Rare-earth Engineering of the Magnetocaloric Effect in RMn6Sn6

We present a comprehensive study of the magnetocaloric effect (MCE) in a family of kagome magnets with formula RMn6Sn6 (R=Tb, Ho, Er, and Lu). These materials have a small rare-earth content and tunable magnetic ordering, hence they provide a venue to study the fundamentals of the MCE. We examine the effect of different types of order (ferromagnetic, ferrimagnetic, and antiferromagnetic) and the presence of a metamagnetic transition on the MCE. We extend the study to a high-entropy rare-earth alloys of the family, and conclude with several guidelines for enhancing the MCE in tunable magnetic materials with a small rare-earth content.Comment: Main Text: 14 pages, 6 figures Supplemental: 3 pages, 2 figures, 1 tabl

Gravitational anomaly in the ferrimagnetic topological Weyl semimetal NdAlSi

Quantum anomalies are the breakdowns of classical conservation laws that occur in quantum-field theory description of a physical system. They appear in relativistic field theories of chiral fermions and are expected to lead to anomalous transport properties in Weyl semimetals. This includes a chiral anomaly, which is a violation of the chiral current conservation that takes place when a Weyl semimetal is subjected to parallel electric and magnetic fields. A charge pumping between Weyl points of opposite chirality causes the chiral magnetic effect that has been extensively studied with electrical transport. On the other hand, if the thermal gradient, instead of the electrical field, is applied along the magnetic field, then as a consequence of the gravitational (also called the thermal chiral) anomaly an energy pumping occurs within a pair of Weyl cones. As a result, this is expected to generate anomalous heat current contributing to the thermal conductivity. We report an increase of both the magneto-electric and magneto-thermal conductivities in quasi-classical regime of the magnetic Weyl semimetal NdAlSi. Our work also shows that the anomalous electric and heat currents, which occur due to the chiral magnetic effect and gravitational anomalies respectively, are still linked by a 170 years old relation called the Wiedemann-Franz law.Comment: 26 pages, 8 figure

Signatures of a Majorana-Fermi surface in the Kitaev magnet Ag3_3LiIr2_2O6_6

Detecting Majorana fermions in experimental realizations of the Kitaev honeycomb model is often complicated by non-trivial interactions inherent to potential spin liquid candidates. In this work, we identify several distinct thermodynamic signatures of massive, itinerant Majorana fermions within the well-established analytical paradigm of Landau-Fermi liquid theory. We find a qualitative and quantitative agreement between the salient features of our Landau-Majorana liquid theory and the Kitaev spin liquid candidate Ag$_3$LiIr$_2$O$_6$. Our study presents strong evidence for a Fermi liquid-like ground state in the fundamental excitations of a honeycomb iridate, and opens new experimental avenues to detect itinerant Majorana fermions in condensed matter systems.Comment: 40 pages, 7 figure

Crystal Chemistry and Phonon Heat Capacity in Quaternary Honeycomb Delafossites: Cu[Li_(1/3)Sn_(2/3)]O)2 and Cu[Na_(1/3)Sn_(2/3)]O_2

This work presents an integrated approach to study the crystal chemistry and phonon heat capacity of complex layered oxides. Two quaternary delafossites are synthesized from ternary parent compounds and copper monohalides via a topochemical exchange reaction that preserves the honeycomb ordering of the parent structures. For each compound, Rietveld refinement of the powder X-ray diffraction patterns is examined in both monoclinic C2/c and rhombohedral R3̅m space groups. Honeycomb ordering occurs only in the monoclinic space group. Bragg peaks associated with honeycomb ordering acquire an asymmetric broadening known as the Warren line shape that is commonly observed in layered structures with stacking disorder. Detailed TEM analysis confirms honeycomb ordering within each layer in both title compounds and establishes a twinning between the adjacent layers instead of the more conventional shifting or skipping stacking faults. The structural model is then used to calculate phonon dispersions and heat capacity from first principles. In both compounds, the calculated heat capacity accurately describes the experimental data. The integrated approach presented here offers a platform to carefully analyze the phonon heat capacity in complex oxides where the crystal structure can produce magnetic frustration. Isolating phonon contribution from total heat capacity is a necessary and challenging step toward a quantitative study of spin liquid materials with exotic magnetic excitations such as spinons and Majorana fermions. A quantitative understanding of phonon density of states based on crystal chemistry as presented here also paves the way toward higher efficiency thermoelectric materials

Plethora of tunable Weyl fermions in kagome magnet Fe3Sn2 thin films

Interplay of magnetism and electronic band topology in unconventional magnets enables the creation and fine control of novel electronic phenomena. In this work, we use scanning tunneling microscopy and spectroscopy to study thin films of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually large number of densely-spaced spectroscopic features straddling the Fermi level. These are consistent with signatures of low-energy Weyl fermions and associated topological Fermi arc surface states predicted by theory. By measuring their response as a function of magnetic field, we discover a pronounced evolution in energy tied to the magnetization direction. Electron scattering and interference imaging further demonstrates the tunable nature of a subset of related electronic states. Our experiments provide the first visualization of how in-situ spin reorientation drives changes in the electronic density of states of the Weyl fermion band structure. Combined with previous reports of massive Dirac fermions, flat bands and electronic nematicity, our work establishes Fe3Sn2 as a unique platform that harbors an extraordinarily wide array of topological and correlated electron phenomena

First demonstration of tuning between the Kitaev and Ising limits in a honeycomb lattice

Recent observations of novel spin-orbit coupled states have generated tremendous interest in $4d/5d$ transition metal systems. A prime example is the $J_{\text{eff}}=\frac{1}{2}$ state in iridate materials and $\alpha$-RuCl$_{3}$ that drives Kitaev interactions. Here, by tuning the competition between spin-orbit interaction ($\lambda_{\text{SOC}}$) and trigonal crystal field splitting ($\Delta_\text{T}$), we restructure the spin-orbital wave functions into a novel $\mu=\frac{1}{2}$ state that drives Ising interactions. This is done via a topochemical reaction that converts Li$_{2}$RhO$_{3}$ to Ag$_{3}$LiRh$_{2}$O$_{6}$, leading to an enhanced trigonal distortion and a diminished spin-orbit coupling in the latter compound. Using perturbation theory, we present an explicit expression for the new $\mu=\frac{1}{2}$ state in the limit $\Delta_\text{T}\gg \lambda_{\text{SOC}}$ realized in Ag$_{3}$LiRh$_{2}$O$_{6}$, different from the conventional $J_\text{eff}=\frac{1}{2}$ state in the limit $\lambda_{\text{SOC}}\gg \Delta_\text{T}$ realized in Li$_{2}$RhO$_{3}$. The change of ground state is followed by a dramatic change of magnetism from a 6 K spin-glass in Li$_{2}$RhO$_{3}$ to a 94 K antiferromagnet in Ag$_{3}$LiRh$_{2}$O$_{6}$. These results open a pathway for tuning materials between the two limits and creating a rich magnetic phase diagram.Comment: 22 pages, 4 figure

Plethora of tunable Weyl fermions in kagome magnet Fe3Sn2 thin films

Interplay of magnetism and electronic band topology in unconventional magnets enables the creation and fine control of novel electronic phenomena. In this work, we use scanning tunneling microscopy and spectroscopy to study thin films of a prototypical kagome magnet Fe3Sn2. Our experiments reveal an unusually large number of densely-spaced spectroscopic features straddling the Fermi level. These are consistent with signatures of low-energy Weyl fermions and associated topological Fermi arc surface states predicted by theory. By measuring their response as a function of magnetic field, we discover a pronounced evolution in energy tied to the magnetization direction. Electron scattering and interference imaging further demonstrates the tunable nature of a subset of related electronic states. Our experiments provide a direct visualization of how in-situ spin reorientation drives changes in the electronic density of states of the Weyl fermion band structure. Combined with previous reports of massive Dirac fermions, flat bands, and electronic nematicity, our work establishes Fe3Sn2 as an interesting platform that harbors an extraordinarily wide array of topological and correlated electron phenomena