Putative quantum critical point in the itinerant magnet ZrFe4_4Si2_2 with a frustrated quasi-one-dimensional structure

The Fe sublattice in the compound ZrFe$_4$Si$_2$ features geometrical frustration and quasi-one-dimensionality. We therefore investigated the magnetic behavior in ZrFe$_4$Si$_2$ and its evolution upon substituting Ge for Si and under the application of hydrostatic pressure using structural, magnetic, thermodynamic, and electrical-transport probes. Magnetic measurements reveal that ZrFe$_4$Si$_2$ holds paramagnetic Fe moments with an effective moment $\mu_{\rm eff}= 2.18~\mu_{B}$. At low temperatures the compound shows a weak short-range magnetic order below 6 K. Our studies demonstrate that substituting Ge for Si increases the unit-cell volume and stabilizes the short-range order into a long-range spin-density wave type magnetic order. On the other hand, hydrostatic pressure studies using electrical-resistivity measurements on ZrFe$_4$(Si$_{0.88}$Ge$_{0.12}$)$_2$ indicate a continuous suppression of the magnetic ordering upon increasing pressure. Therefore, our combined chemical substitution and hydrostatic pressure studies suggest the existence of a lattice-volume-controlled quantum critical point in ZrFe$_4$Si$_2$.Comment: 9 pages, 7 figures, Published version with new title and extended discussio

On the search for the chiral anomaly in Weyl semimetals: The negative longitudinal magnetoresistance

Recently, the existence of massless chiral (Weyl) fermions has been postulated in a class of semi-metals with a non-trivial energy dispersion.These materials are now commonly dubbed Weyl semi-metals (WSM).One predicted property of Weyl fermions is the chiral or Adler-Bell-Jackiw anomaly, a chirality imbalance in the presence of parallel magnetic and electric fields. In WSM, it is expected to induce a negative longitudinal magnetoresistance (NMR), the chiral magnetic effect.Here, we present experimental evidence that the observation of the chiral magnetic effect can be hindered by an effect called "current jetting". This effect also leads to a strong apparent NMR, but it is characterized by a highly non-uniform current distribution inside the sample. It appears in materials possessing a large field-induced anisotropy of the resistivity tensor, such as almost compensated high-mobility semimetals due to the orbital effect.In case of a non-homogeneous current injection, the potential distribution is strongly distorted in the sample.As a consequence, an experimentally measured potential difference is not proportional to the intrinsic resistance.Our results on the MR of the WSM candidate materials NbP, NbAs, TaAs, TaP exhibit distinct signatures of an inhomogeneous current distribution, such as a field-induced "zero resistance' and a strong dependence of the `measured resistance" on the position, shape, and type of the voltage and current contacts on the sample. A misalignment between the current and the magnetic-field directions can even induce a "negative resistance". Finite-element simulations of the potential distribution inside the sample, using typical resistance anisotropies, are in good agreement with the experimental findings. Our study demonstrates that great care must be taken before interpreting measurements of a NMR as evidence for the chiral anomaly in putative Weyl semimetals.Comment: 13 pages, 6 figure

Ising-type Magnetic Anisotropy in CePd2_2As2_2

We investigated the anisotropic magnetic properties of CePd$_2$As$_2$ by magnetic, thermal and electrical transport studies. X-ray diffraction confirmed the tetragonal ThCr$_2$Si$_2$-type structure and the high-quality of the single crystals. Magnetisation and magnetic susceptibility data taken along the different crystallographic directions evidence a huge crystalline electric field (CEF) induced Ising-type magneto-crystalline anisotropy with a large $c$-axis moment and a small in-plane moment at low temperature. A detailed CEF analysis based on the magnetic susceptibility data indicates an almost pure $\langle\pm5/2 \rvert$ CEF ground-state doublet with the dominantly $\langle\pm3/2 \rvert$ and the $\langle\pm1/2 \rvert$ doublets at 290 K and 330 K, respectively. At low temperature, we observe a uniaxial antiferromagnetic (AFM) transition at $T_N=14.7$ K with the crystallographic $c$-direction being the magnetic easy-axis. The magnetic entropy gain up to $T_N$ reaches almost $R\ln2$ indicating localised $4f$-electron magnetism without significant Kondo-type interactions. Below $T_N$, the application of a magnetic field along the $c$-axis induces a metamagnetic transition from the AFM to a field-polarised phase at $\mu_0H_{c0}=0.95$ T, exhibiting a text-book example of a spin-flip transition as anticipated for an Ising-type AFM.Comment: 9 Pages, 8 figure

Thermopower and thermal conductivity in the Weyl semimetal NbP

The Weyl semimetal NbP exhibits an extremely large magnetoresistance (MR) and an ultra-high mobility. The large MR originates from a combination of the nearly perfect compensation between electron- and hole-type charge carriers and the high mobility, which is relevant to the topological band structure. In this work we report on temperature- and field-dependent thermopower and thermal conductivity experiments on NbP. Additionally, we carried out complementary heat capacity, magnetization, and electrical resistivity measurements. We found a giant adiabatic magnetothermopower with a maximum of 800 $\mu$V/K at 50 K in a field of 9 T. Such large effects have been observed rarely in bulk materials. We suggest that the origin of this effect might be related to the high charge-carrier mobility. We further observe pronounced quantum oscillations in both thermal conductivity and thermopower. The obtained frequencies compare well with our heat capacity and magnetization data.Comment: 6 pages, 3 figure

Localized f-electron magnetism in the semimetal Ce3Bi4Au3

Ce$_{3}$Bi$_{4}$Au$_{3}$ crystallizes in the same non-centrosymmetric cubic structure as the prototypical Kondo insulator Ce$_{3}$Bi$_{4}$Pt$_{3}$. Here we report the physical properties of Ce$_{3}$Bi$_{4}$Au$_{3}$ single crystals using magnetization, thermodynamic, and electrical-transport measurements. Magnetic-susceptibility and heat-capacity data reveal antiferromagnetic (AFM) order below $T_N=3.2$ K. The magnetic entropy $S_{\rm mag}$ reaches $R$ln2 slightly above $T_N$, which suggests localized $4f$-moments in a doublet ground state. Multiple field-induced magnetic transitions are observed at temperatures below $T_N$, which indicate a complex spin structure with competing interactions. Ce$_{3}$Bi$_{4}$Au$_{3}$ shows semimetallic behavior in electrical resistivity measurements in contrast to the majority of reported Cerium-based 343 compounds. Electrical-resistivity measurements under hydrostatic pressure reveal a slight enhancement of $T_N$ under pressures up to 2.3 GPa, which supports a scenario wherein Ce$_{3}$Bi$_{4}$Au$_{3}$ belongs to the far left of the Doniach phase diagram dominated by Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Using realistic many-body simulations, we confirm the semi-metallic electronic structure of Ce$_{3}$Bi$_{4}$Au$_{3}$ and quantitatively reproduce its local moment behavior in the paramagnetic state.Comment: 10 pages, 10 figure

Localized <i>f</i>-electron magnetism in the semimetal Ce₃Bi₄Au₃

Ce3Bi4Au3 crystallizes in the same noncentrosymmetric cubic structure as the prototypical Kondo insulator Ce3Bi4Pt3. Here we report the physical properties of Ce3Bi4Au3 single crystals using magnetization, thermodynamic, and electrical-transport measurements. Magnetic-susceptibility and heat-capacity data reveal antiferromagnetic order below TN=3.2K. The magnetic entropy Smag reaches Rln2 slightly above TN, which suggests localized 4f moments in a doublet ground state. Multiple field-induced magnetic transitions are observed at temperatures below TN, which indicate a complex spin structure with competing interactions. Ce3Bi4Au3 shows semimetallic behavior in electrical resistivity in contrast to the majority of reported cerium-based 343 compounds which are semiconducting. Electrical-resistivity measurements under hydrostatic pressure reveal a slight enhancement of TN under pressures up to 2.3 GPa, which supports a scenario wherein Ce3Bi4Au3 belongs to the far left of the Doniach phase diagram dominated by Ruderman-Kittel-Kasuya-Yosida interactions. Using realistic many-body simulations, we confirm the semimetallic electronic structure of Ce3Bi4Au3 and quantitatively reproduce its local moment behavior in the paramagnetic state.</p

Pressure tuning of the anomalous Hall effect in the chiral antiferromagnet Mn3Ge

We report on the pressure evolution of the giant anomalous Hall effect (AHE) in the chiral antiferromagnet Mn$_3$Ge. The AHE originating from the non-vanishing Berry curvature in Mn$_3$Ge can be continuously tuned by application of hydrostatic pressure. At room temperature, the Hall signal changes sign as a function of pressure and vanishes completely at $p=1.53$ GPa. Even though the Hall conductivity changes sign upon increasing pressure, the room-temperature saturation value of 23 ${\rm \Omega^{-1}cm^{-1}}$ at 2.85 GPa is remarkably high and comparable to the saturation value at ambient pressure of about 40 ${\rm \Omega^{-1}cm^{-1}}$. The change in the Hall conductivity can be directly linked to a gradual change of the size of the in-plane components of the Mn moments in the non-collinear triangular magnetic structure. Our findings, therefore, provide a route for tuning of the AHE in the chiral antiferromagnetic Mn$_3$Ge.Comment: 5 pages, 4 figure

The planar triangular S = 3/2 magnet AgCrSe2 : magnetic frustration, short range correlations, and field tuned anisotropic cycloidal magnetic order

Funding: Deutsche Forschungsgemeinschaft (DFG) through the SFB 1143 and the Wurzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter–ct.qmat (EXC 2147, Project No. 390858490), as well as the support of the HLD at HZDR, a member of the European Magnetic Field Laboratory (EMFL). We gratefully acknowledge support from the European Research Council (through the QUESTDO project, 714193), the Leverhulme Trust, and the Royal Society. We thank the Elettra synchrotron for access to the APE-HE beamline under proposal number 20195300. The research leading to this result has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. Part of this work has been performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MUR Italy Progetti Internazionali) project (www.trieste.NFFA.eu).Our studies evidence an anisotropic magnetic order below TN = 32~K. Susceptibility data in small fields of about 1~T reveal an antiferromagnetic (AFM) order for H ⊥ c, whereas for H || c the data are reminiscent of a field-induced ferromagnetic (FM) structure. At low temperatures and for H ⊥ c, the field-dependent magnetization and AC susceptibility data evidence a metamagnetic transition at H+ = 5~T, which is absent for H || c. We assign this to a transition from a planar cycloidal spin structure at low fields to a planar fan-like arrangement above H+. A fully FM polarized state is obtained above the saturation field of H⊥S = 23.7~T at 2~K with a magnetization of Ms = 2.8~μB/Cr. For H || c, M(H) monotonously increases and saturates at the same Ms value at HIIS = 25.1~T at 4.2~K. Above TN, the magnetic susceptibility and specific heat indicate signatures of two dimensional (2D) frustration related to the presence of planar ferromagnetic and antiferromagnetic exchange interactions. We found a pronounced nearly isotropic maximum in both properties at about T* = 45~K, which is a clear fingerprint of short-range correlations and emergent spin fluctuations. Calculations based on a planar 2D Heisenberg model support our experimental findings and suggest a predominant FM exchange among nearest and AFM exchange among third-nearest neighbors. Only a minor contribution might be assigned to the antisymmetric Dzyaloshinskii-Moriya interaction possible related to the non-centrosymmetric polar space group R3m. Due to these competing interactions, the magnetism in AgCrSe2, in contrast to the oxygen based delafossites, can be tuned by relatively small, experimentally accessible, magnetic fields, allowing us to establish the complete anisotropic magnetic H-T phase diagram in detail.PostprintPeer reviewe