Physica C 407 (2004) 89–94

www.elsevier.com/locate/physc

Weak link grain boundaries in intermetallic superconductors

Neeraj Khare a,*, D.P. Singh a, S.B. Samanta a, A.K. Gupta a, P.K. Babu b,G. Ghosh b, R. Nagarajan b, L.C. Gupta b

a National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, Indiab Tata Institute of Fundamental Research, Mumbai 400005, India

Received 22 December 2003; received in revised form 12 April 2004; accepted 23 April 2004

Abstract

Studies of rf-SQUID effect in ternary (YRuB2, LuRuB2, LaRu3Si2) and in binary V3Si intermetallic bulk super-

conductors have been carried out for investigating the nature of grain boundaries in these superconductors. YRuB2(Tc � 7:5 K), LuRuB2 (Tc � 10 K) and LaRu3Si2 (Tc � 7 K) ternary superconductors are found to show rf-SQUIDvoltage–flux modulations from 4.2 K to close to Tc. This indicates that the natural grain boundaries in the ternaryintermetallic superconductors behave as Josephson weak links. Rf-SQUID effect is not observed in V3Si binary

superconductors, which indicates that the grain boundaries in V3Si are not weak link type. STM/STS studies of the

YRuB2 and V3Si grain boundaries (GB) are also reported. The interior of the GB in YRuB2 is found to be of quasi-

insulating type and thus provides a weak link effect. On the contrary the interior regions of the GB of V3Si is found to

be of metallic type and thus provides proximity coupling between the grains of V3Si.

� 2004 Published by Elsevier B.V.

Keywords: rf-SQUID effect; Intermetallic superconductor; Weak link grain boundary

1. Introduction

The discovery of superconductivity in quater-

nary intermetallic Y–Ni–B–C [1] has stimulated

interest in intermetallic superconductors. The

multiphase material Y–Pd–B–C [2] exhibits the

highest (Tc � 23 K) in bulk intermetallic. In ourearlier reports [3–5], we showed the observation ofrf-SQUID effect in several bulk quaternary boro-

* Corresponding author.

E-mail address: nkhare@mail.nplindia.ernet.in (N. Khare).

0921-4534/$ - see front matter � 2004 Published by Elsevier B.V.

doi:10.1016/j.physc.2004.04.027

carbide intermetallic superconductors (YNi2B2C,

ErNi2B2C, DyNi2B2C, LuNi2B2C, YPd5B3C0:35)

which indicates that natural grain boundaries

in the quaternary intermetallic superconductors

behave as Josephson junction type weak links.

Recently the STM studies of YNi2B2C showed

that the grain boundaries are 30 �A wide and quasi-insulating in nature [6]. We have investigatednature of grain boundaries in ternary and

binary intermetallic superconductor through rf-

SQUID studies. This paper reports the results

of our studies in YRuB2, LaRu3Si2, LuRuB2 ter-

nary and in V3Si binary intermetallic supercon-

ductors.

90 N. Khare et al. / Physica C 407 (2004) 89–94

2. Experimental

The YRuB2, LaRu3Si2, LuRuB2 and V3Si

samples used in the present study are prepared by

melting high purity elements of Y, Ru, V, La, Lu,B, Si in an arc furnace and subsequent annealing

at 1050 �C. AC susceptibility (313 Hz, rms field 1.5G) of the samples was measured as a function of

temperature to monitor the diamagnetic response

and, there by, to determine its superconducting

transition temperatures.

For observing rf-SQUID effect in these super-

conductors a small piece of bulk is taken. Thisbulk is surrounded by 8–10 turn coil of copper

wire, which forms a part of the tank circuit of the

SQUID electronics. The resonance frequency of

the tank circuit at 4.2 K is slightly different for

different samples and it lies within 19.1–19.4 MHz.

A rf oscillator was connected to the tank circuit to

provide rf bias to the SQUID. The reflected rf

signal from the tank circuit was amplified by alow-noise amplifier (of 60 dB gain) and detected

using a diode detector. For observing the voltage–

flux characteristics of rf-SQUID an ac signal was

applied to the SQUID through a fifty turn sole-

noid (1.5 cm long, 1 cm diameter) using an audio

Fig. 1. AC susceptibility (v0) vs. temperature curves of (a) YR

frequency (af) oscillator. The field produced by the

solenoid was 3:7� 10�3 T/A. The rf detector out-put was connected to the Y-channel of a CRO

while the output from the af oscillator was con-

nected to the X-channel. Amplitude and frequency

of the rf signal was adjusted for getting goodvoltage–flux SQUID characteristics.

STM/STS studies have been carried out at room

temperature using Nano Scope II (Digital Instru-

ments, USA) work station operating in constant

current mode with bias voltage Vb � �50 mV. APt–Ir tip was used to scan the sample surface and

data were collected with zero input filter and a

moderating feedback gain.

3. Results and discussion

Fig. 1 shows the result of variation of ac sus-

ceptibility (v0) of YRuB2, LaRu3Si2, LuRuB2 and

V3Si samples with temperature. The transition

temperature of these superconductor were foundto be 7.5, 7, 10 and 16 K respectively. Fig. 2 shows

voltage–flux modulations of YRuB2, LuRuB2 and

LaRu3Si2 bulk rf-SQUID at 4.2 K. The peak to

peak amplitude of voltage–flux (V –U) modulations

uB2, (b) LaRu3Si2, (c) LuRuB2 and (d) V3Si samples.

Fig. 2. Voltage–flux modulations of bulk rf-SQUID of YRuB2(top trace), LuRuB2 (middle trace) and LaRu3Si2 (lower trace)

samples at 4.2 K. Peak to peak amplitude of voltage–flux

modulation is 1.4 mV for YRuB2, 2 mV for LuRuB2 and 1 mV

for LaRu3Si2.

N. Khare et al. / Physica C 407 (2004) 89–94 91

was 1.4 mV for YRuB2, 2 mV for LuRuB2 and 1

mV for LaRu3Si2 sample. These voltage–fluxcharacteristics were observed at suitable rf-biasing.

Multiperiodic voltage–flux characteristics were

also observed at some other rf-biasing. Periodic

oscillations in voltage–flux characteristics due to

rf-SQUID effect have been clearly observed in all

the three ternary intermetallic superconductors

upto temperature close to Tc. The observation ofrf-SQUID effect in all the three intermetallicsuperconductors indicates that superconducting

grains in these superconductors are coupled at

grain boundaries through Josephson junction type

weak links. Rf-SQUID loops form inside the bulk

material and these loops pass through several

grain boundaries. Critical currents of these grain

boundaries may be different. The observation of

rf-SQUID voltage–flux modulation in these ter-nary intermetallics ensures that in the supercon-

ducting loop at least one grain boundary junction

is behaving as weak link whereas other grain

boundaries having higher critical currents behave

as a part of single superconducting ring, although

these may also be Josephson junction weak links.

The observation of multiperiodic voltage–flux

characteristics at some rf-biasing indicates that

several rf-SQUID loops of different area were ac-

tive in the ternary bulk superconductors. The ob-

served voltage–flux characteristics correspond torf-SQUID loops for which the coupling between

SQUID loops and the tank circuit coil was opti-

mum. This observation of rf-SQUID behaviour in

ternary bulk superconductors was similar to that

observed in bulk sample of high-Tc superconductor[8,9]. We have performed the same experiment of

rf-SQUID effect in V3Si and did not see any evi-

dence of voltage–flux oscillation from 4.2 K to Tc.This result indicates that grain boundary in the

V3Si is strongly linked.

The voltage–flux modulations in ternary inter-

metallic superconductors shown in Fig. 2 were

observed, when the rf frequency was 0.2 MHz

above the resonance frequency of the tank circuit.

No modulations were observed at resonance fre-

quency (f0). Good voltage–flux characteristicswere also observed when the operating rf-

frequency was kept about 0.2 MHz below the

resonance frequency. A phase reversal in the

voltage–flux modulations was also noticed in all

the three cases, when the rf frequency was changed

from f0 þ 0:2 MHz to f0 � 0:2 MHz. Such phasereversal is a typical feature for a non-hysteritic rf-

SQUID whose b parameter is less than one [7].The parameter is equal to 2pLIc=U0 where L is the

inductance of the SQUID loop, Ic is the criticalcurrent of the Josephson junction and U0ð¼ h=2eÞis the flux quantum. The small value of b para-meter indicates that the Ic and L are small. Thesmall value of L indicates that the superconducting

loop surrounds few grains.

In order to have more information about thegrain boundaries in the intermetallic supercon-

ductors, we have carried out STM/STS study of

one of the ternary (YRuB2) and the binary V3Si

bulk samples. Figs. 3 and 4 shows the STM pic-

tures of one of the natural grain boundary in

YRuB2 and V3Si respectively at ambient temper-

ature. The grain boundary region separating two

crystallite regions is clearly seen in the pictures.The width of the grain boundaries measured in

various STM pictures for YRuB2 is found to vary

Fig. 3. STM image of grain boundary in YRuB2 bulk sample (scan size 30· 30 nm). Image in three-dimensional (3D) mode at 30�pitch.

Fig. 4. STM image of grain boundary in V3Si bulk sample (scan size 70 · 80 nm). Image is in three-dimensional (3D) mode at 60� pitch.

92 N. Khare et al. / Physica C 407 (2004) 89–94

Fig. 6. Conductance spectra for V3Si sample when the tip is (a)

at the grain and (b) at the interior of the grain boundary.

N. Khare et al. / Physica C 407 (2004) 89–94 93

from 15 to 20 �A. For V3Si the grain boundarywidth is found to vary from 60 to 80 �A.For scanning tunneling spectroscopy (STS)

study after stabilizing the STM system for pro-

ducing repetitive real time images of both grain and

grain boundaries, STS was switched on. For this,the feedback loop was opened for a fixed time, bias

voltage was ramped and the change in the tunnel-

ing current was measured as function of the bias

voltage. Normalized conductance fðoI=oV ÞðV =IÞgwas calculated and plotted simultaneously. The

normalized conductance is a measure of local

density of states (LDOS) and it changes sensitively

from the characteristic V-shape to U-shapedepending on whether the probed region is metallic

or quasi-insulating in nature respectively.

Fig. 5 shows the conductance spectra as re-

corded at two positions of the scanning tip for the

YRuB2 sample. When the tip is out side of the

grain boundary (GB) near V shape of conductance

spectra is seen (Fig. 5(a)) which reveals the metallic

nature of the grains. However when the tip is atthe interior of GB non-linearity in conductance

spectra is observed (Fig. 5(b)) indicating the quasi-

Fig. 5. Conductance spectra for YRuB2 sample when the tip is

(a) at the grain and (b) at the interior of the grain boundary.

insulating nature of the GB. Fig. 6 shows the

conductance spectra at two positions of the scan-

ning tip for V3Si sample. Here at both the posi-

tions (at the grain and inside the grain boundary)

the nature of the conductance spectra is V-shape.

This indicates that the interior of the GB in V3Si ismetallic which provides proximity coupling of the

superconducting grains in V3Si sample. This seems

to be the reason of the strongly coupled grain

boundaries in V3Si and for not observing the rf-

SQUID effect in V3Si sample.

4. Conclusion

Rf-SQUID voltage–flux modulations are ob-

served in bulk YRuB2, LaRu3Si2 and LuRuB2ternary intermetallic superconductors from 4.2 K

to near to Tc. The observation of rf-SQUID effectin these ternary intermetallic superconductors

indicates that the grain boundary behave as

Josephson junction type weak links. We have notobserved rf-SQUID effect in the V3Si, a binary

intermetallic superconductor indicating that the

grain boundaries in V3Si are not weak link type.

94 N. Khare et al. / Physica C 407 (2004) 89–94

STM/STS studies shows that the interior region of

the grain boundary in YRuB2 is amorphous and

quasi-metallic, whereas the interior of the grain

boundary of V3Si is metallic. The presence of

amorphous quasi-metallic region at the grain

boundaries in the ternary superconductor seems tobe responsible for the weak link nature of the grain

boundaries and the observed rf-SQUID effect.

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