properties including superconductivity. Now this report [Science, 332:825(2011)] says electron correlation effects can lead to negative compressibility and thus enhance capacitance.

Increases in the gate capacitance of field-effect transistor structures allow the production of lower-power devices that are compatible with higher clock rates, driving the race for developing high-κ dielectrics. However, many-body effects in an electronic system can also enhance capacitance. Onto the electron system that forms at the LaAlO₃/SrTiO₃ interface, we fabricated top-gate electrodes that can fully deplete the interface of all mobile electrons. Near depletion, we found a greater than 40% enhancement of the gate capacitance. Using an electric-field penetration measurement method, we show that this capacitance originates from a negative compressibility of the interface electron system. Capacitance enhancement exists at room temperature and arises at low electron densities, in which disorder is strong and the in-plane conductance is much smaller than the quantum conductance.

Electronic correlations are crucial in 2DEG based on STO

I have highlighted a number of studies on the 2DEG that were created about the interfaces based on SrTiO3 compounds. The 2DEG thus obtained have been shown with various interesting ground states including superconducting ones. Here comes a new work [Science, 331 (6019): 886-889] that demonstrates the importance of electronic correlations in determining the transport properties of this 2DEG. This time the 2DEG was introduced by inserting a RO layer, R=La, Pr,Nd,Sm and Y, in the SrTiO3 matrix. It turns out that, the electronic properties of this 2DEG are crucially hinging on the R element. For La, Pr and Nd, it is conducting while for the rest it is insulating.

The formation of two-dimensional electron gases (2DEGs) at complex oxide interfaces is directly influenced by the oxide electronic properties. We investigated how local electron correlations control the 2DEG by inserting a single atomic layer of a rare-earth oxide (RO) [(R is lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), or yttrium (Y)] into an epitaxial strontium titanate oxide (SrTiO₃) matrix using pulsed-laser deposition with atomic layer control. We find that structures with La, Pr, and Nd ions result in conducting 2DEGs at the inserted layer, whereas the structures with Sm or Y ions are insulating. Our local spectroscopic and theoretical results indicate that the interfacial conductivity is dependent on electronic correlations that decay spatially into the SrTiO₃ matrix. Such correlation effects can lead to new functionalities in designed heterostructures.

2DEG at the surface of STO

Reduced dimension electron systems boast of very interesting physics from both the fundamental and practical point of view. For example, 2DEG houses quantum hall effects, which even today constitutes a rich arena of study [Nature, 469:185–188]. Such 2DEG exists in a diversity of systems, such as semiconductor or transition metal oxide hetero structures. More recently, it defines topological insulators. Now a work in Nature shows that, 2DEG exists also in simple surfaces of strontium titanate, and more interestingly, this 2DEG has similar characteristics with those found at the interfaces of STO with the second compound [doi:10.1038/nature09720]. This offers an easy way to fabricate 2DEG systems. More work is needed in elucidating the mechanism.

As silicon is the basis of conventional electronics, so strontium titanate (SrTiO3) is the foundation of the emerging field of oxide electronics1,2. SrTiO3 is the preferred template for the creation of exotic, two-dimensional (2D) phases of electronmatter at oxide interfaces3–5 that havemetal–insulator transitions6,7, superconductivity8,9 or large negative magnetoresistance10. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs), which is crucial to understanding their remarkable properties11,12, remains elusive. Here we show, using angle-resolved photoemission spectroscopy, that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO3 (including the non-doped insulating material) independently of bulk carrier densities over more than seven decades. This 2DEG is confined within a region of about five unit cells and has a sheet carrier density of 0.33 electrons per square lattice parameter. The electronic structure consists of multiple subbands of heavy and light electrons. The similarity of this 2DEG to those reported in SrTiO3-based heterostructures6,8,13 and field-effect transistors9,14 suggests that different forms of electron confinement at the surface of SrTiO3 lead to essentially the same 2DEG. Our discovery provides a model system for the study of the electronic structure of 2DEGs in SrTiO3-based devices and a novel means of generating 2DEGs at the surfaces of transition-metal oxides.

Polar thin film is not ferroelectric

Epitaxial thin film of strontium titanate on a silicon substrate is now demonstrated non-ferroelectric, despite its spontaneous polarization [PRL 105, 217601 (2010)]. This polarization is not due to spontaneous symmetry breaking, rather it is a property of the interface ground state. The symmetry is absent from the outset in the presence of the substrate, which strains the film. Therefore, the polarization is not switchable. This finding is made by DFT computations and STEM techniques.

We use SrTiO3=Si as a model system to elucidate the effect of the interface on ferroelectric behavior in epitaxial oxide films on silicon. Using both first-principles computations and synchrotron x-ray diffraction measurements, we show that structurally imposed boundary conditions at the interface stabilize a fixed
(pinned) polarization in the film but inhibit ferroelectric switching. We demonstrate that the interface chemistry responsible for these phenomena is general to epitaxial silicon-oxide interfaces, impacting on the design of silicon-based functional oxide devices.

Molecules filtering spins

Using STM with a magnetic tip can be used to probe the magnetic feature of a surface. The tunneling current shall be sensitive to the alignment (collimation) between the spin orientation of the surface and that of the tip. If the tunneling, as is usually the case, is non-magnetic, then parallel alignment yield a bigger current. A very valuable aspect of STM is that this device probes the local properties of a material. This makes it especially useful in investigating defects or impurities of a surface. Now these authors [Phys. Rev. Lett. 105, 066601 (2010)] came to examine what will happen to the signal if the electrons tunnel from the Fe surface into the tip through a single organic molecule with Beneze rings. The result is this: this molecule allows more spin-up electrons to pass. So, it works as a selective valve, which may be tailored to specific applications that needs manipulate spin current. This phenomenon was predicted 3 years ago in Ref.[3], where the computation was implemented in the aid of DFT. However, it may prove more elucidating if a simple model description is prescribed.
For convenience, some references are attested on this subject:

1. Atodiresei, N. et al. Phys. Rev. Lett. 105, 066601 (2010).
2. Brede, J. et al. Phys. Rev. Lett. 105, 047204 (2010).
3. Rocha, A. R. & Sanvito, S. J. Appl. Phys. 101, 09B102 (2007).
4. Barraud, C. et al. Nature Phys. 6, 615–620 (2010).
5. Sanvito, S. Nature Phys. 6, 562–564 (2010).
6. Cinchetti, M. et al. Nature Mater. 8, 115–119 (2009).
7. Drew, A. J. et al. Nature Mater. 8, 109–114 (2009).
8. Szulczewski, G., Sanvito, S. & Coey, J. M. D. Nature Mater. 8, 693–695 (2009)

Surface states

A latest fervent topic in condensed matter physics is the so-called topological insulators. An essential feature of these compounds is their having unique surface states. Surface states are states that decay into the vacuum as well as the bulk, thus qualitatively different from bulk states. These states always show up in the bulk forbidden gap, much the way as impurity states. In a sense they are indeed impurity states, because they are created by breaking periodic boundary conditions. Suppose you have a ring of N identical atoms. Now you break it, and you obtain two end atoms which have different surrounding than other bulk atoms. The breaking bond can be expressed as a perturbation, and thus leading to two impurity states. In history, they are called Shockley-Tamm states.

Introduction to surface states:
[1]http://philiphofmann.net/surflec3/surflec015.html
[2]http://en.wikipedia.org/wiki/Surface_states

Spin-triplet pairs in the proximity of a supercondutor and a ferromagnet

Electrons in a supercondutor below Tc are described by a single wave function, which satifies Schrodinger's equation (dBG equation). The value of this function relates to the effective potential felt by the Copper pairs. The potential is negative in the superconductor but positive in a normal material. Thus, normal mateials make a energy barrier that prevents the peameability of electron pairs , and hence supercurrent to spill much, especially when the normal material is ferromagnetic. A ferromagnet favors spin-triplet, while Copper pairs are singlets. Nevertheless, there are some recent experiments observing a longer-range spilling of supercurrent, which was being put under the context of spin-triplet paring that may arise the perephery of an S-N interaface. Now, it came a paper aiming at this problem[1]. They found further evidences of such paring.

The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. After placing a conical magnet (holmium) at the interface between the two, we detected a long-ranged supercurrent in the ferromagnetic layer. The long-range effect required particular thicknesses of the spiral magnetically ordered holmium, consistent with spin-triplet proximity theory. This enabled control of the electron pairing symmetry by tuning the degree of magnetic inhomogeneity through the thicknesses of the holmium injectors.

[1]Science 2 July 2010: Vol. 329. no. 5987, pp. 59 - 61; DOI: 10.1126/science.1189246

2DEG switchable by electric field ?

Perovskite materials are cool as they frequently exhibit exotic properties and thus offer opportunities to fabricate new electronic components.

Here i talk about a perovskite-based interface structure that traps electrons within a few layers (2DEG). 2DEG has been the focus of extensive investigations for many years, examples concerning cuprate superconductors and transistors.

This structure consists of a NbO2 layer sanwitched by strontium STO on one end and KNO on the other. Electrons shall pool around that NbO2 sheet. As we know, the d orbitals on every Nb atom in bulk KNO are nominally empty. So does the pure NbO2 sheet. As one incorperates this sheet into that structure, due to electronic reconstruction that happens often at interfaces, the d orbitals shall be taken up by electrons, but only partially, which forms the so-called Hubbard layer. For partial filling, these electrons shall conduct electricity, with conductivity proportional to the electron density.

Now that KNO is a ferroelectric (STO is only incipient), one may wonder if the spontaneous polarization appearing in it shall affect the electron density and hence the conductivity. Yes, it is, as recently demonstrated by first-principles computations [1]. The physics is simple: the electric field produced by this polarization shall deplet or enrich electrons (screening effect), depending on the field direction, resembling what takes place to a conventional p-n jucntion in the presence of an ecternal electric field. Hence, by inverting the spontaneous polarization in KNO, one is able to switch the conduction states of the NbO2 layer.

For the moment, it may be interesting to see how this prediction will be confirmed experimentally and to undrstand the switch time required for the polarization reversal. Obviously, this time shall be crucial for applications.

[1]PRL, 103:016804(2009)