The authors have performed inelastic neutron scattering measurements that confirm that the crystal-electric-field split ground state in the heavy-fermion compound PrInAg{sub 2} is a nonmagnetic, non-Kramers doublet. This implies that a quadrupolar Kondo interaction is responsible for the enhanced thermodynamic properties observed at low temperatures. They also observe anomalous broadening of the inelastic peaks and suggest two possible causes for this broadening

Beyermann, W.P.

Canfield, P.C.

Kelley, T.M.

Nakotte, H.

Robinson, R.A.

Trouw, F.

UNT (University of North Texas) Digital Library

LA-UWLA-CP-98- - 3 5 5 4  Approved forpublic release; distribution is unlimited. TITLE: AUTUOR(S): SUBMITTED TO: Los Alamos N A T I O N A L  L A B O R A T O R Y  NONMAGNETIC CRYSTAL-ELECTRIC-FIELD GROUND STATE IN THE HEAVY-FERMION COMPOUND PRINAG2 T.M. Kelly, UC Riverside, LANSCE-12 W.P. Beyermann, UC Riverside R.A. Robinson, LANSCE-12 H. Nakotte, LANSCE-12, NMSU P.C. Canfield, Iowa State Unv., Ames Lab F.Trouw, Argonne Natl. Lab International Conference on Strongly Correlated Electron Systems Paris France, 15-18 July 1998 also published in Physica B MASTER Los Alamos National Laboratory, an afirmative actionlequal opportunity employer, is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free lcense to publish or reproduce the published form of this contribution, or to allow others to do so, for U.S. Government purposes. The Los Alamos National Laboratory requests that the publisher identify this article as work perlormed under the auspices of the U.S. Department of Energy. The Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Form No. 838 R5 sr2629 lrnl DISCLAIMER c DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. Version Date: June 18, 1998 Nonmagnetic Crystal-Electric-Field Ground State in the Heavy-Fermion Compound PrInAg2 T. M. KelleyaVb, W. P. Beyermanna, R. A. Robinsonb, H. NakottebVc, P. C. Canfieldd, and F. Trouwe Department of Physics, University of California, Riverside, CA, USA Lujan Center, Los Alamos National Laboratory, Los Alamos, NM, USA Physics Department, New Mexico State University, Las Cruces, NM, USA Iowa State University and Ames Laboratory, IA, USA Intense Pulsed Neutron Source, Argonne National Laboratory, IL, USA a C e Abstract: We have performed inelastic neutron scattering measurements that confirm that the crystal-electric-field split ground state in the heavy-fermion compound PrInAg2 is a nonmagnetic, non-Kramers doublet. This implies that a quadrupolar Kondo interaction is responsible for the enhanced thermodynamic properties observed at low temperatures. We also observe anomalous broadening of the inelastic peaks and suggest two possible causes for this broadening. Keywords: quadrupolar Kondo interaction, PrInAgz, inelastic neutron scattering, crystal electric field levels. Corresponding author: Prof. W. P. Beyermann Department of Physics University of California Riverside, CA 92521 USA E-mail: ward.beyermann@ucr.edu Tel: (909) 787-3947 Fax: (909) 787-4529 1 The compound PrInAg2 has been found to have a paramagnetic ground state with enhanced thermodynamic and transport properties at low temperatures [ 11. Such renormalized behavior results when the conduction electrons interact with localized electrons having some internal degree of freedom. Usually this is a magnetic degree of freedom (as in the conventional Kondo interaction); however, previous experiments have indicated that the Pr3+ 4f ground state in the presence of a cubic crystal-electric-field (CEF) is a nonmagnetic, non-Krg-ners doublet. If this is correct, then the renormalized behavior at low temperatures could be due to a quadrupolar Kondo interaction [2], in which the internal degree of freedom is the quadrupolar moment of the 4f electron. The correct CEF assignment is crucial in establishing the existence of this novel interaction. Furthermore, discrepancies exist between the predictions of the quadrupolar Kondo model and the measured thermodynamic and transport properties of PrInAg2. Presently, the calculations are based on a single impurity model, so the contradiction may arise from the periodicity of this compound. In addition to determining the CEF level scheme, we want to characterize the dynamic magnetic response of this new type of ground state which is unlike that observed in other correlated-electron compounds. In this paper, results from inelastic neutron. scattering experiments will be presented which f d y  establish a nonmagnetic ground state in PrInAg2. We also observe that the inelastic peaks are anomalously broadened and suggest two possible explanations for this. . Inelastic neutron scattering experiments were performed on the PHAROS chopper spectrometer at the Manuel Lujan, Jr. Neutron Scattering Center at Los Alamos National Laboratory. The sample consisted of 58 g of crushed, polycrystalline PrInAg2; the preparation and characterization of this sample was described elsewhere [l]. The sample was maunted on a three-stage displex refrigerator, and data were collected at two temperatures, 6.6 K and 77 K. At both temperatures, an incident neutron energy of 25.3 meV was used. The spectrometer resolution was measured with a vanadium plate where the elastic peak was very nearly gaussian with M / E i  - 4% (FWHM). Data from the planar array of linear, position- sensitive detectors were summed over the available Q-range (0.07 to 0.47 A-1 at the elastic line) and 2 histogrammed into constant energy bins. Since the transition matrix elements for some CEF level combinations are zero, a measurement was performed with the sample at 77 K where transitions between thermally populated states would be possible. After subtracting the data at 6.6 K, this result is shown in Fig. lb, and two new peaks emerge at approximately 2 meV and 9 meV in the high temperature data. Because the characteristic temperature of this system is so low (TK - 1 K),  higher-resolution data were collected at 4.4 K with the QENS spectrometer at the Intense Pulsed Neutron Source, Argonne National Laboratory. An inverse-geometry spectrometer, QENS uses a fixed final energy of E'= 3.65 meV, and it has a resolution of -0.150 meV at the elastic line. The same sample was used, this time mounted in a helium-4 cryostat. Data from each of the three detector banks on QENS were summed over the available range of Q and histogrammed into constant-energy bins. Only data from the high-angle bank (Q = 2.2 to 2.5 A-' at the elastic line) are presented here in Fig. 2 because sample absorption strongly attenuated the signal in the lower angle banks. Unfortunately, spurious scattering form the cryostat is more problematic at high scattering angles, and the features from this effect are indicated with an arrow between -0.15 and 0.35 meV in Fig. 2. PrInAg2 forms a fcc lattice with a = 7.075 A. The CEF manifold of a J = 4 ion at a cubic symmetry site has four states: a rl singlet, r4 triplet, Ts triplet, and a r3 nonmagnetic, non- Kramers doublet. Lea, Leask, and Wolf [3] determined the energies for these levels as a function of x, the ratio of the fourth to sixth order terms in the CEF Hamiltonian, and W, a parameter which sets the overall energy scale. When neutrons induce transitions between the various states in the manifold, the intensities of the resulting inelastic peaks are determined by matrix elements calculated by Birgeneau [4]. The CEF spectrum can be completely determined from the PHAROS data. Fitting the two inelastic peaks in the 6.6 K data.with gaussians (with widths fixed to the instrument resolution) gives a ratio zs.d.8.3 = 2.13 +- 0.03. From [4], the ratio of the intensity for the r3 + r4 transition to that of the r3 + T5 as 2.33, which is closer to the measured value than other possible 3 combinations. With l-4 or r5 as the ground state, the ratio is 2.67 or 1.14, respectively. If rl were the ground state, then only one inelastic peak would be observed because transitions to this level are only allowed from r4. Therefore r3 is assigned as the ground state, r4 as the first excited level at 5.9 meV (68 K), and T5 at 8.3 meV (95 K). At 77 K, the r4 level is'thermally populated, allowing us to observe additional transitions originating from this level. It follows then that the two new transitions observed at higher temperature correspond to r4 + r5 (2 meV) and r4 + rl (9 meV), which puts the rl level at 16 meV above the ground state. This level scheme is shown in the inset to Fig. la. The energies of the r4 and r5 levels are very close to values observed by Galera et al. [5]. The present level arrangement is also similar to that estimated from the CEF contribution to the specific heat, except that from the specific heat method the energy of the rl level was estimated to be at 20 meV[ 11. , In the QENS data, the elastic peak is resolution limited, but the 5.9 meV peak is significantly broader than the instrument resolution. The inelastic peak at 8.3 meV may also be broadened. We suggest two possible explanations for this broadening. The crystal field symmetry may be lower than cubic, thus splitting the r4 and T5 levels. Alternatively, spin fluctuations involving a conventional, magnetic Kondo interaction between the excited magnetic CEF states and the conduction electrons may lead to a finite lifetime for this state. We are proceeding with further analysis to determine which scenario is correct. This work was supported by the National Science Foundation with grant no. DMR-9624778. Work at Los Alamos National Laboratory, Argonne National Laboratory, and Ames Laboratory was performed under the auspices of the Department of Energy, Office of Basic Energy Research. References [ 11 A. Yatskar et al., Phys. Rev. Lett. 77 (1996) 3637. [2] D. L. Cox, Phys. Rev. Lett. 59 (1987) 1240. [3] K. R. Lea, M. J. M. Leask, and W. P. Wolf, J. Phys. Chem. Solids 23 (1962) 1381. [4] R. J. Birgeneau, J. Phys. Chem. Solids 33 (1972) 59. [5] R. M. Galera et al., J. Less-Common Met. 97 (1984) 151. 4 Figure Captions Fig. 1. Scattering intensity versus neutron energy loss from PHAROS for PrInAg2. (a) The sample temperature is 6.6 K. Inset shows the CEF level scheme determined form these measurements. (b) The sample temperature is 77 K. Note the left axis is expanded by a factor of 8. Fig. 2. Scattering intensity versus neutron energy loss from QENS. This high-resolution data was taken on PrInAg2 at T = 4.4 K. The arrow points to spurious scattering from the cryostat. 5 8000 n % 6000 8 4000 L 0 800 6 0.0 400 20a a a a a a a r34-4 € I i I r l  --1 0 -5 0 5 10 Energy Transfer (mev) 15 Fig. 1; Kelley et a/.; SCES98 0.3 0.0 1 1 I I I I - . . PrlnAg, * t ?d a .  : *  0 .  0 .  I 0 2 4 . 6  8 Energy Transfer (mev) 10 12 Fig. 2; KeIley et a/.; SCES98 

Nonmagnetic crystal-electric-field ground state in the heavy-fermion compound PrInAg{sub 2}

https://digital.library.unt.edu/ark:/67531/metadc675392/m2/1/high_res_d/334328.pdf

Nonmagnetic crystal-electric-field ground state in the heavy-fermion compound PrInAg{sub 2}

Abstract

Similar works

Full text

Available Versions

UNT (University of North Texas) Digital Library