From february 2013 seminars are listed within the ⇒ event pages of the ⇒ Max Planck Institute for the Structure and Dynamics of Matter.


Seminar Archive 2013-2012

CFEL science




Jan. 29th, 14:30, MPSD-Seminar Andrei G. Stepanov: Recent progress in the generation of high-energy ultrashort THz pulses


Oct. 24th, 2012, 11 am - MPSD-Seminar Stefano Bonora: New deformable mirror technologies
Oct. 22nd, 2012, 11:30 am - MPSD-Seminar Michael Fechner: Competing instabilities at paraelectric/superconducting interfaces
Oct. 11th-12th, 2012 - Max Planck Symposium Frontiers in X-Ray Science
Sept. 26th, 2012, 11am - CFEL-MPSD-Seminar Dieter Jaksch: Atom Transport in Optical Lattices
Aug. 27th, 15:15 pm - MPSD-Seminar Jacob Burgess - The energetic landscape of a
magnetic vortex in a disk
July 25th, 2012 - MPSD-Seminar Johan H. Mentink, Magnetism on the timescale of the exchange interaction: explanations and predictions
June 22nd, 2012 - CFEL Colloquium Mark I. Stockman, Solids in Ultrafast and Strong Optical Fields
June 11th, 2012 - MPSD Seminar Alexander W. Holleitner, Picosecond photocurrents and terahertz generation in nanoscale circuits
May 25th, 2012 - MPSD Seminar Deung-Jang Choi, Kondo effect in the presence of a
spin-polarized current
May 11th, 2012 - MPSD Seminar David Gohlke, Tuning Magnetic Interactions in Semiconductors by STM
March 29th, 2012 - CFEL-Seminar Mathieu Le Tacon, Resonant x-ray scattering investigations of
high-temperature superconductors
March 23rd, 2012 - MPSD-Seminar Michael Buchhold, Quasi-particle Theory of Strongly Correlated Lattice Bosons: Application to the Bose-Hubbard Model
March 8th, 2012 CFEL Workshop CFEL Workshop on Nanoscale Imaging
Feb. 20th, 2012 - CFEL-Seminar Marco Aprili: Microwave cooling of the Josephson phase
Feb. 15th, 2012 - MPSD-Seminar Manuel Marks: Formation Mechanism of Metal-Organic Interface States studied by 2-Photon-Photoemission-Spectroscopy
Feb. 7th, 2012 - CFEL-Seminar Martin Eckstein: Theory for strongly correlated systems
out of equilibrium
Feb. 6th, 2012 - MPSD-Seminar Marcin Mierzejewski: Nonlinear current response of interacting fermions
Jan. 31st, 2012 - MPSD-Seminar Timm Rohwer, XUV Photoemission in the fs-regime
Jan. 18th, 2012 - CFEL-Colloquium Immanuel Bloch, Observing and Controlling Quantum Matter at the Single Atom Level

→ Seminar archive: 2011 | 2010 | 2009


January 29th, 2013, 14:30 pm

Recent progress in the generation of high-energy ultrashort THz pulses

Speaker: Andrei G. Stepanov, GAP-Biophotonics, Université de Genève, Switzerland

Location: CFEL Seminar room V (DESY Bldg. 99, 01.109)

During the last 10 years, significant progress has been made in the development of high-power THz sources. Today, near-single-cycle THz pulses with energies up to 100 μJ can be obtained at large-scale accelerator facilities [1]. Moreover, several table-top techniques for high-power THz generation based on the use of femtosecond laser pulses have been developed [2]. High-energy (up to 125 μJ [3]) singlecycle pulses with average frequency below 2 THz were generated through optical rectification of femtosecond laser pulses with tilted pulse fronts. This technique provides a photon conversion efficiency more than 100% due to cascaded χ(2) nonlinear processes [4]. A further increase of the efficiency of THz generation though optical rectification of near infrared femtosecond laser pulses requires to provide a control of the cascaded nonlinear processes.

A few table-top techniques have been developed for the generation of high-energy ultrashort THz pulses with average frequency above 10 THz [2]. Tremendous progresses have been made in the generation of broadband (up to 75 THz) highenergy THz pulses using two-color pumping of a gas plasma [5]. In spite of these progresses, there is still an absence of high-power table-top sources in the 3–9 THz frequency range. High-energy ultrashort THz pulses in this region are desirable for many scientific applications, in particular, for nonlinear probing of latticedynamics in polar semiconductors [6].

Recently we proposed the use of a DAST/SiO2 multilayer structure for efficient generation of near-single-cycle electromagnetic pulses with average frequency of about 6 THz through optical rectification of 800-nm femtosecond laser pulses [7]. This multilayer structure allows compensation for the phase mismatch that appears in THz generation in a bulk DAST crystal. Moreover, it provides an opportunity for an additional increase of the THz generation efficiency by controlling cascaded χ(2) nonlinear processes.

Figure: DAST/SiO2 multilayer structure for efficient generation of near-single-cycle 6 THz pulses.

[1] M. C. Hoffmann, S. Schulz, S. Wesch, S. Wunderlich, A. Cavalleri, and B. Schmidt, Opt. Lett. 36, 4473 (2011).
[2] M. C Hoffmann and J. A. Fülöp, J. Phys. D 44, 083001 (2011).
[3] J. A. Fülöp, L. Pálfalvi, S. Klingebiel, G. Almási, F. Krausz, S. Karsch, and J. Hebling, Opt. Lett. 37, 557 (2012).
[4] M. Cronin-Golomb, Opt. Lett. 29, 2046, (2004).
[5] Ki-Yong Kim, J. H. Glownia, A. J. Taylor, G. Rodriguez, IEEE J. Quantum Electron. 48, 797 (2012).
[6] E. Roman, J. R. Yates, M. Veithen, D. Vanderbilt, and I. Souza, Phys. Rev. B 74, 245204 (2006).
[7] A. G. Stepanov, L. Bonacina, and J.-P. Wolf, Opt. Lett. 37, 2439 (2012).
Host: Stefan Kaiser, MPSD-CMD


October 24th, 2012, 11.00 am

New deformable mirror technologies

Speaker: Stefano Bonora, LUXOR Lab of the Institutes of Phototonics (CNR-IFN Italy)

Location: Seminar Room IV, CFEL (DESY Bldg. 99, 01.111)

Nowadays adaptive optics (AO) is a very powerful technique for several scientific and technological experiments. Successful applications of deformable mirrors were achieved in our laboratory in fields such as: ultrabroadband parametric amplifiers compression and shaping in the NIR and mid-IR, high order harmonics generation, microscopy, high peak power lasers and adaptive time preserving monochromators. Although many researchers have been working in the last decade to the realization of new DMs, the universal component has not been found yet. Therefore each of those applications require an ad-hoc deformable mirror. The properties and design rules of each DM will be described together with the experimental methods and results. The resistive electrodes deformable mirror and the photocontrolled DM recently developed in our laboratory will be described in detail.

Host: Michael Först, MPSD-CMD


October 22nd, 2012, 11:30 am

Competing instabilities at paraelectric/superconducting interfaces

Speaker: Michael Fechner, Materials Theory ETH Zürich

Location: Seminar Room V, CFEL (DESY Bldg. 99, 01.109)

The discovery of the new class of iron based high-Tc superconductors in 2008 lead to an enormous increase of research in this field of solid state physics. In particular the origin of the superconductor pairing mechanism is of high interest, since its identification may allow the designing new superconductors. In order to assist the clarification of this question we follow recent experiments of ultrathin FeSe films on SrTiO32 to investigate from first principles the influence of strain and interface effects on the electronic structure of FeSe. First we compare our calculated electronic properties of coherent FeSe films under different tensile strains, corresponding to SrTiO3 and MgO substrates, and electron/hole doping with experimental findings. The main result is that for moderate applied strain the spin density wave in FeSe is suppressed, whereas there is a sudden strong enhancement for larger strain. Given that superconductivity disappears in highly strained FeSe on MgO, our results thus give an interesting insight in which energy range the SDW still compete with superconductivity. The results of the comparison are further discussed with respect to the possibility of phonon pumping superconductivity in FeSe. In the last part we discuss the FeSe/STO superlattice, focusing on the competition of lattice and magnetic instabilities. The results are then discussed with respect to the conclusions made for coherent films. Finally we argue how only the interface termination of FeSe/SrTiO3 superlattice determines the possibility of superconductivity within this system.

1. Kamihara, Y., Watanabe, T., Hirano, M. & Hosono, H. Iron-Based Layered Superconductor La[O 1-xF x]FeAs ( x= 0.05−0.12) with Tc= 26 K. Journal of the American Chemical Society 130, 3296–3297 (2008).
2. Wang, Q.-Y. et al. Interface-Induced High-Temperature Superconductivity in Single Unit-Cell FeSe Films on SrTiO 3. Chinese Phys. Lett. 29, 037402 (2012).

Host: Andrea Cavalleri, MPSD-CMD

Max Planck Symposium

October 11th-12th, 2012

Frontiers in X-Ray Science

Peter Abbamonte
Giacomo Ghiringhelli
Siegfried Glenzer
Gianluca Gregori
Oliver Gessner
Albert Stolow

Location:  CFEL Seminar Room, DESY-Bldg. 99



September 26th, 2012

Atom Transport in Optical Lattices

Speaker: Dieter Jaksch, Clarendon Laboratory, University of Oxford

Location: Seminar Room, DESY Bldg. 90 (ZOQ)

The transport and coherence of atoms in optical lattices are strongly influenced by their interactions with a background gas which provides a reservoir to dissipate energy [1].
I will discuss two possible resulting applications: (i) quantum transport through lattices with an engineered and well controlled phonon bath, for example enabling the realization of nonmarkovian atom-bath couplings; and (ii) measuring transport properties for distinguishing different quantum phases of the background gas. I will also investigate the influence of dephasing on transport in a strongly interacting fermionic lattice gas and describe a many-body mechanism that leads to negative differential conductivity in these systems.
In addition I will briefly describe the main features of tensor network theory which is our main numerical tool for these studies.

[1] T. H. Johnson, S. R. Clark, M. Bruderer and D. Jaksch, "Impurity transport through a strongly interacting bosonic quantum gas", Phys. Rev. A 84, 023617 (2011).

Host: Andrea Cavalleri, MPSD-CMD

MPSD Seminar

August 27th 2012, 15:15h

Magnetism on the timescale of the exchange interaction: explanations and predictions


Speaker: Jacob Burgess, Department of Physics, University of Alberta + National Institute for Nanotechnology, Edmonton, Canada

Location:  DESY Building 49, seminar room 108

Significant interest has been shown in the interaction of topological defects such as domain walls and magnetic vortices with defects or fabricated pinning sites. With that motivation, the work presented here applies highly sensitive nanomechanical torsional magnetometry in conjunction with magneto-optical techniques and simulation, to develop a robust analytical description of a pinned vortex in a thin magnetic disk. Nanomechanical torsional resonators allow detection of minute magnetization changes as the vortex shifts through pinning potentials. Moreover, the fast acquisition time of the technique permits observation of low speed (~few ms) thermally activated dynamics as the vortex hops from site to site. Description of these results necessitates improvement in analytical modeling of the vortex. Aided by simulation and magneto-optical Kerr susceptometry measurements, a robust model that accurately describes the pinned vortex position and the overall disk magnetization was developed. The model enables description of the thermal dynamics and quantitative extraction of the pinning potential. The technological relevance of torsional magnetometry combined with pinning is then demonstrated by the addition of artificial pinning sites, forming a proof of principle magneto-mechanical logic device.

Host: Stefan Loth, MPSD-DNES, CFEL

MPSD Seminar

July 25th 2012, 15:15h

Magnetism on the timescale of the exchange interaction: explanations and predictions


Speaker: Johan H. Mentink, Radboud University Nijmegen, Institute for Molecules and Materials, Nijmegen, the Netherlands

Location:  DESY Building 49, seminar room 108

Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. However, rather little is known about the behaviour of spins in a magnetic material directly after being excited on a timescale equivalent to or faster than that corresponding to the exchange interaction (10–100 fs), that is, in a non-adiabatic way. After the first demonstration of ultrafast laser-induced demagnetization in ferromagnetic nickel [1], many intriguing observations have been reported on magnets with multiple magnetic sublattices, including ultrafast changes of the anisotropy [2] and magnetization reversal [3]. Nevertheless, the theoretical understanding of ultrafast laser-induced spin dynamics is still a challenge. In particular, so far the role of the exchange coupling between different magnetic sublattices has largely been ignored. In this contribution we present a general theoretical framework for the analysis of ultrafast longitudinal spin dynamics in multi-sublattice magnets [4]. We distinguish relaxation of relativistic and exchange origin and show that when the former dominates, non-equivalent sublattices have distinct dynamics despite their strong exchange coupling. Even more interesting, in the exchange dominated regime sublattices can show highly counter-intuitive transitions between parallel and antiparallel alignment. Moreover, our theory predicts that exchange relaxation enhances the demagnetization speed of both sublattices only when they are antiferromagnetically coupled.

[1] E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996).
[2] A. V. Kimel et al., Nature 429, 850 (2004).
[3] C.D. Stanciu et al., Phys. Rev. Lett. 99, 047601 (2007).
[4] J.H. Mentink et al., Phys. Rev. Lett. 108, 057202 (2012).

Host: Martin Eckstein, MPSD-THEO, CFEL


June 22nd 2012

Solids in Ultrafast and Strong Optical Fields


Speaker: Mark I. Stockman, Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30303, USA

Location:  FLASH Hall Seminar Room, DESY-Building 28c

This talk will consider phenomena in insulator nanofilms and bulk crystals subjected to strong and ultrafast optical fields with carrier frequency much below the bandgap. Such fields cause adiabatic phenomena such as the Wannier-Stark localization, formation of quantum bouncers at the surfaces, and anticrossings of adiabatic levels. In the dielectric nanofilms subjected to sufficiently slow fields, the anticrossings of the quantum-bouncer levels of the valence and conduction bands is predicted to lead to adiabatic metallization of the solid1. In the ultrafast optical fields, a combination of the adiabatic and nonadiabatic effects leads to the increased polarizability of the system making it similar to semiconductors or plasmonic metals2. We will also discuss response of a dielectric solid to near-single cycle strong optical fields, where new theoretical and experimental results have been recently obtained in collaboration with MPQ at Garching.

1 M. Durach, A. Rusina, M. F. Kling, and M. I. Stockman, Metallization of Nanofilms in Strong Adiabatic Electric Fields, Phys. Rev. Lett. 105, 086803-1-4 (2010)

2 M. Durach, A. Rusina, M. F. Kling, and M. I. Stockman, Predicted Ultrafast Dynamic Metallization of Dielectric Nanofilms by Strong Single-Cycle Optical Fields, Phys. Rev. Lett. 107, 0866021-5 (2011)

Host: Andrea Cavalleri, MPSD-CMD, CFEL


June 11th 2012

Picosecond photocurrents and terahertz generation in nanoscale circuits


Speaker: Alexander W. Holleitner, Walter Schottky Institute and Physik-Department, TUM, Garching, Germany

Location:  DESY Building 49, seminar room 108

The time-resolved dynamics of photogenerated charge carriers in nanoscale systems are typically detected by optical methods such as the transient absorption technique and the time-resolved photoluminescence spectroscopy. Many questions remain concerning the separation and transport of photogenerated charge carriers to source and drain leads, when the nanosystems shall be functional modules in (opto-)electronic circuits. Typical propagation times of ballistic photogenerated charge carriers in nanoscale circuits are in the ps-regime. Conventional electronic measurements cannot resolve such ultrafast dynamics because available electronic equipment cannot produce trigger signals and detect transients faster than tens of picoseconds. Furthermore, nanosystems typically exhibit a high impedance of several kilo-ohms, and ultrafast charge-carrier dynamics are therefore obscured by the response time of the high-frequency circuits. We recently introduced an experimental on-chip pump/probe scheme to measure the photocurrent dynamics of electrically contacted nanosystems with a picosecond time-resolution [1]. In the talk, I highlight the picosecond photocurrent dynamics in semiconducting nanowires and graphene [2,3]. In particular, our ultrafast experiments clarify the optoelectronic mechanisms contributing to the photocurrent generation at graphene-metal interfaces. We verify that both built-in electric fields, similar to those in semiconductor-metal interfaces, and a photo-thermoelectric effect give rise to the photocurrent at graphene-metal interfaces at different time scales. Our results open the possibility to design and fabricate graphene-based and nanowire-based ultrafast photodetectors, photoswitches, photovoltaic cells, and THz-sources. [1] L. Prechtel, L. Song, S. Manus, D. Schuh, W. Wegscheider, A.W. Holleitner, Nano Lett. 11, 269 (2011). [2] L. Prechtel, M. Padilla, N. Erhard, H. Karl, G. Abstreiter, A. Fontcuberta i Moral, A.W. Holleitner, Nano Lett. accepted 10.1021/nl300262j (2012). [3] L. Prechtel, L. Song, P. Ajayan, D. Schuh, W. Wegscheider, A.W. Holleitner, Nature Comm. 3, 646 (2012).

Fig.: Graphene is incorporated into a coplanar stripline circuit. A pump laser pulse focused onto the graphene-sheet generates a photocurrent jphoto. The time-resolved photocurrent is measured with a probe pulse focused onto an ultrafast Auston-switch as a function of the time delay.

Host: Stefan Kaiser, MPSD-CMD, CFEL


MPSD Seminar

May 25th 2012, 11:00h

Kondo effect in the presence of a spin-polarized current


Speaker: Deung-Jang Choi, Institut de Physique et Chimie des Matériaux de Strasbourg, CNRS, Université de Strasbourg, France

Location:  DESY Building 49, seminar room 108

The Kondo effect occurs when a localized magnetic moment is screened by the spins of the host metal electrons. Below a typical temperature known as Kondo temperature, this many-body interaction results in the emergence of a resonance in the density of states located near the Fermi level. For more than a decade, such a resonance has been investigated by transport measurements in single Kondo impurities, consisting of magnetic atoms or artificial quantum dots (QD). Of particular interest for the emerging field of spintronics is the interaction of single Kondo impurities with ferromagnetic leads, where a splitting of the Kondo resonance is predicted. Several experimental and theoretical studies have been published on QDs, but are still lacking for single magnetic atoms. Here, we present the first low-temperature STM measurements showing a Kondo splitting of a single atom in the presence of a spin-polarized current. A cobalt atom on the Cu(100) surface presents a Kondo resonance, which we are able to split by approaching a magnetic tip covered by copper. The original aspect of this study is to use copper as a spacer between the magnetic tip and the Co atom to minimize the magnetic direct couplings. With the additional support of equation of motion (EOM) calculations, we show that the splitting is produced mainly by the spinpolarized current flowing across the junction. We also evidence that the Kondo splitting is weakened when a direct ferromagnetic coupling is present. This study demonstrates the impact of magnetic interactions and of the spin-polarized current in the Kondo effect.

Host: Sebastian Loth, MPSD-DNES, CFEL

MPSD Seminar

May 11th 2012, 11:00h

Tuning Magnetic Interactions in Semiconductors by STM


Speaker: David Gohlke, Ohio State University

Location:  DESY Building 49, seminar room 108

Dilute magnetic semiconductors have the electrical properties of semiconductors while allowing for magnetic alignment of spins. In particular, manganese-doped gallium arsenide is a ferromagnetic semiconductor with a Curie temperature of ~200K. These manganese dopants act as electron acceptors while preserving a magnetic moment. Low-temperature (5K) scanning tunneling microscopy (STM) allows us to study these dopants individually. By moving point charges with atomic precision, we adjust the binding energy of single acceptors embedded into the surface, and tune the interaction between multiple acceptors. Because of the anisotropy of the zincblende crystal lattice of GaAs, the magnetic coupling between Mn dopants can be ferromagnetic or antiferromagnetic depending on the orientation of the acceptors. Control of this magnetic interaction will lead to deeper understanding of these dilute magnetic semiconductors, hopefully leading to designs for spintronic materials that can function at room temperature.

Host: Sebastian Loth, MPSD-DNES, CFEL


March 29th 2012, 14:30h

Resonant x-ray scattering investigations of high-temperature superconductors


Speaker: Mathieu Le Tacon, Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany

Location:  FLASH Hall Seminar Room, DESY-Building 28c

In Motivated by the search for the mechanism of high-temperature superconductivity, an intense research effort has been focused on the evolution of the spin excitation spectrumupon doping from the antiferromagnetic (AF) to the superconducting states of the cuprates. Because of technical limitations, however, the experimental investigation of doped cuprates has been largely focused on low energy excitations (<100 meV) in a small range of momentum space. We have used Resonant Inelastic X-ray Scattering (RIXS) to show that a large family of superconductors, encompassing the model compound YBa2Cu3O7, exhibits damped spin excitations - paramagnons - with dispersions and spectral weights similar to those of magnons in undoped AF cuprates over much of the Brillouin zone. This is in excellent agreement with the spin excitations obtained by exact diagonalization of the t-J Hamiltonian on finite-sized clusters. A numerical solution of the Eliashberg equations based on our experimental findings for YBa2Cu3O7 reproduces its TC within a factor of two.


The discovery of a well-defined, surprisingly simple spin excitation branch over a wide range of doping levels thus strongly supports magnetic Cooper pairing models for the cuprates. In parallel to the observation of magnetic excitations, we found, in underdoped compounds, a clear enhancement of elastic scattering around incommensurate wave vectors (0.31,0,L) and (0,0.31,L) indicative of the coexistence of charge ordering and superconducting state. Further investigation revealed that the charge order survives in the pseudogap state of these underdoped compounds, and indicates a competition between the superconducting instability and charge ordering.

Host: Andrea Cavalleri, MPSD-CMD, CFEL


March 23rd 2012, 14:30h

Quasi-particle Theory of Strongly Correlated Lattice Bosons:
Application to the Bose-Hubbard Model


Speaker: Michael Buchhold, Goethe University Frankfurt a.M.

Location:  Seminar Room 13, DESY-Building 222

We present a quasi-particle theory for strongly interacting lattice bosons, which, in contrast to Bogoliubov theory, is also valid for strong depletion of the condensate and in the Mott insulating phase. The derivation is based on the linearization of the equations of motion for a Gutzwiller variational state and a quantization of the classical amplitudes of the resultant excitations.

Within this theory of non-interacting quasi-particles, we calculate the single-particle spectral function of the single-band Bose-Hubbard model. In addition to the gapless Bogoliubov mode, we also clearly resolve the amplitude mode, which becomes particularly relevant in the strongly correlated regime, in the vicinity of the Superfluid-Mott transition.

Additionally, we set up a perturbation theory based on the ladder approximation for dilute Bosonic gases, to obtain the finite lifetime of the quasi-particles as well as the spectral broadening. This theory recovers the well known Beliaev-Popov perturbation theory for Bose condensed systems in the limit of weak interactions.



Host: Martin Eckstein, MPSD-THEO, CFEL


February 20th 2012, 14:30h

Microwave cooling of the Josephson phase


Speaker: Marco Aprili, Laboratoire de Physique des Solides - CNRS, Orsay, France

Location:  FLASH HALL (28c) - Seminar Room


The "friction of light" has been used for cooling atoms, ions and recently mechanical oscillators.Indeed, the radiation pressure in a Fabry-Perrot interferometer in which one of its two mirrors vibrates can reduce the Brownian motion of the vibrating mirror and hence its effective temperature. This is because the radiation pressure on the mirror depends on its position so that the mirror motion and the field in the cavity are dynamically coupled. Negative and positive detuning lead to higher and lower damping of the mirror oscillations, respectively.

As a consequence detuning acts on the mirror amplitude oscillations. Surprisingly, increasing the optical power introduced in the cavity can substantially "cool" or "heat" its vibrating mirror. Although less intuitive the phase difference of the wavefunctions of two weakly coupled superconductors is also a macroscopic degree of freedom. When coupled with a high quality cavity, the oscillations of the Josephson phase produce sidebands in the cavity modes (Fig.b). Vice versa, the intracavity field (Fig.a) in return acts on the phase dynamics as the radiation pressure does on a vibrating mirror.

Therefore, negative or positive detuning lowers or increases the phase effective temperature, respectively (Fig.c). In our experiments the phase effective temperature is obtained directly by measuring the width of the distribution of the junction critical current. As shown in Fig.c the width of the histograms is reduced, and hence the phase temperature by increasing the microwave power in the negative sideband.

Sideband cooling of the Josephson phase associated to an extended Josephson junction which supports electromagnetic cavity modes in the insulating barrier.
a) Artistic view of the junction and of a Fabry-Perot interferometer.
b) The relative change in the switching current Is was measured as a function of the externally imposed microwave frequency around the first cavity mode. This cavity spectroscopy shows sidebands.
c) Shrinking of the
switching histograms with increasing microwave power at negative sideband. Narrower histograms reveal the induced cooling, Teff, of the Josephson phase through anti-Stokes scattering of the microwave photons.

J. Hammer, M. Aprili, and I. Petkovic Phys. Rev. Lett. 107, 017001 (2011)


Host: Andrea Cavalleri, MPSD-CMD, CFEL


February 15th 2012, 15:00h

Formation Mechanism of Metal-Organic Interface States studied by 2-Photon-Photoemission-Spectroscopy


Speaker: Manuel Marks, Philipps Universität Marburg, Fachbereich Physik und Zentrum für Materialwissenschaften, Marburg

Location:  Seminar Room 108, DESY Bldg. 49

For functional devices in organic electronics the charge carrier injection from the metal electrodes into the active organic regions to a large extend determines the device efficiency. These dynamical processes strongly depend among others on the energetic alignment of molecular states and the metallic Fermi level EF as well as newly emerging interface states due to the chemical bonding or the modified symmetry at the interface. They can best be investigated by applying time- and angle-resolved 2-photon photoemission (2PPE) to study structurally well characterized organic thin films adsorbed on single crystal metal surfaces.

Interface states that are located between the Fermi level of the metal and the LUMO level of organic molecules are expected to have a decisive influence on the charge carrier injection across a metal-organic interface. Such an interface state was characterized recently for the interface between monolayer films of the organic semiconductor 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) and a Ag(111) surface [1]. The Shockley State maintains its metal character but is shifted in energy and forms an unoccupied strongly dispersing state with a short inelastic lifetime of 54 fs. In order to gain insight into the mechanisms that lead to the formation of such interface states and to understand their properties we have investigated 1,4,5,8-naphtalene tetracarboxylic acid diandydride (NTCDA) which is strongly related to PTCDA but has a weaker interaction the Ag(111) substrate. The opposite case of a stronger bound molecular layer can be studied for the disordered low-temperature phase of PTCDA [2]. For both interfaces, we observe similar interface states that emerge from the Shockley-surface state due to the interaction of the first molecular layer with the Ag(111) substrate. By combining our 2PPE investigations on the different model systems with density functional theory calculations, we are able to extract the fundamental physical processes that determine the energetic position of these Shockley-derived interface states and their overlap with molecular states [3].

[1] C. H. Schwalb et al., Phys. Rev. Lett. 101, 146801 (2008).
[2] L. Kilian et al., Phys. Rev. Lett. 100, 136103 (2008)
[3] M. Marks et al., Phys. Rev. B 84, 081301(R) (2011)


Host: Sebastian Loth, MPSD, CFEL

CFEL General Seminar:

February 7th 2012, 14:15h

Theory for strongly correlated systems out of equilibrium


Speaker: Martin Eckstein, Theory for strongly correlated systems out of equilibrium group, MPSD, CFEL

Location:  FLASH HALL (28c) - Seminar Room

In correlated materials, exotic quantum states can emerge from the interplay of electrons, spins and lattice at energy scales far below the bare Coulomb repulsion and the Fermi energy. Numerical approaches such as continuous-time quantum Monte Carlo and dynamical mean-field theory have been developed to solve the relevant microscopic models in a wide parameter range. In some cases, they even allow us to make ab-initio predictions for the properties of correlated materials. Recent experimental developments call for a generalization of these methods to nonequilibrium: For example, ultrashort light pulses can be used to drive correlated systems into nonequilibrium phases through the selective excitation of certain phonon modes. In my talk, I report on the effort to develop numerical tools to study the time-evolution of correlated systems within dynamical mean-field theory. I will discuss two applications: photo-doping into a Mott insulator, and the damping of Bloch oscillations in the Hubbard model.



February 6th 2012, 14:30h

Nonlinear current response of interacting fermions


Speaker: Marcin Mierzejewski, University of Silesia, Poland

Location:  Seminar Room 108, DESY Bldg. 49

Nonlinear real-time response of interacting particles will be discussed on the example of a one-dimensional tight-binding model of spinless fermions driven by strong electric field.

It will be demonstrated that for a generic (metallic or insulating) systems at high temperatures the major nonlinear effects can be accounted by internal heating.

For such quasiequilibrium evolution a simple extension of the linear response theory allows one to calculate the real-time current without a formal solution of the real-time problem. For stronger electric field this qausiequilibrium regime terminates and the Bloch oscillations set on.

The anomalous nonlinear response of the integrable systems will also be briefly discussed.

Host: Martin Eckstein, MPSD-THEO, CFEL


January 31st 2012, 11:00h

XUV Photoemission in the fs-regime


Speaker: Timm Rohwer, Institut f黵 Experimentelle und Angewandte Physik, Christian-Albrechts-Universit鋞 zu Kiel

Location:  Seminar Room 108, DESY Bldg. 49


Angle Resolved Photo-Electron Spectroscopy (ARPES) has emerged as a leading technique in identifying static key properties of complex electron systems. In a pumpprobe scheme using femtosecond light pulses this technique can be extended to monitor ultrafast transients in the electronic core levels and at particularly large momenta of the valence band structure. In this contribution I will present two different experimental setups for time-resolved photoemission spectroscopy using femtosecond XUV pulses. The relevant details and specification of the systems such as time- and energy resolution or XUV photon flux will be discussed.

Using this technique we can provide novel insights into the relative roles that the various factors play in charge-density wave (CDW) formation. Charge-density waves are broken-symmetry states of low-dimensional solids that are brought about by strong electron-phonon interaction. They are a classical paradigm of condensed matter physics. Yet, surprisingly, their microscopic origin in real materials is still poorly understood. Apparently, a more successful explanation has to take into account the delicate balance between several factors including not only electronic and phononic structure, but also electron-electron (electron-hole) and electronphonon interactions.

We will focus on three conspicuous CDWs within prominent members of the family of layered transition-metal dichalcogenides: the (2󫎾) CDW in the possible excitonic insulator 1T-TiSe2 [1], the (v13譾13) CDW in the Mott insulator 1T-TaS2[2] and the c(2v34)rect. CDW in the Peierls insulator RbxTaS2 [3]. Our particular goal is to reveal the relative importance of electronic (excitonic) or phononic contributions to each CDW transition by relating measured breakdown and equilibration dynamics of CDWinduced spectral features to typical elementary time scales in layered compounds.

[1] T. Rohwer et al. , Nature 471, 490 (2011)
[2] S. Hellmann et al. Phys. Rev. Lett. 105, 187401 (2010)
[3] K. Rossnagel et al. Phys. Rev. Lett. 95, 126403 (2005)

Host: Andrea Cavalleri, MPSD-CMD, CFEL

CFEL Colloquium:

January 18th 2012, 15:00h

Observing and Controlling Quantum Matter at the Single Atom Level


Speaker: Immanuel Bloch, Max-Planck Institut für Quantenoptik & Ludwig-Maximilians Universität, München

Location:  FLASH HALL, Seminar Room (28c)

The realization of ultracold quantum gases at Nanokelvin temperatures has marked a milestone in modern quantum physics. With the help of laser light, these ultracold atom clouds can be stored in artificial periodic potentials created by laser light - so called optical lattices - that allow us to explore fundamental aspects of strongly interacting fermionic and bosonic quantum matter. In very recent experiments, we have been able to record single snapshots of a quantum fluid in which individual atoms are detected with single lattice site resolution. These open unprecedented novel opportunities for analyzing and manipulating strongly interacting quantum system. In my talk, I will review some of the recent experiments on strongly correlated quantum gases in optical lattices and highlight connections to condensed matter physics, quantum information science and atomic- and molecular physics.

Host: Andrea Cavalleri