Shaken, not stirred: Bose-Einstein condensates under time-periodic forcing
Source: Talk by Prof. Dr. Martin Holthaus, Universität Oldenburg on 10th Feb 2010
In a very persuasive talk delivered by Prof. Dr. Martin Holthaus from Universität Olderburg during the Physics colloquium at Jacobs University, highlighted the major breakthrough in the field of Bose-Einstein (BE) condensates. What is such fascination with Bose-Einsteins condensates? Why BE condensates cooled at ultra low temperature occupy the lowest quantum state? Can we experiment and simulate the BE condensates? What are the approaches towards implementation of quantum simulators? were the major topics of discussion during the talk. Speaker explained the time periodic forcing of ultra cold atoms as the optical technique for answering every such above questions. Prof. Holthaus on collaboration with University of Pisa carried out the dynamic localization of mesoscopic systems, by time-periodic forcing of condensates within a shaken optical lattice. They carried out the experiment by shaking a 3D optical lattice by the laser source and mechanical shaking of mirrors.
When bosons cooled to ultra cold temperature (nearly to 0 °K or -273.16 °C), large fraction of particles occupy the lowest quantum state, we can feel the larger scale quantum effect. Previously, quantum effect was visible only in the particle level and superconducting systems, now whole mass of particle exhibit the quantum effect in ultra cold mesoscopic systems. This effect termed as Bose-Einstein condensation was predicted by Satyendra Nath Bose and Albert Einstein in 1924.
The spreading of a wave packet of cold atoms in an optical lattice is described by Bloch waves, similar to electrons in a crystalline lattice. For Bose-Einstein condensate of 87Rb atoms in a shaken optical lattice gives access to a new form of quantum state engineering. Cold atoms in optical lattices could mimic the physics of solids, and they are often more manageable: because the lattice is clean, tunable, and can be manipulated almost at our own will. For instance, one can apply a driving force that shifts the optical lattice periodically in time. Similar to a lattice that is periodic in space giving rise to a quasi-momentum, a lattice that is periodic in time also introduces quasi-energy. If we can modify the Bloch band of atoms in controlled way then we can make the Bloch band completely flat, which is equivalent to making the effective mass of atom infinite, which will suppress the tunneling of atoms in the lattice. This effect is termed as the “dynamical localization.”
Professor also outlined the huge future scope of mesoscopic quantum as the possible realization of quantum effect in larger scale which will avenue a new era towards the implementation of quantum computation, development of quantum computers and quantum simulators. But the major difficulties which have to be addressed include: Ultracold temperature, frequency parameters in 3D systems, and highly coherent LASERs etc. for the experiment.
Nanostructured Thermoelectrics - Environmentally Friendly Power from New Materials
Source: Talk By Prof. Dr. Kornelius Nielisch, University Hamburg on 24th Feb 2010.
Thermoelectronic (TE) cooling and power generating devices has shown many promising applications like vibration free energy source applied on micrometer scales. Due to their low efficiency their application rage has been limited for quite long period. Fortunately, extensive research in the field of nanostructured TE devices these days has shown relatively higher thermoelectronic efficiencies. Prof. K. Nielsisch from University of Hamburg has reported the enhancement of TE efficiency by twice to thrice fold in low dimensional TE materials, such as 0D (Quantum dots) and 1D (Nanowires) and disordered nanocomposites.
In the TE devices like in the side, TE power generator is made of n-type and p-type composites placed in a row. When there is the temperature difference between the corresponding n and p-composites, electron flows in one direction and holes the other direction taking heat away from one side of the device.
Fig: TE power generation (Peltier effect)
Figure of merit of TE materials is represented as: ZT = S T σT / κ, where, S is the Seebeck coefficient, the electrical Conductivity σ, the specific heat so- conductivity κ, describes the TE efficiency of the device.
During the talk, Prof. Nielisch presented how the presence of nanorods or disordered nanocomposites can increase the ZT of TE devices. From above equation, it is very clear that ZT can be increased by either increasing the electrical conductivity and by decreasing the thermal conductivity (κ). He also proposed many approaches which includes nanorods composites for the solid state materials and doping the existing materials. Nanorod improves elektrischer Leitfähigkeit und ZT-Steigerungen in electrical conductivity and increases in the ZT, since the chargeträger in zwei Dimensionen frei bewegen können, was carriers in two dimensions can move freely. Nanocomposites add another layer of complexity because they contain many interfaces with spacing smaller than the phonon mean free path, which introduces a thermal boundary resistance between different regions of the nanocomposites. While researchers have been able to use thermal boundary resistance to obtain a low thermal conductivity, thermal boundary resistance has also been identified as the key mechanism for the low thermal conductivity in superlattices.
Improved electrical conductivity and reduction of thermal conductivity can be attained by increasing the impurity, which leads to increased ZT. ZT = 0.7 for bulk p-type PbTe which can be increased to ZT = 1.5 for 2% Tl-doped PbTe. TE power generation has huge potential in the nanodevices as the source of energy, next generation solar cells, vibration free coolers and power source in modern electrical vehicles.
Analysis and modeling of charge transport properties in organic field-effect transistors
Source: PHD Defense by Dr. Benedikt Geburek, Jacobs University Bremen on 30th May 2010
In an expressive PHD Defense talk by Dr. Beneditkt Geburek at Jacobs University Bremen, a very Nobel class of electronic device was discussed, which applied organic semiconductors rather than conventional doped Silicon semiconductors. In 1986, when first Organic Field Effect transistors was developed, was an avenue in the field of cheap and flexible electronics which was never been thought of applying conventional semiconductors. It was very fascinating to see tons of application of Organic devices which includes flexible electronics devices, cheap solar cells, printed RFID tags and organic LEDs. These days a new branch of electronics has evolved: namely organic electronics.
Dr. Geburek analyzed charge transport properties in disordered systems applying following standard models like: Multiple trapping and release, Vissenberg Matters Models, Limketkei, Bässler and related model, Coehoorn and Numerical master equation approach models. He prepared number of top-gate p-type Organic FETs, using P3HT (poly(3-hexylothiopene)) as organic semiconductors on the plastic substrate. Then, taking consideration of different models he measured the parameters of the Organic FETs, like carrier mobility and threshold voltage. Finally, those parameters were again measured under variation of temperature and semiconductor layer thickness for comprehensive description. The experimental and theoretical results were compared and verified.
Necessity of clear understanding of working mechanism is the ultimate goal to address the demerits of any system. Understanding the transport property of carriers within an organic semiconductor has always been the major difficulties, which was in some context addressed during the talk. Low efficiency and low stability of organic semiconductors are the major hindrance faced by the researchers in the field of organic semiconductors; Dr. Geburek’s research was entirely dedicated towards development of stable organic device.
He concluded that, the linear increment of output current with gate voltage along the linear regime and quadratic increment of output current with gate voltage along the saturation regime of operation is due to the charge carrier density dependence on the charge carrier mobility. The overall quality of transistor channel as expressed my carrier mobility is exponentially dependent on the randomness of semiconductor or width of hopping of carrier within the channel. Ideally, transistor threshold voltage (channel is totally depleted) is zero, but positive threshold voltage in p-type organic device is due to the presence of residual charge within semiconductor. P-OFETs also exhibits current above the threshold level, which indicates the presence of bulk current, which gives the information of doping of semiconductor and On/Off ratio of current of the device. Larger the On/Off better is the transistor. One major breakthrough from the Dr. Gebureks’s research was the from the finding that mobility of carrier in thin film semiconductor is very low and increases with increase in gate voltage, due to the fact the thin film has very high randomness of material compared with thick film. Hence, OFETs can be improved by increasing the semiconductor thickness. The listeners were exposed to different facets of the transport property of organic semiconductor during the entire talk.
Optical Properties and Application Aspects of Wide-Gap Semiconductor Nanostructures –Quantum Dots, Nanowires, and Laser Structures
Source: Talk By Prof. Dr. Jürgen Gutowski, Universität Bremen on 5th May 2010
In an astounding talk delivered by Prof. Jürgen Gutwowski from University of Bremen mesmerized by his extensive research in Wide Gap Semiconductors during the Physics Colloquium at Jacobs University Bremen. He presented III-V group based semiconductor nanostructures, including quantum dots and epitaxial grown nanowires on Si, which introduces discrete density of states and strong carrier confinement. Nanowire means smaller, faster and single dimensional movement of electrons. When lasers use quantum dots and nanowires as the region of excitation or gain region, superior performance is exhibited.
GaN is the most widely applied III-V group direct band semiconductor having band-gap of 3.4eV, makes its very efficient optoelectronic material. Speaker coined the necessity of Stranski-Krastanow growth method for defect free quantum dot deposition. Many other growth methods can also be applied for self organized quantum dot growth like Molecular Beam Epitaxy (MBE) method or Metal Organic Chemical Vapor Deposition (MOVCD). Nanowires can be grown applying Vapor Liquid Solid (VLS) growth mode.
Professor Gutowski has found astonishing results when LEDs made up of nanowires which were coated with dielectric polymers. Intensity of emitted photon radiation was rapidly increasing in the case of coated nanowires; he described the phenomenon as the surface emission. But, when nanowires were coated with metals the intensity was rapidly decreasing. When metal are attach to the surface of semiconductor they push the carrier away from the band gap (centre of excitation), but polymer rather attracts the free carrier towards the junction of excitation. The interaction of electrons and holes in the junction will lead to high intensity of emitted photon. Speaker concluded that nanowires can be applied as waveguides and LED fabrication on silicon chip called nanophotonics.
Professor and his colleagues at the Institute of Solid State physics, University Bremen fabricated GaN p-n junction diode between the nanowires. GaN being direct band gap material exhibited very good light emission features (LEDs). Professor had also profound interest on II-VI semiconductors like Zinc Oxide (ZnO) and Cadmium Sulphide (CaS) due to their direct band gap features with band gap of 2.42 eV (suitable for quantum dots) and 3.37 eV respectively. His group also observed photonic property in ZnO nanowires which can lead to development of on-chip waveguides, Solar cells and LEDs. It’s quite difficult in doping ZnO with p-type material since it is naturally n-doped. Now-a-days, ZnO as shown promising result as an alternative to poisonous ITO material, for solar cells. The talk was highly acclaimed since entire talk was dedicated towards the actual application of different solid state materials. It was really fascinating talk for all solid state enthusiast.