Pressure-tuned quantum criticality in the large-D antiferromagnet DTN.

Strongly correlated spin systems can be driven to quantum critical points via various routes. In particular, gapped quantum antiferromagnets can undergo phase transitions into a magnetically ordered state with applied pressure or magnetic field, acting as tuning parameters. These transitions are characterized by z = 1 or z = 2 dynamical critical exponents, determined by the linear and quadratic low-energy dispersion of spin excitations, respectively. Employing high-frequency susceptibility and ultrasound techniques, we demonstrate that the tetragonal easy-plane quantum antiferromagnet NiCl2 ⋅ 4SC(NH2)2 (aka DTN) undergoes a spin-gap closure transition at about 4.2 kbar, resulting in a pressure-induced magnetic ordering. The studies are complemented by high-pressure-electron spin-resonance measurements confirming the proposed scenario. Powder neutron diffraction measurements revealed that no lattice distortion occurs at this pressure and the high spin symmetry is preserved, establishing DTN as a perfect platform to investigate z = 1 quantum critical phenomena. The experimental observations are supported by DMRG calculations, allowing us to quantitatively describe the pressure-driven evolution of critical fields and spin-Hamiltonian parameters in DTN.

Field-induced compensation of magnetic exchange as the possible origin of reentrant superconductivity in UTe2.

The potential spin-triplet heavy-fermion superconductor UTe2 exhibits signatures of multiple distinct superconducting phases. For field aligned along the b axis, a metamagnetic transition occurs at µ0Hm ≈ 35 T. It is associated with magnetic fluctuations that may be beneficial for the field-reinforced superconductivity surviving up to Hm. Once the field is tilted away from the b towards the c axis, a reentrant superconducting phase emerges just above Hm. In order to better understand this remarkably field-resistant superconducting phase, we conducted magnetic-torque and magnetotransport measurements in pulsed magnetic fields. We determine the record-breaking upper critical field of µ0Hc2 ≈ 73 T and its evolution with angle. Furthermore, the normal-state Hall effect experiences a drastic suppression indicative of a reduced band polarization above Hm in the angular range around 30° caused by a partial compensation between the applied field and an exchange field. This promotes the Jaccarino-Peter effect as a likely mechanism for the reentrant superconductivity above Hm.

Improved accuracy in high-frequency AC transport measurements in pulsed high magnetic fields.

We show theoretically and experimentally that accurate transport measurements are possible even within the short time provided by pulsed magnetic fields. For this purpose, a new method has been devised, which removes the noise component of a specific frequency from the signal by taking a linear combination of the results of numerical phase detection using multiple integer periods. We also established a method to unambiguously determine the phase rotation angle in AC transport measurements using a frequency range of tens of kilohertz. We revealed that the dominant noise in low-frequency transport measurements in pulsed magnetic fields is the electromagnetic induction caused by mechanical vibrations of wire loops in inhomogeneous magnetic fields. These results strongly suggest that accurate transport measurements in short-pulsed magnets are possible when mechanical vibrations are well suppressed.

Anomalous Lattice Softening Near a Quantum Critical Point in a Transverse Ising Magnet.

We have investigated the elastic response of a transverse Ising magnet CoNb_{2}O_{6} by means of ultrasound velocity measurement. A huge elastic anomaly in the C_{66} mode is observed near a quantum critical point when sweeping a magnetic field perpendicular to the Ising axis. This anomaly appears to become critical only for the Faraday configuration (field parallel to the sound propagation direction) but is much less pronounced for the Voigt geometry (field perpendicular to the sound propagation direction). We propose that the relativistic spin-orbit interaction plays a crucial role in the quantum critical regime resulting in the elastic anomaly, which is enhanced by quantum fluctuations.

Inductive Heating Using a High-Magnetic-Field Pulse to Initiate Chemical Reactions to Generate Composite Materials.

Induction heating is efficient, precise, cost-effective, and clean. The heating process is coupled to an electrically conducting material, usually a metal. As most polymers are dielectric and non-conducting, induction heating is not applicable. In order to transfer energy from an electromagnetic field into polymer induction structures, conducting materials or materials that absorb the radiation are required. This report gives a brief overview of induction heating processes used in polymer technology. In contrast to metals, most polymer materials are not affected by electromagnetic fields. However, an unwanted temperature rise of the polymer can occur when a radio frequency field is applied. The now available high-field magnetic sources provide a new platform for induction heating at very low frequencies, avoiding unwanted thermal effects within the material. Using polycarbonate and octadecylamine as an example, it is demonstrated that induction heating performed by a magnetic-field pulse with a maximum flux density of 59 T can be used to initiate chemical reactions. A 50 nm thick Ag loop, with a mean diameter of 7 mm, placed in the polymer-polymer interface acts as susceptor and a resistive heating element. The formation of urethane as a linker compound was examined by infrared spectroscopic imaging and differential scanning calorimetry.

Synthesis of a Cu-Filled Rh17S15 Framework: Microwave Polyol Process Versus High-Temperature Route.

Metal-rich, mixed copper-rhodium sulfide Cu3-δRh34S30 that represents a new Cu-filled variant of the Rh17S15 structure has been synthesized and structurally characterized. Copper content in the [CuRh8] cubic cluster was found to vary notably dependent on the chosen synthetic route. Full site occupancy was achieved only in nanoscaled Cu3Rh34S30 obtained by a rapid, microwave-assisted reaction of CuCl, Rh2(CH3CO2)4 and thiosemicarbazide at 300 °C in just 30 min; whereas merely Cu-deficient Cu3-δRh34S30 (2.0 ≥ δ ≥ 0.9) compositions were realized via conventional high-temperature ceramic synthesis from the elements at 950 °C. Although Cu3-δRh34S30 is metallic just like Rh17S15, the slightly enhanced metal content has a dramatic effect on the electronic properties. Whereas the Rh17S15 host undergoes a superconducting transition at 5.4 K, no signs of the latter were found for the Cu-derivatives at least down to 1.8 K. This finding is corroborated by the strongly reduced density of states at the Fermi level of the ternary sulfide and the disruption of long-range Rh-Rh interactions in favor of Cu-Rh interactions as revealed by quantum-chemical calculations.

Downscaling Effect on the Superconductivity of Pd3Bi2X2 (X = S or Se) Nanoparticles Prepared by Microwave-Assisted Polyol Synthesis.

Pd3Bi2S2 and Pd3Bi2Se2 have been successfully prepared in the form of nanoparticles with diameters of ∼50 nm by microwave-assisted modified polyol synthesis at low temperatures. The composition and morphology of the samples have been studied by means of powder X-ray diffraction as well as electron microscopy methods, including X-ray intensity mapping on the nanoscale. Superconducting properties of the as-prepared samples have been characterized by electrical resistivity measurements down to low temperatures (∼0.2 K). Deviations from the bulk metallic behavior originating from the submicrometer nature of the samples were registered for both phases. A significant critical-field enhancement up to 1.4 T, i.e., 4 times higher than the value of the bulk material, has been revealed for Pd3Bi2Se2. At the same time, the critical temperature is suppressed to 0.7 K from the bulk value of ∼1 K. A superconducting transition at 0.4 K has been observed in nanocrystalline Pd3Bi2S2. Here, a zero-temperature upper critical field of ∼0.5 T has been estimated. Further, spark plasma-sintered Pd3Bi2S2 and Pd3Bi2Se2 samples have been investigated. Their superconducting properties are found to lie between those of the bulk and nanosized samples.

Nuclear magnetic resonance apparatus for pulsed high magnetic fields.

A nuclear magnetic resonance apparatus for experiments in pulsed high magnetic fields is described. The magnetic field pulses created together with various magnet coils determine the requirements such an apparatus has to fulfill to be operated successfully in pulsed fields. Independent of the chosen coil it is desirable to operate the entire experiment at the highest possible bandwidth such that a correspondingly large temporal fraction of the magnetic field pulse can be used to probe a given sample. Our apparatus offers a bandwidth of up to 20 MHz and has been tested successfully at the Hochfeld-Magnetlabor Dresden, even in a very fast dual coil magnet that has produced a peak field of 94.2 T. Using a medium-sized single coil with a significantly slower dependence, it is possible to perform advanced multi-pulse nuclear magnetic resonance experiments. As an example we discuss a Carr-Purcell spin echo sequence at a field of 62 T.

A star-shaped heteronuclear Cr(III)Mn(II)3 species and its precise electronic and magnetic structure: spin frustration studied by X-ray spectroscopic, magnetic, and theoretical methods.

Molecular magnets incorporate transition-metal ions with organic groups providing a bridge to mediate magnetic exchange interactions between the ions. Among them are star-shaped molecules in which antiferromagnetic couplings between the central and peripheral atoms are predominantly present. Those configurations lead to an appreciable spin moment in the nonfrustrated ground state. In spite of its topologically simple magnetic structure, the [Cr(III)Mn(II)(3) (PyA)(6)Cl(3)] (CrMn(3)) molecule, in which PyA represents the monoanion of syn-pyridine-2-aldoxime, exhibits nontrivial magnetic properties, which emerge from the combined action of single-ion anisotropy and frustration. In the present work, we elucidate the underlying electronic and magnetic properties of the heteronuclear, spin-frustrated CrMn(3) molecule by applying X-ray magnetic circular dichroism (XMCD), as well as magnetization measurements in high magnetic fields, density functional theory, and ligand-field multiplet calculations. Quantum-model calculations based on a Heisenberg Hamiltonian augmented with local anisotropic terms enable us not only to improve the accuracy of the exchange interactions but also to determine the dominant local anisotropies. A discussion of the various spin Hamiltonian parameters not only leads to a validation of our element selective transition metal L edge XMCD spin moments at a magnetic field of 5 T and a temperature of 5 K but also allows us to monitor an interesting effect of anisotropy and frustration of the manganese and chromium ions.