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"Michor, Herwig"
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Anderson transition in stoichiometric Fe2VAl: high thermoelectric performance from impurity bands
Discovered more than 200 years ago in 1821, thermoelectricity is nowadays of global interest as it enables direct interconversion of thermal and electrical energy via the Seebeck/Peltier effect. In their seminal work, Mahan and Sofo mathematically derived the conditions for ’the best thermoelectric’—a delta-distribution-shaped electronic transport function, where charge carriers contribute to transport only in an infinitely narrow energy interval. So far, however, only approximations to this concept were expected to exist in nature. Here, we propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. An innovative approach of continuous disorder tuning allows us to drive the Anderson transition within a single sample: variable amounts of antisite defects are introduced in a controlled fashion by thermal quenching from high temperatures. Consequently, we obtain a significant enhancement and dramatic change of the thermoelectric properties from
p
-type to
n
-type in stoichiometric Fe
2
VAl, which we assign to a narrow region of delocalised electrons in the energy spectrum near the Fermi energy. Based on our electronic transport and magnetisation experiments, supported by Monte-Carlo and density functional theory calculations, we present a novel strategy to enhance the performance of thermoelectric materials.
The mathematical conditions for the best thermoelectric is well known but never realised in real materials. Here, the authors propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario.
Journal Article
Revisiting the Potential Functionality of the MagR Protein
Recent findings have sparked great interest in the putative magnetic receptor protein MagR. However, in vivo experiments have revealed no magnetic moment of MagR at room temperature. Nevertheless, the interaction of MagR and MagR fusion proteins with silica-coated magnetite beads have proven useful for protein purification. In this study, we recombinantly produced two different MagR proteins in Escherichia coli BL21(DE3) to (1) expand earlier protein purification studies, (2) test if MagR can magnetize whole E. coli cells once it is expressed to a high cytosolic, soluble titer, and (3) investigate the MagR-expressing E. coli cells’ magnetic properties at low temperatures. Our results show that MagR induces no measurable, permanent magnetic moment in cells at low temperatures, indicating no usability for cell magnetization. Furthermore, we show the limited usability for magnetic bead-based protein purification, thus closing the current knowledge gap between theoretical considerations and empirical data on the MagR protein.
Journal Article
Porous Silicon Nanocomposites with Combined Hard and Soft Magnetic Properties
Magnetic nanostructures of two ferromagnetic metals have been combined within porous silicon, and the magnetic switching behavior of the resulting porous silicon/metal nanocomposite has been modified by varying the arrangement. The two magnetic materials are Ni and Co, whereas Co is the magnetic harder one. These “hard/soft” magnetic nanocomposites have been achieved by two different routes. On the one hand, double-sided porous silicon has been used whereas one side has been filled with Ni nanostructures and the other one with Co nanostructures. On the other hand, Ni and Co have been deposited within one porous layer alternatingly. The filling of the pores has been carried out by electrodeposition with varying the deposition parameters. In systems which offer two distinct slopes of the hysteresis curves due to the different saturation behavior of the two types of deposited metal, magnetic exchange coupling is not present. For samples which show smooth hysteresis curves exchange, coupling between the Ni and Co nanostructures seems to be present. The aim is to control especially the structure size of the soft and the hard magnetic materials and the distance between them at the nanoscale to optimize exchange coupling resulting in a maximum energy product.
Journal Article
Crystal Chemistry of Ternary Rare Earth Transition Metal Carbides: Studies of the Tb-Fe-C System at 800°C
The isothermal section of the phase diagram of Tb–Fe–C system at 800 °C was studied in the full concentration range using powder X-ray phase and structure analyses, and energy-dispersive X-ray spectroscopy. Six ternary compounds Tb1.88Fe14C, Tb13Fe10C13, TbFeC2, Tb15Fe8C25, Tb5.64Fe2C9, Tb2FeC4 and a limited solid solubility of carbon in the crystal structure of the binary parent compound Tb2Fe17Cх (0≤ х ≤0.8) have been found to exist at 800 °C. The crystal structures of two new ternary carbides have been determined by means of powder X-ray diffraction: Tb15Fe8C25 with structure type Er15Fe8C25, space group P321, a = 11.9706(3) Å, c = 5.1733(2) Å, RB(I) = 0.07, RP = 0.06, RPw = 0.08, and Tb13Fe10C13 with structure type Gd13Fe10C13, space group P3121, a = 9.1800(9) Å, c = 23.703(5) Å, RB(I) = 0.04, RP = 0.16. Both compounds are representatives of the carbometalate class of complex carbides. Tb15Fe8C25 displays an itinerant ferro-or ferrimagnetic ordering of the Fe 3d-moments below TM ≈ 50 K while Tb 4f-moments remain essentially paramagnetic at least down to about 10 K.
Journal Article
The Quasi-Binary System CeCoC2-CeNiC2: Crystal Structure and Physical Properties
The crystal structure of phases in the pseudo-binary system CeCo1–хNiхC2 (x = 0, 0.33, 0.5, 0.67, 0.79, 0.80, 0.83, 1) was investigated by means of X-ray powder diffraction. Co richer solid solutions CeCo1–хNiхC2 (0≤ x ≤0.5) crystallize in the monoclinic CeCoC2-type structure; a = 5.3968(2) Å, b = 5.4013(3) Å, c = 7.4762(3) Å, β = 102.136(3)°, V = 213.06(3) Å3 for x = 0.5. Ni-rich CeNi1–yCoyC2 (0≤ y ≤0.2) are isotypic with the orthorhombic CeNiC2-type structure, a = 3.8486(2) Å, b = 4.5479(2) Å, c = 6.1531(3) Å, V = 107.70(1) Å3 for y = 0.2. In the intermediate region (0.5< x <0.79) both phases, CeCo0.21Ni0.79C2 and CeCo0.5Ni0.5C2, coexist. The non-isoelectronic substitution of Ni by Co in solid solutions CeNi1–yCoyC2 causes a continuous reduction of the Néel temperature and finally, for CeCoC2, results in a paramagnetic Kondo-lattice ground state.
Journal Article
Magnetic Instabilities in Non-Fermi Liquid Ce3Pd4Si4 Driven by Magnetic Dilution
Materials which are close to a quantum critical point are rather prone to magnetic instabilities; even small modifications of relevant interaction mechanisms can be responsible for a magnetic phase transition to occur at finite temperatures. In this work it is demonstrated that actually a substitution of Ce by non-magnetic La in Ce3Pd4Si4 drives a magnetic instability, with ordering temperatures as high as 10 K. To trace the evolution of magnetism and the onset of long range magnetic order in Ce3-xLaxPd4Si4, various bulk property measurements were carried out.
Journal Article
Vacancy-induced pseudo-gap formation in antiferromagnetic Cr\\(_0.86\\)ZnSb
Structural defects are important for both solid-state chemistry and physics, as they can have a significant impact on chemical stability and physical properties. Here, we identify a vacancyinduced pseudo-gap formation in antiferromagnetic Cr\\(_0.86\\)ZnSb. Cr\\(_1-x\\)ZnSb alloys were studied combining efforts of density functional theory (DFT) calculations and experimental methods to elucidate the effect of vacancies. Detailed analyses (X-ray powder and single crystal diffraction, transmission and secondary scanning electron microscopy) of Cr\\(_1-x\\)ZnSb, \\(0
Paper
Competing charge density wave phases in YNiC2
Charge density wave (CDW) orders in YNiC2 are studied by means of combined experimental and computational techniques. On the experimental side, single crystals grown by the floating-zone method were examined by means of X-ray diffraction, as well as transport and thermal techniques. Density functional theory (DFT) calculations founded on the experimentally determined parent and CDW-modified crystal structures provide details of electronic and phononic structures as well as electron-phonon coupling and resolve changes inflicted upon entering the different CDW phases. Thereby, contrasting effects of subsequently emerging CDW states characterized by incommensurate q_1ic and commensurate q_2c modulation vectors are revealed. The former state, on-setting below T_1ic~ 305 K, weakly modifies the electronic structure by opening an almost isotropic gap on a minor part of the Fermi surface (FS). The latter phase, which takes over below T_2c~ 272 K has a more pronounced impact on physical properties via a decomposition of larger parts of the FS. These dissimilar behaviors are directly reflected in the electronic transport anisotropy, which is significantly weakened in the q_2c-type CDW state. As revealed by our DFT studies, CDW phases are very close in energy and their origin is directly related to the anisotropy of electron-phonon coupling, which is linked to a specific orbital character of related FS sheets. Specific heat and thermal expansion studies reveal a nearly reversible first-order phase transition at around T_2c~ 272 K, where both CDW phases co-exist within a T-interval of about 10 K.
Paper
Competing charge density wave phases in YNiC2
Charge density wave (CDW) orders in YNiC2 are studied by means of combined experimental and computational techniques. On the experimental side, single crystals grown by the floating-zone method were examined by means of X-ray diffraction, as well as transport and thermal techniques. Density functional theory (DFT) calculations founded on the experimentally determined parent and CDW-modified crystal structures provide details of electronic and phononic structures as well as electron-phonon coupling and resolve changes inflicted upon entering the different CDW phases. Thereby, contrasting effects of subsequently emerging CDW states characterized by incommensurate q_1ic and commensurate q_2c modulation vectors are revealed. The former state, on-setting below T_1ic~ 305 K, weakly modifies the electronic structure by opening an almost isotropic gap on a minor part of the Fermi surface (FS). The latter phase, which takes over below T_2c~ 272 K has a more pronounced impact on physical properties via a decomposition of larger parts of the FS. These dissimilar behaviors are directly reflected in the electronic transport anisotropy, which is significantly weakened in the q_2c-type CDW state. As revealed by our DFT studies, CDW phases are very close in energy and their origin is directly related to the anisotropy of electron-phonon coupling, which is linked to a specific orbital character of related FS sheets. Specific heat and thermal expansion studies reveal a nearly reversible first-order phase transition at around T_2c~ 272 K, where both CDW phases co-exist within a T-interval of about 10 K.
Paper
Coexistence of charge density wave and field-tuned magnetic states in TmNiC\\(_2\\)
Exploring the relations between coexisting, cooperative, or competing types of ordering is a key to identify and harness the mechanisms governing the mutual interactions between them, and to utilize their combined properties. We have experimentally explored the response of the charge density wave (CDW) to various antiferromagnetic, metamagnetic, and field-aligned ferromagnetic states that constitute the magnetic phase diagram of TmNiC\\(_2\\). The high resolution x-ray diffraction experiment employing synchrotron radiation at low temperature and high magnetic field, allowed to follow the superstructure satellite reflections, being a sensitive probe of CDW. This investigation not only reveals direct evidence that the charge density wave avoids even a partial suppression in the antiferromagnetic ground state but also proves that this state coexists, without any visible signatures of weakening, in the entire dome of the magnetically ordered phases, including the field-aligned ferromagnetic state. The calculations of the electronic and phonon structures support the experiment, revealing that the dominant contribution to the CDW transition stems from momentum-dependent electron-phonon coupling. We conclude that this mechanism prevents the CDW from vanishing, although the nesting conditions within the magnetically ordered phases deteriorate.
Paper