We investigate the influence of magnon decays on the nonequilibrium dynamics of parametrically excited magnons in the magnetic insulator yttrium-iron garnet (YIG). Our investigations are motivated by a recent experiment by Noack et al. [Phys. Status Solidi B 256, 1900121 (2019)] where an enhancement of the spin-pumping effect in YIG was observed near the magnetic field strength where magnon decays via confluence of magnons become kinematically possible. To explain the experimental findings, we have derived and solved kinetic equations for the nonequilibrium magnon distribution. The effect of magnon decays is taken into account microscopically via collision integrals derived from interaction vertices involving three powers of magnon operators. Our results agree quantitatively with the experimental data.

We investigate the structural and magnetic properties of the quantum magnet $\mathrm{Ba}\mathrm{Cu}{\mathrm{Te}}_2{\mathrm{O}}_6$. This compound is synthesized in powder and single crystal form for the first time. Synchrotron x-ray and neutron diffraction reveal a cubic crystal structure $\left(P{4}_{1}32\right)$ where the magnetic ${\mathrm{Cu}}^{2+}$ ions form a complex network. Heat capacity and static magnetic susceptibility measurements suggest the presence of antiferromagnetic interactions with a Curie-Weiss temperature of $\approx -33$ K, while long-range magnetic order occurs at the much lower temperature of $\approx 6.3$ K. The magnetic structure, solved using neutron diffraction, reveals antiferromagnetic order along chains parallel to the $a,$ $b,$ and $c$ crystal axes. This is consistent with the magnetic excitations which resemble the multispinon continuum typical of the spin-1/2 Heisenberg antiferromagnetic chain. A consistent intrachain interaction value of $\approx 34$ K is achieved from the various techniques. Finally the magnetic structure provides evidence that the chains are coupled together in a noncollinear arrangement by a much weaker antiferromagnetic, frustrated hyperkagome interaction.

We theoretically investigate domain-wall motion in an antiferromagnetic-insulator layer caused by thermally generated spin currents in an adjacent spin-split superconductor layer. An uncompensated antiferromagnet interface enables the two crucial ingredients underlying the mechanism—spin splitting in the superconductor and absorption of spin currents by the antiferromagnet. Treating the superconductor using the quasiclassical theory and the antiferromagnet via Landau-Lifshitz-Gilbert description, we find domain-wall propagation along the thermal gradient with relatively large velocities $\sim 100$ m/s. Our proposal exploits the giant thermal response of spin-split superconductors in achieving large spin torques towards driving domain wall and other spin textures in antiferromagnets.

Silicon vacancies in silicon carbide have been proposed as an alternative to nitrogen vacancy centers in diamonds for spintronics and quantum technologies. An important precondition for these applications is the initialization of the qubits into a specific quantum state. In this work, we study the optical alignment of the spin 3/2 negatively charged silicon vacancy in $6H\text{−}\mathrm{SiC}$. Using time-resolved optically detected magnetic resonance, we coherently control the silicon vacancy spin ensemble and measure Rabi frequencies, spin-spin and spin-lattice relaxation times of all three transitions. Then to study the optical initialization process of the silicon vacancy spin ensemble, the vacancy spin ensemble is prepared in different ground states and optically excited. We describe a simple rate equation model that can explain the observed behavior and determine the relevant rate constants.

The optical conductivity contains relevant information on the properties of correlated electron systems. In infinite dimensions, where dynamical mean field theory becomes exact, vertex corrections can be neglected and the conductivity computed from particle-hole bubbles. An interesting question concerns the nature and effect of the most relevant vertex corrections in finite-dimensional systems. A recent numerical study showed that the dominant vertex correction near an ordering instability with wave vector $\pi$ comes from a vertical ladder, analogous to the Maki-Thompson diagram. Since the random phase approximation version of this ladder diagram, dubbed $\pi$-ton, can be easily evaluated, this suggests a simple procedure for incorporating antiferromagnetic or charge density wave fluctuations into dynamical mean field estimates of the optical conductivity and related susceptibilities. We implement this procedure for the half-filled Hubbard model, considering the $\pi$-ton, a double-ladder extension of the $\pi$-ton and Aslamazov-Larkin-type diagrams, and reveal the spectral signatures of these vertex corrections.

Quantum Ising model on a triangular lattice hosts a finite temperature Berezinskii-Kosterlitz-Thouless (BKT) phase with emergent U(1) symmetry, and it will transit into an up-up-down (UUD) phase with $C_3$ symmetry breaking upon an infinitesimal external field along the longitudinal direction, but the overall phase diagram spanned by the axes of external field and temperature remains opaque due to the lack of systematic investigations with controlled methodologies. By means of quantum Monte Carlo at finite temperature and ground state density matrix renormalization group simulations, we map out the phase diagram of triangular quantum Ising model. Starting from the upper BKT temperature at zero field, we obtain the phase boundary between the UUD and paramagnetic phases with its 2D $q=3$ Potts universality at weak field and weakly first order transition at strong field. Originated from the lower BKT temperature at zero field, we analyze the low temperature phase boundary between the clock phase and the UUD phase with Ising symmetry breaking at weak fields and the quantum phase transition between the UUD and fully polarized phases at strong fields. The accurate many-body numerical results are consistent with our field theoretical analysis. The experimental relevance towards the BKT magnet ${\mathrm{TmMgGaO}}_4$ and programmable quantum simulators are also discussed.

We present a comprehensive study on the superconducting properties of ${\mathrm{Re}}_7{\mathrm{B}}_3$ and ${\mathrm{Re}}_3\mathrm{B}$ through specific heat, magnetic susceptibility, resistivity, and transverse and zero-field muon spin rotation/relaxation ($\mu \mathrm{SR}$) experiments on polycrystalline samples. ${\mathrm{Re}}_7{\mathrm{B}}_3$ ($T_C$ = 3.2 K) is a noncentrosymmetric type-II ($\kappa \simeq$ 9.27) superconductor in the weak-coupling (${\lambda }_{e-ph}$ = 0.54) regime. On the other hand, ${\mathrm{Re}}_3\mathrm{B}$ ($T_C$ = 5.19 K) is a centrosymmetric type-II ($\kappa \simeq$ 34.55) superconductor in the moderate-coupling (${\lambda }_{e-ph}$ = 0.64) regime. Our transverse-field $\mu \mathrm{SR}$ measurements show evidence for isotropically gapped BCS-type superconductivity with normalized gap (${\mathrm{\Delta }}_0/k_B{T}_C$) values of 1.69 (${\mathrm{Re}}_7{\mathrm{B}}_3$) and 1.75 (${\mathrm{Re}}_3\mathrm{B}$).

Robustness of helical hinge states of three-dimensional weak second-order topological insulators (WSOTIs) against disorders is studied. The pure WSOTI is obtained from a weak ${\mathbb{Z}}_2$ first-order topological insulator through a surface band inversion. Both bulk states and surface states in the WSOTI are gapped, and in-gap valley-momentum locked helical hinge states are topologically protected by the surface valley-Chern number. In the presence of weak disorders, helical hinge states are robust against disorders while the quantized conductance of the states is fragile due to the intervalley scattering. As disorder increases, the system undergoes a series of quantum phase transitions: from the WSOTI to the weak first-order topological insulator, then to a diffusive metal and finally to an Anderson insulator. Our results thus fully establish the WSOTI phase as a genuine state of matters and open a door for the second-order valleytronics that allows one to control the valley degree of freedom through helical hinge states.

In order to realize the significant potential of optical materials such as metal halides, computational techniques which give accurate optical properties are needed, which can work hand-in-hand with experiments to generate high efficiency devices. In this work a computationally efficient technique based on semiconductor Bloch equations (SBEs) is developed and applied to the material BiSBr. This approach gives excellent agreement with the experimental optical gap, and also agrees closely with the excitonic stabilisation energy and the absorption spectrum computed using the far more computationally demanding ab initio Bethe-Salpeter approach. The SBE method is a good candidate for theoretical spectroscopy on large- or low-dimensional systems which are too computationally expensive for an ab initio treatment.

We study the electroacoustic properties of aluminum scandium nitride crystals ${\mathrm{Al}}_{1-x}{\mathrm{Sc}}_x\mathrm{N}$ with the metastable wurtzite structure by means of first-principles calculations based on density functional theory. We extract the material property data relevant for electroacoustic device design, namely, the full tensors of elastic and piezoelectric constants. Atomistic models were constructed and analyzed for a variety of Sc concentrations $0\le x\le 50$%. The functional dependence of the material properties on the scandium concentration was extracted by fitting the data obtained from an averaging procedure for different disordered atomic configurations. We give an explanation of the observed elastic softening and the extraordinary increase in piezoelectric response as a function of Sc content in terms of an element-specific analysis of bond lengths and bond angles.

A multifunctional two-dimensional (2D) material is theoretically predicted based on systematic ab initio calculations and model simulations for the honeycomb lattice of endohedral fullerene $\mathrm{W}@{\mathrm{C}}_{28}$ molecules. It has structural bistability, ferroelectricity, multiple magnetic phases, and excellent valley characters and can be easily functionalized by the proximity effect with magnetic isolators such as $\mathrm{Mn}\mathrm{Ti}{\mathrm{O}}_3$. Furthermore, we may also manipulate the valley Hall and spin transport properties by selectively switching a few $\mathrm{W}@{\mathrm{C}}_{28}$ molecules to the metastable phase. These findings contribute to integrating different functions in a single 2D material for technological innovations.

We report an angle-resolved photoemission spectroscopy study of the antiferromagnetic Kondo lattice $\mathrm{Ce}{\mathrm{Pd}}_5{\mathrm{Al}}_2$, focusing on the quasi-two-dimensional $k$-space nature of its Fermi surface and, tuning photon energy to the Ce $4d–4f$ on-resonance transition, the hybridization of the Ce $4f$ state. A strong shoulder feature on the $f^0$ peak was detected, suggesting hybridization between conduction and $f$ bands. On-resonance spectra revealed narrow, yet hybridized quasiparticle bands with sharp peaks and $\sim$ 9 meV energy dispersion near the Fermi energy $E_F$. The observed dispersive hybridized $f$ band can be well described by a hybridization-band picture based on the periodic Anderson model (PAM). Hence, the $4f$ electrons in $\mathrm{Ce}{\mathrm{Pd}}_5{\mathrm{Al}}_2$ display a dual nature, with both localized and itinerant features, but with dominantly localized character.

The ground-state properties of CuFeAs were investigated by applying density functional theory calculations within the generalized gradient approximation (GGA) and $\mathrm{GGA}+U$. We find that the bicollinear antiferromagnetic state with antiparallel orbital magnetic moments on each iron which violates the Hund's rule is favored by the on-site Coulomb interaction, which is further stabilized by Cu vacancy. The magnetic ground state can be used to understand weak antiferromagnetism in CuFeAs observed experimentally. We argue that a breakdown of the Hund's rule may be the possible origin for reduced magnetism in iron pnictides, rather than magnetic fluctuations induced by electronic correlations.

Nonlinear optics, and particularly second harmonic generation (SHG), is increasingly used in many modern disciplines from material characterization in physical sciences to bioimaging in medicine and optical signal processing in information technology. We present a theoretical analysis yielding a strong estimate of the energy-integrated SHG response. Compact spherical multipoles are provided for the corresponding natural and magnetic circular dichroic signals. Like symmetry requirements in time and space are traced in the amplitude for magnetic neutron scattering, which includes all axial and polar (Dirac) contributions. Our method of working in terms of, now standard, electronic multipoles and Racah algebra, with full implementation of discrete symmetries, could be of use in a variety of other probes of matter.

We propose a novel realization for a topologically superconducting phase hosting Majorana zero modes on the basis of quantum spin Hall systems. Remarkably, our proposal is completely free of ferromagnets. Instead, we confine helical edge states around a narrow defect line of finite length in a two-dimensional topological insulator. We demonstrate the formation of a new topological regime, hosting protected Majorana modes in the presence of $s$-wave superconductivity and Zeeman coupling. Interestingly, when the system is weakly tunnel coupled to helical edge state reservoirs, a particular transport signature is associated with the presence of a non-Abelian Majorana zero mode.

Graphene is favorable to realize topological nontrivial state by introducing spin-orbit coupling and exchange field through the proximity effect. The recent discovery of van der Waals magnets allows great proximitized modulation on graphene by building atomically clean heterostructures. Here, we realize anomalous Hall effect in graphene coupling with a layered magnetic material, ${\mathrm{Fe}}_x{\mathrm{Sn}}_{1-x}{\mathrm{S}}_2$. The anomalous Hall resistance and the carrier density show a nonmonotonous dependence on the back-gate voltage, indicating emergence of band-structure transformation. Furthermore, low-field quantum interference measurement shows the enhancement of spin-orbit coupling (SOC) in the heterostructure. Our findings confirm that graphene coupling to ${\mathrm{Fe}}_x{\mathrm{Sn}}_{1-x}{\mathrm{S}}_2$ is an ideal platform that is likely to introduce a strong exchange field and SOC simultaneously, which has outstanding potential in realizing topological nontrivial states and spintronics.

We theoretically study the quench dynamics of the local magnetization in a hybrid Majorana-wire–quantum-dot system coupled to external leads. In order to thoroughly understand the origin of the dot magnetization dynamics, we consider either normal metal or ferromagnetic electrodes. In the first case, the magnetization arises exclusively from the proximity to the topological superconductor hosting Majorana zero-energy modes and the associated development of an induced exchange field. We predict a nonmonotonic dependence of the dot's magnetization in the odd-occupation regime and show that the dynamics is governed by the magnitude of the coupling to Majorana wire. However, when the system is coupled to ferromagnetic leads, the ferromagnet and Majorana contributions to the effective exchange field are competing with each other and reveal a nontrivial dynamical behavior. As a result, the time-dependent magnetization can undergo multiple sign changes preceding the relaxation to a new thermal value. We also identify the transport regime, where fine tuning of the coupling to Majorana wire within a narrow range allows one to manipulate the magnetic state of the system. The effect of spin polarization of the leads and influence of the finite overlap between the Majorana edge modes are also examined. Moreover, we analyze the quench in the energy of the quantum dot orbital level and demonstrate that the rather straightforward charge dynamics can disguise nontrivial time evolution of the magnetization. Finally, we compare predicted dynamics with results obtained for quantum dot coupled to spin-polarized fermionic bound state instead of Majorana zero-energy mode.

Contrary to the expected detrimental influence on superconductivity when applying a magnetic field, we predict that the abrupt onset of such a field can temporarily strongly enhance the superconducting order parameter. Specifically, we find that the supercurrent in a Josephson junction with a normal metal weak link can increase more than twentyfold in this way. The effect can be understood from the interplay between the energy dependence of Andreev reflection and the abrupt spin-dependent shift in the distribution functions for excitations in the system. The duration of the increase depends on the inelastic scattering rate in the system and is estimated to be in the range of nanoseconds. We demonstrate this by developing a method which solves the Usadel equation for an arbitrary time dependence. This enables the study of ultrafast time-dependent physics in heterostructures combining superconductors with different types of materials.

Stable excitons in semiconductor monolayers such as transition-metal dichalcogenides (TMDCs) enable and motivate fundamental research as well as the development of room-temperature optoelectronics applications. The newly discovered layered magnetic materials present a unique opportunity to integrate optical functionalities with magnetism. We predict that a large class of antiferromagnetic semiconducting monolayers of the $M\mathrm{P}X{}_3$ family exhibit giant excitonic binding energies, making them suitable platforms for magneto-optical investigations and optospintronics applications. Indeed, our investigations, based on first-principles methods combined with an effective-model Bethe-Salpeter solver, show that excitons in bare Neel-${\mathrm{MnPS}}_3$ are bound by more than 1 eV, which is twice the excitonic energies in TMDCs. In addition, the antiferromagnetic ordering of monolayer samples can be inferred indirectly using different polarization of light.