Jeroen van den Brink

Substrate-induced band gap in graphene on hexagonal boron nitride: Ab initio density functional calculations

We determine the electronic structure of a graphene sheet on top of a lattice-matched hexagonal boron nitride (h-BN) substrate using ab initio density functional calculations. The most stable configuration has one carbon atom on top of a boron atom, the other centered above a BN ring. The resulting inequivalence of the two carbon sites leads to the opening of a gap of 53 meV at the Dirac points of graphene and to finite masses for the Dirac fermions. Alternative orientations of the graphene sheet on the BN substrate generate similar band gaps and masses. The band gap induced by the BN surface can greatly improve room temperature pinch-off characteristics of graphene-based field effect transistors.

Multiferroicity due to charge ordering

In this contribution to the special issue on multiferroics we focus on multiferroicity driven by different forms of charge ordering. We will present the generic mechanisms by which charge ordering can induce ferroelectricity in magnetic systems. There is a number of specific classes of materials for which this is relevant. We will discuss in some detail (i) perovskite manganites of the type (PrCa)MnO3, (ii) the complex and interesting situation in magnetite Fe3O4, (iii) strongly ferroelectric frustrated LuFe2O4 and (iv) an example of a quasi-one-dimensional organic system. All these are ‘type-I’ multiferroics, in which ferroelectricity and magnetism have different origins and occur at different temperatures. In the second part of this article we discuss ‘type-II’ multiferroics, in which ferroelectricity is completely due to magnetism, but with charge ordering playing an important role, such as (v) the newly discovered multiferroic Ca3CoMnO6, (vi) possible ferroelectricity in rare earth perovskite nickelates of the type RNiO3, (vii) multiferroic properties of manganites of the type RMn2O5, (viii) perovskite manganites with magnetic E-type ordering and (ix) bilayer manganites. (Some figures in this article are in colour only in the electronic version)

Electronic Correlations in Oligo-acene and -Thiopene Organic Molecular Crystals

From first-principles calculations we determine the Coulomb interaction between two holes on oligo-acene and -thiophene molecules in a crystal, as a function of the oligomer length. The electronic polarization of the molecules that surround the charged oligomer reduces the bare Coulomb repulsion between the holes by approximately a factor of 2. The effects of relaxing the molecular geometry in the presence of holes is found to be significantly smaller. In all cases the effective hole-hole repulsion is much larger than the valence bandwidth, which implies that at high doping levels the properties of these organic semiconductors are determined by electron-electron correlations.

Kitaev interactions between j = 1/2 moments in honeycomb Na2IrO3 are large and ferromagnetic: insights from ab initio quantum chemistry calculations

Na2IrO3, a honeycomb 5d5 oxide, has been recently identified as a potential realization of the Kitaev spin lattice. The basic feature of this spin model is that for each of the three metal–metal links emerging out of a metal site, the Kitaev interaction connects only spin components perpendicular to the plaquette defined by the magnetic ions and two bridging ligands. The fact that reciprocally orthogonal spin components are coupled along the three different links leads to strong frustration effects and nontrivial physics. While the experiments indicate zigzag antiferromagnetic order in Na2IrO3, the signs and relative strengths of the Kitaev and Heisenberg interactions are still under debate. Herein we report results of ab initio many-body electronic-structure calculations and establish that the nearest-neighbor exchange is strongly anisotropic with a dominant ferromagnetic Kitaev part, whereas the Heisenberg contribution is significantly weaker and antiferromagnetic. The calculations further reveal a strong sensitivity to tiny structural details such as the bond angles. In addition to the large spin–orbit interactions, this strong dependence on distortions of the Ir2O2 plaquettes singles out the honeycomb 5d5 oxides as a new playground for the realization of unconventional magnetic ground states and excitations in extended systems.

Graphene: From strength to strength

I t is one of those seemingly simple questions that kids come up with: “How does a pencil work?” A scientifi cally knowledgeable parent will dutifully explain that the grey stuff inside the pencil is called graphite, that graphite is like a stack of pancakes with each pancake being a layer of carbon atoms, and that when the pencil is moved across a piece of paper, the pancakes peel off to leave marks on the paper. But parents know that such an answer will only provoke more questions such as: “can you see one of the pancakes?” Less than three years ago Andre Geim, Kostya Novoselov and co-workers at Manchester University in the UK and the Institute for Microelectronics Technology in Chernogolovka, Russia, discovered that single layers of graphite — also known as graphene — really can be seen1. Th e technique they used basically involves drawing with a piece of graphite on a silicon substrate. If the substrate has been prepared with an ultrathin layer of silicon dioxide on top, the graphene fl akes, which are just half a nanometre thick, become visible under an ordinary optical microscope. More surprises followed in 2005 when Geim, Novoselov and co-workers and, separately, Philip Kim, Horst Stormer and colleagues at Columbia University in the USA, drove an electrical current through a graphene sheet2,3. In addition to various novel electronic phenomena (see Box 1), they found that the electron mobility of graphene can be as high as 10,000 cm2 V–1 s–1 at room temperature, which is about ten times higher than the mobility of commercial silicon wafers. Mobility is oft en severely limited by structural imperfections, but the structural purity of graphene means that electrons can travel huge distances — 300 nanometres or more — without being scattered. Moreover, this impressive mobility is little aff ected by changes in temperature or by the presence of excess charge on the graphene sheet. Th ere also seems to be ample room for optimizing the manufacturing process of graphene, so it is little wonder that researchers at companies like Intel and IBM have started to work on graphene. But will physicists and engineers be able to convert the undoubted promise of graphene into real-world electronic devices? Th ree recent discoveries indicate that things are moving in the right direction. Writing in Science, Novoselov and co-workers report further evidence of the remarkable electronic properties of graphene4. It has been known for more than two decades that the resistance of a very pure two-dimensional conductor changes in a very particular way when an external magnetic fi eld is applied. In the classical Hall eff ect, which was discovered in 1879, the Hall resistance is proportional to the magnetic fi eld. But in the quantum Hall eff ect, which was fi rst observed in very pure semiconductors in 1980, the Hall resistance is quantized and increases in units of h/νe2 as the magnetic fi eld is increased, where h is Planck’s constant, ν is an integer and e is the charge on the electron. Th is quantum Hall eff ect cannot be observed in conventional semiconductors above about 30 K because thermal fl uctuations wash out the delicate quantum eff ects that are responsible for it. Novoselov et al. have now seen the quantum Hall eff ect in graphene at room temperature. Th is is possible because the energy that sets the scale for the quantum Hall eff ect in ordinary semiconductors is proportional to the inverse eff ective mass of the electrons, but in graphene, owing to its unique electronic structure, the eff ective mass of the conduction electron vanishes. Moreover, in work submitted for publication, Leonid Ponomarenko, Fredrik Schedin, Novoselov and Geim report the fi rst results on an all-graphene singleelectron transistor (SET) that also operates at room temperature (ref. 5; L. A.Ponomarenko, F. Schedin, K. S. Novoselov and A. K. Geim, manuscript in preparation). As its name suggests, the SET is the ultimate transistor — operating at the smallest possible length scales and manipulating the smallest possible currents. Th e SET can be viewed as a quantum dot or ‘box’ for holding electrons, with the electrons being able to enter and leave the quantum dot via separate electrodes. Th e operation of a SET relies on electrons hopping between the quantum dot and the leads. Th e key point is that electrons can only pass through the dot one by one — because of the Coulomb repulsion between the electrons it is simply not possible for two electrons to be on the dot at the same time. If the device is ‘open’, current can fl ow from one lead to With exciting new results appearing every week, graphene is one of the hottest topics in physics, and may also form the basis of a new approach to electronics a decade from now. GRAPHENE

Theoretical Demonstration of How the Dispersion of Magnetic Excitations in Cuprate Compounds can be Determined Using Resonant Inelastic X-Ray Scattering

L.J.P. Ament, G. Ghiringhelli, M. Moretti Sala, L. Braicovich and J. van den Brink Institute-Lorentz for Theoretical Physics, Universiteit Leiden, 2300 RA Leiden,The Netherlands INFM/CNR Coherentia and Soft – Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy Institute for Molecules and Materials, Radboud Universiteit Nijmegen, 6500 GL Nijmegen, The Netherlands Stanford Institute for Materials and Energy Sciences, Stanford University and SLAC National Accelerator Laboratory, Menlo Park, CA 94025. (Dated: March 17, 2009)

Crystal-field splitting and correlation effect on the electronic structure of A2IrO3.

The electronic structure of the honeycomb lattice iridates Na(2)IrO(3) and Li(2)IrO(3) has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split d-d excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to noncubic crystal fields, derived from the splitting of j(eff)=3/2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of j(eff) physics in A(2)IrO(3). We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both Na(2)IrO(3) and Li(2)IrO(3) can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr(2)IrO(4).

Kitaev exchange and field-induced quantum spin-liquid states in honeycomb alpha-RuCl3

Large anisotropic exchange in 5d and 4d oxides and halides open the door to new types of magnetic ground states and excitations, inconceivable a decade ago. A prominent case is the Kitaev spin liquid, host of remarkable properties such as protection of quantum information and the emergence of Majorana fermions. Here we discuss the promise for spin-liquid behavior in the 4d5 honeycomb halide α-RuCl3. From advanced electronic-structure calculations, we find that the Kitaev interaction is ferromagnetic, as in 5d5 iridium honeycomb oxides, and indeed defines the largest superexchange energy scale. A ferromagnetic Kitaev coupling is also supported by a detailed analysis of the field-dependent magnetization. Using exact diagonalization and density-matrix renormalization group techniques for extended Kitaev-Heisenberg spin Hamiltonians, we find indications for a transition from zigzag order to a gapped spin liquid when applying magnetic field. Our results offer a unified picture on recent magnetic and spectroscopic measurements on this material and open new perspectives on the prospect of realizing quantum spin liquids in d5 halides and oxides in general.Ravi Yadav, Nikolay A. Bogdanov, Vamshi M. Katukuri, Satoshi Nishimoto, 2 Jeroen van den Brink, 2, 3 and Liviu Hozoi Institute for Theoretical Solid State Physics, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany Department of Physics, Technical University Dresden, Helmholtzstrasse 10, 01069 Dresden, Germany Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA (Dated: March 5, 2018)

Creating and verifying a quantum superposition in a micro-optomechanical system

Micro-optomechanical systems are central to a number of recent proposals for realizing quantum mechanical effects in relatively massive systems. Here, we focus on a particular class of experiments which aim to demonstrate massive quantum superpositions, although the obtained results should be generalizable to similar experiments. We analyze in detail the effects of finite temperature on the interpretation of the experiment, and obtain a lower bound on the degree of non-classicality of the cantilever. Although it is possible to measure the quantum decoherence time when starting from finite temperature, an unambiguous demonstration of a quantum superposition requires the mechanical resonator to be in or near the ground state. This can be achieved by optical cooling of the fundamental mode, which also provides a method to measure the mean phonon number in that mode. We also calculate the rate of environmentally induced decoherence and estimate the timescale for gravitational collapse mechanisms as proposed by Penrose and Diosi. In view of recent experimental advances, practical considerations for the realization of the described experiment are discussed.

Multiferroicity in rare-earth nickelates RNiO3

We show that charge ordered rare-earth nickelates of the type RNiO3 (R = Ho, Lu, Pr and Nd) are multiferroic with very large magnetically-induced ferroelectric (FE) polarizations. This we determine from first principles electronic structure calculations. The emerging FE polarization is directly tied to the long-standing puzzle of which kind of magnetic ordering is present in this class of materials: its direction and size indicate the type of ground-state spin configuration that is realized. Vice versa, the small energy differences between the different magnetic orderings suggest that a chosen magnetic ordering can be stabilized by cooling the system in the presence of an electric field.