The supercurrent IS of a YBCO bridge can be modulated by the quasiparticle-injection current IG from YBCO/Au or YBCO/PBCO/Au junctions. The behavior of these structures is determined by two effects: 1) summation of the currents IS and IG in the YBCO bridge; 2) nonequilibrium suppression of the supercurrent IS by the quasiparticle-injection. The current gain coefficient ¿IS/¿IG increases linearly with decreasing temperature, reaching a value of 1.5 for YBCO/Au structures at 65 K. The nature of the nonequilibrium state and the effectiveness of the PBCO barrier layer for the formation of the quasiparticles are analyze

Boguslavskij, Yu.M.

Gerritsma, G.J.

Joosse, K.

Roesthuis, F.J.G.

Rogalla, H.

English

The supercurrent IS of a YBCO bridge can be modulated by the quasiparticle-injection current IG from YBCO/Au or YBCO/PBCO/Au junctions. The behavior of these structures is determined by two effects: 1) summation of the currents IS and IG in the YBCO bridge; 2) nonequilibrium suppression of the supercurrent IS by the quasiparticle-injection. The current gain coefficient ΔIS/ΔIG increases linearly with decreasing temperature, reaching a value of 1.5 for YBCO/Au structures at 65 K. The nature of the nonequilibrium state and the effectiveness of the PBCO barrier layer for the formation of the quasiparticles are analyzed

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Quasiparticle-injection effect in YBa2Cu3Ox-based planar structures

Boguslavskij, Y.M.

Rogalla, Horst

University of Twente Research Information

Physica B 194-196 (1994) 85-86 Noah-Holland PHYSICA[ Quas ipa r t i c l e - in j ec t ion ef fec t in YBa2Cu30×-based p lanar s t ructures Yu.M. Boguslavskij*, K. Joosse, F.J.G. Roesthuis, G.J. Gerritsma and H. Rogalla, Department of Applied Physics, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands The supereurrent I s of a YBCO bridge can be modulated by the quasipartiele-injection current I a from YBCO/Au or YBCO/PBCO/Aujunctions. The behavior of these structures is determined by two effects: 1) summation of the currents I s and I o in the YBCO bridge; 2) nonequilibrium suppression of the supercurrent I s by the quasiparticle-injection. The current gain coefficient &Is/AI o increases linearly with decreasing temperature, reaching a value of 1.5 for YBCO/Au structures at 65 K. The nature of the nonequilibrium state and the effectiveness of the PBCO barrier layer for the formation of the quasiparticles are analyzed. 1. INTRODUCTION Injection-controlled three-terminal devices are an interesting object for the study of nonequilibrium superconductivity and for practical application as transistor-like superconducting elements at liquid- nitrogen temperatures. A first study of the quasipar- ticle injection in YBCO/A1 structures shows a current gain of 5-7 at 4.2 K [1]. The number of thermal quasiparticles N v increases with operating temperature. It complicates the creation of the nonequilibrium state (Zk_Ni,j~Nv) at the same injec- tion levels and decreases the gain coefficient. Injection-controlled structures have not been studied yet at high temperatures. In this report we investigate the modulation of the supercurrent of YBCO microbridges under quasiparticle injection from YBCO/Au and YBCO/PBCO/Au planar contacts at temperatures of 60 K to 85 K. 2. DEVICE STRUCTURE Schematic views of the YBCO/PBCO/Au structure are shown in Fig.1. A planar geometry of the structure was chosen to provide uniformity of the injected quasiparticle flow into the YBCO films. The thickness of the YBCO, PBCO and Au layers is 40-80nm, 0-20nm and 30-40nm, respectively. PMMA photoresist covers the edges of the YBCO strip. The junction area varies between 8x20 lain 2 and 20x30 ~ 2 . The critical current density Jc of the YBCO bridges is 5-105-3.106 A/em 2 at 77 K. *On leave from Institute of Metal Physics, Kiev, Ukraine. This research is supported by the Dutch NWO program. • I g I s Is Vs -YBCO I t -PBCO I l-e~lMA -All Fig. 1. Schematic view of YBCO/PBCO/Au structure with indication of the terminals. 3. RESULTS AND DISCUSSION The critical value I c of the controlled current I s of a YBCO bridge is shown in Fig.2 as a function of the injection current IG from the YBCO/Au contact at different temperatures. The same dependencies are measured if both I s and I a are reversed. Ic(Io) dependencies of the YBCO/PBCO/Au structures show similar behavior to that presented in Fig. 2. First we discuss the effect of current summation in the YBCO bridge. Asymmetrical input of the injected current I~ into the structure (Fig.l) creates different distribution of the total current density Js+Jo in the YBCO strip. If Js and JG are in the same direction, their sum Js+JG cannot exceed the Jc value of the YBCO bridge near the entrance of these currents into the junction area. Pure summation results in the dependence Ic(Io)=Ic(0)-Io. For the opposite direction of Js and I o the weakest place is on the other side of the junction and Ic(Io)=Ic(0) due to summation effect. Thus, even at a uniform current flow of IG through the contact area, non- uniformity of the current flow through the bridge 0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0921-4526(93)E0616-O 86 7 O 5O "~" 40 & 3O 20 I0 --6O • - • v v v v v " n o a u = n = D n • tl u • D D ~ v =n u o • D 7 ~ t °° - , , , % =. --40 --20 0 20 40 I o (mA) 6 0 Fig.2. Dependence of the critical current I c of the YBCO bridge on the injection current from YBCO/Au contact at different temperatures. causes different Ic(Ia) dependencies for the parallel and opposite directions of the current flows. The amplification effect of these structures (Ko---Ic(Io=0)/Io(Ic=0)>l) is caused by quasi- particle injection. The injection-affected change of the supercurrent AI c as a function o f Io is shown in Fig.3. Experimental data are plotted as AIc=Ic(0)-Ic(Io) at Io<0 (i.e if I G is opposite to Is) and AIc=Ic(0)-Ic(Io)-IG at Io>0 due to asymmetrical current flow, as was discussed above. The rea- sonable symmetry of the AIc(IG) curves supports the idea that this has a common nature. The spectrum of the injected quasiparticles is almost linear with energy. Thus, the injection can simply raise the effective temperature T*>T~th for both quasipartieles and phonons. At high operating temperatures and small thickness of the films the response to the injected power PG=Io.Vo is determi- 25, 2O 15 i 10 5 °t & / --5--60 ---40 - 0 20 40 60 I o (w~) Fig.3. Injection-suppressed part ofl c as a function of injection current at several temperatures. Simulated curve is a solid line. ned by heat flow through the film-substrate interface, rather than by the heat diffusion time via the layers (xeae--d/DR) [2]. Thus, a simple heating Y*-model can be applied for these structures [3]: Po= IG-V o = c(T .4 -Yba~ 4) (1) The simulated curve is the dependence: AI c =I c (Ybsth) - I c (Y*)=f(Po) (2) plotted by using eq.1) for the effective temperature T* and I c (T) dependence for the values Ic(T* ). A linear increase of the gain coefficient with decreas- ing temperature agrees with the T*- model. The operating speed of the YBCO/Au structures can be estimated as a bolometrie response time x~l-2 ns [2]. Our measurement shows no difference in the gain coefficient Kg up to the frequencies of 50 KHz. A PBCO barrier layer of 0-20 nm thickness has been used to increase the power Po and to vary the spectrum of the injected quasiparticles. In the YBCO/PBCO/Au structures the absolute value of K o decreases with PBCO barrier thickness. The PBCO barrier seems to be effective only at small thicknesesses due to the short hopping distance of the carriers in the c-axis direction and an extra energy loss in the barrier. The largest values of the gate power Po have been applied to the junctions with a thick PBCO barrier, but the K o coefficient is the smallest in these structures. This disagrees with a simple T*-heating by injected power and needs further analysis. 4. C O N C L U S I O N S The behavior of the injection-controlled YBCO/Au and YBCO/PBCO/Au structures is deter- mined by the summation of the transport I s and injection I o currents in the YBCO bridge as well as by nonequilibrium suppression of the supercurrent I s due to quasiparticle injection. The injection effect agrees qualitatively with a simple T*-heating model. A current gain of 1.5 in absolute values of the currents and di s/dI o ~2-3 at 65K can be achieved in YBCO/Au structures. REFERENCES 1. T. Kobayashi et al, IEEE Trans. on Magn., 25, 9 (1986) 1342. 2. S. Zeuner et al, Appl. Phys. Lett., 61 (1992) 973. 3. W.H. Parker, Phys. Re•. B, 12 (1975) 3667.

Quasiparticle-injection effect in YBa2Cu3Ox-based planar structures

https://doi.org/10.1016/0921-4526(94)90372-7

Quasiparticle-injection effect in YBa2Cu3Ox-based planar structures

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University of Twente Research Information

University of Twente Research Information