MEISSNER-OCHSANFELD EFFECT AND A SEARCH FOR. TRANSITION TEMPERATURE FOR YBa 2 Cu 3 O 7. Baris Cetin. Department ofphysics

Transcription

1 MEISSNER-OCHSANFELD EFFECT AND A SEARCH FOR TRANSITION TEMPERATURE FOR YBa 2 Cu 3 O 7 Baris Cetin Department fphysics Purdue University, West Lafayette, In Abstract A simple way f measuring the transitin temperature, T c, f the YBa 2 Cu 3 O 7 high-t c supercnductr by using the Meissner-Ochsanfeld eect will be presented.

2 Cntents 1 Intrductin 3 2 Meissner-Ochsenfeld Eect 3 3 Experiments Sample Preparatin : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : Lading f the Supercnducting Pellet int the Crystat : : : : : : : : : : : : : : : : Experimental Setup : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 6 4 Observatins and Cnclusins 8 2

3 1 Intrductin In this reprt, we try t summarize the details f a simple experiment which is designed t measure the transitin temperature, T c, f the YBa 2 Cu 3 O 7 high-t c supercnductr by using the Meissner- Ochsanfeld eect. In the rst part f the reprt a brief explanatin f Meissner-Ochsanfeld eect will be given. In the secnd part, steps fr making the supercnductr sample and preparing and lading the crystat fr the measurement will be explained in detail. Then, a descriptin f the experimental circuit design and the technique used in the measurements will be given. And in the nal part, a general discussin f the results will be discussed. 2 Meissner-Ochsenfeld Eect A magnetic eld (prvided that it is nt t strng) cannt penetrate int the interir f a supercnductr. This is mst dramatically illustrated by the Meissner-Ochsenfeld (1933) eect: If a nrmal metal in a magnetic eld is cled belw its supercnducting transitin temperature, the magnetic ux is abruptly expelled. Thus the transitin, when it ccurs in a magnetic eld, is accmpanied by the appearance f whatever surface currents are required t cancel the magnetic eld in the interir f the specimen [1]. When a specimen is placed in a magnetic eld and is then cled thrugh the transitin temperature fr supercnductivity, this eect can be bserved easily as shwn in Fig.1 [2]. Supercnductivity ccurs in many metallic elements f the peridic system and als in allys, intermetallic cmpunds, and dped semicnductrs. The range f transitin temperatures best cnrmed at present extends frm 122 K fr the cmpund Tl 2 Ba 2 Ca 2 Cu 3 O 10 t belw 0:001 K fr the element Rh. The transitin temperatures f a number f interesting supercnducting cmpunds are listed in [3]. 3 Experiments 3.1 Sample Preparatin In the experiment we used high-t c supercnductr cmpund YBa 2 Cu 3 O 7. 3

4 Figure 1: Meissner-Ochsenfeld eect in supercnducting sphere cled in a cnstant applied magnetic eld; n passing belw the transitin temperature the lines f inductin B are ejected fm the the sphere Frmula: 1 2 Y 2O 3 +2BaCO 3 + CuO! YBa 2 Cu 3 O 7 + CO 2 All the cmpnuds n the left are 99.9 pure except CuO, which is 99.7 pure. After mixing enugh amunt frm each cmpund ( 1.53 grams f Y 2 O grams f BaCO 3 and 3.22 grams f CuO ) t get 9 grams f the supercnductr material we mix them very carefully and get the gray pwder. Then, we bake this pwder like sample at 975 C fr 24 hurs t make them react well with each ther chemically. After baking, we make the pellet which is actually the nal shape r the frm f the sample which is used t take data in the experiment. The nal shape is given by mixing the pwder with sme ethanl and then pressing hard with the apprpriate tls. Finally, we bake it again at 975 C fr 24 hurs. Then ur supercnductr is ready fr the experiment as in given in Fig.2 with nal weight f 7.97 grams. IMPORTANT NOTE: RECIPE FOR BAKING 1- PR1 = 3 C/min [ increase in the temperature per min ] 2- PL1 = 975 C [ nal temperature that we want t reach ] 3- PD1 = 24 hurs [ ttal stay time f the sample inside ] 4

5 r = 2.5 cm d = 0.4 cm Figure 2: Supercnducting sample pellet 4- PR2 = 1 C [ decrease in the temperature per min ] 5- PL2 = 0 C [ nal temperature ] 6- PD2 = as much as pssible [ ttal time f stay at nal temperature ] 3.2 Lading f the Supercnducting Pellet int the Crystat T make the envirnment cld enugh (which is necessary t bring the sample int the supercnducting state) and islated frm the utside eects like air w, blackbdy radiatin etc. we used a cnventinal crystat (APS Crygenics Inc., HC-2, Helium Charge and Vent) which uses a clsed cycle helium refrigeratr t get desired lw temperature in the sample regin, t take data. T pump ut n the sample a cnventinal turb pump (DRYTEL 31) is used. With this pump we are able get belw 0.1 Trr. Supercnducting pellet is placed n the chrmium cvered cpper munting plate by using silicne vacuum grease which is ging t prvide gd thermal cntact between the cld nger and the sample. Then the cpper cils f 50 turns and radius f 1.27 cm and thickness f 0.4 cm ( made frm 8080 Belden wire ) placed n the supercnductr sample by using GE varnish. Finally, the thermmeter which is ging t measure the temperature f the supercnductr is put nt it. T enhance the thermal cntact between the thermmeter sensr and the supercnductr we again put silicne vacuum grease as given in Fig.3. After, placing all the cmpnents and making the necessary wiring fr the circuit we carefully wrapped them all tgether by a ten tape t make 5

6 Silicn wacuum grease is cvered the space t enhace the thermal cntact between the sample and the temperature sensr Cpper cils Supercnducting sample pellet Tape plug t strengten the mechanical cntact between the sample and the temperature sensr Temperature sensr Chrmuium cvered cpper munting plate Figure 3: This gure shws the side wiew f hw supercnducting sample laded int the crystat them mre islated and mre stable which makes the nise level f data suciently lw. Then we are ready fr measurement. 3.3 Experimental Setup The blck diagram f design which is used in the experiment isgiven in Fig.3. As it can be seen frm the part (a) f the Fig.3. we used a AC signal generatr ( KHORN-HITE MODEL 4300A OSCILLATOR ) t create the current s the magnetic ux thrugh the rst cil r the drive cil. The current thrugh the cil is measured by a multimeter ( HP 3478A ). Then, in part b f the Fig.4. we used anther multimeter ( KEITHLEY 199 SYSTEM DMM / SCANNER ) which is respnsible frm the measurement f induced vltage thrugh the secnd cil r the pickup cil because f the ux change. Finally, there is an scillscpe ( TEKTRONIX MHz OSCILLOSCOPE ) t simultaneus bservatin f the signals thrugh the drive and pickup cil. The magnetic ux which is prduced in part (a) by using AC signal generatr is directly prprtinal t the current that is passing thrugh the drive cil, 8 BI: On the ther hand the magnetic ux change which is induced in the pickup cil can be mea- 6

7 Currentmeter 5 hm Resistr Oscillscpe Drive Cil AC Signal Genaratr Part (a) Pickup Cil Vltmeter Oscillscpe Part (b) Figure 4: Experimental setup blck diagram 7

8 sured directly measuring the vltage acrss the cil r the emf thrugh it, and this vltage is als prprtinal t the driving current, 1V " = d8 dt I: Therefre, if we take the rati f I t 1V at each temperature value we can get a nrmalized data ( data which eliminates the eect f temperature n the resistivity if the cpper cils ) which prvide us t bserve the transitin temperature f the supercnductr YBa 2 Cu 3 O 7 accurately since the ux thrugh the secnd cil dramatically depends n whether the sample is in supercnducting state r nt as we knw frm the Meissner-Ochsenfeld eect. 4 Observatins and Cnclusins In this reprt, nly the best data set, which is given in Fig.5, f will be presented. We knw that the actual value f the transitin temperature is Kswe are able t measure the transitin temperature. One f the main reasns that can cause this errr is the data cllecting way. Data recrding frm the multimeters and the temperature readings frm the temperature cntrller cannt be able dne with cmputer r any ther prfessinal data recrding methd. All three cmpnents are read by three dierent peple as a result it is nt pssible t recrd all three readings at the same time and it is unavidable t have time lags f several secnds between each peple recrding. This type f inability bviusly can cause sme shifts in the nrmalized signal and sample temperature curve. Therefre, fr the future it will be better if ne uses mre prfessinal data recrding methd. Anther reasn is related t the cling dwn prcedure f the sample. There are tw thermmeters respnsible frm the temperature reading. One f them just n the sample and the ther near the cld nger, abut 20 cm belw the sample, which we put the sample at the end f it. It is bserved that during the cling dwn prcedure there is always sme temperature dierences even 50 K, althugh ideally they must be the same. This bring the questins abut accurity f the sample temperature measurement technique. The nly thing which we can d t slve that prblem is putting sme vacuum grease between thermmeter and the sample t enhance the thermal cntact and a tape plug t strenghten the mechanical cntact. Fr the future experiments it is critical t make sme imprvements in thermal cntact either between the sample and the cld nger, r the sample and thermmeter. On the ther hand we shuld be very careful abut smething, which is related t the shape f the curve. Fr a typical supercnductr we usually expect this curve t be like the ne given 8

9 Nrmalized Data Sample Temperature (K) Figure 5: Typical experimental data. Temperature is ramping up frm 80 t 100 K.Fr this graph T c 91:5 C K. At abut 87 K there is an bservable change in the rati f the ux thrugh the cils this means that there is sme change frm nrmal state t supercnducting state. But as it can be seen frm the graph that the change is nt a sudden r sharp ne, which means that there are still sme regins inside the sample in their nrmal state s it takes sme time fr the sample t be unifrmly in supercnducting state. And this the temperature f T c 91:5 C K which we usually dene as transitin temperature fr these type f curves. 9

10 Nrmalized Signal Transitin t supercnducting state T Temperature Figure 6: Expected shape f the curve given in Fig.5. in Fig.6, but bviusly ur data is nt exactly that type. Althugh we can easily cnclude that after T c 91:5 C K, tha sample is perfectly supercnducting there is sme strange behavir in the regin between the 89 K and 91 K in ur curve. The parablic behavir in the curve fr that temperature interval less prbably related t the supercnductr itself like the purity, because the prperties f the supercnductr nly eect the sharpness f the transitin curve and we still d nt expect that increase befre transitn t the supercnducting state is cmpleted. Therefre, it must be smething related t the experimantal setup and the methd f the measurements, like reasns because f the cils, funny temperature uctuatins etc. An explainatin fr that unexpected parablic behavir is pssible if we lk at the behavir f the drive cil itself durin the prcess. As it can be seen frm Fig.7 that there is sme strange behavir f the current thrugh the drive cil. It is smewhat cnstant aswe expected but starts t decrease at abut 85 K and cmpletes decrease at abut 90 K and cnstant after that. This type f behavir bviusly related t the whether the sample is in supercnducting state r nt, since as the sample getting supurcnducting the magnetic ux thrugh the drive cil is changing t. May be the surface currents that are created n the sample induces emf thrugh the drive cil s current changes and mre interestingly the decrease in the current ccurs as the transitin t supercnducting state happens. Finally, when the surface currents reach their nal value as the transitin cmpleted the current is again 10

11 Current Thrugh Drive Cil (ma) Sample Temperature (K) Figure 7: Current thrugh the drive cil versus temperature f the sample. Observe the sudden drp in the current at abut 85 K and then its again steady at abut 90 K. cnstant but less than the initial as we expected since there are n surface currents at all in the begining. If this explanatin is true then we can say that we d nt need pickup cil anymre since transitin temperature is bservable nly by measuring the current thrugh the drive cil. As a result, it culd be a gd future study t investigate that parablic behavir mre carefully and make sme imprvements in the setup and the methd itself. References [1] Neil W. Ashcrft and N. David Mermin. Slid State Physics. Hlt, Reinhart and Winstn, [2] C. Kittel. Intrductin t Slid State Physics. Jhn Wiley and Sns Inc., [3] P. V. E. McClintck and J. K. Wigmre. Lw-Temperature Physics. Blackie and Sn Ltd.,