Array of Graphene dots absorbs light perfectly

http://physics.aps.org/articles/v5/12

http://prl.aps.org/abstract/PRL/v108/i4/e047401

The results may be used in e.g. solar cells ! How to broaden the absorption spectrum?

Casimir force between two sheets of graphene

I had thought about this sometime ago but did not come up with the calculations on this problem, basically i did not get time to do that. Now i'm glad that some people have done this. See the following:
1. http://prb.aps.org/abstract/PRB/v80/i24/e245406
2. http://prb.aps.org/abstract/PRB/v82/i15/e155459
3. http://benasque.org/2011qfext/talks_contr/2021_Sernelius.pdf
4.http://arxiv.org/abs/1102.1757

Edge states in graphene

Graphene offers more [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.107.236806]:

In condensed matter systems, topology often gives rise to gapless excitations at the edge (in 2D) or the surface (in 3D). Such excitations in the 2D fractional quantum Hall state should manifest in the edge behaving as a Luttinger liquid, in which tunneling is determined by a universal power law related to an attribute—the filling factor—of the magnetic flux through, and the number of electrons in, the 2D state.

However, no such behavior has yet been observed at the edges of 2D semiconductor heterostructures, the most-studied quantum Hall systems. Theorists say that in these systems the conflicting interplay between the confinement potential, attracting each electron towards the center, and the Coulomb force, pushing them apart from each other, modifies the edge itself. This process—edge reconstruction—disturbs the universal Luttinger liquid picture in the experimentally accessible distance scales.

In a paper in Physical Review Letters, Zi-Xiang Hu, at Princeton University, and his colleagues tell us that we may, after all, be able to see chiral Luttinger behavior in another system in which fractional quantum Hall effect has been observed—graphene. In graphene, electrons are confined by metallic gates that are placed a specific distance away. By contrast, in semiconductors, electrons are confined by dopants. This one difference should make graphene less susceptible to edge reconstruction and reveal the fractional quantum Hall state. The authors say that experimentalists should therefore finally see the elusive universal edge behavior in the experimentally accessible state with filling factor 1/3. – Sami Mitra

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

Interactions in graphene

The electrons in graphene are particularly nimble and their interactions are usually ignored together with the effects of some defects. However, recently more and more attention have been paid to them, e.g., about superconductivity and other new orders. Here is a brief summary by P. Guinea [http://www.condmatjournalclub.org/wp-content/uploads/2011/09/JCCM_SEPTEMBER_2011_01.pdf].

1. Broken symmetry states and divergent resistance in suspended
bilayer graphene
Benjamin E. Feldman, Jens Martin, Amir Yacoby,
arXiv:0909.2883, Nature Physics 5, 889 (2009)
2. Local Compressibility Measurements of Correlated States in
Suspended Bilayer Graphene
Jens Martin, Benjamin E. Feldman, R. Thomas Weitz, Monica T. Allen,
Amir Yacoby,
arXiv:1009.2069, Phys. Rev. Lett. 105, 256806 (2010)
3. Interaction-Driven Spectrum Reconstruction in Bilayer Graphene
A. S. Mayorov, D. C. Elias, M. Mucha-Kruczynski, R. V. Gorbachev, T. Tu-
dorovskiy, A. Zhukov, S. V. Morozov, M. I. Katsnelson, V. I. Fal'ko, A. K.
Geim, K. S. Novoselov,
arXiv:1108.1742, Science 333, 860 (2011)
4. Transport Spectroscopy of Symmetry-Broken Insulating States
in Bilayer Graphene
J. Velasco Jr., L. Jing, W. Bao, Y. Lee, P. Kratz, V. Aji, M. Bockrath, C.N.
Lau, C. Varma, R. Stillwell, D. Smirnov, Fan Zhang, J. Jung,
A.H. MacDonald
arXiv:1108.1609

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.

More oscillations in cuprate superconductors

http://www.nature.com/nphys/journal/v7/n3/full/nphys1930.html?WT.ec_id=NPHYS-201103

Since its discovery almost 25 years ago, high-temperature superconductivity has led to a wealth of new theoretical ideas and deepened our understanding of complex condensed-matter systems. At the same time, the study of cuprates has been the driving force for tremendous innovations in the experimental methodology of condensed-matter physics, with methods ranging from photoemission, scanning microscopy, optics and neutron scattering to, in the past few years, quantum oscillations. As reported in Nature Physics¹, measurements by Brad Ramshaw et al. of quantum oscillations in the underdoped high-temperature superconductor YBa₂Cu₃O_6.59 typifies these advances in a number of striking ways. First, the samples studied are the result of two decades of intensive development leading to unique levels of purity that would previously have been unimaginable in such complex oxides. Second, the measurements take place in pulsed magnetic fields that reach both a magnitude of field and a quality of signal-to-noise ratio far beyond what could formerly be achieved. And third, the latest innovation of 'genetic algorithms' allows consistent parameters to be extracted from a large data set of quantum oscillations as a function of field direction and temperature. The authors obtain, among other things, a value of the g-factor of the charge carriers near 2, showing that they are surprisingly like free electrons. This result has profound implications for the nature of the ground state that gives rise to these oscillations.

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.

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)