Past Aalto Physics Colloquia
Contents |
Fall 2012
On Friday 30th November 2012 at 15:00 in K building hall 213 (Otakaari 4, Espoo)
Ali Yazdani, Princeton University
Visualizing Topological States of Matter
Soon after the discovery of quantum mechanics it was realized why some solids are insulating (like diamond) and others are highly conducting (like graphite),even though they could be comprised of the same element. Now, 80 years later, the concept of insulators and metals is again being fundamentally revised. During the last few years, it has become apparent that there can be a distinct type of insulator, which can occur because of the topology of electronic wavefunctions in materials comprised of heavier elements. Strong interaction between the spin and the orbital angular momentum of electrons in these compounds alters the sequence in energy of their electronic states. The key consequence of this topological characteristic (and the way to distinguish a topological insulator from an ordinary one) is the presence of metallic electrons with helical spin texture at their surfaces. I will describe experiments that directly visualize these novel quantum states of matter and demonstrate their unusual properties through spectroscopic mapping with the scanning tunneling microscope (STM). These experiments show that the spin texture of these states protects them against backscattering and localization. These states appear to penetrate through barriers that stop other electronic states. I will describe these experiments and our most recent attempts to create and visualize other topological states such as creation of Majorana fermions, which are another instance of boundary state associated with topological order.
On Friday 9th November 2012 at 15:00 in K building hall 213 (Otakaari 4, Espoo)
Christofer Hierold, ETH
Carbon Nanotube Sensors
Carbon nanotubes exhibit a number of excellent mechanical and electronic properties as functional materials in sensors. In particular single walled carbon nanotubes (SWNT) are known for their band gap modulation due to mechanical strain, or electronic property-changes due to interaction with surrounding molecules, but also for their ultra-low power consumption. However, successful technology transfer to production and development of affordable products based on CNTs is threatened by the lack of solutions for fabrication and integration of these materials. We present results on individual SWNTs as functional material in field effect transistors, mechanical and chemical sensors. We discuss the influence of process variations on the properties of SWNT devices, and options for sensor fabrication.
On Friday 5th October 2012 at 15:00 in K building hall 213 (Otakaari 4, Espoo)
Ken Dill, Stony Brook University
How does protein physics limit the behaviors and evolution of cells?
Biological cells have many behaviors, developed through evolution. These behaviors must obey the laws of physics. We combine the knowledge of the physical properties of proteins that are now contained in databases with simple physical models, to make predictions about various physical limits of cells. We consider why cells are so sensitive to temperature, why their internal protein densities are so high, and what are the various speed limits imposed by protein folding, diffusion, and synthesis, for example.
On Friday 14th September 2012 at 15:00 in K building hall 213 (Otakaari 4, Espoo)
Tom Fennell, Paul Scherrer Institut, Switzerland
Coulomb phase and emergent magnetic monopoles in spin ice
A frustrated magnet is one in which all pair-wise interactions cannot be minimized due to competing interactions or geometry (e.g. antiferromagnetically coupled spins on a triangular lattice). Ground states are typically constructed by enforcing a local constraint on every sub-unit of the lattice, usually resulting in an extensively degenerate manifold of ground states. In the case of spin ices, the local constraint is known as the ice rule, and is a magnetic analogue of the Bernal-Fowler ice rules, which originally described degenerate hydrogen bonding configurations in ice crystals. As a consequence of the topological properties of the ice rules, a spin ice can be described by an effective theory in which the ice rule obeying ground states map to a free magnetic field. Then, the excitations (or ice rule defects) are emergent quasiparticles which take the role of magnetic monopoles. These emergent monopoles are deconfined, because the macroscopic ground state degeneracy allows them to diffuse across the lattice with local ground state configurations reestablished in their wake, while the particular interaction scheme of real spin ices such as Ho2Ti2O7 and Dy2Ti2O7 gives them a magnetic Coulomb interaction. I will discuss the origins and extent of the analogies between ice and spin ice, and “emergent” and “fundamental” magnetic monopoles; neutron scattering experiments demonstrating the required form of underlying spin correlations (or monopole vacuum); the role of the monopoles in controlling the dynamics of spin ices at low temperature (by which means they can also be investigated); and the possibility of “quantum spin ice” in Tb2Ti2O7.
Spring 2012
On Friday 11th May 2012 at 15:00, in K building hall 213 (Otakaari 4, Espoo)
Andrew Cleland, University of California
Mechanical resonators in the quantum regime
The superconducting quantum circuits group at UC Santa Barbara has spent the past ten years developing superconducting and nanomechanical systems for fundamental experiments in quantum mechanics; our ultimate goal is to build a superconducting quantum computer. The Josephson junction, a fundamental superconducting device analogous to a transistor, provides an extremely nonlinear electrical circuit element that can be used as an “electronic atom”, enabling the detection and manipulation of single quanta of energy, as well as demonstrations of simple quantum algorithms. We have used this device to demonstrate full quantum control over microwave-frequency photons in electromagnetic resonators, and more recently demonstrate ground-state cooling of a macroscopic mechanical resonator as well as manipulate individual phonons, the quanta of mechanical vibrations.
References:
“Generation of Fock states in a superconducting quantum circuit “, M. Hofheinz et al., Nature 454, 310-314 (2008)
“Synthesizing arbitrary quantum states in a superconducting resonator”, M. Hofheinz et al., Nature 459, 546-549 (2009)
“Quantum ground state and single-phonon control of a mechanical resonator”, A.D. O'Connell et al., Nature 464, 697-703 (2010)
“Implementing the Quantum von Neumann Architecture with Superconducting Circuits”, M. Mariantoni et al., Science 334, 61 (2011)
On Friday 23th March 2012 at 15:00, in K building hall 213 (Otakaari 4, Espoo)
Stefano Liberati, SISSA, Trieste
Black holes, history and mysteries
Black holes, regions of spacetime where the curvature is so strong that even light is not able to escape, are among the most extreme and mysterious manifestations of nature. As the famous astrophysicists Chandrasekhar said they are "the most perfect macroscopic object there are in the universe: the only elements in their construction are our concepts of space and time." Black holes are implicated in a wide range of astrophysical phenomena, including many of the most energetic events in the Universe but they also represent the most striking bridge between classical General relativity and quantum physics. In this colloquium I will review the long history that lead to the discovery and understanding of these extreme spacetimes and discuss how condensed matter physics is now playing a role in further exploring their nature and possibly the very essence of the fabric of reality.
On Friday 24th February 2012 at 15:00, in K building hall 213 (Otakaari 4, Espoo)
Mikhail Katsnelson, Radboud University Nijmegen
Graphene: CERN on the desk
Graphene, a recently (2004) discovered two-dimensional allotrope of carbon (this discovery was awarded by Nobel Prize in physics 2010), has initiated a huge activity in physics, chemistry and materials science, mainly, for three reasons. First, a peculiar character of charge carriers in this material makes it a “CERN on the desk” allowing us to simulate subtle and hardly achievable effects of high energy physics. Second, it is the simplest possible membrane, an ideal testbed for statistical physics in two dimensions. Last not least, being the first truly two-dimensional material (just one atom thick) it promises brilliant perspectives for the next generation of electronics which uses mainly only surface of materials. I will tell about the first aspect of the graphene physics, some unexpected relations between materials science and quantum field theory and high-energy physics.
Electrons and holes in this material have properties similar to ultrarelativistic particles (two-dimensional analog of massless Dirac fermions). This leads to some unusual and even counterintuitive phenomena, such as finite conductivity in the limit of zero charge carrier concentration (quantum transport by evanescent waves) or transmission of electrons through high and broad potential barriers with a high probability (Klein tunneling). This allows us to study subtle effects of relativistic quantum mechanics and quantum field theory in condensed-matter experiments, without accelerators and colliders. Some of these effects were considered as practically unreachable. Apart from the Klein tunneling, this is, for example, a vacuum reconstruction near supercritical charges predicted many years ago for collisions of ultra-heavy ions. Another interesting class of quantum-relativistic phenomena is related with corrugations of graphene, which are unavoidable for any two-dimensional systems at finite temperature. As a result, one has not just massless Dirac fermions but massless Dirac fermions in curved space. Gauge fields, of the central concepts of modern physics, are quite real in graphene and one can manipulate them just applying mechanical stress.
On Friday 15th February 2012 at 15:00 in K building hall 213 (Otakaari 4, Espoo)
Jascha Repp, University of Regensburg
Individual Molecules on Thin Insulating Films
Scanning probe microscopy is an ideal tool to study the properties of individual molecules on the atomic length-scale in a well-defined environment. However, if a molecule is adsorbed onto a metal surface, its molecular identity is partially lost because of the hybridization of its electronic states with the ones of the support. The use of ultra-thin insulating films on metal substrates allows for the almost unperturbed electronic properties of molecules to be studied by means of the scanning probe microscopy techniques as it facilitates an electronic decoupling from the substrate. We investigated different kind of π–conjugated molecules in a combined scanning tunneling (STM) and atomic force microscope (AFM). Whereas both measurement channels show features with sub-molecular resolution, the information they can provide is truly complementary. For example, STM allows the direct imaging of the unperturbed molecular orbitals, whereas the AFM channel directly reveals the bonding geometry in artificial molecular structures and configurational changes in molecular switches.
On Friday 20th January 2012 at 15:00, in K building hall 213 (Otakaari 4, Espoo)
Nicolas Gisin, University of Geneva
Quantum Non-locality
Quantum communications is the art of transferring a quantum state from one location to a distant one. On the application side, quantum communication is already relatively advanced with Quantum Random Number Generators and Quantum Key Distribution (QKD) systems having found niche markets. However, on the academic research side quantum communication has still a long way to go until a functional quantum repeater can extend the distances to continental scales. Quantum repeaters are based on quantum teleportation, the most fascinating application of entanglement. Additionally, quantum repeaters require quantum memories with memory times close to a second; this represents one grand challenge for quantum communication. Another grand challenge is the demonstration of device independent QKD.
Fall 2011
On Friday 11th November 2011 at 15:00, in K building hall 213 (Otakaari 4, Espoo)
Ludwik Leibler, ESPCI ParisTech
Discovery of strong organic liquids
Many aspects of glass formation remain deeply puzzling. During cooling, silica, and a few other inorganic compounds called strong liquids gradually increase their viscosity over a wide temperature range and become so viscous that for all practical purposes they behave like hard solids, glasses. Yet, silica, the archetype of glass, is quite unique. In striking contrast to silica, all organic and polymer glass forming liquids increase their viscosity and rigidify abruptly when cooled. In silica, atoms are linked into a disordered network by chemical bonds. We have designed and synthesized organic networks able to rearrange their topology by exchange reactions without link breaking. Unlike organic compounds and polymers whose viscosity varies abruptly near glass transition, these networks show Arrhenius-like gradual viscosity variations just like vitreous silica. The expansion coefficient studies confirm that topology freezing leads to a glass transition. From materials science point of view, permanently cross-linked polymers, either thermosets or rubbers, have outstanding mechanical properties and solvent resistance, but they cannot be processed and reshaped once synthesized. Non-cross-linked polymers and those with reversible cross-links are processable, but they are soluble. Our materials made by epoxy chemistry can be soft and elastic like rubbers or hard like thermosets. Yet, they are insoluble and processable and thus represent a new class of polymers. Like silica, they can be wrought and welded to make complex objects by local heating without the use of molds. The concept of a glass, made by reversible topology freezing in epoxy networks can be readily scaled up for applications and generalized to other chemistries.
On Thursday 6th October 2011 at 14:00, in Hall B of the main building
Bernardo Huberman, HP Research
Social media and attention
We are witnessing a momentous transformation in the way people interact and exchange information with each other. Content is now co-produced, shared, classified, and rated on the Web by millions of people, while attention is becoming the ephemeral and valuable resource that everyone seeks to acquire. This talk will describe how social attention determines the production and consumption of content within social media, how it can be used to predict future trends, and its role in determining the public agenda.
This colloquium was organized together with the ICS Forum.
Spring 2011
On Friday 13th May 15:00, in K building hall 213 (Otakaari 4, Espoo)
Stuart Parkin, IBM Almaden Research Center:
The Spin on Electronics! - Science and Technology of spin currents in nano-materials and nano-devices
Recent advances in manipulating spin-polarized electron currents in atomically engineered magnetic heterostructures make possible entirely new classes of sensor, memory and logic devices - a research field generally referred to as spintronics [1]. A magnetic recording read head, initially formed from a spin-valve, and more recently by a magnetic tunnel junction, has enabled a 1,000-fold increase in the storage capacity of hard disk drives since 1997. The very low cost of disk drives and the high performance and reliability of solid state memories, may be combined in the Racetrack Memory [2]. The Racetrack Memory is a novel three dimensional technology which stores information as a series of magnetic domain walls in nanowires, manipulated by spin polarized currents. Spintronic devices may even allow for “plastic” devices that mimic synaptic switches in the brain, thereby allowing for the possibility of very low power computing architectures.
[1] S.S.P. Parkin et al. Proc. IEEE 91, 661-680 (2003)
[2] Science 320, 190 (2008); Scientific American (June, 2009).
On Friday 8th April 15:00, in K building hall 213 (Otakaari 4, Espoo)
Marc Mézard, CNRS - Université Paris Sud, France:
Glassy Phase Transitions in Hard Computer Science Problems
Given a large set of discrete variables, and some constraints between them, is there a way to choose the variables so that all constraints are satisfied? This "satisfiability" problem is one of the most fundamental complex optimization problems. It also has very concrete applications, for instance in computer chip testing or in error correcting codes.
There exist deep connections between this fundamental problem in computer science and structural glasses. By increasing the density of constraints in random satisfiability, one meets a phase transition to a glass phase, associated with a structural change in the satisfiability problem at the origin of computational hardness.
This talk will give an introduction to the recent progress, both conceptual and algorithmic, obtained in hard computer science problems using statistical physics concepts and methods.
On Friday 11th March 15:00, hall M of Aalto University Otaniemi main building '
John Pendry, Imperial College, London, the UK:
Invisible Cloaks & a Perfect Lens
Electromagnetism encompasses much of modern technology. Its influence rests on our ability to deploy materials that can control the component electric and magnetic fields. A new class of materials has created some extraordinary possibilities such as a negative refractive index, and lenses whose resolution is limited only by the precision with which we can manufacture them. Cloaks have been designed and built that hide objects within them, but remain completely invisible to external observers. The new materials, named metamaterials, have properties determined as much by their internal physical structure as by their chemical composition and the radical new properties to which they give access promise to transform our ability to control much of the electromagnetic spectrum.
On Friday 11th February 15:00, in K building hall 213 (Otakaari 4, Espoo)
Andrei Varlamov, Institute of Superconductivity and Innovative Materials,
Italian National Research Council
Superconductivity: approaching the century jubilee
The lecture is devoted to discussion of one of the most bright and unusual discoveries of XX century Physics: superconductivity. First we discuss the story of discovery of this phenomenon, hopes and delusions followed it, speak about a long half-century of the search for new superconductors and accumulation of the experimental facts. Then we pass to the remarkable phenomenological theory of superconductivity created by Russian physicists Vitaly Ginzburg and Lev Landau. In those times when this theory was developed, the microscopic origin of this quantum phenomenon still could not be recognized, but even being phenomenological in its nature the Ginzburg-Landau theory allowed to systemize and predict a lot of superconductor’s properties. Basing on it A.A.Abrikosov discovered soon the fundamentally new class of superconductors: superconductors of the second type. At the end of this part of lecture I will present the basic ideas of the microscopic theory of superconductivity, created in 1957 by three American scientists J.Bardeen, L. Cooper and R.Schriffer. This, first period of studies of superconductivity was superseded by the second one: the period of the chase for high critical temperatures and magnetic fields, proposals of the theoretical concepts for alternative to the BCS mechanisms of superconductivity and development first practical applications. At the same time English physicist Brian Josephson predicted the phenomenon of a weak superconductivity, which opened the new fields of applicability of superconductivity. The third period in development of superconductivity started in 1986 with the discovery by Swiss scientists Alex Muller and George Bednortz of the new class of oxide superconductors which critical temperatures in short time overcame crucial for practical applications the “nitrogen limit” - 77 К. The author tells about this last, fascinating, period being its immediate participant.
Fall 2010
On Friday 12th November 15:00, hall M of the Aalto Otaniemi main building (Otakaari 1, Espoo).
Katherine Richardson, University of Copenhagen
Redefining the Human-Earth Relationship: A Scientist’s view on climate change
It is seldom that advances in scientific understanding cause the reverberations in society as a whole as is the case with climate change. Possibly the last time that this happened to such a degree was when Darwin introduced the concept of evolution in 1859. As in the case of evolution, the recognition of human-induced climate change challenges society’s perception of the role of humans in nature. The Bible teaches us that humans are above nature and should dominate nature but Darwin showed us we are a part of nature: a species like all others. Now, science is telling us that that the combined activities of that species influence the way the Earth System functions. This is difficult for many non-scientists to accept but the evidence that humans are influencing the Earth System is overwhelming and it is not only the climate system which is affected. The knowledge that our species is influencing the Earth System brings with it the responsibility to manage our relationship with the planet. In this talk, some of the evidence for human influence on the climate system is reviewed and suggestions as to how management of the human-Earth relationship might be developed are presented.
On Friday 8th October at 15:00, hall A of the Aalto Otaniemi main building (Otakaari 1, Espoo)
Charles Marcus, Harvard University, the USA:
Using Spin as a Quantum Bit
Over the last two decades, our understanding of the uses and limitations of quantum entanglement for efficient information processing and computation has developed remarkably quickly. Theoretically, it appears possible to build machines that remain coherent in a quantum mechanical sense throughout a computation, including error correction, and that in some instances this gives the machine computational power. Despite great progress in our understanding, however, the challenge of building machines (electrical, optical, or mechanical) that can demonstrate these principles remains a profound challenge.
This talk will review recent progress of one approach, which is using electron spin in semiconductor quantum dots as the holder of quantum information. This approach combines semiconductor nanofabrication, electrical measurements at temperatures below 0.1 kelvin, and microwave manipulation of spin and charge. And so far, this is all needed to get a mere one or two quantum bits to work. Extending this approach to several, and ultimately thousands or millions of devices, working together appears almost unimaginably difficult. However, the effort seems justified by the promise, or hope, of building fully controllable quantum "chips."
Work supported by the US DoD, IARPA, DARPA and Harvard University.
On Friday 10th September at 15:00, hall C of the Aalto Otaniemi main building (Otakaari 1, Espoo)
Kurt Binder, University of Mainz, Germany:
Computer simulations of critical phenomena and phase behavior of fluids
Computer simulation techniques such as Monte Carlo (MC) and Molecular Dynamics (MD) methods yield numerically exact information (apart from statistical errors) on model systems of classical statistical mechanics. However, a systematic limitation is the restriction to a finite (and often rather small) particle number N (or box linear dimension L, respectively). This limitation is particularly restrictive near critical points (due to the divergence of the correlation length of the order parameter) and for the study of phase equilibria (possibly involving interfaces, droplets, etc.). Starting out with simple lattice gas (Ising) models, finite size scaling analysis have been developed to overcome this limitation. These techniques work for both simple Lennard-Jones fluids and their mixtures, including generalizations to approximate models for quadrupolar fluids such as carbon dioxide, benzene etc. and various mixtures, whose phase behaviour can be predicted. A combination of MC and MD allows the study of dynamic critical phenomena, and specialised techniques (umbrella sampling plus thermodynamic integration) yield the surface free energy of droplets as function of droplet size. Thus, computer simulation has become a versatile and widely applicable tool for the study of fluids.
Spring 2010
On Wednesday 2nd June 2010 at 15:00, in K building hall 213 (Otakaari 4, Espoo)
Steven M. Girvin, Yale University, the USA:
‘Circuit QED’: Quantum Electrodynamics of Superconducting Circuits and Qubits
‘Circuit QED’ [1] explores quantum optics and cavity quantum electrodynamics in electrical circuits. Josephson junction ‘atoms’ placed inside an on-chip resonant cavity can interact with microwave photons with extremely strong coupling. Even though microwave photons have times less energy than visible photons, very rapid recent experimental progress has led to the ability to see the particle nature of microwaves and create arbitrary superposition states of different numbers of photons. In addition to being a new test bed for quantum mechanics and quantum optics in the ultra-strong coupling regime, the circuit QED paradigm has many promising features for quantum computation. It is now possible to routinely entangle two qubits with high fidelity, and perform simple quantum algorithms on small quantum processors.
[1] ‘Wiring up quantum systems,’ R.J. Schoelkopf and S.M. Girvin, Nature 451, 664 (2008).
On Friday 16th April 2010 at 15:00, hall B of the TKK main building:
Jaw-Shen Tsai, NEC Nano Electronics Research Laboratories & Riken Advanced Science Institute:
Quantum Coherent Behavior in Macroscopic Objects via Superconducting Devices
Can macroscopic object such as a Josephson junction behave like a quantum object with full quantum coherence, and where is the boundary between classical and quantum worlds? The secondary macroscopic quantum effect associated with the Josephson junction produces band structures in the energy spectrum, so in such system, besides the usual BCS solitary ground state associated with superconductivity, there are multiple numbers of macroscopic quantum states with aharmonic energy separations. A concrete demonstration of quantum coherence in such system can be realized by creation of a coherent superposition state involving two of these states. We demonstrated a quantum coherent oscillation between two lowest-energy states in a small Josephson junction system, demonstrating the creation of such coherent state with controls in amplitude and phase [1]. Such object can be considered as an artificial atom. We have been utilizing it to pursue the prospect of quantum information processing. I this direction, dynamical creation of quantum entangled states [2]; quantum controlled-NOT logic gate [3]; simple quantum information manipulation [4] were demonstrated. The Josephson artificial atom can also be used to realize the concept of quantum optics that initially developed for the natural atoms. In this direction, single artificial atom lasing [5], as well as macroscopic quantum scatterings [6] were demonstrated.
[1] Y. Nakamura, Yu. A. Pashkin, J. S. Tsai, Nature, 398, 786, 1999
[2] Yu. A. Pashkin, T. Yamamoto, O. Astafiev, Y. Nakamura, D. V. Averin and J. S. Tsai, Nature, 421, 823, 2003
[3] T. Yamamoto, Yu. Y. Pashkin, O. Astafiev, Y. Nakamura, and J. S. Tsai, Nature 425, 941, 2003
[4] A. O. Niskanen, K. Harrabi, F. Yoshihara, Y. Nakamura, S. Lloyd and J. S. Tsai, Science, 316, 723, 2007
[5] O. Astafiev, K. Inomata, A. O. Niskanen, T. Yamamoto, Yu. A. Pashkin, Y. Nakamura & J. S. Tsai, Nature, 449, 588, 2007
[6] O. Astafiev, A. M. Zagoskin, A. A. Abdumalikov, Jr., Yu. A. Pashkin, T. Yamamoto, K. Inomata, Y. Nakamura, J. S. Tsai, Science, to be published
On Friday 5th March 2010 at 15:00, hall B of the TKK main building:
Tilman Esslinger, ETH Zurich,
Switzerland:
Factories for Quantum Physics
In a cloud of atoms which is cooled to almost zero temperature the particles come practically to a standstill and one may wonder whether anything interesting can happen in such a collection of resting atoms. Yet it turns out that these ultralow temperatures provide an exceptionally clear view on macroscopic quantum phenomena and fascinating quantum effects. Indeed, a large variety of different Hamiltonians can be engineered with quantum gases making their physics accessible. This includes fundamental concepts of statistical physics, condensed matter physics and quantum optics. In my talk I will report on intriguing breakthroughs and address major challenges lying ahead.
Fall 2009
On Thursday 22nd October 2009, at 15:00, hall B of the TKK main building:
Carlo Beenakker, University of Leiden, the Netherlands: What is special about graphene?