2011-2010
21 June Ferruccio Renzoni, University College London
(dia inusual!) Cold atom ratchets
Brownian motors, or ratchets, are devices which ``rectify" Brownian motion, i.e. they can generate a current of particles out of unbiased fluctuations.
We experimentally implemented a Brownian motor using cold atoms in an optical lattice. This is quite an unusual system for a Brownian motor as there is no a real thermal bath, and both the periodic potential for the atoms and the fluctuations are determined by laser fields.
With the help of such a system, we investigated experimentally the relationship between symmetry and transport in a 1D rocking ratchet, both in the periodic and in the quasiperiodic case. We then went beyond 1D rocking ratchets, demonstrating 1D gating ratchets. We also realized 2D rocking ratchets and demonstrated a rectification mechanism
unique to these high-dimensional ratchets.
8 June Heiner Kohler, ICMM (CSIC)
Full Fidelity Statistics of a Doorway state
Exact knowledge of the decay properties of a prepared state weakly coupled to an environment is required for the design of quantum information and quantum storage devices. If the coupling is so weak that it is comparable to the mean level spacing of the random background, corrections to Fermi golden rule become important. We calculate these corrections in the so-called Doorway model, which was originally introduced in nuclear physics. For weak coupling, the probability that the system in still in the prepared state (the fidelity of the state) is finite even in the long time limit. The long time expectation value is the IPR (inverse participation ratio) of the prepared state in the basis of the exact eigenvalues. We study as well numerically the full distribution function of time dependent fidelity. In the long time limit we are able to obtain analytic results. Surprising features like fidelity revival and enhanced non-ergodicity are observed. The role of the coupling coefficients and of complexity of background is studied as well.
25 May German Sierra Rodero, UAM/CSIC
Infinite Matrix Product States and Conformal Field Theory
In this talk we shall introduce a new type of variational ansatzs for the ground state of quantum spin chains using conformal field theory. These ansatzen provide an infinite dimensional version of the so called Matrix Product States. For ansatzs with a rotational SU(2) symmetry, we shall make contact with the Haldane-Shastry model of spin chains
and the Wess-Zumino-Witten model. Finally, we shall discuss analogies between the wave functions obtained with this method, and those proposed for the abelian and non-abelian Fractional Quantum Hall effect.
18 May Isabel Llorente García, Imperial College London
Manipulation of ultracold atoms on atom chips
Atom chips are centimetre-scale devices, often micro-fabricated, which can integrate wires, permanent magnets, optical waveguides, etc. in a flat geometry in order to generate electric and magnetic fields to trap and manipulate cold atom clouds and Bose-Einstein condensates (BECs). In this talk I will report on experiments with ultra-cold rubidium atoms with temperatures down to a few hundred nanoKelvin, confined in microscopic magnetic traps created on two different atom chip experiments.
I will briefly describe experiments on the following topics:
a characterisation of the trapping potential and its anharmonicity effects
a novel transport mechanism whereby we convey cold atoms confined in arrays of magnetic micro-traps over distances as large as 1 cm
interferometry of BECs, with the aim of measuring small magnetic and electric fields, gravity, and Casimir-Polder forces.
11 May Maria Jose Calderon, Instituto de Ciencia de Materiales de Madrid (CSIC)
Magnetic state and anisotropies in iron based superconductors
Fe-based superconductors were discovered early 2008, with the highest superconducting critical temperatures (Tc) ever found after the record holding cuprates. The new superconductors develop the superconducting phase upon doping or applying pressure to a metallic anisotropic antiferromagnetic compound. The similarities with the cuprates (high Tc and antiferromagnetic parent phase) triggered a great deal of attention, which was farther stimulated by the differences (undoped cuprates are Mott insulators, not metals) and by the perceived importance of the multiorbital character of the electronic carriers. The properties of the magnetic phase itself are a subject of a strong debate due to the unexpected low magnetic moment, the different antiferromagnetic orderings encountered (columnar and bi-stripe), and a counter-intuitive resistivity anisotropy. Understanding these properties may be the key to unravel the origin of the superconductivity. I will first present a general overview of these materials and then I will focus on our model studies of the magnetic phase diagram which uncover the importance of the interplay between the spin and orbital degrees of freedom.
References:
[1] M.J. Calderón, B. Valenzuela, and E. Bascones. Phys. Rev. B 80, 094531 (2009).
[2] E. Bascones, M.J. Calderón, and B. Valenzuela. Phys. Rev. Lett. 104, 227201 (2010).
[3] B. Valenzuela, E. Bascones, and M.J. Calderón. Phys. Rev. Lett. 105, 207202 (2010).
20 April No seminar - Semana Santa
6 April Postponed due to speaker's illness
Pablo San-José, Instituto de Estructura de la Materia (CSIC)
"Graphene ratchets"
We propose a quantum ratchet mechanism that is 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 ratchet 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 amplitudes.
Link: http://arxiv.org/abs/1103.5597
9 March Mirta Rodriguez, Instituto de Estructura de la Materia (CSIC)
"Tunable particle current with flashing periodic potentials"
Quantum ratchets are systems that exhibit asymptotic currents when they are driven by a time-periodic potential of zero mean if the proper spatio-temporal symmetries are broken. The current arises from the desymmetrization of
the Floquet cyclic states, eigenstates of the unitary evolution operator for one period of the driving force. There has been recent debate on whether directed currents may arise even with potentials which do not break these symmetries.We show here that crossed terms in the Floquet basis can induce long-lasting directed currents in the presence of structural degeneracies in the quasienergy spectrum, even if the time reversal symmetry is not broken.
We present a highly controllable model which can be realized with ultracold atoms in optical lattices, which shows directed currents with high amplitudes tunable over a large range and where the time scale of the average current can be controlled.
2 March Eugenio Coronado, Instituto de Ciencia Molecular, Universidad de Valencia
"Espintrónica molecular"
La espintrónica es el área más activa del nanomagnetismo actual. Hasta ahora esta área se ha basado en metales y semiconductores inorgánicos convencionales. Sin embargo y gracias a los avances logrados en magnetismo molecular y electrónica molecular esta situación esta cambiando y una nueva sub-área denominada espintrónica molecular está emergiendo. Su objetivo es por una parte diseñar los análogos moleculares a las estructuras espintrónicas inorgánicas (válvulas de espín, por ejemplo), y por la otra medir y manipular el espín de una molécula individual cuando se la conecta a dos electrodos.
En esta charla ilustraré con algunos ejemplos el papel que puede jugar la química en la espintrónica molecular y, en general, en el nanomagnetismo molecular. Me centraré en dos tipos de sistemas moleculares: los materiales moleculares multifuncionales y las moléculas magnéticas.
16 February Jordi Mur Petit, Instituto de Física Fundamental (CSIC)
"Cold molecules for quantum information"
Recent successes in the creation of ground-state, ultra-cold heteronuclear molecules have triggered a growing interest in these systems, leading in particular to theoretical proposals to use them for quantum information processing. In this context, and motivated by the experimental progress in the creation of novel optical lattices, we are studying the controlled formation of cold molecules in these systems, with an emphasis in their potential application to build strongly-correlated many-body states. In this talk I will review some of these theoretical proposals, and discuss our progress in the study of using long-range interactions to build a phase gate between atoms confined in separate traps, and to cool ions with a nearby Bose-Einstein condensate.
27 January Francesca Marchetti, Uinversidad Autonoma de Madrid
(dia inusual!) "Supersolidity in electron-hole bilayers with a large density imbalance"
Pairing phenomena in two-component Fermi systems are a topic of fundamental interest, having relevance to a range of fields spanning superconductivity to QCD. Of particular interest is the case where the densities of the two fermionic species are imbalanced, so that the interspecies pairing is then frustrated. Here, one expects more exotic pairing scenarios such as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) spatially-modulated phase, where fermions pair at finite center-of-mass momentum. The creation of spin-imbalanced Fermi gases in ultracold atomic gases has recently revived the hope of realizing the FFLO state, but, thus far, the atomic system has been dominated by phase separation between superfluid and normal phases, with FFLO only occupying a tiny sliver of the predicted phase diagram.
On the other hand, electron-hole bilayers may provide a better route to achieving the FFLO state. Here, electrons and holes can pair to form excitons which can then in principle condense. Bilayers have already been successfully produced by optically pumping coupled GaAs quantum wells and more recently, they have been fabricated with independently contacted layers, thus allowing the densities in each layer to be controlled individually and providing a means for generating a density imbalance. Moreover, the reduced dimensionality of the bilayer system favors the FFLO phase and, in contrast to the cold-atom system, the intralayer Coulomb repulsion acts to suppress phase separation.
In this seminar, I will provide strong evidence for the existence of the FFLO phase in electron-hole bilayers with a large density imbalance. I will expose an unusual bosonic limit of FFLO, where a dilute gas of excitons forms a condensate with a 2D spatial modulation, a phase otherwise known as a supersolid.
26 January Ivar Zapata, Universidad Complutense de Madrid
"Triplet pair correlations in s-wave superfluids in FFLO-type states"
Triplet pairing is enhanced whenever an effective exchange (Zeeman or population imbalance) field is sufficient to enforce FFLO (Fulde-Ferrel-Larkin-Ovchnnikov) type states. As a result, the pair wave function mixes symmetries. The triplet component uses the same states and is as localized as the polarization. In the case of LO-type states, this turns to be a (narrow) band of Andreev states centred where the s-wave order parameter vanishes. It is also commented why this could be a unifying framework towards a unified explanation of various enhancement of the triplet pairing in FFLO states that have appeared in the literature. Similar triplet mixing which occurs when a balanced two-component system shows FFLO type oscillations due to a spin dependent lattice. A combination of radio-frequency pi/2-pulses and the rapid-ramp technique through a p-wave Feshbach resonance could be used to measure this triplet component. Photo-Association spectroscopy could be used as an independent test.
Note: this seminar will start at 12:30, and not the usual time of 12:00.
19 January M.A. Garcia-March, Colorado School of Mines
"Macroscopic superposition states of ultracold bosons in double well potentials"
We study ultracold bosons in three-dimensional double wells when they are allowed either to condense in single-particle ground states or to occupy excited states. This permits the consideration of angular degrees of freedom in the model, since the second level eigenstates can carry angular momentum. We show that the number of relevant parameters is increased, since new processes, like two-particle hopping between different levels or vortex-antivortex pair creation or annihilation, are considered. We clearly demarcate the new range of dynamical regimes obtained in terms of the new parameters. We show the presence, in the interaction-dominated regimes, of macroscopic superposition states of atoms with non-zero angular momentum. This leads to the study of the dynamics of atoms carrying vorticity while tunneling between wells. Among these new tunneling processes, we find vortex hopping and vortex-antivortex pair superposition along with the sloshing of atoms between both wells.
11 January Postponed due to speaker's illness
Ferruccio Renzoni, University College London
15 December Alfredo Levy Yeyati, Universidad Autonoma de Madrid
"Andreev transport in hybrid carbon based nanostructures"
Great progress in the fabrication of hybrid nanostructures combining graphene or carbon nanotubes with superconducting electrodes has allowed the exploration of Andreev quantum transport in theoretically ideal situations. In this talk I shall present some recent work by our group within this field. In the first part I shall discuss the proximity effect at graphene-superconductor interfaces showing that long-range superconducting correlations are induced due to the appearance of interface bound-states [1]. I shall analyze the possible consequences of these states for non-local transport between weakly coupled probes on the graphene layer. In the second part I shall discuss recent experiments aimed at producing Cooper pair splitting using carbon nanotubes as tunable double quantum dots [2]. After presenting a theoretical analysis based on a phenomenological model I shall discuss how the main ingredients of this model can be derived from microscopic calculations. Finally I shall discuss the appearance of Andreev bound states in carbon nanotubes coupled to superconducting leads and the possibilities opened by its recent experimental detection [3].
[1] P. Burset, W. Herrera and A. Levy Yeyati, Phys. Rev. B 80, 041402(R) (2009).
[2] L. Herrmann, F. Portier, P. Roche, A. Levy Yeyati, T. Kontos and C. Strunk, Phys. Rev. Lett. 104, 026801 (2010).
[3] J.D. Pillet, C.H. L. Quay, P. Morfin, C. Bena, A. Levy Yeyati and P. Joyez, Nature Phys. 6, 965–969 (2010).
1 December Diego Porras, Universidad Complutense
"Matter wave superradiance with ultracold atoms in optical lattices"
La emisión de átomos atrapados inicialmente en una red óptica muestra una fenomonología fascinante, relacionada con la superradiancia de luz emitida por un conjunto de átomos. En particular, hemos demostrado que el proceso de emisión de ondas de materia sigue reglas similares a la emisión de luz por impurezas en un cristal fotónico, y aparecen fenómenos tales como estados ligados, renormalización de los ritmos de emisión, o fases cuánticas con ruptura espontánea de la simetría.
20 October Oliva Garcia Cantu-Ros, Instituto Pluridisciplinar de la UCM
"Some novel and curious properties of electron transport in low dimensional systems"
In this talk I will first discuss the electron dynamics in a 1D chain, where not only the lattice and the electron-lattice interactions, but also the electron hopping term, are affected by anharmonicity. This anharmonicity appears to be responsible for the existence of long living intrinsic localized modes, consisting of a localized electron density and an associated strong lattice distortion which move together through the systems at sub or supersonic velocities. These localized modes are able to maintain coherence between electron and lattice for times that are longer than those of the polaron. The evolution of these localized states suggests their potential as new carriers for fast electric charge transport.
Time permitting I would also like to briefly discuss the general properties of tunnelling current in quantum Hall bilayer systems, where the strong inter and intra layer correlations are responsible for abnormal effects such as negative (tunnelling) resistivity, drag Hall voltage and counter flow superfluidity.
Note: this seminar will start at 12:30, and not the usual time of 12:00.
6 September K. L. Ngai, Polylab, Università di Pisa, Italy
"Universal Relaxation and Diffusion in Many-body Interacting Systems"
A voluminous amount of experimental evidence corroborate to show that relaxation and diffusion in complex interacting systems and materials are universal. Furthermore these universal properties originate from fundamental physical laws governing many-body relaxation and diffusion in interacting systems. Although the Coupling Model of the speaker is not a complete solution of this fundamental problem, it has captured the essence of the solution.