The bulk magnetic measurements (dc magnetization, specific heat, ac-susceptibility) on ${\mathrm{Ho}}_3\mathrm{Co}$ suggest that its magnetic structure changes continuously below the Néel temperature $T_N=21$ K until another antiferromagnetic (AFM) transition takes place at $T_t=9$ K. In the temperature range $3\text{K}\le T<T_N$, the magnetic structure is described by an incommensurate propagation vector $\mathbf{k}=(\alpha ,0,0)$ with $0.1605<\alpha <0.1585$ and a commensurate component ${\mathbf{k}}_0=(0,0,0)$. For $T_t<T\le T_N$, the spin arrangement consists of fanlike and cycloidal modulations for the Ho and Co sublattices, respectively. Below $T_t$, the presence of the 3k and 5k harmonics results in a highly complex anharmonic ground state associated with the squaring up of the modulation. The evolution of the magnetic modulations in ${\mathrm{Ho}}_3\mathrm{Co}$ within $3\text{K}\le T<T_N$ is explained by two active irreducible representations under the $Pn{m}^{\prime }a\left(\alpha 00\right)000$ magnetic superspace group, which allows the change of the structure without further symmetry breaking. A significant moment contribution is detected in Co. Experiments reveal the coexistence of glassy states, probably related to the complex noncollinear AFM structure at low temperature. In addition, externally applied pressure shifts both transitions at $T_N$ and $T_t$ to higher temperatures. Eventually, high pressure strengthens the AFM interactions at low temperature while the weak glassy states persist up to 1.1 GPa.

The stimulated Raman adiabatic passage (STIRAP) is applied to a system coupled to a bath made of fully interacting two-level systems whose dynamics is studied by exploiting the generalized cluster correlation expansion (gCCE) theory. We specialize our analysis to a negatively charged silicon vacancy (${\mathrm{V}}_{\mathrm{Si}}^{-1}$) in nonpurified 4H-SiC to assess the possibility of transferring population between two states of the ground manifold, also taking into account the interaction with a spherical nuclear-spin bath formed by nuclei of ${}^{29}\mathrm{Si}$ and ${}^{13}\mathrm{C}$. For this system, it is demonstrated that the presence of a small- or medium-sized bath has no effect on the protocol, finding in particular a set of parameter values for an efficient STIRAP process.

The compound ${\mathrm{Ba}}_3\mathrm{Ho}{\mathrm{Ru}}_2{\mathrm{O}}_9$ magnetically orders at 50 K ($T_{\mathrm{N}1}$), followed by another complex magnetic ordering at 10.2 K ($T_{\mathrm{N}2}$). The second magnetic phase transition was characterized by the coexistence of two competing magnetic ground states associated with two different magnetic wave vectors ($K_1=0.500$ and $K_2=0.250.250$). The multiferroicity and magnetoelectric coupling were predicted below $T_{\mathrm{N}2}$ in these $4d$-based materials. Here, we have discussed the origin of spin-driven ferroelectricity, which is not known yet, and the nature of magnetoelectric domains. We have investigated the compound through time-of-flight neutron diffraction, synchrotron x-ray diffraction (XRD), ac susceptibility, frequency-dependent complex dielectric spectroscopy, and dc magnetization under external pressure. We have demonstrated that the noncollinear structure involving two different magnetic ions, Ru ($4d$) and Ho ($4f$), breaks the spatial inversion symmetry via inverse Dzyaloshinskii-Moriya (DM) interaction through strong 4d-4f magnetic correlation, which shifts the oxygen atoms and results in nonzero polarization. Such an observation of inverse DM interaction from two different magnetic ions which cause ferroelectricity is rarely observed. The stronger spin-orbit coupling of $4d$ orbital might play a major role in creating DM interaction of noncollinear spins. We have systematically studied the spin and dipolar dynamics, which exhibit intriguing behavior with shorter coherence lengths of second magnetic phase associated with the $k_2$ wave vector. The results manifest the development of finite-size magnetoelectric domains instead of true long-range ordering, which justifies the experimentally obtained low value of ferroelectric polarization. The lattice parameters and volume show a sharp anomaly at $T_{\mathrm{N}2}$ obtained by analyzing the temperature-dependence XRD, which is consistent with the ferroelectric transition, predicting a noncentrosymmetric space group, $P\overline{6}2c$, for this compound. Furthermore, we have investigated the effect of external pressure on this complex magnetism. The result reveals an enhancement of ordering temperature by the application of external pressure (∼1.6 K/GPa). The external pressure might favor stabilizing the magnetic ground state associated with second magnetic phase. Our study shows an unconventional mechanism of spin-driven ferroelectricity involving inverse DM interaction between Ru ($4d$) and Ho ($4f$) magnetic ions due to strong $4d\text{−}4f$ cross coupling.

We have theoretically investigated the spin- and valley-dependent superfluid properties of indirect momentum space dark dipolar excitons in double layers with massive anisotropic tilted semi-Dirac energy bands in the presence of circularly polarized irradiation. An external vertical electric field is also applied to the structure and is responsible for tilting and gap opening of the energy bands. In our numerical calculations, we employed the parameters appropriate for a double layer of ${\mathrm{1T}}^{\prime }$ -${\mathrm{MoS}}_2$. Closed form analytical expressions are presented for the energy spectrum for excitons, their associated wave functions and binding energies. Additionally, we examine the effects which varying the intensity and frequency of circularly polarized irradiation has for $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{MoS}}_2$ on the effective mass of the excitons since it has been demonstrated that the application of an external high-frequency dressing field could tailor the crucial electronic properties including the exciton binding energy as well as the critical temperature for superfluidity. We also calculated the sound velocity in the anisotropic weakly interacting Bose gas of two-component indirect momentum space dark excitons for a double layer of $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{MoS}}_2$. We show that the critical velocity of superfluidity, the spectrum of collective excitations, concentrations of the superfluid and normal component, and mean field critical temperature for superfluidity are anisotropic and formed by a two-component system. The critical temperature for superfluidity is increased when the exciton concentration and interlayer separation are increased. We propose the use of phonon-assisted photoluminescence to experimentally confirm directional superfluidity of indirect momentum space dark excitons in a double layer with massive anisotropic tilted semi-Dirac energy bands.

Topological phases stabilized by crystalline point group symmetry protection are a large class of symmetry-protected topological phases subjected to considerable experimental scrutiny. Here, we show that the canonical three-dimensional (3D) crystalline topological insulator protected by time-reversal symmetry $\mathcal{T}$ and fourfold rotation symmetry ${\mathcal{C}}_4$ individually or the product symmetry ${\mathcal{C}}_4\mathcal{T}$, generically realizes finite-size crystalline topological phases in thin film geometry [a quasi-($3\text{−}1$)-dimensional, or q($3\text{−}1$)D, geometry]: response signatures of the 3D bulk topology coexist with topologically protected, quasi-($3\text{−}2$)D, and quasi-($3\text{−}3$)D boundary modes within the energy gap resulting from strong hybridization of the Dirac cone surface states of the underlying 3D crystalline topological phase. Importantly, we find qualitative distinctions between these gapless boundary modes and those of strictly 2D crystalline topological states with the same symmetry protection and develop a low-energy, analytical theory of the finite-size topological magnetoelectric response.

Lead halide perovskite nanocrystals (NCs) in a glass matrix combine excellent optical properties and stability against environment. The spectral and temporal characteristics of photoluminescence from ${\mathrm{CsPbBr}}_3$ and CsPb(Cl,Br)${}_3$ nanocrystals in a fluorophosphate glass matrix are measured in a temperature range from 6 to 270 K in order to reveal factors that determine their quantum yield and recombination dynamics. At low temperatures, the recombination dynamics is characterized by three decay components with timescales on the order of 1 ns, 10 ns, and 1 $\mu \mathrm{s}$. The relative contributions of the corresponding processes and their characteristic times are strongly temperature dependent. The emission intensity decreases with growing temperature. This effect is stronger in smaller NCs, which highlights the role of surface states. These experimental results are discussed on the basis of a model taking into account the NC energy structure and the presence of electron and hole surface trap states. The photoluminescence dynamics at low temperatures is dominated by charge-carrier radiative recombination and relaxation to shallow traps. At temperatures exceeding 100 K, the dynamics is affected by carrier activation to the excited states.

The phonon thermal conductance of subnanometric vacuum gaps between two in-plane nanoribbons of two-dimensional materials (graphene and silicene) is analyzed using the atomistic Green's function method and by employing the Tersoff and Lennard-Jones potentials for describing the interatomic interactions. It is found that the phonon conductance decays exponentially with the size of the gap. Three exponential regimes have been identified. In the regime where the Lennard-Jones (L-J) potential is driven by the repulsive interatomic forces, caused by the overlap of electronic orbits, there is a sharp exponential decay in conductance as the gap increases ($e^{-10.0d}$ for graphene). When both the repulsive and the attractive (van der Waals) interatomic forces contribute to the L-J potential, the decay rate of the conductance significantly reduces to $e^{-2.0d}$ for graphene and $e^{-2.5d}$ for silicene. In the regime where attractive van der Waals forces dominate the L-J potential, phonon conductance has the slowest exponential decay as $e^{-1.3d}$ for both silicene and graphene. It is also found that the contribution from the optical phonons to the conductance is non-negligible only for very small gaps between graphene nanoribbons ($d<1.6\AA$). The phonon conductance of the gap is shown to vary with the width of the nanoribbon very modestly, such that the thermal conductivity of the gap linearly increases with the nanoribbon widths. The results of this study are of significance for a fundamental understanding of heat transfer in the extreme near-field regime and for predicting the effect of interfaces and defects on heat transfer.

We investigate the properties of a few interacting bosons in a Creutz ladder, which has become a standard model for topological systems, and which can be realized in experiments with cold atoms in optical lattices. At the single-particle level, this system may exhibit a completely flat energy landscape with nontrivial topological properties. In this scenario, we identify topological two-body edge states resulting from the bonding of single-particle edge and flat-band states. We also explore the formation of two- and three-body bound states in the strongly interacting limit, and we show how these quasiparticles can be engineered to replicate the flat-band and topological features of the original single-particle model. Furthermore, we show that in this geometry perfect Aharonov-Bohm caging of two-body bound states may occur for arbitrary interaction strengths, and we provide numerical evidence that the main features of this effect are preserved in an interacting many-body scenario resulting in many-body Aharonov-Bohm caging.

Nonlinear spin polarization occurring in the second order of driving electric current is the dominant source of nonequilibrium magnetization in centrosymmetric or weakly noncentrosymmetric nonmagnetic materials, and induces nonlinear spin-orbit torque in magnets. Up to now, only the intrinsic mechanism based on anomalous spin polarizability dipole, which is the spin counterpart of Berry curvature dipole, has been studied, while disorder-induced mechanisms are still missing. Here, we derive these contributions, which include not only the anomalous distribution function due to skew scattering and coordinate shift, but also interband coherence effects given by disorder-induced spin shift and electric-field-induced anomalous scattering amplitude. We demonstrate these terms and show their importance in a minimal model. A scaling law for nonlinear current-induced spin polarization is constructed, which may help analyze experimental data in the future.

We present a first-principles methodology, within the context of linear-response theory, that greatly facilitates the perturbative study of physical properties of metallic crystals. Our approach builds on ensemble density functional theory as formulated by Marzari, Vanderbilt, and Payne [Phys. Rev. Lett. 79, 1337 (1997)] to write the adiabatic second-order energy as an unconstrained variational functional of both the wave functions and their occupancies. Thereby, it enables the application of standard tools of density functional perturbation theory (most notably, the $2n+1$ theorem) in metals, opening the way to an efficient and accurate calculation of their nonlinear and spatially dispersive responses. We apply our methodology to phonons and strain gradients and demonstrate the accuracy of our implementation by computing the spatial dispersion coefficients of zone-center optical phonons and the flexoelectric force-response tensor of selected metal structures.

There has been an increasing research focus on quantum algorithms for condensed matter systems recently, with a particular emphasis on calculating energy band structures. Here, we propose a quantum algorithm, the powered full quantum eigensolver (P-FQE), by using the power of operators of the full quantum eigensolver. This leads to an exponential increase in the success probability of measuring the target state in certain circumstances where the number of generating elements involved in the power of operators exhibits a log polynomial dependence on the number of orbitals. Furthermore, we conduct numerical calculations for the energy spectrum of the Fermi-Hubbard model and band-structure determination of the twisted double-layer graphene. We experimentally demonstrate the feasibility and robustness of the P-FQE algorithm using superconducting quantum computers for graphene and Weyl semimetal. One significant advantage of our algorithm is its ability to reduce the requirements of extremely high-performance hardware, making it more suitable for energy spectra determination on noisy intermediate-scale quantum devices.

Studying heat transport in nonequilibrium dissipative systems is key to energy modulation. Driving thermal devices have attracted much attention from the perspective of active heat control. In this article, we explore the driving nonequilibrium spin-boson model with noncommutative coupling, which is described by a two-level system coupled to two bosonic baths in the $(xcos\theta +zsin\theta )$ direction and $z$ direction (dephasing channel), respectively. Based on the noncommutativity, we propose a model with quantum thermal chokelike behavior, where the currents flowing into the two thermal baths display distinctly different oscillation amplitudes. This phenomenon leads to a transformation of a largely oscillating current from one bath to nearly nonoscillating one at the other bath. To quantify the asymmetry in the oscillation amplitudes of the currents flowing into the two baths, we introduce a thermal choke ratio. This ratio exhibits a monotonic decrease as the noncommutativity parameter vanishes. We further calculate the time-independent currents via the Floquet theory using the secular approximation and find that they coincide with the time averages of the above oscillating currents. Moreover, we investigate the dependence of these time-independent mean currents on various parameters. Our findings will advance the understanding of externally driven thermal analogs of electronic devices, and contribute to energy transport control in nanoscale systems.

We consider a generation of two-particle quantum states in the process of spontaneous parametric down-conversion of light by a dielectric nanoparticle with a bulk ${\chi }^{\left(2\right)}$ response. As a particular example, we study the generation of surface plasmon polariton pairs with a $\mathrm{GaAs}$ nanoparticle located at the silver-air interface. We demonstrate theoretically that $\mathrm{N}00\mathrm{N}$ states of surface plasmon polariton pairs with $N=2$ can be obtained for certain excitation geometries. The effect can be physically interpreted as the result of a quantum interference between the pairs of induced sources, each emitting either signal or idler plasmons. We then relate the resulting $\mathrm{N}00\mathrm{N}$ pattern to the general symmetry properties of a dyadic Green's function of a dipole emitter exciting surface waves. It renders the considered effect as a general way towards the robust generation of $\mathrm{N}00\mathrm{N}$ states of surface waves using spontaneous parametric down-conversion in ${\chi }^{\left(2\right)}$ nanoparticles.

Bound states in the continuum (BICs) and coalesced states at exceptional points (EPs) represent two anomalous resonant states in non-Hermitian systems, attracting extensive interest due to their theoretical implications and practical applications in fields such as sensing, lasing, and nonlinear light-matter interactions. Understanding the relationship between BICs and EPs is therefore of central importance. Recent studies have demonstrated their coexistence in various optical systems, including gratings, photonic crystal slabs, and individual dielectric inclusions. In this paper, we design a simple coupled-split-ring metasurface to demonstrate the connections between BICs and EPs, providing a direct and intuitive mapping of the theoretical predictions from the coupled-mode theory. Specifically, a pair of EPs with opposite chirality is partitioned by a BIC line in the parameter space, indicating that during the process of adiabatic evolution between EPs, one of the modes transitions through a BIC state. Our findings unveil a more comprehensive picture of mode coupling in metasurfaces, paving the way for innovative non-Hermitian system designs and potentially revolutionizing the integration of BICs and EPs within a unified platform for advanced technological applications.