Publications (34) View all
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Article: Six closely related YbT2Zn20 (T = Fe, Co, Ru, Rh, Os, Ir) heavy fermion compounds with large local moment degeneracy.
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ABSTRACT: Heavy fermion compounds represent one of the most strongly correlated forms of electronic matter and give rise to low temperature states that range from small moment ordering to exotic superconductivity, both of which are often in close proximity to quantum critical points. These strong electronic correlations are associated with the transfer of entropy from the local moment degrees of freedom to the conduction electrons, and, as such, are intimately related to the low temperature degeneracy of the (originally) moment bearing ion. Here we report the discovery of six closely related Yb-based heavy fermion compounds, YbT(2)Zn(20), that are members of the larger family of dilute rare earth bearing compounds: RT(2)Zn(20) (T = Fe, Co, Ru, Rh, Os, Ir). This discovery doubles the total number of Yb-based heavy fermion materials. Given these compounds' dilute nature, systematic changes in T only weakly perturb the Yb site and allow for insight into the effects of degeneracy on the thermodynamic and transport properties of these model correlated electron systems.Proceedings of the National Academy of Sciences 07/2007; 104(24):9960-3. · 9.68 Impact Factor -
Article: Temperature-induced sign change of the exchange bias in Fe0.82Zn0.18F2/Co bilayers
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ABSTRACT: A single crystal, (110)-oriented dilute antiferromagnet (AF) Fe <sub>0.82</sub> Zn <sub>0.18</sub> F <sub>2</sub> film was grown via molecular beam epitaxy on a (110)- MgF <sub>2</sub> substrate by codepositing FeF <sub>2</sub> and ZnF <sub>2</sub>, followed by 1.0 nm pure FeF <sub>2</sub> and 18 nm Co layers. The exchange bias (H<sub>E</sub>) and coercivity (H<sub>C</sub>) of the Co film strongly depend on the cooling field (H<sub> CF </sub>) and temperature. For 0≤H<sub> CF </sub>≪2 kOe, H<sub>E</sub>≪0 in the whole temperature range before reaching zero at the blocking temperature (T<sub>B</sub>). For H<sub> CF </sub>≥15 kOe, H<sub>E</sub>≫0 for T≪T<sub>B</sub>. In both cases, H<sub>C</sub> peaks when T∼T<sub>B</sub>. For 2 kOe≤ H<sub> CF </sub>≪15 kOe, H<sub>E</sub>≪0 at low temperatures, and then suddenly becomes positive at a characteristic switching temperature T<sub>S</sub> (≪T<sub>B</sub>). T<sub>S</sub> decreases as H<sub> CF </sub> is increased and H<sub>C</sub> peaks at both T∼T<sub>S</sub> and T∼T<sub>B</sub>. These results indicate that for intermediate H<sub> CF </sub> unstable domains are created in the AF during the cooling procedure. The domains suddenly disappear as enough thermal energy is a- vailable to switch them when perturbed by the rotation of the ferromagnetic magnetization. © 2003 American Institute of Physics.Journal of Applied Physics 06/2003; · 2.17 Impact Factor -
Article: Commercial apparatus for measuring thermal transport properties from 1.9 to 390 kelvin
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ABSTRACT: We have developed an instrument which simultaneously measures the thermal conductivity κ, Seebeck coefficient α, and electrical resistivity ρ of a sample, thereby determining the thermoelectric figure of merit Z = α ^2 / ρ κ. A Quantum Design Physical Property Measurement System (PPMS) provides the temperature control from 1.9 - 390 K and applied magnetic fields of up to 14 tesla. Two small probes mounted along the sample each contain thin film Cernox chip thermometers as well as electrical contacts to monitor the temperature and voltage drops across the sample. A third probe attached to the end of the sample provides a heater and current source to stimulate the sample both thermally and electrically. The sample's response to cyclic heat pulses is analyzed in real time using DSP techniques. A nonlinear least-squares fit is used, employing a two time-constant model to determine both the thermal conductivity and the Seebeck coefficient for the material. Data acquisition using these ac techniques are expedited since we can sweep both temperature and field during a measurement. Adaptive algorithms continually adjust the thermal and electrical stimuli applied to the sample, optimizing the speed and accuracy of the measurement. We present results from some of the materials measured so far, including Pb in the superconducting state.02/2001; -1:9008. -
Article: Commercial Apparatus for Measuring Thermal Transport Properties from 1.9 to 390 Kelvin
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ABSTRACT: We have developed an instrument which simultaneously measures the thermal conductivity κ, Seebeck coefficient α, and electrical resistivity ρ of a sample, thereby determining the thermoelectric figure of merit Z = α2/(κρ). A Quantum Design Physical Property Measurement System (PPMS) provides the temperature control from 1.9 - 390 K and applied magnetic fields of up to 14 tesla. Two small probes mounted along the sample each contain thin film Cernox chip thermometers as well as electrical contacts to monitor the temperature and voltage drops across the sample. A third probe attached to the end of the sample provides a heater and current source to stimulate the sample both thermally and electrically. The sample's response to cyclic heat pulses is analyzed in real time using DSP techniques. A nonlinear least-squares fit is used, employing a two time-constant model to determine both the thermal conductivity and the Seebeck coefficient for the material. Data acquisition using these ac techniques are expedited since we can sweep both temperature and field during a measurement. Adaptive algorithms continually adjust the thermal and electrical stimuli applied to the sample, optimizing the speed and accuracy of the measurement. We present results from some of the materials measured so far, including thermal conductivity standards and Pb in the superconducting state.MRS Proceedings. 12/2000; 691. -
Article: Imaging high-frequency magnetic and electric fields using a high-T c SQUID microscope
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ABSTRACT: We have used a liquid-nitrogen-cooled scanning SQUID microscope to image room-temperature sources of high-frequency magnetic fields and electric fields. We detect the fields by monitoring their effect on the SQUID modulation depth; high-frequency magnetic fields affect the modulation depth differently than high-frequency electric fields. We briefly describe our system, explain the principle behind detection of magnetic and electric fields, and show images of some simple room-temperature samples in the 0.8-13 GHz frequency rangeIEEE Transactions on Appiled Superconductivity 07/1999; · 1.04 Impact Factor
