The realization of insulating ferromagnetic states in topological insulator (TI) systems, with sufficiently high Curie temperatures ($T_C$) and large magnetically induced gaps, has been the key bottleneck towards the realization of the quantum anomalous Hall effect (QAHE). Despite the limited reports on $3d$ or $4f$ transition-metal (TM)-doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, there remains a lack of systematic studies on $4d$ TMs, which may be potential candidates since the atomic sizes of $4d$ TMs and that of Bi are similar. Here, we report a theoretical work that probes the magnetic behaviors of the $4d$ TM-doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ system. We discovered that among the $4d$ TMs, Nb and Mo can create magnetic moments of 1.76 and $2.96\mu \mathrm{B}$ in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, respectively. While Mo yields a stable gapless antiferromagnetic ground state, Nb favors a strong ferromagnetic order, with the magnetic coupling strength ($T_C$) $\sim 6$ times of that induced by the traditional Cr impurity. Yet, we found that Nb is still unfavorable to support the QAH state in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ because of the reduced correlation in the $t_{2g}$ band that gives a gapless character. This rationale is not only successful in interpreting why Nb, the strongest candidate among $4d$ TMs for achieving ferromagnetism in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, actually cannot lead to QAHE in the $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ system even with the assistance of codoping but also is particularly important to fully understand the mechanism of acquisition of insulating ferromagnetic states inside TI. On the other hand, we discovered that Mo-doped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ favors strong antiferromagnetic states and may lead to superconducting states.

The magnetocaloric properties of single crystalline ${\mathrm{GdFeO}}_3$ have been investigated in the temperature range 2–36 K by magnetization and heat-capacity measurements. Remarkably large and reversible magnetic entropy change, $\mathrm{\Delta }{S}_{\mathrm{m}}=-52.5$ J/kg K, has been observed for a field change of 0–9 T. The adiabatic temperature change, $\mathrm{\Delta }{T}_{\mathrm{ad}}$, is also found to be very large, 22 K, slightly above the antiferromagnetic ordering temperature ($T_N^{\text{Gd}}=2.5$ K) of the ${\mathrm{Gd}}^{3+}$ moment, for a field change of 0–8 T. These magnetocaloric parameters remain large down to the lowest temperature measured and are significantly larger than that reported for the other members of rare-earth ($R$) orthoferrites ($R{\mathrm{FeO}}_3$) and several potential magnetic refrigerants in the same temperature range. Both $\mathrm{\Delta }{S}_{\mathrm{m}}$ and $\mathrm{\Delta }{T}_{\mathrm{ad}}$ are also quite large for a small field change. The large values of magnetocaloric parameters suggest that ${\mathrm{GdFeO}}_3$ could be considered as a potential refrigerant in low-temperature magnetic refrigeration technology, such as liquefaction of hydrogen in the fuel industry. Moreover, ${\mathrm{GdFeO}}_3$ has very low electrical conductivity and exhibits no thermal and field hysteresis in magnetization, fulfilling the necessary conditions for a good magnetic refrigerant.

Huygens sources are elements that scatter light in the forward direction as used in the Huygens-Fresnel principle. They have remained fictitious until recently when experimental systems have been fabricated. In this Rapid Communication, we propose isotropic meta-atoms that act as Huygens sources. Using clusters of plasmonic or dielectric colloidal particles, Huygens dipoles that resonate at visible frequencies can be achieved with scattering cross sections as high as five times the geometric cross section of the particle surpassing anything achievable with a hypothetical simple spherical particle. Examples are given that predict extremely broadband scattering in the forward direction over a 1000 nm wavelength range at optical frequencies. These systems are important to the fields of nanoantennas, metamaterials, and wave physics in general as well as any application that requires local control over the radiation properties of a system as in solar cells or biosensing.

Many-body systems with multiple emergent time scales arise in various contexts, including classical critical systems, correlated quantum materials, and ultracold atoms. We investigate such nontrivial quantum dynamics in a different setting: a spin-1 bilinear-biquadratic chain. It has a solvable entangled ground state, but a gapless excitation spectrum that is poorly understood. By using large-scale density matrix renormalization group simulations, we find that the lowest excitations have a dynamical exponent $z$ that varies from 2 to 3.2 as we vary a coupling in the Hamiltonian. We find an additional gapless mode with a continuously varying exponent $2\le z<2.7$, which establishes the presence of multiple dynamics. In order to explain these striking properties, we construct a continuum wave function for the ground state, which correctly describes the correlations and entanglement properties. We also give a continuum parent Hamiltonian, but show that additional ingredients are needed to capture the excitations of the chain. By using an exact mapping to the nonequilibrium dynamics of a classical spin chain, we find that the large dynamical exponent is due to subdiffusive spin motion. Finally, we discuss the connections to other spin chains and to a family of quantum critical models in two dimensions.

In this work, we present structural, electronic, magnetic, mechanical, and transport properties of equiatomic quaternary Heusler alloy, CoRhMnGe, using theoretical and experimental techniques. A detailed structural analysis is performed using x-ray diffraction and extended x-ray absorption fine structure spectroscopy. The alloy is found to crystallize in $Y$-type structure having space group $F\overline{4}3m$ (no. 216). The ab initio simulation predicts half-metallic ferromagnetic characteristics leading to large spin polarization. The calculated magnetization is found to be in fair agreement with experiment as well as those predicted by the Slater-Pauling rule, which is a prerequisite for half-metallicity. The magnetic transition temperature $\left(T_C\right)$ is found to be $\sim 760$ K. Measured electrical resistivity in the temperature range 2–400 K also gives an indication of half-metallic behavior. Effect of hydrostatic pressure on electronic structure, magnetic, and mechanical properties are investigated in detail. The alloy is found to preserve half-metallic characteristics up to 30.27 GPa, beyond which it transits to metallic phase. No magnetic phase transition is found to occur in the whole range of pressure. The system also satisfies the Born-Huang criteria for mechanical stability up to a limited range of pressure. All these properties make the CoRhMnGe alloy promising for spintronics devices.

Dynamic scaling analyses of linear and nonlinear ac susceptibilities in a model magnet of the long-rang Ruderman-Kittel-Kasuya-Yosida (RKKY) Ising spin glass (SG) ${\mathrm{Dy}}_{0.103}{\mathrm{Y}}_{0.897}{\mathrm{Ru}}_2{\mathrm{Si}}_2$ were examined. The obtained set of critical exponents, $\gamma \sim$ 1, $\beta \sim$ 1, $\delta \sim$ 2, and $z\nu \sim$ 3.4, indicates the SG phase transition belongs to a universality class different from that of either the canonical (Heisenberg) or short-range Ising SGs. The analyses also reveal a finite-temperature SG transition with the same critical exponents under a magnetic field and the phase-transition line $T_{\text{g}}\left(H\right)$ described by $T_{\text{g}}\left(H\right)=T_{\text{g}}\left(0\right)(1-A{H}^{2/\varphi })$, with $\varphi \sim$ 2. The crossover exponent $\varphi$ obeys the scaling relation $\varphi =\gamma +\beta$ within the margin of errors. These results strongly suggest spontaneous replica-symmetry breaking (RSB) with a non- or marginal-mean-field universality class in the long-range RKKY Ising SG.

We demonstrate spin pumping and inverse spin Hall effect (ISHE) in an all oxide heterostructure composed of two oxides with different respective crystal structures. ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG) is used as a spin source, being a perfect candidate because of its low damping and insulating behavior. ${\mathrm{SrRuO}}_3$ (SRO), a perovskite oxide with metallic conductivity, serves as a spin sink. Spin pumping is shown by the increase of the damping in ferromagnetic resonance for the YIG in the presence of an SRO top layer and by the inverse spin Hall effect appearing in the SRO. Transmission electron microscopy shows that the growth of YIG is fully epitaxial, while the SRO grows in islands of several polycrystaline phases, a fact which strongly influences the measured ISHE. The magnetization of the YIG remains unaffected by the SRO layer while the SRO no longer shows the low temperature ferromagnetism typically observed in monocrystalline layers.

We studied timing jitter in the appearance of photon counts in meandering nanowires with different fractional amount of bends. Intrinsic timing jitter, which is the probability density function of the random time delay between photon absorption in current-carrying superconducting nanowire and appearance of the normal domain, reveals two different underlying physical mechanisms. In the deterministic regime, which is realized at large photon energies and large currents, jitter is controlled by position-dependent detection threshold in straight parts of meanders. It decreases with the increase in the current. At small photon energies, jitter increases and its current dependence disappears. In this probabilistic regime jitter is controlled by Poisson process in that magnetic vortices jump randomly across the wire in areas adjacent to the bends.

We investigated the electronic structure of the vacancy-ordered $4d$-transition-metal monoxide $\mathrm{NbO}\left({\mathrm{Nb}}_3{\mathrm{O}}_3\right)$ using angle-integrated soft- and hard-x-ray photoelectron spectroscopies as well as ultraviolet angle-resolved photoelectron spectroscopy. We found that density-functional-based band-structure calculations can describe the spectral features accurately provided that self-interaction effects are taken into account. In the angle-resolved spectra we were able to identify the so-called ‘vacancy’ band that characterizes the ordering of the vacancies. This together with the band-structure results indicate the important role of the very large inter-Nb-$4d$ hybridization for the formation of the ordered vacancies and the high thermal stability of the ordered structure of niobium monoxide.

By means of first-principles calculations, we investigate the thermal properties of silica as it evolves, under hydrostatic compression, from a stishovite phase into a ${\mathrm{CaCl}}_2$-type structure. We compute the thermal conductivity tensor by solving the linearized Boltzmann transport equation iteratively in a wide temperature range, using for this the pressure-dependent harmonic and anharmonic interatomic couplings obtained from first principles. Most remarkably, we find that, at low temperatures, ${\mathrm{SiO}}_2$ displays a large peak in the in-plane thermal conductivity and a highly anisotropic behavior close to the structural transformation. We trace back the origin of these features by analyzing the phonon contributions to the conductivity. We discuss the implications of our results in the general context of continuous structural transformations in solids, as well as the potential geological interest of our results for silica.

Moiré patterns on two-dimensional van der Waals heterostructure can give rise to unique electronic and transport properties. In this work we report a theoretical investigation of Moiré patterns on twisted bilayer black phosphorus (tbBP). It is found that the Moiré pattern has extraordinary effects and leads to significant asymmetry with respect to transport direction and carrier type. The high-symmetry local stacking configurations in the Moiré pattern act as impurities with sizes at the Moiré length scale, and these “Moiré impurities” induce flatbands and localized states in tbBPs. Because both the conduction band minimum and valence band maximum are dominated by these localized states, the deformation potential limited carrier mobility is significantly affected: the electron mobility of tbBPs reduces by almost 20-fold when twisting from zero angle $\left(\sim 2560{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}\right)$ to just $1.8^{\circ }$ $\left(\sim 131{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}\right)$. The microscopic physics behind these effects are revealed by the real-space wave functions.

We report on helicity-dependent terahertz emissions that originate from the helicity-dependent photocurrents in topological insulator $\mathrm{S}{\mathrm{b}}_2\mathrm{T}{\mathrm{e}}_3$ thin films due to ultrafast optical excitation. The polarity of the emitted terahertz radiation is controlled by both the incident angle and the helicity of optical pulses. Using an unprecedented decomposition-recombination procedure in the time domain, the signals of the Dirac fermions are fully separated from bulk contributions. These results provide insights into the optical coupling of topological surface states and open up opportunities for applying helicity-dependent terahertz emission spectroscopy in spintronics.

We report on Hybrid Monte Carlo simulations at finite spin density of the $\pi$-band electrons in monolayer graphene with realistic interelectron interactions. Unlike simulations at finite charge-carrier density, these are not affected by a fermion-sign problem. Our results are in qualitative agreement with an interaction-induced warping of the Fermi contours and a reduction of the bandwidth as observed in angle-resolved photoemission spectroscopy experiments on charge-doped graphene systems. Furthermore, we find evidence that the neck-disrupting Lifshitz transition, which occurs when the Fermi level traverses the van Hove singularity (VHS), becomes a true quantum phase transition due to interactions. This is in line with an instability of the VHS toward the formation of ordered electronic phases, which has been predicted by a variety of different theoretical approaches.

The Andreev transport properties of double quantum dot based Cooper pair splitters with one superconducting and two normal leads are studied theoretically in the Kondo regime. The influence of the superconducting pairing correlations on the local density of states, Andreev transmission coefficient, and Cooper pair splitting efficiency is thoroughly analyzed. It is shown that finite superconducting pairing potential quickly suppresses the $\text{SU}\left(2\right)$ Kondo effect, which can however reemerge for relatively large values of coupling to superconductor. In the $\text{SU}\left(4\right)$ Kondo regime, a crossover from the $\text{SU}\left(4\right)$ to the $\text{SU}\left(2\right)$ Kondo state is found as the coupling to superconductor is enhanced. The analysis is performed by means of the density-matrix numerical renormalization group method.

Low-dimensional half-metallic (HM) systems are invaluable for future spintronics. Yet a definitive experimental demonstration of HM characteristic in two-dimensional (2D) materials remains elusive. Here, we reveal that in recently synthesized graphene/hexagonal boron nitride ($\mathrm{G}\text{/}h\mathrm{BN}$) lateral heterojunctions, pronounced HM can be achieved by applying an in-plane bending. We demonstrate with generalized Bloch theorem that bending has strong influence on interfacial spin states, mimicking the Zeeman effect, which consequently leads to the desired HM phase with a sizable HM gap and excellent magnetic stability. Given recent experimental advances in fabrication of $\mathrm{G}\text{/}h\mathrm{BN}$ heterostructures, this strain-driven HM phase may be practically accessible. The generalized Bloch theorem coupled with self-consistent charge density-functional tight binding is useful to model 2D structures under fundamental deformations, thus may boost the study of strain tunable electronic property of low-dimensional materials with inhomogeneous strain patterns.

Intriguing topological phases may appear in both insulating and semimetallic states. Topological insulators exhibit topologically nontrivial band inversion, while topological Dirac/Weyl semimetals show “relativistic” linear band crossings. Here, we report an unusual topological state of ${\mathrm{Pt}}_3\mathrm{Sn}$, where the two topological features appear simultaneously. Based on first-principles calculations, we show that ${\mathrm{Pt}}_3\mathrm{Sn}$ is a three-dimensional weak topological semimetal with topologically nontrivial band inversion between the valence and conduction bands, where the band structure also possesses type-II Dirac points at the boundary of two electron pockets. The formation of the Dirac points can be understood in terms of the representations of relevant symmetry groups and the compatibility relations. The topological surface states appear in accordance with the nontrivial bulk band topology. The unique coexistence of the two distinct topological features in ${\mathrm{Pt}}_3\mathrm{Sn}$ enlarges the material scope in topological physics, and is potentially useful for spintronics.

We report on spin-dependent phase shift of the electron wave reflected at the film/vacuum and film/substrate interfaces for electrons with energies above the vacuum level. The phase shift is determined within the Fabry-Pérot interferometer model of an absorbing medium, which takes into account the band structure of film and substrate. The results obtained for the model system Fe/W(110) indicate that the phase shift has nonzero values in the entire investigated energy range (up to 20 eV), including the energies of the allowed states in the substrate. This is in contrast to the available models which give zero phase shift for those energy ranges. It is also shown that the spin dependence is much stronger for the Fe/vacuum than for the Fe/W interface.