# Presentation: 185634

## Intellectual Merit

- Strongly Correlated Electron Behavior in
- PrT4As12 (T = Fe, Ru, Os) Single Crystals
***M. Brian Maple, University of California, San Diego — DMR 0335173***

- The Pr-based filled skutterudite compounds with the formula PrT4X12 (where T = Fe, Ru, Os; X = P, As, Sb) exhibit a number of interesting phenomena. Pioneering research in our laboratory has made substantial contributions to further the understanding of these systems. Recent measurements of the Pr-based  filled skutterudite antimonides, have confirmed BCS superconductivity in PrRu4As12 and discovered antiferromagnetic order and heavy fermion behavior in PrOs4As12 as well as ferromagnetic ordering in PrFe4As12.

- The above figure displays the H-T phase diagram for PrOs4As12, indicating two phase transitions in zero field, which are suppressed to 0 K by ~3 T. Neutron diffraction measurements confirmed antiferromagnetic ordering (AFM) for the low field phase.  Further neutron diffraction measurements in high magnetic field will be performed to determine the nature of the second ordered phase (OP2).

**Paramagnetic**

**Notes:**
The lanthanide-based filled skutterudite compounds with the chemical formula RT4X12 (R = lanthanide; T = Fe, Os, Ru; X = P, As, Sb) have properties that are of interest for basic and applied research.  These compounds exhibit a wide variety of strongly correlated electron behavior at low temperatures (~-460F), while a number of them have properties that render them promising candidates for thermoelectric applications at high temperatures (~1100F).  Strong interactions between conduction electrons and f-electrons of the lanthanide ions produce a variety of interesting strongly correlated electron behavior such as magnetic ordering, unconventional superconductivity, and heavy fermion behavior.  At high temperatures, these materials have a high Seebeck coefficient, and are poor conductors of heat while remaining good electrical conductors, making them useful for thermoelectric applications such as producing electricity from the heat generated by motor vehicle exhaust.  Our research efforts on the filled skutterudites are focused on investigating the physical properties of these materials to better understand the fundamental physics associated with the strongly correlated electron behavior they exhibit.  

Most recently, our research effort has focused on the filled skutterudite arsenides, which, unlike the phosphides and antimonides, have not been studied in great detail.  Electrical resistivity, magnetization, and specific heat measurements on PrOs4As12 reveal the existence of two ordered phases at temperatures below ~ 2.3 K (-454F).  In an applied magnetic field, these phase transitions are suppressed to 0 K in fields of 1.75 T and 3.25 T, respectively.  Neutron diffraction measurements have confirmed that the low field ordered phase is antiferromagnetic.  In an antiferromagnet (AFM), the magnetic moments of neighboring ions align antiparallel, resulting in a material with no net magnetic moment.  Additional neutron diffraction measurements in high magnetic fields are planned in order to determine the nature of the second ordered phase (OP2).   Along with these two ordered phases, PrOs4As12 also exhibits heavy fermion behavior where strong electronic correlations lead to an enhancement of the electron (“fermion”) mass at low temperatures.

Measurements of the filled skutterudite compound PrRu4As12 have confirmed the occurrence of conventional BCS superconductivity below a transition temperature Tc ~ 2.4 K.  A material that is superconducting has the remarkable property of conducting electric current without any energy loss (zero resistance).  The term BCS refers to a class of superconductors whose behavior conforms to the BCS theory of superconductivity developed by Bardeen, Cooper and Schrieffer in 1957.  In a BCS superconductor, the electrons form pairs (Cooper pairs) in which an electron with momentum k and spin up () is paired with an electron with momentum -k and spin down (), i.e., in the state (k,-k).  The attractive interaction between electrons that produces the pairing is mediated by phonons (quantized vibrations of the crystal lattice).  Magnetization measurements on the filled skutterudite compound PrFe4As12 have shown that this compound exhibits ferromagnetic ordering below 18 K.  In a ferromagnet, the magnetic moments of the neighboring ions align parallel, resulting in a material with a net magnetic moment.  These measurements illustrate the sensitivity of the ground state of the Pr-based filled skutterudite compounds to the transition metal and pnictogen atoms out of which each compound is composed.  The Pr-based filled skutterudites provide a unique opportunity to study a wide range of strongly correlated electron phenomena and will continue to be an intense area of research for some time to come.

## Strongly Correlated Electron Behavior in 

- Del Mar Pines Elementary School 2005

- Toler Elementary School 2006

**Societal Impact and Community Outreach:**
- We are proud to participate in science day events which promote interest in the sciences in young students and parents. While these events are not directly related to our research, we believe that the development of science awareness in students, through dynamic hands-on experiments and demonstrations, is essential to their continued excitement and interest in science. For this reason we are annually involved in the Del Mar Pines Elementary School Science Day and also recently began to participate in Science Night at Toler Elementary School.

- Strongly Correlated Electron Behavior in
- PrT4As12 (T = Fe, Ru, Os) Single Crystals
***M. Brian Maple, University of California, San Diego — DMR 0335173***