2012-2011

Academic year 2011 - 2012

6 September Mikhail Baranov, University of Innsbruck

"Many-body physics in fermionic dipolar mono and bilayer systems"

Collective modes, stability, and BCS pairing in a quasi-two-dimensional single component Fermi gas of polarized dipolar particles (monolayer) and in a coupled system of two such gases (bilayer) are discussed.

10 May Thomas Brumme, TU-Dresden

(dia inusual!) "Electron transport through helical, biimidazole-based structures"

Molecular electronics and spintronics provide a promising strategy to overcome limitations of semiconductor-based technologies by implementing electronic functionalities at the molecular scale. However, in order to create single molecule spintronics devices one needs to understand the spin-dependent transport through the molecular system, its dependence on different molecular properties and possible mechanisms to change the magnetization of the molecule. Molecular systems with screw symmetry like DNA, are especially interesting for spintronics applications since the

transport through these systems can be spin selective [1, 2]. We investigate the electronic structure of a molecular helix formed by silver atoms and biimidazole units ([Ag(NO)3(H2biim)]n, [3, 4]). First-principles calculations reveal that several molecular orbitals possess screw symmetry and are completely delocalized along the helix. Based on this results we explore the possibility of spin-selective electron transport through this molecular helix.

1] B. Göhler et al., Science 331, 894 (2011)

[2] Xie et al., Nano. Lett. 11, 4652 (2011)

[3] C.A. Hester et al., Polyhedron 16, 2893 (1997)

[4] M. Sowwan et al., Journal of Nanomaterials 2010 (2010)

26 April Rafael Gutierrez, TU-Dresden

(dia inusual!) "Spin selective transport through helical molecular systems"

Highly spin-selective transport of electrons through a helically shaped electrostatic potential is demonstrated in the frame of a minimal model approach. The effect is significant even for weak spin-orbit coupling. Two main factors determine the selectivity: an unconventional Rashba-like spin-orbit interaction, reflecting the helical symmetry of the system, and a weakly dispersive electronic band of the helical system. The weak electronic coupling, associated with the small dispersion, leads to a low mobility of the charges in the system and allows even weak spin-orbit interactions to be effective. The results are expected to be generic for chiral molecular systems displaying low spin-orbit coupling and low conductivity.

17 April Tobias Stauber, Universidad Autonoma de Madrid

(dia inusual!) "On the optical properties of graphene"

One of the hallmarks of the optical properties of (suspended) graphene is that a simply-observable quantity as the optical transparency is defined solely by the fine structure constant. In the first part of this talk, I will give the theoretical explanation to this experiment, i.e., show that even in the visible-optics regime the corrections to the Dirac cone approximation are small (a few percent) and the effect of next-nearest neighbor hopping is negligible. I will also discuss the infrared conductivity of graphene on a substrate where electron-phonon and impurity scattering become important.

In the second part, I will look at the optical properties of double layer graphene with respect to their plasmonic excitations, near-field amplification and extraordinary (perfect) transmission. Also graphene's fluorescence quenching including transverse decay channels and full retardation will be discussed. Finally, the current-current correlation function of the full hexagonal tight-binding model will be derived and I will show that lattice effects lead to a paramagnetic response for graphene with intrinsic doping at low temperatures.

10 April Rafael Sanchez, Instituto de Ciencia de Materiales de Madid (CSIC)

(dia inusual!) "Rectification of thermal fluctuations in electrical conductors"

To demonstrate that the relative stability of non-equilibrium states cannot be found from local criteria, Landauer showed that hot spots in locations of phase space that might be only rarely visited can be decisive. Later, Büttiker[1] and van Kampen[2], investigated noise induced transport generated by hot spots in systems with overdamped Brownian motion dynamics. Recently, these ideas have been applied to electrical circuits in which hot spots occur naturally at places where energy is dissipated. Optimal energy to current conversion can be reached in Coulomb coupled quantum dot systems where both energy and charge transfer are quantized[3].

We present a model of a mesoscopic heat engine that uses the principle of rectifying thermal fluctuations applied to a nonlinear system. The system is a chaotic mesoscopic cavity where the contact transmission to leads is energy-dependent. This energy-dependence is generic in mesoscopic conductors, and leads to an intrinsic nonlinearity. The cavity is coupled capacitively to another conductor, held at a different temperature. This geometry permits separate directions of the heat and current flux. The nonlinear cavity rectifies the thermal fluctuations, leading to a hot spot rectified electrical current that depends on the asymmetry in the energy-dependence of the contacts, and is proportional to the temperature difference[4]. The maximum power produced by the system will be discussed, as well as the efficiency of the engine by comparing it to the heat current that passes between the coupled systems.

Our results might be applied to energy harvesting nano-scale devices which can function independently of an external power supply.

[1] M. Büttiker, Z. Phys. B 68, 161 (1987).

[2] N.G. van Kampen, I.B.M. J. Res. Dev. 32, 107 (1988).

[3] R. Sánchez, and M. Büttiker, Phys. Rev. B 83, 085428 (2011).

[4] B. Sothmann, R. Sánchez, A.N. Jordan, M. Büttiker, arXiv:1201.2796.

22 March Victor Martin-Mayor, Universidad Complutense

(dia inusual!) "Equilibrium fluid-solid coexistence of hard spheres"

Crystallization is a vast field of research, where experiments and theory cross-fertilize. Hard-spheres provide a celebrated example: the numerical finding of a fluid-solid phase transition in the late fifties motivated experiments on colloids (Pusey et al, 1986, 1989). Up to now, numerical simulations of crystallization phase transitions have been well behind their fluid-fluid counterpart (e.g. vapor-liquid equilibria). Actually, hard spheres are the preferred benchmark for numerical approaches to crystallization. Yet, the lack of exact solutions enhances the importance of accurate numerical and/or experimental studies. Here, we describe a new approach of the problem based on constrained free-energies. The method can be regarded as a major simplification of the traditional umbrella sampling, and improves over previous approaches by orders of magnitude. The talk will be intended for a non-specialized audience.

8 February Markus Mueller, Universidad Complutense

"Quantum simulation with Rydberg atoms and ions"

In this talk I will present a scheme for digital quantum simulation, where laser-excited Rydberg atoms in optical lattices provide an efficient implementation of a universal quantum simulator. After a short introduction to some basic concepts of quantum information and atomic Rydberg physics, I will discuss how the proposed simulation architecture allows one to realize coherent Hamiltonian as well as dissipative open-system time evolution of spin models involving n-body interactions, such as e.g. Kitaev's toric code. Our scheme relies on a combination of multi-atom Rydberg gates and optical pumping to implement coherent operations and dissipative processes.

I will also report on recent experiments with trapped ions, where these concepts, and the building blocks of an open-system quantum simulator, have been demonstrated with up to five qubits.

[1] H. Weimer, M. Müller, I. Lesanovsky, P. Zoller and H.P. Büchler. Nature Physics 6, 382 (2010)

[2] J. Barreiro, M. Müller, P. Schindler, D. Nigg, T. Monz, M. Chwalla, M. Hennrich, C. F. Roos, P. Zoller and R. Blatt. Nature 470, 486 (2011)

8 November T. Sagawa, University of Tokyo

(dia inusual!) Nonequilibrium Thermodynamics of Information Processing

Ever since the proposal of "Maxwell's demon" in the 19th century, the relationship between thermodynamics and information has attracted much attention concerning the foundation of the second law of thermodynamics [1]. Due to the recent developments of the experimental technologies of manipulating small thermodynamic systems such as biological molecular motors, the demon has become an experimental issue beyond a classic gedankenexperiment. In this seminar, I'd like to talk about our theoretical and experimental results on the foundation of the relationship between thermodynamics and information. Theoretically, I will focus on a generalized the second law of thermodynamics for information processing processes such as feedback control, measurement, and information erasure [2,3,4]. In our results, information contents and thermodynamic variables are treated on an equal footing. I'd also like to talk about our recent experiment, which has realized the Szilard-type Maxwell's demon [5].

[1] "Maxwell's demon 2: Entropy, Classical and Quantum Information, Computing," H. S. Leff and A. F. Rex (eds.), (Princeton University Press, New Jersey, 2003).

[2] T. Sagawa and M. Ueda, Phys. Rev. Lett. 100, 080403 (2008).

[3] T. Sagawa and M. Ueda, Phys. Rev. Lett. 102, 250602 (2008); ); 106, 189901(E) (2011).

[4] T. Sagawa and M. Ueda, Phys. Rev. Lett. 104, 198904 (2010).

[5] S. Toyabe, T. Sagawa, M. Ueda, E. Muneyuki, and M. Sano, Nature Physics 6, 988-992 (2010).

26 October Pablo San Jose, Instituto de Estructura de la Materia (CSIC)

Single-parameter pumping in graphene

We propose a quantum pump mechanism based on the particular properties of graphene, namely chirality and bipolarity. The underlying physics is the excitation of evanescent modes entering a potential barrier from one lead, while those from the other lead do not reach the driving region. This induces a large nonequilibrium current with electrons stemming from a broad range of energies, in contrast to the narrow resonances that govern the corresponding effect in semiconductor heterostructures. Moreover, the pump mechanism in graphene turns out to be robust, with a simple parameter dependence, which is beneficial for applications. Numerical results from a Floquet scattering formalism are complemented with analytical solutions for small to moderate driving.

Note: This seminar will begin at 12:30

10 October David Guéry-Odelin, Université Paul Sabatier, Toulouse

(dia inusual!) Matter wave probing of dynamical periodic structures

In the first part of this talk, I will discuss our recent experimental realization of a multiple Bragg reflector for guided matter waves. In this experiment, a Bose-Einstein condensate with controlled velocity distribution impinges onto an attractive optical lattice of finite length and directly probes its band structure. I will show the importance of the envelope of the optical lattice, which gives rise to Bragg cavity effects.

In the second part, I will discuss our recent results obtained with modulated optical lattices. The modulation triggers inter-band transitions. With two modulation frequencies we can design a very selective velocity filter whose width corresponds to sub-nK temperatures.

21 September Ennio Arimondo, University of Pisa

Bose-Einstein condensates in optical lattices: optimal quantum control and Rydberg excitation

I will present the basic elements of the most recent activity of the Bose-Einstein condensation research group in Pisa, concentrating on two different topics: the development and test of protocols for optimal quantum control and the excitation of the condensate to Rydberg levels.

In quantum control, it is usually necessary to realize the quantum manipulation in the shortest possible time in order to minimize decoherence, and with a large stability against fluctuations of the control parameters. We have implemented optimal control schemes that achieve nearly perfect fidelity for our system. By suitably tailoring the time-dependence of the system's parameters, an initial quantum state can be transformed into a desired final state through a short-cut protocol reaching the maximum speed compatible with the laws of quantum mechanics. In the opposite limit the transitionless superadiabatic protocols in which the system perfectly follows the instantaneous adiabatic ground state can be implemented.

Rydberg excitations in Bose-Einstein condensates of rubidium atoms, expanded to different sizes in a one-dimensional trap, agree well with the intuitive picture of a chain of Rydberg excitations. The observed time dependence of the condensate excitation agrees with the picture of localized collective Rydberg excitations including nearest-neighbour blockade. For Rydberg excitations in a magneto-optical trap, their counting statistics was obtained by performing a large number of excitations under the same conditions. The observed highly sub-Poissonian distributions with Mandel Q-factors close to -1 indicate the highly collective character of the Rydberg excitations due to the mean interparticle distance's being much smaller than the dipole blockade radius of the Rydberg states (55 < n < 80) used in our experiments.

12 September Harald Haakh, University of Potsdam

(dia inusual!) Fluctuation-induced effects in metallic and superconducting atom chips

Trapping of neutral cold atoms in miniaturized surface-mounted magnetic traps (atom chips) is fundamentally limited by atom surface (Casimir-Polder) forces and losses of trapped atoms due to electromagnetic noise. Theoretical studies of magnetic dipoles trapped near a metallic or superconducting

surface show how the details of charge transport and dissipation in the surface result in characteristic signatures in the trapping lifetime and in the surface-atom interaction. Precise knowledge of these effects is necessary for the design of atom-chips experiments and allows for significant improvement of magnetic trapping. From another point of view, the same phenomena open a window to both atomic physics and material science on the meso- and microscale, and might even shine new light on unresolved problems in the Casimir effect between macroscopic bodies.