UNIVERSITÄT ZU KÖLN

ML4Q Internee - Doga Solmaz

After an eventfull summerbreak including experiments, student retreats, group outings and conferences, the ML4Q internee Doga Solmaz leaves the workgroup to continue her physics undegraduate studies in the Imperial College, London. Doga has been measuring Josephson tunnel junctions comprised of a tri-layer stack of Nb/AlOx/Nb at 4.6K using the cryogenic probe station. The devices have kindly been provided by Dr. Netty Honingh and Dr. Felix Münning of the I Institute of physics and showed superconducting properties up to 8.6K. Do succeeded in measuring the superconducting transition in these junctions and studied junction parameters like the critical current and the normal state resistance as a function of the total junction area. We wish here all the best in her studies and for her future!

Visit: Masayuki Hashisaka, The Institute of Solid State Physics (ISSP), University of Tokyo

The group invited Masayuki Hashisaka from the Institute of Solid State Physics (ISSP) of the University of Tokyo for a Seminar Talk and discussion for future possibilities for collaborations. The visit included scientific discussion with several members of the seconds institute of physics in order to give a broad overview of research activities. After the seminar talk the group of Prof. Erwann Bocquillon, together with Masayuki, went for a dinner at one of colognes traditional "Brauhaus" to celebrate the get-together. 

Group Outing Summer '24

Friday July 12th the group has been canooing for team-building activities. About 10km down the "Sieg" from Eitorf to Blankenberg the group members have been enjoying their ride in groups of two. The weather was sunny and invited to stone skipping during a break at camping Happach and a beautiful view on the caste of Blankenberg at the last stretch of the tour. After the exhausting activity, the day finished with a cosy dinner in Wirtshaus Knott in Köln-Porz.

At the moment of the group outing the group consisted of Prof. Dr. Erwann Bocquillon, Dr. Daniel Rosenbach, PhD students Alina Rupp and Torsten Roeper, Master Students Ricardo Gioia Alvarez and Dominik Hoborka, Bachelor Students Benjamin Roth and Paul Ruhrberg, as well as ML4Q Intern Doga Solmaz from the imperial college of London.

Paper Publication - Propagation, dissipation and breakdown in quantum anomalous Hall edge states probed by microwave edge plasmons

Abstract

The quantum anomalous Hall (QAH) effect, with its single chiral, topologically protected edge state, offers a platform for flying Majorana states as well as non-reciprocal microwave devices. While recent research showed the non-reciprocity of edge plasmons in Cr-doped (BixSb1-x)2Te3, the understanding of their dissipation remains incomplete. Our study explores edge plasmon dissipation in V-doped (BixSb1-x)2Te3 films, analyzing microwave transmission across various conditions. We identify interactions with charge puddles as a primary source of dissipation, providing insights critical for developing improved QAH-based technologies.

DOI: 

Cryogenic Probe Station for radiofrequency characterization of devices at 4.6K

On february 6th, a cryogenic probe station for the characterization of devices at applied radifrequencies up to 18GHz has been installed. The probe station as well as the cryostate have been designed by Janis, where temperatures of down to 4.6K can be reached, when feeding the flow cryostate with liquid helium. The probe station has been transferred from the École Normale Supérieure (ENS) in Paris, from the workgroup of Gwendal Fève. The probe station is equipped with 4 semi rigid coaxial cables on feedthroughs, each mounted on a three-axis linear stage, an optical access on top and a dc line feedthrough. The temperature at the probes is specified for 15K but additional thermalization brings the effective temperature at the probes below T<8K. 

Installation of second cryogenic temperature measurement setup

October 23rd through October 31st the workgroups second cryogenic measurement setup has been installed. We again got valuable support from Bluefors' Cryo Engineer Sajed Rabbani, Senior Site Planning Engineer Marija Macic-Järvinen and many more on- and off-site support from Bluefors. On sunday the 29th of October the LD system with 6-1-1 vector magnet reached 8.9mK base temperature for the first (but not the last) time in our laboratory. The system will be dedicated to ultra-sensitive electronic noise measurements and the d.c. study of magnetic domain switching in quantum anomalous Hall devices. 

Team Outing - Hochseilgarten Blackfoot Forrest and Blackfoot Beach

Celebrating one year after the first PhD student Boris Stanchev arrived, the workgroup Bocquillon went on their second group outing. On the 23rd of June the group tested their teamwork abilities in up to 12m above ground in the climbing parcours of the Blackfoot forrest 'Hochseilgarten'. After this physical exercise at record breaking temperatures, the group enjoyed time at the nearby blackfoot beach, swimming and enjoying cold drinks. The picture shows from left to right: Michael Lee (ML4Q intern, Yale University), Daniel Rosenbach (PostDoc), Torsten Roeper (PhD Student), Boris Stanchev (PhD Student), Prof. Erwann Bocquillon and Hugo Thomas (Master intern, ENS Paris). 

Installation of 7.2mK cryogenic temperature measurement setup

May 15th through May 28th the workgroups first cryogenic measurement setup has been installed. With the help of Bluefors' Cryo Engineer Sajed Rabbani, Senior Site Planning Engineer Marija Macic-Järvinen and many more on- and off-site support the Bluefors LD system reached 7.2mK base temperature. The system is equipped with 48 DC lines separated in two lines of 12 twisted pair looms.  DC lines are filtered using Basel Precision Instruments' Microwave filter and Thermalizer for a guaranteed reduction of -60dB and more for unwanted high frequency noise above 10MHz. Additionally, 4 pairs of radiofrequency lines for the application and the amplification of radiofrequency signals are installed to study the dynamic transport properties of novel quantum materials.  

First team outing, Siebengebirge

On the 22nd of July 2022, the work group of Prof. Erwann Bocquillon enjoyed their first team outing. The group activity included a hike to one of the highest points of the Siebengebirge, the old ruins of Löwenburg (455m). After a refreshing picknick the hike continues to Burg Drachenfels (320m). 

Formation of a new group

Creation of the group, following the appointment of Erwann Bocquillon. The group is from now on looking for new members: post-doc, PhD students or MSc students. If you are interested, please write an e-mail at bocquillonph2.uni-koeln.de.