Nanofibers and nanorods of NiCo- and NiCoFe- oxides and phosphides were synthesized by hydrothermal methods, followed by phosphidation to yield (Ni,Co)P, (Ni,Co)2P, and FeP. The materials were evaluated as electrocatalysts for the oxygen evolution reaction (OER) in water splitting in the presence of a magnetic field in two electrolytes: 1 M KOH and 1 M phosphate buffer saline (PBS) solution. A standard electrochemical cell was equipped with disk magnets directed perpendicular to the electric field. The magnetic field affected the catalyst interface and increased the reaction rate. The best catalyst was found to be NiCoP, and the overpotential (at 10 mA/cm2) was reduced from 330 to 260 mV when a magnetic field of 100 mT was applied and further to 170 mV when a magnetic field of 200 mT was applied. NiCoP has the highest proportion of magnetic domains aligned due to having the highest saturation magnetization (Ms), remanence magnetization (Mr), and the lowest coercivity (Hc). The mixed transition metal phosphide catalysts were found to partly transform into (Ni,Co)3(PO4)2during electrocatalysis; however, they still responded to a change in the magnetic field. The results show that a weak magnetic field can improve the performance of electrocatalysts based on certain transition metals in a neutral pH electrolyte mimicking seawater.

Spin-ice compounds enable the exploration of the dynamics of magnetic monopoles in condensed matter systems. In this study, we use AC calorimetry to probe the dynamic heat capacity response of the classical spin-ice compound Dy₂Ti₂O₇ at low temperatures (0.5–5 K). Using frequencies of 0.01–500 Hz, we observe a strong frequency dependence in the measured heat capacity, allowing us to study thermal-relaxation effects on the corresponding timescales. The relaxation time 𝜏 is determined from the frequency dependence of the heat capacity as the characteristic frequency below which the heat capacity saturates. Specific heat shows a maximum around 1 K. The extracted 𝜏 shows divergent behavior below this temperature, reaching ∼6s at 0.65 K, similar to the relaxation time seen in previous studies. Performing dynamic Monte Carlo simulations, we verify that the specific heat frequency response has its origin in the slow magnetic monopole dynamics indigenous to spin ice. We find a timescale of 20ms per Monte Carlo step at 4K, in contrast to 2.5ms mentioned in previous studies by other techniques.

Knowing the pressure dependence of glass forming liquids is important in various contexts. Here, we study the case of supercooled water, which has at least two different amorphous states with different densities. The pressure dependencies of the two glass transitions are predicted to show opposite behaviour, crossing in the P-T plane at elevated pressure. The experimental identification of the glass transition at elevated pressure and cryo-conditions is technically difficult. Moreover, in the case of amorphous ices, the glass transition is interrupted by crystallization, which makes it even more challenging. We show the feasibility of performing X-ray photon correlation spectroscopy experiments at elevated pressure using a diamond anvil cell at cryogenic temperatures. We observe two dynamic components when approaching the glass transition temperature. For high-density amorphous ice at a pressure of around (0.08 ± 0.02) GPa we determine the glass transition to be at higher temperatures compared to ambient conditions.

Rare-earth elements containing aperiodic quasicrystals and their related periodic approximant crystals can exhibit nontrivial physical properties at low temperatures. Here, we investigate the 1/1 and 2/1 approximant crystal phases of the Ce-Au-Al system by studying the ac susceptibility and specific heat at low temperatures and in magnetic fields up to 12 T. We find that these systems display signs of quantum criticality similar to the observations in other claimed quantum critical systems, including the related Yb-Au-Al quasicrystal. In particular, the ac-susceptibility at low temperatures shows a diverging behavior χ∝1/T as the temperature decreases as well as cutoff behavior in magnetic field. Notably, the field dependence of χ closely resembles that of quantum critical systems. However, the ac susceptibility both in zero and nonzero magnetic fields can be understood from the splitting of a ground state Kramers doublet of Ce3+. The high-temperature Curie-Weiss fit yields an effective magnetic moment of approximately 2.54μB per Ce for both approximant systems, which is reduced to ∼2.0μB at temperatures below 10 K. The low-temperature specific heat is dominated by the Schottky anomaly originating from the splitting of the Ce3+ Kramers doublet, resulting in an entropy of Rln2 at around 10 K.

In strongly correlated systems, interactions give rise to critical fluctuations surrounding the quantum critical point (QCP) of a quantum phase transition. Quasicrystals allow the study of quantum critical phenomena in aperiodic systems with frustrated magnetic interactions. Here, we study the magnetic field and temperature scaling of the low-temperature specific heat for the quantum critical Yb-Au-Al quasicrystal. We devise a scaling function that encapsulates the limiting behaviors as well as the area where the system goes from a temperature-limited to a field-limited quantum critical region, where the magnetic field acts as a cutoff for critical fluctuations. The zero-field electronic specific heat is described by a power-law divergence, Cel/T T-0.54, aligning with previously observed ac-susceptibility and specific-heat measurements. The field dependence of the electronic specific heat at high magnetic fields shows a similar power law Cel/T B-0.50. In the zero-field and low-field region, we observe two small but distinct anomalies in the specific heat, located at 0.7 and 2.1 K.

Recent reports of colossal negative magnetoresistance (CMR) in a few magnetic semimetals and semiconductors have attracted attention because these materials are devoid of the conventional mechanisms of CMR such as mixed valence, double-exchange interaction, and Jahn-Teller distortion. New mechanisms have thus been proposed, including topological band structure, ferromagnetic clusters, orbital currents, and charge ordering. The CMR in these compounds has been reported in two forms: either a resistivity peak or a resistivity upturn suppressed by a magnetic field. Here we reveal both types of CMR in a single antiferromagnetic semiconductor Eu5In2As6. Using the transport and thermodynamic measurements, we demonstrate that the peak-type CMR is likely due to the percolation of magnetic polarons with increasing magnetic field, while the upturn-type CMR is proposed to result from the melting of a charge order under the magnetic field. We argue that similar mechanisms operate in other compounds, offering a unifying framework to understand CMR in seemingly different materials.

High-temperature superconductivity in cuprates emerges upon doping the parent Mott insulator. Key features of the low-doped cuprate superconductors include an effective carrier density that tracks the number of doped holes, the emergence of an anisotropic pseudogap that is characterized by disconnected Fermi arcs and the closure of the gap at a critical doping level. In Sr2IrO4, a spin–orbit-coupled Mott insulator often regarded as a 5d analogue of the cuprates, surface probes have also revealed the emergence of an anisotropic pseudogap and Fermi arcs under electron doping. However, neither the corresponding critical doping nor the bulk signatures of pseudogap closure have yet been observed. Here we demonstrate that electron-doped Sr2IrO4 exhibits a critical doping level with a marked crossover in the effective carrier density at low temperatures. This is accompanied by a five-orders-of-magnitude increase in conductivity and a sixfold enhancement in the electronic specific heat. These collective findings resemble the bulk pseudogap phenomenology in cuprates. However, given that electron-doped Sr2IrO4 is non-superconducting, it suggests that the pseudogap may not be a state of precursor pairing. Therefore, our results narrow the search for the key ingredient underpinning the formation of the superconducting condensate in doped Mott insulators.

High-quality crystals of CoTeO₄ were grown by application of chemical vapor transport reactions in closed silica ampoules, starting from polycrystalline material in a temperature gradient 640°C → 580°C with TeCl₄ as transport agent. Crystal structure analysis of CoTeO₄ from single crystal X-ray data revealed a dirutile-type structure with Co^II and Te^VI atoms at crystallographically distinct sites, each with point group symmetry . The statistical significance and accuracy of the previously reported structural model based on powder data with the ordered arrangement of Co and Te cations was noticeably improved. CoTeO₄ does not undergo a structural phase transition upon heating, but decomposes stepwise (Co₂Te₃O₈ as intermediate phase) to Co₃TeO₆ as the only crystalline phase stable above 770°C. Temperature-dependent magnetic susceptibility and dielectric measurements suggest antiferromagnetic ordering at ∼50 K. Optical absorption spectroscopy and computational studies reveal wide-band semiconductive behavior for CoTeO₄. The experimentally determined band gap of ∼2.42 eV is also found for CdS, which is frequently used in photovoltaic systems but is hazardous to the environment. Hence, CoTeO₄ might be a possible candidate to replace CdS in this regard.

The effect of Eu doping in the Tsai quasicrystal (QC) GdCd_7.88 and its periodic 1/1 approximant crystal (AC) GdCd₆ are investigated. This represents the first synthesis of Eu-containing stable QC samples, where three samples with the final composition Gd_1–xEu_xCd_7.6±α at Eu doping concentrations x = 0.06, 0.13, and 0.19 are obtained (α ∼ 0.2). They are compared to two 1/1 ACs with compositions Gd_1–xEu_xCd₆ (x = 0.12, 0.16). In addition, a new type of 1/1 AC, differing only by the inclusion of extra Cd sites unique to the Eu₄Cd₂₅ 1/1 AC, has been discovered and synthesized for the concentrations Gd_1–xEu_xCd_6+δ (x = 0.25, 0.33, 0.45, 0.69, 0.73, and 0 < δ ≤ 0.085). Due to the preferred cube morphology of its single grains, we refer to them as c-type 1/1 ACs and to the conventional standard ones as s-type. In both QCs and s-type ACs, the Eu content appears to saturate at a concentration of ∼20%. On the other hand, any Gd| Eu ratio is allowed in the c-type ACs, varying continuously between GdCd₆ and Eu₄Cd₂₅. We describe and contrast the changes in composition, atomic structure, specific heat, and magnetic properties induced by Eu doping in the quasicrystalline phase and the s-type and c-type 1/1 ACs. By comparing our results to the literature data, we propose that the occupancy of the extra Cd sites can be used to predict the stability of Tsai-type quasicrystalline phases.

Many thermal measurements in high magnetic fields—including heat capacity, thermal conductivity, thermopower, magnetocaloric and thermal Hall effect measurements—require thermometers that are sensitive over a wide temperature range, are low mass, have a rapid thermal response and have a minimal, easily correctable magnetoresistance. We recently reported the development of a new granular-metal oxide ceramic composite (cermet) for this purpose formed by co-sputtering of the metallic alloy nichrome (Ni0.8Cr0.2) and the insulator silicon dioxide (SiO2). In this earlier work, we found that co-sputtering of NiCr alloy and SiO2 in a reactive oxygen and inert argon gas mixture can produce resistive thin-film thermometers sensitive enough to be used in calorimetry and related measurements from room temperature down to below 100 mK in magnetic fields up to at least 35 T. In this work, we present results for thin cermet films grown with Cu0.55Ni0.45 and Ti0.05Cr0.95. Growth of CuNi-based thin-film cermets generally requires more oxygen in the working gas compared to NiCr and TiCr and yields thermometers that are much less sensitive than comparable NiCr-based thermometers. TiCr-based cermet thin-film thermometers have somewhat higher resistivity for similar sensitivities compared to NiCr-based cermet thin-film thermometers.