What does the time between buses in Cuernavaca have to do with step separations?

While steps on surfaces have many practical uses, they also offer remarkable, quantitatively measurable realizations of some familiar physical models. Step dynamics, e.g., can be viewed as a form of Brownian motion. After a survey of phenomena, I focus on the statistical properties of interstep separations on misoriented ("vicinal") surfaces. Such a terrace-width distribution (TWD) can be related to the spacings of repelling fermions evolving in one dimension. The TWD can then be analyzed simply in terms of undergraduate quantum mechanics and with more sophistication in terms of general properties of equilibrium fluctuations, viz. generalizations of the amazingly broadly applicable Wigner distribution from random-matrix theory. We go beyond equilibrium to describe step relaxation, growth, and capture-zone (Voronoi cell) distributions in island growth, with remarkable experimental implications.

Due to recent advances in fabrication and measurement techniques, new magnetic nanostructures can be synthesized and their quantum properties can be locally probed at the molecular level. Despite this progress, it still remains challenging to characterize interfaces and contacts and to manipulate their properties for specific applications. A understanding of such properties requires theoretical models as well as large-scale computer simulations considering local environmental factors at the atomistic level. Recently, low-dimensional structures made of nanoscale magnetic molecules have drawn attention due to its unique interplay between the internal magnetic degrees of freedom and electronic ones and to possible applications for information storage and materials for quantum computation. In this talk, we consider a monolayer of such magnetic molecules adsorbed on a metallic surface and a single magnetic molecule bridged between electrodes that have been recently synthesized.

We discuss how the electronic, magnetic, and transport properties of the magnetic molecules are influenced by the interactions with the surface or the electrodes, using simulations based on first-principles methods.

Aging and dynamical scaling are generic features in systems far from equilibrium that evolve with slow dynamics. Well known examples can be found in structural glasses, spin glasses, magnetic systems, and colloids. Interestingly, even though physical aging has been observed (and exploited) since prehistoric times, its theoretical understanding is still far from complete. In the first part of the talk I shall discuss some typical experiments and highlight the rich phenomenology of aging as well as its universal features. In the second part of the talk I shall focus on recent theoretical studies of different model systems, as for example disordered ferromagnets and reaction-diffusion systems.

These studies reveal that a robust and rather universal theoretical framework can be developed in order to describe aging phenomena that take place in a large variety of systems. I shall close the talk with a discussion of open questions and an outlook on some future developments.

Over the last several years this department has seem many Colloquia on topics of great current interest in neutrino physics. This talk will take a step back to explain the basics of neutrinos: what they are and how they interact with the rest of the universe.

Biological networks are known to possess functionally specialized modules, which perform tasks almost independently of each other. While proposals have been made for the evolutionary emergence of modularity, it is far from clear that adaptation on evolutionary timescales is the sole mechanism that can lead to functional specialization. Here we show that phenotypic adaptation (non-evolutionary learning) can also lead to the formation of functionally specialized modules in a system exposed to multiple environmental constraints. We use the paradigm of neural networks to represent a task performing system, and use non-evolutionary learning algorithms as mechanisms for phenotypic adaptation. We show that for a network learning to perform multiple tasks, the degree of independence between the tasks dictates the degree of functional specialization emerging in the network. In order to uncover the functional modules, we introduce a method of node knockouts that explicitly rates the contribution of each node to different tasks (differential robustness). Through a specific example we also demonstrate the potential inability of purely topology-based clustering methods to detect functional modules. The robustness of the results in the present study suggests that similar mechanisms might be responsible for the emergence of functional specialization in other multitasking networks as well.

Co-Authors: S. Sreenivasan (UT Austin) and H. Kim (Notre Dame)

Extra spatial dimensions have been employed in theoretical physics for quite some time now. Earth-bound experimental tests for the existence of extra spatial dimensions are currently limited. Astrophysical tests are also possible. I will review some of the basic conceptual ideas that underlie the theories. Then I will discuss some astrophysical observations that could test for the existence of extra spatial dimensions.

Earthquakes can cause considerable loss of life and property. Unlike hurricanes, fires and floods earthquakes strike with almost no warning. To forecast earthquakes will require considerable insight into their physics. Tantalizing clues lie in the fact that fault systems appear to be operating near critical points. The evidence for a critical point is in the appearance of scaling laws such as the Gutenburg - Richter power law distribution of the number of earthquakes of magnitude m. and the Omori law for the number of after shocks as a function of time. However, there is also clustering of earthquake events and evidence of an earthquake cycle in which large events cause a lessening of earthquake activity until the system reloads the stress dissipated in the large event.

In order to understand these processes we have studied simple models of earthquake fault systems with some surprising results. In this talk I will discuss the physical properties of a certain class of models and the relation of these models to real fault systems. The models indicate a mechanism that can produce both scaling and an earthquake cycle and can be seen, in the right range of parameters that control the physics, to be in the same universality class as Ising models and simple fluids.

The 2008 discovery of high temperature superconductivity in doped LaFeAsO by Kamihara and co-workers provided the second class of high Tc materials, the other being the cuprate family discovered in 1986 by Bednorz and Mueller. This discovery was revolutionary in that many of the properties of the iron based superconductors are radically different from those of the cuprates, apparently requiring a new and broader understanding of the physics of high temperature superconductivity. The purpose of this talk is to discuss the chemistry and physics of the new superconductors in relation to cuprates. So far, many puzzles remain. The materials appear to be much more band-like and show much stronger signatures of metallic (Fermi surface related) physics than cuprates, with correspondingly weaker signatures of on-site Hubbard correlations. However, there remain substantial discrepancies between bare band structure calculations and experiment, and interestingly these discrepancies are in the opposite direction from those found in cuprates. These are discussed in the context of spin-fluctuations. A remarkable feature of the iron based materials is that superconductivity can be induced by alloying on the iron site both in LaFeAsO type and in ThCr2Si2 structure materials. The chemistry of these alloys is discussed in terms of their bonding as revealed by electronic structure calculations and experiment. Finally, the possible superconducting pairing is discussed in terms of spin fluctuations based on the electronic structure, and some of the many open questions are laid out.

This work was done in collaboration with I.I. Mazin, M.H. Du, Lijun Zhang, Alaska Subedi and Michelle Johannes. The work was supported by the Department of Energy, Division of Materials Sciences and Engineering.

Our ability to quickly access the vast amounts of information linked in the internet and everywhere else is owed to the miniaturization of magnetic data storage. From the invention of the hard disk drive in 1956 to our present day life, magnetic bits have shrunk from millimeter size to a few tens of nanometers. If magnets shrink further, down to a point where only a few atoms comprise the magnetic structure, they undergo a dramatic shift in their behavior. While classical magnets allow for a continuous rotation of their magnetization direction, small spin systems behave quantum mechanical – their magnetic orientation becomes quantized. Studying the energetically discrete excitations of these nanomagnets and their response to external influences allows insight into basic principles of quantum mechanics.

Here we show how the magnetic properties of individual atoms and artificially created nanostructures can be probed with a low-temperature, high-field scanning tunneling microscope (STM) when the atoms are placed on a thin insulator (see Fig. 1). We find clear evidence of large magnetic anisotropy, a prerequisite for building stable magnetic bits, for individual d-metal atoms embedded in a monatomic layer of Cu2N on copper [1]. The STM allows the determination of all parameters in the corresponding Spin Hamiltonian which describes the quantized energy levels of the spin system in real-space and under application of external magnetic fields. The spin excitations proof to be strongly dependent on the spin orientation of the tunneling electrons. At high current densities, a spin-polarized current can transfer a significant amount of spin angular momentum to the magnetic atoms and nanostructures [2]. The current pumps the atomic spin into highly excited states culminating in the reversal of the spin’s magnetic orientation. This enables exploration of the microscopic mechanisms for spin-relaxation and stability of nanomagnets.

[1] C.F. Hirjibehedin, C.-Y. Lin, A.F. Otte, M. Ternes, C.P. Lutz, B.A. Jones, A.J. Heinrich, Science 317, 1199 (2007)

[2] S. Loth, K.v. Bergmann, M. Ternes, A.F. Otte, C.P. Lutz, A.J. Heinrich, submitted

Modern day electronics is rapidly reaching nanometer dimensions where atomistic, quantum and many-body effects dominate. Textbook concepts such as Ohm's Law for current flow and Fourier's Law for heat flow are now proven to be incomplete, and the future of electronics seems to depend critically on the fascinating world that lies beyond. Fundamental physical and chemical concepts are needed to understand how a graphene or a molecular transistor would behave as a logic element, or how one maybe able to encode information more efficiently for low power device and circuit operation, and most fundamentally, how electrons flow through nanomaterials and interfaces through a combination of interference pathways and localized many-body states. In this talk, I will provide an unified overview on how to model current flow through various nanostructures, how our computational simulations measure up with available experiments, how they advance our fundamental understanding of non-equilibrium quantum physics, and how we could utilize this understanding and quantitative modeling to do potentially useful electronics.

Istvan Hargittai is professor of chemistry at Budapest Univ. of Technology and Economics and is known for his work on gas-phase diffraction studies of molecular structure. He has also written extensively on 20th-century science and has a forthcoming biography of Edward Teller.

Program note by host R. Zallen:

A joke circulated at Los Alamos. The Manhattan Project was benefitting from input by advanced aliens. They looked human, they acted human, BUT, among themselves, they often spoke in a secret language that nobody else could understand. They called it "Hungarian", but they came from Mars.

The Los Alamos Martians were Wigner, von Neumann, Teller, and Szilard. Professor Hargittai has written an excellent book about them: