We theoretically explore nonlinearities of ferromagnets in microwave cavities in the classical and quantum regimes and assess the resources for quantum information, i.e., fluctuation squeezing and bipartite entanglement. The (semi)classical analysis of the anharmonic oscillator (Duffing) model for the Kittel mode when including all other magnon modes, reveals chaotic and limit-cycle phases that do not survive in quantum calculations. However, magnons with nonzero wave numbers that are driven by the Suhl instability of the Kittel mode, form a genuine limit cycle. We subsequently compute bounds for the distillable entanglement, as well as entanglement of formation for the bipartite configurations of the mixed magnon modes. The former vanishes when obtained from a covariance matrix, but can be recovered by injection locking. The predicted magnon entanglement is experimentally accessible with yttrium iron garnet samples under realistic conditions.

In this study, the inelastic neutron scattering probe of SIKA in ANSTO is employed to investigate the magnon dispersion curve in ferromagnetic $\mathrm{SrRu}{\mathrm{O}}_3$ single crystal epitaxial films and to better understand the underlying mechanisms. This report presents the successful measurement of a magnon peak from the $\mathrm{SrRu}{\mathrm{O}}_3$ films which contained an amount of material of only 0.9 mg. We reveal one significant magnon dispersion curve along [002] following the quadratic ($E\propto Q^2$) relation, which shows a magnon gap of 0.32 meV. We have discussed several possible mechanisms, such as the higher symmetry structure and the impurity levels, which may contribute to this smaller gap.

Spin transport of magnonic excitations in uniaxial insulating antiferromagnets (AFs) is investigated. In linear response to spin biasing and a temperature gradient, the spin-transport properties of normal-metal-insulating antiferromagnet-normal-metal heterostructures are calculated. We focus on the thick-film regime, where the AF is thicker than the magnon equilibration length. This regime allows the use of a drift-diffusion approach, which is opposed to the thin-film limit considered by Bender et al. [Phys. Rev. Lett. 119, 056804 (2017)], where a stochastic approach is justified. We obtain the temperature and thickness dependence of the structural spin Seebeck coefficient $\mathcal{S}$ and magnon conductance $\mathcal{G}$. In their evaluation, we incorporate effects from field- and temperature-dependent spin conserving intermagnon scattering processes. Furthermore, the interfacial spin transport is studied by evaluating the contact magnon conductances in a microscopic model that accounts for the sublattice symmetry breaking at the interface. We find that while intermagnon scattering does slightly suppress the spin Seebeck effect, transport is generally unaffected, with the relevant spin decay length being determined by non-magnon-conserving processes such as Gilbert damping. In addition, we find that while the structural spin conductance may be enhanced near the spin flip transition, it does not diverge due to spin impedance at the normal metal magnet interfaces.

We study the emergence and dynamics of in-plane skyrmions in collinear ferromagnetic heterostructures. We present a minimal energy model for this class of magnetic textures, determine the crystal symmetries compatible with it, and propose material candidates, based on symmetries only, for the observation of these topological solitons. We calculate exact solutions of the energy model for in-plane skyrmions in the absence of dipolar interactions at critical coupling, the latter defined by the relations $H=K$ and $D=\sqrt{AK}$ for the strength of the external magnetic field and the Dzyaloshinskii coupling constant, respectively, with $K$ and $A$ being the anisotropy constant and the exchange stiffness of the material. Through micromagnetic simulations we demonstrate the possibility of in-plane skyrmion production via (i) the motion of domain walls through a geometrical constriction and (ii) shedding from a magnetic impurity driven both by spin transfer torques. In-plane skyrmion dynamics triggered by spin-orbit torques are also investigated analytically and numerically. Our findings point towards the possibility of designing racetracks for in-plane skyrmions, whose speed could be tuned by adjusting the angle between the charge current and the uniform background magnetization; in particular, the speed is maximum for currents parallel to the easy axis and becomes zero for currents transverse to it.

We consider a flexible mechanical metamaterial comprising an elastomeric matrix with an embedded square array of circular holes. First, we use the deflated continuation technique of bifurcation analysis to explore its complex energy landscape, characterized by multiple bifurcations from which stable and unstable branches emanate. We then investigate how this landscape can be used to design materials with advanced programmability. We find that the response of the system can be constantly reprogrammed through local manipulation, moving it from one stable branch to another, and that small targeted imperfections can be harnessed to enhance such programmability.

We calculate Heisenberg-type magnetic exchange interactions for ${\mathrm{SrMnO}}_3$ under isotropic volume expansion by using an approach that is based on total-energy variations due to infinitesimal spin rotations around a given reference state. Our total-energy calculations using density-functional theory (DFT) indicate a transition from antiferromagnetic to ferromagnetic coupling for increasing interatomic distances, corresponding to a sign change of the nearest-neighbor exchange interaction. This sign change cannot easily be understood from a standard superexchange mechanism. Furthermore, the exchange interaction strongly depends on the corresponding reference state. This “non-Heisenberg” behavior increases with increasing volume and is also confirmed through noncollinear DFT calculations. An orbital- and energy-resolved decomposition of the exchange coupling suggests that an increased partial occupancy of $e_g$ orbitals near the Fermi level is crucial both for the sign change and the non-Heisenberg behavior of the nearest-neighbor interaction. Furthermore, even though both $e_g$ and $t_{2g}$ contributions to the exchange interactions decay exponentially for large interatomic distances, the $e_g$ contribution remains surprisingly strong over relatively large distances along the crystal axes.

We reconsider the problem of the critical behavior of a three-dimensional $O\left(m\right)$ symmetric magnetic system in the presence of random-anisotropy disorder with a generic trimodal random-axis distribution. By introducing $n$ replicas to average over disorder it can be coarse grained to a ${\varphi }^4$ theory with an $m\times n$ component order parameter and five coupling constants taken in the limit of $n\to 0$. Using a field theory approach we renormalize the model to two-loop order and calculate the $\beta$ functions within the $\varepsilon$ expansion and directly in three dimensions. We analyze the corresponding renormalization group flows with the help of the Padé-Borel resummation technique. We show that there is no stable fixed point accessible from physical initial conditions whose existence was argued in previous studies. This may indicate the absence of a long-range ordered phase in the presence of random-anisotropy disorder with a generic random-axis distribution.

Motivated by the complex behavior of the frustrated magnet $\beta -{\mathrm{TeVO}}_4$, we study an anisotropic Heisenberg model for coupled spin-$1/2$ zigzag chains. Using a cluster mean field approach to capture quantum correlations we find, upon reducing temperature in the absence of applied field, (i) a partially ordered state, (ii) a collinear antiferromagnetic phase, and (iii) an elliptical spiral state characterized by finite vector chirality. For finite fields we find a metamagnetic response close to saturation magnetization. We show via explicit calculations that the quadrupolar order parameter is finite in the metamagnetic regime. The exchange parameters reported in the ab initio study of $\beta -{\mathrm{TeVO}}_4$ are used in our study. We compare our results with those reported in recent experiments on $\beta -{\mathrm{TeVO}}_4$ and highlight similarities as well as differences between experimental results and our cluster mean field calculations.

We report characteristic vortex configurations in $s+id$ superconductors with time-reversal symmetry breaking, exposed to magnetic field. A vortex in the $s+id$ state tends to have an opposite phase winding between $s$- and $d-\mathrm{wave}$ condensates. We find that this peculiar feature together with the competition between $s$- and $d-\mathrm{wave}$ symmetry results in three distinct classes of vortical configurations. When either $s$ or $d$ condensate absolutely dominates, vortices form a conventional lattice. However, when one condensate is relatively dominant, vortices organize in chains that exhibit skyrmionic character, separating the chiral components of the $s\pm id$ order parameter into domains within and outside the chain. Such skyrmionic chains are found stable even at high magnetic field. When $s$ and $d$ condensates have comparable strength, vortices split cores in two chiral components to form full-fledged skyrmions, i.e., coreless topological structures with an integer topological charge, organized in a lattice. We provide characteristic magnetic field distributions of all states, enabling their identification in, e.g., scanning Hall probe and scanning SQUID experiments. These unique vortex states are relevant for high-$T_c$ cuprate and iron-based superconductors, where the relative strength of competing pairing symmetries is expected to be tuned by temperature and/or doping level, and can help distinguish $s+is$ and $s+id$ superconducting phases.

We present a way to quantum-disorder a pair density wave and propose it to be a candidate of the effective low-energy description of the pseudogap metal which may reveal itself in a sufficiently high magnetic field that suppresses the $d$-wave pairing. The ground state we construct is a small-pocket Fermi liquid with a bosonic Mott insulator in the density-wave-enlarged unit cell. At low energy, the charge density is mainly carried by charge $2e$ bosons, which develop a small insulating gap. As an intermediate step, we discuss the quantum disordering of a fully gapped superconductor and its excitation spectrum. A simplified 1D model, which we solve numerically, is used to illustrate the introduced concepts. We discuss a number of experimental consequences. The interplay between the electron and the small-gap boson results in a step-function background in the electron spectral function which may be consistent with existing angle-resolved photoemission spectroscopy data. Optical excitation across the boson gap can explain the onset and the magnitude of the mid infrared absorption reported long ago.

Recently, it was shown that by means of an scanning tunneling microscope it is experimentally possible to stimulate clock transitions between the singlet and the nonmagnetic triplet state of a Heisenberg-coupled spin dimer [Bae et al., Sci. Adv. 4, eaau4159 (2018)]. This leads to more strongly protected clock transitions while ordinary ones only provide first-order protection against magnetic noise. However, large decoherence times of clocklike states normally refer to ensembles of spins which do not dephase. In the cited experiment, only one single dimer is manipulated and not an ensemble. For this reason, we simulate how a single dimer behaves in an environment of other spins which couple to the dimer via dipolar interactions. We perform unitary time evolutions in the complete Hilbert space, including dimer and a reasonably large environment. We will see that for a weak environment, this approach confirms long decoherence times for the clocklike state, but with stronger couplings this statement does not hold. As a reference, we compare the behavior of the dimer with other, non-clock-like, superposition states. Furthermore, we show that the internal dynamics of the bath plays an important role for the decoherence time of the system. In a regime where the system is weakly coupled to the bath, stronger interactions among environmental spins worsen the decoherence time up to a certain degree, while if system and bath are strongly coupled, stronger interactions in the environment improve decoherence times.

The phase diagram of the filled skutterudite ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ has been mapped in fields ${\mu }_{0}H$ of up to 60 T and temperatures $T$ down to 0.5 K using resistivity, magnetostriction, and megahertz conductivity. The valence transition separating the semimetallic low-$H$, low-$T$ $\mathcal{L}$ phase from the metallic high-$H$, high-$T$ $\mathcal{H}$ phase exhibits a very unusual, wedge-shaped phase boundary, with a nonmonotonic gradient alternating between positive and negative. The expected “elliptical” behavior of the phase boundary of a valence transition with $H^2\propto T^2$ originates in the $H$ and $T$ dependence of the free energy of the $f$ multiplet. Here, quantum oscillation measurements suggest that additional energy scales associated with a quantum critical point are responsible for the deviation of the phase boundary of ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ from this textbook behavior at high $H$ and low $T$. The distortion of the low-$H$, high-$T$ portion of the phase boundary may be associated with the proximity of ${\mathrm{CeOs}}_4{\mathrm{Sb}}_{12}$ to a topological semimetal phase induced by uniaxial stress.

While x-ray absorption spectroscopy is mainly governed by electric-dipole transitions, the technique of nonresonant inelastic x-ray scattering (NIXS) offers the possibility to explore higher-order multipole transitions. These transitions obey different selection rules that can reach final states of higher angular momenta, opening complementary spectroscopic perspectives. Here, we investigate the suitability of NIXS to study transuranium compounds. We show that the $K$ edge of the Be encapsulation can be practically fully excluded by using the imaging capabilities of the technique arising from the position of the signals on the multidetector. Experimental results for the multipole transitions at the actinide $O_{4,5}$ edges (90–120 eV) in ${\mathrm{UO}}_2$, ${\mathrm{NpO}}_2$, ${\mathrm{PuO}}_2$, and ${\mathrm{Pu}}_2{\mathrm{O}}_3$ are compared with multielectronic calculations. The spectral features are shown to be very sensitive to the ratio of the triakontadipole and octupole transitions, which could potentially be used to assess the radial expansion of the $5f$ wave function, which is expected to occur in covalent mixing with the O $2p$ states.

The dynamics of band-gap renormalization and gain build-up in monolayer ${\mathrm{MoTe}}_2$-H is investigated by evaluating the nonequilibrium Dirac-Bloch equations with the incoherent carrier-carrier and carrier-phonon scattering treated via quantum-Boltzmann type scattering equations. For the case where an approximately 300-fs-long high-intensity optical pulse generates charge-carrier densities in the gain regime, the strong Coulomb coupling leads to a relaxation of excited carriers on a few-femtosecond timescale. The pump-pulse generation of excited carriers induces a large band-gap renormalization during the timescale of the pulse. Efficient phonon coupling leads to a subsequent carrier thermalization within a few picoseconds, which defines the timescale for the optical gain build-up energetically close to the low-density exciton resonance.

The conductance of bulk metal–insulator–metal junctions is evaluated by the Landauer formula using an ab initio electronic structure calculated using a plane-wave basis set within density functional theory and beyond, i.e., including exact nonlocal exchange using hybrid functional (HSE) or many-body $G_0{W}_0$ and Coulomb-hole screened exchange (COHSEX) quasiparticle schemes. We consider an Ag/MgO/Ag heterostructure model and we focus on the evolution of the zero-bias conductance as a function of the MgO film thickness. Our study shows that the correction of the electronic structure beyond semilocal density functionals goes in the right direction to improve the agreement with experiments, significantly reducing the zero-bias conductance. This effect becomes more evident at larger MgO thickness, that is, in increasing tunneling regime. We also observe that the reduction of the conductance seems more related to the correction of wave functions rather than energies, and thus not directly related to the correction of the band gap of bulk MgO. $G_0{W}_0$ and HSE both provide a band gap in agreement with experiments, but only HSE gives a significant reduction of the conductance. COHSEX, while overestimating the band gap, gives a reduction of the conductance very close to HSE.

We develop a microscopic theory of spin noise in solid-state systems at electron paramagnetic resonance, when the spin dynamics is driven by static and radio-frequency (rf) magnetic fields and the stochastic effective magnetic field stemming from the interaction with environment. The rf field splits the peaks in the power spectrum of spin noise into the Mollow-type triplets and also gives rise to additional spin-spin correlations which oscillate in the absolute time at the rf frequency and the double frequency. Even in systems with strong inhomogeneous broadening, the spin-noise spectrum contains narrow lines insensitive to the dispersion of the effective $g$ factors. Thus, the measurements of spin noise at electron paramagnetic resonance provide an access to the intrinsic spin lifetime of electrons.

Topological quantum computation encodes quantum information nonlocally by nucleating non-Abelian anyons separated by distances $L$, typically spanning the qubit device size. This nonlocality renders topological qubits exponentially immune to dephasing from all sources of classical noise with operator support local on the scale of $L$. We perform detailed analytical and numerical analyses of a time-domain Ramsey-type protocol for noisy Majorana-based qubits that is designed to validate this coveted topological protection in near-term devices such as the so-called “tetron” design. By assessing dependence of dephasing times on tunable parameters, e.g., magnetic field, our proposed protocol can clearly distinguish a bona fide Majorana qubit from one constructed from semilocal Andreev bound states, which can otherwise closely mimic the true topological scenario in local probes. In addition, we analyze leakage of the qubit out of its low-energy manifold due to classical-noise-induced generation of quasiparticle excitations; leakage limits the qubit lifetime when the bulk gap collapses, and hence our protocol further reveals the onset of a topological phase transition. This experiment requires measurement of two nearby Majorana modes for both initialization and readout—achievable, for example, by tunnel coupling to a nearby quantum dot—but no further Majorana manipulations, and thus constitutes an enticing prebraiding experiment. Along the way, we address conceptual subtleties encountered when discussing dephasing and leakage in the context of Majorana qubits.

Kitaev's honeycomb spin-liquid model and its proposed realization in materials such as $\alpha \text{−}{\mathrm{RuCl}}_3$, ${\mathrm{Li}}_2{\mathrm{IrO}}_3$, and ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ continue to present open questions about how the dynamics of a spin liquid are modified in the presence of non-Kitaev interactions as well as the presence of inhomogeneities. Here we use ${}^{23}\mathrm{Na}$ nuclear magnetic resonance to probe both static and dynamical magnetic properties in single-crystal ${\mathrm{Na}}_2{\mathrm{IrO}}_3$. We find that the NMR shift follows the bulk susceptibility above 30 K but deviates from it below; moreover below $T_N$ the spectra show a broad distribution of internal magnetic fields. Both of these results provide evidence for inequivalent magnetic sites at low temperature, suggesting inhomogeneities are important for the magnetism. The spin-lattice relaxation rate is isotropic and diverges at $T_N$, suggesting that the Kitaev cubic axes may control the critical quantum spin fluctuations. In the ordered state, we observe gapless excitations, which may arise from site substitution, emergent defects from milder disorder, or possibly be associated with nearby quantum paramagnetic states distinct from the Kitaev spin liquid.

Layered perovskite iridate ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ shares many similarities with high $T_{\text{c}}$ cuprates and is expected to host novel superconductivity but has never been realized experimentally. Despite the similarities, the prominent ${\mathrm{IrO}}_6$ octahedral rotations and sizable net canted antiferromagnetic moments lying in each ${\mathrm{IrO}}_2$ plane in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ are strikingly different from high $T_{\text{c}}$ cuprates where the octahedral rotations and net canted moment are much smaller or negligible. Here, using reactive molecular beam epitaxy, we demonstrate that the octahedral rotations around the in-plane and out-of-plane axes in epitaxial iridate films can be suppressed step-by-step via interfacial clamping imposed by cubic substrates as the films approach the two-dimensional limit. In situ angle-resolved photoemission spectroscopy and first-principles calculations show a gapped antiferromagnetic ground state with dispersive low-lying bands in 1- and 2-unit-cell-thick ${\mathrm{SrIrO}}_3$ films, providing ideal single- and bilayer analogies of high $T_{\text{c}}$ cuprates.

We report comprehensive temperature and doping dependences of the Raman scattering spectra for ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ ($x=0$, 0.07, 0.24, 0.32, and 0.38), focusing on the nematic fluctuation and the superconducting responses. With increasing $x$, the bare nematic transition temperature estimated from the Raman spectra reaches $T=0$ K at the optimal doping, which indicates a quantum critical point (QCP) at this composition. In the superconducting compositions, in addition to the pair-breaking peaks observed in the $A_{1g}$ and $B_{1g}$ spectra, another strong $B_{1g}$ peak appears below the superconducting transition temperature, which is ascribed to the nematic resonance peak. The observation of this peak indicates significant nematic correlations in the superconducting state near the QCP in this compound.