A magnetic Weyl semimetal is a recent focus of extensive research as it may exhibit large and robust transport phenomena associated with topologically protected Weyl points in momentum space. Since a magnetic texture provides a handle for the configu
ration of the Weyl points and its transport response, understanding of magnetic dynamics should form a basis of future control of a topological magnet. Mn3Sn is an example of an antiferromagnetic Weyl semimetal that exhibits a large response comparable to the one observed in ferromagnets despite a vanishingly small magnetization. The non-collinear spin order in Mn3Sn can be viewed as a ferroic order of cluster magnetic octupole and breaks the time-reversal symmetry, stabilizing Weyl points and the significantly enhanced Berry curvature near the Fermi energy. Here we report our first observation of time-resolved octupole oscillation in Mn3Sn. In particular, we find the giant effective damping of the octupole dynamics, and it is feasible to conduct an ultrafast switching at < 10 ps, a hundred times faster than the case of spin-magnetization in a ferromagnet. Moreover, high domain wall velocity over 10 km/s is theoretically predicted. Our work paves the path towards realizing ultrafast electronic devices using the topological antiferromagnet.
Twisted van der Waals materials open up novel avenues to control electronic correlation and topological effects. These systems contain the unprecedented possibility to precisely tune strong correlations, topology, magnetism, nematicity, and supercond
uctivity with an external non-invasive electrostatic doping. By doing so, rich phase diagrams featuring an interplay of different states of correlated quantum matter can be unveiled. The nature of the superconducting order presents a recurring overarching open question in this context. In this work, we quantitatively assess the case of spin-fluctuation-mediated pairing for $Gamma$-valley twisted transition metal dichalcogenide homobilayers. We construct a low-energy honeycomb model on which basis we self-consistently and dynamically calculate a doping dependent phase diagram for the superconducting transition temperature $T_{mathrm{c}}$. A superconducting dome emerges with a maximal $T_{mathrm{c}}approx$ 0.1-1 K depending on twist angle. We qualitatively compare our results with conventional phonon-mediated superconductivity and discern clear fingerprints which are detectable in doping-dependent measurements of the superconducting transition temperature, providing direct access to probing the superconducting pairing mechanism in twisted Van der Waals materials.
In this work, we show that the same theoretical tools that successfully explain other hydrides systems under pressure seem to be at odds with the recently claimed conventional room temperature superconductivity of the carbonaceous sulfur hydride. We
support our conclusions with I) the absence of a dominant low-enthalpy stoichiometry and crystal structure in the ternary phase diagram. II) Only the thermodynamics of C-doping phases appears to be marginally competing in enthalpy against H$_3$S. III) Accurate results of the transition temperature given by ab initio Migdal-Eliashberg calculations differ by more than 110 K to recently theoretical claims explaining the high-temperature superconductivity in carbonaceous-hydrogen sulfide. A novel mechanism of superconductivity or a breakdown of current theories in this system is possibly behind the disagreement.
Intuitively, doping represents one of the most promising avenues for optimization of best prospect superconductors (SC) such as conventional high-pressure SCs with record critical temperatures. However, doping at high pressure (HP) is very challengin
g, and there is not a proved route to achieve it in a controlled fashion. Aided by computing simulations, we show that it may be plausible to start by alloying primary materials and subsequently incorporate high ratios of hydrogen at moderates pressures ($approx$1.5 Mbar). Our theoretical results evidence the possibility to tune the electronic structure of LaH$_{10}$, increase the density of states at the Fermi level by doping of various elements and hence change their superconducting properties. We found aluminium to increase the occupation at the Fermi level by more than 30 %. Incorporation of other elements such as Si, Ge, H, Ir, Ca, and others with a varying percentage also play in favour to tune the electronic structure. More importantly, these predictions lie in experimentally attainable doping levels. Also, for the first time, we shed light on how the formation of defects and vacancies influence on the electronic structure of a HP-hydride superconductor. The concepts presented in this work can be extended to other high-pressure, hydrogen-based superconductors such as H$_3$S. Arguably, doping is one of the promising paths to reach room-temperature superconductivity, a Holy grail of condensed matter physics.
We report strain engineering of superconductivity in RuO$_2$ singlecrystalline films, which are epitaxially grown on rutile TiO$_2$ and MgF$_2$ substrates with various crystal orientations. Systematic mappings between the superconducting transition t
emperature and the lattice parameters reveal that shortening of specific ruthenium-oxygen bonds is a common feature among the superconducting RuO$_2$ films. Ab initio calculations of electronic and phononic structures for the strained RuO$_2$ films suggest the importance of soft phonon modes for emergence of the superconductivity. The findings indicate that simple transition metal oxides such as with the rutile structure may be suitable for further exploring superconductivity by controlling phonon modes through the epitaxial strain.
Motivated by the recent discovery of superconductivity in doped NdNiO$_2$, we study the magnetic exchange interaction $J$ in layered $d^9$ nickelates from first principles. The mother compounds of the high-$T_{rm c}$ cuprates belong to the charge-tra
nsfer regime in the Zaanen-Sawatzky-Allen diagram and have $J$ larger than 100 meV. While this feature makes the cuprates very different from other transition metal oxides, it is of great interest whether layered $d^9$ nickelates can also have such a large $J$. However, one complexity is that NdNiO$_2$ is not a Mott insulator due to carrier doping from the block layer. To compare the cuprates and $d^9$ nickelates on an equal basis, we study RbCa$_2$NiO$_3$ and $A_{2}$NiO$_{2}$Br$_2$ ($A$: a cation with the valence of $2.5+$), which were recently designed theoretically by block-layer engineering. These nickelates are free from the self-doping effect and belong to the Mott-Hubbard regime. We show that these nickelates share a common thread with the high-$T_{rm c}$ cuprates in that they also have a significant exchange interaction $J$ as large as about 100 meV.
We formulate an efficient scheme to perform Migdal-Eliashberg calculation considering the retardation effect from first principles. While the conventional approach requires a huge number of Matsubara frequencies, we show that the intermediate represe
ntation of the Greens function [H. Shinaoka et al., Phys. Rev. B 96, 035147 (2017)] dramatically reduces the numerical cost to solve the linearized gap equation. Without introducing any empirical parameter, we demonstrate that we can successfully reproduce the experimental superconducting transition temperature of elemental Nb ($sim 10$ K) very accurately. The present result indicates that our approach has a superior performance for many superconductors for which $T_{rm c}$ is lower than ${mathcal O}(10)$ K
Recent development in exact classification of a superconducting gap has elucidated various unconventional gap structures, which have not been predicted by the classification of order parameter based on the point group. One of the important previous r
esults is that all symmetry-protected line nodes are characterized by nontrivial topological numbers. Another intriguing discovery is the gap structures depending on the angular momentum $j_z$ of normal Bloch states on threefold and sixfold rotational-symmetric lines in the Brillouin zone. Stimulated by these findings, we classify irreducible representations of the Bogoliubov-de Gennes Hamiltonian at each $boldsymbol{k}$ point on a high-symmetry $n$-fold ($n = 2$, $3$, $4$, and $6$) axis for centrosymmetric and paramagnetic superconductors, by using the combination of group theory and $K$ theory. This leads to the classification of all crystal symmetry-protected nodes (including $j_z$-dependent nodes) on the axis that crosses a normal-state Fermi surface. As a result, it is shown that the classification by group theory completely corresponds with the topological classification. Based on the obtained results, we discuss superconducting gap structures in SrPtAs, CeCoIn$_5$, UPt$_3$, and UCoGe.
Discoveries of marked similarities to high-$T_{text{c}}$ cuprate superconductors point to the realization of superconductivity in the doped $J_{text{eff}} = 1 / 2$ Mott insulator Sr$_2$IrO$_4$. Contrary to the mother compound of cuprate superconducto
rs, several stacking patterns of in-plane canted antiferromagnetic moments have been reported, which are distinguished by the ferromagnetic components as $-++-$, $++++$, and $-+-+$. In this paper, we clarify unconventional features of the superconductivity coexisting with $-++-$ and $-+-+$ structures. Combining the group theoretical analysis and numerical calculations for an effective $J_{text{eff}} = 1 / 2$ model, we show unusual superconducting gap structures in the $-++-$ state protected by nonsymmorphic magnetic space group symmetry. Furthermore, our calculation shows that the Fulde-Ferrell-Larkin-Ovchinnikov superconductivity is inevitably stabilized in the $-+-+$ state since the odd-parity magnetic $-+-+$ order makes the band structure asymmetric by cooperating with spin-orbit coupling. These unusual superconducting properties are signatures of magnetic multipole order in nonsymmorphic crystal.
We propose a spectroscopic method to identify the nodal gap structure in unconventional superconductors. This method best suits for locating the horizontal line node and for pinpointing the isolated point nodes by measuring polar angle ($theta$) reso
lved zero energy density of states $N(theta)$. This is measured by specific heat or thermal conductivity at low temperatures under a magnetic field. We examine a variety of uniaxially symmetric nodal structure, including point and/or line nodes with linear and quadratic dispersions, by solving Eilenberger equation in vortex states. It is found that (A) the maxima of $N(theta)$ continuously shift from the anti-nodal to the nodal direction ($theta_{rm n}$) as a field increases accompanying the oscillation pattern reversal at low and high fields. Furthermore, (B) local minima emerge next to $theta_{rm n}$ on both sides except for the case of linear point node. These features are robust and detectable experimentally. Experimental results of $N(theta)$ performed on several superconductors, UPd$_2$Al$_3$, URu$_2$Si$_2$, Cu$_x$Bi$_2$Se$_3$, and UPt$_3$, are examined and commented in light of the present theory.
