A. Sidorenko

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

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.

Iron silicide nanoparticles in a SiC/C matrix from organometallic polymers: characterization and magnetic properties

Thermolysis of poly(ferrocenylsilane) at 1000 °C or poly[{(dimethylsilyl)ferrocenyl}diacetylene] at 850 °C resulted in ferromagnetic Fe3Si nanoparticles, well distributed in a SiC/C matrix. The average size of the formed nanoparticles, their size distribution and their agglomeration behaviour depend on the choice of polymer and the thermolysis conditions. The size-dependent magnetic properties of the nanoparticles were analyzed by SQUID measurements. The smallest Fe3Si nanoparticles (10–30 nm) are single magnetic domain particles with superparamagnetic behaviour.

Cooling in rare-earth paramagnets at ultrahigh pulsed magnetic fields due to energy level crossing

Abstract Maxima of differential magnetic susceptibility are observed at 4.2 K in a pulsed field in the tetragonal paramagnets YbPO 4 and PrVO 4 at 280 and 45 T, respectively, caused by crossing of the energy levels of rare-earth ions. Different characters of the level interactions result in different types of magnetocaloric effect in the adiabatic regime. A theory of these phenomena is developed.

ENERGY LEVEL CROSSING AND MAGNETOCALORIC EFFECT IN YBPO4 IN ULTRAHIGH PULSED FIELDS

Abstract The first investigation of the energy level crossing (crossover) effects at ultrahigh magnetic fields generated by an explosive method for paramagnetic YbPO 4 is carried out. A broad maximum of differential susceptibility d M /d H is observed at H c ≈280xa0T. On the assumption of adiabatic magnetization the magnetocaloric effect is calculated, and an essential cooling of a crystal near the crossover takes place.

Kondo Insulator to Semimetal Transformation Tuned by Spin-Orbit Coupling

Recent theoretical studies of topologically nontrivial electronic states in Kondo insulators have pointed to the importance of spin-orbit coupling (SOC) for stabilizing these states. However, systematic experimental studies that tune the SOC parameter λ_{SOC} in Kondo insulators remain elusive. The main reason is that variations of (chemical) pressure or doping strongly influence the Kondo coupling J_{K} and the chemical potential μ-both essential parameters determining the ground state of the material-and thus possible λ_{SOC} tuning effects have remained unnoticed. Here, we present the successful growth of the substitution series Ce_{3}Bi_{4}(Pt_{1-x}Pd_{x})_{3} (0≤x≤1) of the archetypal (noncentrosymmetric) Kondo insulator Ce_{3}Bi_{4}Pt_{3}. The Pt-Pd substitution is isostructural, isoelectronic, and isosize, and it therefore is likely to leave J_{K} and μ essentially unchanged. By contrast, the large mass difference between the 5d element Pt and the 4d element Pd leads to a large difference in λ_{SOC}, which thus is the dominating tuning parameter in the series. Surprisingly, with increasing x (decreasing λ_{SOC}), we observe a Kondo insulator to semimetal transition, demonstrating an unprecedented drastic influence of the SOC. The fully substituted end compound Ce_{3}Bi_{4}Pd_{3} shows thermodynamic signatures of a recently predicted Weyl-Kondo semimetal.

The Vienna Nuclear Demagnetization Refrigerator

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.

Covalent embedding of Ni2+/Fe3+ cyanometallate structures in silica by sol-gel processing.

Compound [Ni(AEAPTS)2]3[Fe(CN)6]2 (AEAPTS=N-(2-aminoethyl)-3-aminopropyltrimethoxysilane), in which Ni2+ and Fe3+ ions are ferromagnetically coupled through cyano bridges, was prepared. Sol–gel processing of the AEAPTS derivative resulted in incorporation of the cyanometallate in silica. The obtained material is magnetically ordered below 22u2005K with an effective magnetic moment μeff of 4.46u2005μB at room temperature, a maximum of 8.60u2005μB at approximately 15u2005K and a narrow hysteresis at 2u2005K, with a saturation remanence of about 300u2005emuu2009mol−1 and a coercitivity of 0.03u2005T.

Quantum Oscillations in Ultra Pure PtSn4

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.

DC Magnetic Susceptibility of CeCoGe 2.36 Si 0.64 under High Pressure

We report an investigation of the magnetic phase diagram of the heavy fermion com-pound CeCoGe2.36Si0.64 using DC magnetic susceptibility measurements under high pressure upto 10 kbar. The antiferromagnetic order that develops at ambient pressure below about 5.5 Kremains essentially unaffected by pressure in the investigated pressure range up to 10 kbar. Onthe other hand, moderate magnetic fields appear to induce a quantum critical point in a sam-ple subject to a pressure of 2 kbar. We discuss the role of disorder in the series of compoundsCeCoGe3−xSix.