|RITA-CT-2003-505313||AR2||11||Spin Current Turbulence||Prof. Yuriy Bunkov is a researcher from Centre de Reserches sur les Très Basses Températures of Centre National de la Reserche Scientifique in Grenoble. He is an expert on nuclear magnetic resonance in the 3He superfluids. One of his current interests is to examine the influence of different order parameter textures and textural defects on various nuclear spin resonance modes of these superfluids. Changes in order parameter textures can be generated in a controlled way by rotating the superfluid. This is the main incentive for prof. Bunkov to perform measurements in uniform rotation. The textural orienting interactions from rotation have two sources: the orienting effect from vortex-free counterflow of the superfluid and normal components or from quantized vorticity, depending on how the situation is prepared.||Prof. Bunkov has been working on a theoretical analysis on the sudden appearance of excessive relaxation losses in a critical temperature regime of an important resonance mode of 3He-B, the so-called Homogenously Precessing Domain (HPD). The HPD mode is currently the most important example of a dynamic coherent order parameter state, where the entire spin magnetization precesses with the drive from the externally applied radio frequency excitation field. The large relaxation in the critical temperature regime is now explained to arise from a mode crossing where the excitation pumping escapes into two new modes. A report on this explanation is under preparation together with two theorists with whom he worked on this problem during his stay at the Low Temperature Laboratory .
At temperatures below the catastrophic loss of the HPD mode a different new resonance mode appears, which is called the persistent spin mode. Prof. Bunkov has also performed measurements on this mode with the rotating cryostat at the Low Temperature Laboratory, to examine the influence of the textural orienting interactions from rotation. It is expected that these studies will reveal the origin of the persistent spin mode in the zero temperature limit. The first results are now under evaluation.
|RITA-CT-2003-505313||AR2||13||New Experimental Techniques for the Investigation of Superfluid Turbulence at Very Low Temperatures||Prof. Wilfried Schoepe has performed extensive research on turbulence in superfluid 4He in the University of Regensburg. His technique to generate turbulence is to oscillate a small magnetic sphere of 100 micrometers in diameter within the superfluid bath above a superconducting plate. If the sphere is charged, it can be driven electrostatically with an oscillating electric field. Prof. Schoepe is interested in the problem of developing a practical method to generate superfluid turbulence in a sample which is at a temperature of 1 millikelvin inside a rotating cryostat. In this way he could study superfluid turbulence as a function of the polarization created by the rotation.||Prof. Schoepe explored different alternatives for a drive of superfluid turbulence in uniform rotation. None of these have yet been tried out, but if the interest in turbulence studies prevails, then one of the possibilities will be selected for further experimenting. Owing to his expertise with oscillating bodies in a bath of superfluid helium, he was immediately interested in ongoing measurements with a quartz oscillator and worked out the interpretation of the measured results, as reported in Ref. .||Physics|
|RITA-CT-2003-505313||AR2||15||Quantized Vortices at the Interface between the A and B Phases of Superfluid 3He||Dr. Richard Haley is a lecturer and researcher from the University of Lancaster. His field of interest is the zero temperature limit of the 3He superfluids. Currently his research is concerned with the physical properties of the AB interface between the A and B phases of superfluid 3He at the lowest possible temperatures. He performs these measurements by creating and manipulating the configuration of the AB interface with specially engineered magnetic field profiles. One of the goals is to understand how a quantized vortex in the B phase interacts with the interface and, in particular, whether a ring incident on the interface is transmitted across, reflected, or remains pinned on the interface. In the Low Temperature Laboratory measurements with known and more regular configurations of vorticles can be carried out in rotating sample becomes. This insight is most useful as a comparison and calibration for Dr. Haley´s research in Lancaster.||Dr. Haley has participated in ongoing studies on the onset of superfluid turbulence. To start turbulence, different energy barriers in series have to be overcome. By cooling to lower temperature and lower vortex damping, the barriers become instabilities and eventually, at sufficiently low temperatures, all barriers disappear and turbulence switches on. Dr. Haley has been examining the first and lowest of these barriers. It controls the onset of the most rapid type of vortex generation via interacting vortex loops. This type of turbulence onset can be triggered by injecting a tight bundle of sufficiently many loops in vortex-free flow. The superfluid Kelvin-Helmholtz instability of the AB interface provides the tools for such injection techniques. These studies are part of Ref. .||Physics|
|RITA-CT-2003-505313||AR2||16||Hydrodynamics of Quantized Vortices in a Long Rotating Column:
Kelvin-Wave Instability and Superfluid Turbulence
|Dr. Rob Blaauwgeers is a young post-doctoral research fellow from the University of Leiden who has participated also at earlier times in joint research projects in the Low Temperature Laboratory. His research concentrates on quantum liquids and solids, but in addition he is particularly interested in low temperature refrigeration and measurement.
The present collaboration is a continuation from his earlier work where he developed a new technique to inject vortex seed loops into vortex–free flow of superfluid 3He-B. This technique is based on the superfluid Kelvin-Helmholtz instability of the first order AB interface between the two stable phases of superfluid 3He, 3He-A and 3He-B, in a two-phase sample under uniform rotation. In a Kelvin-Helmholtz instability event vortex loops are transferred across the AB interface from the A to the B phase. Using the instability one can study the dynamic properties of the injected quantized vortex loops, ie. how the injected vortices evolve as a function of time in the rotating B-phase flow. It was then found that in superfluid 3He-B the vortex damping, the so-called mutual friction dissipation, which has exponential temperature dependence, causes an abrupt hydrodynamic transition of first order. It divides the dynamics of quantized vortex lines into a regular (or laminar) regime at high temperatures above Tonset and to turbulence below. The present studies are a continuation of this work to explore the dependence of the onset temperature on the different controlling parameters.
|We have measured the dependence of Tonset on the applied flow velocity and the number of seed loops injected in the vortex-free superflow. It turns out that Tonset is determined by a single-vortex instability, called the helical Kelvin wave instability, when a curved vortex interacts with its image on the container wall. This instability is at a lower temperature than the threshold for inter-vortex turbulence of interacting loops. We present experimental evidence for power law dependences of Tonset. On the flow velocity and the number of seed loops. The investigation is still in progress, we expect to have a manuscript on the results and their interpretation by the end of the year .
In the meantime a technical report on low temperature thermometry has been submitted for publication , in which Dr. Blaauwgeer’s contribution was integral.
|RITA-CT-2003-505313||AR2||17||Vortex Propagation and Configuration in High Speed Rotation||Rob de Graaf designed and constructed for his M.Sc. thesis at the University of Leiden a magnetically levitated rotating sample container. The plan was to use this device for studying vortices under high speed rotation. The device turned out to be promising, but a second attempt with a number of improvements was needed. For this he started a collaboration in the low temperature laboratory with the aim to demonstrate the operating capabilities of his rotating sample container and to adapt the design to a rotating blade which would be used inside a sample container at a temperature of around 1 millikelvin, to create turbulence in a sample of superfluid 3He-B.||Rob de Graaf built and tested a more developed example of a magnetically supported and driven rotor with superconducting elements. The rotation velocity is measured optically. So far angular velocities of up to 50 Hz have been achieved in liquid helium at 2 K. It is not yet clear how well this design can be transferred to temperatures which are three orders of magnitude lower. Rob de Graaf has also participated in measurements on the onset temperature of turbulence in superfluid 3He-B, in order to acquire more first hand experience about the study of the dynamics of quantized vortices. Two research reports are in preparation on measurements in which he had major responsibility [14,15]. In addition he was involved in the calibration measurements of the thermometer in Ref. .||Physics|
|RITA-CT-2003-505313||AR2||18||Design of a New Optical Cell for Studies of 3He Crystals in a Magnetic Field||Dr. Viktor Tsepelin is a junior scientist working in Lancaster University. His research interests cover studies of both quantum fluids and quantum solids. In the low temperature laboratory of Lancaster University he is able to conduct his superfluid 3He studies. In this project, he is starting a new line of quantum solid studies at high magnetic fields, which he is not able to undertake in Lancaster. The high field experiment requires several improvements in the optical nuclear demagnetization refrigerator, located in the Low Temperature Laboratory, before it can be used for the planed experiments. In this project, the initial improvements of the refrigerator and the design of the new optical cell will be made. This difficult project is a collaborative effort with Leiden University and Helsinki University of Technology.||Dr. Tsepelin worked in the ULTI facility three weeks in the fall of 2005. During that time he completed two main tasks:
(1) Updating the nuclear demagnetization cryostat used for optical measurements:
During 2005 the optical refrigerator at ULTI facility had serious problems as the condensing line of the dilution refrigerator got blocked several times. The decision was made to replace the old condensing line with a new tubing and also to install an extra line with separate impedance. This work was mostly carried out by Viktor Tsepelin who made also all necessary leak tests afterwards. The reason for blocking turned out to be the oil which had collected into tubing from pumps. During 2006 the cryostat has been cold for several months and there has been no problem with blocking condensing line.
(2) Design the capacitive pressure gauge for the high-accuracy melting curve measurements in 4He:
Viktor Tsepelin designed and made drawings for the capacitive pressure gauge which was later assembled and installed to the experimental setup.
During the first half of 2006 the melting curve of 4He was measured very accurately using that gauge. The resolution of the gauge turned out to be better than 1 microbar at 25 bars which is more than an order of magnitude better than achieved earlier. In these measurements it was discovered that below about 0.1 K, instead of the expected T4 dependence, there appears much weaker, almost linear dependence which is not understood yet.
|RITA-CT-2003-505313||AR2||19||Experiments of Mesoscopic SQUID Arrays||The goal of the project is to learn about high frequency properties of Josephson junction and SQUID arrays and to make use of them in experiments on Cooper pair pumps and as switches||Experiments on Cooper pair pumps with SQUID arrays have proven to be difficult due to the arbitrary background charges affecting the small islands between the SQUIDs. No publishable scientific results have been obtained as yet of this effort. Yet, an experiment where SQUIDs are used as switches of photonic heat transport between tiny normal metal islands have worked out beautifully, and we have observed experimentally, as the first group, this heat transport mechanism. We have developed a simple model which explains the observations quantitatively. This effect would be unobservably small, if we would not be able to “chop” the radiative power by applying flux through the SQUIDs. The first set of experiments has been finished, but this project is still continuing.||Physics|
|RITA-CT-2003-505313||AR2||2||Carbon Nanotube/Metal Hybrid Structures at mK-Temperatures||Professor Cristopher Strunk is from University of Regensburg, Germany. His project is directed towards the investigation of quantum transport in carbon nanotubes. Individual multi- and single walled nanotubes will be contacted electrically and investiged at very low temperatures, available at the Low Temperature Laboratory. Special emphasis is given to the interplay between Coulomb blockade and quantum interference. The usability of individual carbon nanotubes for ultrasensitve charge detection in rf-SET schemes will be tested.||Lorenz Lechner, a graduate student of Professor Strunk has visited the LTL for 4 months and the first results on proximity-effect-induced superconductivity have been obtained in multiwalled samples made out of plasma-enhanced CVD nanotubes. This is the first time ever when proximity-effect-induced superconductivity has been observed in multiwalled nanotubes! The observed superconductivity depends strongly on the gate-induced doping of the number of charge carriers in the tube. The products of critical current times the normal state resistance is found to be much lower than predicted for regular, metallic SNS junctions. The results are still unpublished. Publications are under preparation.||Physics|
|RITA-CT-2003-505313||AR2||22||Hydrodynamics of Liquid 3He||Prof. Ladislav Skrbek is a researcher from the Joint Low Temperature Laboratory of the Academy of Sciences of the Czech Republic and of the Charles University in Prague. He is an expert on superfluid hydrodynamics and studies in his home institution vortex dynamics and turbulence in superfluid 4He. The main goal in this work is to understand the coupled two fluid hydrodynamics when both the superfluid and normal components are in turbulent flow. The purpose with the visits to the Low Temperature Laboratory is to establish a comparison to superfluid 3He-B where the normal component is so viscous that it does not become turbulent in any practical experimental setup [ie. the Reynolds number vL/ny<1, where v is the typical scale of flow velocities, L the length scale, and ny the kinematic viscosity]. In this case the dynamics simplifies to that of the superfluid component only.
In addition, measurements of different type and information content can be performed in 3He-B than in the more conventional superfluid 4He.
|Prof. Skrbek and his students have frequently been participating at earlier times in studies on turbulence as a function of vortex damping (or mutual friction dissipation) in superfluid 3He-B at the Low Temperature Laboratory. This work is currently continued with his graduate student Timofey Chagovets who is visiting the Low Temperature Laboratory for three months in 2006. In the meantime measurements have been conducted with commercial quartz tuning fork oscillators, which are in general use as frequency standards in watches. The question of interest is whether they can be employed as sensors for vortices in superfluids. This possibility arose during recent measurements where these devices were studied and calibrated for thermometry both in 3He and 4He liquids in Helsinki, Prague, and Kosice . Studies on vortex detection with quartz oscillators will be continued with prof. Skrbek during his visit in the summer of 2006.||Physics|
|RITA-CT-2003-505313||AR2||23||Frequency Dependent Noise and Higher Cumulants in Mesoscopic Systems||Dr. Samuelsson is a junior theoretician from Lund University, Sweden. The goal of his project is to find a theoretical description of mesoscopic detectors measuring frequency dependent noise and higher cumulants from other mesoscopic devices. Especially he wants to concentrate on the use of a driven Josephson junction as the detector. With these detector models, he would like to find the proper quantum-mechanical formulations of the higher-order correlators measured with the detectors, and calculate these correlators in some generic cases.||Dr Samuelsson visited Low Temperature Laboratory in November 2005. During the short visit, he initiated the project on describing a generic fluctuation detector. This will result in a publication submitted within the next couple of months. The rest of the visiting days will hopefully be used during the remaining of the year, and during this visit we will concentrate on the actual calculation of the frequency dependent higher-order correlators encountered in the theory of mesoscopic detectors.||Physics|
|RITA-CT-2003-505313||AR2||24||Statistics of Electron Transport in Mesoscopic Devices||Professor Edouard Sonin is from Hebrew University, Jerusalem, Israel. The goals of his project are:
1. To study various cases of mesoscopic transport in which counting statistics is not Poissonian. Two particular cases that will be considered are a single junction with current bias and two junctions in series. These studies will pave the way for experiments to probe current statistics using on-chip junction probe detectors.
2. To study not only full counting statistics but also full statistics of voltage and phase fluctuation. This is important since the experimentalists have no direct access to counting statistics measuring voltage or phase fluctuations instead of it.
3. To extend the analysis of full statistics, which up to now has been done only in the low-frequency limit, to high frequencies (short times).
This will be instrumental in order to analyze practical junction noise detectors in the regime of non-Poissonian noise.
4. To study counting statistics in carbon nanotubes. In addition to counting statistics, cross correlations of current fluctuations will investigated in the light of pinpointing the nature of basic charge carriers in single and multiwalled carbon nanotubes.
|A joint analysis (together with P.J. Hakonen and A. Paila from the Low Temperature Laboratory) of statistics of electron tunneling in normal tunnel junctions has been done analytically and numerically taking into account circuit (environment) effects [in Ref (18)]. Full counting statistics, as well as full statistics of voltage and phase have been found for arbitrary times of observation. The theoretical analysis was based on the classical master equation, whereas the numerical simulations employed standard Monte-Carlo methods. The results of the analysis of full counting statistics at long time scales fully agree with previous investigations (Kindermann, Nazarov, and Beenakker) based on the quantum-mechanical approach. This proves that environment effects can be investigated within the classical approach, and the latter approach was exploited for the analysis for short time scales, which has not been done before. The analysis has demonstrated that environment effects become much weaker at short-time scales.
Further challenges to the project are to investigate full statistics for other mesoscopic setups and to analyze possibilities to detect non-Poissonian statistics using on-chip junction probe detectors. The major challenge is to extend the theoretical analysis on noise in single and multiwalled carbon nanotubes. Up to now it is not fully clear whether experimentally detected noise comes from a nanotube itself or mostly from contacts between the nanotube and leads.
|RITA-CT-2003-505313||AR2||26||Quasiclassical Description of Coherent Quantum Systems||Dr. Eschrig is a junior scientist from University of Karlsruhe. He is interested in the studies on the equilibrium supercurrent in a heterostructure containing a piece of strongly ferromagnetic material sandwiched between two singlet superconductors (an SFS structure). Supercurrent flow in such a system can arise because of the proximity-induced pair correlations in the ferromagnet. However, because of the exchange splitting between the two spin bands, conventional spin-singlet proximity effect is confined to a very thin layer (thickness of the order of interatomic distances) inside the ferromagnet. In particular, if the ferromagnet is fully polarized (only one spin band metallic and the other insulating), a half metal, the spin-singlet proximity effect should be entirely absent. However, even in this case there remains the possibility of supercurrent being mediated by (equal-spin) triplet correlations in the ferromagnet, if a mechanism for singlet-triplet conversion exists. Such a situation has been theoretically studied in the dirty limit [Bergeret et al., PRL 86, 4096 (2001)], as well as in the clean limit [Eschrig et al., PRL 90, 137003 (2003)], the latter work dealing especially with a half-metallic ferromagnet. Interestingly, the symmetries of the relevant triplet correlations in these two cases are entirely different: in the dirty limit they are even in the momentum space, and odd in energy, whereas in the clean limit the dominating correlations are odd in momentum and even in energy (these being the two possibilities allowed by the Pauli principle). Dr. Eschrig aims to investigate, together with Dr Juha Kopu from the LTL, more thoroughly the role of these two classes of triplet correlations at arbitrary impurity density - in the intermediate regime between the clean and dirty limit. Their collaborative work is motivated by the recent experimental discovery of supercurrent in a superconductor-half metal-superconductor structure [R. S. Keizer et al., Nature 439, 825 (2006)].||At the moment of writing this report, the project has not yet been finished. For the first time, Dr. Eschrig and Kopu have prepared a numerical routine for computing the pair correlations in an SFS structure (with a strong ferromagnet) with arbitrary impurity density. We are now in the process of collecting data from such calculations, to determine the magnitude of the supercurrent and the type of the relevant triplet correlations for a structure with a realistic amount of impurities. The early results seem to confirm our expectation that the dirty-limit approximation, at least for this particular problem, is too restrictive and eliminates interesting and relevant physics.||Physics|
|RITA-CT-2003-505313||AR2||5||Thermoelectric Effects in Hybrid SN Metal Structure||Dr. Franzesco Giazotto is a senior scientist from University of Pisa.
The goal of his project is to develop quasiparticle non-equilibrium in mesoscopic hybrid structures, and to make use of them in devices, such as cold electron Josephson transistors. Another objective was to finish a review article in Reviews of Modern Physics on electronic microrefrigeration and thermometry.
|Based on prior promising results on creating non-equilibrium quasiparticle distributions in hybrid cooler devices , and the use of non-equilibrium in cold electron transistors , Dr. Giazotto has developed these concepts further in year 2005, in collaboration with Professor Pekola from the LTL, to make use of sharp features in IVs of SIS junctions  (S=superconductor, I=insulator). It turns out that a device based on SIS junctions is superior over the one studied in , as the recent experiments imply. Dr. Giazotto has also completed a common review article with Professor Pekola during this reporting period .||Physics|
|RITA-CT-2003-505313||AR2||9||Quantum Coherent Effects in Josephson Junction Circuits and Quasiparticle Non-Equilibrium||Professor Hekking is from the Joseph Fourier University and CNRS, Grenoble. The goal of his project was to understand theoretically some observations, made in experiments at the Low Temperature Laboratory on Josephson junctions (JJ), in particular those concerning a sudden change of the behaviour at the switching threshold of small junctions, which are, however, still in the regime where Coulomb effects are unimportant. Another goal was to investigate how Josephson junctions could function as threshold detectors of noise and current statistics. The third objective was to understand adiabatic pumping in JJ circuits and its relation to Berry phase. Finally we wanted to understand better the quasiparticle non-equilibrium in electronic microrefrigerators.||The counterintuitive behaviour of switching threshold in JJs could be explained by a model, where the junctions assume phase-diffusion –type dynamics due to moderate dissipation. Experimental results fit this model perfectly [6,7]. Similar observations were reported by two other groups (Chalmers, Stony Brook) almost simultaneously; our observation was the first one, though. Concerning noise detection, we managed to develop a theory, which accounts for high frequency quantum noise of an interacting mesoscopic conductor . In another work [to be published], together with the Pisa group, we developed a measuring scheme of the third cumulant of noise by a JJ detector: under suitable experimental conditions, noise on the level of third order correlators is expected to produce coherent oscillations between the levels in the metastable potential of the JJ. A general formalism based on scattering theory, to find a third order correlator to describe an arbitrary quantum detector was developed [to be published].
Adiabatic pumping and Berry phase were discussed and a measuring scheme in a phase-coherent configuration was developed . Concerning quasiparticle non-equilibrium, we did not produce scientific results within this project, but research proposals were prepared for applying funding for experimental work in this direction.