NOM B Nuclear Instruments and Methods in Physics Research B 89 (1994) 307-314 North-Holland

Bum Interactions with MaterIds & Atoms

Tailored defects in HTSC - What can we learn from swift heavy ion irradiation?

M. Kraus a,*, G. Kreiselmeyer a, J. Daniel a, M. Leghissa a, G. Saemann-Ischenko a, B. Holzapfel a,b, P. Kummeth a,b, R. Scholz ‘, L.Ya. Vinnikov d a Physikalisches Institut III, Universitiit Erlangen - Niimberg, Erwin-Rommel-Strasse I, 91058 Erlangen, Germany b Siemens AG, Research Laboratories, Erlangen, Mailbox 3220, Paul-Gossen-Strasse 100, 91050 Erlangen, Germany ’ Max-Planck-Institut fiir Mikrostmkturphysik, Weinberg 2, 06120 Halle (Saale), Germany d Institute of Solid State Physics, Chemogolouka, Moscow District, 142432, Russian Federation

Since the discovery of superconductivity above liquid nitrogen temperature in copper oxide based compounds many experiments have been performed on the one hand to study their mixed state properties and on the other hand to check their technological applicability. Due to their well defined cylindrical shape latent particle tracks (also known as columnar defects) are a powerful tool to investigate the mixed state in high temperature superconductors (HTSC) as they provide optimized pinning centers. Therefore in this paper we will present experiments performed with HTSC samples (epitaxial thin films on SrTiO, substrates, single crystals and technical tapes) containing columnar defects. We will outline the influence of these linear defects on the resistive phase transition and on transport I-V curves of YBa,Cu,O,_, (123) thin films. All HTSC known up to now exhibit a pronounced structure-related anisotropy. Hence we present magnetic and transport measurements concerning the angular dependence of the critical current in various irradiated HTSC compounds. Furthermore experiments with technical tapes of (BiPb),Sr,Ca,Cu&, (Bi2223) will be shown giving an overview of problems concerning technological applications. Finally in irradiated and nonirradiated Bi,Sr,CaCu,Os (Bi2212) single crystals the flux line lattice is visualized via the high resolution Bitter decoration technique.

1. Introduction: mixed state and defects

All HTSC are extreme type II superconductors and therefore an applied external field H,,, > H,, pene- trates them in the so-called mixed state. The flux penetrates in form of flux lines, each carrying an ele- mentary flux quantum a,, = h/2e. If now a transport current is applied a Lorentz force will act on these flux lines and without being pinned the flux lines will move according to direction and magnitude of the Lorentz force. This movement of flux lines generates an electric field and hence represents a dissipative mechanism which limits the current densities available in the HTSC samples without losses. Due to the high critical tem- peratures the thermally activated separation of the flwc lines from pinning centers is one of the main problems for technical applications of the HTSC, e.g. in the case of Bi2212 the large influence of thermally activated flux creep processes principally limits its applications to low operating temperatures between 4.2 and 30 K [l]. This scenario is further made worse by the pro- nounced structure related anisotropy of the HTSC, and

* Corresponding author.

especially in the case of Bi2212 a variation of j, over about five orders of magnitude can be observed de- pending on the orientation of the external field with respect to the crystal lattice [2].

‘As the superconducting order parameter is sup- pressed within a spatial defect, every defect can act as a more or less effective pinning center. Therefore it is of great interest to study the influence of various defects on the superconducting properties in the mixed state. Such defects may be induced by preparation (twin boundaries, screw dislocations, stacking faults, precipitates, oxygen deficiencies, . . . ) or artificially created via particle bombardment with electrons, neu- trons or ions. Particle bombardment provides the pos- sibility to choose the defect which is to be created by selecting a certain projectile and its energy.

In this paper we will present investigations carried out on HTSC samples of the 123- and the Bi-family irradiated with swift heavy ions. Every high energetic heavy ion creates one latent particle track of disor- dered material in the HTSC along its path, if its electronic energy loss S, exceeds a material dependent threshold value [3-81. As a special feature the cylindri- cal shape of such a columnar defect is very similar to the shape of a flux line and the diameters of these

0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E0769-D

VIII. BEAM MODIFICATION

308 M. Kraus et al. /Nucl. Instr. and Meth. in Phys. Rex B 89 (1994) 307-314

particle tracks are in the same order of magnitude as the coherence lengths tab in the CuO, plane of the various HTSC compounds. Therefore especially these columnar defects should act as optimum pinning cen- ters as they in principle enable to pin long segments of the flux lines. As furthermore the density of the colum- nar tracks is given by the particle fluence these linear defects provide a powerful tool on the one hand to study basic phenomena of the mixed state of HTSC and on the other hand to investigate the current limit- ing dissipation mechanisms, i.e. to find the maximum current densities available in the different HTSC with respect to technical applications.

Due to the cylindrical shape of columnar defects and flux lines the interaction of a single vortex with a particle track can quite easily be derived and at low

temperatures one yields a maximum pinning energy per unit length of

-v

Upin = 4%&,

with the London penetration depth in the CuO, plane A,, [1,9]. Depending on the particle track radius R,

Aab and tab of the HTSC investigatfd the values of upin range between 4 and 10 meV/A. Assuming the flux lines being coherently pinned throughout the whole HTSC thickness (typically at least several 100 nm> even in thin films activation energies in the range of several eV could be expected.

2. Irradiation experiments

The experiments “on beam” have been performed at the low temperature irradiation facilities at VICKSI (HMI, Berlin), at IRASME (GANIL, Caenl, and at UNILAC (GSI, Darmstadt). In the case of thin films all the considered projectiles loose only a few percent of their initial energy within the HTSC and therefore the defect structure is homogeneous throughout the whole film thickness of several 100 nm. For single crystals and technical tapes with thicknesses of about 20 to 50 km this is only true for heavy ions with very high energies of several GeV. In order to provide homogeneous lateral bombardment of the targets the ion beams at IRASME and VICKSI have been scanned in x- and y-direction across the irradiated areas. If not mentioned otherwise the samples have been irradiated at room temperature and parallel to the c-axis.

The projectiles and energies have been chosen in such a way that their electronic energy loss S, in the HTSC exceeds the material dependent threshold for the creation of latet$ particle tracks. Threshold values are S, = 2 keV/A for YBa,Cu,O,-, [S] and

S.5 = 1 keV/A for Bi,Sr,CaCu,O, [lo]. It is worth

Fig. 1. HRTEM image of columnar defects in a YBa,Cu,O, thin film in cross section after irradiation with 340 MeV Xe. The view direction is parallel to the CuO, planes and the

particle tracks are approximately parallel to the c-axis.

mentioning that for all projectiles the electronic energy loss S, exceeds the nuclear energy loss S, by two to three orders of magnitude. Every projectile with an electronic energy loss higher than the threshold value creates one latent particle track of disordered material along its path throughout the sample. It is worth men- tioning that we found such particle tracks also in the classical superconductor NbSe, after irradiation with 1 GeV Pb [ll]. From conversion electron Miissbauer

spectroscopy on YBa,(Cu,,,,Fe,,,,),O,_, we found evidence for the existence of the so-called “green phase” Y,BaCuO, within the particle tracks [12]. The diameters of the columnar defects increas: with grow- ing S, from about 40 A up to about 100 A, depending also on the target material [lo]. Fig. 1 shows an exam- ple for such particle tracks in a YBa,CuaO,-, thin film after 340 MeV Xe irradiation, imaged via HRTEM in cross section. These ions create particlt tracks in YBa,Cu,O,-, with diameters of about 65 A extended throughout the entire film thickness. A typical fluence for the investigation of critical current densities after irradiation is 10” ions/cm2. This corresponds to a T, reduction of only about 1 K and, according to the creation of latent particle tracks, to a disordered vol- ume fraction of only a few percent.

3. Phase transition and transport in YBa,Cu,O,_,

In Fig. 2 the resistive phase transitions of an epitax- ial YBa,Cu,O,_, thin film in zero field and in various external fields (B ]I c-axis) are given in an Arrhenius plot [13]. Also included are the curves after 340 MeV Xe irradiation and a fluence of @t = 2.7 X 10" cm-‘. As expected, irradiation results in an enhanced normal state resistivity at 110 K (p(@t)/p(O) = 1.06, cannot be seen in this plot) due to increased disorder in the sample. In zero field the transition is shifted by 0.4 K to lower temperatures. In applied fields however, upon lowering the temperature the resistivity after irradia- tion drops below its value before irradiation. Within the framework of the flux creep and thermally acti-

M. Kraus et al. /Nucl. I&r. and Meth. in Phys. Res. B 89 (1994) 307-314 309

vated flux flow (TAFF) model the reduced resistivity p a exp[ - U/kT] yields an enhanced activation energy U, Therefore the irradiation-induced defects reduce the dissipative flux motion.

The high efficiency of particle tracks as pinning centers can also be deduced by regarding the shape of transport I-V curves of YBa,Cu,O,_, thin films be- fore and after swift heavy ion irradiation. In order to compare defect density and vortex density it is conve- nient to introduce the so-called dose-equivalent field B,, = @,, x Qt. A sample irradiated up to the fluence @t contains in an external field B,, the same number of vortices and columnar defects. An applied projectile fluence of @t = 2.7 x 10” cm-* therefore corresponds to B,, = 0.55 T. As external fields up to 7 T have been applied, the experimental situation reflects in most cases B >> B,,, thus meaning that the number of vor- tices exceeds the number of latent particle tracks. Assuming that in this high field regime each columnar defect pins one vortex only the remaining excess vor- tices which correspond to a reduced magnetic field Bred = B -Be, will contribute to dissipation.

Fig. 3 presents I-V curves of an YBa,Cu,O,_, thin film as-grown and containing particle tracks corre- sponding to B@, = 0.55 T. Irradiation caused a T, re- duction of about 0.4 K and therefore I-V curves be- fore and after irradiation can be compared directly for a fixed temperature T. From Fig. 3 can be seen that the I-V curves prior to irradiation at magnetic fields B 2 1.5 T coincide with those after irradiation at the magnetic field B + B,,. This “matching effect” indi- cates the existence of an empirical scaling law

E@‘(j, B+B,,) =E’(j, B),

and suggests, that in the high field limit dissipation is

e @cm)

90.9-l 87.0-l 83.3-l 80.0-l

T-l (K-l)

Fig. 2. The resistive phase transition of a YBa,Cu,O, thin film before (open symbols) and after (full symbols) irradiation

with 340 MeV Xe in an Arrhenius plot.

E W-4 100

10-l YBa2Cu30r-6 thin film

= 0.55T; 340 MeV Xe

10-Z

10-s

j Wcm2) Fig. 3. Transport I-V curves of a YBa,Cu,O, thin film before and after irradiation at selected external fields applied parallel to the c-axis and hence aligned with the particle

tracks.

only caused by the motion of the excess vortices. These excess vortices move independent from those vortices pinned at particle tracks. As the vortices in the particle tracks do not depin, the activation energy of the vor- tices pinned by columnar defects exceeds the activation energy of the excess vortices, which may be pinned by background defects being already present before irra- diation.

These findings clearly confirm the high pinning effi- ciency of columnar defects in the case of YBa,Cu,O, and can only be understood by assuming here an almost three-dimensional character of the vortices. Due to this three-dimensional character long segments of the flux lines can be pinned taking advantage of the particle tracks as pinning centers with cylindrical shape.

At lower external fields (B,, I B I 2B,,) the vor- tex-vortex interaction becomes more and more impor- tant, preventing that all columnar defects are occupied. For this reason the I-V curves do not coincide any more.

4. Angle resolved critical currents

As already mentioned all HTSC exhibit a pro- nounced structural anisotropy. This anisotropy strongly affects electronic properties and superconducting pa- rameters like the coherence length, thus invoking dif- ferent values tab and 5, depending on the direction in the HTSC crystal lattice. As a result also the structure of a flux line strongly depends on the anisotropy, on the temperature T, and on magnitude and orientation of the applied external field H,,. Therefore angle resolved measurements of the critical current density j&B, T, 0) (with 0 the angle between external field and c-axis) of samples containing columnar defects

VIII. BEAM MODIFICATION

310 M. Kraus et al. /Nucl. Instr. and Meth. in Phys. Res. B 89 (1994) 307-314

provide insight into the flux line structure of the vari- ous HTSC.

Fig. 4 shows hysteresis loops (measured with a com- mercial DC SQUID magnetometer) of a DyBa, Cu,O,_, single crystal irradiated with 1 GeV Pb un- der an angle cpi = 30” with respect to the c-axis. The applied fluence was 1 x 1O’l cm-2 and the hysteresis loops have been measured at T = 30 K. Full circles represent the orientation of the crystal with the mag- netic field aligned to the columnar defects, and open circles the mirror symmetric orientation to the c-axis, as indicated by the inset. The width of a hysteresis loop at a certain magnetic field is connected with the shield- ing currents inside the sample at that field. Therefore Fig. 4 clearly proves an enhanced critical current den- sity for the external field aligned to the columnar defects as geometrical corrections are the same for both orientations. Similar results have been obtained by Civale et al. [14] on an YBa,Cu,O,_, single crystal, whereas Thompson et al. found almost no orienta- tional dependence for a Bi2SrzCaCu20, single crystal also containing columnar defects [15].

Besides these magnetic measurements of single crystals we also determined the angular dependence of the critical current density of HTSC thin films by means of transport measurements [16]. The films have been irradiated under various angles ‘pi, but always perpendicular to the measurement bar.

In Fig. 5 we present the angular dependence of a YBa,Cu,O,_, thin film as-grown and containing par- ticle tracks under ‘pi = 60” with respect to the c-axis. Prior to irradiation there exist two maxima in j,(O),

M (a.u.) 1 I I I I I

fi ho”/ \3fl”IEi

10

5

0

-5

-10

-6 -4 -2 0 2 4 6

H CT)

Fig. 4. Hysteresis loops of a DyBa,Cu,O, single crystal containing columnar defects under ‘p‘ = 30” with respect to

the c-axis. Closed symbols represent the external field aligned to the particle tracks, open symbols in the symmetrical direc-

tion out of the defects, as indicated by the insets.

1.2

1.0

0.8

0.6

0.4

0.2

j, ( lo6 A/cm2)

Y-123 thin film

W = 8. 10’OPb/cmz

fore irradiation

= l.OT

1

t ‘P, = 60’ -I

0.0 0 0 90 180 270 360

w Fig. 5. Angular resolved transport critical current measure-

ment of a YBa,Cu,O, thin film before and after irradiation

(cp, = 60”).

each representing the external field aligned parallel to the CuO, planes (&direction) and therefore arising from intrinsic pinning. This scenario changes after irra- diation with 770 MeV Pb under cpi = 60”: besides re- duced intrinsic maxima now there exist two additional peaks which now dominate j,(O). Occurring at 0 = 60” = cpi and 0 = 240” = cp, + 180” these new peaks indi- cate to be irradiation induced and represent the exter- nal magnetic field being parallel to the columnar de- fects.

It is worth mentioning that the irradiation induced maxima exceed the peaks for the intrinsic pinning, indicating that the pinning strength of particle tracks in YBa,Cu,O,_, is larger than the intrinsic pinning strength at high temperatures (79 K) and medium external fields (1 T). These results are confirmed by j,(B = 0.5 T, T = 80 K, 0) measurements on YBa,CuaO,_, thin films containing columnar defects in various orientations (cpi = 0”, cp, = 30” and cpL = 60”), as shown in Fig. 6, and can only be explained if there exist rather rigid three-dimensional flux lines in YBa,Cu,O,_,. For the external field aligned parallel to the particle tracks such vortices can be pinned by particle tracks over distances significantly larger than the intrinsic CuO, layer spacing.

In order to test this point of view we also investi- gated the angular dependence j&B, T, 0) of YBa,Cu30,/PrBa2Cu30, superlattices. In Fig. 7 j,(O) of a (3PBC0/3YBCO) X 25 superlattice before and after irradiation with 770 MeV Pb under cpi = 30” are given. The notation indicates that the superlattice con- sists of 25 periods, each with three unit cells nonsuper- conducting PrBa,Cu,O, and three unit cells YBa, Cu,O,. The as-grown multilayer exhibits an angular dependence of the critical current density very similar to that of the highly anisotropic Bi,Sr,CaCu,O, sys-

M. Kraus et al. / Nucl. Instr. and Meth. in Phys. Res. B 89 (1994) 307-314 311

j, ( 105A/cm2)

intrinsic pinnmg

Fig. 6. Angular resolved transport critical current measure- ments of YBa,Cu,O, thin films irradiated under various angles with respect to the c-axis (cpi = o”, cpi = 30” and cpi =

60”).

tern with pronounced peaks for the external field em- bedded in the CuO, planes [2]. The main effect of irradiation under cpi = 30” is a reduction of the intrinsic

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

j, ( lo6 A/cm2)

SxY/SxPr-123 multilaye

Of = 8 1O'O Pb/cm2

o before irradiation l after irradiation

0 90 180 270 360

w Fig. 7. Angular resolved transport critical current measure- ment of a Pr/YESa,Cu,O,_d superlattice before and after

irradiation (cpi = 30”).

peaks together with a strong enhancement of j, within a wide range of 0 by a factor of about 5. Besides this isotropic enhancement there are no additional peaks which might be connected with the external field aligned with the columnar defects.

These results indicate that the nonsuperconducting PrBa,Cu,O, layers decouple adjacent YBa,Cu,O, layers. The flux lines consist of (here 25) quite rigid segments which depin independently from each other from the particle tracks, resulting in an isotropic en- hancement of j&B, T, 0). Therefore PrBa,Cu,O,/ YBa,Cu,O, superlattices alter the structure of the flux lines from a quasi three-dimensional object in YBa,Cu,O, to a nearly two-dimensional object com- parable to the flux lines in Bi,Sr,CaCu,O,, consisting of stacks of two-dimensional pancake vortices, which are defined by the circular system of shielding currents surrounding the core within a CuO, double layer.

5. Technical Bi tapes

Up to recent times the Bi-based compounds have generally been favoured for technical applications: the strong influence of granularity in YBa,Cu,O, conduc- tors implies expensive preparation processes while Bi- based conductors can be economically produced using conventional techniques like the powder-in-tube method or melting processes. Therefore particular at- tention was payed to the critical current densities in these Bi-compounds especially in various external mag- netic fields. In Bi,Sr,CaCu,O, single crystals it was only possible to increase the pinning energies from about 35 meV in as-grown samples to about 70 meV at 10 K even after columnar defects as optimum pinning centers had been induced [l]. Such relaxation measure- ments performed with Bi2212 tapes yielded about the same maximum pinning energies [17,18]. As 70 meV approximately coincide with the theoretical pinning energy of one pancake vortex, depinning in Bi2212 is commonly believed to happen as thermally activated hopping of single pancake vortices in a CuO, double layer 11,191. This independent depinning of single pan- cake vortices limits technical applications of Bi2212 principally to the regime of low magnetic fields or low temperatures. Besides that, the very strong two-dimen- sional character of Bi2212 invokes a sharp drop in j, for external field orientations out of the ab-plane, which further limits the field of possible applications 121.

One possible way out concerning the low activation energies is offered by the Bi2223 compound: in Bi2223 there are triple CuO, layers and therefore higher pinning energies at columnar defects and a stronger coupling of the pancake vortices in c-direction might

VIII. BEAM MODIFICATION

312 M. Kraus et al. /Nucl. Instr. and Meth. in Phys. Rex B 89 (1994) 307-314

be expected. Hence we now present results from irradi- ation experiments on Bi2223 tapes [20].

The polycrystalline (Bi,Pb),Sr,Ca,Cu,O,a+, tapes have been prepared using the powder-in-tube method. The superconducting Bi2223 core of the tapes had a thickness of about 19 urn with a silver top layer of about 14 urn after thinning. Hence we provided a very homogeneous damage production due to irradiation with 2.65 GeV Au throughout the entire superconduct- ing core thickness. Prior to irradiation the tapes exhib- ited a pronounced granular behaviour and according to Angadi et al. [21] we calculated the radius of the grains to be about 60 urn. This grain radius enabled us to calculate the critical current densities from the magne- tization loops obtained using a commercial vibrating sample magnetometer (VSM). Fig. 8 shows the critical current densities of the Bi2223 tapes before and after irradiation for magnetic fields up to 12 T aligned parallel to the c-axis. At T = 60 K we find an optimum fluence of 1 X 10” cm-‘, where about 10” A/cm2 in low external fields up to 1 T were reached.

In order to investigate the pinning properties also relaxation measurements were performed. We ob- tained the effective activation energy U at T= 10 K using the formula of Beasley et al. [22]. The results are given in Fig. 9 for various applied magnetic fields as a function of the applied fluence in terms of the dose- equivalent field B,, . In lower fields (B = 0.125 T) we find the activation energy U to be enhanced from about 35 meV to only about 105 meV. Therefore Clem’s pancake vortex model [19] also applies for the Bi2223 compound with one pancake vortex consisting of the shielding currents of a triple CuO, layer. As a result, also for the Bi2223 compound applications in

.icm (A/cm2)

105

lo4

1n3 i

o unirradiated

0 et = 0.5 x 10” ions/cm2 A w = 1.0 x 10” ions/cm2

v W = 1.5 x IO” mm/cm2

0 W = 2.25 x 10” ions/cm’

M Ot = 2.6 x 10” ions/cm’

x cpt = 3.5 x 10” ions/cm’

1”

10-3 10-Z 10-l 1

BP')

Fig. 8. The critical current densities of Bi2223 tapes before and after irradiation for magnetic fields up to 12 T aligned

parallel to the c-axis.

U(meV)

80

60

40

20

t

open symbols: 2.77 GeV zosPb closed symbols: 2.65 GeV ‘Q7A~

“0 1 2 3 4 5 6

B:t(T;

Fig. 9. The activation energy CI for various applied magnetic fields as a function of the applied fluence in terms of the

dose-equivalent field I?*,.

power technology at liquid nitrogen temperature re- main limited to low external fields.

6. Decoration experiments

One powerful tool to demonstrate the effectiveness of columnar defects as pinning centers is provided by the high resolution magneto-optical Faraday effect [23]. With its resolution of about 0.8 u.rn this technique yields local information about the flux density without being able to resolve single vortices. The visualization of single flux lines is the big advantage of the high-res- olution Bitter technique, which will be discussed in the following. In order to study the arrangement of the flux lines in the virgin sample as well as in a sample containing columnar defects we irradiated Bi,Sr,

CaCu2Gs +x single crystals with 340 MeV Xe ions up to fluences of 1.7 x 10” cme2, but during irradiation the samples have been covered by about one half of their surfaces by a copper absorber [24]. The fluence 1.7 x 10” cmp2 corresponds to a dose-equivalent field B,, = 3.5 kG.

To observe the Bitter pattern the samples have been field cooled down to 4.2 K in an external mag- netic field of 22 G, oriented parallel to the c-axis and therefore also parallel to the columnar defects in the irradiated region. Thereby the vortex arrangement gets frozen and the trapped flux lines could be imaged by decorating the surface of the crystals with fine Fe particles. The Figs. 10a and lob show the Bitter pat- terns for a virgin region and for a region containing columnar defects, respectively.

M. Kraus et al. / Nucl. Instr. and Meth. in Phys. Res. B 89 (1994) 307-314 313

Fig. 10. The flux line lattice of a Bi,Sr,CaCu,O, single crystal before (a) and after (b) irradiation with 340 MeV Xe, visualized using the high resolution Bitter technique.

The image of the unirradiated region shows a hexagonal Abrikosov vortex lattice which extends over several vortex spacings, confirming the good quality of the as-grown crystal. In the case of the irradiated region a completely uncorrelated arrangement of the flux lines can be observed. As the number of columnar defects exceeds the number of flux lines by a factor of about 160 one can assume that almost every flux line is pinned by a columnar defect. Therefore we contribute the arrangement in the irradiated parts to the uncorre- lated impact positions of the high energetic Xe ions and the high pinning efficiency of the latent particle tracks created along their paths.

By performing a quantitative analysis of the vortex patterns the translational order of the flux line ar- rangements can be investigated by calculating the ra- dial correlation functions of the images. In comparison to an undisturbed triangular lattice the translational correlation length R, can be determined. R, denotes a typical length scale within which the correlation fades exponentially. Such an analysis yields a correlation length R, = 1.5~ . . . 2a, for the virgin sample areas and a reduced correlation length R, = OSa, for those parts which contain columnar defects (a,, = dm denotes the vortex spacing of an undisturbed hexago- nal vortex lattice, here in an external field B = 22 G).

7. Conclusion

We presented several experiments investigating the mixed state of Bi-based and 123-HTSC compounds. In order to provide optimum pinning centers for the flux lines we induced particle tracks via swift heavy ion irradiation. The high pinning efficiency of these linear defects was shown by the occurrence of a “matching effect” in transport I-V curves. By visualizing the flux line lattice we studied their influence on the structure of the flux line lattice. Due to the uncorrelated impacts of the projectiles onto the HTSC sample the correla- tion length of the flux line lattice was reduced by the columnar defects. Usually an enhancement of the criti- cal current density j, was achieved, in detail also depending on particle fluence, temperature and exter- nal magnetic field.

In the case of Bi compounds which have been classified as promising candidates for applications, e.g. in power engineering, this enhanced critical currents came along with only a very small enhancement of the activation energies, even at low temperatures and low external fields. This can be understood within the framework of a fragmentation of the flux lines in single and rather decoupled pancake vortices. Huge problems arise for applications, as furthermore the pronounced

VIII. BEAM MODIFICATION

314 M. fiaus et al. /Nucl. Instr. and Meth. in Phys. Res. B 89 (1994) 307-314

quasi two-dimensional electronic structure invokes a sharp drop of j,, if the direction of the external field leaves the &plane.

In contrast to the Bi compounds the flux lines in YBa,Cu,O, prove to be rather rigid. Therefore columnar defects enable strongly enhanced critical cur- rent densities, if the external field is aligned with their direction. PrBa,Cu,O, interlayers reduce the coupling between adjacent CuO, layers in YBa,Cu,O, and therefore angle resolved transport measurements of the critical currents reveal no additional irradiation induced peaks.

Acknowledgement

The authors want to thank H.-W. Neumiiller and G. Ries (both Siemens Erlangen) for helpful assistance and are indebted to S. Klaumiinzer (HMI Berlin), S. Bouffard (GANIL, Caen, France) and G. Wirth and J. Wiesner (both GSI, Darmstadt) for their help during the irradiation experiments. We also thank E.H. Brandt (MPI Stuttgart) for many stimulating discussions, L. Gurevich (ISSP, Chernogolovka) for his help perform- ing the Bitter decoration experiments and the group of Prof. Kronmiiller (MPI Stuttgart) for providing the DyBa,Cu,O, single crystals. This work was financially supported by the Bundesministerium fiir Forschung und Technologie (BMFT), the Bayerische Forschungs- verbund Hochtemperatursupraleiter FORSUPRA, and the DAAD Bonn. We gratefully acknowledge the NATO Scientific Exchange Programme for supporting the collaboration between the Universitlt Erlangen- Niirnberg and the ISSP Chernogolovka.

References

[ll

Dl

[31

[41

El

W. Gerhluser, G. Ries, H.W. Neumiiller, W. Schmidt, 0. Eibl, G. Saemann-Ischenko and S. Klaumiinzer, Phys. Rev. Lett. 68 (1992) 879. P. Schmitt, P. Kummeth, L. Schultz and G. Saemann- Ischenko, Phys. Rev. Lett. 67 (1991) 267. B. Roas, B. Hensel, S. Henke, S. Klaumiinzer, B. Kabius, W. Watanabe, G. Saemann-Ischenko, L. Schultz and K. Urban, Europhys. Lett. 11 (1990) 669. D. Bourgault, S. Bouffard, M. Toulemonde, D. Groult, J. Provost, F. Studer, N. Nguyen and B. Raveau, Phys. Rev. B 39 (1989) 6549. B. Hensel, B. Roas, S. Henke, R. Hopfenglrtner, M. Lippert, J:P. Striibel, M. VildiC, G. Saemann-Ischenko and S. Klaumiinzer, Phys. Rev. B 42 (1990) 4135.

[6] H. Watanabe, B. Kabius, K. Urban, B. Roas, S. KIaumiinzer and G. Saemann-Ischenko, Physica C 179 (1991) 75.

[7] V. Hardy, D. Groult, M. Hervieu, J. Provost and B. Raveau, Nucl. Instr. and Meth. B 54 (1991) 472.

[8] V. Hardy, D. Groult, J. Provost, M. Hervieu, B. Raveau and S. Bouffard, Physica C 178 (1991) 255.

[9] E.H. Brandt, Phys. Rev. Lett. 69 (1992) 1105. [lo] M. Kraus et al., to be published. [ll] P. Bauer, C. Giethmann, M. Kraus, T. Marek, J. Burger,

G. Kreiselmeyer, G. Saemann-Ischenko and M. Ski- bowski, Europhys. Lett., in press.

[12] J. Dengler, G. Errmann, N. Kaner, G. Ritter, B. Hensel, M. Kraus, G. Kreiselmeyer, G. Saemann-Ischenko, S. Klaumiinzer and B. Roas, Hyperfine Interactions 70 (1992) 921.

[13] M. Leghissa, A. Kijniger, M. Lippert, W. Dorsch, M. Kraus and G. Saemann-Ischenko, Z. Phys. B, in press.

[14] L. Civale, A.D. Marwick, T.K. Worthington, M.A. Kirk, J.R. Thompson, L. Krusin-Elbaum, J.R. Clem and F. Holtzberg, Phys. Rev. Lett. 67 (1991) 648.

[15] J.R. Thompson, Y.R. Sun, H.R. Kerchner, D.K. Chris- ten, B.C. Sales, B.C. Chakoumakos, A.D. Marwick, L. Civale and J.O. Thomson, Appl. Phys. Lett. 60 (1992) 2306.

[16] B. Holzapfel, G. Kreiselmeyer, M. Kraus, G. Saemann- Ischenko, S. Bouffard, S. Klaumiinzer ,and L. Schultz, Phys. Rev. B 48 (1993) 600.

[17] H.-W. Neumiiller, W. GerhHuser, G. Ries, P. Kummeth, W. Schmidt, S. Klaumiinzer and G. Saemann-Ischenko, Cryogenics 33 (1993) 14.

[18] P. Kummeth, H.-W. Neumiiller, G. Ries, S. KIaumiinzer and G. Saemann-Ischenko, J. Alloys and Compounds 195 (1993) 403.

[19] J.R. Clem, Phys. Rev. B 43 (1991) 7837; L.N. Bulaevskii, Zh. Eksp. Teor. Fiz. 64 (1973) [Sov. Phys. JETP 37 (1973) 11331.

[20] P. Kummeth, C. Struller, H.-W. Neumiiller, G. Ries, M. Kraus, M. Leghissa, G. Wirth, J. Wiesner and G. Sae- mann-Ischenko, PCMOS, 27.-31.07.1993, Eugene (Oregon), to be published in J. of Superconductivity.

[21] M.A. Angadi, A.D. Caplin, J.R. Laverty and Z.X. Shen, Physica C 177 (19911479.

[22] M.R. Beasley, R. Labusch and W.W. Webb, Phys. Rev. 181 (1969) 682.

[23] M. Leghissa, Th. Schuster, W. Gerhluser, S. Klaumiinzer, M.R. Koblischka, H. Kronmiiller, H. Kuhn, H.-W. Neumiiller and G. Saemann-Ischenko, Europhys. Lett. 19 (1992) 323; Th. Schuster, H. Kuhn, M.R. Koblischka, H. Theuss, H. Kronmiiller, M. Leghissa, M. Kraus and G. Saemann- Ischenko, Phys. Rev. B 47 (1993) 373.

[24] M. Leghissa, L.A. Gurevitch, M. Kraus, G. Saemann- Ischenko and L.Ya. Vinnikov, Phys. Rev. B 48 (1993) 1341.