Showing papers by "Silke Paschen published in 2012"

Destruction of the Kondo effect in the cubic heavy-fermion compound Ce3Pd20Si6

Abstract: How ground states of quantum matter transform between one another reveals deep insights into the mechanisms stabilizing them. Correspondingly, quantum phase transitions are explored in numerous materials classes, with heavy-fermion compounds being among the most prominent ones. Recent studies in an anisotropic heavy-fermion compound have shown that different types of transitions are induced by variations of chemical or external pressure, raising the question of the extent to which heavy-fermion quantum criticality is universal. To make progress, it is essential to broaden both the materials basis and the microscopic parameter variety. Here, we identify a cubic heavy-fermion material as exhibiting a field-induced quantum phase transition, and show how the material can be used to explore one extreme of the dimensionality axis. The transition between two different ordered phases is accompanied by an abrupt change of Fermi surface, reminiscent of what happens across the field-induced antiferromagnetic to paramagnetic transition in the anisotropic YbRh2Si2. This finding leads to a materials-based global phase diagram--a precondition for a unified theoretical description.

Destruction of Kondo effect in cubic heavy fermion compound Ce3Pd20Si6

Abstract: How ground states of quantum matter transform between one another reveals deep insights into the mechanisms stabilizing them. Correspondingly, quantum phase transitions are explored in numerous materials classes, with heavy fermion compounds being among the most prominent ones. Recent studies in an anisotropic heavy fermion compound have shown that different types of transitions are induced by variations of chemical or external pressure [1-3], raising the question of the extent to which heavy fermion quantum criticality is universal. To make progress, it is essential to broaden both the materials basis and the microscopic parameter variety. Here, we identify a cubic heavy fermion material as exhibiting a field-induced quantum phase transition, and show how the material can be used to explore one extreme of the dimensionality axis. The transition between two different ordered phases is accompanied by an abrupt change of Fermi surface, reminiscent of what happens across the field-induced antiferromagnetic to paramagnetic transition in the anisotropic YbRh2Si2. This finding leads to a materials-based global phase diagram -- a precondition for a unified theoretical description.

Phononic filter effect of rattling phonons in the thermoelectric clathrate Ba8Ge40+xNi6-x.

Abstract: One of the key requirements for good thermoelectric materials is a low lattice thermal conductivity. Here we present a combined neutron scattering and theoretical investigation of the lattice dynamics in the type I clathrate system Ba-Ge-Ni, which fulfills this requirement. We observe a strong hybridization between phonons of the Ba guest atoms and acoustic phonons of the Ge-Ni host structure over a wide region of the Brillouin zone, which is in contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host cages. It occurs without a strong decrease of the acoustic phonon lifetime, which contradicts the usual assumption of strong anharmonic phonon--phonon scattering processes. Within the framework of ab initio density-functional theory calculations we interpret these hybridizations as a series of anticrossings which act as a low-pass filter, preventing the propagation of acoustic phonons. To highlight the effect of such a phononic low-pass filter on the thermal transport, we compute the contribution of acoustic phonons to the thermal conductivity of Ba${}_{8}$Ge${}_{40}$Ni${}_{6}$ and compare it to those of pure Ge and a Ge${}_{46}$ empty-cage model system.

Thermoelectric properties of Ba-Cu-Si clathrates

Abstract: In the search for cheaper representatives of thermoelectric clathrates, we have investigated the type-I clathrates Ba${}_{8}$Cu${}_{x}$Si${}_{46\ensuremath{-}x}$ ($3.6\ensuremath{\le}x\ensuremath{\le}7$, $x$ $=$ nominal Cu content) in a combined experimental and theoretical study. The polycrystalline samples are prepared either by melting, ball milling, and hot pressing or by melt spinning, hand milling, and hot pressing techniques. Temperature-dependent electrical resistivity $\ensuremath{\rho}(T)$ and Seebeck coefficient $S(T)$ measurements between 300 and 800 K reveal the behavior of a degenerate semiconductor for all samples. For $x=5$ and 6, density functional theory calculations are performed to derive the enthalpy of formation and the electronic structure. From the latter, we calculate the electrical resistivity and the Seebeck coefficient within Boltzmann's transport theory. For simulating the properties of doped clathrates, the rigid-band model is applied. On the basis of the density functional theory results, the experimentally observed compositional dependence of $\ensuremath{\rho}(T)$ and $S(T)$ is analyzed. The highest dimensionless thermoelectric figure of merit $ZT$ of 0.28 is reached for a melt-spun sample at $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C. The relatively low $ZT$ values in this system are attributed to the too-high charge-carrier concentrations.

Anisotropic electrical resistivity of the Kondo insulator CeRu4Sn6

Abstract: The intermetallic compound CeRu4Sn6 has tentatively been classified as anisotropic Kondo insulator. Here we present electrical resistivity measurements on CeRu4Sn6 single crystals, both along the main directions a and c of the tetragonal crystal structure, and v along the diagonal c' of the a-a plane. This direction was selected because c' = differs from c by only 0.2 %, suggesting that the coumpound might alternatively be regarded as quasi-cubic. Amazingly, strong anisotropy is observed not only between a and c but also between c and c'. We analyse the temperature dependent resistivities both with a simple semiconductor model and within a Kondo insulator picture.

Crystal Growth and Stoichiometry of Strongly Correlated Intermetallic Cerium Compounds

Abstract: Strongly correlated electron systems are among the most active research topics in modern condensed matter physics. In strongly correlated materials the electron interaction energies dominate the electron kinetic energy which leads to unconventional properties. Heavy fermion compounds form one of the classes of such materials. In heavy fermion compounds the interaction of itinerant electrons with local magnetic moments generates quasiparticles with masses up to several 1000 electron masses. This may be accompanied by exciting properties, such as unconventional superconductivity in a magnetic environment, non-Fermi liquid behavior and quantum criticality. Strong electronic correlations are responsible for physical phenomena on a low energy scale. Consequently, these phenomena have to be studied at low temperatures. This, in turn, requires ultimate quality of single crystals to avoid that the low temperature intrinsic properties are covered by extrinsic effects due to off-stoichiometry, impurities or other crystal imperfections. The overwhelming majority of heavy fermion systems are cerium and ytterbium intermetallic compounds. In the present paper we discuss the crystal growth of three cerium compounds, Ce3Pd20Si6, CeRu4Sn6 and CeAuGe. Ce3Pd20Si6 undergoes an antiferromagnetic phase transition at low temperatures and shows a magnetic field induced quantum critical point [Takeda et al (1995), Strydom et al (2006)]. CeRu4Sn6 [Das & Sampathkumaran (1992)] appears to be a Kondo insulator [Aepli & Fisk (1992)] with a highly anisotropic energy gap. CeAuGe is one of a the few cerium compounds showing a ferromagnetic phase transition at low temperatures [Pottgen et al (1998), Mhlungu & Strydom (2008)]. This is of special interest in the context of quantum criticality, since the occurrence of quantum criticality on the verge of a ferromagnetic ground state is a subject of current debate. Much attention in this paper is paid to the problem of stoichiometry. Single crystals of intermetallic compounds are grown at high temperatures, which facilitates the formation of thermal defects realized often as deviation from the stoichiometric composition. Thermal instabilities of some intermetallic phases require the use of flux techniques, i.e., growth from off-stoichiometric melts, which is another source of non-stoichiometry. Sizeable non-stoichiometries can be detected by measuring the composition by chemical and physical analytical techniques, e.g. energy dispersive X-ray spectroscopy analysis (EDX). Tiny deviations from the stoichiometry, on the other hand, can be found only froman analysis of the physical properties of single crystals. Thus physical property measurements are not only the 11

Introducing a Magnetic Guest to a Tetrel-Free Clathrate: Synthesis, Structure, and Properties of EuxBa8-xCu16P30 (0 ≤ x ≤ 1.5).

Abstract: The title compounds are prepared by solid state reactions of the elements (1125 K, 360 h) and characterized by powder and single crystal XRD, XAS, electrical resistivity, specific heat, and magnetic susceptibility measurements.

Melt‐spun Eu8Ga16–xGe30+x Clathrates

Abstract: Intermetallic clathrates are promising materials for thermoelectric applications This is not only due to their low thermal and high electrical conductivities (“phonon glass – electron crystal”) but also due to the expectation that they are semiconductors and thus have large thermopower values Band structure calculations of the stoichiometric compound Eu8Ga16Ge30 indeed yield a semiconducting ground state However, with conventional synthesis methods the exact 8:16:30 stoichiometry could not be reached Herein we use the melt-spinning technique to obtain Eu8Ga16–xGe30+x samples with smaller x than previously realized The quenching procedure and the results of the characterization of the quenched phases by X-ray powder diffraction and electron microscopy are presented The electrical resistivity shows that, in contrast to conventional synthesis procedures, semiconducting compounds can be produced

Structural and Thermoelectric Properties of Ba 8 Cu x Si 23- x Ge 23 (4.5 ≤ x ≤ 7)

Abstract: Intermetallic clathrates are promising materials for thermoelectric applications at elevated temperatures. In the search for cost-competitive representatives, we investigate the thermoelectric properties of quaternary type I clathrates with nominal compositions Ba8CuxSi23-xGe23 (4.5 ≤ x ≤ 7). The specimens were prepared by melting and annealing with subsequent ball milling and hot pressing. X-ray diffraction and energy-dispersive x-ray spectroscopy were used for an accurate structure determination and for an analysis of the phase purity and microstructure, respectively. The transport properties show a systematic variation with the Cu content in the clathrate phase. The highest dimensionless thermoelectric figure of merit ZT of 0.31 is achieved for Ba8Cu6Si17Ge23 at about 250°C (520 K).

Quantum Oscillations in Ultra Pure PtSn4

Abstract: PtSn4 is a non-congruently melting compound in the Pt-Sn binary phase diagram, crystallizing in the centrosymmetric space group Ccca We report on single crystal growth, temperature dependent electrical resistivity, isothermal field-dependent magnetization and magnetostriction, and on band structure calculations of this transition metal compound The high quality of the sample makes it possible to observe clearly resolved quantum oscillations in the magnetization data for temperatures as high as 20 K and in magnetic fields as low as 10 kOe We found several frequencies along the three crystallographic directions and could relate them to several extremal orbits on the Fermi surface

The Vienna Nuclear Demagnetization Refrigerator

Abstract: A new nuclear demagnetization system coupled to a powerful dilution refrigerator and a vector magnet was successfully built and operated. Our aim was to construct a versatile, modular cryostat, with a large experimental space providing an excellent platform for various types of ultralow temperature measurements. A powerful dilution unit allows us to cool the mixing chamber down to 3 mK and to precool a massive copper (~90 mol) nuclear stage in a field of 9 T to 8 mK in 100 h. After demagnetization the lowest temperature of the copper stage measured by a Pt thermometer was 50.9 μK in a field of 20 mT. The cryostat is integrated with a 8 T-4 T vector magnet system. The refrigerator is provided with a 50 mm central clear shot tube allowing the insertion of a top-loading probe to cool down samples for measurements inside the vector magnet bore in a reasonably short time of about 4 hours. The system will be used to study quantum critical behavior of heavy fermion compounds.

Structural, Magnetic and Transport Properties of the Rare-Earth Cage Compound Ce4Pd12Sn25

Abstract: Here we present and discuss structural, magnetic and transport properties of the new ternary cage compound Ce4Pd12Sn25. The isostructural Pt analogue, Ce4Pt12Sn25 has previously been characterized as a metallic Kondo lattice system with a very low Kondo temperature TK ~ 0.25 K and an antiferromagnetic ground state with a Neel temperature TN =0.19 K. In contrast to the expectations that the smaller unit cell volume of Ce4Pd12Sn25 compared to Ce4Pt12Sn25 will weaken the magnetism and tune the system closer to a magnetic instability, we found evidence for a phase transition at a higher temperature Tmag=0.265 K.

High pressure phase diagram of CeCoGe2.2Si0.8

Abstract: We have investigated the temperature-pressure phase diagram of the heavy fermion compound CeCoGe2.2Si0.8 by DC magnetic susceptibility measurements, ?DC(T), under high pressure. The N?el temperature of TN = 4 K in zero pressure is reduced by pressure up to 3 kbar. At higher pressures antiferromagnetic order appears to gradually transform into a spin glass like-state. Magnetic field decreases both TN and the spin glass freezing temperature Tf. At 3 T and 6.5 kbar a divergence of ?DC(T) is observed with a power law that is consistent with a disorder-dominated quantum criticality.

Low-temperature electrical resistivity of antiferromagnetic Ce4Pt12Sn25

Abstract: The compound Ce4Pt12Sn25 has a rather complex cubic crystal structure in which the Ce atoms are encapsulated in cages of Pt and Sn resulting in a large Ce-Ce distance. Recently, specific heat and magnetic susceptibility measurements revealed a phase transition from a paramagnetic to an antiferromagnetic (AFM) ground state with a low Neel temperature of TN = 0.19K. Application of a magnetic field was shown to weaken the AFM phase and ultimately suppress it at a critical field of 0.7 T. Here we investigate the observed antiferromagnetic phase transition by electrical resistivity measurements in applied magnetic fields in the vicinity of this field-tuned quantum critical point.

Inelastic neutron scattering study of Ni-substituted Ce0.5Fe4Sb12 skutterudite compounds

Abstract: An inelastic neutron scattering study of the filled and partially-filled skutterudite compounds RFe4Sb12 and R0.5Fe2.75Ni1.25Sb12 (where R = Ce and La) was carried out to understand the nature of the spin dynamics. Strong magnetic scattering was observed in Ce0.5Fe2.75Ni1.25Sb12 at ~ 5 meV. The integrated intensity of this peak does not follow the Ce3+ form factor, but exhibits a maximum at a momentum transfer (|Q|) of 2 A−1. We attribute this feature to a Ce3+ crystal field excitation in the presence of magnetic exchange interactions. This picture is supported by thermodynamic and magnetic properties. Finally, we confirm the presence of a spin gap in CeFe4Sb12 suggested by our previous work.