Resistivity plateau and extremely large magnetoresistance in NbAs2 and TaAs2

3.4 MB·

by  Yi-Yan Wang

Resistivityplateauand extremely largemagnetoresistance inNbAs and TaAs
2 2
Yi-Yan Wang, Qiao-He Yu, and Tian-Long Xia∗
DepartmentofPhysics,BeijingKeyLaboratoryofOpto-electronicFunctionalMaterials&Micro-nanoDevices,
Renmin University of China, Beijing 100872, P. R. China
(Dated:January19,2016)
Intopologicalinsulators(TIs),metallicsurfaceconductancesaturatestheinsulatingbulkresistancewithde-
creasing temperature, resulting in resistivity plateau at low temperatures as a transport signature originating
frommetallicsurfacemodesprotectedbytimereversalsymmetry(TRS).Suchcharacteristichasbeenfoundin
6
severalmaterialsincludingBi2Te2Se,SmB6etc.Recently,similarbehaviorhasbeenobservedinmetalliccom-
1
poundLaSb,accompanyinganextremelylargemagetoresistance(XMR).Shubnikov-deHass(SdH)oscillation
0
at low temperatures further confirms themetallic behavior of plateau region under magnetic fields. LaSb[1]
2
hasbeenproposedbytheauthorsasapossibletopologicalsemimetal(TSM),whilenegativemagnetoresistance

n is absent at this moment. Here, high quality single crystals of NbAs2/TaAs2 with inversion symmetry have
a beengrownandtheresistivityundermagneticfieldissystematicallyinvestigated.Bothofthemexhibitmetallic
J
behaviorunderzeromagneticfield,andametal-to-insulatortransitionoccurswhenanonzeromagneticfieldis

7 applied,resultinginXMR(1.0×105%forNbAs2 and7.3×105%forTaAs2 at2.5K&14T).Withtempera-
1 turedecreased, aresistivityplateauemergesaftertheinsulator-likeregimeandSdHoscillationhasalsobeen
observedinNbAs2andTaAs2.
]
i
c PACSnumbers:75.47.-m,71.30.+h,72.15.Eb
s
-
l
r INTRODUCTION pounds: NbAs2 andTaAs2. Metal-to-insulatortransitionand
t
m XMR arise at low temperatureand high field. The observed
resistivity plateau is similar to the universalbehavior in TIs,
. The magnetoresistance (MR) of materials is an interest-
t
a ing research topic in condensed matter community. Since inwhichTRSispreserved,whileTRSisbrokeninourmea-
m surements as the case in LaSb. Analysis of SdH effect ob-
the discovery of giant magnetoresistance (GMR) in mag-
- neticmultilayer[2,3]andcolossalmagnetoresistance(CMR) served in NbAs2 and TaAs2 implies the complex Fermi sur-
d facestructureandthetwomajoroscillationfrequenciesreveal
in magnetic oxide materials[4, 5], people have always been
n
theremayexisttwoFermipocketsineachofthesamples.
o looking for materials with higher MR. In general, nonmag-
c netic metals only have small MR. Recently, XMR of about
[ 105% − 106% has been observed in several nonmagnetic

1 metals, such as TX (T=Ta/Nb, X=As/P)[6–14], NbSb2[15],
v LaSb[1], Cd3As2[16, 17] and WTe2[18, 19] et al. In these
9 materials, TX is an important system which is composed of
3
VA elements and pnicogen, all of which are proved to be
2
Weyl semimetals[20–30]. Extraordinary properties such as
4
0 Fermiarc and chiralanomalyobservedin TX have attracted
. greatattention. Theinterestingpropertiesoftransition-metal
1
0 monophosphidesmotivateustoresearchintoothertransition-
6 metalphosphideswith differentcrystal structures. TX2 type
1 compounds, which are made of VA elements and pnicogen,
:
v haveamonoclinicstructurewithinversionsymmetry.Thean-
i timonideNbSb2[15]hasbeeninvestigatedpreviously. Inthis
X
work,wefocusontheTX2typearsenides.
r
a InadditiontoXMR,thefieldinducedresistivityplateauis
also an interesting phenomenon. The plateau is a universal
behavior in topological insulators[31, 32], which originates
fromthe metallic surfaceconductancesaturatingthe insulat-
ingbulkresistance[1]. TRSprotectsthemetallicsurfacestate
andisindispensableinTIs. RecentresearchinpossibleTSM
candidateLaSbshowsthattherealsoexistsplateauwhenTRS
FIG. 1: (Color online) Crystal structure of NbAs2 and TaAs2. (a)
isbroken[1]. Althoughthespecificmechanismofthisfeature
Unit cell of the crystal. A represents Nb or Ta atoms, B1 and B2
is still a mystery, it is meaningfulto find outmore materials representtype-Iandtype-IIatomsofAs,respectively. (b)-(c)Views
withsuchproperty. ofthestructurefromb-axisanda-axisdirections,respectively.
In this work, we have mainly studied the resistivity and
magnetoresistanceoftwosynthesizedsinglecrystallinecom-
2
EXPERIMENTALANDCRYSTALSTRUCTURE Figure1(c). Figure. 2(a)and(b)showpowderX-raydiffrac-
tion patterns and refinements of NbAs2 and TaAs2 crystals.
High-qualitysinglecrystalsofNbAs2 andTaAs2 wereob- ThereflectionsarewellindexedinspacegroupC12/m1. The
tainedbychemicalvaportransportmethod. Firstly,polycrys- refined lattice parametersare given in the figures, consistent
tal of these two compounds was prepared by solid state re- withpreviouslyreportedresults[33–35].
action. The mixturesof Nb/Ta powder and As powder were

stuebaleefdurinnaaceqwuaarstzhetautbeedwtoit8h00a0rCataiondohfe1l:d2f.orF3o0rhN,fboArsT2a,Aths2e 1.6 (a) NbAs2 02 TT 18 (d) TaAs2 02 TT
the tube furnace was heated to 7500C. Secondly, 5 mg/cm3 • cm) 1.2 I//b, H//ac 58 TT • cm) 12 I//b, H//ac 58 TT
oanfdiotdhienetewmapseurasteudreasgtrraadnisepnotrwtaagsesnetttaosg1ro0w50s0iCn-g8le50c0rCys.taAlsll, ( 0.8 1114TT ( 6 1114TT
0.4
theobtainedcrystalsgrowmoreeasilyalongb-axisandform
rod-like crystals. The atomic proportionsdeterminedby en- 0.0 0
0 50 100 150 200 250 300 0 50 100 150 200 250 300
ergydispersiveX-rayspectroscopy(EDS,OxfordX-Max50) Tempera ture (K) Tempera ture (K)
w(XeRreDc)opnastitsetrennstwweirteho1b:t2aifnoerd(fNrobm/Tap)o:wAds.erXof-rsaiyngdleiffcrraycsttiaolns m/K) 0.00(b) N15bAs2 m/K)0.0 (e) 1T5aAs2
orandiaatBiornu.keTrODPA8SA-d4v.2anwcaesXem-rapylodyiefdfrfaocrtothmeerteerfinuesimngenCtsu. RKeα- / dT (• c --00..0021 Gap (meV) 150 / dT (• c--00..42 Gap (meV)12
sistivitymeasurementswereperformedonaQuantumDesign d d 9
physicalpropertymeasurementsystem(QDPPMS-14T). -0.03 T=43K 00 4 H 8(T) 12 16 -0.6 T=29K 0 4 H 8( T) 12 16
0 50 100 150 200 250 300 0 50 100 150 200 250 300
Tempera ture (K) Temper ature (K)
(c) 14T (f) 14T
arb.units ) (a) NbAs2 Rabc e===f i973n...e373d598 451la(((17t7t)))i c ¯¯¯e parameters : ln (• cm) ---0321NbAs2 12851TTTT ln (• cm) 02 TaAs2 12518TTTT
y ( -4 -2
nsit -5 -4
e 0.00 0.05 0.10 0.00 0.05 0.10
Int T-1 (K-1) T-1 ( K-1)
FIG.3: (Coloronline)(a),(d)ResistivityofNbAs2 andTaAs2 plot-
10 20 30 40 50 60 70 80 tedasafunctionoftemperatureunderdifferentmagneticfield(H=0,
Refined lattice parameters : 2,5,8,11,14T).Iisparalleltob-axisandHisparalleltoac-plane.
units ) (b) TaAs2 ab == 93..334826((84)) ¯¯ Tsehrevemdectlaela-troly-i.n(sbu)l,a(teo)r-dlρik/edtTranvseirtsiuosntaenmdpreersaitsutriveiftyorpclaotreraeuspaornedoinbg-
arb. c = 7.762(2) ¯ samples.Theinsetsshowenergygapoftheinsulator-likearea.(c),(f)
y ( Plotsoflnρagainstthereciprocaloftemperature1/T. Thevalues
sit of energygapareobtained byfittingtheinsulator-likeregions (the
nten linearpartinfigures)usingtherelationρ(T) ∝ exp(ξ/kBT). The
I redlinesindicatetheregionsusedinfittings.
10 20 30 40 50 60 70 80
2 (Degree)
FIG.2: (Coloronline)XRDpatternsofNbAs2 andTaAs2 withre- RESULTSANDDISCUSSIONS
finement. Observed curves are in black circle and the calculated
curvesareinredline. Thedifferencecurve isinblueandthevio-
Figure 3(a),(d) plot the temperature dependence of resis-
letverticallinesdenotethepositionsofBraggreflectionsofNbAs2
tivity under different magnetic fields. The electric current
andTaAs2withPDFNo.01-086-0520andNo.01-089-3409. Refined
latticeparametersareshownineachpicture.Rwp=6.69%,8.73%for is parallel to b-axis and the magnetic field is parallel to ac-
NbAs2andTaAs2. plane. For both compounds, the temperature dependent re-
sistivity at zero field exhibits a metallic behavior. The high
NbAs2 and TaAs2 crystallize in a complex structure[33, residualresistivityratio(RRR=75, 83forNbAs2 andTaAs2,
34]. Figure 1 shows the structure of these compoundsfrom respectively)indicateshighqualityofthesamples.Extremely
different views. In the structure, there is one type of Nb/Ta largeMRhasbeenobservedinbothofthem. At2.5Kand14
atoms denoted as A and two types of As atoms denoted as T, MR=1.0×105% and 7.3×105% for NbAs2 and TaAs2,
B1orB2. EachAissurroundedbysixB,includingthreeB1 respectively. With temperature reduced, a magnetic field-
and three B2. Each B2 is coordinatedto three other B2. In inducedmetal-to-insulator-liketransitionisobserved. Anin-
thesecrystals,b-axisistheeasygrowthaxis,andthischarac- teresting feature is that a resistivity plateau appears instead
teristic canbeseen clearlyfroma-axisdirectionasshownin oftheresistivitycontinuestoincreasewithfurtherdecreasing
3
temperature.Temperaturesoftheresistivityplateauappearing dρ/dT. As shownin the figure, T2 remainsunchangedwith
areabout28Kand19KforNbAs2 andTaAs2,respectively. theincreaseofmagneticfield. T1 increasesbutT2 keepsun-
At high field, the plateau becomes distorted because of the changed, which suggests that the range of the insulator-like
effect of SdH oscillation, which will be discussed later. In area becomes larger along with increasing field, meanwhile
general, topologicalinsulatorshave insulatingbulk state and the range of the resistivity plateau keeps invariant. The en-
metallicsurfacestate. TheTRSprotectsthemetallicsurface ergy gaps are shown in the inset of Figure 5(b),(e). All the
state intopologicalinsulators. Alongwiththeappearanceof gapvaluesareobtainedthroughfittingthelinearpartinFig-
themetallicsurfaceconduction,theinsulatingresistivitywill ure3(c),(f).AtthemagneticfieldofH=14T,theenergygaps
reachsaturationresultinginaresistivityplateau.Inoursitua- are 13.2 meV and 15.3 meV for NbAs2 and TaAs2, respec-
tion,theexistenceofmagneticfieldbreakstheTRS,butthere tively.Asawhole,TaAs2hasthelargergapthanNbAs2.
isstillaplateauatlowtemperature. Moreexperimentsarein Figure4(a)and(b)showtheresistanceofNbAs2andTaAs2
greatneedtounderstandtheintrinsicphysics. asa functionoffield. ClearSdH oscillationwasobservedat
lowtemperatureandhighfield. Theinsetsshowtheenlarged
images of oscillating part. In both compounds, the MR-H
4 2.5K 8 K curves(notshown here) all exhibita semiclassical quadratic
(a) behavior(MR∝H2)[36]. Withtheincreaseoftemperature,
3.5K 10 K
theMRbecomessmallerandtheoscillationgraduallydisap-
3.5 5 K
3 pear.
6.5K 30K
) Figure5(a)and(b)plottheoscillationamplitude∆R =
xx
-5 10 2 3.0 Rmxaxgn−etihcRfixexlido1f/HNbaAtsv2aarinodusTateAms2peargaatuinrests.thTehreecaipmropcliatuldoef
( 2.5 50K displaysa complexperiodic behaviorand decreaseswith in-

R 12 14 creasing temperature. There are several peaks in the fast
Fourier transformation (FFT) spectra (Figure 5(c) and (d)),
1
NbAs2 75K but the major peaks are α and β. The complexity of peri-
odic behavior can be attributed to the effect of small peaks
in the FFT spectra. For NbAs2, the major oscillation fre-
0
0 3 6 9 12 quencies are Fα=90 T and Fβ=204 T. For TaAs2, Fα=45 T
and F =158 T. In SdH oscillation, the frequency F is pro-
H (T) β
portionalto the cross sectional area A of Fermi surface nor-
2.5K 8 K mal to the magnetic field and can be described using On-
(b)
12 3.5K 10 K sagerrelationF =(φ0/2π2)A=(h¯/2πe)A[37]. Twomajor
12 5 K peaksinFFT spectraimplytherearetwomajorFermipock-
)
6.5K ets in NbAs2 and TaAs2, which is similar to the situation in
-5 0 9 20K NbSb2[15]. In Figure 5(e)-(h), we display the temperature
1 9 dependence of the relative FFT amplitude of frequencies α
( and β of NbAs2 and TaAs2, respectively. The thermal fac-
R 6 6 30K torRT =(λT)/sinh(λT)inLifshitz-Kosevitchformula[37]
10 12 14 hasbeenemployedtodescribethetemperaturedependenceof
FFT amplitude∆. Intheformula,λ = (2π2k m∗)/(h¯eH¯),
3 B
TaAs2 50K so thecyclotroneffectivemass m∗ canbeobtainedfromthe
fittingresult. Allthedatacanbefittedwellandyieldstheef-
75K
0 fectivemass. ForNbAs2,m∗α = 0.20meandm∗β = 0.27me;
0 3 6 9 12 forTaAs2,m∗α =0.17meandm∗β =0.24me. ARPESexper-
H (T) imentsareneededtoobtainmoreinformationabouttheFermi

surfaceand the collaborativeworkbased onour samplesare
FIG. 4: (Color online) Magnetic field dependence of resistance of
NbAs2 (a)andTaAs2 (b)from2.5Kto75K.Theinsetsshow the alreadyinprocess[38].
enlargedpartofR-Hfrom10-14T,wheretheoscillationcanbeob-
servedmoreclearly.
SUMMARY
Plots of dρ/dT against temperature are displayed in Fig-
ure3(b),(e). Thetemperatureofmetal-to-insulatortransition Insummary,singlecrystalsofNbAs2andTaAs2havebeen
isdeterminedasT1wherethevalueofdρ/dT becomesnega- grownsuccessfully.Resistivityhavebeenmeasuredandmag-
tive. Fromthefigure,itcanbejudgedthatT1 increaseswith netoresistance has been analyzed in detail. Field-induced
the field. The starting temperature of the resistivity plateau metal-to-insulator transition and XMR are observed in both
(T2) can be defined as the temperature at the minimum of samples. The resistivity plateau similar to topological insu-
4

tivityunderparallelfieldwithcurrentisinprocess.
1.5
(a) NbAs 2.5K Note added. While this paper is being prepared, one re-
1.0 2
) 3.5K latedworkonTaSb2 isreportedonline[39], whereresistivity
0.5 5 K plateau, XMR, SdH oscillation and negative MR have been
(
R xx 0.0 observedbytheauthors.
6.5K
-0.5
8 K
-1.0
10 K
ACKNOWLEDGMENTS
-1.5 20 K

0.08 0.10 0.12 0.14
This work is supported by the National Natural Science
9 (b) 2.5K
Foundation of China (No.11574391), the Fundamental Re-
TaAs
6 2 3.5K search Funds for the Central Universities, and the Research
)
3 5 K FundsofRenminUniversityofChina(No. 14XNLQ07).
(xx 0 6.5K
R
-3 8 K
-6 10K
20K ∗ Electronicaddress:[email protected]
-9
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