Mode-locking in a-Mo x Ge 1 － x thin films

1. Mode-locking in a-MoxGe1－x thin films Tibi Sorop, Peter Kes Collaborator: Nobu Kokubo (Tsukuba University, Japan) !!!WORK IN PROGRESS!!! TO DO QUESTIONS

2. PLAN OF THE TALK 1. Introduction to Mode-Locking 2. Motivation 3. Experimental details 4. Results and discussions TO DO QUESTIONS Preliminary conclusions sofar!

3. IINTRODUCTION TO DO QUESTIONS

4. Previously I have presented in this group the work done on mesoscopic channels • Just to remind you few things about that I show you here the drawing of the structure • Channels made out of NbGe and NbN by etching with a proper mask; here is a side view and a top view • The main idea is that one is wek pinning the other strong pinning and when a current is applied the vortices in the weak pinning are moving creating easy flow vortex channels • A lot of phenomena were studied in this system but a breakthrough was achieved when the ML effect was detected • With ML one could get a direct information about the number of moving rows and solve J a Strong pinning Weak pinning Mesoscopic channels - Periodic vs. Random pinning Mode-locking effect - Commensurability and frustration - Incommensurate peaks TO DO QUESTIONS - Field history effects: vortex slips

5. Mode-locking: Channels V VML Ic I • What is ML? • Lets’ suppose we have a system that dc motion: average velocity v dc drive: Idc> Ic + ac modulation fint=v/a mixed RF-DC drive:Iapplied = Idc+Irf sin( 2πf t ) fint = f =>Mode locking interference (ML) TO DO QUESTIONS

6. m J v a a a Channels Washboard potential Equation of motion for a single vortex in a periodic potential  Pinning potential: shearing interaction with static vortices in CE

7. ML in channels Equation Josephson junction: current biased RSJ model Shapiro steps

8. ML in channels Channels vs. Josephson junctions • more realistic model is necessary • disorder in CE • Josephson junction arrays • CDW

9. elastic restoring force What about thin films Pinning centers randomly distributed Can random disorder generate Mode-Locking? Schmid and Hauger (1972) coupling through elastic pinning It’s all done in the 70’s Assumptions • RF fields homogeneously distributed (Thin films) • Weak pinning TO DO QUESTIONS • Straight vortices (2D case) • High velocities Time dependent Ginzburg-Landau equations

10. interference for Modulation velocity: internal oscillations with TO DO QUESTIONS

11. What about experiments? A. Fiory, PRL 27, 501 (1971): thin films of Al (10-100 nm) Surprise: No other examples of ML in thin films !!! J..M. Harris, PRL 74, 3684 (1995) single crystal of YBCO TO DO QUESTIONS Troyanovski 1999 STM detection of washboard peaks in NbSe2

12. Nobu Kokubo (Tsukuba) Mode-Locking in NbSe2 and MoGe Collaboration: MoGe TO DO QUESTIONS

13. V I w NbN NbN NbGe d ML for intrinsic (bulk) potential • ML associated to the channels • However, when a wide channel was measured by Olaf and Rut it appeared as a surprise • In order to understand why it was a surprise let’s have a closer look at what ML means Macroscopic channel, width w=100µm: (Rut Besseling, Nobu Kokubo, Olaf Benningshof) TO DO QUESTIONS

14. III MOTIVATION TO DO QUESTIONS

15. 2D vortex phase diagram Hc2(0) normal state liquid liquid H Vortex solid Tc(0) T Static phase diagram Increasing H or T: Melting transition TO DO QUESTIONS Disorder (pinning) Vortex glass

16. quenched disorder thermal disorder velocity Moving crystal Moving liquid vc Plastic flow Dynamic phase diagram “Dynamic” Melting criterion crystallization velocity TO DO QUESTIONS Koshelev and Vinokur, PRL (1994)

17. It was believed that DC measurements will reveal the right picture Lattice correlations start to increase Dynamic ordering Moving (hexatic) glass Pinned state Plastic flow DC measurements Simulations (Langevin dynamics), Ryu et al. PRL (1996)

18. Smectic flow Coexistence of two phases Bhattacharya 1999 Transverse solid DC measurements ? Dynamic ordering Lattice correlations start to increase ? Moving hexatic glass Pinned state Plastic flow

19. Coherent motion Incoherent motion velocity Moving crystal Moving liquid vc Plastic flow ML vs. ordering (1) ML no ML TO DO QUESTIONS

20. Plastic flow no ML Smectic flow ML only for Transverse solid ML at any ML vs. ordering (2) TO DO QUESTIONS A.B. Kolton et al. PRL (2001)

21. 4 EXPERIMENTAL DETAILS TO DO QUESTIONS

22. First let’s have a look at the sample design • Here is a photo of the sample with the dimensions: important is that the thickness is 330 nm • I show here some of the relevant parameters • a-MoGe is a weakly pinned material Parameters: a-Mo78Ge22 K d = 330 nm 1.7 mWm T/K 4.48 nm Room temp sputtering Tsukuba University, Japan Sample design and parameters • TO DO • QUESTIONS • Is it still a 2D film? • How does it compare with our films?

23. Just to make it clear here is the phase diagram for our MoGe film • And the dependence of the critical current on the magnetic field at T = 4.2 K Weak pinning film a-MoGe-film 1mm Pinning properties of MoGe TO DO QUESTIONS Gertjan van Baarle, STM

24. First let’s have a look at the sample design • Here is a photo of the sample with the dimensions: important is that the thickness is 330 nm • I show here some of the relevant parameters • a-MoGe is a weakly pinned material • dc IV curves V vs. I DC • rf+dc V vs. I RF ｗ = 0.3 mm f = 1 – 100 MHz nV IRF = 0 – 10 mA B = 0 – 8 T l=1.1 mm T = 1.9 – 4.2 K B, T = const Measurements • TO DO • QUESTIONS • Is it still a 2D film? • How does it compare with our films? Current sources d = 330 nm

25. Does MoGe fulfill the criteria? Conditions to detect ML • Weak pinning • Homogeneous application of RF fields: • small samples (thin film) • Good shielding of the RF line • Matching circuit in RF line • Small contact impedance • (steps = 5% of V≈ 1 µV) TO DO QUESTIONS • High enough RF power • Good thermal stability