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Low temperature physics research in China
19911
Physica 1119& 110B (1982'1 lt,~)0 21)ttll North-Holhmd Publishing Company
L O W T E M P E R A T U R E PHYSICS R E S E A R C H IN CHINA Kuan WE1-YEN
Institute of Physics, Academia Sinica, Beijing, ('hina Low temperature physics research in China began in 19611just after the operation of ('hina's lirst helium liqucfier, l'hc range of activities extends from superconducing materials and magnets in the early days to basic superconductivity and h~w temperature properties of solids at present, and is distributed in diverse institutions and laboratories over the count:-y. l'hc content of this article will be limited mainly to topics related to this conference.
!. Introduction
j o u r n a l s , including Acta Physica Temperaturae
Humilis Sinica. T h e I n s t i t u t e of Physics of A c a d e m i a Sinica, Beijing, built the first C h i n e s e h e l i u m liquefier in 1959, half a c e n t u r y later than the first o n e in the world. C h i n a ' s low t e m p e r a t u r e r e s e a r c h s t a r t e d at the b e g i n n i n g of the sixties, mainly on topics r e l a t e d to the a p p l i c a t i o n s of s u p e r c o n d u c t i v i t y . It was at this t i m e that high field s u p e r c o n d u c t o r s a n d the s u p e r c o n d u c t i n g t u n n e l i n g effect were d i s c o v e r e d in the w o r l d , b o t h of t h e m with imp o r t a n t p o t e n t i a l a p p l i c a t i o n s . Since the sixties, r e s e a r c h has b e e n d e w ) t e d to thesc two aspects, especially to the d e v e l o p m e n t of s u p e r c o n d u c ting m a t e r i a l s a n d s u p e r c o n d u c t i n g m a g n e t s . Scientific r e s e a r c h a n d t e a c h i n g were relatively inactive in the ten y e a r s of the C u l t u r a l Revolution. Basic r e s e a r c h was a b a n d o n e d , being c o n s i d e r e d t o o d i v o r c e d f r o m practice. T h i n g s b e g a n to turn b e t t e r only after the o v e r t h r o w of the " g a n g of f o u r " . T h e r e f o r e , we are now still m a p e r i o d of setting up e q u i p m e n t and e x p e r i m e n t a l facilities a n d r e o r g a n i z i n g r e s e a r c h personnel. F r o m 1978, the C h i n e s e A c a d e m y of Sciences a n d the universities b e g a n to e n r o l g r a d u a t e s t u d e n t s . Acta Physica Temperaturae Hutnilis Sinica, a q u a r t e r l y j o u r n a l s t a r t e d in 1979, has a l r e a d y p u b l i s h e d 10 issues. In 1981 the A l P s t a r t e d p u b l i s h i n g Chinese Physics, which selects r e s e a r c h p a p e r s from 9 C h i n e s e Physics ()378-4363/82/0000-()00()/$fl2.75 (~. 1982 N o r t h - H o l h m d
A t p r e s e n t t h e r e are a b o u t 2/) d o m e s t i c a l l y m a d e h e l i u m liquefiers in the low t e m p e r a t u r e l a b o r a t o r i e s of this c o u n t r y , most of t h e m using h e l i u m gas s e p a r a t e d from n a t u r a l gas from Sichuan Province, Seven institutes of the A c a d e m y have their own low t e m p e r a t u r e l a b o r a t o r i e s . T h e s e are: the Institute of Physics, the Institute of Electric E n g i n e e r i n g , thc Institute of H i g h E n e r g y Physics, the Institute of A t o m i c E n e r g y (the liquefier in this institute is the only o n e i m p o r t e d from the Soviet LInion and is preco..'~led with liquid h y d r o g e n ) , the S h a n g h a i Institute of M e t a l l u r g y , the S h e n g y a n g I n s t i t u t e of Metals, a n d the Sichuan S o u t h w e s t crn Institute of Physics. l_ow t e m p e r a t u r e l a b o r a t o r i e s have been scl up in the D e p a r t m e n t s of Physics in four universities to carry out low t e m p e r a t u r e r e s e a r c h a n d to train s t u d e n t s of low t e m p e r a t u r e specialties. T h e s e arc: the U n i v e r s i t y of Science and T e c h n o l o g y of China. Beijing University, N a n j i n g University and F u d a n University. R e s e a r c h institutes and factories of the Ministry of M e t a l l u r g y , such as lhc Beijing R e s e a r c h A c a d e m y of N o n - f c r r o u s Metals, the S h a n g h a i Institute of N o n - f e r r o u s Metals. the C h a n g s h a Institute of Mineral M e t a l l u r g y , a n d the I?,aoji Institute of N o n - f e r rous Metals. In a d d i t i o n , the C h a n g c h u n In-
K. Wei-Yen / Low temperature physics research in China
stitute of Physics and Zhongshan University are building their own low temperature laboratories. Since the downfall of the "gang of four", scientific exchange within China has become much more active. Since 1976, a national "High To" meeting has been held once every two years with a participation of around 100 each time. In recent years, China has increased her contact with foreign countries. Quite a few famous low temperature physicists have visited China with the late K. Mendelssohn of Oxford University in the lead. Dr. Mendelssohn visited China four times before and during the Cultural Revolution. China has sent visiting scholars to Europe, the United States and Japan, and Chinese scientists attended LT15 and other international meetings since then, taking papers for presentation at these meetings.
1991
The new liquefier of the Institute of Physics, with a two-stage expansion engine, is capable of producing 40 1/h using LN2. The plant produced about 10 0001 of liquid helium last year. A turbine expander helium refrigerator has been developed for space simulation chamber application. Tests on a smaller turbine rotor with higher operating speed are in preparation. China's first dilution refrigerator is now also in operation [1]. The refrigeration capacity is 2 4 / z W at 0.l K, and the minimum temperature reached is about 35 inK, with two-stage sintered copper heat exchangers. The heat leak is found to be excessive, being about 10/xW. Dilution refrigerators with silver powder heat exchangers and other technical improvements are under design.
3. Superconducting materials and magnets 2. Cryogenic techniques Our first helium liquefier was based on the Linde cycle with liquid hydrogen precooling. It yielded about 51 of liquid helium per hour and was in service from 1959 to 1965 at the Institute of Physics of the Chinese Academy of Sciences, Beijing. At that time low temperature experiments were restricted to this single laboratory before we developed our version of the expansion-engine-type helium liquefier. In 1964 we succeeded in developing our first helium expansion engine, and a helium liquefier incorporating it was made. The piston is sealed at the room temperature end with a 0.1 mm clearance between the piston and the cylinder. Since 1965, this type of helium liquefier has been adopted by various laboratories and by industry, and over 20 liquefiers have been made, with capacities ranging from 5 to 35 1/h. Expansion engines of the modified D o l l - E d e r type with an automatic inlet valve have been adopted in the helium liquefiers produced by industry, with two sizes of capacities 101/h and 50 l/h, respectively.
In 1962 the Institute of Physics of the Chinese Academy of Sciences started the study of hard superconductors. Investigations were conducted on the superconducting properties of different materials. Development of practical materials and conductor occupied our attention during the subsequent years. Conductors based on NbTi and Nb3Sn are now supplied in batch quantities. Metallurgical and metal physics studies on these materials continue in order to improve their electrical and mechanical properties. The first superconducting solenoids with single core NbTi conductors were made in 1965-66 with field intensities from 3 to 6 T. Since 1972 we have turned to the development of multifilamentary NbTi conductors, following the trend abroad. Most of the present magnets are now made with such conductors, and various forms of composite conductors based on muitifilamentary NbTi are under development for special applications. Studies on the formation of Nb3Sn by diffusion when Nb is immersed in a molten Sn bath began in 1963. Batch production of Nb3Sn tapes based
1992
K. Wei- Yen / Low temperature physics research in ( h i n a
on this technology took place later. The present production of Nb~Sn tapes is based on two different technologies: C V D {Institute of Metallurgy and Mineralogy, Changsha) and low temperature diffusion {Physics D e p a r t m e n t , Beijing University, Beijing). Most of the laboratory magnets with field intensities around 10T are wound with C V D tapes, The tape passes with a rather high speed of about 40 to 80 m/h in the reaction chamber. The high speed adopted and the addition of carbon result in fine crystal grains and raise the J~ of the tape. A magnet with an inner working diameter of 4 0 m m and a central field intensity of 11 T, composed of an inner solenoid of C V D Mb~Sn tape and an outer solenoid of NbTi has been operated in our Institute for several years I21. The low temperature diffusion technology results from a study of the phase diagram of the N b - S n - C u ternary system [3], For the N b - S n binary system, the t e m p e r a t u r e of the diffusion oven for the Sn-covered Nb tape must not be below 930°C, otherwise NbsSn3 or NbSn2 would form. But for the N b - S n - C u ternary system, the case is quite different. From the phase diagram, one comes to the conclusion that low temperature diffusion employing Cu-(30 ~ 50)wt% Sn alloy would result in Nb3Sn layers free of the Sn-rich intermediate phases. A small solenoid with a central field intensity of Ill T was wound with this type of tape in 1975 (Physics Department, Peking University, Beijing) [4]. There seems to be further ground for improving the J~ of Nb3Sn tapes besides optimizing the effect of Cu and ZrO2 in the production technology [5-7]. Measurements of the transition characteristics of Nb3Sn tapes by an inductance method show that the Nb3Sn layer is not uniform [8]. I m p r o v e m e n t s in the uniformity of the layer should bring about a higher J~. Multifilamentary Nb3Sn composite conductors made by the bronze process are in development (Baoji Institute of Non-ferrous Metals Research, Boaji) [9-12]. The in situ method of fabrication
is now also under investigation (Shanghai Institute of Metallurgy, Shanghai) [13]. In 1974, the General Institute for Non-Ferrous Metals, Beijing, took interest in the superconducting properties of V.~Ga which were found to be similar to those of Nb~Sn. Jc was found to be 2.5x 10s A/cm -" at f T . The application of superconductors has been actively developed in line with the development of superconducting materials. In general. emphasis has been placed on the development of magnet systems for laboratory experiments, high energy physics, controlled thermal-fusion reactors, electrical machines, magnetic separators and other different applications.
4. Type I1 superconductivity One m a j o r finding has been obtained during a study of the effect of heat treatment on the superconducting properties of Pb-Sb alloy [14]. Pb and Sb were first melted together and quenched to obtain a uniform supersaturated solid solution. Subsequent heat treatments produced successive precipitation of Sb from the alloy matrix. The critical currents and the upper critical magnetic fields were measured at each stage of precipitation and compared. It is shown in fig. 1 that the Jc(H) curves of
~10 4
.......
H.tl T=4.2K
E u
,,,
210 3 _,o 102 5x10
0
1000 H(G)
2000
Fig. I. J~-H curves of Pb-Sb alloys at 4.2 K at various stages of precipitation. -- For Pb-I wt% Sb specimen: O, quenched; • after ~h aging at 98°C; I , after 11h aging at 98°C; ~), after 54 h aging at 9R°C. - . - For Pb-2.8 wt% Sb specimen: ×. quenched: +, after 54 h aging at 98°C.
1993
K. Wei-Yen / Low temperaturephysics research in China
Pb + 1 wt% Sb in various states differ very much. As the precipitation progresses the critical magnetic field at low current densities continues to decrease, but the low field JcIH..... t is enhanced. The decrease of solute concentration causes an increase of the electronic mean free path, so that the critical magnetic field under low current density decreases. The appearance of precipitates increases the flux pinning sources, so that superconductivity may persist under larger current densities (in the low field region). The decrease of He2 and the increase of J¢ with the advent of precipitation gave us the first direct testimony to the hypothesis that the origin of high mc2 is connected to the short electronic mean free path due to alloying, whereas the origin of high J~ is connected to the large amount of flux pinning sources produced by precipitation. This concept was exploited in our subsequent development of NbTi technology. Cheng Guo-kuang et al. [15] measured the heat capacity of Nb3Sn between 16.0 and 20.3 K. A jump in specific heat at T¢ (17.88 K) is found with A C = 2.21 joule/mole, deg. From the measured A C and by using the thermodynamic relationship, the thermodynamic critical magnetic field at 0 K, tto ~-5300 Oe, was obtained. Using a persistent-current method, Chen Pufun et al. [16] measured the critical characteristic curves J~(H) of cold-worked niobium wire. The "peak effect" in the Jc(H) characteristic was observed (fig. 2). Magnetic flux jumping has been observed in sintered tubular Nb3Sn magnets by Kuan Weiyen et al. [17]. The "frozen" field distributions need not be symmetrical with respect to the mid-plane perpendicular to the cylindrical axis. Theoretical study related to type lI superconductivity has also been carried out in the Institute of Physics. By introducing a paramagnetic energy term into the Ginsburg-Landau equations, Li Hung-cheng [18] deduced a formula for the critical magnetic field for type II superconductors, which is in good agreement with the experimental data of T i - V alloys.
40
3O < u 2O
|0
2
4 6 H (KOe)
8
Fig. 2. Critical characteristic curve Jc(H) of cold worked niobium wire. Hu Su-hui (Shanghai Institute of Metallurgy, Shanghai) has reported on "Superconducting pinning in BCC niobium-base alloys" [19]. The structure dependence of critical current density Jc in superconducting alloys N b - Z r and Nb-Ti was studied by means of X-ray analysis and tensile test. Experimental results indicate that in the absence of second phase particles, annealing increases Jc in deformed alloys due to rearrangement of dislocations into the cell structure and the cell walls are effective pinning centers for magnetic flux. In the precipitation process of second phase particles, new dislocations are formed due to relaxation of the coherent field. These new dislocations increase the dislocation density and the flux pinning ability of the cell walls, which in turn leads to a further increase of Jc. The mechanism that causes precipitates to increase the current-carrying ability in N b - Z r and Nb-Ti alloys is therefore the same as that of cold-work deformation.
5. High Tc superconducting materials The study of superconducting materials and superconducting magnets has been gradually shifted to the industrial laboratories. Research
1994
K, Wei- Yen ,/ l.ow temperature physics re~earch in ('hina
institutions under the Academy and the universities are now devoting their efforts towards basic study but still with much emphasis on superconductivity.
980°C. They obtained high 7[.-~ 22 K film with this mixed phase structure. However, with singlc phase AI5 structure the 7) of the film is not st~ high.
5.1. NbjGe
5.2. Nb,Si
Although high T~ Nb~Ge has been successfully prepared by the thin film technique, there is uncertainty about the formation and stabilization of the metastable high T~- AI5 Nb~Ge phase. The A15 NbjGe with onset Tc 23.08 K has been made by getter sputtering by Li Lin et al. [20]. If the deposited film is non-stoichiometric and Ge is in excess, containing a certain amount of second phase (T)NbsGe> the Tc could be very high (~>22 K). The role of NbsGe3 is to stabilize the A15 NbjGe phase with small lattice parameter (5.14 .,k), which contributes to the high T~'s of the •m. A correlation between deposition temperature TD and Tc in the temperature range 750-101)0°C is plotted in fig. 3. Films were obtained with T~ ~ 22 K when TD is 950-980°C. Li Lin et al. have found from X-ray diffraction results that the AI5 phase increases with increase of TD; if TD is below 800°C the film consists mainly of tetragonal NbsGe3 phase and T~ is low. When TD is above 850°C, the A15 phase increases and NbsGe3 decreases, but with the increase of temperature above 900°C there is still a certain amount of NbsGe3 which persists to
It has been predicted that if A I5 Nb~Si is synthesized, it would have a 1~ higher than 25 K. Some scientists in China have been interested in the AI5 structure of Nb~Si [21]. They have prcpared A I5 Nb~Si with levitation-melting and then annealing (at 750°C for 100h). The superconducting transition temperature T,: is just above 7 K. But X-ray diffraction patterns indicate that the samples include diffraction lines from other coexisting crystalline phases besides A15 Nb~Si. During his stay in Japan as a visiting scholar ol the Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendal, Wang Wenkui and his Japanese collaborators [221 studied the preparation of single-phase A15 NbjSi san> pies. Amorphous N b - 1 9 w t % Si alloy prepared by the rapidly cooling technique was annealed while being subjected to a pressure of 100 kbar. X-ray identification made on the alloy specimens quenched to ambient conditions has shown that pressure greatly alters the crystallization characteristics, and the cubic A15 compound becomes a preferentially forming phase in place of the tetragonal compound at temperatures ranging from 710 to 800°C. Superconducting properties have been measured for the single-phased A I5 structure with a - 5 . 1 5 5 A showing T~ to bc 3.5 K and the temperature coefficient of H,e 15 kOe/K.
25
~20
5.3. NbC
15
750
i
800
i 850
i
900 To (°C)
I 950
1010
0
Fig. 3. The connection between TD and T~ of getter sputtered N b - G e film.
Bl-type NbC was first synthesized by the high pressure-high temperature method by Zhao You-xiang et al. [23]. The reaction time of only a few seconds between niobium and carbon is much shorter than that of other methods. The
K1 Wet-Yen I Low temperature physics research in China
1995
dimensions of the single crystals are of the order of 5 0 - 7 0 0 # m . The lattice parameter range of NbC so prepared is 4.4(I-4.48 A. The maximum superconducting transition temperature Tc is 11.5 K, commencing at 12.5 K. o
6. Fast quenched alloys 6.1. A I - S i
The effect of heat treatment on the microstructure and superconducting properties of the eutectic alloy A l - l l . 3 a t % Si prepared by the splat quenching technique has been studied by Kuan Wet-yen et al. in Beijing [24]. After annealing at 100°C for 50h, the transition temperature of fast quenched alloy was reduced from 4.2 to 1.88 K. We know from the diffraction image obtained by transmission electron microscopy that the state consists of two regions (l and 2 of fig. 4). Region ! is located in region 2 like an island. Selected area diffraction patterns of region 1 show that it is an o~Al solid solution with single-crystal orientation. X-ray analysis results indicate that the lattice parameter of the c~A1 solid solution is smaller than that of pure aluminium. Selected area diffraction patterns of region 2 shows a few diffusion rings. We believe that region 2 is an amorphous phase of aluminium and silicon atoms which are distributed statistically and retain shortrange order. In the amorphous phase a new phase emerges after annealing which appears as a bright point in the dark field image. The results of the analysis show that these dispersive granules, which look like spheres, are Si crystals. Fig. 5 shows the change in the resistance with the intensity of the applied magnetic field. The magnetoresistivity exhibited an anomalous behaviour when the transport current was larger than several mA. Over a certain superconducting-normal transition range the resistance of the sample decreases with increasing magnetic field. In this case the magnetic field did not tend to quench the superconductivity, but to promote it.
[] Fig. 4. AI-I 1.3 at% St, rapidly cooled and then heat treated at I(II)~C for 50h. a - T r a n s m i s s i o n electron micrographs (JIM-1000) 3000 × (dark field); b - s e l e c t e d area diffraction pattern of region 1; c - s e l e c t e d area diffraction pattern of region 2.
100mA
T = 1.zt7 K
/
I
!
1
o. .
. .
20
I
. 40
. . .
60
80
1
1"00
'
H (0e) Fig. 5, The change in the resistance with the intensity of the applied magnetic field for AI-I 1.3 at% Si alloy prepared by splat cooling and annealed at 100°C for 50 h. HI]III the plane of the sample zero of ordinate for each curve is shifted.
K. Wei- Yen / Low temperature physics research in China
1996
The authors suggest that there are two superconducting phases in A 1 - l l . 3 at% Si alloy prepared by rapid cooling and annealed at 100°C for 50h. The interaction between these two phases in a magnetic field leads to an anomalous behaviour. The heat capacity m e a s u r e m e n t s were carried out cooperatively by the authors and the Laboratoire de Physique des Solides in Orsay [25].
7. Out of equilibrium superconductivity
6.2. A I - S i - G e
The same group in Beijing observed a double superconducting transition with both resistancet e m p e r a t u r e (fig. 6) and resistance-magnetic field characteristics in an A1-8.3 at% Si-8.5 at% G e alloy which had been fast quenched and annealed at 100°C for 50 h. An explanation is offered in terms of the existence of two superconducting phases in this alloy [26[. 6.3. T i - P d
Jin Zuo-wen et al. [27] have prepared T i - P d alloys which were arc-melted in a purified argon
1.0i
E: 0.5
0
:--
1.o
__-
I
15
I
.
2o T (KI
Fig. 6. N o r m a l i z e d r e s i s t a n c e vs. t e m p e r a t u r e of A 1 - 8 , 3 a t % S i - 8 . 5 a t % G e alloy fast q u e n c h e d a n d a n n e a l e d at 11)()°C/5()
h.
atmosphere on a cold copper hearth. Magnetic susceptibility measurements showed the 7\, to be 3.67 K for optimum composition (TL.sPd,,5). After that, Liu Zhi-yi et al. [28, 291 made an amorphous superconducting alloy Ti~Pd> by means of the levitation-melt-rapid-cooling method. Low temperature measurements showed T~ to be 2.1 K.
Chen Ying-fei et al. [30] have reported that they have measured the changes of d.c. resistance of superconducting Pb films driven into the nonequilibrium state using tunnel injection of quasi-particles. The specimens were tunnel junctions of A I AIeO~-Pb. In the case of Pb film in contact with He I, the experimental curves at various temperatures are similar to those of lguchi. When the injection current is greater than a certain threshold L,, the d.c. voltage across the Pb film starts to appear and increases gradually on further raising the injection level. In the case of Pb film in contact with He !I, hysteresis begins to appear in the injection characteristic (lig. 7), a p h e n o m e n o n already observed by lguchi. At L - Ic, the Pb film suddenly jumps into the fully resistive state (point b). When the injection level is decreased from point h, the d.c. voltage across the Pb film gradually decreases along the b - c path instead of along the b - a path. After the injection current is decreased to I,~ (point c) the Pb tihn passes into the zero resistance state. (_'hen Ying-fei c t a l . havc found m addition that the resistive transition in the Pb film is r e m e m b e r e d when the injection current is lowered, even after the film has passed into the zero resistance state. Before the injection level is decreased to below point d, the film will always go through the h - c path on further cyclic increase and decrease of the iniection current. This
K. Wei- Yen / Low temperaturephysics research in China
30 E 20;
t3
Apb(4 2) 095mY .
--
.
Rt = ~2 .o.
10
No. 7913 ";7,
L
0.5mY
j
0
V(mV)
Fig. 7. The nonequilibrium voltage curve of Pb film immersed in liquid helium at temperature T < 2 K. r e m e m b r a n c e is completely erased by lowering the injection level to below point d. The threshold corresponding to point d is denoted as It3. In the case of a specimen placed in vacuum, the injection characteristics are similar to those obtained in the case of a Pb film in contact with HeI. No curve indicated any hysteresis phenomenon, even down to 1.54 K. This feature should indicate the importance of excess phonon density in the resistive transition of nonequilibrium superconductors. Chen et al. suggest that the nonequilibrium zero resistance state can be divided into two regions. In the region characterized by I t < I t 3 (section 0-d), the Pb film is in a homogeneous superconducting state. For the region characterized by It ~> It3, the Pb film may be found in one of two kinds of zero resistance states. One of them appears when It is increased from zero to It2 ( d - a ) . T h e other appears when It is lowered from point b to below It1 ( c - d ) . T h e authors assume that the zero resistance state of section ( c - d ) is an inhomogeneous state. -/t3 is the threshold above which the inhomogeneous intermediate state may appear. Once the inhomogeneous state appears, it retains its in-
1997
homogeneous character whether the resistance is zero or not, so long as L is kept above It3. Wei Chong-de et al. [31] have reported the appearance of an inhomogeneous state in Pb film at high injection level. The double tunnel junction is formed by three Pb films of thicknesses -~ 1500 A, evaporated onto a glass substrate. At the beginning the energy gap of the detector decreases continuously with increasing injection current. From the I - V curves of the detector a new structure can be seen to emerge as soon as the injection current reaches some threshold value. This shows that two energy gaps occur in the middle Pb film. The same Beijing University group [32] has reported a preliminary experimental observation at 4 . 2 K on the interaction between two Pb Josephson junctions separated by several thousand A. Recently, Wang Chang-heng and Zhu Xueyuan [33] published a theoretical work about oscillation in a nonequilibrium superconductor. Using the # * - m o d e l and the R o t h w a r f - T a y l o r equations, the authors predict that a superconductor will oscillate between two nonequilibrium superconducting states with different energy gaps when quasiparticles are injected at the edge of the energy gap through the tunneling effect. The main sources of origin of this oscillation are the quasiparticle injection at the gap edge and the existence of recombination phonons with energy /2 = 2A(n). Under certain nonequilibrium conditions, the recombination phonon system does not take part in Cooper pair-breaking processes and can further stimulate the quasiparticles to recombine into pairs.
8. Josephson effect Interest in Josephson effect devices grew in the seventies and now several laboratories are engaged in different aspects of device applications and theoretical studies. Most of the subjects are concerned with the voltage standard,
19tI,R
K. g~'i- Yen
I.ow temln, rature physics research in ("hina
m a g n e t o m e t e r and high frequency device. Somc of these topics will appear in this conference. A general review has been given in ICEC-8 [341. Here I will mention two theoretical works, One is "Effect of a magnetic field on the height of the microwave-induced step" [351. This paper proposes a more advanced theory for the microwave-induced step phenonemon. With this theory, the authors studied in detail the effect of magnetic field and microwave power on the height of the microwave-induced step. The theoretical predictions and the existing experimental facts are consistent. The other paper is titled "Static behavior of variable thickness superconducting microbridges'" [36]. In the case of variable thickness superconducting microbridges, the bridge region is often considered as a one-dimensional problem, and the presence of the bank regions only provides some rigidboundary conditions. In the zero voltage case, the order p a r a m e t e r in the bridge region satisfies a one-dimensional G i n z b u r g - L a n d a u equation. The solutions, which satisfy the Ginzburg-Landau equation, are subject to rigid boundary conditions, and are not unique. In order to determine the solution completely, it ~s realized that one must satisfy the condition of minimum free energy of the system.
9. Theory of superconductivity In ref. 37 a series expansion solution for T~ is derived from the Eliashberg equation. The superconducting critical temperature T, and its convergence are discussed. The results suggest that when it is convergent, the T~. series yields values for the superconducting critical temperature which will be closer to the numerical solution than the ones from the A l l e n - D y n e s formula. From 1979 to 1981, a group from the University of Science and Technology of China made further exploration on the T~ series solution and tried to derive a formula fur T~ from the
Eliashbcrg equation outside the convergence circle of the T,: series solution [38[. For the new mechanism and origin of superconductivity, the "'interface" and "'surface'" problem are very important. There is some work on the calculations of elementary excitation about the surface and interface [3~)1, theoretical studies on the surface of small superconducting particles and clad particles [401, and band structure calculation of I,a. [41]. There is also stud\ on the hybridization of conduction band for close-packed compounds by means of the I.MT() method [42[. The latter work emphasizes thal heterogeneous coordination would be a necessary condition for intensifying hybridization. which is beneficial to high 7",. Various expcrimental results on AI5. BI and other closepacked compounds are in fair agreement with this basic principle. Some theoretical work on two-band superconductors has been published [431 and a new model for excitonic superconductors proposed
1441. 10. Low temperature properties of solids No research is being conducted in China on the superfluidity of liquid aHe or ~He. Apart from research on superconductivity, research on the low temperature properties of solids was started a few years ago by several groups in various laboratories. The following arc some examples [451.
10. 1. Amorphous films Recently the investigation of electronic transport properties in the disordered system has attracted a lot of attention. Han Shun-hui et al. [46] have reported low temperature resistivity anomalies observed in sputtered a m o r p h o u s G d Co alloy films. In uniaxial perpendicular magnetic anisotropic films (with a predominance of G d - C o pairs) the resistivity exhibits a minimum
K. Wei- Yen / Low temperature physics research in China
at about 50 K and from 4.2 to 25 K the resistivity is proportional to - I n T. In order to understand further the electronic transport properties of amorphous alloy films, the authors measured the temperature dependence of the reduced resistivity of a series of TbxFet(Kj-x (20-
1999
10.3. C D W The CDW has been found in several layered compounds, for example, 1T-TaS2 and 2/-ITaSe2. Recently, my colleagues, in cooperation with Prof. C.W. Chu [50], have carried out the resistivity measurements on 1T-TaSz at high pressure and low temperature. They found that at 13kbar the commesurated transition is depressed and at - 2 0 K the resistance begins to decrease but no zero resistance appears down to 1.5K.
10.2. Spin glasses
References
There has been increasing interest in spin glass ever since Cannela and Mydosh first observed the cusp of a.c. susceptibility in AuFe. Zhang Dian-lin et al. have measured the susceptibility and resistivity of the alloy Fe6Mo~sNi76 and found a cusp in its a.c. susceptibility at about 7 K [48]. The cusp is sensitive to background field and will shift to higher temperature as the measuring frequency increases. This is a new kind of spinglass alloy different from canonical ones. From the resistivity measurements no anomaly was detected near Tf, but there was a resistivity minimum at a higher temperature. More recently, Zhan Wen-shah [49] studied the magnetic phase diagram of the amorphous alloys (Pb0~Ag0.10)85-xSilsFex. They measured the a.c. susceptibility of these alloys at a temperature of 1.9 to 150 K by means of mutual inductance. The curves of susceptibility X vs. temperature T have a sharp peak or cusp when )6 = 1, 3, but the peaks gradually become smoother with increasing concentration of Fe. The results show that the alloys exhibit a transition from the spin-glass phase to the paramagnetic phase when )6 = 1, 3, 5, and a transition from the spin glass to the ferromagnetic then to the paramagnetic phase when X = 7. The author has only observed the Curie temperature Tc when )6 = 10.
[1] Ran Qi-ze, Qian Yong-jia and Zhu Yuan-zhen, Acta Physica Temperaturae Humilis Sinica 1 (1979) 18. [2] Yu Kun, Zao Yu-ying, Meng Qing-hui, Han Shi-ii, Han Cui-ying and Cui Chang-geng, Acta Physica Temperaturae Humilis Sinica 2 (19801 305. [3] J. Chinese Assoc. Refrigeration, no. 1 (1979) 3. [4] Low Temperature Physics Group, Physics Department, Beijing University, Acta Physica Sinica 24 (1975) 452. [5] Yang Hong-chuan, Tong Hu-song, Wu Mei-ying and Peng Ying, Acta Physica Temperaturae Humilis Sinica 1 (1979) 284. [6] Cheng Peng-zhu, Wang Gui-rong, Chert Gui-yu, Luo Qi-guang, Ma Xiao-hya, Wang Zu-lun and Liu Zhenxing, Acta Physica Temperaturae Humilis Sinica 3 (1981) 132. [7] Jin Zuo-wen, Guan Zhen-hui, Lin Shu-yuan, Ji Guangda, Zhao Ya-qin and Bi Wen, Acta Physica Temperaturae Humilis Sinica 3 (1981) 136. [8] Wang Shou-zheng, Acta Physica Temperaturae Humilis Sinica 2 (198(I) 27. [9] Physics Department of Beijing University and Baoji Institute of Non-Ferrous Metals Research, Acta Physica Sinica 28 (1979) 406. [10] Tang Xian-de and Zhou Zhong-yi, Acta Physica Temperaturae Humilis Sinica 1 (1979) 221. [ll] Baoji Institute of Non-ferrous Metals Research, Acta Physica Temperaturae Humilis Sinica I (1979) 339, [12] Tang Xian-de, Shan De-qin, Li Ming-sheng and Zhu Ding-si, Acta Physica Temperaturae Humilis Sinica 2 (19801 220. [13] Shanghai Institute of Metallurgy, Academia Sinica and Physics Department, Peking University, Acta Physica Temperaturae Humilis Sinica 2 (1980) 337. [14] Chang Chi-jui, Tsao Hsiao-wen and Kuan Wei-yen, Scientia Sinica 14 (19641 306; Acta Physica Sinica 20 (1964) 568.
2ttlltl
K. Wei- Yen / Low temperature physics research in (-'hina
II5] Cheng Guo-kuang, Liu Ti-hang and Kuan Wei-yen, Acta Physica Sinica 21 (1965) 817. [1~] ('hen Pu-fun, Cheng Guo-kung and Kuan Wei-yen, Acta Physica Sinica 21 (1965) 889. 1171 Kuan Wei-yen, Liu Ti-hang and Cheng Guo-kuang, Acta Physica Sinica 21 (1965) 1345. [18] Li Hung-cheng, Acta Physica Sinica 21 (1965) 5~0. [lg] Hu Su-hui, Acta Physica Temperaturae Humilis Siniea 2 (1980) 9; Chinese Physics I (1961) 181. [201 Li Lin, Zhao Bai-ru, Zhou Ping, Guo Shou-quan and Zhao You-xiang, J. Low Temp. Phys. 45 (19gl) no 3/4. [21] Nb3Si Group, Changsha Institute of Metallurgy and Mineralogy and Low Temperature Laboratory, Institute of Physics, Acta Physica Sinica 27 (1978) (~13. [221 Wei-kui Wang, Hiroshi lwasaki, C. Suryanarayana. Tsuyoshi Masumoto, Naoki Toyoda and Tetsuro Fukuse, to be published. [23] Zhao Yuo-xiang, Xu Shiao-ping, Z.hi Sheng-hui, Bi Wen, Chen Hong and Wang Wen-kui, Kiixue Tongbao 24 (1979) 1014. [241 Kuan Wei-yen, Chcn Sy-san, Yi Sun-sheng and Wang Zu-lun, Acta Physica Sinica 3(1 11981) no. 9. [25] Kuan Wei-yen, Chen Sy-sen, Yi Sun-sheng, Wang Zulun, Cheng Wu and Peere Garoche, to be published. [261 Kuan Wei-yen, Chen Sy-sen, Yi Sun-sheng. Wang Zulun and l..in Ying, Acta Physica Sinica, to be pub fished. [27] Jin Zuo-wen. Ran Qi-ze. Liu Zhi-yi, Jin Duo, Zhao Zhong-xian, Ma Ming-rong and kuo Qi-guang, Acta Physica Temperaturae Humilis Sinica 2 (198(I) 2~1. [28] Liu Zhi-yi, Jin Duo, Jin Duo-wen, Ma Ming-rong, Zhai~ Zhong-xian and Luo Qi-guang, Acta Physica Temperaturae Humilis Sinica 3 (1981) t~. I29[ Liu Zhi-yi, Wang (,hao-guo, Cheng Peng-zhu, Yan Dayu, Ma Xiao-hua and Luo Qi-guang, Acta Phvsica Temperaturae Humilis Sinica 3 (It,~Nl) 13. [30] Chen Ying-fei, Yang Qian-sheng, Tao Hong-jie, Wang Chang-heng, 15u Gui-rong and Shao Li-qin, Aeta Physica Temperaturae Humilis Sinica 3 (1981) 31; ('hinesc Physics, to be published. I31l Wci Chong-de, kuo Xim~ L.an and Meng Xiao-fan, to bc published. [32] Luo Xiao-lan, Wei Chong-De and Meng Xiao-fan, Acta Physica Temperaturae Humilis Sinica 3 (I98 l) 35.
[331 Wang Chang-heng and Zhu Xue-yuan, J. t.ow letup Phys. 42 (1981} 277. [341 ('.S. Hung, Proc. IC'EC'-S (lt,~N0). [351 Wu Hang-sheng, Ji Guang-da and Liu Fu-su, Acta Physica Sinica 26 (1977) 353. [361 Yao Xi-xian, Acta Physica l'emperaturae Humilis Ginica 2 (1980) 1. [371 Wu Hang-sheng. Tsai Chien-hua, Kung Chang-tch, Chi Kuang-ta and Tsai (,hun-tao, Scientia Sinica 20 (1077) 583; 21 (1978} (~2:22 (197t,~) 419: Wu Hang-sheng and Ji Guang-da, Acta Physica Sinica 27 (1978) 746: Scientia Sinica 22 (1979) 513: Wu Hang-shen 27 (1978) 75t~. [38] UST Group, Scientia Sinica 23 (198()) 1378: Acta Physica Sinica 29 (19g(I) 41(1, 844, 11£6. 133S; 30 (It)NI) 277, 783. Chinese Physics 1 (1981)43, 116. [391 Yang Zhan-ru, Gong Chang-dc and Cai Jian-hua, Acta Physica Temperalurac Humilis 2 (1980): Chinese Physics I ( 1 9 g l ) 1£8:
Gong (`hang-dc, Yao Xi-xian and ('ai Jian-hna, Acta Physica Temperaturae Humilis Sinica 2 (19811) 177 [4111 Zhao Zhong-xian, Liu Fu-sui and Han Ru-shan, Acla Physica Siniea 28 (1979) 222. 544. 141] Zhang l,i-yuan. Ying Dao-le and Han Ru-shan, Acta Physica -lemperaturae Humilis Ninica 2 (1~81)) t~: ('hinese Physics 1 (1~}81) 21111. I42] Yin Dao-le and Zhang Li-vuan. Acta t'hysica Smica 29
(IgSO) ,h77. 1431 Tao Rui-bao. Acta Physica Temperaturac ttumilis Smica I (1979) 1. 144] Lei Xiao-lin, Acta Physica lcmpcraturac Humilis Sinica 2 (ITS8()) 98; Chinese Physics 1 (1981) 21)5. 1451 Han Ru-qi Han Ru-shan, l.iu Fu-sui and Chao Chunghsien, J. Non-('rystalline Solids 35 & 3~5 (1980) 141: Kexue Tongbao 24 (lt)7~)) 0SS. 14
Physica 1119& 110B (1982'1 lt,~)0 21)ttll North-Holhmd Publishing Company
L O W T E M P E R A T U R E PHYSICS R E S E A R C H IN CHINA Kuan WE1-YEN
Institute of Physics, Academia Sinica, Beijing, ('hina Low temperature physics research in China began in 19611just after the operation of ('hina's lirst helium liqucfier, l'hc range of activities extends from superconducing materials and magnets in the early days to basic superconductivity and h~w temperature properties of solids at present, and is distributed in diverse institutions and laboratories over the count:-y. l'hc content of this article will be limited mainly to topics related to this conference.
!. Introduction
j o u r n a l s , including Acta Physica Temperaturae
Humilis Sinica. T h e I n s t i t u t e of Physics of A c a d e m i a Sinica, Beijing, built the first C h i n e s e h e l i u m liquefier in 1959, half a c e n t u r y later than the first o n e in the world. C h i n a ' s low t e m p e r a t u r e r e s e a r c h s t a r t e d at the b e g i n n i n g of the sixties, mainly on topics r e l a t e d to the a p p l i c a t i o n s of s u p e r c o n d u c t i v i t y . It was at this t i m e that high field s u p e r c o n d u c t o r s a n d the s u p e r c o n d u c t i n g t u n n e l i n g effect were d i s c o v e r e d in the w o r l d , b o t h of t h e m with imp o r t a n t p o t e n t i a l a p p l i c a t i o n s . Since the sixties, r e s e a r c h has b e e n d e w ) t e d to thesc two aspects, especially to the d e v e l o p m e n t of s u p e r c o n d u c ting m a t e r i a l s a n d s u p e r c o n d u c t i n g m a g n e t s . Scientific r e s e a r c h a n d t e a c h i n g were relatively inactive in the ten y e a r s of the C u l t u r a l Revolution. Basic r e s e a r c h was a b a n d o n e d , being c o n s i d e r e d t o o d i v o r c e d f r o m practice. T h i n g s b e g a n to turn b e t t e r only after the o v e r t h r o w of the " g a n g of f o u r " . T h e r e f o r e , we are now still m a p e r i o d of setting up e q u i p m e n t and e x p e r i m e n t a l facilities a n d r e o r g a n i z i n g r e s e a r c h personnel. F r o m 1978, the C h i n e s e A c a d e m y of Sciences a n d the universities b e g a n to e n r o l g r a d u a t e s t u d e n t s . Acta Physica Temperaturae Hutnilis Sinica, a q u a r t e r l y j o u r n a l s t a r t e d in 1979, has a l r e a d y p u b l i s h e d 10 issues. In 1981 the A l P s t a r t e d p u b l i s h i n g Chinese Physics, which selects r e s e a r c h p a p e r s from 9 C h i n e s e Physics ()378-4363/82/0000-()00()/$fl2.75 (~. 1982 N o r t h - H o l h m d
A t p r e s e n t t h e r e are a b o u t 2/) d o m e s t i c a l l y m a d e h e l i u m liquefiers in the low t e m p e r a t u r e l a b o r a t o r i e s of this c o u n t r y , most of t h e m using h e l i u m gas s e p a r a t e d from n a t u r a l gas from Sichuan Province, Seven institutes of the A c a d e m y have their own low t e m p e r a t u r e l a b o r a t o r i e s . T h e s e are: the Institute of Physics, the Institute of Electric E n g i n e e r i n g , thc Institute of H i g h E n e r g y Physics, the Institute of A t o m i c E n e r g y (the liquefier in this institute is the only o n e i m p o r t e d from the Soviet LInion and is preco..'~led with liquid h y d r o g e n ) , the S h a n g h a i Institute of M e t a l l u r g y , the S h e n g y a n g I n s t i t u t e of Metals, a n d the Sichuan S o u t h w e s t crn Institute of Physics. l_ow t e m p e r a t u r e l a b o r a t o r i e s have been scl up in the D e p a r t m e n t s of Physics in four universities to carry out low t e m p e r a t u r e r e s e a r c h a n d to train s t u d e n t s of low t e m p e r a t u r e specialties. T h e s e arc: the U n i v e r s i t y of Science and T e c h n o l o g y of China. Beijing University, N a n j i n g University and F u d a n University. R e s e a r c h institutes and factories of the Ministry of M e t a l l u r g y , such as lhc Beijing R e s e a r c h A c a d e m y of N o n - f c r r o u s Metals, the S h a n g h a i Institute of N o n - f e r r o u s Metals. the C h a n g s h a Institute of Mineral M e t a l l u r g y , a n d the I?,aoji Institute of N o n - f e r rous Metals. In a d d i t i o n , the C h a n g c h u n In-
K. Wei-Yen / Low temperature physics research in China
stitute of Physics and Zhongshan University are building their own low temperature laboratories. Since the downfall of the "gang of four", scientific exchange within China has become much more active. Since 1976, a national "High To" meeting has been held once every two years with a participation of around 100 each time. In recent years, China has increased her contact with foreign countries. Quite a few famous low temperature physicists have visited China with the late K. Mendelssohn of Oxford University in the lead. Dr. Mendelssohn visited China four times before and during the Cultural Revolution. China has sent visiting scholars to Europe, the United States and Japan, and Chinese scientists attended LT15 and other international meetings since then, taking papers for presentation at these meetings.
1991
The new liquefier of the Institute of Physics, with a two-stage expansion engine, is capable of producing 40 1/h using LN2. The plant produced about 10 0001 of liquid helium last year. A turbine expander helium refrigerator has been developed for space simulation chamber application. Tests on a smaller turbine rotor with higher operating speed are in preparation. China's first dilution refrigerator is now also in operation [1]. The refrigeration capacity is 2 4 / z W at 0.l K, and the minimum temperature reached is about 35 inK, with two-stage sintered copper heat exchangers. The heat leak is found to be excessive, being about 10/xW. Dilution refrigerators with silver powder heat exchangers and other technical improvements are under design.
3. Superconducting materials and magnets 2. Cryogenic techniques Our first helium liquefier was based on the Linde cycle with liquid hydrogen precooling. It yielded about 51 of liquid helium per hour and was in service from 1959 to 1965 at the Institute of Physics of the Chinese Academy of Sciences, Beijing. At that time low temperature experiments were restricted to this single laboratory before we developed our version of the expansion-engine-type helium liquefier. In 1964 we succeeded in developing our first helium expansion engine, and a helium liquefier incorporating it was made. The piston is sealed at the room temperature end with a 0.1 mm clearance between the piston and the cylinder. Since 1965, this type of helium liquefier has been adopted by various laboratories and by industry, and over 20 liquefiers have been made, with capacities ranging from 5 to 35 1/h. Expansion engines of the modified D o l l - E d e r type with an automatic inlet valve have been adopted in the helium liquefiers produced by industry, with two sizes of capacities 101/h and 50 l/h, respectively.
In 1962 the Institute of Physics of the Chinese Academy of Sciences started the study of hard superconductors. Investigations were conducted on the superconducting properties of different materials. Development of practical materials and conductor occupied our attention during the subsequent years. Conductors based on NbTi and Nb3Sn are now supplied in batch quantities. Metallurgical and metal physics studies on these materials continue in order to improve their electrical and mechanical properties. The first superconducting solenoids with single core NbTi conductors were made in 1965-66 with field intensities from 3 to 6 T. Since 1972 we have turned to the development of multifilamentary NbTi conductors, following the trend abroad. Most of the present magnets are now made with such conductors, and various forms of composite conductors based on muitifilamentary NbTi are under development for special applications. Studies on the formation of Nb3Sn by diffusion when Nb is immersed in a molten Sn bath began in 1963. Batch production of Nb3Sn tapes based
1992
K. Wei- Yen / Low temperature physics research in ( h i n a
on this technology took place later. The present production of Nb~Sn tapes is based on two different technologies: C V D {Institute of Metallurgy and Mineralogy, Changsha) and low temperature diffusion {Physics D e p a r t m e n t , Beijing University, Beijing). Most of the laboratory magnets with field intensities around 10T are wound with C V D tapes, The tape passes with a rather high speed of about 40 to 80 m/h in the reaction chamber. The high speed adopted and the addition of carbon result in fine crystal grains and raise the J~ of the tape. A magnet with an inner working diameter of 4 0 m m and a central field intensity of 11 T, composed of an inner solenoid of C V D Mb~Sn tape and an outer solenoid of NbTi has been operated in our Institute for several years I21. The low temperature diffusion technology results from a study of the phase diagram of the N b - S n - C u ternary system [3], For the N b - S n binary system, the t e m p e r a t u r e of the diffusion oven for the Sn-covered Nb tape must not be below 930°C, otherwise NbsSn3 or NbSn2 would form. But for the N b - S n - C u ternary system, the case is quite different. From the phase diagram, one comes to the conclusion that low temperature diffusion employing Cu-(30 ~ 50)wt% Sn alloy would result in Nb3Sn layers free of the Sn-rich intermediate phases. A small solenoid with a central field intensity of Ill T was wound with this type of tape in 1975 (Physics Department, Peking University, Beijing) [4]. There seems to be further ground for improving the J~ of Nb3Sn tapes besides optimizing the effect of Cu and ZrO2 in the production technology [5-7]. Measurements of the transition characteristics of Nb3Sn tapes by an inductance method show that the Nb3Sn layer is not uniform [8]. I m p r o v e m e n t s in the uniformity of the layer should bring about a higher J~. Multifilamentary Nb3Sn composite conductors made by the bronze process are in development (Baoji Institute of Non-ferrous Metals Research, Boaji) [9-12]. The in situ method of fabrication
is now also under investigation (Shanghai Institute of Metallurgy, Shanghai) [13]. In 1974, the General Institute for Non-Ferrous Metals, Beijing, took interest in the superconducting properties of V.~Ga which were found to be similar to those of Nb~Sn. Jc was found to be 2.5x 10s A/cm -" at f T . The application of superconductors has been actively developed in line with the development of superconducting materials. In general. emphasis has been placed on the development of magnet systems for laboratory experiments, high energy physics, controlled thermal-fusion reactors, electrical machines, magnetic separators and other different applications.
4. Type I1 superconductivity One m a j o r finding has been obtained during a study of the effect of heat treatment on the superconducting properties of Pb-Sb alloy [14]. Pb and Sb were first melted together and quenched to obtain a uniform supersaturated solid solution. Subsequent heat treatments produced successive precipitation of Sb from the alloy matrix. The critical currents and the upper critical magnetic fields were measured at each stage of precipitation and compared. It is shown in fig. 1 that the Jc(H) curves of
~10 4
.......
H.tl T=4.2K
E u
,,,
210 3 _,o 102 5x10
0
1000 H(G)
2000
Fig. I. J~-H curves of Pb-Sb alloys at 4.2 K at various stages of precipitation. -- For Pb-I wt% Sb specimen: O, quenched; • after ~h aging at 98°C; I , after 11h aging at 98°C; ~), after 54 h aging at 9R°C. - . - For Pb-2.8 wt% Sb specimen: ×. quenched: +, after 54 h aging at 98°C.
1993
K. Wei-Yen / Low temperaturephysics research in China
Pb + 1 wt% Sb in various states differ very much. As the precipitation progresses the critical magnetic field at low current densities continues to decrease, but the low field JcIH..... t is enhanced. The decrease of solute concentration causes an increase of the electronic mean free path, so that the critical magnetic field under low current density decreases. The appearance of precipitates increases the flux pinning sources, so that superconductivity may persist under larger current densities (in the low field region). The decrease of He2 and the increase of J¢ with the advent of precipitation gave us the first direct testimony to the hypothesis that the origin of high mc2 is connected to the short electronic mean free path due to alloying, whereas the origin of high J~ is connected to the large amount of flux pinning sources produced by precipitation. This concept was exploited in our subsequent development of NbTi technology. Cheng Guo-kuang et al. [15] measured the heat capacity of Nb3Sn between 16.0 and 20.3 K. A jump in specific heat at T¢ (17.88 K) is found with A C = 2.21 joule/mole, deg. From the measured A C and by using the thermodynamic relationship, the thermodynamic critical magnetic field at 0 K, tto ~-5300 Oe, was obtained. Using a persistent-current method, Chen Pufun et al. [16] measured the critical characteristic curves J~(H) of cold-worked niobium wire. The "peak effect" in the Jc(H) characteristic was observed (fig. 2). Magnetic flux jumping has been observed in sintered tubular Nb3Sn magnets by Kuan Weiyen et al. [17]. The "frozen" field distributions need not be symmetrical with respect to the mid-plane perpendicular to the cylindrical axis. Theoretical study related to type lI superconductivity has also been carried out in the Institute of Physics. By introducing a paramagnetic energy term into the Ginsburg-Landau equations, Li Hung-cheng [18] deduced a formula for the critical magnetic field for type II superconductors, which is in good agreement with the experimental data of T i - V alloys.
40
3O < u 2O
|0
2
4 6 H (KOe)
8
Fig. 2. Critical characteristic curve Jc(H) of cold worked niobium wire. Hu Su-hui (Shanghai Institute of Metallurgy, Shanghai) has reported on "Superconducting pinning in BCC niobium-base alloys" [19]. The structure dependence of critical current density Jc in superconducting alloys N b - Z r and Nb-Ti was studied by means of X-ray analysis and tensile test. Experimental results indicate that in the absence of second phase particles, annealing increases Jc in deformed alloys due to rearrangement of dislocations into the cell structure and the cell walls are effective pinning centers for magnetic flux. In the precipitation process of second phase particles, new dislocations are formed due to relaxation of the coherent field. These new dislocations increase the dislocation density and the flux pinning ability of the cell walls, which in turn leads to a further increase of Jc. The mechanism that causes precipitates to increase the current-carrying ability in N b - Z r and Nb-Ti alloys is therefore the same as that of cold-work deformation.
5. High Tc superconducting materials The study of superconducting materials and superconducting magnets has been gradually shifted to the industrial laboratories. Research
1994
K, Wei- Yen ,/ l.ow temperature physics re~earch in ('hina
institutions under the Academy and the universities are now devoting their efforts towards basic study but still with much emphasis on superconductivity.
980°C. They obtained high 7[.-~ 22 K film with this mixed phase structure. However, with singlc phase AI5 structure the 7) of the film is not st~ high.
5.1. NbjGe
5.2. Nb,Si
Although high T~ Nb~Ge has been successfully prepared by the thin film technique, there is uncertainty about the formation and stabilization of the metastable high T~- AI5 Nb~Ge phase. The A15 NbjGe with onset Tc 23.08 K has been made by getter sputtering by Li Lin et al. [20]. If the deposited film is non-stoichiometric and Ge is in excess, containing a certain amount of second phase (T)NbsGe> the Tc could be very high (~>22 K). The role of NbsGe3 is to stabilize the A15 NbjGe phase with small lattice parameter (5.14 .,k), which contributes to the high T~'s of the •m. A correlation between deposition temperature TD and Tc in the temperature range 750-101)0°C is plotted in fig. 3. Films were obtained with T~ ~ 22 K when TD is 950-980°C. Li Lin et al. have found from X-ray diffraction results that the AI5 phase increases with increase of TD; if TD is below 800°C the film consists mainly of tetragonal NbsGe3 phase and T~ is low. When TD is above 850°C, the A15 phase increases and NbsGe3 decreases, but with the increase of temperature above 900°C there is still a certain amount of NbsGe3 which persists to
It has been predicted that if A I5 Nb~Si is synthesized, it would have a 1~ higher than 25 K. Some scientists in China have been interested in the AI5 structure of Nb~Si [21]. They have prcpared A I5 Nb~Si with levitation-melting and then annealing (at 750°C for 100h). The superconducting transition temperature T,: is just above 7 K. But X-ray diffraction patterns indicate that the samples include diffraction lines from other coexisting crystalline phases besides A15 Nb~Si. During his stay in Japan as a visiting scholar ol the Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendal, Wang Wenkui and his Japanese collaborators [221 studied the preparation of single-phase A15 NbjSi san> pies. Amorphous N b - 1 9 w t % Si alloy prepared by the rapidly cooling technique was annealed while being subjected to a pressure of 100 kbar. X-ray identification made on the alloy specimens quenched to ambient conditions has shown that pressure greatly alters the crystallization characteristics, and the cubic A15 compound becomes a preferentially forming phase in place of the tetragonal compound at temperatures ranging from 710 to 800°C. Superconducting properties have been measured for the single-phased A I5 structure with a - 5 . 1 5 5 A showing T~ to bc 3.5 K and the temperature coefficient of H,e 15 kOe/K.
25
~20
5.3. NbC
15
750
i
800
i 850
i
900 To (°C)
I 950
1010
0
Fig. 3. The connection between TD and T~ of getter sputtered N b - G e film.
Bl-type NbC was first synthesized by the high pressure-high temperature method by Zhao You-xiang et al. [23]. The reaction time of only a few seconds between niobium and carbon is much shorter than that of other methods. The
K1 Wet-Yen I Low temperature physics research in China
1995
dimensions of the single crystals are of the order of 5 0 - 7 0 0 # m . The lattice parameter range of NbC so prepared is 4.4(I-4.48 A. The maximum superconducting transition temperature Tc is 11.5 K, commencing at 12.5 K. o
6. Fast quenched alloys 6.1. A I - S i
The effect of heat treatment on the microstructure and superconducting properties of the eutectic alloy A l - l l . 3 a t % Si prepared by the splat quenching technique has been studied by Kuan Wet-yen et al. in Beijing [24]. After annealing at 100°C for 50h, the transition temperature of fast quenched alloy was reduced from 4.2 to 1.88 K. We know from the diffraction image obtained by transmission electron microscopy that the state consists of two regions (l and 2 of fig. 4). Region ! is located in region 2 like an island. Selected area diffraction patterns of region 1 show that it is an o~Al solid solution with single-crystal orientation. X-ray analysis results indicate that the lattice parameter of the c~A1 solid solution is smaller than that of pure aluminium. Selected area diffraction patterns of region 2 shows a few diffusion rings. We believe that region 2 is an amorphous phase of aluminium and silicon atoms which are distributed statistically and retain shortrange order. In the amorphous phase a new phase emerges after annealing which appears as a bright point in the dark field image. The results of the analysis show that these dispersive granules, which look like spheres, are Si crystals. Fig. 5 shows the change in the resistance with the intensity of the applied magnetic field. The magnetoresistivity exhibited an anomalous behaviour when the transport current was larger than several mA. Over a certain superconducting-normal transition range the resistance of the sample decreases with increasing magnetic field. In this case the magnetic field did not tend to quench the superconductivity, but to promote it.
[] Fig. 4. AI-I 1.3 at% St, rapidly cooled and then heat treated at I(II)~C for 50h. a - T r a n s m i s s i o n electron micrographs (JIM-1000) 3000 × (dark field); b - s e l e c t e d area diffraction pattern of region 1; c - s e l e c t e d area diffraction pattern of region 2.
100mA
T = 1.zt7 K
/
I
!
1
o. .
. .
20
I
. 40
. . .
60
80
1
1"00
'
H (0e) Fig. 5, The change in the resistance with the intensity of the applied magnetic field for AI-I 1.3 at% Si alloy prepared by splat cooling and annealed at 100°C for 50 h. HI]III the plane of the sample zero of ordinate for each curve is shifted.
K. Wei- Yen / Low temperature physics research in China
1996
The authors suggest that there are two superconducting phases in A 1 - l l . 3 at% Si alloy prepared by rapid cooling and annealed at 100°C for 50h. The interaction between these two phases in a magnetic field leads to an anomalous behaviour. The heat capacity m e a s u r e m e n t s were carried out cooperatively by the authors and the Laboratoire de Physique des Solides in Orsay [25].
7. Out of equilibrium superconductivity
6.2. A I - S i - G e
The same group in Beijing observed a double superconducting transition with both resistancet e m p e r a t u r e (fig. 6) and resistance-magnetic field characteristics in an A1-8.3 at% Si-8.5 at% G e alloy which had been fast quenched and annealed at 100°C for 50 h. An explanation is offered in terms of the existence of two superconducting phases in this alloy [26[. 6.3. T i - P d
Jin Zuo-wen et al. [27] have prepared T i - P d alloys which were arc-melted in a purified argon
1.0i
E: 0.5
0
:--
1.o
__-
I
15
I
.
2o T (KI
Fig. 6. N o r m a l i z e d r e s i s t a n c e vs. t e m p e r a t u r e of A 1 - 8 , 3 a t % S i - 8 . 5 a t % G e alloy fast q u e n c h e d a n d a n n e a l e d at 11)()°C/5()
h.
atmosphere on a cold copper hearth. Magnetic susceptibility measurements showed the 7\, to be 3.67 K for optimum composition (TL.sPd,,5). After that, Liu Zhi-yi et al. [28, 291 made an amorphous superconducting alloy Ti~Pd> by means of the levitation-melt-rapid-cooling method. Low temperature measurements showed T~ to be 2.1 K.
Chen Ying-fei et al. [30] have reported that they have measured the changes of d.c. resistance of superconducting Pb films driven into the nonequilibrium state using tunnel injection of quasi-particles. The specimens were tunnel junctions of A I AIeO~-Pb. In the case of Pb film in contact with He I, the experimental curves at various temperatures are similar to those of lguchi. When the injection current is greater than a certain threshold L,, the d.c. voltage across the Pb film starts to appear and increases gradually on further raising the injection level. In the case of Pb film in contact with He !I, hysteresis begins to appear in the injection characteristic (lig. 7), a p h e n o m e n o n already observed by lguchi. At L - Ic, the Pb film suddenly jumps into the fully resistive state (point b). When the injection level is decreased from point h, the d.c. voltage across the Pb film gradually decreases along the b - c path instead of along the b - a path. After the injection current is decreased to I,~ (point c) the Pb tihn passes into the zero resistance state. (_'hen Ying-fei c t a l . havc found m addition that the resistive transition in the Pb film is r e m e m b e r e d when the injection current is lowered, even after the film has passed into the zero resistance state. Before the injection level is decreased to below point d, the film will always go through the h - c path on further cyclic increase and decrease of the iniection current. This
K. Wei- Yen / Low temperaturephysics research in China
30 E 20;
t3
Apb(4 2) 095mY .
--
.
Rt = ~2 .o.
10
No. 7913 ";7,
L
0.5mY
j
0
V(mV)
Fig. 7. The nonequilibrium voltage curve of Pb film immersed in liquid helium at temperature T < 2 K. r e m e m b r a n c e is completely erased by lowering the injection level to below point d. The threshold corresponding to point d is denoted as It3. In the case of a specimen placed in vacuum, the injection characteristics are similar to those obtained in the case of a Pb film in contact with HeI. No curve indicated any hysteresis phenomenon, even down to 1.54 K. This feature should indicate the importance of excess phonon density in the resistive transition of nonequilibrium superconductors. Chen et al. suggest that the nonequilibrium zero resistance state can be divided into two regions. In the region characterized by I t < I t 3 (section 0-d), the Pb film is in a homogeneous superconducting state. For the region characterized by It ~> It3, the Pb film may be found in one of two kinds of zero resistance states. One of them appears when It is increased from zero to It2 ( d - a ) . T h e other appears when It is lowered from point b to below It1 ( c - d ) . T h e authors assume that the zero resistance state of section ( c - d ) is an inhomogeneous state. -/t3 is the threshold above which the inhomogeneous intermediate state may appear. Once the inhomogeneous state appears, it retains its in-
1997
homogeneous character whether the resistance is zero or not, so long as L is kept above It3. Wei Chong-de et al. [31] have reported the appearance of an inhomogeneous state in Pb film at high injection level. The double tunnel junction is formed by three Pb films of thicknesses -~ 1500 A, evaporated onto a glass substrate. At the beginning the energy gap of the detector decreases continuously with increasing injection current. From the I - V curves of the detector a new structure can be seen to emerge as soon as the injection current reaches some threshold value. This shows that two energy gaps occur in the middle Pb film. The same Beijing University group [32] has reported a preliminary experimental observation at 4 . 2 K on the interaction between two Pb Josephson junctions separated by several thousand A. Recently, Wang Chang-heng and Zhu Xueyuan [33] published a theoretical work about oscillation in a nonequilibrium superconductor. Using the # * - m o d e l and the R o t h w a r f - T a y l o r equations, the authors predict that a superconductor will oscillate between two nonequilibrium superconducting states with different energy gaps when quasiparticles are injected at the edge of the energy gap through the tunneling effect. The main sources of origin of this oscillation are the quasiparticle injection at the gap edge and the existence of recombination phonons with energy /2 = 2A(n). Under certain nonequilibrium conditions, the recombination phonon system does not take part in Cooper pair-breaking processes and can further stimulate the quasiparticles to recombine into pairs.
8. Josephson effect Interest in Josephson effect devices grew in the seventies and now several laboratories are engaged in different aspects of device applications and theoretical studies. Most of the subjects are concerned with the voltage standard,
19tI,R
K. g~'i- Yen
I.ow temln, rature physics research in ("hina
m a g n e t o m e t e r and high frequency device. Somc of these topics will appear in this conference. A general review has been given in ICEC-8 [341. Here I will mention two theoretical works, One is "Effect of a magnetic field on the height of the microwave-induced step" [351. This paper proposes a more advanced theory for the microwave-induced step phenonemon. With this theory, the authors studied in detail the effect of magnetic field and microwave power on the height of the microwave-induced step. The theoretical predictions and the existing experimental facts are consistent. The other paper is titled "Static behavior of variable thickness superconducting microbridges'" [36]. In the case of variable thickness superconducting microbridges, the bridge region is often considered as a one-dimensional problem, and the presence of the bank regions only provides some rigidboundary conditions. In the zero voltage case, the order p a r a m e t e r in the bridge region satisfies a one-dimensional G i n z b u r g - L a n d a u equation. The solutions, which satisfy the Ginzburg-Landau equation, are subject to rigid boundary conditions, and are not unique. In order to determine the solution completely, it ~s realized that one must satisfy the condition of minimum free energy of the system.
9. Theory of superconductivity In ref. 37 a series expansion solution for T~ is derived from the Eliashberg equation. The superconducting critical temperature T, and its convergence are discussed. The results suggest that when it is convergent, the T~. series yields values for the superconducting critical temperature which will be closer to the numerical solution than the ones from the A l l e n - D y n e s formula. From 1979 to 1981, a group from the University of Science and Technology of China made further exploration on the T~ series solution and tried to derive a formula fur T~ from the
Eliashbcrg equation outside the convergence circle of the T,: series solution [38[. For the new mechanism and origin of superconductivity, the "'interface" and "'surface'" problem are very important. There is some work on the calculations of elementary excitation about the surface and interface [3~)1, theoretical studies on the surface of small superconducting particles and clad particles [401, and band structure calculation of I,a. [41]. There is also stud\ on the hybridization of conduction band for close-packed compounds by means of the I.MT() method [42[. The latter work emphasizes thal heterogeneous coordination would be a necessary condition for intensifying hybridization. which is beneficial to high 7",. Various expcrimental results on AI5. BI and other closepacked compounds are in fair agreement with this basic principle. Some theoretical work on two-band superconductors has been published [431 and a new model for excitonic superconductors proposed
1441. 10. Low temperature properties of solids No research is being conducted in China on the superfluidity of liquid aHe or ~He. Apart from research on superconductivity, research on the low temperature properties of solids was started a few years ago by several groups in various laboratories. The following arc some examples [451.
10. 1. Amorphous films Recently the investigation of electronic transport properties in the disordered system has attracted a lot of attention. Han Shun-hui et al. [46] have reported low temperature resistivity anomalies observed in sputtered a m o r p h o u s G d Co alloy films. In uniaxial perpendicular magnetic anisotropic films (with a predominance of G d - C o pairs) the resistivity exhibits a minimum
K. Wei- Yen / Low temperature physics research in China
at about 50 K and from 4.2 to 25 K the resistivity is proportional to - I n T. In order to understand further the electronic transport properties of amorphous alloy films, the authors measured the temperature dependence of the reduced resistivity of a series of TbxFet(Kj-x (20-
1999
10.3. C D W The CDW has been found in several layered compounds, for example, 1T-TaS2 and 2/-ITaSe2. Recently, my colleagues, in cooperation with Prof. C.W. Chu [50], have carried out the resistivity measurements on 1T-TaSz at high pressure and low temperature. They found that at 13kbar the commesurated transition is depressed and at - 2 0 K the resistance begins to decrease but no zero resistance appears down to 1.5K.
10.2. Spin glasses
References
There has been increasing interest in spin glass ever since Cannela and Mydosh first observed the cusp of a.c. susceptibility in AuFe. Zhang Dian-lin et al. have measured the susceptibility and resistivity of the alloy Fe6Mo~sNi76 and found a cusp in its a.c. susceptibility at about 7 K [48]. The cusp is sensitive to background field and will shift to higher temperature as the measuring frequency increases. This is a new kind of spinglass alloy different from canonical ones. From the resistivity measurements no anomaly was detected near Tf, but there was a resistivity minimum at a higher temperature. More recently, Zhan Wen-shah [49] studied the magnetic phase diagram of the amorphous alloys (Pb0~Ag0.10)85-xSilsFex. They measured the a.c. susceptibility of these alloys at a temperature of 1.9 to 150 K by means of mutual inductance. The curves of susceptibility X vs. temperature T have a sharp peak or cusp when )6 = 1, 3, but the peaks gradually become smoother with increasing concentration of Fe. The results show that the alloys exhibit a transition from the spin-glass phase to the paramagnetic phase when )6 = 1, 3, 5, and a transition from the spin glass to the ferromagnetic then to the paramagnetic phase when X = 7. The author has only observed the Curie temperature Tc when )6 = 10.
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2ttlltl
K. Wei- Yen / Low temperature physics research in (-'hina
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