Coherent manipulation of quasiparticles is a crucial method to realize ultrafast switching of the relating macroscopic order. In this paper, we studied coherent phonon generation under strong light field which allows one to induce nonperturbative nonlinear optical phenomena in solids. The efficiency of coherent phonon generation starts to saturate and deviate from the pronounced linear power dependence when the light intensity goes into the nonperturbative regime. It is clarified that the deviation is not originated from the absorption saturation due to the real incoherent carrier generation. We propose a theoretical model based on the Floquet picture and show that the saturation is due to a limitation of the driving force inherent in nonresonant driving of the electronic system in the nonperturbative regime.

Low-barrier nanomagnets have attracted a great deal of research interest for their use as sources of high-quality true random number generation. More recently, low-barrier nanomagnets with tunable output have been shown to be a natural hardware platform for unconventional computing paradigms such as probabilistic spin logic. Efficient generation and tunability of high-quality random bits is critical for these novel applications. However, current spintronic random number generators are based on superparamagnetic tunnel junctions with tunability obtained through spin transfer torque, which unavoidably leads to challenges in designing concatenated networks using these two terminal devices. The more recent development of utilizing spin orbit torque (SOT) allows for a three-terminal device design, but can only tune in-plane magnetization freely, which is not very energy efficient due to the needs of overcoming a large demagnetization field. In this work, we experimentally demonstrate a stochastic device with perpendicular magnetic anisotropy that is completely tunable by SOT without the aid of any external magnetic field. Our measurements lead us to hypothesize that a tilted anisotropy might be responsible for the observed tunability. We carry out stochastic Landau-Lifshitz-Gilbert simulations to confirm our experimental observation. Finally, we build an electrically coupled network of two such stochastic nanomagnet-based devices and demonstrate that finite correlation or anticorrelation can be established between their output fluctuations by a weak interconnection, despite having a large difference in their natural fluctuation timescale. Simulations based on a dynamical model for autonomous circuits composed of low-barrier nanomagnets show close agreement with the experimental results.

We investigate the mechanisms of nuclear decoupling in synthetically tailored graphenoids, where the electron spin state is introduced by topological manipulation of the lattice. We compare molecular graphenoids containing one and two spin centers, introduced by pentagonal rings in the honeycomb lattice. Exploiting the molecular nature of the systems, we investigate the role of different nuclear species and environments. Variations on the Carr-Purcell-Meiboom-Gill pulse trains are used to prolong the coherence time of the electron spin of the radicaloids, leading to substantial improvements in performance and coherence times up to 300 $\mu \mathrm{s}$ at liquid-nitrogen temperature. The investigation of electron spin coherence as a function of interpulse spacing, with times close to the inverse of the nuclear precession frequency, reveals that a train of pulses in phase with the nuclear precession maximizes the nuclear decoupling. At room temperature the limits imposed by the sample treatment and environment are reached, indicating what amelioration is necessary to further enhance the quantum performance.

We present an application of a high-throughput search of new rare-earth free permanent magnets focusing on $3d\text{−}5d$ transition metal compounds. The search involved a part of the Inorganic Crystal Structure Database, together with tailored search criteria and electronic structure calculations of magnetic properties. Our results suggest that it is possible to find candidates for rare-earth free permanent magnets using a data-mining/data-filtering approach. The most promising candidates identified here are ${\mathrm{Pt}}_2\mathrm{FeNi}$, ${\mathrm{Pt}}_2\mathrm{FeCu}$, and ${\mathrm{W}}_2{\mathrm{FeB}}_2$. We suggest these materials to be a good platform for further investigations in the search of novel rare-earth free permanent magnets.

We study the finite-time dynamics of an initially localized wave packet in the Anderson model on the random regular graph (RRG) and show the presence of a subdiffusion phase coexisting both with ergodic and putative nonergodic phases. The full probability distribution $\mathrm{\Pi }(x,t)$ of a particle to be at some distance $x$ from the initial state at time $t$ is shown to spread subdiffusively over a range of disorder strengths. The comparison of this result with the dynamics of the Anderson model on ${\mathbb{Z}}^d$ lattices, $d>2$, which is subdiffusive only at the critical point implies that the limit $d\to \infty$ is highly singular in terms of the dynamics. A detailed analysis of the propagation of $\mathrm{\Pi }(x,t)$ in space-time $(x,t)$ domain identifies four different regimes determined by the position of a wave front $X_{\text{front}}\left(t\right)$, which moves subdiffusively to the most distant sites $X_{\text{front}}\left(t\right)\sim t^{\beta }$ with an exponent $\beta <1$. Importantly, the Anderson model on the RRG can be considered as proxy of the many-body localization transition (MBL) on the Fock space of a generic interacting system. In the final discussion, we outline possible implications of our findings for MBL.

We use a projective symmetry group analysis to determine all symmetric spin liquids on the stuffed honeycomb lattice Heisenberg model. This lattice interpolates between honeycomb, triangular, and dice lattices, always preserving hexagonal symmetry, and it already has one spin-liquid candidate, ${\mathrm{TbInO}}_3$, albeit with strong spin-orbit coupling not considered here. In addition to the stuffed honeycomb lattice itself, we gain valuable insight into potential spin liquids on the honeycomb and triangular lattices, as well as how they might be connected. For example, the sublattice pairing state proposed on the honeycomb lattice connects to the uniform spinon Fermi surface that may be relevant for the triangular lattice with ring exchange, while there are no spin liquids competitive on both the $J_1-J_2$ honeycomb and triangular lattice limits. In particular, we find three stuffed honeycomb descendants of the U(1) Dirac spin liquid widely believed to be found on the $J_1-J_2$ triangular lattice. We also discuss how spin liquids near the honeycomb limit can potentially explain the physics of ${\mathrm{LiZn}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$.

We have investigated the electron pair emission due to single-photon absorption from Ag(100) and Pd(100) surfaces. We are interested in the energy spectra of pairs in particular near the energy cutoff. The sum energy spectra of Ag display a distinctive photon energy dependence. We also observe some fine structure. Near the high-energy cutoff the coincidence rate is too low to determine the energy position of the cutoff. Nevertheless we observe a finite signal if two $5sp$ electrons near the Fermi level are emitted. For Pd(100) we find sum energy spectra without fine structure and the cutoff region is approached linearly. Within the experimental accuracy the minimum energy to liberate two electrons is twice the work function.

We theoretically study how one can control and enhance nonlinear thermoelectricity by regulating quantum coherence in nanostructures such as a quantum dot system or a single-molecule junction. In nanostructures, the typical temperature scale is much smaller than the resonance width, which largely suppresses thermoelectric effects. Yet we demonstrate one can achieve a reasonably good thermoelectric performance by regulating quantum coherence. Engaging a quantum-dot interferometer (a quantum dot embedded in the ring geometry) as a heat engine, we explore the idea of thermoelectric enhancement induced by the Fano resonance. We develop an analytical treatment of fully nonlinear responses for a dot with or without strong interaction. Based on the microscopic model with the nonequilibrium Green function technique, we show how to enhance efficiency and/or output power as well as where to locate an optimal gate voltage. We also argue how to assess nonlinear thermoelectricity by linear-response quantities.

We examine the presence of nonmagnetic impurities in a hybridization gap model of a Kondo insulator which has band inversion. The model has been used to predict that ${\mathrm{SmB}}_6$ is a topological insulator. We show that there are two types of nonmagnetic impurity states in a Kondo insulator. The type of states can be categorized as deep impurity states, similar to the impurity states in an ordinary metal, and impurity states which have energies within the hybridization gap. Unlike the deep impurity states which only form if the impurity potential exceeds a critical value of the order of the conduction bandwidth, the in-gap impurity states form for exceptionally small values of the impurity potential comparable to the hybridization gap. This result may explain why Kondo insulators are found to be exceptionally sensitive to impurities. We show that these in-gap states are caused by band inversion and have properties similar to those expected for impurities in a topological insulator.

The lattice dynamics in magnetic materials, such as Fe depends on the degree of disorder of the atomic magnetic moments and the time scale of spin fluctuations. Using first-principles methods, we have studied this effect by determining the force constant matrix in two limits: (i) When spin fluctuations are much faster than the atom vibrations, their combined impact is captured by a spin-space averaged force constant matrix, (ii) when individual spin fluctuations are sufficiently slow to scatter the phonon modes, the itinerant coherent potential approximation with spin-pair resolved force constants (i.e., ${\mathbf{\Phi }}_{\uparrow \uparrow },{\mathbf{\Phi }}_{\downarrow \downarrow }$, and ${\mathbf{\Phi }}_{\uparrow \downarrow }$) is employed in this paper. The physical consequences for the vibrational spectral functions are analyzed by systematically modifying the input parameters (magnetization and ratio of force constants betweens atoms with equal and opposite spin directions) and by deriving them for the prototype material system bcc Fe from first-principles calculations. In the paramagnetic regime, the two limits yield identical phonon spectra. Below the Curie temperature, however, there are regions in the parametric study that show qualitative differences, including a broadening of the phonon peaks. For bcc Fe, however, the quantitative modifications of phonon frequencies turn out to be small.

${}^{55}\mathrm{Mn}\mathrm{NMR}$ was used to investigate the effect of carbon doping on the local magnetic anisotropy in ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ epitaxial films (space-group $P{6}_3/mcm$). It was found that carbon enters interstitially in the vicinity of the $6\left(g\right)$ crystallographic positions, occupying the $2\left(b\right)$ octahedral voids. The magnetic properties of the Mn atoms located in the corners of a host octahedron are strongly modified by the presence of carbon. Their magnetic moment is reduced by $0.7{\mu }_B$ with respect to the pristine ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ film and the anisotropy of their orbital moment, measured as a difference between its value along the hexagonal $c$ direction and on the $c$ plane is reduced to $0.058{\mu }_B$, whereas, in the pristine ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ films, it oscillated every ${60}^{\circ }$ between $0.151{\mu }_B$ and 0. These changes are responsible for a significant decrease in magnetocrystalline anisotropy, which was reported to drop by an order of magnitude upon doping the ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ films with carbon.

We consider alignment-dependent spin and heat transport across a magnon spin valve in the tunneling regime, i.e., a junction consisting of two weakly coupled ferromagnetic insulators. We determine the difference in spin and heat conductance between the parallel and antiparallel configuration of the magnetization direction. The dependence of these conductances on both the Gilbert damping and ellipticity is studied. We find that both magnon ellipticity and dissipation open channels for magnons to tunnel through in the antiparallel configuration. Our results highlight an important difference between electronic and magnon spin transport in spin-valve structures and may be important for the development of devices based on magnetic insulators.

Several generalizations of one-dimensional and two-dimensional exactly solvable low-dimensional quantum spin-1/2 models, some of which contain off-diagonal terms of the exchange tensor, are proposed. It is shown that off-diagonal terms of the exchange (symmetric or nonsymmetric with respect to the exchange of spins) yield renormalization of the diagonal ones and phase shifts. The latter can be moved to eigenfunctions of the models and do not affect eigenvalues for open geometries of the lattices. For closed geometries of the lattices the phase shifts manifest themselves in the finite size corrections. The advantage of the proposed models is in their simplicity and in possible realizations of the models in a number of applications, e.g., for the description of the wide variety of correlated electron systems, ultracold atoms, and in the theory of topological quantum computation.

Noncollinear antiferromagnets with a $D{0}_{19}$ ($\mathrm{space}\mathrm{group}=194$, $P{6}_3/mmc$) hexagonal structure have garnered much attention for their potential applications in topological spintronics. Here, we report the deposition of continuous epitaxial thin films of such a material, ${\mathrm{Mn}}_3\mathrm{Sn}$, and characterize their crystal structure using a combination of x-ray diffraction and transmission electron microscopy. Growth of ${\mathrm{Mn}}_3\mathrm{Sn}$ films with both (0001) $c$-axis orientation and ($40\overline{4}3$) texture is achieved. In the latter case, the thin films exhibit a small uncompensated Mn moment in the basal plane, quantified via magnetometry and x-ray magnetic circular dichroism experiments. This cannot account for the large anomalous Hall effect simultaneously observed in these films, even at room temperature, with magnitude ${\sigma }_{xy}\left({\mu }_{0}H=0\mathrm{T}\right)=21{\mathrm{\Omega }}^{-1}\mathrm{c}{\mathrm{m}}^{-1}$ and coercive field ${\mu }_0{H}_c=1.3\mathrm{T}$. We attribute the origin of this anomalous Hall effect to momentum-space Berry curvature arising from the symmetry-breaking inverse triangular spin structure of ${\mathrm{Mn}}_3\mathrm{Sn}$. Upon cooling through the transition to a glassy ferromagnetic state at around 50 K, a peak in the Hall resistivity close to the coercive field emerges. This indicates the onset of a topological Hall effect contribution, arising from a nonzero scalar spin chirality that generates a real-space Berry phase. We demonstrate that the polarity of this topological Hall effect, and hence the chiral nature of the noncoplanar magnetic structure driving it, can be controlled using different field-cooling conditions.

Coupling between $\sigma$-bonding electrons and phonons is generally very strong. To metallize $\sigma$ electrons provides a promising route to hunt for new high-$T_c$ superconductors. Based on this picture and first-principles density functional calculation with Wannier interpolation for electronic structure and lattice dynamics, we predict that trilayer film ${\mathrm{LiB}}_2{\mathrm{C}}_2$ is a good candidate to realize this kind of high-$T_c$ superconductivity. By solving the anisotropic Eliashberg equations, we find that free-standing trilayer ${\mathrm{LiB}}_2{\mathrm{C}}_2$ is a phonon-mediated superconductor with $T_c$ exceeding the liquid-nitrogen temperature at ambient pressure. The transition temperature can be further raised to 125 K by applying a biaxial tensile strain.

We calculate analytically the spectrum of the Andreev bound states in a half-infinite superconducting wire with an arbitrary number of bands crossing the chemical potential. The normal state of the wire is assumed to have an antiunitary symmetry $\mathcal{A}$ (time reversal or its combination with a crystallographic point-group operation) with ${\mathcal{A}}^2=-1$ or $+1$. This symmetry may be broken by the superconducting order parameter and/or the boundary scattering. We present a model-independent proof of the existence of one Majorana mode near the end of the wire with an odd number of bands.