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BULK NON-EQUILIBRIUM SUPERCONDUCTOR
PROBED BY BALLISTIC PHONONS
B. Pannetier, J. Maneval
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplPment au no 8, Tome 39, aolit 1978, page C6-541
B. Pannetier and J.P. Maneval
Groupe de Physique des Solides de Z'Eeole Normale SupSrieure, 24 rue Lhomond, 75231 Paris, France
R6sumg.- On crbe dans un cristal d'btain un domaine de quasiparticules hors d'bquilibre en l'irra- diant soit par une impulsion lumineuse, soit par une impulsion de chaleur. La perturbation locale du gap est analysee par un faisceau de phonons de hautre frgquence.
Abstract.- A domain of non-equilibrium quasiparticles is created in bulk tin by irradiation with ei- ther a light pulse or a heat pulse. A probe beam of high frequency phonons is used to sense the local gap disturbance
.
INTRODUCTION.- Non-equilibrium superconductivity initiated by photon or phonon irradiation has so far been studied mostly for the case of evaporated 'thin-films/]/. Unfortunately, this configuration
involves the film-to-substrate acoustic coupling as an extrinsic parameter. On the other hand, the bulk material is not accessible by electrical means : in this Communication, we propose to probe the inte- rior of a non-equilibrium bulk superconductor with ballistic phonons having energies hv resonant with 2A, the superconducting gap. This is the first ex- tension of a method already applied to bulk super- conductors in equilibrium/2/.
EXPERIMENTAL.- We studied tin single crystal (5 N) grade with millimeter lengths oriented along the
E I ~
or theE o ~
direction. The end faces were mechanically polished, and provided with evaporated transducers insulated from the tin substrate by SiO0
layers (6 000 A). The conditions of formation and structure of the "hot domain" created by irradia- tion are examined in two steps : time-of-flight analysis of the transmitted excitation (A) and at- tenuation of near-gap phonons in the hot domain(B).
A. For this part, we refer to figure 1, which shows as a function of time the signals detected at 1.3 K by an aluminium bolometer opposite to the irradiated face of the sample. The situation of qua- si-equilibrium/2/ is recalled by trace b referred to as low-power heat pulse, in which condition the tin crystal is transparent to the low-energy (hv < 28) phonons. As the input flux is increased abo- ve about 1 w/mm2 (trace c) a broad pulse adds to the phonon signals, characterized by a maximum ve- +work supported in part by the Direction des Recher- c h e ~ et des Moyens dVEssais. Grant No 761486
T I U
l
1.01 mm HEAT PUL i n p u t L. high p o w e r IOW p o w e r LIGHT PULSE 1,
O t l m e ( y s ) 1Fig. 1 : Bolometric detection versus time. Tempera- ture 1.3 K. Trace (a) : photoexcitation (30 ns ar- gon laser pulse) ; (b) low-power heat flux emission and (c) high-power heat flux emission, 10 times lar- ger than for (b).
locity -of about 5 X 106 cm/s. As it has been repor- ted in another context/3/ this new signal is asso- ciated with the domain of quasiparticles liberated by the pair-breaking-phonons present in larger pro- portion in the incident heat flux. Finally, light irradiation (trace a) appears merely as an extreme case of high energy irradiation (light quanta >>2A) same quasiparticles signal as in trace c, with har- dly any phonon signal.
According to the "modified heating theory" /4/, the excess quasiparticles relax very rapidly down to the gap edge, where for longer times they remain in chemical equilibrium with the pair-brea-
if king phonons at an effective temperature T
.
Thus a domain of reduced superconductivity, with an ener-X
gy gap 2A (see figure 2) is created near the irra- diated surface. An appropriate description of our
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786243
Fig. 2 : Hot domain of quasiparticles in a bulk su- perconductor.
situation should include in addition the quasipar- ticles motion in the inhomogeneously disturbed bulk. However, this in not our aim here, and we will ra- ther report a new experimental approach to the non- equilibrium parameter 2AK.
B. Independently of laser irradiation, but from the same point on the sample surface, a low- power heat pulse is generated in a Joule-heated me- tal film. The ballistic transmission window of the non-equilibrium superconductor is limeted by the minimum value of 2nW in the bulk. For a better fre- quency resolution, a tin superconducting junction was used as a high-pass detector (threshold vD), SO
that the detected phonon signals are composed of the frequency band :
<
v
< 2Aw/hv~
- (1)A variable time difference (At) is allowed between the laser emission and the heat pulse. The- refore, since 2A" is a function of time and space, the frequency window (1) is itself a function of At. Figure 3 shows the quasiparticle signal (broad pul- se) detected by the tin junction as well as the transverse phonon signals superposed to it for two values of At. The phonon amplitude can be measured from the background of the broad pulse (see dotted lines) with enough accuracy. The result is plotted in the inset of figure 3 as a function of At (posi- tive times correspond to laser emission preceding the phonon emission). The maximum of phonon atte- nuation occurs for coincidence of the two emissions while the sides of the attenuation curve is a com- bination of the space and time evolutions of the quasiparticle domain. Interpretation of its shape is being studied within the framework of the 'T
T I N m ! 1.35 m m
.g
U T phonons/
'\, 0 1 2 3 t i m e ( p s )Fig. 3 : Interaction of a ballistic transverse (T)
phonon pulse with the quasiparticle domain. Tin tunnel detector. Temperature 0.6 K. Inset shows the transmission of T phonons as a function of the time delay between phonon emission and domain formation (see text).
References
/l/ For a review, see Langenberg,D.N., in Procee- dings of LT XIV, Otaniemi, Finland, edited by M. Krusius and M. Vuorio (North-Holland) 1975 /2/ Pannetier,B., Huet,D., Buechner,J. and Maneval
J.P., Phys. Rev. Lett.
2
(1977) 646131 Pannetier,B., Huet,D., and Maneval,J.P., Bull. Am. Phys. Soc.
