Solid State Communications 159 (2013) 29–31

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Solid State Communications

0038-10

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n Corr

E-m

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Electronic transport minimum in SmCuAs2 at low temperaturesand structural anomalies

K. Sengupta a,n, K.K. Iyer b, R. Ranganathan a, E.V. Sampathkumaran b, Th. Doert c, J.P.F. Jemetio c

a Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, Indiab Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, Indiac Anorganische Chemie, Technische Universitat Dresden, Helmholtzstrasse 10, Dresden D-01062, Germany

a r t i c l e i n f o

Article history:

Received 2 November 2012

Received in revised form

31 December 2012

Accepted 14 January 2013

by P. Shengordering is found to decrease sluggishly with pressure up to 15 kbar. External magnetic field has

Available online 20 January 2013

Keywords:

A. Intermetallics

B. Resistivity

C. HfCuSi2-type structure

D. High pressure

98/$ - see front matter & 2013 Published by

x.doi.org/10.1016/j.ssc.2013.01.014

esponding author: Tel.: þ91 9163920685.

ail address: ksengupta11@gmail.com (K. Seng

a b s t r a c t

Temperature dependent x-ray diffraction and electrical transport under pressure have been reported on

SmCuAs2, which has been known to exhibit an unusual transport behaviour (a pronounced minimum

much before long range magnetic order (Sampathkumaran et al., 2003 [6])) at low temperatures. The

Neel temperature as well as the pronounced resistivity upturn observed before the onset of magnetic

insignificant effect both at ambient and under pressure. Thus the anomalies are quite robust to applied

pressure and magnetic field. Low temperature x-ray diffraction reveals no symmetry change, but the

system exhibits lattice distortions along both a- and c-axes in the temperature region of interest,

thereby suggesting that the resistivity anomalies could be associated with structural anomalies in this

system.

& 2013 Published by Elsevier Ltd.

1. Introduction

In the past decade, the class of ternary rare-earth (R) intermetallicsof RTX2 (T¼transition metal, X¼As, Sb), crystallizing in ZrCuSi2-typelayered tetragonal structure (P4/nmm), has been shown to exhibitinteresting transport, magnetic and thermodynamic properties [1–5].One of the interesting series in this class is RCuAs2 [6–11] adopt theHfCuSi2-type structure. Apart from the members with Ce or Yb, eventhe heavier rare-earth compounds also exhibit anomalous behaviour.Most of the RCuAs2 compounds (except CeCuAs2 which is paramag-netic) order antiferromagnetically at low temperatures (T) with abreakdown of de Gennes scaling. The transport properties of this classof compounds present an interesting scenario. It was found that thereis an unexpected upturn in electrical resistivity (r) in some heavyrare-earth members, R¼Sm, Gd, Tb and Dy, although this feature isabsent in R¼Ho and Er [6]. Occurrence of minima far above the Neeltemperature (b2TN) rules out the possibility of a superzone gapcausing this feature. The origin of the anomaly remains till nowelusive. It is worth to mention that this unusual feature is not onlyfound in this particular class of system, but is also observed in otherstructures like in RAgGe [12] and Gd2PdSi3 [13]. We considered itimportant to carry out crystallographic studies at low temperaturesas well as high pressure studies on one of these compounds.

Elsevier Ltd.

upta).

Here one of the members of the RCuAs2 series, SmCuAs2, hasbeen chosen for further studies. The compound undergoes anantiferromagnetic (AFM) transition at around 13 K with an upturnin resistivity at 35 K [6]. The resistivity near room temperature isreported to exhibit a concave shape as opposed to convex naturefound for other members of the same series. The inverse magneticsusceptibility is non-linear near room temperature. Usually, thisis often the case in Sm-based compounds, and is explained bythe fact that the first excited of the Hund’s rule multiplet (J¼7/2)is very close to the ground state (J¼5/2). Hence, even at roomtemperature, there is a considerable mixing between the twostates.

2. Experimental details

The sample employed in the present investigation is the sameas that used in Ref. [6]. The powder x-ray diffraction (XRD)patterns as a function of temperature till 12 K (the lowesttemperature limit of our instrument) have been taken usingRigaku diffractometer. The resistivity measurements, using thestandard four-probe method, as a function of T and magnetic field(H) were performed employing a commercial clamp type piston-cylinder cell (easyLab Technologies Ltd, UK) with the help of aphysical property measurements system (PPMS, QuantumDesign). The measurements have been taken in a hydrostaticpressure medium using a 1:1 mixture of pentane and isopentane.

K. Sengupta et al. / Solid State Communications 159 (2013) 29–3130

3. Results and discussions

Let us first discuss the effect of applied pressure and externalmagnetic field on the electrical transport properties of SmCuAs2.Fig. 1 shows the electrical resistivity at selected pressures. Inorder to have a better comparison of the qualitative features, wehave normalized the data to the respective values at 200 K. Thereis no qualitative change in the behaviours and the shape ofdifferent r(T) plots appears similar.

Fig. 2 describes some features which are important for aquantitative understanding of the resistivity data describedabove. Different quantities in the Fig. 2 have been derived fromFig. 1. The ordering temperature, TN, has been defined as thetemperature corresponding to the peak below 20 K. From Fig. 2a,TN remains unchanged up to 6 kbar, above which it starts todecrease slowly and the trend continues till 15 kbar. This findingis quite puzzling because an application of pressure often hasbeen found to stabilize the magnetic states (Sm3þ) in Sm-based

0

0.84

0.91

0.98

6

p = 0 kbar

15

SmCuAs2

ρ (T

)/ ρ

(200

K)

T (K)50 100 150 200

Fig. 1. (Colour online) Normalized resistance of SmCuAs2 at selected pressures.

12.0

12.5

36

39

0

12

13

SmCuAs2

T N (K

)T m

in (K

)

p (kbar)

Δρ /

ρ (T

N) %

3 6 9 12 15

Fig. 2. (Colour online) Pressure dependence of (a) Antiferromagnetic ordering

temperature, (b) Temperature corresponding to minimum in resistivity and (c)

Relative changes of the magnitude of the minimum. The continuous lines are

guides to eyes.

compounds [14]. The quantity Tmin which indicates the tempera-ture corresponding to minimum in r(T), shows the oppositetrends compared to TN (Fig. 2b). Initially, Tmin increases slowly,and then it shifts to higher temperatures above 6 kbar. In Fig. 2c,the relative change in the minimum in resistivity (Dr/r(TN)�100%) is shown. The quantity Dr is defined as the differencebetween r(TN) and r (Tmin) (Dr¼r(TN)–r(Tmin)). So Fig. 2c showsthat the magnitude of the upturn below the Tmin decreasesgradually above 6 kbar, but not significant.

SmCuAs2, in Fig. 3, shows negligible field dependence of theelectrical resistivity up to 100 kOe and the behaviour is almostsimilar to that at ambient pressure [6]. The magnetoresistance(MR) has been measured both below and above the temperatureregion where the resistivity upturn appears. MR varies quadrati-cally with H at low fields which is typical of the metallic AFMsystem. The absence of any significant change in r with magneticfield and pressure below Tmin confirms the absence of forma-tion of any pseudo-gap above TN and dominance of metalliccontribution.

Let us now examine structural stability and the role of thelattice at low temperatures. Fig. 4 shows the XRD patterns atselected temperatures. No extra reflections and no peak splitting(within the resolution of our measurement) are visible at lowtemperatures. Intensity of the peaks varies slightly at low tem-perature. However, a tiny shift of peaks is observed below 30 K(see inset of Fig. 4). The key information obtained from Fig. 4 issummarized below in Fig. 5.

Fig. 5 shows the trend of the lattice parameters derived fromthe x-ray powder patterns. It is found that the lattice parameters

0

1.01

1.02

1.03

35 K

SmCuAs2

ρ (H

) / ρ

(H =

0)

H (kOe)

p~ 15 kbar 1.8 K

1.0020 40 60 80 100

Fig. 3. (Colour online) Magnetoresistance of SmCuAs2 at 15 kbar and at 1.8

and 35 K.

20-60

-50

-40

-30

-20

-10

0

10

20

30

48

221

007

106

204

212

21121

0

200

104

112

004

103

11111

0

102

003

100 K

50 K

30 K

Inte

nsity

(a. u

.)

SmCuAs2

T = 12 K

20 K

101

30 40 50 60 702θ

52

Fig. 4. (Colour online) XRD of SmCuAs2 at selected temperatures. Data are shifted

relative to each other for clarity. Inset shows the same figure in an expanded form.

3.92

3.94

3.96

a-la

ttic

e (Å

)SmCuAs2

9.95

10.00

10.05

c-la

ttic

e (Å

)

0

152

156

Vol

ume

(Å3 )

20 40 60 80 100T (K)

Fig. 5. (Colour online) Lattice parameters (a, c) and unit cell volume of SmCuAs2

derived from Fig. 4. The continuous lines are guides to eyes.

K. Sengupta et al. / Solid State Communications 159 (2013) 29–31 31

a, c and volume (V) of the unit cell change smoothly from theroom temperature down to 50 K (not shown here). But severechanges in lattice parameters and volume are obvious below 30 K.There is a pronounced dip observed below 30 K for c down to longrange magnetic ordering temperature, and a correspondinganomaly though weaker for the basal plane parameter a is alsoobserved. This temperature range is exactly coinciding with therange where resistivity minimum is observed.

4. Conclusion

To summarize, the family of materials RCuAs2 clearly presentinteresting transport behaviour. We have tried to understand theorigin of the resistivity minimum and its response to externalinfluences like pressure and magnetic fields in some of the rare-earth members in the series RCuAs2. We have taken up one suchsystem (SmCuAs2) and found that the minimum in resistivity isvery robust to magnetic fields and external pressure within therange of our measurements. Low temperature XRD reveals inter-esting information. The symmetry of the lattice remainsunchanged, but needs to be verified using high resolution XRDmeasurement on a single crystal. There is, however, a latticedistortion in the temperature region of interest in the resistivitydata. Thermal expansion measurements as a function of

temperature and resistivity measurements at higher pressuremay reveal important information. It is not obvious at presentwhether this is a result of a magnetic precursor effect [15] orwhether this is a cause of the resistivity minimum. In any case,the observed lattice anomaly is intriguing and may be helpful infuture for better understanding of such anomalies. Absence ofresistivity upturn in other members of the series (particularly inheavy rare-earths, Ho and Er (exhibits extremely small upturn,only 0.5%)) [16] could possibly be related to critical lanthanidecontraction which plays a crucial role here. Disappearances of theresistivity upturn in the case of DyCuAs2 and TbCuAs2 withapplication of magnetic field most likely indicate a strongmagneto-lattice coupling, whereas in GdCuAs2 and SmCuAs2

these couplings are nominal. One may speculate that the resis-tivity anomalies in the case of RAgGe [12] and Gd2PdSi3 [13] couldalso arise from the lattice distortion, but one needs to verify thispossibility. Employment of different microscopic techniques (e.g.,neutron) will be useful for a better understanding of the transportand magnetic anomalies of this class of systems.

Acknowledgement

One of the authors, KS, is grateful to Anish Karmohapatra forassisting XRD measurement at low temperature.

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