We report temperature dependent resistivity, specific heat, magnetic
susceptibility and thermoelectric power measurements made on the heavy fermion
system YbNi2B2C, for both as grown and annealed single crystals. Our results
demonstrate a significant variation in the temperature dependent electrical
resistivity and thermoelectric power between as grown crystals and crystals
that have undergone optimal (150 hour, 950 C) annealing, whereas the
thermodynamic properties: (c_p(T) and chi(T)) remain almost unchanged. We
interpret these results in terms of redistributions of local Kondo temperatures
associated with ligandal disorder for a small (~ 1%) fraction of the Yb sites.Comment: 5 pages, 4 figures, submitted to PR

Avila, M. A.

Bud'ko, S. L.

Canfield, P. C.

English

arXiv

We report temperature-dependent resistivity, specific heat, magnetic susceptibility, and thermoelectric power measurements made on the heavy fermion system YbNi2B2C, for both as-grown and annealed single crystals. Our results demonstrate a significant variation in the temperature-dependent electrical resistivity and thermoelectric power between as-grown crystals and crystals that have undergone optimal (150 h, 950 degreesC) annealing, whereas the thermodynamic properties [c(p)(T) and chi(T)] remain almost unchanged. We interpret these results in terms of redistributions of local Kondo temperatures associated with ligandal disorder for a small (similar to1%) fraction of the Yb sites.This article is published as Avila, M. A., S. L. Bud’ko, and P. C. Canfield. "Drastic annealing effects in transport properties of single crystals of the YbNi 2 B 2 C heavy fermion system." Physical Review B 66, no. 13 (2002): 132504.
DOI: 10.1103/PhysRevB.66.132504.
Copyright 2002 American Physical Society.
Posted with permission

Bud'ko, Sergey L.

Canfield, Paul

Digital Repository @ Iowa State University (ISU)

PHYSICAL REVIEW B 66, 132504 ~2002!Drastic annealing effects in transport properties of single crystals of the YbNi2B2C heavyfermion systemM. A. Avila,* S. L. Bud’ko, and P. C. CanfieldAmes Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011~Received 21 June 2002; published 8 October 2002!We report temperature-dependent resistivity, specific heat, magnetic susceptibility, and thermoelectric powermeasurements made on the heavy fermion system YbNi2B2C, for both as-grown and annealed single crystals.Our results demonstrate a significant variation in the temperature-dependent electrical resistivity and thermo-electric power between as-grown crystals and crystals that have undergone optimal~150 h, 950 °C) annealing,whereas the thermodynamic properties@cp(T) andx(T)] remain almost unchanged. We interpret these resultsin terms of redistributions of local Kondo temperatures associated with ligandal disorder for a small (;1%)fraction of the Yb sites.DOI: 10.1103/PhysRevB.66.132504 PACS number~s!: 74.70.Dd, 75.30.Mb, 72.15.Qmeonhemfa-ucicedmsisopresveteeaelsa-tioilitisnitinioitienedndm-ed°Cl atre.ht tomplespe-nan-onvicehingereom-tekndthenrs-theirthePMSn aicha-leaAmong the rich variety of physical phenomena displayby the RNi2B2C nickel boro-carbide family,1–4 YbNi2B2C~Yb-1221! is unique so far in its behavior as a heavy fermisystem5,6 with an electronic specific heat coefficientg'500 mJ/mol K2 and a Kondo temperatureTK'10 K: atemperature scale that is conveniently isolated from otcharacteristic temperatures such as superconducting ornetic condensates (Tc ,TN,0.03 K, if they exist at all! andcrystal electric field splitting (Tce f'100 K).7,8 The heavyfermion behavior is understood as a consequence of thethat the ytterbium 4f levels easily hybridize with the electronic conduction band levels near the Fermi surface. Sheavy electron systems often display many unusual physproperties,9 one common example being disorder-relatvariation in the behavior of different samples of the sacompound.9,10Recently the effects of annealing on the electrical retivity of R-1221 single crystals (R5Y, Gd-Lu! werestudied,11 and once again Yb-1221 showed anomalous prerties in comparison to other members of the series. Wheannealing of crystals withR5Y, Lu, Tm-Gd simply leadsonly to changes in the residual resistivity, radical changeR(T) behavior were observed for Yb-1221 over the full~2–300 K! temperature range. The change inR(T) was so dra-matic that comparison of normalized resistance curR(T)/R(300) became highly questionable.To overcome this problem and better understand whahappening to the Yb-1221 crystals as they go through anning processes, in the present work careful resistivity msurements were made on Yb-1221 crystals cut into wdefined geometries, such that no normalization is necesand direct comparison ofr(T) for different samples and annealing procedures is possible. The results, in combinawith measurements of specific heat, magnetic susceptiband thermoelectric power, allow us to propose a mechanin which optimal annealing is essentially affecting the ligadal disorder associated with a small fraction (;1%) of theYb sites. But given that Yb is a hybridizing rare earth ion,is far more sensitive to such disorder than its nonhybridizneighbors. Yb-1221 then presents a model heavy fermsystem that allows for the study of how small changesdisorder can lead to dramatic changes in transport proper0163-1829/2002/66~13!/132504~4!/$20.00 66 1325drag-cthale--asinsisal--l-ryny,m-gnns.Single crystals of YbNi2B2C were grown from Ni2B fluxat high temperatures as described elsewhere.4,5 Single-crystalplates (43430.5 mm3 typical dimensions! were selected,polished, and cut into bars~typically 1–2 mm in length and0.1 mm2 cross section! with the length along the@100# direc-tion using a wire saw. Once cut, the samples were cleawith toluene and methanol, placed in a Ta foil envelope, athen placed into the quartz insert of a high-vacuuannealing furnace. The insert was continuously pumpdown to a pressure of less than 1026 Torr during the anneal-ing schedule which consisted of a one hour dwell at 200to outgas, followed by the chosen number of hours annea950 °C, followed by a 3–4 cooldown to room temperatuOnce the schedule was completed, the insert was brougatmospheric pressure and the samples removed. Each sawas annealed only once, and for the magnetization andcific heat data the same sample was used to collect the unealed and annealed data sets.Electrical resistance measurements were performedQuantum Design MPMS systems operated in external decontrol ~EDC! mode, in conjunction with Linear ResearcLR400/LR700 four-probe ac resistance bridges, allowmeasurements down to 1.8 K. The electrical contacts wplaced on the samples in standard four-probe linear geetry, using Pt wires attached to a sample surface with EpoH20E silver epoxy. For each slab, the sample weight adimensions were carefully measured and an evaluation ofsample densities (8.360.3 g/cm3) was used to estimate aupper limit of 615% for the overall geometry-related erroin calculating resistivityr5RA/ l . Susceptibility measurements were made on these same MPMS systems, usingstandard dc magnetization operating modes, and forsample oriented in bothHuuc and Huua directions. Specificheat measurements were made on a Quantum Design Psystem containing both specific heat and3He options, allow-ing measurements down to 0.35 K.Thermoelectric power measurements were made ocommercial MMR Technologies Seebeck System, whuses a compressed N2-gas-based Joule-Thompson refrigertor allowing temperature control down to 90 K. The sampSeebeck coefficientS(T) is obtained by comparison withreference constantan wire.©2002 The American Physical Society04-1vidthint oealitheindeelFoither toityngderofalsicty0 Kheini-ityughctedts,ig.ce ofr-ely,Theevi-221doheac-litycom-nspre-ginualadtionaveuesre.-r-teshatera-theuresingth ara-olinalorteFilaBRIEF REPORTS PHYSICAL REVIEW B66, 132504 ~2002!The temperature dependence of the electrical resistifor four bars cut from a single crystal~crystal 1! is shown inFig. 1. There is a substantial change in the temperaturependence as well as a reduction in the magnitude ofresistivity as the annealing time is increased. For increasannealing times the resistivity of Yb-1221 approaches thaLuNi2B2C ~shown as the lowest resistivity curve in Fig. 1!12Figure 2 presents a similar set of annealing data fromsecond crystal~crystal 2!. In this case several shorter timanneals were performed in addition to the 50 and 150 hneals. Whereas the longer annealing times lead to quatively similar resistivity curves to those shown in Fig. 1, tshorter annealing times manifest a different behavior: ancrease in the higher-temperature resistivity followed by acrease in the lower-temperature resistivity. Qualitativsimilar behaviors were found for several other crystals.FIG. 1. Temperature dependence of the electrical resistivityseveral pieces of Yb-1221 cut from crystal 1, for different anneatimes at 950 °C. A measurement on unannealed Lu-1221 isincluded for comparison.FIG. 2. Temperature dependence of the electrical resistivityseveral pieces of Yb-1221 cut from crystal 2. Note that for shoannealing times the higher temperature resistivity increases.longer annealing times the resistivity data is quantitatively simto the data shown in Fig. 1.13250tye-egfan-a---yrlow temperatures, resistivity is monotonically reduced wincreasing annealing time, whereas for temperature nearroom temperature there can be an initial rise in resistivfollowed by an ultimate decrease in resistivity for increasiannealing times. Ultimately, for annealing times on the orof 150 h, the resistivity is increasingly approaching thatLuNi2B2C.The temperature-dependent resistivity for 150 h anne~Figs. 1 and 2! is consistent with a Yb-based intermetallthat hasTK'10 K. There is an initial decrease in resistiviwith decreasing temperature. The broad shoulder near 20is very likely associated with the thermal depopulation of tupper CEF levels. There is a beginning of a resistance mmum near 30 K, followed by a sharp decrease in resistivassociated with coherence as the sample is cooled throTK . These data are almost precisely what would be expefrom a textbook example of a low-TK intermetallic.In contrast with the results of resistivity measuremen150 h annealing at 950 °C causes only minor changes~ifany! in the material’s bulk thermodynamic properties. In F3 we show measurements of the temperature dependen~a! specific heat and~b! inverse magnetic susceptibility peformed on two pieces cut from crystals 3 and 4, respectivbefore and after they were annealed for 150 h at 950 °C.general features of these curves are consistent with prously reported measurements on unannealed Yb-1crystals.5 The fact that the bulk thermodynamic propertiesnot change, within experimental limits, gives support to thypothesis that annealing is affecting only a very small frtion of the sample and also helps to rule out any possibiof major structural changes such as phase separation, deposition, corrosion, oxidization, or other such degradatioto the crystalline lattice.Based on the data presented in Figs. 1–3 as well asvious work on the nonhybridizing members or theRNi2B2Cseries~e.g.,R5Y, Lu, Tm-Gd!,11 it appears that annealinsingle crystals ofRNi2B2C can cause significant changesthe electrical resistivity. For the samples of LuNi2B2C orYNi2B2C ~as well as the local moment members! this in-creased perfection simply leads to a reduction of the residresisitivity. On the other hand, for the case of YbNi2B2C,disorder around the hybridizing Yb ion can very easily leto a change of the local Kondo temperature. This observaleads to a simple hypothesis: a small number of Yb sites ha distribution of Kondo temperatures that can have valranging from 10 K or lower up to above room temperatuThis distribution inTK values is associated with a distribution of Yb-ligandal environments. With annealing, this disoder is reduced and a diminishingly small number of Yb sihave Kondo temperatures different fromTK'10 K.The first question that can be simply addressed is, wpercentage of Yb sites would need to have Kondo temptures above room temperature to account for;40 mV cm excess resistivity found near room temperatin the unannealed samples? This can be estimated by uthe expression for the increase in resistivity associated wiKondo impurity for temperatures below the Kondo tempeture associated with the impurity:13fgsofrorr4-2nd-Yth-btivityofthesarlyn-pleaturesn to.,ooferyyigs.21-eofn-furllythen,our-of-x-tricthealea-aftertwo221theithip-mi-tivetedrre.com-terinpeBRIEF REPORTS PHYSICAL REVIEW B66, 132504 ~2002!Drmax5\e24pcpkF~2l 11!, ~1!wherec is the concentration of Yb ‘‘impurities,’’kF is theFermi momentum,p is the number of electrons per atom, al is the ytterbium 4f angular momentum. Usingl 53, p53, andkF'3.73107 cm21 ~Refs. 14 and 15! in this equa-tion, we estimateDrmax530 mV cm for every 1% concentration of disordered Yb sites withTK greater than roomtemperature. This means that only a few percent of theions need to have Kondo temperatures different fromexpected value ofTK'10 K, to produce a significant increase in resistivity that is seen in Figs. 1 and 2. It shouldnoted that the above calculation is at best semiquantitaFIG. 3. ~a! Specific heat of Yb-1221 crystal 3, before and afannealing at 950 °C for 150 h. The inset shows theT2 dependenceof the linear termcp /T at the lowest temperatures.~b! Inverse sus-ceptibility of Yb-1221 crystal 4 for both orientationsHuuc andHuua, before and after annealing at 950 °C for 150 h. Annealdoes not significantly change the crystal’s thermodynamic proties.13250beee~i.e., should be trusted to only within factors of 2!, but it doesdemonstrate that our working hypothesis is a viable one.The evolution of the temperature-dependent resistivcan be understood then by simply invoking a reductionligandal disorder with annealing. For long time annealsnumber of Yb ions that haveTK values higher than 10 K habeen reduced to a negligible amount. This leads to the neclassicalr(T) curves seen in Figs. 1 and 2. For smaller anealing times the higher-temperature resistivity of the samcan either increase or decrease, depending upon the n~quantity and type! of disorder in the as-grown sample. Athe ligandal disorder is annealed away there is no reasoassume that intermediate states~of crystallographic order!will have TK values that monotonically approach 10 K: e.gas seen in crystal 2~Fig. 2! small annealing times appear tbe populating configurations with higherTK values. We haveperformed annealing studies on five different groupssamples cut from crystals from different batches. In evcase the long time anneals (;150 h) approach the resistivitdata shown in Figs. 1 and 2. The behavior illustrated by F1 and 2 appears to be a generic behavior for Yb-12samples prepared in this manner.If indeed the changes inr(T) are due to changes in ligandal disorder in a small fraction of the Yb sites, then wwould anticipate that any bulk, thermodynamic propertythe material should not be significantly affected by the anealing process. Forcp(T), the change in the number oTK'10 K Yb sites~on the order of 98%–100%! that occurswith annealing is so small that it is not resolvable in omeasurements@Fig. 3~a!# on the same piece of sample~asgrown and annealed!. The measured curves are essentiaindistinguishable for 0.35 K<T<30 K. Forx(T), annealingeffects are equally indistinguishable. The vast majority ofYb sites are acting as Yb13 at high temperatures and, agaia change of a percent or two is not distinguishable inmeasurements.Finally, since the assumed disorder affects the densitystates~DOS! profile near the Fermi surface, one should epect significant changes in the material’s thermoelecpower. Figure 4 shows the temperature dependence ofSeebeck coefficientS(T) obtained by measuring severpieces cut out of Yb-1221 crystal 4. Three pieces were msured after cutting, and two other pieces were measuredannealing for 150 h at 950 °C, and the results for thesegroups of data were averaged. An unannealed Lu-1sample was also measured for comparison. Bothunannnealed Yb-1221 and Lu-1221 data agree very well wpreviously reported measurements on these compounds,16 al-though there is a sample-to-sample scatter~as indicated bythe error bars! in S(T) in our data.The limited temperature range of our experimental equment does not allow us to clearly observe the most pronent feature in such measurements, which is a large negapeak inS(T) just below 100 K,16 but the high-temperaturedata of our samples is seen to follow the generally expecbehavior within the framework of the Coqblin-Schrieffemodel17–19 and the disorder-related effects discussed heFor the unannealed samples, the response should be abination of a low-TK Kondo lattice and single-Yb ‘‘impu-gr-4-3oityoatinLu-tionle-ortuencegeb-werl iser ofplyrop-leredhehisandnf theri-tedityp-sic2otCfBRIEF REPORTS PHYSICAL REVIEW B66, 132504 ~2002!rity’’ effects with highTK values, both of which are known tgreatly increase the absolute values ofS(T) for T@TK . An-nealing will then have the effect of reducing the impurcontribution, which in our case results in a reductionroughly 50%~from ;40 to;20 mV/K) in the magnitude ofthe thermopower response near the peak, whose positionseems to shift to slightly higher temperatures. The resulFIG. 4. Absolute thermopower of several samples of Yb-12cut from crystal 4~the unannealed data manifold is the averagedata from three samples, and the annealed data manifold isaverage of data from two samples that were annealed at 950 °150 h!. A measurement on unannealed Lu-1221 is also includedcomparison.Add,t,,Rt,13250flsogcurves for annealed Yb-1221 samples move towards the1221 reference, but still maintain the enhanced contribucoming mostly from the Kondo lattice with CEF splitting.18In conclusion, we have shown how annealing of singcrystal YbNi2B2C leads to dramatic changes in its transpbehavior. These changes can be understood as a conseqof the effects of ligandal disorder that leads to locally larvalues ofTK associated with a small fraction of local Ysites. This small number of high-TK sites leads to large enhancements in electrical resistivity and thermoelectric pothat are very sensitive to the amount of time the crystaannealed. Increased annealing times reduce the numbperturbed Yb sites and lead to more classical, and simunderstood, temperature dependences of the transport perties. To this extent, YbNi2B2C continues to be a modeheavy fermion compound: not only from the fact that othcharacteristic energy scales are well separated fromTK inthis material,5 but also because it allows for more detailinvestigations of exactly how different disturbances in tvicinity of Yb sites affect the hybridization of its 4f levelwith conduction states and the hybridized Kondo lattice. 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Drastic annealing effects in transport properties of single crystals of the YbNi2B2C heavy fermion system

https://doi.org/10.1103/PhysRevB.66.132504

Drastic annealing effects in transport properties of single crystals of the YbNi2B2C heavy fermion system

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