Archive 2016

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In this very informal talk I will first briefly present our view on superconductivity in alkali doped fullerenes. In particular, I will discuss how it is at all possible that M3C60 has an s-wave condensate despite the sizeable intra-molecular Coulomb repulsion relatively to the narrow molecular bandwidth. In the second part of the talk I will review what it is known about the MIR optical properties of doped fullerenes and try to build a link with the remarkable Hamburg observation of a large enhancement of Tc under intense THz pumping. [more]

Phenomena that are connected to quantum mechanics, such as magnetism, transport, and the effect of impurity atoms and disorder, and their relation to material design and energy needs are important for almost every branch of the industry. Density functional theory (DFT) was successful at making accurate Predictions for many materials, in particular compounds which have a metallic behaviour. DFT combines high accuracy and moderate computational cost, but the computational effort of performing calculations with conventional DFT approaches is still non negligible and scales with the cube of the number of atoms. A recent optimised implementation of DFT was however shown to scale linearly with the number of atoms (ONETEP), and opened the route to large scale DFT calculations for molecules and nano-structures. Nonetheless, one bottleneck of DFT and ONETEP, is that it fails at describing well some of the compounds where strong correlations are present, in particular because the computational scheme has to capture both the band-like character of the uncorrelated part of the compound and the Mott-like features emerging from the local strongly correlated centres. A recent progress has been made in this direction by the dynamical mean-field theory (DMFT), that allows to describe the two limits (metal and insulator) in a remarkable precise way when combined with DFT. The ONETEP+DMFT implementation and strategies to overcome the main bottlenecks of this type of calculations will be discussed, and its applications illustrated by a few case of studies, such as the role of quantum entanglement in Myoglobin and heme systems. [more]

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Soft-X-ray photoelectron spectroscopy from liquid microjets has considerably contributed to an understanding of the electronic structure of aqueous solutions. Quantities that can be obtained include valence and core-level electron binding energies of both water solvent and atomic as well as molecular solutes. [more]

The course provides an overview of the working principles of nonlinear optics with a focus on the basic physical concepts. We start from introducing relevant topics in linear optics (e.g., Maxwell equation, Lorentz model, Crystal optics, Optical pulse and dispersion), and continue to discuss key concepts in nonlinear optics such as phase matching and quasi phase matching, second-order nonlinear effects, and third-order nonlinear effects. [more]

Since 2000, we have been developing a real-time, real-space computationalmethod based on time-dependent density functional theory to describe electron dynamics in crystalline solids induced by light pulses. In a microscopic scale, we solve the time-dependent Kohn-Sham equation in a unit cell of solid treating the applied electric field is by the vector potential. We further combine the microscopic calculation with the dynamics of light electromagnetic field in a multiscale modeling, as describe in the figure. In my presentation, I first explain our method including some historical aspects. Then I will show some recent and on-going applications such as energy transfer from a femtosecond laser pulse to electrons in quartz and graphite, and ultrafast changes of dielectric properties of diamond by an intense laser pulse. [more]

The progress in quantum optics utilizes a unique photon state configuration for engineering of the ultimate light-matter interactions with relatively simple material systems. It results in a broad range of photonic applications including radiation sources, quantum communication, information, computing and nanotechnology. The development of the ultrafast multidimensional nonlinear spectroscopy that has been enabled by progress in ultrafast optical technology provides a unique tool for probing complex molecules, semiconductors, nanomaterials by classical light fields. [more]

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This talk will showcase algorithms that I have developed over the course of my PhD suited to X-ray free electron laser data processing. At present, generating merged datasets to sub-atomic resolution can be tricky at LCLS due to the limitations of the photon energy and detector distance. I will present some of the challenges faced by processing diffraction patterns, collected at LCLS, to generate a 1.15 Å resolution dataset, and how these new algorithms have proved useful to solve them. [more]

The new paradigm of topology in condensed matter physics does not only pertain to insulators and semimetals but also to superconductors. Topological superconductors are predicted to show many novel physical properties. [more]

The physics of the “ultra-fast” and “ultra-small” are often closely linked, motivating experiments that access these extremes. In this talk, I will discuss the application of ultrashort light pulses, from THz to X-rays, to the study of dynamics and emergent correlations in quantum materials. [more]

Molecular strong field ionization (SFI)is at the heart of several ultrafast imaging/spectroscopic techniques such aslaser induced electron diffraction and high harmonic spectroscopy. Whileproof-of-principle experiments are underway, it is still a long way before theycan evolve into a standard imaging techniques. The coupled motion of electronsand nuclei under the influence of a non-perturbative external laser field isnot fully understood yet and there is a strong demand for developing many bodymethods that can model these molecular SFI based processes. [more]

When an exciton transition and a resonant optical mode exchange energy faster than any competing dissipation process, it can lead to light-matter strong coupling. This brings about interesting properties possessed by neither the original exciton nor the optical mode and leads to new possibilities such as enhanced conductivity of organic semiconductors. In this case, the enhancement stems from the delocalized nature of the hybrid states over the spatial extent of the optical mode which is also expected to affect energy transport according to recent theoretical studies. [more]

The discovery of graphene established the possibility of obtaining stable two-dimensional solids, and it was soon realized that layered materials other than graphite can be used as bulk parents for novel two-dimensional materials. Among them, layered transition metal dichalcogenides have attracted considerable attention due to interesting physical properties and potential for electronics applications. While most of the research focused on mechanically exfoliated specimens, many proposed approaches to characterizing these materials, as well as potential applications, require large-area and high-quality samples. These can be achieved by epitaxial growth methods developed in our group. [more]

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Light-matter interactions have been extensively studied by physicist in quantum optics and condensed matter physics, [1] but there are only fewer attempts to understand this effect in molecular science. [2, 3] [more]

Due to experimental advances in the preparation and control of ultra-cold atomic gases, there is a widespread interest in the behaviour of quantum systems out of equilibrium. A common way to probe quantum systems for non-equilibrium phenomena is given by sudden quenches. [more]

In this talk, I will first give an introduction to Majorana bound states (MBS), zero-energy modes predicted to appear in exotic spin-polarized p-wave superconductors. MBS satisfy non-Abelian statistics and, in addition, encode quantum information in a topologically protected manner, which makes them highly interesting for quantum computation applications. The required p-wave superconductivity seems to be hard to find in nature, but recent theoretical works have shown that it can instead be artificially engineered, for example in a semiconductor nanowire with strong spin-orbit coupling which is covered by a superconductor and exposed to a magnetic field. I will also discuss the recent experimental progress towards creating and detecting MBS. [more]

The iridium oxide family of correlated electron systems is predicted to host a variety of exotic electronic phases owing to a unique interplay of strong electron-electron interactions and spin-orbit coupling. There is particular interest in the perovskite iridate Sr₂IrO₄ due to its striking structural and electronic similarities to the parent compound of high-Tc cuprates La₂CuO₄. Recent observations of Fermi arcs with a pseudogap behavior in doped Sr₂IrO₄ and the emergence of a d-wave gap at low temperatures further strengthen their phenomenological parallels. [more]

The high-temperature superconductor YBa₂Cu₃O₇ is a very interesting material for many device applications ranging from wiring for energy transmission and high field magnets to superconducting filters and high frequency electronics in the THz regime. [more]

Competition between ordered phases, and their associated phase transitions, are significant in the study of strongly correlated systems. [more]

TiSe₂ is one of the simplest charge density wave (CDW) materials, forming a 2×2 superlattice below a transition temperature T_C = 200 K, but the origin of this phase is controversial. Its nearly inverted band structure led early authors to identify TiSe₂ as an “excitonic insulator,” which is an electronic instability involving spontaneous proliferation of excitons. The problem is that the CDW also exhibits a sizeable lattice distortion, leading later authors to identify it as a conventional Peierls phase. That said, an excitonic phase would also create an incidental lattice distortion, since the interaction with phonons can’t be switched off. The arguments on the matter have gone in circles for decades. [more]

Monolayer transition-metal dichalcogenides such as MoS₂ and WS₂ are prime examples of atomically thin semiconducting crystals that exhibit remarkable electronic properties. They have a pair of valleys that can serve as a new degree of freedom, but these valleys are energetically degenerate, protected by time-reversal symmetry. [more]

A characteristic of ferroic materials is the emergence of a temporally static finite expectation value of an order parameter. Here, we introduce a new mechanism [1] for ferroic order, in which a non-zero quasi-static magnetoelectric quadrupolar order appears, mediated by a strong coupling of spin and phonon fluctuations. We show that our proposed mechanism is consistent, to our knowledge, with many experimental observations for the onset of the pseudo-gap phase in cuprate superconductors and therefore propose the quasi-static magnetoelectric quadrupole as a possible pseudo-gap order parameter. By using first-principles calculations in combination with our recent developed formalism [2,3], to calculate multipole moments within a Berry phase approach, we calculate the magnitude of the effect for the the prototypical cuprate superconductor, HgBa2CuO4+δ. Using these results we finally show that our mechanism embraces several key findings of experimental reports and in addition also aspects of previous theoretical models. [more]

In this talk I will discuss the transient dynamics of a d-wave BCS model after a quantum quench of the interaction parameter. The motivation comes from recent ultrafast pump-probe experiments on high-temperature superconductors. [more]

Detailed understanding and control of the intermolecular forces that govern molecular assembly are necessary to engineer structure and function at the nanoscale. [more]

The charge-induced instabilities of conducting and dielectric drops in an electric field have been studied over a century, starting from the seminar works of Lord Rayleigh, J. Zeleny, G. I. Taylor. The significance of the study is found in the presence of charged droplets in jets, electrospray mass spectrometry methods and atmospheric aerosols. In low temperature physics, applications of the confinement of electrons on the surface of liquid helium add new perspective on the significance of the study of the instabilities. [more]

Recent advances in technology and instrumentation have made it possible to generate vortex beams of electrons and neutrons with phase singularities at their cores, where the beam intensity is zero and the phase is undefined. These new types of beams, apart from the spin angular momentum, carry a quantized orbital angular momentum (OAM) along their axes of propagation resulting in a twisted wavefronts, pretty much like quantum tornadoes. The OAM of electron is capable of fundamentally altering the physics of beams and is already being used for application purposes in electron microscopy. [more]

Fluctuations of the atomic positions are at the core of a large class of unusual material properties ranging from quantum para-electricity and charge density wave to, possibly, high temperature superconductivity. Their measurement in solids is the subject of an intense scientific debate focused on the research of a methodology capable of establishing a direct link between the variance of the ionic displacements and experimentally measurable observables. In this presentation I will introduce our new approach to address quantum and thermal fluctuation in complex materials. By means of non-equilibrium optical experiments performed in shot-noise limited regime we could reveal that the variance of the time dependent atomic positions and momenta is directly mapped into the quantum fluctuations of the photon number of the scattered probing light. A fully quantum description of the non-linear interactions between photonic and phononic fields pave the way for a direct measurement of fluctuation in complex systems. [more]

Three-dimensional structures of membrane proteins are of paramount value for understanding protein function on a molecular level. However, in vitro studies such as structure determination are impaired by the necessity to purify membrane proteins with the aid of detergents that often compromise protein stability and function. [more]

The conformational dynamics of a protein are often essential for its function. In this talk I will present how time-resolved X-ray scattering and vibrational spectroscopy can be used to investigate protein dynamics based on two examples: the light sensing protein phytochrome and green fluorescent protein. In particular, I will show how computational chemistry can support the design and analysis of these experiments and how Molecular dynamics simulations can be used to predict the structural stability of small cyclic peptides. [more]

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